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An AlN–SiC solid solution-bonded alumina composite was successfully synthesized by pre-synthesizing an isostructural AlN mesophase as an induced nucleus at 1600 °C in flowing nitrogen. The phase composition and morphology were characterized by X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy. The reaction mechanis...
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Red mud is a waste discharged from alumina production by Bayer process. It contains iron and titanium resources. Secondary aluminum dross (SAD) is a waste discharged from aluminum smelting and processing. It contains hazardous reactive aluminum nitride (AlN). In this work, iron and titanium in red mud were reduced by AlN in SAD by pyrometallurgy, and slag was manufactured into glass ceramics. Recovery rates of iron and titanium were 97% and 75%, when addition amount of red mud was double of SAD. The reduced iron and titanium were in form of metal iron and titanium nitride (TiN). Reduction process consisted of two steps: AlN, iron and titanium entered aluminosilicate network of melt; AlN reduced iron and titanium. Density, water absorption and microhardness of the glass ceramics were 3.10 g cm−3, 0.03% and 1195 HV. Utilizing 1 ton of SAD to replace carbon for iron reduction could reduce carbon emissions by 407 kg. In addition, co-disposing 400 kg red mud and 200 kg SAD could gain profit of 293 dollars. This work realized the total recycling of both red mud and SAD.
A considerable amount of secondary aluminum dross is generated in the aluminum refining process. Secondary aluminum dross (SAD) is hazardous waste because of its constituent pollutants, such as AlN, fluoride, chloride, and heavy metal pollutants. SAD treatment can be categorized into thermal and wet methods. The thermal process has attracted considerable research attention because of its simplicity and ability to prepare Al–Mg spinels, refractory materials, and ceramics and recover alumina during the treatment. However, at this stage, SAD pyrolysis processes are typically focused on the removal of single pollutants. Limited studies have explored the simultaneous removal of various pollutants and their interactions. This study focused on the pyroprocess in the SAD treatment process and investigated the SAD mechanism in the preparation of Al–Mg spinel, refractories, ceramics, and recycled alumina. Furthermore, the conversion of AlN, fluoride, chloride salts, and heavy metals, which are the main pollutants generated in the thermal reaction of SAD, and their control mechanisms were analyzed. The synergistic mechanism of SAD denitrification\chlorine salt and fluoride fixation\heavy metal was studied through the reaction mechanism of similar component minerals in the pyroprocess for improving the high-value utilization of SAD and scientific prevention and control of secondary pollution. Finally, the feasibility of preparing ceramic pellets from aluminum dross (AD) was evaluated. The results of the study can provide crucial development directions for SAD utilization.
Secondary aluminum dross (SAD) is a hazardous solid waste due to containing aluminum nitride (AlN). In this work, AlN was used to reduce heavy metals in pickling sludge by pyrometallurgy. Reduction extent of Fe, Cr and Ni was up to 97, 91 and 100%. However, it was found that some AlN was oxidized by oxygen. AlN oxidization could be decreased by shortening melting process. The AlN oxidization would be restricted after aluminosilicate was melted. Reduction extent of heavy metals increased from 51% to 86%, when treatment temperature was increased from 1350 to 1400 °C. Meanwhile, AlN oxidization could be exacerbated by increasing CaF2 and soda. The CaF2 and soda could corrode the alumina protective layer on AlN and promote AlN oxidization. Reduction extent of heavy metals decreased from 48% to 19% after adding soda. The AlN oxidization could be restrained by cutting down the air during melting process. After covering the crucible, reduction extent of heavy metals was increased from 48% to 92%, and reduction extent of Fe, Cr and Ni was increased from 58, 18 and 83% to 97, 91 and 100%. After evaluating environment benefit of AlN reducing heavy metals, it was found that utilizing a ton SAD replacing carbon to reduce heavy metals in pickling sludge could reduce carbon emissions of 279 kg. In addition, SAD could be used as reductant to replace Al for chromium metallurgy. Using a ton SAD to replace Al for chromium metallurgy could reduce carbon emission of 3961 kg.
Zirconium nitride is an increasingly attractive material, owning to its wide range of applications. Nitridation with carbon as a reductant is a commonly used process for the preparation of transition metal nitrides. In this study, the nitridation of carbon-containing zirconia gels was studied as a method to synthesize zirconium nitride. The behavior of the gel upon heat treatment was investigated online. The evolution of the microstructure during nitridation was investigated using high-resolution transmission electron microscopy (HRTEM). The relationships between the gaseous species, crystalline phases, microstructure, and chemical elements of the resulting materials were established. The results showed that NH3, CH2O, CO2, CO, H2O, and HCN were the gaseous products in the nitridation process. Nucleation of the nanograins started with zirconium oxynitride. Transformation from oxynitride to nitride led to the occurrence of open-mouth-like defects on the particles. The enrichment of N atoms was accompanied by a core-shell structure. Finally, a systematic nitridation mechanism involving five stages was proposed. This study is important for understanding the nitridation process and controlling the structure and properties of transition metal nitrides.
An Al–Si–Al2O3 composite was prepared with corundum, aluminium powder and silicon powder. A creep test was carried out at 1300°C under 0.2 MPa for 50 hours in air. The results show that the Al–Si–Al2O3 composite performs a low constant creep rate and remain until the end of the 50-hours test. This is attributed to the in-situ formation of the tough non-oxide reinforcements, whisker-like (AlN)x(Al2OC)1-x solid solution and granular β-SiC, by reactions of Al and Si during creep test. The whisker-like (AlN)x(Al2OC)1-x solid solution and granular β-SiC reinforcements are evenly filled in the pores, which play the role of bridging and pinning reinforcement, forming a strong network structure with corundum aggregates. Moreover, these non-oxide phases are not wetted by the liquid phases, which impel the liquid phase shrinks in the network structure in isolation during creep test. Thus, the adverse effect of the liquid phase on the high-temperature strength of the composites is eliminated, so the composites with strong network structure quickly get a stationary low-creep state. A creep mechanism model is established.
The novel Al4O4C–(Al2OC)1-x(AlN)x–Zr2Al3C4–Al2O3 refractories with ultra-low carbon content have been successfully prepared by constructing the core-shell structure of aluminum at 1300–1700°C in nitrogen. The phase composition, microstructure, and properties of the novel refractories are deeply investigated. The cracking temperature on the core-shell structure of aluminum is further explored and the reaction mechanism of Zr2Al3C4 has also added explanation. The results show that the novel refractories have excellent physical properties and cannot be corroded by molten iron. There exist two different Al2OC solid solutions in the novel refractories, Al2OC-rich (Al2OC)1-x(AlN)x and AlN-rich (Al2OC)1-x(AlN)x. The temperatures affect their relative content. When temperatures are less than 1600°C, the relative content of Al2OC-rich (Al2OC)1-x(AlN)x is more than that of AlN-rich (Al2OC)1-x(AlN)x. When temperatures are above 1700°C, the relative content of AlN-rich (Al2OC)1-x(AlN)x is more than that of Al2OC-rich (Al2OC)1-x(AlN)x. The core-shell structure of aluminum fully ruptures at about 1200°C. Zr2Al3C4 begins to form at about 1000°C and generates in large at 1200°C.
Secondary aluminum dross (SAD) from aluminum industry is classified as a hazardous solid waste due to containing aluminum nitride (AlN). In this work, AlN was first used to reduce heavy metals by pyrometallurgy. The reduction rates for iron, chromium and nickel were up to 90%, 80% and 100%, respectively. However, the reduction from AlN and oxygen oxidization of AlN occurred simultaneously. AlN which formed solid solution with alumina could reduce heavy metals, while the rest was oxidized by oxygen. In addition, the reduction rates for iron and chromium could be increased with increasing CaF2 from 6.7 to 9.0 wt.%. CaF2 could decreased viscosity of molten slag, which favored the ion migration, and then increased the reduction rates. After the reduction, glass ceramics were manufactured from the molten slags. The bending strength, microhardness and alkali resistance of the glass ceramics were up to 77 MPa, 1011 HV and 98.7%, respectively. According to XRD and SEM results, glass ceramics with CaAl2SiO6 crystal phase, crosslinked network structure grains and smaller pores exhibited better bending resistance. In addition, glass ceramics with CaAl2SiO6 crystal phase possessed the highest microhardness and alkali resistance. After this process, hazardous pickling sludge and SAD were totally recycled.
An Al–Si–Al2O3 composite was prepared and sintered in air at 1100 °C and 1550 °C for 3 h to investigate the oxidation mechanism. The results show that the anti-oxidation performance of the Al–Si–Al2O3 composite is excellent, with anti-oxidation protection layers formed at both 1100 °C and 1550 °C, whereas the underlying oxidation mechanisms differ completely. At 1550 °C, Al(l), Si(l), and Al2O3(s) react with O2 directly to form a dense mullite solid solution at the surface of the sample, which seals all the diffusion channels of O2 into the sample, resulting in a rapid decrease in PO2 in the sample. At low PO2, Al(l), Si(l), and C(s) from the resin react with N2(g), and an AlN–SiC solid solution reinforcement is formed in the interior of the sample. At 1100 °C, the oxidation behaviour of Al(l), Si(s), and α-Al2O3(s) is different: Al(l) is oxidized to active α-Al2O3, and Si(s) particles are incompletely oxidized to [email protected]2, accompanied by a volume expansion and a slow layer by layer decrease in PO2 in the sample. Then, the active oxidation of Al(l) occurs and a substantial amount of Al2O(g) is formed in the sample, which acts as a gas barrier to prevent O2 from penetrating into the sample, further lowering PO2. Then, the Al2O(g) and C(s) from the resin and the Al(l) in the interior react with N2(g) to form a whisker-like (Al2OC)1-x(AlN)x reinforcement in the sample. The Al–Si–Al2O3 composite shows the ability of self-healing at high temperatures in air atmosphere. The reaction model is established.
