PURPOSE: To execute two-phase sepn. of an org. phase and water phase without inducing an increase in the viscosity of an org. solvent having an increased concn. of metallic ions, the solidification thereof and the elimination of the metallic ions by using an aq. NH4HF2 soln. having a specific concn. in the stage of cleaning said org. solvent.

CONSTITUTION: The aq. soln. of NH4HF2 having the concn. ranging 10W45g/l is used for the aq. soln. for cleaning of the org. solvent having the increased concn. of the metallic ions by an operation for extracting the metallic ions in the aq. soln. using an org. solvent consisting of an ion exchange type extracting agent and petroleum hydrocarbon diluent. The waste water contg. fluoride at a high concn. discharged partly for the purpose of controlling the concn. and compsn. of a fluoride soln. to be used in a metallic ion elimination stage and the waste water contg. dilute fluoride discharged from an org. solvent cleaning stage after the elimination are mixed and the mixture thereof is made usable as the aq. soln. for cleaning, if necessary. The concn. as NH4HF2 is adjusted to the 10W45g/l before use in this case as well.