FIELD OF THE INVENTION The invention relates to a method for fabricating metallic copper in connection with the chloride-based hydrometallurgical production of copper. In the method the copper raw material is leached using copper (II) chloride- alkali chloride solution to obtain a solution of copper (I) chloride. Copper is precipitated from the solution as copper (I) oxide and the alkali chloride solution that is formed is routed to chlorine-alkali electrolysis to produce the chlorine, alkali hydroxide and hydrogen required in raw material leaching and copper recovery. Metallic copper is prepared from copper (I) oxide by reducing it from a clearly alkaline solution using hydrogen at elevated pressure as either a batch or continuous process.

BACKGROUND OF THE INVENTION

US patent publication 6,007,600 describes a method for producing copper in a hydrometallurgical process from a copper-bearing raw material such as copper sulphide concentrate. According to the method, the raw material is leached counter-currently with a strong alkali chloride - copper chloride solution in several stages to form a copper (I) chloride solution. Since both divalent copper and some impurities comprising of other metals are always left in the solution, the divalent copper is reduced and solution purification is performed on the solution. The pure cuprous (I) chloride solution is precipitated by means of alkali hydroxide into copper (I) oxide and the oxide is reduced further into elemental copper. The alkali chloride solution formed during copper (I) oxide precipitation is treated further in chlorine-alkali electrolysis, from where the chlorine gas and/or chloride solution obtained is used for raw material leaching, the alkali hydroxide generated in electrolysis is used for oxidule precipitation and the hydrogen generated is used for the reduction of the copper to elemental copper. US patent publication 6,007,600 relates to the copper recovery method as a whole, and it is mentioned that

copper may be recovered from copper (I) oxide either in a pyrometallurgical process or in an autoclave. Autoclave reduction is not described further in the publication.

The fabrication of metallic copper from cuprite (CU ₂ O), an aqueous slurry of it, is described in the congress presentation by Hevia, J. F. & Wadsworth, M. E.: "Hydrothermal Reduction of Copper Oxides", pp. 93-108 in EPD Congress 1994, The Minerals, Metals & Materials Society, Warrendale, 1994. According to the research they described in the presentation, cuprite i.e. naturally occurring copper (I) oxide, is reduced to metallic copper with hydrogen reduction. Hydrogen reduction takes place in an autoclave at a temperature of approximately 15O ⁰ C and at a hydrogen pressure of 1900 kPa (19 bar). There is what is known as dead time or an induction period related to the reduction before the reaction gets underway. The dead time lasts several hours. The use of certain additives such as hydrazine or anthraquinone may shorten the induction period.

A commercially available copper (I) oxide powder, intended mainly for pigment use, was used in the tests carried out by Hevia and Wadsworth, which is presumably fairly pure. In practice it has been found that the reduction of the copper (I) oxide generated during the hydrometallurgical fabrication is more difficult than that of commercial, pure CU2O. Therefore it has not been possible to estimate the effect of impurities etc. in the reduction tests described in the presentation. In reduction tests, the pH value of the slurry varied between 7 - 9 and in the article it is stated that the pH value was of no significance.

PURPOSE OF THE INVENTION

The purpose of the method according to the present invention is to reduce copper (I) oxide generated during the hydrometallurgical production of copper into pure metallic copper using hydrogen. The induction time related to the reduction of copper (I) oxide generated during hydrometallurgical

production may be cut considerably without additives by applying reduction in strongly alkaline conditions.

SUMMARY OF THE INVENTION The purpose of this invention is to achieve a simple and inexpensive method of producing metallic copper from the copper (I) oxide generated during the hydrometallurgical production of copper. Sulphidic copper concentrate is the preferred feed material used in hydrometallurgical production. Copper concentrate is leached using an alkali chloride - copper (II) chloride solution and the copper (I) chloride solution generated is cleaned from impurities. Copper (I) oxide is precipitated from the copper (I) chloride solution by means of alkali hydroxide by known methods. The alkali chloride solution formed during copper (1) oxide precipitation is routed to chlorine-alkali electrolysis to produce the chlorine, alkali hydroxide and hydrogen required in raw material leaching and copper recovery. The pH of the copper (I) oxide slurry is adjusted to be strongly alkaline by means of alkali hydroxide generated in electrolysis, after which the reduction of copper (I) oxide into metallic copper is carried out in a pressurised space using the hydrogen obtained from electrolysis.

One essential factor of the invention is that the slurry to be fed to reduction is strongly alkaline. The pH value of the slurry is preferably in the range 9.5 - 13.5.

Due to the method of production, the copper (I) oxide always also includes chlorides, which react with alkali hydroxide to form the corresponding alkali salt, which then remains in the solution. The amount of chloride is around 0.1 - 1.5%.

The precipitated Cu ₂ O slurry in a strong alkali chloride solution may be routed to a pressurised space as it is without thickening or the slurry may be thickened. The slurry may also be fed to reduction as thickened, filtered and

washed and reslurried in water. The alkalinity of the slurry is achieved either during Cu ₂ O precipitation, by adding alkali hydroxide to the Cu ₂ O slurry to be fed into the pressurised space or by feeding the alkali hydroxide directly into the pressurised space.

The reduction of copper (I) oxide can be performed either as a batch or continuous process.

The essential features of the invention will become apparent in the attached claims.

DETAILED DESCRIPTION OF THE INVENTION

In the chloride-based hydrometallurgical production of copper, as described in the above-mentioned US patent 6,007,600, it is advantageous to get the copper in solution in monovalent form i.e. as copper (I) chloride. When chlorine-alkali electrolysis is connected to the process, the alkali chloride solution generated in the process stages is routed to chlorine-alkali electrolysis, which produces chlorine gas that is used in the leaching of the raw material, alkali hydroxide, which is used to precipitate copper (I) oxide and hydrogen, used to produce metallic copper.

Copper (I) oxide is reduced into metallic copper in the method according to the invention using hydrogen at an elevated pressure. Current autoclave technology has advanced and utilisation of higher pressure and temperature does not produce any great problems. Copper (I) oxide reduction can therefore be performed at a partial hydrogen pressure of 40 - 50 bar and at a temperature between 140 - 220 ⁰ C, preferably 170 - 220 ⁰ C.

The biggest problem in the hydrogen reduction of copper (I) oxide is what is known as dead time, i.e. the reactions get underway very slowly. It is mentioned in the prior art that this induction time has been shortened by using an additive, e.g. hydrazine or anthraquinone. Hydrazine is classified as

an environmental toxin, so its use is always to be avoided. Now it has been found that the induction time required for reduction can be considerably lessened even without additives, when the solution to be reduced is catalysed by making it strongly alkaline. In contrast to the impression given in the prior art, we have found that raising the pH has a powerful effect on the progression of reduction. According to the method now developed, the pH of the solution entering the autoclave is adjusted preferably to a value of 9.5 - 13.5 by means of the alkali hydroxide generated in the process. The pH adjustment of the slurry can be made in connection with the precipitation of copper (I) oxide that occurs using alkali hydroxide. Alkali hydroxide can also be added to the slurry before it is routed to the autoclave or the alkali hydroxide can be fed directly into the autoclave.

Raising the pH of the solution to be fed to reduction by means of alkali hydroxide is advantageous to the process not only in shortening induction time but also in other regards. In the chloride-based production of copper (I) chloride and further in that of copper (I) oxide, a small amount of chloride, about 0.1 - 1.5%, always remains in the oxide powder. When alkali hydroxide is fed into the oxide slurry entering reduction, the chloride in the oxide powder leaches into the water and is made to react to its salt by means of the hydroxide. When the sodium hydroxide generated in chlorine-alkali electrolysis is beneficially used as the hydroxide, sodium chloride is generated, which does not precipitate with the copper but stays in solution.

The copper powder is separated from the slurry after reduction and the solution is circulated back to copper concentrate leaching. Although we speak of sodium in the description, other alkalis such as potassium may be used in its place.

For process economy it is advantageous that copper (I) oxide reduction can also be performed either directly from the alkali chloride solution or from a thickened slurry without filtration, washing and reslurrying of the precipitate with water. Of course the copper (I) oxide may be filtered, washed and

reslurried in an aqueous solution before reduction within the framework of the method now developed. Copper (I) oxide reduction may be implemented either as a batch or continuous process.

Another benefit of this method that is worthy of mention is the fact that nothing is generated in any of the stages that has to be removed from the internal circulation of the process. Furthermore, the alkali hydroxide used as additive and the hydrogen required in reduction are obtained as products of chlorine-alkali electrolysis, as described in US patent publication 6,007,600.

EXAMPLES

Example 1

In order to clarify the method according to the invention, a series of tests was made, in which an aqueous slurry of copper (I) oxide was fed into an autoclave. The amount of copper (I) oxide in each test was 100 g/l. The partial pressure of hydrogen in every test was 45 bar and the temperature 200 ⁰ C. The chloride content of the starting material was about 1 %. The pH of the solution fed into the autoclave in the tests varied between 8.3 - 13.2. The pH was raised by routing sodium hydroxide into the aqueous solution before it was fed into the autoclave. The other results are presented in Table 1 below. One indicator of the success of the test is the oxygen content of the copper powder produced, in other words, the lower the oxygen content, the more successful the reduction has been.

Table 1

The table shows that the best result was achieved in tests 4-7, when the pH of the solution was high and no additives were fed into the solution.