Intensificación del proceso electroquímico para la remoción de cromo en residuos líquidos
Authors:   , Yáñez Varela Juan Antonio 
Published: May 2022

Chromium is a metal commonly used in the electroplating industry, which generates rinsing water containing hexavalent chromium concentrations above those allowed by environmental regulations. Several treatment alternatives have been proposed to attack this problem, among which electrochemical methods stand out. Among the latter, an attractive process is the one that uses iron electrodes, which are the source for the in situ electrochemical generation of the reducing agent, which chemically reduces the chromium in the liquid from its hexavalent to its trivalent state. The effluent is passed to a mixing tank to promote the precipitation of the metals into metal hydroxides by increasing the pH of the medium in order to separate them from the treated water. In this work, different strategies are presented to intensify this treatment process in order to improve the hydrodynamic performance of the electrochemical reactor and the precipitation stage. To assess the processes performance, in both systems, the electrochemical reactor and in the precipitation reactor, experimental tests were carried out in stirred tank reactors and jars test, which were complemented with studies using computational fluid dynamics to integrate the information on the performance of the processes under the different scenarios tested. Chapter two details the study carried out using computational fluid dynamics tools on the rotating rings electrochemical reactor, which has shown high removal efficiencies of hexavalent chromium in pollulet water. This part of the project shows the most appropriate modeling approach to simulate the hydrodynamics inside the reactor. Comparison of the results predicted by three variants of the κ-ε turbulence model (stadard, RNG and realizable) coupled to the multi-reference frame model to simulate the electrode rotation is performed. The effect of position of the rotational reference frame boundaries relative to the stationary reference frame boundaries is also evaluated. It is shown that the prediction with the κ-ε realizable model in conjunction with the position at 0° generates the results with the closest approach to the experimental mixing time measurements obtaining a 6% error. Chapter three shows the results of the incorporation of a novelty, which consists of a reactor equipped with the static electrode of electro-baffles agitated by two inclined vane impellers, known as PBT. The results of this modification are compared with those obtained with the performance of the electrochemical reactor equipped with the dynamic rotating ring electrode. The comparison is made theoretically to evaluate their differences in terms of hydrodynamic performance, and experimentally to know their efficiency against the reduction of hexavalent chromium. For the comparison, the reactors were operated at the same stirring speed and Reynolds number. The results of the hydrodynamic analysis show that the arrangement of static electro-baffles together with the pair of impellers improves the mixing time by 36%, increasing the hydraulic efficiency by 85% when the reactor is operated at the same Reynolds number. It is evident that the circulation capacity of the reactor directly affects the reduction rate of hexavalent chromium, since the treatment times have a similar trend to the axial circulation times. It is also shown that there is a reduction in energy consumption of at least 21% when the reactor is equipped with the electro-baffles and the two-impeller agitation system. In chapter four, the operating conditions of the electrochemical reactor are evaluated, such as: geometric configuration of the electrode, stirring speed and current intensity. To evaluate the change in the electrode configuration, the static ring electrode and electro-baffles are used. When operating the electrode with static rings, the need to incorporate conventional baffles is also evaluated. The reactor agitation systems are composed of two PBT impellers, of which the separation between them is also evaluated. In performing the simulations, the interaction of the liquid-gas interface was considered. The results reveal that due to the position of the impellers it is necessary to take into account the liquid-air interaction in the model to obtain a more realistic prediction of the flow pattern in the electro-baffles configuration. As a result of the studies, it was found that the configuration with the lowest energy consumption was the electro-baffles with a separation between impellers equal to their diameter, since its geometrical and hydrodynamic characteristics allow it to be more efficient. The effect of agitation speed and current intensity was explored in this system. The agitation speed increases the reduction rate of hexavalent chromium, finding its limit at 300 rpm. At this agitation speed, the effect of the current intensity was explored, from which a linear dependence of the reactor energy consumption on this variable was found in the range evaluated. In chapter five, the precipitation stage is studied. The studies are carried out in an agitated jar system with two types of impellers, one radial and the other axial. The evaluation of the effect of the pH at which the effluent is adjusted to perform precipitation at values of 4, 6, 7 and 9 is carried out. The results show that after precipitating the treated water at pH = 9.0, a clarified product with neutral pH is obtained and all species are precipitated. The hydrodynamic environment of the jars is also evaluated experimentally and numerically, determining that the radial impeller dissipates more turbulent energy than the axial impeller. Therefore, the axial impeller provides a favorable hydrodynamic environment for the development of flocs, which results in higher sedimentation velocities than those achieved when the jar is operated with the radial impeller. In addition, the axial impeller consumes only 50% of the energy consumed by the radial impeller.