172

ALUM.—Fkom Clay.

clay, when employed, requires a considerably longertime than when it is calcined andpulverized previouslyto the addition of the acid; if it be found requisite,the ground material is sifted: 45 per cent, of theweight of the clay of sulphuric acid, of 45° B., ispoured upon it in a cistern which is heated, by thesurplus heat from the calcining furnace, to about158° Fahr. (70° C.). Decomposition ensues, andthe mass assumes greater consistence from the libera-tion of silicic acid; in this state the contents of thecistern are removed to the open air, where they areallowed to remain for a space of some months, toallow of the complete disintegration of the clay bythe acid. The mass is afterwards lixiviated and thealuminium sulphate treated as in ordinary cases.Clay with as little iron and lime as possible shouldbe selected. Some manufacturers employ largeshallow troughs, lined with lead and heated frombelow by flues, for the purpose of mixing the pre-pared clay with sulphuric acid, in which it is fre-quently turned over until the acid has combinedwith its alumina. In this part of the operation animprovement has been lately effected, which consistsof distributing the clay upon a shallow circular

trough, and permitting the proper quantity of acid toflow in several small jets from a cistern placed above,whilst an agitator, worked by machinery, mixes thethe clay and acid thoroughly. In this way a speedycombination is effected. After the termination ofthe action of the acid the contents of the cistern areremoved to the stone vat, a, Fig. 13, where they aretreated with successive quantities of water, to extractthe aluminium sulphate. Each lie is siphoned offinto the pipes, b b, which convey it to the evaporat-ing pans, to be boiled down to the proper strengthfor precipitating with the alkaline salt. When thispoint is attained the liquor is pumped up into thecistern, c, which is similar to a, already described,and called the precipitating vat; here the appro-priate quantity of potassium sulphate is added. Onagitating the solution the “alum powder” precipi-tates ; this is allowed to subside, and the motherliquor is drawn off to the pipes, d d, by a syphon.If the mother liquor be not very impure it is em-ployed for a subsequent precipitation. The precipi-tated alum is washed in the cistern, c, with successivesmall portions of water, each liquor being drawn offrepeatedly in the same way as the mother liquor.

Fig. 13

f nim inl

A >■ n.'

SSI

iiufljijt

i&Sli

OTgSTTvT

iugi'xunivv.

S BJSMK

(maim

§?5i!5

imBsa

asm

When the powder is purified by repeated washings,it is ladled out into the funnel-shaped vessel, e, whereit is very economically dissolved, by injecting steamupon it through the pipe, f, and the saturated solu-tion of alum descends to the crystallizing frame, h,through the pipe, g.

The evaporation of the various liquors is conductedin boilers lined with lead, by means of convolutedsteam pipes placed in the liquid. This plan of con-centration of the liquid is found to be best adaptedfor such liquors, as well as being cheaper.

Tayloe, of Bristol, manufactured alum from pipe-clay by calcining it in the usual way, and then treatingit with its own weight of sulphuric acid, of specificgravity 1-200, in a large tub, upon the bottom ofwhich a steam-pipe is coiled for the purpose of heat-ing the mixture to about 200° or 212° Fahr. (93°-3 to100° C.); furnace clinkers and pieces of pottery, orother matters, are also strewed upon it to facilitatethe filtration of the solution of aluminium sulphate.After the mixture of clay and acid has been heatedfor twelve hours, the solution is drawn off through apipe at the bottom of the tube to the precipitatingvats at a density of 1-300.

Bochin, of Manchester, first prepared alum cakefrom china clay. This product is much esteemed onaccount of the large amount of aluminium sulphateit contains. A clay is chosen which is as free fromiron as possible, and heated gently in a furnace withaccess of air, in order to remove the water and renderthe clay more soluble in acids. By losing its waterthe clay becomes very porous, and takes up the acidwith great avidity by capillarity. The gently ignitedand powdered clay is then gradually added to sul-phuric acid, of specific gravity 1-52, in leaden pans;here the mass thickens, heats violently, and boils; itis then transferred to iron tanks, where it ultimatelysolidifies. The porous mass is perfectly dry, thoughstill retaining a large amount of combined water. Italso contains all the silica originally in the clay, butin an extremely fine state of division. If it is desiredto convert the alum cake into alum, it is lixiviatedwith the washings from alum flour; and the solution,after standing to become clear, treated with potassiumbisulphate, or ammonium sulphate from gas liquor,in the usual way.

Fkoji Felspak.— This mineral, which is a doublesilicate of alumina and potassa — AlKSi 3 O s , or