The Study On The Potential-pH Diagram Of V-P-C1-H₂O System

Potential-pH diagram is a practical and intuitive tool, which shows therelationship between the stability of the components and the potential value of theenvironment. Use the potential-pH diagram can illustrate the range of the potentialvalues and the pH values in the natural environment, determine the type of chemicalreactions which may be present. Furthermore, through the simulation of theenvironment of the metallurgical process, to add the commonly coexisting ions andcoexist ligands, can able to provide guidance on the hydrometallurgical process.China has a rich resources of vanadium, the commonly used extraction method issodium roasting-flooding-ammonium salt precipitation. To study the thermodynamicof the process, identify the influence of potential value and pH value on the chemicalspeciation of vanadium and the advantages region distribution of the solution, it’snecessary to explor a new efficient hydrometallurgical vanadium extraction method.Based on the principles of simultaneously balance and the concentrationcomparison, the predominance diagrams and the Pourbaix diagram of V-P-Cl-H2Osystem and its related sub-systems were studied. Using the experimental methods, theV-P-H2O and V-Cl-H2O system were studied experimentally. The research results hasa positive significance on the thermodynamic study of vanadium extract process, andhas a contribution on the explore the development of a new clean and efficientmethod.The predominance diagram of P-H2O system shows that, at the low potentialareas, there is only proton reaction between the advantages components of H2PO2-andH3PO2. In the high potential area, the equilibrium reaction between the pentavalentphosphorus components H3PO4and H2PO4-mainly occurred in acidic condition. Thereaction between the HPO42-and PO43-is carried out in alkaline solution. Increase thetotal phosphorus concentration is beneficial to precipitate the solid phase of P, whilethe solid phase P4H2precipitation is less affected by the total phosphorusconcentration.The predominance diagram of Cl-H2O system shows that, when the potentialvalue is greater than1.5V, the dominant species of chlorine is ClO4-. In reducingconditions, the chloride ion(Cl-) is the predominance component. In the high potentialregion, when the pH value greater than3, the Cl2(aq.) is the predominance species. In the Pourbaix diagram of this system, the gas phases Cl2(g) and ClO2(g) have the stableregions. Increase of the potential value, the stable region of ClO2(g) will substitute theCl2(g) region.The predominance diagram of V-H2O system at the lower concentration showsthat, the low price vanadium species located in reducing region, while thepredominance region of high price vanadium species is in the oxidation zone. The sizeof the advantage species HVO2+has the smallest area. With the increase of the totalconcentration of vanadium, pentavalent vanadium species prone to polymerize andform polynuclear vanadium species. In the meanwhile, the area of the stable region ofthe solid phase V2O2(s), V2O3(s) and V6O13(s) have increased, but the stable region ofthe solid phase V(s) remains unchanged. This shows that increasing the totalconcentration of vanadium is conducive to solid V2O2(s), V2O3(s) and V6O13(s)precipitation, and favor for the V2O4(s) and V2O5(s) precipitation operation.In the high potential condition of V-P-H2O system, the phosphorus dominantspecies are HPO42-and PO43-, the vanadium dominant species are VO2+, H3V2O7-, andthe vanadium-phosphorus dominant species is (VO)3(PO4)2(aq.). In the neutralelectrical alkaline region, the dominant species of vanadium were HV2O73-, V2O74-,and there were protonation reaction between these two components. In the reductionzone, with the increase of the pH value, the predominance species of phosphorus were:H3PO2, H2PO2-, HPO32-in sequence, and the dominant species of vanadium were V2+,VOH+. In the range of the pH value is less than11.7, and the potential range between-0.6V~2.2V, the predominance species is (VO)3(PO4)2(aq.).In the oxidation environment of V-Cl-H2O system, the predominance species ofvanadium were VO2+, H2V10O284-and HV10O285-, the dominant species of chlorinewas ClO-, the predominance species of vanadium-chlorine was VOCl2(aq.). In theneutral region, the dominant species of chlorine was Cl-, and the predominancespecies of vanadium were V3+, VOH+. In the low potential area with the pH value lessthan5, the V2+is the predominance species. The absolute predominance region ofVOCl2(aq.) is the pH value less than0.9, and the potential value range between1.2V~1.4V.In the oxidation area of V-P-Cl-H2O system, the predominant species of chlorineis ClO4-, the predominance species of vanadium were VO2+and H2V10O284-, thedominant species of phosphorus is HPO42-. In the neutral region, with the increase ofthe pH value, the dominant species of vanadium were V3+, VOH2+, VO+in sequence.In the reduction area, the dominant species of chlorine was ClO-, the dominant species of phosphorus were H2PO3-, HPO32-, and the dominant species of vanadium were V2+,VOH+. The predominance region of VOCl2(aq.) located in the pH value less than2.1and the potential range between-0.3V~0.4V. The predominance region of(VO)3(PO4)2(aq.) is located in the neutral region with the pH value greater than2.1.In the experimental potential-pH diagram of V-Cl-H2O system, there are fivepredominance regions, corresponding to the VO2+, H2VO4-, VO43-, VO2+, HV2O5-respectively. In the oxidation area, the predominance species were pentavalentvanadium, in the reducing area, the tetravalent and trivalent vanadium were theadvantage species. With the increase of the concentration of total vanadium, thepredominance area of VO2+enlarged.In the experimental potential-pH diagram of V-P-H2O system, there are sixpredominance regions, corresponding to the VO2+, H2VO4-, HVO42-, VO43-, VO2+,HV2O5-respectively. In the alkaline region with high electric potential, thepentavalent vanadium anion was in the form of VO43-. In the acid region of lowpotential, the tetravalent vanadium formed as VO2+, in the alkaline region of lowpotential, the tetravalent vanadium existence as HV2O5-. With the decrease of the totalvanadium concentration, the area of HV2O5-has significantly increased, while the areaof HVO42-and VO43-were significantly reduced.