Patent Publication Number: US-6210648-B1

Title: Method for processing refractory auriferous sulfide ores involving preparation of a sulfide concentrate

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application No. 08/735,783, filed Oct. 23, 1996 (now U.S. Pat. No. 5,837,210, issued Nov. 17, 1998), which is a continuation-in-part of U.S. patent application No. 08/423,839, filed Apr. 18, 1995 (now U.S. Pat. No. 5,653,945, issued Aug. 5, 1997. 
    
    
     FIELD OF THE INVENTION 
     The present invention involves a method for processing gold-bearing refractory sulfide ores, including refractory auriferous sulfides, to facilitate recovery of gold. In particular, the present invention involves flotation processing of refractory sulfide ores in a manner that reduces problems associated with conventional flotation of refractory auriferous sulfides in the ore to produce an ore concentrate that is concentrated in the refractory auriferous sulfides and, accordingly, also in the gold within the refractory auriferous sulfides. 
     BACKGROUND OF THE INVENTION 
     Significant amounts of gold occur in ores that are often referred to as refractory sulfides. The gold in these ores is often referred to as being refractory because it cannot be recovered by direct cyanidation. That is because the refractory gold is bound in the mineral lattice of the sulfides, and is therefore unavailable for recovery by traditional gold recovery techniques, such as direct cyanidation of the ore. Therefore, these refractory sulfide ores are commonly treated to chemically destroy the sulfide mineral lattice in which the gold is located and to release the gold for dissolution, such as by cyanidation, during subsequent gold recovery operations. 
     One technique for destroying the sulfide mineral lattice is to subject the ore to an oxidative treatment to oxidize sulfide sulfur, thereby releasing the gold for subsequent recovery. One method for oxidatively treating a refractory sulfide ore is pressure oxidation, in which a slurry of the ore is subjected to oxygen gas in an autoclave at elevated temperature and pressure to decompose the sulfide mineral, freeing the gold for subsequent recovery. Other oxidative treating methods include roasting and bio-oxidation of the ore in the presence of air or oxygen gas. 
     Treating whole ores by pressure oxidation or by oxidative roasting is expensive. Part of the expense is due to energy consumed in heating gold-barren gangue material in the whole ore, and especially the energy required to heat water in which the gangue material is slurried in the case of pressure oxidation. Also, process equipment for treating a whole ore must be sized to accommodate the throughput of gangue material, in addition to the throughput of refractroy auriferous sulfides, thereby significantly adding to the cost of process equipment. Moreover, side reactions may occur involving gangue material which can detrimentally affect the oxidative treating or can produce hazardous materials which require special handling. 
     One way to reduce the high energy and process equipment costs associated with oxidative treating of a whole ore, as well as the potential for problems associated with side reactions, would be to remove gangue material from the ore prior to the oxidative treatment. For example, one method that has been used to remove gangue material from refractory sulfide ores is flotation. In flotation, air is bubbled through a slurry of ore particles which have been treated with reagents and the particles of the ore which are less hydrophilic tend to attach to and rise with the air bubbles, thereby permitting separation of the ore into two fractions. Flotation has been used to prepare concentrates of sulfide minerals from refractory sulfide ores in an attempt to concentrate the gold into the flotation concentrate, thereby avoiding the expense of processing the gangue in subsequent oxidative treating. One problem with flotation of refractory sulfide ores, however, is that a significant amount of refractory gold often reports to the wrong flotation fraction, even when a high percentage of sulfides minerals is recovered in the concentrate. 
     There is a significant need for an improved method for processing refractory sulfide ores that avoids the high costs associated with oxidatively treating whole ores without the significant loss of refractory gold associated with concentrating sulfide minerals by conventional flotation. 
     SUMMARY OF THE INVENTION 
     The present invention involves a method for processing refractory sulfide ores to facilitate gold recovery without the burden of pressure oxidizing or roasting a whole ore and without the substantial loss of gold value associated with preparation of an ore concentrate by conventional flotation. With the present invention, it has been found that much of the refractory gold in refractory sulfide ores is contained in sulfide species referred to herein as refractory auriferous sulfides. Although these sulfide species may represent only a small fraction of the sulfides in a refractory sulfide ore, they typically contain a majority of the refractory gold. 
     As used herein, refractory auriferous sulfides are sulfides containing gold that is not amenable to direct cyanidation except by destruction of the sulfide mineral lattice, which is usually accomplished by oxidation of the refractory auriferous sulfide. Typically, these refractory auriferous sulfides are auriferous arseno ferrous sulfides, having a composition according to the formula Fe x As y S z .Au, with x,y and z being in any relative proportions. The gold contained in the refractory auriferous sulfides is in solid solution in the sulfide mineral lattice and is refractory gold in that it is not amenable to recovery by direct cyanidation except by destruction of the sulfide mineral lattice, which is typically accomplished by oxidation of the refractory auriferous sulfides. Such solid solution gold is also sometimes referred to as atomic gold or structural gold. 
     It has been found that the largest occurrences of refractory gold in refractory sulfide ores occur in these refractory auriferous sulfides. Little, if any, refractory gold occurs in other, more classically recognized pure iron-sulfur containing sulfide species, such as pyrite (FeS 2 ), marcasite (FeS 2 ), and pyrrhotite (Fe (1-x) S, with x ranging from 0 to 0.17). 
     This distinction is important because traditional flotation, involving the use of air as a flotation gas, is often effective for recovering a large fraction of the more traditionally recognized iron-containing sulfide species. It has been found with the present invention, however, that the traditional flotation is not effective for flotation of refractory auriferous sulfides, which contain most of the refractory gold. Therefore, it is often possible with traditional flotation to recover a high percentage of sulfide minerals in the concentrate, and at the same time experience a low recovery in the concentrate of refractory gold. This is because an unacceptably large fraction of the refractory auriferous sulfides, which contain most of the refractory gold, reports to the flotation tails. 
     The refractory auriferous sulfides should not be confused with non-refractory sulfides. Gold in non-refractory sulfides occurs as particulate gold that is amenable to direct cyanidation without destruction of the sulfide mineral lattice. This non-refractory, particulate gold may be exposed, to the extent necessary, for direct cyanidation by mechanical processing, such as comminution to a very fine particle size, prior to direct cyanidation. It should be noted, however, that although gold from non-refractory sulfides may be recovered by direct cyanidation, it is not always economic to do so. For example, particulate gold inclusions are often found in copper and other base metal sulfides. The copper in many copper sulfides, however, has varying degrees of solubility by direct cyanidation. Therefore, cyanide consumption becomes prohibitively large for economic gold extraction by direct cyanidation because of the concurrent cyanide dissolution of the copper. 
     It has been found with the present invention that air, which is used as the flotation gas in conventional flotation, detrimentally affects flotation of refractory auriferous sulfides. According to the present invention, significantly enhanced flotation performance may be obtained by maintaining the ore in an environment substantially free of air until a desired final flotation concentrate has been obtained. 
     It is believed that refractory auriferous sulfides are particularly reactive and susceptible to surface oxidation when in the presence of oxygen gas, such as is present in air. It is this reactivity that can cause a significant fraction of the refractory auriferous sulfides to be depressed during conventional flotation in air. 
     By using a flotation gas that is deficient in oxygen gas relative to air, however, the problems associated with the use of air can be reduced. This results in an increased recovery of refractory auriferous sulfides in the concentrate, and correspondingly, an increase in the recovery of refractory gold in the concentrate. It is also believed that the presence of oxygen promotes increased galvanic interaction, which tends to depress the refractory auriferous sulfides during flotation. 
     In one embodiment, the refractory auriferous sulfides in a refractory sulfide ore are maintained in an environment that is substantially free of oxygen beginning with comminution of the ore and ending with recovery of a desired final concentrate that is enriched in the refractory auriferous sulfides. An oxygen deficient gas can be introduced prior to or during comminution to displace any air that may be present in the ore feed and to prevent air from entering during comminution. Oxygen in the air that would otherwise be present during comminution is, thereby, prevented from oxidizing newly exposed surfaces of refractory auriferous sulfides created during comminution. Although comminution in an atmosphere of the oxygen deficient gas is preferred, an alternative to reduce detrimental effects of oxygen is to seal the entire comminution process to prevent air from entering into the process during comminution. With this alternative, only oxygen initially in the feed to comminution will be present, so that damage to refractory auriferous sulfides will be limited. 
     In addition to reducing the levels of oxygen during comminution and flotation, the use of an oxygen deficient gas tends to decrease galvanic interaction, with a corresponding increase in floatability of refractory auriferous sulfides. In one embodiment, galvanic interaction is further reduced by reducing the amount of iron introduced into the system and/or by removing iron from the system. Iron contamination in the system may be reduced by using comminution media made of stainless steel or a chromium or nickel hardened steel, rather than the normal mild steel, and/or by using a nonmetallic liner for comminution equipment. Iron may be removed from the system prior to flotation by magnetic separation. It has been found that reducing galvanic interaction can significantly improve recovery of refractory auriferous sulfides during flotation, especially when flotation is conducted with an oxygen deficient flotation gas. 
     Possible sources of the oxygen deficient gas include by-product gas from an oxygen plant, a dedicated nitrogen plant, combustion exhaust gases, and on-site delivery of compressed or liquefied gases. In one embodiment to reduce the consumption of the oxygen deficient gas, flotation gas is recycled in the flotation operation. 
     When using an oxygen-deficient flotation gas according to the present invention, adjustment of other flotation operating parameters have been found to be unusually important to maximizing flotation performance. In that regard, it has been found that the flotation should be operated at an acidic pH, preferably below about pH 6. Also, use of a lead-containing activator significantly enhances flotation performance, as does the use of deoxygenated water during comminution and flotation. These additional enhancements are particularly important because, as noted, it has been found that the refractory gold is often most associated with the refractory auriferous sulfides, which are the mineralogical/morphological sulfide species that are generally the hardest to float. Therefore, for example, a flotation enhancement that increases sulfide mineral recovery by just one percentage point may increase recovery of refractory gold in the concentrate by a proportionately larger amount. This is because the incremental sulfide mineral particles that tend to float with each enhancement include those most likely to contain significant quantities of refractory gold. Conversely, the sulfide minerals that are easiest to float, such as pyrite, often contain little or no gold. 
     Another embodiment according to the present invention includes a leach of flotation tails to recover non-refractory gold that may remain in the flotation tail. Many ores contain both refractory and non-refractory gold. Most of the non-refractory gold, if contained in non-refractory sulfides, will float during flotation along with the refractory auriferous sulfides. Other non-refractory gold, such as that associated with oxidized mineral species, may report to the flotation tail, from which this non-refractory gold may be recovered by cyanidation of the flotation tail. For some sulfide ores, the non-refractory gold in the flotation tail may be a significant quantity of gold. A major advantage of the present invention is that the flotation tail is relatively clean of sulfide minerals. This is important to effective cyanide leaching of the tail because of the significant loss of cyanide that would occur if significant quantities of sulfide minerals were present in the flotation tails. 
     In a still further embodiment according to the present invention, a significant operational enhancement is obtained by performing a regrind operation intermediate between two flotation stages or on a sand fraction from the first flotation tail, after size separation to remove fines from the sands. This permits a more coarse initial grind to be used for an initial stage of flotation to recover a significant quantity of sulfide species in the ore, including a significant quantity of the refractory auriferous sulfides. The regrind permits additional liberation of sulfide species locked in middling particles. A very fine grind may be required in this regrind to effectively liberate refractory auriferous sulfides that may be locked in middlings, because the refractory auriferous sulfides tend to be fine grained in character. Such staged processing would not be possible with conventional air flotation because of the detrimental effects of oxygen on refractory auriferous sulfides during conventional grinding and flotation. 
     In one embodiment of the present invention, the flotation operation that is conducted substantially in the absence of oxygen gas, is combined with oxidative treating to decompose the refractory auriferous sulfides, freeing refractory gold for possible subsequent dissolution using a gold lixiviant, such as a cyanide. The preferred oxidative treating is pressure oxidation, although another oxidative treatment such as an oxidizing roast or bio-oxidation may be used instead. Such oxidative treating often requires a source of purified oxygen gas, which is often produced by separation from air in an oxygen plant. A by-product gas from such an oxygen plant is deficient in oxygen gas and rich in nitrogen gas. The by-product gas is, therefore, an ideal source of gas for use during comminution and/or flotation of an ore including refractory auriferous sulfides. This by-product gas is normally vented to the atmosphere in current gold processing operations and is, therefore, wasted. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a flow diagram showing one embodiment of the present invention; 
     FIG. 2 is a flow diagram showing another embodiment of the present invention; 
     FIG. 3 is a flow diagram showing yet another embodiment of the present invention; 
     FIG. 4 is a graph of the grade of concentrate recovered from flotation versus grind size Examples 1-6; 
     FIG. 5 is a graph of the grade of tails from flotation versus grind size Examples 1-6; 
     FIG. 6 is a graph of concentrate weight percent recovery from flotation versus grind size for Examples 1-6; 
     FIG. 7 is a graph of gold recovered in concentrate from flotation versus grind size for Examples 1-6; 
     FIG. 8 is a flow diagram for one embodiment of the present invention relating to a pilot plant for Example 7; and 
     FIG. 9 is a graph of gold recovery in concentrate from flotation versus grind size for Example 8; 
     FIG. 10 is a flow diagram of one embodiment of the present invention including acid pretreatment; 
     FIG. 11 is a process flow diagram of one embodiment of the present invention showing recycle of flotation gas; 
     FIG. 12 is a sectional elevation showing features of one embodiment of a flotation apparatus of the present invention; 
     FIG. 13 is a sectional elevation of another embodiment of a flotation apparatus of the present invention; 
     FIG. 14 is a process flow diagram of a comnminution circuit of one embodiment of the present invention; 
     FIG. 15 is a process flow diagram of one embodiment of the present invention including magnetic separation prior to flotation; 
     FIG. 16 is a process diagram of one embodiment of the present invention including a leach of flotation tails and including the use of deoxygenated water; 
     FIG. 17 is a process flow diagram of one method of the present invention having multiple flotation stages with comminution occurring between flotation stages; 
     FIG. 18 is a graph with plots of sulfide sulfur recovery versus pH for Examples 9-28; 
     FIG. 19 is a graph including plots of gold recovery versus pH for Examples 9-28; 
     FIG. 20 is a graph including plots of incremental gold recovery and incremental sulfide sulfur recovery for Examples 9-28; 
     FIG. 21 is a graph including plots of gold recovery versus flotation time for Examples 29-35; 
     FIG. 22 is a graph of gold recovery versus flotation time for Examples 29-35; 
     FIG. 23 is a graph including plots of gold recovery and oxidation-reduction potential versus flotation time for Example 36; 
     FIG. 24 is a graph including plots of gold recovery and oxidation-reduction potential versus flotation time for Example 37; 
     FIG. 25 is a graph including plots of weight recovery versus flotation time for Example 38; 
     FIG. 26 is a graph including plots of gold-recovery versus flotation time for Example 38; 
     FIG. 27 is a graph including plots of sulfide sulfur recovery versus flotation time for Example 38; 
     FIG. 28 is a process flow diagram of one embodiment of the present invention using gas generated in an acid pretreatment step as a flotation gas. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention provides a method for processing a mineral material including refractory auriferous sulfides to facilitate recovery of the gold from the refractory auriferous sulfides. The method involves preparation of a flotation concentrate in a manner that reduces problems associated with conventional flotation. It has, surprisingly, been found that the problems associated with concentrating refractory auriferous sulfides by conventional flotation may be significantly reduced by the use of a flotation gas which comprises a lower volume fraction of oxygen gas than is present in ambient air. Preferably, the flotation gas should be substantially free of oxygen gas. When air is used as a flotation gas, the oxygen gas in the air appears to detrimentally affect the floatability of the refractory auriferous sulfides. This may be due to surface oxidation of refractory auriferous sulfides caused by the presence of the oxygen gas. The surface oxidation would tend to depress the refractory auriferous sulfides during flotation. Furthermore, the detrimental effects of oxygen gas may be further reduced by maintaining the ore in an environment that is substantially free of oxygen gas during comminution, mixing, pumping and all other processing steps until a final flotation concentrate has been obtained. For example, when multiple flotation steps are used, it is desirable to maintain the ore in an environment that is substantially free of oxygen gas between the flotation steps. 
     By reducing the apparently detrimental effects of oxygen gas, it is possible to recover a greater amount of the sulfide mineral species, including refractory auriferous sulfides, in the flotation concentrate. The present invention, therefore, facilitates the recovery of gold from sulfide mineral material which may have previously been discarded as waste, either with the gangue in a flotation tail or as subgrade ore previously believed to be uneconomical for gold recovery. 
     Enhanced concentration, according to the present invention, of sulfide minerals into the flotation concentrate provides a particular advantage with respect to gold recovery from refractory auriferous sulfides. This is because it has been found that refractory gold in a refractory sulfide ore is often predominantly associated with refractory auriferous sulfides, the sulfide mineral mineralogical/morphological species that are most difficult to effectively float. Therefore, the increase in gold recovery in the concentrate with the present invention will often be a substantially greater percentage increase than the percentage increase in recovery of sulfide minerals. 
     One important class of refractory auriferous sulfides are arsenian iron sulfides, including arsenian pyrite, arsenian marcasite and arsenian pyrrhotite. These refractory auriferous sulfides are different in composition from pyrite, marcasite and pyrrhotite, respectively, in that they include arsenic in the mineral lattice in a manner that also permits the inclusion of gold in the mineral lattice in the form of a solid solution. Inclusion of the arsenic provides some amount of irregularity in the mineral lattice relative to the pure iron-sulfide minerals pyrite. The irregularity provides space within the sulfide mineral lattice to accommodate the presence of gold atoms in the sulfide mineral lattice in the form of a solid solution. This irregularity also increases the susceptibility of the refractory auriferous sulfides to oxidation in the presence of air and to galvanic interaction, both of which are detrimental to flotation. The arsenic content in these arsenian iron sulfides is often in a range of from about 0.01 weight percent to about 10.0 weight percent. These arsenian iron sulfides may have a lattice structure that varies from anhedral to euhedral. Typically, the arsenian iron sulfides have a grain size and/or morphology that is fine grained, amorphous, framboidal, microcrystalline, blastic or rimmed. Not all refractory auriferous sulfides are, however, arsenian iron sulfides. For example, arsenic-rich sulfide species, such as those based on orpiment (As 2 S 3 ) and realgar (A 5 S), may include small quantities of iron in the mineral lattice that provide sufficient irregularity to permit the inclusion within the mineral lattice of gold in the form of a solid solution. Furthermore, arsenopyrite (FeAsS) may contain gold in its mineral lattice in a solid solution, and may be a refractory auriferous sulfide. 
     The arsenic-containing, iron-rich, refractory auriferous sulfides (i.e., arsenian iron sulfides) and the iron-containing, arsenic-rich refractory auriferous sulfides are not classically recognized sulfide species because, strictly speaking, they do not have the classic composition of pyrite, marcasite, pyrrhotite, orpiment or realgar, as the case may be. Most of the refractory auriferous sulfides fall into the category of chemical/structural variances from classical sulfide species that have not been systematically characterized based upon their arsenic, iron and gold contents and their respective floatabilities. A few classically recognized pure sulfide minerals, however, may include some quantities of gold in the mineral lattice and may, therefore, also be refractory auriferous sulfides. These include arsenopyrite and variations thereof, which appear to include some room within their mineral lattices to accept meaningful quantifies of gold in a solid solution form. 
     As an example of concentration of gold in difficult-to-float mineralogical/morphological species, a detailed mineralogical characterization is shown in Table 1 of sulfide species found in two refractory sulfide ore samples. One ore sample is from the Lone Tree Mine and the other ore sample is from the Twin Creeks Mine, both in Nevada, U.S.A. As shown in Table 1, the arsenian pyrite species represent a variety of mineralogical/morphological types. A common theme, however, is that the gold content generally tends to increase as the grain size and/or morphologic character becomes finer. Coarse grained pyrite typically contains insignificant levels of gold, whereas fine grained, amorphous and framboidal arsenian pyrite species all contain much higher levels of gold. The arsenian pyrite species shown in Table 1 are arranged in decreasing coarseness of grain size except for orpiment, marcasite and arsenopyrite. It is these arsenian pyrites having finer grain size and/or morphological character that are particularly benefitted by the flotation of the present invention. Also, as shown in Table 1, arsenian marcasite and iron-containing orpiment also include significant quantities of refractory gold. 
     Table 1 shows the arsenian pyrite refractory auriferous sulfide species including morphologies of coarse grained, blastic, medium grained, fine grained/framboidal, amorphous/framboidal, and framboidal. As seen in Table 1, these arsenian pyrite refractory auriferous sulfides (blastic, medium grained, fined grained/framboidal, amorphous/framboidal, and framboidal) have a much higher gold content than the coarse grained pure pyrite, which is substantially free of arsenic, which has an uninterrupted pyrite mineral lattice that does not contain sufficient space for including gold atoms within the mineral lattice. 
     Unlike coarse grained pure pyrite, the arsenian iron sulfide refractory auriferous sulfides all include sufficient quantities of arsenic to interrupt the mineral lattice of the species and permit gold atoms to be included within the mineral lattice in a solid solution form. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Lone Tree 
                 Twin Creeks 
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Sulfide 
                 Au 
                 As 
                 Au 
                 As 
                 Grain 
               
               
                 Morphology 
                 (ppm) 
                 (%) 
                 (ppm) 
                 (%) 
                 Size 
               
               
                   
               
               
                 Coarse Grained Pyrite 
                  0.35 
                 0.18 
                  0.41 
                 &lt;0.001 
                 Coarse 
               
               
                 Coarse Grained Arsenian 
                  4 
                 2.05 
                  3.1 
                 0.12 
                 Coarse 
               
               
                 Pyrite 
               
               
                 Blastic Arsenian Pyrite 
                  25 
                 0.26 
                 NA 1   
                 NA 1   
                 Coarse 
               
               
                 Medium Grained 
                  48 
                 2.7 
                  9 
                 NI 3   
                 Medium 
               
               
                 Arsenian Pyrite 
               
               
                 Fine Grained/Framboidal 
                 103 
                 6.5 
                  58 
                 NI 3   
                 Fine 
               
               
                 Arsenian Pyritec 
                   
                   
                  29 4   
                 1.45 4   
               
               
                 Amorphous/Framboidal 
                 NA 1   
                 NA 1   
                  96 
                 NI 3   
                 Fine to 
               
               
                 Arsenian Pyrite 
                   
                   
                 187 4   
                 1.41 4   
                 very fine 
               
               
                 Framboidal Arsenian 
                 190 2   
                 5.5 
                 271 
                 NI 3   
                 Very fine 
               
               
                 Pyrite 
               
               
                 Arsenian Marcasite 
                  34 
                 1.6 
                  16 
                 NI 3   
                 NA 1   
               
               
                   
                   
                   
                  1.2 4   
                 0.1 4    
               
               
                 Iron-Containing 
                 NA 1   
                 NA 1   
                  55 
                 7.64 5   
                 NA 1   
               
               
                 Orpiment 
               
               
                 Arsenopyrite 
                  20.5 
                 NI 3   
               
               
                   
               
               
                   1 Not applicable.  
               
               
                   2 Average content from 5 high grade samples from fine grained/amorphous material.  
               
               
                   3 NI = No Arsenic information reported.  
               
               
                   4 Second sampling.  
               
               
                   5 Fe %.  
               
            
           
         
       
     
     Finer grain size and finer morphological character of a sulfide mineral renders the sulfide mineral generally more susceptible to the detrimental effects from the presence of oxygen in a flotation system. To obtain a high recovery of gold in a flotation concentrate it is, therefore, extremely important that the flotation be operated in a manner to maximize the flotation of those mineralogical morphologies that are most difficult to float. To illustrate this problem, calculated gold losses are shown in Table 2 in flotation tailings for each one percent equivalent loss of sulfide mineral to the tail for various sulfide species shown in Table 1. As seen in Table 2, if one percent of sulfide material of the Lone Tree ore is lost to the tail, and that one percent is framboidal arsenian pyrite, then the corresponding loss of gold to the tail is over three percent, or a loss of gold that is proportionately more than three times the loss of the refractory auriferous sulfide. As seen for Twin Creeks, the loss of framboidal arsenian pyrite results in a loss of gold that is proportionately more than ten times the loss of the refractory auriferous sulfide. To further illustrate, experience on the Lone Tree, Twin Creeks and other ores indicates gold recoveries in only the 50 to 80 percent range with conventional flotation recoveries of sulfide minerals in the 75 to 95 percent range. For the Twin Creeks ore, 87 to 90 percent flotation gold recovery in the concentrate is not achieved until sulfide sulfur recovery exceeds about 97 percent. By promoting the flotation of the refractory auriferous sulfides, the most difficult-to-float mineralogical/morphological sulfide species , the present invention addresses the need for extremely high sulfide mineral recoveries in the flotation concentrate to obtain acceptable gold recoveries. 
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Gold Loss (%) per 1.0% Loss Iron Sulfide 
               
            
           
           
               
               
               
            
               
                 Sulfide Morphology 
                 Lone Tree 
                 Twin Creeks 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Coarse Grained Pyrite 
                 0.03 
                 0.08 
               
               
                 Blastic Arsenian Pyrite 
                 0.41 
                 NA (1)   
               
               
                 Medium Grained Arsenian 
                 0.80 
                 0.36 
               
               
                 Pyrite 
               
               
                 Fine Grained/Amorphous 
                 1.71 
                 2.29 
               
               
                 Arsenian Pyrite 
               
               
                 Amorphous/Framboidal 
                 NA (1)   
                 3.79 
               
               
                 Arsenian Pyrite 
               
               
                 Framboidal Arsenian Pyrite 
                 3.15 (2)   
                 10.7  
               
               
                 Arsenian Marcasite 
                 0.56 
                 0.63 
               
               
                 Iron-Containing Orpiment 
                 NA 
                 1.11 
               
               
                   
               
               
                   (1) Not applicable.  
               
               
                   (2) Average content from 5 high grade samples from fine grained/amorphous material  
               
            
           
         
       
     
     One embodiment in accordance with the present invention is shown in FIG.  1 . With reference to FIG. 1, a gold-bearing mineral material feed  102  is provided for processing. The mineral material feed  102  may be any gold-bearing material comprising one or more refractory auriferous sulfides, with which refractory gold is predominantly associated, and from which the refractory gold is difficult to recover because it is not amenable to direct cyanidation. The sulfide species in the mineral material feed  102  could also include one or more nonrefractory sulfide species such as pyrite, marcasite, and pyrrhotite, in addition to the refractory auriferous sulfides, as discussed above. The mineral material feed  102  is typically a whole ore, but may be a residue from other processing or a previously discarded tail. 
     The mineral material feed  102  is subjected to comminution  104  to obtain a particulate mineral material  106  having mineral particles of a size suitable for flotation. The particulate mineral material  106  is preferably sized such that at least 80 weight percent of particles in the particulate mineral material are smaller than about 100 mesh, more preferably smaller than about 150 mesh, and still more preferably smaller than about 200 mesh. The size at which 80 weight percent of a material passes is often referred to as a P 80  size. Any suitable grinding and/or milling operation may be used for the comminution  104 . Wet grinding and/or milling operations are generally preferred due to their relative ease and low cost compared to dry operations. 
     The comminution  104  is conducted in the presence of a blanketing gas  108  which is obtained from a gas source  110 . During, or prior to, the comminution  104 , the mineral material feed  102  is mixed with the blanketing gas  108 , which contains oxygen gas, if at all, at a lower volume fraction of oxygen gas than is present in ambient air, to reduce problems that could be caused by the presence of air during the comminution  104 . During the comminution  104 , it is preferable to maintain a positive pressure of the blanketing gas  108  into any grinding and/or milling apparatus to assist mixing of the mineral material feed  102  with the blanketing gas  108 , and to displace any air which may have been present with the mineral material feed  102 . 
     After the comminution  104 , the particulate mineral material  106  is subjected to flotation  112  to separate sulfide minerals, including refractory auriferous sulfides, from non-sulfide gangue material. During flotation, a slurry of the particulate mineral material  106  is aerated with a flotation gas  114  from the gas source  110 . Any suitable flotation apparatus may be used for the flotation  112 , such as a one or more of a conventional flotation cell or a flotation column. Preferably, however, the flotation apparatus is such that a small positive pressure of the flotation gas  114  may be maintained in the apparatus to prevent the entry of air into the apparatus. The flotation gas  114  has oxygen gas, if at all, at a reduced volume fraction relative to the volume fraction of oxygen gas in ambient air, to reduce the problems associated with using air as a flotation gas. Although not required, the flotation gas  114  will normally be of substantially the same composition as the blanketing gas  108  used in the comminution  104 . Additionally, normal reagents may be added during or prior to the flotation  112  to assist in flotation separation. Such reagents may include frothing agents, activators, collectors, depressants, modifiers and dispersants. The flotation  112  may be conducted at ambient temperature and a natural pH produced by the mineral material. Operating conditions, such as pH and temperature may, however, be adjusted as desired to optimize flotation separation for any particular mineral material. 
     Exiting from the flotation  112  is a flotation concentrate  116 , which is recovered from the flotation froth and which is enriched in sulfide minerals, and particularly in refractory auriferous sulfides and consequently also in the gold in the refractory iauriferous sulfides. Also exiting from the flotation  112  is a flotation tail  118 , which is enriched in non-sulfide gangue materials, and consequently contains low levels of refractory gold. The flotation concentrate  116  may be further processed to recover the gold by any suitable technique, if desired. Alternatively, the flotation concentrate  116  may be sold as a valuable commodity for processing by others to recover the gold. 
     As noted previously, the flotation gas  114  and the blanketing gas  108  each comprise oxygen gas, if at all, at a volume fraction that is less than the volume fraction of oxygen gas in ambient air. Preferably, however, the amount of oxygen gas in the flotation gas  114  and/or blanketing gas  108  is less than about 15 volume percent, and more preferably less than about 5 volume percent. Most preferably, both the flotation gas  114  and the blanketing gas  108  are substantially free of oxygen gas. 
     To aid in the understanding of the present invention, but not to be bound by theory, it is believed that oxygen gas, if present in any appreciable quantity, tends to oxidize the surface of refractory auriferous sulfides, which can have the effect of depressing flotation of particles of those sulfide species during the flotation  112 . By reducing the amount of oxygen gas that comes into contact with sulfide species, it is believed that any surface oxidation effect is reduced, resulting in enhanced flotation of sulfide species, including refractory auriferous sulfides, and a corresponding increase in the amount of sulfides, and therefore gold, recovered in the flotation concentrate  116 . Therefore, it is preferred that the flotation gas  114  and the blanketing gas  108  consist essentially of components which could not oxidize the surface of sulfide mineral particles, and particularly those including refractory auriferous sulfides. 
     It is preferred that the flotation gas  114  and the blanketing gas  108  predominantly comprise one or more gases other than oxygen gas. Suitable gases include nitrogen, helium, argon and carbon dioxide. Preferably, one or more of these gases should comprise greater than about 95 volume percent of the flotation gas  114  and the blanketing gas  108 , and more preferably greater than about 98 volume percent. Still more preferable is for the blanketing gas  108  and the flotation gas  114  to consist essentially of one or more of these gases. Nitrogen gas is particularly preferred because of its relatively low cost. 
     The blanketing gas  108  and/or the flotation gas  114  may be introduced into process apparatus in any appropriate manner. Such gases may be fed under positive pressure or may be induced into the apparatus by creating a suction which pulls the gas in. Preferably, however, the apparatus is designed to substantially prevent introduction of air into comminution and flotation apparatus. 
     In one embodiment, the possible detrimental effects of any surface oxidation of sulfide mineral particles that may be present in a mineral material feed may be counteracted by the addition of a sulfidizing agent, to at least partially replace the oxidized coating with a sulfide coating. Any material capable of reacting to form the desired sulfide coating of the mineral particle could be used. Suitable sulfidizing agents include alkali metal sulfides and bisulfides, such as Na 2 S, NaHS, etc. Such sulfidizing agents could be added during comminution, conditioning just ahead of flotation or during any stage of the flotation  112 . 
     With the present invention, greater than about 80 weight percent of sulfide minerals from the particulate mineral material  106  may be recovered in the flotation concentrate  116 , and preferably greater than about 90 weight percent of those sulfide minerals are recovered in the flotation concentrate  116 . It should be noted that the flotation according to the present invention is substantially not selective as between different sulfide species. Rather, flotation according to the present invention typically promotes flotation of substantially all sulfide species present in the feed material. In this way, the hardest sulfide species to float are adequately concentrated in the flotation concentrate. 
     One major advantage of the process of the present invention is that, in addition to permitting a high recovery of refractory auriferous sulfides in the flotation concentrate  116 , it permits a high rejection of gangue material into the flotation tail  118 . Relative to the use of air as a flotation gas, the present invention permits the same recovery of gold to be obtained in a concentrate of smaller weight. This provides a significant economic advantage because less gangue material is present in the concentrate, from which the gold must ultimately be separated to produce a purified gold product, if desired. 
     The gas source  110  may be any source providing a suitable flotation gas  114  and blanketing gas  108 . One preferred gas source  110  is a facility in which nitrogen gas is separated from air, with the separated nitrogen gas being used as the blanketing gas  108  and the flotation gas  114 . Several processes are known for separating nitrogen from air, including cryogenic separation and membrane separation. 
     One particularly preferred gas source  110  is an oxygen plant, which is commonly found at existing facilities where gold-bearing refractory sulfide ores are processed. An oxygen plant is typically required, for example, when a pressure oxidation operation or an oxidative roasting operation is used in the processing of gold-bearing sulfide ores. In the oxygen plant, oxygen is separated from air, such as by cryogenic separation or membrane separation, and the separated oxygen gas is used in the pressure oxidation or oxidative roasting operation. A by-product of such an oxygen plant is an effluent gas stream which is enriched in nitrogen gas and is suitable for use as the blanketing gas  108  and/or the flotation gas  114 . This by-product stream has previously been vented to the atmosphere and has, therefore, been wasted. With the present invention, however, the by-product stream may be beneficially used to produce the flotation concentrate  116 , in addition to using the oxygen gas product stream for the pressure oxidation or oxidative roasting operation. 
     Another possibility for the gas source  110  is a nitrogen plant dedicated to producing a nitrogen-enriched gas for use as the blanketing gas  108  and/or the flotation gas  114 . A nitrogen plant differs from an oxygen plant in that the main product stream is a nitrogen-enriched gas stream and the by-product stream is an oxygen-enriched gas stream. The oxygen-enriched gas stream from a nitrogen plant, however, is normally of much lower purity in oxygen than an oxygen-enriched stream from an oxygen plant. The nitrogen plant may be based on separation of air into a nitrogen-enriched stream and an oxygen-enriched stream by membrane separation, cryogenic separation or otherwise. 
     Another possibility for the gas source  110  is a burner or other combustion device to produce combustion exhaust gases that are substantially depleted in oxygen. For example, the gas source  110  could be exhaust from an electrical power generator used to generate power for a mine or mineral processing facility. When using combustion exhaust gases as the blanketing gas  108  and/or the flotation gas  114 , it is preferred that the fuel combusted to produce the gases be a clean-burning fuel such as natural gas, propane or another liquefied petroleum gas, or an alcohol such as methanol or ethanol. Although less preferred, other fuels could be used such as coal or fuel oils, including diesel fuel. 
     Yet another possibility for the gas source  110  is gas generated during acid pre-treatment of a mineral material feed  102  comprising carbonate minerals. Carbon dioxide gas is generated from decomposition of the carbonate minerals. The carbon dioxide may be used as the blanketing gas  108  and/or the flotation gas  114 . Such an embodiment is shown in FIG.  10 . As shown in FIG. 10, the mineral material feed  102  is subjected to conmninution  104  to form the particulate mineral material  106 . The particulate mineral material  106  is then subjected to an acid pre-treatment  150  where acid  152  is added to the particulate mineral material  106  to decompose carbonate minerals present in the particulate mineral material  106 . The particulate mineral material  106  remaining following the acid pre-treatment is subjected to flotation  112 , to form the flotation concentrate  116  and the flotation tail  118 . During the acid pre-treatment  150 , a gas that is enriched in carbon dioxide and deficient in oxygen is produced, which is used as the blanketing gas  108  and the flotation gas  114 . An alternative to acid pretreatment of the mineral material feed  102  is to use gases produced during acid pretreatment of another carbonate-containing mineral material, such as a whole ore, prior to pressure oxidation. Such an embodiment is shown in FIG.  28 . 
     A still further possibility for the gas source  110  is to have liquid or compressed nitrogen, carbon dioxide or another gas delivered to the site. 
     In one preferred embodiment of the present invention, the flotation gas  114  is made up, at least in part, of recycled gas from the flotation  112 . One such embodiment is shown in FIG. 11, where a recycle gas  156  from the flotation  112  is used as part of the flotation gas  114 . In this manner, make-up flotation gas  114  from the gas source  110  may be kept to a minimum. This recycling of gas from the flotation  112  provides the benefits of reducing the amount of make-up gas that needs to be supplied by the gas source  110  and reduces emission of oxygen-deficient gas from the flotation  112 . Reducing the emission of oxygen-deficient gas from the flotation  112  is particularly important when the flotation  112  is conducted in an enclosed structure where people are present, so that ambient air in the structure does not become seriously deficient in oxygen. In that regard, oxygen monitors should be placed in any such enclosed structure to ensure that sufficient oxygen is present for human needs. 
     FIG. 12 shows features of one embodiment of a notation apparatus  160  that may be used with the present invention to accomplish recycling of flotation gas. As shown in FIG. 12, the flotation apparatus  160  has a sealed flotation tank  162 , above which is mounted a motor  164  for rotating a shaft  166  extending down into the flotation tank  162  to drive an impeller  168 . When the flotation apparatus  160  is operating, a vapor headspace  170  exists above a liquid column  172 . Gas from the vapor headspace  170  is withdrawn via a conduit  174  by a blower  176  to be forced through a conduit  178  for use as flotation gas. The flotation gas from conduit  178  is forced through an annular conduit  180  to the vicinity of the impeller  168  so that the flotation gas may be adequately distributed and dispersed throughout the liquid column  172 . Make-up flotation gas is provided via conduit  182  to compensate for any system losses of flotation gas. 
     FIG. 13 shows features of another embodiment of the flotation apparatus  160  for effecting recycle of flotation gas. In the embodiment shown in FIG. 13, the flotation apparatus  160  is designed such that a blower is not used. Instead, the conduit  174  collects gases from the overhead vapor space  170  and cycles the gas to the annular conduit  180  for use as a flotation gas. The action of the impeller  168  causes a vacuum in the annular conduit  180  creating sufficient suction to draw the flotation gas through the conduit  174  at a sufficiently high rate. This type of flotation apparatus  160  design, therefore, is self-inducing with respect to the introduction of flotation gas and does not require a blower or other gas compression device. Cycling of the flotation gas without the use of a blower is important because the recycled flotation gas will normally contain a significant amount of acid or corrosive mists or corrosive gases that could significantly corrode interior surfaces of a blower. 
     Referring again to FIG. 1, as noted previously, the flotation  112  may be performed at a natural pH. It has been found, however, that it is preferred that the flotation  112  be conducted at an acidic pH, and preferably at an acidic pH that is lower than about pH 6. Even more preferred is a flotation pH range of from about pH 3 to about pH 6, and most preferred is a pH range of from about pH 5 to about pH 6. Control of the pH may be accomplished by the addition of an acid or a base as necessary to adjust the pH to within the desired range. For example, sulfuric acid and/or any other acid could be added to the flotation  112  to lower the pH and lime, sodium carbonate, caustic or any other base could be added during the flotation  112  to raise the pH. Acid for reducing the pH could come from other mineral processing steps, such as pressure oxidation or bio-oxidation, as discussed below. Furthermore, acidification could be accomplished by introducing sulfur dioxide into the flotation  112 . In its dissolved state in water, sulfur dioxide forms sulfurous acid. The sulfur dioxide could be provided by exhaust gas from a sulfur burner. 
     As noted previously, various reagents may be used during the flotation  112 . It has been unexpectedly found, however, that copper-based activators generally do not perform as well as lead-based activators used during the flotation  112 . Lead-based activators should contain lead in an oxidation state of +2. One preferred activator is lead nitrate. Another preferred activator is lead acetate. One benefit of using a lead-based activator, relative to the use of a copper-based activator such as copper sulfate, is that higher recoveries are experienced in the flotation concentrate  116  for both sulfide minerals and gold. Also, if the flotation tail is subjected to cyanide leaching, as discussed below, the use of a lead-based activator provides the additional advantage of lowering cyanide consumption during the leaching operation relative to a copper-based activator. An added advantage is that cyanide consumption for eventual cyanide leaching of gold contained in the flotation concentrate  116 , such as after pressure oxidation, will be lower with the use of a lead-based activator compared to the use of a copper-based activator. 
     Another reagent that has been found to be particularly useful in the flotation  112  is a xanthate collector. The appropriate xanthate collector may be provided by addition to the flotation  112  of a xanthate salt such as potassium amyl xanthate or sodium isopropyl xanthate. The enhanced performance through use of a xanthate collector reagent is significantly greater than would normally be expected, especially when compared to the performance of other widely used collector reagents. 
     When the mineral material feed  102  contains a significant amount of organic carbon, the organic carbon can significantly detrimentally interfere with recovery of gold and sulfide minerals in the flotation concentrate  116 . To reduce the detrimental effects of organic carbon, when present in the mineral material feed  102 , it has been found to be advantageous to add an oil to the flotation  112 . Examples include an aromatic oil , such as moly collecting oil, commonly referred to as MCO, or a nonaromatic oil such as No. 2 diesel. 
     As discussed above, the comminution  104  is conducted in the presence of the blanketing gas  108 . With reference now to FIG. 14, one embodiment of a comminution circuit is shown. As shown in FIG. 14, the mineral material feed  102  is fed to a first comminution unit  186 , such as a sag mill. In the first comminution unit  186 , the mineral material particles in the mineral material feed  102  are reduced in size. Output from the first comminution unit  186  passes through a trommel screen  188  in a sealed discharge box  190 . Blanketing gas  108  is fed into the discharge box  190  so that the blanketing gas will flow back through the first comminution unit  186  in counter-flow with the mineral material feed  102 , to ensure removal of air from the mineral material feed  102 . Material exiting the discharge box goes to a tank  192  for delivery to a cyclone separator  194  via a pump  196 . The cyclone separator  194  classifies the mineral material by particle size into an overflow  198  of smaller-size particles and an underflow  200  of larger-size particles. The underflow  200  is then fed to sealed feed box  201  of a second comminution unit  202 , such as a ball mill, to further reduce the size of mineral material particles in the underflow  200 . Discharge from the second comminution unit  202  passes through a trommel screen  204  in a sealed discharge box  206 . Blanketing gas  108  is fed into both the feed box  201  and the discharge box  206 . Material exiting the discharge box  206  goes to a tank  208  where it is combined with the overflow  198  from the cyclone separator  194 . Discharge from the tank  208  goes to a cyclone separator  210  via a pump  212 . The cyclone separator  210  makes a size separation of particles into an overflow  214  comprising smaller-size particles and an underflow  216  comprising larger-size particles. The underflow  216  from the cyclone separator  210  is fed to the second comminution unit  202  along with the underflow  200  from the cyclone separator  194 . The overflow  214  from the cyclone separator  210  goes to the tank  218  where particulate mineral material in the overflow  214  may be held for feed to flotation processing. 
     It should be noted that, as shown in FIG. 14, all process equipment is sealed except for feed into the comminution unit  186 . Furthermore, the blanketing gas  108  is introduced at various points in the comminution system to ensure that minimal air makes its way into the system. As shown in FIG. 14, the blanketing gas  108  is specifically fed to the discharge box  190  of the first comminution unit  186 , to the tanks  192 ,  208  and  218 ; to the cyclone separators  194  and  210 ; and to the feed box  201  and the discharge box  206  of the second comminution unit  202 . 
     In addition to maintaining the comminution environment in the absence of any significant quantities of air, it is important to the performance of subsequent flotation operations that the effects of galvanic interaction be reduced as much as possible during comminution of the mineral material feed  102 . Such galvanic interactions are due to different electrochemical activities at different material surfaces. The combination of a cathodic surface (i.e., pyrite, pyrrhotite, arsenian pyrite, etc.) and an anodic surface (i.e., iron from comminution media or from steel walls of mill liners) results in the creation of galvanic cell during the comminution processing. Galvanic cells also exist between different sulfide minerals that may be present in the mineral material feed, with the sulfide mineral with the higher rest potential acting as the cathode and the sulfide mineral with the lower rest potential acting as the anode. For example, the galvanic interaction between grinding iron and various forms of refractory auriferous sulfides, and especially arsenian iron sulfides, can be represented by the following reactions: 
     grinding iron surface (anode): 
     
       
         Fe=Fe +2 +2e −   Eq. 1 
       
     
     refractory auriferous sulfide surface (cathode): 
     
       
         ½O 2 +H 2 O+2e − =2OH −   Eq. 2 
       
     
     For example, during grinding, galvanic cells are created every time a refractory auriferous sulfide surface comes into contact with grinding media, mill liners, abraded tramp iron or any other metal sulfide at a lower or higher rest potential. These galvanic interactions create hydroxide coatings on the surfaces of the refractory auriferous sulfides, depressing their floatability. According to the present invention, such detrimental galvanic interactions can be hindered by reducing the amount of oxygen present during comminution through use of the blanketing gas  108 . 
     It has been found, however, that, in addition to reducing the amount of oxygen present during the comminution operation, improved flotation results may be obtained by further limiting iron contamination of the mineral material being processed. The combination of reducing the amount of iron introduced into the mineral material and the use of the blanketing gas  108  provides an increase in gold recovery in the flotation concentrate that would be unexpected based on the contributions of each alone. 
     One way to reduce the amount of iron available for galvanic interactions during the comminution processing is to use comminution media made of a chromium or nickel hardened steel or a corrosion-resistant steel, such as stainless steel. Although ceramic comminution media could be used, ceramic comminution media are typically not of sufficient density for effective comminution. 
     Another way to reduce detrimental galvanic effects from iron is to provide all comminution equipment, such as grinders and mills, with non-metallic liners, and preferably with rubber liners. 
     A further, and preferred method for reducing detrimental galvanic interactions caused by iron during flotation is to perform a magnetic separation step after comminution and prior to flotation. Referring to FIG. 15, an embodiment is shown including a magnetic separation step after comminution. As shown in FIG. 15, the mineral material feed  102  is first subjected to comminution  104 , followed by magnetic separation  230  to remove particles of magnetic material  232  from the mineral material  106 . 
     As an alternative to the embodiments discussed thus far, it should be noted that many of the advantages of the present invention may be obtained even without the use of the blanketing gas  108 . In that regard, most conventional comminution circuits utilize equipment that freely permits air to enter into the system. With the present invention, however, when it is not practical or economic to use a blanketing gas, enhanced performance may still be obtained by sealing all equipment involved with comminution processing so that substantially the only oxygen entering into the comminution processing enters with the mineral material feed  102  to be processed. In that regard, such a comminution system could be substantially as described with respect to FIG. 14, with all process equipment sealed to prevent the entry of air and with no blanketing gas  108  being fed to the process equipment. Oxygen already present in the mineral material feed  102  would be consumed through oxidation of surfaces of sulfide mineral particles exposed during comminution. Once all of that original oxygen had been consumed, however, the continued detrimental effects of oxygen would be substantially eliminated. Although this mode of operation is not as preferable as using the blanketing gas  108 , it is preferred to a system that it is open to the introduction of air, as is commonly practiced. 
     To further reduce the amount of oxygen introduced in the system to reduce the detrimental effects of surface oxidation of sulfide minerals and of galvanic interactions, it is preferred that process water used to slurry mineral material for comminution processing and/or flotation processing has been deoxygenated. Deoxygenation of process water can significantly enhance recovery in the concentrate of sulfide minerals and gold during flotation processing. The deoxygenation may be performed in any convenient manner, such as by bubbling an inert stripping gas, such as nitrogen or carbon dioxide, through the process water to remove oxygen from the process water or by adding an oxygen scavenger to the water to tie-up the oxygen. It has been found that a flotation tank works well for deoxygenation, with nitrogen or carbon dioxide being introduced into the vessel to perform the oxygen stripping function. Alternatively, the inert stripping gas could be sparged into a talk containing the process water. Preferably, the process water should be deoxygenated to a dissolved oxygen level that is less than about 2.0, most preferably less than about 1.0, and most preferably less than about 0.5, parts per million of oxygen by weight. 
     Also, with the present invention it is possible to use recycled water as process water. When recycled water is used, however, it is important that an activator be used during flotation processing. This is because any cyanide in the recycle water that is available for reaction with sulfide sulfur or sulfide minerals will tend to depress flotation of the sulfide minerals and, accordingly, reduce the recovery of gold in the concentrate. The activators, however, counter the depressing effect that cyanide has on flotation. Also, the recycle water may be treated with a material such as ammonium bisulfite, sulfur dioxide, a peroxide, Caro&#39;s acid or any other known cyanide destruction technology used to destroy the cyanide prior to using the recycle water for flotation. 
     As noted with respect to FIG. 1, one preferred gas source  110  for the blanketing gas  108  and the flotation gas  114  is an oxygen plant. FIG. 2 shows one embodiment of the present invention in which both the oxygen gas product stream and the nitrogen gas by-product stream from an oxygen plant are both used to process gold-bearing sulfide mineral material. Referring to FIG. 2, particulate mineral material  110  is subjected to the flotation  112  to produce the flotation concentrate  116  and the flotation tail  118 , as previously described. The flotation gas  114  is a nitrogen gas enriched by-product stream from an oxygen plant  130 , in which air  132  is separated into an oxygen enriched gas stream (treating gas  128 ) and nitrogen enriched gas stream (flotation gas  114 ). 
     The flotation concentrate  116 , which is enriched in refractory auriferous sulfides, is subjected to pressure oxidation  124  to decompose sulfide minerals, producing an oxidized material  126  from which the gold could be recovered by dissolution using any suitable gold lixiviant, such as a cyanide. The pressure oxidation  124  involves treating a slurry of the flotation concentrate  116  in an autoclave at at temperature of greater than about 150° C. and an elevated pressure in the presence of an overpressure of a treating gas  128 , which is rich in oxygen. It should be noted that other oxidative treating steps could be used instead of the pressure oxidation  124 . For example, an oxidative roasting or bio-oxidation could be used to produce the oxidized material  126  using the treating gas  128 . 
     A further embodiment in accordance with the present invention is shown in FIG. 3 which uses the product and by-product gas streams from an oxygen plant to process a gold-bearing sulfide mineral material provided in two separate feed streams. Referring to FIG. 3, a particulate first mineral material feed  138  is subjected to the flotation  112  to produce the flotation concentrate  116  and the flotation tail  118 , as previously described. The flotation gas  114  is a gas stream enriched in nitrogen from the oxygen plant  130 . A particulate second mineral material feed  140  is combined with the flotation concentrate  116  in a mixing step  142 . The combined stream  144 , in the form of a slurry, is subjected to the pressure oxidation  124  to produce the oxidized material  126 , from which gold could be recovered. 
     One advantage of the embodiment shown in FIG. 3 is that it permits the processing of multiple ores having different characteristics. For example, the first mineral material feed  138  may comprise a lower grade refractory sulfide ore than the second mineral material feed, which may comprise a higher grade refractory sulfide ore. The higher grade ore may be suitable for pressure oxidation in a whole ore form, whereas the lower grade ore must be upgraded to a concentrate form to be suitable for pressure oxidation. 
     Alternatively, the second mineral material feed may comprise a refractory sulfide ore which has a significant amount of carbonate material which would require acid to be added prior to pressure oxidation  124 , and which could, therefore, cause high operating costs compared to ores with low carbonate levels detrimentally interfere with proper operation of the pressure oxidation  124 . A high sulfide sulfur content in the flotation concentrate  116 , however, tends to produce additional acid during pressure oxidation to at least partially offset the acid consuming effect of carbonate material in the second mineral material feed. Most of the carbonate material that may have been present in the first mineral material feed, if any, would ordinarily have been removed during the flotation  112 . 
     With the present invention, most of the gold reports to the flotation concentrate. Gold in the concentrate, which is typically substantially all refractory gold associated with one or more refractory auriferous sulfides , may then be freed for recovery operations by oxidative processing, such as pressure oxidation, oxidative roasting or bio-oxidation. Many mineral material feeds, however, contain a significant amount of gold that is not associated with a sulfide mineral. For example, it is not uncommon for a gold-bearing refractory sulfide ore to also contain some gold in association with oxidized minerals or silica. Up to 50%, and in some instances even more, of the gold in a refractory sulfide ore may be associated with minerals other than sulfide minerals. Also, refractory sulfide ores that have been stockpiled for a significant amount of time, and therefore exposed to air for a significant amount of time, may contain even larger amounts of gold that are no longer held by the refractory auriferous sulfides. This is because a significant amount of a refractory auriferous sulfide may become oxidized so that a significant quantity of the gold is no longer associated with the refractory auriferous sulfide. For example, a refractory sulfide ore stockpiled for several months may oxidize to a degree where 20% to 30% or more of the gold is no longer associated with refractory auriferous sulfide. 
     It has been found that the present invention works very well for the treatment of mineral material feeds having both refractory gold associated with sulfide minerals and non-refractory gold that is not associated with sulfide minerals. Gold that is not associated with sulfide minerals, and especially gold associated with oxidized minerals, may be recovered following flotation processing by leaching of the flotation tail. Although any compatible leaching operation may be used, a preferred leaching operation is cyanide leaching. One embodiment of the present invention involving a leach of the flotation tail is shown in FIG.  16 . 
     With reference to FIG. 16, a mineral material feed  102  is provided having both refractory gold, in refractory auriferous sulfides, and non-refractory gold that is not associated with sulfide minerals, and, therefore, will not float along with sulfide minerals. The mineral material feed  102  is subjected to comminution  104  to prepare the particulate mineral material  106 , which is then subjected to flotation  112 . Following the flotation  112 , the flotation tail  114  is subjected to oxygenation  240  followed by a leach  242  of the flotation tail  114 . Preferred for the leach  242  is a carbon-in-pulp cyanide leach, although a carbon-in-leach cyanide leach could be used instead. Exiting from the leach  242  is loaded carbon  244  that is loaded with gold. Also exiting from the leach  242  is a leach tail  246  that is depleted in gold. The loaded carbon  244  may be processed in any known manner for recovery of the gold. 
     With continued reference to FIG. 16, the comminution  104  and the flotation  112  are performed in the presence of the blanketing gas  108  and the flotation gas  114 , respectively, supplied from the gas source  110 . As also shown in FIG. 16 process water  248  is subjected to deoxygenated prior to using the process water  248  in the process. Therefore, according to the embodiment shown in FIG. 16, the process water  248  is first deoxygenated in the deoxygenation step  250  and, following the flotation  112 , the water with the flotation tail  118  is then oxygenated in the oxygenation step  240  prior to the leach  246 . The oxygenation  240  may be accomplished in any manner suitable for increasing the amount of oxygen dissolved in the liquid of the slurry of the flotation tail  118 . Typically, the slurry of the flotation tail  118  is subjected to sparging or bubbling with air or an oxygen-enriched gas. Oxygenation  240  may be conducted using air or an oxygen-enriched gas, such as would be suitable for pressure oxidation processing, as previously discussed. Although the embodiment described with respect to FIG. 16 includes deoxygenation of process water, such deoxygenation is not required. The use of deoxygenated process water does, however, tend to improve gold recovery from the process. 
     The ability to successfully leach the flotation tail  118 , as shown in FIG. 16, results from the efficient separation of sulfide minerals into the concentrate during the flotation  112 . If a significant amount of sulfide mineral were to report to the flotation tail  118 , then performance of the leach  242  could be significantly impaired because sulfide sulfur from the sulfide mineral would consume cyanide during a cyanide leach. Also, refractory gold in the flotation tail associated with the refractory auriferous sulfides would not be leachable. With the present invention, however, cyanide consumption is reduced during the leach  242  because of the efficient reporting of sulfide minerals to the flotation concentrate  116  and the relative absence of sulfide minerals in the flotation tail  118 . 
     Another significant advantage of the process of the present invention is that it permits interim regrinding of particulate mineral material between flotation stages (to enhance gold recovery in the concentrate). Such intermediate grinding can significantly enhance recovery of refractory gold in refractory sulfide auriferous mineral fragments locked in middling particles. By comparison, with conventional flotation using air as the flotation gas, such intermediate grinding would further reduce the floatability of refractory auriferous sulfides due to the detrimental effects of oxygen. 
     FIG. 17 shows a process diagram for one embodiment according to the present invention involving regrinding of particulate mineral material intermediate between flotation stages. As shown in FIG. 17, a mineral material feed  102  is subjected to a first comminution step  254  to produce a particulate mineral material  106 , which is then subjected to a first flotation stage  256 . The first flotation stage  256  includes rougher flotation  258  and rougher scavenger flotation  260 . In the rougher flotation  258 , the particulate mineral material  106  is separated by flotation into a rougher concentrate  262 , which forms part of a final concentrate  264 , and a rougher tail  266 , that is fed to the rougher scavenger flotation  260 . The rougher scavenger flotation produces a rougher scavenger concentrate  268  and a rougher scavenger tail  270 . The rougher tail  266 , the rougher scavenger concentrate  268  and the rougher scavenger tail  270  often include a substantial amount of middling particles. Such middling particles include refractory auriferous sulfides locked with gangue material, such as silica. Additionally, the rougher scavenger tail  270  will typically include a substantial amount of very fine slime particles. 
     To remove the slime particles and to permit recovery of the sulfide mineral fragments from middling particles, the rougher scavenger tail  270  is subjected to a size separation  274 , such as may be accomplished using a screen or a classifying cyclone. A first fraction  276 , comprising the smaller-size slime particles becomes part of a final tail  278 . For example, a 500 mesh screen may be used in the size separation  274  with all particles passing through the screen being sent to the final tail  278  as slimes. 
     A second fraction  280  from the size separation  274 , which comprises larger-size particles, is sent to a second comminution step  284  along with the rougher scavenger concentrate  268 . In the second comminution step  284 , particles are comminuted to a smaller size to break up middling particles and liberate sulfide mineral fragments, including fragments of refractory auriferous sulfides. The reground mineral material  286  is sent to a second flotation stage  290  for concentration of the sulfide mineral fragments liberated from middling particles. The second flotation stage  290  includes cleaner flotation  292  and cleaner scavenger flotation  294 . In the cleaner flotation  292 , a cleaner concentrate  296  is produced, which is sent to form part of the final concentrate  264 . The cleaner flotation  292  also produces a cleaner tail  298  which is sent to the cleaner scavenger flotation  294 . In the cleaner scavenger flotation  294 , a cleaner scavenger concentrate  300  is prepared, which is recycled to the second comminution step  284  for further processing. The cleaner scavenger flotation  294  also produces a cleaner scavenger tail  302  that is sent to form part of the final tail  278 . 
     Also shown in FIG. 17 is the gas source  110  that supplies blanketing gas  108  to the first comminution step  254 , the second comminution step  284  and the size separation  274 . The gas source  110  also provides flotation gas  114  to the rougher flotation  258 , the rougher scavenger flotation  260 , the cleaner flotation  292  and the cleaner scavenger flotation  294 . Use of the blanketing gas  108  and the flotation gas  114  substantially prevents problems that could occur if comminution and/or flotation operations were conducted in the presence of air. Furthermore, because the detrimental effects from air are reduced, it is possible to have the second comminution step  284  intermediate between the first flotation stage  256  and the second flotation stage  290  without destroying the floatability of sulfide minerals in the reground mineral material  286 . The second comminution step  284  significantly improves performance of the flotation circuit. This is because it will not be necessary to comminute all of the mineral material feed  102  to ia very fine size that may be required for liberating refractory auriferous sulfides from middling particles in the first comminution step  254 . Having a coarser grind for the particulate mineral material  106  is significantly less expensive than comminuting all of the mineral material feed  102  to a size small enough to liberate refractory auriferous sulfide mineral fragments from middling. Also, a coarser comminution to produce the particulate mineral material  106  simplifies operation of the first flotation stage. Middling particles from the first flotation stage  256  are then further comminuted in the second comminution step  284  to liberate the locked refractory auriferous sulfide mineral fragments for recovery in the second flotation stage  290 . 
     The present invention is further described by the following examples, which are intended to be illustrative only and are not intended to limit the scope of the present invention. 
     EXAMPLES 
     Examples 1-6 
     Examples 1-6 demonstrate the use of nitrogen gas as a flotation gas during flotation of a gold-bearing sulfide ore to produce a sulfide enriched concentrate which could be further processed to recover refractory gold in refractory auriferous sulfides, if desired. 
     For each of Examples 1-6, an ore sample is provided from Santa Fe Pacific Gold Corporation&#39;s Lone Tree Mine in Nevada. The ore samples are of a low grade refractory sulfide ore which would be unsuitable for economic pressure oxidation in a whole ore form. A representative assay of an ore sample is shown in Table 3. 
     
       
         
           
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 LONE TREE LOW GRADE REFRACTORY ORE 
               
               
                 REPRESENTATIVE HEAD ANALYSIS 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Gold 
                 0.063 oz/st (1)   
               
               
                   
                 Silver 
                 0.05 oz/st (1)   
               
               
                   
                 Total Sulfur 
                 1.75 wt. % 
               
               
                   
                 Sulfide Sulfur 
                 1.66 wt. % 
               
               
                   
                 Arsenic 
                 1440 ppm. by wt. 
               
               
                   
                   
               
               
                   
                   (1) ounces per short ton of ore  
               
            
           
         
       
     
     For each example, the ore sample is ground to the desired size. A first portion of the ore sample is subjected to flotation in a laboratory-scale flotation cell using air as the flotation gas. A second portion of the ore sample is subjected to flotation under the same conditions, except using a flotation gas which consists essentially of nitrogen gas. During each flotation test, a flotation froth is collected from the top of the flotation cell to recover a flotation concentrate which is enriched in sulfide minerals, including refractory auriferous sulfides, and which is, therefore, also enriched in gold. The flotation tail is that material which is not collected in the froth. For each flotation test, the flotation conditions are substantially as follows: A natural pH and addition of potassium amyl xanthate and mercaptobenzothiazole as collectors, copper sulfate for activation of sulfides and MIBC as a frother. Flotation times range from 20 to 30 minutes. 
     The results for examples 1-6 are shown tabularly in Table 4 and graphically in FIGS. 4-7 and reveal a significant increase in the amount of gold recovered in the concentrate when nitrogen gas is used as the flotation gas, especially at smaller grind sizes. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 LONE TREE LOW GRADE BATCH TESTS 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                 Con- 
                 Gold 
               
               
                   
                   
                 Concentrate 
                   
                 centrate 
                 Reporting to 
               
               
                   
                   
                 Grade 
                 Tail Grade 
                 Recovery 
                 Concentrate 
               
               
                   
                 Grind 
                 oz gold/st (2)   
                 oz gold/st (3)   
                 wt % (4)   
                 % (5)   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Exam- 
                 P80 
                   
                 nitro- 
                   
                 nitro- 
                   
                 nitro- 
                   
                 nitro- 
               
               
                 ple 
                 Mesh (1)   
                 air 
                 gen 
                 air 
                 gen 
                 air 
                 gen 
                 air 
                 gen 
               
               
                   
               
               
                 1 
                 100 
                 0.31 
                 0.35 
                 0.019 
                 0.020 
                 15 
                 15 
                 75 
                 75 
               
               
                 2 
                 150 
                 0.28 
                 0.31 
                 0.021 
                 0.016 
                 15 
                 16 
                 71 
                 79 
               
               
                 3 
                 200 
                 0.33 
                 0.29 
                 0.021 
                 0.016 
                 15 
                 19 
                 74 
                 81 
               
               
                 4 
                 270 
                 0.22 
                 0.25 
                 0.022 
                 0.012 
                 20 
                 24 
                 72 
                 86 
               
               
                 5 
                 325 
                 0.23 
                 0.20 
                 0.022 
                 0.016 
                 20 
                 25 
                 73 
                 81 
               
               
                 6 
                 400 
                 0.14 
                 0.14 
                 0.029 
                 0.012 
                 29 
                 33 
                 67 
                 85 
               
               
                   
               
               
                   (1) 80 weight percent of material passing the indicated size  
               
               
                   (2) ounces of gold per short ton of concentrate  
               
               
                   (3) ounces of gold per short ton of tail  
               
               
                   (4) weight percent of ore sample feed reporting to concentrate  
               
               
                   (5) % of gold in ore sample feed reporting to concentrate  
               
            
           
         
       
     
     FIG. 4 graphically shows the grade of the flotation concentrate (measured as ounces of gold per short ton of concentrate material) as a function of the grind size. As seen in FIG. 4, no identifiable effect on the grade of the concentrate is apparent from using nitrogen gas relative to using air in the flotation. FIG. 5, however, shows that the flotation tail, at smaller grind sizes, contains a significantly lower gold value when using nitrogen gas as a flotation gas than when using air. Therefore, when using nitrogen gas, more of the refractory auriferous sulfides are recovered in the concentrate, apparently without any detrimental effect to the grade of the concentrate recovered. FIG. 6 shows that the amount of material recovered in the concentrate may be significantly higher when using nitrogen gas as a flotation gas than when using air, especially at the smaller grind sizes. FIG. 7 shows that gold recovery in the concentrate may be increased by almost 15% at a P 80  grind of 270 mesh, when using nitrogen gas as a flotation gas as opposed to air, again without detrimental effect to the grade of concentrate recovered. 
     It should be noted that at a P 80  grind of 100 mesh, there is no significant difference in flotation performance when using nitrogen gas as opposed to air as the flotation gas. It is, therefore, surprising and unexpected that the performance using nitrogen gas would improve so markedly relative to air at the smaller grind sizes. Typically, it is expected that flotation performance should improve with a smaller grind size due to a more complete liberation of sulfide minerals from non-sulfide gangue material. As seen in FIG. 7, however, the gold recovery in the concentrate when using air as the flotation gas is flat, at best. When using nitrogen gas, however, gold recovery generally increases with decreased grind size due to increased sulfide mineral particle liberation, including refractory auriferous sulfides, as would normally be expected. 
     One way to explain the unexpectedly poor flotation performance when using air, to assist in the understanding in the present invention but not to be bound by theory, is that some detrimental chemical process may be occurring when air is used as a flotation gas, with the detrimental chemical process counteracting the normally beneficial effects of a smaller grind size. It was observed that when air is used as the flotation gas, the pH of the slurry in the flotation cell drops rapidly for several minutes, sometimes falling by as much as 0.5-2 pH units. Therefore, it appears that oxygen in the air may be oxidizing the surface of sulfide mineral particles, and particularly those of refractory auriferous sulfides, producing sulfuric acid and lowering the slurry pH. Such surface oxidization of the sulfide mineral particles could render them less responsive to flotation. As the grind becomes smaller, the surface area available for oxidation of the sulfides increases significantly and, accordingly, any beneficial effect from more complete liberation of sulfides due to the smaller grind size is offset by increased surface oxidation, further depressing flotation of the refractory auriferous sulfides. Nitrogen gas, however, would not oxidize the surface of sulfide minerals and, therefore, permits better flotation of sulfide mineral particles, resulting in a higher recovery of sulfide minerals at the smaller grind sizes, as would normally be expected. 
     Example 7 
     This example further demonstrates the beneficial use of nitrogen gas in the flotation of gold-bearing refractory sulfide ores, and the use of a rougher-scavenger-cleaner arrangement of flotation to enhance recovery of concentrate. 
     A flotation pilot plant is operated using a low grade refractory sulfide ore from the Lone Tree Mine, as previously described with Examples 1-6. The pilot plant flow is shown in FIG.  8 . 
     With reference to FIG. 8, the ore sample  166  is subjected to comminution  168  in a ball mill to a P 80  size of 270 mesh. The ground ore, in a slurry  170 , is introduced into a rougher flotation step  172 . In the rougher flotation step  172 , an initial flotation separation is made with a rougher concentrate  174  being collected with the flotation froth and a rougher tail  176  being sent to a scavenger flotation step  178 , material collected in the flotation froth of the scavenger flotation step  178  is repulped and introduced, as a slurry  179 , to a cleaner flotation step  180 , where a final flotation separation is made to produce a cleaner concentrate  182  from the froth and a cleaner tail  184 . The cleaner tail  184  is combined with a scavenger tail  186 , from the scavenger flotation step  178 , to produce the final tail  188 . The rougher concentrate  174  and the cleaner concentrate  182  are combined to form a final concentrate  190 . In this example, the rougher flotation step  172  is accomplished in a single dual compartment flotation cell, the scavenger flotation step  178  is accomplished in a series of three dual compartment flotation cells, and the cleaner flotation step  180  is accomplished in a series of three dual compartment flotation cells. As shown in FIG. 8, nitrogen gas  192  is supplied from gas tank  194  and is fed to each of the comminution step  168 , the rougher flotation step  172 , the scavenger flotation step  178  and the cleaner flotation step  180 . The nitrogen gas  192  is used as the flotation gas in each of the flotation steps and is used as a blanketing gas to prevent air from oxidizing ore particles during the comminution  168 . The nitrogen gas is also used to blanket all other process equipment, not shown, such as pumps and mixing tanks. Refractory auriferous sulfides in the ore sample  166  are, therefore, maintained in a substantially air-free environment through the entire pilot plant, until the refractory auriferous sulfides have been recovered in a desired concentrate product. 
     The results of the pilot plant are shown in Table 5, which shows that the final concentrate  190  from the pilot plant is of a higher quality than the concentrates shown in Examples 1-6. Addition of the scavenger flotation step  178  and the cleaner flotation step  180  in the pilot plant significantly improves the grade of concentrate finally recovered, without any appreciable loss of gold recovery. 
     
       
         
           
               
             
               
                 TABLE 5 
               
               
                   
               
             
            
               
                 LONE TREE PILOT PLANT 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                 Final 
                 Tail 
                 Final 
                 Gold Reporting 
               
               
                 Ex- 
                 Grind 
                 Concentrate 
                 Grade 
                 Concentrate 
                 to Final 
               
               
                 am- 
                 P80 
                 Grade 
                 oz  
                 Recovery 
                 Concentrate % 
               
               
                 ple 
                 Mesh (1)   
                 oz gold/st (2)   
                 gold/st (3)   
                 wt % (4)   
                 gold recovery (5)   
               
               
                   
               
               
                 7 
                 270 
                 0.57 
                 .0095 
                 9.4 
                 86.4 
               
               
                   
               
               
                   (1) 80 weight percent of material passing the indicated size  
               
               
                   (2) ounces of gold per short ton of respective concentrate  
               
               
                   (3) ounces of gold per short ton of final tail  
               
               
                   (4) weight percent of ore sample feed reporting to respective concentrate  
               
               
                   (5) % of gold in concentrate relative to feed for the respective flotation step  
               
            
           
         
       
     
     Example 8 
     Laboratory tests are performed on samples of a low grade gold-bearing refractory sulfide ore from Santa Fe Pacific Gold Corporation&#39;s Twin Creeks Mine in Nevada. A representative analysis of an ore sample is shown in Table 6. For each test, a sample is ground to the appropriate size and a portion of each sample is then subjected to flotation using air as a flotation gas and another portion is subjected to flotation using nitrogen as a flotation gas. Substantially the same flotation conditions are used as described for Examples 1-6. 
     
       
         
           
               
             
               
                 TABLE 6 
               
               
                   
               
               
                 Twin Creeks LOW GRADE REFRACTORY SULFIDE ORE 
               
               
                 REPRESENTATIVE HEAD ANALYSIS 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Gold 
                 0.085 oz/st (1)   
               
               
                   
                 Silver 
                 0.28 oz/st (1)   
               
               
                   
                 Total Sulfur 
                 6.45 wt. % 
               
               
                   
                 Sulfide Sulfur 
                 6.27 wt. % 
               
               
                   
                 Arsenic 
                 1630 ppm by wt. 
               
               
                   
                   
               
               
                   
                   (1) ounces per short ton of ore  
               
            
           
         
       
     
     The results of Example 8 are graphically shown in FIG. 9 which shows a plot of gold recovery in the concentrate as a function of grind size. As seen in FIG. 9, the use of nitrogen gas generally results in a significantly higher recovery of gold in the concentrate compared to the use of air as a flotation gas. 
     Examples 9-28 
     These examples demonstrate the importance of flotation pH and the choice of activators for use during flotation with the present invention. 
     A series of laboratory flotation tests are performed using Lone Tree low grade gold-bearing refractory sulfide ore samples. Prior to flotation, each sample is ground to a p 80  size of about 60 microns. One series of tests are performed using a nitrogen atmosphere in the grind and nitrogen flotation gas with varying flotation pH using lead nitrate as an activator. A second series of tests are performed using air as the grinding atmosphere and air as the flotation gas at varying flotation pH&#39;s and using lead nitrate as an activator. A third series of tests are performed using nitrogen as the grinding environment and nitrogen flotation gas with varying flotation pH and using copper sulfate as an activator. The pH is adjusted by either the addition of sulfuric acid or calcium hydroxide, as required. Also, other normal flotation reagents are used in each test. Conditions for the grind and flotation for each example are shown in Table 7 and specific ireagents used with each example are shown in Table 8. 
     Results of the flotation are shown tabularly in Table 9 and graphically in FIGS. 18-20. FIG. 18 has plots of sulfide sulfur recovery in the flotation concentrate versus flotation pH for each of the three test series. FIG. 19 has plots of gold recovery in the flotation concentrate versus flotation pH for each of the three test series. As seen in Tables 7-9 and FIGS. 18 and 19, grinding and flotation using nitrogen gas provides significantly enhanced performance relative to air for all but the highest pH&#39;s. Furthermore, gold recoveries in the concentrate are best at acidic pH&#39;s, and particularly at pH&#39;s below about 6. Moreover, quite surprisingly, lead nitrate as an activator consistently shows a significantly higher gold recovery in the concentrate than the more standard activator of copper sulfate. 
     FIG. 20 includes a plot of the difference in percentage gold recoveries in the concentrate using nitrogen versus air for various flotation pH&#39;s and a plot of the difference in percentage recovery of sulfide sulfur in the concentrate using nitrogen versus air for various flotation pH&#39;s. A dramatic effect of pH is revealed in FIG. 20 at lower pH&#39;s. For example, at pH 6, gold recovery in the concentrate increases by greater than 15 percentage points for nitrogen versus air with less than a 5 percentage point increase in sulfide sulfur recovery. These results further indicate that the refractory gold is primarily associated with sulfide mineral types that are particularly difficult to recover in a concentrate using conventional flotation with air as a flotation gas. These difficult-to-float sulfide mineral types float extremely well, however, with the use of nitrogen, especially when a lead-containing activator is used at an advantageously acidic pH. These results, especially at pHrs below about 6, are particularly surprising. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 7 
               
             
            
               
                   
                   
               
               
                   
                 Grind 
                   
               
            
           
           
               
               
               
            
               
                   
                 P-80 size 
                 Flotation 
               
            
           
           
               
               
               
               
               
            
               
                 Example 
                 Atmosphere 
                 (microns) 
                 Flotation Gas 
                 pH 
               
               
                   
               
               
                  9 
                 nitrogen 
                 62 
                 nitrogen 
                 3 
               
               
                 10 
                 nitrogen 
                 62 
                 nitrogen 
                 4 
               
               
                 11 
                 nitrogen 
                 60 
                 nitrogen 
                 5 
               
               
                 12 
                 nitrogen 
                 60 
                 nitrogen 
                 6 
               
               
                 13 
                 nitrogen 
                 60 
                 nitrogen 
                 7 
               
               
                 14 
                 nitrogen 
                 60 
                 nitrogen 
                 8 
               
               
                 15 
                 nitrogen 
                 60 
                 nitrogen 
                 9 
               
               
                 16 
                 nitrogen 
                 60 
                 nitrogen 
                 10  
               
               
                 17 
                 nitrogen 
                 60 
                 nitrogen 
                 11  
               
               
                 18 
                 air 
                 60 
                 air 
                 3 
               
               
                 19 
                 air 
                 60 
                 air 
                 6 
               
               
                 20 
                 air 
                 60 
                 air 
                 8 
               
               
                 21 
                 air 
                 60 
                 air 
                 9 
               
               
                 22 
                 air 
                 60 
                 air 
                 10  
               
               
                 23 
                 air 
                 60 
                 air 
                 11  
               
               
                 24 
                 nitrogen 
                 64 
                 nitrogen 
                 4 
               
               
                 25 
                 nitrogen 
                 64 
                 nitrogen 
                 5 
               
               
                 26 
                 nitrogen 
                 64 
                 nitrogen 
                 6 
               
               
                 27 
                 nitrogen 
                 64 
                 nitrogen 
                 7 
               
               
                 28 
                 nitrogen 
                 64 
                 nitrogen 
                 8 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                   
                 TABLE 8 
               
             
            
               
                   
                   
               
               
                   
                 Reagents (lb/ton) 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 Sodium 
                   
                   
                 Sulfuric 
                 Calcium 
                   
                 Lead 
                 Copper 
               
               
                 Example 
                 Silicate 
                 MIBC (1)   
                 DF250 (2)   
                 Acid 
                 Hydroxide 
                 PAX (3)   
                 Nitrate 
                 Sulfate 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                  9 
                 0.1 
                 0.14 
                 0.24 
                 8.86 
                 0 
                 0.5 
                 0.2 
                 0 
               
               
                 10 
                 0.1 
                 0.12 
                 0.27 
                 4.22 
                 0 
                 0.5 
                 0.2 
                 0 
               
               
                 11 
                 0.1 
                 0.14 
                 0.25 
                 2.1 
                 0 
                 0.5 
                 0.2 
                 0 
               
               
                 12 
                 0.1 
                 0.14 
                 0.2 
                 1.3 
                 0 
                 0.5 
                 0.2 
                 0 
               
               
                 13 
                 0.1 
                 0.1 
                 0.1 
                 0 
                 0.27 
                 0.5 
                 0.2 
                 0 
               
               
                 14 
                 0.1 
                 0.12 
                 0.15 
                 0 
                 2.49 
                 0.5 
                 0.2 
                 0 
               
               
                 15 
                 0.1 
                 0.11 
                 0.11 
                 0 
                 3.24 
                 0.5 
                 0.2 
                 0 
               
               
                 16 
                 0.1 
                 0.1 
                 0.07 
                 0 
                 3.96 
                 0.5 
                 0.2 
                 0 
               
               
                 17 
                 0.1 
                 0.11 
                 0.11 
                 0 
                 5.56 
                 0.5 
                 0.2 
                 0 
               
               
                 18 
                 0.1 
                 0.2 
                 0.21 
                 6.56 
                 0 
                 0.5 
                 0.2 
                 0 
               
               
                 19 
                 0.1 
                 0.15 
                 0.16 
                 0 
                 1.16 
                 0.5 
                 0.2 
                 0 
               
               
                 20 
                 0.1 
                 0.15 
                 0.13 
                 0 
                 2.84 
                 0.5 
                 0.2 
                 0 
               
               
                 21 
                 0.1 
                 0.15 
                 0.14 
                 0 
                 3.44 
                 0.5 
                 0.2 
                 0 
               
               
                 22 
                 0.1 
                 0.16 
                 0.14 
                 0 
                 4.54 
                 0.5 
                 0.2 
                 0 
               
               
                 23 
                 0.1 
                 0.15 
                 0.13 
                 0 
                 5.82 
                 0.5 
                 0.2 
                 0 
               
               
                 24 
                 0.1 
                 0.20 
                 0.20 
                 4.15 
                 0 
                 0.5 
                 0 
                 0.2 
               
               
                 25 
                 0.1 
                 0.19 
                 0.19 
                 2.17 
                 0 
                 0.5 
                 0 
                 0.2 
               
               
                 26 
                 0.1 
                 0.18 
                 0.18 
                 0.85 
                 0 
                 0.5 
                 0 
                 0.2 
               
               
                 27 
                 0.1 
                 0.16 
                 0.13 
                 0 
                 0.3 
                 0.5 
                 0 
                 0.2 
               
               
                 28 
                 0.1 
                 0.18 
                 0.11 
                 0 
                 2.34 
                 0.5 
                 0 
                 0.2 
               
               
                   
               
               
                   (1) Methyl isobutyl carbanol  
               
               
                   (2) Polyethylene glycol  
               
               
                   (3) Potassium amyl xanthate  
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                   
                 TABLE 9 
               
             
            
               
                   
                   
               
               
                   
                 Recovery in Concentrate 
                   
               
            
           
           
               
               
               
            
               
                 Example 
                 Gold (%) 
                 Sulfide Sulfur (%) 
               
               
                   
               
            
           
           
               
               
               
            
               
                  9 
                 86.7 
                 96.8 
               
               
                 10 
                 86.7 
                 95.3 
               
               
                 11 
                 85 
                 97.3 
               
               
                 12 
                 86 
                 97.3 
               
               
                 13 
                 82.6 
                 97.5 
               
               
                 14 
                 81.1 
                 95 
               
               
                 15 
                 76.5 
                 95.9 
               
               
                 16 
                 67.7 
                 88.6 
               
               
                 17 
                 30.8 
                 67.7 
               
               
                 18 
                 77.7 
                 93.9 
               
               
                 19 
                 70.1 
                 94 
               
               
                 20 
                 63.8 
                 88.2 
               
               
                 21 
                 65.2 
                 89.2 
               
               
                 22 
                 65.8 
                 89.4 
               
               
                 23 
                 59.6 
                 88.1 
               
               
                 24 
                 84.8 
                 96.3 
               
               
                 25 
                 81.6 
                 96.9 
               
               
                 26 
                 77.8 
                 96.7 
               
               
                 27 
                 82.4 
                 96.4 
               
               
                 28 
                 70.8 
                 90.4 
               
               
                   
               
            
           
         
       
     
     Examples 29-35 
     This example demonstrates the importance of the choice of collector reagent in performing the flotation of the present invention. 
     Laboratory flotation tests are performed on samples of Twin Creeks low grade gold-bearing refractory sulfide ore at a pH of from about pH 5 to about pH 6 using various collector reagents in cost equivalent amounts. A list of the different collector reagents, companies that supply the reagents and the amount of each collector reagent used are shown in Table 10. Nitrogen gas is used in the grind and as the flotation gas. Particles are sized at a P 80  size of about 46 microns and flotation is conducted in a slurry with 30% solids. The ore samples are of a low grade gold-bearing refractory sulfide ore having about 0.072 ounces per ton of gold and about 5.58 weight percent sulfide sulfur. Other reagents used during the flotation tests are shown in Table 11. 
     Results of the flotation tests are shown in FIGS. 21 and 22, which plot percent gold recovery in the concentrate versus flotation time for the various collector reagents. As shown in FIGS. 21 and 22, potassium amyl xanthate performs anomalously better than the other collectors with the flotation of the present invention. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 10 
               
               
                   
               
               
                   
                   
                   
                 Amount 
               
               
                 Example 
                 Collector 
                 Source 
                 lb/ton 
               
               
                   
               
             
            
               
                 29 
                 PAX (1)   
                 Kerly Mining, Inc. 
                 0.50 
               
               
                 30 
                 S-703 (2)   
                 Minerals Reagents Inc. 
                 0.14 
               
               
                 31 
                 AP-5100 (3)   
                 Cytec Industries, Inc. 
                 0.17 
               
               
                 32 
                 AP-412 (4)   
                 Cytec Industries, Inc. 
                 0.30 
               
               
                 33 
                 AP-3477 (5)   
                 Cytec Industries, Inc. 
                 0.50 
               
               
                 34 
                 CO-200 (6)   
                 Phillips 66 Company 
                 0.18 
               
               
                 35 
                 Minerec A (7)   
                 Minerec Mining Chemicals 
                 0.17 
               
               
                   
               
               
                   (1) potassium amyl xanthate  
               
               
                   (2) ethyl octyl sulfide, dialkyl dithiophosphate, polyglycol alkyl ether  
               
               
                   (3) alkyl thionocarbonate  
               
               
                   (4) Na-mercapto-benzothiazole and Na-di-iso-amyl dithiophosphate  
               
               
                   (5) Na di-iso butyl dithiophosphate  
               
               
                   (6) t-dodecyl mercaptan  
               
               
                   (7) xanthogen formate  
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                   
                 TABLE 11 
               
             
            
               
                   
                   
               
               
                   
                 Other Reagents (lb/ton) 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Sulfuric 
                   
                   
                 Lead 
                 Sodium 
               
               
                 Example 
                 Acid 
                 MIBC 
                 DF250 
                 Nitrate 
                 Silicate 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 29 
                 7.76 
                 0.12 
                 0.25 
                 0.3 
                 1.0 
               
               
                 30 
                 4.8 
                 0.11 
                 0.07 
                 0.3 
                 1.0 
               
               
                 31 
                 4.37 
                 0.07 
                 0.07 
                 0.3 
                 1.0 
               
               
                 32 
                 5.0 
                 0.09 
                 0.09 
                 0.3 
                 1.0 
               
               
                 33 
                 5.04 
                 0.04 
                 0.04 
                 0.3 
                 1.0 
               
               
                 34 
                 5.74 
                 0.17 
                 0.17 
                 0.3 
                 1.0 
               
               
                 35 
                 5.49 
                 0.11 
                 0.11 
                 0.3 
                 1.0 
               
               
                   
               
            
           
         
       
     
     Example 36 
     This example demonstrates the importance of grind media on operation of the flotation of the present invention. 
     Laboratory flotation is conducted on Lone Tree low grade gold-bearing refractory sulfide ore samples using nitrogen during the grind and during flotation as the flotation gas. All samples were ground to a P 80  size of about 44 microns. One sample is comminuted using stainless steel rods while the other sample is comminuted using conventional mild steel balls. 
     Results of the flotation are shown graphically in FIG. 23, which includes a plot of percent gold recovery versus flotation time for each sample and a plot of oxidation reduction potential versus flotation time for each sample. As shown in FIG. 23, the sample milled with stainless steel rods exhibits substantially higher gold recovery at all flotation times. More importantly, a high gold recovery is achieved in a much shorter flotation time for the sample milled with the stainless steel rods than for the sample milled with the mild steel balls. This distinction is significant because it indicates that flotation times may be reduced with the use of stainless steel or other comminution media, such as a high chromium alloy hardened steel, that would introduce less reactive iron into the flotation system. 
     Example 37 
     This example demonstrates a surprising effect of performing a magnetic separation on a ore sample prior to flotation according to the present invention. 
     Samples of Lone Tree subgrade gold-bearing refractory sulfide ore are subjected to laboratory flotation. Both samples are comminuted to a P 80  size of approximately 270 mesh. Following comminution, one sample is subjected to magnetic separation to remove magnetic iron particles prior to flotation while the other sample is not. 
     Results are shown graphically in FIG. 24, which includes a plot of percent gold recovery in the flotation concentrate versus flotation time and a plot of oxidation-reduction potential in the flotation slurry versus flotation time. As seen in FIG. 24, gold recovery is significantly higher for the sample subjected to the magnetic separation. The effect is particularly pronounced at shorter flotation times, but even after 30 minutes of flotation, the sample having been subjected to magnetic separation exhibits gold recovery that is approximately ten percentage points higher than the sample with no magnetic separation. 
     Example 38 
     This example demonstrates the important effect with the present invention of using deoxygenated process water. 
     Samples of a Twin Creeks low grade gold-bearing refractory sulfide ore are subjected to laboratory flotation. Samples contain about 0.072 ounces per ton of gold and about 5.58 weight percent of sulfide sulfur. Grinding is performed in a nitrogen atmosphere for each sample and flotation is performed using nitrogen as the flotation gas. Both samples are sized at a P 80  size of about 46 microns. One sample is slurried with regular tap water for the flotation. The other sample is slurried with tap water that has been deoxygenated by bubbling nitrogen gas through the water for a sufficient time to remove most of the oxygen previously dissolved in the water. 
     Results of flotation for each sample are shown in FIGS. 25-27. FIG. 25 includes a plot of weight recovery in the concentrate versus flotation time for each sample, and shows that flotation with the deoxygenated water attains a greater weight recovery in the flotation concentrate. FIG. 26 includes a plot of gold recovery in the concentrate versus flotation time for each sample, and shows that at the longer flotation times, gold recovery is higher using the deoxygenated water. FIG. 27 includes a plot of sulfide sulfur recovery in the concentrate versus flotation time, and shows that, at longer flotation times, sulfide sulfur recovery in the concentrate is higher using the deoxygenated water. The increased gold and sulfide sulfur recoveries indicate a significantly higher recovery of refractory auriferous sulfides in the concentrate. 
     Examples 39-57 
     These examples demonstrate the benefit of a tail leach with the present invention. 
     Samples of Lone Tree subgrade gold-bearing refractory sulfide ore are subjected to laboratory flotation. Each sample is ground to a P 80  size of approximately 270 mesh in a nitrogen atmosphere. Flotation is conducted with a nitrogen flotation gas. Following flotation for each sample, the flotation tail is subjected to a carbon-in-leach cyanidation to recover gold remaining in the flotation tail. 
     Results are shown in Table 12, where it is seen that the leach of the flotation tail significantly contributes to gold recovery according to the process of the present invention. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 12 
               
               
                   
               
               
                   
                 Gold Recovery (1)   
                   
                   
               
               
                   
                 Form Flotation 
                 Gold Recovery 
                 Total Gold 
               
               
                 Example 
                 Concentrate 
                 from Flotation Tail 
                 Recovery 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 39 
                 87.3 
                 5.7 
                 93.0 
               
               
                 40 
                 81.6 
                 9.1 
                 90.7 
               
               
                 41 
                 72.9 
                 10.3 
                 83.2 
               
               
                 42 
                 75.6 
                 11.4 
                 87.0 
               
               
                 43 
                 79.0 
                 9.2 
                 88.2 
               
               
                 44 
                 74.5 
                 12.3 
                 86.8 
               
               
                 45 
                 75.5 
                 12.8 
                 88.3 
               
               
                 46 
                 79.9 
                 8.9 
                 88.8 
               
               
                 47 
                 82.4 
                 4.3 
                 86.7 
               
               
                 48 
                 83.6 
                 9.8 
                 93.4 
               
               
                 49 
                 85.5 
                 5.4 
                 90.9 
               
               
                 50 
                 85.6 
                 4.6 
                 90.2 
               
               
                 51 
                 87.9 
                 3.9 
                 91.8 
               
               
                 52 
                 78.8 
                 3.5 
                 82.3 
               
               
                 53 
                 78.4 
                 5.8 
                 84.2 
               
               
                 54 
                 84.4 
                 1.5 
                 85.9 
               
               
                 55 
                 76.6 
                 6.1 
                 82.7 
               
               
                 56 
                 87.1 
                 5.4 
                 92.5 
               
               
                 57 
                 80.7 
                 4.0 
                 84.7 
               
               
                   
               
               
                   (1) Assumes 96% of gold in concentrate removed in CIL leach following pressure oxidation.  
               
            
           
         
       
     
     The present invention has been described with reference to specific embodiments of the present invention. According to the present invention, however, any of the features shown in any embodiment may be combined in any way with any other feature of any other embodiment. For example, any feature shown in any one of FIGS. 1-3,  8 ,  10 - 17  and  28  can be combined with any other feature shown in any of those figures. Furthermore, while various embodiments of the present invention have been described in detail, it is apparent that modifications and adaptations to those embodiments will occur to those skilled in the art. It is to be expressly understood that such modifications and adaptations are within the scope of the present invention, set forth in the following claims.