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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. [ABSTRACT FROM AUTHOR]

Herein, a simple, versatile, and low‐cost approach has been proposed to realize the green utilization of secondary aluminum dross, the hazardous solid waste, namely directly sintering dry‐pressed green bodies from secondary aluminum dross to fabricate porous ceramics according to high‐temperature foaming process spontaneously without adding spare foaming agents. Aluminum nitride (AlN) in secondary aluminum dross was employed to realize high‐temperature foaming due to its oxidation, which makes traditional AlN and salts removal process needless. Moreover, near‐zero shrinkage or even expansion during sintering of porous ceramics have occurred because in‐situ foaming process together with the oxidation of Al particles well offset the sintering shrinkage. After sintering at 1400°C for 2 h, porous ceramics composed of α‐Al2O3 and spinel phases with open porosity of 37.91%, sintering expansion rate of 1.13%, flexural strength of 45.67 MPa, and thermal conductivity of 0.97 W/(m·K) have been prepared. Cenospheres as pore‐forming agents have been added to further improve the porosity, and alumina‐based porous ceramics with open porosity of 28.39%–43.20% and flexural strength of 15.80–52.48 MPa have been obtained. This effective solution for recycling secondary aluminum dross could supply high‐performance porous ceramics, which is expected to be applied in the fields of light‐weight structural components and thermal insulations. [ABSTRACT FROM AUTHOR]

Aluminum dross (AD) and red mud (RM) are waste discharged from aluminum industry. AD and RM contain 5-15 wt% of hazardous aluminum nitride (AlN) and 20–50 wt% of iron oxide, respectively. In this work, alumina-based cermet was in-situ prepared from AD and RM, after reduction of iron oxide with AlN by pyrometallurgy. Cermet was consisted of metal reinforcement phase about 1 μm, alumina matrix phase and low-melting aluminosilicate binder. Mechanism of in-situ preparation was studied after exploring phase composition of the cermet powder. Iron, silicon and titanium oxide from RM was reduced into Fe 3 Si and TiN, which acted metal reinforcement phase in cermet. Reaction was consisted of two steps, Fe 2 O 3 , TiO 2 and AlN dissolving into low-melting aluminosilicate and electron transferring from AlN to Fe 2 O 3 , SiO 2 and TiO 2. Content of Fe in ceramic phase was decreased from 6 at.% to 1 at.% after reduction reaction. Low-melting aluminosilicate acted both reaction solvent for reduction reaction, and binder for metal reinforcement phase and alumina matrix phase. More low-melting aluminosilicate was formed when temperature was increased from 1200 °C to 1400 °C, which made more metal balls be formed. The metal ball and alumina were bonded by low-melting aluminosilicate, which increased the bond strength between reinforcement phase and ceramic phase in cermet. This work achieved a total recycle of AD and RM into cermet. It was proposed a novel idea of in-situ preparing cermet, which was generating metal reinforcement phase in ceramic matrix phase. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Over 6.5 million tons of SAD is discharged on the planet annually. • Recycling SAD to make building materials is reviewed. • Recycling SAD to make ceramics possesses a better carbon reduction. • SAD can be made into metal/alumina ceramics with low carbon emission. Secondary aluminum dross (SAD) is consisted of alumina, aluminum nitride (AlN), salts and heavy metal oxides. Over 6.5 million tons of SAD is discharged in the world annually. Currently, recycling SAD to make building materials is a promising way of waste recovery. Building materials made from SAD can be divided into structure materials and insulation materials. Structure materials inherits the high strength, fire resistance and chemical stability of alumina in SAD. The products include cement, bricks, ceramics and glass ceramics. Insulation materials are manufactured by utilizing reactivity of AlN in SAD. AlN can hydrolyze or be oxidized to release gas. Insulation materials include porous bricks, porous ceramics and porous glass ceramics. Environmental and economic benefits analysis was conducted on structure materials and insulation materials made from SAD. Ceramics and porous ceramics were taken as evaluation objects. Manufacturing a ton of ceramics and porous ceramics from SAD attained profits of 370.2 and 388.2 dollars, and reduced carbon emissions of 746.2 and 261.5 kg. Therefore, recycling SAD to make ceramics possessed a better carbon reduction. In order to further reduce carbon emission and utilize AlN in SAD, we proposed a novel idea of manufacturing metal/alumina composite ceramics from SAD and metallurgical solid waste. AlN in SAD reduced the metal oxides into alloy, which can strengthen the alumina ceramics. We should clarify the thermodynamics and kinetics of AlN reducing metal oxides and strengthening mechanisms of metal/alumina composite ceramics. [ABSTRACT FROM AUTHOR]

Secondary aluminum dross (SAD) and pickling sludge were hazardous wastes. In this work, aluminum nitride (AlN) from SAD was used to reduce heavy metals from pickling sludge by pyrometallurgy. Reaction mechanism was studied after exploring transformation of heavy metals and AlN during reaction process. It was found that reduction reaction between heavy metals and AlN was homogeneous reduction reaction. When mixture was heated to 1000 °C, heavy metals and AlN were distributed independently, and reduction reaction had not occurred. When mixture was heated to 1300 °C, heavy metals and AlN entered aluminosilicate network, and homogeneous reduction reaction occurred. Half of the heavy metals were reduced into metallic state and mixed with the dross. When the mixture was heated at 1400 °C for 1 h, metal block separated from dross. Reduction rates of Fe, Cr and Ni were 94%, 88% and 100%. Like water solution, aluminosilicate melt could dissolve heavy metal oxides, aluminum compound (AlN, Al 4 C 3 , etc.) and silicon compound (Si 3 N 4 , SiC, etc.). This work proposed a novel homogeneous reaction environment, aluminosilicate melt 'solution' environment, which had a significant effect not only on heavy metals reduction, but also on other high temperature oxidation-reduction. [Display omitted] • AlN in SAD was used to reduce heavy metals by pyrometallurgy. • Reduction rates of Fe, Cr and Ni were 94%, 88% and 100%. • Reaction mechanism was studied by exploring AlN and heavy metals transformation. • Reaction between AlN and heavy metals was homogeneous reduction reaction. • Aluminosilicate melt 'solution' was proposed for homogeneous reaction. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fe, Cr and Ni were reduced by AlN in SAD. • Reduction rates of Fe, Cr and Ni were up to 97, 91 and 100% • Using a ton SAD for chromium metallurgy could reduce carbon emission of 3961 kg. 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 CaF 2 and soda. The CaF 2 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. [ABSTRACT FROM AUTHOR]

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 CaF 2 from 6.7 to 9.0 wt%. CaF 2 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 CaAl 2 SiO 6 crystal phase, crosslinked network structure grains and smaller pores exhibited better bending resistance. In addition, glass ceramics with CaAl 2 SiO 6 crystal phase possessed the highest microhardness and alkali resistance. After this process, hazardous pickling sludge and SAD were totally recycled. [Display omitted] • Hazardous pickling sludge and SAD were totally recycled by pyrometallurgy. • AlN in SAD was first used to reduce heavy metals. • Reduction rates for iron, chromium and nickel were 90%, 80% and 100%. • Bending strength and microhardness of the glass ceramics were 77 MPa and 1011 HV. [ABSTRACT FROM AUTHOR]