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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