According to Webster's dictionary, "pyrophorc"[1] means "igniting spontaneously, as certain finely divided materials". The word "pyrophoric" has also been defined[2] as "producing sparks, when rubbed, or burning spontaneously in air, e.g. finely divided metals". Thus, a pyrophoric material is any material that ignites spontaneously or emits sparks when rubbed, scratched, or struck. Chambers' technical dictionary [3] defines pyrophoric powders as "finely divided powders which take fire or oxidize extremely rapidly when exposed to the air; usually a metal or a mixture of a metal and its oxide". By another definition, [4] a pyrophoric material is "any liquid or solid that will ignite in air below 130°F (54.4°C)." In consideration of the many fire and explosion incidents that have occurred involving the handling, transportation, and shipment of many materials in such diversified physical forms as metal machine turnings, chips, pellets, derbies, massive metal, etc., any definition of pyrophoric material should not be restricted to finely divided powders. "Pyrophoricity" (spontaneous ignition or rapid oxidation) as used in its broadest sense, therefore, must also include massive pieces of material, pure metals, alloys, carbides, hydrides, non-metallic materials, and even liquids such as the aluminum alkyls.
A simplified definition of "ignition" is the initiation of combustion, [5] and combustion may be defined as the process by which materials are oxidized at a rapid rate. [6] Combustion may also be defined as "an exothermic, self-sustaining reaction involving a fuel (condensed or gas phase) and an oxidant". Old newspapers tend to turn yellow and brittle with age because of a relatively slow oxidation rate. Since paper is a carbohydrate, the yellowing is a result of the oxidation or slow burning of carbon and hydrogen. Oil is another carbon compound that oxidizes in contact with air. When oily rags are left to accumulate around the house, for example, the heat of oxidation may not be able to escape and after enough heat accumulates, the oily rags may be set aflame. This process is known as spontaneous combustion.
Ignition is the process of initiating self-sustained combustion. If the ignition is caused by the introduction of some small external flame, spark, or glowing object, it is called "pilot-ignition". If ignition occurs without the assistance of an external pilot source, it is termed "autoignition" or "spontaneous ignition". In a chemical context, all elements that are not in their highest oxidation state can undergo oxidation. Some metals oxidize so slowly that the heat generated during oxidation is dissipated so that the ignition temperature is never reached. Under appropriate conditions, some metals oxidize rapidly in the presence of air, oxygen, and/or moisture, and can generate sufficient heat to reach ignition temperatures. Certain metals, such as zirconium and uranium, are described as combustible metals because of their potential for rapid oxidation under appropriate conditions. The use of magnesium as a flashlight powder for photography is a common example of a combustible metal. Ignition is accomplished by raising the temperature of a substance or material to the "ignition temperature". This temperature may be defined as the lowest temperature which will cause a flame or glow, to start and spread through the substance or material. The substance or material need not be a solid, but could be a gas or mechanical mixture of combining substances such as carbon and oxygen. The ignition temperature varies considerably with the substance. Some typical values are:
| Substance | Ignition Temperature °F |
|---|---|
| Phosphorus | 93 |
| Sulfur | 470 |
| Carbon | 750 |
| Hydrogen | 1090 |
| Carbon Monoxide | 1250 |
Ignition has been defined by Setchkin [7] as a "self-perpetuating exothermic reaction that is initiated at the temperature of incipient oxidation, and that increases the temperature of the reactants above the initial air temperature until an explosion, flame, or sustained glow occurs."
In actuality, ignition can occur without an accompanying visible flame, sustained glow, or explosion, such as in metal powers where this type of ignition is termed "thermal ignition". The ignition temperature of a metal has been defined by Pitts [8] as "the temperature at which the exothermic oxidation reaction becomes self sustaining at such a rate as to cause a significant temperature increase above ambient conditions".
For a good discussion of the basic definitions and fundamentals of ignition, combustion, oxidation, reactions of materials, flame, fire, and other terms related to pyrophoricity, the reader is referred to the Fire Protection Handbook.[9]
In a previous review of pyrophoricity in the nuclear materials field, Smith[10] concludes that for a metal powder of a particular size, there exists a definite minimum quantity of metal for spontaneous ignition to occur. Therefore, any metal capable of reacting exothermically with oxygen should also be capable of spontaneously igniting in air if the metal is sufficiently finely-divided and if a sufficient quantity is present.
The minimum temperature at which a substance ignites spontaneously is termed its "autoignition temperature". The autoignition temperature of a substance, whether solid, liquid, or gaseous, is defined [11] as the lowest temperature required to initiate or cause self-sustained combustion in the absence of a spark or flame. This temperature varies considerably depending upon the nature, size, and shape of the igniting surface or container and other factors. The autoignition temperature of a solid is influenced by its physical condition and the rate of heating.
Walker,[12] in a discussion of the ignition of porous solids by gaseous oxidation, states that "the ultimate state of combustion is a state of inflammation or of glowing combustion, maintained by exothermic chemical reaction between atmospheric oxygen and the material under consideration (or its pyrolysis products). This state of combustion is preceded by an ignition mechanism, where exothermic processes raise the temperature to a point where combustion can begin. These exothermic processes may cover a wide range of chemical, biochemical, and physicochemical mechanisms".
The difficulty in defining "ignition" or ignition processes in solids is illustrated by the history of Sub-Committee XVI of the American Society for Testing Materials on the Ignitability of Coal and Coke Formed During 1937 [13] to investigate the ignition characteristics of solid fuels. In 1939 [14] the Committee reached an agreement on a proposed definition of the term "ignitability," as follows:
The work of the Subcommittee was terminated in 1950 [15],[16] "until a better definition of the term (ignitability) was presented, more was known of the relation between coal's reactivity, ignition characteristics, etc. and its use. ..."
Combustion is often described as a "rapid oxidation with the evolution of a large amount of heat," as for example when organic substances such as paper, wood, coal, etc., are burned with atmospheric oxygen, but such a simplified definition may not be adequate. Frank-Kamenetskii [17] defines combustion as "the name given to the occurrence of a chemical reaction under conditions of progressive self-acceleration which are brought about by the accumulation of heat or catalyzing products of the reaction in the system. Such products which catalyze or accelerate the reaction, such as reaction chain carriers, would include free atoms, radicals, organic peroxides, etc. Frank-Kamenetskii then subdivides combustion processes into two main classes (1) thermal combustion, and (2) chain or autocatalytic or diffusive combustion. The rise in temperature in thermal combustion is the main cause of the reaction acceleration. In a detailed discussion of some basic combustion processes, Frank-Kamenetskii describes the basic distinguishing features of various types of combustion, including heterogeneous combustion (e.g., coal) and homogeneous combustion (e.g., a mixture of hydrogen and oxygen gases or a mixture of carbon monoxide and hydrocarbons with oxygen).
Over the years, some very interesting definitions of ignition and ignition temperature have been advanced. Bunsen18 described ignition as "the lowest temperature at which the constituents of a gas mixture combine," Van't Hoff, [19] Gibbs, [20] Schultes, [21] "the temperature at which the rate of generation of heat becomes greater than its rate of dissipation". Byersdorfer [22] stated that "ignition is an occurrence which produces a visible combustion". Nernst [23] said that it is "that temperature to which a point of the system must be heated to cause combustion"; Brown [24] "the temperature in the combustible at which the rate of heat developed by the reactions inducing ignition just exceeds the rate at which heat is dissipated by all causes, under the given conditions".
In a study of the ignition temperature of various papers, woods, and fabrics, Graf [25] had tested numerous samples that had exothermic reactions in which the rate of heat generation in the sample was considerable, but there was no visible combustion until a higher temperature was reached. He defined the ignition temperature of a material as "that temperature at which the rate of heating in the substance being testing exceeds the rate of heating induced by the external source of heat and has visible combustion in the form of a glow or flame as an end result".
It is suggested by Tetenbaum, et.al., [26] that the numerous definitions of ignition could perhaps be distilled into the statement that "Ignition can take place when the rate of heat production by a self-sustaining chemical reaction exceeds the rate of heat lost to the surroundings". The ignition temperature is not an intrinsic property of a substance inasmuch as it depends upon such factors as size, amount, and shape of material, nature of contamination, conditions of heating, etc.
References
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