The alkaline mother liquors obtained by the second process are composed chiefly of acetates, carbonates, ulmates, and caustic alkali. When potash has been employed 'they are suitable for manufacturing prussiate of potash, with animal matters. If soda has been used, a certain quantity of soda acetate may be separated by crystallization, and what remains may be converted into carbonate of soda by calcination, and the ordinary treatment.
Thorn, in a foreign scientific journal, has published the results of an extensive series of experiments on the produetion of oxalic acid from bran, lignose, and sawdust, from which we gather the following :— The mixtures were prepared in a round iron saucer, 2 in. deep, 51 in. in diameter, at the top, and 4 in. in diameter at the bottom. Into the boiling ley at 55° to 75° Tw. the sawdust is introduced, and the whole is heated over an open fire, with continual stirring. if the ley be concentrated to about 80° Tw., it is absorbed by the sawdust and the laborious agitation of the mass is dispensed with. In the course of his experiments the author noticed that the yield differed according to the thickness of the layer of materials acted upon. In consequence another series of experiments was made, in which the compound was heated in flat shallow sheet iron dishes in layers of about in. to in. The sawdust used was from pine wood, and contained moisture about 15 per i cent. Soda alone was tried in t e proportion of 2 to 4 parts of soda-hydrate to 1 part of wood. The results obtained from 50 gr 'ns of sawdust, heated with 100 parts of soda, at 200° (392° F.), 36 per cent. of oxalic acid in the w od ; at 240° (464° F.), per cent. only. But when heated in thin strata, at 200°, per cent. ; and at 240°, but per cent. When 25 parts of sawdust were employed with 100 parts of soda, at 240°, 42.3 per cent., and in thin layers at the same tem perature, per cent. If the heat employed be higher than 200°, great care must be exercised over the process, as with a sudden rise in the temperature there is danger that the oxalic acid already formed may suffer decomposition.
According to general experience, a mixture of potash and soda gives better results than those yielded by potash alone, but as regards the proportions there is much diversity of opinion. Fleck says that at Messrs. Roberts, Dale & Co.'s works at Warrington, li parts of hydrate of potash are used, with one part of soda hydrate. Another authority puts the proportions at 1 equivalent of potash to 2 of soda, which would agree approximately with 1 part of potash hydrate, with 11 parts of soda hydrate. In Kuhneheim's establishment at Berlin, the two alkalies are employed in equal proportions.
With a mixture of 20 parts of potash with 80 parts of soda, a violent decomposition takes place, even with a temperature not exceeding 180° (356° F.). Heat was applied for 45 to 60 minutes.
These results then tend to prove that potash alone is better than any commixture with soda. A considerable difference was perceptible, according to whether it was heated in thick or thin layers. Again, sawdust was added to boiling ley at 80' Tw., in the proportions of 50 parts of wood to 100 of potash. All the ley was absorbed, and the mass heated on iron plates in a stratum of about 0.4 in. The heat was maintained for 1 to hours, during which the compound was fre
quently agitated. The results then were:— When heated in thin strata, and if care be taken to prevent fusion, it is found that a mixture composed of 40 to 60, or 1 equivalent of potash to 2 of soda, is as good, practically, as potash alone.
Next, the experiment was made of passing currents of hot sir over the mass, but the product was not sensibly augmented thereby. Neither was the addition of manganese found to be beneficial. Special teals proved the superior yield of soft wood over hard species. Further, an increase of the proportion of sawdust in relation to tho alkali was also attempted, but there appeared to be serious practical difficulties in the way of heating the mass, and of ultimately extracting the acid from it.
After being heated, the compound is boiled in water till almost completely dissolved, and the mixed decoctions aro coneentrated to about 70° Tw., when the soda oxalate crystallizes out. Either filter presses or centrifugals may be used to free the crystals from the mother liquor, which may be got quite free from oxalic acid, if the fused mass has been properly boiled. The oxalate of soda thus obtained may be transformed into oxalate of limo by dissolving it iu boiling water and adding milk of lime gradually, so that the latter may be slightly in excess. If the liquid be not somewhat diluted the decomposition will be slow. If a filtered sample, made acid by the addition of a little acetic acid, gives a precipitate with chloride of lime, a little more milk of lime must still be added. As soon as the decomposition is fully accomplished, the caustic ley is decanted, and the deposit is repeatedly boiled in water and filtered. A large excess of sulphuric acid is needed in the decomposition of the lime oxalate, even 3 equivalents of acid to one equivalent of the oxalate. The oxalate of litne is reduced to the consistency of thin paste by the addition of water, and then the requisite quantity of sulphuric acid at 25° to 35° Tw. is poured in, while the liquid is kept constantly in commotion. More water is then added, and the whole subjected to gentle heat for one or two hours, without relaxing the stirring. When the decomposition is finished the liquid is strained off, and the deposit of sulphate of lime frequently turned over and washed. The filtered liquid contains oxalic and sulphuric acids, and sulphate of lime. When concentrated to about 25' Tw., the latter crystallizes out when the clear liquid is drawn off, and further concentrated to about 55° Tw. When cool the oxalic acid separates itself from the mass in long crystals, which are further purified by re-crystallization. After the waste alkaline legs have been concentrated to about 75° Tw., they are mixed with sufficient sawdust to absorb them, and are calcined in thin strata on iron plates, or in a reverberatory furnace, but a portion taken out and washed with warm water shows but very little colour. The dark grey calcined mass is lixiviated with the weak ley produced when the soda oxalate is decomposed with lime. The lixivium obtained is causticized with lime, concentrated to about 80° Tw. by evaporation, and is then again ready for use.