JP4923761B2 - Cement manufacturing method - Google Patents

Description

  The present invention relates to a technique for removing water-soluble mercury from a cement manufacturing process in order to suppress the amount of mercury discharged from the cement manufacturing process.

  In recent years, the amount of waste that can be received at the final disposal site has been tightened, and the amount of waste in the cement manufacturing process has increased. Cement factories treat various wastes such as sludge, incineration ash, waste plastic, wood chips, and waste liquid as raw materials for cement, some of which contain mercury. Mercury is also contained in natural mineral cement raw materials such as limestone and clay. Since mercury has a low boiling point and high volatility, the mercury contained in the cement raw material charged into the preheater volatilizes in the preheater and exits the preheater together with the kiln combustion exhaust gas. However, most of the volatilized mercury is condensed due to a decrease in the exhaust gas temperature and adheres to the cement kiln dust. Since this dust is finally collected by a dust collector and used again as a raw material, mercury circulates in the cement manufacturing process. In other words, in the cement manufacturing process, new mercury coming from raw materials and mercury circulating in the process show complex behavior, and some mercury that cannot be collected by the dust collector is discharged outside the system as exhaust gas. It has been discharged. When the carry-in of mercury from waste raw fuel increases, the amount of circulation increases, and as a result, there is a concern that a large amount of mercury is discharged.

Several techniques have been devised to suppress mercury emissions from cement kiln exhaust gas. Techniques have been devised to remove raw materials and dust collection dust used in cement production by heating and volatilizing mercury components (see Patent Documents 1 and 2).
In addition, a technique has been devised that removes mercury in the exhaust gas by extracting a part of the exhaust gas at 350 ° C. or higher after the preheater and cooling it to 100 ° C. or lower (see Patent Document 3).

Japanese Patent Laid-Open No. 2002-355531 JP 2003-55010 A JP 2005-97005 A

  However, when heat-treating raw materials and dust collection dust, enormous facilities are required, and the cost of treating mercury removed by heating is also high. Further, in the case of the method of extracting the exhaust gas, a device for cooling the extracted exhaust gas to 100 ° C. or less has to be installed, and there is a problem of processing of the removed mercury.

  The present invention has been made in view of such a problem, and an object of the present invention is to provide a method for producing a cement that removes water-soluble mercury contained in a cement raw material and further easily treats the removed mercury. And

  As a result of extensive research, the present inventors have found that the above problems can be solved by washing the cement raw material, focusing on the form of mercury contained in the cement raw material, and have completed the present invention.

That is, this invention relates to the manufacturing method of the cement which removes water-soluble mercury by wash-processing coal ash, and uses it as a cement raw material. The cement raw material of the present invention is preferably coal ash, municipal waste incineration ash, and / or sewage sludge incineration ash. Moreover, it is preferable to mix 500-1500 mass parts of water with respect to 100 mass parts of coal ash . Also, wash water is added to the cement finishing mill.

According to the cement production method of the present invention, the mercury content can be reduced by washing the coal ash with water, and there is no need for large-scale equipment.

  Hereinafter, a preferred embodiment of a method for producing a cement according to the present invention will be described with reference to the drawings.

FIG. 1 is a schematic view showing an example of a cement production facility to which a method for producing cement according to the present invention is applied. Cement raw materials are supplied to a preheater after drying, pulverization, and mixing, and then passed through a cement kiln 10. Fired to become cement clinker. The main cement raw materials are limestone, clay and silica stone. In recent years, the use of coal ash, incineration ash such as municipal waste incineration ash, and sewage sludge has increased. Coal is used as cement fired fuel, but in recent years, the amount of waste fuel such as waste plastic and waste tires is increasing.

  A preheater preheats a cement raw material using the residual heat of a cement kiln, and decarboxylates limestone. The preheater is usually composed of four or five-stage cyclones 6a to 6e, and the cement raw material charged into the uppermost cyclone 6e is sequentially sent to the lower cyclone 6a. Meanwhile, the cement raw material is heated and finally supplied to the cement kiln 1 from the kiln inlet hood 4. The temperature of the uppermost cyclone 6e is about 400 ° C., but the temperature rises as it goes to the lower cyclone, and the temperature in the lowermost cyclone 6a becomes 800 ° C. to 900 ° C. Further, in order to further increase the rate of decarbonation of the cement raw material by the preheater, there are many cement factories in which the calcining furnace 5 is installed at the lower part of the preheater. Usually, the temperature inside the calciner is about 1000 ° C.

  The cement raw material passed through the calcining furnace 5 enters the cement kiln 1 from the kiln inlet hood 4 and is fired to become a cement clinker. A rotary kiln is usually used as the cement kiln, and the temperature in the vicinity of the raw material inlet (on the kiln inlet hood) is usually about 1000 ° C., but the maximum temperature in the kiln reaches about 1500 ° C. The cement clinker fired in the cement kiln 1 is cooled by a clinker cooler, and then pulverized with gypsum in a finishing pulverization mill 3 to become cement.

  Cement kiln combustion exhaust gas exits the preheater, passes through the raw material mill 7, the humidity control tower 8, the raw material dryer 9, and the like, and is removed by the dust collector 10 and then released to the atmosphere. The route through which the exhaust gas after the preheater passes varies from factory to factory. The dust collector used for dust removal of cement kiln exhaust gas is generally an electric dust collector (EP), but a bag filter may be used. The temperature of the exhaust gas at the outlet of the preheater is about 400 ° C. However, since the heat is exchanged by the raw material mill 7, the humidity control tower 8, the raw material dryer 9, etc., the temperature of the exhaust gas before entering the dust collector 10 is usually about 100 ° C. is there.

  Mercury is contained in trace amounts in natural minerals and waste. The main cement raw fuels containing mercury are limestone, clay, coal, coal ash, sewage sludge, municipal waste incineration ash, etc., and mercury is newly brought into the cement manufacturing process from these raw fuels. The amount of mercury brought in varies depending on the mercury content of the raw material and the amount of raw material used. In addition, waste materials have a large variation in mercury content, and the amount of mercury brought in varies even with the same raw material formulation.

  While the boiling point of mercury is 357 ° C., the temperature at the outlet of the preheater is about 400 ° C., the mercury contained in the kiln feed material volatilizes in the preheater and is contained in the preheater exhaust gas. The preheater exhaust gas is finally discharged out of the system through the dust collector through the steps of the raw material mill 7, the humidity control tower 8, the raw material dryer 9, and the like. Mercury contained in the exhaust gas is in a gaseous state immediately after the preheater, but due to the subsequent decrease in exhaust gas temperature, most of the mercury is condensed in front of the dust collector and adheres to the dust. For this reason, the mercury adhering to the dust is collected by the dust collector, but a part of the mercury remaining in the gaseous state is discharged out of the system. On the other hand, the dust collected by the dust collector is mixed with the cement raw material and returned to the cement manufacturing process again. Therefore, some of the mercury brought from the raw fuel into the cement manufacturing process is discharged out of the system, but most is circulating in the cement manufacturing process. The balance between the amount of mercury circulated and the amount of emissions varies depending on the raw fuel used, kiln operating conditions, etc. If the amount of mercury circulated exceeds a certain limit, it is considered that the amount of mercury discharged outside the system increases.

  Mercury contained in raw cement fuel is considered to be metallic mercury alone or mercury chloride. Mercury mercury is hardly soluble in water, but mercury chloride is water-soluble and soluble in water. Therefore, the amount of mercury brought into the cement manufacturing process can be reduced by previously washing and removing water-soluble mercury chloride, and the discharge of mercury can be reduced.

The cement raw material of the present invention may be anything as long as it contains mercury, but the one with a high mercury content and the one with a large amount of use are more efficient. In order to improve the cleaning efficiency, it is preferable that the cement raw material is crushed as finely as possible.
The cement raw material is preferably coal ash, municipal waste incineration ash and / or sewage sludge incineration ash. More preferred is coal ash. The reason why coal ash is particularly effective for washing water among these cement raw materials is unclear, but since the porosity is higher than other cement raw materials, it is considered that the area in contact with water is large. Also, coal ash is used in a large amount in cement raw materials, and the total mercury content in the cement kiln can be effectively reduced by reducing the mercury content in the coal ash.

  The water used for cleaning the cement raw material may be normal water such as industrial water, tap water, or distilled water. The amount of water used may be determined in consideration of the concentration of mercury contained in the raw material and dust, the solubility of mercury chloride, and the like. It is better to add agitation so that mercury chloride contained in the cement raw material is easily eluted. It is preferable to use 500 to 1500 parts by mass, preferably 800 to 1200 parts by mass of water based on 100 parts by mass of coal ash. If the amount is less than 500 parts by mass, the removal of mercury may be insufficient. If the amount exceeds 1500 parts by mass, the amount of water used is enormous, leading to an increase in production costs.

  The cement clinker fired in the cement kiln is pulverized in the finishing mill 3 together with gypsum to become the cement of the final product. The temperature at the time of cement finishing is usually 100 ° C. or higher due to frictional heat or the like. The gypsum used for cement finishing is usually dihydrate gypsum, but crystal water dehydrates when the temperature rises. Since the form of gypsum contained in the cement affects the quality of the cement, it may be necessary to suppress the temperature rise of the mill during pulverization. For this reason, water may be sprayed directly on the cement in the mill to cool the cement. The amount of water sprayed at this time varies depending on the amount of cement ground, the clinker temperature, and the like. The water used for cleaning the cement raw material can be used as cooling water for the cement. Mercury contained in the washing water is water-soluble and therefore does not volatilize and is directly taken into the cement. Mercury incorporated into the cement is not released into the environment because it is incorporated into the hardened body by hydration and solidification. Since the amount of water sprayed on the cement varies depending on the type, size, pulverization amount, clinker temperature, and the like of the mill, it may be adjusted to an optimum pulverization temperature. When using cleaning solutions of acid and alkali solutions, the influence on cement quality should be confirmed in advance, and measures such as dilution may be taken as appropriate if they have an adverse effect.

The cleaning process of the cement raw material of the present invention will be described with reference to FIG.
As shown in FIG. 1, the cleaning water in the water supply tank 11 and the raw material of the raw material silo 13 are charged into the mixer 12 and mercury in the raw material is removed by the cleaning liquid. The raw material after mercury removal is fed from the top of the suspension preheater. The washing water of the mixer 12 is put into the finishing crushing mill 3 and used as cooling water for cement.

Examples The contents of the present invention will be described more specifically with reference to examples and comparative examples. However, the present invention is not limited to the following examples.

  5 g each of coal ash (brand A, B, C) and municipal waste incineration ash (brand D) whose mercury sources had been measured in advance were added to a 100 mL beaker, and 50 mL of distilled water was added thereto. Stir with a stirrer for 5 minutes. Immediately after stirring, suspensions of various ash and distilled water were filtered. The filtration residue was dried, and the mercury content analysis of various ashes after the water washing treatment was performed. The mercury content was analyzed according to JCAS I-51-2000 “Method for quantifying trace components in cement by ICP emission spectroscopic analysis and electric heating atomic absorption analysis”. From the mercury content before and after the water washing treatment, the mercury reduction rate due to washing was calculated according to the following formula. The results are shown in Table 1.

  Mercury reduction rate (%) = (Mercury concentration before water washing treatment−Mercury concentration after water washing treatment) / (Mercury treatment concentration before water washing treatment) × 100

  From Table 1, it was found that the mercury content of the ash was reduced by washing with water, although the reduction rate was different depending on the ash brand.

It is the schematic which shows an example of the cement manufacturing equipment with which the manufacturing method of the cement which concerns on this invention is applied.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Cement kiln 2 Clinker cooler 3 Finish grinding mill 4 Kiln inlet hood 5 Calciner 6a-6e Cyclone 7 Raw material mill 8 Humidity control tower 9 Raw material dryer 10 Dust collector 11 Water supply tank 12 Mixer 13 Raw material silo

Claims (2)

  A method for producing cement, comprising: washing coal ash with water to remove water-soluble mercury, and then using the resultant as a raw material for cement; and adding washing water to a cement finishing mill. The method for producing cement according to claim 1, wherein 500 to 1500 parts by mass of the water is mixed and washed with respect to 100 parts by mass of the coal ash .

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