Preparation of Nanoscale Magnesium Oxide from Dross of Magnesium Scrap

by Hsueh, Shu-pei

Abstract (Summary)
The purpose of this study was to recycle the Mg metal from the dross of magnesium alloy refining furnace (magnesium scrap, particle size < 0.85 mm) and convert it to nanoscale MgO. The recycled product further was evaluated for its removal efficiency in treating chlorobenzene in aqueous solution. First, the magnesium scrap was leached by 2N HNO3 using a liquid-to-solid ratio of 50 (mL/g). After two hours of leaching, 99.6 wt% of magnesium was found to be dissolved and the weight decrease of magnesium scrap was 97.31 wt%. Then, the liquid-liquid extraction technology was used to purify the Mg2+ in leached solution. The extractant (10 vol.% of D2EHPA and 5 vol.% of TBP in 85 vol.% of kerosene; the oil-to-aqua ratio of 1:1) was used to extract with leached solution at pH=8 for 10 min, and the above-mentioned procedure of extraction was repeated once. As a result, the impurity ions (e.g., Zn2+, Ca2+ and Mn2+) were found to be extracted to the organic phase, whereas Mg2+ was found to be remained in the aqueous phase. This phenomenon was good for the later synthesis process of nanoscale MgO. The loaded organic phase then was stripped by 6N H2SO4 for the purpose of the organic phase reclamation (i.e., extractant regeneration). The experimental data have shown that the percent differences of the regenerated extractant and fresh extractant in extracting Zn2+, Ca2+, and Mn2+ were 0.01%, <0.01%, and 0.01%, respectively. Therefore, the regenerated extractant could be reused in the original extraction process. The aqueous phase after extraction then was adjusted to pH=6.5 using ammonium solution. Moreover, FeCl3•6H2O with a Fe/P molar ratio of 4.75 was added to the aqueous phase resulting in the formation of phosphate precipitate. In so doing, 99% of phosphate ion was removed. By adjusting the system pH to a value of no less than 6, 99.8% of ferric ion remaining in the aqueous phase was further removed. The filtrate then was used as the source of Mg2+ for synthesis of nanoscale MgO. After the addition of urea and water to the solution containing Mg2+, simply increased the temperature would form the precursor of nanoscale MgO. Under the synthesis condictions of pH 9.2, reaction temperature of 125 ¢J, reaction time of 2 hr, urea-to-Mg ion molar ratio of 20, and [H2O]/[Mg2+] molar ratio of 70, 61.26 wt% of Mg metal was recovered through the use of homogeneous precipitation. The precursor of nanoscale MgO was identified as farringtonite. This species could be converted to nanoscale MgO by calcining at 450 ¢J. The remaining Mg2+ in the supernatant of the above process could be further recovered by titrating with 6N NaOH to yiled Mg(OH)2 precipitate. Again, Mg(OH)2 could be convered to nanoscale MgO by calcining at 450 ¢J. The recovery of Mg metal by the neutral precipitation was determined to be 14.59 wt%. Through a series of treatments (including leaching, liquid-liquid extraction, phosphate ion removal, homogeneous precipitation and neutral precipitation), the overall recovery of Mg metal was found to be 75.85 wt%. Aside from the preparation of nanoscale MgO, MgO further was tested for its capability in treating chlorobenzene in aqueous solution with a dosage of 10 mg/L. After a reaction time of 4 hr, it was found that nanoscale MgO resulting from the homogeneous precipitation process outperformed the commercial MgO in treating chlorobenzene. For nanoscale MgO, however, even a dosage of 23.3 g/L or greater, the removal efficiency of chlorobenzene tended to be stable in the neighborhood of 20%.
Bibliographical Information:

Advisor:Gordon C. C. Yang; Shaw-bing Wen; Tyng-bin Onlin

School:National Sun Yat-Sen University

School Location:China - Taiwan

Source Type:Master's Thesis



Date of Publication:02/12/2006

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