JPH034987A - Defluorination of flue gas desulfurizing waste water - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to waste water discharged from a wet flue gas desulfurization equipment (hereinafter referred to as
The present invention relates to a method for removing fluorine from flue gas desulfurization wastewater (or simply raw wastewater or wastewater).

[Conventional technology]

Flue gas desulfurization wastewater, for example, flue gas desulfurization wastewater discharged mainly from the removal system of wet flue gas desulfurization equipment using the lime-gypsum method for coal-fired boilers, contains several hundred mg/j of fluorine! include. When discharging this wastewater, it is necessary to reduce the concentration of fluorine to below the regulatory value stipulated by the Water Pollution Control Act or local government ordinances.

A treatment method for this purpose is a neutralization coagulation precipitation treatment method in which calcium salts such as slaked lime, calcium chloride, and calcium carbonate are added to the wastewater to remove the fluorine in the wastewater as a calcium precipitate that is hardly soluble in water. is common.

FIG. 5 shows a block diagram of a typical processing method. This treatment method includes a first reaction tank 10, a first flocculation tank 11, a sedimentation tank 1
2, a calcium removal process consisting of a second reaction tank 13, a second flocculation tank 14, a second settling tank 15, a mixing tank 16, a third flocculation tank 17, It consists of a residual fluorine removal step configured with a third settling tank 18.

In the calcium butoxide production, precipitation, and separation process, first, slaked lime a is added to the flue gas desulfurization wastewater in the first reaction tank 10 to adjust the pH.
Adjust to 9 or higher. Normally, flue gas desulfurization wastewater has a pH of 1 to
2, the pH is adjusted by using an alkaline agent such as sodium hydroxide E in addition to slaked lime A, if necessary.

In this reaction, slaked lime and fluorine ions in wastewater undergo a neutralization reaction to produce calcium oxide. Also pH
By setting 7 or more, the magnesium ions and aluminum ions contained in the wastewater will generate hydroxide,
Some of the fluorine is adsorbed to these hydroxides. The first step! ! In the collection tank 11, an appropriate amount of a polymer flocculant is added to flocculate the generated solids that are hardly soluble in water, such as calcium butoxide and magnesium hydroxide. The fraction is precipitated and separated.

In order to increase the fluorine removal rate, it is common knowledge to add calcium salts such as slaked lime in excess of an equivalent amount to the amount of fluorine contained in the wastewater. Furthermore, since there is no established technology to accurately measure the amount of fume rock in wastewater that fluctuates over time in a short period of time, we set the maximum expected amount of fume rock in order to ensure safety. Generally, the amount of calcium salt added is equal to or more than the value. Therefore, in actual operation, the amount of calcium salt added is in the range of an equivalent ratio of 3 to 10 relative to the amount of fluorine in the wastewater.

Therefore, excess calcium ions that did not participate in the neutralization reaction with fluorine combine with sulfate ions in the wastewater to form gypsum. Most of the gypsum produced becomes precipitate, but
The remaining gypsum temporarily remains dissolved in the wastewater in a supersaturated state, and due to changes in pH and liquid temperature over time, various tanks,
It crystallizes on the inner walls of piping, equipment in tanks, and the detection end of meters, causing what is called a scale precipitation phenomenon.

In order to minimize this scale precipitation phenomenon, the following calcium removal step is provided. In this step, the supernatant water from the first settling tank 12 in the previous step is led to the second reaction tank 13, and by reaction with the added sodium carbonate C, calcium ions are converted to calcium carbonate, and the 21st precipitation tank in the next step! Collection tank 1
4. Coagulate, precipitate and remove in the second sedimentation tank 15.

In the residual fluorine removal step, aluminum sulfate d is added to the supernatant water from the second settling tank 15 in the previous step to perform coagulation and sedimentation treatment, thereby reducing the residual fluorine in the treated water to a target value or less.

FIG. 6 shows a block diagram of another conventional treatment method for flue gas desulfurization wastewater. This treatment method is shown in Figure 5, where the first flocculation tank 11 in the calcium fluoride generation, precipitation, and separation steps is shown.
and the first settling tank 12 is omitted, and the solid content such as calcium butoxide produced in the calcium butoxide production process is coagulated and precipitated in the same tank together with the solid content produced in the calcium removal process, and then separated and removed. There are characteristics. Other than this feature, this processing method is the same as the processing method shown in FIG. 5, so each component having the same function is given the same reference numeral as in FIG. 5, and a description thereof will be omitted.

[Problem to be solved by the invention]

However, the conventional technology has the following problems.

(1) A calcium removal step is required and the process is complicated.

(2) Scale generation cannot be completely prevented.

The scale precipitation phenomenon due to excessive calcium ions still occurs in the various types of piping shown with diagonal lines in Figures 5 and 6, and the equipment, instruments, and valves attached to these.
Problems such as valve neck blockage and inability to detect meters may occur, eventually rendering the device inoperable.

(3) The consumption of chemicals and the amount of sludge generated are large.

Since excess calcium salt is added and sodium carbonate is used to remove this excess calcium, the consumption of calcium salt and carbonate increases. Furthermore, since most of these chemicals are separated as precipitates of calcium carbonate, a large amount of sludge is inevitably generated as precipitates, which requires time and effort to process and dispose of the sludge.
Expenses increase.

(4) Fluorine removal performance becomes unstable.

Compared to the method shown in FIG. 5, the treatment method shown in FIG. 6 can omit the first coagulation tank 11 and the first settling tank 12, and has the advantage of simplifying the equipment and operation. But the first
A part of the calcium butoxide produced in the reaction tank 10 is redissolved by the addition of sodium carbonate in the second reaction tank 13,
Residual water in the supernatant water discharged from the second settling tank 15
There is a tendency for the temperature to increase. For this reason, the fluorine concentration in the treated water that has undergone the residual fluorine removal process becomes unstable, and there are cases where the target value is exceeded. In order to eliminate this drawback,
The amount of aluminum sulfate added must be increased in the residual fluorine removal process, and the amount of sodium hydroxide e added for pH adjustment also increases, resulting in lower chemical consumption and sludge generation in the residual fluorine removal process. A new problem arises, which is an increase in

The purpose of the present invention is to provide flue gas desulfurization which eliminates the drawbacks of the above-mentioned conventional techniques, has a simple treatment process, does not generate scale, can reduce the consumption of chemicals and the amount of sludge generated, and can stably remove fluorine. The object of the present invention is to provide a method for removing fluorine from wastewater.

[Means to solve problems]

The method for removing fluorine from flue gas desulfurization wastewater according to the present invention includes a first step of adding hydroxide to the flue gas desulfurization wastewater and adjusting the pH to 9 or higher by adding slaked lime and, if necessary, slaked lime. It consists of a second step in which the precipitate generated in the first step is separated by precipitation, and a third step in which the supernatant water obtained in the second step is coagulated and precipitated, and the calcium ion concentration of the supernatant water in the second step is 40 (1-6,00 mg/l), the amount of slaked lime added in the first step is adjusted.

[Effect]

The calcium ion concentration of the supernatant water in the second step is 400 to 6.
By adjusting the amount of slaked lime added in the 1st process so that it becomes 00mg/i', an appropriate amount of slaked lime will be added in the east 1st process according to the amount of slaked lime in the wastewater. . For this reason, in response to fluctuations in the amount of effluent in wastewater over time, which is the case with conventional technology, the addition of an excessive amount of calcium salt to ensure safety can cause various problems, such as scale formation, increased chemical consumption, and sludge generation. Problems such as increased amount and unstable fluorine removal performance can be solved all at once.

Normally, flue gas desulfurization wastewater is acidic with a pH of 1 to 2, and the production reaction of calcium butoxide is more than 9, especially pH 10 to 1.
0.5, the added slaked lime acts as an alkaline agent that brings the pH close to the above value. However, as mentioned above, the amount of slaked lime added is adjusted depending on the concentration of calcium ions in the supernatant water of the second step, and if the concentration of calcium ions in the raw wastewater is sufficiently high, due to the action of these ions, Sometimes the amount of slaked lime added is zero. Therefore, the pH of wastewater usually does not reach 9 just by adding slaked lime. For this reason, IJum (hydroxide) is added as an auxiliary agent to raise the pH. By adding an appropriate amount of slaked lime and under conditions of pH 9 or higher, the first
In the process, fluorine in the wastewater reacts with the calcium ions contained in the raw wastewater and the calcium ions derived from the added slaked lime, producing hardly soluble fine fragmented calcium. In the second step, the calcium butoxide produced in the first step and various other precipitates are separated by precipitation, for example, after adding a polymer flocculant.

The supernatant water in the second step has a calcium ion concentration of 400 to 6.
00ffig/β, and as a result, the residual fluorine concentration in this supernatant water is 20 to 30 mg/j.
! shows a stable value. For this reason, the coagulation-sedimentation treatment for removing residual fluorine in the third step can be performed stably under constant conditions, and the fluorine concentration in the treated water can be reliably kept below the target value.

〔Example〕

FIG. 1 shows a block system diagram for implementing the present invention.

Slaked lime (a) and hydroxide (IJ) are added as alkaline agents to the reaction tank 20 constituting the first step to adjust the pH to 9 or higher. The amount of slaked lime a is adjusted so that the calcium ion concentration of the supernatant water in the second step is 400 to 600 mg/f, and IJum (hydroxide) is used as the remaining alkaline agent to raise the pH to 9 or higher. . The second step is composed of a flocculation tank 2I and a settling tank 22. In the flocculation tank 21, an appropriate amount of polymer flocculant is added, and the reaction tank 2 of the first step is
Solid content such as calcium butoxide produced in step 0 is aggregated. Next, the flocculated precipitate is separated by sedimentation in the sedimentation tank 22. The third step is composed of a mixing tank 23, a coagulation tank 24, and a settling tank 25. In the mixing tank 23, the precipitate Ff22 of the second step is
Aluminum sulfate d is added to the supernatant water from.

The amount of aluminum sulfate added is because aluminum sulfate also functions as a pH adjuster to lower the pH of liquids with a pH of 9 or higher.
It is preferable to add only the amount necessary to bring the pH of the supernatant water to 6 to 8. If more aluminum sulfate is added, the pH will drop and the aggregation effect of aluminum sulfate will not proceed as expected.

Therefore, in order to maintain the pH within the above range, it becomes necessary to newly add an alkaline agent such as sodium hydroxide.

In the third stage flocculating tank 24, an appropriate amount of polymer flocculant is added, and then the flocculated precipitate is separated by precipitation in the next stage settling tank 25, and the supernatant water is discharged outside the system as treated water.

FIG. 4 shows a modification of the present invention. This modification is characterized in that a part f of the precipitate separated in the settling tank 22 of the second step is returned to the reaction tank 20 of the first step.

According to this modification, as a result of the relative increase in the concentration of precipitates (solids) in the reaction tank 20, flocculation tank 21, and settling tank 22, the production reaction of calcium oxide in the reaction tank 20, and the flocculation tank It has been confirmed that the flocculation action in 21 and the co-precipitation action in the settling tank 22 are promoted, and that even more stable fluorine removal performance can be obtained.

In addition, an immersion type or flow type detector 26 is installed in the sedimentation tank 22 or in the supernatant water communication passage 26 connecting the sedimentation tank 22 and the mixing tank 23 of the third step, and this detector detects the second step. The calcium ion concentration in clear water is detected continuously or intermittently in short cycles, and based on the detection results, the amount of slaked lime added in the reaction tank 20 is controlled by a signal from the controller 27. Good too. As a detector for calcium ion concentration, for example, an ion electrode type detector can be considered. According to this modification, the operation for maintaining the calcium ion concentration in the supernatant water of the 1st and 2nd steps at a predetermined value can be performed automatically without relying on intuition or experience. It is expected that removal performance will be obtained.

Example 1 Wastewater generated when waste casing generated from a coal-fired power generation facility with an output of 175 M11 was subjected to wet desulfurization using the lime-gypsum method was treated in the manner shown in FIG. 1. The amount and quality of treated flue gas desulfurization wastewater were as shown in Table 1.

Table 1 Further, the raw processing conditions were as shown in Table 2.

Table 2 Note that the calcium ion concentration in the supernatant water of the sedimentation tank 22 is determined by sampling the supernatant water once a day, and adjusting the calcium ion concentration in the supernatant water to one of the values shown in Table 2. For example 100.200-800mg every few days
/j! This was done by adjusting the amount of slaked lime added in the reaction tank 20 so that

Figure 2 shows the supernatant water of the second step under the above treatment conditions;
That is, the residual fluorine concentration in the supernatant water of the settling tank 22 and the reaction tank 2
FIG. 2 is a diagram showing the occurrence of scale deposited on a test piece immersed in water.

As is clear from the figure, the calcium ion concentration in the supernatant water in the second step was 400 to 600 mg/j! In the range of , the residual fluorine concentration in the supernatant water is 20 to 30 mg/j! shows stable values in the range of . Therefore, in the mixing tank 23 of the third step at the latter stage, by adding 1000 mg/β of aluminum sulfate, the pH of the supernatant water settles to about 7, which is the preferable setting for the coagulation effect, and no other pH adjusting agent is required. On the other hand, when the calcium ion concentration in the supernatant water in the second step is less than 400 mg/l, the residual fluorine concentration in the supernatant water rapidly increases to 30 mg/l! This shows an unstable value.

Therefore, in the mixing tank 23 of the third stage of the latter stage, even with the addition of 1000 mg/l of aluminum sulfate, the residual fluorine concentration in the treated water shows an unstable value that exceeds the treatment target value of 15a+g/J, and reprocessing is required. did.

In order to achieve the treatment target value, it is necessary to add a larger amount of aluminum sulfate, and as this large amount of addition lowers the pH of the liquid, it is necessary to add an alkaline agent such as sodium hydroxide to adjust the pHtJR. shall be. Therefore, it has been found that such an operation increases the amount of aluminum sulfate and sodium hydroxide used in the third step and the amount of sludge generated.

In addition, the calcium ion concentration in the supernatant water of the second step was 6.
00mg/j! In the above cases, it has been found that as this value increases, the amount of scale generated increases rapidly. Scale was generated not only on the wall surface of the reaction tank 20 in the first step, but also on the liquid contact parts of all the tanks, piping, equipment, and instruments that constitute each treatment step.

Example 2 Based on the results obtained in Example 1, the calcium ion concentration in the supernatant water in the second step was determined using the same apparatus. is 5001
Long-term continuous operation was carried out under the same treatment conditions as in Example 1, except that the treatment was adjusted to the target I1 g/l.

The results are shown in FIG. As is clear from the figure, although the fluorine concentration in flue gas desulfurization wastewater, which is the raw wastewater, fluctuated greatly, the residual fluorine concentration in the second process supernatant water and the third process supernatant water (treated water) remained unchanged. 20-30m each
g/l and 5 to 12 mg/l, and no scale was observed during long-term continuous operation.

Comparative Example The conventional treatment method shown in FIG. 6 was carried out using approximately the same apparatus as in Example 2. The main processing conditions are shown in Table 3.

Table 3 Table 4 Under the above treatment conditions, the residual fluorine concentration in the treated water is the treatment target value of 15mg/j! Operation results were obtained that lowered the limit.

Table 4 shows the main chemical consumption and the total value of sludge separated in each settling tank in comparison with the operation results of Example 2. The values shown in this table are shown as the amount consumed or generated per 1 kg of flue gas desulfurization wastewater, which is raw wastewater.

As is clear from Table 4, the method according to the present invention can reduce the consumption of slaked lime, sodium carbonate and aluminum sulfate compared to the method according to the prior art shown in FIG.

In addition, the amount of slaked lime added to the amount of slaked lime in the raw wastewater was 3.5 equivalents on average in the comparative example, whereas in the example 20
In the method according to the present invention, the average weight was 0.8 equivalents.

According to the calculations made by the present inventors, the drug cost of the method according to the present invention is 30 to 40% lower than that of the method according to the above-mentioned conventional technology.
It has a saving effect. Furthermore, according to the method according to the present invention,
The amount of sludge generated is reduced by approximately 30% compared to the conventional method, and the effort and cost of processing and disposing of sludge can be reduced accordingly.

〔Effect of the invention〕

According to the present invention, various problems that conventionally occurred due to excessive addition of slaked lime, such as scale formation, increased chemical consumption, increased sludge generation, and unstable fluoride removal performance, can be solved. You can solve problems like this all at once. Also,
According to the present invention, the step of removing calcium that was conventionally required can be omitted, so the apparatus and operation can be simplified.

FIG. 6 is a block system diagram showing another method for treating flue gas desulfurization wastewater according to the prior art.

20... Reaction tank, 21... Coagulation tank, 22...
Sedimentation tank, 23... Mixing tank, 24... Coagulation tank,
25... Sedimentation tank, 26... Detector, 27...
- Controller, a... Slaked lime, b... Polymer flocculant, d... Aluminum sulfate, e... Sodium hydroxide.

Claims (4)

[Claims] (1) A first step in which sodium hydroxide and, if necessary, slaked lime are added to the flue gas desulfurization wastewater to adjust the pH to 9 or higher, and a second step in which the precipitate generated in this first step is separated by precipitation. , a third step in which the supernatant water obtained in the second step is coagulated and precipitated, and the calcium ion concentration in the supernatant water in the second step is 400 to 600 mg/l. A method for removing fluorine from flue gas desulfurization wastewater by adjusting the amount of slaked lime added. (2) The method for removing fluorine from flue gas desulfurization wastewater according to claim 1, wherein in the third step, aluminum sulfate is added as a coagulant so that the pH of the liquid becomes 6 to 8, and a coagulation precipitation treatment is performed. . (3) The method for removing fluorine from flue gas desulfurization wastewater according to claim 1 or 2, wherein a part of the precipitate separated in the second step is returned to the first step. (4) A detector for detecting the calcium ion concentration of the supernatant water is provided in the second step, and the amount of slaked lime added in the first step is controlled based on the signal from the detector. The method for removing fluorine from flue gas desulfurization wastewater according to any of Item 3.