RU2535266C2 - Silver refining method - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method involves chemical dilution of an initial raw material, cleaning of a solution from impurities and obtaining pure silver from the cleaned solution. Silver is deposited from the solution in the form of a chloride. Then, silver chloride is mixed in a water solution of an alcohol, and sugar is added so that metallic silver and a hydrochloric acid solution is obtained. A mother solution of deposition of a silver chloride is supplied for dilution of the initial raw material. After silver is recovered, the mother solution is supplied for deposition of the chloride.

EFFECT: simpler technology and increase of speed of separation of pure silver from solutions by 2-3 times.

2 cl, 1 dwg, 1 tbl, 1 ex

Description

The invention relates to the metallurgy of non-ferrous metals, in particular to methods for producing pure silver.

Traditional silver refining technologies are based on electrochemical processes. The starting materials, for example silver-based alloys, are cast in the form of anodes after remelting and subjected to anodic dissolution in nitric acid electrolytes. Silver of commercial purity is obtained at the cathode, insoluble impurities (gold, platinoids) form a slurry, soluble impurities (base metals) pass into the electrolyte and can accumulate on the cathode as they accumulate, contaminating cathode silver. Contaminated electrolyte must be removed from the bath and replaced with fresh. The need for electrolyte regeneration is the main disadvantage of the electrochemical refining method (1. Metallurgy of precious metals: In 2 books. Kn.1 / Yu.A. Kotlyar, MA Meretukov, LS Strizhko. - M .: MISIS. , “Ore and Metals”, 2005., - 432 pp. 2. Maslenitsky I.N., Chugaev L.G. Metallurgy of precious metals. - M .: Metallurgy, 1987. - 366 pp. 3. Meretukov M. A., Orlov AM Metallurgy of precious metals. Foreign experience. - M.: Metallurgy, 1990. - 416).

Known methods for producing pure silver, including chemical dissolution of the starting alloy in nitric acid solutions, precipitation from the resulting silver solution in the form of chloride. To obtain pure silver, chloride that has been thoroughly washed from impurities is melted with soda, reduced in solutions by cementation with zinc, metallic iron (2), hydrogen (RF 2265673 dated December 10, 2005), hydroxylamine (RF 2185452 dated July 20, 2002) and other methods. The noted methods are technologically simpler, the speed of the main processes is higher. The disadvantages of the known methods of recovering silver from chloride are contamination of the product with a reducing agent and the inability to obtain marketable silver, the high cost of the reagents used.

The known method (RF 2100484 from 12.27.1997), selected by the prototype, including dissolving the starting alloy in nitric acid in the presence of ammonium ions, separating sludge, stage-by-stage purification of solutions from platinoids and base metals, electroextraction of silver from the purified solution with simultaneous regeneration of nitric acid and return her to dissolve the alloy. The method allows one-stage production of commercial-grade cathode silver, provides high direct extraction of silver into the product. The most important advantage of the prototype method is the complete turnover of the solutions used, excluding the formation of liquid waste, the absence of circulating anode residues and the ability to process silver alloys with a relatively high content of impurities. The disadvantages of the prototype include technological complexity and low speed of the final stage - electroextraction of silver from a solution purified from impurities. The intensity of electrolysis (current density) in this case is limited by the specificity of the electrode processes at the level of 500-900 A / m ² . Using higher current densities reduces the current utilization factor.

The objective of the invention is to increase the speed of the process, the technical result is to improve the contact of the reacting masses.

The problem is achieved by the use of a refining method, including dissolving the feedstock in nitric acid in the presence of ammonium ions, stage-by-stage separation of impurities from the solution and obtaining pure silver from the purified solution, characterized in that a solution of hydrochloric acid in an amount providing a residual silver content is added to the purified solution in a mother liquor of 1-3 g / l, silver chloride is separated from the mother liquor of precipitation, silver chloride is stirred in an aqueous solution of alcohol and a solution of ca Hara with obtaining metallic silver and mother liquor recovery, moreover, the mother liquor of the deposition of silver chloride is sent to dissolve the feedstock, and the mother liquor of the recovery of silver is sent to precipitate the chloride. In a particular case, silver chloride is mixed in a solution containing 15-20% ethyl alcohol, and sugar syrup containing 70-90% sugar is added to restore silver, and silver is restored at a temperature of 50-70 ° C.

By the totality of features, most electrochemical processes are technologically more complex and proceed at lower speeds than similar processes in reactors using chemical reagents. The fundamental difference between the proposed method of refining from the prototype in the main stage is the production of metallic silver from a purified solution. In the prototype, silver is obtained by electroextraction, in the proposed method, silver chloride is precipitated from a pure solution, and metallic silver is obtained directly from chloride by chemical reduction. From this point of view, with comparable equipment dimensions, the reagent recovery line for silver from chloride is many times more productive than the electroextraction of silver from solutions. Replacing silver electroextraction with precipitation of chloride from solutions and its chemical reduction can increase the rate of silver production. However, the existing methods for the recovery treatment of silver chloride cannot be used in the technology of the prototype due to the inadmissibility of the turnover of mother liquors or because of the high cost of reagents. The main task in improving the prototype is to create a method of reagent reduction treatment of silver chloride to obtain a mother liquor, which can be sent into circulation for the deposition of silver chloride from a new portion of nitric acid solution.

Of the similar and available reagents, alcohol and sugar provide thermodynamic opportunities for the implementation of this process. At atmospheric pressure and moderate temperatures, the activation energy of the reduction of silver from its chloride with alcohol

$$12AgCl+C2H5OH+3H2O=12Ag+12HCl+2CO2\left(\text{one}\right)$$

high enough, the process practically does not go. Heating an alcohol-based mixture to activate the process is technologically undesirable.

Reducing silver from its chloride with sugar

$$118AgCl+2C12H22O\mathrm{eleven}+37H2O=118Ag+118HCl+\mathrm{fourteen}CO2+9CO+C\left(\text{2}\right)$$

at high speeds occurs when the mixture is heated from 50-70 ° C and above. In an alcohol-containing solution, the curdled precipitate of silver chloride is disintegrated into tiny particles. The speed and completeness of silver recovery from such a precipitate is maximally possible. In addition, special measurements have established that the reduction potential of the sugar-alcohol system is higher than when using sugar alone. These circumstances have a positive effect on the heterogeneous process of silver recovery by reaction (2) - the speed and completeness of silver recovery increases.

Thus, the recovery of silver chloride, based on the use of available reagents, provides a solution to the main technical problem. In general, the proposed method of silver refining involves dissolving the feedstock in nitric acid, purifying the solutions from impurities, precipitating silver chloride and its reduction treatment in an alcohol-, sugar-containing solution. When silver chloride is precipitated from a purified nitric acid solution with a reverse hydrochloric acid solution, nitric acid is regenerated:

$$AgNO3+HCl=AgCl+HNO3\left(\text{3}\right)$$

which is sent to the head of the process to dissolve the feedstock. In turn, the mother liquor of silver chloride reduction by reaction (2) is used to precipitate a new portion of silver (Fig. 1). It is noteworthy that in circulating solutions, the formation and accumulation of impurities that can adversely affect the purity of the resulting silver and the speed of the process as a whole does not occur.

The presence of a chlorine ion in a reverse nitric acid solution is unacceptable. To prevent this, the addition of a circulating hydrochloric acid solution during the deposition of silver (3) is carried out with careful monitoring of the residual concentration of silver in the solution. Experiments show that when the silver content in the mother liquor is more than 1-3 g / l, chlorine ion in the liquid phase is practically absent. This silver content in the solution corresponds to the degree of its deposition into the solid phase of chloride by 95-98%. With careless process management, a more complete deposition of silver may be accompanied by a sharp increase in the content of chlorine ion in the circulating solution. While maintaining optimal deposition parameters, negligible amounts of chlorine ion (not more than 2-5 mg / l) do not negatively affect the process of dissolution of raw materials in the process head.

The alcohol content in the chloride reducing solution should be 15-20%. At a lower concentration, the efficiency of the influence of alcohol is small, and at a higher concentration, intense evaporation of alcohol is observed at reduction temperatures, and the process speed does not increase.

The recovery treatment of silver chloride is carried out with a portioned addition of sugar in an alcoholic solution, or rather sugar syrup with a water content of 10-30%. A syrup with a lower water content requires overheating, and this in this case is undesirable - in the reaction zone, alcohol evaporates intensively. The use of more diluted syrup increases the volume of circulating solutions and is accompanied by a decrease in the speed of recovery treatment.

The recovery temperature of silver chloride should be maintained in the range of 50-70 ° C. High temperatures, as already noted, determine the intensive distillation of alcohol. The reduction of silver according to the proposed method is accompanied by the release of gaseous carbon dioxide (oxide) (reaction 2). At elevated temperatures, the process speeds and gas evolution are excessive; solutions of silver and silver chloride from the reactor are possible, especially at the beginning of the processing of the next portion. The end of the recovery treatment is evaluated by the cessation of gas evolution.

In addition, the conduct of recovery treatment at non-optimal parameters is accompanied by incomplete recovery of chloride. Smelting of such a product is complicated.

An example of the implementation of the proposed method are the results of the following experiments.

The starting material was silver-gold alloy with an Ag content of 93.5; 3.7% Au; 1.2% Cu. CVD alloy ingots were dissolved in nitric acid solution according to the conditions of the prototype. First, platinum and palladium were extracted from the silver-containing solution by sorption; in the next stage, copper and base metals were removed by hydrolysis. The purified silver nitrate solution contained 140 g / L silver and 5 g / L free nitric acid. A solution of hydrochloric acid (50 g / l) was added to 2-liter portions of this solution with vigorous stirring and careful control of the silver and chlorine-ion content in the mixture. The resulting silver chloride precipitate was collected by filtration and washed. Silver chloride was stirred in an aqueous solution of alcohol, heated to a predetermined temperature, and sugar syrup was portioned into the resulting suspension. The end of the recovery treatment was fixed by stopping the evolution of gaseous reaction products. For the duration of the process of separation of silver from a nitric acid solution, the total duration of chloride deposition, filtration, washing, and reduction treatment was generally taken. The obtained powdered silver was melted in an induction furnace and the quality of the final product was analyzed by known methods.

To assess the possibility of using reverse solutions, the mother liquor of silver chloride deposition was used to dissolve new portions of the initial alloy, and the solution obtained during the reduction of chloride was used to precipitate silver from a nitric acid solution. After 3-fold use of working solutions, no negative effect on the process speed and purity of the obtained silver was revealed.

From a part of the nitric acid solution purified from impurities, silver was restored by the prototype method by electroextraction at a cathode current density of 500 A / m ² and electrolyte circulation through the bath. In this case, the duration of the deposition was 280 g of silver, which corresponds to the processing of 2 l of the solution.

To correctly compare the speeds of the processes at the stage of silver extraction from identical portions of solutions (2 L), comparable volumes of equipment (stirred tank reactors and electrolysis baths, respectively) were used in the experiments.

Analyzes of the products obtained showed that in all experiments, silver was obtained that met the requirements of Cp-A1 (GOST 28595-90).

The results of the experiments are presented in the table.

Experience number Residual content silver in solution, g / l Recovery Processing Conditions Continued total processing, h Note Contents alcohol in solution,% Concentration sugar in syrup,% Temperature ° C one 0.2 5 fifty 40 2,3 Incomplete restored. chloride 2 1,0 fifteen 70 fifty 2.0 3 2.0 17 80 60 1.7 four 3.0 twenty 90 70 1,5 5 5,0 25 95 80 1.4 Incomplete restored. chloride 6 Prototype method 5,6

Comparative analysis of well-known technical solutions, including the method selected as a prototype, and the alleged invention allows to conclude that it is the totality of the claimed features ensures the achievement of the perceived technical result. The implementation of the proposed technical solution due to the chemical recovery of silver chloride in the recommended modes makes it possible to simplify the technology as a whole and increase the refining rate at the stage of silver extraction from the purified solution by 2–3 times.

Claims (2)

1. The method of refining silver, including dissolving the feedstock in nitric acid in the presence of ammonium ions, stage-by-stage separation of impurities from the solution and obtaining pure silver from the purified solution, characterized in that a solution of hydrochloric acid in an amount providing a residual silver content of a mother liquor of 1-3 g / l, silver chloride is separated from the mother liquor, silver chloride is reduced by stirring in an aqueous solution of alcohol and adding sugar to obtain a metallic about silver and the mother liquor, moreover, the mother liquor obtained after the precipitation of silver chloride is sent to dissolve the feedstock, and the mother liquor obtained after the reduction of silver is sent to precipitate the chloride. 2. The method according to claim 1, characterized in that silver chloride is mixed in a solution containing 15-20% ethyl alcohol, and sugar is added in the form of sugar syrup containing 70-90% sugar, and silver is reduced at a temperature of 50-70 ° C.

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