METHOD FOR PROCESSING PYRITIC CONCENTRATE CONTAINING GOLD, COPPER AND ARSENIC

FIELD OF THE INVENTION The invention relates to a method for processing pyritic copper sulphide concentrate, in which, in addition to copper, there is arsenic and fine gold or invisible gold bound to the sulphide minerals. In the method, concentrate is leached into a chloride-based solution. Raw material leaching is performed in conditions where the copper, iron and arsenic of the raw material dissolve, but the gold-containing material remains undissolved. The chloride solution obtained is routed to neutralisation, in which the iron and arsenic in it are precipitated. The purified copper solution is routed to liquid-liquid extraction. In the stripping stage of extraction, copper is transferred to a sulphate solution, which is routed to copper electrowinning. Gold is recovered from the leaching residue by some known method.

BACKGROUND OF THE INVENTION

The problem with many gold-copper concentrates is that a considerable amount of the gold is bound to pyrite as invisible gold. This is the case for instance in arsenic-bearing gold-copper ores, in which up to 50 % of the gold is in pyrite. The rest of the gold is either metallic, or bound to enargite (Cu ₂ AsS ₄ ), arsenopyrite (FeAsS) and other minerals. These kinds of ores also contain copper, which is most often in the form of enargite or chalcopyrite (CuFeS ₂ ). The beneficiation of the ores in question requires the fabrication of the kind of bulk concentrate that include copper, free gold and the maximum amount of pyrite. Arsenic treatment and the large amount of pyritic sulphur set the major limitations on the creation of a profitable overall process for this type of concentrate.

A method is described in WO patent application 2004/035840, which relates to the recovery of metals, in particular copper, from a copper-bearing raw material, where the material is leached into a chloride-containing solution.

Raw material leaching is performed acidically at a pH value of at least 1.5 and a redox potential of 480 - 500 mV vs. Ag/AgCI, so that the copper in the copper chloride solution exiting leaching is mostly divalent. At the same time the sulphur contained in the concentrate is precipitated in elemental form, and also the iron precipitates. The precious metals included in the concentrate do not dissolve but go into the leaching residue. The leaching residue is subjected to sulphur flotation, whereby a sulphur concentrate is obtained, which also contains precious metals (gold + PGM). The majority of the sulphur is separated from the sulphur concentrate by known methods and thus a PGM concentrate is formed containing precious metals. The chloride solution formed in raw material leaching, which contains copper and other possible valuable metals, is routed to liquid-liquid extraction. In extraction copper is transferred first to the organic phase by means of extraction, and in stripping to a sulphate solution, which is routed to copper electrowinning. There is no mention in the method of how any arsenic that may be contained in the raw material behaves or how gold-containing pyrite is recovered.

US patent 4,023,964 describes a method for fabricating copper. In the method copper sulphide concentrate is leached into a solution of copper (II) chloride and sodium chloride. The NaCI concentration is 100-300 g/l and the pH value at most 1 , so that the iron dissolves. The solution obtained, in which the majority of copper is monovalent, is divided into two parts, one of which is subjected to iron precipitation as goethite by injecting air into the solution. The cupric chloride solution formed during goethite precipitation is routed back to concentrate leaching. The other part of the solution is contacted with the extraction solution. During extraction, air is introduced into the solution in order to oxidise the monovalent copper to divalent. In extraction the copper is bound to the organic phase and the cupric chloride solution that is depleted with regard to copper is routed back to concentrate leaching. In stripping, the organic phase and the copper bound to it are contacted with an aqueous solution of sulphuric acid. The copper sulphate

solution that is formed is routed to fabrication of elemental copper and the organic phase back to the extraction stage.

The method described in US patent 4,023,964 is practical in so far as the copper sulphide concentrate is leached as chloride, which is routed to liquid- liquid extraction and copper is recovered from extraction as copper sulphate solution. The further processing of the copper sulphate solution for example in electrowinning is a well-known technique and produces pure copper. The method is made complicated by the fact that the solution is divided into two branches, in one of which the dissolved iron is precipitated and only the other is routed to extraction. The copper in the copper chloride solution exiting leaching is mainly monovalent, so it has to be oxidised separately during extraction. When oxidation occurs during extraction, there is a danger that the extractant will be oxidised at the same time, making it no longer fit for use. It is recommended in the method that extraction be performed at a temperature of 6O ⁰ C, which in practice is far too high and causes the destruction of the extractant. The processing of arsenic-bearing material is not mentioned in the method, nor the behaviour of the gold contained in pyrite or the behaviour of gold in general.

PURPOSE OF THE INVENTION

Now a method has been developed for processing pyritic copper sulphide concentrate that contains gold, copper and arsenic. Bulk concentrate containing copper pyrite, pyrite, enargite and possibly other arsenic-bearing minerals, such as arsenopyrite, is leached into a chloride-based aqueous solution in conditions where copper, iron and arsenic dissolve, but pyrite and gold remain in the leaching residue.

SUMMARY OF THE INVENTION The invention relates to a method for processing pyritic copper sulphide concentrate containing gold and/or platinum group metals (PGM) and arsenic, whereby the concentrate is leached into an aqueous solution of

copper chloride and alkali chloride containing hydrochloric acid. It is typical of the method that the redox potential of the concentrate leaching is regulated to be in the range of 400 - 600 mV vs. Ag/AgCI electrode by means of the feed of an oxidising agent and the pH is adjusted to the range of 0.2 - 1 , whereupon the copper, iron and arsenic in the copper and arsenic minerals of the concentrate dissolve and the gold and/or PGM bound to these minerals is released. Pyrite and the gold and/or PGM it contains as well as free gold and/or PGM and sulphur remain in the leaching residue. The cupric chloride solution containing iron and arsenic that is formed is neutralised to precipitate the iron and arsenic, after which the neutralised iron- and arsenic- free cupric chloride solution is routed to the liquid-liquid extraction stage, where the copper is recovered into an aqueous solution of sulphuric acid, and is routed to electrolysis for recovery of elemental copper. The raffinate formed in liquid-liquid extraction is fed back to concentrate leaching.

According to one embodiment of the invention the leaching residue is subjected to sulphur flotation. Sulphur concentrate or leaching residue containing gold and/or PGM is preferably routed to pyrometallurgical processing to recover the gold and/or PGM, in which case they are routed to a smelter or roasted. The leaching residue may also be subjected to hydrometallurgical processing in the form of pressure leaching.

The iron- and arsenic-containing cupric chloride solution generated in copper sulphide concentrate leaching in accordance with the invention is neutralised to a pH value of 2 - 2.5 in order to precipitate the iron and arsenic out of the solution. Neutralisation is preferably carried out by means of limestone, lime or lye. Iron and arsenic are precipitated out of the solution as ferric arsenate and goethite or as ferric hydroxide.

The small amount of gold that may have dissolved in the leaching stage is precipitated back into the leaching residue by means of a reducing agent or adsorbent. A typical adsorbent is activated carbon.

LIST OF DRAWINGS

The method according with the invention is described in the appended Figure 1 , which shows a flow chart of one of the preferred embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Pyritic bulk concentrate containing gold, copper and arsenic, is fed to a leaching stage, in which leaching is performed using a solution of hydrochloric acid containing copper chloride and alkali chloride. Leaching takes place generally in several stages, but for the sake of simplicity it is shown in the flow chart as a single stage. Leaching is carried out as countercurrent or concurrent leaching. The redox potential of the leaching is regulated to be in the region of 400-600 mV vs. Ag/AgCI by means of oxygen or some other oxidant. The acid concentration of the leaching stage is adjusted to be in the range of 20 - 100 g/l, so that the pH value is below 1 , but preferably at least 0.2. In these conditions, iron and arsenic dissolve, but sulphur precipitates. In the description of the invention the general term gold refers to gold and/or platinum group metals (PGM).

The amount of alkali chloride in the solution used in leaching is around 100 g/l. The alkali is preferably sodium. The amount of alkali chloride is relatively low, because the copper in solution is divalent, so the required alkali chloride concentration is also low. Leaching conditions are regulated to be such that the iron in the sulphide minerals contained in the concentrate, apart from the pyrite, dissolves, as does the arsenic contained in the concentrate. The gold does not dissolve, but is released as the enargite and other arsenic minerals decompose, and the gold enters the leaching residue.

The reactions occurring in the leaching method accordant with the invention are described by means of the following reaction equations:

2Cu ₃ AsS ₄ +12H ⁺ + 5 ¹ /2 O ₂ = 6Cu ²⁺ + 2H ₃ AsO ₄ + 8S + 3H ₂ O (1 )

CuFeS ₂ + 4H ⁺ + O ₂ = Cu ²⁺ + Fe ²⁺ + 2S + 2H ₂ O (2)

FeAsS + 2O ₂ + 3H ⁺ = S + Fe ²⁺ + H ₃ AsO ₄ (3)

If a small amount of gold has dissolved in the concentrate leaching step, it is precipitated back into the leaching residue by means of a suitable reducing agent or adsorbent, for example activated carbon. After this, liquid/solids separation is performed, whereby the sulphur, pyrite and gold contained in the solids are separated from the solution. The arsenic-free leaching residue thus obtained is treated in an appropriate way to recover the gold. In addition to sulphur, the leaching residue comprises gold, pyrite and the gold contained in it that is released as a result of the decomposition of the minerals, but not iron and arsenic. Therefore the quantity of leaching residue is relatively small and there are many possible ways to process it further.

Sulphur flotation can be performed on the leaching residue (not shown in the drawing), whereby concentrate is obtained that is formed of pyrite and sulphur, containing nearly all the gold of the concentrate used as feedstock. Depending on the composition of the leaching residue or the type of further processing, the leaching residue may be treated without a sulphur concentration stage. The gold content of the generated sulphur concentrate and/or of the leaching residue is so large, that it can be routed to pyrometallurgical processing. This may take place for instance in a smelter, in which case the recovery yield of precious metals is high. The other possible further processing method is roasting, in which pyrite is oxidised and sulphur is burnt and gold is leached from the calcine with cyanide. Leaching waste can likewise be processed hydrometallurgically by pressure leaching in an autoclave, whereupon the pyrite decomposes and the gold can be leached into cyanide with a good yield.

When leaching is performed at the high acid concentration described above, the solution obtained in solids separation has to be neutralised before routing it to the extraction stage. In the method accordant with the invention, iron and arsenic removal from the solution are also carried out during neutralisation by precipitating them as ferric arsenate (FeAsO ₄ 2H ₂ O). Since the amount of iron is generally greater than that of arsenic, the excess iron is precipitated in the same stage as goethite (FeOOH) or ferric hydroxide (Fe(OH) ₃ ). Precipitation is performed by neutralising the solution to a pH value of about 2 - 2.5. Neutralisation is carried out by means of a suitable neutralising agent such as limestone, lime or lye (NaOH).

The divalent copper chloride solution, cleaned of impurities and solids, is routed to extraction. Copper recovery from the organic solution is performed into an aqueous solution of sulphuric acid, which is fed to electrowinning to recover elemental copper. Since the known copper extractants are mostly selective with regard to divalent copper, the copper chloride solution can be fed directly to extraction without an oxidation stage.

The extraction stage is depicted in the drawing as a single step, but it consists of the extraction and stripping stages normally included in extraction. In the extraction stage, an aqueous solution of cupric chloride is contacted with an organic extractant and the copper is made to transfer to the organic phase. The extraction stage includes the normal mixing and settling section, although they are not shown in detail in the drawing. Any known copper extractant is suitable as the extractant, such as oximes, which are diluted with an appropriate solvent, for example kerosene.

The chloride-containing aqueous solution of the extraction stage, the raffinate, which is depleted with regard to copper and has a raised acid concentration, is routed back to concentrate leaching. The organic solution exiting the extraction stages is conveyed via scrubbing to stripping. In stripping, the organic solution containing divalent copper ions is contacted

with an aqueous solution of sulphuric acid and the copper transfers to the aqueous phase as sulphate, from where it is recovered by electrowinning. When copper recovery takes place in copper electrolysis, the electrolysis return acid can be used as the aqueous solution of sulphuric acid in stripping.