Investigations on iron removal process by oxygen leaching in a mechanically agitated contactor

Abstract

Hydro metallurgical routes to eliminate the iron constituent from the mineral ilmenite (FeTiO3) is a crucial step in the manufacture of high value inorganic chemicals such as Titanium dioxide. In contrast to the acid mediated leaching processes, the Becher process has found world wide acceptance due to its environmental advantages since a disposable from of iron oxide and a liquid effluent at near neutral pH are generated. In the Becher process, the metallic iron component in reduced ilmenite is converted to iron oxide by means of an electrochemical reaction with dissolved oxygen followed by oxidation of ferrous to ferric iron hydrolysis and precipitation. Reduced ilmenite (average particle size 200 mm) and precipitated iron oxide particles (average particle size 1.06 mm) suspended in the aqueous electrolyte constitute a unique multi-solid slurry system, whose influence on mass transport of oxygen has not been studied so far. Mechanically agitated, air sparged reactors are normally employed to satisfy the simultaneous requirement of solids suspension and gas dispersion. Notwithstanding its environmental advantages, the Becher process is extremely sluggish and needs significant improvements in its productivity to compete with other processes. The absence of an adequate engineering model has prevented the identification of controlling steps and a systematic effort towards process intensification. This thesis aims to accomplish this objective through the development of an experimentally validated process model based on the rate determining steps, derived from an analysis of the kinetics of sub-processes, namely, mass transfer between coarse active particles (reduced ilmenite) and dissolved oxygen, oxygen mass transfer between gas bubbles and aqueous electrolyte and the homogeneous oxidation of ferrous to ferric iron in the aqueous phase. In view of the unique complexity of the process, each of the rate determining steps were experimentally investigated and interpreted with a view to obtain quantitative expressions which then served as inputs to the overall process model. In order to simulate the process as closely as possible to industrial conditions, the particle sizes, solid loading and the reactor configuration have been chosen accordingly. The effect of high solid loading on mass transfer rates, which has not been adequately studied in the literature, also forms an important subject for study. The objective of process intensification has been accomplished through the addition of ferrous chloride to the reacting solution, which leads to an enhancement of gas-liquid mass transfer rate by shifting the ferrous oxidation reaction from a slow reaction regime to fast reaction regime. Chapter I gives a brief introduction to the subject of investigation, its relevance and scope in the light of the recent trends in the removal and control of iron in mineral to material conversion processes. A critical review of state of art both with respect to the Becher process as well as the rate determining sub - processes served to identify the knowledge gaps in the literature. Chapter II also gives an up-to-date trend in the modelling of iron removal processes with respect to other mineral leaching systems. Chapter III describes the experimental investigations on attenuation of solid-liquid mass transfer due to inert micro-particles and its semi - theoretical interpretation in terms of mass transfer fouling. A range of micro - particles with sizes ranging from 0.3 to 13 mm and density from 2600 to 5400 Kg/m3 were investigated. Characterisation of the colloidal and rheological properties of the micro - particle systems were carried out to aid in the interpretation of the mass transfer attenuation phenomena as being controlled essentially by the hydrodynamics of the system. Chapter IV describes the experimental investigations and quantitative interpretation of gas-liquid mass transfer attenuation due to inert micro-particles. Chapter V and VI describe the development and experimental validation of the models to describe the conversion versus time behaviour of the metallic iron removal process for non-porous (iron particles) and porous (reduced ilmenite) particles respectively. Besides predicting the isothermal and non-isothermal behaviour, the models are able to describe the trends in the variation of pH and oxidation reduction potential during the course of the reaction. Furthermore, the model is also able to predict the conditions under which incipient passivation of iron could occur at higher oxygen partial pressures (greater than 2.03x10 rise to 4 Pa). The significant reduction in iron removal rates beyond a conversion of 0.4 has been explained as being due to hindered diffusion of oxygen in pores filled with micro-particles. Experimental investigation and interpretation of process intensification by enhancement of gas-liquid mass transfer rates is described in Chapter VII. The enhancement factors have been computed based on the oxidation kinetics of ferrous iron determined in Chapter V, and are found to closely agree with the experimentally observed values, the maximum enhancement factor being 1.7. Thus a sound theoretical basis and a validated engineering model has been established for improving the productivity of the Becher process over a wide range of process parameters with relevance to industrial application.