FR3150199A1 - PROCESS FOR RECOVERING A BORON SALT FROM AN ORGANIC SOLVENT COMPRISING BORIC ACID - Google Patents

Abstract

This process for recovering boron in the form of boron salt from boric acid included in an organic solvent, comprises the following steps:– bringing a basic aqueous solution into contact with the organic solvent to extract the boron in the form of solubilized borate towards the basic aqueous solution;– distillation of the basic aqueous solution loaded with borate to concentrate said solution, generating a distillate;– crystallization of the borate in the form of hydrated boron salt;– solid-liquid separation of the crystallized hydrated boron salt, generating crystallization mother liquors;– reintroduction of the distillate and the crystallization mother liquors at the step of bringing the basic aqueous solution into contact with the organic solvent.

Description

PROCESS FOR RECOVERING A BORON SALT FROM AN ORGANIC SOLVENT COMPRISING BORIC ACID Field of invention

The invention relates to the field of recovery of boron contained in industrial waste. More specifically, it relates to a process for recovering a boron salt from an organic solvent comprising boric acid.

Prior art

Boron extraction using organic solvents is used to recover and purify boron from boron-laden aqueous solutions from the treatment of effluents, brines or debris from used permanent magnets. The organic solvent, which is not miscible with water, is used to first extract boron from these aqueous solutions.

The solvent loaded with boric acid is then put in contact with an alkaline solution, which allows the boron to be extracted into an aqueous phase. Sodium hydroxide solutions (NaOH) are the most commonly used, but other sources of hydroxides are possible, such as potash (KOH), lithium hydroxide (LiOH), or ammonia (NH ₄ OH).

From this aqueous phase, the boron derivative obtained is recovered in solid form and will be reused in various sectors of industry, for example in the glass industry (borosilicate glass), ceramics (boron nitride), metallurgy (flux for steel), or medical products (antiseptics).

This extraction process is well known. We can cite document US 3,493,349 which describes a process for solvent extraction of boron from brine, then re-extraction by formation of borate NaB ₂ O ₄ in solution by addition of a sodium hydroxide solution.

Similarly, document US 3,855,392 describes a process for removing boric acid from an aqueous solution of magnesium chloride by liquid-liquid extraction, comprising an extraction step using an organic extraction compound, then a re-extraction step using an alkaline solution whose pH is between 8 and 10.

However, these processes use significant quantities of alkaline solution, which is expensive. In addition, they do not allow the entire aqueous flow to be recycled.

Therefore, there is a need to develop more efficient processes that allow for better management of resources, with reduced environmental impact and are more economical.

The invention provides a process which makes it possible to recover a boron salt from an organic solvent comprising boric acid, by limiting the quantity of alkaline agent used in the process, while making it possible to recycle all of the aqueous streams.

The invention also makes it possible to reduce the quantity of water required for boron recovery.

To achieve this, the process is designed in a closed loop to allow the continuous circulation of aqueous solutions, with a view to their recycling. Thus, the water introduced into the process circulates in a loop within it. In addition, the process does not consume any additional water other than that which is initially introduced and recycled.

At the same time, all traces of organic solvent present in the aqueous phase are fully recycled and revalued.

Thus, the process according to the invention does not result in any loss of solvent and allows the obtaining of a very low residual carbon content, from the organic solvent, in the crystallized boron salt.

For this purpose, the invention provides a process for recovering boron in the form of boron salt from boric acid included in an organic solvent, the process comprising the following steps:
– contacting a basic aqueous solution with the organic solvent to extract the boron in the form of solubilized borate into the basic aqueous solution;
– distillation of the basic aqueous solution loaded with borate to concentrate said solution, generating a distillate;
– crystallization of borate in the form of hydrated boron salt.;
– solid-liquid separation of crystallized hydrated boron salt, generating crystallization mother liquors;
– reintroduction of the distillate and the crystallization mother liquors at the stage of bringing the basic aqueous solution into contact with the organic solvent.

In the present invention, the term "borate" denotes a molecular compound comprising at least one boron atom and at least one oxygen atom. For example and in a non-limiting manner, the metaborate BO ₂ ^- and tetraborate B ₄ O ₇ ^2- ions are borates.

By "boron salt" is included borate salts. For example, and without limitation, sodium metaborate NaBO₂ and sodium tetraborate Na₂B₄O₇ are boron salts. Sodium tetraborate pentahydrate Na₂B₄O₇•5H₂O or decahydrate Na₂B₄O₇•10H₂O which are also called borax, are salts of boron.

By “boron” is included any compound comprising at least one boron atom, present in aqueous or organic phase. For example, boric acid is included in the definition of “boron” according to the invention.

According to the invention, the organic solvent preferably comprises at least one extraction compound and/or at least one modifying compound and/or at least one diluting compound.

An extraction compound allows the transfer of boron from an aqueous solution to the organic solvent. Generally, it comprises at least one alcohol having an aliphatic or aromatic chain of 6 to 18 carbon atoms.

The extraction compound is preferably selected from the group comprising:
– 1-3 diols (β-aliphatic diols), in particular 2-ethyl-1,3-hexanediol (EHD), 2-butyl-2-ethylpropane-1,3-diol (BEPD), 2,2,4-trimethyl-1,3-pentanediol, 2-chloro-4-(1,1,3,3-tetramethylbutyl)-6-methylol-phenol (CTMP) and mixtures thereof;
– monoalcohols of 6 to 18 carbon atoms, in particular decan-1-ol, 2-propylheptanol (2PH), 2-ethylhexanol (2EH), isodecan-1-ol and their mixtures.

A modifier compound is a substance that can change some specific properties of the solvent. For example, the modifier compound can improve the settling of the organic solvent and the basic aqueous solution.

Typically, the modifying compound consists of an alcohol with an aliphatic chain of 6 to 18 carbon atoms.

The modifying compound is preferably selected from the group comprising monoalcohols, in particular decan-1-ol, 2-propylheptanol (2PH), 2-ethylhexanol (2EH), isodecan-1-ol, octan-1-ol, and mixtures thereof.

The diluent compound is a liquid substance or a mixture of liquid substances in which the extracting compound and/or the modifying compound are dissolved. Typically, the diluent compound consists of an aliphatic or aromatic hydrocarbon chain, of 6 to 18 carbon atoms.

Preferably, the diluent compound is selected from the group comprising: paraffins, cycloparaffins, aromatic hydrocarbons, and mixtures thereof.

According to the invention, the organic solvent comprises at least one extraction compound consisting of an alcohol comprising an aliphatic or aromatic chain of 6 to 18 carbon atoms, optionally a modifying compound consisting of an alcohol comprising an aliphatic chain of 6 to 18 carbon atoms and optionally a diluting compound consisting of an aliphatic or aromatic hydrocarbon chain of 6 to 18 carbon atoms.

The organic solvent also includes boric acid at a boron concentration between 1 mg/L and 14 g/L.

Furthermore, the basic aqueous solution is obtained by diluting a concentrated basic solution or by solubilizing a solid basic compound. Preferably, it is a solution of NaOH, KOH, LiOH, or NH ₄ OH. The choice of the base depends on the boron salt to be produced, thus a NaOH solution will allow the formation of sodium borate. Similarly, a KOH solution will be chosen to obtain potassium borate.

Advantageously, the basic aqueous solution is introduced in a stoichiometric quantity relative to the quantity of boron salt to be produced.

Thus, the number of moles of alkali or ammonium added determines the structure of the final product. For example, to produce sodium tetraborate Na ₂ B ₄ O ₇ , 0.5 moles of NaOH are introduced for every 1 mole of boron in the organic solvent. Similarly, 1 mole of NaOH will be introduced for every 1 mole of boron to generate sodium metaborate NaBO ₂ .

The basic solution is therefore not introduced in excess. In doing so, the consumption of alkaline product is reduced.

According to the invention, the ratio between the number of moles of alkali or ammonium in the basic solution and the number of moles of boron in the organic solvent is between 0.3 and 1.

During the step of bringing the basic aqueous solution into contact with the organic solvent, the boric acid contained in the solvent is transferred from the organic phase to the aqueous phase. A basic aqueous solution loaded with borate is then obtained.

The contacting of the basic aqueous solution with the organic solvent is carried out in a liquid-liquid extraction device comprising several stages, where the organic solvent and the basic aqueous solution circulate in countercurrent.

For this purpose, the two liquids are circulated at different flow rates, such that the ratio of the flow rate of the basic aqueous solution and the flow rate of the organic solvent can be between 0.1 and 2, preferably between 0.1 and 1, and even more preferably between 0.1 and 0.5. These flow rate ratios make it possible to minimize the amount of water circulating in the process and to optimize the energy consumption of the process.

According to one variant, this step is carried out at a temperature between 20°C and 70°C, preferably between 40°C and 60°C. These temperature ranges are optimized for a ratio of the flow rate of the basic aqueous solution and the flow rate of the organic solvent between 0.1 and 2.

The basic aqueous solution comprising the solubilized borate is then subjected to a concentration step by distillation generating a distillate. The concentration of the solution makes it possible to obtain a solution saturated with borate.

According to a variant of the invention, the crystallization of the borate is carried out by distillation of the concentrated basic aqueous solution of borate. The crystallized solid is then suspended in the solution. This distillation is carried out until the desired quantity of crystallized solid in suspension is obtained. This step is carried out at a temperature between 20°C and 110°C, preferably between 30°C and 105°C, and at an absolute pressure between 2.10 ⁴ Pa and 10 ⁵ Pa, preferably.

The temperature at which the distillation is carried out determines the degree of hydration of the boron salt produced. For example, for a temperature between 20°C and 60°C, borax decahydrate Na ₂ B ₄ O ₇ .10H ₂ O is obtained. On the other hand, borax pentahydrate Na ₂ B ₄ O ₇ .5H ₂ O is obtained for a temperature between 60°C and 100°C.

According to another variant of the invention, the crystallization of the borate is carried out by cooling the concentrated basic aqueous solution of borate, so that the solubility of the boron salt in solution decreases. In this case, the solution is cooled according to a temperature gradient of 10°C to 40°C per hour. The borate suspension may be reheated to obtain partial re-solubilization of the borate and then resubmitted to cooling to reform a borate suspension, this operation constituting a crystallization cycle. Between 1 and 3 crystallization cycles may be carried out, preferably 1 to increase the purity of the crystallized borate.

These two crystallization methods, respectively distillation and cooling, allow the elimination of traces of solvent remaining in the solid towards the crystallization mother liquors.

By “traces of solvent” we mean small quantities of extraction compound, and/or modifying compound and/or diluting compound.

Furthermore, the distillation carried out during the concentration and possibly crystallization stage generates a distillate, which generally contains less than 1% by mass of traces of solvent.

This distillate is recirculated in the process and/or mixed with the basic aqueous solution during the step of bringing the basic aqueous solution into contact with the organic solvent. In fact, the distillate dilutes the concentrated basic solution or solubilizes the solid basic compound.

After the crystallization step, the borate crystal suspension can be subjected to a maturation step, the duration of which is 30 minutes to 2 hours, preferably 1 hour. This duration allows the solution to reach thermodynamic equilibrium.

To recover the hydrated boron salt crystals, a solid-liquid separation step is carried out. Solid-liquid separation methods include, but are not limited to, distillation, centrifugation and filtration.

According to a preferred embodiment of the invention, the crystallized hydrated boron salt is filtered, generating crystallization mother liquors. These crystallization mother liquors may contain less than 1% by mass of traces of solvent.

Filtration can be carried out under vacuum or under positive pressure between 10 ⁵ and 5.10 ⁵ Pa, preferably 10 ⁵ Pa, or under reduced pressure between 10 ⁴ and 8.10 ⁴ Pa.

Advantageously, the crystallization mother liquors are recirculated in the process during the step of bringing the basic aqueous solution into contact with the organic solvent. They are mixed with the basic aqueous solution during this contacting step.

According to the invention, the solid thus recovered is a hydrated borate, the formula of which may be NaBO ₂ yH ₂ O, Na ₂ B ₄ O ₇ yH ₂ O, Na ₂ B ₅ O ₈ yH ₂ O, Li ₂ B ₄ O ₇ yH ₂ O, K ₂ B ₄ O ₇ yH ₂ O, or (NH ₄ ) ₂ B ₄ O ₇ yH ₂ O, where y is between 0 and 11.

After solid-liquid separation, the recovered solid can be washed to remove all traces of solvent. In this case, the crystallized hydrated boron salt is washed with an aqueous solution and/or with the distillate from the distillation, generating wash water. This wash water may contain less than 1% by mass of traces of solvent.

The wash waters are advantageously recirculated in the process during the step of bringing the basic aqueous solution into contact with the organic solvent. They can be mixed with the basic aqueous solution during this contacting step.

The reuse of the distillate, crystallization mother liquors and/or washing water has the advantage of allowing the recovery of all aqueous flows and all of the solvent.

A drying step may be carried out after washing. In particular, drying may be carried out at a temperature between 50°C and 150°C, at an absolute pressure between 10 ⁴ Pa and 10 ⁵ Pa.

Depending on the drying temperature, hydrated or anhydrous borax will be obtained. For example, a drying temperature between 100°C and 150°C will produce anhydrous borax.

Finally, the process according to the invention makes it possible to recover more than 99% of the boron contained in the organic solvent. The solid borate produced has a purity greater than 99%.

Brief description of the figures

The manner in which the invention can be implemented and the advantages which result therefrom will become clearer from the following example of implementation, given for information purposes only and without limitation, in support of the appended figure.

There is a schematic representation of a battery of four mixer-decanters implementing the method for recovering a solubilized borate according to the invention.

There is a schematic representation of the process for recovering a crystallized borate according to the invention.

Detailed description of the figures

The steps of the process for recovering a borate dissolved in a battery of four mixer-decanters, according to the invention, are shown diagrammatically in the .

The process is carried out in a device comprising several stages, where the organic solvent and the basic aqueous solution circulate in countercurrent.

In particular, the process can be carried out in a battery of mixer-decanters or in liquid-liquid extraction columns. The number of stages of these devices is preferably between 3 and 10, preferably between 5 and 7.

The device of the is a battery of mixer-decanters (e) of four stages (a,b,c,d), of a type known per se. At the level of a first stage (a) are fed an organic solvent loaded with boric acid (11) and a borax solution (12) resulting from the recycling of the crystallization mother liquors and optionally the washing waters.

Furthermore, an aqueous solution of NaOH (13) is fed counter-currently to the fourth stage (d).

A distillate (20) obtained during the implementation of the invention is also fed counter-currently to the fourth stage (d).

The boron-loaded organic solvent (11) from the first stage (a) is then transferred to the second stage (b), then to the third stage (c) and finally to the fourth stage (d), mixed and decanted at each stage.

The aqueous phase that feeds the fourth stage (d), consisting of the aqueous NaOH solution (13), is transferred via stages (c), (b), to the first stage (a) where it combines with the borax solution (12), in each stage the aqueous and solvent phases are mixed and decanted.

The aqueous solution of alkali borate (14) is recovered at the outlet of the first stage (a). The extraction solvent (15) exits, free of boron, at the fourth stage (d).

The steps of the process for recovering a crystallized borate according to the invention are shown schematically in the .

The aqueous borate solution (14) leaving the mixer-decanter battery (e) is recovered and treated to crystallize the borate.

The aqueous borate solution (14) is first distilled (g) by increasing the temperature and/or by modifying the pressure of the medium until a borate-saturated solution and a distillate (20) are obtained. Crystallization (h) is then carried out either by distillation generating distillate (20) again, or by cooling leading in both cases to the formation of a suspension of crystallized borate.

Finally, the boron salt suspension is filtered (i), allowing the recovery of the crystallization mother liquors (21) and the crystallized borate (16). If necessary, the borate can be washed (j), this generating wash waters (22). The distillate (20) is also used to wash the borate.

The distillate (20) is reused to dilute (f) a concentrated basic solution of soda (17) to obtain the aqueous solution of NaOH (13), which will be introduced into the battery of mixer-decanters (e) for the liquid-liquid extraction of boron.

The crystallization mother liquors (21) and the washing waters (22) are mixed to form the borax solution (12) which is reintroduced into the battery (e).

Examples of embodiments of the invention

Example 1: Regeneration of boron from an organic solvent loaded with boric acid and including BEPD

In a battery of five mixer-decanters operating in counter-current, at a temperature of 50°C, the following are supplied:
– on stage 1: an organic solvent consisting of 15% by mass of 2-butyl-2-ethylpropane-1,3-diol (BEPD), 25% by mass of kerosene and 60% by mass of decan-1-ol, the boron concentration of the solvent being 4.40 g/L, with a flow rate of 100 L/h;
– on floor 5: a NaOH solution, concentrated in sodium at 9.40 g/L, with a flow rate of 50 L/h.

We recover:
– on stage 1: an aqueous solution of borate concentrated in boron at 8.78 g/L, and concentrated in sodium at 9.40 g/L, with 0.1% by mass of BEPD, with a flow rate of 50 L/h. The molar ratio Na/B is 0.5, i.e. the stoichiometry of the tetraborate ion Na ₂ B ₄ O ₇ ;
– on stage 5: an organic solvent with a residual boron concentration of less than 0.01 g/L , with a flow rate of 100 L/h.

Thus, 99.8% of the boron included in the extraction solvent is recovered.

Concentrations and flow rates are detailed in Table 1.

[Tab.1]

Example 2: Crystallization of sodium tetraborate pentahydrate or borax (Na ₂ B ₄ O ₇ •5H ₂ O) from a sodium tetraborate solution

4.7 L of aqueous borax solution, or 4.8 kg, are obtained after extraction of boron from an organic extraction solvent consisting of BEPD, kerosene and decan-1-ol, and from a sodium hydroxide solution.

The aqueous solution has a boron concentration of 8.78 g/L, a sodium concentration of 9.40 g/L. It contains 0.1% by mass of BEPD, or 4.8 g of residual BEPD.

The crystallization of borax pentahydrate from aqueous solution is carried out in the following steps:
i. distillation of the aqueous solution at 100°C until 0.7 kg of solution is obtained, with a boron concentration of 6.0% by mass (approximately 60 g/L) and 4.1 kg of distillate;
ii. cooling the solution obtained from 100°C to 65°C over one hour to crystallize the borax pentahydrate;
iii. maturation of the crystals for one hour at 65°C resulting in the formation of a suspension of borax pentahydrate;
iv. filtration of the borax pentahydrate suspension under 10 ⁵ Pa positive pressure;
v. washing the boron salt with 110 g of distilled water at 5°C;
vi. drying the boron salt in a ventilated oven at 65°C until constant mass.

We obtain:
– 0.55 kg of crystallization mother liquors comprising 3.8% by mass of boron and comprising less than 50 ppm of solvent;
– 4.1 kg of distillate comprising 0.12% by mass BEPD, or 4.8 g of BEPD;
– 130 g of wash water comprising 3.6% boron by mass;
– 106 g of borax pentahydrate with a purity greater than 99.8%;
– less than 350 mg/kg of residual organic carbon in borax;
– 24 g of borax pentahydrate impregnation water.

Of the 41.3 g of boron contained in the initial aqueous solution, 15.7 g are recovered in the form of borax pentahydrate. Thus, 38% of the boron initially contained in the aqueous phase is crystallized. The 62% of borate contained in the mother liquors are recycled at the stage of contacting the organic solvent loaded with boron and the basic aqueous solution.

The same result can be obtained by distilling the boron-laden solution at 65°C under reduced pressure, between 10 ⁴ Pa and 7.10 ⁴ Pa, until 106 g of borax pentahydrate is suspended before filtration.

The 4.1 kg of distillate contains more than 99% of the BEPD extraction compound of the initially aqueous-soluble solvent. The distillate is recycled for the production of the basic aqueous solution.

Borax decahydrate can be obtained by the same process, by concentrating the initial aqueous solution under vacuum at 60°C, and cooling it to 5°C over one hour before filtration.

The masses are detailed in Table 2.

[Tab.2]

Example 3: Regeneration of boron from a 2-propylheptanol (2PH) solvent loaded with boric acid

In a battery of six mixer-decanters operating in counter-current, at a temperature of 50°C, the following are supplied:
– at stage 1: an organic solvent comprising an extraction compound consisting of pure 2-propylheptanol, the boron concentration of the solvent being 1.5 g/L, with a flow rate of 100 L/h;
– on floor 6: a concentrated sodium NaOH solution at 12.28 g/L, with a flow rate of 10 L/h.

We recover:
– on stage 1: an aqueous solution of borate concentrated in boron at 14.9 g/L, concentrated in sodium at 12.28 g/L, and concentrated in 2-propylheptanol at 70 mg/L, with a flow rate of 10 L/h. The Na/B molar ratio is 0.39;
– on stage 6: a solvent with a residual boron concentration of less than 0.01 g/L, with a flow rate of 100 L/h.

Thus, 99.3% of the boron included in the extraction solvent is recovered.

Concentrations, flow rates and masses are detailed in Table 3.

[Tab.3]

Example 4: Recovery of borax pentahydrate (Na ₂ B ₄ O ₇ •5H ₂ O) from BEPD

In a battery of five mixer-decanters operating in counter-current, at a temperature of 50°C, the following are supplied:
– on stage 1: an organic solvent consisting of 15% by mass of BEPD, 25% by mass of kerosene and 60% by mass of decan-1-ol, the solvent has a boron concentration of 3.52 g/L and it is introduced with a flow rate of 576 L/h and a borax solution concentrated in boron at 39.7 g/L and in sodium at 42.3 g/L, resulting from the recycling of the mother liquors from the crystallization of the borax produced, the description of which follows, with a flow rate of 39.6 L/h;
– on stage 5: a concentrated sodium NaOH solution at 11.5 g/L introduced at a flow rate of 187.8 L/h. This solution is made from 3.76 kg of solid NaOH and 187.8 L of distillate.

We recover:
– on stage 1: an aqueous solution with a boron concentration of 15.4 g/L, a sodium concentration of 16.4 g/L and comprising 0.1% by mass of solvent, with a flow rate of 233.3 L/h because the conversion of boric acid into borax generates 5.9 L of water. The molar ratio Na/B is 0.5, which is the stoichiometry of the tetraborate ion Na ₂ B ₄ O ₇ ;
– on stage 5: a solvent with a boron concentration of less than 0.01 g/L, with a flow rate of 576 L/h.

Thus, 99.7% of the boron included in the extraction solvent is recovered.

The crystallization of borax pentahydrate from the aqueous solution recovered in stage 1 is carried out according to the following steps:
i. distillation of the aqueous solution recovered in stage 1 under vacuum at 65°C, during which the borax pentahydrate precipitates, until obtaining a solution with a concentration of total boron (solid boron and solubilized boron) of 73.1 g/L and sodium of 77.9 g/L, with a flow rate of 49.2 L/h and 187.8 L/h of distillate;
ii. maturation of the crystals for one hour resulting in the formation of a suspension of borax pentahydrate;
iii. filtration of the borax pentahydrate suspension under 10 ⁵ Pa of positive pressure;
iv. drying the boron salt in a ventilated oven at 65°C until constant mass.

We obtain:
– 187.8 L/h of distillate containing 0.12% solvent, which will be recycled to the battery;
– 39.6 L/h of crystallization mother liquors with a boron concentration of 39.7 g/L and sodium concentration of 42.3 g/L;
– 14.4 kg/h of borax pentahydrate comprising 5% by mass of water or 13.7 kg/h of borax pentahydrate, with less than 50 ppm of BEPD or less than 0.035 g/kg of residual organic carbon.

The borax pentahydrate obtained is more than 99.8% pure.

The crystallization mother liquors contain 44% of the total boron.

The distillate contains all of the aqueous phase soluble solvent contained in the boron solution recovered in stage 1.

The concentrations and flow rates of liquid-liquid extraction and crystallization are detailed in Tables 4 and 5.

[Tab.4]

[Tab.5]

Example 5: Recovery of borax pentahydrate (Na ₂ B ₄ O ₇ •5H ₂ O) from 2-propylheptanol

In a battery of five mixer-decanters operating in counter-current, at a temperature of 50°C, the following are supplied:
– on stage 1: an organic solvent consisting of 2-propylheptanol , with a boron concentration of 1.5 g/L, it is introduced with a flow rate of 1352 L/h and a borax solution concentrated in boron at 39.7 g/L and in sodium at 42.3 g/L resulting from the recycling of the mother liquors from the crystallization of the borax produced, the description of which follows, with a flow rate of 39.6 L/h;
– on stage 5: a concentrated sodium NaOH solution at 11.5 g/L introduced at a flow rate of 187.8 L/h. This solution is made from 3.76 kg of solid NaOH and 187.8 L of distillate.

We recover:
– on stage 1: an aqueous solution with a boron concentration of 15.4 g/L, a sodium concentration of 16.4 g/L and comprising 70 ppm by mass of solvent, with a flow rate of 233.3 L/h because the conversion of boric acid into borax generates 5.9 L of water. The Na/B molar ratio is 0.5;
– on stage 5: a solvent with a boron concentration of less than 0.01 g/L, with a flow rate of 1352 L/h.

Thus, 99.3% of the boron included in the extraction solvent is recovered.

The crystallization of borax pentahydrate from the aqueous solution recovered in stage 1 is carried out according to the following steps:
i. distillation of the aqueous solution recovered in stage 1 under vacuum at 65°C, during which the borax pentahydrate precipitates, until obtaining a solution with a total boron concentration (solid boron and solubilized boron) of 73.1 g/L and sodium of 77.9 g/L, with a flow rate of 49.2 L/h and 187.8 L/h of distillate;
ii. maturation of the crystals for one hour resulting in the formation of a suspension of borax pentahydrate;
iii. filtration of the borax pentahydrate suspension under 10 ⁵ Pa pressure;
iv. drying the boron salt in a ventilated oven at 65°C until constant mass.

We obtain:
– 187.8 L/h of distillate containing 87 ppm of solvent, which will be recycled to the battery;
– 39.6 L/h of crystallization mother liquors with a boron concentration of 39.7 g/L and sodium concentration of 42.3 g/L;
– 14.4 kg/h of borax pentahydrate comprising 5% by mass of water or 13.7 kg/h of borax pentahydrate with less than 50 ppm of 2PH or less than 0.037 g/kg of residual organic carbon.

The borax pentahydrate obtained has a purity greater than 99.8%.

The crystallization mother liquors contain 44% of the total boron.

The distillate contains all of the aqueous phase soluble solvent contained in the boron solution recovered in stage 1.

The concentrations and flow rates of liquid-liquid extraction and crystallization are detailed in Tables 6 and 7.

[Tab.6]

[Tab.7]

Lithium, potassium and ammonium tetraborates as well as sodium tetraborate decahydrate can be manufactured using the process according to the invention.

Since the solubility of these salts in water is different from that of sodium tetraborate pentahydrate, the amount of distillate, mother liquors and boron salt produced during crystallization vary accordingly.

Claims (9)

A process for recovering boron in the form of boron salt from boric acid included in an organic solvent, the process comprising the following steps:
– contacting a basic aqueous solution with the organic solvent to extract the boron in the form of solubilized borate into the basic aqueous solution;
– distillation of the basic aqueous solution loaded with borate to concentrate said solution, generating a distillate;
– crystallization of borate in the form of hydrated boron salt;
– solid-liquid separation of crystallized hydrated boron salt, generating crystallization mother liquors;
– reintroduction of the distillate and the crystallization mother liquors at the stage of bringing the basic aqueous solution into contact with the organic solvent. A process for recovering boron in the form of boron salt according to claim 1, wherein the crystallization of the borate is carried out by distillation of the concentrated basic aqueous solution of borate, or by cooling said concentrated basic aqueous solution of borate. Process for recovering boron in the form of boron salt according to claim 1 or 2, in which the contacting of the basic aqueous solution with the organic solvent is carried out in a device comprising several stages, where the organic solvent and the basic aqueous solution circulate in countercurrent. Process for recovering boron in the form of boron salt according to one of claims 1 to 3, in which the basic aqueous solution is prepared by dilution of a concentrated basic solution or by solubilization of a solid basic compound. A process for recovering boron in the form of boron salt according to claim 4, wherein the distillate dilutes the concentrated basic solution or solubilizes the solid basic compound. Process for recovering boron in the form of boron salt according to one of claims 1 to 5, in which the basic aqueous solution is introduced in a stoichiometric quantity relative to the quantity of boron salt to be produced. Process for recovering boron in the form of boron salt according to one of claims 1 to 6, in which the crystallized hydrated boron salt is washed with an aqueous solution and/or with the distillate, generating wash water. A process for recovering boron in the form of boron salt according to claim 7, in which the washing waters are recirculated in the process during the step of bringing the basic aqueous solution into contact with the organic solvent. Process for recovering boron in the form of boron salt according to one of claims 1 to 8, in which the organic solvent comprises at least one extraction compound consisting of an alcohol comprising an aliphatic or aromatic chain of 6 to 18 carbon atoms, optionally a modifying compound consisting of an alcohol comprising an aliphatic chain of 6 to 18 carbon atoms and optionally a diluting compound consisting of an aliphatic or aromatic hydrocarbon chain of 6 to 18 carbon atoms.