HomeGoldInverse box hole blasting at Kusasalethu Mine

Inverse box hole blasting at Kusasalethu Mine

Inverse box

A mechanically drilled
tunnel for inverse
drop raising

AEL Mining Services recently partnered with EBJ Mining Construction to execute mechanised, inverse box hole blasting at Harmony Gold’s Kusasalethu (formerly Elandsrand) mine, which has resulted in increased safety and production.

Kusasalethu, which employs over 5,000 people, has an estimated life of mine of 28 years, which means operations could continue to 2037. The mine straddles South Africa’s provinces of Gauteng and the North West, near the town of Carletonville. Harmony acquired the mine, which comprises a twin vertical and subvertical shaft system, in January 2001. It subsequently undertook the Elandsrand deepening project, which involved the development of a new mine beneath the original Elandsrand mine. That project targets the southern, deeper portion of the higher-grade Ventersdorp Contact Reef pay shoot at depths of between 3,000 and 3,600 metres.

Inverse box hole blasting has been successfully used in platinum mining. In conventional box hole development in gold mining applications, an incline raise is used to create an ore pass by manually drilling down the host rock to advance to the orebody.

Eugene Fouche, acting mine manager at Kusasalethu, comments, “In the past, we used long hole mechanical (raise boring) and short hole conventional development. With the increase in the reef development rate, this method proved to be slow and labour intensive, resulting in a backlog of box holes. We changed to a mechanised method, combining inverse box holes with drop raising.

“This new mechanised method entails drilling the ore pass from the bottom of the rock upwards at an angle using an automatic drilling machine. This minimises the number of people physically involved in the drilling and blasting of the ore passes.”

Inverse box hole drilling also ensures a more accurate holing point without fracturing the rock, thereby reducing the chances of rock slips and rockrelated injuries. “The current advance rate for conventional box holing is on average 1.2 m per day, compared to inverse blasting of 2.0 m per day,” Fouche says.

Kusasalethu has converted to the DigiShot range of electronic initiation systems supplied by AEL’s sister company, DetNet, which eliminates the need for blast operators. Electronic detonators also ensure that the timing and precision of each blast is accurate.

The hole is blasted with a single blast up to 15 m from the bottom up. The remainder of the ore pass is blasted from the top of the raise. A homogenous emulsion with glutinous properties is used to charge the hole, and ensure that the explosives do not fall out of the predrilled hole when initiating the blast.

Johan Fourie, regional manager at AEL comments on the blasting of the inverse drop raise hole, “With the traditional box hole development method, it would take approximately 12 days to advance 15 metres before loading and blasting the hole. This process could take longer in some of the older gold mines where reef exposure is minimal. With inverse drop raising, we are able to drill the whole length of the tunnel in six days and then blast on the seventh day, reducing the turnaround time from twelve to seven days.”