Cyanide recovery process

A process for removing and recovering cyanide from a cyanide-containing mixture. The process includes the steps of adjusting the pH of the cyanide-containing mixture to between about 6 to about 9.5, volatilizing the HCN contained in the pH adjusted mixture and contacting the volatilized HCN with basic material. Preferably, the cyanide recovery process is performed on tailings slurries resulting from metal recovery processes.

FIELD OF THE INVENTION 
The present invention relates cyanide removal and recovery from 
cyanide-containing mixtures. 
BACKGROUND OF THE INVENTION 
Cyanides are useful materials industrially and have been employed in fields 
such as electro-plating and electro-winning of metals, gold and silver 
recovery from ores, treatment of sulfide ore slurries in flotation, 
tannery processes, etc. Due to environmental concerns, it is desirable to 
remove or destroy the cyanide present in the waste solutions resulting 
from such processes. Additionally, in view of the cost of cyanide, it is 
desirable to regenerate the cyanide for reuse. 
Techniques for cyanide disposal or regeneration (recovery) in waste 
solutions include: ion exchange, oxidation by chemical or electrochemical 
means, and acidification-volatilization-reneutralization (AVR). The term 
cyanide recovery and regeneration are used interchangeably herein. 
U.S. Pat. No. 4,267,159 by Crits issued May 12, 1981, discloses a process 
for regenerating cyanide in spent aqueous liquor by passing the liquor 
through a bed of suitable ion exchange resin to segregate the cyanide. 
U.S. Pat. No. 4,708,804 by Coltrinari issued Nov. 24, 1987, discloses a 
process for recovering cyanide from waste streams which includes passing 
the waste stream through a weak base anion exchange resin in order to 
concentrate the cyanide. The concentrated cyanide stream is then subjected 
to an acidification/ volatilization process in order to recover the 
cyanide from the concentrated waste stream. 
U.S. Pat. No. 4,312,760 by Neville issued Jan. 26, 1982, discloses a method 
for removing cyanides from waste water by the addition of ferrous 
bisulfite which forms insoluble Prussian blue and other reaction products. 
U.S. Pat. No. 4,537,686 by Borbely et al. issued Aug. 27, 1985, discloses a 
process for removing cyanide from aqueous streams which includes the step 
of oxidizing the cyanide. The aqueous stream is treated with sulfur 
dioxide or an alkali or alkaline earth metal sulfite or bisulfite in the 
presence of excess oxygen and a metal catalyst, preferably copper. This 
process is preferably carried out at a pH in the range of 5 to 12. 
U.S. Pat. No. 3,617,567 by Mathre issued Nov. 2, 1971, discloses a method 
for destroying cyanide anions in aqueous solutions using hydrogen peroxide 
(H.sub.2 O.sub.2) and a soluble metal compound catalyst, such as soluble 
copper, to increase the reaction rate. The pH of the cyanide solution to 
be treated is adjusted with acid or base to between 8.3 and 11. 
Treatments based on oxidation techniques have a number of disadvantages. A 
primary disadvantage is that no cyanide is regenerated for reuse. 
Additionally, reagent costs are high, and some reagents (e.g. H.sub.2 
O.sub.2) react with tailing solids. Also, in both the Borbely et al and 
Mathre processes discussed above, a catalyst, such as copper, must be 
added. 
U.S. Pat. No. 3,592,586 by Scott issued July 13, 1971, describes an AVR 
process for converting cyanide wastes into sodium cyanide in which the 
wastes are heated and the pH is adjusted to between about 2 and about 4 in 
order to produce hydrogen cyanide (HCN). The HCN is then reacted with 
sodium hydroxide in order to form sodium cyanide. Although the process 
disclosed in the Scott patent is described with reference to waste 
produced in the electro-plating industry, AVR processes have also been 
applied to spent cyanide leachate resulting from the processing of ores. 
Such spent cyanide leachate typically has a pH greater than about 10.5 
prior to its acidification to form HCN. 
AVR processes employed in the mineral processing field are described in the 
two volume set "Cyanide and the Environment" (a collection of papers from 
the proceedings of a conference held in Tucson, Ariz., Dec. 11-14, 1984) 
edited by Dirk Van Zyl, "Cyanidation and Concentration of Gold and Silver 
Ores," by Dorr and Bosqui, Second Edition, published by McGraw-Hill Book 
Company 1950, and "Cyanide in the Gold Mining Industry: A Technical 
Seminar," sponsored by Environment Canada and Canadian Mineral Processor, 
Jan. 20-22, 1981. Another description of an AVR process can be found in 
"Canmet AVR Process for Cyanide Recovery and Environmental Pollution 
Control Applied to Gold Cyanidation Barren Bleed from Campbell Red Lakes 
Mines Limited, Balmerton, Ontario," by Vern M. McNamara, March 1985. In 
the Canmet process, the barren bleed was acidified with H.sub.2 SO.sub.4 
to a pH level typically between 2.4 and 2.5. SO.sub.2 and H.sub.2 SO.sub.3 
were also suitable for use in the acidification. 
AVR processes take advantage of the very volatile nature of hydrogen 
cyanide at low pH. In an AVR process, the waste stream is first acidified 
to a low pH (e.g. 2 to 4) to dissociate cyanide from metal complexes and 
to convert it to HCN. The HCN is volatilized, usually by air sparging. The 
HCN evolved is then recovered, for example, in a lime solution, and the 
treated waste stream is then reneutralized. A commercialized AVR method 
known as the Mills-Crowe method is described in Scott and Ingles, "Removal 
of Cyanide from Gold Mill Effluents," Paper No. 21 of the Canadian Mineral 
Processors 13 Annual Meeting, in Ottawa, Ontario, Canada, Jan. 20-22, 
1981. 
A process using AVR to recover cyanide values from a liquid is described in 
Patent Cooperation Treaty application PCT/AU88/00119, International 
Publication No. WO88/08408, of Golconda Engineering and Mining Services 
PTY. LTD. The disclosed process involves treating a tailings liquor from a 
minerals extraction plant by adjusting the pH into the acid range to cause 
the formation of free hydrogen cyanide gas. The liquid is then passed 
through an array of aeration columns arranged in stages so that the liquid 
flowing from one aeration column in a first stage is divided into two or 
more streams which are introduced into separate aeration columns in 
successive stages. In a recent paper describing the process, it was stated 
that plant shutdown would occur if pH went above 3.5. In a commonly 
assigned application, PCT/AU88/00303, International Publication No. 
WO89/081357, a process for clarifying liquors containing suspended solids 
is disclosed. The feed slurry is acidified to a pH of 3 or lower. 
Flocculants are added to cause the formation flocs to enable the 
separation of the suspended solids from the liquor. The clarified liquor 
can then be used as a feedstock for the AVR process disclosed in the other 
commonly assigned application. 
The AVR processes described in the Scott patent and the above-mentioned 
texts typically include the step of adjusting the pH of the spent cyanide 
stream to within the range from about 2 to about 4. There are several 
problems with such processes. These AVR processes are expensive due to the 
amount of acidifying agent required to lower the pH to within this range. 
Also, such processes require a substantial amount of base to reneutralize 
the waste stream after the volatilization of HCN and prior to disposal. 
Further, insoluble metal complexes form at the acid conditions employed in 
these processes. The above-mentioned references only disclose a treatment 
of barren bleed which typically results from Merrill-Crowe type 
cyanidation treatment of ore. This bleed does not contain solid tailings. 
Today many ores are treated by a carbon-in-leach or carbon-in-pulp 
cyanidation process. The tailings from such processes include the solid 
barren ore in the spent leachate. Typically the tailing slurries contain 
about 30% to 40% by weight solids and about 100 to 350 parts per million 
(ppm) cyanide. In the past, such tailings were typically impounded and the 
cyanide contained therein was allowed to degrade naturally. Due to 
environmental concerns about cyanide, such impoundment is not a desirable 
alternative in many situations. Therefore, it is often necessary to treat 
the material in some manner to decompose the cyanide. This is expensive 
due to the costs associated with the treatment, as well as the loss of 
cyanide values which results. 
Therefore, it would be advantageous to remove cyanide from a 
cyanide-containing waste stream in an economical manner. Further, it would 
be advantageous to provide a process for treating cyanide-containing 
slurries which also contain ore tailings. It would be advantageous if the 
amount of cyanide present in the waste stream could be reduced. It would 
also be advantageous to regenerate the cyanide for reuse. 
It has now been found that when the HCN is volatilized at pH ranges higher 
than those previously employed, significant advantages are achieved. For 
example, cost savings can be realized due to the reduced amounts of 
reagents required to acidify and subsequently raise the pH of the waste 
stream. Additionally, many insoluble complexes which form under strong 
acid conditions will not form in the pH range employed in the present 
process. Further, the higher pH avoids or minimizes scaling, for example, 
by calcium sulfate and/or metal thiocyanates such as copper thiocyanate. 
The pH ranges successfully employed in the present invention are possible 
because the present invention is preferably conducted on fresh 
carbon-in-pulp (CIP) or carbon-in-leach (CIL) tails. In contrast, previous 
acidification-volatilization-reneutralization (AVR) processes were 
employed on decant water or on barren bleed from Merrill-Crowe gold 
cyanidation processes. In the present process, much of the cyanide in the 
waste stream is in ionic form or only weakly complexed, whereas in barren 
bleed there is significant complexing including insoluble and strongly 
complexed forms. Therefore, previous AVR processes optimized the acidic 
precipitation of some of the metallo-complexes in order to deal with such 
precipitates separately. Use of the instant method for treating 
cyanide-containing slurries has additional advantages when used in 
combination with a CIL or CIP process. Recycling recovered cyanide and the 
low levels of effluent cyanide permits higher cyanide levels to be used in 
the leaching process which provides higher recoveries of precious metal 
values. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a process is provided for 
regenerating cyanide from a cyanide-containing mixture. The process 
includes the steps of: (1) adjusting the pH of the cyanide-containing 
mixture to between about 6 and about 9.5, (2) volatilizing the hydrogen 
cyanide (HCN) contained in the pH adjusted mixture, and (3) contacting the 
volatilized HCN with basic material. 
In another embodiment, the instant invention involves a process for 
regenerating cyanide from alkaline, cyanide-containing solution while 
minimizing equipment fouling due to solids precipitation. The method 
comprises (a) adjusting the pH of the cyanide-containing solution to 
between about 7 and about 9.5 to provide a pH adjusted solution; (b) 
passing a gas through the pH adjusted solution to remove HCN from the pH 
adjusted solution and form a HCN-gas mixture; and (c) contacting the 
HCN-gas mixture with an aqueous alkaline solution to form a 
cyanide-containing solution. 
In another embodiment, the instant invention comprises an apparatus for 
regenerating cyanide values from an alkaline, cyanide-containing slurry. 
The apparatus comprises a zone for adjusting the pH of the slurry to a pH 
of between about 6 and about 9.5 to form a pH adjusted slurry. An HCN 
volatilization zone is adapted to receive the pH adjusted slurry and 
contact the slurry with a volatilization gas to form a HCN-gas mixture. A 
cyanide recovery zone is adapted to receive the HCN-gas mixture and 
contact the mixture with a basic material to form a cyanide salt. 
In another embodiment the instant invention involves an improved method for 
recovering metal values from an ore. The method involves leaching the ore 
with a cyanide-containing solution at a pH of at least about 10 to provide 
a cyanide-containing slurry having dissolved metal values. The 
cyanide-containing slurry is contacted with activated carbon to load the 
carbon with the dissolved metal values. The loaded carbon is separated 
from the slurry to form a barren slurry having reduced dissolved metal 
values. The pH of the barren slurry is adjusted from above about 10 to 
between about 6 and about 9.5 to provide a pH adjusted slurry. A 
volatilization gas is passed through the pH adjusted slurry to form a 
HCN-gas mixture. The HCN-gas mixture is removed from the pH adjusted 
slurry and contacted with a basic solution to form a cyanide-containing 
solution. The cyanide-containing solution is then returned to the leaching 
step.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention concerns a process for regenerating cyanide from 
cyanide-containing waste streams. The process is preferably performed on 
tailings slurries resulting from mineral recovery processes, e.g. gold 
recovery processes employing cyanide leach solutions, such as vat leach, 
carbon-in-leach, and carbon-in-pulp processes. Such tailings slurries 
typically have a pH of greater than about 10, contain about 25% to 40% by 
weight solids and about 10 to 1000, more typically 100 to 600 ppm cyanide. 
The recovery of cyanide from slurries is advantageous for a number of 
reasons. Elimination of sedimentation or clarification steps reduces both 
capital and operating costs for the process. The recovery of cyanide can 
significantly reduce operating costs and the hazards associated with the 
manufacture, transport and storage of the reagent. Reduction of the total 
and weak acid dissociable (WAD) cyanide content entering the tailings 
impoundment minimizes the toxic effects of cyanide on wildlife and 
significantly reduces the potential for generation of leachate containing 
unacceptable levels of metals and cyanide. The requirement for installing 
a lining in the tailings impoundment can be eliminated for many 
applications. The reduction of total cyanide to acceptable levels in mine 
backfill can eliminate the need for wash plants in some circumstances. The 
reduction of total cyanide and metals concentration in the decant water 
and associated cyanide waste waters significantly decreases the costs 
while increasing the reliability and performance of downstream treatment 
processes. The generation of undesirable treatment byproducts such as 
ammonia and cyanate can be minimized thereby reducing significant capital 
outlays required for treatment of such materials. Additionally, the 
recovery and recycle of a substantial amount of cyanide from mineral 
recovery streams particularly from vat leaching, CIL and CIP tailings 
permits higher levels of cyanide to be used in the leach resulting in 
higher and more rapid recovery of precious metal values. 
The cyanide feed streams from minerals recovery processes are typically at 
a pH above 9 and normally above 10. A first step in the cyanide recovery 
process involves adjusting the pH of the stream of the cyanide-containing 
mixture being treated to between about 6 and about 9.5, more preferably 
between about 7 and 9, and most preferably to about 8. This can be 
accomplished through the use of an acidifying agent. Using a near neutral 
or basic pH minimizes problems associated with an increase in sulfate and 
total dissolved solids concentrations which can result in precipitation of 
materials such as calcium sulfate. Proper adjustment of the pH results in 
the formation of HCN in solution. The HCN is volatilized, preferably by 
contacting with air. The volatilized HCN is then contacted with a basic 
material, preferably in a solution having a pH between about 11 and 12, to 
convert the HCN to a cyanide salt. 
The tailings remaining after the HCN volatilization step can be further 
treated to remove remaining cyanide and/or metals and metal complexes. 
Such optional treatment can include metal coagulation, pH adjustment of 
the tailings in order to precipitate metal complexes, and/or further 
cyanide removal by known treatments such as oxidation (e.g. with H.sub.2 
O.sub.2 or SO.sub.2) and/or biological treatments. 
As a result of the process of the present invention, treated ore tailings 
have a greater long-term stability. Potentially toxic species (e.g. 
silver) will be less likely to be mobilized because of the lower cyanide 
concentration in the tailings pond. Discharge concentrations can be 
lowered and management requirements after mine closure reduced. 
Previous cyanide recovery processes have used a low pH precipitation step. 
This is to be contrasted with the present process which instead uses a pH 
in the range of about 6 to about 9.5. An advantage of using a near neutral 
or basic pH is that the formation of solids, such as calcium sulfate, is 
minimized which avoids scaling and fouling of equipment. This can be 
particularly important when packed towers are used to volatilize the HCN. 
Another advantage is that the higher pH reduces the amount of acid 
required to be added to initially acidify the waste stream. The amount of 
base required to subsequently raise the pH of the treated stream is also 
reduced. 
With reference to FIG. 1, a cyanide-containing waste stream 12 is treated 
in a pH adjustment zone 14 in order to obtain a stream having a pH between 
about 6 and about 9.5 and more preferably between about 7 and about 9 and 
most preferably about 8. A cyanide-containing slurry stream from any 
minerals recovery process can be used as a feed for the instant cyanide 
recovery process. In a preferred embodiment, the cyanide-containing waste 
stream is a tailings slurry from a vat leach which can use a precipitation 
method such as with zinc to recover metal values, or, a carbon-in-pulp or 
a carbon-in-leach metal recovery process which tailings normally have a pH 
above about 10 and normally in the range of about 10.5 to 11.5, a solids 
content of between about 20% and 50% by weight, more typically 25% to 40% 
by weight and about 100 to 600 ppm cyanide. Normally, it is not 
advantageous to lower the pH of the feed to below about 6. Based upon 
dissociation constants more rapid recovery of free cyanide and weakly 
bound cyanide e.g., NaCN and Zn(CN).sub.2, can be accomplished at a pH in 
the range of 4.5 to 8.5, whereas for a weak acid dissociable (WAD) 
cyanide, a pH of about 4.0 is optimal. However, it has been found that the 
instant process provides a high recovery of the ionic cyanide and 
unexpectedly, a substantial recovery of the WAD cyanide even at a pH of 6 
or above. For the reasons set forth hereinabove, a near neutral or basic 
pH of between about 6 and about 9.5, more preferably about 7 and about 9, 
is preferred to minimize precipitation problems. Additionally, at pH 
ranges below about 3 or 4, some metal complexes, e.g. Cu(CN).sub.2, will 
precipitate and subsequently resolubilize when the pH is increased. The 
dissolution of metals such as iron, copper, nickel, etc. is also minimized 
when a pH of at least about 6 is used. 
The cyanide-containing stream 12 is acidified in zone 14 by adding an 
acidifying agent 16. The acidifying agent 16 is preferably H.sub.2 
SO.sub.4, but other mineral acids can be used such as hydrochloric acid, 
nitric acid, phosphoric acid, H.sub.2 SO.sub.3, mixtures of H.sub.2 
SO.sub.3 and SO.sub.2, etc. or organic acids such as acetic acid, as well 
as mixtures of acids. The particular acidifying agent of choice depends on 
such factors as economics, particularly the availability of acidic streams 
from other processes, and the composition of the stream being treated. For 
example, if the stream contains materials which are detrimentally affected 
by an oxidizing agent, nitric acid would probably not be useful. A 
potential problem which was anticipated prior to the reduction to practice 
of the present invention was the formation of CaSO.sub.4 precipitates upon 
addition of H.sub.2 SO.sub.4 to slurries containing ore tailings. 
Surprisingly, this problem was not found to be as severe as originally 
anticipated and sulfuric acid can be readily used in connection with the 
packed tower embodiment set forth hereinbelow. The function of the 
acidifying agent 16 is to reduce the pH in order to shift the equilibrium 
from cyanide/metal complexes to CN.sup.- and ultimately to HCN. By 
employing higher pH ranges than those used in prior art AVR processes, the 
amount of acidifying agent 16 required is substantially reduced and the 
other advantages set forth hereinabove can be obtained. 
A pH adjusted stream is then transferred 18 from zone 14 to a 
volatilization zone 20 as shown in FIG. 1. In the volatilization zone 20, 
HCN is transferred from the liquid phase to the gas phase using a 
volatilization gas 19. Air is a preferred volatilization gas although 
other gases such as purified nitrogen can be used. The gas can also 
provide the turbulence required. Air can be introduced into the pH 
adjusted mixture in the volatilization zone 20 by any method well known in 
the art. For example, a diffuser basin or channel can be used without 
mechanical dispersion of the air. Alternatively, an air sparged vessel and 
impeller for dispersion can be employed. Baffles can be arranged in the 
vessel, e.g., radially, to assist in agitation of the slurry. In other 
alternative embodiments, a modified flotation device or a countercurrent 
flow tower with a grid, a plurality of grids, packing, a plurality of 
trays, etc., can be used. 
Volatilization of HCN by gas stripping involves the passage of a large 
volume of low pressure compressed gas through the acidified mixture to 
release cyanide from solution in the form of HCN gas. Alternatively, the 
mixture can be contacted with the volatilization gas, e.g. in a 
countercurrent flow tower. 
When a stripping reactor is used, the pH adjusted mixture is transferred 18 
from the initial pH adjustment zone 14 to the stripping reactor 
(volatilization zone) 20. Incoming volatilization gas 19 is distributed 
across the base of the stripping reactor 20 using gas sparger units 
designed to prevent any solids from entering the gas pipework on cessation 
of gas flow. Preferably, coarse to medium sized bubbles are used to 
provide sufficient gas volume and to minimize clogging of gas ports with 
materials such as clay. The resulting stripping gas stream is continuously 
removed 24 from the enclosed atmosphere above the slurry in association 
with removal of the extracted gas stream 23 which is positively withdrawn 
from the scrubber zone 26 by a device such as a fan. When the 
volatilization gas is air, the preferred flow is from about 250 to about 
1,000 cubic meters of air per cubic meter of pH adjusted mixture per hour, 
more preferably, about 300 to 800 and most preferably, about 350 to about 
700 m.sup.3 /m.sup.3. This flow is maintained for a time sufficient to 
remove the desired level of HCN. The time required to accomplish this 
removal depends on the air flow rate, the slurry feed rate, the slurry 
depth in the stripping reactor, the pH and the temperature of the mixture. 
Normally, the stripping can be accomplished in a period of about 2 to 6 
hours. Preferably, a flow rate of about 300 to 800 m.sup.3 /m.sup.3 is 
used which corresponds to a flux of from 2.8 to 7.4 cubic meters air per 
square meter of pH adjusted mixture per minute, based on a period of 3 to 
4 hours. 
While the key function of air in the system is to provide an inert carrier 
gas and transport, the air also has secondary effects. The first is to 
provide energy to overcome barriers to HCN transfer to the gas phase. 
Although HCN is very volatile, having a boiling point of about 26.degree. 
C., it is also infinitely soluble in water, and HCN solutions have a high 
degree of hydrogen bonding. Thus, there are significant resistances to the 
mass transfer of HCN that can be overcome by using the sparged air to 
provide the necessary energy in the form of turbulence. Furthermore, the 
dissociation equilibrium constants for most of the metal-cyanide complexes 
are low at the desired pH ranges; therefore, it is necessary for the 
CN.sup.- concentration to be close to zero in order to push the 
equilibrium far enough toward CN.sup.- formation in order to substantially 
dissociate the complexes. This can be achieved by efficient formation of 
HCN from CN.sup.-, which is pH dependent, and then by removal of HCN from 
the solution, which is energy dependent. 
As indicated above, preferred retention time in the volatilization zone 20 
is from about 2 to about 6 hours with a stripping reactor. In a stripping 
reactor, the liquid height in the reactor is preferably less than about 3 
meters. This preferred depth is due to the function of air in the system 
and the possibility of bubble coalescence if the depth is greater than 
about 3 meters. The necessary retention time can be achieved by using a 
single reactor or a plurality of reactors arranged in parallel, in series 
or a combination, as is appropriate for the particular feed stream and 
throughput. For example, multiple trains of reactors can be arranged in 
parallel with a plurality of stripping reactors arranged in series in each 
train. 
The stream of volatilized HCN and volatilization gas is removed from zone 
20 and transferred into a cyanide recovery zone 26. The apparatus useful 
in the cyanide recovery zone should provide effective mixing of the basic 
material being added and the stream of volatilized HCN. Suitable apparatus 
includes a gas sparger, preferably in an agitated vessel, which can 
provide effective contact of the HCN containing gas with the basic 
solution. More preferably, a conventional packed countercurrent scrubber 
is used (126 shown in FIG. 2). Basic material, preferably in solution, is 
fed 22 to the recovery zone 26. The recovery solution is preferably at a 
pH of at least about 11 and preferably between about 11 and about 12, in 
order to absorb HCN gas. Any basic material capable of providing a 
solution having the desired pH can be used. Examples of such materials 
include sodium hydroxide, potassium hydroxide, calcium hydroxide, lime, 
calcium carbonate, sodium carbonate, etc. Calcium-containing materials are 
generally not preferred because of the potential for the formation of 
CaSO.sub.4 scale. Sodium hydroxide is generally preferred. The basic 
cyanide solution 30 can be recycled, e.g. to a mineral recovery process 
such as a gold cyanidation process. 
The treated tailings which remain in reactor 20 after the HCN 
volatilization step can be removed 28 and contacted in zone 31 with 
alkaline material 35 to readjust the pH upward to a range of about 9.5 to 
about 10.5 in order to precipitate metals. Generally lime, limestone or 
lime water are preferred basic materials due to cost. The resulting pH 
adjusted tailings 32 can then be impounded 34. Optionally, prior to the pH 
adjustment step 31, complexed metals can be coagulated 36 (shown in 
phantom) by methods known in the art, for example using FeCl.sub.3 or TMT, 
an organic sulfide available from DeGussa Corporation. Additional cyanide 
can also be removed 33 (shown in phantom) from the pH adjusted tailings 
32, for example by known oxidation techniques, e.g. using H.sub.2 O.sub.2 
or SO.sub.2, or by known biological processes. 
A preferred embodiment of the process for removing and recovering cyanide 
values from a slurry is shown in FIG. 2. The pH of an incoming mill 
tailings slurry 112 is adjusted downward from a pH of above about 10 to 
between about 6 and about 9.5. This is accomplished in a sealed, agitated 
reactor vessel 114 normally in approximately a 5 to 20 minute time period. 
The vessel 114 should be constructed of materials compatible with the 
abrasive nature of this process. The acidifying agent 116, preferably the 
H.sub.2 SO.sub.4 shown, is normally added in the form of an aqueous 
solution normally containing about 10 weight percent acid. Once the pH of 
the slurry has been adjusted to the range of about 6 to 9.5, the pH 
adjusted slurry is transferred 118 to the volatilization section 120. 
Preferably, at least one packed tower is used in which the slurry is 
passed in countercurrent flow to the volatilization gas. 
A packed tower useful in the instant process normally has a means for 
distributing the slurry substantially uniformly across the top of the 
packing material. The means is located near the top of the tower and above 
the packing medium. It is preferred that the distributing means minimize 
interference between the slurry and rising volatilization gas to minimize 
the flow disturbance and provide an effective distribution of the slurry 
over a substantial cross-sectional area of the packing material. For 
example, a multiple weir, V-notch assembly can be used. The distributing 
means can be made of any suitable material such as steel or ceramic. The 
tower can also be equipped with a demister. The demister functions to 
suppress or disperse aerosols and can be formed from a fine screen or 
grid, glass wool or other porous media, etc. 
The packing material useful in the tower can be any mass-transfer media 
which provides a high void ratio, i.e., a high surface area to volume 
ratio (e.g. square meter per cubic meter). Preferably, the void ratio is 
above 50%, more preferably above 80% and most preferably above 85%. The 
openings in the packing material must be sufficiently large to allow free 
passage of the particles contained in the slurry. The height of the 
packing is typically 3 to 10 meters, more preferably 4 to 8 meters, most 
preferably about 6 to 7 meters depending on the desired pressure drop. 
To maximize efficiency of the process, it is important to control the 
viscosity of the slurry entering the packed tower. It has been found that 
increasing the viscosity of the slurry within an operative range improves 
the mass transfer and removal of hydrogen cyanide from the solution. 
However, if the viscosity is too high, flow of the slurry through the 
packing can be affected with subsequent operating problems and a decrease 
in removal of the hydrogen cyanide. The viscosity of the slurry is 
affected by the percent solids contained in the slurry, the type of ore 
being treated, and the temperature of the slurry. Normally, the weight 
percent solids in the slurry should not exceed about 60 weight percent. 
Preferably, no more than about 50 weight percent solids should be 
contained in the slurry. More preferably, the slurry should contain 
between about 25 and 45 weight percent solids. 
As set forth hereinabove, the packing material should have a high void 
ratio. The packing can be any material which can withstand the abrasion 
and operating conditions in the packed tower. Preferred materials include 
stainless steel, ceramic materials and plastic materials, for example, 
polyethylene and polypropylene. Examples of packing materials which have 
been found to be effective include 50 mm and 75 mm Pall rings, Rashig 
rings, Tellerette, saddles, and grid, although it is anticipated that 
other packing materials can be used. The tower can be constructed from any 
material capable of withstanding the reaction conditions and the chemicals 
which contact the internal surface of the tower. The preferred materials 
include fiberglass, steel (both mild and stainless) and concrete. 
In an alternative configuration, a stripping reactor 122 can be used as 
discussed for FIG. 1 and as depicted in phantom in FIG. 2. Such a reactor 
would normally be used in place of the stripping tower 120. 
In operation of the stripping tower, the volatilization gas, preferably 
air, is conveyed 119 to the stripping tower 120. Although two towers are 
depicted in FIG. 2, it is contemplated that, depending on the amount of 
slurry to be treated and the size of the tower, a single tower could be 
used. Alternatively, a plurality of stripping towers can be used either in 
parallel as depicted in FIG. 2 or in series or a combination of parallel 
trains with each train containing a plurality of towers arranged in 
series. The towers can be arranged to provide a single pass of the slurry 
as depicted in FIG. 2 or multiple passes with the slurry being recycled. 
In the operation depicted in FIG. 2, air is introduced into the stripping 
tower in countercurrent flow to the slurry. The air can be introduced by 
blower 123 shown in phantom or air can be forced through by negation 
pressure induced by fan 150. The tower is operated under a negative 
pressure with the air-HCN mixture being positively removed through line 
121 and transported to a cyanide recovery section. In the configuration of 
FIG. 2, the fan 150 is operated to exceed the flow of stripping gas so 
that all of the system above the packing in tower 120 through vessel 126 
operates under negative pressure to minimize any leaking of HCN. 
Preferably, the air is recycled as discussed hereinbelow. Sufficient air 
is introduced into the volatilization tower to provide a mean volume to 
volume ratio of air to slurry of about 250 to 1,000, more preferably in 
the range of 300 to 800, and most preferably, in the range of 350 to 700. 
Preferably, a pressure drop of about 15 millimeters (mm) to about 30 mm 
water gauge per meter of packing height is maintained. The pressure drop 
is the difference in pressure between the top and bottom of the tower, the 
air flow or air flux and the cross-sectional area of the tower. The degree 
of flooding is based upon filling all of the void space in the tower being 
considered 100% flooding. 
The slurry is fed to the packed tower at a rate which maintains a desired 
pressure drop over the length of the tower. Normally, the tower is 
operated in the range of about 10% to about 70% of the flooding volume and 
preferably, in a range of about 20% to about 50% of the flooding volume. 
The air-HCN mixture is conveyed 121 to the cyanide recovery section 126. 
Preferably, the cyanide recovery takes place in a packed tower by 
contacting the HCN with a basic solution which is conveyed in 
countercurrent flow to the HCN-containing gas. As discussed hereinabove 
for FIG. 1, any appropriate basic material capable of providing an aqueous 
solution with a pH of at least about 11 can be used. Sodium hydroxide is 
preferred in order to reduce calcium in the circuit and reduce possible 
calcium sulfate precipitation and scale formation. Minimizing such scale 
formation can be particularly important with the packed tower in order to 
minimize packing media fouling. As depicted in FIG. 2, in a preferred 
embodiment, sodium hydroxide solution 128 is added to vessel 125 where it 
is combined with cyanide containing stream 127 from scrubber 126. Caustic 
stream 129 is removed from vessel 125 by pump 140 and conveyed 141 to be 
used to scrub hydrogen-cyanide containing gas in the cyanide recovery 
section 126. The air-HCN mixture is drawn through the scrubber column. As 
depicted in FIG. 2, the scrubber column is vertical but the column can be 
horizontal or any other suitable configuration. Additionally, although a 
single column is depicted, it is recognized that a plurality of columns 
could be used as necessary to effectively scrub the volume of gas. The 
columns can be arranged in series or in parallel as desired. The column is 
preferably packed with a media bed to provide efficient contact between 
the HCN and the basic solution. The media can be any packing capable of 
providing effective contact between a gas and liquid, with such media 
being well-known to those skilled in the art. A proportion of the 
caustic-cyanide solution in vessel 125 bled off 130 to prevent the 
continuous build-up of cyanide removed from the HCN-air mixture introduced 
121. Sodium hydroxide 128 is automatically dosed into the scrubber liquid 
to maintain a constant pH thereby allowing for the portion lost to bleed. 
Cyanide, now in the form of a caustic solution of sodium cyanide bleed 
130, is returned to the mill circuit for reuse. 
Scrubbed air is removed 160 from the scrubber 126 and is conveyed through 
fan 150 to line 162 for recycle or venting to the atmosphere provided the 
air contains a low enough level of hydrogen cyanide. Scrubbed air can be 
discharged to the atmosphere by a line 164. Gas monitoring equipment can 
be installed in connection with line 162 to provide a continuous readout 
of performance and can include detection of levels of cyanide. Preferably, 
the scrubbing unit 126 allows for a minimum of 98% HCN removal from the 
hydrogen cyanide-gas mixture. On this basis, the concentration of HCN 
exiting the scrubber bed is maintained at less than 10 milligrams per 
cubic meter. Preferably, the scrubbed air is recycled to the 
volatilization section gas feed 119 through line 166. 
The stripped tailings slurry is removed 138 from the volatilization tower 
and transported to a reneutralization section 131 which is preferably a 
sealed, agitated vessel. The vessel 131 is constructed of materials 
compatible with the abrasive nature of this process. A basic material 135 
is added to provide the desired pH level for the final slurry. Although 
any suitable base such as sodium hydroxide or potassium hydroxide can be 
used, it is preferred that sodium carbonate, calcium oxide or calcium 
hydroxide be used to minimize the cost. The normal residence time to 
accomplish the reneutralization and retain the desired pH level for the 
slurry is normally about 15 minutes to 1 hour. The necessary time depends 
upon the buffering curve of the components contained in the slurry. 
The adjusted slurry is removed 137 from the reneutralization section and 
transported to a tailings impoundment. Alternatively, the adjusted 
tailings can be treated to remove the remaining cyanide or can be 
transferred to a thickener (not shown) where the coarse material is 
removed and deposited in an impoundment with the decant being additionally 
treated to remove the remaining cyanide. The treatment can be accomplished 
by recycling the whole stream or decant into the feedstream 112 for the pH 
adjustment section. 
Referring to FIG. 3, the use of the instant cyanide recovery process in 
combination with a carbon-in-leach process is depicted. Although the CIL 
process as depicted has no cyanide leach without carbon, it is 
contemplated that some CIL processes can use at least a partial cyanide 
leach prior to introduction of the carbon. The ore slurry 301 suitable for 
treatment by a CIL process is prepared by well-known processes 303. An 
oxidation process can be used to treat refractory ores. The pH of the 
slurry is adjusted in zone 305 preferably to above about 10, more 
preferably in the range of about 10.5 to 11 by adding a basic material 
307, preferably lime. The resulting alkaline slurry is transferred 309 to 
the carbon-in-leach process. A typical CIL process is described in U.S. 
Pat. No. 4,289,532 of Matson et al. (issued 1981) incorporated herein by 
reference. 
In the carbon-in-leach circuit, the slurry is simultaneously contacted with 
cyanide and granular activated carbon in vessel 311. The carbon moves 
countercurrent with the flow of the slurry. Thus, in FIG. 3, stream 309 
enters the first mixing vessel 311 where it contacts a cyanide stream 313 
which can contain cyanide in the amount of between about 0.25 and 2.5 
pounds of cyanide expressed as sodium cyanide per ton of dry ore as 
disclosed in the Matson et al. '532 patent. The cyanide can be added in 
solid form, but it may also be added as a solution, for example, as a 
sodium cyanide solution having between about 10 and about 25 weight 
percent sodium cyanide by weight. Other sources of cyanide such as 
potassium cyanide and calcium cyanide can be used, as is well known in the 
art. Additional lime 307 can be added to maintain the pH above about 10 in 
order to decrease cyanide decomposition. A stream of the slurry is removed 
315 and transferred to a second agitated vessel 317. Activated carbon is 
screened from the slurry being transferred to vessel 317. Fresh activated 
carbon is introduced 319 to vessel 317. A slurry containing cyanide ore 
and activated carbon is transferred 321 back to vessel 311. A slurry 
containing loaded carbon is removed 323 from vessel 311 for subsequent 
recovery of precious metals by methods such as stripping and 
electro-winning which are well known in the art. A slurry which has been 
screened to remove the activated carbon is removed 325 from vessel 317 and 
preferably conveyed to a separation device 327, such as a screen, which 
removes any contained carbon as stream 329. The remaining ore tailings are 
transferred 331 as a feed to the instant cyanide recovery process 333 
which is depicted in detail in FIG. 2. Sodium cyanide containing solution 
(depicted as stream 130 in FIG. 2) is removed 335 from the process and 
recycled to the CIL process. Tailings 337 from the process are disposed of 
as discussed hereinabove. 
Use of the instant cyanide recovery process permits the use of higher 
levels of cyanide in the CIL process. The levels of cyanide used based on 
sodium cyanide can be increased by up to 250%, more typically up to 100%, 
most typically up to 50%. 
Referring to FIG. 4, a carbon-in-pulp process is depicted using the cyanide 
recovery process of the present invention. A typical CIP process is 
described in U.S. Pat. No. 4,578,163 of Kunter et al. (issued 1986). Ore 
is prepared in mill 401 and transferred 403 optionally to a classification 
device 405, such as a cyclone, which classifies the ore into sands and 
slimes. This classification is used where necessary depending on the ore 
and whether the sand is to be used as backfill. The sands are conveyed 407 
to a vat 409 where the pH of the sand is adjusted to the desired pH range 
by the use of a basic material 411 such as lime. The vat can be agitated 
or can be a stationary bed. If a stationary bed of the sand is used, it 
can be leached using a sodium cyanide solution 413 containing about 0.045 
to about 0.055 weight percent sodium cyanide by percolating the solution 
by gravity through the sand. If the vat is agitated, then a solution 
containing about 1 pound of cyanide per ton of ore is used. The sand 
residue from the process is transferred 415 as a feed to the cyanide 
recovery 416 process depicted in FIG. 2. The recovered sodium cyanide 
solution (corresponding to stream 130 of FIG. 2) is recycled 417 to be 
used as feed for leaching the ore in the vat. The tailings are removed 419 
for subsequent treatment as discussed hereinabove. 
The slime which is separated from the sand by apparatus 405 is transferred 
421 to a carbon-in-pulp process. Optionally, the ore slurry 403 can be 
transferred directly from mill 401 to vessel 423 as depicted in phantom. 
The slime is introduced into the pH adjustment vessel 423 to which a basic 
material such as lime is added 425 to increase the pH typically to at 
least about 10 and preferably at least about 10.5. The resulting alkaline 
slurry is transferred 427 to an agitated vessel 429 to which cyanide 431 
is added to provide a final concentration of about 1 pound based on sodium 
cyanide per ton of slurry. The pulp slurry fed to vessel 429 preferably 
has a solids content of about 40 weight percent. Pulp from the cyanidation 
tank 429 is transferred 433 to at least one and normally, a plurality of 
carbon-in-pulp vessels 435 and 439. As depicted in U.S. Pat. No. 4,578,163 
of Kunter et al., normally four or more carbon-in-pulp vessels are 
operated in series to effect a countercurrent extraction with the 
activated carbon. The activated carbon 437 is fed to the final vessel 439 
of the series. A slurry containing activated carbon is transferred 441 
from vessel 439 to vessel 435. Simultaneously, a slurry, from which the 
activated carbon has been separated, is transferred 443 from vessel 435 to 
vessel 439. Loaded activated carbon is removed 445 from vessel 435 and 
precious metal values are subsequently removed from the carbon. A slurry 
stream, from which the activated carbon is substantially removed, is 
transferred 447 from vessel 439 to a separation means 449 which removes 
any remaining activated carbon as a stream 451. The remaining tailings are 
transferred 453 to the cyanide recovery process 455 which is depicted in 
detail in FIG. 2. A sodium cyanide solution (corresponding to stream 130 
of FIG. 2) is transferred 457 to be recycled and used in the 
carbon-in-pulp process. The tailings from process 455 are removed 459 for 
disposal as discussed hereinabove. 
Although two separate cyanide recovery processes are depicted in FIG. 4, a 
single cyanide recovery process can be used if the different sizes of the 
particles in the sand slurry and slime slurry permit. Even if two separate 
processes are used, sodium cyanide solution can, of course, be recycled to 
either portion of the process. 
Use of the cyanide recovery process of the instant invention similarly 
permits higher levels of cyanide to be used particularly in the 
carbon-in-pulp. The level of cyanide can be readily increased by at least 
about 50%, preferably up to 100% and preferably by at least about 250%. 
While not wishing to be bound by any mechanism, it is believed that the 
cyanide recovery process of the present invention operates as follows. 
When the pH of the tailings is adjusted to between 6 and 9.5, the CN.sup.- 
complexes (with the exception of Fe and Co complexes) dissociate to form 
CN.sup.- and ultimately HCN: 
EQU CN complexes ===== CN.sup.- ===== HCN 
These equations represent equilibrium reactions in which the process of the 
present invention shifts the equilibrium to the right-hand side. In the 
volatilization section 20 of FIG. 1, the HCN in solution is volatilized to 
HCN gas: 
EQU HCNsolution ----- HCNgas 
This preferably occurs under an overall pH of about 8 and a high energy 
environment of the volatilization section 20. IN the basic reaction 
chamber 26, the high pH causes the equilibrium to shift back towards HCN 
in solution: 
EQU HCNgas --------- HCNsolution 
Although the process has been described with reference to tailings slurry 
from a carbon-in-leach or carbon-in-pulp mineral recovery process, it is 
to be expressly understood that the process can also be employed on other 
cyanide-containing streams, e.g. from other mineral recovery processes, 
electro-plating processes, etc. 
The following experimental results are provided for the purpose of 
illustration of the present invention and are not intended to limit the 
scope of the invention. 
EXAMPLES 
A. Equipment 
The apparatus employed in Examples 1 and 2 consists of two 3' plexiglass 
columns six inches in diameter, connected in series, and sealed on both 
ends with plexiglass plates. The two columns are connected by tubing to 
permit the flow of air into the bottom of the first column, up through the 
column where it exits at the top, and then enters the bottom of the second 
column, flows through the column and exits at the top of the second 
column. A flow meter was employed to measure the flow of air entering the 
bottom of the first column. The column nearest the flow meter operated as 
the acidification-volatilization column, while the second column operated 
as the absorption column. Tubing was attached to the absorption column and 
ran into a fume hood to vent the air and any cyanide not absorbed. 
The aeration system was capable of producing a continuous flow of air in 
the range of 0-10 scfm at pressures of 10-20 psi. A compressor was 
employed for this purpose. The compressor was attached to the flow meter 
via tubing which was then attached to the first column. A regulator 
between the compressor and the flow meter was employed to regulate and 
record the pressure being applied to the system. 
A pipe was attached in each bottom plate of the two columns to facilitate 
sampling and draining of the columns during and following an experiment. 
B. Procedure 
In Examples 1, 2 and 3, a specific pH and air flow were utilized and the 
extent of cyanide stripping and recovery was evaluated over time. The air 
flow passed from the compressor, through the regulator, the flow meter, 
and the first volatilization column, and finally through the second 
absorption column. The air flow exiting the second column passed into a 
fume hood to vent unabsorbed cyanide. 
EXAMPLE 1 
The ore used in Example 1 was prepared by grinding 25 kilograms of ore 
together with 13.5 kilograms of water (i.e. 65% solids) and 240 grams of 
Ca(OH).sub.2 (i.e. 9.6 kilograms per ton) for 42 minutes in order to 
achieve a particle size distribution of about 85% of the ore less than 45 
microns in size. Twenty kilograms of water were added after grinding in 
order to thin the slurry. The slurry was ground a total of 3 times. Makeup 
water (9.6 kilograms) was added at the completion of the three grinds and 
the pH was adjusted to 10.5. 
The slurry was leached with cyanide. Initially, 83.5 grams of NaCN as a 5% 
solution was added. After 2 hours, 33 additional grams of NaCN (5% 
solution) was added as the cyanide concentration had dropped. The total 
cyanide added to the system was equivalent to 385 parts per million 
cyanide. During leaching, an air flow of 1 liter per minute was 
maintained. The pH and cyanide concentration of the leach slurry was 
monitored hourly. No further additions of NaCN were needed. The final 
cyanide concentration was measured at 210 parts per million. Finally, 
carbon was added after 16 hours. However, the gold and silver 
concentrations were not monitored. After removal of the carbon, the 
composition of the barren leachate was measured prior to stripping. The 
composition is shown in Table I. 
TABLE I 
______________________________________ 
Composition of Barren Leachate Before Stripping 
______________________________________ 
pH 10.3 
Alkalinity 475 
Ammonia-N 1 
Cyanate 23 
Cyanide (Total) 202, 192 
Cyanide (WAD) 200, 190 
Sulphate 320 
Thiocyanate 24 
Arsenic 0.8 
Copper 3.90 
Iron 0.15 
Silver 0.06 
Zinc 2.10 
______________________________________ 
For each of the six runs of Example 1, 10 liters of the slurry prepared as 
described above were placed in the first volatilization column. Initial 
samples of the solution were analyzed for free cyanide (for example, by 
ion selective electrode or by silver nitrate titration), the weak acid 
dissociable cyanide (CN.sub.WAD --by ASTM Method C), and pH. For runs 1 
and 2 the initial pH was not adjusted. For runs 3 and 4 the pH was 
adjusted with H.sub.2 SO.sub.4 to 8.7. For runs 5 and 6 the pH was 
adjusted to 7.6. 
Ten liters of caustic solution was placed in column 2 (the absorption 
column). The caustic solution was prepared by adding sufficient sodium 
hydroxide pellets to bring the pH of the solution to about 11 to about 
11.5. 
Air was then introduced into the columns. In runs 1, 3 and 5, the air flow 
rate was 60 liters per minute (.+-.20%) and in runs 2, 4 and 6, the air 
flow rate was 82 liters per minute (.+-.20%). Table II summarizes the pH 
and air flow rates for each of the runs in Example 1. 
TABLE II 
______________________________________ 
Conditions for Stripping 
Run No. 
1 2 3 4 5 6 
______________________________________ 
pH 10.5 10.5 8.7 8.7 7.6 7.6 
air flow 60 82 60 82 60 82 
(l/min) 
.+-.20% 
______________________________________ 
The amount of total cyanide (CN.sub.T) and Method C cyanide (CN.sub.WAD) 
was measured both in parts per million and in milligrams for the slurry in 
column 1 and the caustic solution in column 2. The results are shown in 
Table III. 
The first column labeled "Hours Stripping" lists the six runs and the time 
each sample was taken. The second column labeled "Kilograms in System" is 
the kilograms of liquor in the first column. Initially, 10 kilograms of 
total slurry was added, made up of liquor and solid tailings. The third 
and fourth columns list the CN.sub.T and CN.sub.WAD measurements in parts 
per million for each run at each time period listed. The fifth and sixth 
columns list the CN.sub.T and CN.sub.WAD in milligrams. The seventh and 
eighth columns list the same measurements as in the sixth and seventh 
columns except they have been adjusted as to account for the samples which 
were removed. 
Columns 2 through 8 list measurements taken from the slurry in column 1. 
Columns 9 through 14 list similar measurements which were performed on the 
caustic solution in column 2 in order to determine the total amount of 
cyanide absorbed. The percent extraction of CN.sub.T and CN.sub.WAD are 
listed in columns 15 and 16. 
The percentage extraction of CN.sub.T is based on the total CN.sub.T figure 
for that particular hour and includes the adjustments. The extraction 
percentages are low because the CN drained from the slurry column is 
actually not available for stripping. A caustic sample was lost in run 
number 4 and therefore there are no corresponding numbers. In runs 1 and 2 
the milligram CN.sub.WAD analysis was not performed on the slurry. 
The 10 liters of initial slurry for runs 3 and 4 required 75 milliliters of 
a 10 volume percent sulfuric acid solution to reduce the pH to 8.7. For 
runs 5 and 6, 115 milliliters of a 10 volume percent H.sub.2 SO.sub.4 
solution was added to the 10 liters of slurry to reduce the pH to 7.6. 
TABLE III 
__________________________________________________________________________ 
Analyses and Balances of Cyanide 
HOURS 
SLURRY CAUSTIC 
STRIP- 
kg.* in 
ppm CN mg CN ADJ. .sup..phi. mg CN 
kg. in 
ppm mg ADJ. 
Total CN 
% Extn 
PING system 
T WAD T WAD T WAD system 
CN CN mg CN 
T WAD T WAD 
__________________________________________________________________________ 
RUN 1 
0 7.91 
163 162 1290 1290 10.0 
0 0 
0 1290 
1 7.91 
158 157 1250 1250 10.0 
9.98 
100 
100 
1350 7.4 
2 7.68 
150 147 1150 1190 9.64 
20.3 
196 
200 
1390 14.4 
3 7.50 
141 143 1060 1120 9.41 
29.0 
273 
281 
1400 20.1 
4 7.20 
134 132 965 1070 9.12 
38.1 
347 
364 
1430 25.5 
RUN 2 
0 7.87 
163 162 1280 1280 10.0 
0 0 1280 
0.9 7.87 
157 158 1240 1240 10.0 
13.0 
130 
130 
1370 9.5 
1.8 7.61 
141 142 1070 1110 9.55 
24.7 
236 
242 
1350 17.9 
2.7 7.38 
136 137 1000 1070 9.22 
34.0 
313 
327 
1400 23.4 
3.6 7.15 
114 114 815 920 8.77 
44.2 
388 
417 
1310 31.8 
RUN 3 
0 7.97 
163 162 1300 
1290 
1300 
1290 
10.0 
0 0 
0 1300 
1290 
0.9 7.97 
50.6 
40 403 319 403 
319 10.0 
91.3 
913 
913 
1320 
1230 
69.2 
74.2 
1.8 7.71 
26.6 
18.3 
205 141 218 
151 9.51 
109 1040 
1080 
1300 
1230 
83.1 
87.8 
2.7 7.44 
20.5 
11.7 
153 87.0 
173 
102 9.08 
116 1050 
1140 
1310 
1240 
87.0 
91.9 
3.6 7.17 
18.0 
8.9 
125 63.8 
155 
82.3 
8.65 
120 1040 
1180 
1330 
1260 
88.7 
93.7 
RUN 4 
0 7.91 
163 162 1290 
1280 
1290 
1280 
10.0 
0 0 
0 1290 
1280 
0.9 7.91 
33.9 
27.2 
268 215 268 
215 10.0 
102 1020 
1020 
1290 
1240 
79.1 
82.2 
1.8 7.63 
18.5 
15.6 
141 119 150 
127 9.64 
112 1080 
1120 
1170 
1250 
95.7 
89.6 
2.7 7.35 
16.3 
11.2 
120 82.3 
135 
94.3 
9.28 
119 1104 
1180 
1220 
1270 
96.7 
92.9 
3.6 7.04 
15.2 
9.8 
107 69.0 
127 
84.5 
8.88 
SAMPLE LOST 
RUN 5 
0 7.54 
163 162 1230 
1220 
1230 
1220 
10.0 
0 0 
0 1230 
1220 
0.9 7.54 
37.2 
31.4 
280 237 280 
237 10.0 
89.3 
893 
893 
1170 
1130 
76.3 
79.0 
1.8 7.24 
22.2 
14.0 
161 101 172 
110 9.55 
105 1000 
1040 
1210 
1150 
86.0 
90.4 
2.7 6.93 
17.4 
10.4 
121 72.1 
139 
85.9 
9.07 
107 970 
1060 
1200 
1150 
88.3 
92.2 
3.6 6.70 
13.6 
8.9 
91 59.6 
113 
75.8 
8.74 
101 883 
1010 
1120 
1090 
90.2 
92.7 
RUN 6 
0 7.85 
163 162 1280 
1270 
1280 
1270 
10.0 
0 0 
0 1280 
1270 
0.9 7.85 
31.7 
23.4 
249 184 249 
184 10.0 
91.8 
918 
918 
1170 
1100 
78.5 
83.5 
1.8 7.55 
22.2 
11.6 
168 87.6 
259 
94.6 
9.60 
112 1075 
1100 
1360 
1190 
80.9 
92.4 
2.7 7.24 
16.1 
9.9 
117 71.7 
132 
82.3 
9.14 
114 1040 
1150 
1280 
1230 
89.8 
93.5 
3.6 6.92 
15.2 
8.6 
105 59.5 
126 
73.3 
8.77 
116 1020 
1190 
1320 
1260 
90.2 
94.4 
__________________________________________________________________________ 
*kg of liquor 
.sup..phi. Adjustments to take into account withdrawal 
EXAMPLE 2 
Following the procedure employed in Example 1, new tests were run on ore 
samples. In the first run, the air flow was 80 liters per minute 
(.+-.20%). In the second run, the air flow was 100 liters per minute 
(.+-.20%). The compositions before and after the runs are shown in Table 
IV. 
TABLE IV 
______________________________________ 
Composition of Barren Leachate Before and After Stripping 
AFTER 
Run No. 
Air Flow 1 2 
(l/min .+-. 20%) 
BEFORE 80 100 
______________________________________ 
pH 10.4 9.7 10.2 
alkalinity 575 170 169 
CN.sub.T 213 29.4 24.6 
CN.sub.WAD 218 7.4 6.8 
hardness 307 2170 2030 
SO.sub.4 360 2525 2350 
SCN 34 37 38 
E.C. (.mu.s/cm 20.degree. C.) 
1710 
As 0.8 0.8 0.7 
Ca 123 869 814 
Cd &lt;0.01 &lt;0.01 &lt;0.01 
Cr 0.02 &lt;0.02 &lt;0.02 
Co 0.16 0.33 0.30 
Cu 4.7 6.0 6.1 
Fe 1.3 8.7 6.7 
Pb &lt;0.1 &lt;0.1 &lt;0.1 
Mn 0.01 0.02 0.02 
Hg 
Ni 0.12 0.43 0.41 
Se 
Ag 0.15 0.04 0.04 
Zn 0.64 0.01 0.06 
______________________________________ 
Reagent consumption to either lower 
or raise pH for 10 l slurry 
final pH 8.1 9.7 10.0 
reagent 10% v/v H.sub.2 SO.sub.4 
Ca(OH).sub.2 
Ca(OH).sub.2 
amount 110 ml 7.7 g 9.0 g 
______________________________________ 
The pH of the initial slurry was 8.1. This pH was achieved by adding 110 
milliliters of 10 volume percent H.sub.2 SO.sub.4 to the 10 liters of 
slurry. After run number 1, 7.7 grams of Ca(OH).sub.2 was added to the 
tails to raise the pH to 9.7. After run number 2, 9.0 grams of 
Ca(OH).sub.2 was added to the tails to raise the pH to 10.0. The results 
for runs number 1 and 2 in Example 2 are shown in Table V. 
TABLE V 
__________________________________________________________________________ 
Analyses and Balances of Cyanide 
__________________________________________________________________________ 
SLURRY 
HOURS kg.* in 
ppm CN mg CN ADJ. .sup..phi. mg CN 
STRIPPING 
system 
T WAD T WAD T WAD 
__________________________________________________________________________ 
RUN 1 
0 7.94 
213 218 1690 
1730 1690 
1730 
1 7.94 
41.7 
16.7 
331 133 331 133 
2 7.66 
36.3 
11.3 
278 86.6 290 91.3 
3 7.36 
33.0 
10.0 
243 73.6 265 81.6 
4 7.05 
25.5 
6.0 
180 42.3 213 53.5 
RUN 2 
0 8.02 
213 218 1710 
1750 1710 
1750 
1 8.02 
37.2 
17.2 
298 138 298 138 
2 7.72 
26.0 
8.2 
201 63.3 212 68.4 
3 7.46 
25.5 
10.2 
190 76.1 208 83.3 
4 7.14 
23.5 
12.4 
168 88.5 194 99.1 
__________________________________________________________________________ 
NaOH 
HOURS kg. in 
ppm mg ADJ. mg 
Total CN 
% Extn 
STRIPPING 
system 
CN CN CN T WAD T WAD 
__________________________________________________________________________ 
RUN 1 
0 10.0 
0 0 
0 1690 
1730 
1 10.0 
95.4 
954 
954 1290 
1090 
74.0 
87.5 
2 9.69 
95.8 
928 
957 1250 
1080 
76.6 
88.6 
3 9.32 
100 932 
997 1260 
1080 
79.1 
92.3 
4 8.94 
98.7 
882 
985 1200 
1040 
82.1 
94.7 
RUN 2 
0 10.0 
0 0 
0 1710 
1750 
1 10.0 
122 1220 
1220 1520 
1360 
80.0 
89.7 
2 9.63 
138 1330 
1380 1590 
1450 
86.8 
95.2 
3 9.28 
133 1230 
1320 1530 
1400 
86.3 
94.3 
4 8.95 
138 1240 
1380 1570 
1480 
87.9 
93.2 
__________________________________________________________________________ 
*kg of liquor 
.sup..phi. adjustments to take into account withdrawals 
EXAMPLE 3 
Five runs were performed in order to test the efficiency of a reactor 
employing air inlets and a turbine to create turbulence. The pH in each 
run was varied as was the air flow rate. In run number 1, the pH was 8 and 
the air flow was 290 liters per minute (2.9 meters.sup.3 /meters.sup.2 
.times.minute). In run number 2, the pH was 7.8 and the air flow rate was 
100 liters per minute (1.0 meters.sup.3 /meters.sup.2 .times.minute). In 
run number 3, the pH was 8.2 and the air flow rate was 50 liters per 
minute (0.5 meters.sup.3 /meters.sup.2 .times.minute). In run number 4, 
the pH was 7.8 and the air flow rate was 200 liters per minute (2.0 
meters.sup.3 /meters.sup.2 .times.minute) In run number 5, the pH was 8 
and the air flow rate was 200 liters per minute. In runs 1 through 5, 30 
liters of solution were tested. Table VI shows the percent CN.sub.WAD 
remaining after 15, 30, 60, 120 and 180 minutes. 
TABLE VI 
______________________________________ 
Run 
Time 1 2 3 4 5 
(minutes) 
Percent CN.sub.WAD Remaining 
______________________________________ 
15 59.6 76.6 96.8 52.1 66.2 
30 36.5 58.5 92.5 33.3 42.1 
60 27.4 46.3 46.2 20.8 24.8 
120 22.1 30.3 35.5 12.5 21.1 
180 19.2 23.4 33.3 13.5 
______________________________________ 
EXAMPLE 4 
The efficiency of a flotation machine and a diffuser column were tested in 
runs 1 and 2 of Example 4, respectively. In run number 1, a flotation 
machine was employed with a 40 liter per minute air flow into a 3 liter 
slurry (1.4 meters.sup.3 /meters.sup.2 .times.minute). In run number 2, a 
diffuser column was employed with 50 liters per minute air introduced into 
a 10 liter slurry (9.4 meters.sup.3 /meters.sup.2 .times.minute). In both 
runs 1 and 2, the pH was 8. The results of these tests are shown in Table 
VII. 
TABLE VII 
______________________________________ 
Run 
Time 1 2 
(minutes) Percent CN.sub.WAD Remaining 
______________________________________ 
15 43 76 
30 20 60 
60 11 46 
120 10 12 
180 8 7 
______________________________________ 
EXAMPLE 5 
A continuous pilot plant was used in which five (5) stirred vessels sealed 
to the atmosphere and each having a volume of 200 liters were connected in 
series with pipes in and out the top of each vessel. The lead reactor was 
connected to a vessel through which tailings slurry could be introduced. 
The lead reactor was also connected to a vessel from which a 10% solution 
of sulfuric acid could be added. Arrangement was also made to introduce 
sodium cyanide as required into the lead reactor in order to maintain a 
desired level of free cyanide in the slurry being leached. The final 
reactor in the series was connected to a sealed aeration basin having a 
coarse bubble flexicap defuser in the bottom region of the basin. The 
aeration basin was divided with plywood baffles into five sections. Each 
plywood baffle had a hole in the top with a drop pipe to the bottom of the 
next section with the pipe sized to the flow of feed into the basin. 
Agitation was accomplished by air flow. The diffuser was connected to a 
source of compressed air with a controller which could provide a range of 
controlled air flow rates. A transfer line was connected from the top of 
the sealed aeration basin to a fan which was capable of providing a 
negative pressure in the aeration basin and conducting the air and 
hydrogen cyanide mixture from the vapor space above the liquid in the 
aeration basin. The exit of the fan was connected to a dilution stack 
which diluted the effluent hydrogen cyanide with air to allow venting. 
Another transfer was connected to the lower portion of the aeration basin 
to allow removal of tailing slurry and transfer to a stirred sealed 
neutralization vessel. A transfer line into the vessel was used to 
introduce sodium hydroxide solution to increase the pH to the desired 
level or a batch basis as necessary. A transfer line allowed removal of 
the reneutralized tailings slurry. Results from runs using this procedure 
are presented in Table VIII and Table IX. 
TABLE VIII 
__________________________________________________________________________ 
Total 
Slurry Feed Influent No. of 
Aeration 
Effluent 
Rate Influent 
WAD CN.sup.- 
Air Flow 
Slurry Depth 
Reactors 
Period 
WAD CN.sup.- 
Run No. 
(m.sup.3 /hr) 
(pH) (mg/L) m.sup.3 /m.sup.2 .multidot. min 
(m) In Series 
(min) 
(mg/L) 
__________________________________________________________________________ 
1 1.7 9.6 230 4.5 1.3 1 138 67 
2 1.7 9.6 150 4.5 1.3 1 138 43 
3 2.2 9.6 228 4.6 1.3 1 106 67 
4 2.2 9.7 228 3.9 1.3 1 106 67 
5 1.7 9.7 198 4.4 1.3 3 138 60 
6 1.8 9.7 195 4.5 1.3 3 130 52 
7 2.2 9.8 168 2.4 1.3 3 106 84 
8 2.2 10.0 182 4.5 1.3 5 92 61 
9 0.5 10.0 207 4.5 1.3 5 312 26 
10 0.5 10.0 157 2.8 1.3 5 312 28 
11 0.5 10.0 198 4.5 1.3 5 312 23 
12 0.5 10.0 170 4.5 1.3 5 312 22 
13 0.5 10.0 203 4.5 1.3 5 312 23 
14 0.5 10.0 179 6.2 1.3 5 312 16 
15 0.5 10.0 171 8.8 1.3 3 187 16 
16 0.5 9.9 161 4.5 1.3 5 312 19 
17 0.5 9.0 176 6.0 1.3 5 312 15 
__________________________________________________________________________ 
TABLE IX 
______________________________________ 
Complete 
Mix Aeration 
Influent Air Flux Reactor 
Period Effluent 
CN.sup.- 
pH m.sup.3 /m.sup.2 .multidot. min 
Stage (min) CN.sup.- 
______________________________________ 
198 6.0 4.5 1 63 33 
2 125 31 
3 187 27 
4 250 25 
5 312 24 
179 8.0 6.2 1 63 21 
2 125 20 
3 187 17 
4 249 18 
5 312 14 
171 8.0 8.8 1 63 16 
2 125 15 
3 187 16 
______________________________________ 
EXAMPLE 6 
A continuous pilot plant was used as in Example 5 except the agitator was 
removed from the final pH adjustor reactor in the series and aeration 
basin was replaced by a packed tower having a diameter of 0.5 meters and a 
height of 6 meters. The tower was packed with about 3 meters of either 50 
millimeter or 75 millimeter plastic Pall rings. The influent distribution 
system consisted of a ceramic multiple weir trough and a demister. The 
packing media was supported by a multiple-beam ceramic gas injector plate. 
The results from this configuration are provided in Table X for 75 mm 
rings and Table XI for 50 mm rings. 
TABLE X 
__________________________________________________________________________ 
Slurry 
Air No. of 
Air/ 
Flow Flow Tower 
Liquid 
Influent 
Effluent 
pH of 
Run No. 
(m.sup.3 /hr) 
(m.sup.3 /hr) 
Passes 
Ratio 
WAD CN.sup.- 
WAD CN.sup.- 
Slurry 
__________________________________________________________________________ 
1 2.37 845 1 357 182 36.6 -- 
2 2.37 845 1 357 182 24.5 -- 
3 1.94 839 1 432 156 45.1 -- 
4 2.17 839 1 387 166.4 22.7 -- 
5 2.54 839 1 330 166.4 22.7 -- 
6 2.10 2126 1 1012 
192.4 15.0 7.9 
7 2.21 2126 1 962 192.4 13.7 -- 
8 2.33 1484 1 637 197.6 18.3 8.0 
2.39 1400 2 586 19.1 5.6 -- 
9 2.36 1615 1 684 223.6 23.9 7.9 
2.45 1615 2 659 22.0 6.0 8.1 
10 4.1 2137 1 571 174.0 29.0 7.6 
4.0 2137 2 534 25.0 7.0 -- 
11 4.17 2581 1 619 193.0 26.0 7.7 
4.0 2581 2 645 22.0 7.0 -- 
__________________________________________________________________________ 
TABLE XI 
__________________________________________________________________________ 
Slurry 
Air No. of 
Air/ 
Flow Flow Tower 
Liquid 
Influent 
Effluent 
pH of 
Run No. 
(m.sup.3 /hr) 
(m.sup.3 /hr) 
Passes 
Ratio 
WAD CN.sup.- 
WAD CN.sup.- 
Slurry 
__________________________________________________________________________ 
12 3.9 1364 1 349 165.0 23.0 7.8 
3.7 1364 2 369 
13 5.0 1682 1 336 186.0 25.0 7.7 
4.6 1682 2 365 
14 4.0 2452 1 613 213.2 17.5 7.5 
4.1 2452 2 598 
15 4.1 1403 1 342 202.8 22.9 7.6 
3.9 1403 2 360 
16 4.18 
2389 1 -- 170.8 14.4 7.9 
17 4.2 2389 1 -- 162.9 14.1 -- 
__________________________________________________________________________ 
While various embodiments of the present invention have been described in 
detail, it is apparent that modifications and adaptations of those 
embodiments will occur to those skilled in the art. However, it is to be 
expressly understood that such modifications and adaptations are within 
the spirit and scope of the present invention, as set forth in the 
following claims.