Ball Mill and Grinding Circuit Preparation for Start-UP
The following items must be checked before the equipment in the Grinding Circuit is started.
- Check the ore slot feeder for obstruction or hang—ups.
- Check the ball mill belt feeder for undue buildup of material.
- Check oil level in ball mill lube oil tanks. Also, check grease level in the mill ring gear spray lube drum.
- Ensure that the plant air shut-off valves are open to the mill clutch and ring gear lube system. Also check the air pressure on the mill clutch air receiver tanks. This reading should be no lower than 90 psi (620 kPa).
- Check the ball mill discharge chute and cyclone overflow and underflow launders for obstructions and debris.
- Check the plug valves in the mill discharge distribution box for correct open or closed position depending on which pump-box compartment is to be used.
- Check that the plant air shut-off valves are open to the cyclone apex valves and to the distribution box plug valves. .
- Check that the cyclone feed pump-box and pump suctions are not sanded.
- Check that the barren solution tank is at least half full and that the barren solution pump is running. Also ensure that the shut-off valves in the barren solution lines to the ball mill feed chute and the cyclone feed pump-box are open.
- Ensure that the gland seal water shut—off valves are open, and that gland seal water is flowing to the appropriate cyclone feed pump.
- Check that the lime slurry pump is in operation, circulating lime slurry around the mill loop. Also ensure that the shut-off valve in the lime line to the ball mill feed chute is open. Check that the lime timer at the control panel is operational.
- Ensure that there is an adequate supply of cyanide solution and that the cyanide metering pump, feeding cyanide to the ball mill, is available to operate.
- Check all annunciator lights with the test push-button.
Ball Mill and Grinding Circuit Start-up Sequence
- Put sump pump into AUTO.
- Start ball mill lubrication system.
- Open air valves to the cyclones and the cylinder operated valves at the Cyclone feed pumpbox.
- Open barren solution isolation valves to cyclone feed pumpbox.
- Open barren solution shut off valve to ball mill.
- Open gland water valve to the appropriate cyclone feed pump.
- Put density controller on MANUAL mode and set the output to 20%. Set the setpoint to 50-55% solids.
- Start the appropriate cyclone feed pump.
- Put flow controller in MANUAL mode and set output to 10.8 cubic meters/hour (60% controller output).
- Start ball mill motor (clutch disengaged).
- Engage ball mill clutch.
- Put weight controller in MANUAL mode and set output to 5-8 tonnes per hour (20% controller output) and slowly raise (2—3 minutes) the feed rate to 22 tonnes per hour (73% controller output).
- Start the ball mill belt feeder.
- When the controllers have reached their desired outputs and the other equipment is operating smoothly, put the weight controller into AUTO mode. When the weight controller has stabilized itself, adjust the flow controller if necessary to meet the weight input, then put the flow controller into AUTO mode.
- Ensure that the lime shut-off valve to the ball mill is open.
- Start ball mill lime timer. The lime control valve should be pulsing open for 3 seconds every 30 seconds (after calibration of all lime timers).
- Open sodium cyanide shut off valve until fully open.
Normal Ball Mill and Grinding Circuit Operation
After the grinding circuit has been brought up to normal operating conditions, the operator must monitor the various process variables and alarms. Most of these variables are monitored in the mill control room, however, the operator is also required to sample and analyse process streams and read local indicators.
The operator must routinely monitor the following conditions:
- The mill power draw by looking at the mill power draw recorder. The recorder should show no major fluctuations and should read approximately 560 kW.
- The indication and alarm lights to ensure that they display the correct status, especially with respect to the ball mill lube system.
- The cyclone feed pressure and cyclone feed density at short intervals.
- Frequently, check the ball mill discharge trommel screen and dispose of any foreign objects on the screen.
The ball mill is susceptible to variations in ore hardness resulting in various grinds at constant throughput, or alternatively, various tonnages at constant grind. The variation in grind is not directly determined. However, a changing cyclone overflow density, at a constant tonnage rate and feed density, would be indicative of changing ore hardness and in that case the tonnage fed to the ball mill should be changed accordingly.
The ore feed rate to the ball mill is controlled by the weightometer located on the mill feed conveyor which can be manually adjusted with in the control room to give a constant weight reading. The signal from the weightometer increases or decreases the belt feeder speed and adjusts the water addition to the ball mill (as a function of the actual weight reading). Both weight control and the proportion of water can be adjusted in the control room.
The water rationing controller must be adjusted programmatically to give the desired ball mill discharge density (normally 60-65% solids). The ball mill discharge density should be checked manually at regular intervals and adjustment made to water ratio controller setpoint to adjust the ball mill discharge density.
The grinding circuit operator must ensure that the ball mill runs properly loaded and gives the correct ore grind. A major practical indication of mill loading is the sound made by the mill. A properly loaded mill will have a deep rhythmic roar, while an under loaded mill will have a metallic rattling type noise and an overloaded mill will be quite silent.
The ore feed rate must be adjusted to suit the mill load condition, for instance, increase feed rate to an under loaded mill.
Another indication of mill loading is the power draw. Overloading and under loading will both reduce power draw. The ball mill must be run to draw maximum power at all times.
The operator must manually measure the cyclone overflow and underflow densities regularly. An increase in overflow density is indicative of softer ore and will soon be accompanied by a lowering of power draw at the mill and a change of sound indicating that the mill is becoming under loaded. To compensate, feed tonnage must be increased. Similarity, a decrease in the cyclone overflow density is indicative of harder ore and this will be accompanied eventually by a coking of the mill. Feed to the ball mill must be reduced.
All changes to the grinding circuit process must be made gradually and the effects of the changes studied before further changes are made.
Normal Shutdown Sequence of Ball Mill and Grinding Circuit
- Shut off cyanide metering pump.
- Put weight controller into MANUAL mode and turn controller output to 0%. Run the ball mill for 15-30 minutes.
- Put density controller into MANUAL mode, set output to 0%.
- Put flow controller into MANUAL mode and set output to 0%.
- Turn off ball mill lime timer.
- Disengage ball mill clutch. The mill feed conveyor will automatically stop due to the interlocks between the clutch or cyclone feed pumps and the conveyor.
- Shut down ball mill motor.
- Shut down ball mill lube system.
- Drain and flush the cyclone feed pump, line and pump box if the shutdown is longer than 1-2 hours. Once the pump and system have been drained, shut off the cyclone feed pumps and close the gland seal water shut off valve to the cyclone feed pump.
- Close the air block valves to the cyclones and the cyclone feed pump box, and shut-off valves on all barren lines.
Ball Mill and Grinding Circuit Emergency Shutdown
In the event of an emergency, the mill feed conveyor is shut down individually or by stopping the operating cyclone feed pump. The ball mill must be shut down separately. All equipment shutdowns are performed locally or from the MCC located in the mill control room.
After the equipment is shut down, shut off the cyanide metering pump and lime timer and close all shut off valves in the air and solution lines.