May 2009

Getting it down to a fine grind

IsaMill™ offers OceanaGold flexibility and efficiency

By M. Eisner

The IsaMill (blue) is in the front, with the Jameson Cell (green) at the top. (Prominent Hill, Australia)

When OceanaGold expanded its gold operations in 2007 to include the Reefton deposit, it was confronted with an interesting challenge: how to process the mine’s refractory gold ore deposit. The company had been operating an open-cut mine at its Macraes site at east Otago in New Zealand since 1990, processing the ore on site. However, the situation was slightly different at the Reefton goldfield. Although ore from that mine is treated at an adjacent 1.0 Mtpa plant, the gold concentrate is sent 800 kilometres by rail to the Macraes pressure oxidation plant for final processing. This proved to be more of a challenge than initially expected, as regrind limitations at Macraes meant the mill could not handle all the ore from Reefton. Without adequate regrinding ahead of pressure oxidation, gold recoveries would be lower than forecast.

Finding the right solution

Processing at Macraes utilizes a grinding and flotation circuit employing SAG and ball mills, followed by sulphide flotation, fine grinding, a pressure oxidation autoclave and then a carbon-in-leach circuit for the gold extraction. While there was some capacity in the autoclave to treat Reefton product, the mill did not have the regrind capacity to handle the mine’s ore. That is when OceanaGold began to look for additional milling equipment to handle the Reefton product for final processing.

The company wanted a mill that could achieve a very fine grind size, was power and energy efficient, and had a low capital cost. Ideally, they also needed something that was simple to operate with a small layout, as space limitations restricted the use of a lot of ancillary equipment normally associated with other grinding methods. Using a traditional ball mill would have required a set of cyclones, pumps for large re-circulating loads and thickeners, which would not have fit at the site. That is when they began to consider the IsaMill™, marketed by Xstrata Technology, and developed with Netzsch.

If the mill fits

“The IsaMill has a lot of benefits, especially for flotation,” said Greg Rasmussen, principal metallurgist at Xstrata Technology, Canada. “The grind is finer and with the inert grinding, you can improve the kinetics in leaching and flotation circuits. Depending on the application, you can grind down to as small as seven microns with a float of 92 to 96 per cent recovery, but the action inside the mill produces a sharp discharge sizing curve, with minimal slimes, even at fine product sizes. As you put more power into it, the IsaMill™ particle size distribution curve steepens without over-grinding.”

The grinding action in the IsaMill™ is based on high intensity stirred milling, in which the shell remains stationary, while inside, discs rotate on a central shaft at speeds up to 20 metres per second. An internal classifier ensures only fines pass out of the mill while the oversize and media stay in the mill for further regrinding.

Compared with conventional grinding, the IsaMill™ reduces the energy usage, media cost and capital cost of fine grinding. Just as importantly, the processing advantages of inert grinding greatly improve metallurgical performance in leaching and flotation circuits, as compared with conventional steel media. Feed sizes on some applications are as coarse as F80 of 300 µm, while product sizes can be as low as 7 µm (P80) for materials including lead and zinc sulphides, copper sulphides, nickel concentrates, platinum concentrates, industrial minerals, iron oxide and refractory gold concentrate. Sequential grinding in the IsaMill™ does not flatten the product size distribution like other grinding methods. Instead, it sharpens it, even in an open circuit, which can have important implications for energy efficiency and downstream processing such as filtering and pumping.

The combination of tight size distribution, small footprint and inert media has profound implications for circuit design. Mills can be easily distributed throughout a flotation circuit, grinding only those particles that need it, and producing a tight size distribution (without cyclones), ideal for subsequent flotation. The use of inert media means that less reagents are required downstream of the mill, with the flotation chemistry better to control.

Similarly the tight size distribution is crucial in leaching applications, where recovery is usually determined by the coarser end of the size distribution (best measured by P98 or P95, not P80).

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