To obtain pure phosphorus, the crude product has to be subjected to distillation, this operation being carried on in iron retorts of a peculiar make, and shaped like the glass retorts used in chemical laboratories. The necks of these retorts dip to a depth of ½ to ¾ inch in water contained in a basin filled to the rim so that any phosphorus which is discharged into this water causes it to overflow. The crude phosphorus having been fused under water is next mixed with 12 to 15 per cent. of its weight of moist sand, and this mixture placed in the retorts, the object of the mixing with sand being to prevent the phosphorus becoming ignited during the filling of the retorts.
Fig. 54.
Fig. 54 shows a distilling apparatus. In consists of a cast-iron retort, K, fitted with a cast-iron dome, H, the joint being made tight by means of clay and screws. The dome, A, tapers to a cone and terminates in a wide glass tube, R, bent at a right angle, and having at the mouth a diameter of about 2⅓ inches.
This dome, A, dips ¾ inch deep in water contained in the copper gutter of the receiver filled to the brim. The copper receiver, P, stands in water. It contracts below in the form of a funnel, and terminates in a pipe closed by a cock, G, to which is fitted a glass tube bent at a right angle.
The retort having been filled with the mixture of crude phosphorus and sand, the dome, H, is placed in position and the apparatus brought into the furnace. The dome is then connected with the condensing apparatus.
The fire is so regulated that the retort is uniformly heated from all sides in order to evaporate as quickly as possible the water still adhering to the phosphorus mixture, since at a higher temperature the water acts upon the phosphorus, and phosphoretted hydrogen is formed. It being scarcely possible to entirely avoid the formation of the latter, the receiver has been given the above-described form, so that the disagreeable vapors formed by the ignition of the phosphoretted hydrogen cannot escape into the workroom. The gas escapes through the conical dome. A, and the glass pipe, R, into the open air, where it burns without molesting the workmen.
At first steam only escapes from the retort, while later on phosphoretted hydrogen passes off. The evolution of the latter, however, soon ceases almost entirely, and the phosphorus distils uniformly over. Heating is continued until the retort shows a slight red heat, all the phosphorus having by that time passed over. The residue in the retort consists only of sand and charcoal.
The phosphorus passing over in the various stages of distillation shows different qualities. The portion which passes over first is perfectly pure, and when cold presents the appearance of bleached wax; the portions passing over later on are of a yellowish-red color, while the last portions are colored brick-red by amorphous phosphorus, and have to be collected by themselves. They are again brought into the retort in the subsequent operation.
In order to be able to separate the phosphorus passing over according to quality, the receiver for the melted mass is fitted with a conical bottom furnished with a glass tube which can be closed by the cock G. This glass pipe leads to a tank filled with warm water, in which the collecting vessel is immersed. The phosphorus collecting in this vessel is from time to time allowed to run into a vessel filled with water, another vessel being substituted when the phosphorus commences to show a yellowish color.