Simulated Phase Diagrams Of The Nickel Slag System

Nickel has become one of the main sources for stainless production in the modern industries.In order to achieve a high percentage yield during the process of nickel smelting,the impurities in the raw nickel ores must be removed and may be recycled for further treatment to be used in other construction areas.During the metallurgical process of nickel,the impurities separated from the liquidus nickel are called nickel slags.Slags are regarded as by-products in the metal purification step,which nickel slag usually contains various metal and non-metal oxides.This paper aims at studying the effects of alkaline oxides in the nickel slag,which can be summarized and simplified to the study of properties of tetra-component system FeO-SiO₂-CaO-Mg O.Computer programming was used to generate a series of phase diagrams of the alkaline oxides system in nickel slag.The purposes of phase diagrams are for data comparison and pattern observation.The computer software used in this study is Fact Sage 7.1,a widely used thermodynamically software with various database packages in material science.Considering the object of this study is nickel slag,database Fact PS and FToxid are chosen as the desired databases because they contain the thermodynamically properties of pure substance and the oxides,along with oxide solutions and metals.For the purpose of this study,Fact Sage 7.1 was set up to calculate liquidus projection of FeO-SiO₂-Ca O-MgO-O₂ with different MgO addition and to generate the simulated phase diagrams.From observing the pattern change of five phase diagrams with different mole fraction of MgO/CaO ranging from 0 to 2,partial conclusion can be drawn that the spinel phase zone is the smallest without the addition of magnesium oxide;however,spinel phase zone expanded as the mole fraction of MgO and CaO is increasing from 0 to 0.75.It worth to mention that the spinel phase zone was almost the same when the mole fraction of MgO and Ca O changes from 0.75 to 1.The spinel phase zone obviously shrinked if the fraction continuously increasing from 1 to 2.Therefore,the spinel phase zone of FeO-SiO₂-Ca O-MgO system is the largest when the fraction is in the range of 0.75 to 1.A change of low liquidus zone at the temperature of 1573Kelvin has also been observed to appear when the mole fraction of MgO/CaO is 0.5,but it vanished if the fraction is up to 1.Besides,the hematite phase zone can be found,but it disappeared if Mg O was added when the spinel phase zone obviously expanded.Similarity,the effect of calcium oxide was also investigated.For the phase diagram module,the effect of CaO addition is positive for spinel phase zone construction in the simulated phase diagram with same condition set up as MgO.The most ideal CaO content is12%to 15%where the spinel phase zone appears to be the largest.With the addition of CaO to 10%,the low melting zone at 1573 K,which is Fe₂O₃,starts to appear similarly to the result of MgO addition.Liquid fraction,which is the ration of liquid slag to the total mixture,is also related to the content of Calcium oxide,where a slag mixture would have a high slag liquid fraction with high temperature and high percent of calcium oxide.Viscosity of the liquid nickel slag as well plays a crucial role in studying the property of Fe O-SiO₂-CaO-MgO slag system.From the analysis of viscosity variation figures,it is to conclude that if a slag mixture with a high percent of calcium oxide content at a low temperature,it has the highest viscosity.Moreover,viscosity is also more sensitive to temperature change than composition because it is directly related to temperature with an inversely proportional relationship.Besides,the effects of basicity,which is the ratio of alkaline oxides to acidic oxides,on the viscosity were also studied.While the slag mixture has a high basicity,usually greater than 1,the viscosity of the mixture is higher than the mixtures that has basicity values smaller than 1 only if the temperature is below 1350 degree Celsius.