Understanding factors affecting the stability of inline polymer-amended tailings

Abstract

Tailings rehabilitation, using soil-like properties with chemical and physical stability, can be influenced by surface forces on a particle scale and microstructure. Non-homogeneous structures, manifesting as particle aggregation, alter the permeability/porosity and improve the balance of aerated and water-filled pores that then favours plant growth. Polymer flocculation in tailings thickeners yields large, weak aggregates that are sheared in beds and on pumping. While low-level residual structure may remain, sizes are small and downstream polymer impacts are minor. However, high polymer dosages added in-pipe to high-solids streams lead to further water recovery on deposition, and while this process is poorly understood, it gives aggregates that are likely denser and stronger. If such structures persist, they could potentially offer closure and rehabilitation benefits in some applications. A high-solids (50 wt%) thickener underflow analogue (kaolin, silt, and fine sand) was treated with anionic polyacrylamide (PAM) polymers at elevated dosages (up to 1,000 g t-1) to establish if aggregate structures created during inline tailings flocculation survive or change over longer-term consolidation. Low and high molecular weight (MW) PAM polymers of fixed anionicity (30%) were tested. Focused beam reflectance measurement (FBRM) provided real-time monitoring of the aggregation state after polymer addition. The compressibility of the inline flocculated tailings was assessed over an eight week period. Variations in the size–density relationship of aggregate structures were determined over this period using an image analysis technique, revealing that the shorter polymer chains produced denser, more compact structures immediately after deposition, whereas the longer chains resulted in a much slower re-conformation of the partially adsorbed polymer, leading to additional densification over time. Eight weeks after deposition, inline flocculated tailings produced with the low MW polymer were characterised by a higher compressive yield stress compared with the high MW polymer, consistent with the formation of smaller, more rigid aggregate structures with the shorter chains. The potential impact of such structures on longer-term rehabilitation properties (e.g. metal/metalloid ion adsorption and surface exchange capacity) of the treated tailings is discussed.