SODA MANUFACTURE

The manufacture divides itself into three branches:—1. The conversion of sea salt, or chloride of sodium, into sulphate of soda. 2. The decomposition of this sulphate into crude soda, called black balls by the workmen. 3. The purification of these balls, either into a dry white soda-ash or into crystals. IMG:4147767755307473660_illo1151.png:Soda furnace IMG:4147767755307473660_illo1152a.png:Soda furnace 1. The preparation of the sulphate of soda. — Figs. 1033 , 1034 , 1035. represent the furnace for converting the muriate of soda into the sulphate. The furnace must be built interiorly of the most refractory fire-bricks, such as are used for gla houses, but of the ordinary brick size; except the bridges C , G , N , which should be formed of one ma , such as what is called a Welsh lump. A is the ash-pit; B , the grate; C , the first bridge, between the fire and the first calcining hearth, D , D ; F , F , is its roof; G , the second bridge, between the calcining hearth and the decomposing hearth I , I , I ; the roof of which is K , K . This hearth I , I , is lined with a lead square pan, 5 or 6 inches deep, sloped at the back opening, in fig. 1035. , marked M ′; which deficient part of the upright side is filled up with two bricks placed one over the other, as shown at m , m , fig. 1034. , and luted with clay, to confine the semi-liquid ma in the pan, I , I . Some manufacturers make this pan 8 inches deep, and line its bottom and sides with bricks or siliceous sandstone, to protect the lead from the corrosive action of the acid. There are others who consider this precaution troublesome, as the points of the pan which become leaky are thereby concealed. In the roof of the decomposing hearth, one or two syphon funnels R , of lead, are inserted when the charge of acid (sulphuric) is to be poured down upon the salt in I , I , to save the risk of any annoyance from the fumes of the muriatic acid. O , O , is a chimney filled with round flint nodules, which are kept continually moist by the trickling of a streamlet of water upon the topmost layer. The muriatic gas meeting this descending film of water upon so extensive a surface, becomes absorbed, and runs out below in a liquid form. When the acid is required in a somewhat concentrated state, this chimney should be made both high and capacious. Such a plan, moreover, is very valuable for abating the nuisance caused by the disengagement of the muriatic acid gas; which is otherwise apt to sterilize the surrounding vegetation. A fire being kindled in the grate B , figs. 1033. and 1034. , 3 cwt. of salt in powder are to be thrown by a shovel into the pan I , through the door M , fig. 1035. , or m , m , fig. 1034. Two hundred weight and a half of oil of vitriol, of specific gravity 1·844 having been diluted with from 25 to 30 per cent. of water, and well mixed, or 3 cwts. at 56° Baumé, are to be slowly poured in by the funnel, and diffused among the muriate of soda, by an occasional stir with an iron rake cased with sheet lead. Fumes of muriatic acid will now plentifully escape, and, pa ing up the condensing-shaft O , will flow down in the form of liquid spirit of salt, and escape by the stoneware stopcock P , into the pipe of a sunk cistern. The fire having been steadily kept up at a moderate degree, the chemical reaction will be tolerably complete in the course of two hours; but as this is relative to the nature of the fuel, and the draught of the furnace, no very precise rule in point of time can be laid down; but it is sufficient for this stage of the proce , when the fumes cease to be very dense and copious, as may be ascertained by opening the door M , and looking in, or by the appearance at the top of the shaft O . Over the door M ′, in the opposite side of the decomposing hearth, fig. 1035. , there must be an arch or hood terminating in a small chimney, 15 or 20 feet high, for the ascent of the muriatic vapours, when the charge is drawn or run out of the hearth, and allowed to fall into a square shallow iron tray, placed on the ground at the back of the furnace. For this discharge, the two bricks which serve as stoppers to that orifice, must be unluted and removed. As soon as that charge is taken out, (the fire being meanwhile checked by opening the door T , fig. 1034. , and shutting partially the ash-pit opening at A ,) a fresh charge must be introduced as above described. The nearly decomposed saline matter during the second charging of the hearth I , will have grown cool and concrete. It must be shovelled into the calcining hearth D , D , fig. 1033. , by the back door Q , fig. 1035. , where it will receive a higher degree of heat; and, by the expulsion of the remaining part of the muriatic acid, it will become a perfect sulphate of soda. It should be finally brought into a state of semi-fusion. When a sample of it, taken out on the end of the rake or trowel-shaped scraper, emits no fumes, the conversion is accomplished. From 3 cwts. of common salt, or muriate of soda, rather more than 3 1 ⁄ 2 cwts. of perfect sulphate should be obtained, quite free from metallic impurity. The next step is the conversion of the sulphate into a crude soda. IMG:4147767755307473660_illo1152b.png:Soda furnace One of the most improved soda furnaces is that, employed in a few factories, represented in figs. 1036 , 1037 , and 1038. In the section fig. 1037. , there are two hearths in one furnace, the one elevated above the level of the other by the thickne of a brick, or about 3 inches. A is the preparatory shelf, where the mixture to be decomposed is first laid in order to be thoroughly heated, so that when transferred to the lower or decomposing hearth B , it may not e entially chill it, and throw back the operation. C is the fire-bridge, and D is the grate. In the horizontal section, or ground plan, fig. 1038. , we see an opening in the front corresponding to each hearth. This is a door, as shown in the side view or elevation of the furnace, fig. 1036. ; and each door is shut by an iron square frame filled with a fire-tile or bricks, and suspended by a chain over a pulley fixed in any convenient place. See Pitcoal, coking of , p. 1041. The workman, on pushing up the door lightly, makes it rise, because there is a counterweight at the other end of each chain, which balances the weight of the frame and bricks. In the ground plan, only one smoke-flue is shown; and this construction is preferred by many manufacturers; but others choose to have two flues, one from each shoulder, as at a , b ; which two flues afterwards unite in one vertical chimney, from 25 to 40 feet high; because the draught of a soda-furnace must be very sharp. Having sufficiently explained the construction of this improved furnace, I shall now proceed to describe the mode of making soda with it. The materials with which the sulphate is decomposed into a rough carbonate of soda, are chalk or ground limestone, and ground coal or charcoal. The proportions in which these three substances are mixed, influence in a remarkable degree the succe of the decomposing proce . I have known a false proportion introduced, and persevered in, at a factory, with the most prejudicial effect to the product; the soda-ash produced, being in a small quantity relatively to the sulphate employed, and being much charged with sulphur. After very numerous trials which I have made on the great scale, and many inquiries at the most succe ful soda-works, both in this country and abroad, I am warranted to offer the following proportions as the most profitable :— Sulphate of soda, 100 parts: carbonate of lime (chalk or limestone), from 110 to 120 parts; if pure, 110; if a little impure or damp, 120: pit coal, 50 parts. These materials must be separately ground by an edge-stone mill, and sifted into a tolerably fine powder. They must be then very carefully mixed. Attention to these particulars is of no little importance to the succe of the soda proce . One hundred parts or pounds of sulphate of soda are equivalent to 75 parts of carbonate, and when skilfully decomposed, will generally yield fully 70 pounds. A charge for the decomposing furnace with the preparatory shelf should not exceed 200 lbs., or perhaps 180; therefore if 75 pounds of ground sulphate of soda, with 80 pounds of chalk or limestone (ground), and 37 pounds of ground coal; be well mixed, they will constitute one charge. This charge must be shovelled in upon the hearth A , or shelf of preparation, ( fig. 1037. ); and whenever it has become hot (the furnace having been previously brought to bright ignition), it is to be transferred to the decomposing hearth or laboratory B , by an iron tool, shaped exactly like an oar, called the spreader. This tool has the flattened part from 2 to 3 feet long, and the round part, for laying hold of and working by, from 6 to 7 feet long. Two other tools are used; one, a rake, bent down like a garden hoe at the end; and another, a small shovel, consisting of a long iron rod terminated with a piece of iron plate, about 6 inches long, 4 broad, sharpened and tipped with steel, for cleaning the bottom of the hearth from adhering cakes or crusts. Whenever the charge is shoved by the sliding motion of the oar down upon the working hearth, a fresh charge should be thrown into the preparation shelf, and evenly spread over its surface. The hot and partially carbonized charge being also evenly spread upon the hearth B , is to be left untouched for about ten minutes, during which time it becomes ignited, and begins to fuse upon the surface. A view may be taken of it through a peep-hole in the door, which should be shut immediately, in order to prevent the reduction of the temperature. When the ma is seen to be in a state of incipient fusion, the workman takes the oar and turns it over breadth by breadth in regular layers, till he has reversed the position of the whole ma , placing on the surface the particles which were formerly in contact with the hearth. Having done this, he immediately shuts the door, and lets the whole get another decomposing heat. After five or six minutes, jets of flame begin to i ue from various parts of the pasty-consistenced ma . Now is the time to incorporate the materials together, turning and spreading by the oar, gathering them together by the rake, and then distributing them on the reverse part of the hearth; that is, the oar should transfer to the part next the fire-bridge the portion of the ma lying next the shelf, and vice versâ . The dexterous management of this transposition characterizes a good soda-furnacer. A little practice and instruction will render this operation easy to a robust clever workman. After this transposition, incorporation, and spreading, the door may be shut again for a few minutes, to raise the heat for the finishing off. Lastly, the rake must be dexterously employed to mix, shift, spread, and incorporate. The jets, called candles , are very numerous, and bright at first; and whenever they begin to fade, the ma must be raked out into cast-iron moulds, placed under the door of the laboratory to receive the ignited paste. One batch being thus worked off, the other, which has lain undisturbed on the shelf, is to be shoved down from A to B , and spread equally upon it, in order to be treated as above described. A third batch is then to be placed on the shelf. The article thus obtained should contain at least 22 per cent. of real soda, equivalent to 37 per cent. of dry carbonate, or to 100 of crystals. A skilful workman can turn out a batch in from three quarters of an hour to an hour, producing a perfect carbonate, which yields on solution an almost colourle liquid, nearly destitute of sulphur, and containing hardly any decomposed sulphate. In some soda-works, where the decomposing furnace is very large, and is charged with a ton of materials at a time, it takes two men to work it, and from five to six hours to complete a batch. Having superintended the operation of the above-described small furnace, and examined its products, I feel warranted to recommend its adoption. The following materials and products show the average state of this soda proce :—