source:  Handbook of Fire Prevention Engineering
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What can cause Spontaneous Combustion?
common list that most people don’t think about
Linseed oil
activated carbon
charcoal briquettes
Alfalfa meal
Animal hides
Castor oil
Charcoal
Coal
Cottonseed oil
Fertilizers
Fish meal
Fish oil
Lanolin
Lard oil
Linseed oil
Manure
Metal powders
Olive oil
Peanut oil
Powdered eggs
Soybean oil
Used burlap
Whale oil
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Diethyl zinc. Diethyl zinc is an organo-metal compound and a dangerous fire hazard. It spontaneously ignites in air and reacts violently with water, releasing flammable vapors and heat. It is a colorless pyrophoric liquid with a specific gravity of 1.2, which is heavier than water, so it will sink to the bottom. It decomposes explosively at 248 degrees Fahrenheit. It has a boiling point of 243F, a flash point of -20F and a melting point of -18F. The four-digit United Nations (UN) identification number is 1366.
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Pentaborane. Pentaborane is a nonmetallic, colorless liquid with a pungent odor. It decomposes at 300F if it has not already ignited and will ignite spontaneously in air if impure. It is a dangerous fire and explosion risk, with a flammable range of 0.46 to 98% in air. The boiling point is 145F, the flash point is 86F and the ignition temperature is 95F, which is extremely low. Any object that is 95F or above can be an ignition source. Ignition sources can be ordinary objects on a hot day in the summer, such as the pavement, metal on vehicles and even the air. The UN identification number is 1380
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Aluminum alkyls. Aluminum alkyls are colorless liquids or solids. They are pyrophoric and may ignite spontaneously in air. Aluminum alkyls are pyrophoric materials in a flammable solvent. The vapors are heavier than air, water reactive and corrosive. Decomposition begins at 350F. The UN identification number is 3051. The NFPA 704 designation is health 3, flammability 4 and reactivity 3. The white space at the bottom of the diamond has a W with a slash through it, indicating water reactivity. They are used as catalysts in polymerization reactions.
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Aluminum phosphide. Aluminum phosphide (AlP) is a binary salt. These salts have the specific hazard of giving off poisonous and pyrophoric phosphine gas when in contact with moist air, water or steam. They will also ignite spontaneously in contact with air. This compound is composed of gray or dark yellow crystals and is a dangerous fire risk. Aluminum phosphide decomposes on contact with water and has a specific gravity of 2.85, which is heavier than water. The UN identification number is 1397.
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Potassium sulfide. Potassium sulfide (K2S) is a binary salt. It is a red or yellow-red crystalline mass or fused solid. It is deliquescent in air, which means it absorbs water from the air, and it is also soluble in water. Potassium sulfide is a dangerous fire risk and may ignite spontaneously. It is explosive in the form of dust and powder. It decomposes at 1,562F and melts at 1,674F. The specific gravity is 1.74, which is heavier than air. The UN identification number is 1382. The NFPA 704 designation is health 3, flammability 1 and reactivity 0. Potassium sulfide is used primarily in analytical chemistry and medicine.
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Sodium hydride. Sodium hydride (NaH) is a binary salt that has a specific hazard of releasing hydrogen in contact with water. It is an odorless powder that is violently water reactive. The UN identification number is 1427. The NFPA 704 designation is health 3, flammability 3 and reactivity 2. The white space at the bottom of the diamond has a W with a slash through it, indicating water reactivity.
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White phosphorus. White phosphorus (P), also known as yellow phosphorus, is a nonmetallic element that is found in the form of crystals or a wax-like transparent solid. It ignites spontaneously in air at 86F, which is also its ignition temperature. White phosphorus should be stored and shipped under water and away from heat. It is a dangerous fire risk, with a boiling point of 536F and a melting point of 111F. The UN identification number is 2447. The NFPA 704 designation is health 4, flammability 4 and reactivity 2. The primary uses are in rodenticides, smoke screens, and analytical chemistry.
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An incident occurred in Gettysburg, PA, involving phosphorus being shipped under water in 55-gallon drums. One drum developed a leak and the water drained off. This allowed the phosphorus to be exposed to air, which caused it to spontaneously ignite. The fire spread to the other containers and eventually consumed the entire truck. The ensuing fire was fought with large volumes of water and in the final stages covered with wet sand. Clean-up created problems because as the phosphorus and sand mixture was shoveled into over-pack drums, the phosphorus was again exposed to air and reignited small fires.
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A train derailment in Brownson, NE, resulted in a tank car of phosphorus overturning and the phosphorus igniting upon contact with air. Phosphorus is shipped under water so there was water inside the tank car. CHEMTREC was called and responders were told correctly that the phosphorous would not explode. However, the water inside the tank car was turned to steam from the heat of the phosphorus fire. The pressure from the steam caused a boiler-type of explosion that had nothing chemically to do with the phosphorus!
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Carbon-based animal or vegetable oils, such as linseed oil, cooking oil, cottonseed oil, corn oil, soybean oil, lard and margarine, can undergo spontaneous combustion when in contact with rags, cardboard, paper or other combustibles.
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Biological processes occur within organic materials such as hay and grass clippings. The activity of biological organisms within the material generates heat that is confined by the materials themselves until the ignition temperature is reached and ignition occurs.
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Slow oxidation is a chemical reaction. Chemical reactions may produce heat; reactions that produce heat are considered exothermic. If the heat is insulated from dissipating to the outside of the material, it will continue to build up. As the heat builds, the material is heated from within. The process continues until the ignition temperature of the material is reached and ignition
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Hydrocarbon compounds usually undergo slow spontaneous combustion.
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Flammable solids, Class 4.2 materials are spontaneously combustible. The DOT defines them as pyrophoric materials.
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When some carbon-based materials, such as activated carbon or charcoal briquettes, are in contact with water, an oxidation reaction occurs between the carbon material, the water, and pockets of trapped air. This type of spontaneous combustion occurs slowly. The reaction is exothermic, which means heat is produced in the reaction and slowly builds up until ignition occurs spontaneously

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facts about oils and fuels that do not spontaneous combust

Spontaneous heating cannot occur in the case of petroleum oils or other hydrocarbon materials that are saturated. Ordinary petroleum products, such as motor oil, grease, diesel fuel and gasoline, do not have a double bond in their chemical make-up. For that reason, the oxidation reaction that occurs with animal and vegetable oils and the oxygen in the air does not occur. Therefore, those materials do not undergo spontaneous combustion!

This fact may come as a surprise to some people because there have been numerous fires blamed on soiled rags with those products on them. The fact is that saturated flammable liquids do not spontaneously ignite and cannot start to burn without some other ignition

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Here is another interesting bit of information from the same source:  Handbook of Fire Prevention Engineering
Thermit Welding

Thermit Welding is a welding process in which heat produced during an exothermic reaction is used to weld two metal pieces (conductors) together. No external heat source is required during this welding process, but only an external heat source is needed to initiate the process. thermite material ( mixture of a metal oxide, aluminium powder, and fuel) is used for the welding process. The chemical reaction that produces heat is an aluminothermic reaction. The reaction of aluminium powder with a metal oxide to produce heat is called an aluminothermic reaction.

This welding process is also known as exothermic welding, exothermic bonding and thermite welding.
It was invented in the year 1898 by Hans Goldschmidt when he improved the aluminothermic reaction.

Working principle

In thermit welding process, a thermite material which is a composition of a metal oxide, aluminium and fuel is used to achieve exothermic reaction. A reaction between the metal oxide (mostly iron (iii) oxide) and aluminium powder is takes place. During reaction the metal oxide reduces to free elemental metal and aluminium get oxidized to aluminium oxide with production of a large amount of heat (about 3310 oC). The heat generated melts the elemental metal ( Fe) and then this molten metal poured into the mold to join the two metal pieces together.

During the aluminothermic reaction, the reduction of iron oxide and oxidation of Al takes place. The large amount of heat generated during the process melts the iron and aluminium oxide and we get molten iron and refractory slag of aluminium oxide. The density of the aluminium oxide slag is much less than the liquid iron, so it floats above the molten iron.

So during the welding process, the molten iron present at the bottom of the crucible and slag floats above it.

If we talk about composition of the metal oxide and aluminium powder, then 5 parts of iron oxide and 3 parts of aluminium powder by weight is mixed before initiating the welding process.

Crucible: It is that part which is capable to withstand in high temperature condition. In thermite welding process, the crucible contains thermite material. The exothermic process during the welding process takes place in the crucible. The molten iron present at the bottom of the crucible and slag of aluminium oxide floats over the molten metal. The crucible contains a taping device to discharge the molten metal to the mold for the welding.
Thermite mixture: It is a pyrotechnic composition of metal oxide, aluminium powder, and fuel.
Mold: A mold is created around the section to be welded. The mold consists of runner, riser, slag basin, heating gate. The molten metal is poured into the mold for the joining of the metal.
Taping device: It is device which is used to discharged or pour molten metal form the crucible to the mold.

Working

A mold is formed over and around the section to be welded. After the mold is formed thermite material is placed in the crucible.
The thermite material is ignited by the use of flint sparker or magnesium ribbon. An exothermic reaction takes place in the crucible called an aluminothermic reaction.
During the reaction metal oxide i.e. iron oxide reduces to iron and Al gets oxidized and forms aluminium oxide. A large amount of heat is generated. This heat generated melts the iron and aluminium oxide and form molten iron and slag of aluminium oxide.
The molten metal so formed is poured into the mold through taping device. As the molten metal reaches the section, it fuses with the metal pieces to be joined. After solidification, it forms a very strong weld.