JP5206455B2 - Cement kiln extraction dust processing method - Google Patents

Description

  In order to reduce the circulation of chlorine, alkali, and sulfur in a cement kiln, the present invention is a dust containing selenium (hereinafter referred to as extraction air) that accompanies extraction of a part of firing gas using an extraction device such as a chlorine bypass. (Referred to as dust).

When cement raw materials containing a large amount of chlorine, alkali, and sulfur are used, the amount of chlorine, alkali, and sulfur contained in the cement clinker increases, which not only adversely affects the quality of the cement, but also chlorine, alkali, and sulfur A compound with a high vapor pressure is formed, and before it is gasified and circulated in the cement equipment, it is condensed at a relatively low temperature in the equipment to form a coating. .
In order to solve this problem, part of the firing gas is extracted from the kiln bottom of the cement kiln to reduce the amount of chlorine, alkali, and sulfur circulating in the cement production.

  However, when such calcination gas extraction is performed, extraction dust with a large content of chlorine, alkali and sulfur is inevitably accompanied, and this dust treatment method becomes a new problem. In other words, extraction dust contains harmful substances regulated by the Water Pollution Control Law, such as lead, cadmium, chromium, manganese, iron, selenium, etc. in addition to chlorine, alkali, and sulfur. Landfilling and disposal can cause environmental pollution and should be handled in an appropriate manner. Even when the extracted dust is not discarded but reused as a cement raw material, it is necessary to reduce the amount of alkali and chlorine contained in the extracted dust and then return to the raw material system.

  Since alkali and chlorine compounds contained in the bleed dust are water-soluble, it is a matter of course that a water washing treatment is most suitable for removal, and is already known (for example, Patent Documents 1 and 2). However, in this document, the method for controlling the addition amount of ferrous chloride is not specified, and in the case of extraction dust in which the amount of dissolved hexavalent selenium varies, there is a possibility that it may be difficult to put it to practical use. .

JP 2002-273455 A JP-A-6-79286

  The present invention is a wastewater treatment method for water pollution treatment, wherein cereal kiln extraction dust is washed with water, the solid content is reused as a cement raw material, and selenium containing selenium that cannot be treated with ordinary trivalent iron ions is contained. It aims at providing the processing method which makes it possible to discharge and discard by removing below a reference value.

As a result of intensive studies, the present inventors have found that the above object can be achieved.
That is, this invention relates to the processing method of the cement kiln extraction dust characterized by including the following processes.
(1) A first step in which water is added to cement kiln extraction dust to form a slurry, followed by solid-liquid separation,
(2) After adding the ferrous salt compound to the liquid phase after solid-liquid separation obtained in the first step (hereinafter referred to as raw water) until the oxidation-reduction potential of the liquid phase becomes −600 mV or less, It is a second step of adjusting the pH to 8 to 10, adding a polymer flocculant, and performing solid-liquid separation , and the ferrous salt compound of the second step is ferrous chloride, ferrous sulfate, Or these hydrates,
(3) The ferric salt compound is added to the liquid phase after the solid-liquid separation obtained in the second step, the pH is adjusted to 8 to 12, and then the polymer flocculant is added to perform the solid-liquid separation. A third step to be performed,
The ferric salt compound in the third step is ferric chloride, ferric sulfate, polyferric sulfate, or a hydrate thereof, and the added amount of the ferric salt compound is 50-300 times the molar amount with respect to all selenium in water,
The polymer flocculant in the second step and the third step is a polymer flocculant containing polyacrylamide or sodium polyacrylate.

When the cement kiln extraction dust processing method of the present invention is carried out, hexavalent selenium, which is extremely difficult to remove, is reduced to tetravalent and collected as a solid phase, and all selenium (tetravalent selenium in the liquid phase after solid-liquid separation is collected. , Hexavalent selenium) can be reduced to a level that sufficiently satisfies the drainage standard value according to the Water Pollution Control Law, and can be discharged.
In addition, since the required amount of the ferrous salt compound used as the reducing agent can be determined on the spot and excessive addition can be avoided, sedimentation separation is facilitated in a short time by adding a small amount of the polymer flocculant. Therefore, the amount of sludge with a high water content which is a solid phase is reduced, and the sludge is easily dewatered.
Furthermore, the solid content can be reused as a cement raw material.

FIG. 1 is a flowchart showing a method of treating cement kiln bleed dust according to the present invention.

The present invention is described in detail below. Moreover, the flow of the processing method of the cement kiln extraction dust which concerns on this invention is shown in FIG.
The first step of the present invention is a step of solid-liquid separation after adding water to the cement kiln bleed dust to form a slurry. In this step, alkali metal salts such as potassium chloride, sodium chloride, and calcium chloride are eluted and collected as a solid phase. Since the main component of the extracted dust is an alkali metal salt, the resulting slurry exhibits a high pH value. The amount of water added is such that the concentration of chlorine in the liquid phase after solid-liquid separation is 2% by mass or less. Generally, the chlorine concentration of seawater is 0.005 to 2% by mass, and the treated wastewater can be discharged as it is if the chlorine concentration of the liquid phase is 2% by mass or less.

In the second step of the present invention, the ferrous salt compound is added to the liquid phase (raw water) separated in the first step until the oxidation-reduction potential of the liquid phase becomes −600 mV or less, and then the pH is adjusted to 8. 10 to 10 and adding a polymer flocculant to perform solid-liquid separation. In this step, hexavalent selenium (selenic acid) in the raw water is reduced to tetravalent selenium (selenous acid) that easily coprecipitates with trivalent iron hydroxide.
Examples of ferrous salt compounds include ferrous chloride, ferrous chloride and dihydrate, ferrous sulfate, ferrous sulfate and tetrahydrate, ferrous sulfate and pentahydrate, Examples thereof include ferrous sulfate and heptahydrate, polyferrous sulfate and the like.
The ferrous salt compound is added until the oxidation-reduction potential of the liquid phase is −600 mV or less, preferably −650 mV or less. If the redox potential of the liquid phase is not −600 mV or less, hexavalent selenium is difficult to be reduced to tetravalent, and hexavalent selenium difficult to coprecipitate with the trivalent iron hydroxide produced in the third step is added to the supernatant. Since it remains, it is not preferable.
Examples of the liquid phase pH adjuster include sodium hydroxide and calcium hydroxide. The pH is adjusted to 8-10, preferably 9-10.

In the third step of the present invention, a ferric salt compound is added to the liquid phase after solid-liquid separation obtained in the second step, the pH is adjusted to 8 to 12, and then a polymer flocculant is added. This is a step of performing solid-liquid separation.
Examples of the ferric salt compound include ferric chloride, ferric chloride and hexahydrate, ferric sulfate, ferric sulfate and trihydrate, ferric sulfate and hexahydrate, And ferric sulfate heptahydrate, polyferric sulfate and the like.
The addition amount of the ferric salt compound is 50 to 300 times mol, preferably 100 to 250 times mol, in terms of iron ion, with respect to the total amount of selenium dissolved in the raw water.
Examples of the liquid phase pH adjuster include sodium hydroxide and calcium hydroxide as in the second step. The pH is adjusted to 8-12, preferably 9-11.

The polymer flocculant added in the second and third steps of the present invention has an effect of facilitating solid-liquid separation by promoting aggregation of precipitates mainly composed of iron hydroxide. As the polymer flocculant, any polymer flocculant containing polyacrylamide or sodium polyacrylate may be used, regardless of nonionic, cationic or anionic. The addition amount of the polymer flocculant is 0.2 to 4 mg / l, preferably 0.5 to 2 mg / l.
As a method for solid-liquid separation performed in the first to third steps of the present invention, a commonly used method, for example, a filtration method, a centrifugal separation method, a sedimentation separation method, or the like can be used.
The liquid phase in which heavy metal and selenium are reduced to below the drainage standard value by solid-liquid separation in the fourth step can be discharged after the pH is lowered to near neutrality. Moreover, since the solid phase obtained in the first to third steps contains iron, it can be reused as a raw material for cement as an iron source.

The present invention will be described in detail with reference to specific examples. The results of the following examples and comparative examples are shown in Table 1.
Example 1
2500 g of water was added to 250 g of extracted dust and stirred for 3 hours to form a slurry. The slurry was subjected to solid-liquid separation with a membrane filter having a pore size of 1 μm, and the pH of the obtained filtrate (hereinafter referred to as raw water 1) was 13.0 and the oxidation-reduction potential was −117 mV.

Total selenium contained in the raw water 1 was quantified by the following method in accordance with JIS K 0102.
That is, sulfuric acid and nitric acid were added to the raw water 1 and heated to generate white sulfuric acid smoke. This sample solution was quantified with a hydrogen compound generation atomic absorption spectrometer. Tetravalent selenium was quantified with a hydrogen compound generation atomic absorption spectrometer without pretreatment of raw water. Hexavalent selenium was obtained by subtracting tetravalent selenium from all selenium.
The contents of total selenium (tetravalent selenium and hexavalent selenium) and hexavalent selenium were 13, 12 mg / l, respectively.

Sodium hydroxide was added so that the pH of the raw water 1 (250 ml) was 8, and ferrous sulfate heptahydrate was added. The ferrous sulfate heptahydrate was added until the redox potential of this liquid phase was -656 mV, and then stirred and mixed for 30 minutes.
Next, 2 mg / l of the above polymer flocculant was added and stirred and mixed for 10 minutes. After 10 minutes of standing, this treatment liquid was filtered with 5 types A filter paper.

To the above filtrate is added 10.2 ml of 1 mol / l ferric chloride hexahydrate (248 times mole amount of 13 mg / l of total selenium dissolved in raw water 1), and sodium hydroxide is added. The pH was adjusted to 10 by mixing for 30 minutes. Subsequently, 2 mg / l of a polymer flocculant was added, and the mixture was stirred and mixed for 10 minutes. After 10 minutes of standing, the treated liquid was filtered with 5 types A filter paper.
The filtrate was quantified by the raw water 1 analysis method. The total selenium was 0.01 mg / l, which cleared the wastewater standard value of 0.1 mg / l according to the Water Pollution Control Law.

(Example 2)
2500 g of water was added to 250 g of extracted dust and stirred for 3 hours to form a slurry. The slurry was subjected to solid-liquid separation with a membrane filter having a pore size of 1 μm. The pH of the obtained filtrate (hereinafter referred to as raw water 2) was 12.0, and the oxidation-reduction potential was −103 mV.

The total selenium contained in the raw water 2 was quantified by the raw water 1 analysis method.
The contents of total selenium (tetravalent selenium and hexavalent selenium) and hexavalent selenium were 49 and 4 mg / l, respectively.

Sodium hydroxide was added so that the pH of the raw water 2 (250 ml) was 10, and ferrous sulfate heptahydrate was added until the redox potential of the liquid phase was -706 mV. Thereafter, the mixture was stirred and mixed for 30 minutes.
Next, 2 mg / l of the above polymer flocculant was added and stirred and mixed for 10 minutes. After 10 minutes of standing, this treatment liquid was filtered with 5 types A filter paper.

15.5 ml of 1 mol / l ferric chloride hexahydrate (100 times mole amount of all selenium 49 mg / l dissolved in raw water 2) is added to the filtrate, and sodium hydroxide is added. The pH was adjusted to 10 by mixing for 30 minutes. Subsequently, 1 mg / l of a polymer flocculant was added and stirred and mixed for 10 minutes. After 10 minutes of standing, this treatment liquid was filtered with 5 types A filter paper.
The filtrate was quantified by the raw water 2 analysis method. The total selenium was 0.04 mg / l, which cleared the wastewater standard value of 0.1 mg / l according to the Water Pollution Control Law.

(Comparative Example 1)
2500 g of water was added to 250 g of extracted dust and stirred for 3 hours to form a slurry. The slurry was subjected to solid-liquid separation with a membrane filter having a pore size of 1 μm, and the pH of the obtained filtrate (hereinafter referred to as raw water 3) was 13.0 and the oxidation-reduction potential was −114 mV.
The total selenium contained in the raw water 3 was quantified by the raw water 1 analysis method.
The contents of total selenium (tetravalent selenium and hexavalent selenium) and hexavalent selenium were 13, 12 mg / l, respectively.
Sodium hydroxide was added so that the pH of the raw water 3 (250 ml) was 8, and ferrous sulfate heptahydrate was added until the redox potential of the liquid phase became −537 mV. Thereafter, the mixture was stirred and mixed for 30 minutes.
Next, 2 mg / l of the above polymer flocculant was added and stirred and mixed for 10 minutes. After 10 minutes of standing, this treatment liquid was filtered with 5 types A filter paper.

To the above filtrate is added 10.2 ml of 1 mol / l ferric chloride hexahydrate (248 times mole amount of 13 mg / l of total selenium dissolved in raw water 1), and sodium hydroxide is added. The pH was adjusted to 10 by mixing for 30 minutes. Subsequently, 2 mg / l of a polymer flocculant was added, and the mixture was stirred and mixed for 10 minutes. After 10 minutes of standing, the treated liquid was filtered with 5 types A filter paper.
The filtrate was quantified by the raw water 1 analysis method. Total selenium was 0.22 mg / l, which did not clear the wastewater standard value of 0.1 mg / l according to the Water Pollution Control Law.

(Comparative Example 2)
2500 g of water was added to 250 g of extracted dust and stirred for 3 hours to form a slurry. The slurry was subjected to solid-liquid separation with a membrane filter having a pore size of 1 μm, and the pH of the obtained filtrate (hereinafter referred to as raw water 4) was 12.0, and the oxidation-reduction potential was −111 mV.
The total selenium contained in the raw water 4 was quantified by the raw water 1 analysis method.
The contents of total selenium (tetravalent selenium and hexavalent selenium) and hexavalent selenium were 49 and 4 mg / l, respectively.
Sodium hydroxide was added so that the pH of the raw water 4 (250 ml) was 10, and ferrous sulfate heptahydrate was added until the redox potential of the liquid phase was −532 mV. Thereafter, the mixture was stirred and mixed for 30 minutes.
Next, 2 mg / l of the above polymer flocculant was added and stirred and mixed for 10 minutes. After 10 minutes of standing, this treatment liquid was filtered with 5 types A filter paper.

15.5 ml of 1 mol / l ferric chloride hexahydrate (100 times mole amount of all selenium 49 mg / l dissolved in raw water 2) is added to the filtrate, and sodium hydroxide is added. The pH was adjusted to 10 by mixing for 30 minutes. Subsequently, 1 mg / l of a polymer flocculant was added and stirred and mixed for 10 minutes. After 10 minutes of standing, this treatment liquid was filtered with 5 types A filter paper.
The filtrate was quantified by the raw water 1 analysis method. The total selenium was 0.25 mg / l, which did not clear the wastewater standard value of 0.1 mg / l according to the Water Pollution Control Law.

1 First process 2 Second process 3 Third process 4 Extracted dust 5 Cement raw material 6 Effluent water

Claims (3)

A cement kiln bleed dust treatment method comprising the following steps.
(1) A first step in which water is added to cement kiln extraction dust to form a slurry, followed by solid-liquid separation,
(2) After adding the ferrous salt compound to the liquid phase after solid-liquid separation obtained in the first step (hereinafter referred to as raw water) until the oxidation-reduction potential of the liquid phase becomes −600 mV or less, It is a second step of adjusting the pH to 8 to 10, adding a polymer flocculant, and performing solid-liquid separation, and the ferrous salt compound of the second step is ferrous chloride, ferrous sulfate, Or these hydrates,
(3) The ferric salt compound is added to the liquid phase after the solid-liquid separation obtained in the second step, the pH is adjusted to 8 to 12, and then the polymer flocculant is added to perform the solid-liquid separation. A third step to be performed,
The ferric salt compound in the third step is ferric chloride, ferric sulfate, polyferric sulfate, or a hydrate thereof, and the added amount of the ferric salt compound is 50-300 times the molar amount with respect to all selenium in water,
The polymer flocculant in the second step and the third step is a polymer flocculant containing polyacrylamide or sodium polyacrylate. The processing method of the cement kiln extraction dust of Claim 1 which adds water so that the chlorine concentration of raw | natural water may be 2 mass% or less in the said 1st process. The method for treating cement kiln bleed dust according to claim 1 or 2, wherein the solid phase after solid-liquid separation obtained in any one of the first to third steps is reused as a cement raw material.

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