JPS62847B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for purifying sodium sulfate containing fluorine impurities. Sodium sulfate is a well-known chemical with a variety of uses. This substance is obtained as a compound of sulfuric acid and sodium hydroxide, but in recent years, it has often been recovered as a byproduct of waste gas treatment containing sulfur oxides due to regulations on sulfur oxide emissions. In the production of such recovered sodium sulfate, the waste gas often contains fluorine, and this fluorine is absorbed together with sulfur oxides into the alkaline solution used for waste gas treatment, resulting in the recovery of sodium sulfate. Fluorine impurities are mixed into the crystal. The presence of such fluorine impurities significantly reduces the commercial value of sodium sulfate. Generally, fluorine ions dissolved in an aqueous solution are removed by precipitation as calcium fluoride. Suitable calcium sources include calcium sulfate and calcium hydroxide. However, it has been found that the method of defluoridation treatment by adding these calcium compounds to an aqueous sodium sulfate solution results in significant contamination of the sodium sulfate.
For example, in order to reduce the fluoride ion concentration to 0.1g/or less by adding calcium sulfate to an aqueous solution containing 400g/of sodium sulfate and 0.95g/fluoride ion, the concentration of calcium ions in the aqueous solution will be 0.27g/. , the sodium sulfate aqueous solution becomes contaminated with calcium salts. The present inventor conducted various studies on methods for purifying sodium sulfate by removing fluorine impurities from an aqueous sodium sulfate solution containing fluorine impurities without contaminating the solution.
The present invention was achieved by discovering that when an aluminum compound is added to an aqueous solution containing the above, most of the fluorine ions present in the solution are converted into poorly soluble sodium fluoroaluminate (Na ₃ AlF ₆ ) and precipitated. It is something. In order to precipitate almost all the fluorine ions in the aqueous solution in the form of sodium fluoroaluminate, it is important that the concentration of sodium sulfate in the aqueous solution is 50 g/min or more. If the amount is slightly less than 50 g, the solubility of sodium fluoroaluminate becomes high, and fluorine ions and aluminum ions remain in the aqueous solution, making it impossible to achieve the purpose of purifying sodium sulfate. Since sodium sulfate itself is a neutral salt, its aqueous solution naturally exhibits neutrality. But as mentioned above
The aqueous solution obtained by alkali treatment of waste gas containing sulfur oxides such as SO ₂ and SO ₃ inevitably contains sodium sulfite and sodium sulfate, and the liquid properties of the treatment liquid vary. For example, if the treatment liquid is acidic, teeth
Various sodium salts such as NaHSO ₃ , NaSO ₃ , NaHSO ₄ and Na ₂ SO ₄ are mixed together, and if the treatment liquid is alkaline, it contains Na ₂ SO ₃ , Na ₂ SO ₄ and free alkali. In order to recover sodium sulfate from the treated liquid of waste gas containing sulfur oxides, sulfite is oxidized by air blowing and stirring, and if acidic salts are present, NaOH is used, and if free alkalis are present, it is oxidized. It is necessary to neutralize each substance with sulfuric acid to convert them all to sodium sulfate, and this is usually done. Therefore, in order to substantially precipitate the fluorine ions in the liquid as sodium fluoroaluminate according to the present invention, after these pretreatments, an aluminum compound must be added to the aqueous sodium sulfate solution at a concentration of 50 g or more. It is achievable if Specifically, when carrying out the present invention, ^SO2-4 , _HSO ^- ₄ , ^SO2-3 _, ^HSO-
The final concentration of sodium sulfate is calculated from the value of ₃ based on the amount of S, and the raw material solution is adjusted so that the concentration (herein referred to as converted concentration) is 50 g/or more. When the solution is neutralized with a dilute aqueous solution of sodium hydroxide or sulfuric acid, the salt concentration of the finished solution is slightly diluted.
The converted sodium sulfate concentration needs to be somewhat higher than 50 g/. If the concentration is 50g/or less, it can be concentrated by evaporation. The raw material aqueous solution whose concentration has been adjusted in this way becomes an aqueous solution containing 50 g or more of sodium sulfate by neutralization and/or oxidation treatment as described above. By adding an aluminum compound to this aqueous solution, fluorine components such as F ^- and SiF ^2- in the solution can be substantially precipitated in the form of sodium fluoroaluminate. The timing of adding the aluminum compound does not necessarily have to follow the above order.
Even if all or part of the neutralization step and oxidation step are performed after adding the aluminum compound, the final concentration of sodium sulfate in the solution is 50 g/or more and the pH of the solution is in the range of 5.5 to 9. As long as it is in the neutralization range, sodium fluoroaluminate can be effectively precipitated. The aluminum compound to be added must be selected from those that do not contaminate the aqueous sodium sulfate solution. For example, Na ⁺ , such as aluminum sulfate, aluminum hydroxide, sodium aluminate,
Compounds composed of Al ³⁺ , AlO ⁻ ₂ , OH ⁻ , _{SO 2-4} ^and materials containing alumina (Al ₂ O ₃ ) are suitable. These compounds may be hydrates, and may contain some impurities that precipitate in the neutralization region. Depending on its solubility, the aluminum compound can be added in the form of an aqueous solution, a fine powder, or a suspension in water. In the case of an aluminum compound that is sparingly soluble in water or sodium sulfate solution, it is convenient to conduct preliminary experiments in advance to determine the optimum amount to be added. In the case of aluminum sulfate, aluminum hydroxide, sodium aluminate, etc. that are soluble in water or sodium sulfate aqueous solution, add the amount theoretically calculated from the amount of Al required to generate sodium fluoroaluminate based on the fluorine content. Then, the fluorine concentration in the solution is
It can be reduced to about 0.03g/. Even if more aluminum compound is added than the theoretical value, no disadvantage will occur. Rather, in the final pH range of 5.5 to 9, excess aluminum precipitates in the form of poorly soluble aluminum hydroxide, so aluminum contamination can be almost ignored.
The presence of excess aluminum makes the production of sodium fluoroaluminate more complete, and a favorable defluorination effect can also be expected. Since the solubility of the precipitated sodium fluoroaluminate in water or an aqueous sodium sulfate solution becomes relatively large when the pH is 5.5 or less or 9 or more, the preferable neutralization range should be a pH of 5.5 to 9.
Furthermore, since this salt has a high solubility in an aqueous sodium sulfite solution, if sulfite ions are present, it is necessary to sufficiently oxidize it by known means such as air blowing. In carrying out the present invention, the treatment temperature in the sodium fluoroaluminate production step and the precipitate separation step is preferably 65° C. or lower. 65℃
Above this, the solubility of sodium fluoroaluminate increases, which affects the effect of removing fluorine impurities. Since the precipitated sodium fluoroaluminate, aluminum hydroxide, etc. are all difficult to settle, practically known flocculants are conveniently used. In the above, the present invention has been mainly explained regarding the defluoridation purification of sodium sulfate, which is a by-product of the treatment of waste gas containing sulfur oxides. It is clear that the present invention can also be applied to sodium sulfate crystals having an extremely low fluorine content by concentrating a sodium sulfate aqueous solution that has been defluorinated by the method of the present invention. The purification method of the present invention will be explained in more detail by the following Examples and Comparative Examples. Examples 1 to 6 Sodium sulfate 400g/, fluoride ion 0.95
14.3 g, 43 g and 185 g of solid aluminum sulfate were added and dissolved in each 10 g/g/aqueous solution. This amount of aluminum sulfate added corresponds to 1, 3, and 13 times the theoretically calculated amount of sodium fluoroaluminate (Na ₃ AlF ₆ ) production. These solutions are kept at 60°C and adjusted to pH 6 by adding sodium hydroxide. After stirring for 1 hour, a flocculant was added to precipitate the precipitate, and the fluorine ion and aluminum ion concentrations in the supernatant were measured. For comparison, 34 g, 68 g, and 102 g of calcium sulfate, corresponding to 1, 2, and 3 times the theoretical amount, were added to each of the 10 aqueous solutions with the above compositions, and the supernatant liquid was treated in the same manner as above. The composition was measured. The results are shown in Table 1.

[Table] Examples 7 to 11 Sodium sulfate concentration is 30, 50, 100, respectively.
Prepare 10 aqueous solutions of 200 and 300g/, each with a fluorine ion concentration of 0.3g/, and add solid aluminum sulfate to each of these aqueous solutions.
Add and dissolve 13.5 g (equivalent to 3 times the theoretical amount), adjust the pH of the liquid to 7, and process in the same manner as Examples 1 to 6, and measure the fluorine ion and aluminum ion concentrations of the supernatant liquid. did. A comparison in Table 2 shows that the sodium sulfate concentration should be 50 g/min or more.

[Table] Example 12 13.5 g of aluminum sulfate (equivalent to 3 times the theoretical amount) was added as a solid to an aqueous solution containing 100 g of sodium sulfate and _0.3 g of SiF ^2-6 fluorine ion, and the pH was adjusted. The fluorine ion and aluminum ion concentrations of the supernatant obtained by adjusting to 8.5 and treating in the same manner as Examples 1 to 6 are as follows:
They were 0.030g/ and 0.002g/. Example 13-15 Acidic sodium sulfate 400g/, fluorine ion
1.5 g, 6.75 g and 13.5 g of aluminum sulfate as a solid were added to each of the 10 aqueous solutions containing 0.05 g/l. This addition amount corresponds to 2 times, 9 times, and 18 times the theoretically calculated amount. In addition to adjusting the pH of each of these solutions to 7, they were treated in the same manner as in Examples 1 to 6, and the fluorine ion and aluminum ion concentrations of the supernatants were measured. The results are shown in Table 3.

[Table] Examples 16-17 Sodium acid sulfite 75g/, fluorine ion
Divide the aqueous solution containing 0.3g/ into two, and add aluminum hydroxide to one sample A in an amount three times the theoretically calculated amount for sodium fluoroaluminate production.
Keep at ℃, adjust pH to 7 with sodium hydroxide, and cool to 1.
Stir for hours. Then, air was blown in with stirring to oxidize the sulfite, the precipitate was settled by adding a coagulant, and the resulting sodium sulfate solution was concentrated to obtain anhydrous sodium sulfate crystals. Another sample B for comparison was neutralized with sodium hydroxide (PH7)
Thereafter, air was blown into the solution for oxidation, and the resulting aqueous sodium sulfate solution was heated and concentrated to obtain crystals of anhydrous sodium sulfate. Regarding sample A, the fluorine ion concentration and aluminum ion concentration of the supernatant liquid of the solution obtained by neutralization treatment and the solution after oxidation treatment after addition of aluminum hydroxide, as well as the concentration of aluminum ions obtained from samples A and B. The ion content of each of the anhydrous sodium sulfate crystals was measured. The results are shown in Table 4.

[Table] Example 18 Sodium acid sulfite 75g/, fluorine ion
After adding sodium aluminate (NaAlO ₂ ) to an aqueous solution containing 0.3 g of
The temperature was maintained at 0.degree. C., the pH was adjusted to 7 with sodium hydroxide, and stirring was continued for an additional hour. The analysis results of the supernatant liquid of this sample and each supernatant liquid of the sample that was subsequently subjected to an oxidation process by blowing air were as follows. Before oxidation treatment F (g/) 0.24 Al (g/) 0.023 After oxidation treatment F (g/) 0.029 Al (g/) 0.002

Claims (1)

[Claims] 1 Contains at least one of fluorine-containing sulfuric acid, acidic sulfuric acid, sulfurous acid, and sodium salt of acidic sulfite as impurities, and the sodium sulfate concentration calculated based on the amount of S in the liquid is at least 50.
The method consists of adding an aluminum compound to an aqueous solution with a concentration of A process for the purification of fluorine-containing sodium sulfate, which consists in removing fluorine-containing ions as a precipitate in the form of sodium fluorine aluminate (Na ₃ AlF ₆ ).