Structure & Properties of Bentonite

How does bentonite viscosify water at <3% solids?

Why does it form filter cakes?

What Are the Special Properties of Bentonite Used in AMCOL

Applications?

• Creates viscosity at low concentrations in water

• Builds filter cake

What Are the Special Properties of Bentonite Used in AMCOL Applications?• Creates viscosity at low concentrations in water• Forms low permeability filter cakes• Has very high surface area per unit mass• Absorbs very large amounts of water• Holds onto water very strongly• Swells in contact with water/creates swelling pressure• Water does not flow through a confined layer of bentonite• Forms stable colloid in water; doesn’t settle over reasonable time• Has very high aspect ratio (l/w)• Platelets impermeable to gases• Has high ion-exchange capacity• Undergoes specific selectivity reaction with K+ ion• Oxide/hydroxide surfaces interact with many adsorbents• Slippery• Available in large quantities; mined• Cheap • Comes in many grades• Can derivatize with cationic molecules

Structure and Properties of Bentonite

1. Diagenesis of volcanic ash2. Mined from sedimentary layers3. Platelet structure4. Embedded negative charge5. Colloidal size 6. Importance of counter-ions, Na+ vs. Ca2+

• Sodium clays vs. sodium-activated calcium clays

• Risks and pitfalls

Structure and Properties of Bentonite

7. Adsorption of water and swelling8. Dispersion into colloidal particles in

fresh water9. Viscosity production at 2-3 vol%

solids– Suspension of solids at rest

10.Comparison to other common clays

Manufacturing of Bentonite

• Core samples taken and testing done to map reserves

• Overburden removed from top of bentonite ore with bulldozers

• Bentonite ore loaded into 10-ton haul wagons and piled near the plants

• Bentonite ore ground and dried– Control of grit only by size of screens that material

passes through

Stages of Mining• Exploration

– Geological mapping– Drill trucks– Lab testing

• Mapping– Surveying (GPS)– CAD

• Permitting– Vegetation, soils, wildlife, cultural resources

• Mining– Topsoil removed & stockpiled, overburden removal,

transport• Reclamation

– Backfill Pit or Build Pond– Re-apply Soils– Seed With Native Grasses– Monitor Revegetation– Apply For Bond Release

Chemical Structure of Bentonite

• Complicated, non-stoichiometric structure– 2[(Al1.67Mg0.33)(Si3.5Al0.5)O10(OH)2]

• It is a 3-layer clay with 1 aluminum oxide sheet surrounded by 2 silicon oxide sheets

• The internal aluminum sheet and external silicon oxide sheets share oxygen atoms

• Such an arrangement would be electrically neutral, but Mg2+ ions often substitute for Al3+ ions, resulting in net negative charge– Chemical “double negative”– Deficiency of positive charge leads to net negative

charge

2:1 Layer Structure of Bentonite

3-Layer Clay Platelets with Net Negative Charge

1. The negative charge in platelet is balanced by counter-ions, usually Na+ and Ca2+ , located between the platelets

2. The source of net negative charge is buried in the platelet structure

3. The charge is dispersed over the clay surface (external silicon oxide layers on both sides of the platelet)

4. Resulting diffusively charged bentonite surfaces adsorb huge amounts of water

Bentonite clay platelet is < 1 nm thickAdsorbed water 10 to 20 nm, maybe 40 nm, thick

Important Properties ofSodium Bentonite

• Mined bentonite is comprised of crystalline packets of montmorillonite platelets

• Packets may expand/disperse to individual platelets in fresh, soft water

• Na+ has a single charge and associates with one platelets and allows complete dispersion

• Ca2+, with 2 charges, associates with two platelets and prevents/slows down dispersion– Addition of soda ash is to replace Ca2+ with Na+

– Ca2+ + 2 Na+ + CO32- → 2 Na+ + CaCO3↓

Hydration & Dispersion ofSodium Bentonite

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Cation Exchange of Bentonite

• Ions can change positions with other ions

• Particular ions have a higher affinity for the exchange sites

• Divalent ions exchange monovalent ions

• High concentrations of monovalent ions can displace divalent ions

• Divalent ions can be removed chemically– Soda ash– Sodium hydroxide

• Cationic surfactants ion-exchange to make organophilic bentonite

Calcium Bentonite

Calcium ion has a special effect on bentonite• The Ca2+ ion can bridge negative charges between two

bentonite faces

• Can prevent dispersion

• Calcium bentonite is much less effective viscosifier than sodium bentonite

• Can lead to face-to-face flocculation

• High temperature and shear can collapse the flocculated structure to calcium bentonite

• The +2 charge is much more effective than +1 in shielding - charges between particles

Hydration of Calcium Bentonite

0

Potassium Ion Has a Special Effect

• Its hydrated ionic diameter is the perfect size to fit into the depression of silicon oxide layer–Hydrated K+ smaller than hydrated Na+

–NH4+ has similar size and effect

• Bentonite in K+ form is resistant to further hydration, swelling and dispersion–KCl often used in drilling fluids to reduce

swelling and dispersion of formation clay

Stability of Colloidal Clay System

• In both fresh water and salt water, interparticle attraction andrepulsion operate simultaneously

– The van der Waal’s attraction is independent of salt concentration

– The electrostatic repulsion decreases with increasing salt concentration

– In fresh water, the charge repulsion predominates• Suspension is largely deflocculated• Only a few particles are interacting

– In salt water, the repulsion is reduced• Attraction begins to predominate• Suspension begins to flocculate

Interactions BetweenBentonite Particles Creates

Viscosity

• Interactions between clay particles give structure or viscosity to the suspension– This structure makes the fluid non-Newtonian– Major effect of structure is to increase Yield Point

• Magnitude of viscosity depends on– Number of particles– Overall energy of interaction between particles

• For untreated bentonite fluids (not extended with polymers), difficult to predict whether number of particles or energy of interaction is more important

• For flocculated bentonite fluids, number of particles is most important

Interactions BetweenBentonite Particles Creates

Viscosity

• Dispersion creates a greater number of particles and more interactions

• Complete dispersion depends on shear history, time and chemical interactions–Quality of the bentonite–Electrolytes in water–Caustic–Soda ash–Dispersants

Dispersion Creates Lots of New Particles with Charged Surfaces

• Water adsorbs onto the new surfaces created by dispersion

• Surface charges are exposed

• These surface charges keep colloidal clay particles suspended– The mud does not settle/separate with time– No clear layer on top

• The adsorbed water also keeps the clay particles apart

Viscosity of Bentonite Slurries Result from Interparticle

Interactions• Positive edges are attracted to negative faces

– Edge-to-face interactions• Face-to-face interactions result from bridging of particles

faces by Ca2+ ions– Also by shielding of negative charges by salts

• These interactions produce viscosity– Mechanical energy is required to break them up

Quiescent and Low Shear Rates

• At rest, interparticle interactions are high.– May increase with time– These interactions produce viscosity

• Low shear rates only break a small fraction of these interactions

• High viscosity

High Shear Rates

• Bentonite particles are moving nearly parallel to each other– Shearing action has broken up the interactions– Faces repel, little edge-face interaction– Low viscosity

Intermediate Shear Rates

• Not all the interparticle interactions are broken up

• Intermediate viscosities

Kaolin Clay

• Simple two-layer structure (1:1)( – Al – Al – Al – Al – )( = Si = Si = Si = Si = )

• Strong bonding between successive sheets

• Hexagonal crystals

• Low cation exchange capacity

• Derived from diagenesis of granite

Mica or Illite

• Two silicon layers to one aluminum( = Si = Fe = Si = Si =)( = Al = Al = Al = Al = )( = Si = Si = Si = Si = )

• Ion substitution in silicon oxide layers

• Layers may be mixed with montmorillonite layers– Mixed layer clays– Some swelling in formations– Low to medium ion-exchange capacity

Summary of Structure and Properties of the most common clay minerals

FlocculatesLittle or noneFlocculatesFlocculatesFlocculatesEffect of salts

LowHighHighLowLowViscosity in water

10-4015-2580-15010-403-15CEC, meq/100g

------200-800------BET - H20, m2/g

14020030-8050-10015-25BET - N2, m2/g

Surface area

1-0.11-0.12-0.1large sheets

to 0.55-0.5

Particle size, microns

plateneedleflakeextensive

plateshexagonal

plateParticle shape

sheetsheetsheetsheetsheetCrystal structure

2:1:12:12:12:11:1Layer type

ChloriteAttapulgiteBentoniteMicaKaolinProperty

Conversion from Oilfield Units to Construction Units

8.63.471.430

7.12.959.525

6.42.653.622.5

4.31.735.715

2.91.123.810

1.40.611.95

Wt % solids

Vol % solids

lb/100 gal

waterlb/bbl

9.03.631.575

8.43.429.470

7.83.127.365

7.22.925.260

6.62.623.155

6.02.421.050

5.42.218.945

4.81.916.840

4.21.714.735

3.61.412.630

3.01.210.525

2.41.08.420

1.80.76.315

Wt % solids

Vol % solidslb/bbl

lb/100 gal

water

Concentration Units Conversion Table

Based on specific gravity of 2.5 for bentonite