Trace metal partitioning in caustic calcined magnesia produced from natural magnesite

Abstract

Caustic calcined magnesia from natural magnesite has been widely employed as a source of magnesium. This mineral, depending on the origin, may contain heavy metals and metalloids that can exceed the regulatory limits in some applications. In most cases, heavy metals and metalloids form solid solutions with the mineral phases of the main impurities, or even magnesium oxide itself, replacing other ions in the crystal lattice. Compared with magnesium oxide, most of these impurities such as silica and silicates are much more chemically stable even in concentrated mineral acids under normal temperature and pressure conditions. In this study, the partitioning of the trace metals was monitored using a sequential extraction procedure (SEP), and their potential solubility was determined using the pH-static leaching test. Only a small fraction of magnesium oxide derived from heavily calcined magnesia is soluble in slightly acidic media. The release of the trace metals and metalloids contained in the soluble fractions was less than 40% as determined by total digestion. It can be concluded that SEP is more accurate than total chemical digestion for setting the maximum limits of the undesirable trace metals.

Keywords:

• Magnesium oxide
• magnesia
• mineral foodstuffs
• trace metals
• metal partitioning.

Acknowledgments

The authors would like to thank Magnesitas Navarras S.A. for its cooperation in financing and supporting the work. The authors also express their thanks to Mrs. Judith Gómez and the Scientific and Technical Services of the University of Barcelona for providing analytical assistance.