Preparation of Phosphorus

By Reduction of Phosphoric Acid

The world's supplies of phosphorus were for nearly a century obtained by a method substantially identical with that of Scheele. Scheele's raw material bone-ash, containing as it does from 34 to nearly 40 per cent, of P₂O₅ and decomposed easily by sulphuric acid, is still the best which can be used. Precipitated phosphate of lime obtained as a by-product in the manufacture of glue from bones also contains nearly 40 per cent, of P₂O₅ and is otherwise suitable. If phosphatic minerals are used for the preparation they must contain less than 10 to 12 per cent, of calcium carbonate, with only small quantities of the oxides of iron, aluminium, magnesium and the alkali metals, since these dissolve in the acid and reappear in the retorts with undesirable effects. The aqueous phosphoric acid liberated by the sulphuric acid is concentrated by evaporation in lead-lined wooden tanks heated by lead pipes through which passes superheated steam. Evaporation is continued with stirring until the density of the liquor is raised to 1.325 or to 1.50, according to the nature of the next process. After the removal of any gypsum, CaSO₄.2H₂O, or anhydrite, CaSO₄, the concentrated acid is mixed with saw-dust or charcoal, the former of which is suitably mixed with an acid of lower density, or the latter with syrupy acid. The mixture is evaporated in a cast-iron pot to expel any sulphuric acid which may remain. The charred mass is then introduced into fireclay bottles, a number of which, say 24, are placed back to back in a galley furnace similar to that used in the distillation of zinc by the Belgian process. The necks of these bottles are luted to malleable iron pipes, which dip beneath water in closed troughs. Gases which are evolved during the distillation are trapped and led away to be burnt. Finally the retorts are raised to a white heat. The phosphorus vapour is completely distilled over in about 16 hours. The liquid which collects under water in the sloping troughs is ladled occasionally into boxes made of malleable iron for transport to the refinery. Here the crude product, which may be buff to brick-red or nearly black, is agitated under water with sulphuric acid and dichromate of potash or soda. The liquid is afterwards siphoned and filtered through a canvas bag, then being remelted and run into tin moulds, where it solidifies in the form of sticks or wedges. The yield is slightly less than 70 per cent., or about two-thirds of the weight of combined phosphorus present in the charge, the loss being due in part to the fact that metaphosphoric acid is volatile above a red heat, and therefore escapes the reduction which is symbolised by equation (3). Consequently the original procedure of Scheele was modified by the addition of only so much sulphuric acid as would form monocalcium phosphate, CaH₄(PO₄)₂, which, on ignition, yields the metaphosphate. When this metaphosphate is strongly heated it loses two-thirds of its phosphoric anhydride, which as it is liberated is reduced by the carbon as follows:—

3Ca(PO₃)₂ = Ca₃(PO₄)₂ + 2P₂O₅
2P₂O₅ + 10C = P₄ + 10CO

The yield in this process is diminished by the production of phosphide, in addition to carbide, thus—

Ca₃(PO₄)₂ + 14C = 3CaC₂ + 2P + 8CO
Ca₃(PO₄)₂ + 8C = Ca₃P₂ + 8CO

It is stated that phosphorus can be made in the ordinary blast furnace at 1300° C., but it rapidly oxidises to the pentoxide, the production of 1 ton of which requires about 4 tons of coke.

Electric Furnace Processes

The intermediate preparation of phosphoric acid and acid phosphates can be avoided by taking advantage of the fact that the less volatile silica can expel the more volatile phosphoric anhydride, which can be reduced by carbon. High temperatures are required for these reactions, and these are best attained by electrical heating, although gas-fired furnaces lined with carborundum have also been employed.

The more acid phosphates of lime melt below 1500° C.—CaO.P₂O₅ at 970° to 980° and 2CaO.P₂O₅ at 1230° —while tribasic and more basic calcium phosphates melt above 1550° C., 3CaO.P₂O₅ at 1670°. This last compound combines with silica at about 1150° C. to form compounds such as 3CaO.3SiO₂.P₂O₅ (m.pt. 1760° C.), which are rather more easily reduced by carbon than the original phosphate, on account of the increase in vapour pressure of the P₂O₅, which also combines with excess of silica to form completely reducible compounds such as 2SiO₂.P₂O₅ and 3SiO₂.P₂O₅. The whole process may be summarised by the equation :

2Ca₃(PO₄)2 + 6SiO₂ + 10C = 6CaSiO₃ + 10CO + P₄

The mixture of bone-ash or ground phosphate rock, sand and coal dust or wood charcoal is introduced continuously into a furnace heated by the electric current, passing between carbon electrodes. The reaction begins at about 1100° C., but a much higher temperature is required to melt the charge and slag of calcium silicate which is drawn off continuously, while the phosphorus distils at about 1300° C. The yield is said to be about 92 per cent, of the theoretical.

Electric Resistance Furnace for the Manufacture of Phosphorus. (Electric Reduction Company.)

The first proposals for the employment of electrical heating in the production of phosphorus were made by Readman, Parker and Robinson. The simultaneous production of an alkali silicate by heating alkali phosphate, silica and carbon in a regenerative furnace was patented by Folie-Desjardins. In the Readman-Parker-Robinson process, as worked later, the phosphate, carbon and fluxes, previously heated to a high temperature, are introduced into the upper part of an electric furnace made of iron lined with refractory bricks and fitted with condensing pipes in its upper part. The gases pass through a series of copper condensers, the first of which contains hot water, the others cold water. It has been found advisable to replace the electric arc originally employed by an electrically heated resister, shown in fig. The current is carried by the graphite rod R, which is packed into the walls of the furnace with blocks of carbon. The rod radiates heat on to a charge C which consists of 100 parts of calcium phosphate and 50 parts of sand and carbon. This charge is introduced through D, is melted on the hearth, and the slag flows away through E. The phosphorus vapour and the gases are drawn off through the pipe P.

The product is said to be more free from impurities than that made atmosphere of carbon monoxide, is condensed in a slanting tube, which is connected by a set of vertical tubes with another slanting tube which conducts the gases to a condensing tower containing flat plates wetted with a solution of a copper salt or other reagent which will remove the last suspended globules of phosphorus. The issuing gas containing carbon monoxide is burnt, and supplies heat for the distillation of the crude condensed phosphorus. The second distillate is condensed in a box under water and may be purified as described later.

An experimental study of the reaction in a graphite tube resistance led to the following conclusions. The mixture was still solid at 1500°. The phosphorus volatilised ranged from 60 to 99.8 per cent, of that present. Volatilisation of phosphorus from mixtures of tri-calcium phosphate and carbon begins at 1150° C., and under favourable conditions the reaction is complete in one hour at 1325° C., or in 10 minutes at 1500° C. Less than 0.2 per cent, of the phosphorus is converted into phosphide at such temperatures. The reaction, in the presence of excess of carbon, is unimolecular between 1250° and 1400° C.

Distillation of Phosphorus

Condensation and Redistillation.—The condensation of the phosphorus vapour, mixed as it is with dust and furnace gases, presents special difficulties. The sketch shows, diagrammatically only, how these have been overcome in the patent of Billandot et Cie.

The phosphorus vapour, escaping from the furnace, passes up a vertical tube which can be cleared of obstructions from time to time by a movable weight. It then passes down an inclined tube D, which is connected with another tube E by a set of vertical tubes T. Both the lower and the upper end of D can be closed by valves. The phosphorus vapour condenses in droplets at about 50° C. Its flow can be further regulated by the introduction of gases from a compressor through the holes in the vertical tubes. The semi-fluid mixture is thus pushed through the valve h into a mixing chamber, and then into one of two distillation retorts H. These are built into the wall and heated by carbon monoxide from the escaping gases. The distillate drops into a double lead-lined and water-cooled box, which is inclined at an angle to the horizontal and furnished with an outflow cock. This condenser is not shown in the elevation (fig).

The gases escape through f and a valve into one of three plate- condensing towers G, and through a fan A which delivers them where required, or through a flue F. The plate towers are supplied with a solution of a copper or other metallic salt, which absorbs the last traces of phosphorus vapour.

Purification is effected mainly by the methods already described. Crude phosphorus may be melted under dilute solutions of chromic acid, nitric acid or chlorine. Arsenic may be removed by distilling in a current of steam and carbon dioxide, and condensing under water. Phosphorus can be granulated if desired by melting under water and shaking until cold.

For many of the purposes to which phosphorus is applied the red form is equally suitable, and when this is the case this form is greatly to be preferred on account of its non-poisonous and non-inflammable character. The conditions of transformation are now accurately known.

On the large scale white phosphorus is heated in an iron pot embedded in a sand-bath which is contained in an external iron vessel with double walls, the space between which is filled with an alloy of tin and lead. The pot containing the phosphorus is lined with porcelain and is provided with a tight cover bearing a bent tube of iron or copper which can be closed by a tap. The end of this tube, which acts as a safety valve against too high a pressure, dips under water or mercury. The external metal pot is heated to 220-250°, and not above 260° C., for about ten days. Transformation proceeds more quickly if the operation is conducted in an autoclave at higher temperatures. The product, a purplish-red mass, is ground and boiled with caustic soda to remove any unchanged white phosphorus, then washed and dried at a steam heat.