METALS AND CYANIDE PROCESSES 235 



oxygen attacks metallic zinc quite vigorously, forming hy- 

 drate of zinc. 



According to Packard, the 60 pounds of zinc shavings 

 required to fill the zinc boxes will precipitate the gold from 

 about 1,500 pounds of 0.2 per cent solution per hour, the 

 solution carrying from 0.1 to 0.8 ounce of gold per ton on 

 entering the zinc box, and from 0.01 to 0.05 ounce on leaving 

 it. The gold in wash waters and weaker solutions is less 

 easily precipitated, a much longer contact with the zinc being 

 required. 



On cleaning up, the zinc shavings are washed and the 

 finely powdered portions, called zinc gold slimes, are screened 

 through sieves varying in mesh, at the different mills, from 

 one fourth inch mesh to 60 mesh, the coarse stuff being 

 returned to the zinc box. In this country, where the zinc 

 gold slimes are treated at the mills, they are subjected to 

 the action of an acid, such as sulphuric, which removes much 

 of the zinc and other soluble bodies, and the residues are then 

 washed, dried, fluxed, and melted. A few mills ship these 

 slimes to smelters and refiners, but the difficulty of obtaining 

 a satisfactory sample, and the almost constant wide disagree- 

 ment between buyer and seller have led many smelters to 

 refuse to handle them. 



Precipitants other than filiform zinc are sometimes 

 employed to throw down the gold. Thus, at the Delamar 

 mine, Nevada, the precipitation is by zinc dust, with agita- 

 tion; MoUoy precipitates with sodium or potassium amalgam, 

 Moldenhauer with aluminum, Johnston with pulverized 

 carbon, Christy with cuprous chloride, and De Wilde with 

 cupric sulphate. Precipitation is effected also by electricity, 

 amalgamated copper plates being used in the Pelatan-Clerici 

 process, and thin lead plates as cathodes, with iron anodes, 

 in the Siemens-Halske process. 



Electro-deposition processes seem to possess an advan- 

 tage over zinc precipitation processes in that the presence 

 of caustic soda makes no difference in the result, and that 

 they are as effective with a w^eak as with a strong solution. 

 Very weak cyanide solutions may therefore be used in the 

 leaching vats. Charles Butters, who has been closely iden- 



