GEOTECHNICAL ENGINEERING & BLASTING: SLURRY EXPLOSIVES

A slurry explosive is a mixture of ammonium nitrate or other nitrates and fuel sensitizers which can either be a hydrocarbon or hydrocarbons and aluminum. In some cases explosive sensitizers, such as TNT or nitrocellulose, along with varying amounts of water are used (Figure 1). An emulsion is somewhat different from water gel or slurry in characteristics, but the composition contains similar ingredients and functions similarly in the blasthole (Figure 2). In general, emulsions have a somewhat higher detonation velocity and in some cases, may tend to be wet or adhere to be blasthole causing difficulties in bulk loading. For discussion purpose, emulsions and water gels will be treated under the generic family of slurries.

Figure 1: Slurry Formations

Figure 2: Emulsion Explosives

Slurries, in general, contain larger amounts of ammonium nitrate and are made water-resistant through the use of gum, waxes, cross linking agents or emulsifiers. A number of varieties of slurries exist, and it must be remembered that different slurries will exhibit different characteristics in the field. Some slurries may be classified as high explosives while others are classified as blasting agents since they are not sensitive to a number 8 blasting cap. Slurries functioned as blasting agents can be delivered as separate ingredients for on-site mixing. The separate ingredients brought to the job site in large tank trucks is non-explosive until mixed at the blasthole. The bulk loading of slurries can greatly reduce the time and cost of loading large quantities of explosives (Figure 3). However, slurries can be broken down into two general classifications, cartridge and bulk.

Figure 3: Slurry Bulk Loading Truck

Cartridged Slurries

Cartridge slurries come in both large and small diameter cartridges. In general, cartridges less than two inches in diameter are normally made cap-sensitive so that they can be substituted for dynamite. The temperature sensitivity of slurries and their lower sensitivity can cause problems when substituted for some dynamite applications. The blaster must be aware of some of the limitations before he tries a one-for-one substitution. The larger diameter cartridge slurries may not be cap-sensitive and must be primed with cap-sensitive explosives. In general, large diameter slurries are the least sensitive. Cartridge slurries are normally sensitized with monometholamine nitrate or aluminum, and air sensitized in the case of emulsions. Air sensitizing is accomplished by the addition of microspheres or entrapping air during the mixing process itself.

Figure 4: Slurry Explosive Manufactured by Gulf Oil Corporatin Limited

Bulk Slurries

Bulk slurries are sensitized by one of three methods. Air sensitizing can be accomplished by the addition of gassing agents which after being pumped into the blasthole produce small gas bubbles throughout the mixture. The addition of powdered or scrap grade aluminum to the mixture also increases sensitivity. The addition of nitrocellulose or TNT to the mixture will sensitize it to initiation. Slurries containing neither aluminum nor explosive sensitizers are the cheapest. They are often the least dense and the least powerful. In wet conditions where dewatering is not practiced, low cost slurries offer competition to ANFO. It should be pointed out that these low cost slurries have less energy than ANFO. Aluminized slurries and those containing significant amounts of other high explosive sensitizers produce significantly more energy and are used for blasting harder dense rock. The alternative to using high energy slurries is pumping blasthole, where possible, with submersible blasthole pumps (Figure 5 & Figure 6) and using polyethylene blasthole liners within the hole with ammonium nitrate as the explosive (Figure 6). In most applications, the use of pumping with polyethylene blasthole liners and ammonium nitrate will produce blasting cost which are significantly less than would result from using higher priced slurries.