PARAMETERS INFLUENCING ORE DILUTION IN UNDERGROUND MINES
Bayram Ercikdi1, Ayhan Kesimal1, Erol Yilmaz1, Recep Kaya1
1Department of Mining Engineering, Karadeniz Technical University, Trabzon, Turkey
ABSTRACT:
This paper provides an overview of the various issues influencing ore dilution in an underground mining. The paper reviews the dilution problem throughout the entire mining process, and provides a rational approach to underground mine design in order to minimize dilution. The stages contributing to dilution include orebody delineation, design and sequencing, stope development, drilling and blasting, and production stages. Vibrations generated by the drilling-blasting operations during the ore production could accelerate the opening of existing natural joints and cracks probably leading to the ore dilution. The effects of drilling-blasting on stability can be determined via measurements of blast vibrations, hole deviation, hole angle and distance of the holes to the exposed stope walls to develop controlled blasting procedures and hence to minimize the ore dilution. Vibration measurements done have shown that overbreaks is increasing with the high vibrations generated by the drilling-blasting operations during the ore production.
Keywords: Ore Dilution, Drilling-Blasting, Orebody Delineation
1. INTRODUCTION
Ore dilution is the addition of waste rock, non-ore material and the material which is below the cut-off grade to the ore during the mining process. In the other words, dilution is defined as the low grade (waste or backfill) material which comes into an ore stream, reducing its value (Diakite, 1999). The addition of waste rock decreases ore grade and increases the mined tonnage for a given geological reserve. The detrimental impact of dilution to the economics of the mining industry has been well documented elsewhere.
Dilution is a source of direct cost as waste or backfill material is blasted, mucked, transported, crushed, hoisted, processed and stored as a tailings. Dilution is also a source of indirect cost as the dilution material may adversely affect the metal recoveries and concentrate grades (Elbrond, 1994).
A survey of underground mines in 1988 reported that a major factor for mine closures has been due to uncontrolled dilution. It has been reported that 40% of open slope operations were experiencing dilution in excess of 20%. In most cases, mining and milling capacity is limited; this capacity is affected by the displacement of ore by waste within the overall mining, and processing facilities. Dilution is always defined and quantified with respect to a planned stope boundary. In order to quantify dilution, an ore body must be properly delineated and the extracted volumes must be effectively measured Puhakka, 1991).
Many attempts at quantification of ore losses and rock dilution have been made and some examples are shown in Table 1. Although they differ widely from each other, it is obvious from these figures that ore losses and rock dilution are significant and have considerable effects on the economical results of a mining operation. Life
length, cost of producing metal and the loss of metal are all affected.
Due to a variety of uncertainties, to inevitable lack of precision in deposit estimation and in mine planning, and due to production constraints, ore
losses and rock dilution occur throughout the many phases of a mining process. In this study, the phases affecting ore dilution in underground are explained.
Table 1. Rock dilution and ore losses (Ingler,1984; Wright, 1983; Arioglu, 1994)
Rock Dilution (%) Ore Losses (%)
Mining Method Ingler Wright Arıoğlu Ingler Wright Arıoğlu
Stoping 5-30 5-10 10-15 5-15 3-5 5-10
Room and Pillar 0-10 - 15-35 5-30 - 10-15
Cut and Fill 5-10 15-30 3-7 5-10 5-7 5-10
Shrinkage - 10-15 10-15 - 5-7 10-15
Sublevel Caving 10-30 10-15 10-20 10-30 12-15 10-20
Block Caving 10-30 15-20 10-20 0-30 15-20 13-15
2. TYPES OF ORE DILUTION
Dilution can be divided into three general categories, namely; internal, external and ore loses (see Figure 1).
Figure 1. Classification of dilution
Internal dilution (planned) usually refers to the low-grade material contained within the boundaries of an extracted stope. It can be caused by insufficient internal delineation of waste pockets within an orebody. It is also occur in situations where the mining method dictates a minimum width of extraction.
External dilution (unplanned) refers to the waste material that comes into the ore stream from sources located outside the planned stope boundaries (Villaescusa, 1995). Low grade material from stope wall overbreak, contamination from backfill, and mucking of waste from stope floors are typical examples of external dilution.
Ore loss refers to the economical material that is left in place within the boundaries of a planned
stope. Planned ore diaphragms (ore skins), unbroken stope areas due to unsufficient blast breakage, non recoverable pillars left to arrest stope wall instability and insufficient mucking of broken ore within stope floors are typical examples of ore loss.
Geological dilution refers to the waste rock or ore-losses incurred during the exploration and orebody delineation stages, where only an estimated model of the orebody can be made. A geological model is based on limited information, and is unlikely to coincide exactly with the real orebody, therefore the delineated orebody boundaries are likely to exclude ore and also to include waste. The magnitude of this problem is a function of the sampling pattern for the mineralization type under study. Geological dilution may comprise up to 1/3 of the total dilution depending upon orebody complexity (Lappalainen and Pitkajarvi, 1996).
Figure 2. Illustration of planned and unplanned ore dilution MINE DILUTION
EXTERNAL INTERNAL ORE LOSS
UNPLANNED PLANNED GEOLOGICAL
INSTABILITY CONTAMINATION MINING METHODS NATURE OF MINERALIZATION MINING METHODS EXPLORATION OREBODY DELINEATION
3. UNDERGROUND MINE DESIGN
Underground mine design is an engineering process in which the key performance indicators are: safety, dilution, recovery, productivity and cost criteria. A safe and economical design may require a combination of physical, analytical, numerical, probabilistic or empirical excavation design tools that must be appropriately calibrated with field observations. (Potvin et al., 1989; Laubscher, 1991).
Figure 2, presents a rational methodology for underground mine design in which three key stages are identified. An initial orebody delineation and rock mass characterization stage, followed by a global and a detailed design stages respectively. Global design issues are relevant and applicable within entire areas of a mine, such an extension of an existing orebody, while detailed design issues are applicable to the extraction of individual stopes.
The methodology proposed involves an integral approach to excavation design (from orebody delineation to stope extraction) in which the interaction among geology, mine planning, rock mechanics and operating personnel is required throughout the entire excavation process. The geometric configuration of an orebody and its spatial grade distribution play a significant role during the selection of a mining method and subsequently influences the amount of dilution experienced during the stoping operations.
The orebody delineation and rock mass characterization stages provide the input for the entire design process. The suggested approach is to obtain representative (mine-wide) rock mass properties likely to be used in the global excavation design and stability analysis. In most cases, this information is obtained from diamond drill holes (core logging) and direct mapping of underground openings. Geophysical tools can also used for orebody delineation and rock mass characterization.
Global design issues are related to the design and stability of large sections of a mine, such a new extension at depth or at an orebody abutment. Global design involves several issues including mine access, infrastructure, pillar and stope span designs.
Detailed design is related to the extraction of individual stopes within a global area. The mine planning engineer uses geological sections from a
mine design package to do a preliminary stope design, while the rock mechanics engineer completes a rock mass characterization program, provides guidelines for dilution control, reinforcement and blast sequencing.
At this stage extraction factors are taken into account. Drill and blast design is undertaken considering the equipment capabilities, to ensure that the designed stope shape is achievable. This is then followed by an economic analysis and finally a stope design document that include plans of sublevel development, sections showing blasthole design concepts and drilling and blasting parameters, ventilation, rock mechanics and overall firing sequence.
Geotechnical measurements are required to assess the response of the rock mass to the excavation process and are a key component of the mine design optimization process required to achieve safe and most economical extractions. The measurements can be classified into three phases: Prior, during and after excavation (Windsor, 1993; Ercikdi et al., 2003).
Measurements prior to an excavation are usually concerned with the characterization of the geotechnical environment as an input to the excavation design. Such measurements include borehole/core logging data to determine rock type, structure, rock material properties and hydrology conditions.
Measurements during excavation are used to provide warning of hazards such as excessive rock stress, deformation and extent of damage envelope around the underground openings. The measurements suggest the type and timing of remedial measures such as modification to extraction rate and sequencing of excavations and to optimize rock support and reinforcement schemes.
Measurements following an excavation are undertaken to obtain data required for optimization of future excavation designs. These measurements are required for dilution control and to minimize ore loses. They are also needed to provide data on long term stability, safety and environmental effects.
NO
YES
NO YES
Figure 3. Underground mine design process
4. PARAMETERS INFLUENCING DILUTION The most common parameters influencing dilution and ore losses in underground mining are listed in Table 2. Five key stages ranging from the initial orebody delineation program to the final extraction stages have been identified within the mine design process. Management issues were also included, given that in some cases they represent the most critical factor controlling dilution (Ashcroft, 1991).
4.1 Orebody Delineation
Ore body delineation is the processes which establish the size, shape, grades, tonnage and mineral inventory for the ensuing mining process. Efficient, effective, and accurate delineation of a deposit is required to design a mine in a manner that maximizes recovery, minimizes dilution and increases safety. Dilution can not be planned or minimized if detailed geological and geotechnical information is not available. Experience indicates that increasing the information density is likely to decrease dilution and ore losses (Braun, 1991). In cases where the stope geology is not well delineated, the presence of waste inclusions is then likely to remain unknown.
4.2 Design and Sequencing
At this stage, several extraction strategies to minimize dilution/ore loss can be studied in advance to choose the best design alternative. Engineering, geology and operating personnel should have a direct input into this stage of the design. Extraction factors that account for dilution, should be applied at this stage. Back analysis from adjacent stopes based on laser (Miller et al., 1992) surveys, drill and blast design and general experience in the area should be used. Proper design means that the planning engineer receives an optimised block thus leaving more time for drilling, blasting and ground support optimization, schedule modifications and other issues.
At this stage, the stable stope and ore outlines are superimposed in order to detect volumes of waste rock inside and ore outside the stope limits. Wall instability and any relevant remedial measures are also identified. A stope shape must be drillable and stable, and the walls must insure proper flow of broken ore to the stope draw point. Economical studies in conjunction with stability analysis can be performed to evaluate different design options (e.g. stope sequencing, dimensioning, etc.).
Table 2. Parameters influencing dilution
Orebody delineation
Under sampling of orebody boundaries Errors in decisions regarding cut-off grades Down hole survey errors
Lack of geotechnical characterization
Orebody Delineation Geology Rockmass characterization Geology Rock mechanics Access & Infrastructure Mine planning Stope & Pillar
size andlocation Stress analysis
(sequencing) Scheduling
Acceptable design
Drill & blast design Economical analysis Rock reinforcement Extractionmonitoring
Acceptable
design Document results
End G L O B A L D E S I G N D E T A I L E D D E S I G N
Design and Sequencing
Poorly designed infrastructure Poor stope design (dimensions) Lack of proper stope sequencing Lack of economical assessment
Stope development
Non alignment of sill horizons
Poor geological control during mining Mining not following geological mark-ups Inappropriate reinforcement schemes
Drilling and Blasting
Poor initial mark-up of holes
Set-up, collaring and deviation of blast holes Incorrect choice of blasting patterns,
sequences and explosive types
Production stages
Mucking of backfill floors
Mucking of fall offs and stope wall failures Contamination of broken ore by backfill Leaving broken ore inside the stopes Poor management of waste rock (tipped into the ore stream)
4.3 Stope Development
Drive location has been shown to be critical for dilution control. Undercut of stope walls by the access drill drives is likely to control the mechanical behaviour at the stope boundaries. Drive shape and size also influence stope wall undercut. Incorrect positioning of sill drive turnouts off access crosscuts may also create stope wall undercut leading to dilution. Cross cuts need to be mapped, sampled and interpreted prior to developing the sill drives along an orebody. In cases where assay information is required prior to sill turnout, a prompt assay turnaround is critical to maintain development productivity. Quality (and quantity) geological face mapping of development is critical to minimize stope wall undercuts. Geologists should highlight any over break beyond an established mining width. Prompt feedback to the operating personnel undertaking the development mining is required. Routine geotechnical mapping of development faces must be also undertaken. Perimeter blasting techniques can be used to reduce wall damage in development access in order to minimize stope wall undercut.
4.4 Drilling and Blasting
If dilution and ore losses can be minimized during the block design stages, drilling and blasting can be done without problems and focused on better fragmentation and damage control within the stope boundaries. Nevertheless, dilution and ore loss can also planned and evaluated during the drilling and blasting stages, where the blasting outlines can be designed to optimize extraction.
The blasting process involves the interaction of the rock mass, the explosives, the initiation sequences and the drill hole patterns. Consequently, a blast design should account for the interaction of the existing development, equipment, ore body boundary and stope outline. Geological, geotechnical, operational and extraction design issues must be considered. Blasting performance is also affected by the ore body geometry and drilling limitations (hole length and accuracy).
The effects of blasting on stability can be determined based on measurements of blast vibrations, hole deviation, hole angle and distance of the holes to the exposed stope walls. A consideration of the most suitable drilling technology for a range of hole sizes and drilling patterns in order to minimize damage and hole deviation is needed. Suggested drilling and blasting patterns for long-hole stoping are presented in Table 3.
Table 3. Drilling-blasting patterns for sublevel stoping
Hole Diam (mm) Burden (m) Stand-Off Distance (m) Drilling
Technology Depth Hole (m) 51 1.0 - 1.5 0.4 rods 10-15 63 1.3 – 1.8 0.6 rods 10-15 73 2.0 - 2.5 0.8 Rods + stabilizers 12-20 76 2.0 – 2.5 1.0 Rods + tubes 20-25 89 2.5-2.8 1.1 Tubes – top hammer 25-35 102 3.0 1.2 Tubes – top hammer 25-40 115 3.0-3.5 1.3 In the hole hammer 40-60 140 3.5-4.0 1.5 In the hole hammer 40-60
Blasting process needs to be performed in a more efficient way to minimize ground and induced structural vibration. Minimization of blast
vibration is restricted by the production requirements, such as good fragmentation and muck pile size. A good option in dipper veins blasting is the stabilization (by pre-installation of cable bolts) of the hanging wall and footwall and then increase the blast vibration energy to obtain the desired fragment size. The blasting damage level is linked to both the rock mass physical-mechanical properties and its structural components as well as the power of explosive used.
The interaction between the explosive material and the geological material creates a peak particle velocity (PPV) which puts in evidence the behaviour of the rock mass under given blasting parameters, and then the blast damage and induced failure are assessed based on the PPV. In most cases, the over break causing dilution is
caused by these blast damage and induced failures. However, it is possible to reduce this problem by controlled blasting. It involve the following components (single or combination), modification of the firing sequence, the use of different explosives, modification of explosives placement procedures and the modification of the geometry of the volume where the explosives are placed. Also the blasting parameters could be changed (firing delay, blast hole pattern, the effective density of the explosive, type of explosive.
Vibration measurements was made to evaluate the over breaks at Bosquet mines in 1997. Vibration measurements done have shown that over breaks is increasing with the high vibrations generated by the drilling-blasting operations during the ore production (Table 4 and Figure 4).
Primary Stope 0 200 400 600 800 1000 1200 1400 1600 1800 2000 0 50 100 150 200 Explosive Charge (kg) PP V ( m m /se c) 3 m 5 m 10 m 20 m Secondary Stope 0 200 400 600 800 1000 0 50 100 150 200 Explosive Charge (kg) PPV (mm/ se c) 3 m 5 m 10 20
Figure 4. Blast vibration in the hanging wall at Bosquet Mine (Henning et al., 1997)
Table 4. Results of over break measurement in Bosquet Mine hanging wall (Henning et al., 1997)
Maksimum linear Overbreak (m) Production Place Hangingwall Footwall Primary 4.0 0.5 Average 3.1 0.5 Secondary 2.0 1.5 Average 2.2 1.0
As can be seen in Figure 4, about the first five meters around the blasthole, vibrations higher than 1000 mm/sec have been measured for the primary stope. Vibrations measured for the second stope
are relatively lowest. This is certainly one of reasons of higher ore dilution in primary stope. As can be noticed from Table 4, the primary stope is
affected by more overbreak volume than the secondary stope.
Therefore, a good approach for reducing ore dilution is to understand the explosive/rock mass interaction. This means good knowledge of both rock mass and explosive characteristics, and the design of optimal fragmentation blast hole pattern. The challence of successful controlled blasting remains still to minimize damages associated with the explosive/rock mass interaction while to optimize the fragmentation produced by explosive energy.
4.5 Production stages
Even at this relatively late stage, dilution and ore losses can still be minimised. Information from percussion blast holes can be used to locate zones of waste within an orebody, thus enhancing orebody delineation. The blast design could be revised based on detailed information regarding zones of ore and waste. Some holes might not be blasted (i.e. leaving a pillar), or additional holes may be drilled. Drill-cutting data can be used to identify the ore-waste contact in production holes. However, these task-intensive operations (sampling, bagging, and assaying) are prone to inaccuracies, and the turn-around time for the data analysis is often too slow for practical use. In practice, information about the ore-waste contact at the production stages is seldom acquired without the use of properly calibrated geophysical tools.
The potential exists for geophysical logging (single hole techniques) of production holes to identify the ore-waste contact for optimal blast design. An advantage of single-hole geophysics is that information would be immediately available; therefore significantly reducing turn-around time. This is particularly beneficial in situations in which severe blast hole deviation is occurring, and the exact location of the ore-waste contact is undefined.
Inspection and floor preparation before firing and mucking commences, minimizes ore contamination during mucking. Mucking units may dig holes and dilute ore with fill. Mucking units may also ramp up and leave broken ore in the stope floors. A training program on draw point inspection for grade, ore contamination and stope status (stability) is required to control dilution. Any significant falloff, over break or under break
should be recorded, given that variations from planned designs could affect stability and place at risk further extraction in adjacent stopes.
Stope performance review must be undertaken following the completion of production blasting. These reviews are needed to improve performance and to determine what lesson can be learnt and what improvements can be made. Geology, mine planning and operations personnel must be involved. The performance review compares the laser (CMS) surveyed void with the planned void. The differences can be due to blasting over break, stope wall failures, pillar failures, insufficient breakage, etc. The variations from the planned volumes are used to determine actual tonnage and to estimate the extraction grade for each stope. These can be used to undertake the final economical analysis and to optimize future extraction in similar conditions.
5. CONCLUSION
Each operation must set the design objectives for dilution control based on the reality of its own particular mining system and its economics. A dilution control action plan must include definition and identification of the dilution sources, including a strategy for measurements and implementation of corrective actions. Realistic targets for dilution reduction over both the short and long term must be set. The success of the program will rely on regular communication of the planned targets and economical importance to all mining personal.
Management must develop performance indicators that are a function of quality rather than quantity. i.e. the focus must be on metal tonnes and dilution control. Mine managers must recognize the potential for improvement within their own mine environment. Most of the understanding of what comprises dilution and the tools to quantify it already exists.
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