JP2002254049A - Method for treating extraction dust in cement manufacturing equipment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a method for treatment capable of performing water washing treatment on an extraction dust accompanied by an extraction gas when a burnt gas is partially extracted to reduce circulations of chlorine, alkali and sulfur and capable of recycling a solid content as a cement raw material and capable of discharging a treated solution outside after removing toxic substances contained therein to values less than effluent standard values in cement manufacturing equipment. SOLUTION: After a slurry is prepared by adding water to the extraction dust accompanied by the extraction gas, the subject is solved by the method for treatment comprising a process separating a solid from a solution, a process reducing hexavalent selenium dissolved into tetravalent selenium by adding calcium polysulfide to a liquid phase obtained and a process performing a second separation of a solid from the solution after sedimenting residual heavy metals together with iron hydroxide in a state that the pH of the slurry ranges from 9.5 to 10.5 by adding ferrous and ferric compounds.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to dust containing harmful substances (hereinafter referred to as "bleeding air") which is entrained when a part of a calcining gas is bled in order to reduce the circulation of chlorine, alkali and sulfur in a cement manufacturing apparatus. (Referred to as dust).

[0002]

2. Description of the Related Art When a cement raw material having a high content of chlorine, alkali and sulfur is 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 increases the chlorine content. , Alkali, sulfur forms compounds with high vapor pressure, and when gasifying and circulating in the cement production equipment, condenses in a relatively low temperature part of the equipment to form a coating,
It also causes problems in cement production. In order to solve this problem, a part of the calcining gas is extracted from the bottom of the kiln of the cement kiln to reduce the amount of chlorine, alkali, and sulfur circulating in the cement manufacturing apparatus. However, when such calcination gas is extracted, extracted dust having a large content of chlorine, alkali, and sulfur is inevitably entrained, and the method of treating the dust becomes a new problem. That is, lead, cadmium, zinc, copper, chromium, manganese,
It contains harmful substances regulated by the Water Pollution Control Law, such as iron, mercury, selenium, and fluorine, and bleed dust is landfilled untreated, and if it is disposed of, it will cause environmental pollution. There is a need. Also, when the dust is reused as a cement raw material instead of being discarded, it is necessary to reduce the amount of alkali and chlorine contained in the dust and then return the dust to the raw material system.

Since the alkali and chlorine compounds contained in the extracted dust are water-soluble, it is a matter of course that washing is the most suitable method for removing alkali and chlorine from the extracted dust. (Eg, JP-A-49-86419 and JP-A-62-252351).
Publication). What is problematic here is a method of treating harmful substances containing heavy metals, which are eluted together with alkalis and salt compounds during the water washing treatment. There is a national standard for hazardous substances in wastewater, and wastewater that cannot be cleared even with one component cannot be released.

[0004] A method is known in which heavy metals contained in industrial effluents are adjusted to a pH at which the solubility of hydroxides of the respective elements is minimized and separated and removed by sedimentation (for example, "Encyclopedia of Environmental Management Equipment"). Industry Research Council, 1985). What applied this to the processing of bleed dust in cement manufacturing equipment,
It is disclosed in JP-A-6-157089. Among them, a method of precipitating and removing cadmium and lead in the slurry after the bleeding dust is washed with water at a pH at which the solubility of each hydroxide is minimized, and a method of adding a precipitation promoting agent such as sulfide. Are described. In the former method, it is impossible to clear the wastewater standard value of 0.1 mg / l for lead, which was revised in 1993 due to the existence of dissolution equilibrium. In the latter method, sodium sulfide and hydrogen sulfide were used. The amount of lead can be reduced to below the wastewater standard value by adding a precipitation accelerator such as the above. However, the amount of selenium unavoidably contained in the bleed dust is 0.1 mg / g, the national wastewater standard value. not drop below l
It is virtually impossible to discharge the wastewater.

[0005]

SUMMARY OF THE INVENTION The present invention is to provide a cement manufacturing apparatus which performs a water washing process on extracted dust accompanying the extracted gas when a part of the calcining gas is extracted in order to reduce the circulation of chlorine, alkali and sulfur. It is another object of the present invention to provide a treatment method in which solid content is reused as a raw material for cement, and a treatment liquid is capable of removing harmful substances contained therein to a level equal to or less than a effluent standard value and enabling discharge disposal. It is an object of the present invention to provide a processing method for reducing lead and selenium, which are difficult to remove, to 0.1 mg / l or less, which is the national standard.

[0006]

SUMMARY OF THE INVENTION The present invention relates to a cement manufacturing method for extracting a part of exhaust gas from a calciner in a cement manufacturing apparatus to reduce the amount of chlorine and alkali in a clinker. Adding water to the bleed dust to form a slurry, then performing solid-liquid separation, adding calcium polysulfide to the obtained liquid phase to reduce dissolved hexavalent selenium to tetravalent, and adding an iron salt to the slurry. pH value 9.
The present invention relates to a method for treating bleed dust, comprising a step of performing a second solid-liquid separation after precipitating residual heavy metals together with iron hydroxide in a state of 5 to 10.5. Hereinafter, the present invention will be described in detail.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, by combining a suitable reducing / precipitating agent with pH adjustment, precipitation, and solid-liquid separation as described below, the harmful substances in the bleed dust slurry are determined by the government. The removal to below the wastewater standard value was achieved. (1) First step: First, high pH is extracted from the extracted dust slurry.
Removing the dissolved metal other than lead and selenium in the region and the cement component containing the undissolved heavy metal salt by solid-liquid separation. (2) Second step: a step of adding calcium polysulfide to the liquid phase after solid-liquid separation to reduce and precipitate dissolved selenium to tetravalent. (3) Third step: Addition of iron salt and pH value of slurry at 9.5
A step of performing solid-liquid separation after co-precipitating dissolved heavy metals with iron hydroxide by combining factors set to 〜10.5.

In the first step, water is added to the extracted dust to form a slurry in order to wash the extracted dust with water to elute the alkali metal salt. Since the main component of the bleed dust is calcium oxide, the resulting slurry shows a high pH value,
The alkali metal salt to be removed shows a sufficient solubility for the purpose of reduction even at a high pH, and the pH value at the time of adding calcium polysulfide in the second step is not particularly limited. Is not required. The slurry is subjected to a first solid-liquid separation.

In the second step, calcium polysulfide is added to the liquid phase after solid-liquid separation to reduce dissolved hexavalent selenium to tetravalent selenium and to precipitate reduced selenium together with other heavy metals. Since it does not largely depend on the pH value, the solid-liquid separation liquid in the first step can be used as it is. In addition, the pH can be set to around 10 which is a pH value set at the time of solid-liquid separation in the next step.

The amount of calcium polysulfide to be added depends on the amount of selenium, but it is sufficient to set the weight ratio so that at least five times the amount of dissolved selenium is present. In addition, calcium polysulfide can be used in any form of solid or liquid without any problem. However, addition of a commercially available 27.5% by mass aqueous solution is most preferable in terms of availability and easy handling. The slurry thus obtained does not need to be subjected to a solid-liquid separation treatment, and can be sent to the next step as it is.

In the third step, since the purpose is to remove iron by adding an iron salt and co-precipitating dissolved selenium and lead with iron hydroxide, the final slurry pH value, that is, the pH value at the time of solid-liquid separation, Needs to be adjusted to 9.5 to 10.5. A particularly preferred pH value is 10. If the condition that the slurry pH value during solid-liquid separation is in this range is satisfied,
The order of the pH adjustment and the addition of the iron salt is not particularly problematic.
There is no particular limitation on the pH value at the time of addition of the iron salt.

Since iron salts do not utilize the redox function of iron salts, both ferrous and ferric salts can be used, but ferric salts have an effect of removing dissolved selenium. In addition to being expensive, it is inexpensive and is a more preferable material. It is needless to say that ferric sulfate or ferric chloride, which is the most common, inexpensive and easily available, is preferably used as the ferric salt. The amount of iron salt added
About 1000 ppm is sufficient. This is because the amount of dissolved metal to be removed does not increase by orders of magnitude if the preceding step is performed reliably. If the amount is too large, it is not only wasteful but also increases the time required for solid-liquid separation, which is not preferable.

In the third step, a polymer flocculant can be added to the slurry before the solid-liquid separation. Polymer flocculants have the effect of facilitating the solid-liquid separation by promoting the flocculation of precipitates containing iron hydroxide as a main component, but commercially available acrylamide polymer flocculants are nonionic, cationic, It can be used effectively regardless of the anionic system. The polymer coagulant may be used alone or in combination with an inorganic coagulant such as polyaluminum chloride and aluminum sulfate (sulfuric acid band). In addition, when adding a polymer flocculant, the addition amount of 1 to 5 ppm is sufficient.

In the present invention, the solid-liquid separation operation is performed at least twice. As a solid-liquid separation method, a generally used method such as a filtration method, a centrifugation method, and a sedimentation method is used. Can be. Among them, the filtration method is the most preferable method in terms of separation efficiency and cost.

The liquid phase in which the amounts of lead and selenium have been reduced to the effluent standard value or less in the third step of solid-liquid separation can be discharged after the pH value has been reduced to near neutrality.

[0016]

The present invention will be described in more detail with reference to specific examples below. 2000 g of water was added to 100 g of dust extracted from the bottom of the kiln of the cement manufacturing apparatus, and the mixture was stirred for 10 minutes to form a slurry. The pH of the resulting slurry was 12.8. The slurry is subjected to solid-liquid separation by filtration. The contents of lead and selenium contained in the obtained filtrate are 180 and 1.7 ppm, respectively, and the amounts of other heavy metals are set for each. It was below the wastewater standard value. For this raw water, removal of lead and selenium was examined by the method described below. The analysis of lead and selenium was performed by the atomic absorption method and the absorption spectrophotometry, respectively, according to JIS K0102. The detection limits are 0.01 ppm and 0.002 ppm, respectively.

A predetermined amount of 27.5 is added to 500 ml of the raw water.
27.5 mg of a mass% aqueous solution of calcium polysulfide was added. Then, after adding 200 mg of ferric sulfate, 1N
The pH of the slurry was adjusted to 10 by adding dilute sulfuric acid. After stirring the resulting slurry for 10 minutes, filtration was performed, and cadmium, lead and selenium in the filtrate were quantified. The respective analysis values were below the detection limit for lead and 0.037 ppm for selenium. This value clears the national effluent standard of 0.1 ppm.

[0018]

According to the method for treating extracted dust of the present invention, the solid content of which the alkali and chlorine contents have been reduced by water washing can be reused as a cement raw material. Hazardous substances, including difficult lead and selenium, are removed before the wastewater standards are cleared, and can be released.

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Claims (3)

[Claims] 1. A method for producing cement in which a part of exhaust gas from a firing furnace in a cement production apparatus is extracted to reduce the amount of chlorine and alkali in a clinker, wherein water is added to extracted dust entrained in the extracted gas to obtain a slurry. After the conversion, a step of solid-liquid separation, a step of adding calcium polysulfide to the obtained liquid phase to reduce dissolved hexavalent selenium to tetravalent, and a step of adding an iron salt to the slurry to obtain a pH value of 9.5 to 10. 5. A method for treating bleed dust, comprising: a step of precipitating residual heavy metals together with iron hydroxide in the state of 5 and then performing a second solid-liquid separation. 2. The iron salt is a ferric salt,
The method for treating bleed dust according to claim 1. 3. The method for treating bleed dust according to claim 1, wherein a polymer coagulant is present when the residual heavy metal is precipitated together with the iron hydroxide.