Oxidation technologies, such as alkaline chlorination and ozonation perform well for free and weak metal–cyanide complexes weak acid dissociable cyanide [WAD] in water, soil slurries, an
Trang 120 Oxidation Technologies for
Treatment of Cyanide
Rajat S Ghosh, Thomas L Theis, John R Smith, and
George M Wong-Chong
CONTENTS
20.1 Alkaline Chlorination Technologies 394
20.1.1 Process Description and Implementation 394
20.1.2 Achievable Treatment Levels 395
20.1.3 Design Considerations 396
20.1.4 Cost of the Technology 398
20.1.5 Technology Status 398
20.2 Oxidation Technologies with Ozone and Hydrogen Peroxide 398
20.2.1 Process Description and Implementation 398
20.2.2 Achievable Treatment Levels 403
20.2.3 Design Considerations 403
20.2.4 Cost of the Technology 403
20.2.5 Technology Status 404
20.3 Photocatalytic Oxidation Technology 404
20.3.1 Process Description 404
20.3.2 Achievable Treatment Levels 404
20.3.3 Design Considerations 405
20.3.4 Cost of the Technology 405
20.3.5 Technology Status 405
20.4 INCO’s Air/SO2Process 406
20.4.1 Process Description 406
20.4.2 Achievable Treatment Levels 407
20.4.3 Design Considerations 408
20.4.4 Cost of the Technology 408
20.4.5 Technology Status 408
20.5 Technology Screening Matrix and Additional Technologies 408
20.6 Summary and Conclusions 408
References 411
Chemical oxidation at ambient temperatures is perhaps the most common treatment technology for cyanide in contaminated waters Oxidation technologies, such as alkaline chlorination and ozonation perform well for free and weak metal–cyanide complexes (weak acid dissociable cyanide [WAD])
in water, soil slurries, and sludges [1–5] However, energy-intensive oxidation technologies, such as
393
Trang 2ambient temperature photocatalytic oxidation are necessary to treat strong metal– cyanide complexes
in water, soil slurries, and sludges [5]
The following ambient temperature oxidation technologies are described in detail in this chapter:
• Ambient temperature alkaline chlorination
• Ambient temperature oxidation with ozone and hydrogen peroxide
• Photocatalytic oxidation technologies
• INCO’s Air/SO2process
These technologies have been applied for the treatment of water, soil slurries, and sludges containing free cyanide, weak metal–cyanide complexes, or strong metal–cyanide complexes Descriptions for the technologies follow, and include the following main features:
• Process description and implementation
• Achievable treatment levels
• Design considerations
• Critical design conditions
• Residuals generated
• Technology complexity
• Cost information
• Status of technology implementation
temperature oxidation technologies
20.1 ALKALINE CHLORINATION TECHNOLOGIES
The most widely used technology for the destruction of free cyanide and certain weak metal–cyanide complexes is chlorine oxidation under alkaline conditions, commonly known as alkaline chlorination Here, free cyanide and certain weakly complexed metal cyanides (i.e., WAD cyanides), such as
(e.g., sodium hydroxide or lime) is used to produce the pH conditions above 9.5 needed to sustain the oxidation reaction When chlorine gas is used as the oxidizing agent, the process chemistry is given by the following reaction [1,6,7]:
The above reaction proceeds at significant rates under alkaline conditions (pH 10 and higher) [8] Addition of alkali is essential to maintain the proper reaction pH and to prevent the generation
cyanide to cyanate is rapid, requiring about 15 to 30 min of contact time and Cl/CN dose of about 3 (on a mass basis) The complete destruction of cyanide can be accomplished by lowering the pH
of the solution after cyanate formation to 9 and addition of excess chlorine This second reaction proceeds as follows [7]:
3Cl2+ 2CNO−+ 4NaOH → 2CO2+ N2+ 2Cl−+ 4Na++ 4Cl−+ 2H2O (20.2)
Trang 3TABLE 20.1
Typical Ope rating Conditions for a Two-StageAlkalineChlorination
Process
StagepH (g Cl/g CN) (g NaOH/g CN) pote ntial (mV) time (min)
Source: Data from Palmer, S.A.K., Breton, M.A., Nunno, T.J., Sullivan, D.M., and
Surprenant, N.F., Metal/Cyanide Containing Wastes: Treatment Technologies, Corp, N.D., Ed.,
Noyes Data Corp., Park Ridge, NJ, 1998.
In cases where a metal–cyanide species is oxidized, the liberated metal generally forms a hydroxide precipitate under the alkaline conditions of the reaction
according to the following reaction:
2SCN−+ 8Cl2+ 20OH−→ 2CNO−+ 2SO−24 + 16Cl−+ 10H2O (20.3) The alkaline chlorination process for free and WAD cyanide can be operated as a one- or two-step process in either batch or continuous flow In the two-step process, the first step is used for oxidation
of cyanide to cyanate; in the second step, cyanate is oxidized to carbon dioxide and nitrogen Cyanate,
the chlorine demand
There is extensive full-scale application of this technology in electroplating and gold mining operations Table 20.1 gives typical operating conditions for a two-stage, full-scale continuous flow alkaline chlorination unit for treating free and WAD cyanide
treatment of cyanide in tailings pond decant water [9] Although the figure shows chlorine gas being used, this can be replaced by hypochlorite solution, which would eliminate the recirculation pump and chlorine eductor; however, a hypochlorite solution feed pump would still be required The hypo-chlorite feed pump or chlorine gas feed would be oxidation–reduction potential (ORP) controlled and effluent quality produced by alkaline chlorination systems at four gold mining operations It should be noted that residual chlorine is toxic to many species in the environment and discharge of effluents with high residual chlorine concentrations can be problematic and, in some instances, will be prohibited For the treatment of certain weak metal–cyanide and strong metal–cyanide complexes, modifica-tions to this process are implemented, including increasing the temperature and retention times in the reaction vessel [6,10,11] Details of high temperature alkaline chlorination technology are provided
20.1.2 ACHIEVABLETREATMENTLEVELS
Weakly complexed metal cyanides are typically reduced to a concentration less than 1 mg/l, while free cyanide concentrations following alkaline chlorination are usually less than 0.2 mg/l These performance levels will depend on chlorine dosage, reaction pH, reaction time, and the general chlorine demand of the waste This technology is not applicable for strongly complexed metal cyanides like iron– or cobalt–cyanide complexes
Figure 20.1presents a schematic flow diagram of a typical alkaline chlorination system for the
Trang 4pH ORP
Reactor tank(s)
0.5–1.5 h
pH 10–11.5
Tailings
Recirculating pump
Eductor
Chlorine gas or hypochlorite
Mixing
Solid tails
Lime slurry
Barren solution or
tailing pond water
FIGURE 20.1 Schematic flow diagram of a typical alkaline chlorination system (Source: Smith, A and
Mudder, T., The Chemistry and Treatment of Cyanidation Wastes, Mining Journal Books, Ltd., London, 1991.
With permission.)
TABLE 20.2
Operating Parameters for Full-Scale Alkaline Chlorination Operations
Giant
overflow Solution rate 3 to 5.5 m3 14.4 m3/day 216 m3/day 6545 m3/day
batches/day
hypochlorite
a Tpd = metric tons (tonnes) per day.
Source: Smith, A and Mudder, T., The Chemistry and Treatment of Cyanidation Wastes, Mining Journal
Books, Ltd., London, 1991 With permission.
The critical design parameters for alkaline chlorination include chlorine/cyanide (Cl/CN) ratio, reaction pH, and reaction time The technology is well suited for treatment up to 5000 mg/l of
Trang 5TABLE 20.3
PerformanceData for Full-ScaleAlkalineChlorination of Gold Mill Effluents
Constituents, mg/l
Baker
Carolin
Mosquito Creek
Giant Yellowknife
Polishing pond O/F 0.15 0.09 — 0.03 <0.1 — <0.1 0.14 9.4 1.1
All samples unfiltered.
a CNT= total cyanide by distillation.
b CN W = weak acid dissociable cyanide by ASTM Method C.
c Analysis not available due to analytical difficulties.
d TRC = total residual chlorine.
e Additional chlorine added with a view to destroying cyanide contained in solid tailings slurry.
Source: Smith, A and Mudder, T., The Chemistry and Treatment of Cyanidation Wastes, Mining Journal Books, Ltd.,
London, 1991 With permission.
free cyanide using batch systems, while continuous processes with flow rates up to 5 gpm can treat
up to 1000 mg/l, with optimal treatment efficiency usually achievable for concentrations below
100 mg/l and influent flow rates up to 100 gpm [6,7,12] Waste chlorine demand greatly influences Cl/CN ratio; chlorine demand does not depend only on cyanide content
The technology is not suitable for waste streams containing strong metal–cyanide complexes,
efficiency is achieved for influents containing less than 100 mg/l of total suspended solids (TSS), less than 1000 mg/l of total dissolved solids (TDS), pH levels between 9 and 13, and ORP greater than 200 mV
As far as residuals are concerned, metal hydroxide sludges can be generated if the influent stream contains appreciable amounts of weak metal–cyanide complexes, or metals in other forms Weaker complexes that dissociate during the process of oxidation will liberate metal cations, leading to the formation of metal hydroxides under alkaline pH conditions Residual chlorine and chloramines are also generated, which, because of their toxic nature, should be removed by dechlorination prior to
Trang 6cyanide to cyanate is a concern Careful control of pH and ORP should be in place to prevent any evolution of CNCl gas
The technology is relatively easy to implement and operate It requires basic wastewater treatment unit operations and continuous monitoring of pH to prevent production of CNCl and HCN Chlorine gas handling and leakage pose possible health hazards If metal hydroxide sludges are generated, they may require additional treatment for stabilization prior to disposal Moreover, the heat of reaction from chlorine and cyanide decomposition may require some form of temperature control before the final effluent can be discharged to the sewer
Capital costs for a typical 500 gpm system for treating waste streams that contain free and WAD complexes has been reported as approximately $300,000 (1990 cost basis), with typical operation and maintenance (O&M) costs varying between $5 and $7 per kilogram of cyanide destroyed [6,9,12,13]
Alkaline chlorination is a well-established, commercially practiced technology with many successful full-scale applications in place in electroplating and gold mining industries [6,9,12,13] Prefabricated chemical feed and monitoring equipment suitable for implementing this technology are commercially available However, some bench-scale testing for a particular application usually is desirable for determination of optimal Cl/CN dose, pH conditions, and reaction time
20.2 OXIDATION TECHNOLOGIES WITH
OZONE AND HYDROGEN PEROXIDE
These processes involve the oxidative destruction of free and WAD forms of cyanide by either
(9< pH < 10) according to the following reaction [14]:
constant for ozone decay as a function of total cyanide concentration As shown in this figure, the cyanide oxidation rate increases with increase in pH Rate expressions for ozone oxidation of cyanide at three different pH values are as follows [3]:
−d[O3]/dt = (2600 ± 700)[CNT]0.55 ±0.06[O3] at pH= 11.2 (20.5)
−d[O3]/dt = (2700 ± 850)[CNT]0.83 ±0.14[O3] at pH= 9.5 (20.6)
−d[O3]/dt = (550 ± 200)[CNT]1.06 ±0.1[O3] at pH= 7.0 (20.7) The presence of copper was found to catalyze the cyanide oxidation process according to the following reaction [15]:
2Cu++ 11CN−+ 3O3→ 2Cu(CN)3 −
Trang 7Phosphate solutions
pH 11.2 1
2.5
2.0
kobs
1.5
1.0
0.5
0
Log [CNT], M
pH 9.5 2
pH 7.0 3
1
2
3
FIGURE 20.2 Observed pseudo-first-order rate constant for ozone decay vs total cyanide concentration on
log scales (Source: Reprinted with permission from Gurol, M.D and Bremen, W.H., Environ Sci Technol.,
19, 804, 1985 Copyright 1985 American Chemical Society.)
In the presence of excess ozone, cyanate is hydrolyzed to bicarbonate and nitrogen according to the following reaction [14]:
This second stage reaction is much slower than the cyanate formation reaction and is usually carried out in the pH range of 10 to 12 where the reaction rate is relatively constant Temperature variation within the ambient range does not have a significant effect on the reaction rates However, the use of ultraviolet (UV) light to enhance radical formation [6] and the presence of copper catalyst [12] have each been shown to increase the rate of the second stage reaction
The metal–cyanide complexes of cadmium, copper, nickel, silver, and zinc are readily oxidized
by ozone For treatment of strong metal–cyanide complexes, such as iron– and cobalt–cyanide, modifications to the existing process are implemented, including prolonged UV light exposure to promote photodissociation [4,5] However, Gurol and Holden [15] reported oxidation of iron–cyanide complexes in the presence of excess ozone (ozone to iron cyanide ratio of 30:1 on a molar basis) under laboratory conditions
pH > 11, SCN− reacts with ozone to form CN− and SO2 −
Trang 8Hydrogen peroxide provides another alternative in treating free and weakly complexed metal
(standard electrode potential of 0.878 V in alkaline solution compared to 1.24 V for ozone under same solution conditions), cyanide can be fully converted by hydrogen peroxide to ammonia and carbonate under alkaline conditions, according to the following reactions:
The first reaction is optimal in the pH range of 9.5 to 10.5 [8] The second reaction, however,
is very slow under alkaline condition and increases as pH decreases [17] The cyanide oxidation rate also depends on the excess hydrogen peroxide concentration, cyanide concentration, and temperature The reaction rates can also be enhanced by the presence of a metal catalyst, such as copper, which ultimately reacts with ammonia to form a tetraamino copper complex that is largely nonreactive [8] Copper-catalyzed hydrogen peroxide oxidation of WAD cyanide complexes in wastewater is prac-ticed commonly in the gold mining industry [9] The destruction of weak metal–cyanide complexes occurs according to the following reactions:
M(CN)−24 + 4H2O2+ 2OH− Cu catalyst−→ 4CNO−+ 4H2O+ M(OH)2(s) (20.12)
CNO−+ 2H2O −→ NH+4 + CO2 −
where M is a metal cation, such as Cu or Zn The copper, which is added as a catalyst or present in
complex according to the following reaction:
It is customary to add copper sulfate pentahydrate as the catalyst to produce a copper concentration
of about 10 to 20% of the WAD cyanide concentration
The peroxide dose needed for successful oxidation of cyanide species may be 200 to 450% of the required amount indicated by stoichiometry [9] The high peroxide dosage rate is reflective of the presence of other oxidizable materials in the wastewater that can compete for the peroxide, as well as the inherent loss of oxidation capacity as some of the peroxide may decompose to oxygen and water:
To reduce these decomposition losses, peroxide stabilizers such as silicate (employed in Degussa’s SILOX process) and sulfuric acid, which forms peroxymonosulphuric acid (Caro’s acid), have been developed and deployed with substantial savings over the conventional peroxide process [18] for cyanide [18] As shown in this figure, hydrogen peroxide is added to the first reaction tank along with the influent solution In the second mixing tank, copper is added as copper sulfate to catalytically promote the cyanide oxidation reaction The supernatant from the second mixing tank then goes to the third tank, where enough settling of solid sludges (copper–iron–cyanide solids; iron hydroxides) and increased residence time causes complete removal of cyanide, and cyanide-free supernatant is discharged into the tailings pond
tinuous tailings slurry treatment system using hydrogen peroxide at the OK Tedi Mine in Papua, Figure 20.3presents a schematic flow diagram of a typical hydrogen peroxide treatment system
Figure 20.4andTable 20.4present a schematic flow diagram and performance data for a
Trang 9con-H2O2 storage
Feed pump
To tailings pond
Reaction tanks
CuSO4 catalysts
(if required)
Tailings pulp
or
Barren solution
FIGURE 20.3 Schematic flow diagram of a typical hydrogen peroxide treatment system for cyanide (Source:
Botz, M et al., Cyanide Monograph, Mining Journal Books, Ltd., London, 1998 With permission.)
Measuring cell
Control unit Multiplier
Reaction tank
H O pumps 2 2
Main tailings stream
Redox pH
H2O2
Control valve
Flow meter
Sample for analysis
1– 10 mg/l CNT
<0.3 mg/l WAD CN Control system
Tailings slurry
1100 m /h
110– 300 mg /l CN
3
T
Activator CN Caroate NaOH
H2SO4
FIGURE 20.4 Schematic flow diagram for the Degussa hydrogen peroxide process at the OK Tedi Mine.
(Source: Smith, A and Mudder, T., The Chemistry and Treatment of Cyanidation Wastes, Mining Journal
Books, Ltd., London, 1991 With permission.)
quickly and accurately enough to allow efficient use of the reagent for treatment of large effluent flows,
a continuous automatic titration is implemented in a small sidestream as depicted in Figure 20.4 The pH of the sidestream is adjusted automatically to a particular value, and a fast-acting strong oxidizing agent is dosed The rate of dosage is controlled by a redox measurement carried out in the presence of a special catalyst (“Activator CN”) Simultaneous to the addition of the strong oxidizing
concentration of 70% by weight, is added to the main tailings stream via a control valve The opening
Trang 10TABLE 20.4 Tailings Slurry Characteristics after Degussa Hydrogen Peroxide Treatment at OK Tedi Mine
Before H 2 O 2 After H 2 O 2
Tailings flow 1100 m3/h 1100 m3/h
Free cyanide 50–100 mg/l Undetectable WAD cyanide 90–200 mg/l <0.5 mg/l
Total cyanide 110–300 mg/l 1–10 mg/l Dissolved Cu 50–100 mg/l <0.5 mg/l
Dissolved Zn 10–30 mg/l <0.1 mg/l
Source: Smith, A and Mudder, T., The Chemistry and Treatment
of Cyanidation Wastes, Mining Journal Books, Ltd., London, 1991.
With permission.
TABLE 20.5
Treatment Performance for Three Hydrogen Peroxide Treatment Plants
Before Treatment (mg/l) After Treatment (mg/l)
Pond overflowa
Case study #2 1350 850 478 178 <5 <1 <5 <2
Barren bleedb
Heap leach solutionc
a Preliminary plant results from pre-operational test runs.
b Typical results during first six months of operation.
c Average of 25 measurements made over 10 days of plant operation.
d Value dropped from 1.0 to 0.4 over 4 days due to coagulation and settling.
Source: Smith, A and Mudder, T., The Chemistry and Treatment of Cyanidation Wastes, Mining Journal
Books, Ltd., London, 1991 With permission.
of this valve is controlled by a signal obtained by multiplying the signal from the control unit by
a second signal obtained from a tailings flow meter
Table 20.5 presents performance data from three other hydrogen peroxide treatment facilities
at gold mining sites While the data in Tables 20.4 and 20.5 show excellent removal of cyanide
by oxidation and precipitation of metals, it must be recognized that these facilities are only used for treatment of primary constituents of concern, like cyanide Hydrogen peroxide treatment does not affect ammonia, nitrate, or thiocyanate; treatment of these constituents will require additional treatment units
Hydrogen peroxide oxidation for free cyanide can also be effective under alkaline conditions, and in the presence of a metal catalyst (Fe, Al, Ni) or formaldehyde The patented Kastone