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Grade Engineering® is focused on extracting metal more efficiently by separating ore from waste before it enters comminution. Early physical rejection of non valuable material through pre-concentration techniques before processing, decreases processing costs and importantly can significantly increase the life of a mine.
Why Grade Engineering® is essential technology
The minerals industry is facing a productivity and investment crisis. The ‘Millennium Super Cycle’ from 2003-11 was an unprecedented period of growth and investment resulting in increased throughput and development of lower grade resources to meet demand. During the boom quantity became more important than quality with throughput the key metric. This was accompanied by a general trend of decreasing feed grades across all commodities which was offset with higher production volumes.
Current industry perception is that declining feed grade is an unavoidable consequence of ore deposit geology and mass mining technologies for increasingly mature mining operations. In typical crush-grind-float operations value recovery only takes place at ~100-micron particle size involving 3-4 orders of magnitude size reduction compared to run of mine feed. For increasingly low grade deposits the cost of energy and capital intensity required to process and reject worthless material at micron scale drives poor productivity. An alternative is to deploy a range of coarse rejection technologies.
What is Grade Engineering®
Grade Engineering® is an integrated approach to coarse rejection that matches a suite of separation technologies to ore specific characteristics and compares the net value of rejecting low value components in current feed streams to existing mine plans as part of a system-view.
Coarse rejection (~10-100 mm) can be used on size distributions ranging from run of mine to comminution mill discharge. Opportunity for Grade Engineering® is based on five rock based ‘levers’ linked to combinations of screening, sensor-based sorting and heavy media separation. These involve (scroll down to the bottom of the page for more information on these five levers):
- Preferential grade deportment by size
- Differential blasting for grade by size
- Sensor based bulk sorting
- Sensor based stream sorting
- Coarse gravity separation
Grade Engineering® is being developed and implemented by a consortium of over 30 mining companies, equipment suppliers and research organisations. Emerging results from collaborative site activities demonstrate potential for generating significant value which can reverse the trend of declining production due to declining feed grades.
Figure1: Vision for early measurement and separation
Concept of Grade Engineering®
A focus on throughput as the main driver of revenue has led to a bulk average mentality with respect to in-situ cut-off grades. In many cases average grades used to define bench or stope scale processing destination decisions such as mill, dump leach, waste, etc. include significant sub-volumes of material outside cut-off specifications. An averaging approach ignores potentially exploitable grade heterogeneity below the scale of minimum mining unit even though significant localised grade heterogeneity is a dominant characteristic of many base and metal deposit styles and ore types.
Localised grade heterogeneity is typically overlooked in favor of maximising extraction rates and loading efficiency. This is coupled with a desire to blend ROM and produce steady state feed in terms of grade and physical properties to optimise and maximise recovery of saleable product particularly in crush-grind-float operations. Where blended supply of ‘averaged’ feed struggles to achieve steady state processing stability, this is a first order indication that significant heterogeneity exists within a resource that could be exploited rather than suppressed.
Grade Engineering® recognises that in many cases out of specification sub-volumes assigned to destinations based on bulk averages can be removed using efficient coarse separation techniques in the ‘dig and deliver’ interface. Coarse separation (~10-100mm) can be used on a range of particle size distributions ranging from ROM to SAG discharge. The earlier this occurs in the conventional dig and deliver mining cycle the higher the potential net value of removing uneconomic material.
Every handling and size transformation interface in the dig and deliver cycle should be considered an opportunity for applying coarse separation. ROM and post primary crushing are obvious intervention points with opportunity for separation conditioning during modified blast design. The decision to intervene is a function of grade heterogeneity in a given parcel of material; the yield-response of a separation device at a specific size reduction point; the ability to change a destination decision for one or more of the new streams following separation; and the net value of the new streams after handling costs.
Grade Engineering® outcomes do not create ‘new’ metal but rather exchange metal from separated components between existing destinations to create improved net value after cost of exchange is taken into account. This involves exchanging a component of separated mill feed with other destinations such as mineralised waste, stockpiles or dump leach with low recovery. The aim is to bring metal forward from destinations that are not delivering maximum current value.
Overall metal exchange balance can be modified to suit operational modes or bottlenecks. This can include keeping the concentrator full with improved grades or deferring the need for expanding installed capacity. Mass pull on separation devices can be used to control accept/reject tonnages and resulting upgrades. While Grade Engineering® does not create ‘new metal’, outcomes improve resource to reserve conversion by potentially separating economic parcels of ore from mineralised waste.
The concept of coarse separation or pre-concentration is not new and has been practiced from the beginning of mining as hand picking. The propensity of some ores to break preferentially during blasting and crushing leading to an increase of valuable phases in finer fractions has also been widely known but rarely exploited at production scale. A notable exception was pre-concentration carried out in the 1980’s at the Bougainville Copper Limited Panguna Cu-Au mine in Papua New Guinea. This involved a screening plant to upgrade marginal low grade ROM ores (0.22 Cu, 0.18 g/t Au) that exhibited preferential grade deportment into fines. The plant had a capacity of 35 Mt p.a. at a <32mm screening size, which produced a 50% Cu-Au upgrade in 38% retained mass.
Additional examples of production scale pre-concentration include the Dense Media Plant at Mount Isa Mines which removes ~35% of coarse and hardest Pb-Zn feed before the fine grinding treatment process. This increases throughput, reduces capital intensity in the comminution circuit, and reduces energy requirement per unit metal in the concentrator by >40%, together with a 15% improvement of grade in the retained stream.
While application of sensor-based sorting has found widespread application in industrial recycling and food quality management, there are limited examples of routine application to pre-concentration in the minerals industry. An exception is the Mittersill tungsten mine in Austria where in response to head grades falling from 0.7% to 0.2% since mining commenced in 1976, X-ray Transmission sensor-based particle sorting units were installed in 2008. The results significantly increased effective head grade and reduced energy intensity while allowing rejected waste to be sold as road aggregate.
Although there are global examples of coarse pre-concentration generating value for some base and precious metal mining operations, there is no coherent system-based industry approach or standard methodology to assess optimal configurations for selecting specific technologies or equipment to deliver maximum value for specific ores and operational constraints.
Grade Engineering® is the first large-scale initiative to focus on integrated methodologies to deliver maximum operational value. The prime aim for CRC ORE is to deliver Grade Engineering® as an industry standard methodology designed to improve productivity and value to mining operations which includes the ability to filter and rank individual operations for highest opportunity.
Coarse Separation Levers and Response Rankings
Within Grade Engineering®, five technology ‘levers’ are recognised that are capable of delivering coarse separation outcomes (>10mm).
1. Natural preferential grade by size deportment: the propensity for some ores to exhibit preferential breakage leading to concentration of minerals in specific size fractions. This typically involves an increase of valuable sulphide mineral phases in finer size fractions. Preferential grade deportment is an interaction function of rock mass properties, texture, ore paragenesis and mineralogy at a range of scales. There is typically no relationship between magnitude of response and head grade, with the main control being textural rather than absolute abundance. Physical separation is a function of screening employed after blasting or primary crushing.
2. Differential blasting for grade: involves conditioning of sub-volumes of material at bench or stope scale using customised blast designs that generate imposed size distributions with higher grade concentrated in finer fractions. Amenability is a function of exploitable grade heterogeneity at blast hole scale linked to the ability to impose and control different energy distributions within a blast design. As for Lever 1 physical separation is a function of screening.
3. Sensor based bulk sorting: involves use of a wide variety of electronic sensors capable of providing on-line information on grade in the dig and deliver material handling interface including shovel buckets, trucks and conveyors. There are many technologies capable of coarse rock sensing ranging from surface based to fully penetrative; and providing elemental to mineralogical resolution. Amenability is a function of the resolution and accuracy of individual sensors; rock interaction times and signal acquisition; and selected scale of resulting separation volume.
4. Sensor based stream sorting: while sensor based bulk sorting involves full particle size distributions, stream sorting involves a modified particle size distribution. This is driven by the requirements of some sensor technologies for a limited size distribution to improve rock interaction and enable individual particle separation using air jets or mechanical actuators. It is also driven by an option to use sensors as ‘cleaners’ on lower volume separated streams derived using other levers.
5. Coarse gravity separation: involves use of heavy media separation and in-line pressure jigs at coarse scale (>10mm) generating individual particle separations based on density. Amenability and separation outcomes are a function of texture and mineralogy at this scale. Compared to Levers 1 to 3, conditioning feed for coarse gravity separation typically requires secondary crushing and screening to deliver a carefully constrained particle size distribution. For this reason, coarse gravity separation in Grade Engineering® applications primarily operates as a ‘cleaner’ in combination with streams derived using other levers.
In order to assess relative merits and resulting value of coarse separation outcomes based on applying individual levers or sets of levers for specific ores and operations, it is necessary to define comparative response attributes through a process of physical testing and simulation.