Large scale equipment can reduce both capital and operating costs for concentrators. Large semiautogenous mills can be effectively utilized in big mills but they must be kept fully loaded to take advantage of their great potential. With varying hardness ores, controlling throughput is difficult and careful design must take this into consideration. A variable speed drive on large primary semi-autogenous mills can overcome this problem to a considerable degree.
A major disadvantage of large semiautogenous mills has been the necessity to keep the mills fully loaded at all times to minimize ball and liner breakage. As ore hardness and grindability change, feed tonnage has to be modified to compensate. This can result in over-grinding on some ores and lower flotation-recovery on other ores unless extra capacity is provided in downstream equipment. A variable speed primary mill provides a method of compensating for variations in ore hardness and gives a much more flexible grinding circuit.
Although there is no such circuit on hand today, we think it is within our grasp the technology is there to make big mills, to use sophisticated control schemes and to vary the speed of huge hp mills. The economics remain to be proved, but we feel that someday an economical 18,650 kW (25,000 hp) single mill grinding section will be a reality.
Variable Speed Drives
Variable speed drives are not new. Many have been used in the cement industry on mills drawing up to 6,525 kW (8,750 hp). There are 39 gearless drive installations worldwide, but only one in North America (St. Lawrence Cement). Recently both Afton and Lornex in British Columbia installed variable speed dc drives on wet grinding semiautogenous mills.
Five methods of providing variable speed drive capability on big grinding mills are as follows:
- Twin DC drives
- Gearless or ring motor drives
- Twin CCV or Cycloconverter
- Twin ICI or Load Commutated Inverters using variable frequency
- Quadramatic II LCI drive.
For power requirements up to approximately 4,476 kW (6,000 hp), it is convenient to drive the mill through a ring gear and single pinion arrangement. The pinion is driven by a low speed brushless synchronous motor driving through an air clutch.
This drive train has been well proven over many years of service as a fixed speed drive and offers major benefits over any other arrangement.
As mill size increased above approximately 4,476 kW (6,000 hp), the limits of gear technology were reached and it was no longer possible to drive through a single pinion. Therefore, two pinions were used, each transmitting half of the power to the mill.
In order for two synchronous motors to share load, it is necessary for two rotors to take up the same load angle. Any difference in mechanical position of the rotors will create a load imbalance between them. The difference can occur in two forms – statically, where the mechanically tied relationship between the rotors shows a continuous error in the average load and dynamically, where errors in gear accuracy and run out create load differences between the two motors during a revolution.
The combination of quadratorque control and pneumatic clutch position adjustment is called the Quadramatic drive.
The Quadramatic drive not only offers all the traditionally accepted benefits of the single pinion air clutch, low speed, synchronous motor drive but the “soft” characteristic of quadratorque reduces the stresses in the pinions and ring gear below the level found in the conventional single pinion drive arrangement.
The twin dc motor drive system with each motor connected to a pinion and each fed from a static thyristor power supply has been successfully installed and operated at the Lornex Mine in British Columbia on a 10.4 m (34 ft) mill rated at 9,325 kW (12,500 hp). Load sharing of two dc motors has been successfully achieved over a very wide range of application and presents no technical problem.
The gearless drive comprises a synchronous motor designed to operate at low variable frequency from a cycloconverter power supply (CCV). The motor rotor is mechanically attached directly to the rotating members of the mill itself, the stator superstructure is floor mounted, encompassing the rotor.
Absence of motor bearings, pinions, gears, clutches, etc., improves its reliability above other drives. Close coordination between the mill and motor manufacturers is essential for a satisfactory drive train to be achieved. Foundation and bearing systems must accept the additional magnetic forces originating from the motor, and specified by the motor manufacturer.
The motors utilize a smaller number of poles and are fed at a low frequency from two cycloconverters. Load sharing is as provided for two dc motors.
This drive combination of the fixed speed Quadramatic drive described earlier in this paper, used in conjuction with two load commutated inverter power supplies to achieve variable speed operation.
The drive enables the mill to run on the utility at fixed speed for optimum efficiency or in the event of malfunction of the LCI.
The gearless drive is probably the most reliable of all drive options mentioned herein. Elimination of gears and motor bearings remove these items from the list of potential problem areas. Furthermore, the carefully designed enclosure and ventilation system maintains a clean, controlled environment within the motor enclosure. Stresses in the motor components are considerably lower than in the other drives considered.
Built-in diagnostic equipment provides rapid trouble shooting of any malfunction in the equipment. Position sensors in the motor will shut the drive down in the event of the air gap reducing below a pre set limit. This ensures that motor pull-over and resultant core damage is prevented. A simple protective circuit is available to prevent a “frozen” mill charge from dropping during start-up. Torque and angle sensors feed an electronic evaluation unit and block the drive if the charge does not cascade at a defined angle.
Grinding Circuit Capital Costs
In order to make a good comparison of capital costs, it was decided to compare total installed costs from semiautogenous mill feed to grinding circuit product (flotation feed) for a typical 36,364 mtpd (40,000 stpd) semiautogenous ball mill circuit treating a copper porphyry ore.
Capital costs were developed for the above two fixed speed drives and for the five variable speed drives that have been described. They include costs of the entire grinding section including mills, drives, conveyors, screens, pumps, cyclones, piping, building, etc.
Basic design criteria used to develop the costs are tabulated in the next column:
Semiautogenous mills must be kept loaded to prevent damage to balls and liners. Hardness varies considerably at most porphyry copper mines and, when the ore gets soft, tonnage is quickly added to keep the mill loaded. This requires a good fast reacting control system and good design dictates that oversized equipment should be installed downstream so that the higher tonnages can be handled without sacrificing recovery.
It is not meant to infer that the above examples of savings will be realized at all operating properties. It is believed that variable speed drives will give some savings in all the areas considered. The savings shown are considered reasonable and attainable and could even be exceeded at some properties. Conversely, such savings would not be so easily realized at properties where ore was of a relatively uniform hardness.
But the savings are real and, if realized, could give significant annual savings as shown below.