PURIFICATION OF SILVER. IO, 



possible 40 per cent of silica and a first -matte or fusible sulphide which assays 

 40 per cent of copper, 400 ounces of silver, and 6 ounces of gold per ton. 



The first matte always contains a certain amount of lead, but the quantity 

 rarely exceeds 10 per cent. The next stage in the process includes the roasting 

 and concentration of the ore-metal, or first matte, to " white metal " containing 

 60 per cent of copper. 



This white metal is now ready for the extraction of the silver, which com- 

 prises the following operations : Rough roasting; fine grinding; fine roasting for 

 sulphate of silver by Ziervogel's process ; leaching, and the precipitation of the 

 silver on plates of copper. 



In the precipitation of the silver a certain amount of copper is found mixed 

 with the silver in the form of cuprous oxide and of small scales and scraps of 

 metallic copper, and a process of refining is necessary previous to melting. This 

 copper is removed by prolonged boiling with water containing a small quantity 

 of sulphuric acid, into which air is injected by means of a small jet of steam. 

 Sulphate of copper is formed, which is carefully washed out of the silver. The 

 silver is then dried and melted into bars of an average fineness of 99.9 per cent. 



The silver came to us in the granulated form obtained by dropping 

 into water, and having been prepared with unusual care, was found to 

 be even purer than Mr. Pearce claimed. After solution in nitric acid, 

 it was crystallized fifteen times from acid solutions with centrifugal 

 draining. The solubility in cold nitric acid is so slight that this train 

 of purification could be effected without serious loss. Even after the 

 second crystallization the salt was free from any observable trace of 

 copper, and all other metallic impurities must have been eliminated 

 before the last crystals were obtained. The pure argentic nitrate was 

 dissolved in pure water and precipitated by hot ammonic formiate, 

 prepared from pure freshly distilled formic acid and ammonia which 

 had been collected in platinum. It is better to use pure ammonic 

 formiate instead of a mixture of ammonic formiate and acetate as Stas 

 did, because in the former case less carbon is included in the cells of the 

 crystals, and, moreover, that which it included occurs in a less danger- 

 ous form. Since the reaction takes place according to the equation 



2AgN0 3 + 2HCOONH 4 = Ag 2 + 2NH 4 N0 3 + C0 2 + HCOOH 



every 170 grams of argentic nitrate need 46 grams of pure formic acid 

 (or about 42 cubic centimeters of 90 per cent acid, with specific gravity 

 1.2) converted into ammonic formiate. It is necessary to take this 

 double amount in order that the ionized hydrogen (nitric acid) may 

 be removed by the salt of the weaker acid. The precipitation was 

 conducted in good glass. The beautiful white precipitate was washed 

 with the purest water, until the washings gave no Nessler test. This 

 silver, V,* although it had not been electrolyzed, was as pure as any 



*The preparation work and analyses of this silver were conducted wholly by 

 T. W. Richards. 



