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? • Crea Create tes s vis visco cosi sity ty at lo low w concentrations in water • Bu Buil ilds ds fi filt lter er ca cake ke
What Are the Special Properties of Bentonite Used in AMCOL Applications? • • • • • • • • • • • • • • • • • •
Create Crea tes s vi visc scos osit ity y at lo low w co conc ncen entr trat atio ions ns in wa wate terr Form Fo rms s low low pe perm rmea eab bil ilit ity y fi filt lter er ca cak kes Has Ha s ver very y hi high su surf rfac ace e are area a per per un uniit mas mass s Abso Ab sorb rbs s ver very y la larg rge e am amou ount nts s of of wat water er Holds on onto wa water ve very st strongly Swel Sw ells ls in co cont ntac actt wit with h wat water er/c /crea reate tes s swe swell llin ing g pres pressu sure re Wate Wa terr doe does s not not fl flow ow th thro roug ugh h a con confi fine ned d lay layer er of be bent nton onit ite e Forms For ms sta stable ble col colloi loid d in in wate water; r; doe doesn’ sn’tt sett settle le ove overr reas reasona onable ble tim time e Has ve very hi high as aspect ra ratio (l (l/w) Pla Pl ate tele lets ts im impe perm rmea eab ble to gas ases es Has Ha s hi high gh io ionn-ex exch chan ang ge ca capa paci city ty Unde Un derg rgoe oes s spec specif ific ic sel selec ecti tivi vity ty rea react ctio ion n with with K+ ion Oxid Ox ide/ e/hy hydro droxi xide de su surfa rface ces s int intera eract ct wi with th ma many ny ad adso sorb rben ents ts Slippery Ava Av ail ila abl ble e in in la larg rge e qua quant ntit itie ies; s; min ined ed Cheap Comes in many grades Can Ca n der deriv ivat atiz ize e wit with h ca cati tion onic ic mo mole lecu cule les s
Structure and Properties of Bentonite 1. 2. 3. 4. 5. 6.
Dia iag gene nes sis of volcani nic c a sh Mine ned d fr from sed sediiment nta ary lay aye ers Platelet st structure Embedded ne negative ch charge Colloidal size Importa tanc nce e of of co coun unte terr-i -io ons, Na Na+ vs. Ca2+ • So Sod diu ium m cl clay ays s vs vs.. so sodi dium um-a -act ctiv ivat ated ed ca calc lciu ium m clays • Risks and pitfalls
Structure and Properties of Bentonite 7. Adsorption of water and swelling 8. Dispersion into colloidal particles in fresh water 9. 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
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 thick Adsorbed water 10 to 20 nm, maybe 40 nm, thick
Important Properties of Sodium 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 of Sodium Bentonite
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 cha bet ticles
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 and repulsion 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 Between Bentonite 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 Between Bentonite 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 Property
Kaolin
Mica
Bentonite
Attapulgite
Chlorite
1:1
2:1
2:1
2:1
2:1:1
sheet
sheet
sheet
sheet
sheet
hexagonal plate
extensive plates
flake
needle
plate
5-0.5
large sheets to 0.5
2-0.1
1-0.1
1-0.1
15-25
50-100
30-80
200
140
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200-800
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meq/100g
3-15
10-40
80-150
15-25
10-40
Viscosity in water
Low
Low
High
High
Low
Layer type Crystal structure Particle shape Particle size, microns Surface area
BET - N 2 , m2 /g BET - H 20, m2 /g , CEC
Effect of
Conversion from Oilfield Units to Construction Units lb/100 gal water
lb/bbl
Vol % solids
Wt % solids
5
11.9
0.6
1.4
10
23.8
1.1
2.9
15
35.7
1.7
4.3
22.5
53.6
2.6
6.4
25
59.5
2.9
7.1
30
71.4
3.4
8.6
