Abstract:
The process of the present invention “jump starts” heap biooxidation of sulfides by incorporating a previously biooxidized material into the heap. The process can be used to recover precious and/or base metals from sulfidic ores and concentrate.

Description:
FIELD OF THE INVENTION 
     The present invention is directed generally to bioleaching of sulfidic ores and specifically to heap and vat bioleaching of sulfidic ores. 
     BACKGROUND OF THE INVENTION 
     A major source of many metals, particularly copper and gold, is sulfidic ores. In sulfidic ores, the metals are either present as or immobilized by stable metal sulfides, which are frequently nonreactive or slow reacting with lixiviants such as cyanide, ferric ion or sulfuric acid. To promote the dissolution of the metals in a lixiviant, the elements compounded with the metal (e.g., sulfide sulfur) are first be oxidized. In one approach, oxidation of the sulfide sulfur is induced by organisms, such as  Thiobacillus Ferrooxidans  and  Thiobacillus Thiooxidans  (commonly referred to as biooxidation or bioleaching). 
     Although biooxidation can be performed in a continuous stirred tank reactor, a common technique is to perform biooxidation in a heap. Compared to biooxidation in a continuous stirred reactor, heap biooxidation generally has lower capital and operating costs but a longer residence time and lower overall oxidation rate for the sulfide sulfur in the feed material. 
     In designing a heap biooxidation process, there are a number of considerations. First, it is desirable to have a relatively high heap permeability and porosity. Fine material can decrease heap permeability and porosity and result in channeling. Channeling can cause a portion of the material in the heap to have a reduced contact with the lixiviant, thereby limiting the degree of biooxidation of the material. Second, it is desirable that the residence time of the feed material in the heap (i.e., the time required for an acceptable degree of biooxidation) be as low as possible. Existing heap leaching processes typically have residence times of the heap on the pad of 12 months or more for an acceptable degree of biooxidation to occur. 
     SUMMARY OF THE INVENTION 
     These and other objectives are addressed by the process of the present invention. The process includes the steps of: 
     (a) biooxidizing a first portion of a feed material containing metal sulfides to form a biooxidized fraction; 
     (b) combining the biooxidized fraction and a second portion of the feed material to form a combined feed material; and 
     (c) thereafter biooxidizing the combined feed material. The metal in the metal sulfides can be copper, gold, silver, nickel, zinc, arsenic, antimony, and mixtures thereof. As will be appreciated, precious metals, such as gold, generally are not compounded with sulfide sulfur but are rendered immobile in the lixiviant by close association with metal sulfides, especially pyrite and arsenopyrite. 
     Because the biooxidized fraction includes large active cultures of organisms, such as  Thiobacillus Ferrooxidans; Thiobacillus Thiooxidans; Thiobacillus Organoparus; Thiobacillus Acidphilus; Sulfobacillus Thermosulfidooxidans; Sulfolobus Acidocaldarius, Sulfolobus BC; Sulfolobus Solfataricus; Acidanus Brierley; Leptospirillum Ferrooxidans ; and the like for oxidizing the sulfide sulfur and other elements in the feed material, the combination of the biooxidized fraction and the second portion of the feed material (which typically has not been biooxidized) “jump starts” the biooxidation of the second portion. In other words, the time required to substantially complete biooxidation of the second portion is significantly reduced relative to existing heap leaching processes, thereby reducing heap pad area and capital and operating costs. 
     The biooxidation in step (a) can be performed in a continuous stirred reactor or on a heap. A continuous stirred reactor is preferred because of the relatively rapid rate of biooxidation in such reactors and the high concentration of microbes on the biooxidized residue. After inoculation of the slurried portion of the feed material, the continuous stirred reactor preferably is sparged with oxygen and supplied with suitable nutrients for the microbes to foster biooxidation. 
     Typically, the second portion of the feed material has not been biooxidized. In one embodiment, the second portion is coarsely sized while the biooxidized fraction (i.e., the first portion) is finely sized. The biooxidized fraction typically has a P 80  size preferably ranging from about 5 to about 200 microns and more preferably from about 10 to about 200 microns while the second fraction has a P 80  size in excess of that of the biooxidized fraction. 
     The combining step can be performed in a number of ways. For example, the biooxidized fraction can be agglomerated with the second portion of the feed material. “Agglomeration” refers to the formation of particles into a ball (i.e., an agglomerate), with or without the use of a binder. Alternatively, the biooxidized fraction can be placed on the conveyor belts along with the second portion of the feed material and be carried by the belts to the heap. 
     In another embodiment, the process includes the step of floating a portion of the feed material to form a tailing fraction and a concentrate fraction. The first portion of the feed material includes the concentrate fraction. A substantial portion of the fine material is discarded in the tailing fraction so that the porosity and permeability of the heap remains unaffected by the fine size of the relatively small quantity of concentrate fraction (which is incorporated into the heap after partial or complete biooxidation of the concentrate fraction). Commonly, the first portion of the feed material constitutes no more than about 15 wt % of the feed material while the concentrate fraction constitutes no more than about 30 wt % of the first portion (i.e., no more than about 4.5 wt % of the feed material). Accordingly, the tailing fraction constitutes at least about 70 wt % of the first portion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts an embodiment of the present invention for the recovery of base metals; and 
     FIGS. 2A and B depict an embodiment of the present invention for the recovery of base and/or precious metals. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, an embodiment of the present invention is depicted for recovering base metals from sulfidic ores. The recoverable base metals include copper, iron, nickel, zinc, antimony, arsenic, and mixtures thereof. The metal generally occurs in the ore as a metal sulfide, such as chalcopyrite (CuFeS 2 ), bornite (Cu 5 FeS 4 ), chalcocite (Cu 2 S), digenite (CU 9 S 5 ), covellite (CuS), and the like. 
     A feed material  10  containing the metal sulfide is comminuted  14  to produce a comminuted material  18 . The P 80  size of the comminuted material preferably ranges from about 5 to about 20 mm. 
     The comminuted material  18  is subjected to primary size separation  22  to form an undersized fraction  26  and an oversized fraction  30 . Primary size separation  22  can be performed by any suitable technique, with screening being most preferred. The preferred screen size ranges from about 1 to about 3 mm. Typically, the undersized fraction  26  represents no more than about 30 wt % of the comminuted feed material  18  while the oversized fraction  30  represents at least about 70 wt % of the comminuted feed material  18 . 
     The undersized fraction  26  is subjected to secondary size separation  34  to produce a sand  38  and a fine portion  42  of the feed material. Secondary size separation  34  can be performed by any suitable techniques such as by cycloning or screening. The secondary size separation  34  is performed such that the fine portion  42  represents no more than about 20 wt % of the undersized fraction  26 . The secondary size separation  34  is typically performed such that the fine portion  42  has a P 80  size ranging from about 5 to about 200 microns. 
     The sand  38  is combined with the oversized fraction  30  to form a coarse portion  46  of the feed material. Preferably, the coarse portion  46  represents at least about 90 wt % of the comminuted feed material  18 . 
     The fine portion  42  is subjected to flotation  50  to produce a tailing fraction  54  and a concentrate fraction  58 . As will be appreciated, the concentrate fraction  58  contains most of the metal sulfide and preferably at least about 80% of the metal sulfide in the fine portion  42 . The collectors and frothers and conditions used during flotation  50  depend, of course, on the particular metal sulfide being recovered. They may include, but are not limited to, xanthates and dithiophosphates. Typically, the concentrate fraction  58  constitutes no more than about 20 wt % of the fine portion  42 . 
     The concentrate fraction  58  is slurried and biooxidized  62  in a series of continuous stirred tank reactors, to produce a biooxidized slurry  66 . Biooxidation  62  is preferably conducted at a slurry temperature ranging from about 20 to about 60° C.; a slurry pH ranging from about pH 1.2 to about pH 2.5; and a sulfuric acid content in the slurry ranging from about 1 to about 20 g/l. During biooxidation  62 , air is sparged through the slurry to provide molecular oxygen for biooxidation. The slurry further includes microbes and suitable energy source and nutrients for the microbes, namely from about 0.1 to about 10 g/l of Fe 2+ ; from about 0.1 to about 10 g/l of ammonium sulfate (NH 4 ) 2  SO 3 , from about 0.05 to about 5 g/l of a phosphate. 
     The microbes that can be used for biooxidation are discussed in U.S. Pat. No. 5,246,486 entitled “Biooxidation Process for Recovery of Gold from Heaps of Low-Grade Sulfidic and Carbonaceous Sulfidic Ore Materials”, which is incorporated herein by this reference. The microbes include  Thiobacillus Ferrooxidans; Thiobacillus Thiooxidans; Thiobacillus Organoparus; Thiobacillus Acidphilus; Sulfobacillus Thermosulfidooxidans; Sulfolobus Acidocaldarius, Sulfolobus BC; Sulfolobus Sulfataricus; Acidanus Brierley; Leptospirillum Ferrooxidans ; and the like. The microbes can be classified as either, (a) facultative thermophile, i.e., the microbe is capable of growth at mid-range temperatures (e.g., about 30° C.) and high (thermophilic) temperatures (e.g., above about 50° C. to about 55° C.) or (b) obligate thermophile which are micro-organisms which can only grow at high (themophilic) temperatures (e.g., greater than about 50° C.). 
     The biooxidized slurry  66  is subjected to liquid/solid separation  70  to form a pregnant leach solution  74  and a biooxidized residue  78 . The pregnant leach solution  74  is subjected to metal recovery  82  to produce a metal product  86 . Metal recovery  82  can be performed by any suitable technique including solvent extraction/electrowinning. 
     The biooxidized residue  78 , which contains active cultures of microbes, is combined with the coarse portion  46  of the feed material to form a combined feed material. The combined material can be agglomerated  90  with or without a suitable binder to form agglomerates  94 . The combined feed material can be contacted with additional microbes prior to agglomeration. In some cases, it may be desirable to introduce different cultures of microbes that flourish at temperatures different from the cultures of microbes present on the biooxidized residue  78 . As will be appreciated, a temperature profile will generally exist in the heap. 
     As shown in FIG. 1, the biooxidized material can alternatively be placed directly on a conveyor belt to the heap along with the second portion of the feed material or on top of the heap formed from the second portion of the feed material. 
     The agglomerates  98  are formed into a heap  102 . The heap  102  is formed on a lixiviant-impervious liner, and an irrigation system for the lixiviant is erected on the heap. A cooling and/or heating system can be installed on the process solution flowstream for temperature control. Air may be introduced to the body of the heap through a pipe network under positive pressure to promote ingress of molecular oxygen through the heap. 
     The heap  102  is biooxidized  106  to produce a solid waste material  110  and a primary pregnant leach solution  114  containing most of the metal values in the comminuted material  18 . Biooxidation is performed by applying a lixiviant, preferably sulfuric acid and containing an innoculate capable of biooxidizing sulfide sulfur and other elements compounded with the metal and/or nutrients for the microbes, to the top of the heap; percolating the lixiviant and nutrients through the heap; and removing the primary pregnant leach solution  114  from the base of the heap  102 . 
     For optimal results, the conditions in the heap  102  are carefully controlled. The lixiviant preferably has a pH less than about pH 2.5 and more preferably ranging from about pH 1.3 to about pH 2.0. The lixiviant can include from about 1 to about 10 g/l of ferric ion sulfate to aid in the dissolution of metals. The lixiviant can also contain an energy source and nutrients for the microbes, such iron sulfate,ammonium sulfate and phosphate. 
     If the combined feed material contains significant amounts of arsenic, the arsenic can be removed by coprecipitation with iron under suitable conditions. Typically, pentavalent arsenic and trivalent iron will coprecipitate when the solution ratio of Fe:As exceeds 4:1 and the solution pH exceeds 3. 
     The primary pregnant leach solution  114  can be subjected to metal recovery  82  to produce the metal product  86 . When biooxidation is complete, the fully biooxidized material in the heap becomes waste material  110 . 
     FIGS. 2A and B depict a second embodiment of the present invention for recovering precious and base metals from a sulfidic feed material. The feed material  100  is comminuted  14  to form a comminuted material  118 . The comminuted material  118  is subjected to primary size separation  22  to form an oversized fraction  130  and an undersized fraction  126 . The undersized fraction  126  is subjected to secondary size separation  34  to produce sand  138  and a fine portion  142  of the feed material. The fine portion  142  is subjected to flotation  50  to form a tailing fraction  154  and a concentrate fraction  158 . The concentrate fraction  158  is biooxidized  62  to form a biooxidized slurry  166 , which is subjected to liquid/solid separation to form a secondary base metal pregnant leach solution  174  and a residue  178 . The residue  178  contains most of the precious metal content of the fine portion  142  of the feed material. 
     The oversized fraction  130  and sand  138  are combined to form a coarse portion  146  of the feed material, and the coarse portion  146  is combined with the residue  178  and the combined material agglomerated  90  to form agglomerates  194 . The agglomerates  194  are formed  98  into a heap  202 . The heap is biooxidized  106  to form biooxidized agglomerates  206  and primary base metal pregnant leach solution  214 . The primary and secondary base metal pregnant leach solution  174  is subjected to base metal recovery  82  to form a base metal product  186  where applicable. 
     The biooxidized agglomerates  206 , which contain most of the precious metal content of the comminuted material  118 , are repeatedly and thoroughly washed  218 , preferably with an aqueous solution, to remove the lixiviant from the agglomerated particles and form washed biooxidized material  222 . During washing, the agglomerates will commonly break apart, thereby facilitating lixiviant removal. 
     The washed biooxidized material  222  is neutralized  226  by contact with a base material to form neutralized material  230  and agglomerated  234  to form agglomerates  230 . The base material, which is preferably lime, limestone, Portland cement, caustic soda, cement dust, or mixtures of these materials, can be utilized as a binder during agglomeration. As will be appreciated, neutralization is important as the washed biooxidized material  222  is fairly acidic and can cause uneconomically high cyanide consumption during cyanidation  246 . 
     The agglomerates  234  are formed into a reconstituted heap  242  which is subjected to cyanidation  246  (using a cyanide lixiviant) to dissolve the precious metal in the agglomerates  234  in a precious metal pregnant leach solution  254 . The precious metal pregnant leach solution  254  can be subjected to precious metal recovery  258  by known techniques to produce a precious metal product  262 . After tyanidation  246  is completed, the agglomerates  234  can be discarded as waste material  250 . 
     While various embodiments of the present invention have been described in detail, it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the scope of the present invention, as set forth in the following claims.