Magnetic Separation for Recovering Ni Compounds from Plating Waste with Use of HTS Bulk Magnets

A characteristic magnetic separation technique has been developed to recycle the Ni resource from the waste fluid of electroless Ni-plating processes. Since the plating waste still contains high concentration of Ni ions after several cycles of plating processes, the magnetic separation experiments were conducted to collect the precipitates containing Ni compounds. After extracting the fine NiHPO₃ precipitate from the waste liquid, we synthesized the NiSO₄ crystals from the NiHPO₃ precipitates through the reaction with the concentrated sulfuric acid. The open-gradient magnetic separation was employed to separate the NiSO₄ crystals from the mixed slurry composed of the NiSO₄ and the NiHPO₃ due to the difference between their magnetic properties. The experiments were practically conducted with use of the Gd123-based HTS bulk magnets with very steep gradient of magnetic field of over 100 T/m, which were activated to the magnetic fields of 1.53 and 1.95 T by the pulsed field magnetization process operated at 35 K. The ratio of NiSO₄ content in the collected slurry attracted to the magnetic poles reached up to 98%, which suggests that the paramagnetic NiSO₄ crystals are preferentially collected by the magnetic force from the mixed slurry in the strong magnetic field generated by the HTS bulk magnet.