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 considered. 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.
Each lever naturally integrates with the mining value chain at one or more points. Some levers can be competitive while others complementary.
One such example of complementary levers is the integration of Particle Sorting with Natural Deportment, referred to as Integrated Screening and Particle Sorting (ISPS) within CRC ORE.