JPS5939373B2 - Method for separating and reusing Cr(6) ions in chlorate production electrolyte - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method for extracting chromate ions and dichromate ions [hereinafter Cr(
■) Collectively called ions.

] and reuse the separated Cr(■) ions for the production of chlorate. In the electrolytic oxidation process of sodium chloride for the production of chlorate, the electrolyte is It mainly consists of sodium chlorate and sodium chloride, but C (■
) add ions.

The toxicity of VO ions to living organisms is well known. Therefore, discharging even a small amount of VO ions as waste fluid will cause major environmental problems and pollution, so Cr(
) It is necessary to take measures to prevent ions from being discharged outside the reaction system. C
An electrolytic solution containing r() ions has a characteristic yellow color, and when sodium chlorate crystals are crystallized from this electrolytic solution, Cr() ions attach to the crystallized crystals and turn yellow, causing chlorate This reduces the product value of sodium crystals. At present, these attached Cr() ions are removed by water washing and recrystallization, and in order to create a purified chlorate aqueous solution, a method is used in which the recrystallized crystals are dissolved in water again, which is a complicated process. Processes and equipment are becoming necessary. In addition, sodium chlorate and hydrochloric acid are continuously supplied to a single reaction tank to react, and chlorine dioxide and chlorine are extracted from the system along with water vapor, and solid sodium chloride is produced as a by-product, which crystallizes as water evaporates. A method of adding a palladium complex as a catalyst in order to efficiently produce chlorine dioxide in a continuous chlorine dioxide generating tank for recovering chlorine dioxide has been proposed in, for example, Japanese Patent Publication No. 55-43401 and Japanese Patent Application Laid-open No. 14296/1983. has been done.

However, when an electrolytic solution consisting of sodium chlorate and sodium chloride containing Cr() ions is supplied to this continuous chlorine dioxide generation tank and reacted with hydrochloric acid,
As the reaction progresses, water evaporates and Cr(VI) ions are concentrated and accumulated, and when the concentrated and accumulated Cr(VI) ions reach a high concentration, their boiling point rises, causing an abnormal temperature rise in the reaction tank. or cause problems such as acting as a catalyst poison for the added catalyst. In this continuous type chlorine dioxide generation tank, it is practically difficult to remove Cr() ions by washing with water or recrystallization. Also,
If Cr() ions are discharged out of the reaction system, their toxicity may cause environmental problems, and Cr(VI) ions must be newly added in the next electrolysis step, which is wasteful. Conventionally, as a method for separating Cr() ions from an electrolytic solution containing concentrated alkali metal chlorates and alkali metal chlorides, Cr() ions are separated from Cr().
) ion and remove it as a water-insoluble Cr(y) ion.

For example, in Japanese Patent Publication No. 47-7692, as a method for separating Cr() from an aqueous solution containing sodium chlorate, a divalent iron salt is added to the solution to remove Cr() ions.
A method is disclosed for reducing the r() ion and separating it as a water-insoluble precipitate of hydrous chromic oxide. However, this method reduces Cr(VD4 ions to Cr() ions, so they cannot be reused as Cr() ions.The method of separating Cr() ions and reusing them is to use anion ions. There is a method using exchange resin, but
In this method, the separation rate of Cr(VO ions was 96.5%, and Cr() ions were not completely separated. Therefore, Cr() ions were removed from the solution after electrolysis for chlorate production. It has been desired to establish a closed recycling system that separates Cr() ions to a trace amount and reuses the separated Cr() ions as an electrolyte.The purpose of the present invention is to The Cr() ions are separated from the chlorate electrolyte containing the () ions to a trace amount (specifically, a few parts per million), and the separated Cr() ions are returned to the inspection process and reused. The aim is to establish a closed recycling system.

The present inventors have conducted various studies to achieve the above object, and have found that it is effective to separate Cr() ions in the chlorate production electrolyte to a few parts per million and reuse them. perfected the method.

That is, a water-soluble barium salt is added to a chlorate production electrolyte containing an alkali metal chlorate, an alkali metal chloride, and Cr(Vl) ions to form a precipitate of water-insoluble barium chromate. The separated barium chromate was dissolved in a dilute hydrochloric acid aqueous solution and passed through a cation exchange resin column to adsorb and separate only the barium ions. taCr()
After the dilute hydrochloric acid aqueous solution containing ions is neutralized and adjusted to a desired pH, it can be used in the electrolytic process for producing chlorate. Then, the barium ions are eluted by passing an aqueous solution of alkali metal chlorate and/or alkali metal chloride through the cation exchange resin tower that has adsorbed the barium ions, and the alkali containing the barium ions is eluted. The metal chlorate and/or alkali metal chloride aqueous solution includes alkali metal chlorate, alkali metal chloride and C
r (can be used to produce a precipitate of water-insoluble barium chromate by mixing with a chlorate production electrolyte containing VD ions. In the present invention, alkali metal chlorate refers to sodium chlorate, Potassium chlorate,
Alkali metal chlorides are sodium chloride and potassium chloride.

Furthermore, water-soluble barium salts include barium chloride and barium nitrate. A flow sheet regarding the separation and reuse of Cr() ions is shown in FIG. 1, and a detailed explanation will be given below in accordance with this flow sheet.

The chlorate production electrolyte after the electrolysis process generally contains 3.2 to 5.2 mol/l of alkali metal chlorate, 1.4 to 2.6 mol/l of alkali metal chloride, and 3 Cr() ions.
．． Cr(
) To separate the ions, the electrolytic solution from electrolysis step 1 is directly mixed with an aqueous alkali metal chlorate and/or alkali metal chloride solution containing a water-soluble barium salt in step 3. In step 3, water-insoluble barium chromate may be generated, or the electrolyte may be passed through the anion exchange resin column 2 to separate most of the Cr() ions, and then water may be generated in step 3. Barium chromate, which is insoluble in water, may be produced by mixing with an aqueous alkali metal chlorate and/or alkali metal chloride solution containing a barium salt that is soluble in water.

According to the latter method, since the amount of precipitated barium taromate produced is small, the operation becomes simple. Barium chromate, which is insoluble in water, has a molecular structure of BacrO4, but Cr() ions have a molecular structure of Cr2O7-2 in an acidic region and CrO,2- in an alkaline region.
Therefore, in order to generate barium chromate precipitate, the electrolyte should be adjusted to be alkaline, and a water-soluble barium salt should be added in an amount equivalent to or more than 8 times equivalent to the number of moles of Cr() ions contained. It is preferable to mix an alkali metal chlorate and/or alkali metal chloride aqueous solution containing the above. Even if more than 8 times the equivalent of barium salt is added, the effect will not be improved.

At this time, adding a commonly used flocculant or a filtration aid will facilitate filtration. Precipitation of barium chromate When the precipitation of barium talomate produced in step 3 is mechanically removed in step 4, a chlorate electrolyte with the concentration of Cr() ions reduced to a few parts per million can be obtained. Obtainable. This chlorate electrolyte can be used as a feed solution to a chlorate crystallization process, a purified chlorate aqueous solution storage tank, or a continuous chlorine dioxide generating tank.
The P-separated barium chromate precipitate can be easily dissolved in step 5 by washing with a dilute aqueous hydrochloric acid solution of 6N or less. If the hydrochloric acid concentration exceeds 6N, it is not preferable because Cr(VI) ions are reduced to Cr() ions during dissolution of the barium chromate precipitate. By passing the dilute hydrochloric acid aqueous solution containing barium ions and Cr() ions through the cation exchange resin column 6, only barium ions can be adsorbed and separated. As a result, the dilute hydrochloric acid aqueous solution flowing out from the cation exchange resin column contains only Cr (N-ion), so in step 7 it is neutralized and adjusted to the desired pH, and after adding an appropriate amount of salt, the chloric acid The barium ions adsorbed in the cation exchange resin tower 6 can be returned to the electrolytic process 1 for salt production.On the other hand, the barium ions adsorbed in the cation exchange resin column 6 can be returned to the electrolysis process 1 using an aqueous solution of alkali metal chlorate and/or alkali metal chloride as an eluent. This eluent can be returned to the barium chromate precipitation step 3 and reused as a source of barium ions.Also, the barium ions adsorbed on the cation exchange resin column can be eluted. When an aqueous solution containing alkali metal chlorate and alkali metal chloride at the same concentration as the electrolytic solution is used as an eluent, the electrolytic solution from which Cr() ions have been separated will contain the alkali metal chlorate and alkali metal chloride. In a continuous chlorine dioxide generating tank, it is desirable that the concentrations of alkali metal chlorate and alkali metal chloride in the feed liquid remain constant. It is desirable to use a chlorate electrolyte from which Cr() ions have been separated as the feed solution.A strongly acidic cation exchange resin can be used as the cation exchange resin.In the chlorate electrolyte of the present invention, Cr() ions are separated in the crystallization process of chlorate crystals using a closed recycling method in which Cr() ions are separated to a trace amount of several parts per million and the separated Cr() ions are returned to the electrolytic process for reuse. It is possible to obtain crystals that do not adhere, and it is also possible to obtain a purified aqueous chlorate solution with a trace concentration of Cr() 7 of a few parts per million.

Furthermore, in a continuous type chlorine dioxide generating tank, it is possible to prevent the boiling point from increasing due to the accumulation of Cr() ions and the disadvantages of catalyst poisoning. Furthermore, the discharge of Cr() ions to the outside of the system is almost completely prevented, and the occurrence of environmental contamination and pollution is prevented. Next, the present invention will be explained with reference to Examples.

Example 1 3.74 mol of NaClO3/1. taken out from an electrolytic cell for chlorate production. ．． NaCl2. OO mol/1. The electrolyzed solution 101, which has a liquid composition of 3.0 x 10-29-ions/l (1600 ppm) of Cr() ions and a pH of 6.9O, is adjusted to pH 2.39 using hydrochloric acid and passed through an anion exchange resin column to form NaClO33. .74 mol/1,. NaC 12.00 mol/1,. An electrolytic solution 101 containing Cr() ions of 1.3×139 ions/l (70 ppm) was obtained.

This electrolyte contains 33.74 mol of NaClO/1. NaCl
2. 1.13× as OO moles/11 barium ions
230 ml of an aqueous solution of BaCl2 with 10-19-ions/l
Add 1 to pH 9. Adjustment to O produced a water-insoluble precipitate of BacrO4. After this precipitation is done, the Cr() ion concentration in the electrolyte is 9.6X10-59-ions/
1 (5 ppm). When NaClO3 crystals are crystallized by a conventional method from the electrolyte from which the Cr() ions have been separated, the amount of Cr() ions attached to the crystals is 0.01 ppm.
It was hot. Sawabetsu BacrO4 is 3.19 and 0
．． It was possible to dissolve it with 220ml of 5N HCl. When this dilute hydrochloric acid solution is passed through a glass column packed with 50ml of cation exchange resin Amberlite (registered trademark) IR-120-B, 9 of the barium ions contained in it are
9% could be separated. The effluent is a 220 ml aqueous solution consisting of Cr() ions 5.5 x 10'''29-ions/l and HCIO. 1..NaClO3l.O9.:f::)R/l, N
aOHO. When regenerated with 8.51 mol of NaCl/1. NaClO3l. O
9 mol/l, Cr() ion 0.03 g-ion/1.
．． NaOHO. An eluent consisting of 1 mol/l was obtained.
Cr() ion 5,5X10-2g-ion/1. H.C.
IO. The effluent of 220 ml consisting of
H was adjusted to 4.6 l mol of NaCl/1. NaC'0
31.04 mol/1. An aqueous solution of 0.039-ion/1.PH6.6 was prepared and returned to the electrolytic cell for chlorate production.In the electrolytic cell, Pt-Ir
The electrode is an anode, SUS is used as a cathode, liquid temperature is 60℃,
When electrolytically oxidized for 48 hours at a current density of 25 A/Dm2 and a current concentration of 10 A/l, 033.70 mol/1. N
aCll. 95 mol/1. Cr() ion 3.0×10
-29-ion/1. An electrolyzed solution with a pH of 6.9O was obtained. N
aClO33.74 mol/1. NaCl2. OO mol/
1. PH2. 230111 using an aqueous solution of IO as eluent
When the liquid is passed through, 95% of the adsorbed barium ions can be eluted, and NaClO33.74 mol/1. ．．
NaCl2. OO mol/11 barium ion 4.9×1
An aqueous solution consisting of 0-29-ions/l was obtained.

BaCl2 is added to this aqueous solution, and barium ion 1.
Adjusted to 13×10-19-ions/l, BacrO4
was reused to produce a precipitate. Example 2 3.70 mol/1,. NaC11.95
Mol/1. The electrolyzed solution 101 having a liquid composition of 3.0 x 10-29-ions/2 (1600 ppm) of Cr() ions and a pH of 6.9O was adjusted to pH 2.39 using hydrochloric acid and passed through an anion exchange resin column to NaClO3. 7Omol/
1. NaCl. 95 mol/1. Cr() ion 1.3
x 10-39-ions/l (70 ppm) electrolyte 10
I got 1.

This electrolytic solution contains 33.70 mol of NaClO/1. NaCl
l. 1.13×1 as 957 rays/11 barium ions
0-19-ion/10BaC12 aqueous solution 230ml
Add to pH 9. Adjustment to O produced a water-insoluble precipitate of BacrO4. When this precipitate is separated, the Cr() ion concentration in the electrolyte is 9.6×10-59-ions/
1 (5 ppm). The electrolytic solution from which the Cr() ions were separated was sent to a storage tank to be supplied to a continuous chlorine dioxide generation tank.

The continuous chlorine dioxide generating tank consists of a glass container with a reaction volume of 31. The reaction solution composition was NaClO3l. O mol/1.
．． NaCl4.8 mol/1. H.C.I.O. 17 mol/l, and the catalyst produced from palladium and glycine was 3,9
X10-4 mol/l was added. At a reaction temperature of 70° C. and a reaction pressure of 185 mmH9, an electrolytic solution from which Cr (k-on) was separated was supplied at a flow rate of 7.2 d/min. At this time, the reaction yield of the chlorine dioxide production reaction was 96.2 (Fb). The separated BacrO4 was 3.1g, and 0.5N-HCl
It was possible to dissolve in 220ml.

When this dilute aqueous hydrochloric acid solution was passed through a glass column filled with 50ml of cation exchange resin Amberlite IR-120-B, 99% of the barium ions contained therein could be separated. Effluent is Cr() ion 5.5×
10-29-ion/1. ．． H.C.I.O. 2 consisting of 5N
20 ml of aqueous solution. On the other hand, the anion exchange resin tower adsorbing Cr() ions contained 4.6 mol of NaCl/1. ．．
3 l of NaClO, 9 mol of O/11 NaOHO. l mol/
When regenerated with regenerant 8.51 consisting of 1, NaCl4,
6 mol/1. ．． NaClO3l, O9 mol/1,. Cr
() ion 0.039-ion/11NaOHO. An eluent consisting of 1 mol/l was obtained. Cr() ion5.
5×10-29-ions/1. H.C.I.O. The effluent of 220 ml consisting of 5N was mixed with the eluate of the anion exchange resin column, NaCl was added, the pH was adjusted with HCl, and NaC
l4.6 l mol/l, NaC7O3 l. O4 mol/1.
Cr() ion 0.039-ion/1. It was made into an aqueous solution with a pH of 6.6 and 8.91, and returned to the electrolytic cell for producing chlorate. In the electrolytic cell, the Pt-1r electrode is used as the anode, and the S
US as cathode, liquid temperature 60℃, current density 25A/D
m2, and electrolytic oxidation for 48 hours at a current concentration of 10 A/l,
NaClO33.70mol/1NaC11.95mol/
l, Cr() ion 3.0×10-29-ion/1.
An electrolyzed solution with a pH of 6.9O was obtained. NaClO33.7 in a cation exchange 5m1 fat tower adsorbing barium ions
O mol/1. ．． NaCPl. 95 mol/1. PH2lO
By passing 230ml of water using an aqueous solution of as an eluent, 95% of the adsorbed barium ions can be eluted,
NaClO33.7Omol/1. NaCl. 95 mol/l, barium ion 4.9X10-29-ion/l
An aqueous solution was obtained.

BaCl2 is added to this aqueous solution, and barium ion 1.
Adjusted to 13×10-19-ions/l, BacrO4
was reused to produce a precipitate. Example 3 and Comparative Example 1 NaC 1033.74 mol/1. NaCl2.
OO mol/1. Electrolysis finished solution 1 having a liquid composition of Cr() ions 1,5X10-39-ions/l, (77Ppm)
00ml, NaClO33.74mol/1. ．． NaC
l2. OO mol/l and 10 ml of an aqueous solution containing the desired amount of BaCl2 were added, and the amount of barium ions mixed with respect to the Cr() ion content was varied to generate a BacrO4 precipitate, and the concentration of the remaining Cr() ions when filtered. were measured and the results shown in Table 1 were obtained.

[Brief explanation of the drawing]

The drawing is a flow sheet for separation and reuse of Cr() ions.

Claims (1)

[Claims] 1. Alkali metal chlorate, alkali metal chloride and C
A water-soluble barium salt is added to an electrolyte for producing chlorate containing r(VI) ions in an amount equivalent to or more than the number of moles of Cr(VI) ions present. A precipitate of Cr(
VI) Obtain a chlorate aqueous solution from which ions have been separated, and the filtered barium chromate is dissolved in a dilute hydrochloric acid aqueous solution of 6N or less, and passed through a cation exchange resin column to adsorb and separate barium ions. (VI) A method for separating and reusing Cr(VI) ions in a chlorate production electrolyte, characterized in that the dilute hydrochloric acid aqueous solution containing ions is used in an electrolysis process for chlorate production after neutralization. 2. Barium ions are eluted by passing an alkali metal chlorate and/or alkali metal chloride aqueous solution as an eluent through the cation exchange resin tower that has adsorbed the barium ions. An aqueous solution of alkali metal chlorate and/or alkali metal chloride containing barium ions is mixed with a chlorate production electrolyte containing alkali metal chlorate, alkali metal chloride and Cr(VI) ions and added to water. A method according to claim 1, which produces a precipitate of insoluble barium chromate. 3. The alkali metal hydrochloride and alkali metal chloride aqueous solution passed as an eluent through the cation exchange resin tower adsorbing barium ions has the same concentration as the chlorate production electrolyte. 3. The method according to claim 2, wherein the aqueous solution is a chloride. 4 Alkali metal chlorate, alkali metal chloride and C
After passing the chlorate production electrolyte containing r(VI) ions through an anion exchange resin tower to separate most of the Cr(VI) ions, Cr(VI) containing barium salts soluble in water is extracted. 2. The method according to claim 1, wherein an amount equivalent to or more than the number of moles of ions is added and mixed to form a precipitate of water-insoluble barium chromate in alkaline conditions. 5. The method according to any one of claims 1 to 4, wherein a water-soluble barium salt is added and mixed in an amount equal to or less than 8 times the number of moles of Cr(VI) ions present.