PHASE EQUILIBRIUM STUDY OF THE MGO–CAO–SIO2 SLAG SYSTEM WITH FERRONICKEL METAL AND SOLID CARBON AT TEMPERATURES 1350-1550°C

• Main Author: Khusnaini P P, Nandhya
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/84853
• Institution: Institut Teknologi Bandung

The commonly used technology to produce nickel alloys from laterite ores through pyrometallurgical route is RKEF. The widespread use of RKEF in Indonesia shows the importance of this technology, especially since there is no other comparable alternative for processing laterite nickel ores on a large scale. One of the important stages in RKEF technology is melting in an electric furnace at a temperature of around 1450–1550ºC. In the area around the carbon electrode of the electric furnace, an equilibrium can be formed between the slag phase, ferronickel metal droplets, and carbon solid content. Metals with high carbon and silica content can form in this area and can cause Si reversion and CO boil phenomena in other areas of the electric furnace. Therefore, the study of the equilibrium phase of MgO–CaO– SiO2 slag with ferronickel metal and carbon solids needs to be studied to determine the composition of the slag liquidus and the composition of the metal formed at the most reductive melting conditions. Phase equilibrium information on the MgO–CaO–SiO2 slag system in equilibrium with ferronickel metal and carbon solids was obtained by conducting a series of experiments at temperatures of 1350, 1400, 1450, 1550, and 1550°C. A synthetic mixture consisting of oxide, metal, and graphite powders was used and melted in a carbon crucible using a vertical tube furnace under inert conditions. After equilibrium was reached, the sample was quickly dropped into water so that analysis could be carried out on the phases formed at the equilibrium temperature using a JEOL NeoScope JCM-7000 SEM–EDS. Phase equilibrium in the slag system which generally requires significant reaction time can be achieved in a shorter time in 2 hours by using the initial mixture composition from the oxidative side. The approach from the oxidative side was carried out by adding iron and silicon in the form of oxides. Iron oxide and silica were found to be reduced faster under reductive melting conditions in this experiment, thereby increasing the Fe and Si content in the alloy. The results of microstructural and sample composition analysis showed that the primary regions that were widely observed in this experiment are olivine and tridymite. The liquidus region of the MgO–CaO–SiO2 slag system which is in equilibrium with ferronickel metal and carbon solids expanded with increasing temperature. The most significant enlargement of the liquidus region occurs at an increase in temperature from 1350 to 1400°C. The expansion of the liquidus area in the pyroxene and olivine regions was consistent at 3 wt% MgO with increasing temperature every 50°C in the temperature range from 1400 to 1550°C. The resulting metal compositions were in the high Fe area with 0-46 atom % Si and 2-58 atom % Ni. Metals with Si content below 10 atom % were generally found at temperatures of 1450 ° C. The equilibrium composition of the metal in this study had a liquidus temperature range of around 1100-1500 ° C. The silicon content in the metal obtained in this experiment did not show clear correlation with metal size compared to temperature, the CaO/SiO2 ratio in the slag, and the Fe/Ni ratio in the metal. Comparison between the experimental results and FactSage 8.0 simulations on the MgO - CaO - SiO2 slag system in equilibrium with ferronickel metal and carbon solids showed a relatively significant difference in the primary olivine phase area. In this study, the slag melting area has been identified at two extreme boundary conditions, namely the olivine solid formation limit and the tridymite / cristobalite solid formation limit. Identification of this melting area is important to ensure that nickel smelting slag can be tapped from the furnace.