WASTE WOOD.
83
leiU to that of the lime used in the decomposition; when two parts of sulphuric, acid are used to one part of lime the sulphuric acid preponderates, and there is obtained a solution of oxalic acid containing free sulphuric acid. However, it is advisable to work in this manner, since the excess of acid iieeeleniteK the decomposition of the oxalate of lime, thi' sulphuric: acid being utilized again in a later operation.
lu order to hasten the, decomposition of the oxalate of lime, steam in continuouHly introduced through a small lead pipe, which reaches to the bottom of the decomposing vessel. The solid compound is thus kept always suspended in the liquid ; the heating also assists the decomposition. After the addition of the whole of the sulphuric acid the liquid is allowed to rest, the solution of oxalic acid drawn off after the <h»position of the gypsum, and the latter washed with water in u lead fiwmel lined with linen, the first washings being added to the oxalic, acid solution ; the remainder are used in place of pun* water to mix with the oxalate of lime in a fresh •de.romjKwitiun. The, residual gypsum is an excellent fertilizer for clover, but may, since it is very pure, be dehydrated and utilised as planter of Paris.
The solution of the oxalic acid is concentrated in a shallow lead pan, standing on a sand-hath, which is heated by an open fire, or better, by steam, until, in the summer, it is Ifi' H»( in the* winter, 10" B. It is then cooled to the ordinary temperature, when the dissolved gypsum separates in crystals. The liquid poured away from the gypsum is then further evaporated to 80" B., and set to crystallize in shallow lend ve8Bel«.
The mother liquors, separated from the crystals, contain the excess of sulphuric acid in addition to the oxalic acid ; the sulphuric acid in estimated, and the liquors added in the next decomposition of oxalate of lime, the amount of sulphuric acid added being decreased by the corresponding quantity.
The oxalic acid crystals are washed with cold water to remove adherent mother liquor, and are then purified by ttolution in boiling water and rapid cooling, so that only small