Chinese name
thermite
Foreign name
thermite
external
Pink powder mixture
App application
Smelting refractory metals, substitute metals and welding metals.
Risk code
Highly flammable
Thermite is a mixture of aluminum powder and high melting point metal oxides (such as iron oxide powder) in proportion. When in use, the oxidant is added and ignited, and the reaction is fierce, so that alumina and simple substance are obtained, and a large amount of heat is released. The temperature can reach about 2500℃, and the generated simple substance can be melted. This reaction is called aluminothermic reaction. The thermite reaction principle can be applied to production, such as welding rails. Use some metal oxides (such as V2O5, Cr2O3, MnO2, etc.). ) can also be used as thermite instead of iron oxide. When aluminum powder reacts with these metal oxides, it generates enough heat to make the reduced metal in a molten state at a higher temperature and separate from the formed slag, thus obtaining more pure metal. This method is often used in industrial smelting of refractory metals such as vanadium, chromium and manganese. It uses the heat released when aluminum is oxidized. Some metal oxides can't react with aluminum or give off little heat, so they can't be used as thermite.
German chemist Hans Goldsmith invented the aluminothermic method in 1893 and applied for a patent two years later. [1] Therefore, this reaction is also called "Goldsmith method" or "Goldsmith process". The original purpose of this research is to prepare high-purity metals without melting carbon, but Goethe Smith is keen to find that aluminothermic method can be used for welding. [2] 1899 In Essen, Germany, thermite method was first applied to welding railway tracks commercially.
burn
The thermite reaction needs high temperature to be initiated, and a magnesium rod can be inserted into the mixture powder as a fuse (an appropriate amount of potassium chlorate can be added to help the magnesium rod burn, and an oxidant such as potassium permanganate and potassium nitrate can also be used for burning; Barium peroxide is ok, but smoke is poisonous. The mixture of potassium permanganate and glycerol releases heat slowly and can also be used as an initiator (the mixture of potassium permanganate and glucose can also react violently after being ignited, triggering an aluminothermic reaction). Propane gun will also increase the high temperature required to initiate the reaction. After the reaction starts, it will release heat violently, with sparks everywhere and extremely high temperature. Pay attention to safety when igniting.
Shared species
Iron oxide/aluminum, [3][4][5] magnetite, the most commonly used thermite, also play a role. Occasionally, other oxides, such as manganese thermite, chromium thermite, silicon thermite or copper thermite, are used, but only for special purposes. All these examples use aluminum as the active metal. Fluoropolymers can be used in special formulations, and PTFE and magnesium or aluminum are relatively common examples. [6]
The combination of dry ice and reducing agents such as magnesium, aluminum and boron follows the same chemical reaction as the traditional thermite mixture, producing metal oxides and carbon. Although the temperature of dry ice aluminum paste mixture is very low, the system can be ignited by flame. When the finely divided dry ice thermite is confined in a tube and ignited like a traditional explosive, it will explode, and some carbon released in the reaction will appear in the form of diamonds.
In principle, any active metal can be used instead of aluminum. But the properties of aluminum are almost ideal for this reaction:
It is by far the cheapest highly active metal.
It forms a passivation layer which is safer than many other active metals. [7]
Its melting point is relatively low (660℃), which means that it is easy to melt metal, so the reaction mainly occurs in the liquid phase, so it proceeds quite quickly.
Its high boiling point (25 19℃) makes the reaction reach a very high temperature, because several processes tend to limit the maximum temperature below the boiling point. This high boiling point is common in transition metals, but not common in highly active metals.
In addition, the low density of alumina formed by reaction tends to make it float on the obtained pure metal. This is especially important to reduce the pollution in the weld.
Although the reactants are stable at room temperature, when they are heated to the ignition temperature, they will have an extremely strong exothermic reaction and burn. Due to the high temperature (up to 2500°C, iron (III) oxide is used), the product looks like a liquid, although the actual temperature reached depends on the ability to quickly dissipate heat to the surrounding environment. Thermite comes with its own oxygen supply and does not need any external gas source. Therefore, as long as it is given enough initial heat, it will not suffocate and can be ignited in any environment. When wet, it burns well and is not easily extinguished by water, although enough water will take away the heat and may stop the reaction. A small amount of water will boil before the reaction. Even so, thermite is also used for underwater welding. [8]
Aluminum alloy is characterized by almost no gas produced during combustion and high reaction temperature. Fuel should have high heat of combustion and produce oxides with low melting point and high boiling point. The oxidant should contain at least 25% oxygen, have high density and low heat of formation, and produce metals with low melting point and high boiling point (so the released energy will not be consumed in the evaporation of reaction products). Organic binders can be added to the composition to improve its mechanical properties, but they tend to produce endothermic decomposition products, resulting in some reaction heat loss and gas generation. [9]
The temperature reached during the reaction determines the result. Ideally, the reaction produces completely separated metal and slag. For this reason, the temperature must be high enough to melt the reaction products, the generated metals and fuel oxides. Too low temperature will lead to the sintering of the mixture of metal and slag, while too high temperature (higher than the boiling point of any reactant or product) will lead to the rapid generation of gas, the dispersion of the burning reaction mixture, and sometimes even the explosion. Too low a reaction temperature can be increased by adding a suitable oxidant (for example, when producing silicon from sand), and too high a temperature can be reduced by using a suitable coolant and/or slag flow rate. The commonly used flux is calcium fluoride, because it only reacts to the lowest degree, its melting point is relatively low, its melt viscosity is low at high temperature (thus increasing the fluidity of slag), and it forms * * * crystals with alumina. However, excessive flux will dilute the reactants to such an extent that combustion cannot be maintained. The types of metal oxides also have a significant impact on the energy generated; The higher the oxidation price, the higher the energy generated. A good example is the difference between manganese (IV) oxide and manganese (II) oxide. The former produces too high temperature, while the latter can hardly maintain combustion. In order to obtain good results, a mixture of two oxides with an appropriate ratio should be used.
The reaction rate can also be adjusted by particle size; Coarse particles burn more slowly than fine particles. This effect is more obvious for particles that need to be heated to a higher temperature to start the reaction.
Under adiabatic conditions, when no heat is lost to the environment, the temperature realized in the reaction can be estimated by Hess's law-by calculating the energy generated by the reaction itself (subtracting the enthalpy of reactants from the enthalpy of products), subtracting the energy consumed by heating products (according to their specific heat, when substances only change their temperature, and when they melt or boil, their melting enthalpy and final evaporation enthalpy). Under practical conditions, the reaction radiates heat to the environment, so the realized temperature is slightly lower. The heat transfer rate is limited, so the faster the reaction is, the closer it is to the adiabatic condition of its operation, and the higher the realized temperature is. [ 10]
Iron thermite
The most common ingredient is iron thermite. The oxidant used is usually iron oxide or iron oxide. The former generates more heat. The latter is easier to ignite, which may be due to the crystal structure of oxides. Adding copper or manganese oxide can make it easier to ignite. [9]
Iron (III) thermite
Copper thermite
Copper thermite can be prepared by using cuprous oxide or copper oxide. The combustion speed is very fast, and the melting point of copper is relatively low, so the reaction produces a large amount of molten copper in a short time. The thermite reaction of copper (Ⅱ) may be so fast that the copper thermite can be regarded as a kind of flash powder. An explosion may occur, and the copper drops will be sprayed to a considerable distance.
Copper (I) thermite has industrial uses, such as welding crude copper conductors and cable splicing.
Aluminothermic combustion agent
Thermite is the thermite of salt-based oxidant (usually nitrate, such as barium nitrate or peroxide). Compared with conventional thermite, thermite has flame and gas release when burning. The presence of oxidant makes the mixture easier to ignite and improves the penetration of combustible materials into the target, because the evolved gas sprays slag and provides mechanical stirring. This mechanism makes thermite more suitable for combustion purposes, while thermosetting materials are more suitable for emergency damage of sensitive equipment (such as password equipment) because the role of thermite is more localized.
App application
The heat released during aluminothermic reaction can make the high melting point metal melt and flow out, so aluminothermic method is widely used in welding emergency projects, such as connecting railway tracks into a long section. In addition, aluminothermic process is also an important means to smelt high melting point metals such as vanadium, chromium and manganese.
Thermite is not only used for welding railway tracks and smelting refractory metals, but also widely used in military affairs. For example, the composition of thermite is loaded into the head and nozzle of the projectile. Because the reaction temperature is extremely high, it can be used to make incendiary bombs, which can melt through armor and greatly improve the lethality. The book Rocket Gun written by scientists in the former Soviet Union says that there is thermite in the rocket warhead charge.
In China People's Revolutionary Military Museum, American helmets and carbines were melted together in the exhibits. Except for rockets, other bombs don't have so much energy.
In addition, after other simple substances are mixed with metal oxides and ignited, a strong redox reaction will also occur, and the effect is similar to aluminothermic reaction. The simple substance can be aluminum, magnesium, calcium, titanium, silicon and boron, and is not limited to metal, but the metal oxide can be boron trioxide, silicon dioxide, chromium trioxide, manganese dioxide, iron trioxide, iron tetroxide, copper oxide and lead tetroxide. Sometimes these reactions are also called "magnesium thermal method", "silicon thermal method", "calcium thermal method" and "carbothermal method" according to the reducing agent in the reaction.