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DOI: 10.1016/j.jmrt.2019.09.043
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Available online 9 October 2019
Mechanism of calcium oxide promoting the separation of zinc and iron in metallurgical dust under reducing atmosphere
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Wei Lv, Min Gan1,
, Xiaohui Fan1,**, Zhiyun Ji, Xuling Chen
School of Minerals Processing & Bioengineering, Central South University, Changsha, Hunan, 410083, PR China
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Abstract

Based on the processes of recovering Zn from metallurgical zinc-bearing dust, it found that CaO could promote the separation of zinc and iron. The effect of CaO on phase evolution and reaction mechanism of zinc and iron oxide was studied. ZnO and Fe3O4 were reacted into zinc ferrite and Fe0.85-xZnxO, restricting the volatilization of Zn without CaO. So only at a high CO proportion, zinc can be reduced and volatilized thoroughly. In the presence of CaO, no significant phase of zinc ferrite was found in the roasting products, while plenty of calcium ferrite phases were generated and the generation of Fe0.85-xZnxO was inhibited. The reaction mechanism can be derived as Fe0.85-xZnxO+CaO→FeO+Ca2Fe2O5+ZnO and ZnFe2O4+CaO→Ca2Fe2O5+ZnO. Both of reactions promoted the volatilization of Zn. Therefore, for separating the zinc and iron in zinc-iron dust, the atmosphere of CO content can be reduced to 50 vol.% from 70 vol.% by adding CaO, and the volatilization rate of Zn reached above 80 wt.%.

Keywords:
Zinc-bearing dust
Zinc volatilization
Reducing atmosphere
Phase evolution
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1Introduction

Iron and steel enterprises produce a large amount of dust in the processes of iron smelting, steelmaking, steel rolling, etc., accounting for about 10% of the total steel output [1]. Zinc content in metallurgical dust was relatively high due to the use of galvanized steel scrap and paragenetic minerals of iron and zinc. These zinc dust contains a large amount of valuable components, such as zinc and iron, which made them important secondary resources. However, the utilization rate of the dust was less than 20% [2–4]. Vast of secondary resources stockpiling was not only harmful to the ecological environment, but also caused a great waste of resources. Therefore, the comprehensive utilization of zinc-bearing metallurgical dust has attracted more and more attention.

Zinc mainly exists as zinc oxide (ZnO) and zinc ferrite (ZnFe2O4) in the zinc-bearing dust, while iron mainly exists in the form of magnetite (Fe3O4), hematite (Fe2O3), wustitie (FeO) and zinc ferrite (ZnFe2O4) [5–7]. How to effectively recover zinc and iron from zinc-bearing dust was the research emphasis, on which lots of researches have been carried out in recent years. Among them, direct-reduction process was to mix the dust, coal powder and some binder to made pellets, then roasted at high temperature. The zinc oxide and iron oxide in the dust were reduced at same time. Thus, Zn was separated and volatilized in the form of steam, then was reoxidized into ZnO and collected at low temperature range. This technology consumed a large amount of coal powder and huge energy, and brought serious environmental pollution [8–10]. Scholars added fluxes, such as limestone and quicklime, into the agglomerate to improve the basicity. It can improve the dezincification rate at constant carbon content [11,12]. In addition, Y. Guo put forward an idea of calcification roasting—mineral phase reconstruction—ammonia leaching, expecting to reconstruct the mineral phase of zinc ferrite at high temperature by adding calcium oxide [13,14]. Scholars in Japan invented a method called LAMS (Lime Addition and Magnetic Separation) to treat zinc-bearing dust. The dust was fully mixed with lime and heated at a high temperature of 1273K, zinc and iron oxide reacted with calcium. Because of the magnetic differences of the products, separation and recovery of zinc was achieved [15]. Researchers also found that CaO was able to promote the decomposition of ZnFe2O4 over 1373K, and lower iron or ZnFe2O4 concentrations in the dust resulted in a lower CaO consumption [16–18]. In addition, some steel plants sent zinc-bearing dust back to sintering ingredients. Materials containing zinc were made into pellets and arranged at the bottom of sintering bed, so that zinc passed into the flue gas in the form of steam. Moreover, increasing the basicity of zinc-bearing pellets could improve the zinc removal rate in the sintering process [19–21]. Studies also found that the removal ratio of Zn improved greatly after adding calcium chloride, and ZnO could be chloridized to produce volatile species like ZnCl2[22–24]. Calcium fluoride had the similar effect. These additives containing calcium could reduce the energy required for zinc reduction reaction [25].

From above, it was found that the adding a certain proportion of calcium oxide was conducive to the separation of zinc and iron in the reduction process of zinc-bearing dust. However, studies at present mainly focused on the technical processes of the utilization of zinc-bearing dust, very few attention were paid on the reaction mechanism.

Therefore, this paper aimed at zinc-bearing dust produced by ironmaking and steelmaking, and conducted atmosphere controlling heat-treatment by a tube furnace. With the help of XRD and Factsage7.1 thermodynamic software, reaction mechanism and phase transformation of the mixture of Fe3O4, ZnO and CaO under reducing atmosphere were studied. The influence mechanism of calcium oxide on zinc and iron separation was clarified, which provided theoretical basis for optimizing and developing the comprehensive utilization technology of zinc-bearing dust.

2Experimental materials and methods

The raw materials of analytical pure reagents(AR) ZnO, Fe3O4 and CaO were used in this study, and all the reagents were pre-ground to 100wt% passing 0.074mm sieve. ZnO and Fe3O4 was mixed as mole ratio of Zn:Fe=1:2 (stoichiometric number of ZnFe2O4). When adding CaO, the mole ratio of Zn:Fe:Ca=1:2:1. The materials were weighed precisely as mole ratio and mixed up gently with an agate mortar and pestle for 30min. After that, the mixed materials were put into a corundum crucible, which was then placed a quartz tube with both ends ventilated, as shown in Fig.1. The quartz tube was put into the horizontal furnace which can be set to the specified temperature. Then the sample was pushed in the high-temperature zone. The total flow rate of the inlet mixed gas was fixed at 4.0L/min. Before starting the test, N2 is pumped into the reactor for about 5min to make sure there was no oxygen. The inject CO2, after about 5min, turn on the CO cylinder switch and adjust the flow meter. The samples were subjected to the predetermined CO/(CO+CO2) atmosphere at a given temperature for 30min. After roasting, the products were rapidly taken out and quenched into liquid nitrogen. Finally, the products were fine ground and prepared for characterization.

Fig. 1.

Diagram of experimental equipment.

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The Zn contents of roasted products were determined by ICP-AES. The volatilization rate of Zn was calculated by the following equation:

Where  η is the volatilization rate of Zn (wt.%);

γ is the mass loss ratio of the products (wt.%);

α is the residual Zn mass contents of the roasted products (wt.%);

β is the original Zn mass contents of the samples (wt.%).

The phase constituents of the roasted samples were identified by X-ray diffraction (XRD, D/max 2550PC, Japan Rigaku Co., Ltd). All the XRD results were tested with the steps of 0.02° at 10°min−1 in ranging from 10° to 80°, and the same instrument, parameters, and the intensity of X-ray in each diffractogram were kept consistent.

3Results and discussion3.1Effect of CaO on phase transformation of zinc and iron3.1.1Phase transformation of zinc and iron under different CO−CO2 atmosphere

Phase transformation of the Fe3O4-ZnO mixture and ZnO-Fe3O4-CaO mixture during the roasting process were studied by XRD phase identification. XRD patterns of the products roasting at different CO contents under 1000°C were shown in Fig. 2.

Fig. 2.

XRD patterns of the products roasted at different CO contents (Temperature 1000°C, roasting time 30min).

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As presented in Fig. 2(a), Fe3O4 and ZnO could react and form ZnFe2O4 under 10% CO atmosphere, indicated that Fe2+ was partly oxidized to Fe3+ during the roasting process under low CO content [26]. When CO content increased to more than 30vol. %, a large amount of iron oxides were reduced to FeO stage, and the lattice of zinc ferrite was destroyed, which was conducive to the volatilization of Zn. However, it was found by carefully observation of 60°–64° in Fig. 2(a) that a kind of iron-zinc compound (Fe0.85-xZnxO) with structure similar to FeO was produced in the roasting product when CO content was 30vol. % [27]. In addition, the peak of the (220) lattice plane shifted to the left gradually as CO content increasing from 30vol. % to 70vol. %, indicated that the phase gradually changed to FeO.

From Fig. 2(b), it is known that in the presence of CaO, calcium ferrite was produced in the roasting products at CO content of 10˜70vol. %, while no obvious phase of zinc ferrite was observed. So calcium ferrite took precedence over zinc ferrite at such temperature. Moreover, the 2θ value of (220) lattice plane decreased significantly, which indicated that zinc in Fe0.85-xZnxO was constantly replaced in the roasting products. Because the higher alkalinity of CaO over ZnO, it is speculated that CaO preferentially combined with iron oxide. Calcium can even replace zinc in zinc ferrite and Fe0.85-xZnxO, and therefore promoted the separation of zinc and iron.

3.1.2Phase transformation of zinc and iron under different roasting temperature

In order to further investigate the phase evolution, the products roasted under 50vol. % of CO content at temperature ranged in 800–1100°C were tested by XRD, and the results are shown in Fig. 3. From Fig. 3(a), it was found that in the roasted products of Fe3O4-ZnO mixture, the main phase was FeO, Fe0.85-xZnxO and ZnO. The diffraction peaks of ZnO decreased gradually with the temperature increased. On the other hand, increasing temperature was in favor of the transformation of Fe0.85-xZnxO to FeO, as presented in the carefully observation of 60–62°. Therefore, increasing temperature was in favor of the separation of zinc and iron.

Fig. 3.

XRD patterns of the products roasted at different temperatures (CO content 50vol.%, roasting time 30min).

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When CaO was added, as shown in Fig. 3(b), the phase of the roasted product was mainly ZnO and a little Fe0.85-xZnxO at 800°C. When the temperature increased to 900°C, calcium ferrite started to generate in a large number, and the diffraction peaks enhanced as the roasting temperature increased. In the meantime, with the increase of temperature, it can be observed that the 2θ value of (220) lattice plane shifted toward FeO. It illustrated that the generation of iron-zinc compounds was inhibited in the presence of CaO, which constantly combined with iron oxides to free zinc.

To summary, CaO can restrain the generation of zinc ferrite and promote the conversion of Fe0.85-xZnxO to FeO above 900°C, thus was conductive to the volatilization of zinc.

3.2Thermodynamics of CaO promoting the zinc-iron separation

Factsage7.1 software was used to further study the phase evolution of Fe3O4-ZnO-CaO under different atmosphere conditions, the results were presented in Fig. 4. When the temperature was 1000°C, calcium ferrite phase always existed under different atmosphere conditions. Fig. 4(a) showed that under air atmosphere, the combining capacity of CaO and Fe3O4 was notably stronger than that of ZnO and Fe3O4. So calcium ferrite was the only new phase under different proportions of Fe3O4, ZnO and CaO.

Fig. 4.

Phase diagram of Fe3O4, ZnO and CaO under different atmosphere (1000°C).

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When there was lower concentration of CO in the system (Fig. 4(b)), calcium ferrite still existed stably. At this time, iron oxide exceeding the proportion of calcium ferrite was partially reduced and might react with ZnO to form spinel. As CO content increased to 10vol.%, shown in Fig. 4(c), the phase composition of the system changed very little compared with that of 1vol. %CO.

However, the case was much different with CO content being 50vol. %, as shown in Fig. 4(d). The phase composition in the system changed significantly with different proportions of Fe3O4, ZnO and CaO. The main phase of the product was calcium ferrite when CaO proportion was high, and the free ZnO was easily reduced and volatilized. When CaO proportion was low, iron oxide would combine with ZnO to form iron-zinc compounds in addition to generate calcium ferrite, which was against the volatilization of zinc. Therefore, the amount of calcium oxide should be adjusted according to the ratio of Zn and Fe in the dust.

From the thermodynamic diagram of Fig. 4, it can derive the reaction of the reduction process in the present of CaO was that: Fe0.85-xZnxO+CaO→FeO+Ca2Fe2O5+ZnO and ZnFe2O4+CaO→Ca2Fe2O5+ZnO. Comparing the reactions and their Gibbs free energy of ZnFe2O4 with or without CaO, it showed in Fig. 5 that the Gibbs free energy of reaction (3) was much lower than that of reaction (1) and (2). This indicated that the reaction of ZnFe2O4 and CaO was far more easily to conduct than the direct reduction of ZnFe2O4.

Fig. 5.

Comparison of Gibbs free energy between the reactions.

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3.3The appropriate condition for CaO to promote the separation of zinc and iron

The volatilization rate of Zn under different roasting temperature was investigated at CO content being 50vol.%, which was illustrated in Fig. 6. It is shown that within 600°C–1000°C, increasing temperature was beneficial to the volatilization of zinc, and the promoting effect of CaO on zinc volatilization strengthened as the temperature increased. However, CaO had little effect on the zinc volatilization when temperature was lower than 800°C. There was tiny difference between the volatilization rates of Zn with or without CaO. When the roasting temperature was increased to 900°C, adding CaO can promote the zinc volatilization of Fe3O4-ZnO mixture, and the auxo-action strengthened with the increasing of roasting temperature. This was consistent with the conclusion in Fig. 3. Therefore, in order to achieve the promoting effect of CaO, the roasting temperature should above 900°C.

Fig. 6.

The effect of temperature on the volatilization rate of Zn (CO content 50vol.%).

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The effect of CO/(CO+CO2) content on the zinc volatilization of Fe3O4-ZnO mixture and Fe3O4-ZnO-CaO mixture was investigated under the condition of roasting at 1000°C for 30min by varying the CO content. The results are shown in Fig. 7. With the increase of CO content, the volatilization rate of Zn in both mixtures showed an increasing trend, while at the same content of CO, the volatilization rate of Fe3O4-ZnO-CaO mixture was higher than that of Fe3O4-ZnO mixture. In the presence of CaO, the volatilization rate of Zn reached above 80wt.% at 50vol.% CO content, even higher than the condition without CaO at 70vol.% CO content.

Fig. 7.

The effect of CO content on the volatilization rate of Zn (Temperature 1000°C).

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However, when CO content was lower than 20vol.% or higher than 70vol.%, the difference value of the two was relatively tiny. This was closely related to the phase transformation of the reaction. The phase evolution of zinc and iron in the zinc-bearing dust during the roasting process was summarized in Fig. 8. When CO content was lower (within 10vol.%), ZnFe2O4 or Fe0.85-xZnxO was generated with lower CO content with or without CaO, the atmosphere at this time was insufficient to reduce zinc-bearing compounds, so the volatilization rate of Zn in both mixtures were relatively low. When CO content was higher than 70vol.%, the reduction atmosphere was very strong, Fe0.85-xZnxO was reduced to FeO in quantity and further converted to Fe, ZnO was easily to be reduced as Zn(g), which narrowed the difference of volatilization rate.

Fig. 8.

Summary of phase evolution of zinc and iron in the roasting process.

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But in the presence of CaO, iron preferentially combined with calcium to form calcium ferrite, which inhibited the combination of ZnO and Fe3O4. Thus, when CO content increased to 20–70vol.%, ZnO was easily to be reduced and volatilized to Zn(g). However, without CaO added, it mainly occurred the conversion of zinc and iron oxides to FeO and ZnO, just a little zinc was freed and reduced. Therefore, the difference of volatilization rate of Zn with and without CaO increased in the range of CO content being 20–70vol.%. Therefore, as shown in Fig. 7, the atmosphere of CO content was reduced from 70vol.% to 50vol.% by adding CaO, and the volatilization rate of Zn reached above 80%. This range of CO had a certain guiding significance for practical application of adding CaO to promote the separation of zinc and iron in metallurgical dust. In the actual system, there may be some deviation in this range due to the presence of other substances and the environmental differences.

4Conclusions

The mechanism of CaO promoting the separation of zinc and iron in zinc-bearing dust was investigated by simulating roasting temperature and reducing atmosphere. The main conclusions are as follows:

  • (1)

    Iron-zinc oxides (Zinc ferrite and Fe0.85-xZnxO) were easily formed by roasting Fe3O4 and ZnO in weak reduction atmosphere, which was inhibited the volatilization of Zn. While in the presence of CaO, no zinc ferrite was observed at the CO content of 0–100vol.%, but calcium ferrite started to generate rapidly when the temperature was above 900°C. It indicated that CaO can constantly combined with iron oxides, promoting the conversion of Fe0.85-xZnxO to FeO and the freeing of zinc.

  • (2)

    Elevation of temperature was beneficial to the combination of Ca and Fe, thereby replaced zinc and promoted its volatilization. From the thermodynamic diagram, it can derive the reaction mechanism of CaO promoting the zinc-iron separation during the reduction process was that: Fe0.85-xZnxO+CaO→FeO+Ca2Fe2O5+ZnO and ZnFe2O4+CaO→Ca2Fe2O5+ZnO, of which the Gibbs free energy was much lower than that of the direct reduction of ZnFe2O4.

  • (3)

    When the CO content was lower than 20vol.% or higher than 70vol.%, the volatilization rate of Zn with or without CaO showed very tiny differences. While the difference increased in the range of CO content being 20–70vol.%. Therefore, for separating the zinc and iron in zinc-iron dust, the atmosphere of CO content can be reduced to 50vol.% from 70vol.% by adding CaO, and the volatilization rate of Zn reached above 80%.

Conflict of interest statement

We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.

Acknowledgements

The research was financially supported by the National Natural Science Foundation of China (U1660206), National Key R&D Program of China (No. 2018YFC1900605), and Hunan Provincial Innovation Foundation for Postgraduate (CX2016B054).

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Both authors contributed equally to this manuscript.

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