This is an abridged version of a paper published in the Tenth International Mining Geology Conference 2017 proceedings. The full paper is available from ausimm.com/shop
The author contends that geoscience is presently under-utilised in the rock fragmentation by blasting process. Before we can set standards in this area, the following is required: mine geologists need to gain a better understanding of the blasting process and the blasting team needs to obtain a better appreciation of geology. Both groups need to be informed of what can be done to improve blasting outcomes, why it is necessary and how it can be achieved. By working closely together, both the geoscience toolkits and blasting expertise can be applied and focused on priority blast sensitive issues, with potential to improve mine economics and safety. Finally, and ideally, mine geologists need to take a leadership role in raw rock mass data collection and data management, and invited to take an active role in rock mass related behaviour data collection, integration and presentation.
This article attempts to provide insight into all these prerequisites for setting a new standard for collaboration and mutual learning in this strategic opportunity area. The potential for blasting to leverage downstream processes is highlighted. Blasting objectives and blast design principles are introduced along with the required rock mass related inputs. Some hazardous geological materials and environmental conditions are described, along with how they can affect operational complexity.
Three features of drill and blast operations make them unique. The first is that the consequences of incidents may be catastrophic, with the direct cause of any incident being beyond doubt. Because of this, explosives manufacture, storage, transport and blasting operations are highly regulated and human error is of particular concern.
Secondly, feedback on design and implementation performance, good or bad, is rapid and can occur in days or weeks. In contrast, for subsidence engineering design, the feedback cycle is weeks to months, and for geotechnical design, feedback can take years or decades. This, coupled with the fact that we are dealing with natural geological materials, means there is still a place for trial and error methods to validate designs and provide information for continuous improvement initiatives.
Finally, drill and blast operations occur early in the value chain and the blast results impact on subsequent processes, and their costs. Because of this, blast designs should be supported by a high level of geoscience input. Blasting objectives and targets should be strongly influenced by internal and external customer requirements and good communication is essential between geology, mine planning, geotechnical, blasting and mineral processing departments.
Geoscience input can assist with these unique aspects of blasting operations. In particular, geoscience input can assist with legislative compliance and preventing serious consequences, with the management of blast result variations and trade-offs, and by driving blast-related downstream opportunities. Figure 1 (see over page) illustrates that for any mining operation to have blasting leverage, it must first have economic leverage and secondly that leverage must be sensitive to blasting.
Figure 1 illustrates this concept which involves the following stages:
- develop a mine and mill process model
- identify site specific economic leverage points
- determine which of these are sensitive to blasting
- formulate blasting objectives
- prioritise any conflicting blasting objectives and set operational targets
- establish continuous improvement plan-do-check-act (PDCA) cycle.
Geoscientific support for blasting operations
The following section provides information for geoscientists to gain an appreciation of some current thinking with regards to rock fragmentation by blasting. This includes: defining primary blasting objectives, operational complexity, the influence of geology and rock mass properties on blasting results, blast engineering approach to design, design modification based on geological changes and data collection and management.
The six main categories of technical blasting objectives are: fragmentation control, profile control, muck pile control, damage control, environmental control and grade control blasting. Note that for each blasting objective the internal customers may be different. Considering the approach outlined, the author believes that blastability should be defined as ‘the ease of achieving the primary blasting objectives’.
Grade control blasting – for grade control blasting, the primary objective is to achieve the target tonnage and grade from economic zones. Grade control geologists are involved with all other aspects of grade control and need to be fully involved with any selective or bulk grade control blasting operations.
Fragmentation control blasting – by definition all blasts fragment the in situ rock mass. For fragmentation control to be a primary blasting objective, the design must attempt to achieve the target post-blast fragmentation distribution.
Profile control blasting – this involves using blasting to create a three-dimensional void with accurate dimensions in a previously solid rock. The primary blasting objective is to achieve the design profile (3D shape) for subsequent use.
Muck pile control blasting – most blasted rock fragments end up in what we call a muck pile. For muck pile control blasting the primary objective is to achieve the targeted three-dimensional muck pile characteristics in terms of swell, shape and location.
Damage control blasting – this objective relates to the avoidance of harm, which may be to reduce the value of the ore feed or the integrity of adjacent wall rocks. In geotechnical terms mine opening walls have an inherent strength and it is not sensible to damage them. In mineral processing terms some excessive fines (damage) can lead to poor recovery and lower value saleable product.
Environmental control blasting – excessive airblast, vibration, fumes, dust, flyrock or water pollution could result in blasting operations being severely restricted. The primary blasting objective here is to design blasts that achieve the agreed blast emission criteria. This will ensure blasting operations can continue without further constraints and to manage community perception.
The degree of operational complexity is dictated by the specific ground conditions and associated blast design. This includes any special explosive loading and firing procedures. The term ‘operational complexity’ relates to the control regime – practices, provisions or procedures – to ensure safe and productive blasting operations.
Rock mass influence on blasting results
Prior to blasting, the knowledge of the rock mass geology and properties is used to select the appropriate explosive product and to design other aspects of the blast. During the blast, the implemented design and the rock mass firstly influence the explosive performance and then in turn the rock response during the dynamic blast event. The results of the blast can then be assessed and used to refine subsequent blast designs.
Blast engineering approach to design and optimisation
Below is an outline of a systematic blasting engineering approach for both design and optimisation.
Planning and design inputs – the blast engineering approach is the primary blasting objective and is blasting domain focused. The inputs are the blasting objectives, the blast domain, the site conditions and constraints, and the tools available at the site.
Blasting domains – the rock masses within a domain are expected to behave similarly with regard to blast results.
Blasting objectives – six primary blasting objectives are proposed above, along with the operating rules. Operational targets also need to be formulated for tracking and are used to measure success.
Basic designs – this is the final stage of planning. The design analysis is based on site experience, design guidelines and any models and simulations undertaken. These are needed for each blast type in each domain. These basic designs are subject to change as conditions vary within each domain by an agreed amount.
Blast implementation – first the drill pattern design is undertaken, next the charging operation is completed, and lastly the tie-in design is implemented. The implementation should be monitored and periodically audited to an agreed standard.
Performance assessment and recording – this is a systematic process of assessing the performance and defining the prevailing conditions. A blastability scheme must be structured to react adequately to the most influential parameters affecting blast performance. These will be blasting objective specific and may be blast domain and blast type specific.
Review and optimisation process – the formal blast engineering approach has several review components and has continuous improvement built in.
Data collection and management
Blast designers need access to raw data collected by the different groups interested in rock mass characterisation. The requirements of the blasting engineers are quite specific and different from the requirements of the resource geologists, geotechnical engineers and process metallurgists. In the longer term, data collection techniques are needed that are less dependent on personal effort and that provide a better understanding of the variation in these properties across the mining area. Future developments should see useful blast engineering data generated from monitoring the performance while drilling, downhole and surface geophysics, and photogrammetry and other advanced scanning techniques.
Ideally mine geologists could take a leadership role in raw rock mass data collection and data management and be invited to take an active role in rock mass related behaviour data collection, integration and presentation.
The following conclusions can be drawn:
- Blasting is unique in at least three important ways and these provide guidance on where geoscience can add value to drilling and blasting operations.
- Some mining operations have economic leverage that are sensitive to blasting; such operations are ideal targets for downstream improvement projects.
- Clear blast design requirements are essential for quality blasting outcomes.
- Some deposits contain hazardous minerals or hostile environments, require specialist geo-knowledge, relevant procedures and additional operational complexity to ensure technical risk reduction.
- The geology and rock mass properties do influence the preblast design process to some degree and then during the blast they directly influence the rock mass response. The rock mass also influences the performance of the explosives in the blasthole and this behaviour in turn influences the rock mass response.
- Ideally mine geologists could take a leadership role in raw rock mass data collection and data management and be invited to take an active role in rock mass related behaviour data collection, integration and presentation.
- Companies should find a way to facilitate close liaison between all mining subdisciplines and conduct awareness programs to ensure mutual appreciation of each other’s abilities, limitations and required inputs.
Feature image: Mark Agnor/Shutterstock.com.