The present research is directed towards processing of slag originating during the yellow phosphorus production. The slag was investigated using physical and chemical treatment methods. The following group of rare earth elements were identified in phosphorus slag such as: Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Ho, Tm, Yb, Lu. Thermodynamic calculation for reactions probability of phosphorous slag components with nitric acid was made. Nitric-acid leaching of slag generated during phosphorus production was investigated for extraction of rare earth metals (REMs). Silicon containing cake obtained after leaching was suitable for precipitated silicon dioxide production. Behaviour of associated components such as calcium, aluminium and iron was studied during the leaching process. The following optimum parameters were selected for leaching studies: nitric acid concentration was 7.5 mol/dm(3); solid-to-liquid ratio was 1:2.6-3; temperature was 50-80 degrees C; process duration was 1 h; pulp stirring rate was 500 rpm and the recovery of rare-earth metals, calcium, aluminum and iron into the solution were seen to be 85%, 98%, 80.7%, and 11.8%, respectively. Cake produced as a result of leaching contained similar to 80-85% of amorphous SiO2. The solution obtained after phosphorus slag leaching was processed through solvent extraction methods to concentrate and separate it from basic macroimpurities. After precipitation of REMs oxalates from strip liquor and calcination of the precipitate a concentrate was obtained, which contained similar to 17% of EREMs oxides.