The extraction of silver from the fixing solution by electrolysis has received extensive attention from all over the world. In the past three decades, there have been dozens of methods and equipment for electrolytic silver extraction that have been tested and recommended by various researchers. Advantage is that the insoluble anode electrolysis can be directly obtained metallic silver purity greater than 90%, and without adding impurity ions in the fixing solution, and therefore does not affect the return of the use solution.
If the amount of the fixing solution is large and the amount of recoverable silver is large, electrolytic cell electrolysis using a thyristor or a silicon rectifier can be used.
Since September 1974, a film studio in China has used the equipment shown in Figure 1 to extract silver from the fixing slag. The 38 batches of industrial production tests are summarized as follows:
Figure 1 Schematic diagram of electrolytic silver extraction equipment system
1-silicon rectifier; 2-silver machine; 3-plastic pump; 4-reservoir;
5-film processor overflow manifold
The silvering machine used in the test was a cylindrical plastic tank with a graphite anode at the center, a diameter of 750 mm, a height of 500 mm and an area of ​​0.395 m 2 . Outside the anode is a stainless steel cathode with a length of 1000 mm and a width of 500 mm. The anode to cathode area ratio was 1.26:1. The pole spacing is 35 to 40 mm.
The electrolyte for the test contained 2.5 to 9.3 g of silver, 240 to 260 g of sodium thiosulfate, 20 g of sodium sulfite, and 20 mg of glacial acetic acid. The fixing solution was treated with 510 L per batch under the conditions of a cell voltage of 2 to 2.2 V, an area current of 175 to 195 A ∕ m 2 , a liquid temperature of 20 to 25 ° C, and an electrolyte circulation linear velocity of 4.82 m/s. After electrowinning for 3~4h (fixing solution containing silver 3~4g∕L) or 5~6h (fixing solution containing silver 5~6g∕L), the solution contains silver to 0.5~0.7g∕L, and returns to the printing workshop. .
Test results: silver recovery rate of 95.76%, electrolytic silver purity of 90% to 93%, current efficiency of 72.51%, electricity consumption per ton of 4100 ~ 4700kW · h, direct production costs of 2000 ~ 2500 yuan / t silver.
Under the above test conditions, the rate of silver precipitation at the anode is very fast within 2 to 4 hours from the start of energization, and the precipitation rate decreases as the silver content in the solution decreases. When the solution contains silver to 0.5 ~ 0.8g ∕ L, it can be returned to the printing workshop. If the silver content of the solution is required to fall below 0.5 g/L, the current density should be greatly reduced during the extended electrolysis time. Because the dissociation of sulfur at a large current density causes the solution to turbid, and the small energy can be returned to use.
Figure 2 shows a small electrolytic device recommended in Japan in 1980. Its cathode is made of stainless steel and has a bell-shaped shape that can vibrate vertically. The cathode has a height of 220 mm, an outer diameter of 102 mm, and an inner diameter of 97 mm. There is a graphite anode inside and outside the cathode. The outer anode is 200mm high, the inner diameter is 142mm, the inner anode is 200mm high, and the outer diameter is 60mm. During the electrolysis process, the fixing solution flows between the outer anode and the anode, passes between the cathode and the inner anode, and finally exits from the inner center of the inner anode. The cell volume is 1.3L. At room temperature and circulating liquid volume of 6.2 L ∕h, current density of 2 ~ 8A ∕ dm 2 under the conditions of electrowinning, silver recovery rate of 98.5% or more, the production capacity of each device is 52 ~ 181g / h.
Figure 2 bell-type electrolyzer
1-cathode; 2-outer anode; 3-inner anode
Some hospitals use electric bell transformers, and the secondary coils are revolved into 1.8V, 1A, and full-wave rectified by a selenium stack or copper oxide to reduce voltage to 0.7V, 1A. The distance between the anode of the stainless steel plate and the anode of the stone plate is 20 mm. The solution is not circulated, and an area current of 100 to 200 mA ∕dm 2 is used. The supply current is regulated by a variable resistor. In general, 0.5 to 1.0 g of silver can be recovered from the fixing solution for each 360 mm x 430 mm chest sheet.
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