How to Recover Silver from Nitrate Solutions

How to Recover Silver from Nitrate Solutions

The usual method of recovering silver from these solutions is to precipitate the silver by means of common salt. From an ordinary alloy the only salts likely to be precipitated are mercurous chloride and lead chloride. The former would be present in traces only, and would be eliminated in subsequent operations. Lead is generally removed before smelting, but assuming any to be present it will be thrown down. The salt should be added from a saturated solution, and should only be in slight excess.

Ag NO3 + NaCl = AgCl + NaNO3

Copper, zinc, and other soluble nitrates and chlorides remain in solution.

The solution should be well mixed and stirred together, for the clots of silver chloride retain some nitrate. The precipitate should then be allowed to settle, and the clear solution decanted off, and, if necessary, run through a filter. The silver chloride is then washed with water until the washings are neutral. It is then reduced by zinc or iron.

On a large scale a lead-lined box or vat serves for the reduction. The silver chloride is introduced, then the zinc required for the reduction added.

Zn + 2AgCl = Zn Cl2 + 2Ag.

At least 65 parts by weight of zinc to 287 of silver chloride. The zinc is added in the form of granules, and these must be well mixed with the chloride. If the action of reduction does not start at once, a small amount of sulphuric must be added. If the chloride is dry a small amount of water must be added to keep the zinc chloride and sulphate in solution. The reduction is complete in about four hours. When the silver is all in the metallic state, as may be tested by withdrawing some of the sludge, washing it with water, and then finding whether it is wholly soluble in nitric acid, enough sulphuric acid is added to dissolve any excess of zinc added. After the action is complete the excess of liquor is decanted; fresh water is run on, the precipitate is allowed to settle, and again decanted. It can be finally washed by being placed on filter cloth laid over the perforated lead bottom of a small vat. Hot water is run on until the washings are wholly free from sulphuric acid. The precipitated silver is then pressed into blocks, and dried in a furnace, after which it is smelted into bars. As a rule it is very nearly pure; any lead present may be removed by adding some silver sulphate crystals when melting.