When Australian Stainless Steel Development Association Accredited Fabricator Nepean Engineering was awarded the tender for the manufacture of the stirring mechanisms for 10 thickeners for the Goro nickel mine in New Caledonia, they had no idea of the enormity of the venture.
But having now completed the two-year undertaking they reflect on what has been their big stainless steel project to date.
Although the nickel mine was a massive development, Nepean Group admits the initial stages of commencement were a little stop-start.
Nepean Group won the tender 2 years before from GLV Australia (Dorr-Oliver Elmco) but the project was cancelled. It then had to re-tender and were lucky enough to win it again.
Manufacturing finally commenced in August 2005 and was completed in March 2007.
According to Nepean Group, the project resulted in 410 tonne of stainless steel product, including an additional 370 tonne of carbon steel.
The contract export value was $10 million. Varying grades were used including 338 tonnes of 316L, 65tonne of 904L and 7 tonne of AL6XN. The thicknesses ranged from 1.6mm up to 80mm.
Six of the thickeners were 70 metres in diameter and required the manufacture of 33 metre long raking arms.
Because a highly corrosion resistant material was needed where the nickel extraction occurred, Nepean Engineering used 904L for its high nickel and chromium content.
The thick sections required meant that 316L and 904L were used to avoid sensitisation and the subsequent risk of intergranular attack.
Super-austenitic grade AL6XN with 6% molybdenum and high nitrogen, offered better corrosion resistance and was used in one of the smaller thickeners, which extracts cobalt. This material was imported from America.
When manufacturing commenced Nepean Engineering experienced quite a few challenges as a large amount of material was non-standard size.
316L angle was unavailable so all angles were pressed from flat plate.
Pressing was performed across the grain. This required joining 2 x 8 metre sheets using sub arc welding so that the longer angles could run across the sheet.
The sheets were then cut to fit the plasma cutter, which could handle 6 x 17 metres. Some of the angles were formed in Nepean’s 1000 tonne press and others were subcontracted for specialist pressing.