Article pubs.acs.org/IECR
Dissolution of Cr2O3(s) and the Behavior of Chromium in Concentrated NaOH Solutions Barbara Zydorczak, Peter M. May, Danielle P. Meyrick,* David Bátka, and Glenn Hefter School of Chemical and Mathematical Sciences, Murdoch University, Murdoch, WA 6150, Australia ABSTRACT: The solubility of Cr2O3(s) in (1 and 6) mol kg−1 sodium hydroxide solutions has been investigated at 25 °C. Even after 75 days of solid/solution contact the apparent solubility continued to increase slowly. Analysis of the solution phase by UV−vis spectrophotometry revealed that all of the dissolved chromium was present as Cr(VI), in the form of CrO42−(aq); no Cr(III) could be detected. Consistent with this finding, Cr(III) dissolved in 6 mol kg−1 NaOH was found to be oxidized to CrO42−(aq) over of period of days. No significant changes to the solid phase, Cr2O3(cr), were found, either by X-ray diffraction or by X-ray photoelectron spectroscopy, as a result of its contact with NaOH(aq). Both CrO42− and Cr(III) were found to be unreactive toward a range of organic and inorganic compounds in 6 mol kg−1 NaOH over many days at ambient temperatures. However, CrO42− catalyzed the decomposition of L-ascorbate, and was rapidly reduced by elemental Zn to Cr(III) in NaOH(aq).
1. INTRODUCTION The Bayer process involves the digestion of bauxitic ores with hot concentrated sodium hydroxide solutions producing supersaturated solutions of sodium aluminate in excess caustic soda. Purified aluminum hydroxide (gibbsite) is then recovered by seeded precipitation at lower temperatures, after which the spent mother liquor is recycled back into the process stream.1 Although this process is well established globally, Bayer plants still face many common operational challenges. One of these is the deportment of trace elements, some of which have significant environmental and/or occupational health implications. Such elements enter the process solutions via the bauxite ore in which they occur naturally, either as small amounts of specific minerals or as impurities in other components. Whereas trace element concentrations in the ores may be very low, such elements can build up over time due to the cyclical nature of the Bayer process. Some elements (e.g., chromium) may also find their way into the process solutions by chemical attack of the hot caustic solutions on plant construction materials such as stainless steel. The behavior of trace elements in Bayer liquors is likely to be complex for a number of reasons. These include the high caustic content, the very high ionic strength, and the presence of a myriad of other species, both inorganic and organic, at widely varying concentrations. It is, therefore, useful to investigate the behavior of such elements in synthetic solutions under welldefined laboratory conditions. Of the many trace elements typically present in Bayer plants, chromium is of particular interest even though its concentration in process solutions is generally thought to be low (
