Effects of Sodium Citrate on the Ammonium Sulfate Recycled Leaching of Low-Grade Zinc Oxide Ores

Materials and equipment

The low-grade zinc oxide ores were obtained from a Yunnan zinc plant and processed with crusting and wet grinding, to sustain particle size at ~76 μm.

The leaching is by means of magnetic agitation with CJJ-93/HJ-65 six-connected magnetic stirrers. Solid–liquid separation applied SHB-Ⅲ-type multi-use of recycled water jet pumps.

Characterizations of raw materials

The zinc raw ores and its leaching slags are characterized by chemical composition analysis, chemical phase method, X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR), and scanning electron microscope/energy dispersive spectrum (SEM/EDS). Its chemical composition, zinc phase, surface element and valence bond structure are shown in Tables 1 and 2 and Figures 1–3.

Figure 1:

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

Figure 2:

XRD pattern of low-grade oxide zinc ore.

Figure 3:

FT-IR spectrum of low-grade oxide zinc ore.

Tables 1 and 2 demonstrate the chemical composition and zinc phase of low-grade zinc oxide ores. From them, it can be concluded that this ore has a low zinc content, only 6.01%, while its oxidation rate is high, being 96.32%. In addition, this zinc oxide ore has drawbacks of high silicon and high alkaline gangue coexisting, [CaO + MgO] approximates 12%, quartz content up to 31.57% and various metals such as Fe, Pb, Al, 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 particle size is primarily at μm level and Figure 1(b) is obtained by the way of area point scanning marked in Figure 1(a) which directly reflects the relative concentration of local element and indicates that oxygen is the most abundant element, followed by Si and Ca, the amount of Zn is a little higher than Fe, Al, Pb, etc.

On the basis of EDS pattern, XRD spectrum of zinc oxide ore is shown in Figure 2. As it is shown, the main phase of low-grade zinc oxide ores are SiO₂ and CaCO₃, a small amount of crystalline phases exist in PbCO₃ and Zn₄Si₂O₇(OH)₂ · H₂O. Meanwhile the crystallinity degree of other zinc compounds may be low, which results in no XRD corresponding phase appearing.

Figure 3 relates to the FT-IR spectrum of low-grade zinc oxide ore, which includes stretching and vibration absorption of frame water, crystal water and Si–O group, etc. The main absorption peaks locate at 3,440.71, 1,631.89, 1,425.10, 1,028.99, 875.74, 797.90, 711.79, 693.43 and 472.26 cm⁻¹. Gadsden [11] and Farmer et al. [12] demonstrate frame water stretching and bending vibration absorption measured at the range of 3,700–3,500 cm⁻¹ and 1,200–600 cm⁻¹, while crystal water at the range of 3,600–3,500 cm⁻¹ and 1,650–1,600 cm⁻¹. The band observed at 3,440.71 cm⁻¹ is a frame water OH stretching mode and the infrared band at 1,631.89 cm⁻¹ assigned to crystal water bending vibration. The range of 1,150–400 cm⁻¹ mainly exists between Si–O stretching and bending vibration, with SiO₄^4– tetrahedron and Si₂O₇^6– as the main corresponding building blocks. Two infrared bands are observed at 1,028.99 and 677.58 cm⁻¹, which are attributed to Si–O_b–Si stretch vibration (O_b represents bridging oxygen) [13]. Band at 940.20 cm⁻¹ should be assigned to Si–O_nb (O_nb represents non-bridging oxygen) stretching vibration and 875.34 cm⁻¹ absorption peak is caused by Si–O₃ symmetrical stretching vibration in the hemimorphite double tetrahedron structure unit. The structure of hemimorphite is generally considered to be a three-dimensional skeletal matrix consisted 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 + Si + 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 stronger 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].

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, extraction 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–Na₃C₆H₅O₇–H₂O solution on a solid-to-liquid ratio of 5. Hold the leaching for 1 h at room temperature and magnetic agitation, followed by solid–liquid separation and recycling filtrate which will be returned to system as raw ore leaching solution. The flow diagram is shown in Figure 4.

Figure 4:

Flow diagram of low-grade oxide zinc ores leaching.

The effects of sodium citrate content on zinc leaching rate

In the experiment, impact of sodium citrate on ammonium sulfide recycled leaching of low-grade zinc oxide ores is accomplished through forming coordination complexes with Zn²⁺. 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 leaching 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 5:

Leaching rates of low-grade zinc oxide ores under different sodium citrate contents.

Chemical and phase analysis of leaching slags

In order to make clear of the impact mechanism of sodium citrate on zinc leaching, choose characterized leaching slag (depending on Na₃C₆H₅O₇ content) to carry out chemical, phase, XRD, SEM/EDS and FT-IR analysis, which is got at the sodium citrate content 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 ammonium 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 fraction value. The mismatch between mass fraction of chemical 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 content is pretty the same.

XRD and SEM/EDS analysis of leaching slags

The SEM images and EDS patterns of leaching slags under different sodium citrate content are displayed in Figure 6 and the EDS patterns are got by area point scanning as marked in SEM images. XRD patterns of leaching slag are shown in Figure 7. By horizontally analyzing leaching slag, it can be concluded that sodium ammonia has a good coordination ability with crystalline hemimorphite (Zn₄Si₂O₇(OH)₂ · H₂O), there being already no sign of hemimorphite in three leaching slags, and the leaching residual phases are comparatively simple, consisting mainly of SiO₂, CaCO₃ and PbCO₃. Hemimorphite is less likely to be refractory phase.

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.

Figure 7:

XRD patterns of leaching slags under different sodium citrate contents (a) [Na₃C₆H₅O₇] 0 mol/L; (b) [Na₃C₆H₅O₇] 0.2 mol/L; (c) [Na₃C₆H₅O₇] 0.35 mol/L.

FT-IR spectra of leaching slags

Figure 8 reveals FT-IR spectra of leaching slags under different sodium citrate content. Through contrasting and analyzing, different absorption peaks mainly locate at 1,029, 876 and 471 cm⁻¹, which are, respectively, caused by tetrahedron structure SiO₄⁴⁻ vibration (1,028.99 cm⁻¹), smithsonite vibration (875.74 cm⁻¹), quartz vibration (472.26 cm⁻¹) and with sodium citrate content increasing, absorption peaks of leaching slag have a tendency of sharpness and then blunting, which demonstrates [Na₃C₆H₅O₇] 0.2 mol/L truly owns the optimum zinc leaching efficiency, Meanwhile there has already no Si₂O₇ group (677.05 and 1,087.28 cm⁻¹) 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.

Figure 8:

FT-IR spectra of leaching slags under different sodium citrate contents: (a) [Na₃C₆H₅O₇] 0 mol/L⁻¹; (b) [Na₃C₆H₅O₇] 0.2 mol/L; (c) [Na₃C₆H₅O₇] 0.35 mol/L.