No more lost and diluted ore

MOUNT Gibson Iron, having jettisoned the poly pipe technique of monitoring blast-related ore movement, is happy with the high-tech monitoring method recently trialled at its Tallering Peak iron ore mine in Western Australia.

Tallering Peak annually produces about 3 million tonnes of ore via bulk and selective hematite mining using five-metre benches, according to company grade control geologist Sandra Pfeifenberger. The bulk ore tended to have higher grades of around 65% iron. In contrast, grades were more variable towards the top of the ore body.

“It consists of pretty narrow strings … So we really need to keep track of where we move our ore to when we blast it, otherwise we just lose it,” Pfeifenberger said.

If movements in the boundaries of an ore block weren’t accounted for, the company would experience ore loss in some parts of the ore block and dilution in others. Pfeiferberger put estimates on the hypothetical losses and dilution on a relatively small ore block using the ore body evaluation software Surpac. The size of the block was 3000 tonnes.

She said the volume of ore loss was about 660 tonnes, which at an estimated ore value of about $70 per tonne, came to a value of some $46,000. The area of the ore block affected by dilution was about 28%.

One argument against bothering to monitor ore block boundary movements might be the practice of spotting ore during digging. However, Pfeiferberger said, lower grade material looked too similar to the high grade ore for that to be a successful approach.

In the past at Tallering Peak, blast monitoring of ore was done using the passive, visual technique of installing lengths of poly pipe into the block before it was blasted.

“We didn’t have very good experiences with poly pipe,” Pfeiferberger said.

Recoveries ranged between zero and 70% and were typically far below 50%. The rest disappeared in the blast heave.

“We had to drill a lot of monitor holes to stick the pipes in and that obviously makes the cost and time of your drilling program a lot bigger.”

Pfeiferberger said that one of the biggest disadvantages of poly pipe was that it only measured movement at the surface of the muckpile, “which is a lot different to the movement [within] the bench”.

“The top of the blast moves less and it also moves more randomly, so you don’t really get any good data.”

She said poor poly pipe recovery and data meant narrow ore zones were quite often lost. Pfeiferberger said another prompt to ditch poly pipes was talk at the mine of moving to 10m blast benches. Concerns about the efficacy of the mooted change led to the search for a more comprehensive monitoring method.

After some shopping around, mine staff contacted Blast Movement Technologies (BMT) and arranged a case study at Tallering Peak to assess the firm’s electronic blast monitoring system.

Use of the BMT system, which originated from work by the Julius Kruttschnitt Mineral Research Centre, involves installing a number of monitors down holes drilled into the ore block to be blasted.

The pre-blast location of each monitor is surveyed, the blast is carried out, and then a walk-over is done on the muckpile to detect where the monitors have moved to.

Pfeiferberger said locating monitors post-blast was quite easy once the operator had become familiar with using the technology.

Post-blast surveying of the monitors is followed by the uploading of all the monitoring data to computer, so that the software can produce three-dimensional movement data in table and diagram format. That data permits adjustments to be made to the ore block boundaries to reflect their new, post-blast positions. With the BMT monitoring system in place, the quality of ore mark-up at Tallering Peak had improved, Pfeiferberger said. She said that as someone who sat on diggers to spot ore, she was able to confirm that result first-hand.

Pfeiferberger also relayed the comment from the mine’s grade reconciliation geologist that mined grades were believed to have improved since the mine began using the BMT monitors. A lot of dilution had been eliminated and mine staff were able to more accurately predict the grade of crushed product.

“The new technique allows us to recover ore from narrow zones that has been excluded from the reserves, which in our case in turn leads to an increase in net present value,” Pfeiferberger said.

Tallering Peak had budgeted on using the monitoring system in 2007-08, particularly for tracking narrow ore zones and the margins of bulk ore, and Pfeiferberger said she was fairly sure the operation would continue using the electronic monitors beyond that.

The mine has chosen to stick with five-metre benches for sampling reasons, but as it turned out, the BMT system revealed that moving to bigger benches would not have been too much of a worry. This was because, compared with five-metre benches, the movement of ore during blasting was in fact more uniform and involved less-random shifts at the top and bottom of the block’s depth profile.

Sandra Pfeifenberger

Mount Gibson Iron

08 9346 0913

Darren Thornton

Principal consultant

Blast Movement Technologies

07 3202 9499

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