OUTOKUMPU Technology’s chief process engineer Sanjeev Latchireddi has written about optimising the grinding mills found in mining plants to maximise uptime and efficiency.
According to the article, the milling and grinding area is central to plant operation. In modern plants, the energy efficiency of a plant rests mostly on the Semi-Autogenous Grinding (SAG) mill operation, resulting in a shift of maintenance emphasis from ball mills.
The load on the mill depends on the characteristics of the ore and the discharge rate of broken particles. The discharge rate in turn is dependent on the efficiency of the discharge pump’s operation. The pulp lifter’s design will affect the discharge capacity and thus the throughput of the grinding mills.
Mills discharge mostly slurry consisted of water and finer particles, or pebbles measuring 20 to 100mm. Single stage mills handle larger amounts of slurry because they are in a closed circuit with classifiers.
Problems with the slurry discharge process include flow-back and carry-over. These problems are due to the geometry of radial and curved pulp lifters, which keep the slurry in contact with the grate until complete discharge.
Slurry carry-over occurs at relatively higher mill speeds with increasing slurry viscosity. The impact of flow-back can be minimised in open-circuit grinding, but can be debilitating in closed-circuit operation, especially when cyclones and fine screens are involved.
These problems cause a slurry pool to form in the mill, absorbing impact energy and decreasing the field of breakage. This in turn leads to less particles being broken and reduces the grinding efficiency.
Multi-stage milling circuits have screens and pebble crushers included in the line with the mills. Slurry discharge is transferred to ball mills for further grinding, and coarse pebbles or rocks are crushed and sent back to the mill.
Most plants use grates with pebble ports instead of normal grate openings. This increases the pebble removal rate. Some operators choose to run the mills at relatively higher speeds to increase capacity. According to this reasoning, high mill speed equates to more impacts and thus more breakage of particles.
The article suggests although mills can operate at higher speeds, the inefficiency of pulp lifers increases with the mill speed. This results increased pebble carry-over and thus a large amount of pebbles trapped inside the pulp lifters.
The pebbles occupy significant volume at the bottom of the pulp lifter, blocking the outer rows of grate slots and reducing the flow gradient across the grate. To maintain the gradient, the load inside the mill increases, resulting in more power being drawn. This leads to a higher rock to ball ratio, insufficient grinding energy, and even more increase in load within the mill.
Another potential is for pebbles to flow back into the mill, facilitated by larger pebble ports. This increases the quantity of critical size material in the mill and cuts milling efficiencies.
To optimise milling power, slurry and coarse pebbles need to be efficiently removed. Some plants use curved pulp lifters, which solve the carry-over problem but does not eliminate the flow-back problem.
Another option is to use a grate, peripheral discharge trunnion supported mill, or an open-ended discharge shell supported mill. These mills do not require pulp lifters but have structural limitations, meaning only a few plants will find them useful.
According to Outokumpu Technology, it has designed a Turbo Pulp Lifter to resolve the discharge problems. The internal design of the product approximates a grate, peripheral discharge or a grate, open-ended discharge to keep the slurry and pebbles away from the grate once they enter the pulp lifter. This is said to eliminate flow-back and carry-over.
The pulp lifter is said to have efficient material transport, allowing the particles to stay inside the mill long enough to be broken into sizes smaller than the grate. It does not require redrilling of the mill head for retrofitting.
By eliminating material transport problems, the mill can operate a maximum capacity, grinding conditions will be improved, resulting in less energy use, and higher mill speeds will see efficient operation.
Add a comment