JPH0729100B2 - Recovery method of EDTA from plating waste liquid - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for recovering valuable EDTA from a waste liquid containing EDTA such as a chemical plating waste liquid.

[Conventional technology]

Traditionally, to recover EDTA from waste solutions containing EDTA, H ₂ SO ₄
After adjusting the pH using, the sodium salt of EDTA existing as Na ₄ Y, Na ₃ HY, Na ₂ H ₂ Y, and NaH ₃ Y was crystallized as insoluble H ₄ -Y, followed by solid-liquid separation. , Had been collected. The crystallization method at that time is to precipitate H ₄ -Y by stirring the EDTA waste liquid and the added H ₂ SO ₄ while lowering the pH to pH 1 to 2 where the H ₄ Y concentration is the minimum. Become.

[Problems to be solved by the invention]

However, in such a method, since the precipitated crystal grain size is as small as 0.05 to 0.2 mm, the solid-liquid separation in the latter stage becomes difficult, and not only the settling tank becomes large, but also the crystals can be reused. In order to prevent crystal leakage in the necessary washing and dehydration steps and improve the washing effect, an expensive acid-resistant high-speed centrifuge has to be used.

Therefore, the present invention overcomes the above-mentioned drawbacks of the prior art,
A stable, continuous and efficient precipitation of H ₄ Y crystals with a large particle size and high purity from waste liquid A simplifies the equipment for the subsequent solid-liquid separation, washing, and dehydration processes and achieves high recovery. and to provide a method of recovering EDTA that can economically recover H ₄ Y crystals at a rate.

[Means for solving problems]

According to the present invention, the problem is, H ₄ Y crystals unsaturated H ₄ Y
This can be solved by using a seed crystal after washing with a solution to remove the micronuclei attached to the crystal surface.

That is, the method of the present invention, the pH was adjusted to 3 or less by adding an acid to the waste solution containing ethylenediaminetetraacetic acid, in the recovery method of EDTA to crystallize the EDTA as the free acid, the separated H ₄ Y
It is characterized in that a part of the crystal is washed with an unsaturated liquid of H ₄ —Y and used as a seed crystal for crystallization, and the H ₄ Y crystal grown to a predetermined grain size is subjected to solid-liquid separation.

As the H ₄ Y unsaturated liquid used for washing the seed crystals, water such as industrial water is used, or the liquid temperature of the treatment liquid is raised, or the pH of the treatment liquid is adjusted to 3 or more, It is also possible to use the one in which the EDTA concentration is unsaturated.

[Action]

When crystallizing the dissolved sodium salt of EDTA as insoluble H ₄ Y, the factor that inhibits the growth of H ₄ Y crystals is that the degree of supersaturation (solution concentration-saturation concentration) locally increases in the crystallization tank. However, a large amount of surplus nuclei are generated by reaching the concentration in the unstable region. Therefore, even if H ₄ Y crystal is added as a seed crystal to the crystallization tank and left floating and crystallization is always performed in the metastable region, excess nuclei are generated in the same manner as in the above case, and the grain size is Could not be increased. The cause is
It was found that the micronuclei attached to the surface of the H ₄ Y crystal added as a seed crystal are desorbed from the crystal in the supersaturated solution and grow as nuclei.

In the present invention, if the H ₄ Y crystal is previously washed with an unsaturated liquid, the micronuclei attached to the crystal surface are dissolved and removed. Therefore, when this is used as a seed crystal, no excess nucleus is generated,
H ₄ Y crystal grows.

〔Example〕

Next, the present invention will be described in detail based on examples, but the present invention is not limited thereto.

Example 1 A plating waste liquid having an EDTA concentration (H ₄ Y) of 15,000 to 30,000 mg / l was introduced into a crystallization tank equipped with a stirrer, pH was adjusted to 1 to 3 with sulfuric acid, crystallization was performed, and then solid-liquid Separated and recovered H ₄
The grain size of Y crystal was measured.

Regarding the addition of seed crystals, solid-liquid separated H ₄ Y crystals washed with industrial water (unsaturated liquid) were returned to the crystallization tank, returned without being washed, and returned. H ₄ Y
The crystal grain size was measured. The results are shown in Table 1.

From the above experimental results, according to the present invention, the average particle size 0.5mm
It has been clarified that the above H ₄ Y crystal can be stably obtained for a long period of time and the grain size can be significantly increased as compared with the grain size of 0.1 to 0.15 mm obtained by the conventional method.

Therefore, it is easy to wash and dehydrate, there is no need to use an acid-resistant high-speed centrifuge as in the past, and it is possible to apply even a simple drainer basket made of a stainless steel net with an opening of about 0.4 mm. Became possible. Also, the recovered H ₄ Y
The crystals were of high purity and of sufficient reusable quality.

Although industrial water was used as the unsaturated liquid of H ₄ Y used for washing the seed crystal in the above-mentioned examples, the treatment liquid was used by heating or the pH was adjusted to 3 or more to adjust the EDTA concentration. Similarly, good results can be obtained even when the is saturated.

〔The invention's effect〕

According to the method of the present invention, a large particle size, high-purity H ₄ Y crystals can be continuously and efficiently recovered, and the subsequent washing,
The equipment for the dehydration step can be made efficient and simple.
Furthermore, according to the method of the present invention, since the generation of micronuclei is small, crystal leakage is extremely small and the recovery rate can be improved.
Equipment costs and collection costs can also be significantly reduced.

Claims (1)

[Claims] 1. An acid is added to a waste liquid containing ethylenediaminetetraacetic acid (hereinafter referred to as EDTA) to adjust the pH to 3 or less, and ED
EDTA that crystallizes TA as a free acid (hereinafter referred to as H ₄ Y)
In the recovery method described above, part of the separated H ₄ Y crystal was washed with an unsaturated solution of H ₄ Y to perform crystallization as a seed crystal, and the H ₄ Y crystal grown to a predetermined grain size was subjected to solid-liquid separation. A method for recovering EDTA from a plating waste liquid, which comprises: