A

71. A. Concentrated Pyrites containing gold, but no silver. - This material is produced in the gold quartz mills, by crushing the rock in a wet stamp battery, in which the free gold is amalgamated with quicksilver, and passing the tailings over concentrating machines, by which the sulphides are almost entirely freed from rock and earth. It generally consists mainly of iron bisulphide, called pyrites, but often contains copper pyrites, sometimes lead and antimony sulphides, etc., and in other cases consists chiefly of arsenical iron pyrites. Tellurium is also met with.

72. Concentrations should be treated as soon as possible after production, or kept in tanks, under water, for, if allowed to lie long exposed to the weather, they oxidize spontaneously to a certain extent, and form hard lumps which must be repulverized for roasting, for which purpose a self-feeding and discharging barrel pulverizer is very suitable.

73. Ores of this class containing talc, or lime, require the addition, during or before the roasting, of from one to five per cent. of salt, to convert those substances into chlorides, otherwise they would consume a great deal of gas in the subsequent chlorination of the gold.

74. The furnace, if new, is dried during five or six days with the aid of a very small fire, after which, or if it has been used before, it is heated for from eight to twelve hours, with a gradually increasing fire, but, for this class of ore, need not be red hot before charging, because there is so much sulphur in the ore that it soon ignites, and assists in heating the furnace. While heating the furnace, a quantity of the ore is dried on the drier, being turned over occasionally with a garden hoe, and a charge is put into the hopper.

75. Many operators put too much ore in the furnace, which increases the work of the roasters, without any corresponding advantage, because the term of roasting is nearly proportional to the thickness of the bed of ore on the hearth. I have found, by repeated experiments, that from ten to twelve pounds to the square foot of hearth is enough. The charge for the furnace described is, therefore, 1,000 pounds on each hearth, or a ton in all, so the charge for the hopper is half a ton.

76. When the furnace is ready, a charge of ore is dropped in on the second hearth, and at once moved, with hoe and spade, to the first, and there spread evenly. Meanwhile, a second charge is put into the hopper, dropped on the second hearth and spread. Care must be taken that none of this charge falls on the first hearth, for which reason the second hearth, as may be seen by referring to Plates I and 2, is made a foot longer, and the first foot has a slight downward inclination, as it is intended to be left bare of ore during the roasting, and to act as a partition between the two charges.

This is very important in roasting ore for chlorina-tion, because the admixture of a little raw ore with that which is half roasted causes great delay in finishing; or, half roasted ore mixed with that which is roasted is injurious in the chlorination. Some operators build the hearths on the same level, and separate them by a low wall, over which the ore is lifted with the spade when transferred from the second hearth to the first.