Soda Recovery.
FIG. 67.
FIG. 68.
The apparatus for accomplishing the evaporation varies with almost every mill. In some, it is very primitive and {181} crude, consisting perhaps of only a furnace for incinerating the residue, and over it a pan containing the liquor, the latter being heated and evaporated by the heat from the furnace. It is obvious that, with such an arrangement, a large quantity of heat must be wasted. To economise as much as possible of this waste heat, various plans have been suggested. That of Rœckner, of Newcastle, appears to be to a great extent efficacious. It consists practically of a series of shallow trays B (Fig. 67) placed in a brick chamber, alternated so as to allow the heated air from the furnace below to play upon the surface of each in succession, on its way to the chimney, with which the whole system is in connection. Above the chamber containing these trays, is a large tank C, containing a store of the liquor to be evaporated, placed there so as still further to economise the heat, and from which the liquor runs on to the trays. The furnace A is of the ordinary reverberatory kind; below it, and connected with it by a kind of damper, is a large chamber J, where the calcined residue from the furnace is put to cool, thus preventing any nuisance from the smell of the burning mass. The chamber is provided with a pipe L, through which the vapours pass into the furnace. Several pipes E from the furnace pass through the {182} tank, to assist in warming the liquor. The residue, when cold, is drawn through doors from the chamber below the furnace. Rœckner has devised an apparatus (Fig. 68), consisting of a small chamber containing a series of pipes A, through which a stream of cold water constantly runs, in connection with the flue from his evaporator, for the purpose of condensing volatile bodies, and thus preventing, to a certain extent, contamination of the surrounding air.
A very economical form of evaporator is that invented by Porion, a French distiller, and named after him. It is shown in sectional elevation and plan in Figs. 69 and 70. It is largely used on the Continent, and also in England and Scotland. It consists of a large chamber k, the floor of which is slightly inclined from the chimney shaft, and through which the waste heat from the furnace a passes.
FIG. 69.
FIG. 70.
The liquor to be evaporated is run in at the end nearest the chimney from the tank placed above the chamber c. A number of cast-iron fanners i, dip into the liquor and revolve rapidly, usually at the rate of about 300 revolutions per {183} minute, producing and filling the chamber with a very fine spray, thus presenting a very large evaporating surface.
Between the furnace and the evaporator are placed the chambers c and f. In c a number of brick walls d are so placed that the flames from the furnace are intercepted and broken up. The object of this is to give time for all the products of combustion to be thoroughly burnt up, which would not be the case without the “smell-consumer,” as these chambers are called. This part is an addition to the original evaporator, and was devised by Messrs. Menzies and Davis. The liquor after having been concentrated in the chamber k runs into a trough placed alongside the doors h and flows into one or other of the furnace beds b where it is still further concentrated, and the residue ignited by the flames from the fires a. The draught can be regulated by the damper g, and also by one placed near the shaft j. The doors e, in the smell-consuming chamber, are for the purpose of cleaning out. The fanners i are worked by a small steam engine, not shown in the drawing. Under properly regulated conditions very excellent results can be obtained with this evaporator. The temperature of the gases near the chimney should not be higher than about 85°. By running the fanners at a very high speed the temperature of the gases may be even further reduced, thus showing the completeness of the evaporation.
This form of evaporator is open to the objection that the whole of the sulphur in the coal employed for the furnaces, finds its way into the recovered soda. It combines with the alkali to form sulphite of soda, part of which is decomposed in the furnace with formation of sodium sulphate, sulphide, and other sulphur compounds. The same objection, of course, applies, though perhaps in a less degree, to all systems of evaporation in which the flame is in contact with the liquor to be evaporated.
The Porion evaporator can be erected at very small cost, and costs but little for maintenance. It is capable of producing 3⁄4 ton of recovered soda per ton of coal with liquors {184} of the usual strength. It has proved itself to be perhaps the most economical evaporator existing.
Some time ago there was erected in Lancashire an evaporator invented and patented by Mr. Alfred Chapman. It is shown in Figs. 71, 72, 73, and 74. The evaporation is effected at a low temperature in three vacuum pans E, and with the unusual result that the concentrated liquor gelatinises after leaving the third vacuum-pan, instead of taking the ordinary form of the concentrated products of other evaporators. It is said that this apparatus gives an excellent product, with great economy of labour and water, and with no drainage of foul liquor from the buildings. Observations extending over three months have proved that it evaporates 22 lb. of water from the liquor per lb. of coal used under the boiler. It is however very costly to erect.
FIG. 71.
FIG. 72.
FIG. 73.
FIG. 74.
The waste liquor is discharged into the tank A, whence it is pumped by the donkey-engine B, through the feed-heater C, {185} into the boiler D, which receives heat from the incinerating furnace H, and, in case of need, from an auxiliary furnace shown on the plan, under the feed-liquor-heater. The steam produced in D is taken to the first vacuum-pan at E, and having heated its contents, the products of evaporation pass over into the tubes of the second pan; this, in its turn, gives up its products of evaporation to the third, whence they go to the condenser of the vacuum-engine F. Thus the heat from the furnace H is used for incinerating the concentrated liquor {186} on its bed, for beating the feed-liquor in the feed-heater pipes, and for making steam out of the liquor itself in the boiler; this steam finally drives the donkey-pump and vacuum-engine, and causes the evaporation in the three vacuum-pans E. One advantage of this evaporator is the fact that the liquor is evaporated out of contact with the furnace gases.
Whatever be the method of evaporating or concentrating the liquor, the final treatment in the furnaces is much the same in every case. The furnaces shown in Figs. 69 and 70 may be taken to represent the ordinary form. The concentrated liquor is run on to either of the beds b, where the last portions of water are driven off by the heat from the fireplaces a, and the residual mass is ignited until all the organic matter contained in the liquors is carbonised and the soda is converted into carbonate of soda. This takes place in about 4 hours, according to the degree of concentration of the liquor as it is run into the furnaces. The running in of liquor should be done with great care, as explosions sometimes occur through the sudden liberation of steam on the liquor coming in contact with the hot beds. The charge should be allowed to remain in the furnace until it is thoroughly carbonised and all volatile matters have been driven off, otherwise a nuisance may be caused when the still burning mass is exposed to the air. Rœckner’s evaporator is provided with a special chamber into which the charge is drawn (J, Fig. 67).
The composition of the recovered soda varies with the nature of the liquors from which it has been obtained, and, as has been already pointed out, with the form of evaporator employed. It consists essentially of carbonate of soda, together with a certain amount of silicate of soda, if derived from liquors in which straw or esparto have been boiled, chloride of sodium, sulphate of soda, sulphite of soda, sulphide of sodium, and other sulphur compounds, the rest being made up of carbon and insoluble impurities. The amount of soda varies from 35 to 45 per cent. (Na2O). The following analysis {187} will give some idea of the composition of Recovered Soda:—
| [*]Sodium carbonate | 72·33 |
|---|---|
| Sodium hydrate | 1·93 |
| Sodium chloride | 8·30 |
| Sodium sulphate | 3·95 |
| Sodium sulphite | ·63 |
| Silica | 7·09 |
| Carbon | 4·70 |
| Oxide of iron and alumina | ·50 |
| Other constituents (by difference) | ·57 |
| 100·00 |
The whole of the soda present as sulphur compounds is not lost, as a large proportion of it is present as sodium sulphite, most of which is converted into caustic soda by the causticising process.
[*] A certain quantity of potash derived from the mineral constituents of esparto and straw is always present in recovered soda.
A certain amount of soda is carried forward, partly mechanically and partly volatilised, to the flue leading to the chimney. This accumulates, and may be from time to time removed in the form of fine dust. It contains, besides carbonate of soda, much sulphate and chloride. In two different samples examined by the authors, the amounts of soda (Na2O) present were 25·0 and 27·1 per cent.