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Aluminum recovery as a product with high added value using aluminum hazardous waste
Journal of Hazardous Materials 261 (2013) 316–324
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Aluminum recovery as a product with high added value using aluminum hazardous waste E. David a,∗ , J. Kopac b a National Institute for Research and Development for Cryogenic and Isotopic Technologies, Street Uzinei, No. 4, P.O. Râureni, P.O. Box 7, 240050 Rm. Vâlcea, Romania b Faculty of Mechanical Engineering, University of Ljubljana, Askerceva 6, SI-1000 Ljubljana, Slovenia
h i g h l i g h t s • • • • •
Granular and compact aluminum dross were physically and chemically characterized. A relationship between density, porosity and metal content from dross was established. Chemical reactions involving aluminum in landfill and negative consequences are shown. A processing method for aluminum recovering from aluminum dross was developed. Aluminum was recovered as an value product with high grade purity such as alumina.
a r t i c l e
i n f o
Article history: Received 30 April 2013 Received in revised form 4 July 2013 Accepted 18 July 2013 Available online xxx Keywords: Aluminum dross Aluminum extraction Acid leaching Alumina
a b s t r a c t The samples of hazardous aluminum solid waste such as dross were physically and chemically characterized. A relationship between density, porosity and metal content of dross was established. The paper also examines the chemical reactions involving aluminum dross in landfill and the negative consequences. To avoid environmental problems and to recovery the aluminum, a processing method was developed and aluminum was recovered as an added value product such as alumina. This method refers to a process at low temperature, in more stages: acid leaching, purification, precipitation and calcination. At the end of this process aluminum was extracted, first as Al3+ soluble ions and final as alumina product. The composition of the aluminum dross and alumina powder obtained were measured by applying the leaching tests, using atomic absorption spectrometry (AAS) and chemical analysis. The mineralogical composition of aluminum dross samples and alumina product were determined by X-ray diffraction (XRD) and the morphological characterization was performed by scanning electron microscopy (SEM). The method presented in this work allows the use of hazardous aluminum solid waste as raw material to recover an important fraction from soluble aluminum content as an added value product, alumina, with high grade purity (99.28%). © 2013 Elsevier B.V. All rights reserved.
1. Introduction Aluminum metal is supplied by two distinct aluminumproduction sectors: primary aluminum producers and secondary aluminum smelters [1–3]. The primary aluminum industry in the Romania produces aluminum mostly from mined ore (bauxite). Some scrap is also used, which is usually from recycled aluminum beverage cans and industrially generated high-grade aluminum scrap. The continuing growth in aluminum can recycling has increased the value of aluminum, yielded energy savings, and resulted in ecological benefits [4–9]. Recycling 1 kg of aluminum
∗ Corresponding author. Tel.: +40 250732744, fax: +40 250 732746. E-mail addresses: [email protected], [email protected] (E. David). 0304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2013.07.042
can save about 4 kg of bauxite, 2 kg of chemicals, and 7.5 kWh of electricity [3,10–12]. Worldwide the aluminum industry produces over 4.5 billion kilograms of aluminum waste each year [3,13–15]. Each type of waste has unique chemical and physical characteristics, and the value of the waste is determined by the level of impurities contained and the cost of recovering of the metal [13,14]. Because the process of recovering aluminum from low-grade scrap requires large quantities of salt, furnaces generate large quantities of salt cake. For every kg of low-grade scrap charged into the rotary furnace, 1 kg of salt flux is typically used [12]. More than 100 flux compositions are available to scrap-aluminum smelters, but the most commonly used fluxes are mixtures of NaCl and KC1 in approximately equal amounts, to which 3–5 weight percent (wt.%) cryolite is added [16,17]. The advantages of this chloride mixture (as a fluxing agent) are its low cost ($100/ton) and low melting point
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Fig. 1. Aluminum scrap recycling process.
[2,7]. Sodium chloride is lower in cost, but potassium salts have lower viscosity and lower surface tension, qualities that increase fluidity. The melting point of this mixture is just below the melting point of aluminum (660 ◦ C). It is important that the melting point of flux is below the melting point of aluminum, because during heating of the charge, the salt melts first and coats the aluminum before the metal melts, thereby protecting the metal from further oxidation. For dross processing, 1 kg of flux is used for each kg of dross concentrate charged to the rotary furnace [8,12]. The salt flux used in scrap aluminum smelting ends up in the black dross and salt cake, which are commonly disposed of in landfills. Fig. 1 is a diagram of the recycling process for scrap aluminum and the various sources of waste generation. Typically, aluminum black dross (ABD) contains aluminum (12–20%), sodium chloride (20–25%), potassium chloride (20–25%), aluminum oxides (20–50%), and other compounds (2–5%) [12]. Black dross is most often further processed to recover as much as 80% of the remaining aluminum. In order to recover the aluminum and make recycling economically justifiable, the dross must be upgraded (usually by means of milling and screening) at least to 50% aluminum concentrate [14,18]. Because the metal is less brittle than the other dross constituents, it will not break in the milling process, and it will tend to be screened out in coarser fractions. The aluminum “concentrate” is processed in a rotary furnace to recovery of the aluminum (Fig. 2). The finer-sized fractions, less than about 1 mm, can represent up to 85% of the milled black dross and can contain as much as 10 wt.% metallic aluminum. A general dross-processing flow diagram is shown in Fig. 3. The salt cake generated from the rotary furnaces ranges up to 75% of the melt charge in rotary furnace.
Fig. 3. Aluminum dross processing flow diagram.
Table 1 lists the estimated total amount of solid residues generated by the secondary aluminum industry from secondary aluminum production operations. Huge quantities of dross and salt cake are generated, of which very little, if any, are recycled, most of them are disposed of in landfills. The salt cake consists of aluminum (1–5%), aluminum oxide (15–30%), sodium chloride (30–55%), potassium chloride (15–30%), and inert compounds (1–4%) [19]. It is very known that due to its properties, the ABD is classified as toxic and hazardous waste (100309), according to the European Catalog for Hazardous Wastes. It is considered as “highly flammable” (H3-A), “irritant” (H4), “harmfull” (H5) and “leachable” (H13). It is also very know that 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 , and H2 S. 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 [3,8]. Worldwide are different ways of recycling of this hazardous waste. Mukhopadhyay et al., shown that the recycling of aluminum dross is very important due to economic and environmental benefits, it is saved raw materials and the waste is not sent to landfill [20]. As show Das et al. [1], Dash et al. [16] the disposal of salt slag and dross is a worldwide problem. In the case of improper disposal, leaching of toxic metal ions into ground water would determine serious problems of pollution. They showed that the processing of these wastes and obtaining useful products is a viable way. A series of convincing experimental results tend to prove that sulfuric Table 1 Major residues from secondary aluminum processing industry.
Fig. 2. Aluminum dross recycling process.
No.
Solid residues
1. 2. 3. 4. 5.
Metal produced Black dross Black dross residuesa Salt cake Baghouse dust
a
Dross – milling residues.
Estimated quantities (wt.%) 75–76 7–8 6–7 7–8 0.6–0.7
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Table 2 Particle size distribution after crushing and milling of primary aluminum dross. Screen aperture (mm)
Weight (%)
+2.000 −2.000 + 1.680 −1.680 + 0.600 −0.600 + 0.400 −0.400 + 0.125 −0.125 + 0.074 −0.074 Initial dross (before crushing and milling) +2.000 −2.000
0.47 2.54 6.17 1.54 6.80 17.25 65.23 28.05 71.95
acid is an effective and less costly acid for metal leaching. Dash et al. have obtained various form of hydrated alumina from three different sources like sodium aluminate liquor, waste aluminum dross and synthetic salt like aluminum sulfate. From the sulfuric acid leach liquor of waste aluminum dross, hydrated alumina was precipitated by varying the pH of the precipitation using 10% aqueous ammonia [21]. Also, several studies have been undertaken where dross, after leaching by H2 SO4 , was utilized for the manufacture of aluminum sulphate [1,16]. Garret [22] presents a method that uses the concentrate sulfuric acid (90–99.4 wt.%) and brine for aluminum leaching from waste to obtain aluminum sulphate. Aluminum sulphate produced by this method was submitted for testing as a flocculant for wastewater treatment process. Huckabay and Skiathas [23] presented a method to prepare aluminum sulfate based on leaching with sulfuric acid of aluminum dross. The aluminum dross prior to reaction with sulfuric acid are pretreated by oxidizing with water and steam at elevated temperatures to reduce the more objectionable contaminants. In this paper, we examine the possibility to use hazardous aluminum dross as raw material to extract aluminum, as an inoffensive product to environment such as alumina, to avoid the chemical reactions involving aluminum dross in landfill and the negative environmental consequences. The presented method refers to a process at low temperature, in more stages: acid leaching, purification, precipitation and calcinations. The paper points out that it can get high purity alumina if the impurity ions from aluminum leaching liquor are removed to the required level. For instance, ferric ions (Fe3+ ) and aluminum ions (Al3+ ) are hydrolyzed and precipitate out at pH level less than 5 [24]. In this case the pH adjustment is ineffective or not quite effective, therefore (Fe3+ ) ions are difficult to be separated of aluminum ions. The solution presented in this paper is the using a selective complexing agent, such as ethylene diamine tetraacetic acid (EDTA) because the complexing ability of each metal (Fe3+ ,Al3+ ) in leaching liquor with EDTA is different, and therefore their separation is possible. Also, to precipitate aluminum it was used NH4 HCO3 , which acts as a buffer and sets an upper limit on pH (max 6.5) and the control on pH level to required of precipitating stage is more easily achieved. 2. Experimental part Granular and compact dross from different smelters sources were examined. After operations of screening and weighing, samples were taken based on particle-size distribution. Table 2 presents the particle size distribution of initial aluminum dross samples and after operations of crushing and milling. Bulk density was measured by weighing in a vessel of known volume (height = 230 mm, diameter = 115 mm [25]. In order to determine the metal content the samples were melted in a graphite crucible at 750 ◦ C with a mixture of 72% NaCl, 26% KCl and 2% CaF2 . After cooling, crushing, and washing, the spent salt flux was screened and particles above 0.5 mm diameter were considered to
Fig. 4. A schematic diagram of an experimental leaching system: 1, electric stirrer; 2, thermometer; 3, sample extraction tube; 4, leaching vat; 5, condenser; 6, cap; 7, thermostat.
be metal. The residue was analyzed by atomic absorption spectroscopy (AAS), using an analyzer type Analytic Yena Nova 300. For leaching tests and chemical analysis were taken additional dross samples. The leaching test was carried out by mixing a 100 g sample with 1000 ml of distilled water to determine the salt content of the dross [26]. Also, a 100 g sample was mixed with distilled water and stirred in a closed vessel to measure the gas evolution. Samples of compact dross were taken by removing small pieces from different block parts and the apparent density was measured by determining the volume of the pieces and their weight using liquid paraffin [27]. Afterward, the dross samples were melted. Also, it was prepared a homogeneous mixture of aluminum dross (particle diameter less than 1 mm) by mechanical mixing for 5 h, time required to obtain homogeneity. The aluminum content in the dross samples was determined by EDTA titration and by atomic absorption spectrophotometry. Other metallic impurities (e.g., Mn, Mg, Fe, Si, Cu, Zn, V, Na, Pb, As) were also determined using the atomic absorption method. For carbon and chlorine contents it was used combustion analysis. Nitrogen was determined by the Kejidhal and spectrophotometrical methods by complexation with Nessier’s reagent. X-ray diffraction analysis of the starting material, the products of the leaching process, and the final aluminum products was carried out using a Philips PW 1710 diffractometer with Ni filter and Cu k␣ radiation and the morphology of the powders was analyzed using scanning electron microscope JSM-7500 F (JOEL-Japan) operated at 10 kV. The recovery of aluminum, first as Al3+ ions and then as alumina (Al2 O3 ) product from aluminum dross is based on application of process with more stages such as acid leaching, purification, precipitation and calcination. Leaching experiments were carried out in a specially designed reactor, a schematic diagram is shown in Fig. 4. For each experiment, 100 g of dross sample was used. The appropriate volume of sulfuric acid, equivalent amount of concentrated H2 SO4 , was used to keep the dross/acid ratio as 5:8. This amount of acid was used in different diluted forms (such as 5, 10 and 15 wt.%). The same reactor, Fig. 4, was used for extraction experiments. The solid purified residue obtained after leaching impurities was used as the starting material for extraction of aluminum. Extraction was carried out using solutions of H2 SO4 (35%, 45%, and concentrated 98% by weight). One hundred grams of sample was used in each experiment with the stoichiometric amount of H2 SO4 calculated from Eq. (1): Al2 O3 + 3H2 SO4 = Al2 (SO4 )3 + 3H2 O
(1)
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Table 3 The physical and chemical properties of aluminum drosses. No.
Properties
Granular dross
Compact dross
1.
Distribution (q, mm−1 )
–
2. 3. 4. 5. 6.
Density (kg/m3 ) Metal content Salt content (%) Lexiviate (pH) Gas evolution (ml/g)
0.08–0.452 (coarse) – (fine) 828–1230 (bulk) 40.9–69.1 (%) 0.18–6.21 9.52–10.14 0.25–1.17
2396–2528 (apparent) 71–93 0.01–0.03 9.03–9.48 No evolution
The analysis of all elements present was performed on the quantitatively collected filtrate and washing solution. On the other hand, one hundred grams of the purified residue dross were placed in a porcelain dish, and the appropriate amount of concentrated H2 SO4 was added carefully to the sample. The acid was mixed well with the sample using a glass rod until a homogeneous paste was obtained. Then the dish was kept in an oven (maintained at the selected temperature for 1 h). The content of the dish was quantitatively transferred to a beaker, boiled for 15 min with 1000 ml of distilled water, and stirred to dissolve the anhydrous aluminum sulfate. The slurry was washed several times with distilled water. The filtrate and washing solution were collected in a measuring flask, which was marked, and consequently a full analysis was made. To perform aluminum extraction, about 1 kg dross was treated in the system presented in Fig. 4 with 3 l of 35% sulfuric acid of commercial grade. The mixture was kept under constant stirring at around a pH 3. As the reaction was exothermic in nature, no further heating was required. After leaving the whole reaction mixture at ambient atmosphere for 8 h, the mixture was decanted and filtered with the help of specially designed filtration system to separate the residue from liquor. The residue was washed with dilute sulfuric acid and finally with water to assure the total recovery of metal content. The process of chemical leaching was repeated until aluminum metal is removed from the residue. The filtrate was concentrated by heating to achieve the final density of liquor to be around 1.35–1.40 kg/dm3 . Immediately after a few minutes, the concentrated liquor converted to hot cake and got hardened. The chemical analysis of this cake was done as per standard. The cake is Al2 (SO4 )3 ·12H2 O with some impurities such as iron. All chemicals used in these experiments were supplied by Aldrich Company. Nitrogen (N2 ), helium, argon and hydrogen with a purity >99.99% were purchased from Linde Company. 3. Results and discussion Table 3 details the range of the measured physical and chemical properties for both dross types analyzed.
Fig. 5. The bulk density of dross as a function of metal content and the discontinuity around 70% metal content.
The results of these experiments were used to identify and characterizing each dross specifically and simplifying pre-analysis for aluminum recovery. Using the content of Table 4 as an example, the characteristics of dross provides details in Table 4. The origin of the dross and the specifications and composition; the particle-size analysis, which is characterized by distribution and the distribution function (q), i.e., dross No. 1 is a granular, medium type; bulk density and metal-content values (the values of 1147 kg/m3 and 43.32%, respectively, for dross No. 1 also characterize granular dross, granular dross have lower density and metal content as compared to compact dross); beside aluminum metal, the content of other metals that are presented in dross composition; the results of the leaching test, conductivity of lixiviate, salt content, and gas evolution, which characterizes the behavior of the dross when coming into contact with water or moisture. The range of variation of these parameters is different between the two dross kinds. Granular dross show higher salt contents and more gas evolution as compared to compact dross. 3.1. Aluminum dross characteristics From experiments a relationship between bulk density and metal content was established (Fig. 5). The bulk density of granular dross decreases with increasing metal content because the specific weight of the aluminum is lower than that of its oxide. Along with the density-metal content relationship, a change in the shape of granules was observed, granular dross with low metal contents appear nodular, but their appearance changes to a dendritic shape with increasing metal content. A lower density is obtained because very irregular granules form
Table 4 Example of a granular dross sample characterizing. No.
1.
2. 3. 4. 5.
6.
Characteristics
Type
Sample 1
Granular dross sample
Al (wt.%) 43.32 Fe Si Cr 4.32 8.58 0.088 Na K Ca 0.45 0.8 0.21 Particle size analysis: q = 0.153 (medium) Bulk density: 1147 kg/m3 Al content: 43.32 (wt.%) Leaching test: pH = 9.80 Conductivity: = 9.12 mS/cm Salt content: 2.8% Gas evolution: 1.7 ml/g dross
Cu 1.17 Cl 0.4
Zn 0.9 F <0.05
Mg 1.85 CO3 0.43
Ti 0.27 NO3 12.66
Mn 0.2 NH4 4.77
Ni 0.087 SO4 0.59
Pb 0.53
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Fig. 6. A schematic representation of the mechanism of filling of voids with liquid metal: (a and b) capillary forces; (c) surface tension that determine the dross morphology (black color represents – liquid metal and white color represents oxide metal).
bridges. A deviation from sphericity form increases the interparticle friction, the flowing properties are affected, and space is incompletely filled. A change in the morphology takes place, at a metal content of 70% and the dross appearance is converted from granular to compact because the liquid fills the space between the particles (Fig. 6). Compact dross contain more than 70% metal and their porosity is much lower than that of granular dross and decreases sharply with the increase of metal content (shown as increasing density in Fig. 5). The chemical analysis of a dross fractions are presented in Table 4. An increasing of metallic aluminum content supports the growth of the dross granules. Theoretically, Al2 O3 contains 52.94% aluminum. Fractions with a lower aluminum content are predominantly oxide. If the aluminum content is above 52.94%, metallic aluminum is present in the dross fraction. The dross samples, both granular and compact, were melted to determine their metal contents. After melting, an ingot and a spent salt flux were obtained. The spent salt flux was screened after washing and drying. Fractions with particle sizes above 0.5 mm were called metal drops, and the undersize material was known as residue. For this material, there is a possibility of recycling the ceramic material after washing and calcination. The fine dross fractions can also be treated together with the salt cake to recycle the salt. The oxide residue can be used as a construction material [7,28].
3.2. Aluminum extracting from dross by leaching process The dross is a complex conglomerate, including metallic oxides (e.g., Al2 O3 , Al2 O3 ·MgO, Al2 O3 ·SiO2 , Al2 O3 ·FeO, CaO, etc.), nitrides (e.g., AlN), chlorides (e.g., AlCl3 , NaCl, KCl), fluorides (e.g., CaF2 , NaF, AlF3 , Na3 AlF6 , etc.), carbides (e.g., Al4 C3 ), sulphides (e.g., Al2 S3 ), phosphides (e.g., AlP), dirt and impurities, apart from metallic aluminum (between 80% and 20%) [14,15,29]. The purpose of the leaching process was to leach the soluble Al ions. Leaching is a method to remove soluble components from a solid matrix [1,30–32]. Leaching is described by a very simple process such as is presented in Eq. (2) and Fig. 7: material (leachee) + leachant → leachate
(2)
The leaching parameters such as the concentration of sulfuric acid, leaching time and leaching temperature were considered to find the optimal extraction efficiency. In Fig. 8 are presented X-ray diffractograms of the as-received aluminum dross (1), residue after leaching (2) and soluble fraction (3). The phases that were identified are: aluminum as metal, corundum (Al2 O3 ), aluminum nitride (AlN), sylvite (KCl), spinel (MgAl2 O4 ), halite (NaCl), diaoyudaoite (NaAl11 O17 ), quartz (SiO2 ). Fig. 9 is a typical view of the as-received aluminum dross (a), milled aluminum dross (b) and SEM micrograph of aluminum dross after
Fig. 7. Schematic model for leaching process.
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Fig. 8. Comparative presentation of X-ray diffractograms for: as-received aluminum dross (1), residue after leaching (2) and soluble fraction (3).
milling (c). The dark regions from SEM micrograph represent nodular and dendritic shapes formed by many granular particles with size equal or less than 5 m. The aluminum content was calculated by EDTA titration to be 44.19% by weight and 43.32% using the atomic absorption method. Nitrogen was calculated by the Kejldhal method as 6.48% by weight and as 6.75% using the spectrophotometric measurements. Combustion analysis gave the extent of chlorine as 4.4% and carbon as 1.8% by weight. All other impurity metals were measured by atomic absorption analysis. The results are given in Table 4. Fig. 10 shows three leaching curves performed at three different temperatures. For each curve, the amount of aluminum leached increases with time until it reaches a constant value, which is maintained to the end of the experiment. The total amount of aluminum leached depends on the leaching temperature. Fig. 10 also shows that the
321
Fig. 10. The aluminum dissolved percent during leaching process of 100 g of sample as a function of leaching time at different leaching temperature using 15 wt.% H2 SO4 .
amount of aluminum leached after 4 h at 90 ◦ C is about 2.5 times greater than leached at 50 ◦ C after the same leaching time. Fig. 11 displays three leaching experiments carried out at three different concentrations of H2 SO4 (5%, 10%, and 15%) and a leaching temperature of 90 ◦ C. It shows that the amount of aluminum leached increases with increasing acid concentrations. At 15% H2 SO4 (curve 3), there is a 4–6% increase in the amount of leached aluminum than that leached with 5% H2 SO4 (curve 1). Because of the complicated composition of the dross sample, it was difficult to exactly calculate the stoichiometric quantity of H2 SO4 with respect to the dross. Therefore, four different solid/liquid ratios (1/12; 1/10; 1/8 and 1/6) were used to study the rate of leaching of aluminum. Fig. 12 shows the results of these studies. Curve 1 (the highest solid/liquid ratio, 1/6) shows an initial rapid increase in the amount
Fig. 9. Black aluminum dross in lumps (a), milled aluminum dross (b) and SEM micrograph of aluminum dross after milling (particles below 5 m).
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Fig. 13. Aluminum extracted from 100 g of dross sample at 90 ◦ C as a function of time using different H2 SO4 solutions. Fig. 11. The aluminum dissolved percent during leaching process of 100 g of sample for different acid concentrations, leaching temperature of 90 ◦ C.
of aluminum leached up to a maximum value. It falls dramatically afterward until it reaches a minimum. The same behavior is seen for curves 2 and 3 (solid/liquid ratios of 1/8 and 1/10, respectively) except that after the maximum value of leached aluminum is attained, these curves fall, but not as sharply as curve 1. This means that the amount of aluminum leached (after reaching the maximum value) decreases as the solid/liquid ratio decreases (i.e., as the concentration of acid gets greater). Curve 4 represents the mixture of lowest solid/liquid ratio (i.e., 1/12). With the highest amount of acid, curve 4 differs from the previous three curves (1–3) in that the percent of aluminum leached increases with time until it reaches a constant value with no further drop. This behavior is attributed to the fact that increasing the amount of H2 SO4 resulted in a dissolution of the basic aluminum sulfate, Al6 (OH)10 (SO4 )4 , thus precipitated in experiments 1–3 which contained a lower amount of acid. This behavior disappeared, however, for curve 4, due to the presence of acid in a quantity greater than the stoichiometric value. Fig. 13 displays two extraction curves using 35% and 45% acid concentrations. It was found that extraction increases rapidly at first, and then slows. Also, extraction increases with increasing acid concentration.
Fig. 12. The percent of aluminum dissolved as a function of time for various solid/liquid (S/L) ratios at 90 ◦ C using 15 wt.% H2 SO4 .
The use of concentrated H2 SO4 (98%) in the leaching and extraction processes showed that the amount of aluminum extracted has increased with time and temperature up to a maximum value, which has remained unchanged. This maximum was always obtained after 60 min, depending on the reaction temperature. Fig. 14 shows that the maximum amount of aluminum extracted reached a maximum at an dross/acid ratio of 0.7. Afterwards, the maximum amount of aluminum extracted decreased with the increasing the dross/acid ratio. The efficiency of the reaction was measured at 90 ◦ C at different dross/acid ratios. Fig. 14 shows, again, the same behavior (i.e., the efficiency increases, reaching its maximum at an dross/acid/ratio of 0.7). The maximum value of aluminum extracted was found to take place at the neutralization point between acid and dross. 3.3. Alumina powder as product obtained by aluminum extraction from dross The leaching liquor obtained from the previous process contains Al3+ , Na+ , Fe3+ , Ca2+ , Mg2+ , and other impurities. In order to obtain high purity Al2 O3 , the impurity ions had to be removed. The selective precipitation of ions by pH value adjustment was found to be unsuccessful in separating Fe3+ and Al3+ [9]. It was chosen ethylene diamine tetraacetic acid (EDTA), a complexing agent, to remove the Fe3+ and other impurity ions. Selectivity arises because the complexing ability of each metal in the leaching liquor with
Fig. 14. The effect of solid/liquid (S/L) ratio on the amount of aluminum extracted and the reaction efficiency, leaching temperature of 90 ◦ C.
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Fig. 17. X-ray diffractogram of residual Al dross after metal recovery.
Table 5 Chemical analysis data of residual dross after leaching process. Element content (wt.%) Fig. 15. XRD pattern for ␣- and ␥-Al2 O3 powders obtained from gelatinous precipitate dried at 120 ◦ C for 30 min and then calcined at 1050 ◦ C for 4 h; (a) represents ␣-Al2 O3 and (b) represents ␥-Al2 O3 .
EDTA is different. The pH value of the leaching liquor was adjusted to 3 before the EDTA was added, and then the solution was continuously stirred for 45 min. The color of the solution turned from dull yellow to bright yellow. NH4 HCO3 was subsequently added into the solution to precipitate the aluminum. At the same time the solution was quickly stirred. After filtration, white precipitates and yellow solution were obtained. These results show that the Fe3+ contained in the leaching liquor was effectively removed. The other soluble impurities absorbed in the precipitates could be removed by four cycles of washing. A white gelatinous precipitate was obtained and then dried in a oven at 120 ◦ C for 30 min and calcinated at 850–1050 ◦ C for 4 h. Active Al2 O3 powder was obtained. The XRD pattern and SEM images of the Al2 O3 produced are shown in Figs. 15 and 16. The purity of the Al2 O3 product was as follows: Al2 O3 99.28%, MgO 0.63%, and ≈0.09% other impurities (i.e., CaO, Fe2 O3 , SiO2 , etc.) These results indicated that further investigation of strategies for Mg removal is necessary to achieve Al2 O3 of higher purity. 3.4. Residual dross analysis The remaining dross after leaching of aluminum metal was washed with water to remove traces of acid, other soluble impurities and then subjected for XRD analysis for examining the availability of metallic aluminum traces. The X-ray diffractogram obtained for dross shown in Fig. 8 has a sharp peak at d-spacing 2.328 that indicates the presence of aluminum metal in initial dross material. The same X-ray diffractogram obtained for the residue
Al traces Cr 0.10 Ca 0.21
Fe 2.60 Na 0.11
Si 10.9 K 0.13
Cu 1.07 Cl 0.02
Zn 0.26 F <0.05
Mg 1.73 CO3 2.12
Ti 0.37 NO3 5.17
Mn 0.17 NH4 3.74
Ni 0.069 SO4 0.55
Pb 0.605
after recovery of aluminum metal using chemical leaching technique is presented in Fig. 17. The disappearance of the peak at d-spacing 2.328 shows that there is almost total recovery of metallic aluminum from dross. This also indicates that the chemical recovery process used is efficiently to extract and to recover total aluminum metal content from the dross material. The chemical composition of the residual dross after aluminum metal recovery presented in Table 5 also shows that there is a significant improvement in the residual dross material composition making it suitable for other applications [7,12]. A comparison of residual composition (Table 5) with that of initial dross (Table 4) indicates that there is an increase in silica contents. The detrimental parameters such as iron and other alkaline impurities have come down significantly in residue. Some of the impurities such as nickel, mangan, titan are found in traces. The reactivity of this residue was very significant reduced and can be thought of possible future applications and therefore, this inert material can open up an attractive line for development. It may be suitable for reuse as a secondary material in refractory industry, for which this would be a interesting line for future research. This study strongly suggests that the dross material of negligible commercial significance could be converted to commercially viable materials and could be used as raw materials to obtain products with high added value such as alumina. 4. Conclusions
Fig. 16. SEM image of Al2 O3 powder obtained from gelatinous precipitate dried at 120 ◦ C for 30 min and calcinated at 1050 ◦ C for 4 h.
The relationship established between density, porosity and metal content of a dross is due to the different densities of the main dross constituents (aluminum, alumina) and the mechanism of the dross formation. A recommendation is to avoid the storage of the dross landfills to prevent reactions with moisture and leading to the formation of toxic, harmful, explosive, poisonous and unpleasant odorous gases and to the pollution of ground water. From experiments, leaching conditions were obtained to permit the maximum aluminum soluble ions extracting. The maximum value of aluminum extracted was found to take place at the neutralization point between acid and dross. These optimum conditions, chosen to be used in the bench scale experiment were specified as follows: temperature: 90 ◦ C; H2 SO4 concentration: 15 wt.%; time of reaction: 5 h; solid/liquid ratio (S/L): 1/12.
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Aluminum waste such as dross can be used as a bauxite substitute in the different processes for alumina production. Among other processes, alumina can be obtained from aluminum hazardous waste such as dross, by a hydrothermal process at low temperature, in more stages: acid leaching, purification, precipitation and calcinations. The method presented in this work allows the use of aluminum waste as raw material for extracting an important soluble aluminum fraction as an added value product, alumina, with high grade purity (99.28%). Acknowledgments With gratitude to the National Agency of Scientific Research from Romania for the financial support provided by the National Plan of R&D in Environmental and Energy Fields. References [1] R.B. Das, B. Dash, C.B. Tripathy, N.I. Bhattacharya, C.S. Das, Production of alumina from waste aluminium dross, Minerals Engineering 20 (3) (2007) 252–258. [2] U. Mikito, T. Shiro, Recovery of aluminum from oxide particles in aluminum dross using AlF–NaFBaCl2 molten salt, Journal of Applied Electrochemistry 35 (2005) 925–930. [3] K. Azom, Aluminum dross recycling – a new technology for recycling aluminium waste products, A to Z of Materials (2002) 346–353. [4] L.E. Macaskie, I.P. Mikheenko, P. Yong, K. Deplanche, A.J. Murray, M. PatersonBeedle, et al., Today’s wastes, tomorrow’s materials for environmental protection, Hydrometallurgy 104 (3/4) (2010) 483–487. [5] F.W.Y. Momade, K. Sraku-Lartey, Studies into the preparation of alum from slime waste from the Awaso Bauxite Washing Plant, Hydrometallurgy 101 (3/4) (2010) 93–98. [6] S. Cao, Y. Zhang, Y. Zhang, Preparation of sodium aluminate from the leach liquor of diasporic bauxite in concentrated NaOH solution, Hydrometallurgy 98 (3/4) (2009) 298–303. [7] H.N. Yoshimura, A.P. Abreu, Evaluation of aluminum dross waste as raw material for refractories, Ceramics International 34 (3) (2008) 581–591. [8] K. Nakajima, H. Osuga, K. Yokoyama, T. Nagasaka, Material flow analysis of aluminum dross and environmental assessment for its recycling process, Materials Transactions 48 (8) (2007) 2219–2224. [9] El. Katatny, E.A. Halany, S.A. Mohamed, Surface composition, charge and texture of active alumina powders recovered from aluminum dross tailings chemical waste, Powder Technology 132 (2003) 137–144. [10] S. Ilyas, C. Ruan, H.N. Bhatti, M.A. Ghauri, M.A. Anwar, Column bioleaching of metals from electronic scrap, Hydrometallurgy 101 (3/4) (2010) 135–140. [11] J. Kim, K. Biswas, W.K. Jhon, Y.S. Jeong, S.W. Ahn, Synthesis of AlPO4 -5 and CrAPO-5 using aluminum dross, Journal of Hazardous Materials 69 (1–3) (2009) 919–925.
[12] M.C. Shinzato, R. Hypolito, Solid waste from aluminum recycling process: characterization and reuse of its economically valuable constituents, Waste Management 25 (1) (2005) 37–46. [13] C. Jirang, Z. Lifeng, Metallurgical recovery of metals from electronic waste: a review, Journal of Hazardous Materials 158 (2/3) (2008) 228–256. [14] J.Y. Hwang, X. Huang, Z. Xu, Recovery of metals from aluminium dross and salt cake, Journal of Minerals & Materials Characterization & Engineering 5 (1) (2006) 47–62. [15] S. Freti, J.D. Bbornand, K. Buxman, Metallurgy of dross formation on aluminium melts, Light Metal (1982) 1003–1016. [16] B. Dash, B.R. Das, Acid dissolution of alumina from waste aluminium dross, Hydrometallurgy 92 (2008) 48–53. [17] J. Aquilar-Santillan, H. Balmori-Ramirez, R.C. Bradt, Sol–gel formation and kinetic analysis of the in-situ/selfseeding transformation of bayerite to ␣alumina, Journal of Ceramics Processing Research 5 (3) (2004) 196–202. [18] O.V. Kravchenko, K.N. Semenenko, B.M. Bulychev, B. Kalmykov, Activation of aluminum metal and its reaction with water, Journal of Alloys and Compounds 397 (2005) 58–62. [19] D.R. Lide, Handbook of Chemistry and Physics, 83rd ed., CRC Press, Boca Raton, FL, 2003. [20] J. Mukhopadhyay, Y.V. Ramana, U. Singh, Extraction of value added products from aluminium dross material to achieve zero waste, Light Metals (2005) 1209–1212. [21] B. Dash, B.C. Tripathy, I.N. Bhattacharya, T. Subbaiah, A comparative study on the precipitation of hydrated alumina from different sources, International Journal of Metallurgical Engineering 1 (5) (2012) 78–82. [22] L.W. Garret, 1982. Process for the Production of Sulfates. US Patent No. 4,337,228. [23] D.A. Huckabay, D.A. Skiathas, 1982. Aluminium Sulfate Manufacture from Aluminum Dross Tailings. US Patent No. 4,320,098. [24] N. Kuyucak, Acid mine drainage – treatment options for mining effluents, Journal of Environmental Management (March) (2001) 14–17. [25] DIN 52110-B, Prüfung von Naturstein; Bestimmung der Schüttdichte von Gesteinskörnungen, Schaufel-Verfahren, Germany, 1985. [26] DIN 38414-Part 4, Schlamm und Sedimente (Gruppe S); Bestimmung der Eluierbarkeit mit Wasser, Germany, 1984. [27] DIN 52102-RE-VA, Prüfung von Naturstein und Gesteinskörnungen; Bestimmung von Dichte, Trockenrohdichte, Dichtigkeitgrad und Gesamtporosität, Auftriebs-Verfahren, Germany, 1988. [28] M.M. Ewais, M.N. Khalil, S.M. Amin, Z.M.Y. Ahmed, A.M. Baraka, Utilization of aluminum sludge and aluminum slag (dross) for the manufacture of calcium aluminate cement, Ceramics International 35 (8) (2009) 3381–3388. [29] E. Biekert, H. Hellmann, H. Ley, et al., in: E. Bartholomé (Ed.), Ullmanns Encyklopädie der Technischen Chemie, Aluminium 4 (B7), Verlag Chemie, Weinheim, Germany, 1974, pp. 64–67. [30] T. Hiraki, S. Yamauchi, M. Iida, H. Uesugi, T. Akiyama, Process for recycling waste aluminum with generator of high-pressure hydrogen, Environmental Science & Technology 41 (12) (2007) 4454–4457. [31] T. Hiraki, M. Takeuchi, M. Hisa, T. Akiyama, Hydrogen production from waste aluminum at different temperatures, with LCA, Materials Transactions 46 (5) (2005) 1052–1057. [32] Z. Yang, M. Rui-lin, N. Wang-dong, W. Hui, Selective leaching of base metals from copper smelter slag, Hydrometallurgy 103 (1–4) (2010) 25–29.
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Aluminum recovery as a product with high added value using aluminum hazardous waste E. David a,∗ , J. Kopac b a National Institute for Research and Development for Cryogenic and Isotopic Technologies, Street Uzinei, No. 4, P.O. Râureni, P.O. Box 7, 240050 Rm. Vâlcea, Romania b Faculty of Mechanical Engineering, University of Ljubljana, Askerceva 6, SI-1000 Ljubljana, Slovenia
h i g h l i g h t s • • • • •
Granular and compact aluminum dross were physically and chemically characterized. A relationship between density, porosity and metal content from dross was established. Chemical reactions involving aluminum in landfill and negative consequences are shown. A processing method for aluminum recovering from aluminum dross was developed. Aluminum was recovered as an value product with high grade purity such as alumina.
a r t i c l e
i n f o
Article history: Received 30 April 2013 Received in revised form 4 July 2013 Accepted 18 July 2013 Available online xxx Keywords: Aluminum dross Aluminum extraction Acid leaching Alumina
a b s t r a c t The samples of hazardous aluminum solid waste such as dross were physically and chemically characterized. A relationship between density, porosity and metal content of dross was established. The paper also examines the chemical reactions involving aluminum dross in landfill and the negative consequences. To avoid environmental problems and to recovery the aluminum, a processing method was developed and aluminum was recovered as an added value product such as alumina. This method refers to a process at low temperature, in more stages: acid leaching, purification, precipitation and calcination. At the end of this process aluminum was extracted, first as Al3+ soluble ions and final as alumina product. The composition of the aluminum dross and alumina powder obtained were measured by applying the leaching tests, using atomic absorption spectrometry (AAS) and chemical analysis. The mineralogical composition of aluminum dross samples and alumina product were determined by X-ray diffraction (XRD) and the morphological characterization was performed by scanning electron microscopy (SEM). The method presented in this work allows the use of hazardous aluminum solid waste as raw material to recover an important fraction from soluble aluminum content as an added value product, alumina, with high grade purity (99.28%). © 2013 Elsevier B.V. All rights reserved.
1. Introduction Aluminum metal is supplied by two distinct aluminumproduction sectors: primary aluminum producers and secondary aluminum smelters [1–3]. The primary aluminum industry in the Romania produces aluminum mostly from mined ore (bauxite). Some scrap is also used, which is usually from recycled aluminum beverage cans and industrially generated high-grade aluminum scrap. The continuing growth in aluminum can recycling has increased the value of aluminum, yielded energy savings, and resulted in ecological benefits [4–9]. Recycling 1 kg of aluminum
∗ Corresponding author. Tel.: +40 250732744, fax: +40 250 732746. E-mail addresses: [email protected], [email protected] (E. David). 0304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2013.07.042
can save about 4 kg of bauxite, 2 kg of chemicals, and 7.5 kWh of electricity [3,10–12]. Worldwide the aluminum industry produces over 4.5 billion kilograms of aluminum waste each year [3,13–15]. Each type of waste has unique chemical and physical characteristics, and the value of the waste is determined by the level of impurities contained and the cost of recovering of the metal [13,14]. Because the process of recovering aluminum from low-grade scrap requires large quantities of salt, furnaces generate large quantities of salt cake. For every kg of low-grade scrap charged into the rotary furnace, 1 kg of salt flux is typically used [12]. More than 100 flux compositions are available to scrap-aluminum smelters, but the most commonly used fluxes are mixtures of NaCl and KC1 in approximately equal amounts, to which 3–5 weight percent (wt.%) cryolite is added [16,17]. The advantages of this chloride mixture (as a fluxing agent) are its low cost ($100/ton) and low melting point
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Fig. 1. Aluminum scrap recycling process.
[2,7]. Sodium chloride is lower in cost, but potassium salts have lower viscosity and lower surface tension, qualities that increase fluidity. The melting point of this mixture is just below the melting point of aluminum (660 ◦ C). It is important that the melting point of flux is below the melting point of aluminum, because during heating of the charge, the salt melts first and coats the aluminum before the metal melts, thereby protecting the metal from further oxidation. For dross processing, 1 kg of flux is used for each kg of dross concentrate charged to the rotary furnace [8,12]. The salt flux used in scrap aluminum smelting ends up in the black dross and salt cake, which are commonly disposed of in landfills. Fig. 1 is a diagram of the recycling process for scrap aluminum and the various sources of waste generation. Typically, aluminum black dross (ABD) contains aluminum (12–20%), sodium chloride (20–25%), potassium chloride (20–25%), aluminum oxides (20–50%), and other compounds (2–5%) [12]. Black dross is most often further processed to recover as much as 80% of the remaining aluminum. In order to recover the aluminum and make recycling economically justifiable, the dross must be upgraded (usually by means of milling and screening) at least to 50% aluminum concentrate [14,18]. Because the metal is less brittle than the other dross constituents, it will not break in the milling process, and it will tend to be screened out in coarser fractions. The aluminum “concentrate” is processed in a rotary furnace to recovery of the aluminum (Fig. 2). The finer-sized fractions, less than about 1 mm, can represent up to 85% of the milled black dross and can contain as much as 10 wt.% metallic aluminum. A general dross-processing flow diagram is shown in Fig. 3. The salt cake generated from the rotary furnaces ranges up to 75% of the melt charge in rotary furnace.
Fig. 3. Aluminum dross processing flow diagram.
Table 1 lists the estimated total amount of solid residues generated by the secondary aluminum industry from secondary aluminum production operations. Huge quantities of dross and salt cake are generated, of which very little, if any, are recycled, most of them are disposed of in landfills. The salt cake consists of aluminum (1–5%), aluminum oxide (15–30%), sodium chloride (30–55%), potassium chloride (15–30%), and inert compounds (1–4%) [19]. It is very known that due to its properties, the ABD is classified as toxic and hazardous waste (100309), according to the European Catalog for Hazardous Wastes. It is considered as “highly flammable” (H3-A), “irritant” (H4), “harmfull” (H5) and “leachable” (H13). It is also very know that 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 , and H2 S. 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 [3,8]. Worldwide are different ways of recycling of this hazardous waste. Mukhopadhyay et al., shown that the recycling of aluminum dross is very important due to economic and environmental benefits, it is saved raw materials and the waste is not sent to landfill [20]. As show Das et al. [1], Dash et al. [16] the disposal of salt slag and dross is a worldwide problem. In the case of improper disposal, leaching of toxic metal ions into ground water would determine serious problems of pollution. They showed that the processing of these wastes and obtaining useful products is a viable way. A series of convincing experimental results tend to prove that sulfuric Table 1 Major residues from secondary aluminum processing industry.
Fig. 2. Aluminum dross recycling process.
No.
Solid residues
1. 2. 3. 4. 5.
Metal produced Black dross Black dross residuesa Salt cake Baghouse dust
a
Dross – milling residues.
Estimated quantities (wt.%) 75–76 7–8 6–7 7–8 0.6–0.7
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Table 2 Particle size distribution after crushing and milling of primary aluminum dross. Screen aperture (mm)
Weight (%)
+2.000 −2.000 + 1.680 −1.680 + 0.600 −0.600 + 0.400 −0.400 + 0.125 −0.125 + 0.074 −0.074 Initial dross (before crushing and milling) +2.000 −2.000
0.47 2.54 6.17 1.54 6.80 17.25 65.23 28.05 71.95
acid is an effective and less costly acid for metal leaching. Dash et al. have obtained various form of hydrated alumina from three different sources like sodium aluminate liquor, waste aluminum dross and synthetic salt like aluminum sulfate. From the sulfuric acid leach liquor of waste aluminum dross, hydrated alumina was precipitated by varying the pH of the precipitation using 10% aqueous ammonia [21]. Also, several studies have been undertaken where dross, after leaching by H2 SO4 , was utilized for the manufacture of aluminum sulphate [1,16]. Garret [22] presents a method that uses the concentrate sulfuric acid (90–99.4 wt.%) and brine for aluminum leaching from waste to obtain aluminum sulphate. Aluminum sulphate produced by this method was submitted for testing as a flocculant for wastewater treatment process. Huckabay and Skiathas [23] presented a method to prepare aluminum sulfate based on leaching with sulfuric acid of aluminum dross. The aluminum dross prior to reaction with sulfuric acid are pretreated by oxidizing with water and steam at elevated temperatures to reduce the more objectionable contaminants. In this paper, we examine the possibility to use hazardous aluminum dross as raw material to extract aluminum, as an inoffensive product to environment such as alumina, to avoid the chemical reactions involving aluminum dross in landfill and the negative environmental consequences. The presented method refers to a process at low temperature, in more stages: acid leaching, purification, precipitation and calcinations. The paper points out that it can get high purity alumina if the impurity ions from aluminum leaching liquor are removed to the required level. For instance, ferric ions (Fe3+ ) and aluminum ions (Al3+ ) are hydrolyzed and precipitate out at pH level less than 5 [24]. In this case the pH adjustment is ineffective or not quite effective, therefore (Fe3+ ) ions are difficult to be separated of aluminum ions. The solution presented in this paper is the using a selective complexing agent, such as ethylene diamine tetraacetic acid (EDTA) because the complexing ability of each metal (Fe3+ ,Al3+ ) in leaching liquor with EDTA is different, and therefore their separation is possible. Also, to precipitate aluminum it was used NH4 HCO3 , which acts as a buffer and sets an upper limit on pH (max 6.5) and the control on pH level to required of precipitating stage is more easily achieved. 2. Experimental part Granular and compact dross from different smelters sources were examined. After operations of screening and weighing, samples were taken based on particle-size distribution. Table 2 presents the particle size distribution of initial aluminum dross samples and after operations of crushing and milling. Bulk density was measured by weighing in a vessel of known volume (height = 230 mm, diameter = 115 mm [25]. In order to determine the metal content the samples were melted in a graphite crucible at 750 ◦ C with a mixture of 72% NaCl, 26% KCl and 2% CaF2 . After cooling, crushing, and washing, the spent salt flux was screened and particles above 0.5 mm diameter were considered to
Fig. 4. A schematic diagram of an experimental leaching system: 1, electric stirrer; 2, thermometer; 3, sample extraction tube; 4, leaching vat; 5, condenser; 6, cap; 7, thermostat.
be metal. The residue was analyzed by atomic absorption spectroscopy (AAS), using an analyzer type Analytic Yena Nova 300. For leaching tests and chemical analysis were taken additional dross samples. The leaching test was carried out by mixing a 100 g sample with 1000 ml of distilled water to determine the salt content of the dross [26]. Also, a 100 g sample was mixed with distilled water and stirred in a closed vessel to measure the gas evolution. Samples of compact dross were taken by removing small pieces from different block parts and the apparent density was measured by determining the volume of the pieces and their weight using liquid paraffin [27]. Afterward, the dross samples were melted. Also, it was prepared a homogeneous mixture of aluminum dross (particle diameter less than 1 mm) by mechanical mixing for 5 h, time required to obtain homogeneity. The aluminum content in the dross samples was determined by EDTA titration and by atomic absorption spectrophotometry. Other metallic impurities (e.g., Mn, Mg, Fe, Si, Cu, Zn, V, Na, Pb, As) were also determined using the atomic absorption method. For carbon and chlorine contents it was used combustion analysis. Nitrogen was determined by the Kejidhal and spectrophotometrical methods by complexation with Nessier’s reagent. X-ray diffraction analysis of the starting material, the products of the leaching process, and the final aluminum products was carried out using a Philips PW 1710 diffractometer with Ni filter and Cu k␣ radiation and the morphology of the powders was analyzed using scanning electron microscope JSM-7500 F (JOEL-Japan) operated at 10 kV. The recovery of aluminum, first as Al3+ ions and then as alumina (Al2 O3 ) product from aluminum dross is based on application of process with more stages such as acid leaching, purification, precipitation and calcination. Leaching experiments were carried out in a specially designed reactor, a schematic diagram is shown in Fig. 4. For each experiment, 100 g of dross sample was used. The appropriate volume of sulfuric acid, equivalent amount of concentrated H2 SO4 , was used to keep the dross/acid ratio as 5:8. This amount of acid was used in different diluted forms (such as 5, 10 and 15 wt.%). The same reactor, Fig. 4, was used for extraction experiments. The solid purified residue obtained after leaching impurities was used as the starting material for extraction of aluminum. Extraction was carried out using solutions of H2 SO4 (35%, 45%, and concentrated 98% by weight). One hundred grams of sample was used in each experiment with the stoichiometric amount of H2 SO4 calculated from Eq. (1): Al2 O3 + 3H2 SO4 = Al2 (SO4 )3 + 3H2 O
(1)
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Table 3 The physical and chemical properties of aluminum drosses. No.
Properties
Granular dross
Compact dross
1.
Distribution (q, mm−1 )
–
2. 3. 4. 5. 6.
Density (kg/m3 ) Metal content Salt content (%) Lexiviate (pH) Gas evolution (ml/g)
0.08–0.452 (coarse) – (fine) 828–1230 (bulk) 40.9–69.1 (%) 0.18–6.21 9.52–10.14 0.25–1.17
2396–2528 (apparent) 71–93 0.01–0.03 9.03–9.48 No evolution
The analysis of all elements present was performed on the quantitatively collected filtrate and washing solution. On the other hand, one hundred grams of the purified residue dross were placed in a porcelain dish, and the appropriate amount of concentrated H2 SO4 was added carefully to the sample. The acid was mixed well with the sample using a glass rod until a homogeneous paste was obtained. Then the dish was kept in an oven (maintained at the selected temperature for 1 h). The content of the dish was quantitatively transferred to a beaker, boiled for 15 min with 1000 ml of distilled water, and stirred to dissolve the anhydrous aluminum sulfate. The slurry was washed several times with distilled water. The filtrate and washing solution were collected in a measuring flask, which was marked, and consequently a full analysis was made. To perform aluminum extraction, about 1 kg dross was treated in the system presented in Fig. 4 with 3 l of 35% sulfuric acid of commercial grade. The mixture was kept under constant stirring at around a pH 3. As the reaction was exothermic in nature, no further heating was required. After leaving the whole reaction mixture at ambient atmosphere for 8 h, the mixture was decanted and filtered with the help of specially designed filtration system to separate the residue from liquor. The residue was washed with dilute sulfuric acid and finally with water to assure the total recovery of metal content. The process of chemical leaching was repeated until aluminum metal is removed from the residue. The filtrate was concentrated by heating to achieve the final density of liquor to be around 1.35–1.40 kg/dm3 . Immediately after a few minutes, the concentrated liquor converted to hot cake and got hardened. The chemical analysis of this cake was done as per standard. The cake is Al2 (SO4 )3 ·12H2 O with some impurities such as iron. All chemicals used in these experiments were supplied by Aldrich Company. Nitrogen (N2 ), helium, argon and hydrogen with a purity >99.99% were purchased from Linde Company. 3. Results and discussion Table 3 details the range of the measured physical and chemical properties for both dross types analyzed.
Fig. 5. The bulk density of dross as a function of metal content and the discontinuity around 70% metal content.
The results of these experiments were used to identify and characterizing each dross specifically and simplifying pre-analysis for aluminum recovery. Using the content of Table 4 as an example, the characteristics of dross provides details in Table 4. The origin of the dross and the specifications and composition; the particle-size analysis, which is characterized by distribution and the distribution function (q), i.e., dross No. 1 is a granular, medium type; bulk density and metal-content values (the values of 1147 kg/m3 and 43.32%, respectively, for dross No. 1 also characterize granular dross, granular dross have lower density and metal content as compared to compact dross); beside aluminum metal, the content of other metals that are presented in dross composition; the results of the leaching test, conductivity of lixiviate, salt content, and gas evolution, which characterizes the behavior of the dross when coming into contact with water or moisture. The range of variation of these parameters is different between the two dross kinds. Granular dross show higher salt contents and more gas evolution as compared to compact dross. 3.1. Aluminum dross characteristics From experiments a relationship between bulk density and metal content was established (Fig. 5). The bulk density of granular dross decreases with increasing metal content because the specific weight of the aluminum is lower than that of its oxide. Along with the density-metal content relationship, a change in the shape of granules was observed, granular dross with low metal contents appear nodular, but their appearance changes to a dendritic shape with increasing metal content. A lower density is obtained because very irregular granules form
Table 4 Example of a granular dross sample characterizing. No.
1.
2. 3. 4. 5.
6.
Characteristics
Type
Sample 1
Granular dross sample
Al (wt.%) 43.32 Fe Si Cr 4.32 8.58 0.088 Na K Ca 0.45 0.8 0.21 Particle size analysis: q = 0.153 (medium) Bulk density: 1147 kg/m3 Al content: 43.32 (wt.%) Leaching test: pH = 9.80 Conductivity: = 9.12 mS/cm Salt content: 2.8% Gas evolution: 1.7 ml/g dross
Cu 1.17 Cl 0.4
Zn 0.9 F <0.05
Mg 1.85 CO3 0.43
Ti 0.27 NO3 12.66
Mn 0.2 NH4 4.77
Ni 0.087 SO4 0.59
Pb 0.53
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Fig. 6. A schematic representation of the mechanism of filling of voids with liquid metal: (a and b) capillary forces; (c) surface tension that determine the dross morphology (black color represents – liquid metal and white color represents oxide metal).
bridges. A deviation from sphericity form increases the interparticle friction, the flowing properties are affected, and space is incompletely filled. A change in the morphology takes place, at a metal content of 70% and the dross appearance is converted from granular to compact because the liquid fills the space between the particles (Fig. 6). Compact dross contain more than 70% metal and their porosity is much lower than that of granular dross and decreases sharply with the increase of metal content (shown as increasing density in Fig. 5). The chemical analysis of a dross fractions are presented in Table 4. An increasing of metallic aluminum content supports the growth of the dross granules. Theoretically, Al2 O3 contains 52.94% aluminum. Fractions with a lower aluminum content are predominantly oxide. If the aluminum content is above 52.94%, metallic aluminum is present in the dross fraction. The dross samples, both granular and compact, were melted to determine their metal contents. After melting, an ingot and a spent salt flux were obtained. The spent salt flux was screened after washing and drying. Fractions with particle sizes above 0.5 mm were called metal drops, and the undersize material was known as residue. For this material, there is a possibility of recycling the ceramic material after washing and calcination. The fine dross fractions can also be treated together with the salt cake to recycle the salt. The oxide residue can be used as a construction material [7,28].
3.2. Aluminum extracting from dross by leaching process The dross is a complex conglomerate, including metallic oxides (e.g., Al2 O3 , Al2 O3 ·MgO, Al2 O3 ·SiO2 , Al2 O3 ·FeO, CaO, etc.), nitrides (e.g., AlN), chlorides (e.g., AlCl3 , NaCl, KCl), fluorides (e.g., CaF2 , NaF, AlF3 , Na3 AlF6 , etc.), carbides (e.g., Al4 C3 ), sulphides (e.g., Al2 S3 ), phosphides (e.g., AlP), dirt and impurities, apart from metallic aluminum (between 80% and 20%) [14,15,29]. The purpose of the leaching process was to leach the soluble Al ions. Leaching is a method to remove soluble components from a solid matrix [1,30–32]. Leaching is described by a very simple process such as is presented in Eq. (2) and Fig. 7: material (leachee) + leachant → leachate
(2)
The leaching parameters such as the concentration of sulfuric acid, leaching time and leaching temperature were considered to find the optimal extraction efficiency. In Fig. 8 are presented X-ray diffractograms of the as-received aluminum dross (1), residue after leaching (2) and soluble fraction (3). The phases that were identified are: aluminum as metal, corundum (Al2 O3 ), aluminum nitride (AlN), sylvite (KCl), spinel (MgAl2 O4 ), halite (NaCl), diaoyudaoite (NaAl11 O17 ), quartz (SiO2 ). Fig. 9 is a typical view of the as-received aluminum dross (a), milled aluminum dross (b) and SEM micrograph of aluminum dross after
Fig. 7. Schematic model for leaching process.
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Fig. 8. Comparative presentation of X-ray diffractograms for: as-received aluminum dross (1), residue after leaching (2) and soluble fraction (3).
milling (c). The dark regions from SEM micrograph represent nodular and dendritic shapes formed by many granular particles with size equal or less than 5 m. The aluminum content was calculated by EDTA titration to be 44.19% by weight and 43.32% using the atomic absorption method. Nitrogen was calculated by the Kejldhal method as 6.48% by weight and as 6.75% using the spectrophotometric measurements. Combustion analysis gave the extent of chlorine as 4.4% and carbon as 1.8% by weight. All other impurity metals were measured by atomic absorption analysis. The results are given in Table 4. Fig. 10 shows three leaching curves performed at three different temperatures. For each curve, the amount of aluminum leached increases with time until it reaches a constant value, which is maintained to the end of the experiment. The total amount of aluminum leached depends on the leaching temperature. Fig. 10 also shows that the
321
Fig. 10. The aluminum dissolved percent during leaching process of 100 g of sample as a function of leaching time at different leaching temperature using 15 wt.% H2 SO4 .
amount of aluminum leached after 4 h at 90 ◦ C is about 2.5 times greater than leached at 50 ◦ C after the same leaching time. Fig. 11 displays three leaching experiments carried out at three different concentrations of H2 SO4 (5%, 10%, and 15%) and a leaching temperature of 90 ◦ C. It shows that the amount of aluminum leached increases with increasing acid concentrations. At 15% H2 SO4 (curve 3), there is a 4–6% increase in the amount of leached aluminum than that leached with 5% H2 SO4 (curve 1). Because of the complicated composition of the dross sample, it was difficult to exactly calculate the stoichiometric quantity of H2 SO4 with respect to the dross. Therefore, four different solid/liquid ratios (1/12; 1/10; 1/8 and 1/6) were used to study the rate of leaching of aluminum. Fig. 12 shows the results of these studies. Curve 1 (the highest solid/liquid ratio, 1/6) shows an initial rapid increase in the amount
Fig. 9. Black aluminum dross in lumps (a), milled aluminum dross (b) and SEM micrograph of aluminum dross after milling (particles below 5 m).
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Fig. 13. Aluminum extracted from 100 g of dross sample at 90 ◦ C as a function of time using different H2 SO4 solutions. Fig. 11. The aluminum dissolved percent during leaching process of 100 g of sample for different acid concentrations, leaching temperature of 90 ◦ C.
of aluminum leached up to a maximum value. It falls dramatically afterward until it reaches a minimum. The same behavior is seen for curves 2 and 3 (solid/liquid ratios of 1/8 and 1/10, respectively) except that after the maximum value of leached aluminum is attained, these curves fall, but not as sharply as curve 1. This means that the amount of aluminum leached (after reaching the maximum value) decreases as the solid/liquid ratio decreases (i.e., as the concentration of acid gets greater). Curve 4 represents the mixture of lowest solid/liquid ratio (i.e., 1/12). With the highest amount of acid, curve 4 differs from the previous three curves (1–3) in that the percent of aluminum leached increases with time until it reaches a constant value with no further drop. This behavior is attributed to the fact that increasing the amount of H2 SO4 resulted in a dissolution of the basic aluminum sulfate, Al6 (OH)10 (SO4 )4 , thus precipitated in experiments 1–3 which contained a lower amount of acid. This behavior disappeared, however, for curve 4, due to the presence of acid in a quantity greater than the stoichiometric value. Fig. 13 displays two extraction curves using 35% and 45% acid concentrations. It was found that extraction increases rapidly at first, and then slows. Also, extraction increases with increasing acid concentration.
Fig. 12. The percent of aluminum dissolved as a function of time for various solid/liquid (S/L) ratios at 90 ◦ C using 15 wt.% H2 SO4 .
The use of concentrated H2 SO4 (98%) in the leaching and extraction processes showed that the amount of aluminum extracted has increased with time and temperature up to a maximum value, which has remained unchanged. This maximum was always obtained after 60 min, depending on the reaction temperature. Fig. 14 shows that the maximum amount of aluminum extracted reached a maximum at an dross/acid ratio of 0.7. Afterwards, the maximum amount of aluminum extracted decreased with the increasing the dross/acid ratio. The efficiency of the reaction was measured at 90 ◦ C at different dross/acid ratios. Fig. 14 shows, again, the same behavior (i.e., the efficiency increases, reaching its maximum at an dross/acid/ratio of 0.7). The maximum value of aluminum extracted was found to take place at the neutralization point between acid and dross. 3.3. Alumina powder as product obtained by aluminum extraction from dross The leaching liquor obtained from the previous process contains Al3+ , Na+ , Fe3+ , Ca2+ , Mg2+ , and other impurities. In order to obtain high purity Al2 O3 , the impurity ions had to be removed. The selective precipitation of ions by pH value adjustment was found to be unsuccessful in separating Fe3+ and Al3+ [9]. It was chosen ethylene diamine tetraacetic acid (EDTA), a complexing agent, to remove the Fe3+ and other impurity ions. Selectivity arises because the complexing ability of each metal in the leaching liquor with
Fig. 14. The effect of solid/liquid (S/L) ratio on the amount of aluminum extracted and the reaction efficiency, leaching temperature of 90 ◦ C.
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Fig. 17. X-ray diffractogram of residual Al dross after metal recovery.
Table 5 Chemical analysis data of residual dross after leaching process. Element content (wt.%) Fig. 15. XRD pattern for ␣- and ␥-Al2 O3 powders obtained from gelatinous precipitate dried at 120 ◦ C for 30 min and then calcined at 1050 ◦ C for 4 h; (a) represents ␣-Al2 O3 and (b) represents ␥-Al2 O3 .
EDTA is different. The pH value of the leaching liquor was adjusted to 3 before the EDTA was added, and then the solution was continuously stirred for 45 min. The color of the solution turned from dull yellow to bright yellow. NH4 HCO3 was subsequently added into the solution to precipitate the aluminum. At the same time the solution was quickly stirred. After filtration, white precipitates and yellow solution were obtained. These results show that the Fe3+ contained in the leaching liquor was effectively removed. The other soluble impurities absorbed in the precipitates could be removed by four cycles of washing. A white gelatinous precipitate was obtained and then dried in a oven at 120 ◦ C for 30 min and calcinated at 850–1050 ◦ C for 4 h. Active Al2 O3 powder was obtained. The XRD pattern and SEM images of the Al2 O3 produced are shown in Figs. 15 and 16. The purity of the Al2 O3 product was as follows: Al2 O3 99.28%, MgO 0.63%, and ≈0.09% other impurities (i.e., CaO, Fe2 O3 , SiO2 , etc.) These results indicated that further investigation of strategies for Mg removal is necessary to achieve Al2 O3 of higher purity. 3.4. Residual dross analysis The remaining dross after leaching of aluminum metal was washed with water to remove traces of acid, other soluble impurities and then subjected for XRD analysis for examining the availability of metallic aluminum traces. The X-ray diffractogram obtained for dross shown in Fig. 8 has a sharp peak at d-spacing 2.328 that indicates the presence of aluminum metal in initial dross material. The same X-ray diffractogram obtained for the residue
Al traces Cr 0.10 Ca 0.21
Fe 2.60 Na 0.11
Si 10.9 K 0.13
Cu 1.07 Cl 0.02
Zn 0.26 F <0.05
Mg 1.73 CO3 2.12
Ti 0.37 NO3 5.17
Mn 0.17 NH4 3.74
Ni 0.069 SO4 0.55
Pb 0.605
after recovery of aluminum metal using chemical leaching technique is presented in Fig. 17. The disappearance of the peak at d-spacing 2.328 shows that there is almost total recovery of metallic aluminum from dross. This also indicates that the chemical recovery process used is efficiently to extract and to recover total aluminum metal content from the dross material. The chemical composition of the residual dross after aluminum metal recovery presented in Table 5 also shows that there is a significant improvement in the residual dross material composition making it suitable for other applications [7,12]. A comparison of residual composition (Table 5) with that of initial dross (Table 4) indicates that there is an increase in silica contents. The detrimental parameters such as iron and other alkaline impurities have come down significantly in residue. Some of the impurities such as nickel, mangan, titan are found in traces. The reactivity of this residue was very significant reduced and can be thought of possible future applications and therefore, this inert material can open up an attractive line for development. It may be suitable for reuse as a secondary material in refractory industry, for which this would be a interesting line for future research. This study strongly suggests that the dross material of negligible commercial significance could be converted to commercially viable materials and could be used as raw materials to obtain products with high added value such as alumina. 4. Conclusions
Fig. 16. SEM image of Al2 O3 powder obtained from gelatinous precipitate dried at 120 ◦ C for 30 min and calcinated at 1050 ◦ C for 4 h.
The relationship established between density, porosity and metal content of a dross is due to the different densities of the main dross constituents (aluminum, alumina) and the mechanism of the dross formation. A recommendation is to avoid the storage of the dross landfills to prevent reactions with moisture and leading to the formation of toxic, harmful, explosive, poisonous and unpleasant odorous gases and to the pollution of ground water. From experiments, leaching conditions were obtained to permit the maximum aluminum soluble ions extracting. The maximum value of aluminum extracted was found to take place at the neutralization point between acid and dross. These optimum conditions, chosen to be used in the bench scale experiment were specified as follows: temperature: 90 ◦ C; H2 SO4 concentration: 15 wt.%; time of reaction: 5 h; solid/liquid ratio (S/L): 1/12.
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