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Vol. 8. Issue 2.
Pages 2336-2349 (April 2019)
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Vol. 8. Issue 2.
Pages 2336-2349 (April 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.03.013
Open Access
Effect of surface dissolution by oxalic acid on flotation behavior of minerals
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Omid Salmani Nuri, Mehdi Irannajad
Corresponding author
iranajad@aut.ac.ir

Corresponding author.
, Akbar Mehdilo
Department of Mining and Metallurgical Engineering, Amirkabir University of Technology, Tehran, Iran
Highlights

  • Surface dissolution by oxalic acid decreases floatability of silicate minerals.

  • Oxalic acid as a pH regulator agent has also depression effect on gangue minerals.

  • Surface dissolution improves selectivity of ilmenite flotation from silicate minerals.

  • Dissolution of Fe ions from surface of silicate minerals is more than that of ilmenite.

  • Improvement of ilmenite flotation selectivity depends on Fe content of gangue minerals.

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Tables (5)
Table 1. Chemical composition (wt%) of purified samples.
Table 2. Results of ilmenite flotation from artificially mixed minerals before and after surface dissolution (pH=6–6.5, 1000g/t sodium oleate, 1000g/t oxalic acid, BS: before surface dissolution, AS: after surface dissolution).
Table 3. Content of dissolved ions from ilmenite, olivine-pyroxene and tremolite-clinochlore measured by ICP mass analysis, before surface dissolution (BS) and after surface dissolution (AS).
Table 4. The characteristics of NaOl adsorption bands on the minerals surfaces.
Table 5. The reactions of the minerals with collector at various pHs.
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Abstract

The surface dissolution pretreatment using oxalic acid as an organic agent was applied for modifying the surface properties of minerals and improving the selectivity of ilmenite flotation from its usual accompanied gangue minerals such as olivine, pyroxene, tremolite and clinochlore. The effect of this pretreatment method was investigated by various techniques including flotation experiments, ICP mass analysis, UV-visible spectroscopy, FTIR analysis and zeta potential measurements. The single mineral flotation tests show that after surface dissolution the maximum flotation recovery of ilmenite occurring at a pH of 6.3 is improved from 73% to 93.4%%, while at this condition the floatability of olivine-pyroxene and tremolite-clinochlore is decreased from 59.6% to 31.5% and 20.1% to 15.4%, respectively. Also, the cell flotation experiments indicate the increase of separation efficiency and selectivity index of ilmenite flotation from 8.37% to 42.04% and 1.18 to 2.6, respectively through the surface pretreatment process. The improvement of flotation selectivity can be due to the more significant removal of active cations from the surface of gangue minerals in comparison with ilmenite which has been shown by ICP mass analysis. By decreasing the surface active sites, as evidenced by UV-visible spectroscopy and FTIR analysis, the collector adsorption density on the surface of gangue phases is considerably reduced. This is also confirmed by zeta potential measurements showing the diminution of negative charge on the surface of pretreated gangue minerals conditioned with sodium oleate collector. The enhancement of collector adsorption on the surface of ilmenite and consequently the improvement of its floatability can be due to oxidation of some Fe2+ ions to Fe3+ ones through the surface dissolution process. Generally, the surface dissolution pretreatment by oxalic acid can easily and effectively improve the selective flotation of ilmenite from iron containing gangue minerals. This organic reagent also assists to this improvement by depressing the gangue minerals when it is used as a pH adjuster agent.

Keywords:
Surface dissolution
Flotation
Surface properties
Ilmenite
Oxalic acid
Full Text
1Introduction

The surface dissolution is a method in which the surface properties of valuable and gangue minerals are transformed via solvent media. This method can change the amount and arrangement of ions on the minerals surfaces. This phenomenon would have some consequences in physicochemical separation methods of minerals, including changing the solution chemistry via entering the ions into the solution [1], impressing the physical and chemical interactions of reagents with cationic ions as a point of the rate and amount of the adsorption [2], altering the surface properties of the minerals such as Iso-Electric-Point (iep), point of zero charge (Pzc), electrophoretic mobility, electrochemical properties (i.e. rest potential and conductivity), hydrophobicity and hydrophilicity. This modification method has been used in the flotation of different minerals such as spodumene [3,4], beryl [4], ilmenite [5–7], proveskite [8], zircon and xenotime [9].

The importance of the minerals is due to containing of metals as a primary resource. Titanium metal is well known for its excellent features such as resistance to corrosion and high strength to weight ratio which makes it suitable for aerospace applications. Titanium is mainly used in the production of white TiO2 pigment [10–13]. By decreasing the rutile (TiO2) resources in the world, ilmenite (FeTiO3) is the most important titanium mineral [10,14–16]. In the titanium resources, ilmenite is usually accompanied by Magnetite, rutile, zircon, apatite and different silicate minerals such as pyroxenes, olivine, quartz, feldspar, etc. Depending on the differences between ilmenite and accompanying minerals properties, the processing of ilmenite is commonly conducted by the combination of gravity, magnetic and electrostatic separation methods [17,18].

In recent years, by decreasing the Ti content in the titanium ores and liberation degree of ilmenite, the flotation technique has become more prominent for separation of ilmenite from gangue minerals [19,20]. According to the scientific and experimental evidences, ilmenite has poor floatability in comparison with magnetite, hematite and rutile minerals. This behavior can be due to the low activity of metallic ions on the surface of ilmenite [20–22].

In order to increase the differences between the surface properties of ilmenite and gangue minerals; and for improving the ilmenite floatability considerable amount of various reagents including collectors [20–23], depressant [23–25] and activators [20,25,26] have been used.

Commonly, the selective separation of ilmenite from accompanied minerals using the flotation method has not been achieved even by adding significant amount of different reagents. Thus, In order to improve the selective flotation of ilmenite, the effects of some auxiliary methods such as microwave irradiation [20–22,27,28], oxidation roasting [29,30], hot flotation [31,32], agglomeration flotation [33], reverse flotation [34] and surface dissolution [5–7,35] have been investigated.

In the surface dissolution method, the sulfuric, nitric and hydrochloric acids have been used for modifying the minerals surfaces and improving the ilmenite flotation recovery in the presence of different gangue minerals such as calcite, titanaugite, olivine-pyroxene, magnetite, enstatite and anortite. All of these researches have reported the positive effect of surface dissolution on the ilmenite flotation [5–7,35].

In the mineral processing operations, oxalic acid as an organic acid has been recently used as a depressant and solvent agents in the flotation and leaching processes, respectively [24,36]. The positive effect of oxalic acid for depressing of some silicate minerals such as olivine and titanaugite has been reported. The reports have shown that the oxalic acid species interact with Mg, Ca, and Fe cations, and inhibit the adsorption of anionic collector on the surface of these minerals [24]. In the other study in which the oxalic acid has been used as solvent agent, it has been depicted that the acidic power of oxalic acid in the iron dissolution is more than that of sulfuric acid. For this reason, the application of oxalic acid in the dissolution processes has been increased in recent years [36].

It seems that the oxalic acid is the most effective complexion carboxilic ligand for iron dissolution [37,38]. The ligand concentration, ligand complex, pH of the solution and capacity of Fe mobilization are the important factors affecting the iron dissolution [36]. Proton-promoted and ligand-promoted dissolution are two main mechanisms of Fe dissolution from oxides and aluminosilicates minerals [39–42]. In particular, it is known that through the dissolution of iron oxides, protonation of surface sites weakens the MeO lattice bonds (Me is a cation), accelerating the rate of detachment [43,44]. Some advantages of oxalic acid in comparison with sulfuric acid which have led to more attention in recent years are as follows:

  • High capacity of Fe mobilization [36].

  • Oxalate ions can form a strong ligand with Lewis acid Fe centers and labilizes the FeO bond, whereas sulfuric acid form weak complexes with iron in the aqueous phase [38].

  • Fe(III) oxalate complex can improve the dissolution of iron ions via facilitating the electron transfer between dissolved Fe(II) and surface Fe(III) [45].

  • Dissolution via sulfuric acid is expensive and the ensuing effluents are environmentally unacceptable while application of organic acid like oxalic acid may be more effective and environmentally acceptable [46].

There are some useful literatures about environmental benefits of organic acids such as citrate oxalate, and malate species in many processes operating in the rhizosphere, including nutrient acquisition and metal detoxification, alleviation of anaerobic stress in roots, mineral weathering and pathogen attraction. Also, methane sulfonic acid by low toxicity, high acid recovery and metal alkane sulfonate salt preparation are also environmentally favorable can be used as an ideal electrolyte for many electrochemical processes of tin and lead. On the other hand, the harmful effect of inorganic acids such as sulfuric acid have been proved through the production of acid mine drainage (AMD) [47–49].

Despite the great benefits, the application of oxalic acid in the surface dissolution of minerals is not significant. Thus, in this work, the effect of surface dissolution by oxalic acid on the surface properties of ilmenite and its accompanied gangue minerals such as olivine, pyroxene, tremolite and clinochlore was investigated. In this regard, the flotation behavior of ilmenite and gangue minerals were investigated by microflotation and mechanical flotation experiments before and after surface dissolution pretreatment. For interpretation of the flotation results, some analytical techniques including zeta-potential measurements, ICP mass analysis, UV-visible spectroscopy and FTIR analysis were also used for analyzing the ilmenite and gangue samples before and after surface modification.

2Materials and methods2.1Mineral samples and reagents

The handpick samples were supplied from Qara Aghaj titanium deposit which is located in West Azerbaijan province, Iran. They were used for preparing the relatively purified samples of ilmenite (Il), olivine-pyroxene (Ol-Px), and tremolite-clinochlore (Tr-Cch). After crushing, grinding and screening to the size of −150 +20μm, the purification processes were carried out using several stages of tabling, low and high intensity magnetic separation methods. Table 1 presents the chemical composition of the purified samples which is determined by XRF. As seen from Table 1, the most important difference between the gangue minerals is in their Fe, Si, Al, Mg, and Ca contents. The XRD patterns (Fig. 1) show that the purified samples are essentially composed by their minerals.

Table 1.

Chemical composition (wt%) of purified samples.

Composition  TiO2  Fe2O3  MnO  V2O5  P2O5  CaO  MgO  SiO2  Al2O3  Na2L.O.I 
Ilmenite  46.2  48.6  1.04  0.29  0.24  0.38  2.53  0.19  0.44  –  0.0 
Olivine-pyroxene (Ol-Px)  0.9  43.0  0.64  0.015  3.34  5.6  15.8  29.5  1.06  –  0.0 
Tremolite-clinochlore (Tr-Cch)  0.74  17.7  0.17  0.059  0.078  8.9  19.8  42.3  4.8  –  5.1 
Fig. 1.

XRD patterns of the purified samples (a) Tremolite-Clinochlore, (b) Olivine-Pyroxene, and (c) ilmenite (IL=ilmenite, Ol=Olivine, Px=Pyroxene, Tr=Tremolite, Cch=Clinochlore, Ap=Apatite).

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Sodium oleate (with 95% purity) was used as a collector agent. The analytical grade of sulfuric acid (97%), oxalic acid (99%) and sodium hydroxide (98%) were applied as a pH regulator. Also, the oxalic acid was used as a surface dissolution media. Sodium silicate (99%) was consumed as depressant agent. All the chemical reagents were supplied by Merck Co., except sodium oleate which was supplied by Sigma Aldrich Co. Additionally, the double distilled water was used throughout the all experiments.

2.2Materials characterization

X-ray fluorescence (XRF, Philips X Unique II) and the XPERT MPD diffractometer (with employing Cu Ka radiation) were used for determining the chemical and phase composition of the samples, respectively.

2.3Surface dissolution

The surface dissolution of the pure samples prior to the flotation experiments was carried out by various concentrations of oxalic acid solution at different dissolution times. After dissolution, the pulp was filtered, and the solid phase was washed for 5min with lukewarm double distilled water in a beaker and dried at the room temperature.

2.4Flotation experiments

The microflotation experiments were conducted in a 300-cm3 Hallimond tube. In each test, 2g of purified sample with a size of −150+20μm were used before and after surface dissolution. After agitation of the sample for 4min, the collector was added to the suspension and conditioned for 8min at a desired pH value. In the experiments for investigating the effects of depressant, the reagent was added to the solution before the addition of collector, with the conditioning times of 5min. The reported recovery for each microflotation experiment is the average of recoveries obtained after 3 times repetitions of the test.

The flotation experiments carried out in a 1.5L Denver cell. In all experiments, 300g of synthetic purified sample was subjected to the cell and conditioned similar to the microflotation tests. Finally, after adding pine oil (100g/t) with a conditioning time of 2min, the froth collection was performed for 4min in 30-s intervals. In all tests, the pH meter was used with an accuracy of ±0.02. After the flotation tests, the concentrate and tailing were filtered, dried, weighed and analyzed for TiO2 content. The conditioning sequences and procedures of microflotation and flotation experiments are shown in Fig. 2a and b, respectively. The flotation recovery (R), separation efficiency (SE) and selectivity index (SI) for flotation of ilmenite from selected gangue minerals are calculated by Eqs. (1)–(3):

where C is the yield of concentrate, c and f are the grade of TiO2 in the concentrate and feed, respectively, Rv is the recovery of TiO2 in the concentrate, and Rg is the recovery of gangue minerals in the concentrate.

Fig. 2.

Timeline and sequence of the (a) microflotation and (b) flotation experiments.

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2.5Zeta potential measurement

The zeta potential of the mineral suspension was measured using a Malvern instrument (Nano ZS, ZEN3600, UK). For determining the zeta potential, 50mg of pure minerals ground to −15μm was added to 100mL of distilled and deionized water containing 2×10−3mol/L KCl as a supporting electrolyte. The time of conditioning and pH adjustment was almost 15min. The NaOH and H2SO4 were used as the pH regulator reagents over the pH ranges of 2–11. The reported results are the average of at least three full repeat experiments. The repeated tests showed a measurement error of ±2mV.

2.6ICP-MS analysis

Inductively coupled plasma mass spectrometry (ICP-MS) is an analytical technique used for elemental determinations. This technique is versatile analytical tool having a wide variety of research applications and routine use in numerous application fields, such as environmental analysis, geochemistry, and biological mater. The CETAC ADX-500 auto-diluter system was tested with ELAN® v 2.1software and the ELAN 6000 ICP-MS instrument to determine on-line automated dilution performance during analysis of standard solutions. In each test, 5g of purified samples were placed in a beaker which was then filled with double distilled water (before surface dissolution) and with the oxalic acid solution (for after surface dissolution) and then agitated with a mechanical stirrer for 15min. After filtration of the suspension, the remaining liquid phase was analyzed by inductively coupled plasma mass spectroscopy (ICP-MS).

2.7The adsorption capacities of collector

The calibration curve was created via standard solutions based on the Beer–Lambert law, ranging from 0.8×10−4 to 4×10−4mol/L of sodium oleate as a collector. The curve is shown in Fig. 3. The adsorption tests were carried out in a quartz conical flask with a specific pH value where 1g of a selected mineral in 50mL of solution agitated for 60min at the room temperature (25°C). Then, the filtrated solution was sent to the UV-Visible spectrophotometer. A UV-1601, Beijing Beifen-Ruili analytical instrument spectrophotometer scanning the wavelength range of 200–700nm was used to measure the residual sodium oleate concentration. The intensity of UV absorption at 192nm was used as a measure of the sodium oleate concentration [50]. At last, the absorbance readout was converted to the real concentration of collector using calibration curve. Eventually, the adsorption amount of collector on the minerals is calculated by Eq. (4):

where C0 and Ct are the initial and final dosages of sodium oleate in the solution (mol/L), respectively, V is the volume of the initial solution (L), M is the mass of adsorbent (g) and qt is the adsorption amount (mol/g) (Fig. 3).

Fig. 3.

Calibration curve for absorbance versus concentration of the NaOL.

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2.8FTIR analysis

The FT-IR analyses have been carried out with NEXU670 FT-IR (Nicolet Corporation, USA) to specify the nature of the interaction between the collector and mineral phases. The sample was ground in a laboratory ring mill to 100% minus 15μm. Preparing the original and treated samples was similar to the flotation experiments. In these experiments, the ratio of KBr to sample was 300:1 (w/w).

3Results and discussion3.1Single mineral flotation3.1.1Surface dissolution parameters

The acid concentration and dissolution time are the important factors affecting the surface dissolution performance. In order to determine the optimal condition of these parameters, various microflotation tests were carried out on ilmenite (as a valuable mineral) in the presence of 3.65×10−4mol/L sodium oleate at a pH of 6.3. For determining the optimal dissolution time, the purified ilmenite was dissolved in the oxalic acid for various times at a constant concentration of 7.5% (w/w). As seen from Fig. 4a, by increasing the time of dissolution, the recovery of ilmenite goes up and reaches to maximum value (93.4%) after 10min. Then, the effect of acid concentration on the ilmenite floatability was investigated after dissolution for 10min. The results are shown in Fig. 4b. It can be seen that the maximum recovery of ilmenite (93.4%) is occurred using 7.5% acid concentration. By exceeding the dissolution time and acid concentration from 10min and 7.5% (w/w), respectively, the flotation recovery of ilmenite decreases. This can be due to dissolution of more active surface ions which are effective ions in the flotation of ilmenite. Thus, 10min surface dissolution time and 7.5% oxalic acid concentration are considered as the optimal conditions for the acid surface dissolution process in the next experiments.

Fig. 4.

Effect of surface dissolution as a function of (a) time and (b) acid concentration on the ilmenite flotation recovery (sodium oleate: 3.65×10−4mol/L, pH=6–6.5)

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3.1.2Effect of pH

The flotation behavior of Il, Ol-Px and Tr-Cch were examined as a function of pH in the presence of 3.65×10−4mol/L sodium oleate before and after surface dissolution at optimal conditions. The results are given in Fig. 5. The occurrence of two recovery peaks for ilmenite at pHs 2.5–4 and 6–6.5 before and after surface dissolution is in good agreement with the previous works [21,22,27,28]. The maximum recovery of ilmenite (73%) is achieved at a pH of 6.3. In the case of Ol-Px and Tr-Cch, the maximum recoveries are occurred at pHs about 7.5 and 9, respectively. This can be due to the reaction of Mg2+ and Ca2+ ions with the oleate species on the surface of these gangue minerals at the mentioned pH ranges [6].

Fig. 5.

Effect of surface dissolution on the flotation recovery of Il, Ol-Px and Tr-Cch as a function of pH value (sodium oleate: 3.65×10−4mol/L, dissolution time: 10min, oxalic acid concentration: 7.5%).

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As seen from Fig. 5, before pretreatment, the recovery of Ol-Px is more than that of ilmenite at the strong acidic and alkaline pHs (2.5<pH<5 and 8<pH<12), while the ilmenite has more floatability at pHs between 5.5 and 7.5. At the acidic pH ranges the flotation recovery of Tr-Cch sample is hardly reached 20%, while at pHs above 7 the recovery is drastically increased and reaches almost 79% at a pH of 9.

According to the evidences, in the flotation of ilmenite, the Ol-Px would be more troublesome gangue mineral in comparison with Tr-Cch one. Fig. 5 shows that unlike the gangue phases, the surface dissolution has a positive effect on the ilmenite flotation recovery at the whole pH ranges. After surface dissolution, the flotation recovery of ilmenite, Ol-Px and Tr-Cch reaches 93.4%, 31.5%, and 15.4%, respectively at a pH of 6.3. In the other words, the surface dissolution increases the differences between the flotation recoveries of ilmenite and two other gangue phases including Ol-Px and Tr-Cch from 13.9% to 61.9% and 53.4% to 78%, respectively (Fig. 5).

3.1.3Effect of collector concentration

Fig. 6 presents the effect of collector dosage on the recovery of purified samples before and after surface dissolution at a pH of 6.3. For all three mineral phases, the flotation recovery is increased as a function of collector dosage before and after surface dissolution. After pretreatment, the recovery curve of ilmenite has gone upward significantly, while it has moved downward in the case of gangue minerals. Thus, the differences between the recoveries of ilmenite and two gangue minerals are enhanced after surface dissolution in the whole dosages of collector but their enhancements in low dosages are more than that of high ones.

Fig. 6.

Effect of surface dissolution on the flotation recovery of Il, Ol-Px and Tr-Cch as a function of sodium oleate dosage (pH=6.3, surface dissolution time: 10min, concentration of acid: 7.5%).

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For example, in the presence of 1.83×10−4mol/L and 2.43×10−4mol/L of collector dosages, the recoveries of pretreated ilmenite are 88.29% and 91.3%, respectively, while before surface dissolution, these values are not achieved even using higher dosages of collector. But, the maximum recoveries for treated and non-treated ilmenite are obtained with 3.65×10−4mol/L, and this dosage is used as an optimal value in the next experiments.

The effect of collector dosage was also investigated on the artificially mixed minerals using the microflotation tests before and after surface dissolution. In these tests, the samples were prepared by mixing 0.5g ilmenite, 0.75g tremolite-clinochlore, and 0.75g olivine-pyroxene. The variation of ilmenite purity and recovery in the flotation concentrate as versus collector dosage has been shown in Fig. 7. As seen from Fig. 7, the surface dissolution simultaneously improves the recovery and grade of ilmenite in the flotation concentrate. The optimal grade and recovery of ilmenite are almost 59% and 90%, respectively. This concentrate is obtained using 3.65×10−4mol/L sodium oleate.

Fig. 7.

Recovery and grade of ilmenite in the concentrate obtained from artificially mixed minerals (0.5g ilmenite, 0.75g tremolite-clinochlore, and 0.75g olivine-pyroxene) as a function of sodium oleate dosage before and after surface dissolution (pH=6.3, surface dissolution time: 10min, the acid concentration: 7.5%).

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3.1.4Effect of depressant and regulator

In most of the flotation processes, sulfuric acid and sodium silicate are commonly used as pH adjusting and gangue minerals depressing agents, respectively. The effects of these reagents on the floatability of ilmenite and both gangue phases are shown in Fig. 8a and b. The results show that the sulfuric acid as the pH regulator has no effect on the recoveries of purified samples before surface dissolution (Fig. 8a). As seen from Fig. 8b, using 6×10−4mol/L of sodium silicate as depressant agent, the flotation recovery of non treated ilmenite, Ol-Px and Tr-Cch are decreased to 35.42%, 26.53% and 8.34%, respectively. The depressing effects of sodium silicate are in good agreement with the previous works [24].

Fig. 8.

Effects of (a) oxalic acid, (b) sulfuric acid and (c) sodium silicate as pH regulator and depressant agents on the flotation recovery of purified samples before and after surface (pH=6–6.5 and sodium oleate: 3.65×10−4mol/L).

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In this research, the oxalic acid was also used as the pH regulator agent before and after surface dissolution. The results are presented in Fig. 8c. By increasing the dosage of oxalic acid, the flotation recoveries of all three phases are decreased. For Ol-Px sample, this decrease is stronger than two other samples. Before surface dissolution, using 6×10−4mol/L of oxalic acid as a regulator agent the recovery of ilmenite is decreased from 73.5% to 64.5%, while the recoveries of Ol-Px and Tr-Cch are extremely decreased from 59.6% to 18.11% and 20.1% to 10.1%, respectively.

After surface dissolution, the recoveries of Ol-Px and Tr-Cch are still decrease, while the floatability of ilmenite is negligibly improved. These results reveal that the oxalic acid not only can be used as the regulator agent, but also it plays the role of a suitable depressant. It can be concluded that the oxalic acid simultaneously plays both depressant and regulator roles even better than sodium silicate and sulfuric acid in the ilmenite flotation.

3.2Flotation of artificially mixed minerals

Based on the positive results of microflotation experiments, the effect of surface dissolution was examined on the artificially mixed minerals as a feed material in the bench-scale flotation at pH=6.3. In these tests, a binary mixture of IL+Tr-Cch and IL+Ol-Px was prepared with the ratio of ilmenite to gangue mineral 20:80wt%. For a trio mixture of IL+Ol-Px+Tr-Cch, the ratio was 20:40:40wt%, respectively. According to the results presented in Table 2, the floatability of ilmenite in the presence of Tr-Cch is much more than that of Ol-Px before and after pretreatment. It should be noted that the surface dissolution improves the metallurgical parameters, including grade, recovery, separation efficiency (SE) and selectivity index (SI) for all three samples. These results are in good agreement with the microflotation results (Table 2).

Table 2.

Results of ilmenite flotation from artificially mixed minerals before and after surface dissolution (pH=6–6.5, 1000g/t sodium oleate, 1000g/t oxalic acid, BS: before surface dissolution, AS: after surface dissolution).

  Yield (wt%)Ilmenite in concentrateGangue in concentrateSE (%)  SI 
  Concentrate  Tailing  Recovery (%)  Grade (%)  Recovery (%)  Grade (%)     
BS
IL+(Tr-Cch)  33.4  66.6  64  38.32  25.75  61.67  38.25  2.26 
IL+(Ol-Px)  36.4  63.6  56  30.76  31.5  69.23  24.5  1.66 
IL+(Ol-Px)+(Tr-Cch)  47.3  52.7  54  22.83  45.62  77.16  8.37  1.18 
AS
IL+(Tr-Cch)  35.85  64.15  87.75  48.95  22.87  51.04  64.87  4.91 
IL+(Ol-Px)  37.4  62.6  83  44.38  26  55.61  57  3.72 
IL+(Ol-Px)+(Tr-Cch)  47.86  52.14  81.5  34.05  39.45  66.93  42.04  2.6 
3.3ICP-MS analysis

The ICP-MS method was used to analyze the liquid phases before and after surface dissolution of purified minerals. The presented results in Table 3 depict that Fe, Mg and Ca ions are the most important ions which are dissolved through the surface dissolution pretreatment. The dissolution amount of Fe and Mg ions from ilmenite surface is expressively less than that of gangue minerals before and after surface dissolution.

Table 3.

Content of dissolved ions from ilmenite, olivine-pyroxene and tremolite-clinochlore measured by ICP mass analysis, before surface dissolution (BS) and after surface dissolution (AS).

Sample  Content of dissolved element (ppm)
  CaFeMgTi
  AS  BS  AS  BS  AS  BS  AS  BS 
Ilmenite  –  –  140.5  21.8  55.63  11.3  0.28  0.01 
Olivine-Pyroxene  642.6  45.7  1503.4  63.4  586.8  40.1  –  – 
Tremolite-Clinochlore  686.2  52.3  437.8  30.5  613.6  43.2  –  – 

Before surface dissolution pretreatment, the dissolution of ions from minerals surfaces is negligible. This means that there are still enough surface active sites for reacting with collector species. After surface pretreatment by oxalic acid, most of the surface active ions (including Fe, Mg and Ca) from gangue minerals are released to the solution phase. Thus, the decrease in the flotation recovery of both gangue samples can be due to absence of Fe, Mg and Ca ions on the surface of Ol-Px and Tr-Cch phases causing the reduction of collector adsorption (Table 3).

The increase in the flotation recovery of ilmenite can be related to conversion of Fe2+ ions to Fe3+ ones [27–29,35]. Because, the adsorption capacity of collector on the surfaces containing ferric ions is more than that of ferrous ones, and more importantly, the stability of ferric oleate species is rather than that of ferrous oleate compounds [35].

It should be noted that the ionic radius and length of cation bonds with oxygen atom are the important factors affecting the dissolution behavior of ions. The investigation on Fe bonds with oxygen in ilmenite and olivine reveals that the less dissolution of iron from ilmenite surface can be due to the shorter length of FeO bonds (2.139Å) in ilmenite in comparison with that's of olivine (2.165Å) [51,52]. Thus, the easily dissolution of some cations from the surface of gangue minerals result in the decrease of the surface active sites to interact with the oleate ions [6,35].

3.4Adsorption capacities of collector

The adsorption capacities of collector on the surface of ilmenite and gangue phases were determined by UV-vis before and after surface dissolution. These analyses were carried out as a function of pH (2.5–11) at ambient temperature using 3.65×10−4mol/L of sodium oleate as initial concentration. The results are shown in Fig. 9 which is in good agreement with the flotation behavior of ilmenite before and after surface dissolution (Fig. 5).

Fig. 9.

Adsorption density of collector on the surface of Ilmenite, Olivine-Pyroxene and Tremolite-Clinochlore as a function of pH before and after surface dissolution (3.65×10−4mol/L sodium oleate, dissolution time: 10min, acid concentration: 7.5%).

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Fig. 9 shows the adsorption capacity of the collector on the surface of ilmenite is increased after treatment and its maxima (qe=11×10−6mol/g) is achieved at a pH of 6.3. This can be due to the conversion of Fe2+ ions to Fe3+ ones through the surface dissolution which results in the formation of more insoluble ferric oleate in comparison with ferrous oleate compound [35]. In the cases of both gangue phases, the adsorption capacities are decreased after surface dissolution. This reduction in collector adsorption is more significant at alkali pH ranges. It is corresponding to the absences of calcium and magnesium ions as the surface active sites on the minerals surfaces for interacting with the oleate species [6,35,53]. By comparing the results presented in Figs. 5 and 9, it is concluded that the variation of flotation recoveries of minerals and collector adsorption capacities as function of pH are in good accordance with each other.

3.5FTIR analysis

The FTIR analysis was used to investigate the adsorption of sodium oleate on the surface ilmenite, gangue phases before and after surface dissolution. Fig. 10 displays the FTIR spectra's of minerals conditioned with sodium oleate at a pH of 6.3. Also, the characteristics of NaOl adsorption bands on the surface of ilmenite and gangue minerals are presented in Table 4. The appeared new bands at around 2851cm−1 and 2923cm−1 are attributed to CH2 stretching of acyclic compounds [4,6]. The appearance of these bands means that the oleate ions have been adsorbed on the surface of ilmenite and gangue minerals before and after surface dissolution. The adsorption band at around 1630–1635cm−1 for all three samples before and after surface dissolution is related to the bending mode of adsorbed water [54]. The new band appeared at about 1467cm−1 for ilmenite is related to the iron oleate which shows the chemisorptions mechanism of collector adsorption. As seen from Fig. 10a, the surface area under spectra at 2923 and 2851cm−1 for treated ilmenite is more than that of non-treated one. This indicates that the adsorption density of sodium oleate on the surface of pretreated ilmenite is more than that of original ilmenite which results in the enhancement of ilmenite flotation recovery after surface dissolution.

Fig. 10.

FTIR spectra of sodium oleate adsorption on the surface of (a) Ilmenite, (b) Olivine-Pyroxene and (c) Tremolite-Clinochlore before and after surface dissolution (3.65×10−4mol/L sodium oleate, pH=6–6.5, dissolution time: 10min, acid concentration: 7.5%).

(0.3MB).
Table 4.

The characteristics of NaOl adsorption bands on the minerals surfaces.

Wave numbers (cm−1Attributed to  Ref. 
1630–1635  Bending mode of adsorbed water  [6] 
2923–2852  CH2 stretching of acyclic compounds  [6] 
1710–1750  Carboxyl (CO) group of sodium oleate  [37] 
1500–1550  Carboxylate group of sodium oleate  [38] 
1467–1585  Iron oleate  [6] 
1541–1577  Calcium oleate  [36] 
1564–1581  Magnesium oleate  [41] 
3.6Zeta potential measurements

The zeta potential of ilmenite and gangue minerals before and after pretreatment was measured at various pHs in the absence and presence of sodium oleate. The results are presented in Fig. 11. As seen from Fig. 11, the isoelectric point (IEP) of the original ilmenite, olivine-pyroxene and tremolite-clinochlore are occurred at pHs 5.3, 4 and 4.2, respectively.

Fig. 11.

Zeta potential of (a) Ilmenite, (b) Olivine-pyroxene, and (c) Tremolite-clinochlore as a function of pH before and after surface dissolution in the presence of KCl (2×10−3mol/L) and KCl+NaOl (3.65×10−4mol/L sodium oleate and 2×10−3mol/L KCl) (BS: surface dissolution and AS: after surface dissolution, (dissolution time: 10min, acid concentration: 7.5%,).

(0.3MB).

After surface dissolution pretreatment by oxalic acid, the IEP values of ilmenite, olivine-pyroxene and tremolite-clinochlore are decreased to pHs of 3.2, 2.9 and 2.8, respectively. This means that the surface dissolution increases the negative charges on the surface minerals at acidic pH ranges. The decrease of ilmenite IEP through surface dissolution can be due to the enhancement of Fe3+ content on its surface. This is in accordance with previous work [55] showing a good negative correlation between IEP and Fe3+ content.

The decrease in the zeta potential and IEP of gangue minerals through the surface dissolution process can be attributed to the removal of some surface ions. The selective dissolution of Fe, Mg and Ca cations from the surface of olivine-pyroxene and tremolite-clinochlore minerals causes that the surface properties of these minerals to be similar to the quartz having low values of IEP and PZC.

In the presence of the sodium oleate as collector, the zeta potential of non-treated minerals has been changed strongly toward the negative values. This can be due to the entrance of oleate ions into the Helmholtz layer of the minerals. The most negative zeta potentials are obtained at pH 6.3 for ilmenite and olivine-pyroxene and at pH 9 for tremolite-clinochlore where the maximum flotation recoveries of minerals are achieved.

After surface dissolution, by adding the collector agent, the zeta potential-pH profile of ilmenite has been shifted toward the more negative values. This means that the surface dissolution which oxidizes the Fe2+ ions to Fe3+ ones [35] increases the entry of oleate ions on the inner stern planes of the electrical double layer. In the case of treated gangue phases the negative surface charge is decreased, and the zeta potential–pH profile has been shifted toward the less negative values.

This reveals that after surface dissolution, the adsorption of oleate ions on the surface of gangue minerals is decreased by removing some surface active sites including Fe2+, Mg2+ and Ca2+ cations. Thus, the enhancement of differences between the zeta potential of ilmenite and gangues through the surface dissolution process being in good agreement with the results of collector adsorption density facilitates the achievement of ilmenite selective flotation from gangue minerals.

4Discussion4.1Flotation behavior of minerals

The surface chemistry of minerals and solution chemistry are the most important factors affecting the flotation behavior of minerals. The distribution diagrams of Ti4+, Fe2+, Fe3+, Ca2+, Mg2+ and Al3+ ions as active cations in the surface of ilmenite and gangue phases, including olivine-pyroxene and tremolite-clinochlore are illustrated in Fig. 12[31]. These ions constitute the hydroxyl species at different pHs which play an important role in the adsorption of flotation agents including activator, depressant and collector on the surface of minerals. The pKsp for the Mg2+, Ca2+, Fe2+, Fe3+, Al3+ and Ti4+ ions as the different metallic ionic hydroxyl complex are 11.15, 5.22, 15.1, 12.6, 33.5 and 58.3, respectively [31].

Fig. 12.

The activity of different ions as a function of pH (a) Ti4+, (b) Fe2+, (c) Mg2+, (d) Ca2+, (e) Al3+, and (f) Fe3+.

(0.38MB).

On the other hand, the oleate species in the solution have a key role in the flotation of minerals. The distribution of oleate species as a function of pH [56–59] is shown in Fig. 13. It indicates that the oleic acid forms various species in the solution such as un-dissociated acid (RCOOH), oleate ion RCOO, oleate dimer (RCOO)22− and the acid-soap (RCOO)2H.

Fig. 13.

Distribution of various oleate species as a function of pH.

(0.07MB).

By placing the minerals in the solution, the first hydroxyl complexes of divalent metalic ions such as FeOH+, CaOH+ and MgOH+ are formed in the surface of minerals which can react with the oleate species [22,27,28]. The collector adsorption on the surface of ilmenite and gangue minerals at different pHs can principally take place through reactions (5)–(12) (Table 5) and affect their flotation behaviors.

Table 5.

The reactions of the minerals with collector at various pHs.

Minerals  Reaction  m, n  pH 
Ilmenite 
mn++3RCCOm(RCOO)n
 
m=Ti(OH), n=1, 2, 3  2–3 
Ilmenite 
m2++2RCOOm(RCOO)2
Fe  5–8 
Ol-Px  Mg, Ca  8–12 
Tr-Cch  FeOH  <7 
Ilmenite 
m2++(RCOO)22−m(RCOO)2
Fe  5–8 
Ol-Px  Mg, Ca  8–12 
Tr-Cch  Fe(OH)2  <7 
Ilmenite 
m2++2(RCOO)2Hm[(RCOO)2H]2
Fe  5–8 
Ol-Px  Mg, Ca  8–12 
Tr-Cch  FeOH  <7 
Ilmenite 
mOH++RCOOmOH.RCOO
Fe  5–8 
Ol-Px  Mg, Ca  8–12 
Tr-Cch  Fe(OH)2  <7 
Ilmenite 
mOH++(RCOO)2H⋯(mOH)[(RCOO)2H]
Fe  5–8 
Ol-Px  Mg, Ca  8–12 
Tr-Cch     
Ilmenite 
2mOH++(RCOO)22⋯(mOH)2(RCOO)2
Fe  5–8 
Ol-Px  Mg, Ca  8–12 
Tr-Cch     
Ilmenite
mn++nRCOOm(RCOO)n
 
Fe, n=3<7
m3++n(RCOO)2Hm[(RCOO)2H]n
 
m(OH)2++(RCOO)2Hm(OH)2·(RCOO)2H
 

Ilmenite flotation is controlled by the activity of Fe2+ and Ti4+ species. The interaction of these ions and their hydroxyl species with oleate ions results in the flotation of ilmenite. The dominant presence of Ti species at strong acidic solutions (Fig. 12a) is responsible for ilmenite flotation while the recovery peak at weak acidic and alkaline solutions (Fig. 5) is attributed to the Fe2+ species (Fig. 12b). The lower value of recovery peak at strong acidic pHs in comparison with that occurred at pHs between 5.5 and 7.5 can be due to the higher pKsp values of Ti4+ ions. The reduction of ilmenite flotation at alkaline pHs can be due to the electrostatic repulsion between OH ions and oleate species [21,27]. The Fe ions have also an important role in the flotation of minerals like olivine, pyroxene, tremolite and clinochlore at acidic pHs [39]. At this pH range the flotation recovery of olivine-pyroxene is greater than that of tremolite-clinochlore. This can be related to the higher content of Fe in the olivine-pyroxene sample.

At alkaline solutions, Ca and Mg species play the most important role in the flotation of olivine-pyroxene and tremolite-clinochlore. At pHs between 8.5 and 10, the higher floatability of tremolite-clinochlore in comparison with olivine-pyroxene can be attributed to the higher amount of Ca and Mg ions in the tremolite-clinochlore phase. The flotation recovery of both phases at alkaline pHs is greater than that of acidic conditions. This can be due to the lower pKsp of Ca and Mg hydroxyl species in comparison with Fe hydroxyl species (Figs. 12 and 13).

As evidenced by FTIR analysis and zeta potential measurements in this work and literatures [60–62], the adsorption of collector on the surface of ilmenite and gangue minerals being responsible for their flotation at various pHs is mainly occurred through chemisorption mechanism by the formation of titanium oleate, iron oleate, magnesium oleate and calcium oleate.

4.2Improvement of flotation selectivity

The dissolution process depends on the surface interactions that take place on the mineral surfaces [39,63]. Through the surface dissolution process, the disassociation of oxalic acid according to reactions (13) and (14) results in the dissolution of Fe, Mg and Ca sites from the mineral surfaces. Dissolution of these cation sites from the surface of minerals can be taken place via reactions (15)–(19)[64]:

Ca2++C2O42−+H2OCaC2O4H2OKsp,ca-ox=2.57×10−9
Mg2++C2O42−MgC2O4K=2.07×105
Fe3++C2O42−Fe(C2O4)+K=5.89×108
Fe(C2O4)++C2O42−Fe(C2O4)2K=3.31×106
Fe(C2O4)2+C2O42−Fe(C2O4)33−K=2.75×104

The surface dissolution pretreatment improves the selectivity of ilmenite flotation from gangue phases. This improvement takes place by increasing the floatability of ilmenite and its reduction for gangue minerals. As shown by ICP mass analysis, the dissolution of Fe ions from the surfaces of ilmenite is less and slower than that of gangue minerals. The contact of oxalic acid with ilmenite oxidizes some of the surface Fe2+ ions to the Fe3+ ones before dissolving, and improves the ilmenite flotation recovery. This improvement can be due to the increase of the formation of ferric iron oleate (Ksp=10−29.7) on the surface of ilmenite which is more stable and insoluble than that of ferrous iron oleate (Ksp=10−15.5) [27–29,35]. The increase of collector adsorption density (as shown by UV/vis analysis) and also the formation of more insoluble compound on the surface of ilmenite improve the stability of bubble–particle attachment, and enhance the flotation recovery.

In the surface dissolution process, when the dissolution time or acid concentration exceeds the optimal values, the floatability of ilmenite is decreased. This may be due to the more dissolution of Fe2+ ions before converting to the Fe3+ ions.

As indicated by ICP-MS analysis, through the surface dissolution pretreatment by oxalic acid, the dissolution and removal of cations including Mg2+, Ca2+, and Fe2+ from the surface of gangue minerals are significant. Thus, the decrease of gangue minerals floatability can be due to the reduction of surface active sites (Mg2+, Ca2+, and Fe2+) reacting chemically by collector ions. At acidic pHs, this decrease can be related to the removal of Fe2+ ions which results in the decrease of collector adsorption and hydrophobicity of minerals. But, the stopping of Ol-Px and Tr-Cch flotation at alkaline pHs can be attributed to the lack of sufficient Mg and Ca cation sites on their surfaces resulting in the reduction of collector adsorption density. This reduction has been proved by UV/vis analysis, zeta potential measurements and FTIR analysis.

Also, the decrease of gangue minerals floatability can be related to the depressing effect of oxalic acid. Because, C2O42− ions of oxalic acid react via a thermodynamically spontaneous reaction with Mg2+, Ca2+, and Fe2+ at pHs between 4 and 9, and hence prevent the adsorption of collector on the surface of gangue minerals [24].

5Conclusion

The surface dissolution by oxalic acid has a good potential for improving the selectivity of ilmenite flotation from gangue minerals. This improvement is achieved by creating significant differences between the surface chemistry of minerals. These differences can be caused by changing in the surface ions arrangement, conversion of some ions to other active ions, removal of some ions and changes in the surface charge of minerals. In this regard, the Fe ions play an important role. The conversion of Fe2+ ions to Fe3+ ones on the surface of ilmenite increases its negative surface charge, and improves ilmenite floatability by enhancing the collector adsorption density. In the case of olivine-pyroxene and tremolite-clinochlore, the significant dissolution of Fe ions decreases their flotation recovery at pH ranges where the maximum recovery of ilmenite is occurred. The decrease of gangue minerals floatability is also taken place by removing the surface ions of Mg and Ca, especially in the alkaline pH ranges. The removal of surface ions including Fe2+, Mg2+ and Ca2+ reduces the floatability of gangue minerals by diminishing their negative surface charge and also surface active sites for reacting with collector species.

The occurrence of above mentioned reactions on the surface of minerals through the pretreatment process by oxalic acid improves the various metallurgical parameters of ilmenite concentrate including grade, recovery, separation efficiency (SE) and selectivity index (SI). The improvement of these parameters has a good negative correlation with the iron content of gangue minerals. The lower Fe content the greater improvement in grade, recovery, SE and SI.

When the oxalic acid is used as a pH adjuster agent in the ilmenite flotation, it acts simultaneously as a depressant for gangue minerals such as olivine, pyroxene, tremolite and clinochlore. This characteristic of oxalic acid beside its higher performance in iron dissolution and lower environmental problems in comparison with sulfuric acid makes it favorable media for surface dissolution pretreatment and desirable agent for pH adjustment in the flotation process.

Conflicts of interest

The authors declare no conflicts of interest.

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Hydrometallurgy, 136 (2013), pp. 15-26
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Journal of Materials Research and Technology

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