  Caustic and Mild. There are two classes of alkalis distinguished by the terms caustic and mild. If a piece of all-wool material is boiled with a solution of caustic soda or potash, it dissolves completely, giving a yellow solution. Mild alkali will not dissolve flannel, though it may have some slight chemical action causing shrinkage. Partly for this reason, and partly because commercial washing soda often contains a little caustic soda, woollen garments must not be boiled or even washed in hot soda water. The disintegrating action of the caustic alkalis is also illustrated by the use of caustic soda in the preparation of wood pulp for paper making. Tree trunks are first torn up and shredded by machinery; but notwithstanding the power of modern machinery, the fibre is not nearly fine enough for the purpose until it has been “beaten” with a solution of caustic soda, whereby the pulp is brought to a smooth and uniform consistency like that of thin cream. Mild Soda and Potash. Until the middle of the eighteenth century, it was thought that the soluble matter extracted from the ashes of all plants was the same. In 1752 it was shown that the substance obtained in this way from plants which grew in or near the sea differed from that from land vegetation by producing a golden yellow colour when introduced into the non-luminous flame of a spirit lamp, while that from land plants gave to the flame a pale lilac tinge. The former substance is now known as mild soda, and the latter as mild potash. At this point it is well to make it clear to the reader that there are two bodies commonly called soda, and two called potash. One of each pair is caustic and one mild. By a simple chemical test it is easy to distinguish a mild from a caustic alkali. When a little dilute acid is added to the former, there is a vigorous effervescence caused by the escape of carbon dioxide, but no gas is given off when a caustic alkali is treated in the same way. The liberation of carbon dioxide on the addition of acids shows that the mild alkalis are carbonates. Washing Soda is so well known, that very little description of its external characteristics is necessary. It is a crystalline substance, easily soluble in water. The crystals, when freshly prepared, are semi-transparent; but after exposure to air for some time, they are found to lose their transparency and to become coated with an opaque white solid which crumbles easily. This change in appearance is accompanied by a loss in weight. Crystals of soda melt very easily on the application of heat and, on continued heating, the liquid seems to boil. When this operation is carried out in a vessel attached to a condenser, the vapour that is given off from the melted soda condenses to a clear colourless liquid which, on examination, proves to be water. When no more water collects in the receiver, the vessel contains a dry, white solid, which by any chemical test that may be applied is shown to be the same as washing soda, but it contains no water of crystallization and has a different crystalline form. This substance is anhydrous sodium carbonate, or soda ash as it is called in commerce. When soda ash is mixed with water, it combines with about twice its own weight of that liquid, forming soda crystals again. Washing soda, then, contains nearly two-thirds of its weight of water. Some of this water is given off spontaneously when the soda is exposed to air; the water may even be said to evaporate. This accounts for the loss of weight observed and also for the formation of the white layer of partially dehydrated soda over the surface of the crystal. The property of losing water in this way is common to most crystals containing a high percentage of water of crystallization. The phenomenon is known as “efflorescence.” It may here be observed that crystals of washing soda which have become coated over in this way contain relatively more soda than those which are transparent. Natural Soda. In Egypt, Thibet, and Utah, there are tracts of country where the soil is so impregnated with soda that the land is desert. The separation of the soda from the earth is a simple operation, for it is only necessary to agitate the soil with water and, after the insoluble matter has settled down, to evaporate the clear solution until the soda crystallizes out. In addition to alkali deserts, there are also alkali lakes. Those in Egypt are small, nevertheless, about 30,000 tons of soda per annum are exported from Alexandria. Owens Lake in California is said to contain sufficient soda to supply the needs of North America; while in the East African Protectorate, beneath the shallow waters of Lake Magadi (discovered in 1910), there is a deposit of soda estimated at 200,000,000 tons. The Leblanc Process. At the present time, the greater part of the world’s supply of soda is made from common salt by two processes. The older of these, which is known as the Leblanc process, was introduced in France towards the end of the eighteenth century. In those days soda was very dear, for the main supply came from the ashes of seaweeds; wherefore the French Academy of Sciences, in 1775, offered a prize for the most suitable method of converting salt into soda on a manufacturing scale. The prize was won by Nicholas Leblanc, who in 1791 started the first soda factory near Paris. These were the days of the French Revolution; the “ComitÉ de SÛretÉ GÉnÉral” abolished monopolies and ordered citizen Leblanc to publish the details of his process. Fig. 12. SALT CAKE FURNACE Fig. 12. SALT CAKE FURNACE The first alkali works were established in Great Britain in 1814. The total amount of soda now made in this country every year is about 1,000,000 tons, of which nearly one-half is still made by the Leblanc process. Salt Cake. The first stage of the Leblanc process consists in mixing a charge of salt weighing some hundredweights with the requisite amount of “chamber” sulphuric acid. The operation is carried out in a circular cast-iron pan (D, Fig. 12) about 9 ft. in diameter and 2 ft. deep. The pan is covered over with a dome of brickwork, leaving a central flue (E) for the escape of hydrochloric acid gas which is produced. At first, the reaction takes place without the application of heat, but towards the end the mass is heated for about one hour. The contents of the pan are then raked out on to the hearth of a reverberatory furnace (a, b) and more strongly heated. More hydrochloric acid gas is given off, and the reaction is completed. The solid product which remains is impure Glauber’s salt (sodium sulphate), and is known in the trade as “salt cake.” Black Ash. In the second stage of the Leblanc process, salt cake is converted into black ash. The salt cake is crushed and mixed with an equal weight of powdered limestone or chalk and half its weight of coal dust. This mixture is introduced into a reverberatory furnace (Fig. 13) by the hopper K, and heated to about 1000° C. by flames and hot gases from a fire at a. During this operation, the mass is kept well mixed, and after some time it is transferred to h where the temperature is higher. The mixture then becomes semi-fluid and carbon monoxide gas is given off. Fig. 13. BLACK ASH FURNACE Fig. 13. BLACK ASH FURNACE The formation of carbon monoxide within the semi-solid mass renders it porous. This is an advantage, because it greatly facilitates the subsequent operation of dissolving out the soluble sodium carbonate. The appearance of the flames of carbon monoxide at the surface of the black ash indicates the end of the process. The product is then worked up into balls and removed from the furnace. The chemical changes which take place in making black ash are probably as follows: Carbon (coal dust) removes oxygen from sodium sulphate, which is thus changed to sodium sulphide. This substance then reacts with the limestone (calcium carbonate), forming sodium carbonate (soda) and calcium sulphide. Extraction of Soda. It now only remains to dissolve out the soda from the insoluble impurities with which it is mixed in the black ash. It is evident that the smaller the amount of water used for this purpose the better, because the water has subsequently to be got rid of by evaporation. The process of extraction is, therefore, carried out systematically. The black ash is treated with water in a series of tanks which are fitted with perforated false bottoms. The soda solution, which is heavier than water, tends to sink to the bottom and, after passing through the perforations, is carried away by a pipe to the second tank, and so on throughout the series. The fresh water is brought first into contact with the black ash from which nearly all the soda has been extracted. The method of finishing off the black ash liquor differs The clarified soda liquor is then evaporated until crystals of soda separate out. The first part of this process is carried out in large shallow pans (P. Fig. 13), using the waste heat of the black ash furnace, and finally in vats containing steam-heated coils. As the crystals separate out, they are removed, drained, and dried. 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 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. The Solvay Process. Soda ash is one of the principal forms of mild alkali used in commerce. Large quantities of this substance are made by heating bicarbonate of soda. We shall now consider another alkali process in which this substance is the primary product. For the greater part of the first century of its existence, the Leblanc soda process had no rival, although another method, known as the ammonia-soda process, was patented as early as 1838. In this case, however, as in many others, expectations based on the experiments carried out in the laboratory were not realized when the method came to be tried under manufacturing conditions. It was not until 1872 that Ernest Solvay, a Belgian chemist, had so far solved the difficulties, that a new start could be made. In that year, about 3,000 tons of soda were produced by the ammonia-soda or Solvay process, as it has now come to be known. Since then, however, the quantity produced annually has been steadily increasing, until at the present time it amounts to more than half of the world’s supply. The Solvay process is very simple in theory. Purified brine is saturated first with ammonia gas and then with carbon dioxide. Water, ammonia, and carbon dioxide combine, forming ammonium bicarbonate, which reacts with salt (sodium chloride), producing sodium bicarbonate and ammonium chloride. The principal reaction is carried out in a tower (Fig. 14 (1), a, a) from 50 to 65 ft. in height and about The perforated plates are necessary for the proper distribution of carbon dioxide through the brine. They are, however, a source of trouble, because the holes quickly become blocked up with sodium bicarbonate, and every ten days or so it is necessary to empty the tower and clean it out with steam or boiling water. Recovery of Ammonia. The production of 1 ton of soda ash by the Solvay process involves the use of a quantity of ammonia which costs about eight times as much as the price realized by selling the soda. It is evident that the success of the process as a commercial venture depends largely on the completeness with which the ammonia can be recovered. During the process, ammonia is converted into ammonium chloride, which remains dissolved in the residual liquor. From this ammonia gas is set free by adding quicklime and by blowing steam through the mixture. It is now claimed that 99 per cent. of the ammonia used in one operation is recovered. Soda Ash. The bicarbonate of soda produced by the Solvay process is moderately pure. For all ordinary purposes, it is only necessary to wash it with cold water to remove unchanged salt, and after drying, it is ready to be placed on the market if it is to be sold as bicarbonate. The greater part of the Solvay product, however, is converted into soda ash by the application of heat. If soda crystals are required, the soda ash is dissolved in water and crystallized. In many ways, the Solvay process compares very favourably with the older method. It is an advantage to start with brine, for that is the form in which salt is very often raised from the mines. The end product is relatively pure; moreover, it is quite free from caustic soda, which for some purposes for which soda ash is used is a great recommendation. There is no unpleasant smelling alkali waste. On the other hand, the efficiency of the Solvay process is not high, for only about one-third of the salt used is converted into soda. This would make the process impossible from the commercial point of view were it not for the cheapness of salt. The Leblanc process, too, has its advantages. In the next chapter we shall see that it is adaptable for the production of caustic as well as mild alkali. The chlorine which is recovered in the Leblanc process is a very valuable by-product. In the Solvay process, chlorine is lost, for hitherto no practicable method has been found for its recovery from calcium chloride. The position with regard to the future supply of alkali is very interesting. The competition between the Leblanc and the Solvay processes for supremacy in the market is very keen. At the same time, both processes are in some degree of danger of being supplanted by the newer electrical methods, which will be mentioned in the last chapter. The following table shows very clearly the rapid progress made by the Solvay process in ten years. The quantities are given in tonnes (1 tonne = 0·9842 ton).
Mild Potash. Potassium carbonate (mild potash) was formerly obtained from wood ashes. The clear aqueous extract was evaporated to dryness in iron pots, and the substance was on this account called potashes; later, potash. A whiter product was obtained by calcining the residue, and this was distinguished as pearl-ash. Chemically pure potassium carbonate was formerly obtained by igniting cream of tartar (potassium hydrogen tartrate) with an equal weight of nitre. It is for this reason that potassium carbonate is sometimes called “salt of tartar.” About the middle of last century, natural deposits of potassium chloride were discovered in Germany. The beds of rock salt near Stassfurt are covered over with a layer of other salts, and for many years these were removed and cast aside as “waste salts” (abraumsalze). When at a later date they were examined more carefully, they were found to contain valuable potassium compounds, notably the chloride. After that discovery, At the outbreak of war, the German supplies of potassium compounds ceased as far as the allied nations were concerned, and an older method of making potassium chloride from orthoclase or potash-felspar was revived. This involves the heating of the powdered mineral to a high temperature after mixing it with calcium chloride, lime, and a little fluorspar. The potassium chloride is then extracted from the fused mass with water. This method has been worked with great success in America, and it is claimed that potassium chloride can be made in that country at a cost which is lower than that formerly paid for the imported article. Mild potash and soda are so very similar in chemical properties that in most cases it is immaterial which compound is used. In all cases in which there is this choice, soda is employed, both because it is cheaper and because it is more economical, for 106 parts of soda ash are equivalent to 138 parts of potash. There are, however, some occasions when soda cannot be substituted, notably for the manufacture of hard glass and soft soap, and for the preparation of caustic potash, potassium dichromate, and other potassium salts. Potassium Bicarbonate. This resembles the corresponding sodium salt in nearly every respect. It is, however, much more readily soluble in water, so much so, that it is not possible to obtain this substance by the Solvay method. It is made from potassium carbonate by saturating a strong aqueous solution of that substance with carbon dioxide. | ||||||||||||||||||||||||||||||||||||||||||||||||||
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