The goal of the current Project 1.1 project is on the development of methods to determine properties of metallurgical slag systems. Slags are by-products generated during smelting operations and hot metal treatment. They should fulfill several metallurgical tasks during production of steel, in which liquid metal melts are involved and, thus, the desired composition of the slag influences efficiency of different process steps during iron and steelmaking up to a great extent. Rapid formation of the reactive slag with the adequate viscosity, density and chemistry facilitates the removal of sulfur and phosphorus from the melt. Therefore, the investigation of physico-chemical properties of the slag contributes to a better process control and an enhanced productivity. Accordingly, the optimization of the metallurgical processes requires precise knowledge regarding the slag formation.
The viscosity of slags has a big influence on the course of metallurgical processes as well as an efficient energy utilization. SiO2, Al2O3, CaO und MgO are the four main components in metallurgical slags, slags from coal-fired power plants, ceramic, glass, magma etc. Therefore, several studies exist regarding the viscosity of slags, which are focused solely on these four oxides. The measurement of slags with additive components, especially, iron oxide is more complex. Since measurements are time consuming and expensive and since slags have different compositions depending on the industrial sector, only limited knowledge is available in literature regarding the viscosity of slags with additional components than the above-mentioned oxides.
To obtain a reactive efficiently working slag, solid materials (slag formers) are charged into the metallurgical aggregates during iron and steelmaking processes. The produced slag acts as a reactive agent with the metal melt to remove and selectively bound elements such as sulfur or phosphorous and beside this, a minimized refractory wear is also important. The dissolution rate of slag formers determines the removal efficiency of impurities. On the other hand, a high dissolution rate of non-metallic inclusions of metal in the slag is essential for high steel purity. Exemplarily, the blowing period in the LD converter is in the range of 15 to 20 minutes, leading to the requirement of a fast proceeding of the slag former dissolution to generate a reactive slag for phosphorous removal. Therefore, the knowledge of the dissolution rate is essential to better understand the course of reactions in the metal-slag zone.
One of the biggest obstacles to describe reactions in metal-slag or slag-gas phase reactions is a rather low description in public literature regarding the thermodynamic activities and diffusivities of reactive species in typical slag systems of iron and steel metallurgy. In this context, elements such as oxygen and different metals are of interest. The activity of species in redox reacting slag systems strongly correlate with the electrical conductivity, which is also in the focus of the current Project 1.1. Finally, slag data such as density, liquidus and solidus temperature as well as the crystallization behavior are necessary to better understand metallurgical reactions and to further develop existing numerical and analytical models.
- Comprehensive competence of the center and his partners regarding measurement of slag properties
- Method for viscosity measurement of metallurgical slags
- Method to determine diffusion coefficients of specific chosen metals in slags
- Measurement method for the activities of oxygen and different metals in slags
- Method for the electrical conductivity of slags
- Methods for liquidus and solidus temperature and the crystallization behavior of specific chosen slag types
Fundamental literature studies represent the starting point of the planned methodology. It must be found out, which slag types can be investigated. Furthermore, an important outcome of the theoretical studies will be the definition, which components i.e. which metallic elements can be measured.
Based on the results from literature studies, existing experimental methods will be adapted i.e. optimum measurement parameters will be defined and new methods will be developed respectively. For viscosity measurements, High-Temperature Rheometry and Air Levitation Technique will be applied. Beside this, a High-Temperature Tube Furnace will be used to determine diffusion coefficients whereas static and dynamic tests (fixed and rotating sample crucibles) will be executed.
The oxygen potential i.e. the activity of oxygen in slag systems can be exemplarily determined electrochemically via the electro motoric force (EMF). The EMF (corresponding to a terminal voltage) can be measured with an electrode cell. It is known from literature that methods such as the cyclic voltammetry or the stepped potential chronoamperometry are applied to measure the electrical conductivity i.e. the ionic mobility of slags at temperatures up to 1,650 °C and defined slag composition and gas atmosphere. The challenge lays in the construction of the electrochemical measurement cell under consideration of slag resistant and non-corrosive material combinations for the electrodes and the cell integration into the heating environment.
To determine the liquidus and solidus temperature and the crystallization behavior, the Differential Thermo Analysis (DTA) and the Hot Thermocouple Technique should be applied to suitable slag system. In this context, it is generally known, that the Hot Thermocouple Technique is restricted to transparent slag systems with low contents of evaporating components and liquidus temperature below 1,400 °C. These and further restrictions must be considered, when choosing slag types and measurement methods.
Measured viscosities, diffusion and activity coefficients will be evaluated by model calculations (software package FactSage®) and will also serve to enlarge the FactSage® database. Beside this, model-based EMF calculations are planned as an accompanying activity to develop a physico-chemical foundation for a description of EMF signal patterns from the blast furnace process.
At the end of the current Project 1.1, a comprehensive competence is expected regarding the statement, which measurement methods can be applied to specific metallurgical slags. The measurable groups of slag systems and material properties should be investigated and defined.
In this context, an important outcome will also be to demonstrate the restrictions i.e. the non-applicability of measurement methods to certain slags. The planned activities will lead to the generation of a competence matrix regarding the investigation and determination of different slag types and material properties. Furthermore, suitable measurement procedures to determine material data for different slag types should be standardized, if possible. Additionally, the feasibility regarding a transfer of measurement sensors and methods from lab-scale environment into the industrial process should be investigated, and necessary adjustments should be demonstrated.
Regarding slag viscosity, a suitability of the Air Levitation Technique as alternative to the Rheometry should be clarified. Furthermore, methods should be developed to determine the dissolution and activity coefficients as well as the electrical conductivity and standardized, if possible. Possibilities and restrictions of DTA and Hot Thermocouple Technique for a determination of liquidus and solidus temperature as well as the crystallization behavior of transparent slags are also expected at the end of Project 1.1. The slag data generated with the developed measurement methods should contribute to increased competences regarding a deeper understanding of metallurgical reactions occurring in different processes.