RU2034063C1 - Method of silver purification - Google Patents

Abstract

FIELD: metallurgy. SUBSTANCE: before silver oxide isolation sodium nitrite is added to silver nitrate solution at heating and stirring up to pH 3.0-3.5. Precipitated silver nitrite is separated, washed and treated with sodium hydroxide solution up to pulp pH 11.5-12.5. Prepared silver oxide precipitate is dried and calcined, and metallic silver is obtained. Mother liquid after silver oxide precipitation is used for silver nitrite precipitation. EFFECT: improved method of silver purification. 1 tbl

Description

 The invention relates to the metallurgy of non-ferrous, rare and trace elements, in particular, to the production of noble metals.

A known method of electrolytic refining of silver [1] by which silver is obtained of high purity, purifying it from impurities of base and noble metals. The method includes conducting electrolysis in three cycles. The electrolyte for the first cycle is prepared by dissolving a sample metal 999.9 in nitric acid. Refined silver is used as anodes of the same purity. The cathode silver of the first cycle is melted, it serves to prepare the anodes and electrolyte of the second cycle. Preparing an electrolyte for the second cycle by dissolving the obtained silver in HNO ₃ . The resulting solution was evaporated to form crystals of salts, they are separated from the mother liquor, melted at 300 ° ^C. The melt was poured into water, stirred and decanted. The solution is filtered and used as an electrolyte, the anodes of the second cycle silver from the first cycle. Anodes and a solution for the third cycle are prepared from silver of the second cycle. The cathode deposit obtained in the third cycle is melted.

 The disadvantages of the method are the high costs and duration of the process cycle.

A known method for the extraction of palladium, gold and silver [2] It includes dissolving the material in aqua regia, separating silver in the form of AgCl, drying and calcining the latter with Na ₂ CO ₃ at 850 ^о С to obtain sponge silver, its dissolution in HNO ₃ , isolation Ag ₂ O NaOH solution, drying and calcining the precipitate at 400 ^about With obtaining high-purity sponge silver.

 The method adopted for the prototype.

The main disadvantage of this method is the lack of the possibility of deep purification of silver from impurities of precious metals. So for silver containing impurities of platinum, palladium, rhodium and gold, their total residual content after treatment is at least 0.2%
The aim of the invention is to increase the degree of purification of silver from impurities of platinum metals.

This goal is achieved by the fact that prior to the isolation of Ag ₂ O, sodium nitrite is introduced into silver nitrate solution while heating and stirring to pH 3.0-3.5, the precipitated AgNO ₂ precipitate is separated, washed and treated with a NaOH solution to a pulp pH of 11.5- 12.5, the mother liquor from the deposition of Ag ₂ O is used to precipitate silver nitrite.

The essence of the proposed method lies in the fact that when sodium nitrite is introduced into a silver nitric acid solution, a sparingly soluble crystalline silver nitrite salt is formed:
Ag ⁺ + NO ₃ ^- + Na ⁺ + NO ₂ ^-
AgNO ₂ + Na ⁺ + No ₃ ^- . (1)
Impurities of metals to pH 3.0-3.5 remain in solution, with platinum metals and copper in the form of stable complex nitrites. Hydrolysis of the precipitate obtained in an aqueous NaOH solution to a pH of 11.5–12.5 leads to the quantitative conversion of nitrite to silver oxide:
2AgNO ₂ + 2Na ⁺ + 2OH ^-
Ag ₂ O + 2Na ⁺ + 2NO ₂ ^- + H ₂ O. (2)
After separation of the oxide, the filtrate contains NaNO ₂ with a slight excess of NaOH, which is sent to the head of the process to precipitate AgNO ₂ .

The pH of the solution during the deposition of AgNO ₂ , equal to 3.0-3.5, guarantees the purification of silver to a residual content of impurities <0.02% of which the sum of Pt, Pd, Rh, Au is below 0.01 with a silver yield of 85-92% A decrease in pH below 3.0 leads to a sharp drop in the yield of refined silver: for example, at a pH of 2.8- to 71.4% at a pH of 2.5-56.5% at a pH of 2.0-31.5%, an increase in pH is higher 3,5 leads to an increase in the content of impurities. So, at pH 3.8, the amount of platinum 0.011% palladium 0.001% rhodium 0.0001% tellurium 0.014% bismuth 0.002% copper 0.013% iron 0.0008% (silver content 99.96%) increases. At pH 4.1, the impurity content is even higher: platinum 0.040% palladium 0.002% rhodium 0.0005% tellurium 0.02% tin 0.001% bismuth 0.003% copper 0.001% iron 0.001% (silver content 99.93%).

The observance of pH when processing the precipitate of AgNO ₂ with alkali equal to 11.5-12.5 is necessary for the complete conversion of AgNO ₂ into Ag ₂ O with a minimal excess of NaOH. With an increase in pH above 12.5, in addition to the unproductive consumption of NaOH, its excess negatively affects the yield of refined silver when reusing this solution to precipitate AgNO ₂ . Since in this case the achievement of the required pH value of the solution occurs due to neutralization with an excess of NaOH and at pH 3.0-3.5 NaNO _{2 is not} enough in the solution to isolate AgNO ₂ . So, if the pH of hydrolysis is 13.0, then when using an alkaline solution of NaNO ₂ in the next cycle to precipitate AgNO _{2, the} yield of refined silver is 62.1%. A decrease in the pH of hydrolysis leads to the incomplete conversion of AgNO ₂ to Ag ₂ O and, accordingly, only part of the ions NO ₂ ^- in solution. As a result, the efficiency of NaNO ₂ recycle decreases, and part of the bound nitrogen is lost in the form of toxic nitrogen oxides upon calcination of a mixture of AgNO ₂ and Ag ₂ O.

PRI me R 1 (prototype). A portion (90.28 g) of technical silver containing 99.68% Ag and the following impurities, according to spectral analysis (SA), 0.08 Pt, 0.16 Pd, 0.017 Rh (Sum of PGM 0.257), 0.009 Au, 0, 02 Te, 0.002 Sb, 0.013 Pb, 0.016 Cu, dissolved in nitric acid (1: 1) with heating and stirring. The volume of the solution is 120 ml. Aliquots (20.00 ml) of this solution are treated with sodium hydroxide to a pH of 6-11. After cooling, the precipitated silver oxide precipitates are filtered off and analyzed by spectral method. The table below shows the extraction efficiency E and the amount of impurities in the silver deposits, depending on the pH of the deposition of Ag ₂ O.

Thus, the total content of impurity elements, regardless of the precipitation pH of Ag ₂ O, remains at the level of 0.27-0.30% and the sum of platinum, palladium, rhodium and gold is not lower than 0.24%. The maximum silver recovery is observed at pH 9- eleven.

PRI me R 2. A portion of 9.21 g of silver containing, according to spectral analysis (CA), 99.70% Ag and the following impurities, Pt 0.050, Pd 0.125, Rh 0.018, Au 0.008, Te 0.05, Sb 0.001, Sn 0.001, Pb 0.023, Bi 0.003, Cu 0.018, Fe 0.001, Co 0.001, are dissolved in nitric acid (1: 1) with heating and stirring. A solution of NaNO ₂ (500 g / l) was added to a hot solution with a volume of 50.0 ml (pH start -0.25) with stirring to a pH of 3.0. The mixture is cooled, filtered. The silver nitrite precipitate on the filter is washed with distilled water, then transferred to a beaker and treated with a NaOH solution (1: 9) to a pH of 11.5. The resulting precipitate of Ag ₂ O is filtered off, washed with water, dried and calcined to silver metal. The mass of refined silver was 7.66 g (yield 83.4%) according to SA, it contained 99.98% Ag and the following impurities, 0.006 Pt, 0.0005 Pd, 0.0003 Rh (Sum of PGM 0.0068), 0.006 Te, 0.002 Cu, 0.002 Bi, 0.0008 Fe.

PRI me R 3. A portion (10.40 g) of technical silver, containing according to the spectral analysis (SA) 99.51% Ag and the following impurities, Pt 0.077, Pd 0.15, Rh 0.021 (Sum of PGM 0.248) , Au 0.014, Te 0.03, Pb 0.009, Cu 0.15, Fe 0.04, dissolved in nitric acid (1: 1) with heating and stirring. In the resulting hot solution, with a volume of 50.0 ml (pH start-0.10), a NaNO ₂ solution (500 g / l) was added with stirring to a pH of 3.5. The mixture is cooled, filtered. The precipitate of silver nitrite on the filter is washed with distilled water, then transferred to a glass and treated with a solution of NaOH (1: 9) to a pH of 12.5. The precipitate of Ag ₂ O formed is filtered off, washed with water, dried and calcined to metallic silver. The mass of refined silver was 9.51 g (yield 91.90%). According to SA, it contained 99.99% Ag and the following impurities, 0.0005 Rh (Total PGM 0.0005), 0.002 Te, 0.003 Pb, 0.0004 Fe.

 The silver content in all examples (1-3) is shown by the difference between 100 and the sum of the determined impurities according to OST 48-78-83. The amount of impurities included: gold, platinum, palladium, rhodium, iron, lead, bismuth, antimony, tellurium, zinc, copper and nickel.

Thus, the proposed method of purification of silver allows to obtain refined silver with a total impurity content of less than 0.02% of which the sum of platinum, palladium, rhodium and gold is below 0.01%
In addition, the method allows to implement the principle of non-waste production and, in particular, allows you to create a recycling of sodium nitrite in the refining.

Claims (1)

 METHOD FOR CLEANING SILVER mainly from noble metal impurities, including dissolving silver-containing material in nitric acid, isolating silver oxide with sodium hydroxide solution, drying and calcining the precipitate to obtain metallic silver, characterized in that the nitric acid solution is treated with stirring and heating before isolation of silver oxide nitrite to pH 3.0 3.5, the precipitated silver nitrite salt is separated from the solution by filtration and subjected to treatment with sodium hydroxide solution to p H pulp 11.5 12.5.

Images