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Better blastholes cut production costs.

Mining of the Bjorkdal gold mine in northern Sweden started in 1985. Blastholes (76 mm) were originally drilled by two Atlas Copco ROC crawler rigs, a 712 and a 748, using T45 extension rods. Initial production of 2 Mt/'y was then doubled by increasing the hole size to 102 mm and using T51 extension rods. Also, souffle blasting was introduced. This raises and fragments the rock mass without dislocating the ore from its original position, thereby allowing very selective loading and minimising dilution.

However, the increase 'in hole diameter was not without problems. It was difficult to clean the holes and drilling was relatively slow. The layer of rock debris that was always present on the benches was the problem. But a change to drilling with Coprod, using an Atlas Copco ROC 942 CR that was delivered in 1993, solved the problem. The Coprod system easily accommodated drilling through the rock debris (1-2 m deep), and helped keep the hole walls stable.

Coprod 89 with 115 mm bits was used first. The 115 mm holes made it possible to increase burden and spacing from a maximum 3 x 3.5 m with 102-mm bits to 3.25 x 4 m. At the same time, the penetration rate in the larger holes rose by some 30%

Coprod also virtually eliminated hole deviation, and it was therefore possible to further increase hole diameter. Coprod 102 was introduced with 127 mm blastholes for production drilling and 140 mm in waste. Consequently, burden and spacing could be further increased to 3.25 x 4.8 m and 3.3 x 6.4 m respectively. The maximum deviation when drilling at maximum hole depth of 18 m is 0.2m.

Two more Coprod rigs were purchased, one ROC 952 CR in 1995 and one in 1996, as production demands increased. The powerful COP 4050 CR rock drills on these two Atlas Copco rigs were prototypes of the COP 4050 ME (Coprod version) released early in 1998. The hydraulic design of the impact system on the COP 4050 ME-CR has been improved resulting in longer wear part life and longer service intervals. One spare rock drill has been sufficient to cover the servicing of the three rigs after every 10,000 m drilled.

Atlas Copco has had service and maintenance agreements covering Bjorkdal's rigs, drills and drilling tools. Between 1993 and last year, consumable life was very satisfactory, considering the relatively hard and abrasive rock, Penetration rates vary from a minimum of 0.7 m/min up to 1.5 m/min. In a 12-month period up to late 1998, 9 Mt of material was mined (ore and waste) from 179,000 m of drilled blastholes. The change from extension equipment to Coprods reduced production cost per tonne by more than 25%, and allowed the mine to increase total production year after year.

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Title Annotation:improved drilling at gold mine in Bjorkdal, Sweden
Publication:Mining Magazine
Date:Feb 1, 1999
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