Investigation of Bath/Freeze Lining Interface Temperature Based on the Rheology of the Slag

Freeze lining is a solidified layer of slag formed on the inner side of a water-cooled pyrometallurgical reactor, which protects the reactor walls from thermal, physical, and chemical attacks. Because of the freeze lining's high thermal resistance, the reactor heat losses strongly depend on the freeze lining thickness. In a batch process such as slag fuming, the conditions change with time, affecting the freeze lining thickness. Determining the freeze lining thickness is challenging as it cannot be measured directly. In this study, a conceptual framework based on the morphology and microstructure of freeze lining and the rheology of the slag is discussed and experimentally evaluated to determine the freeze lining thickness. It was found that the bath/freeze lining interface lies just below critical viscosity temperature. The growth of the freeze lining is primarily controlled by the mechanical and thermal degradation of the crystals forming at the interface. The bath/freeze lining interface temperature for the measured slag lies in the range of 1035–1070°C.

Numerical Analysis of Slag Transfer in the IronArc Process

The IronArc process is a novel approach to ironmaking which aims to reduce the associated $${\hbox {CO}}_{2}$$ CO 2 emissions. By superheating gas using electricity in a plasma generator (PG) the heat required for the process can be supplied without burning of coke. Reduction of hematite and magnetite ores is facilitated by additions of hydrocarbons from liquid natural gas (LNG). The melting and reduction of ore will produce a molten slag containing 90 pct wüstite, which will be corrosive to most refractory materials. A freeze-lining can prevent refractory wear by separating the molten slag from the refractory. This approach is evaluated in CFD simulations by studying the liquid flow and solidification of the slag using the enthalpy–porosity model in two different slag transfer designs. It was found that a fast moving slag causes a high near-wall turbulence, which prevents solidification in the affected areas. The RSM turbulence model was verified against published experimental research on solidification in flows. It was found to accurately predict the freeze-lining thickness when a steady state was reached, but with lacking accuracy for predicting the time required for formation of said freeze-lining. The results were similar when the $$k{-}\omega $$ k - ω SST model was used. A design with a slower flow causes more solidified material on the walls and can protect all areas of the refractory wall from the corrosive slag. A parameter study was done on the effect of viscosity, mushy zone parameter, heat conductivity and mass flow on the amount of solidified material, thickness of solidified material, heat flux, and wall shear stress. In the current geometry, freeze-linings completely protect the refractory for mass flow rates of up to 3 $${\text {kg}} \, {\text {s}}^{-1},$$ kg s - 1 , and are stable for the expected viscosity (0.05 to 0.3 Pa), heat conductivity (2 $${\text {W}}\, {\text {m}}^{-1}\,{\text {K}}^{-1}),$$ W m - 1 K - 1 ) , and used mushy zone parameter (10,000).

Thermal conductivity of titanium slags

In ilmenite smelting furnaces, a freeze lining of solidified slag is used to protect the furnace refractories against the aggressive titanium slag. Freeze lining thickness cannot be measured directly due to harshness of conditions inside the process, thus process modelling is required. Several parameters influence the thickness of the freeze-lining, one of them being thermal conductivity of the frozen slag. However, there is a lack of thermal conductivity values for high titanium slags −especially as a function of temperature. In this study, thermal conductivity of three titanium slag samples and an additional sample of freeze-lining was measured from room temperature to 1100/1400 °C with the laser flash analysis method. In addition, thermal expansion and microstructures of the samples were studied to provide an extensive understanding of how microstructure will affect thermal conductivity. The thermal conductivity of the slag samples was found to increase from 1.2 to a maximum of 2.4 W/(m K) when increasing temperature from room temperature to 1100 °C. An additional experiment at 1400 °C showed that the thermal conductivity increased further as the temperature increased. The freeze-lining sample behaves differently, with conductivity being the highest at room temperature, 2.2 W/(m K).