ISSN 0104-6632 Printed in Brazil

www.abeq.org.br/bjche

Vol. 33, No. 04, pp. 907 - 917, October - December, 2016 dx.doi.org/10.1590/0104-6632.20160334s20150376

*To whom correspondence should be addressed

Brazilian Journal of Chemical Engineering

LEACHING Zn FROM THE LOW-GRADE ZINC OXIDE ORE IN NH3-H3C6H5O7-H2O MEDIA

Ma Ai-yuan1,2,3,4, Peng Jin-hui1,2,3,4, Zhang Li-bo1,2,3,4, Shiwei Li1,2,3,4*,

Yang Kun1,2,3,4 and Zheng Xue-mei1,2,3,4

1Yunnan Provincial Key Laboratory of Intensification Metallurgy, Kunming 650093, China. Phone: + 86 871 65174756; Fax: + 86 871 65138997 E-mail: lswei11@163.com; lishiweikmust@163.com

2National Local Joint Laboratory of Engineering Application of Microwave Energy and Equipment Technology, Kunming, Yunnan 650093, China.

3Key Laboratory of Unconventional Metallurgy, Ministry of Education, Kunming 650093, China. 4Faculty of Metallurgical and Energy Engineering, Kunming University of

Science and Technology, Kunming 650093, China.

(Submitted: March 25, 2015 ; Revised: June 15, 2015 ; Accepted: July 21, 2015)

Abstract - In this research, the effect of different citric acid concentrations, ammonia concentration, temperature, leaching time, stirring speed and liquid-to-solid ratio on the zinc leaching from low-grade zinc oxide ore in a NH3-H3C6H5O7-H2O system were studied. The results showed that the zinc leaching rate is only 4.7% when the citric acid concentration is 0 M, and the leaching efficiency of Zn increased with increasing citric acid concentration. Under the conditions: citric acid concentration of 1.0 M, ammonia concentration of 6 M, temperature of 25 °C, leaching time of 60 min, stirring speed of 300 rpm and the starting solid-to-liquid ratio of 1:5, 81.2% of Zn is leached. The mineralogical changes of the low-grade zinc oxide ores during the processes were characterized by X–ray fluorescence (XRF), X–ray powder diffraction (XRD), Scanning Electron Microscopy associated with Energy Dispersive Spectroscopy (SEM–EDS) and Fourier transform infrared spectroscopy (FT–IR). From the evidence we deduced that citric ions complexed with zinc ions, forming a Zn-citrate complex. As a result, the zinc leaching rate was improved without the risk of pollution or pretreatment. This makes it as a good choice for a more ecological treatment of hemimorphite. Keywords: Low-grade zinc oxide ores; Hemimorphite; Ammonia leaching; Citric acid.

INTRODUCTION

Considering the huge reserves of low grade zinc oxide ores with high alkaline gangue (CaO and MgO) in the world (Li et al., 2010), which is hard to handle by the traditional zinc smelting methods, re-covering zinc from low grade oxide zinc ores has been a matter of discussion recently. The leach-sol-vent extraction-electro-winning process route is the most common method of extraction of zinc from low-grade zinc oxide ores (de Wet and Singleton, 2008). This is a material which can be processed by flotation (Irannajad et al., 2009; Navidi Kashani and

Rashchi, 2008; Li et al., 2013) and acid-leaching methods (Irannajad et al., 2013; Xu et al., 2010; Xu et al., 2012; Li et al., 2010; He et al., 2010; He et al., 2011). However, sulfuric acid leaching could not ef-fectively recover zinc from a low grade oxide ore because of substantial iron dissolution (Yang et al., 2013). Because of the complexities of mineralogical composition and structure, separation of zinc oxide minerals from their gangues by flotation is an ex-tremely complex process (Majid et al., 2014). Hence, with the high consumption of leaching acid, the treat-ment of low-grade oxidized zinc ores by hydrometal-lurgical methods is expensive and complex.

908 Ma Ai-yuan, Peng Jin-hui, Zhang Li-bo, Shiwei Li, Yang Kun and Zheng Xue-mei

Brazilian Journal of Chemical Engineering

The application of alkaline leaching techniques has become an increasingly important aspect in the recovery of base and precious metals from complex low grade zinc oxide ores (Santos et al., 2010; Ding et al., 2010; Yin et al., 2010). However, alkaline leaching processing is often confronted with the problem of low-grade complex ores, especially zinc silicate. The low solubility of these ores does not usually allow for the recovery of the target metal, even by direct chemical leaching in many leaching reagents (Zhao et al., 2009). Extracting zinc from hemimorphite by leaching in sodium hydroxide solu-tion requires high temperature, pressure and alkalin-ity (Chen et al., 2009). Currently, hydrometallurgy in ammonia has been considered as a prospective me-dium for the leaching of complex zinc ores of both oxide and sulfide types (Li et al., 2014).

The use of organic acids for the extraction of metals has been studied by many researchers. It was reported that low molecular weight citric acid has been found to be effective in removing heavy metals by forming soluble complexes and chelates with metal ions (Chen et al., 2003; Hongki et al., 2013), and there is no concern about environmental prob-lems after the treatment. Citric acid was found to be more active than H2SO4. In hydrometallurgical treat-ments, the ability to recover metals or metal oxides depends on their chemical reactivities (Larba et al., 2013). So citric acid seems more appropriate to use as a less costly and more environmentally friendly leachant.

Several papers have been published on the coor-dination of citric acid and transition metals, lead citrate (Kourgiantakis et al., 2000), tungsten citrate (Zhang et al., 2003), aluminum citrate (Matzapetakis et al., 1999) and germanium citrate (Willey et al., 2001). Based on previous work, a new hydrometal-lurgical process for low-grade zinc oxide ore is put

forward. In this work, the effect of the concentration of citric acid, ammonia concentration, temperature, leaching time, stirring speed and liquid-to-solid ratio on the leaching of zinc from low grade zinc oxide ore in the NH3-H3C6H5O7-H2O system was studied. The aim of this work was to develop a new hydro-metallurgical technology, which intends to provide a green and economic method to extract zinc from low-grade zinc oxide ores.

MATERIALS AND METHODS Experimental Materials

The low-grade zinc oxide ores used were given by Lanping County of Yunnan Province, China. The main chemical composition of the low-grade zinc oxide ores obtained by chemical analysis is listed in Table 1. It can be observed that silica (SiO2) and a high content of alkaline gangues (CaO + MgO) are the main components of the ore. The zinc content in this ore, around 6.01%, is considered to be low for a successful pyrometallurgical treatment for zinc re-covery (Dutra et al., 2006), which leads to the greater interest in the hydrometallurgical treatment. The XRD spectrum of the low-grade zinc oxide ore is shown in Figure 1. The XRD analysis shows that silica (SiO2), calcium carbonate (CaCO3), lead carbonate (PbCO3), and hemimorphite (Zn4Si2O7(OH)2·H2O) are the main chemical components. Table 2 for the mineral com-position in zinc shows that the ore contains mainly 67.55% hemimorphite (Zn4Si2O7(OH)2·H2O), 29.78% zinc carbonate (ZnCO3), 1.53% zinc sulfide (ZnS), and 1.05% Franklinite ( ZnFe2O4). Other zinc-con-taining phases, which, on the basis of the chemical analysis, could be present (Table 1) are probably be-low the detection limit.

Table 1: Chemical analysis of the main elements present in the zinc oxide ore.

Element O Si Ca Fe Zn Pb Al C Mg S Weight

(mass %) 46.73 13.03 9.19 8.11 6.01 6.41 4.43 4.30 0.52 0.45

Table 2: Main mineral composition of zinc in ore sample.

Phase composition (Zn4Si2O7(OH)2·H2O) ZnCO3 ZnS ZnFe2O4 Zn Content (mass %) 4.06 1.79 0.092 0.063 Phase occupation ratio (%) 67.55 29.78 1.53 1.05

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Leaching Zn from

Journal of Chem

Figure 1: T

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Figure 3: Floow-grade zin

ThermodynaMedium

Citric acid ural formula

al Method

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M) with diffeaced in the red magneticaleriments we

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Ma Ai-yuan, P

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ore, m (g) t

of the leachleaching.

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nstants of cKa2=4.76 ; pKlution has a sionized citr

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6 8 7 2

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re. The comp, 2005) and al., 1983).

gure 4: Th6H8O7).

RES

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The study fon on the ex

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itric acid areKa3=6.40, andstrong acidityric acid are

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he structural

SULTS AND

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Figure 5: Effecovery fromeaching rate wo steps; theeaching rate

Brazilian

centrations inaverage leacich shows thsed with then. At the lowd 0.05 M, oxtracted in the zinc extr.27% when t

om 0.05 to the citric acitric acid concO5COO- actif zinc contai

were obtainede following le

M in citric ace, 298.15 K ineed. n Figure 5, ancreases the diess of leachmed as it gea possible incitric acid ccovery. An ive the reactioing an increanstream proction of zinc brecipitation ourities.

fect of concem low-graderepresents the first step lin the first st

Leaching Zn from

Journal of Chem

n the 0-1.0 ching rates hat the leache increase ofw citric acid only 4.73% the two-stagraction was the citric aci1.0M. Incre

id concentratcentration efivity, that reining ore. 81d for Zn by tweaching condid concentran temperatur

n increase in issolution raing, citric a

ets convertednsufficient ciconcentrationincrease in con of zinc resase in leachincessing of leby solvent exof Zn2+ after

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m the Low-Grade

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M citric aare shown

ing efficiencf the citric aconcentratioand 24.57%e leaching, improved frd concentrat

easing the ztion increaseffect of increesults in furt1.27% leachwo-stage leadition: 6 M aation, 60 minre, and 300 r

citric acid cte of zinc. D

acid is contind into produitric acid sitns, limiting concentrationsidue with cing. each liquor cxtraction elect

chemical se

C6H8O7 on ze ores. Overhing rate in e represents n in Figure 3.

e Zinc Oxide Ore

g Vol. 33, No. 04,

acid in

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% of re-

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in NH3-H3C6H5O

, pp. 907 - 917,

ffect of Amm

Zinc oxide y two-step lee total ammonc. The leacid concentra °C; stirringtio of 5:1. Th

As shown ionia concentaching effici80.21%. Hoa stable rang

gure 6: Effecovery from

ffect of Temp

An investign zinc extracons were the

effect of amonia concentlity of zinc ir the same clts are shownThe zinc

3C6H5O7-H2Oeased temperained in a cotion under thnc extractionith increased

ffect of Leac

Figure 8 sh

O7-H2O Media

October - Decem

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speed of 30he results arein Figure 6, trations fromency increas

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perature

gation into thction was als

same as thommonia conctration was an the NH3-Hconditions wn in Figure 7

leaching eO system inrature. The zonstant rangehe studied ten was found temperature

ching Time

hows the ef

mber, 2016

entration

s were leachnvestigate thtration on thens were as f75 M; temp00 rpm; and e shown in Fi, with the in

m 2.0 M to 8sed quickly solubility of imately 6 M.

onia concentinc oxide ore

he influence oso performeose detailed centration, eadjusted to 6H3C6H5O7-H2was measure7. efficiency i

ncreased steazinc leachinge, which wastemperature. d to exhibit e.

ffect of leac

9

hed for 60 me influence e extraction follows: citrerature set liquid-to-soligure 6.

ncrease of am.0 M, the zinfrom 23.06zinc remaine.

tration on zines.

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in the sectioexcept the am6 M. The sol

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in the NHadily with ig efficiency rs close to sat

In this studa little impa

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f 0.75 M; amerature set aiquid-to-solidf Zn increaseng time increaction was aemonstrated uration is su

Figure 7: Effeow-grade zin

Figure 8: Efrom low-grad

Effect of Stir

Under the cid concentrion of 6 M, tatio of 5:1, eaching effic

ciency of Znn, maintaininmmonia concat 25 °C; stird ratio of 5:ed from 61.3reased from almost comp that the dur

ufficient to ac

fect of tempernc oxide ores

ffect of leachde zinc oxide

rring Speed

technical paration of 0.7temperature o

the effects ciency were

Ma Ai-yuan, P

n for leachingng citric acidcentration of

rring speed o:1. The leac36% to 79.5320 min to 9

pleted in 60 mration of abouchieve maxim

rature on zincs.

hing time one ores.

arameters as75 M, ammoof 25 °C andof stirring

studied, and

Peng Jin-hui, Zha

Brazilian Jou

g time from d concentratf 6 mol/L; teof 300 rpm; ahing efficien

3% as the lea90 min and min. The resuut 60 to 80 m

mum efficien

c recovery fr

n zinc recov

s follows: citonia concentd liquid-to-so

speed on zd the results

ang Li-bo, Shiwe

urnal of Chemica

20 tion em-and ncy

ach-the ults min ncy.

rom

very

tric tra-olid zinc are

dislea78to a srpmof thicrethiducreBusolficspe

Figfro Ef

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ei Li, Yang Kun a

al Engineering

splayed in Faching effici.44% as the 300 rpm, an

significant efm. This is bhemimorphi

in diffusion ease the stirrickness attac

ucing the thiease the reacut the stirrinlid and the d

ciency cannoeed exceeds

gure 9: Effeom low-grade

ffect of Liqu

The leachining various

ons were as 75 M; tempem; and ammach time of 1

Figure 10 screases signiincreased li

e zinc leachinapproximate

ard a possibleristics of zincstem. At a loe zinc ion copidly. Howevost invariable

and Zheng Xue-m

Figure 9. It ency of Zn istirring spee

nd that the stiffect on the recause the aite on the sulayer at the ring speed reched to the ckness of thction rate anng cannot ddiffusion layeot be substan350 rpm.

ect of stirrine zinc oxide

uid-to-Solid R

ng efficienciliquid-solid follows: cit

erature of 25monia concen

h. The resultshows that thificantly fromquid/solid rang efficiencyely 5:1. This e cause of thc oxide ore iow liquid-soloncentrationver, the zince in the range

mei

can be obseincreased fro

ed increased tirring speed recovery of ammonia leaurface of the

solid-liquidreduces the dsolid surfac

he diffusion nd the dissodestroy adheer, so the zinntially increa

ng speed on ores.

Ratio

ies of zinc wd ratios. Thetric acid con

5 °C; stirringntration as 6ts are shownhe zinc leachm 65.75% toatio from 3 ty remained in observation

he difficult lein the NH3-Hlid ratio (fro

n in the soluc leaching efe of 5:1 to 7:1

erved that thom 63.72% from 200 rpdoes not havzinc after 30

aching proce solid forms

d interface. Idiffusion layce, thereby r

layer, and iolving procesesion betweenc leaching eased when th

zinc recove

were evaluatee leach condncentration

g speed of 306 mol/L andn in Figure 10hing efficienco 80.85% wito 7. Howeven a stable rangn may point taching chara

H3C6H5O7-H2om 3:1 to 5:1ution increasefficiency is a1. The possib

he to

pm ve 00 ess s a In-yer re-in-ss. en ef-he

ery

ed di-of 00

d a 0. cy ith er, ge to-ac-2O 1), ed al-ble

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ause of this ow liquid-soiffusion proc

mainly in they chemical r

Figure 10: Eecovery from

FT-IR Analy

In order totructure, we ynthesized cechniques. Toom tempera

Figure 11: FT

The FT-Initric acid and

were recordedibrationally ic and symm

Brazilian

problem is olid ratio mcess and thee concentratireaction cont

Effect of liqm low-grade z

sis

o obtain morestudied the

compound uThe experimeature are show

T-IR of differe

nfrared spectrd different cid in KBr anactive carboxmetric vibra

Leaching Zn from

Journal of Chem

that the exmight be cone diffusion reion gradienttrol in the lat

quid-to-solidzinc oxide or

e informatioe vibrational using FT-IRental IR specwn in Figur

ent citric acid

ra (400-4000itric acid lea

nd revealed txylate groupations for t

m the Low-Grade

mical Engineering

xtraction ratentrolled by esistance ex, but is maiter stage.

d ratio on zres.

n on the crysspectra of

R spectroscoctra recordedre 11.

d concentratio

0 cm−1) of soaching solutiothe presence

ps. Antisymmhe carboxyl

e Zinc Oxide Ore

g Vol. 33, No. 04,

e at the ists inly

zinc

stal the

opic d at

ons.

olid ons

e of met-late

grosenstrcarThυs13vibwh13ricrevwime(Ctheordmybantheingshiditat droanionvarescoass

tioeffgrocanacicarpo RC M Ch LeXR

duterbypat

in NH3-H3C6H5O

, pp. 907 - 917,

oups of the nt in the sretching vibrrboxylate ca

he correspon(COO−) for 50-1550 cmbration peakhich shifted 97 cm-1) com

c stretching vealed the ioith metal catietric and antOO−)), was e citrate carbdinated to zyrsi et al., 2nd at 3496 ce citrate ligang intermolecifted to lowetion, the com3496 cm−1,

oxyl group od involved inized and thulues (Zárate-spect, it couordination tosembly. Steer and G

on might be fects from Loups attachen play an ids are only rboxylate an

otentially alte

-COO +MO

– metal; R –

haracterizat

eaching ResRF

The X-ray ue was perforrns of the ley 1 M citric attern of leac

O7-H2O Media

October - Decem

coordinated spectra. In ations υas (C

arbonyls in thnding symme

the same grm−1 (Kaliva eks (VC=O: 17

to lower frempared with

vibrations aonized COO−

ions. The difisymmetric sgreater than

boxylate grouinc in a mo

2001). Presencm−1 shows tnd is protonacular H bonder frequency

mpound givesattributed to

of water. Wen coordinatius undergo d-Gutiérrez anuld employ o zinc ion(s)

Griffiths (201better explaewis acid/ba

ed to the carimportant ropartially dis

nion (RCOO−

er the extracti

RCOOM

– organic sub

tion of the L

idue Chara

characterizarmed after th

eached residuacid are showch residue ob

mber, 2016

d citrate ligaparticular,

COO−) were pthe range 15etric stretchroups were pet al., 2006).740 cm−1) oequencies (1free citric ac

and symmet− groups fromfference betwstretches, (

n 200 cm-1, iups were eith

onodentate fance of a brothat the hydrated. Becausd, the absor

y (Che et al.,s a strong abo the vibrati

e found that ion to zinc mdeprotonationnd Lapidus, all of its capossibly pro

13) noted thaained by subase theory. Trboxylic acidrole. Althougssociated ‘we−) shown in

tion capabilit

2M+H O

bstituent grou

Leaching Res

acterization

ation of the he process. Tue obtained wn in Figurebtained was

9

nds were prantisymmetrpresent for th62-1630 cming vibrationpresent aroun

The carboxof citric aci1579 cm-1 ancid. Asymmetric vibrationm AC chelateween the sym(υas (COO−)-indicating thher free or c

fashion (Tsaroad absorptioroxyl group se of the exisrption band , 2005). In absorption banion of the hthe citrate li

may further bn at higher p2014). In th

arboxylates omoting a ne

at zinc extrabstituent grouThe substitued functionaligh carboxyleak acids’, thEq. (5) cou

ty.

(5

up.

sidue

by XRD an

leaching resThe XRD pawith leachin 12. The XRmatched wi

913

re-ric he

m-1. ns nd

xyl id, nd et-ns ed m--υs hat co-ra-on of st-is

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Leaching Zn from the Low-Grade Zinc Oxide Ore in NH3-H3C6H5O7-H2O Media 915

Brazilian Journal of Chemical Engineering Vol. 33, No. 04, pp. 907 - 917, October - December, 2016

Figure 13: (a) SEM image of the leaching residue, EDS mapping of O (b), Si (c), Al (d), Ca (e), Pb (f), Zn (g).

CONCLUSIONS

The dissolution behavior of low grade zinc oxide ores in the NH3-H3C6H5O7- H2O system was investi-gated.

(1) The concentration of citric acid was found to have an important role in low-grade zinc oxide ore dissolution, maximizing at 1 M, and a zinc leaching efficiency of about 81.2% was obtained, under the fol-lowing leaching conditions: ammonia 6 M, tempera-ture of 25 °C, leaching time of 60 min, stirring speed of 300 rpm and solid to liquid ratio of 1:5. The zinc leaching efficiency increased with the increase in cit-ric acid concentration, liquid-solid mass ratio, leach-ing time, stirring speed and ammonia concentration.

(2) The FT-IR spectra reflect the changes in the structural group units and justify the leaching mecha-nism for zinc dissolution, and the complex demon-strated that the Zn (II) ion is coordinated. Three strong FT-IR bands at 1579 cm-1 and 1397cm-1 were

found and they were assigned to a zinc–citrate com-plex in the aqueous solution.

(3) The results of XRD, IR, and SEM/EDS ex-periments indicate that hemimorphite is soluble, and the oxides of Si, Ca, Pb, Fe and Al are the main minerals in the leaching residue that are undissolved.

ACKNOWLEDGEMENTS

The authors are grateful for the financial support from the National Program on Key Basic Research Project of China (973 Program, 2014CB643404), the National Natural Science Foundation of China (51404118), the Yunnan Province Young Academic Technology Leader Reserve Talents (2012HB008), the Yunnan Provincial Science and Technology Inno-vation Talents scheme Technological Leading Talent (2013HA002), and the 2014 PhD Newcomer Award in Yunnan Province (1319880207).

916 Ma Ai-yuan, Peng Jin-hui, Zhang Li-bo, Shiwei Li, Yang Kun and Zheng Xue-mei

Brazilian Journal of Chemical Engineering

NOMENCLATURE

Znη leaching rate of zinc, (%) ZnC zinc concentration of leaching solution

(g/L)V the leaching volume, (L)

0ZnC the zinc content of the low-grade zinc

oxide ore (%)m the mass of the low-grade zinc oxide ore

(G)

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