The Science of Bed Level and Rheology Measurements for Thickener Control

OVERVIEW OF THE PROCESS FLOW

Introduction to the Process

In mineral processing operations, the solid-liquid separation process carries consequences that extend well beyond tailings management alone, and its performance directly constrains the economic and environmental sustainability of a mineral processing operation. Historically, as Pullum (2007) notes, conventional tailings disposal involved pumping low-concentration slurries to large catchment dams, although this approach is generally no longer acceptable, as environmental and economic imperatives are prohibitive. The resulting paradigm shift toward thickened paste has remarkably altered pipeline behaviour. Consequently, the dewatering circuit has evolved into a more complex process, demanding the reliable measurements and monitoring of key variables for effective control and optimisation.

In this article, we will examine the limitations of the conventional methods to manage the dewatering circuit. We will highlight how finer particles in clays can produce considerably different underflow rheology even when density stays the same [Bower et al, 2026], how the laboratory rheology can be non-representative [Sun et al, 2026], and how having a conservative approach can produce inefficient outcomes. We will probe into the relevance of continuous bed level and rheology measurements in thickener control and the cost of not measuring them.

Susceptibility of the Process

The primary dewatering stage in mineral processing typically occurs in thickeners. Inadequate thickener control can cause the underflow solids concentration—and therefore the rheological load on the underflow pumps—to deviate abruptly from design limits. When the settling rate declines, the overflow carries a higher concentration of fine solids, while solids inventory accumulates in the thickener, raising the risk of rake bogging and underflow pump trips.

Moreover, an underflow that develops a high yield stress may exceed the available pump capacity. Overburdened pumps that cannot sustain the required discharge pressure allow the slurry to deposit in the pipeline, leading to line blockages from loss of consistent motion.

ISSUES WITH THE CURRENT CONTROL MEASURES AND ANALYSIS

CONVENTIONAL MEASUREMENTS AND THEIR LIMITATIONS

Inside the Thickener

Most operators monitor the rake torque, overflow turbidity and bed pressure for the thickener’s operations and control. Each has its own limitations; for instance, in some applications, the rake torque remains insensitive to moderate accumulations before spiking suddenly at high loads, leaving operators little time to respond. By the time the overflow turbidity registers higher solids, fine solids have already escaped the tank and contaminated the process water circuit, making it an inherently lagging indicator. Bed pressure, under a set of assumptions, can sometimes estimate the total mass of the inventory, but it cannot denote the vertical distribution.

Each captures a different aspect of thickener performance, yet none fully reveals the overall settling behaviour inside the tank. Having access to the visibility inside the thickener and its stratified zones can help operators.

Outside the Thickener

Beyond the thickener itself, the same information gap extends to how most operators manage slurry once it is pumped from the thickener tank. Most mineral processing sites monitor pressure, temperature, flow, and density—measurements that are necessary but insufficient for characterising the slurry's resistance to flow and its inter-particle bonding strength. Even though the site operators are often aware that a critical variable is missing from their control strategy, one that could help improve their operations and reduce the risk of process upsets, they do not have access to its solution – timely rheological data.

The operators are constrained under proxies and non-representative alternatives, such as:

Density/Solids Concentration Proxy:

Density is at times used as a proxy to understand the viscosity or overall rheological properties of a slurry, such as:

Density vs Viscosity: A common misconception is that higher density leads to higher viscosity. Oftentimes, operators rely on slurry density for impromptu corrective measures, such as dilution of the slurry. This approach can be counter-productive as a minimum viscosity needs to be maintained to keep material suspended in pipelines, and diluting the slurry can result in a reduction in viscosity, which can result in pipe blockages. Similarly, clay-rich or over-flocculated slurries can exhibit a high viscosity, independent of solids concentration, leading to blockages and rake-bogging in thickeners unexpectedly.

Density vs Yield Stress: Two slurries at identical density can have dramatically different yield stresses depending on particle-size distribution, mineralogy, clay content, temperature, and flocculant response.

Figure 1: The samples for Copper and Tin Tailings, with similar density but completely different rheology. Samples A (left) and B (right) have a higher yield stress than their counterparts. Density only measures mass per volume, not inter‑particle bonding strength. Similarly, the bed height may be different even at a similar density. Source: Bower et al. (2026)

Offline (Laboratory) Testing of Slurry Rheology as an Alternative:

Measuring slurry rheology through laboratory testing requires extracting slurry samples from the process for analysis. This method of offline testing rarely reflects true in-process conditions, as temperature, shear rate, mixing intensity, and sample chemical structure change once the slurry is removed from the pipeline and the time taken for analysis. The equipment complexity in the laboratory and skill level required of the technician, for both the test work and data interpretation, is high, and retention of that capability can be challenging.

Any sample transported to a laboratory can undergo an unaccounted shear history during extraction and handling, so even the most careful measurement protocol makes it very challenging to fully recover the in‑process rheological signature.

In a recent study, Sun et al. (2026) showed that in some samples, the true in-situ plastic viscosity was underestimated by a factor of nearly 20 when measured after standard sampling, transportation, and homogenisation — a discrepancy large enough to fundamentally undermine any process decision based on that data.

Online rheology measurement can overcome density proxy and laboratory limitations, tracking rheological changes from ore variability, particle size shifts, chemical dosing, and hydraulic conditions on a continuous basis.

CONVENTIONAL APPROACH AND ITS LIMITATIONS

Conservative sizing as a coping mechanism:

Safety margins throughout the process, such as larger pipes, higher-rated pumps and more conservative pipeline velocities, might theoretically be an alternative approach. The concern is that oversizing to compensate for rheological uncertainty creates its own problems. Slurry transport lines can fail due to over-conservative sizing — if velocity is too low, it can cause pipe blockages due to settling and sedimentation.

As demonstrated by Pallum (2007), the stratified paste flows, which represent most real-world operations once shear is applied, the pressure drop becomes essentially independent of pipe diameter beyond a certain size. Simply installing a larger pipe does not create the expected hydraulic margin and may even promote settling by reducing shear rates that help keep particles entrained. Also, for many slurries, there exists a minimum transport velocity below which solids settle and form stationary beds, leading to blockage.

Pushing tonnes close to breaking-point:

Inversely, running the operations at its maximum capacity, to push more tonnes, without the necessary upgrades to its equipment, can have a toll on the underflow pumps and slurry transport pipes due to excessive wear. Pushing more tonnes precariously close to the equipment’s capacity, coupled with changes in ore chemistry and, in turn, unaccounted slurry rheology, can cause costly process upsets such as lower equipment life and production stoppages.

Continuous, online slurry rheology can track rheological parameters over time, helping operators identify developing trends that signal an approaching process excursion and allowing corrective action to be taken before normal operating limits are exceeded.

NEW SOLUTIONS: CONTINUOUS BED LEVEL AND ONLINE RHEOLOGY DATA

What Bed Level Measurement Reveals

The behaviour of a thickener can be best understood by examining the distinct settling zones that form within a thickener.

The bed level in a thickener comprises four settling zones: (A) clarified liquor, (B) free-settling, (C) hindered settling/mud (often near the “gel point”), and (D) compression/heavy mud, where rakes assist in consolidating the settled solids and facilitating the release of trapped entrained water. Each zone plays an important role in the sedimentation and compaction process. Hence, knowing where these transition zones sit at any given moment can help with valuable insights on how a thickener can be controlled.

Figure 2: Schematics of a thickener tank

In the free-settling zone of a thickener, solids settle under gravity, concentration increases with depth and settling rates progressively slow. Flocculants enhance particle aggregation in this zone, and the height of the zone indicates dosing efficiency.

• Contracting Zone (interface approaching mud) → Faster settling → Reduce flocculant

• Expanding Zone (increasing separation) → Slower settling → Increase flocculant

Furthermore, the height of the mud bed correlates with solids residence time—a key factor in compaction and optimum underflow density. Adequate solids residence time in this zone improves dewatering efficiency.

Operationally, monitoring these bed levels helps ensure the underflow reaches target density before discharge and provides early warning of stratification loss or bed collapse.

What Rheology Measurement Reveals

Viscosity describes how easily a slurry flows, and yield-stress tells you how much force is needed before a slurry starts to move. Many mineral slurries are considered non-Newtonian, meaning that flow characteristics change with changes to various process variables and conditions, such as solids concentration, particle size distribution, ore mineralogy, flocculant response and chemistry. When rheological measurements are performed on underflow slurries, the appropriate constitutive model is the Herschel–Bulkley equation, which separates the true yield stress from non‑Newtonian viscous effects.

Viscoelastic properties govern how efficiently the underflow slurry moves. When key rheological parameters, like viscosity and yield stress, change unexpectedly, the consequences are tangible: blocked slurry pipelines, rake high-torque events in thickeners, pump overloads and inefficient flocculant dosing. Especially, as solids concentration increases, pipeline flow transitions from turbulence-dominated transport to regimes governed by particle interactions and non-Newtonian rheology, fundamentally altering transport mechanisms. Their flow behaviour depends more on rheology than on solids content. Particularly, with conditions such as over-flocculation and high levels of clays and fines.

Unlike density measurements, rheological measurements provide a more direct indication of how the slurry will behave during transport. Particularly in systems containing high clay contents, fines, or over-flocculated material, slurry behaviour may change significantly while density remains relatively constant.

Operationally, continuous rheology measurements can provide actionable insights for process control and help detect early warning signs before process upsets:

• Rising Yield Stress Before Rake Torque Increases → Early warning of rake bogging

• Rising Viscosity → Can result in slurry pipeline blockage and pump overload (high suction head)

• Low Yield Stress and Viscosity → Poor dewatering performance due to flocculant underdosing or insufficient compression

• Changes in Rheology → Root cause identification for variability related to flocculation, feed/ore changes, or underflow imbalance

Continuous online rheology provides a direct, real-time measure of slurry operability, effectively integrating the impacts of thickener variables into a single actionable signal.

How Bed Level and Rheology are Complementary

The case for combining continuous bed level data with continuous rheology measurements in mineral processing is not simply additive but synergistic. Each measurement addresses a different layer of the same operational problem. Combined, these two measurements give operators a more complete picture of thickener performance.

Case Scenario: Bed Level and Rheology for Managing Underflow Pumps

In thickeners, hydrostatic pressure is the gravitational force exerted by the slurry column. However, unlike pure liquids, slurries don't always transmit the entire tank’s load as hydraulic head because of how solids behave at different concentrations. The pressure at the bottom of the thickener does not equal the weight of all the slurry above. Only the hydraulically coupled portion transmits pressure at the bottom of the thickener. This is the key to understanding thickener behaviour because calculating the effective hydraulic head depends on what is inside the thickener and how it operates.

The compacted bed doesn't add its full weight to the hydraulic pressure because compressed solids transition to a mechanically supported regime characterised by direct particle-to-particle contact networks. This structural framework enables the settled solids to transfer their gravitational load directly to the thickener floor through inter-particle friction and compressive forces, effectively decoupling them from hydraulic pressure transmission to the underflow. Consequently, while the upper inventory of hydraulically coupled solids contributes to the available Net Positive Suction Head (NPSH) at the underflow pump inlet through fluid-mediated pressure, the lower compacted bed—despite representing significant mass and vertical height—adds negligible incremental pressure because its weight bypasses the fluid phase entirely.

This dual-regime behaviour explains how the effective pressure comes only from suspended solids, not from the compacted bed where particles form load-bearing networks. Understanding this distinction is essential, as each regime needs to be factored individually, for accurate prediction of underflow pump inlet pressure conditions and, principally, for understanding thickener control strategies in mineral processing operations.

The mud bed height provides operators with the fundamental geometric parameter required to predict a thickener’s hydraulic head. The rheological properties of the slurry will determine what pressure you end up with at the pump inlet.

AVAILABLE TECHNOLOGY AND CLOSING NOTES

Instrumentation in Practice: Science of Bed Level and Rheology Measurements

Commercial instrumentation is now available to provide continuous measurement of both – thickener’s settling behaviour and slurry rheological properties directly in operating plants. These technologies enable the continuous measurement of process variables that were historically inferred from proxies or intermittent laboratory testing under non-representative process conditions.

Bed level measurements provide visibility into what is occurring inside the thickener—how solids are settling, where the transition zones are located, and whether flocculant addition and solids residence time are producing the desired compaction behaviour. Rheology measurements provide visibility into what leaves the thickener at the underflow—whether the discharged slurry remains within the operating limits of pumps and pipelines, and whether changes in ore properties, chemistry, or process conditions are influencing transport behaviour.

Together, these measurements provide complementary information across the dewatering process. Bed-level measurements indicate how material is accumulating and compacting inside the thickener, while rheological measurements indicate how the resulting slurry behaves once transported downstream. Combined, they allow operators to move beyond reactive operation toward a more informed and predictive control strategy.

Figure 3: A process flow for all the available sensors and data points available to optimise dewatering in mineral processing operations.

Examples of commercially available technologies include PLA Process Analysers' SmartDiver® for continuous bed level measurement and ViscoSight™ for online rheology measurement.

The SmartDiver® continuously measures changes in suspended solids through the thickener depth profile and identifies key transition zones such as clarity, interface, mud and heavy mud levels. This information can provide operators with a clearer understanding of thickener behaviour and settling performance.

ViscoSight™ is a side-stream rheometer that continuously determines the rheological characteristics of non-Newtonian slurries. By characterising the relationship between shear stress and shear rate, it provides the key rheological parameters required to manage underflow pumpability and monitor changes in slurry transport behaviour.

Conclusion

The measurement gap of not factoring in the true physical state of the thickener and underflow slurry carries quantifiable operational consequences and monetary implications.

This gap can be closed with continuous and reliable data, representative of the processes – bed level and rheology measurements. Bed levels quantify how the thickener is operating in the settling–compression continuum; rheology quantifies how the underflow slurry behaves.

Neither system eliminates the complexity of the dewatering process in mineral processing. What they offer is essential data — internal bed structure and continuous, representative rheological properties — from which informed decisions can be made.

References

L. Pullum (2007): Pipelining Tailings, Pastes and Backfill. Paste 2007: Proceedings of the Tenth International Seminar on Paste and Thickened Tailings, Perth.

Sun, YS, McGowan, Byrne, Parkinson, Kaminsky, DeJong (2026): In situ measurement of fluid fine tailings rheology. Paste 2026: Proceedings of the 28th International Conference on Paste, Thickened and Filtered Tailings, Australian Centre for Geomechanics, Lisbon.

Bower, M, Matinin, Bonneau (2026): Evaluation of rheology-modifying and conventional flocculants across various tailings through a dynamic thickening test. Paste 2026: Proceedings of the 28th International Conference on Paste, Thickened and Filtered Tailings, Australian Centre for Geomechanics, Lisbon.