BG66925B1 - Method with a closed cycle for extracting gold and silver through halogens - Google Patents

Abstract

The method for extracting precious metals from polymer ore including: - generating hypochlorite from a saline solution; - chlorinating the ore by using hypochlorite under acidic conditions; - filtering for collection of the end solution and processing the end solution as to collect the precious metals; - filtering for separation of the precious metals and depleted saline solution; - purifying the depleted saline solution; and - recycling halogens from the depleted saline solution in the form of hypohalites, formed through electrolysis of the purified saline solution, and containing hypo-chloride and hypo-bromide, where the stage of chlorinating the ore by using the hypochlorites under acidic conditions additionally includes purification of the halogens, using calcium hydroxide; filtering to obtain a solution of precious metals and calcium hypohalites, and recirculation of calcium hypohalites; processing the end solution in stage of the filtering for collection of the end solution and processing the end solution to collect precious metals includes decreasing the oxide reducing potential of the end solution and using calcium hydroxide for neutralization; and rinsing with at least double the volume; and the stage of purifying the depleted saline solution includes an increase of the pH of the depleted saline solution by adding calcium hydroxide, filtering and carbonization of the resulting saline solution.

Description

Field of technology

The present invention relates to a method for extracting gold and silver by halogens. In particular, the present invention relates to an improved closed cycle method for the extraction of gold and silver by halogens.

BACKGROUND OF THE INVENTION

The extraction of gold and silver from polymetallic ores using halogens in brine (NaCl / NaBr in water) is known fromUS 7,537,741.

A closed-loop method for extracting precious metals from ore using halogens has also been developed where halogens (chlorine and bromine) are used as free halogens during the extraction of precious metals and are recycled by electrolysis of the depleted solution in the form of hypohalides. , i.e., sodium hypochlorites NaOCl and sodium hypobromites NaOBr (patent application US 2013/0074655). Free halogens are released from the hypohalide state by treating the leaching under acidic conditions. The following equation summarizes the main steps of this closed loop method, which is illustrated in Figure 1.

Release of free halogens from hypohalides

OC172 OVr + 2 H C1 ₂ / Br ₂ + H ₂ O

Extraction of gold with a mixture of halogens

Vg ₂ + ore (Au) -> 2 AuBr ₃ + ore (depleted)

AuBr ₃ + 3 C1 ₂ AiC1 ₃ + 3 Br ₂

Collect gold

AiC1 ₃ + reducer 2 Au ° + 6 CT

Regeneration of hypohalides from depleted solution

CP + electrooxidation -> 6 OCI

Br + electrooxidation 6 OVg

In these various reactions, the anions, or halides or hypohalides, are carried by sodium as cations, i.e., NaCl, NaBr, NaOBr or NaOCl.

The application of this method involves the recycling of halogens, in particular the more expensive bromine. The release of free halogens under acidic conditions generates an atmosphere rich in chlorine and bromine. The operation is carried out at atmospheric pressure, excess halogens must be purified, usually with sodium hydroxide NaOH, to recover these halogens for recycling. Vapors that accompany the filtration of the productive solution may also contain little, but it is important the amount of halogens that must be recovered for economic and environmental reasons. This is the first time that the addition of a chemical, such as sodium hydroxide NaOH, to the method is required.

A second case of adding a chemical to the process takes place at the time of collecting the gold dissolved in the productive solution, when a reducing agent is used to bring the trivalent gold to the elemental state. This operation is achieved by reducing the redox potential (ORP) from values in the range of 1000 mV to less than 400 mV relative to the reference electrode of Ag / AgCl. The reducing agent, either sodium sulphite Na ₂ SO ₃ or sulfur dioxide gas SO ₂ , involves the addition of sodium directly to OTNa ₂ SO ₃ or by neutralizing the excess and oxidized SO ₂ (to H ₂ SO ₄ ) with sodium hydroxide NaOH.

A third case is after gold recovery, when the depleted salt solution, which has collected impurities, that is, mainly iron from the ore, must be purified before recycling by electrolysis. This purification is performed by adjusting the pH with sodium hydroxide to precipitate iron and other traces of base metals, followed by treatment with sodium carbonate Na ₂ CO ₃ to reduce the calcium content to a level that is acceptable during electrolysis. .

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Descriptions of issued patents for inventions № 08.2 / 30.08.2019

Therefore, in three stages of the above method, namely, during the purification of the waste gases from the chlorination reactor, during the precipitation of gold from the productive solution and during the purification of the depleted solution, the use of sodium hydroxide inevitably leads to sodium accumulation. . This sodium exists as sodium sulphate because the acid used in the reactor is sulfuric acid H ₂ SO _{4 and the} reducing agent for gold deposition is serendioxide SO ₂ (see Figure 1).

Removal of sodium sulfate by crystallization is a known process, but it is relatively expensive for capital and operating costs. In addition, such a step eliminates the nature of the method as a closed loop. Therefore, it is highly desirable to improve those operating conditions that lead to sodium accumulation.

There is still a need in the art for a method and system for extracting gold and silver using halogens.

Technical essence of the invention

In accordance with the present invention, there is provided a method of extracting precious metals from polymetallic ore, comprising:

- generation of hypochlorites from saline solution;

- chlorination of the ore using hypochlorites under acidic conditions;

- filtration to collect the productive solution and treatment of the productive solution to collect precious metals;

- filtration to separate the precious metals and the depleted salt solution;

- purification of the depleted saline solution; and

- recycling of halogens from the purified brine in the form of hypochalites formed by electrolysis of the purified saline solution, including hypochlorites and hypobromites, where the step of chlorinating the ore using the hypochlorites under acidic conditions further comprises purifying the halogens with hydroxy chloride. ; filtration to obtain a solution of precious metals and calcium hypochalites, and recycling of calcium hypochalites; treating the productive solution in the filtration step to collect the productive solution and treating the productive solution to collect precious metals, comprising reducing the oxido-reduction potential of the productive solution and using calcium hydroxide to neutralize; and rinsing with at least double the volume; and the step of purifying the depleted brine comprises increasing the pH of the depleted brine by adding calcium hydroxide, filtering and carbonizing the resulting brine.

Other objects, advantages and features of the present invention will become more apparent upon reading the following non-limiting description of specific embodiments thereof, given by way of example only with reference to the accompanying drawings.

Explanation of the attached figures

Figure 1 is a schematic representation of a method for extracting gold using halogens, as is known in the art; and Figure 2 is a schematic representation of a method for extracting gold / silver using halogens according to an embodiment of the present invention.

Embodiments of the invention

It has been found that in the method of extracting gold and silver using halogens, the accumulation of sodium can be avoided by using a base, such as calcium hydroxide Ca (OH) ₂ instead of sodium hydroxide NaOH by: (i) during purification of the waste gases from the chlorination reactor, n) during the precipitation of gold / silver from the productive solution and iii) during the purification of the depleted solution, thus providing a cost-effective closed-loop method.

Under sulfuric acid conditions, the neutralization obtained results in the production of calcium sulphate (gypsum, CaSO ₄ .2H ₂ O), which has a very low solubility in water and can be easily filtered off.

The use of calcium hydroxide necessary to obtain a suspension of this base,

Descriptions of issued patents for inventions № 08.2 / 30.08.2019 which tends to precipitate, and since the resulting product, gypsum, is insoluble, there is always a coating formed by this gypsum around unreacted calcium hydroxides and other particles in the reaction mixture. This situation implies the use of excess amounts of calcium hydroxide, as the stoichiometric approach would lead to a deficiency of the base and therefore encapsulation of calcium hydroxides or gypsum lime. Also, encapsulation can prevent the interaction of gold-bearing gold particles with chlorinating species.

Another cause for concern with the use of calcium hydroxide as a base is the possibility of co-precipitation of some of the bromide ions in the saline solution with gypsum. Such a phenomenon, even at a very low level, would be extremely detrimental to the economics of the method due to the relatively high cost of bromine or bromides.

Therefore, the use of calcium hydroxide, although desirable to prevent ionic accumulation in the brine, has the potential disadvantage of effectively preventing the extraction of gold and silver and the recycling of bromides.

The three uses of base in the method of extracting gold using halogens have thus been investigated, and conditions for its optimal use have been determined to avoid these potential disadvantages.

First, during the purification of the halogens from the halogenation reactor, a suspension of calcium hydroxide Ca (OH) ₂ with a Ca (OH) _{2 content} in the range between 2 and 10% by weight, preferably 5%, is used to recover the contents. of halogens from the chlorination reactor.

Such a recovery can be described by the following equations:

Br ₂ + 2 Ca (OH) ₂ - + Ca (OVg) ₂ + CaBr ₂ + 2 H ₂ O

C1 ₂ + 2 Ca (OH) ₂ Ca (OC1) ₂ + CaCl ₂ + 2 H ₂ O

During this recovery of the halogen content of the chlorination reactor, half of the calcium thus included exists as calcium hypohalides Ca (OVr) ₂ , Ca (OC1) ₂ . These hypohalides can be recycled to the chlorination reactor to recover the halogen contents. Surprisingly, it was noted that despite the acidic conditions in the chlorination reactor, calcium hypochalides have been shown to be very effective in dissolving gold. Therefore, half of the purified halogen contents are converted into useful hypohalides without the need for electrolysis. This ensures the first stage of halogen recycling.

The expected potential encapsulation problems were not identified, most likely due to the grinding effect of the suspended and well-stirred gypsum ore that occurs in the reactor maintained acidic with sulfuric acid during the addition of the recycled hypohalides from the purification step described. here above. The depleted ore, together with the gypsum, can be filtered to obtain a productive solution with a full content of gold and halogen and, interestingly, without added sodium content.

Many ores contain carbonates and the acidic conditions in the reactor release carbon dioxide, which is evacuated along with the halogen vapors. When washed with lime or calcium hydroxide Ca (OH) ₂ , this vapor gives water-soluble calcium halides or hypohalides, which can be recycled to the reactor, while calcium carbonate CaCO ₃ , which is insoluble, can be filtered off and removed from the system. Thus, the use of calcium hydroxide as a base proves to be much more advantageous than sodium hydroxide, which gives soluble carbonate and does not allow the separation of carbonate from halogen salts.

Second, in the gold precipitation step, in order to recover the gold from the productive solution, the ORP of this productive solution, which is in the range between 0.7 and 1.2 V relative to the reference electrode Ag / AgCl, must be reduced to values lower. of 400 mv to bring trivalent gold to its elemental state. There are various reducing agents that can reduce this ORP.

Sulfur dioxide SO ₂ , which is usually available on site from the oxidation of the primary ore, is the preferred reagent. This reduction in ORP is accompanied by acidification of the solution through the formation of sulfuric acid by the oxidation of the reducing agent SO ₂ . pH of the solution

Descriptions of issued patents for inventions № 08.2 / 30.08.2019 must be regulated by neutralization. Calcium hydroxide can be used in molar equivalent to excess SO ₂ , but precipitation of gypsum should not deprive the saline solution of its halide components, i.e., NaCl and NaBr, by co-precipitation. Proper rinsing, that is, with at least twice the volume (volume of water twice the volume of the solid to be rinsed), prevents this side effect. Oxidized and excess sulfur dioxide SO ₂ are removed as sulfate SO ₄ ^2- and sulfite calcium CaSO ₃ , respectively, these salts with low solubility, leaving a small background of calcium in the brine, ie, reduction to the solubility of CaSO ₄ .

Third, in the depletion solution purification step, the depleted solution, i.e., free of gold / silver, must be purified before regenerating the hypohalides by electrolysis. The main impurity is iron collected in the stage of gold extraction. Also, the small amount of dissolved gypsum, usually 600 ppm Ca ⁺⁺ , must be reduced by a factor of ten in order to preserve the properties of the electrodes in the electrolyte cell.

The removal of iron can be accomplished by increasing the pH of the saline solution in the range between 8 and 9, by adding calcium hydroxide. Other minor contaminants, essentially base metals, arsenic, antimony and bismuth, also precipitate during this treatment.

The reduction of the level of alkaline earth elements (Ca, Mg) can be achieved by carbonization of the salt solution, these carbonates are in equilibrium with less than 50 ppm Ca ⁺⁺ / Mg ⁺⁺ .

These interventions with Ca (OH) ₂ in the brine cycle thus prevent halogen losses while maintaining the desired gold / silver extraction capabilities.

By using calcium hydroxide instead of sodium hydroxide as a neutralizing agent, it has become possible to prevent the accumulation of unwanted ions in the saline solution, to maintain the C1 / Br ratio in the saline solution, to protect halogens from precipitated phases and to maintain the level of calcium at values compatible with the electrochemical recycling of saline.

The following examples illustrate in a non-limiting manner the main features of this invention.

Purification of chlorine, bromine and carbon dioxide with calcium hydroxide.

200 ml of a solution of sodium hypochlorite NaOCl (2%) and sodium hypobromite NaOBr in a 1 liter three-necked flask are acidified to a pH in the range between 0.5 and 3, with a preferred value of 2, by slow addition of sulfuric acid H ₂ SO ₄ 40% over a period of 1 hour. During this period, a stream of nitrogen (100 ml / min) circulates through the flask and is directed to a vertical column, 10 cm in diameter and 2 m long, through which circulates a dispersed suspension of calcium hydroxide with 10% solids (2 Ι / min). Gas analysis of the output shows complete adsorption of chlorine, bromine and carbon dioxide, while in the suspension halogen exists as hypohalides and calcium halides.

Extraction of gold with calcium hypohalides.

During two simultaneous and otherwise identical experiments, gold ore (11.25 g / t Au) was treated with i) a mixture of sodium halide / hypohalide and j) with calcium halide / hypohalide, respectively, using 8% saline with a ratio of NaCl to NaBr in the range between 7/1 and 7 / 3,20% suspension of ore at 30-40 ° C for 4 hours, ie, using an amount of active halogens in the range between 0.5 and 2% of the ore, the redox potential in the reactor is in the range between 0.7 and 1.2 V relative to the reference electrode Ag / AgCl. The gold recovery was found to be identical in both experiments, i.e. at the 95% level, indicating that the replacement of sodium with calcium in the halide / hypohalide reagent did not affect the gold extraction.

Purification of the depleted solution before electrolysis.

A depleted solution of gold reduction by chlorination showed the following composition: CaSO ₄ : 0.99 g / l, CuSO ₄ : 1.637 g / l; Fe ₂ (SO ₄ ) ₃ : 15.98 g / l; H ₂ SO ₄ : 3.67g / 1; MgSO ₄ : 0.10g / l; MnSO ₄ : 0.018g / 1; NaBr: 0.97g / 1; NaCl: 44.8 g / l; At ₂ SO ₃ : 4.15 g / l; Na ₂ SO ₄ : 31.22 g / l; PbSO ₄ : 0.094 g / l. 11 of this depleted solution is treated with excess calcium hydroxide to pH 8.2. The precipitated solid was filtered off and the filtrate, after rinsing (two volumes), contained the initial amounts of NaCl and NaBr. Carbonation of this purified brine brought the Ca ⁺⁺ / Mg ⁺⁺ ratio below 50 ppm.

Descriptions of issued patents for inventions № 08.2 / 30.08.2019

Thus, as illustrated in Figure 2, a method for extracting gold and silver is provided, comprising generating hypochlorites from brine (step 110); chlorination of the ore is the use of hypochlorites under acidic conditions (step 120); filtering the collected productive solution (step 130); treating the productive gold collection solution (step 140) and filtering (step 142) to recover Au / Ag on the one hand and the depleted solution on the other; purification of the depleted gold / silver saline solution, i.e., the depleted solution (step 150), before circulating the purified depleted solution through an electrolyte cell without a halogen regeneration membrane (160) to give a regenerated saline solution of hypohalides. Calcium hydroxide is used instead of sodium hydroxide as a base in three steps of the method, as described below.

In step 120, halogens from the chlorination reactor are purified using a suspension of calcium hydroxide Ca (OH) ₂ with a Ca (OH) _{2 content} in the range between 2 and 10% by weight, preferably 5%, is used to recover the halogen contents of the chlorination reactor (step 122). As described above, the depleted ore, together with gypsum, was filtered off (step 126) to give a solution with gold / silver and calcium hypohalides Ca (OVr) ₂ .Ca (OC1) ₂ which was recycled to the chlorination (step 124), and calcium carbonate CaCO ₃ , which is removed from the system.

In the gold precipitation step, in order to recover the gold from the productive solution, the ORP of the productive solution, which is in the range between 0.7 and 1.2 V relative to the Ag / AgCl reference electrode, must be reduced to values lower than 400 mv to bring trivalent gold to its elemental state. There are various reducing agents that can achieve this reduction in ORP. In step 140, sulfur dioxide SO ₂ , which is usually available on site from the oxidation of the starting ore, is used as a reducing agent. This reduction in ORP is accompanied by acidification of the solution by the formation of sulfuric acid from the oxidation of the reducing agent SO ₂ . The pH of the solution should be adjusted by neutralization. Calcium hydroxide is used in molar equivalent to excess SO ₂ , but the precipitation of gypsum must not deplete the saline solution of its halide components, i.e., NaCl and NaBr, by co-precipitation. Proper rinsing, that is, at least double the volume (volume of water twice the volume of the solid to be rinsed), prevents this side effect. Oxidized and excess sulfur dioxide SO ₂ are removed as sulfate SO ₄ ^2- and calcium sulfite CaSO ₃ , respectively, these salts have a low solubility, leaving a small background of calcium in the brine, that is, reduced to the solubility of CaSO ₄ . Step 150 involves increasing the pH of the salt solution is calcium hydroxide in the range between 8 and 9, which precipitates iron and other minor contaminants, essentially base metals, arsenic, antimony and bismuth, and carbonizing the salt solution, which reduces the level of alkaline earth elements (Ca, Mg).

Thus, it is provided, a closed-loop gold extraction method is the use of brine / sodium bromide as a carrier for hypohalide oxidizing agents, is an improvement that consists of replacing sodium hydroxide with calcium hydroxide for purification processes and neutralizing, thereby preventing the accumulation of sodium in the cycle while maintaining the halogen content of the brine.

In the present method, calcium hydroxide is replaced by sodium hydroxide for the purification of halogen vapors and carbon dioxide emissions, which allows the recycling of the halogen content in the cycle while the carbon dioxide is discarded as calcium carbonate.

Calcium hydroxide is replaced by sodium hydroxide to collect the extracted gold, with calcium hydroxide precipitating the reducing agent as insoluble calcium sulphite / calcium sulphate after reduction of gold chloride to elemental gold.

Calcium hydroxide is replaced by sodium hydroxide to remove impurities from the depleted solution by adjusting the pH, filtering the precipitated impurities, basic iron with traces of other base metals, and reducing residual alkaline earth ions to below 50 ppm by carbonization.

Calcium hydroxide is used as a suspension in water containing from 1 to 20% by weight of calcium hydroxide, preferably 5%.

As those skilled in the art will appreciate, this provides a method for

Descriptions of issued patents for inventions № 08.2 / 30.08.2019 extraction of gold and silver by halogens, chlorine and bromine, where the reagents are recycled and reactivated in a closed cycle. In the present method, the steps where external additions of chemical reagents are required to retain halogens in the cycle, to collect precious metals from the productive solution and to purify the depleted solution before regeneration by electrolysis are achieved using calcium hydroxide as the base, without creating an imbalance in the ionic composition of the saline solution in the circuit.

In a closed loop, the present method for gold / silver extraction involves the use of sodium chloride / sodium bromide saline as a carrier for hypohalide oxidants, and calcium hydroxide for purification and neutralization purposes, thus preventing the accumulation of sodium in the cycle. while preserving the halogen content of the saline solution.

Calcium hydroxide is used to flush out halogen vapors and emit carbon dioxide, which allows halogens to be recycled in the cycle, while carbon dioxide is discarded as calcium carbonate.

Calcium hydroxide is used to collect the extracted gold, calcium hydroxide precipitates the reducing agent as insoluble calcium sulfite / calcium sulfate after reduction of gold chloride to elemental gold.

Calcium hydroxide is used to remove impurities from the depleted solution by adjusting the pH, and precipitated impurities, mainly iron with traces of other base metals, are filtered off and residual alkaline earth ions are reduced to below 50 ppm by carbonization.

Calcium hydroxide is used as a suspension in water containing from 1 to 20% by weight of calcium hydroxide with a preferred value of 5%.

The scope of the claims should not be limited by the preferred embodiments presented in the examples, but the broadest interpretation in accordance with the description as a whole should be considered.

Claims (15)

A method for extracting precious metals from polymetallic ore, comprising: - generation of hypochlorites from saline solution; - chlorination of the ore using hypochlorites under acidic conditions; - filtration to collect the productive solution and treatment of the productive solution to collect precious metals; - filtration to separate the precious metals and the depleted salt solution; - purification of the depleted saline solution; and - recycling of halogens from the purified saline solution in the form of hypochalites formed by electrolysis of the purified saline solution and including hypochlorites and hypobromites; characterized in that: the step of chlorinating the ore using the hypochlorites under acidic conditions further comprises purifying the halogens using calcium hydroxide; filtration to obtain a solution of precious metals and calcium hypochalites, and recycling of calcium hypochalites; and treating the productive solution in the filtration step to collect the productive solution and treating the productive solution to collect precious metals includes reducing the oxido-reduction potential of the productive solution and using calcium hydroxide to neutralize and rinse with at least twice the volume ; and the step of purifying the depleted brine involves increasing the pH of the depleted brine by adding calcium hydroxide, filtering and carbonizing the resulting brine. Process according to Claim 1, characterized in that a calcium hydroxide suspension with a calcium hydroxide content in the range between 2 and 10% by weight is used in the purification of halogens in the step of chlorinating the ore using hypochlorites under acidic conditions. Process according to Claim 1, characterized in that a suspension is used in the purification of halogens in the step of chlorinating the ore using hypochlorites under acidic conditions. Descriptions of issued patents for inventions № 08.2 / 30.08.2019 of calcium hydroxide with a content of calcium hydroxide of 5% by weight. Method according to any one of claims 1 to 3, characterized in that in the step of chlorination of the ore is the use of hypochlorites under acidic conditions, the oxido-reduction potential is in the range between 0.7 and 1.2 V relative to Ag / AgCl reference electrode. Method according to any one of claims 1 to 3, characterized in that in the step of chlorination of the ore is the use of hypochlorites under acidic conditions, the oxido-reduction potential is in the range between 0.7 and 1.2 V relative to Ag / AgCl reference electrode, and the step of filtering to collect the productive solution and treating the productive solution to collect the noble metals involves reducing the oxido-reduction potential to 0.4 V relative to the Ag / AgCl reference electrode. Method according to any one of claims 1 to 5, characterized in that in the step of filtering to collect the productive solution and treating the productive solution to collect precious metals, the reduction of the oxido-reduction potential of the productive solution involves the use of of sulfur dioxide, and the application of calcium hydroxide to neutralize includes an amount of calcium hydroxide that is in molar equivalent to excess sulfur dioxide and rinsing. Process according to any one of claims 1 to 6, characterized in that the step of purifying the depleted brine comprises increasing the pH of the depleted brine to a value in the range between 8 and 9. A process according to any one of claims 1 to 7, wherein the step of chlorinating the ore is to use the hypochlorites under acidic conditions, comprising suspending the ore in a mixed NaCl / NaBr saline solution and acidifying the suspended ore to achieve at a pH in the range between 0.5 and 3. A method according to any one of claims 1 to 8, characterized in that the step of generating hypochlorites from saline comprises generating hypochalites by electrolysis of saline in a cell without a diaphragm. A method according to any one of claims 1 to 9, characterized in that the step of chlorinating the ore is the use of hypochlorites under acidic conditions, comprising contacting the acidic suspended ore with a solution of recycled hypochalites. A method according to any one of claims 1 to 10, characterized in that the step of chlorinating the ore is the use of hypochlorites under acidic conditions, comprising acidifying the suspended ore to a pH of 1.5. The method according to any one of claims 1 to 11, characterized in that the step of chlorinating the ore is the use of hypochlorites under acidic conditions, comprising using an amount of active halogens in the range between 0.5 and 2% of the ore. The method according to any one of claims 1 to 12, characterized in that the step of chlorinating the ore is the use of hypochlorites under acidic conditions, comprising suspending the ore at a temperature of 40 ° C. Process according to any one of claims 1 to 13, characterized in that the precious metals are gold and silver. The method of claim 1, wherein carbonizing the resulting brine in the depleted brine purification step comprises reducing the alkaline earth ion content to less than 50 ppm.