OA18492A - Process for preparing calcium fluoride from fluosilicic acid. - Google Patents
Process for preparing calcium fluoride from fluosilicic acid. Download PDFInfo
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- OA18492A OA18492A OA1201700410 OA18492A OA 18492 A OA18492 A OA 18492A OA 1201700410 OA1201700410 OA 1201700410 OA 18492 A OA18492 A OA 18492A
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- Prior art keywords
- fluoride
- calcium fluoride
- ammonium
- reactor
- solution
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- WUKWITHWXAAZEY-UHFFFAOYSA-L Calcium fluoride Chemical compound [F-].[F-].[Ca+2] WUKWITHWXAAZEY-UHFFFAOYSA-L 0.000 title claims abstract description 112
- 229910001634 calcium fluoride Inorganic materials 0.000 title claims abstract description 109
- 238000004519 manufacturing process Methods 0.000 title claims abstract description 18
- OHORFAFFMDIQRR-UHFFFAOYSA-P Hexafluorosilicic acid Chemical compound [H+].[H+].F[Si-2](F)(F)(F)(F)F OHORFAFFMDIQRR-UHFFFAOYSA-P 0.000 title abstract description 54
- 238000000034 method Methods 0.000 claims abstract description 51
- QGZKDVFQNNGYKY-UHFFFAOYSA-N ammonia Chemical compound N QGZKDVFQNNGYKY-UHFFFAOYSA-N 0.000 claims abstract description 42
- VTYYLEPIZMXCLO-UHFFFAOYSA-L calcium carbonate Chemical compound [Ca+2].[O-]C([O-])=O VTYYLEPIZMXCLO-UHFFFAOYSA-L 0.000 claims abstract description 40
- 238000006243 chemical reaction Methods 0.000 claims abstract description 35
- 239000012065 filter cake Substances 0.000 claims abstract description 32
- LDDQLRUQCUTJBB-UHFFFAOYSA-N Ammonium fluoride Chemical compound [NH4+].[F-] LDDQLRUQCUTJBB-UHFFFAOYSA-N 0.000 claims abstract description 31
- PRKQVKDSMLBJBJ-UHFFFAOYSA-N Ammonium carbonate Chemical compound N.N.OC(O)=O PRKQVKDSMLBJBJ-UHFFFAOYSA-N 0.000 claims abstract description 25
- 239000001099 ammonium carbonate Substances 0.000 claims abstract description 25
- 235000012501 ammonium carbonate Nutrition 0.000 claims abstract description 25
- 239000000706 filtrate Substances 0.000 claims abstract description 25
- 238000001914 filtration Methods 0.000 claims abstract description 22
- 238000005406 washing Methods 0.000 claims abstract description 21
- VHUUQVKOLVNVRT-UHFFFAOYSA-N ammonium hydroxide Chemical compound [NH4+].[OH-] VHUUQVKOLVNVRT-UHFFFAOYSA-N 0.000 claims abstract description 20
- 229910000019 calcium carbonate Inorganic materials 0.000 claims abstract description 20
- 239000000908 ammonium hydroxide Substances 0.000 claims abstract description 18
- 229960003563 Calcium Carbonate Drugs 0.000 claims abstract description 17
- 239000000725 suspension Substances 0.000 claims abstract description 16
- 239000007788 liquid Substances 0.000 claims abstract description 11
- 238000001035 drying Methods 0.000 claims abstract description 7
- 230000002708 enhancing Effects 0.000 claims abstract description 7
- 238000009833 condensation Methods 0.000 claims abstract description 6
- 230000005494 condensation Effects 0.000 claims abstract description 6
- 238000002425 crystallisation Methods 0.000 claims abstract description 6
- 230000005712 crystallization Effects 0.000 claims abstract description 6
- 238000004821 distillation Methods 0.000 claims abstract description 6
- 230000001376 precipitating Effects 0.000 claims abstract description 4
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 claims description 83
- 239000000377 silicon dioxide Substances 0.000 claims description 41
- 239000002002 slurry Substances 0.000 claims description 33
- 235000012970 cakes Nutrition 0.000 claims description 12
- 239000011575 calcium Substances 0.000 claims description 12
- 239000000203 mixture Substances 0.000 claims description 12
- KRHYYFGTRYWZRS-UHFFFAOYSA-M fluoride anion Chemical compound [F-] KRHYYFGTRYWZRS-UHFFFAOYSA-M 0.000 claims description 9
- 239000002367 phosphate rock Substances 0.000 claims description 8
- 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 claims description 7
- 239000002253 acid Substances 0.000 claims description 6
- BPQQTUXANYXVAA-UHFFFAOYSA-N silicate Chemical compound [O-][Si]([O-])([O-])[O-] BPQQTUXANYXVAA-UHFFFAOYSA-N 0.000 claims description 5
- 229910004072 SiFe Inorganic materials 0.000 claims description 3
- 238000005201 scrubbing Methods 0.000 claims description 2
- ATRRKUHOCOJYRX-UHFFFAOYSA-N azanium;hydron;carbonate Chemical compound [NH4+].OC([O-])=O ATRRKUHOCOJYRX-UHFFFAOYSA-N 0.000 abstract 1
- 238000000354 decomposition reaction Methods 0.000 abstract 1
- 239000000243 solution Substances 0.000 description 43
- 229910052731 fluorine Inorganic materials 0.000 description 18
- 239000011737 fluorine Substances 0.000 description 18
- YCKRFDGAMUMZLT-UHFFFAOYSA-N fluorine atom Chemical compound [F] YCKRFDGAMUMZLT-UHFFFAOYSA-N 0.000 description 18
- 239000011541 reaction mixture Substances 0.000 description 18
- 235000011114 ammonium hydroxide Nutrition 0.000 description 17
- 239000002245 particle Substances 0.000 description 14
- ODINCKMPIJJUCX-UHFFFAOYSA-N calcium monoxide Chemical compound [Ca]=O ODINCKMPIJJUCX-UHFFFAOYSA-N 0.000 description 12
- 239000000047 product Substances 0.000 description 11
- 239000000126 substance Substances 0.000 description 11
- AXCZMVOFGPJBDE-UHFFFAOYSA-L Calcium hydroxide Chemical compound [OH-].[OH-].[Ca+2] AXCZMVOFGPJBDE-UHFFFAOYSA-L 0.000 description 10
- 239000000920 calcium hydroxide Substances 0.000 description 10
- 229910001861 calcium hydroxide Inorganic materials 0.000 description 10
- 230000005591 charge neutralization Effects 0.000 description 10
- 230000001264 neutralization Effects 0.000 description 10
- 238000006386 neutralization reaction Methods 0.000 description 10
- NBIIXXVUZAFLBC-UHFFFAOYSA-N phosphoric acid Chemical compound OP(O)(O)=O NBIIXXVUZAFLBC-UHFFFAOYSA-N 0.000 description 10
- 238000004458 analytical method Methods 0.000 description 9
- 235000012239 silicon dioxide Nutrition 0.000 description 9
- 229960005069 Calcium Drugs 0.000 description 8
- OYPRJOBELJOOCE-UHFFFAOYSA-N calcium Chemical compound [Ca] OYPRJOBELJOOCE-UHFFFAOYSA-N 0.000 description 8
- 229910052791 calcium Inorganic materials 0.000 description 8
- 238000001033 granulometry Methods 0.000 description 8
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 8
- KRHYYFGTRYWZRS-UHFFFAOYSA-N HF Chemical compound F KRHYYFGTRYWZRS-UHFFFAOYSA-N 0.000 description 7
- 235000019738 Limestone Nutrition 0.000 description 7
- 229910000040 hydrogen fluoride Inorganic materials 0.000 description 7
- 239000006028 limestone Substances 0.000 description 7
- 238000003756 stirring Methods 0.000 description 7
- 238000002441 X-ray diffraction Methods 0.000 description 6
- 230000003247 decreasing Effects 0.000 description 6
- 235000011007 phosphoric acid Nutrition 0.000 description 6
- NBIIXXVUZAFLBC-UHFFFAOYSA-K [O-]P([O-])([O-])=O Chemical compound [O-]P([O-])([O-])=O NBIIXXVUZAFLBC-UHFFFAOYSA-K 0.000 description 4
- 229910000147 aluminium phosphate Inorganic materials 0.000 description 4
- 239000007864 aqueous solution Substances 0.000 description 4
- 239000012153 distilled water Substances 0.000 description 4
- 239000010452 phosphate Substances 0.000 description 4
- 235000021317 phosphate Nutrition 0.000 description 4
- 238000004064 recycling Methods 0.000 description 4
- 235000008733 Citrus aurantifolia Nutrition 0.000 description 3
- RMAQACBXLXPBSY-UHFFFAOYSA-N Silicic acid Chemical compound O[Si](O)(O)O RMAQACBXLXPBSY-UHFFFAOYSA-N 0.000 description 3
- 235000015450 Tilia cordata Nutrition 0.000 description 3
- 235000011941 Tilia x europaea Nutrition 0.000 description 3
- 239000006227 byproduct Substances 0.000 description 3
- 230000000052 comparative effect Effects 0.000 description 3
- 238000010790 dilution Methods 0.000 description 3
- 239000007789 gas Substances 0.000 description 3
- 238000005469 granulation Methods 0.000 description 3
- 230000003179 granulation Effects 0.000 description 3
- 239000004571 lime Substances 0.000 description 3
- 239000011148 porous material Substances 0.000 description 3
- KLZUFWVZNOTSEM-UHFFFAOYSA-K AlF3 Chemical compound F[Al](F)F KLZUFWVZNOTSEM-UHFFFAOYSA-K 0.000 description 2
- 230000015572 biosynthetic process Effects 0.000 description 2
- CURLTUGMZLYLDI-UHFFFAOYSA-N carbon dioxide Chemical compound O=C=O CURLTUGMZLYLDI-UHFFFAOYSA-N 0.000 description 2
- 229910002092 carbon dioxide Inorganic materials 0.000 description 2
- 238000001816 cooling Methods 0.000 description 2
- 238000005516 engineering process Methods 0.000 description 2
- 230000002349 favourable Effects 0.000 description 2
- 150000002222 fluorine compounds Chemical class 0.000 description 2
- 239000010436 fluorite Substances 0.000 description 2
- 238000005755 formation reaction Methods 0.000 description 2
- XLYOFNOQVPJJNP-ZSJDYOACSA-N heavy water Substances [2H]O[2H] XLYOFNOQVPJJNP-ZSJDYOACSA-N 0.000 description 2
- 239000012535 impurity Substances 0.000 description 2
- 230000001965 increased Effects 0.000 description 2
- 239000006193 liquid solution Substances 0.000 description 2
- 239000002244 precipitate Substances 0.000 description 2
- 230000003134 recirculating Effects 0.000 description 2
- 239000007787 solid Substances 0.000 description 2
- 229940070337 AMMONIUM SILICOFLUORIDE Drugs 0.000 description 1
- ITHIMUMYFVCXSL-UHFFFAOYSA-P Ammonium fluorosilicate Chemical compound [NH4+].[NH4+].F[Si-2](F)(F)(F)(F)F ITHIMUMYFVCXSL-UHFFFAOYSA-P 0.000 description 1
- 210000003491 Skin Anatomy 0.000 description 1
- 238000007792 addition Methods 0.000 description 1
- 229910052782 aluminium Inorganic materials 0.000 description 1
- -1 aluminum ions Chemical class 0.000 description 1
- XKMRRTOUMJRJIA-UHFFFAOYSA-N ammonia NH3 Chemical compound N.N XKMRRTOUMJRJIA-UHFFFAOYSA-N 0.000 description 1
- 229940077747 antacids containing calcium compounds Drugs 0.000 description 1
- 239000007900 aqueous suspension Substances 0.000 description 1
- 239000003637 basic solution Substances 0.000 description 1
- 150000001674 calcium compounds Chemical class 0.000 description 1
- 229940043430 calcium compounds Drugs 0.000 description 1
- YYRMJZQKEFZXMX-UHFFFAOYSA-N calcium;phosphoric acid Chemical class [Ca+2].OP(O)(O)=O.OP(O)(O)=O YYRMJZQKEFZXMX-UHFFFAOYSA-N 0.000 description 1
- UTEFJYPYLXNQGW-UHFFFAOYSA-H calcium;silicon(4+);hexafluoride Chemical compound [F-].[F-].[F-].[F-].[F-].[F-].[Si+4].[Ca+2] UTEFJYPYLXNQGW-UHFFFAOYSA-H 0.000 description 1
- 239000001569 carbon dioxide Substances 0.000 description 1
- 238000007599 discharging Methods 0.000 description 1
- 238000009826 distribution Methods 0.000 description 1
- 239000003651 drinking water Substances 0.000 description 1
- 235000020188 drinking water Nutrition 0.000 description 1
- 238000004334 fluoridation Methods 0.000 description 1
- 239000010440 gypsum Substances 0.000 description 1
- 229910052602 gypsum Inorganic materials 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- 238000009776 industrial production Methods 0.000 description 1
- 238000009434 installation Methods 0.000 description 1
- 239000000463 material Substances 0.000 description 1
- 238000002156 mixing Methods 0.000 description 1
- 239000003973 paint Substances 0.000 description 1
- 239000000123 paper Substances 0.000 description 1
- 239000000376 reactant Substances 0.000 description 1
- 230000035484 reaction time Effects 0.000 description 1
- 238000011084 recovery Methods 0.000 description 1
- 239000005060 rubber Substances 0.000 description 1
- QAOWNCQODCNURD-UHFFFAOYSA-L sulfate Chemical compound [O-]S([O-])(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-L 0.000 description 1
- 239000002426 superphosphate Substances 0.000 description 1
- 229920003002 synthetic resin Polymers 0.000 description 1
- 239000000057 synthetic resin Substances 0.000 description 1
- 239000002699 waste material Substances 0.000 description 1
Abstract
The invention relates to a process for preparing synthetic calcium fluoride (CaF2) (min 90 wt % CaF2) from fluosilicic acid. Said process includes the following steps (a) reacting fluosilicic acid (H2SiFe) with ammonium hydroxide or ammonia in a first reactor (1) so as to obtain a first thick suspension, filtering said first suspension so as to obtain a filtrate containing an ammonium fluoride solution, (b) precipitating the ammonium fluoride solution, obtained in the form of a filtrate in step (a), with calcium carbonate in dry form or in the form of a suspension having a concentration of between 10 and 80 wt %, in a second (3) so as to produce a second suspension containing the calcium fluoride and the ammonium carbonate, filtering said second suspension so as to obtain a filter cake containing calcium fluoride, and a filtrate containing an ammonia carbonate solution, washing and drying said filter cake so as to obtain calcium fluoride and a washing filter cake containing an ammonium hydroxide solution, wherein a portion of the second suspension, of form 10 to 70 wt%, is recycled to the second reactor (3) so as to enhance the crystallization of the calcium fluoride, change in most of the ammonia from the second reactor in step (b) due to partial decomposition of the ammonium carbonate under reactions, then washing and sending said ammonia back into the first reactor, and collecting and treating, by means of distillation and condensation, the ammonium carbonate solutions obtained as filtrate and washing filter cake in step (b), so as to recover the liquid ammonia which is recycled to the first reactor (1).
Description
PROCESS FOR PREPARING CALCIUM FLUORIDE
FROM FLUOSILICIC ACID
FIELD OFTHE INVENTION
The invention relates to a process for the préparation of synthetic calcium fluoride (CaFî) and active silica from fluosilicic acid (HîSÎFe).
BACKGROUND OF THE INVENTION
Phosphate rocks contain 2 to 4 percent of fluorine.
A part of the fluorine from the phosphate rock is precipitated with gypsum, another part is leached out with the phosphoric acid product, and the remaining portion, which represents about 70% ofthe total fluorine, is vaporized in the reactor and the evaporator.
As environmental régulations continue to reduce chemical processing émissions, phosphate plants operators will eventually be required to neutralize fluoride waste materials.
Most phosphate plants currently use direct contact barometric condensera with recirculating cooling pond water that is saturated with fluoride saits.
Most of this fluoride is allowed to precipitate as the recirculating water cools in large cooling pond Systems.
When the fluoride contaminated water treatment is required, phosphate producers typically use lime or limestone neutralization prior to discharging effluent.
A better environmental alternative should use scrubbers to remove fluorides as fluosilicic acid prior to condensing the vapora in barometric condensera.
If the fluosilicic acid could not be used as it is, it could then be neutralized with several calcium compounds such as phosphate rock, lime and limestone, which are not profitable methods.
At the présent time, the most Important outlets of fluosilicic acid are fluoridation of drinking water, manufacturing of silicofluoride, hydrogen fluoride and the aluminium fluoride which Is the most sîgnificant chemical use worldwide of fluosilicic acid.
In the past several processes hâve been developed to make possible the valorization of fluosilicic acid into hydrogen fluoride or aluminium fluoride, however, none of these processes is being suitable to absorb the overall quantifies of fluosilicic acid generated by the phosphate producers.
So there is a need to other ways to convert the fluosilicic acid to more suitable fluorinated products such as synthetic calcium fluoride.
Synthetic calcium fluoride could advantageously substitute the fluorapar which is the most common starting minerai for ail fluorine products, especially when considering the highest cost of the fluorspar as a resuit of the resources decreasing and because of its export limitation ail over the world.
Therefore, some processes hâve been developed to produce calcium fluoride from fluosilicic acid by a reaction with calcium carbonate (CaCO3) or calcium hydroxide
Ca(OH)2.
In this regard, U.S, Pat. No. 2,780,521 and No. 2,780,523 describe the reaction of diluted fluosilicic acid having a concentration between 2.5 and 3.8% with lime. According to these patents if the reaction is carried out in a pH range of 3.5-6.7, calcium fluoride CaF2 is to be separable in an easily filterable form the concurrently produced SiO^sol.
A big disadvantage of the process described in U.S. Pat. No. 2,780,521 is the fact that one obtatns a calcium fluoride product which contains between 4 and 7% of SiO2 as an impurity. Such a material has little suitability for the production of hydrogen fluoride.
In the process described in U.S. Pat. No. 2,780,523, one obtains a calcium fluoride product having only a small content of SiO2, i.e. about 0.5% or less. This is said to be 15 accomplished by adding to the diluted fluosilicic add not more than 85% of the amount of calcium carbonate CaCOa stoichiometrically required for the formation of calcium fluoride. After filtration of the obtained product, the filtrate is once more treated with calcium carbonate to achieve a pH value of between 7 and 7.3. The product yielded in this step constitues a mixture of calcium fluoride and calcium silicofluoride which is not usable.
Besides, a further disadvantage is to be seen in the fact that relatively long reaction periods of approximately 30 minutes to two hours are required. Furthermore, a considérable weakness of both above-described processes is the fact that only very diluted fluosilicic acid (maximum concentration 4-6%) may be employed.
Accordingly, the industrial production of hydrogen fluoride still proceeds today 25 exclusively from natural fluorspar which is reacted in the form of the so-called acid grade spar (acidspar) with more than 97% calcium fluoride CaF2.
Therefore, there is a need to convert into calcium fluoride the fluosilicic acid produced as a by-product in ever increasing amounts during the production of super phosphates and wet phosphoric acid. Accordingly, the fluoride obtained in such a form 30 can be immediately converted to hydrogen fluoride in existing industrial plants. Moreover, calcium fluoride will be seen as a fluorine reserve, since it may be safely stockpiled.
US. Pat. No. 4,031,193 provides a process for the préparation of calcium fluoride from fluosilicic acid which comprises the steps of reacting the fluosilicic acid with calcium carbonate, preferably In the form of an aqueous suspension, at a pH value of between 35 about 2 and 6, preferably between about 3 and 5, and in the presence of sulfate and/or aluminum ions, whereby a calcium fluoride précipitâtes and an aqueous silica sol are formed, and thereafter separating the calcium fluoride precipitate from the aqueous silica sol.
US. Pat No. 3,271,107 provides a method to manufacture high grade silica from fluosilicic acid by the reaction of the fluosilicic acid with ammonium hydroxide in two separate stages which comprise reacting fluosilicic acid with ammonium hydroxide in a reaction zone of high agitation and adjusting the proportions of the reactants to maintain a pH in the range of 6.0 to 8.0, whereby a silica containing slurry is produced, then reacting said slurry in a quiescent reaction zone with additional ammonium hydroxide in a sufficient amount to provide a final pH from 8.3 to 9.0 for a time of about 20 minutes to 120 minutes, and separating precipitated silica from the resulting slurry.
U.S. Pat. No. 6,224,844 provides a process for producing calcium fluoride comprising: (a) mixing phosphoric acid (H3PO4) with aqueous H2SiFe to form a mixture such that the concentration of H3PO4 is at least about 3 moles/liter, (b) adding phosphate rock to the mixture at a rate such that the pH of the mixture is maintained at less than about 1.0 and at a température sufficient to from a second mixture containing calcium fluoride, and (c) separating calcium fluoride from said second mixture.
Documents CN 103073040, CN 102502753 and US 4,915,705 each disclose a process for producing calcium fluoride from fluosilicic acid, comprising the following steps:
(a) reacting fluosilicic acid with ammonium hydroxide or ammonia in a first reactor so as to obtain a first slurry; filtering said first slurry so as to obtain a filtrate containing a solution of ammonium fluoride;
(b) precipitating the solution of ammonium fluoride obtained as a filtrate in step (a) with calcium hydroxide in a second reactor so as to produce a second slurry containing calcium fluoride and ammonium hydroxide; filtering said second slurry so as to obtain a filter cake containing calcium fluoride and a filtrate containing a solution of ammonium hydroxide; washing and drying said filter cake so as to obtain calcium fluoride and a filter cake washing containing a solution of ammonium hydroxide.
However, because of the use of calcium hydroxide to precîpitate calcium fluoride, the chemistry and the kinetics of the reaction are not so favorable to allow easily obtaining calcium fluoride with the desired grade of purity and granulometry for Acidspar market. In addition, the fluorine and ammonia losses during calcium fluoride séparation are enormous due to calcium fluoride crystals form and shape. Furthermore, the ammonium hydroxide filtrate is much concentrated on NH3 which make ammonia recovery very difficult and costly.
BRIEF DESCRIPTION OF THE INVENTION
A goal of the invention is to provide a new process for producing high grade synthetic calcium fluoride (min 90% wt CaF2) at a compétitive cost.
The invention relates to a process for preparing synthetic calcium fluoride (CaF2) min 90% CaF2 from fluosilicic acid, comprising the following steps:
(a) reacting fluosilicic acid (HzSiFe) with ammonium hydroxide or ammonia in a first reactor so as to obtain a first slurry; filtering said first slurry so as to obtain a filtrate containing a solution of ammonium fluoride;
(b) precipitating the solution of ammonium fluoride obtained as a filtrate in step (a) with calcium carbonate as a dry form or as a suspension at a concentration ranging from 10 to 80% by weight in a second reactor so as to produce a second slurry containing calcium fluoride and ammonium carbonate; filtering said second slurry so as to obtain a filter cake containing calcium fluoride and a filtrate containing a solution of ammonium carbonate; washing and drying said filter cake so as to obtain calcium fluoride and a filter cake washing containing a solution of ammonium carbonate ;
(c) evolving the major part of ammonia from the second reactor in step (b) because of the partial décomposition of ammonium carbonate under reactor conditions and then scrubbing and retuming said ammonia to the first reactor, and gathering and treating by distillation and condensation both ammonium carbonate solutions obtained as the filtrate and the filter cake washing in step (b) to recover liquid ammonia which is recycled to the first reactor..
To enhance calcium fluoride crystallization in the second reactor in step (b), a quantity of the second slurry ranging from 10 to 70% is recycled to the second reactor while the remaining second slurry is filtered to recover the calcium fluoride, which is washed and dried.
Advantageously, at step (a) the filtration of the first slurry produces a filter cake containing silica which is precipitated amorphous silica and said silica cake is further washed and dried.
According to an embodiment, the calcium source used is a high grade limestone (CaCO3).
According to an embodiment, the concentration of the fluosilicic acid used in step (a) is between 5 and 40% by weight.
According to an embodiment, the fluosilicic acid used in step (a) is a by-product of phosphate rock acidulation.
Advantageously, the conditions of the reaction canied out in the first reactor in step (a) comprise at least one of:
- agitation of the mixture;
- a température between 20 and 90°C;
- a pH ranging from 5 to 11; and
- a résidence time between 15 and 360 minutes.
According to an embodiment, the filter cake washing obtained in step (a) contains a solution of ammonium fluoride which Is recycled to the first reactor or to the second reactor, and the filtrate obtained in step (a) is sent to the second reactor.
According to an embodiment, the pH of calcium fluoride précipitation in step (b) is In the range of 6 to 12 and the weight ratio Ca/F is greater than 0.5.
Advantageously, the conditions of the reaction carried out in the second reactor in ' step (b) comprise at least one of:
- agitation ofthe mixture;
- a température between 60 and 90*C;
- a résidence time between 30 minutes and 60 minutes.
The ammonia obtained in step (c) may be used in step (a) as it is or as an ammonium carbonate solution.
BRIEF DESCRIPTION OF THE DRAWINGS
Other features and advantages of the invention will be apparent from the following detaiied description in connection with the appended drawings, wherein Figure 1 illustrâtes schematically an installation for carrying out the process according to the invention.
DETAILED DESCRIPTION OF EMBODIMENTS OFTHE INVENTION
This invention provides a process for producing high grade synthetic calcium fluoride min 90% CaFi and active silica which is an amorphous precipitated silica from fluosilicic acid obtained as by-product from phosphate rock acidulation as a resuit of the fluorinated gas treatment
The process is invoiving the ammonia route for the production of synthetic CaF2 and active silica (S1O2) from fluosilicic acid.
The process comprises two main steps.
Fluosilicic acid is provided as an aqueous solution.
The concentration of fluosilicic acid in said aqueous solution is advantageously ranging from 5 to 40% by weight.
The first process step comprises the neutralization of the fluosilicic acid by adding ammonia as liquid solution or gas in a reactor.
Whether ammonia or an ammonium hydroxide solution is used, the pH of this solution is greater than 7, preferably greater than 8, more preferably greater than 9.
If a solution of ammonium hydroxide is provided, the concentration of NH3 in the solution is preferably comprised between 10 and 35% by weight.
In order to Improve the efficiency of the reaction, the reaction has to be carried out with appropriate agitation and/or under suitable température. For example, the reaction température may range from 20* to 90*C.
The reaction between the fluosilicic acid solution and ammonia produces a slurry.
Said slurry is filtered using vacuum filter with a pore size of 100pm or more preferably using press filter, so as to obtain, on the one hand, a filtrate that comprises a solution of ammonium fluoride and, on the other hand, a filter cake that comprises active silica.
The products of the first process step are thus ammonium fluoride (NH4F) (as a liquid form) and active silica S1O2 (as a solid form).
ln view of subsequently using said active silica, the active silica may be further washed by fresh water, dried at 110°C and disposed.
The second process step comprises the précipitation of NH4F by adding a source of calcium which is a high grade limestone (CaCOs).
The calcium source is provided as a suspension of calcium carbonate ln an aqueous solution. The concentration of the suspension ranges from 10 to 80% by weight. Otherwise, the calcium carbonate can be provided as a dry form.
The required quantity of calcium source Is chosen to meet a weight ratio of Ca/F greater than or equal to 0.5, typically equaling 1 in the reaction mixture.
The pH of the reaction mixture is basic, preferably greater than 8, more preferably greater than 9, and even more preferably greater than 10.
In order to improve the efficiency of the reaction which is endothermie, the reaction may be carried out with appropriate agitation and/or under suitable température. For example, the reaction température may range from 60° to 90°C. In particular, contrary to the reaction between calcium hydroxide and ammonium fluoride which is exothermic, the reaction between calcium carbonate and ammonium fluoride Is endothermie. Heating the mixture in the reactor thus allows better soiubilizing the calcium source, decreasing the reaction time and enhancing calcium fluoride crystals shape and form.
The reaction between ammonium fluoride and the calcium carbonate produces a slurry containîng calcium fluoride and ammonium carbonate.
Said slurry is then filtered using vacuum filter with a pore size of 100pm or more preferably using press filter, so as to obtain, on the one hand, a filter cake containîng calcium fluoride and, on the other hand, a filtrate containîng a solution of ammonium carbonate. To enhance calcium fluoride crystallization a determined quantity of the calcium fluoride slurry (typically ranging from 10 to 70% by weight) is retumed to the reactor while the remaining calcium fluoride slurry is separated by filtration using press filter to recover the calcium fluoride product. With this recirculation of the slurry, the supersaturation ratio of calcium fluoride is Increased ln the reactor, which increases the crystallization growth of calcium fluoride particles and allows obtaining big and uniform crystals that will enhance the granulometry distribution of the calcium fluoride particles. The granulometry of calcium fluoride is a key spécification for using synthetic calcium fluoride as substitute of Acidspar.
Then, the filter cake is washed and dried so as to obtain calcium fluoride and a filter cake washing containing a solution of ammonium carbonate.
The products of the second process step are thus calcium fluoride (CaF2) (as a solid form) and ammonium carbonate ((NH^COa) (as a liquid form).
By using calcium carbonate which is an ultrapure calcium source (contrary to calcium hydroxide which contains Impurities), calcium fluoride is easily obtained with a very high purity.
Hence, the second process step produces two ammonium carbonate solutions: the first one Is the filtrate resulting from filtration of the slurry and the second one is the filter cake washing.
As far as ammonia recycling Is concemed, the major part of ammonia and carbon dioxide are evolved from the calcium fluoride reactor because of the partial décomposition of ammonium carbonate under reactor conditions and then are scrubbed and retumed to the silica reactor, both ammonium carbonate solutions are then gathered and treated by distillation and condensation to recover liquid ammonia.
Said ammonia can thus be recycled in the first process step to neutralize fluosilicic acid. This recycling allows minimizing the cost of the process of fabrication of calcium fluoride.
The reactions mechanisms are:
- First process step: neutralization of fluosilicic acid with NH3 or NH*OH:
H2SiFe.2H2O + 6 NH4OH 6 NH<F + SiO2 Φ+ 6 H2O or
H2SiFe.2H2O + 6 NH3 6 NH<F + S1O2 Φ
- Second process step: précipitation of NH<F with limestone (CaCO3):
NH4F + CaCOs CaF2 Φ + CO2 Φ + 2 NH3 φ + H2O
The process of the présent invention makes it possible to use crude fluosilicic acid of high concentration, for example having concentration from 5 to 40% by weight for producing high grade synthetic calcium fluoride and active silica (SiO2).
The calcium fluoride produced is entirely suitable for the production of hydrogen fluoride in accordance with conventional processes.
At the same time, the active silica is produced in such grade that it can be added in phosphoric acid manufacturing to adjust the active silica deficiency in phosphate rock, also because it has excellent properties it can be used for producing synthetic resins, rubber, paints and paper.
This new process allows avoidance of the disadvantages of previously known processes, particularly in the sense that it uses the ammonia route (NH3 or NH4OH) for the neutralization of the fluosilicic acid for having an excellent silica sol précipitation and séparation.
The obtained liquid solution is the ammonium fluoride (NH4F) which can easily precipitate by adding a high grade limestone (CaCOs).
The main advantages are:
- The process can be fed by various grades of fluosilicic acid.
- The process can be fed by either of ammonia forms (liquid or gas).
- The process has high efficiency.
- The séparation of active silica is good.
- High grade active silica Is obtained.
- The séparation of calcium fluoride is good.
- High grade calcium fluoride is obtained.
- The grain size of the calcium fluoride is acceptable in view of the conventional processes requirement for hydrogen fluoride production.
Figure 1 illustrâtes a scheme for carrying out the process according to the invention.
The first process step comprises the production of the active silica by the reaction of an aqueous solution of fluosilicic acid (FSA) at the concentration of 5 to 40% by weight, with a basic solution of ammonium hydroxide containing 10 to 35% by weight as NH3, or with liquid ammonia NH3. The reaction mixture is condueted in reactor 1 under spécifie conditions such as good agitation, favorable température between 20° and 90’C, pH ranging from 5 to 11 and 15 to 360 min of résidence time. The pH is advantageously chosen so as to allow précipitation of ali the fluosilicic acid into amorphous precipitated silica. The résidence time impacts the morphology of the amorphous precipitated silica. The obtained slurry is then separated by filtration in filter 2 using vacuum filter with a pore size of 100pm or more preferably using press filter, and the active silica (silicon dioxide) is washed and dried at 11 O’C. The active silica has a high silicon dioxide content ranging from 50 to 90 by weight, and consists of particles having an average diameter of 32 pm with 67.5% by volume less than 40 pm, and a spécifie surface measured by the laser granufometry of 112m’/g.
The cake washing is a diluted solution of ammonium fluoride which is recycled to the fluosilicic acid stirred reactor 1 or to the reactor 3, and the filtrate which is a solution of the ammonium fluoride is sent to the second process step in which the ammonium fluoride is reading with limestone as a calcium source, to produce high grade calcium fluoride meeting the spécifications for commercial fluorspars.
The précipitation of the ammonium fluoride is performed in a stirred reador 3 in which a suspension of calcium carbonate at the concentration from 10 to 80% by weight is added. The température varies between 60* to 90*C, the pH is of 6 to 12 and the résidence time is about 30 to 60 min. The pH and résidence time are advantageously seleded to allow the précipitation of ali the NHF4 into CaFî which has good crystalline properties. The required quantity of calcium carbonate is chosen to meet a weight ratio of
Ca/F greater than 0.5 in the reaction mixture. To enhance calcium fluoride crystallization a quantity ranging from 10 to 70% of the calcium fluoride slurry is retumed to the reactor 3, the remaining calcium fluoride sluny is separated by filtration in filter 4 to recover the calcium fluoride product, which is washed and dried in drier 5.
The calcium fluoride purity dépends on the purity of the calcium source. It can reach more than 97% of CaF2 by weight since calcium carbonate - which has a high purity - is used. Silicon dioxide content is less than 1.5% by weight because of the good silicon dioxide séparation in the first process step. Regarding the particles of calcium fluoride, the average diameter is about 20 pm with 87% by volume less than 40 pm, and the spécifie area Is 278m’/g, much higher particles diameters will be achieved by adjusting the calcium fluoride slurry recycling or if required by using décantation before calcium fluoride filtration. Also, a granulation of particles can be made after drying.
After the calcium fluoride slurry filtration, both the filtrate and the filter cake washing solution, which are diluted ammonium carbonate solutions, are collected ln a tank 6 and treated by distillation and condensation to obtain a solution of ammonia. Said ammonia is reused, as it is or as an ammonium carbonate solution ranging from 10 to 30% NHa by weight, for feeding the fluosilidc acid neutralization.
In comparison to the known processes, the production of calcium fluoride from fluosilidc acid using ammonia route according to the invention represents a new process in the sense that it uses for the first time the ammonia and calcium carbonate to convert fluosilidc add Into caldum fluoride at a compétitive cost by enhandng reactions efficiency and adopting a strong strategy for ammonia recycling, the caldum fluoride obtained is meeting the spécifications for commercial fluorspars, due to its high degree of purity and its good granulometry.
Furthermore the active silica (silicon dioxide) also called amorphous predpitated silica is easily separated before the calcium fluoride précipitation and séparation
To make the most économie and sustainable process, the major part of ammonia is evolved from the calcium fluoride reactor 3 because of the partial décomposition of ammonium carbonate under reactor conditions and then scrubbed and retumed to the silica reactor, both ammonium carbonate solutions obtained in step (b) as the filtrate and the filter cake washing are then gathered and treated by distillation and condensation to recover liquid ammonia which is recycled to the silica reactor 1.
The foregoing description and the following examples relate to several spécifie embodiments of the process, and it is to be understood that this description is intended to be illustrative only and not in any sense limitative. It is not intended to exclude other possible embodiments which permit carrying out the reaction of the production of the high grade calcium fluoride from fluosilidc add using ammonia route.
Example 1
252 grams of fluosilicic acid solution having a concentration of 32% by weight, which is a commercial fluosilicic add, was fed into a stirred reaction vessel of 1 liter. The solution in the reaction vessel was stirred at a rate of 250 rpm. During stirring, 380 grams of an ammonium hydroxide solution having a concentration of 25% (wt) as NHj was injected just below the liquid surface. The résidence time of the reaction mixture was about 60 minutes and the final pH was about 8.3 while the température decreased from 6Γ to 28°C. The reaction mixture was subsequently filtered, the resulting filter cake washed with distilled water and dried at 110’C. Under these conditions the neutralization yield of fluorine was 81.24%. The chemical analysis and the X-Ray diffractometry of the dried cake showed the production ofthe ammonium silicofluoride and notthe active silica.
Comparative Example
Under the same conditions as Example 1, 350 grams of fluosilicic acid solution having a concentration of 23% by weight, which was prepared by dilution of the commercial fluosilicic acid having 32% (wt). During stirring, 507 grams of an ammonium hydroxide solution having a concentration of 25% (wt) as NHa was used. The final pH of the reaction mixture was about 9.4. Under these conditions, there was no formation of silicofluoride but only active silica, the neutralization yield of fluorine was 97.88%. The chemical analysis of the dried cake showed the obtaining of the active silica (SiO2).
Example 2 grams of ammonium fluoride having a concentration of 8.47% by weight, recovered from the fluosilicic acid neutralization step, was fed Into a reaction vessel provided with stirrer, The solution in the reaction vessel was stirred at rate of 250 rpm. During stirring, 71.4 grams of a suspension (19.5% wt) of calcium hydroxide having a concentration of 74.16% wt was fed to carry out a weight ratio of NHjF/CaO équivalent to 2.1. The résidence time of the reaction mixture was about 30 minutes and the final pH was about 10 while the température decreased from 2Γ to 16eC, The reaction mixture was subsequently filtered, the resulting filter cake washed with distilled water and dried at 110*C. Under these conditions the précipitation yield of fluorine was 87.62%. The chemical analysis and the X-Ray diffractometry of the dried cake showed the obtaining of calcium fluoride.
Comparative Example A
Under conditions identical in ail respects with those of Example 2, 96 grams of ammonium fluoride having a concentration of 8.47% by weight was fed into a réaction vessel provided with stirrer. During stirring, 71.4 grams of a suspension (19.5% wt) of calcium hydroxide having a concentration of 74.16% wt was fed to carry out a weight ratio of NH<F/CaO équivalent to 2.2. The résidence time of the reaction mixture was about 30 minutes and the final pH was about 9.8. Under these conditions the précipitation yield of fluorine was 69.75%. The chemical analysis and the X-Ray diffractometry of the dried cake showed the obtaining of the calcium fluoride.
Comparative Examole B
Under conditions Identical ln ail respects with those of Example 2, 100 grams of ammonium fluoride having a concentration of 6.49% by weight was fed into a reaction vessel provided with stirrer. During stirring 50 grams of a suspension (25% wt) of calcium hydroxide having a concentration of 94.76% wt was fed to carry out a weight ratio of NHiF/CaO équivalent to 2. The résidence time of the reaction mixture was about 30 minutes and the final pH was about 10.7. Under these conditions the précipitation yield of fluorine was 96.66%. The chemical analysis and the X-Ray diffractometry of the dried cake showed the obtaining of calcium fluoride.
Example 3
200 grams of fluosilicic acid solution having a concentration of 23% by weight, which was prepared by dilution of the commercial fluosilicic acid having 32% wt, was fed into a reaction vessel of 1 liter provided with stirrer. The solution in the reaction vessel was stirred at rate of 250 rpm. During stirring, 308 grams of an ammonium hydroxide solution having a concentration of 25% (wt) as NHa was Injected just below the skin of the liquid surface. The résidence time of the reaction mixture was about 60 minutes and the final pH was about 9.6 while the température decreased from 54* to 20’C. The reaction mixture was subsequently filtered, the resulting filter cake washed with distilled water and dried at 110’C. Under these conditions the maximum neutralization yield of fluorine was 98.22% depending of silica morphology and process filtration parameters. The chemical analysis showed the obtaining of active silica which has a high silicon dioxide content ranging from 50 to 90 by weight depending on the filtration technology, and consists of particles having an average diameter of 30 pm with 67% by volume less than 40 pm, and a spécifie surface measured by the laser granulometry of 112 m’/g. The cake washing which Is a diluted solution of ammonium fluoride of 76 grams having a concentration of 6.79% (wt), Is recycled to the fluosilicic acid stirred reactor. The filtered solution of the ammonium fluoride having 286.6 grams and a fluorine content of 7.04% (wt), is sent to the second process step for reacting with 86.7 grams of a suspension (40% wt) of calcium hydroxide having a concentration of 94.76% (wt). At this stage the molar ratio of NhUF/CaO is équivalent to 2. The résidence time of the réaction mixture was about 60 minutes and the final pH was about 10.7. The reaction mixture was subsequently divided into two parts: 60% was retumed to the calcium fluoride reactor and 40 % was filtered; the resulting filter cake was washed with distilled water and dried at 110’C. Under these conditions the précipitation yield of fluorine was 96.66%. The chemical analysis and the X-Ray diffractometry of the dried cake showed the obtaining of calcium fluoride, which meets the spécifications for commercial fluorspars. The calcium fluoride purity was 91% CaF2,
Silicon dioxide content is less than 1.5%. As regards the particles of calcium fluoride, the average diameter is about 12 pm with 87% by volume less than 40 pm, and the spécifie surface measured by the laser granulometry was 276 m’/g, much higher particles diameters being achieved by adjusting the calcium fluoride slurry recyciing or if requîred by using décantation before calcium fluoride filtration. Also, a granulation of particles can be made after drying.
Example 4
400 grams of fluosilicic acid solution having a concentration of 23% by weight, which was prepared by dilution of the commercial fluosilicic acid having 32% wt, was fed into a reaction vessel of 1 liter provided with stirrer. The solution in the reaction vessel was stirred at rate of 250 rpm. During stirring, a stoichiometric quantity of ammonia was injected in the bottom of the silica reactor. The résidence time of the reaction mixture was about 60 minutes and the final pH was about 9.5 while the température decreased from 85e to 45eC. The reaction mixture was subsequently filtered, the resulting filter cake washed with distilied water and dried at 110°C. Under these conditions the neutralization yield of fluorine was 81.69%. The chemical analysis showed the obtaining of active silica which has a high silicon dioxide content ranging from 80 to 90 by weight depending on the filtration technology, and consîsts of particles having an average diameter of 36 pm with 56% by volume less than 40 pm, and a spécifie surface measured by the laser granulometry of 200m’/g. The cake washing which is a diluted solution of ammonium fluoride of 159 grams having 13.44% (wt) fluorine, is recycled to the fluosilicic acid stirred reactor. The filtered solution of the ammonium fluoride having 357 grams and a fluorine content of 17.30% (wt), is sent to the second process step for reacting with 410 grams of a suspension (40% wt) of high grade calcium carbonate having 55.55% CaO content. At this stage, the weight ratio of NHxF/CaO is équivalent to 1.3. The résidence time of the reaction mixture was about 30 minutes and the final pH was about 11.5. The reaction mixture was filtered; the resulting filter cake was washed with distilied water and dried at 110*C. Under these conditions the précipitation yield of fluorine was 96.70%. The chemical analysis and the X-Ray diffractometry of the dried cake showed the obtaining of calcium fluoride, which meets the spécifications for commercial fluorspars. The calcium fluoride purity is 97.95% CaF2. Silicon dioxide content is less than 1%. As regards the particles of calcium fluoride, the granulometry is about 40% under 45 pm and 20% above 150pm, much higher particles diameters being achieved by adjusting the calcium fluoride siurry recyciing or if requîred by using décantation before calcium fluoride filtration, ln addition, a granulation of particles can be made after drying.
REFERENCES
U.S. Pat. No. 2,780,521
U.S. Pat. No. 2,780,523
US. Pat. No. 4,031,193 5 US. Pat. No. 3,271,107
U.S. Pat. No. 6,224,844
US. Pat. No 4,915,705
CN. Pat. No 103073040
CN. Pat. No 102502753
Claims (8)
1. Process for preparing synthetic calcium fluoride (CaF2) from fluosilidc acid, comprising the following steps:
(a) reading fluosilidc acid (H2SiFe) with ammonium hydroxide or ammonia in a first reador (1) so as to obtain a first slurry; filtering said first slurry so as to obtain a filtrate containing a solution of ammonium fluoride;
(b) precipitating the solution of ammonium fluoride obtained as a filtrate in step (a) with calcium carbonate (CaCOs) as a dry form or as a suspension at a concentration ranging from 10 to 80% by weight in a second reador (3) so as to produce a second slurry containing caldum fluoride and ammonium carbonate; filtering said second slurry so as to obtain a filter cake containing calcium fluoride and a filtrate containing a solution of ammonium carbonate; washing and drying said filter cake so as to obtain caldum fluoride and a filter cake washing containing a solution of ammonium carbonate; wherein a portion of the second slurry ranging from 10 to 70% is recycled to the second reador (3) so as to enhance caldum fluoride crystallization ln the second reador (3);
(c) evolving the major part of ammonia from the second reador (3) in step (b) because of partial décomposition of ammonium carbonate under reador conditions and then scrubbing and retuming said ammonia to the first reador (1), and gathering and treating by distillation and condensation both ammonium carbonate solutions obtained as the filtrate and the filter cake washing ln step (b) to recover liquid ammonia which is recycled to the first reador (1).
2. Process according to daim 1, wherein at step (a) the filtration of the first slurry produces a filter cake containing amorphous precipitated silica and said silica cake is further washed and dried.
3. Process according to one of claims 1 to 2, wherein the concentration of the fluosilidc acid used In step (a) is between 5 and 40% by weight.
4. Process according to one of claims 1 to 3, wherein the fluosilidc add used in step (a) Is a by-produd of phosphate rock acidulation.
5. Process according to one of claims 1 to 4, wherein the conditions of the readion carried out in the first reador (1) in step (a) comprise at least one of:
- agitation of the mixture;
- a temperature between 20 and 90’C;
- a pH ranging from 5 to 11 ; and *
- a résidence time between 15 and 360 minutes.
6. Process according to one of claims 1 to 5, wherein the filter cake washing obtained in step (a) contains a solution of ammonium fluoride which is recycled to the first reactor (1) or to the second reactor (3), and wherein the filtrate obtained in step (a) is sent to the second reactor (3).
7. Process according to one of claims 1 to 6, wherein the pH of calcium fluoride précipitation in step (b) is in the range of 6 to 12 and the weight ratio Ca/F is greater than 0.5.
8. Process according to one of claims 1 to 7, wherein the conditions of the reaction carried out in the second reactor (3) in step (b) comprise at least one of:
- agitation ofthe mixture;
- a température between 60 and 90’C;
- a pH ranging from 6 to 12; and
- a résidence time between 30 minutes and 60 minutes.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| EP15164685.8 | 2015-04-22 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| OA18492A true OA18492A (en) | 2018-12-04 |
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