Marek S. Żbik1,2, David J. Williams1

1Geotechnical Engineering Centre, The University of Queensland, Brisbane Qld Australia.

2Centre for Tropical Crops and Biocommodities Faculty of Science & Technology

Qeensland University of Technology Brisbane Qld Australia

Clay suspension voluminous structure, the possible cause of poor

settling and sludge dewatering

Clay-Rich Layers within Coal

Rock layer

Coal with clay layers

Tailings Management

3

Tailings slurry (typically segregating)

Thickened tailings (dewatered, ideally non-segregating

“Wet” filter cake (near - saturated)

“Dry” filter cake (85 to 70% saturated)

Simple water management Efficient water recovery

Process chemical recovery Minimal containment required

Negligible seepage losses Progressive rehabilitation

possible Stable tailings mass

High OpEx and CapEx, but low rehabilitation cost

Complex water management Inefficient water recovery

Containment required Seepage likely

Rehabilitation difficult

Likely low OpEx and CapEx, but high rehabilitation cost Paste tailings

(dewatered, ideally non-bleeding CONTINUUM

Pumpable

Non - Pumpable

Clay-rich tailingsare stuck here!

Na-Bentonite – Effect of Initial % Solids on Settling

CRICOS Provider No 00025B

Initial % Solids >5% will not settle!

Mechanisms of aggregate formation and transformation

At critical concentration, clay particles form spanned network through entire suspension living clear supernatant layer

SEM & AFM IMAGES OF SMECTITE REVEALS FLEXIBLE SHEET

High resolution SEM and AFM images of kaolinite reveals pseudo hexagonal crystals with visible molecular

arrangement on siloxane planes

TEM images show differences between smectite and kaolinite samples

morphology patternsSample 2 Sample 6

Differences in structural ararngement within flocked suspensions

CONTACTS BETWEN CLAY PARTICLES WITHIN 3D NETWORK

Stairstep structure after O”Brien 1971

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 10 20 30 40 50 60 70 80

Time (min.)

D(5

0) m

m

pure water (nat. pH 10.8)

0.05 M CaCl2 (nat pH ~8.9)

CaCl2

addition

conductivity (S/m)

Smectite aggregate forming by Ca2+ cation introduction

2-D & 3-D reconstruction of the montmorillonite gel Na sorption complex (left), Ca sorption complex

(right), sample as seen within the aqueous solution

Force - separation curves for the interaction between Swy-2 on silicon wafer on approach. The dashed line Na+

exchangeable cation form, solid line Ca2+ exchangeable cation form

0.001

0.01

0.1

1

10

-15 185 385 585 785 985 1185

Separation (nm)

Forc

e/2p

R (m

N/m

)

SMECTITE AGGREGATES OF AUSTRALIAN AMCOL BENTONITE WITHIN NaCl SALT 2.5 WT

% SUSPENSION

Mutual arrangement of clay minerals

a- house of cards b- pack of cards

CLAY STRUCTUREA- CELLULAR B- FLOCCULENT

SEM TXM

The formed structure may correspond to the well known Terzaghi “honeycomb” structure, described

for more rigid, platelet shaped minerals such as kaolinite

• Repulsive forces between flakes basal surfaces

• Attractive forces between flakes edges and basal surfaces

1 µm

C O N C L U S I O N• Newly introduced methods of clay soil investigation like

TXM, Cryo-TEM/SEM and FIB/SEM gives new possibility to study and engineering mutual particle orientation in 3-dimensional aqueous clay suspension.

• Results show that clay particles of nano-meter in size liberated smectite particles build spanned network in which most mineral particles and water are arrested.

• This phenomenon may be blamed for poor tailing dewatering and settling behaviour.

• In the inorganic cations treated smectite dense suspensions display severe gelation and form the micelle-like texture of fringe like strong superstructure.

• Future investigations would be focused on primary dense aggregate building rather then flocculating loosely coagulated particles which in effect create extremely voluminous sludge.