Optimization of Wet Ball Milling Parameters of Alumina Powder

In order to obtain the raw material powder with reasonable particle size distribution which can be used for the sintering of ceramic film support body, alumina powder with particle size of 500M was used as raw material in the experiment to explore the optimal conditions of wet ball milling. Under the conditions of different milling time and pellet material ratio, the ball milling effect was compared, the particle size and distribution of the powder after ball milling was determined and the cost factor was considered comprehensively. The experimental results show that the optimal milling conditions are ball-material ratio (9 ~ 11)∶1 and ball-milling time (9 ~ 10h). Subsequent experiments show that the particle size of the powders obtained under the optimal milling parameters is relatively uniform, which can be directly used for sintering ceramic film supports with high temperature resistance and high strength.

With the rapid development of membrane separation technology, ceramic membrane has been widely used in environmental protection, food, chemical, pharmaceutical, biological engineering and other industries, but the research on wet preparation technology of alumina powder, its common raw material, has not been reported. Particle size, particle size distribution, sphericity and other parameters of the powder can directly affect the structure and performance of the ceramic membrane support, and reducing the preparation cost of ceramic membrane support is one of the focuses of ceramic membrane research in recent years. Therefore, it is very important to optimize the preparation conditions and reduce the cost to prepare powders with shape and properties that meet the sintering needs of ceramic film support.

In today’s ceramic film production, α -al2o3 is one of the most important raw materials for ceramic membrane support system, and α -al2o3 is the most stable in a variety of alumina crystals, the largest lattice energy, high melting point, hardness, grinding difficulty, so the commodity α -al2o3 powder grinding into suitable particle size powder, ball grinding time is generally more than 12h, Therefore, this process has high energy consumption, which can account for about 50% of the cost of ceramic membrane preparation. Therefore, it is necessary to optimize ball milling conditions, improve production efficiency, reduce energy consumption, reduce the comprehensive production cost of ceramic membrane, and further improve the feasibility of industrial promotion of ceramic membrane. Although after dry ball mill is superior to the wet particle spherical degree, but the efficiency is better than that of dry wet ball mill, so this experiment adopts the circulating wet ball mill technology, polyacrylic acid (PAA) as dispersing medium, optimize the influence on the effect of ball mill is the most significant ball grinding time, material ratio and other parameters, and by comparing the ball mill parameters such as particle size, particle size distribution of powders after experimental verification.

Ⅰ. The Experiment

1. Experimental Materials

2. Main Experimental Instruments and Equipment

  • Vertical circulation ball mill, model LM-70, total power 5.5KW, wuxi Xinbiao Powder Machinery Manufacturing Co., LTD.
  • Laser particle size analyzer, model Winner2000S, test accuracy 0.1 ~ 300μm, produced by Jinan Micro nano Particle Instrument Co., LTD.
  • Air compressor, double pneumatic diaphragm pump, oven, electronic day equality.

3. Experimental Process

Using wet ball milling technology, the raw alumina powder with the particle size of 500M, at 3∶1, 5∶1, 7∶1, 9∶1, 13∶1 different ball/material ratio (W/W) grinding into the target particles (particle size less than 10μm) percentage (wt%) of more than 90%, can be directly used to prepare ceramic film support alumina powder. In order to reduce the influence of ball grinding pollution, zirconium composite beads with a diameter of 6mm were used as the grinding medium for 15h. 500mL of suspended alumina samples were taken after grinding for 2.5h, 5.0h, 7.5h, 8.5h, 10.0h and 15.0h, respectively, and placed in the oven to dry. Their particle size and distribution were determined by laser particle size analyzer.

Ⅱ. Results and Analysis

1. Experimental Results

The grinding results under different grinding time and material ratio are shown in Table 2.

2. Comparison and Analysis

(1) Influence of Ball Material Ratio on Ball Grinding Effect

Taking the target particle percentage as the vertical axis and the ball/material ratio as the horizontal axis, the effect of grinding for the same time was compared, and the influence of ball/material ratio in different time periods was analyzed. Figure 1 shows the target particle size after grinding for 2.5h, 5.0h, 7.5h, 8.5h, 10.0h and 15.0h. Figure 1A shows that, no matter the size of pellet to material ratio, the raw material particles are still in the range of 10-40 μm after grinding for 2.5h, which makes it difficult to reach the target particle size. Figure 1 b shows that, after 5.0 h, ball material ratio can meet the target particles reach more than 50%, the highest ball material residency is not detected target particles, that early grinding, within 0 ~ 5.0 h ball material ratio as the main influence factors of ball material ratio, the more the higher the efficiency of ball mill, determines the target particles generated stage or early material crushing, However, the influence of pellet/material ratio did not show a good regular change.

It can be seen from Figure 1C that after grinding for 7.5h, the target particle completion proportion increases continuously with the increase of pellet/material ratio, and tends to be stable when the pellet/material ratio is higher than 10∶1, indicating that when the pellet/material ratio increases to a certain value, it is no longer the main factor, and the crushing and quantity increase of raw material particles have become the main factor to improve the effective collision rate. Figure 1D, E, and F show that those with higher pellet material have completed the grinding process before the end of 15h (the percentage of target particles is more than 90%). In the later period, “reverse grinding” phenomenon may occur, that is, agglomeration phenomenon, so that the pellet material ratio is high and the percentage of target particles increases slowly.

(2) The Influence of Ball Grinding Time on Ball Grinding Effect

With the percentage of target particles as the vertical axis and the ball grinding time as the horizontal axis, the change of grinding effect over time under the condition of the same ball-material ratio was compared, as shown in Figure 2, to analyze the law and characteristics of particle collision in the grinding process.

In general, with the increase of the ball material ratio, the contact probability of the material and the ball grinding medium increases, and the grinding efficiency shows a trend of improvement compared with the same period of last year, but the resulting grinding effect has a significant change trend. Figure 2 a and b indicates the low ball material ratio when grinding 15 h, the target particle percentage is less than 90%, and showed a trend of accelerated, show that the ball material ratio under the condition of grinding during the 15 h, has been in the raw material in the early broken (10 ~ 40 microns) and part of the target particle formation stage, still need more time to finish grinding target, high energy consumption. When the ball/material ratio reaches 7∶1 or above, the increasing trend of the target particle percentage is no longer accelerated, but basically in a straight line. However, when the ball/material ratio is 9∶1, the increasing speed gradually slows down. When the ball/material ratio is 13∶1, the ball grinding target is reached after about 8h, which greatly reduces the grinding time and energy consumption. The percentage of measured target particles did not reach 100%, indicating that although the particle size was in micron level, there was still agglomeration phenomenon.

Goal according to the grinding particle size less than 10 microns particle percentage above 90%, can be made of the ball than 7:1, 9:1, 13:1 three sets of grinding data trend line, fitting calculation, get the lowest must milling time were 13.7 8.5 8.7 h, h, h, show that the ball material ratio is more than 9:1, the lowest must milling time change is not big, In other words, the ball-material ratio is no longer a controlling factor. According to engineering practice, the optimal ball-material ratio is (9 ~ 11)∶1. Based on the results in Figure 1 and Figure 2, the optimal ball-grinding time is 9 ~ 10h in order to reduce grinding energy consumption and ensure grinding effect.

3. Sintering Experiment and Analysis

The sphericity of powder has a direct effect on the sintering of ceramic film support: the regular spherical Al2O3 powder can form support in the form of spherical accumulation, and the porosity is lower, the contact surface between spherical particles is smaller, and the strength of support is slightly lower. The advantage is that the shape of the pore formed by the spherical powder is more regular, the flow resistance is reduced, and the overall strength of the support body is more uniform.

Using the ball material ratio: 1, 10 h the powder grinding time, according to certain firing system for ceramic membrane support system, the income of the ceramic membrane support body porosity was 31.7 ~ 35.6%, the largest aperture for 9 ~ 12 microns, bending strength is 3.9 ~ 4.5 kN, specific parameters are shown in table 3, reach has been industrialized ceramic membrane support body parameters, The results show that the powder obtained under the optimal ball milling condition meets the requirements of the support system preparation, which is consistent with the experimental results of particle gradation.

Ⅲ. Conclusion

1. The sphericity of the powders obtained by wet ball milling is good, which can meet the needs of direct preparation of sintered ceramic membrane support system;

2. The experimental results show that the optimal wet milling conditions are as follows: the milling time is 9 ~ 10h, the ratio of ball to material is (9 ~ 11)∶1, under which the accumulated particle size of the obtained powder smaller than 10μm reaches more than 90%, the milling time is about 30% lower than the optimal time reported in related literature, the energy saving effect is obvious, and the preparation cost of the powder is reduced.

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