Our EXAMPLE Grinding and Classification Circuit is designed to grind 500 tonnes of ore per day, operating 24 hours per day, with an availability of 95%. This circuit will grind -5/8″ material from the Crushing Plant, classify the slurry in one of two cyclones, and pass 70% of the minus 200 mesh material to the mill feed Thickener Circuit at a rate of 22 mtph.
This section is intended to be read in conjunction with the Flowsheet and Piping and Instrument Diagrams.
The Grinding and Classification Circuit is designed to the following criteria:
F80 Feed Size 80% minus 7/16″ (11,000 microns)
P80 Product Size 70% minus 200 mesh (75 microns)
Ore’s Bond Work Index 17.7
Circulating Load 400%
Ball Mill Power 800 HP
Detailed Process Description and Control: Crushed ore in the fine ore bin flows through a slot feeder onto a 30″ variable speed belt feeder. The
bin low level alarm on the mill control panel annunciator will inform the mill that there is a shortage of feed material. The belt feeder has level and
pullcord switches identical to the other three conveyors. In addition, the belt feeder is equipped with two speed switches; one at each pulley, in order to measure and alarm the difference in pulley speeds as a low speed may be normal for this drive. A belt weigh scale under the conveyor measures the weight of material passing over the idlers and coupled with the belt speed measures the total weight passing on the belt. The ball mill feed rate is set via the weight controller at the mill control panel which adjusts the speed of the belt. A digital counter or totalizer displays the results as total tonnes of ore passed and a recorder maintains a permanent record of ball mill feed.
Regular feed or head samples are taken from the belt feeder with an air operated belt cutter sampler.
Ore from the belt feeder enters a 10′-6″dia. x 13′-0″ ball mill, which grinds the -5/8” ore to 70% minus 200 mesh. Barren solution is added to the ball mill to produce a slurry of 60% solids. The solids content of the ball mill discharge is controlled by a ratio control loop, whereby barren solution addition is proportional to the ball mill feed. The set point of the controller is the percent solids desired in the ball mill. An orifice plate measures the barren solution flow rate and the controller calculates how much barren solution is required to make the desired slurry, based on the weight of material coming through, and modulates a motorized butterfly flow control valve in the barren solution line accordingly.
Lime slurry and sodium cyanide solutions are also added to the ball mill feed chute at a rate of 250 litres per hour and 80 litres per hour respectively.
The ball mill is an overflow type discharge mill, and is designed for a 45% ball charge. Balls should be added on a regular basis as they are consumed in the mill. The balls are added with a one ton ball bucket, lifted by an overhead crane, and discharged into the ball mill feed chute. The ball charge is self-limiting – a greater than 45% ball charge will cause the excess balls to discharge over the trommel screen and out of the mill.
Ground ore passes through the trommel screen onto a chute and is fed to a distributor box above the cyclone feed pumpbox. The pumpbox has two distinct compartments, each equipped with its own 30 HP cyclone feed pump at its discharge. Normal operation requires the use of only one compartment at a time. Air cylinder valves on the distributor box are manually operated depending on which pumpbox compartment is being used. On changing compartments the distributor valves and cyclone feed pumps must be manually switched over, and the now unused pumpbox compartment must be pumped dry. Gland seal water to the pumps must also be switched over with their associated valves.
Barren solution is used as gland seal water for the two cyclone feed pumps. Each gland seal water line to the pumps is equipped with a sight flow glass; to ensure gland water is flowing to each pump, a dole valve; to control the flowrate of gland water to each pump; and a common pressure gauge; to ensure that sufficient gland seal water pressure is applied to the pump seals.
Although the cyclone feed pumps will self-regulate flow according to the level in the pumpbox, high and low level switches in each compartment will alarm at the control panel annunciator should a problem occur. The 10 HP 2″ grinding area sump pump and the crushing area sump pump discharge into the distribution box, along with barren solution. A control loop regulates the flow of barren solution to the cyclone feed pumpbox.
The discharge of each cyclone feed pump is piped separately to its own 15″ diameter cyclone, at a normal rate of 122 cubic meters per hour and 52% solids. Should both pumps shut down simultaneously, or the ball mill clutch be disengaged, the feed conveyor will be shut down. The cyclones separate the slurry particles by size; the coarse particles flow through the cyclone underflow to an underflow collection launder, and are then fed back to the ball mill for regrinding. Cyclone overflow (70% minus 200 mesh) is collected in an overflow collection launder, and flow by gravity to a 16 meter diameter thickener. The pH level in the overflow collection launder is monitored, and an indicating transmitter sends an alarm signal to the mill control panel annunciator if the pH level drops below 10.5 (basic).
In order for the cyclones to separate the particles by size effectively, the density of the cyclone feed slurry must be consistent, approximately 52% solids. This is controlled by measuring the slurry density in either cyclone feed line and controlling the amount of barren solution to the cyclone feed pumpbox. The density measurement is achieved using a nuclear source and detector/transmitter, saddle mounted to each pipe. Each transmitter feeds its signal to the controller, which decides which signal to use depending on which pump is running. The output of the controller modulates a butterfly control valve, in the barren solution line, adjusting the barren flow accordingly. The cyclone feed density is recorded on a recorder which also records the thickener underflow density. Adjustment of the cyclone apex, utilizing air operated pinch valves, will also shift the particle size separation point coarser or finer.
The 15 HP 2″ grinding area sump pump is designed to pump 35 cubic meters per hour, and in AUTO regulates its sump level with high and low float level switches. This pump can feed material to the cyclone feed pump-box or to the tailings box.
Two overhead cranes, a 2 tonne and a 15 tonne, are available for service and maintenance in the grinding area and the main operating levels in the rest of the mill.