FIELD OF THE INVENTION The invention relates to a method for recovering gold from an intermediate product or residue containing sulphur and iron generated in the leaching of a copper raw material in connection with the hydrometallurgical production of copper. The recovery of both copper and gold takes place in a chloride environment. The gold contained in the intermediate product or residue is leached with divalent copper and chlorine in a solution of copper (II) chloride - sodium chloride, in conditions where the oxidation-reduction potential is in the range of 650-750 mV and the pH in the range 1 -1.6. The acid generated during the chlorine feed is neutralised with the copper (II) oxychloride generated in the process. Neutralisation prevents the costs that arise from the dissolution of iron.

BACKGROUND OF THE INVENTION US patent 6,007,600 describes a method for the hydrometallurgical production of copper from a copper-bearing raw material such as copper sulphide concentrate. According to the method, the raw material is leached as a countercurrent leaching with a solution of sodium chloride - copper chloride in several stages to form monovalent copper (I) chloride solution. Since there are always both divalent cupric chloride and impurities formed from other metals remaining in solution, reduction of the divalent copper and solution purification is performed on the solution. The pure cuprous chloride solution is precipitated by means of sodium hydroxide into copper oxidule and the oxidule is reduced further to elemental copper. The sodium chloride solution formed during copper oxidule precipitation is further treated in chlorine-alkali electrolysis, from which the chlorine gas and/or chloride solution obtained is used for raw material leaching, the sodium hydroxide formed in electrolysis is used for oxidule precipitation, and the hydrogen produced for the reduction of elemental copper. The recovery of gold from the leaching residue is not described separately in the method.

Several methods are known in the prior art, which are used to leach gold from material containing sulphur and iron in connection with a chloride-based copper recovery process.

US patent 4,551 ,213 describes a method in which gold can be leached from sulphur-containing material, in particular the residue from hydrometallurgical processes. The preferred starting material for the method is the residue from the CLEAR process. The CLEAR process is a hydrometallurgical copper recovery process, which occurs in a chloride environment and at elevated pressure. The gold-containing residue is elutriated into water and the chloride concentration of the resulting suspension is adjusted so that it is 12 - 38 weight percent. The oxidation-reduction potential is adjusted to the range of 650 - 750 mV and the pH to below 0. Copper (II) chloride or iron (III) chloride are added to the suspension to oxidize the gold contained in the raw material, so that it dissolves.

EP patent 646185 relates to the recovery of copper from sulphidic concentrates with chloride leaching in atmospheric conditions. The gold from the leach residue is dissolved into electrolyte, which includes two or more halides, such as sodium chloride and sodium bromide. The purpose is to store oxidising energy in the bromine complex on the copper electrolysis anode, and thereby leach the gold from the residue.

There are certain drawbacks to the above-mentioned methods. In the method of US patent 4,551 ,213 the leaching conditions are very severe. The patent mentions that sulphur is not dissolved in the patent conditions, but the mention is not universally applicable, since the dissolving tendencies of elemental sulphur and the iron compounds mentioned in the patent depend on the manner in which the sulphur and the compounds in question are generated. In the tests we carried out it was found that when leaching residues formed in atmospheric conditions are treated in conditions in accordance with the said patent, the dissolution of sulphur and iron is considerable. Since, according to the publication, the sulphur and iron do not dissolve, there is no mention either of how to recover them from the solution. The gold leaching method used in EP patent 646185 using a bromine complex is not advantageous from an environmental point of view because harmful bromine emissions may be generated in the concentrate leaching stage.

WO patent application 03/091463 describes a method for leaching gold from leaching residue or intermediate product containing iron and sulphur, generated in the atmospheric chloride leaching of copper sulphide concentrate. The publication states that gold may be leached from an iron- and sulphur-containing material into an aqueous solution of copper (II) chloride and sodium chloride with copper and oxygen in conditions where the oxidation-reduction potential is below 650 mV and the pH value of the solution in the range 1 - 3. In these conditions iron does not dissolve and the sulphur remains undissolved to a large extent. Thus the costs that arise when iron and sulphur are removed from the solution are avoided. The recovery of gold from the solution is made by one of the methods of the prior art such as electrolysis or active carbon. The method in question is in itself quite good, but in practice however it is somewhat slow.

WO patent application 03/035916 describes a method where the divalent copper chloride (CuCI2) produced in the leaching of copper-bearing raw material is removed from the solution by precipitating the copper with the aid of a suitable reagent as basic copper (II) chloride, that is as copper (II) oxychloride. Copper (II) oxychloride is leached with hydrochloric acid and the copper (II) chloride solution that is generated is circulated to copper sulphide concentrate leaching. PURPOSE OF THE INVENTION Now a method has been developed for the recovery of gold from an intermediate product or residue that contains sulphur and iron, that is generated in the leaching of a sulphidic copper raw material in connection with the hydrometallurgical production of copper. Raw material leaching is carried out using a concentrated aqueous solution of alkali chloride - copper (II) chloride in atmospheric conditions. As a result of leaching, a solution is obtained where copper is mainly present as monovalent copper chloride CuCI, but partly also as divalent CuCI2. Divalent copper is precipitated as basic copper (II) chloride. When oxygen or oxygen-containing gas is fed into the copper concentrate leaching stages, iron is oxidized and precipitates as oxide or hydroxide and the valuable metals, with the exception of gold, dissolve. The leaching of gold from the remaining residue is carried out with the aid of an alkali chloride - copper (II) chloride aqueous solution and chlorine in atmospheric conditions. The oxidation-reduction potential of the leaching stage is raised to the range of 650-750 mV by means of chlorine. A high oxidation-reduction potential enables the elemental sulphur in the residue to dissolve and as a consequence, acid is formed in the stage. Preferably, at least the basic copper (II) chloride generated in connection with the precipitation of divalent copper is used to neutralize the acid. The simultaneous neutralisation of the acid keeps the pH at a value of 1.0 - 1.6, whereby the dissolution of iron is prevented. The dissolved gold is recovered by some method known as such in the prior art.

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

SUMMARY OF THE INVENTION A gold-bearing intermediate product or residue is leached into a sodium chloride solution containing copper (II) chloride forming a suspension and the Dxidation-reduction potential required for gold leaching is obtained Darticularly by means of divalent copper and chlorine gas. If the feed to gold leaching still contains undissolved copper sulphide, oxygen-containing gas can be fed to the start of the leaching stage in order to dissolve the copper. Chlorine gas is obtained advantageously from chlorine-alkali electrolysis connected with the hydrometallurgical recovery of copper concentrate. The oxidation-reduction potential is measured with Pt- and Ag/AgCI electrodes and the potential is kept at a value of 650-750 mV. The amount of divalent copper, Cu2+, in the solution is preferably 20 - 80 g/l and the amount of sodium chloride in the region of 200 - 330 g/l. Gold dissolves as a chlorine compound in accordance with the following reaction: Au + 3 Cu2+ + 6 CI" ■» AuCI4" + 3 Cu+ + 2 Cr (1 )

Leaching occurs in atmospheric conditions at a temperature, which is between room temperature and the boiling point of the suspension, preferably, however, between 800C and the boiling point of the suspension.

Thus, tests have now revealed that raising the redox potential of the reacting slurry with chlorine gas accelerates the dissolution of gold. There is however a drawback to this acceleration. Raising the redox potential increases the dissolution of the elemental sulphur (S°) in the material to be leached, which probably occurs in accordance with the following reaction (2):

S0 + 3 Cl2 + 4 H2O -» H2SO4 + 6 HCI (2)

Reaction (2) shows that a lot of acid is generated (8 mol H+/ mol S°). The acid generated in the solution must however be neutralised, since at pH values under 1 the iron in the solids begins to dissolve. The dissolution of iron causes process costs, as dissolved iron is circulated and is precipitated during Cu2+ precipitation and consumes reagents. The preferred pH region to keep iron in the residue is between 1.0 and 1.6.

According to our invention, the hydrochloric acid that is generated is neutralised at least by the basic copper (II) chloride generated during divalent copper precipitation, which has the mineral name of atacamite and the chemical formula Cu2(OH)3CI. This procedure produces considerable cost savings in comparison with the conventional procedure. The basic copper (II) chloride dissolves and the copper (II) chloride generated can be used for raw material leaching. If copper (II) oxychloride is not routed to this stage, acid is however required for leaching it, and said acid must be produced some other way. The neutralization of hydrochloric acid and copper (II) oxychloride leaching occur according to the following reaction: 3 Cu(OH)2 CuCI2 + 6 HCI ■» 4 CuCI2 + 6 H2O (3)

The sulphuric acid generated can be neutralised for example with lime:

H2SO4 + CaCO3 ■» CaSO42H2O + CO2 (4)

Naturally some other alkali can be used instead of lime such as sodium or potassium hydroxide. The recovery of gold from the solution takes place by some method known as such in the prior art, for instance active carbon, electrolysis or chemical precipitation.

LIST OF DRAWINGS. The method of the invention is further described in the flow chart of Fig 1 , where the recovery of gold is combined with the hydrometallurgical recovery of copper.

DETAILED DESCRIPTION OF THE DRAWINGS According to Fig. 1 , a copper sulphidic raw material such as copper sulphide concentrate 2 is fed to the first leaching stage 1 , and solution 3 from a later process stage, which is an aqueous solution of copper (II) chloride - sodium chloride, is also circulated to this stage. The thicker arrows denote solids and the thinner arrows the flow of solution. The stage always includes at least one or more reactors and thickening. The copper of the copper concentrate dissolves mainly into the process solution and the resulting solution 4 includes copper chloride, in which about 70 g/l of copper is mainly monovalent. The divalent copper of the solution is precipitated in precipitation stage 5 by means of some suitable alkali or alkali earth such as e.g. lye NaOH or lime CaCO3. The solution, in which all the copper is now monovalent, is routed to solution purification 6 and after this to the cuprous oxide precipitation stage 7. The sodium chloride solution from cuprous oxide precipitation is routed to chlorine-alkali electrolysis 8, where the chlorine and hydrogen gas formed and sodium hydroxide are used as reagents in the process. Hydrogen can be used for instance in the cuprous oxide reduction stage 9, and if required, also with chlorine in the preparation of hydrochloric acid according to the following reaction:

H2 + Cl2 = 2 HCI (5)

The leaching of solids 10 from the first leaching stage is continued in the second leaching stage 11 with solution 12, which is taken from a later process stage. Air is fed into the reactors at this stage in order to enhance leaching. Thickening is done at the end of this stage.

The solution 3 from the second stage is routed to the first leaching stage 1 to leach the concentrate. The leaching of the solids 13 from the second leaching stage is continued in the third stage 14 in order to leach the rest of the copper and the gold. In the third leaching stage, i.e. the gold leaching stage, the residue is leached with copper (II) chloride - sodium chloride solution 15, in which the Cu2+-content is 20 - 80 g/l and the sodium chloride content 200 - 330 g/l. If the residue entering this leaching stage still contains undissolved copper sulphide, oxygen, preferably in the form of air, can also be routed to the first reactor at the beginning of the stage. Copper sulphide should be leached out of the residue before the gold dissolves. In order to raise the redox potential to the range of 650-750 mV, chlorine gas 16 is also fed into the reactor, obtained from the chlorine-alkali electrolysis 8 connected to the process. Because of the high potential, sulphur starts to dissolve and as a result acid is formed in the stage. So that the pH of the stage does not fall below 1.0, some basic copper (II) chloride 17 is fed into it, which is formed during the precipitation of divalent copper. If necessary some other alkali could also be added here.

The gold-chloro complex solution 18 obtained from the leaching stage is routed either as it is or filtered to gold recovery, which in this case occurs in a carbon column 19 by means of active carbon. The gold product 20 is obtained from the column. The solution exiting column 19 is a gold-free solution 21 , which is circulated to the second stage of leaching 11 and sodium chloride solution is routed there as required to achieve a suitable copper (II) chloride content for leaching. The residue of the gold recovery stage, after normal after-treatment such as filtration and washing (not shown in detail in the diagram), becomes the final leach residue 22, which contains nearly all the sulphur and iron of the concentrate. The residue filtrate and rinse water are returned for example to the concentrate leaching process.

The flow chart in Fig. 1 presents a gold leaching method in connection with the leaching of copper-bearing raw material, but the method of the invention is not exactly limited to the copper-bearing raw material leaching process shown in the chart. The crux of our method is that the leaching of gold- bearing material is performed with divalent copper and chlorine in conditions where the redox potential of the solution is raised to a value of 650-750 mV, and the acid formed during the dissolution of sulphur is neutralised so that the pH is minimum 1 , preferably at least 1.0 - 1.6.

EXAMPLES The invention is described further by means of the appended examples, of which example 1 is in accordance with the state of the art and example 2 describes the method according to the invention. Example 1 Gold was leached from the leaching residue of a chalcopyrite concentrate in the laboratory in a one-litre reactor equipped with a mixer. The residue was obtained by leaching ordinary chalcopyrite copper concentrate according to US patent 6,007,600 in a laboratory-scale three-stage countercurrent leaching. The residue contained 3.7 % Cu, 28.9 % Fe, 32.4 % S and 5.8 ppm Au. A synthetic solution was used for leaching that contained approx. 40 g Cu2+ as chloride and approx. 300 g NaCI / L. The test temperature was 40 0C and the redox-potential was kept at a level of 690-700 mV (AgCI/Ag vs. Pt) by means of chlorine gas. The test lasted 12 hours and the gold leaching yield amounted to approx. 83 %. During the test the pH of the solution and the solution analyses changed as follows:

The results show a strong formation of acid, which led to the plentiful dissolution of iron.

Example 2 A laboratory-scale continuous three-stage countercurrent leaching was performed according to US patent 6,007,600, which included a Cu2+ removal stage. In the removal stage, divalent copper was precipitated out of the product solution as an basic copper (II) chloride, and in addition Fe was also precipitated out completely and SO42" almost completely. The size of the leaching reactors was 5 L. The third leaching stage in which gold leaching took place, consisted of two reactors, R4 and R5. Their operating temperature was 90 0C, their redox potential (AgCI/Ag vs. Pt) was adjusted with chlorine gas and the pH value was adjusted with the residue from Cu2+ precipitation. A synthetic solution was used as solvent, which contained 40 g Cu2+ chloride and 280 g NaCI/L. The solution delay time of the stage was in the region 10 h. The feedstock was the residue from the preceding reactor R3. The solids analyses from that particular week were as follows:

It can be seen that good Cu and Au leaching yields were achieved, of around 98% and 87%, respectively. The wet analyses and pH and redox values were as follows:

It can be seen that, despite the high redox potential, the Fe and S contents of the solution from Au leaching entering the preceding leaching stage were fairly reasonable. Therefore the residue containing basic copper (II) chloride had done its job and dissolved quite well.