676 



PYRITES 



mineral materially increased from that date, and for many years the supply was prin- 

 cipally derived from the mines of Cornwall and from those of Wicklow in Ireland. 



About the year 1853 pyrites containing a small percentage of copper began to be 

 imported from Spain and Portugal, and considerable quantities of ordinary pyrites are 

 derived from Norway. The increased importance, however, of this branch of industry 

 will be appreciated when it is stated that the annual importation into the United 

 Kingdom of pyrites for the manufacture of sulphuric acid is about 450,000 tons, of 

 which at least 321,500 tons contain a sufficient amount of copper to render its treat- 

 ment, for that metal, commercially advantageous. 



The principal mines from which the chemical trade is now supplied with cupreous 

 pyrites are those of Mr. James Mason at San Domingos in Portugal, and those of the 

 Tharsis Sulphur and Copper Company in the south of Spain. The ores from these 

 localities do not differ very materially from each other ; and the following analysis, 

 made by Mr. Claudet, of a specimen from Mr. Mason's mines, may be taken as repre- 

 senting an average sample : 



Analysis of Mason's Pyrites. 



Total 



100-00 



In the manufacture of sulphuric acid this pyrites is burnt in kilns supplied with a 

 limited amount of air; the products of combustion being thence conducted into leaden 

 chambers, as in the case of vitriol manufactured from ordinary brimstone. The residue 

 or 'burnt ore' was formerly, to a great extent, smelted for copper, and from the large 

 amount of oxide of iron present acted as a valuable flux for more siliceous ores. 



Burnt ore, resulting from the treatment of San Domingos pyrites, may be taken as 

 having the following average composition : 



Analysis of Burnt Ore from the San Domingos Mines. 



Sulphur 



Arsenic 



Iron 



Copper 



Zinc 



Cobalt 



Lead 



Lime 



Insoluble matter 



Moisture 



Oxygen and loss 



3-66 



25 



58-25 = 83-00 Fe 2 3 

 4-14 



37 



traces 

 1-24 



25 



1-06 



3-85 



26-93 



Total. 



100-00 



Silver 18 dwts. per ton. 



Soluble in water; sulphate of copper, 4-12 per cent. = 1 '65 of copper. 



The cinder, or burnt ore, which remains in the pyrites-kilns of the sulphuric-acid 

 makers, although sometimes still smelted as a flux for other ores, is now generally 

 treated by a process of wet extraction, conducted in the following way : 



The ore from which the principal portion of sulphur has been removed by burning, 

 but which still retains from 3 to 5 per cent, of that body, and a nearly equal ameunt 

 of copper, together with a little silver, is first mixed with 15 per cent, of rock-salt, 

 and then reduced to a state of fine division, by being passed between powerful crushing 

 rollers to which proper sieving apparatus is attached. 



The ore thus ground, and ultimately mixed with salt, is next calcined in a reverbe- 

 ratory furnace at a low-red heat. By this calcination sulphur is oxidised, and the 

 resulting sulphuric acid, combining with the sodium of the common salt, forms sodium 

 sulphate ; the copper, on the other hand, becomes converted into chloride by the action 

 of liberated hydrochloric acid, and a highly soluble salt of that metal is the result. 



This roasting with salt is sometimes conducted in revolving mechanical furnaces, 

 somewhat similar to those employed for the calcination of tin ores in Cornwall ; in 



