IMPROVEMENTS IN THE PRODUCTION OF PHOSPHORIC ACID AND PHOSPHATES AND IN THE UTILISATION OF SLAGS
Year: 1883

page_text: LETTERS PATENT to Thomas Twynam of 22 Cromwell Houses Fellows Road
S. Hampstead for an Invention of IMPROVEMENTS IN THE PRODUCTION OF
PHOSPHORIC ACID AND PHOSPHATES AND IN THE UTILISATION OF SLAGS
PROVISIONAL SPECIFICATION left by the said Thomas Twynam at the
Office of the Commissioners of Patents on the 19th February 1883.
THOMAS TWYNAM of 22 Cromwell Houses Fellows Road S. Hampstead IMPROVE-
MENTS IN THE PRODUCTION OF PHOSPHORIC ACID AND PHOSPHATES AND IN THE
UTILISATION OF SLAGS.
My invention relates to an improved method of extracting and utilizing the
phosphoric acid which is contained in several metallurgical slags, more especially
the slags obtained by treating phosphoric pig in the basic Bessemer or Siemens
processes.
The slag is ground, preferably after having been first broken up by being
thrown into water whilst still hot, any shots of metallic iron may be separated
by the aid of magnets or otherwise The ground slag is then treated with a
sufficient quantity of hydrochloric acid to dissolve out the phosphoric acid it
contains, the acid used may be of the ordinary strength of commerce but in most
cases I prefer to use a more dilute acid, one containing from five to ten per cent
of real acid I have found to answer well. The solution thus obtained is run off
from any undissolved residue and any ferrous salt oxidized to the ferric state by
the aid of manganese dioxide, air, or other convenient oxidizing agent.
I then carefully add to the somewhat acid solution a sufficient quantity of
carbonate of lime or other suitable carbonate to precipitate all the ferric oxide in
combination with a certain quantity of the phosphoric acid present, it will be
found that any phosphoric acid in excess of that required to combine with the iron
will remain in solution.
The phosphate of iron precipitate having been separated the remainder of the
phosphoric acid may be precipitated as a phosphate of lime by adding to the

page_text: solution sufficient lime or magnesia, the phosphate of lime having been separated
from the solution and washed is then ready for sale or for conversion into super-
phosphate; the solution from which the phosphate of lime has been separated may
be treated for the recovery of any magnesia it may contain by digesting it with
lime or dolomitic lime.
The phosphoric acid which is thrown down in combination with the iron I prefer
to recover by treating the precipitate with sufficient sulphuric acid to cause the
formation of free phosphoric acid and of sulphate of iron which latter is insoluble
in excess of sulphuric acid, as described in the joint Specification of S. G. Thomas
and myself, No 438 of the year 1883 The mixed solution of sulphuric acid and
phosphoric acid thus obtained after having been separated from the insoluble
Bulphate of iron may be utilized by mixing it with the precipitated phosphate of
lime obtained as above or with any of the ordinary phosphates of commerce, in
suitable proportion for producing superphosphate, the phosphoric acid contained
in-it may replace some of the sulphuric acid ordinarily employed. Or the acid
solution may be evaporated and the sulphuric acid separated from the phosphoric
by the aid of heat.
In some cases I precipitate with chalk without previous oxidation, and after
separating the resulting precipitate, add to the solution a sufficient amount of
lime to cause the precipitation of the remaining phosphoric acid as a phosphate
of lime.
Instead of precipitating the phosphoric acid left in solution by lime or magnesia
hoonhoto of lime obtained from a

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent filed by
the said Thomas Twynam in the Great Seal Patent Office on the 16th August
1883.
THOMAS TWYNAM of 22 Cromwell Houses Fellows Road, S. Hampstead IMPROVE-
MENTS IN THE PRODUCTION OF PHOSPHORIC ACID AND PHOSPHATES AND IN THE
UTILISATION OF SLAGS
My invention relates to an improved method for the production of pure phos-
phate of lime and for extracting and utilising the phosphoric acid contained in
several metallurgical slags especially in the slags obtained by treating phosphoric
pig in the basic Bessemer and Siemens processes.
The phosphoric slag is ground preferably after having been first broken up by
being thrown into water whilst hot. Any shots of metallic iron may then be
separated by the aid of magnets. The ground slag is treated in a suitable tank
with hydrochloric acid (preferably in the cold) in sufficient quantity to dissolve
out all the phosphoric acid which the slag contains. The strength of the hydro-
chloric acid may be varied with advantage according to whether it is desired to
produce chiefly phosphoric acid or phosphate of lime. In the former case acid of
the ordinary strength of commerce (30 per cent) or thereabouts may be advan-
tageously employed, in the latter case I always prefer to use an acid of a less
strength, from 5 to 10 per cent strength I find to give good results as it leaves the
bulk of the iron insoluble. In order to keep as much iron as possible out of
solution the ground slag may be first roasted in a current of air before being
treated with the weak acid so as to convert the ferrous oxide into ferric oxide as
described in the Specification of Letters Patent granted to S. G. Thomas, No 4452
in the year 1878, but this preliminary roasting is not essential. As sulphuretted
hydrogen gas is given off when the slag is dissolving the tank should be covered
and connected with a flue. When the digestion with acid is complete the solution
is run off from the insoluble residue, which residue when weak acid has been used,
contains a considerable percentage of oxide of iron together with some oxide of
manganese and gelatinous and undissolved silica, and forms a valuable material in
some metallurgical operations. The ferrous oxide contained in the solution is
oxidized into the ferric state by any of the known means of oxidation, chlorine
gas being the oxidant most conveniently employed, either by passing the gas into
the solution or by adding manganese dioxide and some free hydrochloric acid to
the solution. Air may also be used as an oxidant by forcing it through the
solution though I do not find it so efficacious as chlorine. I have also found it
very convenient for the purpose of oxidizing the ferrous oxide contained in the
slag to roast it in a current of air in a reverberatory or other calciner, or to blow
air or air and steam through the molten slag.
The ferric oxide in solution is then precipitated in combination with part of the
phosphoric acid as ferric phosphate by carefully adding carbonate of lime to the
somewhat acid solution The amount of carbonate required varies according to
the amount of free acid in the solution and according to the amount of ferric oxide
present, it should be added in powder and kept well agitated in the solution.
When the ferric oxide is nearly all precipitated the carbonate should be added
more gradually, care being taken to avoid as far as possible any excess otherwise
the ferric phosphate precipitated will contain also phosphate of lime, the precipi-
tation should be performed in the cold or at most a gentle heat. The precipitate
of ferric phosphate having been separated from the solution containing phosphate
of lime, is washed and dried and may then be utilised to recover the phosphoric

page_text: acid contained in it by the process described in the Specification of Letters Patent
N° 438 of the year 1883 granted to S. G. Thomas and myself, that is it is treated
with sufficient strong sulphuric acid to cause the formation of free phosphoric acid
and sulphate of iron which is insoluble in the excess of sulphurio acid employed,
the mixed solution of phosphoric and sulphuric acids is then utilised in one of the
ways described, I prefer however to use it in the treatment of the phosphate of
lime subsequently obtained from the solution filtered from the ferric phosphate.
The phosphoric acid remaining in the solution separated from the ferric phos-
phate is precipitated by lime or chalk as a phosphate of lime or if there was much
magnesia in the slag as a mixed phosphate of lime and magnesia; in place of
ordinary lime or chalk dolomitic lime or dolomite may be employed. The precipi-
tation by chalk is not so readily accomplished as when lime is employed, the
powdered chalk should be kept well agitated and the solution heated. The
phosphate of lime may be obtained either in the bibasic or tribasic state according
-to-the-amount-of-the-precipitant employed-in-either-case it should-be-well-washed to-
free it from calcium chloride. If there be any magnesia left in solution it may
afterwards be precipitated by digesting the solution with lime or dolomitic lime,
or the magnesia may be thrown down with the phosphate of lime, as it forms a
valuable manuring agent. Sometimes especially when the amount of ferrous oxide
in solution is not large I precipitate all the ferric oxide present as ferric phosphate
by means of chalk as before described but without previous oxidation of the
ferrous oxide present in solution, and then precipitate the phosphate of lime,
preferably by chalk, as before described, keeping the solution as far as possible
from the air to prevent oxidation of the ferrous oxide.
I do not claim merely dissolving the slag, roasted or otherwise, in weak acid for.
the purpose of taking up phosphate of lime and afterwards précipitating it leaving
all the iron insoluble, but I claim ;-
The hereinbefore described process of obtaining separately phosphoric acid
and phosphate of lime from metallurgical slags containing oxide of iron by
precipitating ferric oxide in combination with part of the phosphoric acid
by means of chalk and subsequently precipitating the phosphate of lime
remaining in the solution.
In Witness whereof I the said Thomas Twynam have hereunto set my hand
and seal this fourteenth day of August in the year of our Lord 1883.