December 2017

Achieving and sustaining productivity benefits through a mine to mill approach

  • By Kristy-Ann Duffy, Process Consultant - Mining and Mineral Processing, Hatch

The mining industry is facing low metal prices, lower grade and complex deposits that are difficult to exploit, as well as growing challenges associated with the cost and supply of energy and water. To make these projects economically viable, improved resource efficiency is needed.

Mine to mill optimisation aims to develop and implement integrated mining and processing strategies that are tailored for a specific operation. The objective is to maximise production with the available assets while minimising the overall cost per ton treated, thus maximising company profit in a sustainable manner. Mine to mill involves looking at the complete operation, including the mine and processing plant, rather than optimising each in isolation.

The benefits of mine to mill optimisation are well-recognised within the industry, including the potential for optimised blasting fragmentation to improve downstream processing performance. Unfortunately, in some cases, this has resulted in over-simplification of the mine to mill concept to the single idea that increasing explosive consumption in the mine results in optimisation of the entire value chain (mining and processing). However, for truly holistic (ie mine to mill) optimisation, blast intensity is not always increased, but rather adjusted to best suit the different types of ores and also the circuit configuration, equipment, installed comminution power and the separation process. A properly implemented mine to mill operation is a fully integrated effort, with many aspects of optimisation in the mine, comminution and separation processes.

The specific targets and objectives vary for each project depending on prevailing site, orebody and economic conditions. A comprehensive mine to mill approach recognises that every orebody and mining operation is different. Understanding the orebody, and the characteristics of the ore within, allows the process to be tailored to suit the ore properties and key business drivers of the operation.

Various mine to mill initiatives have been implemented over the past twenty years, with varying degrees of success. There is a long list of well-documented successes; for example, at Antamina, throughput was more than doubled for certain hard ore types with the existing equipment and installed power, therefore reducing the specific energy consumption (Valery et al, 2012). This involved detailed ore characterisation, development of optimised blasting designs according to ore types (domains) and optimisation and adjustment of the downstream comminution circuit to achieve the energy and capacity benefits while meeting grind size targets as required for the subsequent flotation process.

However, there are also anecdotal references of failures; stories of how ‘mine to mill didn’t work for us.’ These failures can often be attributed to a number of common reasons, such as:

  • powder factor (kg of explosive per ton of ore) increased indiscriminately with no consideration for variation in ore characteristics and without optimisation carried through the downstream processes to realise the full benefits
  • lack of a necessary structured and integrated methodology
  • results were not properly measured, followed through and well documented
  • implementation of integrated optimisation strategies in the mine and plant were not fully supported and/or properly incentivised by management
  • costs and performance were measured in isolation in the mine versus the mill, instead of the overall mine and mill costs and benefits.

To be successful, mine to mill projects require a structured methodology supported by measurements with extensive auditing, surveys and data analysis (Figure 1). Furthermore, training, incorporation of strategies into site procedures, identification and measurement of appropriate key performance indicators (KPIs) or metrics are essential in order to maintain benefits over the long term.

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The performance in mining and mineral processing activities is governed by in situ ore properties. Therefore, mine to mill optimisation should start with proper ore characterisation. Ore domains should be defined based on blasting, comminution and metallurgical properties and the spatial distribution of these domains should be mapped across the orebody. Detailed data from blasting and processing operations should be collected through audits and surveys, along with historical operating and benchmarking data. After this, site-specific predictive mathematical models can be developed for each unit operation (blasting, comminution, separation). Together, these models can be used to indicate how the whole process will respond to different ore types and operating practices and conditions in the mine and the plant.

Blasting can be the cheapest and most energy efficient rock breaking stage. The size distribution of the blasted material has a significant impact on the entire process, especially the throughput and energy consumption in the downstream crushing and grinding operations. These, in turn, impact on the performance of the subsequent separation processes.

Blast fragmentation is affected by the inherent structure and strength of the rock. Harder, massive zones of the orebody need more blasting energy, while softer, more jointed and naturally fractured zones need less. Rock structure and strength are measured across the deposit, which is delineated into domains. A site-specific blast fragmentation model is calibrated and used to determine the optimum blast design for each domain. The complete blasting design is optimised according to the ore properties of each domain and downstream circuit requirements. The result is a set of blasting guidelines, and this generates consistent run-of-mine (ROM) fragmentation with appropriate size distribution for the downstream processes to optimise the overall performance of the operation. Excessive blasting in softer ore domains is avoided, thus reducing energy consumption and costs, while also preventing the production of excessive ultrafine material that can be detrimental to some downstream processes (eg in heap leach operations or flotation).

The optimum size distribution from blasting depends on the downstream processes. The ideal ROM size distribution, which will result in maximum throughput and performance, will depend on the breakage characteristics of the ore, equipment type, circuit arrangement and operating conditions. The optimum feed size requirements for autogenous (AG) mill, semi-autogenous grinding (SAG) mill, multi-stage crushing, and high pressure grinding rolls (HPGR) are very different. For example, SAG mills benefit from having the finest possible top size, minimisation of intermediate (critical) sized material and increased fines (-10 mm). On the other hand, fully AG mills require larger rocks in the feed to act as grinding media and often do well with a bimodal feed size distribution (larger rocks and fines).

Comminution flowsheets using technologies such as HPGR and stirred mills are becoming more common due to growing challenges with regards to supply and cost of grinding media, energy and water. The holistic mine to mill approach is equally relevant for different technologies and flowsheet arrangements. It requires detailed understanding of the ore characteristics and how this affects the processes and technologies employed at the operation.

To realise the full benefits of improved ROM fragmentation from changing blasting practices, optimisation must be carried through the downstream processes. It is not sufficient to optimise the energy in the blast without adjusting and optimising the downstream process.

Comminution is very energy intensive and costly. Changes in feed (hardness and fragmentation from the blast) and comminution circuit operating strategy can result in a reduction in power usage or an increase in circuit throughput. Site-specific models of comminution processes allow simulations of different operating conditions, alternative circuit configurations, expansion options, etc. This facilitates evaluation of a large number of scenarios avoiding trial and error experimentation in the mine and processing plant, which is risky and expensive due to lost production. Trend and variability analysis of historical operating data, power calculations and benchmarking with similar operations can also be used to determine the bottlenecks and identify opportunities for improvement.

Just as the size of blasted material affects crushing and grinding, the particle size and liberation that comminution produces strongly affects the recovery of the valuable component in the separation processes. The energy of comminution increases exponentially as particle size decreases. So, it is important to understand the trade-off between the higher costs, energy consumption and possibly lower throughput of producing a finer grind size versus the improved liberation and recovery of the valuable component resulting from the finer size. This varies considerably for different ore types and also breakage mechanisms, so the trade-off needs to be understood for the particular conditions in each case.

In addition to the important interaction with preceding grinding circuits, the separation processes, which may include ore sorting, flotation, leaching, magnetic separation, gravity concentration, etc, should also be optimised. Comprehensive sampling of these processes, conducted together with blast and comminution audits and surveys, allow the impact of each to be understood. Survey data is used along with historical operating data in modelling and simulations to evaluate alternative strategies. These techniques, together with extensive industrial and consulting experience and databases, are used to highlight opportunities for performance improvement.

This integrated approach allows a range of operating strategies to be simulated and evaluated for different ore types in the mine and plant. To ensure the expected results are achieved and maintained, the recommended changes must be incorporated into managerial decisions and site-operating procedures, and training is crucial to ensure everyone understands and is on board. This approach has helped many operations significantly increase their metal production with no or very little capital expenditure. Costs are cut, energy is saved and overall process efficiency improves.

Furthermore, this integrated approach can be applied over the life-of-mine (LOM). The models can be combined with the mine plan to generate production forecast and extended to develop geometallurgical models. This provides an understanding of ore variability and its effect on the mining and processing over the LOM. Capital equipment purchases can be predicted well in advance, long-term strategic planning becomes easier and more accurate and can be used to reduce risks and maximise profitability.
Current and future trends in mine to mill optimisation

The mining industry is facing low metal prices, lower grade and complex deposits that are difficult to exploit, as well as growing challenges associated with the cost and supply of energy and water. To make these projects economically viable, we need to improve resource efficiency. Therefore, increasing productivity through optimisation of the entire value chain, from mine to mill, is more important than ever.

Many strategies for improving the efficiency and sustainability of mining operations are not new, but rather involve novel application of existing technologies and tailored solutions based on understanding of the process and ore. Existing technologies from other industries can be used, or technologies can be applied in novel arrangements to maximise efficiency. Also, understanding the impact of each step on preceding and subsequent stages allows the optimisation of the process as a whole. Mining operations need to operate more efficiently to meet environmental targets, but improving the resource efficiency also increases the economic return of the project, and can make the difference between a project being viable or not. A more resource and eco-efficient mining process of the future may incorporate:

  • high intensity selective blasting (HISB) to improve blast fragmentation in order to decrease energy consumption and increase throughput in downstream comminution circuits
  • in-pit crushing and conveying (IPCC) is a more efficient method of transporting ore and waste than conventional truck and shovel operations, and can eliminate the use of diesel
  • elevated high angle conveying (HAC) facilitates removal of material from deep pits via the shortest route for IPCC solutions
  • preconcentration using screening, gravity separation and/or bulk ore sorting could be implemented to
    discard barren material, consequently reducing haulage and downstream processing requirements per tonne of product
  • alternative efficient comminution technologies such as HPGR, vertical roller and stirred mills may be applied in novel flowsheet arrangements with higher efficiency classification (such as fine screens)
  • improvements to coarse particle flotation could allow coarser grind sizes to be targeted for the first stages of separation to reduce the amount of material requiring fine grinding
  • filtration and dry stacking of tailings can be implemented to reduce water consumption, with a much higher recovery of water than can be achieved from a traditional tailings dam as well as reducing tailings footprint and eliminating the risk of tailings dam failure.

Feature image: Steve Lovegrove/Shutterstock.com.

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