A compilation of slag foaming phenomenon research: Theoretical studies, industrial experiments and modelling

A literature study was made on the foaming phenomenon in the modern electric arc furnace (EAF) stainless steelmaking process. Slag foaming has become an important feature of the EAF process. The chemical and physical conditions of the slag, which affect the foaming phenomenon of the slag-steel system, appear to be relatively complicated to control in the manufacture of stainless steel. The foaming index is a parameter that quantifies the ability of slag to generate foam from either injected gas or gas that is generated within the slag or metal. In the case of ideal slagging, the foaming index is equal to the average foam life. The foaming index decreases with increasing viscosity and increases with decreasing viscosity. Various techniques based on dimensional analysis of the kinetic properties of the slag have been applied in order to find the relationship describing the foaming index. In industrial experiments, the control methods of the slag foaming operations are based on the visual observation or on noise emitted by the EAF vessel. The lower Cr2O3 content slags and large initial FeO content slags are more foaming. The foaming is also observed to promote chromium recovery. The foaming index for the slags is observed to be relatively similar, due to the similarities between viscosity, density and surface energy values, although their compositions are quite different. The foamy slag provides protection for the melt against nitrogen pick-up. The foaming capability of slags can be enhanced by the addition of appropriate materials such as limestone and calcium nitrate. In the modelling studies, the aim is to understand and predict foaming in EAF steelmaking. The slag foaming caused by slag-graphite reaction is dependent mainly on the CO-gas evolution rate. Decarburisation and slag formation models consider the rate phenomena for decarburisation and the carbon-FeO reaction in the slag, and the mass balance for each phase with the melting behaviour of pig iron scrap and fluxes. A recent thermodynamic model of the EAF process for stainless steel calculates the evolution of temperature and the composition of slag and metal phases with time. In the slag foaming in the EAF model the potential foaming is very high when using high amounts of input materials. The actual foam is limited by the height at which the slag is flushed out or by the volume of slag. At the end of the process the foam decays due to lower CO-gas generation.