sulphate of soda (salt-cake) with about the same weight of crushed limestone and half its weight of coal, until the materials are fluxed and a reaction has taken place, the principal phase of which is expressed by the equation Na2SO4 + CaCO3 + 2C = 2CO2 + Na2CO3 + CaS. A number of secondary reactions, however, occur, owing partly to the excess of calcium carbonate and coal and partly to the impurities present, so that the solid product of the process, which is called “black-ash,” has a somewhat complicated composition. Its principal constituents are always sodium carbonate and calcium sulphide, which are separated by the action of water, the former being soluble and the latter insoluble.
The furnace in which the reaction takes place is shown in fig. 6 in a sectional plan. It is called a “black-ash” furnace, and belongs to the class of reverberatory furnaces. A large fire-grate (ab), having a cave (c) to facilitate stoking and stepped back at (d), is bounded on one side by a fire-bridge (e); on the other side of this, separated by an air-channel (g), there is first the proper fluxing bed (h), and behind this the “back-bed” (i) for pre-heating the charge. The flame issuing from the furnace by (o) is always further utilized for boiling down the liquors obtained in a later stage, either in a pan (p) fired from the top and supported on pillars (qq) as shown in the drawing, or in pans heated from below. The charge of salt-cake (generally 3 cwt.), limestone and coal is roughly mixed and put upon the back-bed; when the front-bed has become empty it is drawn forward and exposed to the full heat of the fire, with frequent stirring. After about three-quarters of an hour the substances are so far fluxed or softened that the reaction now sets in fully, as shown by the copious escape of gas. This is at first colourless carbon dioxide, but later on inflammable gases come out of the mass, which at this stage has turned into a thicker, pasty condition, showing that the end of the reaction is near. The inflammable gas is carbon monoxide, which, however, does not burn with its proper purple flame, but with a flame tinged bright yellow by the sodium present. This carbon monoxide is formed by the action of coal on the lime formed at this stage from the original limestone. When the “candles” of carbon monoxide appear, the pasty mass is quickly drawn out of the furnace into iron “bogies,” where it solidifies into a grey, porous mass, the “black-ash.” Care must be taken to heat it no longer than necessary, as it otherwise turns red and yields bad soda.
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| Fig. 7.—Revolving Black-ash Furnace. (Elevation.) |
The hand-wrought black-ash furnace has been mostly superseded in the large factories by the revolving black-ash furnace, shown in fig. 7. These furnaces possess a large cylindrical shell (e), lined with fire-bricks, and made to revolve round its horizontal axis by means of a toothed wheel fixed on its exterior; (ff) are tire-seats holding tires (gg), which work in friction rollers (h). The flame of a fixed fireplace (a) enters through an “eye” (b) in the centre of the front end of the cylinder and issues in the centre of the back end, first into a large dust-chamber (m), and then over or under boiling-down pans (p). These mechanical furnaces do the work of from four to ten ordinary furnaces according to their size. with comparatively very little expense for labour, but they must be very carefully managed and the black-ash from them is more difficult to lixiviate than that from hand-wrought furnaces, because it is less porous. The lixiviation of the black-ash requires great care, as the calcium sulphide is liable to be changed into soluble calcium compounds, which immediately react with sodium carbonate and destroy a corresponding quantity of the latter, rendering the soda weaker and impure. This change of the calcium sulphide may be brought about either by the oxidizing action of the air or by “hydrolysis,” produced by prolonged contact with hot water, the use of which, on the other hand, cannot be avoided in order to extract the sodium carbonate itself. The apparatus which has been found most suitable for the purpose was devised by Professor H. Buff of Giessen, and first practically carried out by Charles Dunlop at St Rollox. It consists of a number of tanks or “vats,” placed at the same level and connected by pipes which reach nearly to the bottom of one tank and open out at the top into the next tank. The vats are also provided with false bottoms, outlet cocks, steam pipes and so forth. Tepid water is run in at one end of the series, where nearly exhausted black-ash is present; the weak liquor takes up more soda from the intermediate tanks and at last gets up to full strength in the last tank, charged with fresh black-ash and kept at a higher temperature, viz. 60° C. When the first tank has been quite exhausted, the water is turned on to the next, the first tank is emptied by discharging the “alkali-waste,” and is filled with fresh black-ash, whereupon it becomes the last of the series. In spite of all precautions a certain quantity of impurities is always formed, but this should be kept down as much as possible by strictly watching the temperature in the vats and by taking care that the black-ash in the wet state is never exposed to the air. The unavoidable contamination with muddy particles of vat-waste is removed by allowing the vat-liquor to rest for some hours in a separate tank and settling out the mud.
The clear vat-liquor, if allowed to cool down to ordinary temperature, would separate out part of the sodium carbonate in the shape of decahydrated crystals. As these do not come out sufficiently pure, they would not be marketable and therefore they are not allowed to be formed, but the liquid, while still hot, is either run into the boiling-down pans, or submitted to one of the purifying operations to be described below. If it is boiled down without further purification, the resulting soda-ash is not of the first quality, but it is sufficiently pure for many purposes. The boiling down is most economically performed by means of large iron pans covered with a brick arch and heated from the top by the waste flame issuing from the black-ash furnaces (see figs. 6 and 7). It is continued until the contents of the pan have been converted into a thick paste of small crystals of monohydrated sodium carbonate, permeated by a mother-liquor which is removed by draining on perforated plates or by a centrifugal machine, and is always returned to the pans. The drained crystals are dried and heated to redness in a reverberatory furnace; when “finished,” the mass is of an impure white or light yellow colour and is sold as ordinary “soda-ash.” It is not easy to make it stronger than 92% of sodium carbonate, which is technically expressed as “52 degrees of available soda” (see next page). If purer and stronger soda-ash is wanted, the boiling down must be carried out in pans fired from below, and the
