Low output from ball mill & moisture issue in classified fines

We have a 4x16 ceramic lined ball mill and we are milling precipitated calcium carbonate. We are trying to to get to a gradation of D50: 3 micron and D90: 8 micron, however, we are having trouble getting the gradation down. Currently we are at a D50 of 5-6 micron and D90 of ~15 micron. We are using grinding aid and running the mill at 28 rpm. Our air classifier's rotor is running at 2200 rpm. We are only getting approximately 50-60 lbs an hour, when our mill manufacturer said our mill has the capability of 3,000 lbs per hour. Clearly, there is a massive issue here.

We first thought that static may be the problem, causing the material to not grind effectively and possibly clumping and registering as a larger particle size in the air classifier, causing fines to be sent to the coarse regrind. We then added ionized air to help with the issue, which didn't result in any increase in output. Neither grinding aid nor ionized air have helped.

Could we have the wrong type of mill? Do we need a steel mill with steel media to effectively crush calcium carbonate to increase our output problem and fix our gradation issue? We are at a loss since our mill manufacturer said that we would be able to effectively grind using a ceramic mill and our output would be 3,000 lbs an hour, not 50-60 lbs.

Additionally, moisture content is an important part of our process, moisture MUST be below 0.20%. We dry the raw material and we test at different points in our process to ensure moisture content is below 0.20%. Entering the ball mill, the feed is below 0.20% moisture. Exiting the ball mill, the moisture content is below 0.20%. The coarse regrind is the same as the ball mill exit feed at below 0.20%. HOWEVER, the fines are registering at well above 0.20% at around 0.9% to 1%. How is this possible? Why are our fines registering at such a high moisture content when the ball mill exit and coarse regrind are all below 0.20%? Again, we are at a loss. Could our air classifier be creating that much condensation during the downtime in the evenings to affect the fines?

If anyone has any suggestions, we would greatly appreciate it. ■