A study of calcining aluminum oxide

- Mar 22, 2019-

Calcination of fused aluminum oxide

Brown fused alumina is one of the most basic abrasives. It has strong resistance to crushing, oxidation, and corrosion, and has good chemical stability. In China, the production process of brown corundum is mature (the smelting method currently used in China has the fixed furnace frit method and the tilting furnace smelting method), the production cost is lower than other abrasives, and the cost performance is better than other abrasives. Therefore, after more than 60 years of development, brown fused alumina sector has gradually formed an industrial material system of a certain industrial scale, which has become an irreplaceable basic material for industrial development in China and an indispensable and important component of the national economy. In the manufacture of coated abrasives and bonded abrasives and the production of refractory materials, play an important role.

With the rapid development and structural adjustment of China's machinery manufacturing industry, the quality requirements of brown fused alumina products in the abrasive industry are getting higher and higher, prompting abrasive production enterprises to continuously carry out tentative exploration experiments and find new practical treatments. To improve the quality of brown fused alumina. At present, the treatment technologies for improving the quality of brown fused alumina abrasive products mainly include: improving the shape of abrasive particles, heat treatment or chemical coating. Only improving the shape of the particles to improve the performance of the abrasives cannot meet all requirements of the abrasive tools, and the chemical treatment also has the disadvantages of high cost and environmental pollution. Therefore, the heat treatment of the brown corundum abrasives has become the main method to improve the quality of the abrasives.

As a kind of heat-treated abrasive, brown corundum calcination has been used in foreign countries for more than 20 years, and it has been used in China for more than ten years. Yannuo was one of the first manufacturers to implement the production of calcined brown fused alumina abrasives.

(1) Calcination methods

Calcination kiln Includes shuttle kiln, tunnel kiln, rotary kiln and vertical kiln. The heat source is mainly coal, gas, natural gas, liquefied gas, methane gas and electricity.

(2) Calcination process

The brown fused alumina abrasive is preheated in a special container and then gradually heated to 800~1350 °C for 2 to 4 hours. After natural cooling, it is washed and sieved. According to different calcining temperature, the color of the abrasive might be dark brown, brownish, blue, light blue, and blue-gray.

(3) Advantages of calcined brown fused alumina abrasive

1. The abnormal expansion of brown fused alumina abrasive is eliminated.

In the smelting process of fused corundum, the abrasive particle which contains low-valent titanium oxide (Ti2O3, TiO) and titanium carbide, titanium nitride (TiC, TiN), will be further calcined in a temperature of 800 ° C -1350 ° C incurring Oxidation, as a result, the volume increases correspondingly, causing the abrasive volume to change beyond its normal expansion, which is abnormal expansion. The abnormal expansion of the abrasive will cause the brown corundum ceramic wheel to have a net structural crack on the surface when it is baked, resulting in a large amount of defective.

The calcination treatment is the most effective means to eliminate the abnormal expansion of the brown corundum abrasive.

2. Reduce the bulk density of the abrasive and improve the toughness of the abrasive.

During the calcination Ti3+ which is originally solidified in the corundum crystal is oxidized to cause lattice shrinkage, thereby improving the compressive strength of the abrasive. Second, calcination also compensates for the internal stresses caused during the granulation process. Therefore, the use of calcined abrasives to produce various abrasive tools is not only durable but also sharp, which improves the grinding performance of the abrasives, especially the production of resin sheets and heavy-duty grinding wheels.

Table 3 calcined abrasive impact toughness


Mesh

Original

toughness

1000oC calcined

1350oC

calcined

#16

#20

#24

#30

#36

#46

#60

#80

54

62

46

56

71

67

54

78

67

80

63

67

83

79

66

83

76

80.1

73

74

88

88

68

84.6


3. Improved the hydrophilicity of the brown corundum abrasive. After the brown fused alumina abrasive was baked at 800 ° C - 1350 ° C for 2 - 4 hours, the hydrophilic properties of the abrasive increased by 3.5 times. This is mainly because the impurities such as volatiles on the surface of the abrasive are removed, the low-melting substance is burned off, and the capillary phenomenon on the surface of the abrasive increases. The abrasive bulk density is correspondingly reduced, the hydrophilicity is improved, and the abrasive is used for coating abrasives, and the durability can be improved by about 30%. Table 4:

Table 4 Effect of Calcination on Abrasive Hydrophilicity

Particle Size


Time (minutes)

Hydrophilic Height (mm)



Uncalcined

Calcination

150

      3

5

10

20

30

  49

60

80

103

125

   116

139

190

250

300

60

    3

5

10

20

30

45

47

50

52

50

120

128

157

160

167

4. Increased strength of the tool.

After the calcination treatment, the abrasive removes impurities such as surface volatiles, thereby improving the purity and cleanliness of the abrasive, thereby improving the bonding force of the abrasive particles and the binder during the manufacture of the abrasive tool, reducing the amount of the bonding agent, and improving the strength of the abrasive.

5. Improved the appearance and cutting ability of the abrasive tool.

After the calcination treatment, the magnetic materials can be easily removed by magnetic sorting due to the expansion and rupture of the iron-encapsulated corundum abrasive grains. Which reduces the magnetic content of the abrasive to avoid rust spots on ceramic grinders; When the abrasive is baked, it improves the appearance of the abrasive tool and Cutting ability.

(4) Disadvantages of calcined abrasive

1. The fluidity deteriorates. Since impurities such as volatiles on the surface of the calcined abrasive are removed, the low-melting substance is burned off, the capillary phenomenon on the surface of the abrasive increases, the bulk density of the abrasive decreases, and the specific gravity decreases, thereby causing deterioration of fluidity.

2. Poor moisture resistance. As the capillary phenomenon increases, the specific surface area of the abrasive increases, the hydrophilicity increases, and the moisture absorption capacity increases, which may adversely affect the sanding of the coated abrasive.

(5) The pros and cons of calcined abrasive.

Although the calcined brown fused alumina abrasive relative to the uncalcined, whether its toughness, strength, hydrophilicity, grinding ability, etc., is greatly improved compared with the uncalcined brown fused alumina abrasive. However, after all, its relative price is relatively high, and it has some shortcomings. In particular, the 150-mesh medium-temperature calcined abrasive has a significant increase in moisture absorption capacity, resulting in poor fluidity, which will inevitably cause some difficulties for planting abrasive sand on adhesives. Therefore, it is recommended that abrasive production enterprises should be systematic and standardized as much as possible in the production process, to ensure the stability and durability of their products, and to create conditions for downstream customers. Secondly, abrasives manufacturer who use calcined abrasives for the first time, a trial should be conducted as mandated. With a gradual improvement and adaptability of the equipment, it can then be used in batch; If usage of the medium temperature calcined abrasive is satisfiable, as far as possible not to use high temperature calcined abrasive. After all, the high-temperature abrasive price is significantly higher than the medium temperature calcined abrasive.

In short, the use of calcination to treat brown fused alumina abrasives is an effective measure to save resources, protect the environment, and improve product performance. Production enterprises and downstream enterprises must work together to constantly improve, and effectively promote the overall improvement and healthy development of China's abrasives manufacturing industry.