=69. History and Manufacture.=—The basic process for the manufacture of
Bessemer steel is known in Europe as the Thomas or Thomas and Gilchrist
process, and the slags rich in phosphate, one of the waste products
of the process, are known by the same name. In this country all the
phosphatic slags which have been made in the manufacture of steel have
been obtained working under the patents of Reese, and, when prepared
for the market, are known as odorless phosphate. The only place where
these slags have been made in this country is Pottstown, Pennsylvania.
In Europe they are extensively manufactured, in England, France, and
Germany, and their use for agricultural purposes has increased until it
is quite equal to that of superphosphates.
The quantity of basic slag manufactured in Germany in 1893 was
750,000 tons; in England 160,000; in France 115,000, making the total
production of central Europe about 1,000,000, a quantity sufficient to
fertilize nearly 5,000,000 acres.
=70. Process of Manufacture.=—The principle of the process depends upon
the arrangement of the furnaces, by means of which the phosphoric acid
in the pig iron is caused to combine with the lime which is used as a
flux in the converters. A general outline of the process is as follows:
The pigs, which contain from two to four per cent of phosphorus, are
melted and introduced into a Bessemer converter lined with dolomite
powder cemented with coal-tar, into which has previously been placed
a certain quantity of freshly-burned lime. For an average content of
three per cent of phosphorus in the pig iron, from fifteen to twenty
pounds of lime are used for each 100 pounds of pig iron. As soon as the
melted pig iron has been introduced into the converter, the air-blast
is started, the converter placed in an upright position, and the
purification of the mass begins. The manganese in the iron is converted
into oxid, the silicon into silica, the carbon into carbon dioxid and
oxid, and the phosphorus into phosphoric acid.
By reason of the oxidation processes, the whole mass suffers a rise of
temperature amounting in all to about 700° above the temperature of the
melted iron. At this temperature the lime which has been added, melts,
and, in this melted state, combines with the phosphoric acid, and the
liquid mass floats upon the top of the metallic portion, which has, by
this process, been converted into steel.
As soon as the process, which occupies only about fifteen minutes, is
completed, the fused slag is poured off into molds, allowed to cool,
broken up, and ground to a fine powder. For each five tons of steel
which are made in this way, about one ton of basic slag is produced.
In another process, in order to make a slag richer in phosphoric
acid, a lime is employed which contains a considerable percentage of
phosphate. Although the slag thus produced is richer in phosphoric
acid, it is doubtful whether it is any more available for plant growth
than that made in the usual way with lime free from phosphoric acid. In
other words, when a basic slag is made with a lime free from phosphoric
acid, nearly the whole of the phosphoric acid is combined as tetrabasic
calcium phosphate. On the other hand, when the lime employed contains
some of the ordinary mineral phosphate the basic slag produced becomes
a mixture of this mineral phosphate with the tetracalcium salt. The
mineral phosphate is probably not rendered any more available than it
was before.
It is easily seen from the above outline of the process of manufacture,
that basic slags can have a very widely divergent composition. When
made from pig iron poor in phosphorus, the slag will have a large
excess of uncombined lime and consequently the content of phosphoric
acid will be low. When made from pigs rich in phosphorus there may
be a comparative deficiency of lime, and in this case the content of
phosphoric acid would be unusually high.
It is found also that the content of iron in the slag varies widely. In
general, the greater the content of iron the harder the slag and the
more difficult to grind. If the pig iron contain sulfur, as is often
the case, this sulfur is found also in the slag in combination with the
lime, either as a sulfid or sulfate.
No certain formula can therefore be assigned to basic slags and the
availability of each one must be judged by its chemical composition.
=71. Composition of Basic Slag.=—The slags produced by the method above
outlined may be amorphous or crystalline. When large masses are slowly
cooled the interior often discloses a crystalline composition. In some
samples analyzed in this laboratory the crystals were found to be of
two forms; _viz._, acicular and tabular.[62] They had the following
composition:
CALCULATED PER CENTS AS
CaO. Fe₂O₃. Al₂O₃. MgO. V₂O₂. P₂O₅. SiO₂.
Acicular crystals 42.69 20.98 3.71 0.49 0.18 27.06 4.96
Tabular crystals 53.61 9.64 0.91 0.08 —— 33.92 1.75
These data show that the two sets of crystals belong to two distinct
mineral forms. The presence of vanadium in one of the samples is
worthy of remark, and leads to the suggestion that in the slags made
of phosphoriferous pigs may be found any of the rare metals which
may exist in the ores from which the pigs were made. The amorphous
portions may have a widely varying composition and consequent content
of phosphoric acid. In all good slags, however, whether in crystalline
form or as amorphous powder, the lime and phosphoric acid will be found
combined as tetracalcium phosphate (Ca₄P₂O₉).
=72. Molecular Structure of Tetracalcium Phosphate.=—Several theories
have been advanced in respect of the atomic arrangement of the elements
contained in a molecule of tetracalcium phosphate. It must be confessed
that so little is known concerning the reactions of this body as to
make theories of its constitution largely visionary. But the existence
in definite crystalline form of this salt shows that it is not merely
an intimate mechanical mixture, but a true molecular form. As a type of
the supposed arrangement of its particles the graphic formula proposed
by Kormann may be consulted; _viz._,
[Illustration]
The crystals of this salt, as may be seen by inspection of the
analytical data, contain other bodies than calcium, oxygen, and
phosphorus. It would be of interest to push the investigation of their
constitution further and see if crystals of pure tetracalcium phosphate
could be obtained, and under what conditions they would be contaminated
by other metallic oxids. Usually, by the color of the crystals, it will
be easy to determine something of the nature, if not the extent of the
contamination.
=73. Solubility of Phosphatic Slags.=—The high agricultural value of
basic slags led to an early study of their solubility in ammonium
citrate, citric acid, and other organic solutions. Even finely ground
mineral phosphates and bones are soluble to some extent in ammonium
citrate, as was pointed out as long ago as 1882.[63] The most common
solvents used for basic slags are ammonium citrate and citric acid. The
ammonium citrate should be the same as that used for the determination
of reverted phosphoric acid and the citric acid solution commonly
used contains five grams in a hundred cubic centimeters. The slags of
different origin and even of different age vary greatly in respect of
the quantity of soluble matter they contain. It is believed, however,
that a very fair idea of the agricultural value of a slag may be
obtained by determining its degree of solubility in one of the menstrua
named.
=74. Separation by Sifting.=—The relative availability of a slag, as
in the case of a mineral phosphate, is determined by the percentage
of fine material it contains. Sieves of varying apertures are used
to determine this percentage. A one-half millimeter or a one-quarter
millimeter circular aperture is best, and the percentage of the total
material passing through is determined. A method used in Germany
consists in sifting the slag in a sieve twenty centimeters in diameter
the meshes of which are from 0.14 to 0.17 millimeter square and which
measure diagonally from 0.22 to 0.24 millimeter.
=75. Solution of Phosphatic Slags.=—Sulfuric acid has been found to be
an excellent solvent for basic slags preparatory to the determination
of phosphoric acid. There is, however, no unanimity of opinion
concerning the best method or means of solution. Aqua regia and
nitric acid are objected to because they may convert any phosphorus
in combination with the iron into phosphoric acid and thus increase
the quantity present.[64] But iron phosphid is seldom or ever found
in slags and therefore this objection is not always tenable. Sulfuric
acid has also been deemed objectionable because the gypsum separated is
likely to carry with it some of the other substances to be determined.
Hydrochloric acid is also excluded by some from the list of solvents
because it dissolves so many of the foreign elements in the slag and
thus tends to complicate the subsequent determination, especially
of magnesia. Further than this a hydrochloric acid solution is not
suited to the use of the citrate method now so commonly employed.
When hydrochloric acid is used, moreover, the dissolved silica must
be removed and thus the time required for making a phosphoric acid
determination is much increased.
If the sample be sufficiently fine the occlusion of undissolved
phosphate particles by the gypsum formed when sulfuric acid is used
is not to be feared and the disturbance of volume by the gypsum is
pretty nearly constant and can be allowed for. When five grams of slag
are used the mean volume of gypsum in the solution is about two cubic
centimeters.
=76. Estimation of Total Acid.=—In the determination of total
phosphoric acid in a slag, twenty-five cubic centimeters of the
strongest sulfuric acid are placed in an erlenmeyer having a wide
neck, and with careful shaking five grams of the fine slag meal
gradually added. The flask is heated over a naked flame until solution
is complete. When the mass is cold it is washed into a quarter liter
flask; again allowed to cool, filled with water to the mark, and two
cubic centimeters of water corresponding to the volume of gypsum
undissolved, are added, well mixed, and filtered. In fifty cubic
centimeters of the filtrate the phosphoric acid is determined by either
the molybdic or citrate methods already described.
=77. Alternate Method.=—The following method may also be used: Ten
grams of the substance are heated with fifty cubic centimeters of
concentrated sulfuric acid until white vapors have been evolved for
some time. The operation lasts for about fifteen minutes and can be
carried on in a half liter flask or in a porcelain dish. Without
regarding the undissolved material the volume of the liquid is now
made up to half a liter and filtered. The filtered liquid becomes
turbid after some time through the separation of calcium sulfate, but
this turbidity should not be regarded. To fifty cubic centimeters of
the solution, corresponding to one gram of substance, twenty cubic
centimeters of citric acid solution (500 grams citric acid to the
liter) are added, and it is afterwards nearly neutralized by the
addition of ten per cent ammonia and the liquid, which is warmed
by this operation, cooled. There are now added twenty-five cubic
centimeters of the ordinary magnesium chlorid mixture and the solution
stirred until turbidity is produced, one-third of its volume of ten per
cent ammonia added, and again stirred for about a minute.
Instead of the addition of the citric acid and ammonia the ammonium
citrate prepared as follows, may be added: 1,500 grams of citric acid
are dissolved with water, made up to three liters and five liters of
twenty-four per cent ammonia and seven liters of water added. The rest
of the operation is carried on in the usual manner.
=78. Halle Method for Basic Slag.=—The total phosphoric acid is
estimated at the Halle Station by the following process:[65]
Ten grams of the substance are moistened in a porcelain dish with a
few drops of water and about five cubic centimeters of a one to one
solution of sulfuric acid added, and after the mass has hardened,
which takes place very soon, fifty cubic centimeters of concentrated
sulfuric acid are added and stirred with a glass rod until it is evenly
distributed throughout the whole mass. In stirring this mixture the
greatest care must be taken, otherwise the substance would remain
attached to the sides of the dish, which during later heating would
cause loss through spurting. The complete solution now takes place
after a few hours’ heating on a sand-bath. During the cooling the
jelly-like mass must be stirred with a glass rod, and after it is cool,
by means of a washing-bottle, gently along the sides of the dish,
water is added, and when the mixture becomes hot it is again cooled
and washed into a half liter flask, which is made up to the mark at a
temperature of 17°.5 and filtered. When the acid filtrate stands for
some time there is often a separation of gypsum which, however, does
not in any way influence the subsequent analysis, which is made in the
usual manner.
Fifty cubic centimeters of the filtrate, representing one gram of the
original substance, are placed in an erlenmeyer. In the case of double
superphosphates which often contain large quantities of pyrophosphates,
twenty-five cubic centimeters of the filtrate just obtained, equivalent
to five grams of the substance, are diluted with seventy-five cubic
centimeters of water, ten cubic centimeters of nitric acid of 1.42
specific gravity added, and heated on a sand-bath to convert the pyro
into orthophosphates. The heating should be continued until the liquid
is reduced to its original volume of twenty-five cubic centimeters. The
strongly acid liquid is saturated with ammonia and with the addition
of a drop of rosolic acid as an indicator, again acidified with nitric
acid, and treated as with superphosphates.
=79. Dutch Method for Basic Slag.=—Heat ten grams of the sample with
fifty cubic centimeters of sulfuric acid (1.84 specific gravity)
till white vapors are evolved, shaking or stirring constantly. After
cooling make the fluid up to 500 cubic centimeters with water, taking
no account of the undissolved substance. Filter, and to fifty cubic
centimeters of the filtrate add 100 cubic centimeters of the ammoniacal
citrate solution, and after cooling, twenty-five cubic centimeters of
magnesia mixture. Stir or shake for a sufficient time. After the lapse
of two hours the precipitate is to be separated by filtration and
treated in the usual manner.
=80. Estimation of Citrate-Soluble Phosphoric Acid in Basic
Slag.=—Experience has shown that the manurial value of basic slags
does not depend alone on their content of phosphoric acid. Slags may
contain tri- as well as tetracalcium phosphate, and even this latter
salt may exist in states of differing availability. In determining the
availability of basic slag for manurial purposes its solubility in
ammonium citrate is considered the best standard. But this solubility
will evidently be influenced by the basicity of the sample, or in other
words, by the quantity of lime present. A slag rich in calcium oxid
would deport itself differently with a given ammonium citrate solution
from one in which the lime had been chiefly converted into carbonate.
If possible, therefore, all samples should be reduced to the same state
of basicity before the action of any given solvent is determined.
Wagner proposes to neutralize the basicity of a slag in the following
manner:[66] Five grams of the slag are placed in a half liter flask
which is then filled up to the mark with a one per cent solution
of citric acid and shaken for half an hour. After filtering, fifty
cubic centimeters are titrated with a standard soda solution using
phenolphthalein as indicator. This gives the quantity of citric acid
necessary to neutralize the slag. To a second portion of five grams of
the sample in a half liter flask are added 200 cubic centimeters of
water and enough five per cent citric acid solution to neutralize the
lime and then 200 cubic centimeters of acid ammonium citrate made as
indicated below. After filling to the mark with water it is shaken for
half an hour and filtered. To fifty cubic centimeters of the filtrate
are added 100 cubic centimeters of molybdic solution and heated to 80°.
After cooling, the precipitate is filtered and the phosphoric acid
estimated in the usual way.
The acid ammonium citrate solution used is made as follows: Dissolve
160 grams of citric acid with enough ammonia to represent about
twenty-eight grams of nitrogen and make up with water to one liter. The
exact method is given in =82=.
The molybdic solution is made by dissolving 125 grams of molybdic
acid in a slight excess of two and a half per cent of ammonia, adding
400 grams of ammonium nitrate, diluting to one liter and pouring the
solution into one liter of nitric acid having a specific gravity of
1.19. After allowing to stand at room temperature for one day the
mixture is filtered and is then ready for use.
=81. Wagner’s Shaking Apparatus.=—The latest directions given by
Wagner for determining the phosphoric acid in slags and raw phosphates
soluble in citrate solutions, are the following:[67] Five grams of
the material as it is sent into commerce without grinding or sifting,
are placed in a half liter flask and covered with nearly a quarter
liter of water, and then 200 cubic centimeters of citrate solution
added, prepared as described below. The flask is filled to the mark
with water. The flasks, which are of the shape shown in the figure,
are closed with rubber stoppers, and without delay placed for half an
hour in a rotating apparatus, (Fig. 6) which is turned on its axis from
thirty to forty times a minute. If a shaking apparatus be used instead
of the one mentioned, 200 cubic centimeters of the citrate solution
should be placed in a half liter flask, filled to the mark with water,
and the contents poured into a liter flask containing the phosphate.
This flask should be placed in a nearly horizontal position in the
apparatus and the agitation be continued for half an hour. On removal
from the apparatus the mixture is filtered and fifty cubic centimeters
thereof treated with double the quantity of molybdic solution at
80° and the precipitate separated after cooling. The precipitate is
carefully washed with one per cent nitric acid mixture, after which
the filter is broken and the precipitate washed into a beaker with two
per cent ammonia and the filter washed therewith until about 100 cubic
centimeters have been used. If the solution is turbid from the presence
of silicic acid it should be precipitated a second time by addition of
molybdic solution until the acid reaction is restored. The ammoniacal
solution of the yellow precipitate is treated, drop by drop, with
constant stirring, with fifteen cubic centimeters of magnesia mixture,
and set aside for two hours. The precipitate is collected, washed,
ignited, and weighed in the usual manner. The direct precipitation of
the phosphoric acid by the magnesia solution in presence of citrate is
not advisable because of the almost general presence of silicic acid
which would cause the results to be too high.
[Illustration: FIGURE 6.