Abstract

In underground mining, rock fragment size is a key variable for productivity because it impacts equipment selection and mining structure design, such as for drawbells and ore passes. During ore flow, those structures are usually affected by interferences such as hang-ups and issues associated with oversized rocks or fine materials. Fine materials could imply problems with mudrush and/or inrush of fines. The rock fragment sizes must be estimated, in particular, in block caving where fragmentation develops naturally while the broken ore is being drawn. Nowadays, estimation tools and models are available to estimate fragmentation, but some variables have yet to be studied to understand their effect on the fragmentation process. One such variable whose mechanisms have yet to be studied is the residence time, which in block caving directly depends on the distance that fragments travel from their original position in the broken ore to the drawpoint. A greater degree of fragmentation is expected for longer distances. In this study, granular material fragmentation was characterized during its flow to understand its characteristics and mechanisms. Tests were done using a laboratory setup built to emulate various heights where material is drawn. The fragment-size distribution was measured before and after tests for clay, concrete fragments, and gravel. It was found that for 6 m of travel distance, fragmentation mainly occurs in minor sizes where d(10) decreases by 4.7% for gravel, 6.2% for concrete, and by 63.1% for clay. The breakage of fragment singularities occurred mainly within medium and smaller sizes, increasing fine material. Thus, the main fragmentation mechanism in experiments was identified as abrasion between rock fragments where fine material continuously increases as distance increases in spite of the high rock strength and the piston flow movement used. Then, in block caving mines, the higher the draw column, the more fine material is expected to be generated.