Many technologies classify powders. The most common are screening, airflow, and centrifugal classification. Their effects on powder purity vary.
Sieving method
The sieving method is to use a sieve to grade the powder, which is suitable for coarse powder grading. If the sieve is intact, it can remove large impurities. It will improve purity. However, if the sieve is worn or has holes, large particles will mix into the fine powder. This will reduce its purity. In quartz sand screening, a damaged sieve lets larger particles into the final product. This reduces its purity.
A batch of calcium carbonate powder has an initial purity of 85%. It contains 10% large particles and 5% impurities. This powder is then screened and graded. A standard, undamaged sieve can remove over 90% of large impurities after screening. After screening, the powder’s purity can be increased to 94%. This assumes the distribution of other impurities remains unchanged. If the sieve is damaged, it may only remove 50% of large impurities. The powder’s purity can then only reach about 87.5%.
Airflow classification
This method is based on the difference in sedimentation speed of particles in the air flow. It can accurately separate particles of different sizes. It has a high impurity removal rate and can significantly improve purity. However, if the classification system has turbulent air or local eddies, some particles may mix, affecting purity. For example, in making lithium battery cathodes, airflow classification can remove large impurities and clumps. This improves material purity.
Take the production of titanium dioxide that requires a purity of more than 99% as an example. Under ideal airflow classification conditions, titanium dioxide has 95% purity. It contains 4% coarse-grained impurities and 1% other impurities. By controlling airflow speed, temperature, and the classifier’s structure, we can remove about 98% of the coarse impurities. The final product’s purity can reach 99.2%. If the airflow unevenness exceeds 20% in the system (it should be within 5%), the coarse particle removal rate may drop to 90%. This will result in a product purity of only 98.6%.
Centrifugal classification
The centrifugal classification method classifies particles according to their centrifugal force. It has a good classification effect on tiny particles and helps to improve purity. However, wear and corrosion will generate impurities that will contaminate the powder. Also, improper control of operating parameters will reduce purity by worsening the classification effect. For example, when preparing high-purity nano calcium carbonate, if the centrifuge speed is unstable, it will affect the purity of the product.
For alumina micropowder with 90% initial purity (8% mixed impurities and 2% other), the following applies. Under normal centrifugal classification, a stable speed is key. If the centrifuge runs at that speed, and the equipment is unworn, it can remove 95% of impurities. This increases the powder’s purity to 97.6%. If the centrifuge is worn from long use, it causes 0.5% impurities from wear. The unstable centrifugal force reduces the impurity removal rate to 90%. Then, the final powder purity may be only 96.1%.
These data show that, when working well, different classifiers can boost powder purity. However, the final powder depends on three factors. They are the equipment’s operating status, process parameter control, and equipment stability. Purity is crucial. These factors must be tightly controlled in production to get high-purity powder products.