On the utilization of mica waste: The pore-fluid chemistry of mica soils and its implication for erosion susceptibility

Abstract

Soil is a vital resource which is limited in availability and must be adequately managed for sustainable development. Despite the importance of soil and its limited availability, a huge amount of soil is classified as waste. As one of the world’s largest soil waste generators, the mining industry must transition to a circular economy model to meet sustainability requirements. As a readily available, large-volume waste material (about 10 million tonnes each year), waste mica has been considered for re-use as an alternative soil potassium source for plants, as landscaping material etc. One of the problems with mica waste is its susceptibility to erosion. To use mica as an agricultural or landscaping material, it is therefore important to understand the erodibility of mica and how its erodibility can be reduced. This study presents an experimental campaign to characterize mica erosion susceptibility. Mica particle-to-particle interaction forces and their effect on the macroscopic material behaviour were systematically investigated by changing pore fluid pH and ionic concentration. Sedimentation and liquid limit tests were first carried out to inform a conceptual model of the mica fabric. Triplets of 12 mica samples, compacted at different water contents and with different pore chemistry, were tested for erosion susceptibility using a Jet Erosion Test (JET) apparatus. The particle configuration of mica samples consistently varied with the pore water chemistry, regardless of whether the samples being tested were suspension sediments or compacted samples. For mica samples formed with neutral water at low ionic concentration, the particles are in a dispersed configuration. This implies a poor mechanical behaviour and high erosion susceptibility, as particles are eroded one by one. For porewater samples formed with an increased ionic strength or within the acidic pH range, particles tend to cluster together and organize in a non-dispersed configuration. This results in an improved mechanical behaviour and less erosion susceptibility, as group of particles must be eroded as opposed to individual ones. Similarly, the Jet erosion test results reinforced these observations showing that mica erodibility varies with porewater chemistry. Considering that mica erodibility varies with pore fluid chemistry and mica waste derived from mining operations are often slightly acidic, this paper paves the way for tailored assessments of individual mica waste materials to determine the appropriateness of their use.