removal of thiocyanate from solutions by precipitation

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REMOVAL OF THIOCYANATE FROM SOLUTIONS BY PRECIPITATION E Yazıcı1, R Üçüncü2 and H Deveci3 ABSTRACT In this study, the removal of thiocyanate from aqueous solutions by precipitation in the presence of thiosulfate and copper ions was studied. Taguchi L9 (34) experimental design was adopted for the study. The effects of concentration of thiocyanate (10-50 mg/L SCN-), copper (10-50 mg/L Cu2+), thiosulfate (20 - 150 mg/L S2O32-) and pH (4.5-10.5) on the extent of thiocyanate removal from synthetic solutions were investigated at three levels. Concentration of thiocyanate and copper in the residual solution were also determined. The experimental data was analysed using a statistical software. The findings have revealed that the concentration of thiosulfate (S2O32-) and pH were the significant factors affecting the removal of thiocyanate, while concentration of thiocyanate (SCN-) and copper (Cu2+) were found to be insignificant under the conditions tested. The increase in pH was found to promote the removal of thiocyanate, which had a tendency to decrease with increasing the concentration of thiosulfate. Keywords: thiocyanate, Taguchi design, cyanide effluents, wastewater treatment, precipitation

INTRODUCTION Metallurgical operations e.g. gold/silver leaching and metal finishing, often produce effluents containing cyanide and cyanide-related compounds such as thiocyanate (SCN-). These industrial effluents can pose environmental threat due to toxic characteristics of cyanide species with free cyanide (CN-, HCN) being the most toxic form. Despite its relatively low toxicity (7 times less) when compared with hydrogen cyanide, thiocyanate (SCN-) is also of concern from environmental point of view (Boening and Chew, 1999; Mudder et al, 2001; The International Cyanide Management Institute, 2009; Üçüncü, 2009). Although SCN- dissociates under weak acidic conditions, it is not classified as a weak acid dissociable (WAD) cyanide due to its complexing capabilities similar to free cyanide (Young and Jordan, 1995). Thiocyanate (SCN-), is one of the primary specie that often occurs in cyanide leaching effluents in the range of concentration of 30) (Yazıcı, 2005; Deveci

1. Research Assistant, Department of Mining Engineering, Karadeniz Technical University, Trabzon 61080, Turkey. Email: [email protected] 2. Mining Engineer, Department of Mining Engineering, Karadeniz Technical University, Trabzon 61080, Turkey. Email: [email protected] 3. Associate Professor, Department of Mining Engineering, Karadeniz Technical University, Trabzon 61080, Turkey. Email: [email protected]

XXV INTERNATIONAL MINERAL PROCESSING CONGRESS (IMPC) 2010 PROCEEDINGS / BRISBANE, QLD, AUSTRALIA / 6 - 10 SEPTEMBER 2010

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E YAZICI, R ÜÇÜNCÜ AND H DEVECI

et al, 2006). Some widely used chemical oxidation processes i.e. SO2/air and H2O2, are inadequate for the removal of thiocyanate and SAD cyanide from waste solutions. Only 10.5-11 to avoid the release of toxic HCN gases. At high pHs, the oxidation of SCN- by ozonation adversely affected due to decomposition of ozone (O3) at pH>11 resulting in the consumption of excess reagent (Mudder et al, 2001; Sharma et al, 2008). Sharma et al (2008) suggest ferrate (VI) as an effective oxidant for the removal of thiocyanate in terms of high oxidation kinetics compared with conventional methods. Biological degradation methods (particularly aerobic bio-treatment) offer a low-cost and effective solution i.e. readily oxidation of SCN- with limited by-product generation. On the other hand, it is kinetically a slow process and adversely affected by the decrease in temperature (Mudder et al, 2001; Mudder and Botz, 2001). Although many industrial recovery/recycling methods are employed for the recovery of cyanide from free and WAD cyanides, there has been no reported industrial process focused on the recovery of cyanide from thiocyanate (Botz et al, 2001; Mudder et al, 2001). Botz et al (2001) discussed the potential methods for regeneration of cyanide from thiocyanate. They reported that regeneration of SCN- (particularly through ozonation) is advantageous compared with oxidative treatments due to its relatively low cost. However, in recovery/recyling methods, cupric copper ions (Cu2+) present in the effluent (>10 mg/l) promote the decomposition of SCN-, thus hinder the efficiency of the regeneration processes (Jara et al, 1996; Botz et al, 2001). Given that, cyanidation effluents may contain copper (0.1-400 mg/l) which originated from dissolution of copper minerals during leaching stage (Gaydardjiev, 1998), regeneration of SCN- seems to be unfavorable for cyanide effluents containing high concentration of copper (>10 mg/l). Removal of thiocyanate from solutions by precipitation, although has drawn limited attention, is possible under certain conditions. It has been known that, in the presence of cuprous copper (Cu+), thiocyanate precipitates as copper(I) thiocyanate (CuSCN, logK=16.8) (Gündüz, 1993). However, a reducing agent such as thiosulfate (S2O32-), sulfur dioxide (SO2) or ferrous iron (Fe2+) is required to generate cuprous copper (Cu+) by the reduction of cupric copper (Cu2+) in the medium. The precipitation of thiocyanate takes place mainly in two stages: Cupric copper ions (Cu+2) is reduced to cuprous copper (Cu+) in the presence of a reducing agent e.g. thiosulfate (4) resulting in the precipitation of thiocyanate (SCN-) as copper(I) thiocyanate (CuSCN) (5): 2Cu2+ + 2S2O32- → S4O62- + 2Cu+

(4)

Cu+ + SCN- → CuSCN (s)

(5)

Priyadarshan (2000) investigated the removal of free/WAD/total cyanide from solutions in the presence of thiosulfate and copper sulfate, through the formation of CuSCN. Keller (1945) proposed a method for the recovery of copper from copper-bearing solutions with the addition of SO2 as a reductant and thiocyanate to produce copper(I) thiocyanate (CuSCN). This can be also exploited to the removal of thiocyanate from solutions. The method (4-5) can be a potential alternative to oxidation/regeneration methods. In this study, the removal of thiocyanate from synthetic solutions by precipitation in the presence of thiosulfate and copper sulfate was studied. The effects of concentration of thiocyanate, copper, thiosulfate and pH on the extent of removal of thiocyanate from solutions were investigated. Taguchi experimental design (L9-34) was used to determine the mode of effects of the parameters on the removal of thiocyanate. The experimental data was analysed using a statistical software.

EXPERIMENTAL Material and method Reagent grade potassium thiocyanate (KSCN), sodium thiosulfate (Na2S2O3.5H2O) and copper sulfate (CuSO4.5H2O) were used in the study. All solutions were prepared using deionised-distilled water. XXV INTERNATIONAL MINERAL PROCESSING CONGRESS (IMPC) 2010 PROCEEDINGS / BRISBANE, QLD, AUSTRALIA / 6 - 10 SEPTEMBER 2010

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REMOVAL OF THIOCYANATE FROM SOLUTIONS BY PRECIPITATION

Precipitation tests were performed in 50-ml borosilicate erlenmeyer flasks. Solutions (50 ml) were prepared at different strengths of SCN- (10-50 ppm), S2O32- (20-150) and Cu2+ (10-50 mg/l) at pH 4.5-10.5. pH of the solutions were adjusted using H2SO4 and/or NaOH.The flasks were then placed in a temperature controlled (20°C) reciprocal shaker operating at 140 rpm. Over the experimental period, the top of the flasks were kept covered to avoid diffusion of air into the solution. Following the reaction period (1 hour) 10-ml aliquots were removed for sampling. All samples were then filtered by 0.45 μm cellulose nitrate filters prior to analysis of residual thiocyanate and copper. Residual SCN- in the solution was analysed by colorimetric method using a UV-vis spectrophotometer (Jenway 6405) at a wavelength of 640 nm (Patnaik, 1997). Residual copper was also analysed using an atomic absorbtion spectrophotometer (Perkin Elmer AAnalyst 400).

Experimental design Taguchi design, a widely used statistical methodology, allows to evaluate the effect of parameters on the response of an experimental study with a limited run of experiments (Roy, 2001). In the current study, Taguchi L9 (34) orthogonal array was implemented. The effects of initial concentration of thiocyanate (10-50 mg/l SCN-), copper (10-50 mg/l Cu2+), thiosulfate (20-150 mg/l S2O32-) and pH (4.5-10.5) on the extent of removal of thiocyanate from synthetic solutions were investigated at three levels within nine experiments. The parameters and their levels of the design were presented in Table 1. Table 2 shows the Taguchi L9 (34) array adopted for the current study. Main effects and ANOVA table were evaluated using Minitab (2004) statistical software. TABLE 1 Factors and their levels adopted for the experimental design. Parameters

Level 1

Level 2

Level 3

-

(A) SCN (mg/l)

10

20

50

2+

10

20

50

(B) Cu (mg/l) 2-

(C) S2O3 (mg/l)

20

80

150

(D) pH

4.5

8.5

10.5

TABLE 2 Taguchi L9 (34) array implemented for the study. Exp. no

Levels of Parameters (A) SCN- (mg/l)

(B) Cu2+ (mg/l)

(C) S2O32- (mg/l)

(D) pH

1

1

1

1

1

2

1

2

2

2

3

1

3

3

3

4

2

1

2

3

5

2

2

3

1

6

2

3

1

2

7

3

1

3

2

8

3

2

1

3

9

3

3

2

1

RESULTS AND DISCUSSION Extent of removal of thiocyanate (%) and precipitation of copper (%) for each experiment were presented in Table 3. Table 4 shows mean values for the removal of SCN- (%) for parameters at each level. Delta values were calculated by substracting maximum and minimum average values, which help to compare the relative magnitude of effect of the parameters. Concentration of S2O32-, pH, concentration of SCN- and Cu2+ were the most significant factors affecting the process. XXV INTERNATIONAL MINERAL PROCESSING CONGRESS (IMPC) 2010 PROCEEDINGS / BRISBANE, QLD, AUSTRALIA / 6 - 10 SEPTEMBER 2010

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TABLE 3 Removal of SCN– (%) and precipitation of Cu (%) for the experiments. Exp. no

(A) SCN (mg/l)

(B) Cu (mg/l)

(C) S2O3 (mg/l)

(D) pH

Removal of SCN(%)

Precipitation of Cu (%)

1

10

10

20

4.5

79.0

82.7

2

10

20

80

8.5

90.3

64.5

3

10

50

150

10.5

3.2

59.1

4

20

10

80

10.5

12.1

90.5

5

20

20

150

4.5

8.9

80.7

6

20

50

20

8.5

59.7

24.0

7

50

10

150

8.5

7.1

77.4

8

50

20

20

10.5

29.0

89.5

9

50

50

80

4.5

73.2

80.7

-

2-

2+

TABLE 4 Mean values of SCN removal for parameters at each level and delta statistics for parameters. -

Removal of SCN- (%)

Parameters Level 1

Level 2

Level 3

Delta (Max.-Min.)

Rank

-

(A) SCN (mg/l)

57.53

26.88

36.45

30.64

3

(B) Cu2+ (mg/l)

32.74

42.74

45.38

12.64

4

(C) S2O32- (mg/l)

55.91

58.55

6.40

52.15

1

(D) pH

53.71

52.37

14.79

38.92

2

Analysis of variance (ANOVA) were performed for statistical evaluation of the results (Table 5). In order to calculate error term in ANOVA table, the parameter with the lowest rank value i.e. Cu2+ (mg/l) (Table 4), was pooled (Table 5). P values have been used to test the significance level of parameters. The P value is the probability that the test statistic will take on a value that is at least as extreme as the observed value of the statistic when the null hypothesis (H0) is true. To assign a parameter as significant, P value should be lower than the alpha (α) value determined for the significance test (Montgomery, 2001). Considering the P values as shown in Table 5, S2O32- (mg/l) was found to be statistically significant at 95% (α=0.05), while pH was only at 90% (α=0.10) confidence interval. Their contributions on the extent of removal of thiocyanate were determined to be as 52.58% and 29.74%, respectively (Table 5). However, effect of concentration of thiocyanate (SCN-, mg/l) was found to be insignificant even at 90% (α=0.1) confidence interval. Contribution ratios (%) also reflect the relative importance of each factor (Table 5). It can be concluded from the findings that concentration of S2O32- and pH are the most important parameters in decreasing order of magnitude, for the precipitation of SCN-. TABLE 5 Results of ANOVA for the parameters. Source of deviation

Degree of freedom

Sum of squares

Mean squares

F value

P value

Contribution (%)

(A) SCN- (mg/l)

2

1474.7

737.3

5.53

0.153

14.97

2-

(C) S2O3 (mg/l)

2

5178.1

2589.1

19.42

0.049

52.58

(D) pH

2

2929.1

1464.5

10.99

0.083

29.74

2+

(2)

(266.6)

(133.3)

Pooled

Pooled

-

133.3

(B) Cu (mg/l) Residual Error

2

266.6

Total

8

9848.5


2.71 100

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S CN 40

20

0 Level1

Level2

Level3

S 2O3

60

40

20

0 Level1

Level2

Level3

Mean values for the removal of SCN (%)

60

40

Cu

20

0 Level1




The effects of parameters on thiocyanate removal (%) in each level were depicted in Figure 1 using the mean values from Table 4. Copper seemed to have a positive effect (Figure 1), but it has the least impact on the process (Table 4). Figure 2 illustrates the surface and contour plots of removal of thiocyanate (%) vs. levels of statistically significant parameters i.e. S2O32- and pH. These plots graphically show the simultaneous effect of these parameters on the response i.e. removal of SCN- (%). Accordingly, to achieve high SCN- removal (>90%), levels of S2O32- and pH should be selected at values between level of 1 and 2 (Figure 2). At the lowest and highest levels of concentration of S2O32- and pH, significant reduction in SCN- removal occurred (Figure 2). In the experiments, maximum removal of SCN- i.e. ~90%, was achieved in the 2nd run (Table 3). Although a reducing agent (S2O32-) was essential to keep copper in monovalent state, the increase in the concentration of S2O32- adversely affected SCN- removal (Figure 1), probably due to the formation of insoluble copper sulfide species such as CuS (logK=36.5) and/or Cu2S (logK=54.1), as suggested by speciation calculations (MEDUSA, 2004). In fact, removal of SCN- occur favourably at near acidic

Level2

Level3

60 pH

40

"

20

0 Level1

Level2

Level3

FIG 1 - The effects of parameters on the removal of thiocyanate (%).

S2O3 level

3

2

1

(a)

Removal of SCN (%) < 30 30 - 50 50 - 70 70 - 90 90 - 95 > 95

1

(b)

2 pH level

3

FIG 2 - Surface (a) and contour (b) plot of removal of thiocyanate (%) vs. levels of concentration of S2O32- and pH. XXV INTERNATIONAL MINERAL PROCESSING CONGRESS (IMPC) 2010 PROCEEDINGS / BRISBANE, QLD, AUSTRALIA / 6 - 10 SEPTEMBER 2010

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(pH 4.5) and neutral (pH 8.5) conditions (Figure 1). Keller (1945) reported high removal of copper as CuSCN (>99%) from acidic solutions. Adverse effect of high pH can be ascribed to the relatively low stability of CuSCN and formation of cuprous- and/or cupric copper-oxides in alkaline thiocyanate solutions (Richardson, 1997). These findings also confirmed by the speciation calculations using MEDUSA software (2004) indicating that CuSCN is stable at 80 ppm. Acidic (pH 4.5) and neutral solutions (pH 8.5) favor the precipitation of SCN- while alkalinity (pH 10.5) produces adverse effect. The method seems to offer an effective solution for the treatment of effluents containing thiocyanate with no by-product generation. Further investigations should be carried out thoroughly to evaluate and compare the process with oxidation and regeneration methods in terms of effectiveness and economics.

ACKNOWLEDGEMENT The authors would like to express their sincere thanks and appreciation to the Research Foundation of Karadeniz Technical University for the financial support (Project No: 2002.112.8.3).

FIG 3 - pH-dependent speciation of copper (0.79 mM Cu2+) in the presence of thiocyanate (0.86 mM SCN-) and thiosulfate (0.18 mM S2O32-) (s:solid, c:crystalline). XXV INTERNATIONAL MINERAL PROCESSING CONGRESS (IMPC) 2010 PROCEEDINGS / BRISBANE, QLD, AUSTRALIA / 6 - 10 SEPTEMBER 2010

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REFERENCES Boening, D W and Chew, C M, 1999. A critical review: General toxicity and environmental fate of three aqueous cyanide ions and associated ligands, Water, Air and Soil Pollution, 109:67-79. Botz, M M, Dimitriadis, D, Polglase, T, Phillips, W and Jenny, R, 2001. Processes for the regeneration of cyanide from thiocyanate, Minerals & Metallurgical Processing, 18(3):126-132. Deveci, H, Yazıcı, E Y, Alp, I and Uslu, T, 2006. Removal of cyanide from aqueous solutions by plain and metal-impregnated granular activated carbons, Int. J. Miner. Process., 79(3):198-208. Gaydardjiev, S S, 1998. Hydrometallurgy of precious metals: effects on the environment, in Mineral Processing and the Environment (eds: G P Gallios and K A Matis), pp 257-280 (Kluwer Academic Publishers:Dardrecht) Gündüz, T, 1993. Kantitatif Analiz Laboratuar Kitabı, 312 p (Bilge Yay:Ankara) (in Turkish). Jara, J, Soto, H and Nava, F, (Canadian Liquid Air Ltd.), 1996. Regeneration of cyanide by oxidation of thiocyanate, US Patent 5482694. Keller, C H, (The Dow Chemical Company), 1945. Recovery of copper from copper-bearing solutions, US Patent 557383. Lottermoser, B G, 2007. Mine Wastes – Characterization, Treatment, Environmental Impacts, 340 p (Springer: New York) Marsden, J O and House, C I, 2006. The Chemistry of Gold Extraction, 651 p (SME:USA) MEDUSA, 2004. Software for Chemical Equilibrium Diagrams, Version 18. Royal Institute of Technology, Sweden. Minitab, 2004. Minitab Inc, Statistical Software, Release 14.12.0 Montgomery, D C, 2001. Design and Analysis of Experiments, 684 p (John Wiley&Sons Inc:New York) Mudder, T I and Botz, M M, 2001. Overview of water treatment methods for thiocyanate removal, in Cyanide Monograph (eds:T I Mudder, M M Botz), pp 509-517 (Mining Journal Books Ltd:London) Mudder, T I, Botz, M M and Smith, A, 2001. Chemistry and Treatment of Cyanidation Wastes, 373 p (Mining Journal Books Ltd:London). Patnaik, P, 1997. Handbook of Environmental Analysis: Chemical Pollutants in Air, Water, Soil and Solid Wastes, 604 p (Lewis Publishers:New York). Priyadarshan, G, 2000. Removal and stabilization of cyanide from process waters, MSc thesis, University of Nevada, Reno, 82 p. Richardson, H W, 1997. The manufacture of copper compounds, in Handbook of Copper Compounds and Applications (ed: H W Richardson), pp 53-92 (CRC Press:New York). Roy, R K, 2001. Design of Experiments Using The Taguchi Approach: 16 Steps to Product and Process Improvement, 560 p (Wiley-Interscience:New York). Sharma, V K, Yngard, R, Cabelli, D E and Baum, J Y, 2008. Ferrate(VI) and ferrate(V) oxidation of cyanide, thiocyanate, and copper(I) cyanide, Radiation Physics and Chemistry, 77(6):761-767. The International Cyanide Management Institute (ICMI), 2009. Cyanide Facts: Cyanide Chemistry, International Cyanide Management Code for the Manufacture, Transport, and Use of Cyanide in the Production of Gold [online]. Available from: [Accessed: 04 October 2009]. Üçüncü, R, 2009. Removal of thiocyanate from wastewaters, BSc thesis, Karadeniz Technical University, Trabzon (in Turkish). Yazıcı, E Y, 2005. Removal of cyanide from waste waters using hydrogen peroxide, activated carbon adsorption and ultrasonic waves, MSc thesis, Karadeniz Technical University, Trabzon (in Turkish) Young, C A and Jordan, T S, 1995. Cyanide remediation: current and past technologies, in Proceedings of the 10th Annual Conference on Hazardous Waste Research, pp 104-129 (Kansas State University:Kansas).


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