Alkali Waste. Black ash contains less than half its weight of soda, so that for every ton of soda produced there is from a ton and a half to two tons of an insoluble residue which collects in the lixiviating and settling tanks. This residue is known as alkali waste.
Alkali waste is of no particular value. It is not even suitable as a dressing for the land, and since it is not soluble in water there is no convenient means of disposing of it. Consequently, it is just accumulated at the works and, as the heap grows at an alarming rate, it cumbers much valuable ground. Moreover, it contains sulphides from which, under the influence of air and moisture, sulphuretted hydrogen is liberated. Alkali waste, therefore, has a very unpleasant odour.
The whole of the sulphur which was contained in the sulphuric acid used in the first stage of the process remains in the alkali waste, mainly as calcium sulphide. A plant for the recovery of this sulphur is established in some of the larger works. The alkali waste is mixed with water to the consistency of a thin cream, in tall, vertical cylinders. Carbon dioxide under pressure is forced into the mixture, and this converts the calcium sulphide into calcium carbonate and sets free hydrogen sulphide, which, when burnt with a limited supply of air, yields sulphur.
By this process, the most unpleasant feature of alkali waste, namely, the smell, is removed. The calcium carbonate which remains is of very little value. Some of it is used in making up fresh charges for the black ash process and some for preparing Portland cement, for which finely-ground calcium carbonate is required; the remainder is thrown on a heap.
Bicarbonate of Soda. Bicarbonate of soda can be easily distinguished from washing soda. It is a fine, white powder similar in appearance to the efflorescence on soda crystals. It does not contain any water of crystallization.
When bicarbonate of soda is heated, it does not melt, and, as far as its external appearance is concerned, it does not seem to undergo any change. If, however, suitable arrangements are made, water and carbon dioxide gas can be collected, and the sodium bicarbonate will be found to have lost 36·9 per cent. of its weight. The substance which remains is identical with that obtained by heating soda crystals, that is, anhydrous sodium carbonate. Sodium bicarbonate is, therefore, a compound of sodium carbonate and carbonic acid.
The most familiar use of this compound is indicated by its common names “baking-soda” and “bread-soda.” It is mixed with dough or other similar material in order to keep this from settling down to a hard solid mass in baking. The way in which bicarbonate of soda prevents this will be readily understood when it is remembered that an ounce of this substance liberates more than 2,300 cu. in. of carbon dioxide when it is heated. When the bicarbonate of soda is well mixed with the ingredients of the cake or loaf and disseminated throughout the mass, each particle will furnish (let us say) its bubble of gas. Since these cannot escape, a honey-combed structure is produced.
Fig. 14. THE SOLVAY PROCESS
Baking powder is a mixture of bicarbonate of soda and ground rice; the latter substance is merely a solid diluent.