Estimation of the Correlation between Selected Factors of Elution of Gold Cyanide from Carbon: A Plant-Wise Study

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1 International Journal of Engineering and Technology Volume 7 o.9, September, 2017 Estimation of the Correlation between Selected Factors of Elution of Gold Cyanide from Carbon: A Plant-Wise Study Gideon A. Ocran and ana Y. Asiedu Department of Chemical Engineering, Kwame krumah University of Science and Technology, Kumasi, Ghana. ABSTRACT The elution of gold from carbon depends on various factors such as eluant flow rate, ionic strength of the eluant, ph, cyanide strength, cyanide loading and decomposition or adsorption of cyanide on the carbon surface as well as the temperature at which the elution is conducted. The stripping efficiency of gold cyanide and ionic strength of the eluant is strongly correlated (R 2 = ). Also, the stripping efficiency of gold cyanide is strongly correlated with ph (R 2 = ) and cyanide strength (R 2 = 0.99). The decomposition of cyanide and eluant flow rate are highly correlated with cyanide loading with coefficient of determination (R 2 ) values of and 0.99 respectively. The elution of gold cyanide is found to be independent of eluant flow rate with R 2 value of Keywords: Elution, Stripping Efficiency, Gold Cyanide, Correlation, Coefficient of Determination. 1. ITRODUCTIO Activated carbon is used in carbon-in-pulp or leach (CIP/L) process for extraction of gold cyanide from leached pulps. During the past years, the kinetics and mechanism of adsorption of gold cyanide onto activated carbon have been studied thoroughly [1], auxiliary process step such as elution has been given less attention [2]. Consequently, opportunities exist for research on the mechanism of the elution process. The study is based on the Zadra elution process. In essence, the process consists of circulating warm caustic cyanide solution through an elution column and an electrowinning cell (a U.S. Bureau of Mines invention) [3]. Other methods of elution are used in industry namely [4]: the Anglo American Research Laboratory (AARL) process and the Micron elution process. The Anglo American Research Laboratory (AARL) process involves pre-soaking of the loaded carbon with hot caustic cyanide solution followed by elution with hot deionized water (a South African invention). The Micron elution process involves soaking the loaded carbon in a solution of sodium cyanide followed by elution with acetonitrile, ethanol or methanol vapour (Australian invention). According to Adams and icol [5], temperature, cyanide and hydroxide concentrations, and the ionic strength of the eluant constitute the most significant effects on elution. Davidson [6] considered the temperature and reagent cyanide addition to be the most important factors affecting gold elution. Davidson and Veronese [7] stated that where the loaded carbon contain high amounts of calcium carbonate, the temperature and acid washing of the carbon are considered to be most important for efficient elution. Van Deventer and Van der Merwe [8] summarize the most important factors that influence the rate of elution, in order of importance are as follows: temperature, concentration of cyanide and caustic and ionic strength of the eluant, ph and acid treatment. The objective of this paper is to find the correlation (if any) between selected factors of elution of gold cyanide from loaded carbon. 2. TOOLS AD METHODS 2.1. Model for Prediction of Cyanide Profile The plug flow model of Van Deventer and Liebenberg, (2003) is adapted. A plug flow model to describe the profiles are based on the following assumption: the oxidation and hydrolysis reactions of the cyanide in the pores of the carbon can be combined and described by a single first-order reaction; the concentration of cyanide inside the pores of the carbon is homogeneous; mass transfer between the bulk solution and the pore liquid occurs via film diffusion; the combined decomposition rate of cyanide in the pores will decrease with an increase in the cyanide loading which means that the carbon surface becomes passivated [9]. A mass balance of cyanide in the pore liquid is given by C p t 6k V d C Cp Kp Cp 1 s P c # ISS: IJET Publications UK. All rights reserved. 542

2 where C p the carbon pores, interparticle solution, is the cyanide concentration in the liquid phase of C is the cyanide concentration in the K p is the reaction rate constant for the decomposition of cyanide in the carbon pores, C -QC 6k 1 t h d k s is the film transfer coefficient for cyanide, the carbon, d c is the apparent density of is the average diameter of a carbon particle, is the specific pore volume and t is the time variable. A mass balance of cyanide in the interparticle solution in the elution column is given by s = C Cp K C 2 a c V P h is the axial variable in the column and Q is the volumetric flow rate of eluant. where K reaction rate constant for the decomposition of cyanide in the interparticle solution, is the void fraction of the carbon in a packed column, a is the flow area of the column, It can be expected that temperature will affect the reaction rate constant for the decomposition of cyanide in the K interparticle solution through an Arrhenius relationship - E K b exp 3 RT g where b is an empirical constant, E is the activation energy for the decomposition of cyanide in the interparticle solution, T is the temperature of elution and R g is the ideal gas constant. According to Van der Merwe [9] the effect of cyanide loading on K p is given by f Kp 4 g Q 1 L where Q L is the cyanide loading of the carbon, f and g are empirical constants relating to the decomposition of cyanide in the carbon pores and the passivation of the carbon respectively. The change in the cyanide loading of the carbon is calculated from the amount of decomposition of cyanide in the pores of the carbon as t QL V P K p C p 5 The values of parameters used are 1404 hr -1, E hr -1, 540 kg/m 3, k s = m/hr, f = = kJ/mol, g = 3.5, b = VP 4 m 3 /kg, = 0.188, a = 3.01, d c 2.0 mm, h = m, column diameter of 1.96 m and the elution data of the mine. It is assumed that the cyanide concentration in the pore liquid will be lower than the cyanide concentration in the interparticle solution at the end of the pre-treatment step. The initial 2.2.Analysis of Data The correlation between the selected factors of elution of gold cyanide were determined by finding the Pearson correlation coefficient (R) values between the chosen parameters of the plant and model data using Microsoft excel spreadsheet. The strength of the correlation coefficient is ascertained from the coefficient of determination (R 2 ). The correlation coefficient (R) is expressed mathematically as Q L 0.96 g/kg, C 0 kg/m 3 and Cp 1400 kg/m 3. The prediction of cyanide loading and its rate of decomposition during elution are modelled using equation (1) to (5) in Matlab program. ISS: IJET Publications UK. All rights reserved. 543

3 x x y y 1 R n (6) 1 x y Where n is number of pairs of data, for x and y respectively, x and x y and deviations for the x and y values respectively. y are the means represent the standard 3. RESULTS AD DISCUSSIO y = x R² = Eluant Flow Rate (m 3 /hr) Fig. 1: Correlation between Stripping Efficiency and Eluant Flow Rate for Elution of Gold Cyanide from Carbon at 120 C. ISS: IJET Publications UK. All rights reserved. 544

4 y = -4.x R² = Ionic Strength of the Eluant (%) Fig. 2: Correlation between Stripping Efficiency and Ionic Strength of the Eluant for Elution of Gold Cyanide from Carbon at 120 C. y = x R² = ph Fig. 3: Correlation between Stripping Efficiency and ph for Elution of Gold Cyanide from Carbon at 120 C. ISS: IJET Publications UK. All rights reserved. 545

5 Cyanide Loading (kg/t) y = x R² = Cyanide Strength (%) Fig. 4: Correlation between Stripping Efficiency and Cyanide Strength for Elution of Gold Cyanide from Carbon at 120 C y = x R² = Eluant Flow Rate (m 3 /hr) Fig. 5: Simulation and correlation between Cyanide Loading and Eluant Flow Rate for Elution of Gold Cyanide from Carbon at 120 C. ISS: IJET Publications UK. All rights reserved. 546

6 Rate of Decomposition of Cyanide (1/hr) y = x R² = Cyanide Loading kg(c)/t Fig. 6: Simulation and correlation between First-order rate constant for the Decomposition of Cyanide and Cyanide Loading for Elution of Gold Cyanide from Carbon at 120 C. The stripping efficiency of gold cyanide and the eluant flow rate is poorly correlated as shown in Fig. 1. The claim is confirmed by the nature of the low coefficient of determination (R 2 = ). The reason for the result could be that stripping efficiency of gold cyanide is independent of the eluant flow rate in bed volume (BV) of 3.6 per hour. A similar trend was reported elsewhere [8]. Also, the observation could be that the elution is a more diffusion controlled mechanism. The desorption of gold cyanide is known to be influenced by ionic strength of the eluant as cations accelerate the rate of ionpair formation. High ionic strength of the eluant therefore leads to low stripping efficiency of gold cyanide. As shown in Fig. 2, there is a strong correlation between stripping efficiency of gold cyanide and ionic strength of the eluant. About % of the variation in stripping efficiency of gold cyanide may be explained by variations in the ionic strength of the eluant. The control of ph in elution is vital in view of the effect it has on downstream operation. A relatively high correlation (R 2 = 0.865) exist between stripping efficiency of gold cyanide and ph as shown in Fig. 3. This could be attributed to the stability of some base metal complexes under high ph conditions, thus decreasing stripping efficiency of gold cyanide from loaded carbon. A similar trend between stripping efficiency of gold cyanide and ph is observed by Lunga [10]. However, ph values above 13 are used in some Electrowinning cells to minimize corrosion of anode caused the localized drop in ph. Cyanide is used to control the selective desorption of gold cyanide during the elution. There is a strong correlation between stripping efficiency of gold cyanide and cyanide strength as shown in Fig. 4. However, there is no substantial increase in the stripping efficiency of gold cyanide from the loaded carbon when cyanide strength is increased. The observation could be assumed that cyanide is essential for faster elution rates. An increase in cyanide loading on carbon may correspond to low eluant flow rate. Although there is a general negative correlation between the cyanide loading and eluant flow rate as shown in Fig. 5, the eluant flow rate is strongly correlated to the cyanide loading with a high coefficient of determination (R 2 = 0.997). Variation in cyanide loading may be accounted for by variations in eluant flow rate. The observed trend in cyanide loading and eluant flow rate is expected as seen in the foregoing analysis. The eluant flow rate may be there only for uptake of cyanide (Fig. 1 and Fig. 5). Generally, an expected inverse relation exists between the decomposition of cyanide and cyanide loading [9]. The decomposition of cyanide during elution is known to affect the rate of elution under certain condition. The cyanide loading is strongly correlated to the decomposition of cyanide. About 98% of the variation in the decomposition of cyanide can be explained by variations in the cyanide loading. However, the elution process could be equilibrium transport based (Fig. 1, Fig. 5 and Fig. 6). 4 COCLUSIO Studies on some selected factors of elution of gold cyanide such as eluant flow rate, ionic strength of the eluant, ph, cyanide strength, cyanide loading and decomposition of cyanide from plant and model simulation data gave an indication of a very strong correlation between stripping efficiency of gold cyanide and ionic strength of the eluant (R 2 = ). Also, a high correlation exist between stripping efficiency of gold cyanide and cyanide strength (R 2 = 0.99), cyanide loading and eluant flow rate (R 2 = 0.99) and decomposition of cyanide and cyanide loading (R 2 = ) respectively. A strong correlation exist between stripping efficiency of gold cyanide and ph (R 2 = ). The stripping efficiency of gold cyanide is found to be independent of the eluant flow rate (R 2 = ). ISS: IJET Publications UK. All rights reserved. 547

7 ACKOWLEDGEMET The researcher would like to appreciate the management of Goldfields Ghana Limited the mine where the data was obtained for the study. REFERECES Van Deventer J.S.J and Van der Merwe P.F. (1994), The Mechanism of Elution of Gold Cyanide from Activated Carbon, Metallurgical and Materials Transaction, Volume 25B, Adams M.D and icol M.J. (1986), Gold : Proceedings of the International Conference on Gold, C.E. Fivaz and R.P. King, (Eds.), South African Institute of Mining and Metallurgy, Johannesburg, South Africa, Vol. 2, Davidson, R.J. (1986), Lecture 15, C.I.P. School, South African Institute of Mining and Metallurgy, Johannesburg, South Africa. Davidson, R.J and Veronese, V. (1979), Further Studies on the Elution of Gold from Activated Carbon using Water as the Eluant, J. S. Afr. Inst. Min. Metall., 79, Preez Du A.F, Van Deventer J.S.J, Petersen K.R.P and Lorenzen L. (1997), The Modelling of Mass Transfer Controlled Competitive Elution from Activated Carbon, Proceedings of the XX IMPC- Aachen. Zadra, J.B., Eagel, A.L. and Aeimen, H.J. (12), Process for the recovery of gold and silver from activated carbon by leaching and electrolysis, US, Bureau of Mines, Report of investigations, o Bailey P.R. (1987) in G.G. Stanley (Ed.), The Extractive Metallurgy of Gold in South Africa, South African Institute of Mining and Metallurgy, Johannesburg, South Africa, Vol. 1, 379. Van Deventer J.S.J and Van der Merwe P.F. (1993), Factors influencing the Elution of Gold from Activated Carbon, Department of Metallurgical Engineering, University of Stellenbosch, South Africa. Van der Merwe P.F. (1991), Fundamentals of the Elution of Gold Cyanide from Activated Carbon, Ph.D. Thesis, University of Stellenbosch, South Africa. Lunga, A.L. (2006), Optimizing the operating conditions of Gold Elution and Electrowinning for Tau Lekoa Stream at Kopanang Gold plant, Msc. Thesis Dissertation, ISS: IJET Publications UK. All rights reserved. 548