These same principles apply to lower speed (80% C.S.) low aspect mills. However, for higher speed mills in this category (most of which run at 91% C.S.) grid liners are used with hardly any lift except a rough surface of pebbles that are packed into steel grids. Grinding by abrasion against this surface at near centrifugal mill speeds creates a wholly different dispersive action within the mill which allows the flow of pulp with high circulating loads through a discharge screen (grate) or, alternatively, a peripheral discharge.

**Feed Chute Design**

Although details of the various types of feed chute designs are outside the scope of this paper, it is important to mention that maximum feed size (e.g., run-of-mine feed without primary crushing) and rock trajectories (with or without rock boxes) impact on the size of these feed chutes and also on the feed bearing trunnion liner diameter, and mill diameter: length ratio for optimum hydraulic gradient, and mill diameter.

**Diameter: Length Ratio**

The iterative process of sizing primary mills implies testing the sensitivity of mill length (EGL), motor rated power output, torque output, and rated speed (expressed as mill speed), and shell liner/lifter wear so that the expected range of operating conditions can be accommodated once the mill diameter has been set In this respect, the ratio of mill diameter to mill length (EGL) is important as it affects selection of the rated speed.

In general for high aspect mills, higher ratios of D:L permit operation at higher mill speeds for a given rated power output Lower ratios of D:L are commonly rated at 72% C.S. or 74% C.S. but, as has been seen in the above example (D:L = 1.88), there is an advantage to be gained in raising the rated power output and increasing the rated speed to 76% C.S. Both MacPherson and Turner predicted optimum ratios of 3:1 for D:L, whether for wet or dry operation. At the time (1964), they were dealing principally with single-stage mills processing iron ore to final product. For two-stage circuits, D:L has been progressively moving toward 2:1 or less as perceived and time-dependent limitations on mill diameter have been exceeded. The D:L ratio for 32 ft dia. mills decreased from 3.00:1 to 2.17:1 over 35 years until the advent of 36 ft dia. mills in 1973. The D:L ratio for 36 ft dia. mills decreased from 2.53:1 to 2.06:1 over 23 years until the first 38 ft dia. mill was purchased in 1996. The 38 ft dia. mills are 1.87:1, with special cases at Freeport (2.08:1) and Olympic dam (1.60:1), and the Cadia mill (40 ft dia.) is 1.97:1 and rated at 74% C.S. and 20000 kW (Jones 2001).

For low aspect mills, in which D:L can vary from 1:1 to 1:2, the optimum ratio is governed by:

· The selection of a single-stage or two-stage circuit

· Mill speed, £ 80% C.S., or 91% C.S.

· Liner/lifter design, Hi-Hi or grid

· Pebble porting, survival rate of pebbles, breakage rates

· Slurry flowrate, including circulating load

· Discharge arrangement, grate or peripheral, open area, and effective hydraulic gradient.

For single-stage mills at lower mill speeds ( <, 80% C.S.) with conventional shell liner/lifters, D:L ranges from 1:1.5 to 1:2; at the higher mill speed (91% C.S.) with grid shell liners, D:L is generally 1:2. Lower speed primary mills are found to be within 1:1 to 1:1.1.

**SECONDARY MILL SIZING AND ASPECT
RATIO**

The standard method for sizing secondary ball mills (usually the overflow type) in AG/SAG circuits follows published mill supplier information with aspect ratios in the range 1:1.5 to 1:2.0, depending upon product sizing and pulp flow; i.e., the highest expected circulating loads (Morrell 2001). The applied net power, calculated from testwork and power-based modelling, will have been based on mill speed and shell liner/lifter simulations and, particularly, ball charge volume and ball top size, which is dependent upon the contingency applied to the transfer size in the SAG circuit product Iterations of these parameters can be performed in some models (e.g., "GRINDPO WER") to determine the desired motor power and drive rating, the maximum design mill speed, and the maximum design ball charge volume (Barratt 1989).

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