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Removal of arsenic from Waelz zinc oxide using a mixed NaOHNa2S leach

Yuhu Li , Zhihong Liu, Qihou Li, Zhongwei Zhao, Zhiyong Liu, Li Zeng

Metallurgical Science and Engineering School, Central South University, Changsha 410083, China

a b s t r a c ta r t i c l e i n f o

Article history:

Received 4 January 2011

Received in revised form 2 April 2011

Accepted 5 April 2011

Available online 13 April 2011

Keywords:

Waelz zinc oxide

Arsenic leaching

Sodium hydroxide

Sodium sulde

Calcium arsenate

Zinc sulde

Arsenic was selectively leached from Waelz zinc oxide with a mixed NaOH+Na2S solution followed by

hydrogen peroxide oxidation and lime precipitation. The effects of temperature, L/S ratio, leaching time and

reagent concentration on the leaching of arsenic were investigated. With the mixed solution of 25 g/L NaOH

and 25 g/L Na2S9H2O and L/S ratio of 4:1, more than 90% As was extracted at 30 C in 3 h, while theconcentrations of lead and zinc in the leach solution were below 0.005 and 0.02 g/L, respectively. After

recycling the leach solution and doubling the concentration of arsenic, the leach solution was treated by

hydrogen peroxide oxidation followed by lime precipitation which removed 99.86% As, leaving 2 mg/L As in

the solution. The ltrate was recycled with additional NaOH and Na2S9H2O and the precipitate of calcium

arsenate was solidied with cement and treated in landll.

2011 Elsevier B.V. All rights reserved.

1. Introduction

In many non-ferrous metallurgical processes, arsenic co-exists with

metal values as complex by-products, which should be properlydisposed and comprehensively utilized to obtain the maximum

economic and environmental benets (Montenegro et al., 2010).

Arsenic-containing materials can be treated by pyrometallurgical or

hydrometallurgical processes. The pyrometallurgical-process common-

ly involves anoxic roasting at 300600 C, in which arsenic is volatilized

as As2O3 in ue dust and then collected (Shibayama et al., 2010).

However, the dust-collection is often unsatisfactory and the volatilized

arsenic may form a source of secondary pollution. In addition, As2O3is

notsuitable forsolidicationand disposal (Leistet al., 2000; Drahota and

Filippi, 2009). Due to these disadvantages, arsenic removal is now

mainly focused on hydrometallurgical processes, such as solvent

extraction (Demirkiran and Rice, 2002), selective alkali leaching

(Brostow et al., 2010; Tongamp et al., 2009, 2010), pressure leaching

(Xu et al., 2010), mechanical activation leaching and otation (Balaz

et al., 2000; Welham, 2001).

The most widely used method is acid leaching in which both metal

values and arsenic dissolve into solution and arsenic is then separated

andnally solidied as calcium arsenate or ferric arsenate (Ke et al.,

1984; Nunez et al., 1985; Bolin and Sundkvist, 2008). However, the

stability of calcium arsenate and ferric arsenate is a concern and various

methods of increasing the crystallinity and stability of these arsenates

have been investigated, such as the hydrothermal process (Monhemius

and Swash, 1999) and calcination (Riveros et al., 2001).

Waelz zinc oxide is generated by reductive volatilization in the

metallurgical processing of lead and zinc. It often contains lead andarsenic (typically Zn +Pb N50%, As:515%) and it is generally returned

to sintering for the recovery of lead and zinc. However, arsenic may

accumulate in the metallurgical system and pose a pollution and

potential safety hazard. Some research on the removal of arsenic from

Waelz zinc oxide has been reported.Tan (1998)employed a process of

caustic soda roasting followed by water leaching to extract N90% As

which was nally precipitated as crude sodium arsenate, while zinc

remained in the leach residue and was re-leached using acid. Zhang

(1997) used an acid leach-oxidation-hydrolysis process in whichAs(III)

was oxidized to As(V) and then precipitated as iron and manganese

arsenate. Other methods, such as sulfation-roasting-leach (Chen et al.,

2001) and ammonia leaching (Yi, 2001) have been also reported, but

these methods were complex and had low economic benet. Moreover,

arsenic could not be completely solidied, which caused pollution

problems. To solve these problems, a novel process of selective arsenic

leaching with alkaline sodium sulde followed by oxidative precipita-

tion of calcium arsenate was applied in this study, leaving zinc and lead

sulde in the leach residue which could be returned to the pyromet-

allurgical process to recover zinc and lead.

2. Experimental

2.1. Materials

The Waelz zinc oxide sample was obtained from the Shaoguan

Smelter, Guangdong Province, China and was used as received. Particle

Hydrometallurgy 108 (2011) 165170

Corresponding author. Tel.: +86 0731 88830478; fax: +86 0731 88830478.

E-mail addresses:[email protected],[email protected](Y. Li).

0304-386X/$ see front matter 2011 Elsevier B.V. All rights reserved.

doi:10.1016/j.hydromet.2011.04.002

Contents lists available at ScienceDirect

Hydrometallurgy

j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / h yd r o m e t
http://dx.doi.org/10.1016/j.hydromet.2011.04.002http://dx.doi.org/10.1016/j.hydromet.2011.04.002http://dx.doi.org/10.1016/j.hydromet.2011.04.002mailto:[email protected]:[email protected]:[email protected]://dx.doi.org/10.1016/j.hydromet.2011.04.002http://www.sciencedirect.com/science/journal/0304386Xhttp://www.sciencedirect.com/science/journal/0304386Xhttp://dx.doi.org/10.1016/j.hydromet.2011.04.002mailto:[email protected]:[email protected]://dx.doi.org/10.1016/j.hydromet.2011.04.002


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analysis found that N 90% particles were less than 44 m with a specic

surface area of 2.39 m2/g. The chemical composition of the sample that

is shown inTables 1 and 2shows the chemical phases of arsenic in

Waelz zinc oxide determined by selective dissolution procedure (Luo,

1990; Neaman et al., 2004). The XRD of the Waelz zinc oxide sample,

presented in Fig. 1, shows onlytwo mainphases of ZnO and As2O3 with

no evidence of lead phases, due to their amorphous states. The SEM

(Fig. 2) indicated that the sample contained a mixture of micron sizedpolyhedral crystals and aggregates of spherical nano-particles. The EDS

analysis (Fig. 3) found that the crystalline phase was ZnO and the

aggregates were mainly a mixture of As2O3and PbO.

Reagents of NaOH and Na2S9H2O were analytical grade and

deionized water was used in all experiments.

2.2. Experimental procedures

In batch tests, 100 g Waelz zinc oxide wasmixed with leach reagent

and stirred at 500 rpm at a certain temperature. After leaching and

ltration, the residue was washed by hot water three times and then

dried and recycled to the factory's sintering process to recover zinc andlead. Theleachsolution wasmixedwith thewash water andoxidizedby

hydrogen peroxide and then calcium arsenate was precipitated by the

addition of lime.

2.3. Characterization and analyses

The phases of Waelz zinc oxide and the leach residue were analyzed

by X-ray diffraction (Siemens D5000, CuKa, =1.5421010 m), and

thecontent of arsenic in theleach residue wasdeterminedby thesodium

hypophosphite reduction-iodometric method. The content of As(III) and

As(V) in the leach solution was determined by the iodometric method

and the content of zinc and lead was determined by EDTA complex

titrationor by ICP-MS (IRISInterpid II XSP,ThermoElectronCorporation)

when present in trace amount. The morphologies of Waelz zinc oxide

were observed using a JSM-6360LV SEM-EDS instrument.

3. Results and discussion

3.1. Thermodynamic calculation of ZnOZnSAs2O3H2O system

Waelz zinc oxide was mainly composed of ZnO and As2O3(Fig. 1),

however, ZnS may be formed in the alkaline sulde leach system.

The main reactions with the alkaline sulde system are as follows:

As2O3 2NaOH 2NaAsO

2 H

2O 1

As2O3 NaOH NaHAsO22 2

ZnO

2NaOH

Na

2

ZnO2

H2

O

3

Table 1

Chemical components of Waelz zinc oxide sample.

Element Zn Pb As Ge

Content (%) 51.2 11.8 7.1 0.03

Table 2

Chemical phase analysis of arsenic in Waelz zinc oxide.

Chemical phase As2O3 As2O5 A rsenite Arsenate

Content (%) 6.44 0.52 0.31 0.12

Proportion (%) 87.1 7.1 4.2 1.6

Fig. 1.XRD of Waelz zinc oxide sample.

Fig. 2.SEM of Waelz zinc oxide sample.

Fig. 3. EDS analysisresultsof Waelz zincoxide sample. A coarse polyhedral particles.

Bspherical nano-particles.

166 Y. Li et al. / Hydrometallurgy 108 (2011) 165170
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Na2ZnO

2 Na

2S 2H

2O ZnS 4NaOH 4

Therefore, the equilibrium concentration diagram of ZnOZnS

As2O3H2O system was determined as shown in Fig. 4 based upon

thermodynamic relationships described inTable 3which shows all the

possible species (Dean, 2003; Kuchar et al., 2006). The total zinc(II)

and arsenic(III) concentrations at two different concentrations of

sulde ion (Cs) between pH 0 and 14 are described by the curves.

Regionsenclosed by curve L1andL2arestable regionsfor As2O3andZnO,

respectively in the absence of sulde ion. As seen from Fig. 4, the

following conclusions were made:

(1) It is better to employ an alkaline system in the range pH 811

for the selective leaching of arsenic and the recovery of zinc

oxide than an acid system.

(2) Theadditionof sulde ion effectivelydecreasesthe concentration

of zinc (as well as lead) in the leach solution ensuring a high

recovery of metal values which can be returned to pyrometal-lurgical operations.

3.2. Effect of leaching system on the leaching of arsenic and the loss of

lead and zinc

Both single and mixed alkaline leaching systems were studied. The

effects of the leaching system on the extraction of arsenic, zinc and

lead are shown inFigs. 5 and 6.

As seen fromFig. 5, the total alkali concentration was the most

important factor. The leaching of arsenic in NaOH alone was a little

higher than that in the mixed NaOHNa2S system when the total

alkali concentration was below 39 g/L, otherwise the leaching of

arsenic was a little higher in the alkaline sulde system. However,

with the NaOH system, a large amount of lead and zinc were alsoextracted, as shown in Fig. 6, resulting in the precipitation of insoluble

lead arsenite and zinc arsenite. Therefore, complete arsenic extraction

into solution was inhibited at high NaOH concentration (Fig. 5),

whereas in the mixed alkaline sulde system, the dissolved Pb2+ and

Zn2+ preferred to precipitate with S2 rather than AsO2 which not

only reduced the loss of metal values, but also promoted the leaching

of arsenic, as shown inFig. 6. Therefore, the mixed alkaline sulde

leaching system is clearly better for the selective removal of arsenic

from Waelz zinc oxide.

3.3. Effect of temperature on the leaching of arsenic

The effect of temperature on the extraction of arsenic, shown in

Fig. 7, indicates that the leaching of arsenic increased only slightlywith the increase of temperature. About 92% As was leached out at

30 C in comparison to 96% As at 90 C. However, the loss of metal

values increased rapidly with the elevation of temperature. At 30 C,

the concentration of lead and zinc in the leach liquor was 0.02 g/L and

0.04 g/L respectively, in comparison to 0.11 g/L and 0.18 g/L at 90 C,

respectively. Therefore, given the considerations on the cost and

energy consumption, 30 C was determined to be the optimum

reaction temperature.

Table 3

Balance relationships of ions in ZnOZnSAs2O3H2O system.

Balance relationship log K

1 Zn2++ OH=Zn(OH)+ 4.40

2 Zn2++2 OH=Zn(OH)2(aq) 11.30

3 Zn2++3 OH=Zn(OH)3 14.14

4 Zn2++4 OH=Zn(OH)42 17.66

5 AsO++ OH= HAsO2(aq) 14.33

6 AsO++ 2OH= H2AsO3 18.73

7 AsO++ 3OH= H3AsO42 20.60

8 AsO++ 4OH= H4AsO53 21.20

9 H2S(aq)= HS+ H+ 6.97

10 HS= H++ S2 12.90

11 H2O = H++ OH 13.995

12 ZnS=Zn2+ + S2 23.80

10 20 30 40 50 60 70 80

81

84

87

90

93

96

99Leaching with NaOH

Leaching with NaOH and Na2S

Theleachingofarsenic(%)

Alkali concentration (g/L)

Fig. 5.Effect of leaching system on the leaching of arsenic. Mixed alkaline leach was

conductedwith a mass ratio of Na2S9H2O/NaOHof 3:1 and L/S of4 at30 Cfor3 h.The

single alkali leach was carried out with only NaOH under the same conditions.

10 20 30 40 50 60 70 80

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5[Pb], leaching with NaOH and Na

2S

[Zn], leaching with NaOH and Na2S

[Pb], leaching with NaOH[Zn], leaching with NaOH

TheconcentrationofPbandZ

n(g/L)

Alkali concentration (g/L)

Fig. 6.Lead and zinc concentrations in different leaching systems. (Conditions as Fig.5).

0 2 4 6 8 10 12 14-20

-16

-12

-8

-4

0

L4

L2

L3logC

pH

As2O

3

ZnOL1

Fig. 4. Log CpH relationship in ZnOZnSAs2O3H2O system. L1:CAs(CS=0 mol/L).

L2:CZn(CS=0 mol/L). L3:CZn(CS= 106 mol/L). L4:CZn(CS=1 0

1 mol/L).

167Y. Li et al. / Hydrometallurgy 108 (2011) 165170
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3.4. Effects of leaching time on the extraction of arsenic

The effect of time on the leaching of arsenic was carried out with

the same alkaline sulde solution at 30 C and the results in Fig. 8

show 90% As was easily leached out after 1 h and only increased to

94% after 4 h. It can be inferred that arsenic mainly existed as single

phase in the Waelz zinc oxide rather than as an inclusion or

encapsulated. In addition, the concentration of lead and zinc in the

leach liquor decreased slightly before 3 h, which attributed to the

consumption of alkali by the dissolution of arsenic. Therefore, the

optimum leach time was determined to be 3 h.

3.5. Effect of L/S ratio on the leaching of arsenic

The effect of L/S ratio on the leaching of arsenic is presented inFig. 9 and shows that arsenic extraction increased from 84% to 95%

with the increase of L/S ratio from 2 to 8. Clearly, when the L/S ratio

was below 4, the leaching of arsenic was not complete with the low

amount of total alkali. Although the lead and zinc concentration was

almost constant, the increase volume of leach liquor meant the

increase loss of metal values. Therefore the optimum L/S ratio of 4 was

determined.

3.6. Effect of Na2S9H2O on the leaching of arsenic

The effect of varying the amount of Na2S9H2O on the leaching of

arsenic was shown inFig. 10. With 10 g/L Na2S9H2O, about 96% As

was extracted and little change in arsenic extraction was observed

when the amount of Na2S9H2O was increased from 10 g/L to 55 g/L.

In the leach system, it appears that the minimum concentration of

Na2S9H2O required to precipitate Pb2+ and Zn2+ was about 10 g/L.

Since excess Na2S addition would cause negative effects, such as the

formation of As2S3 (Delni et al., 2003), theoptimum concentration of

Na2S was chosen as 25 g/L.

3.7. Optimum operating conditions

Based on the above results, the optimal leaching conditions were

determined to be a mixed alkaline sulde solution of 25 g/L NaOH and

25 g/L Na2S9H2O with L/S ratio of 4 at 30 C for 3 h. Under theseconditions 9296% As was selectively extracted to give a leach

solution containing 5.45 g/L As, b0.005 g/L Zn and 0.02 g/L Pb. The

XRD of the leach residue showed only the diffraction lines of ZnO with

no lines associated with As2O3, ZnS or PbS. The chemical analysis

found that the leach residue contained 0.47% As, 54.7% Zn and 12.6%

Pb as mainly oxide phases with only 0.65% ZnS and 1.92% PbS as

amorphous phases.

30 40 50 60 70 80 9080

84

88

92

96

100

The extration of arsenic

Zn concentration

Pb concentration

Temperature (C)

Theextractionofarsenic(%)

0.00

0.04

0.08

0.12

0.16

0.20

0.24

0.28

PbandZnconcen

trationinleachliquor(g/L)

Fig. 7. Effect of temperature on the leaching of arsenic with mixed alkaline solution.

(75 g/L Na2S9H2O and 25 g/L NaOH and L/S of 4 for 3 h).

1 2 3 480

84

88

92

96

100

The extraction of arsenic

Zn concentration

Pb concentration

Time (h)


0.00

0.04

0.08

0.12

0.16

0.20

PbandZnconcentrationinleachliquor(g/L)

Fig. 8.Effect of leaching time on the leaching of arsenic with mixed alkaline solution.

(75 g/L Na2S9H2O and 25 g/L NaOH and L/S of 4 at 30 C).

2 4 6 880

82

84

86

88

90

92

94

96

98

100


Zn concentration

Pb concentration

L/S ratio


0.00

0.04

0.08

0.12

0.16

0.20


Fig. 9.Effect of L/S ratio on the leaching of arsenic with mixed alkaline solution. (75 g/L

Na2S9H2O and 25 g/L NaOH at 30 C for 3 h).

0 10 20 30 40 50 6080

84

88

92

96

100


Zn concentration

Pb concentration

Na2S concentration (g/L)

Theextractionofarsen

ic(%)

0.0

0.1

0.2

0.3

0.4

0.5


Fig. 10. Effect of Na2S9H2O amount on the leaching of arsenic with 25 g/L NaOH

solution. (L/S of 4 at 30 C for 3 h).



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When the same leach solution wasrecycledto treat a second batch

of Waelz zinc oxide, very similar extraction results were obtained and

the concentration of arsenic in solution increased from 5.45 g/L to

10.37 g/L As while the concentration of zinc and lead remained low

(0.005 and 0.03 g/L, respectively). Therefore, recycling of the leach

liquor was feasible, with reagent make-up as required and a bleed

circuit to precipitate arsenic from the more concentrated arsenic

solution.

4. Arsenic precipitation

Hydrogen peroxide oxidationlime precipitation is a classic

technique for the removal of arsenic and has been widely used in

practical applications (Moon et al., 2004; Camacho et al., 2009). The

toxicity and mobility of As(III) is much higher than that of As(V) and

the solubility of As(III) salts is also higher than that of the

corresponding As(V) insoluble salts (Driehaus, 1995; Suzuki et al.,

1997; Deschamps, 2003), Therefore, it is essential to convert As(III) to

As(V). Many oxidants for As(III) have been investigated, including

chlorine (Sorlini and Gialdini, 2010), ozone (Kim and Nriagu, 2000),

manganese oxides (Tournassat et al., 2002), TiO2/UV (Lee and Choi,

2002) andhydrogenperoxide(Pettine et al., 1999). These oxidants are

powerful for the transformation of As(III) to As(V), but the treatment

costs were high. Undoubtedly, catalytic air oxidation is the most

promising cost effective method (Zhang et al., 2000), but it is not

suitable for concentrated arsenic solutions.

In this study, hydrogen peroxide was chosen to oxidize As(III) to

As(V) in view of its advantages of a clean and simple reagent with noimpurity introduced. At room temperature, As(III) can be completely

oxidized to As(V) using 1.2 times the stoichiometric amount of

hydrogen peroxide in 0.5 h.

4.1. The effect of lime/As mole ratio and temperature on arsenic

precipitation

The effect of lime/As mole ratio on the removal of arsenic was

carried out at 80 C for 2 h. The results inTable 4show that although

the theoretical lime/As mole ratio for the precipitation of Ca5(AsO4)3OH is 1.67, it is necessary to use an excess amount of lime

up to a mole ratio of 3 to remove 99.5% As.

Elevating the temperature from 40 to 90 C not only increased the

removal of As (Table 4), but also improved the crystallinity of the

calcium arsenate precipitate. The removal of arsenic was 65.6% at 40 C,

with no XRD diffraction lines for arsenic species, and increased to 99.9%

at 90 C with XRD diffraction lines of crystallite Ca5(AsO4)3OH clearly

evident (Fig. 11). The nal solution contained only 2 mg/L As together

with 18 g/L NaOH which can be recycled and returned to the leaching

process with extra addition of NaOH and Na2S.

Based on the investigation above, the conceptual ow sheet of

removing arsenic from arsenical Waelz zinc oxide is proposed in

Fig. 11.

Table 4

Effect of lime/As mole ratio and temperature on the removal of arsenic from solution as

calcium arsenate.

Factors Removal of arsenic (%)

Lime/As mo le ratio (at 80 C for 2 h) 1.7 72.3

2.4 88.4

3 99.5

4 99.6

Temperature (lime/As mole ratio of 3 for 2 h) 20 C 56.1

4 0 C 65 .67 0 C 85 .3

9 0 C 99 .8

Waelz zinc oxide

51.2% Zn11.8% Pb7.1% As

NaOH

25 g/L

Na2S9H2O

25g/L

Selective leaching

30C3h

Return to pyrometallurgical processing

First leachingAs3+:5.45

Zn2+:0.02g/L

Pb2+: 0.005g/L

First leaching:94.1% As

Second leaching:91.3% As

Second leachingAs3+:10.37

Zn2+:0.03 g/L

Pb2+: 0.005g/L

H2O2 Ca(OH)2

lime/As=3:1

90C

2h

Tailings

Removal of As: 99.5%

Final solution

As5+:2.09 mg/LNaOH:18 g/L

Fig. 11.The proposed process scheme.

169Y. Li et al. / Hydrometallurgy 108 (2011) 165170


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5. Conclusions

A novel process of treating Waelz zinc oxide containing a

high content of arsenic(III) oxide is proposed, involving a selective

arsenic(III) leach with mixed alkaline sulde, hydrogen peroxide

oxidation of As(III) to As(V) and lime precipitation of calcium

arsenate. Sulde ion passivates the zinc oxide surface and precipitates

any soluble Zn(II) and Pb(II) as ZnS and PbS.

(1) N90% As was selectively leached from Waelz zinc oxide with25 g/L NaOH and 25 g/L Na2S9H2O at30 Cfor3 h using a liquid/

solid ratio of 4, to give a solution containing 5.45 g/L As(III)

together with b0.005 g/L Pb and 0.02 g/L Zn. The leach residue

was mainly zinc and lead oxide together with 0.6% ZnS and 1.9%

PbS.

(2) The recycled leach solution gave similar leach results and

doubled the concentration of arsenic.

(3) The second round leach solution was oxidized by 1.2 times

stoichiometric hydrogen peroxide at room temperature.

(4) 3 moles lime/mole As were required to precipitate 99.9% As at

90 C as Ca5(AsO4)3OH leaving only 2 mg/L As in the nal

solution. The nal solution can be returned to the leaching step

withextraaddition of alkaliand sulde, andthe calcium arsenate

can be treated in landll after solidication with cement.

This preliminary investigation offers a low cost process with low

loss of metal values and no secondary contamination. Therefore it

warrants further larger scale work to make it practical and viable for

the industry.

Acknowledgments

This work was nancially supported by the National Science

Foundation of China, namedThe Applied Basic Research on

The Treatment of High Arsenic Bearing Materials in Nonferrous

Metallurgy(50874121).

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http://dx.doi.org/10.1016/j.mineng.2010.03.020http://dx.doi.org/10.1016/j.jhazmat.2010.05116http://dx.doi.org/10.1016/j.jhazmat.2010.05116http://dx.doi.org/10.1016/j.mineng.2010.03.020

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