RU2848032C1 - Method for extracting copper from cyanide solutions and regenerating free cyanide - Google Patents

Abstract

FIELD: chemical industry.

SUBSTANCE: invention relates to the regeneration of cyanide process solutions and can be used to extract copper from cyanide solutions in gold mining and electroplating industries. The method involves treating the cyanide solution with reagents to precipitate copper, forming a copper-containing precipitate, and regenerating the cyanide solution. Copper (II) compounds are used as reagents for the precipitation of copper with the formation of a copper-containing precipitate. The regenerated cyanide solution is separated from the copper-containing precipitate and returned to cyanidation. The resulting copper-containing precipitate is treated with a sulphuric acid solution to obtain a copper (II) sulphate solution. At the same time, alkali is added to part of the copper (II) sulphate solution and copper hydroxide is precipitated, which is used to precipitate copper in the form of a copper-containing precipitate from a new portion of the cyanide solution, and the remaining copper (II) sulphate solution is processed to obtain metallic copper.

EFFECT: simplification of equipment design, minimisation of volatile hydrocyanic acid formation into the atmosphere, reduction of acid and alkali consumption.

3 cl, 1 dwg, 10 ex

Description

The invention relates to methods for recovering free cyanide from process solutions containing cyanides and heavy metals. Specifically, the proposed method can be used to extract copper from cyanide solutions and may find application in non-ferrous metallurgy, gold mining, and electroplating industries.

During the processing of gold-copper ores using cyanide leaching, large quantities of copper and other non-ferrous metals, along with precious metals, are released into the productive solution in the form of cyanide complexes, resulting in increased cyanide consumption. As the cyanide solution is recycled, the copper concentration gradually increases and can reach 1-5 g/L. Consequently, with the overall high cyanide concentration in the recycled solution, most of the reagent is in the form of a copper-cyanide complex. The functional properties of the cyanide solutions deteriorate, an effect known as "fatigue." This fatigue is most often caused by the accumulation of copper in the recycled solutions. To restore the leaching properties, copper is extracted from the solutions, and the cyanide solutions are regenerated.

The most well-known regeneration method is acidification of cyanide solutions followed by absorption of volatile hydrogen cyanide by an alkali solution (Maslenitsky I.N., Chugaev L.G. Metallurgy of Noble Metals. - Moscow: Metallurgy, 1987.-366 p.). Sulfuric acid or, if possible, sulfur dioxide is used for acidification, and a solution of lime or caustic soda is used to absorb hydrocyanic acid. The following chemical reactions occur during acidification of cyanide solutions:

neutralization of protective alkali:

Ca(OH) ₂ +H ₂ SO ₄ =CaSO ₄ + 2H ₂ O (1)

decomposition of free cyanide to produce hydrocyanic acid:

2NaCN+H ₂ SO ₄ =Na ₂ SO ₄ +2HCN↑ (2)

decomposition of complex cyanide salts:

Na ₂ Cu(CN) ₄ +2H ₂ SO ₄ =4HCN+CuSO ₄ +Na ₂ SO ₄ (3)

The released hydrocyanic acid is absorbed by the alkali solution:

HCN + NaOH = NaCN + H ₂ O (4)

As follows from the given reactions, cyanide is completely regenerated under the condition of complete decomposition of free cyanide and cyanide salt in the entire volume of the process solution.

There are known variations of the method for extracting copper from cyanide solutions based on acidification of the initial solution and precipitation of copper sulfide with the addition of water-soluble sulfides and with distillation of hydrocyanic acid vapors with air, steam or carbon dioxide from the acidified solutions and subsequent capture of the distilled hydrocyanic acid with alkaline solutions of sodium or calcium hydroxide for reuse (US No. 1050303; US No. 5169615; US No. 4587110; RF No. 2443791; RF No. 2 285734; RF No. 2141538).

The disadvantages of known methods are:

- operations of distilling hydrocyanic acid with its subsequent capture, which makes this method complex both in terms of equipment design and operation;

- the transfer of hydrocyanic acid (a highly toxic compound) into the gas phase makes the process potentially dangerous.

Known methods for extracting copper from cyanide solutions, based on extraction and sorption (RF No. 2427656), are lengthy and require the use of bulky equipment.

The closest in technical essence to the claimed solution is the method chosen as a prototype for the regeneration of free cyanide from cyanide solutions and the removal of heavy metals, including copper, which includes the treatment of cyanide solutions with reagents to form a precipitate of poorly soluble compounds of simple cyanides without the formation of gaseous hydrocyanic acid, the separation of the copper-containing precipitate from the solution and the regeneration of free cyanide (RF No. 2285734).

According to the prototype formula, the initial cyanide solution is treated with mineral acid. To prevent the formation of volatile hydrocyanic acid, the treatment is carried out in a sealed reactor without exposure to the atmosphere. To regenerate the cyanide, alkali is added to the solution after the precipitate of poorly soluble compounds is separated.

Free cyanide is regenerated directly in the treated solution without converting hydrocyanic acid into the gas phase. The technical result is the ability to recycle most of the cyanide, reduce its consumption, reduce fresh water consumption, and simplify the process flow. In fact, when using the prototype, the entire volume of the initial alkaline cyanide solution is converted to an acidic state, and the cyanide is converted to hydrocyanic acid, which entails a corresponding consumption of acid. During the regeneration stage, the entire volume of the solution is converted from an acidic state back to an alkaline state with a higher alkali consumption.

The main disadvantage of the prototype is the inevitable formation of toxic hydrocyanic acid, the complexity of hermetically sealed equipment that prevents the release of volatile hydrocyanic acid into the atmosphere, and the increased costs of mineral acid and alkali.

The technical problem that the proposed invention is aimed at solving is the high consumption of acid and alkali and the complexity of the equipment design.

The technical result consists in changing the conditions for the regeneration of the cyanide solution and the precipitation of copper from it.

This objective is achieved by using a method for extracting copper from cyanide solutions and regenerating free cyanide, which involves treating cyanide solutions with reagents and precipitating copper, separating the copper-containing precipitate, and regenerating the cyanide. The method is characterized in that copper (II) compounds are added to the cyanide solution, resulting in the formation of a precipitate of sparingly soluble copper cyanide. The cyanide solution is separated from the copper-containing precipitate and returned for cyanidation. The precipitate of sparingly soluble copper cyanide is treated with a sulfuric acid solution to obtain a copper (II) sulfate solution. An alkali is added to part of the copper (II) sulfate solution to form copper hydroxide, which is used to precipitate copper from a new portion of the cyanide solution. Copper hydroxide, oxide, or carbonate are used to precipitate copper from cyanide solutions. The consumption of copper (II) compounds at the copper precipitation stage is adopted at a level that ensures a residual copper content in the cyanide solution of 50-100 mg/l.

Evidence of the decisive influence of the distinctive features of the proposed method on the achievement of the technical result is provided by a combination of theoretical foundations and the results of special studies.

As noted above, when using known methods, including the prototype method, the target transformation is described by reaction (2).

It is important that only after the complete transformation of cyanide in the entire volume of the cyanide solution (hundreds and thousands of cubic meters) do impurities precipitate in the form of poorly soluble compounds.

According to the theoretical foundations, soluble complex compounds of metals with ligands are formed with an excess of the complexing agent. In a cyanide solution, several cyanide anions are required for dissolution of one metal cation; in particular, gold passes into solution as the compound Au(CN) ₂ ^- . A peculiarity of the interaction of oxidized Cu(II) compounds with a cyanide solution is the reduction of copper to monovalent copper due to the oxidation of CN- ions, which are then oxidized with the formation of cyanogen (CN) ₂ . The main forms of copper in a cyanide solution are Cu(CN) ₃ ^2- and Cu(CN) ₄ ^3- . The stability index of these complexes, the instability constants, are equal to 2.6*10 ^-29 and 5*10 ^-31 , respectively. Ultimately, this is manifested in the fact that with an excess of cyanide, most copper compounds (hydroxide, carbonate, etc.) are highly soluble. At the same time, when the molar concentrations of copper and cyanide are equal, the poorly soluble simple cyanide Cu(CN) is formed. It has been established that the formation of simple cyanide occurs when copper compounds—oxide, hydroxide, or carbonate—are added to the process copper-containing cyanide solution in the required quantity. The chemistry of this process is described by the equations

Cu(CN) ₄ ^2- + Cu(OH) ₂ = 2CuCN↓ + 2OH ^- + (CN) ₂ (5)

Cu(CN) ₄ ^2- + CuCO ₃ = 2CuCN↓ + CO ₃ ^2- + (CN) ₂ (6)

A similar result is observed when hydroxide interacts with free cyanide anions:

Cu(OH) ₂ + 2CN ^- = CuCN↓ + 2OH ^- + 1/2(CN) ₂ (7)

This transformation is accompanied by excessive consumption of copper(II) compounds and the undesirable formation of cyanogen.

Thermodynamically, reactions (5 and 6) are more probable than reaction (7), which is reflected in their sequence - copper hydroxide and carbonate will react with the copper cyanide complex first.

Dicyanogen (CN) ₂ is formed in acidic and neutral environments via reactions (5,6) and is a highly toxic gas. In an alkaline environment, typical for cyanide leaching of gold, dicyanogen is transformed into cyanide and cyanate:

(CN) ₂ + 2NaOH = NaCN + NaCNO + H ₂ O (8).

The main parameter determining the completeness of copper precipitation in this case is the amount of copper compound added to the cyanide solution. The required consumption of copper hydroxide or carbonate is determined by the properties of the initial solution, in particular, the copper content. Given the return of the cyanide solution after copper precipitation as simple cyanide, it is advisable to prevent reaction (7). This condition is most easily met with incomplete copper precipitation. Therefore, the consumption of copper hydroxide added to the "tired" copper-containing solution is taken such that it is insufficient to bind copper from the solution to simple cyanide according to reaction (7). The residual copper content in the cyanide solution can serve as a criterion for the consumption of copper (II) compounds; the optimal value for this parameter is 50-100 mg/l. At lower residual copper concentrations, some of the free cyanide can bind to simple copper cyanide according to reaction (7), which is accompanied by unproductive losses of this reagent.

Remarkably, during the copper precipitation stage, unlike known methods and the prototype, the free cyanide in the feed solution remains unchanged, and after separating the CuCN precipitate, the leaching solution can be recycled. The mass of copper-containing CuCN precipitate recovered from 1 cubic meter of feed cyanide solution does not exceed 1-2 kg. The chemistry of copper-containing precipitate treatment and cyanide regeneration is described by the following reactions:

CuCN+ H ₂ SO ₄ +O ₂ = CuSO ₄ + HCN↑ +H ₂ O (9)

CuSO ₄ + 2NaOH = Cu(OH) ₂ + Na ₂ SO ₄ (10)

When economically feasible, hydrocyanic acid vapors (reaction 9) are captured with an alkaline solution, similar to similar methods. The alkaline cyanide solution is sent for cyanidation.

For processing a small mass of copper-containing precipitate, the required equipment volume is proportionally smaller, and it is easier to ensure a seal and prevent the release of hydrocyanic acid. The reagents—sulfuric acid and alkali—are consumed only for processing the CuCN precipitate, which is significantly less than regenerating the entire volume of the original copper-containing solution using the prototype method.

Copper(II) compounds such as hydroxide, oxide, and carbonate can be used to precipitate copper from cyanide solutions. Theoretically, the use of copper sulfate is undesirable due to the accumulation of sulfate ions in the circulating solutions, which negatively impact cyanidation.

Thus, the set of distinctive features of the proposed method:

- the use of copper (II) compounds to precipitate copper from a cyanide solution;

- consumption of copper (II) compound, ensuring a residual copper content in the cyanide solution of 50-100 mg/l;

- treatment of CuCN precipitate with a solution of sulfuric acid to obtain a solution of copper (II) sulfate;

- treatment of part of the copper (II) sulfate solution with an alkali solution to obtain hydroxide, which is used for copper precipitation;

Overall, it simplifies the hardware design, minimizes the formation of volatile hydrocyanic acid in the atmosphere, and reduces the consumption of acid and alkali compared to the prototype.

The results of the following experiments serve as an example of the implementation of the proposed method.

The mother liquor from Berezovsky Rudnik JSC, obtained after cementation precipitation of gold with zinc powder, was used in the experiments. At pH = 10.7, the initial solution contained, in mg/l: 1530 CN ^- ; 1050 Cu ²⁺ . Copper precipitation was carried out using reagents of divalent copper grade "pure".

Specified amounts of copper compounds were added to 1-liter aliquots of the stock solution with stirring. The solution was stirred at room temperature for an hour, after which the copper-containing precipitate was separated from the cyanide solution by filtration. The residual copper content in the solution was determined by atomic adsorption, the residual concentration of free cyanide was determined by titration, and the degree of copper precipitation and unproductive precipitation of free cyanide were calculated.

The copper-containing precipitate was treated with a sulfuric acid solution (50 g/l) in a fume hood until completely dissolved. Copper was precipitated from a portion of the resulting copper sulfate solution as hydroxide or carbonate by neutralization with alkali and/or soda. The resulting compounds were used to precipitate copper from the initial solution. The results of the comparative experiments are presented in Fig. 1 (table).

From the bulk of the copper sulfate solution, a precipitate of metallic copper with a purity of 95% was obtained by cementation.

A comparative analysis of the technical solutions, including the method presented as a prototype, and the proposed invention, allows us to conclude that the combination of the claimed features ensures the achievement of the desired technical result. Implementation of the proposed method, based on copper precipitation from cyanide solutions with hydroxide, oxide, or carbonate, minimizes acid and alkali consumption and simplifies equipment design.

Claims (3)

1. A method for extracting copper from cyanide solutions and regenerating free cyanide, comprising treating the cyanide solution with reagents to precipitate copper to form a copper-containing precipitate and regenerating the cyanide solution, characterized in that copper (II) compounds are used as reagents for precipitating copper to form a copper-containing precipitate, the regenerated cyanide solution is separated from the copper-containing precipitate and returned to cyanidation, the resulting copper-containing precipitate is treated with a sulfuric acid solution to obtain a copper (II) sulfate solution, wherein alkali is added to part of the copper (II) sulfate solution and copper hydroxide is precipitated, which is used to precipitate copper in the form of a copper-containing precipitate from a new portion of the cyanide solution, and the remaining copper (II) sulfate solution is processed to obtain metallic copper. 2. The method according to claim 1, characterized in that the precipitation of copper with the formation of a copper-containing precipitate is carried out with CuO oxide, Cu(OH) ₂ hydroxide, or copper carbonate CuCO ₃ . 3. The method according to paragraph 1 or 2, characterized in that the precipitation of copper with the formation of a copper-containing precipitate with reagents in the form of CuO oxide, Cu(OH) ₂ hydroxide or copper carbonate _CuCO3 is carried out until the residual concentration of copper in the cyanide solution is reduced to a level of 50-100 mg/l.