JP4426923B2 - Cement kiln exhaust gas treatment method - Google Patents

Description

  The present invention relates to a method for treating exhaust gas containing heavy metals such as mercury discharged from a cement kiln that uses various types of waste as raw materials and fuels.

In recent years, it has become difficult to secure a final disposal site for waste (landfill, etc.), and wastes such as various sludges and incinerated ash are used as raw materials or fuel for cement production. .
However, heavy metals such as mercury, zinc, and selenium contained in these wastes volatilize in the cement kiln and are discharged in a state of being contained in the exhaust gas as volatile components or solids. Since heavy metals such as mercury in the exhaust gas are harmful substances, it is necessary to remove them using an appropriate method.
Under such circumstances, various methods have been proposed as a method for removing heavy metals and the like from the exhaust gas of a cement kiln.

For example, the dust collection dust collected from the exhaust gas from the cement manufacturing process is guided to a heating furnace and heated to a temperature higher than the volatilization temperature of the volatile metal component (for example, mercury) contained in the dust collection dust, and the volatile metal component is gasified. There has been proposed a method for treating exhaust gas from cement production, characterized in that dust collection dust that has been converted to a volatile metal component is used as part of the cement raw material (Patent Document 1).
Japanese Patent Laid-Open No. 2002-355531

The method for treating cement production exhaust gas described in the above-mentioned document is to heat the collected dust dust and volatilize and remove volatile metal components such as mercury. A large amount of solids that do not require heating other than the metal component are heated, which is uneconomical from the viewpoint of the volume of the heating furnace and the amount of heat energy used.
On the other hand, when the collected dust collection dust is used as a part of the cement raw material without heating, heavy metals contained in the dust collection dust are volatilized again in the cement kiln, and then the cement production facility. It will circulate inside. In addition, with the supply of new raw materials into the cement kiln, the amount of heavy metal circulating in the cement manufacturing facility will gradually increase. Therefore, it is not preferable to use the dust collection dust as a cement raw material without any treatment.
Then, an object of this invention is to provide the method which can remove heavy metals, such as mercury contained in the waste gas of a cement kiln efficiently, without providing a heating means.

As a result of intensive studies to solve the above problems, the present inventor found that dust contained in the exhaust gas of the cement kiln includes dust having a high heavy metal content and dust having a low heavy metal content. Then, by selectively recovering dust having a high heavy metal content from these dusts using a predetermined means, and putting only dust having a low heavy metal content into the cement kiln, The present invention has been completed by finding out that the object can be achieved.
That is, the present invention is to provide a method of the following [1].
[1]
The dust contained in the exhaust gas from the cement kiln is separated and recovered into dust with a high content of heavy metals and dust with a low content of heavy metals using the specified means, and then the content of heavy metals obtained. A method for treating exhaust gas from a cement kiln that supplies low-dust dust to the cement kiln , wherein the predetermined means is means for recovering unburned carbon as dust having a high content of heavy metals. Cement kiln exhaust gas treatment method.

According to the method of the present invention, a method of heating and volatilizing a heavy metal such as mercury contained in the exhaust gas of the cement kiln as a solid component (in other words, a method of consuming a large amount of heat energy) is not used. In addition, it can be removed efficiently.
Further, according to the method of the present invention, dust with a low content of heavy metals is returned to the cement kiln and used as a raw material for cement clinker production, so that the amount of heavy metals circulated in the cement production facility is reduced to a small amount. Can be suppressed. However, the conventional method of heating and volatilizing heavy metals can be used in combination with the method of the present invention, and if the heavy metal content is reduced by processing using the method of the present invention in the previous stage, It is possible to reduce the consumption of heat energy in the later stage (conventional method).
Furthermore, according to the method of the present invention, in order to recover both dust having a high content of heavy metal and dust having a low content of heavy metal, heavy metal is contained in the exhaust gas discharged out of the cement manufacturing facility. It is not substantially contained and can prevent the diffusion of heavy metals into the surrounding environment.

According to the exhaust gas treatment method of the present invention, dust contained in the cement kiln exhaust gas is separated and recovered into dust having a high heavy metal content and dust having a low heavy metal content using a predetermined means. Thereafter, the obtained dust having a low content of heavy metal is supplied to the cement kiln.
Here, the “cement kiln” refers to a heating furnace for firing cement clinker, such as a rotary kiln.
“Cement kiln exhaust gas” means exhaust gas generated by firing or calcining (decarbonation reaction) of a cement clinker raw material in a cement kiln and related equipment (for example, a calcining furnace).
In this specification, “dust” is a general term for solids in exhaust gas, and includes coarse particles (particles having a particle size exceeding 60 μm), fine particles (particles having a particle size of 10 to 60 μm), And fine particles (having a particle size of less than 10 μm).
"Predetermined unit", as will be described later, is "a high heavy metal content of the dust, means for recovering unburned carbon."
In the present invention, “heavy metal” is mainly a highly volatile heavy metal (having a relatively low temperature during volatilization), and specifically includes mercury, zinc, selenium, thallium and the like.

The above “predetermined means” will be described below.
"Predetermined unit" as a high content of heavy metals dust is a means for recovering unburned carbon.
We have obtained knowledge that unburned carbon has a particularly high ability to adsorb heavy metals such as mercury among dust, so let's efficiently remove heavy metals in exhaust gas by separating and collecting the unburned carbon. That's it.
The purity of the unburned carbon recovered is preferably 60% by mass or more, more preferably 70% by mass or more, and particularly preferably 80% by mass or more. Most of the unburned carbon is fine particles having a particle size of 10 μm or less.
In this case, examples of means for collecting unburned carbon include dry means such as a wind separator and a vibration sieve, and wet means such as a separation means using an organic solvent such as heavy oil and kerosene and water.
Here, the wind separator is blown from below using a blower, and at the same time, dust is thrown in and dropped from above, and a part of the dust (mainly composed of unburned carbon) is moved upward, And it is an apparatus which collect | recovers unburned carbon from dust by moving the remainder (thing which does not contain unburned carbon) of dust downward.

The vibration sieve is a method in which dust is contained in a predetermined container, and then the container is automatically vibrated so that a part of the dust (mainly composed of unburned carbon) forms a lower layer, and The apparatus is configured so that the remaining portion (without unburned carbon) forms an upper layer, and then only the lower layer portion (with unburned carbon as a main component) is recovered.
The separation means using an organic solvent and water is an organic solvent layer in which an organic solvent such as toluene, water, and dust are accommodated in a predetermined container, shaken for a predetermined time, and then allowed to stand. Then, the organic solvent layer separated and recovered from this liquid is subjected to mixing, shaking and separation treatment with water several times, and the organic solvent layer is undissolved in the organic solvent. This means is configured to extract the fuel carbon. In addition, the unburned carbon in the organic solvent can be easily recovered as a solid component by heating the organic solvent as necessary to evaporate.
Separation means using an organic solvent and water is that unburned carbon is oleophilic and dust (other than unburned carbon) is hydrophilic, so that dust (other than unburned carbon) is in the water layer, This is because the unburned carbon is separated into the organic solvent layer.

Hereinafter, the exhaust gas treatment method of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing an exhaust gas treatment system used in the method of the present invention.
In FIG. 1, first, cement clinker raw materials (specifically, waste such as limestone, clay, silica stone, iron slag, foundry sand, slag, alumina sludge, municipal waste incineration ash, etc.) are dried (dryer). After drying at 1, the mixture is pulverized and mixed by a raw material mill (pulverizer) 2, and fly ash is further added as required, and then supplied to the suspension preheater 4 via the raw material supply path 3.
Here, the suspension preheater 4 is for preheating the cement clinker raw material while performing heat exchange, and is composed of a plurality of cyclones 4a, 4b, 4c, and 4d.
In the suspension preheater 4, the cement clinker raw material is preheated while being sequentially moved from the upper cyclone 4 a to the lower cyclone and supplied to the rotary kiln 5. The rotary kiln 5 is a heating furnace (cement kiln) for firing cement clinker.
A calcining furnace 6 may be provided between the suspension preheater 4 and the rotary kiln 5. The calcination furnace 6 is provided to promote calcination (decarbonation reaction) of the cement raw material.

The cement clinker raw material supplied into the rotary kiln 5 is fired under a temperature condition of 1,300 to 1,600 ° C. to become a cement clinker. The cement clinker discharged from the rotary kiln 5 is finely pulverized in the finishing mill 7 after adding gypsum and the like to become cement.
Exhaust gas generated by combustion in the rotary kiln 5 and calcining furnace 6 and exhaust gas generated when the cement raw material is thermally decomposed (decarbonation reaction) flow into the suspension preheater 4 from the rotary kiln 5 and calcining furnace 6. Then, after moving upward in the suspension preheater 4 and reaching the uppermost cyclone 4a, the exhaust gas path 8 connected to the uppermost cyclone 4a (indicated by a one-dot chain line in FIG. 1). Flows in. The exhaust gas passes through the raw material mill 2 and the dryer 1 in order to become a heat source, flows into the dust collector 9 from the exhaust gas path 8, and is subjected to a process for removing dust in the exhaust gas in the dust collector 9, and then processed. The gas is finally discharged from the chimney 11 to the atmosphere via the spent gas discharge passage 10.

The dust collector 9 is for collecting dust containing heavy metals (for example, mercury) in the exhaust gas of the cement kiln. As the dust collector 9, an electric dust collector is usually used, but other dust collectors (for example, a gravity dust collector, an inertial force dust collector, a centrifugal dust collector, a filtration dust collector, etc.) may be used.
The place where the dust collector 9 is installed is preferably determined at a point where the temperature of the exhaust gas discharged from the suspension preheater 4 is lowered to 30 to 150 ° C. (particularly 50 to 120 ° C.). When dust is collected at a relatively low temperature in this way, the amount of heavy metal (for example, mercury) contained in the dust increases, so the concentration of heavy metal contained in the exhaust gas after processing by the dust collector 9 is reduced. Can be convenient.
The dust collected by the dust collector 9 usually includes about 0.1 to 100 mg / kg of mercury and other heavy metals, although it varies depending on the type of waste constituting a part of the cement raw material. Yes.

The dust collected by the dust collector 9 is subjected to a process of removing dust having a high content of heavy metals such as mercury (removal process of a high concentration portion), and then as a dust having a low content of heavy metals, the raw material mill 2 and the suspension. At a predetermined point between the preheaters 4, the raw material is supplied to the raw material supply path 3 and becomes a part of the cement clinker raw material.
As described above, the removal step of the high concentration part is specifically performed by selective removal of unburned carbon.
In FIG. 1, the path indicated by an arrow A indicates a case where a high concentration portion removal step is not provided (when not corresponding to the present invention), and the route indicated by an arrow B includes a high concentration portion removal step. The case (when applicable to the present invention) is shown.

[Example 1]
100 g of dust collected by an electric dust collector in a cement factory, 200 ml of toluene, and 200 ml of distilled water were placed in a 1 liter separatory funnel and shaken for 30 minutes, and then the toluene layer and the water layer were separated. . This operation was repeated several times to extract unburned carbon into the toluene layer. Next, the toluene layer was recovered, and this toluene was naturally evaporated at room temperature to obtain unburned carbon (purity: 90%) as a solid content. After this solid content was dried, the mass was measured to determine the amount of unburned carbon.
Next, for each of dust containing unburned carbon (dust before treatment), unburned carbon, and dust not containing unburned carbon (dust after treatment), a dedicated mercury measurement device (trade name: SP-3D, Japan) The amount of mercury was measured using Instruments, Inc.
The results are shown in Table 1. From Table 1, it can be seen that removing unburned carbon from the dust collected by the electric dust collector significantly reduces the amount of mercury contained in the dust.

It is a figure which shows the processing system of the waste gas used with the method of this invention.

Explanation of symbols

1 dryer (dryer)
2 Raw material mill (pulverizer)
DESCRIPTION OF SYMBOLS 3 Raw material supply path 4 Suspension preheater 4a, 4b, 4c, 4d Cyclone 5 Rotary kiln 6 Calcining furnace 7 Finishing mill 8 Exhaust gas path 9 Dust collector 10 Drain path for processed gas 11 Chimney A Path not belonging to the method of the present invention B Present invention Routes belonging to the method

Claims (1)

The dust contained in the exhaust gas from the cement kiln is separated and recovered into dust with a high content of heavy metals and dust with a low content of heavy metals using the specified means, and then the content of heavy metals obtained. A method for treating exhaust gas from a cement kiln that supplies low-dust dust to the cement kiln ,
The method for treating exhaust gas from a cement kiln, wherein the predetermined means is means for collecting unburned carbon as dust having a high content of heavy metals .

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