JPH11116299A - Artificial lightweight aggregate and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the quality at a low cost by mixing a melting point lowering agent, a binder and a foaming agent with a coal ash, pulverizing the mixture to have a specific particle diameter and firing a molding obtained by adding water to the pulverized material in a rotary kiln at a specific temp. to have specific range of bone dry specific gravity. SOLUTION: Bentonite of the binder to be 0.2-5 pts.wt. in external ratio per 100 pts.wt. coal ash, the melting point lowering agent to be 2-6 wt.% in the fired material in total quantity expressed in terms of Na2 O and K2 O and further the foaming agent are added. The resultant mixture is pulverized to have <=15 μm average particle diameter. Water is added to the pulverized material to mold and the molding is fired at 1000-1250 deg.C. As a result, a lightweight aggregate having bone dry specific gravity of 1.0-0.5 in is obtained. As the melting point lowering agent, one prepared by mixing sodium carbonate, potassium carbonate and the coal ash, fusing by heating to make vitreous to be 30-50 wt.% in total quantity of Na2 O and K2 and cooling and pulverizing is used.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method for recycling coal ash (fly ash) generated from a coal-fired boiler such as a coal-fired power plant as an artificial lightweight aggregate for construction and civil engineering, and to effectively utilize it. The present invention relates to an artificial lightweight aggregate prepared by the method described above.

[0002]

2. Description of the Related Art Coal has abundant resources and lower price per unit calorific value than petroleum. Therefore, domestic energy policy has planned or implemented a significant increase in the use of coal, especially as fuel for power generation. It is getting. As a result, the amount of coal ash (fly ash) generated from coal-fired boilers such as coal-fired power plants will increase almost in proportion to the amount of coal used, making effective use of the rapidly increasing coal ash a major issue. ing.

[0003] As an effective use of coal ash, utilization as artificial lightweight aggregate is in great demand, so that it is suitable for mass processing.

[0004] However, the use of coal ash generated from coal-fired boilers such as coal-fired power plants as aggregate is extremely small in Japan, although a part of the coal ash is formed by a sinter-great method.

[0005] The reason for this is that in coal-fired boilers such as coal-fired power plants, coal that generates ash with a high melting point is selected and used in order to reduce the adhesion of ash to water pipes and boiler walls of the boiler. There. In other words, coal ash generated from a coal-fired boiler such as a coal-fired power plant generally has a high melting point, and must be calcined with a large amount of low-melting clay or shale mixed into a lightweight aggregate. . It is difficult to secure a large amount of these clays and shale, and
Mining, transporting, pre-treating, and mixing these clays and shale require a lot of money, which raises the cost of manufacturing artificial lightweight aggregates. Further, since the usage rate of coal ash per unit product is low, it is not preferable for effective utilization of coal ash.

The absolute dry specific gravity of artificial lightweight aggregate using coal ash is about 1.2 to 1.4, and the absolute dry specific gravity is 1.0 to 1.0.
There is no technology for producing a light artificial lightweight aggregate of about 0.5 to about 0.5, which limits its use.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a technique for producing a high-quality artificial lightweight aggregate at low cost by adding an easily available and low-cost additive. .

[0008] Another object of the present invention is to reduce the amount of additives used, thereby increasing the use rate of coal ash per unit product, and improving the utilization efficiency of coal ash.

Another object of the present invention is to provide a very light artificial lightweight aggregate by increasing the lightness, and to expand its use.

[0010]

The artificial lightweight aggregate of the present invention is obtained by mixing a coal ash generated from a coal-fired boiler with a melting point depressant, a binder, and a foaming agent to obtain a mixture. A pulverized product is obtained by pulverizing so that the average particle size becomes 15 μm or less, and a water compact is added to the pulverized material to obtain a molded product.
It is manufactured by firing so that the absolute specific gravity becomes 1.0 to 0.5 within a temperature range of 1250 ° C.

The method for producing an artificial lightweight aggregate according to the present invention is characterized in that a coal ash generated from a coal-fired boiler is mixed with a melting point depressant, a binder and a foaming agent to obtain a mixture, and the mixture is averaged. A pulverized product is obtained by pulverizing so that the particle size becomes 15 μm or less, and a molded product is obtained by adding water to the pulverized product to obtain a molded product.
Firing in a temperature range of 1250 ° C. In addition, before baking, drying is performed as needed.

The blowing agent comprises iron oxide in an amount such that the amount of Fe ₂ O ₃ in the coal ash is in the range of 1% to 10% by weight, and 0.2% to 10% by weight of the coal ash. It consists of an amount of carbonaceous material in the range and an amount of silicon carbide in the range of 0% to 1% by weight based on the coal ash. In particular, the absolute specific gravity is 1.0-0.5
When the calcining is performed so that the amount of Fe ₂ O ₃ in the coal ash is in the range of 3% by weight to 10% by weight,
0.1% to 1% by weight of silicon carbide based on coal ash;
It is preferable to add 0.2% by weight to 10% by weight of carbonaceous material to coal ash. The carbonaceous material is typically coal or coke.

The melting point depressant is obtained by mixing an alkali metal compound and coal ash so that the total amount of each of Na ₂ O and K ₂ O or both is within a range of 30% by weight to 50% by weight.
It is produced by heating and melting in a temperature range of 1000 ° C. to 1200 ° C. to form a glass, followed by cooling and pulverization. The melting point depressant is preferably added to the coal ash such that the total amount of Na ₂ O and K ₂ O is in the range of 2% by weight to 6% by weight in the fired product. Preferably, the alkali metal compound is sodium carbonate or potassium carbonate.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have mixed an alkali metal compound and coal ash so that the total amount of each of Na ₂ O and K ₂ O, or both, is 30 to 50% by weight. Melted at 1000 ° C. to 1200 ° C. to form a glass, cooled and pulverized, as a melting point depressant Na ₂ O, K ₂
By adding to the coal ash such that the total amount of O 2 conversion amounts is 2% by weight to 6% by weight in the fired product, the melting point of the coal ash is lowered to a temperature that is easily industrially fired at 1000 ° C. to 1250 ° C. And by adding a carbon material such as iron oxide, silicon carbide, coal or coke having an average particle size of 10 μm or less as a foaming agent, the specific strength of the absolute dry specific gravity of about 0.5 to 1.5 is high, and the water absorption rate is high. The inventors have found a method of firing a small amount of artificial lightweight aggregate, and have reached the present invention. Here, the iron oxide is Fe in the coal ash.
_It is added to coal ash such that the amount of ₂ O ₃ is 1% by weight to 10% by weight. In particular, when the absolute specific gravity is 0.5 to 1.0,
The content of Fe ₂ O ₃ is 3% by weight or more, and the content of silicon carbide is 0.1%.
It is added to the coal ash to be 1% by weight to 1% by weight. The carbon material is used in an amount of 0.2% by weight to 10% by weight based on the coal ash. The carbonaceous material also functions to adjust the reduced state inside the granulated pellets during firing.

[0015] In a specific embodiment, the melting point depressant is obtained by mixing sodium carbonate and potassium carbonate with coal ash.
Melted by heating at 2,000 ° C to 1200 ° C and Na ₂ O and K ₂ O
And glass-cooled and cooled and pulverized so that the total amount of each or both becomes 30% to 50%.

According to one aspect of the method for producing an artificial lightweight aggregate, first, bentonite as a binder is 0.2 to 5 parts by weight based on 100 parts by weight of coal ash as a raw material. Further, the above melting point depressant is added so that the total amount in terms of Na ₂ O and K ₂ O is 2% by weight to 6% by weight in the fired product. Further, a foaming agent is added in the ratio as described above.

The mixture obtained in this way has an average particle size of 15
Pulverize to a size of μm or less. Next, water is added to the obtained pulverized product to form a molded body, and after drying if necessary, the molded body is fired at 1000 ° C to 1250 ° C.

The molding method used in the present invention is not particularly limited as long as it can be molded so as to have a predetermined diameter. However, it is convenient to use a pan pelletizer or an extrusion molding machine. In addition, it is preferable to use a rotary kiln for firing in consideration of continuous operation and uniformity of quality.

The melting point depressant will be described in detail below.

Coal ash often has an extremely high temperature of 1400 ° C. to 1500 ° C. at which a liquid phase is formed to start sintering.
Firing artificial lightweight aggregates at 1400 ° C. to 1500 ° C. is not practical because fire resistance of the firing equipment, energy cost, and difficulty in selecting a foaming agent are not practical. Conventionally, when firing such a high refractory raw material, a method of adding a large amount of waste minerals such as clay and shale and low glass containing a large amount of alkali metals as a melting point depressant and waste glass such as bottle glass is generally used. It was a target. The present inventors have conducted various studies on the effects of adding clay and shale, and as a result, it has been confirmed that a small amount of alkali metals among these constituents significantly lowers the liquid phase formation temperature. However, when an industrial chemical having a high alkali metal content is added to coal ash, only the surface of pellets obtained by granulating the coal ash is melted and the inside cannot be fired. This is an industrial product of alkali metals that is effective in lowering the melting point. Most of the sodium and potassium salts have a water bath property, and the alkali metals are concentrated on the pellet surface at the stage of drying the granulated pellets. This is because only the pellet surface melts during firing and the inside cannot be fired. As a result of studying a method for preventing the concentration of industrial alkali metal compounds on the pellet surface, a mixture of an alkali metal compound such as sodium carbonate or potassium carbonate and coal ash was mixed at 1000 ° C.
1200 what the total amount of each or both ℃ in heat melting to Na ₂ O and K ₂ O is 30% to 50% and so as glassy cooled grinding, which Na ₂ O, K ₂ O
Is added to the coal ash so that the total amount of the ash becomes 2% by weight to 6% by weight in the fired product, a high-strength artificial lightweight aggregate uniformly foamed from the inside can be obtained by firing at 1000 ° C to 1250 ° C. I found that.

In the present invention, alkali metal compounds such as sodium carbonate and potassium carbonate used as melting point depressants are mass-produced at low cost as industrial chemicals, so that the present invention is advantageous in cost. Further, carbonates and bicarbonates of alkali metals are preferable because they do not generate harmful gases when heated. By adding this alkali metal compound to coal ash, a glass which is hardly soluble in water is formed by the alkali metal and silica. Since coal ash is used as a silica source for glass production, coal ash can also be used as a melting point depressant,
This is preferable because the processing rate of coal ash can be improved and new resources are not required.

In the melting point depressant, when the total amount of each of Na ₂ O and K ₂ O or both is 30% by weight or less, the melting temperature for vitrification exceeds 1200 ° C., and equipment and maintenance costs become expensive. , Energy costs are also high.
In addition, since the content of alkali decreases, the amount of the melting point depressant used becomes large, which is not preferable. Na ₂ O and K ₂ O
If each or both of them exceed 50% by weight, the water solubility of the produced glass increases, only the surface of the granulated pellets tends to soften, and the inside of the pellets cannot be fired, which is not preferable.

In the artificial lightweight aggregate of the present invention, Na ₂
The reason why the melting point depressant is added so that the total amount in terms of O and K ₂ O is 2% by weight to 6% by weight in the fired product is that the chemical composition of coal ash differs depending on the type of coal, but SiO ₂ : 50 wt% to 55 wt%, AI ₂ O _3: 25 wt% to 30 wt%, Na ₂ O: 0.2 wt% to 2 wt%, K ₂ O: 0.2
This is because the melting point is greatly reduced and the solubilization temperature range is extended by the addition of the alkali metal in the above amount.

If the total amount of Na ₂ O and K ₂ O in the artificial lightweight aggregate is less than 2% by weight, the firing temperature becomes 1250 ° C. or more, which is not practical. Further, if it is more than 6% by weight, the effect of lowering the melting point is small, and the production cost increases due to an increase in additives, which is not preferable.

The blowing agent will be described in detail below.

When granulated by adding water to coal ash, the bulk specific gravity of the dried pellets is about 1.5 to 1.9, depending on the granulation method. The specific gravity becomes about 1.5 to 2.0. Therefore,
A blowing agent is added to coal ash in order to make the absolute specific gravity of the artificial lightweight aggregate about 0.5 to 1.5.

Hematite having a high degree of oxidation is preferred as the iron oxide as a foaming agent. The reason why the particle size of the iron oxide is set to 10 μm or less is to promote a deoxygenation reaction by a carbon material or silicon carbide during firing. Further, the reason why the amount of Fe ₂ O ₃ of the artificial aggregate during firing is set to 1% by weight or more is that the effect as a foaming agent is small below this, and the absolute dry specific gravity of the artificial aggregate is about 1.0 to 1.5. This is because it cannot be reduced to. Further, in order to set the absolute specific gravity to 0.5 to 1.0, the amount of Fe ₂ O ₃ is set to 3% by weight.
Thus, it is necessary to sufficiently react with silicon carbide. The weight reduction is remarkable due to the foaming action of carbon generated by decomposition of silicon carbide. Conversely, the amount of Fe ₂ O ₃ in the fired aggregate is 1
If it exceeds 0% by weight, the effect of weight reduction by foaming does not increase. The specific gravity of iron oxide is significantly larger than that of coal ash, and if foaming is not promoted, the absolute specific gravity of the artificial lightweight aggregate will increase.

When the granulated pellets generate a large amount of liquid phase by heating, the silicon carbide efficiently reacts with iron oxide (Fe ₂ O ₃ ) to generate CO and CO ₂ gas. The CO and CO ₂ gas are captured, and the foam swelling of the pellet is promoted. When the amount of silicon carbide is less than 0.1% by weight, the effect of reducing the weight of the absolute dry specific gravity to 0.5 to 1.0 is not sufficient, and the absolute dry specific gravity of 1.0 or less cannot be achieved. Conversely, 10
Even if the amount exceeds the weight percentage, the weight reduction effect does not increase.

The carbonaceous material has a great effect of adjusting the degree of reduction inside the pellets during sintering, and also has a foaming action by reacting with iron oxide.

If the carbon material addition ratio is 0.2% by weight or less, the inside of the pellet cannot be kept in a reduced state, and the effect of reducing the weight by foaming cannot be obtained. Further, even if the addition ratio of the carbonaceous material exceeds 10% by weight, the effect of reducing the weight by foaming expansion does not increase, and unburned carbon may remain inside the pellet and reduce the strength of the artificial lightweight aggregate. Not preferred.

[0031]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to embodiments.

Table 1 shows the chemical compositions of coal ash, bentonite, hematite, silicon carbide, and coke used in the experiment. The melting point depressant is obtained by mixing the coal ash shown in Table 1 with the first grade reagents of sodium carbonate and potassium carbonate as alkali metal raw materials and heating the mixture in an electric furnace under the conditions shown in Table 2 at a predetermined temperature. After holding for 10 minutes, it was taken out of the furnace, allowed to cool, and then pulverized.

[Examples 1 to 31: Foaming with hematite and carbonaceous material] The above-mentioned raw materials were weighed and collected in the composition shown in Table 3, and were pulverized and mixed by a ball mill. The particle size distribution of the ground material is
Table 3 shows the results measured with a laser diffraction particle size distribution analyzer.

While adding water to the obtained pulverized raw material, it was granulated into a spherical shape having a diameter of about 5 to 15 mm by a pan pelletizer, dried, and then supplied to a rotary kiln (brick inner diameter 500 mm × length 4800 mm) for firing. Table 3 shows the chemical composition of the alkali metal in the artificial lightweight aggregate after firing.

The absolute specific gravity and the water absorption of the fired artificial lightweight aggregate were measured based on JIS A 1110, and the crushing strength was measured for an artificial lightweight aggregate having a diameter of about 10 mm. Table 4 shows the obtained results and firing temperatures. Absolute dry specific gravity is about 1.
0 to 1.5, the absolute specific gravity of commercially available artificial lightweight aggregate
An artificial lightweight aggregate equivalent to 2-1.4 was obtained. In addition, the crushing strength is 5 N to 6 N of commercially available artificial lightweight aggregate,
An artificial lightweight aggregate having an extremely high specific strength of 11N to 15N at an absolute dry specific gravity of 1.2 to 1.3, and 7N to 8N even at an absolute dry specific gravity of around 1.0 was obtained. The 24-hour water absorption also showed a low value of about 5%.

[Comparative Examples 1, 7, 13] In the case where the melting point depressant is small and the total amount of the alkali metal compounds in the baked artificial lightweight aggregate is less than 2% by weight, the calcination temperature is from 1210 ° C. to 1
Even at a high temperature of 260 ° C., the pellets were insufficiently fired, the absolute specific gravity exceeded 1.55 and higher than the target value (1.5), the crushing strength was low, and the water absorption was high.

[Comparative Examples 2, 8, 14] When the amount of the melting point depressant is large and the total amount of the alkali metal compounds in the fired artificial lightweight aggregate exceeds 6% by weight, the pellet surface melts at a low temperature. The firing temperature is as low as 1050 ° C. to 1120 ° C., the inside of the pellet cannot be sufficiently fired, and the crushing strength is 2N.
４4N and the water absorption increased to 10% to 11%.

[Comparative Examples 3, 9, 15] Even when the total amount of alkali metal compounds is 2% by weight to 6% by weight, when the amount of hematite added is small, the strength is high and the water absorption is low. The absolute dry gravity exceeded 1.55 and became higher than the target value (1.5), and the weight reduction became insufficient.

[Comparative Examples 4, 10, 16] When the total amount of alkali metal compounds is 2 to 6% by weight, even if the amount of hematite exceeds 10% by weight, there is no improvement effect on the specific gravity, strength and water absorption. Was.

[Comparative Examples 5, 11, 17] When no carbon material was added, sintering did not proceed, the absolute density was high, the strength was low, and the water absorption was high.

[Comparative Examples 6, 12, 18] When the amount of carbonaceous material exceeds 10% by weight, the absolute dry specific gravity is as high as about 1.65.
The specific strength decreased to 6N to 8N.

[Examples 32 to 65: Synergistic weight reduction of carbon material and silicon carbide]
It was pulverized and mixed by a ball mill. The particle size distribution of the pulverized raw material was measured with a laser diffraction type particle size distribution meter and is shown in Table 5.

While adding water to the obtained pulverized raw material, the mixture was granulated into a spherical shape having a diameter of about 5 to 15 mm with a pan pelletizer, dried, and then supplied to a rotary kiln (brick inner diameter 500 mm × length 4800 mm) for firing. Table 5 shows the alkali metal content and the iron content in terms of Fe ₂ O ₃ in the artificial lightweight aggregate after firing.

The specific gravity and water absorption of the fired artificial lightweight aggregate are as follows:
The crushing strength was measured based on JIS A 1110, and the crushing strength was measured on an artificial aggregate having a diameter of about 10 mm. Table 6 shows the obtained results and firing temperatures. Absolute dry specific gravity is about 0.5 ~
1.0, and an extremely light artificial lightweight aggregate was obtained.
The crushing strength was about 3N near the absolute dry gravity of 0.5, but was about 7N to 8N near the absolute dry specific gravity of 1.0, and an artificial lightweight aggregate having extremely high specific strength was obtained. The 24-hour water absorption was 12% to 13% when the absolute dry specific gravity was 0.5, and was about 6% when the absolute dry specific gravity was 1.0.

[Comparative Example 19] The amount of the alkali metal compound in the fired artificial lightweight aggregate was low, because the melting point depressant was small.
If the amount is less than 1% by weight, the pellets are insufficiently fired even when the firing temperature is increased to 1300 ° C., and despite the addition of a foaming agent, the absolute dry specific gravity is as high as 1.21 and the crushing strength is 2%. .2N, and the water absorption increased to 14.2.

[Comparative Example 20] When the melting point depressant is large and the total amount of the alkali metal compounds in the fired artificial lightweight aggregate exceeds 6% by weight, the pellet surface is melted at a low temperature and the firing temperature is lowered. The temperature was lowered to 1000 ° C., and the inside of the pellets could not be fired sufficiently.
0), the crushing strength was as low as 2.7 N, and the water absorption was as high as 13.1%.

Comparative Example 21 The total amount of the alkali metal compound was 2% by weight to 6% by weight, and the amount of hematite added was increased so that the Fe ₂ O ₃ content after firing exceeded 10%. Also, there is no significant change in absolute dry gravity or strength, and no effect of increasing hematite is observed.

[Comparative Example 22] In the case where silicon carbide was not added, the absolute dry specific gravity was 1.15, and did not fall to the target value (1.0). This example belongs to an example in which hematite was deoxygenated using only the carbonaceous material.

[Comparative Example 23] Even when the addition amount of silicon carbide exceeded 1% by weight, the effect of lowering the absolute dry specific gravity was not improved.

[Comparative Example 24] When no coke as a carbon material was added at all, the oxidation inside the artificial lightweight aggregate proceeded,
The foaming expansion was small, and the absolute specific gravity was 1.44, and the target value (1.0) could not be achieved.

[Comparative Example 25] Coke as a carbonaceous material was used for 1.
When added in an amount of more than 0% by weight, the degree of oxidation of the surface of the artificial lightweight aggregate was reduced, the firing temperature could not be increased, the specific gravity increased, and the strength decreased.

[0052]

[Table 1]

[0053]

[Table 2]

[0054]

[Table 3]

[0055]

[Table 4]

[0056]

[Table 5]

[0057]

[Table 6]

[0058]

According to the present invention, as described above, high-quality artificial lightweight aggregates such as extremely light and high in specific strength can be produced at a low cost by using coal ash generated from a coal-fired boiler as a raw material. Can be produced efficiently. Therefore, it is possible to recycle industrial waste into building materials that require weight reduction without landfill treatment, which greatly contributes to environmental conservation and stable supply of energy.

Claims (6)

[Claims] 1. A mixture is obtained by mixing a coal ash with a melting point depressant, a binder and a foaming agent, and the mixture has an average particle size of 15 μm.
m or less to obtain a pulverized product, and water is added to the pulverized product to form a molded product. The molded product is dried in a rotary kiln within a temperature range of 1000 ° C to 1250 ° C. An artificial lightweight aggregate produced by firing such that the specific gravity becomes 1.0 to 0.5. 2. A mixture is obtained by mixing a coal ash with a melting point depressant, a binder and a blowing agent, and the mixture has an average particle size of 15 μm.
m to obtain a pulverized product, water is added to the pulverized product to obtain a molded product, and the molded product is fired by a rotary kiln in a temperature range of 1000 ° C to 1250 ° C. A method for producing an artificial lightweight aggregate, characterized in that: 3. A blowing agent comprising iron oxide in an amount such that the amount of Fe ₂ O ₃ in the coal ash is in the range of 1% by weight to 10% by weight, and 0.2% by weight to 10% by weight based on the coal ash. % Of the carbonaceous material and silicon carbide in an amount of 0% to 1% by weight with respect to the coal ash. Production method. 4. A melting point lowering agent, by mixing an alkali metal compound and coal ash so that the total amount of each or both of Na ₂ O and K ₂ O is in the range of 30% to 50% by weight, The method for producing an artificial lightweight aggregate according to claim 2, wherein the artificial lightweight aggregate is produced by heating and melting in a temperature range of 1000 ° C. to 1200 ° C. to form a glass, followed by cooling and pulverization. 5. A melting point lowering agent, by mixing an alkali metal compound and coal ash so that the total amount of each or both of Na ₂ O and K ₂ O is in the range of 30% to 50% by weight, It is produced by heating and melting in a temperature range of 1000 ° C. to 1200 ° C. to form a glass, followed by cooling and pulverization,
As the total amount of Na ₂ O and K ₂ O is in the range of 2% to 6% by weight of the baked good, according to claim 2, characterized in that the addition of melting point depressants in coal ash Production method of artificial lightweight aggregate. 6. The method for producing an artificial lightweight aggregate according to claim 4, wherein the alkali metal compounds are sodium carbonate and potassium carbonate.