More recently, this kiln has been improved by letting the spent pyrites fall into the ash pit. For this purpose, four-sided iron furnace bars are used to support the mineral. These are 1-1/ in. in diameter, and have circular pieces turned on them at each end lying in semicircular hollows so that they may be made to revolve. In order that they may be turned easily by an iron key, their ends project somewhat from the kiln wall. The space between each two bars is naturally greater when they lie square to each other than when at an angle. Thus the pyrites is crushed through by the revolution of the bars. With this arrangement the ashpit is made very deep in order to accommodate a great mass of spent ore at a time. Its mouth is closed with an iron plate pierced with holes for admitting tho air. The plate is fixed and luted into the wall at each operation.
In many French works such kilns are fitted at top with leaden pans for concentrating acid to about 140° Tw. by the beat of the gases. With this disposition the ore must be introduced at a lateral opening. Such a kiln, with a grating area of 6 ft. x 3 ft. 9 in., is charged every three hours, or eight times daily with 1 cwt. of ore.
At Cheesy, near Lyons, in France, a great deal of copper pyrites is smelted, and the sulphurous acid is utilized by means of a kiln consisting of a hollow vault, 4i ft. long, 3 ft. wide, and 3} ft. high, enclosed by strong walls. Low down in one of the long side walls are four holes, equidistant from each other, each 8 in. wide and 11 in. high, shut by cast-iron doors, provided with numerous holes, of less than in. diameter, for admitting air. An opening is left in the upper part of one of the short side walls through which the gases escape into the channel or flue leading to the chambers. This can be shut at will by a damper.
For feeding each chamber system sixteen kilns are needed, the gases from which are conveyed to the first chamber by a single flue, as the kilns are grouped into one body. This flue is fitted with a cast-iron pipe that takes to pieces for convenience in removal, and is only used for carrying away the smoke, &c., when lighting up the kiln.
To start the kilns each partition is furnished with such an amount of broken ore as will evenly cover the grating surface from the doors, up to a level of about 21 in.
After disconnecting the kilns from the chambers they are heated to redness with an ordinary fire. The ashes, are then quickly raked out, each kiln is supplied with 1 cwt. of ore, and communication with the chambers is re-made. Three hours later, another 1 cwt. is introduced, and this is continued regularly, so that °lel kiln receives 8 cwt. per twenty-four hours. This quantity
can be roasted daily with pmper working. After the kilns have been brought into condition the charge is only made once in twelve hours, each time with 4 cwt., and the spent ore is dravrn out at the same. time. As the mines yield ores varying considerahly in the proportion of sulphur they contain, the shah samples are mixed with the poor, so that the mass burnt may be as uniform as possible. Mil the kiln is going well it is good to use only rich ores.
Much tho same form of kiln is used also in Belgium, but they are larger and are funaished with grates. These are 22 ft. long, 6 ft. 6 in. wide, and 11 ft. 4 in. high ; the grate, formed of iron bars about 1 in. apart, has a surface of 113 square ft. and lies at about 6 ft. 6 in. above the sole of the Uhl, so thst it divides into two portions, an upper of about 5 ft. in height and a lower of about 6 ft. 6 in, Into the lower space the air is admitted, ttnd thence also the spent pyrites ia withdrawn hy means of a rake. The spent ore lies in the lower part till it ceases to glnw, and is then with drawn. In the top of the kiln are several holes through which the ore is introduced and is spread over the surface of the kiln with iron tools to an equable depth of 8 to 12 ins The gases leave the kilns through a broad fine in which the nitre and sulphuric acid are put for decomposition, and thence the mixed gases pass to the chambers. In such a kiln from 40 to 60 ewt. are burnt per twenty-four hours, introduced in charges from four to eight times daily.
Fig. 65 shows a large set of kilns fitted with a nitre oven in the main flue, as devised and manu factured by Messrs. R. Dogfish and Co., St. Helen's Foundry, Lancashire. The long iron pipe is for the purpose of coolnig the gases beore they enter the chambers.
Fig. 60 shows the front of a pyrites kiln as most commonly constructed ; these fittings are, how ever, subject to endless variation, as few manuffictur. ernploy kilns of the same height and shape. The size and position of tho charging and working doors are also widely different indifferent works.
Fig. 67 shows in detail the construction of the nitre pot, which remains stationary in the oven, it being periodically supplied with nitro and warm acid through a hopper, as seen in the preeeding figure. Tho plug in the bottom of the pot admits of the sulphate of soda being run out, the acid supplied being always somewhat in excess, so as to ensure the sulphate of soda being sufficiently soft to flow out easily.