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Recycling of hazardous waste from tertiary aluminium 1 industry in a value-added material 2 3 Laura Gonzalo-Delgado 1 , Aurora López-Delgado 1 *, Félix Antonio López 1 , Francisco 4 José Alguacil 1 and Sol López-Andrés 2 5 6 1 Nacional Centre for Metallurgical Research, CSIC. Avda. Gregorio del Amo, 8. 28040. 7 Madrid. Spain. 8 2 Dpt. Crystallography and Mineralogy. Fac. of Geology. University Complutense of 9 Madrid. Spain. 10 *Corresponding author e-mail: [email protected] 11 12 CORE Metadata, citation and similar papers at core.ac.uk Provided by Digital.CSIC

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  • Recycling of hazardous waste from tertiary aluminium 1

    industry in a value-added material 2

    3

    Laura Gonzalo-Delgado1, Aurora López-Delgado1*, Félix Antonio López1, Francisco 4

    José Alguacil1 and Sol López-Andrés2 5

    6

    1Nacional Centre for Metallurgical Research, CSIC. Avda. Gregorio del Amo, 8. 28040. 7

    Madrid. Spain. 8

    2Dpt. Crystallography and Mineralogy. Fac. of Geology. University Complutense of 9

    Madrid. Spain. 10

    *Corresponding author e-mail: [email protected] 11

    12

    CORE Metadata, citation and similar papers at core.ac.uk

    Provided by Digital.CSIC

    https://core.ac.uk/display/36055163?utm_source=pdf&utm_medium=banner&utm_campaign=pdf-decoration-v1

  • Abstract 13

    14

    The recent European Directive on waste, 2008/98/EC seeks to reduce the 15

    exploitation of natural resources through the use of secondary resource management. 16

    Thus the main objective of this paper is to explore how a waste could cease to be 17

    considered as waste and could be utilized for a specific purpose. In this way, a 18

    hazardous waste from the tertiary aluminium industry was studied for its use as a raw 19

    material in the synthesis of an added value product, boehmite. This waste is classified as 20

    a hazardous residue, principally because in the presence of water or humidity, it releases 21

    toxic gases such as hydrogen, ammonia, methane and hydrogen sulphide. The low 22

    temperature hydrothermal method developed permits the recovery of 90% of the 23

    aluminium content in the residue in the form of a high purity (96%) AlOOH (boehmite). 24

    The method of synthesis consists of an initial HCl digestion followed by a gel 25

    precipitation. In the first stage a 10% HCl solution is used to yield a 12.63 g.l-1 Al3+ 26

    solution. In the second stage boehmite is precipitated in the form of a gel by increasing 27

    the pH of the acid Al3+ solution by adding 1M NaOH solution. Several pH values are 28

    tested and boehmite is obtained as the only crystalline phase at pH 8. Boehmite was 29

    completely characterized by XRD, FTIR and SEM. A study of its thermal behavior was 30

    also carried out by TG/DTA. 31

    32

    Key words: hazardous waste, tertiary aluminium industry, recycling, boehmite, 33

    hydrothermal process 34

    35

  • 1. Introduction 36

    37

    Aluminium is the most widely used non-ferrous metal. More than 60 million tons of this 38

    metal are consumed in packaging, cars, aircraft, buildings, machinery and thousands of 39

    other products. Primary aluminium is obtained from its main raw material, bauxite, by 40

    electrolytic reduction of alumina dissolved in a cryolite bath. Although few compounds 41

    of aluminium are classified as toxic according to Annex 1 of the European Economic 42

    Union Council (EEC) Directive 67/1548, the industry itself is considered to be one of 43

    the most polluting, and aluminium production has been classified as carcinogenic for 44

    humans by the International Agency for Research on Cancer (IARC 1987). When 45

    aluminium products reach their end of their useful life, they become scrap, and as a 46

    result, a new industry has grown up over the last few decades. This is the secondary 47

    industry, which has developed the tools and technology to use this scrap as its raw 48

    material. Thus, secondary aluminium is obtained by processes involving the melting of 49

    aluminium scrap and other materials or by-products containing this metal, and the 50

    technologies used vary from one plant to another depending on the type of scrap, the 51

    oxide content, and the impurities, etc. According to the International Aluminium 52

    Institute (IAI 2010) in 2006, 16 million tones of total world production of aluminium 53

    came from the secondary industry. In Europe, about 40% of aluminium demand is 54

    satisfied by recycled material. In Spain, secondary aluminium production (refiners) 55

    practically doubled in only ten years, from 173 kt in 1997 to 345 kt in 2007 (OEA 56

    2010). 57

    As there is a market for the slag produced in the secondary industry, a tertiary 58

    industry has come into being, whose business is to obtain marketable products from slag 59

    milling. In this industry, slag is treated by milling, shredding and granulometric 60

  • classification processes to yield different size fractions which are commercialised 61

    according to their aluminium content. The coarser the fraction, the higher the 62

    aluminium content, and thus the more valuable the product. The finest fraction from the 63

    milling process constitutes the “Hazardous Aluminium Waste” named hereinafter as 64

    HAW, which is kept in secure containers in special waste storage facilities. HAW is a 65

    very fine grey-coloured powdery solid with a characteristic odour (due to its aluminium 66

    nitride, carbide and sulphide contents). It also contains other aluminium compounds 67

    (metallic aluminium, corundum, spinel), quartz, calcite, iron oxide and other metal 68

    oxides, chlorides and salts. Its chemical composition varies depending on the process 69

    used to melt scrap (López et al. 2001; López-Delgado et al. 2007). 70

    The United States Environmental Protection Agency (U.S. EPA 1980) and the 71

    European Union (Council Directive 1994) classify this material as a hazardous residue, 72

    principally because of its poor chemical stability. European legislation defines the waste 73

    as H12 (Donatello et al. 2010), because in the presence of water or humidity, HAW 74

    releases toxic gases such as hydrogen, ammonia, methane and hydrogen sulphide, based 75

    on the following reactions (López-Delgado et al. 2007): 76

    77

    2Al + 3H2O(l) 3H2(g) + Al2O3 ΔG025ºC=-208.0 kcal (1) 78

    2AlN + 3H2O(l) 2NH3(g) + Al2O3 ΔG025ºC =-78.7 kcal (2) 79

    Al4C3 + 6H2O(l) 3CH4(g) + 2 Al2O3 ΔG025ºC =-405.7 kcal (3) 80

    Al2S3 + 3H2O(l) 3H2S(g) + Al2O3 ΔG025ºC =-61.4 kcal (4) 81

    82

    HAW has commonly been stockpiled in secure storage facilities in accordance with 83

    safety regulations 67/548/EC (79/831/EC) and Directive 99/31/EC (European Directive 84

    1999). Processes developed for the treatment of HAW related wastes have generally 85

  • sought to render them inert, e.g. by very high temperature thermal treatment (Lindsay 86

    1995) or by stabilisation/solidification with cement or gypsum (López-Gómez and 87

    López-Delgado, 2004; Shinzato and Hypolito, 2005). Other procedures were developed 88

    to obtain valuable products, using synthesis and crystallization methods (Lin & Lo 89

    1998; López-Delgado et al. 2010). Vitrification has also been studied as a way of 90

    obtaining valuable glasses from waste similar to HAW, as an aluminium oxide source 91

    (López-Delgado et al. 2009). However, much more research needs to be done to find 92

    commercial uses for this waste, in accordance with the recent European Directive on 93

    waste, 2008/98/EC (European Directive 2008), which seeks to reduce the use of natural 94

    resources by means of secondary resource management. This Directive also includes a 95

    new status for waste, which is defined as “end-of-waste”. Several requirements have to 96

    be fulfilled by the waste in order for it to attain that status. These are: i) the substance is 97

    commonly used for specific purposes, ii) markets exist for the substance, iii) the 98

    substance meets the technical requirements for the specific purposes and fulfils the 99

    existing legislation and standards applicable to products, and iv) the use of the 100

    substance will not lead to an overall adverse impact on the environment or human 101

    health. Recycling is therefore, acknowledged to be the best rational way of making use 102

    of resources that would otherwise end up being dumped and/or discarded. 103

    Boehmite, an aluminium oxyhydroxyde described by the general formula 104

    AlOOH.nH2O, which crystallized within an orthorhombic system with a lamellar 105

    structure, is an important chemical used in many industries such as ceramics, 106

    composites, cement and derived-materials, paints and cosmetics. One of its most 107

    important applications is as a precursor of the major industrial catalysts supports: γ-108

    alumina (Raybaud et al., 2001). Applications of boehmite depend mostly on its physic-109

    structural properties, and these are very sensitive to the synthesis parameters (Okada et 110

  • al. 2002).The hydrothermal process is a traditional method of boehmite synthesis in 111

    which inorganic aluminium salts of analytical grade (high purity) are commonly used as 112

    raw materials (Mishra et al., 2000, Li et al. 2006, Liu et al. 2008). This paper reports on 113

    the recycling of hazardous waste in an added value material. The purpose is then, to use 114

    hazardous waste as the raw material for the synthesis of boehmite. This means working 115

    towards the transformation of a waste into an “end-of-waste”, the new concept defined 116

    in the European Directive on waste, as mentioned above (European Directive 2008). 117

    118

    2. Experimental 119

    120

    2.1. Materials 121

    122

    A hazardous aluminium waste (HAW) sample was obtained from a slag milling 123

    installation of a tertiary aluminium company in Madrid (Spain). Slag is shredded in 124

    autogenous mills, and different size fractions were obtained. HAW is the powdery solid 125

    (d90 < 100 µm) which is captured by suction systems and treated in sleeve filters. 126

    From the point of view of the chemical and mineralogical composition, this 127

    waste is highly heterogeneous because it is entirely dependent on the type of slag that is 128

    milled. So, a preliminary homogenisation process was carried out. The as-received 129

    waste (25 kg) was homogenised in a mixer and then successively quartered to obtain a 130

    representative sample of 1 kg. This sample was then placed in an automatic sample 131

    divisor to obtain 100 g fractions. HCl 37% (Panreac) and NaOH pellets 98% (Panreac) 132

    were used as reactants. 133

    134

    2.2. Methods 135

  • 136

    A low temperature hydrothermal method was used to recycle HAW into an 137

    added-value material. The process consisted of two stages: the first one aimed at 138

    recovering the aluminium content of the HAW by dissolving it in an acid medium; and 139

    the second one, dealt with the value of the aluminium recovered in the first stage by 140

    means of its precipitation as boehmite. In the first stage (acid digestion) 50 g of HAW 141

    were digested in 500 ml of boiling HCl solution (120 ºC) in an open glass vessel to 142

    dissolve the acid-soluble aluminium compounds. Two HCl solutions of concentrations 4 143

    and 10 %, were used to study the effect on aluminium recovery. The reaction time 144

    varied from 15 up to 180 min and the Al3+ concentration in solution was determined by 145

    AAS, at the different reaction times. The suspension obtained was centrifuged and the 146

    Al3+ solution was separated from the insoluble solid. This solid was characterised by 147

    chemical analysis and XRD. The acidic Al3+ solution obtained at this stage was then 148

    subjected to the second stage (gel precipitation), in which a 1M NaOH solution was 149

    added dropwise to a magnetically stirred 100 ml of Al3+ solution until reaching pH 150

    values of 7, 8 and 9. This second stage was performed at room temperature and 151

    pressure. A gel started to precipitate from the first drop and was aged with continuous 152

    stirring for 24 hours, after which it was separated by centrifugation and washed with 153

    deionised water until the total chloride was removed. The gel was dried at 60ºC for 4 154

    days and then crushed in a mortar to obtain a fine powder. 155

    156

    2.3. Analysis and Characterisation techniques 157

    158

    For the characterization of HAW, the next chemical analysis were performed: 159

    Al, Fe, Mg and Ca were determined in extracted acid solution, by atomic absorption 160

  • spectroscopy (AAS, Varian Spectra model AA-220FS). The AlN content was 161

    determined by analysing the nitrogen content, using the Kjendhal vapour distillation 162

    method (Velp Scientifica UDK 130). The SiO2 was measured by a gravimetric method 163

    (ASTM E 34-88) and C and S were determined by oxygen combustion in an induction 164

    furnace (Leco model CS-244). Other elements such as Ti, Cu, Zn, Cr, etc., were 165

    determined by X-ray fluorescence (XRF, Philips, modelo PW-1480 dispersive energy 166

    spectrophotometer, anode Sc/Mo, in working conditions of 80 kV and 35 mA). 167

    The crystalline phases of HAW, the insoluble solid obtained from the acid 168

    digestion and the boehmite samples were identified by X-ray diffraction (XRD, Bruker 169

    D8 advance difractrometer, CuKα radiation). The morphological characterisation of the 170

    samples was performed by scanning electron microscopy (SEM, Field emission Jeol 171

    JSM 6500F). For observations, the powdered sample was embedded in a polymeric 172

    resin. A graphite coating was used to make the sample conductive. 173

    The skeletal density of boehmite was measured in a He displacement 174

    pycnometer (Micromeritics Accupyc Mod 1330). The specific surface area (SBET) was 175

    determined from the N2 adsorption isotherms (Coulter Mod SA3100) at a relative 176

    pressure range of 0.04 to 0.2, after previously vacuum degassing the sample at 200 ºC. 177

    The characterisation of boehmite was completed by Fourier transform infrared 178

    spectroscopy (FTIR, Nicolet Magna-IR 550). Spectra were recorded through a disk 179

    obtained by mixing the sample with CsI. Thermal analysis was performed by 180

    simultaneous TG/DTA (SETARAM DTA-TG Setsys Evolution 500) at a heating rate of 181

    20 ºC min-1, in a He atmosphere up to 1200 ºC. Alumina crucibles with 20 mg samples 182

    were used. 183

    184

    3. Results and Discussion 185

  • 186

    3.1 Waste characterization 187

    188

    The chemical composition of the HAW is given in Table 1. The soluble aluminium 189

    content refers to all the aluminium compounds which dissolve in hydrochloric acid, e.g. 190

    metallic aluminium, aluminium nitride, sulphide and different types of salts. The total 191

    aluminium content also refers to other compounds such as insoluble oxides. Figure 1 192

    shows the XRD pattern for HAW, identifying the principal crystalline phases, such as 193

    metallic aluminium, spinel, corundum, aluminium nitride. From the chemical analysis 194

    and XRD study, the mineralogical distribution of the different major chemical elements 195

    constituting HAW is shown in Table 2, along with their composition expressed as wt %. 196

    Figure 2 shows a SEM image of HAW in which the morphology of the different phases 197

    can be observed. Grains of metallic aluminium or alloyed aluminium of different size 198

    and shape can be observed along with grains of spinel; small particles of corundum are 199

    observed in disaggregation zones surrounding the metallic aluminium grains. 200

    The hazardousness of HAW, as mentioned above, comes from the release of 201

    gases in the presence of water, according to Eq. (1-4). From the contents of metallic 202

    aluminium, and aluminium nitride and sulphide (Table 2) the calculated emission of H2, 203

    NH3 and H2S, per ton of HAW are 388.3, 45.9 and 3.1 Nm3 respectively. 204

    205

    3.2. Acid digestion stage 206

    207

    The total aluminium content of HAW is 47.5 %, but only the part in the form of 208

    hydrochloric acid soluble compounds is recovered as an Al3+ solution in the HCl 209

    digestion stage. This value represents 78 % of the total aluminium content and comes 210

  • mainly from compounds such as Alº and AlN and other soluble compounds. The 211

    recovery of aluminium increases with the reaction time. Thus when 10 % HCl was used 212

    for a reaction time of 45 min, 73 % of the aluminium was dissolved and for a time of 213

    150 min, 90 %. For the same time, an increase in the acid concentration yields a greater 214

    aluminium recovery. So, when digestion was carried out with 4 % HCl the dissolved 215

    aluminium percentage was lowerand for 45 min, only 54 % of the aluminium was 216

    recovered. This indicates that the kinetics of aluminium dissolution from HAW with 217

    HCl is faster with an acid concentration of 10 % than that of 4 % acid. The variation in 218

    the Al3+ concentration in the acidic solution with time, using a 10 % HCl, is shown in 219

    Figure 3. The [Al3+] concentration in the solution was 7.52 g.L-1 at a reaction time of 45 220

    min, and the value increased up to 12.63 g.L-1, at 150 min. 221

    The hydrochloric acid also partially dissolves iron oxide along with the 222

    aluminium compounds. The variation in the Fe3+ concentration over time is also shown 223

    in Figure 3. The curves of both metals exhibit similar behaviour. Therefore it is very 224

    important to control the experimental conditions of acid digestion in order to achieve an 225

    aluminium solution with low iron content. Nevertheless, several procedures such as 226

    solvent extraction can be used to increase the purity of the Al3+ solution (Alguacil et al. 227

    1987). In the present research, the Al3+ solution obtained directly from acid digestion 228

    was used for the next stage (gel precipitation), i.e., to obtain boehmite. On the other 229

    hand, the Al3+ solution obtained from the acid digestion of HAW may be used for many 230

    other purposes, in order to obtain added-value material, apart from boehmite. 231

    The composition of the solution obtained in the acid digestion stage (HCl 232

    concentration: 10 % and reaction time: 150 min) and subjected to gel precipitation was 233

    as follows: [Al3+] of 12.63 g.L-1 and [Fe3+] of 0.49 g.L-1. 234

  • The solid residue generated in this stage is composed of the insoluble 235

    compounds of HAW. Its chemical analysis yielded: 69.50 % Al2O3, 20.19 % SiO2, 4.00 236

    % MgO, 2.44 % TiO2, 0.51 % Fe2O3, 0.64 % CuO and 0.64 % Cl-. By XRD, only the 237

    crystalline phases such as, corundum, spinel and quartz were identified. This solid may 238

    therefore be considered as an inert material which could be suitable for using, for 239

    example, in the cement industry. 240

    241

    3.3. Gel precipitation stage 242

    243

    Figure 4 shows the XRD patterns of the different gels obtained at pH 7, 8 and 9. 244

    The pattern of the pH 7 gel shows several poorly defined broad diffraction bands around 245

    28, 39, 49 and 65 º (2θ), but special attention must be paid to the band observed at a low 246

    angle, around 7-8 º (2θ), which is common for lamellar structure compounds and seems 247

    to indicate the formation of a transitional phase prior to boehmite formation. Different 248

    authors (Okada et al. 2002, Ishikawa et al. 2006) have reported transitional phases in 249

    aluminium hydroxide formation with very similar diffraction patterns to the pH 7 gel , 250

    which are described as aluminium hydroxychloride. Chemical analysis of this gel 251

    suggests a stoichiometry of Al(OH)2.8Cl0.2.1.2H2O for this phase. 252

    The XRD pattern of pH 8 gel consists of five diffraction bands centred at 12.9, 253

    28.1, 38.6, 49.5 and 64.1 º (2θ) which fit well with the diffraction pattern corresponding 254

    to boehmite (AlOOH JCPDS no. 03-0066). The XRD pattern of pH 9 gel shows the 255

    same bands as the pH 8 gel and two others centred at 18.44 and 20.66 (2θ), indicating 256

    the presence of nordstrandite (Al(OH)3 JCPDS nº18-0050) along with the boehmite. 257

    According to these results, the value of 8 seems to be the best pH to obtain boehmite 258

    from HAW, as the only crystalline phase. For their part, the products obtained at pH 7 259

  • and pH 9 may be suitable for obtaining alumina by thermal treatment, although this 260

    possibility is not studied in the present work. The percentage of aluminium recovered 261

    from the acidic solution, increases with the pH, from the value of 95 % for pH7 to 96.2 262

    % for pH8. In the case of pH 9, this value (94.1 %) is slightly lower than that of pH 8. 263

    Then, this last value of pH is the best for obtaining the highest recovery of aluminium. 264

    265

    3.4. Characterization of boehmite 266

    267

    The crystallographic parameters of boehmite were determined from the XRD 268

    pattern of gel pH 8. Table 3 shows the values of 2θ, d and hkl index. From reflection 269

    020, which best characterises boehmite, the following unit cell parameters were 270

    calculated: a=3.582 Å; b=13.678 Å; c=2.902 Å, in a face-centered rombic symmetry 271

    and the space group Amam with Z (number of molecules per unit cell) of 4 (Tsukada et 272

    al. 1999; Okada et al. 2002, Liu et al. 2008). The crystallite size, calculated from 273

    Scherrer’s equation (Music et al. 1999) is 4.7 nm and indicates that boehmite is a 274

    nanocrystalline sample. This low crystallinity is typical of products obtained by 275

    precipitation in aqueous media. The aluminium oxy-hydroxides have low solubility and 276

    this causes quick supersaturation and massive precipitation, which hinders the 277

    crystalline developed (Tsukada et al. 1999, Li et al. 2006). 278

    From chemical analysis of gel pH 8, the stoichiometry of the boehmite can be 279

    formulated as AlOOH·0.8H2O, with a purity grade of 96%. Iron oxide, probably in the 280

    form of FeOOH, and other minor compounds make up the rest of the composition. 281

    The density of boehmite, determined in a helium pycnometer was 2.59 g.cm-3, 282

    which fits well with bibliographic values ranging between 2.3-3.03 g.cm-3 (Gevert & 283

    Ying 1999, Mishra et al. 2000). The SBET of sample was 205.2 m2.g-1, this value fits 284

  • well with that reported by Okada et al. (2002) for the same crystallite size boehmite, 285

    obtained by a hydrothermal procedure using a pure aluminium nitrate solution as a 286

    starting material. 287

    The FTIR spectrum of pH 8 gel is shown in Figure 5. Three different zones can 288

    be seen which correspond to the following wavenumber intervals: 1) 400-1200 cm-1, 2) 289

    1300-2500 cm-1 and 3) 2800-4000 cm-1. Zones 1 and 3 show the typical sample 290

    molecular vibrations and zone 2 provides information about the structural interconexion 291

    of the crystalline cell. 292

    Zone 1: the band corresponding to bending of O-H (ν3) appears at 1165 cm-1 as a 293

    shoulder of the very strong band at 1068 cm-1, which corresponds to stretching vibration 294

    ν2 Al=O. That weak vibration is characteristic of boehmite and its position is related to 295

    the crystallinity of the sample (Ram 2001). The vibration ν4 Al=O appears between 880 296

    and 740 cm-1 and is also very sensitive to the crystallinity of the sample. At 630 cm-1 the 297

    band corresponding to bending in the plane of the angle HO-Al=O (ν5) is observed as a 298

    very broad, strong band. 299

    Zone 2: In this area, three bands are observed at 1649, 1515 and 1413 cm-1. 300

    Their relative position and amplitude depend primarily on the degree of crystallinity of 301

    the sample and they are only observed in amorphous or nanocrystalline boehmite. Thus 302

    the first band is stronger than the others in nanocrystalline boehmite, and the second 303

    band is the strongest in amorphous samples (Ishikawa et al. 2006). The assignation of 304

    these bands is not clear because they cannot be assigned to overtones or combination 305

    bands. Ram (2002) suggested that these bands are derived from the formation of 306

    amorphous superficial structures in nanocrystalline hydrated boehmite. And this is the 307

    case of the boehmite obtained from HAW. 308

  • Zone 3: This area corresponds to stretching vibrations of O-H bonds. A very 309

    broad, strong band is observed at 3446 cm-1 with a shoulder at 3155 cm-1. A difference 310

    of nearly 300 cm-1 between both bands is also found in the nanocrystalline structures. 311

    Figure 6 shows the morphology of boehmite obtained at pH 8, revealing an 312

    agglomerate of very fine spherical particles of 7-15 nm size. The grains are aggregates 313

    of amorphous particles which coalesce without any long distance order. Grains of 314

    crystalline appearance were not observed. This morphology agrees with the results of 315

    the physical and structural properties. Fine particle compounds are commonly used to 316

    obtain high densification ceramic products. 317

    The TG and DTA curves of boehmite are shown in Figure 7. The DTA curve 318

    shows a first endothermic effect which appears as a very broad, strong band between 319

    45- 280 ºC with the maximum centered at 129 ºC. The mass loss associated with this 320

    effect is 15.3 % and corresponds to a dehydration process of interlaminar water 321

    according to eq. (5): 322

    Al2O2(OH)2.nH2O Al2O2(OH)2 + nH2O (5) 323

    The value of n calculated by mass loss is 0.60 which is slightly lower than that 324

    obtained by chemical analysis and is consistent with the presence of a certain amount of 325

    absorbed water. 326

    A second endothermic effect is observed between 281-548ºC with an associated 327

    mass loss of 14.0%. If all the water molecules had been lost at this stage in order to 328

    form aluminium oxide, a mass loss of 15% would have been attained. This means that 329

    the dehydratation process is not completed at this temperature and an intermediate 330

    phase can be formulated according to eq. (6). 331

    332

    333

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  • 360

    The authors thank the MEC for financing project CTM2005-01964 and the company 361

    Recuperaciones y Reciclajes Roman S.L. (Fuenlabrada, Madrid, Spain) for supplying 362

    HAW. Laura Delgado-Gonzalo is grateful to the CSIC (Spanish National Research 363

    Council) for an I3P contract. 364

    References 365

    366 Alguacil, F.J., Amer, S., & Luis, A. (1987) The Application of Primene 81R for the 367 Purification of Concentrated Aluminium Sulphate Solutions from Leaching of Clay 368 Minerals. Hydrometallurgy 18, 75-92 369

    Alphonse, P., & Courty, M. (2005). Structure and thermal behaviour of 370 nanocrystalline boehmite. Thermochimica Acta 425, 75-89 371

    Council Directive of 12.12.1991 on hazardous waste 91/689/EEC, Council Directive 372 94/31/CE of 27.6.1994 amending Directive 91/689/EEC. 1994 373

    Directive 2008/98/EC of the European parliament and of the council on waste and 374 repealing certain Directives, 2008 375

    Donatello, S., Tyrer, M., & Cheeseman, C.R. (2010) EU landfill waste acceptance 376 criteria and EU hazardous Waste Directive compliance testing of incinerated sewage 377 sludge ash. Waste Management 30, 63-71 378

    European Directive. Council Storage Directive 99/31/CE on the landfill of waste. 379 1999 380

    European Directive. Dangerous Substances Directive (67/548/EEC), later amending 381 by Directive 79/831/EEC “relating to the classification, packaging and labelling of 382 dangerous substances” 383

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    IAI, International Aluminium Institute www.world-aluminium.org 2010 386

    IARC Monographs on the Evaluation of Carcinogenic Risks to Humans. Suplement 387 7 pag 89-91 (1987) 388

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    Lin, S.H., & Lo, M.C. (1998). Recovery of aluminum alum from waste anode-394 oxidizing solution. Waste Management 18, 281-286 395

    Lindsay. R.D. Process for treatment of reactive fines; U.S. patent nº US5613996; 396 1995 397

  • Liu, Y., Ma, D., Han, X., Bao, X., Frandsen, W., Wang, D., & Su, D. (2008) 398 Hydrothermal synthesis of microscale boehmite and gamma nanoleaves alumina. 399 Materials Letters 62, 1297-1301 400

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  • U.S. EPA, 1980. Definition of hazardous waste, EPA/40 CFR/Part. 261.3, Federal 441 Register 442

    443

    444

    445

  • Figure legends 446

    Fig.1. XRD pattern of hazardous aluminium waste (HAW). (1: Alº, JCPDS 04-0787; 2: 447

    AlN, JCPDS 76-0565; 3: Al2O3, JCPDS 81-2266; 4: MgAl2O4, JCPDS 21-1152; 5: 448

    SiO2, JCPDS 85-0865, 6: CaCO3, JCPDS 85-1108). 449

    Fig.2. SEM image of HAW. (1: Alº, 2: Al alloyed with Fe/Cu, 3: MgAl2O4, 4: Al2O3) 450

    Fig.3. Variation in [Al3+] and [Fe3+] in the acid solution obtained with 10% HCl. 451

    Fig.4. XRD patterns of gel obtained at pH 7 (a), pH 8 (b) and pH 9 (c). (1: 452

    Al(OH)2.8Cl0.2.1.2H2O; 2: boehmite, AlOOH, JCPDS no. 03-0066; Norstrandite, Al(OH)3, 453

    JCPDS nº18-0050). 454

    Fig.5. FTIR spectrum of boehmite obtained from HAW. 455

    Fig.6. SEM micrograph of boehmite obtained from HAW. 456

    Fig.7. DTA/TGA curves of boehmite obtained from HAW. 457

    458

    459

  • Table1. Chemical composition of HAW. 460

    Element Weigh ( %)

    Altotal 47.45±0.05

    Alsoluble 36.87±0.02

    SiO2 8.0±0.5

    C 1.06±0.01

    N 2.88 ± 0.02

    S 0.14±0.01

    Cl 1.54±0.06

    F 0.59±0.09

    CaO 4.6±0.2

    MgO 4.2±0.1

    Fe2O3 1.8±0.06

    Na2O 1.67±0.06

    TiO2 1.53±0.06

    CuO 0.68±0.03

    K2O 0.46±0.03

    ZnO 0.32±0.02

    PbO 0.12±0.01

    Cr2O3 0.11±0.01

    461

    462

  • Table 2.- Mineralogical distribution and weight percentage of the major chemical 463

    elements present in HAW. 464

    Elements Mineralogical phases Weight %

    Al Al0

    Corundum, Al2O3

    Spinel, MgAl2O4

    Aluminium Nitride, AlN

    Aluminium Sulfide, Al2S3

    31.2

    20.0

    15.0

    8.4

    0.7

    Si Quartz, SiO2 8.0

    Mg Spinel, MgAl2O4 15.0

    Ca Calcite, CaCO3 8.2

    Fe Hematite, Fe3O4 1.8

    465

    466

    Table 3.- XRD reflections of the pH 8 gel diffractogram 467

    Reflections 2θ d(Ǻ)

    020 12.934 6.839

    120 28.097 3.173

    031 38.674 2.326

    051 49.505 1.839

    002 64.121 1.451

    468

    469

    470

  • 471

    472 473 474 Fig. 1 475 476

    10 20 30 40 50 60 70 800

    50

    100

    150

    200

    1

    1

    1

    1

    2

    2

    2

    2

    2

    2

    2

    2

    3

    3

    3

    3

    3

    3

    3

    6

    6

    6

    3,5

    5

    55

    54

    2,4

    4,5

    5

    4

    4

    4

    cou

    nts

    2θ (º)

    4

  • 477

    478 Fig 2. 479

    480

  • 481 482 483 484

    Fig. 3 485 486 487

    Al

    (g L

    -1)

    Al

    (g L

    -1)

    Al

    (g L

    -1)

    6.00

    8.00

    10.00

    12.00

    14.00

    15 30 45 60 90 120 180

    Time (min)

    0

    0.1

    0.2

    0.3

    0.4

    0.5

    0.6

    AlFe

    [Al]

    (g L

    -1)

    [Fe]

    (g L

    -1)

  • 488

    489 Fig. 4 490

    491 492

  • 4000 3500 3000 2500 2000 1500

    40

    60

    80

    100zo ne 3 zone 2

    Tran

    smitt

    ance

    (%)

    W avenum ber (cm -1)

    3446

    1649

    3155

    1515

    1165

    1068

    1413

    493 494

    495 496 497 498

    Fig. 5 499 500

    501

  • 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521

    Fig. 6 522 523

    524

  • 0 200 400 600 800 1000 1200

    -25

    -20

    -15

    -10

    -5

    0

    -30

    -25

    -20

    -15

    -10

    -5

    0 D T A

    Hea

    t Flo

    w (µ

    V)

    T em perature (ºC )

    E xo

    E n d o

    T G

    Mas

    s Var

    iatio

    n (w

    t,%)

    525 526

    Fig. 7 527