MX2008010507A - Method for recovering gold - Google Patents

Abstract

The invention relates to a method for recovering gold in connection with the hydrometallurgical production of copper from a waste or intermediate product containing sulphur and iron that is generated in the leaching of the copper raw material. The recovery of both copper and gold occurs in a chloride environment. The gold contained in the waste or intermediate is leached by means of divalentcopper, oxygen and alkali bromide in a solution of copper (II) chloride and alkali chloride, in conditions where the oxygen-reduction potential is a maximum of 650 mV and the pH a minimum of 0.5. The bromide accelerates the dissolution of the gold.

Description

METHOD FOR GOLD RECOVERY Field of the Invention The invention relates to a method for the recovery of gold in relation to the hydrometallurgical production of copper from a waste or an intermediate product containing sulfur and iron that is generated in the leaching of copper raw material. . The recovery of both copper and gold takes place in a chloride environment. The gold contained in the waste or intermediate is leached by means of divalent copper, oxygen and bromide in a solution of copper chloride (II) -sodium chloride, under conditions in which the oxygen reduction potential is a maximum value of 650 mV and the pH between 0.5 - 2.5.

Background of the Invention The method of Hydrocopper ™ for the manufacture of copper in hydrometallurgical form from a copper-containing raw material such as a copper sulfide concentrate is described in US Pat. No. 6,007,600. According to the method, the raw material is leached in countercurrent with a solution of copper chloride-alkali chloride in several stages to form a solution of copper chloride (I) monovalent. Part of the solution that is formed is brought to oxidation which is carried out with chlorine gas, after which the copper (II) chloride generated is recirculated back to the leaching of the concentrate. The purification of the solution is carried out on the rest of the solution formed in the leaching of the raw material. The pure cuprous chloride solution is precipitated by means of an alkaline hydroxide, to copper (I) oxide and the oxide is subsequently reduced to elemental copper. The alkali chloride solution formed during the precipitation of the copper (I) oxide is further processed in the chlor-alkali electrolysis, from which the chlorine gas and / or the chloride solution obtained is used for the oxidation of the chloride. copper (I) and / or the leaching of the raw material, the alkaline hydroxide generated in the electrolysis is used for the precipitation of oxide and the generated hydrogen is used for the reduction of elemental copper. The recovery of gold from the leach residue is not described separately in connection with the method. Various methods are known in the prior art, which are used for the leaching of gold from materials containing sulfur and iron in connection with a copper recovery process based on chloride. U.S. Patent 4,551,213 discloses a method according to which gold can be leached from sulfur-containing materials, particularly from hydrometallurgical process residues. The preferred starting material for the method is the residue of the CLEAR process. The CLEAR process is a recovery process of hydrometallurgical copper, which takes place in a chloride environment and at an elevated pressure. The gold-containing residue is purified by washing with water and the suspension obtained is adjusted to contain 12-38 weight percent chloride. The oxidation-reduction potential is adjusted to the range of 650 - 750 mV and the pH value below 0. Copper (II) chloride or iron (III) chloride is added to the suspension to oxidize the gold contained in the raw material, after which he dissolves. It is mentioned in the publication that the oxidation-reduction potential should not rise above 750 mV, since above this value the sulfur will dissolve. In the publication there is no information regarding the amount of dissolved sulfur or iron. Patent EP 646185 refers to the recovery of copper from sulphide concentrates by leaching with chloride under atmospheric conditions. In the final stage of countercurrent leaching, gold is leached directly into the electrolyte by the electrolysis of copper, zinc and lead with a high oxidation potential. It is an essential characteristic of the method that the high oxidation potential is achieved by means of a halide complex such as BrCl "2, which is formed in electrolysis. According to Example 4, which describes the leaching of gold, the gold is dissolved at an oxidation-reduction potential of approximately 700 mV vs Ag / AgCl. The patent application WO 03/091463 describes a method for leaching gold from a leaching residue or intermediate containing iron and sulfur, which is generated in the leaching of chloride under atmospheric conditions from a copper sulfide concentrate. The publication states that it is possible to leach gold from a material containing iron and sulfur in an aqueous solution of copper (II) chloride and sodium chloride by means of divalent copper and oxygen in conditions where the oxidation-reduction potential is below 650 mV and the pH value of the solution is in the range of 1 - 3. Under these conditions the iron it does not dissolve yet and the sulfur remains largely undissolved, thus avoiding the costs incurred when removing the iron or sulfur from the solution. The recovery of gold from the solution is carried out by means of one of the prior art methods such as electrolysis or activated carbon. The method in question is pretty good in itself, but in practice it is a bit slow.

Objectives and Compendium of the Invention A new method has now been developed to leach gold from a leaching or intermediate residue containing iron and sulfur, which is generated in leaching with chloride under atmospheric conditions of copper sulphide concentrate and which is essentially copper free. We have found that when gold is leached from a material containing iron and sulfur in an aqueous solution of copper (II) chloride and alkaline chloride and an oxygen-containing gas is fed into the solution, a small amount of Bromide accelerates the time required for the dissolution of gold. The leaching takes place by means of divalent copper in conditions where the oxidation-reduction potential is regulated with oxygen in the range of 600 - 650 mV vs. an Ag / AgCl electrode and the pH value of the solution is adjusted in the range 0.5-2.5, preferably 0.5-1.5. The bromide fed accelerates the dissolution of gold without causing the oxidation-reduction potential of the leaching to rise above the value of 650 mV. The gold-containing or intermediate residue is purified by washing with an alkali chloride solution containing copper (II) chloride forming a suspension, and the oxidation-reduction potential required for gold leaching is achieved only by divalent copper and oxygen. To improve leaching, some alkaline bromide such as sodium or potassium bromide is fed to the suspension which is formed so that the concentration of Br ion in the gold leaching stage is 0.5-30 g / 1, preferably of 8 - 15 g / 1. After the gold leaching stage the solution containing gold is taken to the gold recovery stage, after which the solution is recirculated back to the leaching stage. The leaching occurs under atmospheric conditions at a temperature that is in the range of between ambient temperature and the boiling point of the suspension, preferably however at a temperature between 80 ° C and the boiling point of the suspension. Recovery of gold from the solution is done using some method known in the prior art such as electrolysis or by means of activated carbon. The remaining residue is a disposable waste. When the gold has been recovered from the solution, the solution is recirculated back to the gold leaching stage. The essential features of the invention will become apparent in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS The method of the invention is described in more detail in the drawings, in which: Figure 1 is a flow diagram in which the recovery of gold is combined with a leaching process of the copper sulfide concentrate . Figure 2 is a graphical presentation of the effect of bromide addition as a function of the yield of the gold dissolution rate, the oxidation-reduction potential according to example 1, and Figure 3 is a graphical presentation of the effect of the addition of the bromide on the gold dissolution rate and the oxidation-reduction potential according to example 2.

Detailed Description of the Preferred Modes of the Invention It is advantageous to join the method developed now with a process of leaching copper concentrate chloride as a sub-process. As was previously mentioned, one such process is written in for example the patent of the United States of America 6,007,600. In the method in question, a raw material containing copper sulphide such as a concentrate is leached countercurrently with a solution of alkali chloride and copper (II) chloride, NaCl-CuCl 2, in several steps to form a chloride solution. copper (I) monovalent, CuCl. The alkaline chloride solution formed in the process is processed in chlorine alkali electrolysis and the alkali hydroxide, chlorine and hydrogen formed in the electrolysis are used in several stages of the process. process. After the leaching of the concentrate a residue remains, which contains mainly sulfur and iron of the starting material as well as the gold contained in the raw material. The method developed now focuses on the gold leaching residue, which is formed in the type of process mentioned above. The leaching step of a waste or intermediate containing gold occurs in principle separately from the leaching stage of the actual concentrate, since the solution from which gold is separated, instead of being returned to the concentrate leaching circuit, is recirculated back to the leaching of gold. The oxidation-reduction potential in the gold leaching stage is measured with Pt and Ag / AgCl electrodes and the potential is maintained at a maximum value of 650 mV, preferably at a maximum of 640 mV. When the oxidation potential reduction is maintained below a value of 650 mV, the sulfur does not yet dissolve from the residue, and remains as elemental sulfur. The preferred pH range is between 0.5 and 1.5, so the iron in the residue remains almost undissolved. The oxidation gas used can be air, air enriched with oxygen, or oxygen. The amount of divalent copper, Cu, in the solution is preferably 40-100 g / 1 and the amount of sodium chloride in the range of 200 - 330 g / 1. If the chloride solution used in the gold leaching is a sodium chloride solution, the alkali metal bromide to be preferably fed is also sodium bromide. Sodium is of a lower price than potassium, so its use is therefore justified. Most of the bromide to be fed in the gold leaching stage is in circulation within the leaching stage. A small part of this, 0.5 - 10%, however, is removed with the filtrate, which is formed during the filtration performed on the gold leaching residue. To avoid losses of bromine, the filtrate is taken to the oxidation stage belonging to the leaching stage of the concentrate. In the oxidation stage, part of the copper chloride (I) formed in the leaching of the concentrate, is oxidized back to copper (II) chloride by means of the chlorine formed in the alkali electrolysis of chlorine, which is fed into of the final stage of the concentrate leaching process. The chlorine gas also oxidizes the bromide in the filtrate to bromine gas, which is recovered in a scrubber connected to the oxidation stage, where it dissolves into the wash fluid. As for the washing fluid of the scrubber, it is taken to the stage of leaching of gold. The wash fluid from the scrubber is recirculated back to the gold leaching stage, where the slurry from the leach stage reduces the bromine back to bromide. The flow chart according to Figure 1 is an example of an embodiment of our invention. The solid arrows in Figure 1 describe the flow of solids and the dotted arrows the flow of the solution. A copper sulfide raw material, such as a copper sulfide concentrate, is fed into the first leach stage 1, within which a solution 3, which is an aqueous solution of copper chloride (II), is recirculated. ) and alkaline chloride leaving the second stage 2 leaching. When we talk about alkaline chloride later, for reasons of simplicity, only sodium chloride is mentioned, although some other alkali can be used instead if necessary. Each stage of leaching is presented as a single block, but it is clear that each stage generally consists of several reactors and thickeners. In the leaching stages of the copper concentrate, the copper dissolves in the solution of the process, and the solution is directed to thickening. After thickening of the first leach stage, the overflow solution 4 contains copper chloride, in which there is about 70 g / 1 of copper, mainly monovalent, and this is directed according to the Hydrocopper1 process to copper recovery (not shown in detail in the drawing). The leaching of the solids contained in the lower flow 5 is continued in the second leaching step 2 with chloride solution 6. The chloride solution is formed from the sodium chloride solution which is obtained from the chlorine alkali electrolysis belonging to the Hydrocopper ™ process, and the copper (II) chloride solution which is formed in the oxidation 7 by the oxidizing part of the copper (I) chloride 4 formed in the first leaching step. The solution 3 leaving the thickening of the second leaching stage 2 is directed to the first leaching stage 1 to leach the concentrate. The leaching of the residue 8 leaving the second leaching stage is continued in a separate leaching step 9 to leach the gold concentrate therein. The gold leaching stage 9 also generally takes place in several reactors, but for purposes of simplicity, the entire stage is illustrated as a single unit. The precipitate in the Gold leaching stage is leached with a concentrated solution of copper (II) chloride and sodium chloride 10, in which the concentration of Cu is 40 - 100 g / 1 and the concentration of sodium chloride is 200 - 330 g / 1 and the amount of bromide calculated as bromine ions is 0.5 - 30 g / 1. Oxygen is also directed within the leaching stage, which allows the oxidation-reduction potential of the solution to be raised to a level suitable for leaching gold, ie, a range of 600 - 650 mV vs. an Ag / AgCl electrode. The alkali metal bromide is preferably potassium or sodium bromide and in the first stage of leaching the bromide is directed as a finely ground solid within the gold leaching stage. Since there is a closed circulation of the solution in the gold leaching stage, the continuous addition of bromide is not required, and only the small amount that is consumed in the process is subsequently replaced with a bromide feed. The bromide feed shortens the time of gold leaching, because the gold is dissolved as a result of the bromide remaining in solution and not being precipitated back. Gold also dissolves possibly as a bromide complex more easily than as a chloride complex. At the end of the gold leaching step 9, the separation of solids is carried out. The overflow 1 1 that is formed is directed as it is or is filtered for the recovery of gold 12, which takes place for example by means of activated carbon in carbon columns. A gold product 13 is obtained from the columns. The solution removed from the columns is a gold-free solution, which is recirculated back to stage 9 of gold leaching. The lower thickening flow of the gold recovery stage, ie, the precipitate, after the further normal treatment such as filtration and washing 14 comprises the final residue 15, which includes almost all of the sulfur in the concentrate and most of the iron . The filtered residue and wash water 16 contain dissolved iron and a small amount of the recirculating bromide in the gold leaching stage. The filtrate and the washing water 16 are directed to the oxidation step 7 of the concentrate leaching process. The chlorine fed in the oxidation step oxidizes the bromide to bromine gas. The gas generated in the oxidation stage is directed to the gas scrubber that belongs to the stage, where the bromine that is generated dissolves in the wash fluid of the scrubber. The wash fluid 17 from the scrubber is directed to the gold leach stage, and the slurry from the leach stage reduces joke back to bromide. This ensures that the bromide circulates only at this stage. The invention is further described by means of the appended examples.

Example 1 In a test a residue containing an average of 7 g / t of gold, which was formed in the leaching of a starting material containing copper sulfide, was leached as a batch test. The residue was leached in batch tests in a 5 liter reactor, which was equipped with in-line electrodes for pH measurement and oxidation-reduction potential. The tests were carried out at a temperature of 95 ° C. The estimated gold leaching time was 30-40 h. At the beginning of leaching the pH was adjusted to a value of 2.0 by means of the oxygen and hydrochloric acid feed, after which the pH was allowed to fall freely, but not below the value of 0.5. The pH should be below a value of 2.5 at the beginning of leaching as well, to prevent the copper in the solution from precipitating as atacamite. During leaching the redox potential was gradually raised (over 5 - 8 hrs.) To a value of 580 mV and subsequently 15 g / 1 of sodium bromide were added to improve gold leaching. The addition of bromide was repeated at 10 hour intervals and at that point the amount was 10 g / 1. The results are illustrated in diagram 2. The addition of bromide has a beneficial effect on the dissolution of gold particularly at lower potential levels. When there was sodium bromide in the solution, the gold dissolved at a potential of 630 mV, which is an easy potential to achieve with an oxygen feed.

Example 2 The effect of bromine in a continuous leach circuit was tested in a laboratory pilot test. The leaching circulation of the gold circuit was not connected to the leaching circuit of the copper concentrate; only the copper-free solids in the copper concentrate leaching circuit were transferred to the gold leaching stage. There were two 5 liter leach reactors in the stage of leaching of gold, in both of which there were deflectors, mixers and electrodes in line for the measurement of pH and redox potential. The leach circuit temperature in the leach reactors was 95 ° C. The retention time of the solids in the reactors was 10-15 hours. The pH of the first leach reactor was maintained at a value of 1.0 and the pH of the second reactor was 0.9. The redox potential in leaching was a maximum of 630 mV. The Cu ion concentration of the solution was 60 g / 1 and the NaCl concentration was 250 g / 1. The advance of leaching is illustrated in diagram 3. 8 - 10 g / 1 of bromine was added to the solution of the gold circuit at point number 5. As shown in the diagram, after the addition of bromine the concentration of gold of the solution doubled.

Claims (12)

1. Claims 1. A method for the recovery of gold from a leach residue essentially free of copper or an intermediate containing iron and sulfur, which is generated in a chloride leaching process under atmospheric conditions of a copper starting material sulphurous, by means of which the gold is leached from the waste or intermediate in an aqueous solution of copper (II) chloride and alkali chloride under atmospheric conditions by means of a gas containing oxygen and the divalent copper contained in the solution , and the oxidation - reduction potential of the suspension formed is adjusted to a maximum value of 650 mV vs. an Ag / AgCl electrode, characterized in that to improve the leaching, the alkali bromide is directed to the solution so that the amount of bromine ions in the solution is 0.5 - 30 g / 1, the pH is regulated to a value of 0.5 - 2.5; the gold dissolved in the leaching is recovered by a method known per se and the undissolved precipitate formed in the gold leaching stage is a waste containing sulfur and iron.
2. 2. A method, according to claim 1, characterized in that the amount of bromine ion in the solution is 8-15 g / 1.
3. 3. A method, according to claim 1 or 2, characterized in that the alkali bromide is sodium bromide or potassium bromide.
4. 4. A method according to claim 1, characterized in that the precipitate formed in the gold leaching step is filtered and the filtrate and wash water are directed to an oxidation step belonging to a leaching process of copper sulfide concentrate, in whose oxidation stage the bromide contained in the filtrate and the washing water is oxidized by means of chlorine gas to bromine gas, which is recovered in the washing fluid of the gas scrubber of the oxidation step, and the wash fluid is recirculated to the gold leaching stage. A method, according to claim 4, characterized in that the amount of bromide removed with the filtrate and with the wash water from the gold leaching step is 0.
5. 5-10%.
6. 6. A method, according to claim 4 or 5, characterized in that the slurry in the gold leaching stage reduces the bromine contained in the wash fluid of the gas scrubber to bromide.
7. 7. A method, according to claim 1, characterized in that the oxidation-reduction potential of the gold leaching stage is maintained in the range of 580-640 mV.
8. 8. A method, according to claim 1, characterized in that the pH of the suspension in the gold leaching step is maintained at a value of 0.5-1.5.
9. 9. A method, according to claim 1, characterized in that the amount of divalent copper in the suspension is 40-100 g / 1.
10. 10. A method, according to claim 1, characterized in that the amount of alkali chloride in the suspension is 200-330 g / 1.
11. 11. A method, according to claim 1, characterized in that the temperature is maintained in the range between 80 ° C and the boiling point of the suspension.
12. 12. A method according to claim 1, characterized in that the oxygen-containing gas is one of the following: air, air enriched in oxygen and oxygen.