Air classifiers do a great job of removing undesirable fines from quarried crushed stone and heavy mineral sands in a dry process without using any water.
In order to effectively remove the fine dust, the moisture in the stone sand must be very low, 1-2% is best but some fines can be removed even at 2.5-3%. Higher moisture reduces an air classifier’s efficiency and requires a larger machine. Excessive moisture can make air classification totally ineffective.
Low surface moisture is an important point to stress when air classifying because it directly affects the final products that are being processed.
Here is what happens when the moisture goes up:
- More fines remain in the dedusted product
- Since the fines stick to the coarse material a larger air classifier with more air flow may be necessary for the machine to be effective
- When the moisture of a material is excessive (3.5% or more), the water centrifuges out and also leads to equipment clogging
If the moisture levels rise, the following adjustments are necessary to improve the machine’s performance:
- First, remove some or all rejector blades to try to maintain the desired dedusted product
- Next, significantly reduce the feed rate to try to maintain the desired dedusted product
- Finally, monitor the machine for vibration and scrape off any build-ups on the blades and on the inner housing
Even with all the above adjustments, the maximum moisture allowed will depend on the % fines in the feed and the % fines allowed in the desired dedusted product.
So, to summarize, make sure the feed material that is being air classified has the lowest moisture possible.
*When we talk about moisture affecting air classification, we’re only worried about surface moisture, not ‘inherent’ moisture’. Inherent moisture is naturally found inside particles of minerals or stone sand after naturally air drying. This inherent moisture does not affect an air classifier’s ability to remove fine powder or dust from coarse particles.
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