118 REPORT — 1861. 



XIII. Alum. 



One of the most important improvements introduced into our chemical 

 manufactures during the last twenty years is the new process of making 

 alum, first patented by INIr. Spence in IS^S, and carried out on a large scale 

 by Messrs. Spence and Dickson since 1847. Before that time the alum 

 manufactured in this district was confined to a small quantity made from 

 pipeclay, our chief supplies being derived from Whitby. By the old process, 

 60 tons of the oolitic shale of Yorkshire were required in order to produce 

 1 ton of potash alum and 1 ton of Epsom salts. By Mr. Spence's process 

 50 tons of shale yield 65 tons of ammonia-alum. Mr. Spence employs the 

 shale found underlying the seams of coal in this district. This shale, which 

 is black from the organic matter contained in it, is piled up in heaps about 

 4- or 5 feet high, and slowly calcined at a heat approaching to redness. Before 

 calcination the alumina of the shale will not dissolve in sulphuric acid ; and, 

 on the other hand, if the heat be raised too high, so as to induce a partial 

 vitrification of the clay, the alumina is again rendered quite insoluble in acid. 

 The calcination lasts ten days, the heaps being supplied daily with fresh shale. 

 When suflficiently calcined, the material is soft and porous, and of a pale brick- 

 red colour. The calcined shale is then placed in covered pans, each capable 

 of holding 20 tons of the material, and is there digested from thirty-six to 

 forty-eight hours with sulphuric acid of sp. gr. 1'35. The liquid is kept at 

 a temperature of 230° Fahr., partly by fire underneath the pans, and partly 

 by the introduction of vapour from a boiler containing gas-liquor. This 

 part of the process was patented by Mr. Spence in 1858-59, it having been 

 found unnecessary to treat first with acid and then with alkali, the com- 

 bined treatment answering quite as well, provided there is an excess of acid 

 present. The volatile ammonia-salts of the gas-liquor pass over into the pans 

 and are decomposed by the acid ; the ammonia of the remainder is liberated 

 by the addition of lime. The liquor is now run off into cisterns, and kept 

 continually agitated while it cools, in order to promote the formation of 

 small crystals. The crystals are allowed to drain, and washed with the 

 liquor which runs off from the blocks of alum. No iron is found in the 

 crystals, though there is an abundance in the mother-liquor in the shape 

 of persulphate of iron. To this succeeds the so-called Hoching process, 

 which simply consists in rapidly recrystallizing. This is effected by Mr. 

 Spence through the agency of steam, without the addition of water. The 

 crystals are thrown into a hopper, at the bottom of which they come into 

 contact with a current of steam, which dissolves them rapidly, fresh crystals 

 being successively added in quantities sufficient to prevent the escape of the 

 steam. By this means 4 tons of crjstals may be dissolved in one half to three 

 quarters of an hour. The solution runs immediately into a leaden tank, 

 where it is allowed to settle for three hours, and deposits a quantity of matter 

 insoluble both in water and acid, supposed to be a compound consisting of, or 

 containing subsulphate of alumina. The clear liquor is now allowed to run 

 into tubs, the bottoms of which are formed of Yorkshire flags each 6 feet in 

 diameter, and the sides of moveable staves 6 feet long, which are kept in their 

 places by hoops and screws. After standing from five to eight days, the 

 hoops are unscrewed and the staves removed, when a mass of crystallized 

 alum of the form of the tub appears. After standing eight days longer, a 

 hole is made at 8-10 inches from the bottom, and a quantity of liquor runs 

 out. The mass is generally 18 inches thick at the bottom, and 1 foot at the 

 sides, and contains 3 tons of marketable alum, while the liquor contains 1 ton, 

 which goes back to the pans. 



