KR0128133B1 - Method for treatment of wastewater generated during stainless manuracturing - Google Patents

Abstract

Process for purification of waste water from stainless manufacture, which decreases the quantity of sludge and concentration of residualfluorine greatly, is composed of i)pouring 100,000-200,000 ppm of magnesium hydroxide into a first reaction tank, ii)adjusting to pH 8.0-9.0 with caustic soda in a second reaction tank, iii)putting 500-1,000 ppm of calcium hydroxide and 300-700 ppm of alum followed by adjusting to pH 7.0-7.5 with sulfuric acid in a third reaction tank, iv)pouring an aggregating agent to form a floc in a fourth reaction tank.

Description

Wastewater Treatment Method in Stainless Steel Manufacturing Process

1 is a process chart of the conventional method.

2 is a process diagram of the present invention.

3 is a graph showing the pH change according to the drug injection.

The present invention relates to a method for treating wastewater generated in a stainless steel manufacturing process. The stainless steel manufacturing process uses heavy acids such as hydrofluoric acid, sulfuric acid, and nitric acid for surface treatment, and contains a lot of heavy metals. Since wastewater is a strong acid, discharge of water without treatment causes water pollution. Therefore, it is essential to purify the wastewater generated in the stainless manufacturing process (hereinafter referred to as stainless wastewater).

The conventional stainless wastewater treatment method is the same as FIG. That is, in FIG. 1, the wastewater enters the first and second reaction tanks through the adjustment tank, in which the lime is added in the first and second reaction tanks. By adding slaked lime, the stainless wastewater is neutralized, with the heavy metals becoming insoluble. Insoluble solids are precipitated in the gravity settling tank, and the sludge is discharged to the outside through the sludge storage tank and dehydrator. On the other hand, the supernatant from the settling tank is introduced into the third reaction tank. At this time, since the pH of the supernatant is high, hydrochloric acid is added and neutralized, and a polymerizer (coagulant) is added. When the suspended solids are floc in the floc generating tank by the polymer, they are sent to the settling tank for precipitation. The supernatant of the settling tank is discharged through a sand filter. At this time, the back washing water (or washing water) of the sand filter goes back to the neutralization tank.

However, according to the conventional method, there is a problem that a lot of sludge occurs and the fluorine concentration in the wastewater exceeds the legal standard value (15 ppm).

An object of the present invention is to solve the drawbacks of the conventional method, to provide a method in which sludge is generated less and the fluorine concentration is treated below the legal standard.

Hereinafter, the present invention will be described.

The process of the present invention is as shown in FIG.

Referring to the present invention according to FIG. ₂ , 100,000-150,000 mg / l of Mg (OH) ₂ is added to the wastewater introduced into the first reactor after the adjustment tank, and Na (OH) is added to the pH of 8.0 in the second reactor. Adjust it to -9.0. At this time, the stainless wastewater is neutralized, and the insoluble solids generated in this process are sedimented and separated in the gravity settling tank, and the precipitated sludge is discharged to the outside through the sludge storage tank and the dehydrator.

On the other hand, the supernatant from the settling tank is introduced into the third reaction tank, 500-1000ppm calcined lime and 300-700ppm of Alum (Alum) was added and stirred, after adjusting the pH to 7.0-7.5 with sulfuric acid and stirred again. In the fourth reaction tank, a polymer flocculant (polymer) is added and stirred, and when the suspended solids are floc in the floc production tank by the polymer, they are sent to the precipitation tank for precipitation. The supernatant of the settling tank is discharged through a sand filter.

The difference between the method of the present invention and the conventional method is that in the conventional method, slaked lime is added to the first and second reaction tanks, whereas in the present invention, Mg (OH) _{2 is added} to the first reaction tank and Na (OH) is added to the second reaction tank. ), And in the conventional method, while adding HCl in the third semi-bath, in the present invention, slaked lime, alum and sulfuric acid are added.

In the present invention, the use of Mg (OH) ₂ in place of Ca (OH) ₂ , which is a conventional medicine, is performed in the first reactor for the following reasons. Figure 3 shows the pH change according to the NaOH, Ca (OH) ₂ dose. According to FIG. 3, below pH 7.5, the addition of NaOH and Mg (OH) ₂ showed greater neutralization capacity for stainless wastewater than the addition of Ca (OH) ₂ . In particular, Mg (OH) 2 has a higher neutralization capacity than Ca (OH) ₂ because of its low molecular weight. That is, Mg (OH) ₂ is 58.3, while Ca (OH) ₂ is 74.1, and Mg (OH) ₂ has a neutralization capacity of 1.3 times greater than Ca (OH) ₂ when the same amount is added.

Also, in FIG. 3, the difference in neutralization capacity by the addition of Mg (OH) ₂ and Ca (OH) ₂ was found to be the largest between pH 4.5-7.5. On the other hand, neutralization of the stainless wastewater with alkali generates precipitates. In neutralization with calcination, CaF ₂ is generated while CaSO ₄ is generated, whereas when neutralized with Mg (OH) ₂ , only MgF ₂ is generated. This is because CaSO ₄ in neutral is KSp 2.5 × 10 ^{-11dlsep qksgo} MgSO ₄ is soluble.

The introduction of NaOH into the second reactor is to raise the pH to 8.5 for complete removal of heavy metals. In FIG. 3, it can be seen that it is difficult to raise the pH above 7.5 by adding Mg (OH) ₂ .

The reason why slaked lime, alum, sulfuric acid, etc. are added in the third reactor is to remove fluorine in the wastewater generated in the second reactor. That is, the MgF ₂ produced in the first and second reactors had a neutral solubility of about 87 mg / l, so the theoretical minimum fluorine concentration in the influent of the third reactor was about 53 metal oxides or 53 ppm, exceeding the legal standard of 15 ppm. As a result, calcined lime and alum are removed to remove sulfuric acid, and sulfuric acid reacts with calcined lake to produce CaSO ₄ , and CaSO ₄ serves as fluorine removal.

The fourth reactor serves to agglomerate suspended solids similar to the third reactor of the conventional method. The difference from the conventional reactor 3 is that no hydrochloric acid is added for neutralization, which results in better floc formation. That is, floc formation occurs well around pH 7.0, and better flocs are produced in the present invention which performs them separately than the third reactor of the conventional method in which neutralization and aggregation of stainless wastewater, which is alkaline wastewater, are performed at once.

The invention will be described based on the following examples.

Example 1

Slurry was added to stainless wastewater having a pH of 2.01 and a fluorine concentration of 16,000-17,000 ppm, and the fluorine purification treatment was repeated several times. The results are shown in Table 1.

TABLE 1

As can be seen from Table 1, even when 200,000 ppm of lime is added, the residual fluorine does not fall below 15 ppm, which is a legal standard.

Example 2

The stainless wastewater was neutralized to pH 8.5 using the conventional method (calcined lime injection) and the method of the present invention (magnesium hydroxide and caustic soda), respectively, and the amount of sludge produced therein was investigated and the results are shown in Table 2.

TABLE 2

As shown in Table 2, in the case of using the present invention, the sludge amount was reduced by 25.6% compared with the conventional method.

Example 3

When the pH was adjusted from 8.0 to 9.0 with caustic soda after adding 100,000 to 150,000 mg / l of magnesium hydroxide to the stainless wastewater, the residual fluorine ion concentration of the stainless wastewater was 60.5-151.2 ppm. Using the supernatant of the wastewater treated as raw water, 500-1000 ppm of hydrated lime and 300-700 ppm of alum were added and stirred, the pH was adjusted to 7.0-7.5 with sulfuric acid, and then stirred again. Add flocculant. After stirring, the precipitate was separated and the fluorine concentration of the supernatant was measured. The fluorine concentration of the used solution was 3-10 ppm.

In the conventional method, as shown in Example 1, the residual fluorine concentration in the wastewater was 54-92 ppm, which far exceeded the legal standard value (15 ppm). However, as shown in Example 3, the waste water remained in the case where all the processes of the present invention were passed. Fluoride concentration was 3-10ppm, which satisfies the legal standard (15ppm).

As such, the present invention is said to be very useful to greatly reduce the amount of hazardous wastes that are a problem in the stainless wastewater treatment process and to significantly reduce the residual fluorine concentration in the wastewater, which is a water pollutant.

Claims (1)

In the wastewater treatment process generated in the stainless steel manufacturing process, 100,000-200,000 ppm of magnesium hydroxide is added to the first reactor, the pH is adjusted to 8.0-9.0 with caustic soda in the second reactor, and slaked lime 500- in the third reactor. 1000 ppm, 300-700 ppm of alum and pH adjusted to 7.0-7.5 with sulfuric acid, and then flocculant is added in the fourth reactor to produce a floc, wastewater treatment method generated in a stainless manufacturing process.