ferro-reduction of zno using concentrated solar energy

Energy 29 (2004) 745–756

www.elsevier.com/locate/energy

Ferro-reduction of ZnO using concentrated solar energy

Michael Epstein a,�, Koebi Ehrensberger b, Amnon Yogev a

a Solar Research Facilities Unit, Weizmann Institute of Science, Rehovot 76100, Israelb AAC Infotray AG, Winterthur, Switzerland

Abstract

In recent years, the production of zinc from its oxide using solar energy has been attracting increasinginterest. This is a promising process for the conversion of solar radiation to its chemical form and stor-age. When carbon is used as a reducing agent, the main product gases are zinc, CO, and CO2. A majorproblem is the need for rapid cooling and separation of the zinc to avoid back reaction and reoxidation.In industry, lead splash condenser is used to remove the zinc vapor rapidly. The lead is circulated out ofthe condenser and chilled, so that the solubility of the dissolved zinc is reduced and part of the moltenzinc is separated and floats on the lead. Because lead is capable of dissolving only a small amount of zinc,the amount of lead to be circulated is about 400 times as much as the amount of produced zinc. Thistechnology is complicated and cumbersome. This paper describes a new approach for two-step processfor the reduction of ZnO that can potentially solve this problem. The two-step process can be charac-terized by the following equations:

ZnOðsÞ þ FeðlÞ ! ZnðgÞ þ FeOðlÞ; DH0 ¼ 177 kJ=mol;

FeOðlÞ þ CðsÞ ! FeðlÞ þ COðgÞ; DH0 ¼ 153 kJ=mol:

Both steps are endothermic, require temperatures in the range of 1300–1600vC, and can be carried out

using concentrated solar energy. In the first step, iron reduces ZnO and Zn vapors are distilled out (zincis miscible in iron, but at the relevant temperatures, and at atmospheric pressure it volatilizes). In thesecond step, carbon is injected into the FeO melt and reduces it back to iron. The CO obtained in thesecond step is separated from the zinc vapors. Basically, this is a gasification process. The carbon is con-verted to CO. When using coal, the ashes form slag on the surface of the melt and can be removed. Theadvantages of this process compared to the direct carboreduction are: (i) high rates of heat and masstransfer mechanisms between the iron melt and the ZnO powder; (ii) avoiding the necessary preparationof the feed as required in the direct process, mixing ZnO and carbon in a measured proportion, prep-aration of briquettes; if ZnO is recycled from zinc/air batteries, there is no need for size reduction and, inprinciple, large-size fractions can be processed; (iii) avoiding the major difficulty of separation of the COand providing the possibility for simpler zinc condenser compared to the lead splash condenser. The pro-

� Corresponding author. Tel.: +972-934-3804; fax: +972-934-4117.E-mail address: [email protected] (M. Epstein).

0360-5442/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.doi:10.1016/S0360-5442(03)00181-6

cess has higher thermodynamic gain and higher contribution of solar radiation. This paper analyzes thethermodynamics of the two-step process and compares it to the direct carboreduction of ZnO. The kinet-ics and mechanism of the reactions are discussed. Experimental results with solar energy for the first stepare described. A mixture of ZnO and iron was heated at the Weizmann Institute of Science (WIS) solarfurnace. At 1600

vC, 90% of the theoretical yield was obtained after 5 min of testing. The zinc vapors

were condensed and X-ray diffraction analysis showed very high purity of zinc and crystalline. Thesecond step is known in the literature. Finely divided carbonaceous materials, e.g. coal are injected intothe FeO melt (which floats above the iron as it forms). The coal is dissolved in the FeO and reduces it,thus creating CO gas. The gasification of coal takes place very rapidly owing to the high temperature, thecarbon content of the melt and the mixing created by the evolving gas. A conceptual scheme of the solarreactor is shown.# 2003 Elsevier Ltd. All rights reserved.

1. Introduction

In recent years, the thermal production of zinc from its oxide with the aid of concentrated

solar radiation has been attracting increasing interest. Results of the work on direct decompo-

sition of ZnO at high temperatures [1], reduction of ZnO with CH4 [2,3] and with carbon [4]

have been published in the last few years. Zinc can be reoxidized with steam to generate hydro-

gen [5] or be used in zinc/air batteries. The direct thermal decomposition of ZnO requires very

high temperatures. Only at above 2000vC, a significant yield can be obtained (see Fig. 1 for

equilibrium yield of zinc as a function of the temperature at a pressure of 1 bar absolute) [6].

However, the vapor also contains oxygen, which needs to be separated before the reverse reac-

tion takes place. Quenching of the gases at this temperature is difficult, unless rapidly diluted

with cold inert gas.Reduction of ZnO can be also achieved with zinc sulfide [7]. Zinc sulfide is the main ore

found in nature (sphalerite). It is relatively an inert material, which is unaffected when heated

Fig. 1. Thermodynamic equilibrium compositions of thermal decomposition of ZnO(s) [6].

M. Epstein et al. / Energy 29 (2004) 745–756746

with carbon. The following reaction between sulfide and oxide

ZnSðsÞ þ 2ZnOðsÞ ! 3ZnðgÞ þ SO2ðgÞ; DH01820 K ¼ 918 kJ=mol; (1)

does not take place spontaneously until a temperature of 1547vC (DG ¼ 0) is reached (see Fig.

2 for thermodynamic equilibrium) [6].In addition, the separation of SO2 has to be performed at high temperatures or through a

rapid quenching process. For these reasons, the first stage in the recovery of zinc by all commer-cial processes is the conversion of the sulfide into a more reactive oxide, according to the fol-lowing equation:

ZnSðsÞ þ 1:5O2ðgÞ ! ZnOðsÞ þ SO2ðgÞ; DH01200 K ¼ 431 kJ=mol: (2)

The reduction of ZnO with solid carbon is especially important because the gasification ofcarbon is part of the process. The source of carbon can be different carbonaceous materials,such as coal and biomass. The reaction can be described as follows:

ZnOðsÞ þ CðsÞ () ZnðgÞ þ COðgÞ; DG0 ¼ 352:8 0:29T kJ=mol: (3)

This reaction proceeds in two successive and reversible gas–solid reactions [8], as follows:

ZnOðsÞ þ COðgÞ () ZnðgÞ þ CO2ðgÞ; (4)

CðsÞ þ CO2ðgÞ () 2COðgÞ: (5)

Eq. (5) is the gasification step. The amount of the CO2 in the products is mainly dictated bythis reaction. High temperature of the product vapors leaving the reactor and excess of carbonwill maintain a low content of carbon dioxide. The product gas consists of roughly equal pro-portions of zinc vapor and carbon monoxide, with a small amount of carbon dioxide. As thetemperature of these gases drops and the dew point of this mixture (830

vC) is reached, zinc

starts to condense. This cooling process is complicated by the fact that Eq. (5) proceeds rapidlyin the reverse direction. When this occurs, a large proportion of the condensing zinc will be oxi-dized and the excess ‘blue powder’, a dust consisting of particles of metallic zinc coated with

Fig. 2. Thermodynamic equilibrium compositions for Eq. (1) [6].

747M. Epstein et al. / Energy 29 (2004) 745–756

ZnO, will be formed. These particles will not coalesce to form liquid metal. Therefore, the con-

denser is a critical equipment to the process—it has to be capable of cooling the vapors so rap-

idly that reoxidation is eliminated. Two industrial solutions are utilized. In the most common

way, the condenser volume contains a pool of molten lead. By means of rotating blades, a dense

spray of lead droplets fills the condenser. The product hot gas from the reactor is forced to flow

through this spray. It is thoroughly scrubbed and its temperature drops from above 1000vC to

less than 500vC at the exit. The zinc vapors are absorbed and rapidly dissolved in the lead

droplets. Hot lead at a temperature of 550vC contains about 2.5% zinc. The lead is circulated

out of the condenser and chilled to a temperature of 450vC. At this temperature, the solubility

of zinc in lead is 2.25%. The excess zinc is released and it forms a layer of molten zinc floating

above the lead. Because the lead is able to dissolve only small amounts of zinc, the amount of

lead to be circulated must be 400 times as much as the produced amount of zinc. In the second

type of condenser, the vapors leaving the reactor pass through a chamber holding a bath of

molten zinc, which is held at a controled temperature of about 500vC by cooling coils. A

motor-driven graphite impeller is immersed in the zinc bath. The impeller rotates and fills the

chamber with a spray of zinc droplets. The zinc vapors condense as a metal on the droplets and

only a small chance of reoxidation is possible [7]. The present paper describes a two-step cyclic

solar process that eliminates the complication of rapid cooling of the product gases, since the

zinc vapor and the carbon monoxide are obtained separately. The process is shown schemati-

cally in Fig. 3. At the first step, particles of ZnO are injected into a bath of molten iron. The

iron reduces the solid ZnO and the zinc is volatilized as metallic vapors, according to the fol-

lowing reaction:

ZnOðsÞ þ FeðlÞ ! ZnðgÞ þ FeOðlÞ; DH0 ¼ 177 kJ=mol: (6)

The second step involves the feeding of carbonaceous material such as charcoal into the mol-

ten FeO, reducing it back to iron, according to the following reaction:

CðsÞ þ FeOðlÞ ! COðgÞ þ FeðlÞ; DH0 ¼ 153 kJ=mol: (7)

The CO gas is obtained separated from the zinc and can be used directly as a fuel or through

a reaction with steam to generate hydrogen and carbon dioxide, as follows:

COþH2O ! H2 þ CO2; DH0 ¼ 41:16 kJ=mol: (8)

Eq. (7) is a gasification process that takes place very quickly due to the high temperature, the

high heat and mass transfer between the FeO medium and the carbon, and the mixing created

by the evolving gas. An additional advantage of this two-step process is the omission of the

requirement for feed preparation. In a direct carbothermal reduction, this requires mixing of

ZnO with carbon in a measured proportion and preparation of briquettes from the powder.

When ZnO from zinc/air batteries is recycled, size reduction is usually required. In the present

process, relatively large-size fractions can be reduced, due to the contact with the iron, and the

high heat and mass transfer mechanisms.


2. Thermodynamic analysis

The equilibrium compositions [6] as a function of the temperature for Eqs. (6) and (7) areillustrated in Figs. 4 and 5, respectively [6]. These calculations demonstrate that above 1535

vC,

the melting point of iron (the melting point of FeO is 1430vC), the conversion of both reactions

is complete. The energy balance of the entire process at 1900 K is summarized in Table 1.The solar energy could be used in steps 1–4 of this process. The total solar input in this

example is:

RDHsolar ¼ DH1 þ DH2 þ DH3 þ DH4 ¼ 447 kJ=mol of ZnO:

The theoretical total heat output in this process can be summarized as follows:

RDHheat out ¼ DH5 þ DH6 þ DH7 þ DH8 þ DH9 þ DH10 ¼ 826 kJ=mol of ZnO:

Fig. 3. Scheme of a two-step process for the production of pure zinc.


Thus, the solar contribution is

RDHsolar

RDHheat out¼ 0:54:

The enthalpy gain (Fgain) is defined as the theoretical total heat output divided by the

enthalpy of Eq. (9) between the amount of carbon used in the process and oxygen (possible heat

output without the process), as follows:

CðsÞ þO2ðgÞ ! CO2ðgÞ; DH01900 K ¼ 396 kJ=mol of carbon: (9)

In this case, Fgain will be 826=396 ¼ 2:09.

Fig. 4. Thermodynamic equilibrium compositions of the reduction of ZnO(s) by Fe(l) (Eq. (6)) [6].

Fig. 5. Thermodynamic equilibrium compositions of the recovery of Fe(l) from FeO(l) using solid carbon (Eq. (7))[6].


3. Reduction of ZnO with Fe

An alternative to the use of carbon as a reducing agent is the metallothermic reduction ofZnO with either solid or liquid iron (e.g. at 1538

vC—the melting point of Fe), according to the

following reaction:

ZnOðsÞ þ Feðs or lÞ ! ZnðgÞ þ FeOðs or lÞ; DH01811 K ¼ 208 kJ=mol: (10)

This process has been receiving increasing attention recently, since electric arc furnace (EAF)dust contains significant amounts of zinc, mostly in the form of ZnO, which can be recovered.ZnO in the dust can react with either solid or liquid iron. The kinetics of the reduction of ironZnO powder by solid iron powder, formed into cylindrical briquettes at the temperature rangeof 1073–1423 K, shows that the reaction is chemically controled [8] with activation energy of230 kJ/mol. Once a product layer of zinc is formed, the reaction is limited by the diffusion ofzinc gas away from the reaction interface. It was found [9] that at around 1400

vC, ZnO is

reduced rapidly in the presence of an iron bath until the zinc concentration reaches about 3%,and subsequently reduction is slow. Stirring the iron bath increases the rate of reduction of zincvery significantly.

3.1. Experimental results of solar reduction of ZnO with Fe

Experiments have been conducted at the WIS solar furnace. Two grams (0.03 mol) of ZnOpowder and 4 g (0.07 mol) of Fe were heated under a flow of argon in an alumina tube (99.7%Al2O3, 22 cm length, 2.5 cm diameter), partially placed inside a cavity box with a side aperturediameter of 6 cm, through which the concentrated solar radiation enters, impinging directly onthe walls of the lower part of the tube (Fig. 6). The average solar concentration at the apertureof the cavity was about 3000 and the input power to the cavity was 6 kW of heat. The top partof the tube was protruding from the cavity box. This part is equipped with a ‘cold finger’ con-denser and a proper water-cooled sealing. The experiments have been conducted at 1473 K, cor-responding to the solid–solid process, and at 1863 K, where the iron was in a liquid phase. Inthe first case, 0.5 g of zinc was condensed after about 5 min of heating, corresponding to about

Table 1Energy balance for the entire cycle

Number Steps Tn (K) DHn (kJ/mol)

1 ZnOðsÞ ! ZnOðsÞ 300 ! 1900 +842 ZnOðsÞ þ FeðlÞ ! ZnðgÞ þ FeOðlÞ 1900 +1773 CðsÞ ! CðsÞ 300 ! 1900 +334 FeOðlÞ þ CðsÞ ! FeðlÞ þ COðgÞ 1900 +1535 COðgÞ ! COðgÞ 1900 ! 1180 186 ZnðgÞ ! ZnðgÞ 1900 ! 1180 457 ZnðgÞ ! ZnðlÞ 1180 ! 1180 1168 ZnðlÞ ! ZnðlÞ 1180 ! 700 159 COðgÞ þ 0:5O2! CO2ðgÞ 1180 282 (oxidation)10 ZnðlÞ þ 0:5O2! ZnOðsÞ 700 350 (oxidation)


30% of the theoretically possible yield. In the case of the high temperature, 1.5 g of zinc were

condensed at the ‘cold finger’ after 5 min of heating (corresponding to about 90% yield). Pow-

der X-ray diffraction analyses (Rigaku, Rotaflex RU200B with graphite monochromator and

rotating anode, CuK/-radiation) of the material settled on the ‘cold finger’ show that the zinc

is of high purity and crystallinity (Fig. 7). The residues inside the alumina reactor tube were also

analyzed and FeO, Fe and ZnO were found in most of the cases (Fig. 7).

Fig. 7. Powder X-ray diffraction of the product material of the ZnO reduction obtained on the ‘cold finger’ and theresidue in the alumina tube reactor.

Fig. 6. A scheme of the experimental solar cavity reactor used for the reduction of ZnO by iron.


4. Reduction of FeO with carbon

The second step of the process is the reduction of FeO with carbon and recovery of the ironwith CO as a product gas (Eq. (7)). The equilibrium compositions at different temperatures areshown in Fig. 5. Thermodynamically, at 1000

vC, 100% conversion of FeO to iron can be

obtained. This process is widely used in the industry [10,11]. Industrial scale reduction of ironore containing solid FeO and carbon monoxide takes place, according to the following equa-tion:

FeOn þ nCO ! Feþ nCO2: (11)

Carbon monoxide is produced in a blast furnace from the reaction of oxygen in the blast airwith hot coke and other reducing agents such as injected coal or oil, according to the followingreaction:

Cþ 0:5O2 ! CO; DH01000

vC¼ 113:7 kJ=mol: (12)

Carbon monoxide is also produced from CO2, the product of Eq. (11), with carbon by theBoudouard reaction (Eq. (5)). If FeO is present as a liquid, the main reaction that takes place isthat given in Eq. (7). If coal is used as the reducing agent, the gases actually participating in thereduction process originate from the coal pyrolysis (i.e. hydrogen and hydrocarbons) and coalgasification. Coal also contains ash, moisture (10% typically, which reacts endothermally in thewater–gas reaction) and sulfur. The kinetics and reaction mechanism of the reduction process ofFeO, especially in slag, either in solid phase ore in a smelting reduction, are reported in theliterature [12–14]. The reduction of FeO can also be achieved with hydrogen, as follows:

FeOþH2 ! FeþH2O; (13)

and by a mixture of hydrogen and carbon monoxide resulting, for example, from methanereforming or coal gasification processes.In the presence of CO, in addition to H2, (PCO2

=PCO) and (PH2O=PH2), partial vapor pressure

ratios are coupled at any given temperature by the gas phase equilibrium [10] as follows:

COþH2O () CO2 þH2; K ¼ PCO2=PCO

PH2O=PH2

� �equilibrium: (14)

5. Concept of a solar reactor

The beam-down optics of a solar tower [15,16] can be used to provide concentrated solarradiation in a ground reactor containing a liquid matter from its ceiling through a terminal con-centrator (known as compound parabolic concentrator—CPC). The concept of a solar reactor isconceived in Fig. 8. A central compartment is used for the reduction of ZnO and production ofzinc. ZnO powder is injected into the iron melt creating a liquid layer of FeO, which floats onthe iron layer. The zinc vapors, the only product in this compartment, exit at the top of thiscompartment directly to a condenser where the vapors first cooled from the reaction tempera-ture (1500

vC typically) to about 907

vC, condensed to zinc liquid, and the liquid is further


cooled to about 500vC. If the final goal of the zinc is to produce hydrogen via reaction with

steam [5], the most suitable condition of the zinc is a liquid phase at around 500vC. Part of the

zinc produced during the day can be stored as a liquid for the night, when production of hydro-gen can take place. This results in a chemical storage of solar energy.The central compartment is surrounded by peripheral compartments (i.e. six hexagonal per-

ipheral compartments surrounding a central hexagon) (Fig. 8), where the reduction of FeO(l) toFe(l) takes place. There are openings at a certain height in the compartments through whichFeO(l) flows gravitationally to the peripheral compartments and at the bottom, Fe(l) flows backto the center. The reduction of FeO(l) can be executed by the injection of carbon, charcoal orcoal into the FeO(l) layer. If carbon is used, the main product is CO(g). CO is cooled and canbe converted in a conventional process to CO2 and H2, according to Eq. (8). CO2 can be sepa-rated, recycled and used to inject the carbon into the reactor. At the temperature of the reactor,

CO2 reacts with the solid carbon to produce CO, according to the Boudouard reaction [17].According to Eq. (5), the enthalpy of this endothermic reaction, which equals DH0 ¼ 172:5 kJ/mol, can be obtained from the sun, resulting in CO2 gasification of the carbonaceous material.Both C(s) and CO(g) can reduce FeO(l) and the product gas will be a mixture of CO and CO2.Instead or in addition to the recycled CO2, steam can be injected as well and used to feed thecarbon. In this case, the carbon is gasified according to Eq. (15) [17], and both CO and H2 canbe used to reduce FeO as follows:

CðsÞ þH2OðgÞ () COðgÞ þH2ðgÞ; DH0 ¼ 131:4 kJ=mol: (15)

Finally, the peripheral compartments can be equipped with means to inject oxygen in casesolar energy is not available to avoid freezing of the Fe and FeO melts. The oxygen reacts with

Fig. 8. Schematic concept of a solar reactor for the two-step reduction of ZnO(s) with Fe(l).


the carbon in an exothermic reaction to provide heat for maintaining the temperature of thebath and for reduction of FeO(l), if necessary, according to Eq. (12).

6. Conclusions

The use of iron as a reducing agent for ZnO may have an interesting alternative to the directreduction by carbon or hydrocarbon. The main advantages are the pure zinc that is obtainedand the possibility to refrain from a complicated and expensive quencher and separator requiredin the direct carboreduction process, such as the lead splash condenser. The reaction rates of thereduction of solid ZnO by liquid iron are very high and this phenomenon was demonstrated bythe use of concentrated solar energy. In addition, even solid iron can be used as a reducingagent at temperature ranges of 1100–1300

vC, with lower conversion rates. Containment materi-

als of construction for a large-scale solar reactor can be adopted from the vast experience of theiron production industry. The newly developed beam-down optics for a solar tower plant can bespecifically suitable for this application. The concept for the solar reactor revealed in this paperenables a good practice for the maintenance of the liquid iron/iron oxide bath at a constanttemperature during periods of solar radiation fluctuations, clouds and night hours, by injectionof oxygen when needed.

References

[1] Palumbo R, Lede J, Boutin O, Elorza Ricart E, Steinfeld A, Moller S, et al. The production of Zn from ZnO ina high-temperature solar decomposition quench process—the scientific framework for the process. Chem Eng Sci1998;53(14):2503–17.

[2] Steinfeld A, Frei A, Kuhn P, Wuillemin D. Solar thermal production of zinc and syngas via combined ZnO-reduction and CH4-reforming processes. Int J Hydrogen Energy 1995;20(10):793–804.

[3] Steinfeld A, Brack M, Meier A, Weidenkaff A, Wuillemin D. A solar chemical reactor for coproduction of zincand synthesis gas. Energy 1998;23(10):803–14.

[4] Berman A, Epstein M. The kinetic model for carboreduction of zinc oxide. J Physique IV 1999;9/3:319–24.[5] Berman A, Epstein M. The kinetics of hydrogen production in the oxidation of liquid zinc with water vapor. Int

J Hydrogen Energy 2000;25(10):957–67.[6] Roine A. Outokumpu HSC chemistry for Windows—chemical reaction and equilibrium software with extensive

thermochemical database. Pori: Outokumpu Research Oy; 1999.[7] Morgan SWK. Sintering and roasting. In: Morgan SWK, editor. Zinc and its alloys and compounds. Chichester:

Ellis Horwood Ltd; 1985, p. 39–55.[8] Donald JR, Pickles CA. A kinetic study of the reaction of zinc oxide with iron powder. Metall Mater Trans B

1996;27B(Jun):363–74.[9] Rankin WJ, Wright S. The reduction of zinc from slag by an iron–carbon melt. Metall Mater Trans B

1990;21B(Oct):885–97.[10] Ottow M, Meiler H, Wessiepe K, Koltermann M, Yagi J-I, Formanek L, et al. Iron. Elviers B, editor. Ullmann’s

encyclopaedia of industrial chemistry, vol. A14. Weinheim: VCH; 1989, p. 461–590.[11] von Bogdandy L, Engell H-J. The reduction of iron ores. Berlin: Springer-Verlag; 1971, p. 100.[12] Fruehan RJ. Reduction smelting processes—technology and economics. In: Feinman J, Donald R, Mac Rae R,

editors. Direct reduced iron—technology and economics of production and use. Warrendale: Iron & SteelSociety; 1999, p. 163–72.


[13] Min DJ, Han JW, Chung WS. A study of the reduction rate of FeO in slag by solid carbon. Metall Mater TransB 1999;30B(Apr):215–21.

[14] Lee J-C, Min D-J, Kim S-S. Reaction mechanism on the smelting reduction of iron ore by solid carbon. MetallMater Trans B 1997;28B(Dec):1019–28.

[15] Yogev A, Kribus A, Epstein M, Kogan A. Solar ‘‘tower reflector’’ systems—a new approach for high-tempera-ture solar plants. Int J Hydrogen Energy 1998;23(4):239–45.

[16] Segal A, Epstein M. The optics of the solar tower reflector. Solar Energy 2000;69(Suppl):229–41.[17] Ottow M, Meiler H, Wessiepe K, Koltermann M, Yagi J-I, Formanek L, et al. Gas production. Elviers B, edi-

tor. Ullmann’s encyclopaedia of industrial chemistry, vol. A12. Weinheim: VCH; 1989, p. 179.


ferro-reduction of zno using concentrated solar energy

Documents