| abstract |
The invention relates to a method for purifying aqueous solutions containing at least one impurity element from the group consisting of arsenic, phosphorus, mercury and other heavy metals and solid matter by precipitation in at least two stages with the aid of precipitation reagents possessing ions capable of forming not-readily dissolved hydroxide precipitate. In accordance with the invention the main part of the precipitation reagent is introduced to the stage or stages following the first stage. The impurity element-containing precipitate formed in the aqueous solution subsequent to adding the reagent is separated therefrom substantially completely, and is returned so as to be present in the first precipitation stage, while that part of the solution which has been freed from precipitate in a later stage is withdrawn from the system. Substantially all the impurity element amount present in the ingoing aqueous solution are separated therefrom in the form of a sludge prior to the second precipitation stage. The amount of reagent introduced to the precipitation stages is suitably adapted so that the ratio of equivalence Me to a mole of impurity elements, expressed as X is at least 2, preferably 10-15, in the last stage, and at least 0.8, preferably 1-3, in the first stage. |