Complete recycling of valuable metals from electroplating sludge: Green and selective recovery of chromium

The recovery of chromium (III) is a key step for complete recycling of heavy metals from electroplating sludge. Therefore, thermodynamic equilibrium diagrams of Me-PO43--H2O[Me: Cr (III), Zn (II), Cu (II), Fe (II), Fe (III), Ni (II)] systems at 298 K have been established, the thermodynamic analysis for the precipitation of chromium (III) using phosphates and the conversion of CrPO4 to Cr(OH)3 using caustic soda were conducted. The results showed that phosphate seemed preferentially combined with Cr (III) rather than other metal cations in order of Cr (III) > Fe (III) > Fe (II) > Zn (II) at the pH range of 1.0 ∼ 5.0. The verification experiment results indicated that about 94.1% Cr (III), 5.51 %Fe (II), and 0.33% Zn (II) reached were precipitated by adding 110% theoretical amount of Na3PO4·12H2O at equilibrium pH equal to 2.0. As consistent with the thermodynamic analysis, alkali conversion of CrPO4 with NaOH achieved a leaching efficiencies of Cr (III) and phosphorus were 5.1% and 90.6%, which realized phosphate recycling and the recovery of leached Cr (III). 99.7% Zn was deeply separated and recovered from Fe (II/III) by D2EHPA by two stages of counter current extraction with an O/A ratio of 1:2, 5 min stirring at 20 °C. And the optimal conditions for Zn stripping were considered as mixing 1 mol/L H2SO4 and loaded D2EHPA with O/A ratio of 5:1 at room temperature for 5 min. The proposed process shows a promising industrial application in view of its low cost, complete recycling, and wide adaptability. Electroplating sludge will cause serious pollution to the environment if not properly treated. This study reports a new treatment method of electroplating sludge to achieve comprehensive recovery of all valuable metals. Compared with traditional process, this process exhibits a promising industrial application due to its no emissions, lower cost, complete reagent cycle, purer product, higher quality, broader adaptability, and greener production.