Removal of Residual Elements in the Steel Ladle by a Combination of Top-Slag and Deep Injection

Performed by: Department of Materials Science and Engineering,
McMaster University
Hamilton, Ontario L8S 4K1
Project Leaders: DrK. S. Coley (coleyk@mcmaster.ca)
Dr G. A. Irons (ironsga@mcmaster.ca)
AISI Contract Manager: J. R. Vehec(aisiap@aol.com)
AISI Technical Manager: B.V. Lakshminarayana (Blakshmi@steel.org)
Industrial Sponsors: DOFASCO Inc: Bruce Farrand ( Bruce_Farrand@dofasco.ca)
Weirton Steel: Dr D. Aichbhaumik (debo.aichbhaumik@weirton.com)
U.S. Steel: Don Zhung (tel: 412-825-2111)

Project Overview

The level of undesirable residual elements (Cu, Sn, Ni , Mo, P) is increasing to an unacceptable level and it is anticipated that other elements such as (Pb, As, Sb, Bi) will become a problem in the future. At present this problem is avoided by steelmakers by careful selection of scrap, but this may not be possible in the future.

There are a number of processes proposed in the past, for the removal of residuals, but none have been seen as sufficiently attractive for adoption by the industry. One such process involves the use of high calcium fluoride fluxes under highly reducing conditions for the removal of Sn, Pb, Sb, As and Bi. The thermodynamics of this process have been thoroughly investigated and successful removal of the element M, can be achieved by the reaction:

xCaO + yM = CaxMy + x/2O2

This study will employ calcium injection to ensure an extremely low local oxygen potential and rapid reaction kinetics, in combination with a top slag to absorb the reaction products.

Each of the following critical components will be studied independently:

  • Reaction Kinetics
  • Slag Composition
  • Oxygen Potential.

The rate of reversion from the top slag at higher oxygen potentials is seen as critical to the process and will comprise a major part of this study. Data from this phase of the investigation will then be used in conjunction with kinetic data from injection experiments to develop a model of the entire process. With positive results from this phase of the work, plant trials will be proposed.

Project Objectives

To investigate the feasibility of tin removal using a combination of, deep injection of calcium, and a basic top-slag, and ultimately to develop a process that will allow a greater proportion of high tin scrap to be used by steelmakers. In order to evaluate such a process the following questions must be answered:

1. Are the kinetics of such a process sufficiently fast? Whilst extensive thermodynamic investigations indicate that the process is feasible there has been no study of the kinetics.

2. Is it necessary to employ high fluoride fluxes?

3. Can a sufficiently low oxygen potential be achieved? Very high partition ratios have been measured but these required an oxygen potential of 10-24 and to achieve acceptable partitions 10-22 would be required.

4. Can magnesium be used as a substitute for, or in addition to calcium and what are the relative advantages?

Benefits of Proposed Research

The "problem" of residuals has been discussed for a number of years, but in general steelmakers have managed to avoid the problem by selecting appropriate scrap. Whether the problem can be circumvented for the foreseeable future is not clear, nor is the cost of avoiding the problem as opposed to confronting it. Therefore, currently it is not possible to put a dollar value on the outcome of this proposal but it is clear that current trends will serve to make the problem worse, and a solution of the type proposed more attractive.

As the percentage of the worlds steel from electric steelmaking increases, the ability of steelmakers to pick and choose scrap will decrease, and the capacity to control residuals will be required to provide a level of flexibility. In addition, the proposed process will increase the percentage of total product that can be recycled, a requirement that is continually being increased by governments.

Research Program

The research program will be conducted in three phases, two laboratory based and one in-plant. The details of the plant study are dependent on the results from the laboratory work but will essentially be a validation stage. The industrial sponsors have expressed a willingness to carry out full-scale plant trials. The possibility of using pilot plant facilities available at Argonne National Labs is also being considered.

The laboratory studies to be carried out in parallel, will comprise:

1) a fundamental study of slag metal reversion kinetics under a controlled atmosphere, as a function of slag composition, and oxygen potential;

and 2) a study of reaction kinetics during calcium injection with and without a top slag.

Schedule

Milestone Time to complete from project start
Part 1.
Equipment assembly and testing 0.5 year
Reversion tests with tin with various slags 1.0 year
Reversion test with Pb, As, Sb and Bi individually and various slags 2.0 year
Reversion tests with multiple elements 3.0 year
Part 2.
Equipment assembly and testing 0.5 year
Tin removal experiments with Ca and CaSi and various top slags 1.0 year
Removal of Pb, As, Sb and Bi by Ca and CaSi injection 2.0 year
Plant trials at AISI member company 3.0 year