Or, for every 142 parts of phosphoric acid, there are 224 parts of lime. Contrary to what we might expect, this phosphate is less insoluble than the ordinary tribasic or bone phosphate. This may be owing to the fact that, in the tetrabasic phosphate, there is more lime present than that which the phosphoric acid can retain with strong chemical affinity.[227] In the manufacture of superphosphate the tribasic phosphate is converted into the soluble phosphate—the lime, which was formerly in combination with the phosphoric acid, uniting with the sulphuric acid, and forming gypsum.[228] It was till recently supposed that soluble phosphate and gypsum were the only two resulting products of this decomposition. It has been recently shown, however, by Ruffle and others, that this is not, strictly speaking, the case, and that probably a large proportion of free phosphoric acid is formed; in fact, it seems probable that in the first stage of the reaction, only phosphoric acid is produced, and that this subsequently acts upon the undecomposed phosphate, with the production of monocalcic phosphate.[229] The amount of sulphuric acid which experience has shown it is necessary to add for the successful and economical manufacture of superphosphate, depends on the composition of the raw material employed. The larger the percentage of tribasic phosphate, the larger the quantity of sulphuric acid required for its decomposition; but sometimes even a poor phosphate consumes a large amount of sulphuric acid. This is the case where much calcium carbonate or fluoride is present in the raw phosphate, as both of these compounds require a quantity of acid for their decomposition, which takes place before the decomposition of the phosphate. Hence phosphates rich in carbonate of lime are not well suited as economical materials from which to manufacture superphosphate.
Reverted Phosphates.
A change which is apt to take place in superphosphate after its manufacture is what is known as reversion of the soluble phosphate. Thus it is found that on keeping superphosphate for a long time the percentage of soluble phosphate becomes less than it was at first. The rate at which this deterioration of the superphosphate goes on varies in different samples. In a well-made article it is practically inappreciable, whereas in some superphosphates, made from unsuitable materials, it may amount to a considerable percentage. The causes of this reversion are twofold. For one thing, the presence of undecomposed phosphate of lime may cause it. This source of reversion, however, is very much less important than the other, which is the presence of iron and alumina in the raw material. When a soluble phosphate reverts, what takes place is the conversion of the monocalcic phosphate into the dicalcic. Now in the first case, where reversion is due to the presence of undecomposed phosphate, the action taking place may be represented as follows:—
| Lime | } | } { | lime | } | } |
| Lime | } phosphoric acid | } { | water | } phosphoric acid | } |
| Lime | } | } + { | water | } | } = |
| (One molecule of insoluble phosphate) | } { | (One molecule of soluble phosphate) | } | ||
| Lime | } | } { | lime | } | } |
| Lime | } phosphoric acid | } { | lime | } phosphoric acid | } |
| Water | } | } + { | water | } | } = |
| (One molecule of reverted phosphate) | } { | (One molecule of reverted phosphate) | } | ||
It may be mentioned, however, that reversion from this cause probably takes place to a very slight extent in practice.[230] Where reversion is due to the presence of iron and alumina in the raw material, the nature of the reaction is not well understood, and is consequently not so easily demonstrated as in the former case. Where iron is present in the form of pyrites, or ferrous silicate, it does not seem to cause reversion. It is only when it is present in the form of oxide—and in most raw phosphatic materials it is generally in this latter form[231]—that it causes reversion in the phosphate.
The value of reverted phosphate is a subject which has given rise to much dispute among chemists. That it has a higher value than the ordinary insoluble phosphate is now admitted; but in this country, in the manure trade, this is not as yet recognised. At first it was thought that it was impossible to estimate its quantity by chemical analysis. This difficulty, however, has been overcome, and it is generally admitted that the ammonium citrate process furnishes an accurate means of determining its amount. Both on the Continent and in the United States reverted phosphate is recognised as possessing a monetary value in excess of that possessed by the ordinary insoluble phosphate. The result is, that raw phosphates containing iron and alumina to any appreciable extent are not used in this country, although they do find a limited application in America and on the Continent.
Composition of Superphosphates.
Superphosphates as manufactured may be divided, generally speaking, into three classes—viz., low class, medium, and high class. The ordinary or medium class contains from 25 to 27 per cent of soluble phosphate; and here it may be pointed out that by soluble phosphate is meant the percentage of tribasic phosphate which has been dissolved—not, as might at first sight be supposed, the percentage of monocalcic phosphate. The lower-class superphosphates are those containing less than 25 per cent, generally 23 to 25 per cent, of soluble phosphate; while the high-class superphosphate may contain from 30 to 45 per cent. For the manufacture of high-class superphosphate only a certain number of raw phosphates are available, such as Curaçao and Somme phosphates, phosphatic guanos, bone-char, &c. Certain processes have been patented for the manufacture of even more concentrated superphosphates, and by them phosphates containing as much as 40 per cent of soluble phosphoric acid—i.e., equal to 87 per cent of soluble phosphate—have been prepared. To this class belongs the so-called double superphosphate, manufactured at Wetzlar in Germany. Such a concentrated form of manure is naturally very expensive to manufacture, and is hardly to be recommended for home consumption. Where, however, manures have to be conveyed long distances, and the freight is consequently very high, such a concentrated article may be found most economical.
Action of Superphosphates.