The first consideration when discussing pebble crushing is why there is a need for the unit operation. Secondly, the configuration of the overall pebble circuit merits discussion. Pebble crushing, almost a standard for SAG circuits today, was controversial early in the development of autogenous grinding (AG) SAG milling. This was largely due to the fear of failing to efficiently separate grinding steel from the recycle load, with subsequent crusher damage. Magnet and metal detector manufacturers have minimized this difficulty, and today, more SAG circuits are constructed with pebble-crushing circuits than without. Making an efficient steel–magnetite separation, however, remains problematic for some producers.
The need for pebble crushing stems from two factors: a depression in SAG grinding rates at certain particle sizes, and the accumulation of a harder fraction in the mill load. These factors typically result in a mill throughput increase with the installation of pebble crushing that is larger than would be expected purely on the basis of the additional power. Overall, pebble crushing can increase throughput as well as decreasing the total power required to grind to a given size. Typically, pebble crushing also coarsens ball-mill circuit feed, a consideration if a ball-mill circuit is already taxed.
The definition of critically sized material is often misunderstood. Critical size particles are those where the product of the mill feed-size distribution and the mill breakage rates result in a build-up of a size range of material in the mill load; this critical size can be of any dimension. That said, the concept of critical size has become almost synonymous with pebble-crusher feed, and therefore it is typically referenced as the size range of 13–75 mm. Such a definition ignores larger critical-size material that cannot pass the mill grates– such a size often results with very hard ore types that have received insufficient breakage in blasting and primary crushing. Throughput with these ores can benefit from improved blast fragmentation, primary crushing, or SAG pre-crushing. Without such feed-size reduction, however, additional pebble crushing power may be of little benefit, because pebble generations can be quite low.
There are several critical design elements of a successful pebble-crushing circuit, including: material handling/diversion capabilities, metal removal, belt loading, pebble-crusher feeding, and return of crushed material to the circuit.
After classification, the SAG discharge oversize is conveyed to the pebble crushing circuit. The conveyors should have provision for returning the oversize to SAG feed (during pebble-crusher maintenance, for example, or during periods when metal is detected). Additionally, the ability to reject oversize material can also be useful. This is useful for diverting material after metal detects (discussed below), for sampling, or in cases where metallurgical work confirms grade depletion, and allows rejection of the stream to waste. A travelling chute (as opposed to flop gates) to separate the stream between pebble-crusher feed and return to SAG feed offers the greatest flexibility.
The design of an efficient metal-removal system is critical. The risk of inefficient metal removal from the pebble-crusher feed is obvious, and allowing excessive mill balls to the pebble crusher will rapidly damage both crusher manganese and other crusher components. The present industry standard for metal removal is the cross-belt magnet. In design of cross-belt systems, sufficient belt capacity should be designed so that belts can be run with lower volumetric loading. In other words, at a fixed belt size and loading, metal separation is better with relatively faster belt speeds versus relatively higher belt loadings. Designing to a Conveyor Equipment Manufacturers Association (CEMA) belt loadings of 65% or less has worked well at PTFI, with peak loadings of up to 85%. PTFI has used belt speeds up to 3.8 m/s (750 fpm) successfully. As an alternative to cross-belt magnets, manufacturers have recently fielded magnets fitted to remove tramp metal from directly screening oversize as the oversize material is loaded onto belts. After the magnets for steel removal, metal detectors should be installed to detect any metal that bypassed the magnet(s). Such detectors should be upstream of a diverter gate, so that a metal detect results in diversion of the material back to the SAG feed.
For the most efficient operation of a pebble-crusher, provision for a surge bin should be included. The use of a surge bin to allow full-choke feeding improves crusher performance and helps ensure that crusher components wear more evenly. Far steadier operation (in terms of maintaining high power draw without power spiking) can be maintained with a surge bin than without. Perhaps the ultimate ‘surge bin’ is a pebble stockpile with reclaim feeders. In addition to the advantages of surge bins, the use of a stockpile of sufficient capacity can allow the benefit of not having to recycle pebbles back to SAG feed during periods of crusher maintenance, and can allow mill throughput to be maintained at high levels even if the pebble-crushing circuit capacity cannot keep up. The pebble accumulation can then be worked through during periods of increased SAG capacity (due to softer, finer ore, or other reasons). Obviously, the pebble stockpile must stay in balance.
The last major decision for a pebble-crushing circuit is where to put the pebble-crusher product. Conventionally, crusher product was returned to SAG feed. Some designs, however, now allow pebble-crusher product to be returned to the SAG screens/discharge (allowing operation of the pebble crusher in closed circuit) or even to the ball-mill circuit. There is no ‘right’ answer for where the crusher product should be put 100% of the time. Sending the pebble-crusher product to the SAG discharge allows the material to be classified prior to going to the ball-mill circuit, and relieves the SAG mill of loading. Sending the pebble-crusher product back to the SAG circuit allows for attaining a finer SAG circuit product, and can relieve the ball-mill circuit. Sending the pebble-crusher product directly to the ball-mill circuit can reduce SAG discharge screening requirements, but if pebble-crusher product size is not well controlled, ball-mill scatting problems could result. Given sufficient screen capacity, perhaps the best combination is to allow for directing the crusher product either back to the SAG feed, or to the SAG discharge screens.
