JPS594378B2 - Manufacturing method of high purity sodium sulfate - Google Patents

Description

[Detailed Description of the Invention] In recent years, there has been remarkable progress in flue gas desulfurization technology as an environmental protection measure due to regulations on the emission of sulfur oxides into the atmosphere. The treatment of immobilized substances is becoming a major problem, and there is an urgent need to develop recovery methods that can be used effectively in various fields.

As a result of research on a useful by-product recovery method that can be adopted in the desulfurization process, the present inventors have produced sodium sulfate, especially neutral anhydrous sulfuric acid, which can be used in detergent builders and food additive grades. We succeeded in establishing a technology that can do this.

Conventionally, one of the flue gas desulfurization methods has been an absorption process in which flue gas containing sulfur oxides is blown into an aqueous sodium hydroxide solution to convert the sulfur oxides into sodium sulfite or acidic sodium sulfite, and an oxidation process in which air or ozone is blown into this aqueous solution. process, a purification process to remove heavy metals contained in flue gas, a crystallization process to concentrate and crystallize sodium sulfate, and finally a process to separate and dry the crystallized product from the aqueous solution. A manufacturing method has been adopted.

However, the sodium hydroxide aqueous solution used in the absorption process is
In most cases, it is an aqueous sodium hydroxide solution obtained by the diaphragm method, which inevitably contains chlorate ions.Furthermore, chlorate ions remain almost undecomposed even if the pH is lowered in the oxidation process. In the crystallization process, a double salt is formed with sodium sulfate, resulting in a decrease in the purity of anhydrous sodium sulfate.

Incidentally, anhydrous sodium sulfate containing 30 PPM or more of sodium chlorate is not suitable as a builder for detergents or as a food additive grade.

That is, an object of the present invention is to provide a method for producing high-purity sodium sulfate that is suitable for the above-mentioned uses and which reduces the amount of contaminated sodium chlorate as much as possible.

Reducing agents such as hydrazine and ferrous salts are usually used to decompose chlorate ions, but in the case of hydrazine and ferrous salts, treatment tanks for these different types of chemicals must be installed and treatment solutions must be adjusted. In addition to requiring equipment, it is undesirable to use expensive chemicals such as hydrazine, and in the case of ferrous iron,
This requires an extra iron removal process, which tends to become more complicated.

The present inventors installed a chlorate ion decomposition tank immediately after the oxidation process, and used sulfite ions or acidic sulfite ions obtained from the outlet of the absorption process before or after adjusting the pH to low (acidic) to increase the reaction rate. It has been found that chlorate ions can be almost completely decomposed by adding an appropriate amount of the aqueous solution containing them.

Note that the pH of the oxidation step can be either acidic or basic, so it is conceivable to adjust the pH to the acidic side and place the chlorate ion decomposition step before the oxidation step, or to make it an integrated step.
A considerable amount of acid such as sulfuric acid used to adjust the acidity is consumed to convert sulfite ions into acidic sulfite ions, which is undesirable as it significantly lowers the basic unit of acid such as sulfuric acid.

Now, the aqueous solution in which the chlorate ions have been decomposed is subjected to a purification process as necessary, and then sent to a second oxidation process to convert residual sulfite ions and/or acidic sulfite ions into sulfate ions, thereby producing a high-purity sodium sulfate aqueous solution. I was able to do it.

That is, the gist of the present invention is a method for producing sodium sulfate from an aqueous sodium sulfate solution containing a few sodium chlorides, in which sulfite ions or acid sulfite ions are added to the aqueous sodium sulfate solution, chlorate ions are decomposed under acidic conditions, and then chlorate ions are decomposed under acidic conditions. The method for producing high purity sodium sulfate is characterized in that sulfite ions or acid sulfite ions remaining in the liquid are oxidized.

Next, the method of the present invention will be explained in detail.

Boiler flue gas containing sulfur oxides is passed through a sodium hydroxide aqueous solution produced by the diaphragm method to form an aqueous solution containing sodium sulfite, acidic sodium chloride, and a small amount of sodium chlorate, and most of the Air or ozone is blown into the solution to oxidize the sulfite ions (rarely acidic sulfite ions) to sulfate ions, then transferred to a decomposition tank, lowered the pH to 4 or less with sulfuric acid, heated preferably to 40°C or higher, and then A part of the absorption liquid obtained in the absorption process is introduced into the
The amount is 803 (or H3O3-) mol introduced/C in the decomposition tank.
lO3-mol is 3 mol/1 mol or more, preferably 5 mol/mol
It is preferable to introduce it in an amount of 1 mol or more.

Further, it is preferable that the pH of the decomposition tank be maintained at 4, and it is preferable to use sulfur dioxide gas or sulfuric acid for adjustment.

The higher the temperature of the decomposition tank, the better the decomposition efficiency, and it is not preferable for the temperature to drop below 40°C.

If chlorate ions are no longer observed while stirring the decomposition tank liquid, it is moved to the second oxidation step, but since flue gas often contains heavy metals, carbon, etc., it is desirable to go through the purification step once.

In the second oxidation step, residual sulfite ions and other reducing ions are oxidized with air or ozone and almost completely converted to sulfate ions.

After going through the above steps, the pH is adjusted to about 7, and high purity neutral anhydrous sodium sulfate can be produced through normal concentration, separation, and drying steps.

Next, the effects of the present invention will be shown with examples.

Example Boiler flue gas containing sulfur oxides 200,000 N7i/
The desulfurization liquid obtained by introducing Hr exhaust gas into a commercially available diaphragm method caustic soda aqueous solution containing sodium chlorate and desulfurizing it (generation rate 51/hr, composition Na2
8032.1 mol/, l, Na2S 040.
2 mol/hr, NaC1038
The liquid obtained at the outlet has a pH of 7.5, a liquid temperature of 75°C, and a Na2SO35X10-
5 mol/l, Na2SO42,3 mol/l, NaClO
37×10 mol/11 others.

This liquid was introduced into the chlorate ion decomposition tank, and the desulfurization liquid was
Feed and mix at i/hr, 98% H2SO4
to pH 2.0 to 2.5 and temperature to 70 to 80℃.
The liquid composition at the outlet of the decomposition tank is N
a25OsO1035mol/tlN a2S 042
．． 3 mol/11 NaClO31 x 10-5 mol/l, indicating that most of the chlorate ions were decomposed.

This treated solution is then led to a purification process, where the pH is adjusted to 8 to 11 with a pure sodium hydroxide aqueous solution, and heavy metals are removed.
Purification was performed using a precoat filter, and this purified liquid was introduced into the second oxidation step.

As in the first oxidation step, the sulfite ions in the solution were oxidized using air to obtain a purified sodium sulfate solution with a sulfite ion concentration of 1 x 10-4 mol/l at a rate of about 5-/hr. Ta.

The pH of the aqueous solution was adjusted to about 7 with 98% concentrated sulfuric acid,
It was led to a crystallization step, concentrated 15 times at 500C, and the precipitated neutral anhydrous sodium sulfate was continuously extracted and separated and dried. As a result, a neutral product with a concentration of sodium chlorate IPPM or less that can be used as a food additive was obtained. Anhydrous sodium sulfate is about 1
．． Obtained at a rate of 5 tons/hr.

Note that the common salt contained in the sodium hydroxide was not crystallized in the crystallization step, and could be separated and removed as an aqueous solution.

Claims (1)

[Claims] 1. In the method of producing sulfuric acid from an aqueous solution of sodium sulfate containing sodium chlorate, sulfite ions or acidic sulfite ions are added to the aqueous solution of sodium sulfate, the chlorate ions are decomposed under acidic conditions, and then the solution is A method for producing high purity sodium sulfate, which comprises oxidizing residual sulfite ions or acidic sulfite ions.