INTRODUCTION TO

EVALUATION, DESIGN A N D
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 t h e 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 without permlssion of the publlsher.

Library ofCongress Catalog Card Number 88-60773

ISBN 0-87335-074-X
Beyond Design: Considerations
Critical to Project Success
 Chapter 13

Cyanide Geochemistry and Detoxification Regulations

Adrian Smith and Debra W. Struhsacker

13.1 INTRODUCTION

Cyanide i s a g e n e r i c t e r m i n d i c a t i n g t h e presence o f t h e cyanide i o n

(CN-). Cyanide i s a v e r y common, n a t u r a l l y o c c u r r i n g compound produced by many
b i o c h e m i c a l r e a c t i o n s . Many p l a n t s p e c i e s s y n t h e s i z e o r g a n i c compounds which
c o n t a i n c y a n i d e i n t h e form o f c y a n o g e n i c g l y c o s i d e s (Knowles, 1976). For
example, t r a c e amounts o f c y a n i d e a r e p r e s e n t i n t h e seeds and leaves o f many
members o f t h e Rosaceae ( r o s e ) f a m i l y ( K i n g s b u r y , 1 9 6 4 ) .

Many common i t e m s such as l e t t u c e , m a i z e , sweet p o t a t o e s , k i d n e y beans,

(Oke, 1969) alrnonds, and c i g a r e t t e smoke c o n t a i n c y a n i d e . As examples, a
r e c e n t chemical a n a l y s i s o f chocol a t e - c o v e r e d almonds measured a t o t a l c y a n i d e
c o n t e n t o f a b o u t two ppm ( S t e f f e n R o b e r t s o n and K i r s t e n , 1987); and t h e U.S.
Surgeon General ( U .S. D e p a r t ~ i i e n t o f H e a l t h , 1964) has shown t h a t c i g a r e t t e
smoke c o n t a i n s up t o 1,600 ppm t o t a l c y a n i d e .

The c h e m i s t r y o f c y a n i d e s o l u t i o n s i s compl i c a t e d because t h e c y a n i d e

i o n forms compounds and complexes w i t h many e l e m e n t s . Some c y a n i d e species a r e
h i g h l y t o x i c whereas o t h e r s a r e r e l a t i v e l y i n e r t and h a r ~ n l e s s . M o l e c u l a r
h y d r o g e n c y a n i d e (HCN) i s t h e most t o x i c f o r m o f c y a n i d e . Under most
c o n d i t i o n s , HCN e x i s t s as a gas w h i c h r e a d i l y d i s s i p a t e s o r r e a c t s w i t h t h e
e n v i r o n m e n t t o f o r m l e s s t o x i c o r n o n t o x i c f o r m s o f c y a n i d e . Thus HCN i s an
ephemeral t o x i n , and many n a t u r a l 1 y o c c u r r i n g geochemical processes reduce t h e
HCW c o n c e n t r a t i o n o f a heap system w i t h t i m e .

As d i s c u s s e d below, f r e e c y a n i d e i n c l u d e s t h e two s p e c i e s , i o n i c c y a n i d e
(CN-) and m o l e c u l a r hydrogen c y a n i d e . F r e e - c y a n i d e t o x i c i t y i n man, mammal S,
a n d a q u a t i c s p e c i e s i s we1 1 documented ( D o u d o r o f f , 1976; E c o l o g i c a l A n a l y s t s ,
.
1979; T o w i l l e t a1 , 1978). The 1 e t h a l doses r e p o r t e d f o r human a d u l t s v a r y
w i t h t h e t y p e o f exposure as f o l l o w s :

One t o t h r e e mg/kg body w e i g h t i f i n g e s t e d ;

One hundred t o 300 ppm i f i n h a l e d ; and
One hundred mg/kg o f body w e i g h t i f absorbed.

Acute t o x i c i t y o f f r e e c y a n i d e t o f r e s h w a t e r i n v e r t e b r a t e s ranges from

0.028 t o 2.295 mg/l, depending o n s p e c i e s and t e s t c o n d i t i o n s . G e n e r a l l y ,
f r e e - c y a n i de c o n c e n t r a t i o n s g r e a t e r t h a n 0.1 mg/l a r e expected t o k i 11
s e n s i t i v e s p e c i e s i n f r e s h w a t e r o r m a r i n e e n v i r o n m e n t s (Doudoroff, 1976;
275
276 PRECIOUS METAL HEAP LEACHING PROJECTS

Cardwell e t a1 ., 1 9 7 6 ) . C o n c e n t r a t i o n s o f HClV as l o w as 0.05 mg/l c a n be

l e t h a l t o f i s h ( S c o t t and I n g l e s , 1 9 8 1 ) . Several s i t e s p e c i f i c v a r i a b l e s
i n c l u d i n g t h e s o l u t i o n pH and t e m p e r a t u r e , oxygen c o n t e n t o f t h e water, i o n i c
s t r e n g t h , s p e c i e s a c c l i m a t i o n , and body s i z e can a f f e c t t h e degree o f f r e e -
cyanide t o x i c i t y .

13.2 CYANIDE TERMINOLOGY AND ANALYTICAL METHODS

One o f t h e d i f f i c u l t i e s i n d i s c u s s i n g c y a n i d e and t h e e n v i r o n m e n t and

e s t a b l i s h i n g c y a n i d e r e g u l a t o r y s t a n d a r d s i s t h a t t h e t e r m i n o l o g y used t o
d e s c r i b e c y a n i d e s p e c i e s i s c o n f u s i n g . To add t o t h e c o n f u s i o n , t h e r e a r e a
v a r i e t y o f a n a l y t i c a l methods used t o t e s t f o r t h e v a r i o u s c y a n i d e s p e c i e s ,
and some o f t h e s e methods a r e p r o n e t o a n a l y t i c a l i n t e r f e r e n c e problems. Thus
comparing a n a l y t i c a l d a t a f r o m one l a b o r a t o r y t o a n o t h e r may n o t be v a l i d
(Conn, 1981). The f o 1 l o w i n g d i s c u s s i o n w i l l a t t e m p t t o c l a r i f y c y a n i d e
t e r m i n o l o g y and r e v i e w t h e m o s t c o n ~ m o n l yused a n a l y t i c a l methods.

13.2.1 Free C y a n i d e

The t e r m " f r e e c y a n i d e " means t h e t w o s p e c i e s , i o n i c c y a n i d e (CN') and

m o l e c u l a r hydrogen c y a n i d e o r h y d r o c y a n i c a c i d (HCN). I n a c i d i c and w e a k l y -
a1 k a l i n e s o l u t i o n s , t h e d o m i n a n t f r e e c y a n i d e s p e c i e s i s m o l e c u l a r h y d r o g e n
c y a n i de.

Most a n a l y t i c a l t e c h n i q u e s f o r m e a s u r i n g f r e e c y a n i d e i n v o l v e s o l v e n t
e x t r a c t i o n o r s p a r g i n g t h e HCN f r o m s o l u t i o n and m e a s u r i n g t h e HClV (Conn,
1981). Free c y a n i d e c a n a l s o b e a n a l y z e d u s i n g a s p e c i f i c i o n e l e c t r o d e by
comparing t h e p o t e n t i a l r e a d i n g s o f t h e e l e c t r o d e a g a i n s t a c y a n i d e s t a n d a r d
c a l i b r a t i o n c u r v e . The s p e c i f i c i o n e l e c t r o d e t e c h n i q u e i s one o f t h e e a s i e s t
and most e c o n o m i c a l methods f o r d e t e r m i n i n g f r e e c y a n i d e i n a f i e l d
1 a bora t o r y .

However, t h e r e i s a s i g n i f i c a n t b o d y o f d a t a w h i c h i n d i c a t e i n t e r f e r e n c e
problems w i t h f r e e c y a n i d e a n a l y s e s . These d a t a i n c l u d e many examples where
t h e a p p a r e n t f r e e c y a n i d e 1 eve1 s w e r e h i g h e r than t h e t o t a l c y a n i d e measured
i n t h e same sample due t o t h e i n t e r f e r e n c e o f t h i o c y a n a t e (CNS-) o r o t h e r i o n s
i n s o l u t i o n . The s p e c i f i c i o n e l e c t r o d e method, f o r example, i s p a r t i c u l a r l y
s e n s i t i v e t o t h e p r e s e n c e o f ~ 2 - , C 1 - , and ~ g + . Thus, f r e e c y a n i d e a n a l y s e s
may be q u e s t i o n a b l e d e p e n d i n g upon t h e c h e m i c a l c o r r ~ p o s i t i o n o f t h e s o l l ~ t i o n
b e i n g analyzed.

Even though HCN i s t h e s p e c i e s t h a t i s measured and i s t h e d o m i n a n t

t o x i c species of concern, f r e e c y a n i d e i s g e n e r a l l y expressed i n t h e
l i t e r a t u r e as "CN-". To add t o t h e c o n f u s i o n , f r e e c y a n i d e i s sometimes
w r i t t e n as "CN" (U .S. E n v i r o n m e n t a l P r o t e c t i o n Agency, 1980).

13.2.2 Total Cyanide

T o t a l c y a n i d e r e f e r s t o t h e sum, i n terms o f c y a n i d e i o n (CN-), o f

m o l e c u l a r hydrogen c y a n i d e ( H C N ) , c y a n i d e i o n (CN-1, and most c y a n i d e bound as
m e t a l l i c c o ~ ~ p l e x eand
s compounds. A c i d r e f 1 u x / d i s t i l l a t i o n u s i n g a c a t a l y s t t o
b r e a k down most m e t a l 1 i c c y a n i d e s p e c i e s i s t h e a n a l y t i c a l t e c h n i q u e commonly
used t o measure t o t a l c y a n i d e (ASTM, 1 9 8 1 ) . T h i s method cannot measure
c o m p l e t e l y t h e c y a n i d e c o n t e n t o f g o l d - , c o b a l t - , p l a t i n u m - , and some i r o n -
 CYANIDE GEOCHEMISTRY AND DETOXIFICATION REGULATIONS

c y a n i d e species. The a c c u r a c y and p r e c i s i o n o f t h e method a r e a1 so compromised

by i n t e r f e r e n c e from t h i o c y a n a t e s (CNS-) . The t h i o c y a n a t e i n t e r f e r e n c e
problem, c o u p l e d w i t h v a r i a t i o n s i n a n a l y t i c a l p r o c e d u r e s , has l e d t o g r e a t
d i f f i c u l t i e s i n c o m p a r i n g t o t a l c y a n i d e d a t a p r o d u c e d by t h e m i n i n g i n d u s t r y
(Conn, 1981).

Because t o t a l c y a n i d e i s e x p r e s s e d i n t e r m s o f (CN'), t o t a l c y a n i d e d a t a
can be d i f f i c u l t t o d i s t i n g u i s h f r o m f r e e c y a n i d e d a t a , a l s o l a b e l e d (CN').
The use o f t h e words " f r e e " and " t o t a l " c a n p r e v e n t t h i s c o n f u s i o n .

Some c y a n i d e r e g u l a t i o n s a r e e x p r e s s e d i n terms o f t o t a l c y a n i d e r a t h e r
than f r e e cyanide. M o s t b i o l o g i s t s and e n v i r o n m e n t a l s c i e n t i s t s would p r e f e r
regulations expressed as free cyanide, because total cyanide is
t o x i c o l o g i c a l l y m e a n i n g l e s s (Conn, 1 9 8 1 ) . However, as m e n t i o n e d above, t h e r e
a r e s i g n i f i c a n t a n a l y t i c a l problems w i t h the a n a l y s i s o f free cyanide.

13.2.3 Other Methods f o r D e t e r m i n i n g C y a n i d e

There a r e v a r i o u s methods f o r m e a s u r i n g f r e e c y a n i d e p l u s t h e c y a n i d e
e v o l v e d from c e r t a i n m e t a l c y a n i d e complexes u n d e r spec1 f i e d c o n d i t l o n s . Some
c y a n i d e r e g u l a t i o n s a r e based upon t h e s e d e t e r m i n a t i o n s w h i c h measure 1 e s s
t h a n t o t a l c y a n i d e . The c y a n i d e amenable t o c h l o r i n a t i o n method i s based upon
t h e d i f f e r e n c e between t o t a l c y a n i d e d e t e r m i n a t i o n s on a sample b e f o r e and
a f t e r a l k a l i n e c h l o r i n a t i o n . The a l k a l i n e c h l o r i n a t i o n p r o c e s s o x l d i z e s a l l
c y a n i d e s except t h e i r o n complexes and t h l o c y a n a t e . The d i f f e r e n c e between t h e
two t o t a l c y a n i d e v a l u e s i s r e p o r t e d as c y a n i d e amenable t o c h l o r i n a t i o n , T h i s
method i s s u b j e c t t o t h e same drawbacks and t h i o c y a n a t e i n t e r f e r e n c e p r o b l e m s
as t h e t o t a l c y a n i d e method (Gannon, 1 9 8 1 ) .

The weak a c i d d i s s o c i a b l e method (WAD), o t h e r w i s e known as Method "C"

from t h e ASTM d e s i g n a t i o n , i n v o l v e s a p r o c e d u r e s i m i l a r t o t h e t o t a l c y a n i d e
t e c h n i q u e e x c e p t t h a t i t uses d i f f e r e n t r e a g e n t s . T h i s method r e c o v e r s a l l t h e
c y a n i d e from z i n c - and n i c k e l - c y a n i d e c o m p l e x e s , b u t o n l y r e c o v e r s a b o u t 7 0
p e r c e n t from copper- and 30 p e r c e n t f r o m cadmlum-cyanide complexes. I t does
n o t r e c o v e r any c y a n i d e f r o m f e r r o - , f e r r i - , and c o b a l t - c y a n l d e complexes.
There i s no t h i o c y a n a t e i n t e r f e r e n c e p r o b l e m w i t h t h i s method (Gannon, 1 9 8 1 ) .
T h i s method i s c u r r e n t l y f a v o r e d b y some e n g i n e e r s and geochemists because i t
o b v i a t e s t h e problems a s s o c i a t e d w i t h f r e e c y a n i d e a n a l y s e s and i t i s n o t
subject t o thiocyanate interference.

13.3 THE GEOCHEMIS'IRY OF CYANIDE I N AN ABANDONED HEAP

One approach t o u n d e r s t a n d i n g c y a n i d e b e h a v i o r i n a decommi s s l o n e d heap
l e a c h o p e r a t i o n i s t o i d e n t i f y t h e c y a n i d e r e a c t i o n s l i k e l y t o t a k e p l a c e , and
t h e c y a n i d e s p e c i e s l i k e l y t o be p r e s e n t i n t h e v a r i o u s geochemical
environments w i t h i n t h e heap, t h e pad, and t h e u n d e r l y i n g s e d i m e n t s and
bedrock. F i g u r e 13.1 i s a s c h e m a t i c d i a g r a m o f an abandoned heap s y s t e m
showing t h e m a j o r components o f t h e s y s t e m and t h e p r e v a i l i n g geochemical
c o n d i t i o n s w i t h i n t h e heap e n v i r o n m e n t . 'These c o n d i t i o n s v a r y f o r d i f f e r e n t
p a r t s o f t h e system. The upper p o r t i o n s o f t h e system, i n c l u d i n g t h e heap
i t s e l f , t h e pad, and t h e u n d e r l y i n g w e a t h e r e d bedrock a r e l i k e l y t o be
o x i d i z e d , and r e 1 a t i v e l y d r y o r a t l e a s t u n s a t u r a t e d . The u n d e r l y i n g
unweathered bedrock i s more l i k e l y t o be a r e d u c e d and s a t u r a t e d e n v i r o n m e n t .
 T Y PlCAL CYANIDE PRECIPITATION
REACTIONS
EQUATION No.
(SEE TEXT) UNSATURATED

No. 1-7, 10, 11

58 IF SULFIDE SPENT ORE HEAP
PRESENT

NO. 2-7, 10, 11

IF OXIDIZED

N a 9, 10, 12, 13
IF REDUCED
-... -... -

No. 1, 4-6, 8,10,1 1

FIGURE 13.1
PREVAILING GEOCHEMICAL CONDITIONS AND TYPICAL CYANIDE
REACTIONS IN THE ABANDONED HEAP ENVIRONMENT
 CYANIDE GEOCHEMISTRY AND DETOXIFICATION REGULATIONS 279

O b v i o u s l y t h e s p e c i f i c geocheniical c o n d i t i o n s w i t h - i n a heap l e a c h s y s t e m
1 1 be s t r o n g l y i n f l u e n c e d by s i t e c o n d i t i o n s s u c h as t h e p o s i t i o n o f t h e
t e r t a b l e , amount o f p r e c i p i t a t i o n f a l l i n g on t h e heap, t h e m i n e r a l o g y o f
t h e spent o r e and o f t h e u n d e r l y i n g b e d r o c k , a n d t h e f r a c t u r e d e n s i t y and
p e r m e a b i l i t y o f t h e bedrock. For purposes o f t h i s d i s c u s s i o n , however, we w i l l
assume t h a t t h e geochemical c o n d i t i o n s shown o n F i g u r e 13.1 a r e t h e most
common s c e n a r i o .
The f o l l o w i n g d i s c u s s i o n i d e n t i f i e s t h e c y a n i d e s p e c i e s m o s t 1 ik e l y t o
be p r e s e n t w i t h i n v a r i o u s components o f t h e abandoned heap e n v i r o n m e n t . The
behavior o f these cyanide species i s discussed i n terms o f t h e i r r e l a t i v e
t o x i c i t y and t h e i r p o t e n t i a l e f f e c t upon t h e e n v i r o n m e n t .

13.3.1 H y d r o l y s i s and V o l a t i l i z a t i o n

R e a c t i o n between w a t e r and t h e c y a n i d e i o n ( h y d r o l y s i s ) r e s u l t s i n t h e
f o r m a t i o n o f m o l e c u l a r hydrogen cyanide (HCN) a s shown i n E q u a t i o n ( 1 ) and
F i g u r e 13.2.

CN' + Hz0 +HCN + OH- (1)

T h i s r e a c t i o n i s s t r o n g l y dependent on pH. A t a pH o f 9.36, t h e pKa o f t h e

h y d r o l y s i s r e a c t i o n , t h e c o n c e n t r a t i o n s o f CN- and HCN a r e e q u a l ( F i g u r e
13.2). A t a l l l o w e r pH values, hydrogen c y a n i d e i s t h e d o m i n a n t s p e c i e s ; a t pH
7, 99 p e r c e n t o f t h e c y a n i d e e x i s t s i n t h i s f o r m ( H u i a t t e t a l . , 1 9 8 2 ) . Most
heaps have an o p e r a t i o n a l pH o f about 10.5. F o l l o w i n g d e c o m m i s s i o n i n g and
abandonment, t h e r e w i l l be a gradual d e c r e a s e i n pH w i t h t i ~ n e due t o
n e u t r a l i z a t i o n o f t h e a1 k a l i n e e n v i r o n m e n t d u r i n g r i n s i n g p r o c e d u r e s ,
i n f i l t r a t i o n o f r a i n w a t e r , and carbon d i o x i d e u p t a k e . Thus, HCN w i l l be
produced b y most n e w l y abandoned heaps.

M o l e c u l a r hydrogen cyanide (HCN) h a s a h i g h vapor p r e s s u r e and r a p i d l y

v o l a t i l i z e s i n t o a gas. T h i s v o l a t i 1 iz a t i o n p r o c e s s i s m o d e r a t e l y t e m p e r a t u r e
s e n s i t i v e . The HClV produced by h y d r o l y s i s o f CN- i s thus r e a d i l y v o l a t i l i z e d
i n t o a gas and i s c o n t i n u o u s l y evolved f r o m t h e heap e n v i r o n m e n t t h r o u g h
d i s s i p a t i o n o r d e s t r o y e d by o x i d a t i o n a s d i s c u s s e d below. The n e t r e s u l t i s
t h u s a l o s s o f c y a n i d e from t h e system.

As shown on F i g u r e 13.1, h y d r o l y s i s o f i o n i c c y a n i d e i s m o s t l i k e l y t o
o c c u r w i t h i n t h e upper p o r t i o n s o f t h e heap, u n d e r o x i d i z e d and u n s a t u r a t e d
c o n d i t i o n s . However, t h i s r e a c t i o n i s n o t Eh dependent and c a n a l s o o c c u r i n
t h e reduced and s a t u r a t e d p o r t i o n s o f t h e heap e n v i r o n m e n t .

I t s h o u l d be n o t e d t h a t i f t h e heap c o n t a i n s a g g l o m e r a t e d o r e , t h e l i m e
a g g l o m e r a t i n g medium may b u f f e r t h e h y d r o l y s i s r e a c t i o n by k e e p i n g t h e pH
above 9.36. T h i s w i l l t e m p o r a r i l y p r e v e n t h y d r o l y s i s and t h e p r o d u c t i o n o f
HCN. With t i m e , i n f i l t r a t i o n o f water and c a r b o n d i o x i d e i n t o t h e heap w i l l
n e u t r a l i z e t h e cement a g g l o m e r a t i n g a g e n t s . The heap i s t h e n no l o n g e r
b u f f e r e d a t a n e l e v a t e d pH, and t h e h y d r o l y s i s o f CN- t o HCN w i l l t a k e p l a c e .

13.3.2 O x i d a t i o n o f HCN and CN'

The o x i d a t i o n o f e i t h e r HCN o r CN' i s g e n e r a l l y r e s t r i c t e d t o t h e upper,

o x i d i z e d p o r t i o n s o f t h e heap environment as shown o n F i g u r e 13.1. The
280 PRECIOUS METAL HEAP LEACHING PROJECTS

o x i d a t i o n o f HCN produces hydrogen cyanate (HCNO) as shown i n Equation 2

(Owenback, 1978).

The d i r e c t o x i d a t i o n o f CN- (Equation 3) r e q u i r e s a m i n e r a l o g i c a l ,

b a c t e r i o l o g i c a l , o r photochemical (sun1 ig h t ) c a t a l y s t , and produces cyanate
i o n s (CNO - ) .

2CN- + 02 + catalyst --t 2CNO' (3

Hydrogen c y a n a t e and c y a n a t e i o n s are much l e s s t o x i c than HCN. W i t h i n

t h e heap e n v i r o n m e n t , HCNO and CNO- r e a d i l y hydrolyze t o form ammonia and

-
c a r b o n d i o x i d e w h i c h a r e e v o l v e d from t h e system as gases as i l l u s t r a t e d by
E q u a t i o n 4.

HCNO + Hz0 NH3(g) + COZ(~)

The ammonia g e n e r a t e d i n Equation 4 can e i t h e r form ammonium compounds o r be

o x i d i z e d t o form n i t r a t e s depending upon t h e pH [Hendrickson and Daignault,
1 9 7 3 ) . In t h i s manner, o x i d a t i o n o f HCN o r CN- lowers t'he o v e r a l l cyanide
c o n t e n t o f t h e system.

13.3.3 Hydrolysis/Saponification o f HCN

As t h e system pH f a l l s , HCN can be hydrolyzed by a d i f f e r e n t r o u t e t o
form f o r m a t e , e i t h e r f o r m i c a c i d o r ammonium formate by:

HCN + 2H20 +- NH4.COOH (ammonium formate) (5)

or
HCN + 2H2O +- NI-13 + H.COOH ( f o r m i c a c i d ) (6)
'The system pH w i l l d e t e r m i n e t h e e x t e n t o f f o r m a t i o n o f each compound, a l o w e r
pH f a v o r i n g f o r m i c a c i d f o r m a t i o n . T h i s form o f h y d r o l y s i s has been r e f e r r e d
t o a s " s a p o n i f i c a t i o n " by some workers, although t h i s term n o r m a l l y r e f e r s t o
t h e f o r m a t i o n o f soapy compounds o r f a t t y soaps [ s t e a r a t e s , e t c . ) .

A e r o b i c B i o d e g r a d a t i o n o f HCN

Under a e r o b i c c o n d i t i o n s w i t h i n t h e upper, o x i d i z e d p o r t i o n s of t h e
heap, b i o l o g i c a l processes may consume hydrogen cyanide and generate hydrogen
.
c y a n a t e as shown i n E q u a t i o n 7 ( T o w i l l e t a1 , 1978). The hydrogen cyanate i s
i n t u r n h y d r o l y z e d i n t o ammonia and carbon d i o x i d e (Equation 4 ) .

~HCN + 02 + enzyme -2HCNO (7)

HCNO + Hz0 +NH3 + CO2 (4)

As i n d i c a t e d i n E q u a t i o n 7, t h i s process r e q u i r e s an enzyme, but i s otherwise

i d e n t i c a l t o t h e o x i d a t i o n r e a c t i o n shown i n Equation 3.

13.3.5 F o r m a t i o n o f Thiocyanates

T h i o c y a n a t e i o n s (CNS-) can be formed by t h e r e a c t i o n between cyanide

and a n y s u l f u r s p e c i e s such as s u l f i d e , hydrogen s u l f i d e , o r t h i o s u l fate.
 CYANIDE GEOCHEMISTRY AND DETOXIFICATION REGULATIONS 28 1

~ h i o c y a n a t ew i l l be a common c y a n i d e s p e c i e s i n a heap c o n t a i n i n g s u l f i d e o r e ,
o r i n heaps p l a c e d upon s u l f i d e - r i c h bedrock.

AS such, t h i o c y a n a t e f o r m a t i o n depends more upon m i n e r a l o g i c c o n d i t i o n s

than on pH and Eh, and can o c c u r anywhere i n a heap environment w i t h a v a i l a b l e
sulfur species. Exalnples o f t h i o c y a n a t e f o r m i n g r e a c t i o n s i n c l u d e t h e
following equations:

I n o x i d i z e d p o r t i o n s o f t h e heap environment, t h e ~ 0 ~ 2formed

- .in
Equation 9 w i l l r e a c t w i t h oxygen t o f o r m s u l f a t e SO^^-).

T h i o c y a n a t e i s a r e l a t i v e l y s t a b l e and l e s s t o x i c form o f c y a n i d e , and

t h i o c y a n a t e f o r m a t i o n i s an e f f e c t i v e way o f removing c y a n i d e from t h e heap
environment. However, i n heaps w i t h e l e v a t e d s u l f i d e c o n c e n t r a t i o n s , a c i d i c
c o n d i t i o n s can f o r m w h i c h i n t u r n can i n f l u e n c e t h e c y a n i d e g e o c h e m i s t r y . As
described above, a c i d i c c o n d i t i o n s w i l l f a v o r t h e h y d r o l y s i s o f CN' t o HCN.
A c i d i c c o n d i t i o n s may a1 so cause t h e d i s s o c i a t i o n o f heavy m e t a l - c y a n i d e
complexes and compounds, r e s u l t i n g i n t h e i n c r e a s e d m o b i l i t y o f some heavy
metals and c r e a t i n g p o t e n t i a l heavy m e t a l c o n t a m i n a t i o n problems.

13.3.6 Simp1 e Cyanide Compounds

Cyanide forms s i m p l e compounds (i.e., simple s a l t s ) w i t h s i n g l e i o n s o f

c e r t a i n m e t a l s . I n s o l u t i o n , t h e s e s i m p l e c y a n i d e compounds i o n i z e and produce
free m e t a l c a t i o n s and c y a n i d e . An example o f t h i s t y p e o f r e a c t i o n i s shown
i n E q u a t i o n 10.

NaCN +~ a + + CN- (10)

Some o f t h e m e t a l c y a n i d e compounds commonly found i n a heap e n v i r o n m e n t

are l i s t e d i n T a b l e 13.1. O b v i o u s l y t h e presence and r e l a t i v e abundance o f any
o f t h e s e compounds depends upon t h e c o m p o s i t i o n and m i n e r a l o g y o f t h e o r e i n
the heap and t h e u n d e r l y i n g bedrock.

As i n d i c a t e d on T a b l e 13.1, t h e s o l u b i l i t y o f s i m p l e c y a n i d e compounds
ranges from r e a d l l y s o l u b l e t o f a i r l y i n s o l u b l e . G e n e r a l l y speaking, a l l
simp1 e c y a n i d e compounds a r e r e l a t i v e l y n o n - t o x i c , and t h e f o r m a t i o n o f s i m p l e
cyanide compounds i s a n o t h e r n a t u r a l l y o c c ~ r r r i n g geochemical process t h a t
removes t o x i c c y a n i d e s p e c i e s fro111 t h e heap environment. However, t h e s o l u b l e
simple c y a n i d e compounds such as NaCN, KCN, c ~ ( c N )and ~ H ~ ( c N )d ~i s s o c i a t e
r e a d i l y i n s o l u t i o n and produce c y a n i d e i o n s q qua ti on 10). A t pH v a l u e s l o w e r
than 9.36, t h i s i o n i c c y a n i d e w i l l h y d r o l y z e and produce t o x i c hydrogen
cyanide ( E q u a t i o n 1).

13.3.7 M e t a l - C y a n i d e Complex I o n s

Cyanide can a l s o r e a c t w i t h m e t a l s t o form m e t a l - c y a n i d e complexes.

Metal-cyanide complex i o n s form as t h e p r o d u c t s o f t h e r e a c t i o n between t h e
i n s o l u b l e c y a n i d e compounds and excess c y a n i d e i o n s . Equation 11 i s an example
of t h i s t y p e o f r e a c t i o n .
 PRECIOUS METAL HEAP LEACHING PROJECTS

TABLE 13.1
RELATIVE STABILITY OF METAL CYAN1DE COMPOUNDS AND COMPLEXES I N WATER
(LISTED I N APPROXIMATE ORDER OF INCREASING STABILITY)

C Y A N I D E SPECIES EXAMPLES PRESENT I N THE HEAP ElVV IRONMENT

1. Free Cyanide CN', HCN

2. Simple Cyanide Conipounds
a) Readily s o l u b l e NaCN, KCN, Ca(CN)2, Yg(CN)2
b ) Re1a t iv e l y in s o l u b l e Zn (CN) 2, CuCN, N i (CN) 2, AgCN
3. Weak Metal-Cyanide z ~ ( c N ) ~ Cd(CNI3-,
~-, c ~ ( c N ) ~ ~ -
Complexes
4. Moderately Strong Metal
Cyan1 de Compl exes
- Cu(CN)'2, Cu(CN)3 2-, N i ( c N ) ~ ~ -Ag(CN)-2
,

5. S t r o n g Metal-Cyanide F ~ ( c N ) ~ F~ ~- ,( c N ) ~ C
~O- ,( C N ) ~ ~AU- ,( c N ) ~ ~H, ~ ( c N ) ~
Compl exes

M o d i f i e d a f t e r H u i a t t e t a1 ., 1982; and B r i c k e l l , 1981.

As shown on Table 13.1, some o f these m e t a l - c y a n i d e complexes a r e

r e l a t i v e l y t o e x c e p t i o n a l l y s t a b l e ( i .e., i n s o l u b l e ) , whereas o t h e r s i o n i z e
r e a d i l y and form CN', which i n t u r n h y d r o l y z e s t o form HCN (Equation 1 ) .

As w i t h t h e s i m p l e c y a n i d e compounds, t h e t o x i c i t y o f metal c y a n i d e
complexes i s due t o p r o d u c t i o n o f HCN as a d i s s o c i a t i o n and h y d r o l y s i s
product. However, copper- and s i l v e r - c y a n i d e cornpl exes i n t h e i r u n d i s s o c i a t e d
forms appear t o be t o x i c t o f i s h (Gannon, 1981). A1 though t h e i r o n - c y a n i d e
complexes a r e q u i t e i n s o l u b l e , t h e y may be d e s t r o y e d through p h o t o l y s i s
( r e a c t i o n w i t h u l t r a v i o l e t r a d i a t i o n ) t h e r e b y r e l e a s i n g i o n i c cyanide (CN').
The r a t e o f p h o t o l y s i s i s h i g h l y v a r i a b l e depending upon s i t e c o n d i t i o n s .
Photodecomposi t i o n o f iron-cyanide complexes may be negl ig i b l e i n deep,
t u r b i d , o r shaded waters, o r w i t h i n t h e i n t e r i o r o f a heap.

Data developed r e c e n t l y , however, i n d i c a t e t h a t t h e " t . r a d i t i o n a 1 "

e x p l a n a t i o n o f weak and s t r o n g metal complexes, w h i l e t h e o r e t i c a l l y c o r r e c t ,
may n o t be t o t a l l y a p p l i c a b l e i n n a t u r e . Work by Ridgeway M i n i n g Company
(Smith, 1987) i n d i c a t e s t h a t t h e s t a b i l i t y o f copper and c o b a l t c y a n i d e s
appears t o be a f u n c t i o n o f t h e c o n c e n t r a t i o n o f c y a n i d e i n s o l u t i o n , a s
suggested by t h e d a t a on Table 13.2.

The o r i g i n a l , h i g h pH s o l u t i o n has copper and c o b a l t as c y a n i d e

complexes, b o t h i o n s b e i n g i n s o l u b l e compared t o t h e i r observed 1 e v e l s o f
occurrence i n such a l k a l i n e c o n d i t i o n s . As t h e cyanide l e v e l s decrease due t o
degradation, t h e copper and cobal t v a l u e s d r o p d r a m a t i c a l l y as t h e s o l u b i 1 i t y
c o n t r o l changes from cyanide cornpl e x a t i o n t o s o l u t i o n pH. 'These changes c a n n o t
be e x p l a i n e d by d i l u t i o n a l o n e . The copper and c o b a l t complexes appear n o t t o
be s t a b l e under t h e s e c o n d i t i o n s .
 CYANIDE GEOCHEMISTRY AND DETOXIFICATION REGULATIONS 283

-
FIGURE 13.2
T H E R E L A T I O N S H I P BETWEEN H C N AND C N W I T H P H
284 PRECIOUS METAL HEAP LEACHING PROJECTS

TABLE 13.2
TIME/CYANIDE CONCENTRATION DEPENDENCY O f COPPER AND COBALT C Y A N I D E
I N THE TAILINGS SUPERNATANT SOLUTION

ORIGINAL TAILINGS RINSED SAMPLE RINSED SAMPLE

PARAMETER SUPERNATANT TIME 1 TIME 2

T o t a l cyanide

Free cyanide

Cobal t

Copper

13.3.8 Anaerobic Biodegradation

Anaerobic biodegradation o f cyanide and hydrogen cyanide i s r e s t r i c t e d

t o t h e moderately t o s t r o n g l y reduced p o r t i o n s o f t h e heap environment ( F i g u r e
13.21, and can o n l y occur if HS- o r H z S ( ~ ~a r)e present. (The s u l f u r s p e c i e s
p r e s e n t w i l l depend on pH. At a pH v a l u e g r e a t e r than 7, HS' i s t h e dominant
s p e c i e s . At a lower pH, HzS(,~) w i l l be p r e s e n t ) . Equations 12 and 13
i l l u s t r a t e t h e anaerobic biodegradation o f cyanide.

HCN + HS--HCNS + H' (13)

The HCNS w i l l the n hydrolyze t o form NH3, H2S and C02 (Schmidt e t a1 ., 1981).
13.3.9 Formation o f HCN Polymers

There i s a growing body of data i n d i c a t i n g t h a t , under n a t u r a l

c o n d i t i o n s i n o r below heap l e a c h o p e r a t i o n s o r t a i l i n g s f a c i l i t i e s , HCN can
form polymers. A1 though t h i s may be based on t h e o r e t i c a l c o n s i d e r a t i o n s o f t h e
b e h a v i o r o f l i q u i d HCN, f t i s known t h a t HCN i n t h e presence o f t r a c e amounts
o f ammonia can form adenine (CgHgNg), a b i o c h e m i c a l l y i m p o r t a n t molecule t h a t
o c c u r s i n DNA and RNA. This r e a c t i o n i s n o t shown on F i g u r e 13.1. However, t h e
environmental e f f e c t of cyanide p o l y m e r i z a t i o n would be t o remove cyanide from
s o l u t i o n by f o r m i n g i n s o l u b l e polymers.
 CYANIDE GEOCHEMISTRY AND DETOXIFICATION REGULATIONS

13.4 SUMMARY OF THE ENVIRONMENTAL EFFECTS OF CYANIDE IN AN ABANDONED HEAP

As d i s c u s s e d i n t h e p r e c e d i n g s e c t i o n s and a s shown on F i g u r e 13.1,

nunierous c y a n i d e s p e c i e s e x i s t w i t h i n an abandoned heap e n v i r o n m e n t . Many o f
t h e c y a n i d e s p e c i e s p r e s e n t a r e n o t h i g h l y t o x i c and a r e r e l a t i v e l y s t a b l e
complexes o r compounds under m o s t c o n d i t i o n s . Some c y a n i d e s p e c i e s , however,
a r e n o t s t a b l e and r e a c t w i t h t h e e n v i r o n m e n t t o p r o d u c e HCN, t h e t o x i c form
o f cyanide.

As d e s c r i b e d above, t h e r e a r e a number o f p o t e n t i a l n a t u r a l l y o c c u r r i n g
geochemical r e a c t i o n s w i t h i n t h e h e a p e n v i r o n m e n t w h i c h d e g r a d e HCN i n t o l e s s
t o x i c o r n o n t o x i c compounds. Thus w i t h t i m e , t h e heap e n v i r o n m e n t t e n d s t o be
sel f - n e u t r a l i z i n g , and t h e HCN c o n c e n t r a t i o n w i l l d e c r e a s e .

E n g l e h a r d t (1985) documented t h a t i n an a c t i v e heap c o n t a i n i n g s i l v e r -

l e a d o r e t a i l i n g s , t h e m o l e c u l a r h y d r o g e n c y a n i d e (HCN) c o n t e n t i n t h e
s o l u t i o n s e n t r a i n e d i n t h e heap d e c r e a s e s r a p i d l y due t o n a t u r a l d e g r a d a t i o n .
Based upon d a t a 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 f o r one and one-ha1 f y e a r s ,
r o u g h l y 85 p e r c e n t o f t h e o r i g i n a l c y a n i d e c o n t e n t d i s s i p a t e d o r degraded by
n a t u r a l processes w i t h i n 18 m o n t h s . C y a n i d e d e s t r u c t i o n i n t h i s heap o c c u r r e d
as a r e s u l t o f f i r s t o r d e r c h e m i c a l r e a c t i o n s p r o p o r t i o n a l t o t h e c o n c e n t r a t d o n
o f t h e r e s i d u a l c y a n i d e i n t h e heap. E x t r a p o l a t i o n o f t h e s e d a t a s u g g e s t t h a t
t h e c y a n i d e i n t h i s heap w o u l d b e t o t a l l y d e s t r o y e d I n l e s s t h a n f o u r y e a r s
following closure.

More r e c e n t l y , Chatwin e t a l , ( 1 9 8 7 ) o n b e h a l f : o f Dupont, has p r o d u c e d a

Phase 1 r e p o r t o f a s t u d y o n t h e a t t e n u a t i o n o f c y a n i d e , T h e i r i n t e r i m
c o n c l u s i o n s suggest t h e m a j o r mechanisms f o r c y a n i d e r e d u c t l o n I n t h e vadoze
( u n s a t u r a t e d ) zone w i l l be v o l a t i z a t i o n , c h e m i c a l r e a c t l o n o r a d s o r p t l o n w d t h
s o i l s / r o c k s i n t h i s zone and, p o s s i b l y , b i o d e g r a d a t i o n , I n a d d i t i o n , t h o s e
m a t e r i a l s w h i c h a t t e n u a t e d c y a n i d e i n t h e i r t e s t s c o n t i n u e d t o be e f f e c t f v e a t
the 50 pore volume s t a g e when t h e t e s t s were h a l t e d .

Cyanide N e u t r a l i z a t i o n R e q u i r e m e n t s f o r S p e n t Heaps

R e g u l a t o r y o f f i c i a l s f r o m 1 4 s t a t e s were questioned a b o u t t h e i r s t a t e ' s

r e g u l a t i o n s c o n c e r n i n g maximum r e s i d u 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 s p e n t heaps.
Table 13.3 l i s t s t h e s t a t e s a n d t h e r e g u l a t o r y a g e n c l e s c o n t a c t e d a n d T a b l e
13.4 summarizes t h e r e s u l t s o f t h e s u r v e y .

A wide d i v e r s i t y o f r e s p o n s e s was r e c e i v e d i n t h l s s u r v e y , No t w o s t a t e s
have i d e n t i c a l r e g u l a t i o n s . However, some generalizations a b o u t t h e s u r v e y
data can be made. Most s t a t e s t o o k o n e o f t h e f o l l o w i n g a p p r o a c h e s :

S t r i c t adherence t o a u n i v e r s a l l y a p p l i c a b l e c y a n i d e d e t o x i f i c a t i o n
s t a n d a r d ( g e n e r a l l y t h e EPA d r i n k i n g w a t e r g u i d e l i n e o f 0.2 mg/l
t o t a l c y a n i d e ) ; and
E s t a b l i s h p e r m i t c o n d i t i o n s on a c a s e - b y - c a s e b a s l s d e p e n d i n g on
s i te-speci f i c conditions.

I n m o s t s t a t e s , c y a n i d e n e u t r a l i z a t i o n c r i t e r i a a r e specified i n t h e
" z e r o - d i s c h a r g e " w a t e r qua1 it y p e r m i t r e q u i r e d t o b u i 1 d and o p e r a t e a heap
l e a c h f a c i l i t y . I n a d d i t i o n t o n e u t r a l i z a t i o n r e q u i r e m e n t s , t h e s e p e r m i t s a1 so
specify design, c o n s t r u c t i o n , o p e r a t i o n a l , and m o n i t o r i n g c r i t e r i a .
 PRECIOUS METAL HEAP LEACHING PROJECTS

TABLE 13.3
STATE AGENCIES CONTACTED FOR THIS SURVEY

STATE ORGANIZATION

Alaska D e p t . o f E n v i r o n m e n t a l C o n s e r v a t i o n (907/425-1714)

Arizona S t a t e Mine I n s p e c t o r ' s O f f i c e ( 6 0 2 / 2 5 5 - 5 9 7 1 )

C a l if o r n i a S o u t h e r n ~ a h o n f t a n Region Water Qua1 it y C o n t r o l Board

(619/245-6583)
N o r t h e r n Lahontan Region Water Q u a l i t y C o n t r o l Board
( 916/544- 3481 )

C o l o r a d o R i v e r B a s i n Region Water Qua1it y C o n t r o l B o a r d

(619/346-7491)

Colorado D e p t . o f N a t u r a l Resources/Mined Land R e c l a m a t i o n D i v .

( 303/866-3567)

Idaho Dept. o f H e a l t h and W e l f a r e D i v i s i o n o f E n v i r o n m e n t

(208/334-4784)

Montana Dept. o f S t a t e Lands (406/444-2074)

Nevada Dept. o f C o n s e r v a t i o n and N a t u r a l R e s o u r c e s / D i v i s i o n

o f Environmental P r o t e c t i o n (702/885-4670)

New Mex ico Groundwater S e c t i o n New M e x i c o E n v i r o n m e n t a l Improvement

Division

N o r t h Dakota H e a l t h Dept. - Water S u p p l y and P o l l u t i o n C o n t r o l

( 701/224-2354)

Oregon Dept. o f E n v i r o n m e n t a l Q u a l i t y - Water Q u a l i t y D i v i s i o n

(503/229-5325)

South Carol ina S o u t h C a r o l i n a Land Resources Commission D i v i s i o n o f

M i n i n g and E n v i ronment (803/758-2823)

S o u t h Dakota Department o f Water and N a t u r a l R e s o u r c e s

(605/773-3151)

Utah D i v . o f E n v i r o n m e n t a l Heal t h / W a t e r P o l l u t i o n C o n t r o l
(801/533-6146)

Washington D i v . o f Geology and E a r t h Resources/Dept. o f Natural

Resources (206/459-6372)

Wyomi n g Water Q u a l i t y D i v i s i o n (307/777-7756)

 CYANIDE GEOCHEMISTRY AND DETOXIFICATION REGULATIONS 287

TABLE 13.4
SUMMARY OF STATE CYANIDE REGULATIONS SURVEY

CYANIDE
STATE REGULATIONS COMMENTS

A1 as ka No s p e c i f i e d No e x p e r i e n c e w i t h c y a n i d e heap
standards leach mining.

Arizona 0.2 mg/l f r e e CN' State health standards only.

Assume z e r o d i s c h a r g e . D i s c o u r a g e
hypochlorit e neutral i z a t i o n .

Cal if o r n ia
N o r t h e r n Lahontan 0.4 m g / l t o t a l CN' Standards a p p l y t o r e m a i n i n g
Region 0.2 m g / l f r e e CN- l i q u i d i n ponds, heaps, e t c . A l s o
have s o l i d s e x t r a c t i o n t e s t o f
l e s s t h a n 4.0 mg t o t a l CN'/kg
s p e n t o r e and l e s s t h a n 1.6 mg/kg
f r e e CN'/kg s p e n t o r e .

S o u t h e r n Lahontan 0.2mg/l f r e e CN' Standard a p p l i e s t o r e g i o n

Region r e m a i n i n g l i q u i d s i n ponds, heaps,
e t c . A l s o have s o l i d s e x t r a c t i o n
s t a n d a r d o f 10 mg t o t a l CNm/kg
tailings.

Colorado R i v e r No s p e c i f i e d S t a t i s t i c a l a p p r o a c h : 90 p e r c e n t
B a s i n Region standard (precedent o f a t l e a s t 10 samples must
o f 1 0 m g / l f r e e CN' c o n t a i n l e s s t h a n 10 mg/l f r e e
f o r Mesquite) CN'; no sample can c o n t a i n more
t h a n 20 mg/l f r e e CN-. T e s t i n g and
sample p r o c e d u r e s s p e c i f i e d f o r
5 : l e x t r a c t i o n p e r f o r m e d on a
100 g sample o f l e a c h e d o r e .

Colorado No s e t s t a n d a r d s E v a l u a t e each s i t e f o r q u a l i t y o f
( w i l l be r e d r a f t i n g r e c e i v i n g s u r f a c e and g r o u n d w a t e r .
regulations) Emphasize z e r o - d i s c h a r g e d e s i g n
c o n t r o l s t o ensure containment.

Idaho New r e g u l a t i o n s R e g u l a t i o n s do n o t s p e c i f y one,

r e c e n t l y adopted. u n i v e r s a l CN' s t a n d a r d . W i l l
Neutralization e v a l u a t e each s i t e - p r o x i r n i t y
r e q u i r e m e n t based t o s u r f a c e w a t e r , qua1 i t y o f
upon s i t e c h a r a c t e r - r e c e i v i n g w a t e r , e t c . New
i s t i c s expressed i n r e g u l a t i o n s i n c l u d e a bonding
t e r m s o f pH r a n g e o r r e q u i rernent .
f r e e a n d / o r WAD
c y a n ide
 PRECIOUS METAL HEAP LEACHING PROJECTS

TABLE 13.4 ( c o n ' t )

SUMMARY OF STATE CYANIDE REGULATIONS SURVEY

CYANIDE
STATE REGULATIONS CCIMMENTS

Montana No s e t s t a n d a r d s Determined by q u a l i t y o f r e c e i v i n g
(nondegrada t i on w a t e r . Nondegradation imp1 i e s a
rule) d r i n k i n g w a t e r s t a n d a r d (0.2 m g / l
f r e e CN-) i f high-qua1 it y
.
r e c e i v i n g w a t e r s P e r m i t s c o u l d be
more l e n i e n t f o r l i n e d f a c i l i t i e s
w i t h caps and d i v e r s i o n s t o
p r e v e n t d r a i n a g e i n t o streams.

New Mex ico 0.2 mg/l t o t a l CN- Not an e f f l u e n t s t a n d a r d p e r se.

W i l l take i n t o consideration
a t t e n u a t i o n and d i l u t i o n ( i .e.,
can have more t h a n 0.2 m g / l t o t a l
CN' i n e f f l u e n t ) . O r t i z r i n s e
t e s t : 100 l b sample r i n s e d w i t h
one g a l l o n f r e s h water; f i l t r a t e
must be l e s s t h a n 100 ppm f r e e
CN'. M i g h t a c c e p t t h i s f o r o t h e r
lined facilities.

Nevada No s e t s t a n d a r d s Eva1 u a t e each s i t e f o r p r o x i m i t y

0.2 mg/l f r e e CN- t o v a l u a b l e s u r f a c e w a t e r and
i s a " t a r g e t " con- modify "target" concentration
c e n t r a t i o n (most a c c o r d i n g l y . R i s k i n g and t e s t i n g
o p e r a t o r s can t procedures d i f f e r f o r agglomerated
meet ) vs. nonagglomerated o r e . D i s -
courage h y p o c h l o r i t e a n d / o r
peroxide.

N o r t h Dakota No s e t s t a n d a r d s No heap l e a c h mines. Would

p r o b a b l y impose s u r f a c e w a t e r
q u a l i t y standards.

Oregon I n t h e process o f d r a f t i n g .

South Carol ina No s e t s t a n d a r d s W i l l e v a l u a t e each s i t e . A f t e r

(Hai 1 e p r e c e d e n t c l o s u r e heaps a t H a i l e w i l l d r a i n
o f 10 mg/l f r e e i n t o stream w/pH o f 4 and e l e v a t e d
CN-) CN' c o n t e n t .

South Dakota S t a t e standard o f Measured i n e f f l u e n t coining o f f o f

0.50 WAD c y a n i d e heap - any method o f d e t o x i f i -
c a t i o n a c c e p t a b l e . Mean o f s e v e r a l
samples must be l e s s t h a n o r e q u a l
t o 0.50 WAD c y a n i d e .
 CYANIDE GEOCHEMISTRY AND DETOXIFICATION REGULATIONS 289

T A B L E 13.4 ( c o n ' t )
SUMMARY OF S T A T E C Y A N I D E R E G U L A T I O N S SURVEY

- -

CYANIDE
STATE REGULATIONS COMMENTS

Utah No s e t standards Would l o o k a t Mercur p e r m i t as a

(Mercur precedent precedent, b u t n o t a s t a n d a r d .
o f 5 ppm f r e e CN-) Require double-1 i n e d pads w / l eak
d e t e c t i o n system. Mercur uses
perrnanent/expandable pads ( i .e.,
heaps wasted i n s i t u ) .

Washington No s e t standards W i l l probably e v a l u a t e each s i t e ,

( i n process o f no measurable CN' a l l o w e d i n
d r a f t i n g regula- e f f l u e n t from r e c l a i m e d
tions) facilities.

Wyomi rr g .02 mg/l f r e e CN' Based on qual it y o f r e c e i v i n g

water, e f f l u e n t c a n ' t exceed
ambient water qual i t y . Would be
more l e n i e n t f o r z e r o - d i s c h a r g e
f a c i l i t i e s (i.e., i f no s u r f a c e
o r groundwater i m p a c t s ) .
290 PRECIOUS METAL HEAP LEACHING PROJECTS

G e n e r a l l y speaking, most o f t h e s t a t e s a d o p t i n g a d r i n k i n g w a t e r
g u i d e l i n e a s t h e i r c y a n i d e n e u t r a l i z a t i o n c r i t e r i o n do n o t have many a c t i v e
c y a n i d e heap l e a c h o p e r a t i o n s and t h u s do n o t have much d i r e c t e x p e r i e n c e w i t h
heap l e a c h t e c h n o l o g y . I n c o n t r a s t , t h e s t a t e s w i t h s e v e r a l a c t i v e c y a n i d e
heap l e a c h p r o j e c t s have o p t e d f o r t h e more s i t e s p e c i f i c approach, g i v i n g
some f l e x i b i l i t y i n d e t e r m i n i n g p e r m i t c o n d i t i o n s .

Those states which do consider site specificity in setting

n e u t r a l i z a t i o n r e q u i r e m e n t s and o t h e r p e r m i t c o n d i t i o n s general l y t r y t o
s e l e c t r e c l amat i o n p r o c e d u r e s and n e u t r a l i z a t i o n and abandonment r e q u i r e m e n t s
a p p r o p r i a t e f o r t h e p r o j e c t s i t e . When s i t e c o n d i t i o n s a r e t a k e n i n t o
c o n s i d e r a t i o n , i t becomes a p p a r e n t i n many cases t h a t i t i s i m p r a c t i c a l and
u n n e c e s s a r y t o impose d r i n k i n g w a t e r g u i d e l i n e s o n t h e r i n s a t e f r o m a
d e t o x i f i e d heap. The e n v i r o n m e n t a l s e n s i t i v i t y o f t h e p r o j e c t s i t e may n o t
w a r r a n t s u c h s t r i n g e n t r e q u i r e m e n t s . F u r t h e r m o r e , i t may be c o s t l y , if n o t
imposs-ible, t o n e u t r a l i z e t o a d r i n k i n g water g u i d e l i n e .

In e v a l u a t i n g a p r o j e c t and d e t e r m i n i n g p e r m i t c o n d i t i o n s , most o f t h e
s t a t e s w i t h e x p e r i e n c e i n heap l e a c h o p e r a t i o n s were more concerned w i t h
cyanide containment than w i t h s p e c i f i c cyanide concentrations remaining i n t h e
heap. These states emphasize engineering parameters such as 1i n e r
s p e c i f i c a t i o n s and h y d r o l o g i c ( f l o o d c o n t r o l ) d e s i g n c r i t e r i a . Some o f t h e
more e x p e r i e n c e d s t a t e s a l s o s p e c i f y d e t a i l e d p r o c e d u r e s f o r heap r i n s i n g ,
r i n s a t e sampl i n g and a n a l y t i c a l t e s t i n g .

There i s a l s o no consensus among t h e s t a t e s surveyed c o n c e r n i n g t h e

c y a n i d e s p e c i e s s p e c i f i e d i n t h e r e g u l a t i o n s o r p e r m i t c o n d i t i o n s . Some o f t h e
s t a t e s e x p r e s s t h e i r c y a n i d e r e q u i r e m e n t s i n terms o f f r e e c y a n i d e , whereas
o t h e r s have s t a n d a r d s d e f i n e d i n t e r m s o f t o t a l c y a n i d e . A t l e a s t one s t a t e
( I d a h o ) i n c l u d e s weak a c i d d i s s o c i a b l e c y a n i d e i n i t s r e g u l a t i o n s . Some s t a t e s
a l s o e x p r e s s n e u t r a l i z a t i o n r e q u i r e m e n t s as a r a n g e o f pH v a l u e s i n l i e u o f a
specified cyanide l e v e l .

As shown on T a b l e 13.4, some s t a t e s a r e i n t h e process o f e s t a b l i s h i n g

o r r e d r a f t i n g c y a n i d e n e u t r a l iz a t i o n r e q u i r e m e n t s f o r s p e n t cyanided heaps. I n
most cases, t h i s a c t i o n has been prompted b y t h e i n c r e a s i n g number o f p e r m i t
a p p l i c a t i o n s f o r p r e c i o u s m e t a l c y a n i d e heap l e a c h o p e r a t i o n s . The c h a n g i n g
r e g u l a t o r y atmosphere i s i n d i r e c t response t o t h e i n c r e a s i n g use o f heap
.
1e a c h t e c h n i q u e s

Coupled w i t h t h e r e g u l a t i o n s i n s e v e r a l s t a t e s i s a bonding a u t h o r i t y
g o v e r n i n g heap l e a c h p r o j e c t s . Heap l e a c h o p e r a t o r s i n t h e s e s t a t e s a r e
r e q u i r e d t o o b t a i n a bond t o c o v e r t h e c o s t o f r i n s i n g and d e t o x i f y i n g t h e
heaps, and r e l e a s e o f t h i s bond i s t i e d t o n e u t r a l i z i n g t h e s p e n t heaps t o a
s p e c i f i e d standard.

An u n d e r s t a n d i n g o f c y a n i d e g e o c h e m i s t r y i s a p r e r e q u i s i t e t o
e s t a b l i s h i n g r e a l i s t i c c y a n i d e d e t o x i f i c a t i o n s t a n d a r d s f o r abandoned heap
leach projects. Environmental regulations should consider the s e l f-
n e u t r a l i z a t i o n p o t e n t i a l o f an i n a c t i v e heap, t h e ephemeral n a t u r e o f
~ i i o l e c ual r hydrogen c y a n i d e , and t h e e n v i r o n m e n t a l s e n s i t i v i t y o f a g i v e n
p r o j e c t . More s t r i n g e n t r e q u i r e m e n t s s h o u l d be r e s e r v e d f o r p r o j e c t s w i t h
n e a r b y , d o w n g r a d i e n t f i s h a b l e s t r e a m s o r p o t a b l e s u r f a c e and g r o u n d w a t e r
suppl i e s .
 CYANIDE GEOCHEMISTRY AND DETOXIFICATION REGULATIONS 29 1

A p p l i c a t i o n o f a c y a n i d e d r i n k i n g w a t e r g u i d e l i n e on t h e r i n s a t e from a
n e u t r a l i z e d heap i s n e i t h e r r e a l i s t i c a l l y a c h i e v a b l e n o r n e c e s s a r y i n o r d e r t o
p r o t e c t s u r f a c e and g r o u n d w a t e r r e s o u r c e s i n n e a r l y a l l s i t u a t i o n s i n o u r
e x p e r i e n c e . Based upon a c o n s i d e r a t i o n o f s i t e c o n d i t i o n s i n c l u d i n g t h e
a t t e n u a t i n g p r o p e r t i e s o f s i t e s u b s o i l s and bedrock, t h e d i s t a n c e t o t h e
nearest p o t e n t i a l s u r f a c e demonstrated t h a t water g u i d e l i n e s a r e m a i n t a i n e d
f o r t h e s e r e c e i v i n g w a t e r s when h i g h e r r e s i d u a l c y a n i d e l e v e l s a r e a l l o w e d i n
t h e d e t o x i f i e d heaps.

As more heap l e a c h m i n i n g p e r m i t a p p l i c a t i o n s a r e r e v i e w e d and p r o c e s s e d

b y r e g u l a t o r y a g e n c i e s , more changes i n t h e r e g u l a t o r y e n v i r o n m e n t a f f e c t i n g
p r e c i o u s m e t a l heap 1 each p r o j e c t s can be e x p e c t e d . W i t h i n c r e a s e d e x p e r i e n c e
i n heap l e a c h o p e r a t i o n s , and a b r o a d e n i n g awareness o f t h e s h o r t - and l o n g -
term e f f e c t s o f c y a n i d e and t h e b e h a v i o r o f c y a n i d e i n t h e heap l e a c h
environment, more s t a t e s h o p e f u l l y w i l l a d o p t t h e s i t e - b y - s i t e a p p r o a c h t o
establ i s h i n g cyanide n e u t r a l i z a t i o n requirements.

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