Fixer for recycling silver
The method for recovering silver from the fixing solution includes a precipitation method, a displacement method, a hypochlorite method, a sodium borohydride method, a sodium dithionite method, and an electrolysis method.
In the waste fixing solution, silver is often present in the form of a complex such as Ag(S 2 O 3 ) 2 3 -, and the concentration of silver is 0.5 to 9 g/L.
First, the precipitation method
Fixer sodium sulfide was added to the Ag (S 2 O 3) 2 3 - and S 2- plasma reaction silver sulphide precipitate was separated from the solution:
2Ag(S 2 O 3 ) 2 3- +S 2- =Ag 2 S↓+4S 2 O 3 2-
There are several methods for recovering silver from a silver sulfide black precipitate:
Nitric acid dissolution method Silver sulfide is dissolved with dilute nitric acid ( acid: water = 1 : 2 to 3 ) to produce a silver nitrate solution and elemental sulfur, and filtered to obtain a silver nitrate solution.
Ag 2 S+4HNO 3 =2AgNO 3 +S 0 +2H 2 O+2NO 2 ↑
Method for firing a silver smelting sulfide in 700 ~ 800 ℃ roasting, sulfur oxidized to sulfur dioxide into the gas phase, silver oxide and silver is generated subsequently decomposed to silver metal.
Ag 2 S+1.5O 2 =Ag 2 O+SO 2 ↑
2Ag 2 O=4Ag+O 2 ↑
Substitution method in hydrochloric acid solution at room temperature, the silver may be displaced from the silver sulfide, iron scrap:
Ag 2 S+Fe+2HCl=2Ag+FeCl 2 +H 2 S↑
Sulfide precipitation is simple and easy, silver recovery is complete, suitable for small-scale use, but there is an excess of sodium sulfide in the post-silver solution, and the fixer cannot be regenerated.
Second, the replacement method
Available iron powder, zinc powder, aluminum powder as a reducing agent, silver fixing solution reduced to metallic silver. This method is efficient and simple, but the main disadvantage of the metal replacement method is that the replacement metal dissolves into the solution, making the fixing solution difficult to regenerate. For example, iron replacement is the addition of iron or iron filings or iron powder to the acidic fixing solution, and the silver is replaced by a reduction precipitate:
2Ag(S 2 O 3 ) 2 3- +Fe=2Ag↓+Fe 2+ +4S 2 O 3 2-
The replacement product contains silver powder, iron powder, silver sulfide, etc., and is black. After further purification, crude silver powder can be obtained. In an acidic medium, S 2 O 3 2- in the fixing solution is decomposed to generate elemental sulfur and sulfur dioxide.
Third, hypochlorite method
Hypochlorite decomposes the silver-thiosulfate complex in the spent fixer, while silver forms a silver chloride precipitate and separates from the solution.
8ClO - +Ag(S 2 O 3 ) 2 3- +4OH - =AgCl↓+4SO 4 2- +2H 2 O+7Cl -
For example, when a fixing solution containing 6 g/L of silver is treated, it is treated with a mixed solution containing 10% to 15% NaClO and 1 to 1.5 mol/L NaOH. The sodium hypochlorite method consumes a large amount of sulfurized sulfate and requires a large amount of chemicals.
Fourth, sodium dithionite method
This method is a simple and effective method for extracting silver from waste fixing liquid. First adjust the pH of the solution to near neutrality, adjust with glacial acetic acid and sodium hydroxide, or use ammonia water.
Then, solid or liquid sodium dithionite (Na 2 S 2 O 4 ) was added to the waste fixing solution, heated to 60 ° C and vigorously stirred, and silver was precipitated from the solution. However, if the pH is too low, even sodium disulfite or sodium thiosulfate will decompose to produce elemental sulfur, and the same phenomenon occurs when the temperature exceeds 60 °C. The method is simple in process and high in efficiency; the fixing solution can also be recycled.
Five, sodium borohydride method
Sodium borohydride (NaBH 4 ) is a strong reducing agent that was widely used in the chemical analysis field in the early days and was later applied to the separation and extraction process of precious metals. Some factories in foreign countries have replaced the traditional zinc powder, iron powder replacement method and sodium sulfide precipitation method with this method, and have shown their advantages when dealing with small batches and low concentration waste liquids. The silver in the spent fixing solution is recovered by sodium borohydride, mostly under the condition of pH=6-7, and the amount of sodium borohydride added is determined according to the silver content in the solution. Generally: Ag: NaBH 4 = 1: 0.45 (mass ratio). The following reaction occurred:
8Ag(S 2 O 3 ) 2 3- +NaBH 4 +8OH - =NaBO 2 +6H 2 O+16S 2 O 3 2- +8Ag
Sodium borohydride and sodium hydroxide can be firstly formulated into a solution containing NaBH 4 12%, NaOH 40%, diluted 10 times when used, and slowly added to the waste liquid according to the silver content in the fixing solution to form a black precipitate of silver. .
6. Electrolysis
The electrolytic recovery of silver in the silver-containing waste liquid and the fixing liquid exhibits many advantages both technically and economically. Many studies, improvements, and developments have been made in various forms of electrolyzers, electrolyzers, or silver machines.
According to the structure of the equipment, in summary, the electrolytic silvering machine can be divided into two categories: a slotted electrolytic silvering machine and a closed-cell circulating electrolytic silvering machine. Among them, the slotted type silvering machine has been basically eliminated due to low efficiency, large floor space, harmful gas production and environmental pollution, and it is generally used to conduct silver extraction by a closed mechanical stirring electrolytic silver machine.
In combination with domestic reality, China has developed a closed-cell high-efficiency silver feeder. Its outer casing is a plastic cylinder and has a sealing cover. The inner lining has a thickness of 0.5 to 1 mm stainless steel as a cathode, and a hollow graphite cylinder stands in the center of the cylinder as an anode. The anode-anode distance is 35 to 40 mm, and the solution is closed in the machine. The fixing liquid enters between the cathode and the anode at a high speed from the lower part of the cylinder in the tangential direction, spirally rises rapidly, flows out from the upper part in the tangential direction, and enters the liquid storage tank. The waste fixer is continuously circulated at high speed with a pump, and a higher current density can be used.
Electrolytic technical conditions are as follows: tank voltage 2 ~ 2.2V; liquid temperature: 20 ~ 35 ° C; current density 175 ~ 193A / m 2 ; circulating fluid volume: 510L; electrolysis time: containing silver 3 ~ 4g / L, 3 ~ 4h 5~6g/L of silver, 5~6h; silver after electrolysis: 2.5~9.3g/L of silver in the stock solution and 0.5~0.7g/L of silver in the back liquid (when the tail liquid is not regenerated, it contains silver) Can be reduced to 0.15g / L); silver grade: 90% ~ 93%, after casting can reach 98% ~ 99%; silver recovery rate of about 95%.
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