Air decking in blasting enables explosive energy to act repeatedly in separate pulses on the surrounding rock mass, rather than instantly, as in concentrated charge blasting. Varidrill director Steve Gilmartin reveals how air decking can reduce carbon emissions and provide other benefits in blasting.
The issue of climate change and reducing our carbon footprint is high on everyone’s environmental agenda. This article explains how the carbon dioxide (CO2) produced in a quarry blast can be calculated and offers some ideas on how it can be reduced. For simplicity, ammonium nitrate/fuel oil (ANFO) has been used as the main explosive. Bulk and cartridge explosives will give slightly different results, but as ammonium nitrate forms a large percentage of most commercial explosives, the following will provide a reasonable base to work from.
Explosives work by producing large quantities of gases. ANFO mixed in the standard proportion of 94 per cent ammonium nitrate (AN) to six per cent fuel oil (FO) will produce water in the form of steam, nitrogen and CO2. The quantity of CO2 produced can be calculated by adding up the molecular weights of the chemical reaction between the FO and the AN. Thus, every tonne of ANFO used produces 176kg of CO2.
As a practical example, a quarry producing one million tonnes of rock per year, using ANFO with a blast ratio (BR) of seven tonnes per kilogram, would require 143 tonnes of ANFO, producing around 25 tonnes of CO2.
However, if the BR was at the lower end, eg four tonnes per kilogram, that would require 250 tonnes of ANFO, producing some 44 tonnes of CO2, a 76 per cent increase in emissions. Thus, the BR can have a significant impact on the carbon footprint as well as fragmentation.
Any well run quarry should already be maximising its BR to ensure optimum fragmentation, good floor conditions, face stability and to reduce its carbon footprint. But has air decking been considered?
When explosives are detonated in a blasthole they produce a single high amplitude stress wave which crushes the blasthole and moves further into the surrounding rock, producing a crack mechanism. During or after the stress wave propagation, high temperature/pressure gases assist and extend the crack formation and produce the expansion and movement of the rock mass. The blasthole pressure produced by commercial explosive is far in excess of that required to fracture the rock, so if explosive weight can be reduced without compromising fragmentation, floor conditions or final wall stability, this would offer advantages to the quarry operator. By incorporating an air column (air deck) above or within the explosive column, secondary or multiple stress waves are produced which extend the duration of their action, thus increasing the extent of crack propagation.
The reduced blasthole pressure caused by the air deck is still capable of creating an extended fracture system plus sufficient high pressure gas to obtain the desired amount of ground movement. The lower peak blasthole pressure reduces the loss of explosive energy associated with excessive crushing of the rock adjacent to the blasthole.
Air decks can be created at any point down the length of the blasthole simply and effectively by using gasbags. Gasbags are high strength balloons capable of “locking off” a blasthole and bearing heavy explosive loads for a long period of time. They consist of an outer polypropylene bag, and an inner “balloon” containing an aerosol. They are activated by pressing a button on the aerosol (easily accessible through the wall of the balloon) and lowered to the desired depth in the borehole using a tape or string hooked onto a plastic loop on the bag. A built-in delay allows sufficient time to get it into position. A sharp tug on the string breaks the loop so the hook can be retrieved.
As an environmental aid, a simple air deck between the top of the explosive column and the stemming is easily achieved by stopping explosive delivery into the hole, for example two metres below the normal column length. The hole is sealed with a gasbag at normal stemming height then stemmed off on top of the gasbag. In the case of ANFO in a 115mm hole, this will reduce the explosive by about 16kg per hole or about 18 per cent in a 15m hole with a corresponding reduction in CO2 emissions.
Another topical consideration in these days of changing weather patterns is the considerable effort being put into sea defences, which means an increase in the requirement for armoured stone. Armoured stone production can be successfully achieved using a base charge of high explosive and creating a large air gap by positioning a gasbag at stemming height. Typically, this achieves rock sizes from six to 15 tonnes. If necessary, two decks with air gaps can be created to achieve intermediate rock sizes (four to eight tonnes). A few trial holes will establish the optimum charge and stemming height.
Environmental considerations are not the only reasons for using gasbags. A similar technique to that used to create armoured stone can be employed when pre-splitting to create smooth, stable final walls. A simple, quick and cheap method is to use air deck techniques where about 10 per cent of the hole is filled with explosive and a gasbag is placed two metres from the top (depending on the competence of the rock). Experience has shown that the spacing between holes can also be increased compared with traditional detonating cord methods.
Water is often a problem in blastholes, particularly if bulk ANFO is being used. Sealing off up to two metres of static groundwater in a borehole can be very effective. The shockwave from the explosive creates a pulsed infusion effect through the water which breaks out the toe.
Gasbags can also help in maximum instantaneous charge reduction. Instead of decking the holes using chippings, a gasbag can create an air deck. The explosive will work within this air deck instead of having “dead” areas within the hole as would happen when chippings are used. Gasbags can also be used to seal off cavities or to create extra confinement within holes as an added precaution in sensitive areas, such as close to power lines.
Steve Gilmartin is the director of Varidrill Ltd, based in the United Kingdom. Article courtesy of Quarry Management (UK). Visit agg-net.com