p exit > p amb
2.2 Operating practice
2.2.4 Slag practice
, (17)
wherepdenotes the partial pressure. Some models [165–169] employ the concept ofoptical basicity[170] to relate slag composition to its sulphide capacity. Other approaches include theFlory polymerisation model[171] and theion and molecule coexistence theory[172, 173], while the so-called KTH model is based on computational thermodynamics [174–177].
2.2.4 Slag practice
Table 7 shows typical chemical compositions of AOD slags after the decarburisation stage and after the reduction stage. It can be seen that the main constituents of the AOD slag are CaO, SiO2, and Cr2O3, most of which is reduced during the reduction stage. It should be noted that X-ray fluorescence (XRF) analyses, such as those shown in Table 7, do not account for the different oxidation states of the oxide species. For example, chromium may be present in both divalent (CrO) and trivalent (Cr2O3) oxidation states.
The fraction of CrO decreases with increasing total chromium content, increasing basicity, increasing oxygen partial pressure, and decreasing temperature [178, 179].
With respect to basicity, the constituents of the AOD slag can be categorised into basic oxides (CaO, CrO, Cr2O3, FeO, MnO, MgO), acid oxides (SiO2), amphoteric oxides (Al2O3, Fe2O3), and salts (CaF2, CaS) [160, 180, 181].
Table 7. Typical chemical composition of AOD slags. Adapted from [73].
Composition [wt-%]
Sample CaO SiO2 MgO Cr2O3 Fe2O3 MnO NiO
After the decarburisation stage 35.5 4.6 4.0 25.0 26.9 0.6 0.9
After the reduction stage 60.9 27.7 7.1 0.6 1.0 0.1 0.2
Note: austenitic grade 1.4301 (18 wt-% Cr, 8.5 wt-% Ni).
Table 8 shows the reported phases and minerals in solidified AOD slags [156, 158].
The decarburisation slags consist mainly of a chrome-spinel phase, a silicate matrix, and metal droplets [156, 158]. The silicate matrix is based on networks of silicon tetrahedra, which consist of Si cations surrounded by four oxygen anions [180, 182]. The silicon tetrahedra are connected to each other by bridging oxygens, which can also connect other cations with tetrahedral coordination (e.g. Al3+and Fe3+) and can be broken
by network-modifying cations (e.g. Fe2+, Ca2+, and Mg2+) to form non-bridging oxygens [180, 182]. The chrome-spinel phase is reduced in the reduction stage and the resulting final slag consists mainly of dicalciumsilicate [156, 158]. Because the β→γdicalciumsilicate transformation causes a volume increase, it is necessary to use stabilising compounds, typically boron, to avoid disintegration of the slag in storage [183].
Table 8. Phases and minerals in solidified AOD slags.
Slag type Mineral group Mineral Chemical formula
Rubens and co-authors [156, 158]
Carry-over
Chrome-spinel Picrochromite MgO·Cr2O3
Silicate matrix Merwinite 3CaO·MgO·2SiO2
Metal alloy
Decarburisation
Chrome-spinel Picrochromite MgO·Cr2O3
Chrome-spinel Calcium chromite CaO·Cr2O3
Silicate matrix β-Dicalciumsilicate 2CaO·SiO2
Metal alloy
Reduction
Silicate matrix Merwinite 3CaO·MgO·2SiO2
Silicate matrix γ-Dicalciumsilicate 2CaO·SiO2
Other Fluorspar CaF2
Other Calciowüstite (Ca,Fe)O
Other Periclase MgO
Other Cuspidine 3CaO·2SiO2·CaF2
Ternstedtet al.[184]
Decarburisation
Chrome-spinel Calcium chromite CaO·Cr2O3
Garnet Uvarovite 3CaO·Cr2O3·3SiO2
Silicate matrix Rankinite 3CaO·2SiO2
Amorphous phase Metal alloy Lindstrandet al.[185]
Decarburisation
Chrome-spinel Calcium chromite CaO·Cr2O3
Silicate matrix Dicalciumsilicate 2CaO·SiO2
Silicate matrix Rankinite 3CaO·2SiO2
Metal oxide Various Cr2O3, FexOy, and MnxOywith
parts of CaO and/or MgO Amorphous phase
Metal alloy
One of the objectives of the slag practice during the decarburisation stage is to minimise chromium oxide solubility in the slag [10]. On the other hand, refractory lining wear is a considerable cost factor in AOD processing and therefore it is necessary to ensure that the slag practice and the employed refractory lining material are compatible [156, 186]. In order to meet these two goals, the basicity of the slag is adjusted with the addition of basic oxides, particularly lime [10, 139, 186]. The recommended basicity ratios for decarburisation slags are shown in Table 9. The total amount slag former additions are typically in the range of 3–7% of total bath weight [5]. The dissolution of lime in decarburisation slags is hindered by the formation of a calcium silicate layer, which surrounds the lime particles [156, 158]. For example, Münchberget al.[158]
suggested a dissolution time of at least 16 minutes.
Table 9. Basicity ratios for decarburisation slags. Adapted from [186].
Basic oxide addition Minimum ratios for refractory combatibility
Typical steel grades∗
Lime addition only (%CaO)
(%SiO2)≤1.6
Doloma or doloma/lime (%CaO) + (%MgO)
(%SiO2) ≤2.0
Special steel grades∗∗
Doloma or doloma/lime (%CaO) + (%MgO)
(%SiO2) + (%Al2O3) + (%Nb2O5)≤2.0
Basic oxide addition Recommended ratios for solid slag practice
Typical steel grades∗
Lime addition only (%CaO)
(%SiO2)≤2.0
Doloma or doloma/lime (%CaO)
(%SiO2)≤2.0 or (%CaO) + (% MgO)
(%SiO2) ≤3.33
Special steel grades∗∗
Doloma or doloma/lime (%CaO) + (%MgO)
(%SiO2) + (%Al2O3) + (% Nb2O5)≤3.33
* low Al2O3and Nb2O5; ** high Al2O3and Nb2O5. All concentrations in weight-percent.
Earlier it was common to have separate slags for both reduction of slag and subsequent desulphurisation, but modern plants employ a single slag practice, in which desulphurisation takes place during the reduction stage [147, 159]. The advantages of the single slag practice include higher chromium yields, decreased refractory lining
wear, as well as reductions in process time and lime additions [4, 158].
Because the viscosity of the AOD slag increases with increasing chromium oxide content [187], it becomes highly viscous towards the end of the decarburisation stage.
For this reason, the basicity and viscosity of the slag are adjusted by additions of fluxes and slag formers to provide sufficient preconditions for efficient reduction of slag and desulphurisation [55]. Nevertheless, it has been suggested that even after the reduction stage the slag can have a small amount of solid CaO·Cr2O3precipitates [162].
According to Songet al.[161], the slag basicity should be adjusted to (%CaO/%SiO2)>
2 for efficient reduction of slag and desulphurisation. Furthermore, it was recommended that the metal bath temperature should be higher than 1943 K (1670◦C) to increase slag fluidity [161].
A common flux employed in the AOD process is calcium fluoride (CaF2), also known as fluorspar, which is an efficient flux, but very aggressive towards the refractory lining [188]. Moreover, it may leach fluor, which contaminates ground water [189].
The melting point of the slag can also be reduced with the addition of bauxite, which is an aluminium ore consisting mainly of gibbsite, boehmite, and diaspore minerals mixed with goethite and hematite [188]. In normal operating practice the removal of phosphorous is inefficient. However, laboratory experiments [190] suggest that lime-based slags can be modified with BaO and NaF to improve their phosphate capacity.