Article pubs.acs.org/EF
Acidic Peroxidation of Brazilian Coal: Desulfurization and Estimation of the Forms of Sulfur Vera L. V. Fallavena,* Taísi D. Inácio, Cristiane S. de Abreu, Carla M. N. Azevedo, and Marçal Pires Faculty of Chemistry, Pontifical Catholic University of Rio Grande do Sul, Av. Ipiranga, 6681, Partenon, Porto Alegre − RS, Brazil 90.619-900 ABSTRACT: Existing standard test methods for determining forms of sulfur take a long time, consume a significant amount of reagents, and generate waste. Faced with this problem, an alternative screening method is proposed using hydrogen peroxide in acid medium for the desulfurization of coal and estimation of forms of sulfur in Brazilian raw coal. The detailed characterization of the extracts and the solids was performed at different reaction times. Hydrogen peroxide solutions at different concentrations (3, 10, and 30%) were tested in the presence and absence of the HCl or HNO3 (0.1 mol L−1). Best results were obtained using 10% H2O2/0.1 mol L−1 HCl with removals of 88−98% of pyritic sulfur and 3.5−18% of ash present in the coal samples. The concentration of total dissolved iron measured in extracts approaches the maximum concentration of iron expected for total oxidation of pyrite to sulfate at the end of reaction (240 min). Thus, it can be used to estimate the content of pyritic sulfur, with maximum percent error of 12%. Peroxidation also causes the dissolution of sulfate minerals, which together with sulfate from the oxidation of pyritic sulfur allows estimating the sum of these forms of sulfur (pyrite + sulfate) from the sulfate measured in the leaching extracts with errors 10%) and mild acid medium (0.1−0.5 mol L−1). However, no study has evaluated both dissolved iron and sulfate ions during the peroxidation process that would allow faster and easy estimation of the efficiency of desulfurization. Moreover, the estimate of the forms of sulfur in the coal, using these data, has not been evaluated. Furthermore, in our knowledge the chemical desulfurization of Brazilian coal was not yet studied. The aim of this study was to evaluate the behavior of Brazilian coals during peroxidation in acid medium. To do this, samples of raw coals from Brazil’s main mines and coal Standard Reference Materials (SRM), selected based on their sulfur contents, were evaluated. Concentrations of both dissolved iron and sulfate were monitored in the leaching and used to estimate the forms of sulfur. The optimized procedure was tested on different types of coal and their advantages and limitations are also discussed.
Nitric acid primarily reacts with pyritic sulfur and oxidizes it to water-soluble sulfates and nitrates in addition to nitrogen monoxide.6,19 3FeS2 + 18HNO3 → Fe2(SO4 )3 + Fe(NO3)3 + 3H2SO4 + 15NO + 6H2O
(5)
2FeS2 + 10HNO3 → Fe2(SO4 )3 + H2SO4 + 10NO + 4H2O
(6)
The mineral acids alone have low sulfur removal efficiency when compared to peroxidation. Due to low acid concentrations used in most studies of peroxidation the reaction of the sulfur compounds directly with the mineral acids are less likely. Several authors have evaluated the desulfurization of coal in oxidative media using acid media/hydrogen peroxide. Smith17 evaluated the desulfurization of the American coals using sulfuric acid and/or H2O2. By varying the concentration of H2SO4 (0.05−0.15 mol L−1) and H2O2 (7−17%), he concluded that peroxide concentrations above 9% showed no significant differences in the removal of pyrite. The best results were obtained using 10% H2O2/0.15 mol L−1 H2SO4 with 91% removal of pyritic sulfur and only 7% of organic sulfur after 420 min of reaction. Boron et al.20 investigated the peroxidation (30% H2O2) in American coal sample obtaining removal of 97% for pyritic sulfur, 8% of organic sulfur, and 60% of the total sulfur, after 384 h. Later, Boron et al.21 investigated acidic peroxidation followed by treatment with H3PO4 at reflux. According to the authors, the organic sulfur in coal might be first oxidized to sulfonic acids and then removed through known desulfonation methods. In these conditions, removals >90% and up to 32% of the pyritic sulfur and organic sulfur were obtained, respectively. A synergistic effect was observed by Vasilakos and Clinton1 in the chemical beneficiation of coal with the combination of H2O2/H2SO4. Almost complete removal of the sulfate and pyritic sulfur was observed with the use of 15% H2O2/0.05 mol L−1 H2SO4. The authors verified that H2SO4 concentrations above an optimal value have an adverse effect on pyritic sulfur oxidation by H2O2. The possible explanation of this effect is that sulfate ions adsorb on the surface of coal particles and thus impede the peroxide reaction as observed by Antonijević et al.16 for the leaching of pyrite with H2O2/H2SO4. Karaca and Ceylan3 studied the chemical cleaning of two Turkish lignites by leaching with aqueous hidrogen peroxide in sulfuric acid medium. High removals for pyritic sulfur (>92%) and sulfate sulfur (>80%), and low organic sulfur reduction (
