Emulsion Explosives: The wave of future

For a long time nitroglycerine (NG) based gelatine explosives were the only products available for blasting in hard rock. Alfred Nobel developed NG gelatine in the 19th Century. They suffered from defects like headache during usage, excessive fumes taking long time to clear after blasting, high hazard of accidental detonation and in some cases very poor water resistance. In 1950’s slurry explosives were developed which could overcome all these defects – they were non-headache causing, had low fumes, excellent water resistance and lower sensitivity to accidental detonation. However, the blasting performance of slurry explosives did not match that of NG gelatines. Therefore, many blasters continued to use NG gelatines. During last one decade or so, emulsion explosives have replaced slurries.

Slurry Explosives

Slurry (Water gel) explosives are based on saturated aqueous solutions of AN(usually with sodium nitrate) in which fuels, sanitizers are dispersed. The liquid phase is thickened with gums and gelled with cross-linking agent to keep the solids in suspension, provide a satisfactory degree of cohesiveness and maximize water resistance. When completely gelled, water gels have a rubbery, porride-like consistency.

Typically slurries are made following the following procedure: Pre-dissolving the ammonium nitrate in a small amount of water. Thickening the solution with a guargum or starch. Adding fuel components as soluble or finely divided insoluble materials

(solid1). Adding dry oxidizers to reduce the overall water content (solid2). (Optionally) Crosslinking the gum thickeners to produce a gelled product.

Watergels are sensitized by air/gas bubbles and sometimes with other volatile fuel sensitizers. Energy fuels include aluminium powder and distillate.

The presences of solids (solid 1 and solid 2) as well as the ammonium nitrate crystals that precipitated upon cooling of the formulation lead to the general designation of 'slurries' for these composite blasting agents/explosives.

Emulsion ExplosivesDue to the presence of solid ingredients in the explosive mixture in case of Slurries, the intimacy between the fuel and the oxidiser and consequently the efficiency of the reaction, had still got a lot of room for improvement. Hence Emulsion-Based products were developed replacing the solid ingredients by liquid ones. This led to an explosive whose performance was a class above slurries. An emulsion-Based explosive consists of the following ingredients

Oxidizers that are responsible for "Burning" the mixture generally consist of a blend of Nitrates viz. Ammonium Nitrate (maximum percentage), Sodium

Nitrate (for lowering the fudge point temperature at which emulsification occurs), Calcium Nitrate and Zinc Nitrates. Ammonium Nitrate (NH4NO3), upon combustion breaks down into Nitrogen, Hydrogen and Oxygen, all of which are gases. Hence, the chemical reaction is much more complete when oxidizers such as these are used.

Fuels that are the molecules that "Burn" in the mixture comprises of Hydrocarbons like Paraffin and Microcrystalline Waxes and Oils that form a continuous medium/phase in the intimate mixture.

Emulsifiers are the agents/surfactants, which keep the above two components in an intimate bond. The emulsifier molecules are responsible for keeping the small size oxidizer droplets (typically less than 5 microns) surrounded firmly and uniformly by the Fuel phase so that the efficiency of the reaction is maximum.

Figure: Emulsion matrix

Sensitizers are the agents inside an Emulsion-based explosive that render the hitherto insensitive emulsion an explosive. During the production stage, this element is added just before the final cartridging of the explosive. Typical chemical sensitizers used in Emulsions are Nitrites, which fill the voids between the Fuel-coated Oxidizer droplets with minute air bubbles. The adiabatic compression of these air bubbles upon the initiation of the detonation reaction create the "hotspots" which result in the release of intense amounts of heat and pressure within very short time periods which both keep the reaction going as well as send enough shock outside the confinement to break the surrounding rock. Physical sensitizers used in Emulsion-Based explosives include Glass Micro Balloons.

Dopants are the solid, inert elements added to the Emulsions to render specific requirements into the mixture. For example, aluminum powder is added to increase the Energy/Strength of the explosive, but reduces the Velocity of Detonation (VOD) of the chemical reaction front. The following table shows the characteristic sizes of oxidizers and their effect on explosive performance

Emulsion explosives are characterised by very intimate mixing between oxidisers and fuels. Oxidisers, which are solutions of ammonium nitrate, calcium nitrate, sodium nitrate, etc, are dispersed in continuous fuel medium comprising of oils, waxes and other hydrocarbons. The result of such intimate mixing is reaction, which is very rapid and complete. It means high velocity of detonation (VOD), high-energy efficiency and lesser fumes than slurry explosives. Typical values for slurry and an emulsion are given in table 1.

Table – 1: Slurry and Emulsion Comparison:

TypePowerex 80

SlurryPowergel 801

EmulsionRelative Wt. Strength % Anfo

110 118

Energy Efficiency % 85 93Noxious Fumes , l/kg 46 45Impact Sensitivity*, cm 40 90

( * Impact Sensivity is measured by dropping 5 kg weight steel on explosives sample kept on sand paper on a steel anvil )

Emulsion explosives are highly water-resistant also, making them most suitable in watery conditions.

Figure-1 : Structure of emulsion under Electron Microscope

Explosive Size (mm)

Form VOD (Km/s)

ANFO 2.00 All Solid 3.2

Slurry 0.20 Solid/Liquid 3.5-4.0

Emulsion 0.001 Liquid 4.0-5.0

3500 times expanded 500 times expanded Av oxidiser droplet size 5 micron Av gas bubble size 50 micron

3500 times expanded picture of emulsion under Electron Microscope