Improved drill-and-blast designs free $3.6 million of ore for surface copper mine

Capstone Copper’s Pinto Valley Mine faced challenges in recovering rock and ore on final walls because of geotechnical constraints that require restrictions on methods for loading blastholes adjacent to the pit boundaries to preserve wall stability. The operation had been using the same drill-and-blast designs for years. With changing geology, such as harder rock, as mining progressed, material was being left unmined in final walls because of poor breakage. The amount of unrecovered, unprocessed ore was estimated at 345 kt/a (380,000 stpy) generated from an average of 60 final wall blasts performed annually. The engineering team from Capstone and Dyno Nobel engaged to minimize the amount of material left on the final pit walls. This article presents how data were collected and design standards reviewed, and the opportunities that were found to improve ore recovery from pit final walls without increasing drilling and blasting costs. The results collected from implementing the new design on two benches showed the potential for extra revenue of approximately $3.6 million generated annually from reducing final wall unrecoverable underbreak by nearly 71 percent on the mine’s highwalls. Figure 1: Visual evidence of unrecovered material attached to the highwall on the south side of the Capstone Copper Pinto Valley Mine (left), and schematic cross-section illustrating unrecoverable underbreak and reduced catch-benches (right). Introduction The Pinto Valley Mine is located approximately 123 km (80 miles) east of Phoenix, in Globe, AZ, and has been in operation since 1972. Pinto Valley is a copper porphyry deposit. The primary host rock is the Precambrian-age Lost Gulch Quartz Monzonite (equivalent to the Oracle Granite or Ruin Granite).The deposit’s economic mineralization is a hypogene ore body with chalcopyrite, pyrite and minor molybdenite as the only significant primary sulfide minerals. The openpit mine extracts and processes copper ore locally with the capacity to process 60 kt/d (66,000 stpd). The operation produces a primary copper sulfide concentrate and a byproduct molybdenum concentrate (Capstone Copper, 2024).The mine uses traditional hard-rock mining methods (drilling, blasting, loading and hauling) to extract the ore and transport it to the processing facilities. A series of drill-and-blast standards and rules were developed over the years of operation to meet requirements for fragmentation (diggability and processing) and slope stability (vibration limits). The drill-and-blast bench heights at the mine are 13.7 m (45 ft). Where slope stability allows, single 27.4-m (90-ft) benches are mined with two passes of 13.7-m (45-ft) benches. Single-pass, pre-split lines help prevent damage and keep a cleaner highwall. With the primary objective of maintaining highwall stability near final pit walls, the mine faced challenges associated with material unrecovered on the walls over the years. This generated loss of revenue from unrecovered, unprocessed ore. Other issues such as extra time spent on trying to recover the hard material on the walls using secondary breakage techniques (for example, hydraulic hammers and excavators) that were often unsuccessful; coarse fragmentation near the wall; reduced catch bench functionality; and wall stability problems (such as overhangs) were also common and visible on the walls mined years prior (Fig. 1). Figure 2: Coarser fragmentation observed near the wall on the top 13.7 m (45 ft) of a bench that has been pre-split down to the full 27.4-m (90-ft) depth. Notice coarser material near the bench crest. Methodology To address the problem, the Capstone Copper Pinto Valley Mine engineering team and Dyno Nobel engaged to perform a detailed evaluation of the trim-blast designs used over the years at the operation and conduct field measurements to verify if the performance observed was aligned with the drilling and blasting plans. The steps taken in the project entailed the following: (1) Review drill-and-blast designs to evaluate if any immediate opportunities exist to enhance them and improve fragmentation, and therefore recovery of the material left on the walls. (2) Collect field data, including drill operators’ feedback, to evaluate the quality of pre-splitting techniques and potential influence in the results. (3) Test and assess design alternatives in two benches of the mine for a period of approximately six months. Figure 3: Plan and cross-section views of the traditional trim-blast designs used at Capstone Copper Pinto Valley Mine for 27.4-m (90-ft) final bench heights. Results The results obtained during this project are summarized as follows: The review of the trim-blast designs revealed two main concerns: The low powder factor and resulting potential and usually observed poor fragmentation on the top, back side of the blasts, near the highwall (see coarse material in Fig. 2, generated from the blue circle area of low powder factor in cross-section of Fig. 3). The proximity of the toes of the trim row holes to the pre-split line of approximately 0.9 m (3 ft) (see pink circles in cross-section of Fig. 3). The Capstone Copper Mine engineering and DynoConsult teams conducted pre-split drill hole deviation (inclination and orientation) measurements and interviews with drilling operators to evaluate the potential impact of pre-split drilling quality on trim-blasting operations. The results are shown in Table 1. The results showed that the spectrum of pre-split hole deviation could interfere directly with the design and execution of trim blasts in front of them — especially for the bottom, second 13.7-m (45-ft) pass, where the deviation (walk of the pre-split holes) was more accentuated (see Fig. 4 for a visual reference and Fig. 5 for detailed schematics of the issue). On one hand, the pre-split holes can intersect trim row holes, if drilled later than the trim blast holes in front of them. On the other hand, if drilled and blasted earlier, pre-split holes can directly affect the area where the trim holes will be drilled in the future by creating a pre-fractured surface that can work as an “energy-loss conduit.” The second case is the most common sequence of events at the Pinto Valley Mine. The behavior observed in Fig. 5 matches two important observed behaviors: The behavior observed in Fig. 1, where the nonbroken knobs on the wall were generally observed at the bottom 13.7 m (45 ft) of the 27.4-m (90-ft) full bench, and less frequently but still possible on the top 13.7 m (45 ft). Occurrences of pre-split holes venting out of trim row holes when the pre-split lines were drilled after trim blast holes had already been drilled ahead of them. From this point forward, the team working on the project identified that two paths could be pursued to mitigate the issue of unrecovered material being left on the walls: Adjust trim-blast designs to accommodate inherent pre-split drilling accuracy limitations. Improve pre-split hole drilling quality (minimize drill hole deviation) as much as possible. The focus of the project team was to create an alternate design option while coaching the pre-split drillers on how important the drilling quality was for the overall results achieved. The new design created for the trim blasts is shown in Fig. 6. Figure 7 shows a visual comparison of results achieved where the new design was used with the previous results obtained in multiple benches above with the traditional designs. Figure 4: Image of pre-split hole traces on 27.4-m (90-ft) bench face showing significant deviation on bottom portion compared to the top 13.7 m (45 ft). To quantify the monetary impact of the improvements, the final wall shown in Fig. 7 was used as a sample. The volume and average copper grades of the knob-like shapes were extracted with the aid of Maptek’s Vulcan mine planning software. The results showed an average 71.2 percent decrease in unrecoverable underbreak on the benches where the new design was used. Figure 8 illustrates the observed improvements in material recovery obtained with the alternate design. Table 1: Boretracking data collected from pre-split holes showing spectrum of average angle of inclination from 14.8 to 18.7 degrees and corresponding changes in design length (depth) and bottom walk from collar (horizontal distance between collar and bottom of pre-split hole). For an average of 60 final-wall blasts per year recorded in the recent years at the mine, we estimated unrecoverable tons to be near 345 kt/a (380,000 stpy). With that, the amount of extra material to be recovered with the alternate design tested was estimated at 245 kt (270,000 st) of ore at average grade of 0.187 percent copper (Cu, average grade of the knobs quantified in the study). Figure 5: Cross-section of traditional trim-blast design with the addition ofpre-split holes deviation spectrum. The gain with improved ore recovery was estimated at $3.6 million yearly with the average numbers considered in the study for final-wall blasts conducted yearly and expected average grade of 0.187 percent Cu (Table 2). Furthermore, the new design is estimated to save approximately 2 percent on drilling and blasting costs near the highwalls. This cost reduction comes from a reduction in drilling footage per ton of rock extracted due to using short holes in the trim row, and a much-reduced need for gas bags as accessories for air-decking. This was done without altering the pre-established geotechnical constraints on maximum pounds per delay allowable near geotechnically concerning areas. Table 2: Estimated gains in tons, copper pounds and extra revenue generated from three average-grade scenarios. Figure 6: Alternate cross-section design to accommodate drill-hole deviation. Conclusion and future work The main findings of this work were the cause-and-effect relationships between pre-split drilling quality and its potential impact in traditional designs practiced at Capstone Copper’s Pinto Valley Mine. The team was able to decrease by 71.2 percent the amount of material losses by using alternate drill-and-blast designs that were created considering field-collected drilling-accuracy data. The increase in material recovery is estimated to bring into production nearly one million pounds of copper into the mine’s yearly production, considering an average grade after all recoveries applied of 0.187 percent Cu. Using a currently conservative price of $3.6/lb of copper, this equates to approximately $3.6 million in extra revenue a year in benefit for the operation. The positive monetary impact comes with a series of other harder-to-quantify benefits, such as cleaner walls and more effective catch benches. Figure 7: Visual evidence of the decrease in unrecoverable underbreak in the areas of the bench where the new design was used successfully. Trim blasts’ drilling and blasting costs were decreased with the new design, by drilling less per ton blasted using short trim holes and decreasing the need for accessories (blast bags) by nearly two-thirds on those blasts. Enhancements to the alternate design presented here remain a potential for future work. Further effort to improve pre-split drilling accuracy can and should still be investigated. The use of different drill bit types, stabilizing drill pipes, and tools for angle setup could also be evaluated to decrease drill-hole deviation. Figure 8: Schematics of observed improvements in material recovery obtained with alternate design. Acknowledgments The authors would like to recognize the value added by multiple professionals that supported this work, including, but not limited to, the Capstone Copper’s Pinto Valley Mine management and engineering teams, Mike Kotraba and Sid Erikson. Reference Capstone Copper (2024) Capstone Copper - Pinto Valley Mine. Accessed July 17, 2024: https://capstonecopper.com/operations/pinto-valley/

Improved drill-and-blast designs free $3.6 million of ore for surface copper mine

Authored by: Tácio Ferreira and Liz Diaz

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