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Evaluation of Zeolite/Backfill Blend for Acid Mine Drainage Remediation in Coal Mine Vera L. V. Fallavena, Marçal J. R. Pires, Suzana Frighetto Ferrarini, and Ana Paula Buss Silveira Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03322 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018
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Evaluation of Zeolite/Backfill Blend for Acid Mine Drainage Remediation in Coal Mine Vera L. V. Fallavenaa*, Marçal Piresa, Suzana Frighetto Ferrarinia, Ana Paula Buss Silveiraa a
Faculty of Chemistry, Pontifical Catholic University of Rio Grande do Sul, Porto
Alegre, Brazil; Keywords: Backfill; zeolite; tailing, acid mine drainage; remediation.
Abstract: Brazilian coal exhibits high ash and sulfur content, making it more susceptible to acid mine drainage (AMD). Furthermore, combustion of this coal generates a significant amount of ash containing amorphous silicon and aluminum oxides, allowing alkaline hydrothermal processing of this waste via zeolites. These versatile compounds have been suggested as an alternative material for AMD remediation. This study aims to evaluate the use of zeolite Na-P1 in backfill blends for application in coal mines. Laboratory scale leaching experiments were performed and several techniques (major and trace elements, pH and conductivity) were monitored over time, in order to assess AMD remediation and the mobilization of ions in leachates using different zeolite:tailing blends. The results indicate that the addition of zeolites, obtained from coal ash, promotes the remediation of metal content in water generated by AMD and is therefore a more beneficial process in the removal of metal ions. With regard to the 50:50 backfill:zeolite blend, reductions of 100, 98, 39, 55, 94 and 41% were observed in aluminum, iron, calcium, magnesium, zinc and manganese content, respectively, after 7 days of leaching. The increase in pH caused by zeolite addition promoted the precipitation of both metal ions and sulfate ion. Addition of 50% or 25%
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of zeolite in the backfill sample increased pH from 2.36 to 8.38 or 4.66, respectively, over a leaching period of 7 days. Keywords: Backfill; zeolite; tailing; acid mine drainage; remediation.
1. INTRODUCTION Mining has always been, and remains, an essential activity for humanity, since a predominant portion of industry and energy production uses raw materials excavated from mines.1 In Brazil, approximately 1.5% of the electricity produced is generated in seven coal-fueled power plants in the southern states.2,3 Around 85% of the country`s coal is consumed in the production of thermoelectricity, 6% in the cement industry, and the remainder in other industries.3,4 The largest coal deposits are located in the states of Rio Grande do Sul and Santa Catarina. In situ coal reserves in Brazil total 32 billion metric tons, 89.25% in Rio Grande do Sul and 10.41% in Santa Catarina.5 Coal in Brazil contains on average 50% ash, consists largely of clay minerals, has a high sulfur content, and is usually classified as low-rank.5−7 Solid waste and tailings consisting primarily of carbonaceous and mineral materials are discarded during coal mining and beneficiation. There is growing concern regarding the oxidation of sulfide minerals (mainly pyrite, the most abundant sulfide on the planet) in sulfide-rich coal, resulting from exposure to oxygen and water or bacteria (Thiobacilus ferroxidans, Gallionella ferruginea). This oxidation promotes acidification of the environment and consequently the dissolution of metals in abandoned or operational mines, leading to acid mine drainage (AMD).8−9 This prompts the mobilization of iron ions and sulfate to the medium in a wide pH range.10-13 In the absence of alkaline materials, acid production reactions can continue indefinitely.9
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In the year 2016, approximately 5.3 million metric tons of solid waste and tailings were generated by Brazilian mining companies through coal beneficiation.14 As previously mentioned, a typical mine generates about 3 million m³ of AMD annually. The continued generation of large volumes of AMD over extended periods of time (centuries) has prompted an ongoing search for AMD mitigation processes.15 Additionally, electricity generation through the burning of bituminous and subbituminous coal and lignite, produces solid waste including fly ash, bottom ash, boiler slag and flue gas desulphurization materials. In many countries, these by-products of coal burning represent the second largest waste stream after municipal solid waste.16 In Brazil, about 3 million metric tons of ash is produced every year from coal burned to generate thermoelectricity.5 Leaching of the ash gives toxic elements potential access to groundwater, contaminating current or future water sources.17 Another byproduct of coal combustion is flue gas desulphurization (FGD) gypsum. An electric power station block can generate up to 20 tons of FGD gypsum per hour and there are insufficient applications for such a high amount.16 In light of growing concern about preserving the environment and the proximity of urban settlements and mines, research into the environmental impact of mining and coal use is ongoing.1,18-20 This includes backfilling mined areas with tailings as a means of using high amounts of industrial waste. The use of by-products such as fly ash, slag and metallurgical silica waste to produce backfill materials has been widely reported.21 In the development of low cost products, the use of slag and fly ash has been reported in the production of alkali-activated cements in aggressive environments for application in paste backfill.22 Backfill refers to any material (waste) used to refill underground mines to improve their ventilation, prevent fires and explosions, and increase rock stability, as well as for
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engineering purposes related to structural stability, economic and environmental factors.23-25 According to Masniyom25, three main aspects should be considered when characterizing backfill material, namely it physical, mechanical and chemical properties. The author investigated the effect of different parameters on backfill behavior and permeability, including grain size, age of specimens, binder factor and water to binder, fly ash and filter dust content. Different blends were used in the backfill composition (synthetic anhydrite, natural anhydrite, cement, filter dust, tailings, flue gas desulfurization gypsum and fly ash). The author noted that backfill mixes using fly ash leached often and exhibited lower trace mineral concentrations than cement-only backfill. According to Ward et al.26, the use of coal ash in backfill and mine remediation is gaining attention from energy and mining industries as a beneficial process for the disposal of ash. The most commonly reported benefits of using ash as backfill in mines typically result from interaction between alkaline ash and solids from mines, mine water, or open spaces in mines that promote acid drainage conditions. Yao and Sun21 investigated the use of a silica alumina-based backfill material using large amounts of coal combustion by-products. The results indicated that milled fly ash with a large specific surface area significantly enhanced the performance of the silica alumina-based backfill, which was capable of stabilizing and/or solidifying hazardous elements. The production of a commercially valuable by-product derived from coal ash for use in effluent treatment is attractive.15 It is important to note that coal ash consists primarily of silica and alumina and can be transformed to zeolitic materials via heat treatment in an alkaline medium.27 Zeolites have been suggested as an alternative
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material, due to their high ion exchange capacity and mechanical stability in effluent treatment.9,27-30 According to Fungaro and Izidoro9, synthetic zeolite can be used as a low-cost absorber, exhibiting a cation exchange capacity of 100 to 396 meq 100 g-1. Synthetic zeolites are hydrated aluminosilicates synthesized by hydrothermal treatment with strong alkali (NaOH) of Si and Al precursors. They are crystalline and microporous, formed by type TO4 tetrahedrons (T = Si, Al). Zeolites exhibit structural cavities and interconnected channels of molecular dimensions containing water molecules, compensation ions or other adsorbates. This type of structure provides a large internal surface compared to the outer surface and allows the transfer of matter in the intracrystalline space, but is limited by the diameter of zeolite pores.27,31 The Na-P1 zeolite used in this experiment was obtained by integrated synthesis. In this process, in addition to obtaining a high purity zeolite (4A), a second low grade (NaP1) and low cost product is formed. Zeolite Na-P1, considered waste in this process, can then be used as a viable alternative to form a blend with the backfill material in order to minimize AMD. The zeolite confers desirable properties on the backfill/zeolite blend, such as alkalinity and high metal adsorption capacity. According Cardoso et al.27 and Ferrarini et al.30, integrated synthesis can produce 82% pure zeolite 4A and low grade zeolite Na-P1 (57 - 61 %). Moderate conditions and residues (coal ash and aluminum metal) were used in this process. To the best of our knowledge, there is no study on the use of zeolites in backfill blends formed by residues from coal beneficiation. The backfill of mine voids seeks minimize to environmental impact through avoidance of leachate generation and mobilization of toxic chemical species that can potentially pollute groundwater sources. This study aimed to characterize coal beneficiation tailings traditionally used to fill abandoned mines and its blends, using alternative materials such as coal fly ash and low
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grade zeolite Na-P1. This is an attempt at formulating backfill material using two waste steams namely coal processing waste and zeolites synthesized from coal ash. The study evaluated, on a laboratory scale, the release of metal ions and sulfate, and other parameters, in the water leachates of the raw backfill and its blends over time. The work also examined the remediation of ion mobilization in leachates using a zeolite Na-P1 and backfill blend.
2. EXPERIMENTAL SECTION 2.1. Materials. The backfill used in this study was collected in a coal beneficiation plant that washes ROM (Run of Mine) coals from the northern portion of the Southern Santa Catarina Coalfield. The backfill of the galleries and coal mining panels consisted of thick tailings from a beneficiation plant and can be considered a rock-fill type material. It was transported into the mine by trucks and spread using loaders and track tractors. The mining of the coal is continuous and carried out with disassemble by detonation with explosives and followed of method of chamber and pillars and methods of coal mining with ceiling fall. The removal of the fragmented coal is carried out by wagons and treadmills until the exit of the mine.32,33 Washed coal and tailings were separated in a jig using gravity separation. The tailings are used as backfill in nearby coalmines. The main objective of the mining companies was to dispose of the large amount of tailings generated by using them as backfill. This practice improves mine structural stability, with the potential to enhance mining recovery and reduce the environmental impacts caused by disposing of the tailings on the surface. The filling of already mined galleries was motivated by the risk of collapse.33
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In the present study, fly ash generated in Unit B of the President Médici Thermoelectric Power Plant (UTPM) in Candiota (Rio Grande do Sul state, Brazil), NaOH (Merck, 99.5%) and metallic Al (Synth, commercial grade) were used to synthesize zeolite Na-P1 in an integrated process.27,30 The first stage of synthesis involved extracting Si from fly ash (3.0 mol L−1 NaOH, L/S ratio of 6 L kg−1, 100 ◦C, 24 h). The resulting product was separated by filtration and the Si rich extract, used to produce zeolite 4A (higher purity). Zeolite Na-P1 was characterized at a purity of 61%, exhibiting some phases (quartz and mullite) originally present in the ash as contaminants.27 The use of low grade zeolite Na-P1 zeolite makes integrated synthesis more competitive. The chemical composition of the Candiota fly ash was measured by FRX and presented in Table 1. These results are compared with previous studies about this ash performed at our laboratory.27,30 The SEM images of these fly ash and Na-P1, synthetized using this ash, are presented in Figure S1 (see Supporting Information). 2.2. Leaching tests. The study of the elements leached by backfill in the presence of water followed the methodology proposed by Ward et al.26 Tests in this study were conducted by adding 32 g of the sample (particle size . acessed: Dec 6, 2017
