Method of controlling a grinding mill process
A grinding circuit comprises a grinding mill having a shaft in a grinding chamber, a cyclone for receiving circuit feed material and for feeding said circuit feed material further in the grinding circuit, and monitoring devices. The grinding mill process comprises pumping circuit feed material from a feed tank to the cyclone upstream of the mill, and pumping the circuit feed material forming slurry into the grinding mill for grinding the slurry into finer particles. The method of controlling the grinding mill process comprises monitoring at least one operational parameter, and controlling particle fineness online by adjusting at least one of the following operational parameters: flow rate of the circuit feed material, density of the circuit feed material, shaft speed, filling rate in the grinding mill, milling density and retention time.
1. A method of controlling a grinding mill process in a grinding circuit, wherein said grinding circuit comprises:
a feed tank;
a grinding mill having a mill shaft in a grinding chamber;
a cyclone for receiving feed material and for feeding the feed material further in the grinding circuit;
a pump; and
feed material monitoring devices, wherein the method of controlling the grinding mill process in a grinding circuit further comprises the steps of:
pumping feed material from the feed tank to the cyclone upstream of the grinding mill; and
adding water to the feed material to form a feed slurry and pumping the feed slurry into the grinding mill for grinding the feed slurry into finer particles;
measuring particle size distribution of the feed material coming from the cyclone in the grinding circuit using at least one of the feed material monitoring devices;
comparing specific grinding energy measurements measured by at least one of the feed material monitoring devices to a target specific grinding energy per total feed material flow value; and
controlling mill shaft speed of the mill shaft provided in the grinding chamber of the grinding mill to reach the target specific grinding energy per total feed material flow.
2. The method according to claim 1 , wherein the method further comprises the following steps:
monitoring a feed material rate by measuring flow and density of the slurry using the feed material monitoring devices;
determining a specific grinding energy as a set point for the grinding circuit for achieving a target particle size;
controlling the operation of the grinding mill with the mill shaft speed such that the specific grinding energy set point is reached in spite of changes in the feed material rate; and
adjusting grinding in the grinding mill based on measurements by the feed material monitoring devices about particle size in the grinding circuit by comparing measurements by the feed material monitoring devices to target values and if the particle size differs from target values, a new set point is given for the grinding circuit.
3. The method according to claim 1 , wherein the method further comprises a step of controlling particle size by controlling a power input into the feed material by a variable speed drive of the grinding mill to control the mill shaft speed.
4. The method according to claim 1 , wherein the grinding mill comprises make-up grinding beads which are fed continuously to the grinding mill along with the slurry.
5. The method according to claim 1 , wherein the method further comprises a step of increasing or decreasing a bead charge to a new optimal level if there is a change in throughput or in a particle size distribution target level.
6. The method according to claim 1 , wherein the method further comprises a step of measuring a power output of the grinding mill.