CHAPTER VI.

MANUFACTURE OF PHOSPHORUS.

In some instances the preparation of phosphorus is carried on in conjunction with other industries, for instance, glue-boiling, the preparation of sal ammoniac, yellow prussiate of potash, etc. Bone-ash is the chief material used by phosphorus makers. Many manufacturers do not burn the bones to ashes, but purchase bone-ash, large quantities of which are brought from South America, especially from the Argentine Republic.

The ordinary method of preparing phosphorus includes the following operations:

1. Burning the bones and grinding the bone-ash to powder.

2. Decomposition of the bone-ash by sulphuric acid, and evaporation of the acid phosphate previously mixed with charcoal.

3. The distillation of the phosphorus.

4. The refining and purifying the phosphorus.

_Burning the bones to ash._ The object of the ignition of the bones is the complete destruction of the organic matter. The operation is conducted in a kiln very similar to those in use for burning lime. A layer of brush-wood having been put on the bottom of the kiln, bones form the next stratum, and so on alternately. The wood having been lighted, the combustion of the bones ensues. In order to carry off the fumes, the smell of which is very offensive, a hood made of boiler-plate is placed on the kiln, and either connected with a tall chimney, or the smoke and gases are conducted into the fire of the kiln and burnt. The white burnt bones are withdrawn through an opening reserved in the wall on purpose, the kiln being kept continuously in operation, as in the case with some lime kilns.

This kind of kiln, however, possesses many disadvantages, and an improved form, as proposed by Fleck, is shown in Fig. 50.

The actual combustion chamber consists of a shaft, _A_, composed of two inverted cones. In the lowest portion of the lower cone are four or six apertures, _b_, which terminate in inclined channels and serve as air-flues, as well as for withdrawing the burnt bones. Through the aperture _a_ in the upper portion of the shaft additional bones may be introduced. This aperture is covered with a heavy iron lid.

As will be seen from the illustration, the shaft contracts towards the top in the form of a retort and passes into a horizontal channel _B_ which is provided, near its beginning, with an ordinary fire-place _d_. The gases and fumes escaping from the burning bones must pass over the flame of the fire-place _d_, and are thereby so completely consumed to water, carbonic acid and free nitrogen that no odor is perceptible even in the immediate neighborhood of the kiln.

In order not to lose the heat yielded by the fire-place _d_ and the burning vapors, the channel _B_ is covered with shallow pans _P_, for the evaporation of such fluids as have to be subjected to this treatment in the factory.

The mode of operation with a kiln of this construction is as follows: The shaft is filled two-thirds full with bones and dry wood split in small pieces is placed in the channels _b_ and simultaneously ignited. Four or six long hot flames thus strike the bones, the latter becoming in a short time so highly heated that they commence to burn briskly and ignite fresh portions of bones introduced through the aperture _a_.

The white burnt bones in the lower portion of the kiln are withdrawn, while in a glowing state, by means of iron hooks; the next layer of bones sinks down and fresh material is introduced through _a_, the kiln being thus kept continuously in operation.

The quantity of substance which remains after burning the bones depends of course on the quality of the material used. Tubular bones of old animals contain the largest quantity of mineral substance, and give a much more abundant yield of bone-ash than the spongy bones of younger animals. On an average 100 parts by weight of fresh bones yield 55 parts by weight of bone-ash. The composition of the latter is as follows:

Basic calcium phosphate, 80 to 84 per cent. Basic magnesium phosphate, 2 to 3 per cent. Calcium carbonate, } 10 to 14 per cent. Calcium fluoride, }

The bone-ash thus obtained is converted into a coarse powder by means of machinery, a bone-mill being best suited for the purpose. Experience has shown that the granules obtained by grinding should be the size of lentils. With the use of larger pieces the acid, with which the ash is treated later on, does not penetrate the entire thickness of the bone mass and a portion of the latter remains undecomposed. If the granules are too small, lumps are formed when the ash is brought together with the acid, and the mass would have to be constantly stirred in order to, make the action of the acid effective.

_Decomposition of the bone-ash by sulphuric acid._ When the basic calcium phosphate—the constituent of the bone-ash which comes here into consideration—is brought in contact with an acid of sufficient strength to effect its decomposition, calcium sulphate (gypsum) is formed, and a solution of acid calcium phosphate. If the latter be mixed with powdered charcoal, evaporated to dryness, and the mixture exposed, with the exclusion of air, to a strong red heat, the acid calcium phosphate is first converted into calcium metaphosphate, water being yielded. At this high temperature the calcium metaphosphate is by the action of the carbon decomposed to basic calcium phosphate and phosphorus; the latter escapes in the form of vapor and may be caught in suitable condensing vessels.

Hence three separate processes have to be distinguished: 1. The formation of acid calcium phosphate from the basic calcium phosphate contained in the bone-ash. 2. The conversion of the acid calcium phosphate into calcium metaphosphate. 3. Decomposition of the calcium metaphosphate, phosphorus being liberated, while basic calcium phosphate remains behind.

Expressed in chemical symbols these processes may be embodied in the following equations:

I. Ca_{3}(PO_{4})_{2} + 2H_{2}SO_{4} = 2CaSO_{4} + CaH_{4}(PO_{4})_{2} Basic calcium Sulphuric Calcium Acid Calcium phosphate. acid. sulphate phosphate. (gypsum).

II. CaH_{4}(PO_{4})_{2} = 2H_{2}O + Ca(PO_{3})_{2} Acid calcium Water. Calcium phosphate. metaphosphate.

III. 3Ca(PO_{3})_{2} + 10C = 10CO + Ca_{3}(PO_{4})_{2} = P_{4} Calcium Carbon. Carbon Basic calcium metaphosphate. monoxide. phosphate.

If the processes mentioned under II. and III. would pass off in the practice exactly as there stated, two-thirds, or 13.3 per cent., of the total quantity of phosphorus contained in the basic calcium phosphate originally present would be obtained. However, besides these processes, others take place which cause a loss of phosphorus. By the action of the red heat upon the acid calcium phosphate, reciprocal action takes place between the latter, the water and carbon, so that a portion of the water is decomposed, and in addition to carbon monoxide, phosphoretted hydrogen is formed; the phosphorus contained in the latter must be considered as lost. Furthermore, a portion of the phosphorus is lost in the form of vapor, even with the use of the best condensing contrivances. In consequence of these losses the actual yield of phosphorus is between 8 and 11 per cent.

The formation of acid calcium phosphate may be effected either cold or with the assistance of heat, less time being required in the latter case. The process without the assistance of heat is as follows:

The bone-ash is brought into a lead-lined wooden tank, and enough boiling water to cover it poured in. It is then thoroughly mixed with the water by vigorous stirring with wooden rakes, and the necessary quantity of sulphuric acid is then run in with constant stirring. When an intimate mixture has been effected, the tank is covered with a well-fitting lid and allowed to stand for a few hours. As heat has been liberated by the introduction of the sulphuric acid into the mixture prepared with hot water, the entire mass acquires a high temperature.

Decomposition is accelerated by stirring the contents of the tank every six hours, and the process may be supposed to be finished in 48 hours. With the use of fresh burnt ash no special phenomena are observed, but if the ash has been prepared for some time the caustic lime formed in burning the bones has been completely converted into carbonate of lime, and the carbonic acid escapes, causing a slight foaming of the mass. In addition to carbonic acid, there also escapes a certain quantity of hydrogen fluoride gas, which is liberated by the decomposition of the calcium fluoride present in the ash. This gas being, even in very small quantities, very injurious to health, the tanks should be placed in a thoroughly ventilated room.

When decomposition is complete, enough water is admitted for the mass to acquire by stirring a thick milky appearance, when it is allowed to rest until it clarifies and a perfectly clear solution of acid calcium phosphate stands over the precipitate of gypsum. The clear solution is drawn off, and the sediment washed with water to obtain the solution retained by it. For this purpose the gypsum is stirred up with water, and the thick fluid discharged into a filtering tank. Upon the bottom of the latter is a four-inch-deep layer of coarse quartz sand; upon this is placed a false bottom, and upon the latter is spread a linen cloth. The liquor first running off being milky is poured back into the tank. However, it runs off clear so soon as the pores of the filtering cloth have become somewhat contracted by the gypsum.

As a rule, the contents of several filtering vats are run into a common filter, and the mass is repeatedly allowed to drain off. The dilute solutions thus obtained are evaporated with the first liquor. A third lixiviation of the sediment yields a fluid which is used instead of water in a subsequent operation.

The residue of gypsum is taken from the filtering tanks, and may be used as a fertilizer.

In the warm way the decomposition of the bone-ash is effected by providing the decomposing tanks with lead pipes through which steam is introduced, decomposition being complete in 24 hours, and the first run of solution of acid calcium phosphate reaches the evaporating pan in a very hot state. The lixiviation of the gypsum residue is also effected with water heated by steam, the object of separating the acid calcium phosphate as much as possible from the gypsum being thus obtained more completely with a comparatively small quantity of water than is possible by washing with cold water.

A suitable apparatus for hot lixiviation is shown in Figs. 51 and 52 in cross section and profile. A lead-lined tank, 13 to 16 feet in diameter and 3½ feet deep, is fitted with a stirrer furnished with two or four paddles, and closed by a well-fitting lid. The stirrer is kept in motion during the entire operation.

A lead steam pipe _D_ furnished with several narrow flat outlet pipes placed in the direction in which the stirrer revolves, lies about 4 inches above the bottom of the tank. _W_ is the pipe for the admission of water, _S_ the lead pipe connected with the sulphuric acid reservoir, and _A_ an outlet of boards for carrying off the vapor evolved from the mass in admitting the sulphuric acid. _R_ is a wooden hopper, which serves for the introduction of the bone-ash, and is removed when the tank has been filled, the aperture being closed with a well-fitting wooden lid. The bottom of the tank is furnished with a lead discharge-cock.

Water is run into the tank, the bone-ash being simultaneously introduced through the hopper _R_, and the stirrer allowed to revolve slowly to effect an intimate mixture. Sulphuric acid and steam are then at the same time admitted. The steam heats the fluid very quickly to the boiling-point, and assists the action of the stirrer, it passing out from the outlet pipes in the same direction in which the stirrer revolves.

When the required quantity of sulphuric acid has been introduced, the admission of steam is interrupted, the stirrer, however, being constantly kept in motion. To keep the mass hot, steam is for a few minutes admitted every hour. When the sulphuric acid has acted for 24 hours, decomposition is complete, and the liquor is discharged through the cock on the bottom of the tank.

For the evaporation of the liquor leaden pans are used, and this operation is continued until the fluid has attained a specific gravity of 1.45. The pans rest upon cast-iron plates covered with a layer of clay or sand, to prevent them from being injured by the fire gases. For heating the pans, the fire gases escaping from the distilling furnace or the bone kiln are utilized.

The fluid having been evaporated to the above-mentioned specific gravity is now mixed with charcoal powder, or rather granulated charcoal of the size of small peas, in the proportion of 20 to 25 parts of charcoal to 100 of liquor. The mixture is then quickly dried in shallow cast-iron pans heated by a direct fire. Much sulphurous acid is evolved during this operation, and provision must be made for carrying off the vapors arising from the pans.

When the mass has been dehydrated so far that it balls together, it is taken from the pan by means of shovels and brought into a copper cylinder which is furnished with a sheet-iron sieve-like bottom through which it is forced into another pan. In this second pan the mass is moderately heated until a sample of it still evolves slight vapors and when pressed with the hand, after cooling somewhat, appears still moist without, however, being sticky. The material is now ready for distilling and 100 parts of concentrated solution of 1.45 specific gravity and 20 to 25 parts of charcoal yield about 77 parts by weight of so-called distilling mass.

It is best to bring the hot mass as taken from the pan at once into the retorts, as it is very hygroscopic and, if allowed to lie exposed to the air, would absorb moisture and require to be again dried. If it cannot be immediately subjected to distillation, it is advisable to keep it in sheet-metal boxes tightly closed.

The liquor obtained in treating bones for the manufacture of glue with hydrochloric acid may, as mentioned in Chapter V, be advantageously utilized in the manufacture of phosphorus. In order to obtain in crystallized form the acid calcium phosphate contained in it, the liquor has to be concentrated by evaporation and as during this operation hydrochloric acid vapors constantly escape, provision must be made for their removal from the workroom. The operation is conducted as follows: The flue of the furnace for distilling the phosphorus, through which the fire gases usually escape to the chimney, is made to communicate with a long low chamber, which can be tightly closed and connects at the other end with a high chimney. The flue is fitted with a slide and by opening the latter, the fire gases are forced to pass through the chamber before reaching the chimney.

In the chamber are placed large, well-glazed, earthenware vessels which contain the liquor to be evaporated, the vapors evolved being carried off by the fire-gases through the chimney. Evaporation proceeds quite rapidly, and fresh liquor is from time to time introduced through an earthenware pipe, the operation being repeated until a sample taken from the full vessels shows on cooling the presence of an abundance of crystals of acid calcium phosphate.

The admission of the fire-gases into the chamber is then interrupted, and the contents of the vessels are brought into a wooden tank furnished with a stirrer, which is kept in constant motion so that when the liquor cools, only small crystals will be formed. When crystallization is complete, the mother-liquor is drawn off and again evaporated. By this operation more crystals of acid calcium phosphate are obtained, which are, however, less pure than those from the first liquor. The mother-liquor drawn off from this second yield of crystals might, on being again evaporated, give more crystals which, however, would be too impure to be used to advantage.

To obtain the calcium phosphate contained in the last mother-liquor, the latter is exactly neutralized with burnt lime, a white precipitate of basic calcium phosphate being thereby obtained. The precipitate is repeatedly washed in water and allowed to settle, and added in small portions to the acid liquors obtained by extracting the bones. As these liquors always contain a considerable quantity of hydrochloric acid in excess, the basic calcium phosphate, being in a finely divided state, is readily and completely dissolved.

The crystals of acid calcium phosphate are removed from the crystallizing tanks by means of wooden shovels and brought into baskets covered inside with stout sack-cloth. They are left in the baskets until no more mother-liquor drains off, when the cloths are folded together and the crystals further freed from liquor by pressure. They are then heated, with constant stirring, in shallow stoneware pans until they are so dry as to crumble of their own accord. In this manner small crystals of a mother-of-pearl lustre are obtained, which feel like sharp quartz sand and consist of pure acid calcium phosphate.

This mass is mixed with 25 per cent. of its weight of granulated charcoal. The mixture is heated until it is pulverulent, and then treated in the same manner as the distilling mass from bone-ash.

In place of stoneware vessels, shallow lead pans may be used for evaporating the liquor containing calcium phosphate. To prevent the melting of the lead the pans are bricked in under a very flat arch, so that the fire gases are forced to pass close over the liquor, the pans being kept constantly full. When crystallization is complete the liquor is drawn off and the pans are refilled.

In the manufacture of phosphorus there is left after every distillation a residue of basic calcium phosphate, and it is advisable to decompose it with hydrochloric acid, this being effected in a vat lined with lead or coated with paraffine. The mass is completely dissolved, and the black sludge remaining on the bottom of the vat consists of charcoal, which had been added to the distilling mass.

_Distillation of the Phosphorus._—The distilling mass consists of acid calcium phosphate, charcoal and about 4 to 6 per cent. water. By heating in the retorts, the acid calcium phosphate is first converted into calcium metaphosphate, water being eliminated, according to the following equation:

CaH_{4}(PO_{4})_{2} = Ca(PO_{3})_{2} + 2H_{2}O.

By further heating to a white heat the calcium metaphosphate is so far reduced as to yield two-thirds of its content of phosphorus, while one-third remains behind as calcium phosphate, corresponding to the following equation:

3Ca(PO_{3})_{2} + 10C = Ca_{3}(PO_{4})_{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.

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.

Under certain conditions enameled cast-iron may be used as a material for receivers, but the enamel must be of such a nature as not to be attacked by the phosphorus vapors, otherwise the receivers would in a short time be destroyed.

The retorts having been filled with the required quantity of mixture are placed in the furnace and the brickwork is restored. The fire is then kindled and kept up very gently for some time in order to dry the fire clay used in joining the bricks. The receivers are filled with water and fitted to the retorts. In each receiver a small iron spoon is placed fastened to iron wire which serves as a stem. After six to eight hours of firing the heat has been so much increased as to cause the expulsion of any moisture left in the material placed in the retorts, while quantities of hydrocarbon gases and oxide of carbon are formed and expelled with the sulphurous acid. Subsequently other gases are given off, and because they contain some hydrogen phosphide are spontaneously inflammable. As soon as this phenomenon is observed the joints of the receivers and apparatus connecting it with the retorts are luted with clay, care being taken to leave, by the insertion of an iron wire, a small opening for the escape of the gases, which are as speedily as possible removed by well arranged ventilators from the building in which the furnace is placed. The appearance of amorphous phosphorus at the small opening indicates the commencement of the distillation.

The spoon is then placed in the receiver in such a direction that any phosphorus coming over may collect in it. During the progress of the operation, and as long as any phosphorus distils over, the evolution of combustible gases continues, and consequently a small blue-colored flame is observed at the opening in the lute. The water in the receiver is kept cool during the operation. After forty-six hours, with greatly-increased firing, a full white heat is reached, and the quantity of phosphorus coming over has decreased so much as to make a continuation of the ignition process wasteful. The receivers are therefore disconnected from the retorts.

The receivers are taken to a special room and entirely submerged in large wooden troughs filled with water in order to drive off inflammable gases still contained in them and to cover the phosphorus with water. They should be opened only after this has been done, and every manufacturer should rigidly enforce the rule of carrying on the operation in the above-described manner. Crude phosphorus is very inflammable, and when carelessly handled by the workmen may inflict horrible burns and, as the phosphorus as a rule causes blood-poisoning, such injuries generally cause death.

The phosphorus is then removed from the receivers (always under water). The trough in which this operation is effected should be provided, a few inches above the actual bottom, with a perforated false bottom upon which the receivers are placed. The larger pieces of phosphorus taken from the receivers are collected, under water, in special vessels, while the smaller pieces fall through the perforations of the false bottom to the actual bottom. When all the receivers have been emptied, the water in the trough is discharged into a large barrel in which it remains until the particles of phosphorus have subsided. The water is then drawn off, with the exception of a sufficient quantity to cover the phosphorus in the barrel.

The water from the receivers as well as from the troughs shows a quite strong acid reaction due to phosphoric acid, which has been formed by the combustion of phosphorus and passed into solution. In order not to lose this phosphoric acid, the water is partly used for filling the receivers and partly for mixing the bone-ash before adding the sulphuric acid.

Crude phosphorus is a mixture of crystalline (ordinary) phosphorus with amorphous phosphorus, the reddish color of the mass being due to the latter. It further contains phosphorus in various stages of oxidation, free carbon, and if impure sulphuric acid has been used, arsenic in combination with phosphorus.

_Refining and purifying the phosphorus._ The crude phosphorus was formerly purified by forcing it through the pores of stout wash leather by means of a machine. The crude phosphorus contained in a tightly tied piece of wash leather is placed on a perforated copper support situated in a vessel filled with water at 122° to 140° F. As soon as the phosphorus is molten, there is placed on the wash leather a wooden plate which by the aid of a mechanical arrangement and a lever can be forced downwards so as to cause the fluid phosphorus to pass through the pores of the leather, the impurities being retained. The phosphorus in the form of a slightly yellowish fluid collects on the bottom of the vessel and is immediately moulded into the shape in which it is brought into commerce. The residue in the wash leather consists chiefly of charcoal dust and amorphous phosphorus. The wash leather can, as a rule, be only used once, and only small quantities of phosphorus can be worked at one time.

A more suitable process of purification is as follows: Porous, unglazed porcelain or earthenware plates are fixed in an iron cylinder connected with a steam boiler. The cylinder having been hermetically closed is placed in a vessel containing water at 140° F. When the phosphorus is molten, steam of a few atmospheres’ pressure is admitted into the cylinder, the phosphorus being thus forced through the earthenware plates.

The phosphorus obtained by either of these methods is free from mechanically admixed particles of charcoal and amorphous phosphorus, but it is by no means pure, as all the substances dissolved in it (oxides of phosphorus) pass through the filter. The loss of phosphorus amounts to from 5 to 6 per cent. of the weight of the crude product. The masses taken from the filter plates are therefore collected and subjected by themselves to distillation in order to obtain the phosphorus contained in them.

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 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.

_Moulding the refined phosphorus._ It has long been the custom to mould phosphorus into the shape of sticks formed by the aid of a glass tube open at both ends, one of these being placed in molten phosphorus covered by a stratum of warm water. The liquid phosphorus is sucked by the operator into the tube until it is quite filled. The lower opening of the tube being kept under water is closed by the finger of the operator; the tube is instantly transferred to a vessel filled with very cold water by which the phosphorus is solidified. It is removed from the glass tube by pushing it out with a glass rod or iron wire while being held under water.

Independent of its danger, the method of moulding above described is not suitable for the manufacture on a large scale, and various contrivances have been introduced for this purpose; the apparatus constructed by Seubert being much used. It consists of a copper boiler fitted on a furnace. To the flat bottom of this boiler is fitted by hard solder an open copper trough communicating with a water-tank. In the boiler is fitted a copper funnel provided with a horizontal tube. This portion of the apparatus is intended for the reception of the phosphorus. At the end of the horizontal tube is placed a stopcock, while the portion of the projecting mouth of the tube beyond the cock is widened out and fitted by means of bolts and nuts, with a flange-like copper plate, into which are inserted two glass tubes. Into the copper trough is let a wooden partition, which serves the purpose of supporting the glass tubes as well as of preventing the communication of the hot water in the boiler and a portion of the trough with the cold water of the tank and the portion of the trough nearest to it. The phosphorus having been introduced in the boiler, the water is gently warmed so as to cause the fusion of the phosphorus. As the warm water reaches to the wooden partition, it is evident that on opening and closing the cock at the end of the horizontal tube, some phosphorus will pass through and flow out of the glass tubes, but that remaining in these tubes will solidify, and on opening again the cock at the end of the horizontal tube, the solid sticks of phosphorus may be removed from the glass tubes by taking hold of the piece of projecting phosphorus, the phosphorus being immediately immersed under water in the tank, and kept there protected from the action of the light.

Notwithstanding its apparently very practical arrangement, Seubert’s apparatus possesses many disadvantages, the principal drawback to its use being that the phosphorus-sticks frequently stick so firmly in the glass tubes that the operation of moulding has to be interrupted, the tubes removed, and the phosphorus stick pushed out with a stout wire. Furthermore, the melted phosphorus in flowing in frequently causes the glass tubes to crack.

Hence many factories have returned to the old method of moulding by sucking the fused phosphorus into glass tubes. To render this operation perfectly free from danger, the apparatus shown in Fig. 55 has been devised, by means of which a larger quantity of phosphorus can in a short time be moulded into sticks.

A hollow prism, _P_, of stout sheet-iron is fitted at its lower end with 8 to 12 short tubes. In the latter are inserted air-tight, by means of rubber, 12 glass tubes, _G_, each about 3¼ feet long and somewhat contracted at the lower end. Two iron rods, _E_, are fitted to the prism and, by means of suitably-shaped pieces of cork, serve to hold the glass tubes in their proper position. To the back of the prism is secured a rubber tube, _L_, which communicates with a small air-pump, and to the upper surface of the prism is fixed a handle.

The phosphorus to be moulded is fused in a shallow vessel of such a shape that a portion of it is covered only about 2 inches deep with water. The glass tubes are placed in the fused phosphorus and the air is sucked from them by means of the air-pump; the external air-pressure forcing the fused phosphorus into the glass tube.

The tubes are now sufficiently raised to allow of a rubber plate being pushed under their mouths in the shallower portion of the vessel. The rubber plate is pressed against the tubes and the entire apparatus placed in a vessel filled with cold water. The phosphorus solidifies very rapidly in the lower narrower portions of the tubes, and the latter are immediately detached from the prism and replaced by others. The phosphorus when entirely cold is pushed from the glass tubes by means of a wire or wooden stick.

In some factories the phosphorus is moulded in wedge-shaped sheet-metal boxes. In packing two such wedges are laid together with their longitudinal sides so as to form a prism.

Phosphorus is stored either in strong sheet-iron tanks or in wooden boxes lined with tinned sheet-iron and covered with a stratum of water fully 1¼ inches deep. For shipping smaller quantities of phosphorus, the sticks are packed in tinned sheet-iron boxes and the latter having been filled up with water, the lid is soldered on. To prevent the water from freezing in winter, it is advisable to mix it with spirits of wine.

_Manufacture of phosphorus with the assistance of electricity._ Readman and Parker have recently devised a process for the manufacture of phosphorus on a large scale in a continuously working apparatus by means of a powerful electric current, such as is yielded by a larger dynamo. The mixture used for the operation differs from the ordinary one of calcium phosphate and charcoal, in that it contains in addition a slag-forming body—a flux—silicic acid (quartz sand) being at first used for the purpose. Numerous experiments, however, have shown kaolin or pipe clay, _i. e._, aluminium silicate, to be more suitable.

When a mixture of calcium phosphate, charcoal and aluminium silicate is exposed to the action of the voltaic arc the following process takes place: By the extraordinarily high temperature prevailing in the proximity of the arc, the reduction to phosphorus of the phosphoric acid contained in the calcium phosphate is very rapidly effected. The liberated calcium combines immediately with the aluminium silicate to a calcium-aluminium-silicate, _i. e._, to a glass fusible with the greatest difficulty which, however, at the high temperature possessed by the voltaic arc becomes fluid like water.

The apparatus employed is, generally speaking, similar to the electric furnaces now in use. The mass to be treated is contained in a carbon crucible in which the two electrodes are placed opposite to one another, so that the electric current must pass through the mass. However, since the phosphorus at the moment of liberation would, on coming in contact with oxygen, immediately burn again to phosphorus pentoxide, the apparatus has to be so arranged that the entire process passes off in an entirely indifferent gas, and the condensation of the phosphorus vapors takes place under the same conditions.

Fig. 56 shows the apparatus employed for the electrolytic manufacture of phosphorus.

The carbon crucible, _a_, is enclosed by a clay jacket, which serves as an insulator of heat, and is closed by a graphite cover, _c_. Through the bottom and the cover of the crucible pass the two electrodes, _k k_, which conduct the current, and between which the voltaic arc is formed. To prevent the upper electrode from becoming too highly heated during the operation, it is constantly cooled by water admitted at _g_, and running off at _f_. Through the pipes _h_ and _l_ an indifferent gas—as a rule, illuminating gas—is conducted into the apparatus and escapes together with the phosphorus vapors through the pipe _d_.

In operating with the apparatus, the reduction soon takes place, and the phosphorus vapors escape through _d_, while a thinly-fluid slag remains behind in the crucible. The slag is discharged through a pipe not shown in the illustration, and a fresh charge of the mixture brought into the crucible _a_, the process of reduction being thus carried on without interruption. The phosphorus vapors escaping from _d_ are conducted through a cooled pipe and condense to a liquid in water heated to between 122° and 140° F. contained in a receiver.

Although the electrolytic method of manufacturing phosphorus is of quite recent origin, a considerable portion of the phosphorus brought into commerce is now prepared in that manner, it being cheaper than the older processes which involve a very large consumption of fuel.