Basic Research For Electrocatalysis Synthesis Process Of Chromic Anhydride In Integrated Technology Of Clean Chromic Salts Production

Chromic salts are widely used in metallurgy, electroplating, tanning, printing and dyeing, fuel, rubber, and wood-corrosion protection, etc., as a kind of important inorganic chemical products, and called one of eight resource materials with the most competitive in the international. With the sustained development of economy, the need for chromic anhydride as an essential feed of multiple industries is yearly increased both in China and abroad, however the supply is insufficient. The research for new synthesis methods of chromic anhydride is quite active. The electrochemical synthesis of chromic anhydride, called as "Green Technology", is characterized by high use ratio of resource, high-class product, mild reaction conditions, and reduce waste. Hence, the study on the selection and electro-catalytic behaviour of anode, technological conditions optimization, mutative law of working voltage, macrokinetics, fouling and cleaning processes of ionic membrane, etc., have a great science and applied significance for the electrochemical synthesis of chromic anhydride from sodium chromate. The research contents of this paper are one of subjects funded by the National Natural Science Foundation of China (20676136), and the National High Technology Development Program of China (2005AA647010).The anode is the catalyst and a reaction field for electrocatalysis reaction. Base on the properties of the electrosynthesis process of chromic anhydride from sodium chromate and with the methods of dynamic cyclic voltammetry, steady-state polarization curve, direct electrocatalysis reaction, and characterizing in XRD and SEM, it was investigated for the electrochemical behaviours of anode in the anolyte for electrosynthesis of chromic anhydride, the electrocatalysis activity and electrochemical stability of anode, the kinetics of electrode reaction, XRD and SEM of anodic surface. It was indicated that the Ti/IrO₂/TiO₂-RuO₂-IrO₂ anode exhibited a lower overpotential to oxygen evolution, better electrocatalysis activity and electrochemical stability, and can be applied in the electrosynthesis of chromic anhydride. The active constituents on anodic surface are TiO₂, RuO₂ and IrO₂.Using Ti/IrO₂/TiO₂-RuO₂-IrO₂ as anode, stainless steel as cathode, Aciplex^?-F4602 cation-exchange membrane (the electrosynthesis of sodium dichromate), and Nafion?324 cation-exchange membrane (the electrosynthesis of chromic anhydride), the technology conditions were optimized in a self-made two-chamber bipolar type electrolyzer. Under the conditions of stir speeds 400 r·min^-1 and distance between anode and cathode 1 mm, by using the orthogonal design, the optimum technological conditions were found that the current density was 0.3 A·cm^-2, initial NaOH concentration in catholyte was 50 g·L^-1, reaction times of two steps were all theoretical electrochemical reaction times. In the first electrosynthesis, initial Na₂CrO₄ concentration in anolyte was 650 g·L^-1, and reaction temperature was 80℃. In the second electrosynthesis, initial Na₂Cr₂O₇ concentration in anolyte was 700 g·L^-1, and reaction temperature was 85℃. Under the optimum technological conditions, the five times repeat experiments were tested, the results show that, in the electrosynthesis of sodium dichromate, the current efficiency is about 94%, the fractional conversion is about 95%, and the electric consumption is about 550 kW·h·t^-1; In the electrosynthesis of chromic anhydride, the current efficiency and fractional conversion are about 62% and 63%, respectively, and the electric consumption is about 1210 kW·h·t^-1.According to the electrochemical reaction principle and inherent reaction causes leading to the variation of working voltage with reaction time, it is indicated that the electrochemical synthesis process may be quantitatively formulated by the variation of working voltage measured macroscopically with reaction time. The working voltages were experimentally measured on-line at different reaction times under conditions of different temperatures, different current densities, and different initial anolyte concentrations in the electrochemical synthesis process of sodium dichromate and chromic anhydride from sodium chromate and sodium dichromate, respectively. The mathematical models of variation of cell voltage with reaction time and the change rate equations of cell voltage were established. The imitative law of cell voltage with reaction time was elementarily discussed from the physical process and chemical change in the electrcatalysis synthesis process.According to the electrochemical synthesis reaction principle of chromic anhydride from sodium chromate, it is shown that the reactions of two steps are all complex irreversible inhomogeneous reaction system of variable volume. The macrokinetics was investigated by relating these essential cell processes and their interaction, as well as their synergistic effect to the whole electrochemical synthesis process from an engineering application point of view. The kinetic data such as anolyte volume, fractional conversion, etc., were experimentally measured at different reaction times, different temperatures and different initial anolyte concentrations. The mathematical equation of dependence of anolyte volume on fractional conversion, the expression of reaction rate, and the macro-kinetic model were established, and the macro-kinetic parameters were calculated. The experimental results show that the electrochemical synthesis process of sodium dichromate from sodium chromate presents zero order reaction kinetic characteristics, and the electrochemical synthesis process of chromic anhydride from sodium dichromate presents first order reaction kinetic characteristics. The dependences of apparent reaction rate constant on reaction temperature for the two reactions are in aggreement with the Arrhenius equation, and the apparent activation energies are 7.88 kJ·mol^-1 and 6.04 kJ·mol^-1, respectively.In view of the relative seriousness that undissolved by-products polluted the ionic membrane in the electrocatalysis synthesis process of chromic anhydride from sodium chromate with two steps, the initial working voltages and the average current efficiencies at different service times of Aciplex?-F4602 membrane (the first electrocatalysis synthesis) and Nafion^?324 membrane (the second electrocatalysis synthesis) were experimentally measured. Further, from the viewpoint of engineering application, the fouling ratio of membrane and the cleaning degree of fouled membrane, characterized with the initial working voltages, were defined in this paper, and the fouling rate equations of membrane were presented. According to the characteristics of the electrocatalysis synthesis process, and based on the causes of ionic membrane pollution, a kind of effective cleaning method in-situ of polluted membrane was presented.The research results not only provide foundations for the searching investigation and industrialization of the electrochemical synthesis process of chromic anhydride from sodium chromate, but also boost the integrated technology progress of clean chromic salts production and the development of relevant science to a certain degree.