WO2007109841A1

WO2007109841A1 - Improved processing of metal values from concentrates

Info

Publication number: WO2007109841A1
Application number: PCT/AU2007/000370
Authority: WO
Inventors: Ian Christopher Ritchie; Victor John Ketcham; Karel John Osten
Original assignee: Dundee Precious (Barbados) Inc
Priority date: 2006-03-28
Filing date: 2007-03-23
Publication date: 2007-10-04
Also published as: PE20080231A1; AU2007231537A1; US20090293680A1; EP1999284A1

Abstract

The present invention relates to an improved method for the recovery of metal values, in particular copper and gold, from a metal value-bearing material containing arsenic and/or antimony and a source of sulphate ions, by means of a high temperature pressure oxidation process followed by cyanidation of the resultant high temperature pressure oxidation residue.

Description

Improved processing of metal values from concentrates Field of the invention

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 røetal-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. which the Oxygen Reduction Potential (ORP) of the reaction slurry in the first compartment, and typically approximately the first 50% of the total volume, of a pressure vessel such as a substantially continuously operated autoclave, is kept below about 425 mV, and preferably below about 400 raV, 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. 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

In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date:

(i) part of common general knowledge; or (ϋ) known to be relevant to an attempt to solve any problem with which this specification is concerned.

Many base metals are sourced from sulphide ores. For example, copper sulphide minerals such as chalcopyrite [CuFeS₂] contribute to the majority of global copper production. There are also many other deposits that contain copper in the form of arsenic-bearing minerals, primarily enargite [Cu₃AsS₄] and tennantite [Cu₁₂As₄S₁₃], and/or antimony-bearing minerals such as tetrahedrite [Cu₁₂Sb₄S₁₃]. Included in such deposits is die enargite-containing copper-gold resource at Chelopech, Bulgaria.

Processes that involve the recovery of metal values from arsenic-containing sulphide minerals such as those indicated above generally require consideration of the form or forms in which the arsenic component reports, and the environmental impact upon the disposal of such arsenic-containing residues. Relevant national and international discharge regulations specify the maximum allowable dissolution of arsenic from such arsenic-containing residues under appropriate disposal regimes. Pyrometallurgical treatment of arsenic-containing metal sulphide minerals is generally regarded as technically and economically undesirable, as most of the arsenic reports as a flue dust and as a speiss phase. Safe disposal of these arsenic-containing materials involves considerable cost and technical disincentives.

By contrast, many 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 [FeAsO₄2H₂O]. 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 kinetics of the copper leaching stage of many such pressure oxidation processes are frequendy slow and there is generally co-precipitation of an iron-copper- arsenate-sulphate compound or compounds, leading to copper losses to the leach stage solid residue and thus to the overall process. Various means have been proposed to overcome the slow leach kinetics, including finer grinding of the feed material, although these sometimes result in substantially increased capital and operating costs. Many copper sulphide materials that contain arsenic and/or antimony often also contain metal values including precious metal values such as those of gold and/or silver, and any process to treat such materials must also employ economically viable treatment stages to recover the gold and/or silver contents. 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)SO₄] 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. In other words, 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.

In summary, many of the hydrometaUurgical processes currently employed to treat arsenic- and/or antimony-containing copper sulphide materials suffer from unacceptable copper losses to the leach residue. Moreover, if the feed material also contains precious metal values such as those of gold and/or silver, then current processing conditions also lead to the generation of solid residues that result in unacceptably high lime and cyanide consumption.

The present invention seeks to overcome at least some of the aforementioned disadvantages.

Summary of the invention In the following description of the invention, except where the context requires otherwise due to express language or necessary implication, the words "comprise" or variations such as "comprises" or "comprising" are used in an exclusive sense, ie., to specify the presence of stated features, but not to preclude the presence or addition or further features in various embodiments of the invention. Before the invention and preferred embodiments thereof are described, it is to be understood that this invention is not limited to the particular materials described, as these may vary. It is also to be understood that die terminology used herein is for the purpose of describing particular embodiments only, and is not intended to limit the scope of the present invention in any way. It must also be noted that as used herein, the singular forms of "a", "an" and "the" include plural reference unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms herein have the same meanings as commonly used by one of ordinary skill in the art to which this invention belongs. According to one aspect of the present invention there is provided 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, the process comprising the steps of:

(a) providing a feed stream comprising a metal value-bearing material containing arsenic and/or antimony and a source of sulphate ions;

(b) subjecting the feed stream to oxidative conditions under elevated temperature and pressure conditions thereby forming a slurry comprising a metal value-containing leach solution and a solid residue;

(c) separating the metal value-bearing leach solution from the solid leach residue;

(d) recovering the metal value(s) from the metal value-bearing leach solution; and

(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 70⁰C to about 100⁰C 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.

According to another aspect of the present invention there is provided 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, the process comprising the steps of:

(b) subjecting the feed stream to oxidative conditions under elevated temperature and pressure conditions in the presence of at least one component selected to decrease the effective free acid concentration during the pressure oxidation step and promote the formation of pH-stable iron(III) sulphate conditions, thereby forming a slurry comprising: (i) a metal value-containing leach solution and a solid residue containing pH-stable iron(HT) sulphate products; and

(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;

(d) recovering the metal value (s) from the metal value-containing leach solution; and (e) recovering precious metals such as gold and/or silver values, if present, in the solid leach residue by cyanide leaching.

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 7O⁰C to about 100⁰C 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.

According to another aspect of the present invention there is provided 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, the method comprising the steps of:

(a) providing a feed stream comprising a metal value-bearing material containing arsenic and/or antimony and a source of sulphate ions; (b) subjecting the feed stream to oxidative conditions under elevated temperature and pressure conditions in the presence of certain iron- containing compounds and/or other chemical agents selected to decrease the effective free acid concentration during the pressure oxidation step and promote the formation of pH-stable iron(III) sulphate conditions, thereby forming a slurry comprising:

(i) a metal value-containing leach solution and a solid residue containing pH-stable iron(ΗI) sulphate products; and

(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

425 mV, so that the soluble ferric to ferrous molar ratio is above about 1:1, and typically substantially above 1:1 to facilitate the precipitation of the pH- stable iron(III) products and ensure a substantial proportion, and preferably substantially all, of the sulphide sulphur is oxidised to sulphate; (c) separating the metal value-bearing leach solution from the solid leach residue;

(d) recovering the metal value (s) from the metal value-containing leach solution; and

(e) recovering precious metal values such as gold and/or silver values, if present, in the solid leach residue by cyanide leaching.

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 7O⁰C to about 100⁰C 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.

The terms "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. For convenience, 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. In addition, 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.

Preferably, 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. Typically the metal value-containing material is an ore or concentrate that contains arsenic and/or antimony, and include but is not limited to:

(a) an ore or concentrate that contains recoverable base and other metals including but not limited to copper, nickel, cobalt, zinc, and the platinum group metals; (b) an ore or concentrate that contains recoverable precious metals, especially gold and silver;

(c) an ore or concentrate that contains recoverable base and other metals including but not limited to copper, nickel, cobalt, zinc and the platinum group metals, as well as precious metals, especially gold and silver. Typically, 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. In one preferred embodiment of the present invention, the pH stable iron(III) sulphate product is hydronium and/or sodium jarosite. While it is common for a metal value-bearing material containing arsenic and/or antimony such as a copper sulphide ore or concentrate containing arsenic and/or antimony to also contain at least trace amounts of iron compounds, 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. Preferably, 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. Thus by ensuring that the Fe:(As+Sb) molar ratio in the feed material to step (b) is greater than about 1:1 and 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 inventors have found that 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.

Provided diat die requirement for rapid solubilisation under the high temperature/high pressure leach conditions is met, the chemical valency of the iron compounds added to the feed material to adjust the Fe: (As +Sb) molar ratio to the required level is not thought to be critical. This is because, under the operating conditions of the high temperature/high pressure leach step (b), substantially all ferrous

[Fe(II)] will be rapidly oxidised to the ferric [Fe(III)] state. In other words, the iron compounds may be ferrous or ferric compounds, or mixed ferrous/ferric compounds.

However, it is preferred diat die iron compounds are in the ferric state since diis reduces the oxygen consumption during the high temperature/high pressure leach step.

In one preferred embodiment of the invention, the iron compounds are derived from pyrite, in particular calcined pyrite produced under conditions that favour the formation of FeS, FeO, Fe₃O₄ or gamma-Fe₂O₃ over the formation of alpha-Fe₂O₅, since the former iron compounds are more readily solubilised compared with the latter iron compound.

During the high temperature/high pressure leach step there are many competing chemical reactions relating to the formation and precipitation of different iron-containing species, such as, for example, basic ferric sulphates, hematite, and jarosite. Promotion of precipitation of jarosite and/or hematite over basic iron sulphate is favoured by the presence of suitable reactions that decrease the effective concentration of free acid generated during the high temperature/high pressure leach step.

In one preferred embodiment of the invention, die chemical agents added to the feed material being subjected to the high temperature/high pressure leach step comprise metal salts which directly participate in the formation of jarosite-type compounds, in particular soluble alkali metal ion salts such as those of sodium or potassium, and ammonium salts, all of which form stable jarosite-type minerals of the general formula MFe₃ (SO₄) ₂ (OH)₆ where M = Na, K and NH₄, respectively. The formation of these jarosite-type minerals decreases the effective concentration of free acid under the prevailing high temperature/high pressure leach conditions. The addition of such soluble alkali metal ion salts also increases the temperature at which jarosite-type minerals tend to form in preference to basic iron sulphate type minerals during the pressure oxidation process at any given acid concentration. The ability to operate at higher temperatures while promoting the formation of pH-stable iron(III) sulphate products over basic iron sulphate type minerals provides economic advantages in the form of enhanced leaching reaction kinetics and shorter required residence (retention) times.

In another preferred embodiment of the invention, 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. In another preferred embodiment of the invention, the soluble zinc salt may be introduced by the leaching of zinc sulphide minerals that may be present in the feed material.

In a further preferred embodiment of the invention, 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. In the case of copper sulphides containing arsenic and/or antimony, 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.

By means of limiting the copper content of the solid leach residue and efficient separation of the soluble copper form the solid leach residue, the economics of gold and/or silver recovery from the leach residue by cyanidation is enhanced as the extent of the reaction between copper and the cyanide leachant is significantly reduced, thereby lowering the overall cyanide consumption.

Detailed Description of the Invention In accordance with various aspects of the present invention, 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.

For convenience, however, the description of the preferred embodiments of the invention is restricted to copper-containing materials that also contain arsenic and/or antimony. 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 (Cu₁₂As₄S_1J), enargite (Cu₃AsS₄) and tetrahedrite (Cu₁₂Sb₄S₁₃), and to other ores or concentrates containing copper sulphide minerals such as, for example, chalcopyrite (CuFeS₂), chalcocite (Cu₂S), bornite (Cu₅FeS₄) and covellite (CuS), when contaminated with arsenic- and/or antimony-bearing material.

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. Thus, the metal value-bearing material is preferably a mixed gold/silver/copper ore or concentrate containing arsenic and/or antimony. Typically 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.

After the metal value-bearing concentrate stream has been suitably prepared as a slurry, the slurry is fed to an agitated pressure vessel, preferably an autoclave, and subjected to pressure oxidation. Typically the high temperature/high pressure leaching process is carried out at a temperature in the range of from about 180°C to about 250⁰C, preferably from about 190⁶C to about 230^βC. The optimum temperature depends on many factors including, but not limited to, the πύneralogical composition of die feed, the sulphide sulphur content of the feed, the particle size distribution of the feed, and the pulp density. As a general rule, 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.

In one of the preferred embodiments of the present invention, it has been found advantageous to control the Oxygen Reduction Potential (ORP) of the slurry in the first compartment of the autoclave and more preferably in approximately the first 50% of the total autoclave volume, to a value below about 425 mV, and more preferably below about 400 mV, when measured against a standard platinum (Pt) electrode against a standard silver/silver chloride (Ag/AgCl) reference electrode. In this instance, the ORP is recorded within one hour using a filtered slurry sample withdrawn from the autoclave that had been rapidly cooled to ambient temperature. 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. 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. Under the high temperature/high pressure leaching process conditions, 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).

MS(soUd) + 2O₂(gas) -> MSO₄ (solution) (1)

The arsenic and antimony components of the feed material are oxidized to the arsenate (AsO₄ ³) and aαtimonite (SbO₄ ^3") species, respectively.

Some of the 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 (FeS₂), a significant component of many metal sulphide concentrates, can be represented by reaction (2).

4FeS₂ + 15O₂ + 6H₂O -^■» 4Fe(OH)SO₄ + 4H₂SO₄ (2) The reaction of pyrite to form hematite (alpha Fe₂O₃) is favoured by high temperatures and low free acidity concentrations according to reaction (3).

4FeS₂ 4- 17O₂ + 8H₂O -> 2Fe₂O₃ + 8H₂SO₄ (3)

The formation of jarosite is favoured by low operating temperatures and the presence of cations such as Na⁺, K⁺ or NH₄ ⁺, according to reaction (4) where M = Na, K or NH₄.

12FeS₂ +45O₂ + 30H₂O + 2M₂SO₄ -> 4MFe₃(SO₄),^!!),_; + 18H₂SO₄ (4)

Hydronium jarosite, in which M = H₃O⁺, the hydronium ion of free acid, takes the place of Na, K or NH₄ is also favoured by low operating temperatures in the absence of such cations.

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(iπ) arsenate, FeAsO₄2H₂O, 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.

Under typical prior art operating conditions for the high temperature/high pressure leaching of mixed copper/gold metal sulphide concentrates containing arsenic and/or antimony, formation of basic iron sulphate and hematite are favoured. The basic iron sulphate and hematite report to the solid residue resulting from the high temperature/high pressure leach process. When the solid residue is washed, repulped and then subjected to cyanidation in order to extract the gold and/or silver values therein, there is an uneconomically high consumption of lime and cyanide. This is because the lime reacts directly with the basic iron sulphate during the adjustment of the pH to a value of 10 or higher diat is required for the gold and/silver cyanidation step.

In the present invention, 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. Under these conditions the jarosite phase acts as an efficient scavenger for any soluble arsenate and/or antimonate formed during the pressure oxidation reactions. Moreover, the jarosite phase does not itself reac_t with lime when the gold and/or silver are recovered from the leach residue by cyanidation.

Preferably, 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. Apart from facilitating the formation of arsenic- and antimony-containing jarosite phases which do not react with lime during cyanidation, 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). In other words, 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. However, 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.

In a preferred embodiment of the present invention 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, Fe₅O₄ or gamma-Fe₂O₃, rather than alpha-Fe₂O_} produced in a conventional pyrite roaster, or the higher sulphide containing FeS₂ or uncalcined pyrite.

In a further preferred embodiment of the present invention, 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. Alternatively, 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. Typically 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. Typically, 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. Typically, 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_ts may be separately transferred to the autoclave before or after introduction of the feed stream to the autoclave.

During the high temperature/high pressure leach step metal values, in particular copper, may be solubilised to form a metal value-containing solution, It is envisaged that the metal values will be recovered from the metal value-containing solution by well understood methods and techniques. For example, where the metal value is copper, copper is typically recovered from the copper-containing solution by a combination of solvent extraction and electrowinning. However, other metal recovery processes such as cementation or precipitation of an intermediate product such as a hydroxide or sulphide could be employed. In a preferred embodiment of the present invention it has been found to be advantageous to maintain the slurry discharged from the autoclave at a temperature above about 70⁰C, 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. This compares with prior art that incorporates rapid cooling of the autoclave discharge slurry to ambient temperature by means of a series of flash vessels and subsequent solid/liquid separation processed generally conducted below about 70^PC The advantage of this slow cooling or digestion-conditioning step disclosed in the present invention relates to the fact that any remaining basic ferric sulphate and/or copper-iron-sulphate-arsenate- antimonate in the leach slurry will be converted into a pH stable iron(HI) sulphate and/or redissolve, which in the case of copper-iron-sulphate-arsenate-antimonate will release soluble copper, respectively. By this means, the lime consumption required and, in the case of copper-containing feed materials, the cyanide consumption required for gold and/or silver cyanidation should be reduced, while any copper losses to the solid leach residue should also be reduced.

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.

By application of the preferred embodiments of the present invention, 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.

In summary, the advantages of the present invention compared with prior art include but are not limited to the following:

(a) enhanced recovery of metal values, typically copper and gold, by preventing the co-precipitation of metal values in the solid residue discharged from the high temperature/high pressure leach step;

(b) prevention of the formation of unstable basic iron sulphate species in the solid residue discharged from the high temperature/high pressure leach step that consume excessive lime during the recovery of the gold and/or silver by cyanidation; and

(c) generation of arsenic- and/or antimony-containing residues that can be stored in conventional residue storage impoundments without causing unacceptable environmental outcomes.

Description of the Preferred, Embodiments of the Invention Preferred embodiments of die present invention are now described by reference to the following example. The process conditions reflected therein are intended to exemplify various aspects of the invention, and are not intended to limit the scope of the claimed invention. Numerous variations and modifications will suggest themselves to persons skilled in the relevant art, in addition to those already described, without departing from the basic inventive steps. All such variations and modifications are to be considered within the scope of the present invention, die nature of which is to be determined from the foregoing description.

Example

This example outlines the general scope of the preferred embodiments of the present invention as applied to the continuous processing of a run-of-mine tennantite- enargite-chalcopyrite-pyrite ore containing on average 1.5% Cu and 3-8 g/t gold derived in from the Chelopech (Bulgaria) resource. A simplified flowsheet of one preferred embodiment of the present invention is shown in Figure 1. After crushing and grinding, a copper concentrate typically containing 15.5% Cu,

24.8% Fe, 38.1% S, 4.7% As and 30 g/t Au is produced by rougher, scavenger and cleaner flotation banks using the appropriate flotation reagent regime. 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 MgSO₄ and 15 g/L ZnSO₄ at pH 8.5, prior to regrinding to a P₈₀ of 25 micron. 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. 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 19O⁰C to about 230 "C, is continuously maintained. 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_tors 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. 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. Following the required retention time, typically about 60-80 minutes, 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). As the

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

2-5 g/L (pH approximately 1.5). After solid/liquid separation to remove precipitated solids, principally gypsum, the neutralised PLS is clarified before the copper is recovered by conventional solvent extraction and electrowinning techniques.

The raffinate from the solvent extraction circuit is then subjected to an impurity removal (IR) step by addition of limestone and lime slurries. After removal of the precipitated solids, which are recycled to the autoclave feed slurry preparation circuit, the clarified neutral barren solution (NBS) is used in a variety of appropriate duties noted above, including repulping of the incoming dewatered concentrate and as autoclave quench water.

Modifications and improvements to the invention will be readily apparent to those skilled in the art. Such modifications and improvements are intended to be widiin the scope of this invention.

Claims

Claims:

1. 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, comprising the steps of: (a) providing a feed stream comprising a metal value-bearing material containing arsenic and/or antimony and a source of sulphate ions;

(b) subjecting the feed stream to oxidative conditions under elevated temperature and pressure conditions diereby forming a slurry comprising a metal value-containing leach solution and a solid residue; (c) separating the metal value-bearing leach solution from the solid leach residue;

(e) recovering any precious metal values in the solid leach residue by cyanide leaching.

2. The method according to claim 1, wherein the slurry from step (b) is maintained at a temperature in the range of from about 70⁰C to about 10O⁰C 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.

3. 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, comprising the steps of:

(a) providing a feed stream comprising a metal value-bearing material containing arsenic and/or antimony and a source of sulphate ions; (b) subjecting the feed stream to oxidative conditions under elevated temperature and pressure conditions in the presence of at least one component selected to decrease the effective free acid concentration during the pressure oxidation step and promote the formation of pH-stable iron(IH) sulphate products, thereby forming a slurry comprising: (i) a metal value-containing leach solution and a solid residue containing pH-stable iron (III) sulphate products; and

(ii) environmentally stable iron-arsenic and iron-antimony products, (c) separating the metal value-bearing leach solution from the solid leach residue;

(d) recovering the metal value(s) from the metal value-containing leach solution; and (e) recovering any precious metals in the solid leach residue by cyanide leaching.

4. The method according to claim 3, wherein the oxidation conditions in the vessel used in step (b) provide the slurry, in at least a first part of the vessel, with an Oxygen Reduction Potential (ORP) of below about 425 mV, when measured with a standard platinum (Pt) electrode against a standard silver/silver chloride (Ag/AgCl) electrode, and a soluble ferric to ferrous molar ratio of below about 1:1, and wherein the oxidation conditions provide the slurry, in at least a second part of the vessel, with an OPR of above about 425 mV and the soluble ferric to ferrous molar ratio of above about 1:1, to facilitate the precipitation of the pH-stable irαn(HI) products and oxidation of the sulphide sulphur to sulphate.

5. The method according to claim 4, wherein the ORP in the reaction slurry in said first part of the vessel is below about 400 mV.

6. The method according to claim 4 or claim 5, wherein said first part of the vessel encompasses up to about 50% of the total volume of the vessel used in step (b) .

7. The method according to any one of claims 4 to 6, wherein said second part of the vessel encompasses up to about 50% of the total volume of the vessel used in step (b).

8. The method according to any one of claims 4 to 7, wherein the oxidation conditions are controlled by limiting the rate of oxygen injection into the first and/or second part of the vessel.

9- The method according to any one of claims 4 to 8, wherein the vessel of step (b) is a pressure vessel, preferably an autoclave, and more preferably, a substantially continuously operated autoclave.

10. The method according to any one of claims 3 to 9, wherein the slurry from step (b) is maintained at a temperature in the range of from about 70⁰C to about 100⁰C 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.

11. 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, the method comprising the steps of: (a) providing a feed stream comprising a metal value-bearing material containing arsenic and/or antimony and a source of sulphate ions;

(b) subjecting the feed stream to oxidative conditions under elevated temperature and pressure conditions in the presence of certain iron-containing compounds and/or other chemical agents selected to decrease the effective free acid concentration during the pressure oxidation step and promote the formation of pH-stable iron(III) sulphate products, thereby forming a slurry comprising:

(i) a metal value-containing leach solution and a solid residue containing pH-stable iron (111) sulphate products; and (ii) environmentally stable iron-arsenic and iron-antimony products;

(d) recovering the metal value (s) from the metal value-containing leach solution; and (e) recovering any precious metal values in the solid leach residue by cyanide leaching.

12. The method according to claim 11, wherein the oxidation conditions in the vessel used in step (b) provide the slurry, in at least a first part of the vessel, with an Oxygen Reduction Potential (ORP) of below about 425 mV, when measured with a standard platinum (Pt) electrode against a standard silver/silver chloride (Ag/AgCl) electrode, and a soluble ferric to ferrous molar ratio of below about 1:1, and wherein the oxidation conditions provide the slurry, in at least a second part of the vessel, with an OPR of above about 425 mV and the soluble ferric to ferrous molar ratio of above about 1:1, to facilitate the precipitation of the pH-stable iron(III) products and oxidation of the sulphide sulphur to sulphate.

13. The method according to claim 12, wherein the ORP in the reaction slurry in said first part of the vessel is below about 400 mV.

14. The method according to claim 12 or claim 13, wherein said first part of the vessel encompasses up to about 50% of the total volume of the vessel used in step (b).

15. The method according to any one of claims 12 to 14, wherein said second part of the vessel encompasses up to about 50% of the total volume of the vessel used in step (b).

16. The method according to any one of claims 12 to 15, wherein the oxidation conditions are controlled by limiting the rate of oxygen injection into the first and/or second part of the vessel.

17. The method according to any one of claims 12 to 16, wherein the vessel of step (b) is a pressure vessel, preferably an autoclave, and more preferably, a substantially continuously operated autoclave.

18. The method according to any one of claims 11 to 17, wherein the slurry from step (b) is maintained at a temperature in the range of from about 70⁰C to about 100⁰C 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.

19. The method according to any one of claims 11 to 18, wherein the chemical agents added to the material in the feed stream include metal salts, preferably soluble alkali metal ion salts, more preferably sodium, potassium and ammonium salts.

20. The method according to any one of claims 11 to 19, wherein the chemical agents added to the material in the feed stream include a source of soluble sulphate salts, preferably magnesium and/or zinc sulphate.

21. The method according to claim 20, wherein the source of soluble sulphate salts include carbonate and/or hydroxide salts of magnesium and/or zinc formed in situ under the oxidative conditions of step (b) or by the leaching of zinc sulphide minerals present in the material in the feed stream.

22. The mediod according to any one of claims 11 to 21, wherein the chemical agents added to the material in the feed stream include a base and/or carbonate, preferably limestone or lime.

23. The method according to any one of claims 3 to 22, wherein the pH stable iron(IH) sulphate products formed are composed of one or more jarosite-type minerals including hydronium, sodium, potassium or ammonium jarosite, preferably hydronium and/or sodium jarosite.

24. The method according to any one of claims 1 to 23, wherein the metal value- bearing material containing arsenic and/or antimony is a copper-bearing material containing arsenic and/or antimony, preferably a copper sulphide containing arsenic and/or antimony, and more preferably a mixed copper-gold sulphide containing arsenic and/or antimony.

25. The method according to any one of claims 1 to 24, wherein the metal value- containing material is an ore or ore concentrate that contains arsenic and/or antimony, and includes one or more recoverable metals selected from the group consisting of copper, nickel, cobalt, zinc, palladium and platinum.

26. The method according to any one of claims 1 to 25, wherein the metal value- containing material is an ore or ore concentrate that includes one or more recoverable precious metals, preferably gold and silver.

27. The method according to any one of claims 1 to 26, wherein die material in the feed stream includes iron compounds, preferably iron (IU) compounds.

28. The method according to claim 27, wherein the molar ratio of Fe:(As+Sb) in the material in the feed stream in step (b) is greater than about 1:1, and preferably greater than about 2:1.

29. The method according to claim 27 or claim 28, wherein the iron compounds are derived from pyrite, preferably calcined pyrite produced under conditions that favour the formation of more solubilzable forms of iron compounds including FeS, FeO, Fe₃O₄ or gamma-Fe₂O_j over the formation of alpha-Fe₂O₃.