32 BULLETIN 42, UNITED STATES NATIONAL MUSEUM. 



off sulphuric anhydride, and sulphate of copper will be formed, at 

 a still higher temperature this will decompose, giving off sulphuric 

 anhydride, and sulphate of silver will be formed. Sulphate of silver 

 being soluble in water, while the oxides of iron and copper, formed by 

 the decomposition of the sulphates, are insoluble, it can be readily 

 leached out by warm water. 



Metallic silver can be precipitated from the solution of silver sulphate 

 by letting it flow over metallic copper plates. The residues are fre- 

 quently treated a second time for silver, and if the ores contain gold 

 they are always treated for the recovery of this metal, after which the 

 copper is recovered by smeltiug. The copper in the solution after jjre- 

 cipitating the silver is precipitated by scrap iron. 



Boston and Colorado Smelting Works. — The Ziervogel process is illus- 

 trated by a collection from the Boston and Colorado Smeltiug Works 

 situated at Argo, near Denver, Colorado. They were built chiefly for 

 the purpose of extracriug silver and copper from argentiferous copper 

 ores, but the process has been exteuded so as to include gold ores in 

 the treatment. The works are so situated that they can draw their 

 supplies from an extended area of country, and thus command a large 

 variety of ores, and especially refractory ores. 



Besides the strictly argeutiferous copper ores for which the Ziervogel 

 process was devised, many rebellious ores, especially those containing 

 zinc, which can not be readily treated in any other way, and in fact 

 almost any gold or silver ore, except those containing large amounts of 

 lead, are treated successfully. This treatment of rebellious ores is ren- 

 dered possible by having a good supply of docile ores for mixing. 



Ores consisting esseutially of sulphides are lirsc roasted to remove the 

 excess of sulphur. Lumppres are roasted in kilns and the sulphur re- 

 duced to 7 per cent. Fine ores are roasted in reverberatory furnaces and 

 the sulphur reduced to 5 per cent. The roasted ores are mixed with raw 

 ores not requiring roasting, and the mixture smelted in a reverberatory 

 furnace for the production of matte. The ore mixture is made up so 

 that 16 tons will produce 1 ton of matte of the following composition: 



Matie. 



Per cent. 



Copper (Cu) - 38. 78 



Lead(Pb) 24.52 



Iron ( Fe ) T. 49 



Zinc(Zu) 3. 42 



Manganese (Mn) 0. 32 



Silver (Ag) 2.48 



Gold(An) 0.05 



Antimony (Sb) 0.08 



Sulphur (S) 22.26 



Insolublo residue - 0. 40 



