Oxidative ammonia leaching of sphalerite

Int. J. Miner. Process. 66 (2002) 241 – 254 www.elsevier.com/locate/ijminpro

Oxidative ammonia leaching of sphalerite Part I: Noncatalytic kinetics M.K. Ghosh a,*, R.P. Das a, A.K. Biswas b b

a Regional Research Laboratory, Bhubaneswar 751013, India Chemical Engineering Department, Indian Institute of Technology, Kharagpur 721302, India

Accepted 31 May 2002

Abstract The results of oxidative leaching kinetics of sphalerite concentrate in ammonia solution are presented. The effect of oxygen partial pressure (0.5 – 10 atm), ammonia concentration (1.54 – 5.35 M), pH (10.4 – 11.2), leaching temperature (90 – 130 jC), particle size (
90 + 75 to
53 + 45 Am), and agitation speed on the leaching rate were determined. For the generation of kinetic data, 1% of slurry density was used. Leaching rate was nearly independent of agitation and pH in the studied range. From the comparison of experimental reaction rate constant with calculated mass transfer coefficients, it was established that chemical reaction rate is the slowest process. Shrinking core model for surface reaction fitted the experimental data as well. Reaction order with respect to PO2 and [NH3] were 0.2 and 0.6, and the activation energy was determined to be 44.3 kJ/mol. D 2002 Elsevier Science B.V. All rights reserved. Keywords: leaching; sphalerite; kinetics; ammonia leaching

1. Introduction Zinc ranks as the third largest nonferrous metals in the world and is conventionally produced from sphalerite by roast – leach – electrowin (RLE) process. Strict environmental restrictions imposed on the sulphide smelters and the need to utilize small and complex deposits stimulated the development of alternative methods especially hydrometallurgical routes that avoid the production of SO2, a pollutant.

*

Corresponding author. Fax: +91-674-581750. E-mail address: [email protected] (M.K. Ghosh).

0301-7516/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 7 5 1 6 ( 0 2 ) 0 0 0 6 8 - 6

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During the last four decades, attention has been given to the leaching of zinc sulphide ores/concentrates by aqueous sulfuric acid (Forward and Veltman, 1959; Parker, 1961; Demopoulos and Baldwin, 1999), nitric acid (Bjorling, 1954), hydrochloric acid (Mizoguchi and Habashi, 1981; Majima et al., 1981), and acidified solutions containing ferric ions (Bobeck and Su, 1985; Palencia Perez and Dutrizac, 1991). Ammonia has a number of inherent advantages as a leaching agent due to its low cost, low toxicity, low corrosivity, and easy regeneration due to its low vapour pressure and above all good complexing ability. The application of ammonia leaching for dissolution of copper concentrates has been widely investigated (Evans et al. 1964; Stanzyk and Rampacek, 1966). Umetsu et al. (1967) studied the ammonia leaching of complex Cu – Zn sulphide concentrates (19.8% Zn and 10.1% Cu) under elevated temperature and pressure. Extraction of Cu, Zn, and S was influenced by temperature, O2 partial pressure, agitation, pulp concentration and NH3/(Cu + Zn) ratio. It was found that oxygen partial pressure had little effect above 115 jC. Rao et al. (1992) investigated the kinetics of copper and zinc dissolution from complex sulphide in ammonia solution. It was observed that in the temperature range of 70 – 100 jC, zinc dissolution followed the shrinking core model, whereas copper dissolution followed the diffusion control model. Majima and Peters (1966) have shown that presence of complexing agents like ammonia increases the oxidation rate of sulphide in comparison to the absence of it. Although ammonia complexes with zinc, there was no apparent effect of ammonia on the oxidation rate of sphalerite. This inertness may be due to its covalent crystal structure as evidenced by the low coordination number for zinc and sulfur in the zinc sulphide lattice. Other possibility may be due to low electrical conductivity of the ZnS lattice, which is not directly connected to its covalent structure. In comparison to copper sulphides and complex sulphides, literature on ammonia leaching of zinc sulphide is inadequate. Nelen and Sobol (1959) had reported the study on the kinetics of sphalerite oxidation in ammonia. Effect of partial pressure of oxygen, temperature, composition of solution, intensity of mixing, and catalytic effect of copper were studied on a polished sample of almost pure sphalerite mineral. Increase in rate was observed with increase in ammonia concentration, temperature, PO2, and Cu content. In absence of copper, activation energy reported was 56.5 kJ/mol (i.e. 13.5 kcal/mol). The main objective of the present investigation was to develop rate equation for the reaction of sphalerite with oxygen in ammonia solutions. The effect of variables such as oxygen partial pressure, ammonia concentration, temperature, and particle size on the reaction rate were analyzed and kinetic models for solid –liquid reactions were tested with the extraction data in order to establish the rate expression.

2. Experimental Composition of the Hindustan Zinc (Udaipur, India) sphalerite concentrate used for the present study is given in Table 1. Concentrate was wet-sieved into different narrow size fractions from
90 + 75 to
53 + 45 Am. After washing with acetone and air-dried, concentrate was preserved in airtight bottles and kept inside desiccator.

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Table 1 Chemical composition of sphalerite concentrate Zn Fe Cu Pb S

53.5% 10.2% 0.05% 1.1% 34%

Leaching experiments were carried out in PARR 2-l capacity stainless steel autoclave. Temperature was controlled through a PID controller with digital read-out for temperature, pressure, and agitation speed. For each run, 1000 ml of aqueous ammonia solution of predetermined molarity along with ammonium sulfate was charged into the reactor and 10g sphalerite concentrate was added to it, and the reactor closed properly. Prior to heating the autoclave, gas volume was purged with nitrogen three to four times to make the environment almost oxygen-free during heating. Reactor was heated to the desired temperature with mild agitation and when the set temperature was reached, oxygen was introduced and full agitation was placed and the reaction time counts from this point. Zinc analysis in the leach liquor was made volumetrically by EDTA titration using Eriochrome Black T as indicator.

3. Results In order to generate the kinetic data, 1% of slurry density was used for the leaching experiments. Standard experimental conditions were: 120 jC, [NH3] = 3.39 M,
53 + 45 Am particle size, 2 atm PO2, pH 10.75, and 3 h time. Before studying the effect of other variables, particular speed of agitation was fixed after observing the effect of agitation. Stirring speed varied from 300 to 750 min
1. Above 450 min
1, the rate of agitation did not influence the amount of zinc extracted, indicating that sphalerite particles were well suspended and that there is adequate distribution of dissolved oxygen in the solution. In all the subsequent experiments, stirring speed was maintained at 650 min
1 to assure independence of this variable. Experimental results on the leaching of sphalerite are presented in Figs. 1– 4. Fig. 1 shows the results of the experiments conducted to determine the effect of ammonia concentration on the leaching rate. The concentrations of NH3 and OH
are related through the following equilibrium:
NH3 þ H2 O ¼ NHþ 4 þ OH

K25

jC

¼ 1:78  10
5

In order to keep the pH constant, ammonium sulfate was also added during leaching because of its buffering action. To investigate the effect of ammonia concentration, pH was kept constant at 10.75 and ammonia varied from 1.54 to 5.35 mol/l. It is obvious in Fig. 1 that increasing the NH3 concentration of the solution increases the leaching rate of sphalerite. Similarly, by keeping the total ammonia concentration constant at 3.39 mol/l

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Fig. 1. Effect of NH3 concentration on Zn extraction. (Conditions: temperature 120 jC, PO2 2 atm, particle size
53 + 45 Am, and pH 10.75.)

Fig. 2. Effect of oxygen partial pressure on Zn extraction. (Conditions: temperature 120 jC, particle size
53 + 45 Am, [NH3] 3.39 M, and pH 10.75.)

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245

Fig. 3. Effect of temperature on Zn extraction. (Conditions: PO2 2 atm, particle size
53 + 45 Am, [NH3] 3.39 M, and pH 10.75.)

Fig. 4. Effect of particle size on Zn extraction. (Conditions: temperature 120 jC, pH 10.75, PO2 2 atm, and [NH3] 3.39 M.)

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and changing the ratio of [NH3/[(NH4)2SO4], pH was allowed to vary from 10.4 to 11.2. It was observed that the leaching rate was almost independent of pH in the studied range. The next series of experiments were carried out to determine the effect of oxygen partial pressure on the leaching rate as the concentration of dissolved oxygen is directly proportional to the partial pressure of oxygen over the solution. The oxygen partial pressure varied from 0.5 to 10 atm. Fig. 2 shows that increasing the oxygen partial pressure increases the sphalerite-leaching rate. The variation in leaching rates with various temperatures is shown in Fig. 3. It is seen from the figure that by increasing the leaching temperature from 90 to 130 jC, zinc extraction increases from 14.8% to 54% after 3 h of leaching. Fig. 4 shows the variation in leaching rate with various initial particle sizes at 120 jC. It is seen that the smaller the particle is, the faster the reaction rate becomes.

4. Discussion From a global reaction standpoint rather than a mechanistic one, the following equation represents the pressure oxidation of sphalerite in ammoniacal medium (Tozawa et al., 1976): ZnS þ 4NH3 þ 2O2 ¼ ZnðNH3 Þ4 SO4

ð1Þ

where NH3 and O2 are actually aqueous, NH3 and dissolved oxygen, respectively. 4.1. Comparison between mass transfer and reaction resistances In heterogeneous reaction-like leaching, three resistance are of considerable importance, as far as rate-controlling step is concerned: (1) gas – liquid mass transfer resistance, (2) liquid –solid mass transfer resistance, and (3) reaction resistance. 4.1.1. Gas – liquid mass transfer resistance For the gas –liquid mass transfer, there will be resistance in both the gas side and the liquid side of the interfacial area with mass transfer coefficient as kG and kL, respectively. In the present case, as the gas used is pure oxygen, resistance in the gas side will be negligible, hence, it is not being considered. In order to calculate the volumetric mass transfer coefficient (kLa), correlation proposed by Chaudhury et al. (1987) for a flat bottom cylindrical autoclave is applied. According to them, kLa can be calculated as: kL a ¼ 1:48  10
3 ðN Þ2:18 ðVg =VL Þ1:88 ðdI =dT Þ2:16 ðh1 =h2 Þ1:16

ð2Þ

In the present case, Vg = 1010 cm3, VL = 1000 cm3, dI = 5.8 cm, dT = 10.2 cm, N = 10.8 Hz (650/min), h1 = 12.5 cm and h2 = 13.5 cm. The calculated value of kLa is, thus 7.1  10
2/s. Mass transfer coefficient value calculated here is the minimum value to be expected since it has been calculated at room temperature. The mass transfer coefficient depends on the diffusivity of the dissolved gas, kL~D according to the film model (Whitman, 1923;

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Lewis and Whitman, 1924) or kL ~MD according to surface renewal model (Danckwerts, 1951, 1955). The specific gas – liquid interfacial a depends on the reactor geometry, configuration, and hydrodynamic conditions. So kLa is expected to give the same temperature dependence as D since hydrodynamic conditions are not significantly affected by variation in temperature. 4.1.2. Liquid –solid mass transfer In the case of liquid – solid mass transfer, the minimum mass transfer coefficient kc, min can be calculated on semi-theoretical basis for monosize spheres from the physical properties of aqueous, solid phase and diffusion coefficient of the reactant. Basically, the semi-theoretical correlation (Harriot, 1962) takes the form, Sh ¼ 2 þ 0:6Re1=2 Sc1=3 which can be rewritten as, kc ¼ D=d ½2 þ 0:6ðqL dvs =lÞ1=2 ðl=qL DÞ1=3 

ð3Þ

A major difficulty in using Eq. (3) to describe the kinetics of particulate leaching lies in the choice of correct slip velocity, which appears in the particle Reynolds number term. If the density of particles is much greater than that of fluid density and the particles are present at low concentration, then terminal-settling velocity is a good approximation of the slip velocity. Haider and Levenspiel (1989) presented the following useful approximation for the direct evaluation of the terminal velocity of particles: !
1 18 2:335
1:744/s u* ¼ þ ; 0:5 < /s < 1 ð4Þ ðd*Þ0:5 ðd*Þ2 where, /s is the shape factor. Assuming spherical particles, Eq. (4) reduces to !
1 18 0:591 u* ¼ þ ðd*Þ2 ðd*Þ0:5

ð5Þ

where u* and d* are dimensionless terminal velocity and particle diameter, respectively, and defined as:  u* ¼ ut

q2L glðqs
qL Þ

1=3 ð6Þ

and  d* ¼ d

gqL ðqs
qL Þ l2

1=3 ð7Þ

With the help of Eqs. (5) – (7) and by using the following values: d = 4.88  10
3 cm, l = 8.57  10
3 g/cm s, qL = 0.997 g/cm3, qs = 4.102 g/cm3, D (for oxygen in water at 25

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jC) = 2.5  10
5 cm2/s, and g = 981 cm/s2, the mass transfer coefficient value calculated from Eq. (3) as, kc;min ¼ 2:1  10
2 cm=s The specific solid– liquid interfacial area ac can be calculated as: ac ðcm2 =cm3 of liq:Þ ¼ 6ms =dqs The calculated value of ac is 3/cm (considering 1% solid loading) and the value of kcac is, thus 6.3  10
2/s. 4.1.3. Reaction resistance (kS) The reaction rate constant or reaction resistance can be calculated assuming the rate control by chemical reaction at the surface. The rate of a heterogeneous reaction between a solid reactant B and an aqueous reactant A can be written as:
dNB =dt ¼ bSB k½CA m

ð8Þ

where, NB is the moles of solid reactant, b is stoichiometric factor, i.e. moles of B reacting with 1 mol of A, SB is the surface area of solid, CA is the concentration of aqueous reactant and m is the reaction order. In the present case, Eq. (8) is modified to account for the effect of ammonia, and rate law can be written as:
dNZnS =dt ¼ bkSZnS ½PO2 n ½NH3 m

ð9Þ

where NZnS and SZnS are moles of sphalerite and surface area of sphalerite, respectively. Concentration of oxygen here is replaced by partial pressure by incorporating Henry’s coefficient in the rate constant k. Eq. (9) can be rewritten in terms of fraction of extraction a by considering SZnS to be the external surface area of spherical particle as: da ¼ kS ð1
aÞ2=3 dt

ð10Þ

For the initial kinetic response, Eq. (10) becomes 

da dt

 ¼ kS t¼0

where kS is given by Eq. (11). kS ¼

6bMk½PO2 n ½NH3 m qs d0

ð11Þ

In order to determine initial rate, second-order polynomial regression was performed to fit the experimental a vs. time data. From the regression equation slope has been determined at t = 0. Highest rate was found at 130 jC and the corresponding initial rate was determined to be 0.24/h, i.e. 6.67  10
5/s.

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It is observed that even at the highest temperature, chemical reaction is the slowest process and about three orders of magnitude less than the transport processes. 4.2. Kinetic model The objective of the kinetic study was primarily the development of rate equation suitable for reactor design and process modeling. For this purpose, experimental data have been analyzed with the help of shrinking core model (Levenspiel, 1972). Disregarding the diffusion control through the external boundary layer as per previous section, sphalerite leaching is expected to be controlled by either the surface reaction or the diffusion through the product layer. In the case of isometric monosize, spherical particles following shrinking core models are applicable. Product layer diffusion: 1
2=3a
ð1
aÞ2=3 ¼ kD t

ð12Þ

Surface reaction: 1
ð1
aÞ1=3 ¼ kS t

ð13Þ

where, kD and kS are the apparent rate constants. Attempt to fit the model Eq. (12) with the experimental data failed to give the linear plots. Under the present experimental conditions, pyrite remains mostly unattacked as observed from the residue colour as well as from the XRD patterns. It can be mentioned here that for surface reaction control, shrinking core and shrinking particle model lead to same model equations (Levenspiel, 1972).

Fig. 5. Plot of 1
(1
a)1/3 against time for various temperatures. (Data correspond to Fig. 3.)

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Fig. 6. Arrhenius plot for sphalerite oxidation.

Application of Eq. (13) to the experimental data obtained at different temperatures resulted in linear plots as shown in Fig. 5. From the slopes of the 1
(1
a)1/3 vs. t plots, the kS values were obtained, and the Arrhenius plot is constructed as shown in Fig. 6. Since kS values are directly proportional to intrinsic rate constant and the data represent experimental results of the same particle size, the activation energy is directly related to intrinsic kinetics. Activation energy value of 44.3 kJ/mol supports the view that leaching reaction is controlled by chemical reaction at the particle surface.

Fig. 7. Plot of 1
(1
a)1/3 against time for various oxygen partial pressures. (Data correspond to Fig. 2.)

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251

Fig. 8. Plot of log kS vs. logPO2 for the estimation of reaction order.

Fig. 7 shows the linear kinetics plots of 1
(1
a)1/3 vs. time at various oxygen partial pressures. From the slopes of the plots, apparent rate constant (kS) values were determined, and log(kS) vs. log( PO2) plot (Fig. 8) is constructed to determine the order of dependency with respect to oxygen partial pressure. The empirical reaction order with respect to PO2 is 0.2. Similarly, shrinking core model for surface reaction fitted the extraction data well at various ammonia concentrations. From the slopes of the straight lines, experimental kS values obtained and a plot of log(kS) vs. log[NH3] is made, as shown in Fig. 9. Reaction order with respect to ammonia concentration is 0.6. In a similar way, apparent rate constant (kS) values calculated from 1
(1
a)1/3 vs. time plots for different particle sizes. Rate

Fig. 9. Plot of log kS vs. log[NH3] for the estimation of reaction order.

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Fig. 10. Plot of rate constant vs. inverse of particle diameter in support of surface reaction control.

constants (kS) are plotted against inverse of initial particle radii (r0) and the resultant plot is linear and passes through the origin (Fig. 10) supporting once more the rate controlling process as surface reaction. Initial particle diameter or mean particle size is the geometric mean of the corresponding size fractions. From the slope and intercept of Arrhenius plot in Fig. 6 and Eqs. (11) and (13), the following shrinking core model equation is proposed to represent the ammonia leaching of sphalerite. ( )3 PO0:22 ½NH3 0:6 a ¼ 1
1
83:6 exp½
5290=T t ð14Þ d0 Furthermore, assuming the stoichiometry of Eq. (1) and a sphalerite density of 4.102 g/ cm3, the rate law was formulated with the help of Eq. (11) as: RZnS ¼ 1:76PO0:22 ½NH3 0:6 exp½
5290=T 

ð15Þ 2

where RZnS is rate of sphalerite leaching in mol/cm h.

5. Conclusions Oxidative ammonia leaching of sphalerite is influenced by ammonia concentration, temperature, particle size, and oxygen partial pressure. Under the studied range, leaching rate is independent of solution pH and agitation has no effect beyond 450 min
1. Comparison of calculated mass transfer rates both gas –liquid and liquid – solid with the experimentally observed reaction rate shows that reaction rate is the slowest. Besides, from shrinking particle model for surface reaction, it is found that surface reaction is the ratecontrolling process. The apparent activation energy is 44.3 kJ/mol, and empirical orders of reaction with respect to ammonia concentration and oxygen partial pressure are 0.6 and 0.2, respectively.

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Nomenclature a fraction of extraction a, ac specific gas – liquid and solid– liquid interfacial area (cm2/cm3) b stoichiometric factor dI, dT impeller and tank diameter, respectively (cm) D diffusion coefficient (cm2/s) d particle diameter (cm) d0 initial particle diameter (cm) h1, h2 heights of the upper impeller and liquid level, respectively, from the bottom of vessel (cm) k true rate constant kc solid –liquid mass transfer coefficient (cm/s) kL gas – liquid mass transfer coefficient (liquid side) (cm/s) kD and kS apparent rate constants for product layer diffusion and surface reaction control M molecular weight (g) ms pulp density (g/cm3) n, m orders or reaction N frequency of rotation (Hz) qs, qL density of solid and liquid, respectively (g/cm3) l viscosity (g/cm s) RZnS leaching rate of sphalerite (mol/cm2 h) Re particle Reynolds no. (qLdvs/l) Sc Schmidt no. (l/qLD) Sh Sherwood no. (kcd/D) ut terminal velocity (cm/s) vs slip velocity (cm/s) Vg, VL volume occupied by gas and liquid, respectively (cm3)

Acknowledgements The authors are thankful to Dr. V.N. Misra, Director of the Regional Research Laboratory, Bhubaneswar, for his kind permission to publish the paper. We would also like to thank Dr. (Mrs.) S. Anand, Scientist, Hydrometallurgy Department, Regional Research Laboratory for the suggestions during the preparation of the manuscript. References Bjorling, G., 1954. Lixiviation of sulphidic minerals under oxygen pressure. Metallurgie 8, 781 – 784. Bobeck, G.E., Su, H., 1985. The kinetics of dissolution of sphalerite in ferric chloride solutions. Metall. Trans. 16B, 413 – 424. Chaudhury, R.V., Gholap, R.V., Emig, G., Hofmann, H., 1987. Gas – liquid mass transfer in ‘‘dead-end’’ autoclave reactors. Can. J. Chem. Eng. 65, 744 – 751 (Oct.). Danckwerts, P.V., 1951. Significance of liquid-film coefficients in gas absorption. Ind. Eng. Chem. 43 (6), 1460 – 1467.

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Danckwerts, P.V., 1955. Gas absorption accompanied by chemical reaction. AIChE J. 1, 456 – 463. Demopoulos, G.P., Baldwin, S.A., 1999. Stoichiometric and kinetic aspects on the pressure leaching of zinc concentrates. In: Mishra, B. (Ed.), TMS Annual Meeting, San Diego, 567 – 583. Evans, D.J.I., Romanchuk, S., Mackiw, V.N., 1964. Treatment of copper – zinc concentrates by pressure hydrometallurgy. Can. Min. Metall. Bull. 57, 857 – 866. Forward, F.A., Veltman, H., 1959. Direct leaching of zinc sulphide concentrate by Sherrit Gordon. J. Met. 11, 836 – 840. Haider, A., Levenspiel, O., 1989. Drag coefficient and terminal velocity of spherical and nonspherical particles. Powder Technol. 58, 63 – 70. Harriot, P., 1962. Mass transfer to particles—Part 1. Suspended in agitated tanks. AIChE J. 8 (1), 93 – 101. Levenspiel, O., 1972. Chemical Reaction Engineering, 2nd ed. Wiley, New York, pp. 357 – 400. Lewis, W.K., Whitman, W.G., 1924. Principles of gas absorption. Ind. Eng. Chem. 16, 1215 – 1220. Majima, H., Peters, E., 1966. Aqueous oxidation at elevated temperatures. Trans. Metall. Soc. AIME 236, 1409 – 1413 (Oct.). Majima, H., Awakaura, Y., Misaki, N., 1981. A kinetic study on nonoxidative dissolution of sphalerite in aqueous hydrochloric acid solution. Metall. Trans. 12B, 645 – 649. Mizoguchi, T., Habashi, F., 1981. The aqueous oxidation of complex sulfide concentrates in hydrochloric acid. Int. J. Miner. Process. 8, 177 – 193. Nelen, I.M., Sobol, S.I., 1959. Kinetics of the oxidation of sphalerite under conditions of ammoniacal leaching under pressure of sulphide concentrate. Sb. Tr., Gos. Nauchno-Issled. Inst. Tsvetn. Met. 15, 447 – 475. Palencia Perez, I., Dutrizac, J.E., 1991. The effect of the iron content of sphalerite on its rate of dissolution in ferric sulphate and ferric chloride media. Hydrometallurgy 26, 211 – 232. Parker, E.G., 1961. Oxidative pressure leaching of zinc concentrate. CIM Bull. 74 (5), 145 – 150. Rao, K.S., Anand, S., Das, R.P., Ray, H.S., 1992. Kinetics of ammonia leaching of multimetal sulphides. Miner. Process. Extr. Metall. Rev. 10, 11 – 27. Stanzyk, M.H., Rampacek, C., 1966. Oxidation leaching of copper sulfides in ammoniacal pulps at elevated temperature and pressures. U.S. Bur. Mines, R.I. No. 6808. Tozawa, K., Umetsu, Y., Sato, K., 1976. On chemistry of ammonia leaching of copper concentrate. In: Yannopoulos, J.C., Agarwal, J.C. (Eds.), Extractive Metallurgy of Copper, vol. II. AIME, New York, pp. 706 – 721. Umetsu, Y., Tozawa, K., Sasaki, K.J., 1967. Ammonia pressure leaching of complex copper/zinc sulfide concentrate. Nippon Kogyo Kaishi 83 (8), 1016 – 1022. Whitman, W.G., 1923. Preliminary experimental confirmation of the two-film theory of gas absorption. Chem. Metall. Eng. 29, 146 – 148.