Abstract

This paper focuses on introducing the use of digital imaging processing techniques to obtain the overall size distribution of rock blocks at drawpoints. A photogrammetric analysis code (WipFrag) and an automatic photograph & analysis tool (PortaMetrics) are used to characterise secondary size distribution. Preliminary tests on performance of WipFrag and PortaMetrics in capturing particle size distribution were carried out in this paper.

To the authors’ knowledge, no significant research has been carried out to date relating caved ore size and hardness on mill performance, also referred to as a Mine-to-Mill approach. It is anticipated that the results of this work will feed into a larger initiative at the University of British Columbia pursuing research on Cave-to-Mill, a proposed new approach to integrate cave mining with milling processes to improve production and forecasting capabilities. The variable and relatively uncontrollable nature of cave fragmentation is considered to be a key distinguishing feature of the suggested Cave-to-Mill approach when compared with typical Mine-to-Mill strategies for open-pit mines.

1. INTRODUCTION

Block caving is one of the most cost effective underground mining techniques, typically employed to mine massive low grade deposits. A general block caving mine layout is shown in Figure 1, the ore material being mined sequentially in large blocks, with base areas of thousands of square meters. Caving is initiated by blasting an underground tunnel (called undercut) under the block of ore that is to be excavated. As caving of the ore is initiated, the undercut is connected with the production level by blasting bell-shaped ore passages (drawpoints). The broken ore material falls into the production level through the drawpoints, then transported to the crushers and, subsequently, brought to the surface. As broken ore is removed from the drawpoints, the ore above continues to break and cave in by gravity.

Ore fragmentation in a block caving process includes three stages: i) in-situ fragmentation; ii) primary fragmentation and iii) secondary fragmentation. In-situ fragmentation is the original condition of rock fragmentation before the undercut is blasted. Loading conditions imposed on the rock mass by the caving process leads to primary fragmentation, as a combination of failure along existing discontinuities and fracturing of the intact rock. Secondary fragmentation is the reduction in size of primary fragmentation blocks as they move down through the ore column to the drawpoints.

Secondary fragmentation has long been recognized as a key parameter for evaluating block caving productivity and assessing the possibility of hang-ups, which is caused by large boulders becoming stuck within inside the drawpoints. Whereas fragmentation controls the overall success and profitability of a block caving operation, it is extremely difficult to measure in a reliable and routinely manner. For example, in-situ fragmentation can be estimated based on the characterization of the natural fracture network and the use of Discrete Fracture Network (DFN) techniques [1], while secondary fragmentation can be potentially

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