Purification of boric acid with ion exchange process

Abstract

The present invention relates to the process of purification of boric acid by ion exchange method. Boric acid is dissolved in hot demineralized water. The hot solution is pressure-filtered. The hot saturated solution, which is purified from water-insoluble, is passed through a column containing strong cation exchange resin, followed by a column containing weak anion exchange resin at the same temperature and cooled afterwards. The crystals settling by cooling are separated from the mother liquor, the amount of aqueous solution within them is reduced and then dried. The waste solution formed during crystallization and filtrate formed after separation of crystals from aqueous solution are mixed and used in boric acid dissolving process. The developed method enables the reduction of sodium, sulfate, chloride and iron impurities of technical grade boric acid to less than 1 ppm and is more economic and environmental friendly than current methods.

Claims

1. A boric acid purification process by ion exchange method comprising the following steps: (A) dissolving a boric acid in demineralized water at a temperature in a range of 60 to 70° C.; (B) pressure-filtering; (C) passing through a strong cation exchange resin column; (D) passing through a weak anion exchange resin column; (E) crystallizing a precipitation; (F) separating settled wet crystals from a waste solution; (G) separating crystals from the settled wet crystals and an aqueous solution; (H) drying the crystals; (I) sieving the crystals dried in step (H) and feeding coarse crystal grains obtained in step (I) back to step (A); (J) combining the waste solution and a filtrate from step (G) into a combined stream and feeding the combined stream back to step (A).

2. The boric acid purification process by ion exchange method according to claim 1, wherein the boric acid used in step (A) is dissolved by heating and mixing using a jacketed and agitated reactor.

3. The boric acid purification process by ion exchange method according to claim 1, wherein the boric acid used in step (A) is at a minimum 99.96% purity and contains a maximum of 300 ppm sulfate, 5 ppm chloride and 4 ppm iron.

4. The boric acid purification process by ion exchange method according to claim 1, wherein the boric acid used in step (A) is a saturated boric acid solution and prepared at a concentration in a range of 12% to 14% by weight.

5. The boric acid purification process by ion exchange method according to claim 1, wherein step (B) is carried out under a pressure in a range of 3 to 5 bar.

6. The boric acid purification process by ion exchange method according to claim 1, wherein step (B) is performed using a membrane filter of less than 1 micron pore size.

7. The boric acid purification process by ion exchange method according to claim 1, wherein in step (C) and step (D), a temperature of the strong cation exchange resin column and the weak anion exchange resin column is maintained at a range of 60° C. to 70° C.

8. The boric acid purification process by ion exchange method according to claim 1, wherein a boric acid solution saturated with the boric acid is used in step (C) and step (D), and the boric acid solution is passed through the strong cation exchange resin column and the weak anion exchange resin column at a flow rate of 20-25 m.sup.3/m.sup.3.Math.h (BV/h).

9. The boric acid purification process by ion exchange method according to claim 1, wherein in step (C), a jacketed column containing a strong cation exchange resin is used as the strong cation exchange resin column, and in step (D), a jacketed column containing a weak anion exchange resin is used as the weak anion exchange resin column.

10. The boric acid purification process by ion exchange method according to claim 1, wherein step (E) is carried out at a temperature range of 35-40° C.

11. The boric acid purification process by ion exchange method according to claim 1, wherein step (F) is carried out using a hydrocyclone.

12. The boric acid purification process by ion exchange method according to claim 1, wherein in step (G), centrifuge is used for separating the crystals from the aqueous solution.

13. The boric acid purification process by ion exchange method according to claim 1, wherein step (H) is carried out by means of air flow.

14. The boric acid purification process by ion exchange method according to claim 1, wherein step (H) is carried out in a temperature range of 45-55° C.

15. The boric acid purification process by ion exchange method according to claim 1, wherein the boric acid is obtained from colemanite, tinkal, kernite and ulexite minerals, and the boric acid is an industrial grade boric acid with minimum 99.96% purity, containing maximum 300 ppm sulfate, 5 ppm chloride and 4 ppm iron content.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) FIGURE. Boric acid purification process by ion exchange process.

EXPLANATION OF REFERENCES IN FIGURE

(3) A. Boric acid dissolving step 1. Boric acid 2. Demineralized water 3. Heating (A) 4. Hot saturated solution B. Pressure-filtering step 5. Pressure 6. Insoluble impurities 7. Hot filtrate solution C. Passing through strong cation exchange resin column step 8. Heating (C) 9. Cation-impurity-reduced solution D. Passing through weak anion exchange resin column step 10. Heating (D) 11. Impurity-removed solution E. Crystallizing precipitation step 12. Vacuum 13. Cooling 14. Impurity-removed crystalline solution F. Separation of the settled wet crystals from waste solution step 15. Settled wet crystals 20. Waste solution G. Separation of crystals from aqueous solution step 16. Wet crystals 21. Filtrate H. Wet crystals drying step 17. Dried crystals I. Sieving of dried crystals step 18. Boric acid with a minimum of 99.99% purity, whose main impurities are reduced below 1 ppm. 19. Coarse crystal grains from the sieve J. Combining filtrate with waste solution step 22. Combined stream

DETAILED DESCRIPTION OF THE EMBODIMENTS

(4) The process flow diagram created by the developed method is given in FIGURE.

(5) Boric acid (1) with a minimum of 99.96% purity, containing 300 ppm max. sulfate, 5 ppm chloride, 4 ppm iron, is dissolved in demineralized water (2) by heating (3) and mixing (A). 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). Micron porous membrane filters can be used for filtration. The hot filtrate (7), which is purified from not-fully-water-soluble impurities (6), is passed through a heated (8) column containing strong cation exchange resin (C). The cation-impurity-reduced solution (9) is passed through a heated (10) column containing weak anion exchange resin (D). In passing through strong cation exchange resin column (C) and passing through weak anion exchange resin column (D) steps, the solution can be purified by passing through jacketed columns. The impurity-removed solution (11) is cooled (13) and boric acid is precipitated by crystallization (E). The settled wet crystals (15) in the impurity-removed crystalline solution (14) are separated (F) from the waste solution (20). In the step of separation of settled wet crystals from waste solution, impurity-removed crystalline solution can be fed to a hydrocyclone to separate settled wet crystals. Settled wet crystals (15) are subjected to crystal-solution separation process (G) in order to reduce solution amount. The centrifugation technique can be used to remove crystals from solution. The obtained wet crystals (16) are dried (H) and the dried crystals (17) are sieved (I). Airflow can be utilized for drying during wet crystal drying step. Boric acid (18) with a minimum 99.99% purity, whose main impurity levels are reduced below 1 ppm, is obtained. The coarse crystals grains (19) coming out of the sieve are fed back to the first stage of the process namely boric acid dissolving step (A). The waste solution (20) formed during the separation of settled wet crystals from waste solution step and the filtrate (21) that formed during separation of crystals from aqueous solution step are combined (J). This combined stream (22) is fed back to the first stage of the process, the boric acid dissolving step (A).

(6) With the method developed, technical grade boric acid of 99.96% purity, which is preferably produced from colemanite mineral and which contains 300 ppm max. sulfate, 5 ppm chloride and 4 ppm iron; can be purified to a such degree where it contains less than 1 ppm of sulfate, iron, chloride and sodium. In the mentioned method, the step of dissolving boric acid in water is carried out at a temperature in the range of 60 to 70° C., and the acidic solution formed after dissolution contains boric acid between 12-14% by weight. The resulting hot saturated solution is filtered at a pressure range of 3 to 5 bar. The filtered solution is passed through respectively the column containing cation exchange resin which is maintained at a temperature of 60-70° C., with a flow rate of 20-25 BV/h and column containing a weak anion exchanging resin maintained at a temperature of 60-70° C. with a flow rate in the range 20-25 BV/h. The purified solution is cooled by cooling crystallization under vacuum at temperature range of 35 to 40° C. and the boric acid is settled by crystallization. The wet crystals are separated from the aqueous solution and dried at a temperature between 45° C. and 55° C. After sieving the dried crystals, boric acid with a purity of 99.99% min. containing less than (<) 1 ppm content of sulfate, sodium, chloride and iron, is obtained. During the process, the waste solution formed after crystallizing precipitation step and the filtrate solutions formed during separation of crystals from aqueous solution step are mixed and fed back to the first stage of the process that is boric acid dissolving step.

(7) Example: 1750 g demineralized water is transferred to a jacketed and agitated reactor and heated to 60° C. 250 g of boric acid having 99.96% purity, whose impurity content is given in Table 1, is weighed and added to the reactor and mixed for 1-2 hours. The solution, which is at 60° C. and contains 12.5% by weight boric acid is filtered at 3 bar pressure through a membrane having 1 micron and less pores. The hot filtrate solution, which is purified from water-insoluble impurities, is passed through a jacketed column containing strong cation exchange resin and jacketed column containing weak anion-exchanging resin, respectively, at 60-70° C. and at a flow rate of 25 BV/h. The solution, whose impurities are removed via purification is cooled to 35-40° C. by applying vacuum. Wet crystals that are crystallized and settled with effect of cooling, are removed from waste solution. The amount of waste solution in the wet crystals is reduced by centrifugation, which is applied for the separation of the settled wet crystals from the waste solution. The wet crystals are dried and sieved by airflow at a temperature range of 45-55° C. The waste solution coming from crystallization and the filtrate coming from the separation of settled wet crystals fromwaste solution step are mixed and fed back to boric acid dissolving step. After the process a high purity boric acid containing less than 1 ppm content of sulfate, sodium, chloride and iron impurities, is obtained. Anion-cation resin pairs used in the experimental study; Purolite Puropack PPC 100H-Purolite A100, Relite CF-Relite A100 and Amberlite IR120Na-Dowex Marathon WBA Amberlite. The results of the analysis of the products obtained are given in Table 2.

(8) TABLE-US-00001 TABLE 1 Impurity values of technical grade boric acid Impurity type Content (ppm) Na 10.81 Mg 28 Ca 21 K <2.5 Li <0.2 SO.sub.4 229 Cl 1.80 Fe 2.34 Non-soluble 27

(9) TABLE-US-00002 TABLE 2 Impurity values of high purity boric acid products obtained in experimental studies (ppm) Impurity Purolite 100H - Relite CF- Amberlite IR120Na- type Purolite A100 ReliteA100 Marathon Dowex Na 0.45 0.49 0.45 Mg <0.58 <0.61 <0.56 Ca <1.92 <1.68 2.80 K <1.16 <1.22 <1.13 Li <0.1 <0.1 <0.1 SO.sub.4 0.35 0.37 0.34 Cl 0.07 0.07 0.07 Fe 0.30 0.43 0.45

(10) The Way of Application of the Invention to the Industry

(11) The 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 forms resistance to thermal 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 due to being 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.