Observation of ore pass system performance at Brunswick mine
URANIUM 2010, Saskatoon, SK
J. Hadjigeorgiou, K. Esmaieli, and R. Harrisson
The Brunswick mine, operated by Xstrata Zinc, is an underground lead-zinc-copper-silver mine that has been in operation since 1964 and produces almost 10,000 t/d. The orebody consists of close to 10 sub-parallel massive sulphide lenses striking north-south and dipping 75º west; the overall strike length is 1,200 m with a width of up to 200 m. The orebody extends from surface to a depth of close to 1,200 m.
Brunswick mine covers a wide area resulting in the development and operation of multiple ore pass systems. Historically, the mine has constructed close to 25 ore pass systems, with a total length of 7,200 m. Currently, only nine systems are still in operation, with the remaining abandoned either due to depletion of material to be transferred or due to operational failures (critical expansion of the ore pass section).
This paper reports on work carried out to develop a site-specific database of ore pass systems and their performance. Information is provided on the ore pass systems including excavation methods, configuration, etc. At Brunswick, the length of ore pass sections varies from under 25 m to over 300 m. It was also noted that 41 sections of ore passes were rectangular, 44 were square and 13 were circular. Ore pass inclination is critical as it controls material flow. Shallow sections may restrict flow, especially if a high proportion of fine material is present, while steeper excavations result in higher material velocities and compaction. At Brunswick, ore pass inclination varied between 45o and 90o, with the mean inclination being 66.2o. Material flow is hindered at inclinations less than 70o. Chutes with doors or fingers are the most popular flow control infrastructures at Brunswick mine, although the majority of ore pass sections at the mine are not equipped with flow control infrastructures.
The paper also presents two case studies of ore pass degradation. The first case study refers to the 1000 South Fill Raise (1000SFR ore pass) commissioned in the early 1990s. It was developed as a 3 m diameter, unsupported raise bored ore pass comprised of two long sections. The ore pass had displayed significant degradation in the upper section along the hanging and foot wall. Although the induced stresses around the ore pass were comparatively higher than the strength of the rock mass, there was no significant degradation in the lower ore pass section.
The upper section of the 1000SFR was developed sub-parallel to the host rock foliation and bedding. This unfavourable orientation was made worse by the high induced stresses and resulted in wear and abrasion of the ore pass. The lower part of the ore pass was developed in a favourable orientation and has maintained its integrity. Another explanation was that it was easier to maintain the lower part of the ore pass full, thus favourable ore pass geometry and good practice mitigated ore pass wear.
The second case study relates to the 18-21 ore pass system serving mining zones 20 and 21. This is a high-stress area displaying considerable seismic activity. Ore passes 19 and 21 were abandoned in 2004 due to considerable enlargement of their cross-sections. Ore passes 19 and 19A merged, thus necessitating the backfilling of ore pass 19 in order to inhibit further expansion. Damage to ore pass 18 was, however, considerably smaller than other ore passes. This was explained by the presence of an adequate distance between the ore pass and other infrastructure and because the expansion of nearby ore passes resulted in a stress shadow region for ore pass 18. This ore pass complex was constructed to a large degree in a competent rock mass in massive sulphide rocks. The north and south walls of ore passes 19, 19A and 21 were further damaged by the combination of high-stress conditions and the impact of material transferred through ore passes 19 and 21.
Ore pass systems, Database, Degradation, Underground mining