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Recovery of valuable materials from aluminium salt cakes
Int. J. Miner. Process. 93 (2009) 1–5
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Int. J. Miner. Process. j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i j m i n p r o
Recovery of valuable materials from aluminium salt cakes W.J. Bruckard ⁎, J.T. Woodcock CSIRO Minerals, Box 312, Clayton South, Vic. 3169, Australia
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Article history: Received 17 February 2009 Received in revised form 29 April 2009 Accepted 13 May 2009 Available online 19 May 2009 Keywords: Salt cake Aluminium Waste processing
a b s t r a c t Salt cakes, which are nominally waste products derived from aluminium dross melting furnaces, are complex mixtures of some 20 different compounds made up of many different elements. Normally they are regarded as waste products and they are disposed of in toxic waste dumps. However, it is shown here that some components are readily recoverable as high-grade products for recycling or sale and that the residues thus generated can be non-toxic. Recoverable components include metallic aluminium, salt (halite (NaCl) and sylvite (KCl)), alumina-containing compounds, and possibly hydrogen gas. Metallic aluminium is soft and malleable and is not reduced in size by crushing and grinding, whereas the other components in salt cake are soft and brittle or are readily dissolved in water. Hence the coarse metallic aluminium can be readily recovered by crushing and screening and the finer metallic aluminium can be recovered by fine grinding and screening, froth flotation, or possibly electrostatic separation. Aqueous acid or alkaline leaching has also been proposed to recover aluminium metal from salt cake. The halite and sylvite are easily extracted by leaching ground salt cake with cold water and filtering off the saline solution. This solution can be sent to solar evaporation ponds where the water is evaporated and the dry salt harvested for recycling to dross treatment furnaces or other markets. Some of the water-insoluble or oxide aluminium compounds present are soluble in Bayer-type leach solutions and could possibly be sent to a Bayer-type leach plant for production of high-grade alumina for aluminium production. Alternatively, because the oxide aluminium compounds are inert they could be sent to a non-toxic dump. The possibility of integrated flowsheets to recover more than one product in a single plant is also discussed. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved.
Contents 1. 2. 3. 4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . General characterisation of salt cakes . . . . . . . . . . . . . . . . . Recovery of sodium and potassium chlorides . . . . . . . . . . . . . Recovery of metallic aluminium . . . . . . . . . . . . . . . . . . . . 4.1. Aluminium metal recovery by screening . . . . . . . . . . . . 4.2. Aluminium metal recovery by flotation . . . . . . . . . . . . . 4.3. Aluminium metal recovery by electrostatic separation . . . . . . 4.4. Aluminium metal extraction by aqueous acid or alkaline leaching 5. Recovery of oxide aluminium compounds . . . . . . . . . . . . . . . 6. Hydrogen gas recovery . . . . . . . . . . . . . . . . . . . . . . . . 7. Decomposition of reactive compounds in salt cakes . . . . . . . . . . 8. Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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1 2 3 3 3 4 4 4 4 4 4 5 5 5
1. Introduction
⁎ Corresponding author. Tel.: +61 3 9545 8500; fax: + 61 3 9562 8919. E-mail address: [email protected] (W.J. Bruckard).
Aluminium salt cakes are solid waste products derived from the smelting of aluminium drosses under a salt cover to recover some of the metallic aluminium entrained in the dross. However, the solidified
0301-7516/$ – see front matter. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2009.05.002
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W.J. Bruckard, J.T. Woodcock / Int. J. Miner. Process. 93 (2009) 1–5
salt cake itself contains metallic aluminium as well as many other substances, some of which have an intrinsic value, some of which are toxic, and some of which react with water to give off toxic gases. Whilst a number of workers have considered the recovery of valuables, mostly metallic aluminium, from aluminium dross, e.g., Zhang (2006), Peterson and Newton (2002), Schlesinger (2000), and Bruckard et al. (2004), few studies have been conducted on the salt cakes derived from them. Those that have, e.g., Bahr and Kues (1978) and Gil (2005, 2007), have noted the complexity of salt cakes and the relatively low tonnages produced in most areas. These two factors have contributed to the slow progress in developing economic treatment routes for the re-processing of aluminium salt cakes. There are, however, several commercial operations treating salt cake or high-salt aluminium dross (for example JBM International Ltd, UK; Engite Technologies, Italy; Alustockach, Germany; KVS-Ekodivize, Czech Republic; Alumitech Inc, USA), with each operation using different technologies to cope with the different feed stocks. It is the main purpose of this paper to review and discuss the various ways in which some of the more valuable materials can be recovered from salt cakes. It may be possible to recover more than one material in a single processing plant. These materials include metallic aluminium, sodium and potassium chlorides, and an alumina-containing solid or solution which could be suitable as Bayer process plant feed. In addition, leach conditions which decrease the toxicity caused by compounds such as aluminium nitrides are proposed so that final residues can be safely sent to a non-toxic waste dump. 2. General characterisation of salt cakes Salt cakes contain many different components, and most of the elements present are listed in Table 1 for two separate Australian samples. These components have been determined by many different analytical techniques as discussed by various investigators such as Bruckard and Woodcock (2004, 2007), Hagni (2002), and Manfredi et al. (1997). Although the two samples are broadly similar in composition there are many subtle differences in detail. These differences presumably result from differences in feedstocks and operating practices. Table 2 lists the various phases identified in the two Australian salt cakes studied in this work using qualitative X-ray diffraction (XRD),
Table 1 Chemical analyses of two Australian salt cakes. Element
Sample A
Major elements (N 1%) Al (total) Al (metallic) Cl Na N F K Mg Si Fe
37.2 1.22 9.39 8.52 7.53 5.50 3.18 2.59 2.07 0.82
36.8 2.79 6.79 5.20 1.96 5.50 3.74 0.70 1.03 5.85
0.72 0.55 0.13
0.50 0.42 0.50
Intermediate elements (0.1–1.0%) Ca C Ti Minor elements (b 0.1%) Zn P Mn Ba S Pb V a
Table 2 Alphabetical list of phases identified in typical Australian salt cakes by qualitative XRD and other methods.a Phase
Formula
Relative abundanceb
Aluminium calcium Aluminium metal Aluminium nitride (25–1133)c Aluminium oxide Aluminium oxide nitride (48–1581)c,d Aluminium oxide nitride (48–686)c,d Aluminium nitride (00-46-1200)c Bayerite Corundum Cryolite Diaoyudaoite Elpasolite Fluorite Gibbsite Halite Iron metal Silicon metal Sodalite Sylvite Villiaumite
AlCa Al AlN Al2O3 Al5O6N Al5O6N AlN Al(OH)3 Al2O3 Na3AlF6 NaAl11O17 K2NaAlF6 CaF2 Al(OH)3 NaCl Fe Si Na6[Al6Si6O24].2NaCl KCl NaF
bDL Trace Medium bDL – trace bDL – trace bDL – trace Medium bDL – minor Major Trace – minor Minor Trace – medium bDL bDL Major bDL bDL bDL Minor – medium bDL
a Other methods included SEM examination, electron probe micro analysis (EPMA), chemical analysis by various methods, and hand sorting. b The decreasing order of relative abundance is as follows: major, medium, minor, trace, and bDL, where bDL was below the detection limit by the XRD conditions used (say 3–5%). This phase was detected by one or more of the other analytical methods. c Bracketed numbers are the International Center for Diffraction Data (ICDD) file numbers. d These phases have the same composition but different crystal forms.
scanning electron microscope (SEM) examination, electron probe micro analysis (EPMA), various chemical techniques, and hand sorting of the coarser size fractions of crushed salt cake to remove relatively coarse pieces of metallic aluminium. Of particular note is the method of determination of the fine-sized metallic aluminium by cold bromine methanol extraction followed by analysis of the extract solution. It is known that particles of metallic aluminium up to about 2.5 mm in size can occur in salt cakes (Bruckard and Woodcock, 2007) and that particles down to 38 μm can occur in aluminium drosses from which salt cakes are produced (Bruckard et al., 2004). Table 3 gives an estimate of the contribution of the various aluminium-bearing phases present in the two salt cake samples to the total aluminium assay. These results together with those in Table 1 indicate that between 1% and 3% of the feed is possibly recoverable as metallic aluminium.
Sample B Table 3 Estimated contribution of the various aluminium-bearing phases present in the salt cakes to the total aluminium assay. Aluminium-bearing phase
Sample A (% Al in feed)
Sample B (% Al in feed)
+ 20 mm metallic Ala − 20 + 0.85 mm metallic Ala − 0.85 mm metallic Ala Total metallic Alb Carbide aluminiumc Nitride aluminiumd Water-soluble aluminiume Other aluminiumf
0.50 0.36 0.36 1.22 1.59 4.8 0.28 29.3
0.88 0.09 1.82 2.79 1.21 1.26 0.30 31.24
Total aluminium in feed
37.2
36.8
a
0.089 0.065 0.057 0.017 0.020 0.019 0.013
Less than the detection limit by the method used.
0.012 0.020 0.047 0.018 0.020 0.024 b DLa
Determined by direct analysis of the screen size fraction. b Sum of aluminium in the three coarsest size fractions. c Calculated by assuming that all of the carbon present is there as aluminium carbide (Al4C3). d Calculated by assuming that all the nitrogen present is there as aluminium nitride (AlN), but some aluminium oxide nitride (Al5O6N) is also present. e Present in an unknown form, but calculated as the Al2O3 equivalent. f Calculated by the difference between the aluminium accounted for here and total aluminium content of the feed.
W.J. Bruckard, J.T. Woodcock / Int. J. Miner. Process. 93 (2009) 1–5
Table 3 also shows that reasonably high levels of aluminium carbide (Al4C3), aluminium nitride (AlN), and aluminium oxide nitride (Al5O6N) are present. These and other compounds present can react with water to give off noxious gases such as hydrogen, ammonia, methane, and gaseous sulphides and phosphides, and this must be taken into account in any proposed wet treatment process as discussed later. Very little water-soluble aluminium is present, and this can probably be ignored in any treatment process. However, as shown in Table 2, appreciable levels of water-soluble halite (NaCl) and sylvite (KCl) are present and these could be relatively easily extracted by water washing and recovered by solar evaporation for recycling to the dross smelting furnace. Reasonably high levels of aluminium oxide, bayerite, corundum, and gibbsite were present. Whilst corundum is not soluble in strongly alkaline solution, some of the other phases are, and these could potentially be recovered for feeding to a Bayer process plant (Davies et al., 2008). With respect to magnesium, data from Table 1 indicate one of the salt cake samples contained 2.59% Mg. No magnesium-bearing phases were identified by XRD, but in general magnesium in salt cakes has been known to be present as oxide phases or alloyed with aluminium. On the basis of the potential recoverable products outlined above, it seemed that it was worth reviewing the various methods suggested for recovering these and other compounds from salt cakes and this is done below.
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In this circuit, cold salt cake from the melting furnace is first crushed in closed circuit with a double deck screen fitted with 20 mm and 10 mm aperture screens. The screen sizes here are nominal and may be varied to suit the particular salt cake feed (it is proposed that material less than 10 mm may be a suitable size to wet mill for salt cake). Any + 20 mm material is mostly aluminium metal with some entrained salt cake and can be recycled to the remelting furnace. The −20 + 10 mm material is returned to the crusher for crushing to −10 mm. The −10 mm material is then wet ground in an open circuit tube mill with the aim of flattening any metallic aluminium particles present, so that they are more readily screened out, and grinding the non-metallic particles, which are relatively brittle, to a fine size. Simultaneously the water-soluble particles are dissolved. The ground
3. Recovery of sodium and potassium chlorides Recovery of sodium and potassium chlorides from salt cakes for recycling back to the smelting process could be economically advantageous and would reduce the amount to be disposed of in toxic waste dumps. Bruckard and Woodcock (2007) have shown that simple water washing of −2 mm crushed salt cake lowers the level of these chlorides to below the level of detection by XRD. Whilst the chlorides are efficiently removed by water washing, the process does not remove all of the sodium and potassium because these elements are also present in other, non-water-soluble compounds. No other watersoluble compounds are known to be present. It should then be possible to recover the pure dissolved salts by solar evaporation as practised commercially in many operations throughout the world (e.g., Gilbert (1993) and Garrett (2001)). The solids residue from the water leach stage may be suitable for disposal in a non-toxic dump, depending on the absolute levels of elements such as fluorine in the residue. Appropriate TCLP tests would need to be conducted to validate this. 4. Recovery of metallic aluminium Methods that can be used for the recovery or extraction of metallic aluminium from a salt cake include screening, flotation, electrostatic separation, and dissolution in strong alkali. These methods are discussed briefly in turn below. 4.1. Aluminium metal recovery by screening As shown in Table 3, aluminium salt cakes contain metallic aluminium pellets ranging in size up to at least 2 cm in diameter and down to an unknown, but very fine size. Because aluminium is a soft, malleable metal it cannot be crushed or ground to a finer size. Fortunately, the bulk of the salt cake itself is brittle and is readily crushed and ground to liberate the metal so that most of it can be recovered by screening. The sodium and potassium chlorides present are readily soluble in water and their dissolution can also help liberate metallic aluminium. A circuit to accomplish this is proposed by the authors and is shown in Fig. 1. The flowsheet is a good example of recovering two products in one unit operation.
Fig. 1. Proposed flow sheet for recovering valuable materials from aluminium salt cakes.
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W.J. Bruckard, J.T. Woodcock / Int. J. Miner. Process. 93 (2009) 1–5
product is fed to a DSM (Dutch State Mines) screen having 150 μm wide slots. According to Pryor (1965) this is about the finest size that can be used commercially for wet screening. The +150 μm material is mainly aluminium metal and can be dried and sent back to the melting furnace. The − 150 μm pulp goes to a salt recovery circuit where solar evaporation is used to recover the salt as discussed later in this paper. It should also be noted that the grinding behaviour of aluminium metal will depend on its content of alloying elements. It is likely, for example, that an aluminium-silicon alloy would be more brittle in comparison to pure aluminium. The grinding behaviour of the metallics would also be influenced by the time of grinding and the type of grinding device used. 4.2. Aluminium metal recovery by flotation Only a few metals occur in nature in metallic form. These include gold, silver, platinum, and more rarely mercury and copper. Metallic aluminium does not occur in nature and so no work has been published on its flotation from ores. Furthermore, no work has been published on its flotation from salt cakes. However, Soto and Toguri (1986) and Bruckard and Woodcock (2004) conducted work on floating aluminium from aluminium dross. Soto and Toguri used a Hallimond tube initially, and subsequently a Wemco laboratory cell. Bruckard and Woodcock used a 3 L Denver laboratory flotation cell and obtained similar results to those of Soto and Toguri. It was found that metallic aluminium did not float with xanthatetype collectors alone, but when copper sulphate was added the aluminium was activated and floated readily. The activation process in this instance was in fact copper cementation on the aluminium metal surfaces. Best results were obtained on particles in the size range of 110–275 μm, and good recoveries in good grade concentrates were achieved. These results need to be confirmed on salt cakes, but it seems reasonable to expect that they would be. Coarse aluminium that does not float could be recovered by screening, and, as noted previously, dissolved halite and sylvite could be recovered by solar evaporation. 4.3. Aluminium metal recovery by electrostatic separation Electrostatic recovery of metallic aluminium from dross has been studied by Mah et al. (1986) who developed a circuit which involved first crushing the dross to −840 μm. They achieved a recovery of 70% of the metallic aluminium at a concentrate grade of 70% Al. However, dross usually contains much more aluminium than salt cake and at a much greater size, partly because it also often contains scrap aluminium, so it was relatively easy to achieve a good result. It remains to be shown if a similar result could be obtained on salt cake, but it could be worthwhile investigating this possibility. 4.4. Aluminium metal extraction by aqueous acid or alkaline leaching As Habashi (1997) has pointed out, aluminium metal is amphoteric and can dissolve in acid solutions (Eq. (1)) or alkaline solutions (Eq. (2)) with the generation of complex aluminium-containing ions and the generation of hydrogen gas. þ
2Al þ 6H3 O þ 6H2 O→2½AlðOHÞ6 −
3þ
−
þ 9H2
2Al þ 2OH þ 6H2 O→2½AlðOHÞ4 þ 3H2
ð1Þ ð2Þ
Either acid or alkaline leaching would also dissolve many of the other oxide aluminium compounds present in salt cakes. Recovery of aluminium in metallic form from acid solutions is not easy, whereas with alkaline leaching it may be that the solution is suitable for feeding directly to a Bayer-type process plant. This might be viable,
as discussed later, and could be worth further investigation. It would be necessary, of course, to first dissolve and remove the chlorides and some other elements before conducting an alkaline leach because these elements are not wanted in Bayer-type solutions. 5. Recovery of oxide aluminium compounds Davies et al. (2008) studied the treatment of salt cakes by Bayertype leaching and found that about 43% of the total aluminium present was dissolved in a typical two-stage process with a leach at 100 °C in the first stage and 140 °C in the second stage. Evidently, much of the aluminium is present as phases which are not soluble in Bayer-type solutions. This was examined in detail by Davies et al. (2008), and are not discussed here. Although the amount extracted may be worth recovering in a combined process as shown in Fig. 1, it is also possible that the tonnage may be too small to be of interest to bauxite mining and treatment companies, which commonly treat several million tonnes per year. 6. Hydrogen gas recovery It may be possible to produce hydrogen gas from salt cake using a complex process analogous to that developed by Owada et al. (2008) for producing “green hydrogen” from aluminium dross. They proposed that hydrogen could be generated from the reaction of metallic aluminium and an alkaline aqueous solution according to Eq. (3). 2Al þ 6H2 O→2AlðOHÞ3 þ 3H2
ð3Þ
The hydrogen could be used as a fuel and/or feed to fuel cells and the aluminium hydroxide sold in the market. Furthermore, the reaction is exothermic and the heat of reaction could be utilized for drying the aluminium hydroxide product. Whether or not such a process could be economic is unknown. Certainly there are numerous methods currently available for commercial hydrogen production (Anon, 1995), e.g. steam reforming of natural gas. Moreover, as shown elsewhere in this paper, when salt cakes are mixed with water other gases are given off and these may contaminate any hydrogen produced. 7. Decomposition of reactive compounds in salt cakes Various compounds in salt cakes react with water in the atmosphere, and especially in aqueous pulps, to give off ammonia and other noxious gases. These gases need to be properly vented from any treatment plant. Only one venting point is shown for example in Fig. 1, but a number may be required in practice. Aluminium nitrides react with water according to Eqs. (4) and (5) to give either ammonium hydroxide or ammonia. Eq. (4) is more likely to occur in more alkaline solutions (say above pH 8) and Eq. (5) is more likely to occur below pH 8. AlN þ 4H2 O→AlðOHÞ3 þ NH4 OH
ð4Þ
AlN þ 3H2 O→AlðOHÞ3 þ NH3
ð5Þ
Aluminium oxide nitride (Al5O6N) reacts with water according to Eq. (6) to give aluminium oxide and ammonia. 2Al5 O6 N þ 3H2 O→5Al2 O3 þ 2NH3
ð6Þ
Aluminium carbide reacts with water according to Eq. (7) to give aluminium oxide and methane gas. Al4 C3 þ 6H2 O→2Al2 O3 þ 3CH4
ð7Þ
W.J. Bruckard, J.T. Woodcock / Int. J. Miner. Process. 93 (2009) 1–5
Beckman (1991) has reported that gaseous sulphides and phosphides can be generated from some German salt cakes, and these would need special care in their removal. Based on the ratio of aluminium nitride to metallic aluminium in the salt cakes considered in this study, one would expect about 5 times more ammonia to be generated via Eqs. (4) to (7), than hydrogen via Eq. (3). 8. Conclusion Methods proposed and tested for recovering valuable materials from salt cakes, waste products derived from the treatment of aluminium dross, are presented and discussed. Whilst the chemical and physical complexity of salt cakes make re-processing or recycling challenging, many workers have shown that valuable components can be extracted in simple flowsheets which have both technical and possibly economic merit. Coarse metallic aluminium can be recovered by crushing and screening, intermediate size aluminium can be recovered by grinding and screening or electrostatic separation, and fine aluminium can be recovered by flotation. Recovery of aluminium metal by aqueous acid or alkaline leaching has also been proposed. Sodium and potassium chlorides present can be extracted by cold water leaching and recovered by solar evaporation after removing the insoluble components by thickening and filtration. Some of the oxide aluminium components are soluble in Bayertype leach solutions and could be sent to a Bayer-type leach plant to extract alumina. Alternatively, since the water-insoluble phases are inert, they could possibly be disposed of in a non-toxic dump, given they pass appropriate TCLP testing criteria. Hydrogen gas has also been generated from salt cakes, but, to date, only in laboratory studies. It seems possible that integrated flowsheets could be developed whereby one or more of the above valuable components are recovered, assisting positively with the process economics, and helping to close the loop in terms of overall waste disposal. Acknowledgements The Analytical Services and XRD Services Groups of CSIRO Minerals in Melbourne and Perth are thanked for conducting the many chemical analyses and XRD scans required in this work.
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Int. J. Miner. Process. j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i j m i n p r o
Recovery of valuable materials from aluminium salt cakes W.J. Bruckard ⁎, J.T. Woodcock CSIRO Minerals, Box 312, Clayton South, Vic. 3169, Australia
a r t i c l e
i n f o
Article history: Received 17 February 2009 Received in revised form 29 April 2009 Accepted 13 May 2009 Available online 19 May 2009 Keywords: Salt cake Aluminium Waste processing
a b s t r a c t Salt cakes, which are nominally waste products derived from aluminium dross melting furnaces, are complex mixtures of some 20 different compounds made up of many different elements. Normally they are regarded as waste products and they are disposed of in toxic waste dumps. However, it is shown here that some components are readily recoverable as high-grade products for recycling or sale and that the residues thus generated can be non-toxic. Recoverable components include metallic aluminium, salt (halite (NaCl) and sylvite (KCl)), alumina-containing compounds, and possibly hydrogen gas. Metallic aluminium is soft and malleable and is not reduced in size by crushing and grinding, whereas the other components in salt cake are soft and brittle or are readily dissolved in water. Hence the coarse metallic aluminium can be readily recovered by crushing and screening and the finer metallic aluminium can be recovered by fine grinding and screening, froth flotation, or possibly electrostatic separation. Aqueous acid or alkaline leaching has also been proposed to recover aluminium metal from salt cake. The halite and sylvite are easily extracted by leaching ground salt cake with cold water and filtering off the saline solution. This solution can be sent to solar evaporation ponds where the water is evaporated and the dry salt harvested for recycling to dross treatment furnaces or other markets. Some of the water-insoluble or oxide aluminium compounds present are soluble in Bayer-type leach solutions and could possibly be sent to a Bayer-type leach plant for production of high-grade alumina for aluminium production. Alternatively, because the oxide aluminium compounds are inert they could be sent to a non-toxic dump. The possibility of integrated flowsheets to recover more than one product in a single plant is also discussed. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved.
Contents 1. 2. 3. 4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . General characterisation of salt cakes . . . . . . . . . . . . . . . . . Recovery of sodium and potassium chlorides . . . . . . . . . . . . . Recovery of metallic aluminium . . . . . . . . . . . . . . . . . . . . 4.1. Aluminium metal recovery by screening . . . . . . . . . . . . 4.2. Aluminium metal recovery by flotation . . . . . . . . . . . . . 4.3. Aluminium metal recovery by electrostatic separation . . . . . . 4.4. Aluminium metal extraction by aqueous acid or alkaline leaching 5. Recovery of oxide aluminium compounds . . . . . . . . . . . . . . . 6. Hydrogen gas recovery . . . . . . . . . . . . . . . . . . . . . . . . 7. Decomposition of reactive compounds in salt cakes . . . . . . . . . . 8. Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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1 2 3 3 3 4 4 4 4 4 4 5 5 5
1. Introduction
⁎ Corresponding author. Tel.: +61 3 9545 8500; fax: + 61 3 9562 8919. E-mail address: [email protected] (W.J. Bruckard).
Aluminium salt cakes are solid waste products derived from the smelting of aluminium drosses under a salt cover to recover some of the metallic aluminium entrained in the dross. However, the solidified
0301-7516/$ – see front matter. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2009.05.002
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W.J. Bruckard, J.T. Woodcock / Int. J. Miner. Process. 93 (2009) 1–5
salt cake itself contains metallic aluminium as well as many other substances, some of which have an intrinsic value, some of which are toxic, and some of which react with water to give off toxic gases. Whilst a number of workers have considered the recovery of valuables, mostly metallic aluminium, from aluminium dross, e.g., Zhang (2006), Peterson and Newton (2002), Schlesinger (2000), and Bruckard et al. (2004), few studies have been conducted on the salt cakes derived from them. Those that have, e.g., Bahr and Kues (1978) and Gil (2005, 2007), have noted the complexity of salt cakes and the relatively low tonnages produced in most areas. These two factors have contributed to the slow progress in developing economic treatment routes for the re-processing of aluminium salt cakes. There are, however, several commercial operations treating salt cake or high-salt aluminium dross (for example JBM International Ltd, UK; Engite Technologies, Italy; Alustockach, Germany; KVS-Ekodivize, Czech Republic; Alumitech Inc, USA), with each operation using different technologies to cope with the different feed stocks. It is the main purpose of this paper to review and discuss the various ways in which some of the more valuable materials can be recovered from salt cakes. It may be possible to recover more than one material in a single processing plant. These materials include metallic aluminium, sodium and potassium chlorides, and an alumina-containing solid or solution which could be suitable as Bayer process plant feed. In addition, leach conditions which decrease the toxicity caused by compounds such as aluminium nitrides are proposed so that final residues can be safely sent to a non-toxic waste dump. 2. General characterisation of salt cakes Salt cakes contain many different components, and most of the elements present are listed in Table 1 for two separate Australian samples. These components have been determined by many different analytical techniques as discussed by various investigators such as Bruckard and Woodcock (2004, 2007), Hagni (2002), and Manfredi et al. (1997). Although the two samples are broadly similar in composition there are many subtle differences in detail. These differences presumably result from differences in feedstocks and operating practices. Table 2 lists the various phases identified in the two Australian salt cakes studied in this work using qualitative X-ray diffraction (XRD),
Table 1 Chemical analyses of two Australian salt cakes. Element
Sample A
Major elements (N 1%) Al (total) Al (metallic) Cl Na N F K Mg Si Fe
37.2 1.22 9.39 8.52 7.53 5.50 3.18 2.59 2.07 0.82
36.8 2.79 6.79 5.20 1.96 5.50 3.74 0.70 1.03 5.85
0.72 0.55 0.13
0.50 0.42 0.50
Intermediate elements (0.1–1.0%) Ca C Ti Minor elements (b 0.1%) Zn P Mn Ba S Pb V a
Table 2 Alphabetical list of phases identified in typical Australian salt cakes by qualitative XRD and other methods.a Phase
Formula
Relative abundanceb
Aluminium calcium Aluminium metal Aluminium nitride (25–1133)c Aluminium oxide Aluminium oxide nitride (48–1581)c,d Aluminium oxide nitride (48–686)c,d Aluminium nitride (00-46-1200)c Bayerite Corundum Cryolite Diaoyudaoite Elpasolite Fluorite Gibbsite Halite Iron metal Silicon metal Sodalite Sylvite Villiaumite
AlCa Al AlN Al2O3 Al5O6N Al5O6N AlN Al(OH)3 Al2O3 Na3AlF6 NaAl11O17 K2NaAlF6 CaF2 Al(OH)3 NaCl Fe Si Na6[Al6Si6O24].2NaCl KCl NaF
bDL Trace Medium bDL – trace bDL – trace bDL – trace Medium bDL – minor Major Trace – minor Minor Trace – medium bDL bDL Major bDL bDL bDL Minor – medium bDL
a Other methods included SEM examination, electron probe micro analysis (EPMA), chemical analysis by various methods, and hand sorting. b The decreasing order of relative abundance is as follows: major, medium, minor, trace, and bDL, where bDL was below the detection limit by the XRD conditions used (say 3–5%). This phase was detected by one or more of the other analytical methods. c Bracketed numbers are the International Center for Diffraction Data (ICDD) file numbers. d These phases have the same composition but different crystal forms.
scanning electron microscope (SEM) examination, electron probe micro analysis (EPMA), various chemical techniques, and hand sorting of the coarser size fractions of crushed salt cake to remove relatively coarse pieces of metallic aluminium. Of particular note is the method of determination of the fine-sized metallic aluminium by cold bromine methanol extraction followed by analysis of the extract solution. It is known that particles of metallic aluminium up to about 2.5 mm in size can occur in salt cakes (Bruckard and Woodcock, 2007) and that particles down to 38 μm can occur in aluminium drosses from which salt cakes are produced (Bruckard et al., 2004). Table 3 gives an estimate of the contribution of the various aluminium-bearing phases present in the two salt cake samples to the total aluminium assay. These results together with those in Table 1 indicate that between 1% and 3% of the feed is possibly recoverable as metallic aluminium.
Sample B Table 3 Estimated contribution of the various aluminium-bearing phases present in the salt cakes to the total aluminium assay. Aluminium-bearing phase
Sample A (% Al in feed)
Sample B (% Al in feed)
+ 20 mm metallic Ala − 20 + 0.85 mm metallic Ala − 0.85 mm metallic Ala Total metallic Alb Carbide aluminiumc Nitride aluminiumd Water-soluble aluminiume Other aluminiumf
0.50 0.36 0.36 1.22 1.59 4.8 0.28 29.3
0.88 0.09 1.82 2.79 1.21 1.26 0.30 31.24
Total aluminium in feed
37.2
36.8
a
0.089 0.065 0.057 0.017 0.020 0.019 0.013
Less than the detection limit by the method used.
0.012 0.020 0.047 0.018 0.020 0.024 b DLa
Determined by direct analysis of the screen size fraction. b Sum of aluminium in the three coarsest size fractions. c Calculated by assuming that all of the carbon present is there as aluminium carbide (Al4C3). d Calculated by assuming that all the nitrogen present is there as aluminium nitride (AlN), but some aluminium oxide nitride (Al5O6N) is also present. e Present in an unknown form, but calculated as the Al2O3 equivalent. f Calculated by the difference between the aluminium accounted for here and total aluminium content of the feed.
W.J. Bruckard, J.T. Woodcock / Int. J. Miner. Process. 93 (2009) 1–5
Table 3 also shows that reasonably high levels of aluminium carbide (Al4C3), aluminium nitride (AlN), and aluminium oxide nitride (Al5O6N) are present. These and other compounds present can react with water to give off noxious gases such as hydrogen, ammonia, methane, and gaseous sulphides and phosphides, and this must be taken into account in any proposed wet treatment process as discussed later. Very little water-soluble aluminium is present, and this can probably be ignored in any treatment process. However, as shown in Table 2, appreciable levels of water-soluble halite (NaCl) and sylvite (KCl) are present and these could be relatively easily extracted by water washing and recovered by solar evaporation for recycling to the dross smelting furnace. Reasonably high levels of aluminium oxide, bayerite, corundum, and gibbsite were present. Whilst corundum is not soluble in strongly alkaline solution, some of the other phases are, and these could potentially be recovered for feeding to a Bayer process plant (Davies et al., 2008). With respect to magnesium, data from Table 1 indicate one of the salt cake samples contained 2.59% Mg. No magnesium-bearing phases were identified by XRD, but in general magnesium in salt cakes has been known to be present as oxide phases or alloyed with aluminium. On the basis of the potential recoverable products outlined above, it seemed that it was worth reviewing the various methods suggested for recovering these and other compounds from salt cakes and this is done below.
3
In this circuit, cold salt cake from the melting furnace is first crushed in closed circuit with a double deck screen fitted with 20 mm and 10 mm aperture screens. The screen sizes here are nominal and may be varied to suit the particular salt cake feed (it is proposed that material less than 10 mm may be a suitable size to wet mill for salt cake). Any + 20 mm material is mostly aluminium metal with some entrained salt cake and can be recycled to the remelting furnace. The −20 + 10 mm material is returned to the crusher for crushing to −10 mm. The −10 mm material is then wet ground in an open circuit tube mill with the aim of flattening any metallic aluminium particles present, so that they are more readily screened out, and grinding the non-metallic particles, which are relatively brittle, to a fine size. Simultaneously the water-soluble particles are dissolved. The ground
3. Recovery of sodium and potassium chlorides Recovery of sodium and potassium chlorides from salt cakes for recycling back to the smelting process could be economically advantageous and would reduce the amount to be disposed of in toxic waste dumps. Bruckard and Woodcock (2007) have shown that simple water washing of −2 mm crushed salt cake lowers the level of these chlorides to below the level of detection by XRD. Whilst the chlorides are efficiently removed by water washing, the process does not remove all of the sodium and potassium because these elements are also present in other, non-water-soluble compounds. No other watersoluble compounds are known to be present. It should then be possible to recover the pure dissolved salts by solar evaporation as practised commercially in many operations throughout the world (e.g., Gilbert (1993) and Garrett (2001)). The solids residue from the water leach stage may be suitable for disposal in a non-toxic dump, depending on the absolute levels of elements such as fluorine in the residue. Appropriate TCLP tests would need to be conducted to validate this. 4. Recovery of metallic aluminium Methods that can be used for the recovery or extraction of metallic aluminium from a salt cake include screening, flotation, electrostatic separation, and dissolution in strong alkali. These methods are discussed briefly in turn below. 4.1. Aluminium metal recovery by screening As shown in Table 3, aluminium salt cakes contain metallic aluminium pellets ranging in size up to at least 2 cm in diameter and down to an unknown, but very fine size. Because aluminium is a soft, malleable metal it cannot be crushed or ground to a finer size. Fortunately, the bulk of the salt cake itself is brittle and is readily crushed and ground to liberate the metal so that most of it can be recovered by screening. The sodium and potassium chlorides present are readily soluble in water and their dissolution can also help liberate metallic aluminium. A circuit to accomplish this is proposed by the authors and is shown in Fig. 1. The flowsheet is a good example of recovering two products in one unit operation.
Fig. 1. Proposed flow sheet for recovering valuable materials from aluminium salt cakes.
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product is fed to a DSM (Dutch State Mines) screen having 150 μm wide slots. According to Pryor (1965) this is about the finest size that can be used commercially for wet screening. The +150 μm material is mainly aluminium metal and can be dried and sent back to the melting furnace. The − 150 μm pulp goes to a salt recovery circuit where solar evaporation is used to recover the salt as discussed later in this paper. It should also be noted that the grinding behaviour of aluminium metal will depend on its content of alloying elements. It is likely, for example, that an aluminium-silicon alloy would be more brittle in comparison to pure aluminium. The grinding behaviour of the metallics would also be influenced by the time of grinding and the type of grinding device used. 4.2. Aluminium metal recovery by flotation Only a few metals occur in nature in metallic form. These include gold, silver, platinum, and more rarely mercury and copper. Metallic aluminium does not occur in nature and so no work has been published on its flotation from ores. Furthermore, no work has been published on its flotation from salt cakes. However, Soto and Toguri (1986) and Bruckard and Woodcock (2004) conducted work on floating aluminium from aluminium dross. Soto and Toguri used a Hallimond tube initially, and subsequently a Wemco laboratory cell. Bruckard and Woodcock used a 3 L Denver laboratory flotation cell and obtained similar results to those of Soto and Toguri. It was found that metallic aluminium did not float with xanthatetype collectors alone, but when copper sulphate was added the aluminium was activated and floated readily. The activation process in this instance was in fact copper cementation on the aluminium metal surfaces. Best results were obtained on particles in the size range of 110–275 μm, and good recoveries in good grade concentrates were achieved. These results need to be confirmed on salt cakes, but it seems reasonable to expect that they would be. Coarse aluminium that does not float could be recovered by screening, and, as noted previously, dissolved halite and sylvite could be recovered by solar evaporation. 4.3. Aluminium metal recovery by electrostatic separation Electrostatic recovery of metallic aluminium from dross has been studied by Mah et al. (1986) who developed a circuit which involved first crushing the dross to −840 μm. They achieved a recovery of 70% of the metallic aluminium at a concentrate grade of 70% Al. However, dross usually contains much more aluminium than salt cake and at a much greater size, partly because it also often contains scrap aluminium, so it was relatively easy to achieve a good result. It remains to be shown if a similar result could be obtained on salt cake, but it could be worthwhile investigating this possibility. 4.4. Aluminium metal extraction by aqueous acid or alkaline leaching As Habashi (1997) has pointed out, aluminium metal is amphoteric and can dissolve in acid solutions (Eq. (1)) or alkaline solutions (Eq. (2)) with the generation of complex aluminium-containing ions and the generation of hydrogen gas. þ
2Al þ 6H3 O þ 6H2 O→2½AlðOHÞ6 −
3þ
−
þ 9H2
2Al þ 2OH þ 6H2 O→2½AlðOHÞ4 þ 3H2
ð1Þ ð2Þ
Either acid or alkaline leaching would also dissolve many of the other oxide aluminium compounds present in salt cakes. Recovery of aluminium in metallic form from acid solutions is not easy, whereas with alkaline leaching it may be that the solution is suitable for feeding directly to a Bayer-type process plant. This might be viable,
as discussed later, and could be worth further investigation. It would be necessary, of course, to first dissolve and remove the chlorides and some other elements before conducting an alkaline leach because these elements are not wanted in Bayer-type solutions. 5. Recovery of oxide aluminium compounds Davies et al. (2008) studied the treatment of salt cakes by Bayertype leaching and found that about 43% of the total aluminium present was dissolved in a typical two-stage process with a leach at 100 °C in the first stage and 140 °C in the second stage. Evidently, much of the aluminium is present as phases which are not soluble in Bayer-type solutions. This was examined in detail by Davies et al. (2008), and are not discussed here. Although the amount extracted may be worth recovering in a combined process as shown in Fig. 1, it is also possible that the tonnage may be too small to be of interest to bauxite mining and treatment companies, which commonly treat several million tonnes per year. 6. Hydrogen gas recovery It may be possible to produce hydrogen gas from salt cake using a complex process analogous to that developed by Owada et al. (2008) for producing “green hydrogen” from aluminium dross. They proposed that hydrogen could be generated from the reaction of metallic aluminium and an alkaline aqueous solution according to Eq. (3). 2Al þ 6H2 O→2AlðOHÞ3 þ 3H2
ð3Þ
The hydrogen could be used as a fuel and/or feed to fuel cells and the aluminium hydroxide sold in the market. Furthermore, the reaction is exothermic and the heat of reaction could be utilized for drying the aluminium hydroxide product. Whether or not such a process could be economic is unknown. Certainly there are numerous methods currently available for commercial hydrogen production (Anon, 1995), e.g. steam reforming of natural gas. Moreover, as shown elsewhere in this paper, when salt cakes are mixed with water other gases are given off and these may contaminate any hydrogen produced. 7. Decomposition of reactive compounds in salt cakes Various compounds in salt cakes react with water in the atmosphere, and especially in aqueous pulps, to give off ammonia and other noxious gases. These gases need to be properly vented from any treatment plant. Only one venting point is shown for example in Fig. 1, but a number may be required in practice. Aluminium nitrides react with water according to Eqs. (4) and (5) to give either ammonium hydroxide or ammonia. Eq. (4) is more likely to occur in more alkaline solutions (say above pH 8) and Eq. (5) is more likely to occur below pH 8. AlN þ 4H2 O→AlðOHÞ3 þ NH4 OH
ð4Þ
AlN þ 3H2 O→AlðOHÞ3 þ NH3
ð5Þ
Aluminium oxide nitride (Al5O6N) reacts with water according to Eq. (6) to give aluminium oxide and ammonia. 2Al5 O6 N þ 3H2 O→5Al2 O3 þ 2NH3
ð6Þ
Aluminium carbide reacts with water according to Eq. (7) to give aluminium oxide and methane gas. Al4 C3 þ 6H2 O→2Al2 O3 þ 3CH4
ð7Þ
W.J. Bruckard, J.T. Woodcock / Int. J. Miner. Process. 93 (2009) 1–5
Beckman (1991) has reported that gaseous sulphides and phosphides can be generated from some German salt cakes, and these would need special care in their removal. Based on the ratio of aluminium nitride to metallic aluminium in the salt cakes considered in this study, one would expect about 5 times more ammonia to be generated via Eqs. (4) to (7), than hydrogen via Eq. (3). 8. Conclusion Methods proposed and tested for recovering valuable materials from salt cakes, waste products derived from the treatment of aluminium dross, are presented and discussed. Whilst the chemical and physical complexity of salt cakes make re-processing or recycling challenging, many workers have shown that valuable components can be extracted in simple flowsheets which have both technical and possibly economic merit. Coarse metallic aluminium can be recovered by crushing and screening, intermediate size aluminium can be recovered by grinding and screening or electrostatic separation, and fine aluminium can be recovered by flotation. Recovery of aluminium metal by aqueous acid or alkaline leaching has also been proposed. Sodium and potassium chlorides present can be extracted by cold water leaching and recovered by solar evaporation after removing the insoluble components by thickening and filtration. Some of the oxide aluminium components are soluble in Bayertype leach solutions and could be sent to a Bayer-type leach plant to extract alumina. Alternatively, since the water-insoluble phases are inert, they could possibly be disposed of in a non-toxic dump, given they pass appropriate TCLP testing criteria. Hydrogen gas has also been generated from salt cakes, but, to date, only in laboratory studies. It seems possible that integrated flowsheets could be developed whereby one or more of the above valuable components are recovered, assisting positively with the process economics, and helping to close the loop in terms of overall waste disposal. Acknowledgements The Analytical Services and XRD Services Groups of CSIRO Minerals in Melbourne and Perth are thanked for conducting the many chemical analyses and XRD scans required in this work.
5
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