PROCESS FOR RECOVERY OF METAL

Abstract: A process for recovery of metal from a feedstock comprising a metal selected from the group comprising gold, platinum, silver, copper, nickel, cobalt, zinc, lead, antimony, molybdenum, chromium, mercury, manganese, tin, other members of the platinum series and combinations thereof is provided. The process comprises the steps of: (a) contacting the feedstock with an aqueous mixture comprising ammonium nitrate having a concentration greater than about 1.5 M and sulphuric acid having a concentration greater than about 6 M at a temperature of at least about 105 DEG C, to produce an insoluble residue and a leachate comprising the metal dissolved therein; (b) separating the residue from the leachate; and (c) recovering the metal from the leachate. The process is particularly advantageous for the recovery of gold and other precious metals, in addition to being useful to recover copper.

Description: PROCESS FOR RECOVERY OF METAL FIELD OF THE INVENTION The present invention relates to a novel process for recovery of metal from a feedstock comprising a metal selected from the group comprising gold, platinum, silver, copper, nickel, cobalt, zinc, lead, antimony, molybdenum, chromium, mercury, manganese, tin, other members of the platinum series and combinations thereof. More particularly, the present invention relates to a novel process for recovery of such a metal from an ore, a concentrate or a combination thereof. BRIEF DESCRIPTION OF THE PRIOR ART The use of acidic leach media to leach sulphur, arsenic and telluride compounds of metals such as copper, iron, zinc and the like is known. To date, much of the prior art has been devoted to the use of leach media comprising nitric acid and sulphuric acid at temperatures generally below 100°C. Unfortunately, when such leach media are utilized under these conditions, a solution containing 100% of the metal can not be obtained. United States patent 3,888,748 (Brennecke) discloses a process for recovering metal values from ore concentrates. Essentially, the process comprises contacting the ore concentrate with an aqueous solution of nitric and sulphuric acids to produce a mixture comprising a residue and a leachate. The leachate is further processed to recover copper and iron precipitated as jarosite. This process is deficient in that it requires the use of highly corrosive acids as the leach medium. Moreover, the solution of nitric and sulphur acids does not completely dissolve the concentrate. United States patent 3,910,636 (Hard) discloses a process for in situ mining. Specifically, the process comprises the use of an acidic leaching medium comprising nitrate ions and having a pH of from about 0.2 to about 2.0. The process comprises drilling at least one well into an underground formation or ore deposit and thereafter sealing at least a portion of the well. The leaching solution is then introduced into the wellbore and SUBSTITUTE SHEET thereafter into the zone in the formation immediately surrounding the wellbore. Since the process involves in situ mining, a relatively dilute concentration of nitrate ion and sulphuric acid is required. Not surprisingly, the dilute nature of the leaching medium necessitates relatively long residence time to obtain maximum copper loading in the leach medium. Even with such long residence time, the leaching medium does not completely dissolve the contacted copper ore. United States patent 3,912,330 (Carnahan et al) discloses chemical mining of copper porphyry ores. Specifically, it is disclosed that the addition of catalytic amounts of nitrate ion added to an oxygenated sulphuric acid leach medium purportedly improves the rate of copper extraction from copper sulphides contained in porphyry ores. It is disclosed that the nitrate ion has the catalytic effect when present in concentrations of from 0.05 to 0.50% of the leach medium. During the process, any codissolved iron is precipitated as jarosite. Thus, the pregnant leach solution recovered in the process purportedly comprises very low iron concentration. "Oxygen pressure" is disclosed as being an important variable in the process. Preferred oxygen pressures are in the range of from 25 to 200 psi. The preferred temperature in the leaching step is 100°C. It is noteworthy that the patentee acknowledges that the subject process is not feasible for use in conventional surface heap leaching. Indeed, the limitations of temperature and oxygen overpressure in conventional in situ mining processes effectively exclude such processes from use in surface heap leaching applications. It would be desirable to have a process for recovery of metal from a feedstock comprising a metal selected from the group comprising gold, platinum, silver, copper, nickel, cobalt, zinc, lead, antimony, molybdenum, chromium, mercury, manganese, tin, other members of the platinum series and combinations thereof. In cases where the feedstock comprises copper and iron, it would be desirable if the recovered iron was in a form other than jarosite which is a product of little or no value. It would also be desirable if such a process, when used in a batch-wise manner, was time efficient (e.g. minutes instead of hours). DISCLOSURE OF THE INVENTION It is an object of the present invention to provide a novel process for recovery of metal from a feedstock comprising gold, platinum, silver, copper, nickel, cobalt, zinc, lead, antimony, molybdenum, chromium, mercury, manganese, tin, other members of the platinum series and combinations thereof It is another object of the present invention to provide a novel process which mitigates or obviates at least one of the above-mentioned deficiencies of the prior art. Accordingly, the present invention provides a process for recovery of metal from a feedstock comprising a metal selected from the group comprising gold, platinum, silver, copper, nickel, cobalt, zinc, lead, antimony, molybdenum, chromium, mercury, manganese, tin, other members of the platinum series and combinations thereof, the process comprising the steps of: (a) contacting the feedstock with an aqueous mixture comprising ammonium nitrate having a concentration greater than about 1.5 M and sulphuric acid having a concentration greater that about 6 M at a temperature of at least about 105°C, to produce a insoluble residue and a leachate comprising the metal dissolved therein; (b) separating the insoluble residue from the leachate; and (c) recovering the metal from the leachate. A key aspect of the present process is the temperature at which Step (a), the leaching step, is conducted. Specifically, it has been discovered that the use of a defined temperature in combination with defined reagents leads to a surprisingly and unexpectedly enhanced level of metal removal from the feedstock. Thus, the temperature in Step (a) of the present process is greater than about 105°C. For most practical applications of the process, the temperature will be in the range of from about 105° to about 170°C. Below about 105 °C, the process is somewhat inefficient in that a significant amount SUBSTITUTE S of the metal in the feedstock is not leached therefrom. Above about 170°C, there is substantially no improvement in the process; in fact the process becomes expensive to operate in that it becomes energy intensive and Step (a) should be conducted in a vessel capable of withstanding high temperatures. Preferably the temperature in Step (a) of the present process is in the range of from about 105°C to about 130°C, preferably from about 110°C to about 120°C, most preferably from about 112°C to about 115°C. Under such conditions the leaching step (i.e. Step (a)) is very time efficient. Preferably, this step in the process is conducted in a period of time in the range of from about 10 to about 40 minutes, more preferably from about 15 to about 25 minutes, most preferably about 20 minutes. Indeed, one of the significant advantages of the present process over the prior art is that the overall efficiency of recovery of the metal is significantly improved by maximizing the fraction of metal recovery while substantially minimizing the time required to conduct the process. BRIEF DESCRIPTION OF THE DRAWING Embodiments of the present invention will be described with reference to the accompanying drawing which illustrates a schematic of the present process. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The feedstock suitable for use in the present process is not particularly restricted provided that it comprises a metal (elemental or ionic form) selected from the group comprising gold, platinum, silver, copper, nickel, cobalt, zinc, lead, antimony, molybdenum, chromium, mercury, manganese, tin, other members of the platinum series (e.g. osmium, rhodium, iridium and palladium) and combinations thereof. As will be appreciated various of these metals may be found alone or in combination, and thus, the present process is applicable to feedstocks comprising one or more of this metals. Preferably, the feedstock is selected from at least one of an ore, S a concentrate or a combination thereof. As is well known in the art, an ore is typically mined from the ground and may contain a desired metal in a relatively dilute concentration (e.g. 0.5% by weight or less). It is quite common to initially process the ore to produce a concentrate (also known as an ore concentrate) wherein the concentration of the metal is increase by a factor of twenty or more with respect to the concentration of the metal in the original ore. The present process is applicable to a feedstock which is an ore, a concentrate or a combination of these. A particularly preferred feedstock is one which comprises at least one metal selected from the group comprising gold, platinum and silver. As is well known, each of these metals is considered a precious metal. Gold is generally mining as a free milling gold ore or a refractory gold ore. As is known a free milling ore is one which may be subjected to mechanical processing to physically separate the gold from the ore. A non-limiting example of a useful free milling gold ore is placer gold. A refractory gold ore is one which must be subject to more severe processing to release the gold from the ore (i.e. the gold is "locked into" the ore and usually is not obtainable by mechanical process). Non-limiting examples of useful refractory gold ores include auriferous sulphide ores (or the oxides produced by their alteration, especially pyrite, arsenopyrite, chalcopyrite, spalerite, marcasite and the like) and telluride ores (these are the only minerals containing gold in combination and include sylvanite, calaverite and the like). Concentrates of free mill and refractory gold ores may also be used in the present process. As is known in the art, conventional gold recovery techniques are dangerous since the employ the use of cyanide extraction techniques and inefficient since they require of contact time of many hours or even days. The present process is particularly advantageous as a replacement for these conventional gold recovery processes since it obviates the need to employ cyanide and, in many case, can be used to recover a substantial amount of the gold in an hour or less. Platinum is naturally available in a number of mineral and other forms. Examples of minerals which contain platinum are olivine (and SUBSTITUTE SHEET serpentine derived from alteration thereof), chromite, magnetite, zircon, corundum and the like. Copper is naturally available in a number of mineral and other forms. Typically, copper is mined from the ground as an ore. This copper ore, a concentrate thereof or a combination of the ore and the concentrate may be used in the present process. Non-limiting examples of suitable copper concentrates include chalcopyrite (comprises copper and iron), chalcocite (comprises copper), bornite (comprises copper and iron), molybdenite, pyrite, arsenopyrite, sphalerite, cuprite, native copper and tetrahedrite. Preferably, the concentrate is chalcopyrite. Alternatively, the feedstock may comprise copper in the form of an ore selected from sulphide ore and arsenide ore. Non-limiting examples of suitable arsenide ores include iron diarsenide and cobalt diarsenide. Non-limiting examples of suitable sulphide ores are those comprising, inter alia, at least one member selected from the group comprising chalcopyrite, chalcocite, bornite, molybdite, pyrite, arsenopyrite, sphalerite and tetrahedrite. Further the feedstock comprising copper may be a telluride precious metal ore also containing the uncombined metals (e.g. silver and gold). It will be appreciated that, in certain cases, the feedstock may comprise both copper and iron. For example, in the case of sulphide ores containing minerals such as chalcopyrite (CuFeS^ and bornite (Cu 3 FeS 4 ), a preferred aspect of the present process may be used to recover both the copper and the iron as useful products. Additionally, aspects of the present process may be utilized to separate and recover other metals which may be in the feedstock, such as zinc, silver and gold. Nickel and cobalt are generally found in nature in mineral form. Examples of such minerals include pyrrhotite and pentlandite. Other metals which may be recovered by the present process are typically found in nature in mineral form as follows: SUBSTITUTE SHEET The feedstock suitable for use in the present process is typically in paniculate form. Preferably, the feedstock is ground to a size of from about 250 mesh to about 350 mesh, more preferably from about 275 mesh to about 325 mesh, prior to contact with the aqueous mixture in the first step of the present process. In the initial step (i.e. Step (a)) of the present process, the feedstock is contacted with an aqueous mixture comprising ammonium nitrate having a concentration greater than about 1.5 M, preferably in the range of from about 2.5 to about 4 M, and sulphuric acid having a concentration greater than about 6 M, preferably in the range of from about 7 to about 10 M. The reaction between this aqueous mixture and the feedstock results in the production of an insoluble residue and a leachate comprising the desired metal (or metals, depending on the feedstock) dissolved therein. It is difficult to generalize the nature of the insoluble residue since, for virtually all feedstocks, this will be dictated by the nature of the feedstock. Step (a) is conducted at a temperature of at least about 105°C, preferably in the range of from about 105°C to about 175°C, more preferably in the range of from about 105°C to about 130°C, still more preferably from about 110°C to about SUBSTITUTE SHEET 120°C, most preferably from about 112°C to about 115°C. In a particularly preferred embodiment of the present process, Step (a) comprises contacting the feedstock with a chloride salt in addition to the sulphuric acid and the ammonium nitrate. It has be surprisingly and unexpectedly discovered that the use of a chloride salt significantly improves the yield of metal from the feedstock. The choice of chloride salt is not particularly restricted. Non-limiting examples of suitable chloride salts include potassium chloride, calcium chloride and sodium chloride. The preferred chloride salt is sodium chloride owing to its relatively low cost and abundant availability. In Step (b) of the present process, the insoluble residue is separated from the leachate comprising the desired metal (or metals depending on the nature of the feedstock) dissolved therein. The manner by which this separation is accomplished is not particularly restricted. Generally, the separation may be effected using conventional techniques such as solid bowl centrifuging, cycloning, decantation and other techniques for physical separation of the residues from liquids containing them. Alternatively, the insoluble residue from Step (a) may be washed out of the leach tank. Once the insoluble residue is separated from the leachate in Step (b), the former may be washed and discarded in any conventional manner, usually in compliance with local governmental regulations. The separated leachate may then be subjected to conventional techniques to recover the metal or metals therefrom. The particular technique employed is usually dictated by the nature of the metal being recovered. For example, if the leachate comprises copper, it is possible to recover the copper using conventional cementation techniques involving the use of one or both of zinc or iron powder, as preferred. Alternatively, for some metals it is possible to utilize conventional membrane separation techniques (e.g. electrodialysis, filtration, reverse osmosis, ultrafiltration and the like) which rely on selectively permeable (e.g. selection based on metal ion size) membranes to separate the metal from the leachate. The preferred, and for most metals, convenient recovery technique is to subject the leachate to conventional electrowinning techniques (e.g. electroplating, electrodeposition and the like) wherein the individual metals may be recovered on the basis of their unique electrode potentials. Of course, it will be appreciated that one more of these or other conventional recovery techniques may be used to obtain the metal or metals originally present in the feedstock. Although not wishing to be bound by any particular theory or mode of action, the Applicant believes that the present process is particularly advantages due to the combined effects of temperature and the reagents used in Step (a) of the process. For example, it is known that the following metals typically may be found in chalcopyrite: gold silver copper lead zinc iron mercury It is also known that chalcopyrite comprises non-metallic elements such as sulphur and chlorine. The order of reactivity of these metals during Step (a) of the process is generally related to their relative positions in the electrochemical series, as follows: Zn 2+ > Fe 2+ > Pb 2+ > Cu 2+ > Ag I+ > Hg 2+ > Au 3+ . These are believed to be the most likely oxidation states produced under the moderately oxidizing conditions in Step (a) of the process. As is known in the art, sulphuric acid (one of the reagents added used in Step (a) of the present process) is a strong acid whose dissociation may be represented by the following equilibrium: H 2 SO 4 ** 2H + + SO 4 2" which substantially to the right. The reaction energy for this equilibrium generally lies between lead and copper in the electrochemical series described above. Accordingly, it is believed that the sulphuric acid reacts with zinc, BSTITUTE SHEET iron and lead present in the chalcopyrite to form the corresponding water soluble sulphates. When the ammonium nitrate (the other of the reagents added in Step (a) of the present process), in a preferred embodiment of the process, chloride salt are added, the strong acid conditions and the presence of the chloride ion is believed to result in dissociation of the ammonium nitrate to ammonium ion (NH 4 + ) and nitrate ion (NO 3 " ). The nitrate ion is believed to react with the remaining metals in the chalcopyrite to form the corresponding water soluble nitrates and the ammonium ion is believed to react with ferrous sulphate produce above to form ferric ammonium sulphate (FeNH^SO^), a water soluble compound. Thus, in the case of chalcopyrite, the metals therein remain substantially in solution after Step (a) of the process. After separation from the insoluble residue according to Step (b) of the process, the leachate comprising the dissolved metals is subjected to one or more conventional recovery techniques. As described hereinabove, the preferred technique for recovery of the metal from the leachate comprises electrowinning. As is known in the art, this technique may be suitably used to electrolytically recover one or more metals from the leachate on the basis of the unique electrode potentials for the metals. The electrode potential (E°) for various metals is as follows: Reaction Electrode Potential (V, at 25°) Zn 2+ + 2e - Zn(s) -0.763 Fe 2+ + 2e ** Fe(s) -0.440 Cu 2+ + 2e - Cu(s) +0.337 Ag + + e ** Ag(s) +0.799 Au + + e ^ Au(s) +1.68 Pb 2+ + 2e ^ Pb(s) -0.1263 Hg 2+ + 2e ** Hg(s) +0.27 Au 3+ + 3e ** Au(s) +1.359 SUBSTITUTE SHEET As described hereinabove, when the present process is applied to chalcopyrite, one the products of the process is a ferric salt, namely ferric ammonium sulphate (Fe(NH 4 ) 2 (SO 4 ) 2 ). In this application of the present process the formation of jarosite (generally MFe 3 (S0 4 ) 2 (OH) 5 wherein M is a cation selected from sodium, potassium and ammonium ions) is substantially minimized or, in some cases, completely eliminated. Jarosite is a relatively worthless by-product formed in many prior art processes. In contrast ferric ammonium sulphate is a component of conventional fertilizers and thus, this aspect of the present process is capable of recovering iron originally present in the feedstock as a saleable compound. It is believed that the use of a leach medium comprising sulphuric acid and ammonium nitrate is important in the production of ferric ammonium sulphate in preference to jarosite in this particular application of the present process. In this regard, it is also believed that the relatively high concentrations of the sulphuric acid and ammonium nitrate used in the leach medium help prevent jarosite formation. With reference to the drawing there is illustrated a schematic of the present process when applied to a particular feedstock. It will be appreciated that the schematic is for illustrative purposes only and should not be used to limit the scope of the present invention or specifically any feedstocks suitable for use therewith. The drawing illustrates a schematic for a process which is capable of treating a feedstock such as chalcopyrite which comprises copper and, optionally, a wide variety of other metals. A feedstock tank 1, a sulphuric acid tank 2 and an ammonium nitrate tank 3 are provided and are capable of metering predetermined amounts of their contents to a leaching tank 4 for reaction of the components. The reaction of the components is conducted at substantially atmospheric pressure. It will of course be appreciated that a pressurized system (not shown) may also be used. The reaction is conducted at a temperature of at least about 105°C, preferably in the range of from about 105°C to about 175°C, more preferably in the range of from about 105°C to about 130°C, still more preferably from about 110°C to about 120°C, most preferably from about 112°C to about 115°C. The SUBSTITUTE SHEET products of the reaction are an insoluble residue and a leachate comprising a number of metals dissolved therein as discussed hereinabove. The major metal components (in the sense of desired recoverability) dissolved in the leachate are typically iron, copper, and zinc. The insoluble residue and the leachate are fed to and separated in a solids/liquids separation zone 5. In a preferred aspect of the present process, zone 5 further includes an intercooler capable of cooling the leachate to a temperature of from about 50° to about 90°C, more preferably from about 55° to about 65°C. Typically, such temperatures favour precipitation of iron (as ferric ammonium sulphate) in this zone. As is well known in the art, it is preferred not to have iron present in the leachate going to the electrowinning cells. The leachate (liquid) from zone 5 is fed to a zinc electrowinning zone 7 wherein zinc is electro-deposited in an electrowinning cell and removed from zone 7. The electrowon solution in zone 7 still contains copper and is thereafter fed to a copper electrowinning zone 6 wherein copper is electro-deposited in an electrowinning cell and removed from zone 6. The electrowon solution from zone 6 essentially comprises sulphuric acid and ammonium nitrate, which may recycled to leaching zone 4 or fed to a waste tank (not shown) via a valve 12. If the electrowon solution from zone 6 is recycled to zone 4, it is preferred to preheat the solution to a temperature of at least about 105°C, preferably in the range of from about 105°C to about 175°C, more preferably in the range of from about 105°C to about 130°C, still more preferably from about 110°C to about 120°C, most preferably from about 112°C to about 115°C prior to introduction into zone 4. During the reaction of the components in leaching tank 4, nitrogen oxides (NOJ are produced and are allowed to be introduced to the anode compartment of the electrowinning cells in zones 6 and 7. The primary purpose for sparging the NO x in this fashion is to reoxidize it to NO 3 " . Further, the nitrate ion may be recycled to the initial steps of the process. The insoluble residue produced in zone 4 typically contains sulphur (one third of which may be milk of sulphur), iron in the form of ferric ammonium sulphate and gangue, and may be "blown down" in solids/liquids separation zone 5 to a sulphur removal zone 8. In sulphur removal zone 8, the first residue is heated to a temperature in range of from about 200°C to about 250°C to produce liquid sulphur having a relatively high vapour pressure. The sulphur vapour may be drawn off the first residue and condensed and collected (not shown) for future burning and/or sale. After the sulphur vapour is removed, the remaining residue is passed to ferric ammonium sulphate removal zone 9. In zone 9, the remaining residue is contacted with hot water (temperature of at least 60°C) which results in extraction of the ferric ammonium sulphate from the remaining residue. The resulting solution may be separated from the remaining components of the remaining residue and thereafter subjected to boiling to remove water thereby yielding ferric ammonium sulphate. The remaining residue typically comprises gangue and may be disposed of in any convenient way, usually in compliance with local governmental regulations. If desired, the gangue may washed prior to being discarded. Aspects of the present invention will be described with reference to the following Examples which are provided for illustrative purposes only and thus, the following Examples should not be used to limit the scope of the invention. Example 1 - Recovery of Gold Recovery of gold was conducted in a bench scale experiment using two samples of placer gold. Sample 1 had a mass of 0.3080 g and was used as it was received. Sample 2 had a mass of 0.3202 g and was flattened on an anvil prior to use. Sample 1 was contacted in a beaker with 10.00 mL of 18 M sulphuric acid and then heated to a temperature of 105°C. Thereafter, 12 g of ammonium nitrate and 5.0 g of sodium chloride were added to the beaker of reactants. The reaction in the beaker was allowed to proceed for 40 minutes at a temperature of 110°C. SUBSTITUTE SHEET Sample 2 was contacted in a beaker with 15 mL of 18 M sulphuric acid and heated to 105°C. Thereafter, 12.0 g of ammonium nitrate, 5 g of sodium chloride were slurried in 10 mL of water, and the so formed slurry was added to the beaker of reactants. The head grade (i.e. the theoretical amount of gold in the ore sample) of the placer gold sample was determined for comparative purposes. Specifically, a sample having a mass of 0.2792 g was flattened to be substantially paper thin and contacted in a beaker with 25 mL of concentrated aqua-regia (a 3:1 mixture of hydrochloric and nitric acid). After digestion under refluxing conditions of 110°C for 30 minutes, 10.0 mL of HC10 4 and 3.0 mL of hydrogen peroxide were added and refluxing digestion was continued for 45 minutes. In the case of treatment of Samples 1 and 2, and in the determination of the head grade, after the reaction was terminated, an insoluble residue and a leachate comprising dissolved gold resulted. After separation of the insoluble residue from the leachate, the later was introduced to an electrowinning cell wherein gold was recovered in a conventional recovery step. The results of gold recovery were as follows. The head assay experiment provided an indication that the placer gold ore comprised 48.32% . In other words 0.1349 g of pure gold was recovered from the actual sample tested in the head assay experiment. Accordingly, this was related to the gold recovered from Samples 1 and 2 as follows: Thus, the results of this Example illustrate at least two advantages of the present process. First, it has been shown that the present process is useful for recovering a substantial amount of the gold present in the placer gold ore. Second, it has been shown that the use of sodium chloride is result effective in enhancing the amount of gold recovered from the ore. Example 2 - Recovery of Copper In this Example, a number of samples of chalcopyrite ore obtained from Westmin Mines were tested for recovery of copper therefrom. The commercial supplier of the ore also provided the head grade of the ore. Three samples of the Westmin ore, each having a mass of 10.000 g, where used in the experiment. The general procedure used to treat the samples involve contacting each sample with 40 mL of 18 M sulphuric acid (indicated percentages related to molar concentration) as follows: at a temperature of in the range of from 50 °C to 60°C. Thereafter, sodium chloride was added to ore/acid mixtures in Sample 2 (3 g NaCl) and Sample 3 (5 g NaCl). At this point Sample 1 comprised a clear liquid whereas Samples 2 and 3 comprised a green liquid. In the case of each Sample, 20 g of ammonium nitrate was added to the reaction mixture of a period of 5 to 10 minutes, and resulting mixture was heated at a temperature of 112°C for 30 minutes. This resulted in a white and black precipitate in a green liquid in Sample 1 and a white precipitate in a green solution for Samples 2 and 3. After separation of the precipitate from the green liquid for each Sample, a green solution result and was introduced to an electrowinning cell wherein copper metal was recovered in a conventional manner. The results of copper recovery (compared to a head assay conducted in a conventional SUBSTITUTE SHEET manner on the Westmin ore) were as follows: As in Example 1, the results of this Example illustrate the applicability of the present process to recovery of a substantial amount of copper metal from an ore. Further, the recovery of copper metal is significantly enhanced by using a chloride salt during the leaching step. Although the present invention has been described hereinabove with reference to specific steps of the process thereof, it will be appreciated that various modifications and changes may be made therein without departing from the true spirit and scope of the invention, and thus, it is not intended to limit the invention by the specific steps described hereinabove. SUBSTITUTE SHEET

Claims: WHAT IS CLAIMED IS: 1. A process for recovery of metal from a feedstock comprising a metal selected from the group gold, platinum, silver, copper, nickel, cobalt, zinc, lead, antimony, molybdenum, chromium, mercury, manganese, tin, other members of the platinum series and combinations thereof, the process comprising the steps of: (a) contacting the feedstock with an aqueous mixture comprising ammonium nitrate having a concentration greater than about 1.5 M and sulphuric acid having a concentration greater that about 6 M at a temperature of at least about 105°C, to produce a insoluble residue and a leachate comprising the metal dissolved therein; (b) separating the insoluble residue from the leachate; and (c) recovering the metal from the leachate. 2. The process defined in claim 1, wherein Step (a) is conducted at a temperature in the range of from about 105 °C to about 175 °C. 3. The process defined in claim 1, wherein Step (a) is conducted at a temperature in the range of from about 105°C to about 130°C. 4. The process defined in claim 1, wherein Step (a) is conducted at a temperature in the range of from about 110°C to about 120° C. 5. The process defined in claim 1, wherein Step (a) is conducted at a temperature in the range of from about 112°C to about 115°C. 6. The process defined in claim 1 , wherein Step (a) contacting said feedstock with said aqueous mixture and, additionally, a chloride salt. 7. The process defined in claim 2, wherein Step (a) contacting said feedstock with said aqueous mixture and, additionally, a chloride salt. 8. The process defined in claim 6, wherein said chloride salt is selected from sodium chloride, potassium chloride, calcium chloride and combinations thereof. 9. The process defined in claim 7, wherein said chloride salt is sodium chloride. 10. The process defined in claim 1, wherein said feedstock is selected from at least one of an ore, a concentrate or a combination thereof. 11. The process defined in claim 1, wherein said feedstock comprises gold and selected from the group comprising free milling gold ore, refractory gold ore, mixtures of free milling and refractory gold ores and concentrates thereof. 12. The process defined in claim 11 , wherein said free milling gold ore is placer gold. 13. The process defined in claim 11, wherein said refractory gold ore is a sulphide ore, a telluride ore or a mixture thereof. 14. The process defined in claim 1, wherein said feedstock is a copper concentrate. 15. The process defined in claim 14, wherein said copper concentrate is selected from the group comprising chalcopyrite, chalcocite, bornite, molybdenite, pyrite, arsenopyrite, sphalerite and tetrahedrite. 16. The process defined in claim 2, wherein said feedstock is chalcopyrite. 17. The process defined in claim 1, wherein said feedstock BSTITUTE SHEET comprises copper in the form of an ore selected from sulphide ore, arsenide ore and mixtures thereof. 18. The process defined in claim 17, wherein said arsenide ore is selected from iron diarsenide, cobalt diarsenide and mixtures thereof. 19. The process defined in claim 17, wherein said sulphide ore comprises at least one member selected from the group comprising chalcopyrite, chalcocite, bornite, molybdenite, pyrite, arsenopyrite, sphalerite and tetrahedrite. 20. The process defined in claim 1, wherein said feedstock ground to a size of from about 250 mesh to about 350 mesh prior to contact with the aqueous mixture. 21. The process defined in claim 2, wherein said feedstock is ground to a size of from about 275 mesh to about 325 mesh prior to contact with the aqueous mixture. 22. The process defined in claim 2, wherein the concentration of said sulphuric acid is in the range of from about 7 to about 10 M and the concentration of said ammonium nitrate is in the range of from about 2.5 to about 4 M. 23. The process defined in claim 1 , wherein Step (c) comprises one or more of the following: a cementation step, membrane separation step and an electrowinning step. 24. The process defined in claim 23, wherein said cementation step comprises the addition of a metal powder to the leachate. SUBSTITUTE SHEET 25. The process defined in claim 24, wherein said metal powder is zinc, iron or a mixture thereof. 26. The process defined in claim 23, wherein said membrane separation step comprises one or more of electrodialysis, filtration, reverse osmosis and ultrafiltration. 27. The process defined in claim 23, wherein said electrowinning step comprises electroplating. 28. The process defined in claim 23, wherein said electrowinning step comprises electrodeposition. 29. The process defined in claim 2, wherein Step (c) comprises electrowinning the metal from the leachate. 30. The process defined in claim 29, wherein Step (c) is conducted at a temperature of from about 50° to about 70°C. 31. A process for recovery of gold from a feedstock comprising free milling gold ore, refractory gold ore and combinations thereof, the process comprising the steps of: (a) contacting the feedstock with an aqueous mixture comprising ammonium nitrate having a concentration greater than about 1.5 M, sulphuric acid having a concentration greater that about 6 M and a chloride salt, at a temperature in the range of from about 105°C to about 150°C, to produce a insoluble residue and a leachate comprising gold dissolved therein; (b) separating the insoluble residue from the leachate; and (c) subjecting the leachate to at least one electrowinning step to recover gold metal. SUBSTITUTE SHEET

Inventors: MCEWAN THOMAS D, O'BRIEN ROBERT N, MCEWAN, THOMAS, D., O'BRIEN, ROBERT, N.
Applicants: ENVIRONCHIP TECHNOLOGIES LTD, ENVIRONCHIP TECHNOLOGIES LTD.
Application year: 1992
Main classification: C22B3
IPC codes: C22B3, C25C1