| In the industrial use of water, many processes, from the cooling system with thelargest water usage to the electronics industry with high precision, are limited bysilica in water. Thus different industries all have strict standards for silica content.However, advanced treatment of silica is difficult to achieve for the complex forms ofsilica. The chemical methods to remove silica usually have inefficient removal rates,cause secondary pollution and cannot be used as advanced treatment. Ion exchangeand membrane system usually have high cost and high requirement for water inlet.Therefore, an efficient, economical and clean technology must be found to removesilica, which is suitable for different industries. The paper used electrocoagulationsystem as an advanced treatment to remove silica from source water. The mechanismof this technology was explored, and the effects of all kinds of factors werecomprehensively studied. Then electrocoagulation was compared with chemicalflocculation method. Finally, a novel combined electrochemical system wasdeveloped to remove hardness species as a pretreatment for silica removal.In a continuous process of silica removal, there was a key time point when thevoltage went up sharply under constant current density. And these time points werebrought forward with the current density increasing. The study showed that it had thesame electric quantity at the time points. And further study showed that the rise ofvoltage was mainly due to the change of the anode surface. The overpotential ofanode controlled the whole potential of the electrochemical system. Adding Cl-couldprevent the rise of voltage. When the concentration of Cl-was fixed, the type of cationdid not affect much on the change of voltage. By comparing with theelectrode-reverse-method, adding Cl-was more economic, but had no effect on thesilica removal efficiency. The effect of co-existing anions on the activity of the Al flocwas studied also, which concluded that the system with NO3-(10mM)+SO42-(10mM)consumed the lest electrode mass but had highest removal rate.The effects of different factors were studied by response surface methodology.The optimal condition was60mg/L of initial silica concentration1.0cm of electrodespacing，71.67min of time，7.5of pH value，3.18mA/cm2of current density，4.00cm of submerged electrode height, under which the removal rate was92.8%and thetotal cost was1.64RMB/t. After real experimental verification, the removal rate was91.9%and the total cost was1.68RMB/t. Seven kinds of reaction kinetics model were tested. It was indicated that the first order kinetics model and intraparticlediffusion model were the two applicative models for silica removal byelectrocoagulation. The correlation coefficients (R2) under different initial silicaconcentration were all above0.99. The effects of temperature showed that the removalof silica by electrocoagulation was a spontaneous, endothermic and entropy increasingprocess, because the Gibbs free energy change was negative, enthalpy change was45.11kJ/mol and entropy change was0.18kJ/mol.After comparison of electrocoagulation and chemical flocculation, we knew thatelectrocoagulation technology was an advanced treatment for silica removal, withlower sludge production, neutral pH of the solution and smaller conductivity.Therefore, electrocoagulation is an effective and environment friendly technology. Forthe mechanism, the difference between electrocoagulation and chemical flocculationlied in the characteristics of aluminum floc. During the whole process, the aluminumfloc in electrocoagulation had a more negative Zeta potential and larger grain size. Foreconomic aspect, the cost of electrocoagulation may little higher than that of chemicalflocculation, but chemical flocculation cannot remove silica in depth. The two-stepelectrochemical system developed in this study applied electrocoagulation cell under alow current density as a pretreatment for electrochemical precipitation cell. It couldavoid large volume of sludge and have lower operating cost. |
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