Effects of Cations/Anions in Recycled Tailing Water on Cationic Reverse Flotation of Iron Oxides

Abstract

It is well known that reverse flotation performance of iron oxides is affected by water quality. Since many potential variations among water sources recycling in a mineral processing plant bring unpredictable effects on the flotation system of iron oxides: disturbing ions/compounds, pH, hardness, residual reagents, etc. In this study, the recycled tailing water from a local plant, characteristically constituting of Ca²⁺, Mg²⁺, Na⁺, K⁺, Al³⁺, Fe³⁺, Cl⁻, SO₄²⁻ etc., was introduced into the cationic reverse flotation process of an iron ore. A series of bench flotation tests using iron ores, micro-flotation tests using pure fine quartz, water chemical analyses, and zeta potential measurement were conducted with the objective of identifying the possible influences of both cations and anions in the recycled tailing water on the flotation performance. The flotation results pointed out that the cation with higher valency had more severe influences on the recovery of iron oxides. The formation of the pH-dependent surface complexes on mineral surfaces, for example, Fe(OH)⁺, Fe(OH)²⁺, and Fe(OH)₃ resulted from Fe³⁺ ions adsorption, contributed to the less negative zeta potentials of the quartz, and consequently weakened its interaction with the amine collector. It is worthy to note that SO₄²⁻ ions seem to have a more positive effect on the recovery of iron oxides than Cl⁻ ions. This is probably attributed to the formation of inner/outer- sphere surface complexes on the iron oxides, inhibiting the dissolution of the iron ions/species, and the coordination with these cations from the recycled tailing water, shielding their disturbances in the flotation.

1. Introduction

It is essential for a mineral processing plant to reduce plant-operating costs and minimize impact on the local ecosystem by increasing water reuse. The recycled water is usually enriched with many dissolved ions/compounds [1,2,3]. And a gradual built-up of some dissolved cations/anions such as K⁺, Na⁺, Ca²⁺, Mg²⁺, Fe³⁺, Al³⁺, Cl⁻, No₃⁻, SO₄²⁻ etc., possibly occurs during water recycling in a flotation process. This could bring an unexpected change into the physicochemical/electrochemical atmosphere of flotation pulp, affecting mineral floatability as well as dewatering and pumping [3].

Table 1. Some literature about the effects of cations/anions in the water on the reverse/direct flotation of iron oxides [4,5,6,7,8,9,10,11,12,13].

Cations/Anions	Type of Collector	Minerals Floated	Main Results or Conclusions	References
Ca2+, Mg2+, Fe3+, Al3+/Cl−	Anionic	Quartz	(i) Promotive adsorption of an anionic collector on quartz by Ca2+ at near 10.5 or Mg2+ at near 9.5; (ii) The order of the activation abilities on quartz: Ca2+ ≥ Mg2+ > Fe3+ > Al3+.	Ruan et al. 2018 [4]
Ca2+, Mg2+/Cl−	Cationic		Competitive adsorption of ester amine on quartz.	Ren et al. 2018 [5]
Ca2+, Mg2+, K+, Na+/Cl−	Non		Dissolution rate of quartz in near neutral pH salt-free solutions: MgCl2 < CaCl2 < NaCl~KCl.	Dove 1999 [6]
Fe3+, Pb2+, Mg2+, Al3+, Mn2+, Co2+/Cl−	Anionic		(i) Reversal of the zeta potential; (ii) The surface complexes: Fe(OH)2+, Pb(OH)+, Mg(OH)+, Al(OH)2+, Mn(OH)+, or Co(OH)+.	Fuerstenau and Palmer 1976 [7]
Na+, Ca2+/Cl−	Anionic		(i) Promote the adsorption of anionic surfactants on quartz; (ii) Little difference on this adsorption between Na+ and Ca2+.	Nevskaia et al. 1998 [8]
Ca2+, Mg2+, Pb2+, Al3+, Fe3+/Cl−	Anionic		Promotive adsorption of sulfonate on quartz.	Fuerstenau et al. 1963 [9]
Al3+/SO42−	Anionic	Silica	Al ions/species adsorbed and hinder its reaction with NaOl.	Chen et al. 2018 [10]
Mn2+/Cl−	Cationic		Competitive adsorption of Mn(OH)2.	Duarte et al. 2015 [11]
Na+, Mg2+, Ca2+, La3+/Cl−	Polymer		(i) Promotive adsorption with an increase of concentration and valency strength; (ii) Little effect by NaCl.	Flood et al. 2006 [12]
Ca2+, Mg2+, Al3+, Sn4+/NO3−, SO42−, PO43−	Anionic	Hematite	(i) Depress the adsorption of oleate; (ii) Anions have a greater depression than cations of the same charge; (iii) A drop on adsorption density with increasing charge of cation or anion	Ofor 1996 [13]

shows the positive or negative effects of ions, especially cations, Ca²⁺, Mg²⁺, Fe³⁺, Al³⁺, K⁺, Na⁺ etc., in the reverse/direct flotation of iron oxides [4,5,6,7,8,9,10,11,12,13]. It seems to be complicated to figure out specific effects of these cations on the effectiveness of flotation since they may be detrimental in some cases [5,11,13], while in other cases they may be beneficial to the flotation process [4,7,8,9,10,12]. But it is closely related to the properties of minerals floated, type of flotation reagents, especially collector, and valence/concentration of cations as well. Since the promotive effects of these cations mainly occur in the anionic reverse flotation of iron oxides [4,7,8,9,10] while the competitive effects of them obviously take place in the cationic reverse flotation or anionic direct flotation [5,11,13]. And either promotive or competitive effects of these cations in the flotation of iron oxides largely depend on their valences and concentrations based on the data from Table 1 [4,7,9,12,13].

There are a few reports focusing on the influences of the metal ions in flotation by their valences and concentrations. Flood et al. (2006) reported that the adsorption of Poly(ethylene oxide) on the silica can be enhanced with the metal ions at a higher valence or concentration [12]. Identical observations have been made by Choi et al. (2016), indicating that Ca²⁺ or Mg²⁺ ions have a more promotive effect on the adsorption of sodium oleate on malachite than K⁺ or Na⁺ ions at the same concentration [14]. Ofor (1996) and Wang et al. (2019) also found that the metal ions with a higher valence tend to have a more depressive effect in the anionic flotation of molybdenite or hematite [13,15]. But Ruan et al. (2018) reported that the activation abilities on the quartz of the divalent cations (Ca²⁺ or Mg²⁺) are greater than the trivalent cations (Fe³⁺ or Al³⁺) during the anionic flotation [4].

A change or even reversal of the zeta potential resulted from the adsorption of cations/their compounds on mineral surfaces probably contributes to their promotive or competitive effects in flotation [7,9,16,17,18]. According to the previous research performed by Fuerstenau and Palmer (1976), Ca²⁺ or Mg²⁺ ions at a high concentration induce a reversal of the surface charge on the silica from negative to positive and hinder the adsorption of the amine collector on the mineral surfaces [7]. This potential reversal on mineral surfaces is largely attributed to the formation of Ca²⁺ or Mg²⁺ hydrolyzed products, such as Ca(OH)⁺, CaCO₃, Mg(OH)⁺, Mg(OH)₂, MgCO₃ etc. [16]. The findings by Rao (2004) also show that the potential reversal hinders the floatability of the minerals if collector adsorption dominantly depends on electrostatic interaction [17]. This is well in agreement with the observations made by Ren et al. (2018), indicating that Ca²⁺ ions reverse the zeta potential of the quartz by physical adsorption and inhibit the adsorption of an ester mine on it [5]. In addition, Shortridge et al. (1999) found that the presence of ions such as Mg²⁺ and Ca²⁺ ions in the flotation system can enhance the depressive activity of the CMC polymer due to its strong negative charge density [18].

Only a couple of studies placed emphasis on the role of anions in plant water. Ofor (1996) found that an anion in water has a greater depression than a cation at the same charge in the flotation of hematite. And an increase in the valence of anion leads to a drop on the adsorption of oleate on hematite [13]. Ikumapayi and Rao (2015) also reported that the presence of SO₄²⁻ ions influence the recovery of the chalcopyrite and galena due to the competitive adsorption between CaCO₃ and PbSO₄ [19].

Although the role of some cations, for example, Fe³⁺, Al³⁺, Ca²⁺, Mg²⁺, K⁺ or Na⁺ ions in water during anionic reverse flotation of iron oxides is well established, the possible effects of those cations, and the typical anion, Cl⁻ or SO₄²⁻ ions, on cationic reverse flotation of iron oxides are not sufficiently understood. In this study, the influences of the cation, Fe³⁺, Al³⁺, Ca²⁺, Mg²⁺, K⁺ or Na⁺, and anion, Cl⁻ or SO₄²⁻ from the recycled tailing water on the cationic reverse flotation performance of iron oxides were investigated. A series of micro-flotation tests using pure fine quartz, bench flotation tests using an iron ore fed in the field, water chemical analyses, and zeta potential measurement were also conducted with the objective of identifying how the cations/anions in the water influence the cationic reverse flotation system of the iron oxides was also made.

2. Materials and Methods

2.1. Materials

An iron oxide ore sample was collected and sampled from an iron mine in Yunnan, China. It was at a size range of about 87.6% passing 74 microns. The ore contains 55.6% Fe, 10.5% SiO₂, 1.89% CaO, 3.28% MgO, 1.36% Al₂O₃. The main iron oxides are magnetite, accounting for 67%. And part of them is associated with the silica gangues. The main gangues containing silica are quartz, talc, chlorite etc. Several barrels of the recycled tailing water sample were collected, sealed and transported from the local tailing ponds, about 25 km from the local concentrator in Yunnan, China. The removal of the slimes and residues of the organic reagents from the tailing water sample were also conducted before water chemical analyses and flotation tests. The pure quartz and all reagents used for zeta potential measurement and flotation tests were shown in

Table 2. The materials and reagents used in this study.

Materials or Reagents	Serial Number/Company	Purity, %
Corn starch	S-4180/Sigma-Aldrich, St. Louis, MO, USA	99.0
DDA	124-22-1/Shandong Chemical Technology Co., Laizhou, China	/°C 155.0~158.0 (2.0)
Sodium hydroxide	S318-1/Thermo Fisher Scientific, Shanghai, China	98.8
Sodium chloride	S271-500/Thermo Fisher Scientific, Shanghai, China	99.8
Calcium chloride	10043-52-4/Kunming Minerals Co., Kunming, China	96
Magnesium chloride	7791-18-6/Kunming Minerals Co., Kunming, China	99
Ferric chloride	10025-77-1/Kunming Minerals Co., Kunming, China	99
Aluminum chloride	446-70-0/Kunming Minerals Co., Kunming, China	≥98.5
Sodium sulfate	7757-82-6/Sigma-Aldrich, St. Louis, MO, USA	≥99.0
Hydrochloric acid	7647-01-0/Thermo Fisher Scientific, Shanghai, China	36.5–38.0
Sulfuric acid	7664-93-9 Sigma-Aldrich, MO, USA	95–98
Quartz (–37 μm, 90%)	Kunming Minerals Co., Kunming, China	95.0

DDA, Dodecylamine.

.

2.2. Methods

2.2.1. Bench Flotation Tests

To prepare the sample to do bench flotation tests, 500 g of the iron ore sample was weighted and mixed with tap water at room temperature. The flotation test was conducted by using a 1.5 L cell (XF-D) for roughing and cleaning stages at an impeller speed of 1200 rpm for a certain time based on the flowsheet shown in Figure 1a. A froth depth (FH) of 15.2 cm, superficial gas velocity (J_g) of 0.75 cm/s and collecting time (t) of 10 min was employed for the roughing stage, but a froth depth of 11.5 cm and collecting time of 4 min at the same superficial gas velocity for the cleaning stage. Dodecylamine (DDA) as a collector, caustic-digested corn starch as flocculants, and sodium hydroxide or hydrochloric acid as pH modifiers were introduced in the test. It was conducted at a natural pH (near 9.0 (±0.3)) for using tap water, or at a similar pH range adjusted by hydrochloride acid for using recycled tailing water. The concentrate, middling, and tailing from the bench flotation tests had been filtrated, dried, weighted, and then assayed the grade of Fe and SiO₂, respectively. Each result plotted to the curve was chosen directly from a series of five repeating tests, which was a highly repeatable one. The error bars covered the total range of these values instead of the standard deviation.

A five-day closed-circuit flotation test was performed in the local plant using the recycled tailing water as shown in Figure 1b. It employed the same flotation reagents as the ones shown in Figure 1a. Hydrochloride acid was used in the close-circuit as a pH modifier. The cumulative content of the main cations/anions in the flotation process water of the feed, concentrate, and tailing were also analyzed after settling, centrifuging, and filtrating.

2.2.2. Micro-Flotation Tests

To prepare the sample for a flotation test using a Hallimond cell, a 1 g quartz was weighed and mixed with 100 mL of distilled water. A 10 mg/L DDA as a collector was used for each test and its conditioning time was 1 min. Calcium chloride, magnesium chloride, aluminum chloride, or ferric chloride at a concentration of 1 mmol/L was added and conditioned for 5 min if needed before the addition of the collector. All micro-flotation tests were performed at an air rate of 0.2 m³/min at room temperature, and the flotation time was 1 min.

2.2.3. Zeta Potential Measurement

The zeta potentials of quartz were measured with the use of a ZetaPALS zeta potential and particle size analyzer manufactured by Brookhaven Instruments Corporation, Holtsville, NY, USA. The technique is based on electrophoretic light scattering (ELS), also known as Laser Doppler Velocimetry (LDV). To prepare samples for the zeta potential measurements, a 100 mL 0.5% (5 g/L) quartz solution was mixed with 100 mL of a metal ion at a different concentration at room temperature. The mixed suspension was stirred at room temperature for 10 min. Five mL of the mixed suspension was withdrawn and diluted to 100 mL with a 10⁻³ mol/L NaCl solution. The pH of the diluted suspension was adjusted using HCl or NaOH, and a small aliquot of the suspension was transferred to the sample cell of the ZetaPALS for zeta potential measurements.

2.2.4. Water Chemical Analyses

The contents of the metal ions in the recycled tailing water sample were analyzed by an ICP-AES instrument purchased from Thermo Fisher Scientific, Shanghai, China, while the contents of Cl⁻ and SO₄²⁻ ions, and total hardness in the water sample were detected by titration in terms of standards.

3. Results and Discussion

3.1. Water Chemistry of the Recycled Tailing Water and Its Reuse in Flotation

3.1.1. Effect on Flotation Performance by Using the Recycled Tailing Water

Figure 2 presents the comparison on the grade/recovery of Fe and content of SiO₂ in concentrates as a function of % recycling tailing water reported into the flotation system at a pH range of 8.5–9.3. The data showed that there is an upward trend on the recovery of Fe and content of SiO₂ with increasing % recycling tailing water. An increase by 1.5% on the content of SiO₂ and 6.0% on the recovery of Fe in the concentrate by using the recycled tailing water only occurs in comparison by using the fresh tap water only. It seems to be beneficial to improve the recovery of Fe in concentrates by using the recycled tailing water, but at a cost of the quality of concentrates by dramatically increasing the content of SiO₂ and lowering the grade of Fe. The disturbing inorganic ions, especially cations in the recycled tailing water may play an important role in influencing the interactions between DDA and silica by modifying the surface zeta potential of the minerals [1,2,3,4,5,6,7,8,14,15,16,17].

3.1.2. Water Chemistry of the Recycled Tailing Water

The main elements in the recycled tailing water before/after entering the flotation process and tap water from the lab were shown in

Table 3. Chemical analysis of the tap water in the lab and recycled tailing water before/after its reuse in the flotation process.

Item	Tap Water from Lab	Recycled Tailing Water before Its Reuse	Recycled Tailing Water after Its Reuse
			Water Filtrated from Concentrate	Water Filtrated from Tailing
pH	7.90	9.80	9.90	9.50
Al3+, mg/L	<0.020	0.035	0.020	0.038
TFen+, mg/L	0.009	5.361	16.12	8.57
Ca2+, mg/L	-	196.5	278.3	235.4
Mg2+, mg/L	-	134.7	186.9	268.9
Total Hardness (CaCO3), mg/L	158.7	1254	1587	1871
Cl−, mg/L	5.52	258	589	347
SO42−, mg/L	12.7	463	972	417

. The content of the main cations, Al³⁺, TFeⁿ⁺, Ca²⁺ and Mg²⁺ ions, as well as the dominant anions, Cl⁻ and SO₄²⁻ ions in the tailing water before entering into the flotation system was obviously much higher than the one in the tap water. The total hardness of the recycled tailing water was approximately 10 times the hardness of the tap water. The comparison of the accumulative content of the main cations/anions in concentrates/tailings after a five-day closed-circuit process by using the recycled tailing water alone was also provided from Table 3. It indicates that the accumulative rates of the content of Ca²⁺, Mg²⁺, and total hardness in the tailing water were much higher than in the concentrate water. But the built-up on the contents of TFeⁿ⁺, Cl⁻, and SO₄²⁻ ions is the opposite, which is distinctly faster in the water from the concentrate than from the tailing. A sharp increase on the recovery of Fe and content of SiO₂ in the concentrate from the flotation tests using the recycled tailing water as shown in Figure 2 may be closely related to the built-up of these ions, especially the cations, either in the tailing or concentrate. Since these cations/their species could adsorb on the surface of the silica, modifying the zeta potential and triggering the disfunction of the amine collector on the minerals [14,15,16,17,18,19].

3.2. Flotation Performance of Iron Oxides in the Presence of Cations

As shown in Figure 3a–c, the recovery/grade of Fe and content of SiO₂ in the concentrate change a little if Na⁺ or K⁺ ions are present in the tap water. But the multivalent cation, especially Al³⁺ or Fe³⁺ ions, has a significant influence on the flotation performance. The results show that there is a substantial downward trend on the grade of Fe and an obvious upward trend on the content of SiO₂ with increasing concentration of Al³⁺ or Fe³⁺ ions in the tap water. An increase by approximately 6.0% on the recovery of Fe, but at a cost of a drop by near 2.5% on the grade of Fe, and an increase by 3.7% on the content of SiO₂ occurs if the concentration of Al³⁺ ions reached 800 mg/L. A different story was observed in the presence of Ca²⁺ or Mg²⁺ ions. High concentration of Mg²⁺ only induces a slight increase in the recovery of iron with a little change on the grade of iron and content of SiO₂. But it seems to be beneficial to upgrade the concentrate in the presence of Ca²⁺ ions because an upward trend on the grade of iron was observed with increasing its concentration. This is inconsistent with the previous research [7,12], indicating that floatability of silica has little change in the presence of sodium chloride, while the promotive adsorption of an anionic collector on quartz can be harvested by increasing the concentration and valency of cation [7,8,9,10,12]. In terms of the accumulative content of the main cations as shown in Table 3, an obvious build-up of TFeⁿ⁺, Ca²⁺ or Mg²⁺ ions either in the concentrate or tailing water after the closed-circuit flotation test occurs, which may contribute to the increase in the content of SiO₂ and the drop in the grade of Fe in concentrates from Figure 2.

3.3. Flotation Recovery of Quartz in the Presence of Cation

Figure 4 illustrates the changes on the recovery of pure quartz in the absence/presence of cation, Ca²⁺, Mg²⁺, Al³⁺ or Fe³⁺ ions at a concentration of 1 mmol/L as a function of pH. The data indicate that the flotation recovery does not change a lot in the presence of Ca²⁺ or Mg²⁺ ions at a pH range of 2–8. But it was significantly influenced by the addition of Al³⁺ or Fe³⁺ ions. A severe drop by more than 80% or 50% on the recovery of quartz occurs at a pH of 7–9 by adding Al³⁺ or Fe³⁺ ions, respectively. This is well in accordance with the results as shown in Figure 3, which show that at this pH range the flotation performance of the iron oxides was deteriorated with increasing the concentration of Al³⁺ or Fe³⁺ ions. According to the previous research performed by Araujo and Coelho (1991), it indicates that the quartz is greatly depressed at a pH range of 7–9 in the presence of Al³⁺ ions during amine flotation, which strongly depends on the adsorption density of Al³⁺ ions on the quartz [20].

Based on the data shown in Table 3, a large amount of Al³⁺ ions accumulated from the flotation process by using the recycled tailing water was not observed while a dramatic increase on the content of TFeⁿ⁺ ions occurred in the concentrate water. The built-up of TFeⁿ⁺ probably contributes to the unexpected entrainment of the silica into the concentrate, and inducing a substantial increase on the content of SiO₂.

There is also a slight downward trend on the flotation recovery at a pH range of over 8 in the presence of Ca²⁺ or Mg²⁺ ions in comparison with the original one without any addition of cation at the same pH range. The decrease on the recovery of quartz is probably attributable to the hydrolyzed species of them at this range of pH. A built-up of the total hardness, and content of Ca²⁺/Mg²⁺ ions as well, of course, cannot be ignored as shown in Table 3. But it does not seem to help the improvement on either the recovery of Fe or the content of SiO₂ in the concentrate by using the recycled tailing in the process based on the results as shown in Figure 3 and Figure 4.

3.4. Zeta Potentials of Quartz in the Presence of Cations

The zeta potential values of the quartz mixed with Ca^2+, Mg²⁺, Fe³⁺ or Al³⁺ ions as a function of pH were observed in Figure 5, indicating that there is a reversal of the surface charge at a pH range of approximately 2–8 when the quartz interacted with Al³⁺ or Fe³⁺ ions. It shows a reversal of the surface charge for the quartz treated with Mg²⁺ ions at a very strong alkali atmosphere, but no reversal at all for the quartz treated with Ca²⁺ ions at a pH range of 1–12. This is inconsistent with the results from Figure 4, which show that the surface adsorption of these metal ions/their complexes on quartz is strongly pH dependent. Since the difference in these isoelectric point (IEP) values is associated with differences in the surface complex formation at the mineral-liquid interfaces.

No reversal, but only the slightly less zeta potential on quartz occurs in the presence of Ca²⁺ ions, which is probably due to its low concentration based on the literature [7,16,17,18]. In terms of the Ca²⁺ species distribution diagram as shown in Figure 6, the predominant species in water are Ca²⁺ ions at a pH range of less than 10, but Ca(OH)⁺, and Ca(OH)_2(s) at a pH range of over 10. This is well explained by the data as shown in Figure 4. The precipitate of Ca(OH)₂ partly contributes to the drop on flotation recovery of quartz at this pH due to hindering the interaction of the amine collector on mineral surfaces while the disturbance of Ca²⁺ ions on mineral surfaces at a pH range of less than 10 is possibly relevant to the layer structure of the silica and its exchange rate with NH₃⁺ [21].

In terms of the Al³⁺, Fe³⁺species distribution diagram as shown in Figure 6, at pH 8.5–9.5 the predominant species in water are FeOH²⁺, Fe(OH)²⁺, and Fe(OH)₃ for Fe³⁺ ions, or Al(OH)₃, Al(OH)²⁺, and Al(OH)⁴⁻ for Al³⁺ ions. The formation of these species on the quartz probably contributes to its less negative zeta potential, compared with the ones without the addition of any cation. The precipitate of Fe(OH)₃ or Al(OH)₃ on the quartz may also act as metal oxide coating disturbs the interaction with the amine collector. This is well in agreement with the data from Figure 2, which demonstrate that the enrichment of the cations, especially Al³⁺ or Fe³⁺ ions in the recycled process water, contributes to the increase in the content of SiO₂ in concentrates with increasing the percent of the recycled tailing water used in the flotation. Based on the previous literature [7,22,23,24,25], the possible competitive adsorption mechanism of those cations on the silica in the reverse cationic flotation of the iron oxides at weak alkali pH may involve the specific pH-dependence of the metal binding and surface complexes interactions shown as follows (Modified from Fuerstenau et al. (1963) and Stumm (1992) [7,25]), in the water:

$$\mathrm{Fe}{({\mathrm{H}}_2\mathrm{O})}_6^{3+}\leftrightarrow \mathrm{Fe}{({\mathrm{H}}_2\mathrm{O})}_5{\mathrm{OH}}^{2+}+{\mathrm{H}}^{+}$$

(1)

$$\mathrm{Fe}{({\mathrm{H}}_2\mathrm{O})}_5{\mathrm{OH}}^{2+}\leftrightarrow \mathrm{Fe}{({\mathrm{H}}_2\mathrm{O})}_4{{(\mathrm{OH})}_2}^{+}+{\mathrm{H}}^{+}$$

(2)

$$\mathrm{Fe}{({\mathrm{H}}_2\mathrm{O})}_4{(\mathrm{OH})}_2^{+}\leftrightarrow \mathrm{Fe}{({\mathrm{H}}_2\mathrm{O})}_3{(\mathrm{OH})}_3+{\mathrm{H}}^{+}$$

(3)

$$\leftrightarrow \mathrm{Fe}{(\mathrm{OH})}_3+3{\mathrm{H}}_2\mathrm{O}+{\mathrm{H}}^{+}$$

(4)

on the surface of quartz:

$$>\mathrm{Si}-\mathrm{OH}+\mathrm{Fe}{({\mathrm{H}}_2\mathrm{O})}_6^{3+}\leftrightarrow >\mathrm{Si}-\mathrm{OFe}{({\mathrm{H}}_2\mathrm{O})}_5{\mathrm{OH}}^{+}+2{\mathrm{H}}^{+}$$

(5)

$$>\mathrm{Si}-\mathrm{OH}+\mathrm{Fe}{({\mathrm{H}}_2\mathrm{O})}_5{\mathrm{OH}}^{2+}\leftrightarrow >\mathrm{Si}-\mathrm{OFe}{({\mathrm{H}}_2\mathrm{O})}_4\mathrm{OH}+2{\mathrm{H}}^{+}$$

(6)

$$>\mathrm{Si}-\mathrm{OH}+\mathrm{Fe}{({\mathrm{H}}_2\mathrm{O})}_4{(\mathrm{OH})}_2^{+}\leftrightarrow >\mathrm{Si}-\mathrm{OFe}{({\mathrm{H}}_2\mathrm{O})}_3\mathrm{OH}+{\mathrm{H}}^{+}$$

(7)

The species after the adsorption of the cations on the minerals can modify the surface potentials and substantially influence the electrostatic adsorption between minerals and DDA [7,14,15,16,17,18,19].

3.5. Effect on Flotation Performance in the Presence of Cl⁻ or SO₄²⁻

The content of Cl⁻ or SO₄²⁻ in the recycled tailing water is much higher than in the tap water as shown in Table 3. The influence of these anions on the flotation behavior was presented in Figure 7. It shows that Cl⁻ ions have less influence on the recovery of the iron oxides than SO₄²⁻ ions. At a concentration of Na₂SO₄ (1500 mg/L), a concentrate at a grade of 60.1% Fe and 5.7% SiO₂ with a recovery of 83.4% Fe was harvested. The data indicate that it is beneficial to enhance the recovery of the iron oxides in the presence of SO₄²⁻. According to the previous research reported by Ahmed and Masimov (1968) and Hesleitner et al. (1987), residual chloride ion can adsorb to the iron oxide surfaces, therefore lowering its IEP by the formation of a positively charged chlorocomplex [27,28]. Wantanabe and Seto (1986) also found that the adsorption of sulfate ions can lower the IEP of iron oxides [29]. The schematic surface complex structure of the hydrous iron oxide surface in the presence of Cl⁻ and SO₄²⁻ ions was shown in Figure 8, which indicates that Cl⁻ ions are considered to adsorb mainly in outer-sphere complexes and as diffuse ion swarm while SO₄²⁻ ions are more likely to form inner and outer sphere binuclear or polynuclear surface complexes on hematite at near-neutral pH values [24,29]. They can provide less suitable leaving groups for detachment into the water, therefore successfully inhibiting the dissolution of iron ions from the mineral surfaces [30,31,32,33]. The coordination of this anion with a multi-valent cation from the pulp, of course, needs to be considered as well.

It is worthwhile to note that the content of SO₄²⁻ ions as shown in Table 3 was dramatically built up by approximately 2 times in the concentrate water, compared with the one in the feed if the recycled tailing water was fully introduced into the entire flotation process. The formation of both outer-sphere inner and outer sphere binuclear or polynuclear surface complexes of sulfate ions on iron oxides may attribute to it. The less presence of the cations with high valency in the pulp, for instance, Fe³⁺ ions, due to either the inhabited dissolution from the surface of the minerals or coordination of sulfate ions in the pulp, responsibly enhance the selectivity of the cationic reverse flotation, which is well in accordance with the flotation results as shown in Figure 2, Figure 3 and Figure 4. However, the influences of these cations discussed have been considered independently of each other. In the flotation process, those cations can influence simultaneously, and the extent of their impact can be largely different.

4. Conclusions

The flotation results pointed out that the presence of multi-valent cation, Fe³⁺, Al³⁺, Ca²⁺, or Mg²⁺, tends to have more significant influences on the selectivity of the flotation than mono-valent cation, Na⁺ or K⁺. A sharp increase by 1.5% on the content of SiO₂ in concentrate, coupled with an increase of 6.0% recovery of Fe, was also observed by using the recycled tailing water only in comparison by using fresh tap water only. A build-up of the cations, especially TFeⁿ⁺, Ca²⁺ or Mg²⁺, in the recycled process water contributes to it. But SO₄²⁻ ions seem to have a more positive effect on the recovery of iron oxides than Cl⁻ ions. This is probably due to the formation of the inner and outer sphere binuclear or polynuclear surface complexes on mineral surfaces at near-neutral pH values, providing less suitable leaving groups for detachment into the water, therefore successfully inhibiting the dissolution of metal ions with high valence. It can also coordinate the dissolved cations in the pulp and lower the effect of these cations in the flotation.