File:Reverberatory furnace for manufacture of sodium carbonate.png

English: Reverberatory furnace transforming Sodium sulfate into Sodium carbonate, in two steps.

The next step in the manufacture is the decomposition of the sulphate of soda into sulphide of sodium, and its subsequent conversion into carbonate of soda. This is effected in the following manner: — The dry sulphate of soda, obtained by the process above described, is mixed with small coal and chalk, or limestone, in about the following proportions; sulphate of soda 3 parts, chalk 31 parts, and coal 2 parts. It is necessary that these materials should be first separately ground, and sifted into a tolerably fine powder, and then carefully mixed, as a great deal depends on the attention to these points. The mixture is then subjected to heat in a reverberatory furnace, figs. €.09, 610, 611,

In the section fig. 610, there are two hearths in one furnace, the one elevated above the level of the other by the thickness of a brick, or about three 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 essentially 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. 611, we see an opening in the fiont corresponding to each hearth. This is a door, as shown in the side view or elevation of the furnace, fig. 609; 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. The workman, on pushing up the door lightly, makes it rise, because there is a counter weight at the other end of each chain, which balances the weight of the fi ame and bricks. In the ground plan, only one smoke flue is shown ; and this constructon is preferred by many manufacturers; but others choose to have two flues, one fiom each shoulder, as at a, b; which two flues afterward 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, we shall now piocctd to desciibe the mode of making soda with it.

The quantity of this mixture required for a charge depends, of course, on the size of the furnace. This charge must be shovelled in upon the hearth a, or shelf of prepaiation, (fig 610) 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 las 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 with a garden hoe at the end ; and another, a small shovel, consisting of a long iron rod terminated like 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, andvevenly spread over the surface.

The hot and partially carbonized charge being also evenly spread tipon the hearth B, is to be left untouched for aliout 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, whith should be shut immediately, in order to prevent the reduction of the temperature. When the mass is seen to lie in a state of incipient fusion, the workman takes the oar and turns it over breadth by breadth in regular layers, till he has reverseil the position of the whole mass, 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 fire or six minutes, jets of flame begin to issue from various parts of the pasty-consistenced mass. 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 mass lying next the shelf, and vice versa. 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 mass 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 product thus obtained is called " black balls," which of course vary in their composition. The following is the composition, according to Eichardson, of the Newcastle " black balls " from the balling furnaces :

Carbonate of soda 9"89, hydrate of soda 25'64, sidphide of calciun 35"57, carbonate of lime 15.67, sulphate of soda 3'64, chloride of sodium 0'60, sulphide of iron 122, silicate of magniesia 0"88, carbon 4'28, sand 0'44, and water 2'17.

The principal changes which take place in this process may be represented by the following equations :

NaSO4 + 4C = NaS + 4C0

then —

NaS + CaCO3 = NaCO3 + CaS

In the first place, the sulphate of soda is deoxidized by the coal, with the formation of sulphide of sodium and carbonic oxide, which latter takes fire and forms the candles above mentioned ; in the next place, the sulphide of sodium and carbonate of lime (chalk) decompose each other, forming carbonate of soda and sulphide of calcium ; and from the fact of some of the chalk being converted into caustic hme by the heat of the furnace, there is also formed by it some caustic soda ; the sulphide of calcium itself is only sparingly soluble in water, but is rendered still less so by the excess of lime which is present, forming with it an oxysulphide, which is much less soluble than the sulphide of calcium alone.