RECOVERY or THE A CID.—When cloth that is partially impregnated with oxalic acid (or tartaric acid) is passed through the discharge vat, containing chloride of lime, with a greater or smaller excess of lime, decomposition ensues and the acids are converted into their calcium salts, and theae being insoluble settle with the excess of undissolved lime at the bottom of the vat. If the slimy deposit so obtained be allowed to remain for some weeks in the working vat and be frequentry atirred, it will become continually richer in the salts, and poorer in free lime, till it attains even such a percentage of ,the former as 65.5 per cent. of tartrate of lime and 18 per cent. of oxalate.
From this mixture the acids may be liberated by washing it, and partially diluting it with water, and then adding a previously determined quantity of sulphuric acid of 1.568 sp. gr., and after stirring and boiling, with as much more water as is equal to the weight of tbe original depoait, the liquid is separated from the sulphate of lime by means of a centrifugal machine. Any excess of sulphuric acid contained in the liquid is removed by boiling with barium tartrate.
This plan affords still better results when chloride of soda—Eau de Labarraque—is employed in the discharging vat in place of the chloride of lime. As the tartrate and oxalate remain. insoluble it is only necessary that a quantity of chloride of lime solution be added from time to time, aufficient to precipitate all the tartaric and oxalic acids as lime salts, and to restore the original working strength of the vat liquor. In this case the precipitate will contain neither hydrate nor carbonate of calcium and hence a great saving will be effected in the consurnption of aulphuric acid. Also another advantage is found to follow the use of the soda solution, which is that the whites are whiter and the reds are less injured than when the chloride of calcium is employed.
In 1873 Messrs. Dale, of Manchester, procured a patent for concentrating or evaporating the acid in memo. As we have seen, in the process of the manufacture, it is necessary that a mixture of sulphuric and oxalic acids shall be concentrated together, in order that it may reach a strength at which the latter acid will crystallize out of the solution; and it is of great importance that the concentration be conducted at the lowest possible temperature, in order to provide against decomposition setting in, and the loss ensuing in consequence. The operation is
therefore conducted in suitable leaden vessels, preferably by means of steam, which may be con ducted into the vessels by means of a series of pipes in connection with the steam chest of n boiler, while above the vessels are placed vacuum pumps, or other suitable apparatus for rapidly removing the steam produced in the vessels by the evaporation. The patent refers equally to the concen tration of other solutions, but is found to be especially applicable to oxalic acid.
A simple method of obtaining dehydrated acid, HO, from the ordinary crystallized acid, consists in drying the latter acid at a temperature not under 100° and not above 120°.
Reichhardt dissolves the ordinary oxalic acid in concentrated sulphuric acid, from which solution tho rnonohydrated acid crystallizes out in course of time. • The first crystals that form are of the ordinary trihydrate, then a billydrate separates and, after a week, Eanull rhombic octahedral crystals of HO deposit. They quickly attract moisture, and must bo isolated and dried with great care.
The process given by somo authors for the preparation of pure oxalic acid is inaccurate. It is recommended to purify by repeated crystallization, replacing the mother liquors by distilled water, and tiro last crystallization will, it is said, be the most pure. In reality, however, the contrary is the case, however little alkali the acid may contain, the successive crystallizations become richer and richer in it, which may readily be understood when the less solubility of the aid• oxalates is con sidered. Maumen4 dissolved a kilogramme of oxalic acid in three litres of hot distilled water. The filtered solution deposited an abundance of very white crystals. 63 grammes of these were then dissolved in n litre of water to make a standard solution of the acid. The weather was very cold and next day crystals were found to be deposited. On calcination, 3.71 grin. of these dried crystals gave a residue of 0.64 corresponding to nearly of their weight of quadroxalate of potash, while 4•95 grm. of crystals obtained from the mother liquor of tho latter, yielded equivalent to of the total weight, or 1 KO to 88 Thus, it is evident that the first crystals are richest in alkali ; and successive crystallization from pure water does not affect the purification of the acids.