Reducing Dilution With Narrow-vein Mining
A plan that considers drive size, blast design and quality control provides the best results
By Paul Salmenmaki
The dilution study began with vein characterization. The broad aim was to identify the general variability in vein width, geometry, and lateral extent (strike and dip) and to identify geotechnical conditions that may determine the applicability of various mining methods. The findings included:
• Consistent vein strike 1,500 m;
• Vein width ranging from 0.5 m to more than 5 m;
• Ground condition ranging from poor to good, across strike and depth extents;
• Cemented rockfill and waste rock considered to aid ground support; and
• LHOS and CAF used at the mine, with potential to optimize sublevel spacing and mining sequence.
Optimized Dilution Scenarios
Using the results of the vein characterization study, it was concluded that the minimization of dilution for LHOS and CAF would be based on:
• Vein widths of 1.5 m, 2.25 m, 3.5 m and 5 m;
• Varied ore drive dimensions, and rectangular and shanty-back profiles; and
• Orebody dips of 61°, 66°, 68° and 72°.
AMC utilized the following formulae to determine dilution. Dilution ratios were calculated for planned and unplanned dilution. Total dilution is the sum of planned and unplanned dilution (Figure 1.1 and Figure 1.2). This method has been selected from numerous alternatives for calculating dilution. The optimum dilution is a trade-off against ore recovery. Given the value of the ore for this case study, recovery was given priority over dilution, with a target recovery of 100%.
AMC examined the existing fleet of stope equipment (jumbos, LHDs, longhole drills, and bolters) at the mine and established optimal drill-and-blast designs for the selected vein widths, dips, drift profiles (square and shanty), drift size (height and width), and mining methods, which could immediately be implemented to reduce dilution, without additional equipment purchases.
AMC also identified the optimal fleet of stope mining equipment for dilution minimization. This requires the purchase of small, low-profile equipment and reduced development sizes.
Optimized Stope and Drive Dilution
AMC completed a comprehensive analysis to determine the minimized dilution for both LHOS and CAF using the formulae discussed. All of the selected vein widths, dips, drift profiles, and drift sizes were examined to minimize dilution and maximize recovery. The drift sizes considered had to accommodate either existing or optimal equipment.
In general, minimum dilution is achieved using the optimal equipment and smaller ore drives for the LHOS methods, or the lowest practicable drift heights for CAF. Minimum dilution for LHOS ranged from 66% (1.5 m stope width, 0.5 m vein width) to 9%. For a 5-m vein width, a minimum dilution of 9% is achievable in good ground with a 20-m sublevel interval and either a 5-m wide by 3-m high ore drive or a 5-m wide x 5-m high ore drive. In fair ground, the dilution projection increases to 10%. Key results for CAF method was 28% for a 1.5-m vein width and 11% for a 5-m vein width.
AMC used the ideal stope and drive dilution results for the basis to create drilland- blast designs, which included slot raise, production rings and powder factors (PFs) for LHOS. The drill-and-blast designs for CAF included drill patterns, suggested explosive loading and PFs. An example of LHOS design for an irregular vein shown in Figure 1.3.
To target idealized dilution, AMC recommends closely monitoring three areas: drive dimension optimization, drill-andblast practices, and overall quality control. For the ore drives, engineers should consider the narrower veins and purchase equipment that fits into 2.7-m x 3-m drift. They should also consider the use of shanty ore drive backs. Development into orebody hanging wall should be minimized. Technical support processes for designing narrow vein stopes should be reviewed.
And, when it comes to ore quality control, all departments (planning, surveying, geology, geotechnical, ventilation and operations) need to provide input and signoff for stope and mine designs. The stope markup and reference line should be set by surveyors. A surveyed drill setup process should be implemented to improve drill hole accuracy. Blasting engineers should be involved continuously with blasting crews. The cavity should be monitored and surveyed for every stope blast. Engineers need to perform regular, quantitative reconciliations between the design and the blast results. The results should be shared with stakeholders to justify implementing changes.
Paul Salmenmaki is a senior mining engineer working with AMC Consultants. He can be reached at psalmenmaki@ amcconsultants.com.