Gold ore plant grinding media size
Several operations have experimented with progressively larger ball sizes in efforts to improve SAG throughput, particularly with hard, coarse ores. While the energy of individual impacts increases with larger balls, the number of impacts for a given load (or volume filling of ball charge) decreases. The optimum ball size for a given operation is a function of feed size, ore size, and competency, as well as mill operation conditions (speed, steel charge, and total mill volume). There is little consensus in this area. This may be a function of the difficulty in conducting and evaluating plant trials with ore of various feed characteristics (a relatively small performance benefit), or relate to the ore-specific nature of the optimum steel size.
Mill relines
No discussion of milling, and particularly SAG milling, would be complete without some mention of relining. Unlike a concentrator with multiple grinding lines, conducting SAG mill maintenance shuts down an entire concentrator, so there is rightly a tremendous focus on minimizing required maintenance time; reline time represents the majority of scheduled maintenance requirements.
Reline times are a function of the number of pieces to be changed, and the time required per piece. Advances in casting and development of progressively larger lining machines have allowed larger and larger liners. Like many operations, PTFI has devoted substantial resources to a liner development program to reduce the total number of liner pieces and fasteners.
SAG mill discharge classification
There are two primary mechanisms for classifying SAG mill discharge: screens and trommels. Due to a reduction in the required capital costs and space required for screens, trommels enjoyed a period of popularity, but the most favourable method of preparing SAG circuit oversize for pebble crushing is screening. All things being equal in terms of ore character and pulp density, screening produces a cleaner, drier oversize with much less fines carryover than does a trommel. This is particularly true for large SAG installations. As mill diameter increases, the volumetric throughput increases substantially faster than the effective screening area of a trommel. Trommel oversize like pebble-crusher feed has caused problems with pebble crushing at a number of operations. This is due to carryover of fines and moisture, which results in crusher chamber packing and ring bounce.
As most SAG circuits are now designed for the inclusion or subsequent addition of a pebble-crushing circuit, a design that incorporates screens allows maximum future flexibility. The method of returning the classified oversize to the mill requires mention. Some large SAG operations have been designed and built with water cannon return. With this system, the trommel oversize is returned directly to the mill with a water jet. Of course, the ability for these plants to retrofit pebble crushing into the circuits is significantly more complicated than if an external method of recycling oversize had been employed. It goes without saying that the oversize from the SAG mill classifier oversize must be returned to the mill via a belt system to use a pebble-crushing circuit. The use of screens is not without complications. Attaining even feed distribution to multiple screens can be challenging, the maintenance requirements of screens requires that stand-by units (either installed stand-by, for use in a rotating spare program, or both) be used.
