Ndue Kanari

Kinetics of reduction of iron oxides by H2: Part II. Low temperature reduction of magnetite

This study deals with the reduction of Fe3O4 by H2 in the temperature range of 210–950 °C. Two samples of Fe3O4 produced at 600 and 1200 °C, designated as Fe3O4(600) and Fe3O4(1200), have been used as starting material. Reduction of Fe3O4(600) by H2 is characterized by an apparent activation energy ‘Ea’ of 200, 71 and 44 kJ/mol at T 390 °C, respectively. The important change of Ea at 250 °C could be attributed to the removal of hydroxyl group and/or point defects of magnetite. This is confirmed during the reduction of Fe3O4(1200). While transition at T ≈ 390 °C is probably due to sintering of the reaction products as revealed by SEM. In situ X-rays diffraction reduction experiments confirm the formation of stoichiometric FeO between 390 and 570 °C. At higher temperatures, non-stoichiometric wustite is the intermediate product of the reduction of Fe3O4 to Fe. The physical and chemical modifications of the reduction products at about 400 °C, had been confirmed by the reduction of Fe3O4(600) by CO and that of Fe3O4(1200) by H2. A minimum reaction rate had been observed during the reduction of Fe3O4(1200) at about 760 °C. Mathematical modeling of experimental data suggests that the reaction rate is controlled by diffusion and SEM observations confirm the sintering of the reaction products. Finally, one may underline that the rate of reduction of Fe3O4 with H2 is systematically higher than that obtained by CO in the explored temperature range.

Thermal decomposition of zinc carbonate hydroxide

This study is devoted to the thermal decomposition of two zinc carbonate hydroxide samples up to 400 °C. Thermogravimetric analysis (TGA), boat experiments and differential scanning calorimetry (DSC) measurements were used to follow the decomposition reactions. The initial samples and the solid decomposition products were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) and laser particle size analyzer. Results showed that zinc carbonate hydroxide decomposition started at about 150 °C and the rate of decomposition became significant at temperatures higher than 200 °C. The apparent activation energies (Ea) in the temperature range 150–240 °C for these two samples were 132 and 153 kJ/mol. The XRD analyses of the intermediately decomposed samples and the DSC results up to 400 °C suggested a single-step decomposition of zinc carbonate hydroxide to zinc oxide with not much change in their overall morphologies.

A low temperature chlorination-volatilization process for the treatment of chalcopyrite concentrates

Abstract Chlorination of two chalcopyrite concentrates with Cl 2 +N 2 was investigated under isothermal conditions in the temperature range of 20–750°C using boat experiments. The effects of gas flow rate, chlorine content of the gas mixture and residence time on the reaction rate were also investigated. Scanning electron microscopy (SEM), X-ray diffraction (XRD), and chemical analysis are used for the physico-chemical characterization of the reaction products. The chlorination of chalcopyrite concentrates started at room temperature generating chlorides of Cu, Pb, Zn, Fe, and S. The reaction of chlorine with sulfides is almost complete at about 300°C and the overall reaction is exothermic. At this temperature, the valuable metal chlorides were concentrated in the chlorination residues, while those of iron and sulfur were volatilized. A flow sheet is proposed for the selective chlorination of chalcopyrite concentrates at low temperatures. Such flow sheet could be considered as an attractive route for the sulfide concentrates’ treatment without SO x emissions. It could be also regarded as energy saving with respect to the classical pyrometallurgical routes.

Kinetics of oxychlorination of chromite: Part II. Effect of reactive gases

Abstract The effects of Cl2/O2 ratio, P(Cl2+O2), PCl2, and PO2 on the oxychlorination rate of the chromite mineral were determined between 750 and 1000°C using isothermal TGA measurements. The effect of the gases’ composition on the oxychlorination rate of chromite were compared with those resulting from the oxychlorination of simple oxides of the chromite (Cr2O3, Fe2O3 and MgO). The apparent reaction orders with respect to Cl2+O2, Cl2, and O2 for the chromite oxychlorination at 750°C were about 0.94, 1.24, and −0.30, respectively. At 1000°C, apparent reaction orders with respect to the reactive gases changed significantly as the reaction progressed. Boat experiments were carried out to oxychlorinate a chromite concentrate between 600 and 1000°C. The reaction products were analyzed by SEM, XRD and chemical analysis. The oxychlorination of a chromite concentrate at about 800°C led to the partial elimination of iron increasing the Cr/Fe ratio in the treated concentrate. A part of chromium was also oxychlorinated and it was recovered as chromium oxychloride (CrO2Cl2).

Reactions of wüstite and hematite with different chlorinating agents

Chlorination of wustite (Fe(1−x)O) and hematite (Fe2O3) with Cl2 + CO and Cl2 + N2 was studied by thermogravimetric analysis using non-isothermal conditions up to about 1000°C. The wustite started to react with the carbochlorinating gas mixtures at low temperatures producing FeCl3 and Fe2O3 as final reaction products. The presence of carbon monoxide, during non-isothermal tests, enhanced the chlorination of wustite at temperatures higher than 550°C when the produced hematite started to react with carbochlorinating gas mixture. The separate treatment of the two oxides under isothermal conditions in Cl2 + CO for 2 h led to their full reaction at about 550°C. An apparent activation energy of about 53 kJ/mol was obtained for the carbochlorination of hematite between 350°C and 550°C. Reaction of wustite with FeCl3 was also studied by thermogravimetric analysis using non-isothermal conditions. Higher oxides of iron and ferrous chloride were the main reaction products at 600°C, even in the presence of carbon monoxide.

Use of chlorination for chromite upgrading

The chlorination of a chromite concentrate was studied between 600 and 1000°C. The reaction products were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), and chemical analyses. Chlorination of a chromite concentrate at about 700°C allowed the extraction of about 50% of the iron, thus doubling the Cr/Fe ratio in the treated concentrate. Aluminum chloride was tested as a chlorinating agent in the presence of a reducing atmosphere. The effect of temperature on the kinetics of chromite chlorination was studied between 900 and 1040°C using thermogravimetric analysis (TGA). Temperature effects changed significantly with reaction extent. The initial stage of the chlorination was characterized by an apparent activation energy of about 112 kJ/mol, while a value of about of 269 kJ/mol was found for reaction extents greater than 0.4.

Kinetics of chlorination and carbochlorination of lead sulfate

The kinetics of chlorination and carbochlorination of PbSO4 with Cl2 + N2 and Cl2 + CO + N2 gas mixtures has been studied using thermogravimetric measurements in the range 700–900°C. The chlorination reaction rate of PbSO4 with Cl2 + N2 increases with rise in the chlorine content in the gas mixture. The reaction order is about 0.66 with respect to chlorine. The chlorination rate of PbSO4 is controlled by a chemical reaction mechanism with an apparent activation energy of about 174 kJ/mol. In the same temperature range, the apparent activation energy of the carbochlorination of lead sulfate by Cl2 + CO + N2 gas mixture is about 114 kJ/mol. The reaction order is about 0.72 with respect to Cl2 + CO. The maximum reaction rate is obtained by using a carbochlorinating gas mixture having a Cl2(Cl2 + CO) ratio equal to about 0.6.

Synthesis of potassium ferrate using residual ferrous sulfate as iron bearing material

This paper summarizes the results obtained during potassium ferrate (K2FeVIO4) synthesis which is a high added value material. This compound that contains iron in the rare hexavalent state is becoming a substance of growing importance for the water and effluent treatment industries. This is due to its multi-functional nature (oxidation, flocculation, elimination of heavy metals, decomposition of organic matter, etc.). The most well known synthesis methods for potassium ferrate synthesis are those involving the chemical and/or electrochemical oxidation of iron (II) and (III) from aqueous solutions having a high alkali concentration. These methods are generally characterized by a low FeVI efficiency due to the reaction of the potassium ferrate with water, leading to the reduction of FeVI into FeIII. Concerning the work pertinent to this paper, the synthesis of K2FeVIO4 was achieved by a simultaneous reaction of two solids (iron sulfate and KOH) and one gaseous oxidant (chlorine). The synthesis process is performed in a rotary reactor at room temperature and the global synthesis reaction is exothermic. The effects of different experimental parameters on the potassium ferrate synthesis are investigated to determine the optimal conditions for the process.