Green synthesis of mesoporous γ-Al2O3 from coal fly ash with simultaneous on-site utilization of CO2

Analysis of enrichment, correlation, and leaching patterns of rare earth elements in coal fly ash assisted by statistical measures

Coal fly ash (CFA) is a typical industrial solid waste, which has recently been reported to contain rare earth elements (REEs). REEs are important materials in many industrial fields. Therefore, extracting REEs from CFA becomes a win-win strategy to both make full use of CFA and reclaim REEs. However, the stable crystalline structure of CFA is hard to break, which limits the extraction of REEs. The inter-correlation and the leaching patterns of the REEs in CFA also remain unclear. In this work, REEs were enriched by desilication, and the correlation and the influences of multiple acids of the leached REEs were investigated. It was found that desilication could increase the leachable amount of REEs from 137.37 ppm to 346.12 ppm. The light rare earth elements (LREEs) were less inter-correlated than heavy rare earth elements (HREEs) and desilication enhanced the leaching of LREEs more than that of HREEs. The ratio and type of the leaching acids both influenced the extraction of REEs from CFA: HCl and HF played important roles in the extraction from the untreated CFA while HNO3 and HF were more decisive for the desilicated CFA. In addition, we used statistical analysis to quantificationally confirm that desilication and acids both significantly influenced the extraction of REEs. This work provides evidence for the enrichment of REEs in CFA and acid choosing when leaching REEs from CFA.

Kinetics Study of Al Extraction from Desilicated Coal Fly Ash by NaOH at Atmospheric Pressure

The most promising source of alumina in the 21st century is the coal fly ash (CFA) waste of coal-fired thermal plants. The methods of alumina extraction from CFA are often based on the pressure alkaline or acid leaching or preliminary roasting with different additives followed by water leaching. The efficiency of the alumina extraction from CFA under atmospheric pressure leaching is low due to the high content of acid-insoluble alumina phase mullite (3Al₂O₃·2SiO₂). This research for the first time shows the possibility of mullite leaching under atmospheric pressure after preliminary desilication using high liquid to solid ratios (L:S ratio) and Na₂O concentration. The analysis of the desilicated CFA (DCFA) chemical and phase composition before and after leaching has been carried out by inductively coupled plasma optical emission spectrometry (ICP-OES) and X-ray diffraction (XRD). The morphology and elemental composition of solid product particles has been carried out by scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX). An automated neural network and a shrinking core model (SCM) were used to evaluate experimental data. The Al extraction efficiency from DCFA has been more than 84% at T = 120 °C, leaching time 60 min, the L/S ratio > 20, and concentration of Na₂O-400 g L^-1. The kinetics analysis by SCM has shown that the surface chemical reaction controls the leaching process rate at T < 110 °C, and, at T > 110 °C after 15 min of leaching, the process is limited by diffusion through the product layer, which can be represented by titanium compounds. According to the SEM-EDX analysis of the solid residue, the magnetite spheres and mullite acicular particles were the main phases that remained after NaOH leaching. The spheric agglomerates of mullite particles with non-porous surface have also been found.

On the state of the art of crystalline structure reconstruction of coal fly ash: A focus on zeolites

Coal fly ash (CFA) is fine particles generated from coal combustion, and large amount of CFA causes environmental pollution. Traditionally, CFA is added into construction materials, which has realized effective reduction. As the exploration of CFA properties goes deeper, finer utilization has been studied to maximize the recycling of CFA. Summarized from plenty of investigations, structure reconstruction has become the most crucial part for re-production as well as pre-treatments. Various zeolites and other complex materials have been synthesized by structure reconstruction. In this work, the state of the art of structure reconstruction were technically collated in the order of pre-treatments, mechanisms, specific techniques, and novel optimizing strategies. It has been found the crystalline types are closely related to the reaction conditions, that certain types of products could be obtained via accurate condition controls, especially the ratio of Si to Al. The current as-synthesized products were listed as well as their crystalline structure characteristics. Recently, combined materials and techniques have been innovatively investigated. However, the challenge remains as low purity, not only impurities in CFA but also different types of zeolites formed in one process.

Biogas Upgrading via Cyclic CO2 Adsorption: Application of Highly Regenerable PEI@nano-Al2O3 Adsorbents with Anti-Urea Properties

Solid amine adsorbents are among the most promising CO₂ adsorption technologies for biogas upgrading due to their high selectivity toward CO₂, low energy consumption, and easy regeneration. However, in most cases, these adsorbents undergo severe chemical inactivation due to urea formation when regenerated under a realistic CO₂ atmosphere. Herein, we demonstrated a facile and efficient synthesis route, involving the synthesis of nano-Al₂O₃ support derived from coal fly ash with a CO₂ flow as the precipitant and the preparation of polyethylenimine (PEI)-impregnated Al₂O₃-supported adsorbent. The optimal 55%PEI@2%Al₂O₃ adsorbent showed a high CO₂ uptake of 139 mg·g^-1 owing to the superior pore structure of synthesized nano-Al₂O₃ support and exhibited stable cyclic stability with a mere 0.29% decay per cycle even under the realistic regenerated CO₂ atmosphere. The stabilizing mechanism of PEI@nano-Al₂O₃ adsorbent was systematically demonstrated, namely, the cross-linking reaction between the amidogen of a PEI molecule and nano-Al₂O₃ support, owing to the abundant Lewis acid sites of nano-Al₂O₃. This cross-linking process promoted the conversion of primary amines into secondary amines in the PEI molecule and thus significantly enhanced the cyclic stability of PEI@nano-Al₂O₃ adsorbents by markedly inhibiting the formation of urea compounds. Therefore, this facile and efficient strategy for PEI@nano-Al₂O₃ adsorbents with anti-urea properties, which can avoid active amine content dilution from PEI chemical modification, is promising for practical biogas upgrading and various CO₂ separation processes.

Clean production of porous-Al(OH)3 from fly ash

Fly ash is one of the largest solid waste and causes serious environment problems. Extraction of Al(OH)₃ from fly ash is beneficial to environment and economy. We developed a clean electrolysis method to generate hydroxyl groups in situ to extract Al(OH)₃ from fly ash leachate without adding chemicals or using expensive membranes, avoiding the introduction of new impurities, secondary pollutants generation, and membrane limitations. Batch experiments yielded porous electrolytic products with BET surface areas from 11.7610 to 25.5267 m²/g, pore volumes from 0.1935 to 0.1643 cm³/g and pore sizes from 65.7960 to 25.7434 nm. The composition of the electrolytic products was 86.43 wt% Al(OH)₃, 9.00 wt% SO₃, 1.67 wt% Fe(OH)₃, and 0.29 wt% Ca(OH)₂. The current efficiency was 90.51 % under optimized conditions of c (Al³⁺) = 0.1 M, t =2 h, and J = 750 A/m². Mean particle size was from 24.1-98.1 μm. Impurities mainly affected the composition of the electrolytic products. The OH- generated by H₂O reduction reacted with Al³⁺, Fe³⁺, and Ca²⁺ to generate a hydroxide. Fe³⁺ preceded Ca²⁺ into the hydroxide. H₂ released continuously from H₂O reduction, resulting in a porous hydroxide. The wastewater was reused as a leaching reagent to promote zero-pollution discharge.

Production of Gamma Alumina Using Plasma-Treated Aluminum and Water Reaction Byproducts

High purity hydrogen and solid-state byproducts are produced using a proposed plasma-activated aluminum and water reactions approach. These byproducts could be transformed into pure gamma Al2O3 powder material, while hydrogen can be used for electricity generation. Various chemical methods can be used for the synthesis of gamma alumina, but most could result in high levels of remaining impurities. Boehmite is a cost-effective starting material for the production of high-purity Al2O3. Herein, we present a novel method for the synthesis of boehmite and its transformation into high-specific-surface-area γ-alumina. Specifically, this method implicates the direct reaction between distilled water and plasma-treated aluminum powder. The results show the structural and morphological changes of the byproduct of the aluminum/water reaction to boehmite and γ-Al2O3 after a simple heating procedure (at 280 and 500 °C respectively). The high-purity hydrogen produced during the aluminum/water reaction can be used for the high-efficiency and environmentally friendly production of electrical energy.