If we compare this method with that of incineration in pits, it will be seen - 1. That the drying is done away with under circumstances where it is very difficult - on the seashore and in the winter season. 2. That we may substantially count upon getting all the iodine contained in the weeds. Thus, if in the summer we only obtain 5 per 1000, or 1/2 per cent, of iodine, in winter we get 14 or even 15 per 1000, = 1 2/5 or even 1 1/2 per cent., at a time when the yield is greater and when drying would be impracticable. 3. The extraction of iodine and of potash from the light ashes obtained by this process is much easier than from the residues of the common process, which are hard as stones, and require a long time and much hot water for lixiviation. (Bull. Soc. Chem.)
At the beginning of the present century, Highland kelp realised 20/. to 22/. per ton. Those were lucrative days for the Highland proprietors. The small island of Colonsay yielded 200 tons of kelp annually, to the value of about 4000/. Long Island yielded 4000 tons, with a net revenue from this source alone, free of all charges, of about 30,000/. per annum; the sum paid to the burner being only 35s. per ton. The Hebrides afford a very large supply, roughly estimated at 6000 tons annually. Kelp as it comes to Glasgow occurs in large pieces, somewhat resembling iron slag. It is first broken up into pieces the size of ordinary road metal, and lixiviated in vats heated by steam. The liquor thus obtained is evaporated, and the salts which deposit on evaporation are fished out. They consist chiefly of potassium and sodium chlorides, and potash sulphate. After the removal of the mother-liquor from the salts, some manufacturers add to the liquor 1/7 of its volume of sulphuric acid, and having agitated the whole, it is left to repose for about 24 hours, during which period the sulphurous compounds are decomposed; sulphates are produced, whilst the sulphides, hydrogen, and sulphurous acid undergo mutual decomposition, and sulphur is thrown down.
At the end of this time, the liquor is put into an iron still, which is adapted to a series of stoneware receivers; a quantity of dry binoxide of manganese is added, and the iodine distils over by applying a strong heat. The binoxide of manganese, by acting on the chlorides present, under the influence of the excess of sulphuric acid, liberates chlorine, which in turn displaces the iodine. To obtain iodine pure, it is necessary to subject it to sublimation. The following is the process pursued by one Glasgow firm in the preparation of this article. The kelp is lixiviated in large water vats, and the liquor is boiled down to required strengths, by which the muriates and sulphates of potash and alkaline salts are separated by alternate precipitation and crystallization. The remaining mother-liquor, which possesses a specific gravity of 1.5, contains the soluble iodides, bromides, sulphates, and hyposulphites, and by adding sulphuric acid the latter two are decomposed, and sulphur is precipitated, which may be collected. The clear liquor is then treated with excess of sulphuric acid and black oxide of manganese in retorts, when the iodine distils over and is collected in earthenware receivers.
The bromine, which is obtained from the same source as iodine, is subsequently separated by a further addition of manganese, and collected in suitable receivers. The iodine is subsequently subjected to the process of sublimation it necessary. The iodine and bromine may be converted into iodide and bromide of potassium, the two compounds which are chiefly used, by dissolving them in potassium hydrate, eating to decompose the iodate and bromate which are formed, and crystallizing the salts. (Cochrane.)
. 2. From Caliche. - Iodine occurs in caliche or raw nitrate deposit, as iodate of sodium, in quantities varying from mere traces to 50 per cent.
The aqua vieja, or mother-liquor, of these works contains about -
Nitrate of sodium, NaNO2 ..
Chloride of sodium, NaCl
Sulphate of sodium, Na2S04 ..
Sulphate of magnesium, MgS04
Iodate of sodium, NaIO3
The mother - liquor is conducted through the pipe for mother-water to the precipitators, which are constructed of 2-in. tongued and grooved timber, lined with sheet lead, to prevent leakage by warping and shrinking; they are stayed transversely by 3/4-in. bolts. The reagent for precipitating the iodine is the run-off from the tanks for acid deposits in sufficient quantity to precipitate the iodine held in solution, which is determined by measuring previous to precipitation. The wings, or fans, which are also of wood, are then turned by hand until the liquor becomes thoroughly mixed with the acid.
This causes most of the iodine to fall to the bottom of the precipitators in slimes and flakes, and some to rise to the surface as a black froth. The iodine on the surface is skimmed off by large wooden spoons and placed in clarifying tanks, and the mother-liquor is then drawn off to the tank for mother-water after precipitation. Thence it is returned to the nitrate of sodium department, where it is again used, again becomes impregnated with iodine, and again goes through a similar process.
The deposit of iodine left in the bottom of the precipitators is taken out and placed in the clarifying tanks, where it undergoes a series of washings with pure water. It is then filtered and partially dried in a filter press, whence it is taken and pressed in the forming press, and is removed from the movable bottom of the press in blocks of cheese form, 8 in. in diameter by 6 in. thick. The blocks are next placed in a cast-iron retort, to which are attached 8 earthenware receivers, each 3 ft. long by 2 ft. 6 in. diameter. The last or end receiver is stopped by a wooden end and clay joint. The joints of the receivers are also made of clay. When the retort is charged, the crude iodine is sublimed by a slow fire. After sublimation, the retort is allowed to cool, the joints of the receivers are broken, the receivers taken down and emptied, and the contents placed in tarred kegs for exportation.
The crude iodine, previous to sublimation, contains: iodine, 80 to 85 per cent.; non-volatile matter, 6 to 10 per cent.; the remainder being water. The reagent for the precipitation of iodine is the acid sulphite of sodium, NaHSO3 formed by saturating the aqueous solution of " salnatron " (impure carbonate of sodium, Na2CO3) with fumes of burning sulphur. " Salnatron" is formed by burning coal-dust with nitrate of sodium, thus: 2NaNO3 + C = Na2CO3 + N2O3. Its impurities consist of chloride and sulphate of sodium, earthy matters, and unburnt coal; the latter are eliminated by dissolving the salnatron in water, and settling.
The fumes from the burning sulphur are generated in a fire-brick oven, and are drawn by an ejector from the oven, to the drainer, which catches the particles of partly burned sulphur, and from the drainer to the cylindrical fume receivers, which are charged with " salnatron " solution, and are traversed with perforated pipes for the passage of the fumes.
The steam for the ejector is taken from a small horizontal boiler at the extremity of the building. The building is well ventilated, and is made of wood and corrugated iron. The apparatus employed is illustrated in Fig. 14. The cost of this plant was 23,000 dollars Chilian currency. During the months of October and November 1881, the author exported from Iquique 7560 lb. of sublimed iodine, manufactured by this plant. (R. Harvey, in Mm. Proc. Inst C.E., vol. Ixix., session 1881-2, part iii., paper No. 1850.)
The method invented by Thiercelin for use in Chili and Peru is as follows: - The mother-liquors resulting from the manufacture of sodium nitrate are treated with a mixture of sulphurous acid and sulphite of soda, in proper proportions, and the iodine is precipitated as a black powder. The precipitated iodine is put into earthen jars, on the bottom of which there are layers of quartz aand, line at the top and coarse at the bottom; from this, it is removed by earthen spoons into boxes lined with gypsum, and a great port of the water is thus removed. It is sometimes sold in this impure state, or further purified by sublimation.