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COMMON PART

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,

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.

PROJECT CONTENT

Annotation
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.

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