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


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”


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


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


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


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


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


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


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


Paste Properties

� Solids suspension

� Non-settling

� Non-segregating

� High concentration

� Holds a shape

� High viscosity

ferrochrome tailings

50 wt% solids

75% -20 µm


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


Yield Stress

� A pressure is required to make the paste flow

� Pressure is “yield stress”

� Measured in Pascal


Overcome Yield Stress for Flow

� Slurry assumes the

shape of a container

� Paste Holds a shape


Flow requires overcoming yield stress


Yield Stress Curve

% solids

Yield St ress

Filter Cake

Paste

Thickened Tailings


16

Yield Stress Curves

David Boger,

University of

Melbourne


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


Factors Affecting Yield Stress

� Solids concentration

� Particle size distribution

� Mineralogy

� Water chemistry

� Flocculant

� Temperature


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


Paste Deposit Slope?


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


Yield Stress and Deposition Slope

Low yield stress 1- 2% slope

Medium yield stress 2- 3% slope

High yield stress 3- 5% slope


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


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


Yield Stress Curves Depend on Mineralogy


Shearing Determines Yield Stress

� Shear history changes yield stress

�Agitation

�Pumping

�Pipeline flow

� How is a sample collected without shear?


Effect of Pump and Pipeline Shear on the Yield Stress Curve

% solids

Yield Stress

Sheared

Unsheared


Shearing Example


Inside thickener

150 Pa 35%

Thickener

discharge

80 Pa 35%Deposition

50 Pa 35%

Shear Reduces Yield Stressfor Each Phase of Flowsheet


Sheared and Unsheared Tailings


The Paste System Design ProcessIntegrates Rheology


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


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


Same Yield Stress, Different % Solids

Specific

Gravity

of Solid


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


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%


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


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


Particle Size Effect on Yield Stress


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


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)


Paste Thickener Types

� Deep Cone

� High Density


Deep Cone Paste Thickener


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


High Density Paste Thickener


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


Paste Thickener Yield Stress Curve


Criteria for Choosing a Paste Thickener


Gracias

steve slottee, pastethick associates salt lake city, utah · salt lake city, utah. disposicion de...

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