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Experimental investigation of the effect of broken ore properties on secondary fragmentation during block caving Dorador, Leonardo

Abstract

Block cave mining is experiencing a global growth in importance as new large, lower grade and deeper ore bodies favouring underground mass mining methods are developed. With block caving, the rock mass fragmentation process is decisive in the design and success of the operation. The last stage of this fragmentation process known as secondary fragmentation, plays a major role in the design and success of a caving operation. Despite this, it is the least understood fragmentation stage due in part to the complex mechanisms and the numerous variables involved in this phenomenon. The broken ore density (BOD) and the inter-block friction angle (ϕ') are comprehensively investigated here. A conceptual framework describing the BOD distribution and a procedure to evaluate this parameter under both an isolated movement zone and interactive flow are proposed, and an approach to evaluate ϕ' under different broken ore properties and draw column conditions is developed to be applied to early stage feasibility studies and design. A comprehensive laboratory testing program was carried out using concrete cuboids, controlling their size, shape and compressive strength. These are used as a proxy for broken ore fragments. These results were used to develop empirical design charts for assessing secondary fragmentation and hang-ups potential. Several factors influencing the secondary fragmentation for feasibility and advanced engineering assessments have been investigated including: air gap thickness, BOD, segregation of large blocks due to draw column surface topology, broken ore strength heterogeneity, block strength damage and crushing under high confining stresses, water within draw columns, and cushioning by fines. This new knowledge will contribute to more accurate secondary fragmentation predictions at the drawpoints. Finally, a new empirical approach to predict secondary fragmentation and drawpoint block size distribution (BSD) directed at early-stage conceptual and feasibility engineering design studies is developed. This methodology, built with relevant data from related fields and supplemented by generated data, was tested against field data from the El Teniente mine, Chile, confirming satisfactory predictions for stronger rocks and mixtures of strong and weak broken ore materials. The results were not as reliable for predicting drawpoint BSD for weak rocks.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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