Crystallization Behaviors Concerned with TTT and CCT Diagrams of Blast Furnace Slag Using Hot Thermocouple Technique

Abstract

Currently, the most of slags are recycled and reused by taking the advantages of the feature of respective slags (e.g. the BF slag is used for cement and roadbed material, LD slag is also used for the roadbed material and marine resources, etc.). The quality and quantity of recycle of slags changed greatly within recent ten years and it will be expected as an important resource in the future. Whereas there is a remained possibility on the recycle of slags, most of slags have a high temperature more than 1 500, when it was exhausted.In general, there are two cooling processes adopted into the slag treatment, one is an air-cooling process and the other is a water quenching process. However, those slag cooling processes does not utilize the sensible and the latent heat related to high temperature melt. If there were a TTT (Time-Temperature-Transformation: isothermal transformation) diagram and a CCT (Continuous-Cooling-Transformation: continuous cooling curve) diagram, the property of the final slag could be estimated by the designed cooling path. Furthermore, it could be possible or easier to recover the sensible and the latent heat through a given cooling path.In the present study, using SHTT (Single Hot Thermocouple Technique), TTT and CCT diagrams of BF slags were measured. Crystallization behavior in the TTT and CCT diagrams of BF slag were clarified by XRD analysis, SEM observation and EDS analysis together with the in situ observation. Crystal phases in the TTT diagram for BF slag used were Gehlenite (2CaO·Al2O3·SiO2) and Merwinite (3CaO·MgO;·2SiO2). The Merwinite precipitated faster than the Gehlenite. The nose position of Merwinite was 4 s at 1 090 and the nose of Gehlenite was 8 s at 1 230. CCT diagram had wider glass region than TTT diagram and the temperature of crystal region decreased to 1 340 at 1 000 s and 1 160 at 14 s.