JP4479116B2 - Cement production equipment extraction dust processing method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for treating dust containing harmful substances (hereinafter referred to as extracted dust) that is entrained when a part of a firing gas is extracted in order to reduce the circulation of chlorine, alkali, and sulfur in a cement manufacturing apparatus. It is about.
[0002]
[Prior art]
When cement raw materials with a high chlorine, alkali and sulfur content 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 chlorine, alkali and sulfur When a compound with high vapor pressure is formed and gasified and circulated in the cement manufacturing equipment, it forms a coating by condensing at a relatively low temperature in the equipment, causing a problem in cement production. ing. In order to solve this problem, a part of the firing gas is extracted from the kiln bottom portion of the cement kiln to reduce the amount of chlorine, alkali, and sulfur circulating in the cement manufacturing apparatus. However, when such calcination gas extraction is performed, extraction dust with a high 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, zinc, copper, chromium, manganese, iron, mercury, selenium, and fluorine in addition to chlorine, alkali, and sulfur. If the extracted dust is reclaimed untreated and discarded, it causes environmental pollution. Therefore, it is necessary to treat it with an appropriate method. Also, when dust is not discarded but reused as a cement raw material, it is necessary to return to the raw material system after reducing the amount of alkali and chlorine contained in the dust.
[0003]
Since the alkali and chlorine compounds contained in the extraction dust are water-soluble, it is natural that the water washing treatment is most suitable as a method for removing alkali from the extraction dust and the chlorine removal compound, and is already known ( For example, JP-A-49-86419 and JP-A-62-2252351). The problem here is a method for treating harmful substances including heavy metals that are eluted together with alkalis and salt compounds during the water washing treatment. There are national standards for hazardous substances in wastewater, and wastewater that cannot be cleared even with one component is not allowed to be discharged.
[0004]
A method for separating and removing heavy metals contained in industrial wastewater by adjusting the pH so that the solubility of each element's hydroxide is minimized is known (for example, “Environmental Management Equipment Encyclopedia” Sangyo Kenkyukai Co., Ltd.). 1985). Japanese Laid-Open Patent Publication No. 6-1557089 discloses an application of this to extraction dust extraction in a cement manufacturing apparatus. Among them, a method of precipitating and removing cadmium and lead in the slurry after the extraction dust is washed with water at a pH at which the solubility of each hydroxide is minimized, and a method of adding a precipitation accelerator such as sulfide Two are described. In the former method, it is impossible to clear the 0.1 mg / l effluent standard value for lead revised in 1993 due to the presence of dissolution equilibrium. In the latter method, sodium sulfide, hydrogen sulfide The amount of lead can be reduced to below the effluent standard value due to the addition of a precipitation accelerator such as selenium, but the amount of selenium inevitably contained in the extracted dust is also the national effluent standard value of 0.1 mg / It does not drop below 1 and it is virtually impossible to discharge the waste water.
[0005]
[Problems to be solved by the invention]
In the present invention, in order to reduce the circulation of chlorine, alkali, and sulfur in a cement production apparatus, when a part of the calcination gas is extracted, the extraction dust accompanying the extraction gas is washed with water, and the solid content is reused as a cement raw material. In addition, the treatment liquid has an object to provide a treatment method that enables discharge disposal by removing harmful substances contained in the treatment liquid to below the effluent standard value. An object of the present invention is to provide a treatment method for reducing lead and selenium, which are difficult to remove, to a national standard value of 0.1 mg / l or less.
[0006]
[Means for Solving the Problems]
The present invention relates to a cement manufacturing method for extracting a part of exhaust gas from a firing furnace in a cement manufacturing apparatus to reduce the amount of chlorine and alkali in a clinker, and adding slurry to extraction dust accompanied by extraction gas to form a slurry. After that, the step of solid-liquid separation, the step of reducing the hexavalent selenium dissolved by adding calcium polysulfide without adjusting the pH to the obtained liquid phase, the salt pH value of 9 by adding iron salt The present invention relates to a method for treating bleed dust, comprising the step of precipitating residual heavy metal together with iron hydroxide in a state of 0.5 to 10.5 and then performing a second solid-liquid separation. The present invention is described in detail below.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, by combining appropriate reducing / precipitating agents as shown below, pH adjustment, precipitation, and solid-liquid separation, harmful substances in the extracted dust slurry can be removed to a level equal to or lower than the national wastewater standard value. Is achieved.
(1) First step: First, a step of removing cement components containing lead, dissolved metals other than selenium, and undissolved heavy metal salts from the extracted dust slurry in a high pH region by solid-liquid separation.
(2) Second step: A step of adding calcium polysulfide to the liquid phase after solid-liquid separation, and reducing and precipitating dissolved selenium to tetravalent.
(3) Third step: combining the addition of iron salt and the factor for setting the slurry pH value to 9.5 to 10.5, coprecipitating dissolved heavy metal with iron hydroxide, and then performing solid-liquid separation Process.
[0008]
In the first step, since the extracted dust is washed with water to elute the alkali metal salt, water is added to the extracted dust to form a slurry. Since the main component of the extracted dust is calcium oxide, the resulting slurry exhibits a high pH value, but the alkali metal salt to be removed exhibits sufficient solubility for reduction purposes even under high pH, and Since the pH value when adding the two-stage calcium polysulfide is not particularly limited, no special pH adjustment is necessary.
The slurry undergoes a first solid-liquid separation.
[0009]
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 precipitate the reduced selenium together with other heavy metals. Does not depend greatly, the solid-liquid separation liquid in the first step can be used as it is. Moreover, it can also set to 10 vicinity which is the pH value set at the time of solid-liquid separation in the next process.
[0010]
The amount of calcium polysulfide added depends on the amount of selenium, but it is sufficient to set it so that there is at least 5 times the amount of dissolved selenium by weight.
Calcium polysulfide can be used in a solid or liquid form without any problem, but addition with a commercially available 27.5% by mass aqueous solution is most preferable from the viewpoint of easy availability and handling.
The slurry obtained here does not need to be subjected to a solid-liquid separation process, and can be sent to the next step as it is.
[0011]
In the third step, iron salt is added to remove dissolved selenium and lead by coprecipitation with iron hydroxide, so that the final slurry pH value, that is, the pH value during solid-liquid separation is set to 9. There is a need to adjust to 5-10.5. A particularly preferred pH value is 10.
If the conditions that the slurry pH value during solid-liquid separation is within this range are satisfied, the order of pH adjustment and iron salt addition is not particularly problematic. Moreover, there is no restriction | limiting in particular also in the pH value at the time of iron salt addition.
[0012]
Since iron salt does not use the redox function that it has, either ferrous or ferric salt can be used, but ferric salt is highly effective in removing dissolved selenium. In addition, it is inexpensive and is a more preferable material. Needless to say, it is preferable to use ferric sulfate or ferric chloride which is the most common, inexpensive and easily available as the ferric salt.
About 1000 ppm is sufficient for the amount of iron salt added. This is because the amount of dissolved metal to be removed does not increase by an order of magnitude if the pre-process is performed reliably. Too much is not preferable because it is not only useless but also increases the time required for solid-liquid separation.
[0013]
In the third step, a polymer flocculant can be added to the slurry before solid-liquid separation. The polymer flocculant has the effect of facilitating the aggregation of precipitates mainly composed of iron hydroxide to facilitate solid-liquid separation. However, commercially available acrylamide-based polymer flocculants are nonionic, cationic, It can be used effectively regardless of anionic type. The polymer flocculant may be used alone or in combination with an inorganic flocculant such as polyaluminum chloride or aluminum sulfate (sulfuric acid band). In addition, when adding a polymer flocculant, 1-5 ppm is sufficient for the addition amount.
[0014]
In the present invention, the solid-liquid separation operation is performed at least twice. As a solid-liquid separation method, a commonly used method such as a filtration method, a centrifugal separation method, a sedimentation separation method, or the like can be used. Among these, the filtration method is the most preferable method from the viewpoint of separation efficiency and cost.
[0015]
The liquid phase in which the amounts of lead and selenium are reduced to below the drainage standard value in the solid-liquid separation in the third step can be discharged after the pH value is lowered to near neutrality.
[0016]
【Example】
Hereinafter, the present invention will be described in more detail with reference to specific examples.
To 100 g of dust extracted from the kiln bottom of the cement manufacturing apparatus, 2000 g of water was added and stirred for 10 minutes to form a slurry. The pH of the product slurry was 12.8. The slurry is subjected to solid-liquid separation by filtration. The content of lead and selenium contained in the obtained filtrate is 180 and 1.7 ppm, respectively, and the amount of the other heavy metals is set for each. It was below the effluent 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 carried out by atomic absorption method and absorptiometric method according to JISK0102. The detection limits are 0.01 ppm and 0.002 ppm, respectively.
[0017]
A predetermined amount of 27.5 mg of a 27.5 mass% calcium polysulfide aqueous solution was added to 500 ml of the raw water. Next, 200 mg of ferric sulfate was added, and 1N diluted sulfuric acid was added to adjust the pH of the slurry to 10. The resulting slurry was stirred for 10 minutes and then filtered, and cadmium, lead and selenium in the filtrate were quantified. Each analytical value was below the detection limit for lead and 0.037 ppm for selenium. This value clears the national wastewater standard of 0.1 ppm. [0018]
【The invention's effect】
When the method for treating extracted dust of the present invention is carried out, the solid content whose alkali content and chlorine content have been reduced by washing with water can be reused as a cement raw material. Including toxic substances will be removed by the time the wastewater standard value is cleared, allowing discharge.

Claims (5)

In a cement manufacturing method in which a portion of the exhaust gas from the firing furnace in a cement manufacturing apparatus is extracted to reduce the amount of chlorine and alkali in the clinker, water is added to the extracted dust accompanied by the extracted gas, Step of liquid separation, Step of reducing dissolved hexavalent selenium to tetravalent by adding calcium polysulfide without adjusting pH to the obtained liquid phase, Adding iron salt to slurry pH value of 9.5-10 A process for treating bleed dust, comprising the step of precipitating residual heavy metal together with iron hydroxide in the state of .5, followed by a second solid-liquid separation. The method for treating extracted dust according to claim 1, wherein the iron salt is a ferric salt. The method for treating extracted dust according to claim 1 or 2, wherein a polymer flocculant is present when the residual heavy metal is precipitated together with iron hydroxide. The processing method of the extraction dust of any one of Claim 1 to 3 whose addition amount of the said calcium polysulfide is 5 times or more of the amount of dissolved selenium by weight ratio. The method for treating extracted dust according to any one of claims 1 to 4, wherein the solid content after the second solid-liquid separation is reused as a cement raw material.