JP5250486B2 - Purification process for steel manufacturing wastewater - Google Patents

Description

  The present invention relates to a purification method for steel manufacturing wastewater, and more particularly to a purification method that can more efficiently treat oil and organic matter in rinse washing wastewater in the steel manufacturing industry.

  In the production process of the steel manufacturing industry, a large amount of water is used for cooling, degreasing, cleaning for the purpose of removing rust and scale, etc., and wastewater containing various components is generated. For example, an alkaline liquid used in an alkaline degreasing process in a rolling process using rolling oil contains a large amount of organic substances due to oil, surfactants, and the like. This alkaline solution is generally circulated and reused, but when repeatedly used, SS (floating substance) such as iron is contained and the quality deteriorates. For this reason, it is regularly replaced, and the entire amount is discarded as alkaline dump drainage. On the other hand, a large amount of rinse cleaning wastewater generated in the rinse cleaning process performed after the alkaline degreasing process also includes oil and organic substances resulting from the alkaline liquid. Usually, the oil concentration in wastewater (hereinafter referred to as oil) is indicated by the amount of n-Hex (normal hexane) extract, while the organic matter concentration is indicated by the COD value (chemical oxygen supply amount). In the above-described alkaline dump drainage in the steel manufacturing industry, the oil content is about 600 to 4,500 mg / L, and the COD is about 1,500 to 4,500 mg / L. Even in the rinse cleaning wastewater, the oil content is 10 to 50 mg / L. L and COD show values of about 15 to 40 mg / L.

  On the other hand, from the standpoint of environmental protection, it regulates the discharge of wastewater containing environmental pollutants into public water areas as they are, and there are emission control values for each environmental pollutant. Although the oil content and COD value in the various alkaline cleaning wastewaters described above are also subject to regulation, in recent years the importance of environmental protection has been recognized more than ever, and emission regulations have been further strengthened. It has become. For example, the current treated water discharge reference value in a closed sea area is set to a very strict treated water discharge reference value with COD <10 ppm and the oil content of 3 ppm or less. For this reason, the alkaline cleaning wastewater as described above in the steel manufacturing industry is subjected to purification treatment so that the oil content and the COD value are less than or equal to this emission regulation value, and then the treated water is discharged into public water bodies. .

  As one of the purification methods for alkaline washing wastewater in steel production that has been performed with respect to the conventional treated water discharge reference value, there is a two-stage agglomeration treatment. Purifying wastewater by connecting. Specifically, first, an aggregating treatment is performed by a combination of an inorganic flocculant (for example, Al-based or iron-based one) and an organic flocculant, and the agglomerated water is again supplied to the inorganic flocculant and the organic flocculant. By aggregating with an aggregating agent, the oil and organic matter in the alkaline washing wastewater are aggregated in a good state, and the agglomerates are mainly floated under pressure and separated by solid-liquid separation to be removed.

  The above-mentioned conventional two-stage agglomeration treatment method can reduce the oil content of treated water treated with alkaline washing wastewater to 10 mg / L or less and COD to 20 mg / L or less. The effluent standard value can be observed at the station. However, for example, when stricter effluent standards are applied, such as establishments facing closed waters, it is difficult to stabilize the oil content and COD value of treated water below the treated water discharge standard value. There was a problem. In contrast, the present applicants can reliably and economically reduce the oil content and COD value in the treated water below the emission regulation value when the alkaline cleaning wastewater discharged in the steel production is purified. A method for purifying steel manufacturing wastewater has been proposed (see Patent Document 1).

JP 2007-252969 A

  When a large amount of rinse washing wastewater generated in the rinse washing process performed after the alkaline degreasing process is purified by the above method, in establishments located in general sea areas where the water quality standards are relatively mild, the treatment is performed in good condition. The treated water can be maintained below the discharge standard value. However, in establishments located in closed waters, it is necessary to satisfy the stricter treatment water discharge standard values described above. It is difficult to always carry out the process stably, and further processing may be required to release it. In the case of the above method as well, the purification treatment is basically performed by agglomeration by the aggregating agent and removal of the agglomerates. At that time, the optimum conditions for efficient purification treatment are found, and in particular, the treatment It is desirable that the oil content of water is less than 2 ppm, for example, 1 ppm or less, and can be made extremely clean. Further, since the rinse washing wastewater is large, development of more economical, simple and efficient treatment technology is demanded.

  Therefore, the object of the present invention is to allow the rinse washing wastewater discharged from the steel manufacturing process to be purified more efficiently, more easily and reliably, the oil content and the COD value are always stable, It is providing the purification processing method of the steel manufacture waste water which becomes.

The above object is achieved by the present invention described below. That is, the present invention is a method for purifying rinse washing wastewater from an alkaline degreasing process in the steel manufacturing industry, wherein the raw water to be treated is at least separate from the rinse washing wastewater, relative to the rinse washing wastewater. was added to and mixed with waste water containing magnesium oxide, magnesium concentration in the raw water, on a molar basis, to be in the range of ^{0.05 × 10 -3 ~0.4 × 10-3mol /} L Provided is a method for purifying steelmaking wastewater, which is prepared and characterized by adding a flocculant to the raw water and stirring, and then solid-liquid separation of the agglomerates by pressure flotation.

Moreover, the following are mentioned as a preferable form of this invention. The said steel manufacture waste water purification processing method which performs the said stirring at a slow speed | rate of Gt = 30,000-120,000 after rapid stirring. The above steel production wastewater purification method, wherein 20-40% of the treated water is returned to the inlet of the treatment tank when the agglomerate is subjected to pressure levitation treatment. Wastewater containing the magnesium oxide comprises a surface treatment waste water discharged from the iron and steel manufacturing process, cleaning acid waste water, or, at least one of neutralizing wastewater discharged by neutralizing the washing acid waste water A method for purifying the steel manufacturing wastewater described above.

  According to the present invention, it is possible to more efficiently and stably purify the rinse cleaning wastewater discharged from the steel manufacturing process, and to control the oil content and COD value in the treated water more easily and stably. Provided is a method for purifying steel production wastewater that can be made clean at a value below the value. According to the present invention, the above-described excellent effect can be obtained by a very simple means of adding and mixing other waste water into the rinse washing waste water, and its practical value is extremely high.

Relationship between magnesium concentration in raw mixed water and oil concentration in treated water. The relationship between the slow stirring conditions of raw mixed water and the COD concentration in treated water.

  Hereinafter, the present invention will be described in more detail with reference to preferred embodiments. As a result of intensive investigations to solve the above-described problems of the prior art, the present inventors have determined that other wastewater such as rinse-washed wastewater, for example, surface treatment wastewater discharged from the steel manufacturing process, at a specific rate. As a result of adding and mixing and performing purification treatment using the mixture as raw water, it was found that the treatment efficiency was remarkably improved as compared with the case where the rinse-washed wastewater was treated alone as raw water. More specifically, when drainage other than rinse washing wastewater such as surface treatment wastewater is added so that the magnesium concentration or calcium concentration in the raw water is within a specific range, the treatment efficiency Has been found to be significantly improved, leading to the present invention. As will be described later, according to the present invention, when the rinse washing wastewater is treated, it is possible to stably obtain an extremely clean treated water having an oil content of 1 ppm or less and a COD of 10 ppm or less, whereas magnesium in raw water When the concentration or the calcium concentration is not adjusted, even if the treatment is performed under the same conditions, the oil content is about 5 ppm and the COD is about 20 ppm, and there is a clear significant difference in the treatment efficiency.

In the present invention, the waste water containing magnesium component or calcium component, which is different from the rinse washing waste water, with respect to the rinse washing waste water 100 on a molar concentration basis, the magnesium concentration or calcium concentration is 0.05 × 10 5. ^{-3 to} 0.4 × 10 ⁻³ mol / L, more preferably 0.1 × 10 ^{−3 to} 0.3 × 10 ⁻³ mol / L. Purify wastewater as raw water. According to the study by the present inventors, when the concentration of magnesium or calcium in the raw water is less than 0.05 × 10 ⁻³ , the effect of the addition is small, and the effect cannot be obtained stably. There is a case. On the other hand, if the concentration of magnesium or calcium in the raw water is higher than 0.4 × 10 ⁻³ mol / L, the coagulability tends to be deteriorated, leading to an increase in the coagulant injection rate. In addition, since the amount of waste water other than the rinse water in the mixed raw water increases, properties other than the concentration of magnesium or calcium may affect the raw water, and it is necessary to change the treatment condition settings. There are also practical problems.

  The present inventors consider the reason why the treatment efficiency is improved by setting the magnesium concentration or the calcium concentration in the raw water in the above-described range when treating the rinse washing waste water. By constituting the raw water as in the present invention, magnesium oxide and the like in the wastewater added to the rinse washing wastewater, and particles such as oil droplets in the rinse washing wastewater are neutralized by charge, and there is no charge repulsion, It is thought that intermolecular attraction due to Van der Waals force is superior to charge repulsion, and molecular assembly, that is, aggregation is facilitated.

  The following are mentioned as another waste_water | drain containing a magnesium component or a calcium component different from the rinse washing | cleaning waste_water | drain used and mixed with the rinse washing | cleaning waste_water | drain used by this invention. For example, surface treatment wastewater discharged from a steel manufacturing process and acid cleaning treatment in a steel sheet rolling process, which occurs when a zinc steel sheet is subjected to chromium-free surface treatment or surface insulation coating treatment of a magnetic steel sheet Acid cleaning waste water discharged at the time, or neutralized waste water neutralized with an alkaline agent such as lime for stabilizing the pH. One type of these waste waters may be added, or two or more types may be added. What is important in the present invention is to adjust the magnesium concentration or calcium concentration in the raw water within the specific range defined in the present invention by adding these waste waters. Among the above-mentioned, the surface treatment waste water mentioned above containing a small amount of oil and containing an appropriate amount of magnesium oxide component or calcium component is preferable. That is, depending on the concentration, in order to make the magnesium concentration in the raw water within the range defined in the present invention, for example, about 3 to 8 with respect to the rinse washing wastewater 100 on a volume basis, more preferably 5 The desired raw water can be obtained by adding and mixing the surface treatment wastewater at a ratio of about ˜6. As described above, the present invention can be achieved by simply adding a simple operation of adding and mixing a small amount of other wastewater to the rinse water that is the raw water without changing the basic purification process. It is a very economical method that can improve. Considering the influence on the treated water, it is particularly preferable that the magnesium and calcium components are contained as solid calcium carbonate and magnesium oxide.

  Furthermore, the present inventors have conducted a detailed study on the optimum flocculation conditions and procedure when raw water is used as the above mixed solution. As a result, it was found that it is preferable to use an inorganic flocculant and a polymer flocculant in combination as the flocculant to be used. As the inorganic flocculant, an aluminum-based or iron-based flocculant can be used. More specifically, polyaluminum chloride (hereinafter abbreviated as PAC) or ferric chloride can be used. As the polymer flocculant, it is preferable to use a strong anionic polymer flocculant rather than a nonionic polymer flocculant. Examples of commercially available products include KEA-455 (manufactured by Nippon Steel Environmental Engineering Co., Ltd., ionic strength = 2 to 3 CEQ) for nonionic products, and KEA-545 (Nittetsu Environmental Engineering Co., Ltd.) for strong anionic products. Manufactured, ionic strength = 38-42CEQ) and the like. For example, the amount of PAC used may be about 100 to 200 mg per liter of raw water, and the amount of the polymer flocculant used may be about 1 to 2 mg per liter of raw water.

  Furthermore, in the present invention, after these flocculants are added to the raw water, the oil and organic matter in the raw water are sufficiently agglomerated, and then a purification treatment is performed by separating the agglomerates by pressure floating treatment. At that time, it is preferable to perform the treatment under the following conditions because the aggregation efficiency and the separation efficiency can be further improved. After adding the flocculant to the raw water, first, it is rapidly stirred in a paddle type stirrer at 80 to 150 rpm, more preferably in a flash mixer for several minutes. And after that, it is good to stir slowly for several minutes under the condition of Gt = 30,000 to 120,000, more preferably Gt = 30,000 to 90,000. Furthermore, if the stirring is stopped and the mixture is allowed to stand for several hours and the aggregate is separated by the pressure levitation treatment, cleaner treated water can be obtained. By doing in this way, the treated water whose oil content is 1 ppm or less and COD is 10 ppm or less can be obtained stably.

  In the present invention, the agglomerates are subjected to solid-liquid separation by pressure levitation treatment. In that case, the treated water pressurization method is preferable to the raw water pressurization method. Specifically, it is preferable that 20 to 40%, preferably about 30% of the obtained treated water is returned to the treatment tank and pressurized. According to the study by the present inventors, in this way, the oil concentration of the treated water is significantly reduced compared to the case where the pressurized water flotation treatment is performed with the raw water pressurizing method that treats the treated water without returning it. Can be made.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples.
<Raw water adjustment>
The surface treatment wastewater in the coating treatment step containing magnesium oxide was added to the rinse washing wastewater discharged from the cold rolling step, and nine types of wastewaters having different magnesium concentrations were prepared as raw water. At this time, the rinse washing waste water having a pH of 9.2, SS of 28 mg / L, oil of 23 mg / L, and T-COD of 18 mg / L was used. As the surface treatment waste liquid, one having properties of pH 10.6, SS 260 mg / L, oil content <0.5 mg / L, and T-COD 40 mg / L was used. The raw water was adjusted using the above-mentioned surface treatment waste liquid and its diluted solution so that the volume-based mixing ratio was about 5 for the waste water to be added to the rinse washing waste water 100. And the magnesium density | concentration (x10 < ^-3 > mol / L) of obtained mixed raw water is 0, 0.05, 0.1, 0.15, 0.2, 0.3, 0.4, 0.5. And different raw water was adjusted in 0.6 and 9 stages.

<Aggregation test>
A jar test was performed using each raw water adjusted as described above. Specifically, a 1 L beaker is used for the jar test, 820 cm ^{3 of} each raw water is added, 150 mg / L of PAC as an aluminum flocculant, and KEA-545 as a strong anionic polymer flocculant. 2 mg / L was injected and agglomeration was performed. The agglomeration treatment was carried out under conditions of rapid stirring at 100 rpm for 5 minutes and then slow stirring at 50 rpm for 5 minutes. After the treatment, the oil content of each treated water was measured. Table 1 and FIG. 1 show the correlation between the magnesium concentration in the mixed raw water and the measurement result of the oil content in the treated water. As can be seen from Table 1 and FIG. 1, the magnesium concentration in the mixed raw water is 0.05 × 10 ^{−3 to} 0.4 × 10 ⁻³ mol / L, more preferably 0.1 × 10 ^{−3 to} 0. It was confirmed that the range of 3 × 10 ⁻³ mol / L was effective in improving the processing efficiency. In addition, as a result of examining the calcium concentration in the mixed raw water, it was confirmed that there was the same effect as magnesium.

<Test on aggregation conditions>
Next, the mixed raw water having a magnesium concentration of 0.2 × 10 ⁻³ mol / L, which gave a preferable result in the coagulation test, was examined for coagulation treatment conditions. Specifically, the jar test was performed in the same manner as described above except that the conditions of the slow stirring were changed by changing the number of revolutions of the stirrer. As a result of measuring the COD of the treated water obtained after the test, it was found that the COD value of the treated water was different depending on the conditions of slow stirring, and the treatment efficiency was significantly different. That is, as shown in Table 2 and FIG. 2, Gt = 30,000 to 120,000, and more preferably, the mixture is gently stirred for several minutes under the condition of Gt = 30,000 to 90,000. Has been confirmed to be effective in improving the aggregation removal efficiency.

<Test on solid-liquid separation conditions>
The conditions of the step of subjecting the agglomerates to pressure-flotation and solid-liquid separation were examined. When agglomerates in raw water that had been agitated by adding a flocculant were subjected to pressure levitation treatment, the effect of solid-liquid separation while returning a part of the treated water to the treatment tank again was verified. Specifically, the conventional method (raw water pressurization method) in which the aggregate in the raw water is pressurized and floated without returning the treated water and the addition of the aggregate while returning 30% of the treated water to the treatment tank. A comparison was made with the treated water pressurization system that performs pressure levitation treatment. At that time, the pressurizing pressure was set to 5 kg / cm ² , which is the same as conventionally performed. As a result, in the treatment performance, the oil concentration in the treated water by the raw water pressurization method was 3 mg / L, whereas in the treated water pressurization method, it was 1 mg / L, and its effectiveness could be confirmed.

<Confirmation test for actual drainage>
(Raw water adjustment)
On the basis of volume, a beaker test similar to that described above was performed using the rinse-washed wastewater 100 discharged from the cold rolling process and the surface treatment wastewater 5 containing magnesium oxide as mixed raw water. The wastewater was sampled three times on different days, mixed to prepare raw water, and the obtained mixed raw water was tested. The magnesium concentration of these mixed raw waters is surely within the range of 0.1 × 10 ^{−3 to} 0.3 × 10 ⁻³ mol / L.

(Test results)
When the properties of the drainage water were examined each time, the rinse washing wastewater had a pH of 8.8 to 9.4, an SS of 13 to 50 mg / L, an oil content of 15 to 30 mg / L, and a T-COD of 15 to 31 mg / L. L. Further, the surface treatment drainage liquid had pH of 10.5 to 10.8, SS of 160 to 410 mg / L, oil content <0.5 mg / L, and T-COD of 29 to 67 mg / L. The resulting treated water has a pH of 7.0 at any time, SS is <1.5 mg / L at all times, and the oil content is <0.5 mg / L at the first time and at the second time, respectively. 0.6 mg / L and the third time were 0.5 mg / L. The T-COD was 10 mg / L for the first time, 7.9 mg / L for the second time, and 7.9 mg / L for the third time. For comparison, when the same test was performed using only rinse washing wastewater as raw water, the pH was 7.0, the oil content was 5.0 mg / L, and the T-COD was 16.0 mg / L. These facts indicate that even in actual wastewater whose drainage characteristics fluctuate, if the rinse-washed wastewater is treated by the method of the present invention, the oil content in the treated water is clearly compared with the case where the rinse-washed wastewater is treated as raw water. And T-COD can be reduced, and the effect can be obtained stably.

Claims (4)

A method for purifying process the rinsing waste water from the alkaline degreasing step in the steel manufacturing industry, raw water to be processed, with respect to the rinsing waste water, separate from the said rinsing waste water, containing at least magnesium oxide drainage was added and mixed, those magnesium concentration in the raw water, on a molar basis, was adjusted to 0.05 × 10 ^-3 ~0.4 × range of 10 ^-3 mol / L A method for purifying steelmaking wastewater, comprising: adding a flocculant to the raw water and stirring the mixture, followed by solid-liquid separation of the agglomerates by pressure flotation.   The method for purifying steelmaking wastewater according to claim 1, wherein the stirring is performed at a slow speed of Gt = 30,000 to 120,000 after rapid stirring.   The method for purifying steelmaking wastewater according to claim 1 or 2, wherein when the agglomerates are subjected to pressure levitation treatment, 20 to 40% of the treated water is returned to the inlet of the treatment tank. Wastewater containing magnesium oxide is surface treated waste water discharged from the iron and steel manufacturing process, cleaning acid waste water, or, according to at least one of neutralizing wastewater discharged by neutralizing the washing acid waste water Item 4. A method for purifying steel manufacturing wastewater according to any one of Items 1 to 3.

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