(1) Chloride of Potash from Potassic Minerals.—The head-quarters of the chloride of potash extraction are at the Stassfurth, Leopoldshall, and Dougleshall works in Saxony and the Duchy of Anhalt. A mixture of carnallite and other potsssic minerals is here found in exten sive deposits, varying from 150 to 400 ft. in thickness, and at depths of from 400 to 800 ft. from surface. Shafts have been sunk upon these deposits, supplying to some thirty-five works upwards of 500,000 tons of the crude minerals—technically called "potash salts," or " Abraum salts "—per annum. Au analysis of these " potash salts" is given in the following table, and may be compared with the analysis of pure carnallitc already set forth :— Chloride of potassium .. 16.50 ,, sodium .. . • .. 20.83 22 magnesium .. 21.99 Sulphate of magnesium .. 21 calcium .. . • • • .. 1 08 Insoluble .. • • .. The mineral is first broken up and lixiviated with water, heated by steam to about 110°, the proportions being so arranged that the solution shall stand at least at 53° Tw. (32° B.). After settling, the clear liquor is run off from all insoluble residue to shallow vessels, in which, upon cooling, crystals of chloride of potassium separate out, the double salt (carnallite) only forming in the presence of an excess of chloride of magnesium. These crystals are removed and thoroughly The water is then driven off by drying the crystals at a gentle heat. Sometimes the crude carnallite is dissolved in a hot strong solution of chloride of maguesium and recrystallized. These crystals arc then subjected to the ordinary process desei ibed. The mother liquors, and the strongest of the wash waters, arc evaporated to 36° B., when chloride of sodium separates out, and the chlorides of potassium and magnesium remain in solution. The latter being in excess, crystals of carnallite separate out when the solution is cooled, and are treated like the fresh potash-salt. The weaker wash waters are used to dissolve further quantities of the raw material. Various other processes are carried on at the Stassfurth works, more particularly the preparation of saltpetre, by decomposing the chloride with nitrate of sodium. The residue that is left from the first solution of the potash salts contains up to 75 per cent. of chloride of sodium, and is occasionally used in the manufacture of sulphate of soda. The presence, however, of 10 or 12 per cent. of insoluble matter, and the absence of the usual crystalline form of chloride of sodium as used in the decomposing process, render it somewhat unsuitable. Its only virtue is an extremely low cost.
The Stassfurth deposits were first worked on in the year 1861. At tho present time about 70,000 tuns of chloride of potassium are produced in the district. In 1868, the Kalutz (GoMein) deposit was discovered, and a chloride of potassium of very great purity is now turned out from the works established iu the district. The raw material hero is somewhat richer than that at Stassfurth, containing from 22 to 24 per cont. of chloride, without the admixture of magnesium salts. and often iu the state of pure sylvinc. The process followed is an exceedingly simple one. The potash salt is broken up and digested with a hot saturated solution of chloride of potassium. The chloride is thereby completely dissolved, the chloride of sodium and the rest of the impurities being deposited. The strong solution is run into shallow crystallizing vessels and allowed to cool. A remarkably pure chloride separates out, the crystals, after draining and drying, giving the following composition :— Chloride of potassium 98.83 „ sodium •82 Insoluble •10 99 • 94 The mother liquors are returned to dissolve a fresh box of mineral. The Kalutz chloride is held in high esteem on account of its excellent quality, an absence of magnesia salts being especially desired.
(2) Preparation from the Ashes of Marine Plants.—Siuce the year 1730, when the industry was first introduced into Scotland, the treatment of kelp to obtain potash salts has become of con siderable importance. Before this time, it had been a recognized source of profit in France and the Channel Isles, and also iu Ireland, and the terms " vraic " and " varec" have an equivalent in our word "wreck," applied to vsrious forms of sea-weed. For a long period, the manufacture was carried ou chiefly for the sake of obtainiug carbonate of soda, reaching its greatest prosperity at the commencement of the present century, when there were about fifteen works in the United Kingdom, Scotland alone producing 20,000 tons of finished products per annum. Since that time, owiug to the
discovery of new and better sources of potash salts, the kelp trade has dwindled into comparatively small compass. The British turn-out of kelp salts is now probably nut more than 6000 to 7000 tons per annum.
Two kinds of weed are recognized : that which clings to the foot of the rocks and has to he detached at low spring tides, technically termed " cut-weed," and the loose plants that float upon the surface of the water, or are thrown upon the shore, to which the name of " drift-weed " is given. As a rule the latter is the richer in potassium salts.
The different varieties of sea-weed, too, inter se, give widely differing products when burned, and the selection and proper admixture of the material is a matter requiring considerable care and experience.
The burning is conducted in kilns of various forms and of rude description, built upon the level ground, with sides and ends of rough stone or brickwork. Sometimes pits in the ground form the kilns, 3 ft. in diameter and 18 to 20 in. deep. More commonly, however, the kiln is rectangular in shape, built with stonework sides and ends about 12 in. high, and varying in length from 6 ft. to 16 ft., and in width from 2 to 3 ft. The object in keeping the kiln narrow is to allow of the ends of the weed overlapping the sides, and so to admit the air freely throughout the mass. The weed is carefully arranged upon a bedding of dried heather or straw, and the whole mass set on fire. After burning for six or eight hours the ash begins to show signs of melting. It is then well stirred about to produce an equal flux, and finally allowed to harden into a rough cake, varying in thick ness from 3 to 6 in., which is broken up by throwing water upon it, and removed. The operation is then recommenced with a fresh lot of weed. The fragments of cake are broken down somewhat further by hand labour, occasionally in a mill of edge stones or fluted rollers, and thrown into tanks arranged after the manner of the black ash vats of the Le Blanc soda process, to which the reader is referred. Here the kelp is lixiviated with water at 43° (110° F.), the first and strongest liquors run off to the settlers, and the subsequent weaker lye run upon the second tank, now filled with fresh kelp. This operation is continued without intermission over a round of four tanks, the fresh water being always run upon the " weakest" tank and the second liquor, after the best has been removed to the settlers, being turned upon fresh kelp. The strong lye is drawn off as soon as a freshly filled tank is covered with it. It then stands at about 50° to 55° Tw., and is drawn away till it falls to 35°. It is then turned upon the next tank. Sometimes a tank is considered finally exhausted when the lye standing upon it registers 5° Tw., but more usually lixiviation is continued down to 2°. After being allowed to settle, the liquors are pumped into an evaporating pan and concentrated, by waste heat where practicable, up to 60° or 65°, hot. During this concen tration, various impurities, sulphate of potassium, &c., separate, and are fished out, the drainings being allowed to run back. The purified lye is drawn off and run into settlers, where chloride of potassium crystallizes out. These crystals arc removed and drained. The mother liquor is again concentrated, and the operation of crystallizing and separating impure salts repeated. In this way, three crops of chloride crystals are obtained, the second being usually the best. The first test up to 90 per cent. of pure potassium chloride, the second to 98 per cent., whilst the third does not average 82 or 83 per cent. The salts that are separated from the lyes during concentration consist of chloride of sodium, sulphate of sodium, sulphate of potassium, and carbonate of sodium, and command a somewhat slow sale to glass manufacturers and to alkali makers, the latter using them for the purpose of manipulating their soda ash and refined alkali so as to produce the strengths required by their various contracts. The use of " kelp-salt," as it is called, for " reducing " purposes is, however, on the wane, a readier and cheaper material being found in common salt.