Copper Ore Reduction Processing
Caiman Crushers Machine
Figure 1: schematic of ROM bin, sublevel conveyor, and mineral sizer
The supply of crushed ore comes by truck transportation from a distant open cut mine site. Large hall packs transport the mined ore to a ROM bin situated close to the process plant. The ROM bin is effectively a large storage bin positioned in a raised location with an exit passage and chute at its ground level. Referring to figure 1, the crusher (3), also referred to as a mineral sizer, is fed from a sublevel conveyor (2) beginning at the base of the ROM bin (1).
The mechanics of a mineral sizer can be viewed as two parallel corkscrews rotating in opposite directions towards each other. These ‘corkscrews’ are driven by two 110kw three phase, squirrel cage, induction motors. Ore fed to the mineral sizer can only pass through to the conveyor when the particle size is small enough to fit through the pre-calibrated gap size. The sizer used for the kin severe project is rated to output crushed ore with a diameter no greater than 625mm.
CaimanTM Conveyors
Figure 2: site picture of a belt conveyor
There are two main sections of the conveyor system employed in the kin severe project. The first is a tie-in conveyor, intended to selectively transport crushed ore from the existing crusher to the new process plant. The second section of conveyor system is the mill feed conveyor which takes the course from new crusher to new ball mill.
The conveyor model employed in both situations is a belt conveyor: a long, continuous, rubber belt, looping the entire length of the conveyor, supported by rollers, with the ore sitting on top, and two drive motors positioned at each end of the conveyor to provide movement. Both conveyors are driven by three phase squirrel cage induction motors.
Ball mill
Briefly mentioned in the copper extraction overview was the ball mill’s electrical connection scheme. The key electrical components are:
1. A 3mva wound rotor induction motor 2. A liquid resistance starter (LRS). 3. a slip energy recovery (ser) unit
Wound rotor motor
The 3mva motor is the main drive of the mill during operation. It is an ac induction motor with a six pole stator and its rotor circuit brought out for resistance insertion. Connection to the stator is at 11kv and during normal operation the rated current is 160a. The principle of operation of this machine is the same as the squirrel cage motor and has been explained in depth in previous sections.
Liquid resistance starter
A simplified description of an LRS is a variable resistor placed in series with the rotor circuit. Initially the resistance is at its maximum and is gradually reduced as the motor starts up. Physically, the device is more like a battery i.e. Terminals (electrodes) submerged in battery solution (electrolyte). How the LRS differs from a battery is it possesses the mechanical means to raise and lower its electrodes in and out of the electrolyte.
In essence, it is the amount of contact between the electrode and electrolyte which determines the resistance. Practically, as the rotor is three phase, the LRS has three electrodes, with the resistance being seen between the phases. Starting position is when the electrodes are fully submerged in the electrolyte and at this point the resistance is at its greatest. As the motor runs up to full speed the LRS raises its terminals, lifting the electrodes out of the electrolyte, and reducing the resistance. When in its maximum vertical position the LRS is completely shorted and effectively removed from the rotor circuit.
A major advantage of the LRS over other methods such as a staged resistances technique is its controllability of the resistance value during operation. For a staged resistance implementation the steps which occur when resistance change is requested are fixed at the minimum resistance size installed which can be shorted from the circuit. With an LRS implementation, the resistance change is smooth and analogue, based on the height of the electrode removed from the electrolyte. The implementation design for height adjustment of the LRS electrodes is via a worm drive configuration, a squirrel cage motor, and a VVVFdrive controlling the speed of operation of the motor. In this method of height adjustment any desired resistance change characteristic during operation of the LRS can be programmed to occur.
Also, as the current is being controlled via resistance in the rotor circuit, a consequence of this method of control is the generation of heat in the resistors. For metallic resistance starters this heat would have to be absorbed by the resistors installed without causing damage, therefore high power ratings would be required and fast and efficient cooling methods included. However with the LRS, the mass of electrolyte can store this heat and dissipate it after the starting procedure via natural ventilation.
Slip energy recovery
A slip energy recovery unit is installed on the rotor circuit to control the speed of the mill between 80% and 110% of synchronous speed, while also utilising the rotor current by feeding this energy back into the grid.
Last update: February 21, 2012
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