Two basic forms of energy are released when high explosives react. The first type of energy will be called shock energy. The second type will be called gas energy. Although both types of energy are released during the detonation process, the blaster can select explosives with different proportions of shock or gas energy to suit a particular application. If explosives are used in an unconfined manner, such as mud capping boulders (commonly called plaster shooting) or for shearing structural members in demolition, the selection of an explosive with high shock energy would be advantageous. On the other hand, if explosives are being used in boreholes and are confined with stemming materials, an explosive with a high gas energy output would be beneficial.
Figure 1: Pressure Profiles for Low and High Explosives
(Source: Konya and Walter, 1990)
To understand the difference between the two types of energy, compare the difference in reaction of a low and high explosives. Low explosives are those which deflagrate or burn very rapidly. These explosives may have reaction velocities of two to five thousand feet per second (600 to 200 m/s). Such explosives produce no shock energy. They produce energy only from gas expansion. A very typical example of low explosive would be black powder. On the other hand, high explosives detonate and produce not only gas pressure, but also another energy or pressure which is called shock pressure. Figure 1 shows a diagram of a reacting cartridge of low and high explosive.
High explosives are explosive materials that detonate, meaning that the explosive shock front passes through the material at a supersonic speed ranging from 3 to 9 km/s. A high explosive detonation exhibits shock pressure at the reaction front which travel through the explosive before the gas energy is released. Generally the shock energy is higher pressure than the gas energy. After the shock energy passes, gas energy is released. The gas energy in detonating explosives is much greater than the gas energy released in low explosives. In a high explosives, there are two distinct and separate pressures. The shock is estimated to account for only 10% to 15% of the total available useful work energy in the explosion. The gas pressure accounts for 85% to 90% of the useful work energy and follows thereafter. However, the gas energy produces a force that is constantly maintained until the borehole ruptures.
Figure 2: Underwater Test Record Sample
The shock energy is commonly believed to result from the detonation pressure of the explosion. The detonation pressure is a function of the explosive density times the explosion detonation velocity squared and is a form of kinetic energy. Determination of the detonation pressure is very complex. There are a number of different computer codes written to approximate this pressure. Unfortunately, the computer codes come up with widely varying answers. Until recently, no method existed to measure the detonation pressure. Until that time occurs, one could use this formula to achieve a number that may approximate the detonation pressure.
The detonation pressure or shock energy can be considered similar to kinetic energy and is maximum in the direction of travel, which would mean that detonation pressure would be maximum in the explosive cartridge at the end opposite that where initiation occurred. It is generally believed that the detonation pressure on the sides of the cartridge are virtually zero, since the detonation wave does not extend to the edges of the cartridge. To get maximum detonation pressure effects from an explosive, it is necessary to place the explosives on the material to be broken and initiate it from the end opposite that in contact with the material. Laying the cartridge over on its side and firing in a manner where detonation is parallel to the surface of the material to be broken reduces the effects of the detonation pressure. To maximize the use of detonation pressure, maximum contact area between material and explosive is required.
Figure 3: Mud Cap Blasting
When the charges confined are detonated in boreholes, then the expanding gas energy cause majority of the rock to break. The gas pressure, often called explosion pressure, is the pressure that is exerted on the borehole walls by the expanding gasses after chemical reaction has been completed. Explosion pressure results from the amount of gases liberated per unit weight of explosive and the amount of heat liberated during the reaction. The higher the temperature produced, the higher the gases pressure. If more gas volume is liberated at the same temperature, the pressure will also increase. For a quick approximation, it is often assumed that explosive pressure is approximately one-half of the detonation pressure.
It should be pointed out that this is only an approximation and conditions can exist where the explosion pressure exceeds the detonation pressure. This explains the success of ANFO which yields a relatively low detonation pressure, but relatively high explosion pressure. Explosion pressures are calculated from computer codes or measured using underwater tests. Explosion pressures can also be measured directly in boreholes, however, few of the explosive manufacturers use the new technique in rating their explosives. A review of some very basic explosives chemistry helps one to understand how powdered metals and other substances effect explosion pressures.
Please remember that the action of explosion pressure is important in loosening, expanding and throwing material.
Figure 4: Nomograph of Detonation and Explosion Pressure
Chemical explosives are materials which undergo rapid chemical reactions to release gaseous products and energy. These gases under high pressure exert forces against borehole walls which causes rock to fracture.
The elements which comprise explosives, are generally considered either fuel elements or oxidizer elements (Table 1). Explosives use oxygen as the oxidizer element. Nitrogen is also a common element in explosives and is in either a liquid or solid state, but once it reacts it forms gaseous nitrogen. Explosives sometimes contain ingredients other than fuels and oxidizers. Powdered metals such as powdered aluminum are used in explosives. The reason for the use of the powder metals is that, upon reaction, powdered metals give off heat. The heat formed heats up the gases, which result from other ingredients, causing a higher explosion pressure.
Table 1: Explosive Ingredients
Explosive may contain other elements and ingredients which really add nothing to the explosives energy. The other ingredients are put into explosives to decrease sensitivity or increase surface area. Certain ingredients such as chalk or zinc oxide serve as an antacid to increase the storage life of the explosive. Common table salt actually makes an explosive less efficient because it functions as a flame depressant and cools the reaction. On the other hand, the addition of table salt allows the explosive to be used in explosive methane atmospheres because the cooler flame and shorter flame duration makes it less likely that a gas explosion would occur. This is the reason that permissible explosives are used in coal mines or in tunneling operations in sedimentary rock where methane is encountered.
This basic elements or ingredients which directly produce work in blasting are those elements which forms gases when they react, such as carbon, hydrogen, oxygen, and nitrogen.
When carbon reacts with oxygen, it can either form carbon monoxide or carbon dioxide. In order to extract the maximum heat from the reaction, we want elements to be completely oxidized or in other words for carbon dioxide to form rather than carbon monoxide. Table 2 shows the difference in heat released when one carbon atom forms carbon monoxide versus the case where one carbon atom form carbon dioxide. In order to release the maximum energy from the explosive reaction, the elements should react and form the following products:1.) Carbon reacts to form carbon dioxide
2.) Hydrogen reacts to form water (Figure 5)
3.) Liquid or solid nitrogen reacts to form gaseous nitrogen (Figure 6)
Table 2:Heats of Formation for Selected Chemical Compounds
Figure 5: Hydrogen-Oxygen Ideal Reaction
Figure 6: Nitrogen-Nitrogen Ideal Reaction
If only the ideal reactions occur from the carbon, hydrogen, oxygen, and nitrogen, there is no oxygen left over or any additional oxygen needed. The explosive is oxygen balanced and produces the maximum amount of energy.
If two ingredients are mixed together, such as ammonium nitrate and fuel oil, and an excess amount of fuel oil is put into mixture, the explosive reaction is said to be oxygen negative. Thus means that is not enough oxygen to fully combine with the carbon and hydrogen to form the desired end products. Instead, what occurs is that free carbon (soot) and carbon monoxide will be liberated (Figure 7).
Figure 7: Non-Ideal Carbon-Oxygen Reaction
If too little fuel is added to a mixture of ammonium nitrate and fuel oil, then the mixture has excess oxygen which can't react with carbon or hydrogen. This is called an oxygen positive reaction. What occurs is that the nitrogen which is normally an inert gas will be changed from nitrogen gas to an oxide of nitrogen (Figure 8). If oxides of nitrogen are formed, they will form rust colored fumes and reduce the energy of the reaction.
Figure 8: Non-Ideal Nitrogen-Oxygen Reaction
The energy is reduced because nitrogen oxides absorb heat in order for them to form. This can be seen in Table 2. Water and carbon dioxide have a negative sign which means they give off heat when they form. On the other hand, the nitrogen oxides have a plus sign meaning that they take in heat when the form.
Please remember that the net result is that the reaction will occur at a lower temperature. The gas pressure is lowered if the reaction temperature is lowered.
Figure 9: Identification of Problem Mixture of ANFO and Fuel Oil
Figure 10: Energy Loss in ANFO