:: Global Technology from Outokumpu gives Oxiana Leading-edge Grinding Mill for Sepon

The latest Outokumpu grinding mill just commissioned at Oxiana’s Sepon copper project in Laos is a technological tour de force.

Global Outokumpu Group contributions - and a high level of engineering within Australia - have produced a design uniquely suitable for extreme environments, incorporating metals technology common to nuclear power reactors.

This technology welds a protective layer of stainless steel onto the inside of the carbon steel substrate of the 9m long grinding mill. The new mill, with a 5.2m inside diameter, was successfully commissioned in February 2005. Sepon has proven resources of 3.6 million oz of gold and more than a million tonnes of copper so far and resources are currently being expanded.

Outokumpu Technology’s Grinding Manager, Oskar Gustavson, says combining the protective stainless steel with a more durable carbon steel structure will offer very reliable service against the high-acid environment at Sepon.

"Oxiana faced an interesting technological challenge in providing a copper mill which needed to operate in a very aggressive processing environment. The pulp in the mill contains large quantities of sulfuric acid with a pH level less than 1.0"

"They could have put in multiple small vertical mills and scrubbers, but the number of machines required would have been uneconomic and produced a complicated plant. Engineering multiple machines into a circuit like this is not a preferred option. It’s best to keep it simple, for reliability and efficiency" he said.

Mr Gustavson project-managed the international engineering of the single stage high-efficiency pebble mill. It uses flint rock media (as opposed to normal steel balls), against which the mill’s interior is protected by a 75 mm rubber lining over a metal sandwich of 6 mm stainless steel, welded to 60-100 mm carbon steel plate.

Capitalising on the Outokumpu Group’s wide expertise, its Outokumpu Stainless (formerly Avesta Polarit) operations were also used in the project. Engineers from Outokumpu Stainless, one of the largest producers of stainless steel, contributed expertise in welding the metals involved, having specialist knowledge of metal types and heat treatments to produce the ideal result.

"They had extensive experience in stainless projects, not only involving copper and gold, but also the demanding nuclear environment, where stainless steel and titanium are commonly welded into carbon steel vessels", said Mr Gustavson.

Work on the new mill was very much based on a partnership approach with Oxiana joint venture Khanong Development Group, which comprises Bateman Engineering and Ausenco Limited. The success of this unique partnership resulted from the mutual confidence built up over several years during previous mill installations, said Mr Gustavson.

Outokumpu Technology engineered a shell-supported fabricated mill design, rather than a more conventional mill using cast heads and trunnions. This design ideally accommodated the welded sandwich components required for the site.

Evolved in conjunction with Oxiana, the $A5 million mill incorporates a stainless feed chute with ceramic interior lining and a stainless steel trommel.

More than half the mill components were fabricated in Australia. To simplify future spare part requirements on-site, the mill was designed with the same drives as an earlier Outokumpu mill delivered to the Sepon gold plant.

"Experience on-site has been a major reason why the project has proceeded so smoothly, even with the technical challenges," says Mr Gustavson. "Oxiana Project Manager Phil Dunstan’s team have built up an excellent rapport with our Australian and international staff. Under the direction of Outokumpu’s Installation Manager Peter Nilsson, we now have supplied Sepon with five mills, three for the gold plant and two for the copper" said Mr Gustavson, who is based in Australia but responsible internationally for Outokumpu grinding technology.