Why does wood burn but not iron

Pyrophoric and self-igniting substances

Pyrophores (Greek for fire-bearing) in the real sense are substances that spontaneously ignite in air even in small quantities at room temperature. Their ignition temperature is below, at or at most just above room temperature. When air comes in, they glow almost instantly and ignite. These substances and bodies typically have a very large specific surface (often - similar to a sponge - a fissured structure) that absorbs oxygen from the air so quickly that the heat of oxidation that develops in the process leads to inflammation.

In the case of self-igniting substances in general, a silent oxidation takes place, the reaction heat of which cannot be dissipated to the environment due to poor heat dissipation, but remains in the material and therefore leads to self-heating and possibly self-ignition after a certain storage time and above a certain size of the substance accumulation. Typically, these are flammable solid substances that are finely divided with a relatively large surface area.

The pyrophores include, for. B. the following substances:

phosphorus

White or yellow phosphorus (tetraphosphorus) can quickly ignite in a finely divided state in air; as a rod, the ignition temperature in air is 45 ° C to 60 ° C. Inflammation also occurs from friction or impact. Phosphorus is therefore stored under water and also cut under water. With oxygen carriers there is an explosive reaction. Extinguishing media: water; after extinguishing cover with damp sand or earth.

Pyrophoric metals and metal compounds

Pyrophoric sulfur-iron compounds, which are formed from iron oxide (rust) by organic sulfur compounds, are particularly dangerous. B. on the walls of containers with sulfur-containing flammable liquids (crude oil, benzene and the like.). Pyrophoric iron sulphides or iron mercaptides glow when exposed to air and can then ignite fuel / air mixtures. Therefore, special care should be taken when opening and cleaning such devices and containers. Containers may only be opened after they have cooled down sufficiently.

During the warm storage of bitumen, deposits of iron sulfur compounds and coke-like compounds can form on the inner surfaces of the tank roof. Under certain conditions, these can be self-igniting or pyrophoric, glow when exposed to air and thus ignite any potentially explosive atmosphere. Product can self-ignite on the insulation material, which is why it must be structurally ensured that no bitumen or product condensate gets into the tank insulation. Powders and shavings of light metals are particularly pyrophoric, with moisture increasing the risk of spontaneous combustion. Fine iron powder is also pyrophoric and can only be stored in the absence of air. Powder that trickles out when an ampoule breaks glows instantly and burns.

Metal alkyls

Aluminum compounds ignite immediately in air and also react very violently with water to form flammable gases. They cause very severe burns on the skin. The vapors irritate the mucous membranes and cause bronchial damage. Water, open flames and sparks must be kept away. Magnesium alkyls as well as the unpleasant-smelling liquids dimethyl zinc and diethyl zinc ignite in the air and cause burns and chemical burns. In the case of syntheses in the laboratory, work must be carried out in the fume hood; flammable substances that are not immediately required for continuing work must be removed from the fume cupboard. Suitable extinguishing agents are to be kept ready.

Nickel carbonyl

Chemical formula: Ni (CO) ~ 4. The very toxic liquid is created when carbon monoxide is passed over nickel powder. The surface of the liquid is sufficient, e.g. B. if spilled on the table or floor, to immediate spontaneous combustion. In such cases, dilute immediately with water. The vapors are heavier than air and, when mixed with air, are explosive within certain limits. Keep container tightly closed and store in a cool place. Keep open flames and sparks away. Do not inhale vapors. Wear protective clothing and respiratory protection (CO filter).

Hydrazine

Chemical formula: N ~ 2H ~ 4. Hydrazine is a toxic liquid compound of nitrogen and hydrogen, also called diamide. It is very easy to oxidize and, when dripped onto large-area materials (e.g. diatomite, rust), decomposes with the appearance of fire. It has such a strong oxidizing effect on organic substances that they ignite in the air. The vapors are slightly heavier than air and explosive when mixed with air (explosion limits 4.7-100 vol%; flash point 40 ° C; boiling point 113 ° C). Protective clothing, gloves, safety glasses and respiratory protection are required when working with hydrazine.

Gases that self-ignite on contact with air: Examples of corresponding raw gases are: Diborane (hydrogen boron), dichlorosilane, phosphine (hydrogen phosphide; in the case of fumigation, preparations that develop hydrogen phosphide must contain an additive that prevents self-ignition), monosilane (hydrogen silicon ; its self-ignition is reduced by adding hydrogen). Self-igniting residues must be expected in raw gas pipes: 1,3-butadiene can form a white, popcorn-like polymer ("popcorn") in the gas phase (preferably in storage tanks and dead spaces in pipelines), which is self-igniting in the air. It must be kept moist with water until disposal.

Self-igniting substances and groups of substances that are not pyrophores, but pose a considerable fire hazard if handled incorrectly, are e.g. B .:

Metal powder

Coarser powders of aluminum, lead, iron or magnesium can easily be oxidized. The powder form also means a large surface area of ​​the mass, which glows due to the heat of oxidation and finally ignites. Iron filings that are wetted with oils that tend to self-ignite also glow. Zinc dust in storage can self-ignite when exposed to moisture. In the Ordinance on Hazardous Substances, the following are specified as spontaneously flammable in air: magnesium powder, aluminum powder, zinc powder and zirconium powder. Accumulated aluminum powder can lead to self-ignition due to post-oxidation and thus to fires and explosions.

coal

Fine-grained and pulverulent coal, especially in bulk storage, can be dangerous if the heat generated during the oxidation cannot flow off sufficiently. Coal must not be stored near hot pipes, warm walls, etc. Temperatures inside coal deposits must be constantly monitored, the stratification must be kept as low as possible. Inerting with protective gas is recommended in bunkers.

Pyrite

Pyrite (sulfur or iron pebbles) in finely divided form can ignite relatively easily, especially in the presence of moisture. Easily flammable substances, e.g. B. wood, can quickly catch fire due to the heat of oxidation released by the pyrite. Storage should therefore take place in as small heaps as possible and only for a short period of time; containers should be blown through with inert gas.

Used cleaning rags, cleaning wool: Oily cleaning wool and cleaning rags, fibers soaked with drying oils and certain metal soaps (metal salts of organic acids), which are supposed to accelerate the drying of paints, can tend to self-ignite. The oil that is absorbed in the fiber provides a large surface area for the oxygen in the air to react; the temperature can rise to the point of ignition of the fiber. Oily cleaning rags and the like must be collected in special, non-flammable and labeled containers with automatically and tightly closing lids, removed from the workrooms at the end of the day and stored in a fire-safe place until they are destroyed or transported away.

Paint residue

Paint residues from oil paints, from synthetic resin paints that contain oil-modified or oil-combined alkyd or epoxy resins, and from polyester paints tend to self-ignite. Reason are the unsaturated organic compounds (unsaturated fatty acids, linoleic acids, maleic acid and the like.) The z. B. can react exothermically with atmospheric oxygen. Such waste, like oily cleaning rags, must therefore be kept in appropriate containers (protection from the effects of heat, daily emptying). In addition, during cleaning work, self-ignition can occur if the cleaning agent and coating material come into contact; furthermore, electrostatic charging must be expected during cleaning work (ignition sparks).

If nitrocellulose-containing paint residues or solutions are worked up by distillation, the distillation residue (sump) must be drained into lockable metal containers after cooling, in which water is provided because of the risk of self-ignition. The metal containers should be kept in a place where no hazard can arise even if the residue ignites, e.g. B. outdoors.

Oil coal (etc.)

On air heaters and on the pressure vessels connected to them, which are charged with compressed air from compressors with oil-lubricated pressure chambers, self-igniting deposits, e.g. B. oil carbon occur. This is why an expert must carry out a relevant test after the first 500 hours of operation. If necessary, samples must be taken and their self-ignition temperature determined. If the self-ignition temperatures of the deposits are in the range of the operating temperature of the heated compressed air, the operator must take further measures (lowering the temperature of the heated compressed air, use of suitable types of oil) to avoid fires and explosions.

Hay, straw, flax, tobacco

These and other protein-containing natural products tend to self-ignite if they are insufficiently dried and stored in high layers. The temperature increase due to oxidation is preceded by a warming by bacterial activity, which z. For example, hay is heated to 70-80 ° C and so develops so much heat that air oxidation sets in, which leads to inflammation.

literature

  • Ordinance on protection against hazardous substances (Hazardous Substances Ordinance - GefStoffV) (CHV 5)
  • UVV production and processing of aluminum powder (BGV D 13)
  • UVV hazardous substances (VSG 4.5)
  • Operation of work equipment (BGR 500)
  • Handling magnesium (BGR 204)
  • Organic peroxides (BGI 752)
  • Safety-related parameters - determination and evaluation (BGI 747)
  • Thermal safety of chemical processes (BGI 828)
  • Warm storage of bitumen (BGI 5041)
  • Leaflet M 035 "Aluminum alkyls", BG Chemie
  • GESTIS substance database

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