Introduction
Due to the strong increasing crude steel production, the occurring by-products and residues from the iron - and steel industry are rising as well. This fact in conjunction with an environmental aspect is crucial to intensify the investigation in the field of recycling wastes and by-products from the iron - and steel industry. Some successful applications of utilizing materials from this industry sector are already in operation like the standardized cement from blast furnace slag because of its latent-hydraulic properties [1, 2]. Furthermore, the basic oxygen furnace slag can be used for road construction in different layers, due to its hardness [3]. Some other by-products cannot be used directly in a further application, because of their properties or chemical composition. One example is the desulphurization slag , which is the product at hot metal desulphurization. Such a sulphur removal is mandatory in the iron and steel industry, because sulphur is well known as steel parasite [4, 5].
Sulphur in the Iron- and Steel Industry
This short chapter describes the influence of sulphur in the steel product and the possibilities for a successful removing.
Influence of Sulphur and Its Input Material
Binary system of sulphur and iron [6]
Manganese forms together with sulphur a compound, which leads to problems during welding processes, and lamellar fractures occur. Only at some special steel types the sulphur is needed, like free-cutting steels. In earlier times lead was used in such steel types, which was prohibited in the last years and was substituted by sulphur, since of similar properties. Typically, sulphur contents are 0.15–0.3% in free-cutting steels [4, 5, 7].
The main source for sulphur in hot metal is the used coke at the blast furnace process. About 80% of the total sulphur input is contributed by this material. The remaining part of the sulphur feed is coming from ores and additives [5, 8].
Desulphurization Technologies for Hot Metal
Different methods can be used for a successful desulphurization. Examples are the removal via the gas phase, through diffusion equalization, with sulphur affine metals or the usage of the partition equilibrium between a slag and iron . Within the hot metal or steel production different desulphurization technologies take place during different processing steps. The first removal is done in the blast furnace itself by the slag , where about 4% of the fed sulphur remains in the pig iron . The main step is done between the blast furnace and the steel plant in a pig iron ladle or torpedo ladle. To reach the defined level, the secondary metallurgy of steel is the last possibility to perform a desulphurization [5].
Overview of possible desulphurization concepts an additives
Group | Process | Place of treatment | Type of desulphurization agent |
|---|---|---|---|
casting stream mixing process | soda desulphurization | Ladle/mixer | Soda |
Mechanical mixing process | Diverse mixer; Hoesch mixing process | blast furnace gutter; pig iron ladle | CaO ; CaC2; soda/lime (1:1) |
Pneumatic mixing process | Injection lance; injection process | Torpedo ladle | CaC2 + CaO ; Mg and Mg + CaO |
Electromagnetically mixer | Electrochemical gutter | Pre-melted slag |
Depending on the operated desulphurization process various amounts of desulphurization additives are needed and also the required time together with a temperature loss is varying [5].
Fundamental of the Roasting for Sulphur Removing
For special types of desulphurization slags some processes are available [9, 10], but in most cases a successful concept is not available and therefore it gets dumped. This information is also mentioned in the IRC Reference Report Best Available Techniques (BAT) Reference Document for Iron and Steel Production from March 2012. One main idea is the separation of sulphur without generating of a new waste. This can be achieved through a removal of sulphur in the off-gas linked to a sulphuric acid generation.
In the field of metallurgy, the roasting process is typically used to generate metal oxides from sulphide ores. This leads to the fact that such a roasting step can be used to treat sulphur containing slags as well. Different roasting technologies are available like the multiple hearth furnace , the fluidized bed roaster, a rotary hearth furnace , a shaft furnace or a sinter belt. All these processes are working in solid state [11–14].
Recycling of Desulphurization Slag from the Pig Iron Desulphurization
The considered desulphurization slag is generated in a hot metal ladle desulphurization through an injection lance. The obtained slag contains the sulphur and gets removed from the ladle. Here, a separation in three different grain size fraction can be achieved. The biggest fraction are so called “skulls” (>120 mm) which can be used as scrap in the basic oxygen furnace or an electric arc furnace . With respect to the sulphur content, in the most cases it is possible to recycle the “middle” sized-fraction (10–120 mm) in the blast furnace , but too high sulphur levels avoids such a reuse. In this case the middle fraction has to be treated together with the “fine” fraction (0–10 mm), which has the highest sulphur content. At the moment, no satisfying concepts are available to remove the sulphur from these fractions. Therefore, intensive research in the past leads to the following described process design.
Characterization and Parameter Definition in a Hot Stage Microscope (Lab Scale Trials)
For this investigation the fine fraction of the desulphurization slag was used, that typically contains 4–6% S, CaO , MgO, C, SiO2 , FeO, Fe2O3 and Femetallic. The first step was a detailed chemical analysis of the used material to obtain information about the detailed composition.
To clarify the possibility for a roasting process, a hot stage microscope was applied. Usually such a device is used to determine the softening behaviour of slags, dust, ashes or other materials. Therefore, a shadow image of a cylindrical sample gets detected at the continuous increasing temperature . Through geometrical changes of the cylinder during the heating, which can be recognized by the projected picture of the sample, it is possible to define the softening point and/or melting point from the considered sample. Such practical results are needed for pyrometallurgical process developments. In the case of desulphurization slag , the hot stage microscope is used to perform lab scale trials. This means, that the cylindrical sample (diameter: 3 mm; height: 3 mm) gets heated (10 K/min.) to defined temperatures and different gas atmospheres. The different continuously added gases, to generate a specific atmosphere in the furnace , were pure oxygen, synthetic air and pure carbon monoxide. To interpret the success of the treatment, the continuously released off-gas composition was analysed and furthermore, a scanning electron microscope analysis from the treated sample was used.
Main results from the lab scale trials (example for 1400 °C and pure oxygen atmosphere)
The results were applied in order to define useful process parameter for a successful treatment of the desulphurization slag . This analysis showed, that it is needed to perform such a process in liquid state. This is necessary since lower temperatures (in solid state ) lead to a not satisfying sulphur removal . Regarding the detected gases via a gas analyser attached to the hot stage microscope , it was possible to define the best conditions. A representative example is shown in Fig. 2. After a heating up to 1400 °C in an oxidizing atmosphere (provided by the usage of pure oxygen), the carbon from the liquid slag starts to combust and leaves the reaction room with the off-gas stream. After a complete removal of carbon, the sulphur starts to react and forms SO2, which is gaseous as well. The light microscope picture of the treated slag at the mentioned conditions shows a lot of pores, which indicates the gas formation. A subsequent performed chemical analysis of the remaining slags shows sulphur amounts which are lower than 0.01%.
These successful investigation and parameter definition in such a small scale leads to an up-scale of the trial size into a technical scale device, which is mentioned in the following chapter.
Recycling Process Development in Technical Scale Size
The lab-scale experiments showed, that oxygen in the furnace atmosphere is mandatory to reach a very low level of sulphur in the slag at the end of the treatment. Therefore, a furnace should be used which can supply oxygen with a well mixing effect to homogenize the slag together with the supplied oxygen. Regarding these demands, a TBRC (Top Blown Rotary Converter) is a useful furnace design. Such a TBRC, with a reaction room of about 70 l and an oxygen-methane burner is available at the Chair of Nonferrous Metallurgy, Montanuniversitaet Leoben.
Off-gas analysis from a TBRC trial at 1400 °C
Tapping of the TBRC after the treatment
Due to the very satisfying results this process design was verified by pilot scale trial, which was the next step in the development.
Pilot Scale Trial for the Treatment of Desulphurization Slag
As next step in this investigation pilot scale trials with 200–1000 kg desulphurization slag were performed in a short drum furnace , which has a similar facility design to the TBRC. To optimize the process itself and also to minimize the treatment time, an additional oxygen lance was installed additionally to the natural gas -oxygen burner. With the individual control of the oxygen flow it is possible to optimize the defined process conditions.
Different performed trials in this scale confirmed the results from the former executed investigations. It was possible to verify the feasibility of this process design and the pilot scale trial allowed to define the most useful process parameter.
Conclusion
Possible process flowsheet for the treatment of desulphurization slag
It is possible to reach sulphur levels in the treated slag which are lower than 0.01% and this can be seen as very good results. This led to a patent which was applied for this process. Due to the changings in the composition and viscosity of the slag different additives were evaluated and the optimum was defined.