Aluminium ash is a waste produced in the primary and secondary aluminum industry. There are three main sources. One is the aluminum ash produced by the electrochemical smelting of aluminum, which is 30-50kg/t aluminum. The second is the aluminum ash produced during the process of ingot casting, multiple remelting, alloy preparation, and part casting, which is 30-40kg/t aluminum. The above aluminum ash is called primary aluminum ash, also known as white ash. At present, most companies recycle the primary aluminum ash with higher metal aluminum content. The third refers to the secondary aluminum industry, which is about to be discarded aluminum products and the waste generated from processing, and the waste generated in the process of recycling primary aluminum ash. It is called secondary aluminum ash, also called black ash. The current recovery rate is generally 75 %~85%, 150~250kg/t aluminum. It is estimated that my country produces more than 2.5 million tons of aluminum ash every year. The process technology of primary aluminum ash recycling metal aluminum has matured and has been put into industrial production, but the recycling or utilization of secondary aluminum ash is still in the research stage, and a large amount of aluminum ash is accumulated or landfilled. In Europe, aluminum ash is classified as a hazardous waste. The main hazard is exudation or reacts easily in water and humid air to generate harmful and toxic gases, such as ammonia, methane, hydrogen, etc. Untreated aluminum ash is harmful Groundwater and air can cause pollution and occupy land.
The chemical composition of aluminum ash has obvious differences due to the composition of raw materials and processes, and it is mainly composed of a mixture of metallic aluminum, alumina, and salt flux. Specifically: Al10%~30%, Al©O320%~40%, Si, Mg, Fe oxide 7%~15%, K, Na, Ca, Mg chloride and a small amount of fluoride 15%~30%. Some of the oxides and chlorides adhere to the surface of metal aluminum.
Refractory materials are a resource-based industry, with various chemical compositions and types, and have space to accommodate various raw materials. The chemical composition of aluminum ash is similar to bauxite, the main raw material of refractory materials. It can be considered as refractory raw material directly or after processing, which opens up a new way for the effective use of aluminum ash, which not only protects the environment, but also reduces the production of refractory enterprises. Cost is helpful to the sustainable development of enterprises.
Application of aluminum ash added to refractory ingredients
1.1 As an explosion-proof agent
The substance that can improve the air permeability of the unshaped refractory lining body and prevent the lining body from bursting due to the excessive vapor pressure generated during the baking process is called an anti-explosive agent, also known as a quick-drying agent (a fast-baking additive ). The anti-explosion agent of unshaped refractories includes active metal aluminum powder. The aluminum powder reacts with H©O to form Al(OH)© and release H©. Before the castable has solidified, H© will form capillary when it escapes from the castable. Vent holes to improve its exhaust. Wang Liwang [1] uses aluminum ash instead of aluminum powder as an anti-explosive agent for iron trough castables. The chemical composition of aluminum ash is: Al31.63%, Al©O©18.15%, AlN9.25%, MgO6.16 %, SiO©12.21%, Fe©O©7.27%, CaO2.23%, Na©O2.15%, K©O1.03%, TiO©2.04%, Cr©O©0.58%, others 7.33%. Among them, Al and AlN can hydrate and release gas. The test shows that adding w (aluminum ash) 4% to the iron ditch castable can play a good anti-explosion effect. Too much aluminum ash will cause bulging and cracking. The aluminum ash can also promote the hardening of the iron ditch material and shorten the construction time. .
1.2 Join the blast furnace taphole mud
Huang Zhaohui and others invented the addition of 0.4% to 40% aluminum ash to replace aluminum and siliceous raw materials in blast furnace taphole mud. Other raw materials are: industrial grade corundum, silicon carbide, medium temperature asphalt granular powder, Suzhou soil fine powder, coke powder, etc., using tar, modified asphalt and phenolic resin as binders, mixing and mixing uniformly, and then extruding through a vacuum mud mill , That is, get the gun mud. Its performance is stable, can meet production requirements, and can reduce production costs.
1.3 instead of calcined bauxite
It has been studied to add aluminum ash to replace calcined bauxite in castables, preforms and refractory clay products. Aluminum ash does not need to be calcined and can be used directly as raw materials, with an approximate amount of 5%.
Preparation of refractory materials by aluminum ash processing
As we all know, raw materials are the basis of refractory materials, and only high-quality refractory materials can produce good products. The basic requirement for refractory raw materials is fire resistance, that is, raw materials with a refractoriness above 1580℃ can be used as refractory raw materials. In addition to Al©O©, aluminum ash also contains more impurities with lower refractory properties. Therefore, it is generally not possible to directly prepare refractory materials with aluminum ash. Further processing is required to remove impurities and increase the content of Al©O©. Consider using as a refractory material. The following is a brief introduction to aluminum ash processing methods.
2.1 Flotation purification of aluminum ash
Liu Ruiqiong and others used sodium oleate as the collector. When the pH value was fixed at about 8.6 and the amount of collector was 1000g/t, the content of aluminum ash w(Al©O©) after flotation increased from 43.14% to 86.41%. , The recovery rate is 68.89%. It can replace bauxite to smelt alumina-based fused materials.
2.2 Preparation of α-Al©O©
α-Al©O© is the main raw material for advanced refractory materials such as corundum. The basic principle of extraction with aluminum ash is: at a temperature of 400~600℃, the metallic aluminum and alumina in the aluminum ash react with NaOH and NaNO© to form water-soluble metal salts, which are dissolved with water to achieve aluminum After being separated from other impurities, the aluminum-containing solution is treated by the seed decomposition method, and α-Al©O© is finally obtained. The obtained preparation conditions are: alkali-to-ash ratio (mNaOH/m aluminum ash) 1.3, salt-to-ash ratio (mNaNO©/m aluminum ash) 0.7, matched according to the requirements, mixed uniformly, smelted at 500 ℃, smelting time 60min; The smelted product is leached with deionized water in a constant temperature water solution at 60°C. The leaching time is 30min, the solid-liquid ratio is 1:4, and the aluminum leaching rate* is as high as 92.71%. After leaching, suction filtration, solid-liquid separation, the leaching solution is purified, and the caustic ratio is adjusted. The seed crystal is decomposed and calcined to obtain alumina.
Xie Gang and others used pressure alkaline leaching and microwave activation assisted methods to recover alumina from aluminum ash. Firstly, the aluminum ash was crushed, sieved, washed with water, mixed with NaOH solution according to the solid-liquid ratio of 1:7 and stirred uniformly, and then reacted in an autoclave at 140°C and 1.15MPa for 6 hours, and then further solid-liquid separation, acid neutralization, After water washing and separation, the product is placed in a microwave device with an output power of 5W/g to dry and activate for 7 minutes, and the exhaust speed is 30m/min. *Al©O© products can finally be obtained.
Some people prepare high hardness γ-Al©O© by aqua regia leaching method and adding yttrium oxide. First, the aluminum ash is dissolved in aqua regia at room temperature, and then precipitated under the condition of pH 9~10, 0~20% yttrium oxide particles are added, and after compaction, it is calcined at 1550~1650℃ to obtain high hardness γ-Al© O©.
2.3 Preparation of nano alumina
Introducing α-Al©O© powder into corundum refractory products to reduce the sintering temperature, save energy, and improve its performance. For example: in the ingredients of fused corundum (Al©O©99.5%), add 4%~8% α-Al©O© micropowder and 1%~2% α-Al©O© nanopowder, products The sintering temperature is reduced from 1700~1800℃ to 1400℃.
Liu Xiaohong and others use sulfuric acid to leaching aluminum ash to prepare nano-alumina. The process is: first, under stirring conditions of 80 ℃, leaching aluminum ions in aluminum ash with sulfuric acid solution for multiple times, filtering and separating to obtain aluminum sulfate solution, and then carbonic acid The ammonium hydrogen solution was added to the aluminum sulfate solution, and the reaction was stirred at 40°C for 60 minutes to generate the precursor ammonium aluminum carbonate precipitation and ammonium sulfate solution. After aging, the vacuum filtration was separated, and the aluminum ammonium sulfate precipitation was washed and dried at 1200°C. Calcined for 1 hour to obtain α-Al©O© powder with a particle size of about 70nm.
2.4 Use aluminum ash to smelt brown corundum
Brown corundum for refractories is generally smelted from special-grade bauxite, with an Al©O© content of 94.5%-97%. It is the main raw material for medium and high-grade refractories, especially amorphous refractories. In recent years, in order to save energy and protect the environment and reduce production costs, some people are studying the use of aluminum ash to smelt brown corundum. Among them, Liu Ruiqiong et al. [5] experimented with low-temperature smelting to produce brown corundum. The production process is: put 1 part of aluminum ash (less than 0.10mm) into 2~5 parts of 90~100℃ hot water, soak for 6~10h, drain the water, and add the same quality of 90~100℃ hot water. Soak for 2~14h, soaking is an exothermic reaction, keep stirring, keep the water temperature at 90~100℃ to ensure that the aluminum ash does not deposit, separate the aluminum ash after soaking and rinse with running water, the rinse water flow is 3~6m/min, Then it is filtered with a vacuum filter, and then dried at 80~110℃ until the moisture content is less than 20%, and the pretreatment is completed. Smelting in an electric arc furnace: adding 0.5% to 4% of iron filings as a precipitant to the aluminum ash, smelting in the furnace at 1700 to 1800°C for 6 to 8 hours, smelting and reducing the SiO©, Fe©O©, and TiO© in the aluminum ash After the oxides are cooled, they are crushed, magnetically separated and sieved to obtain brown corundum products. The chemical composition of the aluminum gray and brown corundum products tested is shown in Table 2.
2.5 Synthetic Sialon powder
Sialon ceramics are a kind of high-temperature structural materials that developed rapidly after the 1970s. Sialon materials are considered to be one of the most promising high-temperature ceramic materials due to their superior mechanical properties, thermal properties and chemical stability. Sialon is a solid solution of Si©N4-AlN-Al2O©-SiO© system, which is made of pure chemical raw materials and has a high cost. Li Jiajing et al. [6] used aluminum ash, carbon black and fly ash as raw materials, and prepared Sialon powder by a carbothermic aluminothermic composite reduction nitridation process. The chemical composition of aluminum ash and fly ash is shown in Table 3. Weigh the good material, ball mill it for 12h (using Si©N4 balls, absolute ethanol as the medium), then dry, sieving, pressing into discs, and then calcining, natural cooling and grinding into powder, study the composition and synthesis of raw materials The effect of temperature on the resulting phase. The results show that when Si/Al is 1 (33% for aluminum ash and 50% for fly ash) in the raw material, 17% carbon black is added and the synthesis temperature is 1450℃, the main phase obtained is Si©Al©O The product of ©N(5β-Sialon, Z=3) and SiAl4O©N(415R); when Si/Al is 1.5, adding 80% fly ash, 1450℃ can prepare relatively pure Si©Al©O©N5 powder.
2.6 Preparation of magnesium aluminum spinel
Magnesia-aluminum spinel is an important refractory raw material. It is used as granules and magnesia as fine powder to prepare castable for ladle with corundum. Li Xiaona [7] uses aluminum ash, bauxite and fused magnesia as raw materials, iron filings as precipitating agent, and coke as reducing agent, and uses high-temperature fusion method to synthesize rich aluminum-magnesium-aluminum spinel. The test shows that the comprehensive index of magnesia-aluminum spinel produced by adding 20%, 40%, and 60% alumina ash exceeds the technical index of bauxite-based magnesia-aluminum spinel; when 40% alumina ash is added, the comprehensive index is the best It contains Al©O©82.48%, SiO©0.35%, MgO14.10%, CaO1.12%, Fe©O©0.5% (mass fraction), apparent porosity 0.9%, bulk density 3.48g/cm©, refractory Degree>1800℃; 40% aluminum ash is added, and 60% of the spinel produced contains calcium hexaaluminate (CA6) phase.
2.7 Preparation of TiN-Al©O© multiphase refractory raw materials
TiN-Al©O© composite material has excellent high temperature stability, wear resistance and mechanical properties, and is an excellent refractory material. Liu Haitao et al. [8] used rutile and aluminum ash as raw materials, and used metallic aluminum in the aluminum ash as a reducing agent to synthesize TiN-Al©O© composite powder by thermite reduction nitridation method. The chemical composition of aluminum ash and rutile used in the test are shown in Table 4.
The principle is: According to the reaction formula 6TiO©+8Al+3N©=6TiN+4Al©O©, the theoretical mass ratio of aluminum ash and rutile is calculated as 16:27. The specific method is: first weigh the good material, put it in a ball mill, dry-grind it for 12 hours, dry-press the green body at a pressure of 40 MPa, and then put it in a graphite crucible, and calcine it in flowing ammonia at 600-1400°C for 5 hours. The products calcined at 1300 ℃ and synthesized according to the theoretical amount are mainly TiN, α-Al©O©, a small amount of feldspar and MgAl©O4. After calculation, TiN is 30.4%, α-Al©O© is 45.8%, with the increase of aluminum ash, α-Al©O© increases, and TiN decreases. When the aluminum ash excess is 50%, TiN is 26.4%, α-Al© O© is 55.0%. The flexural strength of TiN-Al©O© composite material reaches 520.2MPa.
2.8 Fused Mullite
Chen Hai et al. [9] used aluminum ash to melt mullite. The specific steps are: The first step is the aluminum ash pretreatment process. First, the aluminum ash is calcined at 1100°C to convert the metal aluminum into Al©O©, and then the calcined aluminum ash is put into the water tank, and hydrochloric acid is added for cleaning. Then dry; the second step is electric melting, according to the mass fraction ratio of aluminum ash, bauxite and silica: 30%~80%: 0~50%: 10%~20%, mix well and add In the electric arc furnace, smelting, pouring, cooling, crushing, sorting, and obtaining mullite.
Using aluminum ash to make refractory bond
3.1 Synthetic polyaluminum chloride
Polyaluminum chloride, also known as basic aluminum chloride, or PAC for short, is a hydrolysis product between AlCe© and Al(OH)©. Its general chemical formula is (Al(2OH)nCe6-n)m, where m <10, n=1~5. Polyaluminum chloride is divided into two types: solid and liquid. The solid is usually a yellow or colorless resin-like product. The content of Al©O© is 40%~50%; the liquid is colorless, yellow-brown or black, and the content of Al©O© 10% or more.
Polyaluminum chloride can be used as a binder for shaped refractory products, refractory plastics, ramming materials and castables. It has certain requirements for alkalinity and density. Generally, alkalinity is 46%~72% and density is 1.17~1.23. g/cm©.
Xie Yinghui et al. [10] studied the preparation of polyaluminum chloride from aluminum ash. Among them, the neutralization method is to react caustic soda and hydrochloric acid with aluminum ash to produce sodium aluminate and aluminum trichloride, and then synthesize polyaluminum chloride with a suitable ratio. The acid solution method is to directly produce liquid polyaluminum chloride by reacting aluminum ash and hydrochloric acid once. The specific operation is: water-washing method to remove water-soluble salts, after treatment, the aluminum ash Al©O© content is about 30%, then industrial hydrochloric acid and a certain amount of water are put into the reactor, stirred and heated in a water bath, and the aluminum ash is weighed Gradually add the hydrochloric acid solution, the reaction is exothermic, the reaction temperature is 96℃, and the time is 6~12h. At the end of the reaction, add a certain amount of water to dilute the material. The experiment thinks that the aluminum ash:HCe:water ratio is 3:1:3, and the reaction time is 6~8h. , Adjust the pH value to 3.5~4.5, aging for 15~24h to obtain liquid polyaluminum chloride product.
3.2 Preparation of aluminum sulfate
Aluminum sulfate is dissolved in water and can be used as a binding agent for shaped and unshaped refractories. Since aluminum sulfate solution is acidic, it is mainly used as a binder for acidic and neutral refractory materials. Kang Wentong et al. [11] studied the process of preparing aluminum sulfate with aluminum ash as raw material: aluminum ash—adding sulfuric acid and water for reaction—filtering to remove filter cake—filtrate to remove impurities—concentration—cooling and crystallization—aluminum sulfate product. The reaction time is 3h, the sulfuric acid concentration is 30%, the sulfuric acid dosage is 1.05 (expressed as the ratio of the actual dosage of sulfuric acid to the theoretical dosage), the pH value is 3, and the yield is 93.2%.
Concluding remarks
The process technology of primary aluminum ash recycling metal aluminum has gradually become mature and put into industrial production, but the aluminum ash slag after the extraction of metal aluminum, especially the recovery or comprehensive utilization of secondary aluminum ash is still in the research stage. Scholars at home and abroad have carried out a lot of in-depth research on the process technology of alumina recovery. They have successfully recovered the alumina in the aluminum ash slag by using hydrolysis, acid leaching, alkaline leaching, alkaline smelting, and radio frequency plasma methods. Polyaluminum chloride, aluminum sulfate and fused brown corundum, combined with other materials to synthesize magnesium aluminum spinel, Sialon material, TiN-Al©O© composite materials, etc. This provides a prerequisite for the continuation of refractory materials, and in the future, research results should be transformed into enterprise production. It is recommended that the aluminum production department and the refractory material enterprise jointly establish an aluminum ash recycling company to produce the above-mentioned related products required by the refractory material department. This will not only achieve effective use of waste, but also save energy and reduce emissions, protect the environment, and reduce production costs.
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