7

Click here to load reader

Simulated Small-scale Pilot Plant Heap Leaching of Low-grade Oxide

Embed Size (px)

Citation preview

Page 1: Simulated Small-scale Pilot Plant Heap Leaching of Low-grade Oxide

This article is also available online at:

www.elsevier.com/locate/mineng

Minerals Engineering 20 (2007) 694–700

Simulated small-scale pilot plant heap leaching of low-grade oxidezinc ore with integrated selective extraction of zinc

Qin Wen-qing *, Li Wei-zhong, Lan Zhuo-yue, Qiu Guan-zhou

School of Resource Processing and Bioengineering, Central South University, Changsha 410083, China

Received 9 July 2006; accepted 5 January 2007Available online 21 February 2007

Abstract

The leaching of low-grade oxide zinc ore and simultaneous integrated selective extraction of zinc were investigated using a small-scaleleaching column and laboratory scale box mixer-settlers. Di-2-ethythexyl phosphoric acid (D2EHPA) dissolved in kerosene was used asan extractant. The results showed that it was possible to selectively leach zinc from the ores by heap leaching. The zinc concentration ofthe leach liquor in the first leaching–extraction circuit was 32.57 g/L, and in the 16th cycle the zinc concentration was 8.27 g/L after thesolvent extraction. The leach liquor was subjected to solvent extraction, scrubbing and selective stripping for the enrichment of zinc andthe removal of impurities. The pregnant zinc sulfate solution produced from the stripping cycle was suitable for zinc electrowinning.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Zinc; Low-grade zinc oxide ores; Heap leaching; Solvent extraction; Di-2-ethythexyl phosphoric acid (D2EHPA)

1. Introduction

Zinc oxide ore is an important source of zinc metal afterzinc sulfide ores. With the escalating depletion of zinc sul-fide ores, the zinc-bearing minerals such as willemite(Zn2SiO4), hemimorphite [Zn4(Si2O7)(OH) Æ H2O] andsmithsonite (ZnCO3), etc., have become an importantsource of zinc (Abdel-Aal, 2000). In China, zinc oxidizedores are relatively abundant, and are mainly founded insouth-west and north-west China, in places such as Yun-nan, Sichuan, Guangxi and Gansu provinces, etc. (Duanand Luo, 2000).

Many studies have been done on concentration of zincoxide ores, yet very little progress has been made. Usuallyzinc oxide ores are concentrated by flotation or gravity,where the metal recovery is low and the operating cost ishigh (Duan and Luo, 2000). Zinc oxide ores can be treatedby acidic leaching processes (Bodas, 1996). However, zincis dissolved following the dissolution of many other metals

0892-6875/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.mineng.2007.01.004

* Corresponding author.E-mail address: [email protected] (W.-q. Qin).

such as Fe, Ca, Mg, and SiO2, etc. As a result, the acid con-sumption is high, and complex purification processes arerequired. The large quantity of silica may transform intoa gel and prevent the separation of zinc sulfate from theslurry. Furthermore acidic leaching processes are uneco-nomic for treating low-grade zinc oxide ores.

Zinc oxide ores can also be treated by pyrometallurgicalprocess (Chen and Qu, 1998). The high-grade zinc concen-trate is produced from low-grade ores by volatilizationtechniques at high temperature in blast furnaces orWaelz-type kilns, and then subjected to a hydrometallurgi-cal process. Though the pyrometallurgical process can treatlow-grade ores, it is not acceptable because of heavy pollu-tion and high capital investment (Choi et al., 1993).

Solvent extraction is regarded as a highly effectivetechnique of separation and purification, which hasbeen used in the extraction of gold, copper, cobalt andnickel, etc. (Qiu et al., 2002, 2003; Hsu and Harrison,1995). The extraction of zinc by various reagents includingdi-2-ethylhexyl phosphoric acid was studied in detail byRice and Smith (1975). Conventional zinc extractantsbelong to the group of acidic reagents. Alkylphosphoricacids have been used for many years, and among them

Page 2: Simulated Small-scale Pilot Plant Heap Leaching of Low-grade Oxide

W.-q. Qin et al. / Minerals Engineering 20 (2007) 694–700 695

di-2-ethythexyl phosphoric acid (D2EHPA) is the extract-ant most used (Bart et al., 1992; Forrest and Hughes,1978; Sato et al., 1978). The recovery of zinc from the leachliquors of the CENIM-LNETI process by solvent extrac-tion with di-2-ethylhexyl phosphoric acid is performed byAmer and Luis (1995).

Kongolo efficiently performed the removal of zinc andmanganese from industrial cobalt sulphate solutions bysolvent extraction and they also investigated the recoveryof Cobalt and zinc from copper sulphate solution by sol-vent extraction with D2EHPA, which has been successfullyused to separate cobalt and zinc into their respective solu-tions (Kongolo et al., 2000, 2003).

The solvent extraction of zinc and cadmium from phos-phoric acid solutions using di-2-ethylhexyl phosphoric acid(D2EHPA) in treated kerosene as diluent has been investi-gated by Mellah and Benachour (2006). The dimerizationconstant of D2EHPA is about 106 in polar solvents andthe equation of the dimerized form can be written in thefollowing way:

2ðHRÞ ! ðHRÞ2The mechanism of extraction by the D2EHPA and the

nature of the formed metal complexes depend on severalfactors such as concentration of the metal cations, the nat-ure of organic solvent, the acidity of the aqueous phase andthe type of extracted cations (Pinipenko, 1974).

Column Lea

L

Solv

S

Organic feed

Scrubbing water waste

Organic recycling

Organic

Stripping HCl

Organic

Stripping H2SO4

HCl

H2SO4

Fig. 1. Schematic diagram of the le

For zinc extraction with di-2-ethylhexyl phosphoric acid(D2EHPA) as extractant dissolved in aliphatic diluents, theequilibrium is given by the following scheme reactions(Mansur et al., 2002):

Zn2þðaqÞ þ 1:5ðRHÞ2ðorgÞ � ZnR2RHðorgÞ þ 2HþðaqÞat the liquid–liquid interface

2ZnR2RHðorgÞ � 2ZnR2ðorgÞ þ ðRHÞ2ðorgÞin the extract phase

where RH represents the extractant species D2EHPA thatacts like a liquid cationic ion exchanger, and subscripts (aq)and (org) refers to aqueous and organic species, respec-tively (Mellah and Benachour, 2006).

Solvent extraction may be used for the selective extrac-tion of impurities or valuable metal from process streamsinto an organic phase. The loaded organic is often strippedwith an acid (aqueous) solution to back extract the Zn2+.The organic is regenerated, reconditioned and recycled.

In this paper, the solvent extraction of zinc from leachliquor using D2EHPA in treated kerosene as diluent hasbeen investigated. It presents the results of a study carriedout to determine if solvent extraction could be used toselectively extract zinc from the leach liquor of low-gradezinc oxide ore, which was treated in a large diameter col-umn (4 m high · 0.2 m diameter) simulating a unit cell ofa commercial operation. The organic extractant used was

ching of zinc oxidized ore

each liquor

ent extraction Raffinate

crubbing Scrubbing water

H2SO4Stripping

Pregnant zinc electrolyte

ach-solvent extraction process.

Page 3: Simulated Small-scale Pilot Plant Heap Leaching of Low-grade Oxide

696 W.-q. Qin et al. / Minerals Engineering 20 (2007) 694–700

di-2-ethylhexyl phosphoric acid (D2EHPA). The pregnantzinc sulfate solution produced from the stripping cycle wassuitable for electrowinning. Fig. 1 shows the schematic dia-gram for the leaching of low-grade zinc oxide ore, the zincsolvent extraction process in this paper.

2. Experimental

2.1. Zinc oxide ore

The zinc oxide ore was obtained from Gaofeng mine inGuangxi province in China, whose chemical composition isgiven in Table 1. The Pb, Cd, Cu, Ni, Ag and Co contentwere analyzed by an atomic absorption spectrophotometer(Japan, Shimadzu AA-6800), and the Zn, Fe, MgO, CaO,Mn, Al2O3 were analyzed by titrimetric analysis. The Swas analyzed by combustion analysis. The As was analyzedby colorimetric analysis and the SiO2 were analyzed bygravimetric analysis. Microscopic examination in thinslides of the groundmass showed that the zinc mineralsare mainly willemite (Zn2SiO4), hemimorphite [Zn4-(Si2O7)(OH) Æ H2O] and smithsonite (ZnCO3), and gangueminerals are quartz, gypsum, dickite, and sericite (finegrained mica) in both crystalline and microcrystallineforms. Table 2 shows the mineral composition.

2.2. Column leaching

Leaching of zinc oxide ore by sulfuric acid can be illus-trated as

ZnCO3 þH2SO4 ! ZnSO4 þH2Oþ CO2 ð1ÞZn4Si2O7ðOHÞ2 �H2Oþ 4H2SO4

! 4ZnSO4 þ 2SiðOHÞ4 þ 2H2O ð2ÞZn2SiO4 þ 2H2SO4 ! 2ZnSO4 þ SiðOHÞ4 ð3Þ

With high acidity both the zinc leach rate and the zincextraction rate increases. However, iron and silica are alsodissolved in large quantities (Tang and Ouyang, 1998;Doepker and O’Connor, 1990). Silica forms a gel, whichdecreases the filtration rate or even results in the collapseof the operation. Controlling the acidity of leaching can

Table 1Chemical composition of the zinc oxide ore

Element Zn Pb Cd Cu Fe MgO CaO SiO2

w/% 14.24 0.25 0.035 0.046 23.12 1.35 3.64 36.29Element Mn Al2O3 Co Ni Ag S Asw/% 1.42 8.52 Mim. 0.032 0.0028 2.55 0.23

Table 2Minerals composition of the zinc oxide ore

Mineral Willemite Hemimorphite Smithsonite Quartz GypsumComposition

(%)10.24 8.15 8.35 32.12 12.08

reduce the dissolved silica and other impurities or makethem precipitate in the leach residues. Column leachingwas adopted for the sulfuric acid leaching of zinc oxideores.

The size distribution of the zinc oxide ore was 90% pass-ing 20 mm and 42% passing 3 mm. The chemical analysis isshown in Table 1. The original concentration of H2SO4 inthe lixiviant was 0.306 mol/L. The leach liquor was recy-cled without addition of acid after solvent extraction ofzinc.

2.2.1. Column construction

The experiments were carried out in a column reactorthat was fabricated from 5 mm thick 304 L stainless steel.The column was 4 m high with an internal diameter of0.2 m, and it stood in a shallow tank with a capacity of0.1 m3, which collected the PLS solution draining fromthe column. The liquor level was maintained at a sufficientheight to provide a seal, forcing the air upwards throughthe column charge. Solution was applied to the surface ofthe column charge using a simple garden sprinkler headof the type used in drip irrigation systems.

Support rock selection for use in column leaching pro-cess requires the rock be competent, low in carbonateand other acid-consuming minerals, and sized at �30 +10 mm. A granite rock was sourced from a local supplier.Mineralogical analysis showed that the major mineralswere quartz. Sizing was 80% passing 25–30 mm with only0.6% passing 10 mm. The rock contained <0.03% Zn,1.7% Fe.

Fig. 2 shows the schematic of the heap reactor.

2.2.2. Column operation

The original concentration of H2SO4 was 0.306 mol/L.A tank with a capacity of 0.1 m3 was used to collect thePLS solution draining from the column. Leach liquorwas sampled from this tank, and was analyzed to determinesolution concentrations, metal dissolution and the acid bal-ance by an atomic absorption spectrophotometer.

Ore was coated onto the support rock by tumblingweighed batches. A layer of uncoated support rock,

Fig. 2. Schematic of the heap reactor. (1) Peristaltic pump; (2) water bath;(3) mixer; (4) feed vessel; (5) liquid distributor.

Page 4: Simulated Small-scale Pilot Plant Heap Leaching of Low-grade Oxide

W.-q. Qin et al. / Minerals Engineering 20 (2007) 694–700 697

100 mm deep, was placed in the bottom of the columnbefore the coated ore was loaded. The leach liquor waspassed through the ore sample by gravity and re-circulatedthrough a side loop with a peristaltic pump. In the leachingexperiments, the column system was comprised of 100 kgof ore. For all of the tests the initial temperature and pHof the feed were 25 �C and 1.2, respectively.

Solution and raffinate were fed to the top of the columnfrom a feed container by means of a peristaltic pump at arate of approximately 0.5 L/min, and collected at the baseinto the PLS stock tank by a pump controlled by a levelsensor in the solution reservoir. The volume of liquid inthe actual column was 80 L. From this and the flow rate,the apparent residence time was calculated to be about160 min.

The zinc from the PLS was recovered by solvent extrac-tion with D2EHPA. Evaporation losses and sampleremoval were made up with water (pH 1.2) to a total circu-lation volume of 80 L per column. After leaching, liquor inthe column was allowed to drain, and the column contentswere then rinsed to remove residual zinc and other solublespecies. The column charge was rinsed first with dilute sul-phuric acid solution (pH 1.2) followed by a water rinse.The column was then unloaded through the samplingports, and the residue was separated from the support rockby wet screening. The residue was filtered, dried, and pre-pared for final chemical analysis with similar chemicalanalysis methods used for zinc ore (Section 2.1).

2.3. Extraction and stripping processes

Di-2-ethylhexyl phosphoric acid was obtained fromTianjing Chemistry Reagent Factory in China. It had apurity of 95%. Kerosene (260#) was obtained from SinopecShanghai Gpc Oil Refinery in China and distilled to collectthe fraction distilling over 260 �C. It was mostly aliphaticin nature. The pH-extraction isotherms of zinc, calcium,copper, cadmium, cobalt and nickel were determined by

E1 E2 SB1

Raffinate

Organic feed

S1 S2 S3

Pregnant

zinc electrolyte

H2SO4

HCl, FeCl3

Scrubbing water wastLeach liquor

Fig. 3. The schematic diagram for th

shakeout test at different equilibrium pH values. The metalconcentration in aqueous solution was 3 g/L separately,which was prepared with AR grade sulfate, and was eachstudied separately. Extraction experiments were carriedout in mechanically agitated beakers. The solution was agi-tated by a mechanical stirrer with a constant stirring rate.After shaking beakers for 10 min, the organic phase wasseparated from the aqueous phase. The pH of the aqueoussolution was adjusted to the desired value by adding asmall amount of HNO3 or NaOH. After phase disengage-ment, the aqueous phase was separated and its equilibriumpH was measured with a pH meter. The D2EHPA concen-tration was 10%, the phase ratio (VO/VA) was 1:1, and thecontact time was 10 min.

Fig. 3 shows the schematic diagram for the zinc solventextraction process. The extraction-stripping process of zincsulphate with D2EHPA had been studied at laboratoryscale according to the following operating variables: pHof the aqueous phase, concentration of D2EHPA, equili-bration time and O/A volumetric ratio (Qin et al., 2003).

The D2EHPA concentration was about 30% v/v dilutedin a kerosene-type diluents, the phase ratio (VO/VA) was1.5:1, and the contact time was 5 min. The concentrationof H2SO4 in the stripping aqueous feed is 1.53 mol/L.

The mixer-settler cascade used in this work was com-posed of box-type mixer-settlers made of Teflon with simi-lar internal arrangement and dimension (width = 860 mm,depth = 250 mm and height = 500 mm). The active volumeof one mixer-settler or stage was 6.0 L, while the ratio of themixer and settler volumes was 1:4 (1.2 and 4.8 L, respec-tively). The stirring speed used was 1200 rpm. The connec-tion between mixer and settler units was made by a centralhole. Each mixer unit was provided with a pump-mixerimpeller made of Teflon. The mixer-settlers were tightlyconnected into three sections of two stages each, one forextraction, one for scrubbing and other for stripping ofzinc. The residence time of each phase in the mixer was5 min, so the flow rate for the feed and solvent streams

SB2

S4

HCl

S5 S6

H2SO4

H2SO4

Scrubbing watere

e zinc solvent extraction process.

Page 5: Simulated Small-scale Pilot Plant Heap Leaching of Low-grade Oxide

698 W.-q. Qin et al. / Minerals Engineering 20 (2007) 694–700

was 120 mL/min in the extraction section, and 30 mL/minfor the stripping solution stream in the stripping section.

When the zinc loaded organic was stripped by sulfuricacid, zinc transfers from organic phase to aqueous phaseand iron(III) remains in the organic phase, which is directlystripped by concentrated hydrochloric acid in two-stages.After the iron(III) is removed, the organic phase is againstripped by sulfuric acid in two-stages. A similar mixer-set-tler cascade was used for stripping. It was composed ofbox-type mixer-settlers that were connected into two sec-tions of two stages each, one for hydrochloric acid strip-ping and other for sulfuric acid stripping. The activevolume of one mixer-settler was 6.0 L while the ratio ofthe mixer and settler volumes was 1:4.

Table 3Chemical components of leach liquor (leach time 1 day)

Composition Zn Fe SiO2 CaO MgOContent/(g/L) 32.57 0.38 2.84 1.17 0.60Composition Cu Cd Ni Sb AsContent/(mg/L) 84.34 0.85 0.02 0.16 3.01

Table 4Chemical components of leach liquor (leach time 16 days)

Composition Zn Fe SiO2 CaO MgOContent/(g/L) 12.36 0.22 2.45 1.00 0.43Composition Cu Cd Ni Sb AsContent/(mg/L) 82.43 0.45 0.02 0.11 2.84

3. Results and discussion

3.1. Column leaching of zinc oxidized ores

In these experiments, the column was set up initiallywith pH 1.2 medium, but no pH control was exercised dur-ing the experiment. And the leach liquor was recycled with-out adding compensating acid after the solvent extractionprocess. After about 16 days, solution pH level for theexperiment was typically about pH 1.6. Zinc extractionfrom the column was determined from solution assays,mass of concentrate filled and the initial head grade, as wellas on the basis of assays of the residual solids. The extrac-tions of zinc, iron and silica as a function of time are shownin Fig. 4a and b, respectively. The results of column leach-ing are shown in Fig. 4. In the 16th day (cycle) the recoveryof zinc was above 95% (Fig. 4a).

It can be seen in the 16-cycle closed circuit leaching, thatthe pH of solution where less iron and silica were extractedwas from pH 1 to 2. The recovery of iron and silica were0.6% and 2.4%, respectively.

From Fig. 1, some leach liquor was recovered by solventextraction, and the raffinate was recycled back to leaching.So the zinc extraction % values become lower. The chemi-cal components of the leach liquor were analyzed by anatomic absorption spectrophotometer (Japan, Shimadzu

0 2 4 6 8 10 12 14 160

20

40

60

80

100

Rec

over

y/%

Leaching time/d

Fig. 4. Column leaching of zinc oxide ores (a) % rec

AA-6800). The results are shown in Tables 3 and 4 respec-tively, when the leach time is one or 16 days. The zinc con-tent was between 12 g/L and 33 g/L, and iron; calcium andsilica content were lower than 0.40, 1.20 and 3.00 g/Lrespectively. The other impurities such as copper, cad-mium, nickel, antimony and arsenic, etc., which are delete-rious to zinc electrowinning, were at very low level. Hence,the leach liquor obtained from column leaching was suit-able to the next process.

Compared with agitation leaching, the leaching time ofcolumn (heap) leaching is longer. In this case the optimumrecovery of zinc was observed after 10 days. The reasonsare, the particle size of ores is relatively larger and the acid-ity of lixiviant is moderate, which decreases the speed ofleaching. However, heap leaching of zinc oxide ores hassome advantages, such as low acid consumption and lowdissolved impurities. Above all, the filtration process isnot needed in heap leaching, which is free from the negativeeffect of silica on solid liquid separation. In the leaching cir-cuit, the accumulation of iron, silica and other impuritieswas not observed. This is probably because of the absorp-tion in the accumulated ores. In fact, the pH value of thesolution soaked into the ores particles increases while theleaching reaction occurs, which results in iron and silicaprecipitating in the residues.

0 2 4 6 8 10 12 14 160.0

0.5

1.0

1.5

2.0

2.5

3.0

SiFeR

ecov

ery/

%

Leaching time/d

overy of zinc; (b) % recovery of iron and silica.

Page 6: Simulated Small-scale Pilot Plant Heap Leaching of Low-grade Oxide

W.-q. Qin et al. / Minerals Engineering 20 (2007) 694–700 699

3.2. Extraction and stripping

The results for the extraction of various metals with 10%D2EHPA are shown in Fig. 5.

At the equilibrium pH values about 2.0, the %zinc andcalcium extraction were 90% and 70%, respectively, whileboth of the %copper and cadmium extraction were lessthan 5%, and the %cobalt and nickel extraction were neg-ligible. Therefore, it may be seen that zinc is easily sepa-rated from copper, cadmium, cobalt and nickel at anequilibrium pH value of 2.0. However, zinc cannot be eas-ily separated from calcium. At an equilibrium pH value oflow than 1.5, Ca concentration will be around its satura-tion value under the conditions in the liquor owing to pre-cipitation of gypsum on the bed. Low calcium wasextracted than zinc. Fortunately, calcium at less concentra-tion has no effect on zinc electrowinning.

The solvent extraction process is shown in Fig. 3. Oneextraction process was carried out in each leaching circuit,and about 30 L leach liquor was used in each extractionprocess. The data for a closed loop leaching–extraction cir-cuit are shown in Table 5. The pH value of the extractionsolution was less than 2.0, after zinc extraction the aqueouspH value decreases to about 1.0, so the raffinate can be

1 2 4 63 50

20

40

60

80

100

Zn Ca Cu Cd Co Ni

Ext

ract

ed /

%

Equilibrium pH

Fig. 5. D2EHPA pH-extraction isotherms for six elements at 25.

Table 5Closed loop of leaching–extraction circuit

Cycle pH q(Zn)/g/L

Leach liquor Raffinate Leach liquor

1 1.84 0.98 32.572 1.80 0.97 28.623 1.70 0.98 24.874 1.75 0.96 22.495 1.60 1.00 22.206 1.82 0.98 24.807 1.63 0.99 22.798 1.68 0.98 21.239 1.55 1.05 18.1810 1.63 1.03 17.8711 1.58 0.97 16.0212 1.49 0.98 14.9413 1.56 0.97 13.6414 1.70 0.96 13.9515 1.55 1.00 12.6316 1.60 0.98 12.36

recycled to leaching without the addition of acid, whichreduces the acid consumption.

The zinc concentration of the leach liquor in the firstleaching cycle was 32.57 g/L. From Fig. 1, some of theleach liquor was treated by solvent extraction, and the raff-inate is recycled back to leaching section. After the zincextraction operation, a amount of zinc (about 8.27 g/l) isnot extracted and remains in the raffinate when an O/Aratio close to 1.5 is chosen. So the zinc concentration ofleach liquor was decreased to 12.36 g/L in the 16th cycle.

The co-extraction of impurity metals with D2EHPA isinevitable particularly at high concentrations. Thereforeto obtain a pregnant solution with low contaminationthe loaded organic phase must be scrubbed to removeimpurities.

The scrubbing solution was zinc sulfate solution at a pHvalue of 1.5. To maintain the water balance, a high phaseratio (O/A) was necessary. It was found that the co-extracted calcium and the entrainment of silica in loadedorganic can be removed by scrubbing, and a phase ratio(O/A) of 10:1 was acceptable. Iron cannot be removed byscrubbing.

Hence, zinc was separated from iron by selective strip-ping. The stripping is the reverse process of extraction, inwhich zinc transfers from organic phase to aqueous phase,the equation is as follow:

ZnR2 � 2HRorg þ 2Hþaq () 2H2R2ðorgÞ þ Znþaq ð4Þ

Hence the aqueous phase in stripping must be of highacidity to keep the equilibrium move rightwards. The zincloaded organic phase contained iron(III), and minim ofcalcium and silica. The percentage stripping increased withthe increase of acidity. It was found the iron in the loadedorganic phase was hardly stripped by 1.53 mol/L sulphuricacid. When the zinc loaded organic was stripped by sulfuricacid, zinc transfers from organic phase to aqueous phasewhile iron(III) remains in the organic phase.

Recovery of zinc (%) Extraction of zinc (%)

Raffinate

22.15 30.51 32.0019.99 36.51 30.1417.40 41.14 30.0015.36 45.91 31.7015.65 51.94 29.5017.36 58.98 30.0016.34 63.99 28.0015.24 68.56 28.2013.36 71.32 26.5012.69 75.55 29.0011.05 78.67 31.0010.76 82.31 28.009.71 85.01 28.809.54 88.98 31.608.80 91.88 30.308.27 95.21 33.10

Page 7: Simulated Small-scale Pilot Plant Heap Leaching of Low-grade Oxide

Table 6Chemical components of pregnant solution q/(mg/L)

Zn Fe Cu Mn Co Cd CaO MgO68000.00 0.30 0.03 25.00 0.03 0.11 0.48 0.13Ge F Cl Ni Sb As SiO2 H2SO4

0.02 <5.00 <10.00 0.09 0.02 0.04 0.18 150.00

700 W.-q. Qin et al. / Minerals Engineering 20 (2007) 694–700

Though the iron concentration in the leach liquor waslow, it accumulated in the recycle organic, which decreasedthe loading capacity of the extractant. Hence the recycleorganic must be treated periodically to remove the iron(III).The conventional method is that the Fe3+ is directlystripped by strong hydrochloric acid. It was found that astrong acid solution (6 mol/L) was required to strip Fe3+

from D2EHPA (Gu et al., 2000).The long run test indicated that the performance of

organic phase is stable in the recycle process and neitheremulsion nor third phase formation was observed in thesolvent extraction process.

The chemical analysis of the pregnant zinc sulfate pro-duced from solvent extraction is shown in Table 6. Zincelectrowinning is dependent on high hydrogen over poten-tial on the zinc metal; a small quantity of impurities canresult in the decrease of this hydrogen overpotential, whichdecreases the current efficiency and the quality of cathodezinc. The impurities in the electrolyte were at levels as fol-lows (mg/L): Cu < 0.10, Ni < 0.30, Co < 0.30, Fe < 10.00,Cd < 0.30, Sb < 0.03, As < 1.00, Ge < 0.03, Sn < 0.10, theelectrolyte is suitable to electrowinning (Peng, 1992; Yangand Liu, 1988). It can be seen that the impurities in thepregnant solution are less than the above standards.

4. Conclusions

This leaching-solvent extraction study has indicated thepotential to selectively extract Zn from a low-grade zincoxide ore. The zinc concentration in leach liquor in the firstleaching–extraction circuit was 32.57 g/L, and in the 16thcycle the zinc concentration was 8.27 g/L after solventextraction. The pregnant zinc sulfate solution obtained mustbe suitable to electrowinning. Therefore, the leach liquorwas subjected to solvent extraction with D2EHPA asextractant, scrubbing and selective stripping for the enrich-ment of zinc and the removal of impurities. Fig. 1 shows theconceptual flow sheet for treating a typical a low-grade zincoxide ore using the process described in this paper.

Acknowledgements

This work was aided by the National Natural ScienceFoundation of China (Project No. 50321402) and NationalBasic Research Program of China (2004CB619205).

References

Abdel-Aal, E.A., 2000. Kinetic of sulfuric acid leaching of low-grade zincsilicate ore [J]. Hydrometallurgy 39 (2), 247–254.

Amer, S., Figueiredo, Luis, A., 1995. The recovery of zinc from the leachliquors of the CENIM-LNETI process by solvent extraction withdi(2-ethylhexyl)phosphoric acid. Hydrometallurgy (37), 323–337.

Bart, H.H., Marr, R., Scheks, J., Koncar, M., 1992. Modelling of solventextraction equilibria of Zn(II) from sulfate solutions with bis(-2-ethylhexyl)phosphoric acid. Hydrometallurgy (31), 13–28.

Bodas, M.G., 1996. Hydrometallurgical treatment of zinc silicate ore fromThailand [J]. Hydrometallurgy 35 (1), 37–49.

Chen, Shi-ming, Qu, Kai-liu, 1998. On the treatment of oxidized zinc orein lanping [J]. Yunnan Metallurgy 27 (5), 31–35.

Choi, W.K., Torma, A.E., Ohline, R.W., 1993. Electrochemical aspects ofzinc sulphide leaching by Thiobacillus ferrooxidans. Hydrometallurgy33 (1), 137–152.

Doepker, R.D., O’Connor, W.K., 1990. Column leach study II.Heavy metal dissolution characteristics from selected lead–zinc minetailings. In: Proceedings of the Western Regional Symposium onMining and Mineral Processing Wastes, May 30, Berkeley, CA, USA,pp. 69–80.

Duan, Xiu-mei, Lou, Lin, 2000. Review on present situation of theflotation of oxidized zinc ore [J]. Mining and Metallurgy 9 (4), 47–51.

Forrest, C., Hughes, M.A., 1978. The separation of Zn from Cu byDZEHPA – an equilibrium study. Hydrometallurgy (3), 327–342.

Gu, H., Chang, C.-M., et al., 2000. Preliminary design of a solventextraction processing for the galvanic stripping of iron from D2EHPA[J]. Mineral and Metallurgical Processing 17 (1), 16–22.

Hsu, C.H., Harrison, R.G., 1995. Bacterial leaching of zinc and copperfrom mining wastes. Hydrometallurgy 37 (2), 169–179.

Kongolo, K., Mwema, D.M., Kyony, P.M., Mfumu, K., 2000. Manganeseand zinc removal from cobalt sulphate solutions by means ofsolvent extraction. In: Proceedings of the 21st IMPC, July 23–27,Rome, Italy.

Kongolo, K., Mwema, M.D., Banza, A.N., Gock, E., 2003. Cobalt andzinc recovery from copper sulphate solution by solvent extraction.Minerals Engineering 16 (12), 1371–1374.

Mansur, M.B., Slater, M.J., Biscaia Jr., E.C., 2002. Equilibrium analysisof the reactive liquid–liquid test system ZnSO4/D2EHPA/n-heptane.Hydrometallurgy (63), 117–126.

Mellah, A., Benachour, D., 2006. The solvent extraction of zinc andcadmium from phosphoric acid solution by di-2-ethyl hexyl phospho-ric acid in kerosene diluent. Chemical Engineering and Processing (45),684–690.

Peng, Rong-qiu, 1992. Nonferrous Metal Extraction Metallurgy Manual-Zn, Cd, Pb, Bi [M]. Metallurgical Industry Press, Beijing.

Pinipenko, A.T., 1974. Use of diphenyldabiocarbazone (dithizone) inanalysis. III. Dissociation of thallium and cadmium dithizonate andzinc dithizonate. Zhur. Anal. Khim. (5), 14–20.

Qin, Wen-qing, Lan, Zhuo-yue, Li, Weizhong, 2003. Selective extractionof zinc from sulfate leach solution of zinc ore. Trans. Nonferrous Met.Soc. China 13 (6), 1435–1439.

Qiu, Guan-zhou, Wu, Bo-zeng, Qin, Wen-qing, Lan, Zhuo-yue, 2002.Bioleaching of marmatite in high concentration of iron. Trans.Nonferrous Met. Soc. China 12 (6), 1435–1439.

Qiu, Guan-zhou, Qin, Wen-qing, Lan, Zhuo-yue, Wu, Bo-zeng, 2003.Elective culture of bacteria in bioleaching on pyrrhotite. Trans.Nonferrous Met. Soc. China 13 (1), 175–179.

Rice, N.M., Smith, M.R., 1975. The recovery of zinc, cadmium andmercury(II) by solvent extraction. J. Appl. Chem. Biotechnol. 25, 379–402.

Sato, T., Kawamura, M., Ueda, M., 1978. The extraction of divalentmanganese, iron, cobalt, nickel copper and zinc from hydrochloric acidsolutions by di(-2-ethylhexyl)phosphoric acid. J. Appl. Chem. Bio-technol. (28), 85–94.

Tang, Mo-tang, Ouyang, Min, 1998. Preparation of grade zinc oxide byusing ammonium sulfate and ammonia complex leaching process.Zhongguo Youse Jinshu Xuebao/Chinese, Journal of NonferrousMetals 8 (1), 118–121.

Yang, Jiao-yong, Liu, Da-xing, 1988. Extraction [M]. Metallurgy IndustryPress, Beijing.