CA1045339A - Process and apparatus for purifying wet-processed phosphoric acid - Google Patents

Process and apparatus for purifying wet-processed phosphoric acid

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Publication number
CA1045339A
CA1045339A CA235,803A CA235803A CA1045339A CA 1045339 A CA1045339 A CA 1045339A CA 235803 A CA235803 A CA 235803A CA 1045339 A CA1045339 A CA 1045339A
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CA
Canada
Prior art keywords
phosphoric acid
calcium sulphate
suspension
acid
reactor
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired
Application number
CA235,803A
Other languages
French (fr)
Inventor
Klaus P. Ehlers
Gero Heymer
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Hoechst AG
Original Assignee
Hoechst AG
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Filing date
Publication date
Priority to DE2447390A priority Critical patent/DE2447390C3/en
Application filed by Hoechst AG filed Critical Hoechst AG
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Publication of CA1045339A publication Critical patent/CA1045339A/en
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Classifications

    • CCHEMISTRY; METALLURGY
    • C01INORGANIC CHEMISTRY
    • C01BNON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B25/00Phosphorus; Compounds thereof
    • C01B25/16Oxyacids of phosphorus; Salts thereof
    • C01B25/18Phosphoric acid
    • C01B25/234Purification; Stabilisation; Concentration
    • C01B25/237Selective elimination of impurities

Abstract

ABSTRACT OF THE DISCLOSURE
Process and apparatus for purifying crude wet-processed phosphoric sold containing free sulphuric acid. To this end, the wet-processes phosphoric acid is treated in a reaction zone, at a temperature of 70°C to 100°C, with a calcium compound and with calcium sulphate seed crystals;
the sulphate acid is precipitated in the form of calcium sulphate semihydrate with the resultant formation of a suspension; a proportion of the suspension, with the calcium sulphate semihydrate seed crystals present therein, is recycled to the reaction zone; the balance of the sus-pension is treated for separation of purified phosphoric acid from the accompanying precipitate; the precipitate is transformed into an aqueous suspension and the calcium sulphate semihydrate in said suspension is converted into calcium sulphate dihydrate; the aqueous phase of the sus-pension is separated from the solid matter therein, and the solid matter is water-washed; a proportion of the aqueous phase so separated in recycled together with a pro-portion of the wash water, for use in suspending and con-verting calcium sulphate semihydrate to dihydrate; and the balance of the aqueous phase and wash water are com-bined with the said purified phosphoric acid.

Description

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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 .. . .
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 . --1-- ~ . .
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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 ~ ~
' ' ,:

., -53~3 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 :! .
~j^ matter; recycling a proportion of the aqueous phase so separated9 and a pro- :
, 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 ;~ ~
. 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. ~`
.. ... ~
Fluorine is precipitated in the form of sodium silicofluoride by _5_ :' 5i33i9 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 :, , 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 i339 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 -7- `
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~5339 ... :
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 .1 ,,~ .
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 ~, .
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, .

~gS33~

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 _g_ .

"

` ~L63~339 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 , .
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.

, .' ' ~

5~
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.
,

Claims (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. In the process for purifying crude wet-processed phosphoric acid containing free sulphuric 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 alkali metal-, silicic acid- or sulphide-compounds as precipita-ting agents for the said further contaminants together with an absorbent for organic contaminants; and separating the phosphoric acid so purified sub-sequently from solid matter present in the suspension obtained; the improve-ment which comprises: adding to the crude wet-processed phosphoric acid containing from 45 to 55 weight% of P2O5 in a reaction zone, at a temperature of 80 to 100°C, the calcium compound, the calcium sulphate seed crystals, in the form of calcium sulphate semihydrate in the presence of the said precipi-tating agents and active carbon as adsorbent; precipitating thereby the sulphuric acid in the form of calcium sulphate semihydrate together with the further contaminants with the resultant formation of a suspension, passing off the suspension from the reaction zone; recycling a proportion of this sus-pension to the reaction zone; separating the balance of the suspension into purified phosphoric acid and solid matter; forming from the solid matter an aqueous suspension containing up to 30 weight% of phosphoric acid and convert-ing the calcium sulphate semihydrate in the aqueous suspension at 20 to 70°C
into calcium sulphate dihydrate; separating the aqueous phase of the aqueous suspension from the solid matter therein, and water-washing the separated solid matter; recycling a proportion of the aqueous phase so separated, and a proportion of the wash water, for forming the aforementioned aqueous sus-pension containing the calcium sulphate semihydrate and combining the balance of the aqueous phase and wash water with the purified phosphoric acid.
2. The process as claimed in claim 1, wherein the wet-processed phosphoric acid to be purified is one which contains from 45 to 55 weight%
of P2O5.
3. The process as claimed in claim 1, wherein a proportion of the first-mentioned suspension is recycled to the reaction zone after a period of 45 to 60 minutes.
4. The process as claimed in claim 1, wherein the calcium sulphate semihydrate is converted to calcium sulphate dihydrate at a temperature of 40 to 50°C.
5. The process as claimed in claim 1, wherein the proportions of the aqueous phase separated from the calcium sulphate dihydrate and of washwater which are recycled for use in suspending and converting calcium sulphate semi-hydrate are proportions which provide a suspension having a solids content of 10 to 20 weight%.
6. The process as claimed in claim 5, wherein the said proportions are proportions which provide a suspension having a solids content of 12 weight%.
7. The process as claimed in claim 1, wherein the proportions of the calcium compound, alkali metal compound(s), silicic acid, and sulphide compound(s), and the active carbon, which are employed are proportions which provide a ratio of calcium sulphate semihydrate to other solid matter in the first-mentioned suspension, after precipitation, of approximately 7 : 1.
8) The process as claimed in claim 1, wherein the sulphide compound is hydrogen sulphide, and the phosphoric acid to be purified is treated with the hydrogen sulphide before either of them reaches the reaction zone.
9) The process as claimed in claim 1, wherein the active carbon is added in a proportion sufficient to establish in the first-mentioned suspension, after precipitation, a ratio of calcium sulphate semihydrate to active carbon within the range 10 : 1 to 15 : 1.
10) The process as claimed in claim 1, wherein the phosphoric acid to be purified contains at least 6.5 weight % of H2SO4, calculated on its P2O5 content, or an equivalent proportion (i.e. equivalent with respect to the sulphuric acid) of alkali metal sulphate, its fluorine content being approximately 1 weight %
calculated on its P2O5 content.
11) The process as claimed in claim 1, wherein the phosphoric acid to be purified is mixed with the respective calcium compound and the other precipitant, and the seed crystals, in the reaction zone, at a temperature of 80 to 100°C.
12) An apparatus suitable for purifying wet-processed phosphoric acid by the process as claimed in claim 1, comprising: a first agitator-containing reactor provided with a first feed 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 metering admission means for active carbon, all three feed pipes and the metering means opening into the reactor, and the reactor being 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 conveying line, a second reservoir, a filter cake conveying line, a second agitator-containing reactor having an outlet line, and a rotating filter; the 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 cooperating 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 another branch line running to the second reservoir.
13) An apparatus as claimed in claim 12, wherein the first feed pipe or the first reactor is additionally provided with a hydrogen sulphide or alkali metal sulphide inlet.
CA235,803A 1974-10-04 1975-09-18 Process and apparatus for purifying wet-processed phosphoric acid Expired CA1045339A (en)

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FR2447347B1 (en) * 1979-01-24 1983-02-04 Rhone Poulenc Ind
SE418845B (en) * 1979-05-10 1981-06-29 Boliden Ab PROCEDURE FOR PURIFICATION OF VAT PROCESS PHOSPHORIC ACID
DE3134847A1 (en) * 1981-09-03 1983-03-17 Hoechst Ag, 6000 Frankfurt "METHOD FOR PRODUCING CADMIUM-FREE RAW PHOSPHORIC ACID"
DE3202659A1 (en) * 1982-01-28 1983-08-04 Hoechst Ag, 6230 Frankfurt METHOD FOR PURIFYING WET METHOD PHOSPHORIC ACID
DE3360853D1 (en) * 1982-02-12 1985-10-31 Hoechst Ag Process for the reduction of the heavy metal content in raw phosphate
IL69100A (en) * 1982-07-21 1986-11-30 Hoechst Ag Process for removing heavy metal ions and arsenic from wet-processed phosphoric acid
DE3325862A1 (en) * 1983-07-18 1985-01-31 Hoechst Ag, 6230 Frankfurt METHOD AND DEVICE FOR REMOVING ORGANIC IMPURITIES FROM WET PROCESS PHOSPHORIC ACID
DE19703884C2 (en) * 1997-02-03 1999-04-29 Bk Giulini Chem Gmbh & Co Ohg Process for purifying wet process phosphoric acid

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FR1472520A (en) * 1966-03-23 1967-03-10 Marchon Products Ltd Phosphoric acid desulfation process

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FR2286790B1 (en) 1979-10-12
EG11849A (en) 1978-06-30
NL170839B (en) 1982-08-02
DE2447390A1 (en) 1976-04-08
JPS5163394A (en) 1976-06-01
ES441121A1 (en) 1977-03-16
NL7511568A (en) 1976-04-06
FI752749A (en) 1976-04-05
DE2447390C3 (en) 1983-11-17
JPS537319B2 (en) 1978-03-16
TR19680A (en) 1979-10-05
GB1524552A (en) 1978-09-13
SE401668B (en) 1978-05-22
DD120417A5 (en) 1976-06-12
IL48236D0 (en) 1975-12-31
DK137229B (en) 1978-02-06
BE834003A (en) 1976-03-30
DK137229C (en) 1978-07-10
FR2286790A1 (en) 1976-04-30
CH599900A5 (en) 1978-06-15
BR7506423A (en) 1976-08-10
IT1047666B (en) 1980-10-20
AT358525B (en) 1980-09-10
ZA755820B (en) 1976-08-25
NL170839C (en) 1983-01-03
ATA753175A (en) 1980-02-15
SE7511066L (en) 1976-04-05
DE2447390B2 (en) 1977-10-20
DK447575A (en) 1976-04-05
IL48236A (en) 1978-04-30
SU671716A3 (en) 1979-06-30

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