JP2009028594A - Method of manufacturing granular product utilizing coal ash containing harmful chemical substance and granular product and resource material obtained thereby - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a treatment method enabling one-process treatment of coal ash possibly containing at least one of hexavalent chromium, arsenic, selenium, fluorine and boron and fixing of these harmful substances in a hardly leaching-out state. <P>SOLUTION: A method of manufacturing a granular product utilizing coal ash comprises adding a reducing agent, lime, cement, a heavy-metal-fixing agent and water to coal ash possibly containing the harmful substance(s), e.g. hexavalent chromium and kneading and granulating the resultant mixture. The granular product and a resource material comprising the product are also provided. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for producing a granular material useful as a recycling material using coal ash such as fly ash generated in a thermal power generation facility using coal as a fuel, the granular material thus produced, and a resource comprising the granular material Regarding materials.

  Thermal power generation burns various types of coal (coal species) collected from coal production sites around the world in order to generate economically and efficiently. For this reason, some coal types may contain hexavalent chromium, arsenic, selenium, fluorine, and boron, and coal ash containing these harmful chemical substances can be generated by combustion. These harmful chemical substances need to be regulated because they can cause environmental pollution of the soil. Therefore, the “Environmental Standard for Soil Contamination (August 23, 1991, Environment Agency Notification No. 46)” is stipulated. Therefore, when the content of harmful chemical substances exceeds the environmental standard, there is a demand for a method of processing in a chemically stabilized state in consideration of economy so as to satisfy the standard.

  The best way to treat toxic chemicals is to remove the toxic chemicals by washing coal ash with acid or alkali. However, this method complicates the processes such as washing of coal ash, neutralization of coal ash, and installation of wastewater treatment containing toxic chemicals, resulting in a huge facility. Therefore, vast land is required, and the number of man-hours for maintenance from construction of the treatment facilities increases, so that recycled products of coal ash become expensive.

  When incinerating paper sludge, coal is sometimes used for auxiliary combustion, and the above-mentioned harmful chemical substances are also contained in the paper sludge incineration ash generated in that case. Water, quicklime, and cement are added to such paper sludge incineration ash, mixed, granulated, and then subjected to steam curing under high temperature and high pressure in an autoclave to produce a hydrothermal solidification reaction to produce a solidified body. A method is known (Patent Document 1). Although this method is said to be effective in suppressing elution of the harmful chemical substances, there is a disadvantage that it is necessary to perform curing under high temperature and high pressure using an apparatus such as an autoclave.

JP 2005-313032 A

  In addition, as a method for treating these regulated hazardous chemical substances, methods and techniques for individually treating each hazardous chemical substance have been published in the literature. However, combining such individual processing methods is not suitable as a fly ash recycling technique because it complicates the process and increases the cost. Accordingly, there is a need for a treatment method that can treat all of these plurality of harmful chemical substances in one step, and a fly ash recycled product manufactured by applying this method.

  Therefore, the problem of the present invention is that a special apparatus such as an autoclave and a hydrothermal reaction are not required for coal ash that may contain at least one kind of hexavalent chromium, arsenic, selenium, fluorine, and boron, and simple. It is an object of the present invention to provide a treatment method that can be processed even in a single step and that can immobilize these harmful chemical substances in a state where it is difficult to elute them.

The present invention provides a means for solving such a problem,
Adding a reducing agent, lime, cement, heavy metal fixative and water to coal ash which may contain at least one of hexavalent chromium, arsenic, selenium, boron and fluorine, kneading and granulating A method for treating coal ash is provided.

The present invention provides a means for solving such a problem,
It is characterized by adding reducing agent, lime, cement, chelating agent and water to coal ash which may contain at least one of hexavalent chromium, arsenic, selenium, boron and fluorine, kneading and granulating A method for treating coal ash is provided.

  This invention also provides the granular material obtained by the said processing method.

  The present invention further provides a resource material comprising the granular material.

  According to the present invention, a special apparatus such as an autoclave and a hydrothermal reaction are not required for coal ash containing hexavalent chromium, arsenic, selenium, fluorine, and boron, and can be processed even in a simple process. In addition, these harmful substances can be immobilized in a state where it is difficult to elute them. The granular material obtained by this method is useful as a safe resource material.

  Since the treatment method of the present invention is not a method of washing and removing, no waste water is generated and no other waste is generated. Since it is not heated, the energy consumed in production is only the power of the granulation stirrer. In addition, slaked lime, iron powder, and ferrous sulfate are inexpensive, and in particular, if iron powder is not oxidized, it can be used from waste.

  Granules produced by this method satisfy the environmental standards for hexavalent chromium, arsenic, selenium, fluorine, and boron when subjected to a dissolution test and analysis by the method specified in “Environmental standards for soil contamination”. Moreover, even if it exposes to water and shakes and stirs, a particle | grain does not collapse or re-mud, and even if it uses a granular material for a backfill material, for example, it does not cause the turbidity of water.

  “Coal ash” treated by the method of the present invention includes fly ash, cinder ash, and clinker ash. The method of the present invention is particularly useful as a method for treating fly ash.

  There are 27 hazardous chemical substances defined in the “Environmental Standards Concerning Soil Contamination”, but fly ash contains hexavalent chromium, arsenic, selenium, fluorine and boron, and includes the other 22 items. There are almost no examples. For this reason, if the above five harmful elements can be rendered harmless, it is sufficient for the reuse of coal ash. Hereinafter, each harmful element will be described in detail.

-Hexavalent chromium Hexavalent chromium is reduced to harmless trivalent chromium by a reducing agent. The most suitable reducing agent for hexavalent chromium is a highly reducing organic compound and inorganic compound. Preferred reducing organic compounds include, for example, carbohydrates having a reducing end group such as dextrin, oligosaccharide, and monosaccharide, and organic compounds having an alcoholic hydroxyl group. Preferred specific examples include glucose, Examples of fructose and oligosaccharide include maltose and lactose. Further, as the organic compound having an alcoholic hydroxyl group, primary alcohols such as methanol, ethanol, 1-propanol, 1-butanol, and 2-methyl-1-propanol are preferable. Secondary alcohols such as 2-propanol and 2-butanol are also included. Preferable reducing inorganic compounds include, for example, iron powder, ferrous sulfate, and sodium bisulfite. Among these reducing agents, iron powder and ferrous sulfate are particularly preferable in that they have an action of adsorbing arsenic and selenium after reduction.

  The iron powder is preferably as fine as possible and has a large specific surface area so that it can sufficiently react with hexavalent chromium, arsenic, selenium and the like. Since ferrous sulfate dissolves in water, a reaction field with hexavalent chromium, arsenic, and selenium is formed by the water added during kneading, and a quick reaction can be expected. In comparison, iron powder is more effective for selenium and ferrous sulfate is more effective for arsenic, so it is desirable to use both in combination.

  Trivalent chromium is an essential nutrient for the human body, and is said to be involved in the function of insulin. However, absorption in the intestinal tract is very poor, and there are no excess cases. Therefore, even if trivalent chromium diffuses into the environment, it is unlikely that it will cause any harm to human health. In addition, in the natural world on the earth's surface, it is considered that trivalent chromium is hardly oxidized to hexavalent chromium. Considering the conditions under which chromium is oxidized, an artificial thing such as an acid having strong oxidizing power (such as aqua regia) or a strong heat of 1000 degrees Celsius or more can be considered. Therefore, it can be said that the purpose of the treatment can be achieved if hexavalent chromium is completely reduced and can be reduced to a certain limit (for example, below the regulation value of the soil environment standard, specifically 0.05 ppm or less).

  However, when it is necessary to consider the influence on the surrounding environment and reputation, it is necessary to further prevent metals such as trivalent chromium from eluting after the reduction treatment. For the treatment in this case, an immobilization treatment with a heavy metal fixing agent is effective. Examples of the heavy metal fixing agent include organic heavy metal fixing agents and inorganic heavy metal fixing agents.

A typical example of an organic heavy metal fixing agent is an organic chelating agent having a heavy metal fixing action (hereinafter simply referred to as a chelating agent). Examples of the chelating agent include sodium dialkyldithiocarbamic acid such as diethyldithiocarbamic acid and dibutyldithiocarbamic acid. All dithiocarbamines such as salts and potassium salts, sodium and potassium salts of high molecular weight dithiocarbamic acid with synthons of polyethyleneimine and polyamine, and sodium and potassium salts of dithiocarbamic acid with synthons of heterocyclic amines such as pyrrolidine, piperazine and morpholine And acid salt compounds. Furthermore, xanthate is also mentioned. These chelating agents can be effective in any form such as an aqueous solution, powder or granule, and slurry, but an aqueous solution product is optimal in view of good handling. In particular, potassium diethyldithiocarbamate is optimal because the concentration of the chemical solution can be increased to 60% and the reaction with the metal is very fast.

  Since metal compounds chelated with dithiocarbamate do not dissolve in water, diffusion into the environment by water is small. Therefore, when the granular material is embedded in the soil after the treatment according to the present invention, the trivalent chromium remains in the fixed state in the soil and no elution occurs.

Examples of the inorganic heavy metal fixing agent include silicic acid, phosphoric acid, sodium sulfide (Na ₂ S), sodium hydrosulfide (NaSH), and the like.
Of these heavy metal fixing agents, organic chelating agents are preferred.

・ Arsenic, selenium, and fluorine In wastewater treatment, coprecipitation with iron powder, iron salt, and calcium salt has been used to remove arsenic and selenium. Fluorine combines with lime calcium to form calcium fluoride (fluorite component) that is insoluble in water. Lime also adsorbs arsenic and selenium. When iron powder or ferrous sulfate is used as a reducing agent in the above hexavalent chromium treatment, arsenic and selenium are adsorbed on the reduced iron component. Therefore, it is effective to use iron powder or ferrous sulfate for the treatment of hexavalent chromium, arsenic and selenium, and to treat arsenic, selenium and fluorine with lime (slaked lime, quicklime), especially slaked lime. . Since slaked lime reacts as long as it contains calcium hydroxide and calcium oxide, it may be of any grade and inexpensive.

  Furthermore, since the aforementioned chelating agent can immobilize arsenic and selenium co-precipitated with iron, a more stable elution preventing effect can be obtained. In particular, diethyldithiocarbamate is highly effective and can react directly with chromium and arsenic.

Boron Boron does not exist as a simple substance, and it is considered that borate or boride is formed in fly ash. Free boric acid and alkali metal salts of boric acid are water-soluble, and other borides and borates are water-insoluble. In order to make a water-soluble borate water-insoluble, it must be melted or heat-treated with a metal or the like, and does not react immediately at room temperature and pressure. For this reason, solidification with cement is performed to prevent elution.

  The cement used in the present invention may be of any type, but it is preferable to use a blast furnace cement that has a lower hexavalent chromium content and is equivalent in price and performance compared to ordinary Portland cement.

  In the treatment method of the present invention, the ratio of each treatment agent is usually 2 to 15 parts by weight (preferably 5 to 10 parts by weight) and 2 to 5 parts by weight (preferably a reducing agent) per 100 parts by weight of fly ash. 2 to 3 parts by weight), lime 2 to 15 parts by weight (preferably 5 to 10 parts by weight), water 10 to 20 parts by weight (preferably 15 to 20 parts by weight), heavy metal fixing agent 0.5 to 2 Part by weight (preferably 1 to 1.5 parts by weight) may be used.

  Implementation of the treatment method of the present invention is carried out so that the chemical reaction for treating the harmful chemical substances listed above occurs, coal ash, cement, reducing agent (preferably iron powder and / or ferrous sulfate). Then, lime (preferably slaked lime), water, and heavy metal fixing agent are put into, for example, a granulating stirrer and kneaded for several minutes, and then granulated to a desired particle size and cured. Hydrothermal reaction is not necessary.

  Various specific mixing or kneading procedures are possible when carrying out the treatment method of the present invention. For example, first, powder materials such as fly ash, cement, and lime are charged into a granulator. If the reducing agent is in powder form, it is also added together. Next, these powder materials are mixed with stirring to obtain a mixed powder. The mixing time is not limited, but it may usually be about 2.5 to 5 minutes. Next, a heavy metal fixing agent and water are added to the mixed powder. The heavy metal fixing agent and water may be added to the granulation stirrer in the state of a mixed chemical solution. Thereafter, the mixture is kneaded and granulated for about 2.5 minutes to 5 minutes. After that, it is cured for one week under normal temperature and pressure conditions to complete the granular material.

  In another example, the order of adding the materials may be a procedure in which fly ash, cement, lime, reducing agent and water are put into a granulation stirrer and kneaded, and a heavy metal fixing agent is added to the obtained kneaded product. Various other variations are acceptable.

  There is no restriction | limiting in the shape and dimension of the granular material obtained by said processing method, What is necessary is just to select according to the use of recycling. Irregular shapes are common, but regular shapes such as spherical and square shapes may be used. When a specific shape is not required for application, it may be an irregular shape, and when a specific regular shape is desired, it may be formed into a required shape during granulation or after granulation. The particle size is usually 0.5 mm to 40 mm, preferably 2 mm to 20 mm.

  The granular material thus obtained can be used as a recycled material, and can be used as a resource material such as a backfill material, a roadbed material, and a banking material.

  In order to confirm the optimum conditions for producing toxic chemicals and backfilling materials, the following experiment was conducted.

(1) Treatment of fly ash:
As described in Table 1, in each example of the control, Comparative Examples 1-6 and Example 1-8, each material was used in the ratio shown in the table.

-Description of Table 1-
[Amount of material used]
In Table 1, the amount of each material is described as follows.

  The fly ash (abbreviated as FA) is 100 parts by weight, and the blast furnace cement, slaked lime, reducing agent (iron component), chelating agent and water added to the fly ash are expressed as relative amounts (parts by weight) to the FA amount.

[Addition order]
“1” indicates the stage of addition and stirring in the first stage. “2” indicates the stage of addition and stirring in the second stage.

  Therefore, from the description in Table 1, for example, in Example 1, FA, cement, iron powder and lime are charged into the granulation stirrer in the first stage and stirred to obtain a powder mixture, and then the second stage. Thus, it is understood that water and a chelating agent are added in an aqueous solution state, and are kneaded and granulated by stirring. In Example 3, FA, cement, iron powder, lime and water are added to the granulation stirrer in the first stage, and are kneaded and kneaded to obtain a kneaded product, and then the chelating agent is added in the second stage. And kneaded and granulated by stirring.

[Stirring time]
“First time” indicates the stirring time in the first stage. “Second time” indicates the stirring time in the second stage.

[Materials used]
Fly ash (FA): Fly ash containing harmful chemical substances shown in Table 2 was used. In addition, the leaching amount of harmful chemical substances shown in Table 2 was measured by the analysis method shown in Table 3 defined in “Environmental Standards Concerning Soil Contamination (August 23, 1991, Environmental Agency Notification No. 46)” It is the value. The content is a value measured according to the method described in Table 4 above.

  Looking at the results shown in Table 2, the amount of elution of harmful chemical substances in the raw fly ash exceeds the regulation value (see Table 6 described later). That is, when buried in the soil without treatment, these harmful chemical substances may diffuse into the environment and contaminate the soil and groundwater.

  The “control” of sample number 0 is a sample that was used to determine the amount of water added.

・ Cement: Blast furnace cement ・ Chelating agent: Dithiocarbamate chelating agent (trade name “Metal Blocker”, manufactured by Nippon Health Industry Co., Ltd.)
・ Slaked lime: Commercial slaked lime

  As described above, the materials were kneaded with a granulation stirrer, granulated, and then cured for one week under normal temperature and pressure conditions to complete a granular material.

(2) Dissolution test from granular materials:
For the particulate matter obtained by the treatment in each Example and each Comparative Example as described above, the elution amount of harmful chemical substances was determined as “Environmental Standard on Soil Contamination (August 23, 1991, Environment Agency Notification No. 46 No.) ”and measured by the analysis method shown in Table 3 above.

  Table 5 shows the measurement results. In addition, the elution amount regulation value of hazardous chemical substances according to “Environmental standards for soil contamination (August 23, 1991, Environment Agency Notification No. 46)” is as shown in Table 6.

                                                    (Unit: mg / L).

  From the results shown in Table 5 above, Sample No. At 0 (“control”), no slaked lime or iron was added, but all hazardous chemicals were below regulatory limits. However, in the long term, these chemical substances may be eluted. Sample No. with slaked lime added. In 5 and later (Comparative Examples 5 and 6 and Examples 1-8), both fluorine and boron were significantly below the regulation values. From this, it can be seen that slaked lime contributes to the fixation of boron.

  After the samples of each example were allowed to stand for 3 months after production, some examples were again subjected to a dissolution test in the same manner as described above. The results are shown in Table 7.

  From the results shown in Table 7, the elution amount of selenium increased in the control and comparative example 4, but the elution amount did not exceed the regulation value in Examples 1, 4 and 7, and the long-term stability was high. did.

(3) Soil test:
Next, the granular material of Example 3 obtained above was subjected to a soil test as a backfill material.

[Method of soil test]
When obtaining indices for determining soil classification, the survey test method for determining soil classification shown in Table 8 is used as a standard.

＊「ＪＧＳ」は、地盤工学会基準を表す。

* “JGS” represents the Geotechnical Society standard.

  As a result, the results shown in Table 9 were obtained.

  In the above, the corn index of 2120+ is classified as “Second-class improved soil of second-class construction soil” in the soil classification criteria, and corresponds to sandiness, gravelly soil, and the like. .

  The use is defined in the “Applicable standard” in “Regarding the use of generated soil (issued on August 10, 2006)” issued by the Ministry of Land, Infrastructure, Transport and Tourism. Is backfilling of works, backfilling of buildings, backfilling for civil engineering structures, river embankment (high standard embankment, general embankment), land preparation (residential land, park / green space), railway embankment, airport It can be used for embankment and landfill.

Claims (3)

Adding a reducing agent, lime, cement, heavy metal fixative and water to coal ash which may contain at least one of hexavalent chromium, arsenic, selenium, fluorine and boron, kneading and granulating A method for producing a granular material using coal ash as a feature. The granular material obtained by the manufacturing method of Claim 1. A resource material comprising the granular material according to claim 2.