Preparation of high purity zinc from zinc oxide ore by vacuum carbothermic reduction

Accepted Manuscript Preparation of high purity zinc from zinc oxide ore by vacuum carbothermic reduction L.Z. Xiong, Y.H. Xiang, X.W. Wu, Z.Q. He, Z.L. Yin PII:

S0042-207X(17)30832-1

DOI:

10.1016/j.vacuum.2017.09.050

Reference:

VAC 7626

To appear in:

Vacuum

Received Date: 15 July 2017 Revised Date:

30 September 2017

Accepted Date: 30 September 2017

Please cite this article as: Xiong LZ, Xiang YH, Wu XW, He ZQ, Yin ZL, Preparation of high purity zinc from zinc oxide ore by vacuum carbothermic reduction, Vacuum (2017), doi: 10.1016/ j.vacuum.2017.09.050. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Preparation of High Purity Zinc from Zinc Oxide Ore by Vacuum Carbothermic Reduction1 L.Z. Xionga, b,c, Y.H. Xiang c, X.W. Wuc, Z.Q. He b,c,*, Z.L. Yina,** School of Chemistry and Chemical Engineering, Central South University, Changsha,

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a

Hunan, 410083, china b

College of Materials & Chemical Engineering, Hunan City University, Yiyang, Hunan,

c

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413000, China

The Collaborative Innovation Center of Manganese-Zinc-Vanadium Industrial Technology

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(the 2011 Plan of Hunan Province), Jishou, Hunan, 416000, China

Abstract: Effects of the yield and quality of Zn were investigated by vacuum carbothermic reduction from zinc oxide ore. Results indicate that initial moles ratio of C/ZnTotal, pressure of system, reaction time affect significantly the Zn yield from zinc oxide ore, while sintering

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temperature and sintering time have important influences on the quality of Zn. The optimal operating conditions are obtained as follows: the sintering temperature 973 K, the sintering time 50 mins, the molar ratio of C/ZnTotal 2.5, the reaction temperature 1173 K, the pressure system lower than 20 kPa, the reaction time 50 mins. Under the optimal conditions, the yield and purity of

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Zn are more than 95% and 99.995% respectively, suggesting the vacuum carbothermic reduction

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is a promising method to extract high-purity zinc from zinc oxide ore.

Keywords: Vacuum; Carbothermic reduction; Zinc oxide ore; Zinc yield; High-purity zinc;

Metallurgy

1 Introduction With the expanding of the application scope of zinc, zinc consumption increases rapidly, resulting in a shortage of zinc sulphide increasingly. Therefore, how to develop and utilize zinc 1

*Corresponding author. Tel./fax: +86 743 8564416. **Corresponding author. Tel./fax: +86 731 88879616. E-mail addresses: [email protected] (Z. He), [email protected] (Z. Yin) 1

ACCEPTED MANUSCRIPT oxide ore effectively has attracted more and more attention[1]. Zinc oxide ore is a refractory ore whose mineral composition is complex and mineral disseminated grain size is fine power[2-3]. There are two main methods to deal with zinc oxide ore. One is hydrometallurgical technology, such as acid leaching, alkaline leaching, ammonia leaching[4]. The processes have their own

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advantages. But there are some technical difficulties in the processes, such as many impurities in leaching liquor, the complex process of purification, the difficult technology of solid-liquid separation[5]. The product is electrolytic zinc whose purity is about 99%[5-7]. The other one is the pyrometallurgical technology. The pyrometallurgical route is gradually becoming less attractive on

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account of many links, long process flow, high energy consumption and heavy pollution[8]. The product is zinc oxide powder (the purity is under 95%) or zinc (the purity is lower than

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99%)[9-11]. The main methods of purification for zinc are electrolytic refining, vacuum distillation, zone melting and so on[12-16]. The purity of product is higher than 99.995%, while the raw material with purity higher than 99.96% zinc is needed[17-19]. Vacuum metallurgical technology has high-efficiency on improving metal recovery, shortening process flow and thus meets the environmentally-friendly and resource-saving requirements[20-24]. Vacuum distillation

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is a good method for the purification of metals[13,25-26]. Zinc oxide ore has lots of association with other metals such as Pb, Cd. The physicochemical properties of Pb, Cd are similar to Zn, so Pb, Cd as impurities usually enter the product of Zn from zinc oxide ore. Zinc has the lower

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boiling point (907 ) and thus is distilled easily. Therefore, vacuum carbothermic reduction which can be operated at lower temperatures could be an energy-saving method to evapourate zinc from

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the oxide ore. The metals of Zn, Pb and Cd have different boiling points, and they have their own vapour pressure and saturated vapour pressure at same temperature. In the present work, a new process in the particular vacuum furnace is used to prepare high-purity zinc from zinc oxide ore by vacuum carbothermic reduction. The particular vacuum furnace has condensator with many condensate pans whose the temperature is inequable. In the process, the after-heat is used to the full to separate zinc and the other metal by the particular vacuum furnace. So the high purity zinc and other metal are collected as condensate in the condensator at different condensate pans by the process. So the process has shorter process flow, high energy ratio, high metal recovery and little enviroment pollution, and can produce high value-added products.

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ACCEPTED MANUSCRIPT 2 Experimental 2.1Experimental materials Zinc oxide ore from Lanping County of Yunnan province was used in the experiments. Table 1 and Table 2 illustrate the chemical content and phase compositions of the original mineral. Table

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1 and Table 2 show that the content of zinc is 33.46% and the major mineral is smithsonite (ZnO) in the original mineral. Coking coal from Douli Mountain Town of Lianyuan City, Hunan province in China, is used not only as reductant but also as binder in this study. The chemical composition of the coal is shown in Table 3. As shown in Table 3, the content of carbon is 70.28%

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in the coking coal used. 2.2 Experimental facilities

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The reduction reaction was carried out in an experimental setup designed by ourselves. It is composed of a vacuum furnace with classification condensation (Fig.1), a temperature control, a vacuum acquisition and a gas-supply system. 2.3 Experiment process

The experiment process is described as follows:

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1)Zinc oxide ore and Coking coal were sifted through a 0.214 mm（70 meshes）sieve. Zinc oxide ore, coking coal and water were well mixed according to the specified proportion that water accounts for 12 per cent, the mass of zinc oxide ore and coking coal is followed initial moles ratio

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of C/Zntotal in Table 4, then the cylindrical samples are manufactured through compacting and pressure working by special mould with a height and radius of about 10 mm, respectively.

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2)The samples were dried at 100℃ for 2h. 3)The dried samples were sintered in the vacuum furnace in order to ensure their strength.

The sintering process can be described as follows: At first, the vacuum furnace was sealed after loading the samples and quickly heated to a certain sintering temperature. Then, the vacuum furnace was maintained at the temperature for a certain sintering time and vacuum acquisition system began to work. At last, the heating system was cut out. When the temperature of the vacuum furnace was below 50℃, the furnace lids were opened and the condensate pans were replaced. 4)The vacuum furnace was sealed again and quickly heated to the experimental temperature at the required pressure for reaction time. 3

ACCEPTED MANUSCRIPT 5)Vapour of zinc and other volatile substances enter into the condensator while Zn and other products were collected in the condensator at different condensate pans. 6) After cooling for a few hours, the products and slag were weighed and detected. The relevant experimental conditions are listed in Table 4.

Zinc yield, YZn, at t = t can be calculated according to Eq. 1. t =0 t =t − mslt =t × C Zn msat =0 × CZn ] ×100% t =0 msat =o × CZn

t =0

Where t is the reaction time, min; msa

(1)

t =t

represents mass of the samples at t=0 , g；msl is the

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YZn = [

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2.4 Calculation of Zn yield and analysis of content of impurities in product Zn

t =0

t =t

mass of slag at t=t, g; CZn is content of Zn in the samples at t=0, g·g-1, CZn is the content of Zn

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in slag at t=t, g·g-1.

The content of Zn in the samples was analyzed according to GB/T 8151.1-2000. Due to the very low content of Zn, ICP-OES (Intrepid II xsp, American hermo Electron Corporation ) were used to determine the Zn content in the slag.

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The content of impurities in product of Zn was determined by ICP-MS[27] according to GB/T 470-2008 that the impurities content of Pb, Cd, Fe, Cu, Sn and Al require analysis in purity of Zn product from 99.95% to 99.995%. The purity of Zn product, PZn is calculated according to Eq. 2.

PZn = 100% − ∑ Ci

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(2)

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Where Ci is the content of impurities, wt %; i is impurity element in product of Zn. 3 Results and discussions

3.1 The influence of various factors on the zinc yield The results of the initial molar ratio of C/ZnTotal on the zinc yield are shown in Fig.2. The

higher the initial molar ratio of C/ZnTotal, the higher the zinc yield gets. The samples with higher initial molar ratio of C/ZnTotal have higher carbon content, and the surface contact between C and ZnO will strengthen, which is benificial to the reduction reaction of ZnO [28-31]. When the initial molar ratio of C/ZnTotal surpasses 2.5, the zinc yield enhances slowly. To reduce the power consumption and production costs, the initial molar ratio of C/ZnTotal should be nearly 2.5. The effect of the system pressure is presented in Fig.3. It can be seen from Fig.3, the change 4

ACCEPTED MANUSCRIPT of the zinc yield may be ignored when the system pressure is in the range from 10 Pa to 2.0 k Pa. This may be due to the existence of a critical pressure for Zn evaporation at certain temperature and the rate of evaporation does not change with the increasing or decreasing system pressure when the system pressure is less than the critical pressure[20, 32]. However, the higher the system

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pressure, the lower the zinc yield is obtained over 2.0 k Pa. As the system pressure becomes higher, the diffusion resistance of Zn vapor increases in the samples, resulting in a lower yield of zinc[20].

Fig.4 shows the effect of reaction time on the zinc yield from 10 mins to 70 mins. It can be

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seen that the zinc yield increases with the prolonging of reacting time, but the growth rate of zinc yield gets less with reaction time from 50 mins to 70 mins. In order to reduce energy consumption

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and production costs, the suitable reaction time is 50 mins.

Experiments are done at four different temperatures from 1073 K to 1223 K for 50 mins, and the results are shown in Fig.5. It is found that the increase in reaction temperature enhances the zinc yield, but when the reaction temperature is higher than 1173 K, the zinc yield is raised slightly. The suitable temperature is 1173 K. The reaction temperature of extraction of zinc is

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1303-1423 K from zinc oxide[29-31, 33] and is about 1303 K from zinc sulfide with additives by the non-vacuum carbothermic reduction [31, 34-35]. The reaction temperature of the non-vacuum carbothermic reduction is 130-350 K higher than the present work. In summary, the vacuum

oxide ore.

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carbothermic reduction technology may be a more energy efficient route to produce Zn from zinc

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3.2 The influence of various factors on the zinc quality The content of impurities in product of Zn was analyzed using ICP-MS and the results were

given in Table 5. From Table 5, the purity of Zn was about 99.95%, the content of Cd has reached the standard of 99.95% high-purity zinc, while that of Pb has reached to the standard of 99.995% zinc according to GB/T 470-2008. The other impurities content of Fe, Cu, Al, Sb and Sn have reached the standard of 99.999% high-purity zinc according to YS/T 920-2013. Therefore, the impurity of Cd must be removed to improve the quality of Zn. Fig.6 shows the effect of sintering temperature on content of Cd with sintering time of 30 mins. Obviously, the higher the sintering temperature is, the lower the content of Cd gets in the product Zn, because Cd has the lower boiling point (767 ) and can volatilize during sintering the 5

ACCEPTED MANUSCRIPT samples, the content of Cd is decreased with the rise of sintering temperature. However, the strength of samples will reduce seriously when the sintering temperature is higher than 973 K[36]. According to experimental results 973 K is the suitable sintering temperature. The effect of sintering time on content of Cd is depicted in Fig.7. With the increase of the

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sintering time, the content of Cd in the product Zn reduces significantly. However, the content of Cd changes little when the sintering time from 50 mins to 60 mins. Therefore, the sintering time is controlled in 50 mins.

Fig.8 shows the effect of reaction time from 40 mins to 70 mins on the content of Cd in

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product Zn. It can be seen from Fig.8 that reaction time has a negligible influence on the content of Cd that is due to high volatilizing of Cd at the reaction temperature.

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Fig.9 shows the effect of reaction temperature on the content of Cd in product Zn. The higher the reaction temperature is, the lower content of Cd in product Zn. The product Zn is condensate of Zn vapour which is reduced by carbon from the samples in the vacuum furnace with condensator. The temperature of condensator is far lower than reaction temperature, the temperature of each condensate pan is inequable and constantly lowered from bottom to top. The

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temperature of condensate pan at the top is lower than 300 K in the experiment. Schematic diagram of condensation process of vapours in the classification condensation of the vacuum furnace is shown in Fig.10. Vapours generated from samples by carbothermic reduction enter into

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condensator during experiment. The vapour of metal can be condensed if the vapour pressure is greater than the saturated vapour pressure of the metal. Because the temperature of condensator is

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constantly lowered, the vapour pressure of metal will be higher than Saturated vapour pressure which decreases with temperature then the metal is condensed at some condensate pans. Saturated vapour pressures of metals are different at same temperature, such as Zn, Cd, Pb. Saturated vapour pressures of Zn is higher than that of Pb but lower than that of Cd, hence Pb is first condensed, second Zn, third Cd at different condensate pan. So Zn, Cd, Pb can be separated from another. In our experiment, Pb was collected at 873K-1273K, Zn at 673K-873K, and Cd at 473K-673K. Moreover the condensate pan is not only place of condensed matter but also of volatilized substance. The higher the reaction temperature is, the higher the temperature of condensate pan. With the reaction temperature increasing, it help to volatilize of Cd at condensate pan, so the content of Cd in product Zn decreases. The content of Cd in product is more than 80 µg/g when 6

ACCEPTED MANUSCRIPT the reaction temperature is 1073K. However, the value of Cd content clearly decreases to about 40 µg/g when the temperature increases to 1123K, but the content of Cd is slightly reduced from 1123 K to 1223 K, especially from 1173K to 1223K. Therefore, 1173K is chose as the reaction temperature.

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3.3 The content of impurities in product Zn The samples with the initial molar ratio of C/ZnTotal 2.5 were sintered at 973 K for 50 mins in 10 Pa in the vacuum furnace, then reacted at 1173K for 50 mins by the vacuum carbothermic reduction. The content of impurities in product of Zn was detected and the result was showed in

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Table 6.

As shown from Table 6, the purity of the product Zn was about 99.995% which is higher

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purity than that of other pyrometallurgical process of zinc oxide which is less than 99% [9-11, 37]. All of the impurities of Fe, Cu, As, Cd, Al, Sb, Sn and Pb have reached the standard of 99.995% high-purity zinc according to GB/T 470-2008, indicating the vacuum carbothermic reduction is a good route to obtain high-purity zinc under the given experimental conditions. 4 Conclusion

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Initial moles ratio of C/ZnTotal, pressure of system, reaction time are the main influence factors on the zinc yield from zinc oxide ore and the sintering temperature and time have important influences on the quality of product Zn. The optimal conditions are controlled as

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follows: the sintering temperature 973 K, the sintering time 50 mins, the molar ratio of C/ZnTotal 2.5, the reaction temperature 1173 K, the pressure system lower than 20kPa, the reaction time 50

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mins. The zinc yield is about 95% from zinc oxide ore and 99.995% high-purity zinc is obtained at the optimal conditions. Acknowledgement

The authors are deeply grateful to the National Natural Science Foundation of China

(51672104, 51364009, 51472107), Natural Science Foundation of Hunan Province, China (2017JJ2216), China Post-doctoral Science Foundation (2015M572262), Postdoctoral Science Founded of Central South University (149930), the Construct Program of the Key Discipline in Hunan Province, China (JSU0713), the Aid Program for Science and Technology Innovative Research Team in HigherEducation Institutions of Hunan Province ([2014]207), for their financial support. 7

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[13]K.F.L. Raymond, R.M. Danil, US patent 5698158, 1997. [14]M. Kanda, E. Kimura, JP8295957,1996.

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[16]H. Nagasaki, JP58171536,1983.

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ACCEPTED MANUSCRIPT [27]J.J. Ge, J. Liu, Metallurgical Analysis. 36(9)(2016)37-41. [28]L.Z. Xiong, Q.Y. Chen, Z.L.Yin, P.M. Zhang, the Chinese Journal of Process Engineering. 10(1)(2010)133-137. [29]C.H. Huang, C.I. Lin, H.K. Chen, Journal of the Chinese Institute of Chemical Engineers.

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[37]Editoria board of metallurgy of lead and zinc, Science Press, China. 2003.

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ACCEPTED MANUSCRIPT Table 1 The chemical composition of the original mineral, wt % Element

Zn

Pb

Cd

Mg

Fe

S

O

Si

Mn

Ca

Content

33.46

0.09

0.432

0.201

17.72

0.584

25.1

2.24

1.306

1.51

Table 2 The phase compositions of the original mineral, wt % ZnO

ZnS

Zn2SiO4·H2O

ZnO·Fe2O3

Total Zinc

Zinc content

32.29

0.11

0.65

0.41

33.46

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Phase

Table 3 The chemical composition of coking coal, wt% Moisture in

Ash in

Volatile in

Fixed carbon in

Total sulphur as

as received, Mt

air-dried basis,

air-dried

air-dried basis,

air-dried basis,

received, S (%)

(%)

Mad (%)

basis, Aad (%)

Vad (%)

FCad (%)

0.50

1.51

6.39

21.82

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Total moisture

70.28

0.75

Variable Sintering time/min Sintering temperature/K Initial moles ratio of C/Zntotal Pressure of system/kPa Reaction time/min Reaction temperature/K

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Table 4 Operating variables in the vacuum carbothermic reduction experiments Values

20, 30, 40, 50*, 60 823, 873, 923, 973* 0.5, 1, 1.5, 2, 2.5*, 3 0.01*, 0.5, 1.0, 1.5, 2.0, 2.5, 3.5 10, 20, 30, 40, 50*, 60, 70 1073, 1123, 1173*, 1223

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Note: underlined value is standard operating variable before the single factor test. “*” is standard operating variable after the single factor test

Table 5 The chemical composition of the previous product of Zn, wt % Fe

Cu

As

Cd

Sb

Al

Pb

Sn

Zn

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Element

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Content 0.00020 0.00024 0.00008 0.02832 0.00001 0.00009 0.00274 0.00001 99.96832

Table 6 The chemical composition of the later product of Zn, wt %

Element

Fe

Cu

As

Cd

Al

Pb

Sn

Zn

Content 0.00014 0.00007 0.00001 0.00270 0.00007 0.00180 0.00004 99.99537

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Fig.1 Schematic diagram of the vacuum furnace with classification condensation

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1-Furnace lids, 2-Cooling water outlet, 3-Condensator, 4-Furnace shaft, 5-Thermocouple, 6-Heat shield, 7-Cooling water inlet, 8-Graphite crucible, 9-Graphite heater, 10-Electrode, 11-Furnace bottom, 12-Condensate pan, 13-To vacuum pump

90

Zinc yield, YZn(%)

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80

70

60

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50

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40

0.5

1.0

1.5

2.0

2.5

3.0

Initial moles ratio of C/Zntotal

Fig.2 Effect of the initial molar ratio of C/Zntotal on the zinc yield

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Zinc yield, YZn(%)

89

82 81 80

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Pressure of system (kPa)

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Fig.3 Effect of system pressure on the zinc yield

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100

Zinc yield, YZn(%)

90

80

70

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60

50

10

20

30

40

50

60

70

Reaction time(min)

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Fig.4 Effect of reaction time on zinc yield

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100

Zinc yield, YZn(%)

80

60

40

20

0

1073

1123

1173

1223

Reaction temperature(K)

Fig.5 Effect of reaction temperature on zinc yield

160 140 120 100 80 60 40

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The content of Cd of product Zn (µg/g)

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20 0

823

873 923 Sintering temperature(K)

973

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100 80

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The content of Cd of product Zn (µg/g)

Fig.6 Effect of sintering temperature on content of Cd in product Zn

60 40 20 0

20

30

40

50

60

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sintering time(min)

30 25

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The content of Cd of product Zn (µg/g)

Fig.7 Effect of sinter time on content of Cd in product Zn

20

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15 10 5 0 35

40

45

50

55

60

65

70

75

Reaction time(min)

Fig.8 Effect of reaction time on content of Cd of product Zn

80

60

40

20

0 1073

1123

1173

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The content of Cd of product Zn (µg/g)

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1223

Reaction temperature(K)

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Fig.9 Effect of reaction temperature on content of Cd of product Zn

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Fig.10 Schematic diagram of condensation process of vapours in the classification condensation

ACCEPTED MANUSCRIPT Highlights of the paper In this paper entitled "Preparation of High Purity Zinc from Zinc Oxide Ore by Vacuum Carbothermic Reduction", it has many highlights which were as follows:

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1.Preparing high-purity zinc from zinc oxide ore by vacuum carbothermic reduction in an experimental setup designed by ourselves with classification condensation. 2.The reaction temperature is 1173 K which is 150-350℃lower than that of the non-vacuum carbothermic reduction of zinc oxide or zinc sulfide (the addition of additive). 3.The purity of the product Zn was about 99.995% which is higher purity than that of other pyrometallurgical process of zinc oxide. 4.The yield of Zn is more than 95% from zinc oxide ore by vacuum carbothermic reduction.