Study On Nonlinear Kinetics During Electro-synthesis Of Metal Mn And KMnO₄

Electrolysis is a widely applied technology in the chemical metallurgical industry.Metal manganese (Mn) and potassium permanganate (KMnO4) are obtained fromnatural mangnese ore with intermediate valence by electrolysis. Metal Mn is preparedfrom Mn2+solution by electrochemical reduction, while KMnO4is prepared fromMnO42–solution by electro-oxidation. However, high resource consumption, lowresource utilization and environmental pollution are the problems during electrolysisprocess. Therefore, to achieve energy-saving and emission-reduction has attracted muchattention.Most industrial electrolysis processes are operating with high concentration andhigh current density on the conditions of open and flow, which is a typical non-equilibrium system. It is possible to have some non-equilibrium and nonlinearphenomenon during industrial electrolysis process, such as electrochemical oscillationand fractal growth. Despite a lot of researches have been done to improve the currentefficiency in the electrolysis so far, the effect of non-equilibrium and nonlinear behavioron long-time electrolysis is often ignored. In this paper, on the base of non-equilibriumnonlinear physical chemistry theory, the behavior and the mechanism of nonlinearkinetics were scrutinized during the electro-synthesis of metal Mn and KMnO4. Firstly,during the electro-synthesis of metal Mn process, the impact of chemical synthesizedmanganese oxide on nonlinear kinetics of anode and the mechanism of additives oncathode were studied. Secondly, electrochemical oscillation on the anode in the alkalinesolution of potassium permanganate was first discovered, and the mechanism ofnonlinear kinetics was analyzed. Finally, we developed a flotation technology forremoving impurities from ore or slag and made mineral impurities resource utilization.It is beneficial to achieve energy-saving and emission-reduction during theelectro-synthesis of metal Mn and KMnO4, and to promote the development of novelgreen hydro-electrometallurgy technologies.Works could be listed as follows:(1) The effect of chemosynthetic manganese oxide on nonlinear kinetics behaviorof anodic reaction during the electro-synthesis of metal Mn was studied. The anodeduring the electro-synthesis of metal Mn was simulated by Pt-MnO2combined electrode,which was prepared by pressed disc method. In electrolyte including Mn2+, obviouscurrent oscillation was detected by the two methods of cyclic voltammetry and constant potential. We found that electrochemical oscillations would be more obvious withthinner δ-MnO2sheet and longer steeping time for the composite electrode; When Mn2+was not added into the electrolyte, periodicity oscillation would become chaoticoscillation and gradually disappear; neutral and weakly acidic electrolyte has fewinfluences on oscillation. It is further evidence that the layered nano-MnO2can increaseelectrode catalytic activities, and promote catalyzed electro-oxidation reaction of Mn2+and intermediate products Mn(Ⅲ), in order to cause current oscillation on the electrode.(2) The mechanism of additives on the cathode during the electro-synthesis ofmetal Mn was studied. In the presence of the SO2-class main additive, the effect of threetypes of the amino-class auxiliary additives on the electrolysis was investigated. Wefound that ammonium thiocyanate as an auxiliary additive decreased current efficiency,and caused100μm spherolitic dendrites growing on the surface of Mn produces;Thiourea, as an auxiliary additive, could increase current efficiency, and the whole Mnproduces was made up of5μm small punctuated dendrites; Polyacrylamide could resultin white and lightweight deposited manganese. Moreover, polyacrylamide-thioureacomposite as an auxiliary additive could inhibit dendrites which caused by the nonlinearfractal growth, and current efficiency was increased to67.1%, with both auxiliaryadditives working synergistically. In summary, the reason for “extrusive growing”dendrites of metal Mn was explained by the theory of the nonlinear fractal growth, andthe mechanism of amino-class polar groups during the electro-synthesis of metal Mnwas studied. Results provided a theoretical basis for selecting auxiliary additives duringthe metal electrolysis.(3) The mechanism of nonlinear kinetics on the anode during the electro-synthesisof KMnO4was studied. Representative current oscillation was reported for the first timein the strong alkaline electrolyte of potassium manganate. It was found that the energyconsumption was obviously afected by the electrochemical oscillation. The relativeadditional energy consumption could reach up to19.0%. With disturbing, the relativeadditional energy consumption would decrease by77%. At the start of Electrolysis,because of anodic dissolution, hydroxide gradually formed on the electrode surface,electrochemical oscillation was mainly affected by cyclical alternation of hydroxidedissolution and precipitation on the electrode surface. After a period of time, a thicklayer of oxide was covered on the electrode. MnO42–, MnO4–, and H2O2produced bywater oxidation would form a complicated dynamic balance, and it has a significantimpact on electrochemical oscillations. We planed to build simplified systemic kinetics mathematical model and to simulate time-current oscillation curve which is basicallyconsistent with experiment results, in order to explore the mechanism of nonlinearkinetics for periodic oscillation. It will provide new theoretical guidance for industrialhigh current electrolysis to achieve energy-saving and emission-reduction. Byoptimizing electrolysis process, current efficiency could maintain40.0%.(4) Flotation technology for removing impurities from ore or slag, and resourceutilization of mineral impurities was developed. An ore-dressing terrace was built in thelaboratory. By optimizing flotation technology and screening flotation medicaments, acombinatorial flotation medicament for industrial ore-dressing was found:0.10g·L–1xanthate (ethyl-xanthate: butyl xanthate=1:1)+0.12mL·L–1terpenic oil+0.07g·L–1copper sulfate+0.07g·L–1tannic acid. After only one flotation， the recovery ofbeneficiated ore was up to81%, and the content of Fe was44.8%. Moreover, we selectsemiconductor iron sulfide nanocrystals directly out of ore or slag dust through anorganic phase selection process, which have potential for applications in optoelectronicdevices. It will provide a technology for removing impurities of iron sulfide frommanganese ore and manganese slag by reverse flotation, and for resource utilization ofmineral impurities.