Aluminium recovery during black dross hydrothermal treatment

Journal of Environmental Chemical Engineering 1 (2013) 23–32

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Aluminium recovery during black dross hydrothermal treatment P.E. Tsakiridis *, P. Oustadakis, S. Agatzini-Leonardou Department of Mining and Metallurgical Engineering, Laboratory of Metallurgy, National Technical University of Athens, 9, Iroon Polytechniou Street, 157 80 Zografou, Athens, Greece1

A R T I C L E I N F O

A B S T R A C T

Article history: Received 2 February 2013 Accepted 12 March 2013

The purpose of present research work was to present a process for the recovery of Al, by treating aluminium black dross (ABD), a by-product formed during aluminium scraps melting. The proposed process consists of the following four (4) unit operations: a. Crushing the initial ABD by a jaw crusher to ‘‘1 mm’’ and recovery of metallic Al by sieving and screening (distorted or flattened particles). b. Further grinding of the material by ball mill to ‘‘100 mm’’, and recovery of the residual metallic Al. The total quantity of metallic Al recovered was estimated at 10.25% of the ABD. c. Recovery of soluble salts (NaCl, KCl), at atmospheric pressure, by water leaching of the grinded ABD at 90 8C. During leaching, about half of the nitrides were also decomposed. d. Aluminium recovery from the water leached residue, by pressure leaching at 240 8C, with NaOH strong solution (260 g/L). The total aluminium leaching efficiency on the basis of ABD weight reached 57.5%. ß 2013 Elsevier Ltd All rights reserved.

Keywords: Secondary aluminium Al black dross Characterization Leaching

Introduction Aluminium is a light, conductive and corrosion resistant metal with a strong affinity for oxygen, properties that have made it a widely used material, with applications in the aerospace, architectural construction and marine industries, as well as many domestic uses. Today aluminium is produced via two different routes: primary aluminium production from bauxite ore and recycling aluminium (secondary production) from process scrap and used aluminium products. In 1990, total aluminium production was around 28 million tonnes, (with over 8 million tonnes recycled from scrap), while in 2010 the total was close to 56 million tonnes (with close to 18 million tonnes recycled from scrap). By 2020 metal demand throughout the world is projected to have increased to around 97 million tonnes (www.world-aluminium.org/), with 31 million tonnes recycled from scrap [1]. The secondary industry is dependent on sources of scrap aluminium as the feedstock. Typical sources of aluminium scrap are process scrap, used beverage cans (UBCs), foils, extrusions, commercial scraps, turnings, and old rolled or cast metal [2]. The scrap feed, which is a complex combination of all types of aluminium scraps collected, is loaded into melting furnaces (reverberatory or rotary furnaces).

* Corresponding author. Tel.: +30 210 7722181; fax: +30 210 7722218. E-mail address: [email protected] (P.E. Tsakiridis). 1 www.hydrometallurgy.metal.ntua.gr. 2213-3437/$ – see front matter ß 2013 Elsevier Ltd All rights reserved. http://dx.doi.org/10.1016/j.jece.2013.03.004

The molten aluminium in the furnace is covered with molten salt flux, which protects the metal from the reactive atmosphere and facilitates agglomeration and separation of the metal. The nonmetallic components from the raw mix are completely absorbed by the liquid flux. Before the molten metal is trapped, the top layer, a dark-coloured material called black dross, is tapped or removed by skimming. More than a million tonnes of aluminium black dross (ABD) are reported throughout the world each year, and around 95% of this material is landfilled [3]. Typically, ABD contains aluminium metal (10–20%), a salt-flux mixture (40–55%), aluminium oxide (20–50%) and other contaminants and is one of the major waste by-products in the production of secondary aluminium. Depending on the market price of aluminium and the cost of transportation, ABD can be processed to recover approximately 20% of the remaining metal by means of salt bath rotary furnaces or hammer mills, which physically separate aluminium from the dross. When the recovery of aluminium from dross is not economically justifiable, it is disposed of in landfills [4,5]. Due to its properties, the ABD is classified as toxic and hazardous waste (100309), according to the European Catalogue for Hazardous Wastes [6]. It is considered as ‘‘highly flammable’’ (H3-A: substances and preparations which, in contact with water or damp air, evolve highly flammable gases in dangerous quantities), ‘‘irritant’’ (H4: non-corrosive substances or preparations which through immediate prolonged or repeated contact with the skin or mucus membrane can cause inflammation), ‘‘harmfull’’ (H5: substances and preparations which, if they are inhaled or ingested or if they penetrate the skin, involve limited

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health risk) and ‘‘leachable’’ (H13: Substances and preparations capable by any means, after disposal, of yielding another substance) [6,7]. The main problem is its leachability (H13) and its high reactivity with water or even humidity in air (H3-A), leading to the formation of toxic, harmful, explosive, poisonous and unpleasant odorous gases, such as NH3, CH4, PH3, H2, H2S, etc. As a result, when ABD is disposed in hazardous waste landfills, pollution of ground water (e.g. F, Cl, NH4+, CN, high pH) and ambient air (e.g. CH4, H2, NH3) can be observed [7–14]. In the last few years, public awareness and concerns about the quality of the environment have forced federal and local lawmakers to pass strict regulations on air pollution and on the disposal of various industrial wastes. Besides, its disposal costs (operational cost) are very high, as it requires controlled landfills. Today, the increasing number of environmental regulations has forced the secondary aluminium industries in Europe and the United States to consider recycling technologies for the reduction of waste. Furthermore, the recovery of aluminium oxide became increasingly important as landfill costs increase. Processes that could convert the oxide content of ABD to value-added alumina products would be necessary for the profitability of salt cake recycling [15,16]. The conventional ABD treatment consists of grinding the dross, sieving, to recover the metal value, followed by water leaching at ambient or higher temperature, to dissolve the salt in water from residue oxide [17–20]. The salt is recovered by filtering and evaporation techniques. The residue, which contains primarily alumina and other alloying elements, can be used, after washing (or calcination) in various industries (cement, ceramic, building industries) [21,22]. Alternative processes have also been proposed for ABD treatment, in order to recover aluminium in various forms. There are two possibilities to manage black dross: hydrometallurgical and pyrometallurgical processes. Pyrometallurgical processes face the problems of high energy consumption. They require some reducing agents and relatively high temperatures [23–25]. On the other hand hydrometallurgical processes are still a promise for the future and they could offer an interesting alternative for ABD recycling. Hydrometallurgical leaching processes can be carried out either by alkaline or acid leaching. Regarding acid leaching, a lot of work has been carried out on the utilization of ABD to produce a pure aluminium sulphate, which is known as paper-makers’ alum or filter alum [Al2(SO4)312H2O] and it is usually used in water treatment, cellulosic insulation, dying, fire-proofing fabrics etc. [26–29]. Other similar processes are targeting to the recovery of pure aluminium hydroxide by neutralization of the sulphuric leach liquor and the recovery of pure alumina by heat treatment of the produced aluminium hydroxide [21,30]. On the other hand, the alkali leaching process is based on Al and Al2O3 dissolution in strong sodium hydroxide solution at atmospheric or high pressure conditions. Aluminium can be recovered as aluminium hydroxide by crystallization/precipitation. High-grade metallurgical a-alumina can be then produced by calcination at high temperature (1100 8C) [31,32]. Al2 O3 þ 2NaOHþ ! 2NaAlO2 þ H2 O

(1)

2Al þ 2NaOH þ 2H2 O ! 2NaAlO2 þ 3H2 "

(2)

NaAlO2 þ 2H2 O ! AlðOHÞ3 # þ NaOH

(2 )

2AlðOHÞ3 þ heat ! Al2 O3 þ 3H2 O

(3)

0

Alpha-alumina has been used as raw materials for polishing abrasives, catalyst supports for high temperature reactions, cutting tools and advanced ceramics. Alumina has got some of the special

properties such as high hardness, high mechanical strength, high thermal conductivity and good thermal shock resistance. El-Katatny et al. investigated methods of recovering highsurface area aluminas, for catalytic and other purposes [33,34]. Extraction of the aluminium was carried out via atmospheric alkaline leaching (T = 100 8C, t = 150 min S/L = 0.3), with maximum aluminium recovery at 50%, and high-pressure alkaline leaching (T = 180 8C, t = 100 min S/L = 0.2), with maximum aluminium recovery at 73%. Aluminium was then separated from the sodium aluminate solution, using six different methods of precipitation via: H2O2, CO2, (NH4)HCO3, (NH4)2CO3, (NH4)Al(SO4)2 and aluminium hydroxide seed. The precipitates were filtered, washed, dried and calcined at 600 8C for the final alumina production. Miskufova et al. examined the leachability of Al from dross fines, after mechanical pre-treatment in alkaline solution at atmospheric conditions [35]. The maximum Al extraction (42% of total Al content and around 98% of leachable Al from dross) was achieved by leaching with 10% NaOH solution at 96 8C for 120 min. Al in the form of aluminium hydroxide was produced by precipitation, using various agents such as NH4HCO3, H2O2 and NH4Al(SO4)2, whereas alumina, suitable for utilization as a catalysts carrier or a sorbent in water treatment industry, was produced by aluminium hydroxide calcination at 600 8C. The possibility of alkaline leaching of ABD was also investigated by Lucheva et al. [36]. The leach liquor consist of 10% (v/v) NaOH, with molar ratio (alkaline module) Na2O/Al2O3 = 2. According to the results about 42% of the Al was recovered together with other minor elements. Al in the form of aluminium hydroxide was produced by carbonation of the solution. The sodium aluminate solution was neutralized by blowing CO2 and the produced aluminium hydroxide was used in the production of the Al2(OH)5Cl coagulant. The non metallic residue, after calcinations at 1000 8C, consists of 80% alumina, 5% MgO and 6% SiO2 and it was proposed as a raw material for alumina refractory materials. Atmospheric and high pressure alkaline leaching have also been performed for the recovery of aluminium from aluminium salt cakes, a similar waste with ABD but with lower metallic aluminium content [11,37,38]. The variables studied were the leach feed size, leach time and temperature. According to the results, over 40% of the residual alumina present in the leach residues was readily extracted using Bayer digestion conditions. Decomposition of the non metallic residue, after the removal of salt from dross with water leaching, was also carried out by NaOH leaching for 8 h, at 98.8 8C [16,32]. The precipitation of aluminium hydroxide was achieved by adjusting the pH (pH = 7) with sulphuric acid, whereas aluminium oxide with Al2O3 content 99.75% was recovered after drying. Leaching residue consisted of spinel (MgAl2O4) and some of the non-leached aluminium oxide. Hiraki et al. proposed a process for the synthesis of zeolite-X from Al(OH)3 precipitated from ABD in a NaOH solution and silicon sludge [39]. When metallic aluminium in the dross reacted with water in the NaOH solution, it changed into Al(OH)4. This was accompanied by the generation of hydrogen. The residual liquid was repeatedly used as raw material to save Si and Na sources. During the synthesis, the effect of the cyclic use of residual liquid on the property of zeolite was examined and it was compared with the product synthesized from reagents of Al(OH)3, Si, and NaOH. Although the original raw materials used were industrial wastes, all products showed the same mineralogical characteristics, Si/Al molar ratio (1.0–1.5) and BET surface area (500–600 m2/g), similar to the commercially available product. Finally, ABD powder and an extracted precipitate from Al dross in an alkali solution were also used as an aluminium source, in order to produce microporous aluminophosphate and its chromium-containing analogue [40]. Al dross extraction in an alkali medium was carried out with 10 wt.% NaOH solution at 100 8C for

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48 h, whereas Al precipitation took place via the addition of aluminium hydroxide seed, by stirring for 2 h. The aim of the present research work was to investigate the possibility of aluminium recovery from ABD by alkaline pressure leaching. More specifically, metallic aluminium was recovered by screening and sieving, after crushing and grinding the original ABD. The grinded ABD was water leached at 90 8C in order to recover the soluble salts. The soluble salt fraction could be recovered by evaporation and returned as a molten salt flux to secondary aluminium smelter, together with metallic Al. About 57.5% of the aluminium was leached from the washed residue by pressure leaching at 240 8C, using 260 g/L NaOH. The resulting leach liquor contained mainly aluminium which could be recovered by crystallization, in the form gibbsite (Bayer process). Experimental Aluminium black dross characterization The initial ABD, collected from an aluminium melting plant in Greece, as well as the leached residues were analysed chemically by X-ray Fluorescence analyzer (Spectro-Xepos) and atomic absorption spectrophotometer (Perkin Elmer 4100). Mineralogical analysis was carried out by X-ray diffraction (XRD), using a Bruker D8-Focus diffractometer with nickel-filtered Cu Ka radiation (l = 1.5405 A˚), 40 kV and 40 mA. The morphology of the initial dust and leached residues was also examined by scanning electron microscopy (SEM) using a Jeol 6380LV Scanning Electron Microscope. Experimental conditions involved 20 kV accelerating voltage. Chemical composition of the samples particles was carried out by an Oxford INCA Energy Dispersive Spectrometer (EDS) connected to the SEM. Grain size separation – metallic aluminium removal The original ABD dross consisted of rounded lumps up to about 30 mm in size with many small fragments. In order to obtain information about its grain size distribution, dross was fractionated by physical separation by sieving through a series of sieves (ISO 565) of specific diameters. A freshly produced 10 kg sample was dry-sieved into the following size fractions (mm): 63, 90, 150, 300, 600, 1000, 1400, 2360, 4750, 8000 and was weighed. The above procedure allowed a better identification of the different mineralogical phases in the fractions. In order to separate and recover the metallic aluminium from the original ABD, a dried sample was crushed using jaw crushers to ‘‘1 mm’’. Particles of distorted or flattened metallic aluminium were removed and weighted. A grinding stage was further chosen for the extraction of the rest of metallic Al by screening. Dross was grinded in a laboratory rotating ball mill to ‘‘100 mm’’, using steel balls as grinding medium. Particle size distribution of the final grinded ABD was measured by a CILAS-Model 1064 laser scattering particle size distribution analyzer. An amount of 0.1 g of powdered sample was put in 100 ml of ethanol and underwent dispersion treatment by an ultrasonic dispersion unit for 60 s. Aqueous leaching procedure All water leaching experiments were conducted in a 1 L fivenecked, round bottomed split reactor, which was fitted with a glass stirrer, a vapour condenser and a thermometer. In all the experiments, a constant stirring speed (700 rpm) was used to ensure suspension of the particles. Heating was provided by an electrical mantle and the temperature of the liquid was controlled by a Pt100 digital temperature controller. A typical run was carried out as follows: specific amounts of the solution and Al black dross

25

were loaded into the glass reactor and heated. The stirring speed was kept constant by means of a mechanical stirrer with digital display. At the end of the runs, the content of the reactor was filtered under vacuum. The resulting leached residues dried at 110 8C and weighed. Pressure leaching tests Aluminium was dissolved as sodium aluminate from black dross by treatment with NaOH at high pressure. Alkali leaching experiments were carried out in an Inconel autoclave reactor of 600 mL maximum capacity with pure nickel protective linings. Temperature was controlled using an external heating mantle with a PID controller. Cooling of the reactor was performed by water flow through an internal cooling tube. The pressure in the autoclave, which is necessary for the leaching process, was obtained as the result of the temperature increase in the closed vessel. The slurry was agitated by four blade impellers on a shaft connected to a rotating magnetic stirrer drive. The main test parameters for the hydrothermal treatment of Al black dross were Temperature (160–260 8C) and NaOH concentration (180–280 g/ L). Stirring speed and solid to liquid ratio were kept fixed at 500 rpm and 30% respectively. At the end of the experiment, the autoclave was rapidly cooled by circulating cold water through the cooling coil and the solid residues were filtered, washed and dried overnight at 110 8C. Results and discussions Aluminium black dross characterization The chemical and mineralogical composition of ABD depends mainly on the quality of Al scrap processed, the operating conditions and the type of technology and furnace applied for Al-metal production [7–12]. The dross used in this work was collected from a domestic dross producer and it consisted of rounded lumps up to about 30 mm in size. A representative sample was crushed and intensively grinded and the metallic aluminium was collected and weighted by firstly screening and then sieving to ‘‘100 mm’’, considering that the passing consist of the non metallic product. The as-received ABD chemical composition is presented in Table 1. It is composed of soluble salts (of Na, K and Cl), aluminium and scraps alloying elements (Mg, Si, Fe, Ti, etc.). Metallic aluminium in dross reached 10.25%. The X-ray diffraction pattern of the ABD, after the removal of metallic aluminium, is shown in Fig. 1. The quantitative determination of the principal phases was carried out by Rietveld Table 1 Chemical analysis of the as received ABD. Al black dross composition Element basis (%) Si Al AlMetallic Fe Ca Mg Na K Mn Cu Zn Ti S Cl LOI1000

Oxide basis (%) 0.65 33.25 10.25 0.65 1.74 2.12 5.06 0.95 0.12 0.53 0.49 1.15 0.23 5.95 8.28

SiO2 Al2O3 – Fe2O3 CaO MgO Na2O K2O MnO CuO ZnO TiO2 SO3 – LOI1000

1.39 62.81 – 0.93 2.44 3.52 13.64 2.29 0.15 0.66 0.61 1.92 0.58 – 8.28

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Fig. 1. Mineralogical phases of the as-received ABD.

analysis technique and the results are given in Table 2. The principle of Rietveld analysis is to iteratively compare the experimental pattern with a pattern simulated based on the presumed amounts, crystal parameters, and equipment parameters of a mixture of known phases. Six major phases were identified: spinel (MgAl2O4), aluminium nitride (AlN), diaoyudaoite (NaAl11O17), aluminium oxide nitride (Al5O6N), halite (NaCl) and corundum (Al2O3). Furthermore, hibonite (CaAl12O19), fluoride (CaF2), calcite (CaCO3) and sylvite (KCl) are also present as minor constituents. NaCl, KCl and CaF2 are coming from the molten salt flux, which had been used during the melting process, in order to protect the metal from the reactive atmosphere. The corundum (also known as a-alumina) comes from the reaction of the atmospheric oxygen with metallic aluminium molten at high temperatures. As aluminium, during melting in air, does not only combine with oxygen but also with nitrogen, aluminium nitride and aluminium oxide nitride are also produced. MgAl2O4 is the result of oxidation of magnesium present in the aluminium scrap as an alloying element, during the metal melting. The presence of CaCO3 should be attributed to the partial carbonization of portlandite Ca(OH)2, previously formed during hydration of CaO (impurity from the raw material). Metallic Al was not detected as it had been removed by grinding and sieving. ABD was also subjected to scanning electron microscopy (SEM) using a backscatter detector, in conjunction with energy dispersive spectroscopy (EDS). SEM was performed in polished sections, which were produced by vacuum impregnation, of the selected

Table 2 ABD phase composition by Rietveld analysis. Phases

Al black dross phases composition (wt.%)

1. MgAl2O4 – spinel 2. AlN – aluminium nitride 3. Al2O3 – corundum 4. NaAl11O17 – diaoyudaoite 5. NaCl – halite 6. CaAl12O19 – hibonite 7. Al5O6N – aluminium oxide nitride 8. CaF2 – fluoride 9. CaCO3 10. KCl – sylvite

22.61 20.55 11.64 14.73 8.56 6.18 9.58 2.39 2.05 1.71

sample, in a low viscosity epoxy resin. As it is shown in Fig. 2, the dross is characterized by the presence of metallic aluminium occluded in the inside of the oxide plates. The oxides in the dross exhibit the form of a long continuous network where metallic Al stays entrapped. It contained lath-shaped corundum along with euhedral spinel crystals, which form an irregular and often discontinuous chain-like structure. Aluminium nitrides are detected with the form of both fibrous and elongated crystals. The fibrous occur as a lump with many pores, whereas most of the elongated crystals are chains of roundish beads. Sizable euhedral crystals of calcium aluminate were detected in all cases. Its formation can be attributed to the reaction of alumina in the spinel with CaO impurity from the raw material. Finally, sodium chloride forms a mass with regularly shaped large cubic crystals, which are compact with sharp edges. Grain size separation – metallic aluminium removal The original ABD, which consisted of rounded lumps up to about 30 mm in size with many small fragments, was fractionated by the physical separation through a series of sieves (ISO 565) of diameters (mm): 63, 90, 150, 300, 600, 1000, 1400, 2360, 4750, and 8000. Its particle size distribution is presented in Fig. 3. The ABD was a mixture of fine particles with large agglomerated particles. The coarse fractions (+20 mm) contained about 60% of metallic aluminium, while the medium fraction (20 mm +1 mm) contained about 35% of it. The fine fraction (1 mm), which mainly consists of oxides, nitrides and salts, was about 40% of the ABD, whereas the amounts of fines with d < 100 mm reached 15%. The sieve analysis was also carried out in order to determine the mineralogical phases of the dross retained on the different sieves, after the removal of metallic Al (Fig. 4). X-ray diffraction analysis suggests that all the fractions of ABD were mineralogically similar, presenting almost the same peak patterns, when ground to comparable sizes. The recovery of metallic Al was carried out by sieving and screening, after crushing the original ABD to ‘‘1 mm’’ in a jaw crusher. A grinding stage was further used for the total extraction of the remaining metallic Al. The final particle size distribution (cumulative passing and individual retained) of ABD, after grinding in ball mill to ‘‘100 mm’’ and removing metallic Al, is given in

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Fig. 2. Scanning electron micrographs of the as-received ABD. a: metallic Al, b: Al2O3, c: MgAl2O4, d: AlN/Al5O6N, e: NaCl, f: CaF2, and g: CaAl12O19.

Fig. 5. Grinding resulted in a reasonably smooth and continuous shift of the particle size distributions to finer sizes. It was found that 50% of it was below 25 mm, whereas the majority (90%) of the particles were below 60 mm. Aqueous leaching procedure A water-leaching stage is always required before alkaline leaching under pressure. It is known that a high pressure alkaline

Fig. 3. Size distribution of the initial ABD used.

leaching process (Bayer process) is sensitive to impurity elements (soluble salts such as sodium and potassium chlorides) and compounds which can give explosive gases during digestion [37]. After grinding and removal of metallic Al, black dross was leached with water to separate water-soluble phases (salts, nitrides) from the insoluble residues, so the salt fraction could be recovered and returned (as a molten salt flux) together with previously separated metal granules, to secondary aluminium smelters. Preliminary experiments showed that the temperature increase did not have any considerable effect on the solubility, especially in case of nitrides. However, since the increase of temperature led to the kinetics reaction improvement the value of 90 8C was chosen as the optimum. The X-ray diffraction data of the residues, washed at different temperatures is presented in Fig. 6. Equilibrium time was examined by employing different periods and the experimental results indicated that 60 min (at 90 8C) is enough for the dissolution process. Further increase in the washing time up to 10 h did not lead to further dissolution. The chemical composition of the leached residue at 90 8C is given in Table 3. Its principal phase analysis, carried out by Rietveld method, is presented in Table 4. According to the results, the washing treatment results in significant salt removal. Effectively all of the chloride has been dissolved together with 80% of the sodium and 85% of the potassium during water leaching at 90 8C, whereas only small amounts of the other elements were dissolved. The decrease of the black dross weight reached 11%. The solution pH values

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28

Fig. 4. Mineralogical phases of different fractions of the ABD used.

indicated an increase in the area of pH = 11.0, due to the partial dissolution of nitrides and the production of NH3 or NH4OH. Chloride phases were below the XRD detection level. However, nitrides are still present in high quantities and, from the environmental point of view, the possibility of these species being leached could generate serious risks. The XRD data indicated a total decrease of about 50% of AlN, after the normalization of the final removal percentages (due to the NaCl and KCl absence from the leached residue). However Al5O6N seems to be undissolved during the aqueous treatment. Water leaching did not remove sodium and potassium completely because these elements are also present in other, non-water-soluble compounds. The detection of aluminium hydroxide was attributed, firstly, to the partial leaching of some of the aluminium phases, contained in black dross, and then to its precipitation, mainly due to the pH increase. The above results were also confirmed by electron microscopic observations. SEM micrographs of ABD water leached residue are shown in Fig. 7. Although the washed residue presented similar microstructure with the as received ABD, the detection of aluminium hydroxide should be noted. This was attributed to the partial precipitation of Al dissolved in the leach liquor. Al(OH)3, consisted of needle like and prismatic crystals of 5–10 mm in 50

100

ABD water leached residue Element basis (%) Si Al Fe Ca Mg Na K Mn Cu Zn Ti S Cl LOI1000

Oxide basis (%) 1.58 35.75 0.85 2.02 3.05 1.03 0.12 0.13 0.55 0.52 1.17 0.25 0.11 12.25

SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O MnO CuO ZnO TiO2 SO3 – LOI1000

3.39 67.53 1.22 2.83 5.06 2.78 0.29 0.17 0.69 0.65 1.95 0.63 – 12.25

length and 1–2 mm in diameter. Aluminium oxides and spinels were detected with the form of aggregates of fine crystallites. The aggregates had an irregular shape and size of about 10–20 mm. Aluminium nitrides were also found in lower proportions mainly with the form of fibrous crystals. No discrete Cl-bearing phases were readily identified under the SEM, although small amounts of Cl were present.

40

80 70

60

30

50 40

20

Total Particles (%)

Cumulative Number of Particles (%)

90

Table 3 Chemical composition of the leached residue after water leaching.

30 20

10

10 0 0,01

0,1

1

10

100

0 1000

Particle Diameter (μm)

Fig. 5. Particle size distributions of the grinded ABD by a laser scattering analyzer.

Table 4 ABD water leached residue phase composition by Rietveld analysis. Phases 1. 2. 3. 4. 5. 6. 7. 8. 9.

MgAl2O4 – spinel AlN – aluminium nitride Al2O3 – corundum NaAl11O17 – diaoyudaoite Al(OH)3 – bayerite CaAl12O19 – hibonite Al5O6N – aluminium oxide nitride CaF2 – fluoride CaCO3

ABD water leached residue phases composition (wt.%) 28.63 11.37 15.68 14.90 5.09 8.23 10.98 2.74 2.35

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Fig. 6. Mineralogical phases of the ABD water leached residues at different temperatures.

Pressure leaching tests After water leaching the residue was leached at elevated temperature, under pressure, using concentrated NaOH solution. Stirring speed and solid to liquid ratio were kept fixed at 500 rpm and 30% respectively. Experiments were designed to investigate the effect of temperature and that of NaOH concentration and the results are presented in Fig. 8. The sodium hydroxide concentration, during pressure leaching of the washed residue, varied from 180 g/L to 280 g/L. All the experiments were performed under constant temperature (220 8C). The results showed that the extraction of aluminium increased from 31% to 53% with NaOH concentration increase from 220 g/L to 260 g/L. When the NaOH

concentration was increased further, aluminium extraction remained almost the same. Thus, 260 g/L of NaOH was chosen as the optimal leaching concentration for Al extraction. The study of the relationship between leaching temperature and aluminium recovery was carried out with constant NaOH concentration (250 g/L). As can be seen (Fig. 8), a temperature increase within the range of 160–240 8C caused an aluminium content increase in the leach liquor. Further temperature increase at 260 8C did not cause additional leaching of aluminium, indicating that the system had reached equilibrium. Therefore, 240 8C was chosen as the optimal leaching temperature. In all cases the rate of extraction was relative fast and the dissolution reactions were completed in less than 2 h.

Fig. 7. Scanning electron micrographs of the ABD water leached residue. a: Al2O3, b: MgAl2O4, c: AlN/Al5O6N, d: AlOOH, and e: CaAl12O19.

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Fig. 8. Aluminium extraction vs. temperature and NaOH concentration.

Fig. 9. Mineralogical phases of the ABD pressure leached residue.

The chemical composition of the leached residue, at the optimum leaching conditions is given in Table 5. Maximum Al recovery (57.5%), achieved after 100 min leaching at 240 8C using 260 g/L of NaOH, is comparable with that from typical bauxites. After pressure leaching about 28% of the washed residue was dissolved. The main remaining mineralogical phases, detected by X-ray diffraction analysis, and the phase’s analysis, carried out by Rietveld method, are presented in Fig. 9 and Table 6 respectively. Only spinel, aluminium oxide, diaoyudaoite and calcium aluminate were detected. SEM micrographs of ABD pressure leached residue are shown in Fig. 10. According to the results, no aluminium nitrides phases were detected, a fact that indicates their compete dissolution. Although aluminium hydroxide was below the detection limit, when X ray diffraction analysis was performed, its presence in the residue was confirmed by the scanning electron microscopy, with

Table 5 Chemical composition of the leached residue after pressure leaching. ABD alkaline leached residue Element basis (%) Si Al Fe Ca Mg Na K Mn Cu Zn Ti S LOI1000

Oxide basis (%) 1.42 35.85 0.89 3.85 5.03 0.85 0.05 0.16 0.58 0.53 1.85 0.05 5.75

SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O MnO CuO ZnO TiO2 SO3 LOI1000

3.04 67.72 1.27 5.39 8.34 2.29 0.12 0.21 0.73 0.66 3.09 0.13 5.75

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Fig. 10. Scanning electron micrographs of the ABD pressure leached residue. a: Al2O3, b: MgAl2O4, c: CaAl12O19, d: Al(OH)3, and e: amorphous silica.

the form of prismatic crystals. Calcium aluminate and spinel phases, which have not been leached, were also detected with the form of sizable euhedral crystals. Furthermore, small quantities of amorphous silica were also found. The average composition of the sodium aluminate pregnant solution produced from ABD pressure leaching with NaOH strong solution was as follows: Al3+ = 28.64 g/L, Ca2+ = 0.28 g/L, Mg2+ = 0.12 g/L, Si4+ = 0.18 g/L, K+ = 0.044 g/L and Fe3+ = 0.007 g/ L. Alumina can be recovered by firstly crystallizing/precipitating aluminium hydroxide from the leach liquor by different precipitating agents [H2O2, CO2, NH4HCO3, (NH4)2CO3, (NH4)Al(SO4)2, fresh aluminium hydroxide jelly] and subsequent calcination at about 1100 8C. The pressure leached residue could be landfilled or further reused. However, processes that convert the oxide content of the residue to value-added alumina products are necessary for the profitability of the total process of recycling. The leached residue Table 6 ABD alkaline leached residue phase composition by Rietveld analysis. Phases 1. 2. 3. 4.

MgAl2O4 – spinel a-Al2O3 – alpha-aluminium oxide NaAl11O17 – diaoyudaoite CaAl12O19 – hibonite

ABD pressure leached residue phases Composition (wt.%) 36.88 23.22 20.70 19.20

could be used to introduce alumina into the clinker burning process of the cement kiln, as its high alumina content is important for the formation of calcium aluminate phases. It could be also act as a source of alumina in ceramic and refractory applications (building bricks, pavers, firebricks). Other possible uses of the final non metallic leached residue are the following [3]:  Civil works: inert filling for constructions, pavements and mortar components  Chemical industry: production of hydrate aluminium oxide and aluminium salts, epoxy resin mortar and inert load in polymers  Metallurgical industry: Synthetic steel refining slags to remove sulphur, phosphorus and aluminium oxide from molten steel  Agriculture: artificial soil and fertilizers Conclusions Based on the data presented in this paper, a hydrometallurgical process of several stages for treating aluminium black dross (ABD), a by-product formed during aluminium scraps melting, was suggested in order to maximize aluminium recovery and minimize waste volumes and toxicity. The characterization study indicated that ABD is a complex mixture of metallic Al and various oxides and salts. All the metallic Al (10.25%) was recovered, with the form of distorted or flattened particles, by screening and sieving, after crushing the initial ABD in

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a jaw crusher to ‘‘1 mm’’ and then grinding in a ball mill to ‘‘100 mm’’. Washing treatment of the grinded ABD at 90 8C led to the salt removal. Chloride was completely dissolved, whereas 80% of the sodium and 85% of potassium were leached. At this stage only 50% of nitride phases were dissolved. The decrease of the black dross weight reached 11%. The aluminium recovery from the ABD washed residue by 57.5%, was accomplished by alkaline high pressure leaching, using 260 g/L of NaOH at 240 8C for 100 min. At those conditions about 28% of the residue was dissolved. The main mineralogical phases of leached residue were a-Al2O3, MgAl2O4 (spinel), NaAl11O17 (diaoyudaoite) and CaAl12O19. All aluminium nitrides phases were dissolved. The average composition of the produced sodium aluminate pregnant solution was: Al3+ = 28.64 g/L, Ca2+ = 0.28 g/L, Mg2+ = 0.12 g/L, Si4+ = 0.18 g/L, K+ = 0.044 g/L and Fe3+ = 0.007 g/L. References [1] IAI, Global Aluminium Recycling: A Cornerstone of Sustainable Development, Global Aluminium Recycling Committee, International Aluminium Institute, London, 2009. [2] J.A.S. Tenorio, D.C.R. Espinosa, Effect of salt/oxide interaction on the process of aluminum recycling, Journal of Light Metals 2 (2002) 89–93. [3] P.E. Tsakiridis, Aluminium salt slag characterization and utilization – a review, Journal of Hazardous Materials 217–218 (2012) 1–10. [4] J.Y. Hwang, X. Huang, Z. Xu, Recovery of metals from aluminium dross and salt cake, Journal of Minerals and Materials Characterization and Engineering 5 (2006) 47–62. [5] B.J. Jody, E.J. Daniels, P.V. Bonsignore, D.E. Karvelas, Recycling of aluminum salt cake, Journal of Resource Management and Technology 20 (1992) 38–49. [6] European Waste Catalogue and Hazardous Waste List, Environmental Protection Agency, Ireland, Valid from 1 January 2002. [7] K.E. Lorber, H. Antrekowitsch, Treatment and disposal of residues from aluminium dross recovery, in: 2nd International Conference on Hazardous and Industrial Waste Management, Crete, (2010), p. B.2.1. [8] Y. Xiao, M.A. Reuter, U. Boin, Aluminium recycling and environmental issues of salt slag treatment, Journal of Environmental Science and Health Part A 40 (2005) 1861–1875. [9] G.V. Calder, T.D. Stark, Aluminum reactions and problems in municipal solid waste landfills, Practice Periodical of Hazardous, Toxic, and Radioactive Waste Management 14 (2010) 258–265. [10] M.C. Shinzato, R. Hypolito, Solid waste from aluminum recycling process: characterization and reuse of its economically valuable constituents, Waste Management 25 (2005) 37–46. [11] W.J. Bruckard, J.T. Woodcock, Recovery of valuable materials from aluminium salt cakes, International Journal of Mineral Processing 93 (2009) 1–5. [12] K.E. Lorber, Disposal of dross on landfill. A case study, in: Proceedings of International Solid Waste Association (ISWA), World Environment Congress, Istanbul, Turkey, 2002. [13] A. Gil, Management of the salt cake from secondary aluminium fusion processes, Industrial & Engineering Chemistry Research 44 (2005) 8852–8857. [14] H. Shen, E. Forssberg, An overview of recovery of metals from slags, Waste Management 23 (2003) 933–949. [15] D. Graziano, J.N. Hryn, E.J. Daniels, The economics of salt cake recycling, in: Light Metals, Proceedings of Sessions, TMS Annual Meeting, Warrendale, PA, (1996), pp. 1255–1260. [16] J.W. Pickens, Assuring the benefits of aluminum recycling: engineering economical environmental solutions to the issues of black dross & saltcake, in: 4th

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