3Ca(PO3)2 + 10C = Ca3(PO4)2 + 10CO + 4P.
The mixture of acid calcium phosphate and charcoal is distilled in glazed fire-clay retorts, 12 to 18 of them being placed on each side of a so-called galley-furnace. The bodies of the retorts are placed on the side of the fire, while the necks pass through openings in the walls of the furnace, those portions of the wall being only lightly bricked up, as the retorts, after distillation is finished and the furnace cooled, have to be removed in order to clear out the residue and introduce fresh mixture. Between each pair of retorts is left a space of 5 to 6 inches for the passage of the flames.
Fig. 53.
Experience, however, has proved the advisability of modifying the galley-furnace by reducing its length and increasing its height, and placing the retorts in two or three rows, one above the other. Two such furnaces are placed together with their narrow sides, so that the fire-gases of both meet in a common chamber, and are conducted from the latter under the evaporating pans. Four such furnaces may also be arranged in the form of a cross, and their fire-gases conducted into a common chamber. By arranging the furnace, as is most frequently done, for three double rows of seven retorts each, it will hold 42 retorts, a double furnace 84, and one in the form of a cross 168. The arrangement of a double furnace is shown in Fig. 53.
The wall C, which separates the two fire-places, serves for supporting the lowest row of retorts, while the second and third rows rest upon intermediate pieces. The fire gases pass through flues into the space over the furnace, the top of which may be directly formed by the evaporating pans. It is, however, more suitable to place the evaporating pans on one side and not run the collecting chamber for the fire gases directly into the chimney. For the introduction of the retorts into, and their removal from, the furnace, a narrow door is provided between each two vertical rows of retorts. After placing the retorts in the furnace, this door is closed with stones and the joints luted with clay.
Every three retorts lying one above the other have a common receiver, p, for the collection of the phosphorus distilled off. The necks, r, of the retorts terminate in the collecting pipe, o.
The galley-furnaces, previously described, require the use of a fuel which yields a very long flame, and can, therefore, be heated only with wood or very fat coal.
In order to render possible the use as fuel of coal yielding a short flame, and especially of coke, furnaces have been constructed which hold only a small number of retorts, generally five, placed in two rows, by twos and threes, one above the other. The retorts are cylindrical in form, and have a capacity equal to that of several smaller retorts.
The receivers for collecting the phosphorus distilling over from the retorts are made of clay, and should be well glazed and smooth inside. Each receiver consists of two parts, one of which is a cylindrical vessel open at the top, into which the other part fits, and is fixed by means of a rim, which is prolonged so as to form a neck, between which and the first part is inserted a tube fitted on the neck of the retort, while the other end of this tube dips for about 4 inches into the receiver, the latter being filled with water.