Thermodynamic Study of Trace Elements in the Blast Furnace and Basic Oxygen Furnace

Abstract: The iron ore based steel producers in Sweden and Finland operates mainly on pellets produced by LKAB. The introduction of new mining sites is believed to influence the future pellet chemistry. Furthermore, environmental and economic factors act as a driving force towards increasing the material- and energy efficiency by increasing the recirculation of in plant by products. All together this amounts to a realized change in feed chemistry. Therefore, it is of interest to study how trace elements behave in the process to be able to follow the material flow of trace elements within the integrated steel plant.
In this thesis it was attempted to describe the distribution of trace elements between metal, slag and gas phase in the blast furnace and basic oxygen furnace (BOF) process using thermodynamic equilibrium calculations. The work was focused on developing an approach to calculate the distribution of zinc in the blast furnace and chromium in the BOF. The same approach was then utilized for lead in the blast furnace and cobalt in the BOF to determine if it was applicable on other trace elements as well.
The blast furnace calculations were divided into three different scenarios representing different parts of the furnace; namely, the hearth, thermal reserve zone and the section above the thermal reserve zone. The results showed that for elements having a cyclical behavior in the furnace, such as zinc and lead, the assumed recirculation rate is directly decisive for the calculated output through the tap hole. And, that more data is needed to confidently estimate a probable recirculation rate that fits the calculations. Furthermore, it was shown that, from a thermodynamic standpoint, no lead or zinc leaves the blast furnace through the top. To describe the output of these elements through the off gas it was argued that a thorough study of the connection between furnace operating parameters and the dust and sludge amount and zinc and lead contents of the dust and sludge is required.
The BOF calculations were executed by adding the oxygen in increments to an open system, allowing the gas to leave between each calculation step. The calculations were carried out for and compared to results of a pilot plant scale converter and an industrial scale converter. From the results it was concluded that the distribution of chromium could be described for the pilot plant scale converter although the comparison of the overall composition of the slag and crude steel was not satisfactory. Furthermore, the distribution of chromium for the industrial scale converter could not be described using the method at hand. It was argued that the failure to describe the outcome resided in the fact that thermodynamic calculations were employed on a process where kinetics is known to play an important part. Cobalt could be described using the method. However, a simple mass balance with the assumption that essentially all cobalt reports to the crude steel phase would give the same results.