RU2819540C1 - Synthetic fluorite of high purity, method of its production and installation for implementation of method - Google Patents

Abstract

FIELD: chemical industry.

SUBSTANCE: invention can be used in producing hydrofluoric acid. Acid-grade synthetic fluorite CaF₂ has the shape of granules, is characterized by loss on ignition (LOI) 0.3–1.2 and/or BET value measured in samples after drying at 800 °C, which is 50–100 m²/g. Average particle size with respect to the weight of synthetic fluorite, wt.%: more than 10 mm—zero, more than 5 mm—1–10, more than 1 mm—40–80, more than 0.05 mm—10–30, less than 0.05 mm—1–20. Concentration of the following impurities in synthetic fluorite, measured on samples fired at 800 °C, is, wt.%: SiO₂—less than 1; MgO—less than 0.5.

EFFECT: content of CaF₂ in the end product is not less than 95 wt.%, which is an indicator of its high purity.

5 cl, 8 dwg, 5 tbl

Description

The present invention relates to synthetic fluorite (CaF ₂ ) of high purity. Also, the present invention relates to a method for producing the specified synthetic fluorite (CaF ₂ ) of high purity, related to the acid grade, from fluorosilicic acid H ₂ SiF ₆ (FSC). Furthermore, the present invention relates to the use of said high purity synthetic fluorite (CaF ₂ ) in the industrial production of hydrofluoric acid. Finally, the present invention relates to a device for producing said synthetic fluorite.

Fluorosilicic acid H ₂ SiF ₆ (FSA) is a by-product of industrial phosphoric acid production, obtained by absorbing silicon tetrafluoride (SiF ₄ ) with water, formed by the reaction between silicon dioxide and fluorine, which are present in the phosphate mineral used as raw material, and sulfuric acid, used to produce phosphoric acid.

During the phosphoric acid concentration step, SiF ₄ can be absorbed into the aqueous solution to produce FCC having a concentration of 23% to 35%.

Known methods for producing synthetic fluorite are often limited to the synthesis of fluorite in wet form with a particle size of 5 microns, and this is not always of optimal purity. These types of synthetic fluorite cannot be used to produce hydrofluoric acid HF.

Unfortunately, the available known methods for producing synthetic fluorite cannot eliminate the presence of contaminants or compounds or impurities such as silicon dioxide SiO ₂ and/or aluminum oxide (Al ₂ O ₃ ), oxides of magnesium (MgO), iron (Fe ₂ O ₃ ) and sodium (Na ₂ O), which are also present in natural fluorite CaF ₂ acid grade in varying quantities. In the case of, for example, silicon dioxide SiO ₂ , natural fluorite CaF ₂ may contain it in a variable amount from 0.5 to 1.5 wt. %. It is well known that silica present as an impurity in fluorite used as raw material has a negative effect on the HF formation process according to the following reaction:

SiO ₂ +3H ₂ SO ₄ +3CaF ₂ →H ₂ SiF ₆ +2H ₂ O+3CaSO ₄

Fluorite loss can be calculated from stoichiometry and is approximately 3.9% per 1% SiO₂, and the loss of sulfuric acid is 4.9% per 1% SiO₂. In the absence of silica, the reaction to form HF from synthetic fluorite is:

H ₂ SO ₄ +CaF ₂ →CaSO ₄ +2HF.

The presence of magnesium, such as magnesium oxide, in fluorite causes some problems during reaction with sulfuric acid. Gypsum CaSO ₄ produced in the presence of magnesium usually forms scale on the walls of HF furnaces. This effect can lead to a complete cessation of the reaction, thus resulting in undesirable downtime or, in any case, a large increase in the specific fluorite consumption (the loss of fluorite in the gypsum increases).

Therefore, from an economic and technological point of view, it is necessary to be able to reduce the amount of magnesium (in the form of magnesium oxide) present in the fluorite to an amount of at least below 0.5%.

Therefore, there remains a need for high purity synthetic fluorite that is substantially free of contaminants or compounds or impurities such as, for example, silica SiO ₂ (in an amount of less than 1 wt.%) and/or aluminum oxide (Al ₂ O ₃ ) and/or magnesium oxide (MgO) (in an amount less than 0.5 wt.%) and/or oxides of iron (Fe ₂ O ₃ ) and/or sodium (Na ₂ O) to reasonably ensure the use of the specified synthetic fluorite in method for producing hydrofluoric acid.

Moreover, there remains a need for a method and apparatus for producing synthetic fluorite, and the method should be simple, efficient and have a high yield, so that it can be used to obtain synthetic fluorite of high purity from fluorosilicic acid, which does not contain contaminants or compounds or impurities, such as, for example, silicon dioxide SiO ₂ and/or aluminum oxide (Al ₂ O ₃ ) and/or oxides of magnesium (MgO) and/or iron (Fe ₂ O ₃ ) and/or sodium (Na ₂ O). The specified synthetic fluorite can be reasonably used in the method for producing hydrofluoric acid.

The present invention relates to high purity synthetic fluorite (CaF ₂ ) belonging to the "acid grade" and having the characteristics specified in the attached claims. Said CaF ₂ is obtained in granules having an average size of more than 1 mm for at least 50 wt. % of these granules, which have good mechanical strength and a large specific surface area (according to BET). This particle size imparts significant reactivity to synthetic CaF ₂ , making it competitive as a replacement for acid-grade natural fluorite in the production of HF.

The present invention relates to a method and apparatus for producing the specified synthetic fluorite (CaF ₂ ) of high purity, belonging to the “acid grade”, characterized by the features specified in the attached claims.

The present invention relates to the use of said synthetic fluorite (CaF ₂ ) of high purity, referred to as "acid grade", in the industrial production of hydrofluoric acid (HF), characterized by the features defined in the attached claims.

Preferred embodiments of the present invention are disclosed in the following detailed description.

In FIG. 1 is a flow diagram of a process for producing high purity synthetic fluorite according to one embodiment of the present invention, including purifying an NH ₄ F solution, converting the NH ₄ F to ammonium bifluoride (NH ₄ HF ₂ ), and using CaCO ₃ .

In FIG. 2 is a flow diagram of a process for producing high purity synthetic fluorite according to one embodiment of the present invention, including purifying an NH ₄ F solution and using Ca(OH) ₂ .

In the context of the present invention, synthetic fluorite (CaF ₂ ) of “high purity” means synthetic fluorite having a concentration equal to or greater than 95 wt. % relative to dry weight, preferably equal to or greater than 97 wt. % relative to dry weight, preferably equal to or greater than 99 wt. % relative to dry weight.

In the context of the present invention, high purity synthetic fluorite (CaF ₂ ) referred to as “acid grade” means fluorite having a CaF ₂ content of more than 95 wt. %, for example, more than 97 wt. %, relative to the dry weight, measured according to modern technologies and based on the knowledge that: 1) fluorite at 100 ° C, a pressure of 100 kPa (1 atm.) after 60 minutes has a water content of approximately 4 wt. % and 2) fluorite at 800°C, a pressure of 100 kPa (1 atm) after 80 minutes has a water content of approximately 0 wt. %.

In the first embodiment R1, the method of the present invention is shown by way of example, without limiting the protection scope of the present invention, in a flowchart of FIG. 1 (a simplified flow diagram of the main steps of the process of the present invention, which comprises purifying the NH ₄ F solution, converting the NH ₄ F to ammonium bifluoride (NH ₄ HF ₂ ), and using CaCO ₃ ).

Briefly stated, the first embodiment of R1 (FIG. 1 and FIG. 5) includes the following steps.

1) Decomposition of fluorosilicic acid H ₂ SiF ₆ (FCC) with ammonia and separation of precipitated silicon dioxide from a solution of ammonium fluoride NH ₄ F (R1F1).

2) Purification of the NH ₄ F solution by dosing suitable reagents that ensure the elimination, by precipitation and subsequent separation, of the silica still present in the NH ₄ F solution (R1F2).

3) Conversion of NH ₄ F into ammonium bifluoride NH ₄ HF ₂ by distillation under reduced pressure (according to reaction B, see below) and subsequent recovery of the NH ₃ fraction by absorption with an aqueous solution or condensation (R1F3).

4) Synthesis and precipitation of the thus obtained synthetic fluorite CaF ₂ by reaction of calcium carbonate or calcium hydroxide Ca(OH) ₂ with NH ₄ HF ₂ and simultaneous distillation of free ammonia to recover the remaining NH ₃ fraction by absorption with an aqueous solution or condensation (R1F4) .

In the second embodiment R2, the method of the present invention is shown by way of example, without limiting the scope of protection of the present invention, in a flowchart of FIG. 2 (a simplified flow diagram of the main steps of the method of the present invention, which includes purification of the NH ₄ F solution and use of Ca(OH) ₂ ).

Briefly, the second embodiment of R2 (FIG. 2 and FIG. 6) contains the following steps.

1) Decomposition of fluorosilicic acid H ₂ SiF ₆ (FCC) with ammonia and separation of precipitated silicon dioxide from a solution of ammonium fluoride NH ₄ F (R2F1).

2) Purification of the NH ₄ F solution by dosing suitable reagents which ensure the elimination, by precipitation and subsequent separation, of the silica still present in the NH ₄ F solution (R2F2).

3) Synthesis and precipitation of synthetic fluorite CaF ₂ directly from NH ₄ F in the presence of calcium hydroxide Ca(OH) ₂ or calcium carbonate (R2F3).

The method of the present invention (in each of its embodiments) enables the production of synthetic fluorite having the qualitative/quantitative composition described below.

In one embodiment, the composition of the synthetic fluorite after drying at 110°C to constant weight is as follows:

CaF ₂ = 95-96%

CaCO ₃ (or Ca(OH) ₂ ) = 0.7-1.2%

SiO ₂ = 0.01-0.2%

MgO = 0.05-0.2%

Al ₂ O ₃ = 0.05-0.2%

LPP (H ₂ O - loss on ignition) = 4-5%

In another embodiment, the composition of the above fluorite, already dried, after firing at 800°C for 30 minutes is as follows:

CaF ₂ = 98-99%

CaCO ₃ (or Ca(OH) ₂ ) = 0.7-1.3%

SiO ₂ = 0.01-0.2%

MgO = 0.05-0.2%

Al ₂ O ₃ = 0.05-0.2%

LLP (H ₂ O loss on ignition) = 0.5%

This first embodiment of R1 (FIG. 1) is described in detail below and includes the following steps.

The first stage of R1F1 includes the decomposition of fluorosilicic acid H ₂ SiF ₆ (FSC) with ammonia and the separation of precipitated silicon dioxide from a solution of ammonium fluoride NH ₄ F according to reaction (A):

A) H ₂ SiF _6(aq) +6NH _3(aq) +2H ₂ O→6NH ₄ F _(aq) +SiO _2(solid)

Then, the second stage R1F2 involves purifying the NH ₄ F solution by dosing suitable reagents selected from nitrate salts such as iron nitrate and/or magnesium nitrate, which ensure the elimination, by precipitation and subsequent separation, of silica still present in the NH solution ₄ F. The use of chloride salts such as ferric chloride is not recommended.

Then, the third stage R1F3 involves the conversion of NH ₄ F to ammonium bifluoride NH ₄ HF ₂ by distillation under reduced pressure (according to reaction (B)) and subsequent recovery of the NH ₃ fraction by absorption into an aqueous solution or condensation.

Reaction (B) is schematically as follows:

B) 2NH ₄ F _(aq.) →NH ₄ HF _{2 (aq.)} + NH _{3 (gas)}

Then, the fourth stage R1F4 involves the synthesis and precipitation of fluorite CaF₂ via NH reaction₄HF_2(aq) with calcium carbonate and simultaneous distillation of free ammonia to extract the remaining NH fraction₃ by absorption into an aqueous solution or condensation (reaction (C).

Reaction (C) is schematically as follows:

C) NH ₄ HF _{2 (aq.)} + CaCO _{3 (tv.)} → CaF _{2 (tv.)} + CO _{2 (gas)} + NH _{3 (gas)}

Alternatively, said fourth stage R1F4 involves the synthesis and precipitation of fluorite CaF ₂ by reacting NH ₄ HF _2(aq) with calcium hydroxide. The reaction can be expressed schematically as follows:

NH ₄ HF _2(aq) +Ca(OH) _2(solid) →CaF _2(solid) +2H ₂ O _(aq) +NH _3(gas) .

This is followed by a drying step to produce synthetic fluorite suitable for use in the industrial production of hydrofluoric acid.

At this first stage R1F1, fluorosilicic acid FCC is reacted, having a concentration of 15 to 30 wt. %, preferably from 20 to 25 wt. %, with constant mechanical stirring, with an aqueous solution of NH ₃ having a concentration of 10 to 35 wt. %, preferably from 15 to 25 wt. %. The reaction is exothermic and the temperature can reach 90°C, so to avoid excess NH ₃ loss, the reaction temperature is kept constant at 50-70°C.

NH ₃ is dosed in a stoichiometric excess of approximately 20-30 wt. % to FCC relative to the theoretical value (6 moles of NH ₃ per mole of FCC). During the hydrolysis step, reactants are added such that the pH of the solution remains stable at about 9. In a preferred embodiment, FCC is added to the NH ₃ to maintain a stable pH, resulting in easily filterable silica.

The efficiency of the hydrolysis process is highly dependent on the rate of addition of reagents, or FCC, to NH ₃ . The total estimated time to obtain high quality filterable silica and complete the hydrolysis reaction is generally from 2 to 6 hours, preferably from 3 to 5 hours in total, for example 4 hours in total, taking into account the addition rate of about 0.01 l/min per 1 liter 18% NH ₃ or 23% FCC.

The addition sequence produces two different reaction environments, start and end environments, which have different effects on the quality of the resulting silica, particularly with respect to structural and surface properties. In fact, the pH of nucleation, aggregates and agglomerates changes from acidic to basic depending on whether NH ₃ is added to the FCC or vice versa. Accordingly, the environment in which crystallization nuclei, aggregates or agglomerates are formed is different in one case and in another. Different environments affect the formation of crystallization nuclei, aggregates and agglomerates of the amorphous silicon dioxide produced in it.

After a few minutes of reaction, silica is formed and a white-colored suspension is formed. After completion of the reaction, the silica present in the suspension is preferably separated from the solution containing ammonium fluoride NH ₄ F and a slight excess of NH ₃ . The separation of silica can be accomplished by filtration, for example by means of a filter press or strainers, or by centrifugation. The first wash water after washing the silicon dioxide is extracted into the NH ₄ F solution, the water from subsequent washing stages is sent for cleaning. Preferably, the final solution is clear and still contains a small proportion of dissolved silica, ranging from 1 to 5 g/l. In fact, if the solution is left undisturbed for approximately 2-4 hours, additional precipitation of silica can be observed.

In this second R1F2 step, the silica present in the NH ₄ F solution must be removed before producing synthetic fluorite in order to reduce the SiO ₂ content in the final product. The purification process involves adding a small amount of an aqueous solution of ferric nitrate and/or magnesium nitrate. Preferably, the optimal amount to add in grams is from 0.010 (eg 0.015) to 0.10, preferably from 0.020 (eg 0.025 or 0.030) to 0.080 (eg 0.050) Fe(NO ₃ ) ₃ per 1 g of SiO ₂ present in NH ₄ F solution, 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(NO ₃ ) ₂ per 1 g of SiO ₂ present in the solution NH ₄ F. The pH of the solution should be above 8.5, preferably 9 to 11, and the reaction time should be 45-90 minutes, preferably 60 minutes. Under these conditions, the metals precipitate as hydroxides, including silica still present in solution in the flocs.

The yield of the purification process is more than 90% (in an experimental test, which was carried out with a solution containing NH ₄ F and SiO ₂ from the said first stage R1F1 containing 0.66% SiO ₂ , after purification a solution containing 0.04% SiO ₂ was obtained (stage R1F2)). The reaction can be carried out at room temperature or, in any case, at the final temperature of the first stage, advantageously without the need for a cooling step of the NH ₄ F solution. The silica present in the resulting suspension is separated by filtration (eg using a strainer).

This first embodiment of R1 includes the distillation process necessary to convert NH ₄ F to (NH ₄ )HF ₂ which is more reactive. In fact, compared to calcium hydroxide, carbonate does not react spontaneously with NH ₄ F, and this is necessary in order to rely on the distillation method to direct the reaction towards the formation of fluorite.

In this third R1F3 step, the NH ₄ F solution, which has previously been purified from SiO ₂ , is distilled under reduced pressure to promote the decomposition of the NH ₄ F compound, which is not very stable, and its conversion into the more stable form NH ₄ HF ₂ (reaction IN). Decomposition involves the removal of one mole of NH ₃ per mole of NH ₄ F, and the NH ₃ already present in free form in solution is added to this amount. Distillation is performed by raising the system temperature from 30°C to 130°C at slightly negative pressure (approximately 6 kPa (60 mbar) below ambient pressure). In the distillation process, carried out at 130°C and 6 kPa (60 mbar) below ambient pressure, the small loss of fluorite that occurs is returned by recycling the distilled ammonia into the process during said first stage. The distillation residue is also a solution, although ammonium bifluoride can be crystallized and isolated in solid form, even if this is not advantageous for the purposes of the process.

Fluorite synthesis then continues (mentioned fourth stage R1F4) with the addition of calcium carbonate (reaction C) in a stoichiometric amount relative to the fluorite present in the above NH ₄ HF ₂ solution (1:2 molar ratio) to avoid the presence of excess carbonates in the final product. Preferably, the calcium carbonate used should be dry or contain moisture in an amount of less than 10 wt. %, preferably less than 5 wt. % and is in the form of a fine powder.

The chemical quality of the calcium carbonate should be high, with a CaCO ₃ concentration greater than 97%, preferably greater than 99%, and low inorganic contaminants (SiO ₂ , MgCO ₃ and other metals and metal oxides). Preferably, calcium carbonate is used having an average particle size distribution of 50 to 400 microns, preferably 100 to 200 microns, the use of larger particle sizes is not advisable as this increases the reaction time. The reaction can take place at a temperature of 20° C., but preferably, in order to improve the recovery of ammonia, it is advisable to operate at temperatures of about 60-70° C. and always at slightly negative pressure. The stirring speed should be such as to prevent solid material from settling at the bottom of the reactor.

Advantageously, although the reaction is practically instantaneous, the best yields are obtained by continuously stirring the thus obtained fluorite suspension for at least 30-60 minutes. The fluorite thus obtained is separated from the suspension by filtration.

The filtered product is washed and takes on a suspension-like consistency with an average residual moisture of approximately 40%.

This second embodiment of R2 (FIG. 2) is described in detail below and includes the following steps.

This first stage R2F1 involves the production of NH ₄ F by basic hydrolysis of H ₂ SiF ₆ in an aqueous solution with a concentration of 15 to 30 wt. %, preferably from 20 to 25 wt. %, with an aqueous solution of NH ₃ , with constant mechanical stirring, with a concentration of 10 to 35 wt. %, preferably from 15 to 25 wt. %.

The reaction is as follows:

H ₂ SiF _6(aq) +6NH _3(aq) +2H ₂ O→6NH ₄ F _(aq) +SiO _2(solid) ↓

Said first stage R2F1 is performed under the same conditions as stage R1F1.

For example, to a container with a capacity of, for example, 500 ml, containing ammonia in an amount of from 200 to 250 g, for example, 237 g of ammonia (for example, 30% excess relative to the calculated stoichiometric amount for a given reaction), add FCC in an amount of from 150 to 250 g, preferably 200 g.

Preferably, the dispersion resulting from the above reaction is vigorously stirred, for example for about 20-40 minutes with a mechanical stirrer, eg VELP, while controlling the pH and temperature. During this time, the pH remained stable at between 8.5 and 9.5, preferably about 9. The temperature increased from 25°C to about 60-65°C.

After mixing, the precipitated silica is preferably separated by filtration, for example by vacuum filtration, preferably at a relative pressure of about 5-15 kPa (50-150 mbar), even more preferably at a pressure of about 10 kPa (100 mbar). Preferably, the solid thus obtained was redispersed 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 X-ray fluorescence.

According to the above operating conditions, three quantitative analyzes were performed and it was observed that from the theoretical calculated amount of silica (17.62 g), 15.60 g of silica was obtained in the first analysis, 16.40 g in the second analysis, and 16.94 g in third analysis.

Preferably, the first water from the silica wash is added to the initial NH ₄ F solution.

Said second stage of R2F2 involves purification of NH ₄ F from silica.

The NH ₄ F containing solution obtained after filtration is treated/purified (the said second stage R2F2) under the same operating conditions as described for stage R1F2.

For example, a solution containing NH ₄ F obtained after filtration is treated/purified with a solution containing iron (III) nitrate having a concentration of 20 to 60 w/v. %, preferably from 30 to 50 wt./vol. % and/or magnesium (II) nitrate having a concentration of 40 to 80 wt./vol. %, preferably from 50 to 70 wt./vol. %.

For example, the NH ₄ F solution obtained after filtration is treated with Fe(NO ₃ ) ₃ in an amount of 0.02 g to 0.08 g, preferably 0.04 g to 0.06 g (43.3 wt./ vol.% aqueous solution of ferric nitrate nine-water - Fe(NO ₃ ) ₃ ⋅9H ₂ O) and Mg(NO ₃ ) ₂ in an amount of from 0.05 g to 1 g, preferably from 0.07 g to 0.09 g (64.4 wt/vol.% aqueous solution of magnesium nitrate - Mg(NO ₃ ) ₂ ). The solution thus obtained is kept under stirring for a period of 10 to 90 minutes, preferably 60 minutes, at a temperature of 20°C to 25°C.

This solution is then filtered to obtain a substantially silica-free aqueous NH ₄ F solution.

Then proceed to said third step (R2F3), which involves treating said substantially silica-free aqueous solution of NH ₄ F directly with calcium hydroxide in an excess amount of 0.01 to 0.5% relative to the stoichiometric amount to obtain a dispersion, which is maintained with stirring for a period of time from 10 to 60 minutes at a temperature of from 40 to 90°C. Finally, the last solution is filtered to obtain synthetic fluorite.

The solution is preferably filtered under vacuum at a pressure of, for example, 5 kPa to 15 kPa (50 mbar to 150 mbar), preferably 10 kPa (100 mbar), using a 0.45 μm cellulose acetate filter.

Quantitative analysis of silicon dioxide is performed using an inductively coupled plasma atomic emission spectrometer on NH ₄ F solution samples taken before and after treatment. It was found that on average the SiO ₂ concentration decreases by at least 70 wt. % in samples treated with nitrates, for example, the content of 2.5 g/l of silica present in the sample was reduced to 0.3 g/l.

This third stage of R2F3 involves the synthesis of CaF ₂ starting from NH ₄ F in the presence of calcium hydroxide.

The reaction can be schematically represented as follows:

2NH ₄ F _(aq) +Ca(OH) _2(solid) →CaF _2(solid) ↓+2 NH _3(gas) +2H ₂ O _(aq)

For example, the amount of NH ₄ F is from 250 g to 350 g, preferably 300 g, for example, an aqueous solution of 9.5 wt. % ammonium fluoride NH ₄ F was placed in a 500 ml PTFE (polytetrafluoroethylene) flask with three necks and it reacted with calcium hydroxide Ca(OH) ₂ (97.8%).

Quantitative analyzes were performed using an excess amount of approximately 0.3% relative to the stoichiometric amount. In all analyzes performed, the dispersion was left under mechanical stirring for a period of 20 to 60 minutes, preferably 30 minutes, in an oil bath at a temperature of 80-90°C.

The precipitate (CaF ₂ ) is filtered by vacuum filtration at a relative pressure, for example 5 kPa to 15 kPa (50 mbar to 150 mbar), preferably 10 kPa (100 mbar), using a filter, for example Whatman 42 filter paper, washed and oven dried at 110°C and analyzed by X-ray fluorescence.

The reaction yield is over 95% and quantitative analysis of the solid shows a very low percentage of residual silica (less than 0.2% SiO ₂ ). Fluorite wash water contains no residual fluorite and ammonia is recovered 100% in a closed system.

Alternatively, said third step R2F3 involves the synthesis of CaF ₂ starting from NH ₄ F in the presence of calcium carbonate.

The reaction can be expressed schematically as follows:

2NH ₄ F _(aq) +CaCO ₃ ( _sol. →CaF _{2 (sol.)} ↓+2 NH _{3 (gas)} +CO _{2 (gas)} +H ₂ O _(aq.)

Calcium carbonate is used in an excess amount of 0.01 to 0.5% relative to the stoichiometric amount to produce a dispersion, which is maintained under stirring for a period of 10 to 60 minutes, preferably 30 minutes, at a temperature of 60 to 90°C, preferably 80°C.

For example, NH ₄ F in an amount from 250 g to 350 g, preferably 300 g, for example, an aqueous solution of 9.5 wt. % ammonium fluoride NH ₄ F was placed in a 500 ml PTFE flask with three necks, and it reacted with calcium carbonate.

Quantitative analyzes were performed using an excess amount of approximately 0.3% relative to the stoichiometric amount. In all analyzes performed, the dispersion was left under mechanical stirring for a period of 20 to 60 minutes, preferably 30 minutes, in an oil bath at a temperature of 80-90°C.

The precipitate (CaF ₂ ) is filtered by vacuum filtration at a relative pressure, for example 5 kPa to 15 kPa (50 mbar to 150 mbar), preferably 10 kPa (100 mbar), using a filter, for example Whatman 42 filter paper, washed and oven dried at 110°C and analyzed by X-ray fluorescence.

The reaction yield is over 95% and quantitative analysis of the solid shows a very low percentage of residual silica (approximately 0.1% SiO ₂ ). Fluorite wash water contains no residual fluorite and ammonia is recovered 100% in a closed system.

In FIG. 3 is a flow diagram of a method for producing high purity synthetic fluorite, similar to the method shown in FIG. 1, however, without purifying the NH ₄ F solution.

In FIG. 4 is a flow diagram of a process for producing high purity synthetic fluorite, similar to the method shown in FIG. 2, however, without purifying the NH ₄ F solution.

Fig. 5, 6, 7 and 8 are schematic views of an apparatus of the present invention for producing said high purity synthetic fluorite (CaF ₂ ) in accordance with embodiments (R1, R2, R3 and R4) exemplified in FIG. 1, 2, 3 and 4, respectively.

In the third embodiment R3, the method of the present invention is presented as an example, without limiting the scope of protection of the present invention, in the form of a flowchart in FIG. 3 (a simplified flow diagram of the main steps of the process of the present invention, which includes the conversion of NH ₄ F to ammonium bifluoride (NH ₄ HF ₂ ) and the use of CaCO ₃ ).

Briefly, this third embodiment of R3 (FIG. 3 and FIG. 7) includes the following steps.

1) Decomposition of fluorosilicic acid (FSA) H ₂ SiF ₆ with ammonia and separation of precipitated silicon dioxide from a solution of ammonium fluoride NH ₄ F (R3F1).

2) Conversion of NH ₄ F into ammonium bifluoride NH ₄ HF ₂ by distillation under reduced pressure (according to reaction B) and subsequent recovery of the NH ₃ fraction by absorption in an aqueous solution or by condensation (R3F2).

3) Synthesis and precipitation of the synthetic fluorite CaF ₂ thus obtained by reacting calcium carbonate or calcium hydroxide Ca(OH) ₂ with NH ₄ HF ₂ and simultaneous distillation of free ammonia to extract the remaining NH ₃ fraction by absorption in an aqueous solution or by condensation (R3F3 ).

In the fourth embodiment R4, the method of the present invention is presented as an example, without limiting the scope of protection of the present invention, in the form of a flowchart in FIG. 4 (a simplified flow diagram of the main steps of the process of the present invention, which includes purification of the NH ₄ F solution and use of Ca(OH) ₂ ).

Briefly, this fourth embodiment of R4 (FIG. 4 and FIG. 8) includes the following steps:

1) Decomposition of fluorosilicic acid (FSA) H ₂ SiF ₆ with ammonia and separation of precipitated silicon dioxide from a solution of ammonium fluoride NH ₄ F (R2F1).

2) Purification of the NH ₄ F solution by dosing suitable reagents which ensure the elimination, by precipitation and subsequent separation, of the silica still present in the NH ₄ F solution (R2F2).

3) Synthesis and precipitation of synthetic fluorite CaF ₂ , starting directly from NH ₄ F in the presence of calcium hydroxide Ca(OH) ₂ or calcium carbonate (R2F3).

The plant of the present invention can produce, in accordance with the method described and claimed herein, high purity synthetic fluorite (CaF ₂ ) of the "acid grade" which can be used in the industrial production of hydrofluoric acid (HF) starting from fluorosilicic acid ( FKK) and calcium hydroxide (Ca(OH) ₂ ).

The plant of the present invention can produce synthetic fluorite (CaF ₂ ) of high purity in accordance with the method described in the specified fourth embodiment R4 (Fig. 4 and Fig. 8). Installation contains:

Reagent loading and storage area

D x01 (fluorosilicic acid storage tank)

Polypropylene tank for storing fluorosilicic acid, equipped with a valve for devacuuming. The tank is installed inside an insulated polyethylene storage tank of suitable size, equipped with inlet and outlet connections and an inspection window.

G x01 (pump for extracting FCC from D x01)

Horizontally mounted centrifugal pump with casing and impeller made of polypropylene or coated steel. Equipped with a double mechanical seal with barrier fluid supply.

D x02 (ammonia storage tank)

A tank made of carbon steel suitable for storing ammonia in aqueous solution. Maintained under reduced pressure by P x08 and equipped with a devacuum valve, an insulated polyethylene storage tank of suitable size, equipped with inlet and outlet connections and an inspection window.

G x02 (pump for extracting NH ₃ from D x02)

Horizontally mounted centrifugal pump with casing and impeller made of polypropylene or coated steel. Equipped with a double mechanical seal with barrier fluid supply.

First reaction zone

R x01 (FKK hydrolysis reactor)

Batch reactor made of coated steel (ebonite) or polypropylene, temperature controlled by E x01 and stirred by P x01. Maintained under reduced pressure using P x08 and equipped with reagent supply and product extraction connections.

E x01 (cooling coil R x01)

Made from SANICRO 28 (or similar material) and supplied with cooling water from the Z x01 tower. It is sized to maintain the temperature within 50-60°C.

R x01 (mixing means R x01)

Made from SANICRO 28 (or similar material), driven by an electric motor. Designed and sized to provide suitable contact between reagents.

G x04 (pump for extracting suspension from R x01)

Horizontally mounted centrifugal pump with casing 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 polypropylene, temperature controlled by E x02 and stirred by P x02. Maintained under reduced pressure using P x08 and equipped with inlet and outlet connections.

E x02 (cooling coil D x03)

Made from SANICRO 28 (or similar material) and supplied with cooling water from the Z x01 tower. It is sized to maintain the temperature within 30°C.

P x02 (mixer D x03)

Made from SANICRO 28 (or similar material), driven by an electric motor. Sized to provide suitable contact between reactants (nitrates) and to prevent silica precipitation.

First filtration zone

G x05 (pump supplying F x01)

Pneumatic diaphragm pump supplied with air from the pneumatic system.

F x01 (first filter press)

Automatic filter press with polypropylene plates and filter cloth. Equipped with a tray for collecting moisture, a system for squeezing and blowing the membrane, washing the filter cloth and washing the cake. Also equipped with an exhaust hood connected to P x08.

G x06 (cake washing pump)

Horizontally mounted centrifugal pump with casing and impeller made of cast iron, equipped with a simple mechanical seal.

D x04 (intermediate tank)

Made from coated steel or polypropylene. Maintained under reduced pressure using P x08 and equipped with inlet and outlet connections.

Second reaction zone

G x07 (pump supplying R x02)

Horizontally mounted centrifugal pump with casing and impeller made of polypropylene or coated steel, equipped with a simple mechanical seal.

D x05 (bin for storing calcium hydroxide)

Made of carbon steel, equipped with a ventilation duct, equipped with a dust collector with a bag filter for dust removal and a fan. It is fed from a tanker truck through a pneumatic conveyor.

T x01 (rotating valve)

T x02 (screw conveyor for calcium hydroxide extraction) Made of steel, equipped with a scale capable of weighing calcium hydroxide in stoichiometric quantities into the R x02 reactor.

R x02 (reactor for fluorite formation)

The reactor is a continuously stirred reactor type, made of coated steel (ebonite) or polypropylene and stirred with P x04. Maintained under reduced pressure using P x08 and equipped with connections for supplying reagents and extracting the product.

R x04 (reactor stirrer R x02)

Made from SANICRO 28 (or similar material), driven by an electric motor. Sized to provide suitable contact between reagents and prevent fluorite precipitation.

G x08 (pump from R x02 to D x06)

Horizontally mounted centrifugal pump with casing and impeller made of coated steel or polypropylene, equipped with a simple mechanical seal.

D x06 (intermediate tank)

Buffer tank made of coated steel or polypropylene, stirred P x05. Maintained under reduced pressure using P x08 and equipped with inlet and outlet connections.

P x05 (mixer D x06)

Made from SANICRO 28 (or similar material), driven by an electric motor. It is sized to prevent fluorite precipitation.

Second filtration zone

G x09 (pump supplying F x02)

Pneumatic diaphragm pump supplied with air from the pneumatic system.

F x02 (second filter press)

Automatic filter press with polypropylene plates and filter cloth. Equipped with a tray for collecting moisture, a system for squeezing and blowing the membrane, washing the filter cloth and washing the cake. It must also be equipped with an exhaust hood maintained at reduced pressure using P x08.

D x07 (hopper for collecting wet synthetic fluorite)

D x11 (intermediate tank)

Made from carbon steel. Maintained under reduced pressure using P x09 and equipped with inlet and outlet connections. It serves as a buffer for C x01.

P x07 (mixer for tank D x11)

G x14 (pump for cake washing)

Horizontally mounted centrifugal pump with casing and impeller made of cast iron, equipped with a simple mechanical seal.

Ammonia extraction zone

G x11 (pump supplying mother liquors to C x01)

Horizontally mounted centrifugal pump with casing and impeller made of cast iron (steel), equipped with a simple mechanical seal.

S x01 (ammonia stripper)

Made of carbon steel, equipped with inlet nozzles (2.54 mm (1 inch) metal Raschig rings) at the top and bottom and a system for supplying mother liquors to the top. Maintained at reduced pressure using P x09.

E x03 (capacitor)

Made of carbon steel, horizontal type with condensation in the inter-tube area. It is supplied in the pipe zone with cooling water coming from the water tower Z x01.

D x12 (storage tank)

Made from carbon steel or polypropylene. Maintained at reduced pressure using P x09.

G x12 (pump supplying condensed ammonia to D x02)

Horizontally mounted centrifugal pump with casing and impeller made of polypropylene or coated steel. Equipped with a double mechanical seal with barrier fluid supply.

P x09 (fan)

Axial fan. The pressure used is such as to ensure suction at C x01.

Ventilation duct cleaning area

P x08 (fan)

Axial fan.

D x10 (H ₂ SO ₄ storage tank)

Small polyethylene tanks with a volume of 1 m ³ .

G x10 (pump supplying H ₂ SO ₄ to C x02)

Dosing pump.

S x02 (wet scrubber)

Made of polypropylene, plastic nozzle (2.54 mm (1 inch) Raschig rings), equipped with a pH-adjustable recirculation system tank. System for supplying acidic water at the top. Maintained at reduced pressure using P x08.

Z x01 (cooling tower)

G x13 (pump supplying the cooling circuit from Z x01)

Fluorite drying zone

B x01 (dryer)

A rotary kiln equipped with a burner, a wet cleaning zone for removing smoke particles (cyclone), a feeding hopper, screw conveyors for feeding and extracting fluorite, a gear motor for changing the rotation speed and hydraulic pistons for changing the inclination.

T x03 A/B (screw conveyors for feeding wet fluorite into a rotary kiln)

Carbon steel screw conveyors equipped with an electric motor, suitable for feeding fluorite into a rotary kiln at a constant flow rate.

T x04 (removal screw conveyor)

Carbon steel screw conveyor for extracting granular fluorite from a rotary kiln and feeding the T x05 bucket elevator. Equipped with an electric motor that operates at a constant speed.

T x05 (bucket elevator)

Bucket elevator made of steel, equipped with an electric motor. It receives granular fluorite from the T x04 screw conveyor and feeds the D x08 storage bin.

T x06 (rotating valve)

Rotating valve made of steel. It regulates the discharge of particles entrained and captured by cyclone D x09 into a storage tank located below.

P x06 (fan)

Axial fan equipped with an electric motor. It supplies the air flow required for combustion in burner B x01.

D x08 (bunker for storing granular fluorite)

A steel (or aluminum) bin suitable for holding granular product. It is fed from a height using a T x05 bucket elevator. Equipped with a fan and a dust collector with bag filters for release into the atmosphere.

D x09 (cyclone)

Made of steel. It receives hot flue gases coming from the rotary kiln. It aims at wet cleaning and trapping, using centrifugal force, small particles captured by the turbulence present inside the oven.

1. The ammonia present in the storage tank D x02 is recovered by means of a centrifugal pump G x02 and sent to the batch reactor R x01. Fluorosilicic acid is then sent to the reactor from tank D x01 using a centrifugal pump G x01. The R x01 reactor, equipped with a stirring device R x01 and a cooling coil E x01, ensures complete hydrolysis of the acid, which leads to the formation of NH ₄ F and the precipitation of SiO ₂ according to reaction A.

A) H ₂ SiF ₆ +6NH ₃ +2H ₂ O→6NH ₄ F+SiO ₂ ↓

1) The suspension obtained in R x01 is extracted using a centrifugal pump G x04 and sent to the intermediate tank D x03. The suspension is kept stirred with P x02 and cooled with E x02. The pH of the solution can be adjusted to a suitable level by adding ammonia from D x02 using pump G x03.

2) The suspension is removed from D x03 using a centrifugal pump G x05 and sent to the filter press F x01. Silicon dioxide is retained in it, and the mother liquors are sent to the intermediate tank D x04, mixed with a stirrer P x03. The SiO ₂ cake is washed with process water directed through a G x06 centrifugal pump to extract the fluorite present in the cake. Rinse water is also sent to tank D x04.

3) Mother liquors extracted from D x04 are sent using a centrifugal pump G x07 to a continuous reactor R x02 (mixed with P x04). At the same time, the weighing screw conveyor T x02 transports the calcium hydroxide extracted from the hopper D x05 using the rotating valve T x01 into the reactor R x02, in which reaction B takes place.

B) 2NH ₄ F+Ca(OH) ₂ →CaF ₂ ↓+2H ₂ O+2NH ₃ ↑

4) The suspension obtained in R x02 is removed using a centrifugal pump G x08 and sent to the intermediate tank D x06 (mixed using R x05).

5) The suspension is removed from the intermediate tank using a centrifugal pump G x09 and sent to the filter press F x02. Fluorite is retained in it, and the mother liquors are sent to the buffer tank D x11, mixed using a stirrer P x07. The fluorite cake is washed with process water using a G x14 pump to extract the ammonia present in the cake. Wash water is also sent to Dx11. Wet fluorite cake is unloaded from filter F x02 and transported to hopper D x07 for collection.

6) Stock solutions in D x11 consist of NH ₃ in 10 wt. % solution. To reuse them as a reagent in A, their concentration must be increased to 25 wt. %. Consequently, they are removed using a centrifugal pump G x11 and sent to the desorber C x01, maintained at reduced pressure by pump P x09. This apparatus is a packed column into which mother liquors containing the ammonia to be concentrated are fed from above and a steam stream at 200 kPa (2 bar) is supplied from below, and is sized such that the overhead is vapor containing 25% NH ₃ , which then condense into E x03 and accumulate into D x12. The bottom stream consists of excess water, which is sent to a water treatment plant.

25% ammonia, condensed at a temperature slightly above 30°C, is removed from D x12 using a centrifugal pump G x12 and sent to tank D x02 for storage. Cooling water for E x03, as well as for E x01 and E x02, is supplied from the cooling tower Z x01 and supplied to the circuit using pump G x13.

7) Devices R x01, D x02, D x03, F x01, D x04, R x02, D x06, F x02 and Dx11 are maintained at reduced pressure using the fan P x08 to prevent the dispersion of NH ₃ vapors into the atmosphere. The ventilation flows are directed to the lower part of C x02, a wet scrubber, into which a solution of H ₂ SO ₄ stored in tank D x10 is supplied from above using a pump G x10, which serves to capture the ammonia present in the flow. The ammonia-free vent streams are thus released to the atmosphere and the solution containing ammonium sulfate formed within the scrubber is released from the bottom of the column.

8) Wet fluorite, unloaded from hopper D x07 for collection, is transported to the rotary furnace B x01 using a feed screw conveyor T x03 A/B. Drying occurs by direct contact of hot flue gases with wet fluorite. The rotation and high temperature of the oven promote the formation of dried fluorite granules. The air flow required for combustion is supplied by fan P x06, while the fine product entrained in the flue gases leaving furnace B x01 is collected by cyclone D x09 before the gases are sent to scrubber C x02. The dried fluorite in fine powder form is then discharged from the D x09 cyclone using the T x06 rotary valve and can be recovered at the top of the drying process. The dried fluorite in the form of granules is then unloaded from the furnace onto the T x04 extraction screw conveyor and transported to the T x05 bucket elevator to then be stored in the D x08 storage hopper.

The plant of the present invention can produce synthetic fluorite of high purity (CaF ₂ ) in accordance with the method described in the specified third embodiment R3 (Fig. 3 and Fig. 7). Installation contains:

Reagent loading and storage area

D x01 (fluorosilicic acid storage tank)

Polypropylene tank for storing fluorosilicic acid, equipped with a valve for devacuuming. The tank is installed inside an insulated polyethylene reserve yap tank of suitable size, equipped with inlet and outlet pipes and an inspection window.

G x01 (pump for extracting FCC from D x01)

Horizontally mounted centrifugal pump with casing and impeller made of polypropylene or coated steel. Equipped with a double mechanical seal with barrier fluid supply.

D x02 (ammonia storage tank)

A tank made of carbon steel suitable for storing ammonia in aqueous solution. Maintained under reduced pressure by P x08 and equipped with a devacuum valve, an insulated polyethylene storage tank of suitable size, equipped with inlet and outlet connections and an inspection window.

G x02 (pump for extracting NH ₃ from D x02)

Horizontally mounted centrifugal pump with casing and impeller made of polypropylene or coated steel. Equipped with a double mechanical seal with barrier fluid supply.

First reaction zone

R x01 (FCC hydrolysis reactor)

Batch reactor made of coated steel (ebonite) or polypropylene, temperature controlled by E x01 and stirred by P x01. Maintained under reduced pressure using P x08 and equipped with connections for supplying reagents and extracting the product.

E x01 (cooling coil R x01)

Made from SANICRO 28 (or similar material) and supplied with cooling water from the Z x01 tower. It is sized to maintain the temperature within 50-60°C.

R x01 (mixing means R x01)

Made from SANICRO 28 (or similar material), driven by an electric motor. Designed and sized to provide suitable contact between reagents.

G x04 (pump for extracting suspension from R x01)

Horizontally mounted centrifugal pump with casing 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 polypropylene, temperature controlled by E x02 and stirred by P x02. Maintained under reduced pressure using P x08 and equipped with inlet and outlet connections.

G x03 (pump for supplying ammonia to D x03)

E x02 (cooling coil D x03)

Made from SANICRO 28 (or similar material) and supplied with cooling water from the Z x01 tower. It is sized to maintain the temperature within 30°C.

P x02 (mixer D x03)

Made from SANICRO 28 (or similar material), driven by an electric motor. Sized to provide suitable contact between reactants (nitrates) and to prevent silica precipitation.

First filtration zone

G x05 (pump supplying F x01)

Pneumatic diaphragm pump supplied with air from the pneumatic system.

F x01 (first filter press)

Automatic filter press with polypropylene plates and filter cloth. Equipped with a tray for collecting moisture, a system for squeezing and blowing the membrane, washing the filter cloth and washing the cake. Also equipped with an exhaust hood connected to P x08.

G x06 (pump for cake washing)

Horizontally mounted centrifugal pump with casing and impeller made of cast iron, equipped with a simple mechanical seal.

D x104 (evaporator)

Made from coated steel. Maintained under reduced pressure using P x08 and equipped with inlet and outlet connections. An increase in temperature (up to 130°C) ensures the conversion of ammonium fluoride into ammonium bifluoride, which has greater reactivity relative to CaCO ₃ .

P x03 (evaporator stirrer D x104)

E x04 (heating jacket D x104)

Made of carbon steel and supplied with steam. It is sized to bring the internal temperature D x104 up to approximately 130°C.

Second reaction zone

G x07 (pump supplying R x02)

Horizontally mounted centrifugal pump with casing and impeller made of polypropylene or coated steel, equipped with a simple mechanical seal.

D x105 (carbonate storage bunker)

Made of carbon steel, equipped with a ventilation duct, equipped with a dust collector with a bag filter for dust removal and a fan. It is fed from a tanker truck through a pneumatic conveyor.

T x01 (rotary valve)

T x102 (screw conveyor for carbonate extraction)

Made of steel, equipped with scales capable of weighing carbonate in stoichiometric quantities into the R x02 reactor.

R x02 (reactor for fluorite formation)

The reactor is a continuously stirred reactor type, made of coated steel (ebonite) or polypropylene and stirred with P x04. Maintained under reduced pressure using P x08 and equipped with connections for supplying reagents and extracting the product.

R x04 (reactor stirrer R x02)

Made from SANICRO 28 (or similar material), driven by an electric motor. Sized to provide suitable contact between reagents and prevent fluorite precipitation.

G x08 (pump from R x02 to D x06)

Horizontally mounted centrifugal pump with casing and impeller made of coated steel or polypropylene, equipped with a simple mechanical seal.

D x06 (intermediate tank)

Buffer tank made of coated steel or polypropylene, stirred P x05. Maintained under reduced pressure using P x08 and equipped with inlet and outlet connections.

P x05 (mixer D x06)

Made from SANICRO 28 (or similar material), driven by an electric motor. It is sized to prevent fluorite precipitation.

Second filtration zone

G x09 (pump supplying F x02)

Pneumatic diaphragm pump supplied with air from the pneumatic system.

F x02 (second filter press)

Automatic filter press with polypropylene plates and filter cloth. Equipped with a tray for collecting moisture, a system for squeezing and blowing the membrane, washing the filter cloth and washing the cake. It must also be equipped with an exhaust hood maintained at reduced pressure using P x08.

D x07 (hopper for collecting wet synthetic fluorite)

D x11 (intermediate tank)

Made from carbon steel. Maintained under reduced pressure using P x09 and equipped with inlet and outlet connections. It serves as a buffer for C x01.

P x07 (mixer for tank Dx11)

G x14 (pump for cake washing)

Horizontally mounted centrifugal pump with casing and impeller made of cast iron, equipped with a simple mechanical seal.

Ammonia extraction zone

G x11 (pump supplying mother liquors to C x01)

Horizontally mounted centrifugal pump with casing and impeller made of cast iron (steel), equipped with a simple mechanical seal.

C x01 (ammonia stripper)

Made of carbon steel, equipped with a nozzle (2.54 mm (1 inch) metal Raschig rings), upper and lower supply pipes and a system for supplying mother liquors to the upper part. Maintained at reduced pressure using P x09.

E x03 (capacitor)

Made of carbon steel, horizontal type with condensation in the inter-tube area. It is supplied in the pipe zone with cooling water coming from the water tower Z x01.

D x12 (storage tank)

Made from carbon steel or polypropylene. Maintained at reduced pressure using P x09.

G x12 (pump supplying condensed ammonia to D x02)

Horizontally mounted centrifugal pump with casing and impeller made of polypropylene or coated steel. Equipped with a double mechanical seal with barrier fluid supply.

P x09 (fan)

Axial fan. The pressure is such as to guarantee a reduced pressure in C x01.

Ventilation duct cleaning area

P x08 (fan)

Axial fan.

D x10 (H ₂ SO ₄ storage tank)

Small polyethylene tanks with a volume of 1 m ³ .

G x10 (pump supplying H ₂ SO ₄ to C x02)

Dosing pump.

S x02 (wet scrubber)

Made of polypropylene, plastic nozzle (2.54 mm (1 inch) Raschig rings), equipped with a pH-adjustable recirculation system tank. System for supplying acidic water at the top. Maintained at reduced pressure using P x08.

Z x01 (cooling tower)

G x13 (pump supplying the cooling circuit from Z x01)

Fluorite drying zone

B x01 (dryer)

A rotary kiln equipped with a burner, a wet cleaning zone for removing smoke particles (cyclone), a feeding hopper, screw conveyors for feeding and extracting fluorite, a gear motor for changing the rotation speed and hydraulic pistons for changing the inclination.

T x03 A/B (screw conveyors for feeding wet fluorite into a rotary kiln)

Carbon steel screw conveyors equipped with an electric motor, suitable for feeding fluorite into a rotary kiln at a constant flow rate.

T x04 (removal screw conveyor)

Carbon steel screw conveyor for extracting granular fluorite from a rotary kiln and feeding the T x05 bucket elevator. Equipped with an electric motor that operates at a constant speed.

T x05 (bucket elevator)

Bucket elevator made of steel, equipped with an electric motor. It receives granular fluorite from the T x04 screw conveyor and feeds the D x08 storage bin.

T x06 (rotating valve)

Rotating valve made of steel. It regulates the discharge of particles entrained and captured by cyclone D x09 into a storage tank located below.

P x06 (fan)

Axial fan equipped with an electric motor. It supplies the air flow required for combustion in burner B x01.

D x08 (bunker for storing granular fluorite)

A steel (or aluminum) bin suitable for holding granular product. Feeding is carried out from a height using a T x05 bucket elevator. Equipped with a fan and a dust collector with a bag filter for release into the atmosphere.

D x09 (cyclone)

Made of steel. It receives hot flue gases coming from the rotary kiln. It serves for wet cleaning and collecting, by centrifugal force, small particles captured by the turbulence present inside the oven.

1. The ammonia present in the storage tank D x02 is recovered by means of a centrifugal pump G x02 and sent to the batch reactor R x01. Fluorosilicic acid is then sent to the reactor from tank D x01 by means of a centrifugal pump G x01. Reactor Rx01, equipped with mixing means P x01 and cooling coil E x01, ensures complete hydrolysis of the acid, which leads to the formation of NH ₄ F and the precipitation of SiO ₂ according to reaction A.

A) H ₂ SiF ₆ +6NH ₃ +2H ₂ O→6NH ₄ F+SiO ₂ ↓

1) The suspension obtained in R x01 is extracted using a centrifugal pump G x04 and sent to the intermediate tank D x03. The suspension is kept stirred with P x02 and cooled with E x02. The pH of the solution can be adjusted to the required level by adding ammonia from D x02 using pump G x03.

2) The suspension is removed from D x03 using a centrifugal pump G x05 and sent to the filter press F x01. Silicon dioxide is retained in it, and the mother liquors are sent to the heat exchanger/evaporator D x104, consisting of a stirrer P x03 and a heating jacket E x04. At this stage, ammonium fluoride, brought to about 130°C by a stream of steam, decomposes into ammonium bifluoride (more reactive relative to calcium carbonate), releasing a mole of NH ₃ (reaction B). The SiO ₂ cake is washed with process water directed using a G x06 centrifugal pump to extract the fluorite present in the cake. Rinse water is also sent to tank D x104.

B) 2NH ₄ F→NH ₄ HF ₂ +NH ₃

3) Mother liquors extracted from D x104 are sent using a centrifugal pump G x07 to a continuous reactor R x02 (mixed with P x04). At the same time, the weighing screw conveyor T x102 transports calcium carbonate, extracted from the hopper D x105 using the rotating valve T x01, into the reactor R x02, in which reaction C takes place.

C) NH ₄ HF ₂ +CaCO ₃ →CaF ₂ ↓+CO ₂ ↑+NH ₃ ↑

4) The suspension obtained in R x02 is removed using a centrifugal pump G x08 and sent to the intermediate tank D x06 (mixed using R x05).

5) The suspension is removed from the intermediate tank using a centrifugal pump G x09 and sent to the filter press F x02. Fluorite is retained in it, and the mother liquors are sent to the Dx11 buffer tank, mixed using a P x07 stirrer. The fluorite cake is washed with process water using a G x14 pump to extract the ammonia present in the cake. Wash water is also sent to Dx11. The wet fluorite cake is unloaded from the filter and transported to hopper D x07 for collection.

6) Stock solutions in D x11 consist of NH ₃ in 10 wt. % solution. To reuse them as a reagent in A, their concentration must be increased to 25 wt. %. Consequently, they are removed using a centrifugal pump G x11 and sent to the desorber C x01, maintained at reduced pressure by pump P x09. This apparatus is a packed column into which mother liquors containing the ammonia to be concentrated are fed from above and a steam stream at 200 kPa (2 bar) is supplied from below, and is sized such that the overhead is vapor containing 25% NH ₃ , which then condense into E x03 and accumulate into D x12. The bottom stream consists of excess water, which is sent to a water treatment plant.

25% ammonia, condensed at a temperature slightly above 30°C, is removed from D x12 using a centrifugal pump G x12 and sent to tank D x02 for storage. Cooling water for E x03, as well as for E x01 and E x02, is supplied from the cooling tower Z x01 and supplied to the circuit using pump G x13.

7) Devices R x01, D x02, D x03, F x01, D x104, R x02, D x06, F x02 and Dx11 are maintained at reduced pressure using the fan P x08 to prevent the dispersion of NH ₃ vapors into the atmosphere. The ventilation flows are directed to the lower part of C x02, a wet scrubber, into which a solution of H ₂ SO ₄ stored in tank D x10 is fed from above using a pump G x10, which serves to capture the ammonia present in the flow. The ammonia-free vent streams are thus released to the atmosphere, and the solution containing ammonium sulfate formed within the scrubber is released from the bottom of the column.

8) Wet fluorite, unloaded from hopper D x07 for collection, is transported to the rotary furnace B x01 using a feed screw conveyor T x03 A/B. Drying occurs by direct contact of hot flue gases with wet fluorite. The rotation and high temperature of the furnace promotes the formation of dried fluorite granules. The air flow required for combustion is supplied by the fan P x06, while the fine product entrained in the flue gases leaving the furnace B x01 is collected by the cyclone D x09 before the gases are sent to the scrubber C x02. The dried fluorite in fine powder form is then discharged from the D x09 cyclone using the T x06 rotary valve and can be recovered at the top of the drying process. The dried fluorite in the form of granules is then unloaded from the furnace onto the T x04 extraction screw conveyor and transported to the T x05 bucket elevator to then be stored in the D x08 storage hopper.

The plant of the present invention can produce synthetic fluorite of high purity (CaF ₂ ) in accordance with the method described in the specified second embodiment R2 (Fig. 2 and Fig. 6). Installation contains:

Reagent loading and storage area

D x01 (fluorosilicic acid storage tank)

Polypropylene tank for storing fluorosilicic acid, equipped with a valve for devacuuming. The tank is installed inside an insulated polyethylene reserve yap tank of suitable size, equipped with inlet and outlet pipes and an inspection window.

G x01 (pump for extracting FCC from D x01)

Horizontally mounted centrifugal pump with casing and impeller made of polypropylene or coated steel. Equipped with a double mechanical seal with barrier fluid supply.

D x02 (ammonia storage tank)

A tank made of carbon steel suitable for storing ammonia in aqueous solution. Maintained under reduced pressure by P x08 and equipped with a devacuum valve, an insulated polyethylene storage tank of suitable size, equipped with inlet and outlet connections and an inspection window.

G x02 (pump for extracting NH ₃ from D x02)

Horizontally mounted centrifugal pump with casing and impeller made of polypropylene or coated steel. Equipped with a double mechanical seal with barrier fluid supply.

First reaction zone

R x01 (FCC hydrolysis reactor)

Batch reactor made of coated steel (ebonite) or polypropylene, temperature controlled by E x01 and stirred by P x01. Maintained under reduced pressure using P x08 and equipped with connections for supplying reagents and extracting the product.

E x01 (cooling coil R x01)

Made from SANICRO 28 (or similar material) and supplied with cooling water from the Z x01 tower. It is sized to maintain the temperature within 50-60°C.

R x01 (mixing means R x01)

Made from SANICRO 28 (or similar material), driven by an electric motor. Designed and sized to provide suitable contact between reagents.

G x04 (pump for extracting suspension from R x01)

Horizontally mounted centrifugal pump with casing 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 polypropylene, temperature controlled by E x02 and stirred by P x02. Maintained under reduced pressure using P x08 and equipped with inlet and outlet connections.

E x02 (cooling coil D x05)

Made from SANICRO 28 (or similar material) and supplied with cooling water from the Z x01 tower. It is sized to maintain the temperature within 30°C.

P x02 (mixer D x05)

Made from SANICRO 28 (or similar material), driven by an electric motor. Sized to provide suitable contact between reactants (nitrates) and to prevent silica precipitation.

D x203 and D x204 (tank for storing Mg(NO ₃ ) ₂ and Fe(NO ₃ ) ₃ in solution)

Small polyethylene tanks with a volume of 1 m ³ .

G x205 and G x206 (pumps for nitrate extraction)

Small dosing pumps.

First filtration zone

G x05 (pump supplying F x01)

Pneumatic diaphragm pump supplied with air from the pneumatic system.

F x01 (first filter press)

Automatic filter press with polypropylene plates and filter cloth. Equipped with a tray for collecting moisture, a system for squeezing and blowing the membrane, washing the filter cloth and washing the cake. Also equipped with an exhaust hood connected to P x08.

G x06 (pump for cake washing)

Horizontally mounted centrifugal pump with casing and impeller made of cast iron, equipped with a simple mechanical seal. D x04 (intermediate tank)

Made from coated steel or polypropylene. Maintained under reduced pressure using P x08 and equipped with inlet and outlet connections.

P x03 (tank stirrer D x04)

Second reaction zone

G x07 (pump supplying R x02)

Horizontally mounted centrifugal pump with casing and impeller made of polypropylene or coated steel, equipped with a simple mechanical seal.

D x05 (silo for calcium hydroxide storage)

Made of carbon steel, equipped with a ventilation duct, equipped with a dust collector with a bag filter for dust removal and a fan. It is fed from a tanker truck through a pneumatic conveyor.

T x01 (rotary valve)

T x02 (screw conveyor for calcium hydroxide extraction) Made of steel, equipped with a scale capable of weighing calcium hydroxide in stoichiometric quantities into the R x02 reactor.

R x02 (reactor for fluorite formation)

The reactor is a continuously stirred reactor type, made of coated steel (ebonite) or polypropylene and stirred with P x04. Maintained under reduced pressure using P x08 and equipped with connections for supplying reagents and extracting the product.

R x04 (reactor stirrer R x02)

Made from SANICRO 28 (or similar material), driven by an electric motor. Sized to provide suitable contact between reagents and prevent fluorite precipitation.

G x08 (pump from R x02 to D x06)

Horizontally mounted centrifugal pump with casing and impeller made of coated steel or polypropylene, equipped with a simple mechanical seal.

D x06 (intermediate tank)

Buffer tank made of coated steel or polypropylene, stirred P x05. Maintained under reduced pressure using P x08 and equipped with inlet and outlet connections.

P x05 (mixer D x06)

Made from SANICRO 28 (or similar material), driven by an electric motor. It is sized to prevent fluorite precipitation.

Second filtration zone

G x09 (pump supplying F x02)

Pneumatic diaphragm pump supplied with air from the pneumatic system.

F x02 (second filter press)

Automatic filter press with polypropylene plates and filter cloth. Equipped with a tray for collecting moisture, a system for squeezing and blowing the membrane, washing the filter cloth and washing the cake. It must also be equipped with an exhaust hood maintained at reduced pressure using P x08.

D x07 (hopper for collecting wet synthetic fluorite)

D x11 (intermediate tank)

Made from carbon steel. Maintained under reduced pressure using P x09 and equipped with inlet and outlet connections. It serves as a buffer for C x01.

P x07 (mixer for tank Dx11)

G x14 (pump for cake washing)

Horizontally mounted centrifugal pump with casing and impeller made of cast iron, equipped with a simple mechanical seal. Ammonia extraction zone

G x11 (pump supplying mother liquors to C x01)

Horizontally mounted centrifugal pump with casing and impeller made of cast iron (steel), equipped with a simple mechanical seal.

C x01 (ammonia stripper)

Made of carbon steel, equipped with a nozzle (2.54 mm (1 inch) metal Raschig rings), upper and lower supply pipes and a system for supplying mother liquors to the upper part. Maintained at reduced pressure using P x09.

E x03 (capacitor)

Made of carbon steel, horizontal type with condensation in the inter-tube area. It is supplied in the pipe zone with cooling water coming from the water tower Z x01.

D x12 (storage tank)

Made from carbon steel or polypropylene. Maintained at reduced pressure using P x09.

G x12 (pump supplying condensed ammonia to D x02)

Horizontally mounted centrifugal pump with casing and impeller made of polypropylene or coated steel. Equipped with a double mechanical seal with barrier fluid supply.

P x09 (fan)

Axial fan. The pressure is such as to guarantee a reduced pressure in C x01.

Ventilation duct cleaning area

P x08 (fan)

Axial fan.

D x10 (H ₂ SO ₄ storage tank)

Small polyethylene tanks with a volume of 1 m ³ .

G x10 (pump supplying H ₂ SO ₄ to C x02)

Dosing pump.

S x02 (wet scrubber)

Made of polypropylene, plastic nozzle (2.54 mm (1 inch) Raschig rings), equipped with a pH-adjustable recirculation system tank. System for supplying acidic water at the top. Maintained at reduced pressure using P x08.

Z x01 (cooling tower)

G x13 (pump supplying the cooling circuit from Z x01)

Fluorite drying zone

B x01 (dryer)

A rotary kiln equipped with a burner, a wet cleaning zone for removing smoke particles (cyclone), a feeding hopper, screw conveyors for feeding and extracting fluorite, a gear motor for changing the rotation speed and hydraulic pistons for changing the inclination.

T x03 A/B (screw conveyors for feeding wet fluorite into a rotary kiln)

Carbon steel screw conveyors equipped with an electric motor, suitable for feeding fluorite into a rotary kiln at a constant flow rate.

T x04 (removal screw conveyor)

Carbon steel screw conveyor for extracting granular fluorite from a rotary kiln and feeding the T x05 bucket elevator. Equipped with an electric motor that operates at a constant speed.

T x05 (bucket elevator)

Bucket elevator made of steel, equipped with an electric motor. It receives granular fluorite from the T x04 screw conveyor and feeds the D x08 storage bin.

T x06 (rotating valve)

Rotating valve made of steel. It regulates the discharge of particles entrained and captured by cyclone D x09 into a storage tank located below.

P x06 (fan)

Axial fan equipped with an electric motor. It supplies the air flow required for combustion in burner B x01.

D x08 (bunker for storing granular fluorite)

A steel (or aluminum) bin suitable for holding granular product. Feeding is carried out from a height using a T x05 bucket elevator. Equipped with a fan and a dust collector with a bag filter for release into the atmosphere.

D x09 (cyclone)

Made of steel. It receives the hot flue gases coming out of the rotary kiln. It serves for wet cleaning and trapping, by centrifugal force, of small particles captured by the turbulence present inside the oven.

1) The ammonia present in the storage tank D x02 is recovered through centrifugal pump G x02 and sent to the batch reactor Rx01. Fluorosilicic acid is then sent to the reactor from tank D x01 by means of a centrifugal pump G x01. Reactor Rx01, equipped with mixing means P x01 and cooling coil E x01, ensures complete hydrolysis of the acid, which leads to the formation of NH ₄ F and the precipitation of SiO ₂ according to reaction A.

A) H ₂ SiF ₆ +6NH ₃ +2H ₂ O→6NH ₄ F+SiO ₂ ↓

2) The suspension obtained in R x01 is extracted using a centrifugal pump G x04 and sent to the intermediate tank D x03. The suspension is kept stirred with P x02 and cooled with E x02. At this stage, a solution of Mg(NO ₃ ) ₂ and Fe(NO ₃ ) ₃ is also supplied from D x203 and D x204 using pumps G x205 and G x206, respectively. The pH of the solution can be adjusted to the required level by adding ammonia from D x02 using pump G x03.

3) The suspension is removed from D x03 using a centrifugal pump G x05 and sent to the filter press F x01. Silicon dioxide is retained in it, and the mother liquors are sent to an intermediate tank D x04, maintained with constant stirring using a stirrer P x03. The SiO ₂ cake is washed with process water directed using a G x06 centrifugal pump to extract the fluorite present in the cake. Rinse water is also sent to tank D x04.

4) Mother liquors extracted from D x04 are sent using a centrifugal pump G x07 to a continuous reactor R x02 (mixed with P x04). At the same time, the weighing screw conveyor T x02 transports the calcium hydroxide extracted from the hopper D x05 using the rotating valve T x01 into the reactor R x02, in which reaction B takes place.

B) 2NH ₄ F+Ca(OH) ₂ →CaF ₂ ↓+2H ₂ O+2NH ₃ ↑

5) The suspension obtained in R x02 is removed using a centrifugal pump G x08 and sent to the intermediate tank D x06 (mixed using R x05).

6) From the intermediate tank D x06, the suspension is removed using a centrifugal pump G x09 and sent to the filter press F x02. Fluorite is retained in it, and the mother liquors are sent to the buffer tank D x11, maintained with constant stirring using a stirrer P x07. The fluorite cake is washed with process water using a G x14 pump to extract the ammonia present in the cake. Wash water is also sent to Dx11. Wet fluorite cake is unloaded from filter F x02 and transported to hopper D x07 for collection.

7) Stock solutions in D x11 consist of NH ₃ in 10 wt. % solution. To reuse them as a reagent in A, their concentration must be increased to 25 wt. %. Consequently, they are removed using a centrifugal pump G x11 and sent to the desorber C x01, maintained at reduced pressure by pump P x09. This apparatus is a packed column into which mother liquors containing the ammonia to be concentrated are fed from above and a steam stream at 200 kPa (2 bar) is supplied from below, and is sized such that the overhead is vapor containing 25% NH ₃ , which then condense into E x03 and accumulate into D x12. The bottom stream consists of excess water, which is sent to a water treatment plant.

25% ammonia, condensed at a temperature slightly above 30°C, is removed from D x12 using a centrifugal pump G x12 and sent to tank D x02 for storage. Cooling water for E x03, as well as for E x01 and E x02, is supplied from the cooling tower Z x01, which supplies the circuit using pump G x13.

8) Devices R x01, D x02, D x03, F x01, D x04, R x02, D x06, F x02 and Dx11 are maintained at reduced pressure using the fan P x08 to prevent the dispersion of NH ₃ vapors into the atmosphere. The ventilation flows are directed to the lower part of C x02, a wet scrubber, into which a solution of H ₂ SO ₄ stored in tank D x10 is supplied from above using a pump G x10, which serves to capture the ammonia present in the flow. The ammonia-free vent streams are thus released to the atmosphere, and the solution containing ammonium sulfate formed within the scrubber is released from the bottom of the column.

9) Wet fluorite, unloaded from hopper D x07 for collection, is transported to the rotary furnace B x01 using a feed screw conveyor T x03 A/B. Drying occurs by direct contact of hot flue gases with wet fluorite. The rotation and high temperature of the furnace promotes the formation of dried fluorite granules. The air flow required for combustion is supplied by the fan P x06, while the fine product entrained in the flue gases leaving the furnace B x01 is collected by the cyclone D x09 before the gases are sent to the scrubber C x02. The dried fluorite in fine powder form is then discharged from the D x09 cyclone using the T x06 rotary valve and can be recovered at the top of the drying process. The dried fluorite in the form of granules is then unloaded from the furnace onto the T x04 extraction screw conveyor and transported to the T x05 bucket elevator to then be stored in the D x08 storage hopper.

The apparatus of the present invention can produce high purity synthetic fluorite (CaF ₂ ) according to the method described in said first embodiment R1 (FIG. 1 and FIG. 5). Installation contains:

Reagent loading and storage area

D x01 (fluorosilicic acid storage tank)

Polypropylene tank for storing fluorosilicic acid, equipped with a valve for devacuuming. The tank is installed inside an insulated polyethylene reserve yap tank of suitable size, equipped with inlet and outlet pipes and an inspection window.

G x01 (pump for extracting FCC from D x01)

Horizontally mounted centrifugal pump with casing and impeller made of polypropylene or coated steel. Equipped with a double mechanical seal with barrier fluid supply.

D x02 (ammonia storage tank)

A tank made of carbon steel suitable for storing ammonia in aqueous solution. Maintained at reduced pressure by P x08 and equipped with a valve for vacuum regulation, an insulated polyethylene storage tank of suitable size, equipped with inlet and outlet connections and an inspection window.

G x02 (pump for extracting NH ₃ from D x02)

Horizontally mounted centrifugal pump with casing and impeller made of polypropylene or coated steel. Equipped with a double mechanical seal with barrier fluid supply.

First reaction zone

R x01 (FCC hydrolysis reactor)

Batch reactor made of coated steel (ebonite) or polypropylene, temperature controlled by E x01 and stirred by P x01. Maintained under reduced pressure using P x08 and equipped with connections for supplying reagents and extracting the product.

E x01 (cooling coil R x01)

Made from SANICRO 28 (or similar material) and supplied with cooling water from the Z x01 tower. It is sized to maintain the temperature within 50-60°C.

R x01 (mixing means R x01)

Made from SANICRO 28 (or similar material), driven by an electric motor. Designed and sized to provide suitable contact between reagents.

G x04 (pump for extracting suspension from R x01)

Horizontally mounted centrifugal pump with casing 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 polypropylene, temperature controlled by E x02 and stirred by P x02. Maintained under reduced pressure using P x08 and equipped with inlet and outlet connections.

E x02 (cooling coil D x03)

Made from SANICRO 28 (or similar material) and supplied with cooling water from the Z x01 tower. It is sized to maintain the temperature within 30°C.

P x02 (mixer D x03)

Made from SANICRO 28 (or similar material), driven by an electric motor. Sized to provide suitable contact between reactants (nitrates) and to prevent silica precipitation.

D x203 and D x204 (tank for storing Mg(NO ₃ ) ₂ and Fe(NO ₃ ) ₃ in solution)

Small polyethylene tanks with a volume of 1 m ³ .

G x03 (pump for supplying ammonia to D x03)

G x205 and G x206 (pumps for nitrate extraction)

Small dosing pumps.

First filtration zone

G x05 (pump supplying F x01)

Pneumatic diaphragm pump supplied with air from the pneumatic system.

F x01 (first filter press)

Automatic filter press with polypropylene plates and filter cloth. Equipped with a tray for collecting moisture, a system for squeezing and blowing the membrane, washing the filter cloth and washing the cake. Also equipped with an exhaust hood connected to P x08.

G x06 (pump for cake washing)

Horizontally mounted centrifugal pump with casing and impeller made of cast iron, equipped with a simple mechanical seal.

D x104 (evaporator)

Made from coated steel. Maintained under reduced pressure using P x08 and equipped with inlet and outlet connections. An increase in temperature (up to 130°C) ensures the conversion of ammonium fluoride into ammonium bifluoride, which is more reactive relative to CaCO ₃ .

E x04 (heating jacket D x104)

Made of carbon steel and supplied with steam. It is sized to bring the internal temperature D x104 up to approximately 130°C.

P x03 (evaporator stirrer D x104)

Second reaction zone

G x07 (pump supplying R x02)

Horizontally mounted centrifugal pump with casing and impeller made of coated steel or polypropylene, equipped with a simple mechanical seal.

D x105 (carbonate storage bunker)

Made of carbon steel, equipped with a ventilation duct, equipped with a dust collector with a bag filter for dust removal and a fan. It is fed from a tanker truck through a pneumatic conveyor.

T x01 (rotary valve)

T x102 (screw conveyor for carbonate extraction)

Made of steel, equipped with scales capable of weighing carbonate in stoichiometric quantities into the R x02 reactor.

R x02 (reactor for fluorite formation)

The reactor is a continuously stirred reactor type, made of coated steel (ebonite) or polypropylene and stirred with P x04. Maintained under reduced pressure using P x08 and equipped with connections for supplying reagents and extracting the product.

R x04 (reactor stirrer R x02)

Made from SANICRO 28 (or similar material), driven by an electric motor. Sized to provide suitable contact between reagents and prevent fluorite precipitation.

G x08 (pump from R x02 to D x06)

Horizontally mounted centrifugal pump with casing and impeller made of coated steel or polypropylene, equipped with a simple mechanical seal.

D x06 (intermediate tank)

Buffer tank made of coated steel or polypropylene, stirred P x05. Maintained under reduced pressure using P x08 and equipped with inlet and outlet connections.

P x05 (mixer D x06)

Made from SANICRO 28 (or similar material), driven by an electric motor. It is sized to prevent fluorite precipitation.

Second filtration zone

G x09 (pump supplying F x02)

Pneumatic diaphragm pump supplied with air from the pneumatic system.

F x02 (second filter press)

Automatic filter press with polypropylene plates and filter cloth. Equipped with a tray for collecting moisture, a system for squeezing and blowing the membrane, washing the filter cloth and washing the cake. It must also be equipped with an exhaust hood maintained at reduced pressure using P x08.

D x07 (hopper for collecting wet synthetic fluorite)

D x11 (intermediate tank)

Made from carbon steel. Maintained under reduced pressure using P x09 and equipped with inlet and outlet connections. It serves as a buffer for C x01.

P x07 (mixer for tank Dx11)

G x14 (pump for cake washing)

Horizontally mounted centrifugal pump with casing and impeller made of cast iron, equipped with a simple mechanical seal.

Ammonia extraction zone

G x11 (pump supplying mother liquors to C x01)

Horizontally mounted centrifugal pump with casing and impeller made of cast iron (steel), equipped with a simple mechanical seal.

C x01 (ammonia stripper)

Made of carbon steel, equipped with a nozzle (2.54 mm (1 inch) metal Raschig rings), upper and lower supply pipes and a system for supplying mother liquors to the upper part. Maintained at reduced pressure using P x09.

E x03 (capacitor)

Made of carbon steel, horizontal type with condensation in the inter-tube area. It is supplied in the pipe zone with cooling water coming from the water tower Z x01.

D x12 (storage tank)

Made from carbon steel or polypropylene. Maintained at reduced pressure using P x09.

G x12 (pump supplying condensed ammonia to D x02)

Horizontally mounted centrifugal pump with casing and impeller made of polypropylene or coated steel. Equipped with a double mechanical seal with barrier fluid supply.

P x09 (fan)

Axial fan. The pressure is such as to guarantee a reduced pressure in C x01.

Ventilation duct cleaning area

P x08 (fan)

Axial fan.

D x10 (H ₂ SO ₄ storage tank)

Small polyethylene tanks with a volume of 1 m ³ .

G x10 (pump supplying H ₂ SO ₄ to C x02)

Dosing pump.

S x02 (wet scrubber)

Made of polypropylene, plastic nozzle (2.54 mm (1 inch) Raschig rings), equipped with a pH-adjustable recirculation system tank. System for supplying acidic water at the top. Maintained at reduced pressure using P x08.

Z x01 (cooling tower)

G x13 (pump supplying the cooling circuit from Z x01)

Fluorite drying zone

B x01 (dryer)

A rotary kiln equipped with a burner, a wet cleaning zone for removing smoke particles (cyclone), a feeding hopper, screw conveyors for feeding and extracting fluorite, a gear motor for changing the rotation speed and hydraulic pistons for changing the inclination.

T x03 A/B (screw conveyors for feeding wet fluorite into a rotary kiln)

Carbon steel screw conveyors equipped with an electric motor, suitable for feeding fluorite into a rotary kiln at a constant flow rate.

T x04 (removal screw conveyor)

Carbon steel screw conveyor for extracting granular fluorite from a rotary kiln and feeding the T x05 bucket elevator. Equipped with an electric motor that operates at a constant speed.

T x05 (bucket elevator)

Bucket elevator made of steel, equipped with an electric motor. It receives granular fluorite from the T x04 screw conveyor and feeds the D x10 storage bin.

T x06 (rotating valve)

Rotating valve made of steel. It regulates the discharge of particles entrained and captured by cyclone D x09 into a storage tank located below.

P x06 (fan)

Axial fan equipped with an electric motor. It supplies the air flow required for combustion in burner B x01.

D x08 (bunker for storing granular fluorite)

A steel (or aluminum) bin suitable for holding granular product. Feeding is carried out from a height using a T x05 bucket elevator. Equipped with a fan and a dust collector with a bag filter for release into the atmosphere.

D x09 (cyclone)

Made of steel. It receives hot flue gases coming from the rotary kiln. It is designed to wet clean and collect, by centrifugal force, small particles captured by the turbulence present inside the oven.

1) The ammonia present in the storage tank D x02 is recovered by means of a centrifugal pump G x02 and sent to the batch reactor R x01. Fluorosilicic acid is then sent to the reactor via a centrifugal pump G x01. Reactor R x01, equipped with mixing means P x01 and cooling coil E x01, ensures complete hydrolysis of the acid, which leads to the formation of NH ₄ F and the precipitation of SiO ₂ according to reaction A.

A) H ₂ SiF ₆ +6NH ₃ +2H ₂ O→6NH ₄ F+SiO ₂ ↓

2) The suspension obtained in R x01 is extracted using a centrifugal pump G x04 and sent to the intermediate tank D x03. The suspension is kept stirred with P x02 and cooled with E x02. At this stage, a solution of Mg(N0 ₃ ) ₂ and Fe(N0 ₃ ) ₃ is supplied from D x203 and D x204 using pumps G x205 and G x206, respectively. The pH of the solution can be adjusted to the required level by adding ammonia from D x02 using pump G x03.

3) The suspension is removed from D x03 using a centrifugal pump G x05 and sent to the filter press F x01. Silicon dioxide is retained in it, and the mother liquors are sent to the heat exchanger/evaporator D x104, consisting of a stirrer P x03 and a heating jacket E x04. At this stage, ammonium fluoride, brought to about 130°C by a stream of steam, decomposes into ammonium bifluoride (more reactive relative to calcium carbonate), releasing a mole of NH ₃ (reaction B). The SiO ₂ cake is washed with process water directed using a G x06 centrifugal pump to extract the fluorite present in the cake. Rinse water is also sent to tank D x104.

B) 2NH ₄ F→NH ₄ HF ₂ +NH ₃

4) Mother liquors extracted from D x104 are sent using a centrifugal pump G x07 to a continuous reactor R x02 (mixed with P x04). At the same time, the weighing screw conveyor T x102 transports calcium carbonate, extracted from the hopper D x105 using the rotating valve T x01, into the reactor R x02, in which reaction C takes place.

C) NH ₄ HF ₂ +CaCO ₃ →CaF ₂ ↓+CO ₂ ↑+NH ₃ ↑

5) The suspension obtained in R x02 is removed using a centrifugal pump G x08 and sent to the intermediate tank D x06 (mixed using R x05).

6) The suspension is removed from the intermediate tank using a centrifugal pump G x09 and sent to the filter press F x02. Fluorite is kept in it and the mother liquors are sent to the buffer tank D x11, mixed using a stirrer P x07. The fluorite cake is washed with process water using a G x14 pump to extract the ammonia present in the cake. Rinse water is also sent to D x11. The wet fluorite cake is unloaded from the filter and transported to hopper D x07 for collection.

7) Stock solutions in D x11 consist of NH ₃ in 10 wt. % solution. To reuse them as a reagent in A, their concentration must be increased to 25 wt. %. Consequently, they are removed using a centrifugal pump G x11 and sent to the desorber C x01, maintained at reduced pressure by pump P x09. This apparatus is a packed column into which mother liquors containing the ammonia to be concentrated are fed from above and a steam stream at 200 kPa (2 bar) is supplied from below, and is sized such that the overhead is vapor containing 25% NH ₃ , which then condense into E x03 and accumulate into D x12. The bottom stream consists of excess water, which is sent to a water treatment plant.

25% ammonia, condensed at a temperature slightly above 30°C, is removed from D x12 using a centrifugal pump G x12 and sent to tank D x02 for storage. Cooling water for E x03, as well as for E x01 and E x02, is supplied using the cooling tower Z x01 and fed into the circuit using pump G x13.

8) Devices R x01, D x02, D x03, F x01, D x04, R x02, D x06, F x02 and Dx11 are maintained at reduced pressure using the fan P x08 to prevent the dispersion of NH ₃ vapors into the atmosphere. The ventilation flows are directed to the lower part of C x02, a wet scrubber, into which a solution of H ₂ SO ₄ stored in tank D x10 is supplied from above using a pump G x10, which serves to capture the ammonia present in the flow. The ammonia-free vent streams are thus released to the atmosphere and the solution containing ammonium sulfate formed within the scrubber is released from the bottom of the column.

9) Wet fluorite, unloaded from hopper D x07 for collection, is transported to the rotary furnace B x01 using a feed screw conveyor T x03 A/B. Drying occurs by direct contact of hot flue gases with wet fluorite. The rotation and high temperature of the furnace promotes the formation of dried fluorite granules. The air flow required for combustion is supplied by the fan P x06, while the fine product entrained in the flue gases leaving the furnace B x01 is collected by the cyclone D x09 before being sent to the scrubber C x02. The dried fluorite in fine powder form is then discharged from the D x09 cyclone using the T x06 rotary valve and can be recovered at the top of the drying process. The dried fluorite in the form of granules is then unloaded from the furnace onto the T x04 extraction screw conveyor and transported to the T x05 bucket elevator to then be stored in the D x08 storage hopper.

Said embodiments R1 (FIGS. 1 and 5), R2 (FIGS. 2 and 6), R3 (FIGS. 3 and 7) and R4 (FIGS. 4 and 8) comprise a substantial drying and granulating step to produce synthetic fluorite, which can be used in existing industrial technology. In fact, as is well known, fluorite used as a raw material should be supplied to HF production lines in the form of a dry powder with an appropriate particle size. In the process of the present invention, the drying step not only removes the water contained in the product, but also produces grains of aggregated material that can be easily handled (transported, binned, dosed and crushed) by conventional common industrial processes. The drying-granulation stage is performed with a suspension of synthetic fluorite containing 30-50 wt. % of moisture leaving D x07 and directed to T x03 A/B so that it enters the loading funnel. Using a belt extraction conveyor equipped with a tipping device, it is possible to regulate the loading of wet fluorite entering the B x01 furnace. This charge is varied to have a flow rate of dried slurry and granules of 180 to 300 kg/h (motor frequency is approximately 9.34 Hz to 15 Hz). The suspension removed from the conveyor belt enters a funnel, which, in turn, feeds it to an underlying screw conveyor, which delivers the fluorite to the furnace. The screw conveyor has a constant rotation speed.

The burner, equipped with an automatic regulator of the air-fuel mixture, operates with such a flow rate of liquefied hydrocarbon gas to have a temperature in the combustion chamber ranging from 700°C to 800°C, for example, 784°C. This temperature is measured using a thermocouple connected to the diagnostic communication system, adjusting the fuel flow. The fuel flow setting was changed, for example, from 4 to 8 m ³ /h (flow meter at the burner inlet). The temperature of the flue gases leaving the furnace, also measured by a thermocouple connected to the diagnostic communication system, shows a value of, for example, 120°C to 220°C. For example, the tilt of a rotary kiln was kept constant using a piston at a height of 91 cm above the floor so as to give the kiln a tilt of 1.58°.

The rotation speed of the furnace is controlled using a gear motor equipped with an inverter. In the tests carried out, the inverter changes its frequency, for example between 9.5 and 30 Hz, to ensure that the oven rotates at a speed of, for example, between 8 and 23 rpm. The dried granular slurry is removed from the kiln using a screw conveyor at the top of the kiln. The final product, synthetic fluorite, is stored and weighed, and small particles, carried away by the turbulence of the flue gases and captured in the cyclone (at high flow rates of the final product, above 300 kg/h, a flow rate of 80-100 kg/h of product from the cyclone was observed), are also stored and weighed. The amount of fluorite granulated and processed in a cyclone is added to the loss of moisture during the process, which thus makes it possible to close the material-energy balance of the process. The granulated synthetic fluorite is then sent to the laboratory for careful analysis of the resulting characteristics (LOI (loss on ignition) and particle size distribution).

The tables below summarize the results obtained from several trials.

According to one embodiment of the present invention, in the installation of the present invention, a reactor may be provided at the outlet of the storage tank (D x07) for receiving said synthetic fluorite in the form of a suspension from said storage tank (D x07) and receiving an aqueous solution of acids, preferably 5% or 10% HCl or H ₂ SO ₄ , and a filter device to remove excess oxides, carbonates or calcium hydroxide.

In the method of the present invention, implemented, for example, using the indicated embodiments R1 (Figs. 1 and 5), R2 (Figs. 2 and 6), R3 (Figs. 3 and 7) and R4 (Figs. 4 and 8), involve the use of washing steps and subsequent filtration using aqueous solutions of dilute acids, such as 5% or 10% HCl or H ₂ SO ₄ , at the outlet of the synthetic fluorite from F x02, prior to the drying stage, in order to reduce the concentration of oxides present in the synthetic fluorite, for example, MgO, or Al ₂ O ₃ , or Fe ₂ O ₃ , and/or excess carbonates, for example CaCO ₃ , and/or excess calcium hydroxide. Reducing the concentration of eg MgO prevents problems during the sulfuric acid reaction step. This reaction is carried out in a jacketed rotary furnace heated by flue gases circulating in the furnace jacket. In particular, it has been noted that the resulting gypsum typically forms a deposit on the walls of HF furnaces, thus inhibiting heat exchange between the hot flue gases (furnace exterior) and the reaction mass (furnace interior). This effect can cause the reaction to stop completely, resulting in unwanted plant downtime, or, in any case, significantly increases the specific fluorite consumption (the amount of fluorite lost in the gypsum increases).

The synthetic fluorite of the present invention, obtained, for example, through the specified embodiments R1 (Figs. 1 and 5), R2 (Figs. 2 and 6), R3 (Figs. 3 and 7) and R4 (Figs. 4 and 8), has the following physico-chemical characteristics that characterize it as a new product:

- an SPP value (measured in accordance with procedures and technologies known to the person skilled in the art, in samples from the outlet side D x08), ranging from 0.3 to 1.2, preferably from 0.5 to 0.8, even more preferably from 0, 6 to 0.7,

- BET value (measured in accordance with procedures and technologies known to the skilled person, in samples from the outlet side D x07 after drying at 800°C in the laboratory), ranging from 20 m ² /g to 100 m ² /g, preferably from 40 m ² /g to 80 m ² /g, even more preferably from 50 m ² /g to 60 m ² /g,

- average particle size (measured in accordance with procedures and technologies known to the specialist), given below in mass. % relative to the mass of fluorite:

more than 10 mm: zero,

more than 5 mm, ranges from 1 to 10, preferably from 1 to 5, even more preferably from 1 to 3,

more than 1 mm, ranges from 40 to 80, preferably from 50 to 70, even more preferably from 55 to 65,

more than 0.05 mm, ranges from 10 to 30, preferably from 15 to 25, even more preferably from 20 to 25,

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

The fluorite of the present invention also has:

- a concentration of silicon dioxide SiO ₂ less than 1%, preferably less than 0.7%, even more preferably less than 0.35% (measured in accordance with procedures and technologies known to the specialist, on samples fired at 800 ° C),

- MgO concentration value less than 0.5%, preferably less than 0.3%, even more preferably less than 0.2% (measured in accordance with procedures and technologies known to the specialist, on samples fired at 800°C).

Table 2 shows the composition of synthetic fluorite obtained in the form of a suspension coming from F x02.

Table 3 shows the main compounds.

Table 4 shows the composition of the resulting synthetic fluorite, dried in the form of granules coming from B x01.

Table 5 shows the main connections.

Claims (20)

1. Synthetic fluorite CaF ₂ acid grade in the form of granules, where the specified fluorite has: - loss on ignition (LOI) value measured in the samples, ranging from 0.3 to 1.2, and/or - BET value measured in samples after drying at 800°C, ranging from 50 m ² /g to 100 m ² /g, and/or - average particle size given below, in wt.% relative to the mass of synthetic fluorite: more than 10 mm - zero, more than 5 mm is from 1 to 10, more than 1 mm ranges from 40 to 80, more than 0.05 mm is from 10 to 30, less than 0.05 mm is from 1 to 20. 2. The acid-grade synthetic fluorite according to claim 1, wherein said fluorite has a PPP value of 0.5 to 0.8, preferably 0.6 to 0.7. 3. The acid grade synthetic fluorite according to claim 1, wherein said fluorite has a BET value of 50 m ² /g to 80 m ² /g, preferably 50 m ² /g to 60 m ² /g. 4. The acid-grade synthetic fluorite according to claim 1, wherein said fluorite has an average particle size given below, in wt.% relative to the weight of the synthetic fluorite: more than 10 mm - zero, more than 5 mm is from 1 to 5, preferably from 1 to 3, more than 1 mm is from 50 to 70, preferably from 55 to 65, more than 0.05 mm is from 15 to 25, preferably from 20 to 25, less than 0.05 mm is from 5 to 15, preferably from 5 to 10. 5. Synthetic acid-grade fluorite according to any of the preceding paragraphs, wherein said fluorite has, in wt.%: - a concentration of silicon dioxide SiO ₂ less than 1%, preferably less than 0.7%, even more preferably less than 0.35%, measured on samples fired at 800°C, and/or - MgO concentration less than 0.5%, preferably less than 0.3%, even more preferably less than 0.2%, measured on samples fired at 800°C.

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