Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B11/00—Obtaining noble metals
  • C22B11/08—Obtaining noble metals by cyaniding
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B1/00—Preliminary treatment of ores or scrap
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B11/00—Obtaining noble metals
  • C22B11/04—Obtaining noble metals by wet processes
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B15/00—Obtaining copper
  • C22B15/0063—Hydrometallurgy
  • C22B15/0065—Leaching or slurrying
  • C22B15/0067—Leaching or slurrying with acids or salts thereof
  • C22B15/0071—Leaching or slurrying with acids or salts thereof containing sulfur
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B15/00—Obtaining copper
  • C22B15/0063—Hydrometallurgy
  • C22B15/0084—Treating solutions
  • C22B15/0089—Treating solutions by chemical methods
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B3/00—Extraction of metal compounds from ores or concentrates by wet processes
  • C22B3/04—Extraction of metal compounds from ores or concentrates by wet processes by leaching
  • C22B3/06—Extraction of metal compounds from ores or concentrates by wet processes by leaching in inorganic acid solutions, e.g. with acids generated in situ; in inorganic salt solutions other than ammonium salt solutions
  • C22B3/08—Sulfuric acid, other sulfurated acids or salts thereof
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B30/00—Obtaining antimony, arsenic or bismuth
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P10/00—Technologies related to metal processing
  • Y02P10/20—Recycling

Definitions

• the present invention relates to an improved process for the recovery of metal values, in particular copper and gold, from metal-bearing concentrates.
• the present invention also relates more particularly to an improved process for the recovery of metal values, in particular copper and gold, from metal-bearing concentrates by means of a high temperature pressure oxidation process followed by cyanidation of the resultant high temperature pressure oxidation residue.
• the present invention also relates more particularly but not exclusively to a process of maximising copper and gold extraction from r ⁇ etal-bearing concentrates that also contain significant amounts of arsenic and/or antimony, and that substantially simultaneously results in the formation of environmentally stable iron-arsenic and/or iron- antimony compounds in the process residues that can be discharged to tailings dams or the like such that strict environmental regulations are complied with.
• the present invention also relates more particularly but not exclusively to a high temperature pressure oxidation process in which there is controlled oxygen addition to die first compartment of a pressure vessel such as a substantially continuously operated autoclave, and also more particularly relates to controlled oxygen addition to approximately the first 50% of the total volume of the continuously operated autoclave.
• the present invention also relates more particularly but not exclusively to a high temperature pressure oxidation process in.
• the ORP and ferric and ferrous iron assays referred to above are those obtained by rapid cooling to room temperature of a sample of slurry withdrawn from the autoclave within one hour and then filtered for assay purposes. Background of the invention
• Copper sulphide minerals such as chalcopyrite [CuFeS 2 ] contribute to the majority of global copper production.
• copper sulphide minerals such as chalcopyrite [CuFeS 2 ] contribute to the majority of global copper production.
• copper sulphide minerals such as chalcopyrite [CuFeS 2 ] contribute to the majority of global copper production.
• copper sulphide minerals such as chalcopyrite [CuFeS 2 ] contribute to the majority of global copper production.
• copper sulphide minerals such as chalcopyrite [CuFeS 2 ] contribute to the majority of global copper production.
• arsenic-bearing minerals primarily enargite [Cu 3 AsS 4 ] and tennantite [Cu 12 As 4 S 13 ] and/or antimony-bearing minerals such as tetrahedrite [Cu 12 Sb 4 S 13 ]. Included in such deposits is die enargite-containing copper-gold resource at Chelopech, Bulgaria.
• hydrometallurgical processes for treating copper sulphide minerals that also contain arsenic are directed towards the generation of an acidic copper sulphate solution containing soluble copper, which is typically recovered therefrom by a combination of solvent extraction and electrowinning.
• the arsenic component of the feed material is converted into an insoluble arsenic-containing phase such as hydrated ferric arsenate [FeAsO 4 2H 2 O].
• This particular phase also occurs in nature as the mineral scorodite.
• the hydrated ferric sulphate produced by the hydrometallurgical processes can be safely disposed of in a conventional tailings impoundment.
• Most of the hydrometallurgical processes for treating copper sulphide minerals generally fall within the general designation of pressure oxidation processes.
• the gold and/or silver In the pressure oxidation process described above, the gold and/or silver generally report to the solid residue generated by the leach process.
• the gold and/or silver are usually recovered by repulping the residue and cyanide leaching under the appropriate alkaline pH conditions.
• Meta-stable iron compounds such as basic ferric sulphate [Fe(OH)SO 4 ] and any copper-containing precipitates such as an iron-copper-arsenate-sulphate in the residue will decompose (break down) under the alkaline pH conditions required for gold/silver cyanidation and thus bring about an increase in the lime and cyanide consumption, thereby decreasing the economic efficiencies of the overall process.
• the abovementioned solid components present in the leach residue break down during the cyanidation step, generating excess acid and reactive sulphate compounds that must be subsequently neutralised.
• the present invention seeks to overcome at least some of the aforementioned disadvantages.
• step (e) recovering precious metal values such as gold and/or silver values, if present, in the solid leach residue by cyanide leaching.
• the slurry from step (b) may be maintained at a temperature in the range of from about 70 0 C to about 100 0 C for a period in the range of from about 15 minutes to about 4 hours prior to separating the metal value-containing solution from the solid leach residue.
• a method for the recovery of metal values from a metal value-bearing material containing arsenic and/or antimony and a source of sulphate ions such as a sulphide ore or concentrate comprising the steps of:
• (ii) environmentally stable iron-arsenic and iron-antimony products such oxidative conditions comprising the provision that the Oxygen Reduction Potential (ORP) of the reaction slurry in at least part of the vessel used for step (b) is kept below about 425 mV, when measured with a standard platinum (Pt) electrode against a standard silver/silver chloride (Ag/AgCl) electrode, and the soluble ferric to ferrous molar ratio is below about 1:1, and wherein in at least another part of the vessel the OPR is allowed to increase above about 425 mV and typically substantially above about 425 raV so that the soluble ferric to ferrous molar ratio is above about 1:1 and typically substantially above about 1:1 to facilitate the precipitation of the pH-stable iron (HI) products and ensure a substantial proportion, and preferably substantially all, of the sulphide sulphur is oxidised to sulphate;
• ORP Oxygen Reduction Potential
• the vessel of step (b) will typically be a pressure vessel such as an autoclave, and more typically a substantially continuously operated autoclave.
• Up to approximately the first 50% of the total volume of the vessel of step (b) may be kept below about 425 mV and typically below about 40OmV. Up to approximately 50% of the remaining volume of the vessel of step (b) may be allowed to increase above about 425mV and typically substantially above about 425mV.
• the slurry from step (b) may be maintained at a temperature in the range of from about 7O 0 C to about 100 0 C for a period in the range of from about 15 minutes to about 4 hours prior to separating the metal value-containing solution from the solid leach residue.
• a method for the recovery of metal values from a metal value-containing feed material containing arsenic and/or antimony and a source of sulphate ions such as a sulphide ore or concentrate comprising the steps of:
• (ii) environmentally stable iron-arsenic and iron-antimony products such oxidative conditions including the provision that the Oxygen Reduction Potential (ORP) of the reaction slurry in at least part of the vessel used for step (b) is kept below about 425 mV, when measured with a standard platinum (Pt) electrode against a standard silver/silver chloride (Ag/AgCl) electrode, and the soluble ferric to ferrous molar ratio is below about 1:1, and wherein in at least another part of the vessel the OPR is allowed to increase above about 425 mV, and typically substantially above
• ORP Oxygen Reduction Potential
• the vessel of step (b) will typically be a pressure vessel such as an autoclave, and more typically a substantially continuously operated autoclave. Up to approximately the first 50% of the total volume of the vessel of step(b) may be kept below about 425 mV and typically below about 40OmV. Up to approximately 50% of the remaining total volume of the vessel of step (b) may be allowed to increase above about 425mV and typically substantially above about 425mV.
• the slurry from step (b) may be maintained at a temperature in the range of from about 7O 0 C to about 100 0 C for a period in the range of from about 15 minutes to about 4 hours prior to separating the metal value-containing solution from the solid leach residue.
• pressure oxidation or "pressure oxidation step” or “oxidative conditions under elevated temperature and pressure” used herein refer to a high temperature/high pressure leach process operated under acidic oxidising conditions.
• One particular aspect of the present invention is based upon die realisation that it is possible to adjust the processing conditions such that they prevent die formation of insoluble copper-containing precipitates during the high temperature pressure leaching process to extract metal values such as copper from a metal value-containing material such as a sulphide ore that also contains arsenic and/or antimony.
• Another particular aspect of die present invention is based upon the realisation that it is possible to adjust the processing conditions to promote the formation of solid iron(m) sulphate containing-products in the residue derived from the pressure leaching process that are stable under the alkaline pH conditions at ambient temperature that are used to recover die gold and/or silver values from die said residue.
• this solid iron(III) sulphate containing-product is referred to as a "pH stable iron(III) sulphate".
• Included in the means of promoting the formation of the pH stable iron(III) sulphate product are means of controlling (decreasing) the free acid generated during die pressure oxidation step by addition of certain additives and/or control of the slurry ORP in typically about die first 50% of the total volume of die vessel such as a continuous autoclave used for the pressure oxidation step. This latter means is achieved by limiting the rate of oxygen injection into about die first 50% of the total volume of the continuous autoclave.
• the result of the correct selection of the high temperature/high pressure leaching conditions for treating metal value-bearing materials containing arsenic and/or antimony is that die majority of die arsenic and/or antimony reports to a solid residue as an environmentally stable mixed iron-arsenic and/or iron-antimony solid species mixed with pH stable iron(III) sulphate products.
• copper losses to the residue are minimised by prevention of precipitation of a copper-iron-sulphate-arsenate, while cyanidation of the gold and/or silver content of the leach residue is enhanced because of the promotion of precipitation of pH stable iron(III) sulphate products such as jarosite- type minerals rather than basic iron sulphate.
• the present invention is accordingly concerned with the development of economically viable conditions that can at least partially achieve one or more of (a) minimizing copper losses to the leach residue, (b) ensuring that the arsenic and/or antimony components of the feed material report to the residue in an environmentally stable form, and (c) preventing the formation of solid residues that break down during the gold and/or silver cyanidation step and a concomitant increase in lime and cyanide consumption in the case where the initial feed material contains recoverable gold and/or silver.
• the metal value-bearing material containing arsenic and/or antimony is a copper-bearing material containing arsenic and/or antimony, in particular a copper sulphide containing arsenic and/or antimony, and even more particularly a mixed copper- gold sulphide containing arsenic and/or antimony.
• the metal value-containing material is an ore or concentrate that contains arsenic and/or antimony, and include but is not limited to:
• the pH stable iron(QI) sulphate product formed in the abovementioned pressure leach step is composed of one or more jarosite-type minerals, such as hydronium, sodium, potassium or ammonium jarosite.
• the pH stable iron(III) sulphate product is hydronium and/or sodium jarosite.
• the inventors have advantageously found that the presence of additional iron compounds in the feed material subjected to the pressure leaching process also promotes the formation of copper-free secondary ferric sulphate minerals that also contain arsenic and/or antimony.
• the molar ratio of Fe:(As+Sb) in the feed material to step (b) of the preferred embodiments described above is greater than about 1:1, and more preferably greater than about 2:1.
• the bulk of the arsenic and/or antimony in die feed material reports to the residue as an environmentally stable iron-arsenate and/or iron-antimonate phase, radier than as a copper-iron-sulphate- arsenate/antimonate.
• the iron compounds suitable for the abovementioned modification to the Fe: (As +Sb) molar ratio in the feed material are such compounds that are readily solubilised under the acidic high temperature/high pressure leach conditions of the invention.
• the particle size of the suitable iron compounds will typically be such that the solubilisation kinetics are compatible with die retention time if the high temperature/high pressure leach stage.
• the iron compounds may be ferrous or ferric compounds, or mixed ferrous/ferric compounds.
• diat die iron compounds are in the ferric state since diis reduces the oxygen consumption during the high temperature/high pressure leach step.
• the iron compounds are derived from pyrite, in particular calcined pyrite produced under conditions that favour the formation of FeS, FeO, Fe 3 O 4 or gamma-Fe 2 O 3 over the formation of alpha-Fe 2 O 5 , since the former iron compounds are more readily solubilised compared with the latter iron compound.
• soluble alkali metal ion salts such as those of sodium or potassium
• M Na, K and NH 4
• the chemical agents also comprise soluble sulphate salts whose cations are merely spectator ions and as such do not participate in any precipitation reactions.
• the preferred chemical reagents particularly include magnesium and/or zinc. Addition of a suitable soluble sulphate increases the concentration of the bisulphate ion present in the high temperature/high pressure leach slurry and decreases the effective concentration of free acid at temperature from that which would otherwise be experienced at a given feed solids composition and concentration (% solids).
• the soluble sulphate salts may be added directly to the high temperature/high pressure leach step or generated by reacting carbonate and/or hydroxide salts of magnesium and/or zinc in the high temperature/high pressure leach step.
• the soluble zinc salt may be introduced by the leaching of zinc sulphide minerals that may be present in the feed material.
• the chemical reagents may also comprise bases or carbonates, in particular limestone or lime, which directly consume acid and decrease the effective concentration of free acid in the high temperature/high pressure leach step.
• copper dissolution in the high temperature/high pressure leach step is optimised by addition of iron compounds to the reaction vessel, typically an autoclave, in sufficient quantities to favour precipitation of environmentally stable secondary iron-arsenate and/or iron- antimonate and/or iron-arsenate-sulphate and/or iron-antimonate-sulphate phases within the autoclave rather than the precipitation of copper-containing arsenate-antimonate residues, thereby limiting the copper content of the leach residue and maximising the soluble copper content of the resultant liquid stream available for copper recovery by a combination of solvent extraction and electrowinning or by means of another suitable recovery method.
• a metal value-bearing material containing arsenic and/or antimony that also constitutes a source of sulphate ions is provided for processing.
• the metal value-bearing material may be an ore, concentrate, or any other material from which metal values, in particular copper and gold and/or silver values, may be recovered.
• the invention is equally applicable to other metal value-bearing materials containing arsenic and/or antimony such as ores and concentrates containing other valuable metals such as nickel, cobalt, zinc and the platinum-group metals.
• the copper-containing material is preferably a copper sulphide ore or concentrate that contains arsenic and/or antimony, and particularly applies to ores and/or concentrates that contain tennantite (Cu 12 As 4 S 1J ), enargite (Cu 3 AsS 4 ) and tetrahedrite (Cu 12 Sb 4 S 13 ), and to other ores or concentrates containing copper sulphide minerals such as, for example, chalcopyrite (CuFeS 2 ), chalcocite (Cu 2 S), bornite (Cu 5 FeS 4 ) and covellite (CuS), when contaminated with arsenic- and/or antimony-bearing material.
• CuFeS 2 chalcopyrite
• Cu 2 S chalcocite
• Cu 5 FeS 4 bornite
• CuS covellite
• Geologically gold and/or silver are frequently associated with metal sulphide ores such as, for example, pyrite, chalcopyrite, galena, arsenopyrite and stibnite. Gold and/or silver are also often present in sulphide concentrates produced from such ores. Accordingly, a preferred embodiment of the present invention is particularly advantageous in connection with the recovery of copper and gold and/or silver from mixed gold/silver/copper ores or concentrates containing arsenic and/or antimony.
• the metal value-bearing material is preferably a mixed gold/silver/copper ore or concentrate containing arsenic and/or antimony.
• the mixed gold/silver/copper ore is a tennantite-enargite-chalcopyrite-pyrite ore.
• the metal value-bearing material typically undergoes comminution, flotation, blending and/or slurry formation, as well as chemical and/or physical conditioning to afford a feed stream which, in turn, is subjected to a high temperature/high pressure oxidative leach step and a series of downstream unit stages to afford recovery of the contained metal values.
• the specific conditions applicable to the comminution, flotation and conditioning stages are determined by the chemical and physical properties of the metal value-bearing ore material. As a general rule, these specific conditions are designed to yield a concentrate that optimises recovery versus grade. These specific conditions do not have a direct bearing on the application of the preferred embodiments of the present invention. As such, the present invention is primarily concerned with the treatment of a dewatered concentrate exiting the comminution, flotation and conditioning circuits.
• the slurry is fed to an agitated pressure vessel, preferably an autoclave, and subjected to pressure oxidation.
• an agitated pressure vessel preferably an autoclave
• the high temperature/high pressure leaching process is carried out at a temperature in the range of from about 180°C to about 250 0 C, preferably from about 190 6 C to about 230 ⁇ C.
• the optimum temperature depends on many factors including, but not limited to, the ⁇ neralogical composition of die feed, the sulphide sulphur content of the feed, the particle size distribution of the feed, and the pulp density.
• the higher temperatures in the above ranges provide for shorter retention times and/or a reduction and/or elimination of the need for regrinding of the feed material prior to the high temperature/high pressure leach step.
• the high temperature/high pressure leaching process is typically carried out at a total pressure sufflciendy high to provide an oxygen partial pressure inside the autoclave of between about 100 kPa and about 1500 kPa, preferably in the range of from about 400 kPa to about 1000 kPa, taking into account the partial pressure of steam and other non- condensable gases within the autoclave such as nitrogen and carbon dioxide.
• Oxygen is typically delivered to the autoclave by bottom entry spargers entering beneath the autoclave agitators at a pressure above that inside the autoclave.
• the autoclave agitators are designed to maximise oxygen mass transfer from the gas phase to the feed slurry.
• Pt platinum
• Ag/AgCl silver/silver chloride
• Control of the ORP is achieved by limiting the rate of oxygen injection into the first compartment and more preferably approximately the first 50% of the total autoclave volume.
• the ORP In the remaining autoclave compartments and/or approximately the second 50% of the total autoclave volume the ORP is allowed to increase above about 500 mV by increasing the rate of oxygen injection into the autoclave.
• the inventors have found that control of the ORP in the above manner permits regulation of the oxidation of ferrous iron to the ferric state as the slurry moves through the autoclave and assists in the generation of solid pH stable iron(HI) sulphate products.
• the high temperature/high pressure leach step is typically conducted over a period of from about 20 minutes to about 4 hours, and more preferably to about 2 hours, with higher operating temperatures and a finer feed particle size facilitating shorter reaction times.
• solid metal sulphide minerals within the feed material are oxidised to the corresponding soluble metal sulphates. That is, the metal values are released into solution.
• the actual oxidation/dissolution reactions for each metal sulphide mineral are a reflection of the chemical composition of that mineral as well as the temperature and free acidity of the leach slurry, but the overall reaction can be simplified as shown in reaction (1).
• the arsenic and antimony components of the feed material are oxidized to the arsenate (AsO 4 3 ) and a ⁇ timonite (SbO 4 3" ) species, respectively.
• solubilised metal values then re-precipitate within the autoclave and report to the solid phase component of the autoclave slurry as metal oxides and/or metal mixed hydroxyl-sulphates and/or metal-sulphate-arsenate-antimonate species.
• Iron may report to the solid phase component of the autoclave slurry as one or more different iron-containing compounds during the high temperature/high pressure leach process, the identity of such phases being determined by a specific set of operating conditions. For example, the formation of basic iron sulphate is favoured by high operating temperatures and high free acid conditions. Under such conditions, the oxidation of pyrite (FeS 2 ), a significant component of many metal sulphide concentrates, can be represented by reaction (2).
• Arsenate and antimonate species formed by die oxidation of the arsenic and antimony components of the feed material may precipitate as the respective iron(IH) arsenate and iron (III) antimonate phases, but may also substitute for sulphate in, for example, the jarosite phase.
• the precipitation of arsenate as hydrated iron(i ⁇ ) arsenate, FeAsO 4 2H 2 O, also known as scorodite, and the partial replacement of sulphate by arsenate in various jarosite phases is well documented in the scientific literature. Jarosite is sometimes referred as a scavenger for both arsenate and antimonate.
• the formation of hydrated iron(HI) arsenate and/or arsenic-containing jarosite materials in the present invention is of considerable environmental benefit since these materials are known to be environmentally stable and can be safely discharged into and stored in conventional residue storage impoundments.
• additional iron compounds are added to the feed material to the high temperature/high pressure leach step in order to promote the formation of jarosite rather than basic iron sulphate.
• the jarosite phase acts as an efficient scavenger for any soluble arsenate and/or antimonate formed during the pressure oxidation reactions.
• the jarosite phase does not itself reac t with lime when the gold and/or silver are recovered from the leach residue by cyanidation.
• the total iron content of the feed material to the high temperature/high pressure leach process is such that the molar ratio of Fe: (As+ Sb) is greater than about 2:1 and more preferably at least about 4:1.
• the high Fe: (As +Sb) molar ratio reduces and/or prevents the formation and precipitation of a mixed copper-iron-arsenate-antimonate-sulphate phase.
• the iron compounds added to the metal value-bearing feed material in order to adjust the molar ratio of Fe: (As+ Sb) to the desired level are of a mineral/chemical composition and particle size such that they are readily solubilised under the acidic high temperature/high pressure leach conditions.
• the valency of the iron in the iron compounds is not thought to be critical because under the operating conditions of the high temperature/high pressure leach process, substantially all iron(IT) will be oxidised to iron(IIT).
• the iron compounds may be ferrous or ferric compounds or mixed ferrous/ferric compounds, provided that they are soluble under the high temperature/high pressure leach conditions.
• it is preferred that the iron compounds are pre-treated to maximise the ferric content and minimise any sulphide content in order to lower the overall oxygen consumption required during the high temperature/high pressure leach step.
• the iron compounds are derived from pyrite, in particular calcined pyrite produced by oxidative conditions with the calciner operated in such a fashion as to produce a calcined pyrite with a significant portion of the iron present in a form readily capable of being solubilised in the autoclave under the high temperature/high pressure conditions, such as for example, FeS, FeO, Fe 5 O 4 or gamma-Fe 2 O 3 , rather than alpha-Fe 2 O ⁇ produced in a conventional pyrite roaster, or the higher sulphide containing FeS 2 or uncalcined pyrite.
• pyrite in particular calcined pyrite produced by oxidative conditions with the calciner operated in such a fashion as to produce a calcined pyrite with a significant portion of the iron present in a form readily capable of being solubilised in the autoclave under the high temperature/high pressure conditions, such as for example, FeS, FeO, Fe 5
• the iron compounds may be sourced from recycled process solutions containing iron sulphate, preferably in the ferric form, although the process solutions may also carry minor amounts of ferrous iron as well.
• the iron compounds may be iron-containing precipitates from various other parts of the overall process, such as the iron-containing precipitate produced during minor impurity removal ahead of or subsequent to metal value recovery steps such as copper recovery by a combination of solvent extraction and electrowinning.
• the iron compounds may be mixed with the metal value-bearing feed stream before it is transferred to the high temperature/high pressure autoclave leach vessel, or the iron compounds may be separately transferred to the autoclave before or after introduction of the feed stream to the autoclave.
• One of die preferred embodiments of the present invention incorporates the addition of specific chemical agents which decrease the effective concentration of free acid generated during the high temperature/high pressure leaching process thereby affording the precipitation of pH stable iron (HI) sulphate compounds and avoiding the precipitation of a basic ferric sulphate.
• One group of chemical agents includes metal salts mat directly participate in the formation of jarosite-type compounds, in particular sodium, potassium and ammonium jarosites. Such metal salts include soluble alkali metal (sodium and potassium) and ammonium sulphate.
• the molar ratio of the added metal salt per mole of iron present in the feed should be at least 1:3 and preferably at least about 1:2, diat is, an excess of metal salt above the stoichiometric requirement.
• Another group of chemical agents that have the ability to decrease the effective concentration of free acid generated during the high temperature/high pressure leaching process comprise soluble sulphate salts whose cations are merely spectator ions and which do not participate in any precipitation reactions. Addition of soluble sulphate increases the concentration of the bisulphate ion present at the operating high temperature and decreases the effective concentration of free acid that would otherwise be expected at the given temperature, feeds solids composition and pulp density.
• the soluble sulphate salts may be direcdy added to the high temperature/high pressure leaching step or generated by reacting carbonate or hydroxide salts of the appropriate metals.
• the inventors have established that the appropriate metal sulphate salts include those of magnesium and zinc.
• magnesium is added as magnesium carbonate (magnesite), magnesium oxide, dolomite, or mixtures thereof.
• the soluble sulphate salts once added to or generated by the overall process, may be conveniently recycled in process water used for feed preparation and/or autoclave quench water once the copper or other dissolved metal values have been recovered from the leach solution.
• the chemical agents may also comprise carbonates and other bases, in particular limestone and lime, which directly consume acid and decrease the effective concentration of free acid during the high temperature/high pressure leach process.
• bases are added in an amount necessary to yield less than about 60 g/L sulphuric acid in solution in the product from the high temperature/high pressure leach step, as measured by titration of slurry samples at ambient temperature.
• the chemical agents may be mixed with the feed stream before it is transferred to the autoclave for the high temperature/high pressure leach step, or the chemical agen t s may be separately transferred to the autoclave before or after introduction of the feed stream to the autoclave.
• metal values in particular copper
• metal values may be solubilised to form a metal value-containing solution
• the metal values will be recovered from the metal value-containing solution by well understood methods and techniques.
• the metal value is copper
• copper is typically recovered from the copper-containing solution by a combination of solvent extraction and electrowinning.
• other metal recovery processes such as cementation or precipitation of an intermediate product such as a hydroxide or sulphide could be employed.
• a temperature above about 70 0 C and preferably in the range of about 85-100° C for a period in the range of from about 15 minutes to about 4 hours in an agitated tank or series of tanks before it is subsequently cooled to ambient temperature and subjected to solid/liquid separation by counter current decantation and thickening ahead of the metal recovery from solution and gold and/or silver recovery from the solid residue.
• Precious metal values such as gold and/or silver values contained in the feed material will report to the solid residue formed during the high temperature/high pressure leach process. It is envisaged that the gold and/or silver values will be recovered from the solid residue by washing to remove entrained acid and soluble metal values, repulping and treating the consequent slurry by a combination of conventional cyanidation, activated carbon, stripping, electrowinning and smelting techniques.
• copper recoveries in excess of 95% and lime consumption of less than 15 kg/t of solid residue can be expected form a wide range of copper/gold sulphide ores and concentrates that also contain appreciable arsenic and/or antimony contents.
• the copper concentrate is directed to a copper concentrate dewatering circuit where the free moisture is reduced to about 10%.
• the copper concentrate is repulped in neutral barren solution (NfBS) derived from the downstream copper recovery circuit (solvent extraction and electrowinning) that typically contains about 42 g/L MgSO 4 and 15 g/L ZnSO 4 at pH 8.5, prior to regrinding to a P 80 of 25 micron.
• NfBS neutral barren solution
• the reground concentrate is thickened to approximately 55% solids and transferred to the agitated autoclave feed tank. To this tank are added controlled amounts of underflows from the final impurity (IR) stages of the SX raffinate and mine water treatment circuits, as well as a limestone slurry sufficient to achieve the desired carbonate:sulphur ratio in the feed.
• IR final impurity
• the relative amounts of limestone slurry and impurity removal underflow added to the reground concentrate are controlled to ensure that the free acidity and Fe:(As+Sb) molar ratio of the feed slurry are sufficient to prevent the precipitation of unstable basic ferric sulphate and copper-iron-sulphate-arsenate phases in the autoclave discharge slurry.
• the solid component of the blended reground concentrate, limestone and impurity removal slurry typically contains about 8.5% Cu, 13.1% Fe, 24.5% S, 2,7% As and 8.8 g/t Au, which is pumped into the high temperature/high pressure leach autoclave as a 45% solids slurry.
• the combined slurry is directed to the first compartment of a multi-compartment high pressure autoclave fitted with a plurality of agitators by means of a centrifugal pump feeding a positive displacement, piston driven diaphragm pump at an operating pressure of over the steam saturation pressure at the operating temperature, which will generally be over 2000 kPa.
• High pressure steam is supplied to the autoclave for initial heat-up and on as- needed basis.
• Each compartment of the autoclave is fitted with a quench water system by which a controlled flow of quench water, typically neutral barren solution (NBS), can be directly injected into each compartment such that the desired operating temperature, typically in the range of from about 19O 0 C to about 230 "C, is continuously maintained.
• NBS neutral barren solution
• the use of NBS as quench water assists with maintaining the overall process flowsheet water balance, and since it also contains appreciable magnesium and zinc sulphate contents, also assists with the control of the autoclave slurry chemistry.
• Oxygen at 94% or greater purity is delivered from a cryogenic oxygen plant to the autoclave by bottom entry spargers entering beneath each of the autoclave agita t ors at a pressure greater than about 2000 fcPa.
• the bottom impeller on the agitators is of the Rushton turbine design in order to maximise oxygen mass transfer to the feed slurry.
• the rate of oxygen injection into the first 50% of the total autoclave volume is controlled such that the Oxygen Reduction Potential (ORP), as previously defined and measured, is maintained at or below about 400 mV.
• ORP Oxygen Reduction Potential
• the rate of oxygen injection into the remaining 50% of the total autoclave volume is increased so that the ORP increases to above about 500 mVto enhance the oxidation of ferrous iron to the ferric state.
• the processed slurry is discharged from the autoclave via a single stage flash vessel at approximately 100°C.
• Flashed slurry flows by gravity through two agitated discharge tanks connected in series with a total retention time of about 2 hours, where the temperature is maintained at 85-100°C. From there the conditioned autoclave slurry is subjected to solid/liquid separation via a series of five conventional counter current thickeners.
• the thickened underflow is washed to remove entrained leach solution, washed and the resultant cake forwarded to a conventional gold cyanidation circuit.
• the final thickener overflow contains the dissolved content of the feed and is directed to a primary neutralisation (PN) circuit as pregnant leach solution (PLS).
• PN primary neutralisation
• PLS pregnant leach solution
• PLS contains a relatively high sulphuric acid concentration, typically 30-60 g/L, excess acid is neutralised by addition of a limestone slurry to achieve a final PLS free acidity of about
• the raffinate from the solvent extraction circuit is then subjected to an impurity removal (IR) step by addition of limestone and lime slurries.
• IR impurity removal
• the clarified neutral barren solution is used in a variety of appropriate duties noted above, including repulping of the incoming dewatered concentrate and as autoclave quench water.

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