Rajiv Shekhar

Electroremediation of Cr(VI) contaminated soils: kinetics and energy efficiency

Experiments have been conducted on Cr(VI) contaminated kaolin to determine the kinetics of electroremediation (ERM) and the associated rate controlling mechanism. ERM experiments of potassium dichromate contaminated kaolin show that approximately 15% chromium was removed in the first 20 h. Thereafter, there was a drastic reduction in the chromium removal rate. After 144 h, only 31% of Cr(VI) was removed from kaolin. Migration of the acid and base fronts leading to acid-base neutralization appears be the rate controlling step in the ERM of chromium. A significant fraction of chromium could not be remediated due to acidic conditions in kaolin. Results also show that the extent of both remediated and adsorbed (unremediated) chromium and power consumption increased with voltage.

Physical modeling studies of electrolyte flow due to gas evolution and some aspects of bubble behavior in advanced hall cells: Part I. Flow in cells with a flat anode

The need for energy reduction in the electrolytic production of aluminum led to the concept of advanced Hall cells that can be operated at lower interelectrode gaps compared to existing cells. However, gas bubbles generated by the anodic reaction increase the resistivity of electrolyte and cancel out part of the reduction in interelectrode resistance expected from bringing the electrodes closer together. Therefore, the primary objective of this work was to determine a cell design in which flow can be managed to promote the removal of anode gas bubbles from the interelectrode gap. In particular, this article focuses on advanced Hall cells equipped with “flat” anodes, similar to those used in existing cells. The principal experimental tool has been a “water” model consisting of a large tank in which simulated anodes can be suspended in either the horizontal or near-horizontal configurations. Gas was generated by forcing compressed air through porous graphite, and the fine bubbles characteristic of inert anodes used in advanced Hall cells were produced by adding butanol to water. Velocities were measured using a laser-Doppler velocimeter (LDV). This study indicates that the existing cell configuration might not be the optimum configuration for advanced Hall cells. The results also show that operation of an advanced Hall cell with a fully submerged anode should give rise to higher electrolyte velocities and thus rapid removal of bubbles. The bubble effect should be further lowered in a near-horizontal configuration; however, the flow pattern could have an adverse effect on current efficiency and alumina distribution in the cell. It has also been shown that the bubble size, and, therefore, the physical properties of the electrolyte, can have a significant effect on the electrolyte flow pattern in the interelectrode gap.

Fluid flow in pachuca (air-agitated) tanks: Part I. Laboratory-scale experimental measurements

Gas-agitated reactors are used in a number of process industries, including the metallurgical industry, where they are known as “Pachuca” tanks. In spite of the fact that it is the circulation (i.e., velocity and turbulent kinetic energy distribution) within these tanks that governs the main process requirements,i.e., mass transfer and particle suspension, very little attention has been paid to the question of fluid flow. In the present study, velicity measurements made in a laboratory-scale Pachuca tank have suggested the importance of the fluid flow pattern in governing the performance of air-agitated tanks and have shed some light on the efficient operation of these tanks. Full-center-column tanks with large tank height-to-diameter ratios have a “near-stagnant zone” in the lower section of the annulus. The stagnant zone is a region of low turbulent kinetic energy and is undesirable, since it costs energy and is likely to provide very little in return in terms of mass transfer. An increase in the draft tube diameter, for a given tank diameter, leads to higher velocity and turbulence levels in the annulus, which, in turn, should promote mass transfer. Free-airlift tanks seem to be more vigorously agitated than full-center-column tanks. The present study shows that operating a full-center-column Pachuca tank with the liquid surface at or below the same level as the draft tube top would be disadvantageous in terms of particle suspension and mass transfer and also illustrates that it is erroneous to correlate the turbulence on the liquid surface with the turbulence level within the tank.

Physical modeling studies of electrolyte flow due to gas evolution and some aspects of bubble behavior in advanced Hall cells: Part III. predicting the performance of advanced Hall cells

For the effective removal of bubbles from the anode-to-cathode gap (ACG), an earlier study had suggested that advanced Hall-Heroult cells be operated with a “grooved” anode in the near-horizontal electrode configuration. However, it was observed that a large number of bubbles did not enter the grooves readily; the avoidance of this phenomenon could further lower the interpolar resistance. To circumvent this problem, an “improved” anode design which would promote the entry of gas bubbles into the grooves was conceived. A further objective was to analyze the performance of advanced Hall cells equipped with this improved anode. For this purpose, measurements of (simulated) electrolyte velocities, interpolar resistance, and mass transfer coefficient at the cathode were carried out in a “water” model. An important suggestion emerges from this work: energy savings of 2 to 2.5 kWh/kg aluminum may result when a conventional Hall cell is replaced by an advanced Hall cell equipped with this improved anode.

Physical modeling studies of electrolyte flow due to gas evolution and some aspects of bubble behavior in advanced hall cells: Part II. Flow and interpolar resistance in cells with a grooved anode

This article illustrates the role of anode design in increasing the energy efficiency of advanced Hall-Héroult cells. In this study, a “novel” anode that promotes the removal of gas bubbles, generated by the anodic reaction, from the interelectrode gap has been simulated. The efficacy of this anode has been judged with the help of electrolyte flow and interpolar resistance measurements in the horizontal, near-horizontal, and near-vertical electrode configurations. The experimental arrangement was similar to that used in Part I, the companion article. Velocities were measured with a laser-Doppler velocimeter (LDV). Both velocity and interpolar resistance measurements indicate the superiority of this novel anode over a flat anode. Use of this novel anode (vis-à-vis the flat anode) should lead to a reduction (approximately 40 Pct) in interpolar resistance at current density levels used in the industry. Furthermore, electrolyte flow in the anode-to-cathode gap (ACG) is uniform, thereby minimizing the possible problem of reoxidation of aluminum which might be present in cells operated with flat anodes. This study also highlights the major drawbacks of advanced Hall cells operated in the near-vertical configuration: existence of recirculating flow and very high volume fraction of bubbles near the top of the ACG. Both of these factors could lead to increased oxidation of aluminum and, therefore, to reduced current efficiency. Finally, comparison of results with Part I suggests that maximum energy efficiency should be obtained in retrofitted advanced Hall cells operated with this novel anode in the near-horizontal electrode configuration.

Effect of Hot Rolling on Microstructure and Texture Evolution of Mg-Li Based Alloy

Mg-Li based alloy, namely Mg-9%Li-5%Al-3%Sn-1%Zn (LATZ9531) was cast and consequently hot rolled at ~573K. In the present study, as cast and hot rolled structural features has been investigated by scanning electron microscope and electron back scattered diffraction (EBSD). Phase analysis revealed presence of major Mg-rich α-phase and Li-rich β-phase. Significant crystallographic texture evolution due to conventional hot rolling process was observed due to the increased activity of the non-basal slip mode.

Molten salt electrolysis of neodymium: electrolyte selection and deposition mechanism

Abstract Molten salt electrolysis of neodymium from Nd2O3 can be a viable alternative to calciothermic reduction. Two important issues merit attention: selection of an appropriate electrolyte and the mechanism of the neodymium deposition reaction. Cyclic voltammetry experiments and melting point considerations point to the eutectic LiF–CaF2 as an appropriate electrolyte. The presence of CaF2 polarises the Li deposition reaction to more cathodic potentials, thereby providing a larger potential window for the deposition of neodymium. Experiments with the LiF–CaF2–NdF3–Nd2O3 and LiF–CaF2–LaF3–Nd2O3 melts suggest that both [NdF6]3− and [NdOF5]4−, depending on their molar ratios, can be reduced to neodymium.

Electrochemical cleaning of soil contaminated with a dichromate lixiviant

Abstract Experiments are conducted on kaolin contaminated with a lixiviant obtained from a sodium dichromate plant residue to demonstrate the efficacy of electrochemical cleaning. After 120 h, only 36% chromium is removed from the soil. Chromium removal is high in the first 20 h, after which it falls drastically. Acid–base neutralization near the cathode appears to be the rate controlling mechanism. Unremediated chromium in kaolin are either present as Cr(VI) in the pore fluid or as strongly adsorbed species because of acidic conditions in kaolin.

Optimal load shedding: An economic approach

In this paper an economic criterion based optimal load shedding algorithm has been developed. The objective is to minimize the sum of curtailed load and also the total system interruption cost within the frame-work of system operational and security constraints. The load shedding problem is formulated as a non-linear optimization model that is optimized using Genetic Algorithm (GA). A test system designated as the RBTS [30, 31] is used to test the technique. The system includes two generator busbars with eleven generating units, nine transmission lines and five load busbars where six distribution systems are connected.

Design of a Cyclone Separator for Cleaning of Dust from Volumetric Air Receiver

Arid regions, like Rajasthan, receive abundant solar energy and are prone to dust/sand storms. Line and point focusing technologies are used for harnessing this energy. At IIT Jodhpur, open porous volumetric air receiver based point focusing technology is considered for process heat application. This system uses air as heat transfer fluid and is expected to achieve a temperature as high as 800 °C. This receiver on account of volumetric heating, is exposed to a high heat flux, even, up-to 1000 suns (1sun = 1 kW/m2). As this receiver is open to atmosphere, the dust storms in these regions can block the absorber pores and enter the system. Due to lower thermal conductivity of sand in comparison to absorber material, high temperature gradients and thermal stresses are expected on the absorber. It can result in failure of the system. In view of this the current activity aims at cleaning and collection of the removed dust from pores of receiver. A 2D2D geometry of cyclone separator is proposed for deposited dust collection. The experiments on collection efficiency and pressure drop are performed and compared with empirical model. Pressure drop is estimated using CFD and experimentally validated for an improved relation for pressure drop coefficient.