EP3126290A2 - High purity synthetic fluorite and process for preparing the same - Google Patents

High purity synthetic fluorite and process for preparing the same

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Publication number
EP3126290A2
EP3126290A2 EP15771700.0A EP15771700A EP3126290A2 EP 3126290 A2 EP3126290 A2 EP 3126290A2 EP 15771700 A EP15771700 A EP 15771700A EP 3126290 A2 EP3126290 A2 EP 3126290A2
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EP
European Patent Office
Prior art keywords
nh4f
solution
weight
silica
process according
Prior art date
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EP15771700.0A
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German (de)
French (fr)
Inventor
Luca Pala
Michele LAVANGA
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Fluorsid SpA
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Fluorsid SpA
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Publication of EP3126290A2 publication Critical patent/EP3126290A2/en
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    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F11/00—Compounds of calcium, strontium, or barium
    • C01F11/20—Halides
    • C01F11/22—Fluorides
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/51—Particles with a specific particle size distribution

Definitions

  • the present invention relates to high purity synthetic fluorite (CaF 2 ). Furthermore, the present invention relates to a process for preparing said high purity synthetic fluorite (CaF2), classified as acid grade, starting from fluorosilicic acid H2S1F6 (FSA). Moreover, the present invention relates to the use of said high purity synthetic fluorite (CaF2) in the industrial production of hydrofluoric acid.
  • Fluorosilicic acid H2S1F6 is a by-product of the industrial production of phosphoric acid, obtained by absorption in water of silicon tetrafluoride (SiF 4 ) generated by the reaction between silica, fluorine, which are present in the phosphatic mineral as raw material, and sulfuric acid used for the production of phosphoric acid.
  • SiF can be absorbed in an aqueous solution generating FSA with a concentration varying between 23% and 35%.
  • Fluorite loss can be calculated stoichiometrically and is about 3.9% per 1 % of S1O2, and sulfuric acid loss is about 4.9% per 1% of Si0 2 .
  • the reaction of HF formation from synthetic fluorite is as follows: H 2 S0 + CaF 2 CaS0 4 + 2HF.
  • magnesium e.g. as magnesium oxide
  • the chalk CaSC produced in the presence of magnesium tends to build scales on the walls of HF production furnaces. This effect can lead to a complete shutdown of the reaction, thus causing an unwanted plant standstill or anyhow to a large increase in specific fluorite consumption (the amount of fluorite lost in chalk increases). Therefore, from the economic and process point of view, it is necessary to be able to reduce the amount of magnesium (expressed as magnesium oxide) in fluorite to an amount below at least 0.5%.
  • An object of the present invention is a high purity synthetic fluorite (CaF2), classified as "acid grade", having the characteristics as defined in the appended claims.
  • An object of the present invention is a process for the preparation of said high purity synthetic fluorite (CaF2), classified as "acid grade", having the characteristics as defined in the appended claims.
  • An object of the present invention is the use of said high purity synthetic fluorite (CaF2), classified as "acid grade", in the industrial production of hydrofluoric acid (HF), having the characteristics as defined in the appended claims.
  • CaF2 high purity synthetic fluorite
  • HF hydrofluoric acid
  • Figure 1 represents a block diagram of the process for the preparation of high purity synthetic fluorite, according to an embodiment of the present invention including the purification of the solution of NH4F, the transformation of NH4F into ammonium bifluoride (NH4HF2) and the use of CaCCb.
  • Figure 2 represents a block diagram of the process for the preparation of high purity synthetic fluorite, according to an embodiment of the present invention including the purification of the NH4F solution and the use of Ca(OH) 2 .
  • "high purity" synthetic fluorite means a synthetic fluorite having a concentration of 95% by weight or above with respect to dry weight; preferably of 97% by weight or above with respect to dry weight; still more preferably of 99% by weight or above with respect to dry weight.
  • high purity synthetic fluorite (CaF2), classified as "acid grade" means a fluorite having a CaF2 content above 95% by weight, e.g. above 97% by weight, with respect to dry weight, as measured according to current techniques and based on the knowledge that 1) fluorite at 100°C, pressure 1 atmosphere, after 60 minutes, has a water content of about 4% by weight; and that 2) fluorite at 800°C, pressure 1 atmosphere, after 80 minutes, has a water content of about 0% by weight.
  • the process according to the present invention is represented by way of example and therefore in a manner not limiting the scope of the present invention, in the block diagram of Figure 1 (simplified block diagram of the main steps of the process according to the present invention, including the purification of the solution of NFUF, the transformation of NH 4 F into ammonium bifluoride (NH 4 HF 2 ) and the use of CaC0 3 ).
  • said first embodiment R1 ( Figure 1) includes the following steps:
  • the process according to the present invention is represented by way of example and therefore in a manner not limiting the scope of the present invention, in the block diagram of Figure 2 (simplified block diagram of the main steps of the process according to the present invention, including the purification of the solution of NH4F and the use of Ca(OH)2).
  • composition of the synthetic fluorite, after drying at 110°C until a constant weight is obtained is as follows:
  • composition of the above synthetic fluorite, after calcination at 800°C for 30 minutes is as follows:
  • a first step R1 F1 includes the decomposition of fluorosilicic acid (FSA) h1 ⁇ 2SiF6 with ammonia and the separation of silica precipitated from the solution of ammonium fluoride NH4F, according to reaction A):
  • FSA fluorosilicic acid
  • a second step R1 F2 includes the purification of the solution of NH4F by dosing suitable reagents selected among nitrate salts such as iron nitrate and/or magnesium nitrate, which enable the elimination, by precipitation and following separation, of silica still present in the solution of NH4F. It is not advisable to use chlorinated salts such as iron chloride.
  • a third step R1 F3 includes the transformation of NH4F into ammonium bifluoride NH4HF2 by means of a distillation under reduced pressure (according to reaction B) and following recovery of a fraction of NH3 by absorption in an aqueous solution or condensation.
  • Reaction B is schematized as follows:
  • a fourth step R1 F4 includes the synthesis and precipitation of fluorite CaF2 by reaction of NH4HF2 ( aq ) with calcium carbonate and simultaneous distillation of free ammonia so as to recover the remaining fraction of NH3 by absorption in an aqueous solution or condensation (reaction C).
  • Reaction C is schematized as follows:
  • the process includes a drying step until a synthetic fluorite suitable for use in the industrial production of hydrofluoric acid is obtained.
  • fluorosilicic acid FSA having a concentration of 15 to 30% w/w (weight/weight), preferably of 20 to 25% w/w is reacted under constant mechanical stirring with an aqueous solution of NH3 having a concentration of 10 to 35% by weight, preferably of 15 to 25% by weight.
  • the reaction is exothermic and temperature can reach 90°C, therefore to avoid excessive losses of NH3 the reaction temperature is kept constant at 50-70°C.
  • NH3 is dosed in a stoichiometric excess of about 20-30% by weight on FSA with respect to the theoretical value (6 moles of NH3 per mole of FSA).
  • the reagents are added so that the pH of the solution remains stable at a value of about 9.
  • FSA in NH3 is added, thus ensuring to obtain a silica that can be easily filtered.
  • the effectiveness of the hydrolysis process is strictly related to the speed of addition of the reagents, i.e. FSA in NH3.
  • the total estimated time for obtaining a filterable, high quality silica and completing the hydrolysis reaction is 2 to 6 total hours, preferably 3 to 5 total hours, e.g. 4 total hours considering a speed of addition of 0.01 l/min per 1 liter of NH 3 18% or FSA 23%.
  • the sequence of addition produces two different reaction environments, an initial and a final environment, which differently affect the quality of the silica obtained, in particular as far as the structural and surface properties are concerned.
  • the pH of the formation of nuclei, aggregates and agglomerates switches from acid to basic depending on whether NH3 in FSA is added or vice versa.
  • the environment in which the nuclei, aggregates and agglomerates is different in one case and in the other.
  • the different environment affects the nucleation, aggregation and agglomeration of the amorphous silica here produced.
  • silica is formed and a white-colored suspension is generated.
  • the silica present in the suspension is preferably separated from the solution containing ammonium fluoride NH4F and a slight excess of NH3.
  • the separation of silica can be carried out by filtration, e.g. by means of a filter press or basket strainers, or by centrifugation.
  • the first washing water of silica is recovered in the solution of NH4F, water from the following washing steps is sent to water purification.
  • the final solution is clear and still contains a small fraction of dissolved silica of 1 to 5 g/l. As a matter of fact, by letting the solution rest for about 2-4 hours a further formation of precipitated silica can be observed.
  • the silica present in the solution of NH4F should be eliminated before producing the synthetic fluorite so as to reduce the content of S1O2 in the finished product.
  • the purification process includes the addition of small amounts of an aqueous solution of iron nitrate and/or magnesium nitrate.
  • the optimal dosage in grams is of 0.010 (e.g. 0.015) to 0.10, preferably 0.020 (e.g. 0.025 or 0.030) to 0.080 (e.g. 0.050) of Fe(N0 3 ) 3 per 1 g of Si0 2 present in the solution of NH 4 F, and 0.010 (e.g. 0.015) to 0.10, preferably 0.020 (e.g.
  • the reaction can be carried out at room temperature or anyhow at the final temperature of the first step, without advantageously including a cooling step of the solution of NH4F.
  • the silica present in the obtained suspension is separated by filtration (e.g. basket strainer).
  • Said first embodiment R1 includes a distillation process, which is necessary for the conversion of NhUF into more reactive (NH4)HF 2 .
  • NH4HF 2 more reactive
  • carbonate does not react with NH4F spontaneously and it is mandatory to implement a distillation process so as to lead the reaction to fluorite formation.
  • the solution of NH4F which was previously purified from S1O2, is distilled under reduced pressure so as to promote the decomposition of the compound NH4F, which is not very stable, and its transformation into the more stable form NH4HF2 (reaction B).
  • the decomposition includes the removal of a mole of NH3 per mole of NH4F, to this amount NH3 already present in free form in the solution is added.
  • the distillation is carried out by increasing system temperature from 30°C to 130°C) under a slight vacuum (about 60 mbar lower than ambient pressure).
  • the synthesis of fluorite goes on (said fourth step R1 F4) by adding calcium carbonate (reaction C) in stoichiometric amounts with respect to fluorine present in the solution of NH4HF2 obtained above (molar ratio 1 :2), so as to avoid the presence of an excess of carbonates in the finished product.
  • the calcium carbonate used should be dry or with a moisture below 10% by weight, preferably below 5% by weight, and as a fine powder.
  • the chemical quality of calcium carbonate should be high with a concentration of CaCC>3 above 97%, advantageously above 99%, and with a low content of inorganic contaminants (S1O2, MgC03 and other metal).
  • the reaction can occur at a temperature of 20°C; advantageously. In order to improve ammonia recovery it is advisable to work with temperatures of about 60-70°C and always under a slight vacuum.
  • the stirring speed should be such as to prevent the deposition of solid material onto the reactor bottom.
  • the reaction is practically instantaneous, the best yields are obtained by leaving the fluorite suspension thus obtained under constant stirring for at least 30-60 minutes. The fluorite thus obtained is separated from the suspension by filtration.
  • the filtered product is washed and takes a muddy consistency with an average residual moisture of about 40%.
  • Said first step R2F1 includes the production of NhUF by basic hydrolysis of H SiFe in an aqueous solution having a concentration of 15 to 30% by weight, preferably of 20 to 25% by weight, with an aqueous solution of NH3, under constant mechanical stirring, having a concentration of 10 to 35% by weight, preferably of 15 to 25% by weight.
  • Said first step R2F1 is carried out under the same conditions as for the step R1 F1.
  • a container e.g. a 500 ml container, containing an amount of 200 to 250 g of ammonia, e.g. 237 g of ammonia (e.g. 30% excess with respect to the estimated stoichiometric amount for the reaction), an amount of FSA of 150 to 250 g, preferably 200 g, is added.
  • the dispersion obtained from the reaction above was intensively stirred e.g. for about 20-40 minutes with a mechanical stirrer, e.g. Velp, monitoring pH and temperature. During this time the pH remained stable at a value of 8.5 to 9,5, preferably around 9. The temperature rose from 25°C to about 60-65°C.
  • a mechanical stirrer e.g. Velp
  • the precipitated silica was preferably separated by filtration, e.g. by filtration under vacuum, preferably at a relative pressure of about 50-150 mbar, still more preferably at a pressure of about 100 mbar.
  • the solid thus obtained was re-dispersed in water and filtered under the same operating conditions as described above.
  • the solid thus obtained was dried, preferably in an oven at about 105-110°C, and weighed.
  • the dried solid was analyzed by XRF. According to the above operating conditions, the Applicant executed three assays and observed that of the theoretical estimated amount of silica (17.62 g) 15.60 g of silica were obtained in the first assay, 16.40 g in the second assay, and 16.94 g in the third assay.
  • the first silica washing water is added to the initial solution of NH4F.
  • Said second step R2F2 includes the purification of NH4F from silica.
  • the solution containing NH4F, obtained after filtration, is treated/purified (said second step R2F2) under the same operating conditions as described for step R1 F2.
  • a solution containing NH4F, obtained after filtration, is treated/purified with a solution comprising iron (III) nitrate having a concentration of 20 to 60%by weight/volume, preferably of 30 to 50% by weight/volume, and/or magnesium (II) nitrate having a concentration of 40-80% by weight/volume, preferably of 50 to 70% by weight/volume.
  • iron (III) nitrate having a concentration of 20 to 60%by weight/volume, preferably of 30 to 50% by weight/volume
  • magnesium (II) nitrate having a concentration of 40-80% by weight/volume, preferably of 50 to 70% by weight/volume.
  • the solution containing NH4F, obtained after filtration, is treated with an amount of 0.02 to 0.08 g, preferably 0.04 to 0.06 g of Fe(N0 3 ) 3 (ferric nitrate nonahydrate - ⁇ ( ⁇ 0 3 ) 3 ⁇ 9 ⁇ 2 0 -43.3% weight/volume aqueous solution) and with an amount of 0.05 g to 1 g, preferably of 0.07 to 0.09 g of Mg(NC>3)2 (magnesium nitrate - Mg(NOa)2 - 64.4% weight/volume aqueous solution).
  • the solution thus obtained is kept under stirring for a time of 10 to 90 minutes, preferably 60 minutes, at a temperature of 20C° to 25°C.
  • the third step (R2F3) includes the treatment of said aqueous solution of NH4F basically without silica directly with calcium hydroxide in an excess amount of 0.01 to 0,5% with respect to the stoichiometric amount, thus obtaining a dispersion which is kept under stirring for a time of 10 to 60 minutes at a temperature of 40 to 90°C. Finally, the latter solution is filtered thus obtaining the synthetic fluorite.
  • the solution is preferably filtered under vacuum at a pressure of 50 mbar to 150 mbar, preferably at 100 mbar, e.g. with a 0.45 ⁇ filter made of cellulose acetate.
  • Quantitative analyses by ICP-AES are carried out on the samples of solution of NH 4 F taken before and after treatment. It was found that on average, the concentration of Si0 2 decreased of at least 70% by weight in the samples treated with nitrates, e.g. from an initial value of 2.5 g/l to 0.3 g/l.
  • Said third step R2F3 includes the synthesis of CaF2 starting from NhUF in the presence of calcium hydroxide.
  • the reaction can be schematized as follows:
  • NH4F e.g. ammonium fluoride - NH4F - 9.5 by weight aqueous solution
  • the assays were made using an excess amount of about 0.3% with respect to the stoichiometric amount. In all the assays that were made, the dispersion was left under mechanical stirring for a time of 20 to 60 minutes, preferably 30 minutes in an oil bath at a temperature of 80-90°C.
  • the precipitate (CaF 2 ) was filtered by filtration under vacuum at a relative pressure of 50 mbar to 150 mbar, preferably 100 mbar with a filter, e.g. a Whatman 42 paper filter, washed and dried in an oven at a temperature of 110°C and analyzed by XRF.
  • a filter e.g. a Whatman 42 paper filter
  • the yield of the reaction is above 95% and the quantitative analysis of the solid shows a very low percentage of residual silica (below 0.2% of Si0 2 ). Fluorite washing water does not exhibit residual fluorine and ammonia is recovered at 100% in a closed system.
  • said third step R2F3 includes the synthesis of CaF 2 starting from NH4F in the presence of calcium carbonate.
  • the reaction can be schematized as follows:
  • Calcium carbonate is used in an excess amount of 0.01 to 0.5% with respect to the stoichiometric amount to give a dispersion which is kept under stirring for a time of 10 to 60 minutes, preferably 30 minutes at a temperature of 60 to 90°C, preferably 80°C.
  • an amount of 250 g to 350 g, preferably of 300 g of NFUF, e.g. ammonium fluoride - NH 4 F - 9.5 by weight aqueous solution was placed in a 500 ml PTFE three-neck flask and reacted with calcium carbonate.
  • the assays were made using an excess amount of 0.3% with respect to the stoichiometric amount. In all the assays that were made, the dispersion was left under mechanical stirring for a time of 20 to 60 minutes, preferably 30 minutes in an oil bath at a temperature of 80-90°C.
  • the precipitate (CaF2) was filtered by filtration under vacuum at a relative pressure of e.g. 50 mbar to 150 mbar, preferably 100 mbar with a filter, e.g. a Whatman 42 paper filter, washed and dried in an oven at a temperature of 110°C and analyzed by XRF.
  • a filter e.g. a Whatman 42 paper filter
  • the yield of the reaction is above 95% and the quantitative analysis of the solid shows a very low percentage of residual silica (about 0.1% of S1O2). Fluorite washing water does not exhibit residual fluorine and ammonia is recovered at 100% in a closed system.

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  • Chemical & Material Sciences (AREA)
  • Organic Chemistry (AREA)
  • Life Sciences & Earth Sciences (AREA)
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  • Inorganic Chemistry (AREA)
  • Compounds Of Alkaline-Earth Elements, Aluminum Or Rare-Earth Metals (AREA)

Abstract

The present invention relates to a high purity synthetic fluorite (CaF2). Moreover, the present invention relates to a process for the preparation of said high purity synthetic fluorite (CaF2), classified as acid grade, starting from fluorosilicic acid H2S1F6 (FSA) and calcium carbonate (CaCO3). Finally, the present invention relates to the use of said high purity synthetic fluorite (CaF2) in the industrial production of hydrofluoric acid.

Description

DESCRIPTION of the invention entitled:
High purity synthetic fluorite and process for preparing the same.
The present invention relates to high purity synthetic fluorite (CaF2). Furthermore, the present invention relates to a process for preparing said high purity synthetic fluorite (CaF2), classified as acid grade, starting from fluorosilicic acid H2S1F6 (FSA). Moreover, the present invention relates to the use of said high purity synthetic fluorite (CaF2) in the industrial production of hydrofluoric acid.
Fluorosilicic acid H2S1F6 (FSA) is a by-product of the industrial production of phosphoric acid, obtained by absorption in water of silicon tetrafluoride (SiF4) generated by the reaction between silica, fluorine, which are present in the phosphatic mineral as raw material, and sulfuric acid used for the production of phosphoric acid.
During the step of phosphoric acid concentration, SiF can be absorbed in an aqueous solution generating FSA with a concentration varying between 23% and 35%.
Unfortunately, available known methods for the preparation of synthetic fluorite cannot eliminate the presence of contaminating substances or compounds or impurities such as silica S1O2 and/or aluminum (AI2O3), magnesium (MgO), iron (Fe2C>3) and sodium (Na20) metal oxides, which are also present in natural acid grade fluorite CaF2 in a variable amount. In the case of silica S1O2, for instance, natural fluorite CaF2 can contain an amount varying between 0.5% and 1.5%, by weight. It is well known about the negative effect of silica, which is present as impurity in fluorite used as raw material, on the HF formation process according to the following reaction:
S1O2 + 3H2S04 + 3CaF2 -=> H2SiF6 + 2H20 + 3CaS04
Fluorite loss can be calculated stoichiometrically and is about 3.9% per 1 % of S1O2, and sulfuric acid loss is about 4.9% per 1% of Si02. In the absence of silica, the reaction of HF formation from synthetic fluorite is as follows: H2S0 + CaF2 CaS04 + 2HF.
The presence of magnesium, e.g. as magnesium oxide, in fluorite causes some problems during the reaction with sulfuric acid. The chalk CaSC produced in the presence of magnesium tends to build scales on the walls of HF production furnaces. This effect can lead to a complete shutdown of the reaction, thus causing an unwanted plant standstill or anyhow to a large increase in specific fluorite consumption (the amount of fluorite lost in chalk increases). Therefore, from the economic and process point of view, it is necessary to be able to reduce the amount of magnesium (expressed as magnesium oxide) in fluorite to an amount below at least 0.5%.
Therefore, there is still the need to have a high purity synthetic fluorite, basically without contaminating substances or compounds or impurities such as e.g. S1O2 (in an amount below 1% by weight) and/or aluminum (AI2O3) and/or magnesium (MgO) metal oxides (in an amount below 0.5% by weight) and/or iron (Fe203) and/or sodium (Na20) metal oxides, so as to validly enable the use of said synthetic fluorite in a process for the production of hydrofluoric acid.
Furthermore, there is still the need to have a process for the production of synthetic fluorite which is simple, efficient and with high yields and which, starting from fluorosilicic acid, can therefore provide a synthetic fluorite with a high purity and without contaminating substances or compounds or impurities such as e.g. silica S1O2 and/or aluminum (AI2O3) and/or magnesium (MgO) and/or iron (Fe2C>3) and/or sodium (Na20) metal oxides. Said synthetic fluorite can be validly used in a process for the production of hydrofluoric acid.
An object of the present invention is a high purity synthetic fluorite (CaF2), classified as "acid grade", having the characteristics as defined in the appended claims.
An object of the present invention is a process for the preparation of said high purity synthetic fluorite (CaF2), classified as "acid grade", having the characteristics as defined in the appended claims.
An object of the present invention is the use of said high purity synthetic fluorite (CaF2), classified as "acid grade", in the industrial production of hydrofluoric acid (HF), having the characteristics as defined in the appended claims.
Preferred forms of the present invention will be disclosed hereinafter in the following detailed description.
Figure 1 represents a block diagram of the process for the preparation of high purity synthetic fluorite, according to an embodiment of the present invention including the purification of the solution of NH4F, the transformation of NH4F into ammonium bifluoride (NH4HF2) and the use of CaCCb.
Figure 2 represents a block diagram of the process for the preparation of high purity synthetic fluorite, according to an embodiment of the present invention including the purification of the NH4F solution and the use of Ca(OH)2. In the framework of the present invention, "high purity" synthetic fluorite means a synthetic fluorite having a concentration of 95% by weight or above with respect to dry weight; preferably of 97% by weight or above with respect to dry weight; still more preferably of 99% by weight or above with respect to dry weight.
In the framework of the present invention, high purity synthetic fluorite (CaF2), classified as "acid grade", means a fluorite having a CaF2 content above 95% by weight, e.g. above 97% by weight, with respect to dry weight, as measured according to current techniques and based on the knowledge that 1) fluorite at 100°C, pressure 1 atmosphere, after 60 minutes, has a water content of about 4% by weight; and that 2) fluorite at 800°C, pressure 1 atmosphere, after 80 minutes, has a water content of about 0% by weight.
In a first embodiment R1 , the process according to the present invention is represented by way of example and therefore in a manner not limiting the scope of the present invention, in the block diagram of Figure 1 (simplified block diagram of the main steps of the process according to the present invention, including the purification of the solution of NFUF, the transformation of NH4F into ammonium bifluoride (NH4HF2) and the use of CaC03).
Summarizing, said first embodiment R1 (Figure 1) includes the following steps:
1 ) Decomposition of fluorosilicic acid (FSA) FbSiFe FSA with ammonia and separation of silica precipitated from the solution of ammonium fluoride NhUF (R1 F1).
2) Purification of the solution of NH4F by dosing suitable reagents enabling the elimination, by precipitation and following separation, of silica still present in the solution of NH4F (R1 F2).
3) Transformation of NH4F into ammonium bifluoride NH4HF2 by distillation under reduced pressure (according to reaction B, see below) and following recovery of a fraction of NH3 by absorption in an aqueous solution or condensation (R1 F3).
4) Synthesis and precipitation of the synthetic fluorite thus obtained CaF2 by reaction with calcium carbonate or calcium hydroxide Ca(OH)2 with NH4HF2 and simultaneous distillation of free ammonia so as to recover the remaining fraction of NH3 by absorption in an aqueous solution or condensation (R1 F4).
In a second embodiment R2, the process according to the present invention is represented by way of example and therefore in a manner not limiting the scope of the present invention, in the block diagram of Figure 2 (simplified block diagram of the main steps of the process according to the present invention, including the purification of the solution of NH4F and the use of Ca(OH)2).
Summarizing, said second embodiment R1 (Figure 2) includes the following steps
1) Decomposition of fluorosilicic acid (FSA) H2S1F6 FSA with ammonia and separation of silica precipitated from the solution of ammonium fluoride NH4F (R2F1).
2) Purification of the solution of NH4F by dosing suitable reagents enabling the elimination, by precipitation and following separation, of silica still present in the solution of NH4F (R2F2).
3) Synthesis and precipitation of the synthetic fluorite CaF2 starting directly from NH4F in the presence of calcium hydroxide Ca(OH)2 or calcium carbonate (R2F3).
The process according to the present invention (in each of its embodiments) enables to obtain a synthetic fluorite having a qualitative/quantitative composition as disclosed below.
In an embodiment, the composition of the synthetic fluorite, after drying at 110°C until a constant weight is obtained, is as follows:
CaF2 = 95-96%
CaC03 (or Ca(OH)2) = 0.7-1.2%
Si02 = 0.01-0.2%
MgO = 0.05-0.2%
LOI (H20 - loss on ignition) = 4-5%
In another embodiment, the composition of the above synthetic fluorite, after calcination at 800°C for 30 minutes, is as follows:
CaF2 = 98-99%
CaC03 (or Ca(OH)2) = 0.7-1.3%
Si02 = 0.01-0.2%
MgO = 0.05-0.2%
LOI (H20 - loss on ignition) = 0.5%
Said fist embodiment R1 (Figure 1 ) is described in detail below and comprises the following steps. A first step R1 F1 includes the decomposition of fluorosilicic acid (FSA) h½SiF6 with ammonia and the separation of silica precipitated from the solution of ammonium fluoride NH4F, according to reaction A):
A) H2SiF6 (aq) + 6NH3 (aq) + 2H20→ 6NH4F(aq) + Si02 (solid)
Then, a second step R1 F2 includes the purification of the solution of NH4F by dosing suitable reagents selected among nitrate salts such as iron nitrate and/or magnesium nitrate, which enable the elimination, by precipitation and following separation, of silica still present in the solution of NH4F. It is not advisable to use chlorinated salts such as iron chloride.
Then, a third step R1 F3 includes the transformation of NH4F into ammonium bifluoride NH4HF2 by means of a distillation under reduced pressure (according to reaction B) and following recovery of a fraction of NH3 by absorption in an aqueous solution or condensation.
Reaction B) is schematized as follows:
B) 2NH4F (aq)→ H4HF2 (aq) + NH3 (gas)
Then, a fourth step R1 F4 includes the synthesis and precipitation of fluorite CaF2 by reaction of NH4HF2 (aq) with calcium carbonate and simultaneous distillation of free ammonia so as to recover the remaining fraction of NH3 by absorption in an aqueous solution or condensation (reaction C).
Reaction C) is schematized as follows:
C) NH4HF2 (aq) + CaCC>3 (solid)→ CaF2 (solid) + C02 (gas) + NH3 (gas)
Then, the process includes a drying step until a synthetic fluorite suitable for use in the industrial production of hydrofluoric acid is obtained.
In said first step R1 F1 , fluorosilicic acid FSA having a concentration of 15 to 30% w/w (weight/weight), preferably of 20 to 25% w/w is reacted under constant mechanical stirring with an aqueous solution of NH3 having a concentration of 10 to 35% by weight, preferably of 15 to 25% by weight. The reaction is exothermic and temperature can reach 90°C, therefore to avoid excessive losses of NH3 the reaction temperature is kept constant at 50-70°C.
NH3 is dosed in a stoichiometric excess of about 20-30% by weight on FSA with respect to the theoretical value (6 moles of NH3 per mole of FSA). During the hydrolysis step the reagents are added so that the pH of the solution remains stable at a value of about 9. In a preferred embodiment, in order to maintain a stable pH, FSA in NH3 is added, thus ensuring to obtain a silica that can be easily filtered.
The effectiveness of the hydrolysis process is strictly related to the speed of addition of the reagents, i.e. FSA in NH3. The total estimated time for obtaining a filterable, high quality silica and completing the hydrolysis reaction is 2 to 6 total hours, preferably 3 to 5 total hours, e.g. 4 total hours considering a speed of addition of 0.01 l/min per 1 liter of NH3 18% or FSA 23%.
The sequence of addition produces two different reaction environments, an initial and a final environment, which differently affect the quality of the silica obtained, in particular as far as the structural and surface properties are concerned. Indeed, the pH of the formation of nuclei, aggregates and agglomerates switches from acid to basic depending on whether NH3 in FSA is added or vice versa. As a result, the environment in which the nuclei, aggregates and agglomerates is different in one case and in the other. The different environment affects the nucleation, aggregation and agglomeration of the amorphous silica here produced.
After few minutes of reaction, silica is formed and a white-colored suspension is generated. When the reaction is completed, the silica present in the suspension is preferably separated from the solution containing ammonium fluoride NH4F and a slight excess of NH3. The separation of silica can be carried out by filtration, e.g. by means of a filter press or basket strainers, or by centrifugation. The first washing water of silica is recovered in the solution of NH4F, water from the following washing steps is sent to water purification. Preferably, the final solution is clear and still contains a small fraction of dissolved silica of 1 to 5 g/l. As a matter of fact, by letting the solution rest for about 2-4 hours a further formation of precipitated silica can be observed.
In said second step R1 F2, the silica present in the solution of NH4F should be eliminated before producing the synthetic fluorite so as to reduce the content of S1O2 in the finished product. The purification process includes the addition of small amounts of an aqueous solution of iron nitrate and/or magnesium nitrate. Advantageously, the optimal dosage in grams is of 0.010 (e.g. 0.015) to 0.10, preferably 0.020 (e.g. 0.025 or 0.030) to 0.080 (e.g. 0.050) of Fe(N03)3 per 1 g of Si02 present in the solution of NH4F, and 0.010 (e.g. 0.015) to 0.10, preferably 0.020 (e.g. 0.025 or 0.030) to 0.080 (e.g. 0.050) of Mg(N03)2 per 1 g of Si02 present in the solution of NH4F. The pH of the solution should be above 8.5; preferably of 9 to 11 and the reaction time is of about 45-90 minutes, preferably 60 minutes. Under these circumstances, metals precipitate as hydroxides incorporating into the flakes the silica still present in the solution. The yield of the purification process is above 90% (in an experimental assay on a solution comprising NH4F and S1O2 from said first step R1 F1 containing 0.66% of S1O2, a solution comprising 0.04% of S1O2 is obtained after purification (step R1 F2)). The reaction can be carried out at room temperature or anyhow at the final temperature of the first step, without advantageously including a cooling step of the solution of NH4F. The silica present in the obtained suspension is separated by filtration (e.g. basket strainer).
Said first embodiment R1 includes a distillation process, which is necessary for the conversion of NhUF into more reactive (NH4)HF2. As a matter of fact, with respect to calcium hydroxide, carbonate does not react with NH4F spontaneously and it is mandatory to implement a distillation process so as to lead the reaction to fluorite formation.
In said third step R1 F3, the solution of NH4F, which was previously purified from S1O2, is distilled under reduced pressure so as to promote the decomposition of the compound NH4F, which is not very stable, and its transformation into the more stable form NH4HF2 (reaction B). The decomposition includes the removal of a mole of NH3 per mole of NH4F, to this amount NH3 already present in free form in the solution is added. The distillation is carried out by increasing system temperature from 30°C to 130°C) under a slight vacuum (about 60 mbar lower than ambient pressure). During the distillation process, carried out at 130°C e 60 mbar lower than ambient pressure, the small fluorine losses which occur are recovered by recycling ammonia distilled in the process during said first step. The residue of distillation is again a solution, although it is possible to crystallize and isolate ammonium bifluoride in solid form, even though this is not advantageous for the process.
Then, the synthesis of fluorite goes on (said fourth step R1 F4) by adding calcium carbonate (reaction C) in stoichiometric amounts with respect to fluorine present in the solution of NH4HF2 obtained above (molar ratio 1 :2), so as to avoid the presence of an excess of carbonates in the finished product. Advantageously, the calcium carbonate used should be dry or with a moisture below 10% by weight, preferably below 5% by weight, and as a fine powder.
The chemical quality of calcium carbonate should be high with a concentration of CaCC>3 above 97%, advantageously above 99%, and with a low content of inorganic contaminants (S1O2, MgC03 and other metal). Advantageously, a calcium carbonate with an average granulometric distribution of 50 to 400 microns, preferably of 100 to 200 microns, is used; higher granulometries are not advisable since they increase reaction times. The reaction can occur at a temperature of 20°C; advantageously. In order to improve ammonia recovery it is advisable to work with temperatures of about 60-70°C and always under a slight vacuum. The stirring speed should be such as to prevent the deposition of solid material onto the reactor bottom. Advantageously, although the reaction is practically instantaneous, the best yields are obtained by leaving the fluorite suspension thus obtained under constant stirring for at least 30-60 minutes. The fluorite thus obtained is separated from the suspension by filtration.
The filtered product is washed and takes a muddy consistency with an average residual moisture of about 40%.
Said second embodiment R2 (Figure 2) is described below in detail and comprises the following steps.
Said first step R2F1 includes the production of NhUF by basic hydrolysis of H SiFe in an aqueous solution having a concentration of 15 to 30% by weight, preferably of 20 to 25% by weight, with an aqueous solution of NH3, under constant mechanical stirring, having a concentration of 10 to 35% by weight, preferably of 15 to 25% by weight.
The reaction is as follows:
H2SiF6 (aq) + 6 NH3 (aq) + 2 H20→ 6 NH4F (aq) + Si02 (s)|
Said first step R2F1 is carried out under the same conditions as for the step R1 F1.
For instance, in a container, e.g. a 500 ml container, containing an amount of 200 to 250 g of ammonia, e.g. 237 g of ammonia (e.g. 30% excess with respect to the estimated stoichiometric amount for the reaction), an amount of FSA of 150 to 250 g, preferably 200 g, is added.
Preferably, the dispersion obtained from the reaction above was intensively stirred e.g. for about 20-40 minutes with a mechanical stirrer, e.g. Velp, monitoring pH and temperature. During this time the pH remained stable at a value of 8.5 to 9,5, preferably around 9. The temperature rose from 25°C to about 60-65°C.
Once the stirring time was over, the precipitated silica was preferably separated by filtration, e.g. by filtration under vacuum, preferably at a relative pressure of about 50-150 mbar, still more preferably at a pressure of about 100 mbar. Preferably, the solid thus obtained was re-dispersed in water and filtered under the same operating conditions as described above.
The solid thus obtained was dried, preferably in an oven at about 105-110°C, and weighed. The dried solid was analyzed by XRF. According to the above operating conditions, the Applicant executed three assays and observed that of the theoretical estimated amount of silica (17.62 g) 15.60 g of silica were obtained in the first assay, 16.40 g in the second assay, and 16.94 g in the third assay.
Preferably, the first silica washing water is added to the initial solution of NH4F.
Said second step R2F2 includes the purification of NH4F from silica.
The solution containing NH4F, obtained after filtration, is treated/purified (said second step R2F2) under the same operating conditions as described for step R1 F2.
For instance, a solution containing NH4F, obtained after filtration, is treated/purified with a solution comprising iron (III) nitrate having a concentration of 20 to 60%by weight/volume, preferably of 30 to 50% by weight/volume, and/or magnesium (II) nitrate having a concentration of 40-80% by weight/volume, preferably of 50 to 70% by weight/volume.
For instance, the solution containing NH4F, obtained after filtration, is treated with an amount of 0.02 to 0.08 g, preferably 0.04 to 0.06 g of Fe(N03)3 (ferric nitrate nonahydrate - Ρβ(Ν03)3·9Η20 -43.3% weight/volume aqueous solution) and with an amount of 0.05 g to 1 g, preferably of 0.07 to 0.09 g of Mg(NC>3)2 (magnesium nitrate - Mg(NOa)2 - 64.4% weight/volume aqueous solution). The solution thus obtained is kept under stirring for a time of 10 to 90 minutes, preferably 60 minutes, at a temperature of 20C° to 25°C.
Then, said solution is filtered thus obtaining an aqueous solution of NH4F basically without silica.
Then, the third step (R2F3) includes the treatment of said aqueous solution of NH4F basically without silica directly with calcium hydroxide in an excess amount of 0.01 to 0,5% with respect to the stoichiometric amount, thus obtaining a dispersion which is kept under stirring for a time of 10 to 60 minutes at a temperature of 40 to 90°C. Finally, the latter solution is filtered thus obtaining the synthetic fluorite.
The solution is preferably filtered under vacuum at a pressure of 50 mbar to 150 mbar, preferably at 100 mbar, e.g. with a 0.45 μιη filter made of cellulose acetate. Quantitative analyses by ICP-AES are carried out on the samples of solution of NH4F taken before and after treatment. It was found that on average, the concentration of Si02 decreased of at least 70% by weight in the samples treated with nitrates, e.g. from an initial value of 2.5 g/l to 0.3 g/l.
Said third step R2F3 includes the synthesis of CaF2 starting from NhUF in the presence of calcium hydroxide.
The reaction can be schematized as follows:
2 NH4F(aq) + Ca(OH)2(s) -→ CaF2(s) j + 2 NH3(gas) + 2 H20(aq)
For instance, an amount of 250 g to 350 g, preferably of 300 g of NH4F, e.g. ammonium fluoride - NH4F - 9.5 by weight aqueous solution, was placed in a 500 ml PTFE three-neck flask and reacted with calcium hydroxide Ca(OH)2 (97.8%).
The assays were made using an excess amount of about 0.3% with respect to the stoichiometric amount. In all the assays that were made, the dispersion was left under mechanical stirring for a time of 20 to 60 minutes, preferably 30 minutes in an oil bath at a temperature of 80-90°C.
The precipitate (CaF2) was filtered by filtration under vacuum at a relative pressure of 50 mbar to 150 mbar, preferably 100 mbar with a filter, e.g. a Whatman 42 paper filter, washed and dried in an oven at a temperature of 110°C and analyzed by XRF.
The yield of the reaction is above 95% and the quantitative analysis of the solid shows a very low percentage of residual silica (below 0.2% of Si02). Fluorite washing water does not exhibit residual fluorine and ammonia is recovered at 100% in a closed system.
As an alternative, said third step R2F3 includes the synthesis of CaF2 starting from NH4F in the presence of calcium carbonate.
The reaction can be schematized as follows:
2 NH F(aq) + CaC03(s)→ CaF2(s) |+ 2 NH3(gas) + C02(gas) + H20(aq)
Calcium carbonate is used in an excess amount of 0.01 to 0.5% with respect to the stoichiometric amount to give a dispersion which is kept under stirring for a time of 10 to 60 minutes, preferably 30 minutes at a temperature of 60 to 90°C, preferably 80°C. For instance, an amount of 250 g to 350 g, preferably of 300 g of NFUF, e.g. ammonium fluoride - NH4F - 9.5 by weight aqueous solution, was placed in a 500 ml PTFE three-neck flask and reacted with calcium carbonate.
The assays were made using an excess amount of 0.3% with respect to the stoichiometric amount. In all the assays that were made, the dispersion was left under mechanical stirring for a time of 20 to 60 minutes, preferably 30 minutes in an oil bath at a temperature of 80-90°C.
The precipitate (CaF2) was filtered by filtration under vacuum at a relative pressure of e.g. 50 mbar to 150 mbar, preferably 100 mbar with a filter, e.g. a Whatman 42 paper filter, washed and dried in an oven at a temperature of 110°C and analyzed by XRF.
The yield of the reaction is above 95% and the quantitative analysis of the solid shows a very low percentage of residual silica (about 0.1% of S1O2). Fluorite washing water does not exhibit residual fluorine and ammonia is recovered at 100% in a closed system.

Claims

1. A process for the preparation of a synthetic fluorite comprising the following steps:
- preparing a solution of NH4F, having a concentration of 15 to 30% by weight, by means of basic hydrolysis at a pH value of 8.5 to 9.5 of H2SiF6 with an aqueous solution of NH3 having a concentration of 10 to 25% by weight;
- treating the solution of NhUF with an amount of 0.01 g to 0.10 g of iron nitrate (I I I) per 1 g of S1O2 in said solution of NH4F and with an amount of 0.01 g to 0.10 g of magnesium nitrate per 1 g of S1O2 in said solution of NH4F ;
- filtering said solution to give an aqueous solution of NH4F which is basically without silica;
- treating said aqueous solution of NH4F basically without silica with calcium hydroxide in an excess amount of 0.01 to 0.5% with respect to the stoichiometric amount to give a dispersion which is kept under stirring for a time of 10 to 60 minutes at a temperature of 40 to 90°C;
- filtering the latter solution obtaining synthetic fluorite.
2. The process according to claim 1 , wherein said aqueous solution of NH4F, obtained after filtration and basically without silica, is treated with calcium carbonate CaCC>3 or calcium hydroxide Ca(OH)2 in an excess amount of 0.01 to 0.5% with respect to the stoichiometric amount to give a dispersion which is kept under stirring for a time of 10 to 60 minutes, preferably 30 minutes at a temperature of 60 to 90°C, preferably at 80°C.
3. The process according to claim 1 , wherein said aqueous solution of NH4F, obtained after filtration and basically without silica, is distilled under reduced pressure so as to transform ammonium fluoride into ammonium bifluoride according to the following reaction:
2NH4F (aP)→NH4HF2 (aq) + NH3 (gas)
4. The process according to claim 3, wherein ammonium bifluoride is reacted with calcium carbonate to give synthetic fluorite CaF2, according to the following reaction:
NH4HF2 (aq) + CaC03 (solid)→ CaF2 (solid) + C02 (gas) + NH3 (gas)
5. The process according to claim 1 , wherein said basic hydrolysis is carried out at a pH value of 9 for a time of 45 to 60°C and at a temperature of 25°C.
6. The process according to one of the preceding claims, wherein calcium carbonate should have a moisture content below 10% by weight, preferably below 5% by weight, a CaC03 concentration above 97%, preferably above 99%, and a low content of inorganic contaminants selected among Si02l MgCC>3, metal oxides and metals.
7. The process according to claim 6, wherein calcium carbonate has an average granulometric distribution of 50 to 400 microns, preferably of 100 to 200 microns, and wherein ammonia is recovered at a temperature of about 60-70°C and always under a slight vacuum.
8. The process according to claim 1 , wherein the synthetic fluorite obtained after filtration has an average moisture content of about 40% by weight.
9. A synthetic fluorite that can be obtained with the process according to any one of the preceding claims having the following composition:
CaF2 = 95-96%,
CaC03 (or Ca(OH)2) = 0.7-1.2%,
Si02 = 0.01-0.2%,
MgO = 0.05-0.2%,
LOI (H20 - loss on ignition) = 4-5%,
after drying at 110°C up to a constant weight;
or:
CaF2 = 98-99%,
CaC03 (or Ca(OH)2) = 0.7-1.3%,
Si02 = 0.01-0.2%,
MgO = 0.05-0.2%,
LOI (H20 - loss on ignition) = 0.5%,
after calcination at 800°C for 30 minutes.
10. Use of the synthetic fluorite according to claim 9 for the production of hydrofluoric acid.
EP15771700.0A 2014-04-04 2015-04-02 High purity synthetic fluorite and process for preparing the same Pending EP3126290A2 (en)

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JOHN W ANTHONY ET AL: "Fluorite", HANBOOK OF MINERALOGY, 1 January 2001 (2001-01-01), XP055553681, Retrieved from the Internet <URL:http://www.handbookofmineralogy.org/pdfs/fluorite.pdf> [retrieved on 20190207] *

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