March 2001Analytical Instrumentation & Environmental Monitoring
The operational difficulties within a mining environment have severe implications such as blockages within an ore pass which not only have financial repercussions but also the risk of loss of life. Further to this, the control of dust emission levels in the mining shaft is vitally important in order to minimise health hazard risks. One of the greatest contributors to this problem is when ore is passed down between levels and further increased tenfold when several ore passes are in use simultaneously. Should the skip or hoist be nonoperational or the chute (flask) be shut to control production outflow, the dust levels in each higher level can rise dramatically.
The solution was to provide noncontact level control in these passes which addresses all the problems above with the added downtime cost saving and the reduced visits of inspection crews suspended into the ore pass to investigate a blockage.
Installation and commissioning
The Krohne BM70A had to be installed in one of the most arduous environments. The measurement window was a 100 m long ore pass having a diameter up to 3 m. The only available positioning access was the mouth of the ore pass, which is in close proximity to the rail tracks of the diesel locomotives which pull the ore-laden carriages. The carriages are emptied at the ore pass mouth by a forwards and backwards 'jerking' movement over a cantilever mechanism. At this point any protruding instruments should be highly susceptible to severe damage. Previously installed instrument protective covers or shrouds were also found to be highly susceptible to damage and were therefore not used. The BM70A solution entailed suspending the instrument by four chains from a steel deck mounted over the mouth. The suspension height had to be varied such that the BM70A would be prevented from damage by the falling ore from the carriage and any adjoining ore passes. The BM70A local display was removed from the housing and retrofitted into a panel-mounted enclosure attached adjacent to the steel deck. This allowed operators to do in situ programming and commissioning of the BM70A and have a continuous local level indication.
Focusing the BM70A into the 'dog leg'- shaped ore pass proved more difficult than anticipated, due to zero light conditions and more so the varying diameter of the ore pass due to the rock outcrops. The Krohne software program 'Pc-Cat' was used as an alignment tool, to accommodate the varying degrees of reflections from the outcrops, which were made visible in the power spectrum as the BM70A was repositioned by varying the chain linkages attached to the steel deck.
Result
The maximum depth measured was 91 m, with an acceptable uncertainty due to the level status in the ore pass at the time of commissioning.
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