The energy efficiency of the blasting process can be improved by using Varistem stemming plugs from Midrand-based mining consumables supplier ERG Industrial.

For mining operations and blasting contractors, explosive energy that escapes into the surrounding environment is energy that is not contributing to breaking and moving rock, explains ERG Industrial MD Eugene Preis.

Varistem stemming plugs are designed to change that equation by improving energy retention within the blast, allowing a greater proportion of the explosive energy to be directed into the rockmass.

By helping the blast energy work harder in the rock, Varistem plugs can contribute to improved fragmentation and productivity, while reducing unwanted blast impacts including airblast, flyrock, noise and vibration, he says.

Varistem is currently used by mining operations for all major commodities, quarrying or civil operations in 68 countries across the globe, with the highest concentration of users being in Africa. A higher-than-average concentration of Varistem users are in South Africa, Zambia, the Democratic Republic of Congo, Mali, Burkina Faso, Guinea, Côte d’Ivoire and Senegal.

Particularly in South Africa, Varistem stemming plugs are used by 38 surface mining operations, five underground mining operations and 71 quarrying sites.

The vast majority of Varistem stemming plug users use the product for productivity gains, on-the-bench procedural improvements and to reduce flyrock, airblast, noise and vibration, or a combination of these impacts.

“Different types of operations have different overall blasting objectives. For example, one of the objectives when blasting manganese and iron-ore is to reduce the amount of fines generated from blasting,” explains Preis.

He elaborates that, for example, fine iron-ore and manganese reach significantly lower prices on the market as compared to the lump product.

However, one of the objectives when blasting gold or platinum group metals ores is the opposite, whereby the rock needs to be fragmented as fine as possible to reduce the crushing and milling energy requirements downstream.

“The core principle of improved energy retention is universally true for the Varistem stemming plugs, regardless of the commodity being mined. The site-specific blasting objectives define the most valuable way in which to apply the Varistem to a mining operation,” he states.

Development, Deployment

The development of Varistem stemming plugs was driven by a simple objective: to reduce the wasting of blast energy in conventional production blasting, explains Preis.

“As drilling and blasting are the first steps in the mining production chain, even incremental improvements in blast efficiency can deliver downstream benefits across loading, hauling and crushing.

“While eliminating energy loss entirely is not possible, ERG Industrial set out to reduce a portion of this wastage, with the expectation that even modest gains in blasting efficiency could deliver significant returns across the full value chain,” he adds.

Varistem stemming plugs are manufactured from a combination of different vinyl plastisols through a process called dip moulding. The combination of the raw material and the manufacturing process produces a flexible, pliable plastic with a high tear strength.

Varistem works in the critical moments immediately after blast initiation and before detonation occurs. As pressure rapidly builds within the blasthole, the pressure wave travels down the explosive column towards the Varistem plug positioned above the explosives.

The plug’s pliable plastic is forced into the small gaps between the plug, blasthole wall and stemming material, creating a temporary pressure seal. This increases resistance to energy escaping upwards and redirects more of the pressure into the surrounding rockmass, activating existing microfractures and weakening the rock ahead of detonation.

“Once the explosive fully detonates, the plug is destroyed; but its work is already done. By retaining more of the initial blast energy within the blasthole and rockmass, Varistem helps the subsequent detonation transfer a greater proportion of its chemical energy into breaking the rock,” states Preis.

The use of Varistem stemming plugs is easy, he notes, adding that operators can be trained to install the plugs correctly in a “very short time”.

To support installation, ERG Industrial supplies capped and sleeved aluminium poles fitted with depth indicators, available in lengths ranging from 1.5 m to 9 m, in increments of 1.5 m and 3 m owing to the use of connector pieces.

The poles are manufactured from aluminium to meet the requirement for non-ferrous tools when working on live blasting benches, ensuring they are suitable for use in these environments.

“There are no additional safety or skill requirements for the use of Varistem,” says Preis, adding that Varistem is one of the only stemming devices that complies to the South African National Standards’ 120:2009 standard for stemming use in blasting, which is a legislative requirement of the Department of Mineral and Petroleum Resources.