The chief point to be attended to is to deal with the scutch as early as possible after its removal from the glue-pan. An excellent apparatus for inoffensively extracting the fat from scutch is shown in Fig. 939. The scutch, mixed with acid, is heated by free steam in a large pan A, provided with a rim a, containing water, into which dips the edge of a conical cover B, so as to form a water-lute when the cover is let down. A short length of pipe closed at the top, rises the apex of the cover, and is surrounded by a ring of perforated pipe c, from which cold water constantly flows over the outside of the cover, into the grooved rim of the pan, whence a waste-pipe d carries it away. By this means, the steam within is condensed, and runs down the inside of the cover into the rim. The fat is ladled out, and the residue is run off into a covered tank outside the works to cool.
Another form of apparatus is shown in Figs. 940, 941. The scutch and sulphuric acid are intro duced into a dose cylindrical leaden vessel A, encased in wood, and charged at the upper part through a circular opening a, about 18 in. wide, which is closed while working, by an iron cover screwed down. Within is a stirrer B, the shaft of which passes through the centre of the top of the vessel. When the cylinder has been charged and closed, steam at about 105° (220° F.) is admitted for 2 hours at d. Very little vapour escapes by the safety-valve b, and its odour is not perceptible outside the works. At the end of 2 hours, the steam is shut off, and the material is left to settle till next day. The fat is then ladled out through the charging door, and the residue is run off through an opening c, about 14 in. wide by 5 in. deep, near the bottom of the vessel. Another method in use is as follows. The scutch, quite fresh from the pane, whence it is brought in closed vessels, is put into a tub with water and sulphuric acid ; steam is injected to separate the fat, which is taken off, and the scutch is enclosed in coarse bags, and strongly pressed in a hydraulic press, to which steam is admitted. The fat flows off from the bottom of the press, and is collected and refined by remelting by steam-heat. The pressed scutch has no odour,and can be kept under shelter, without heating or becoming offensive.
Manufacture of Artificial Manures.
—The next consideration is the means adopted for preparing raw materials for use as artificial fer tilizers. It has been already re marked, that the greater part of the phosphate of lime existing in mineral phosphates is in a state that defies solution in ordinary water, and that, in order to render that phosphate soluble in water, and immediately available for the plant, it is dissolved by the action of sulphuric acid. The decomposition of the mineral would occupy such a great length of
time, however, if the acid were to be applied to the unbroken nodules, that they are universally reduced to a fine state before being mixed with the acid.
Crushing.—The first process in the preparation of the mineral is that of reducing it to such a consistency as to admit of its being fed regularly into the mills which grind it to powder. This may be effected by the " crusher," shown in Fig. 942, consisting of a pair of chilled cast-iron rollers. Sometimes two pairs are arranged in the same machine, the lower ones being set somewhat closer them the upper. The mill may be fed with material about the size of road-metal. It is usually worked in conjunction with the grinding-stones, and is then driven from the same lay-shaft, to which the pinion and disengagiog-clutch are attached, as shown. The method of driving may, however, be made suitable for any required position. Pressure is applied to the rollers by means of weighted levers, which may be varied according to the nature of the material under operation. These crushers are manufactured by E. R. and F. Turner, of Ipswich, in two sizes : the smaller, capable of crushing about 2 tons an hour, requires about 3 II.-P.; the larger needs double that power to reduce about 5 tons an hour. In some cases, so-called " edge-runner " mills are used, instead of the specially constructed crusher.
Grinding.—The material issuing from the crusher is taken by elevators, or by other suitable means, to be fed as required into the hoppers on the grinding-mill. These do not differ materially from an ordinary flour-mill. The bed-stones should be firmly secured in cast-iron coned pans, fitted with adjustment-screws ; and the driving-wheels on the lay-shaft should be geared with hard and well seasoned wooden cogs. These wheels may be made in halves, for facilitating the renewal of the cogs by means of a duplicate wheel. They work with iron pinions on the stone spindles.
The stones are usually best French burrs, and are 4 ft. 6 in. in diameter. Each mill requires about 6 horse nominal engine power to reduce 10 cwt. an hour of the material from the crusher to a fine powder. It is very important that the phosphate shall be reduced to an exceedingly fine powder, espe cially when the mineral is very hard, as otherwise the acid will not have a fair opportunity of acting upon it, and consequently the proportion of soluble phosphate produced will be less than it might be.