Phosphorous Recovery from Ca2SiO4–Ca3P2O8 Solid Solution By Carbothermic Reduction
Huafang Yu, Xin Lu, Takahiro Miki, Yasushi Sasaki, Tetsuya Nagasaka
Abstract
Steelmaking slag is considered as a promising phosphorus resource as it contains the phosphorus in a condensed phase of Ca2SiO4–Ca3P2O8 solid solution (C2S–C3P)ss. In the present study, the carbothermic reduction of (C2S–40 mol% C3P)ss at 1573 K was carried out to understand the phosphorus removal mechanism by varying reduction temperature, external gas flow rate, amount of graphite, and concentration of Ca3P2O8 in (C2S–C3P)ss. The carbothermic reduction proceeded as a zero-order reaction when the temperature was 1573 K, despite the P2O5 concentration decrease with the reduction time. At temperatures less than 1473 K, the reduction did not occur. Based on the XRD analysis of the reduced (C2S–C3P)ss samples, it was found that carbothermic reduction of the C2S–C3P solid solution proceeded by the decomposition of the C2S–C3P solid solution into CaO and C2S with the removal of phosphorus, but not by a uniform decrease of the phosphorous throughout the C2S–C3P solid solution. The overall reduction degrees were found to be independent of the reduction rate (zero-order reaction) and the P2O5 concentration in the (C2S–C3P)ss. From these independencies, the rate-controlling step of the carbothermic reduction of (C2S–C3P)ss was concluded to be the Boudouard reaction.