Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a process and an apparatus for
purifying crud0 wet-processed phosphoric acid containing free sulphuric acid,
at an elevated temperature, by treating the acid to be purified with a
calcium compound the nature and proportion of which are such as to precipitate
the sulphuric acid in the form of calcium sulphate, the acid to be purified
being treated also with calcium sulphate seed crystals and with one or more
alkali metal compounds, silicic acid or a derivative thereof and one or more
sulphide compounds and with one or more agents for adsorbing organic contam-
inants, and the phosphoric acid so purified being subsequently separated
from solid matter present in the product of the treatment.
It is already known that phosphoric acid containing sulphuric acid
can be ~reated with a calcium compound so as to make it possible for the
sulphuric acid to be partially eliminated by the precipitation of calcium
sulphate and separation of the precipitate.
It is also known that the fluorine content in phosphoric acid can
be reduced by the addition of substances providing silicic acid and sodium,
which are added to the phosphoric acid either when removing sulphuric acid
therefrom or in a separate treatment, and that heavy metals present in the
phosphoric acid can be precipitated in sulphide form by the addition of
substances liberating hydrogen sulphide.
i Attempts have also been made to free phosphoric acid from organic
contaminants by means of active carbon or other adsorbents.
German Patent Specification ("Auslegeschrift") No. 1,265,725~ for
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example, describes a process for making aqueous phosphoric acid solutions of
improved purity, wherein the acid is treated with a calcium compound to
effect the precipitation of excess sulphuric acid and also with CaSO4.2H2O
seed crystals and an alkali metal fluosilicate, and the resulting suspension
~ is cooled down to temperatures of 0 to S0C. The process also provides for
3 the calcium sulphate dihydrate to be precipitated in the presence of adsorbents `;
,l 30 and sulphides.
This known processJ however, is less than fully satisfactory, for
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the following reasons. Firstly, it can only be used for the purification of -
phosphoric acid of a rather low concentration (containing 300-420 g of
P205 per litre, i.e. 20-30 % of P205), in which calcium sulphate dissolves
more easily than in more concentrated phosphorlclacid; accordingly it yields
solutions having relatively high proportions of sulphuric acid therein. In
addition to this, following the addition of the precipitant and seed crystals
to the suspension, it is necessary for the latter to be cooled, which is a
difficult and especially a time-consuming step when the process is carried
out on an industrial scale. Even more serious, however, is the loss of
phosphoric acid which arises from the fact that considerable quantities of
` CaHPO4.2H20, which is isomorphous with CaS04.2H20, go into the precipitate
in all those cases in which sulphuric acid is precipitated with the formation
of calcium sulphate dihydrate.
Similar processes have been disclosed in French Patent Specifica-
tions Nos. 1,334,532 and 1,334,533, wherein phosphoric acid containing
approximately 30 weight % of P2O5 is admixed with the usual reagents for
freeing it simultaneously from sulphuric acid, fluorine and organic contami-
nants.
The temperatures used in these known processes for effecting
precipitation and allowing the material to mature are again low, e.g. 45C.
.. .
' Despite this, the resulting purified acid contains S04 and F in minimum
j proportions which can be as high as 2 and 0.6 weight %, respectively. These ;~
two known processes thus present disadvantages similar to those described
hereinabove in connection with the process of German Patent Specification
(!'Auslegeschrift") No. 1,265,725. In other words, their use is practicallY
limited to those cases in which crude phosphates are processed to obtain
,l phosphoric acid containing not more than approximately 30 weight % of P205,
for use as either an intermediate or a final product. Commercial phosphoric
l acids, however, generally contain 50 to 54 weight % of P205. Such high
30 P205 acids have almost exclusively been used heretofore for purposes where
the degree of contamination with sulphuric acid, fluorine, heavy metals or
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533~
organic material is not a very critical factor, and more particularly for
the production of all kinds of phosphatic fertili~ers.
Highly concentrated acids are, however, gaining increasing in-
terest for the production of commercial phosphates in general, i.e. not
merely in the fertilizer field, and this interest is growing with the in-
creasing availability of technically improved and commercially attractive
methods for the purification especially of phosphoric acid which has been
wet-processed by extraction.
Especially in those cases in which it is desirable to have rather
pure material, it is necessary for the phosphoric acids to be prepurified or
freed from other acids, heavy metals and organic contaminants. As a result
the art is in great need of a process permitting concentrated wet-processed
phosphoric acid to be purified or prepurified under commercially really at-
tractive conditions.
According to the present invention there is provided in the pro-
cess ~or purifying crude wet-processed phosphoric acid containing free sul-
phuric acid and further contaminants by adding to the acid to be purified
at an elevated temperature a calcium compound the nature and proportion of
which are such as to precipitate the sulphuric acid in the form of calcium
sulphate; adding further to the said acid calcium sulphate seed crystals, and
alkall metal-, silicic acid- or sulphide-compounds as precipitating agents
for the said further contaminants together with an absorbent for organic con-
taminants; and separating the phosphoric acid so purified subsequently from
solid matter present in the suspension obtained; the improvement which com-
prises: adding to the crude wet-processed phosphoric acid containing from
45 to 55 weight % of P205 in a reaction zone, at a temperature of 80 to 100C,
;,.
the calcium compound, the calcium sulphate seed crystals, in the form of cal-
cium sulphate semihydrate in the presence of the said precipitating agents
and active carbon as adsorbent; precipitating thereby the sulphuric acid in r~ :
the form of calcium sulphate semihydrate together with the further contami-
nants with the resultant formation of a suspension~ passing off the suspen-
sion from the reaction ~one; recycling a proportion of this suspension to the ~ ~
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reaction zone; separating the balance of the suspension into purified phos-
phoric acid and solid matter; forming from the solid matter an aqueous sus-
pension containing up to 30 weight % of phosphoric acid and converting the
calcium sulphate semihydrate in the a~ueous suspension at 20 to 70C into
calcium sulphate dihydrate; separating the aqueous phase of the aqueous sus-
pension from the solid matter therein, and water-washing the separated solid
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~j^ matter; recycling a proportion of the aqueous phase so separated9 and a pro- :
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portion of the wash water, for forming the aforementioned aqueous suspension
. containing the calcium sulphate semihydrate and combining the balance of the
aqueous phase and wash water with the purified phosphoric acid.
The process of the present invention enables concentrated wet-
processed phosphoric acids, particularly those which contain 45 to 55 weight
% of P2O5, to be purified. With respect to the first-mentioned suspension,
it is preferable for a proportion of it to be recycled to the reaction zone
after a period of 45 to 60 minutes, and for the calcium sulphate semihydrate
-~ to be converted to calcium sulphate dihydrate at a temperature of 40 to 50C~ -
The proportions of the aqueous phase separated from the calcium sulphate di-
hydrate and of wash water which are recycled for use in suspending and con-
verting calcium sulphate semihydrate are preferably proportions which provide -
a suspension having a solids content of 10 to 20 weight %, and more preferably
, approximately 12 weight %. A further preferred feature which facilitates
filtration of the semihydrate and later of the dihydrate comprises adding the
calcium compound, alkali metal compound(s), silicic acid or derivative thereof,
and sulphide compounds~s), and the active carbon, in proportions which provide
a ratio of calcium sulphate semihydrate to active carbon within the range 10:1 .
~ to 15:1, and a ratio of calcium sulphate semihydrate to other solid matter in
.j the first-mentioned suspension, after precipitation, of approximately 7:10
In the event of the process being carried out continuously, it is particular-
ly advantageous for the one or more sul- ..
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~533~3
phide compounds to comprise hydrogen sulphide, and for the phosphoric acid
~ to be purified to be treated with the hydrogen sulphide before either of them
reaches the reaction zone. It is also advantageous in the present process
that, for a fluorine content of approximately 1 weight %~ the phosphoric acid
to be purified should contain at least 6.5 weight % of H2S04, calculated on
its P205 content, or an equivalent proportion (i.e. equivalent with respect
to the sulphuric acid of alkali metal sulphate).
The process of the present invention offers a series of technically
beneficial effects. To begin with, it enables the loss of phosphoric acid
to be kept to a minimum by precipitating the sulphuric acid in the form of
calcium sulphate semihydrate, recrystallizing the calcium sulphate semi-
hydrate in the form of the dihydrate3 and thoroughly washing ~he dihydrate;
- it further enables the concentration of the incoming phosphoric acid to be
- only slightly reduced by the recycling of the filtrate from the separation
of the calcium sulphate dihydrate. In addition to this, the separation of
normally difficultly filterable silicofluoride and sulphide precipitates is
less of a technical problem than in the prior art, as these compounds can be
1 entrained in a relatively easily filterable calcium sulphate semihydrate
j sludge or slurry if they are precipitated jointly with the semihydrate.
Phosphoric acids purified by the process of the invention can contain very
minor proportions of sulphate and fluorine and can be substantially free
from organic contaminants. The fact that it is not necessary for the prèsent
reaction medium to be cooled intermediarily and thereafter reheated is a
further considerable improvement in the art, having regard both to the
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. technical and to the commercial aspects of the purification process.
The calcium compound employed to precipitate the sulphuric acid in ;-
the form of calcium sulphate should preferably be ground lime, hydrate of
~'~ lime~ calcium carbonate or crude calc`ium phosphate. It may conveniently be
;` added in stoichiometric proportions~ based on the sulphate, so as to avoid
the presene of an unnecessary excess of calcium in the solution. ~`
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Fluorine is precipitated in the form of sodium silicofluoride by
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means of the silicic acid or derivative thereof. A particularly convenient
silicic acid derivative is water glass. It is possible to employ silicic
` acid in the form of ~e.g.) kieselguhr or phosphorus furnace slag, which may
be used in admixture with an alkali liquor or alkali metal sulphate, and
; which is preferably added in stoichiometric proportions. No silicic acid
or derivative thereof need be added in those cases in which the crude phos-
phoric acid itself has sufficient silicic acid therein, and the reference
herein to treatment in the presence of silicic acid or a derivative thereof
are to be construed accordingly. -
: 10 ~ith respect to heavy metal sulphides, it is possible for them to
be precipitated from the crude phosphoric acid by the addition of H2S to
the acid before it reaches the above-mentioned reaction zone, or by the
addition of an alkali metal or alkaline earth metal sulphide thereto, within
the reaction zone. Organic contaminants in the crude phosphoric acid are
adsorbed by the active carbon, of which 0.1 to 1.0 % calculated on the P2O5,
is usually needed, depending on the particular phosphoric acid used.
The separation of the various precipitates from the phosphoric
acid may conveniently be achieved by means of one or more pressure filters.
The filtration capacity required will depend upon the various factors dis-
cussed above and upon the residence time of the material therein. Underordinary filtration conditions, i.e. under pressures within the range 0 to
5 atmospheres gauge, the filtration capacity should preferably be approxi-
mately 180 litres of filtrate per m2 per hour for short residence times of
0.5-1 hour. The advisable filtration capacity increases to 300 litres per -~ `
m2 per hour for residence times within the range 5 to 7 hours. ~ -
It is possible for the dihydrate to be filtered off continuously
with the use of a rotating vacuum drum filter. Under the above-mentioned
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ordinary filtration conditions, the average filtration capacity of this
type of filter should preferably be 650 litres of filtrate per m2 per hour,
or 100 kg of filter cake ~moist with adhering water) per m2 per hour.
The filter cake is preferably washed continuously on the filter
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with fresh water, the filtrate from this washing being combined with the
primary filtrate from the vacuum filter, so that it is impossible for the
P2O5 concentration in the resulting solution to increase beyond a certain
value. Approximately 1/3 of the combined filtrates is preferably added
continuously to the purified phosphoric acid, the balance (i.e. approximately
2/3~ thereof being recycled and re-used for converting semihydrate to di-
hydrate. In a typical case, the P2O5 loss associated with the separation of
calcium sulphate dihydrate is approximately 0.5 %, under these conditions.
If the present process is applied to phosphoric acid containing
; 10 about 51 % of P205, the filtrate from the semihydrate filtration stage, to
- which has been added a propor~ion of filtrate from the dihydrate filtration
stage, has in a typical case an approximate composition as follows: 47 %
of P2O5; 0.~ % of SO4, based on P205; 0.24 % of Ca, based on P205; and 0.5 %
of fluorine, based on P2O5.
In this typical case the phosphoric acid contains altoge~her
approximately 1 ppm of heavy metal contaminants and between 150 and 300 ppm
~ of C, depending on the origin of the crude acid.
i In addition to a process as defined earlier, the present invention
includes an apparatus suitable for use in carrying out that process, com-
prising: a first agitator-containing reactor provided with a first feed i~
pipe for wet-processed phosphoric acid, a second feed pipe for a calcium
compound, a third feed pipe for at least one alkali metal and/or silicic acid
compound, and a metering admission means for active carbon, all three feed
pipes and the metering means opening into the reactor, and the reactor being t~,
provided also with an overflow; an agitator-containing first reservoir
connected to the first reactor by means of said overflow; an outlet line -
from the first reservoir, a branch line and a pressure filter, the outlet
line running back from the first reservoir to the first reactor and the
branch line connecting the outlet line with the pressure filter; a filtrate - -
, 30 conveying line, a second reservoir, a filter cake-conveying line, a second
agitator-containing reactor having an outlet line and a rotating filter; the
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filtrate conveying line connecting the pressure filter to the second
reservoir, the filter cake-conveying line connecting the pressure filter to
the second reactor, and the outlet line of the second reactor running to the
rotating filter; a wash water feed line opening on to the rotating filter;
a filter cake discharge means co-operating with the rotating filter; a
', second filtrate conveying line, and a washed filtrate-conveying line, both
running from the rotating filter to, and opening into a collecting tank
having a bottom outlet, and the said bottom outlet leading to a pipe which
is bifurcated to feed one branch line running back to the second reactor and
~; 10 another branch line running to the second reservoir. The first feed pipe
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or the first reactor may additionally be provided with a hydrogen sulphide ;~-
, or alkali metal sulphide inlet.
A preferred embodiment of the apparatus of the present invention
l will now be described with reference to the accompanying drawing, the single ;
i Figure of which is a diagrammatic side view of the said embodiment.
;I In the case to which the drawing relates, phosphoric acid which
' contains 50 weight % of P2O5, which is at a temperature of 80C, and which
¦ has been treated with H2S is conveyed through a line 1 to a first reactor 4,
in which it is admixed with a recycled suspension having precipitated CaS04
semihydrate suspended therein. The suspension comes from a reservoir 7 and
! is recycled through a line 3. Lime or another calcium-containing reagent
coming from a reservoir 24, through a line 5, is added in metered quantities
I by means of a conveyor-belt weighing device the use of which is diagrammatic-
ally indicated at 25. At the same time, reactor 4 is charged with sodium
hydroxide solution, or a solution of sodium sulphate in admix~ure with water
glass or another silicic acid-containing substance, through a line 2, and
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with active carbon, through a metering device 6. The resulting suspension
is conveyed from the first reactor 4 to the reservoir 7, in which it is
allowed to remain for ton average) a period of 4 hours, with agitation, at
85C. Line 3 is used not only for the above-mentioned recycling but also to
deliver a portion of the suspension through a line 8 to a pressure filter 9,
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in which the phosphoric acid is separated from the CaSO~i semihydrate. The
filtered phosphoric acid flows through a filtrate line 10 into a reservoir
11, whereas the CaSO4 semi-hydrate separated from it in filter 9 is delivered
to a second reactor 13J by means of a line 12. In the second reactor 13,
the filter cake from the filter 9 is treated with a mixture of another
filtrate and wash water, this mixture being brought from a collecting vessel
19, through a return line 14, and is thereby converted at 40C into CaS04
dihydrate. The conversion product accumulating in the lower portion of the
reactor 13 is pumped through a line 15 to a rotating filter 16, and the
resulting filtrate is conveyed to the collecting vessel 19 through a line 17.
The filter cake from the filter 16 is washed with fresh water coming from
line 21, and thereafter passes to discharge means 20. The filtra~e resul~ing
from this washing leaves filter 16 through a line 18, and is united in the -
collecting vessel 19 with the primary filtrate coming from the rotating
filter through ~he line 17. Approximately two-thirds of the resulting
solution is recycled through the return line 14 to the second reactor 13 and
re-used for the conversion of CaS04 semihydrate to dihydrate therein, and
~, the balance, i.e. approximately one-third, is conveyed through a line 22 to
the reservoir 11, and is there united with the purified phosphoric acid
containing filtrate from the filter 9. The combined phosphoric acid contain- ;~
ing solution is discharged through a line 23.
The pipe 1, or the first reactor 4, may be additionally provided ,;:
with a hydrogen sulphide or alkali metal sulphide feed pipe.
The following Examples further illustrate the invention, which is
not, however, limited thereto.
EX~MPLE 1:
An apparatus as shown in the accompanying drawing was used.
156.29 kg/h of crude phosphoric acid (from Khouviga phosphate~ which contained
51.26 % of P205, 3.32 % of H2SO4, 0.51 % of F, 0.16 % of Ca and traces of
copper and arsenic, which was at a temperature of 80C, and which had been
treated earlier with hydrogen sulphide gas, was introduced into the first
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reactor (4) and mixed therein with 0.56 kg/h of active carbon, 1.52 kg/h of
water glass, 0.8 kg/h of a 50% sodium hydroxide solution, and recycled
suspension from the reservoir 7. 2.97 kg/h of unslaked lime from the respec-
tive reservoir (24) was added at the same time, at a maximum temperature
-; of 95C. The suspension travelled continuously from the first reactor (4)
to the reservoir 7, in which it was allowed ~o remain for 2 hours at a mean
~ temperature of 85C. By means of lines 3 and 8, the suspension was delivered
i to the pressure filter (9) and filtered therein under a pressure which was
- increased gradually from O to at most 6 atmospheres gauge.
138.65 kg/h of filtrate, containing 52.0 % of P205, 0.30 % of S04,
^ 0.13 % of Ca and 0.17 % of F, was delivered from the pressure filter to the
~;~ reservoir 11, through the line 10. 23.0 kg/hr of sludge, comprising CaS04
semihydrate, sodium silicofluoride, active carbon, adherîng organic contami-
^~ nants, and arsenic and copper sulphides, was separated, and was delivered - `
i from the pressure filter to the second reactor (13) and mixed therein with
65 kg/h of filtrate (from the second filtration step) coming from the
collecting vessel 19. The recycled filtrate contained 21.9 % of P205, 0.66 %
of S04, 0.27 % of Ca and 0.56 % of F. The second reactor (13) was operated
continuously, and was found to contain sufficient CaS04 dihydrate crystals
for the purpose of seeding, these being crystals introduced thereinto after
crystallizing out from previously treated suspension. After a mean residence
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time for 45 minutes, during whlch the CaS04 semihydrate underwent conversion
, to dihydrate, the suspension was filtered by means of a rotating vacuum drum
1! filter ~16). On the filter, which worked continuously, the retained di-
3 hydrate sludge was washed with 20.85 kg/h of fresh water. The primary
filtrate and the filtrate resulting from this washing were united in the -~
collecting vessel (19) and mixed therein. 29.25 kg/h of the resulting
solution was pumped into the reservoir 11 and mixed with the filtrate from
the pressure filter 9. 167090 kg/h of purified solution, containing 46.75%
3Q of P205, 0.37 % of S04, 0.15 % of Ca, and 0.24 % of F, was obtained as the
final product.
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This product contained altogether 1 ppm of heavy metals and 160
ppm of C. Dihydrate sludge was discharged from the rotating filter 16 at a
rate of 14.60 kg/h, this sludge containing 28.9 % of adhering water with
~per hour) 0.4 kg of P203, 0.65 kg of NaSiF6, 0.56 kg of acti~e carbon and
organic substances adhering thereto, 7.66g ofCus, and 3.13 % of As2S3.
EXAMPLE 2:
. The apparatus was as used in Example 1. 100 l/h of crude phos-
; phoric acid (prepared from Youssoufia phosphate) which contained approximate-
. .
ly 51% of phosphoric acid, 2.5 % of H2SO4, 0.51 % of F, 0.2 % of C, and
~races of copper and arsenic, which was at a temperature of 80C, and which
had been treated earlier with gaseous hydrogen sulphide, was introduced into
the first reactor (4) and mixed therein with 1.91 kg/h of NaS04. Simultane-
ously there was recycled from the reservoir 7 a proportion of a phosphoric
acid suspension containing precipitated CaSO4 semihydrate, sodium silico-
fluoride, heavy metal sulphides, and active carbon with adsorbed organic
contaminants. 5.46 kg/h of CaCO3 coming from the respective reservoir (24)
was added by means of the conveyor-belt weighing device (25) to the mixture
of material in the first reactor (4) to effect the formation of additional
CaS04 semihydrate. 0.71 kg/h of silicic acid and 0.5 kg/h of active carbon
were added at the same time. The resulting suspension was delirered to
the reservoir 7 and from there to the pressure fil~er ~9), where it was `~
filtered. CaSO4 semihydrate was filtered off, recrystallized to dihydrate
as in Example 1, separated in the rotating vacuum filter (16) from its
mother liquor, and washed with water. The mother liquor and wash water were
united in the collecting vessel (19). 2/3 of the resulting solution was
recycled to the second reactor (13), and the balance ~1/3) was united in the
reservoir 11 with filtrate from the pressure filter 9. The solution contained
47.5 % of P205, 0.31 % of S04, 0.19 % of F, and organic contaminants and
heavy metals in similar quantities to those indicated in Example 1.
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