Case Studies

Horizontal grinding mills have been used in the mining industry for more than a century, and due to their operational and maintenance simplicity they remain a widely used technology for various stages of comminution. The relative ‘efficiency’ of horizontal grinding mills compared to other comminution equipment is often debated, and efforts to improve the energy efficiency of these mills is an on-going area of attention for process engineers. The discharge arrangement design affects the material transport characteristics within a horizontal grinding mill. Material transport plays a significant role on efficiency of horizontal grinding mills. The magnitude of benefits that can be achieved with efficient material transport in AG/SAG mills has been demonstrated in a Greenfield milling operation at Mirabela Mineração’s SantaRita Nickel operation in Brazil. This grinding circuit was commissioned in October 2009, which was designed as a typical SABC circuit to grind 4.6 mtpa (575tph) to produce a P80 of 125um with 10-12% ball charge in SAG mill. The nameplate design capacity has been achieved in few days in ABC circuit without grinding media in SAG mill, and has been operating in ABC mode. The through has gradually increased to an average of 800 tph and Recently 7% ball charge has been introduced to increase the throughput to +900tph. This paper investigates improved material transport using Outotec’s TPL™ (Turbo Pulp Lifter) technology and the opportunities this technology presents when designing horizontal grinding mills. Operational data, with particular reference to energy and throughput optimization, is presented.

Deteriorating ore grade and high demand of metals has tremendously influenced the unit capacity of SAG mills. Pebble circuits have been added to the existing SAG circuits to increase their unit capacity. To make the pebble circuits justifiable, the grate openings are increased almost to 4.5 inches to draw large quantity of pebbles. The inherent transportation problems associated with SAG mills in closed circuit with pebble fraction are more complicated than the single stage SAG mill operation, because the coarse pebbles behave significantly different to that of slurry in pulp lifters. In addition, operation of mills at higher speed, to take advantage of precise design of high release angled shell lifters significantly reduces the discharge efficiency of the conventional pulp dischargers. The presence of a sizable quantity of pebbles in the pulp lifter limits the space available and reduces the flow gradient through the grate thus increases the load inside the mill and hence the mill power-draw. This paper analyses all the problems associated in discharge of slurry and pebbles in SAG mills with pebble circuits. The Outokumpu patented Turbo Pulp Lifter (TPLTM) aims at eliminating the inherent material transport problems to ensure good grinding conditions. Recent installation of TPLTM in a 26-ft SAG mill has proven these claims by reducing the specific power consumption while increasing the mill capacity.

The Cortez SAG mill liner design was recently modified to improve the mill operating energy efficiency. In addition to the expected benefits, the design change unexpectedly played a significant role of verifying the proposed grinding circuit expansion for the Cortez Hills project as well as illustrating current SAG mill bottlenecks. This paper will discuss the evaluation that addressed operator and maintenance observations, historical PLC data, and “crash stop” observations to calculate charge motion, packing between lifters, and slurry transport through grate and pulp lifters. The performance of the new shell and discharge end liner design will also be discussed as well as their role in the possible grinding circuit expansion.

The comminution process indicates that the most energy-efficient comminution system would be one where the particles leave the breakage field as soon as they reach product size. Contrary to the free falling gravitational (vertical) flow in crushers, the grinding mill’s energy efficiency essentially depends on ore characteristics and the discharge rate of broken particles, which in turn depends on how efficiently the discharge pump (grate and pulp lifters) operates. The essential function of a pulp lifter is to transport the broken material and slurry from the discharge grate out of the mill. Hence, the design optimisation of pulp lifters affects not only the energy efficiency and throughput of autogenous grinding/semi-autogenous (AG/SAG) grinding mills, but also contributes other process benefits such as improved wear life and operator-friendly steady and smooth mill operation. Following its introduction in 2006, Outotec’s patented Turbo Pulp Lifter (TPL™) design has been retrofitted at existing sites and also installed in some new ones around the world. This paper will provide operational experience and data on how the TPL™ design has allowed AG and SAG mills to operate efficiently.

Alcoa operates 9 sag mills at two alumina refineries in Western Australia. All of these mills run as single-stage units in closed circuit with DSM screens and provide a product that is sent directly to the digestion stage of the Bayer process. Historically Alcoa has run its mills at maximum throughput as dictated by the maximum load level that could be achieved without the mills spilling from the feed end trunnion. At maximum throughput mill performance was typified by a large mill inventory of slurry, which caused the formation of a large slurry pool and a depressed power draw. Research at the JKMRC and operational experience at one of the refineries showed that the mills were severely limited in their pulp lifter capacity and that increasing this capacity would improve mill performance. Installing a new concept of pulp lifter, which came out of the JKMRC research program, subsequently increased pulp lifter capacity. This resulted in large gains in throughput and power utilization efficiency. This paper documents the experiences at Alcoa from recognition of the initial problem, the research carried out at the JKMRC, the engineering difficulties that had to be overcome in designing the new concept pulp lifter and the gains that were subsequently made in milling efficiency.

Sandfire Resources NL commenced commissioning of its 1.5Mtpa DeGrussa copper processing plant in September 2012. The processing plant includes primary crushing, primary closed circuit milling, secondary closed circuit milling, conventional three stage flotation including concentrate regrind and tailings disposal. The grinding circuit has a 7.30m x 3.35m variable speed Outotec SAG mill with an installed power of 3.4MW. This mill was installed with Turbo Pulp Lifter (TPL) technology and was designed to operate in closed circuit with 500mm cyclones to deliver a transfer size of 180µm to the next stage of grinding. A 4.7m x 7.5m Outotec grate discharge ball mill with an installed power of 2.6MW and Turbo Pulp Lifters is operated in closed circuit with 250mm cyclones to provide a 45m product to flotation. The primary mill was initially commissioned as an Autogenous Grinding (AG) mill. During this period the milling circuit operated at lower than design throughput, whilst the AG mill operated with excessive pebble production rates and a finer transfer size to the ball mill circuit (60-80µm). Production requirements inevitably led to the addition of 8% graded ball charge with a top size of 100mm. The change improved throughput to design expectations (187tph). The addition of the ball charge increased the transfer size to 80-100µm, however the pebble generation rate remained unchanged. With the continued addition of steel to the mill (12%) the SAG mill transfer size remained finer than design expectations resulting in a very fine flotation feed particle size distribution. The over ground sulfides in the flotation feed led to issues maintaining selectivity during flotation, and hence the grinding circuit was operated without the ball mill circuit for a period until other process changes were made to improve transfer size. This paper discusses commissioning issues in the grinding circuit during the early stages of processing and highlights remedial actions undertaken to achieve key design parameters. It also discusses the outcomes of grinding surveys and the subsequent modelling work completed on the data collected.

Sandfire Resources’ (Sandfire) DeGrussa copper comminution circuit has been designed to produce, by today’s standards, a very fine primary grind size of 45 microns. The grinding circuit is comprised of two stages of milling – primary and secondary, to process 1.5Mt per annum (187tph) of primary sulphide ore. The primary grinding circuit consists of a 7.30m x 3.35m variable speed SAG mill in closed circuit with 500mm cyclones designed to deliver a 180m transfer size, the secondary grinding circuit consists of a 4.7m x 7.5m ball mill in closed circuit with 250mm cyclones to provide a 45m product to flotation. Initially the primary mill was commissioned as an autogenous grinding mill (AG mill) at lower than designed throughput (~150tph) and to achieve the design capacity of 187tph steel grinding media was added. Even with the steel addition, high pebble generation with finer transfer size (<100µm) and excessively fine flotation feed have remained an issue. The over-ground sulfides in the flotation feed led to issues in maintaining mineral selectivity and optimised copper recovery. Several process and operational changes were carried out to reduce the circulation of critical size material and generation of ultra-fines, however stable throughput rates could not be maintained at the design rate of 187tph. Besides the process issues, the variable speed drive (Slip Energy Recovery - SER) limited the available power to the system to ensure the driveline was not over loaded, and the mill vibration at higher mill speeds limited the upper operating range to 74% critical speed. Further to these issues it became difficult to maintain a high ball charge in the SAG mill, to maintain throughput, due to concern over damaging the grates and shell liners (pushing the mill to the design boundaries) . This paper discusses the journey from discovery through comminution design and into commissioning of the DeGrussa concentrator, the difficulties faced in achieving design throughput rates, maintaining a consistent throughput rate and the inability to achieve design flotation feed particle size distribution while transitioning from partial open pit ore (transitional) treatment to solely underground ore (primary sulphide) treatment. The paper also focuses on the need to run a high ball charge in the primary mill and the damage to the lining system due to this elevated ball charge. The paper also outlines the modelling and simulation work carried out to debottleneck the primary mill. The detailed process modelling identified the presence of elevated proportions of critical size material in ore feed and highlighted the need to include a pebble crusher and primary classifying screen to overcome the problems. Post pebble crusher operational results are also discussed due to the system having been commissioned in February 2015.