Patent Publication Number: US-2002001552-A1

Title: Treatment of roasted metal sulphide ores and ferrites by leaching with peroxysulphuric acid

Description:
[0001] The present invention relates to a process for the treatment of roasted metal sulphide ores, and especially to a process for the treatment of roasted ores containing copper and zinc sulphides for the separation of copper, zinc and other metal values therefrom. The present invention also relates to a process for the treatment of ferrites (as defined herein) in roasted metal sulphide ores and from other sources for separation of metals (metal values) therefrom.  
       [0002] The current industrial methods for separation of base metals and precious metals from metal sulphides generally employ smelting. However, metal sulphide smelters are a major source of industrial pollution, and of serious environmental concern. Such concerns have led to the shutting down of base metal development projects that rely on smelting techniques for the recovery of base metals and precious metals.  
       [0003] Extraction and refining of base metals from sulphide ores may be carried out by smelting, a pyrometallurgical process, which is usually followed by metal electrowinning and metal refining. Base metal sulphides are generally calcined i.e. roasted in air, to produce a metal oxide that is smelted to produce crude metal, for example blister nickel. The crude metal is subsequently re-melted, and cast into anodes which are used as feed material for metal electrowinning processes. Precious and rare metals, e.g. selenium and tellurium, do not dissolve in the electrolyte that is used and fall to the bottom of the electrolyte cell as anode slimes.  
       [0004] The slimes may be digested with sulphuric acid to remove the residual base metals. The liquor obtained is then added to a stage for recovery of secondary base metals. Arsenic usually remains in the electrolyte and causes environmental problems in the aqueous state. Residual slimes are smelted to produce a precious metal (dore) bar which may contain a residue of base metals plus antimony, bismuth, tellurium and some rare earth metals. The dore bar usually contains between 10-50 percent by weight of gold, and is usually shipped to the mint for gold extraction and refining. The refining of such dore bars is usually expensive, using for example the chloride and cyanidation processes that are used in many precious metal refineries for the separation of metals from gold. Both processes tend to be very hazardous environmentally because of the waste that is emitted.  
       [0005] In other processes, zinc may be recovered by roasting, leaching and electrowinning. Lead, which is frequently associated with zinc, is processed by oxidation of lead sulphide to oxide, which is then reduced to metallic lead. The resultant material is heated and impure copper in the form of copper dross is skimmed off from the top of the heated material. Addition of zinc produces insoluble intermetallic silver. Silver, gold and other precious metals are usually recovered by dore liquation i.e. addition of borax or chloride as fluxing materials followed by vacuum distillation and cupel (oxygen) treatment to obtain dores. Such a process may be referred to as a pyrometallurgical and cupellation process.  
       [0006] In lead refining, addition of calcium or magnesium (alkaline earth metals) usually results in the formation of an intermetallic compound with bismuth and oxidation slags containing arsenic or antimony. Such a process also involves pyrometallurgic procedures.  
       [0007] A process for the recovery of, in particular, copper and zinc from metal sulphide ores that does not involve the smelting of the ores would be useful, especially to eliminate the resulting environmental hazards associated with smelting and other processes.  
       [0008] Many ores contain significant amounts of iron. In processes involving application of heat, especially under oxidizing conditions, complex multiple oxides of ferric oxide are formed, which may be referred to as ferrites and ferrates; as used herein, all such complex multiple oxides of ferric oxide are referred to as ferrites, with further complexes or compounds with other metals being referred to as metal ferrites.  
       [0009] In particular, processes involving dead or sulphation roasting followed by treatment with sulphuric acid to recover e.g. zinc as soluble sulphates, generally result in failure to recover metal values from ferrites in the absence of additional subsequent severe leach conditions with high acid concentrations and elevated temperature. For instance, typical weak acid leaching to recover zinc values from zinc oxide formed during roasting does not recover zinc values from zinc ferrites. The latter zinc values must be recovered under more severe leaching conditions, with higher acid concentration and higher temperatures. However, under the severe leaching conditions, iron values are not only recovered from zinc ferrite but also from other iron oxides in the roasted ore. Consequently, solutions containing zinc contain higher concentrations of iron, which affects subsequent steps to recover zinc and other dissolved metal values. In addition, the higher acid concentrations must be neutralized, thereby requiring use of greater quantities of lime or other bases, which affects steps to recover metal values from tails and in other downstream processes.  
       [0010] Various ferrites are used in the ceramics and electronics industries e.g. so-called ceramic ferrites, and methods of recycling of metals therefrom are required.  
       [0011] GB 1 594 851 discloses the use of sulphuric acid containing peroxysulphuric acid in the extraction of un-roasted zinc sulphide ores. EP 0 039 837 discloses the selective removal of iron from a sulphate leach solution by addition of peroxysulphuric acid to the leach solution to oxidize ferric ions. U.S. Pat. No. 4,452,706 discloses the oxidation of iron and arsenic in a leach solution to ferric and arsenic ions using peroxysulphuric acid.  
       [0012] Processes with the potential for recovery of metal values from sulphide ores in a more efficient manner, with less potential environmental impact, and for recovery of metal values from ferrites and the potential use of such ferrites are required. Such a process has now been found.  
       [0013] Accordingly, one aspect of the present invention provides a process for the treatment of a ferrite for the separation of metals therefrom, comprising:  
       [0014] (a) leaching said ferrite with peroxysulphuric acid; and  
       [0015] (b) separating a solution containing metals therefrom.  
       [0016] Another aspect of the present invention provides a process for the treatment of a roasted metal sulphide ore, for the separation of metals therefrom, comprising:  
       [0017] (a) leaching said roasted ore with peroxysulphuric acid; and  
       [0018] (b) separating a solution containing metals therefrom.  
       [0019] In preferred embodiments of the processes of the present invention, the ore has been subjected to a dead roast or to a sulphation roast.  
       [0020] In further embodiments, the roasted ore contains ferrite.  
       [0021] In another embodiment, a ferrite is added to the ore, before or after roasting.  
       [0022] In a further embodiment, a solution containing copper and zinc is obtained.  
       [0023] In another embodiment, the ferrite is being recovered from ceramic ferrites or from the electronics industry.  
       [0024] In particularly preferred embodiments of the processes of the invention, leaching or treatment is carried out with peroxysulphuric acid (Caro&#39;s acid) in the presence of gaseous chlorine.  
       [0025] In one aspect, the present invention relates to a process for the treatment of metal sulphide ores, and in particular to a process for the treatment of metal sulphide ores that contain substantial proportions of copper and zinc sulphides. It is understood that the ores will likely also contain iron sulphides, and may also contain at least minor amounts of a large variety of other metallic compounds especially in the form of sulphides. For example, the ore could contain aluminum, arsenic, bismuth, barium, calcium, cadmium, potassium, magnesium, manganese, sodium, nickel, phosphorous, lead, titanium, tellurium and vanadium, as well as other metals. Many such ores also contain gold.  
       [0026] The leaching process is operable on ore or ore concentrate formed from an ore, which has previously been subjected to a roasting process. The roasted ore or concentrate should be in a particulate form, especially particles of a size suitable for use in a fluidized bed, rotary kiln or torbed process for treatment of turbulent masses of material, as is known. Thus, the ore or concentrate may have been subjected to a grinding process.  
       [0027] Methods for roasting of ores are known. In one method, known as dead roasting, the particulate ore is roasted in the presence of air, with sufficient air being present for oxidation of the sulphide to the corresponding oxide and formation of sulphur dioxide. Good circulation of air through the particulate during the roasting process is believed to be important, both to expedite the roasting of the sulphide to the oxide and to prevent the formation of local hot spots within the ore during the roasting process. A fluidized bed or other techniques may be used for the roasting of the ore as is known.  
       [0028] The roasting of the ore is conducted at a temperature to effect oxidation of the metal sulphide to the corresponding metal oxide, particularly copper sulphide and zinc sulphide to the corresponding oxides, without agglomeration of the particulate. It is known that oxides are be formed at relatively low roasting temperatures e.g. 700° C. or higher, but that ferrite is also formed, if iron is present as is normally the case, at similar temperatures. Thus, ferrites will normally be present in roasted ore, even in those instances where steps are taken to try to reduce formation of ferrities.  
       [0029] As an alternative to dead roasting of the ore in the presence of air, sulphur dioxide may be introduced into the roasting chamber to cause the formation of metal sulphates during the roasting process. Metal sulphates are water soluble and therefore the roasting material should require less quantity of acid in order to solubilize metal species i.e. solubilize metal oxides, during the leaching step. The use of sulphation roasting will result in a lower operating temperature than required for an oxidation roasting.  
       [0030] The roasting may be carried out in the presence of both oxygen and sulphur dioxide, as such a mixture of gases will result in conversion of metal oxides to the corresponding sulphate by reaction with sulphur trioxide, and the conversion of metal sulphides to the corresponding metal sulphates by reaction with oxygen. The first reaction tends to be endothermic and the second reaction exothermic, thereby permitting control of the temperature in the process.  
       [0031] Both dead roasting and sulphation roasting are known per se, and not part of the process of the present invention. The present invention may be applied to roasted ore from both processes, but in aspects it also applies to combinations of roasting and leaching as described herein.  
       [0032] According to one aspect of the present invention, roasted ore is subjected to leaching with peroxysulphuric acid. Caro&#39;s acid is peroxysulphuric acid, also known as persulphuric acid, which has the formula H 2 SO 5  or HOSO 2 OOH. Such an acid is both highly acidic and a strong oxidizing agent. In embodiments of the present invention, the leaching is carried out in multiple steps, with the acid concentration in the first step being relatively dilute compared to the concentration in a subsequent step.  
       [0033] The process provides a leach solution, which is a solution of leached metals. The metals are in a soluble form, which would normally be in the form of a sulphate of the metal. The peroxysulphuric acid is converted to sulphuric acid during the leaching process, and many metal sulphates especially those of copper and zinc are soluble in the solution.  
       [0034] Leach solutions may be diluted, if necessary or desirable, and then be treated for recovery of metal values therein. In particular, the leach solutions are treated for recovery of copper and zinc. Techniques for the recovery of copper and zinc from acid solutions, especially sulphuric acid solutions, are known.  
       [0035] It is to be anticipated that the leach solutions will contain relatively high concentrations of one or more metals, and that such metals would normally be the first to be recovered from the leach solution. The leach solution subsequently remaining would normally then be further treated for recovery of other metal values. Techniques for the recovery of such other metal values from such solutions are known.  
       [0036] In the roasting process, the presence of both iron and zinc in a metal sulphide ore lead to the formation of zinc iron ferrite compounds which are generally resistant to weak acid leaching conditions. Other ferrites are also known. However, ferrites are leachable in the process of the present invention. In particular, embodiments of the invention, ferrites may be added to roasted ore or treated separately according to the invention.  
       [0037] Ferrites are multiple oxides of ferric oxide with another oxide. A variety of techniques are known for the formation of ferrites, including heating metal oxides with ferric oxide. This occurs naturally in roasting of ores as roasting tends to form metal oxides e.g. metal sulphides are converted to the corresponding oxides. Many ores contain significant quantities of iron, which tends to be converted to iron oxide, especially ferric oxide, during roasting. Thus, ferrites tend to be formed during roasting even under controlled temperature conditions. As noted above, temperatures used in roasting are similar to those for formation of ferrites.  
       [0038] Other sources of ferrites are known. For instance, ceramic ferrites are obtained by sintering or firing mixtures of the oxides. Ferrites are used in rectifiers, recording tapes, permanent magnets, semiconductors, insulating materials and dielectrics, e.g. in the computer, television, radio, radar and other industries, especially electronics industries.  
       [0039] In the treatment of ferrites with Caro&#39;s acid, it is believed that iron is converted to higher oxidation states such as the Fe +6  state, in which it is a strong oxidizing agent. In particular, it is believed that it oxidizes ferrites to effect separation of the metal value from the iron, thereby facilitating recovery of the metal value. While formation and use of Fe +6  is believed to be important in the leaching of ferrites from a variety of sources, it is believed to be particularly so in leaching of metal values from roasted sulphide ores.  
       [0040] In preferred embodiments of the invention, leaching or treatment is carried out using both Caro&#39;s acid and gaseous chlorine. As exemplified herein, use of combinations of Caro&#39;s acid and chlorine can provide further improvements in the degree of extraction.  
       [0041] Arsenic oxides tend to vaporize and be removed from the roasting chamber. It is believed to be advantageous to remove arsenic during roasting, and prevent arsenic from entering the aqueous phase during the leaching step. There may be difficulty in controlling the presence of arsenic in aqueous solutions, with the consequent environmental hazard.  
       [0042] Tails obtained from the leach solution may be treated for separation of gold and other metals from the tails. A preferred method for the leaching of gold from the tails is to subject the tails to treatment with Caro&#39;s acid, as is described in WO 97/05294 of Protium Metals Inc., published Feb. 13, 1997.  
       [0043] The process of the present invention provides a method of recovery of a variety of metal values, including copper and zinc, from roasted sulphide ores without requiring the smelting of the ore. This has substantial advantages in reducing environmental hazards associated with smelting processes. The present invention also provides a versatile process for recovering copper, zinc and other metals, including precious metals, from roasted metal sulphide ores and concentrates. In addition, the present invention provides for the recovery of metals from ferrites obtained from a number of sources including roasted ores and recycled ferrites used in other industries.  
       [0044] It is believed that the process of the present invention will lead to less consumption of acid cf. use of weak acid/strong acid leaching in sequence, less base to neutralize acid, less slime and/or less material sent to settling ponds.  
       [0045] The process of the present invention is illustrated by the following examples. 
     
    
    
     EXAMPLE I  
     [0046] A metal sulphide ore concentrate containing copper, zinc and iron was subjected to a roasting step at a temperature of about 950° C.  
     [0047] Samples of the calcined ore obtained were subjected to leaching with aqueous solutions of sulphuric acid or Caro&#39;s acid at temperatures of 25° C. or 90° C. Details of the calcined ore and leaching conditions are given in Table 1. Other leaching conditions, especially leaching time, were the same for all leaching steps.  
     [0048] The results obtained are given in Table 1.  
                                   TABLE 1                                   Weight   Cu (%)   Zn (%)   Fe (%)                                                Run A: Leach with H 2 SO 4  at pH 1.5, 25° C.                                         Concentrate   100   17.35   18.65   22.9           Residue   63.5   18.9   7.99   36.6           Extraction (%)       49.4   83.7   11.4                         Run B: Leach with H 2 SO 4  at pH 1.0, 90° C.                                         Concentrate   100   17.7   19.1   24.2           Residue   49.3   11.9   7.5   38.1           Extraction (%)       67.3   80.9   23.1                         Run C: Leach with Caro&#39;s acid, 90° C.                                         Concentrate   100   17.7   19.1   24.2           Residue   27.8   2.9   7.5   34.1           Extraction (%)       96.3   90.2   70.7                         Run D: Leach with Caro&#39;s acid plus chlorine, 90° C.                                         Concentrate   100   17.7   19.1   24.2           Residue   25.2   1.9   6.7   32.5           Extraction (%)       97.2   93.3   73.5                      
 
     [0049] The results show that leaching with Caro&#39;s acid, at 90° C., was substantially more effective in separation of copper, zinc and iron values from the concentrate than leaching with strong sulphuric acid at a pH of 1.0 at the same temperature or with weaker acid at 25° C. For instance, copper extraction increased from 67.3% to 96.3% with the residue of copper decreasing from 11.9% to 2.9%. Copper extraction was further increased to 97.2% and the residue decreased to only 1.9% by using Caro&#39;s acid in the presence of chlorine.  
     [0050] The results for both copper and iron indicate that complex iron oxides with copper e.g. copper ferrites are extracted in the process and/or that the use of Caro&#39;s acid is causing formation of Fe +6  or other highly oxidized states of iron, which act as oxidizing agents and further increase extraction of copper.  
     [0051] Similar results were obtained for zinc.