permissible explosive[pər′mis·ə·bəl ik′splō·siv]
an explosive designed for use underground when there is a possibility of the formation of explosive mixtures of natural gas or combustible dust with air. Permissible explosives contain mainly ammonium nitrate, trinitrotoluene, or esters of nitric acids. Their special features result from the introduction of significant amounts (12 to 75 percent by weight) of components that inhibit the oxidation of methane and other combustibles and lower the temperature of explosion. These components include potassium chloride, sodium chloride, and ammonium chloride. The action of the chlorides is enhanced when they are used in finely ground form; if the chlorides are formed during the explosion, their effect is increased as a result of ion exchange between salts, for example, between ammonium chloride and potassium nitrate.
Safety properties are greater in selectively detonating permissible explosives, which contain 10 percent nitroglycerin and 90 percent ion-exchange salts. In blastholes with strong walls, such explosives detonate with full release of energy. In an open charge, only the nitroglycerin detonates, resulting in a very low energy release and temperature of the explosion products; this eliminates the danger of gas ignition.
In the USSR, there are six classes of explosives. The first two classes include nonpermissible explosives, which are used in open work and in mines where there is no danger of the accumulation of gas or dust; permissible explosives are placed in classes III-VI. The velocity of detonation of class III permissible explosives including AP-4ZhV, AP–5ZhV, and PZhV-20 ammonites and VP–4 Pobedit, ranges from 3.6 to 4.6 km/sec, with a heat of explosion of about 200 joules/g. Uglenit no. 7, with a velocity of detonation from 1.6 to 1.8 km/sec and a heat of explosion of about 90 joules/g, is in class VI. The use of each class of permissible explosives is strictly regulated according to the specific conditions of the underground mining operation.
REFERENCEDubnov, L. V., N. S. Bakharevich, and A. I. Romanov. Promyshlennye vzryvchatye veshchestva. Moscow, 1973.
L. G. BOLKHOVITINOV