This article presents the results of a study conducted to investigate the recovery of rare earth metals and precipitated silicon dioxide from phosphorus slag. To determine the possibility and completeness of the studied processes, thermodynamic data was obtained through the determination of the Gibbs energy and equilibrium constant of the reactions of calcium metasilicate with different reagents, namely sodium hydroxide and carbonate as well as nitric, hydrochloric, and sulphuric acids. This article also presents the results of studies on the treatment of phosphorus slags via hydrometallurgical methods using alkaline agents (sodium hydroxide and sodium carbonate) and nitric acid. The recovery of silicon in solution by the autoclave leaching of phosphorus slag using solutions of sodium hydroxide and sodium carbonate resulted in recovery efficiencies of 1.1% and 16.6%, respectively. The nitric acid treatment of phosphorus slag was studied, and the recovery efficiencies of various elements were the following: rare earth metals, 98.3-98.6%; aluminium, 96.5-98.6%; iron, 94.9-96.5%; and calcium, 99.1-99.5%. Nitric acid (46.5%) was selected as the phosphorus slag recovery agent. The cake produced after the nitric acid treatment of phosphorus slag was leached using two processes based on the use of a sodium hydroxide solution: (1) in a temperature-controlled cell under normal conditions and (2) in an autoclave. The process of leaching under normal conditions was determined to be the most effective process, resulting in an efficiency of silicon recovery into solution of 97.7%. (C) 2015 Elsevier Ltd. All rights reserved.