Detection of Free-lime in Steelmaking Slag by Cathodoluminescence Method

Abstract

In steelmaking industry of Japan, one-third steelmaking slag is reused for road constructions. Steelmaking slag contains a few weight percent of calcium oxide, which is termed as free-lime. Free-lime expands its volume by the reactions with water and carbon dioxide, leading to collapse of road substrates. To prevent the road collapse, free-lime in steelmaking slag is now treated to be transformed to Ca(OH)₂ and CaCO₃ by an aging process; however, the residual free-lime still causes the road collapse. Therefore, it is necessary to detect free-lime in steelmaking slag to safely reutilize it for road constructions. In the present study, we carried out a cathodoluminescence (CL) analysis for identification of free-lime in steelmaking slag. For this purpose, we prepared a synthesized slag sample including free-lime by heating mixtures of reagents. A part of the sample was soaked in water at 70°C for 3 h (aging process). Crystallized free-lime in the sample illuminated orange due to an emission peak of Mn²⁺ at 600 nm regardless of the precipitation forms of free-lime. Undissolved free-lime illuminated blue due to an emission peak of oxygen vacancy at 460 nm. The sample after the aging process illuminated orange because of CaCO₃ having with an emission peak of Mn²⁺ at 620 nm. We could identify free-lime and CaCO₃ by camera detecting light over 680 nm selectively because the luminescence of CaCO₃ also appeared on longer wavelength side over 690 nm. Thus, we could detect free-lime in steelmaking slag by capturing CL images within 30 s.