ELISHA MITCHELL SCIENTIFIC SOCIETY. 69 



centrators for the purpose of separating the sulphurets from the 

 gangue; the proportion of sulphurets to gaugue varies from teu 

 to thirty-five per cent. The concentrates contain from 25 to 30 

 per cent, sulphur, with a value of $20 per ton, and contain from 

 1 to 2 per cent, copper, with small amount of silver. These are 

 dead roasted in a revolving hearth furnace, with frequent rab- 

 bling; each furnace will roast about one ton in twelve hours. 

 It is upon the thoroughness of this roast that the success of the 

 succeeding chlorination depends; it is the purpose to free the ore 

 as far as possible from sulphur compounds. Assay value of 

 roasted ore is about §30 per ton. The thoroughly roasted ore 

 is then charged into a lead-lined iron cylinder, 42 in. x 60 in., 

 provided with discharge valve, with heads securely bolted on 

 each end ; by suitable gearing these cylinders are caused to revolve 

 horizontally at the rate of 20 revolutions per minute. Charge 

 for each cylinder is as follows: 



Roasted ore, ... 1 ton. 



Water, . . . . 100 to 125 gallons. 



Bleach, . . . . 40 to 50 lbs. 



Sulphuric acid 66°, . . 50 to 60 5bs. 



The valve is closed, the cylinder set in motion and continued 

 so from 8 to 10 hours. The chemical action within the cylinder 

 is the evolution of free chlorine by the action of the sulphuric acid 

 on the bleach with formation of sulphate of lime; the free chlorine 

 attacks the oxide of copper formed in roasting, with formation 

 of chloride of copper, the free gold with formation of gold 

 chloride aud some of the oxide of iron with formation of iron 

 chloride. It is in the chlorinator that the necessity of a dead 

 roast becomes apparent, for the action of sulphuric acid upon the 

 undecom posed sulphides would yield hydrogen sulphide and 

 precipitate the gold from the gold chloride and any ferrous sul- 

 phate left in the ore would likewise cause a precipitation of the 

 gold. In neither case, then, could the gold be leached out in the 

 form of chloride. 



It has been found by actual practice that it is better to divide 

 the charge of bleach and acid aud add them at least in two 



