Kelp-salt contains, too, as a rule, appreciable quantities of insoluble matter, and the greatest objection to its use as a reducing agent is that it is apt to " fleck " the alkali, from its containing a certain amount of carbonaceous impurity.
In France, the kelp is roughly ground, and the tanks are usually built of wood, of much smaller dimensions than those in use in this country. The chlorides of potassium and sodium are extracted together by running fresh water, or weak liquor, upon the unexhausted tank, concentrat ing the lye and separating the chloride of sodium by fishing it from the evaporating pans in the manner already described. This deposit, carefully washed and drained, yields a very fair salt for many manufacturing and agricultural purposes. To purify it from an admixture of sulphate of potassium, it is sometimes washed in weak lye, in boxes fitted with false bottoms. A jet of steam is introduced, the mass thoroughly agitated, and, after settling, the supernatant liquor is run off, and the residue drained and dried. The lye from the evaporating pans, after the chloride of sodium is removed, is concentrated further and run into coolers, where the chloride of potassium crystallizes out in the manner already described.
It will be readily apparent that these methods arc all of them exceedingly laborious, an enormous mass of material having to be treated to obtain even a small result. The process of incineration iu the open, too, is manifestly faulty, as a great loss of volatile products and heat must ensue. Moreover, both waste and injury result from the inevitable admixture of the ash with sand and dirt, and the whole process is liable to be seriously interfered with by bad weather, &c.
Many methods have been devised with a view to remedy these defects ; those of Lamont and Kemp may be mentioned. Stanford's suggestions, however, are more deserving of notice. He has proposed to submit the marine plants to destructive distillation in an iron retort, obtaining various inflammable gases, water, naphthas, and tar as volatile products of distillation, and a light porous charcoal left in the retort, which may be lixiviated and otherwise treated as ordinary kelp.
By this method, an exceedingly pure product has been obtained. It will be noted that this process analogous to that of Billet for the treatment of " vina.sses," already described. It is claimed for
:he method that, in addition to the usual products, there are obtained, from one tou of kelp, tho following valuable substances :— Volatile oil .. 61 gals. Colouring matter .. 6} lb.
Paraffin oil .. .. 9 gals. Pure charcoal .. 13 cwt. 39 lb.
Naphtha .. 3} gals. Gas (approximative) .. 4456 cubic feet.
Sulphate of ammonia .. .. 2 cwt. 48 lb. Iodine .. .. 5 lb.
Acetate of lime .. 37 lb.
These figures of course represent something like an average possible addition to the results usually obtained.
Another method of treating sea-weed has been devised by Schmidt, of the chemical works at Aalborg, in Jutland, Denmark, where the new ammonia-soda process is in operation. When this process is carried out completely, the ammonia is recovered, by boiling the ignore with lime or magnesia. The sea-weed, which abounds in the neighbourhood, is carefully stacked under cover and dried. It is then burnt in any convenient way, and a strong solution of the ash or " cake " is made. This is added to the waste liquor from the ammonia-recovery process, which contains chlorides of calcium and sodium. The potassium, magnesium, and sodium sulphates contained in the kelp are thereby decomposed, and hydrated sulphate of lime and magnesia precipitated, the operation being facilitated by the addition of a small quantity of chloride of barium. These precipitates are utilized in the manufacture of " pearl-hardening." The clear liquor is drawn off and the iodine precipitated with nitrate of lead as iodide of lead, which is separated by filtration and utilized for the production of iodine, &c. The liquor is then concentrated, and nitrate of soda added, to convert the potassium chloride into nitrate. Run into coolers, this salt crystallizes out of the mixture, leaving, as mother liquor, a solution of chloride of sodium, containing traces of ammonia and chloride of potassium, which is used again in the first process of the soda production—the decomposition with ammonia and carbon dioxide. It may be mentioned that, so far at least, the ammonia-recovery process has not been a great success, and in fact has been abandoned in the English works which manufacture soda by the Solvay method.