Carbothermic Reduction Kinetics of Phosphorous Vaporization from Tri-calcium Phosphate (TCP) Under Microwave Rapid Heati
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INTRODUCTION
THE sustainable supply of phosphorous to the industry is an important aspect of our daily lives because phosphorous is one of the primary indispensable elements for human bodies, fertilizers of plants, and raw materials for industrial chemical products. However, phosphorous production from resource-rich mines is running low.[1] Therefore, steel-making slag[2] and sewage sludge[3] are considered important resources of phosphorous through the recycling process.
NOBORU YOSHIKAWA and SHOJI TANIGUCHI are with the Graduate School of Environmental Studies, Tohoku University, 6-602 aza-Aoba, Aramaki, Aoba-ku, Sendai, 980-8579 Japan. Contact e-mail: [email protected] MANAMI SUNAKO and KEITA KAWAHIRA are with the Graduate School of Environmental Studies, Tohoku University, Sendai, Japan. KOKI SUZUKI and KAZUNORI MIYAMOTO are with the Department of Materials Science and Engineering, Tohoku University, Sendai, Japan. Manuscript submitted August 21, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
In the steel-making process, dephosphorization is one of the important processes to produce high-quality steels and is accomplished by the refining process of blowing oxygen into molten pig iron and oxidizing the dissolved phosphorous (impurity) in a converter operation (BOP) or during pretreatment. For the purpose of maintaining a high-basicity slag for dephosphorization, calcium oxide (CaO) is charged and a so-called converter slag is formed with the proper composition for incorporating the phosphorous oxides.[4] Such a slag generally contains phosphorous oxide and excess calcium oxide after dephosphorization. When it is cooled to room temperature, it generally exists in a form of tri-calcium phosphate (TCP) along with silica and other constituents such as MgO. On the other hand, in the recycling of phosphorous from a steel-making slag (containing TCP), the reduction of phosphorous is performed at an elevated temperature where phosphorous is mostly removed in a vapor state by carbon. In this removal process, understanding the carbothermic reduction kinetics of TCP is of primary importance. A report on the reaction kinetics can be found[5] and the role of phosphorous vapor is discussed[6,7] for these reduction processes. In
this study, our first objective is to investigate the dependence of the reduction kinetics of TCP on the heating rate. In the steel-making slag recycling process, the coexistence of iron oxides and their relative reduction rates is important. Phosphorous absorption into a solid phase of reduced iron is not significantly observed; however, when the reduced iron is melted, it is expected that more phosphorus is absorbed that results in less extraction. It is expected that one of the effective methods for phosphorous removal from steel-making slag can be achieved by applying a high heating rate to the reduction reaction temperature, meaning phosphorous vapor removal is completed before the reduced liquid iron appears. There have been attempts at dephosphorization by heating using an induction furnace a
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