Role of sodium citrate in leaching of low-grade and multiphase zinc oxide ore in ammonia–ammonium sulfate solution

Accepted Manuscript Role of sodium citrate in leaching of low-grade and multiphase zinc oxide ore in ammonia–ammonium sulfate solution

Kun Yang, Libo Zhang, Chao Lv, Jinhui Peng, Shiwei Li, Aiyuan Ma, Weiheng Chen, Feng Xie PII: DOI: Reference:

S0304-386X(16)30467-4 doi: 10.1016/j.hydromet.2017.03.013 HYDROM 4546

To appear in:

Hydrometallurgy

Received date: Revised date: Accepted date:

20 July 2016 11 January 2017 17 March 2017

Please cite this article as: Kun Yang, Libo Zhang, Chao Lv, Jinhui Peng, Shiwei Li, Aiyuan Ma, Weiheng Chen, Feng Xie , Role of sodium citrate in leaching of low-grade and multiphase zinc oxide ore in ammonia–ammonium sulfate solution. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Hydrom(2017), doi: 10.1016/j.hydromet.2017.03.013

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ACCEPTED MANUSCRIPT Role of sodium citrate in leaching of low-grade and multiphase zinc oxide ore in ammonia–ammonium sulfate solution Kun Yang a, b, c, Libo Zhang a, b, c, Chao Lvd, Jinhui Peng a, b, c, Shiwei Li a, b, c*, Aiyuan Ma a, b, c, Weiheng Chen a, b, c, Feng Xie a, b, c a

Yunnan Provincial Key Laboratory of Intensification Metallurgy, Kunming, Yunnan 650093,

b

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China National Local Joint Laboratory of Engineering Application of Microwave Energy and

c

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Equipment Technology, Kunming, Yunnan 650093, China

Faculty of Metallurgical and Energy Engineering, Kunming University of Science and

d.

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Technology, Kunming, Yunnan 650093, China

Faculty of Land Resource Engineering, Kunming University of Science and Technology,

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Kunming 650093, Yunnan, China

Abstract: In this paper, a novel system for leaching of low-grade and multiphase

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zinc oxide ore is presented, in which ammonia–ammonium sulfate is chosen as leaching agent, meanwhile adds a small amount of sodium citrate to strengthen the

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complexation behavior. The thermodynamics is studied by using chemical equilibrium

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modeling to predict the zinc species distribution diagrams for the Zn (II)-NH3-Cit3system. Through employing various analytical technologies, such as X-ray diffraction (XRD), chemical analysis, and Fourier transform infrared spectroscopy (FT-IR),

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compositions, bond structures and chemical states are obtained. Moreover, effect of sodium citrate is studied by X-rays photoelectron spectroscopy (XPS) analysis, and it

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can be established that addition of sodium citrate is favorable for zinc leaching by inhabiting the formation of low complexation compound Zn(NH3)i2+ (i=1, 2) and lowering the fraction of Zn(NH3)32+. The leaching rate can reach 96.91 % with a cit3concentration of 0.2 M/L. Once above it, diffusion rate of zinc would decrease, which causes the dropping of zinc leaching. Key words: Sodium citrate; Low-grade and multiphase zinc oxide ore; Complexation; Hemimorphite 1 Introduction With the depletion of natural sphalerite and increasingly stringent environmental 1

ACCEPTED MANUSCRIPT legislation, zinc oxide ore such as willemite (Zn2SiO4), smithsonite (ZnCO3), zincite (ZnO) and hemimorphite (Zn4Si2O7(OH)2·H2O) have become important alternative resources for sulphide ore (Abdel-Aal, 2000). Much effort has been expended in an attempt to exploit unconventional zinc resources by hydrometallurgical and pyrometallurgical methods (Chen and Qu, 1998; Chen et al, 2009).

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Nowadays, hydrometallurgy in ammonia solutions has been considered as a prospective medium in the extraction industry of zinc for advantages of low vapor

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pressure, low cost, low toxicity, good complexation ability and easy regeneration [Wang, 2008; Feng, 2007].What’s more, large parts of wasteful components neither

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chelate with ammonia nor are soluble in ammonia solutions, which allow selective extraction of desired metals.

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The attempts of applying ammonia solutions in recovering zinc ore began in 1880 as ‘Schnabel Process’ (Schnabel, 188). Since then, many researches had

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explored the application to specific feedstock. There are three widely used ammonia solutions: ammonium carbonate, ammonium chloride and ammonium sulfate. Frenay

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(1985)studied the leaching of various oxidized zinc ore in different solvents and

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confirmed that ammonium carbonate could enhance the leaching of hemimorphite. Moghaddam et al. (2005) applied Taguchi method in experiment designing to determine the optimum conditions for high dissolution of nonsulfide zinc ore in

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ammonium carbonate solution, and to minimize the extraction of iron and lead. They got a ~92 wt-% zinc leaching rate under optimum conditions. Yang et al (2016)

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proposed a novel combination of microwave roasting assisted phase transformation and leaching in ammonium chloride solution to recover zinc from an oxide-sulphide zinc ore, and indicated both microwave heating and the Na2O2 level having significant efforts on phase transformation and zinc leaching. Yuan et al (2010) demonstrated that both dry milling and mechano-chemical milling can enhance zinc extraction rate and shorten the leaching time of hemimorphite in NH4OH&NH4Cl leaching medium. Yang et al (2016) improved the leaching of amorphous smithsonite (ZnCO3) and zinc silicate (Zn2SiO4) in ammonium sulfate solution through adding sodium citrate. Ding et al (2010) compared the effect of ammonium anions on the dissolution kinetics of 2

ACCEPTED MANUSCRIPT zinc silicate (hemimorphite) and showed that the reactivity order of ammonium salts toward hemimorphite is NH4HCO3>NH4NO3>(NH4)2SO4> (NH4)2CO3>NH4Cl. Maraszewska and Zembura (1990) found the dissolution kinetics of zinc in ammonia solutions can be divided into two stages: steady and decreasing according to ammonia concentration. Though reports about zinc oxide ore leaching in ammonia solution are

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abundant, research on synergistic complexation is rare. In this investigation, we attempt to leach low-grade and multiphase zinc oxide

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ore in ammonia–ammonium sulfate solution, and add sodium citrate ligand to improve the leaching rate. Meanwhile, by analyzing the changes of phase

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compositions, functional groups and chemical states, optimum level of sodium citrate is determined and its detail function mechanism is discussed.

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2 Materials and processing 2.1 Materials and equipment

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The low-grade and multiphase zinc oxide ore used in this study was obtained from Lanping County of China. Its main chemical compositions and mineralogical analysis

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are presented in Tables 1 and 2. It can be referred that this low-grade zinc ore is

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consisted of 31.75 wt-% SiO2, 6.01 wt-% zinc and a high alkaline content 12.40 wt-% (CaO+MgO). The content of ferrous is beyond 8 wt-%, and can be categorized as high ferrous zinc ore. What’s more, it can be seen that zinc mainly exits as zinc oxides.

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This low-grade zinc ore has a high content of ferrous, alkaline and SiO2, which is not suitable for acid leaching, and highly selective ammonium complexation has obvious

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advantage in treating this ore. Table 1 Main chemical composition of low-grade and multiphase zinc oxide ore (mass fraction, wt-%)

ZnT

SiO2

CaO

Fe

Pb

Al2O3

S

MgO

6.01

31.75

12.05

8.11

6.41

5.98

0.45

0.35

ZnT-total zinc Table 2 Mineralogical analysis of low-grade and multiphase zinc oxide ore Zinc phase

Zinc sulphate

Zinc oxides

3

Zinc sulphide

Franklinite et al

ZnT

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1.79

4.06

0.092

0.063

6.01

Distribution/ wt-%

29.78

67.55

1.53

1.05

100

XRD pattern of low-grade zinc oxide ore is shown in Fig. 1. It reveals the main crystalline phases in this low-grade ore include SiO2, Zn4Si2O7(OH)2·H2O, CaCO3

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and PbCO3, and the primary zinc oxide phase is hemimorphite (Zn4Si2O7(OH)2·H2O).

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Fig. 1. XRD pattern of low-grade and multiphase zinc oxide ore

Fig. 2. FT-IR spectrum of low-grade and multiphase zinc oxide ore

The FT-IR spectrum of low-grade and multiphase zinc oxide ore is shown in Fig. 2. The results are interpreted on the basis of published data. Bands at 3440.71 cm-1 corresponds to stretch vibration of O-H, and band at 1631.89 cm-1 corresponds to 4

ACCEPTED MANUSCRIPT bending vibration of O-H in crystal water, which may indicates the existing of Zn4Si2O7(OH)2·H2O in this zinc oxide ore ((Makreski, 2007; Nakamoto, 1970; Goswami and Sen, 2004). The two relatively weak absorption peaks at 2513.52 and 1797.48 cm-1 shall be ascribed to calcium carbonate ((Li et al, 2011; Farmer, 1974; Gadsden, 1975; Ferraro, 1982; Jones and Jackson, 2012). The FT-IR spectroscopy

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also exhibits the characteristic absorption peaks of calcite at 875.74 and 711.79 cm-1 ((Deng, 2012; Xie, 2006; Busca, 1994). The stretch vibrations of Si2O7 unit can be

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divided into SiO3 vibration and Si-O-Si bridge vibration (Gabelica-Robert and Tarte, 1979). Peaks at 1092.93, 693.42 and 526.06 cm-1 are attributed to Si-O-Si stretch

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vibrations, while the bands at 936.71 and 1028.99 cm-1 should be attributed to SiO3 vibration (Hofmeister, 1987). Peaks at 797.90 and 472.26 cm-1 are ascribed to the

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characteristic peaks of SiO2 (Chen et al, 2011). Assignment of bands in IR spectra of low-grade and multiphase zinc oxide ore is as stated in Table 3. From the analysis of

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FT-IR, conclusion can be drawn as the mineral existing in the low-grade zinc oxide ore includes CaCO3, SiO2 and hemimorphite, and the hemimorphite can be divided

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into two structure: one bridge and two bridge.

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Table 3 Assignment of bands in IR spectra of low-grade and multiphase zinc oxide ore

group

cm-1

group

OH

2513.52

CaCO3

CaCO3

1631.89

OH

CO32-

1092.93

Si-O-Si

1028.99

SiO3

936.71

SiO3

875.74

CO32-

797.90

SiO2

711.79

CO32-

693.42

Si-O-Si

526.06

Si-O-Si

472.26

SiO2

Bands identification

3440.71 1797.48

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1425.10

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cm-1

Bands identification

XPS survey spectrum of low-grade and multiphase zinc oxide ore is shown in Fig. 3, and its XPS detail spectra of Si2p is displayed in Fig. 4.

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Fig. 3. XPS survey spectra of low grade and multiphase zinc oxide ore

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Fig. 4. XPS detail spectra of Si2p of low grade zinc oxide ore

One important feature to be noted in Fig. 3 is the highly dominant O1s as well as C1s peak, while signals of other elements of interest are quite weak. Elements on the surface of low-grade and multiphase zinc oxide ore include Zn, Fe, O, Pb, Ca and Si. In order to view the spectra associated with each element presented in Fig. 3, the XPS spectra over 108-98 eV regions specific to the interested elements Si are recorded. Results was shown in Fig. 4. XPS studies show silicon peaks at 103.1 eV which might be attributed to SiO2 and silicon peaks at 102.2 eV which shall be attribute to hemimorphite. From the XPS Si2p spectrum of raw ore, it can be seen that Si element 6

ACCEPTED MANUSCRIPT compose two kind of ore, i.e., silica and hemimorphite. The mass fraction of silica is obviously higher than hemimorphite, which is accordance with the chemical analysis. 2.2 Analysis techniques X-ray diffraction (XRD) analysis is utilized to characterize the composition phases. Fourier transform infrared spectroscopy (FT-IR) analysis is used to identify

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the functional group and track the course of chemical reactions. The FT-IR spectra are acquired on a Thermo Fisher Scientific Nicolet IS 10 FT-IR spectrometer with a

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DTGS detector, a KBr beam splitter spread with Ge, an interferometer driven by plane mirrors electromagnetic force and longer lifetime middle/far infrared source. Its

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ASTM linearity is lower than 0.1 wt-%, spectral resolution is beyond 0.4 cm-1, and sensitivity is superior to 45000:1. X-rays photoelectron spectroscopy (XPS) analysis

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can give the information of surface species and their chemical state. The XPS spectra are recorded using a PHI5000 Versaprobe-II Scanning XPS Microprobe system with

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an Al K-alpha X-ray source (hm=1486.6 eV) at a normal take off angle and a band-pass energy of 46.95 eV. The vacuum pressure in the chamber is sooty to 10-7

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Torr. The x-rays are nonmonochromated and the instrument is calibrated against the

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C1s peak binding energy (284.8eV) (Kosova et al, 2007; Buckley et al, 1989). The charge compensation is adjusted for each sample to obtain the maximum signal possible for Zn2p3/2 at Eb=464 eV.

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2.3 Experimental procedure

Leaching experiments are carried out in a CJJ-93/HJ-65 six-connected magnetic

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stirrers. In each case, a sample of 10 g is injected in 100 ml ammonia solution. The leaching solutions are prepared by mixing up desired amount of sodium citrate (0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35 and 0.4 M/L) with ammonia-ammonium sulphate agent, in which the molar ratio of NH3 and (NH4)2SO4 is 2:1 and total concentration of NH3 (c(NH3)+ c(NH4+)) is 7.5 M/L. Experiments are performed at 25 ◦C and stirred speed at 300 rpm. At selected time intervals, a fix amount of slurry (5 mL) is withdrawn and filtered for performing analysis. 3 Result and discussion 3.1 Reaction process 7

ACCEPTED MANUSCRIPT In this leaching system, there may exist the following zinc chelation phases: Zn(NH3)i2+ (i=1, 2, 3, 4), Zn(OH)j2-j (j=1, 2, 3, 4), ZnCit-, ZnHCit. A function of zinc fraction to pH value calculated from reaction Gibbs free energy ( ∆Gθ ) and equilibrium constants (Kθ ) is shown in Fig. 5, and content of sodium citrate are 0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35 and 0.4 M/L, respectively. From it, it can be

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concluded sodium citrate would inhibit the formation of low chelation Zn(NH3)i2+ (i=1, 2), and lower the fraction of Zn(NH3)32+ (no Zn(NH3)22+ existing, when [cit3-]

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being 0.15 M/L), which are favorable for zinc recovery. What’s more, the stability

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constant (log10K1) of ZnCit- (6.25), ZnHCit (10.26) is higher for that of zinc-ammonia complex (2.3, 4.8, 7.1 and 9.3, i=1, 2, 3 and 4, respectively) (Burgess et al, 2011). All

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these factors can improve the leaching and chelation behavior of zinc oxide ore. Compared the nine diagrams, once [cit3-] adds above 0.15 M/L, the decrease trend of

M/L is more reasonable. [c i t3 ]TO T = [Z n 2+ ]TO T =

N [ H

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ZnN ( H

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7 .5 0M 3 )4

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Z nO (c r)

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1 .0

0 .8

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0 .6

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F rac tion

0 .0 0 9 2 .0 0 mM

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Zn(NH3)32+ is not obvious, thus the adding amount of sodium citrate being about 0.15

ZnN ( H 3 )32+ ZnN ( H 3 )22+ Z nNH 32+

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ZnO ( H )42 

0 .0

0

2

4

6

8 pH

8

10

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5 0 .0 0 mM 9 2 .0 0 mM

N [ H

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Z n 2+

1 .0

=

ZnN ( H

7 .5 0M 2+ 3 )4 Z nO (c r)

0 .8

Z n (c it) 

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ZnH ( c it)

0 .4

ZnN ( H 3 )32+ ZnN ( H 3 )2 2+ Z nNH 32+

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ZnO ( H )42 

0 .0 0

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[c i t3 ]TO T = 1 0 0 .0 0 mM [Z n 2+ ]TO T = 9 2 .0 0 mM 1 .0

N [ H

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Z n 2+

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Z n (c it) 

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pH

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ZnO ( H )42  10

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1 .0

N [ H

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Z n (c it) 

0 .8

=

7 .5 0M

ZnN ( H

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Z nO (c r)

ZnH ( c it)

0 .2

ZnN ( H

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0 .4

2+

ZnO ( H )42 

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[c i t3 ]TO T = 2 0 0 .0 0 mM [Z n 2+ ]TO T = 9 2 .0 0 mM

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3 ]TO T

Z n (c it)  ZnH ( c it)

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ZnN ( H

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7 .5 0M 3 )4

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Z nO (c r)

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ZnN ( H

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N [ H

Z n 2+

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pH

10

2+

ZnO ( H )42  10

12

14

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N [ H

Z n 2+

1 .0

3 ]TO T

=

7 .5 0M

ZnN ( H

Z n (c it) 

3 )4

2+

Z nO (c r)

ZnH ( c it) 0 .8

0 .2 ZnN ( H

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0 .4

2+

ZnO ( H )42 

0 .0 0

2

4

6

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[c i t3 ]TO T = 3 0 0 .0 0 mM [Z n 2+ ]TO T = 9 2 .0 0 mM

N [ H

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Z n 2+ ZnH ( c it)

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ZnN ( H

Z n (c it) 

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7 .5 0M 3 )4

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Z nO (c r)

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0 .2 0 .0

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ZnN ( H 6

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pH

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ZnO ( H )42  10

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14

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N [ H

Z n 2+

1 .0

3 ]TO T

=

ZnN ( H

Z n (c it) 

7 .5 0M 3 )4

2+

Z nO (c r)

ZnH ( c it) 0 .8

0 .4

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0 .6

0 .2 3 )3

2+

ZnO ( H )42 

0 .0 0

2

4

6

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[c i t3 ]TO T = 4 0 0 .0 0 mM [Z n 2+ ]TO T = 9 2 .0 0 mM

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N [ H

Z n 2+

1 .0

=

ZnN ( H

14

7 .5 0M 3 )4

2+

Z nO (c r)

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0 .8

12

3 ]TO T

Z n (c it)  ZnH ( c it)

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0 .2 0 .0

2

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10

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pH

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ZnN ( H

4

ZnN ( H 6

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8

2+

ZnO ( H )42  10

12

14

pH

Fig. 5. Zn (II) species distribution diagrams in the solution with different sodium citrate levels

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3.2 Leaching rate

Fig. 6 reveals the role of sodium citrate on leaching of low-grade zinc oxide ore in ammonia- ammonium sulfate solution. It can be seen that with Na3C3H4OH(COO)3 addition, leaching rate of zinc appears the trend of rising first and then gradually dropping. Leaching rates of points a~I corresponding to sodium citrate addition content being 0, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35 and 0.40 M/L are 78.48, 80.12, 85.64, 94.55, 96.91, 94.89, 78.48, 75.70 and 72.39 %, respectively. The maximum leaching rate is obtained at [Cit3-]=0.2 M/L, which is accordance with Zn (II) species distribution diagrams (the error is acceptable ). And the followed decease trend may 12

ACCEPTED MANUSCRIPT be ascribed to low diffusion rate of zinc in high concentration solution. With the increasing of [cit3-], concentration of ion chelated with cit3- would increase, which is not benefit for the diffusion of zn2+, thus lowers the leaching rate. The pH value of experimental solution is measured at about 9, and the assumption of forming zinc-cit3chelation can be eliminated. The role of sodium citrate thus is limited to inhibit the formation of low chelation Zn(NH3)i2+ (i=1, 2), and lower the fraction of Zn(NH3)32+.

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To investigate the impact behavior of cit3-, XRD patterns, XPS spectra and FT-IR

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spectra of leaching slags are performed, and the results are shown in Fig. 7-9.

Fig. 6. Leaching degrees of low-grade and multiphase zinc oxide ore with different sodium citrate

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levels ((a) 0 M/L; (b) 0.05 M/L; (c) 0.1 M/L; (d) 0.15 M/L; (e) 0.2 M/L; (f) 0.25 M/L; (g) 0.3 M/L; (h) 0.35 M/L; and (i) 0.4 M/L)

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3.3 XRD patterns of leaching slags It can be referred from the XRD patterns of leaching slags (Fig. 7) that, main phases in these leaching slags include SiO2, CaCO3 and PbCO3, and peaks of these phases show no significant differences, while the peaks of hemimorphite decrease first and then remain at a certain level. Combined these characteristics with Zn(II) species distribution diagrams (Fig. 5), it can be deduced that addition of cit3- would promote the leaching of refractory hemimorphite by formation of more stable zinc-ammonia complexation -Zn(NH3)42+.

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Fig. 7. XRD patterns of leaching slags with different sodium citrate levels

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((a) 0 M/L; (b) 0.05 M/L; (c) 0.1 M/L; (d) 0.15 M/L; (e) 0.2 M/L; (f) 0.25 M/L; (g) 0.3 M/L; and (h) 0.35 M/L)

3.4 FT-IR spectra of leaching slags

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The FT-IR spectra of leaching slags are shown in Fig. 8, and the right one is

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obtained by enlarging the left one at the range of 400-1200 cm-1.

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Fig. 8. FT-IR spectra of leaching slags with different sodium citrate levels ((a) 0 M/L; (b) 0.05

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M/L; (c) 0.1 M/L; (d) 0.15 M/L; (e) 0.2 M/L; (f) 0.25 M/L; (g) 0.3 M/L; and (h) 0.35 M/L)

From Fig. 8 (A) and (B), it can be known that hardly no change occurs in the

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FT-IR spectra under different adding content of sodium citrate. Vibration peaks at 2921.52, 2358.12 and 2340.31 cm-1, are corresponding to symmetric vibration of C-H,

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and C-H asymmetric vibration usually results in peak at 2847.69 cm-1 (Chirea et al.,

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2010; Chen et al., 2008; Azzam et al., 2013; Ujihara et al., 2014). The C-H stretch modes should be assigned to cit3- (Lin et al, 2004). The presence of C=O is confirmed by two bands locating at 2360.12 cm-1 and 2340.31 cm-1 in agreement with previous

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reports, which may be caused by absorbed CO2 in the air or intermediary molecular CO2 decomposition of carbonate (Cervantes-Uc et al, 2007; Gärd et al, 1995; Yeung et

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al, 2003). When no sodium citrate adds, no C-H vibration peaks appears. The FT-IR spectrum with no essential difference once again affirms that, zinc would not chelated with cit3- in this leaching solution (pH≈9), which is consistent with Fig. 5. 3.5 XPS Si2p of leaching slags Fig. 9 reveals the Si2p photoelectron spectra of leaching slags with different sodium citrate levels. As can be seen, XPS-peak-differentation-imitating results are well fitted. With the addition of sodium citrate, ratio of silica and hemimorphite increases firstly until adds sodium citrate 0.2 M/L. And then the ratio decreases, which demonstrates sodium citrate can truly improve zinc leaching rate within limited 15

ACCEPTED MANUSCRIPT addition level, while excess addition lowers the effect. The XPS analysis results once again affirmed the assumption of adding sodium citrate inhabiting the formation of low complexation compound Zn(NH3)i2+ (i=1, 2) and lowering the fraction of Zn(NH3)32+, and improve the leaching of refractory zinc phase, resulting the fraction of hemimorphite dropping. Nevertheless, excessive addition is harm to the diffusion

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of zinc ion, resulting the fraction of hemimorphite rising.

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Fig. 9. Si2p photoelectron spectra of leaching slags with different sodium citrate levels

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((a) 0 M/L; (b) 0.05 M/L; (c) 0.1 M/L; (d) 0.15 M/L; (e) 0.2 M/L; (f) 0.25 M/L; (g) 0.3 M/L; and (h) 0.35 M/L)

4 Conclusions

A novel technique for leaching of low-grade and multiphase zinc oxide ore is discussed in this paper. The thermodynamics is studied using chemical equilibrium modeling to predict the zinc species distribution diagrams for the Zn(II)-NH3-Cit3system. By analyzing the XRD patterns, FT-IR spectra and XPS patterns of raw ore and leaching slags, it can be established that addition of sodium citrate is favorable for zinc leaching by inhabiting the formation of low complexation compound Zn(NH3)i2+ 19

ACCEPTED MANUSCRIPT (i=1, 2) and lowering the fraction of Zn(NH3)32+. The leaching rate can reach 96.91 % with a cit3- concentration of 0.2 M/L. Once above it, diffusion rate of zinc would decrease, which causes the dropping of zinc leaching. Acknowledgement This work was supported by National Natural Science Foundation of China

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(51604135). References

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Abdel-Aal, E.A., 2000. Kinetics of sulfuric acid leaching of low-grade zinc silicate ore. Hydrometallurgy 55(3), 247-254.

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alkaline leaching of hemimorphite. Hydrometallurgy 104 (2), 136-141.

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Highlights 1. In this paper, a novel system for leaching of low-grade and multiphase zinc oxide ore is presented, in which a small amount of sodium citrate is added to strengthen

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the complexation behaviour. 2. The thermodynamics were studied using chemical equilibrium diagram software

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(HYDRA-MEDUSA) to predict the zinc species distribution diagrams for the Zn(II)-NH3-Cit3- system.

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3. Addition of sodium citrate is favourable for zinc leaching by inhabiting the formation of low complexation compound Zn(NH3)i2+ (i=1, 2) and lowering the

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fraction of Zn(NH3)32+.

4. The extent of leaching reached 93.4 % with a cit3- concentration of 0.2 M/L. High

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levels of sodium citrate doped were consumed (NH3)T through the generation of

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NH4(Cit)2-, which suppressed the leaching of hemimorphate.

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