Fig. 23.
Filter Presses.
In recent years the use of filter presses for separating liquids from precipitates and for washing the latter has become general. A filter press consists, as shown in [Fig. 23], of a number of frames, between which are perforated plates and sheets of filter cloth, and which can be pressed tight together by a screw. A powerful pump forces the liquid containing the precipitate into the hollow spaces of the frames (known as chambers), in which the solid body remains, whilst the liquid goes through the filter cloth. By afterwards pumping fresh water through, the solid remaining in the chambers is soon completely washed.
CHAPTER X.
ZINC WHITE.
Zinc oxide, known as a pigment under the name of zinc white, is one of the most important white pigments. Although not absolutely permanent, yet it has, in common with enamel white, the valuable property of preserving its whiteness in air containing sulphuretted hydrogen. Its low price has brought it into general use.
Although zinc white is a most important pigment, it is very seldom made in colour works, because on account of its origin it is a product of metallurgical processes. Zinc white in chemical composition is pure zinc oxide; it is formed when zinc vapour burns in air. In zinc-smelting works zinc white is obtained by putting zinc in tubes which are heated to whiteness; the zinc vapours burn when they come in contact with the air, and the zinc oxide is caught by special arrangements.
The retorts used for this purpose are similar to those employed in the manufacture of coal gas. From 8 to 18 of these retorts are arranged in a furnace, in two rows, one above the other. In the lid of the retorts is an opening, which serves for charging and for carrying off the zinc vapours. When the operation is commenced, the retorts being heated to a white heat, two zinc plates are brought into each, the metal is soon volatilised, and the vapours pass through the above-mentioned openings. A current of air, heated to 300° C., is blown in to meet the zinc vapours, which take fire and burn with a blinding white flame, producing a very fine white powder, which is carried by the current of air through a series of chambers in which it deposits.
Fig. 24.
C. Freitag recommends for the production of zinc white the use of retorts of oval section, A, [Fig. 24]. These retorts, containing crude zinc, are heated to a white heat, and then a mixture of generator gas from coke and air is introduced by B and the pipe running through B. The zinc burns completely in the flame of the generator gas, which contains excess of oxygen. A product of faultless nature is said to be obtained in this way.