Surface Characterization in Mineral Processing - ACS Publications

12 Surface Characterization in Mineral Processing D. W. FUERSTENAU aud S. CHANDER

Downloaded by UNIV LAVAL on April 11, 2016 | http://pubs.acs.org Publication Date: September 22, 1982 | doi: 10.1021/bk-1982-0199.ch012

University of California, Department of Materials Science and Mineral Engineering, Berkeley, C A 94720

Surface and i n t e r f a c i a l phenomena of importance i n mineral processing are reviewed. Examples of a fundamental and an applied nature are taken from the recent literature to illustrate how the use of several different surface characterization techniques makes it possible to delineate a detailed molecular-scale picture of interfaces. Lack of techniques to study solid/liquid interfaces in-situ has led to the development of many ex-situ and indirect methods for surface characterization. The results of such measurements are often interpreted through use of models requiring various underlying assumptions. It has been often necessary to use a combination of two or more techniques to delineate the physico-chemical i n t e r f a c i a l phenomena. Mineral processing research has extensively involved determination of electrokinetic behavior, voltammetry, gas adsorption, aqueous-phase adsorption, infrared and UV-VIS spectroscopy, measurement of contact angles, and, more recently, ESCA analysis to delineate surface composition. Correlations between processing behavior and surface chemical characteristics are demonstrated.

The recovery o f v a l u a b l e minerals and metals r e q u i r e s s e v e r a l stages o f s e q u e n t i a l p r o c e s s i n g o p e r a t i o n s . The mined ore must be crushed and ground to f i n e s i z e s p r i o r to treatment by such benef i c i a t i o n processes as heavy-medium s e p a r a t i o n , t a b l i n g , magnetic separation, electrostatic separation, flotation, selective f l o c c u l a t i o n , e t c . Since most o f these processes are c a r r i e d out i n aqueous media, s o l i d - l i q u i d separations by such operations as t h i c k e n i n g and f i l t r a t i o n are an i n t e g r a l p a r t o f the b e n e f i c i 0097-6156/82/0199-0283$08.25/0 © 1982 American Chemical Society

Casper and Powell; Industrial Applications of Surface Analysis ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


284

SURFACE ANALYSIS

ation flowsheet. Sometimes, the b e n e f i c i a t e d minerals are too fine t o be u s e d d i r e c t l y and must u n d e r g o s i z e e n l a r g e m e n t by agglomeration a n d / o r s i n t e r i n g before subsequent u t i l i z a t i o n . The a b o v e p r o c e s s e s i n v o l v e s e p a r a t i o n b a s e d e i t h e r on b u l k p r o p e r t i e s ( f o r example, s i z e , d e n s i t y , shape, e t c . ) d i r e c t l y or by subtle control of the chemistry o f the narrow i n t e r f a c i a l r e g i o n between the m i n e r a l p a r t i c l e and t h e a q u e o u s s o l u t i o n in w h i c h it i s s u s p e n d e d . In the p r o c e s s i n g o f c e r t a i n o r e s , such as those o f uranium, g o l d or o x i d i z e d c o p p e r , c h e m i c a l a l t e r a t i o n o f the m i n e r a l s may be r e q u i r e d to r e c o v e r the v a l u a b l e metals. These t e c h n i q u e s are not d i s c u s s e d h e r e , e x c e p t to i n c l u d e t h o s e a s p e c t s w h i c h a r e d i r e c t l y r e l a t e d t o s u r f a c e s and i n t e r f a c e s . B e c a u s e m i n e r a l s e p a r a t i o n s a r e g e n e r a l l y made a t f i n e p a r t i c l e s i z e s , p r o p e r t i e s o f s u r f a c e s and i n t e r f a c e s p l a y a d o m i n a n t r o l e i n t h e p e r f o r m a n c e o f many o f t h e m i n e r a l p r o c e s s i n g unit operations. The v a r i o u s t y p e s o f i n t e r f a c e s i n v o l v e d i n m i n e r a l p r o c e s s i n g o p e r a t i o n s a r e summarized i n T a b l e I . Alteration of surfaces by s u r f a c e c h e m i c a l methods is often used to modify mineral properties i n o r d e r to affect t h e i r response to such s e p a r a t i o n t e c h n i q u e s as f l o t a t i o n o r s e l e c t i v e f l o c c u l a t i o n . By c o n t r o l o f i n t e r f a c i a l c h e m i s t r y and p h y s i c s , it is possible to c o n t r o l ore breakage, s e q u e n t i a l s e p a r a t i o n s o f v a l u a b l e m i n e r a l s , and, when n e c e s s a r y , r e - a g g l o m é r a t i o n to r e c o v e r s o l i d or l i q u i d products. In t h i s r e v i e w more e m p h a s i s has b e e n g i v e n t o the solid/liquid interface because of its greater importance in understanding the response o f s p e c i f i c m i n e r a l s to a p r o c e s s i n g stage. T h e r e a r e v e r y few t e c h n i q u e s f o r in~situ analysis of solid/liquid interfaces and it i s t h e r e f o r e , often necessary to o b t a i n i n d i r e c t i n f o r m a t i o n through s e v e r a l different types o f measurements. F o r e x a m p l e , s o l i d / l i q u i d i n t e r f a c e s have b e e n Table

Two

I:

Interfacial Interactions Mineral Processing

Phase

L/V S/L S/V

of

Importance

in

Interact!ons Foam s e p a r a t i o n Wet c o m m i n u t i o n , l e a c h i n g , f i l t r a t i o n Dry comminution, electrostatic separation, photometric sorting

T h r e e Phase S/L-|/L,2 S/L/V S-J/S2/L

F o u r Phase S^Sg/L/V S/L /L /V 1

2

Interactions Liquid-phase agglomeration F l o t a t i o n , agglomeration, f i l t r a t i o n Flocculation, selective flocculation, slime coatings, thickening Interactions Floe f l o t a t i o n Oil flotation, liquid-phase agglomeration, c a r r i e r f l o t a t i o n


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286

ANALYSIS

s t u d i e d by i n v e s t i g a t i n g t h e s o l i d a f t e r it h a s been s e p a r a t e d from the l i q u i d . The s t u d y o f t h e s o l i d / v a c u u m ( o r g a s ) interface in s u c h c a s e s i s p r i m a r i l y an ex~ situ method t o characterize s o l i d / l i q u i d i n t e r f a c e s and f o r c o m p l e t e n e s s s e v e r a l ex- situ t e c h n i q u e s have been i n c l u d e d i n t h i s r e v i e w .


Surface

Properties

of

Importance

in Mineral

Processing

The m i n e r a l p r o c e s s i n g u n i t o p e r a t i o n s i n w h i c h s u r f a c e s or interfaces p l a y an i m p o r t a n t r o l e a r e l i s t e d i n T a b l e I I . Also g i v e n i n t h e t a b l e a r e t h e more i m p o r t a n t i n t e r f a c i a l p r o p e r t i e s on w h i c h t h e p e r f o r m a n c e o f p a r t i c u l a r u n i t o p e r a t i o n d e p e n d s . In o r d e r t o a c q u i r e a d e t a i l e d p i c t u r e o f t h e c h e m i c a l and p h y s i c a l p r o c e s s e s on a m o l e c u l a r s c a l e a t v a r i o u s i n t e r f a c e s , it i s n e c e s s a r y to a s c e r t a i n the f o l l o w i n g : (i) t h e p h y s i c a l and c h e m i c a l n a t u r e o f the m i n e r a l s u r f a c e , ( i i ) the s u r f a c e c o n c e n t r a t i o n o f a t o m i c , i o n i c o r m o l e c u l a r s p e c i e s and t h e i r i d e n t i t y , (iii) the geometric or structural arrangement of these surface species i n c l u d i n g t h e i r m o b i l i t y , and ( i v ) t h e c h a r g e d i s t r i b u t i o n and the energy l e v e l d i s t r i b u t i o n o f v a l e n c e e l e c t r o n s i n the a d s o r b a t e and t h e s u r f a c e . T h i s i n f o r m a t i o n i s n o t e a s y t o o b t a i n and f r e q u e n t l y a combination o f s e v e r a l measuring techniques i s needed. Techniques for the measurement of surface properties of m i n e r a l s i n g a s o r vacuum a r e s u m m a r i z e d i n T a b l e I I I . As s t a t e d in the previous section, many o f these techniques are used indirectly to study processes occurring at mineral/solution interfaces. Methods to s t u d y s o l i d / s o l u t i o n i n t e r f a c e s a r e l i s t e d

Table

III.

Techniques Mineral

Surface

surface

size

Measurement o f

Technique

Property

Specific Pore

for

Surface

Properties

of

Particles

area

distribution

Gas

of

Measurement

adsorption

Mercury p o r o s i m e t e r ,

gas

adsorption/

desorption Surface

Optical

topography

m i c r o s c o p y , SEM

Surface chemical a n a l y s i s (including surface region)

Auger s p e c t r o s c o p y , ESCA, Moessbauer s p e c t r o s c o p y , e l e c t r o n m i c r o p r o b e , EDS

Surface chemical microdistribution

scopy

(EDS)

LEED,

SEM-electron

Phase a n a l y s i s o f or s u r f a c e r e g i o n Surface Surface

functional energy

surface

groups

SEM

IR

with energy

spectroscopy,

dispersive

diffraction

chemical

D i r e c t measurement r e q u i r e d to c r e a t e

spectro-

methods

o f energy a new s u r f a c e


12.

FUERSTENAU AND CHANDER

Mineral

Processing

287

i n Table I V , and p r o p e r t i e s o f m i n e r a l particle/fluid/fluid systems are l i s t e d i n Table V . Since the u n i t o p e r a t i o n o f f l o t a t i o n has been most widely i n v e s t i g a t e d as f a r as i n t e r f a c i a l behavior i s concerned, s e v e r a l examples i n t h i s review are taken from fundamental and a p p l i e d s t u d i e s o f v a r i o u s aspects o f flotation.


The E l e c t r i c a l Double Layer a t Mineral/Water

Interfaces

Since the e l e c t r i c a l double l a y e r a t mineral/water i n t e r f a c e s i s known to c o n t r o l the a d s o r p t i o n o f i o n s , on which depend the response o f s e v e r a l u n i t o p e r a t i o n s , i t s c h a r a c t e r i z a t i o n i s o f utmost importance. The most important parameter t h a t d e s c r i b e s the e l e c t r i c a l double l a y e r i n mineral/water systems i s the p o i n t of-zero-charge (PZC). This i s the c o n d i t i o n i n the aqueous s o l u t i o n a t which the surface charge i s z e r o . For semi-soluble s a l t type minerals (such as b a r i t e , BaSO^, or i o d y r o r i t e , A g i ) i n which ions are free to pass between the two phases, the surface charge a r i s e s from the preference o f the l a t t i c e ions f o r the s o l i d r e l a t i v e to the aqueous phase. E q u i l i b r i u m i s a t t a i n e d when the e l e c t r o c h e m i c a l p o t e n t i a l o f the ions c o n s t i t u t i n g the c r y s t a l l a t t i c e i s constant throughout the system. Those p a r t i c u l a r ions which are f r e e to pass between both phases and t h e r e f o r e e s t a b l i s h the e l e c t r i c a l double l a y e r are c a l l e d p o t e n t i a l - d e t e r m i n i n g i o n s . The s u r f a c e charge, 0 , i s expressed i n terms o f the a d s o r p t i o n d e n s i t y o f p o t e n t i a l - d e t e r m i n i n g c a t i o n s , (Γ ) and anions, ( Γ ) , as Q

+

σ ο

=

zF (Γ - Γ ) + -

where F i s the Faraday constant and ζ the valence f o r a symmetri c a l ( 1 - 1 or 2 - 2 ) s a l t . For o x i d e s , i n which the metal c a t i o n s and oxygen ions are not free to move from s o l i d to s o l u t i o n and v i c e v e r s a , H and OH" have long been considered t o be p o t e n t i a l determining (1). These ions i n t e r a c t w i t h the oxides to e s t a b l i s h the v a r i o u s e q u i l i b r i a . The surface p o t e n t i a l , Ψ , o f the m i n e r a l (measured w i t h reference to the s o l u t i o n ) i s u s u a l l y considered to be zero a t the PZC. The value o f the surface p o t e n t i a l a t any a c t i v i t y o f the p o t e n t i a l - d e t e r m i n i n g c a t i o n , a , i s given by +

0

+

ψ

=

RT

I n a /a (PZC) + + + ζ F where a (PZC) i s the a c t i v i t y o f the p o t e n t i a l - d e t e r m i n i n g e l e c t r o l y t e a t the PZC, R i s the gas constant, Τ i s the absolute temperature and z i s the valence o f the p o t e n t i a l - d e t e r m i n i n g c a t i o n i n the e l e c t r o l y t e . Ions i n s o l u t i o n adsorb a t m i n e r a l / s o l u t i o n i n t e r f a c e s to balance the s u r f a c e charge and maintain e l e c t r o n e u t r a l i t y . These ions are known as the counter i o n s . In c o n t r a s t to the s i t u a t i o n ο

+

+


288

SURFACE ANALYSIS

Table IV: Techniques f o r Measurement o f P r o p e r t i e s o f M i n e r a l / P a r t i c l e Solution Interfaces


Surface

Property

Technique o f Measurement

I n t e r f a c i a l area

Adsorption or negative a d s o r p t i o n o f ions or molecules o f known s i z e

Point-of-zero-charge

Adsorption o f p o t e n t i a l - d e t e r m i n i n g i o n s , many i n d i r e c t methods based on a c h a r a c t e r i s t i c response t o m i n e r a l processing operation

Surface charge

Adsorption o f i o n s

Zeta p o t e n t i a l

E l e c t r o p h o r e t i c m o b i l i t y , streaming potential,etc.

Electrode p o t e n t i a l

D i r e c t measurement w i t h a standard electrode

Surface f u n c t i o n a l groups

S p e c i f i c a d s o r p t i o n o f ions or molecules, in-situ spectroscopic techniques

Adsorption o f s u r f a c t a n t s

"Loss from s o l u t i o n " method, d i r e c t measurement u s i n g r a d i o a c t i v e t r a c e r s , e l l i p s o m e t r y , nephelometry

Concentration o f o x i d i z a b l e / Voltammetry r e d u c i b l e species a t surface Surface energy

Table V:

S o l u b i l i t y , heat o f s o l u t i o n

Techniques f o r Measurement o f P r o p e r t i e s o f M i n e r a l P a r t i c l e / F l u i d / F l u i d Systems

I n t e r f a c i a l Properties

Technique f o r Measurement

Wettability

Contact angles, displacement o f a f l u i d by another on the s o l i d surface

Wetting

E q u i l i b r i u m t h i c k n e s s measured by o p t i c a l methods ( i n t e r f e r e n c e f r i n g e s , and r e f l e c t a n c e techniques)

f i l m thickness

Film v i s c o s i t y

Contact time, drainage and rupture of l i q u i d f i l m


12.

FUERSTENAU AND CHANDER

Mineral

Processing

289

i n w h i c h the potential-determining i o n s are s p e c i a l for each system, any ions present in the solution can function as counter i o n s . As h a s been w e l l e s t a b l i s h e d , t h e c o u n t e r ions o c c u r i n a d i f f u s e l a y e r t h a t e x t e n d s from t h e i n t e r f a c e o u t i n t o t h e s o l u t i o n . The c l o s e s t d i s t a n c e o f a p p r o a c h o f c o u n t e r i o n s t o the s u r f a c e i s one h y d r a t e d i o n i c r a d i u s away, shown a s the d i s t a n c e

Surface Characterization in Mineral Processing - ACS Publications

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