A Study on the Flocculation and Sedimentation of Iron Tailings Slurry Based on the Regulating Behavior of Fe³⁺

Abstract

Based on the regulating behavior of Fe³⁺, flocculation and sedimentation tests of iron tailings slurry flocculated using 2 mg/L anionic polyacrylamide (APAM) were studied, including the tests of Fe³⁺ dosage, regulating time, and pH. Flocculation–sedimentation tests showed that: a recommended addition of 56 mg/L Fe³⁺ produced a surprisingly bad flocculation effect; sedimentation ability decreased with the increase of regulating time, however, the regulating ability during the first 1 min was low; Fe³⁺ displayed a high regulating effect at pH 6–7, and then decreased with the increase of slurry pH. Size analysis demonstrated that the regulating ability of Fe³⁺ was related to the change of floc size, which increased with the decrease of size. Zeta potential analysis and calculation showed that small concentrations of Fe³⁺ and certain hydroxyl complex ions (such as Fe(OH)²⁺ and Fe(OH)₂⁺) adsorbed onto the APAM molecular chains, resulting in a decreased charge density of the APAM molecules, and this electrostatic adsorption was able to make the polymer curl more easily. Fourier Transform Infrared Spectroscopic (FTIR) indicated the APAM on the hematite was slightly degraded into a monomer-like short-chain polymer while adding certain concentration of Fe³⁺. Scanning Electron Microscope (SEM) imaging showed that the network structure of APAM molecules treated by Fe³⁺ collapsed, and the APAM adsorption amount on hematite was significantly reduced. Therefore, the effect of Fe³⁺ on the APAM could be recommended as a new method for desorption and degradation of the APAM contained in the tailings slurry or flocs.

1. Introduction

Polyacrylamide (PAM) and derivative products are well-known flocculants widely applied in various fields, such as wastewater treatment [1,2], fine sedimentation [3,4], and agglomeration flotation [5] in mineral processing. There are three main types of PAM: anionic, cationic, and nonionic, and amongst them, the anionic PAM (APAM) usually exhibits a strong flocculating effect and has been used effectively for the flocculation–sedimentation process of tailings slurry [4,6]. The APAM polymer usually contains a large number of polar groups such as acyl and carboxyl groups, resulting in a high adaptability and strong adsorption capacity at alkaline pH. APAM adsorbs well onto suspended particles in solution to form flocs, leading to an increased settling rate [4] for fine particles and producing good performance for fine flotation [7,8].

As a water-soluble synthetic organic polymer, PAM is able to produce large flocs for water treatment or flotation by hydrogen bonding adsorption, chemisorption, and bridging adsorption [7]. However, PAM usually takes a long time to degrade naturally in the water system [9,10], and sometimes a large addition of PAM could lead to an adverse effect on the following flotation separations of the target minerals [11] due to agglomerates containing many gangue particles. Chang [12] reported that a large concentration of 10–15 mg/L APAM used for the bauxite tailings sedimentation produced bad flotation selectivity in the following flotation process when the wastewater after the flocculation is recycled in the flotation.

In order to improve the flotation performance for using PAM as flocculants and reduce the adverse effect of residual PAM in the recycle wastewater, dispersants [12,13] could be applied in the slurry after flocculation, such as sodium hexametaphosphate (NaPO₃)₆, and water glass, among others. Dispersants can produce a high degree of dispersion for fine suspension; however, it does not help affect the PAM molecular structure and its adsorption on particles. Therefore, methods for the PAM degradation on the particles can be proposed, such as mechanical degradation by stirring or ultrasonic degradation [14], biodegradation [15], thermal degradation [16,17], photocatalytic degradation [18,19,20], and oxidative degradation [21,22], etc. Among them, the oxidative degradation is a universal and practical method using oxidizing agents, such as O₂, Fenton agent, KMnO₄, and some metal ions, producing reactive free radicals during redox reactions and then leading to linked oxidation reactions for degradation [23].

Due to the presence of polar groups in the molecular chains, there might be electrostatic attraction and a cross-linked reaction between PAM and metal ions (especially including those high valence metal ions) [24,25,26,27]. Furthermore, the interaction between them could decrease the charge density on the APAM molecules, and then make the APAM molecular chains curl greatly and release “bound water” [28,29]. Studies [26,28] revealed that the PAM viscosity can be affected by metal ions via the electrostatic adsorption between metal ions and polar groups of PAM molecules, and the order of the interaction capacity using 10 mg/L metal ions was: Fe²⁺ > Mg²⁺ > Ca²⁺ > Na²⁺ ≈ K⁺. These also suggested that high valence metal ions could produce stronger electrostatic interaction due to their positive charges, such as Al³⁺ and Fe³⁺, among others. Liu [30] found that the APAM adsorption amount on the magnetic seeds recycled from wastewater was reduced after adding metal ions of Fe²⁺, Ca²⁺, Mg²⁺, K⁺, and Na⁺. The study also claimed that the zeta potentials were increased via the adsorption of these metal ions on the magnetic seeds and the APAM polymer on the magnetic seeds was degraded into the water.

In this study, effects of the Fe³⁺ on the flocculation–sedimentation tests of iron tailings slurry flocculated using 2 mg/L APAM were investigated. The regulating behavior of Fe³⁺ on the slurry was characterized in terms of the decreased settling ability and the factual damage to the flocs’ size in the presence of different concentration, regulating time, and pulp pH. Then the regulating mechanisms were explored by flocs’ size analysis, zeta potential study, FTIR, and SEM analysis.

2. Materials and Methods

2.1. Materials

The iron mine tailings collected from a hematite ore mine in Jiayuguan City, China, containing 32.82% Fe and 38.66% SiO₂, were tested for flocculation and sedimentation studies. The sample with an average particle size of 17.11 μm displayed a bad kind of sedimentation capacity without flocculants, and the main iron mineral in the tailings was hematite. Moreover, the pure hematite sample (98.13% Fe₂O₃, minus 30 μm) was used for the mechanism studies. Chemical compositions of materials used are given in

Table 1. Chemical compositions of material used.

Sample	Fe2O3 (%)	SiO2 (%)	Al2O3 (%)	CaO (%)	MgO (%)	LOI (%)
Hematite ore slimes	44.86	38.66	6.17	2.75	3.15	2.45
Pure hematite	98.13	0.68	0.48	0.06	0.05	0.47

LOI = loss on ignition.

.

APAM ((C₂H₃CONH₂)_n(C₂H₃COONa)_m) with a 5 million molecular weight was prepared into a 1 g/L solution as the flocculant for iron tailings slurry, and then a reagent of 100 mg/L Fe³⁺ solution was prepared using Fe(III) salts (FeCl₃·6H₂O). Reagents of 50 g/L NaOH and HCl solutions were prepared as the pH regulators.

2.2. Methods

2.2.1. Flocculation and Sedimentation Tests

The tailings slurry (20 g/L) using 2 mg/L APAM was conditioned for 1 min at the stirring speed of 350 rev/min. After the flocculation, a certain concentration of Fe³⁺ was subsequently added into the suspension for conditioning for a certain time at a stirring speed of 350 rev/min. The tailings pulp was finally introduced into a graduated cylinder for sedimentation tests.

2.2.2. Flocs’ Size Study

The tailings slurry (20 g/L) flocculated using 2 mg/L APAM was treated for certain time with the addition of Fe³⁺, and finally transferred into a smaller beaker for size measurements using a Mastersizer2000 analyzer (Malvern Corporation, Worcestershire, UK). It was worth noting that the stirring speed of the analyzer should be less than 1000 rev/min due to the unsteady or fragile agglomeration, and the ultrasonic function of the analyzer should be turned off, too.

2.2.3. Zeta Potential Analysis

Firstly, 30 mg hematite (minus 5 µm) was cleaned in the deionized water via an ultrasonic cleaner for 5 min. The suspension was subsequently conditioned in the presence of reagents (APAM and Fe³⁺) and stirred for a certain time for a pH adjustment using the pH regulators of NaOH and HCl solution. After the equilibrium of pH, a supernatant of the suspension was obtained for zeta potential measurements using a Coulter Delsa-440SX zeta potential analyzer (Brookhaven Corporation, Long Island, NY, USA).

2.2.4. Fourier Transform Infrared Spectroscopic (FTIR) Study

Hematite (less than 2 μm) conditioned with APAM and hematite-APAM in the presence of Fe³⁺ solution was prepared first. The precipitates were dried at 50 °C and then the resultant samples were used for FTIR measurements using an IRAffinity-1 spectrometer (Shimadzu Corporation, Kyoto, Japan). In addition, samples of APAM and APAM in the presence of Fe³⁺ solution were also measured using the transmission method.

2.2.5. Scanning Electron Microscope (SEM) Imaging

Samples of 2 g pure hematite in the size range of 20–30 μm were prepared first using the elutriation method, and then dispersed in 50 cm³ deionized water for 5 min. Then suspensions of hematite coated with the APAM and hematite-APAM in the presence of Fe³⁺ were prepared. In addition, the 1 g/L APAM solution and the APAM solution conditioned with 56 mg/L Fe³⁺ were directly prepared. After conditioning, the dried samples with gold powder were prepared for SEM imaging using the Model JSM-6360LV (JEOL Corporation, Tokyo, Japan).

3. Results and Discussion

3.1. Flocculation and Sedimentation Tests

Figure 1 indicates that the settling rate of the slurry decreased rapidly after adding 20 mg/L Fe³⁺, and then the settling ability decreased slightly with the increased dosage of Fe³⁺ in the range of 20–56 mg/L. However, it was a surprise in Figure 1 that the settling ability increased with the increase of Fe³⁺ in the range of 56–150 mg/L, and the settling effect using 150 mg/L Fe³⁺ was still worse than that without adding Fe³⁺. These demonstrated that a low concentration of 20–56 mg/L Fe³⁺ could produce a positive effect on the regulating behavior in terms of the reduced settling ability, but a large concentration of 56–150 mg/L Fe³⁺ produced an adverse effect of regulating behavior due to the hetero-coagulation between Fe(OH)₃ colloids and tailings [31]. Therefore, a moderate dosage of Fe³⁺ could be suggested as 56 mg/L here. Figure 2 shows that the settling rate of the slurry using Fe³⁺ decreased with the increase of regulating time; however, the regulating efficiency was low during the first 1 min. These suggested the interaction kinetics between Fe³⁺ and the flocs appeared slow at first and then fast, which might be attributed to the Fe³⁺ dispersion in the slurry and collision efficiency between the active species and the polymer molecules on the flocs. Figure 3 indicates that a high regulating effect was produced in the presence of Fe³⁺ at pH 6–7, and then decreased with the increase of slurry pH. The pulp pH had a critical influence on the chemical species’ distributions of Fe(III) [32]. Concentration of Fe³⁺ and related hydroxyl complex ions (Fe(OH)²⁺ and Fe(OH)₂⁺) decreased with the increase of pulp pH, resulting in a decreasing regulating degree. In this study the addition of 56 mg/L Fe³⁺ could produce a bad flocculation–sedimentation effect with strong regulating behavior for the flocs at pH 6.4. It can be inferred from Figure 4 that the slurry only adding Fe³⁺ produced a better settling effect than that without flocculants (APAM and Fe³⁺) at pH 6.4, and the increased pH (alkaline slurry) was not beneficial to the settling effect might due to the higher slurry dispersion and the precipitate of coagulant. However, it was a surprise that the settling effect of the slurry using APAM first and then Fe³⁺ was worse than that using Fe³⁺ first and then APAM, indicating that the adding order of reagents was critical to the settling effect. Therefore, the method using APAM first and then Fe³⁺ at pH 6-7 could produce a good regulating effect on the slurry.

Based on the flocculation–sedimentation tests, it was concluded that a recommended addition of 56 mg/L Fe³⁺ at neutral pH produced a good regulating effect in terms of a surprisingly bad flocculation effect; this regulating behavior might be beneficial to weaken or reduce the adverse effect of the flocs and the residual APAM in the slurry on the following flotation process, such as the flotation selectivity.

3.2. Flocs’ Size Analysis

Table 2. Effect of Fe³⁺ on the flocs’ size based on its regulating behavior.

Iron Tailings Slurry	d (0.1) (µm)	d (0.5) (µm)	d (0.9) (µm)	Average Size (µm)	Specific Area (m2/g)
Tailings slurry	4.672	15.324	32.083	17.114	0.172
Slurry with 2 mg/L PAM	9.618	28.266	69.768	41.277	0.087
2 mg/L PAM and 20 mg/L Fe3+	6.456	22.064	60.086	32.491	0.117
2 mg/L PAM and 40 mg/L Fe3+	5.574	18.437	50.085	28.023	0.136
2 mg/L PAM and 56 mg/L Fe3+	5.298	17.901	48.675	25.077	0.141
2 mg/L PAM and 90 mg/L Fe3+	5.557	18.341	49.487	27.620	0.139
2 mg/L PAM and 120 mg/L Fe3+	5.784	20.405	55.080	29.288	0.125
2 mg/L PAM and 150 mg/L Fe3+	7.596	23.305	58.971	33.044	0.104
2 mg/L PAM, 56 mg/L Fe3+, t = 0.5 min	7.147	23.854	80.920	39.170	0.107
2 mg/L PAM, 56 mg/L Fe3+, t = 1.0 min	6.649	22.272	75.358	37.291	0.114
2 mg/L PAM, 56 mg/L Fe3+, t = 2.0 min	5.971	20.262	60.075	31.813	0.126
2 mg/L PAM, 56 mg/L Fe3+, t = 3.0 min	5.722	18.725	56.098	29.847	0.132
2 mg/L PAM, 56 mg/L Fe3+, t = 4.0 min	5.347	17.915	51.215	26.404	0.140
2 mg/L PAM,56 mg/L Fe3+, t = 5.0 min	5.298	17.901	48.675	25.077	0.141
2 mg/L PAM, 56 mg/L Fe3+, pH 6.5	5.298	17.901	48.675	25.077	0.141
2 mg/L PAM,56 mg/L Fe3+, pH 8.8	6.211	20.330	51.681	27.414	0.124
2 mg/L PAM, 56 mg/L Fe3+, pH 9.8	6.960	22.421	55.813	31.361	0.112
2 mg/L PAM, 56 mg/L Fe3+, pH 10.5	7.689	24.194	61.992	33.665	0.104
2 mg/L PAM, 56 mg/L Fe3+, pH 11.2	7.905	24.455	62.396	34.066	0.102

t = regulating time.

presents the flocs’ size change while adding certain Fe³⁺ solution under different conditions. It can be inferred that the regulating ability of Fe³⁺ was related to the change of floc size, which increased with the decrease of size. As shown in Table 2, the flocs’ size decreased with the increasing Fe³⁺ dosage first and then increased with the further increase of Fe³⁺; the flocs’ size decreased by 39.25% with the increase of regulating time in 5 min, but the size was only decreased by 9.66% (from 41.28 to 37.29 μm,) during the first 1 min; and the flocs’ size decreased with the increase of slurry pH and exhibited a fine size at neutral pH. The flocs’ size change agreed well with the sedimentation effects, i.e. the regulating behavior could be explained as the “damage” to the flocs.

3.3. Zeta Potential and Solution Chemistry Studies

It was demonstrated that the pulp pH had an important effect on the regulating behavior, therefore, zeta potential and solution chemistry studies of hematite-APAM in the presence of Fe³⁺ were investigated, as given in Figure 5 and Figure 6.

As shown in Figure 5, the iso-electric point of hematite has a negative shift after adding 2 mg/L of APAM, indicating that the APAM contained a large number of polar groups, such as acyl and carboxyl groups, which adsorbed onto the hematite. The hematite flocculated by APAM has a positive potential below pH 7 while adding Fe³⁺ and the absolute values of hematite potential were raised between pH 4 and 12, suggesting that the electrostatic adsorption and complexation happened between Fe³⁺ (related active species) and the polar groups of APAM. Furthermore, competitive adsorption between the Fe³⁺ ions and APAM molecules on the hematite also had an important influence on the potential.

Figure 6 presents the logC–pH diagram of Fe³⁺. The species of Fe(OH)_3(s) and Fe(OH)₄⁻ display a high concentration at neutral and alkali pH. The main species of Fe³⁺, Fe(OH)²⁺, and Fe(OH)₂⁺ are widely distributed at pH 0–10. Fe³⁺ and its hydroxyl complex ions in the solution at pH 7 were calculated to be 1 × 10⁻¹⁸ mol/L Fe³⁺, 1 × 10⁻¹³ mol/L Fe(OH)²⁺, and 1 × 10⁻⁹ mol/L Fe(OH)₂⁺, respectively, but the concentration of them were “small”, which mainly interacted with the APAM. Studies reported that the interaction between them decreased the charge density on the APAM molecules, resulting in great contractility of the PAM molecules [28,29]. Therefore, the APAM molecules adsorbed on the hematite particles might be affected via adding certain Fe(III) salts.

3.4. FTIR Spectra of Hematite-APAM Treated with Fe³⁺

Figure 7 presents the FTIR spectra of APAM and hematite-APAM conditioned with Fe³⁺. As shown in the spectrum of APAM that the adsorption around 3200 cm⁻¹ was related to the –NH₂ (associative) stretching vibration, and the band at 1662 cm⁻¹ was attributed to the carbonyl group corresponding to the C=O stretching vibration of the amide group, and then the –COO adsorption characteristic was also observed around 1540 cm⁻¹. The peak at 3350–3400 cm⁻¹ was the –NH₂ (free) stretching vibration. Compared to the spectrum of APAM-Fe³⁺, a small band at 1040 cm⁻¹ referred to the external bending vibration peak of olefins produced by double bonds, which could be called the characteristic peak of acrylamide monomer, suggesting that a type of weak degradation of APAM took place in the presence of Fe³⁺. As shown in the FTIR spectra of hematite-APAM, the –COO adsorption at 1540 cm⁻¹ and C–N stretching vibration nearing 1400 cm⁻¹ were observed, demonstrating the chemisorption and hydrogen bonding adsorption of APAM onto hematite. Then it can be inferred from the hematite-APAM in the presence of the Fe³⁺, the intensities of C–N stretching vibration band at 1400 cm⁻¹, and the C=O stretching vibration band around 1662 cm⁻¹ were improved, suggesting the adsorption of APAM molecules on hematite was intensified due to small acrylamide molecules by degradation.

3.5. Surface Characterization (SEM imaging) and Proposed Model of APAM in the Presence of Fe³⁺

Figure 8a shows the crystal of APAM solution in the natural state. It can be seen from the crystal that the APAM presents large molecules, forming into a type of spatial network structure. Then Figure 8b shows the crystal of APAM in the presence of Fe³⁺, which appears like a type of colloidal flocs releasing “bound water” by making the polymer curl. Therefore, the APAM in terms of colloidal flocs adsorbed on the hematite might be cleaned via mechanical stirring action in the slurry, which was confirmed in Figure 8d. Figure 8c presents the hematite conditioned with the APAM solution, it can be observed that there are large amounts of APAM coated on the hematite particles. Figure 8d reveals the adsorption characteristics of APAM on the hematite particles after conditioning with Fe³⁺, and the surface of hematite was cleaner and the APAM adsorption amount was smaller than that in Figure 8c. These characteristics agree well with the APAM desorption from hematite in the presence of Fe³⁺.

Based on the zeta potential analysis and SEM imaging, a proposed model of the APAM in the presence of Fe³⁺ is illustrated in Figure 9. In the simplified model, a small concentration of free Fe³⁺ (as calculated in Figure 6 at pH 7: 1 × 10⁻¹⁸ mol/L) only played an auxiliary effect on the electrostatic adsorption, which was mainly produced by the complex ions (such as 1 × 10⁻¹³ mol/L Fe(OH)²⁺ and 1 × 10⁻⁹ mol/L Fe(OH)₂⁺). In addition, the hydrogen bonding adsorption [33,34] of Fe(OH)²⁺ and Fe(OH)₂⁺ on the polymer molecular chain should also be considered due to a large number of polar groups in the polymer molecules. This model only proposes that the interaction between active species and the APAM molecular chain lies in the electrostatic adsorption and complexation in the electric double layer of the polymer molecular chain, suggesting that this interaction could decrease the negative charge density and then affect the stretch degree of the APAM molecules (confirmed in Figure 8a,b).

4. Summary

There were three critical factors investigated in the flocculation–sedimentation tests, including the addition of Fe³⁺, slurry pH, and the regulating time. The use of Fe³⁺ produced a positive regulating effect on the sedimentation under a certain dosage of Fe³⁺ reagent, but the high addition of Fe³⁺ led to a negative regulating effect. The high regulating ability using Fe³⁺ reagent was obtained at a neutral pH, and the increase of slurry pH was not beneficial to the regulating effect. The regulating ability was low during the first 1 min, but increased rapidly in the following 2 min. In addition, the adding sequence of the APAM and Fe³⁺ affected the regulating ability, which showed a bad result while using Fe³⁺ first and then APAM. The regulating ability of Fe³⁺ agreed well with the floc size change, which increased with the decrease of flocs’ size. So, the process using a certain concentration of Fe³⁺ at neutral pH could be recommended as a method to treat the wastewater containing APAM or the tailings flocculated by APAM.

Behind the high regulating ability of Fe³⁺, there were two main reasons: the desorption of the APAM and the weak degradation of APAM. The introduction of Fe³⁺ into the slurry produced related hydroxyl complex ions for electrostatic adsorption, complexation, and hydrogen bonding adsorption, which could decrease the charge density of the APAM molecules and then make the polymer chain curl more easily in terms of colloidal flocs, the colloid flocs of APAM were cleaned via stirring action, and this process could be called “desorption”. The weak degradation of APAM took place in the presence of Fe³⁺, and the degradation could reduce the flocculating ability of APAM.

SEM studies confirmed that the network structure of APAM molecules treated with Fe³⁺ collapsed, and then the APAM adsorption amount on hematite was significantly reduced. A proposed model of the APAM molecular chain in the presence of Fe³⁺ was presented. It was noticed that the active species of the regulator (Fe³⁺ solution in the slurry) were mainly its hydroxyl complex ions including Fe(OH)²⁺ and Fe(OH)₂⁺.