Commercial chlorine dioxide generators are operated in an about 4.5-mol/L sulfuric acid solution. The chlorous acid disproportionation may be an important pathway to generate chlorine dioxide under such a condition. On the basis of the product determination, we found that, in the absence of a chloride ion, chloric acid, hypochlorous acid, and chlorine, rather than chlorine dioxide, are the products from the chlorous acid disproportionation. However, chloride addition to the chlorous acid solution under otherwise the same condition results in the generation of chlorine dioxide. The underlying mechanism of the chloride effect on the chlorous acid disproportionation is discussed.

An alkaline extraction method has been developed for the determination of total sulfate in mine tailings containing secondary sulfate minerals formed by the oxidation of primary sulfides. Oxidized tailings were extracted with a 0.3 M NaOH solution at a liquid/solid ratio of 30 at room temperature for 16 h. The sulfate concentration in the extracts was determined by ion chromatography (IC). The coefficient of variation for sulfate determinations ranged from 1.9 to 3.2% for five tailings samples collected at two tailings impoundments. Mineralogical analysis of the tailings by scanning electron microscopy/X-ray energy dispersive spectrometry (SEM/EDS) demonstrated that the extraction of sulfate was complete, with the exception of extremely insoluble barite. The proposed method is simple, yields an accurate yet rapid measurement of sulfate, and involves a safer laboratory operation than conventional methods that make use of strong HCl acid solutions. Moreover, this method allows the specific measurement of sulfate in the extract, whereas conventional methods are generally limited to the measurement of total S by inductively coupled plasma atomic emission spectrometry (ICP-AES) due to the interference of chloride with sulfate in IC.

The chlorous acid reaction with hypochlorous acid (chlorine) may be responsible for the chlorine dioxide formation in a methanol based chlorine dioxide generation process. In this paper, the reaction between chlorous acid and hypochlorous acid in a 4.5 mol/L sulfuric acid solution was studied. It was found that chlorine dioxide is generated only if chloride is present. The underlying mechanism associated with the effect of chloride on the reaction was discussed.

The generation of chlorine dioxide from the reaction between hypochlorous acid and chlorite with or without an initial chloride addition has been studied under slightly acidic conditions. Chloride (Cl-), one of the pfoducts from the reaction, not only changes the reaction stoichiometry, but also alters the rate law. It was found that the formation of chlorine dioxide from the HOCI-CD-system consists of two distinct parts, one is promoted by chloride, the other is independent of chloride. The overall kinetics of the chlorine dioxide generation from the reaction is: La production de bioxyde de chlore a partir de la reaction entre I'acide hypochloreux et la chlorite avec ou sanc ajout initial de chlorure a ete etudiee dans des conditions Iegerement acides. Le chlorure (Ct), I'un des produits de la reaction, non seulement modifie la stoechiometrie de la reaction, mais altere egalement la loi de vitesse. On a trouve que la formation de bioxyde de chlore avec le systeme HOCI-CI0;comprend deux parties distinctes, l'une due au chlorure, l'autre etant independante du chlorure. La cinetique globale de la production de bioxyde de chlore a partir de la reaction est:Ce modele peut parfaitement predire la reaction dans les conditions suivantes: 273-303K, [CIO,-]

Flooding partially oxidized mine tailings for the purpose of mitigating further oxidation of sulfide minerals and generation of acid drainage is generally preceded by treatment with alkaline amendments to prevent releasing previously accumulated acidity to the water cover. This work compares the ability of calcite (CaCO 3 ) and quicklime (CaO), two common amendments, to establish and maintain pH conditions and dissolved metal concentrations within environmentally acceptable ranges over long time periods. Although higher initial pH values were obtained with quicklime, the pH of quicklime treated tailings decreased over time. This was attributed to the low buffering capacity of quicklime treated tailings and to the consumption of hydroxide ions by incongruent dissolution of water-insoluble iron oxyhydroxysulfate minerals. In contrast, the pH of tailings treated with calcite increased initially and then remained stable at pH ≈ 6.7. This pH behavior was due to the lower reactivity of iron oxyhydroxysulfates with calcite, the increased buffering capacity provided by bicarbonate ions, and the incomplete dissolution of calcite. Overall, calcite was found preferable to quicklime for maintaining long-term neutral pH conditions in the treated tailings. With the exception of zinc, acceptable dissolved metal concentrations were achieved with calcite treated tailings.