OA16788A - Dihydrate-hemihydrate process for producing phosphoric acid. - Google Patents
Dihydrate-hemihydrate process for producing phosphoric acid. Download PDFInfo
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- OA16788A OA16788A OA1201300485 OA16788A OA 16788 A OA16788 A OA 16788A OA 1201300485 OA1201300485 OA 1201300485 OA 16788 A OA16788 A OA 16788A
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- Prior art keywords
- slurry
- phosphoric acid
- weight
- attack
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- NBIIXXVUZAFLBC-UHFFFAOYSA-N phosphoric acid Chemical compound OP(O)(O)=O NBIIXXVUZAFLBC-UHFFFAOYSA-N 0.000 title claims abstract description 92
- 229910000147 aluminium phosphate Inorganic materials 0.000 title claims abstract description 46
- 238000000034 method Methods 0.000 title claims abstract description 6
- JIUIIWVYHWSFAZ-UHFFFAOYSA-N AC1O0V2M Chemical compound O.O.O.O.O JIUIIWVYHWSFAZ-UHFFFAOYSA-N 0.000 title description 3
- 239000002002 slurry Substances 0.000 claims abstract description 63
- QAOWNCQODCNURD-UHFFFAOYSA-N Sulfuric acid Chemical compound OS(O)(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-N 0.000 claims abstract description 61
- OSGAYBCDTDRGGQ-UHFFFAOYSA-L Calcium sulfate Chemical compound [Ca+2].[O-]S([O-])(=O)=O OSGAYBCDTDRGGQ-UHFFFAOYSA-L 0.000 claims abstract description 22
- 229910052731 fluorine Inorganic materials 0.000 claims abstract description 13
- 239000011737 fluorine Substances 0.000 claims abstract description 13
- YCKRFDGAMUMZLT-UHFFFAOYSA-N fluorine atom Chemical compound [F] YCKRFDGAMUMZLT-UHFFFAOYSA-N 0.000 claims abstract description 13
- 235000012970 cakes Nutrition 0.000 claims abstract description 11
- 230000015572 biosynthetic process Effects 0.000 claims abstract description 10
- 238000005755 formation reaction Methods 0.000 claims abstract description 10
- 238000010438 heat treatment Methods 0.000 claims abstract description 10
- 239000008346 aqueous phase Substances 0.000 claims abstract description 9
- 238000001953 recrystallisation Methods 0.000 claims abstract description 5
- 238000004519 manufacturing process Methods 0.000 claims description 25
- 238000006243 chemical reaction Methods 0.000 claims description 24
- 239000011435 rock Substances 0.000 claims description 21
- 239000002253 acid Substances 0.000 claims description 13
- 238000007792 addition Methods 0.000 claims description 8
- PASHVRUKOFIRIK-UHFFFAOYSA-L calcium sulfate dihydrate Chemical compound O.O.[Ca+2].[O-]S([O-])(=O)=O PASHVRUKOFIRIK-UHFFFAOYSA-L 0.000 claims description 8
- 239000000203 mixture Substances 0.000 claims description 8
- ZOMBKNNSYQHRCA-UHFFFAOYSA-J calcium sulfate hemihydrate Chemical compound O.[Ca+2].[Ca+2].[O-]S([O-])(=O)=O.[O-]S([O-])(=O)=O ZOMBKNNSYQHRCA-UHFFFAOYSA-J 0.000 claims description 6
- CPLXHLVBOLITMK-UHFFFAOYSA-N magnesium oxide Chemical compound [Mg]=O CPLXHLVBOLITMK-UHFFFAOYSA-N 0.000 claims description 6
- PNEYBMLMFCGWSK-UHFFFAOYSA-N AI2O3 Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 claims description 4
- 238000011084 recovery Methods 0.000 claims description 4
- 238000005063 solubilization Methods 0.000 claims description 4
- JEIPFZHSYJVQDO-UHFFFAOYSA-N iron(III) oxide Inorganic materials O=[Fe]O[Fe]=O JEIPFZHSYJVQDO-UHFFFAOYSA-N 0.000 claims description 3
- 150000001674 calcium compounds Chemical class 0.000 claims description 2
- 230000005591 charge neutralization Effects 0.000 claims description 2
- 230000001264 neutralization Effects 0.000 claims description 2
- 238000006386 neutralization reaction Methods 0.000 claims description 2
- 150000002222 fluorine compounds Chemical class 0.000 claims 1
- 150000004683 dihydrates Chemical class 0.000 abstract description 22
- 238000005530 etching Methods 0.000 abstract 2
- VSIIXMUUUJUKCM-UHFFFAOYSA-D pentacalcium;fluoride;triphosphate Chemical compound [F-].[Ca+2].[Ca+2].[Ca+2].[Ca+2].[Ca+2].[O-]P([O-])([O-])=O.[O-]P([O-])([O-])=O.[O-]P([O-])([O-])=O VSIIXMUUUJUKCM-UHFFFAOYSA-D 0.000 abstract 2
- 239000002367 phosphate rock Substances 0.000 abstract 2
- -1 etching Chemical compound 0.000 abstract 1
- 239000000725 suspension Substances 0.000 abstract 1
- 229910052602 gypsum Inorganic materials 0.000 description 28
- 239000010440 gypsum Substances 0.000 description 28
- 229940032330 Sulfuric acid Drugs 0.000 description 25
- 238000002425 crystallisation Methods 0.000 description 15
- 230000005712 crystallization Effects 0.000 description 15
- 238000001914 filtration Methods 0.000 description 15
- 235000021317 phosphate Nutrition 0.000 description 13
- 238000009434 installation Methods 0.000 description 11
- NBIIXXVUZAFLBC-UHFFFAOYSA-K [O-]P([O-])([O-])=O Chemical compound [O-]P([O-])([O-])=O NBIIXXVUZAFLBC-UHFFFAOYSA-K 0.000 description 10
- 239000010452 phosphate Substances 0.000 description 10
- 239000012535 impurity Substances 0.000 description 9
- 238000010561 standard procedure Methods 0.000 description 8
- 238000005406 washing Methods 0.000 description 8
- 238000000605 extraction Methods 0.000 description 7
- ODINCKMPIJJUCX-UHFFFAOYSA-N calcium monoxide Chemical compound [Ca]=O ODINCKMPIJJUCX-UHFFFAOYSA-N 0.000 description 6
- 239000000047 product Substances 0.000 description 6
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 5
- VTYYLEPIZMXCLO-UHFFFAOYSA-L calcium carbonate Chemical compound [Ca+2].[O-]C([O-])=O VTYYLEPIZMXCLO-UHFFFAOYSA-L 0.000 description 4
- 230000005070 ripening Effects 0.000 description 4
- 229960005069 Calcium Drugs 0.000 description 3
- OYPRJOBELJOOCE-UHFFFAOYSA-N calcium Chemical compound [Ca] OYPRJOBELJOOCE-UHFFFAOYSA-N 0.000 description 3
- 229910052791 calcium Inorganic materials 0.000 description 3
- 239000011575 calcium Substances 0.000 description 3
- 239000000292 calcium oxide Substances 0.000 description 3
- 239000000706 filtrate Substances 0.000 description 3
- 239000003295 industrial effluent Substances 0.000 description 3
- 150000003013 phosphoric acid derivatives Chemical class 0.000 description 3
- 229960003563 Calcium Carbonate Drugs 0.000 description 2
- 238000002441 X-ray diffraction Methods 0.000 description 2
- 229910052925 anhydrite Inorganic materials 0.000 description 2
- 239000007864 aqueous solution Substances 0.000 description 2
- 229910000019 calcium carbonate Inorganic materials 0.000 description 2
- 239000007788 liquid Substances 0.000 description 2
- 239000007791 liquid phase Substances 0.000 description 2
- 238000002156 mixing Methods 0.000 description 2
- 239000012071 phase Substances 0.000 description 2
- 238000004064 recycling Methods 0.000 description 2
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 description 2
- QAOWNCQODCNURD-UHFFFAOYSA-L sulfate Chemical compound [O-]S([O-])(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-L 0.000 description 2
- WUKWITHWXAAZEY-UHFFFAOYSA-L Calcium fluoride Chemical compound [F-].[F-].[Ca+2] WUKWITHWXAAZEY-UHFFFAOYSA-L 0.000 description 1
- 241000196324 Embryophyta Species 0.000 description 1
- QORWJWZARLRLPR-UHFFFAOYSA-H Tricalcium phosphate Chemical compound [Ca+2].[Ca+2].[Ca+2].[O-]P([O-])([O-])=O.[O-]P([O-])([O-])=O QORWJWZARLRLPR-UHFFFAOYSA-H 0.000 description 1
- 240000008042 Zea mays Species 0.000 description 1
- 235000002017 Zea mays subsp mays Nutrition 0.000 description 1
- 229910001634 calcium fluoride Inorganic materials 0.000 description 1
- 239000001506 calcium phosphate Substances 0.000 description 1
- 229910000389 calcium phosphate Inorganic materials 0.000 description 1
- 235000011010 calcium phosphates Nutrition 0.000 description 1
- 229910052681 coesite Inorganic materials 0.000 description 1
- 150000001875 compounds Chemical class 0.000 description 1
- 235000005822 corn Nutrition 0.000 description 1
- 235000005824 corn Nutrition 0.000 description 1
- 230000000875 corresponding Effects 0.000 description 1
- 229910052906 cristobalite Inorganic materials 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- XLYOFNOQVPJJNP-ZSJDYOACSA-N heavy water Substances [2H]O[2H] XLYOFNOQVPJJNP-ZSJDYOACSA-N 0.000 description 1
- 239000002440 industrial waste Substances 0.000 description 1
- 238000002347 injection Methods 0.000 description 1
- 239000007924 injection Substances 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 238000006011 modification reaction Methods 0.000 description 1
- 238000011017 operating method Methods 0.000 description 1
- 229910052904 quartz Inorganic materials 0.000 description 1
- 238000005070 sampling Methods 0.000 description 1
- 239000000377 silicon dioxide Substances 0.000 description 1
- 239000007787 solid Substances 0.000 description 1
- 239000011343 solid material Substances 0.000 description 1
- 229910052682 stishovite Inorganic materials 0.000 description 1
- 238000000844 transformation Methods 0.000 description 1
- 230000001131 transforming Effects 0.000 description 1
- 229910052905 tridymite Inorganic materials 0.000 description 1
- 230000004580 weight loss Effects 0.000 description 1
Abstract
The invention relates to a process for producing phosphoric acid, including etching, in an aqueous medium, phosphate rock using sulfuric acid, resulting in the formation of a first dihydrate slurry, in suspension in an aqueous phase, having a free P2O5 content of between 38 and 50% and a free SO3 content lower than 0.5%, converting said first slurry by means of heating, resulting in the recrystallization of the solubilized calcium sulfate so as to obtain a second hemihydrate slurry, and separating the second slurry into industrial phosphoric acid and a hemihydrate cake, characterized in that said process includes, during the etching, adding a fluorine source into the first slurry with a content of 1% to 5% of F relative to the P2O5 contained in the phosphate rock.
Description
“A method for producing phosphoric acid of the dlhydrate/hemihvdrate type11
The présent invention relates to a method for producing phosphoric acid by attack of phosphated rock by means of sulfuric acid.
The standard method of this type consists of reacting phosphated rock with sulfuric acid under conditions giving rise to crystallization of calcium sulfate dihydrate or gypsum (CaSO4.2H2O). The gypsum slurry obtained in a first reactor may then be subject, in a second reactor, to ripening allowing enlargement of the sulfate grains obtained, and this in order to increase filterability. The ripened slurry is then filtered, with phosphoric acid being obtained, having a free P2O5 content of the order of 25 to 35% by weight. The obtained gypsum still contains enough P2O5l either non-attacked, or co-crystallized, i.e. set in the crystalline lattice of gypsum. This limits the extraction yield of P2O5 contained in the phosphate and makes the gypsum unsuitable for certain applications.
Methods for producing phosphoric acid by attack with sulfuric acid giving, at higher températures and at P2O5 and/or SO3 concentrations, a calcium sulfate slurry in the form of a hemihydrate (CaSO4.1/2H2O) or anhydrite, are also known. These methods generally give a concentrated phosphoric acid and a well filterable sulfate, but the P2O5 extraction yield of these methods is less than the standard method. In certain cases, after this attack, it is also proceeded with conversion of the obtained calcium sulfate hemihydrate into calcium sulfate dihydrate (c.f. Schrôdter et al., Phosphoric Acid and Phosphates, Ullman’s Encyclopedia of Industrial Chemistry, 2008, pages 8 and 9).
Methods comprising triple crystallization of calcium sulfate first as a hemihydrate and then as a dihydrate and finally again as a hemihydrate, are also known (c.f.. US-A-4,588,570).
With view to improving the results of the standard method, provision was made, after separating the production phosphoric acid from the gypsum slurry, for mixing the remainder of the latter with concentrated sulfuric acid and optionally for heating the whole, in order to couvert gypsum into calcium sulfate hemihydrate. The thereby formed second slurry is in turn filtered and
-2the acid filtrate is recycled to the attack step (see FR 1485940). This method clearly improves the P2Os extraction yield. Indeed, P2O5 co-crystallized in gypsum during the attack is released during solubilization of gypsum crystals, while calcium sulfate recrystallized as a hemihydrate, is very pure and extremely well filterable. However this method requires two filtrations, i.e. two filters, whence enormous congestion of the installation on an industrial scale.
According to an alternative of this method, provision was made, after having applied the conditions of the standard method for forming a gypsum slurry, for directly subjecting the latter to mixing with sulfuric acid and optîonally to heating, without separating beforehand the production phosphoric acid therefrom. The obtained hemihydrate slurry is filtered while obtaining a very pure hemihydrate cake, but a filtrate formed with a mixture of phosphoric acid and sulfuric acid. In order to obtain a quality production phosphoric acid, this mixture has then to be subject to a desulfation installation, which therefore does not solve the congestion problems. Other methods for producing phosphoric acid very similar to the previous one are further known, comprising a conversion of dihydrate into hemihydrate and having the same drawbacks (GB-1,164,836; US-A-3,984,525).
Finally a method is known in which the phosphated rock is again subject to attack conditions of the standard method so as to obtain a first sluny in which the formed gypsum has a grain size allowing good filtration. A portion of this first slurry is then sampled and subject to conditions under which the gypsum is converted into a hemihydrate, while thereby forming a second slurry. The remainder of the first slurry is then mixed with the second and the whole is filtered (see WO 2005/118470).
A major problem in the production of phosphoric acid lies in the déplétion of P2O5rich phosphate ore deposits. These deposits hâve been exploited. Presently it is necessary to turn towards ores for which the P2Os concentration is considered as poor, for example P2O5 contents of 30% by weight or less as compared with phosphated rock, and in certain cases 20% or less.
A method allowing exploitation of such ores and extraction therefrom of a high quality production phosphoric acid was described in the international patent application PCT/EP2010/068709 not yet published. The attack conditions of this method provide a substantially stoichiometric reaction between the introduced sulfuric acid and the calcium contained in the phosphated rock, while the free P2O5 content in the crystallization slurry is maintained high between 38 and 50% by weight and the température between 70 and 90°C. Surprisingly, these conditions give rise to very fine crystals of stable dihydrate. This slurry is then subject to an increase in température during which the dihydrate grains solubilize and release the non-attacked or cocrystallized P2O51 while crystallization of calcium sulfate hemihydrate which is well filterable and a production phosphoric acid with very low free SO3 content are obtained.
-3It should be noted that frequently these ores poor in P2O$ further hâve increasingly high contents of impurities. The content of impurities is commonly expressed by the ratio (AI2O3 + Fe2O3 + MgO)/P2O5 X100, also noted as MER (Minor Element Ratio). So-called standard phosphates are characterized by an MER ratio from about 5 to 8. Beyond 10, the impurity content is so large that it begins to negatively influence the crystallization of calcium sulfate as gypsum during the attack ofthe ore by sulfuricacid. At these impurity contents, the production of phosphoric acid becomes a problem, notably because of difficulties in crystallization of calcium sulfate dihydrate and in filtration of the latter. This therefore has a great disadvantage in ail the methods where filtration directly occurs after attack of the phosphated rock.
In a method as described in the non-published patent application PCT/EP2010/068709, gypsum crystallization is also affected by the impurities, but given that this gypsum is not intended to be filtered, this does not hâve any conséquence.
However, the step for converting the dihydrate slurry into a hemihydrate slurry proves to be laborious in the case of a phosphate ore with a high impurity content, notably in the case of MER ratios of more than 10, and in particular of more than 20 or even 30. Crystallization of the hemihydrate cornes of increasingly poor quality, which required an increase in the total acidity (P2O5 + SO3) in the conversion slurry in order to remedy this poor crystallization. Inevitably this causes an increasingly high content of free SO3 in the slurry and the formation of a production phosphoric acid increasingly strongly polluted with sulfuric acid.
The object of the présent invention is to develop a method for producing phosphoric acid by attack of poor quality phosphated rock by means of sulfuric acid which allows a quality production phosphoric acid and a good P2O5 extraction yield from the rock to be obtained. This method should further be able to be easily applied in an existing standard installation and therefore not requiring costly and indefensible transformations, economically speaking.
In order to solve these problems, provision is made according to the invention for a method for producing phosphoric acid, comprising
- attack in an aqueous medium of phosphated rock with sulfuric acid at a first température comprised between 70 and 90°C with formation of a first slurry of calcium sulfate dihydrate crystals suspended in an acid aqueous phase, the acid aqueous phase of this slurry having a free P2O5 content comprised between 38 and 50% by weight and a free SO3 content of less than 0.5% and greater than 0.05% by weight,
- conversion of this first slurry by heating to a température above 90°C, with solubilization of the calcium sulfate dihydrate crystals and recrystailization of the solubilized calcium sulfate giving
-4rise to a second slurry formed with calcium hemihydrate crystals suspended in an aqueous phase based on phosphoric acid, and
- séparation in the second slurry between a production phosphoric acid, having a free SO3 content of less than 2% and greater than 0.05% by weight, and a séparation cake based on calcium sulfate hemihydrate, this method further comprising, during the attack, an addition of a fluorine source in the first slurry with a 1% to 5% weight content of F relatively to the P2O5 contained in the phosphated rock.
In the présent patent application, it should be understood that the free phosphoric acid and free sulfuric acid contents are expressed in free P2O5 and free SO3.
Quîte unexpectedly, as this will be explained subsequently, the method according to the invention gives rise during the attack step under the specified conditions to a slurry of dihydrate crystals, while one skilled in the art would expect the formation of hemihydrate. The dihydrate crystallization is not optimum, the crystals are small, they hâve a grain size having a dæ of less than 20 pm. In the presence of a phosphated rock having a (AI2O3 + Fe2O3 + MgO)/P2O5 x 100 ratio of more than 10, in particular more than 20, notably of the order of 30, it is noted that crystallization of gypsum becomes a problem. It is further noted that after adding the fluorine source, advantageously directly in the tank for attack of the phosphated rock, crystallization of dihydrate is affected, the crystals becoming not only small but further heterogeneous. Such a slurry would hâve a very low filtration coefficient and therefore would not be able to be exploited industrially in a process requiring its filtration. This aspect is however of no importance according to the invention since this first slurry is not intended to be filtered.
The attack conditions are such that they provide a substantially stoichiometric reaction between the introduced sulfuric acid and the calcium contained in the phosphated rock, mainly in the form of calcium carbonate and phosphate. The acid aqueous phase of this first slurry resulting from the attack contains no or extremely little free sulfuric acid and its free P2O5 content is quite high.
Advantageously, the attack température may be comprised between 70 and 80°C.
Preferentially, the free P2O5 concentration resulting from the attack step may be from 40 to 45% by weight.
The free SO3 concentration of the acid aqueous phase of the first slurry may be from 0.1 to 0.4% by weight.
This first slurry is then directly subject, in its totality, to a conversion step which consists of heating it to a température above 90°C, preferably comprised between 90 and 105°C.
This heating in a known way causes solubilization of the gypsum crystals, release of P2O5
-5co-crystallized from gypsum during the attack step and recrystallization of the calcium sulfate as hemihydrate.
It is noted that after adding a fluorine source, advantageously directly into the tank for attacking the phosphated rock, the subséquent conversion of the first slurry of dihydrate into hemihydrate becomes easier, which is expressed by a réduction in the acidity required for this conversion.
A second slurry is thus obtained in a simple and easy way, wherein the hemihydrate crystals hâve a spherical shape and are of a common size, for example having a of 60 pm, which gives a filtration cake having an excellent filtration coefficient.
By adding a fluorine source, it is possible, even under the conditions of a phosphated rock with a high rate of impurities, to obtain proper crystallization of the hemihydrate without having to necessarily add sulfuric acid.
The production acid has an extremely low free SO3 content advantageously of the order of 0.05% to less than 1% by weight, which makes it a quality phosphoric acid. It is possible to advantageously obtain a production phosphoric acid having a free P2O5 content from 35 to 45% by weight.
According to an embodiment of the invention, the step for converting the first slurry does not comprise any addition of sulfuric acid. However it may be contemplated that the method comprise during the step for converting the first slurry, introduction of sulfuric acid into the latter. The amount of added sulfuric acid should however be metered accurately so that, after filtration of the second slurry, the production phosphoric acid contains a free SO3 content of less than 2% by weight, preferably of the order of 0.05 to 1% by weight, in particular of the order of 0.05 to less than 1% by weight. It is important not to exceed this content in order not to pollute phosphoric acid with sulfuric acid which would make it unsuitable for use as a phosphoric acid and would require a desulfation step.
By fluorine source, should be understood according to the invention any composition capable of releasing fluorine into the slurry of the attack. For example CaF2 or further a product from recovery of a fluorinated effluent may be contemplated. It is possible to contemplate the metered addition of a compound obtained as a product from the neutralization of fluorinated effluents, which has been precipitated by a calcium compound such as calcium carbonate or oxide. The solid material decanted and separated from the effluent may be directly introduced into the attack tank.
According to a preferred embodiment of the invention, the method comprises the attack step in a first reactor, and a transfer of the first slurry from the first reactor to a second
-6reactor, in which is carried out the conversion step with formation of the second slurry, the séparation step being carried out on a filter. This method has the advantage of being able to be used in an existing installation for producing phosphoric acid by standard attack with sulfuric acid. The first reactor is the attack reactor of the standard installation, wherein different attack conditions are applied. The second reactor is the ripening reactor of the standard installation. As an enlargement of the gypsum grains resulting from the attack is not necessary according to the invention, it is possible to use this ripening reactor as a conversion reactor. Finally the filter of the standard installation may be used for filtering the hemihydrate instead of gypsum. This filter may be any known suitable filtration device, for example a band filter, a device with filtering cells arranged as a carrousel, inter alia.
The method according to the invention is advantageously carried out continuously. Preferably the dwelling time in the first reactor is from 2 to 4 hours and the dwelling time in the second reactor is from 0.5 to 1.5 hours. The dwelling times correspond to the dwelling times in the attack and ripening reactors of a standard installation.
Otherembodiments ofthe invention are indicated in the appended claims.
Other details and particularities of the invention will become apparent from the description given hereafter, not as a limitation, but with reference to the appended drawings.
Fig. 1 illustrâtes as a flow diagram, an exemplary installation applying a method according to the invention.
Fig. 2 is a graph illustrating régions for the formation of dihydrate and of hemihydrate versus température, P2O5 concentration and SO3 concentration.
Fig. 3 is a graph which illustrâtes the different processes applied for producing phosphoric acid by sulfuric attack.
The installation illustrated in Fig. 1 comprises an attack reactor 1 in which crushed phosphated rock having an MER ratio of the order of 20-32 is introduced in 2 and sulfuric acid in 3, for example 98-99 wt% concentrated sulfuric acid. A composition including 52% by weight of CaO and 21% of F is for example advantageously introduced in 17 into the reactor 1.
The operating conditions applied in this reactor are the following:
Température | 70-80°C |
Free P2O5 % | 38-50 % by weight |
Free SO3 %: | 0.05 % - < 0.5 % by weight |
Dwelling time | 2-4 h. |
-7Unlike what is expected by one skilled in the art, very fine heterogeneous but stable dihydrate crystals are obtained.
Indeed, Fig. 2 may now be consulted. The illustrated graph is extracted from A.V. Slack, Phosphoric Acid, Vol. 1, Part I, ed. Marcel Dekker Inc., 1968, New York. The température in ordinates is indicated in °C, the P2O5 and SO3 concentrations in abscissae. The curves illustra te the equilibrium between dihydrate and hemihydrate. The région of the graph located below these curves corresponds to the conditions for forming dihydrate, those located above these curves to those for forming hemihydrate. The hatched ellipse illustrâtes the area meeting the preferential conditions of the attack according to the présent invention. Unlike what is provided by examining this graph, a stable slurry of dihydrate and not hemihydrate crystals is obtained according to the invention.
The same conclusion may be drawn upon examining the graph of Fig. 3 extracted from P. Becker, Phosphates and Phosphoric Acid. 2nd ed., Marcel Dekker Inc., 1989, New York Basle. In this graph, the température is indicated in ordinates and the P2O5 concentration in abscissae. In the lower left corner of the graph is found the région for forming the dihydrate, in the right upper corner is found the région for forming anhydrite and between both, the région for forming hemihydrate. The area A corresponds to the conditions of the standard method with formation of gypsum. The areas B and C, connected by an arrow, and D and E connected by an arrow, correspond to the conditions of two methods first forming hemihydrate and then converting the latter into dihydrate. The areas F and G connected by an arrow illustrate the conditions of a method first forming dihydrate and then converting the latter into hemihydrate.
The ellipse marked with an asterisk meets the attack conditions according to the invention and the ellipse marked with two asterisks connected to the first by an arrow, fits the conversion conditions according to the invention.
According to the general knowledge of one skilled in the art, the formation of hemihydrate could therefore be expected in both areas corresponding to these two ellipses, which is not the case and this is already surprising per se.
The totality of the gypsum slurry obtained in the reactor 1 is then transferred through the conduit 6, provided with transfer means known per se, to a conversion reactor 7. In this reactor, the following operating conditions are applied:
Température | 90-105°C |
Ρΐθδ% | 35-45 % by weight |
so3% | 0.1 % - <1.0 % by weight |
Dwelling time | 0.5-1.5 h. |
-8In order to heat this reactor, common means may be applied, for example direct heating, injection of steam into the conversion medium or a combination of both. Any other suitable heat source may of course be used for this purpose. Under the effect of the increase of heat, the crystals of gypsum solubilize, the co-crystallized P2O5 is released and the calcium sulfate recrystallizes as relatively pure hemihydrate crystals.
According to an alternative method, it is possible in order to further improve the conversion of gypsum into hemihydrate, to add in the conversion reactor 7 a small metered amount of sulfuric acid. This amount should however be such that the free SO3 content in the production acid be less than 2% by weight, preferably 1%. The production acid cannot be polluted by this addition of sulfuric acid.
The illustrated method comprises a transfer of the slurry of the reactor 7 through the conduit 9 to a filter 10 of a common type.
In a first section of the filter, a filtrate is obtained in 11, which is production phosphoric acid and a filtration cake.
The illustrated method comprises two steps for washing the cake. The second washing is carried out with a washing liquid which is preferably water, supplied in 12.
The product of this washing obtained in 13 is an aqueous solution with a low phosphoric acid content which is used as washing liquid supplied in 14 to the first washing step. The product of this first washing, obtained in 15, is an aqueous solution of phosphoric acid which may be recycled in 4 to the attack reactor 1, via a recycling conduit 16.
As compared with the so-called standard method, the method according to the invention provides the advantage of an improved extraction yield, given the recrystallization and the production of an acid with a high P2O5 content, with a content of more than 35% by weight, instead of the 25 to 30% obtained with the standard method, and this in spite of the high content of impurities of the initial rock. The dry hemihydrate cake is well recoverable in industry. Its packing properties are excellent, since in the presence of water or humidîty, the hemihydrate reconverts into gypsum. These improvements are obtained in a simple way in an existing installation, without increasing the congestion thereof. The method further allows recycling of an industrial waste, of a fluorinated industrial effluent, but also taking out a function therefrom, which is bénéficiai for the method.
The method according to the invention will be described below in a more detailed way by means of non-limiting exemplary embodiments.
Example 1
-9A pilot test was conducted with a sedimentary phosphate from Central Asia containing 17% of P2O5, 29% of CaO, 2% of F, 34.7% of SiO2, and a MER of 32.
The grain size distribution of the crushed rock was close to the one generally used in phosphoric acid plants (99.8% passing through 500pm, 60% passing through 150pm and 40% passing through 75pm),
Two cases are to be distinguished:
a) in the attack tank, the phosphate is introduced without any external fluorine source.
b) in the attack tank, a mixture of phosphate with an external fluorine source (advantageously a product from the recovery of formulated industrial effluents) is introduced in order to facilitate the dihydrate-hemihydrate conversion.
a) The phosphate is introduced into the attack reactor with a volume of 30 liters by a feeding screw and the throughput (3.2kg/h) is controlled by a weight loss system. Two metering pumps inject sulfurîc acid (1.1kg/h) and the recycled acid from the washing of the filtration cake of the hemihydrate slurry (12kg/h).
The attack of the phosphate with sulfuric acid was carried out under the following conditions:
P2O5 content | 39.1% by weight |
SO3 excess | 0.46% by weight |
Température | 80°C |
Solids content | 32% by weight |
The output flow rate of the attack pulp is of about 11 l/h. The average dwelling time in the attack reactor (calculated as being the ratio between the volume of the reactor and the output flow rate of the dihydrate slurry) is therefore of about 30 1/11 l/h = 2.7 hours. Under these conditions, a stable slurry of small size dihydrate crystals (d60 less than 20 pm) is obtained, with a monomodal size distribution, These are mainly isolated crystals but cross- and star-shaped crystals are also observed. These crystals, which would hâve very poor filterability, are not filtered in the relevant method.
The gypsum slurry obtained during this first step is then heated up to 101°C in a second 15 liter reactor. A second addition of sulfuric acid (about 892 g/h) is carried out so as to be able to obtain, in the presence of a high rate of impurities in the attack slurry, conversion of the gypsum crystals into hemihydrate. The free SO3 content required for this conversion is 7% by weight. It is only under these extreme SO3 conditions, characterized by a « total acidity »
-10(% P2O5+% SO3) of 45% by weight, that recrystallization of the gypsum into hemihydrate was able to be obtained.
Crystals with a spherical shape and characterized by d50 = 42 pm were obtained.
The thereby produced phosphoric acid contains 38% by weight of P2O5 and 7% by weight of free SO3. If such an operating procedure was used, a desulfation step for the acid would be required, which should be avoided according to the invention.
b) In the second case, the working parameters are similar to the previous case, but a mixture of the phosphate with 10% by weight (relatively to P2O5) of a product from the recovery of fluorinated industrial effluents (équivalent to 5% by weight of F) is introduced into the attack tank. The dihydrate - hemihydrate conversion is clearly easier and, at the same température of 101°C, it is attained for a « total acidity » (% P2O5+% SO3) of 38% by weight, which allows observance of an SO3 content in the production phosphoric acid of less than 2% by weight. Under these conditions, the method of this invention is applicable.
The produced phosphoric acid contains 36.2% by weight of P2OS and 1.8% by weight of free SO3. The filtration cake of the obtained hemihydrate contains a P2O5 total content of 0.54% by weight which corresponds to a total P2O5 extraction yield (attack and filtration) of 97.8% by weight. The free water content of the hemihydrate cake is 22.9%. The crystallization water is of 6.4%, which ïs typical for hemihydrate, and X-ray diffraction actually confirms that this is hemihydrate.
Example 2
A slurry formed with calcium sulfate and phosphoric acid titrating by weight 4.16% of P2Os, 0.4% of SO3) 0.24% of AI2O3, and 0.99% of F is obtained by attack of a North African phosphate.
It is stable, at least for a duration of 8 hours, as dihydrate when the température is 78°C.
Next, it is subject to addition of sulfuric acid in steps of 0.2%. When the SO3 content in the liquid phase is 0.6%, both phases (dihydrate and hemihydrate) are stable and co-exist. When the SO3 content in the liquid phase is 0.8% of SO3, the conversion into hemihydrate is complété, the hemihydrate phase is the stable form from now on. Conversion is confirmed by observing crystals under the microscope, by measuring the crystallization water (6.18%) and X-ray diffraction (XRD).
It should be understood that the présent invention is by no means limited to the embodiments described above and that many modifications may be provided thereto within the scope of the appended claims.
Claims (11)
1. A method for producing phosphoric acid, comprising
- attack in an aqueous medium of phosphated rock with sulfuric acid at a first température comprised between 70 and 90°C, with formation of a first slurry of calcium sulfate dihydrate crystals suspended in an acid aqueous phase, the acid aqueous phase of this slurry having a free P2O5 content comprised between 38 and 50% by weight and a free SO3 content of less than 0.5% and greater than 0.05% by weight,
- conversion of this first slurry by heating to a température above 90°C, with solubilization of the calcium sulfate dihydrate crystals and recrystallization of the solubilized calcium sulfate giving rise to a second slurry formed with calcium sulfate hemihydrate crystals suspended in an aqueous phase based on phosphoric acid, and
- séparation in a second slurry between a production phosphoric acid, having a free SO3 content of less than 2% and greater than 0,05% by weight, and a séparation cake based on calcium sulfate hemihydrate, characterized in that It further comprises during the attack, an addition of a fluorine source in the first slurry in a content of 1% to 5% by weight of F relatively to the P2O5 contained in the phosphated rock.
2. The method according to claim 1, characterized in that the phosphated rock has a weight ratio (AI2O3 + Fe2O3 + MgO)/P2O5 X 100 greater than 10.
3. The method according to one of claims 1 and 2, characterized in that the fluorine source is a fluorine compound which may release fluorine in the attack slurry.
4. The method according to claim 3, characterized in that said fluorine source is a product from the recovery of a fluorinated effluent obtained by neutralization and précipitation of the latter with a calcium compound, this precipitated composition being introduced into said slurry.
5. The method according to one of claims 1 to 4, characterized in that it comprises, during the step for converting the first slurry, introduction of sulfuric acid into the latter.
6. The method according to one of claims 1 to 4, characterized in that the step for converting the first slurry does not comprise any addition of sulfuric acid.
7. The method according to any of claims 1 to 6, characterized in that it comprises the attack step in a first reactor and a transfer of the first slurry from the first reactor to a second reactor in which is carried out the conversion step, with the formation of the second slurry, the séparation step being carried out on a filter.
8. The method according to claim 7, characterized in that heating occurs by heating the second reactor, by injectîng steam into the second slurry, or by both of these processes simultaneously.
9. The method according to either one of claims 7 and 8, characterized in that the
5 dwelling time in the first reactor is from 2 to 4 hours and the dwelling time in the second reactor from 0.5 to 1.5 hours.
10. The method according to any of claims 1 to 9, characterized in that the production of phosphoric acid has a P2O5 content from 35 to 45% by weight.
11. The method according to any of claims 1 to 10, characterized in that it is carried 10 out continuously.
Applications Claiming Priority (1)
Application Number | Priority Date | Filing Date | Title |
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EPPCT/EP2011/059128 | 2011-06-01 |
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OA16788A true OA16788A (en) | 2016-01-04 |
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