Using Rheology to Predict Unstable Flow for Laterite Slurry

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Using Rheology to Predict Unstable Flow for
Laterite Slurry

• Don Hallbom P.Eng.
• Ph.D. Candidate
• Advisor Dr. Bern Klein

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Overview

• What is Unstable Flow?
• Problems Predicting Unstable Flow
• Rheometry
• Unstable Flow Diagrams

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Unstable Flow of Laterite Slurry

• 3 new pressure acid leach nickel plants recently
  opened in Australia
• Unstable flow problems were encountered at each
  plant
• Operating solids content limited by problems at
  various choke points

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What is Unstable Flow?

• Yield Stress Related Unstable Flow
• Rat-holing
• Plugging
• Pulsing (build-up and break down)
• Other Types of Unstable Flow
• Turbulence
• Settling
• Transients (water hammer)

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What is Unstable Flow?
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Why Does it Matter?

• Slurry needs to be handled at a low solids
  content to obtain stable flow
• More water required
• Chemical additives may be required
• More acid for leaching
• More limestone for neutralization
• More tailings to dispose of

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How Can Unstable Flow Conditions Be Predicted?

• Experience
• Pilot Plants
• Loop Tests
• Analytically (Design Equations)

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Variables Affecting Slurry Flow

• Mineral types
• Particle sizes
• Solids content
• Pipe diameter
• Flow rate
• Temperature
• pH
• Pipe roughness

• Impurities
• Water hardness
• Salt content
• Flocculants
• Surfactants
• Shear history
• Heterogeneity
• Clay content

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Rheometry Basics

• Cylinder is rotated in a cup of fluid at known
  speeds
• Shaft torque is measured
• Shear stresses and shear rates are calculated
• Flow curve plotted

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Simple Rheometer in Action!
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Flow Curves for Laterite Slurry

• Shear
• Stress
• Shear Rate

Bingham
Nickel Laterite
Herschel-Bulkley
Power Law
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Unstable Flow of Wood Slurry

• Similar unstable flow problems occur in pulp
  paper mills
• Bodenheimer (unstable flow) diagrams have been
  used for 30 years in mill design and operation
• Operating solids have been increased from 3 to
  15 w/w

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Wood Slurry Flow Curve
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Research Objectives

• To derive methods to create/modify Unstable Flow
  Diagrams for laterite slurry based on bench scale
  experiments (i.e., rheometry)
• To confirm the validity of the methods by flow
  loop testing

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Rheometry and Analysis

• APPROACH
• Model the flow curves of laterite slurry based on
  rheometry
• Use the model to predict unstable flow in pipe
• Relate the model parameters to physical parameters

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Rheometry and Analysis

• ASSUMPTIONS
• A large number of physical parameters can be
  reduced to a small number of model parameters
• Slurries with similar flow curves (models) will
  behave similarly
• Unstable flow regions can be related to the
  slurry flow curves

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Time Dependency and Instability

• Shear
• Stress
• Shear Rate

Unstable!
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Time Dependency and Instability
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• Shear
• Stress
• Shear Rate

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30
Unstable!
Normal
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Low Shear Flow Regimes

• NORMAL FLOW
• Slurry flows as a stable fluid
• Annular flow (unyielded core)
• PLUG FLOW
• Slurry flows as a semi-solid mass
• Shears at the wall
• UNSTABLE FLOW

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Unstable Flow Diagram (Flow in a Pipe)
30C
45
70C
Log(dH/L) Shear Stress
Plug
70C
40
35
30C
30
Unstable!
12k tpd
8k tpd
Normal
Log (Q) Shear Rate
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Summary

• Some mineral slurries can exhibit unstable flow
  at low shear rates
• Unstable Flow Diagrams can be used to predict
  whether flow will be stable
• UFDs can aid plant design, trouble shooting, and
  operation

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Summary

• Bench scale rheometry and analysis can be used to
  construct UFDs quickly and at a fraction of the
  cost of loop testing or pilot plants
• A similar approach may be used in the design of
  other slurry system components (e.g. tanks)

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Acknowlegements

• SPONSORS
• CERM3
• NSERC
• Kvaerner
• Fluor Daniel Wright
• Highlands Pacific
• Lakefield Research

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Using Rheology to Predict Unstable Flow for
Laterite Slurry