Quantitative analysis of static yield stress variation in thickened tailings within the compaction zone based on fine structure

Abstract

The poor flowability of high-concentration tailings slurry often leads to slurry hardening and rake blockages in thickeners. To address this, the study employed computed tomography and rheological measurement techniques to investigate the effect of slurry concentration on static yield stress (τB), and a comparative analysis was conducted between thickened tailings and freshly mixed slurry. Results show that the concentration, coarse particle content, and pore structure of thickened tailings are nonhomogeneous. Slurry concentration and the proportion of coarse particles (75–300 μm) increase with decreasing slurry height, while pores in the 50–250-μm range serve as the primary storage space for water. The τB of thickened tailings is 5.3–61.3 times higher than that of freshly mixed slurry. Furthermore, τB decreases with decreasing coefficient of variation (CV) of slurry porosity. It is proposed to use CV to quantify differences in τB between thickened tailings and freshly mixed slurry. Field application at an iron ore mine in China validated the results, providing insights to mitigate slurry hardening in silos.

1. Introduction

Substantial volumes of low-concentration tailings slurry are produced during mining operations. If not handled properly, such slurry can cause severe environmental disasters [1–2]. In fact, dam failures in tailings ponds and the discharge of tailings wastewater have led to significant ecological damage, fatalities, and extensive property losses [3–4]. Consequently, the reasonable, effective, and safe management of low-concentration tailings slurry is a critical concern for mining enterprises worldwide [5–7]. Tailing thickening is a technique used to produce high-concentration tailings slurry [8–11]. It provides a dense underflow for slurry storage and paste backfill in mines, reduces water consumption in mining operations, and lowers the cost of fill materials [7,10,12–14]. Vertical sand silos and thickeners are widely used in the industry for tailings thickening, offering operational simplicity, high processing capacity, and cost efficiency [15–16].

Researchers have studied the effects of flocculation, floc settling behavior, and underflow concentration improvement of tailings in vertical sand silos and thickeners [11,17]. Factors like slurry temperature [18–20], pH, flocculant properties, and tailing properties have been examined for their influence on floc structure and density [19,21–24], with findings showing that higher fluid shear significantly disrupts floc structures [25–27]. Regarding floc settling behavior, studies have analyzed changes in equivalent floc diameter and breakage mechanisms during settling [17,28], as well as the effects of floc breakage on fractal dimensions [29–30]. Additionally, research has explored the evolution of floc breakage and strength factors [31–32], and the interactions between fluid flow fields and floc transport during settling [33–34]. Scholars have also investigated how physical properties evolve under various flocculation conditions, focusing on how changes in floc density and settling velocity affect flocculation outcomes [17,28]. In terms of underflow concentration, studies have examined the effects of slurry height, residence time, and rake shear conditions [35–36], showing that underflow concentration without shear is lower than that with shear [35,37]. At the microstructural level, it was found that large drainage channels rupture into smaller ones, reducing pore volume and increasing slurry concentration [9,38–39].

Although significant research has been devoted to flocculation effects, settling behavior, and variations in underflow concentration, few studies have addressed slurry rheology in thickening equipment. Poor flowability of the high-concentration slurry leads to hardening of the vertical sand silo and blockage of the thickener rake, which negatively impacts the smooth discharge of the thickened slurry [20,40–41]. Therefore, rheological experiments are urgently needed to investigate how static yield stress (τB) varies with slurry concentration. Previous rheological experiments involving tailings thickening, mixing, and pipeline transport have typically used freshly mixed slurry [42–45]. However, high-speed mixer shearing improves slurry homogeneity and does not reflect the structural characteristics of thickened slurry in silos.