A NEW PROCESS OF PRODUCING ECONOMICALLY CONCENTRATED ASSIMILABLE PHOSPHATES FOR MANURE, AND SOLUBLE PHOSPHATES FOR THE PREPARATION OF PHOSPHORIC ACID AND PHOSPHORUS
Year: 1884

page_text: I, MICHEL CAHEN, Engineer, of Brussels in the Kingdom of Belgium do
hereby declare the nature of my said Invention for "A NEW PROCESS OF
PRODUCING ECONOMICALLY CONCENTRATED ASSIMILABLE PHOSPHATES FOR
MANURE, AND SOLUBLE PHOSPHATES FOR THE PREPARATION OF PHOSPHORIC
ACID AND PHOSPHORUS" and in what manner the same is to be performed, to
be particularly described and ascertained in and by the following statement :-
Carbonic acid in aqueous solution at the ordinary pressure of the atmosphere is
considered one of the weakest of acids; it scarcely reddens blue litmus paper, but
when it is compressed, its strength improves and the aqueous solution acts upon
litmus in the same way as the stronger acids, turning it red. And yet, even with
a solution obtained at a low pressure, it reacts in such a manner upon limestone
and upon tribasic phosphate of lime dissolving both as has long been well known.
Thus under high pressure at some hundreds of metres below the surface of the
earth, carbonic acid reacts in an extraordinary inanner, disintegrating and dissolving
limestone rocks. The carbonic acid frees itself from this solution when it comes
to the surface of the earth, leaving a considerable deposit of carbonate of lime,
witness the formation in many localities of mounds and even entire hills produced
in this manner.
At the ordinary pressure, but in an atmosphere of carbonic acid gas, a cubic
metre of water dissolves 2 kilogrammes of carbonic acid, and this proportion
increases at first in a manner proportionato to the pressure as with all liquefiable
gases; at bigh pressures carbonic acid gas dissolves in water in increasing propor-
tions up to 50 atmospheres when its solubility becomes almost indefinite.
At these high pressures carbonic acid has a most powerful solvent action upon
tribasic phosphate, the carbonates of lime, iron, magnesia et cetera.

page_text: High pressure is seldom made use of in chemistry in order to hasten the re-action
except just for the liquefaction of gas. I believe the day is not far off when, as
I have already pointed out in a treatise, chemical industry will utilise the force of
pressure in order to obtain many new results; for it must be borne in mind that
pressure derived from force is only a development of the power of heat; but to
return to our subject, taking 1,000 litres of water in which say 100 kilogrammes
of carbonic acid gas have been dissolved under considerable pressuro, if this
solution be mixed with carbonate of lime, it is evident that the proportionate
equivalent of carbonate will be transformed into bicarbonate soluble in the water.
The equivalent of carbonate of lime being 625 and that of carbonic acid 275: the
100 kilogrammes before mentioned will dissolve nearly 228 kilogrammes of
carbonate of lime producing 328 kilogrammes of bicarbonate of lime in solution.
The saine effect would be reproduced upon other carbonates, and also upon tribasic
phosphates of lime, the insoluble gangue remaining behind as a residue, which is a
great advantage. In a natural tribasic phosphate invariably containing more or
less carbonate of lime, the carbonic acid solution will dissolve in solution the
phosphate rendered soluble; then the natural carbonate of lime and 2 equivalents
of the line of the phosphate in the form of soluble bicarbonate of lime.
If now this-solution be collected, apart from the gangue, at ordinary atmospheric
pressure, the entire-content-of bicarbonate of lime will-decompose_and_neutral
carbonate of lime will be precipitated from a solution of soluble phosphate. This.
solution can then easily be separated by decantation and by pressure in one of the
numerous filter-press apparatuses, centrifugal etcetra, adapted to similar purposes,
-such for instance as in the manufacture of sugar. On releasing the solution from
pressure we shall recover-half-tho-carbonic acid employed to dissolve two
equivalents of the lime of the phosphate together with all the carbonic acid=
employed to dissolve the carbonate of lime accompanying the native phosphate..
But the operation can be carried further. On heating to 100 degrees the mixture
of soluble phosphate and the precipitated carbonate-of-lime,-neutral phosphate of
line as assimilable as the acid phosphate is produced, another equivalent of carbonic
acid is recovered, and a manure composed of a mixture of carbonate of lime and
neutral phosphate is obtained, very easily assimilated by plants, and excellent for
clay soils or those in which chalk is deficient. The filtration and separation of the
soluble phosphate is in this case unnecessary. On these principles and on the
employment of powerful pressure, this Patent is based; this point should be always
kept in mind, that at low pressures similar effects may be produced, but only very
weak solutions will be obtained and it would cost much more to extract either the
soluble phosphate or the phosphate and lime manure. Thus supposing a water
charged at five atmospheres only with 10 kilogrammes of carbonic acid to the
1,000 litres, the solution would be 10 times weakor than with 100 kilogramines,
and it would take ten times more fuel either to extract the soluble phosphate or to
precipitate the manure. The force of compression is much less expensive. The
following details explain the principle of the Invention.
It should be borne in mind that the high pressures employed which could
formerly not have been attained in practice are no longer disputable, they are
employed under many circumstances especially for the transmission of force to a
distance, by means of compressed water, and even of compressed air. And it is to
be noted that the compression is facilitated, and the carbonic acid prevented from
becoming too hot, by the presence in the barrel of the force pump at each stroke of
the piston of the quantity of cold water necessary for the complete solution of the
carbonic acid. In this manner the solution arrives all ready at the apparatus,
where the phosphate is being treated. The presence of the water having prevented
excessive heating.
As to the apparatus, vessels are now constructed to resist the highest pressure
especially when, as in our case, they are only to be used at the ordinary
temperature

page_text: Total 1684,0 (4)
To sum up 1000 kilogrammes of phosphate of lime require for their complete
solution 568 kilogrammes of carbonic acid in place of 1000 kilogrammes of
sulphuric acid at 52 degrees Baumé. Before treating of the carbonate of line to
be dissolved by the carbonic acid, let us consider the question of the phosphate
in the two processes. The treatment of the tribasic phosphate by sulphuric acid
is effected as completely in practice as in theory, and by treating 1000 kilogrammes
of pure tribasic phosphate with 1000 kilogrammes of acid at 52 degrees 2000 kilo-
grammes of superphosphato are obtained from which it is not advantageous to
extract the soluble phosphate.
By the treatment by carbonic acid it is quite otherwise, there are even two
methods as has been already. explained. 1st. The separation of the soluble
_phosphate leaving carbonate of lime as a residue. 2 The conversion-of-tho
soluble phosphate into noutral phosphate, equally assimilable in agriculture. In-
either caso the greater part of the carbonic acid employed can be recovered, and
even more by the second method than by the first.

page_text: OLD PROCESS SO³.
1. Treatment by the old process with sulphuric acid at 52 degrees Beaumé.
700
kos pure phosphate
700
SO at 52° B.
100
=
Co Ca O
1655 mineral
200 ganguo
1855 kilo
44 CO set free.
155
So at 52°.
Finished product 1,811 kilog of superphosphate
1811 kilos containing 3200 G PO³
100
">
"
177
33
5 This percentage of phosphoric acid is but low considering that it is derived from
a rich phosphate.
NEW PROCESS.
Treatment with CO' of the same rich phosphate of percentage given above.
1st case soluble phosphate obtained by CO²
700 pure phosphate
PO³, Ca O, 2 HO. (154-8 per cent.)
397-6a. carbonic
Then 700 kon
-
100. Co³ Ca O
Products
44 CO°
528.4 PO*
100-
-
-
528 kos.
320.6
60

page_text: CO' actually employed '/, of 397-6 = 198 km 8 CO³ used for the carbonate recovered
entirely.
ATL

page_text: showing that in the manufacture of soluble phosphate by the new process the
result is completely independent of the composition of the raw phosphate which
is a great advantage.
Claneumation of I have employed in all.CO² = 397'6 + 176 :
= 573-6

page_text: .
By the sulphuric acid process the acid all remains in the product which is not
the case with CO
C. O {2nd case CO
A Rich PO 1st case CO2 used 441-6. recovered 242-8 consumed 198-8
441 -6
342-2
99-4
"
"
33
"
"
B."
1st case CO³
Poor 2nd case CO₂
573.6
374-8
198-8
33
"
"
"
573.6
474.2
99-4
-
33
"
"
"
"
'
A rich PO³. SO³ at 52° Baumé 855. Recovered 0.- consumed 855 kilo
Old process 1 poor PO
"
»
1320
0
"
"
"
1320 kilo
It now romains to apply the above calculations to a practical working: in order
to illustrate clearly the advantages of the carbonic over the sulphuric acid process,
the general expenses and cost of labour are omitted, being taken to be the same
in both processes; the price of the raw phosphates also does not enter into the
account primarily.
Old process with SO³ cost account for a consumption of 50 tons of rich phosphate
k
16-030 PO always contained in 157-200 = 100* = 10* P.0°
Price of the 100 PO*
-
F. 51-70
It is obvious that the use of rich phosphates in the production of super-
phosphate is an enormous advantage as compared with the poorer qualities, a
fact well known to the trade. Thus it has been shown that to produce the
same quantity of phosphoric acid (which alone counts) say 16,030 kilos, would
cost with rich phosphate 5,093,50 francs
and with DOO 8.288 20 francs
difference 3,194,70
"
So that 100 kilos of phosphoric acid would cost in the first case 31-77 francs
in the second case 51-70
"
Neglecting the cost price of the raw material. Now taking the price of rich
phosphate at 35 francs per ton and of poor phosphate at 10 francs per ton the
results would be as follows
Rich phosphate Cost of working
5093.5
-1750.0
4
4

page_text: NEW PROCESS WITH CARBONIC ACID.
Fertiliser of neutral phosphate. This branch of the carbonic acid process is alone
of primary importance, the other product of the carbonic acid treatment being only
available for the-manufacture of phosphorus. With respect to the carbonic acid
process, the expenses involved are threefold.
1st. The price of the carbonic acid actually consumed.
2nd. The cost of compressing the-CO³ to the degree requisite for giving the best
practical results.
3rd. The cost of the fuel requisite to transform the soluble phosphate into
neutral phosphate forming a perfectly assimilable fertiliser.
Let us consider these three questions with a view of ascertaining the cost price,
say for a consumption of 50 tons of high grade or 100 tons of low grade phosphate
per diem.
Cost price of the carbonic acid.
The CO' which is now used in large quantities in the sugar works and in the
manufacture of carbonate of soda does not cost more than 1 to 11 centime the
kilogramme; it is obtainable puro at this price from the combustion of carbon.
Nevertheless to allow margin for losses etcetera let this cost be reckoned at
4 centimes the kilo at which price the cubic metre of CO³ at atmospheric pressure
will cost 8 centimos.
Cost of compressing the carbonic acid.
The cubic metre of water at 4 atmospheres pressure dissolves 4 cubic metres of
carbonic acid gas or 8 kilogramines. Now it has been already shewn that in order
to dissolve the phosphate and the carbonate of a rich phosphate rock 441-6 kilos
CO are requisite. At this rate 55 cubic metres of water would have to be
handled for every ton of rock treated. This quantity of water is excessive and
demonstrates the necessity of using higher pressures. At 50 atmospheres for
example the solubility of CO2 is practically unlimited. But it is only requisite to
have 736 cubic metres of carbonic acid dissolved in 1000 litres of water say 147.2
kilos per cubic metre. With 3000 litres of water thus saturated the figure given
above as requisite for the solution of 1000* of phosphate rock is attained. Making
all needful calculations-the force necessary to compress 50 cubic metres to
50 atmospheres, giving actually 73-6 cubic metres of carbonic acid dissolved
amounts to two inillion kilogrammetres and for three cubic metres of water and
441-6 kilos of carbonic acid to six million kilogrammetres. From this force that
realised by the recovered carbonic acid should be deducted which would not be less
than one half. This recuperation however does not appear in the calculations and
270000
ditto
may be set against the friction etcetera-Hence 6,000000 of kilogrammetres equal 23 horse
power of steam power at work for 1 hour-with a good machine this involves a
consumption of 50 kilos of coal in all which at 25 francs the ton = = 1.25 francs
cost even reckoning this item at 2 francs the ton of rich phosphate rock treated,
the cost is trifling.
3rd. Cost of fuel for precipitating the neutral phosphate.
This precipitation is effected by heating to 100 degrees the 3000 kilos of water
holding in solution the mixture of phosphato and of carbonate of lime. To obtain
this result a consumption of coal of 150 kilos or 3.75 francs is sufficient.

page_text: Finally in practical work, there must be calculated for the treatment of ric
natural phosphate.
1. Carbonic acid requires 4 centimes per kilo.
2. Compression of same costs 2 francs per ton of phosphate.
3. Fuel used costs
3.75 „ "
33
"
"
Cost of treating 50 tons of rich phosphate with carbonic acid to produce fertilise
of neutral phosphate.
Costs. 50 tons breaking at 200.0
Carbonic acid 99*.4 is 100* at 4 conts = 4F × 50 tons
Compressing CO at 2. per ton × 50 tons
"
3.75
"
"
× 50
Packages 1166 × 50 tons = 58.300* at 10.
Fuel at 100°
Carriage at 100 kilo
=
-
-
100,,
200,
-
100,
188,
583
"
233
"
-
-
00
00
58.300 at 400/00
-
1,000 of manure costs
100 kilo of phosphoric acid
24,50
"
11,23
»
The same treatment of a low grade phosphate with SO, would cost per diem
8288 francs and the 100 kilos phosphoric acid 5170 francs that is to say about
4 times as much. This however is not of primary importance because it is
admittedly unprofitable to manufacture superphosphate from rock of this quality.
Let the comparison therefore be made with high grade phosphate.
This expense has been estimated at 5093-50 frs. per diem.
New process (low grade phosphate) 1801.00
"J
"
"
33
Saving per diem 3,292.50
nearly as much as with rich phosphate which would give 3689.5 francs.
Comparing the cost price per 100 kilos of phosphoric acid according to the
4 divisions mentioned

page_text: These figures speak for themselves-low grade phosphates unfit for making super-
phosphates are moreover of very little value: thus when the 1000 kilos high grade
phosphate cost say 35 francs the 1000 kilos low grade are obtainable at 10 francs
and in this case the carbonic acid process becomes more profitable than even the
treatment of high grade phosphate by the same method
T
cost
-
F. 6.487.80
4.042, 50
F. 353
The abundance of low grade phosphates gives this process a great importance :
it should also be noted that as the CO is readily producible anywhere, the
process can be carried on at the place whence the rock is obtained thus effecting
considerable saving in carriage. With the old process on the contrary it was
necessary either to carry the phosphate to the sulphuric acids works, or else to
transport the vitriol to the source of the phosphate which is in general even more
costly and inconvenient, on account of the fragility of the packages. The high
pressure of 50 atmospheres is not absolutely necessary in the manufacture of
neutral phosphate. Suppose the quantity of CO³ dissolved in water be
proportionate to the use of 6000 litres per ton of phosphate in place of 3000:
this would involve double expense and fuel say 376 francs instead of 188 franca,
This increase of expense changes the actual results but little, seeing that the force
required for compression would be materially diminished though not in the same
proportion

page_text: Passing now to the question of the soluble phosphate PO* Ca O, HO. This
article is of daily increasing importance for the manufacture of phosphorus. The
production of soluble phosphates from bones by means of sulphuric acid is too
expensive or on account of the first cost of the materials: it is now customary to
dissolve out the soluble phosphates from a rich superphosphate with water, and
evaporate the solution to the consistency of treacle: reference will be made further
on to the cost of this method which will not bear comparison with my process.
My process for preparing PO³ Ca O, HO is as follows:
Instead of heating the solution of soluble phosphate and of carbonate of lime
mixed to precipitate the neutral phosphate: the lime is separated at once in the 1
cold having become insoluble on the release of the carbonic acid-this separation
is effected by filtration and pressure in this case it is true an equivalent of CO*
is lost and it is necessary to evaporate nearly all the 3000 kilos of water per ton
of phosphate instead of simply heating to 100 degrees. But on the other hand
this evaporation is indispensable whatever be the process employed, in order to 1
reduce the phosphate solution to treacle like consistency. Moreover this phosphate
has a greater value being directly available for the production of phosphorus.
Calculating the cost of working 50 tons of raw high grade phosphate and of
100 tons of low grade, including cost of the material.
-
Fr
1.50 Fr.

page_text: Old process
Low grade phosphate
and the prico of 100 kils. of pure phosphoric acid
New process CO2 high grade phosphate
low grade phosphate
Old process SO³ high grade phosphate
-
-
-
10780
280
13
2160
17-70
47 -0
Practical Results.-The yield of product is decidedly in favour of the new
process a single example will make this clear-that of the neutral phosphate for
fertilising produced by the CO³ process.
Let it be assumed that 10 per cent. of the phosphoric acid pass away unutilised
in the residue of a low grade phosphate working 100 tons per diem―
12V "
The other expenses remaining the same the 100 kilos PO will cost 18-30 francs
instead of 1770 francs: this difference is trifling compared with the 47-00 cost
of the 100 kilos PO by the sulphuric acid method. But suppose on the other
hand that there remains unattacked by the sulphuric acid 10 per cent. of the crude
phosphate, as often happens in practice and of which amount the analyses give no
account either as soluble or as neutral phosphate. This 10 per cent. is really lost
to the inanufacturer because to render them soluble the whole would have to be
manufactured over again. Thus 50 tons of crude phosphate treated per dien
42 750 kilos of superphospate but which contain, not 16-030 kilos of PO
but 1th loss
1-603
Nett
which makes the 100 of POs cost 47 francs.
14.427 kilos costing 6843 frs.
Then a difference of 10 per cent. by the new process raises the price
of 100 of PO³
and by the old process
0.90 francs
4.30 "
A comparison of the two processes with regard to the soluble phosphate would
show similar results.
In all cases the new carbonic acid process is cheaper than the sulphuric acid.
process. The condensed statements below shew this clearly. In these tables cost
of labour and standing expenses are not included, as already explained. The crude
phosphate is charged at 35 francs for high grade an! 10 francs for low grade, The.
results with low grade phosphates are especially striking.
,

page_text: acid so that at the end of their stroke the pumps C C¹ will force into E and E¹
only a saturated solution of carbonic acid. The water is delivered by the pipes a
and a¹ and the injection into C and O' by the pipes b and b'. E and E' collecting
pipe for the carbonic acid solution, F F F F annular pipe which receives the
compressed solution and delivers it as required into the phosphate dissolving
tanks 1, 2, 3, 4. At the moment shewn in the Drawing, this new CO, solution is
passing through the open tap G into the first tank H. which contains a quantity of
rock almost exhausted that is having already been subjected to two charges of CO³
solution. Passing out of N° 1 or H the solution enters the second tank 2 or H¹
the tap I being opened then into H" through tap J. This last tank contains fresh
phosphate and the liquor already mostly saturated with phosphate and carbonate
having passed through tho fresh phosphate is discharged continuously or
intermittently through tap M' into the annular pipe N N N. A tap O being
opened admits it into the reservoir P. In this cistern P the solution sets free
gradually the compressed carbonic acid which passes through tap kept nearly
shut into gasometer A as soon as the cistern P is sufficiently full the tap O is
closed, the tap is completely opened and the carbonic acid which is not in
permanent combination is set free being then only under atmospheric pressure.
During this time the contents of reservoir P¹ have been discharged and this
becomes in turn the receiving reservoir. In the same manner tank No. 4 after
having been emptied is recharged with crude phosphate and now becomes No. 3;
No. 3 becomes No. 2 and No. 2, No. 1, No. I becomes last in the series. The
taps M³, 1, G, O are shut and the taps G¹ K M³ and O¹ are opened and so on. It
is in fact a continuous and systematic apparatus for extraction in the cold.
G G G G taps which admit the C O, solution in turn into the tanks 1, 2, 3
and 4 H H H H³ extracting tanks for the phosphates, 3 of which are constantly
in operation and 1 being emptied of its insoluble residues in order to be recharged
with fresh phosphate. I, J, K L taps and pipes connecting the dissolving tanks
with each other. M M M M³ taps which discharge in turn the liquors saturated
with phosphates from tanks H H H H³ N N N annular pipe receiving said
liquors. O taps and pipes taking the saturated liquors into the receiving
cisterns P and P¹. Q and Q' taps and pipes conveying the recovered gas and
delivering it into the gasometer A.
When it is desired to produce neutral phosphate of lime as has been explained
in the preceding pages the phosphate solution is heated in the reservoirs P and P¹
or in other suitable vessels by means of a jet of steam blown into the liquid. The
phosphate soluble at 100 degrees retakes an equivalent of lime and is precipitated
as a neutral phosphate liberating simultaneously an equivalent of carbonic acid.
The soluble phosphate remains mixed with carbonate of lime just as in the
sulphuric acid process it is mixed with all the sulphate of lime.
For manurial purposes-This process is especially applicable to crude tribasic
phosphates mixed with silicious gangue or other impurity insoluble in carbonic
acid.
Thus taking a crude phosphate containing s
1000
By the sulphuric acid process this would yield:
1 equivalent tribasic phosphate
requires 2 equivalent SO, at 52°
Gangue 60%
=
1937.5 kilos
1937.5 "
= 2906.0
"

page_text: By the carbonic acid process, the gangue being eliminated the composition of
the remainder would be
1 equivalent PO³
887.5
of PO³ 2325: 887.5 :: 100 = test 38%.
2325 kilos containing 887-5*
In order to obtain the syrup of soluble phosphate, the contents of reservoir P
or P¹ consisting after the liberation of the carbonic acid of a mixture of soluble
phosphate and of insoluble carbonate of lime, are discharged into a filter press
which retains the carbonate of lime. The clear filtrate is evaporated in leadon
vessels to a syrup in the usual manner,
Referring again to the Drawings, Fig. 1 represents in plan all the phases of the
dissolving of crude phosphate by carbonic acid in an apparatus with continuous
circulation. For this arrangement however another may be substituted, that
of a high column of iron Figs. 1 and 2. This column is charged with crushed
-phosphate, the carbonic solution continuously injected by the pump permeates the
lower part of the column traverses it from bottom to top, meeting and dissolving
rich and richer layers of phosphato and the saturated solution flows continuously
at O into a reservoir P as shewn at Fig. 1. When requisite the contained gas
is let off into the gasometer and the column refilled with phosphate through the
man-hole T. In the same manner the-silicious gangue-in-the bottom of tho
column from which the phosphate has been dissolved is withdrawn through the
manhole T. With-this-simple-apparatus, the-manufacture-may-be-commenced
with only a single column, moro being added as the demand requires.
As-carbonic-acid-is-the main re-agent employed in this process, and as there will
be losses by leakage etcetora in addition to the consumption of gas caused by the
production of carbonate of lime with tho 2 equivalents of lime from the tribasic
phosphate, it is indispensable that the cost price of the carbonic acid be the lowest
possible. I have already touched on this question but will add a further
explanation.
If necessary the products of combustion might be pumped directly into the
dissolving tanks together with the water to dissolve them: carbonic acid being
much more soluble than nitrogen the latter gas would escape into the atmosphere
at the pressure at which the contents of the tanks are maintained. It is however
preferable to prepare the gaseous solution beforehand and to send it into the
gasometer where it mixes with the recovered gas. To obtain pure CO, I make use
of ordinary chimney gases. These usually contain about 1 of their volume of
carbonic acid, which may be dissolved out under a pressure of 3 to 4 atinospheres.
Upon diminishing the pressure on the vessel all the carbonic acid beyond what is
soluble at atmospheric pressure is set free again and passes into the gasometer.
Separate investigations have shown that the cost of the carbonic acid need not
exceed 1 centime the kilo, but the cost has been estimated at 4 centimes as a safo
figure.
The high pressure mentioned is not indispensable because carbonic acid being
cheap and the cost of compressing it relatively unimportant I can charge the
phosphatic liquor by causing it to pass 2 or 3 times into the dissolving tanks
recharging it cach time with carbonic acid the consumption of which will not be
materially increased thereby.
Having now particularly described and ascertained the nature of my said
Invention, and in what manner the same is to be performed I declare that what
I claim is:
21. The process of manufacturing neutral phosphate of lime from crude tribasic
phosphate of dissolving out the phosphate of lime from the other matters contained

page_text: in the crude phosphate by carbonic acid solution under high pressure and
subsequently heating the solution to 100 degrees in order to precipitate the neutral
phosphate of lime, after having set free the carbonic acid gas present in the
solution, substantially as hereinbefore described.
2. The process of preparing soluble phosphate of lime for the manufacture of
phosphorus and for other purposes, by dissolving out the phosphoric acid and the
lime from crude tribasic phosphate by means of solution of carbonic acid gas undor
high pressure, whereby a solution of mono-calcic phosphate and of bicarbonate of
lime is obtained: this solution on being released from pressure gives up the excess
of carbonic acid contained and the lime salt becomes insoluble carbonate, is
separated by filtration, whilst the filtrate contains the soluble phosphate and may
be concentrated by evaporation-all substantially as hereinbefore described.
3. The process of dissolving out the phosphate of lime from crude natural
phosphate by carbonic acid solution under moderate pressure only, by drawing off
the solution recharging the same with carbonic acid and returning it into the
dissolving tank, repeating the operation until the liquid is completely or sufficiently
saturated with phosphate of lime.
4. The combination of parts forming the apparatus for the purpose of dissolving
phosphate of lime by carbonic acid solution under pressure, consisting of gasometer A
pumps C C and D D¹ dissolving tanks HH HH³, reservoirs P P¹ together with
the requisite connecting pipes gas tubes and stop cocks substantially as described,
and shewn at Fig. 1 of the annexed Drawings.
5. The combination of parts forming the dissolving column for the purpose of
dissolving out the phosphate of lime from crude natural phosphate by means of
carbonic acid solution under pressure substantially as described and shewn in the
annexed Drawings, Figs. 2 and 3.