Fig. 170 is a longitudinal section of the filter. It consists of a wooden vessel a, from the bottom of which a pipe 1 passes to a vessel c, and a vacuum is maintained in this vessel by an air—pump in connection with the pipe d, which enters the vessel. The vessel c should be made of such a capacity as to contain all the liquid which it is required to filter at one time ; it is fitted with a glass tube gauge 0, to indicate the quantity of liquid in it. At a short distance above the bottom of the vessel a, a layer of bricks e is laid, supported on other bricks f; the joints between the bricks e are left open, no mortar or cement being used in laying them ; on the bricks is placed a layer of pebbles g, over this a layer of fine sand, and on the sand a grating of wood h, the intervals between the bars of the grating being also filled with sand which just covers the bars. The liquid to be filtered is run into the vessel ,/, and a vacuum being formed in the vessel c, it speedily passes through the layer of sand. The solid matter separated may be removed by shovels from the filter, which is prepared to be again used by spreading a thin layer of fresh sand on the wooden grating, or the precipitate may, if desired, be stirred up with a small quantity of water by means of an agitator with which the vessel a may be furnished.
Fig. 171 shows a vertical section of a similar filter, constructed to work by artificial pressure at the top, as already mentioned, in place of by vacuum below ; this filter is, as will be seen, fitted with an agitator. The filt. rs nro prepared for use by washing them first with water, then with dilute hydioehloric acid, and again with water. We would remark that although we have spoken of the use of and to form the filter bed, other finely divided and insoluble matters may be employed, se, for example, the sulphate of baryte, which indeed, when it can be readily obtained in a suitable form, is to be preferred to sand, as it is difficult to obtain the latter free from alumina, which acts injuriously on solutions of the acids. Raw cotton fibre may also be conveniently employed when filtering concentrated solutions which might be acted on injuriously by sand.
When a precipitate of tartrate or citrate of lime, or other insoluble tartrate or citrate, has been ob tained by any process ordinarily pruptised, and which requires to be washed to free it from impurities, it is mixed with water and run into the vessel a of the filter. The pre ,cipitato is allowed to subside, and the supernatant water is run off, when the remaining water ie made to pass through tho filter, either by vacuum or pressure. By means of nn agitator, it is again mixed with water and the operation is repeated, and so on till the precipitate is sufficiently purified ; it is then again mixed up with a small quantity of water, and run into another vessel to bo decomposed with sulphuric acid to sot the tartaric or citric acid free in the usual manner. This
process of decomposition produces a precipitate of sulphate of lime (if lime he the base with which the citric or tartaric acid was previen,ly combined), and this is also sepa rated by filtration through a filter such as already described. This filtered liquid is, by preference, evaporated and granulated by keep ing it agitated while it 000ls.
More recently, How has pro posed further improvements in the manufacture, by submitting the crude argol or tartar to preparations capable of purifying them to a greater extent than is usually done, at the same time using less water in washing, thereby saving loss in solution, and, which is most important, rendering the acid liquors produced so pure and of such a strength that great cost of evaporation and repeated crystallization may be avoided.
In treating crude argol or tartar in the preparation of tartrate of lime, or other base for the manu facture of tartaric acid, to the mixture of whiting or other base that may be used to neutralize the free acid existing in the argol or tartar, as much ferrocyanide of potassium is added as is found neces sary to precipitate all the iron existing therein as prussian blue, the tartrate of potash being decom posed with a salt of lime, either sulphate, nitrate, or muriate (according to the current value of the potash salts), formed in the usual manner, and a ith the usual apparatus for mingling and treating the mixture. The ferrocyanide of potassium may be used to precipitate the iron from the solution at any stage of the process, and whether such solution be strong or weak. The tartrate compound being formed as before mentioned, the precipitate is first allowed to subside, and as much as possible of the clear liquid is drawn off into suitable reservoirs. Then, after well mixing np the precipitate that remains, with or without the addition of cold water as may be necessary, the whole contents of the "generator" are run into a vessel, which we now proceed to describe.
Figs. 172 and 173 show the apparatus in perspective and in sectional elevation. a is a wooden cistern lined with lead (called the " decomposing tank") of any convenient size, the bottom of which is slightly rounded so as to facilitate the action of the stirrers h, as shown in the section. The stirrers consist of splints of wood fixed into a square spindle occupying one-third more or less of the whole N 2 length, the spindle when in action being turned by a band on a pulley at the end of it, and actuated by any suitable motive power. c is a false bottom made of wood covered with flannel or any other filtering material ; d is an outlet pipe for the filtered liquor, and e is an outlet pipe for the air contained in the chamber. f is a leaden pump to produce a partial vacuum when necessary, and g is a cover to prevent splashing from the stirrers.