Variable speed drives improve mining’s high-pressure grinding rolls
Variable speed drives are adding to the energy efficiency and productivity of HPGRs, as one plant in Mexico has discovered.
In the mining industry, high-pressure grinding rolls (HPGRs) frequently replace conventional semiautogenous (SAG) grinding mills, primarily to reduce energy consumption and costs within crushing and grinding circuits. First used in 1985 in the cement industry for grinding cement clinker, HPGRs reduced energy consumption by up to 50%. Today, variable speed drives are adding to the energy efficiency and productivity of HPGRs, as one plant in Mexico has discovered.
HPGRs consist of a pair of counter rotating rolls, one fixed and the other one floating. The feed is introduced to the gap in between the rolls and is crushed. Variable speed drives can adjust the speed of the rotating rolls and thus the throughput of the units, which is advantageous for grinding or crushing inhomogeneous types of ore. As such, HPGRs can optimize the grinding process for ores with varying grinding properties.
HPGRs require drives systems with high dynamics and strong mechanical characteristics to sustain heavy loads and stress on the machinery. They can reach short-term overload conditions up to 180% for 500 ms. In steady state operation, while material is being fed into the rolls, torque oscillates between +/- 20% around the nominal torque of 80%. In addition, several factors influence the correct configuration of the drive system, with particular attention paid to the impact of harmonics on the supply network, environmental conditions and the control interface.
Despite these challenges, HPGRs have been used with great success in mining diamonds in 1987, iron in 1994, and hard rock (copper, gold and platinum) in 2005 with improvement of the wear protection of the rolls.
Recently, ABB signed a contract with Polysius Corporation USA for the Peñasquito HPGR extension project. ABB will add to the existing energy efficiency and productivity of HPGRs by optimizing the grinding process with its ACS1000 variable speed drive systems. These drives will increase the HPGR’s throughput efficiency by up to 5%.
At the Peñasquito ore grinding plant in Mexico, which is located in the mining state of Zacatecas,. ABB will supply a medium voltage (MV) variable speed drive system that includes two motors, two transformers, and two MV ACS1000 water cooled converters for the high-pressure grinding rolls. The drive systemhas a power capacity of 2 x 2500 kW. Start-up of the HPGR project is scheduled for Q4 2010.