Abstract
An experimental study on the time-dependent rheology of highly concentrated and flocculated suspensions of bauxite residue (red mud) is presented. Both the thixotropic breakdown with shear and recovery at rest have been quantitatively examined using a vane-shear instrument and a capillary rheometer. It is demonstrated that both the yield stress and the apparent viscosity of the material can be drastically reduced, by orders of magnitude, by shear-induced agitation with a simple anchor impeller. The rate of thixotropic decay is a function of solids concentration, agitation time and speed. With prolonged agitation, the flow properties are brought to an equilibrium level characterized by a finite yield stress and a shear rate-dependent (shear-thinning) viscosity. In the absence of shear, the yield stress gradually increases with time but at a rate considerably slower than the rate of decay in the shear field. Even after an extended rest period of several months, only a fraction of the initial yield stress can be regained. The observed behaviour has been interpreted in terms of a time-dependent but non-reversible transformation of a network structure of flocculated particles initially developed in the red mud. Using a structural kinetic approach, models have been developed for correlating the experimental kinetic data. The paper concludes with a discussion on the practical consequences of the work.
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Abbreviations
- D :
-
Diameter of capillary tube
- k 1 :
-
Breakdown rate constant in eq. (2)
- k ′1 :
-
Modified rate constant in eq. (7)
- k 2 :
-
Recovery rate constant in eq. (8)
- N :
-
Impeller rotational speed
- P :
-
Pressure
- t :
-
Agitation (mixing time)
- t′:
-
Resting (aging) time
- V :
-
Average velocity
- \(\dot \gamma _m \) :
-
Average mixing shear rate
- η′ :
-
Apparent viscosity (τ w/(8V/D))
- τ w :
-
Wall shear stress
- τ y :
-
Yield stress
- τ E y :
-
Yield stress at infinite resting time
- ψ :
-
Dimensionless structural parameter
- e :
-
Equilibrium condition (t → ∞)
- o :
-
Initial condition (t = 0)
- ∞:
-
Conditions att → ∞ and\(\dot \gamma _m \to \infty \)
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Nguyen, Q.D., Boger, D.V. Thixotropic behaviour of concentrated bauxite residue suspensions. Rheol Acta 24, 427–437 (1985). https://doi.org/10.1007/BF01333970
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DOI: https://doi.org/10.1007/BF01333970