Patent EA-005306-B1 (EA)
Title: A method for the removal of metals from an aqueous solution using lime precipitation
Inventor: NYMAN BROR
IPC: C23G1/36
Filed: 2002-06-14
Abstract: 1. A method for the removal of metals from their aqueous solution as metallic hydroxides and gypsum using lime precipitation, characterized in that the precipitation is made in at least two neutralization stages, where the majority of the metal-containing feed solution is routed to the first neutralization stage, where the pH is adjusted using a lime compound to a value of at least 10.5 and in the second stage the pH is adjusted using the feed solution to at least 9.5; after the final neutralization stage the precipitate is settled in a settling stage, from where the metallic hydroxide-gypsum slurry is recirculated to the first neutralization stage. 2. A method according to claim 1, characterized in that the pH value of the first neutralization stage is 10.5-11.5. 3. A method according to claim 1, characterized in that the pH value of the second neutralization stage is 9.5-10.5. 4. A method according to claim 1, characterized in that the aqueous solution contains fluorides, which are removed during neutralization. 5. A method according to claim 1, characterized in that 5-30 % of the metal-containing feed solution is routed to the second neutralization stage. 6. A method according to claim 1, characterized in that neutralization is performed with lime milk Ca(OH)2. 7. A method according to claim 1, characterized in that the amount of gypsum is adjusted to be 10-50g/l in the neutralization stages. 8. A method according to claim 1, characterized in that neutralization takes place at a temperature between 5-95 degree C. 9. A method according to claim 1, characterized in that the neutralization stages are equipped with pH sensors. 10. A method according to claim 1, characterized in that the neutralization stages are located in series in relation to the gypsum containing underflow and lime compound. 11. A method according to claim 1, characterized in that both the feed solution and the lime compound are fed to the surface of the solution in the reactor in the neutralization stages. 12. A method according to claim 1, characterized in that the feed solution and the lime compound are fed to opposite sides of the reactor. 13. A method according to claim 1, characterized in that the metallic hydroxide-gypsum slurry is premixed into the lime compound before being fed into the neutralization reactors. 14. A method according to claim 1, characterized in that each neutralization stage takes place in a reactor that is equipped with a helix-type mixing element. 15. A method according to claim 14, characterized in that the diameter of the mixing element is 50-80% that of the neutralization reactor. 16. A method according to claim 1, characterized in that a polymer to flocculate solids is fed into the settling stage. 17. A method according to claim 16, characterized in that the amount of the flocculating polymer is 50-150 g per tonne of solids fed into the settling stage. 18. A method according to claim 1, characterized in that a metal containing aqueous solution is formed from the electrolyte solution used in pickling steel and the pickling flush water.