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Development Towards Sustainable Ironmaking

The IronArc Process

Time: Fri 2020-12-18 14.00

Location:, Stockholm (English)

Subject area: Materials Science and Engineering

Doctoral student: Jonas Svantesson , Processer

Opponent: PhD Robert Eriksson, Jernkontoret

Supervisor: Universitets lektor Mikael Ersson, Materialvetenskap

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The IronArc process is a novel process for a more sustainable production of liquid pig iron using electricity for heating and hydrocarbons for reduction. This thesis aims to facilitate its use by investigating possible refractory solutions and the gas blowing in the process which is done by a plasma generator.

The process involves a slag with a high FeO content of 90 wt % and gangue content of approximately 5 wt % SiO2 and 5 wt % CaO. The interaction between such a slag and refractories of MgO, Al2O3, Cr2O3, SiC, ASZ,and C was investigated by high temperature experiments at 1700 K and by thermodynamic calculations in Thermo-calc and FactSage. In the high temperature experiments it was found that all of the studied refractory materials experienced signicant wear after 3 h, but the MgO-Al2O3 spinel refractories were the least affected. The thermodynamic calculations show fair agreement to the experiments, with the exception for the Cr2O3-spinel refractory which performed much worse than predicted by thermodynamic equilibrium calculations. It was concluded that the thermodynamic equilibrium calculations in Thermo-calc and Factsage can be used as an indicator for the stability of a refractory material, but with varying accuracy depending on the quality of the data in the database used.

Since industrial refractory materials are not viable as refractory for the IronArc process a freeze-lining approach was evaluated by using CFD in ANSYS Fluent. The flow of a slag was simulated through two different designs of slag runner to investigate how well a freeze-lining protects the walls in a region with rapid flow and the cooling required to form and maintain said freeze-lining. It was found that the enthalpy porosity model in ANSYS Fluent in combination with the RSM turbulence model accurately predicts the thickness of a freeze lining when validated against experiments in the CaCl2-H2O system. For optimal protection of the refractory walls the reactor and runner should be designed to minimize the movement close to the walls as high near-wall turbulence will reduce the thickness and stability of the freeze-lining, leading to greater cooling requirements to maintain afreeze-lining.

The IronArc process uses a plasma generator to supply heat to the reactor using electricity. By blowing gas and hydrocarbons through an electric arc, superheated gas is formed which when injected into the reactor provides both stirring and heating for the process. To study the behavior of the injected gas a simulation model was developed in OpenFOAM. The model for simulating gas blowing was tested in both incompressible and compressible simulations in the air-water system which were veried against an experimental study in the air-water system and found good agreement. The simulations of the plasma generator blowing were done in the compressible model to account for the high temperature and pressure present in the IronArc process.

It was found that the stability of the gas blowing is dependent on the Froude number where low values cause an unstable and pulsating plume and higher values produce a more stable jet. It was also found that the empirical equation for penetration length is only valid for gas blowing with suciently high Froude numbers to produce a jetting behavior. It was found that the transition from pulsating to steady jetting in the IronArc system occurred around Froude numbers of 300 and higher values further increased the stability of the jet. For gas blowing below the transition region, the penetration length of the unstable and pulsating jet will be severely underpredicted by the empirical equation. This behavior must be considered when designing the gas blowing system for the IronArc process as the gas penetration length will signicantly influence the stirring in the reactor. Additionally, a pulsating and unstable jet produces large bubbles which risk coming in contact with the refractory walls which in previous studies has been shown to be very detrimental to the refractory lifetime. A decrease of the inlet diameter for the gas blowing increases the Froude number and the stability of the jet.

By implementing the proposed refractory protection by freeze-lining and the small changes to the plasma generator inlet diameter the IronArc process can be developed into a promising industrial process capable of producing liquid pig iron in a more sustainable way.