JPH085676B2 - A method of recovering high-purity iron sulfate from the sulfuric acid pickling waste liquid of stainless steel - Google Patents

Description

Detailed Description of the Invention [Industrial applications]

The present invention relates to a method for recovering high-purity iron sulfate having low Cr from a sulfuric acid pickling waste liquid of stainless steel.

[Prior art]

Currently, a pickling treatment with sulfuric acid is performed as a surface treatment method for stainless steel. By pickling, Fe and Cr in the steel material become H ₂ S
When it is dissolved in O ₄ and the concentration of these increases, the pickling rate decreases significantly, so it must be discharged out of the system and replaced with fresh acid. Therefore, a large amount of waste sulfuric acid solution having a relatively high H ₂ SO ₄ concentration is discharged. On the other hand, in the case of ordinary steel, the sulfuric acid pickling waste liquid is evaporated under reduced pressure to evaporate water, and the dissolved FeSO ₄ is precipitated as iron sulfate monohydrate by increasing the free-H ₂ SO ₄ concentration to 45-60%. after having allowed, which was separated from the H ₂ SO ₄ solution, H ₂ SO ₄ solution how to re-use back into the pickling tank is being performed. Further, since the separated iron sulfate monohydrate contains almost no heavy metals such as Cr, it is used as a raw material for pigments and ferrites. (See Japanese Examined Patent Publication No. 50-2879) As an alternative method, the H ₂ SO ₄ concentration is adjusted as it is by using waste sulfuric acid as it is, or by preconcentrating or adding concentrated sulfuric acid, and then cooling it to 20 ° C. or less. By doing FeSO
₄ · 7H ₂ O to separate it after allowed precipitate, the separated liquid is after adjusting the H ₂ SO ₄ concentration by addition of water, is utilized to repeatedly pickling as recovered acid. (See Japanese Patent Publication No. 47-40629)

[Problems to be Solved by the Invention]

However, in the sulfuric acid pickling waste liquid of stainless steel, a large amount of Cr is contained in addition to Fe. When this is treated by the above method, iron sulfate is contained in both the concentration method and the cooling method. To deposit. The amount of Cr contained in this is initially small, but the Cr concentration in the mother liquor in the can rises with the passage of time, and the Cr concentration contained in the crystals increases accordingly, and the mother liquor is pickled. When it is reused in the tank, there is a problem that the pickling rate will decrease, so it cannot be reused as the recovered acid, so there is no example of installing a recovery device. Therefore, although the method of treating by lime neutralization is performed,
Since the hydroxide of Cr is mixed in the generated iron hydroxide and cannot be discarded as it is from the standpoint of pollution, it is possible to make it insoluble such as solidification by cement, or to make it harmless by air oxidation and ferrite during neutralization before discarding. Currently being conducted. Therefore, as described in JP-A-63-295442, as an efficient purification method of an iron salt aqueous solution when producing high-purity iron oxide or iron hydroxide, iron or base is added to waste acid. PH
A method has been proposed in which the hydroxides produced by adjusting to 3 to 7 are separated and removed by natural sedimentation, filtration, centrifugation, or the like. In the case of this method, waste sulfuric acid containing 100 ppm or less as Cr is neutralized with iron or a base to PH of 5.0 or more, and when Cr is separated and removed as hydroxide, the Cr concentration in the filtrate is 10 ppm.
Fall to. However, if the Cr concentration in the waste sulfuric acid is several hundred ppm or more, the residual Cr will be 80 ppm or more even if the PH is neutralized to 5.0 or more. Even if the pH is 6.0 or more, 20 ppm or more of Cr remains. However, Cr in waste sulfuric acid discharged when pickling stainless steel with sulfuric acid is significantly high at 4000 ppm or more, so de-Cr is effective in neutralization with a base or neutralization with iron scrap and base. And there is a problem that it is not performed sufficiently. The present invention has been made to solve this problem.

[Means for Solving the Problems]

In the present invention, waste sulfuric acid having a high Cr concentration is first treated with a diffusion dialyzer to remove most of the free sulfuric acid to obtain a deoxidized liquid having a residual sulfuric acid concentration of several% or less. Metallic iron or mill scale is added to this deoxidation liquid, and this is heated at 50 to 100 ° C.
The solid-liquid contact is strongly effected by mechanical means such as stirring, mixing, and shaking to perform the substitution reduction reaction.
Is deposited as a metal substance. After separating and removing this by filtration or centrifugation, sulfuric acid is added to adjust the pH to 2 or less, and the adjusted Cr-free solution is directly or concentrated by FeSO.
₄ After increasing the concentration to, for example, 20% or higher, cool it to 20 ° C or lower to precipitate iron sulfate crystals, and separate this from the mother liquor to convert low Cr high-purity FeSO ₄・ 7H ₂ SO ₄ into sulfuric acid of stainless steel. It is collected from the washing waste liquid.

[Work]

The operation of each step of the present invention will be described in detail below. Most of the free-H ₂ SO _{4 in} the waste sulfuric acid is separated and removed by applying the waste sulfuric acid to a diffusion dialysis device, and Fe,
It is possible to obtain a deoxidized liquid having a low concentration of free-H ₂ SO ₄ that is left as it is, such as Cr. Next, metallic iron or mill scale is added to this, the liquid temperature is heated to 50 to 100 ° C., mechanical action such as stirring, mixing, and shaking is promoted to promote the contact action between the solid and the liquid, and Cr ions are removed. It replaces the added metallic iron or mill scale, reduces it, and precipitates and precipitates it as a metal substance to effectively remove Cr. In this case, it is preferable to add the metallic iron or mill scale to be added sufficiently. The main reaction mechanism is believed that reducing ^{Cr 3+ + Fe → Fe 2+ +} Cr of Cr ³⁺ by (1) increasing the PH due to dissolution of ^{Fe 2H + + Fe → Fe 2+} + H 2 ↑ (2) Fe. The reaction of (2) occurs even when the pH is low regardless of the acid concentration, but when the concentration of sulfuric acid is high, the precipitated Cr is redissolved, so the reaction of (1) must proceed until the pH rises. Substantially no progress. Therefore, it is desirable that the deoxidation rate in the first step be close to 1 or 2 in PH value, but increasing the deoxidation rate requires an increase in the membrane area of the dialysis device, which leads to an increase in equipment costs. , PH1 or less. Almost all of the Cr in the deoxidation solution is deposited by the substitution reduction reaction with metallic iron or mill scale, but the Cr concentration cannot be determined simply by PH based on the above reaction mechanism, but the lower limit of PH is 4 for the time being. Use as a guide. In the Cr-free treatment liquid, metallic iron or mill scale and
Since precipitates such as Cr are contained, these are filtered off, and H ₂ SO ₄ is added to the filtrate to adjust the pH to 2 or less. This is because if the treatment liquid is left as it is, dissolved Fe ²⁺
This is to prevent the precipitation of Fe (OH) ₃ because it is easily oxidized to form Fe ³⁺ . The pH adjustment liquid can be cooled to 10 ° C or below as it is or concentrated to produce FeSO 4.
₄ By cooling to 20% or higher, preferably 30% or higher, and then cooling to 20 ° C or lower, high-purity iron sulfate crystals can be precipitated and recovered, but if it is desired to increase the amount of crystal precipitation, the temperature should be 10 ° C or lower. Cool it down. On the other hand, most of the free-H ₂ SO _{4 in} the waste sulfuric acid migrates to the recovered liquid side, but Fe, Cr, etc. are slightly (3% of the concentration in the stock solution).
Since it contains only ~ 4%), there is an advantage that it can be reused for pickling by adding supplemental sulfuric acid to this and adjusting the concentration. Also, the residual Cr concentration in the third step is 20 ppm for PH4 and 1 ppm for PH5.
Cr content of FeSO ₄ · 7H ₂ in O was separated from the solution crystallized析分is 1ppm
Fe crystallized and separated from the Cr removal solution with a residual Cr concentration of 1 ppm
Cr content of SO ₄ · 7H ₂ in O becomes 0.1ppm or less.

【Example】

Examples of the present invention will be described below. Liquid composition: Free-H ₂ SO ₄ 91 g /, Fe 90 g /, Cr 6 g / waste sulfuric acid 260 / h by diffusion dialysis device (Tokuyama Soda Co., Ltd., TSD
-50-550 type). On the other hand, an appropriate amount of water 2
Liquid composition by passing 60 / h: free-H ₂ SO ₄ 22.4 g /
A deoxidizing solution of Fe70g /, Cr4.7g / 324 / h was obtained. At the same time, liquid composition: free-H ₂ SO ₄ 83.2g /, Fe3.5g /, Cr0.2g /
The recovered acid 200 was obtained. 130 kg of metallic iron is added to the deoxidizing solution,
Do not heat the liquid temperature to 85 ° C, and circulate with the pump. PH after 5 hours
After the reaction was stopped at 4, the pressure was immediately filtered with a filter press to obtain 316 kg of filtrate. The Conc-H ₂ SO ₄ 250g prevent oxidation of the ferrous added. Residual Cr in the filtrate
Was 20 ppm. Next, this liquid is divided into two equal parts, and 158 kg of a part of the liquid is divided into 0 ° C.
After cooling down to 23 kg of FeSO
₄ · 7H ₂ O was recovered. The Cr content in the crystal was 0.9 ppm. The remaining 158 kg of the filtrate was concentrated to a FeSO ₄ concentration of 30%, cooled to 20 ° C., and the precipitated crystals were centrifuged to recover 31 kg of FeSO ₄ .7H ₂ O. The Cr content in the crystal was 1.5 ppm. On the other hand, by adding Conc-H ₂ SO ₄ 60 kg and H ₂ O 45 kg to the recovered acid, liquid composition: H ₂ SO ₄ 24%, Fe 0.2%, Cr 0.01%
315 kg of the adjusted sulfuric acid solution was obtained, and this was reused for pickling again. As can be seen from the examples, according to the method of the present invention, Cr contained in the stainless steel sulfuric acid pickling waste liquid at 6000 ppm is PH,
= 4 in the Cr removal solution = 4, the Cr removal rate is 99.
7%. By the way, in comparison with the method disclosed in the above-mentioned Japanese Patent Laid-Open No. 63-295442, in Example 4 of the same,
In the case of PH = 4.2, the Cr removal rate is 42.3%, and even in the case of PH = 6.2, the Cr removal rate is 94.3%, which shows that the method of the present invention provides extremely effective Cr removal.

【The invention's effect】

The method of the present invention has the following effects. (1) For the ferrite from the sulfuric acid pickling waste liquid of stainless steel, which has been conventionally treated and disposed of by the lime neutralization method,
High-purity iron sulfate, which is useful as a pigment and has a high merit and a low Cr content, can be obtained. (2) Sulfuric acid having almost the same purity as the new acid can be recovered and reused in the pickling step, which is extremely useful for effective use of resources and prevention of pollution. (3) A very high Cr removal rate can be obtained as compared with the conventional technology.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor, Yasuo Morimoto, 3-3 Suehiro-cho, Kita-ku, Osaka City, Osaka Prefecture Daido Chemical Engineering Co., Ltd. (72) Kiyo Tatsuma, 3 Suehiro-cho, Kita-ku, Osaka-shi, Osaka No. 3 Daido Chemical Engineering Co., Ltd.

Claims (2)

[Claims] 1. A waste sulfuric acid having a high Cr concentration, which is discharged when pickling stainless steel with sulfuric acid, is treated with a diffusion dialysis device to remove free-H ₂ SO _{4 and} to remove a deoxidizing solution. In the first step of obtaining and adding metallic iron or mill scale to the deoxidation liquid,
By heating to ~ 100 ℃ and further promoting the contact action between solid and liquid by mechanical means, the second step of precipitating Cr and the liquid treated in the second step are filtered to remove the impurities of precipitated Cr. After the removal, the third step of adding sulfuric acid to the filtrate to adjust the pH to 2 or less, and cooling the pH-adjusted Cr-free solution obtained in the third step to precipitate iron sulfate crystals and separate this from the mother liquor A method of recovering high-purity iron sulfate from a sulfuric acid pickling waste liquid of stainless steel, which comprises four steps. 2. Concentrating the pH-adjusted Cr-free solution after the third step
The purity higher than that of the sulfuric acid pickling waste liquid of stainless steel according to claim 1, comprising a fourth step of cooling the FeSO ₄ concentration to 20% or more, precipitating iron sulfate crystals, and separating this from the mother liquor. A method of recovering iron sulfate.