Extraction And Separation Of Molybdenum, Vanadium,Nickel, Cobalt From Acid Leach Solution

Spent hydrodesulphurisation(HDS) catalysts contain molybdenum, cobalt, nickel and/or vanadium on an alumina carrier. These catalysts are regarded as the most important catalysts for recycling these metals due to not only their large amounts and enormous economic values, but also environmental concerns if disposed. This paper develops an effective and clean process to recover molybdenum, vanadium, nickel and cobalt from a sulphuric acid leach solution of spent HDS catalysts supplied by a Korean plant using combination of solvent extraction and ion exchange. The study mainly focused on the separation of molybdenum and vanadium over nickle, cobalt, iron and aluminum by solvent extraction, the extraction and stripping isotherms of molybdenum and vanadium, the separation of nickle and cobalt over iron and aluminum using Dowex M4195resin, the stucture identification of Mo-LIX63and V-LIX63complexed, the extraction kinetics study of molybdenum and vanadium using Lewis cell. The main experimental results are as follows:A commercial hydroxyoxime extractant LIX63can be used to selectively extract molybdenum(Ⅵ) and vanadium(V) from a synthetic acid leach solution of spent HDS catalysts containning Mo(Ⅵ), V(Ⅴ), Ni(Ⅱ), Co(Ⅱ), Fe(Ⅲ) and Al(Ⅲ) in the pH range of1.0-2.0. After a single contact, the extraction of molybdenum(VI) and vanadium(V) reached>99.9%, and their separation factors over iron(III) reached over25000and87000, respectively. A solution of NaOH was used to strip loaded organic with>95%stripping of both molybdenum and vanadium in a single contact. After ammonium salt precipitation of vanadium from the loaded strip liquor, a quaternary ammoniumsalt Aliquat336was then employed for the further separation of vanadium(V) from molybdenum(VI) in the pH range of8.0-8.5with the extraction of vanadium reached>97%. The loaded organic was stripped using mixed solution of1mol/L NaOH and0.5mol/L NaCl. The stripping of vanadium reached>99%.The extraction and stripping distribution isotherms of molybdenum and vanadium and their McCabe Thiele diagrams showed that three-stage counter-current operation is needed to extract all of the molybdenum and vanadium. Only one theoretical stripping stage is needed to strip most of the extracted molybdenum and vanadium from the loaded organic solution. The extraction kinetics of vanadium was very fast with92%V extracted in the first60seconds. However, only about72%Mo was extracted in the first60seconds. After3mins of mixing, the vanadium and molybdenum extraction reached98%and82%, respectively. The increase in temperature and stirring tip speed had little or no effect on the extraction kinetics of the two metals. The stripping kinetics of molybdenum and vanadium showed that More than94%V was stripped after one minute of stripping and95%Mo stripped after three minutes of stripping. The test results of phase disengagement time(PDT) revealed that under both extraction and stripping conditions, phase separation with both organic and aqueous continuities was fast in terms of PDT values. This indicates that although aqueous continuity is the preferred operating mode especially in extraction for ease of pH control if required, both continuities may be used for extraction and stripping as far as phase disengagement is concerned.With the solution containing Fe(Ⅲ) at pH=3, the metal adsorption with Dowex M4195was found in the order (adsorption constant K): Cu (3402)>> Ni (452)>Zn (444)>Co (89)>Fe(Ⅲ)(55)>>Cr, Mn, Mg,Al,Ca (17-23).Cobalt cannot be effectively separated from Fe(Ⅲ) due to their similar adsorption constant and, therefore, poor selectivity.With the solution containing Fe(Ⅱ) at pH=3, the metal adsorption was found in the order (adsorption constant K): Cu (258)>>Ni (74)>Zn (41)>Co (26)>>Al, Fe(Ⅱ), Cr, Mn, Mg,, Ca (<1.1).Dowex M4195resin can be used to effectively separate nickle and cobalt over iron(Ⅱ) and aluminum. These results indicate that a pre-reduction of Fe(Ⅲ) to Fe(Ⅱ) is necessary for potential application of Dowex M4195for the solution containing Fe(Ⅲ). This is particularly important for the recovery of cobalt.A preliminary column test was carried out to obtain continuous loading data using Dowex M4195and the synthetic solution containing22g/L Fe(II). The The breakthrough orders agreed with the adsorption order as obtained in the batch tests. The loaded resin was firstly desorbed using100g/L H2SO4to desorb Ni, Co, Zn and followed by desorbed using12.5%NH3·H2O to desorb copper.The molar ratio between molybdenum and the anti oxime isomer of LIX(?)63in the extraction reaction was inferred to1:2using saturation extraction method. The extraction reaction of molybdenum with the LIX(?)63oxime (H2ox) in sulphate solutions can be expressed as: MoO22++2H2ox→MoO2(Hox)2+2H+The molar ratio between vanadium and the anti oxime isomer of LIX(?)63in the extraction reaction was determined to be1:1, suggesting that vanadium was predominant in the form of VO2+in the solution. The extraction reaction of vanadium with the LIX(?)63oxime in sulphate solutions can be expressed as: VO2++H2ox→VO2(Hox)+H+According to the NMR and IR analysis of Mo-LIX63and V-LIX63complexes, the structure of two complexes was firstly proposed as follows: The results of extraction kinetics for molybdenum and vanadium using CSIRO Lewis cell demonstrated that the Lewis cell worked well in terms of maintaining constant interfacial area under turbulent flows. UV-Vis was firstly used to analyse the concentration of vanadium online in the kinetics study, the results obtained by UV-Vis and ICP-AES analytical methods were in good agreement, demonstrating that both methods are reliable and can be used in solvent extraction kinetic studies with the Lewis cell.At stirring speed of150rpm in both organic and aqueous phases, the molybdenum extraction with the LIX63system was proposed to shift to transient regime in the temperature range of30-50℃from kinetic regime in the temperature range of20-30℃, supported by the change of apparent activation energy from57.5kj/mol to25.4kj/mol. The vanadium extraction with the LIX63system was shifted from kinetic regime in the temperature range of20-30℃to transient regime in the temperature range of30-50℃, supported by the change of apparent activation energy from90.3kj/mol to31.9kj/mol.