Recovery of Hf and Zr from slurry waste of zirconium purification plant using solvent extraction

• Process for recovery of Zr and Hf from raffinate stream of zirconium purification plant has been developed.
• In-house synthesised MAPO ligand is capable of extracting Zr and Hf from lean solution at low acidity.
• The process was applied in 5 stage mixer settler set up in counter current mode streams were for concentration build up.
• About 9.5 g/l of hafnium nitrate solution with 95% purity was obtained with Zr as impurity.

Hafnium is a very important valuable metal with numerous applications in electronics, refractory and nuclear industry. Its source is the secondary waste streams of zirconium purification plant. The waste raffinate stream generated from Zr extraction plant contains about 4 kg of Hf per ton of zircon sand processed. The conventional solvent extraction process with Tri Butyl Phosphate (TBP) as extractant is not suitable to recover Hf value lost in this low acidic lean stream. Presence of high concentration of silica, about 2 g/L poses additional challenge for application in solvent economic continuous contactors. A novel alkyl phosphine oxide based ligand having high separation factor for Zr over Hf, Mixed Alkyl Phosphine Oxide (MAPO) has been synthesised indigenously which is applicable for this lean stream. Batch equilibrium data were generated for this novel solvent to optimise the process conditions for application in continuous contactor. The developed process was successfully applied in five stage mixer settler set up in counter current mode with MAPO in mixed diluents as extractant, zirconium nitrate as the scrubbing agent and oxalic acid as the strippant. Recycle of extractant, scrub and strippant with concentration make up has been attempted to increase metal concentration in product stream and to reduce plant waste volume. The work has been successful in achieving not only complete extraction of the Hf value from the plant stream but also recovery of the metal as a concentrated solution ~ 10 g/L with 95% purity which was efficiently converted to oxide. The Zr value is also recovered from the waste stream completely during stripping. Studies were also conducted on the characteristics of silica present in the waste stream and a technique comprising of flocculation followed by ultrafiltration was applied to make the slurry feed solid-free to achieve trouble free operation. The present studies show the potential of the developed process for application to Zr purification plant for recovery of the Hf as well as the Zr value from its raffinate waste.