Calcium extraction from unmilled BOF slag for energy-efficient CO₂ sequestration using graphical analysis and application for AMD treatment

ABSTRACT

Production of precipitated calcium carbonate (PCC) by indirect carbonation of BOF (Basic Oxygen Furnace) slag usually requires a calcium extraction stage to promote the dissolution of calcium. Previous works have shown that the grinding of BOF slag to smaller particle size accelerates calcium leaching at the expense of a high energy input and further environmental constraints such as dust management. In order to minimize the excessive consumption of energy due to grinding, this study focused on the optimization of calcium extraction from unmilled BOF slag using ammonium chloride (NH₄Cl) and ammonium nitrate (NH₄NO₃). The study was conducted with a solvent-to-slag mass ratio of 10/1, to avoid the dissolution of impurities such as iron, silicon, magnesium and manganese and guarantee minimum equipment costs. The results obtained using umilled BOF slag show that the efficiency of calcium extraction reached ~75% for NH₄NO₃ and ~69% for NH₄Cl in two extraction stages. A calcium carbonate precipitation yields of 80% was reached at 25°C and 6.5bars. Scanning Electron Microscopy (SEM) analysis of the dry PCC obtained showed that it is mostly dependent on the flow rate of CO2 and the carbonation pressure. Finally, the PCC was evaluated for the treatment of acid mine drainage.

KEYWORDS:

• Calcium extraction
• BOF slag
• mass balance
• carbonation
• calcium carbonate
• acid mine drainage
• graphical analysis

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Notes on contributors

G. Kahilu Mwengula

G. Kahilu Mwengula is a PhD candidate at the University of Witwatersrand with a strong academic background and industrial experience in Metallurgical processes and applications mainly in mineral processing and hydrometallurgy of different South Katanga’s minerals in the Congo, Democratic. Kahilu hold a MSc in Chemical Engineering from the University of Witwatersrand.

Jean Mulopo

Jean Mulopo is currently an associate professor in the school of Chemical Engineering at the University of Witwatersrand. He is the Coordinator and founder of the Sustainable Energy and Environment Research Unit (SEERU) within the School of Chemical and Metallurgical Engineering. One of the key focus areas of his research is on understanding the opportunities and constraints provided by the general and hazardous waste generation towards growing a green economy particularly in the context of water and energy challenges. Prior to joining Wits University, Jean was a Senior Engineer/Scientist in the Council of Science and Industrial Research (CSIR) Natural Resources and the Environment competency area (Waste Research Group). Jean holds a PhD in Chemical Engineering from the University of Witwatersrand, a postgraduate diploma in higher education from the Wits School of education and a certificate in Advanced Management and Business Leadership from the Wits Business School.