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Project Number18-79-00305

Project titleComplex processing of fly ash from coal power plants by hydrochloric acid

Project LeadValeev Dmitry

AffiliationBaikov Institute of Metallurgy and Material Science, Russian Academy of Sciences,

Implementation period 07.2018 - 06.2020 

Research area 09 - ENGINEERING SCIENCES, 09-204 - Equilibrium and kinetics of processes within chemically reactive systems

KeywordsCoal fly ash, hydrochloric acid, autoclave leaching, flotation, reduction roasting, spray drying, alumina, coagulant, scandium, silica, iron, portland cement.



Every year in Russia more than 123 million tons of solid fuel is burnt, this produces ~ 25 million tons of coal fly ash (CFA), only 2 million tons per year is received for processing. Ash dumps occupy large areas within urban areas (more than 22 thousand ha), and transportation of coal fly ash due to the high water consumption from 10 to 100 m3/t, thus the poisoning of surrounding areas occurs, making them unsuitable for economic use. The arrangement and operation of ash dumps require a significant investment that leads to the increase of cost of electricity [1]. Complex processing of ash will improve the environment by eliminating the source of environmental pollution and save thousands hectares of fertile land due to the lack of fossil remains. The ash contains up to 33 % of aluminum oxide, which makes this kind of raw materials the most promising as alternatives to bauxite during production of alumina, the main material used to produce aluminum metal [2]. Currently, the main part of bauxite (~60%) comes from abroad (Australia, Jamaica, Guyana, and Guinea). The supply of aluminum ores from remote fields are marginal due to the constantly rising costs for the freight of ships and transportation by rail. Large extent of the logistics flows make the problem even more severe [3]. Unlike natural raw materials, CFA does not require the costs of exploration, extraction and transport from abroad, which significantly reduces the cost of processing [4]. Utilization of ash for alumina solves the problem of raw material security of the country in the field of production of aluminum metal. Aluminium in the ash is in the form of the mineral mullite. This is aluminosilicate which processing by traditional alkaline technology is possible with the use of the preliminary calcination of the raw material with limestone, for conversion of silica into the alkali-insoluble calcium silicate. Calcination is an optional, sophisticated in technology and hardware design stage, requiring significant consumption of heat [5]. The use of alternative methods allows the conversion of aluminium into the solution and conversion of silica into sediment [6]. However, they have several drawbacks. For example, the use of sulfuric acid leads to the deposition of calcium sulfate on the surface of ash particles, which prevents the interaction of aluminum with sulfate ion [7], and the use of the fluoride method associated with noxious emissions of ammonia and fluorine [8]. Hydrochloric acid method is the most promising, as it allows to use of raw materials of different composition and regenerates of HCl for re-use [9-10]. Due to the low reactivity of mullite [11], this project will first be used for autoclave leaching of CFA by hydrochloric acid. In addition to alumina, the CFA contains a significant amount of iron oxides, which can be separated by reduction roasting followed by magnetic separation. However, the magnetic phases formed during the roasting process are too small and it’s difficult to separate them from the gangue. To enlarge the magnetic phases during roasting, a small amount sulphates or carbonates of alkali metals are added. This method has found application by utilize the waste of the alumina industry - red mud. [12-15]. Since ash and red mud are highly dispersed anthropogenic wastes of similar composition and physico-chemical properties, so this approach can therefore be applied with high efficiency to recover iron powder from CFA. In addition to aluminium, rare earth metals (REM) convert into solutions after leaching, the total content of ashes can be up to 600 g/t. Sorption of REM from acid solutions is extensively used in ferrous metallurgy [16]. The use of this approach will allow you to select individual REM in the target product that will significantly increase the overall profitability of the developed technological process. After separation of REM, solutions are subjected to spray-drying. Depending on the process temperature, it is possible to obtain polyaluminum chloride (PACH) or amorphous aluminum oxide. PACH is a reagent-coagulant, which is used for cleaning potable and waste water. The use of PACH in comparison with the currently used aluminum sulfate allows working at lower temperatures (down to -22 °C), with simultaneous reduction of reagent consumption and residual aluminium in treated water [17]. Amorphous aluminium oxide can be further processed with alkaline technology (Bayer method) with obtaining of smelter grade alumina brand G-0. Solid waste for hydrochloric acid technology - amorphous silica. It can be used as an active mineral additive in Portland cement. The strength of Portland cement stone will increase by 10-12% at 28-day age [18-19]. This project will reduce the negative impact of CFA on the environment and will allow you to obtain different types of products (alumina, polyaluminum chloride, scandium concentrate, silica, iron powder, active mineral additive in cement) with lower cost than existing technologies.

Expected results
Upon completion of the project, it is expected to obtain the following main results: 1st year: – the optimal parameters of the process of electromagnetic separation of magnetite from the ash, depending on the magnetic field strength and flotation process for separation of carbon concentrate (underburning); – thermodynamic analysis of the behavior for components of the magnetic fraction of CFA in the reduction roasting process; – optimal conditions of low-temperature reduction roasting (roasting temperature, amount of additives, roasting duration); – optimal conditions of autoclave leaching of ash (temperature, concentration of hydrochloric acid, liquid to solid ratio and leaching duration); – kinetics of interaction for the mullite with hydrochloric acid in autoclave conditions; – the effect assessment of amorphous silica additives for the strength properties of Portland cement. 2nd year – characteristics of the process of spray drying solutions (temperature, power, ultrasonic spraying); – evaluation of coagulating ability of polyaluminum chloride to reduce turbidity, color and permanganate oxidation of natural waters containing suspended and organic matters; – balance of rare earth metals in semi-products of the technological process; – conditions of REM sorption depending on the acidity of the medium and types of ion exchangers, the optimal reagents for washing and desorption, as well as data on distribution of main impurities (aluminium and iron) at all stages of the sorption process; – characteristics of the process for leaching of amorphous alumina with subsequent formation of aluminum hydroxide by carbonization method; – characteristics of the physical properties of the alumina obtained by the calcination of aluminium hydroxide (phase composition, specific surface area, average particle size). Expected results meet the international level of research and will significantly expand the ashes processing means. The implementation of this technology will not require significant costs for design of new metallurgical apparatus, as separate conversions have been successfully implemented in the chemical industry (autoclave decomposition of aluminium hydroxide in hydrochloric acid in the production of polyaluminum chloride and production of dry coagulant spray drying of solutions at AURAT JSC (Moscow). Alkaline branch of the developed technology can be used in existing plants for the production of alumina, which will not only allow to refuse the import of raw materials, but also to prevent the closure of the core enterprises due to the low profitability of the existing technologies. The use of amorphous silica as an active mineral additive in Portland cement will allow the utilization of the entire volume of ash without the formation of additional waste. Implementation of the developed process will give impetus to the integrated use of coal fly ash, will significantly reduce the cost of alumina and other by-products that will increase the competitiveness of steel mills of Russia in the world market.



Annotation of the results obtained in 2019
In the second year of the project, systematic research on the use of an aluminum chloride solution obtained after autoclave leaching of the non-magnetic fraction of ash from TPP-4 in Omsk by hydrochloric acid was carried out. The optimal doses of the liquor (coagulant) were determined and its effectiveness was studied to reduce the turbidity, chromaticity, and permanganate index of natural waters containing suspended and organic substances. A comparison of the coagulant obtained by ash leaching with the industrial sample polyaluminum chloride Aqua-Aurat-18 were made. It was shown that the efficiency of the obtained coagulant is comparable with the industrial analogue, and the content of Al, Fe and heavy metals in purified water corresponds to the WHO requirements. The regularities of the scandium sorption kinetics from hydrochloric acid liquors by the Purolite S-957 resin were studied. The influence of the S:L ratio and the liquor temperature on the Sc and Ti extraction degree was studied. It was shown that with optimal technological parameters (S: L = 1:100, duration 24 h), the Sc extraction degree was 90%, titanium 80%. The sorption mechanism was studied and the activation energies for scandium were determined: Ea = 32.87 kJ/mol and for titanium Ea = 36.97 kJ/mol. Analysis of the sorption isotherms using the Langmuir and Sips equation allowed to show that the internal diffusion is the limiting stage at the initial stage of sorption; as the concentration of scandium in the liquor decreases, the mechanism changes to external diffusion; unlike titanium, scandium can be sorbed in several layers. The resin capacities were determined: for Sc - 0.472 mg/g and Ti - 296.293 mg/g. The dispersed composition of the spray torch of ultrasonic atomization of aluminum chloride liquors to obtain a dry coagulant was determined. It is shown that this powder consists of fragile spherical particles with an average size of 39.96 um. A innovate new acid-alkali method for sandy grade alumina production from coal fly ash has been developed. The salting-out process for precipitate ACH crystals from an aluminum chloride liquor in the temperature range of liquor 0 °С - 95 °С was studied; the influence of the liquor temperature on the yield of ACH crystals and the behaviour of impurity elements was shown. The calcination of ACH crystals in air atmosphere was studied. It was shown that calcination at 400 °С allows obtaining amorphous alumina with a specific surface area of 36.9 m2/g, which can subsequently be recrystallized in an alkaline solution on the surface of coarse aluminum hydroxide. It was determined that during recrystallization, the particle size of gibbsite increases from 76 μm to 99 μm. The alumina obtained after calcination corresponded to GOST 30558-2017 “Metallurgical alumina. Specifications" on the chemical and physical composition. The chlorine content in alumina did not exceed 0.004 wt. % The author of the project was attended in the following scientific events with the report about complex utilization of coal fly ash : 1. Complex utilization of ash from coal-fired power plants by autoclave hydrochloric acid leaching // XXI Mendeleev Congress on General and Applied Chemistry; St. Petersburg, 09-13 of September 2019. Type of report: presentation. 2. Extraction of alumina from the coal fly ashes by hydrochloric acid // International Congress and Exhibition "Non-Ferrous Metals and Minerals 2019" and XXXVII Conference ICSOBA (International Committee for Study of Bauxite, Alumina & Aluminum); Krasnoyarsk, 16-20 of September 2019. Type of report: presentation. 3. Recycling of ashes from coal-fired power plants by hydrochloric acid to produce alumina // XVI Russian annual conference of young researchers and graduate students "Physical chemistry and technology of inorganic materials" (with international participation); Moscow, 01-04 of October 2019. Type of report: presentation. 4. Enrichment of coal fly ash by wet magnetic separation and flotation methods // V Interdisciplinary scientific forum with international participation "New materials and advanced technologies"; Moscow, 30.10.2019 - 01.11.2019. Type of report: presentation.



1. Valeev D.V., Kunilova I.V., Alpatov A.V., Mikhailova A.B., Goldberg M.A., Kondratiev A.V. Complex utilisation of ekibastuz brown coal fly ash Iron & carbon separation and aluminum extraction Journal of Cleaner Production, Volume 218, Pages 192-201 (year - 2019)

2. Valeev D.V., Kunilova I.V., Alpatov A.V., Varnavskaya A.D., Ju Dianchun Magnetite and Carbon Extraction from Coal Fly Ash Using Magnetic Separation and Flotation Methods Minerals, 2019, 9(5), 320 (year - 2019)

3. Valeev D.V., Kunilova I.V., Shoppert A.A., Salazar-Concha K., Kondratiev A.V. High-pressure HCl autoclave leaching of coal ash to extract Al into a chloride solution with further use as a coagulant for water treatment Journal of Cleaner Production, - (year - 2020)

4. Valeev D.V., Shoppert A.A., Mikhailova A.B., Kondratiev A.V. Acid and Acid-Alkali Treatment Methods of Al-Chloride Solution Obtained by the Leaching of Coal Fly Ash to Produce Sandy Grade Alumina Metals, V. 10 (5), P. 585 (year - 2020)

5. Valeev D.V., Varnavskaya A.D. Изучение способов обогащения и солянокислотное выщелачивание зол угольных электростанций Труды Кольского научного центра РАН, №1 (3), Том 10, с.37-43 (year - 2019)

6. Valeev D.V., Shoppert A.A. Extraction of Alumina from the Coal Fly Ash by Hydrochloric Acid Proceedings of the 37th International ICSOBA Conference and XXV Conference «Aluminium of Siberia», TRAVAUX 48, AA30, p.417-426 (year - 2019)

7. Valeev D.V., Shoppert A.A. Извлечение глинозема из зол угольных электростанций с помощью соляной кислоты Цветные Металлы и Минералы - 2019, С. 153-162 (year - 2019)

8. - Ученые из России сделали угольные ТЭС более экологичными и выгодными РИА Новости, 11:22 | 27.05.2019 (year - )

9. - Ученые извлекли 99% несгоревшего углерода из угольной золы с ТЭС Газета.ru, 12:24 | 27.05.2019 (year - )

10. - Ученые ИМЕТ РАН разработали новый способ утилизации отходов угольных электростанций — золошлаков Открытая Наука, 04.05.2020 (year - )

11. - Отходы угольных станций помогут алюминиевой промышленности Научно-информационный портал “Поиск”, 04.05.2020 (year - )

12. - В России создали новый способ утилизации отходов угольных электростанций Indicator.Ru, 13:43 | 06.05.2020 (year - )

13. - Отходам угольных электростанций нашли суперприменение Метеовести, 17:27 | 07.05.2020 (year - )

Annotation of the results obtained in 2018
In the first year of the project, systematic studies of the gravitational enrichment of the ashes of TPP-4 in the city of Omsk followed by hydrochloric acid leaching of an aluminosilicate concentrate were carried out. The influence of the magnetic field on the output of the magnetic fraction during dry and wet ash separation has studied. The thermodynamic analysis of the Al2O3 – SiO2 – Na2O ternary diagram with the addition of CaO and MgO in the process of reduction roasting of magnetite concentrate was carried out using the program FactSage v.7.1. The optimum temperature and fluxing additive for obtaining grain of metallic iron of maximum size have determined. The process of ash flotation was investigated, and the effects of the collector consumption: kerosene and diesel fuel on the yield, content, extraction of the carbon fraction and the enrichment efficiency indicator are determined. The optimal technological parameters of autoclave leaching of ash with hydrochloric acid (temperature, HCl concentration, T: W ratio and particle size) and kinetics characteristics (activation energy and order of the reagent) were determined. The effect of the addition of amorphous silicon dioxide, obtained after leaching of ash with hydrochloric acid on the strength properties of Portland cement, was studied. In order to test the results of the research, participation in the following scientific events taken: 1) VII International Scientific and Technical Conference of Young Scientists, Postgraduates and Students "High Technologies in Modern Science and Technology", Section 2: Rational use of natural and human-made mineral raw materials and water resources, November 26-30, 2018 National Research Tomsk Polytechnic University (TPU), Tomsk. 2) XIII Interregional Scientific and Technical Conference of Young Scientists, Specialists and Students of Universities "Scientific and Practical Problems in the Field of Chemistry and Chemical Technologies", April 17-19, 2019 I.V. Tananaev Institute of Chemistry and Technology of Rare Elements and Mineral Raw Materials of the Russian Academy of Sciences Kola Science Center



1. D.Valeev, I.Kunilova, A.Alpatov, A.Mikhailova, M.Goldberg, A.Kondratiev Complex utilisation of ekibastuz brown coal fly ash Iron & carbon separation and aluminum extraction Journal of Cleaner Production, Volume 218, Pages 192-201 (year - 2019)

2. - Ученые из России выяснили, как полностью переработать угольную золу РИА Новости, - (year - )

3. - Найден способ снизить количество токсичных отходов при сжигании угля Индикатор, - (year - )

4. - Металлурги придумали, как снизить количество токсичных отходов при сжигании угля Газета.ру, - (year - )