Regeneration of Spent Phosphoric Acid Leaching Solutions by Solvent Extraction

Solvent extraction was employed to regenerate spent phosphoric acid leaching solutions used for cleaning galvanized pole line hardware. Initial laboratory tests showed that a cationic extractant, such as di-2-ethylhexyl phosphoric acid (DEHPA), could regenerate the H3P04 and remove the contaminating zinc. Subsequently, a pilot plant campaign demonstrated the regeneration process in a continuous circuit. About 400 L of spent phosphoric acid were processed. The phosphoric acid was regenerated to 79%-82% of its original concentration, and 68%-78% of the contaminating zinc was removed. The stripping circuit produced > 200 L of solution containing about 110 g Zn/L in a sulphuric acid medium that was suitable for zinc production by electrowinning. Attempts were also made to remove the iron and organic contaminants present in the spent phosphoric acid solutions. No effective extractant for iron was found, but the iron could be controlled by precipitation from a bleed stream. Although the organic contaminants could be effectively removed with activated carbon, this may not be necessary as the solvent extraction process appears to be able to tolerate their presence.