JPS582164B2 - Reductive separation method of hexavalent chromium - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for reductively separating hexavalent chromium contained therein from an alkaline aqueous solution.

Conventionally, various methods for separating hexavalent chromium from acidic or neutral aqueous solutions are known, such as an ion exchange resin method, a solvent extraction method, and a reductive separation method.

However, no effective method for separating hexavalent chromium from an alkaline aqueous solution is known.

When chromium-containing solids such as chromium-containing sludge and laterite are leached directly or after roasting with an alkaline aqueous solution such as sodium hydroxide, some of the chromium becomes hexavalent chromium.
Cr(VI)] is eluted.

In this case, if aluminum is present, it will also be eluted as sodium aluminate.

When separating hexavalent chromium from such an alkaline aqueous solution, the ion exchange resin method and solvent extraction method, which are effective for acidic aqueous solutions, are ineffective.

The present inventors have conducted various studies on the separation of hexavalent chromium from alkaline aqueous solutions, and found that methods using metals such as zinc and iron among reductive separation methods were not effective, but when using aluminum, It has been found that hexavalent chromium in an alkaline aqueous solution is reduced to trivalent chromium and precipitated and separated as Cr(OH)3.

Although hexavalent chromium was reduced to trivalent chromium when metallic zinc was used, it did not precipitate as a hydroxide.

The metal aluminum used in the present invention is used in powder form, and its particle size is usually 30 mesh or less, and the finer the particle size, the stronger its reactivity.

To carry out the method of the present invention, aluminum powder is added to an alkaline aqueous solution containing hexavalent chromium, such as a sodium chromate aqueous solution or a mixed aqueous solution of sodium chromate and sodium aluminate, and the mixture is stirred.

By adding and stirring this aluminum, aluminum is dissolved in the alkaline aqueous solution with hydrogen generation, and sodium chromate is reduced by this aluminum and Cr(O
H) It precipitates as 3.

This reaction proceeds even at room temperature, but in order to improve the sedimentation and filterability of chromium hydroxide, it must be carried out at a temperature of 40°C or higher, usually at a temperature of 40 to 90°C.
It is best to select a temperature of 5°C.

This reaction also depends on the concentration of hexavalent chromium in the solution and free NaO.
It is not affected much by H concentration.

The reduction reaction of hexavalent chromium by aluminum in the present invention is considered to be expressed by the following formula.

Na2CrO4+Al+2H2O→Cr(OH)3+N
aAlO2+NaOH (1) According to this reaction formula, the amount of aluminum added in the present invention is 0.5 g per 1 g of chromium, but in the case of the present invention,
At least 1 g of aluminum per 1 g of chromium, preferably 1.
It was found that 5g or more was required.

That is, it is considered that the added aluminum is consumed as a by-product in the following reaction.

Al+NaOH+H2O→NaAlO2+3/2H2
(2) In the present invention, hexavalent chromium in the solution decreases linearly in proportion to the amount of metal aluminum added, but when the remaining hexavalent chromium concentration reaches about 0.5 g/l, the reduction efficiency decreases. It will gradually decrease.

However, by increasing the amount of aluminum powder added, the residual hexavalent chromium concentration can be reduced to 1 ppm or less.

According to a preferred embodiment of the present invention, a method for separating hexavalent chromium contained in an aqueous sodium aluminate solution produced in the Bayer process is provided.

In the case of the Bayer process, bauxite powder is treated with a hot solution of sodium hydroxide, and the alumina contained in the bauxite is eluted as sodium aluminate.

Next, this solution is filtered to remove red mud from the solution, and then alumina is precipitated from this solution.

Solution (mother liquor) after precipitated alumina is separated and recovered from the solution
is used again in the alumina elution process from bauxite.

In such an alumina manufacturing method, the concentration of hexavalent chromium contained in the circulating mother liquor gradually increases depending on the number of cycles.

The invention applies to the separation of hexavalent chromium contained in such circulating alkaline solutions.

In this case, the metal aluminum to be added is the reaction formula (
As is clear from 1) and (2), sodium aluminate, which is an alumina raw material, is formed by reaction with an alkali, so there is no possibility of contaminating the circulating solution.

In the present invention, the chromium hydroxide produced in the solution has good sedimentation and filterability, so it is easy to separate from the solution, and the chromium hydroxide separated and recovered as a precipitate can be used as a green pigment or Used as an abrasive.

In the present invention, if zinc ions are mixed into the alkaline solution, it becomes difficult to separate them from aluminum and chromium. Therefore, it is recommended that the zinc be separated in advance according to a conventional method before the treatment according to the present invention. Good.

Next, the present invention will be explained in more detail with reference to Examples.

Example 1 Aluminum powder (4
When 14 g of particles with a particle size of 8 mesh or less (the same applies hereinafter) was added, the reduction reaction was completed in 30 minutes, and the hexavalent chromium concentration in the supernatant was 1 g/l.

Example 2 When 2 g of aluminum powder was added to 1 liter of an aqueous solution containing 2.5 g/L of hexavalent chromium and 150 g/L of NaOH at a liquid temperature of 60°C and a reduction reaction was performed, the hexavalent chromium concentration in the supernatant was 1. It showed 5g/l.

In addition, when the same reaction was carried out by changing the amount of aluminum powder added, it was found that when the amount of aluminum powder added was 4 g, 6
When the valent chromium concentration is 0.5 g/l, 8 g, hexavalent chromium concentration is 0.2 g/l, and when 10 g, hexavalent chromium concentration is 0.05.
g/l.

Example 3 Hexavalent chromium concentration was maintained at 0.5 g/l, NaOH concentration was varied from 50 to 150 g/l, and 2 g of aluminum powder was added to 1 liter of solution at a liquid temperature of 60°C to perform a reduction reaction. However, the concentration of hexavalent chromium after reduction was 0.15 to 0.
．． 25 g/l, and no significant influence of NaOH concentration on the reaction was observed.

Example 4 Hexavalent chromium concentration 0.5 g/l and NaOH concentration 100 g
When 8 g of aluminum powder was added to 1 liter of an aqueous solution of 60° C. and a reduction reaction was carried out, the concentration of hexavalent chromium in the supernatant became 1 ppm or less.

The green chromium hydroxide produced by this reaction had good sedimentation and filterability, and could be easily separated from the aqueous solution by filtration.

Claims (1)

[Claims] 1. A method for producing hexavalent chromium, which is characterized by bringing metal aluminum into contact with an alkaline aqueous solution containing hexavalent chromium, and separating the hexavalent chromium contained therein as a precipitate of chromium hydroxide. Reductive separation method. 2. The method according to claim 1, in which metal aluminum is brought into contact with the mother liquor after alumina precipitation in the Bayer process, and hexavalent chromium contained therein is separated as a chromium hydroxide precipitate.