Development of Device for Measuring Zeta Potential Using

Sedimentation MethodSalah Uddin

Preliminary Examination, 2009

McGill University Department of Mining and Materials Engineering

Outline

Zeta potential (ZP)Origin of surface chargeElectrical double layer and ZPImportance of ZP measurement

Sedimentation potential (SP)PrincipleCell design and instrument controlResults

Single oxidesOre

Possible explanationFuture workContribution to original knowledge

Origin of surface charge

= Si

= O

= H

– Si+ + H2O ↔ – Si – OH + H+

– O- + H2O ↔ – O – H + OH-

= M

= S

– M+ + H2O ↔ – M – OH + H+

– S- + H2O ↔ – S – H + OH-

1. Surface hydration

Origin of surface charge (cont.)

2. Reaction with H+ and OH-

Silica– Si – OH + H+ ↔ – Si+ + H2O– O – H + OH- ↔ – O- + H2O

Metalsulphide

– M – OH + H+ ↔ – M+ + H2O

– S – H + OH- ↔ – S- + H2O

Electrical double layer (EDL)

Surface-----+-

Mineral Solution

Net surface charge is negative

-

+

+

+

+ +

+

+

+

+

-

--

-

Inner bound layerDiffuse layer

-+

Inner bound layerCounter ions firmly attached

Considered to be immobile

Diffuse layerCounter ions loosely bonded

Can move by external force

Electrical double layerSpatial non-uniform distributionof counter ions

Zeta potential

Zeta potential Potential at the surface of inner layer

surface

potential, mV

distance fromsurface, nm

ψo

ζ = zeta potential= surface potential

ζ

surface of inner layer

+

0

_ψo

Zeta potential and flotation

2 4 6 8 10 12

pH

ZETA

PO

TEN

T IA

L , m

V

60

40

20

0

- 20

- 40

- 60

10-4 M NaCl

10-2 M NaCl

iep ~ pH 6.5

+ + + +

_ _ _ _

surface

surface

100

80

60

40

20

00 2 4 6 8 10 12 14

FLO

TATI

ON

REC

OV

ERY

, %

pH

iep= R-SO3

-

R-SO

3 -

+ + + + _ _ _ _

= R-NH3+

R-N

H3 +

Below iep, positively charged mineral surface attracts collector with negative reactive group

Above iep, negative mineral surface adsorbs collector with positive reactive group

ZETA

PO

TEN

TIA

L, m

V60

40

20

0

- 20

- 40

- 60

Silica, SiO2

Silica + Ca

2 4 6 8 10 12

pH- 80

ZETA

PO

TEN

TIA

L, m

V60

40

20

0

- 20

- 40

- 60

Silica, SiO2

Silica + Ca

2 4 6 8 10 12

pH- 80

iep

with CaCharge reversal

no Ca

Zeta potential and selective separation

Zeta potential Reveals insight into mechanisms of aggregation and dispersion

Zeta potential measurement

Zeta potential

Applied fieldgenerates movement

Applied force generates potential

Electrophoresis Particles move, liquid

phase stationary

Electro-osmosis Liquid phase moves, particles stationary

Sedimentation potential

Particles move, liquid phase stationary

Streaming potentialLiquid phase moves, particles stationary

Sedimentation method Can handle concentrated systemsApplicable to mixed mineral systemsPotential values can be correlated with visual observations

++

+

+

+

+

+

+

--

--

- - -

-

--

-

--

-

_

+

∑=i

iEEgrvr

Sedimentation potential: principle

h

VVE a−= 0

g

E

r )( 00 ρρφεεηλζ

−=

Volume fraction of solidCalculated from measured

resistance data using Maxwell’s model

Suspension potential difference

Measured directly

Background potential difference

Measured directly

Specific conductivityCalculated from

measured resistance dataZeta

potential

Sedimentation and zeta potential

Settling column

pH meter

pH electrode

Ag/AgCl electrode(potential difference)

Timer relay

Vacuum pump

Multimeter

Ag/AgCl electrode(resistance)

Cell design

Measurements

Instrument control and data acquisition

Interfacing VXI pnp driver and serial communication

Command format SCPI (Standard Code for Programmable Instruments)

Measurement sequence and data processing MatLab R2008a program

Sample Alumina Silica Specific surface area (cm2/cm3) 5270.40 1240.2Median size (µm) 28.14 76.23 Mean size(µm) 27.85 83.06 Variance (µm2) 53.68 1551.5S.D. (µm) 7.32 39.38 CV 26.30 47.41 Mode (µm) 28.43 72.89

Sample Alumina Silica Density 3.97 2.65

Melting point (°C) 2040 1700 Chemical analysis Al2O3 > 99.0

Na2O < 0.4 Fe2O3 < 0.03 SiO2 < 0.03

SiO2 > 99.5 TiO2 ~ 0.1 K2O ~ 0.1 CaO ~ 0.03 Fe2O3 ~ 0.03 Al2O3 ~ 0.01

Validation test samples

0.0141

0.0144

0.0147

0.015

0 5 10 15 20No. Reading

Pote

ntia

l diff

eren

ce (V

)BackgroundSuspension

Sample Silica at pH 7

Potential difference

47000

49000

51000

53000

0 5 10 15 20

No. Reading

Res

ista

nce

(ohm

) Background(polarity 1)Suspension(polarity 1)Background(polarity 2)Suspension(polarity 2)

Sample Silica at pH 7

Resistance

0

0.01

0.02

0.03

10 12 14 16 18 20 22No. Reading

Volu

me

fract

ion

(-)

silicaalumina

S

Lp R

RCC

C=

+−

= ,12/

1φ

Volume fraction of solid (solid holdup)

-80

-70

-60

-50

-40

-30

-20

-10

0

2 4 6 8 10 12pH

Zeta

pot

entia

l (m

V)

SP methodElectrophoresis

-60

-40

-20

0

20

40

60

2 4 6 8 10 12pH

Zeta

pot

entia

l (m

V)

SP methodElectrophoresis

Silica

Alumina

Zeta potential vs. pH

-0.004

-0.002

0

0.002

0.004

0.006

0.008

2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5 10.5

pH

SP (V

)

Ore 1: visual observation vs. SP

Dispersed Partially dispersed Agglomerated

pH

Visual observation (cont.)

-0.5-0.4-0.3-0.2-0.1

00.10.20.30.40.5

0 2 4 6 8 10 12 14

pH

SP (m

V)

-0.6-0.5-0.4-0.3-0.2-0.1

00.10.20.30.40.5

0 2 4 6 8 10 12 14

pH

SP (m

V)

-0.5-0.4-0.3-0.2-0.1

00.10.20.30.40.5

0 2 4 6 8 10 12 14

pH

SP (m

V)

Heavy fraction Light fraction

Middle fraction

Ore 2: gravity separated fractions

SP vs. pH of the gravity separated fraction of an oreThe polynomial shows the trend for ore itselfLight fraction, the predominating part in ore, closely follows the ore trend

Effect of Effect of CationsCations on Oxideson Oxides

-40

-20

0

20

40

pH

Z.P.

(mV) CR1 CR2 CR3

With hydrolyzable metal ions

Alone

M2+ MOH+ M(OH)2

CR: Charge reversal

ZP of oxides: cation effect

Typical for silicates

Interaction: is it Mg?

Mg2+ OH-

_+

Mg(OH)2(s)

mineral

60

40

20

0

- 20

- 40

- 60

Silica, SiO2

Silica + Mg

2 4 6 8 10 12

pH

Z.P. Mg(OH)2(s)

iep ~ pH 12.5

ZETA

PO

TEN

TIA

L, m

V

V-4

H.I Voltameter

Computer

Fixed FritMedium

Fritted Gas Dispersion Tube

Medium

Ag/Ag Cl Electrodes

Drainage Stopcocks

Teflon Quick Fit Joints

V-5

Fixed FritFine

Future work: bubble swarms

M

PM

PC

T

G

S

F E

(M) Mulitimeter, (E) Ag/AgCl electrode, (PE) pH electrode, (PM) pH meter, (P) Vacuum pump, (B) Suspension beaker, (S) Magnetic stirrer, (F) Timer relay (DPDT), (PC) Personal computer, (G) Air inlet (T)

Frit and (S) Suspension tube

Future work: bubbles and particles

Contribution to original knowledge

A fully integrated ZP device using SP method has been described and validated

Analysis of real mineral systems using this technique should result in new insights to assist design of flotation systems

Surface characterization of mixed mineral systems will be possible with this technique

Concentrated slurry, similar to real systems, can be studied

Thank You