Next figure shows the simplest case in closed grinding circuit. There are two parameters to control, ore feed rate and water flow. It is convenient and productive to fix the amount of water to be added as a constant value so that the ore feed rate can be the only variable to control. Normally, operators are used to vary the water rather than the ore feed rate which produces considerable uncertainty as to regularity of tonnage milled and in flotation and/or cyanidation, considerable variation in reagent additions. In this way is a good criterion to talk to operators about the problems generated if the water flow is manipulated constantly. It the adjustment is necessary, it should be changed so that this variation does not affect in the least the total quantity of water in the circuit.
Closed Grinding Circuit
For example, consider a gold flotation operation whose daily tonnage is 1000 t/d and the hydrocyclone overflow dilution must be 1.86 parts of water to 1 part gold ore. Valve 1 is opened until the water flow meter register a flow of water of 77.4 cubic meters per hour. The grinding operator is not authorized to alter this setting. It is clear that all this water has to get a balance in the hydrocyclone and if the operator adjust the ore feed rate until the hydrocyclone overflow reaches a value of 35% solids, the tonnage must be 41.6 tph. Obviously, some allowance must be considered in the water entry to compensate for moisture in the ore.
In real operations, the operator has to take several frequent percent solids tests in the overflow since the operation start and during the operation in order to assure stability. Slurry samples can be taken at 30 or 60 minutes intervals. Nowadays, conveyor belts are equipped with variable speed motors and the right tonnage can be obtained by changing the velocity of the conveyor belt. Then, it will be important to check the weight reported by the scale by taking samples in the same conveyor belt.
According to the figure, the grinding operator may adjust Valve 2 at times to provide the best percent solids distribution in the grinding circuit. The manipulation of Valve 2 does not affect the water added in the circuit. The drawing includes a flow meter which is useful, but not totally necessary if the water flow can be measure by other device. If the supply tank remains at constant head, the water could be employed directly from this if there is a protection screen in the piping system. The water valve to the grinding circuit may be adjusted by weighing and checking the ore feed rate and adjusting the water addition until get the right percent solid in the overflow.