Settling behavior and macro–mesoscopic response characteristics of viscous phosphate tailings slurry under quiescent conditions

Introduction Fine-grained viscous phosphate tailings slurry contains a high proportion of fine particles and exhibits weak plastic consistency and pronounced structural sensitivity during settling, resulting in behavior different from that of conventional coarse-grained tailings. This behavior has important implications for tailings thickening, clarified-water recycling, and subsequent safe storage. Methods The phosphate tailings fraction passing a 75 micrometre sieve was used to prepare suspensions with different solid mass concentrations for quiescent settling tests. Macroscopic settling indicators were integrated with mesoscopic image-based observations. Results Without flocculant addition, apparent-interface height, interval-averaged apparent-interface velocity, clarified-zone turbidity, normalized apparent-interface migration, and image-derived two-dimensional pore-area ratios showed a strongly stage-dependent response. Three stages were identified: rapid apparent-interface formation at 0-5 min, transition-zone adjustment and aggregate rearrangement at 5-30 min, and gradual interface stabilization with apparent sediment-layer densification after 30 min. Higher-concentration groups showed faster early apparent-interface migration but higher final interface positions and higher 120-min turbidity. Discussion Apparent-interface migration, supernatant clarification, transition-zone adjustment, and lower sediment-layer development are related but non-equivalent aspects of the settling response. Their combined interpretation provides an experimental basis for evaluating the natural settling and clarification behavior of fine-grained phosphate tailings slurry under quiescent low-solids conditions.