2019 Volume 59 Issue 1 Pages 201-207
Blast furnace sludge (BFS) is an industrial waste generated during ironmaking, which contains approximately 10% Zn, 25% Fe and 24% C and is a valuable secondary resource of zinc. In this study, ZnSO4·7H2O was produced using BFS by the combination of acid leaching, iron and calcium precipitation, concentration and crystallization. The effects of different factors and operation steps on zinc recovery were investigated, and the optimum parameters of the treatment process were obtained as follows: Acid leaching was carried out at 60°C for 10 min using sulfuric acid with the concentration of 150 g/L and a liquid to solid ratio at 3 mL/g; Iron precipitation was performed at 25°C using the liquid solution with pH value in a range of 4.0–4.4 and the H2O2 dosage of 60 mL/L; Calcium precipitation was conducted at 60°C for 40 min with the ZnF2 dosage of 7.5 g/L. The recovery efficiency of zinc was approximately 95% and the purity of ZnSO4·7H2O in the product reached 99.57%. The obtained results indicate that the proposed process in this work can successfully recover Zn from BFS with high efficiency and short production route.
Blast furnace sludge (BFS) is a by-product obtained by the wet dust removal in the process of blast furnace ironmaking.1,2) The main components of BFS are iron, carbon and zinc. In addition, BFS also contains some harmful ingredients, such as lead, arsenic et al.3) BFS has the characteristics of fine particle size and heterogeneous, surface asperities, low density, unique crystal phases, difficult separation, easy reaction and strong corrosion.4) With the rapid development of the global iron and steel industry, the output of BFS increases constantly with increasing blast furnace volume and iron production. BFS is a toxic solid waste, disposal of which would cause a great waste of resources and serious environmental pollution.5,6) Therefore, government and research organization have paid attention to the treatment and comprehensive utilization of BFS.
Previously, large amount of BFS was discharged and stockpiled, which not only occupy much land but also cause pollution of the environment and waste resources. Traditional technology to treat BFS was to use it as sintering burden or ironmaking raw materials.7) However, because of the high temperatures in the process of blast furnace ironmaking, zinc volatilization occurred in the furnace and subsequently condenses on the walls of the furnace at lower temperature, resulting in upper of BF heeling and gas pipe blocking. Some researchers studied the zinc removing from BFS by carbothermal reduction,8,9) leaching with hydrometallurgical process10) and hydrocyclone separation.11) Moreover, some researchers used BFS to produce binders and catalysts.12,13) However, all above-mentioned processes have the problems, such as high-energy consumption, low metal recovery efficiency and serious pollution.
Affords have been made by some researchers to recover zinc by leaching, iron precipitation, solvent extraction and electro-deposition process, and the obtained results showed that the extraction efficiency of zinc could only reach 80%.14,15) It was reported that ZnO in BFS could react with NH4SCN to form Zn(SCN)2 dissolved easily in water16) and zinc from BFS was recovered by chlorination roasting with CaCl2 for the sublimation of ZnCl2 under oxidizing atmosphere.17) Zinc recovery from BFS using rotary kiln was also reported in reference. In the process, zinc is volatilized in the form of ZnO at high temperature.18) Some studies were carried out to recover iron concentrate and coke through ore dressing, such as magnetic separation, gravity separation and flotation, but the recovery efficiency of zinc is low.19) In addition, BFS was found to be a good adsorbent for Pb2+, Cu2+, Cr2+, Cd2+ and Zn2+ from aqueous solutions.
ZnSO4·7H2O is an important inorganic chemical material with wide application in various industrial fields, such as manufacturing lithopone, preservation of wood and leather, bone glue clarification and artificial fiber precipitation agent. In addition, it is widely used in ore dressing, medicine, electroplating, agriculture, etc.20) The ZnSO4·7H2O was traditionally produced through the procedure of sphalerite roasting and leaching with sulfuric acid. However, the process has many disadvantages such as long production route, high-energy consumption and serious environmental pollution.21) Other method to produce ZnSO4·7H2O is direct leaching of metallic zinc using acid. The process is simple with less environmental pollution and high production efficiency, but there are also some problems such as high production cost and low economic efficiency.22) Compared with other processes of ZnSO4·7H2O preparation and Zn recovery from BFS, ZnSO4·7H2O produced from blast furnace sludge achieved comprehensive utilization of resources, and the process was less environmental pollution and higher economic efficiency.
In the present work, a novel hydrometallurgical process is proposed to produce ZnSO4·7H2O from the waste product of blast furnace ironmaking. The proposed process could recover Zn from blast furnace sludge with high metal recoveries and short process route. The effects of different parameters on zinc recovery in each stage of the treatment will be investigated in detail, and the optimal parameters for the preparation of ZnSO4·7H2O will be discussed.
The BFS was obtained from Panzhihua iron & steel Co., located in Sichuan Province, China. The chemical composition of BFS is shown in Table 1. It can be seen from the table that the main chemical components of BFS include iron, carbon, zinc, SiO2, Al2O3 and alkaline earth oxides. The XRD analysis is shown in Fig. 1. The characteristic peak intensity positions show that the main phases presented in this sample are hematite (Fe2O3), magnetite (Fe3O4), graphite (C), zinc oxide (ZnO) and quartz (SiO2).
| Component | TFe | C | TZn | TPb | SiO2 | Al2O3 | MgO | CaO |
|---|---|---|---|---|---|---|---|---|
| Content | 25.21 | 24.34 | 10.08 | 1.69 | 6.24 | 3.33 | 1.08 | 2.13 |
XRD pattern of the blast furnace sludge.
The flow sheet for the production of ZnSO4·7H2O from BFS was given in Fig. 2. The whole process consists of four main stages: (1) pretreatment of BFS; (2) acid leaching; (3) iron and calcium precipitation by oxidation and neutralization; (4) concentration by evaporation, and crystallization following by cooling, filtration and drying.
Flow chart for preparing ZnSO4·7H2O using zinc bearing blast furnace sludge.
BFS sample was crushed, dried at 60°C, ground, and sieved (100 mesh) through a metal sieve. In the leaching process, H2SO4 was used to leach BFS. The effects of various parameters such as H2SO4 concentration (90 g/L–220 g/L), leaching temperature (25°C–80°C), the ratio of liquid to solid (1 mL/g–6 mL/g) and leaching time (5 min–40 min) were examined. All preliminary leaching experiments were carried out in a 250 mL conical flask with an agitation rate of 200 rpm at a water-bath using a raw material mass of 30 g. After leaching, the slurry was separated by filtration. The filter residues were washed with distilled water, and the leaching solutions were analyzed.
After acid leaching, the filter liquor was subjected to H2O2 (mass percentage, 30%) oxidizing and CaCO3 neutralizing to precipitate iron. The removing iron experiments were carried out in a 500 mL beaker. The pH meter was used to monitor the pH value of the solution. The effects of pH value (3.2–5.4), temperature (25°C–80°C) and H2O2 dosage (0–75 mL/L) were investigated. Finally, the solution after removing iron was separated from solid residue by filtration, and the obtained solution was analyzed.
The calcium precipitation was conducted by the addition of ZnF2 powder. The experiments were carried out in a beaker with continuous stirring (an agitation rate of 200 rpm) in a water-bath. The main influence factors, such as ZnF2 dosage (1.5–9.0 g/L), reaction time (5 min–60 min) and reaction temperature (25°C–80°C) were studied. Finally, the separation between calcium residues and purified zinc sulfate solution was completed using vacuum filtration.
The purified zinc sulfate solution was concentrated under boiling, and then the solution was further concentrated at lower temperatures in water-bath until the saturation of the zinc sulfate was reached. The solution saturated with ZnSO4 was then crystallized by vaporization at room temperature. The final crystal slurry was separated to produce pure ZnSO4·7H2O.
2.4. Analysis MethodsThe chemical composition of BFS was analyzed by X-ray fluorescence method; the mineralogical composition was determined by powder X-ray diffraction method using a Japan Science D/max-R diffractometer with Cu Kα radiation (λ = 1.5406 Å) from 10° to 90°, an operating voltage of 40 kV and a current of 40 mA. The morphology of the sample was observed using Scanning Electron Microscopy (HITACHI-S3400N). The concentrations of Zn, Ca and Fe in solution were analyzed by Atomic Absorption Spectroscopy (PinAAcle900).
During the leaching process of the BFS using sulphuric acid, the following chemical reactions may occur:23,24)
| (1) |
| (2) |
| (3) |
| (4) |
Iron oxides (Fe2O3, Fe3O4) react with sulphuric acid to generate Fe2+ and Fe3+, which then move into the leaching solution. This makes the iron separation from leaching solution difficult and it also increases acid consumption. Calcium oxide can rapidly react with sulphuric acid to generate calcium sulphate that has a limited solubility in water, and most of them remain in the solid residues.
The effect of H2SO4 concentration (90–220 g/L) on leaching efficiency of Zn and Fe was investigated with a leaching time of 10 min, liquid to solid ratio of 2 mL/g, and a stirring speed of 200 rpm at 25°C. As shown in Fig. 3(a), the leaching efficiency of Zn increases with increasing H2SO4 concentration up to about 150 g/L, and the leaching efficiency of Fe gradually decreases with decreasing the H2SO4 concentration. It was reported that increasing sulphuric acid concentration could improve the activity of hydrogen ion and thus increase the rate of chemical reaction. The excess sulphuric acid could significantly increase acid consumption and make the purification operation difficult. In this case, the optimum concentration of H2SO4 is selected as 150 g/L.
Effect of different conditions on leaching efficiency of Zn from BFS. (a) H2SO4 concentration; (b) liquid to solid ratio; (c) leaching time; (d) leaching temperature.
As shown in Fig. 3(b), the effect of liquid to solid ratio on the leaching efficiency of Zn and Fe was studied using 150 g/L H2SO4 at 60°C for 10 min. It is shown that the leaching efficiency of Zn increases with increasing liquid to solid ratio up to about 3 mL/g and then the curve remains constant. When the liquid to solid ratio increases, the viscosity of the slurry decreases, which will facilitate the mixing and reduce the mass transfer resistance. This is beneficial to improve the zinc extraction.25) Therefore, the higher liquid to solid ratio leads to a high consumption of reaction reagents and low concentration of Zn. The optimum liquid to solid ratio should be selected as 3 mL/g.
The effect of time on leaching efficiency of Zn and Fe is shown in Fig. 3(c), with H2SO4 concentration of 150 g/L, liquid to solid ratio of 3 mL/g, and stirring speed of 200 rpm at 25°C. It can be observed that the leaching efficiency of Zn and Fe increases with increasing leaching time to 10 min. There is no significant increase found when the leaching time is over 10 min, and this indicates that the adequate leaching time should be 10 min. Longer leaching time will result in higher energy consumption and bigger leaching equipment required for a given amount of BFS to be treated.
The effect of leaching temperature (25–80°C) on leaching efficiency of Zn and Fe was studied with H2SO4 Concentration of 150 g/L, leaching time of 10 min, leaching stirring speed of 200 rpm and liquid to solid ratio of 3 mL/g, as shown in Fig. 3(d). The leaching efficiency of Zn and Fe gradually increases with increasing the leaching temperature. This can be attributed to that the relatively high temperature will improve the atomic/molecular collisions and mass transfer rate.26,27) Higher leaching temperature can increase the extraction of zinc, but it will result in the increase in the impurities and energy consumption. Thus, the optimum temperature is selected as 60°C.
3.2. Solution Purification 3.2.1. Precipitation of Iron by Oxidation and NeutralizationThe leaching liquor of BFS mainly contains Zn2+, Fe2+ and Fe3+. The iron in the solution is not helpful to ZnSO4·7H2O crystallization. Thus, it is necessary to remove iron before crystallization. Figure 4 shows the E–pH diagram of the Zn–Fe–H2O systems at 25°C. According to Fig. 4, the order of the pH value for three ions initial precipitation is Fe3+<Zn2+< Fe2+. When the Zn2+ starts hydrolysis, the Fe3+ has been hydrolyzed extremely while the Fe2+ has been not hydrolyzed. Therefore, the Fe3+ ions can be removed by the formation of Fe(OH)3 precipitation. However, the Fe2+ cannot be removed by means of neutralizing acid directly, so Fe2+ must be oxidized to Fe3+ by added oxidative reagent.28) The XRD analysis in Fig. 5 showed that the iron residue is mainly composed of CaSO4·2H2O and Fe(OH)3.
E–pH diagram of the Zn–Fe–H2O system at 25°C.
XRD pattern of the iron residue.
During the process of iron removing, the main chemical reactions can be described are as follows:29)
| (5) |
| (6) |
| (7) |
According to Eq. (5), the reaction should be carried out in acidic condition. Fe3+ will precipitate with Eq. (6). Since sulfuric acid produced in the hydrolysis process, the acidity of the solution will increase with this process. Therefore, it is necessary to neutralize the acid during the hydrolysis to ensure that iron was completely precipitated with Eq. (6).
The effect of pH on the precipitation of iron ions was studied using 60 mL/L H2O2 at 25°C, and the results are presented in Fig. 6(a). It can be seen that Fe removal efficiency increases with the increase of pH and then it remains constant when pH increases to 4.0. More than 99.7% of iron ions can be removed when the pH is higher than 4.0. As the pH of the solution is increased, the acid formed during the hydrolysis was neutralized, and the equilibrium of Eq. (6) should shift to the right side, resulting in an increase in the removal rate of iron. On the other hand, the zinc loss efficiency increases slowly with the increase of pH from 3.2 to 4.4, and then it increases rapidly with the pH increase from 4.4 to 5.4. When the pH value is greater than 4.0, part of Zn2+ can hydrolyze and participate into residue. Meanwhile, the adsorption rate of Zn2+ on ferric hydroxide decreases with increasing pH, which results in a decrease of zinc loss efficiency. Considering the higher reagent consumption and larger zinc loss at high pH values, it should be appropriate to keep pH value in the range of 4.0–4.4.
Effect of different conditions on the precipitation of iron during oxidation and neutralization. (a) pH value; (b) H2O2 dosage; (c) temperature.
The effect of H2O2 dosa ge on the precipitation of iron was investigated in the range of 0–75 mL/L using pH value of 4.0–4.4 at 25°C, as shown in Fig. 6(b). As can be seen in the figure, the Fe removal efficiency increases to the maximum (99.76%) with increasing the H2O2 dosage from 0 to 60 mL/L. This may be attributed to the fact that increasing H2O2 dosage increases the activity of reactants, resulting in the shift of the equilibrium of Eq. (5) to the right side. When the H2O2 dosage increased over 60 mL/L, no significant change on the removal efficiency can be observed. However, the loss efficiency of zinc remained stable during this process, which indicates that H2O2 dosage has no significant effect on the zinc loss. Thus, the optimal H2O2 dosage for iron removing is selected as 60 mL/L.
The effect of temperature on the precipitation of iron was studied in the temperature range of 25–80°C using pH value of 4.0–4.4 and 60 mL/L H2O2, as shown in Fig. 6(c). It shows that the Fe removal efficiency remains steady (about 99.9%), increase of temperature has no significant effect on iron removal. The loss efficiency of zinc increases with increasing temperature, and this behavior may be attributed to the increase of Zn2+ adsorption by ferric hydroxide. Thus, considering its high energy consumption and high zinc loss at elevated temperature, the optimal temperature for iron removal is selected as room temperature (25°C).
3.2.2. Precipitation of CalciumAccording to thermodynamic data, the solubility of calcium fluoride in the solution is low, which indicates that the Ca2+ could combine with F− to form calcium fluoride, which is insoluble in water. Thus, the Ca2+ can be removed by ion precipitation.30)
| (8) |
Figure 7(a) shows the E–pH diagram of the Ca–F–H2O systems at 60°C, and the system allocates the stability area of CaF2 in a wide range of pH values from 1.43 to 13.52. The CaF2 could be dissolved only under strong acid (pH<1.43) condition. The Ca2+ in solution could react with the HF or F− to generate CaF2 in the pH range of 1.43 to 13.52.29,31) Figure 7(b) shows the E–pH diagram of the Zn–F–H2O system at 60°C, and the system allocates in the stable area of ZnF2 in a narrow range of pH values from 4.03 to 5.00. The ZnF2 can be dissociated into Zn2+ and F−, and F− enters into the solution when pH is less than 4.03. The ZnF2 would be hydrolyzed to F− and F− enters into the solution when pH values is more than 5.00, which must lead to higher F− concentration in the solution. Thus, it may be better that the reaction of ZnF2 dissolution and hydrolysis was not occurred. If using ZnF2 as calcium precipitation reagent, the pH value should be controlled at 4.03 to 5.00 in the calcium precipitation process; the Ca2+ precipitation and F− accumulation in the solution should be avoided.32)
E–pH diagram of different systems at 60°C. (a) Ca–F−–H2O; (b) Zn–F−–H2O.
The effect of ZnF2 dosage on the precipitation of calcium was investigated in the range of 1.5–9.0 g/L for 40 min at 60°C, and the results are showed in Fig. 8(a). It can be observed that the removing efficiency of calcium increases with increasing ZnF2 dosage. Thereafter, when ZnF2 dosage increases to 7.5 g/L, the curve becomes steady. The removing efficiency of calcium is 98.53% when ZnF2 dosage is 7.5 g/L. Similarly, when the dosage of ZnF2 increases, the activity of reactant increases and the equilibrium constant of the reaction also increases. This is beneficial to improve the conversion efficiency of calcium. Finally, the optimal ZnF2 dosage is 7.5 g/L.
Effect of different conditions on the precipitation of calcium. (a) ZnF2 dosage; (b) temperature; (c) time.
The effect of temperature on the precipitation of calcium was studied by using the solution with 7.5 g/L ZnF2 for 40 min with a stirring rate of 200 rpm, and the results are presented in Fig. 8(b). It can be seen that the precipitation efficiency of calcium increases with increasing temperature, and the precipitation efficiency of calcium is 98.53% when the temperature increases to 60°C. With increasing temperature, the activity and movement velocity of reactants as well as the diffusion coefficient of the components are all increased. This is beneficial to the production of calcium fluoride.33) The optimal temperature obtained is 60°C.
The effect of reaction time on the precipitation of calcium was studied with the conditions of ZnF2 dosage of 7.5 g/L at 60°C, and a stirring speed of 200 r/min. As shown in Fig. 8(c), the precipitation efficiency of calcium increases with increasing reaction time, and after 40 min, the precipitation efficiency of calcium will reach its maximum at 98.53%. This indicates that 40 min is sufficient for the maximum precipitation efficiency of calcium.
3.3. Crystallization of Zinc Sulfuric by Evaporation and CoolingThe purified zinc sulfuric solution was concentrated by evaporation until the saturation of zinc sulfate reached in the solution. The cooling crystallization was completed at 25°C within 60 min. After filtration and drying, the ZnSO4·7H2O product was obtained. The result of chemical analysis showed that the content of the ZnSO4·7H2O in the final product was 99.57%. The recovery efficiency of zinc was 94.84% in the whole process. The X-ray diffraction pattern and SEM micrograph of the ZnSO4·7H2O product are shown in Figs. 9 and 10. The diffraction peaks of the product match well with the standard diffraction peaks of ZnSO4·7H2O, indicating that the product of pure ZnSO4·7H2O was obtained. It is observed that the particle size of product is larger and the granularity is relatively homogeneous.
XRD pattern of the ZnSO4·7H2O product.
SEM image of the ZnSO4·7H2O product.
A novel process for the treatment of blast furnace sludge (BFS) was studied through the combination of acid leaching, iron and calcium precipitation, concentration and crystallization. A new product, ZnSO4·7H2O, with higher added value and extensive market demand was produced. The optimal parameters for ZnSO4·7H2O production were obtained as follows: (1) Acid leaching on the BFS was carried out at 60°C for 10 min using sulfuric acid with the concentration of 150 g/L and a liquid to solid ratio of 3 mL/g; (2) Iron precipitation was performed at 25°C with the liquid solution with pH value in a range of 4.0–4.4 and the H2O2 dosage of 60 mL/L; while Calcium precipitation was conducted at 60°C for 40 min with the ZnF2 dosage of 7.5 g/L; (3) Crystallization of Zinc sulphate solution was carried out at 25°C for 60 min.
The quality of final product obtained, ZnSO4·7H2O, was found to be higher than the requirement of national standard. The recovery efficiency of zinc was 94.84% and the purity of ZnSO4·7H2O in the product reached 99.57%. The results obtained suggest that proposed process could achieve the goals of comprehensive use of waste product of blast furnace and recovery of Zn from its secondary resource with the advantages of high efficiency.
Financial support for this study was supplied from the National Natural Science Foundation of China (Project Nos. 51764035).
