Grinding mills perform a critical role in the productivity and efficiency of ore processing circuits. Effective and efficient comminution in AG (autogenous), SAG (semi-autogenous) or ball mills means higher volumes through the mill and a finer product, leading to improved recovery.
A poorly designed mill liner, however, can reduce the efficiency of the entire process, particularly if the liner wears quickly or unevenly. With price tags up to $2 million per liner, together with the downtime required to install a new liner, frequent liner replacement is not a fiscally responsible option.
“The importance of a mill liner is often overlooked,” said Malcolm Powell, a professorial research fellow at the University of Queensland, who has conducted applied surveying and site optimization studies on over 60 mineral processing plants worldwide and published more than 170 articles on sustainable comminution. “It is only seen as a wear component. You need to look at it as a wearing component that affects productivity. Do your homework and invest in the best solution available. You can’t outsource productivity.”
Powell and Lawrence Nordell, president of Conveyor Dynamics, Inc. (CDI), joined forces in late 2017 to form Comminution & Transportation Technologies Inc. (CTTI) to help mining companies optimize the efficiency of their mills and to design custom solutions to maximize mill performance. Although the partnership is new, Nordell and Powell have spent decades studying, modelling, and optimizing how to grind rocks effectively. Both are frustrated by the incremental changes the industry has made over the last 30 years and the surprisingly low uptake of redesigning processes despite the advanced computational techniques available.
Back to basics
The liner inside of a mill housing serves two purposes. The first is to protect the drum so the mill does not wear out, and the second is to transfer energy from the rotary mill into the grinding medium, like the ore itself in an AG or SAG mill or the grinding balls in a ball mill. Typically, a new mill comes from the supplier complete with all the components, including the liner. Mine managers, said Powell, only start to think about the suitability of the liner and other consumable components after they start to wear and need to be replaced, and often forget the secondary purpose of the liner.
Related: Malcolm Powell on establishing a single comminution model
“The two actually go hand-in-hand,” said Powell. “If I optimize the way I transfer energy using that liner into the grinding media, I reduce the stress on the liner and reduce the wear of both the liners and ball grinding media. If I do something that is the most efficient at pumping the energy into the grinding media, it actually reduces the stress on the liner. It’s a double win. It’s always assumed it’s either liner life or performance. No. They go together.”
The Discrete Element Method
Nordell and Powell use the Discrete Element Method (DEM) to generate detailed simulations of the wear on the liners and the interactions between the steel balls and the rocks being crushed. Nordell first applied DEM modelling to mill liner wear on two AG mills at the Palabora copper mine in South Africa’s Limpopo Province in the mid-1990s. Initially, he had used DEM to design a solution to prevent excess belt damage on a conveyor chute at the site.
“The chute project showed an increase in belt life by a factor of better than 10 times the two- to three-year wear life previously experienced,” said Nordell. Impressed with this result, Nordell’s client at Palabora asked if the DEM concept might be applied to the AG mills and CDI developed a comminution team to study the mills.
“DEM showed most mill designs were misunderstood on how comminution worked within the mill. CDI also developed a new concept of wear mechanics using the same DEM modelling tools,” said Nordell. After successfully applying the method to the AG mills at Palabora, CDI further developed the mathematical modelling tools and created the Rocky software package of DEM codes, which Nordell later sold to global industrial equipment company, Metso.
In 2000, CDI was awarded the contract to evaluate and improve the liners in the 40-foot diameter SAG mill – the biggest mill in the world at the time – at the Cadia gold and copper mine in New South Wales, Australia. Nordell’s DEM simulations showed that the shape of the traditional trapezoidal, angular lifters on the liner – designed to scoop the rocks and balls from the base of the mill, carry them up and drop them at the top of the cycle – created high stress points on the liner and wasted energy grinding the liner instead of grinding the rock. He designed a liner with lifters that were curved on one side, similar in shape to a shark fin, that transferred more energy to the rocks and balls and prolonged the life of the liner. The through-life stress caused by the mounting bolts was also modelled and modified to prevent liner cracking.
Related: a new generation of high-pressure grinding rolls are working their way into the comminution circuit
It took a year to design, refine and install the new “Larry Liners” and new bolts to attach the liners to the mill. At first, a trial set of 12 liner rows were installed to replace existing worn liners at a scheduled change-out. After a few months, Cadia was convinced to replace all the liners in the mill. The custom liners Nordell designed remained in use for 15 years. “When you get down to looking at the actual science, you need a tool such as DEM modelling to assess the interactions between the rocks and the distribution of the particles from the original rock,” said Nordell, who has now used DEM to evaluate the performance of about 30 mills around the world.
Powell presented one of his more recent case studies at the 14th AusIMM Mill Operators’ Conference in August. At the Pustynnoye gold mine in Kazakhstan, Powell and a team of liner specialists, site personnel and liner suppliers worked together to increase mill liner life by around 50 per cent and mill throughput by more than 20 per cent. The team used a design approach to liner optimization that follows a logical path of initial design and step-wise improvements based on liner wear monitoring.
A unified solution
Powell and Nordell are currently working with five mining clients to find the best mill liner and circuit solutions to suit the unique conditions at each site, rather than rushing to buy an off-the-shelf liner design. New mills, they say, often have inappropriate liners installed in them, and companies undergo many years of incremental improvement to find an optimum liner solution. Additionally, the significant energy draw at the conical ends of the mills are not appreciated, so their potential contribution to improved energy utilization is overlooked.
Comminution is the most energy intensive process in almost all mines, using on average 36 per cent of the overall energy consumed at a mine. If a liner costs between $1 million and $2 million and lasts only three to six months, investing in a custom solution is both energy and financially efficient. Powell’s message to mine managers is this: “Invest wisely once and don’t spend 10 years frittering money away unnecessarily. Do your homework and find the best solution available. It will pay you back in just a year.”
Nordell and Powell assess the mill’s current performance requirements and what the company wants the mill to achieve and use historic production data to set a baseline for performance. Then they use a laser scanner, like those used by surveyors, to survey the mill in 3D when the liner is installed, and another two to three times during its life, to build up a progressive wear profile.
Nordell and Powell then work with the mining company’s preferred liner supplier to engineer a solution to make the liner last longer and transfer more energy through the liner to increase performance. “Mining companies need to invest in the best possible liners for their mill,” said Powell. “We take a systems approach and ask the mining company to invest in their own destiny.”
