Bureau of Mines Information Circular/l9?8
Processing Gold Ores Using Heap
Leach-Carbon Adsorption Methods
UNITED STATES DEPARTMENT OF THE INTERIOR
Information Circular 8770
Processing Gold Ores Using Heap
Leach-Carbon Adsorption Methods
By H. J. Heinen, D. G. Peterson, and R. E. Lindstrom
UNITED STATES DEPARTMENT OF THE INTERIOR
Cecil D. Andrus, Secretary
BUREAU OF MINES
This publication has been cataloged as follows:
Heinen, Harold J
Processing gold ores using heap leachsarbon adsorption
methods / by H. J. Heinen, D. G. Peterson, and R. E.
Lindstrom. [Washington] : U.S. Dept. of the Interior. Bureau
of Mines, 1978.
21 p. : ill. ; 27 cm.
8770)
(Information circular
-
Bureau of Mines ;
Bibliography: p. 19-21.
-
1. Gold
Merallurgy. 2. Cyanide process. 3. Gold ores. I.
Peterson, David G., joinr author. 11. Lindstrom, Roald E., joinr
author. 111. United Srates. Bureau of Mines. 1%
.'
Title. V. Series: Unired States. Bureau of Mines. Informarion circular Bureau of Mines ; 8770.
-
TN23.U71
no. 8770
622.06173
U S . Depr. of the Inr. Library
CONTENTS
................................................................
............................................................
Abstract
Introduction
Chemistry of c y a n i d a t i o n .................................................
Mineralogy of gold o r e s ..................................................
Amenability t e s t i n g ......................................................
Cyanide heap-leach o p e r a t i o n s ............................................
G o l d - s i l v e r recovery from p r o c e s s s o l u t i o n s .........................
Economics of heap-leach o p e r a t i o n s ..................................
Cyanide handling and d i s p o s a l
Recent i n n o v a t i o n s i n g o l d - s i l v e r r e c o v e r y from cyanide p r o c e s s s o l u t i o n s
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References ...............................................................
............................................
ILLUSTRATIONS
Laboratory column l e a c h i n g u n i t ......................................
P i l o t - s c a l e heap leach ...............................................
Heap-leach c y a n i d a t i o n
Leaching o r e heaps by t h e pending method .............................
H y p o t h e t i c a l d i s t r i b u t i o n of gold i n a continuous carbonadsorption operation ...............................................
Recovery of gold from a c t i v a t e d carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Proposed t r e a t m e n t of cyanide e f f l u e n t s high i n s i l v e r c o n t e n t . . . . . . .
Proposed heap leach-carbon a d s o r p t i o n process f o r gold o r e s low
i n silver ..........................................................
...............................................
TABLE
Estimated c o s t of a commercial heap-leach c y a n i d a t i o n o p e r a t i o n
.
PROCESSING GOLD ORES USING HEAP LEACH-CARBON
ADSORPTION hiETHODS
by
H. J. Heinen,'
D. G. Peterson,' and R. E. Lindstrom2
ABSTRACT
This Bureau of Mines r e p o r t reviews t h e r e c e n t developments i n t h e heap
l e a c h i n g of gold o r e s w i t h d i l u t e cyanide s o l u t i o n s and t h e recovery of gold
and a s s o c i a t e d s i l v e r from t h e pregnant e f f l u e n t s by a carbon a d s o r p t i o n d e s o r p t i o n method. M i n e r a l o g i c a l c o n s i d e r a t i o n s , e x p e r i m e n t a l work, f i e l d
s t u d i e s , and commercial o p e r a t i o n s a r e d e s c r i b e d . Also p r e s e n t e d a r e r e c e n t
process advances developed by t h e Bureau, i n c l u d i n g t h e a p p l i c a t i o n of
(1) a l c o h o l a d d i t i v e s t o t h e c o n v e n t i o n a l h o t a l k a l i n e cyanide s t r i p s o l u t i o n
t o enhance gold d e s o r p t i o n from precious metal-laden a c t i v a t e d carbon, and
( 2 ) Na,S p r e c i p i t a t i o n of t h e s i l v e r from g o l d - s i l v e r cyanide s o l u t i o n s a s a
means of reducing t h e i n v e n t o r y of carbon r e q u i r e d f o r p r e c i o u s - m e t a l a d s o r p t i o n from heap l e a c h e f f l u e n t s and f o r t h e p r o d u c t i o n of l o w - s i l v e r b u l l i o n
from a c t i v a t e d carbon s t r i p p i n g s o l u t i o n s .
INTRODUCTION
Gold m i n e r a l i z a t i o n i s widespread i n t h e United S t a t e s . I n 1967 t h e
Bureau of Mines conducted a n engineering a p p r a i s a l of more t h a n 1,300 lode and
p l a c e r d e p o s i t s , r e p r e s e n t i n g a l m o s t a l l o f t h e N a t i o n ' s known gold r e s e r v e s , t o
determine t h e i r gold product p o t e n t i a l ,(28).3 These d e p o s i t s were estimated
t o c o n t a i n over 400 m i l l i o n ounces of g o l d ; however, only 2 pct of t h i s amount
was found t o be e x p l o i t a b l e a t $35 p e r ounce. A 400-pct i n c r e a s e i n t h e p r i c e
of gold w i t h i n a period of 1 o r 2 y e a r s caused a remarkable r e e v a l u a t i o n of
t h e type of m a t e r i a l t h a t c o n s t i t u t e s o r e . Low-grade m a t e r i a l s c o n t a i n i n g a s
l i t t l e a s 0.03 ounce of gold p e r ton, providing s u f f i c i e n t tonnage i s a v a i l a b l e , a r e now being f i e l d t e s t e d f o r precious-metal r e c o v e r y . The Bureau of
Mines and s e v e r a l mining companies have conducted e x t e n s i v e h y d r o m e t a l l u r g i c a l
s t u d i e s t o e x p l o i t low-grade o r e s and mine waste m a t e r i a l . The innovations
developed i n c l u d e heap-leach c y a n i d a t i o n followed by precious-metal r e c o v e r y
-
lMetallurgist.
" ~ u ~ e r v i s ochemical
r~
engineer.
Authors a r e with t h e Reno Metallurgy Research Center, Bureau of Mines, Reno,
Nev
3 ~ n d e r l i n e dnumbers i n parentheses r e f e r t o items i n t h e l i s t of r e f e r e n c e s a t
t h e end of t h i s r e p o r t .
.
from t h e r e s u l t a n t s o l u t i o n s by carbon a d s o r p t i o n o r by p r e c i p i t a t i o n on zinc,
and new procedures f o r more e f f i c i e n t s t r i p p i n g of gold from a c t i v a t e d carbon
and s e p a r a t e recovery of gold and a s s o c i a t e d s i l v e r from enriched s t r i p s o l u t i o n s . The purpose of t h i s r e p o r t i s t o review t h e s t a t e - o f - t h e - a r t o f heapl e a c h processing of p r e c i o u s metal o r e s and t h e a p p l i c a t i o n of g r a n u l a r
a c t i v a t e d c h a r c o a l f o r precious-metal recovery from cyanide s o l u t i o n s .
Heap l e a c h i n g may be d e f i n e d a s t h e p e r c o l a t i o n leaching of p i l e s o f lowg r a d e o r e s o r mine waste t h a t have been stacked o r p i l e d on s p e c i a l l y prepared
w a t e r t i g h t drainage pads f o r pregnant l i q u o r c o l l e c t i o n . This p r o c e s s i n g conc e p t d a t e s back t o about 1752 when t h e Spanish miners p e r c o l a t e d a c i d s o l u t i o n s through l a r g e heaps of oxide copper o r e on t h e banks of t h e Rio T i n t o
R i v e r . Since t h e n , t h i s p r o c e s s has been used e x t e n s i v e l y throughout t h e
world t o leach oxide copper v a l u e s from mine s t r i p m a t e r i a l from open p i t mini n g of porphyry copper d e p o s i t s . Uranium producers have a l s o used heap l e a c h i n g s i n c e t h e l a t e 1950's f o r e x t r a c t i n g uranium from sub-mill-grade o r e s and
f o r e x p l o i t i n g r e l a t i v e l y s m a l l low-grade o r e d e p o s i t s . Heap-leach cyanidat i o n i s a comparatively r e c e n t development suggested by Bureau of Mines m e t a l l u r g i s t s i n 1967 a s a p o t e n t i a l l o w - c a p i t a l means f o r p r o c e s s i n g limestone and
dolomite s i l t s t o n e - t y p e o r e s c o n t a i n i n g submicrometer p a r t i c l e s of gold i n a
-* -a19 22).
porous h o s t r o c k (15
CHEMISTRY OF CYANIDATION
The b a s i c p r i n c i p l e of t h e c y a n i d a t i o n process i s t h a t weak a l k a l i n e
c y a n i d e s o l u t i o n s have a p r e f e r e n t i a l d i s s o l v i n g a c t i o n on t h e gold and s i l v e r
c o n t a i n e d i n a n o r e . The r e a c t i o n ( E l s n e r ' s e q u a t i o n ) g e n e r a l l y accepted f o r
s e v e r a l decades a s r e p r e s e n t i n g t h e d i s s o l u t i o n of gold by cyanide s o l u t i o n i s
Recent r e s e a r c h on t h e mechanism of c y a n i d a t i o n , however, i n d i c a t e s t h i s r e a c p . 305). Most of t h e gold d i s s o l v e s by t h e
t i o n proceeds i n two s t a g e s
r e a c t ion
(1,
and a s m a l l b u t s i g n i f i c a n t p r o p o r t i o n d i s s o l v e s v i a r e a c t i o n 1. The gold
d i s s o l u t i o n r a t e i s dependent on t h e c o n c e n t r a t i o n of NaCN and t h e a l k a l i n i t y
o f t h e s o l u t i o n , t h e optimum pH being 1 0 . 3 ( 3 ) . For e f f i c i e n t leaching, t h e
g o l d should occur a s f r e e , f i n e - s i z e , c l e a n p a r t i c l e s i n an o r e t h a t c o n t a i n s
no "cyanicides" o r i m p u r i t i e s t h a t might d e s t r o y cyanide o r otherwise i n h i b i t
t h e d i s s o l u t i o n r e a c t i o n . An adequate supply of d i s s o l v e d oxygen must be
p r e s e n t i n t h e cyanide s o l u t i o n throughout t h e r e a c t i o n p e r i o d .
The chemistry involved i n t h e d i s s o l u t i o n of gold i n t h e heap-leach c y a n i d a t i o n treatment i s t h e same a s t h a t f o r t h e a g i t a t i o n - c y a n i d a t i o n p r o c e s s .
I n heap leaching, t h e oxygen, e s s e n t i a l f o r t h e d i s s o l u t i o n of gold, i s i n t r o duced i n t o t h e cyanide s o l u t i o n a s i t i s being s p r i n k l e d upon t h e o r e heap.
The adsorbed oxygen and carbon d i o x i d e from t h e a i r may a l s o cause chemical
l o s s e s of cyanide according t o t h e following r e a c t i o n s :
NaCN
+
CO,
+
H20
-
HCN
+
NaHCO,.
(4
I n heap l e a c h i n g h i g h l y oxidized o r e s , t h e decomposition of cyanide by carbon
dioxide may be a s g r e a t a s t h a t caused by t h e a c i d c o n s t i t u e n t s of t h e o r e .
The decomposition of cyanide by carbon d i o x i d e , a s w e l l a s by ground a c i d s , i s
minimized by u s i n g s u f f i c i e n t a l k a l i such a s lime (CaO) o r c a u s t i c soda (NaOH)
i n t h e l e a c h s o l u t i o n t o m a i n t a i n t h e a l k a l i n i t y a t a pH r a n g e of 9 t o 11.
The m i n e r a l c o n s t i t u e n t s of t h e o r e and o t h e r f o r e i g n substances c a n
i n f l u e n c e t h e c y a n i d a t i o n process i n a number of d i f f e r e n t ways
pp. 284351; 1 2 ) . S i l v e r normally occurs w i t h g o l d e i t h e r a s a n i m p u r i t y i n t h e p a r t i c l e r o f n a t i v e gold o r a s s i l v e r - b e a r i n g m i n e r a l s . The occurrence o f s i l v e r
i n gold o r e s may range from l e s s t h a n 1 ppm t o s e v e r a l hundred times t h e l e v e l
of t h e gold p r e s e n t i n a n o r e . Generally, s i l v e r i s a l s o d i s s o l v e d by t h e
cyanide s o l u t i o n and follows t h e gold through t h e p r o c e s s sequence. I r o n s u l f i d e m i n e r a l s , which a r e common c o n s t i t u e n t s o f gold o r e s , a r e o x i d i z e d t o
some e x t e n t d u r i n g t h e cyanide leach, t h u s r e s u l t i n g i n t h e formation of a c i d .
These a c i d s a r e n e u t r a l i z e d by t h e lime used i n t h e cyanide l e a c h sequence.
Copper m i n e r a l s may be d i s s o l v e d by t h e cyanide leach s o l u t i o n and t h u s consume l a r g e q u a n t i t i e s of NaCN and oxygen. Arsenic-bearing m i n e r a l s may a l s o
i n t e r f e r e w i t h c y a n i d a t i o n . Realgar (Asas,) and orpiment (As,S,) r e a c t
r a p i d l y w i t h t h e cyanide s o l u t i o n and i n h i b i t t h e d i s s o l u t i o n o f g o l d .
Arsenopyrite (FeAsS), however, g e n e r a l l y o x i d i z e s v e r y slowly i n a n a e r a t e d
cyanide s o l u t i o n and has v e r y l i t t l e a d v e r s e e f f e c t on t h e l e a c h i n g of g o l d .
S t i b n i t e (Sb2S,) s t r o n l y i n h i b i t s c y a n i d a t i o n . The presence of b a s e m e t a l
ions such a s Fea+, Fe3 , ~ i " ,cu2+, 2n2+, and ~ n i n~ t h+e cyanide l e a c h s o l u t i o n w i l l r e t a r d t h e c y a n i d a t i o n of g o l d . I n some c a s e s t h e n a t u r a l l y occurr i n g carbonaceous m a t e r i a l s i n sedimentary-type gold o r e s a c t a s a d s o r b e n t s
f o r t h e gold d i s s o l v e d by cyanide s o l u t i o n s . Charred mine timbers have simil a r p r o p e r t i e s and cause premature p r e c i p i t a t i o n of g o l d . Organic substances
such a s decayed wood, o i l , g r e a s e , and f l o t a t i o n r e a g e n t s slow down cyanidat i o n of gold by consuming t h e d i s s o l v e d oxygen i n t h e l e a c h s o l u t i o n and a l s o
i n h i b i t s subsequent gold recovery from l e a c h s o l u t i o n by p r e c i p i t a t i o n of t h e
gold on z i n c d u s t .
(1,
I:
Generally, gold and s i l v e r a r e recovered from pregnant cyanide s o l u t i o n s
e i t h e r by p r e c i p i t a t i o n on z i n c d u s t o r by a d s o r p t i o n on a c t i v a t e d carbon.
For p r e c i p i t a t i o n of gold on z i n c , c l a r i f i c a t i o n of t h e pregnant s o l u t i o n i s
required t o e l i m i n a t e t h e suspended c l a y e y c o n s t i t u e n t s t h a t can c o a t t h e zinc
p a r t i c l e s and r e t a r d p r e c i p i t a t i o n of t h e p r e c i o u s m e t a l s . E l i m i n a t i o n of t h e
dissolved oxygen from t h e pregnant s o l u t i o n i s e s s e n t i a l t o p r e v e n t t h e r e d i s s o l u t i o n of t h e p r e c i p i t a t e d gold v i a r e a c t i o n 1 o r 2, and e x c e s s i v e z i n c
consumption through i t s i n t e r a c t i o n w i t h t h e oxygen remaining i n s o l u t i o n .
The p r e c i p i t a t i o n of gold on z i n c i s g r e a t l y improved by adding s o l u b l e lead
s a l t s , such a s l e a d a c e t a t e o r lead n i t r a t e , t o cyanide s o l u t i o n s t o form a
zinc-lead couple o f g r e a t e r a c t i v i t y . The r e a c t i o n f o r p r e c i p i t a t i p n of gold
by z i n c may be r e p r e s e n t e d by t h e f o l l o w i n g e q u a t i o n :
Na2Zn(CN),
+
Au
+
1/2H,
+ NaOH.
(5)
Activated carbon has t h e c a p a b i l i t y of adsorbing t h e gold cyanide complex
from c y a n i d a t i o n r e a c t i o n pulps and u n c l a r i f i e d cyanide e f f l u e n t s , t h e r e b y
e l i m i n a t i n g t h e l i q u i d - s o l i d s s e p a r a t i o n , c l a r i f i c a t i o n , and d e a e r a t i o n p r o c e s s i n g s t e p s t h a t a r e employed i n a n a l l s l i m e 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 z i n c p r e c i p i t a t i o n p l a n t . Although a c t i v a t e d carbon has been used i n
g o l d - s i l v e r r e c o v e r y from cyanide s o l u t i o n s f o r s e v e r a l decades, t h e mechanism
of gold a d s o r p t i o n on a c t i v a t e d carbon i s s t i l l n o t f u l l y understood. One
t h e o r y i s t h a t t h e gold i s adsorbed a s t h e gold cyanide complex AU(CN)~-and
t h a t t h i s a d s o r p t i o n t a k e s p l a c e by i o n exchange (1,p. 341). Some f a c t s t h a t
support t h i s t h e o r y a r e (1) CN- ions a r e a l s o adsorbed by t h e a c t i v a t e d c a r bon, (2) a d s o r p t i o n of AU(CN)~-i o n i s n o t accompanied by a d s o r p t i o n of a d
e q u i v a l e n t q u a n t i t y of caa+ o r ~ a ions
+
from s o l u t i o n , and ( 3 ) t h e adsorbed
g o l d can be desorbed o r d i s p l a c e d w i t h OH- i o n s provided by h o t a l k a l i n e s o l u t i o n s . A r e c e n t s t u d y (7) i n d i c a t e d t h a t t h e gold adsorbed o n t o a c t i v a t e d
carbon i s e i t h e r i n t h e calcium aurocyanide or hydrogen aurocyanide form,
depending on t h e pH of t h e cyanide s o l u t i o n and t h e c o n c e n t r a t i o n and c h a r a c t e r of t h e " s p e c t a t o r " c a t i o n s p r e s e n t .
MINERALOGY OF GOLD ORES
The manner o f gold occurrence and i t s a s s o c i a t i o n w i t h t h e gangue minera l s d i c t a t e whether o r n o t t h e o r e can be processed by t h e cyanide heap-leach
method. From a m e t a l l u r g i c a l s t a n d p o i n t , gold o r e s may be roughly c l a s s i f i e d
a s (1) simple o x i d e o r e s c o n t a i n i n g f i n e p a r t i c l e s of n a t i v e gold i n a q u a r t z
o r limestone gangue, (2) simple s u l f i d e o r e s i n which t h e gold i s a s s o c i a t e d
~ i t minor
h
amounts of p y r i t e o r a r s e n o p y r i t e , (3) p l a c e r o r alluvium m a t e r i a l ,
( 4 ) complex o r r e f r a c t o r y o r e s i n which t h e gold-bearing m i n e r a l s p e c i e s a r e
n o t r e a d i l y s o l u b l e i n cyanide s o l u t i o n , (5) complex b a s e m e t a l o r e s i n which
t h e precious m e t a l s a r e important economic c o n s t i t u e n t s , and (6) base m e t a l
o r e s i n which t h e p r e c i o u s metals a r e of minor v a l u e and a r e byproducts o f
t r e a t m e n t (27). Of t h e s e , only t h e simple oxide and s u l f i d e o r e s and c e r t a i n
p l a c e r s a r e s u i t a b l e f o r heap l e a c h i n g . These m a t e r i a l s must p o s s e s s t h e f o l lowing c h a r a c t e r i s t i c s : (1) Gold and s i l v e r v a l u e s a r e l e a c h a b l e by cyanidat i o n , (2) s i z e of t h e gold p a r t i c l e s i s extremely s m a l l , ( 3 ) t h e h o s t r o c k i s
porous t o cyanide s o l u t i o n , and remains permeable during t h e r e l a t i v e l y long
l e a c h c y c l e , (4) gold p a r t i c l e s i n o r e s o f low p o r o s i t y a r e l i b e r a t e d o r
exposed by f r a c t u r i n g and c r u s h i n g , (5) t h e o r e i s f r e e o f carbonaceous mater i a l which has t h e c a p a b i l i t y of adsorbing gold cyanide and c a u s e s premature
p r e c i p i t a t i o n of t h e gold, (6) t h e o r e i s r e l a t i v e l y f r e e of "cyanicides" o r
substances t h a t d e s t r o y cyanide o r i n t e r f e r e with t h e g o l d - c y a n i d a t i o n r e a c t i o n , such a s p a r t i a l l y oxidized s u l f i d e s of antimony, z i n c , i r o n , copper, and
a r s e n i c c o n t a i n i n g m i n e r a l s , (7) t h e o r e does n o t c o n t a i n e x c e s s i v e amounts of
"fines" o r c l a y e y c o n s t i t u e n t s t h a t w i l l impede s o l u t i o n p e r c o l a t i o n , and
( 8 ) t h e o r e i s r e l a t i v e l y f r e e of acid-forming c o n s t i t u e n t s t h a t cause high
l i m e consumption.
Types of gold-bearing d e p o s i t s found amenable t o heap leaching a r e a s
follows:
(1) Limey s i l t s t o n e c o n t a i n i n g submicrometer-size gold p a r t i c l e s and
minute amounts of p y r i t e , g a l e n a , c i n n a b a r , and s t i b n i t e , ( 2 ) s i l i c i f i e d s i l t s t o n e s c o n t a i n i n g micrometer-size p a r t i c l e s o f g o l d , o f t e n a s s o c i a t e d w i t h
r e s i d u a l i r o n oxides, (3) sanded dolomite o r e i n which f i n e gold p a r t i c l e s
occur on t h e i n t e r g r a n u l a r s u r f a c e s , ( 4 ) v e i n q u a r t z o r e i n which gold occurs
i n l i m o n i t e - r i c h c a v i t i e s and f r a c t u r e s , ( 5 ) igneous h o s t m a t e r i a l c u t w i t h
small q u a r t z v e i n s c o n t a i n i n g f r e e gold and minor amounts of p y r i t e , ( 6 ) a
s c h i s t c o n t a i n i n g f r e e gold i n t h e l a m i n a t i o n of t h e r o c k s , and (7) s i l i c e o u s
q u a r t z s i n t e r of h o t - s p r i n g o r i g i n .
AMENABILITY TESTING
Pump
Barren cyanide
solution
FIGURE 1. - Laboratory colunin leaching u n i t .
Even though t h e minera l o g i c a l s t u d i e s may i n d i c a t e t h a t t h e gold
mineralization i s associated
w i t h h o s t rocks t h a t a r e
s u i t a b l e f o r heap l e a c h i n g ,
it i s prudent t o conduct l a b o r a t o r y and p i l o t t e s t s f o r
verification.
The t e c h n o l ogy f o r determining whether
o r n o t a gold o r e i s amena b l e t o p e r c o l a t i o n leaching
w i t h weak cyanide s o l u t i o n s
is well established. I n i t i a l l y , b o t t l e cyanidation
t e s t s a r e conducted on 200t o 500-gram charges of
ground o r e t o determine t h e
degree o f gold e x t r a c t i o n
and r e a g e n t consumption. I f
t h e gold c o n t e n t of t h e o r e
i s leachable a t a r e l a t i v e l y
c o a r s e g r i n d , column p e r c o l a t i o n t e s t s a r e made on o r e
crushed t o v a r i o u s s i z e s ,
ranging from minus 2- t o
minus 1/4-inch f e e d . The
t e s t apparatus i s shown i n
f i g u r e 1. About 60 pounds
of crushed o r e i s mixed with
s u f f i c i e n t lime t o provide
p r o t e c t i v e a l k a l i n i t y during
c y a n i d a t i o n . This ore-lime
mixture i s t r a n s f e r r e d t o a
column about 6 inches i n
diameter t o make a bed about
!, f e e t h i g h .
Twelve l i t e r s
of s o l u t i o n c o n t a i n i n g 1 . 0
pound NaCN per t o n i s pumped t o t h e t o p of t h e column, and t h e flow r a t e i s
c o n t r o l l e d so t h a t t h e s o l u t i o n t r i c k l e s slowly downward through t h e o r e . The
pregnant e f f l u e n t from t h e column i s t h e n passed upward through t h r e e s m a l l
columns c o n t a i n i n g g r a n u l a r a c t i v a t e d carbon and placed i n s e r i e s . The b a r r e n
s o l u t i o n from t h e a d s o r p t i o n columns i s r e c y c l e d f o r a d d i t i o n a l l e a c h i n g a f t e r
making any n e c e s s a r y adjustments i n t h e cyanide and lime c o n c e n t r a t i o n . Leachi n g i s continued u n t i l no f u r t h e r s i g n i f i c a n t amount of gold i s e x t r a c t e d from
t h e o r e . P i l o t - s c a l e heap-leach t e s t s may be conducted on a tonnage s c a l e i f
i t i s d e s i r a b l e t o confirm r e s u l t s obtained i n l a b o r a t o r y column-percolation
The p e r c o l a t i o n
t e s t s , or t o f u r t h e r q u a n t i f y r e a g e n t requirements ( f i g . 2 ) .
r a t e of s o l u t i o n through t h e o r e can b e s t be measured by conducting tonnages c a l e experiments i n columns c o n t a i n i n g a bed of o r e 15 t o 20 f e e t high.
FIGURE 2.
-
Pilot-scale heap leach.
CYANIDE HEAP -LEACH OPERATIONS
Cyanide heap l e a c h i n g i s a comparatively r e c e n t h y d r o m e t a l l u r g i c a l d e v e l opment f o r e x p l o i t i n g low-grade gold o r e s , mine waste m a t e r i a l , o r d e p o s i t s
too s m a l l t o j u s t i f y c o n s t r u c t i o n of m i l l i n g f a c i l i t i e s . Although t h e process
i s analogous t o heap l e a c h i n g of copper o r e s , i t i s s t i l l considered t o be i n
t h e developmental s t a g e because many p r o c e s s v a r i a b l e s i n f l u e n c i n g s o l u t i o n
p e r c o l a t i o n and d i s s o l u t i o n of t h e gold a r e n o t f u l l y understood. Each comm e r c i a l i n s t a l l a t i o n has developed an o p e r a t i n g technique f o r pad and heap
p r e p a r a t i o n and leaching procedure t h a t meets t h e requirements of t h e o r e .
The technique s e l e c t e d depends l a r g e l y on t h e manner of occurrence o f t h e
gold, p h y s i c a l c h a r a c t e r i s t i c s and m i n e r a l composition of t h e o r e , and t h e
s c a l e of o p e r a t i o n .
A l l commercial cyanide heap-leach o p e r a t i o n s a r e being conducted on mater i a l stacked on impermeable pads. W a t e r t i g h t pads o r bases a r e r e q u i r e d t o
c o l l e c t t h e pregnant s o l u t i o n s and t o e l i m i n a t e t h e p o s s i b i l i t y of l o s i n g gold
and s i l v e r cyanide s o l u t i o n s t o t h e ground and contaminating l o c a l streams and
underground water r e s o u r c e s . The types o f m a t e r i a l s used f o r c o n s t r u c t i n g t h e
impervious pads i n c l u d e (1) compacted t a i l i n g s mixed w i t h b e n t o n i t e ,
(2) a s p h a l t ( b l a c k t o p ) o r l i g n i n s u l f o n a t e mix placed on compacted g r a v e l and
covered w i t h a n a s p h a l t s e a l e r , ( 3 ) r e i n f o r c e d c o n c r e t e p a d s , and (4) p l a s t i c
or rubber s h e e t i n g l a i d on a smooth excavated a r e a and covered w i t h 2 t o 3
f e e t of washed sand and g r a v e l .
B a s i c a l l y , two methods of heap-leach c y a n i d a t i o n a r e used commercially:
(1) s h o r t - t e r m l e a c h i n g of crushed o r e , and ( 2 ) long-term l e a c h i n g of r u n - o f mine m a t e r i a l .
\
I n p r a c t i c i n g s h o r t - t e r m , heap-leach c y a n i d a t i o n , t h e o r e i s crushed t o a
small s i z e , s t a c k e d 4 t o 8 f e e t high on permanent pads, each w i t h a c a p a c i t y
ranging from 1,000 t o 10,000 t o n s , and leached by s p r i n k l i n g t h e top of t h e
heap w i t h d i l u t e cyanide s o l u t i o n ( f i g . 3 ) . Cyanide s o l u t i o n p e r c o l a t e s
through t h e heap d i s s o l v i n g t h e gold and s i l v e r v a l u e s .
I t i s subsequently
c o l l e c t e d on t h e w a t e r t i g h t pad, which i s sloped t o permit t h e pregnant s o l u t i o n t o flow i n t o channels f o r t r a n s f e r t o a s t o r a g e pond o r tank. The feed
m a t e r i a l i s crushed t o a s i z e t h a t w i l l g i v e good l i b e r a t i o n o r exposure of
the gold m i n e r a l i z a t i o n t o cyanide s o l u t i o n and s t i l l o b t a i n r e a s o n a b l e percolation rates.
I n t y p i c a l o p e r a t i o n s , o r e s a r e crushed t o minus t h r e e - f o u r t h s
inch, and a s f i n e a s minus one-fourth i n c h i n t h e c a s e of a gold-quartz o r e .
The leach c y c l e i s measured i n days, g e n e r a l l y from 7 t o 30. When t h e leach
cycle i s completed, t h e waste i s removed from t h e pad and a new b a t c h of
crushed o r e i s a p p l i e d . The C a r l i n Gold Mining Co. (21) s t a r t e d t h i s t y p e of
leaching on mine c u t o f f m a t e r i a l crushed t o t h r e e - f o u r t h s inch i n 1971. The
Smoky Valley Mining Co. has completed t h e i n s t a l l a t i o n of a 8,000-tonlday
short-term heap leach-carbon a d s o r p t i o n - e l e c t r o w i n n i n g f a c i l i t y a t Round
29).
Mountain t h a t went on-stream December 1976 ( Long-term l e a c h i n g i s used p r i m a r i l y t o e x t r a c t gold from uncrushed,
porous, sub-mill-grade m a t e r i a l from open p i t o p e r a t i o n s . The o r e charges
a r e run-of-mine m a t e r i a l generated by b l a s t i n g , and may c o n t a i n some l a r g e
Barren
cyanide
solution
Pregnant
cyanide
effluent
FIGURE 3.
- Heap-leach cyanidation.
boul:l..rs, b u t most of t h e feed i s minus 6 inches i n s i z e . The tonnage of t h e
heaps under t r e a t m e n t u s u a l l y ranges from 10,000 t o 2,000,000 t o n s . Most
l e a c h heaps a r e shaped t o resemble a t r u n c a t e d pyramid 20 t o 30 f e e t high, a s
d e p i c t e d i n f i g u r e 3 . The h e i g h t i s governed by f a c t o r s such a s o r e permeab i l i t y and t h e maintenance of p r o t e c t i v e a l k a l i n i t y , cyanide s t r e n g t h , and
s u f f i c i e n t d i s s o l v e d oxygen i n t h e l e a c h s o l u t i o n a s i t p e r c o l a t e s downward
through t h e heap. The leach c y c l e i s measured i n months. These heaps a r e
leached u n t i l i t i s no longer p r o f i t a b l e t o c o n t i n u e t h e o p e r a t i o n , which may
be f o r y e a r s . Upon t e r m i n a t i o n of t h e l e a c h , t h e r e s i d u e i s l e f t on t h e pad.
Cortez Gold Mines has completed heap l e a c h i n g approximately 2 m i l l i o n t o n s of
Cortez operated t h e f i r s t
run-of-mine c u t o f f m a t e r i a l (8-9; 20, pp. 40-42).
known i n t e g r a t e d heap-leach cyanidation-carbon a d s o r p t i o n - e l e c t r o w i n n i n g p l a n t
a t i t s Gold Acres p r o p e r t y , about 8 m i l e s from t h e main cyanide p l a n t .
Generally, t h e cyanide l e a c h s o l u t i o n s a r e introduced o n t o t h e heaps by
s p r a y i n g from p e r f o r a t e d p l a s t i c p i p e s , by s p r i n k l i n g u s i n g p l a s t i c s p r i n k l e r
heads, o r by ponding. Ty i c a l r a t e s of a p p l i c a t i o n of t h e l e a c h s o l u t i o n s
r a n g e from 5 t o 75 g a l / f t of s u r f a c e a r e a p e r day. Pond l e a c h i n g , a s i l l u s t r a t e d i n f i g u r e 4 , was a p p l i e d s u c c e s s f u l l y by t h e Idaho Mining Corp. (30)
t o e x t r a c t t h e r e a d i l y l e a c h a b l e gold v a l u e s from a sanded dolomite o r e t h a t
n
e x h i b i t s slow 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 . one hundred and s i x t y thousand t o n s
of o r e was stacked about 25
f e e t h i g h and l e v e l e d ; t h e
f l a t s u r f a c e of t h e p i l e was
d i v i d e d by s m a l l d i k e s i n t o
n i n e ponds t o f a c i l i t a t e
c o n t r o l of l e a c h i n g s o l u t i o n s . The heap was washed
w i t h b a r r e n s o l u t i o n s from
t h e carbon a d s o r p t i o n p l a n t .
Barren
cyanide
solution
In commercial a p p l i c a tion, solution strengths
range from 0 . 5 t o 1 . 0 pound
of sodium cyanide p e r t o n of
s o l u t i o n , and t h e pH i s
maintained a t about 10 w i t h
t h e a p p l i c a t i o n of lime
(CaO) o r c a u s t i c soda (NaOH)
Impervious pad
Pregnant
cyanide
effluent
Gold-Silver Recovery From
Process S o l u t i o n s
- Leaching ore heaps by the ponding method.
Based on t h e a v a i l a b i l i t y of e x i s t i n g c o n v e n t i o n a l
cvanide m i l l f a c i l i t i e s w i t h
a z i n c p r e c i p i t a t i o n c i r c u i t , t h e gold and s i l v e r contained i n heap-leach
pregnant s o l u t i o n s may be recovered from s o l u t i o n s by c o n v e n t i o n a l M e r r i l l Crowe p r e c i p i t a t i o n . I f m i l l f a c i l i t i e s a r e n o t a v a i l a b l e o r i f t h e concent r a t i o n of gold i n s o l u t i o n i s below a nominal 0.05-0z/t0n s o l u t i o n , t h e
p r e f e r r e d method f o r r e c o v e r i n g t h e p r e c i o u s metal v a l u e s from heap-leach
e f f l u e n t s i s by a d s o r p t i o n on a c t i v a t e d carbon. Not only i s t h e carbon
a d s o r p t i o n process more e f f i c i e n t t h a n z i n c p r e c i p i t a t i o n f o r t r e a t i n g d i l u t e
g o l d - s i l v e r cyanide s o l u t i o n s , b u t t h e method a l s o e n t a i l s lower c a p i t a l and
operating c o s t s (5-5,24).
FIGURE 4.
The carbon a d s o r p t i o n method became p r a c t i c a l on development of a u s a b l e
method f o r s t r i p p i n g t h e p r e c i o u s metal v a l u e s from t h e a c t i v a t e d carbon s o it
could be r e c y c l e d i n t h e system. Today, t h e u s e of a c t i v a t e d carbon i n con...junction w i t h heap l e a c h i n g i s a simple, economical p r o c e s s t h a t i s s u i t a b l e
f o r e x p l o i t i n g l e a n r e s o u r c e s o r s m a l l o r e b o d i e s . The c a p i t a l investment f o r
t h i s p r o c e s s i s e s t i m a t e d t o b e about 20 t o 25 p c t of t h e c o s t of a convent i o n a l 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 cyanide p l a n t , and o p e r a t i n g c o s t s a r e about
40 p c t (6-5, 24) of a c o n v e n t i o n a l cyanide p l a n t .
I n t y p i c a l continuous carbon-adsorption o p e r a t i o n , t h e gold cyanide
e f f l u e n t s from heap leaching a r e pumped upward through from t h r e e t o f i v e
columns o r tanks i n s e r i e s which c o n t a i n g r a n u l a r a c t i v a t e d carbon. The
harder carbons, manufactured from coconut s h e l l s , a r e p r e f e r r e d f o r u s e
because t h e r e i s l e s s tendency f o r breakage o r a b r a s i o n . The amount of gold
and s i l v e r t h a t can be loaded on t h e carbon w i l l v a r y g r e a t l y , depending on
t h e t y p e of o r e and t h e t e n o r of t h e cyanide l e a c h s o l u t i o n . Some f a c t o r s
t h a t i n f l u e n c e t h e loading a r e ( 1 ) t h e c o n c e n t r a t i o n of gold and s i l v e r i n
cyanide l e a c h s o l u t i o n s , ( 2 ) r a t i o of gold t o s i l v e r , ( 3 ) pH of leach s o l u t i o n , (4) c o n c e n t r a t i o n of i m p u r i t i e s , (5) flow r a t e , and ( 6 ) t y p e and p a r t i c l e s i z e of g r a n u l a r carbon employed. T y p i c a l loadings o b t a i n e d
commercially range from 200 t o 800 ounces of gold, o r combination of gold and
s i l v e r , per t o n of carbon. However, t h e a d s o r p t i o n phenomena i s n o t
r e s t r i c t e d t o gold and s i l v e r cyanide complexes. A c t i v a t e d carbon i s c a p a b l e
of adsorbing a l a r g e v a r i e t y of o r g a n i c s u b s t a n c e s , and i n o r g a n i c c o n s t i t u e n t s
s u c h a s s i l i c i c a c i d , and m e t a l l i c i o n s such a s calcium, copper, n i c k e l , merc u r y , and i r o n , t h u s r e d u c i n g t h e number of s i t e s a v a i l a b l e f o r t h e p r e c i o u s
m e t a l s . The p r e f e r r e d pH of t h e l e a c h s o l u t i o n t o o b t a i n e f f i c i e n t gold
a d s o r p t i o n ranges from 9 t o 11. The c o n c e n t r a t i o n of f r e e cyanide i n t h e
l e a c h s o l u t i o n should be maintained a t a v a l u e t h a t i s a s low a s p o s s i b l e and
s t i l l be c o n s i s t e n t w i t h s a t i s f a c t o r y metal e x t r a c t i o n from t h e o r e . F i n e r
s i z e carbon p a r t i c l e s adsorb g r e a t e r amounts of gold and a t a f a s t e r r a t e t h a n
c o a r s e carbons; however, t h e mesh s i z e should n o t be so s m a l l t h a t it r e s u l t s
i n p a r t i c u l a t e carbon l o s s e s t o t h e process s o l u t i o n s d u r i n g t h e a d s o r p t i o n
s t e p . The a d s o r p t i v e c a p a c i t y o f a c t i v a t e d carbon manufactured from coconut
s h e l l s f o r gold cyanide i s g e n e r a l l y g r e a t e r t h a n t h a t of carbons prepared
from petroleum coke, wood, o r c o a l . Also, h i g h e r c o n c e n t r a t i o n s of gold i n
pregnant cyanide s o l u t i o n s produces h i g h e r gold loading on t h e carbon. Simil a r l y , h i g h e r gold loading on a c t i v a t e d carbon and t h e p r o d u c t i o n of lower
gold-bearing b a r r e n s a r e achieved by using lower flow r a t e s .
There a r e two methods f o r loading a c t i v a t e d carbon. I n one method, t h e
g o l d - b e a r i n g cyanide s o l u t i o n i s p e r c o l a t e d downward through a f i x e d bed of
a c t i v a t e d carbon. I n t h e o t h e r , t h e pregnant cyanide s o l u t i o n i s pumped
upward through t h e carbon a t a v e l o c i t y s u f f i c i e n t t o m a i n t a i n t h e bed o f c a r bon i n a f l u i d i z e d s t a t e o r suspended i n t h e s o l u t i o n stream without being
c a r r i e d o u t of t h e system. The choice of loading technique depends on t h e
amount of t u r b i d i t y or s l i m e s p r e s e n t i n t h e heap-leach e f f l u e n t s . The f i x e d
beds o r gacked carbon columns a r e l i m i t e d t o a maxirnum s o l u t i o n flow of about
5 gpmlft of c r o s s - s e c t i o n a l a r e a . The feed s o l u t i o n s must be f r e e of p a r t i c u l a t e m a t t e r because t h e bed of carbon behaves a s a s a n d - f i l t e r and w i l l
e v e n t u a l l y become plugged i f s l i m e s a r e p r e s e n t . The o n l y advantage of t h e
fixed-bed carbon i s t h a t t h e amount of carbon r e q u i r e d i s l e s s t h a n t h a t
r e q u i r e d f o r a f l u i d i z e d system processing t h e same amount of s o l u t i o n .
The f l u i d i z e d - b e d a d s o r p t i o n system i s g e n e r a l l y used i n commercial
p r a c t i c e f o r adsorbing gold cyanide v a l u e s from u n c l a r i f i e d l e a c h s o l u t i o n s
c o n t a i n i n g minor amounts of s l i m e s . Four important process v a r i a b l e s t o be
c o n s i d e r e d i n t h e d e s i g n of a f l u i d i z e d bed a c t i v a t e d carbon loading system
a r e (1) flow r a t e of t h e feed s o l u t i o n , (2) average d a i l y production of
p r e c i o u s metals, ( 3 ) maximum amount of gold t h a t can be loaded on t h e carbon,
and (4) t h e p a r t i c l e s i z e of carbon employed. Item 1 i s based on t h e volume
of pregnant s o l u t i o n generated d a i l y from t h e heap-leach o p e r a t i o n .
Item 2
may be based on a weighed maximum gold c o n c e n t r a t i o n i n s o l u t i o n s obtained
from s e v e r a l heaps placed i n o p e r a t i o n a t s t a g e d i n t e r v a l s . The inventory of
I
carbon r e q u i r e d i n t h e p l a n t and t h e s i z e of a d s o r p t i o n and s t r i p p i n g equipment can be minimized by u t i l i z i n g t h e a d s o r p t i v e c a p a c i t y of t h e carbon f o r
gold t o t h e f u l l e s t e x t e n t t h a t i s p r a c t i c a l . A loading of 400 ounces of
p r e c i o u s metals per t o n i s considered d e s i r a b l e by t h e i n d u s t r y . However, i f
t h e incoming s o l u t i o n c o n t a i n s l e s s than 0.05 ounce of gold per ton, t h e l o a d i n g u s u a l l y w i l l n o t exceed 200 o z / t o n of carbon. The p a r t i c l e s i z e of carbon
used c o m e r c i a l l y i n heap-leach c y a n i d a t i o n i s e i t h e r minus 6 p l u s 16 o r minus
12 p l u s 30 mesh. Generally, t h e s o l u t i o n flow r a t e r e q u i r e d t o m a i n t a i n
f l u i d i z a t i o n i n a bed of carbon c o n t a i n i n g minus 6- p l u s 16-mesh p a r t i c l e s i s
about 25 gpm/fta of c r o s s - s e c t i o n a l a r e a o f t h e column, whereas f o r t h e minus
12- p l u s 30-mesh carbon, t h e r e q u i r e d flow r a t e i s 15 gpm/f$.
Under t h e s e
c o n d i t i o n s , t h e carbon bed expands about 50 p c t . The depth of t h e bed of c a r bon a t r e s t should n o t be more t h a n t h r e e times t h e diameter o f t h e column.
The h e i g h t of t h e column should be 2.5 t o 3 times t h e h e i g h t of t h e carbon a t
r e s t t o allow f o r proper expansion of t h e bed and t o provide enough f r e e board
t o a l l o w f o r s o l u t i o n s u r g e s . The amount of carbon r e q u i r e d f o r a f l u i d i z e d bed a d s o r p t i o n system employing minus 12- p l u s 30-mesh carbon p a r t i c l e s i s
about 10 times t h e d a i l y amount of carbon s t r i p p e d . For t h e minus 6- p l u s 16mesh carbon, t h e carbon i n v e n t o r y should be about 15 times t h e d a i l y amount of
carbon s t r i p p e d because of higher s o l u t i o n v e l o c i t y and s h o r t e r c o n t a c t time.
I n d u s t r i a l experience has shown t h a t t h e carbon charge should be e q u a l l y d i s t r i b u t e d through f o u r o r f i v e columns o r tanks i n s e r i e s f o r a n e f f i c i e n t c a r bon a d s o r p t i o n system.
During s t a r t u p o f a c o u n t e r c u r r e n t carbon a d s o r p t i o n system f o r p r o c e s s ing heap-leach cyanide e f f l u e n t s , t h e f i r s t o r lead column of carbon may
adsorb a l l of t h e p r e c i o u s metal v a l u e s . A f t e r t h e carbon has been i n s e r v i c e
for some time and c a r r i e s about 20 ounces o f gold p e r ton, a g r a d u a l b r e a k through of gold w i l l occur i n t h e e f f l u e n t from t h e f i r s t column and be
adsorbed by t h e succeeding bed of carbon. As t h e carbon i n t h e f i r s t column
becomes loaded w i t h more gold and i t s a d s o r p t i v e c a p a c i t y d e c r e a s e s , t h e gold
content of t h e column e f f l u e n t s o l u t i o n g r a d u a l l y i n c r e a s e s . It i s obvious
from monitoring t h e progress o f a d s o r p t i o n t h a t t o o b t a i n e f f i c i e n t u t i l i z a t i o n of t h e carbon g o l d , it i s advantageous t o use s e v e r a l charges o f carbon
i n s e r i e s . E f f l u e n t samples should be c o l l e c t e d a t r e g u l a r i n t e r v a l s and analyzed, p r e f e r a b l y by t h e atomic a b s o r p t i o n technique t o o b t a i n r a p i d gold and
s i l v e r a s s a y s a t t h e p l a n t s i t e . When t h e a n a l y s e s i n d i c a t e t h a t t h e l e a d
charge of carbon has adsorbed t h e d e s i r e d amount of gold o r becomes s a t u r a t e d ,
a p o r t i o n of t h i s carbon charge ( r e p r e s e n t i n g t h e d a i l y production of loaded
carbon) i s removed, and an e q u i v a l e n t amount from each of t h e o t h e r columns i s
advanced one column. An e q u i v a l e n t amount o f f r e s h a c t i v a t e d carbon i s added
t o t h e l a s t column t o i n s u r e t h e p r o d u c t i o n of b a r r e n s o l u t i o n s low i n gold
content f o r r e c y c l i n g t o t h e heap. F i g u r e 5 i l l u s t r a t e s t h e h y p o t h e t i c a l
d i s t r i b u t i o n of gold a t t a i n a b l e i n p r o c e s s i n g 5,000 tons of cyanide s o l u t i o n
c o n t a i n i n g 0.10 ounce of gold per t o n and employing f o u r 1-ton beds of carbon
in series.
The p r e c i o u s metal v a l u e s p r e s e n t l y a r e recovered from loaded carbon by
two r a d i c a l l y d i f f e r e n t t e c h n i q u e s . I n one p r a c t i c e , adopted by some s m a l l
companies w i t h l i m i t e d c a p i t a l f o r investment i n t h e r e q u i r e d equipment, t h e
loaded carbon i s shipped t o s m e l t e r s where t h e product i s burned t o recover a
dore' b u l l i o n by s m e l t i n g t h e
ashed r e s i d u e . T y p i c a l
s m e l t e r charges i n c l u d i n g
t h e nonpayment f o r 7 . 5 p c t
o f t h e t o t a l gold c o n t e n t
Barren
w i l l average about $3,500
Solution
p e r d r y t o n of carbon cont a i n i n g 300 ounces o f gold
per t o n . A d d i t i o n a l c o s t s
t o be considered a r e t h e
Pregnant solution
purchase
p r i c e of t h e a c t i 0.10 oz/fonPu
NoCN
CaO
v a t e d carbon and haulage
c h a r g e s . Thus, a r e a s o n a b l e
c o s t f i g u r e f o r processing
a t o n of loaded carbon by
t h e s m e l t i n g technique i s
u
$5,500 t o $7,000. I n t h e
Solution storage
p r e f e r r e d method, gold
ond makeup tank
recovery from a c t i v a t e d c a r FIGURE 5. Hypothetical
distribution of gold in a con.
bon i s a c c o m ~ l i s h e dbv
~.
desorbing t h e gold anb s i l tinuous carbon-adsorption operation.
v e r v a l u e s from t h e carbon
and e l e c t r o w i n n i n g t h e v a l u e s from t h e r e s u l t a n t s t r i p s o l u t i o n s . This p e r m i t s repeated r e u s e o f t h e carbon, which g r e a t l y enhances t h e economics of t h e
carbon c y a n i d a t i o n p r o c e s s . The f i r s t p r a c t i c a l d e s o r p t i o n process was d e v e l oped by t h e Bureau o f Mines i n 1952 (26
-3
3 1 ) . The process (commonly r e f e r r e d
t o a s t h e Zadra p r o c e s s ) employs a 1 . 0 p c t NaOH-0.1 p c t NaCN s t r i p s o l u t i o n a t
93" C and a t atmospheric p r e s s u r e t o desorb t h e gold and s i l v e r from t h e c a r bon. The g o l d - s i l v e r d e s o r p t i o n s t e p , which i s dependent upon t h e c o n c e n t r a t i o n of a l k a l i (NaOH) and t h e temperature o f t h e s t r i p s o l u t i o n , i s g r e a t l y
enhanced by t h e a d d i t i o n of a n a l c o h o l such a s e t h a n o l o r methanol t o t h e
d e s o r b e n t o r s t r i p p i n g s o l u t i o n . The p r e c i o u s metal v a l u e s contained i n t h e
s t r i p s o l u t i o n a r e electrowon from t h e s o l u t i o n , and t h e b a r r e n s o l u t i o n
r e c y c l e d f o r a d d i r i o n a l s t r i p p i n g . Current i n d u s t r i a l a p p l i c a t i o n of t h e
- - i n d i c a t e s t h a t 50 t o 100 hours a r e r e q u i r e d
Zadra d e s o r p t i o n process (10-11)
t o s t r i p t h e carbon from a loading of 300 ounces of gold t o l e s s t h a n 5 ounces
o f gold per t o n of carbon. The Bureau a t t h e S a l t Lake C i t y Metallurgy
Research Center developed a p r e s s u r e s t r i p p i n g method t h a t g r e a t l y reduces
s t r i p p i n g time and r e a g e n t consumption (3,25).
00002 oz Au
I
-
Economics of Heap-Leach Operations
P i z a r r o d e s c r i b e d i n d e t a i l t h e 10,000-ton-per-month heap-leaching operat i o n a t t h e C a r l i n Gold Mine, C a r l i n , Nev. (21). The heap-leach e f f l u e n t s
j o i n t h e pregnant s o l u t i o n s from t h e C a r l i n mill f o r c l a r i f i c a t i o n followed by
c o n v e n t i o n a l z i n c d u s t p r e c i p i t a t i o n f o r gold recovery. Reagent consumption
was minimal, 0 . 1 pound NaCN and 1 . 0 pound CaO per t o n of o r e . P i z z a r o (21)
and McQuiston (20, pp. 19-25) presented s e v e r a l e x c e l l e n t t a b l e s o f c a r l i n t s
o p e r a t i n g costsBased on t h e s e d a t a , t h e c a l c u l a t e d heap-leaching o p e r a t i o n
c o s t s a t C a r l i n were $0.96 per t o n of o r e i n 1973 and $ 1 . 2 1 per t o n i n 1974.
Mountain S t a t e s Research and Development, Tucson, A r i z . (4-5J,
made a n
economic e v a l u a t i o n on four p r o c e s s e s co-nly
employed f o r r e c o v e r i n g gold
from low-grade o r e s . The e x t r a c t i o n p r o c e s s e s e v a l u a t e d were (1) heap l e a c h ing w i t h carbon a d s o r p t i o n , ( 2 ) v a t leaching w i t h carbon a d s o r p t i o n , (3) conv e n t i o n a l 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 , and (4) carbon-in-pulp c y a n i d a t i o n .
Bhappu (4) i n d i c a t e d t h a t t h e heap-leach process i s economically j u s t i f i e d i n
t h e p r o c e s s i n g of l e a n o r e s averaging 0.04 ounce of r e c o v e r a b l e gold p e r t o n
a t gold p r i c e s o f $80 per ounce f o r m u l t i m i l l i o n - t o n o r e b o d i e s . For t h e
s m a l l e r o p e r a t i o n s , l a b o r c o s t s would be p r o p o r t i o n a t e l y much l a r g e r p e r t o n ,
and t h e c o s t s f o r equipment and f a c i l i t i e s would d i f f e r widely depending on
t h e a c c e s s i b i l i t y of t h e mine.
Chisholm ( 6 ) r e p o r t e d t h a t a s m a l l mining company i n New Mexico brought
i n t o production-a 500-ton-per-day open p i t gold mine and heap-leach o p e r a t i o n
a t a t o t a l c o s t of $150,000. A breakdown of t h e c o s t s f o r items of equipment
purchased and f o r mine f a c i l i t i e s was p r e s e n t e d .
A d d i t i o n a l c o s t f a c t o r s t h a t appear t o s u b s t a n t i a t e t h e foregoing c o s t s
were developed a s a r e s u l t of information g a t h e r e ? from s e v e r a l s o u r c e s .
Based on a 500,000-ton heap-leach o p e r a t i o n , t y p i c a l c o s t s f o r p r e p a r i n g an
a s p h a l t pad, i n c l u d i n g p r e p a r a t i o n of s i t e , subbase, s e a l e r , and covering t h e
a s p h a l t w i t h a l a y e r of g r a v e l a r e $0.50 per square f o o t . This i s e q u i v a l e n t
t o $0.50 per t o n of o r e , assuming t h a t a t o n of o r e i s s t a c k e d p e r square f o o t
of pad s u r f a c e . The s t a c k i n g of o r e on a pad r e p o r t e d l y c o s t s about $0.10 per
ton. The l e a c h i n g c o s t s a r e dependent on r e a g e n t consumption and l e n g t h of
period of t r e a t m e n t , and t h u s can v a r y a p p r e c i a b l y . A r e a s o n a b l e l e a c h i n g
c o s t r e p o r t e d l y i s $0.50 per t o n o f o r e . Operation of t h e carbon d e s o r p t i o n
u n i t c o s t s about $0.02 per t o n of o r e , assuming t h a t t h e o r e c o n t a i n s 0.05
ounce of r e c o v e r a b l e gold per t o n and t h a t t h e carbon i s loaded w i t h 400
ounces of gold per t o n . P r e v a i l i n g p r a c t i c e i s t o r e g e n e r a t e t h e leached c a r bon b e f o r e r e u s e by h e a t i n g t h e carbon i n a steam atmosphere a t -700" C i n a
-Based on c o s t f i g u r e s s t a t e d i n a r e c e n t a r t i c l e on c a r r o t a r y k i l n (8-10).
bon r e g e n e r a t i o n (161, t h e c o s t f o r r e a c t i v a t i n g t h e s t r i p p e d carbon i s e s t i mated t o be $0.02
t o n of o r e . A sunnnary of t h e c o s t e s t i m a t e s , excluding
mining c o s t s , i s shown i n t a b l e 1.
per
TABLE 1.
-
Estimated c o s t of a commercial heapleach cyanidation operation
Item
1 Cost
C o n s t r u c t i o n of pad . . . . . . . . . . . . . . . I
Stacking o r e on pad ...............
Heap leaching . . . . . . . . . . . . . . . . . . . . .
Carbon d e s o r p t i o n . . . . . . . . . . . . . . . . .
Carbon r e g e n e r a t i o n . . . . . . . . . . . . . . .
r o t a 1. . . . . . . . . . . . . . . . . . . . . . . .
per t o n
of o r e
$0.50
I
.02
1.14
CYANIDE HANDLING AND DISPOSAL
The p r e c i o u s - m e t a l s mining i n d u s t r y f o r many y e a r s h a s promoted t h e
h e a l t h and s a f e t y o f i t s employees r e g a r d i n g t h e h a n d l i n g and u s e o f c y a n i d e .
The i n d u s t r y h a s d e m o n s t r a t e d t h a t , w i t h p r o p e r t r a i n i n g and i n s t r u c t i o n s ,
cyanide c a n b e used r o u t i n e l y i n leaching g o l d - s i l v e r o r e s w i t h l i t t l e r i s k
t o t h e w o r k e r . However, growing c o n c e r n a b o u t o c c u p a t i o n a l h a z a r d s and
e n v i r o n m e n t a l p o l l u t i o n has r e s u l t e d i n t h e p r o m u l g a t i o n of r e g u l a t i o n s t h a t
r e q u i r e i n d u s t r y t o comply w i t h s t a n d a r d s and g u i d e l i n e s e s t a b l i s h e d b y
F e d e r a l , S t a t e , and c o u n t y r e g u l a t o r y b o d i e s . The O c c u p a t i o n a l S a f e t y and
H e a l t h A d m i n i s t r a t i o n (OSHA) i n O c t o b e r 1976 p u b l i s h e d i t s recommendations
d e s i g n e d t o p r o t e c t t h e h e a l t h o f employees working w i t h c y a n i d e s a l t s . The
p u b l i c a t i o n , " C r i t e r i a f o r a Recommended S t a n d a r d - 4 c c u p a t i o n a l Exposure t o
Hydrogen Cyanide and Cyanide S a l t s [ N ~ c N , KCN, and c ~ ( c N ) , ] " c a n be p u r c h a s e d
f r o m t h e S u p e r i n t e n d e n t o f Documents, U.S. Government P r i n t i n g O f f i c e ,
Washington, D . C . 20402.
Employees working a t a h e a p - l e a c h c y a n i d a t i o n i n s t a l l a t i o n may be exposed
t o c y a n i d e i n t h e form o f d u s t and s o l u t i o n s , e s p e c i a l l y d u r i n g mixing o f c o n c e n t r a t e d s t o c k cyanide s o l u t i o n s .
I n g e s t i o n o f a s l i t t l e a s 0.20 gram o f
sodium c y a n i d e i s c o n s i d e r e d t o be l e t h a l f o r human b e i n g s ( 1 7 ) . The h e a p l e a c h o p e r a t i o n i t s e l f i s c o n s i d e r a b l y l e s s hazardous b e c a u s e t h e leach i s
c o n d u c t e d i n a n open a r e a w i t h maximum v e n t i l a t i o n .
By m a i n t a i n i n g t h e a l k a l i n i t y o f t h e l e a c h s o l u t i o n a t pH 1 0 t o 11, t h e p o s s i b i l i t y o f g e n e r a t i n g
hydrogen c y a n i d e g a s (HCN) i s minimized, and o n l y t r a c e amounts of HCN c a n be
r e l e a s e d by i n t e r a c t i o n of NaCN and COa i n t h e e n v i r o n m e n t . Measurements made
b y Mining Enforcement and S a f e t y A d m i n i s t r a t i o n (MESA) i n s p e c t o r s (18) show
t h a t t h e HCN c o n c e n t r a t i o n i n t h e a i r c l o s e t o a working heap i s c o n s i s t e n t l y
2)
o n l y 2 t o 3 ppm. T h i s i s s i g n i f i c a n t l y l e s s t h a n t h e l i m i t o f 1 0 ppm ( e s t a b l i s h e d b y OSHA f o r s u s t a i n e d b r e a t h i n g o f g a s e o u s c y a n i d e .
In a well-designed heap-leach i n s t a l l a t i o n , t h e pregnant cyanide s o l u t i o n
s e t t l i n g pond, which c a t c h e s t h e d r a i n a g e from t h e heap, s h o u l d b e d e s i g n e d s o
t h a t t h e c a p a c i t y of t h e pond i s s u f f i c i e n t t o accommodate t h e maximum r a i n f a l l and r u n o f f t h a t c a n b e e x p e c t e d f o r t h a t p a r t i c u l a r l o c a l i t y , t h u s p r e v e n t i n g t h e discharge of cyanide s o l u t i o n t o t h e watershed during o p e r a t i o n
a n d a f t e r abandonment of t h e l e a c h e d o r e heap. The s e t t l i n g ponds a r e gene r a l l y earth f i l l structures t h a t a r e lined with watertight polyvinyl chloride
o r p o l y e t h y l e n e s h e e t i n g . Ponds h o l d i n g c y a n i d e s o l u t i o n s s h o u l d b e a d e q u a t e l y p o s t e d and f e n c e d t o r e s t r i c t a c c e s s t o t h e a r e a .
Because o f t h e a p p r e c i a b l e e v a p o r a t i o n l o s s e s t h a t o c c u r d u r i n g heap
l e a c h i n g , most o p e r a t o r s a r e a b l e t o m a i n t a i n c o m p l e t e r e c y c l i n g o f t h e l e a c h
and wash s o l u t i o n s . Thus t h e need t o d i s c h a r g e p o t e n t i a l l y h a z a r d o u s s o l u t i o n s t o maintain the water balance f o r the leach o p e r a t i o n i s circumvented.
I f a b l e e d o f f s y s t e m i s r e q u i r e d , i n t h e e v e n t o f a n a b n o r m a l l y heavy r a i n f a l l ,
c y a n i d e removal t e c h n i q u e s must be c o n s i d e r e d . The most w i d e l y used method
f o r r e d u c i n g f r e e c y a n i d e and h e a v y - m e t a l c y a n i d e c o n c e n t r a t i o n s i n w a s t e
s t r e a m s i n v o l v e s c h e m i c a l t r e a t m e n t w i t h c h l o r i n e o r h y p o c h l o r i t e . The
r e a c t i o n mechanism i s b e l i e v e d t o be a s follows:
NaCN
+
2NaOH
+
C1,
-. NaCNO
+
2NaC1
+ H,O,
(6)
The a v a i l a b l e c h l o r i n e may be f u r n i s h e d a s c h l o r i n e gas o r a h y p o c h l o r i t e
s o l u t i o n . Approximately 1 pound of calcium h y p o c h l o r i t e , Ca(OCl),, w i l l o x i d i z e 1 pound of f r e e c y a n i d e . The term 'heavy m e t a l s " g e n e r a l l y denotes t h o s e
metals t h a t a r e o f p a r t i c u l a r concern i n cyanide t r e a t m e n t such a s copper,
s i l v e r , z i n c , cadmium, mercury, a r s e n i c , chromium, manganese, i r o n , and n i c k e l .
The main method employed f o r removal of t h e heavy metals a f t e r t h e d e s t r u c t i o n
of cyanide i s by a d d i t i o n of lime t o p r e c i p i t a t e t h e metals and t o promote
f l o c c u l a t i o n of t h e p r e c i p i t a t e s . The heavy-metal p r e c i p i t a t e s would be f i l t e r e d and shipped t o a n approved t o x i c - s u b s t a n c e l a n d f i l l f o r d i s p o s a l . The
U.S. Environmental P r o t e c t i o n Agency (EPA) r e g u l a t i o n s f o r l i m i t a t i o n s of
cyanide d i s c h a r g e i n waste s o l u t i o n s i s 0.02 ppm C N .
It i s c l e a r l y advantageous from a n economic s t a n d p o i n t f o r t h e mine opera t o r t o lower t h e s o l u b l e gold l o s s e s i n t h e heap t o a minimum by thorough
washing of t h e leached o r e w i t h f r e s h w a t e r . The washing s t e p r e s u l t s i n
recovery of most o f t h e d i s s o l v e d gold and a l a r g e p o r t i o n of o t h e r cyanides
remaining i n t h e heap a s f r e e cyanide o r complexed w i t h heavy m e t a l s . Heavymetal cyanide s a l t s a r e known t o p e r s i s t f o r s e v e r a l y e a r s , b u t r e s i d u a l f r e e
cyanide i n abandoned heaps i s b e l i e v e d t o e x i s t no more t h a n 1 month, depending on c l i m a t i c c o n d i t i o n s ; however, s c i e n t i f i c d a t a t o s u p p o r t t h i s content i o n a r e l a c k i n g . The r e t e n t i o n and f a t e of r e s i d u a l cyanide i n heap leached
r e s i d u e s i s being s c r u t i n i z e d t o an i n c r e a s i n g e x t e n t by r e g u l a t o r y a g e n c i e s .
Abandoned heaps a r e l e s s s u s c e p t i b l e t o wind and water e r o s i o n t h a n
f i n e l y ground t a i l i n g s impounded behind a dam. I n t h e semiarid r e g i o n s of t h e
Western United S t a t e s , where most of t h e heap l e a c h c y a n i d a t i o n i s being p r a c t i c e d , i n v a s i o n of t h e abandoned heaps by n a t i v e d e s e r t f l o r a has been
observed t o occur w i t h i n 1 o r 2 y e a r s .
Under p r o v i s i o n s of t h e F e d e r a l Metal and Nonmetallic Mine S a f e t y Act of
1966 ( P u b l i c Law 89-577), MESA i s r e s p o n s i b l e f o r t h e enforcement of t h e
h e a l t h and s a f e t y s t a n d a r d s p r e s c r i b e d t o p r o t e c t t h e workers a t a l l mine
s i t e s , which i n c l u d e s heap-leach c y a n i d a t i o n o p e r a t i o n s . With t h e s i g n i n g of
the F e d e r a l Mine S a f e t y and Health Act of 1977 ( P u b l i c Law 91-173), i t became
the r e s p o n s i b i l i t y of t h e S e c r e t a r y of Health, Education, and Welfare and t h e
S e c r e t a r y of Labor t o develop and promulgate improved h e a l t h and s a f e t y s t a n dards f o r persons working a t mining p r o p e r t i e s . New and/or r e v i s e d r e g u l a t i o n s w i l l be implemented by t h e Mining S a f e t y and Health ~ d m i n i d t r a t i o n
(MSHA) under t h e Department of Labor. The e f f e c t i v e d a t e o f t h e s e r u l e s and
r e g u l a t i o n s i s expected t o be March 9, 1978.
Under p r o v i s i o n s of t h e Resource Conservation and Recovery Act of 1976
(Public Law 94-580), EPA i s r e s p o n s i b l e f o r t h e development of r e g u l a t i o n s and
g u i d e l i n e s f o r d i s p o s a l and management of a l l s o l i d w a s t e s , i n c l u d i n g mining
wastes. EPA c u r r e n t l y i s conducting a d e t a i l e d s t u d y on t h e adverse e f f e c t s
on t h e environment of s o l i d wastes from a c t i v e and abandoned s u r f a c e and
underground mines. Guidelines f o r t h e d i s p o s a l of mining wastes a r e expected
following t h e completion of t h e s t u d y i n June 1978. Meanwhile, S t a t e and/or
county gwernmentswhere heap-leach c y a n i d a t i o n o p e r a t i o n s e x i s t r e g u l a t e t h e
d i s p o s a l o r abandonment of cyanided o r e heaps.
RECENT INNOVATIONS I N GOLD-SILVER RECOVERY
FROM CYANIDE PROCESS SOLUTIONS
Recently, e f f o r t s have been d i r e c t e d toward t h e development of lower c o s t
gold-processing procedures t h a t w i l l improve t h e economics of t r e a t i n g lower
grade o r e s and d e p o s i t s t o o s m a l l t o warrant c o n s t r u c t i o n of c o n v e n t i o n a l m i l l
facilities.
The Bureau o f Mines (14) developed a more e f f i c i e n t p r o c e s s f o r r e c o v e r ing gold and s i l v e r from laden a c t i v a t e d carbon. F a c t o r s t h a t c a n a f f e c t t h e
performance of t h e modified s t r i p s o l u t i o n i n c l u d e temperature, and c o n c e n t r a t i o n s of NaOH and e t h a n o l . The s t r i p s o l u t i o n , which i s w a t e r c o n t a i n i n g
1 wt-pct NaOH and 20 v o l - p c t e t h a n o l , i s heated t o 80" C and c i r c u l a t e d
through a bed of loaded carbon t o desorb p r e c i o u s metals and cyanide i o n s .
The precious metals a r e recovered by e l e c t r o l y s i s , and t h e d e p l e t e d e l e c t r o l y t e i s r e c y c l e d t o t h e carbon d e s o r p t i o n column. F i v e t o s i x hours of
simultaneous s t r i p p i n g and e l e c t r o w i n n i n g w i l l desorb up t o 99 p c t of t h e
precious metal values c a r r i e d by t h e a c t i v a t e d c a r bon. A flow diagram f o r t h e
p r o c e s s i s shown i n f i g u r e 6 .
le
"
FIGURE 6.
- Recovery of gold from activated carbon.
Activated carbon, which
had been used f o r c a r b o n - i n pulp c y a n i d a t i o n of g o l d
ores f o r several years, is
now p l a y i n g a n important
r o l e i n t h e recovery of gold
from heap-leach c y a n i d a t i o n
e f f l u e n t s . However, a p p l i c a t i o n of t h i s technology t o
t h e t r e a t m e n t of gold o r e s
that are rich i n silver
g r e a t l y i n c r e a s e s t h e amount
of a c t i v a t e d carbon needed
t o adsorb t h e e q u i v a l e n t
d o l l a r v a l u e of s i l v e r from
a n e q u a l volume of l e a c h
solution. A processing
sequence was developed t h a t
circunwents t h e handling of
l a r g e q u a n t i t i e s of a c t i v a t e d carbon ( 1 3 ) . It cons i s t s of r e c o v e r i n g t h e
s i l v e r s e l e c t i v e l y a s a AgzS
p r e c i p i t a t e u s i n g sodium s u l f i d e , and a f t e r f i l t r a t i o n , r e c o v e r i n g t h e gold
from t h e f i l t r a t e by carbon a d s o r p t i o n . The key t o t h i s p r o c e s s i s t h e format i o n of r e a d i l y f i l t e r a b l e AgzS f l o c s by a g i t a t i n g t h e s l u r r y w i t h 0.5 pound
of CaO p e r t o n of s o l u t i o n f o r about 1 hour. The p r o c e s s sequence i s shown i n
f i g u r e 7 . The Ag,S p r e c i p i t a t i o n - c a r b o n a d s o r p t i o n processing method i s being
evaluated a t a heap-leach c y a n i d a t i o n i n s t a l l a t i o n by Sand Springs Co., F a l l o n ,
Nev., f o r p r o c e s s i n g a s i l v e r o r e low i n gold c o n t e n t .
The sodium s u l f i d e p r e c i p i t a t i o n procedure was a l s o shown t o be e f f e c t i v e
f o r s e l e c t i v e l y p r e c i p i t a t i n g s i l v e r from h i g h l y c o n c e n t r a t e d g o l d - s i l v e r
e f f l u e n t s obtained i n t h e i n i t i a l s t a g e s of s t r i p p i n g p r e c i o u s metal-laden
a c t i v a t e d carbon. The method provides a means f o r t h e subsequent production
of a low-silver gold b u l l i o n . S i l v e r i s p r e c i p i t a t e d s e l e c t i v e l y from s t r i p
s o l u t i o n by t h e same procedure used p r e v i o u s l y f o r s i l v e r p r e c i p i t a t i o n from
pregnant cyanide l e a c h s o l u t i o n s . F i l t r a t e s c o n t a i n i n g l e s s than 1 ppm Ag and
UD t o 800 m m Au were o b t a i n e d .
The ~ r e c i ~ i t a t e swhich
,
c o n t a i n diatomaceous
e a r t h , lime, s l i m e s , e t c . , a s w e l l a s Ag,S, a s s a y 3,000 t o 6,000 ounces of
s i l v e r p e r t o n and r e p r e s e n t 99-pct s i l v e r r e c o v e r y . The e n t r a i n e d gold
amounts t o l e s s t h a n 0 . 1 p c t of t h e t o t a l p r e c i o u s m e t a l c o n t e n t . The
L .
NaAg(CNI2 soluticIn from heap leach
Gold recovery
I
precipitotion
Retention
.....
Corbon
adsorption
Loaded
carbon
I
I
Corbon
stripping
IL,
I
u
Filtration
Silver product
Gold product
wren
solution
FIGURE 7.
-
Proposed treatment of cyanide effluents high in silver content.
Corbon columns
l Loaded
I
Stripped carbon
L---------------------
I
mrbon
II
17
s i l v e r - f r e e f i l t r a t e can
t h e n be e l e c t r o l y z e d t o win
t h e gold by d e p o s i t i o n on
s t e e l wool cathodes. Barren
e l e c t r o l y t e recycled t o t h e
d e s o r p t i o n u n i t can be used
t o desorb more v a l u e s from
t h e carbon. F i n a l l y , t h e
gold-laden s t e e l wool c a t h odes when mixed w i t h proper
f l u x e s c a n be r e f i n e d i n t o
gold b u l l i o n low i n s i l v e r .
Figure 8 shows a conceptual
flow diagram d e p i c t i n g t h e
heap leach-carbon a d s o r p t i o n d e s o r p t i o n system and t h e
precious m e t a l s s e p a r a t i o n
and recovery c y c l e .
Desorplion
unit
1
Strip
roluliOn
tank
1
SUMMARY
The r i s i n g free-market
gold p r i c e has given a new
.
Au cyonida solution
l e a s e on l i f e t o t h e gold
-+-+-...-+
mining
i n d u s t r y . The heap
Filtration
IH
y
leach c y a n i d a t i o n - c a r b o n
E 1 e c l r o l ~ t i c Recycled r l r i p
soIution
adsorption-electrowinning
SilvFr product
Gold product
process developed by t h e
F e d e r a l Bureau of Mines has
FIGURE 8. Proposed heap leach-carbon adsorption
?roved t o b e a n economical
process for gold ores low in silver.
method f o r e x p l o i t i n g lowgrade gold o r e s and s m a l l i s o l a t e d d e p o s i t s n o t s u i t a b l e f o r t r e a t m e n t by conv e n t i o n a l c y a n i d a t i o n procedures. Heap l e a c h i n g may be t h e most p r o f i t a b l e
method f o r p r o c e s s i n g s e l e c t e d gold and s i l v e r o r e s t h a t do not r e q u i r e f i n e
g r i n d i n g and a r e r e a d i l y amenable t o c y a n i d a t i o n .
precipitation
Digestion
-%
-
-
I
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