The following Table shows the Imports of Phosphates into the United Kingdom,
and the Countries of Production, during the Years 1885-92.

1885.1886.1887.1888.1889.1890.1891.1892.
Tons.Tons.Tons.Tons.Tons.Tons.Tons.Tons.
United States138,844144,623165,275111,369122,554177,283*131,084*201,465
Canada 21,484 18,069 19,194 12,423 23,297 21,089 15,918 7,814
Dutch West Indies (Curaçao, Aruba) 11,588 12,581 9,505 10,736 14,730 14,763 8,851 6,648
British West Indies (Sombrero, &c) 7,727 3,351 6,451 11,010 1,880 3,970 1,960 2,473
Spain and Portugal 19,282 5,825 15,612 6,978 1,326 — 320 971
Belgium 35,405 31,551 45,322 54,261 64,643 82,096 70,723 65,079
Holland 865 2,194 4,778 4,137 2,270 2,428 3,434 6,627
France 2,276 1,503 11,140 39,059 65,490 35,659 18,325 18,239
Australia — 200 350 — 1,250 — — —
Germany 704 — — — — — — —
Hayti (San Domingo) — 2,175 3,044 6,238 4,094 992 1,639 2,965
Brazil — — 1,200 — — — — —
Venezuela and Guiana — — 405 — — — 540 —
Norway — — — — — 4,151 1,495 305
Other countries 397 1,039 1,139 1,675 390 1,070 1,483 1,594
*Florida phosphate — — — — — — 35,203 66,327
Carolina phosphate — — — — — — 96,881135,138

CHAPTER XIII.

SUPERPHOSPHATES.

As was mentioned in the chapter on Bones, Liebig in the year 1840 discovered that the effect of adding oil of vitriol, or sulphuric acid, to bones was to render the phosphate they contain soluble. This discovery marked an epoch in the history of artificial manures, and laid the foundation of the now enormous manufacture of superphosphate. In 1862 the juries of the London International Exhibition published an elaborate report containing an interesting article on the manure trade of Great Britain, in which it was stated that the annual quantity of superphosphate then made amounted to from 150,000 to 200,000 tons. Now it may be placed not far short of a million tons. Probably that made in the United States is considerably more. In the first instance, superphosphate was manufactured by Sir John Lawes from spent bone-char. This was superseded by coprolites and Estremadura phosphorite, Suffolk coprolites being for many years the chief material employed. This in turn was succeeded by the richer Cambridge coprolites, but of late years coprolites have practically ceased to be a source of superphosphate, the other mineral phosphates mentioned in the previous chapter—such as the South Carolina, Belgian, Somme, &c., phosphates—taking their place.

Manufacture of Superphosphate.

The manufacture of superphosphate is of too technical a nature to permit of discussion in a work of this kind. It is important, however, that the general principles underlying the process of manufacture and the chemical changes in the phosphate taking place during the process be clearly understood. In the first place, great importance attaches in the manufacture of the superphosphate to the fineness of division of the raw material, and much ingenuity has been spent on apparatus designed for this purpose. The difficulty of grinding the phosphate varies, of course, with the nature of the material used—apatite, for example, being much more difficult to reduce to the necessary fineness than phosphatic guano. The finer the state of division, the more complete will be the decomposition of the phosphate by the acid. Mr Warington recommends that for first-class work the powder should be so fine as to admit of it passing through a sieve of eighty wires to the inch. After the phosphate is reduced to powder, it is mixed with acid. This takes place in the mixer, which is generally in the form of an iron cylinder furnished in the centre with a revolving shaft, the sulphuric acid used being the ordinary chamber acid (sp. gr. 1.57). Whatever strength of acid is used, there must be a certain quantity of water present to form gypsum. It is to the formation of gypsum in the resulting product that the dryness of the superphosphate is due. The proportion of sulphuric acid used depends on the composition of the phosphate; and here it may be pointed out that the presence of much carbonate of lime is a most important factor in determining the quantity of acid required. The reason of this is, that where carbonate and phosphate of lime are present together, sulphuric acid first acts upon the carbonate, and it is not till this is wholly decomposed that the phosphate can be acted upon. Hence mineral phosphates with a large percentage of carbonate of lime do not constitute such an economical material for the manufacture of superphosphate as those in which the percentage of carbonate is small.[225] A certain amount of heat is necessary for the purpose of enabling a quick decomposition to take place. For this purpose the sulphuric acid added has been previously heated. In the ordinary manufacture of superphosphate, however, this is not considered necessary, as the heat developed by the chemical action between the phosphate and the acid is sufficiently great. The phosphate, after being thoroughly mixed with the acid, is discharged into what is technically known as the pit, a chamber built of brick or concrete. The mixture, which is in a fluid state when it enters the pit, very soon hardens, and is dug out in a day or two. It is next reduced to powder in a disintegrator, and is then ready for use as a manure.

Nature of the Reaction taking place.