METHOD FOR THE HYDROMETALLURGICAL PROCESSING OF SULPHIDIC MATERIAL CONTAINING ZINC AND COPPER

FIELD OF THE INVENTION The invention relates to a method whereby valuable metals are recovered from a sulphidic raw material such as concentrate, containing at least copper and zinc, by means of hydrometallurgical processing. Copper sulphide and zinc sulphide are components of the raw material, and are leached by means of an alkali chloride - copper (II) chloride solution. The divalent copper chloride is removed from the solution that is formed, and the solution is routed to zinc extraction to remove the zinc from the alkali chloride - copper (II) chloride solution. The solution exiting zinc extraction, which is depleted with regard to zinc, is routed to final solution purification and copper recovery. The reagents required in the process are obtained primarily as products of the chlorine-alkali electrolysis that is related to the process.

BACKGROUND OF THE INVENTION

US patent publication 6,007,600 describes the method developed by

Outokumpu Oyj for the hydrometallurgical fabrication of copper from a copper-containing raw material such as copper sulphide concentrate. In accordance with the method, the raw material is leached countercurrently with a concentrated alkali chloride - copper (II) chloride solution in several steps to form a copper (I) chloride solution. Since there always remain in the solution both divalent copper and impurities formed from other metals, the solution is subjected to precipitation of the divalent copper and solution purification. Pure copper (I) chloride solution is precipitated by means of alkali hydroxide as copper (I) oxide and the oxide is reduced further to elemental copper. The alkali chloride solution formed in connection with copper (I) oxide precipitation is processed further in chlorine-alkali electrolysis, from which the chlorine gas and/or chloride solution obtained is used for raw material leaching, the alkali hydroxide generated in electrolysis is used for copper (I) oxide precipitation and the hydrogen that is generated

for reducing copper to elemental copper. This method is called the HydroCopper™ process. US patent publication 6,007,600 describes the copper recovery method in detail, but relates mainly to pure copper sulphide concentrates.

US patent publication 5,487,819 describes a method for the hydrometallurgical production of copper from a raw material such as sulphide concentrate containing copper and possibly other valuable elements. According to the method, the raw material is leached countercurrently with a sodium chloride - copper chloride solution in several stages. If there are in fact sulphides other than copper sulphide in the raw material, it is described in the method that the other sulphides are leached in the first leaching step, from which the solution is removed for further processing. The leaching of the undissolved raw material is continued in order to form a copper (I) chloride solution and residue containing iron and sulphur. The further processing of the solution removed from the first leaching step is after thickening and filtration, for example the removal of silver and mercury. In the second step, iron, arsenic, bismuth, mercury, antimony etc. are removed by known methods. When the solution contains lead and zinc, the lead is recovered first in its own electrolysis, and afterwards the zinc is recovered from the solution in a second electrolysis. According to the publication, in both electrolysis treatments the metal is produced at the cathode, which is wiped so that both lead and zinc are recovered in particulate form from the floor of the cell.

The recovery of zinc and lead in electrolysis, in which a particulate metal is produced, is probably not in commercial use. Its implementation in practice in any case seems problematic.

A method is known from WO patent application 2006/070052, in which valuable metals are recovered from sulphidic concentrate, whereby the concentrate is leached first in a sulphate milieu to leach some valuable metal

other than copper and the copper is leached from the remaining residue by chloride leaching. Thus the recovery of other valuable elements, such as zinc or nickel, takes place clearly in a sulphate milieu.

A method is known from US patent publication 4,288,304, in which a sulphide concentrate containing zinc, and also other valuable metals such as copper, is routed to chloride-based leaching in order to recover the valuable metals. The leaching of valuable metals from the concentrate occurs by means of divalent copper and there may also be alkali metal chlorides in the solution to improve the solubility of the monovalent copper. After leaching, the copper of the solution is primarily in monovalent form. The solution containing valuable metals is first routed to liquid-liquid extraction of zinc, which takes place by means of an organophosphorus acid such as D2EHPA. In zinc extraction, hydrogen ions are generated in the solution and the pH of the solution falls, but in the method the decrease of pH to outside the operating range of 0.3 - 1.5 is prevented by feeding oxygen into the extraction phase, whereby the monovalent copper of the solution is oxidised to divalent and simultaneously water is formed. If the amount of copper in the concentrate is small, it is recycled back to leaching and part of it may be recovered by cementation. If there is a greater amount of copper, the solution exiting zinc extraction, in which copper is divalent, is routed to copper extraction.

It is characteristic of the method accordant with US patent 4,288,304 that the conditions of the extraction stage are controlled by feeding oxygen into it.

However, it has been found that feeding oxygen also oxidises the organic extraction solution, which is an undesirable phenomenon and prevents effective extraction. In the method, copper is oxidised back to divalent during zinc extraction, but the reduction of divalent copper to metal in electrolytic treatment requires more energy than the reduction of monovalent copper. It has also been found in practical tests that the oxidation of copper during

extraction when using organophosphorus acid causes the formation of troublesome sediment, which makes extraction more difficult.

PURPOSE OF THE INVENTION The purpose of the method accordant with the invention is to recover zinc in addition to copper from sulphidic raw material containing copper and zinc. The recovery of zinc takes place as a sub-process connected to the copper recovery process, whereby the solution exiting copper (II) chloride - alkali chloride leaching is routed to zinc liquid-liquid extraction. Conditions in zinc extraction are maintained in which the copper of the solution remains monovalent and the oxidation of the extraction solution is avoided.

SUMMARY OF THE INVENTION

The essential features of the method accordant with the invention are presented in the patent claims.

The invention relates to a method for recovering copper and zinc from sulphidic raw material containing them by means of hydrometallurgical treatment. The raw material is leached with a concentrated solution of alkali chloride - copper (II) chloride and the generated copper (I) chloride solution, which also contains dissolved zinc, is routed to liquid-liquid extraction to recover the zinc from the solution. The hydrogen ions generated in the extraction stage of liquid-liquid extraction are neutralised by means of alkali hydroxide. Neutralisation enables the adjustment of the extraction pH to the region of 2 - 4, thereby preventing the oxidation of the copper (I) chloride in the solution to copper (II) chloride. Zinc is recovered from the organic solution by known methods into a solution containing sulphuric acid and metallic zinc is formed preferably by means of zinc electrolysis. Copper (I) oxide is precipitated by means of alkali hydroxide from the copper (I) chloride solution that is removed from zinc extraction and being depleted with regard to zinc, and is reduced in a suitable way to metallic copper.

According to one preferred embodiment of the invention, the sulphidic raw material is sulphide concentrate.

According to another embodiment of the invention, the sulphidic raw material is zinc-containing copper sulphide precipitate.

It is typical of the method accordant with the invention that the alkali chloride concentration of the alkali chloride - copper (II) chloride solution is at least 200 g/L.

It is typical of the method accordant with the invention that the alkali to be used is sodium.

In the method accordant with the invention, liquid-liquid extraction includes extraction, scrubbing and stripping stages. The scrubbing solution used in the scrubbing stage of liquid-liquid extraction is an aqueous solution containing a zinc salt, where the pH is adjusted to be in the range of 1 - 3.

The zinc concentration of the scrubbing solution of the scrubbing stage is preferably regulated to be in the order of 10 - 30 g/L and the ratio of the streams of extraction solution and scrubbing solution in the order of 5 - 40:1.

It is typical of the method accordant with the invention that the copper recovery process includes chlorine-alkali electrolysis.

It is advantageous for the method that the neutralisation of the extraction stage and the precipitation of copper (I) oxide from the copper (I) chloride solution use the kind of alkali hydroxide that is generated in chlorine-alkali electrolysis. The alkali chloride solution formed in the precipitation of copper (I) oxide is routed to chlorine-alkali electrolysis to produce the chlorine, alkali hydroxide and hydrogen needed for raw material leaching, extraction neutralisation and copper recovery.

LIST OF DRAWINGS

Figure 1 presents a flow chart of such a method accordant with the invention, in which copper-zinc sulphide concentrate is processed.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a method whereby zinc is recovered from zinc- and copper-containing sulphidic material by means of extraction before copper recovery. The copper- and zinc-containing material is leached into an alkali chloride - Cu (II) chloride solution, when alkali chloride is obtained from the chlorine-alkali electrolysis belonging to the process. Zinc- and copper- containing material means mainly zinc- and copper-containing sulphide concentrate or sulphide precipitate containing the above-mentioned metals generated in some process step.

When copper-zinc sulphide concentrate is concerned, the amount of copper is generally about double, even triple that of the amount of zinc. This kind of concentrate has for example the following composition: Cu 14 %, Zn 3.4 %, Fe 35 %, S 42 %, Pb 0.5 %, As 0.3 % and Sb 0.1 %. Leaching occurs in atmospheric conditions in the way outlined in US patent 6,007,600. By atmospheric conditions we mean a temperature, which is at maximum the boiling point of the solution i.e. approx. 105 ⁰ C and the pressure that is formed when using unpressurised reactors. Figure 1 presents a flow diagram of the leaching of zinc-containing copper-zinc concentrate. Bulk concentrate i.e. Cu-Zn concentrate 1 is routed to leaching stage 2, where concentrate leaching occurs countercurrently with concentrated alkali chloride - copper (II) chloride solution 3. The amount of alkali chloride in the solution is at least 200 g/l. The alkali chloride is generally sodium chloride. In the reactions that take place during leaching the compounds contained in the concentrate break down and elemental sulphur, and compounds of iron and arsenic, as well as gold and other precious metals (PGM) generated as a result of the reactions remain in the leaching residue. The leaching residue is treated as necessary in the desired fashion, for instance for recovery of gold and other

precious metals. An oxidising gas such as air or oxygen is fed into leaching stage 2. Zinc dissolves as zinc chloride ZnCI ₂ and copper mostly as monovalent cuprous chloride CuCI.

The solution exiting leaching stage 2 is a copper chloride - alkali chloride solution 4 containing zinc chloride. The amount of copper in the solution is in the order of 50 - 100 g/L. The majority of the copper chloride is monovalent cuprous chloride CuCl, but some is also cupric chloride CuCb, for which reason it is preferable to subject the solution to precipitation 5, so that the divalent copper is precipitated for example as basic copper (II) chloride Cu ₂ CI(OH) ₃ by means of an alkali hydroxide, such as sodium hydroxide NaOH. The precipitation of copper (H) chloride from a solution is described for example in EP patent 1 438 439. The basic copper chloride is leached either in a separate leaching step and the copper (II) chloride that is formed is fed back to concentrate leaching or it is fed directly back to concentrate leaching (shown in the drawing by a broken line).

Zinc can be separated selectively from the copper (I) chloride solution 6 obtained from HydroCopper leaching by a liquid-liquid extraction process. Liquid-liquid extraction enables the production of a pure zinc solution, which is suitable for zinc electrolysis whereby zinc cathodes are produced. If necessary, however, a conventional zinc sulphate solution purification process can also be connected to the process before zinc electrolysis, but this is not shown in detail in the flow chart.

In zinc extraction step 7 a commercial extractant can be used, such as organophosporus acid D2EHPA (di-(2-ethylhexyl)phosphoric acid) or organic phosphinic acid, which is known for example by the trade name Cyanex 272. The concentration of the extractant in extraction solution 8 is selected according to the zinc content of feed solution 6 and it varies between 10-50 vol.%. Extractants are diluted in a suitable hydrocarbon solvent, for example Shellsol D70. Zinc extraction is performed by using one or several extraction

steps connected in series. The extraction equipment used may be mixer- settler apparatuses, which are connected countercurrently. In the drawing the organic solution is depicted by a dashed line and the aqueous solution by a solid line.

In the extraction steps, the acidity of the aqueous phase is adjusted to the pH range of 2 - 4 by neutralising the hydrogen ions generated in the extraction reaction with sodium hydroxide, which is obtained as a product of the chlorine-alkali electrolysis included in the HydroCopper process. The use of sodium hydroxide, i.e. lye, in the process is preferred because it can be obtained as a product of the chlorine-alkali electrolysis belonging to the process. Even though lye is a more expensive product than lime, its use in neutralisation is still preferred, because in the pH range used calcium is extracted with the zinc and causes problems in later stages of the process. Raffinate solution 19 from the extraction process, from which zinc has been removed, continues onward to the copper recovery steps.

For example, when extracting zinc from a feed solution containing 25 g/L Zn, 45 vol. % D2EHPA extraction solution can be used. In this case a ratio of approx. 1.3 - 1.5:1 volume flow of the organic extraction solution to aqueous solution volume flow can achieve a zinc yield of 95-100%.

The organic extraction solution phase containing zinc LO (Loaded Organic) 9 contains impurities after extraction. The impurities are either extracted chemically or transported physically in the extraction solution in water droplets as a result of imperfect phase separation. The impurity content is reduced to a sufficiently low level by routing the extraction solution to scrubbing stage 10, which comprises one or several scrubbing steps connected in series. The scrubbing solution used for the scrubbing step is aqueous solution 11 , which has a zinc salt as well as acid, whereby the acidity of the solution is adjusted to a pH value of about 1-3. When the zinc concentration of the scrubbing solution is sufficiently high, e.g. 10-30 g/L Zn,

zinc is prevented from being stripped into the acidic scrubbing solution. The impure elements are stripped into acidic scrubbing solution 12 and are replaced by zinc, which transfers into the organic solution. The extraction solution phase can be made sufficiently pure by adjusting the ratio of the flow of extraction solution and scrubbing solution, which may be 5 - 40:1.

If desired, an additional scrubbing stage can be used after the scrubbing step, in which the unsettled residual droplets of water containing impurities originating from the previous scrubbing step are removed from the extraction solution with clean water. The solution used in this scrubbing stage can be used in the preparation of the solution for the previous scrubbing step (not shown in detail).

Zinc is transferred from CLO (Clean Loaded Organic) extraction solution 13 into the aqueous solution in stripping stage 14. When the extraction solution is brought into contact with LE (Lean Electrolyte) 15 in the stripping stage, said LE being typically a solution containing sulphuric acid from zinc electrolysis, zinc is released from the extraction solution into the acidic solution as zinc sulphate in the reverse reaction to the extraction reaction. Stripping can be performed in one or several stripping steps, in which zinc can be removed practically speaking fully from organic phase 8, which is returned to extraction stage 7. The rich zinc electrolyte RE 16 obtained from stripping contains 50 - 150 g/L of zinc as zinc sulphate, depending on the electrolysis process. In zinc electrolysis 17, zinc is recovered from the solution and precipitated on cathodes.

When using D2EHPA extractant, the iron contained in feed solution 6 is extracted so effectively into the extraction solution, that it is not removed from the extraction solution apart from small amounts in the scrubbing and stripping steps. This causes the accumulation of iron in the extraction solution, which reduces its mass transfer capacity. Iron can be removed by

routing a small side stream from the extraction solution to a separate acidic iron removal step 18, in which concentrated hydrochloric acid is used.

The aqueous solution exiting extraction stage 7, i.e. raffinate 19 from which zinc has been removed, is routed to copper recovery. The raffinate solution includes, in addition to copper, a small amount of other dissolved metals, which are recovered from the solution with known methods in solution purification stage 20 before copper precipitation. One known method is described in EP patent 1497474, in which the separation of impurities is performed by ion exchange.

During solution purification the lead dissolved along with the copper is precipitated out of the solution by known methods. Copper (I) oxide is precipitated from pure copper (I) chloride solution 21 by mean of alkali hydroxide in precipitation step 22. The alkali chloride solution formed in copper (I) oxide precipitation is routed to chlorine-alkali electrolysis 23 to produce the chlorine, alkali hydroxide and hydrogen required in raw material leaching and copper recovery. The generated copper (I) oxide 24 is reduced to metallic copper in an appropriate manner in reduction stage 25. If precious metals, particularly gold, are part of a multicomponent concentrate, it is also possible to recover them, for example by the method described in WO patent 03/091463 in relation to the HydroCopper process.

Although the invention is described as a method in which zinc and copper are recovered from concentrate, the method is also suitable for the further processing of sulphidic precipitate containing zinc and copper.

EXAMPLES Example 1 A copper-zinc sulphide concentrate, with 14% copper, 28.5% iron, 40.4% sulphur, 2.4% zinc and 1.6% arsenic, is leached in atmospheric conditions, in which concentrate leaching occurs countercurrently with a concentrated

solution of alkali chloride and copper (II) chloride. The solution exiting the leaching stage is a copper chloride - alkali chloride solution containing zinc chloride. The solution is routed to solution purification, where divalent copper is precipitated as basic copper (II) chloride Cu ₂ CI(OH) ₃ by means of alkali hydroxide, sodium hydroxide NaOH, and the copper (II) chloride that is formed is routed back to concentrate leaching. Subsequently zinc is separated selectively from the copper (I) chloride solution obtained from HydroCopper leaching by a liquid-liquid extraction process. Liquid-liquid extraction enables the production of a pure zinc solution, which is suitable for zinc electrolysis in which zinc cathodes are produced. The aqueous solution or raffinate exiting the extraction stage is routed to copper recovery. The raffinate solution contains not only copper but also a small amount of other dissolved metals, which are recovered from the solution, for example by ion exchange.

Example 2

The example used is a HydroCopper installation, which is located in the vicinity of a zinc plant. Copper-zinc sulphide concentrate, 14.5% copper, 34.1% iron, 40.3% sulphur, 5.3% zinc and 0.2% arsenic, is leached in atmospheric conditions, where concentrate leaching takes place countercurrently with a concentrated solution of alkali chloride and copper (II) chloride. Various copper- and zinc-containing precipitates from the zinc plant may also be fed into leaching. The solution exiting the leaching stage is a copper chloride - alkali chloride solution containing zinc chloride. The solution is routed to solution purification, where divalent copper is precipitated as basic copper (II) chloride Cu ₂ CI(OH)S by means of sodium hydroxide NaOH, and the copper (II) chloride that is formed is routed back to concentrate leaching. Subsequently zinc is separated selectively from the copper (I) chloride solution obtained from HydroCopper leaching by a liquid- liquid extraction process. Liquid-liquid extraction enables the production of a pure zinc solution, which is fed to the zinc plant located in the industrial park area. The aqueous solution or raffinate exiting the extraction stage is routed

to copper recovery. The raffinate solution contains not only copper but also a small amount of other dissolved metals, which are recovered from the solution, for example by ion exchange.