S. P. Mehrotra

Optimal-Suboptimal Synthesis and Design of Flotation Circuits

The integrated approach of process synthesis and design has been applied to determine the optimal-suboptimal configuration and design parameters of a flotation circuit for separation of mineral species. Mean residence time of parti-culate species in each cell is the design parameter to be optimized, which along with the optimal structure is extracted simultaneously from a generalized circuit by direct search. The validity of the method has been demonstrated by comparing results obtained by using this method with real optimal structure and parameter values obtained by optimizing all feasible structures, enumerated one at a time, for the case of a two cell circuit using two different objective functions of recovery, grade, and profit. The method is then extended to more realistic four cell circuit and feed comprising valuable mineral, gangue, and middlings.

Experimental investigation of mixing phenomena in a gas stirred liquid bath

Mixing phenomena in a room temperature water bath, agitated by injecting air through a straight circular nozzle fitted axially at the bottom of the vessel, were characterized by experimentally measuring mixing time(tmix) by electrical conductivity technique. It was found thattmix defined at 99.5 pct homogenization did not depend on location and size of conductivity probe, location of tracer injection, and the amount of tracer injected. tpet decreased with increasing gas flow rate and bath height, but decreasing nozzle diameter. Visual observations of the two-phase plume and flow conditions in the bath revealed that the plume swirled above a certain gas flow rate which enhanced the mixing rates in the bath. The transitions in Intmix vs In εb curves were found to correspond to onset of swirling; εb is the rate of buoyancy energy input per unit bath volume. Systematic analysis of experimental data revealed that a fraction of gas kinetic energy contributed to mixing in the bath. It was a function of bath height, being negligible at lower bath heights and almost 1 at larger bath heights. Further, it was experimentally found thattmix decreased with increasing bath height only up to a certain value, beyond which it started increasing. Visual observations of the bath revealed that the height at whichtmix started increasing corresponded to a transition in which the bath was converted into a bubble column. The experimental data, for a particular bath height, were fitted into two separate straight lines of the formtmix =cε−n wherec andn are empirical constants and ε is the rate of energy input per unit bath volume.

Simulation of Particle Stratification in Jigs

A mathematical model employing the discrete element method is used to study the stratification of particles during jigging. This model is used to simulate jigging of binary as well as ternary particles. In particular, effect of amplitude and frequency of pulsation on the stratification behavior of the particles is analyzed. The existence of an optimal condition in terms of amplitude and frequency of pulsation is established. Furthermore, the pulsation of fluid is controlled to generate different waveforms and its effect in turn on the stratification of the particles is investigated. It is shown that the numerical results correlate well with the available data in the literature.

Characterization of two-phase axisymmetric plume in a gas stirred liquid bath—A water model study

AbstractPlume profile and plume cone angle (θc) were determined by still photographic technique in a cold model set-up where air was blown into a water bath through an axial nozzle located at the bottom of the vessel. Effects of various operating variables,e.g., gas flow rate(Q), bath height(H), and nozzle diameter(dn) on the plume profile were investigated. θc increased with increasingQ, decreased with increasingH, but was approximately independent ofd. Based on the experimental data the following empirical correlation was developed:

Magnetohydrodynamics in advanced Hall-Heroult cells: physical modelling of flow in a laboratory scale cell

(\frac{{\theta _c }}{{180}}) = 0.915\;Fr_m^{0.12} (\frac{H}{D})^{ - 0.254} (\frac{{d_n }}{D})^{0.441}

Role of mathematical modelling in metallurgy

whereD is bath diameter, Frm is modified Froude number (= 16Q2/π2gd4nH⋅G/(pL-pG)), g is acceleration due to gravity, andpL andpG are densities of liquid and gas, respectively.

Mixing in liquid baths by gas injection

A macroscopic, steady state energy balance model has been formulated to describe mixing phenom-ena in a liquid bath stirred by injecting gas through a straight nozzle fitted axially at the bottom of the vessel. This, along with experimental data on a water model previously reported, was employed to make predictions. Input energy terms considered in the model consist of buoyancy energy and empirically determined fraction of gas kinetic energy. Dissipation of energy was attributed to liquid circulation and bubble slip. The two-phase plume was assumed to be a truncated cone whose dimen-sions depended upon operating conditions. Numerical solution of model equations gave liquid velocity and gas hold-up inside the plume as well as liquid circulation rate and liquid velocity in the region outside the plume. Influence of process variables, e.g., gas flow rate, bath height, and nozzle diameter, have been predicted. Validity of the model has been established by comparing some pre-dicted entrainment ratios with those experimentally measured by other investigators. Empirical cor-relations to predict circulation time and circulation number have been proposed. Circulation number was found to vary between 2 and 12 in contrast to the existing assumption in the literature of a con-stant value of 3. Usefulness of these correlations in predicting mixing time for industrial vessels has been demonstrated.

Particle suspension in (air-agitated) pachuca tanks

Abstract The need to reduce the anode to cathode gap (ACG)† has led to the concept of advanced Hall–Heroult cells. Electrolyte flow will have a major influence on the dynamics of advanced Hall cells. However, studies on the fluid flow pattern in the ACG of advanced Hall cells are limited. Although both electromagnetic forces and the bubble buoyancy effects cause electrolyte flow, the effect of the former has not been sufficiently investigated. Hence electromagnetically driven flow in advanced Hall–Heroult cells has been simulated in a laboratory scale cell with Woods metal as the electrolyte. A thin, solid aluminium layer mimics the cathode. Velocity measurements suggest that electromagnetically driven velocities, ∼40–50 mm s-1, are of the same order as bubble buoyancy driven flow, as expected in industrial Hall cells. This investigation also sheds light on the effect of important process parameters such as the anode to cathode distance and current density on flow in advanced Hall cells.