ISSN 20700504, Catalysis in Industry, 2010, Vol. 2, No. 1, pp. 67–71. © Pleiades Publishing, Ltd., 2010. Original Russian Text © A.A. Batoeva, M.R. Sizykh, D.G. Aseev, 2010, published in Kataliz v Promyshlennosti.
CATALYSIS AND ENVIRONMENTAL PROTECTION
Catalytic Oxidation of Thiocyanates A. A. Batoeva, M. R. Sizykh, and D. G. Aseev* Baikal Institute of Nature Management, Siberian Branch, Russian Academy of Sciences, UlanUde, 670047 Russia *Email:
[email protected] Received July 21, 2009
Abstract—The current liquid waste neutralization technology in the hydrometallurgical processing of gold concentrates includes thiocyanate oxidation with calcium hypochlorite, which is carried out after the sepa ration of cyanides and heavy metals as precipitates of complex compounds. Here, we consider the possibility of replacing calcium hypochlorite with hydrogen peroxide, an environmentally friendly oxidizer. Hydrogen peroxide is superior to calcium hypochlorite because the latter needs pretreatment before use and, in addi tion, thiocyanate and cyanide oxidation with calcium hypochlorite can release toxic compounds of chlorine and yields large amounts of gypsum waste. The objects of this study are model thiocyanate solutions, as well as real solutions from the neutralization department of a hydrometallurgical refinery (Kholbinskii mine, OAO Buryatzoloto) after acidification and cyanide removal by stripping. The oxidative destruction of thiocyanates with hydrogen peroxide in the model and real solutions was studied in the presence of iron ions. Hydrogen peroxide offers promise as an environmentally appropriate and sufficiently efficient oxidizer for neutraliza tion of liquid waste from the hydrometallurgical processing of gold concentrates. DOI: 10.1134/S2070050410010113
We examined model thiocyanate solutions and real recycled water of the hydrometallurgical refinery of a goldmining plant after scarification, acidification, and cyanide volatilization in a centrifugal bubbler. The reactor was a Drechsel bottle. Air was fed into the reactor with a compressor at a rate of approxi mately 1 l/min. The working volume of the liquid in the reaction compartment was 150 ml. In the experi ments on model solutions, the thiocyanate concentra tion was varied between 125 and 1000 mg/l. The cata lysts were Fe(II) and Fe(III) salts. The SCN– concen tration in solutions was determined photometrically as Fe(III) thiocyanate on a KFK3 spectrophotometer at a wavelength of 460 nm [3]. One of the main factors in the oxidative destruction process is the pH of the medium. We found that thio cyanate oxidation is most efficient at pH 2.5–3.0 (Fig. 1), which is in agreement with the data of others [4]. Other significant factors in the oxidative destruc tion of thiocyanates are the initial concentrations of the reactants, as is evident from the nonlinear surface of extents of thiocyanate destruction in the [H2O2]– [Fe3+]–[SCN–] coordinates (Table 1, Fig. 2). According to our experimental data, the reactant molar ratios [H2O2] : [SCN–] ≥ 3.5 and [Fe3+] : [H2O2] = 0.14–1.00 are optimal for rapid and complete rhodanide destruction. The best catalysts for this process are Fe3+ salts, as is clear from the experimental data presented in Fig. 3. It is likely that use of Fe(II) salts leads to extra hydro gen peroxide consumption for their oxidation. This is indicated by the fact that the colorless solutions con
INTRODUCTION Although sodium cyanide is highly toxic, cyaniding of goldcontaining ores is still the most common way of recovering noble metals. The wastewater and recy cled water of gold extraction plants, which contain simple and complex cyanides of heavy metals and thiocyanates, need to be finely cleaned to remove these highly toxic components before being dis charged or reused. We have developed a liquid waste treatment tech nology for hydrometallurgical processing of gold con centrates [1, 2]. This technology includes efficient HCN stripping and absorption processes and the oxi dative destruction of the residual impurities: the recy cled solution from the hydrometallurgical refinery is acidified, HCN is removed from the solution in cen trifugal bubblers and is absorbed into an alkali solution in a jet scrubber or a centrifugal bubbler, and the pre cipitates of heavy metal compounds are then separated out (AVR process). After the removal of cyanides as HCN and the separation of heavy metals as precipi tates of coordination compounds, thiocyanates are oxidized with calcium hypochlorite. Here, we demonstrate the possibility of replacing calcium hypochlorite with hydrogen peroxide, an environmentally friendly oxidizer. Hydrogen peroxide is preferable for the following reasons: firstly, calcium hypochlorite needs pretreatment before use, secondly, thiocyanate and cyanide oxidation with calcium hypochlorite can yield toxic compounds of chlorine, and, finally, this process yields large amounts of gyp sum waste. 67
68
BATOEVA et al. ϕ, % 100
Extent of destruction, % 90 pH = 2.5 2.0 70 3.0 50
80 60
3.5
30 10
40
4.0 0
10
20
20 30
40
50
60 τ, min
Fig. 1. Effect of the pH of the medium on the extent of destruction of thiocyanates at reactant molar ratios of [H2O2] : [Fe3+] : [SCN–] = 7 : 1 : 2 and an SCN– concen tration of 0.25 g/l.
0 0
20
40
(X, Y, Z)
60
80
100 Fe3+
80
taining thiocyanates and Fe(II) turned bright red as hydrogen peroxide was added. The overall reaction in SCN– oxidation with hydrogen peroxide in an acid medium (pH ≤ 3) is as follows [5]: –
–
(1) 3H 2 O 2 + SCN = HSO 4 + HCN + 2H 2 O. For the process to be efficient, it is necessary to remove the resulting HCN from the reaction mixture by stripping in an intensive masstransfer apparatus. In our technology, hydrogen peroxide decomposi tion is likely initiated by an Fe(III) thiocyanate com plex. After the addition of H2O2 to a solution contain ing the colored iron thiocyanate complex, the color intensity begins to decrease and the solution gradually becomes colorless. This indicates the decomposition of the thiocyanate complex and the intermediate products of its oxidation. The mechanism of catalysis by the iron complexes can be represented as –
HO 2 + H+,
H2O2 – HO 2
LFe3+ + LFe2+ + H2O2
OH• + H2O2
LFe2+ + LFe3+ + OH + OH–, •
HO 2 + H2O, •
•
–
H+ + O2 ,
HO 2
•
• HO 2 , •
–
LFe3+ + O2 • HO 2 •
(2) (3) (4) (5) (6)
LFe2+ + O2, – OH 2 , –
(7)
LFe3+ + (8) LFe2+ + 2+ + OH 3+ LFe + OH , (9) LFe oxidation products of the L = SCN–.(10) L + OH• Thiocyanate oxidation in real solutions needs a larger amount of hydrogen peroxide because the latter is wasted on the oxidation of all admixtures, including thiosulfates, sulfides, and sulfites (Table 2). It was established experimentally that the process can be made more efficient by adding the oxidizer in portions and
From 4 : 5 : 1 to 8 : 1 : 1
60
40
20
H2O2 20 0 (X, Y, Z)
40
60
80
100 SCN−
Fig. 2. Nonlinear surface of extents of rhodanide destruc tion in the [H2O2] : [Fe3+] : [SCN–] coordinates. T = 20°C, pH 2.5, and τ = 5 min.
that the optimal reactant molar ratios for the real solu tions are [H2O2] : [Fe3+] : [SCN–] = 6 : 1 : 1 (Fig. 4). Based on the kinetics and mechanism of thiocyan ate oxidation with hydrogen peroxide in acid media, we developed a conceptual process design for treat ment of liquid waste from the cyanide leaching of gold concentrates (Fig. 5). After HCN stripping in a centrifugal bubbler and precipitate separation in a vertical settler, the acidic cyaniding solution is directed to a bubble column and H2O2 and a thiocyanate oxidation catalyst are dosed there. Air is simultaneously fed into the bubble col umn through a flow disperser for removing HCN resulting from SCN– oxidation. The solution leaving the column is alkalified to pH 8.0–8.5 and is directed to a settler to precipitate and then separate iron hydroxide. The spent cyaniding solution, now free of cyanides, thiocyanates, and heavy metal ions, is returned into the closedloop waterrecycling system. CATALYSIS IN INDUSTRY
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Table 1. Extents of oxidative destruction of rhodanides under various experimental conditions Molar ratios
Extent of destruction, %, at various times (τ), min
Initial concentration, mg/l
H2O2
Fe3+
SCN–
H2O2
Fe3+
SCN–
1
1
8
73
120
1000
1
2
7
73
240
2
1
7
146
1
3
6
2
2
3
15
30
5
5
5
875
8
8
8
120
875
8
8
12
73
361
750
9
9
12
6
146
240
750
11
12
14
1
6
220
120
750
4
11
12
1
4
5
73
481
625
12
13
14
2
3
5
146
361
625
18
19
19
3
2
5
220
240
625
22
22
22
4
1
5
293
120
625
18
20
21
1
5
4
73
601
500
14
15
17
2
4
4
146
481
500
21
22
23
3
3
4
220
361
500
25
26
26
4
2
4
293
240
500
25
27
27
5
1
4
366
120
500
27
27
28
1
6
3
73
721
375
18
19
20
2
5
3
146
601
375
27
28
28
3
4
3
220
481
375
29
30
31
4
3
3
293
361
375
32
33
33
5
2
3
366
240
375
33
34
34
6
1
3
439
120
375
25
26
26
1
7
2
73
841
250
20
24
26
2
6
2
146
721
250
34
35
40
3
5
2
220
601
250
39
42
46
4
4
2
293
481
250
46
46
50
5
3
2
366
361
250
52
51
54
6
2
2
439
240
250
59
62
64
7
1
2
512
120
250
53
71
72
1
8
1
73
961
125
23
31
31
2
7
1
146
841
125
37
56
57
3
6
1
220
721
125
50
78
82
4
5
1
293
601
125
71
94
96
5
4
1
366
481
125
99
100
100
6
3
1
439
361
125
100
100
100
7
2
1
512
240
125
96
100
100
8
1
1
586
120
125
91
100
100
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70
BATOEVA et al. Extent of destruction, % 100
1 2 3
80
4
60
40
20
0
10
5
15 τ, min
Fig. 3. Extent of destruction of thiocyanates in the presence of (1, 3) Fe3+ and (2, 4) Fe2+ at [H2O2] : [Fe] : [SCN–] = (1, 2) 6 : 1 : 2 and (3, 4) 6 : 2 : 2.
Thiocyanate concentration, mg/l 1600 (a) 1400
Thiocyanate concentration, mg/l 1600 (b) 1400
1200
1200
1000
1000
800
800
600
600
400
400
1 2
200 3 0 10
30
3 1
200 50
70
90
110 τ, min
0 10
30
50
70
90
110 τ, min
Fig. 4. Time profiles of the thiocyanate concentration for the peroxide treatment of the recycled water of the hydrometallurgical refinery: (a) portionwise and (b) onetime addition of the oxidizer. [H2O2] : [Fe3+] : [SCN–] = (1) 4 : 1 : 1, (2) 6 : 1 : 1, and (3) 8 : 1 : 1; pH 2.5.
H2SO4
H2O2
FeCl3 HCN stripping
Recycled water of a hydrometallurgical refinery
Clarification
Thiocyanate oxidation
HCN absorption into an alkali
NaCN return
NaOH
Neutralization
Discharge of the flotation tailings into a tailing pit
Fig. 5. Conceptual liquid waste treatment flowsheet for the cyanide leaching of gold concentrates. CATALYSIS IN INDUSTRY
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Table 2. Results of neutralization of the recycled water of a hydrometallurgical refinery Concentration, mg/l Sample
pH
Recycled water of a hydrometallurgical refinery After stripping in a centrifugal bubbler treatment with hydrogen peroxide
CN–
CNS–
9.9
774
2.5 3.3
99 40
2–
2–
S2 O3
SO 3
1597
436
256
1391 44
194 48
56 32
Use of H2O2 as the oxidizer instead of calcium hypochlorite allows the existing equipment at gold extraction plants to be employed without any signifi cant modification. The results of neutralization of waste from cyanide leaching of gold concentrates using the above technol ogy are presented in Table 2.
Hydrogen peroxide offers promise as an environ mentally appropriate and efficient oxidizer for neu tralization of liquid waste from the hydrometallurgical processing of gold concentrates. The results of this study can be used in the development of wastewater and recycled water aftertreatment methods for gold extraction plants.
CONCLUSIONS The oxidative destruction of thiocyanates with hydrogen peroxide in the presence of iron ions was studied in model and real solutions. The most significant factors in thiocyanate destruction are the pH of the medium, the initial con centrations of the reactants, and the reactant molar ratios. The optimal reactant molar ratios for rapid and complete destruction of thiocyanates at concentra tions of up to 1 g/l are [H2O2] : [SCN–] ≥ 3.5 and [Fe3+] : [H2O2] = 0.14–1.00. The most efficient thio cyanate oxidation is observed at pH 2.5–3.0.
REFERENCES
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1. Ryazantsev, A.A., Asalkhanov, A.A., et al., RF Patent 2310614; Byull. Izobret., 2007, no. 32. 2. Kachanov, A.A., Ryazantsev, A.A., et al., Khim. Interes. Ust. Razv., 2004, vol. 12, pp. 445–450. 3. Lahti, M., Viipo, L., and Hovinen, J., J. Chem. Educ., 1999, vol. 76, no. 9, p. 1281. 4. Sychev, A.Ya. and Isak, V.G., Gomogennyi kataliz soedineniyami zheleza (Homogeneous Catalysis by Iron Compounds), Chisinau: Shtiintsa, 1988. 5. Figlar, J.N. and Stanbury, D.M., Inorg. Chem., 2000, vol. 39, pp. 5089–5094.