Low carbon ferrochromium production from Eti krom concentrate via self propagating high temperature synthesis

The main usage area of chromium in metallurgy is ferrochromium, and ferrochromium covers %95 of the total chromium production. Ferrochromium, classified by chromium and carbon content. %90 of this ferrochromium is used as an alloying addition in stainless and heat-resistant steels. Although Turkey does not have large reserves of chromite, these ores are preferred to be used in metallurgical sector due to the fact that they have high quality. In generally, high carbon ferrochromium is producing by consuming 2700 kWh/t energy. In the case of the low carbon ferrochromium production the energy consumption can increase up to 8200 kWh/t. Instead of the electrical energy, aluminum scrap and powder are used for reduction and smelting of low carbon ferrochromium as the concept of the energy efficiency. Thus, energy consumption and pollution is decreased. In this study, production conditions of ferrochromium alloys via Self Propagating High Temperature Synthesis were investigated. Experiments were conducted in two stages including thermochemical investigations and SHS synthesis. Before Self Propagating High Temperature Synthesis thermochemical investigations were performed to estimate the possible product compositions, required mixtures of the raw materials, generated energy and maximum adiabatic temperatures of SHS reactions by using FactSageTM 6.3 thermochemical databases software. Thermodynamic calculations were made by using the advanced "Phase Diagram", "Reaction" and "Equilib" module of FactSageTM 6.3 with Fact, FS, SGTE, BINS databases. Binary and ternary phase stability diagrams of selected alloy groups were plotted by using "Phase Diagram" module with SGTE (Scientific Group Thermodata Europe) database and estimated alloy compositions were selected due to the solidification graphs of these alloy systems. Amount of the raw materials used to obtain the estimated alloy compositions were calculated and "Reaction" and "Equilib" modules of FactSageTM 6.3 were used to determine the reaction characteristics. The amount of possible products and their compositions, also generated energy and adiabatic temperature of reactions were calculated by using "Equilib" module utilizes the Gibbs Energy Minimization method. Self-propagating high temperature synthesis is a relatively novel and simple method for producing certain advanced ceramic, composites, metals and its alloys and intermetallic materials. Borides, carbides, nitrides, aluminides, silicides, hydrides, intermetallics, carbonitrides, cemented carbides, chalgogenides, binary compounds and composites have been synthesizing by this process. These materials can be applied in industry as resistive heating elements, powders for further processing, abrasives, cutting tools, polishing powders, shape memory alloys, steel processing additives, high temperature intermetallic compounds, electrodes for electrolysis of corrosive media. However metallothermic reactions were discovered by Beketov in 1865 and Goldschmidt in 1895 first gasless metallothermic combustion experiments were conducted by Merzhanov et al. in 1967. This process has received considerable attention as alternative to conventional furnace technology. When compared this process for synthesis ferrolalloys like conventional arc furnace process SHS method has advantages as following; simple operation, low cost, low energy requirement, high quality of product with low carbon. Specific heat is the main process parameter for Self Propagating High Temperature Synthesis reactions and it shows whether a Self Propagating High Temperature Synthesis reaction is self sustaining or not. Determining the heat evolved during the reaction and to estimate if the temperature achieved is sufficient to smelt the metal and the slag and to separate them due to the different density. Spesific heat is calculated by dividing the enthalpy of the reaction at 25 °C by the sum of the molecular weights of the reaction products. If specific heat is between 2250-4500 J/g Self Propagating High Temperature Synthesis reaction is available for the reactants. Under this range, the reaction will not occur and over these range violent reaction will occur and this reaction may even be explosive. In the present study, specific heat was calculated and it is between the value of 2250-4500 J/g. Another way to estimate the required reaction conditions of the process is to calculate the adiabatic temperature. It also shows whether the reaction is self-sustaining or not. It is suggested that reaction will self propagate when adiabatic temperature is higher than 1527 °C. Adiabatic temperature was calculated by using FactSage Thermodynamic Databases and it is between the value of 1850 – 3100 °C. Low-carbon ferrochromium production from chromite was aimed and Eti Krom A.Ş. concentrate containing % 47,16 Cr2O3, % 16,00 Fe2O3 , % 6,5 SiO2, % 10,63 Al2O3, % 0,32 CaO, % 19,05 MgO ; % 98 purity Al powder was used as a reductant and minimum % 99,7 CrO3 containing chromic acid was added into the charge. Grinded CrO3 and chromite concentrate were dried at 105 0C in an oven for about 30 minutes. Ratio of concentrate and chromic acid are fixed by using FactSageTM 6.3 Thermochemical Software data. Addition of Al used as reductant was calculated according to calculation mixture scale with using assay balance. Then base mixture was prepared in turbula mixer. The reaction mixtures were mixed thoroughly for 15 minutes in a turbula mixer and powder mixtures were charged into Cu crucible and compacted. W (tungsten) wire was placed at the top of copper crucible and the reaction was realized by passing current through the wire. After initiation, a highly exothermic reaction became self-sustaining and propagated throughout the metallothermic mixture. The obtained metallothermic products were discharged from the crucible after cooling. In the first series of experimental studies, effects of chromic acid were investigated. Although the stoichiometric ratio of Al was remained constant at % 100, the amount of Al was changed in the initial mixtures because of chromic acid and concentrate ratio variation. The highest chromium recovery was % 59,15 and achieved with using % 65 concentrate , % 35 chromic acid and % 100 Al powders. The next series of experimental studies, effects of initial mixtures amount was investigated. Chromium recovery, total metal recovery and scattered ratio were examined by increasing the the first experimental series initial mixtures three times more. Increasing the charge amount contributed positevely to the realization of reaction. The highest chromium recovery was achieved at the same concentrate-chromic acid ratio with the first experimental series. Chromium recovery increased % 64,19 by increasing the amount of charge. In the last experimental series, effects of different stoichiometric Al amounts were investigated, initial mixtures were prepared with neccessary stoichiometric Al amount between % 100-120 for reaction and none of flux additon was used in the initial mixtures. As stoichiometric aluminium amount increased in the initial mixture till % 110 Al, total metal recoveries increased and scattered ratios decreased considerably. On the other hand when the stoichiometric Al was % 120 adiabatic temperature was increased and scattered ratio was increased accordingly. The highest total metal recovery was % 93,76 and recovery of Cr metal was obtained % 66,46 by adding % 110 stoichiometric Al addition into the initial mixtures. Consequently, the study shows that production of low-carbon ferrochromium is possible by self propagating high temperature synthesis process. Optimum condition was detected in % 65 concentrate, % 35 CrO3 and stoichiometric % 110 Al mixture. In this experiment Cr recovery was calculated % 66,46 and metal recovery was % 93,76. Meanwhile carbon content of this experiment was % 0,25.