Note: Descriptions are shown in the official language in which they were submitted.
635
The present invention pertainstoa pTocess for purifying a wet-
process phosphoric acid and, more particularly, pertains to a defluorination
process for producing a phosphoric acid of alimentary quality.
It is known that the acid attack on phosphate-bearing natural rocks
provides, after filtration, a crude phosphoric filtrate containing numerous
anionic and cationic impurities, including metal ions such as Al, Fe, etc.,
and anions such as those from the acid, and fluoride, and fluosilicate result-
ing from impurities in the rock. This impure acid is unsuitable for certain
applications, and it is for this reason that different purification processes
have been proposed. In particular, the presence of the fluorine, irrespective
of its form, such as fluoride ionl fluosilicic ion or a metal-fluorine com-
plex, prevents it from being used in the alimentary industry which requires a
phosphoric acid having a fluorine content by weight, relative to the content
of P2O5 of the aqueous solution of lower than 10 parts per million ~ppm).
Different processes for removing the fluorine have been proposed,
which comprise precipitating the fluorine from the crude phosphoric filtrate
in the form of alkali or alkaline-earth metal fluosilicate. Another type of
process comprises entraining the fluorine from the crude acid by steam or a
hot gas in the form of the volatile species HF or SiF4. However, such pro-
cesses provide at best a phosphoric acid having a fluorine content by weight
to P2O5 of 500 ppm, which renders it unsuited for use in the food industry.
The present invention seeks to provide a process for purifying wet-
process crude phosphoric acid.
The present invention provides a process for purifying and de-
fluorinating an inpure phosphoric acid produced by a sulphuric acid treatment
of a phosphate-bearing rock which comprises the counter-current extraction of
the impure aqueous phosphonic acid solution with an organic solvent having a
low miscibility with water in the presence of concentrated sulfuric acid,
washing of the resulting phosphoric acid organic phase with water or purified
phosphoric acid and counter-current re-extracting the phosphoric acid con-
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tained in the organic phase with water, separating the resulting dilute
aqueous solution of decationised and partially defluorinated phosphoric acid,
and treating said solution with steam, hot gas, or a mixture thereof to obtain
a concentrated and defluorinated aqueous phosphoric acid solution in which the
F/P2O5 ratio by weight is lower than 10 ppm.
The present invention also provides a further embodiment of the
above process characterized in ~hat the phosphoric acid organic phase result-
ing from the extraction of the crude aqueous acid is further treated before
washing with water or with pure phosphoric acid with an aqueous solution con-
taining phosphate ions and calcium ions prepared from calcium phosphate, phos-
phoric acid and water, in respective amounts such that the content of calcium
ions in solution is from 2 to 6% by weight expressed as CaO, so as to pre-
cipitate the sulphate ions in the form of calcium sulfate and to put them in
suspension in said solution.
The final product obtained is a concentrated solution of defluorin-
ated phosphoric acid whose F/P2O5 ratio by weight is lower than 10 ppm.
One embodiment, of the invention comprises a process uherein an im-
pure phosphoric acid solution resulting from an acid attack on phosphate
containing rock is continuously extracted in the form of a purified solution
having a concentration greater than 90%, by treating the impure phosphonic
acid solution in a first series of counter-current extraction apparatus, with
a solvent selected from the class of alkylated derivatives of phosphoric acid,
and adding concentrated strong acid, such as sulfuric acid, at one or more
points in said first series of extraction apparatus; extracting the resulting
solution of phosphoric acid in said solvent in the return direction with an
aqueous solution in a second series of counter-current extraction
apparatus, the thus resulting solution of phosphoric acid in said solvent is
then washed in a third series of counter-current extraction apparatus by a
fraction of the aqueous solution from the second series of counter-current
apparatusJ and returning the fraction of aqueous solution, upon discharge from
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the third series of counter-current apparatus, to the first series of extrac-
tion apparatus to the initial impure phosphoric acid solution, returning the
solvent issuing from the second series of extraction apparatus, to the first
series of extraction apparatus and collecting at the discharge from the first
series of extraction apparatus a solution which has little or no phosphoric
acid and which contains the strong acid added, and at the discharge from the
second series of extraction apparatus the purified solution of phosphoric acid.
The resulting aqueous solution of phosphoric acid, is substantially
free from cationic impurities and is partially defluorinated. The aqueous
phosphoric acid solution which has been purified in this way, and which issues
from the second series of counter-current extraction apparatus, is additionally
subjected to a treatment by means of steam or hot gas, whereby the final pro-
duct collected is a concentrated solution of defluorinated phosphoric acid,
in which the F/P205 ratio by weight is lower than 10 ppm~
In this embodiment, the aqueous phosphoric acid solution issuing
from the second series of extraction apparatus for extraction of the organic
phase by water in counter-current generally has a phosphoric acid concentra-
tion expressed as P2O5 of from 15 to 30%, preferably from 20 to 26%, a fluorine
concentration expressed as the F/P2O5 ratio by weight of from 1000 ppm to
7000 ppm, with the fluorine being substantially present in the form of hydro-
fluoric acid and fluosilicic acid.
In a further e~odiment,the invention comprises a process wherein the
phosphoric acid contained in the impure solutions resulting from the acid
attack on phosphate containing rocks is continuously extracted a purified
solution having a concentration of greater than 95%, wherein the impure phos-
phoric acid is subjected to counter current extraction in the presence of con-
centrated strong acid such as sulfuric acid and water with a solvent having a
low miscibility with water, and separating an aqueous phase containing most
of the impurities and an organic solvent phase containing the phosphoric acid
which is then re~extracted. In this emhodiment, in a first step, the impure
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phosphoric acid is subjected to counter-current extraction in the presence of
sulfuric acid and water by means of solvent which has a low miscibility with
water selected from aliphatic alcohols containing less than ~ carbon atoms in
the molecule. In the second stepl the separated organic phase containing the
phosphoric acid and sulfuric acid is contacted with an aqueous solution con-
taining phosphate ions and calcium ions. Such a solution may be prepared
from calcium phosphate, phosphoric acid and water, in respective amounts such
that the content of calcium ions in solution is from 2 to 6% by weight ex-
pressed as CaO.
The second step results in the formation of a suspension of calcium
sulfate reducing or eliminating the sulfate ion concentration in selection.
In a third step the organic phase containing the phosphoric acid is
washed with water, and in a fourth step, the phosphoric acid is extracted from
the organic phase with water. The organic solvent is separated from a purified
aqueous solution which forms the product. The resulting aqueous phosphoric
acid solution is substantially free from cationic impurities and partially de-
~; fluorinated. The aqueous phosphoric acid solution which is purified in this -
way is additionally subjected to a treatment by means of steam or hot gas,
whereby the final product collected is a concentrated solution of defluorinated
phosphoric acid in which the F/P2O5 ratio by weight is lower than 10 ppm.
In this second embodiment, the aqueous phosphoric acid solution re-
sulting from re-extraction of the organic phase by water in counter-current
generally has a phosphoric acid concentration expressed as P205 of from 15 to
30% and preferably from 20 to 26%, and a fluorine concentration expressed in
the form of the F/P2Q5 ratio by weight of from 1000 ppm to 7000 ppm, the
; fluorine being substantially present in the form of hydrofluoric acid and
fluosilicic acid.
In its most general aspect, the process of the invention comprises
treatment of the preceding aqueous solution which is substantially free from
cations, by means of steam or hot gas. Taking into account the fluorine-
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bearing species present in the solution, under these conditions it is possible
to entrain the fluorine in the vapour phase in the form of hydrofluoric acid
and SiF4, which is separated from the aqueous solution, and to exhaust the
solution in respect of fluorine until the value of the ratio F/P205 is lower
than 10 ppm. When this is done, the aqueous solution of phosphoric acid which
is generally in the diluted condition undergoes concentration which may be up
to 55% of P205-
In a defluorination method of the invention, the aqueous phosphoricacid solution is subjected to a treatment with steam or hot gas in a single
apparatus, whereby said solution is concentrated and defluorinated. The treat-
ment may be a discontinuous operation comprising an evaporator of known type
into which the acid solution is introduced, and heated to boiling temperature,
or else a flow of steam or hot gas is passed into the solution which has been
pre-heated to a given temperature. The concentration-defluorination treatment
may be performed continuously with the streams of aqueous acid solution and
steam or hot gas flowing in the same direction or opposite directions, the
counter-current system being preferred. The continuous treatment apparatus
may have one or more stages. A preferred apparatus of the continuous counter-
current type comprises a multi-stage column into which the aqueous solution to
be concentrated and defluorinated is introduced at the top, while ~he flow of
steam or hot gas is introduced into the column at the bottom, to collect the
concentrated and defluorinated phosphoric acid at the column bottom. Knowledge
of the separation isotherms in respect of fluorine in the form of HF, H2SiF6,
and SiF4, as between a phosphoric acid solution and its vapour makes it pos-
sible to determine the selection of the parameters of the apparatus and its
mode of operation as regards the number of stages, temperatures, pressures and
,
flow rates, for pre-established values of the final concentration of the acid
` and its degree of exhaustion in respect of fluorine. Other types of continu-
ous apparatus, with a plurality of stages, comprise well-known gas-liquid
contactor means such as a filled column, a column ~ith plates, a spraying
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column, etc.
The temperature in the apparatus is from 105C to 135C and more
particularly from 120C to 130C, and the pressure may be a pressure which is
equal to or lower than atmospheric pressure, and is generally from 0.6 bar to
1 bar, these two parameters being linked.
In a second defluorination method, the regenerated aqueous phosphoric
acid solution which is substantially free from cations is first concentrated
in a first step to the final desired concentration by means of steam or hot
gas; during this concentration step a part of the fluorine present in the
solution passes into the vapour phase, taking into account the operating condi-
tions, but the F/P205 weight ratio produced after this concentration step does
not attain the low value required in respect of an acid of alimentary quality.
It is for this reason that the solution produced in this first step is then
subjected to a final treatment of defluorination proper, by means of steam or
hot gas.
The apparatus for concentrating the acid in the first step of this -
second embodiment comprises well-known types of evaporators, operating dis-
continuously or continuously, in co-current or in counter-current. To reduce
the energy involved in the process, an apparatus of the triple-action counter-
current type is preferred. The separation isotherms in respect of water as ~-
between an aqueous phosphoric acid solution and its vapour make it possible
to determine the operating conditions to obtain a predetermined P205 concentra-
tion, starting from a given dilute solution. In general, operation is under
conditions of an increasing pressure in the direction of the flow of phosphoric
acid. The pressure is preferably from 0.07 bar to 1.3 bar, and under condi-
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tions of increasing temperature in the direction of the flow of phosphoric acid,
of from 50C to 145C. Under these conditions, introducing a dilute solution
of a P205 concentration of about 20%, results in a solution having a P205
concentration of about 50%.
,
In the second step of this second embodiment, the aqueous phosphoric
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acid solution which has been concentrated to the desired value, and which is
produced as the result of the second stepJ is subjected to advanced defluorina-
tion by contact with steam or a hot gas. The apparatus used for carrying out
this step is a known co or counter-current gas-liquid contacting means operat-
ing discontinuously or continuously. It is preferred however to use a counter-
current apparatus uith a plurality of stages, such as a column with plates.
The phosphoric acid solution which enters the multi-stage counter-current
apparatus is generally pre-heated to the temperature at which it begins to
boil at the pressure in the apparatus, and is maintained at that temperature.
The stream of steam is also heated to said temperature so that there is no -
substantial variation in the P205 concentration of the solution. Knowledge
of the separation isotherms in respect of the fluorine as between an aqueous
phosphoric acid solution and its vapour phase makes it possible to define the
selection of the parameters as regards the number of stages of the apparatus
and its operating conditions, in order to arrive at a predetermined degree of
exhaustion in respect of fluorine, starting from a given initial acid. The
temperature in the contacting means is generally higher than or equal to 130C
and the pressure is higher than or equal to 1 bar.
The process of the invention makes it possible to remove the
fluorine present in the aqueous regeneration solution of the phosphoric acid,
to give an F/P205 weight ratio, in dependence on the selection of the operat-
ing conditions and the form of the apparatus. In particular an exhaustion in
respect of fluorine such that an F/P205 ratio of lower than 10 ppm can easily
be obtained.
The phosphoric acid obtained in accordance with the process can be
used in the food industry and in particular in human foodO
The following examples are given by way of illustration and are not
intended to limit the scope of the invention.
In the drawing Figures 1 and 2 are schematic diagrams of the pro-
cesses of the present invention.
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EXA~PLE 1
The phosphoric acid solution to be purified is derived from the
filtration of slurries produced by reaction of sulfuric acid on natural
phosphate-bearing rock. The fluosilicic acid which it contains is partially
separated by a treatment with sodium carbonate and a filtration operation.
This solution then has a P205 concentration of 33%, and its content
~- in respect of various impurities relative to the P205 content is:
H2S4 3.9%
Fe 1.56%
V 247 ppm
Mn 298 ppm
Ca 4050 ppm
Mg 1920 ppm
F 9700 ppm ~
Al 6600 ppm -
SiO2 4000 ppm
~-~ Cr 208 ppm
T-R and Y 250 ppm
Reference will be made to Figure 1 to follow the course of treatment
of this solution as described below.
A battery of 10 counter-current extractors 1 is supplied with the
stream 4 of phosphoric acid solution to be purifiedJ having the composition
set out above, at a rate of 9.4 m3/h, and with stream 5Ofpuretributylphosphate
at a rate of i5.6 m3/h. In addition, a stream 6 of 2040 kg/hour of 98% sul-
furic acid is introduced at the fifth stage of the battery 1. The tributyl-
phosphate which contains the phosphoric acid leaves the battery of counter-
current exractors 1 at 7 and passes into a second battery 3 consisting of four
extractors for washing with a branch flow 8 of 4.8 m3/h of a solution of pure
phosphoric acid taken from the discharge 9 of the 10-stage extractor battery
2.
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The flow of tributylphosphate which contains the phosphoric acid
and from which some entrained impurities are removed in the battery 3 passes
at 10 into the battery 2 in which it is treated with a counter-current flow of
deionised water 11 at a rate of 13.6 m3/h.
After the branch flow has been drawn off at 8 from the discharge
9 of purified phosphoric acid solution, the remaining stream 12 at rate of
12.6 m3/h of 26% P2O5 acid. The phosphoric acid solution used for the washing
operation issues from the battery 3 at 13 and is fed to the solution 4 to be
purified. The flow 5 of tributylphosphate which supplies the battery 1 issues
from the battery 2. This flow is purified continuously in an extractor (not
shown), taking off a flow of 3.2 m3/h which is treated by counter-current flow
of 1.6 m3/h of a solution of half-normal sodium hydroxide. An exhausted acid
solution titrating 1.3% of P2O5 also containing all the sulfuric acid added
and the various metal impurities issues from the battery 1 at 14. The purified
phosphoric acid solution issuing at 9 from the battery 2 is of the following
composition, the proportions of impurities being expressed relative to the
P2O content:
H2 4 0 5% Mg 37 ppm
Fe 60 ppm F 1500 ppm
V 4 ppm Al 16 ppm
Mn 1.8 ppm SiO2 1500 ppm
Ca 44 ppm Cr 8 ppm
T-R and Y 40 ppm
The degree of extraction of the phosphoric acid is 96%.
The purified phosphoric acid flow 12 is then treated in a decoloura-
'~ tion column by activated carbon. The resulting solution is introduced at the
top of a counter-current column with plates, which has four theoretical stages.
The initial temperature of the acid is 110C. A flow of steam heated at 300C
is introduced at the base of the column, under a pressure of 1 bar, the ratio
between the flow of steam and the flow of phosphoric anhydride being 40/1.
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At the column bottom there is collected a phosphoric acid solution at a tem-
perature of 135C, in which the P205 content by weight is 55% and the F/P205
ratio by weight is 10 ppm.
EXAMPLE 2
Figure 2 shows the general diagram of an installation for continu-
ously performing the process in its second embodiment.
Reference numeral 15 denotes the wet-process phosphoric acid to be
treated, a portion 16 of which passes into a reactor 17. The reactor receives
a stream of sulfuric acid at 18 and an aqueous suspension issuing from the sub-
sequent desulfation step at 19. The products from reactor 17 are passed into
a filtration zone 20 in which the insoluble products are separated at 21, ~-
while the liquid is passed by line 22 into an extraction contacting means 23
into which a solvent, which may be mixed with water, is also introduced at 24.
If required a portion of the sulfuric acid at 18, may be introduced by line 25
at any stage of the contacting means 23. The residual aqueous phase is re- ~ -
moved at the discharge 46.
An organic phosphoric acid extract is collected at the discharge 27
' of the extraction contacting means 23, and passed into a desulfation contact-
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~ ing means 24 which also receives an aqueous solution containing calcium ions
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at 25. The c~lcium containing solution is prepared in the reactor 26. This
solution is prepared by contacting tricalcium phosphate introduced at 27 with
a portion of the wet-process phosphoric acid introduced at 28 into the process.
The flow of this aqueous solution of calcium ions is combined with a flow 29
of calcium-containing suspension taken from the product issuing at 30 from the
contacting means 24.
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An organic extract is collected at the discharge 31 of the contact-
ing means 24 and passed into the washing contacting means 32 for washing with
~ater introduced at 33. The water exits the washing contacting means at 24
in the form of an aqueous solution, while the organic extract after washing
is introduced at 35 into a contacting means 36. A flow of water 37 is passed
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to the last stage of the contacting means 36, and a solution 40 of aqueous
purified phosphoric acidl containing 96% of the acid, is collected at the
first stage.
In this example~ the calcium-rich solution is formed by reaction
between the ore and all of the wet-process phosphoric acid which is introduced
into the process.
The flow of the liquid at 16 is then zero. The flow in the conduit
15 is equal to that in the conduit 28, namely 1 tonne per hour of an impure
wet-process phosphoric acid containing by weight: P2O5 25%; CaO 0.25%; and
SO4 1-5%; F 0.2%; MgO 0.2%; Fe2O3~A12O3=0.3%.
The liquid passed through the conduit 18 comprises H2SO4 at a flow
rate of 97 kg/h. The flow at 22 is a solution titrating P2O5 22%; soluble
CaO 0.02%; free H2SO4 4%, at a flow rate of 1100kg/hour. The contacting means
23 comprises 12 theoretical stages.
Isobutanol containing 8.3% of water is introduced at 24, at a flow
of 8 t/h.
The flow at 25 is zero.
, 280 kg/h of a solution titrating P2O5 4% and H2SO4 0.5%, and contain-
; ing all the metallic impurities of the initial impure acid is removed at 26.
... .
The contacting means 24 comprises one theoretical stage. 1080 kg/h
of a slurry titrating P2O5 26%, CaO 3% and CaSO4 2% is introduced at 25. Ore ;
titrating P2O5 35% and CaO 50% is introduced at 27 at a flow rate of 80 kg/h.
The flow rate at 30 is equal to the flow rate at 19, namely llOO kg/h of slurry
titrating P2O5 21%, insoluble gypsum 8% and soluble CaO 1%. The contacting
means 32 comprises one theoretical stage. 210 kg/h of water is introduced at
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,` 33. A washing solution titrating P2O5 15% and containing the metallic im-
,, purities which had been extracted with the phosphoric acid is introduced at
34 at a flow rate of 95 kg.
Water is passed to the eighth stage of the contacting means 36 by
the flow 37 at a flow rate of 1.4 t/h.
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A purified aqueous phosphoric acid solution containing 96% of the
acid entering and titrating 15% of P2O5 is collected at the first stage of
the contacting means 36.
The solvent 42 is dehydrated by known means before being re-
introduced into the contacting means 23 at 24.
The second mode of the invention is then performed, comprising
preliminary concentration of the aqueous phosphoric acid solution resulting
from regeneration with water of the organic phase, jointly with partial de-
fluorination, then a subsequent step of advanced defluorination, the P205
concentration of the acid remaining substantially unaltered. -
In the first step of concentrating and partially defluorinating the
solution of decationised phosphoric acid, the apparatus used is a triple-
action contacting means operating in the counter-current mode, the first ex-
changer being ofthe parallel~piped stainless steel shell type which is 7
metres in height, while the second and third exchangers are of the graphite
shell and tube type, ~eing 6.1 metres in height. Phosphoric acid from the
preceding regeneration operation is introduced thereinto at a flow rate of
16.7 tonnes/hour, the concentration and the fluorine and silica contents of
which are as follows:
- P2O5 content by weight ................................. 15%
- F/P205 ratio by weight ................................. 6150 ppm
- SiO2/P205 ratio by weight .............................. 2300 ppm
The steam is introduced in a ratio of 2.6 tonnes per tonne of P205
introduced. The operating parameters of the apparatus and the results achieved
are set out in the following table.
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Temp Absolute Content of the phosphoric
C pressure acid s olution
mm Hg P205 content %F/P2O5 SiO2 mg/l
Inlet Outlet
Exchanger 3 50 100 15 20.5 6150 800
Exchanger 2 77 300 20.5 25 6050 1140
_
Exchanger 1 130 760 . 50.5 800 190
The acid solution issuing from exchanger No 1 of the triple-action
contacting means is pre-heated to a temperature of 164C and is then continu-
ously introduced at the head of a plate column comprising fi~e theoretical
stages, while at the base of the column steam is introduced at a flow rate of
0.8 tonne/tonne of P2O5, at a temperature of 164C, the pressure in the column
; being 2.5 bars. The production is drawn off at the column bottom. The solu-
tio= produced contalns Sl~ of P205, and its F/P205 ratio by weight is 7 ppm.
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