INTRODUCTION TO
EVALUATION, DESIGN AND
OPERATION OF PRECIOUS METAL
HEAP LEACHING PROJECTS
Editors
Dirk J.A. van Zyl
Welsh Engineering Inc.
Denver, Colorado
Ian P.G.Hutchison
Steffen Robertson and Kirsten (Colorado)Inc .
Denver, Colorado
Jean E. Kiel
J .E. Kiel & Associates
Denver, Colorado
Society of Mining Engineers, Inc., Littleton, Colorado
1988
Copyright O 1988 by the
Society of Mining Engineers, Inc.
Printed in the United States of America
by Cushing-Malloy, Inc., Ann Arbor, Michigan
All rlghts resewed. This book, or parts thereof, may not be
reproduced In any form wlthout permlsslon of the publisher.
Library of Congress Catalog Card Number 88-60773
ISBN 0-87335-074-X
Chapter 5
Ore Preparation: Crushing and Agglomeration
Gene E. McClelland and Dirk van Zyl
5.1
INTRODUCTION
Heap 1each c y a n i d a t i o n t e c h n i q u e s possess c o n s i d e r a b l e p o t e n t i a l f o r
e x p l o i t a t i o n o f low-grade ores, s m a l l o r e bodies, mine s t r ' i p waste and t a i l i n g
m a t e r i a l s where f i n e g r i n d i n g i s n o t necessary f o r good e x t r a c t i o n . For heap
1 eaching t o be s u c c e s s f u l , o r e must e x h i b i t c e r t a i n c h a r a c t e r i s t i c s , as
previously
discussed
i n Chapter 1. Primary among t h e necessary o r e
c h a r a c t e r i s t i c s i s t h a t t h e o r e must have good p e r n i e a b i l i t y a f t e r being
crushed o r t r e a t e d and stacked i n t o heaps i f u n i f o r m d i s t r i b u t i o n o f the
c y a n i d e l e a c h s o l u t i o n i s t o be achieved.
Gold and s i l v e r o r e s c o n t a i n i n g excessive amounts o f c l a y s o r f i n e s
generated b y c r u s h i n g a r e some o f t h e most d i f f i c u l t t o t r e a t s u c c e s s f u l l y by
heap 1eaching. The presence o f excessive amounts o f s l i m e s ( g e n e r a l l y
c l a s s i f i e d as minus 50-micron o r 270 T y l e r mesh s i e v e s i z e p a r t i c l e s ) i n t h e
heap l e a c h f e e d w i l l slow t h e p e r c o l a t i o n f l o w o f t h e l e a c h s o l u t i o n , cause
channeling, o r produce dormant o r unleached areas w i t h i n t h e heap. This may
r e s u l t i n u n r e a s o n a b l y l o n g l e a c h i r l g p e r i o d s and poor e x t r a c t i o n s . I n extreme
cases, t h e c l a y s o r s l i m e s can c o m p l e t e l y seal t h e o r e heap, c a u s i n g t h e l e a c h
s o l u t i o n t o r u n o f f t h e s i d e s o f t h e heap r a t h e r than t o p e n e t r a t e t h e o r e
heap.
The problem o f heap l e a c h i n g o r e s c o n t a i n i n g f i n e s can be aggravated
d u r i n g s t a c k i n g and p r e p a r a t i o n o f t h e o r e heaps, because n a t u r a l s o r t i n g o f
coarse and f i n e m a t e r i a l occurs d u r i n g these steps (Johanson, 1978). This
phenomenon r e s u l t s i n a c o n c e n t r a t i o n o f o r e f i n e s a t t h e c e n t e r o f i n d i v i d u a l
o r e p i l e s and a c o n c o m i t a n t c o n c e n t r a t i o n o f l a r g e r r o c k fragments on t h e
l o w e r s l o p e s and base o f t h e p i l e s . When t h e i n d i v i d u a l p i l e s w i t h i n t h e heap
a r e l e v e l e d o f f p r i o r t o i n s t a l l a t i o n o f t h e s p r i n k l i n g system which d e l i v e r s
t h e l e a c h s o l u t i o n , a d d i t i o n a l s e g r e g a t i o n occurs as t h e f i n e s s i f t through
t h e c o a r s e r o r e p a r t i c l e s . The s e g r e g a t i o n r e s u l t s i n l o c a l i z e d areas o r zones
w i t h marked d i f f e r e n c e s i n permeabil it y . As a consequence, t h e 1each s o l u t i o n s
f o l l o w t h e p a t h o f l e a s t r e s i s t a n c e , p e r c o l a t i n g downward through t h e coarse
o r e r e g i o n s and b y p a s s i n g o r b a r e l y w e t t i n g areas t h a t c o n t a i n l a r g e amounts
o f f i n e s o r s l i m e s . E f f e c t i v e u t i l i z a t i o n o f m a r g i n a l gold and s i l v e r
resources t h r o u g h heap l e a c h processing r e q u i r e s t h e development o f new
methods i n o r d e r t o a c h i e v e more uniform s i z e d i s t r i b u t i o n d u r i n g o r e heap
p r e p a r a t i o n and b e t t e r s l ime c o n t r o l d u r i n g 1eaching.
68
ORE PREPARATION: CRUSHING AND AGGLOMERATION
69
As
briefly
discussed
in
the
preceding
chapter,
agglomeration
p r e t r e a t m e n t has been shown t o be e f f e c t i v e f o r t h e e x p l o i t a t i o n o f l o w - g r a d e
m a t e r i a l s which d i s p l a y p o o r p e r c o l a t i o n c h a r a c t e r i s t i c s . I n some cases,
a g g l o m e r a t i o n - h e a p 1e a c h i n g i s t h e o n l y v i a b l e p r o c e s s i n g t e c h n i q u e f o r a
problem f e e d . However, c r u s h i n g c i r c u i t s and a g g l o m e r a t i n g systems a r e c a p i t a l
i n t e n s i v e , and s h o u l d n o t be i n c o r p o r a t e d i n t o a commercial o p e r a t i o n u n l e s s
a b s o l u t e l y necessary. C a r e f u l a t t e n t i o n s h o u l d be g i v e n t o d e t e r m i n i n g t h e
requirement f o r agglomeration p r e t r e a t m e n t f o r a p a r t i c u l a r feed before
c o m m i t t i n g i t t o t h e c o m m e r c i a l o p e r a t i o n . T h i s c h a p t e r p r o v i d e s some
g u i d e l i n e s f o r t h e e f f e c t i v e and economic use o f a g g l o m e r a t i o n systems, as
w e l l as a b r i e f h i s t o r y o f t h e d e v e l o p m e n t o f a g g l o m e r a t i o n t e c h n o l o g y .
A g g l o m e r a t i o n and b a l l i n g o f c r u s h e d o r e t o produce a porous and more
u n i f o r m f e e d m a t e r i a l f o r heap l e a c h i n g i s a v i a b l e method f o r t r e a t i n g c l a y e y
o r e s . I n t h e m i d - 1 9 7 0 ' s t h e U.S. Bureau o f Mines (USBM) i n Reno, Nevada, began
an a g g l o m e r a t i o n p r e t r e a t m e n t r e s e a r c h program t o a1 l o w t h e commercial
e x p l o i t a t i o n o f these p o o r l y p e r c o l a t i n g feed m a t e r i a l s . P r i o r t o t h i s , l i t t l e
a t t e n t i o n had been g i v e n t o such m e t h o d s f o r i m p r o v i n g u n i f o r m p e r c o l a t i o n
f l o w o f l e a c h s o l u t i o n s t h r o u g h s i l v e r a n d g o l d o r e s . However, t h e g e n e r a l
i d e a o f o r e a g g l o m e r a t i o n was advanced and, t o some e x t e n t , r e s e a r c h e d w e l l
b e f o r e t h e USBM r e s e a r c h program.
I n 1905, T.C. S c r u t t o n d e v e l o p e d a u n i q u e t e c h n i q u e f o r o b t a i n i n g r a p i d
v a t l e a c h i n g o f a c l a y e y o r e i n w h i c h t h e g o l d was f i n e l y d i s s e m i n a t e d ( D o r r
and Bosqui, 1950). S c r u t t o n ' s t e c h n i q u e c o n s i s t e d o f r o l l i n g t h e o r e down a
c h u t e i n c l i n e d a t 60 d e g r e e s t o f o r m a g g l o m e r a t e s o r b a l l s r e a d i l y permeable
t o t h e cyanide s o l u t i o n .
However, t h e s e a g g l o m e r a t e s l a c k e d r i g i d i t y and, t o
e n s u r e good p e r c o l a t i o n l e a c h i n g and washing, t h e y c o u l d n o t be bedded i n
l a y e r s more than t h r e e f t deep. I f t h i s d e p t h was exceeded, d i f f i c u l t i e s would
be e x p e r i e n c e d i n o b t a i n i n g u n i f o r m l e a c h i n g and washing, r e s u l t i n g i n r e d u c e d
g o l d r e c o v e r y . Shepard and S k i n n e r ( 1 9 3 7 ) , s t u d i e d t h e a d d i t i o n o f l i m e and
carbon d i o x i d e o r c a l c i u m c a r b o n a t e t o gold-bearing t a i l i n g s t o form
a g g l o m e r a t e s s u i t a b 1 e f o r v a t 1 e a c h i ng. S a t i s f a c t o r y p e r c o l a t i o n fl ow r a t e s
were a c h i e v e d i n 90-gram s c a l e e x p e r i m e n t s , b u t t h e r e a g e n t r e q u i r e m e n t s were
prohibitive.
A g g l o m e r a t i o n and p e l l e t i z i n g have been used i n o t h e r segments o f t h e
m i n e r a l i n d u s t r y . T h i s t e c h n i q u e was f i r s t used s u c c e s s f u l l y around 1 9 1 1 f o r
p e l l e t i z i n g i r o n o r e s ( D a v i s and Wade, 1951; E n g l i s h and Frans, 1977). S i n c e
t h a t t i m e , a g g l o m e r a t i o n and p e l 1 e t i z i n g ( o r b r i q u e t t i n g ) have been w i d e l y
adopted f o r c o n s o l i d a t i o n o f f i n e s f o r many o t h e r m a t e r i a l s such as manganese,
f l u o r s p a r , and p h o s p h a t e s . These m a t e r i a l s have been p e l 1 e t i z e d t o c o n s o l i d a t e
f i n e p a r t i c l e s i n t o 1 a r g e , dense, and m e c h a n i c a l l y s t r o n g masses, p r i n c i p a l l y
t o prevent dusting problems d u r i n g f u r n a c i ng operations.
The o b j e c t i v e o f t h e USBM's i n v e s t i g a t i o n s i n t h e 1 9 7 0 ' s was t o e v a l u a t e
p a r t i c l e a g g l o m e r a t i o n p r o c e d u r e s f o r i m p r o v i n g f l o w r a t e s i n t h e heap l e a c h
c y a n i d a t i o n process. The c o n c e p t was t o i n v e s t i g a t e p r o c e d u r e s whereby 1 oosek n i t , polymer1 ike a g g l o m e r a t e s c o u l d be formed t h a t were h i g h l y permeable t o
s o l u t i o n f l o w and y e t m e c h a n i c a l 1 y s t a b 1 e d u r i n g t h e 1 e a c h i n g sequence.
T h r e e i m p o r t a n t a g g l o m e r a t i o n p a r a m e t e r s were d e t e r m i n e d from t h e USBM's
work f o r s u c c e s s f u l a g g l o m e r a t i o n p r e t r e a t m e n t o f crushed o r e s w i t h poor
percolation characteristics.
The p a r a m e t e r s were: 1 ) q u a n t i t y o f b i n d e r
70
PRECIOUS METAL HEAP LEACHING PROJECTS
( p o r t l a n d cement) added t o t h e d r y feed; 2 ) amount o f m o i s t u r e added t o t h e
b i n d e r / o r e m i x t u r e ; and 3 ) t h e c u r i n g p e r i o d r e q u i r e d t o form c a l c i u m s i l i c a t e
b r i d g i n g . The i n v e s t i g a t i o n r e v e a l e d t h a t most crushed ores ( f i n e r t h a n 1
i n c h ) c o u l d be e f f e c t i v e l y agglomerated by m i x i n g 10 'Ibs o f p o r t l a n d cement
per t o n ( 5 kg/mt) o f d r y feed, w e t t i n g w i t h water o r cyanide s o l u t i o n t o a
f i n a l m o i s t u r e c o n t e n t o f about 12 w e i g h t percent, m e c h a n i c a l l y t u m b l i n g t h e
w e t t e d m i x t u r e t o e f f e c t agglomeration, and c u r i n g f o r more than e i g h t h o u r s
before a p p l y i n g c o n v e n t i o n a l heap l e a c h i n g techniques (Heinen, e t a1 , 1979;
M c C l e l l and and E i s e l e , 1982).
.
The a g g l o m e r a t i o n technol ogy developed by t h e USBM was r a p i d l y adopted
by t h e p r e c i o u s metal p r o d u c t i o n i n d u s t r y . The f i r s t p i l o t - s c a l e c o l urnn
p e r c o l a t i o n l e a c h i n g t e s t s on agglomerated feeds were performed i n 1978. The
i n i t i a l p i l o t - s c a l e heap l e a c h t e s t on crushed and agglomerated o r e was
conducted i n e a s t e r n Nevada i n 1979.
The f i r s t commercial agglomeration-heap
l e a c h o p e r a t i o n poured b u l l i o n i n l a t e 1980. Many o p e r a t o r s have s i n c e adopted
t h e t e c h n o l ogy f o r p r o d u c i n g p r e c i o u s metal s from t h e i r low-grade o r e
d e p o s i t s , mine wastes, and m i l l e d t a i l i n g s m a t e r i a l s .
5.2
AGGLOMERATION PRINCIPLES
The b a s i c o b j e c t i v e o f a g g l o m e r a t i q g p r e c i o u s metal o r e s f o r heap
l e a c h i n g i s t o produce porous o r e which w i l l be s t a b l e when handled, s t a c k e d ,
and p e r c o l a t e d w i t h l i x i v i a n t . As p r e v i o u s l y discussed, excessive f i n e s i n an
o r e can l e a d t o r e d u c t i o n i n heap p e r m e a b i l i t y , as w e l l as i n c r e a s e d
channeling, and b l i n d i n g .
A somewhat s i m i l a r a p p l i c a t i o n o f t h e general p r i n c i p l e o f a g g l o m e r a t i o n
i s i n t h e proven procedure o f s o i l s t a b i l i z a t i o n f o r r o a d c o n s t r u c t i o n [i.e.,
t h e m i x i n g o f road pavement m a t e r i a l s w i t h s t a b i l i z i n g agents t o improve t h e i r
s t r e n g t h and c o n s t r u c t i o n b e h a v i o r ) . Cement, l i m e , f l y - a s h , a s p h a l t , and o t h e r
c a l c i u m s i l i c a t e ) have been used f o r s t a b i l i z a t i o n .
Amounts
chemicals (e.g.,
used i n r o a d c o n s t r u c t i o n u s u a l l y range from 5 t o 10 p e r c e n t b y weight. I t i s
c l e a r t h a t such h i g h percentages o f a d d i t i v e s a r e n o t economical f o r heap
l e a c h purposes. O p t i m i z a t i o n i s t h e r e f o r e i m p o r t a n t .
W i t h r e g a r d t o o r e agglomeration, s i m i l a r m a t e r i a l s t o t h o s e used i n
cement and f l y - a s h ) have been used w i t h some
road c o n s t r u c t i o n work (i.e.,
success. A l l o f t h e s e b i n d e r s have i n common two i m p o r t a n t p h y s i c a l / c h e m i c a l
e f f e c t s which account i n l a r g e p a r t f o r t h e success w i t h which t h e y have been
used :
The exchange o f sodium c a t i o n s i n t h e o r e c l a y particles w I t h
c a l c i u m c a t i o n s from t h e b i n d e r , t h e r e b y i m p r o v i n g t h e w o r k a b i 1 it y
and permeabil it y o f c l a y m a t e r i a l s ; and
The cementing ( o r p o z z o l a n i c ) a c t i o n o f b i n d e r , t h e r e b y a d d i n g
s t r e n g t h t o t h e agglomerated m a t e r i a l s .
I t s h o u l d be noted here t h a t t h e d i f f e r e n t b i n d e r s may v a r y i n
e f f e c t i v e n e s s , depending on t h e o r e i n q u e s t i o n and t h e p a r t i c u l a r needs o f
each a g g l o m e r a t i n g system. For example, l i m e i s v e r y e f f e c t i v e i n o b t a i n i n g
t h e sodium c a t i o n exchange r e a c t i o n ; however, t h e s t r e n g t h o b t a i n e d from l i m e
and t h e r a t e a t which s t r e n g t h i n c r e a s e s are b o t h l o w e r f o r l i m e t h a n f o r
cement and some t y p e s o f f l y - a s h .
Depending on t h e o r e type, t h e c a t i o n
ORE PREPARATION: CRUSHING AND AGGLOMERATION
71
exchange r e a c t i o n may o r may n o t be more i m p o r t a n t t h a n s t r e n g t h and r a t e o f
It i s , therefore, important t o c a r e f u l l y investigate t h e behavior
reaction.
o f ore w i t h respect t o binder.
Two b a s i c pathways f o r successful a g g l o m e r a t i o n can be i d e n t i f i e d f o r
p r e c i o u s metal ores. These a r e :
Agglomeration o f f i n e s onto coarse crushed o r e and waste; and
Agglomeration o f f i n e s i n t o s t a b l e b a l l s .
F i g u r e 5.1 shows schematics o f t h e a g g l o m e r a t i o n e f f e c t s . I n t h e l a t t e r
i n s t a n c e ( f i n e s agglomerating i n t o s t a b l e b a l l s ) t h e f i n e s can be s i l t and
sand p a r t i c l e s o r t h e y can c o n t a i n h i g h percentages o f c l a y (examples b and c
on F i g u r e 5.1). Moderate t o moderately h i g h q u a n t i t i e s o f b i n d e r a r e r e q u i r e d
f o r t h e agglomeration o f f i n e s and t a i l i n g s i n t o s t a b l e b a l l s. S p e c i f i c
p r o j e c t s exempl i f y i n g t h i s t y p e o f aggl omeration a r e discussed i n Subchapters
5.6 t h r o u g h 5.8.
When f i n e m a t e r i a l such as t a i l i n g s w i t h no o r l i t t l e c l a y i s
agglomerated, t h e f i n e s a r e bound t o g e t h e r w i t h a b i n d e r such as p o r t l a n d
cement. Clayey o r e i s p a r t i c u l a r l y d i f f i c u l t t o l e a c h e f f e c t i v e l y because o f
t h e l o w p e r m e a b i l i t y o f c l a y . B e n t o n i t i c c l a y s (sodium m o n t m o r i l l o n i t e ) a r e
e s p e c i a l l y a problem. I n t h e general C i v i l E n g i n e e r i n g i n d u s t r y , s u c h c l a y s
have been " s t a b i l i z e d "
( i e.,
w o r k a b i l it y and permeabi 1 it y have been
i n c r e a s e d ) , successful 1 y u s i n g 1 ime. C a t i o n exchange t a k e s p l a c e and t h e
sodium r i c h c l a y changes t o a c a l c i u m r i c h c l a y . I t i s , t h e r e f o r e , suggested
t h a t c l a y e y ores can be agglomerated b y u s i n g l i m e and p o r t l a n d cement as
b i n d e r s . 'The l i m e w i l l " s t a b i l i z e " t h e c l a y m i n e r a l s , w h i l e t h e p o r t l a n d
cement w i l l form a s t r o n g b i n d e r .
I n order t o
the clay
e f f e c t i v e l y , t h e l i m e must come i n c l o s e c o n t a c t w i t h t h e c l a y m i n e r a l s .
I n t h e case o f agglomeration o f crushed o r e and wastes c o n t a i n i n g h i g h
percentages o f f i n e s , t h e f i n e s a r e bound t o t h e c o a r s e p a r t i c l e s ( s e e example
a on F i g u r e 5.1). T y p i c a l l y , p o r t l a n d cement can be used as a b i n d e r and low
t o m o d e r a t e l y l o w q u a n t i t i e s a r e s u f f i c i e n t . Subchapter 5.6 d i s c u s s e s s p e c i f i c
p r o j e c t s which a r e examples o f t h i s t y p e o f a g g l o m e r a t i o n .
5.3
AGGLOMERATORS
T h i s subchapter discusses agglomeration d e s i g n f a c t o r s and t h e n goes on
t o d e s c r i b e t h e v a r i o u s k i n d s o f agglomerators w h i c h have been s u c c e s s f u l l y
used on p r e c i o u s metal o r e s . Agglomeration d e s i g n f a c t o r s i n c l u d e ( M i l 1 igan,
1983) :
Feed c h a r a c t e r i s t i c s , e.g. p a r t i c l e s i z e and presence o f f r a c t u r e s
i n p a r t i c l e s . The r e l a t i o n s h i p between t, t h e t i m e t o a g i v e n
e x t r a c t i o n and D, t h e p a r t i c l e d i a m e t e r , c a n be s t a t e d a s :
where C 1 i s a constant. For r a p i d u n i f o r m h i g h p r e c i o u s metal
recovery, the f i n e s t crush i s t h e r e f o r e desirable. Fine g r i n d i n g i s
i n c o n f l i c t w i t h t h e u n i f o r m l y h i g h permeabil it y r e q u i r e m e n t s o f
heap l e a c h i n g . Agglomeration makes t h e s e t w o more c o m p a t i b l e ;
72
PRECIOUS METAL HEAP LEACHING PROJECTS
AFTER AGOLOMERATION
BEFORE AOQLOMERATION
r) COARSE M A T E R I A L WlTH LARQE
P E R C E N T A Q E FINES.
b) A Q Q L O M E R A T E S A R E F O R M E D B Y
B l N D l N Q F I N E S TOQETHER W l T H
BINDER B U C H AS P O R T L A N D CEMENT.
b ) F I N E M A T E R I A L . e.g. T A l L l N Q 8
WITH N O OR L I T T L E C L A Y
C)
a) F I N E S A R E A Q Q L O M E R A T E D ONTO
C O A R S E PARTIC.LES, BINDER SUCH
A S P O R T L A N D C E M E N T I S USED.
CLAY MATERIAL WITH M E T A L
'LOCKED' I N L O W P E R M E A B I L I T Y
MEDIUM.
C)
A Q Q L O M E R A T E S ARE FORMED BY
BINDINQ F I N E S TOQETHER AFTER
MODlFYlNQ CLAY PROPERTIES,
L I M E A N D P O R T L A N D CEMENT CAN
B E USED.
FIGURE 5.1
AGGLOMERATION EFFECTS
ORE PREPARATION: CRUSHING AND AGGLOMERATION
73
Process c o n t r o l , e.g. f e e d r a t e and t y p e o f o r e and b i n d e r a d d i t i o n
rates.
Optimum c o n d i t i o n s
are obtained during steady-state
c o n d i t i o n s o f optimum feed s i z e and r a t e and a t optimum m o i s t u r e
and b i n d e r a d d i t i o n r a t e s (as d i s c u s s e d below). O p e r a t o r c o n t r o l i s
v e r y i m p o r t a n t and c o n t i n u o u s m o n i t o r i n g may be r e q u i r e d i f u p s e t s
a r e f r e q u e n t ; and
Equipment s e l e c t i o n . The a g g l o m e r a t o r must p r o v i d e t h e a g i t a t i o n
and m o t i o n needed t o make t h e necessary green s t r e n g t h f o r
c o n s t r u c t i o n o f t h e heap. The t h r e e m a j o r t y p e s o f a g g l o m e r a t o r s
f o r g o l d and s i l v e r o r e s a r e b e l t , drum and pan. These t h r e e a r e
d i s c u s s e d below. The d i s c u s s i o n i s t a k e n from M i l 1 i g a n ( 1 9 8 3 ) .
O t h e r methods o f a g g l o m e r a t i o n i n c l ude s t o c k p i l e a g g l o m e r a t i o n ,
v i b r a t i n g deck, and s t e e p l y i n c l i n e d conveyor be1 t s (Chamberlain,
1986).
5.3.1
Be1 t Aggl o m e r a t o r s
Be1 t - t y p e a g g l o m e r a t o r s use conveyors t o o b t a i n t h e necessary tumbl i n g
and compaction. T h i s a g g l o m e r a t o r t y p e produces t h e l e a s t degree o f c o m p a c t i o n
and a g g l o m e r a t i o n o f any a g g l o m e r a t o r c u r r e n t l y i n use. The b e l t may be
s t e e p l y i n c l i n e d t o i n d u c e r o l l i n g on t h e be1 t o r may have mu1 t i p l e t r a n s f e r
p o i n t s a t w h i c h a g i t a t i o n and compaction o c c u r .
'The h i g h e r t h e p e r c e n t a g e o f
f i n e s , t h e more t r a n s f e r p o i n t s a r e needed ( h a r d s i l i c e o u s o r e w i t h f i v e
100 mesh, two t o t h r e e t r a n s f e r s , and w i t h 10 p e r c e n t o r 15 p e r c e n t
percent
- 100 mesh, f o u r t o f i v e t r a n s f e r s ) (Chamberlain, 1986). A g g l o m e r a t i o n may
a l s o be i n d u c e d dur-ing d i s c h a r g e where t h e o r e r o l l s down a s t e e p i n c l i n e .
F i g u r e 5.2, B e l t A g g l o m e r a t o r S i z i n g , i l l u s t r a t e s t h e c a p a c i t y o f a t y p i c a l
t h r e e - d r o p conveyor a g g l o m e r a t i o n . S i z i n g i s based on t h e c a r r y i n g c a p a c i t y o f
Be1 t Agglomerator Cost,
t h e be1 t n e a r t h e maximum speed. F i g u r e 5.3,
i l l u s t r a t e s an e s t i m a t e d budget comparison c o s t o f such an agglomerator.
-
T h i s t y p e o f a g g l o m e r a t o r i s s u i t a b l e where t h e o r e c o n t a i n s v e r y 1 i t t l e
f i n e m a t e r i a l and a c o a r s e c r u s h y i e l d s good p r e c i o u s metal e x t r a c t i o n .
5.3.2
Drum A g g l o m e r a t o r s
The drum-type a g g l o m e r a t o r uses a drum t o o b t a i n t h e r e q u i r e d t u m b l i n g
and c o m p a c t i o n f o r a g g l o m e r a t i o n . T h i s t y p e o f equipment produces a f a i r l y
wide s i z e d i s t r i b u t i o n o f agglomerates. The green s t r e n g t h can be e x c e l l e n t
depending on equipment s i z e and p r o d u c t i o n r a t e .
F i g u r e 5.4,
Drum
Agglomeration S i z i n g ,
i l l u s t r a t e s a t y p i c a l r a t i n g capacity for
drum
a g g l o m e r a t o r s . The f o l l o w i n g e q u a t i o n can a1 so be used t o d e t e r m i n e r e s i d e n c e
t i m e i n an a g g l o m e r a t o r . Rate o f g r o w t h o f agglomerate reaches a maximum a t
t h e optimum m o i s t u r e c o n t e n t :
where T
material
(rpm), L
s i z e and
i s r e q u i r e d residence time (minutes), A i s t h e angle of repose o f
( d e g r e e s ) , S i s t h e s l o p e o f s h e l l (degrees), N i s speed o f drum
Agglomerate
i s l e n g t h o f drum ( f t ) , and D i s d i a m e t e r o f drum ( f t ) .
s t r e n g t h increases w i t h longer residence time ( r e v o l u t i o n s ) .
PRECIOUS METAL HEAP LEACHING PROJECTS
BELT WIDTH-IN.
FIGURE 5.2
BELT AGGLOMERATOR SIZING
( from Mllllgan, 1983 1
ORE PREPARATION: CRUSHING AND AGGLOMERATION
BELT W IDTH-IN.
FIGURE 5.3
BELT AQGLOMERATOR COST
( from MHIIgan, 1983 )
PRECIOUS METAL HEAP LEACHING PROJECTS
DRUM DIAMETER-FT.
FIGURE 5.4
DRUM AGGLOMERATOR SIZING
( from Mllllgan, 1 9 8 3 )
ORE PREPARATION: CRUSHING AND AGGLOMERATION
T y p i c a l l y f o r scale-up,
77
t h e L t o D r a t i o i s kept constant (between 2 and
4 ) , t h e slope o f t h e s h e l l i s c o n s t a n t near 7 degrees, and t h e residence t i m e
remains c o n s t a n t as does t h e degree o f l o a d i n g . F i g u r e 5.5, Drum Agglomerator
Cost, illu s t r a t e s t h e t y p i c a l budget c o s t o f drum agglomerators.
Recent improvement i n t h e drum a g g l o m e r a t o r i n c l u d e s the use o f f l e x i b l e
r u b b e r l i n e r s t o r e d u c e c a k i n g w i t h i n t h e drum and t o e l i m i n a t e l a r g e o v e r s i z e
material.
5.3.3
Pan Agglomerators
The pan-type ( d i s k ) agglomerator uses a f l a t i n c l i n e d pan t o o b t a i n t h e
r e q u i r e d tumbl i n g and compaction.
T h i s equipment
produces a u n i f o r m
F i g u r e 5.6,
Pan Agglomerator
agglomerate w i t h e x c e l 1 e n t green s t r e n g t h .
S i z i n g , i l l u s t r a t e s a t y p i c a l r a t i n g f o r pan agglomeration. The f o l l o w i n g
e q u a t i o n can a l s o be used t o s c a l e up d e s i g n :
C
=
,,Em
Constant;
where C i s Y c a p a c i t y and D i s d i a m e t e r o f a s h a l l o w d i s k o r pan.
The m a j o r o p e r a t i n g parameters i n t h e pan agglomerator a r e : 1) angle o f
d i s k ; 2 ) speed o f d i s k ; 3) p o i n t o f m a t e r i a l feed; and 4 ) l o c a t i o n and amount
o f water spray. I f t h e w a t e r i s sprayed upon t h e l a r g e r agglomerate, t h e
agglomerate w i l l t e n d t o i n c r e a s e i n s i z e . I f t h e feed m a t e r i a l and the water
s p r a y l o c a t i o n a r e on o r n e a r f i n e m a t e r i a l , t h e agglomerate w i l l tend t o be
small
Larger p e l 1 e t s a r e formed i f t h e f e e d and s p r a y are brought c l o s e r
t o g e t h e r . Budget c o s t s o f t h i s t y p e o f equipment are i l l u s t r a t e d i n F i g u r e
5.7, Pan Aggl omera t o r Cost.
.
Table 5.1 i s a p a r t i a l l i s t o f s u p p l i e r s f o r both drum and pan
agglomerators. Most have t h e i r own t e s t i n g f a c i l i t i e s . Be1 t agglomerators a r e
i n n o v a t i o n s by o p e r a t o r s and as such a r e n o t a v a i l a b l e as predesigned u n i t s .
5.4
OPTIMUM WATER CONTENT FOR AGGLOMERATION
The p r o p e r m o i s t u r e c o n t e n t f o r agglomeration i s determined by t h e
p a r t i c l e s i z e , c l a y c o n t e n t , w e t t i n g c h a r a c t e r i s t i c s , and d e s i r e d degree o f
compaction d u r i n g a g g l o m e r a t i o n . The mass must n o t be saturated, however, f o r
t h i s destroys t h e green s t r e n g t h . Green s t r e n g t h i s a term a p p l i e d t o t h e
s t r e n g t h o f agglomerates t o s t a n d up t o p r o c e s s i n g and handling. A s u f f i c i e n t
green s t r e n g t h i s r e q u i r e d t o r e s i s t c r u s h i n g d u r i n g Reap b u i l d i n g . Green
s t r e n g t h i s made u p o f s t r e n g t h i m p a r t e d by a b i n d e r , b u t m o s t l y due t o t h e
tension o f the water menisci.
A simple method o f e s t i m a t i n g t h e proper m o i s t u r e c o n t e n t f o r
agglomeration i s t o n i i x a s l u r r y o f t h e o r e i n water and f i l t e r on a vacuum
f i l t e r . The dewatered f i 1 t e r cake w i l l c o n t a i n s l i g h t l y more water than
necessary f o r p r o p e r agglomeration.
Another t e c h n i q u e f o r e s t i m a t i n g t h e proper m o i s t u r e c o n t e n t i s t o
o b t a i n t h e w a t e r c o n t e n t vs. d r y d e n s i t y r e l a t i o n s h i p f o r t h e s o i l by u s i n g a
Standard P r o c t o r Compaction t e s t . The optimum m o i s t u r e content i s a good
estimate f o r t h e proper agglomeration moisture content.
PRECIOUS METAL HEAP LEACHING PROJECTS
04
1
4
6
I
8
DRUM DIAMETER-FT.
FIGURE 5.5
DRUM AGGLOMERATOR COST
( from M l l l l ~ a n ,1983 )
I
1
10
12
ORE PREPARATION: CRUSHING AND AGGLOMERATION
400
-
300 -
200 -
100-
0
5
I
I
I
1
10
16
20
25
DIAMETER-FT.
FIGURE 5.6
PAN AGGLOMERATOR COST
( from Mllllgan, 1883 )
PRECIOUS METAL HEAP LEACHING PROJECTS
0
5
10
15
P A N DIAMETER-FT.
FIGURE 5.7
PAN AGGLOMERATOR SIZING
( from Mllllgan, 1 9 8 3 )
20
25
ORE PREPARATION: CRUSHING AND AGGLOMERATlON
TABLE 5.1
AGGLOMERATOR SUPPLIERS
Acme I n t e r n a t i o n a l L t d .
A t t n . Richard Stollman
100 T.S. West Ave.
J e n k i n t o w n , PA
215-885-7750
G l a t t A i r Technique
A t t n . Steve Abbely
20-T Spear R d t .
Ramsey, NJ
201-825-8700
A l l i s - C h a l m e r s M i n i n g Systems D i v .
A t t n . George S k o r o n s k i
Box 512
Milwaukee, W I 53201
414-475-2437
Legend M e t a l 1 u r g i c a l L a b o r a t o r y
Attn. Larry
125-T Manuel S t .
Reno, NV 89502
702-786- 3003
Bepex Corp.
A t t n . Dave P h i l l i p s
Minneapol i s , MN
612-331-4370
MMC/MARS M i n e r a l s
A t t n . Mark C a t h e r / B o b H i n k l e
Box 128
V a l e n c i a , PA 16059
412-898-1551
C a l i f o r n i a P e l l e t M i l l Co.
1800 Fol som S t .
San F r a n c i s c o , CA 94103
415-431-3800
O ' B r i e n I n d u s t r i a l E q u i p m e n t Co.
A t t n . Andy W h i t a k e r
1596 Hudson Avenue a t N e w h a l l
San F r a n c i s c o , CA
415-826-3033
Dravo Company
A t t n . D.W. K e s t n e r
One 01 iv e r P l aza
P i t t s b u r g , PA 15222
412-566-3000
Renneburg & Sons Co.
A t t n . John R e n n e b u r g / G a r r y
2635 B o s t o n S t .
B a l t i m o r e , MD 21224
302-732- 1666
D u s t Suppression System I n c .
A t t n . J a y Reading
2459-A C h a r l o t t e
Kansas City, MO
816-421-4739
Feeco I n t e r n a t i o n a l I n c .
Attn. Larry C l i v e r
3913 Algoma Rd. D e p t . A
Green Bay, W I 54301
414-468- 1000
Ferro-Tec h
Attn. Carl H o l l y
Eureka and 5 Ave.
(467 Eureka Rd.)
Wyandotte, M I 48192
313-282-7300
82
PRECIOUS METAL HEAP LEACHING PROJECTS
5.5
AGGLOMERATION OF CRUSHED ORES AND WASTES
Most p r e c i o u s m e t a l o r e s and wastes r e q u i r e c r u s h i n g t o m i n u s one i n c h
( 2 5 mm) o r f i n e r b e f o r e a g g l o m e r a t i o n . C r u s h i n g t o t h e s e s i z e s l i b e r a t e s
p r e c i o u s m e t a l v a l u e s and i m p r o v e s o v e r a l l r e c o v e r y . C r u s h i n g t o a b o u t 3 / 4
i n c h ( 1 9 mm) u s u a l l y r e q u i r e s two s t a g e s o f c r u s h i n g . A f e e d s i z e o f about 3 / 8
i n c h ( 9 mm) i s c o n s i d e r e d t h e economic minimum, and r e q u i r e s t h r e e c r u s h i n g
s t a g e s . P r i m a r y c r u s h i n g i s u s u a l l y a c c o m p l i s h e d b y a jaw o r a s t a n d a r d cone
c r u s h e r . S e c o n d a r y and t e r t i a r y c r u s h i n g i s u s u a l l y done u s i n g s h o r t - h e a d cone
crushers.
Crushed o r e s can, i n g e n e r a l , be a g g l o m e r a t e d b y m i x i n g 5 t o 10 I b s o f
p o r t l a n d cement p e r t o n (2.5 t o 5 k g / p e r t o n ) o f d r y feed, w e t t i n g w i t h 8 t o
1 6 p e r c e n t m o i s t u r e as e i t h e r w a t e r o r s t r o n g c y a n i d e s o l u t i o n , m e c h a n i c a l l y
t u m b l i n g t h e w e t t e d m i x t u r e , and c u r i n g t h e a g g l o m e r a t e d feed d u r i n g heap
b u i l d i n g p r o c e d u r e s . The q u a n t i t y o f cement added d u r i n g a g g l o m e r a t i o n usual 1 y
p r o v i d e s t h e p r o t e c t i v e a1 k a l i n i t y r e q u i r e d f o r c y a n i d e l e a c h i n g . A f t e r
a g g l o m e r a t i o n and heap b u i l d i n g , l e a c h i n g i s c o n d u c t e d w i t h c o n v e n t i o n a l heap
1e a c h i u g t e c h n i q u e s .
D u r i n g a g g l o m e r a t i o n , t h e c l a y and f i n e p a r t i c l e s c o n t a i n e d i n t h e o r e
a d h e r e t o t h e c o a r s e r p a r t i c l e s and c r e a t e a c o a t i n g o f f i n e s around t h e
c o a r s e p a r t i c l e s The a g g l o m e r a t e s produced a r e o f s u f f i c i e n t green s t r e n g t h
a f t e r c u r i n g t o w i t h s t a n d h a n d l i n g and w e t t i n g w i t h m i n i m a l d e g r a d a t i o n .
Agglomeration
overcomes
the
major
problems
associated
with
particle
s e g r e g a t i o n d u r i n g heap b u i l d i n g , f i n e s m i g r a t i o n , and s o l u t i o n c h a n n e l i n g
d u r i n g l e a c h i n g b y p r o d u c i n g a p o r o u s , permeable f e e d .
.
A p e r m e a b l e f e e d m a t e r i a l s t a c k e d i n a heap p e r m i t s t h e u n i f o r m f l o w o f
1 e a c h i n g s o l u t i o n and c o n t a c t o f t h e c y a n i d e 1 e a c h i n g s o l u t i o n w i t h t h e
exposed p r e c i o u s m e t a l p a r t i c l e s , and d e c r e a s e s t h e l e a c h i n g t i m e r e q u i r e d t o
o b t a i n t a r g e t e d precious metal recovery.
'The f o l l o w i n g i s a b r i e f d e s c r i p t i o n o f o n e c o n v e n t i o n a l heap l e a c h
o p e r a t i o n ( n o a g g l o m e r a t i o n ) and t h r e e commercial o p e r a t i o n s where c r u s h i n g
and a g g l o m e r a t i o n p r e t r e a t m e n t was n e c e s s a r y f o r t h e s u c c e s s f u l heap l e a c h i n g
t r e a t m e n t o f t h e o r e . The t h r e e were s e l e c t e d f o r d i s c u s s i o n because o f t h e
d i v e r s i t y o f t h e a g g l o m e r a t i n g equipment used f o r p r e t r e a t i n g t h e o r e . Two o f
t h e o p e r a t i o n s have been p r o d u c i n g b u l l i o n s i n c e t h e e a r l y 1 9 8 0 ' s and t h e
i n f o r m a t i o n g i v e n h e r e i n d e s c r i b e s t h e i r o p e r a t i o n s h o r t l y a f t e r s t a r t - u p . The,
t h i r d o p e r a t i o n began p r o d u c t i o n i n e a r l y 1985. A l l t h r e e a r e c o n s i d e r e d
m o d e r a t e s i z e d heap 1 e a c h i n g o p e r a t i o n s and t h e i r p r o d u c t i o n r a t e s range f r o m
2,000 t o 3,500 t o n s o f o r e p e r day.
5.6
EXAMPLES OF CRUSHED ORE AGGLOMERATION
5.6.1
C e n t r a l Nevada G o l d C o n v e n t i o n a l Heap Leach: 20,000 Tons P e r D a l
A c o n v e n t i o n a l g o l d heap l e a c h i n g o p e r a t i o n i s l o c a t e d near t h e
g e o g r a p h i c a l c e n t e r o f t h e s t a t e o f Nevada. The s i l i c e o u s o r e i s mined a t a
r a t e o f a b o u t 20,000 t o n s p e r d a y (TPD) u s i n g o p e n - p i t m i n i n g methods. M i n e d
o r e i s t r u c k e d t o an i n - p i t u n d e r g r o u n d p r i m a r y s t a n d a r d cone c r u s h e r .
D i s c h a r g e f r o m t h e p r i m a r y c r u s h e r ( a p p r o x i m a t e l y f o u r i n c h e s , 100 mrn) i s
conveyed t o a s e c o n d a r y s h o r t head cone c r u s h e r where i t i s reduced i n s i z e t o
n o m i n a l m i n u s o n e i n c h . The s e c o n d a r y c r u s h e d f e e d i s conveyed a s h o r t
ORE PREPARATION: CRUSHING AND AGGLOMERATION
89
d i s t a n c e t o two s h o r t head cone crushers and i s reduced t o an 80 p e r c e n t
passing 3/8 i n c h ( 9 mm) feed s i z e .
The o r e is v e r y s i 1 ~ C ~ O Uand
S
few f i n e s a r e generated by c r u s h i ng t o 318
i n c h ( 9 ~nm) Consequently, agg1omeration p r e t r e a t m e n t i s n o t necessary f o r
c o n t r o l l i n g f i n e s m i g r a t i o n d u r i n g l e a c h i n g . Gold grade averages 0.045 ounces
per t o n (1.5 g l i n t ) , and some v i s i b l e g o l d i s p r e s e n t i n t h e feed.
Crushed o r e i s conveyed t o an underground o r e s t o c k p i l e . Lime i s added
t o t h e o r e a t t h e conveyor before i t reaches t h e o r e s t o c k p i l e . The ore-1 irne
m i x t u r e i s conveyed from t h e s t o c k p i l e t o a t r u c k l o a d o u t b i n and i s
t r a n s p o r t e d t o t h e a s p h a l t pad by t r u c k . Heaps a r e b u i l t t o a h e i g h t o f 40 f t
(12 m) by pushing up t h e a n g l e of repose w i t h a t r a c k e d dozer. O v e r a l l
dimensions o f t h e heap a r e 4,800 f t x 300 f t x 40 f t (1,460 m x 91 m x 12 m ) .
his s i z e heap w i l l a l l o w f o r t h e continuous l e a c h i n g o f 1.2 m i l l i o n tons o f
ore.
.
i s conducted by a p p l y l n g cyanlde s o l u t i o n c o n t a i n i n g 1.0 l b
0.5 k g l m t ) o f s o l u t i o n o v e r t h e heaps a t a r a t e o f 0.005 gprn/ft
(0.003 l / s / m 2 ) o f heap surface area. Wobbler s p r i n k l e r s a r e used f o r a p p l y i n g
s o l u t i o n i n a p a t t e r n t o e n s u r e maxlmum coverage and minimum evaporation. A 52
t o 55 day l e a c h c y c l e i s r e q u i r e d t o o b t a i n t a r g e t e d g o l d recovery, Leached
o r e i s removed from t h e l e a c h pad and i s t r u c k e d a s h o r t d i s t a n c e t o t h e
t a i l i n g s d i s p o s a l area.
!!
Pregnant s o l u t i o n d r a i n s from t h e heap and f l o w s by g r a v i t y t o
c o l l e c t i o n d i t c h e s . S o f u t l o n i n t h e d i t c h e s f l o w by g r a v i t y t o a pregnant
sol u t l o n r e s e r v o l r and i s pumped t h r o u g h a f i ve-stage carbon a d s o r p t I o n
c i r c u i t a t a r a t e o f 2,600 gpm (164 1
s The carbon a d s o r p t l o n c i r c u i t
c o n t a i n s from 15 t o 18 t o n s o f carbon. A t o t a l a f 2.7 volume tons o f carbon I s
educted t o t h e s t r l p p l n g r s i r c u l t each day, Loaded carbon (120 t o 150 OPT Au)
I s desorbed i n about e l g h t b u r s us1 ng pressure s t r i p p i n g technology. Desorbed
carbon I s e c l d washed and t h e m a l l y regenerated b e f o r e b e i n g r e c y c l e d t o t h e
a d s o r p t i o n c i r c u i t. Gold values s t r i p p e d from t h e carbon a r e e l ectrowon o n t o
s t e e l wool cathodes. Cathodes a r e r e f i n e d o n s i t e t o produce dore b u l l ion, Q s r e
b u l l f o n I s about 92 p e r c e n t gold, and 1s shipped t o a custom r e f i n e r y f o r
purl f i c a t l o n .
Low-grade o r e from the mjna I s dump leached a t a run-of-mlne feed s i z e .
c l a y pad, The dump
About 600,000 t a n s o f I o n - g r a d e i s Isached a t a t i m e on
leach s i t e has I t s own carbon ~ d s e r p t i o n c l r c u i t , b u t loaded carbon i s
processed a t t h e mfne p l a n t s l t a , B a r r e n s o l u t i o n from t h e carbon c i r c u i t i s
r e c y c l e d t o the dump, M u l t i p l e l i f t s o f low-grade o r e w i l l u l t i m a t e l y be
processed an t h e clay pad, Each I l f t I s 60 F t (18 rn) i n h e i g h t , L i f t s a r e
b u i l t by t r u c k dumping from the s u r f a c e o f t h e dump.
Few problems a r e exparlanced w l t h
primarily because t h e o r e fs v e r y " c l @ a n "
I t I s r e p o r t a d t h a t t h e key t o t h e success
s t r l p p l n g and r e a c t i v a t i o n o f t h e carbon,
expansion t o 40,000 TPI) o f crushed feed.
heap l e a c h i n g i n t h l s o p e r a t l o n ,
and c o n t a l n s almost no c y a n i c i d e s .
o f t h l s operation i s the e f f i c i e n t
The o p e r a t i o n i s I n the process o f
5.6.2 A r f t o n a S l l v e r Heap L e a c h t n s 2,000 Tons Per Day
I\ s i l v e r argglomeratlon-heap l e a c h i n g o p e r a t f o n i s
Tombstone, Arfzona (Anon, 1981; McClelland, e t a1 ., 1983).
located near
Two types o f
84
PRECIOUS METAL HEAP LEACHING PROJECTS
m a t e r i a l s a r e m i n e d : o l d waste m a t e r i a l which was used f o r mine b a c k f i l l ; and
v i r g i n o r e a d j a c e n t t o t h e w a s t e m a t e r i a l . The waste m a t e r i a l i s mined w i t h
f r o n t - e n d l o a d e r s t o expose t h e v i r g i n o r e . The v i r g i n o r e i s d r i l l e d and
b l a s t e d and moved b y f r o n t - e n d l o a d e r s . The s i l v e r c o n t e n t of t h e two feed
m a t e r i a l s v a r i e s . The c u t o f f g r a d e o f f e e d t o t h e heaps i s 1.0 t r o y ounce of
s i l v e r p e r t o n ( a b o u t 34 g / m t ) . A p p r o x i m a t e l y 2,000 t o n s per day o f o r e a r e
m i n e d and a g g l o m e r a t e d
.
The m i n e d o r e and m i n e r a l i z e d waste a r e moved f r o m a s t o c k p i l e t o t h e
c r u s h i n g p l a n t where t h e y a r e c r u s h e d t o a nominal 1 / 2 i n c h (12 mm) w i t h t h r e e
s t a g e s o f c r u s h i n g and s c r e e n i n g . Run-of-mine o r e i s f e d t o t h e p r i m a r y jaw
and i s d i s c h a r g e d a t a b o u t a 2-1/2 i n c h (37 mm) s i z e . Secondary and t e r t i a r y
c r u s h i n g i s done w i t h a s c r e e n i n g c i r c u i t and s h o r t head cone c r u s h i n g system.
L i m e ( 7 I b s , 3 kg, p e r t o n o f o r e ) i s used as t h e b i n d e r f o r a g g l o m e r a t i o n and
i s m i x e d w i t h t h e o r e d u r i n g s e c o n d a r y c r u s h i n g . The crushed o r e - 1 ime m i x t u r e
i s conveyed t o an u n d e r g r o u n d o r e s t o c k p i l e . Some m o i s t u r e i s sprayed o n t o t h e
o r e o n t h e c r u s h e r d i s c h a r g e c o n v e y o r t o decrease d u s t i n g .
The o r e f r o m t h e u n d e r g r o u n d s t o c k p i l e i s agglomerated on a r e v e r s e b e l t
c o n v e y o r d e s i g n e d b y t h e o p e r a t o r s . The 4 f t (1.2 m) x 25 ft (7.6 m) b e l t
a g g l o m e r a t e s o r e a t a r a t e o f 20% t o n s p g r hour. The a g g l o m e r a t i n g conveyor
c a n be s e t a t an a n g l e between 35 and 45 , and t h e b e l t t r a v e l s upward w h i l e
t h e o r e lnoves down t h e be1 t. The a n g l e and speed o f t h e be1 t can be v a r i e d t o
p r o v i d e t h e d e s i r e d r e t e n t i o n t i m e o f o r e on t h e b e l t . Water i s sprayed a t
s e v e r a l l o c a t i o n s a l o n g t h e l e n g t h o f t h e b e l t and g i v e s t h e agglomerated feed
a m o i s t u r e c o n t e n t o f between 10 and 12 p e r c e n t . A s m a l l amount o f moistened
f i n e s a d h e r e s t o t h e b e l t and r i d e s u p t h e conveyor. A s c r a p e r a t t h e bottom
s i d e o f t h e d r i v e r o l l e r e l i m i n a t e s e x c e s s i v e f i n e s b u i l d-up.
The a g g l o m e r a t e d o r e i s t r a n s p o r t e d t o a s t o c k p i l e b y a r a d i a l arm
s t a c k e r . The m a t e r i a l f r o m t h e s t o c k p i l e i s t r u c k e d 500 f t (150 m) t o a 3/4
a c r e (4,000 m2) l e a c h i n g pad and i s a1 lowed t o c u r e d u r i n g heap b u i l d i n g . F i v e
heaps, each c o n t a i n i n g 6,000 t o n s o f agglomerated o r e s t a c k e d 10 t o 11 f t ( 3
t o 3.4 m) h i g h , a r e l e a c h e d o n t h e pad. Three heaps a r e a t d i f f e r e n t stages o f
l e a c h i n g , w h i l e t h e o t h e r two a r e e i t h e r b e i n g p r e p a r e d f o r l e a c h i n g o r b e i n g
removed f r o m t h e pad.
The heaps a r e s p r a y e d w i t h pH 10.5 s o l t i o n c o n t a i n i n g two I b s NaCN per
t o n ( 1 k g / m t ) a t a r a t e o f 0.0075 gpm/ftY (0.005 l / s / m 2 ) .
The l e a c h i n g
s o l u t i o n p e r c o l a t e s t h r o u g h t h e heap, i s c o l l e c t e d o n t h e i m p e r v i o u s l e a c h i n g
pad, and d r a i n s i n t o t h e p l a s t i c - l i n e d s o l u t i o n t r e n c h e s . The l e a c h i n g and
w a s h i n g c y c l e i s seven days. The l e a c h e d r e s i d u e i s t r a n s f e r r e d t o an
a u x i l i a r y l e a c h i n g pad and sprayed w i t h c y a n i d e s o l u t i o n one day p e r month f o r
a d d i t i o n a l p r e c i o u s m e t a l e x t r a c t i o n . P r e c i o u s m e t a l v a l u e s i n t h e pregnant
s o l u t i o n a r e r e c o v e r e d b y M e r r i l 1-Crowe z i n c p r e c i p i t a t i o n t e c h n o l o g y . The
p r e c i o u s - m e t a l - b e a r i n g z i n c p r e c i p i t a t e s a r e r e f i n e d on s i t e and y i e l d d o r e
b u l l i o n . The d o r e i s s h i p p e d t o a n o t h e r f a c i l i t y f o r r e f i n i n g . The b a r r e n
s o l u t i o n s a r e r e c y c l e d t o t h e heaps.
Heap l e a c h i n g was unsuccessfu1 before a g g l o m e r a t i o n p r e t r e a t m e n t was
a p p l i e d t o t h e o r e . C o n v e n t i o n a l heap l e a c h i n g r e c o v e r e d o n l y 37 p e r c e n t o f
t h e l e a c h a b l e s i l v e r f r o m t h r e e i n c h (75 mm) feed m a t e r i a l t r e a t e d i n 90 day
1 eac h i n g c y c l es. Severe p e r c o l a t i o n problems were encountered. Agglomeration
p e r m i t t e d f i n e r c r u s h i n g , w h i c h 1 ib e r a t e d a d d i t i o n a l s i l v e r v a l u e s f o r
ORE PREPARATION: CRUSHING AND AGGLOMERATION
85
d i s s o l u t i o n by cyanide.
F i n e r c r u s h i n g and a g g l o m e r a t i o n - h e a p
leaching
i n c r e a s e d s i l v e r r e c o v e r y t o 90 p e r c e n t o f t h e l e a c h a b l e s i l v e r a n d d e c r e a s e d
t h e l e a c h i n g t i m e t o seven days.
5.6.3
N o r t h e r n Nevada Gold Heap L e a c h i n g : 2,500
Tons Per Day
An o p e r a t i o n i n n o r t h e r n Nevada, w h i c h has p r o d u c e d g o l d b y a g i t a t i o n
c y a n i d a t i o n and c o u n t e r c u r r e n t d e c a n t a t i o n f o r s e v e r a l y e a r s , d i s c o v e r e d a new
o r e d e p o s i t s e v e r a l m i l e s from t h e w o r k i n g mine. H i g h e r g r a d e o r e f r o m t h e new
d e p o s i t i s t r a n s p o r t e d t o t h e o r i g i n a l m i l l f o r g o l d r e c o v e r y . Ore c o n t a i n i n g
l e s s t h a n 0.07 t r o y ounce g o l d p e r t o n (2.4 g / m t ) i s heap l e a c h e d a t t h e new
s i t e . Average o r e g r a d e f o r t h e heap m a t e r i a l i s 0.034 t r o y o u n c e g o l d p e r t o n
(1.2 g / m t ) . The o r e i s mined by o p e n - p i t methods and i s t r u c k e d t o t h e heap
l e a c h i n g s i t e a p p r o x i m a t e l y o n e - h a l f m i l e ( 0 . 8 k m ) f r o m t h e new p i t . A b o u t
2,500 t o n s p e r day o f o r e a r e mined and heap l e a c h e d .
T h e o r e i s c r u s h e d t o minus 518 i n c h ( 1 6 mm) b y a p r i m a r y j a w c r u s h e r
and a s e c o n d a r y cone c r u s h e r . P o r t l a n d cement ( t y p e 11), a t t h e r a t e o f 7 t o
1 0 l b s p e r t o n (3.5 t o 5 k g / m t ) o f o r e , i s added t o t h e o r e a t t h e p r i m a r y j a w
c r u s h e r d i s c h a r g e c o n v e y o r , and i s m i x e d w i t h t h e o r e d u r i n g s e c o n d a r y
c r u s h i n g . The o r e - b i n d e r m i x t u r e f r o m s e c o n d a r y c r u s h i n g i s c o n v e y e d t o a
r a d i a l arm s t a c k e r . Water i s s p r a y e d o n t o t h e m i x t u r e a t t h e d i s c h a r g e end o f
t h e s t a c k e r and r e s u l t s i n a f i n a l m o i s t u r e c o n t e n t o f 9 t o 13 p e r c e n t . The
o r e i s a g g l o m e r a t e d by c a s c a d i n g down t h e s i d e s o f t h e c o n i c a l 1y-shaped
a g g l o m e r a t e d o r e s t o c k p i 1e.
Additional
tumbl ing,
sufficient
t o effect
a g g l o m e r a t i o n , o c c u r s when t h e f r o n t - e n d l o a d e r l o a d s t h e r e a r dump t r u c k w i t h
a g g l o m e r a t e s f r o m t h e s t o c k p i l e and when t h e t r u c k dumps t h e a g g l o m e r a t e d
f e e d o n t o t h e a s p h a l t l e a c h i n g pad. The a g g l o m e r a t e d o r e I s c u r e d f o r t w o t o
t h r e e d a y s w h i l e t h e heap i s b e i n g b u i l t .
F i v e 17,000-ton heaps, a p p r o x i m a t e l y 12 f t (3.7 m ) h f g h , a r e b u i l t o n
t h e l e a c h i n g pad. Three heaps a r e i n d i f f e r e n t s t a g e s o f t h e l e a c h i n g c y c l e
w h i l e t h e r e m a i n i n g two a r e e i t h e r b e i n g p r e p a r e d f o r l e a c h i n g o r b e i n g
removed f r o m t h e pad.
The a g g l o m e r a t e d heaps a r e l e a c h e d by s p r a y i n g 0.004 t o 0.005 g p m / f t 2
( a b o u t 0.003 l / s / m * ) o f pH 10-11 s o l u t i o n c o n t a i n i n g 1.0 1 b NaCN p e r t o n ( 0 . 5
k g / m t ) o f s o l u t i o n . The p o r t l a n d cement added d u r i n g a g g l o m e r a t f o n p r o v i d e s
most o f t h e a1 k a l i n l t y r e q u i r e d d u r i n g l e a c h i n g , b u t s m a l l q u a n t i t i e s o f NaOH
a r e a d d e d t o t h e b a r r e n s o l u t i o n t o m a i n t a i n t h e d e s i r e d pH v a l u e . The
l e a c h i n g and washing c y c l e i s 20 days. Leached r e s l d u e s a r e t r a n s p o r t e d t o a
t a i l i n g s d i s p o s a l area.
The p r e g n a n t s o l u t i o n d r a i n i n g f r o m t h e l e a c h i n g pad f l o w s b y g r a v i t y t o
a r e s e r v o i r and i s pumped t h r o u g h a s e r i e s o f f i v e c a r b o n a d s o r p t i o n t a n k s f o r
r e c o v e r y o f d i s s o l v e d g o l d v a l u e s . The g o l d - l o a d e d , 12 x 3 0 mesh, c o c o n u t
s h e l l a c t i v a t e d c a r b o n i s t r a n s p o r t e d t o t h e o r i g i n a l m i l 1s i t e f o r d e s o r p t i o n
b y an a l k a l i n e a l c o h o l s o l u t i o n . The v a l u e s d e s o r b e d f r o m t h e c a r b o n a r e
e l e c t r o w o n on s t e e l wool c a t h o d e s , w h i c h a r e r e f i n e d o n s i t e t o p r o d u c e d o r e
bullion.
T h i s o p e r a t i o n i n i t i a l l y t r i e d c o n v e n t i o n a l heap 1 e a c h i n g t o p r o c e s s t h e
l o w - g r a d e o r e . C o n v e n t i o n a l heap l e a c h i n g was u n s u c c e s s f u l because o f t h e h i g h
c l a y c o n t e n t o f t h e o r e , p a r t i c l e s e g r e g a t i o n , and f i n e s m i g r a t i o n , w h i c h
r e s u l t e d i n l e a c h s o l u t i o n channel i n g . A g g l o m e r a t i o n p r e t r e a t m e n t i n c r e a s e d
86
PRECIOUS METAL HEAP LEACHING PROJECTS
g o l d r e c o v e r y by 60 p e r c e n t , w h i l e d e c r e a s i n g t h e l e a c h i n g c y c l e ft-om 50 days
t o 20 days. Even w i t h agglomeration, some o r e r e t a i n s a p p r o x i m a t e l y 30 Percent
m o i s t u r e . and l o n g washing p e r i o d s a r e r e q u i r e d t o r e c o v e r t h e dissolved
v a l u e s f r o m t h e agglomerates. The l o n g e r washing c y c l e extends the t o t a l
1 caching c y c l e t o 30 days f o r some heaps.
5.6.4 West C e n t r a l Nevada Gold Heap Leach: 3,500 Tons Per Day
An o p e r a t i o n i n west c e n t r a l Nevada processes 3,500 TPD o f crushed and
agglomerated g o l d o r e . Ore i s mined u s i n g o p e n - p i t methods. D r i l l e d and
b l a s t e d o r e i s loaded by a f r o n t - e n d l o a d e r o n t o t r u c k s and i s hauled about
0.5 m i l e s (0.8 km) t o t h e c r u s h i n g c i r c u i t . A p r i m a r y jaw crusher produces a
minus s i x i n c h (150 mm) feed. The p r i m a r y c r u s h e r d i s c h a r g e i s conveyed a
s h o r t d i s t a n c e t o a secondary s h o r t head cone c r u s h e r where i t i s reduced i n
s i z e t o a n o m i n a l minus one i n c h .
The c r u s h e d o r e i s v e r y c l a y e y and c o n t a i n s about 25 weight percent
minus 100 mesh f i n e s . The crushed o r e i s agglomerated by adding 10 I b s
p o r t l a n d cement per t o n ( 5 kg/mt) t o t h e secondary c r u s h e r . W e t t i n g a t several
l o c a t i o n s w i t h b a r r e n s o l u t i o n c o n t a i n i n g cyanide t o a f i n a l m o i s t u r e content
o f 10 t o 12 w e i g h t percent, m e c h a n i c a l l y t u m b l i n g i n a t h r e e - s t a g e drop b e l t
a g g l o m e r a t i o n c i r c u i t , and c u r i n g i n a s t o c k p i l e f o r t h r e e t o four days before
b e i n g p l a c e d on t h e pad.
L e a c h i n g i s conducted on t h e p r i m a r y heaps by a p p l y i n g cyanide s o l u t i o n
c o n t a i n i n g 0.2 I b s NaCN per t o n (0.1 kg/mt) o f s o l u t i o n a t a r a t e o f 0.003 gpm
per sq f t (0.002 1/s/m3) o f heap s u r f a c e area. P r i m a r y heaps a r e constructed
on a s p h a l t pads t o a h e i g h t o f 12 f t (3.7 m) by l i f t i n g w i t h a front-end
l o a d e r . A t o t a l o f f i v e heaps a r e processed a t once. Three heaps are i n
d i f f e r e n t s t a g e s o f l e a c h i n g w h i l e one i s b e i n g removed from t h e pad, and the
o t h e r i s b e i n g b u i l t . A 21-day l e a c h i n g and washing c y c l e i s r e q u i r e d f o r each
p r i m a r y heap t o a c h i e v e 75 p e r c e n t g o l d recovery.
Leached r e s i d u e from t h e p r i m a r y heaps i s t r u c k e d t o t h e releach pad
area. R e l e a c h heaps a r e b u i l t b y t r u c k dumping from t h e heap surface t o a
h e i g h t o f 100 t o 120 f t (30 t o 37 m). Several months o f r e l e a c h i n g i s required
t o i n c r e a s e t h e o v e r a l l g o l d r e c o v e r y from t h e crushed o r e t o 85 percent.
Low-grade o r e from t h e mine i s processed by dump l e a c h i n g a t a run-of mine feed s i z e . Dumps a r e b u i l t on c l a y pads t o a h e i g h t o f 120 ft by t r u c k
dumping f r o m t h e heap surface. Low-grade o r e remains on t h e pad u n t i l i t
becomes uneconomical t o process and r e c y c l e s o l u t i o n s . Dump 1each cycles
u s u a l l y a r e f r o m 9 t o 14 months.
Each t y p e o f heap ( p r i m a r y , r e l e a c h and durr~p) has i t s own pregnant
s o l u t i o n r e s e r v o i r s . A l l pregnant s o l u t i o n s u l t i m a t e l y a r e processed i n a
s i n g l e z i n c p r e c i p i t a t i o n c i r c u i t . S O ~ U ~ ~from
O ~ Sany o f t h e pregnant ponds
can be t a k e n d i r e c t l y t o t h e z i n c c i r c u i t o r can be d i v e r t e d t o o t h e r heaps t o
i n c r e a s e p r e g n a n t s o l u t i o n g o l d c o n c e n t r a t i o n s . Pregnant s o l u t i o n s d r a i n by
g r a v i t y t o a s u r g e t a n k f o r p r o c e s s i n g t h r o u g h t h e z i n c c i r c u i t . S o l u t i o n s are
c l a r i f i e d and deareated before a d d i n g t h e M e r r i l l i t e z i n c d u s t . The z i n c
c i r c u i t c a n p r o c e s s 600 gpm ( 3 7 - 8 11s) o f pregnant s o l u t i o n . P r e c i p i t a t e s are
r e c o v e r e d i n f i l t e r presses and a r e r e f i n e d o n s i t e t o produce dore b u l l i o n .
ORE PREPARATION: CRUSHING AND AGGLOMERATION
87
A t o t a l o f 2,000 gpm (126 1 1 s ) o f p r e g n a n t s o l u t i o n d r a i n s f r o m t h e
v a r i o u s heaps. The z i n c p r e c i p i t a t i o n c i r c u i t can process o n l y 600 gpm (37.8
l / s ) . Consequently, i t i s d i f f i c u l t t o o b t a i n a good s o l u t i o n b a l a n c e f o r t h e
o p e r a t i o n . Even t h o u g h s o l u t i o n s can be d i v e r t e d from any heap a t a s p e c i f i c
s o l u t i o n grade, i t i s s t i l l d i f f i c u l t t o o b t a i n a s o l u t i o n balance f r o m a n y
g i v e n heap because a1 1 s o l u t i o n s a r e processed t h r o u g h a s i n g l e z i n c c i r c u i t .
The o p e r a t i o n does n o t have t o %hut down d u r i n g t h e w i n t e r months
because b a r r e n s o l u t i o n s a r e heated t o 30 F. S o l u t i o n s a r e heated w i t h a h e a t
exchanger c i r c u i t .
Some problems a r e i n h e r e n t t o t h i s o p e r a t i o n . Organic compounds
c o n t a i n e d i n t h e o r e caused problems i n t h e z-inc p r e c i p i t a t i o n c i r c u i t . Most
s e v e r e p r e c i p i t a t i o n problems were encountered e a r l y i n t h e p r o j e c t . Go1 d
p r e c i p i t a t i o n problems were m i n i m i z e d by p r e c i p i t a t i n g t h e d i s s o l v e d o r g a n i c s
w i t h b a r i u m c h l o r i d e . S c a l i n g has been a m a j o r problem t h e d u r a t i o n o f t h e
p r o j e c t . S c a l i n g has been c o n t r o l l e d by t h e use o f d e s c a l a n t c h e m i c a l s . I n
severe cases, 1 i n e s have t o be p h y s i c a l l y unplugged u s i n g a h y d r o j e t . S o l u t i o n
b a l a n c e remains t h e m a j o r problem. T h i s problem can o n l y be s o l v e d b y
i n c r e a s i n g t h e c a p a c i t y o f t h e z i n c c i r c u i t o r by b u i l d i n g s e p a r a t e s o l u t i o n
r e c o v e r y systems f o r t h e r e l e a c h and dump heap l e a c h c i r c u i t s . The o p e r a t i o n
has been v e r y s u c c e s s f u l even w i t h t h e problems encountered.
5.7
AGGLOMERATION OF F I N E L Y GROUND TAILINGS
The Western U n i t e d S t a t e s has many t a i l i r l g s m a t e r i a l s from f o r m e r m i n i n g
o p e r a t i o n s t h a t c o n t a i n s i g n i f i c a n t p r e c i o u s m e t a l v a l u e s . Most o f t h e s e
t a i l i n g s r e s o u r c e s a r e t o o low-grade o r t o o s m a l l t o w a r r a n t t h e c a p i t a l
expenditure t o c o n s t r u c t a conventional a g i t a t e d cyanide l e a c h i n g c i r c u i t
( M c C l e l l a n d , e t a1 ., 1 9 8 5 ) . I n most cases, t h e o n l y v i a b l e p r o c e s s i n g
t e c h n o l o g y f o r t h e s e f e e d s i s ,qglomeration-heap
1 eaching.
A g g l o m e r a t i n g parameters w h i c h a r e i m p o r t a n t f o r s u c c e s s f ~ ~t lr e a t m e n t o f
crushed o r e s a r e e q u a l l y i m p o r t a n t f o r t a i l i n g s a g g l o m e r a t i o n w i t h some
m o d i f i c a t i o n . The b i n d e r normal l y r e q u i r e d f o r t a i 1 i n g s a g g l o m e r a t i o n i s a
c o m b i n a t i o n o f l i m e and cement, u s u a l l y 10 - 15 I b s o f each p e r t o n o f d r y
feed. M o i s t u r e a d d i t i o n s a r e u s u a l l y h i g h e r , a t from 16 t o 22 w e i g h t p e r c e n t
f i n a l m o i s t u r e . C u r i n g t i m e s i n t h e range o f 72 hours o r l o n g e r a r e r e q u i r e d .
Two m e c h a n i c a l p a r a m e t e r s a r e a1 so i m p o r t a n t f o r agglomeratirlg t a i l i n g s :
M o i s t u r e s h o u l d be added as a n o n - a t o m i z i n g s p r a y o r as d r o p l e t s ;
and
A r o l l i n g , r a t h e r t h a n a bouncing o r t u m b l i n g , a c t i o n s h o u l d be
'imparted t o t h e t a i l i n g s by t h e a g g l o m e r a t o r .
S o l u t i o n s h o u l d be added as d r o p l e t s o r c o a r s e s p r a y because t h e w a t e r
d r o p i m p a c t i n g t h e d r y f e e d i m m e d i a t e l y forms a s m a l l b a l l which a c t s as t h e
n u c l e u s f o r a g g l o m e r a t e g r o w t h . I f f i n e s p r a y s a r e used, no nucleus i s formed.
A g g l o m e r a t o r s such as drunis, d i s k s , and pug m i l l s i m p a r t a r o l l i n g
r a t h e r t h a n a bouncing ( b e l t s ) a c t i o n t o t h e t a i l i n g s b e i n g agglomerated.
These
types
of
aggl omerators
are
required
for
effective
t a i l ings
agglomeration.
PRECIOUS METAL HEAP LEACHING PROJECTS
88
S e v e r a l commercial-scale t a i l i n g s agglomeration-heap l e a c h operations
have been s u c c e s s f u l . T h r e e s u c c e s s f u l o p e r a t i o n s a r e b r i e f l y d e s c r i b e d below.
The o p e r a t i o n s , however, a r e n o t i n c u r r e n t p r o d u c t i o n . The go1 d t a i l i n g s
p r o j e c t s w e r e c o m p l e t e d and a l l t h e t a i l i n g s were l e a c h e d . The s i l v e r t a i l i n g s
p r o j e c t was s h u t down because o f t h e l o w s i l v e r p r i c e ( l e s s t h a n $7.00 p e r
o u n c e ) . None o f t h e t a i l i n g s f e e d m a t e r i a l s c o u l d be e c o n o m i c a l l y processed b y
means o t h e r t h a n a g g l o m e r a t i o n - h e a p 1 eaching.
5.8
EXAMPLES OF T A I L I N G S AGGLOMERATION
Go1 d A g g l o m e r a t i o n - H e a p L e a c h i n g i n S o u t h - C e n t r a l Nevada
A t a i l i n g s m a t e r i a l from t h e s o u t h - c e n t r a l Nevada G o l d f i e l d D i s t r i c t was
p r o c e s s e d b y a g g l o m e r a t i o n p r e t r e a t m e n t and heap 1each c y a n i d a t i o n . 'The
t a i l i n g s r e s u l t e d f r o m a c y a n i d e m i l l i n g o p e r a t i o n t h a t was a c t i v e j u s t a f t e r
t h e t u r n o f t h e c e n t u r y . The o r i g i n a l o r e was h i g h i n s u l f i d e s and g o l d
r e c o v e r i e s w e r e l o w . The t a i l i n g s o x i d i z e d f o r a p p r o x i m a t e l y 70 years by
n a t u r a l w e a t h e r i n g and r e s i d u a l s u l f i d e s o x i d i z e d t o s o l u b l e s u l f a t e . The
n a t u r a l pH o f a 50 p e r c e n t s o l i d s s l u r r y was 1.7 because o f t h e s o l u b l e
s u l f a t e s . The t a i l i n g s were 65 p e r c e n t minus 200 mesh (0.074 rnm) and conta'ined
0.08 ounce Au p e r t o n (2.7 g/mt) o f t a i l i n g s . The maximum g o l d r e c o v e r y b y
a g i t a t e d c y a n i d a t i o n was 83 p e r c e n t .
The t a i l i n g s were moved t o t h e a g g l o m e r a t i n g p l a n t by a f r o n t - e n d l o a d e r
and dumped i n t o a hopper. The t a i l i n g s were conveyed t o a 8.5 f t (2.6 m) x 22
f t (6.7 m) drum a g g l o m e r a t o r , w h i c h was a m o d i f i e d a s p h a l t k i l n . The drum
r o t a t e d a t 10.5 rpm, had a s l o p e t o t h e d i s c h a r g e end o f 4 d e g r e e s and was
l i n e d w i t h l o o s e l y f i t t i n g c o n v e y o r b e l t m a t e r i a l . A s p r a y b a r was s i t u a t e d
l e n g t h w i s e i n t h e drum and d e l i v e r e d a f a n d r o p l e t s p r a y t h a t c o v e r e d t h r e e f o u r t h s o f t h e l e n g t h o f t h e drum. A lime-cement s l u r r y was a p p l i e d t h r o u g h
t h e s p r a y s y s t e m t o add b i n d e r and b r i n g t h e f i n a l m o i s t u r e c o n t e n t o f t h e
a g g l o m e r a t e s t o between 12 and 14 w e i g h t p e r c e n t . The t o t a l b i n d e r a d d i t i o n
was 50 l b s ( 2 5 k g ) l i m e and 1 0 I b s ( 5 k g ) cement p e r t o n ( m t ) o f d r y feed. The
l a r g e l i m e a d d i t i o n was r e q u i r e d t o a d j u s t t h e pH o f t h e t a i l i n g s f r o m 1.7 t o
10.5. A 1 2 i n c h ( 3 0 cm) w e i r o n t h e i n s i d e o f t h e drum was f o u r f t from t h e
d i s c h a r g e end t o i n c r e a s e f e e d r e t e n t i o n t i m e and t o p r e v e n t d i s c h a r g e
s u r g i n g . The a g g l o m e r a t e s d i s c h a r g e d from t h e drum t o a t r a n s f e r p o i n t f e e d i n g
a r a d i a l arm s t a c k e r . The green agglomerates were g e n t l y p l a c e d o n t h e heap by
t h e s t a c k e r a n d c u r e d d u r i n g heap b u i l d i n g .
The l e a c h pad was c o n s t r u c t e d by compacting b a r r e n t a i l i n g s i n s i x i n c h
l a y e r s and c o v e r i n g them w i t h a t h i n PVC l i n e r . The heaps were b u i l t b y
a d j u s t i n g t h e r a d i a l - a r m s t a c k e r t o i t s l o w e s t a n g l e , sweeping a c r o s s t h e
w i d t h o f t h e pad, r a i s i n g t h e s t a c k e r d i s c h a r g e end one f o o t ( 3 0 cm), and
sweeping t h e o p p o s i t e d i r e c t i o n a c r o s s t h e w i d t h o f t h e pad. T h i s p r o c e d u r e
c o n t i n u e d u n t i l t h e heap was 16 ft ( 5 m) high. The s t a c k e r remained a t 16 f t
( 5 m), and new a g g l o m e r a t e s were added t o t h e heap by sweeping t h e s t a c k e r
a c r o s s t h e w i d t h o f t h e pad and a l l o w i n g t h e agglomerates t o cascade down t h e
heap. Heaps b u i l t i n t h i s manner avoided cornpacting t h e a g g l o m e r a t e s . The
a g g l o m e r a t i n g e q u i p m e n t and t h e s t a c k e r were moved as a u n i t b y a dozer b e f o r e
a new row o f a g g l o m e r a t e s was added t o t h e l e a c h i n g pad, A w i d t h o f PVC l i n e r
was r o l l e d o u t t h e w i d t h o f t h e pad and welded as necessary, t o c o n t i n u e heap
b u i l d i n g . The a g g l o m e r a t e s c u r e d f o r s e v e r a l days w h i l e t h e 6,400 t o n heap was
b u i l t . 'The hea was l e a c h e d b y s p r a y i n g a c y a n i d e s o l u t i o n c o n t a i n i n g 2.0 I b s
NaCN p e r t o n
kg/rnt) o f s o l u t i o n on i t a t a r a t e o f 0.003 gpm/ft2 (0.002
el
ORE PREPARATION: CRUSHING AND AGGLOMERATION
89
l/s/m3).
The p r e g n a n t s o l u t i o n t h a t d r a i n e d f r o m t h e s l o p e d l e a c h i n g pad
c o l l e c t e d i n l i n e d d i t c h e s and f l o w e d b y g r a v i t y t o a p r e g n a n t s o l u t i o n pond.
Gold was r e c o v e r e d from t h e p r e g n a n t s o l u t i o n by c a r b o n a d s o r p t i o n - d e s o r p t i o n e l ectrowinning.
Go1 d r e c o v e r y b y a g g l o m e r a t i o n - heap 1e a c h i n g was 78 p e r c e n t . C y a n i d e
consumption was 0.7 1 b NaCN p e r t o n (0.35 kg/mt) o f t a i l i n g s . The l e a c h i n g washing c y c l e was a b o u t 2 4 days.
5.8.2
S i l v e r Agglomeration-Heap L e a c h i n g i n S o u t h e a s t e r n C a l i f o r n i a
A t a i l i n g s p i l e from a former f l o t a t i o n o p e r a t i o n i n southeastern
Cal if o r n i a was processed by a g g l o m e r a t i o n - h e a p l e a c h i n g t o r e c o v e r t h e
c o n t a i n e d s i l v e r v a l u e s ( M i l 1 i g a n and Engel h a r d t , 1 9 8 3 ) . The t a i l i n g s w e r e 90
p e r c e n t minus 200 mesh and c o n t a i n e d an average o f 1.4 ounces Ag p e r t o n ( 4 8
g/nit) o f t a i l i n g s . 'The o p e r a t o r s o f t h e p r o p e r t y c o n d u c t e d b o t h b e n c h - and
p i l o t - s c a l e experiments t o e v a l u a t e agglomeration-heap l e a c h i n g f o r r e c o v e r i n g
s i l v e r from t h e t a i l i n g s . B e n c h - s c a l e r e s u l t s showed t h a t 63 p e r c e n t o f t h e
s i l v e r was r e c o v e r e d b y a g i t a t e d c y a n i d a t i o n and 72 p e r c e n t o f t h e s i l v e r was
r e c o v e r e d b y a g g l o m e r a t i o n - h e a p l e a c h i n g . The l o n g e r 1 e a c h i n g c y c l e f o r heap
leaching allowed the higher recovery.
The p l a n t was designed t o a g g l o m e r a t e and heap l e a c h 1,000 t o n s p e r d a y
o f d r y t a i l i n g s . The t a i l i n g s were mined and c r u s h e d t o b r e a k u p t h e l a r g e
chunks o f cemented m a t e r i a l . The c r u s h e r d i s c h a r g e was conveyed t o t w o m i x e r s
where 35 'Ibs o f p o r t l a n d cement p e r t o n (17.5 k g l m t ) o f d r y f e e d w e r e added a s
a b i n d e r . The b i n d e r and f e e d m i x t u r e were conveyed t o a 1 0 f t ( 3 m) x 30 f t
( 9 m) drum a g g l o m e r a t o r . A s c r a p e r , w h i c h r o t a t e d i n t h e o p p o s i t e d i r e c t i o n t o
t h e drum, was s i t u a t e d a l o n g t h e l e n g t h o f t h e drum t o p r e v e n t a g g l o m e r a t e
b u i l d u p . Water was a p p l i e d as a c o a r s e s p r a y t h r o u g h a s p r a y b a r s i t u a t e d
l e n g t h w i s e i n t h e drum. The w a t e r was added a t t h e b o t t o m o f t h e drum and
m i x e d w i t h t h e t a i l . i n g s t o b r i n g t h e f i n a l m o i s t u r e c o n t e n t t o 1 2 t o 15 w e i g h t
p e r c e n t . The a g g l o m e r a t e s d i s c h a r g e d f r o m t h e drum t o a t r a n s f e r p o i n t f e e d i n g
a r a d i a l - a r m s t a c k e r . The g r e e n a g g l o m e r a t e s were p l a c e d on a s t o c k p i l e b y t h e
s t a c k e r and c u r e d f o u r days b e f o r e b e i n g t r a n s p o r t e d t o t h e l e a c h i n g pad.
The 1e a c h i n g pad was c o n s t r u c t e d f r o m m o i s t e n e d c l a y e y s i l t m a t e r i a l
compacted i n t h r e e l a y e r s . The m i d d l e l a y e r was m i x e d w i t h b e n t o n i t e t o e n s u r e
i m p e r m e a b i l i t y o f t h e l e a c h i n g pad ( v a n Z y l , 1 9 8 4 ) . The heaps w e r e b u i l t t o a
h e i g h t o f 13 f t ( 4 m) b y a f r o n t - e n d l o a d e r t o a v o i d d r i v i n g o n t h e
agglomerates.
The heaps were sprayed w i t h a s o l u t i o n c o n t a i n i n g 2.0 l b s NaCN p e r t o n
( 1 kg/mt) o f s o l u t i o n and a t a r a t e o f 0.01 g p m / f t 2 (0.007 l / s / m 2 ) ( M i l l i g a n ,
1983). The p r e g n a n t s o l u t i o n c o l l e c t e d i n l i n e d d i t c h e s and d r a i n e d b y g r a v i t y
t o a p r e g n a n t s o l u t i o n pond. The p r e g n a n t s o l u t i o n c o n t a i n e d an a v e r a g e o f 1.0
ounce Ag p e r t o n (34.2 g/mt) o f s o l u t i o n and was pumped t o t h e p r e c i p i t a t i o n
c i r c u i t a t a r a t e o f 150 gp~n (9.5 l / s ) . The s o l u t i o n was c l a r i f i e d , d e a e r a t e d ,
and c o n t a c t e d w i t h z i n c d u s t t o r e c o v e r t h e s i l v e r .
S i l v e r r e c o v e r y was 72
p e r c e n t a f t e r a 25 day l e a c h i n g and 40 d a y washing c y c l e .
5.8.3
Gold Agglomeration-Heap L e a c h i n g i n E a s t C e n t r a l C a l i f o r n i a
A s m a l l t a i l i n g s p i l e i s b e i n g processed b y a g g l o m e r a t i o n - h e a p l e a c h i n g
methods f o r r e c o v e r y o f g o l d and s i l v e r v a l u e s . The o p e r a t i o n i s l o c a t e d i n
PRECIOUS METAL HEAP LEACHING PROJECTS
90
e a s t c e n t r a l C a l i f o r n i a n e a r t h e h i s t o r i c M o t h e r Lode D i s t r i c t . A p p r o x i m a t e l y
130,000 t o n s o f o l d m i l l e d t a i l i n g s c o n t a i n i n g 0.070 o u n c e s o f g o l d (2.4 g / m t )
and 0.75 o u n c e s o f s i l v e r (25.7 g / m t ) p e r t o n was a v a i l a b l e f o r p r o c e s s i n g .
The t a i l i n g s a r e 90 p e r c e n t m i n u s 200 mesh and c o n t a i n e x c e s s i v e q u a n t i t i e s o f
c l a y e y m i n e r a l s . The t a i l i n g s a r e mined b y f r o n t - e n d l o a d e r s and a r e
t r a n s p o r t e d b y t h e l o a d e r a v e r y s h o r t d i s t a n c e t o a s t o c k p i l e . T a i l i n g s from
t h e s t o c k p i l e a r e pushed by a t r a c k e d d o z e r i n t o a hopper. The f e e d i s
conveyed from t h e h o p p e r t o a s i n g l e one i n c h s c r e e n deck. The p l u s one i n c h
( 2 5 mm) c l u m p s o f c l a y a r e pushed t o t h e s i d e and a r e a l l o w e d t o a i r d r y f o r
s e v e r a l d a y s . The a i r d r i e d c l u m p s a r e b r o k e n u p b y d r i v i n g o v e r them w i t h t h e
t r a c k e d d o z e r , and a r e t h e n p l a c e d back i n t o t h e h o p p e r .
Minus one i n c h m a t e r i a l i s conveyed t o a 6 f t x 22 f t (1.8 m x 6.7 m)
r u b b e r 1 i n e d drum a g g l o m e r a t i o n . P o r t 1 and cement ( 2 0 1 b / t o n , 1 0 kg/mt, o f
f e e d ) i s added from a cement s i l o t o t h e c o n v e y o r . The b i n d e r i s m i x e d w i t h
t h e t a i l i n g s i n t h e f i r s t 1 / 3 o f t h e drum. Water i s added i n t h e m i d d l e 1 / 3 o f
t h e drum t o a c h i e v e a f i n a l m o i s t u r e c o n t e n t f o r t h e a g g l o m e r a t e s o f 15 t o 18
w e i g h t p e r c e n t . M e c h a n i c a l f r e e t u m b l i n g i n t h e l a s t 1 / 3 o f t h e druni produces
s t r o n g and s t a b 1 e a g g l o m e r a t e s .
A s i n g l e heap o f 75,000 t o n s o f a g g l o m e r a t e d t a i l i n g s was b u i l t on a
d o u b l e l i n e d c l a y pad u s i n g a s e r i e s o f c o n v e y o r s a n d a r a d i a l arm s t a c k e r .
The heap was b u i l t t o a h e i g h t o f 24 f t (7.3 m). The a g g l o m e r a t e d t a i l i n g s
were c u r e d i n t h e heap f o r a b o u t o n e week b e f o r e a p p l y i n g l e a c h i n g s o l u t i o n .
L e a c h i n g was c o n d u c t e d b y a p p l y i n g c y a n i d e s o l u t i o n c o n t a i n i n g 2.0 I b s
NaCN e r t o n ( 1 k g / m t ) o f s o l u t i o n a t a r a t e o f 0.003 gpm p e r s q f t (0.002
l / s / m ) o f heap s u r f a c e a r e a . A l e a c h c y c l e o f a b o u t t h r e e months was r e q u i r e d
t o o b t a i n t a r g e t e d precious metal recovery. Pregnant s o l u t i o n s drained by
g r a v i t y f r o m t h e heap t o a p r e g n a n t s o l u t i o n pond. P r e g n a n t s o l u t i o n was
pumped f r o m t h e pond t o a s u r g e t a n k o u t s i d e t h e z i n c p r e c i p i t a t i o n c i r c u i t
building.
P
S o l u t i o n s w e r e c l a r i f i e d and d e a r e a t e d b e f o r e a d d i n g z i n c d u s t . The z i n c
c i r c u i t p r o c e s s e d 1 6 0 gpm ( 1 0 l / s ) o f p r e g n a n t s o l u t i o n . P r e c i p i t a t e s were
r e c o v e r e d i n f i l t e r p r e s s e s and w e r e p r o c e s s e d o n s i t e t o p r o d u c e d o r e b u l l i o n .
The d o r e b u l l i o n p r o d u c e d c o n s t i t u t e d a b o u t 65 p e r c e n t g o l d and 30 p e r c e n t
silver.
The o p e r a t i o n i s c u r r e n t l y b u i l d i n g a n o t h e r pad t o p r o c e s s t h e r e m a i n i n g
t a i l i n g s . F u t u r e p l a n s a r e t o process waste m a t e r i a l from t h e o l d m i n i n g
o p e r a t i o n b y a g g l o m e r a t i o n heap l e a c h i n g u s i n g t h e p r e s e n t f a c i l i t y .
5.9
CONCLUSION
A g g l o m e r a t i o n p r e t r e a t m e n t o f c r u s h e d o r e s and wastes, and f i n e l y g r o u n d
t a i l i n g s h a s been a p p l i e d b y t h e i n d u s t r y f o r t h e s u c c e s s f u l heap l e a c h i n g o f
f e e d s t h a t d i s p l a y p o o r p e r c o l a t i o n c h a r a c t e r i s t i c s . Many f e e d m a t e r i a l s
p e r c o l a t e we1 1 w i t h o u t a g g l o m e r a t i o n .
The need f o r a g g l o m e r a t i o n f o r a
p a r t i c u l a r f e e d s h o u l d be d e t e r m i n e d b e f o r e c o m m i t t i n g a commercial o p e r a t i o n
t o t h i s processing technology.
ORE PREPARATION: CRUSHING AND AGGLOMERATION
5.10
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