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 The white burnt bones are withdrawn through an opening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.
[Illustration: 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.
[Illustration: FIG. 51.]
[Illustration: FIG. 52.]
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.
[Illustration: 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.
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.
[Illustration: 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.
_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.
[Illustration: FIG. 55.]
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.
[Illustration: FIG. 56.]
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.