Synthesis and characterization of tamarugite from aluminium dross: Part I

Particle sorting to improve the removal of fluoride and aluminum nitride from secondary aluminum dross by roasting

It is important to remove active substances from secondary aluminum dross (SAD) to meet the reuse of SAD. In this work, the removal of active substances from different particle sizes of SAD was studied using roasting improvement with particle sorting. The results showed that roasting after particle sorting pretreatment can effectively remove fluoride and aluminum nitride (AlN) from SAD, while getting the high-grade alumina (Al₂O₃) crude materials. The active substances of SAD mainly contribute to AlN, aluminum carbide (Al₄C₃), and soluble fluoride ions. AlN and Al₃C₄ mainly exist in particles of 0.05-0.1 mm, while Al and fluoride are mainly in particles of 0.1-0.2 mm. The SAD of particle size ranging 0.1-0.2 mm has high activity and leaching toxicity; the gas emission was reached 50.9 mL/g (limit value of 4 mL/g), and the fluoride ion concentration in the literature was 137.62 mg/L (limit value of 100 mg/L) during the identification for reactivity and leaching toxicity according to GB5085.5-2007 and GB5085.3-2007, respectively. Roasting at 1000 °C for 90 min, the active substances of SAD were converted to Al₂O₃, N₂, and CO₂; meanwhile, soluble fluoride converted to stable CaF₂. The final gas release was reduced to 2.01 mL/g while soluble fluoride from SAD residues was reduced to 6.16 mg/L, respectively. The Al₂O₃ content of SAD residues was determined at 91.8% and has been classified as category I solid waste. The results suggested that the roasting improvement with particle sorting of SAD can meet the reuse of valuable materials at full scale.

Removal of nitrides and fluorides from secondary aluminum dross by catalytic hydrolysis and its mechanism

Secondary aluminum dross (SAD) refers to hazardous waste from secondary aluminum refinement. It contains a large amount of aluminum nitride and fluorides that cause serious environmental pollution for direct discharge and hinder the resource utilization of SAD. However, it is difficult to remove nitride and fluoride simultaneously for their complicated phases. In this paper, the catalytic hydrolysis of SAD using NaOH as a catalyst to remove nitrides and fluorides synchronously was investigated systemically through single factor and response surface experiments. In addition, the chemical speciation and transformation of nitrides and fluorides were analyzed systematically. The catalytic hydrolysis removal mechanism was summarized. The optimal conditions for catalytic hydrolysis were established as follows: reaction temperature 96.60 °C; reaction time 2.85 h; liquid-solid ratio 9.28 mL/g and catalyst addition 12.62 wt %; and removal efficiency of nitrides and fluorides reached 99.03% and 81.93%, respectively. The mechanism of nitrides removal was that aluminum nitride was hydrolyzed to Al(OH)₃ and NH₃. NaOH reacting with Al(OH)₃ covering on the surface of AlN and the rapid escape of NH₃ promoted the hydrolysis of AlN under the catalysis of NaOH. The mechanism of fluorides removal was that the encapsulated fluoride particles were opened by catalytic hydrolysis to be dissolved in the solution. In this research, nitrides and fluorides were removed efficiently and synchronously. The hydrolysis residues can be used to prepare polyaluminum chloride (PAC) and ceramic materials. The hydrolysate can be prepared NH₃·H₂O by evaporative in alkaline solution. Then the solution without NH₄ ⁺ was prepared Al(OH)₃ by precipitation of adjusting pH value using HCl. And the remained liquid after removing NaAlO₂ was used to prepare refining agent by evaporative crystallization. The work in this paper was beneficial for the utilization of SAD.

Preparation of Sintered Brick with Aluminum Dross and Optimization of Process Parameters

Aluminum dross is produced in the process of industrial production and regeneration of aluminum. Currently, the main way to deal with aluminum dross is stacking and landfilling, which aggravates environmental pollution and resource waste. In order to find a green and environmental protection method for the comprehensive utilization, the aluminum dross was used as raw materials to prepare sintered brick. Firstly, the raw material ratio, molding pressure and sintering process were determined by single factor test and orthogonal test, and the mechanism of obvious change of mechanical strength of sintered brick was studied by XRD and SEM. The experimental results show that, the optimal formula of sintered brick is 50% aluminum dross, 37.50% engineering soil and 12.50% coal gangue. The optimum process parameters are molding pressure 10 MPa, heating rate 8 °C/ min, sintering temperature 800 °C, holding time 60 min. The samples prepared under the above formula and process parameters present outstanding performance, and the compressive strength, flexural strength and water absorption rate are 16.21 MPa, 3.42 MPa and 17.12% respectively.

Extraction of alumina from aluminum dross by a non-hazardous alkaline sintering process: Dissolution kinetics of alumina and silica from calcined materials

The aluminum dross (AD), which causes numerous problems of its management and disposal to environment is a useful resource to extract alumina. This study explains a novel process to extract highly pure alumina (Al₂O₃) from AD at a high extraction rate without producing the residues and exhaust gases. An experimental set up was designed to perform the grinding of AD for the decomposition of aluminum nitride (AlN) and the removal of salts. Thereby, the desalted dross was used to detect the optimum alkaline (NaOH) calcination parameters and leaching conditions, as well as the dissolution kinetics of alumina and silica. The leaching residues were used to produce Ettringite mineral with calcium-based compounds (including CaO and CaSO₄) to avoid the problems of solid waste disposal from the leaching process. Moreover, to purify the alumina, slightly soluble CaSO₄ was added in leaching solution to precipitate silicate and the optimum additive/solution ratio (g/mL) was determined. The aluminum hydroxide (Al(OH)₃), precipitated after the carbonization was calcinated at 900.0 °C for 2 h to produce γ-alumina. The morphological and mineralogical characterizations of AD, γ-Al₂O₃ and the synthesized Ettringite mineral were studied by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and X-ray fluorescence (XRF). It was observed that activation temperature of 1000.0 °C, Na₂O/Al₂O₃ molar ratio of 1.4, leaching temperature of 60.0 °C, leaching time of 40.0 min, and the leaching liquid/solid ratio (mL/g) of 25/1 were the optimal parameter conditions to extract alumina with the extraction rate at 86.7% and purity of more than 98%. The results of leaching kinetics' study showed that the dissolution of alumina and silica were both controlled by layer diffusion process with the apparent activation energy of 11.4010 kJ·mol^-1 and 2.0556 kJ·mol^-1, respectively.

Harmless disposal and resource utilization for secondary aluminum dross: A review

Secondary aluminum dross (SAD) is solid waste of primary aluminum dross extracted aluminum, which contains approximately 40-60 wt% alumina, 10-30 wt% aluminum nitride (AlN), 5-15 wt% salts and other components. The salts include sodium chloride, potassium chloride and fluorine salts. SAD has dual attributes as resource and pollutant. SAD landfill disposal has the disadvantages of occupying land, wasting resources, a high cost and great environmental impact. SAD utilization methods are currently pyrometallurgy and hydrometallurgy. In pyrometallurgy, AlN is oxidized and the salts are evaporated at high temperature. After mixing, molding and calcination, firebricks and ceramics can be manufactured from SAD. In hydrometallurgy, AlN is hydrolyzed and salts are dissolved in water. After dissolving, filtrating, precipitating, washing and calcination, γ-Al₂O₃ can be prepared from SAD. Resource consumption and emission from both utilization methods were assessed. A ton of magnesium aluminum titanate based ceramics by pyrometallurgy consumes 1043 kg raw materials and releases 69 kg of waste gas, 4.17 t of waste water and no solid waste. A ton of γ-Al₂O₃ by hydrometallurgy consumes 3389 kg raw materials and releases 111 kg of waste gas, 12.98 t of waste water and 267 kg of solid waste. Therefore, the resource consumption and emission of SAD utilization by pyrometallurgy is lower than that by hydrometallurgy. We should focus on reducing the emission of the three wastes from pyrometallurgy. We are sure that SAD can be utilized for glass ceramics by pyrometallurgy. AlN and salts can be transformed into alumina and glass phases at high temperature with no emission. We should clarify mechanisms for SAD composition adjustment to lower the glass ceramics' melting point, AlN and salts transformed into alumina and glass phases respectively, and nucleation and crystal growth of glass ceramics at high temperature.

Synthesis of Cryolite (Na3AlF6) from Secondary Aluminum Dross Generated in the Aluminum Recycling Process

Secondary aluminum dross (SAD) is regarded as a solid waste of aluminum recycling process that creates serious environmental and health concerns. However, SAD can also be used as a good source of aluminum, so that utilizing the SAD for the production of valuable products is a promising approach of recycling such waste. In the present work, a novel eco-friendly three-step process was proposed for the synthesis of cryolite (Na3AlF6) from the SAD, and it consisted of (1) water-washing pretreatment of SAD, (2) extraction of Al component via pyro-hydrometallurgy, including low-temperature alkaline smelting, water leaching and purification of leachate in sequence, (3) precipitation of cryolite from the purified NaAlO2 solution using the carbonation method. By analysis of the parameter optimization for each procedure, it was found that the maximum hydrolysis efficiency of aluminum nitride (AlN) in the SAD was around 68.3% accompanied with an extraction efficiency of Al reaching 91.5%. On this basis, the cryolite of high quality was synthesized under the following optimal carbonation conditions: reaction temperature of 75 °C, NaAlO2 concentration of 0.11 mol/L, F/(6Al) molar ratio of 1.10, and 99.99% CO2 gas pressure, and flow rate of 0.2 MPa and 0.5 L/min respectively. The formation of Na3AlF6 phase can be detected by XRD. The morphological feature observed by SEM revealed that the as-synthesized cryolite had a polyhedral shape (~1 μm size) with obvious agglomeration. The chemical composition and ignition loss of the as-synthesized cryolite complied well with the requirements of the Chinese national standard (GB/T 4291-2017).