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Open AccessArticle

Aluminum Extractions by the Alkali Method Directly from Alkali-Acid (NaOH-HCl) Chemical Deashing of Coals

by Lijun Zhao

Materials 2025, 18(15), 3661; https://doi.org/10.3390/ma18153661 - 4 Aug 2025

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An advanced alkali-acid (NaOH-HCl) chemical method was used to deash aluminum-rich coals (ARCs) with a high ash content of 27.47 wt% to achieve a low ash content of 0.46 wt%. In the deashing process, aluminum in the coal ashes was dissolved in both alkali solutions and acid solutions. The deashing alkali solutions with dissolved coal ashes were regenerated by adding CaO, and the resulting precipitates were added with sodium bicarbonate for aluminum extraction. High temperatures increased aluminum extraction, and excessive sodium bicarbonate addition decreased aluminum extraction. The deashing acid solutions were concentrated by evaporation, and silica gels formed during the process. The obtained mixtures were calcinated at 350 °C for the decomposition of aluminum chlorides, and soaked with water at 60 °C to remove the soluble chlorides. For the insoluble oxides after soaking, diluted alkali solutions were used to extract the aluminum at 90 °C, and aluminum extraction failed due to the formation of albite in the presence of sodium, aluminum and silicon elements as proved by XRD and SEM/EDS. When silica gels were separated by pressure filtering, aluminum extraction greatly increased. Aluminum extractions were accordingly made in the form of sodium aluminate from the deashing solutions of coals, which could be advantageous for sandy alumina production. Full article

(This article belongs to the Section Materials Chemistry)

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18 pages, 16102 KB

Open AccessArticle

Study on the Deashing of Lignite with Hydrochloric Acid/Sodium Fluoride Leaching, Assisted by Microwave and Ultrasonic Waves

by Xinming Ran and Jie Yuan

Materials 2024, 17(14), 3537; https://doi.org/10.3390/ma17143537 - 17 Jul 2024

Cited by 1 | Viewed by 1253

Abstract

This study was aimed at investigating the effects of adding sodium fluoride (NaF) and using the assistance of ultrasonic and microwave energy on the removal efficiency of ash content during the hydrochloric acid (HCl) chemical leaching process of lignite samples from Zhaotong, Yunnan, China. Chemical leaching was conducted on lignite samples from Zhaotong, Yunnan, China, under the experimental conditions of time (30–120 min), temperature (55–95 °C), microwave power (240–800 W), ultrasonic power (25–100%), and NaF addition concentration (0.2–1.2 M). The addition of NaF greatly improved the removal efficiency of ash content from lignite. Under optimized conditions, the addition of NaF increased the removal rate of ash content from lignite from 25% to 65.27%. The microwave-assisted deashing of lignite can significantly improve the deashing efficiency, with positive implications for the microstructure regulations of lignite. Ultrasonic-assisted deashing can lower the temperature for coal powder burnout and enhance the combustion performance of coal. Full article

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16 pages, 8611 KB

Open AccessArticle

Effect of Deashing Treatment on Ash Fusion Characteristics of Biochar from Bamboo Shoot Shells

by Hao Ren, Qi Gao, Liangmeng Ni, Mengfu Su, Shaowen Rong, Shushu Liu, Yanhang Zhong and Zhijia Liu

Molecules 2024, 29(6), 1400; https://doi.org/10.3390/molecules29061400 - 21 Mar 2024

Cited by 3 | Viewed by 2160

Abstract

To investigate the influence of deashing on fusion characteristics, a combined method of water and acid washing with different sequences (water washing followed by acid washing, and acid washing followed by water washing) was used to treat the biochar of bamboo shoot shells (BBSSs). The results show that deashing decreased the K content of the biochar from 50.3% to 1.08% but increased the Si content from 33.48% to 89.15%. The formation of silicates and aluminosilicates from alkali metal oxides with silicon was an inevitable result of ash phase transformation at the high temperatures used to improve the fusion temperature (>1450 °C). The thermochemical behavior of ash mainly occurs at 1000 °C. The deashing treatment significantly reduced the reaction intensity during the high-temperature process. This significantly increased the thermal stability of the ash. The adjustment of the washing sequence had a slight impact on the chemical compositions, but the differences in ash micromorphology were obvious. Deashing treatments with different washing sequences can significantly improve ash fusion properties effectively and reduce the risk of scaling, slagging, and corrosion. This study provides a new and reasonable strategy for the deashing of biochar to commercially utilize bamboo shoot shell resources. Full article

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14 pages, 3306 KB

Open AccessEditor’s ChoiceArticle

Deashed Wheat-Straw Biochar as a Potential Superabsorbent for Pesticides

by Irmina Ćwieląg-Piasecka, Elżbieta Jamroz, Agnieszka Medyńska-Juraszek, Magdalena Bednik, Bogna Kosyk and Nora Polláková

Materials 2023, 16(6), 2185; https://doi.org/10.3390/ma16062185 - 9 Mar 2023

Cited by 19 | Viewed by 5492

Abstract

Biochar activation methods have attracted extensive attention due to their great role in improving sorptive properties of carbon-based materials. As a result, chemically modified biochars gained application potential in the purification of soil and water from xenobiotics. This paper describes changes in selected physicochemical properties of high-temperature wheat-straw biochar (BC) upon its deashing. On the pristine and chemically activated biochar (BCd) retention of five pesticides of endocrine disrupting activity (carbaryl, carbofuran, 2,4-D, MCPA and metolachlor) was studied. Deashing resulted in increased sorbent aromaticity and abundance in surface hydroxyl groups. BCd exhibited more developed meso- and microporosity and nearly triple the surface area of BC. Hydrophobic pesticides (metolachlor and carbamates) displayed comparably high (88–98%) and irreversible adsorption on both BCs, due to the pore filling, whereas the hydrophilic and ionic phenoxyacetic acids were weakly and reversibly sorbed on BC (7.3 and 39% of 2,4-D and MCPA dose introduced). Their removal from solution and hence retention on the deashed biochar was nearly total, due to the increased sorbent surface area and interactions of the agrochemicals with unclogged OH groups. The modified biochar has the potential to serve as a superabsorbent, immobilizing organic pollutant of diverse hydrophobicity from water and soil solution. Full article

(This article belongs to the Special Issue Biochar and Carbon-Based Materials: Properties and Applications)

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11 pages, 1548 KB

Open AccessArticle

Alumina and Silica Extraction and Byproduct Development Directly from Chemical Deashing of Coals

by Lijun Zhao

Minerals 2022, 12(2), 179; https://doi.org/10.3390/min12020179 - 29 Jan 2022

Cited by 5 | Viewed by 3383

Abstract

Coal ashes (minerals) could be chemically removed to produce ultraclean coals for advanced utilizations such as oil substitutes and electrode materials. To eliminate secondary pollution and reduce deashing cost, chemicals should be recycled and valuable byproducts developed, in addition to ultraclean coals. In this work, an advanced alkali–acid (NaOH–HCl) chemical method featuring submolten salts was used to deash coals with high ash of 27.95%, and ultraclean coals were prepared with low ash of 0.62%. The alkali solutions after treating coals were regenerated by adding CaO, and the resulting precipitates were transformed into CaSO₄ by adding dilute H₂SO₄, while alumina and silica were dissolved in acid solutions. The hydrochloric acid (HCl) after treating coals could be largely regenerated by evaporation. From concentrated solutions after evaporation, silica gels occurred with high purity, which were then filtered for the production of silicate fertilizer, highlighting low heavy metal content and Na₂O. Concentrated H₂SO₄ was added into the remaining acid filtrate, and sulfates were precipitated and redissolved to remove CaSO₄. By further two-step calcinations, alumina of high purity (98.6%) could be produced. Alumina and silica extraction and byproduct development from directly deashing the coals were compared with those from fly ashes. Full article

(This article belongs to the Special Issue Critical Material Recovery from Coal and Coal Byproducts)

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