Modelling: Transfer size limits

The transfer size, T₈₀, is the characteristic size of the stream between the primary (SAG) and second (ball mill) stages of grinding. It is an important tuning factor for making the circuit work effectively.

How is the automatic T₈₀ calculated?

The software looks at the primary and secondary grinding stages and sees how much power is available in each stage. For example, the SAG mill may have 10 MW of power draw available and the two ball mills each of 7.5 MW of power draw available for a total of ratio of 10:15 (1:1.5). The software uses the particular power model (Bond/Barratt, Morrell Mi, etc.) to determine what is the transfer size where the ratio of E_SAG to E_ball matches the ratio of mill power (1:1.5 in this example). This is the 'maximum efficiency' transfer size where the mills will both operate at full power.

If this transfer size is outside the limits set for a particular flowsheet, then the limit is imposed and the circuit specific energy consumptions will recalculate based on that T₈₀ limit. A warning message is displayed to tell the model operator that a particular sample will be inefficient in a particular circuit.

Primary mill limited condition

The software generates a "Primary mill limited" warning when the ratio of primary:secondary power available has too much secondary power (or insufficient primary power). This means either your SAG mills are too small (or you don't have enough SAG mills) or your ball mills are too big (or you have too many of them) for a particular sample to grind efficiently.

The model responds to this situation using the operating strategy specified in the Circuit Settings:

• "vary P80" is for fixed-speed or variable-speed ball mills. The extra secondary stage power is consumed by over-grinding the ore.
• "vary speed" is only for variable-speed ball mills. The ball mill speed is reduced so that the secondary power draw is exactly what is necessary to achieve the target circuit P₈₀.

Secondary mill limited condition

The software generates a "Secondary mill limited" warning when the ratio of primary:secondary power available has too much primary power (or insufficient secondary power). This means either your SAG mills are too big (or you have too many SAG mills) or your ball mills are too small (or you have too few of them) for a particular sample to grind efficiently.

The model responds to this situation by reducing the circuit throughput to match the ball mill limit, and then reducing the total filling in the primary mill until the power draw matches the power draw dictated by the T₈₀ limitation. This is a "grind-out" situation in a SAG mill where the feed rate is insufficient to keep the SAG mill at the specified load level. The reduced total filling causes the power draw to drop until the necessary power draw is achieved.

The grind-out condition cannot decline below the ball charge level of the SAG mill; if this happens, another warning message is displayed saying the SAG mill speed must be reduced to achieve the target power draw. The model assumes all SAG mills are variable-speed; if a fixed-speed mill encounters this condition, it will completely grind out and operators will probably be forced to stop the SAG mill.

Pick your mills sizes to avoid secondary mill limited conditions.

This transfer size is NOT a prediction

This is the transfer size you must design the circuit to achieve in order for the circuit to be efficient! The actual factors that affect transfer size, such as grate dimensions, screen apertures and ore breakage rates, are not part of the power-model toolkit. Other models (such as population balance models) are required to figure out exactly how to achieve the required transfer sizes.

Transfer sizes are SYNTHETIC

Bond and Morrell type specific energy models are based on a synthetic "Bond-compatible" particle size distribution which is different from the actual measurements one will make in a SAG mill circuit. A phantom cyclone calculation is one way to convert a SAG survey particle size distribution and make it "Bond-compatible".

Typical transfer size ranges

Efficient grinding is achieved only when circuit transfer sizes are within a certain range. That efficient range is determined empirically and varies depending on the type of ore and the circuit flowsheet.

Some example transfer size ranges:

• Canadian Shield gold SAB operation, use 500 µm to 1500 µm (typical target 800 µm)
• Porphyry copper SABC-A operation, use 1000 µm to 3500 µm (typical target 2000 µm)
• Porphyry copper SABC-B use a range of 3000 µm to 7000 µm (greater pebble flow means you should use a coarser T₈₀).

Fine transfer sizes (E.g. 250 µm) might require the SAG mill to be in closed circuit with a hydrocyclone. The circuit diagram won't show this primary hydrocyclone, but the simulation expects it to be there and the transfer size is the hydrocyclone overflow that is passed to the ball mill.