This document discusses enhancing cyanide recovery from industrial processes using an air-sparged hydrocyclone (ASH). The ASH can be used as a reactor to treat cyanide solutions, reducing the cyanide concentration from 250 ppm to below 20 ppm with over 80% recovery in a single stage. Strict environmental regulations require improving cyanide recovery and recycling from mining and industrial processes due to its toxicity.

1. Full Paper
Enhance Cyanide Recovery by Using Air-Sparged Hydrocyclone
By JosØ R. Parga Torres* and David L. Cocke
Human health and environmental concerns dictate that industrial processes be improved or replaced. Recovery or recycling is an
important activity that allows cyanide residue from the industrial processes to be re-used, reducing its production cost and
disposal problems. In this regard, the air-sparged hydrocyclone (ASH) has been used as a reactor for the treatment of cyanide
solutions for cyanide recycling by acidification/volatilization using the Mexican modification of the Mills-Crowe process.
Aqueous cyanide-ion concentration can be reduced from 250 ppm to below 20 ppm in the ASH with recoveries greater than 80 %
in a single stage.
1 Introduction Au + 2 CN± = Au(CN)2± + e± (2)
A variety of industrial effluents are known to contain O2 + 2 H2O + 2e± = 2 OH± + H2O2 (3)
cyanides. These various waste streams arise from different
process industries, such as those wastes from manufacturing
H2O2 + 2e± = 2 OH± (4)
synthetic fiber (acrylonitrile), coal conversion wastes or
coking effluents (from the iron and steel industries), electro-
In this mechanisms cyanide ion is the complexing agent or
plating waste and wastes from the petroleum industry. The
ligand, and oxygen is the oxidant [2].
most significant source of hazardous cyanide waste is the
After extraction and recovery of the precious metals,
metal finishing industry and wastes from the processing of
substantial amounts of cyanide are delivered to tailings ponds,
precious metals resources by cyanidation. All these wastes
which create environmental problems due to the toxicity of
have varied characteristics and are therefore subject to
cyanides. Therefore, the recycling of cyanide is a matter of
different processing and treatment strategies that depend
interest from both an economical aspect and to protect the
upon the concentration of cyanides and the flow rate of the
receiving water from potentially harmful cyanide, as was
waste stream.
shown by White [3].
The cyanide process has been in large-scale use in many
Due to the widespread use of cyanide in mining operations,
parts of the world for more than 110 years and is likely one of
the recycling or destruction of cyanide is important both from
the most thoroughly studied and well-understood industrial
the environmental aspects of wastewater and effluent treatment,
chemical processes, as was shown by McNultty [1]. In mining
and from the economic aspects associated with the high reagent
operations, cyanidation is the predominant method by which
consumption by the process itself, for example, the use of a
gold and silver are recovered from their ores. In practice, the
procedure to recover cyanide may be a good option since the
dissolution of gold and silver in aqueous cyanide solution is
market price of cyanide is between US$ 1.00 and US$ 1.50 on
typically carried out with 0.03±0.3 % NaCN and it is usually
average [4]. This latter situation was the case for a cyanidation
the most significant reagent cost. Lime is added as a pH
process developed at Bacís mine (in Durango, Mexico) for the
modifier to increase the pH and prevent as much as possible
recovery of gold and silver from a pyrite concentrate [5]. The
the hydrolysis of the cyanide ion to hydrogen cyanide. Also
process comprises the following steps: leaching the complex
aeration is necessary to keep the pulp or solution saturated
sulfide concentrate by a one-stage pressure oxidation in a
with oxygen (> 7 ppm). The overall reaction for the dissolution
highly alkaline cyanide solution (1 % cyanide), filtration and
of gold and silver in dilute, aerated, and alkaline cyanide
washing to separate the solid, and precipitation of gold and
solutions may be expressed by the classic Elsner equation
silver with zinc dust from the filtrate. The formation of metal
complexes (copper, iron and zinc) with thiocyanate during
4 Au + 8 CN± + O2 + 2 H2O = 4 [Au(CN)2±] + 4 OH± (1)
pressure leaching results in species, which are particularly
toxic. Since ultraviolet light decomposes thiocyanate to form
which has the following mechanism:
cyanide, it is then possible that sunlight may liberate cyanide,
which would be toxic to aquatic life. Also, it should be noted
that cyano-complexes of moderate stability could be readily
decomposed by acidification or oxidation.
±
The US Environmental Protection Agency has proposed
[*] J. R. Parga Torres (author to whom correspondence should be addressed,
e-mail: drjrparga@hotmail.com), Institute of Technology of Saltillo, a limit of 0.2 mg/l cyanide in drinking water. The German
Department of Metallurgy and Materials Science, V. Carranza 2400, and Swiss regulations have set limit of 0.01 mg/l for cyanide
C.P. 25000, Saltillo Coah. MØxico; D. L. Cocke, Lamar University,
Chemistry and Chemical Engineering Department, Beaumont Texas
for surface water and 0.5 mg/L for sewers [6]. In Mexico,
77710, USA. the Secretary of Environmental and Natural Recourses
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2. Full Paper
(SEMARNAT) has set the limit for cyanide as 0.2 mg/L. In
view of these considerations, cyanide recycling is a necessary
processing step. HCN
Air
2 Air-Sparged Hydrocyclone Reactor ASH
Waste Air
The ASH technology was originally developed at the solution with
University of Utah for the fast and efficient flotation of fine cyanide
particles from suspension [7,8]. Also, recent studies indicate
that the fluid flow conditions inside the ASH system can be
SO2
effectively exploited for air stripping of VOCs from con- pH=2
Ca(OH)2 NaOH
taminated water [9].
Tank Underflow Overflow
Results reported in the literature [10,11] indicate that the
greatest mass transfer of compounds from water to air is Figure 1. Schematic drawing of the Ash system used for stripping of cyanide.
achieved for those compounds, which have high values of
Henry's law constants and are relatively insoluble in water. It is
safe to say that when compound properties favor air stripping,
maximum mass transfer will occur in air strippers that
± maintain the greatest possible interfacial area between bulk 3 Cyanide Recycling
liquid that contains the HCN(g) and the stripping air;
± increase the magnitude of the liquid mass-transfer coeffi- Free cyanide exists as the uncomplexed cyanide ion, CN±,
cient by providing sufficient turbulence to minimize the and molecular hydrogen cyanide, HCN. These species are
boundary layer thickness. related by the acid dissociation of HCN:
The ASH reactor is one of the new, emerging stripping
technologies, which can fulfill both requirements for max- HCN(aq) = CN± + H+ (5)
imum mass transfer. The ASH unit consists of two concentric
right-vertical tubes and a conventional cyclone header at the The concentration of free cyanide is the sum of the CN± and
top. The porous inner tube is constructed of any suitable HCN concentrations, and the equilibrium diagram shown in
material, such as plastic, ceramic or stainless steel, and allows Fig. 2 illustrates the distribution. This figure shows the
for the sparging of air or any other gas or steam. The outer proportions of free cyanide as CN±, and HCN as a function
nonporous tube simply serves to establish an air jacket and of pH at 25 C. At pH values below 7, cyanide is predominantly
provides for the even distribution of the air through the porous present as the un-ionized HCN molecule, which is easily
tube. Thus, the ASH can be used for air stripping where volatilized because of its high vapor pressure. The equilibrium
volatile species, such as HCN(g) which has a high vapor is displaced in favor of cyanide ion formation at pH values
pressure and volatilizes as a gas (Henry's law of constant of above 7.
6.4 atm/mole) [12], can be displaced from solution by air,
which is considered in this paper. The cyanide solution is fed
tangentially at the top through the cyclone header to develop a
swirl flow adjacent to the inside surface of the porous tube, 100
leaving an empty air core centered on the axis of the ASH unit. 90
The high-velocity swirl flow shears the sparged air to produce
80
a high concentration of small bubbles and intimate interaction
Percentage of HCN
between these numerous fine bubbles and the cyanide 70
solution. Gaseous products are then transported radially to 60
the center of the cyclone. The major portions of the gas phase
50
move towards the vortex finder of the cyclone header and are
vented into an appropriate post-treatment device. The specific 40
capacity of the ASH system is at least 1,500 liters per minute 30
per .028 cubic meter of equipment volume, 100±600 times that
20
of conventional air-stripping equipment. The ASH equipment
requires an operating space significantly less than that of a 10
packed tower or other air stripping devices, which result in a 0
significant savings in capital cost. A schematic drawing of the 6 7 8 9 10 11 12
ASH unit as used for air stripping of cyanide is presented in pH
Fig. 1. Figure 2. Equilibrium distribution diagram for cyanide as a function of pH.
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Hydrogen cyanide (HCN), also known as hydrocyanic acid, It has been almost 80 years since the Mills-Crowe process
is a colorless gas or liquid with a boiling point of 25.7 C, a for cyanide regeneration was developed by the Company
vapor pressure of 100 kPa at 26 C and Henrys law constant of Beneficiadora de Pachuca, Mexico (England Pat. No. 241669,
6.4 atm/mole [12], this makes HCN very volatile. Thus, low 3.9.24) [14] and until today no significant changes to the
pH, high temperature, low pressure, and intimate contact with process have been made. The simplest process for cyanide
air, all tend to increase the rate of dissipation of cyanide from recycling involves acidifying the clarified solution, then
solution as hydrogen cyanide. volatilizing of the HCN(g) formed and reabsorbing it from
In addition to free cyanide, other complexes, such as the the air stream with a caustic or milk of lime spray to produce
metal cyanide complexes formed with gold, mercury, zinc, aqueous NaCN.
cadmium, silver, copper, nickel, iron and cobalt, must be Recently, the company Minera Real del Monte [15]
considered. These are classified into five general categories, as acidified the clarified cyanide solution with sulfuric and
shown in Tab. 1 [13]. hydrochloric acid (to avoid gypsum formation). In the
volatilization stage a series of four stripping towers packed
Table 1. Classification of cyanide and cyanide complexes on the basis of stability with wooden grids are used. The towers are constructed of 316
[13].
stainless steel and measure about 95 cm of diameter by 8 m in
Classification Compound height. A total cyanide recovery of about 95 percent is
Free cyanide CN±, HCN
achieved with about 50 percent removal realized in each of the
four stripping stages.
Simple compounds
a) readily soluble Zn(CN)2, Cd(CN)2, CuCN, Ni(CN)2, AgCN
b) neutral insoluble salts NaCN , KCN, Ca(CN)2, Hg(CN)2
4 Experimental Procedure
Weak complexes Zn(CN)42±, Cd(CN)32±, Cd(CN)42±
Moderately strong Cu(CN)2±, Cu(CN)32±, Ni(CN)42±, Ag(CN)2± Experiments for cyanide recycling by air stripping at the
complexes
Institute of Technology of Saltillo pilot plant included
Strong complexes Fe(CN)64±, Co(CN)64±, Au(CN)2±, Fe(CN)63± acidification of the cyanide solution by bubbling SO2 gas to
the 2±7 pH range for HCN(g) formation and stripping with air
in a 2-inch diameter ASH unit. Chemical analysis for cyanide
The term total cyanide is used for all cyanide, (free as well as in the effluent streams was accomplished with a reflux
coordinated cyanide), present in a sample. The concentration distillation method. Important aspects of the distillation step
of free cyanide in a solution depends on the pH value of the are the elimination of interferences and the decomposition of
solution and its content of heavy metals capable of forming stable metal-cyanide complexes. The collected cyanide was
cyanide complexes. Weakly complexed metal cyanides de- quantified by titration with silver nitrate standard solution
compose at pH values lower than 4, with the evolution of and/or the ion-selective electrode
hydrogen cyanide. Strong metal-cyanide complexes are During the experiments two streams had to be delivered to
usually unaffected at room temperature because these the ASH: the cyanide solution and the air. Cyanide solution
complexes are very stable and resist oxidation, however, was provided by a sump pump mounted on a 300 liter retention
partial decomposition can occur with increasing temperature tank. The cyanide solution flow rate was adjusted using a
and acid content. regulated return flow to the tank. Using an air compressor,
The cyanide recycling process utilizes the volatility of airflow was evenly distributed between the upper and lower
HCN(g) at low pH to strip free cyanide from solution or slurry sections of ASH and all parts were sealed with gaskets.
with air and recover it in a caustic solution. The simplified Cyanide solution acidified to pH = 2±7 in the tank was fed at
chemistry of the process is represented by the following different flow rates to the top of the ASH. The exit pipe was
reactions: located at the bottom of the closed regenerated cyanide tank
to prevent release of HCN(g). The HCN-laden air was
CN± + H+ = HCN(aq) (6) collected in the absorber where reaction with sodium
hydroxide 10 % v/v regenerated the NaCN aqueous solution.
Air was in closed circuit at slightly reduced pressure for
HCN(aq) = HCN(g) (7) volatilization of hydrogen cyanide from the acidified waste
and absorption of hydrogen cyanide in a solution of sodium
hydroxide. The use of air in closed circuit prevents introduc-
HCN(g) + OH± = CN± + H20 (8) tion of atmospheric carbon dioxide which would neutralize
lime in the absorption solution. Operators were provided with
In the final step, the HCN(g) diffuses into the stripping personal HCN gas monitor/alarm units (DrägerSensor-
solution of concentrated sodium hydroxide and reacts to form XSECHCN-68 09 150 is a trademark of the Drägerwerk
aqueous NaCN. Aktiengesellschaft, registered in Germany).
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5 Results and Discussion 80
70
Cyanide recovery (%)
Experiments were performed using 500 ppm of CN± 60
prepared from a 50 g/L stock solution of aqueous sodium 50
cyanide from plant in a 300 liter fiberglass vessel at ambient 40
temperature (24 C). Fig. 3 presents the experimental 30 Air = 150 l/min.
conditions and the results obtained regarding cyanide deple- 20
Air = 130 l/min.
tion with the acidification of the feed solution with SO2(g). 10
Also, Fig. 3 shows the variation with time of the cyanide 0
0 5 10 15 20 25 30 35
concentration for the two flow rates of SO2(g). As may be seen, Time of distribution air (sec.)
the formation of volatile HCN(g) is a fast reaction and changes
Figure 4. Effect of time of air distribution on cyanide recovery.
in the concentration of free cyanide are a function of the
gradual acidification of the solution.
85
Cyanide Regeneration, %
75 Air flow rate = 210 l/min
1000
65
100 pH=2
Free Cyanide (PPM)
55
10 0.5 l/min of SO2 45 pH=5
2 l/min of SO2 35
1 pH=7
25
0.1
15
0.01 15 20 25 30 35 40 45
0.001 Solution Flow rate, l/min
0 5 10 15 20
Figure 5. Cyanide recovery and regeneration with ASH.
Time Of Feed SO2(Minutes)
Figure 3. Variation with time of the cyanide concentration for two flow rates of
SO2(g). Based on these test results, an initial economic comparison
with current Mills-Crowe processes is summarized in Tab. 2.
Typical results collected at pH 2.0 for cyanide regeneration All of these processes for cyanide recovery are current
are presented in Fig. 4. All experiments were made at the same versions of the original Mills-Crowe process [12,15,16]. With
solution flow rate of 20 liters/minute and two air flow rates. two stages, the cyanide-ion concentration can be reduced to
Also, the data in Fig. 4 show that cyanide regeneration below 0.2 mg/L with recoveries than 99 %.
increases because air flow is rate-dependent. Finally, in Tab. 3, based on the pilot plant results, estimates
Also, as seen in Fig. 5, the pH and solution flow rate of the cost of cyanide recovery have been prepared per
influence cyanide recovery. Thus, at a low pH value when the kilogram of cyanide recovery, and the performance of the
concentration of CN± is very small, a high recovery is achieved ASH compares favorably to the packed-bed stripping tower
due to the easy volatilization of HCN(g). On the other hand, at technology.
pH = 5, the recovery is significantly lower (52 % at 20 liters/ The advantage of the ASH technology over packed towers
minute). These tests indicate that stripping of volatile HCN(g) is the residence time. In packed towers, the residence time for
with air and regeneration of cyanide with sodium hydroxide is stripping varies from 7 to 20 minutes, whereas the ASH
pH-dependent. operates with a retention time of only 4 seconds [7].
Table 2. Comparison of the experimental results with results from traditional Mills-Crowe operations
[12,15,16].
Mine Reactor Air/Liq % Rec. CNIN CNOUT Streams
Flin Flon Mine 4 towers 521 92 560 44 solution
Real del Monte (Mex.) 4 towers 340 93 220 3 solution
AVR (Canmet) 2 towers 330 95 330 2 solution
C. R. P. (Tasmania) ± ± 95 200 5 solution
Cyanisorb
(NERCO DeLamar, US) 2 towers 300 95 600 30 slurry
ASH (Bacis-MØxico) 1 ASH 10 80 250 50 solution
ASH (Bacis- MØxico) 1 ASH 100 90 250 25 slurry
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5. Full Paper
Table 3. Comparison of cost and performance
Cost Ratio % Recovery
Mine Remarks
(US$/kg CN) Air/solution Single stage
Build-up gypsum and
Real del Monte
1.00 200±350 64 copper thiocyanate
(packed towers)
precipitates
Bacis Mine
0.85 20±100 80 Free of precipitates
(ASH)
6 Conclusions References
The application of the gas-sparged hydrocyclone reactor for [1] T. McNulty, Mining Mag. 2001, 5, 256.
[2] M. I. Jeffrey, I. M. Ritchie, J. Electrochem. Soc. 2000, 147, 3257.
cyanide recycling is a new and potentially inexpensive [3] D. M. White, T. A. Pilon, C. Woolard, Wat. Res. 2000, 34, 2105.
approach for cyanide recovery. The ASH reactor has been [4] G. L. Miltzarek, C. H. Sampaio, J. L. Cortina, Minerals Eng. 2002, 15, 75.
tested in bench and pilot-plant scale applications and has been [5] J. R. Parga, H. Mercado, Precious Metals Extraction by Direct
Oxidative Pressure Cyanidation of Bacís Concentrates, Proc. Randol
proven effective for the recycling of cyanide in solution and Gold Forum, Beaver Creek 1993, 209.
slurries. [6] J. D. Desai, C. Ramakrishna, P. S. Patel, J. Awasthl, Chem. Eng. World
1998, 33, 115.
Experiments performed show that the ASH reactor is very [7] J. D. Miller, Ye Yi, Min. Proc. and Extract. Metall. Rev. 1989, 3, 307.
competitive with other technologies and that single-stage [8] D. Lelinski, R. Bokotko, J. Hupka, J. D. Miller, Min. Metall. Proc. 1996,
cyanide recovery exceeding 80 % can be achieved. 5, 87.
[9] J. D. Miller, D. Lelinski, J. R. Parga, Final Report-CX 823711, Advance
Process Technology for the Wastepaper Recycling Plants and Pulp/
Paper Plants, Southwest Center for Environmental Research and Policy,
Acknowledgements [10]
1996.
D. F. LaBranche, M. R. Collins, Wat. Environ. Res. 1996, 68, 348.
[11] W. J. Parker, H. D. Monteith, Environ. Progress 1996, 15, 73.
The authors wish to express their gratitude to CONACYT, [12] Smith, T. Mudder, The Chemistry and Treatment of Cyanidation Wastes,
Mining Journal Books Ltd., London 1991, 277.
COSNET (701.95-P) and Grupo Minero Bacis for financial [13] W. Hoecker, D. Muir, Res. Dev. in Extractive Metallurgy 1996, 5, 29.
support and permission to publish the results. Many thanks go [14] C. W. Lawr, Cyanide Regeneration as Practiced by the Compaæía
to Lamar University for support and assistance. Beneficiadora de Pachuca, Mexico, Technical Publication AIME No. 208
(06) 1929, 1±37.
Received: July 31, 2002 [CET 1668] [15] Report Compaæía Minera de Real del Monte, Pachuca, Mexico, Process
for the Recovery of Cyanide, 1997, 1±13.
[16] M. Botz, J. Stevenson, Eng. Mining J. 1995, 6, 44.
Abbreviations
ASH air-sparged hydrocyclone
aq aqueous phase
g gas phase
_______________________
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