We helieve that Messrs. Roberts, Dale & Co., who are, perhaps, the largest manufacturers of the acid in this country, adopt the following plan :— 2 equivalents of potash are mixed with 3 equivalents of soda and after the alkalies hays been partially caustieized, the ley is concentrated up to about sp. gr. Then 30 to 40 Nils of sawdust are added for every 100 parts of real alkali in the concentrated ley. The sawdust is spread evenly and carefully over iron plates by means of an iron rake, and the ley is gradually and completely incorporated with it. When the mixing is completed the temperature is raised by means of heated flues beneath the plates, and the heating is maintained for 4 to 6 hours. The first effect of the heat is to evaporate the water contained in the mixture, and when all the moisture has been driven off, the heat is continued with constant stirring of the mass till it reaches 177° (350' F.) to 205° (400° F.), at which point it must be carefully kept while the compound is regularly agitated until the sawdust has disappeared.
The crude mass is now composed of a mixture of oxalate of soda with carbonated and caustic alkali. Owing to the slight solubility of the oxalate of soda, and the very minute state of division in which this salt exists when obtained by the usual process of concentration, the ordinary plan of separating two soluble salt@ by dissolving and crystallizing them is inapplicable to this compound.
When quite cold, therefore, the mixture is placed in a, row of tanks such as are used in soda manufacture and submitted to the action of cold or warm water, so that the partially saturated solution from the first tank flows into the second, or that containing a salt more recently subjected to the washing process, and so on as in the manufacture of alkali. Cold water is run over the oxalate until the liquor flowing from the vessel has a sp. gr. of 1.030, when the oxalate of soda will ho left in the tank in an almost pure condition. In this simple manner the potash and other foreign substances are dissolved away from the soda oxalate, and by evaporating the water from the washings the potash may be recovered from them for re-use. The oxalate of soda obtained may be employed to produce either oxalic acid or binoxalate of soda.
Possoz prefers to prepare his caustic ley by reducing alkaline sulphate to sulphide by the agency of some form of carbon, treating the solution of sulphide with black oxide of copper and concentrating the caustic alkaline liquor up to strength. The sulphide of copper produced is roasted to reform black oxide, ready for further use. When it is intended to employ the same caustic alkali repeatedly, and to avoid the formation of acetates, carbonates, formates and ulmates, it is recommended to make the caustic potash in lixivium and to concentrate it until its boiling point is about 193° (380° F.), and whilst keeping it at a temperature between 160° (320° F.)
and 205° (400° F.) to add for every 100 parts genuine caustic potash, 100 parts of bran or other organic matter, mixed with 500 parts of mangauats of potash which has been previously produced by melting 250 parts of genuine caustic potash with 250 parts peroxide of manganese. After the mixture is made it should be heated and stirred until the mass becomes a thick paste, easily detaching itself from the implement, care being taken that the temperature does not rise heyond 260° (500° F.).
It appears to be an advantageous plan, however, to operate each time with new alkali instead of repeatedly using the same. In this case 250 to 300 parts of genuine caustic potash, concentrated to a boiling point, of about 205° (400° F.), or into 400 to 500 parts of genuine caustic soda, con centrated to about 149° (300° F.) boiling point are introduced 100 parts of organic matter—bran, ley, straw, guano, dried meat, blood—the whole mass being subjected to a gentle heat. When it is intended to collect and recover the ammonia which is liberated, the process is best conducted in a distilling apparatus provided with an agitator and the ammoniacal vapours may be condensed in pure or acidulated water. When nearly all the ammonia has been disengaged the mass may be heated in an oven by a current of warm air, or by other suitable apparatus, great care being observed in the regulation of the heat. It has been noticed that less heat is required with pure soda than with potash, and that the heat must be well distributed among the whole mass. The best heat for soda is from 149° (300° F.) to 205° (400° F.), and for potash from 205° (400° F.) to 260° (500° F.). The heating of the mass is continued and the stirring repeated until it becomes a paste or even dry, according to the substances used.
When this point has been reached by either of the two processes the mass must be dissolved in warm water in order to obtain saturated solutions, and then left to cool and crystallize. The mother liquors are concentrated to sp. gr. for potash, and to 1.320 sp. gr. for soda. The potash or soda oxalates, when crystallized and separated from the mother liquors, are converted either into oxalic acid, or acid oxalates, by any well-known process. The solutions of the potash and soda oxalates are heated with milk of lime, either in the cold or under the influence of heat, by which an insoluble oxalate of lime is produced, and the alkali is recovered as caustic ley for further use ; while the oxalate of lime is decanted, thoroughly washed, and afterwards decom posed by sulphuric acid to eliminate the oxalic acid.