A novel leaching process of zinc ferrite and its application in the treatment of zinc leaching residue

Zinc ferrite is a refractory phase generated in the pyrometallurgical process of zinc and steel production. Much energy is invested in the decomposition of zinc ferrite to recycle zinc since zinc ferrite is difficult to leach. In this work, a novel leaching process targeted at decomposition of zinc ferrite was proposed to save energy and improve metal recovery efficiency. The key of this novel leaching process was the use of copper powder as the reductant. Leaching of zinc ferrite in the presence of copper powder was investigated. The extraction of zinc was 100% when molar ratio of copper to zinc ferrite was 1.5 while the extraction of zinc was only 19.3% without copper. Effects of leaching temperature, acid concentration, the ratio of liquid to solid and reaction time were studied. Under the conditions: 60 °C, ≥ 70 g·L1 H2SO4, the ratio of liquid to solid ≥ 25 mL/g and the molar ratio of copper to zinc ferrite ≥ 1.5 at the open system, the synthetic zinc ferrite was dissolved completely within 60 min. Besides, the zinc ferrite-bearing zinc leaching residue from a roast-leach-electrowin plant was analyzed and leached under the optimal conditions. It was found that most of zinc ferrite in the zinc leaching residue was removed, remaining unreacted lead sulphate in the leached residue. The leaching efficiency of zinc in zinc leaching residue reached 94.2%. The results suggest that copper powder enhanced facile and efficient zinc extraction from zinc ferrite without concentrated acid, high temperature, long reaction time and specific investment. The recyclability of copper and its intrinsic commodity value showed the potential application in industrial zinc and iron containing wastes.

Zinc and iron are important metals with various applications in vehicles, buildings, machinery and infrastructure [1].In the processes of production and regeneration of zinc or iron, pyrometallurgical methods are necessary.Since zinc and iron tend to be companions in common use and zinc-bearing minerals, co-treatment of massive zinc and iron containing materials at the furnace brings about zinc ferrite, which is a stable phase causing trouble in recycling zinc [2,3].Many solid wastes such as zincbearing flue dust and zinc leaching residue are reported to contain considerable amounts of zinc ferrite [4].To realize resource recycle, carbothermal reduction method has been long used, followed by obvious drawbacks including coal consumption, slag stockpile and loss of non-volatile metals [5].Hot-acid leaching is another industrial practice targeted at zinc ferrite destruction while this intensive investment plays a limited role because of kinetics retard [6].Thus, developing a new method to decompose zinc ferrite is key to sustainable and effective treatment of zinc and iron containing wastes.
Many methods are proposed to extract zinc from zinc ferrite.Among them, the pyrometallurgical processes combined with leaching are effective on the phase transformation of zinc ferrite.For instance, reducing gases including CO and H2 change zinc ferrite into weak acid soluble zinc oxide and magnetically separated iron oxides [7,8].Roasting by sulphate makes zinc in zinc ferrite be water soluble zinc sulphate [9,10].However, these ways have no apparent advantages over carbothermal reduction method because the resulting residue or slag demands further safe and effective utilization.Besides, they are energy-consuming.Hydrometallurgical processes have the potential to save energy and reach complete extraction.In particular, reductive leaching has attracted wide interest in enhancing leaching efficiency of zinc ferrite with shorter time.As shown in Table 1, in the presence of the reductant, the leaching efficiency of metals in the zinc ferrite could reach more than 85% within 120 min.Without the reductant, the leaching efficiency reduces by more than 20% under the same leaching conditions.However, the mentioned reductants rarely succeed in practical application due to material cost, impurity input, new residue generation and other reasons.Besides, hot acid seems to be a must though the reductant exists, which improves the leaching conditions slightly.To further improve the economics of the reductive leaching process, copper which much decreases the reaction temperature as well recyclable and reusable was proposed as the reductant in the sulfuric acid leaching system.In this study, the high and fast destruction efficiency of the synthetic zinc ferrite under mild conditions was achieved.Effect of parameters including temperature, acid concentration, dosage of the reductant, ratio of liquid-solid and leaching time was investigated in detail.Moreover, zinc leaching residue containing zinc ferrite was leached coupled with copper to verify the performance.
Zinc leaching residue was collected from a zinc smelter using the traditional roast-leach-electrowin process.The sample was dried at 105 ℃ for 12 hours in a draught drying cabinet then ground.The zinc ferrite and zinc leaching residue were screened using a 200-mesh sieve to obtain fractions below 74 μm size as the experiment materials.

Leaching experiments
Batch tests were conducted in 50 ml centrifuge tubes with perforated membranes in a thermostatic waterbath oscillator with a speed of 260 r/min for 60 min.6 ml of the leaching solution was put in a centrifuge tube and heated to the set temperature followed by adding the mixture of zinc ferrite and copper powder.The leached residue at 60 min was collected via a centrifuge and the cake was washed and dried in a vacuum freeze dryer for 12 h.The addition of copper powder (k) was evaluated by the formulas: (1) where M is the molecular weight and m is the mass.
For the experiment of time, 0.5 ml sample of the solution was extracted using the pinhole filter membrane of 0.22 μm at 10, 20, 30, 40, 50, 60 min from 12 ml of the leaching solution.The solution was chemically analyzed to calculate the leaching rate (w).The computational formula was as follows: where VL is the calibrated volume of the leaching solution; CL is the concentrations of metals in the leaching solution; GS is the mass of the solid used in the experiment; WS is the content of metals in the solid.
All experiments were conducted at the open system.

Analytical methods
X-ray diffraction (XRD, D8 Advance, Bruker) and scanning electron microscopy (SEM-EDX, JSM-IT300, JEOL) were utilized to study the characteristics of the solids.Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES, Agilent 5100) was used to determine the total Zn, Fe and Cu in the solution.

Characterization of zinc ferrite, zinc leaching residue and copper powder
As indicated in XRD pattern of Figure 1-a, the synthetic solid was totally zinc ferrite.The zinc ferrite was constituted by piled particles in both nano size (~500 nm diameter) and micro size (~1.5 μm diameter) as illustrated in Fig. 1d, which agreed with the published study [15].The main phases in zinc leaching residue are zinc ferrite and lead sulphate as shown in Figure 1-b.The particle size of zinc ferrite in zinc leaching residue was similar to that of the synthetic one including nano size and micro size particles in Figure 1-e.X-ray diffraction pattern and SEM image of copper powder were presented as Figure 1-c and Figure 1-f, respectively.It could be seen that the copper powder was composed with independent coral-like particles, which were stacked by smaller granules (~5 μm diameter).The micro particle size of the copper powder was over 20 μm.
The zinc leaching residue sample contains 16.9% Zn and 24.8% Fe analyzed by ICP-OES.The mass of zinc ferrite in zinc leaching residue was calculated as 53.4% according to the ratio of Fe in zinc ferrite and in zinc leaching residue.

Leaching of zinc ferrite in the presence of copper
The effect of copper powder dosage on the leaching of zinc ferrite was studied under the conditions: liquid-solid ratio of 30, sulfuric acid concentration of 100 g/L and 60 ℃.As shown from Figure 2-a, the leaching efficiency of zinc were strongly dependent on the dosage of copper.When zinc ferrite was leached without copper, zinc was extracted only 19.3%.The leaching efficiencies of zinc increased to 43.2% and 76.4% as molar ratio of copper to zinc ferrite was 0.5 and 1.0, respectively.Zinc ferrite was completely dissolved when molar ratio of copper to zinc ferrite was 1.5 and 2.0. Figure 2-b illustrated the phase transformation of the residues.Without copper, the peak intensity of the residual zinc ferrite was stronger, indicating that pure acid leaching made more crystalline zinc ferrite stay in the leached residue.With copper, the peak intensities of the residual zinc ferrite were weaker as the of copper was more, demonstrating that copper helped destruction of zinc ferrite.When molar ratio of copper to zinc ferrite was 1.5, the residue was only copper.Notably, a little copper was found in the residue along with the main phase zinc ferrite when molar ratio of copper to zinc ferrite was 1.0.It could be speculated that excess copper was a must to make zinc ferrite disappear.The effect of temperature on the leaching of zinc ferrite was studied under the conditions: liquid solid ratio of 30 mL/g, sulfuric acid concentration of 100 g/L and molar ratio of copper to zinc ferrite = 1.5.As shown in Figure 3-a, the temperature put a positive effect on zinc leaching.The leaching efficiencies of zinc increased as the temperature increased from 28.8% at 30 ℃ to 78.6% at 50 ℃.When the temperature was over 60 ℃, the zinc extraction reached 100%.

Acid concentration.
The effect of acid concentration on the leaching of zinc ferrite was studied under the conditions: liquid-solid ratio of 30 mL/g, molar ratio of copper to zinc ferrite = 1.5 and temperature of 60 ℃.It could be seen that in the weak acid solution like 30 g/L sulfuric acid, facile leaching of zinc ferrite was realized.The leaching efficiency of zinc was 51.4% and 85.2% as the sulfuric acid concentration was 30 g/L and 50 g/L, much higher than that without copper when the sulfuric acid concentration was 100 g/L in Figure 3-b.Zinc ferrite was totally dissolved as the sulfuric acid concentration was over 70 g/L.

Ratio of liquid to solid.
The effect of liquid to solid ratio on the leaching of zinc ferrite was studied under the conditions: molar ratio of copper to zinc ferrite = 1.5, sulfuric acid concentration of 100 g/L and temperature of 60 ℃.As shown in Figure 3-c, zinc leaching efficiency increased proportionally to the ratio of liquid to solid when the ratio of liquid to solid was below 20 mL/g.The leaching efficiencies of zinc were 25.6%, 47.7% and 91.3% as ratio of liquid to solid was 5 mL/g, 10 mL/g and 20 mL/g.When ratio of liquid to solid was more than 25 mL/g, zinc ferrite was completely dissolved.It is indicated that sufficient acid is a must for the dissolution of zinc ferrite, which is an acid-consuming process.However, excessive acid is unnecessary because zinc ferrite has been completely dissolved, from results of acid concentration (Figure 3-b) and liquid-solid ratio (Figure 3-c).

Leaching of zinc leaching residue
The experiment of zinc leaching residue was studied under the conditions: molar ratio of copper to zinc ferrite = 1.5, ratio of liquid to solid (zinc ferrite in zinc leaching residue) = 30 mL/g, sulfuric acid concentration of 100 g/L and 60 ℃.As shown in Figure 4-a, the leaching behavior of zinc in zinc leaching residue was similar to that in the synthetic zinc ferrite.The leaching efficiency of zinc climbed up within 30 min from 40.7% at 10 min to 83% at 30 min and reached a plateau after 40 min at 94.2%.

Conclusions
In this study, copper powder was used as a reductant in the sulfuric acid system to realize the facile and efficient dissolution of the man-made zinc ferrite as well as zinc ferrite-bearing zinc leaching residue.Zinc ferrite could be completely dissolved within 60 min at 60 ℃ without concentrated acid.Under the conditions: molar ratio of copper to zinc ferrite = 1.5, ratio of liquid to solid (zinc ferrite in zinc leaching residue) = 30 mL/g, sulfuric acid concentration of 100 g/L and temperature of 60 ℃, the extraction of zinc from zinc leaching residue reached 94.2%.Lead sulphate was the main phase in the leached residue.

Figure 1 .
Figure 1.X-ray diffraction patterns and SEM images of the synthetic zinc ferrite (a) (d), zinc leaching residue (b) (e) and copper powder (c) (f).

Figure 2 .
Figure 2. Leaching of zinc ferrite in the presence of copper (a) zinc leaching efficiencies; (b) X-ray diffraction patterns of the leached residues

3. 3 . 4
Time.The experiment of time was conducted under the conditions: molar ratio of copper to zinc ferrite = 1.5, sulfuric acid concentration of 100 g/L, ratio of liquid to solid=30 mL/g and 60 ℃.As shown in Figure3-d, the rate of zinc extraction maintained at a high level within 30 min as the reaction proceeded.The leaching efficiencies of zinc was 13.6%, 43.3% and 79.5% at 10 min, 20 min and 30 min, respectively.When the reaction time was 40 min, the extraction of zinc was 97.1%.Zinc extraction reached 100% after 50 min, indicating that zinc ferrite could be completely dissolved within 60 min.

Figure 3 .
Figure 3. Effects of factors on zinc leaching (a) temperature; (b) sulfuric acid concentration; (c) ratio of liquid to solid; (d) time

Figure 4 -
b compared the constituents of the leached residue with those of the pristine zinc leaching residue.It could be seen that zinc ferrite almost disappeared after leaching and lead sulphate was the main phase in the leach residue, demonstrating that copper worked for the industrial zinc ferrite and very efficient for zinc extraction from zinc ferrite-bearing zinc leaching residue.

Figure 4 .
Figure 4. Leaching of zinc leaching residue (a) zinc leaching efficiencies; (b) X-ray diffraction patterns of the leached residue

Table 1 .
Previously reported reductive leaching conditions and results of zinc ferrite.