OA18423A

OA18423A - High purity synthetic fluorite, process for preparing the same and apparatus therefor.

Info

Publication number: OA18423A
Application number: OA1201700382
Authority: OA
Inventors: Luca Pala; Michele LAVANGA
Original assignee: Fluorsid S.P.A.
Priority date: 2015-04-02
Filing date: 2016-03-31
Publication date: 2018-11-02

Abstract

The present invention relates to a high purity synthetic fluorite (CaF2). The present invention further relates to a process and an apparatus for preparing said high purity synthetic fluorite (CaF2), classified as acid grade, starting from fluorosilicic acid H2SiF6 (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

The présent invention relates to a high purity synthetic fluorite (CaF₂). Furthermore, the présent invention relates 5 to a process for the préparation of said high purity synthetic fluorite (CaF₂), classified as acid grade, starting from fluorosilicic acid H₂SiF6 (FSA). In addition, the présent invention relates to the use of said high purity synthetic fluorite (CaF₂) in the industrial production of hydrofluoric acid. Finally, the présent invention relates to an apparatus for the production of said synthetic fluorite.

Fluorosilicic acid H₂SiF₆ (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 and fluorine, which are présent in the phosphate minerai used as a raw material, and sulphuric acid used for the production of phosphoric acid.

During the step of phosphoric acid concentration, S1F4 can be absorbed into an aqueous solution, producing FSA with a concentration varying between 23% and 35%.

The known methods for preparing synthetic fluorite are limited, at times, to the synthesis of fluorite in wet form with a particle size of 5 microns and not always optimal purity. These types of synthetic fluorite cannot be used for the purpose of producing hydrofluoric acid HF.

Unfortunately, available known methods for the préparation of synthetic fluorite are not able to eliminate the presence ofcontaminating substances or compounds or impurities such as silica SiO₂ and/or aluminium (AI2O3), magnésium (MgO), iron (Fe₂O3) and sodium (Na₂O) oxides, which are also présent in natural acid grade fluorite CaF₂ in a variable amount. In the case of silica SiO₂, for example, natural fluorite CaF₂ can contain a variable amount of between 0.5% and 1.5% by weight. Silica, présent as an impurity in fluorite used as a raw material, is well known to hâve a négative impact on the HF formation process according to the following reaction:

SiO₂ + 3H₂SO4 + 3CaF₂ H₂SiF₆ + 2H₂O + 3CaSO₄

Fluorite loss can be calculated stoichiometrically and is about 3.9% per 1% of SiO2 and sulphuric acid loss is about 4.9% per 1% of SiO2. In the absence of silica, the reaction of HF formation from synthetic fluorite is: H₂SO₄ + CaF₂ CaSO₄ + 2HF.

The presence of magnésium, for example as magnésium oxide, in fluorite causes some problème during the reaction with sulphuric acid. The gypsum CaSO₄ produced in the presence of magnésium tends to form scale on the walls of fumaces for producing HF. This effect can lead to a complété interruption of the reaction, thus resulting in an unwanted plant downtime or in any case a large increase in spécifie fluorite consumption (the amount of fluorite lost in the gypsum increases).

Therefore, from an économie and processing standpoint, it is necessary to be able to reduce the amount of 35 magnésium (expressed as magnésium oxide) présent in the fluorite to an amount below 0.5% at least.

Hence, there remains a need to hâve a high purity synthetic fluorite which is substantially free of contaminating substances or compounds or impurities such as, for example, silica SiO₂ (in an amount of less than 1% by weight) and/or aluminium (AI2O3) and/or magnésium (MgO) métal oxides (in an amount of less than 0.5% by weight) and/or iron (Fe₂O3) and/or sodium (Na₂O) métal oxides, so as to validly enable the use of said synthetic 5 fluorite in a process for the production of hydrofiuoric acid.

Moreover, there remains a need to hâve a process and an apparatus for the production of synthetic fluorite which is simple, efficient and has high yields and which, starting from fluorosilicic acid, is therefore capable of providing a synthetic fluorite with a high purity and without contaminating substances or compounds or impurities such as, for example, silica SiO₂ and/or aluminium (AI₂O₃) and/or magnésium (MgO) and/or iron 10 (Fe₂O3) and/or sodium (Na₂O) métal oxides. Said synthetic fluorite can be validly used in a process for the production of hydrofiuoric acid.

The présent invention relates to a high purity synthetic fluorite (CaF₂), classified as 'acid grade, having the characteristics as defined in the appended claims. Said CaF₂ is produced in granules with an average particle size greater than 1 mm for at least 50% by weight of the same, with granules that show good mechanical stability 15 and a large spécifie surface area (BET). Said particle size imparts marked properties of reactivity to the synthetic

CaF₂, making it compétitive as a replacement for acid grade natural fluorite in the production of HF.

The présent invention relates to a process and an apparatus for preparing said high purity synthetic fluorite (CaF₂), classified as “acid grade, having the characteristics as defined in the appended claims.

The présent invention relates to the use of said high purity synthetic fluorite (CaF₂), classified as “acid grade, 20 in the industrial production of hydrofiuoric acid (HF), having the characteristics as defined in the appended claims.

Preferred embodiments of the présent invention will be illustrated in the detailed description that follows below. Figure 1 represents a block diagram of the process for preparing high purity synthetic fluorite, according to an embodiment of the présent invention comprising the purification of a solution of NH4F, the transformation of 25 _NH4F into ammonium bifiuoride (NH4HF2) and the use of CaCO₃.

Figure 2 represents a block diagram of the process for preparing high purity synthetic fluorite, according to an embodiment of the présent invention comprising the purification of a solution of NH4F and the use of Ca(OH)₂.

In the context of the présent invention, “high purity synthetic fluorite (CaF₂) means a synthetic fluorite having a concentration equal to or greater than 95% by weight relative to the dry weight; preferably equal to or greater 30 than 97% by weight relative to the dry weight; preferably equal to or greater than 99% by weight relative to the dry weight.

In the context ofthe présent invention, high purity synthetic fluorite (CaF2), classified as “acid grade, means a fluorite having a CaF₂ content greater than 95% by weight, for example greater than 97% by weight, relative to the dry weight, as measured according to current techniques and based on the knowledge that: 1) fluorite at

100°C, pressure of 1 atmosphère, after 60 minutes, has a water content of about 4%, by weight; and that 2) fiuorite at 800°C, pressure of 1 atmosphère, after 80 minutes, has a water content of about 0%, by weight.

In a first embodiment R1, the process according to the présent invention is represented, by way of example, without limiting, therefore, the scope ofthe présent invention, in the block diagram of Figure 1 (simplified block 5 diagram of the main steps of the process according to the présent invention, which comprises the purification of the solution of NH₄F, the transformation of NH4F into ammonium bifluoride (NH4HF2) and the use of CaCO3).

In summary, said first embodiment R1 (Figure 1 and Figure 5) comprises the following steps:

1) Décomposition offluorosilicic acid (FSA) H2S1F6 FSA with ammonia and séparation ofthe silica precipitated from the solution of ammonium fluoride N H₄ F (R1F1 )

2) Purification of the solution of NH4F by dosing suitable reagents which enable the élimination, by précipitation and subséquent séparation, of the silica still présent in the solution of NH4F (R1F2).

3) Transformation of NH4F into ammonium bifluoride NH4HF2 by distillation under reduced pressure (according to reaction B, see below) and conséquent recovery of a fraction of NH3 by absorption in an aqueous solution or condensation (R1F3).

4) Synthesis and précipitation of the synthetic fiuorite CaF2 thus obtained by reaction of 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 (R1F4).

In a second embodiment R2, the process according to the présent invention is represented, by way of example, without limiting, therefore, the scope ofthe présent invention, in the block diagram of Figure 2 (simplified block 20 diagram of the main steps of the process of the présent invention, which comprises the purification of the solution of NH4F and the use of Ca(OH)2).

In summary, said second embodiment R2 (Figure 2 and Figure 6) comprises the following steps:

1) Décomposition offluorosilicic acid (FSA) H2S1F6 FSA with ammonia and séparation ofthe silica precipitated from the solution of ammonium fluoride NH4F (R2F1).

2) Purification of the solution of NH₄F by dosing suitable reagents which enable the élimination, by précipitation and subséquent séparation, of the silica still présent in the solution of NH₄F (R2F2).

3) Synthesis and précipitation of the synthetic fiuorite CaF2 starting directly from NH₄F in the presence of calcium hydroxide Ca(OH)2 or calcium carbonate (R2F3).

The process according to the présent invention (in each ofthe embodiments thereof) makes it possible to obtain 30 a synthetic fiuorite having a qualitative/quantitative composition as disclosed below.

In one embodiment, the composition of the synthetic fiuorite, after drying at 110°C until a constant weight is obtained, is the following:

CaF₂ = 95-96%

CaCO₃ (orCa(OH)₂) = 0.7-1.2%

SiO₂ = 0.01-0.2%

MgO = 0.05-0.2%

AI2O3 = 0.05-0.2%

LOI (H2O - loss on ignition) = 4-5%

In another embodiment, the composition of the above synthetic fluorite, once dried after calcination at 800°C 5 for 30 minutes, is the following:

CaF₂ = 98-99%

CaCO₃(orCa(OH)₂) = 0.7-1.3%

SiO₂ = 0.01-0.2%

MgO = 0.05-0.2%

AI₂O₃ = 0.05-0.2%

LOI (H2O - loss on ignition) = 0.5%

Said first embodiment R1 (Figure 1) is described in detail below and comprises the following steps.

A first step R1F1 comprises the décomposition of fluorosilicic acid (FSA) H₂SiF6 with ammonia and séparation of the silica precipitated from the solution of ammonium fluoride NH₄F, according to reaction A):

A) HzSiFe (aq) + 6NH₃ (aq) + 2H2O —> 6NH₄F(aq) + SÎO₂ (solid)

Subsequently, a second step R1F2 comprises the purification of the solution of NH4F by dosing suitable reagents selected from among nitrate salts such as iron nitrate and/or magnésium nitrate, which enable the élimination, by précipitation and subséquent séparation, of the silica still présent in the solution of NH4F. It is inadvisable to use chlorinated salts such as iron chloride.

Subsequently, a third step R1F3 comprises the transformation of NH4F into ammonium bifluoride NH4HF2 by distillation under reduced pressure (according to reaction B) and conséquent recovery of a fraction of NH₃ by absorption in an aqueous solution or condensation.

Reaction B) is schematized as follows:

B) 2NH₄F_(aq) -> NH₄HF₂(aq) + NH₃₍gas)

Subsequently, a fourth step R1F4 comprises the synthesis and précipitation 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) + CaCO₃ (solid) —> CaF₂ (solid) + CO2(gas) + NH₃(gas)

Then follows a drying step until a synthetic fluorite suitable for use in the industrial production of hydrofluoric acid is obtained.

In said first step R1F1, fluorosilicic acid FSA having a concentration comprised from 15 to 30% w/w ( weight/weight), preferably from 20 to 25% w/w, is reacted under constant mechanical stirring with an aqueous solution of NH₃ having a concentration comprised from 10 to 35% by weight, preferably from 15 to 25% by

weight. The reaction is exothermic and the température can reach 90°C; thus, in order to avoid excessive losses of NH3, the reaction température is maintained constant at 50-70°C.

The NH3 is dosed in a stoichiometric excess of about 20-30% by weight on FSA relative to the theoretical value (6 moles of NH3 per mole of FSA). During the hydrolysis step the reagents are added in such a manner that the 5 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, which assures the obtainment of an easily filtrable silica.

The efficiency of the hydrolysis process closely dépends on the speed of addition of the reagents, or FSA in NH3. The total estimated time for obtaining a filterable, high quality silica and completing the hydrolysis reaction is comprised from a total of 2 to 6 hours, preferably from a total of 3 to 5 hours, e.g. a total of 4 hours considering 10 a speed of addition of about 0.01 l/min per 1 litre of 18% NH3 or 23% FSA.

The sequence of addition produces two different reaction environments, an initial and a final environment, which differently influence the quality of the silica obtained, in partïcular with respect to the structural and surface properties. In fact, the pH of the formation of nuclei, aggregates and agglomérâtes switches between acid and basic depending on whether NH3 in FSA is added or vice versa. Accordingly, the environment in which the 15 nuclei, aggregates and agglomérâtes form is different in one case and in the other. The different environment influences the nucléation, aggregation and agglomération ofthe amorphous silica produced hère.

After few minutes of reaction, silica is formed and a white-coloured suspension is generated. Upon completion of the reaction, the silica présent in the suspension is preferably separated from the solution contaîning ammonium fluoride NH4F and a slight excess of NH3. The séparation of the silica can be carried out by filtration, 20 for example by means of a filter press or strainer filters, or by centrifugation. The first washing water of the silica is recovered in the solution of NH4F; the water from subséquent washing steps is sent off for water purification. Preferably, the final solution is clear and still contains a small fraction of dissolved silica comprised from 1 to 5 g/l. In fact, if the solution is allowed to rest for about 2-4 hours, an additional formation of precipitated silica can be observed.

In said second step R1F2, the silica présent in the solution of NH4F must be eliminated before the synthetic fluorite is produced in order to reduce the content of S1O2 in the finished product. The purification process comprises adding small amounts of an aqueous solution of iron nitrate and/or magnésium nitrate. Advantageously, the optimal dosage in grams is comprised from 0.010 (for example 0.015) to 0.10, preferably from 0.020 (for example 0.025 or 0.030) to 0.080 (for example 0.050) of Fe(NÛ3)3 per 1 g of S1O2 présent in the 30 solution of NH4F, and from 0.010 (for example 0.015) to 0.10, preferably from 0.020 (for example 0.025 or 0.030) to 0.080 (for example 0.050) Mg(NÛ3)2 per 1 g of S1O2 présent in the solution of NH4F. The pH of the solution should be greater than 8.5, preferably comprised from 9 to 11, and the reaction time about 45-90 minutes, preferably 60 minutes. Under these conditions the metals precipitate as hydroxides incorporating the silica still présent in the solution into the flakes.

The yield of the purification process is greater than 90% (in an experimental trial conducted on a solution comprising NH4F and S1O2 from said first step R1F1 containîng 0.66% S1O2, a solution containîng 0.04% S1O2 was obtained after purification (step R1F2)). The reaction can be carried out at room température or in any case at the final température of the first step, advantageously without requiring a step of cooling the solution of NH4F.

The silica présent in the suspension obtained is separated by filtration (for example strainer filter).

Said first embodiment R1 includes a distillation process, necessary for the conversion of NH4F into (NH4)HF2, which is more reactive. In fact, compared to calcium hydroxide, carbonate does not react spontaneously with NH4F, and it is indispensable to rely on a distillation process in order to drive the reaction towards the formation of fluorite.

In said third step R1F3, the solution of NH4F, which was previously purified from S1O2, is distilled under reduced pressure so as to promote the décomposition of the compound NFLF, which is not very stable, and the transformation thereof into the more stable form NH4HF2 (reaction B). The décomposition involves the removal of one mole of NH3 per mole of NH4F; the NH3 already présent in free form in the solution is added to this amount. The distillation is carried out by increasing the system température from 30°C to 130°C under a slight négative pressure (approximately 60 mbar below ambient pressure). During the distillation process, carried out at 130°C and 60 mbar below ambient pressure, the small losses of fluorine which occur are recovered by recycling the ammonia distilled in the process during said first step. The residue of the distillation is likewise a solution, though it is possible to crystallize and isolate ammonium bifluoride in solid form, even though this is not advantageous for the purposes of the process.

Subsequently, the synthesis of fluorite proceeds (said fourth step R1F4) with the addition of calcium carbonate (reaction C) in stoichiometric amounts relative to the fluorine présent 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 must be dry or with a moisture of less than 10% by weight, preferably less than 5% by weight, and in the form of a fine powder.

The chemical quality of the calcium carbonate must be high, with a CaCOs concentration greater than 97%, advantageously greater than 99%, and a low content of inorganic contaminants (S1O2, MgCOs, and other metals). Advantageously, a calcium carbonate having an average particle size distribution comprised from 50 to 400 microns, preferably from 100 to 200 microns, is used; larger particle sizes are not advisable, as they increase reaction times. The reaction can take place at a température of 20°C; advantageously, in order to improve the recovery of ammonia, it is advisable to work with températures of about 60-70°C and always under a slight négative pressure. The stirring speed should be such as to prevent solid material from depositing on the bottom of the reactor.

Advantageously, although the reaction is practically instantaneous, the best yields are obtained by leaving the fluorite suspension thus produced under constant stim’ng for at least 30-60 minutes. The fluorite thus obtained 35 is separated from the suspension by filtration.

The filtered product is washed and takes on a slurry-like 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 comprises the production of NH₄F by basic hydrolysis of H2S1F6 in an aqueous solution having a concentration comprised from 15 to 30% by weight, preferably from 20 to 25% by weight, with an aqueous solution of NH3, under constant mechanical stirring, having a concentration comprised from 10 to 35% by weight, preferably from 15 to 25% by weight.

The reaction is the following:

HîSÏFe (aq) + 6 NH3 (aq) + 2 H2O —» 6 NH4F (aq) + SiOî (s)J.

Said first step R2F1 is carried out under the same conditions as in step R1F1.

For example, in a container, e.g. a 500 ml container, containing an amount of ammonia comprised from 200 to 250 g, e.g. 237 g of ammonia (for example, a 30% excess relative to the estimated stoichiometric amount for the reaction) an amount of FSA comprised from 150 to 250 g, preferably 200 g, is added.

Preferably, the dispersion obtained from the above reaction was vigorously stirred, for example for about 20-40 minutes with a mechanical stirrer, e.g. a VELP, monitoring pH and température. During this time, the pH remained stable at a value comprised from 8.5 to 9.5, preferably around 9. The température rose from 25°C to about 60-65° C.

Once the stirring time was over, the precipitated silica was preferably separated by filtration, for example by 20 vacuum filtration, preferably at a relative pressure of about 50-150 mbar, even 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 performed 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 comprises 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 R1F2.

For example, a solution containing NH4F, obtained after filtration, is treated/purified with a solution comprising iron (III) nitrate having a concentration comprised from 20 to 60% by weight/volume, preferably from 30 to 50% by weight/volume, and/or magnésium (II) nitrate having a concentration comprised from 40-80% by 3 5 weight/volume, preferably from 50 to 70% by weight/volume.

For example, the solution containing NH4F, obtained after filtration, is treated with an amount comprised from 0.02 g to 0.08 g, preferably from 0.04 g to 0.06 g of FefNOah (ferrie nitrate nonahydrate - Fe(NO3)3-9H2O 43.3% weight/volume aqueous solution) and with an amount comprised from 0.05 g to 1 g, preferably from 0.07 to 0.09 g of Mg(NO₃)2 (Magnésium Nitrate - MgfNChh - 64.4% weight/volume aqueous solution). The solution thus obtained is maintained under stirring for a time comprised from 10 to 90 minutes, preferably 60 minutes, at a température comprised from 20 C° to 25°C.

Then said solution is filtered so as to obtain a substantially silica-free aqueous solution of NH4F.

Subsequently, one proceeds with said third step (R2F3), which comprises treating said substantially silica-free 10 aqueous solution of NH4F directly with calcium hydroxide in an excess amount comprised from 0.01 to 0.5% relative to the stoichiometric amount so as to obtain a dispersion which is maintained under stirring for a time comprised from 10 to 60 minutes at a température comprised from 40 to 90’C. Finally, the latter solution is filtered so as to obtain the synthetic fiuorite.

The solution is preferably filtered under vacuum at a pressure comprised from 50 mbar to 150 mbar, preferably 15 at 100 mbar, for example with a 0.45 pm filter made of cellulose acetate.

Quantitative analyses of the silica are performed by ICP-AES on the samples of solution of NH4F taken before and after treatment. It was found that, on average, the concentration of S1O2 is decreased by at least 70% by weight in the samples treated with nitrates, for example a content of 2.5 g/l of silica present in a sample was reduced to 0.3 g/l.

Said third step R2F3 comprises the synthesis of CaF2 starting from NH4F in the presence of calcium hydroxide.

The reaction can be schematized as follows:

NHLFfaq) + Ca(OH)2(s) —» CaF2(s) 1+2 NH3(gas) + 2 FfeOfaq)

For example, an amount comprised from 250 g to 350 g, preferably 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 25 hydroxide Ca(OH)₂ (97.8%).

The assays were performed using an excess amount of about 0.3% relative to the stoichiometric amount. In ali the assays performed, the dispersion was left under mechanical stirring for a time comprised from 20 to 60 minutes, preferably 30 minutes in an oil bath at a température of 80-90°C.

The precipitate (CaF2) was filtered by vacuum filtration at a relative pressure comprised for example from 50 30 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 température of 110°C and analyzed by XRF.

The yield of the reaction is greater than 95% and the quantitative analysis of the solid shows a very low percentage of residual silica (less than 0.2% of S1O2). Fiuorite washing water does not exhibit residual fluorine and ammonia is recovered at 100% in a closed system.

Altematively, said third step R2F3 comprises the synthesis of CaF₂ starting from NH4F in the presence of calcium carbonate.

The reaction can be schematized as follows:

NHifiaq) + CaCO3(s) —♦ CaF₂(s) J.+ 2 NH3(gas) + CO₂(gas) + H₂O(aq)

The calcium carbonate is used in an excess amount comprised from 0.01 to 0.5% relative to the stoichiometric amount to yield a dispersion which is maintained under stirring for a time comprised from 10 to 60 minutes, preferably 30 minutes, at a température comprised from 60 to 90 °C, preferably 80°C.

For example, an amount comprised from 250 g to 350 g, preferably 300 g of NH4F, e.g. ammonium fluoride 10 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 performed using an excess amount of 0.3% relative to the stoichiometric amount. In ail the assays performed, the dispersion was left under mechanical stirring for a time comprised from 20 to 60 minutes, preferably 30 minutes, in an oil bath at a température of 80-90°C.

The precipitate (CaF₂) was filtered by vacuum filtration at a relative pressure comprised for example from 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 température of 110°C and analyzed by XRF.

The yield of the reaction is greater than 95% and the quantitative analysis of the solid shows a very low percentage of residual silica (about 0.1% of SiO₂). Fluorite washing water does not exhibit residual fluorine and 20 ammonia is recovered at 100% in a closed system.

Figure 3 represents a block diagram of the process for preparing high purity synthetic fluorite, like the process of Figure 1, but without purification ofthe solution of NH4F.

Figure 4 represents a block diagram of the process for preparing high purity synthetic fluorite, like the process of Figure 2, but without purification of the solution of NH4F.

Figures 5,6,7 and 8 are schematic views of the apparatus according to the présent invention for preparing said high purity synthetic fluorite (CaF₂) in accordance with the embodiments (R1, R2, R3 and R4), as exemplified in Figures 1,2,3 and 4, respectively.

In a third embodiment R3, the process according to the présent invention is represented, by way of example, without limiting, therefore, the scope of the présent invention, in the block diagram of Figure 3 (simplified block 30 diagram of the main steps of the process of the présent invention, which comprises the transformation of NH4F into ammonium bifluoride (N H4HF2) and the use of CaCO3).

In summary, said third embodiment R3 (Figure 3 and Figure 7) comprises the following steps:

1) Décomposition of fluorosilicic acid (FSA) H₂SÏF6 FSA with ammonia and séparation of the silica precipitated from the solution of ammonium fluoride NH4F (R3F1).

2) Transformation of NH4F into ammonium bifluoride NH4HF2 by distillation under reduced pressure (according to reaction B) and conséquent recovery of a fraction of NH3 by absorption in an aqueous solution or condensation (R3F2).

3) Synthesis and précipitation of the synthetic fluorite CaF₂ thus obtained by reaction of calcium carbonate or 5 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 (R3F3).

In a fourth embodiment R4, the process according to the présent invention is represented, by way of example, without limiting, therefore, the scope of the présent invention, in the block diagram of Figure 4 (simplified block diagram of the main steps of the process of the présent invention, which comprises the purification of the 10 solution of NH4F and the use of Ca(OH)j).

In summary, said fourth embodiment R4 (Figure 4 and Figure 8) comprises the following steps:

1) Décomposition offluorosilicic acid (FSA) HjSiFe FSAwith ammonia and séparation ofthe silica precipitated from the solution of ammonium fluoride NH4F (R2F1).

2) Purification ofthe solution of NH4F by dosing suitable reagents which enable the élimination, by précipitation 15 and subséquent séparation, of the silica still présent in the solution of NH4F (R2F2).

3) Synthesis and précipitation of the synthetic fluorite CaF₂ starting directly from NH4F in the presence of calcium hydroxide Ca(OH)₂ or calcium carbonate (R2F3).

The apparatus according to the présent invention is capable of producing, in accordance with the process here disclosed and claimed, a high purity synthetic fluorite (CaF2), classified as “acid grade, which can be used in 20 the industrial production of hydrofluoric acid (HF), starting from fluorosilicic acid (FSA) and calcium hydroxide [Ca(OH)j].

The plant according to the présent invention is capable of producing a high purity synthetic fluorite (CaF2) in accordance with the process described in said fourth embodiment R4 (Figure 4 and Figure 8). The plant comprises:

Reaqent storage and feed section • D xO1 (storage tank for the fluorosilicic acid):

Polypropylene tank for storing the fluorosilicic acid, provided with a vacuum breaker valve. The tank is installed inside a suitably sized, insulated polyethylene rétention basin equipped with feed and extraction nozzles and a manhole.

· G xO1 (pump for extracting FSA from D xO1 ):

Horizontally installed centrifugal pump, with a body and impeller made of polypropylene or coated steel. Equipped with a flushed double mechanical seal.

• D x02 (ammonia storage tank):

Tank made of carbon steel, suîtable for storing ammonia in aqueous solution. Kept under suction by P x08 and provided with a vacuum breaker valve, a suitably sized, insulated polyethylene rétention basin equipped with feed and extraction nozzles and a manhole.

• G x02 (pump for extracting NH3 from D x02):

Horizontally installée! centrifugal pump, with a body and impeller made of polypropylene or coated steel. Equipped with a flushed double mechanical seal.

First Reaction Section • R xO1 (FSA hydrolysis reactor):

BATCH-type reactor, made of coated steel (ebonite) or PP, thermoregulated by E X01 and stirred by P x01. 10 Kept under suction by P x08 and provided with reagent feed and product extraction nozzles.

• E xO1 (cooling coil R x01):

Made of SANICRO 28 (or the like) and supplied with cooling water coming from the tower Z xO1. Sized so as to maintain the temperature within 50-60°C.

• Px01 (stirrer R x01):

Made of Sanicro 28 (or the like), powered by an electric motor. Designed and sized so as to enable suîtable contact between the reagents.

• G x04 (pump for extracting slurry from R xO1 ):

Horizontally installed centrifugal pump, with a body and impeller made of polypropylene or coated steel. Equipped with a simple mechanical seal.

· D x03 (intermediate tank):

Buffer tank, made of coated steel or PP, thermoregulated by E x02 and stirred by Px02. Kept under suction by P x08 and provided with feed and extraction nozzles.

G x03: (pump for supplying ammonia to D x03) • E x02 (cooling coil D x03):

Made of Sanicro 28 (or the like) and supplied with cooling water coming from the tower Z x01. Sized so as to maintain the temperature within 30°C.

• P x02 (stirrer D x03):

Made of Sanicro 28 (or the like), powered by an electric motor. Sized so as to enable suîtable contact between the reagents (nitrates) and prevent sédimentation ofthe silica.

First Filtration Section • G x05 (pump supplying F x01 ):

Pneumatic diaphragm pump, supplied with instrument air.

• F xO1 (first filter press):

Automatic filter press, with polypropylene plates and cloths. Provided with drip tray, membrane wringing and 35 blowing, cloth washing and cake washing system. Further equipped with a suction hood connected to P x08.

• G x06 (cake washing pump):

Horizontally installed centrifugal pump, with a cast-iron body and impeller, equipped with a simple mechanical seal.

• D x04 (intermediate tank):

Made of coated steel or PP. Kept under suction by P x08 and provided with feed and extraction nozzles.

P x03 : (stirrer of D x04)

Second Reaction Section • G x07 (pump supplying R x02):

Horizontally installed centrifugal pump, with a body and impeller made of coated steel or PP, equipped with a 10 simple mechanical seal.

• D x05 (storage silo for calcium hydroxide):

Made of carbon steel, equipped with a vent provided with bag filter for dust removal and fan. Fed from tanker trucks via a pneumatic conveyor.

• T xO1 (rotary valve) · T x02 (screw conveyor for extracting calcium hydroxide):

Made of steel, equipped with a balance capable of weighing the calcium hydroxide in a stoichiometric amount to the reactor R x02.

• R x02 (reactor for the formation of fluorite):

CSTR-type reactor, made of coated steel (ebonite) or PP and stirred by P x04. Kept under suction by P x08 and 20 provided with reagent feed and product extraction nozzles.

• P x04 (stirrer reactor R x02):

Made of Sanicro 28 (or the like), powered by an electric motor. Sized so as to enable suitable contact between the reagents and prevent sédimentation ofthe fluorite.

• G x08 (pump from R x02 a D x06):

Horizontally installed centrifugal pump, with a body and impeller made of coated steel or PP, equipped with a simple mechanical seal.

• D x06 (intermediate tank):

Buffer tank, made of coated steel or PP, stirred by Px05. Kept under suction by P x08 and provided with feed and extraction nozzles.

· P x05 (stirrer D x06):

Made of Sanicro 28 (or the like), powered by an electric motor. Sized so as to prevent sédimentation of the fluorite.

Second Filtration Section • G x09 (pump supplying F x02):

Pneumatic diaphragm pump, supplied with instrument air.

• F x02 (second filter press):

Automatic filter press, with polypropylene plates and cloths. Provided with drip tray, membrane wringing and blowing, cloth washing and cake washing system. A hood kept under suction by P x08 should also be provided. D x07: silo for collecting wet synthetic fluorite · Dx11 (intermediate tank):

Made of carbon steel. Kept under suction by P x09 and provided with feed and extraction nozzles. It serves as a buffer for Cx01.

P x07: (stirrer tank D x11 ) • G x14 (cake washing pump):

Ammonia Recovery Section • G x11 (pump supplying mother liquors to C xO1 ):

Horizontally installed centrifugal pump, with a cast-iron (steel) body and impeller, equipped with a simple 15 mechanical seal.

• C xO1 (ammonia stripper):

Made of carbon steel, provided with packing (1 in. métal Raschig rings) of supply nozzles on the head and bottom and a system for feeding the mother liquors to the head. Kept under suction by P x09.

• E x03 (condenser):

Made of carbon steel, of the horizontal type with condensation on the shell side. Supplied on the tube side with cooling water coming from the water tower Z xO1.

• D x12 (accumulation tank):

Made of carbon steel or PP, Kept under suction by P x09.

• G x12 (pump supplying condensed ammonia to D x02) :

• P x09 (fan):

Axial fan. Usable pressure such as to ensure suction in C x01.

Vent Cleaninq Section · P x08 (fan):

Axial fan.

• D x10 (storage tank for H2SO4):

Small 1 m³polyethylene tanks.

• G x10 (pump supplying H2SO4 to C x02) :

Metering pump.

• C x02 (wet scrubber):

Made of PP, packing of plastic material (1 in. Raschig rings), equipped with pH-controlled recirculation tank. System forfeeding sour water at the head. Kept under suction by P x08.

Zx01: (cooling tower)

G x13: pump supplying cooling circuit from Z xO1

Fluorite drying section • B xO1 (dryer):

Rotary oven, provided with a bumer, fume particle scrubbing section (cyclone), loading hopper, screw conveyors for feeding and extracting fluorite, gear motor for varying the rotation speed 10 and hydraulic pistons for varying the inclination.

• T x03 A/B (screw conveyors for feeding wet fluorite to the rotary oven):

carbon steel screw conveyors, equipped with an electric motor, suitable for feeding fluorite to the rotary oven at a constant flow rate.

• T x04 (extracting screw conveyor):

carbon steel screw conveyor, for extracting the granular fluorite from the rotary oven and feeding the bucket elevator T x05. Equipped with an electric motor that runs at constant speed.

• T x05 (bucket elevator):

steel bucket elevator, provided with an electric motor. It receives the granular fluorite from the screw conveyor T x04 and feeds the storage silo D x08.

· T x06 (rotary valve):

steel rotary valve. It régulâtes the discharge ofthe fine particles entrained and entrapped in the cyclone D x09 to a storage réservoir below.

• P x06 (fan):

axial fan, provided with an electric motor. It delivers the air flow necessary for combustion in the bumer of B 25 x01.

• D x08 (storage silo for granular fluorite):

steel (or aluminium) silo, suitable for containing the granular product. Fed from a height by the bucket elevator T x05. Provided with a fan and bag filter for venting into the atmosphère.

• D x09 (cyclone):

made of steel, it receives the hot fumes coming out of the rotary oven. It has the purpose of scrubbing and entrapping, by centrifugal force, the fine particles entrained by the turbulence présent inside the oven.

1. The ammonia présent in the storage tank D x02 is extracted by means of the centrifugal pump G x02 and sent to the batch reactor R x01. Subsequently, the fluorosilicic acid is sent to the reactor from the tank D xO1 by means of the centrifugal pump G xO1. The reactor R xO1, provided with the stirrer P xO1 and cooling coil E xO1, enables the hydrolysis of the acid to take place completely, leading to the formation of NH4F and the précipitation of S1O2 according to reaction A.

A) H2S1F6 + 6 NH₃ + 2H2O-> 6 NH4F + S1O2I

1) The slurry produced in R xO1 is extracted by means of the centrifugal pump G x04 and sent to the intermediate tank D x03. The slurry is maintained under stimng with P x02 and is cooled by E x02. The pH of the solution can be appropriately adjusted through the addition, by means of the pump G x03, of ammonia from Dx02.

2) The slurry is extracted from D x03 by means of the centrifugal pump G x05 and sent to the filter press F x01. Here the silica is retained and the mother liquors are sent to the intermediate tank D x04 stirred by the stirrer P x03. The S1O2 cake is washed with process water, sent by means of the centrifugal pump G x06, in order to recover the fluorine présent inside the cake. The washing water is likewise sent to the tank D x04.

3) The mother liquors, extracted from D x04, are sent by means of the centrifugal pump G x07 to the continuous reactor R x02 (stirred by P x04). Simultaneously, the weighing screw conveyor T x02 conveys the calcium hydrate extracted from the silo D x05 by means of the rotary valve T xO1 into the reactor R x02, where the reaction B takes place:

B) 2NH₄F + Ca(OH)₂ -> CaF₂l + 2H₂O + 2NH₃Î

4) The slurry produced in R x02 is extracted by means of the centrifugal pump G x08 and sent to the intermediate tank D x06 (stirred by means of P x05).

5) From the intermediate tank the slurry is extracted by means of the centrifugal pump G x09 and sent to 20 the filter press F x02. Here the fluorite is retained and the mother liquors are sent to the buffer tank D x11, stirred by means of the stirrer P x07. The fluorite cake is washed with process water, by means of the pump G x14, to remove the ammonia présent in the cake. The washing water is also sent to D x11. The wet fluorite cake is discharged from the filter F x02 and conveyed to the collecting silo D x07.

6) The mother liquors in D x11 are composed of NH₃ in a 10% wt solution. In order to be reused as a 25 reagent in A they must be concentrated to 25% wt. Hence, they are extracted by means of the centrifugal pump

G x11 and sent to the stripper C x01, kept under suction by the pump P x09. This apparatus is a packed column, fed at the head with the mother liquors containing the ammonia to be concentrated and at the bottom with a vapour stream at 2 bar, and sized so that the overhead product will be vapours containing 25% NH₃₁ which will then be condensed in E x03 and accumulated in D x12. The bottom product consists of the excess water, which 30 will be sent to the water treatment plant.

The 25% ammonia, condensed at a température of just above 30°C, is extracted from D x12 by means of the centrifugal pump G x12 and sent to the storage tank D x02. The cooling water for E x03, as well as for E xO1 and E x02, is supplied by the cooling tower Z x01, and delivered into the circuit by means of the pump G x13.

7) The apparatus R xO1, D x02, D x03, F xO1. D x04, R x02, D x06, F x02 and D x11 are kept under 35 suction by the fan P x08, to prevent the dispersion of NH₃ vapours into the atmosphère. The vent streams are

sent to the bottom of C x02, a wet scrubberfed overhead with a solution of H2SO4, stored in the tank D x10, by means of the pump G x10, which has the purpose of entrapping the ammonia présent in the stream. The vent streams free of ammonia will thus be emitted into the atmosphère and a solution containing ammonium sulphate formed inside the scrubber will be discharged from the bottom of the column.

8) The wet fluorite outfed from the collecting silo D x07 is conveyed inside the rotary oven B x01 by means of the feed screw conveyor T xO3A/B. Drying takes place by direct contact of the hot combustion fumes with the wet fluorite. The rotation and high température of the oven promote the formation of dried fluorite granules. The air flow necessary for combustion is supplied by the fan P x06, whilst the fine product entrained by the fumes coming out of the oven B x01 is entrapped by the cyclone D x09 before being sent to the scrubber C x02. The 10 dried fluorite in the form of a fine powder is then discharged from the cyclone D x09 by means of the rotary valve T x06, and can be recovered at the overhead of the drying process. The dried fluorite, in the form of granules, is then unloaded from the oven on the extraction screw conveyor T x04 and conveyed to the bucket elevator T x05 in order to then be stored in the storage silo D x08.

The plant according to the présent invention is capable of producing a high purity synthetic fluorite (CaF2) in 15 accordance with the process described in said third embodiment R3 (Figure 3 and Figure 7). The plant comprises:

Reaoent storage and feed section • D x01 (storage tank for the fluorosilicic acid):

Polypropylène tank for storing the fluorosilicic acid, provided with a vacuum breaker valve. The tank is installed 20 inside a suitably sized, insulated polyethylene rétention basin equipped with feed and extraction nozzles and a manhole.

• G x01 (pump for extracting FSA from D x01 ):

Horizontally installed centrifugal pump, with a body and impeller made of polypropylène or coated steel. Equipped with a flushed double mechanical seal.

· D x02 (ammonia storage tank):

Tank made of carbon steel, suitable for storing ammonia in aqueous solution. Kept under suction by P x08 and provided with a vacuum breaker valve, a suitably sized, insulated polyethylene rétention basin equipped with feed and extraction nozzles and a manhole.

• G x02 (pump for extracting NH3 from D x02):

First Reaction Section • R x01 (FSA hydrolysis reactor):

BATCH-type reactor, made of coated steel (ebonite) or PP, thermoregulated by E X01 and stirred by P x01.

Kept under suction by P x08 and provided with reagent feed and product extraction nozzles.

• E xO1 (cooling coil R x01):

Made of SANICRO 28 (or the like) and supplied with cooling water coming from the tower Z xO1. Sized so as to maintain the température within 50-60°C.

• Px01 (stirrer R x01):

Made of Sanicro 28 (or the like), powered by an electric motor. Designed and sized so as to enable suitable contact between the reagents.

• G x04 (pump for extracting slurry from R x01):

· D x03 (intermediate tank):

Buffer tank, made of coated steel or PP, thermoregulated by E x02 and stirred by P x02. Kept under suction by P x08 and provided with feed and extraction nozzles.

G x03: (pump for supplying ammonia to D x03) • E x02 (cooling coil D x03):

Made of Sanicro 28 (or the like) and supplied with cooling water coming from the tower Z xO1. Sized so as to maintain the température within 30°C.

• P x02 (stirrer D x03):

Made of Sanicro 28 (or the like), powered by an electric motor. Sized so as to enable suitable contact between the reagents (nitrates) and prevent sédimentation of the silica.

First Filtration Section • G x05 (pump supplying F xO1 ):

Pneumatic diaphragm pump, supplied with instrument air.

• F xO1 (first filter press):

Automatic filter press, with polypropylene plates and cloths. Provided with drip tray, membrane wringing and 25 blowing, cloth washing and cake washing system. Further equipped with a suction hood connected to P x08.

• G x06 (cake washing pump):

Horizontally installed centrifugal pump, with a cast-iran body and impeller, equipped with a simple mechanical seal.

• D x04 (evaporator):

Made of coated steel. Kept under suction by P x08 and provided with feed and extraction nozzles. The increase in température (up to 130°C) enables ammonium fluoride to be converted into ammonium bifluoride, which is more reactive toward CaCO3.

P x03 : (stirrer evaporator D x04) • E x04 (heating jacket D x04):

Made of carbon steel and supplied with steam. Sized so as to bring the temperature inside D x04 up to about 130°C.

Second Reaction Section • G x07 (pump supplying R x02):

• D x05 (storage silo for carbonate):

· T xO1 (rotary valve) • T x02 (screw conveyor for extracting carbonate):

Made of steel, equipped with a balance capable of weighing the carbonate in a stoichiometric amount to the reactor R x02.

• R x02 (reactor for the formation of fluorite) :

CSTR-type reactor, made of coated steel (ebonite) or PP and stirred by P x04. Kept under suction by P x08 and provided with reagent feed and product extraction nozzles.

• P x04 (stirrer reactor R x02):

Made of Sanicro 28 (or the like), powered by an electric motor. Sized so as to enable suîtable contact between the reagents and prevent sédimentation of the fluorite.

· G x08 (pump from R x02 a D x06) :

• D x06 (intermediate tank):

Buffer tank, made of coated steel or PP, stirred by Px05. Kept under suction by P x08 and provided with feed 25 and extraction nozzles.

• P x05 (stirrer D x06):

Second Filtration Section · G x09 (pump supplying F x02):

Pneumatic diaphragm pump, supplied with instrument air.

• F x02 (second filter press):

Automatic filter press, with polypropylene plates and cloths. Provided with drip tray, membrane wringing and blowing, cloth washing and cake washing system. A hood kept under suction by P x08 should also be provided. 35 D x07: silo for collecting wet synthetic fluorite

• D x11 (intermediate tank):

Px07: stirrer tank D x11 · G x14 (cake washing pump):

Ammonia Recovery Section • G x11 (pump supplying mother liquors to C x01):

Horizontally installed centrifugal pump, with a cast-iron (steel) body and impeller, equipped with a simple mechanical seal.

• C xO1 (ammonia stripper):

Made of carbon steel, provided with packing (1 in. métal Raschig rings), overhead and bottom feed nozzles and a system for feeding the mother liquors to the head. Kept under suction by P x09.

· E x03 (condenser):

Made of carbon steel, ofthe horizontal type with condensation on the shell side. Supplied on the tube side with cooling water coming from the water tower Z xO1.

• D x14 (accumulation tank):

Made of carbon steel or PP, Kept under suction by P x09.

· G x12 (pump supplying condensed ammonia toDx02):

• P x09 (fan):

Axial fan. Head such as to guarantee suction in C x01.

Vent Cleaninq Section • Px08(fan):

Axial fan.

• D x10 (storage tank for H2SO4):

Small 1 m³ polyethylene tanks.

· G x10 (pump supplying H2SO4 a 0x02):

Metering pump.

• C x02 (wet scrubber):

Made of PP, packing made of plastic material (1 in. Raschig rings), equipped with pH-controlled recirculation tank. System for feeding sour water at the head. Kept under suction by P x08.

Z x01: (cooling tower)

G x13: pump supplying the cooling circuit from Z xO1

Fiuorite dryinci section • B xO1 (dryer):

Rotary oven, provided with a bumer, fume particle scrubbing section (cyclone), loading hopper, screw conveyors for feeding and extracting fiuorite, gear motor for varying the rotation speed and hydraulic pistons for varying the inclination.

• T x03 A/B (screw conveyors for feeding wet fiuorite to the rotary oven):

carbon steel screw conveyors, equipped with an electric motor, suitable for feeding fiuorite to the rotary oven at a constant flow rate.

· T x04 (extracting screw conveyor):

carbon steel screw conveyor, for extracting the granular fiuorite from the rotary oven and feeding the bucket elevator T x05. Equipped with an electric motor that runs at constant speed.

• T x05 (bucket elevator):

steel bucket elevator, provided with an electric motor. It receives the granular fiuorite from the screw conveyor 15 T x04 and feeds the storage silo D x08.

• T x06 (rotary valve):

steel rotary valve. It régulâtesthe discharge ofthe fine particles entrained and entrapped in the cyclone D x09 to a storage réservoir below.

• P x06 (fan):

axial fan, provided with an electric motor. It delivers the air flow necessary for combustion in the bumer of B x01.

• D x08 (storage silo for granular fiuorite):

· D x09 (cyclone): made of steel, it receives the hot fumes coming out of the rotary oven. It has the purpose of scrubbing and entrapping, by centrifugal force, the fine particles entrained by the turbulence present inside the oven.

) The ammonia present in the storage tank D x02 is extracted by means of the centrifugal pump G x02 and sent to the batch reactor R xO1. Subsequently, the fluorosilicic acid is sent from D xO1 to the reactor by 30 means of the centrifugal pump G xO1. The reactor R xO1, provided with the stirrer P xO1 and cooling coil E xO1, enables the hydrolysis ofthe acid to take place completely, leading to the formation of NH4F and the précipitation of S1O2 according to reaction A.

A) H₂SiF₆ + 6 NH3 + 2H₂O-> 6 NH₄F + SiO₂|

1) The slurry produced in R xO1 is extracted by means of the centrifugal pump G x04 and sent to the 35 intermediate tank D x03. The slurry is maintained under stirring with P x02 and is cooled by E x02. The pH of

the solution can be appropriately adjusted through the addition, by means of the pump G x03, of ammonia from Dx02.

2) The slurry is extracted from D x03 by means of the centrifugal pump G x05 and sent to the filter press F xO1. Here the silica is retained and the mother liquors are sent to the exchanger/evaporator D x04, consisting of a stirrer P x03 and a heating jacket E x04. In this step, the ammonium fluoride, brought to about 130°C by a vapour stream, dégradés into ammonium bifluoride (more reactive toward calcium carbonate), releasing a mole of NH3 (reaction B). The S1O2 cake is washed with process water, sent by means of the centrifugal pump G x06, in order to recover the fluorine présent inside the cake. The washing water is likewise sent to the tank D x04.

B) 2NH4FNH4HF2 + NH₃

3) The mother liquors, extracted from D x04, are sent by means of the centrifugal pump G x07 to the continuous reactor R x02 (stirred by P x04). Simultaneously, the weighing screw conveyor T x02 conveys the calcium carbonate extracted from the silo D x05, by means of the rotary valve T x01, into the reactor R x02, where the reaction C takes place:

C) NH4HF₂ + CaC0₃ -» CaF₂j + CO2Î + NH₃|

5) From the intermediate tank the slurry is extracted by means of the centrifugal pump G x09 and sent to the filter press F x02. Here the fluorite is retained and the mother liquors are sent to the buffer tank D x11, stirred by the stirrer P x07. The fluorite cake is washed with process water, by means of the pump G x14, to remove the ammonia présent in the cake. The washing water is also sent to D x11. The wet washed fluorite cake is discharged from the filter and conveyed to the storage silo D x07.

6) The mother liquors in D x11 are composed of NH₃ in a 10% wt solution. In order to be reused as a reagent in A they must be concentrated to 25% wt. Hence, they are extracted by means ofthe centrifugal pump G x11 and sent to the stopper C xO1, kept under suction by the pump P x09. This apparatus is a packed column, fed overhead with the mother liquors containing the ammonia to be concentrated and at the bottom with a vapour stream at 2 bar, and sized so that the overhead product will be vapours containing 25% NH₃₁ which will then be condensed in E x03 and accumulated in D x14. The bottom product consists of the excess water, which will be sent to the water treatment plant.

The 25% ammonia, condensed at a température of just above 30°C, is extracted from D x14 by means of the 30 centrifugal pump G x12 and sent to the storage tank D x02. The cooling water for E x03, as well as for E xO1 and E x02, is supplied by the cooling tower Z xO1 and introduced into the circuit by means of the pump G x13.

7) The apparatus R x01, D x02, D x03, F x01, D x04, R x02, D x06, F x02 and D x11 are kept under suction by the fan P x08, to prevent the dispersion of NH₃ vapours into the atmosphère. The vent streams are sent to the bottom of C x02, a wet scrubber fed overhead by means of the pump G x10 with a solution of H2SO4, stored in the tank D x10 and which has the purpose of entrapping the ammonia présent in the stream. The vent

streams free of ammonia will thus be emitted into the atmosphère and a solution containing ammonium sulphate formed inside the scrubber will be discharged from the bottom of the column.

8) The wet fluorite outfed from the collecting silo D x07 is conveyed inside the rotary oven B x01 by means of the feed screw conveyor T xO3A/B. Drying takes place by direct contact of the hot combustion fumes with the wet fluorite. The rotation and high température of the oven promote the formation of dried fluorite granules. The air flow necessary for combustion is supplied by the fan P x06, whilst the fine product entrained by the fumes coming out of the oven B xO1 is entrapped by the cyclone D x09 before being sent to the scrubber C x02. The dried fluorite in the form of a fine powder is then discharged from the cyclone D x09 by means of the rotary valve T x06, and can be recovered at the overhead of the drying process. The dried fluorite, in the form of granules, is then unloaded from the oven on the extraction screw conveyor T x04 and conveyed to the bucket elevator T x05 in order to then be stored in the storage silo D x08.

The plant according to the présent invention is capable of producing a high purity synthetic fluorite (CaF2) in accordance with the process described in said second embodiment R2 (Figure 2 and Figure 6). The plant comprises:

· G xO1 (pump for extracting FSA from D xO1 ):

• D x02 (ammonia storage tank):

• G x02 (pump for extracting NH3 from D x02):

First Reaction Section • R xO1 (FSA hydrolysis reactor):

BATCH-type reactor, made of coated steel (ebonite) or PP, thermoregulated by E xO1 and stirred by P x01.

• E xO1 (cooling coil R x01):

• Px01 (stirrer R x01):

Made of Sanicro 28 (or the like), powered by an electric motor. Designed and sized so as to enable suitable 5 contact between the reagents.

• G x04 (pump for extracting slurry from R x01 ):

• D x05 (intermediate tank):

• E x02 (cooling coil D x05) :

Made of Sanicro 28 (or the like) and supplied with cooling water coming from the tower Z x01. Sized so as to maintain the température within 30°C.

· P x02 (stirrer P x05):

• Dx03 and Dx04 (storage tank for Mg(NÛ3)₂ and Fe(NO3)3 in solution):

Small 1 m³ polyethylene tanks.

G x03: (pump for supplying ammonia to D x05) • G x05 and G x06 (nitrate extraction pumps):

Small metering pumps.

First Filtration Section • G x07 (pump supplying F xO1 ):

Pneumatic diaphragm pump, supplied with instrument air.

• F xO1 (first filter press):

Automatic filter press, with polypropylene plates and cloths. Provided with drip tray, membrane wringing and blowing, cloth washing and cake washing system. Further equipped with a suction hood connected to P x08.

• G x08 (cake washing pump):

• D x06 (intermediate tank):

P x03 (stirrer tank D x06)

Second Reaction Section

• G x09 (pump supplying R x02):

• D x07 (storage silo for calcium hydroxide):

• T xO1 (rotary valve) • T x02 (screw conveyor for extracting calcium hydroxide):

Made of steel, equipped with a balance capable of weighing the calcium hydroxide in a stoichiometric amount 10 to the reactor R x02.

• R x02 (reactor for the formation of fiuorite):

• P x04 (stirrer reactor R x02):

Made of Sanicro 28 (or the like), powered by an electric motor. Sized so as to enable suitable contact between the reagents and prevent sédimentation of the fiuorite.

• Gx10(pumpfromRx02toDx08):

· D x08 (intermediate tank):

• P x05 (stirrer D x08):

Made of Sanicro 28 (or the like), powered by an electric motor. Sized so as to prevent sédimentation of the 25 fiuorite.

Second Filtration Section • G x11 (pump supplying F x02):

Pneumatic diaphragm pump, supplied with instrument air.

• F x02 (second filter press):

Automatic filter press, with polypropylene plates and cloths. Provided with drip tray, membrane wringing and blowing, cloth washing and cake washing system. A hood kept under suction by P x08 should also be provided. D x09: silo for collecting wet synthetic fiuorite • D x13 (intermediate tank):

Made of carbon steel. Kept under suction by P x09 and provided with feed and extraction nozzles. It serves as 35 a buffer for 0x01.

Px07: stirrer tank D x13 • G x16 (cake washing pump):

Ammonia Recovery Section • G x13 (pump supplying mother liquors to C xO1 ):

• C xO1 (ammonia stripper):

• E x03 (condenser):

· D x14 (accumulation tank):

Made of carbon steel or PP, Kept under suction by P x09.

• G x14 (pump supplying condensed ammonia to D x02):

· P x09 (fan):

Axial fan. Head such as to guarantee suction in C x01.

Vent Cleaninq Section • P x08 (fan):

Axial fan.

· D x12 (storage tank for H2SO4):

Small 1 m³ polyethylene tanks.

• G x12 (pump supplying H2SO4 to C x02):

Metering pump.

• C x02 (wet scrubber):

Zx01: (cooling tower)

G x15: pump supplying cooling circuit from Z xO1

Fluorite drvina section · B xO1 (dryer):

Rotary oven, provided with a bumer, fume particle scrubbing section (cyclone), loading hopper, screw conveyors for feeding and extracting fluorite, gear motor for varying the rotation speed and hydraulic pistons for varying the inclination.

• T x03 A/B (screw conveyors for feeding wet fluorite to the rotary oven):

• T x04 (extracting screw conveyor):

· T x05 (bucket elevator):

steel bucket elevator, provided with an electric motor. It receives the granular fluorite from the screw conveyor T x04 and feeds the storage silo D x10.

• T x06 (rotary valve):

steel rotary valve. It régulâtes the discharge of the fine particles entrained and entrapped in the cyclone D x11 15 to a storage réservoir below.

• P x06 (fan):

• D x10 (storage silo for granular fluorite):

steel (or aluminium) silo, suitable for containîng the granular product. Fed from a height by the bucket elevator T x05. Provided with a fan and bag filter for venting into the atmosphère.

• Dx11 (cyclone):

1 ) The ammonia présent in the storage tank D x02 is extracted by means of the centrifugal pump G x02 and sent to the batch reactor R x01. Subsequently, the fluorosilicic acid is sent from D xO1 to the reactor by means of the centrifugal pump G xO1. The reactor R xO1, provided with the stirrer P x01 and cooling coil E xO1, enables the hydrolysis ofthe acid to take place completely, leading to the formation of NH4F and the précipitation of S1O2 according to reaction A.

A) H2SÎF6 + 6 NH3 + 2H₂O-> 6 NH4F + S1O2J,

2) The slurry produced in R xO1 is extracted by means of the centrifugal pump G x04 and sent to the intermediate tank D x05. The slurry is maintained under stirring with P x02 and is cooled by E x02. In this step a solution of Mg(NÛ3)2 and Fe(NO₃)3 is also fed from D x02 and D x04 by the pumps G x05 and G x06, respectively. The pH of the solution can be appropriately adjusted through the addition, by means of the pump 35 G x03, of ammonia from D x02.

3) The slurry is extracted from D x05 by means of the centrifugal pump G x07 and sent to the filter press F x01. Hère the silica is retained and the mother liquors are sent to the intermediate tank D x06, maintained under constant stim'ng by the stirrer P x03. The SiO₂ cake is washed with process water, sent by means of the centrifugal pump G x08, in order to recover the fluorine présent inside the cake. The washing water is likewise sent to the tank D x06.

4) The mother liquors, extracted from D x06, are sent by means of the centrifugal pump G x09 to the continuous reactor R x02 (stirred by P x04). Simultaneously, the weighing screw conveyor T x02 conveys the calcium hydrate extracted from the silo D x07 by means of the rotary valve T xO1 into the reactor R x02, where the reaction B takes place:

B) 2NH₄F + Ca(OH)₂ -» CaFzl + 2H₂O + 2ΝΗ₃|

5) The slurry produced in R x02 is extracted by means of the centrifugal pump G x10 and sent to the intermediate tank D x08 (stirred by means of P x05).

6) From the intermediate tank D x08 the slurry is extracted by means of the centrifugal pump G x11 and sent to the filter press F x02. Hère the fluorite is retained and the mother liquors are sent to the buffer tank D x13, maintained under constant stirring by the stirrer P x07. The fluorite cake is washed with process water to remove the ammonia présent in the cake, by means of the pump G x16. The washing water is also sent to D x13. The wet fluorite cake is discharged from the filter F x02 and conveyed to the collecting silo D x09.

7) The mother liquors in D x13 are composed of NH₃ in a 10% wt solution. In order to be reused as a reagent in A they must be concentrated to 25% wt. Hence, they are extracted by means of the centrifugal pump

G x13 and sent to the stripper 0x01, kept under suction by the pump P x09. This apparatus is a packed column, fed overhead with the mother liquors containing the ammonia to be concentrated and at the bottom with a vapour stream at 2 bar, and sized so that the overhead product will be vapours containing 25% NH₃₁ which will then be condensed in E x03 and accumulated in D x14. The bottom product consists of the excess water, which will be sent to the water treatment plant.

The 25% ammonia, condensed at a température just above 30°C, is extracted from D x14 by means of the centrifugal pump G x14 and sent to the storage tank D x02. The cooling water for E x03, as well as for E xO1 and E x02, is supplied by the cooling tower Z xO1, which supplies the circuit by means of the pump G x15.

8) The apparatus R xO1, D x02, D x05, F x01, D x06, R x02, D x08, F x02 and D x13 are kept under suction by the fan P x08, to prevent the dispersion of NH₃ vapours into the atmosphère. The vent streams are sent to the bottom of C x02, a wet scrubber fed overhead with a solution of H₂SO4, stored in the tank D x12 and fed by means of the pump G x12, which has the purpose of entrapping the ammonia présent in the stream. The vent streams free of ammonia will thus be emitted into the atmosphère and a solution containing ammonium sulphate formed inside the scrubber will be discharged from the bottom of the column.

9) The wet fluorite outfed from the collecting silo D x09 is conveyed inside the rotary oven B x01 by means of the feed screw conveyor T xO3A/B. Drying takes place by direct contact of the hot combustion fumes with the

wet fluorite. The rotation and high température of the oven promote the formation of dried fluorite granules. The air flow necessary for combustion is supplied by the fan P x06, whilst the fine product entrained by the fumes coming out of the oven B xO1 is entrapped by the cyclone D x11 before being sent to the scrubber C x02. The dried fluorite in the form of a fine powder Is then discharged from the cyclone D x11 by means of the rotary valve T x06, and can be recovered at the overhead of the drying process. The dried fluorite, in the form of granules, is then unloaded from the oven on the extraction screw conveyor T x04 and conveyed to the bucket elevator T x05 in order to then be stored in the storage silo D x10.

The plant according to the présent invention is capable of producing a high purity synthetic fluorite (CaF2) in accordance with the process described in said first embodiment R1 (Figure 1 and Figure 5). The plant comprises:

Polypropylene tank for storing the fluorosilicic acid, provided with a vacuum breaker valve. The tank is installed inside a suitably sized, insulated polyethylene rétention basin equipped with feed and extraction nozzles and a 15 manhole.

• G xO1 (pump for extracting FSA from D xO1):

• D x02 (ammonia storage tank):

• G x02 (pump for extracting NH₃ from D x02):

Horizontally installed centrifugal pump, with a body and impeller made of polypropylene or coated steel. 25 Equipped with a flushed double mechanical seal.

First Réaction Section • R xO1 (FSA hydrolysis reactor):

BATCH-type reactor, made of coated steel (ebonite) or PP, thermoregulated by E xO1 and stirred by P x01. Kept under suction by P x08 and provided with reagent feed and product extraction nozzles.

· E xO1 (cooling coil R x01 ):

• Px01 (stirrer R x01):

Made of Sanicro 28 (or the like), powered by an electric motor. Designed and sized so as to enable suitable 35 contact between the reagents.

• G xO4 (pump for extracting slurry from R xO1 ):

• D x05 (intermediate tank):

Buffer tank, made of coated steel or PP, thermoregulated by E x02 and stirred by Px02. Kept under suction by

P x08 and provided with feed and extraction nozzles.

• E x02 (cooling coil D x05):

· P x02 (stirrer P x05):

• Dx03 and Dx04 (storage tank for Mg(NÛ3)2 and Fe(NÛ3)3 in solution):

Small 1 m³ polyethylene tanks.

Small metering pumps.

First Filtration Section • G x07 (pump supplying F x01):

Pneumatic diaphragm pump, supplied with instrument air.

• F xO1 (first filter press):

• G x08 (cake washing pump):

• D x06 (evaporator):

Made of coated steel. Kept under suction by P x08 and provided with feed and extraction nozzles. The increase in temperature (up to 130°C) enables ammonium fluoride to be converted into ammonium bifluoride, which is 30 more reactive toward CaCO3.

• E x04 (heating jacket D x06):

Made of carbon steel and supplied with steam. Sized so as to bring the temperature inside D x06 up to about 130°C.

P x03 : (stirrer evaporator D x06)

Second Reaction Section

• G x09 (pump supplying R x02):

• D x07 (storage silo for carbonate):

• T xO1 (rotary valve) • T x02 (screw conveyor for extracting carbonate):

Made of steel, equipped with a balance capable of weighing the carbonate in a stoichiometric amount to the 10 reactor R x02.

• R x02 (reactor for the formation of fluorite):

• P x04 (stirrer reactor R x02):

• Gx10(pumpfromRx02toDx08):

· D x08 (intermediate tank):

• P x05 (stirrer D x08):

Made of Sanicro 28 (or the like), powered by an electric motor. Sized so as to prevent sédimentation of the 25 fluorite.

Second Filtration Section • G x11 (pump supplying F x02) :

Pneumatic diaphragm pump, supplied with instrument air.

• F x02 (second filter press):

Automatic filter press, with polypropylene plates and cloths. Provided with drip tray, membrane wringing and blowing, cloth washing and cake washing system. A hood kept under suction by P x08 should also be provided. D x09: silo for collecting wet synthetic fluorite • Dx13 (intermediate tank):

Made of carbon steel. Kept under suction by P x09 and provided with feed and extraction nozzles. It serves as 35 a buffer for C xO1.

P x07: stirrer tank D x13 • G x16 (cake washing pump):

Ammonia Recovery Section • Gx13 (pump supplying motherliquorstoCxOl):

• C xO1 (ammonia stripper):

• E x03 (condenser):

· D x14 (accumulation tank):

Made of carbon steel or PP, Kept under suction by P x09.

• G x14 (pump supplying condensed ammonia to D x02) :

· P x09 (fan):

Axial fan. Head such as to guarantee suction in C xO1.

Vent Cleaninq Section • P x08 (fan):

Axial fan.

· D x12 (storage tank for H2SO4):

Small 1 m³ polyethylene tanks.

• G x12 (pump supplying H2SO4toCx02):

Metering pump.

• C x02 (wet scrubber):

Zx01: (cooling tower)

G x15: pump supplying cooling circuit from Z xO1

Fluorite drying section · B xO1 (dryer):

• T x03 A/B (screw conveyors for feeding wet fluorite to the rotary oven):

• T x04 (extracting screw conveyor):

· T x05 (bucket elevator):

• T x06 (rotary valve):

• P x06 (fan):

• D x10 (storage silo for granular fluorite):

• Dx11 (cyclone):

1 ) The ammonia présent in the storage tank D x02 is extracted by means of the centrifugal pump G x02 and sent to the batch reactor R xO1. Subsequently, the fluorosilicic acid is sent to the reactor by means of the centrifugal pump G x01. The reactor R xO1, provided with the stirrer P xO1 and cooling coil E xO1, enables the hydrolysis of the acid to take place completely, leading to the formation of NH4F and the précipitation of S1O2 according to reaction A.

A) H₂SiF₆ + 6 NH₃ + 2H₂O 6 NH₄F + SiO₂J,

2) The slurry produced in R xO1 is extracted by means of the centrifugal pump G x04 and sent to the intermediate tank D x05. The slurry is maintained under stirring with P x02 and is cooled by E x02. In this step, a solution of Mg(NO₃)2 and Fe(NO₃)₃ is fed from D x02 and D x04 by the pumps G x05 and G x06, respectively.

The pH of the solution can be appropriately adjusted through the addition, by means of the pump G x03, of ammonia from D x02.

3) The slurry is extracted from D x05 by means of the centrifugal pump G x07 and sent to the filter press

F xO1. Here the silica is retained and the mother liquors are sent to the exchanger/evaporator D x06, consisting of a stirrer P x03 and a heating jacket E x04. In this step, the ammonium fluoride, brought to about 130°C by a vapour stream, dégradés into ammonium bifluoride (more réactivé toward calcium carbonate), releasing a mole of NH₃ (reaction B). The S1O2 cake is washed with process water, sent by means of the centrifugal pump G x08, in order to recover the fluorine présent inside the cake. The washing water is likewise sent to the tank D x06.

B) 2NH4F-> NH4HF2 + NH₃

4) The mother liquors, extracted from D x06, are sent by means of the centrifugal pump G x09 to the continuous reactor R x02 (stirred by P x04). Simultaneously, the weighing screw conveyor T x02 conveys the calcium carbonate extracted from the silo D x07 by means of the rotary valve T xO1, in the reactor R x02, where the reaction C takes place:

C) NH4HF2 + CaCO₃ -> CaF₂l + CO₂î + NHtf

6) From the intermediate tank the slurry is extracted by means of the centrifugal pump G x11 and sent to the filter press F x02. Here the fiuorite is retained and the mother liquors are sent to the buffer tank D x13 stirred by the stirrer P x07. The fiuorite cake is washed with process water, by means of the pump G x16, to remove the ammonia présent in the cake. The washing water is also sent to D x13. The wet fiuorite cake discharged from filter F x02 is conveyed to the storage silo D x09.

7) The mother liquors in D x13 are composed of NH₃ in a 10% wt solution. In order to be reused as a reagent in 4 they must be concentrated to 25% wt. Hence, they are extracted by means of the centrifugal pump G x13 and sent to the stripper C x01, kept under suction by the pump P x09. This apparatus is a packed column, fed overhead with the mother liquors containing the ammonia to be concentrated and at the bottom with a vapour stream at 2 bar, and sized so that the overhead product will be vapours containing 25% NH₃, which will then be condensed in E x03 and accumulated in D x14. The bottom product consists of the excess water, which will be sent to the water treatment plant.

The 25% ammonia, condensed at a température of just above 30°C, is extracted from D x14 by means of the centrifugal pump G x14 and sent to the storage tank D x02. The cooling water for E x03, as well as for E x01 and E x02, is supplied by the cooling tower Z x01 and delivered into the circuit by means of the pump G x15

8) The apparatus R x01, D x02, D x05, F x01, D x06, R x02, D x08, F x02 and D x13 are kept under suction by the fan P x08, to prevent the dispersion of NH₃ vapours into the atmosphère. The vent streams are sent to the bottom of C x02, a wet scrubber fed overhead, by means of the pump G x12, with a solution of

H2SO4, stored in the tank D x12, which has the purpose of entrapping the ammonia présent in the stream. The

vent streams free of ammonia will thus be emitted into the atmosphère and a solution contaîning ammonium sulphate formed inside the scrubber will be discharged from the bottom of the column.

9) The wet fluorite outfed from the collecting silo D x09 is conveyed inside the rotary oven B xO1 by means of the feed screw conveyor T xO3A/B. Drying takes place by direct contact of the hot combustion fumes with the 5 wet fluorite. The rotation and high température of the oven promote the formation of dried fluorite granules. The air flow necessary for combustion is supplied by the fan P x06, whilst the fine product entrained by the fumes coming out of the oven B xO1 is entrapped by the cyclone D x11 before being sent to the scrubber C x02. The dried fluorite in the form of a fine powder is then discharged from the cyclone D x11 by means of the rotary valve T x06, and can be recovered at the overhead of the drying process. The dried fluorite, in the form of 10 granules, is then unloaded from the oven on the extraction screw conveyor T x04 and conveyed to the bucket elevatorT x05 in order to then be stored in the storage silo D x10.

Said embodiments R1 (Figure 1 and 5), R2 (Figure 2 and 6), R3 (Figure 3 and 7) and R4 (Figure 4 and 8) comprise an essential drying and granulation step serving to obtain a synthetic fluorite that can be used by existing industrial technology. In fact, as is well known, the fluorite used as a raw material must be fed to HF 15 production lines in the form of a dry powder with an appropriate particle size. In the process of the présent invention, the drying step not only enables the water contained in the product to be eliminated, but also produces grains of aggregate material that can be easily managed (transportée!, stored in silos, dosed and ground) with common, widely disseminated industrial technologies. The drying-granulation step is carried out on a synthetic fluorite slurry, contaîning 30-50% moisture by weight, output from D x07 or D x09 and directed toward T xO3A/B 20 so as to enter into a loading hopper. Using an extraction belt equipped with an inverter it is possible to modulate the load of wet fluorite entering the oven B x01. This load is made to vary so as to hâve a flow rate of dried slurry and granulate comprised from 180 to 300 kg/h (the motor frequency ranges approximately from 9.34 Hz to 15 Hz). The slurry extracted from the belt goes on to feed a hopper, which in turn feeds the underlying screw conveyor that takes the fluorite into the oven. The screw conveyor has a constant rotation speed.

The bumer, equipped with an automatic modulator of the air-fuel ratio, works with a flow rate of LPG such as to hâve, in the combustion chamber, a température comprised from 700^eC to 800^eC, for example 784°C. This température is sensed by means of a thermocouple connected to a DCS system, as is the adjustment of the fuel flow rate. The adjustment of the fuel flow rate was varied, for example, between 4 and 8 m³/h (flow meter at the bumer inlet). The température of the fumes coming out of the oven, likewise measured by a thermocouple 30 connected to DCS, shows a value comprised, for example, from 120°C to 220^eC. For example, the inclination of the rotary oven was maintained constant, with the piston at a height of 91 cm above floor level, such as to give the oven an inclination of 1,58^e.

The rotation speed of the oven is regulated by means of an electric gear motor equipped with an inverter. In the tests carried out, the inverter varies its frequency, for example, between 9.5 and 30 Hz, such as to enable the 35 oven to rotate with a speed comprised, for example, between 8 and 23 RPM. The dried granulated slurry is

extracted from the oven by means of a screw conveyor positioned on the head of the oven. The finished product, synthetic fluorite, is stored and weighed, and the fine particles entrained by the turbulence of the fumes and entrapped in the cyclone (with a high flow rate of finished product, over 300 kg/h, a flow rate of 80-100 kg/h of cyclone product was observed) are likewise stored and weighed. The amounts of granulated and cyclone5 treated fluorite, added to the moisture lost during the process, thus make it possible to close the matter-energy balance of the process. The granulated synthetic is then sent to the laboratory for a careful assessment of the characteristics obtained (LOI and particle size distribution).

The table below summarizes the results obtained in several tests:

The process of the présent invention, implemented, for example, by said embodiments R1 (Figure 1 and 5), R2 10 (Figure 2 and 6), R3 (Figure 3 and 7) and R4 (Figure 4 and 8) envisages, in order to reduce the concentration of the oxides présent in the synthetic fluorite, e.g. MgO or AI2O3 or Fe₂O3, and/or excess carbonates, e.g. CaCO3 and/or excess calcium hydroxide, împlementing the washing steps and subséquent filtration with an aqueous solution of diluted acids such as 5% or 10% HCl or H2SO4 on the synthetic fluorite output from F x02, prior to the drying step. Reducing MgO, for example, prevents problems during the step of reaction with sulphuric acid.

This reaction is conducted in a jacketed rotary oven heated with combustion fumes circulating in the oven jacket. In particular, it is noted that the gypsum produced tends to form scale on the walls of the ovens for the production of HF, thus inhibiting the heat exchange between the hot fumes (external part of the oven) and the reaction mass (internai part of the oven). This effect can cause the reaction to corne to a complété hait, producing an undesired plant downtime, or in any case considerably increasing the spécifie consumption of fluorite (the 20 amount of fluorite lost in the gypsum increases).

The synthetic fluorite according to the présent invention obtained, for example, through said embodiments R1 (Figure 1 and 5), R2 (Figure 2 and 6), R3 (Figure 3 and 7) and R4 (Figure 4 and 8) has the following physicochemical characteristics that characterize it as new product:

- LOI value (measured in accordance with the procedures and the techniques known to the person skilled in the 25 art, in samples from the outlet side of D x08 and D x10) comprised from 0.3 to 1.2, preferably comprised from

0.5 to 0.8, even more preferably from 0.6 to 0.7;

- BET value (measured in accordance with the procedures and the techniques known to the person skilled in the art, in samples from the outlet side of D x07 or D x09 after drying at 800°C in the laboratory) comprised from 20 m²/g to 100 m²/g, preferably comprised from 40 m²/g to 80 m²/g, even more preferably from 50 m²/g to

60 m²/g;

- average particle size (measured in accordance with the procedures and the techniques known to the person skilled in the art) as follows in % by weight relative to the weight of the fluorite:

- greater than 10 mm, zéro ;

- greater than 5 mm, comprised from 1 to 10, preferably from 1 to 5, even more preferably from 1 to 3;

- greater than 1 mm, comprised from 40 to 80, preferably from 50 to 70, even more preferably from 55 to 65;

- greater than 0.05 mm, comprised from 10 to 30, preferably from 15 to 25, even more preferably from 20 to 25;

- less than 0.05 mm, comprised from 1 to 20, preferably from 5 to 15, even more preferably from 5 to 10.

Table 1

		Average values
> 10 mm	%	0
> 5 mm	%	2.4
> 1 mm	%	63.9
>0.05 mm	%	19.2
<0.05 mm	%	10.9
LOI	%	0.8

The fiuorite of the present invention also has:

- a concentration of silica SiO₂ of less than 1 %, preferably less than 0.7%, even more preferably less than 0.35% (measured in accordance with the procedures and the techniques known to the person skilled in the art, in samples calcined at 800’C);

- a value of concentration of MgO less than 0.5%, preferably less than 0.3%, even more preferably less than

0.2% (measured in accordance with the procedures and the techniques known to the person skilled in the art, in samples calcined at 800°C).

Table 2 shows the composition of a synthetic fiuorite obtained in the form of slurry coming out from F x02. Table 3 shows the main compounds.

Table 4 shows the composition of a synthetic fiuorite obtained dried in the form of granules coming out from B x01.

Table 5 shows the main compounds.

Wet fiuorite In the form of slurry

	U.M.
CaF₂	%	96.06
F	%	46.80
CaO	%	47.80
SiO₂	%	0.22
Na₂O	%	0.11
SO₃	%	0.02

ai₂o₃	%	0.12
P2O5	%	0.06
Fe₂O3	%	0.04
MgO	%	0.35
K₂O	%	0.02
CO₂	%	0.01
Ca(OH)₂	%	0.20
LOI 800°C	%	3.50
Others	%	0.75
Sum	%	100.00
Table 2

	U.M.
CaF₂	%	96.06
CaSOi	%	0.03
SiO₂	%	0.22
P2O5	%	0.06
C8CO3	%	0.02
Others	%	0.10
LOI	%	3.50
Total	%	100.00

able 3

Dried fluorite in the form of granules

	U.M.
CaF₂	%	98.42
F	%	47.95
CaO	%	49.80
SiO₂	%	0.26
Na₂O	%	0.23
SO₃	%	0.05
AI₂O₃	%	0.17
P2O5	%	0.06

Fe₂O₃	%	0.04
MgO	%	0.40
K₂O	%	0.01
CO₂	%	0.00
Ca(OH)₂	%	0.00
LOI 800°C	%	0.54
Others	%	0.49
Sum	%	100.00
Table 4

	U.M.
CaF₂	%	98.42
CaSO4	%	0.09
S1O2	%	0.26
p₂o₅	%	0.06
CaCCh	%	0.00
Others	%	0.63
LOI	%	0.54
Total	%	100.00

able 5

Claims

1. A process for preparing a synthetic fluorite CaF2 comprising the following steps:

- preparing a solution of NH4F, having a concentration comprised from 15 to 30% by weight, by basic hydrolysis, at a pH value comprised from 8.5 to 9.5, of FUSiFe with an aqueous solution of NH3 having a concentration comprised from 10 to 25% by weight;

- filtering said solution of NH4F so as to yield a substantially silica-free aqueous solution of N H4F;

- treating said substantially silica-free aqueous solution of NH4F with calcium hydroxide in an excess amount comprised from 0.01 to 0.5%, relative to the stoichiometric amount, so as to yield a dispersion which is maintained under stirring for a time comprised from 10 to 60 minutes at a température comprised from 40 to 90°C;

- filtering the latter solution so as to obtain the synthetic fluorite in the form of a slurry having a moisture content comprised from about 30% to about 50% by weight;

- subjecting said slurry to a drying-granulation step so as to yield an acid grade synthetic fluorite CaF2 in the form of granules with an average particle size greater than 1 mm for at least the 50% of the same.

2. The process according to claim 1, wherein said solution of NH4F obtained after the first step is treated with an amount of iron nitrate (III) comprised from 0.01 g to 0.10 g per 1 g of S1O2 présent in said solution of NH4F and/or with an amount of magnésium nitrate comprised from 0.01 g to 0.10 g per 1 g of S1O2 présent in said solution of NFUF, so as to yield a solution of NH4F purified with the precipitated silica.

3. The process according to claim 2, wherein said solution of NH4F purified with the precipitated silica is subjected to filtration to separate the precipitated silica from the purified solution of NH4F.

4. The process according to one of daims 1 or 2 or 3, wherein said substantially silica-free aqueous solution of NH4F obtained after filtration is treated with calcium carbonate in an excess amount comprised from 0.01 to 0.5%, relative to the stoichiometric amount, so as to yield a dispersion which is maintained under stirring for a time comprised from 10 to 60 minutes, preferably 30 minutes at a température comprised from 60 to 90 °C, preferably 80°C.

5. The process according to claim 4, wherein the calcium carbonate has an average particle size distribution comprised from 50 to 400 microns, preferably from 100 to 200 microns, and wherein the ammonia is recovered at a température of about 60-70°C and always under slight négative pressure.

6. The process according to claim 4 or 5, wherein said substantially silica-free aqueous solution of NH4F obtained after filtration is distilled under reduced pressure so as to transform the ammonium fluoride into. ammonium bifluoride according to the following réaction:

2NH₄F_(aq)-> NH4HF_2(aq) + NHaoas)

7. The process according to claim 6, wherein the ammonium bifluoride obtained is reacted with calcium carbonate so as to yield the synthetic fluorite CaF2, according to the following reaction:

NH4HF2 (aq) + CaCÛ3 (solid) —♦ CaF2 (solid) + CO2(gas) + NH3 (gas) or else it is reacted with calcium hydroxide so as to yield the synthetic fluorite CaF2, according to the following 10 reaction:

NH4HF2 (aq) + Ca(OH)2 (solid) —♦ CaF2 (solid) + 2H2O (aq) + NH3 (gas).

8. The process according to one of the preceding claims, wherein the calcium carbonate must hâve a moisture content of less than 10% by weight, preferably less than 5% by weight, a CaCCh concentration greater than

15 97%, preferably greater than 99% and a low content of inorganic contaminants selected from among SiCfe,

MgCO₃, métal oxides and metals.

9. An apparatus for preparing an acid grade synthetic fluorite according to any one of claimsl-8, comprising:

- a hydrolysis reactor (R x01), equipped with a heating means (E x01) and a stirring means (P x01), so as to 20 receive ammonia from the storage tank (D x02) by means of the pump (G x02), and to receive fluo rosilicic acid • from the storage tank (D x01 ) by means of the pump (G x01 ), so as to yield a slurry comprising NH4F and silica;

- a filter press (F x01) for receiving said slurry comprising NH4F and silica so as to yield an aqueous solution comprising substantially silica-free NH4F;

- a reactor (R x02) for the formation of the synthetic fluorite, for receiving said aqueous solution comprising 25 substantially silica-free NH4F and receiving the calcium hydroxide Ca(OH)2 from a storage silo by means of a rotary valve (T x01) and a screw conveyor (T x02) so as to yield a slurry comprising the synthetic fluorite, water and ammonia;

- a filter press (F x02) for receiving the slurry comprising the synthetic fluorite, water and ammonia so as to yield the synthetic fluorite in the form of a slurry with a moisture comprised from 30% to 50% by weight, which is

30 collected in a storage tank (D x07);

- a rotary oven (B x01) provided with a bumer, for receiving said synthetic fluorite in the form of slurry from said storage tank (D x07) by means ofthe screwconveyor (Tx03 A/B) so as to yield the acid grade syntheticfluorite in granules.

10. The apparatus according to claim 9, wherein there is an intermediate tank (D x05) for receiving the slurry comprising NH4F and silica from the hydrolysis reactor (R x01 ) by means of the pump (G x04) and for receiving the magnésium nitrate and the iron nitrate from the tanks (D x03 and D x04) by means of the pumps (G x05 and G x06) respectively, so as to yield said solution of NH4F purified with the precipitated silica to feed the filter

5 press(FxOI).

11. The apparatus according to claim 9 or 10, wherein a reactor (not shown) is provided at the outlet of the storage tank (Fig. 8, D x07) or (Fig. 6, D x09) in order to receive said synthetic fluorite in the form of slurry from said storage tank (D x07) and to receive an aqueous solution of acids, preferably 5% or 10% HCl or H2SO4, and

10 a filtering unit to remove the excess of oxides, carbonates or calcium hydroxide.

12. An acid grade synthetic fluorite CaF2 in the form of granules with an average particle size greater than 1 mm for at least the 50% of the same, said fluorite being obtained by the process for the préparation thereof in accordance with any one of claims 1-8.

13. The acid grade synthetic fluorite in accordance with claim 12, wherein said fluorite has:

- a LOI value (measured in accordance with the procedures and the techniques known to the person skilled in the art, in samples from the outlet side of D x08 and D x10) comprised from 0.3 to 1.2, preferably comprised from 0.5 to 0.8, even more preferably from 0.6 to 0.7; and/or

20 - a BET value (measured in accordance with the procedures and the techniques known to the person skilled in the art, in samples from the outlet side of D x07 or D x09 after drying at 800°C in a laboratory) comprised from 20 m²/g to 100 m²/g, preferably comprised from 40 m²/g to 80 m²/g, even more preferably from 50 m²/g to 60 m²/g; and/or

- average particle size (measured in accordance with the procedures and the techniques known to the person 25 skilled in the art) as follows in % by weight relative to the synthetic fluorite:

- greater than 10 mm, zéro ;

30 - less than 0.05 mm, comprised from 1 to 20, preferably from 5 to 15, even more preferably from 5 to 10.

14. The acid grade synthetic fluorite in accordance with claim 12 or 13, wherein said fluorite has:

- a concentration of silica S1O2 of less than 1 %, preferably less than 0.7%, even more preferably less than 0.35% (measured in accordance with the procedures and the techniques known to the person skilled in the art, in

35 samples calcined at 800°C); and/or

- a concentration of MgO of less than 0.5%, preferably less than 0.3%, even more preferably less than 0.2% (measured in accordance with the procedures and the techniques known to the person skilled in the art, in samples calcined at 800°C).

5 15. A use ofthe acid grade syntheticfluorite in granules according to any one of claims 12-14 for the production of hydrofluoric acid.