A LUM.—Lixiviation. 1C1

accidents causes a deficiency in the produce of alum.Those bituminous schists which have been used asfuel under steam boilers have suffered such a violentcombustion that their ashes scarcely yield any alum.Even the best regulated calcining piles are apt toburn too briskly in high winds, and under such cir-cumstances should have their draught-holes carefullystopped. It may be laid down as a general nilc thatthe slower the combustion the richer will the roastedore be in aluminium sulphate. When the calcinationis complete the heap is diminished to one half itsoriginal bulk; it is covered with a light reddish ash,and is open and porous in the centre, so that the aircirculates freely throughout the mass. In order tofavour access of air the masses should not be tooelevated; and in dry weather a little water should beoccasionally sprinkled on them, which, by dissolvingsome of the saline matter, renders the interior moreopen to the atmosphere.

When the calcined mineral is thoroughly cold it istaken to the lixiviating vats. Since many weeks, oreven months, may elapse from the first constructionof the piles or beds till their complete calcination,care should be taken to provide a sufficient numberof them in different stages, so as to have an adequatesupply of material for carrying on the lixiviating andcrystallizing processes during the course of the year.The complete decomposition of the beds is judged ofby the efflorescence of the salt, by the strong alumin-ous taste of the ashes, and by the appropriate chemi-cal test of lixiviating an aliquot average portion ofthe mass, and seeing how much alum it will yieldwith a solution of potassium sulphate or chloride.

Spence calcines the shale by forming on the grounda number of air-channels, on which are laid parallellines of common bricks at the distance of four inchesapart, on these others are placed crosswise; thus thechannel formed is about four inches square. Thetransverse bricks arc placed on loosely, so as to allowthe air to pass freely upwards; burning coals arc laidon the channels, and a layer of the shale which ismost bituminous, broken into small pieces; and asthe combustion proceeds other layers of the shalefragments, less bituminous than the preceding, areput on continuously, but not in too great a quantity.The thickness of each layer is regulated by thebriskness of the combustion, which should never gobeyond a low red heat, as a higher temperature wouldbe apt to partially flux tire materials, thus renderingthe alumina less soluble in the sulphuric acid. Theblaze, candle-slates, and other bituminous shales andfire-clay stones, are first broken into lumps aboutthe size of the stones used in road-making. If claystone be the substance under treatment small coal, orsawdust, must be mixed through it to insure perfectcalcination. An examination of the preceding figure,exhibiting the heaps or mounds in various stages ofprogress, will show that a method similar to this ispractised at Ilurlet.

The heaps may be made of any convenient dimen-sions, but the height most appropriate is 4 to 5 feet.The mass will burn out, and be cool enough for usein eight or ten days. I

vol. r.

Wilson selects shale as free as possible from im-purities, and after exposing it for three or four monthsto the action of the atmosphere, by which it breaksdown and assumes a pulverulent form, roasts it in alime kiln of the ordinary construction, in which con-tinued removals of roasted shale are effected below,and continued additions of new shale arc made inthe upper part of the kiln, care being taken to.avoid fluxing.

Lixiviation of the Roasted Ores .—This part of theoperation for dissolving out the aluminium sul-phate and other soluble components, is one whichrequires much attention. The amount of watermust be so regulated as to extract the whole ofthe soluble bodies, without having a superabund-ance of liquid to eliminate when the solutions are tobe concentrated for precipitation or crystallization.When an excess of water has been employed toexhaust the calcined shale much time and labour arerequired for the evaporation. Hence, only so muchwater should bo employed as is necessary to exhaustthe ore of its soluble saline ingredients. Vessels forthe lixiviation are either wooden tanks lined withsheet-lead, or cisterns made of stone; the latter aremore durable, but more expensive. The lixiviatingvessels are in England and France placed at differentlevels, for the purpose of facilitating the exhaustionof the material in them, while in Scotland they areconstructed upon the same plane. The form usuallygiven to these vessels is that of a square or oblong.

When the exhaustion of the roasted shale is effectedby the first of these arrangements, as at Whitby, awooden or iron sluice carries the lie from the seriesof tanks to a large cistern to be again returned uponthe upper troughs, recharged with fresh portions ofthe burned ore, in order to bring it to the properdegree of strength, when it is drawn off to be con-centrated by heat. Water is let into the uppertroughs through several inlets, till it covers theburned material to the depth of one or two inches.After six to twelve hours, according to the facilitywith which the sulphates arc extracted, the liquorfrom this tank is drawn off at the openings at thebottom iuto the cistern next beneath it, where it issuffered to remain for an equal length of time beforeit is run into the clarifying vessel, whence it is con-ducted to the pans for evaporation. No more thanhalf the water employed in the first washing can bedrawn off, as nearly half is retained by the shale;and in consequence of this only half the quantity ofwater used in the first is added to make the secondlie, which, after twelve hours, is drawn off into thelower cistern, like the preceding, and a third quantityof water is poured on in order to extract the wholeof the soluble ingredients. This third washing beingof a low strength is pumped up to form the firstwatering of fresh quantities of the material, to bringit to the desired strength for exaporating, whichshould bo from T113 to 1T57 specific gravity.

At the Ilurlet and Campsic Works, near Glasgow ,the process pursued is the following:—After theore is calcined it is wheeled from the ridges intolarge stone cisterns, called steeps, occupying the same

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