Silver recycling

The separated nitric acid solution, in addition to containing silver nitrate and other base metal nitrates metal, but also contains a small amount of micro-fine gold mud, so before treating solution, must be recovered by the process of precipitation or filtration, to reduce the loss of gold .
Saline or hydrochloric acid was slowly added to the solution and stirred to obtain a silver chloride precipitate until no white precipitate appeared after the addition of saline. The reaction is as follows:
AgNO 3 +NaCl=====AgCl↓+NaNO 3
When adding saline or hydrochloric acid, the amount of addition and the rate of addition must be strictly controlled. If added in excess, a silver chloride complex is produced which reverses the silver:
AgCl(solid)+Cl - =====[AgCl 2 ] -
Sometimes silver chloride particles are formed in the solution, which cannot be precipitated for a long time. At this time, the water can be diluted by heating, and the solution can be quickly clarified. However, sometimes the solution is turbid, which is generally caused by the fact that the solution contains more Fe 3+ . In this case, if the dilution water or flocculant is not able to clarify the solution, the caustic soda solution may be added to precipitate various metals in the solution as oxides or hydroxides:
2Ag + +2OH-=====2AgOH=====Ag 2 O↓+H 2 O
Fe 3+ +3OH - =====Fe(OH) 3
It should be noted that in this case, silver is precipitated in the form of silver oxide. Since some base metal oxides or hydroxides are simultaneously precipitated, the quality of silver after smelting will be greatly affected.
There are many treatments for silver chloride. Some factories and mines wash the silver chloride to neutral, and mix it with sodium carbonate after drying. Since silver chloride has a strong volatility at high temperatures (above 750 ° C), it will increase the loss of silver. At the same time, the corrosion resistance is also relatively large, so this method should not be used when handling large quantities of silver chloride. Some silver chloride is an overseas factories flat ingot and then cast into layered thin iron wire was added 5% dilute hydrochloric acid and replaced, after silver sponge is smelted to obtain:
AgCl+Fe+Cl - =====Ag+FeCl 2
The silver chloride is leached with ammonia water to obtain a silver chloride ammonia complex solution, and then the silver is reduced with 20% hydrazine hydrate, washed, dried and then smelted: [next]
AgCl+2NH 3 ·H 2 O=====Ag(NH 3 ) 2 Cl+2H 2 O
4Ag(NH 3 ) 2 Cl+N 2 H 4 ·H 2 O=====4Ag↓+N 2 ↑+4NH 3 ↑+4NH 4 Cl+H 2 O
This method is relatively expensive and has not been widely promoted.
Zinc powder replacement is a relatively mature one method, which is widely used. The grade and recovery of the obtained silver are relatively high:
2AgCl+Zn=====2Ag+ZnCl 2
AgCl+Zn+H 2 O=====Ag↓+Zn(OH)Cl+0.5H 2 ↑
Before the zinc powder is replaced, the silver chloride is washed with hot water for 4 to 5 times in advance to make it weakly acidic. The replacement of silver cyanide powder is an exothermic reaction. If there is too little water, it will cause splashing and the reaction will not be thorough; if too much water is added, the zinc powder will consume a lot. Generally, it is advisable to adjust the liquid-solid ratio of silver chloride to 2:1 before adding zinc powder.
The addition of zinc powder should be carried out slowly and continuously, and stirring is required. After the reaction, the silver mud is washed with hot water for 5-6 times to remove the soluble matter and other salts. Finally, the silver mud is filtered, dried and then smelted with a flux.

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