MANUFACTURE OF SODIUM
CARBONATE The manufacture of alkali and chlorine by modern methods re quires complicated plant ; but the relatively few plants in opera tion are highly efficient and, eco nomically placed as regards car riage of raw materials and ship ping of products, and work on a very large scale. It is to be em phasized that the industry has been very considerably standardized in recent years; the old Le Blanc "black ash" process has been replaced by the Solvay "am monia-soda" process, the basis of which is the mutual decomposi tion, in water, of ammonium bicarbonate and sodium chloride (common salt).
Theraw materials required are coal (to sup ply power and heat), salt and limestone of good quality. Salt is the source of the sodium in the finished products. It occurs as a mineral, widely distributed over the globe, in subterranean beds of great thickness. For the purpose of alkali manufacture it is not mined, but extracted in the form of brine by pumping. This brine occurs naturally in many places and has been formed by the percolation of underground springs through the salt beds; when of a sufficient degree of saturation it is used direct. In other cases water has to be admitted artificially to the salt deposits and then, after saturation, pumped again as brine.
The limestone serves a double purpose ; it is the source of the carbon dioxide gas which when combined with the sodium of the salt produces sodium bicarbon ate; and it is also the source of the lime with which, as we shall see presently, the ammonia (from which the process takes its name) is recovered and returned to the cycle of operations. Only the better class of limestone is used for alkali. The stone is quarried and crushed; it is then burnt, intimately mixed with coke, in tall kilns to which a reg ulated quantity of air is admitted (see fig. 1). These kilns are peri odically charged at the top; and as the charge travels downwards the lime is extracted at the bot tom and the kiln gases contain ing the carbon dioxide, are drawn off at the top. Ammonia, which forms the key to the whole opera tion and is obtained principally from gas-works liquor (a residue from the distillation of coal), cannot properly be regarded as a raw material.
Theproduction of crude sodium bicarbonate is the central operation. The brine, usually of ter purification to remove undesirable magnesium and calcium compounds, is pumped into the top of tall vertical iron absorption towers. Simulta neously ammonia gas and some carbon dioxide are pumped in at the bottom. The thorough con tact of the falling brine and the rising gases is ensured by the in ternal partitioning of the vessel in such a way that the whole of the ammonia is absorbed (see fig. 2). This absorption gives rise to a considerable evolution of heat, so the ammoniated brine is passed through a series of coolers to reduce its temperature.
From the coolers the liquor is pumped to the top of a system of iron towers, subdivided inter nally, at the bottom of which carbon dioxide gas enters; this carbon dioxide is obtained partly from the limekilns as already de scribed and partly from a later stage of - the process in which sodium bicarbonate is heated to form the carbonate. The inter action of this carbon dioxide with the ammoniated brine produces a precipitate of sodium bicarbonate and at the same time disen gages a considerable quantity of heat which has to be counteracted by a system of coolers in the lower part of the towers; the reaction is reversible, and is greatly influenced by the temperature, so that the maintenance of the liquor at the correct temperature is of the first importance (see fig. 3).
The magma of bicarbonate and mother liquor is taken con tinuously from the towers and fed on to rotary vacuum filters (see fig. 4). Here the mother liquor is removed and the bicar bonate washed with a little water. The crude bicarbonate is then conveyed automatically to the so-called "finishing machines" for conversion to carbonate. These machines consist of hemi-cylindri cal iron vessels, with mechanical agitators, and are heated from below either with producer gas or by direct coal-firing. The crude bicarbonate is fed in at one end, and in its passage along the length of the machine loses carbon dioxide, water, and any ammonia These gases together with those given off in the preliminary steam treatment are cooled and then pass to the absorption system to undergo a further round of the process. Under modern methods the loss of ammonia is extremely small. The liquor leaking the bottom of the distiller contains calcium chloride and about one third of the salt that originally entered the process : some calcium chloride is occasionally extracted and sold, but in general it is a waste product. It also contains excess lime, grit and pebbles, the economical disposal of which is a problem still to be solved. Fig. 6 is a diagram that will serve to connect in the reader's mind the foregoing operations with some of those about to be described.
Part of the crude bicarbonate ob tained by the ammonia soda process is converted into "refined that it may contain. These gases are then returned to the towers. The product issuing from the finishing machines is sodium car bonate, and is usually referred to as "soda ash." The purity is very high, averaging 98% sodium carbonate, the remainder being mostly salt.
Almost every manufactured commodity has at some stage of its production made use of soda ash or its derivatives, the yearly world production of which has now reached some three and a half millions of tons. For example, glass of all kinds contains from io to 20% by weight of soda and accounts for roughly one quarter of the total consumption. There are indeed certain qualities of glass which do not use soda ash, but these are for special use and do not affect the general statement. All hard soap has been made primarily from animal or vegetable fats and caustic soda, the latter being derived directly from soda ash; this accounts for another quarter of the consumption. The textile industry accounts for one-sixth, employed in wool-scouring, cotton-scouring, calico printing, flax boiling and so on. The gen eral chemical industry takes about one-tenth, and paper one twentieth. The remainder is consumed in comparatively small quantities by a number of indus tries, a full list of which is as tonishing in its length and variety.
Fig. 5 gives this distribution, which is of course only approxi mate, in a graphic form.
Recovery of Ammonia.— Apart from the small amount carried forward with the crude bicarbonate, which is recovered separate, the bulk of the ammonia is contained in the mother liquor separated at the filters and is in the form of ammonium chloride.
This liquor is therefore distilled with lime. As, however, the liquor still contains some ammonia in combination with residual carbon dioxide it is first of all subjected to treatment with steam to eliminate these compounds.
The liquor is then fed, together with milk of lime produced by mixing lime from the kilns with water, into the top of the distiller, a tall iron tower with internal divisions : exhaust steam is ad mitted to the bottom, the temperature of the liquor is raised, and the ammonia distils off accompanied by some carbon dioxide.
bicarbonate of soda" which is largely used in the preparation of food and drugs. The trade in it is very steady, and does not vary appreciably from year to year. It is principally used as a source of carbon dioxide gas for the manufacture of artificial mineral waters. Another source of consumption is the baking indus try: baking powders contain sodium bicarbonate and a weak vegetable acid (citric, tartaric, etc.) which are prevented from reacting on one another when dry by a film or glaze of starch round the individual grains : on moistening in the process of dough making, these compounds react, carbon dioxide is evolved, and the dough "rises." The action of yeast is similar in that carbon dioxide is produced but dissimilar in that the production of the gas is due to organic fermentation. Bicarbonate of -soda is also used largely in medicine and in proprietary compounds for daily use. It is one of the principal ingredients of "blue" for laundry purposes, and a large amount is consumed in the manufacture of dry soap.
This is the form in which soda occurs in nature, mixed of course with more or less impurity. In composition it is intermediate between sodium carbonate and bicarbonate, and it contains some combined water :-- (Sesquicarbonate) It is manufactured directly from crude bicarbonate, the process of heating to drive off carbon dioxide not being carried so far as for the manufacture of soda ash. It has the advantage of being a mild alkali well suited for laundry detergent purposes and is sometimes known as "concentrated soda crystals." Soda Crystals (Washing Soda).—If soda ash be dissolved in water and the solution allowed to crystallize slowly, large trans parent crystals are produced of the composition
i oH2O. In this form the product is familiar to everyone as "washing soda." In practice the manufacture is not quite so simple. Soda ash is first dissolved in hot water. This solution frequently contains small amounts of iron and organic matter which are removed and the liquor is rendered colourless by the addition of small quantities of bleaching powder. The clear settled liquor is then transferred to large steel crystallizing pans; where it gradually cools and deposits the crystals. After a few days these are fished out, dried, graded for size and packed. Their principal use is in domestic and laundry work, in the manufacture of dyestuffs, varnishes and photographic materials, and in tanning.
Thisfamiliar product is manufactured by fusing topther soda ash and clean pure sand in a furnace. The latter is built of fireclay blocks with an arched roof of silica, and is heated internally by a direct flame produced by the combustion of producer gas or oil. The mixed charge of ash and sand is fed in at one end of the furnace and the molten glass is drawn off at intervals at the other. The glass may either be broken up by jets of water as it runs in a molten stream from the furnace, or it may be allowed to cool and be subsequently broken up by hand. It is then fed into a dissolver which may be either of a horizontal and rotary type, or stationary and vertical. In either case the glass is treated with a mixture of water and steam and goes into solution. The latter is then bleached, if neces sary, and clarified; and is then concentrated in evaporators of modern type. The resultant viscous fluid is the "water-glass" of commerce. A familiar but minor use is that of preserving eggs; the effect is produced by the closing of the pores of the egg shell by the silicate, which results in the exclusion of air and the prevention of putrefaction. It is an excellent adhesive for all purposes and particularly for machine work in box making. In the United States particularly this application has become very widespread. It is use with remarkable effect to harden and con solidate the surface of concrete : the effect is produced by the interaction of the silicate with the lime of the concrete whereby hard calcium silicate is formed. The silicate may be applied either during the process of concrete making or to the finished concrete surface. It often forms an ingredient of soap of inferior quality. Other applications include the lining of casks, ceramics, fireproofing of wood, preservation of wood against the attacks of insects, ink making, flotation of ores, paints, silk weighting, and so on. For all these various uses there is usually available a partic ular grade of silicate, dependent upon the ratio of silica to soda best suited to the purpose, the degree of viscosity required and the rate of drying.
