By using the heat of chemical reaction released by reducing certain metal oxides with aluminum, the oxide reduction reaction can be completed and the separated alloy and slag can be obtained without the need to add heat from the outside.
Thermite method is commonly used in industry to produce ferrotitanium, ferromolybdenum, ferroniobium, ferroboron, ferrovanadium, ferrotungsten, metallic chromium, metallic manganese and nickel-based, titanium-based, aluminum-based master alloys.
A metal thermal reduction method using aluminum powder as the reducing agent. When aluminum powder reacts with metal oxides, enough heat is generated to melt and separate the reduced metal and the formed slag to obtain metal or alloy. It is widely used in industrial production of pure metals (such as manganese, chromium, vanadium, etc.), carbon-free or low-carbon iron alloys, and metal welding. Basic introduction Chinese name: thermite process Foreign name: thermite process Usage: metal smelting, welding, etc. Advantages: simple equipment, many products, short production cycle Safety hazards: high-heat reactions are prone to fires and explosions Principle: aluminum powder is used as the reducing agent Introduction to metal thermal reduction, advantages, brief history, principles, reducing the loss of low-priced oxides in slag, methods to increase reaction heat, production technology, raw material preparation, ingredients, smelting, finishing, production of ferroniobium by thermite reduction method, Thermite reduction method outside the furnace, thermite reduction method in electric furnace, safety, brief introduction to the metal thermal reduction method using aluminum powder as reducing agent. When aluminum reacts with metal oxides, sufficient heat is generated to melt and separate the reduced metal and the formed slag to obtain a metal or alloy. It is widely used in the production of industrial pure metals (such as manganese, chromium, vanadium, etc.), carbon-free or low-carbon ferrous alloys and metal welding. The thermite method and the silothermal method using silicon iron as the reducing agent are both methods that use autothermal reactions to produce ferroalloys. They are called metal thermal methods and are also called out-of-furnace methods. They use aluminum particles, ferrosilicon powder or aluminum-magnesium alloy powder as reducing agents. Thermite method is mainly used to produce iron alloys, master alloys, chromium and manganese containing high melting point metals and difficult-to-reduce elements. The product is characterized by extremely low carbon content (generally <0.05). Advantages: Thermite production equipment is simple, the floor space is small, the production scale can be determined according to the task, there are many product varieties, and the production cycle is short. Brief History In 1859, Russian scientist H.H. Beketov (H.H. BeKeTOBy) mentioned in "On Some Reduction Phenomena" that "barium oxide is reduced with aluminum to obtain 24Ba and 33Ba barium aluminum alloys." This was the earliest report of experiments with the thermite method, but it was not used industrially at the time. In 1893, H. Goldschmidt discovered that a mixture of metal oxide powder and powdery reduced metal (basically aluminum), after igniting to initiate the reaction, can automatically continue until the charge reaction is completed. In 1898, Goldschmidt gave a report on the metal thermal reduction method at the German Electrochemical Society. Only then did people know that the thermite method has achieved good results in industrial production and can produce carbon-free metals economically and in large quantities. Ferrous alloys and pure metals. This year should be the starting point for the thermite method to be used in industrial production. The ferroalloys produced by the thermite method in industry mainly include: ferrotitanium, ferromolybdenum, ferroniobium, ferroboron, ferrovanadium, ferrotungsten, metallic chromium, metallic manganese, and master alloys such as nickel-based, titanium-based, and aluminum-based. The industrial production of ferroalloys by thermite method in China began with the production of ferromolybdenum by Jilin Ferroalloy Plant at the end of 1957. Principle The reaction of aluminum reducing oxides is a displacement chemical reaction and releases heat ΔH°298 (reaction). The thermochemical reaction formula is expressed as: 2/yM x O y 4/3Al =2x/y M 2/3Al 2 O 3 ΔH° 298 (reaction) The reaction heat ΔH° 298 is calculated using the data in the chemistry manual. That is, ΔH°298 (reaction) = 2/3ΔH° 298 (Al 2 O 3 )-2/yΔH° 298 (MxOy) The standard enthalpy of formation of oxide ΔH° 298 , commonly known as the standard heat of formation. Figure 1 The △F°-T relationship diagram of oxide formation. Figure 1 The △F°-T relationship diagram of oxide formation. Whether the thermite reduction reaction can proceed can be judged based on the relative stability of the oxide.
The stability of the oxide is judged based on the free energy of oxide formation ΔF°=-kTlnpo 2. All oxides decompose more readily with increasing temperature and are therefore more susceptible to reduction. The oxygen potential difference of various oxides becomes smaller at high temperatures. The restoration situation can be estimated from Figure 1. In the ΔF°-T diagram, elements with lower positions can reduce oxides with higher positions. The larger the distance between the two △F°-T lines, the more heat is generated by the reduction reaction. The prerequisite for the thermal reduction reaction of aluminum (or silicon) is △F°≤0, that is, the greater the negative value of the reaction free energy, the easier it is for the aluminothermic reduction reaction to proceed. When analyzing the thermal reduction reaction of aluminum (or silicon) from the △F°-T diagram, the kinetic process is not considered, so this judgment is qualitative. For all metallothermal reduction reactions, the △F° at a lower temperature is more negative than the △F° at a higher temperature. Therefore, under the conditions where the reaction can proceed, the reaction temperature should be controlled as low as possible, so that It is advantageous for the reduction reaction to proceed to the right. Some thermite reduction reactions can replace all metals from related oxides, such as iron, tungsten, molybdenum, etc.; while others can only proceed until the alloy liquid and the oxides in the slag are close to equilibrium, and part of the oxides remain. In the slag. Some oxides are reduced to low-valent oxides during the thermite reduction process, such as TiO 2 is reduced to TiO, converted from acidic oxides to alkaline oxides, and combined with Al 2 O 3 produced during the reduction process to form aluminate And remaining in the slag increases the loss of titanium. To reduce the loss of low-priced oxides in the slag (1) is to increase the amount of reducing metal added to avoid the generation of low-priced oxides under the condition of excess reducing agent; (2) to add alkaline oxides such as CaO, MgO, BaO It can reduce the content of TiO, MnO, etc. in the slag and improve the recovery rate of metal elements. Alkaline oxides can also lower the melting point of the slag and improve the fluidity of the slag. The amount of alkaline oxide added should be as small as possible to avoid increasing the amount of slag and affecting the reaction process. Because of the rapid reaction, it is difficult to achieve equilibrium conditions. Part of the reduced metal is not used for reduction and remains in the alloy, forming intermediate compounds such as TiAl, TiAl 3, etc., which makes the alloy contain high aluminum content and makes it difficult to obtain high-grade alloys. In order to bring the reaction closer to equilibrium, a third element, such as iron, is sometimes added to absorb the metal produced by the reaction and make the reaction proceed to the right. This method is feasible when producing ferroalloys and can also reduce the melting point and reaction temperature of the alloy. To obtain products with low aluminum content, the aluminum dosage can be slightly lower than the calculated amount. Figure 1 can provide a reference for selecting the type of reducing agent and type of oxide. The commonly used reducing agents in ferroalloy smelting are mainly aluminum and ferrosilicon, and occasionally a small amount of magnesium (added in magnesium-aluminum alloy) is used. The reaction result of the thermite method must make the metal and slag have good fluidity, that is, be heated above their melting points, so that the produced alloy and slag can be clearly separated; and a higher metal yield can be obtained before it can be considered The reaction proceeds automatically and is adopted in industrial production. This problem requires analysis of the heat balance of the thermite smelting process. In the process of thermite reduction reaction, the reduction of reactants, the generation of products, the generation of reaction heat, and the heating of reactants (alloy and slag) are all completed at the same moment and in the same system. Therefore, the heat is concentrated, the reaction speed is fast, the time is short, and the thermal efficiency is high. The surface of the reaction melt is always covered by the added charge, so when the reaction proceeds, the heat loss caused by heat conduction and heat radiation in the reactor has little impact on the reduction process. Due to the short reaction time, the evaporation loss of the charge and reactants is small, so the evaporation heat is also small. The main heat source of the thermite method is the reaction heat ΔH° 298 (reaction) generated by the thermochemical reaction, which can be obtained through calculation methods. In 1914, the Russian chemist Zhemchuzhny proposed that "the heat content of the obtained metal and slag, and the heat loss accompanying the reaction process, are approximately the same for various alloys" and proposed "the thermite process. To proceed normally, the heat generated per gram of charge during the reaction must be no less than 550cal." That is, the heat generated by the unit charge is used to judge whether the thermite reduction process can be carried out automatically. Zhemchuzhne's law can be used as a reference in production or as a preliminary estimate when developing new varieties.
The reason is that there are different regulations on the degree of reduction of oxides, different melting points of alloys and slags, different smelting scales, different phase structures of ores, etc. Therefore, after the proportion of the charge is obtained through batching calculation, small-scale smelting equipment must be used first. Test it and then make appropriate adjustments before it can be used for production. In a normal production factory, when the ore is changed, trials are also required to correct the ingredient list. The total amount of charge for production should include the quality of reducing agents such as aluminum and ferrosilicon, oxides (or ores) and impurities (or gangue), fluxes, etc. The heat of reaction is calculated based on the enthalpy of formation (△H° 98) data in the manual. Due to different years and versions, there are varying degrees of differences, and the calculated heat of reaction is also different. Practical workers should select a batch of data, use it regularly, and derive correction coefficients based on practice. Through calculation, if the calorific value per unit charge is lower than 550cal/g, the thermite reaction cannot proceed automatically, and the ingredients need to be adjusted to increase the reaction heat. Methods to increase the heat of reaction (1) Adjust the ratio of high-valent and low-valent oxides in the oxide to increase the total amount of oxygen in the oxide. The concept of active oxygen is used in the production of manganese metal by thermite method. The so-called active oxygen refers to the oxygen that is not combined with Mn after calculating manganese oxide as MnO. For example, the active oxygen content of Mn3O4 is 7, while that of Mn2O3 is 10. This is an example of using the ratio of high- and low-valent oxides to adjust the calorific value of the thermite reduction reaction. (2) When producing iron alloys, hematite (Fe 2 O 3 ) or iron scale (Fe 3 O 4 ) can be added to replace part of the steel scraps. They generate a large amount of heat after reacting with aluminum or silicon. For example, NiO is used to replace part of nickel when producing nickel-based alloys. (3) Add BaO 2 or NaClO 3 , KClO 3 , NaNO 3 , KNO 3 and other heat-increasing agents that can release a large amount of heat after reacting with aluminum to increase the unit calorific value of the charge to the desired value. This is a commonly used method. However, it should be noted that when using NaNO 3 or KNO 3, the alloy will contain high nitrogen content and will emit gases such as nitrogen oxide that pollute the environment. (4) Preheat the charge to increase the sensible heat of the charge. Generally speaking, every 100℃ increase in charge preheating temperature can increase the unit calorific value by about 30cal/g. (5) Deliver electric energy into the reactor to form the electric thermite method. When the calorific value per unit charge is too high, the thermite reaction will be violent, even reaching an explosive level; severe splashing during smelting will increase the loss of charge and products, and the alloy and slag will be mixed and unclearly separated. In serious cases, it will damage the equipment and endanger the safety of operators. An effective way to reduce the unit calorific value of the charge is to add inert substances and increase the amount of charge. Commonly used inert materials include alloy chips produced by alloy finishing, slag, lime, magnesia, etc. produced by smelting. In addition, increasing the particle size of aluminum particles and charge can inhibit the reaction speed. The generation of metal compounds and the slagging reaction when Al 2 O 3 and other oxides form composite oxides all generate heat, but they are not considered when calculating the unit calorific value of the charge. Production process The reaction of thermite smelting of ferroalloys is automatic once triggered and cannot be controlled, so there are strict requirements for charge preparation. Ingredient calculation and weighing must be accurate. The prepared furnace materials must be mixed evenly before being loaded into the reactor. The particle size of the charge directly affects the reaction speed. The reaction speed is slow when the particle size is coarse, and the reaction speed is fast when the particle size is fine. Only by properly matching the particle sizes of ores or oxides and reduced metals to achieve optimal coordination between heat concentration and reaction speed can a higher metal yield be obtained. For example, the reaction between vanadium pentoxide and aluminum is very violent, and the particle size can be increased to control the reaction process between them. There are few oxides on the surface of coarse aluminum particles, which reduces the oxygen content of the aluminum particles and generates large high-aluminum alloy droplets during the reaction. The density of the high-aluminum alloy droplets increases to the point where the droplets sink. During the dropping process, they continue to react with the metal oxides in the slag, causing most of the aluminum to be consumed. The high-aluminum alloy sinking to the surface of the alloy layer and the metal oxide in the covering slag continue to undergo replacement reactions at high temperatures. The oxides generated on the surface of fine aluminum particles make the aluminum particles contain high oxygen, which is unfavorable to the reduction reaction. Therefore, the number of aluminum particles used in the thermite method should be less than 0.1mm and should be less than 5.
As mentioned above, the process of smelting ferroalloys by thermite method can be divided into four main processes: raw material preparation, batching, smelting, and finishing. Raw material preparation: The main work is to thoroughly dry the ore, oxides and fluxes (lime, fluorite), etc., and remove attached water, crystal water and volatile matter (such as mineral processing reagents). Then processed into the particle size required for production. The equipment used is all general equipment, such as rotary kiln, drying furnace, crusher and ball mill. Aluminum granules are manufactured by the ferroalloy factory itself. Granularity requirements are specified. It is generally used immediately after production and should not be stored for a long time. The spray method for producing aluminum particles is to heat and melt the aluminum ingot and pressurize it with compressed air. When it is sprayed from the aluminum melting pot through a nozzle, the atomizer uses compressed air to crush the aluminum flow to form aluminum particles. Aluminum particles of the required particle size can be obtained by adjusting the spray pressure or changing the atomizer. Another method is to roll aluminum ingots into aluminum foil and then mechanically cut them into aluminum chips. Ingredients This is a key process in thermite production. No mistakes are allowed, otherwise adverse consequences will occur and the product may not even be obtained. Batching takes place at the batching station. The main equipment of the batching station includes storage bins, scales, mixing barrels and transport hoppers. Calibrate the scale before batching. Ingredients must be weighed in the specified order. The weight of the charge added to the mixer and the mixing time are specified by the mixer capacity. Manual mixing can be used for small-volume production. Methods to reduce losses are as shown in the diagram of the thermite batching station layout: 1—storage silo; 2—mixing cylinder; 3—transport hopper; 4—scale; 5—fluorite bin. Smelting Thermite reduction reaction is carried out in the reactor. The reactor is also called a smelting furnace. Reactors are divided into two types: fixed reactors (Figure 5) and mobile reactors (Figure 6). Figure 5a is a reactor for upper ignition smelting, without feeding equipment, placed on a sand base; Figure 5b is a reactor for lower ignition, also placed on a sand base; Figure 5c is a fixed reactor with bricks at the bottom. All three types of reactors have slag outlets, and most of the slag is released after the smelting reaction. The mobile reactors are placed on mobile trolleys and pushed under the fume hood, or in the reaction chamber. a—Fume hood type: 1—tiltable iron furnace shell; 2—knotting material containing magnesia; 3—driving Frame; 4—exhaust hood; 5—the retractable part of the exhaust hood (to facilitate the entry of the vehicle and manual loading) b—reaction chamber type: 1—silo; 2—screw conveyor; 3—water cooling material Pipe; 4-Bricklaying upper reactor body; 5-Bricklaying crucible; 6-Magnesia lining; 7-Bricklaying chamber; 8-Cyclone dust collector is fed by a feeder or manually. The reactor consists of two parts: the upper part is a hollow cylinder, the outer shell is welded with steel plates, the upper and lower edges are reinforced with angle steel, and there are lifting rings on the upper edge. The lining is made of refractory bricks, magnesia bricks or high alumina bricks. It can also be made of produced slag that is crushed and knotted. It can also be cast with liquid slag, or cast iron can be cast into pieces and assembled into pieces without building a refractory lining. The bottom is a crucible made of quartz sand (only used in silica thermal method), magnesia or magnesite bricks, which contains the alloy produced by the reaction. The smelting operation is divided into two types according to the ignition method, namely the upper ignition method and the lower ignition method. a—Upper ignition reactor: 1—furnace shell; 2—clay brick lining; 3—slag mouth; 4—sand base; 5—furnace material; 6—igniting agent; 7—fume hood; b—lower ignition reactor: 1— Sand base (magnesia); 2-furnace barrel; 3-furnace hood; 4-fume hood; 5-silo; 6-chute; 7-bottom material; 8-igniting agent; 9-adding charge c-brick crucible Reactor: 1 - magnesia brick layer; 2 - crucible made of magnesia bricks; 3 - knotting material; 4 - magnesia; 5 - axle lugs for lifting; 6 - fume hood; 7 - ignition at the top of the gate for manual feeding The method is to load all the mixed materials in the batching station into the reactor. Then put the igniting agent on the upper part of the charge. After the igniting agent is ignited, the smelting reaction starts. After the reaction of all the charge is completed, the slag is released after sedation. After the alloy is condensed, take it out and cool it. In industrial production, ferromolybdenum is smelted using the upper ignition method. This method is also used to produce ferroalloys in small batches. Lower ignition method smelting is to first add partially mixed charge to the bottom of the reactor, add igniting agent to the upper part of the charge layer to initiate the reaction, and then add the mixed charge from the upper bunker one after another. The feeding speed should be such that there is a thin layer of charge on the surface of the melt and the reaction continues stably. Artificial feeding can also be used. Compared with the upper ignition method, the lower ignition method can make full use of the melting furnace volume and save refractory materials. The bottom ignition method is mostly used in industrial production.
When smelting some ferroalloys, refining materials are added after the reaction. The refining material is composed of iron ore powder, aluminum powder, and ferrosilicon powder, which can release a large amount of heat and keep the slag in a molten state for a certain period of time, which is beneficial to the sinking of the alloy particles mixed in the slag; the reaction product of the refining agent is Iron droplets, when falling through the slag layer, can absorb the "metal mist" in the slag layer and condense it into larger droplets that sink, which can increase the yield of metal elements. There is a fixed reactor equipped with an alloy discharge hole and a slag discharge port. After the reaction is completed, the slag is first released from the slag discharge port, and then the alloy is released from the alloy discharge port. This kind of reactor can save refractory materials and improve thermal efficiency. Of course this is only useful for mass production of alloys with lower melting points. (See ferroniobium) When smelting precious metal ferroalloys by the aluminothermic method, a certain amount of metal often remains in the slag, which can be remelted and recycled in an electric furnace. The slag produced by the thermite method contains high aluminum oxide and is a useful refractory and abrasive material. Al 2 O 3 gt; 90 slag can be used as raw material for making high alumina bricks. After finishing, the smelted alloy ingot is cooled in the air until solidified and then lifted out of the crucible. It is sent to the cooling chamber and sprayed with water for rapid cooling to cause cracks in the alloy ingot for crushing. The water-cooled alloy ingots are sent to the shot blasting room to remove the slag and refractory materials attached to the surface. Some elements segregate greatly in alloy ingots, and chemical analysis samples must be collected according to prescribed sampling methods. The alloy ingots are crushed to a specified size and then packaged for sale. Thermite reduction method is used to produce ferroniobium, a process in which metallic aluminum is used to reduce niobium concentrate or niobium oxide to produce ferroniobium master alloy. Niobium has a high melting point and is difficult to reduce. However, if iron is present, the reduced niobium will form an alloy with iron, which is not only easy to reduce, but also because the melting point of ferro-niobium is lower than that of niobium, it is more suitable as an additive for steelmaking or high-temperature alloys. The production of ferroniobium generally uses two raw materials: pure Nb 2 O 5 and niobium concentrate. Ferro-niobium produced from pure Nb 2 O 5 has low impurity content and high purity. It is called high-grade ferro-niobium and is mainly used for refining high-temperature alloys. Ferroniobium produced from niobium concentrate is called standard grade ferroniobium, containing 60 to 65% niobium, and is mainly used as an additive in steelmaking. According to the niobium content in the alloy, ferroniobium can be divided into high-grade ferroniobium (Nbgt; 65), medium-grade ferroniobium (Nb about 50) and low-grade ferroniobium (Nblt; 30). According to the equipment used in reduction smelting, it can be divided into the outside-furnace thermite reduction method and the electric furnace thermite reduction method. Thermite reduction method outside the furnace is a process in which the reduction smelting reaction is realized in a furnace without external heating. After the reaction is completed, the smelting furnace is disassembled, the reaction product is taken out, and the alloy and slag are separated outside the furnace. The characteristic of the outside-furnace thermite reduction method is that no liquid reaction products are released from the smelting furnace during the smelting process, so the process is relatively simple. Generally, a detachable cylindrical smelting furnace with low construction cost is used. In the outside-furnace thermite reduction method, since the liquid metal and slag generated by the reaction are solidified and crystallized in the same reactor, and the optimal solidification and crystallization conditions of the two products are different, it is difficult to obtain high technical and economic indicators, raw materials and There are problems such as high consumption of refractory materials, intermittent operation, high labor intensity in building furnaces, dismantling furnaces and removing slag and refractory materials mixed in alloys. To this end, an inclined smelting furnace has been developed that can discharge liquid metal or slag respectively. The outside-furnace thermite reduction method is only suitable for processing niobium concentrate or niobium oxide with low impurity content. In particular, the content of harmful impurities phosphorus, sulfur, lead, arsenic, antimony, tin, and bismuth in the raw materials must be strictly limited. In order to ensure that the reduction reaction is complete, both the raw material and the reducing agent aluminum need to be ground to a fine particle size and mixed evenly to maximize the reduction reaction contact area between the materials. Only when the materials are fully mixed can a high reduction reaction rate and high niobium recovery rate be obtained. In addition to ensuring the reduction of niobium and iron, the amount of reducing agent aluminum powder should also calculate the aluminum consumed by the reduction of impurities, which is generally 110 of the theoretical amount. Using too much aluminum powder not only fails to improve the recovery rate of niobium, but also causes the reaction to be too violent, increasing the residual aluminum content in niobium. When iron concentrate is used as an additive, its silicon and phosphorus content should be low. Depending on the purity requirements of ferroniobium, iron filings or electrolytic iron powder additives are often used. The amount of iron added is appropriate to make the alloy close to the low melting point of Fe2Nb. When using pure Nb 2 O 5 raw material, the amount of iron used is preferably 30 to 40% of the mass of niobium oxide. Reducing the viscosity of the slag allows easy separation of the alloy and slag.
Usually, fluxes such as hydrated lime, barium oxide, magnesium oxide, and fluorspar are added during the smelting process to reduce the viscosity of the slag. The amount of flux added must be moderate. If too much hydrated lime is added, calcium niobate will easily be formed, which will increase the loss of niobium. Too much flux will also corrode the furnace lining refractory material. In addition, in order to supplement heat, it is sometimes necessary to add exothermic agents such as sodium chlorate. Commonly used strong oxidants to initiate reactions include chlorate, saltpeter, magnesium powder, etc. Capacitor wires can also be used to ignite. The materials must be dried in advance, and the reactor and sand nest must be kept dry to prevent explosion. Electric furnace thermite reduction method is a production method that uses electric energy compensation heating during the thermite reduction process. This method can more easily control the speed of the reduction reaction, obtain higher quality products, and save aluminum powder. The technical and economic indicators are also higher than those of the out-of-furnace thermite reduction method. There are one-stage method and two-stage method for producing ferroniobium by the electric furnace aluminothermic reduction method. In the one-stage method, the material completes the reduction reaction under the action of arc to produce ferroniobium. The two-stage method uses an electric arc furnace to melt the material first, and then performs aluminothermic reduction smelting. In order to properly insert the electrode deeply and maintain stable furnace conditions, the furnace resistance must be strictly controlled. The furnace resistance is affected by factors such as the composition of the furnace charge, the particle size and quantity of aluminum powder, the chemical composition of the slag, the furnace size and electrode spacing, and the temperature distribution in the furnace. Some factories in China use columbium ore as raw material and use three-phase electric arc furnaces to produce medium and high-grade ferroniobium. The smelting temperature is 1973~2073K, the niobium oxide recovery rate is 96, the tantalum oxide recovery rate is 83, and the niobium content in ferroniobium is 50~ 70. Safety Thermite reduction is an automatic reaction, so special attention should be paid to safety issues to avoid accidents such as fire, explosion, and burns. Furnace charges should be stored separately, and aluminum particles, exothermic agent and oxide powder should not be stacked together. The mixed smelting charge must be smelted immediately and cannot be stored. The mixing site must not be wet or have stagnant water, so as to avoid accidentally causing the reaction of the mixed materials and causing an explosion accident. During the smelting process, operators should be located in a safe area and wear labor protection equipment to avoid burns. The scene must be cleaned up in time to prevent dust from existing to avoid causing fire accidents. Be careful when lighting a fire. During smelting, the ventilation system must be started to discharge smoke, dust and exhaust gas in time to avoid polluting the working environment.