164 ALUM.— Evaporation.

saturated with aluminium sulphate, which is availablefor the production of alum.

When, on the contrary, the copperas is not inexcess, the process is reversed, and begins with theproduction of alum from the crude liquor. For thispurpose the lie is pumped at once from the crude

liquor cisterns, in which it has been clarified, into theevaporating pans to be brought to the proper degreeof concentration. If the pans arc made of tinnediron they will be corroded by the lie, with evolutionof hydrogen and the precipitation of basic aluminiumsulphate ; this action is not prevented by the addi-

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tion of the basic mud before mentioned. Whencast-iron or leaden pans are employed, which arcnot thus affected, the sediment frequently getsburnt on the bottom, and causes difficulty and damage;but the former are brittle, and the latter are tooeasily melted by the heat. To prevent this burningmetal evaporating pans are now, except under specialcircumstances, universally discarded, and the flameis carried over the surface of the liquid instead ofbelow the pan. When, however, the liquors containmagnesium sulphate surface evaporation cannot beused, as when the concentration has proceeded to acertain extent the salts form a crust onthe surface of the liquid which protectsit from the heated air passing over it.

At the Ilurlet works the evaporatingbasins are protected by a shed-roofing,Fig. 4.

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as represented in Fig. 3. They are long cisterns ofbrickwork well cemented together, and arched overwith the like materials; they have a length ofGO feet and a breadth of 6, with a depth of about

4 ; arc capable of evaporating about 4500 to 5000gallons in twenty-four hours. The cisterns areheated by a furnace, which is placed at one end,and on a level almost with the surface of theliquid when the boiler is full, as has been alreadydescribed; a powerful chimney at the opposite endof the boiler creates a strong draught that sweepsthe flame and heat from the fire over the surface ofthe liquor, producing a very high temperature, andrapid evaporation takes place. Fig. 4 representsa longitudinal section of one of these evaporatingcisterns.

Hie liquor is placed in the basin, A A, the walls ofwhich are made of brick, placed upright, and wellcemented with mortar composed of lime and lixivi-ated alum-shale waste. In order that the current ofair passing through the fire at D may not carry anyappreciable quantity of theashes into the liquor, thefire bars are fixed low down;the flame passes below theflat arch, B B, to thechimney. This arch con-tains apertures, C C, throughwhich to observe the dif-ferent parts of the interior.Fresli liquors, or the mother liquors from a previousoperation, are introduced tlirough the openings,C C, as the water is evaporated from the contentsof the furnace, until the solution acquires a densitv