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Boliden Tara mine is an underground mine, Europe’s largest zinc mine. The world’s ninth largest. It has been in operation since 1977 and produces approximately 2.6 million tons of zinc and lead concentrate annually. With proven reserves of 17 million tons, and an extensive exploration program, Boliden Tara mines plans to operate for many more years. Hence the need for upgraded ventilation.
It fell upon raiseboring contractor Bergteamet Europe AB to pilot and ream the fresh air intake no:5 raise. Bergteamet has vast experience in raiseboring. Their history stretches back to the mid-70’s when Skanska raiseboring where formed. Bergteamet acquired Skanska raiseboring in 2005. Later the raiseboring part of NCC was incorporated into the company. Lately Master Drilling, based in South Africa, has stepped in as a major shareholder in Bergteamet. The head office of Bergteamet is located in Boliden, Sweden. The same town where the Boliden group has its roots.
Bergteamet utilized the Atlas Copco Robbins 91RH-C raiseboring machine, the RXL reamer and MVKC6 cutters to complete the raise.
The Robbins 91RH-C
The Atlas Copco Robbins 91RH-C is a low profile raise drill designed for larger raises. It features a hydraulic drive system, ground loading pipe loader and semi-automatic wrenching. In order to be “soft” on both man and drill string. Operational capacity is 6700kN of thrust and a maximum running torque of 450kNm. Nominal capacity for the 91RH-C is 4.5m diameter by 600m long raises. However under the right circumstances it can do far more.
The RXL reamer
The RXL reaming system is modular and versatile. The diametrical capacity ranges from 2.5m up to 5.0m as standard. By stretching the envelope, diameters of up to 6.6m have been drilled using this system. One of our most versital reaming systems where the design focus has been on user friendliness and handling.
The MVKC6 or 6-row cutter
What makes the 6-row cutter unique is its double tracking. The whole reamer can be dressed with only one cutter type. Since the insert rows on the cutters in the same row on the head work the same rock kerf, it acts similar to a reamer running at double the rpm. This feature, together with enhanced carbide and 38mm (1-1/2”) kerf spacing makes this cutter a winner in all applications where enough loading capacity is available. Advantages seen are: longer service life of cutters, increased penetration per reamer revolution and decreased torque requirements to turn the reamer.
Piloting sequence
During the piloting sequence. A RVDS guide tool from Micon was used. Normally deviation over the length of a pilot when such a tool is used should be less than 0.1%. In this case, the pilot hole was straight and accurate down 780m, then the tool started to malfunction. Since the requirement from Boliden was that the deviation should be less than 1.5% over the entire length of the raise, the decision was taken that the pilot hole should be completed using the faulty tool. Over the rest of the way down to the 848 meter break through the pilot hole deviated a meter.
Reaming the raise
At the start of the raise the initial loading was less than 8 tons per cutter, but the Atlas Copco 91RH-C raisedrill still managed to ream an average of 0.5m/hour at 5 rpm. As the weight of the drill string was reduced the rate of penetration picked up. The average penetration rate for the full raise length was 0.88 meter/hour at just over 9 tons of load per cutter on average. The greatest advancement in reaming achieved in a 24 hour period was 26 meters. In the harder sections loading was increased to 15 tons /cutter to keep the rate of advancement.
The 848 meters was completed in a short period of time. The raise was reamed from November 28th 2015 to February 9th 2016. This time period included a break during the Christmas period and some down time required to change out a gearbox.
Apart from the inner cutters of the reamer showing evidence of the long raise, the rest of the cutting structure looks great, with an average of more than 80% life left on the cutting structure. There was some matrix wear on some of the cutters due to running in cuttings but, In general, it looks like these cutters will be re-greased, and sent off to the next stage for this rock star.
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