It is normal to get different outputs from the mill with materials from different sources. This will depend on the hardness of individual materials. There are ways to increase the output from a vertical mill. These involved justments to the separator and the airflow through the mill. A number of case studies have been reported by Siam Cement where major increases in output were achieved by these means.

If you are adequately controlling the kiln feed residues to 15 per cent I am not sure what the grit return weigher is going to tell you. Will you be able to remotely adjust the vanes of the mechanical separator from the control room while the mill is running? If not I would suggest periodic sampling and measuring residues around the circuit is all you need to do to determine the rejects rate. What do you mean by "optimise" the raw mills? Do you need more raw mix output? Is there sufficient drying capacity in the mills? I presume they are swept by preheater exhaust gases?

The first stage is to maximise the output with the existing mill. You should conduct axial tests and recirculating load tests to make sure that the ball charge and lining are optimal. The next stage might be to conduct trials with grinding aids to try to boost the output of the mill by up to 15 per cent. Beyond that you will need to consider modifications to the milling circuit by adding further equipment. There are two options to do that: (i) add a pregrinding facility such as a hammer mill or roll press, or (ii) add a regrind mill. Adding further equipment will be significantly more expensive than introducing grinding aids but has the potential to boost the output to higher levels.

Our plant has four cement mills (closed circuit with first generation separator) - we use about 20 per cent slag, the output is 100tph ,the dimension of every mill is 15.5 x 4.4, the liners in the first chamber are lifting and in second chamber classifier. Are there any ways to increase mill production? How about for a raw mill?