Production of g-alumina from waste aluminium dross

Transcription

1 Minerals Engineering 20 (2007) This article is also available online at: Production of g-alumina from waste aluminium dross B.R. Das, B. Dash, B.C. Tripathy *, I.N. Bhattacharya, S.C. Das Regional Research Laboratory, Bhubaneswar , India Received 31 May 2006; accepted 11 September 2006 Available online 31 October 2006 Abstract Processing of aluminium dross is one of the most challenging tasks because of its toxic nature. The dross generated while melting at various facilities is generally remelted with salts to recover residual metal values. The remaining residue dross contains mostly aluminium oxide, alloying elements and salts such as NaCl or KCl. This residue dross while stock piling creates pollution of the adjoining area as salts leach out to water stream and also emits harmful gases. In the present study domestic aluminium dross was treated for developing a suitable process flow sheet to obtain g-alumina a high valued product. Initially H 2 SO 4 leaching was carried out for both un-washed and washed dross. With un-washed dross the leaching efficiency achieved was 71% but washing of dross followed by leaching raised the recovery to 84%. Washing of dross is essential to have higher alumina recovery and also to recover salt for recycling. The liquor obtained after treatment of the dross with acid was further processed to obtain aluminium hydroxide of amorphous nature by hydrolyzing aluminium sulphate with aqueous ammonia. The aluminium hydroxide was then subjected to calcinations which resulted in the formation of g-alumina at 900 C. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Hydrometallurgy; Leaching; Reclamation; Recycling; Waste processing 1. Introduction Disposal and recycling of dross produced during aluminium melting is a worldwide problem. Majority of dross is disposed off in landfill sites, which is likely to result in leaching of toxic metal ions into ground water causing serious pollution problems (Unger and Beckmann, 1992). In addition to this when aluminium dross comes in contact with water it emits harmful gases such as NH 3, CH 4, PH 3,H 2,H 2 S, etc. (Unger and Beckmann, 1992). In India, a rough estimate shows that 75,000 tons of dross is generated annually and most of it is used for making crackers, impure chemicals and low quality refractory bricks or stock piled due to want of proper treatment options. Aluminium dross is formed by natural oxidation of molten aluminium. The metal when comes in contact with air * Corresponding author. Tel.: ; fax: address: bankimtripathy@gmail.com (B.C. Tripathy). forms oxide of aluminium at the outer surface of the melt. The residual metal part in the dross is removed by remelting it by adding salt flux to minimize the oxidation. The oxide in the dross exhibits the form of a long continuous net where aluminium stays entrapped. The molten flux also breaks this framework and facilitates the coalescence of aluminium drops that sinks to the aluminium bath (Tenorio and Espinosa, 2002). In majority of the cases salt bath is used to maximize the recovery of aluminium. By this process though oxide generation is less the dross becomes toxic because of its salt content and makes the disposal of dross and recovery of aluminium from the dross more complex. The increasing demand of valuable materials and environmental standard enforcement has forced the development of suitable treatment facilities for industrial wastes. Further, for sustainable development, replacement of primary resources with secondary resources has also become very essential. Scanning of literature shows that attempts have already been made to utilize aluminium dross by adopting either /$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi: /j.mineng

2 B.R. Das et al. / Minerals Engineering 20 (2007) pyro- or hydrometallurgical methods. The conventional dross oxide treatment consists of grinding the dross, sieving to recover the metal value followed by water leaching to dissolve the salt in water from residue oxide. The salt is recovered back by filtering and evaporation technique. The residue which contains primarily alumina and other alloying elements is then calcined. In another process aluminium is mechanically separated from the oxide part and the remaining oxide fines are blended into a product that can be used for steel industries. In recent times Portland cement industries use certain quantities of alumina for the production of cement (Zuck, 1995). As the requirement of aluminium oxide is around 5%, many producers in US utilize this dross oxide for production of cement. Alcan (Lavoie et al., 1991; Lavoie and Lachance, 1995), since 1990 has been using plasma dross processing facility for its treatment plant where aluminium recovery is said to improve to around 90%. The dross generated is utilized for manufacture of other products like sodium aluminate (Na 2 Al 2 O 4 ) spinel based ceramic, brown alumina, calcium aluminate etc. In a Korean process (Park et al., 1999), the dross was leached with NaOH to extract the aluminium as sodium aluminate and then precipitated in the form of aluminium hydroxide. The residue in the leaching was roasted to oxidize the remaining metals to oxide. This roasted oxide is used for making high castable refractories by mixing with aggregates and alumina cement. El-Katatny et al. (2003) described a process where aluminium is recovered from the dross by precipitating out aluminium hydroxide using NaOH solution. This aluminium hydroxide powder is then activated at 600 C to obtain c-al 2 O 3. Various studies (Osborne, 1995; Garret, 1982; Huckabay, 1984; Huckabay and Skiathas, 1982; Amer, 2002) have been undertaken where H 2 SO 4 was utilized for manufacture of aluminium sulphate. In most of the works (Osborne, 1995; Garret, 1982; Huckabay, 1984; Huckabay and Skiathas, 1982) aluminium sulphate produced was having various contaminants, thus making the sulphate impure. Amer (2002) produced two types of alums (aluminium sulphate) by dividing the alumina leaching process in to two steps. In the first step leaching was carried out with dilute H 2 SO 4 to remove impurities and in the second step alumina was extracted from the purified dross tailings using concentrated H 2 SO 4. In India no significant effort has been made to reclaim the dross and add value to the waste. The dross is treated mostly in unorganized sectors for preparation of impure chemicals, crackers and castable refractories as mentioned earlier. In the present study an attempt has been made to develop a process flow sheet to recover salt and alumina value to produce g-alumina. 2. Experimental Aluminium dross is a waste material obtained from aluminium melting plants whose composition differs due to the various alloying elements used during melting. The dross used in this work was collected from a domestic dross producer, which contains mainly 65% alumina, 4% SiO 2 and oxides of Mg, Ca and Fe along with some salts such as NaCl and KCl. This dross contains bigger particles of size 2 5 mm, which are mostly aluminium or its alloy and the finer fractions are mostly alumina (Al 2 O 3 ). Screening of the above material was undertaken to separate out the aluminium part from the oxide. The <850 lm (Tyler 20) size particles were taken for the study. The analysis of this fraction is given in Table 1. Philips X-ray Fluorescence (XRF) analytical equipment was used for the dross analysis. The X-ray diffraction analysis (XRD) was carried out on a Philips Powder diffractometer Model PW 1830 X pert system. The XRD pattern of the dross is shown in Fig. 1 with the possible phases present. Carbon and sulphur were analysed on a LECO instrument. The XRF and X-ray diffraction analysis of the dross indicated the presence of a-al 2 O 3, CaF 2, MgAl 2 O 4, Fe 2 O 3, CaO, carbon, sulfur and salts like KCl and NaCl. The dissolution experiments of dross in H 2 SO 4 medium were carried out in a flat-bottomed glass reactor, which was placed on a Remi make hot plate cum magnetic stirrer. During leaching the temperature was maintained at around 90 C. A similar system was used for washing of dross with water. As the dissolution reaction is exothermic, the temperature of the leaching medium automatically increases up to C without any input of heat from out side. The hot plate was utilized to maintain the temperature at 90 ± 2 C. For each experiment 10 g of dross was taken unless otherwise specified. Each experiment was carried out for 3 h followed by alumina analysis of the leached liquor by conventional EDTA ZnSO 4 method. The leach solution obtained after filtration was subjected to iron removal followed by aluminium hydroxide precipitation using 10% solution of aqueous ammonia that was added drop wise into the solution using a burette. The product aluminium hydroxide was heat treated at various temperatures from 200 to 1100 C in a muffle furnace to observe the phase transformation and to identify the temperature for g-alumina transformation. Table 1 Chemical composition of aluminium dross Compound/element Percent (w/w) Al 2 O CaO 0.93 SiO Fe 2 O MgO 3.2 Na 2 O 2.75 K 2 O 0.51 Cl 3.9 C 1.25 S 0.22

3 254 B.R. Das et al. / Minerals Engineering 20 (2007) Alumina recovery, % % 20 % 30 % 40 % Fig. 1. XRD pattern of original dross of particle size less than 850 lm (Tyler 20). 3. Results and discussion 3.1. Dissolution of dross oxide Leaching of both un-washed and washed dross oxide in sulfuric acid medium was carried out to evaluate the leaching behavior Dissolution of un-washed dross After screening, the <850 lm size dross was subjected to sulphuric acid leaching. During sulphuric acid leaching of the dross following reactions take place and water-soluble aluminium sulphate is formed. Al 2 O 3 +3H 2 SO 4 =Al 2 (SO 4 ) 3 +3H 2 O ð1þ 2Al + 3H 2 SO 4 =Al 2 (SO 4 ) 3 +3H 2 2Al + 6H 2 SO 4 =Al 2 (SO 4 ) 3 + 3SO 2 +6H 2 O Fig. 2 shows variation of alumina recovery as a function of amount of H 2 SO 4 at different percentage of its concentrations. Here at each level of H 2 SO 4, 10 g of dross was taken. The amount of acid added varied between stoichiometric amount to almost three times of the stoichiometric amount. The temperature during leaching was maintained at around 90 C. The experiments mentioned were carried out to know the exact amount (ml) and concentration of acid (%) required for the maximum recovery of alumina from the dross. This is required because solubility of aluminium sulphate is limited to concentrated H 2 SO 4 solution. Fig. 2 also shows the alumina recovery at a fixed percentage of acid with change in acid quantity. The different percentages of acid used were 15%, 20%, 30% and 40%, whereas its amount varied from 15 to 50 ml. With 15% acid marginal improvement in recovery was observed when amount of acid increased to 20 ml but the recovery remained almost constant (75%) with further increase in acid quantity. The recovery was found to be 74% with 15 ml of 20% acid. On increasing the acid quantity to ð2þ ð3þ Volume of H 2 SO 4, ml Fig. 2. Variation of alumina recovery as a function of H 2 SO 4 amount at various acid concentrations. 30 ml maximum recovery of 78% was obtained but with further increase in the acid quantity the recovery started falling. Similar observation was made for 30% acid where 30 ml acid showed maximum recovery of 88% and with further increase in acid quantity recovery showed a declining trend. However with 40% acid the maximum recovery was obtained at 40 ml of acid amount. The reason behind decrease in recovery with increase in acid is exactly not known. Though the decrease in recovery is not appreciable but it happened in all the cases. Generally in any dissolution/leaching operation stoichiometric quantities of leachant does not show the optimum level of recovery. Invariably some more acid would be required to have the optimum value. In this case the role of acid quantity, quantity of water, salts like KCl and NaCl, formation of aluminium sulphate etc. will influence the leaching behaviour. It has been mentioned that where washed dross is taken for dissolution studies, with stoichiometric quantity of acid, alumina recovery is higher. It has been found, therefore that the optimum recovery of alumina for original dross is 88%, with 30 ml of 30% acid. But this 30 ml of acid is almost double the stoichiometric amount that is required for 100% solubilisation of available alumina. Fig. 3 shows percentage of recovery of alumina plotted as a function of solids (dross) concentration at various acid quantities. It has been found that solid concentration of 10% showed maximum recovery. Similar results were also obtained with other percentages of acid considered in the present study. Therefore, pulp density of 10% may be taken as the optimum solid concentration to obtain maximum alumina recovery. It has been observed from the above results that maximum recovery of 88% can be achieved with 30 ml of

4 B.R. Das et al. / Minerals Engineering 20 (2007) Alumina Recovery, % Solid (dross) Concentration, % Fig. 3. Efficiency of alumina recovery as a function of pulp density at 30% acid concentration. 30% acid for 10 g of dross. However as mentioned earlier, this 30% acid is almost double of the stoichiometric amount. With 30% acid leaching, the concentration of acid in the leach liquor would be too high and would require significant amount of alkali for its neutralization during hydroxide precipitation. Considering this aspect it was decided to use stoichiometric quantity of acid i.e. 15% but in this case the recovery would be limited to 70% Washing of dross followed by leaching of washed dross The leaching of original dross with stoichiometric amount of acid resulted in only 70% alumina dissolution. In addition the salts will contaminate the leach liquor. These salts in the leach liquor will contaminate the hydroxide in the precipitation stage and thus the ultimate product. In order to improve the leaching efficiency and prevent contamination of leach liquor from the soluble salts, some leaching experiments were carried out after washing the dross. This has improved the leaching efficiency as well as brought almost all the salts into the washed solution. It was then decided to introduce a washing step to remove the soluble salts followed by its recovery. So a detailed study on water washing of the dross followed by leaching was taken up Washing of dross and recovery of salts for recycling. As discussed earlier the salts used in the dross during melting are water-soluble salts like KCl and NaCl. The total quantity of the salt present in the dross was found to be around 8%. Prior to acid leaching the dross was subjected to water washing for salt recovery. The variables considered in these set of experiments were time and washing temperature. All these experiments were carried out under stirring condition with 20 g of dross maintaining a solid to liquid ratio of 10%. The effect of time on washing was carried out from 1 to 24 h. Washing at room temperature for 1 h yielded about 65% of the total salt present in the dross. Further increase in the washing time up to 24 h has practically no effect. However when the residue obtained after 1 h washing was again washed with fresh water for 1 h the salt recovery was improved by another 5 7%. To improve the salt recovery efficiency, washing of the dross was carried out at 80 ± 2 C. It has been found that kinetics of salt recovery was enhanced with rise in temperature. At 80 C around 90% salt is washed with water in 1 h. Similar to room temperature washing, the residue obtained after 1 h washing at 80 C if washed with fresh water for another 1 h at 80 C the recovery improved by further 10%. Thus almost 100% recovery of salt could be possible in two step washing at 80 C. This washed solution was evaporated to obtain KCl and NaCl crystals, which can be recycled during melting of dross. Fig. 4 shows the XRD phase analysis of recovered salt. In actual plant operation a washing scheme has to be involved so as to obtain a wash solution of high salt content for economical recovery of salts Leaching of washed dross. Acid leaching studies were carried out with both washed dross, and washed and dried dross. In these cases, 10 g dross was taken each time and was washed with 100 ml of demineralised (DM) water under optimum conditions to recover salts. The slurry was filtered and the residue was taken for leaching with 15% acid under the conditions established above. In another case the residue obtained after washing and filtration was dried and the dried residue was taken for leaching under the same conditions. The acid leaching efficiencies (recovery of Al 2 O 3 ) of the three different drosses i.e. original, washed and washed & dried were compared. It was found that the alumina recovery efficiency has improved considerably with washing. The washed dross showed maximum recovery of 84% with 15% (v/v) acid and 10% dross (w/v). The improvement in leaching efficiency for washed dross may be due to the removal of salts from the alumina surfaces. This finding has good implication, as inclusion of a washing step is essential to recover the salts from the filtrate as well as removal of washable impurities, which might otherwise end up with the final product. X-ray diffraction data (Fig. 1) showed original dross contains mainly a-al 2 O 3,MgOÆ Al 2 O 3, SiO 2, CaF 2,KCl, NaCl etc. There is also possibility of formation of sillimanite (Al 2 O 3, SiO 2 ) as 100% relative intensity is observed at 26.5 (2h) as well as at 38.5 (2h) coincides with certain a- alumina reflections. Probably these ceramic oxides do not respond to the present leaching conditions thus limiting the alumina recovery to 84%.

5 256 B.R. Das et al. / Minerals Engineering 20 (2007) Fig. 4. X-ray diffraction pattern of salts obtained from dross after water washing Aluminium hydroxide precipitation The leach liquor obtained from sulphuric acid leaching contains g/l of Al 2 O 3 as aluminium sulphate. It also contains considerable amount of iron i.e. around mg/l. Since presence of iron in the aluminium hydroxide imparts colour and contaminates the final product, it was removed by controlled addition of aqueous ammonia at ph 4.4. Aluminium hydroxide was then precipitated by raising the ph of the iron free solution to 7.0. The aluminium hydroxide thus obtained is rich in water content ðal 2 O 3 :42%; H 2 O and SO 2 4 : 58%Þ Production of activated alumina The amorphous aluminium hydroxide during calcination undergoes phase transformation and produces different forms of transition oxides. The phase transformation depends on the precursor material. The different transition aluminas available are v, g, c and q alumina in low temperature range ( C) and d, j and h at higher temperature range (Goodboy and Downing, 1990). These oxides form a large group of activated aluminas. In the present study the precipitated hydroxide is subjected to calcinations in the range of C. This temperature range was chosen because major transformations are obtained in this range only. The activated alumina is obtained from aluminium hydroxide by controlled heating to eliminate most of the water of constitution. The XRD study of the product aluminium hydroxide carried out at different temperatures showed amorphous behaviour till 800 C but at 900 C it transformed to g-alumina (Fig. 5) and at 1100 C a-alumina was obtained. A similar observation was also made in our previous study (Bhattacharya et al., 2004) where aluminium hydroxide precipitated from pure aluminium sulphate was transformed to g-alumina at 900 C. TGA study of the precipitated aluminium hydroxide indicated two distinctive high rates of weight loss zones i.e. at 250 C and 900 C. The first zone was referred to as due to dehydration or dehydroxylation and the second zone due to desulphurisation reaction. These effects result in the increased porosity. It was also observed that BET surface area of the calcined product at 900 C was maxi- Fig. 5. X-ray diffraction pattern of g-alumina.

6 B.R. Das et al. / Minerals Engineering 20 (2007) Fig. 6. Typical flow sheet for aluminium dross processing. mum (112 m 2 /g). Higher surface area and increased porosity of g-alumina indicated that this can be used as a very good adsorbent and also as catalytic material. The surface area can further be increased by treating/doping the material with inorganic acids and/or various metal ions. 4. Processing routes Fig. 6 represents a flow sheet proposed from the present study for aluminium dross processing to recover aluminium value and processing of leach liquor for producing value added product like g-alumina. 5. Conclusions The process developed for the treatment of aluminium dross is unique in nature. The treatment options available mostly aimed to optimize the recovery of metal part from oxide part and rest was land filled. In this study only alumina part was subjected to the treatment after recovering the metal part by screening. A flow sheet has been developed where initially water-soluble salt is recovered by water washings. H 2 SO 4 dissolution process has been optimized and found that 30% acid at 10% pulp density of dross will leach out 88% Al 2 O 3 and 15% acid at 10% solid concentration showed around 71% recovery. Alternatively, it was found that when the leaching was carried out with washed dross and 15% acid around 84% recovery was obtained. The aluminium containing leached solution was further treated with aqueous ammonia to obtain amorphous aluminium hydroxide. This aluminium hydroxide was heat treated at 900 C to obtain g-al 2 O 3, which is a high valued activated alumina and can be used as an adsorbent or can be used for catalytic purpose. Acknowledgements The authors are grateful to Ministry of Environment and Forests, Govt. of India, New Delhi, for the financial support to carry out the work. They are also grateful to M/s Agarvanshi Aluminium, Secunderabad for supplying the aluminium dross. The authors also like to thank the Director, Regional Research Laboratory, for permission to publish this paper. They are also thankful to Dr. P.S. Mukherjee and Dr. Rajeev for XRD/XRF analyses. References Amer, A.M., Extracting aluminum from dross tailings. J. Metals 54, Bhattacharya, I.N., Gochhayat, P.K., Mukherjee, P.S., Paul, S., Mitra, P.K., Thermal decomposition of precipitated low bulk density basic aluminium sulfate. Mater. Chem. Phys. 88, 32. El-Katatny, E.A., Halany, S.A., Mohamed, M.A., Zaki, M.I., Surface composition, charge and texture of active alumina powders recovered from aluminum dross tailings chemical waste. Powder Technol. 132, Garret, L.W., Process for the Production of Sulfates. US Patent No. 4,337,228. Goodboy, K.P., Downing, J.C., In: Hart, L.D. (Ed.), Production, Process, Properties and Applications of Activated and Catalytic Aluminas. The American Ceramic Society Inc., Wessterville, Ohio, p. 93.

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