6. From Cryolite and Bauxite—Alum may be prepared from the mineral cryolite in the following way (Sauerweio'a method) :—Very finely ground eryolite is boiled with water and lime, the latter being as pure and as free from iron as possible, in a leaden pan. By this means a solution of aluminate of soda and insoluble fluoride of lime are formed. The latter is allowed to deposit, and the clear liquor is decanted and the residue washed, the first washing being added to the decanted liquor, and the second and third used instead of pure water in the subsequent operations. Excess of finely powdered eryolite is now added to the solution for the purpose of separating the alumina from the aluminate of soda, the result of the decomposition becoming alumina and fluoride of aoda. When no more caustic soda can be detected in the liquid, it is allowed to become clear, when the solution of fluoride of soda is run off. The alumina is then dissolved in dilute sulphuric acid, and the solution is treated with potash or ammonia in order to produce the required alum. The fluoride of lime occurring as a bye-product has recently been used in glass-making.
When bauxite is employed as a aource of alum, it is first ignited with carbonate of soda in order to disintegrate it, or with a mixture of sulphate of soda and charcoal. In either case, lixiviation of the ignited mass yields aluminate of soda, from which alum may be prepared by processes described in the above paragraph.
7. The Alum-process for Removing Potash from Saccharine Solutions.—A process, bearing the above title, has recently been patented by James Duncan, John A. R. Newlanda, and Benj. E. R. Newlands, for converting the potash present in saccharine solutions obtained from the beet into alum by treating it with a solution of sulphate of alumina. Thia process, by which potash alum is produced as a bye-product, is now largely employed in sugar refineries. It consists of two parts : (1) precipitation of the potash in the form of alum, and (2) neutralization of the residual acid liquor by means of lime.
(1) Precipitation.—Thia is accomplished by adding to the cold syrup solution of sulphate of alumina, in quantity sufficient to form an alum with the whole of the potash present. It is convenient to work with syrup at a density of 38° B., and solution of sulphate of alumina at 27° B., or thereabouts. If the density of the syrup be much over 38° B., the alum cannot easily separate out. The mixture is well stirred for about fifteen minutes to one hour, and the whole allowed to repose for four or five hours, until the deposit—which consists of small crystals of alum, technically known as " alum flour "—has completely subsided. The tank in which this operation is performed is provided with mechanical stirring gear, and may be called the "alum tank."
The three principal points to be attended to in this part of the process, in order to obtain the best results and to prevent the formation of glucose, are :—(a) To work at the lowest attainable temperature, (b) to employ solutions as dense as possible, and (c) to perform the whole operation as quickly as possible, consistently with due separation of the alum.
The amount of potash present in syrups is generally equal to two-fifths of the ash. The ash is determined in the usual way, by addition of concentrated sulphuric acid, followed by incineration and weighing, one-tenth being deducted from its weight. It is sufficient for most practical purposes to assume that two-fifths of the ash is potash. Every 1 part of potash requires for conversion into alum about 9i parts of sulphate of alumina, out of which 2i parts are required to convert the potash into sulphate, and the remaining to combine with the sulphate of potash, so as to form alum. If the liquor contain any sulphuric acid, either free or combined, or if the solution of sulphate of alumina contain any free sulphuric acid, the 2i parts of sulphate of alumina required to convert the potash into sulphate may be partly or entirely dispensed with. For practical purposes, it is sufficient to determine tho percentage of ash, to assume two-fifths of this to be potash, then to multiply the percentage of potash by 9.5, which gives the dry sulphate of alumina, and, lastly, to ascertain the amount of solution corresponding to this by trawls of the following table:— (2) Neutralization.—The alum-Iauk is provided with several taps, at different heights, and when the alum has well settled down, the clear acid liquor is run off, by means of these taps, into another tank placed on a lower level, and also provided with mechanical stirring-gear. This tank may be called the "lirneing-tank." As soon as the acid liquor has been thus decanted into tho limeiug-tank, a little finely-divided chalk, previously made into a pastel with water, is added, so ae to produce a slight effervescence. Milk of lime is then added at frequent intervals, until the froth has nearly, but not entirely, disappeared ; the gradual abatement of the froth serves to indicate when the neutralization is nearly complete. This operation takes from ono to two hours. The point at%Ilia the neutralization is practically complete may be known by three simple observations: —To The absence of any large amount of froth ; (b) the absence of any taste of aluminuus compounds; (c) the liquor should give only a dull-red tinge to blue litmus paper (see Sugar).