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Thermodynamic Properties of Chromium Bearing Slags and Minerals

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Thermodynamic Properties of Chromium Bearing Slags and Minerals tKK-V-- SUP Helsinki University of Technology Faculty of Process Engineering and Materials Science Department of Materials Science and Rock Engineering Laboratory of Metallurgy Thermodynamic properties of chromium bearing slags and minerals Research programme: SULA II Research project: Chemistry of Chromium Bearing Processes Key words: chromium oxides, slag chromite, oxidation state, activities, phase relations, chromium recovery, FeCr, stainless steel Vuorimiehentie 2 K FIN-02150 Espoo, Finland DismaunoN of im document is unlimited ISSN 0785-5168 ISBN 951-22-3170-0 2B ABSTRACT In this report, the thermodynamic properties of chromium bearing slags and minerals were reviewed based on the available information in the literature. It includes the ana ­ lysing methods for oxidation state of chromium in slags, oxidation state of chromium and activities of chromium oxides in slags and minerals. The phase diagrams of chro ­ mium oxide systems and chromium distributions between slag and metal phases are also covered in this review. Concerning the analysing methods, it was found that most of the available approaches are limited to iron free slag systems and the ‘sample preparation is very sensitive to the analysing results. In silicate slags under reducing atmosphere, divalent and trivalent chromium co-exist in the slags. It is agreed that the fraction of divalent chromium to total chromium increases with higher temperature, lower slag basicity and oxygen po ­ tential. For the slags under oxidising atmosphere, trivalent, pentavalent and hexavalent states were reported to be stable. The activities of CrO and CrO, 5 were concluded to have positive deviation from ideal solution. Slag basicity has a positive effect and tem­ perature has a negative effect on the activities of chromium oxides. The phase diagrams of the Cr-O, binary, and ternary chromium containing oxide sys­ tems have been examined systematically. The analysis shows that the data on the quar­ ternary and quinary systems are insufficient, and require further investigation. The most important features of the chromium containing silicate slags are the large miscibility gaps and the stability of the chromite spinel. The phase diagrams are affected signifi ­ cantly by system oxygen potential due to the different possible oxidation states of chromium. In addition, the factors influencing the chromium recovery in FeCr and stainless steel production processes were also inspected, including the effects of slag basicity and temperature as well as the compositions of the melt. The knowledge indi ­ cates that increasing slag basicity is very effective to increase the chromium recovery from slag. The recovery of chromium to the metal phase is enhanced by the presence of MgO and A1203 in the slag. TABLE OF CONTENTS 1 Introduction. 1 2 Oxidation states of chromium under different conditions ............................. ..2 2.1 Analysing method for chromium oxides in slags .................................. ..2 2.2 Oxidation state of chromium in slags................................................... ..4 2.2.1 Chromium containing slags in high oxygen partial pressure (air) ..5 2.2.2 Chromium containing slags in low oxygen partial pressure ........ ..5 2.3 Oxidation state of chromium in chromite solid solutions ..................... ..8 3 Activities of chromium oxides in slags......................................................... 11 3.1 Activities of chromium oxides in literature under different conditions 11 3.2 Comparison of activities of chromium oxides in slags and discussion. 16 3.3 Activities in chromite solid solution ..................................................... 18 4 Phase diagrams of chromium containing oxide systems.............................. 19 4.1 Cr-0 system......................................................................................... 19 4.2 Binary oxide systems........................................................................... 20 4.2.1 Fe-Cr-0 system........................................................................... 20 4.2.2 Ca-Cr-0 system............................................................................ 21 4.2.3 Mg-Cr-0 system........................................................................... 23 4.2.4 Al-Cr-0 system........................................................................... 23 4.2.5 Si-Cr-0 system............................................................................ 23 4.3 Ternary oxide systems......................................................................... 24 4.3.1 Si-Ca-Cr-0 system...................................................................... 24 4.3.2 Si-Mg-Cr-0 system..................................................................... 26 4.3.3 Si-Al-Cr-0 system....................................................................... 27 4.3.4 Ca-Mg-Cr-0 system.................................................................... 27 4.3.5 Ca-AI-Cr-0 system...................................................................... 28 4.3.6 Mg-Al-Cr-0 system.................................................................... 29 4.3.7 Fe-Mg-Cr-0 system.................................................................... 29 4.3.8 Fe-Si-Cr-0 system....................................................................... 30 4.3.9 Fe-Al-Cr-0 system...................................................................... 31 4.4 Quaternary oxide systems................................................................... 32 4.4.1 Ca-Mg-Si-Cr-0 system............................................................... 32 4.5 Quinary oxide systems......................................................................... 4.5.1 Ca-Mg-Al-Si-Cr-0 system.......................................................... 4.6 Chromite raw materials........................................................................ 5 Distribution of chromium between slag and metal...............................................................39 5.1 Distribution of chromium between slag and FeCr.......................................................39 5.2 Distribution of chromium between slag and metal (steel)............ '..............................42 6 Summary...............................................................................................................................45 7 References 47 DISCLAIMER Portions of this document may be Illegible in electronic image products. Images are produced from the best available original document 1 1 Introduction The steadily growing consumption of stainless steel in the world is the impact for increasing the production of both ferrochromium and stainless steel. Also the R&D work in these fields has been greatly stimulated at present. From economic point of view, the optimisation of chromium yield in the related smelting and refining processes in the FeCr and stainless steel production industries is very important. A successful operation in a large extent depends on the chromium yield in the metal phase, i.e., chromium distribution controlled by the reactions between slag and metal phases. High chromium recovery would be beneficial in saving raw materials and energy as well as in reducing eventual chromium pollution from chromium containing slags and wastes. Thermodynamic approach can quantitatively define the effects of the various process variables on the chromium distribution between the slag and the metal, and further on the chromium yield. To carry out an effective thermodynamic evaluation of chromium distributions, thermodynamic properties in the slag phase are needed. The knowl ­ edge of the thermodynamic properties of slags including activities of chromium oxides and its phase relations will definitely help to evaluate the equilibrium distribution, to promote ther ­ modynamic process modelling, and further to better understand the process to achieve a higher recovery of chromium from the minerals. Chromium, the fourth element in the first transition series of the periodic table, has five elec­ trons on the orbital 3d and one electron on 4s, thus exists in a variety of oxidation states. The behaviour of chromium oxides in metallurgical slags is, therefore, very complex due to the co-existence of multivalent chromium ions, the high melting point of chromium oxides con­ taining slags, the characteristics of chromium oxides volatilisation, and the sophisticated structures of chromite raw materials. Physically, Cr2+ colour is blue, Cr3+ colour is green, Cr6+ colour is yellow to red. These three chromium ions are identified to exist in the different raw materials under the different environmental conditions. In chromite minerals as well in py- rometallurgical processing, CrJ+ is the dominant valence. Cr203 is moderately stable with re­ spect to its constituent elements. For comparison, Cr203 is less readily reduced than FeO, CoO and NiO, but somewhat more readily reduced than MnO, and is more easily reduced than the very stable oxides Si02, A1203, CaO and MgO. Mainly because of the tremendous technological importance of iron and its compounds, the equilibrium thermodynamic rela­ tions pertaining to this element at high temperatures are well known. In contrast, quantitative data on the thermodynamics of chromium oxide in silicate melts are practically very few, and until recently phase-equilibrium data in systems containing chromium were available mostly at high oxygen pressures (air), under poorly defined reducing conditions or under certain lim­ ited experimental conditions,
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