JPH09309752A - Manufacturing method of lightweight aggregate - Google Patents

Abstract

(57) [Abstract] The present invention uses 0.5 g / cm ^{3 of} coal ash as a raw material.
It is an object of the present invention to provide a method for stably producing a lightweight aggregate having the following bulk density. SOLUTION: Al ₂ O ₃ / [Fe ₂ O ₃ + CaO + MgO +
After adding and mixing SiC as a foaming agent to the raw material system in which the molar ratio of [Na ₂ O + K ₂ O] is adjusted, firing and foaming are performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a foamed lightweight aggregate mainly composed of coal ash and having a bulk specific gravity of 0.5 g / cm ³ or less.

[0002]

2. Description of the Related Art As a method for producing a foam-type lightweight aggregate, a method of independently firing shale, clay, volcanic glass, etc., which has a driving force for foaming, and a method of adding a foaming agent to glassy powder There is a method of baking. In the latter method of adding a foaming agent, closed pores are formed by sintering glassy powder, and a gas is generated from the foaming agent in the closed pores to obtain a foam. Therefore, it is necessary that the foaming agent remains unreacted up to the temperature at which the closed pores are formed. For example, if low melting glass is used,
Although calcium carbonate having a decomposition temperature lower than 0 ° C can be used, it cannot be used when a material that forms closed pores at a high temperature of 900 ° C or higher is used as a raw material. On the other hand, it is known that silicon carbide that reacts with an oxygen source to generate gas only at a high temperature of 900 ° C. or higher can be used as a foaming source of a material that forms closed pores at a high temperature. For example, Japanese Patent Application Laid-Open No. 49-73409 discloses a method of producing foam glass by adding silicon carbide to a high melting point glass powder and then heating it to 950 ° C. or higher.

In order to produce a lightweight aggregate having a bulk density of 1 g / cm ³ or less by adding a foaming agent, it is desirable that the viscosity at the foaming temperature is in the glass molding viscosity range (10 ³ to 10 ⁷ P). However, the crystal is the lower limit of the definition range of solids, which is 10
^Although it melts at ^14.6 P, it is difficult to manufacture a foam-type lightweight aggregate using a crystalline material because the viscosity of the melt rapidly decreases with temperature. Therefore, the materials used for the raw material of the lightweight aggregate foamed by silicon carbide are mainly volcanic glass (shirasu, anti-fire stone) and shale that vitrifies by heating (for example, Japanese Patent Laid-Open No. 58-140365).
JP-A-60-145941 and JP-A-7-187737). However, mining of shale is being restricted due to environmental protection.

In recent years, as an alternative to natural resources such as shale, an artificial lightweight aggregate manufacturing method using coal ash as a main raw material, which is discharged in large quantities from combustion furnaces of coal-fired power plants and various plants, has been attracting attention. However, since coal ash is composed of crystalline minerals such as mullite and quartz and glassy material and has an intermediate property between crystalline material and glassy material, it is difficult to control foaming by silicon carbide with coal ash alone. A method of adding glass having a low softening point has been carried out. For example, Japanese Patent Laid-Open No. 7-
Japanese Patent No. 41343 discloses a method of manufacturing a lightweight aggregate by adding glass and silicon carbide that soften around 600 ° C. to coal ash. However, in this method, the amount of glass added is as large as 30 to 33 parts by weight, and since the firing temperature is as low as 1175 ° C., foaming is not sufficient and the bulk density remains at 0.68 g / cm ³ .

Further, since the composition, softening point, and melting point of coal ash differ depending on the type of coal used for combustion, it is difficult to manufacture a lightweight aggregate of constant quality using the coal ash. That is, depending on the type of coal ash, its composition is 40 to 75% by weight of SiO ₂ , 14 to 35% by weight of Al ₂ O ₃ , and Fe.
₂ O ₃ is 2-15% by weight, CaO is 0.1-11% by weight
, MgO 0.2-5 wt%, Na ₂ O 0.2-3
% By weight and K ₂ O fluctuate in the range of 0.2 to 3% by weight, and the softening temperature of the coal ash is accordingly increased by 1100 to 1650.
A range of ° C occurs. Therefore, there is a problem in that the firing temperature in the case of adding silicon carbide to coal ash for foaming also causes a significant difference depending on the type of coal ash, and a large difference in the bulk density of the obtained lightweight aggregate.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for producing a lightweight aggregate obtained by foaming coal ash as a main raw material with silicon carbide. More specifically, it is an object of the present invention to provide a method for stably producing a lightweight aggregate having a bulk density of 0.5 g / cm ³ or less regardless of the composition of coal ash.

[0007]

The present invention is directed to Al ₂ O ₃ /
Coal Ash _{[Fe 2 O 3 + CaO +} MgO + Na 2 O + K 2 O] molar ratio is in the range of 0.8 to 2.5, or a mixture consisting of coal ash and the inorganic material, silicon carbide 0.1-5
The present invention relates to a method for producing a lightweight aggregate, which comprises adding, by weight%, mixing, granulating, and then firing and foaming.

Further, the content of SiO ₂ is 58% by weight or more, and Al ₂ O ₃ / [Fe ₂ O ₃ + CaO + MgO + Na
₂ O + K ₂ O] molar ratio is in the range of 1.0 to 1.8, or 0.1 to 2 wt% of silicon carbide is added to a mixture of coal ash and an inorganic substance and mixed and granulated. Then, the present invention relates to a method for manufacturing a lightweight aggregate, which is characterized by firing and foaming.

Here, Al ₂ O ₃ / [Fe ₂ O ₃ + CaO +
MgO + Na ₂ O + K ₂ O] molar ratio is Al, F contained in coal ash or a mixture of coal ash and an inorganic substance.
e, Ca, Mg, Na and K are each Al ₂ O ₃ ,
It is calculated as existing in the state of Fe ₂ O ₃ , CaO, MgO, Na ₂ O and K ₂ O, and is hereinafter referred to as an alumina / fluxing agent ratio.

As mentioned above, the main components of coal ash are SiO ₂ and Al ₂ O ₃ , and Fe ₂ O ₃ , CaO and M as fluxing agents.
gO, Na ₂ O, K ₂ O and the like are included, and the constituent phases at room temperature are a glass phase and a crystal phase such as quartz and mullite. Therefore, coal ash is a solid in a non-equilibrium state at room temperature, but in the temperature range that is important for foaming, softens by heating, or produces a melt, the equilibrium of the SiO ₂ —Al ₂ O ₃ —fluxing agent system It is considered that the state will be similar to the state.

The relationship between the composition and the liquidus formation temperature of a system in equilibrium can be investigated by a phase diagram. SiO ₂ -A
The phase diagram of a system composed of 1 ₂ O ₃ -metal oxide (where metal oxide is an oxide of any one metal of Fe, Ca, Mg, Na, and K) is a liquid phase in this system. It is shown that the generation start temperature increases as the molar ratio of Al ₂ O ₃ to the metal oxide increases, regardless of the type of metal. It also shows that the influence of the content of SiO ₂ is small in the range of 40 to 75% by weight. In the present invention, with this as a clue, the relationship between the softening point and melting point of coal ash and the SiO ₂ —Al ₂ O ₃ / fluxing agent ratio of coal ash was investigated.

[0012] As a result, in a typical SiO ₂ content is 40 to 75 wt% of the coal ash, the softening point, although the small effect of the SiO ₂ content with respect to the melting point of alumina /
It has been found that there is a very strong correlation between the flux ratio and its softening point and melting point, with the softening point and melting point increasing with increasing alumina / fluxing agent ratio.

The temperature at which the glass phase in the coal ash has an optimum viscosity for foaming and the bulk density of the foam is minimized, that is,
It was also found that the optimum firing temperature is near the softening point of coal ash. Further, it was also found that when the alumina / fluxing agent ratio of coal ash is 0.8 or more, an extremely small foam having a bulk density of 0.5 g / cm ³ or less can be obtained. After adding and mixing silicon carbide, which is a foaming agent, to coal ash having this composition range, by firing and foaming at a temperature near the softening point according to the alumina / fluxing agent ratio, a bulk density of 0.5 g / It is possible to stably produce a lightweight aggregate having a size of cm ³ or less.

When the alumina / fluxing agent ratio is less than 0.8, the coal ash begins to soften at 1150 ° C., but the amount of liquid phase at the optimum firing temperature is large, and the viscosity is suitable for foaming with silicon carbide. No glass phase is formed. Therefore, it is difficult to produce a lightweight aggregate having a bulk density of 0.5 g / cm ³ or less using coal ash having this composition. On the other hand, when the alumina / fluxing agent ratio exceeds 2.5, the main crystalline phase is mullite (3A
1 ₂ O ₃ .2SiO ₂ ), and the temperature at which a glass phase having a viscosity suitable for foaming is formed becomes extremely high, which is not preferable.
From these facts, the coal ash used in the present invention has an alumina / fluxing agent ratio of 0.8 to 2.5, preferably 1.0 to 1.8.
Range. The firing temperature depends on the alumina / fluxing agent ratio, but is in the range of 1200 to 1600 ° C.

In the present invention, when the alumina / fluxing agent ratio of the coal ash exceeds 2.5, an inorganic substance acting as a fluxing agent is added and mixed prior to firing to adjust the composition suitable for foaming. Here, the inorganic substances to be added are Fe ₂ O ₃ and Ca.
It is necessary to contain one or more components selected from O, MgO, Na ₂ O and K ₂ O, and examples thereof include iron concentrate, steelmaking furnace dust, limestone, dolomite, blast furnace slag, gasification slag, sodium silicate. , Clay having a high alkali content, and volcanic glass can be preferably used. Further, the coal ash composition can be adjusted by mixing other coal ash having an alumina / fluxing agent ratio of 2.5 or less, preferably 1.5 or less.

On the other hand, the coal ash has an alumina / fluxing agent ratio of 0.8.
When the amount is less than the above, the composition is adjusted by adding and mixing an inorganic substance containing Al ₂ O ₃ . Here, the inorganic substances that can be used are
Alumina, mullite, acid clay, and coal ash having an alumina / fluxing agent ratio of 0.8 or more, preferably 1.5 or more.

In order to obtain a foam having a low water absorption, it is necessary to prevent the foam shell from bursting during foaming and suppress the formation of open pores. For that purpose, it is important that a viscous glass phase is present at the time of foaming. The viscosity of glass is
High if Al ₂ O ₃ is present as a network-forming oxide.
Al ₂ O ₃ / R ₂ O is used for ordinary glass that does not have a crystalline phase.
When (R is an alkali metal) is around 1, Al ₂ O ₃ becomes a network-forming oxide, and the glass has the highest viscosity. However,
In coal ash having Al ₂ O ₃ incorporated in a crystal phase such as mullite (3Al ₂ O ₃ .2SiO ₂ ), or a mixture of coal ash and an inorganic substance, the alumina / fluxing agent ratio is 1.0 to In the range of 1.8, the viscosity of the glass phase is the highest. In this case, the so-called intermediate oxide A
It is considered that l ₂ O ₃ acts as a network-forming oxide.

Further, the viscosity of the glass is also dependent on the content of SiO _2, the viscosity becomes higher when the SiO ₂ increases. In the present invention, silicon carbide is added to and mixed with a raw material system having an alumina / fluxing agent ratio in the range of 1.0 to 1.8 and a SiO ₂ content of 58 wt% or more, and 1200 to 1600.
By firing at 0 ° C., it is possible to obtain a lightweight aggregate having a bulk density of 0.5 g / cm ³ or less and a water absorption rate of 10% or less.

When the alumina / fluxing agent ratio of coal ash exceeds 1.8, an inorganic substance acting as a fluxing agent is added and mixed prior to firing to adjust the composition suitable for foaming. here,
The inorganic substances to be added are Fe ₂ O ₃ , CaO, MgO, Na
It is necessary to contain one or more components selected from ₂ O and K ₂ O, such as iron concentrate, steelmaking furnace dust, limestone,
Dolomite, blast furnace slag, gasification slag, sodium silicate,
Clay and volcanic glass having a high alkali content can be preferably used. Further, the coal ash composition can be adjusted by mixing other coal ash having an alumina / fluxing agent ratio of 1.8 or less, preferably 1.5 or less.

On the other hand, the coal ash has an alumina / fluxing agent ratio of 1.0.
When the amount is less than the above, the composition is adjusted by adding and mixing an inorganic substance containing Al ₂ O ₃ . Here, the inorganic substances that can be used are
Alumina, mullite, acid clay, and coal ash having an alumina / fluxing agent ratio of 1.0 or more, preferably 1.5 or more.

The SiO ₂ content of coal ash is usually 40 to 75
Although it is in the range of wt%, if the SiO ₂ content is less than 58 wt%, the SiO ₂ content is adjusted by adding and mixing an inorganic substance containing SiO ₂ prior to firing. Inorganic substances that can be used here are silica, glass powder, mullite, acid clay, and other types of coal ash having a SiO ₂ content of 58 wt% or more, preferably 65 wt% or more.

The amount of silicon carbide added as a foaming agent in the present invention is 0.1 to 5% by weight. 0.1
If the content is less than wt%, the amount of gas that serves as a driving force for foaming is small, and a lightweight aggregate having a bulk density of 0.5 kg / cm ³ or less cannot be obtained. On the other hand, if the amount of silicon carbide added exceeds 5% by weight, abnormal foaming will occur, which is not preferable. Further, in the case of manufacturing a lightweight aggregate having a water absorption rate of 10% or less,
The amount of silicon carbide added is 2% by weight so that the foam shell does not burst.
It is necessary to keep below.

Silicon carbide acts as a foaming agent by reacting with a transition metal oxide such as ferric oxide (Fe ₂ O ₃ ) contained in coal ash to generate CO or CO ₂ . Therefore, the atmosphere in the firing furnace is preferably kept in an oxidizing atmosphere until the closed pores are formed and the silicon carbide is shielded from the atmosphere in the firing furnace. Transition metal oxides such as ferric oxide,
This is because if it is fired in a reducing atmosphere, it is reduced and reaction with silicon carbide is less likely to occur. When the atmosphere in the firing furnace is a reducing atmosphere, silicon nitride may be used instead of silicon carbide. Silicon nitride generates nitrogen gas at a high temperature regardless of the atmosphere in the furnace.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION

EXAMPLES The present invention will be described more concretely with reference to the following examples. Table 1 shows the chemical composition of the three types of coal ash used.
Table 2 shows the chemical composition of dolomite used for adjusting the alumina / fluxing agent ratio of coal ash.

[0025]

[Table 1]

[0026]

[Table 2]

The bulk density and 24-hour water absorption of the obtained foam were measured by the following methods. Bulk density: JIS A 1135
Measured according to

24-hour water absorption rate: measured in accordance with JIS A 1135

EXAMPLE 1 6.7 parts by weight of dolomite having the composition shown in Table 2 was added to 93.3 parts by weight of coal ash A having an alumina / fluxing agent ratio of 3.4 and mixed to obtain alumina / melting agent. The agent ratio was adjusted to be 1.5. Further, 0.5 part by weight of silicon carbide powder was added and mixed, and then a granule having a diameter of 10 mm was prepared.
Baking was performed at 00 ° C. for 10 minutes. The bulk density of the obtained fired body was 0.5 g / cm ³ or less (Table 3).

Comparative Example 1 Here, an example is shown in which coal ash A was used but the alumina / fluxing agent ratio of the raw material system was not adjusted. Coal ash A: 100
After adding and mixing 0.5 part by weight of silicon carbide powder to parts by weight, a granulated body having a diameter of 10 mm was prepared and fired at 1350 ° C. for 10 minutes in the atmosphere. The bulk density of the obtained fired body is 0.5 g
/ Cm ³ (Table 3).

Example 2 Coal ash A: 6.4 parts by weight of dolomite and 4.8 parts by weight of ferric oxide were added and mixed with 88.8 parts by weight of the coal ash A, and the raw material system had an alumina / fluxing agent ratio of 1. Adjusted to 3. Further, 0.5 part by weight of silicon carbide powder was added and mixed, and then a granulated body having a diameter of 10 mm was prepared and calcined in the air at 1300 ° C. for 10 minutes. The bulk density of the obtained fired body was 0.5 g / cm ³ or less (Table 3).

Example 3 A fired body was obtained in the same manner as in Example 2 except that the firing temperature was changed to 1250 ° C. The bulk density of the obtained fired body is 0.5 g / c
It was less than m ³ (Table 3).

[0033]

[Table 3]

EXAMPLE 4 Coal ash B having an SiO ₂ content of 64.6% by weight and an alumina / fluxing agent ratio of 1.5: 100 parts by weight of silicon carbide powder 0.5
After adding and mixing parts by weight, a granule having a diameter of 10 mm was prepared and fired at 1325 ° C. in the atmosphere for 10 minutes. The obtained fired body had a bulk density of 0.5 g / cm ³ or less and a water absorption rate of 10% or less (Table 4).

Example 5 Coal ash B: 3.2 parts by weight of dolomite was added to 96.8 parts by weight and mixed, and the raw material system had a SiO ₂ content of 62.7% by weight and an alumina / fluxing agent ratio of 1. Adjusted to 1. Furthermore, after adding 0.5 parts by weight of silicon carbide powder and mixing,
Granules having a size of mm were prepared and calcined in air at 1275 ° C. for 10 minutes. The obtained fired body had a bulk density of 0.5 g / cm ³ or less and a water absorption rate of 10% or less (Table 4).

Example 6 A fired body was obtained in the same manner as in Example 5 except that the firing temperature was 1225 ° C. The bulk density of the obtained fired body is 0.5 g / c
It was m ³ or less and the water absorption rate was 10% or less (Table 4).

Comparative Example 2 Coal ash B: 13.5 parts by weight of dolomite was added to 86.6 parts by weight to adjust the SiO ₂ content to 56.9% by weight and the alumina / fluxing agent ratio to 0.5. Further, 0.5 part by weight of silicon carbide powder was added and mixed, and then a granulated body having a diameter of 10 mm was prepared and calcined at 1225 ° C. in the atmosphere for 10 minutes. The bulk density of the obtained fired body was greater than 0.5 g / cm ³ (Table 4).

Example 7 50 parts by weight of coal ash A and 50 parts by weight of coal ash C were mixed and adjusted so that the SiO ₂ content was 58.4% by weight and the alumina flux ratio was 1.1. . Further, 0.5 part by weight of silicon carbide powder was added and mixed, and then a granule having a diameter of 10 mm was prepared and fired at 1300 ° C. for 10 minutes in the air. The obtained fired body had a bulk density of 0.5 g / cm ³ or less and a water absorption rate of 10% or less (Table 4).

Comparative Example 3 0.5 parts by weight of silicon carbide powder was added to 100 parts by weight of coal ash C having an alumina / fluxing agent ratio of 0.4 and mixed, and then a granulated body having a diameter of 10 mm was prepared, and the mixture was placed in the atmosphere. Baking was performed at 1200 ° C for 10 minutes. The water absorption of the obtained fired product was 10% or less, but the bulk density was more than 0.5 g / cm ³ (Table 4).

[0040]

[Table 4]

[0041]

The composition, softening point, and melting point of coal ash differ according to the type and combustion method of coal.
It has been difficult to produce a constant quality lightweight aggregate.
For this reason, in order to manufacture a product using coal ash, it has been necessary to use coal ash in which the type of coal and the generation location are determined. However, in the present invention, even when coal ash having a different composition, softening point, and melting point is used, the alumina / fluxing agent ratio Al ₂ O ₃ / [F
By adjusting the molar ratio of [e ₂ O ₃ + CaO + MgO + Na ₂ O + K ₂ O] in the range of 0.8 to 2.5, it was possible to form a glass phase having a good viscosity for foaming.
As a result, it becomes possible to produce a lightweight aggregate having a bulk density of 0.5 g / cm ³ or less, even if coal ash having different types of coal and different generation points is used.

Claims (2)

[Claims] 1. Al ₂ O ₃ / [Fe ₂ O ₃ + CaO + MgO +
[Na ₂ O + K ₂ O] molar ratio in the range of 0.8 to 2.5, or a mixture of coal ash and an inorganic substance,
A method for producing a lightweight aggregate, which comprises adding 0.1 to 5% by weight of silicon carbide, mixing and granulating, and then firing and foaming. 2. A SiO ₂ content of 58% by weight or more and Al ₂ O ₃ / [Fe ₂ O ₃ + CaO + MgO + Na ₂ O +
K ₂ O] coal ash having a molar ratio in the range of 1.0 to 1.8,
Alternatively, 0.1 to 2% by weight of silicon carbide is added to a mixture of coal ash and an inorganic substance, mixed, granulated, and then fired to foam, thereby producing a lightweight aggregate.