Green Recycling of Goethite and Gypsum Residues in

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Green Recycling of Goethite and Gypsum Residues in Hydrometallurgy with #-Fe3O4 and #-Fe2O3 Nanoparticles: Application, Characterization and DFT Calculation Tong Yue, Zhen Niu, Hongbiao Tao, Xiao He, Wei Sun, Yuehua Hu, and Zhenghe Xu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06142 • Publication Date (Web): 11 Mar 2019 Downloaded from http://pubs.acs.org on March 17, 2019

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ACS Sustainable Chemistry & Engineering

Green

Recycling

Hydrometallurgy

of

Goethite

and

Gypsum

with

α-Fe3O4

and

γ-Fe2O3

Residues

in

Nanoparticles:

Application, Characterization and DFT Calculation

Tong Yue†,‡, Zhen Niu‡, Hongbiao Tao‡, Xiao He‡, Wei Sun*†, Yuehua Hu†, and Zhenghe Xu*‡,§ †: School of Minerals Processing and Bioengineering, Central South University,

Changsha 410083, China ‡: Department of Chemical and Materials Engineering, University of Alberta,

Edmonton, Alberta, Canada T6G 1H9. §: Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen, Guangdong 518055, China

Corresponding authors: Wei Sun Address: 310 Shengwu Building, 932 Lushannan Road, Changsha 410083, China Email: [email protected] Zhenghe Xu Address: 12-354, Donadeo Innovation Centre for Engineering, 9211-116 Street, Edmonton, Alberta, Canada T6G 1H9 Email: [email protected]

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ACS Paragon Plus Environment

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Abstract Millions of tons of hazardous iron oxide residues are produced during the iron purification

process

of

sulphate

leaching

solutions

in

nonferrous

metals

hydrometallurgy industry per year. The generated iron oxide residues, which mainly contain goethite and gypsum precipitates, pose great threats to the local ecological environment and human health. We proposed a novel method, treatment and recovery of goethite and gypsum by the synthetic magnetic nanoparticles (MNPs) such as αFe3O4 and γ-Fe2O3, to treat the residues efficiently and cost-effectively. MNPs served as the magnetic crystal nuclei of the goethite precipitates during the iron purification process, and the goethite and gypsum precipitates formed under this condition can be separated in magnetic field for recycling purposes. The separation efficiency of the goethite and gypsum precipitates was much higher when γ-Fe2O3 was used as the crystal nuclei, indicating that the surface of γ-Fe2O3 was more favorable for the formation of goethite particles than α-Fe3O4, which has also been verified by SEM, FBRM, XRD, TEM and XPS analysis. DFT calculations suggested that the binding energy between the MNPs and iron hydroxide plays a critical role and is responsible for the distinguished collecting efficiencies of α-Fe3O4 and γ-Fe2O3 towards goethite. Keywords: Hazardous residues; Waste treatment; Magnetic nanoparticles; DFT calculation; Green recycle Synopsis:

Green separation and recycle of the hazardous iron oxide residues from

hydrometallurgy industry by introducing magnetic nanoparticles to realize the sustainable development of environment and resources. 2


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Introduction For the green production and sustainable development of environment, the treatment of industrial hazardous solid waste has received more and more attention.1-5 In nonferrous metals hydrometallurgy industry, millions of tons of hazardous iron oxide residues are produced during the iron removal process in sulphate leaching solutions.6, 7

The iron oxide residues are highly undesirable as they are normally contaminated with

heavy and hazardous elements, such as Zn, Ni, Cu, Pb, As, Co, In and S, which pose great threats to the environment and human health.8-10 Iron can be removed from leaching solutions by forming goethite precipitates using the slurry of calcium hydroxide, calcium oxide or calcium carbonate as the pH neutralizer because they are low in cost and they do not introduce impurity ions in the sulphate system. Therefore, the iron residues produced by the goethite process mainly consist of goethite and gypsum precipitates. However, the treatment of the goethite and gypsum mixed residues was rarely reported previously due to the fact that the low content of Fe (at around 15 wt.%) and Ca (at around 20 wt.%) in the residues makes the recovery difficult. To recycle the goethite and gypsum mixed residues, the goethite precipitates and gypsum precipitates are usually separated first during the iron removal process, and then the hazardous elements (such as As and S) from the precipitates will be removed using reductive roasting. The treated residues can be reused as building materials, raw materials of iron-making and painting, etc.11-13 Magnetic separation techniques have long been in use, and intensive research into superparamagnetic nanomaterials has

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accelerated the development of magnetically recoverable catalysts.14-16 In our previous paper,17 we proposed a novel method, using maghemite fine particles as the magnetic seeds to induce the formation of goethite surface precipitates in the iron removal process, followed by separating the goethite and gypsum residues in the magnetic field and recycling these residues after As and S removal. This method can not only realize simultaneous recovery of iron and calcium, but also eliminate potential environmental risks caused by the iron oxide residues. The formation of goethite and gypsum precipitates in aqueous solution is a crystallization process involving nucleation and crystal growth.18 The addition of crystal nuclei in the crystallization system in advance is important for manipulating the crystal form and accelerating crystallization.19, 20 We found that the goethite precipitates preferred to form on the surface of maghemite nanoparticles rather than that of magnetite nanoparticles during the iron removal process. The adsorption data of ferric ions on magnetite and maghemite nanoparticles were fitted by the surface complexation/precipitation model to reveal the mechanism of goethite surface precipitates formation on magnetite and maghemite nanoparticles. The fitting results demonstrated a transformation process of polynuclear surface complexes to surface precipitates and revealed the different mechanisms of goethite surface precipitates formation on magnetite and maghemite.18 In order to reveal the mechanism of goethite surface precipitates systematically, we used the synthetic magnetic nanoparticles (MNPs) α-Fe3O4 (magnetite) and γ-Fe2O3 (maghemite) as the magnetic crystal nuclei of goethite precipitates and applied 4


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magnetic field to separate goethite and gypsum in this research. The separation efficiency of goethite and gypsum precipitates as well as the magnetization of the concentrates with the two MNPs were investigated. In addition, the feasibility of cycle use of the MNPs was studied. The properties of the goethite precipitates formed on the surface of α-Fe3O4 and γ-Fe2O3 were characterized thoroughly by an array of techniques, such as FBRM, XRD, TEM and XPS. DFT calculations were used to verify the experimental work and explore the mechanism of goethite precipitates formation on the surface of the MNPs at molecular level. Material and methods Synthetic procedures of α-Fe3O4 and γ-Fe2O3 nanoparticles The nano-sized α-Fe3O4 (magnetite) particles were prepared in an aqueous solution at room temperature. 5 mL mixed solution of 8.92g FeCl3·6H2O and 3.28g FeCl2·4H2O was added into 50 mL NH3·H2O solution (1 mol/L) dropwise with slightly stirring. The resulting solution was centrifuged first, and then the obtained precipitates was washed and dried at 50℃ for 48h to prepare the α-Fe3O4 nanoparticles. The reaction equation of this synthetic procedure was shown in Eqs 1. 2Fe3+ + Fe2+ + 8NH3·H2O → α-Fe3O4 + 8NH4+ + 4H2O

(1)

Half amount of the obtained α-Fe3O4 nanoparticles was roasted at 250℃ for 1h in atmosphere to prepare the γ-Fe2O3 nanoparticles (Eqs 2). The change of colour from black to brown indicated the lattice transformation of the particles from α-Fe3O4 to γFe2O3. 5



4α-Fe3O4 + O2 → 6γ-Fe2O3

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(2)

Iron removal experiments with magnetic nanoparticles 200mL FeSO4 (0.1 mol/L) and ZnSO4 (1 mol/L) solution at pH 2.0 was prepared in lab to simulate the sulphuric acid leach solution for zinc calcination. The mixed solution was heated to 85℃ by water bath. The magnetic nanoparticles (α-Fe3O4 or γ-Fe2O3) with concentration ranging from 1 to 7 g/L were added into the solution in advance with stirring at 250 rpm. Oxygen gas (0.3 m3/min) and Ca(OH)2 slurry were then injected into the suspension simultaneously to maintain the pH within the range of 3.5-4. Goethite and gypsum precipitates were formed during this process. After precipitation, the suspension was separated into concentrate and tailing by pouring it into a magnetic tube with a magnetic field of 1000 Gs. All samples were filtered, washed and dried. The reaction equations in the iron removal experiments were listed as follows: 4Fe2+ + O2 + 4H+ → 4Fe3+ + 2H2O

(3)

Fe3+ + 3OH- → FeOOH(s) + H2O

(4)

Ca2+ + SO42- + 2H2O → CaSO4·2H2O(s)

(5)

The Fe recovery (RFe) and Ca recovery (RCa) in the concentrates was calculated as follows: (6) (7) Where 𝛽Fe, 𝛽FeM and 𝛽Fe’ are the content of Fe element in the concentrates, MNPs and tailings, which were obtained by the XRF analysis; m, mM and m’ in Eq (6) are the mass 6


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of the concentrates, MNPs and tailings. Similarly, 𝛽Ca and 𝛽Ca’ are the content of Ca element in the concentrates and tailings; m and m’ in Eq (7) are the mass of the concentrates and tailings. Recovery and cycle use of magnetic nanoparticles To recycle the magnetic nanoparticles from the obtained concentrates, a 2L corundum rod mill was used to grind the two concentrates with water (wt. 25%) for up to 10 mins to dissociate goethite precipitates with the magnetic nanoparticles, followed by collecting the magnetic nanoparticles at different magnetic fields (500 Gs, 750 Gs and 1000 Gs). Subsequently, the obtained magnetic nanoparticles were reused as the magnetic seeds in a new iron removal process as described above to investigate the cycle efficiency of the two magnetic nanoparticles. Analytical techniques Fourier transform infrared spectroscopy (FTIR, Spectrum One, Perkin Elmer, US) was used for the identification of the magnetic nanoparticles, while the size distribution was analyzed by a laser particle analyzer (Mastersizer 3000, Malvern, UK). The in-situ particle size distribution was obtained from a focused beam reflectance measurement (FBRM, S400A, Mettler Toledo, UK). The elementary compositions and morphologies of the obtained concentrates and tailings in the magnetic separations were analyzed by the X-ray fluorescence (XRF, AxiosmAX, Panalytical B.V., Netherlands) and scanning electron microscopy (SEM, HELIOS 600i, FEI, US), respectively. The magnetometer 7



(PPMS, Quantum Design, US) was used for the magnetization measurement and the the X-ray diffraction (XRD, SIMENS D500, Bruker, Switzerland) was used to analyze the crystal structures of all the samples. Transmission electron microscope (TEM, JEMF200, JEOL, Japan) and X-ray photoelectron spectroscopy (XPS, PHI-5702, PerkinElmer, US) analyses were performed to study the surface microstructure and elemental speciation of the concentrates and tailings, respectively.

Results and discussion Characterization of the MNPs The FTIR spectra and images of the two synthetic magnetic nanoparticles (MNPs) are shown in Figure 1a. The peaks at 1627.72cm-1 and 3428.23 cm-1 were assigned to the bending and stretching vibrations of the adsorbed H2O molecules, respectively.21 The characteristic peak of α-Fe3O4 at 565.52 cm-1 was associated with the stretching vibration of the Fe-O bond, while the characteristic peak of the Fe-O bond in γ-Fe2O3 splited into two peaks at 635.9 cm-1 and 556.1 cm-1, which agrees well with literature sources.22-24 The specific colour of the two MNPs (black for α-Fe3O4 and brown for γFe2O3) suggested that the targeted magnetic nanoparticles were synthesized. The particle size of the prepared α-Fe3O4 mainly fell into the range of 70 to 400 nm, while the size of the prepared γ-Fe2O3 was smaller, ranging from 25 to 110 nm(Figure 1b).

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Figure 1. FTIR spectra and images (a) of the prepared α-Fe3O4 and γ-Fe2O3 NPs; particle size distribution (b) of the prepared α-Fe3O4 and γ-Fe2O3 NPs.

Magnetic separation of the goethite and gypsum precipitates We added either α-Fe3O4 or γ-Fe2O3 NPs into the FeSO4 and ZnSO4 solution as the crystal nuclei of goethite in advance during the iron removal process. After the formation of the goethite and gypsum precipitates, the suspension was separated into concentrates and tailings by a magnetic tube. The recoveries of Fe and Ca in the concentrates and tailings with different dosages of α-Fe3O4 or γ-Fe2O3 NPs are shown in Figure 2a. The Fe recovery in the concentrates increased from 15.01% to 52.43% / 29.29% to 96.29% with increasing the dosage of αFe3O4 / γ-Fe2O3 from 1 to 7 g/L, which indicated that the amount of goethite precipitates formed on the surface of γ-Fe2O3 was much larger than that on α-Fe3O4 surface. When used as the crystal nuclei of the goethite precipitates, γ-Fe2O3 NPs had much better performance than that of α-Fe3O4 NPs. It can be also observed that, the losses of Ca in the concentrates were almost the same with different MNPs addition, and increased slightly with increase in the dosage of the MNPs, suggesting that gypsum precipitates 9



lost in the concentrates was not affected by the MNPs. The slight increase in the loss of Ca in the concentrates would be attributed to the entrainment of gypsum by the magnetic particles in the magnetic separation. The microtopography of the concentrates and tailings with the two MNPs is shown in the SEM images (Figure 2b-2e). In the concentrates, the goethite particles with αFe3O4 as the crystal nuclei were dispersive and less than 1 µm in diameter, while the goethite particles with γ-Fe2O3 as the crystal nuclei aggregated into large particles. The newly formed goethite precipitates on the surface of the MNPs have low crystallinity and can be used as the flocculants between different goethite precipitates. Hence, it is hard for the α-Fe3O4 particles with a small amount of goethite attached on the surface to aggregate with each other. As can be seen from the SEM image of tailing for αFe3O4, there were many goethite particles on the surface of the gypsum particles. These goethite particles, which were formed in the solution rather than on the α-Fe3O4 surface, were non-magnetic and therefore could not be collected during magnetic separation. Instead, those non-magnetic goethite particles entered the tailings with gypsum particles. Only gypsum particles appeared in the tailing for γ-Fe2O3, which suggested that almost all the goethite particles were collected into the concentrate for γ-Fe2O3. To probe the separation abilities of the goethite and gypsum precipitates by the magnetic tube, the magnetization of the prepared α-Fe3O4 nanoparticles, γ-Fe2O3 nanoparticles, and the concentrates with 7 g/L α-Fe3O4 or γ-Fe2O3 was measured by a magnetometer at room temperature (Figure 2f). The saturation magnetization of bare α-Fe3O4 nanoparticles was 84.9 emu/g, which is consistent with the reported value in 10


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literature.25 The saturation magnetization of bare γ-Fe2O3 nanoparticles was slightly lower than that of α-Fe3O4 as the γ-Fe2O3 nanoparticles were prepared by oxidizing roasting of α-Fe3O4 at 250 ℃ . When the MNPs were coated by goethite precipitates, the saturation magnetization was 54.7 emu/g for the concentrate with α-Fe3O4 and 34.5 emu/g for the concentrate with γ-Fe2O3, decreased by 35.6% and 55.4% compared with the corresponding bare MNPs. Because goethite is antiferromagnetic and could not be magnetized, the magnetization reduction ratios of MNPs and the corresponding concentrates reflected the mass ratio of goethite and MNPs in the concentrates. The values of the reduction ratios were in excellent agreement with the recovery of Fe in the concentrates (Figure 2a).

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Figure 2. Recovery of Fe and Ca (a) in the concentrate and tailing after magnetic separation, respectively; SEM images (b-e) of the concentrate and tailing with 7 g/L MNPs; and magnetization (f) of α-Fe3O4, γ-Fe2O3 and the concentrates with 7 g/L α-Fe3O4 or γ-Fe2O3 at room temperature.

Cycle use of the magnetic nanoparticles In order to reduce the cost of the magnetic nanoparticles, we investigated the recovery of the two magnetic nanoparticles from the concentrates and the cycle use efficiency of the MNPs as the magnetic seeds in the goethite process. Figure 3(a) and 3(b) show the magnetic collection yield of the concentrates for αFe3O4 and γ-Fe2O3 under different milling time and magnetic fields. It can be seen that all the yields decreased significantly with increasing in the milling time and almost reached equilibrium after 5 mins, indicating that the nonmagnetic goethite particles can be dissociated with the magnetic seeds almost completely by grinding the concentrates using a rod mill for 5 mins. A decreased trend was observed for the yields of both αFe3O4 and γ-Fe2O3 with the same milling time by reducing the magnetic field intensity. The yields of the concentrates for α-Fe3O4 and γ-Fe2O3 under 5 mins milling time at 750 Gs magnetic field intensity were almost the same as the theoretical yields of the αFe3O4 and γ-Fe2O3 nanoparticles in the corresponding concentrates. The theoretical yields of the α-Fe3O4 and γ-Fe2O3 nanoparticles in the concentrates were 57.54% and 42.46%, respectively, which were calculated by weighing the mass of the obtained dried concentrates with 7 g/L α-Fe3O4 or γ-Fe2O3. Therefore, 5 mins of the milling time and 12


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750 Gs of the magnetic field intensity were optimum for collecting the magnetic nanoparticles from the concentrates. The obtained magnetic nanoparticles were reused as the magnetic seeds for a new iron removal process and their performance were investigated. The recovery of Fe and loss of Ca in the multiple cycle use of the α-Fe3O4 or γ-Fe2O3 are shown in Figure 3(c) and 3(d), respectively. For γ-Fe2O3, the recovery of Fe was around 99% during the 5 cycles, which is much higher than that of α-Fe3O4 (around 60 %), while the loss of Ca for both magnetic nanoparticles maintained at around 15%~20%. The results indicated the feasibility of recycling magnetic seeds in the iron removal process, which would effectively reduce the cost of magnetic seeds in the industrial application.

Figure 3. Magnetic recovery of α-Fe3O4 (a) and γ-Fe2O3 (b) from the concentrates after rod 13



milling; and recovery of goethite precipitates in the cycle use of α-Fe3O4 (c) and γ-Fe2O3 (d).

Goethite forming on the surface of MNPs In order to further study the formation of goethite on the surface of α-Fe3O4 and γFe2O3 NPs, 0.1 mol/L NaOH solution was used as the neutralizer to avoid the interference of gypsum precipitates formation. The real time particles number counts and mean size of the suspensions in the goethite precipitation process were measured by FBRM, as shown in Figure 4a and 4b, respectively. Without any nucleus addition, the counts of the particles within the size range of

Green Recycling of Goethite and Gypsum Residues in

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