PURIFICATION OF BORIC ACID Technical Field of the Invention

This invention relates to a process for boric acid purification. The developed method relates to the technical field of inorganic chemistry and describes all steps of the process for removing inorganic impurities in boric acid.

State of the Art

The methods developed for the purification of industrial boric acid, called technical grade, are grouped into two main groups as“Recrystallization” and“Adsorption on Ion Exchange Resin”.

In U.S. Pat. No. 2113248, the analysis of commercially produced boric acid indicates that there is arsenic, heavy metal, chlorine, sulfate and a small amount of borax as impurities. In this method, pH value is adjusted by adding strong inorganic acid solution to concentrated boric acid solution, which is prepared at high temperature, and then the crystals are removed by cooling. In the study, 136 kg technical grade boric acid is dissolved in 568 kg hot water at 93 °C. The solution is filtered and then pH of the solution is adjusted to 3-4 by adding sulfuric acid solution. The solution is cooled to 30 °C, the boric acid is crystallized, centrifuged, and the crystals are washed with cold distilled water. In U.S. Pat. No. 5084260, technical grade boric acid, which has a purity of 99%, is dissolved in water and purified to 99.99% by a single-stage recrystallization process. In the first stage, technical grade boric acid is dissolved in water in the range of 88 °C to 92 °C and saturated solution containing 18 to 22% by weight boric acid is prepared. Following filtration, saturated solutions are crystallised by cooling to 40 °C under vacuum. After the solution is sent to thickener, the wet crystals are separated by centrifugation and dried. After separation of the crystals, the 8% boric acid and impurity-containing mother liquor is purified within the columns containing strong cation exchange resin and weak anion exchange resin and then back-fed to the dissolution process. In CN Pat. No.104743564, industrial boric acid is used as raw material. The saturated solution prepared at a temperature of 30 °C is passed through a strong cation exchange resin followed by a strong anion exchange resin, respectively. After evaporation at 90 °C, the crystals are separated from the solution by cooling crystallization. The separated crystals are vacuum filtered, washed and dried.

In Pat. No. CN101412519, high purity boric acid is prepared by ion exchange and recrystallization. Boric acid solution between 50 °C and 60 °C is passed through a column containing strong acidic cation exchange resin and the metal ions are removed. The solution is cooled to room temperature and the boric acid is crystallized. In Pat. No. CN104386704, unsaturated solution of technical grade boric acid is dissolved by heating at a temperature of 50-60 °C. The hot solution is quartz sand filtered and then passed through an acid-based mixed bed ion exchanger column at a flow rate of 8 ~ 10 ml_ / min. After the ion exchange process, the solution is heated to a temperature of 40 - 55 °C and filtered through a microporous membrane, cooled, centrifuged and boric acid is obtained. On the other hand, the crystal is washed and dried in vacuum filter to give the second pure boric acid.

Pat. No. CN105347353 provides a method of preparing high purity boric acid. Industrial boric acid with 95% purity and higher is added in the main solution containing inorganic acid and alcohol and dissolved in the temperature range of 80-95 °C for 1 to 2 hours. It is cooled and centrifuged to obtain wet boric acid crystals. The mother liquor is purified by passing through a cation exchange resin column and an anion exchange resin column, respectively, and the purified solution is used in the crystal washing process.

The Technical Problem that The Invention Aimed for Solving Boric acid production is based on the reaction of borate minerals such as tinkal, colemanite, cernite, ulexite with strong inorganic acid solutions. The impurities, resulting from the mineral and the acid solution used in the process, pass to the solution and remain in the boric acid structure after the drying phase. Alkali metal oxides, calcium, magnesium, iron, sulfate and chlorine are the main impurities. Boric acid, which contains high amounts of impurities, cannot be used in the production of TFT-LCD panel glass and in nuclear power plants. The main impurities specifically undesired in the boric acid that is preferred in the production of TFT -LCD panel glasses are alkali metal oxides and sulfates. Alkaline metal oxides such as sodium, potassium and lithium are dispersed to the liquid crystal by diffusion, leading to instability and deterioration of the TFT structure. Sulfate content transforms to sulfur dioxide gas (SO2) in a short time by the effect of high temperature and causes a rough structure on the glass surface by forming bubbles. In nuclear power plants, the impurities contained in boric acid, which are used as neutron absorbers, have a corrosive effect on the cooling lines and reduce the efficiency of neutron absorption.

In order to prevent such problems, it is necessary to purify industrial boric acid before use. In previous studies, boric acid with 99.99% and higher purity was prepared by recrystallization and ion exchange methods (US5084260, US21 13248, CN104743564, CN101412519, CN104386704, CN105347353).

In the purification method developed in US. Pat. No. 5084260, the saturated solution prepared from technical grade boric acid is crystallized by cooling and then boric acid crystals are separated from the mother liquor by thickening and centrifugation. The impurities that passed to the mother liquor were discharged from the process by adsorbing them to the ion exchange resins.

The type and amount of impurities vary depending on the boron mineral used in boric acid production (tinkal, colemanite, kernite, ulexite), the inorganic acid used, the purification method and the efficiency of the equipment in the purification process. While initial impurity values determine the entire purification process, the sulfate content in the product can be high and variable. Since the impurity content of the technical boric acid used in the method developed in the patent developed US5084260 is not specified, the type and amounts of the initial impurities optimized are unclear. In addition, a process step about the removal of water-insoluble impurities by the crystal washing step is not described. It has been proved by modeling and experimental studies within the context of the invention that boric acid, containing < 1 ppm sulfate, calcium and magnesium, cannot be obtained without crystal washing process. Furthermore, it is stated that the total amount of impurities in the pure boric acid obtained in U.S. Pat. No. US5084260 is stated to be below 100 ppm while the chemical impurity content is not stated in detail. In the boric acid products preferred for the LCD sector and for the nuclear field as well, it is particulary desired that the sulfate impurities be below 1 ppm for the prevention of bubble formation in the produced glasses.

In US. Pat. No. 21 13248, borax containing technical grade boric acid was dissolved in water, the solution was filtered, and cooled using sulfuric acid. The technical problem in this method is the inability of recovering the post-crystallization solution, which contains boron and impurities not compatible for enviromental discharge.

In US. Pat. No. CN104743564, the saturated boric acid solution prepared at 30 °C was passed through ion exchange resin columns and the crystals obtained after cooling to 20 °C were subjected to washing. The low solubility difference suggests that the rate of boric acid production will be low. It was also stated that boric acid was dried at 90 °C. In industrial applications, the drying temperature of boric acid is between 40 ~ 50 ° C and high temperature drying leads to decomposition.

The technical problem in Pat. No. CN101412519 is that 99.9999% purity is guaranteed without anion removal from boric acid. In boric acid, anionic compounds such as sulfates and chlorides, , are the major impurities along with cations and must be removed in order to reach high purity. The type and amount of impurities in boric acid, which were dissolved at the beginning of the process and purified, were not explained.

In Pat. No. CN104386704, it was reported that high purity boric acid was obtained using 10-12% boric acid solution, yet the type and amount of impurities present in the purified boric acid and the product purity as well, are not mentioned. Continuous use of water is required since it is not stated whether the mother solution is cycled back to re-dissolving following crystallization or not. This increases the cost of the process.

The technical problem in Pat. No. CN105347353 is that the inorganic acids added to the solution during dissolving lead to the accumulation of chloride, sulfate, fluoride and nitrate impurities in boric acid. Likewise, the alcohols used in dissolving lead to the increase of organic contaminants in boric acid. When the results of the analysis are evaluated, it is seen that the purity of the boric acid with a minimum of 95% purity is increased to 99.99% and the amount of anionic impurities is uncertain.

With the process developed; sulfate, chloride, ammonium, iron, sodium, magnesium, calcium, potassium and lithium impurities of technical grade boric acid, having max. 300 ppm sulfate, are reduced below 1 ppm. Since the concentration of the boric acid used is high (17.5%), the rate of pure boric acid production in unit time is about 1 .5 to 2 times higher than those of state-of-art resin studies. In addition, the designed process is environmentally-friendly and economical since it makes the re-use of waste solutions possible.

Description of Figures

The flow diagram of the purification process designed to achieve the object of the present invention is shown in the appended form.

Figure 1 . Block flow diagram of boric acid purification process.

Explanation of References in Figures

A. Boric acid dissolving step

1. Boric acid

2. Demineralized water

3. Heating (A)

4. Hot saturated solution

B. Pressure-filtering step

5. Pressure

8. Impurities not fully soluble in water

7. Hot filtrate

C. Crystallization step

8. Vacuum

9. Cooling

10. Crystalline solution

D. Seperation of wet crystals in crystalline solution step

1 1. Settled wet crystals

12. Waste solution formed in separation process

E. Crystal washing step

13. Washed crystals

20. Waste solution formed in washing process

21. Combining

F. Wetting crystals step

14. Demineralized water being at maximum room temperature

15. Crystalline solution

G. Separation of crystals from aqueous solution step

18. Filtrate solution

17. Wet crystals

H. Crystal drying step

18. Dried crystals

I. Sieving dried crystals step

19. Boric acid having a minimum 99.99% purity

J. Mixing waste solutions step

22. Heating (J)

23. Combined waste solutions K. Passing the combined waste solutions through ion exchange resin columns step

24. Heating of columns (K)

25. Hot solution purified from anion and cation impurities

28. Coarse crystal grains from the sieve

Disclosure of the Invention

The process flow diagram created by the developed method is given in Figure 1.

Boric acid (1 ) containing 300 ppm max. sulfate is dissolved in demineralized water (2) by heating (3) and dissolved (A) by mixing. In boric acid dissolving step, heating and mixing can be performed in a jacketed and agitated reactor having demineralized water. Hot saturated solution (4) is filtered (B) by using pressure (5). Filters having micron porous membrane can be used for filtration. The hot filtrate (7) separated from impurities that are not fully soluble in water (6) is crystallized (C) by cooling (9) under vacuum (8). The settled wet crystals (1 1 ) present in the crystalline solution (10) are separated (D) from the waste solution (12) formed in the separation process. In order to separate wet crystals from the solution, wet crystals may be settled by feeding the crystalline solution to a hydrocyclone. The settled wet crystals (1 1 ) are washed (E) after the separation from crystalline solution. Staged washing by counter-current principle on a vacuum belt filter can be performed for washing step of the settled wet cystals. The washed crystals (13) are wetted (F) by mixing with demineralized water being at maximum room temperature (14). The crystalline solution (15) is subjected to separation (G) of crystals from aqueous solution. Centrifuge technique can be used to separate crystals from solution. The filtrate solution (16) leaving separation of crystals from aqueous solution step (G), is fed onto the wet crystals (1 1 ) that settles during wet crystal separation from crystalline solution. The wet crystals (17) obtained during crystal separation from solution step are dried (H) and the dried crystals (18) are sieved (I). In the drying step, airflow can be used to dry the wet crystals. Boric acid (19), having minimum 99.99% purity and less than 1 ppm content of sulfate, alkali metals, chlorides and iron, is obtained. The coarse crystal grains (26) coming out of the sieve are fed back to the first stage of the process, namely boric acid dissolving step (A). The waste solution that is formed in separation of wet crystals in crystalline solution (12) and waste solution that is formed in the crystal washing step (20) are combined together (21 ) and mixed (J) while heating (22). The heated combined waste solutions (23) are purified by passing through heated (24) strong cation exchange resin and weak anion exchange resin columns, respectively (K). The hot solution, purified from anion and cation impurities (25), is fed back to the dissolving step (A), which is the first stage of the process. In step K where the heated combined waste solutions are passed through columns, strong cation exchange resin made of macroporous styrene divinylbenzene copolymer and weak anion-exchange resin made of divinylbenzene crosslinked macroporose polystyrene can be used.

With the method developed, technical grade boric acid containing maximum 300 ppm sulfates can be purified so that its sulfate, alkali metals, iron and chloride content fall below 1 ppm. In the aforementioned method, the mixing of technical grade boric acid with demineralized water step is carried out in the temperature range of 90 °C to 95 °C and the acidic solution, formed after dissolution, contains 17 % to 18 % by weight of boric acid. The resulting solution is filtered under a pressure range of 3 to 5 bar. During the cooled crystallization under vacuum step, the solution is cooled to a temperature in the range of 35 to 45 °C and the boric acid is crystallized. The wet crystals are separated from the mother liquor and the separated crystals are washed.

The solution used in the washing process is saturated with boric acid at room temperature and it is about 1/3 of the wet crystal weight. The washed crystals are mixed with demineralized water being at maximum room temperature, to bring the crystal percentage in solution to 45 % to 50 % by volume. After separation from aqueous solution, the wet crystals are dried at a temperature in the range of 45-55 °C.

After sieving the dried crystals, boric acid, with less than 1 ppm content of sulfate, alkali metal, chloride and iron, is obtained. Over the course of the process, the waste solutions formed during washing and separation steps are mixed and heated to a temperature in the range of 60 - 80 °C. The combined hot waste solution is purified by first passing through strong cation exchange resin column and then through weak anion exchange resin column, that are at 60 - 80 °C, at a flow rate in the range of I Q- 15 BV/h. The hot solution, whose impurities are removed, is fed back to the first step of the process, namely the boric acid dissolving step.

Example: 6 kg of demineralized water is transferred to a jacketed and agitated reactor and heated to 90 °C. 1.273 kilograms of boric acid, which is produced from colemanite mineral and which has 99.96% purity with an impurity content as given in Table 1 , is added to the reactor. It is dissolved at a constant temperature of 90 °C. The hot saturated solution containing 17.5% by weight of boric acid, is filtered through a filter containing a porous membrane of less than 1 micron under 3 bar pressure. During filtration, inorganic impurities such as calcium sulfate and clay, which are not fully soluble in water, are removed from the solution by being withheld on the membrane filter. A vacuum is applied to the reactor where the filtrate at a temperature of 90 °C is collected, the solution is cooled to 40 °C and boric acid is crystallized. The settled crystals are separated from the mother liquor by vacuum filtration. The wet crystal slurry remaining on the sieve is washed under vacuum. The solution used in the washing process is saturated with boric acid at room temperature and it is about 1/3 of the wet crystal weight. In the final stage, demineralized water is added to the washed crystals to bring the crystal percentage in the solution to 50 % by volume, followed by centrifugation to separate wet crystals from the solution. The wet crystals are dried by airflow at a temperature of 50 °C. The analysis results of the high purity boric acid obtained after the sieving step are given in Table 2. The coarse crystal grains coming out of the sieve are fed back to the boric acid dissolving step. The waste solutions formed during vacuum-filter washing and post-crystallization separation processes are combined and heated to a minimum temperature of 60 °C in the column and mixed. The combined hot waste solution is passed through jacketed columns having strong cation exchange resin (Amberlite IR120H, copolymer of styrene-divinyl benzene) and weak anion exchange resin (Purolite A100-macroporous polystyrene crosslinked with divinylbenzene) respectively; at 70 °C with a flowrate of 10 BV/h . The solution, whose anion and cation impurities adsorbed on the resin, is cycled back to the first stage of the process, namely boric acid dissolving step.

Table 1. Impurity values measured in industrial boric acid produced from colemanite mineral

Impurity Value (ppm)

Na 10.81

Mg 28

Ca 21

K < 2.5

Li <0.2

S04 229

Cl 1 .80

Fe 2.34

Non-solubles 27

Table 2. Impurity values measured in the developed high purity boric acid

Impurity Value (ppm) Na < 1

Mg < 1

Ca < 1

K < 1

Li < 0.1

S04 < 1

Cl < 0.4

Fe < 1

NH ₄

< 0.5

Non-solubles < 5 The Way of Application of the Invention to the Industry

Boric acid, purified by the method developed in the context of the present invention, can be used in the production of TFT-LCD panel glasses and in nuclear power plants. The reason for the use of boric acid in the production of TFT-LCD panels is that it provides resistance to heat and mechanical impact by forming a network within the glass structure, thereby creating resistance to scratching and chemical wear. In addition to increasing the transparency and optical properties of the glass, it also reduces the melting temperature and thereby reduces production costs. Boric acid is used in nuclear plants since it is a water-soluble and chemically stable neutron absorber. It is included in the main cooling lines for the control of nuclear fusion speed in pressurized water reactors (PWR). It allows the reactor operator to get control values in the reactor for a longer time. It helps to minimize corrosion and damage to the parts in contact with cooling water.