Purification of solid from secondary aluminum smelting dross

In Mexico alone, as much as 10,000 tons of dross is annually available. The secondary aluminum dross generally requires specially designed disposal facilities, as this material is considered toxic by environmental legislation in many countries. As it can be inferred, the dross treatment is important for two main reasons: the necessity of recovering aluminum from the dross, and the possibility to transform some of the dross components in by-products for industrial use, avoiding the necessity of building disposal facilities. Secondary aluminum dross, which has been exposed for several years to atmospheric conditions, was characterized. Chemical analysis of the dross was performed by using atomic absorption spectrophotometry, atomic emission spectrometry, and gravimetric methods. Chemical analysis of the as-received dross showed that the main components were alumina (34.7 wt%), aluminum (17.7 wt%), and silica (5.98 wt%). However, Fe (1.35 wt%), K (1.53 wt%), and Na (1.75 wt%) contents were at levels that could limit its use as a raw material in high temperature applications. The alumina content in the original dross was similar to that in kaolin or hallocite. Since dross usually contains amounts of water-soluble salts, the material was washed in boiling distilled water for two hours. After the water washing process, the calcium (0.21 wt%), sodium (0.10 wt%), and potassium (0.11 wt%) contents decreased considerably, indicating that some water-soluble salts were dissolved. Some of the metallic aluminum trapped in the dross was separated and bar-milled for five minutes. The aluminum particles were re-melted and an alloy with an aluminum content of 96 wt% was obtained. About 10 wt% of metallic aluminum was recovered from the dross after performing this process.
In order to recover the remaining aluminum and to reduce the content of iron and alkalis of the dross, lixiviation experiments were performed. The effect of three different lixiviation solutions was evaluated: sulphuric, nitric, and hydrochloric acid solutions at the following experimental conditions: 80°C, 180 min., and 20% solids. After the test, the solids were washed with distilled water and dried at 100°C.
In general, aluminum was dissolved in these three acid solutions, but alkalis and iron were removed more efficiently from the dross by using the HCl lixiviating solution. The alumina content increased considerably after lixiviation in HCl leading to a solid product containing 68 wt% of alumina. The content of metallic aluminum was reduced to 5.40 wt%. The SEM analysis of the washed dross showed that an alumina layer covered most of the metallic aluminum particles (14 mm to 36 mm.). This layer has an effect on the lixiviation rate and efficiency. The XRD diffraction pattern corresponding to the solid products after lixiviation with HCl is shown in the figure. The main components detected were MgAl2O4, SiO2, Al2O3, AlN, Si, and Na2Al22O34·2H2O.
The alumina content in the dross after the lixiviation process was similar to that in sillimanite mineral. Addition of silica to form mullite during the sintering process can be easily performed. Another potential application is the use of the refined dross as a reinforcing material in the manufacture of aluminum-dross composites.
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