Geochemical modelling of environmental processes in rare earth element mining

Rare earth elements (REE) are considered indispensable and non-replaceable in many optical, electronic, catalytic and magnetic applications. Indeed they are viewed as critical metals due to the monopoly China has over their supply chain and to their ever increasing demand. Ion adsorption-type deposits (IAD) have attracted much attention in recent years as the relative ease of REE recovery makes these deposits economically significant. At present IAD are only mined in China, which has led to considerable environmental damage. This study considers an IAD in NW Madagascar which is under active exploration. The research aim was to develop a numerical modelling approach to simulate the mobilisation of REE from IAD. To achieve this, REE exchange reactions with Madagascar IAD minerals in batch reactor tests were modelled, and thermodynamic equilibrium constants were estimated for these reactions. Reactive transport modelling of flow-through laboratory column experiments was undertaken to test the estimated REE exchange constants. A single exchange constant of log K 2.29 ± 0.5 (experimental standard deviation) was able to describe all the REE exchange reactions with the Madagascar IAD in both batch and column datasets. Exchange constants were also estimated for each individual REE to account for any variation in REE behaviour (e.g. due to the lanthanide contraction). The determined constant only varied to 2 decimal places and the difference between the objective functions for individual log K model (5.71 x 10-11) and for the single log K model (5.72 x 10-11) was minimal. Without further application to different IAD and different sample materials from Madagascar, the exchange constants estimated in this study should not be considered as the standard equilibrium constants for all IAD-REE exchange reactions.