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New mill boosts efficiency

Grange Resources owns and operates Australia’s largest integrated iron ore mining and pellet production facility and is the country’s oldest magnetite producer. Located 100km southwest of Burnie in north-western Tasmania, is the company’s Savage River magnetite iron ore mine and concentrator.

An overview of the from-mine-to-metal process at Savage river, starts with the crushed, stockpiled magnetite ore being transported via a tunnel system and fed into the concentrator. Here the ore is initially ground in two Hardinge 9.75m x 3.66m autogenous (AG) mills, followed by two Nordberg 8.84m x 3.96m 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 at Port Latta.

“From an operation perspective each of these factors have to be consistently met, with little margin for error,” said Grange Resources’ engineering superintendent Wayne Peck. “Starting from the pit, right through to the end of the ship loader, our customers’ requirements are our focus. Reliability of supply depends on the performance of our personnel and the processing equipment they use; while remaining price competitive depends on keeping our costs in check. Energy is our biggest cost, and an area where even small changes can have a big impact.”

The two Hardinge AG mills were originally installed in the late 1960s, but 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 there was always the risk of catastrophic failure. With the mine’s operation expected to extend till at least 2034, Grange’s management team decided that in order to ensure concentrator reliability and to improve output efficiency, these mills would need to be upgraded. In 2011, the company awarded Metso an $8 million contract to engineer, supply, install, and commission the first of two planned 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. This specific requirement introduced a great deal of complexity to the design. A high level of detailed analysis to assess the impact of increased stress due to the new larger rotating element was required. A key to the success of 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 had lasted more than 45 years, the new design had to be stronger to ensure trouble-free operation beyond the mines expected lifespan. So in the first instance, Metso’s new design replaced the single cone-shell with a two-piece structure that was 3.6 metres wide (from feed to discharge) and featured long-life polymet rubber liners. To provide sufficient space for these liners, the new mill had to be 10m in diameter, 30cm larger than before. The larger shell, together with thicker stronger steel construction, meant that the new mill was 20 per cent heavier than the old one.

In order to avoid the need for additional civil works and to keep costs down, the new mill had to be installed in the same position as the old one. To ensure that the existing structure could accommodate the extra weight and size of the new mill, Metso had to perform extensive FEA’s (finite element analysis) and carefully consider alternate designs.

Key to the successful design was the ability of the mill’s bearings to cope with the increased weight. An overview of the bearing components helped to explain how important this aspect was. Starting from the centre and working outwards, first was the trunnion, the shaft that extended out on both sides of the mill and which passes through its axis. The trunnion rested on a bronze bush that provided the bearing with its crucial lubrication. The bronze bush was supported inside the bearing housing by the rocker. As its name suggests, the rocker allowed the mill small lateral movements from side-to-side along its spin axis. Important to note was that the trunnion rolled in a 0.2-0.4mm film of oil that was pumped at high pressure through ports in the bronze bush.

Because 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 (pressure) increases. Then of course there was the additional weight that also had to be taken into account.

“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,” said Metso’s grinding product manager John Aran. “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.

“Just imagine pumping oil at 10,000 kPa at a rate of 15 litres per second to achieve a 0.4mm-thick film that needs to lift and support a rotating element 10 metres in diameter and weighing over 500 tonnes on each bearing. There are so many interdependent variables, providing a solution to this unique arrangement gave us all a great sense of achievement,” said Aran.

The base plates that the mill’s bearings sit on also presented an engineering challenge — they had a flatness tolerance of 0.13mm over the entire surface, with a 0.025mm tolerance over an area of 300mm x 300mm. The parallelism tolerance between the top and the underside surface was just 0.13mm. Such precise tolerances are even difficult to achieve in a controlled environment like a machining workshop.

“Because all the work would be carried out in the field rather than in a workshop, I was keen to see how closely the team would achieve these tolerances in an operational site environment where the base plates had to be positioned using site surveying equipment,” said Metso’s QA engineer Brian Bunch. “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.”

Dan Tonks, Grange Resources’ projects design engineer, was involved in the design aspects of the plant’s infrastructure upgrade, which had to facilitate and match the new mill’s design. “It might sound straight forward, but the infrastructure upgrade consisted of components sourced from different manufacturers located all over the world,” said Tonks. “Together with Metso, we invested in a lot of interactive and iterative planning to make sure everything matched up. An error in the most minor detail – for example bolt-hole sizes –  could be a disaster for the project. So there was a sense of pride, and relief too, when everything came together and matched up so perfectly.”

Work space during the dismantling of the old mill and the construction of the new mill was a challenge. “Originally the mill was built first and then the building was built around it,” said Frank Lovell, Grange Resources’ engineering manager. “So working on removing the old mill and installing the new mill within the existing building presented major restrictions. We had lifting weight and size limits, as well as difficult work access.

“The fact that the whole project was completed without a single safety incident is a credited to the way Metso’s design addressed these construction complexities.”

“Metso didn’t just supply the components and technical expertise for the installation, we were an integral part of Grange’s engineering team,” said Metso’s regional manager for mining services Steve Searle. “This approach makes such a big difference on a project like this. We worked through all of the issues together to ensure that the installation ran smoothly and on time. The installation process was developed with the Grange team, and we performed each operation as a unified team. The outcome is a new mill, which together with the new drive system, will provide the plant with 19 per cent more capacity.”

Peck also highlighted how the close inter-company team work maximised efficiency and cost reductions. “Working so closely together on the project meant that Metso could assist us to look at the design of the project as a whole, not just to consider the quickest or lowest cost solution, but to look at improving quality and throughput, reducing running costs, and overall long-term maintenance.”

“Efficiency is a big benefit that has been gained from this project,” said Tonks. “The mill is now grinding more, using the same power. Equally important for Grange, are the safety benefits that the new mill has provided. In particular, both our staff and contractors have praised the new mill’s better overall access and larger size work area.

“The guarding on the rotating elements is greatly improved and easier to handle. In my view, the project receives big ticks for safety, efficiency, and cost reduction.”

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