High Temp. Mater. Proc. 2016; 35(3): 275–281

Kun Yang, Shi-wei Li*, Li-bo Zhang, Jin-hui Peng, Ai-yuan Ma and Bao-bao Wang Effects of Sodium Citrate on the Ammonium Sulfate Recycled Leaching of Low-Grade Oxide

Abstract: The effects of sodium citrate on ammonium Keywords: sodium citrate, low-grade ores, sulfate recycled leaching of low-grade zinc oxide ores ammonium sulfate, recycled leaching were studied. By applying various kinds of detection PACS® (2010): 81.40.Wx and analysis techniques such as chemical composition analysis, chemical phase method, scanning electron microscopy and energy dispersive spectrum (SEM/ DOI 10.1515/htmp-2014-0215 EDS), X-ray diffraction (XRD) and Fourier-transforming Received November 20, 2014; accepted February 26, 2015 infrared spectrum (FT-IR), zinc raw , its leaching slag and the functional mechanism of sodium citrate were investigated. Based on a comprehensive analysis, it can Introduction be concluded that in contrast to hemimorphite (Zn4Si2O7 (OH) ·HO), amorphous (ZnCO ) and zinc 2 2 3 As the third largest non-ferrous metal, zinc is regarded as silicate (Zn SiO ) prove to be refractory phases under 2 4 an important anticorrosive, energy and magnetic material ammonium sulfate leaching, while sodium citrate has and is widely used in traffic, transportation, construction, a better chelating action with the refractory phases, energy and other industries. At present, most of zinc resulting in a higher zinc leaching rate. Under condi- productions are extracted from sulfide ores, which tions of [NH ]/[NH ] molar ratio being 0.5, [NH ] being 3 3 T 3 T makes it more difficult to meet demands for foreign and 7.5 mol/L, [Na C H O ]being0.2mol/L,S/Lratiobeing 3 6 5 7 domestic production as world rapidly growing demand 1:5, temperature being 303 K, holding time being 1 h in for zinc and lowing of zinc sulfide ore exploitation value; each of the two stages, and stirring rate being 300 rpm, thus, utilization of zinc oxide ores has become an inevi- the leaching rate of zinc reached 93.4%. In this article, table way for non-ferrous metallurgy development and sulfate ammonium recycled technology also reveals its plays a vital role in ensuring the economic development. unique advantage in processing low-grade zinc oxide The pyrometallurgical process of zinc oxide ores has ores accompanied by high silicon and high alkaline almost been eliminated for its high raw material require- gangue. ment, poor raw material adaptability, low resource utilization rate, high energy consumption and serious envir- onmental pollution, and it is widely accepted that hydro- *Corresponding author: Shi-wei Li, State Key Laboratory of Complex leaching, especially alkaline leaching which can avoid Nonferrous Metal Resources Clean Utilization, Kunming University of forming of gel SiO and alleviate subsequent solid–liquid Science and Technology; Yunnan Provincial Key Laboratory of 2 Intensification Metallurgy, Kunming, Yunnan 650093, China; separation load [1, 2], is the trend of times and broad in National Local Joint Laboratory of Engineering Application of prospect. What’s more, ammonia leaching is evaluated as Microwave Energy and Equipment Technology, Kunming, Yunnan the optimal advanced clean zinc production technology for 650093, China; Faculty of Metallurgical and Energy Engineering, the following advantages [3–6]: (1) abundance in raw mate- Kunming University of Science and Technology, Kunming, Yunnan rials; (2) short process; (3) leaching solution easily purify; 650093, China, E-mail: [email protected] Kun Yang, Li-bo Zhang, Jin-hui Peng, Ai-yuan Ma, Bao-bao Wang, (4) more production varieties and (5) high selectivity. State Key Laboratory of Complex Nonferrous Metal Resources Clean Many researches have shown that citric acid can Utilization, Kunming University of Science and Technology; Yunnan improve the recovery rate of and secondary Provincial Key Laboratory of Intensification Metallurgy, Kunming, valuable metal resources. Tzeferis et al. [7] compared Yunnan 650093, China; National Local Joint Laboratory of several organic acids’ ability of solubilizing nickel, such Engineering Application of Microwave Energy and Equipment as oxalic, lactic, formic, acetic, citric and salicylic acids, Technology, Kunming, Yunnan 650093, China; Faculty of Metallurgical and Energy Engineering, Kunming University of and they found that citric acid proves to be the most Science and Technology, Kunming, Yunnan 650093, China effective organic acid for nickel extraction. Sonmez 276 K. Yang et al.: A New Technology for Zinc Leaching et al. [8] developed a new process leaching and desulfur- Table 2: Zinc phase of low-grade oxide zinc ores (mass fraction, %). ization of PbSO4 by citric acid and sodium citrate solu- tion, which can achieve effective recovery of Zinc phase Zinc Oxide Sulfide Franklinite ZnT precursor as lead citrate while simultaneously removing sulfate et al. sulfur as Na2SO4. Li et al. [9] investigated the recovery of Zinc content/% . . . . . cobalt and lithium from spent lithium ion batteries using distribution/% . . . . . citric acid and hydrogen peroxide as leachant, which is found to be simple, environmentally friendly and ade- quate for the recovery of valuable metals from spent lithium ion batteries. But literature about the effects of content, only 6.01%, while its oxidation rate is high, citric acid on leaching of the low-grade zinc oxide ores is being 96.32%. In addition, this zinc oxide ore has very limited [10]. Sodium citrate was chosen as auxiliary drawbacks of high silicon and high alkaline gangue leaching agent in this study, and its effects on low-grade coexisting, [CaO + MgO] approximates 12%, quartz con- zinc oxide ores leaching and corresponding leaching tent up to 31.57% and various metals such as Fe, Pb, Al, mechanism were studied. Mg, Ca, etc. symbiosis, which aggravate the burden of ore dressing and metallurgical processing. Figure 1 reveals morphology and element content of this low-grade zinc oxide ores: Figure 1(a) proves the ore Experimental particle size is primarily at μm level and Figure 1(b) is obtained by the way of area point scanning marked in Materials and equipment Figure 1(a) which directly reflects the relative concentra- tion of local element and indicates that oxygen is the The low-grade zinc oxide ores were obtained from a most abundant element, followed by Si and Ca, the Yunnan zinc plant and processed with crusting and wet amount of Zn is a little higher than Fe, Al, Pb, etc. grinding, to sustain particle size at ~76 μm. On the basis of EDS pattern, XRD spectrum of zinc The leaching is by means of magnetic agitation with oxide ore is shown in Figure 2. As it is shown, the main CJJ-93/HJ-65 six-connected magnetic stirrers. Solid–liquid phase of low-grade zinc oxide ores are SiO2 and CaCO3,a separation applied SHB-Ⅲ-type multi-use of recycled small amount of crystalline phases exist in PbCO3 and water jet pumps. Zn4Si2O7(OH)2 ·H2O. Meanwhile the crystallinity degree of other zinc compounds may be low, which results in no XRD corresponding phase appearing. Characterizations of raw materials Figure 3 relates to the FT-IR spectrum of low-grade zinc oxide ore, which includes stretching and vibration The zinc raw ores and its leaching slags are characterized absorption of frame water, water and Si–O group, by chemical composition analysis, chemical phase etc. The main absorption peaks locate at 3,440.71, method, X-ray diffraction (XRD), Fourier transform infra- 1,631.89, 1,425.10, 1,028.99, 875.74, 797.90, 711.79, −1 red spectra (FT-IR), and scanning electron microscope/ 693.43 and 472.26 cm . Gadsden [11] and Farmer et al. energy dispersive spectrum (SEM/EDS). Its chemical com- [12] demonstrate frame water stretching and bending position, zinc phase, surface element and valence bond vibration absorption measured at the range of 3,700– −1 −1 structure are shown in Tables 1 and 2 and Figures 1–3. 3,500 cm and 1,200–600 cm , while crystal water at −1 −1 Tables 1 and 2 demonstrate the chemical composi- the range of 3,600–3,500 cm and 1,650–1,600 cm . The −1 tion and zinc phase of low-grade zinc oxide ores. From band observed at 3,440.71 cm is a frame water OH −1 them, it can be concluded that this ore has a low zinc stretching mode and the infrared band at 1,631.89 cm assigned to crystal water bending vibration. The range of 1,150–400 cm−1 mainly exists between Si–O stretching Table 1: Main chemical composition of low-grade oxide zinc ores 4– (mass fraction, %). and bending vibration, with SiO4 tetrahedron and 6– Si2O7 as the main corresponding building blocks. Two −1 ZnT Fe Pb S SiO AlO MgO CaO infrared bands are observed at 1,028.99 and 677.58 cm ,

which are attributed to Si–Ob–Si stretch vibration (Ob . . . . . . . . represents bridging oxygen) [13]. Band at 940.20 cm−1 – Note: ZnT, total Zn content. should be assigned to Si Onb (Onb represents non- K. Yang et al.: A New Technology for Zinc Leaching 277

(a) (b)

Figure 1: SEM image and EDS pattern of low-grade zinc oxide ore.

structure unit. The structure of hemimorphite is generally considered to be a three-dimensional skeletal matrix con- sisted of Zn–O tetrahedron and Si–O double tetrahedron connected by angular and point, and these tetrahedrons connect with each other in three vertex angle to form six atom ring (2Zn þ Si þ 3O) [14]. Moreover, these rings connect and form an infinite net layer in 101 surface [15]. There also appear characteristic absorption bands of quartz. The peaks at 797.90 and 772.58 cm−1, the medium intensity sharp doublet, characterized by a slightly stron-

ger high frequency peak, owe to SiO2 vibration. Quartz also has weak bands in 693.43, 472.26 and 520.29 cm−1. The bands at 1,425.10, 875.74 and 711.79 cm−1 are ascribed to calcite [16] and smithsonite [17]. Figure 2: XRD pattern of low-grade oxide zinc ore.

Experiment

The ammonium sulfide recycled leaching of low-grade zinc oxide ores can be divided into two steps: step 1, leaching of raw ore with recycled filtrate; step 2, extrac-

tion of leached residue with prepared NH3-(NH4)2SO4-

Na3C6H5O7-H2O solution. The procedure is as follows: weight accurately 20 g zinc oxide ores, add it into the recycled filtrate and adopt magnetic agitation leaching at a speed of 300 rpm. After atmospherically leaching for 1 h, separate solid and liquid through vacuum filtration.

Meanwhile, leached residue is washed with 20 mL NH3–

(NH4)2SO4–Na3C6H5O7–H2O solution. Dry the leached

residue and leach it with 100 mL NH3–(NH4)2SO4– Figure 3: FT-IR spectrum of low-grade oxide zinc ore. Na3C6H5O7–H2O solution on a solid-to-liquid ratio of 5. Hold the leaching for 1 h at room temperature and mag- netic agitation, followed by solid–liquid separation and −1 bridging oxygen) stretching vibration and 875.34 cm recycling filtrate which will be returned to system as raw – absorption peak is caused by Si O3 symmetrical stretch- ore leaching solution. The flow diagram is shown in ing vibration in the hemimorphite double tetrahedron Figure 4. 278 K. Yang et al.: A New Technology for Zinc Leaching

ores is accomplished through forming coordination com- plexes with Zn2 þ . When ammonia volatilization and leaching efficiency are both considered, it is appropriate

to choose a mixed leaching solution of [NH3]/[NH3]T

molar ratio being 0.5, [NH3]T being 7.5 mol/L. The leach- ing rates of low-grade zinc oxide ores are shown in Figure 5. As can be seen from it, increasing sodium citrate molar content will lead to leaching rate gradually

increasing, and till [Na3C6H5O7] being 0.2 mol/L, the leaching rate reaches maximum value, 93.4%. However, further increasing sodium citrate molar content causes a decrease in the leaching rate.

Figure 4: Flow diagram of low-grade oxide zinc ores leaching. Chemical and phase analysis of leaching Results and discussions slags In order to make clear of the impact mechanism of The effects of sodium citrate content on zinc sodium citrate on zinc leaching, choose characterized leaching rate leaching slag (depending on Na3C6H5O7 content) to carry out chemical, phase, XRD, SEM/EDS and FT-IR In the experiment, impact of sodium citrate on ammo- analysis, which is got at the sodium citrate content nium sulfide recycled leaching of low-grade zinc oxide being 0, 0.2 and 0.35 mol/L. Chemical compositions and zinc phase of leaching slags are illustrated in Tables 3 and 4. Table 3 also indicates the residual contents of leaching slags, which suggests different behaviors of component in the ammo- nium sulfide recycled leaching process. In accordance with Figure 5, mass fraction of zinc in leaching slag under sodium citrate content 0.2 mol/L is the lowest, followed by sodium citrate content 0 mol/L, and no sodium citrate added sample has the biggest mass frac- tion value. The mismatch between mass fraction of che- mical composition and residual content of leaching slag should be own to analysis error. It is clear from the illustration of Table 4 that sodium citrate plays a special role in the ammonium sulfide recycled leaching of oxides (mainly including smithsonite and hemimorphite), while its contribution to zinc sulfate, sulfide and Franklinite et al. is not obvious, whose con- Figure 5: Leaching rates of low-grade zinc oxide ores under different tent is pretty the same. sodium citrate contents.

Table 3: Main chemical composition of leaching slags (mass fraction, %).

NaCHO ZnT Fe Pb S SiO AlO CaO Leaching slag content (mol/L) weight (g)

. . . . . . . . .. . . . . . . . . . . . . . . . . K. Yang et al.: A New Technology for Zinc Leaching 279

Table 4: Zinc phase of leaching slags (mass fraction, %). analyzing leaching slag, it can be concluded that sodium ammonia has a good coordination ability with NaCHO Zinc Oxide Sulfide Franklinite ZnT crystalline hemimorphite (Zn4Si2O7(OH)2 ·H2O), there content (mol/L) sulfate et al. being already no sign of hemimorphite in three leaching . . . . . slags, and the leaching residual phases are compara-           ...... tively simple, consisting mainly of SiO2, CaCO3 and . . . . . . PbCO3. Hemimorphite is less likely to be refractory phase. XRD and SEM/EDS analysis of leaching slags FT-IR spectra of leaching slags The SEM images and EDS patterns of leaching slags under different sodium citrate content are displayed in Figure 8 reveals FT-IR spectra of leaching slags under Figure 6 and the EDS patterns are got by area point different sodium citrate content. Through contrasting scanning as marked in SEM images. XRD patterns of and analyzing, different absorption peaks mainly locate leaching slag are shown in Figure 7. By horizontally at 1,029, 876 and 471 cm−1, which are, respectively,

a1 a2

b1 b2

c1 c2

Figure 6: SEM images and EDS patterns of leaching slags under different sodium citrate contents (a1) and (a2): [Na3C6H5O7] 0 mol/L; (b1) and (b2): [Na3C6H5O7] 0.2 mol/L; (c1) and (c2): [Na3C6H5O7] 0.35 mol/L. 280 K. Yang et al.: A New Technology for Zinc Leaching

Figure 8: FT-IR spectra of leaching slags under different sodium −1 citrate contents: (a) [Na3C6H5O7] 0 mol/L ; (b) [Na3C6H5O7]

0.2 mol/L; (c) [Na3C6H5O7] 0.35 mol/L.

optimum zinc leaching efficiency, Meanwhile there −1 has already no Si2O7 group (677.05 and 1,087.28 cm ) in all slags. Figure 8 hints that the coordinations of ammonia with amorphous smithsonite and zinc silicate are relatively difficult, and sodium citrate has a better chelating action with them resulting in a higher zinc leaching rate.

Conclusion

Based on the discussion above, conclusions can be drawn as follows:

(1) Sodium citrate can enhance the leaching rate of the low-grade zinc oxide ores. Under conditions of

[NH3]/[NH3]T molar ratio being 0.5, [NH3]T being 7.5

mol/L, [Na3C6H5O7] being 0.2 mol/L, S/L ratio being 1:5, temperature of being 303 K, holding time 1 h at each stage of the two stages, stirring rate and being 300 rpm, the leaching efficiency of zinc could reach Figure 7: XRD patterns of leaching slags under different sodium as high as 93.4%. citrate contents (a) [Na3C6H5O7] 0 mol/L; (b) [Na3C6H5O7] 0.2 mol/L; (2) Hemimorphite with good crystallinity is easy to leach in (c) [Na C H O ] 0.35 mol/L. 3 6 5 7 ammonium sulfate recycled process. In contrast, amor- phous smithsonite and zinc silicate prove to be refrac- tory under ammonium sulfate leaching and sodium 4− caused by tetrahedron structure SiO4 vibration citrate has a better chelating action with the refractory (1,028.99 cm−1), smithsonite vibration (875.74 cm−1), phase resulting in a higher zinc leaching rate. quartz vibration (472.26 cm−1) and with sodium citrate (3) Sulfate ammonium recycled technology reveals content increasing, absorption peaks of leaching slag its unique advantage in processing high silicon have a tendency of sharpness and then blunting, which and high alkaline gangue low-grade zinc oxide demonstrates [Na3C6H5O7] 0.2 mol/L truly owns the ores. K. Yang et al.: A New Technology for Zinc Leaching 281

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