KR20010063524A - Method for improving the quality of the discharged water in the lime stone baking process - Google Patents

Abstract

PURPOSE: An improving method of water quality of discharge water from lime calcination process is provided, which can improve the quality of discharge water from pH 12.5 to pH 6-7 by reacting CO2 in effluent gas with discharge water of primary settling treatment of the settling tank through bubbling of a part of kiln furnace and can improve Ca hardness, thereby reducing effluent of dust from calcining furnace and CO2 that is discharged to atmosphere. CONSTITUTION: The method comprises as follows: (i) collect water discharged from lime calcination process at a settling tank (7); (ii) settle the sediments from the collected water to separate supernatant; and (iii) neutralize the separated supernatant by feeding the discharge gas.

Description

Method for improving the quality of the discharged water in the lime stone baking process}

The present invention relates to a method for improving the water quality of effluent generated in the process of calcining limestone (CaCO ₃ ) to quicklime (CaO) in a lime kiln (rotary kiln, shaft kiln). The present invention relates to a method for improving the effluent water quality by reducing the pH (pH) and reducing the Ca hardness of the effluent by blowing and reacting the effluent generated in the calcining process using 20% of exhaust gas.

In general, the calcining process is a process for producing quicklime by heating limestone in a rotary kiln or shaft kiln. At this time, the limestone used for the preparation of quicklime is divided into a lump of about 10 to 40 mm and a lump of about 40 to 80 mm, depending on its particle size. Typically, calcined limestone is calcined in rotary kilns and lumped limestone is calcined in shaft kilns to produce quicklime.

Referring to FIG. 1, in which a quicklime manufacturing process is schematically illustrated, limestone 1 mined and deposited in a mine is charged directly to a rotary kiln 2 or a shaft kiln 3, depending on its particle size, in good quality. However, limestone that is mixed with gangue components during mining and transport or is finely granulated by collision, is washed before being charged into a kiln, such as a rotary kiln (2) or a shaft kiln (3). It is charged to the kiln in a removed state.

Limestone charged in the kiln is calcined by the heat of combustion of coke oven gas (COG) and converted to quicklime. The calcined quicklime is classified in the screen 4 through cooling and then stored in the steelmaking hopper 5 or the sintering hopper 6. The stored quicklime is then supplied to the required process.

On the other hand, carbon dioxide (CO ₂ ) gas generated during the reaction with the dust generated when the heat generated by burning the coke gas to react with the limestone in order to fire the limestone, preheated the limestone, and is discharged to the atmosphere through the pipe After collecting dust using water to remove dust in the process, the flue gas is released into the atmosphere.

Then, the water washed with limestone and dust collected are collected in a thicker tank and recycled to be sprinkled to collect the exhaust gas during the firing process, or sent to a drainage treatment process, followed by a water treatment process. It is recycled or sent to sea for road sprinkling and yard dust scattering.

At this time, the calcined effluent from the sedimentation tank is strongly alkaline and contains a large amount of suspended Ca ⁺⁺ ions not precipitated.

Such a conventional process of treating flue gas and flush water has the following problems.

First, if ionic floats are present in the water sprayed to remove dust from the kiln, the dust collection efficiency of dust in the flue gas will be reduced, and the dust concentration emitted to the atmosphere will rise above the legal limit, causing environmental pollution. do.

Secondly, in order to neutralize the strongly alkaline lime calcined effluent, the corrosion of the equipment due to the use of sulfuric acid is intensified and the chemical cost of the sulfuric acid is required.

In order to solve such a conventional problem, the present invention blows the flue gas containing carbon dioxide generated in the kiln to the lime discharged water to neutralize the alkalinity of the discharged water to improve the water quality, as well as to lower the floating matters in the atmosphere. The purpose of the present invention is to provide a method for improving the discharge water quality of the calcining process to improve the atmospheric environment by using CO ₂ in the flue gas so as to reduce the dust concentration in the flue gas discharged to the furnace.

According to the present invention, in order to achieve the above object, the method for improving the discharge water quality of the lime firing process in which the flue gas containing CO ₂ and lime discharge water is generated, collecting the discharge water generated in the limestone firing process in the sedimentation tank, It is characterized by consisting of a precipitation step of sedimenting the precipitate by sedimentation from the collected discharge water and the neutralization step of supplying the exhaust gas to the sedimentation water separated from the sedimentation to neutralize the sedimentation water.

1 is a process for preparing quicklime in lime kiln

2 is a view showing the configuration of the discharge water neutralization treatment according to an embodiment of the present invention

Figure 3 is a graph showing the pH change of lime effluent by pure carbon dioxide.

Figure 4 is a graph showing the pH change of the effluent by lime flue gas CO ₂ .

Figure 5 is a graph showing the pH change at the time of lime discharge water flow rate by lime flue gas CO ₂ .

<Description of Symbols for Major Parts of Drawings>

2: rotary kiln

3: shaft kiln

4: screen

5: steelmaking hopper

6: sintering hopper

7: settling tank

8: Drainage terminal treatment room

9: neutralization tank

Hereinafter, with reference to the accompanying drawings, preferred embodiments of the present invention will be described in detail as follows, the same configuration adopts the same reference numerals.

Figure 2 shows the configuration of adjusting the pH and turbidity of the effluent using CO ₂ in the flue gas generated in the calcining according to an embodiment of the present invention.

According to an embodiment of the present invention, the method for improving the effluent water quality may include effluent generated in the process of calcining limestone, for example, sediment contained in the calciner exhaust flue gas collection and limestone wash water in the sedimentation tank (7), The neutralization treatment is carried out by bubbling carbon dioxide gas in the off-gas generated in the limestone firing process in the precipitated water overflowed from the 7 to the neutralization tank 9 to lower the pH of the precipitated water.

On the other hand, calcium ions, which are ionic substances contained in the precipitated water, are precipitated, and the precipitated water is recycled to exhaust gas collection or reprocessed in the wastewater treatment chamber 8.

Firing limestone by calcining limestone may be represented by the following formula.

Heat (coke gas combustion)

↓

CaCO ₃ (limestone) → CaO (limestone) + CO ₂ (carbon dioxide)

On the other hand, dust is included in the flue-gas generated in the process of burning limestone into the kiln and burning the limestone for burning the coke oven gas (COG). At this time, dust is removed by spraying water on the exhaust gas, and the exhaust gas from which the dust is removed is discharged to the atmosphere.

The dust collected in the spray water is precipitated in the sedimentation basin, and then the sediment is recycled as lime sludge after dehydration. In addition, after the dust is settled, the sedimented water is recycled to be used for collecting lime flue gas or supplied to a drainage treatment step for reprocessing.

The exhaust gas component of the lime kiln contains about 15 to 20% of CO ₂ gas, as shown in Table 1 below. In addition, it can be seen that the water quality of the effluent after the precipitation treatment is strongly alkaline with a pH of about 12 and also contains a large amount of a solid substance such as Ca.

Table 1

Flue gas Waste water CO ₂ (%) O ₂ (%) pH SS (ppm) Cl (ppm) Ca hardness (mg / ㎡) 15 to 20 9.5 to 11 12 to 12.5 110 to 120 150 to 250 1560

When CO ₂ gas reacts with water to generate hydrogen ions, it can be expressed as follows.

_{H 2 O + CO 2 → H} 2 CO 3 → H + + HCO 3 - → 2H + + CO 3 2-

In other words, carbon dioxide reacts with water to produce hydrogen ions in the water, which neutralize the alkalinity of the discharged water. When neutralizing wastewater by CO ₂ gas, 2 mol of H ⁺ is generated per 1 mol of CO ₂ to neutralize the effluent. In addition, the effluent contains a large amount of Ca ²⁺ ions, and foreign matter reacts with CO ₂ to form CaCO ₃ to precipitate and improve turbidity of the effluent.

When the discharged water with improved turbidity is recycled for exhaust gas dust collection, the recycled water does not contain solid matter, so that the concentration of dust in the exhaust gas discharged to the atmosphere can be reduced by increasing the solubility of dust in the sprayed water. In addition, even when supplied to the wastewater treatment equipment, the neutralization treatment using sulfuric acid is omitted, thereby shortening the process to improve water quality.

Hereinafter, embodiments of the present invention will be described in more detail.

Effluent and flue gas flow rate can be known and equipment for measuring the pH change of the discharged water was installed at the site, and the flow rate of the discharge water was adjusted to the collection time of the collection tank to meet the conditions of the site. The pH change of the effluent was tested by varying the ratio of effluent and flue gas to the extent allowed by the capacity.

As shown in Table 1, the water quality of the lime-calcined plant effluent is considerably high as the pH is 12.5, so it can be expected that the alkalinity is so high that it has a strong buffer against acid.

Usually, if the alkalinity is 150 or more, a considerable amount of acid is required for neutralization. In this case, it is close to 40,000, so the amount of acid required for neutralization is very large and sulfuric acid is required.

Figure 3 is a graph showing the pH change of lime effluent water by pure carbon dioxide, the pH change of the effluent water is shown when the 100% CO ₂ in 500ml effluent bubbled ₂ minutes per minute.

In this case, since the CO ₂ content of the calcined flue gas is about 20%, the amount of flue gas required for the neutralization of waste water can be calculated as follows. That is, it took about 7 minutes to neutralize to pH 7 when neutralized with 100% CO ₂ , the total CO ₂ amount was blown 14L.

Therefore, when calculating about 1500 tons of daily discharged water from lime, the amount of calcined flue gas required to neutralize it is about 29 m ³ / min per minute, as described below.

That is, 14 l / 500 ml x 1000 ml / 1 l x 1000 l / 1 m ³ x 1500 m ³ / day = 42000 m ³ / day = 29 m ³ / min.

And, when converted to 20% flue gas takes 29 × 100/20 = 145m ³ / min flue gas.

After filling the discharge tank 200 liters in the sump and measuring the time when the pH is reduced to 7 or less without supplying the raw water, the results are shown in FIG.

At this time, the supply of flue gas was 13 l / min per minute, and the ratio of flue gas and raw water was five times. 4, it can be seen that after about 50 minutes, the pH decreases to 7 or less, and the decreasing tendency gradually decreases until 11 and then gradually starts to decrease from 11. In addition, after the pH was reduced to 6.7, the effluent was started to be supplied, and the pH was stable. These results indicate that the flue-gas can maintain the raw water in the sump at pH 7-8 if it is about five times the raw water flow rate.

5 shows a change in pH that is stabilized while changing the flow rate of the discharged water while maintaining a constant flow rate of the exhaust gas.

That is, initially, the flow rate of raw water was added at half the field conditions. As a result, the pH was reduced to 7 or less after about 50 minutes. Afterwards, when the flow rate of the discharged water was increased to 2.6 L / min, the pH was stabilized at 7 or less, and the ratio of the most suitable raw water and flue gas for neutralization was shown.

The ratio is an exhaust gas flow rate of 30 m ³ per minute. The flow rate was increased again to reduce the ratio of raw water / exhaust gas to 3.25. In this case, although there was an increase in pH, it was not significant and stable at about 8 degrees.

Table 2 shows the results of measuring the total hardness and the Ca-hardness to determine how much the Ca component of the flue-gas treated water was changed.

TABLE 2

division Total Hardness (mg CaCO ₃ ) Ca hardness (mg CaCO ₃ ) Raw water (pH 12.5) 1574 1560 Flue gas treated water pH 11.6 844 844 pH 6.4 568 568

Referring to Table 2, it can be seen that the dust collection process of the calcining step shows the hardness by most calcium. This is because the total data shows little difference between the total hardness and the calcium hardness.

Therefore, the total hardness of the discharged water was very high at 1574. When the flue gas treated water pH decreased to 11.6, the hardness was removed by 444% by 844. When the pH was reduced to 6.4, the hardness was 568 by the total hardness. It can be seen that 64% has been removed.

This means that the CO ₂ gas in the flue gas reacts with Ca ions to reduce Ca hardness and produce CaCO ₃ as precipitate. As a result, dust in the calcined flue gas can be reduced.

That is, as shown in Table 3, the concentration improvement effect of dust discharged to the atmosphere before and after neutralizing the flue gas with CO ₂ is recycled to improve the water quality of the discharged water by using CO ₂ in the flue gas. It can be seen that the dust concentration is reduced.

TABLE 3

division Conventional example Inventive Example Dust concentration in rotary kiln flue gas (mg / N㎡) 48.0 35.0

Therefore, according to the present invention, by bubbling a part of the exhaust gas into the kiln in the sedimentation tank effluent, the carbon dioxide (CO ₂ ) in the flue gas reacts with the effluent and the water quality of the effluent is alkaline at pH 12.5. Neutral levels of 6 to 7 improve water quality and improve Ca hardness to reduce dust emissions from the kiln flue-gas by recycling. In addition, CO ₂ emissions can be reduced by recycling CO ₂ gas to the atmosphere.

The foregoing is merely illustrative of preferred embodiments of the present invention and those skilled in the art to which the present invention pertains may make modifications and changes to the present invention without departing from the spirit and spirit of the invention as set forth in the appended claims. It should be recognized.

Claims (2)

In the method of improving the effluent water quality of the calcining process in which flue gas and lime effluent are generated, A collecting step of collecting the discharged water generated in the limestone firing process into the sedimentation tank, A precipitation step of separating the precipitated water by precipitating the precipitate from the discharged water collected in the settling tank, Impurity water quality improvement method comprising the step of neutralizing the precipitated water by supplying the exhaust gas to the precipitated water separated from the precipitate. The method of claim 1, wherein the exhaust gas contains CO ₂ .