The mass has next to be boiled, while constantly agitated, for about 2 hours, at the end of which time the second equivalent of the tartaric acid will have been deposited as tartrate of lime, while the sulphate of potash only is left in solution. Care must be taken that the contents of the the generator bo allowed to become quite cool before the supernatant solution is decanted, as tartrate of lime is not absolutely insoluble, and is more soluble in hot than in cold liquids. The solution is evaporated to recover the sulphate of potash. The tartrate of lime is subjected to several washings with cold water, and is then decomposed by the addition of sulphuric acid, thus yielding free tartaric acid, and a deposit of sulphate of lime. Some of the latter is used to decom pose the neutral tartrate of potash, as already described, and the remainder may be employed as a manure. The quantity of sulphuric acid required may be calculated directly from the amonnt of chalk used, and the previously ascertained percentage of lime, present as a tartrate, with crude tartars employed. The separation of the gypsum will be promoted if the sulphuric acid be allowed to bo a trifle in excess, say • 2 per cent.
The whole compound is next transferred to a filtering apparatus, consisting sometimes of only a shallow reservoir with a perforated false bottom, over which is spread thick flannel, while iu other works a deep vessel is employed, and the filtration is promoted by the application of pressure. Thu sulphate of lime deposit is washed repeatedly, and with great care to free it entirely from tartaric acid.
The solution of tartaric acid, and the washings of the gypsum, are evaporated together in shallow leaden pans, heated externally by water baths, or internally by cells of steam pipe. This plan, however, is accomplished by a large loss, owing to the decomposition set up by the action of any temperature exceeding 60 (140° F.) to 65° (150° F.), especially in the presence of sulphuric acid.
Pontifex, a large manufacturer of this acid, has devised a plan of concentrating the solutions ui vacuo, thus not only reducing the temperature, but offering other great advantages in shutting out all air, decreasing the time occupied in concentration to about one-tenth of what it was by the old system, and keeping the liquid constantly in agitation. The last is a most important considera tion, as when evaporating with a steam coil, there is so little motion occasioned in the liquid that a portion of it will remain in contact with the coil, whose temperature is about 105° (220° F.), for an inconveniently long time. By the modern plan, however, the temperature of the first solutions never need rise above 49° (120° F.) to 51° (130° F.), and though some difficulty was at first experi enced from the disposition of the sulphate of lime to deposit itself in a flocculent form, which it was troublesome to separate, this obstacle has been overcome by careful attention.
Pontifex first separates or filters the liquors, from which tartaric and citric acids, and tartrate of potash and soda are obtained, from so much of the sulphate of lime, or baryta, &e., as is not in solu tion, as well as from other impurities, by pressure through textile fabrics. The liquor is then evaporated in vacuo nearly to concentration, and, if necessary, the concentrate] liquors are allowed to dep,:sit any sulphate of lime or baryta, or other matters that may have been previously held in solution by the weak aeid-liquor, and afterwards the concentration is completed to the point of crystallization. When the crystals have formed they aro remelted, and, after the liquor has been decolorized in the usual way, it is evaporated en racuo and crystallized, the operation to be repeated if required, till the crystals are sufficiently pure. The melting in the various re-crystallizations is also best conducted in the vacuum pan.
The apparatus employed consists of a vessel in which the liquors are to be evaporated, and this vessel is placed within a second vessel, which is exhausted, by which means the pressure of the atmosphere is removed both from the outside and the inside of the inner vessel, and thus the most convenient metal, lead, may be used to contain the liquors. The lead may also be dressed down upon the outer vessel with some pigment, or cement, between the surface, so as to expel the air, and the joints so arranged that no air can enter between them ; or glass, earthenware, or enamelled iron, or other material not affected by acids, may be used as the evaporating vessel, without the addition of another vessel to sustain the pressure. The overflow vessel contains at the bottom a quantity of chalk, milk of lime, or other alkaline or car bonated solution, through which the vapour from the vacuum pan is conducted by means of a pipe terminating in a rose. Above the surface of this alkaline or carbonated solution, a priming plate, perforated for the escape of the steam, is fixed in the overflow vessel, to prevent the solution being earl ied away by the overflow of steam through it. The overflow vessel is fitted with suitable taps and pipes for the supply of fresh, or discharge of spent solution. The object of employing the solution here is to collect the acetic, butyrio, sulphurous, and other acids, that would otherwise pass over and injure the apparatus. The overflow vussel would also collect any tartaric acid that might accidentally boil over from the vacuum pan. The steam may be condensed by the ordinary injection condenser ; but the patent condenser that bears the mune of Edmund Pontifex is preferable, not only on account of its economy in air-pump power and quantity of condensing water required, but because hy means of it the whole of the products of evaporation can be more conveniently colleeted, and the escape or loss of tartaric aeid he detected.