708 



ROASTING ORES 



wood arranged as in Jig. 1714, for the great roastings in the open air, is cal- 

 cined with the disengagement of much sulphur, which finds more facility in escaping 

 Ly the lateral conduits in the walls, than up through the whole mass, or across the 

 upper surface covered over with earth ; whence it passes into the chambers of conden- 

 sation. In this way upwards of a thousand tons of pyrites T/y be roasted at once, 

 and a large quantity of sulphur obtained. See PYRITES. 



The reverboratory furnace affords one of the best means of ustulation where it is 

 requisite to employ the simultaneous action of heat and atmospheric air to destroy 

 certain combinations, and to decompose the sulphides, arsenides, &c. See COPPER. 



In every case where it is desired to have a very perfect roasting, as for blende 

 from which zinc is to be extracted, for sulphide of antimony, &c., or oven for ores 

 reduced to a very fine powder and destined for amalgamation, it is proper to perform 

 the operation in a reverberatory furnace. When very fusible sulphurous ores are 

 treated, the workmen charged with the calcination must employ much care und 

 experience, chiefly in the management of the fire. It will sometimes, indeed, happen 

 that the ore partially fuses, when it becomes necessary to withdraw the materials 

 from the furnace, to let them cool and grind them anew, in order to recommence 

 the operation. The construction of these furnaces demands no other attention 

 than to give to the solo or laboratory the suitable size, and so to proportion to 

 this the grate and chimney that the heating may be effected with the greatest 

 economy. 



The reverberatory furnace is always employed to roast ores of the precious metals, 

 and especially those for amalgamation ; as the latter often contain arsenic, antimony, 

 and other volatile substances, they must be disposed of in a peculiar manner. 



The sole, usually very spacious, is divided into two parts, of which the one farthest 

 off from the furnace is a little lower than the other. Above the vault there is a space 

 or chamber in which the ore is deposited, and which communicates with the laboratory 

 by a vertical passage, which serves to allow the ore to bo pushed down when it is dried 

 and a little heated. The flame and the smoke which escape from the sole or laboratory 

 pass into condensing chambers before entering into the chimney, so as to deposit in 

 them the oxide of arsenic and other substances. When the ore on the part of the 

 sole farthest from the grate has suffered so much heat as to begin to be roasted, has 

 become less fusible, and when the roasting of that in the nearer part of the sole is 

 completed, the former is raked towards the fire-bridge, and its ustulation finished by 

 stirring it over frequently with a paddle, skilfully Avorked, through one of the doors 

 left in the side for this purpose. The operation is considered to be finished when 

 the vapours and the smell have almost wholly ceased ; its duration depending obviously 

 on the nature of the ores. 



When this furnace is employed to roast very arsenical ores, the chambers of con- 

 densation for the arsenious acid are much more extensive than in furnaces commonly 

 used. 



Compared with the German Eostbfen, the mechanical furnaces are less efficient for 

 the calcination of silver ores, particularly when the ores operated on are very 

 damp and contain much sulphur ; in which case the excessive production of lumps 

 becomes a serious inconvenience to contend with. 



1715 



n 



1716 



But in the treatment of the silver ores of Hiendelaoncina, they possess the advan- 

 tage of calcining a largo quantity of ore in a given time, and require no further 

 attendance than is necessary for supplying them with ore and fuel. The supply of 

 fuel is, however, subject to great neglect. The management of the fires is neverthe- 

 less a matter of much importance, for should they be forgotten, and the heat get much 



