The Effect of Grinding System on The Ultra-fine Grinding Effect of Soapstone

Soapstone is a branch of the muciolite family, which is an expansible water aluminum silicate, which is a small and unique mineral resource. The number of cations and the species of cationic in the octahedron are different from those of his subites, so they have a unique three-octome structure. The special structure has many superior sexual energy, such as the swelling, the contact, the thickening and the glue. These properties make soapstone successfully applied to pharmaceuticals, cosmetics and his industry.

 

Ultrafine powder has excellent physical and chemical properties such as large specific surface area, high surface activity, high covering rate, good filling and reinforcing performance, fast chemical reaction, etc. Therefore, almost all the powder mineral raw materials required by high-tech industry need ultrafine processing, and industrial applications have endless requirements for material refinement. As a scarce mineral resource, saponite is widely used in various industrial fields. It is of great significance to study superfine grinding of saponite, but there are few reports about it at present.

 

As a kind of high-efficiency grinding equipment, planetary mill has been paid attention in recent years, and some achievements have been made in basic research and application research. The problem of the existing studies is that the planetary grinding efficiency is generally low, with grinding time ranging from 1 h to dozens of hours. CJXXM type planetary mill is a new type of high energy density planetary mill, the whole machine integration, simple operation, high grinding efficiency, grinding time from a few minutes to dozens of minutes. In order to study the effect of CJXXM superfine grinding of saponite by planetary grinding dry method, dry superfine grinding of saponite was studied in this paper. Through particle size analysis, the influences of mass ratio of medium to material, grinding speed, type and dosage of grinding aid and grinding time on the particle size of saponite were investigated.

Ⅰ. The Experiment

1.  Materials and Reagents

  • D50 and D97 (d50 and D97 are the corresponding particle sizes when the cumulative mass fraction is 50% and 97%, respectively) are 6.59 and 32.87 μm, respectively. The particle size distribution is shown in Table 1.
  • Yttrium stabilized zirconia ball, grinding medium, particle size 0.6~2 mm, Beijing Zhonglightweight Stone Import and Export Co., LTD.
  • Polyethylene glycol 1000, grinding aid, chemical pure, pH 4.0~7.0, Guangdong Xilong Chemical Co., LTD.
  • Sodium stearate, chemical pure, Guangdong Xilong Chemical Co., LTD.

2. Equipment and Instrument

  • CJXXM high ability living star mill, zhejiang jiaxing weapons new machine weapons co., LTD.
  • The bt-1500 type centrifugal sedimentation scale distribution measuring instrument, the baites instrument co., LTD. In dandong, liaoning province.

3. Experimental Steps and Characterization Methods

Put 25 g soapstone powder into two grinding tanks, add grinding medium according to a certain medium to material mass ratio (medium to material ratio for short), weigh a certain quality grinding aid, pour into the grinding tank, stir evenly, grind continuously for a certain time, and take samples. In this paper, D50 and D97 are used as the basis for comparative analysis.

Ⅱ. Results and Discussion

1. Effect of Medium to Material Mass Ratio on Particle Size of Ore Powder

The mass of raw ore was 25 g, the grinding speed was 600 r/min, and the grinding aid sodium stearate was added with a mass fraction of 0.5%. The grinding conditions were as follows: 1, 2, 3, 4 and 5, respectively, and the samples were taken for testing. The influence of medium to material mass ratio on particle size is shown in Figure 1. It can be seen from the figure that the particle size of ore powder decreases first and then increases with the increase of medium material ratio, and finally remains stable. When the medium material ratio is less than 3, the medium mass is too small, and only part of the large particles produce fracture and dissociation of the weak point and weak plane. With the increase of medium to material ratio, the energy provided by medium to ore powder gradually increases, and the probability of fracture and dissociation of large particles is greater, while small particles can overcome the internal bond energy and produce fragmentation. When the medium to material ratio reaches 3, the grinding effect is the best. When dielectric material ratio greater than 3, instead of grinding effect, the reason is that the quality is too big, the medium effect on the ores in the force is too large, which has a larger surface to small particles reunited together, at the same time, has broken the surface of the small particles unsaturated bond under the action of mechanical force chemistry generates a small amount of water, adding reunion phenomenon. In conclusion, the suitable medium/material ratio is 3, and the D50 and D97 of the product are 1.63 and 6.6 μm, respectively.

2. Influence of Mill Speed on Particle Size of Ore Powder

The mass of raw ore was 25 g, the medium to material ratio was 3, and the grinding aid sodium stearate was added with a mass fraction of 0.5%. The grinding was performed continuously for 24 min at the grinding speed of 400, 500, 600, 700 and 800 r/min, respectively, and the samples were tested. The influence of mill speed on particle size of ore powder is shown in Figure 2.

It can be seen from the figure that the particle size of ore powder decreases first and then increases with the increase of the mill speed. When the speed is less than 600 r/min, with the increase of the speed, the force of medium on the ore powder increases gradually, the probability of fracture of large particles at the fragile point and fragile surface also increases, and the particle size of ore powder decreases correspondingly. When the speed is 600 r/min, the force of medium on the ore powder is large enough. At this speed, not only the brittle point and fragile surface of large particles fracture, but also the fracture process of small particles overcome the internal bond energy, and the grinding effect is the best. When the speed is greater than 600 r/min, the grinding effect becomes worse. The reason is that the speed is too large, the medium action on the force of ore powder is too large, so that the small particles with large surface energy agglomeration; At the same time, due to the increase of speed, the heat generated in the grinding process increases sharply, resulting in part or all of the water of crystallization between the layers of ore powder unit, aggravating the agglomeration phenomenon; Moreover, under the condition of high heat and high rotational speed, the ore powder will be subjected to mechanochemical action, and the unsaturated bond at the edge will also produce a small amount of water under this action, further aggravating the agglomeration phenomenon. In conclusion, the grinding effect is not always optimized with the increase of the mill speed, but there is a critical value. In this experiment, the mill speed is appropriate to 600 r/min, and the D50 and D97 of the product are 1.63 and 6.6 μm, respectively.

3. Effect of Grinding AIDS on Particle Size of Ore Powder

The mass of raw ore was 25 g, the medium to material ratio was 3, and the mill speed was 600 r/min. Polyethylene glycol 1000 and sodium stearate were used as grinding AIDS, respectively, and the mass fraction was 0.5%. Samples were taken every 6 min, grinding for 30 min in total, and five samples were taken in total. The influence of grinding AIDS on particle size of ore powder is shown in Figure 3. The figure shows that the grinding aid effect of sodium stearate is significantly better than that of polyethylene glycol 1000. The reason is that sodium stearate is ionic organic matter. Under the action of mechanochemistry, the cationic group of stearate combines with the unsaturated bond at the fragile point and fragile surface of mineral powder particles on the one hand, neutralizes the unsaturated bond energy and accelerates the grinding process of mineral powder. On the other hand, by closely combining with the surface unsaturated bonds of the fragmented small particles, the steric hindrance effect of organic macromolecules is utilized to prevent the occurrence of agglomeration, so as to achieve a smaller crushing limit. Polyethylene glycol 1000 is non-ionic organic matter, can only use van der Waals force and mineral particles physical adsorption, play a simple lubrication role, so the grinding efficiency is low, large particles slowly become smaller with the extension of grinding time, the formation of small particles can not be further refined, so the crushing limit is large. In conclusion, organic cationic surfactants are more suitable as grinding AIDS for saponite than non-ionic surfactants.

4. Influence of Grinding Aid Dosage on Particle Size of Ore Powder

The mass of raw ore was 25 g, the medium/material ratio was 3, the grinding speed was 600 r/min, and sodium stearate was selected as grinding aid. The grinding experiments were carried out under the conditions that the mass fractions of grinding aid were 0, 0.2%, 0.3%, 0.4%, 0.5%, 0.6% and 0.7%, respectively. Continuous grinding for 24 min, sampling detection. The particle size test results of samples are shown in Figure 4. According to the figure, when the mass fraction of sodium stearate is less than or equal to 0.4%, the grinding aid effect is not significant. When the mass fraction of sodium stearate was 0.4%~0.6%, the particle size of ore powder decreased significantly with the increase of grinding aid dosage, and reached the minimum value when the mass fraction was 0.6%, the D50 and D97 of the product were 1.28 μm and 7.00 μm, respectively. When the amount of grinding aid continues to increase, not only can not play the role of grinding aid, but also adversely affect the grinding process. The reason is that the grinding aid dosage is too small, can not form a complete monolayer adsorption on the fragile point and fragile surface of mineral powder particles and the outer surface, so the grinding aid effect is not obvious; When the dosage is appropriate, the grinding aid group just forms a complete monolayer adsorption at the fragile point and fragile surface, which not only neutralizes the unsaturated bond energy, but also plays a role of steric hindrance, so as to accelerate the fragmentation process of ore powder and effectively prevent the occurrence of agglomeration phenomenon. When the amount is too large, the “anti-adsorption” effect occurs, so that the fragile point and fragile surface of the mineral particles and the outer surface are re-charged, in an unsaturated state, easy to occur agglomeration phenomenon. In conclusion, the suitable mass fraction of sodium stearate as grinding aid for dry grinding of soapstone is 0.6%.

5. Effect of Grinding Time on Particle Size of Ore Powder

The raw ore mass was 25 g, the medium/material ratio was 3, the grinding speed was 600 r/min, 0.6% grinding aid sodium stearate was added, and samples were taken every 2 min for 30 min in total. FIG. 5 shows the effect of grinding time on particle size of ore powder. As can be seen from the figure, when the time is less than 24 min, the sample granularity shows significant fluctuation with the change of time, and then tends to be stable. Due to the small sampling time interval, the curve can well reflect the complex dynamic changes in the grinding process, that is, in the first 24 min, the ore powder is in an unbalanced state of crushing and agglomeration, and after 24 min, the crushing and agglomeration basically reach a dynamic balance. The final grinding time was 30 min, and the D50 and D97 of the product were 1.55 and 7.38 μm, respectively.

Ⅲ. Conclusion

1. Dry ultrafine grinding of saponite was carried out with CJXXM high-energy activated planetary mill, zirconia ball with particle size of 0.6~2 mm was used as grinding medium.

The appropriate process conditions were as follows: The mass ratio of medium to material was 3, the grinding speed was 600 r/min, 0.6% sodium stearate was added as grinding aid, and the grinding time was 30 min.

2.CJXXM high energy active star grinding tool has higher grinding efficiency. Under suitable grinding conditions, the D50 and D97 of ore powder are 1.55 and 7.38 μm respectively for 30 min.

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