Grange Resources has successfully reduced costs and boosted the efficiency of its Savage River iron ore operation in Tasmania through a mill upgrade project.
The miner, winner of the 2017 Vega Minerals Processing of the Year award, owns and operates Australia’s largest integrated iron ore mining and pellet production facility on the state’s west coast.
Savage River required a mill upgrade to improve the site’s operational performance and to ensure its future. Grange engaged Metso to work with the company on the project.
An overview of the “from mine to metal” process at Savage River starts with the crushed, stockpiled magnetite ore being transported via a tunnel and fed into the concentrator.
Here, the ore is initially ground in two Hardinge 9.75 x 3.66 metre autogenous (AG) mills, followed by two Nordberg 8.84 x 3.96 metre ball mills.
Magnetic separators then isolate the magnetite from the gangue (valueless adhering rock), with the fine-particle gangue being pumped to tailings dams. The rich iron concentrate slurry is pumped via an 83km pipeline to the pellet plant.
Grange ranks the reliability of product supply, quality, and volume, combined with aggressive pricing, as vital interdependent ingredients to successfully operate in the competitive iron ore market.
The two Hardinge AG mills were installed in the late 1960s, and by 2005 maintenance personnel noticed cracks starting to appear in their shells and cones. The mills required ongoing repairs, which resulted in plant down time and the risk of catastrophic failure.
With the mine’s operations expected to extend until at least 2034, Grange’s management decided that to ensure reliability and improve output efficiency, these mills would need to be upgraded.
In 2011, Metso was awarded an $8 million contract to engineer, supply, install, and commission the first of two new mills.
The project scope involved increasing production volume with a new rotating element that would accommodate a charge weight of 344 tonnes, but with the requirement to use the existing footprint and some of the components from the original 1966 mill.
A high level of detailed analysis was needed to assess the impact of increased stress due to the new larger rotating element thats was required.
Key to the project was the review of the bearing housing structure and ensuring adequate lubrication. The original mill was based on a single shell design, and although it lasted more than 45 years, the new design had to be stronger to ensure trouble-free operation beyond the mine’s expected lifespan of 2034.
In the first instance, Metso’s new design replaced the single cone-shell with a two-piece structure that was 3.6 metres wide (distance from feed to discharge) and featured long-life Polymet rubber liners.
To provide sufficient space for these liners, the new mill had to be 10 metres in diameter, 30cm larger than before. The larger shell, together with thicker stronger steel construction, meant the new mill was 20 per cent heavier than the old one.
The new mill had to be installed in the same position as the old one to avoid the need for additional civil works and to keep costs down.
To ensure the existing structure could accommodate the extra weight and size of the new mill, Metso performed extensive finite element analysis and carefully considered alternate designs. Key to the successful design of the new mill was the ability of the mill’s bearings to cope with the increased weight.
A simple overview of the bearing components helps to explain how important this aspect was.
Starting from the centre and working outwards, first is the trunnion, the shaft that extends out on both sides of the mill and which passes through its axis. The trunnion rests on a bronze bush, which is supported inside the bearing housing by the rocker.
As no civil work could be considered, stiffening the bearing could only be achieved by thickening the rocker and the brass bush.
The increase in load and reduction in bearing clearance meant that the force per unit of area increases. The additional weight also had to be considered.
Metso grinding product manager John Aran explained: “Making it strong enough was the easy part, next we had to design a lubrication system that would keep the trunnion suspended above the bronze bush with a 0.4mm thick oil film.
“There are no off-the-shelf designs for this, so designing an effective and reliable lubrication solution that included a custom-built cooling and filtering system involved input from Metso’s experts across the globe.”
The base plates that the mill’s bearings sit on also presented a challenge; they had a flatness tolerance of 0.13mm over the entire surface, with a 0.025mm tolerance over an area of 300 x 300mm. The parallelism tolerance between the top and the underside surface was just 0.13mm.
Metso QA engineer Brian Bunch commented: “The attention invested into these fine details by the project team resulted in an almost effortless installation. Once the sub-sole plates were aligned correctly, the base plates were lowered into position and there was no need for any shimming or additional ‘tweaking’ to attain the required flatness.”
This article also appears in the November edition of Australian Mining.