The Henderson Mine is a large underground panel cave molybdenum mine located near Empire, Colorado. Secondary blasting requirements have varied from one shot per 27 tonnes (30 tons) to one shot per 900 tonnes (1,000 tons) of ore. Geology, structure, cave abutment loads, and location of the ore in the column all affect the amount of oversize boulders (greater than three-fourths cubic meter or one cubic yard) that require secondary breakage.
Factors Affecting Secondary Blasting Requirements
Abutment loads are generated in a new panel cave operation as undercut rings are shot and the mining operation develops an ever increasing horizontal dimension to the cave. Vertical cave growth at Henderson occurs at the rate of about 0.3 m (1′) per day during production within the orebody. Unbroken, in-place ore directly above a caved drawpoint is supported as a cantilever load by solid, uncaved rock along the cave line.
As the cave area increased in size on 7700 and vertical growth continued, the cave breached the 8100 level cave. This minimizes the 7755 undercut level abutment load as the cantilever beam is “broken” at the 8100 level interface that is overlain by broken rock. The 8100 cave breakthrough to the surface occurred over 900 m (3,000′) above the production level, compared to just 122 m (400′) between the 7700 level and 8100 level cave.
Ore that is located just above the region blasted during bell and undercut ring development travels about 21 m (70′) vertically down to the drawpoint.
Secondary Breakage Techniques
Henderson utilizes an LHD system of draw, and the entire production area is designed to utilize rubber tire equipment. This allows the use of mobile drill tractors for secondary blasting. Drilling allows the most efficient use of the explosive charge, and minimizes blast damage due to lower explosive requirements vs “bombs”.
“Cone charges” became the normal method for blasting about three-fourths of the oversize rock in the mine. The timing for implementation of this new method of blasting oversize rock was perfect as the new 7700 level was producing about one oversize rock per every 27 to 54 tonnes (30 to 60 tons) of ore drawn.
The basic difference between a cone charge and bomb is that a cone charge is a shaped charge, with the primer located at the apex of the cone. Detonation energy from the explosion is directed into the rock the cone is placed upon. With a typical bomb, primer location and charge shape are not considered, and much energy is lost to concussion, which also can cause extensive damage to ventilation control doors in the production area.
Emulsions and water gels from several explosive manufacturing companies were tested. Final selection was based on price, availability, packaging, and most importantly, a viscous, sticky consistency that allows the loaded cones to adhere to a “clean” rock at any angle from horizontal to approaching vertical. All explosives broke rocks when tested as cone charges, however some were only useable when placed on a horizontal surface.
The secondary blasting crew (2-3 people per shift) has a large powder truck, as well as a tractor they use to transport their supplies. To load funnels, they place about 25 funnels apex down on a sheet of plywood with circular cut-outs. A double overhand knot is tied at the end of a 0.7 m (2.5′) piece of 25 grain E-cord which is then dropped knot-last into each funnel. Emulsion chubs are cut into proper lengths and packed with the plastic skin into the funnels.
Rocks larger than three-fourth m³ (one yd³) that fit in a 5.4 m³ (7 yd³) LHD bucket and can readily be pulled from under the drawpoint brow are moved to a drawpoint that has already been drawn for the day, or is developed but not on draw. Rocks may be moved 16 m to 122 m (53′ to 400′) depending on the availability of a storage drawpoint.
The 35 mm (1-3/8″) drill holes are loaded with a partial stick of 31 mm (1¼”) diameter gelatin dynamite. Detonation is by 25 grain E-cord, connected to a 10.5 or 25 grain E-cord trunk-line. The trunk- line is used to detonate all secondary shots that occur in the production area. The trunk-line is detonated by a nonel cap which is detonated by a conventional shotgun shell primer. The primer fires when struck by a blow from a solenoid contained in a “remote initiator” that is energized by an electric current from the capacitor discharge blasting station.
Despite the fact that bomb shots are not as efficient as cone charges or drilled shots, they comprise about 1% of the secondary blasting at Henderson. Bombs are 2.7 kg (6 lb) emulsion chubs tied onto 25 mm x 50 mm x 4.9 m (1″ x 2″ x 16′) (or two connected together to increase the reach) wooden “bomb sticks” that are used to place and hold the emulsion in position prior to blasting.
Very high hangups (over 20′ from the brow) or those that are comprised of very large boulders (0.5% of total hangups) are moved down to the drawpoint area with ANFO shots. A primer, made up of E-cord and a partial stick of emulsion is placed in an empty burlap 23 kg (50 lb) ANFO bag.