In order to prevent leakage in the valves or taps attaehed to the vacuum pan, caused by the action of the acid in the substance of which they are composed, the body of the tap or valve should be of cast iron lined with lead, the valve free and disc, gland of iudiarubber, and the spindle must be secured from the action of the acid by a c,overing of lead.
Fig. 168 shows a sectional elevation of the apparatus employed ; the vacuum pan a is formed of a cast-iron case b, and a leaden lining c ; d is a manhole ; e is a leaden steam coil ; f is a valve for the admiasion "of st,eam to a ; and g a valve for discharging the contents of the vacuum pau.
This valve g is composed of a spindle h, carrying the leaden piece 1, which is covered with india-ruhber k; I is an india-rubber disc or diaphragm ; m is a nut and washer, and n is a screw spindle, which is fitted with a handle by which the valve is worked ; o is a pipe leading from the valve g ; pp are pipes for exhausting the space between the cast-iron case and the lining c; q q is the passage to the overflow receiver which contains the carbonated or alkaline solution at the bottom, through which the vapour from the vacuum pan is conducted by the pipe q, which terminates in the rose r ; r' a supply tap and pipe; r2 and r2 are dmw-off taps ; s is the perforated priming plate ; and s', s2 aro gauge glasses ; t, the upper part of the overflow vessel, may be called a safety vessel, and is connected hy a passage u, with a second safety vessel w ; r is a valve for cutting off communica tion between the pan, &c., and the condenser, and y is a pipe which leads to the condenser and air-pump. The valve g is found to be well adapted to all parts of the apparatus where such is necessary.
The first crop of crystals must always' be re-dissolved and freed from the colouring matter that is always present in some degreo. This is accomplished by boiling them with animal charcoal, front .which the earthy phosphates and carbonates have been removed by treating it with hydrochlorio acid.
Firmin has devoted much attention to the filtration and decolorization of the resultant liquids. First, the solution is separated from the earthy sulphate by pas.sing it over an exhausted surface, provided with numerous perforations, while water, steam, or both, are forced through the sulphate, so as to insure the removal of all the acid. For this purpose a vessel fitted with a per forated cover, over which an endless cloth is made to pass, is placed in connection with an air-pump, and upon this cloth the earthy sulphate, mixed with the acid solution, is gradually placed and the vessel exhausted by mcans of the pump. The cloth is caused to move slowly along over the per forated cover, by means of rollers, placed at each end during its passage. A finely divided shower of water is made to fall on to the sulphate and is drawn through it hy the pump, or a second method of causing the aulphate and acid to pass over an exhausted surface is shown in Fig. 169.
An iron cylinder lined with lead a is connected by a pipe 1), with a receiver that communicates with an air-pump and is fitted with a gauge to show how much liquor is in the receiver, and with a tap to empty it, which is of lead. Inside a placed a perforated leaden or copper cylinder d, which is fixed at each end by the rings c c. Another perforattd copper cylinder e, having a worm attached to it revolves in d, the ends being carried through the plates f where they are fixed in stuffing boxes'. This cylinder e is hollow and at the end .t, taps' are inserted for the regulated admission of air to the cylinder c, which air is allowed to pass as far as the division g, which divides' the lower from the upper part of the cylinder at the end f'. A stationary chamber h is fixed, to which are attached taps communicating with a water cistern placed above, also taps for the admission of air or steam. The driving spindle i is brought through this chamber with a stuffing box and connected to the power with a slow motion ; k is the hopper which may be con nected with a vessel containing the salts to be operated upon, the latter being kept in a state of thick ei eatu ; and, finally, / is the exit valve.
In using this arrangement the sulphate and acid solutions are passed through the hopper k, where they fall on the screw c, which, revolving, carries them to the exit /, but in their passage water and steam are admitted at f', and air at f. The air-pump exhausting the cylinder a, and the nceiver causes the water and steam to pass through the sulphates during their passage through the cylinder to g, while the air which admitted at f effectually forces the remaining liquor from the earthy sulphates, which are delivered at 1 free from the acid solution. The two perforated cylinders aro about in. apart, and the sorew fits so as to clear the outer cylinder, which is covered with a Slit ring cloth. The solution of tartaric acid is then run into a vessel lined with le,ad, placed near the evapomting pan. The capacity of this vessel is known and a given quantity is run into the evaporating or vacuum pan at each charge. This evaporating pan is preferably of iron, having an iron dome or cover, well painted and lined with lead. In it is an opening, closed by a movable cover to enable the workman to remove the gypsum, which will be largely deposited upon the pipes, sides, and bottom of the pan, aad which, unless removed, would collect and stop the progress of evaporation. The pan is heated by means of coils of steam pipes inside and an iron jacket for steam or hot water outside ; in the bottom is a pipe for discharging the contents into a suitable receiver, which pipe is closed by a top or valve. From the top of the evaporating pan a pipe is carried to a receiver of iron lined with lead, which will retain any of the liquor which may boil over. This vessel must be fitted with a gauge to show how much liquor has been passed into it, and with a tap at the bottom to draw it off. From the top of this vessel a pipe is conducted to the condenser, where the steam meets a stream of injected water, and thence to an air-pump for exhausting the vessels. The solution being sufficiently concentrated is run into the receiver, where it is kept warm, until any gypsum which may be carried from the pan with the liquor is deposited. The degree of concentration of the solution may be ascertained by examining a small quantity by means of the sampler, a copper bath containing about fitted near the bottom of the evapo rating pan, having a communication with the vacuum space above and a tap for emptying it. The liquor is then drawn from the receiver into a suitable vessel, where it is kept in agitation till it. granulatea and cools. It is then placed upon a vessel connected with the air-pumps, having the top covered with perforated aheet lead over which is placed a cloth through which the liquor from the salt is drawn by the action of the pump into the vessel, which may be large enough to retain it, or may be connected with an air-tight receiver, or, in place of this vessel, a cylinder, similar to that described for separating the earthy sulphates from the acid solutions, may be used, admitting air and finely divided water. Or, the concen trated solutions, when granulated and cool, are placed in a centrifugal machine, in a similar manner to that in which the crystals of sugar are frequently separated from the mother liquor. The salts obtained are, if necessary, dissolved, filtered through properly prepared animal charcoal, and again sub jected to evaporation, as before, and the colour less concentrated solution is either treated as before described, or drawn into leads to crystallize, as is now practised. The mother liquor from the first evaporation, consisting of sulphuric and tartaric acids, is treated with tartrate of lime to remove a portion of the sulphuric acid, and after filtration is subjected to evaporation as Firmin, also a large manufacturer, has improved upon the preceding method of fil tering, and forces the liquids through layer of finely divided, insoluble matters by means of a vacuum under the filter bed, and allowing the pressure of the air to exert itself on the surface of the liquids, or pressure may be obtained by closing the filter at the top and forcing the liquid into the vessel by a pump.