Lignite ash: Waste material or potential resource - Investigation of metal recovery and utilization options

    Lignite ash: Waste material or potential resource - Investigation of metal recovery and utilization options Ren´e Kermer, Sabrina Hedrich, S¨oren Bellenberg, Beate Brett, Daniel Schrader, Petra Sch¨onherr, Martin K¨opcke, Karsten Siewert, Nils G¨unther, Tilman Gehrke, Wolfgang Sand, Konstantin R¨auchle, Martin Bertau, Gerhard Heide, Lars Weitk¨amper, Hermann Wotruba, Horst-Michael Ludwig, Roswitha Partusch, Axel Schippers, Susan Reichel, Franz Glombitza, Eberhard Janneck PII: DOI: Reference:

S0304-386X(16)30411-X doi: 10.1016/j.hydromet.2016.07.002 HYDROM 4397

To appear in:

Hydrometallurgy

Received date: Revised date: Accepted date:

29 February 2016 22 June 2016 5 July 2016

Please cite this article as: Kermer, Ren´e, Hedrich, Sabrina, Bellenberg, S¨oren, Brett, Beate, Schrader, Daniel, Sch¨onherr, Petra, K¨opcke, Martin, Siewert, Karsten, G¨ unther, Nils, Gehrke, Tilman, Sand, Wolfgang, R¨ auchle, Konstantin, Bertau, Martin, Heide, Gerhard, Weitk¨ amper, Lars, Wotruba, Hermann, Ludwig, Horst-Michael, Partusch, Roswitha, Schippers, Axel, Reichel, Susan, Glombitza, Franz, Janneck, Eberhard, Lignite ash: Waste material or potential resource - Investigation of metal recovery and utilization options, Hydrometallurgy (2016), doi: 10.1016/j.hydromet.2016.07.002

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ACCEPTED MANUSCRIPT Lignite ash: waste material or potential resource Investigation of metal recovery and utilization options René Kermer1,*, Sabrina Hedrich2, Sören Bellenberg3, Beate Brett4, Daniel Schrader5,

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Petra Schönherr6, Martin Köpcke7, Karsten Siewert8, Nils Günther9, Tilman Gehrke3,

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Wolfgang Sand3, Konstantin Räuchle4, Martin Bertau4, Gerhard Heide5, Lars Weitkämper7, Hermann Wotruba7, Horst-Michael Ludwig8, Roswitha Partusch10, Axel Schippers2,

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Susan Reichel1, Franz Glombitza1 and Eberhard Janneck1 1

G.E.O.S. Ingenieurgesellschaft mbH, 09633 Halsbrücke, Germany; 2Federal Institute for

Geosciences and Resources (BGR), 30655 Hannover, Germany; 3University Duisburg-Essen,

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Aquatic Biotechnology, 45141 Essen, Germany; 4Freiberg University of Mining and Technology, Institute of Chemical Technology, 09599 Freiberg, Germany; 5Freiberg University of Mining and

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Technology, Institute of Mineralogy, 09599 Freiberg, Germany; 6Loser Chemie GmbH, 08056 Zwickau, Germany; 7RWTH Aachen, Mineral Resources Processing Unit (AMR), 52064 Aachen, Germany; 8Bauhaus-University Weimar, F.A. Finger-Institute of Building Materials Science (FIB),

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99421 Weimar, Germany; 9Nickelhütte Aue GmbH, 08280 Aue, Germany; 10Vattenfall Europe

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Mining AG, Lignite Mining & Generation Production, 03050 Cottbus, Germany;

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* Corresponding author: René Kermer, [email protected], +49-(0)3731-369 270 Keywords: lignite ash; ash utilization; ash exploitation; chemical leaching; bioleaching; pozzolan

Abstract. Ashes from lignite combustion for power generation contain strategic metals, metalloids

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and rare earth elements (REE) and may thus be a potential source of industrially demanded metals. The presented project focused on the assessment and utilization of this potential raw material. Lignite ash assessment showed that the largest ash amounts for a potential utilization in Germany are available in the Lusatia region and that these ashes have a high value potential. A stabilized ash taken from the landscape building “Spreyer Höhe”, Lusatia, served as the main sample. For enrichment, separation and mobilization of valuable substances from the lignite ashes mechanical and thermal pre-treatment methods as well as chemical and biological leaching approaches were applied. Mechanical ash pre-treatment provided enriched fractions by different methods but still suffered from low yields of enriched fractions. Thermal ash processing showed multiple significant phase changes compared to original ash. Digestion with sc-CO2 and chemical leaching using HClaq of untreated and thermally treated ash provided high extraction for the metals Al, Ca, Fe, Mg, with the highest values achieved for thermally treated ash. Alternatively, bioleaching was applied using acidophilic Fe/S-metabolizing microorganisms (MO) as well as heterotrophic MO. The results indicated likewise high and partly specific metal mobilizations, e.g. for the elements Al, Ca, Fe, Mg, Mn, V, Zn, Zr and for some REE. A potential utilization was investigated for the original stabilized ash (not treated otherwise) as well as for ash fractions and

ACCEPTED MANUSCRIPT leaching residues. Two potential utilization routes were identified i) partial substitution of the original resource by original stabilized ash or ash fractions in the production of Al-Fe-solutions applicable for water treatment and ii) usage of original stabilized ash or residues from ash leaching

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as (reactive) supplement in cement, concrete and mortar production.

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1. Introduction

In recent years the intensified worldwide demand and utilization of metal resources and the partly existing dependence on a low number of producing countries lead to uncertainties in supply and prices [1]. Several EU countries and the European Commission (EC) have undertaken steps to

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guarantee a proper supply of mineral resources for Europe and to maintain international economic competitiveness [2, 3]. Following this idea strategy documents were published and a number of

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R&D projects were approved which focus on the utilization of secondary raw materials still containing different amounts of residual critical and strategic elements. Among these resources are low grade ores and ores hard to process by conventional technology, as well as dumps, heaps, slags and ashes [4, 5]. Ashes from lignite combustion for power generation, e.g., contain varying

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amounts of critical and strategic metals, metalloids and rare earth elements (REE) and may thus

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be a potential source for the industry.

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During the past decades huge amounts of ashes have been accumulated through combustion of gas, oil and coal for heat and energy generation which is still ongoing to date. Thus, metal recovery from power plant ashes has been discussed for decades, initially with special interest on iron, aluminium and also titanium in the 1980s [6]. The topic was regularly discussed, however, always

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with the result of an insufficient profitability [7]. While ashes from oil, fuel oil and refinery residues containing high iron, vanadium and nickel concentrations are already utilized today [8, 9], metals present in lignite and hard coal are not materially recovered. The main reasons in the past were low resource prizes, unsuitable processing technologies and low profitability. In addition, the awareness of resource sustainability was not as developed as it is today. An overview of studies on element recovery from coal ashes (in particular Ge, Ga, Be, Mo) is given in [10, 11]. Besides, iron recovery from fly ashes was intensively investigated [11, 12]. The studies were proven for application at the industrial scale. However, no industrial application was started due to economic reasons. In contrast, a study by Gilliam et al. proved a high profitability of metal recovery from fly ashes by consequent consideration of all valuable components [13]. Although the proposed technology suffered from a too high complexity as it consists of a high number of necessary process steps, it still underlines the high economic potential. In recent years metal recovery from coal ashes moved more into focus because of the dramatically increasing interest in trace metals and REE. Metal recovery from ashes in general turned out to be feasible under laboratory conditions despite varying metal contents and different evaluations [14, 15, 16]. However, high acid

ACCEPTED MANUSCRIPT consumption, cost-intensive reagents and a number of necessary process steps prevented the technology to be successfully transferred to industrial scale.

Beside chemical leaching experiments also biological approaches (biomining or bioleaching) on

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metal recovery from ashes were undertaken by using microorganisms. Acidophilic Fe/S-oxidizing

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species are the most prominent bioleaching microorganisms mediating the conversion of insoluble

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metal sulfides into a water soluble form, e.g. metal sulfates [17, 18]. Bioleaching was also investigated for non-sulfidic minerals and residues like mining dumps, ashes, sludges and slags as well as for electronic waste. Typically, autotrophic acidophilic bacteria (e.g. Acidithiobacillus ferrooxidans) and heterotrophic bacteria as well as fungi were applied [19, 20]. In a number of

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studies using acidophilic species the feasibility of bioleaching for recovery of different metals and trace elements from ashes was shown in lab-scale and partly in pilot-scale, although these were

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restricted to ashes from waste incineration [21, 22, 23, 24, 25, 26]. One of the first study was carried out by Bosecker [21] which showed the successful leaching of different metals from fly ash by sulfuric acid produced by At. ferrooxidans. Later, a patent for a fly ash leaching process in 50L scale with At. ferrooxidans was published [26]. In studies by Brombacher et al. [22, 23] the

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development of a plant for fly ash leaching with At. ferrooxidans and At. thiooxidans was described

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enabling the recovery of more than 50 % of the containing Zn, Al, Cu and Ni. In experiments by Krebs et al. [24] leaching of 80 % Cu, Cd, Zn, 60 % Al and 30 % Fe and Ni was achieved using At.

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thiooxidans. Ishigaki et al. [27] investigated a mixed culture of At. ferrooxidans and At. thiooxidans during ash leaching and found correlations between leached metal contents of Cu, Cd, Cr and Zn and the supplementation of different concentrations of Fe(II), sulfur and organic carbon. In addition, the recovery of electronic metals like Ga and Ge was shown, however, the described processes

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are not yet industrially applied [28, 29]. Besides bioleaching with autotrophs, silicate, oxide and carbonate ores are leachable by heterotrophic microorganisms requiring an organic substrate [18, 20]. Bioleaching with heterotrophic organisms was investigated for fly ashes and slags from waste incineration plants as well as for fly ashes from lignite power plants. Bosshard et al. [30] could extract 60 – 70 % of Cu and Pb and 80 – 100 % of Al, Mn and Zn from waste incineration fly ash by leaching with citric acid produced by Aspergillus niger. Xu and Ting [31] conducted optimization studies on leaching of fly ash from waste incineration with A. niger and found optimal conditions at a pulp density of 2.7 % and at a sucrose concentration of 150 g/L which was used as organic substrate. A study by Singer et al. [32] focused on the leaching of aluminum-rich lignite fly ashes by microbial and commercial citric acid and reported an extraction of around 12 % aluminium oxide. In addition, a dependence of extraction efficiency on acid concentration and extraction temperature was reported. Torma and Singh [33] investigated acidolysis of fly ashes from lignite power plants by citric acid and oxalic acids produced by A. niger. An extraction of around 93 % of aluminium was reported. A study by Krejcik [34] concentrated on metal mobilization from a lignite fly ash by leaching with organic acid producing microorganisms like Acidomonas methanolica. An

ACCEPTED MANUSCRIPT increased extraction was observed e.g. for iron, however, metal extraction decreased with increasing ash supplement due to a constant amount of produced acid.

Because of a missing processing technology for the recovery of elemental resources lignite ashes

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are mainly used i) as supplemental component in dumps and heaps, ii) as hydraulic barrier against

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rising groundwater or for neutralisation purposes in abandoned open cast pit mines or iii) as

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supplemental stowing material in the mining industry [35, 36, 37]. In addition, the use of lignite ashes as supplemental material for construction was intensively investigated [38] but mainly suffered from their varying composition [11, 39].

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Ashes from lignite combustion are being produced in lignite power plants to date. In Germany, for example, about 178 million tons of lignite are produced and about 160 million tons thereof are combusted for power generation annually [40]. Considering an ash content of ca. 10 % in the coal,

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an amount of ca. 16 million tons of ash is produced in Germany per year. Until 1990 about 30 – 60 million tons of ash were generated each year in the eastern federal states of Germany (production of about 300 million tons of lignite per year), which were utilized for the above mentioned dumping

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purposes or as supplemental components. These ashes contain high amounts of currently

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demanded metals like aluminium, iron, magnesium and manganese as well as trace elements and REE. The metal concentrations are varying essentially in dependence of the deposit from which

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the lignite originated from. The major metal proportion is bound in the inorganic part of lignite which directly depends on the geologic formation. Only a small part of metals is bound to the organic part of the ash for which transfer reactions by redox and ion exchange mechanism from the surrounding rock or soil layers are important [41]. During the combustion process in the power

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plant metals bound to the lignite are either directly transferred into the ash (mainly from inorganic part and partly co-mined surrounding material) or end up in the flue gas from which they are separated afterwards together with other ash particles by different cleaning/washing procedures (e.g. electro-filters, flue gas desulfurization) [42]. Later in the process the wash solution from flue gas desulfurization is added to the generated fly ash resulting in a substance which contains the transferred metals and is termed “stabilized ash”. The flue gas cleaning can thus be operated as a zero liquid discharge facility [43]. In Germany an amount of ca. 10 million tons of stabilized ash is produced per year containing a high amount of the so enriched valuable metals.

The presented study focused on the potential raw material lignite ash and had the following aims: i) evaluation of the value potential of lignite ash by assessment of the ash amounts that are available for utilization in Germany and analysis of valuable metal contents in the considered ashes and ii) development of a processing technology for comprehensive recovery and utilization of raw materials from lignite ashes. In the first process steps mechanical and thermal pre-treatment techniques were investigated for their suitability to produce ash concentrates and to allow a

ACCEPTED MANUSCRIPT targeted enrichment of specific valuable metals. The follow-up steps included the use of biological and chemical leaching methods for metal extraction from original ash and ash fractions. Finally, the original ash, produced fractions, concentrates as well as solutions and residues from leaching were

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investigated for options to separate valuable metals and for potential industrial utilization routes.

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2. Material and methods

2.1 Assessment of ash amounts and sample material

The assessment of lignite ash amounts was based on data from all relevant lignite mining districts

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in Germany (Rhenish, Lusatian, Middle-German, Helmstedt). The considered ash quantities included annually generated and already deposited amounts. This means also ash quantities used

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for re-cultivation purposes or land filling/forming were considered. In cooperation with the Vattenfall Europe Mining AG an amount of 2 t of stabilized ash material from the landscape building “Spreyer Höhe” located in Lusatia, East Saxony, Germany, was sampled for comminution, homogenization and subsequent distribution to all project partners. An

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RFA analysis of the sampled homogenized ash was conducted and provided to the project

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partners. All experiments mentioned in this article were carried out with this homogenized

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stabilized ash. The designations “original ash” or “non-treated ash” always refer to this ash. 2.2 Mechanical pre-treatment

Mechanical processing methods were applied to produce ash concentrates with specific metal contents. Test work included investigations on classified and crushed ash samples (< 500 µm).

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Applied processing methods were density separation, magnetic separation, electrostatic separation as well as flotation. Density separation was conducted via Falcon centrifuge (< 63 µm) as well as on a Holman shaking table as one stage as well as a two stage sorting process. The particle size fractions sorted on the shaking table were 500 – 250 µm, 250 – 125 µm, 125 – 63 µm, < 250 µm and < 125 µm. Pre-tests on the technical feasibility of magnetic separation were conducted on high- (gaustec separator, wet) and low-intensity (Sala separator, wet and dry) magnetic separators with particle sizes of < 500 µm. Electrostatic separation was conducted at particle sizes of 500 800 µm and 250 – 500 µm and field intensities of 13 kV, 17 kV and 22 kV on a Krupp corona roll separator. Flotation test series were done with different reagents (such as Flotanol, diesel, sodium phosphate or Hydroxamat) and different pH-values (between pH 1 – 11) at particle sizes between 100 – 30 µm. In addition, detailed investigations on the technical feasibility of magnetic separation were conducted. For the test work, an inductive roll separator from Carpco was used. The material was separated at various field intensities (input coil currents 0 – 3 A) and different particle size fractions (500 – 250 µm, 250 – 125 µm, 125 – 100 µm, 63 – 45 µm, 45 – 32 µm). Generated fractions were analysed by ICP for metal composition.

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2.3 Thermal ash pre-treatment Thermal ash processing targeted a specific metal enrichment on the particle surfaces due to recrystallization processes after tempering. For optimal crystallization behaviour 40 g of stabilized

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ash were filled in corundum trays and put into muffle furnace. Different temperature-duration

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variations were investigated (900 °C, 1000 °C, 1057 °C; 1 h, 4 h and 9 h). Previously, the

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transformation-point at 1057°C was determined by heating microscopy according to Scholze [44]. After soaking the ash was cooled down to room temperature and the mineral phase analyses were carried out by XRD and REM.

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2.4 Chemical treatment

Studies were conducted with native and thermally pre-treated ash (1000 °C, 1 h). Leaching

combination of sc-CO2 and mineral acid.

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experiments were conducted with supercritical carbon dioxide (sc-CO2), mineral acids or a Digestion studies with sc-CO2 (150 bar, 100 °C) were carried out in a temperature adjustable pressure autoclave (Berghof, Hastelloy C, 400 mL) with 10 % (w/v) ash over 24 h, while the

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suspension (100 mL) was stirred (300 rpm). Leaching with HClaq (31 %) was done in a three-

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necked flask equipped with sealed precision glass (KPG) stirrer (600 rpm) and Dimroth condenser. The suspension (100 mL) with 10 % (w/v) ash was kept at 100 °C for 4 h. Metals in the leaching

Freiberg).

Biological

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2.5 Bioleaching

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solution were analysed by AAS and leaching residues by XRD (Institute of Mineralogy, TU

leaching

experiments

were

conducted

with

acidophilic

chemolithoautotrophic

microorganisms as well as acidophilic or neutrophilic heterotrophic bacteria. Acidophilic cultures comprised mesophilic, moderate-thermophilic and thermophilic Fe-/S-oxidizing strains. Shake flask experiments with increasing ash supplements (1 – 40 %) were carried out at respective temperatures (28, 45 or 65 °C) and pH 1.2 – 3.5 in mineral salt medium described by Mackintosh [45]. The pH of the suspension had to be adjusted by H2SO4 addition to the appropriate value at the beginning of each experiment, due to the alkaline reaction of the ash. Furthermore, oxidative (aerobic conditions) and reductive (anaerobic conditions) leaching experiments were carried out in 2 L pH- and temperature-controlled bioreactors at 30 °C in a mineral salt medium described by Wakeman et al. [46] supplemented with 1 % (w/v) sulfur at pH 2.0. All experiments were performed with 10 % (w/v) fly ash over 28 d and pH was controlled via automated sulfuric acid addition. The oxidative bioleaching reactor was inoculated with a mixed culture of mesophilic Fe-/S-oxidizing acidophiles and continuously aerated with sterile air (1 L/min) for the aerobic experiments, whereas reductive leaching experiments were performed with iron-reducing acidophiles and the reactors

ACCEPTED MANUSCRIPT were regularly flushed with nitrogen. Control experiments were carried out with the same setup but under sterile conditions. The gluconic acid-producer Acidomonas methanolica and the silicate-solubilizer Bacillus circulans served for bioleaching with heterotrophs. Experiments with A. methanolica were carried out in a

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7 L-stirred bioreactor containing 165 g/L glucose dissolved in distilled water at a pH of 3.6

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according to the method described by Iske et al. [47]. The mixture was inoculated to a final cell

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number >109 cells/mL with a pre-cultured A. methanolica strain (pre-culture in mineral growth medium, [48]). Titration and measurement of free gluconic acid by an enzyme assay (Megazyme, Ireland) were applied to follow the subsequent conversion of glucose to gluconic acid. After a conversion of ~85 % was reached, ash was supplemented stepwise up to 10 % (w/v) within 3 d.

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The mixture was further incubated for 8d. Chemical control experiments at pH 4 (adjustment by HCl or commercial gluconic acid) were carried out through addition of 0.05 % NaN3. Bacillus circulans was grown in shake flasks containing nitrogen-free mineral medium with glucose

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(20 g/L) at pH 7 – 7.5 and 30 °C (internal lab method). The medium was supplemented with 10% (w/v) ash and pH adjusted with HCl. Semi-continuous bioleaching experiments involved repeated exchange of the pregnant leach solution (PLS) every 3 – 5 d, with an overall leaching duration of

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47 d, corresponding to 11 exchange cycles. The leaching residues were treated with 5 g/L EDTA to

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re-dissolve metals which had precipitated again during cultivation. Analysed metal concentrations of each exchanged PLS and those resulting from re-dissolution via EDTA-treatment were summed

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up for final data analysis. Batch leaching approaches were performed for 63 d without solution exchange. Sterile control experiments were performed by addition of 0.05 % of NaN3. Samples were withdrawn regularly from each bioleaching experiment for the analysis of mobilized metals in the supernatant by ICP-MS and residues were analysed by X-ray fluorescence and X-ray

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diffraction analysis.

2.6 Metal separation from polymetallic solutions Because of minor metal concentrations and for the purpose of specific separation and/or enrichment from solution, possible separation/enrichment options of valuable substances and rare earth elements from ash leaching were tested by application of ion exchange and liquid-liquidextraction methods. All experiments were carried out with a polymetallic solution which was produced by acidic leaching of a thermally pre-treated slagged ash (1500 °C, 3 h) at a pH of 0.5 using HClaq. The approaches on ion exchange involved addition of 10 % (w/v) of the cation exchanger Dowex 50 W X 8 (Dow Chemicals) to the polymetallic solution, slow stirring for 30 min and subsequent separation and analysis of the solution. For the experiments on liquid-liquid-extraction the extractants Diethylhexylphosphinic acid (D2EHPA, 20 % in Exxsol D100, Rhodia) and Tributylphosphate (TBP, 40 % in Exxsol D100, Obermeier) were applied. The approaches were set up with organic/aqueous phase ratio (O/A

ACCEPTED MANUSCRIPT ratio) of one and at a pH of 0.5. The mixtures were stirred for 30 min. After phase separation each organic phase was re-extracted. In case of D2EHPA phase HClaq was used for re-extraction, in case of TBP-phase water was applied. Serial extraction experiments were carried out by applying the extractant TBP in the first step and the extractant D2EHPA with the resulting raffinate in the

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second step. Afterwards re-extracted organic phases were analysed for metal contents.

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2.7 Conversion to water treatment reagents

Non-treated ash, enriched fractions and solutions from ash leaching were investigated for their suitability to be utilized in the production of water treatment agents (substitution, in-part substitution and/or supplementation). The approaches were carried out by a two-step acidic conversion. The

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first step involved stirring at room temperature and used HClaq in a concentration of ≤ 0.27 mmol/g ash (corresponding to a HClaq solution between 1 – 5 %). In the second step concentrated HClaq or

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H2SO4 were used under reflux at normal pressure or autoclave reaction at 155 °C. Ash residues that had not reacted were separated from the solution by filtration.

2.8 Utilization in building material production

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The pozzolanic reactivity can be identified by indirect or direct techniques. A commonly used

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indirect method is the determination of the activity index. The activity index is the ratio (in percent) of the compressive strength of standard mortar bars, prepared with 75 % test cement plus 25 % fly

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ash by mass, to the compressive strength of standard mortar bars prepared with 100% test cement, when tested at the same age [49]. That’s similar to the American standard [50], where the 7 d and 28 d compressive strengths of mortar cubes with a 20 % mass replacement of cement by pozzolan are compared to those of a control without pozzolan, at constant flow conditions. The

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disadvantage of indirect methods is that they give no information about the pozzolanic material itself, because the result depends on the characteristics of the cement and the mix design. Direct methods describe the chemical reaction of calcium hydroxide (portlandite) with the pozzolan. The portlandite consumption can be determined by x-ray diffractometry (XRD), thermogravimetry (TG) or by Chapelle’s test. Chapelle’s test is a standard method in France for metakaolin [51] and in Brazil for pozzolanic materials in general [52]. For experimental purpose the Chapelle’s test [53] was modified as follows: 1 g pozzolan was added to 250 mL distilled water. After adding 2 g CaO the vessel with the suspension was sealed and the suspension was stirred for 16 hours at 90 °C. The consumption of hydroxyl ions was determined by titration with HCl 0.1 M.

ACCEPTED MANUSCRIPT 3. Results and discussion

3.1 Assessment of ash amounts and sample material Lignite ash assessment showed that the largest ash amounts for a potential utilization in Germany

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are available in the Lusatia district. In the German lignite mining districts about 10 million tons of

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lignite ash are generated each year. An amount of ca. 5 million tons of lignite ash per year is

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generated in the Lusatia district. The bulk of this is dumped or used for re-cultivation purposes and land forming because processing technologies are lacking. The remaining 5 million tons of lignite ash are produced in the other lignite mining districts and are largely not available for utilization. Elemental analysis of the sampling material (stabilized ash) from the landscape building “Spreyer

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Höhe” in the Lusatia district (Tab. 1) indicated Al, Ca, Fe, Mg, Mn, Ti and Si as the main ash components. In addition, increased contents of several trace metals (Cr, Cu, V, Zn, Zr) and REE (Ce and La) were found to be present. From the analytical data and the large available amounts

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(ca. 20 million tons deposited since the middle of the 1990’s and ca. 5 million tons produced annually) the sampled ash was evaluated to contain a high value potential. A theoretical metal value of ca. 640 €/t of ash was calculated (based on metal prices from 22.15.2015) which is mainly

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made up by the contents of Si, Ti, Al, Ni, Mg and Fe. It has to be noted that this is a solely

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theoretical value at the end of the value added chain and does not contain any costs for metal

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separation or refining.

Table 1: Chemical composition of the stabilized fly ash used in this study. Si 190 Sr mg/kg 1465 Nb mg/kg 26 W mg/kg 6

Fe 113 Ba 856 Th 26 Sm 5.8

Ca 110 Zr 158 As 26 Mo 4

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unit g/kg

Al 59 Zn 116 Ga 24 Cs 5.1

Mg 28 V 107 Pb 18 Bi <3

K 6.6 Cr 85 Sc 18 Sb 2.0

Ti 4.8 Cu 84 Sn 16 Ag 1.1

P 1.8 Ce 76 Nd <15 Cd 0.5

Na 1.5 Rb 55 Co 12 In < 0.5

Mn 1.5 La 46 U 8 Te < 0.5

Y 41 Hf 8

Ni 31 Ta 6

3.2 Mechanical ash pre-treatment Results of physical processing tests showed that an enrichment of ferrous and non-ferrous metals is not feasible, due to the low and homogeneous dissemination of the metals in the ash. During the physical separation test work, focus was put on the production of iron-rich concentrates. However, most of the iron present in the samples was also disseminated and only a small part was present in iron-rich particles, which could be separated by magnetic and density separation. By applying a two stage density separation on a shaking table a concentrate with a Fe-grade of 80%, at a recovery of 2.4% and a yield of 1.5%, was produced. In addition, detailed investigations on the technical feasibility of magnetic separation were conducted by separating different particle size fractions of the sample material at various field intensities. A potential use of products from

ACCEPTED MANUSCRIPT magnetic separation for further leaching experiments was evaluated. Most promising results from magnetic separation were achieved for the particle size fraction 125 – 250 µm, by which a concentrate with a Fe-grade of 70% was produced at a recovery of 2% and a yield of 0.3%. A production of Fe-rich concentrates for further use in hydrometallurgy or pyrometallurgy is

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technically feasible. However, the results of this study have shown that recoveries of valuable

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contents are low.

3.3 Thermal ash pre-treatment

Investigations on thermal pre-treatment showed that targeted tempering resulted in a number of mineral phase changes compared to the original stabilized ash (Fig. 1). For the purpose of visibility

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only the most prominent mineral phases were depicted.

Figure 1: Mineral phase analysis (XRD) of ash without pre-treatment (first column) and after different variants of thermal pre-treatment. After thermal pre-treatment a significant decrease of amorphous fractions depending on temperature and soak was visible in the phase distribution (Fig. 1). This behavior is due to partial crystallization. Quartz, for instance, showed an increasing trend due to the re-crystallization of the amorphous part. Hematite, periclase, and rutile showed no significant changes (not shown). Magnetite arose only negligibly at 1000 °C. Calcite decomposed at a temperature around 850 °C

ACCEPTED MANUSCRIPT under release of CO2 to calcium oxide [11] (not shown). The phase distribution at 900 °C and 1000 °C gave evidence for a formation of åkermanite from CaO and diopside (equation 1). Furthermore at temperatures around 1057 °C and longer soak åkermanite dissociated (equation 1). T

Ca2 MgSi2 O7

2 CaO

MgO

2 SiO2

(1)

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CaO

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CaMg Si2 O6

Diopside was found in all phase distributions and there is an obvious evidence that diopside is

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formed during higher temperature and soak from the amorphous part. Anorthite and wollastonite were formed from gehlenite and SiO2 during increasing temperature and soak (equation 2). This process is in relation to a decreasing part of gehlenite. lSiO7

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2 SiO2

Ca2 l2 Si2 O8

CaSiO3

(2)

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Ca2 l

2 CaO

l2 O3

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SiO2

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Gehlenite is formed at around 800 °C from calcium-, aluminum- and silicon oxide [54] (equation 3). Ca2 l

(3)

lSiO7

As expected, gypsum and ettringite were not detected after thermal pre-treatment (not shown). An

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explanation is the dissociation of ettringite at low temperatures (equations 4 – 6). In addition,

process [55, 56] (equation 7).

12

(SO4

Ca4 l2 [(OH

12

(SO4

12

CaSO4 2 H2 O

26 H2 O

12 H2 O

6 H2 O

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Ca3 l2 (OH

3

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T

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Ca6 l2 [(OH

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gypsum can form hemihydrate under water release and finally anhydrite after another dewatering

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Ca4 l2 [(OH

Ca3 l2 (OH

3 CaO

l2 O3

CaSO4 0,5 H2 O

12

12

(SO4 6 H2 O

12 H2 O

2 CaSO4

CaSO4

6 H2 O

12 H2 O

1,5 H2 O

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14 H2 O

(4) (5) (6)

CaSO4

0,5 H2 O

(7)

Mullite showed the highest stability at 1000°C due to a high nucleation. Cristobalite was only detected at 1057 °C. This is similar to albite, which was formed at a temperature of 1057 °C and a soak of 4 h and 9 h (not shown). Brownmillerite showed a decreasing trend with increasing temperature and soak, whereas augite showed a significant increase in the phase distributions. As a result of these investigations following thermal pre-treatment, experiments to provide sample material, e.g. for subsequent chemical leaching approaches, were conducted at a temperature of 1000 °C and a soak of 1 h. These conditions were selected for the following reasons: i) adequate crystallization of the amorphous part, ii) sufficient transformation of low soluble to readily soluble compounds and iii) economic reasons.

ACCEPTED MANUSCRIPT 3.4 Chemical treatment The recovery of metals from ashes in hydrometallurgical processes is complicated due to the low solubility of silicates in mineral acids [57]. One approach is the pre-treatment of ashes according to the concept of enhanced weathering. The decomposition of the silicates occurs under

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hydrothermal conditions by the reaction with supercritical CO2 (sc-CO2) [58, 59].

Figure 2: Mineral phase analysis (XRD) after sc-CO2-digestion and/or HClaq-leaching of A) untreated ash and B) thermally pre-treated ash (1000 °C, 1 h). Experiments (Fig. 2) showed that silicate (diopside, gehlenite, mullite), sulfate (anhydrite, ettringite, gypsum) and oxide (brownmillerite, quartz, rutile) ash components could be converted into acid

ACCEPTED MANUSCRIPT soluble carbonates (calcite, aragonite) and bicarbonates by sc-CO2-treatment (150 bar, 100 °C, 24 h). In the applied reaction system, ettringite is completely consumed in favour of amorphous aluminium hydroxide, Al(OH)3, gypsum, CaSO4 · 2 H2O, and aragonite, CaCO3 [60] (equation 8). However, as a result of sc-CO2-treatment, gypsum was promptly converted to CaCO3 (equation 9).

12

(SO4

CaSO4 2 H2 O

26 H2 O

3

CO2

H2 O

3 CO2

H2 O

CaCO3

3 CaCO3 3 CaSO4 2 H2 O

H2 SO4

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Ca6 l2 [(OH

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equilibrium between CaSO4 · 2 H2O and CaCO3 was also inverted.

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The same applied to gypsum that had been present in the genuine ash material, where the

2 l(OH

2 H2 O

3

24 H2 O

(8) (9)

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These findings were supported by liquid phase analyses. Aluminium was not detected, while

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calcium was present in concentrations up to 380 mg/L as Ca(HCO3)2 (Fig. 3).

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Figure 3: Metal concentrations in liquid phase after sc-CO2-digestion and/or HClaq-leaching of A) untreated ash and B) thermally pre-treated ash (1000 °C, 1 h). Other mineral phases, such as hematite and magnetite showed minor response, while brownmillerite as well as gehlenite seemed more prone to react. Due to their acid solubility, there is no hindrance for subsequent metal recovery processing. Under the prevailing conditions, sc-CO2-treatment releases metal cations from the digested mineral phases under formation of amorphous aluminium and iron hydroxides and/or oxides. This is due to the instability of their carbonates [59, 61]. These in turn are easily extractible by acid leaching. Mg from periclase and åkermanite was successfully transferred into the liquid phase by applying the concept of enhanced weathering, too. However, carbonic acid acidity was too weak for extracting Mg from ash components, such as diopside, augite and the amorphous fraction. An improved carbonisation was achieved by thermal pre-treatment. While comparing original and tempered ashes (1000 °C, 1 h), e.g. mullite content was markedly reduced in the residue by approx. 50 %. Similarly improved reaction behaviour was found for the acid solubility. A combined sequence of tempering, digestion with sc-CO2 and leaching with HClaq (31 %, 100 °C, 4 h) resulted in a further increased metal mobilisation of Ca, Fe, Al and Mg (Fig. 3), where thermal treatment

ACCEPTED MANUSCRIPT has a predominant influence. After leaching with HClaq (with or without pre-treatment) the residue mainly contained amorphous remainders and quartz, as well as minor parts of diopside, mullite and rutile (Fig. 2). The responses of the individual mineral phases to thermal and chemical treatment are complex.

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They proceed along complex reaction pathways and multiple acid-base interactions [59]. For

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instance, the reaction of diopside or mullite with HClaq or sc-CO2 resulted in the formation of quartz.

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In addition, there was a significantly increased mobilisation of aluminium by HClaq-leaching after ash pre-treatment with sc-CO2.

Further investigations on chemical digestion and leaching behaviour revealed that specific surface area and particle size only have a minor effect on metal mobilisation from lignite ash. In fact

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mineral phase composition clearly dominates reaction behaviour of ash constituents upon chemical

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treatment [59].

Figure 4: Schematic illustration of the synergetic CO2 and acid treatment. While conventional acid leaching takes effect on crystal lattice components only, CO 2-treatment results in anion exchange and therefore causes a surface-near conversion of the crystal lattice (Fig. 4), thus provoking tensions within the crystal structure, which facilitate proton attack [59, 62]. In consequence there is an enhanced leaching result obtained by combining both HClaq and scCO2 treatment. 3.5 Bioleaching The

chemolithoautotrophic

acidophilic

and

heterotrophic

microorganisms

catalysed

the

solubilization of several elements from the stabilized ash [63]. Sulfuric acid-producing cultures

ACCEPTED MANUSCRIPT tolerated pulp densities of up to 40 % (w/v) ash, but metal bioleaching was more efficient at solid loads below 20 % (w/v). Even so the rate of biological sulfur oxidation and bioleaching kinetics were enhanced with increasing temperature, the final metal recovery was similar for all test

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conditions and only Fe solubilization was more efficient at 45 °C and 65 °C than at 28 °C (Fig. 5).

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Figure 5: Metal recovery from ash during bioleaching with acidophilic, sulfur-oxidizing microorganisms in shake flasks depending on pulp density (A: 5 % (w/v) and B: 10 % (w/v)) and temperature (Mes.: mesophilic mixed culture, 28 °C, Mod. Therm.: moderate-thermophilic mixed culture, 45 °C, Therm.: thermophilic mixed culture, 65 °C). In Table 2 the recovery rates in bioleaching assays is presented in respect to the aqua-regia leachable metal content of a variety of elements of the periodic table. General characteristics of

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sulfuric acid based metal leaching are represented by good mobilization of metals, excluding those metals that form not readily soluble sulfate salts, such as alkaline earth metals and lead. As well as a trend describing increasing solubility of lanthanides by bioleaching with their mass was observed.

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Table 2: Fraction of metals solubilized by bioleaching at an ash pulp density of 10 % in cultures of mesophilic (28 °C, n1 = 3), moderate thermophilic (45 °C, n2 = 4) and thermophilic (65 °C, n3 = 3) consortia of sulfur-oxidizing microorganisms. Solubilized metal content in bioleaching assays referred to the total leachable metal content after aqua-regia treatment is depicted bold in percent. Below corresponding standard deviations (n = 10) are presented. Metal recoveries of 0 %, 50 % and 100 % are marked gradually from red to yellow and green, respectively. For an optimal interpretation of the colours in this table the reader is referred to the online version of this article. Be

77.6

86.3

7.9

5.9

Na

Mg

33.8

79.2

30.9

8.3

K

Ca

Sc

Ti

V

Cr

Mn

Fe

13.8

7.5

56.2

6.0

16.7

42.3

74.8

11.0

17.2

2.1

18.6

6.6

15.5

14.4

8.9

Rb

Sr

Y

Zr

Nb

Mo

Tc

12.5

12.6

76.6

25.4

32.2

5.3

n.a.

21.3

5.8

6.2

17.0

20.5

9.2

Cs

Ba

La-Lu

Hf

Ta

W

45.6

2.0

36.1

77.4

2.6

17.1

1.1

25.3

33.3

3.3

La

Ce

Pr

Nd

43.7

43.5

44.0

7.0

6.4

Ac n.a.

Lanthanides

Actinides

CR

Li

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O

77.0

n.a.

n.a.

n.a.

Al

Si

P

Se

75.0

n.a.

n.a.

n.a.

8.0 Ni

Cu

Zn

Ga

Ge

As

Se

60.6

62.8

49.2

60.6

26.2

41.9

13.4

10.6

9.7

10.3

10.9

14.1

15.4

15.9

11.6

16.8

6.1

Ru

Rh

Pd

Ag

Cd

In

Sn

Sb

Te

n.a.

n.a.

n.a.

6.4

83.1

27.8

4.8

34.5

53.3

2.6

12.4

14.4

4.9

22.7

26.4

Os

Ir

Pt

Au

Hg

Tl

Pb

Bi

Po

n.a.

n.a.

n.a.

n.a.

5.0

17.3

1.6

1.4

n.a.

2.0

19.7

1.6

1.8

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

45.1

50.7

56.1

62.5

66.8

70.0

73.2

75.6

77.9

79.2

6.7

7.4

7.7

7.8

7.6

7.3

7.1

6.9

6.4

6.1

5.9

Th

Pa

U

Np

Pu

Am

…

35.2

n.a.

77.6

n.a.

n.a.

n.a.

n.a.

12.4

Pm

N

5.7

US n.a.

C

Co

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Re

B

7.0

n.a.: not analysed

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Figure 6: Metal recovery during bioleaching with A) chemolithoautotrophic acidophilic microorganisms in pH-controlled 2 L stirred bioreactors after 28 d at pH 2.0, B) gluconic acidproducing A. methanolica after 8 d at pH 3.6, and C) silicate-solubilizing B. circulans after 47 d (continuous) or 63 d (non-continuous) at pH 7. All approaches contained 10 % (w/v) ash. In addition, chemical control experiments are shown. Reductive bioleaching under anaerobic conditions led to the increased recovery of Fe, Mg, Zn, Al, Ca, Si (Fig. 6A) and several rare earth elements compared (not shown) to bioleaching under aerobic conditions. Extraction for most of the considered elements reached values between

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30 – 60 %, for the metals Cr, Mg, Mn, Zn even 60 – 70 %. XRD analysis of the residues confirmed

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that the amorphic fraction had decreased in all samples, which is a sign for the dissolution of poorly-crystalline minerals, but was still higher in samples from biological leaching than from

schwertmannite).

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chemical control experiments probably due to the formation of secondary minerals (e.g.

Bioleaching with the gluconic acid-producing A. methanolica led to increased mobilization of several metal ions (Al, Ca, Fe, Mg) compared to the chemical control with HCl (Fig. 6B). The

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observed extraction ranged from 30 to 60 % and up to 80 % for Ca. Improved mobilization was also observed for further metal ions (Ce, Sr, Ti, V and Zr, 50 – 80 % extraction, not shown). However, chemical approaches by adding commercial gluconic acid (no microbial production within the approach) showed an almost equal metal mobilization compared to biological approaches indicating a quite low impact of the microorganisms on metal mobilization. Semi-continuous bioleaching with the silicate-solubilizing B. circulans led to a considerably improved extraction of Ca and Mg (~70 %) but not for the other elements considered (Fig. 6C) compared to batch leaching experiments. EDTA-treatment of the residues showed that obviously only low amounts of metals were leached, subsequently precipitated and afterwards available for re-dissolution through EDTA treatment. In contrast, higher concentrations of metals could be redissolved by EDTA in residues of non-continuous experiments (Fig. 6C).

3.6 Metal separation from polymetallic solutions Due to low concentrations of valuable metals and rare earth elements in the ash and for the purpose of specific separation and/or enrichment from solution, first tests were performed targeting

ACCEPTED MANUSCRIPT a possible metal separation/enrichment from ash leaching solutions by ion exchange and liquidliquid-extraction. The experiments were based on a polymetallic solution produced by acidic leaching of stabilized ash after thermal pre-treatment. The cation exchanger Dowex 50 W X 8 from Dow was applied with special focus on the recovery

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of rare earth elements. Unfortunately, only accompanying elements like calcium, potassium or

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silicon were extracted in a quite unspecific way. No enrichment of valuable metals or rare earth

The

experiments

on

liquid-liquid-extraction

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elements was achieved. were

based

on

the

extraction

agents

Diethylhexylphosphinic acid (D2EHPA, 20 % in Exxsol D100) and Tributylphosphate (TBP, 40 % in Exxsol D100). With D2EHPA ca. 20 % of the dissolved iron and over 90 % of the containing

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titanium were extracted. Unfortunately, only iron was re-extractable from the organic phase. By application of the extractant TBP both iron and titanium were nearly completely extracted.

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Remaining iron and titanium contents could be extracted in serial approaches using TBP in the first and D2EHPA in the second step with the result of a complete iron and titanium extraction. Likewise only iron could be re-extracted. Titanium (and other extracted metal ions) irreversibly remained on the extraction agent.

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It has to be highlighted that the conducted experiments were not in the immediate focus of the

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project and thus can be regarded as pre-experiments. More work would be necessary on metal

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separation from ash leaching solutions.

3.7 Conversion to water treatment reagents The use of lignite ash for the production of water treatment agents mainly arises from the Al and Fe components (and some others such as Ca and Mg) present in the ash. These can be applied in the

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production process instead of the original resource which is usually Bauxite (Al(OH)3). The investigations of non-treated stabilized ash and enriched fractions/solutions for their applicability in the production of water treatment agents (substitution, in-part substitution and/or supplement) involved a two-step acidic conversion. After reaction of the ash with HClaq (1 to 5 %) in the first step, Ca-Mg-solutions were obtained as products. In the second step the residues were subjected to concentrated HCl or H2SO4 resulting in Al-Fe-solutions. Substitution or partial substitution of the original raw material Al(OH)3 by ashes/fractions and/or bioleached Al-solutions have also been investigated and resulted in similar Al-Fe-solutions. All produced solutions showed the same behaviour in water treatment like commercial Al-Fe-solutions. However, their commercial applicability would be restricted as they also contain certain metal contaminations like Cr and Ni. Accordingly, the results would allow a potential application of the produced solutions in industrial waste water treatment but not in e.g. drinking water treatment.

ACCEPTED MANUSCRIPT 3.8 Utilization in building material production If the original stabilized ash or residues from chemical or biological ash leaching have pozzolanic properties there will be interesting utilization options. Pozzolanic means, that in alkaline medium silica and aluminium oxide dissolved away out of the vitreous phase and react with the calcium

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hydroxide which is released during hydration of cement to form comparable hydration products as

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you see at the hydration of cement. Those are in general calcium silicate- (CSH) and calcium

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aluminate hydrate phases (CAH). In principle, it would be possible to use the ashes as reactive main constituent in cements or as reactive addition at the making of mortars or concrete. Therefore it can be possible to replace Portland cement clinker or to reduce the cement content in concrete. Both ways of utilization would improve the productivity of resources and would contribute to a

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reduction of greenhouse gas emissions. The pozzolanic reactivity of non-treated stabilized ash and residues from chemical or biological ash leaching was investigated by determination of the activity index, a common applied indirect method, and by analysis of the calcium hydroxide (portlandite)

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consumption through Chapelle’s test, a suitable direct test method. A replacement of 25 % cement by mass by an inert material will drop the activity index by 25 %. Thus, an activity index above 75 % indicate that the ash takes an active part in the hydration

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process, has an own hardening potential and cannot be regarded as inert material. Figure 7

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presents the ratio of compressive strengths of standard mortar bars (activity index according to

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[49]) as well as ratio of compressive strengths of concrete cubes with an edge length of 150 mm.

Figure 7: Ratio of compressive strengths of specimens with and without ash. The minimum requirement after 90 days stated above is according to German national technical approvals for additions [64].

ACCEPTED MANUSCRIPT Pozzolans can react with dissolved calcium hydroxide and thereby contribute to strength development in mortar or concrete. The higher the lime consumption is the higher the reactivity of the pozzolan. The lime consumption was calculated by the difference between the added (lime suspension) and the remaining lime (lime-ash suspension). Our results and results from literature

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[53] are presented in figure 8.

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Figure 8: Calcium consumption of the original stabilized ash (OA), bioleached (aerobic and anaerobic) ash (BLA), HCl-leached (31 %, 100 °C, 2 h) (CLA) and five different pozzolanic reference materials (SCBA: sugar cane bagasse ash, RHA: rice husk ash, FA: hard coal fly ash, MK: metakaolin, SF: silica fume) taken from literature [53]. Vertical lines represent the scattering of the results; mean values are shown as dot on the lines. The results show, that both original stabilized ash (OA) and the two residues from ash leaching (BLA, CLA) have pozzolanic activities which are comparable to the activities of the reference

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materials (SCBA, RHA, FA, MK and SF, Fig. 8). To characterize the pozzolanic properties of the leached ashes in more detail further investigations will be conducted, e.g. by measuring the dissolved silica and aluminium contents in 10 % NaOH solution.

4. Conclusions The investigations showed that it is possible to mobilize metals from stabilized lignite ash. The generation of ash fractions with enriched metal contents by mechanical and thermal pre-treatment steps is possible in principle but challenging. Low yields of the concentrates and a rather homogenous distribution of desired elements in the ash particles are major issues that may only be improved by a more intense thermal pre-treatment. As a positive outcome of such pre-treatment low soluble compounds present in the ash are sufficiently transformed to readily soluble compounds. In turn the treated ash is more susceptible to acid attacks in leaching experiments. However, from an economic point of view it is clear that thermal pre-treatment is not appropriate, due to the necessary high energy input. Metal extraction from the original ash is possible by chemical and also biological leaching methods. Because of the partly very low metal contents in

ACCEPTED MANUSCRIPT the ash, the corresponding concentrations of such in the resulting solutions were quite low. The separation of mobilized valuable metals, albeit small, was investigated but it was a challenge, mainly due to the quite high content of accompanying elements like silicon, calcium or iron which disturbed separation reactions. In conclusion from all investigated ash utilization and processing

Partial substitution of the original resource by untreated stabilized ash (without mechanical

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i.

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options evaluated, the following two options appear to be most promising:

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or thermal pre-treatment) in the production of water treatment agents. The produced Al-Fesolutions have potential to be applied as waste water treatment reagents. Other potential water treatment applications, e.g. for drinking water, are not feasible, due to metal contaminations.

Usage of non-treated stabilized lignite ash or leaching residues as reactive main constituent

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ii.

in cements or as reactive addition in the making of mortars or concrete. Therefore it can be

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possible to replace Portland cement clinker or to reduce the cement content in concrete. Both ways of utilization would improve the productivity of resources and contribute to the reduction of greenhouse gas emissions.

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From the mentioned potential utilization options for the stabilized ash further (environmental)

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benefits may arise. A major environmental aspect is due to the savings of conventional resources needed for e.g. the production of building materials or water treatment agents. The usage of ash

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instead of these original resources would mean less mining of these resources, less demand in mining areas and, accordingly, less potential negative impact on the environment in these areas. Further minor benefits may arise from the deposition itself. In Germany ash deposition underlies different governmental regulations. In the federal state Brandenburg, for example, ash is regarded

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as inert and non-hazardous waste and is deposited on dumps. In that case an ash utilization, e.g. in the production of building materials, would create less ash “waste” material which has to be dumped. Consequently, less area would be needed for ash deposition and these areas could be used for other purposes instead. In general the deposition of ash and also its utilization for recultivation and land-forming purposes (e.g. landscape building “Spreyer Höhe” in the Lusatia mining district, Germany) does not create environmental risks or threats. However, experience in the field of long-term ash deposition (e.g. for more than 50 – 100 years) is not available. In principle, the release of substances from deposited ashes cannot be completely excluded because absolutely insoluble substances do not exist and the conditions of the deposit may change in the future (e.g. climatic changes, severe weather situations). In this consideration ash utilization should be favored over ash deposition.

ACCEPTED MANUSCRIPT 5. Acknowledgement We would like to thank the German Federal Ministry for Education and Research (BMBF) for financial support of the project (project “Lignite ash”, funding number 033R099A). In addition, we thank our associated project partners, the SolarWorld Solicium GmbH in Freiberg (Prof. Dr. Armin

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Müller) and the Vattenfall Europe Mining AG in Cottbus, for a comfortable cooperation during the

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project and for providing test material.

6. References

[1] German Resource Agency (DERA), German Resource Situation, 2013.

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[2] BMWi German Federal Ministry for Research and Technology, Resource Strategy of the German Federal Government – Safeguard a sustainable supply of Germany with non-energetic mineral ressources, Berlin, 2010. [3] European Commission, Ad-hoc Working Group on defining critical raw materials: Report on critical raw materials for the EU., Bruxelles, 2014.

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[4] Saxon Federal Ministry for Economy and Labour: Saxon Resource Strategy, 2012.

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[5] H. Alwast, B. Birnstengel, A. Häusler and B. Ross, Secondary Resource Atlas, 2009.

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[6] L. Bohdan, „Method for extraction of iron, aluminum and titanium from coal ash“. US Patent 4567026, 1986. [7] F. J. Calzonetti and G. . Elmes, „Metal recovery from power plant ash: n ecological approach to coal utilisation,“ Geo. J., Nr. 3, pp. 59-70, 1981.

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[8] N. Hopf, . Bimüller, E. Poehlmann and S. Sattelberger, „Energieeinsparung in der Metallurgie am Beispiel der energieeffizienten Aufschmelzung von Vanadiumkonzentraten, Abschlussbericht des Forschungs- und Entwicklungszentrum für Sondertechnologien,“ 2006. [9]

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[10] N. Smirnova, „Seltene Elemente in Kohlen und den Produkten ihrer Verarbeitung,“ Z. angew. Geol., Nr. 23, pp. 42-43, 1977. [11] U. Münch, Zu Konstitution, Elutionsverhalten und Kathodolumineszenz von Braunkohlenaschen, Freiberg, TU Bergakademie Freiberg, Dissertation, 1996. [12] R. Zenger, Method. Untersuchungen zur Schwefel- und Eisenführung in Braunkohlen u. Braunkohlefilteraschen des niederlausitzer Braunkohlenreviers (I); Literaturzusammenstellung: Versuche der Eisenkonzentratgewinnung aus Braunkohlefilteraschen in der ehem. DDR (II), Freiberg, TU Bergakademie Freiberg, Diplomarbeit, 1991.

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[14] K. Fytianos, B. Tsaniklidi and E. Voudrias, „Leachability of heavy metals in Greek fly ash from coal combustion,“ Environ. Int., Nr. 24, pp. 477-486, 1998.

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[15] J. F. Llorens, J. L. Fernandez-Turiel and X. Querol, „The fate of trace elements in a large coal-fired power plant,“ Environ. Geol., Nr. 40, pp. 409-416, 2001. [16] T. Okada, Y. Tojo, N. Tanaka and T. Matsuto, „ Recovery of zinc and lead from fly ash from ash-melting and gasification-melting processes of MSW-comparison and applicability of chemical leaching methods.,“ Waste Manage., Nr. 27, pp. 69-80, 2007.

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[17] Geobiotechnologie – Status und Perspektiven, Ein Statuspapier des Temporären Arbeitskreises Geobiotechnologie in der DECHEMA e.V., Dechema, 2013.

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[18] . Schippers, S. Hedrich, J. Vasters, M. Drobe, W. Sand and S. Willscher, „Biomining: Metal Recovery from Ores with Microorganisms,“ in Geobiotechnology, A. Schippers, F. Glombitza and W. Sand, Eds., Berlin-Heidelberg, Springer-Verlag, 2014.

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[19] J. C. Lee and B. D. Pandey, „Bio-processing of solid wastes and secondary resources for metal extraction – a review,“ Waste Management, Nr. 32, pp. 3-18, 2012.

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[20] F. Glombitza and S. Reichel, „Metal-containing residues from industry and in the environment - Biotechnological urban mining,“ in Geobiotechnology, A. Schippers, F. Glombitza and W. Sand, Eds., Springer-Verlag, Berlin-Heidelberg, 2014. [21] K. Bosecker, „Microbial recycling of mineral waste products,“ Acta Biotechnol., Bd. 7, Nr. 6, pp. 487-497, 1987.

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[22] C. Brombacher, R. Bachofen and H. Brandl, „Biohydrometallurgical processing of solids: a patent review,“ Appl. Microbiol. Biotechnol., Nr. 48, pp. 577-587, 1997. [23] C. Brombacher, R. Bachofen and H. Brandl, „Development of a laboratory-scale leaching plant for metal extraction from fly ash by Thiobacillus strains". Appl. Environ. Microb., Nr. 64, p. 1237–1241, 1998. [24] W. Krebs, R. Bachofen and H. Brandl, „Growth stimulation of sulfur oxidizing bacteria for optimization of metal leaching efficiency of fly ash from municipal solid waste incineration". Hydrometallurgy, Nr. 59, pp. 283-290, 2001. [25] N. Jain and D. Sharma, „Biohydrometallurgy for Nonsulfidic Minerals – Geomicrobiol. J., Nr. 21, pp. 135-144, 2004.

review,“

[26] R. Fass, J. Geva, Z. P. Shalita, M. D. White and J. C. Fleming, „Bioleaching method for the extraction of metals from coal fly ash using Thiobacillus“. US Patent 5278069, 1994. [27] T. Ishigaki, . Nakanishi, M. Tateda, M. Ike and M. Fujita, „Bioleaching of metal from municipal waste incineration fly ash using a mixed culture of sulfur-oxidizing and iron-

ACCEPTED MANUSCRIPT oxidizing bacteria,“ Chemosphere, Nr. 60, pp. 1087-1094, 2005.

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[28] G. L. A. Bowers-Irons, J. R. Pease, Q. K. Tran, T. Gibb, R. J. Pryor and S. Haddad, „Biomining of gallium and germanium containing ores“. US Patent 5030426, 09 07 1991.

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[29] . E. Torma and K. Bosecker, „Bacterial leaching,“ in Progress in Industrial Microbiology, Bd. 16, A. L. Bull, Hrsg., Elsevier, 1982, pp. 77-118.

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[30] P. B. Bosshard, R. Bachofen and H. Brandl, „Metal leaching of fly ash from municipal waste incineration by Aspergillus niger.,“ Environ Sci Technol, Nr. 30, p. 3066–3070, 1996.

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[31] T. J. Xu and Y. P. Ting, „Optimization study on bioleaching of municipal solid waste incineration fly ash by Aspergillus niger,“ in Biohydrometallurgy: a sustainable technology in evolution (First edition 2004), Proceedings of the 15th international biohydrometallurgy symposium (IBS), M. Tsezos, A. Hatzikioseyian und E. Remoundaki, Hrsg., National technical University of Athens, Athens, Greece, 2003, pp. 329-336.

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[32] . Singer, J. Navrot and R. Shapira, „Extraction of aluminium from fly-ash by commercial and microbiologically-produced citric acid,“ Eur. J. Appl. Microbiol. Biotechnol., Nr. 16, p. 228–230, 1982.

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Graphical abstract

ACCEPTED MANUSCRIPT Lignite ash: waste material or potential resource Investigation of metal recovery and utilization options

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René Kermer1,*, Sabrina Hedrich2, Sören Bellenberg3, Beate Brett4, Daniel Schrader5,

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Petra Schönherr6, Martin Köpcke7, Karsten Siewert8, Nils Günther9, Tilman Gehrke3, Wolfgang Sand3, Konstantin Räuchle4, Martin Bertau4, Gerhard Heide5, Lars Weitkämper7,

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Hermann Wotruba7, Horst-Michael Ludwig8, Roswitha Partusch10, Axel Schippers2,

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Susan Reichel1, Franz Glombitza1 and Eberhard Janneck1

Highlights

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construction industries

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Investigations on lignite ash utilization based on a stabilized ash from Germany Combination of ash pre-treatment, leaching and utilization approaches High extraction of the elements Al, Ca, Fe, Mg, Mn, V, Zn, Zr and for some REE achieved Ash and residues from ash leaching are potentially utilizable in water treatment and

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