The Evolution of Crushing and Grinding: Changes in the Industry by Damian
Crushing and grinding processes have undergone significant changes over the last 20 years. These adjustments have focused on lowering costs and increasing production and energy efficiency, mainly due to the fact that the mining industry has seen much larger projects and consequently has required substantial crushing equipment.
Today, mining companies want lower capital costs with fewer pieces of equipment and higher capacity from each unit, even if the equipment is disposable after a reasonable service life. This is why the three stages of crushing, which was the norm a couple of decades ago, gave way to the two new stages of crushing.
Operating cost efficiency is another important factor driving change. The globalization of the mining industry and rationalization of equipment vendors has also seen technology development and diffusion on a global scale. Metal prices have fallen in real terms, ores being mined are lower grade with higher throughputs and, at the same time, power and labor costs have increased significantly.
There is also a trend for vendors to work closely with the end user and provide technical and after-sales support, at the same time feeding operating data back into the design of new equipment.
The uptake of Semi Autogenous Grinding (SAG) eliminated the need for crushing plants other than a primary crusher. The use of pebble milling has seen resurgence in the use of cone crushers. Crushing plants were dusty, high maintenance and required additional manpower to operate. The Mine to Mill concept (looking at the relationship between blast fragmentation and crushing and comminution energy) has changed the way engineers look at the total project comminution.
High Pressure Grinding Rolls (HPGR) crushers have been adopted and have renewed the need for secondary crushing, which has taken the place of SAG mills for very hard ores. The energy savings are an important factor in driving this change.
The public demand for reduced CO2 emissions, sustainability in the mineral industry and higher energy efficiency ensures the push for the development of new technology will continue. Some of these equipment items include:
Primary Gyratory Crushers
There has been very little change in the basic design of primary gyratory crushers other than increased size, improved lubrication and simplified lower cost installations.
Jaw crushers have increased significantly in size and now compete with smaller gyratory crushers for large capacity primary crushing.
Jaw crushers are now made from fabricated steel plate and have increased significantly in size. The lubrication has been improved and the use of rock breakers above the jaws has improved operability through easier removal of blockages.
Old Symons Cone Crushers – Metso
The transition from the standard Symons cone crusher to modern cone crushers that use hydraulic hold-down clamping with nitrogen cylinder tramp iron relief has seen modern crushers with much more power and capacity. The automatic control of cone crushers has eliminated bogged crushers and maximized power draw and through put.
The largest cone crusher available was an 895 kW motor (120 kW 20 years ago) and the development of a 1 MW crusher is not too far away. The basic design has not changed but finite element analysis, improved lubrication and hydraulic clamping have seen significant increases in throughput for the same machines.
Two stage crushing is very common with some three stage crushing where a fine product is required. Liner wear is still an issue as it relates directly to costs and bowl and mantle change-out times of 24 hours are still common. Cone crushers dominate the hard rock crushing market; however, other technologies are being used on soft rocks. These are crushers such as the MMD sizer and impact crushers.
Autogenous impact crushers of the Barmac type have advantages in energy efficiency and reduction ratios and can handle abrasive ores such as Banded Iron Formation (BIF) in a pebble crushing duty. The breakage mechanism is one of rock impact against rock, so wear rates of metal components are greatly reduced. The Canica crusher uses impacting ore against a solid steel anvil to achieve breakage, such as the Yandicoogina mine in Western Australia.
The concept of in-pit crushing has been around for manyyears but, in the current market, with high energy and fuel cost, labour shortages, tire shortages and emission standards, the trend towards in- pit crushing is greater than ever.
Krupp was early into in-pit crushing but others such as MMD, Metso and Sandvik have followed. Some in-pit crushing and conveying installations include Grassberg, CVRD, Minera Dona Inés and Collahuasi in Chile and China Shougang, among others.
Pebble crushing is an arduous duty in a SAG mill circuit. The trend to installing larger pebble crushers capable of crushing up to 70-100 percent of the new feed rate has been observed. The improvements in SAG milling efficiency with a pebble crusher for some ores are now well established.
In Australia, these were pioneered at Argyle on diamond ores and proved problematic. HPGRs are emerging as an important new comminution process in mineral processing circuits, primarily because they offer substantial energy savings. There is good evidence from previous work that HPGR technology is more energy-efficient than the typical tumbling comminution machines, including autogenous, semi-autogenous and ll mills. HPGR technology has been widely used in the cement industry and, to a lesser extent, in the diamond industry and on iron ores, but rarely in the wider mineral industry. This is set to change in the future environment of high energy costs.
In addition, the use of HPGR technology has the potential to provide significant capacity increases in existing plants because there is evidence that the HPGR product has a significantly lower Bond Work Index in downstream ball milling and, therefore, will grind to the required size more quickly and with reduced energy. Furthermore, HPGR technology may allow a simpler upstream process compared with AG/SAG mills. Machines have capacities up to 3000 t/h and can do similar work to an SAG mill at half the energy consumption.
Feeders and Conveyors
Apron feeders have come back into favor with units that are more mechanically reliable and designed for no spillage. A combination of vibratory and apron feeders is used under stockpiles for example.
There is more care with sample selection and representivity and comminution testing has become mandatory. There is less reliance on manufacturer’s catalogues and a greater use of conditional simulation. The comminution tests available include JK Drop Weight, Abrasive index and unconfined Compressive Strength (UCS) tests. There have been project failures and the industry as a whole has learnt from these. The uses of “variability” testing and “geometallurgy” have also been important advances.
Simulation packages have been developed that allow simulation and optimization of crushing and grinding circuits. JKSimMet is an award-winning, general-purpose computer software package for the analysis and simulation of comminution and classification circuits in mineral processing operations. It incorporates industrial strength models developed at the Julius Kruttschnitt Mineral Research Centre (JKMRC). The package is designed for plant and development metallurgists who wish to apply process analysis techniques to characterize plant behavior, and design engineers who require process simulation models to assess design alternatives.
Using the software minimizes process risk going forward and provides confidence at an early stage in the comminution circuit design.
Damian Connelly is a Director/ Principal Consulting Engineer at Mineral Engineering Technical Services (METS).
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