Dissolution of magnetite and hematite in acid mixtures

Abstract

As the quality of raw materials decreases and legislation tightens, interest in various alternative materials has grown. For example, raw materials often contain iron as an impurity, which must be removed before the actual material can be utilized. Dissolution is a good option because it can be selective and can be used even at room temperature and in a normal atmospheric pressure. The dissolution of iron oxides has been extensively studied in individual acid systems, but a full understanding of the prevailing phenomena has not yet been achieved. In addition, dissolution in acid mixtures is a less understood phenomenon, but a two-acid system may offer advantages over a single acid system. An in-depth understanding of dissolution mechanisms and kinetics provides tools for manipulating various dissolution-based, i.e. leaching, processes.

The aim of this thesis was to investigate how the addition of oxalic acid to sulphuric or nitric acid affects the dissolution kinetics, mechanisms and thermodynamics of magnetite and hematite. The work is based on solubility and kinetic experiments in different acid systems. It was hypothesised that mixing oxalic acid with sulphuric or nitric acid could accelerate dissolution. Pure synthetic magnetite and hematite were selected for this thesis in order to study the dissolution of iron. Industrial minerals often contain other impurities that can compete with iron dissolution reactions, but these substances were excluded from this thesis.

The results showed that even low amounts of oxalic acid in sulphuric or nitric acid and at a higher temperature accelerated the dissolution but did not automatically lead to a higher level of solubility. The effect of temperature and acid mixtures can be preliminarily studied with special cube models. The dissolution kinetics of magnetite and hematite followed the Kabai model well throughout the extent of the reaction. The solid specific constant a of the Kabai model varied, which was not similar to the finding Kabai made. Therefore, it is suggested that the variation was due to changes in the solid phase during dissolution and that the constant is not solid-specific but is a dissolution-related constant describing changes in the dissolution mechanism.

The dissolution mechanisms of magnetite and hematite in oxalic acid included theadsorption of oxalate on a solid surface, the reduction of Fe(III) to Fe(II), and finally the liberation of Fe(II) into a solution, which catalysed the rate of dissolution. In pure sulphuric acid, sulphate and bisulphate accelerated the rate of dissolution, while in nitric acid systems the rate was slowest because the dissolution occurred via a slow protonation mechanism. Dissolution mechanisms were more complex in acid mixtures, as was evidenced by the need for a more complex statistical model to describe the dissolution system compared to individual acid systems. The formation of humboldtine, Fe(II)(C2O4)∙2H2O, took place in pure oxalic acid. Density functional theory calculations (DFT) showed that the adsorption of oxalate on the surface of hematite and the reduction of Fe(III) to Fe(II) were the key steps in the formation of humboldtine. This finding can help improve practical applications where solid precipitates can be a real problem. An addition of even low amounts of sulphuric and nitric acid is sufficient to inhibit the formation of humboldtine. Another way is to gradually add oxalic acid to the system or shift the reaction to form carbon dioxide.

Neither the specific surface area of BET or the oxalate and nitrate concentrations do not correlate with the dissolution mechanisms. However, the pH of the solution describedwell the dissolution degree. In addition, DFT calculations combined with experimental results provided additional information needed to visualize and rationalize the reaction steps during dissolution.