Steve Slottee, PasteThick Associates Salt Lake City, Utah · Salt Lake City, Utah. DISPOSICION DE...
Transcript of Steve Slottee, PasteThick Associates Salt Lake City, Utah · Salt Lake City, Utah. DISPOSICION DE...
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Paste Thickener Rheology
Steve Slottee, PasteThick
Associates
Salt Lake City, Utah
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Objectives
� Teach the basis of paste thickener design
�Slurry systems - designed on slurry density
�Paste systems – designed on rheology
� End users/engineering companies make better decisions by understanding “Paste Thickener Rheology”
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Introduction
� This presentation reviews:
�Advances in paste thickener process design
�Where to find paste thickening knowledge
�Rheological basis for paste thickener design
�Why wt% solids does not apply
�Importance of particle size distribution
�Criteria for choosing paste thickeners
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History
� Before 1996 paste thickening limited to the alumina industry
� Experience with copper, gold, lead and zinc, iron ore, others developed by:
�Increasing number of full-scale installations
�Across variety of mineral types
�Extensive pilot and bench-scale testing as applications were evaluated
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Development of Paste Thickener Design
� Development by paste thickener manufacturers such as Eimco (now FL Schmidth)
� Since 1996, paste thickening used in mineral tailings growing worldwide
� Applications
�Surface disposal (stacking)
�Water recovery
�CCD, backfill, increase pond capacity
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Paste Thickener Suppliers
� Major paste thickener suppliers
�FL Schmidth
�Outotech
�WesTech Engineering (since 2002)
� The few number of suppliers limited by
�Knowledge and experience base required
�High torque drives required for paste
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Paste Thickening Expertise
� End-users and engineering companies do not have history of paste experience
� Paste system designs from:
�Thickener manufacturers
�Paste pipeline designers
�Geotechnical paste designers
� Within these paste design companies there are “true experts” – find them
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Future
� Expertise in paste will be developed by:
�Engineering companies
�End-users
� Thickener manufacturers will focus on:
�Larger diameters
�Methods of rheology control
� Flocculant manufacturers can be expected to develop more rheology modifiers
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Rheology Basis of Thickener Design
� Slurry thickeners based on particle concentration
� Paste thickeners designs are based on inter-particle attraction forces
�Created by high solids concentration
�Particle to particle interactions produce non-Newtonian characteristics
�High viscosity
�Yield stress
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Paste Properties
� Solids suspension
� Non-settling
� Non-segregating
� High concentration
� Holds a shape
� High viscosity
ferrochrome tailings
50 wt% solids
75% -20 µm
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Paste is a Network of Fine Particles
� Fine particles bond
� Hold coarse particles in suspension
� Approx ≥20% particles <20 µm
makes paste
iron ore tailings
80 wt% solids
70% -20 µm
10% +200 µm
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Yield Stress
� A pressure is required to make the paste flow
� Pressure is “yield stress”
� Measured in Pascal
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Overcome Yield Stress for Flow
� Slurry assumes the
shape of a container
� Paste Holds a shape
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Yield Stress Curve
% solids
Yield St ress
Filter Cake
Paste
Thickened Tailings
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16
Yield Stress Curves
David Boger,
University of
Melbourne
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Yield Stress of Materials
Substance Yield Stress (Pa)
Tomato sauce 15
Yogurt 80
Toothpaste 110
Peanut butter 1900
Paste tailings (thickened tailings) 30-100
Mine paste backfill 250-300
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Factors Affecting Yield Stress
� Solids concentration
� Particle size distribution
� Mineralogy
� Water chemistry
� Flocculant
� Temperature
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Yield Stress Ranges
� <30 Pa Newtonian settling slurry
� 50 Pa produces 1-2% slopes
� >>100 Pa for mine paste backfill
� ≤200 Pa for centrifugal pumping
� 30-400 Pa for paste thickener underflow
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Yield Stress Ranges are Approximate
� Mineralogy determines the yield stress
� Some materials will be paste at <30 Pa
� Some materials will be >2% slope at 50 Pa
� Rheology testing with a pilot plant, viscometer or pipe loop is required
�Identify the yield stress curve
�Identify paste properties (non-settling, slope, etc) and the yield stress curve
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Yield Stress and Deposition Slope
Low yield stress 1- 2% slope
Medium yield stress 2- 3% slope
High yield stress 3- 5% slope
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Yield Stress Curve with Filter Cake
% solids
Yield St ress
Filter Cake
Paste
Thickened Tailings
� Yield stress exponential
with concentration
� Approaches filter cake
without drying,
asymptotically
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Yield Stress Is Exponential
� Paste thickeners often operate in this exponential rise
�1-2 wt% concentration differences have large changes in yield stress
�A paste thickener designed to operate on the steep portion of the curve
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Shearing Determines Yield Stress
� Shear history changes yield stress
�Agitation
�Pumping
�Pipeline flow
� How is a sample collected without shear?
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Effect of Pump and Pipeline Shear on the Yield Stress Curve
% solids
Yield Stress
Sheared
Unsheared
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Inside thickener
150 Pa 35%
Thickener
discharge
80 Pa 35%Deposition
50 Pa 35%
Shear Reduces Yield Stressfor Each Phase of Flowsheet
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The Paste System Design ProcessIntegrates Rheology
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Wt% Solids Does not Describe Paste
� Conventional tailings designed using slurry density
� For paste, solids density is less significant
� Rheology becomes the major design
�Paste thickener discharging paste
�Pump/pipeline transporting paste
�Deposition slope determination
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Solids S.G. Determines Location of Yield Stress
Curve (other characteristics equal such as particle size, mineralogy)
% solids
Yield Stress
solids s.g.= 2.6 solids s.g.= 2.7
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Same Yield Stress, Different % Solids
Specific
Gravity
of Solid
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Paste Thickeners Produce a Specific Rheology
� Paste thickeners produce a specific rheology (yield stress), not a solids concentration
� 50 Pa may be
�25 wt% thickener underflow for clay tailings
�65 wt% thickener underflow for copper tailings
� Specifying a solids concentration is not relevant to the thickener designer1
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Volume Percent is a Better Description for Paste?
� Vol % recognizes surface area (electrical charges)
� Aggregates or voids make vol% less descriptive
� Industry standard is wt% to describe paste
Concentration
Iron ore tailings, s.g. 3.5
65 wt% 35 vol%
Coal tailings, s.g. 2.0
52 wt% 35 vol%
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Best Description of Paste Design Basis
� Yield stress curve
�Sheared and unsheared
�Concentration
�Recognize concentration is tied to a specific particle size
�If wt% used, recognize tied to a solids density
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Particle size affect on rheology
� Small particles - high surface areas
� More small particles, more electrical attractions
� H2O molecule bonds with particles
� For same yield stress, lower solids concentration compared to coarser particles
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Same yield stress, different concentrations
� Iron ore tailings – 50 Pa yield stress
�20% particles <20 microns
�70 wt% concentration
� Kaolin clay – 50 Pa yield stress
�90% < 20 microns
�30 wt% concentration
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Measuring Particle Size
� ≤ 20 micron fraction determines rheology
� Small particle size analysis very important for thickener design
� Often sub-sieve particle size not available
� Screens suggest <20 micron fraction
�Minus 44 micron (US 325 mesh screen)
�Minus 37 micron (US 400 mesh screen)
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Paste Thickener Types
� Deep Cone
� High Density
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Deep Cone Features
� Height : diameter > 1
� Unit areas (m2/tph) << high-rate thickener
� Mud bed height >> high rate thickener
�Produces high underflow solids concentration
� 30 degrees cone discharges paste
� Diameters to 24 m and higher
� High torque
� No rake lift
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High Density Features
� Height : diameter < 1
� 10o to 15o cone
� Bed depth > high rate thickener
� On ground construction
� Diameter > 50 m
� Large diameter allows high feed flow rates
� High torque