The pressure effect on the kinetics of ilmenite reduction in hydrogen

Abstract

A new process for ilmenite upgrading in the pigment industry has been proposed. The process involves reduction of ilmenite in hydrogen under pressure. Past research in this field of ilmenite reduction under pressure reveals a lack of understanding of the rates and mechanisms involved. A twin reactor, symmetrical-beam pressurised thermogravimetric microbalance was designed and commissioned by the author. The system enabled the kinetics of hydrogen reduction of ilmenite to be studied at high gas flow rates, where gas transport resistances were minimised. The application of the micro-balance to mass loss determinations, coupled with microscopic examinations of quenched reduction products, were used to determine the mechanisms of oxygen removal from ilmenite between temperatures of 823 and 1173 K up to pressures of 10 atm and 13 atm at the lower temperatures. Natural ilmenite was initially investigated in a packed bed arrangement but changes in experimental strategies led to in-situ gravimetric examinations of synthetic ilmenite discs. Polishing the discs prior to reduction eliminated the problem reported in an earlier study (Brigss and Sacco, 1991) of nucleation and growth of iron metal over the surface of the discs to form a barrier to the gas. The reaction in a hydrogen atmosphere in the experimental conditions used had a significant contribution from crystallographic control with parallel bands of polycrystalline iron metal forming within the ilmenite grains of varying orientation. The bands were parallel to the (0 0 0 1) basal plane of the parent ilmenite. Conversion of ilmenite to rutile, Ti02, in the inter-band regions was also found to be crystallographically controlled, with the b-axis of the rutile parallel to the c-axis of the ilmenite, and the c-axis of rutile parallel to one of the three equivalent [1 1 0 0 ] ilmenite directions. The hexagonal structure of ilmenite implies three possible orientations of the rutile, giving triply-twinned rutile in the reduction products. The twinned rutile grew perpendicular to the iron bands, consistent with fast solid-state diffusion of metal ions along the c-axis of rutile. Although the metallisation within the individual ilmenite grams was crystallographically controlled and was in the form of bands, the overall macroscopic reduction within the poly-granular ilmenite discs was of the shrinking core topochemical type. A shrinking core reduction model, modified to account for the partial pressure build-up of reactant and product gases within the sample and for the growth of pore size during reduction, was capable of predicting conversion-time relationships of ilmenite samples. The change in reduction kinetics with increase in applied pressure of the reactant gas was strongly influenced by the adsorption of product gas onto the product solid. This resulted in a sharp increase in reaction rates up to approximately 3 - 4 atm followed by a slow rate increase with any further pressure mcrease. The results of the modelling confirmed that at the lowest pressure used, 1.2 atm, and at temperatures up to 1 023 K, the reduction reaction was predominantly under chemical reaction control, particularly in the early stages of conversion. However the microscopy studies indicated that solid-state diffusion is contributing in parallel with the interface reaction. Knudsen diffusion is the major contributor to pore diffusion at the low pressures. At 1123 K and 1.2 atm, the reduction is under mixed control with the interface reaction and molecular pore diffusion both contributing. As the pressure is increased, in addition to adsorption effects, molecular pore diffusion increases its contribution to the control in the reduction kinetics. Show more