Benchmarking: Bond - Cadia East

Benchmarking: Bond/Barratt - Cadia

Engelhardt, D., Robertson, J., Lane, G., Powwel, M.S. and Griffin, P., Cadia Expansion - From open pit to block cave and beyond. Proceedings of MetSoc 2012.

Design criteria and plant trial of underground Cadia East ore

Ore characterization

• blased underground ore, secondary crushed to 80 mm.
  • Wi_C = 30 (metric)
  • Wi_RM = 26.5 (metric)
  • Wi_BM = 21.4 (metric)
  • DWI = 9.9
  • density = 2.7 t/m³

The ore was reported blasted undergrond (block caving had not yet started). Particle size distribution given as two points which confirm a Bond-compatible "root-2" PSD.

• 80% passing 80 mm
• 20% passing 5 mm

Modelling

Details of the mill operating conditions are not given, but the power draw at the shell is given.

• 15.737 MW SAG mill power draw
• 19.509 MW ball mill power draw

Circuit operating conditions:

• F₈₀ = 80 mm
• P₈₀ = 140 µm
• instantaneous throughput = 1482 t/h

The mill load, ball charge and pulp density was tweaked to mimic these power draws, corresponding to:

• SAG operating 9% v/v balls, 25.5% v/v total filling and 78% of critical speed (Austin model has fixed %solids)
• ball mills operating 37% v/v total filling and 70% w/w solids (fixed speed mills, 72% of critical speed)

The ball load required to match the reported power draw is very high, and could only be achieved with a significant reduction in the trunnion diameter (using ball retaining rings). Moreover, the motors are predicted to be operating at 100.5% of their rated output power to achieve the indicated power draw.

Model results

	ESAG	Eball	Etotal	Throughput
Model	12.2 kWh/t	15.1 kWh/t	27.7 kWh/t	1294 t/h
Measured	10.6 kWh/t	13.2 kWh/t	23.8 kWh/t	1482 t/h
Difference	1.6 kWh/t	1.9 kWh/t	3.9 kWh/t	-188 t/h
Difference	15%	14%	16%	-13%

The predicted transfer size needed to balance the power draw between primary & second mills is very coarse, 5 mm.

Discussion

There is a substantial difference between the survey and the predictions. Some observations:

• The rod mill work index was determined on an apparatus with smooth liners. This can cause a difference of 2-3 kWh/t versus the wave-liner apparatus specified by Bond. Using a rod mill work index of 24.0 kWh/t gives E_total = 26.8 kWh/t; still 13% high.
• The E_ssbm predicts the energy required for a "standard" secondary crushing & ball milling plant that is believed to be one of the most energy efficient types of grinding circuits. This ore has E_ssbm = 24.3 kWh/t which is still less than the survey E_total. Very odd result that a SABC circuit is more efficient than an SSBM (ergo, HPGR) circuit on such a hard ore.