Innovative process solutions towards recycling of salt cake from secondary aluminum smelting

Abstract: To offer better solutions for the recycling of salt cake from secondary aluminum melting, several innovative investigations were carried out based on hydrometallurgical and pyrometallurgical views.Thermal diffusivity measurements as a function of temperature on salts-Al composites having various compositions (0, 2, 4, 6, 8, 10, 12wt pct metallic Al) were carried out. Its attempt to derive theoretical relationships between aluminium contents in the salt-Al composites and the thermal conductivities so that these would serve as calibration curves for industrial samples taken out from secondary aluminium re-melting at a later stage.The AlN hydrolysis behavior in NaCl solution was investigated by immersing pure AlN powder in deionized water, 0.3mol/l NaCl aq, 0.6mol/l NaCl aq and NaCl solution respectively with CO2 bubbling at 291K. The results showed that AlN powder underwent enhanced hydrolysis in NaCl aq than that in deionized water, while, the introduction of CO2 was found to hinder the hydrolysis even in the presence of NaCl. The characterization of the products after hydrolysis was carried out using XRD, SEM and TEM analyses. It was shown that the hydrolysis process included a slow-reaction period involving the dissolution of aluminum hydroxide layer around raw AlN particle, followed by the precipitation of aluminum hydroxide gel and the crystallization of boehmite, bayerite and gibbsite. The effects of sodium chloride concentration on the hydrolysis behavior are presented.The leaching process in CO2-saturated water showed that, at a solid to liquid ratio of 1:20 and 3h at 291K, the extraction of Na and K from the dross could be kept as high as 95.6% and 95.9%, respectively. At the same time, with continuous CO2 bubbling, the mass of generated NH3 during the leaching process decreased significantly, also the escaping NH3 gas decreased from 0.25mg in pure water down to <0.006mg. The above results showed that the introduction of CO2 causes hindrance to the hydrolysis of AlN, meanwhile, effective absorption of ammonia. The plausible mechanisms for the observed phenomena are discussed. The concept of the leaching of the salt cake by carbonated water and the consequent retention of AlN in the leach residue opens up a promising route towards an environment-friendly recycling process for the salt cake viz. recovery of the salts, utilization of CO2 and further processing of the dross residue, towards the synthesis of SiAlON from the leach residues.  In alternative route to the processing of salt cake, the ammonia gas evolved by hydrolysis of AlN was collected by CO2-saturated water during water leaching at 373K. The products, i.e. ammonium carbonates which are free of chlorides, has application as a fertilizer, besides that, this method also has the advantage of fixing carbon from CO2, which is the subject of many investigations around the world.The oxidation behavior of composites SiMgAlON phases (?-SiAlON, MgAlON and 15R) synthesized from the residue during the leaching treatment of salt cake and corresponding synthetic samples was investigated in air by thermogravimetric measurements. The oxidation studies reveal the effects of impurities, namely, Fe2O3 and CaO present in the salt cake residue. From the view of kinetics, the addition of Fe2O3 brings a lower activation energy and more aggressive oxidation. The additive of CaO caused the shrinkage during the synthesis and liquid formation during the oxidation above 1673K, thus retard the oxidation rate. The impurities of CaO and Fe2O3 in the leaching residue can result in an aggressive oxidation at low temperature and a protective oxidation at temperatures above the eutectic point. From the view of phase evolution, with the progress of oxidation, the composition of the material being oxidized moved towards the Al2O3-rich corner of MgO-Al2O3-SiO2 or CaO-MgO-Al2O3-SiO2 phase diagrams relevant to SiMgAlON composite. At lower temperatures, the addition of Fe2O3 and CaO facilitated the formation of cordierite and anorthite, respectively. With the increasing of temperature, islands of silicate melt were formed dissolving these oxides, with the liquidus temperature getting lowered as a consequence. The liquid phase formed engulfed the adjacent solid phases providing strong mobility for the cations and enabling the crystal growth. As a result, intermediate products, i.e. cordierite, anorthite, spinel, which formed earlier during oxidation are found to get dissolved in the liquid phase.