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 sulphidic raw material. The recovery of gold and the other valuable metals in the raw material takes place in a chloride environment. The gold contained in the intermediate product or residue is leached with divalent copper and chlorine in a copper (II) chloride - sodium chloride solution in conditions where the oxidation-reduction potential is in the range of 650-750 mV and the pH between 1 and 1.6. The acid generated during the feed of chlorine is neutralized with a suitable alkali. Neutralization avoids the costs of dissolving the iron.

BACKGROUND OF THE INVENTION 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.

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, generated in the leaching of a sulphidic raw material. Raw material leaching is carried out using a concentrated aqueous solution of alkali chloride and copper (II) chloride in atmospheric conditions. When oxygen or oxygen-containing gas is fed into the sulphidic concentrate leaching stages, iron is oxidised and precipitated 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 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, which is neutralised by some suitable alkali. 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 sodium chloride solution containing copper(ll) chloride forming a suspension and the oxidation-reduction potential required for gold leaching is obtained particularly by means of divalent copper and chlorine gas. If the feed to gold leaching still contains undissolved copper- or other sulphide, oxygen- containing gas can be fed to the start of the leaching stage in order to dissolve it. 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 chloro complex in accordance with the following reaction:

Au + 3 Cu2+ + 6 Cl" -» AuCI4" + 3 Cu+ + 2 Cl" (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 consumes reagents. The preferred pH region to keep iron in the residue is between 1.0 and 1.6.

Some suitable alkali from the group NaOH, KOH, CaO, CaCO3 or MgO is used to neutralise the acid. If the process is combined with a chloride-based hydrometallurgical method of producing copper, in which basic copper (II) chloride is generated in the precipitation of divalent copper from leaching, the use of the copper (II) chloride is the best option. When basic copper (II) chloride dissolves, the copper (II) chloride generated can be used for raw material leaching. The neutralization of hydrochloric acid and copper (II) oxychloride leaching occur according to the following reaction:

3 Cu(OH)2CuCI2 + 6 HCI -* 4 CuCI2 + 6 H2O (3)

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

H2SO4 + CaCO3 ■» CaSO4-2H2O+ CO2 (4)

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 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 and 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 one or more reactors and thickening. The copper and other valuable metals of the concentrate mainly dissolve into the process solution and the resulting solution 4 includes copper chloride, in which about 70 g/l of copper is mainly monovalent. The further treatment of the copper chloride solution is not presented in more detail here. The leaching of the solids 5 from the first leaching stage is continued in the second leaching stage 6 with solution 7, which is taken from a later process stage. Air is fed into the reactors at this stage in order to enhance leaching of the valuable metals and to precipitate the iron. 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 8 from the second leaching stage is continued in the third stage 9 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 10, 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 sulphide, oxygen, preferably in the form of air, can also be routed to the first reactor at the beginning of the stage. Copper and other sulphides 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 11 is also fed into the reactor. 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 alkali 12 is fed into it such as NaOH, KOH, CaO, CaCO3 or MgO. If the process is combined with a chloride-based hydrometallurgical method to produce copper, in which basic copper (II) chloride is generated in the precipitation of divalent copper from leaching, the basic copper (II) chloride is used.

The gold-chloro complex solution 13 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 14 by means of active carbon. The gold product 15 is obtained from the column. The solution exiting column 14 is a gold-free solution 16, which is circulated to the second stage of leaching 6 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 17, 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 copper-bearing raw material leaching, but the method of the invention is not 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 neutralized so that the pH is minimum 1 , preferably at least 1.0 - 1.6.