YAO Haiwei, MA Han, MAO Rui, QIU Jiayong, CHEN Chunyu, JU Dianchun＊

(1.School of Metallurgy and Materials Engineering, Jiangsu University of Science and Technology, Zhangjiagang 215600, China; 2.Shagang Iron & Steel Research Institute of Jiangsu Province, Zhangjiagang 215625, China)

Abstract: To solve the problem of the low added value Zn-containing rotary hearth furnace (RHF)dust, two deep eutectic solvents (DESs) were employed, such as choline chloride-urea (ChCl-urea) and choline chloride-oxalic acid dihydrate (CC-OA) solvent and Zn-containing RHF dust (water-washed) as the research target. Then, we prepared ZnO nanoparticles using two DESs or their combination, namely, ChCl-urea (Method A), CC-OA (Method B), first CC-OA and then ChCl-urea (Method B-A) and first ChCl-urea and then CCOA (Method A-B), respectively. The effects of these methods on the properties of as-obtained precursors and ZnO nanoparticles were investigated in detail. The results indicated that the precursor obtained by Method A was Zn4CO3(OH)6·H2O, and those by Methods B, B-A, and A-B were all ZnC2O4·2H2O. Moreover, the decomposition steps of the last three methods were similar. The ZnO contents of 95.486%, 99.768%, 99.733%,and 99.76% were obtained by Methods A, B, B-A, and A-B, respectively. Methods A, B, and B-A led to the formation of spherical and agglomerated ZnO nanoparticles with normal size distributions, where Method B showed the best distribution with an average diameter 25 nm. The ZnO nanoparticles obtained by the Method A-B did not possess good properties.

Key words: rotary hearth furnace; Zn-containing dust; deep eutectic solvent; ZnO nanoparticle

1 Introduction

Dusts could be generated during the production process of steel, which accounts to 8%-12% of the total yield. Meanwhile, the proportion of electric-furnace steel-making and zinc-plated steel scrap process has been increased. These increased processes produce large amounts of Zn, Pb, K, and Na elements in the above dusts. The industrial dust could cause severe pollution issues if it is not handled correctly. Moreover, it could bring economic losses to the steel factories if the metallic elements in the dust are not recovered[1-4]. As a technique developed in the recent decade, the rotary hearth furnace (RHF) method can recycle and process Zn-containing dust efficiently. However, the Zn-containing RHF dust collected in the steel factories is usually sold directly, instead of being paid attention to its added value.

ZnO nanoparticles have exciting optical and electrical properties, exhibiting great potential in piezoelectric devices, non-linear optics, anti-bacterial applications, photo-catalysis or biomedicine[5-9]. Therefore, the controllable synthesis and renovation on the preparation of ZnO nanoparticles become the research focus, which is mainly manifested from approaches:preparation method, the applied surfactant, and reaction source. The first two aspects are developing faster,especially the preparation method, such as sol-gel, hydrothermal synthesis, microwave heating, electrochemical routes, chemical vapour deposition and solid-phase reaction[10-14].

In recent years, deep eutectic solvents (DESs)as a group of new, green solvents have drawn the attention of researchers. Owing to their relatively lower volatility[15,16], higher electric conductivity[17,18]and better chemical stability[19,20]and solubility[21,22]compared to the traditional ionic solvents, DES exhibits high potential and enhanced results regarding isolation and purification[23-25], electrochemistry[26-28], materials chemistry[29-32]. Abbottet al[33-34]found that the choline chloride-urea (ChCl-urea) DES with a melting point of 12 ℃ can be obtained by heating the mixture of choline chloride and urea with a molar ratio of 1:2 at 80 ℃ till converting it into a transparent solution. The as-obtained ChCl-urea DES can dissolve metal oxides selectively. It has good solubility toward ZnO, Cu2O,and PbO2while it cannot deal with Al2O3, SiO2, MgO,CaO, and Fe2O3. Zhen Leiet al[35]reported that the DES based on choline chloride and oxalic acid dihydrate (CC-OA) with a molar ratio of 1:1 only showed good solubility toward ZnO, Fe2O3and a part of their chlorides. Meanwhile, ChCl-urea and CC-OA are both ideal solvents for the preparation of ZnO nanoparticles,since they can be used as reaction sources and their additional surfactant properties could facilitate the preparation of nanomaterials with unique morphologies without further addition of surfactant.

We extracted and prepared ZnO nanoparticles from Zn-containing RHF dust by four processing methods using (A) ChCl-urea, (B) CC-OA, (C) first CCOA and then ChCl-urea, and (D) first ChCl-urea and then CC-OA.

2 Experimental

2.1 Raw materials and instruments

The raw materials and agents included choline chloride (analytical reagent, 98%, Shanghai Aladdin Bio-chem Technology Co., Ltd.), urea (analytical reagent, 99%, Shanghai Aladdin Bio-chem Technology Co., Ltd.), Zn-containing dust, (Shagang Group) and oxalic acid dihydrate (analytical reagent, 98%, Shanghai Aladdin Bio-chem Technology Co., Ltd.).

The characterizing methods included X-ray diffractometry (XRD, Ulitima IV, Rigaku Corporation)with a scanning range of 10°-90° and a scanning rate of 5°/min, X-ray fluorescence spectroscopy (XFS, EDX-7000, Shimadzu), cold field-emission scanning electron microscope (FE-SEM, SU8010, Hitachi), laser particle size analysis (LA950, Horiba) and thermogravimetric differential thermal analysis (TG-DTA, D-09123,Mettler-Toledo) in N2atmosphere，with a temperature range of 50-600 ℃ and a heating rate of 10 ℃/min.

2.2 Materials preparation

Choline chloride, urea and oxalic acid dihydrate were dried at 80 ℃ for 10 h before further modification. Choline chloride and urea were mixed with a molar ratio of 1:2 while choline chloride and oxalic acid dihydrate were mixed with a molar ratio of 1:1. The two mixtures were heated in oil-bath and stirred at 80℃ until the solutions became transparent, in order to obtain the ChCl-urea and CC-OA DESs, respectively.5 g of the Zn-containing RHF dust (washed by water with a solid-to-liquid ratio of 1:3 for 3 min at 50 ℃)was added into the DESs in a volume of 50 mL, and heated and stirred in oil-bath at 80 ℃ until the solutions became transparent once more. After 12 h, the supernatants of the two solutions were added to 300 mL de-ionized water and stirred for 1 h before filtration.The filtrates were dried at 105 ℃ for 2 h to obtain the precursors and then calcinated at 450 ℃ for 2 h to form ZnO nanoparticles. The precursors obtained by the Ch-Cl-urea and CC-OA DESs were added back to the two DESs, respectively, and heated and stirred in oil-bath at 80 ℃ until the solutions became transparent. After 12 h, the supernatants were added to 300 mLde-ionized water, respectively and stirred for 1 h before filtration.The filtrates were dried at 105 ℃ for 2 h to obtain the precursors, then calcinated at 450 ℃ for 2 h to form ZnO nanoparticles.

3 Results and discussion

3.1 Characterization of the precursors prepared by different methods

3.1.1 Phase composition

The XRD patterns of the as-obtained precursors are shown in Fig.1. The precursor by Method A is mainly composed of Zn4CO3(OH)6·H2O, while those by Methods B, B-A and A-B are all zinc oxalate dihydrate(ZnC2O4·2H2O).

The possible reactions of ZnO in Method A are shown in Eqs.(1)-(5)[15]:

The possible reactions of ZnO in Method B are shown in Eqs.(6) and (7)[35]:

In Method B-A, ZnO first reacts and forms Zn-C2O4·2H2O in Step B described by Eqs.(6) and (7),while Eqs.(7) and (8) show the possible reactions of ZnC2O4·2H2O in the following Step A.

In Method A-B, ZnO first reacts and forms Zn4CO3(OH)6·H2O in Step A described by Eqs.(1)-(5),and Eqs.(7) and (9) show the possible reactions of Zn4CO3(OH)6·H2O in the following Step B.

3.1.2 Differential thermal analysis of the precursors prepared by different methods

The TG and DTG diagrams of the precursor prepared by Method A are shown in Fig.2(a). The decomposition of Zn4CO3(OH)6·H2O mainly consists of three phases: (1) desorption of water from the surface of Zn4CO3(OH)6·H2O at about 150 ℃; (2) thermal decomposition of Zn4CO3(OH)6·H2O at about 250 ℃;(3) secondary nucleation of the intermediates, formed during the thermal decomposition of Zn4CO3(OH)6·H2O at about 500 ℃.

The decomposition of the precursor prepared by Method A, Zn4CO3(OH)6·H2O, could be described by the following equation:

The TG and DTG diagrams of the precursor prepared by Method B are shown in Fig.3. Two phases mainly describe the decomposition of ZnC2O4·2H2O: (1)desorption of water from the surface of ZnC2O4·2H2O powder at about 150 ℃; (2) thermal decomposition of ZnC2O4·2H2O at about 400 ℃.

The decomposition of the precursor prepared by Method B, ZnC2O4·2H2O, is shown below:

The TG and DTG diagrams of the precursors prepared by Methods B-A and A-B are shown in Figs.4 and 5. From the XRD analysis, it was already confirmed that the as-obtained precursor was ZnC2O4·2H2O in both of the cases, and the decomposition was similar to the process mentioned above for ZnC2O4·2H2O.Small deviations exist just in the second phase of the decomposition, which could be due to the morphological differences of the precursors prepared by Methods B, B-A, and A-B. Fig.6 shows the DTA curves of four precursors, which indicates consistent decomposition behaviour with the TG/DTG results. The secondary nucleation of decomposition intermediates of Zn4CO3(OH)6at 500 ℃ is exothermic reaction while all the other reactions are all endothermic.

3.1.3 Precursor morphologies prepared by different methods

SEM images of the precursors prepared by different methods are shown in Fig.7. The precursor prepared by Method A shows an agglomeration of particles,while in the case of Method B is an agglomeration of uniform and spherical particles with a small average diameter of about 50 nm. The precursor prepared by Method B-A shows a relatively uniform particle size distribution and spherical particles with an average diameter of 1 μm; that prepared by Method A-B has similar size with that prepared by Method B-A, while the agglomeration is more accentuated in comparison with the previous method.

3.2 Characterization of the ZnO nanoparticles prepared by different methods

3.2.1 Composition analysis of the ZnO nanoparticles

Table 1 Fractions of the constituents of the Zn-containing RHF dust/%

Since the Zn-containing RHF dust had a certain amount of (other) salts, the raw material was firstly washed with water. As shown in Table 1, after washing,the salt content in the raw material had been almost removed, and the ZnO content increased from 76.597%to 92.281%. The other constituents were not removed efficiently by washing. After adding the water-washed raw material into the DES, the solution in Method A had dissolved a certain amount of ZnO, Cu2O, PbO2and a few chlorides, while that in Method B had dissolved a certain amount of ZnO, Fe2O3, with some chlorides. Therefore, it can be affirmed that SiO2, CuO,and Fe2O3in the raw material could be removed by Method A but not PbO2or the chlorides. On the other hand, SiO2, CuO, and the chlorides in the raw material could be removed by Method B while Fe2O3does not precipitate during the hydrolysis phase until a long settling of about 7 days, forming FeC2O4·2H2O as confirmed by the XRD result[35]. Therefore, ZnO with a concentration of 99.768% can be obtained by Method B,while ZnO with contents of 99.733% and 99.76% can be achieved by Methods B-A and A-B, respectively.

3.2.2 Phase analysis of the ZnO nanoparticles

According to the XRD measurements, the main phases in the Zn-containing RHF dust included ZnO,NaCl, KCl, PbO2and SiO2. After washing with water,NaCl and KCl were entirely removed, leaving the ZnO,PbO2, and SiO2. After processing with DES, the final product was just mainly ZnO. The crystal planes of(100), (002), (101), (102), (110), (103), (200), (112),and (201) corresponded to the hexagonal wurtzite structure of ZnO.

3.2.3 Morphologies and particle size analysis of the

ZnO nanoparticles

Table 2 Comparison of ZnO samples prepared by different methods

The morphologies and particle size analysis of the as-prepared ZnO nanoparticles are shown in Fig.9 and Table 2. The ZnO nanoparticles prepared by Method A were non-uniform and spherical, forming agglomerates with an average diameter of 39 nm. In contrast, those prepared using Method B were uniform and spherical,forming agglomerates, with a normal distribution of particle sizes and an average diameter of 25 nm. The samples prepared by Method B-A were similar to the samples obtained by Method B but with an average diameter of 32 nm, while the particle size distribution of those prepared by Method B-A was not so homogeneous. The samples prepared by Method A-B showed a broader size distribution with an average diameter of 259 nm. Compared with the samples prepared by Methods B and B-A, the morphology of ZnO nanoparticles prepared by Method A was not uniform, probably because of the lower ZnO content of ZnO. The results of Methods B-A and A-B could be interpreted by adding the ZnC2O4·2H2O into the product of Method A or adding Zn4CO3(OH)6·H2O into the product of Method B,respectively. The size distribution of ZnO nanoparticles prepared by Method A-B is not good, This distribution may result from the Zn source of Method B, which was Zn4CO3(OH)6·H2O.

4 Conclusions

By employing ChCl-urea and CC-OA DESs as the solvent system and Zn-containing RHF dust (water-washed) as Zn-source, the effects of four processing methods (Methods A, B, B-A and A-B) on the properties of precursors and ZnO nanoparticles were investigated.

a) The precursor obtained by Method A was Zn-4CO3(OH)6·H2O and those by Methods B, B-A, and A-B were ZnC2O4·2H2O. The decomposition during the last three processing methods was similar.

b) The ZnO with contents of 95.486%, 99.768%,99.733%, and 99.76% were obtained by Methods A, B,B-A, and A-B, respectively.

c) Methods A, B and B-A all lead to the agglomeration of the spherical ZnO nanoparticles with a normal distribution of particle size. Among them, Method B showed the most promising result, with an average diameter of 25 nm. The particle size distribution of ZnO nanoparticles obtained by Method A-B was broad and not good, with an average diameter of 259 nm.