Process for recovering components from pickling acid residue
A novel process for treating and recovering valuable metals and other components from pickling acid residue (PAR) has been developed. The metals and other components are recovered by neutralizing the pickling acid residue using a magnesium compound or a mixture of magnesium compounds, and separating components of the resulting mixture (metals and sulfates) into products that can be reused, such as magnesium sulfate, nickel sulfate, iron and chromium phosphate, or various metal hydroxides or oxides.
1. A method for recovering sulfates and metals from a sulfuric acid containing pickling acid residue comprising the steps of:
a) mixing the sulfuric acid containing pickling acid residue with at least one magnesium compound selected from the group consisting of magnesium oxide, magnesium hydroxide and magnesium carbonate to obtain a modified pickling acid residue having a pH in the range of 7-10,
b) heating the modified pickling acid residue to a temperature of 70-90° C. and bringing magnesium into solution from the modified pickling acid residue as magnesium sulfate at a stable pH in the range 5.5-7, optionally adjusting the pH with sulfuric acid,
c) recovering the magnesium sulfate as a magnesium sulfate solution by filtration, and
d) obtaining a FeNiCr residue as a precipitate.
2. The method according to claim 1 , wherein the method further comprises the steps of:
a) slurrying the FeNiCr residue in water to obtain FeNiCr slurry,
b) heating the slurry to 70-90° C. and lowering the pH of the FeNiCr slurry using sulfuric acid to a range of 2.5-3.5, bringing nickel into solution from the FeNiCr slurry as nickel sulfate,
c) recovering the nickel sulfate as a nickel sulfate solution by filtration, and
d) obtaining a FeCr residue as a precipitate.
3. The method according to claim 2 , wherein the method further comprises the steps of:
a) slurrying the FeCr residue in water to obtain FeCr slurry,
b) heating the slurry to 70-90° C., adding sulfuric acid to the FeCr slurry to obtain pH 0.5-1 and to dissolve solids,
c) cooling the slurry to a temperature of 3-10° C.,
d) adding a stoichiometric amount based on expected iron content of phosphoric acid to the FeCr slurry,
e) precipitating iron as iron phosphate by adjusting the pH with at least one magnesium compound selected from the group consisting of magnesium oxide, magnesium hydroxide and magnesium carbonate to the range of 2-3,
f) recovering the iron phosphate as a precipitate by filtration, and
g) obtaining a residual solution.
4. The method according to claim 3 , wherein the method further comprises the steps of:
a) adding a stoichiometric amount based on expected chromium content of phosphoric acid to the residual solution,
b) adjusting pH to 4-6 with at least one magnesium compound selected from the group consisting of magnesium oxide, magnesium hydroxide and magnesium carbonate to precipitate chromium phosphate at a temperature of 50-60° C., and
c) recovering the precipitated chromium phosphate by filtration.
5. The method according to claim 1 , wherein the magnesium compound is magnesium oxide.
6. The method according to claim 1 , wherein the magnesium compound is magnesium hydroxide.
7. The method according to claim 1 , wherein the magnesium compound is magnesium carbonate.
8. The method according to claim 1 , wherein the magnesium compound is a combination of magnesium oxide, magnesium hydroxide and magnesium carbonate.
9. The method according to claim 1 , wherein the at least one magnesium compound or the combination of magnesium oxide, magnesium hydroxide and magnesium carbonate is added in dry form.
10. The method according to claim 1 , wherein the at least one magnesium compound or the combination of magnesium oxide, magnesium hydroxide and magnesium carbonate is added as a water slurry.