Excerpt from the article ‘Mining to Energy Efficiency and Beyond’ by Carly Leonida, in the June edition of Engineering and Mining Journal. Reprinted with permission.
According to the report from Weir Minerals, comminution in mining consumes up to 1% of total final energy consumption globally — equivalent to the power consumed by 221 million typical U.K. homes. This makes comminution circuits, both new and existing, a priority area for optimization, as small improvements can lead to large energy savings.
Grant Ballantyne, global director for technical solutions at Ausenco, and Malcolm Powell, emeritus professor, University of Queensland, joined E&MJ to discuss potential operational improvements.
“There are three main things that we can do to improve energy efficiency in existing milling circuits,” Ballantyne explained. “We can reduce the mass, reduce the intensity or reduce the production of fines. In terms of reducing the mass, progressive recovery strategies such as preconcentration and early rejection of waste can reduce the amount, and increase the grade of material fed to the comminution plant.
“To reduce the intensity, we can recover the metals at a coarser grind size using coarse particle flotation (CPF) or another alternative separation technique. Reducing the fines can be achieved via improved external classification using screens and cyclones, as well as improved internal classification within the comminution unit, which some people refer to as the ‘selection function.’ Reducing the mass, intensity and fines can reduce energy consumption in milling by 50%.”
Ausenco has performed a number of studies recently on preconcentration, specifically bulk ore sorting, as well as coarse particle recovery (CPR). The company designed and commissioned the world’s first commercial CPR plant in base metals at Newcrest’s Cadia mine in Australia, and Ballantyne said these techniques/technologies are now routinely included in the firm’s studies.
Powell weighed in: “It’s also possible to use improved predictive control to stabilize the load on comminution equipment, the most critical generally being the filling of semi-autogenous grinding (SAG) mills, as this dictates downstream stability of product size distribution and throughput,” he said. “The predictive control should be based on mechanistic/phenomenological models as opposed to black-box fitting of responses (the standard basis of ‘model-based’ control).”
Improving measurement of the work balance across the circuit to distribute breakage more evenly across the crushing and grinding stages can also help prevent one section becoming overloaded while another is underutilized — a common issue in SAG-ball-crusher (SABC) circuits.
Powell explained: “The work balance is based on keeping each stage of comminution in the best envelope of operation e.g., avoiding feeding oversized material to ball mills, and recycling crusher top-size to suit the desired crushed product size. This, in turn, is set at the ideal recycle feed-size range of -10 mm, which is finer than applied on most sites.”
He added that the efficiency of cyclone classification is often overlooked, and these units are routinely operated outside of their natural range i.e., at high density to coarsen up the final product size. This leads to high recycling of final product sized material and sliming of the flotation/leach product.
“This wastes a considerable amount of grinding energy, with case studies showing improved throughput for the same energy input of 15%-20% in poor operations,” he said. “Reducing the hidden embodied energy cost of steel grinding media and maximizing the use of the rocks in the feed as grinding media rather than crushing it all to below media size is another strategy worthy of attention.”
The benefits of these types of improvement programs can be significant, but instigating them can be dependent upon the operator’s mindset and business structures. Powell said inadequate education in the principles of comminution equipment, classification systems and control mean that process operating staff often fall back on tried and tested methods rather than innovating.
“I see education as the key,” he said. “This can be applied through technical courses at a range of levels, not just academic, that provide industry-certified levels of skill.”
Ballantyne agreed but was sympathetic to mining companies’ attitudes toward risk. “There’s so much risk in mining in general, primarily related to the orebody,” he said. “It’s understandable that these companies can be reticent to take on additional risk associated with technology.
“As engineers, when we address energy efficiency in mining, we need to keep in mind the capital cost challenge and the possibility of subtraction rather than addition. Do we really need two cyclone feed pumps or could we manage with a single duty pump by concentrating on preventative maintenance? There’s no point designing an energy-efficient circuit that is so costly that it never gets built or results in a low economic return.”
Read more of Grant and Malcolm’s interview on designing energy-efficient circuits and innovation in comminution technologies.
Questions? Contact Grant Ballantyne.