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Case Study: Disintegration of Air-Cooled Slags - Coggle Diagram
Case Study: Disintegration of Air-Cooled Slags
PROBLEM
However, a similar disintegration is still frequently observed for electric arc and ladle refining slags from steelmaking.
Blast furnace ironmaking slags were known to exhibit a peculiar disintegration upon air-cooling.
The problem for blast furnace slags has been eliminated by changes in slag compositions.
CAUSE
The disintegration of the slag is caused by the presence of (2CaO·SiO2 or C2S) or dicalcium silicate in the form of calcium silicate.
When the mineral undergoes athermal transformation, it becomes amorphous polymorph, and the expansion of the volume of the slag causes the disintegration.
SOLUTION
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The oxides refer to their respective weight fractions. If the slag conforms to either one of these conditions, the slag should not contain large amounts of C2S and, as a consequence, should not disintegrate.
CaO ≤ 0.93SiO2 +0.55Al2O3 +1.75S
CaO + 0.8MgO ≤ 1.20SiO2 +0.39Al2O3 +1.75S
Compositional limits for disintegrating slags were defined, based on the stability field of C2S in the CaO–MgO–SiO2–Al2O3 slag system, with an adjustment for the calcium sulphide content:
These criteria were primarily used to keep stable and disintegrating blast furnace slags separate at the slag yard.
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One pathway is to prevent slag disintegration, either by inhibiting the β- to ɣ-transformation of C2S or by outright avoiding the presence of C2S.
The first option was the development of a borate-based stabiliser for stainless steel slag.
For pure C2S, it was previously demonstrated that borates stabilize the higher temperature polymorphs to ambient temperatures by forming a solid solution.
The crystallographic mechanism is believed to be the partial replacement of SiO4 4− units by BO3 3−units Because of the large difference in ionic radius between Si4+ and B3+, this replacement suppresses the Ca2+ migrations and SiO4 4− rotations required for the β- to ɣ- transformation, even with only 0.13 wt% of B2O3
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Other compounds, such as phosphorus, can also stabilize the slag. A qualitative criterion has been developed to predict the stability of an additive. Although the results were satisfactory, the larger amounts needed for rapid cooling were not met. This method is mainly used for the stabilization of C2S. The rapid cooling method is capable of producing a uniform cooling rate of 5 C/s.
The second option is to modify the composition of the slag. However, this method can only be used if the slag has the appropriate high temperature properties. The composition of the slag should be adjusted after the metal separation process to avoid making promises. Also, adding a large amount of SiO2 to the slag can help avoid C2S precipitation.
The composition of the slag should be adjusted following the separation of metal. It is generally suggested that a large concentration of SiO2 is added to prevent C2S precipitation. The operation would require a relatively low cost. However, it is difficult to formulate due to the limited heat content and the heat conductivity of the slags. By injecting oxygen, a large volume of quartz sand is dissolved in a basic steelmaking slag. The resulting Fe2O3 is formed and the added heat is required to produce the desired dissolution.