COLLECTIONS IN ECONOMIC GEOLOGY AND METALLURGY. 33 



Tlie eurtliy imi)uiities of the ore tbriii a islag of the followiiig compo- 

 sitiou : 



Slag 



Per cent. 



Silica (SiO:) TO. 70 



Alniuina(Al.iO:!) 5.71 



Protoxide of iron (FeO) :U. i>6 



Protoxide of inan,<j;anose (MuO) 4.00 



Oxide of lead(Pl)O) :?.66 



Lime (CaO) 2.54 



Magnesia (MgO) 0.07 



Sulphur (S) 0.75 



Copper(Cu) 0.30 



The specific gravity of this slag varies from 3.6 to 3.8, but occasion- 

 ally, especially when lead is absent, it falls as low as 3.4. The slag 

 from each charge is drawn off as soon as the charge is thoroughly 

 smelted, but the matte is allowed to accumulate in the furnace, and is 

 drawn off once in 24 hours. The slag drawn oflf towards the end of the 

 skimming frequently contains shot of matte, and it is therefore returned 

 to the furnace, as are also abnormal slags which are rich in metal. 



This first matte is mixed with rich siliceous silver ores, and subjected 

 to a second fusion for the production of concentrated matte. This sec- 

 ond matte is coarsely crushed and roasted, it is then ground very tine 

 and roasted again for the production of sulphates. This sulphate 

 roasting is carried just as far as possible, in order to convert all or nearly 

 all the silver into sulphate, but it must be stopped before nil the sul- 

 I3hate of copper has been decomposed, since the last traces of sulphate 

 of copper can not be decomposed without decomposing a portion of the 

 sulphate of silver, which would render the silver insoluble in water. 

 The sulphate of iron first formed is completely decomposed during 

 the latter stage of the operation. 



The product of this roasting is leached in vats with hot water, which 

 dissolves the sulphate of silver. This solution is allowed to run over 

 plates of copper which precipitate the silver, and after depositing the 

 silver, the solution flows over scrap iron, which removes both the cop- 

 per dissolved out of the ore as sulphate, and also that which was dis- 

 solved in preci])itating the silver. The precipitated metallic silver is 

 thoroughly washed, dried, and melted, and then cast into bars. 



The residues Mt after dissolving out the sulphate of silver are mixed 

 with rich sulphide and telluride gold ores and smelted ; this gives a rich 

 matte containing gold and silver. It is subjected to a second smelting, 

 which gives copper bottoms containing the gold, and pimple metal con- 

 taining the silver ; this latter is roasted for sulphate of silver in the 

 same manner as the concentrated matte, and the residues from this pro- 

 cess are smelted for the production of copper. 

 9110— No. 42 3 



