Deep separation of critical metals of Mo and Re from waste solution by stepwise precipitation flotation: Selective chelation underlying separation mechanism

Conventional selective precipitation method has disadvantage in the low-efficiency filtration of fine precipitated particles during the treatment of waste solutions. To overcome this shortcoming, a stepwise precipitation flotation process was proposed to deeply separate Mo and Re from Mo-Re mixing waste solution with low concentrations in this work. Chelating agents are critical for the selective separation of Mo and Re. Experimental results show that the rhenate can be firstly separated from mixing solution by the synergistic effect of cetyl trimethyl ammonium bromide (CTAB) and dodecyl trimethyl ammonium bromide (DTAB). In the weak alkaline solutions with pH range of 6–10, CTAB shows a stronger competitive adsorption capacity with rhenate than molybdate through electrostatic interactions and coordination reactions. Meanwhile, DTAB has a great inhibition effect on the chelation of molybdate by CTAB in this pH range, which can further enhance the selective chelation of rhenate by CTAB. After Re separation, molybdate can be further recovered by CTAB from the residual solution with pH range of 3–5. In this pH range, Mo mainly exists in the form of isopolymolybdates acid, and molybdate shows a stronger competitive adsorption ability with CTAB than DTAB. Molybdate reacts with CTAB to form precipitate, and the fine precipitated particles are readily separated via flotation. The turbidity of the residual liquid after flotation is less than 5 NTU, meeting the wastewater discharge standard. In the Re precipitation flotation procedure, the comprehensive efficiencies of Re and Mo are 96.6% and 0.14% under the main conditions of temperature of 25°C, n(CTAB)/n(Re) molar ratio of 2:1, n(DTAB)/n(CTAB) molar ratio of 2:1, and pH of 6. In the further Mo flotation separation, Mo comprehensive recovery of 97.9% is achieved by using CTAB and KH2PO4 as chelation agent and flocculant, respectively. The technological parameters of the conventional selective precipitation method and precipitation flotation method as well as the Mo and Re distributions in these two processes were compared in this study. Moreover, compared with the conventional selective precipitation method using inorganic precipitators, the precipitation flotation process shows a unique advantage in the easy purification of obtained products due to the utilization of organic agents.