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Issue
Metall. Res. Technol.
Volume 117, Number 1, 2020
Article Number 102
Number of page(s) 9
DOI https://doi.org/10.1051/metal/2019047
Published online 13 January 2020

© EDP Sciences, 2020

1 Introduction

During beneficiation of iron ore, a huge quantity of tailings was generated and stored in the pond. These tailings occupy massive storage place and cause environmental hazards because of the presence of harmful elements in the tailings. Due to depletion of high-grade reserves, it has become necessary to reutilize the low-grade ores and tailings to meet the future market demand. To mitigate these issues, the government of India had fixed the cut-off limit and any ore containing iron values above this cut-off value has to be utilized [1]. Review shows presently iron ore tailings are further reutilized either by beneficiation route or by agglomeration process [2]. Investigation illustrates many attempts were taken to beneficiate these low grade iron ore and tailings with conventional methods i.e. gravity separators, magnetic separators, flotation, etc. But these processes are not effective for handling the ultrafine particles present in the tailings. For Indian iron ore, alumina is the major concern, which causes critical problems in the blast furnace [3,4]. Extensive studies were conducted to beneficiate the iron ore to meet the specification of blast furnace grade by enriching the Fe (%) and reducing the Al2O3 (%). Selective flocculation is one of the physicochemical method used for beneficiation of the ultra-fines. For establishing the selective flocculation process, many attempts were taken using synthetic mixtures of iron ore and its associated gangue minerals using different reagents i.e. natural and synthetic reagents, etc. [514]. Very few attempts were taken using natural iron ore tailings. Gururaj et al. [7] attempted to enrich the Bersuan iron ore tailing using selective flocculation, they achieved very poor selectivity index ∼ 2 [7]. Rao and Narasimhan [15] achieved better selective index 4.5 using natural tailings. Their investigation shows better process performance at pH ∼ 10.5 [15]. For further enhancement of the selectivity, Singh and Singh [16] attempted ultra-sonication prior to the selective flocculation process. They achieved more than 65% Fe with 1.6% alumina and 1.2% silica from the feed assay of 50.5% Fe, 7.2% alumina, and 7.8% silica [16]. Ahmed et al. [17] reported that for smaller particle size, liberation will be better and selective flocculation will perform better [17]. Mukherjee et al. [4] attempted a selective flocculation process for beneficiation of ultrafine iron ore (< 10 μm) by classifying the feed in a hydrocyclone in the laboratory scale [4]. Though the process was successfully established in the laboratory scale, the commercial application of this technique is very limited. Literature shows few pilot scale trials i.e. Camdag mines, Wadii Sawain Mines, Kudremukh mines, Noamundi mines, etc. were attempted for establishing the process [18,19]. Only at Tilden Iron ore plant, the selective flocculation process was applied prior to flotation [10,18]. Till now, the commercial application of this process is very limited because of a very complex mechanism. Panda et al. [14] attempted to study selective flocculation behaviour using natural tailings and they compared the results with the synthetic mixtures study. Detailed work was done to remove alumina from natural iron ore tailings of Joda area, India using selective flocculation process. The process mechanism was investigated using FTIR and zeta potential analysis.

2 Experimental

2.1 Materials and reagents

Approximately 200 kg of iron ore tailing was collected from representative points of tailing pond of iron ore beneficiation plant, Joda. The collected sample was subjected for natural drying and mixed properly by coning quartering method. Few quantities of the representative samples were taken out from the representative samples and subjected for size analysis. From the size analysis, it was observed that approximately 95% of the particles are below 37 μm, given in Table 1a. These ultrafine particles were used as a feed in the selective flocculation process. The feed sample (< 37 μm) was subjected to different characterization studies i.e., mineralogical and physicochemical property analysis, etc. Assay analysis of the samples is given in Table 1b. Assay analysis shows that the sample contains 50.98% Fe, 8.86% Al2O3, 9.64% SiO2 with 7.12% LOI, etc. Optical image analysis is given in Figure 1. From the plot, it was noted that the sample is enriched with haematitic phase in the free form. XRD analysis is shown in Figure 2. XRD analysis infers that the sample is enriched with mostly hematitic phase. Along with hematite few other phases i.e., goethite and kaolinite are also associated in the sample. SEM analysis shown in Figure 3, illustrates that along with iron phase, few quantities of kaolinite and silicates phases are present in the sample. The zeta potential analysis of iron ore tailings is shown in Figure 4. The figure shows the point of zero charge (pzc) of the iron ore tailings is ~ 4.03. The pzc value of the iron ore tailing deviates from the reported pzc of pure hematite mineral because of the association of few other gangue minerals in the sample [20]. The FTIR analysis of iron ore tailings in the range of 4000 to 400 cm−1 is shown in Figure 5. The wave number of 3696 cm−1 indicates stretching due to presence of kaolinite group [21]. Similarly, the peaks at 488.4 cm−1 is due to the presence of iron ore group [22] and the peak at 1633 cm−1 is for −OH stretching [23].

Table 1a

Size analysis of as received Joda tailings.

Table 1b

Assay analysis of the iron ore tailings.

thumbnail Fig. 1

Optical image analysis of the as received iron ore tailing.

thumbnail Fig. 2

XRD analysis of the as received iron ore tailing.

thumbnail Fig. 3

SEM analysis of iron ore tailings.

thumbnail Fig. 4

Zeta potential of the iron ore tailings.

thumbnail Fig. 5

FTIR of natural iron ore tailings.

2.1.1 Reagents

Degraded wheat starch solution was used as the flocculant. The flocculant was prepared by mixing sodium hydroxide and starch with (2:1) ratio by maintaining temperature [1113]. Analytical grade of sodium hexametaphosphate solution was used as dispersant. For pH adjustment, analytical grade hydrochloric acid or sodium hydroxide solution was used.

2.2 Methods

2.2.1 XRD analysis

For analysing different mineral phases present in the sample, XRD analysis was carried out in a PAN-Analytical instrument with copper tube. The diffractogram shows different peaks, based on which the mineral phases present in the sample can be identified.

2.2.2 True density measurement

True density was measured by pycnometer (AccPucII1340). Helium gas was used for maintaining the required pressure (19-20PSI). By using the automatic system analyzer, density was measured.

2.2.3 FTIR analysis

FTIR was applied for characterizing the surface property of the feed sample and the treated products (before and after adsorption of the reagent on feed sample). Change in surface property was detected by the variation in the peaks in different wave numbers. FTIR analysis was carried out with a NICOLET 6700 FTIR instrument and Thermo Electron corporation probe.

2.2.4 Zeta potential measurements

The zeta potential was measured with a Colloidal Dynamics zeta probe auto-titrator instrument.

2.2.5 Selective flocculation

In this process, the required amount of dispersant dose (sodium hexametaphosphate solution) was added to the system to form a disperse homogeneous phase in the system. After addition of the dispersant dose, mechanical stirrer speed was maintained at a high rate approximately 2000 RPM continuously for one hour to mix the dispersant homogeneously on each particle surface and all the particles remains in the dispersed phase. Subsequently, the required pH of the slurry was maintained by adding hydrochloric acid or sodium hydroxide solution drop by drop. After addition of the flocculant, mechanical stirrer speed was decreased up to 100 RPM, to prevent breaking of the floc. During all the experiments, 12% of solids were maintained. Experiments were conducted by varying the flocculant dose and the dispersant dose and pH, whereas the other parameters i.e. solid concentration (%), mechanical stirrer speed remains constant for all experiments.

The effects of different operating parameters i.e. flocculant dose, dispersant dose and pH for evaluation of separation performances of selective flocculation in terms of grade (%) and Fe recovery (%) was evaluated. In addition to Fe (%), Al2O3 (%) and SiO2 (%) of the products also were evaluated. For the clear depiction of the effects of the operating parameters, a 2D graph was plotted. All reagents addition levels were fixed based on initial experimental experiences [1113].

3 Results and discussion

The effects of operating parameters on selective flocculation performance was conducted by varying the operating parameters i.e. pH (7, 9, 11), flocculant (41.67 g/t, 125 g/t, 208 g/t), dispersant (361.25 g/t, 444.58 g/t, 527.92 g/t), etc. During all the experiments, 12% of solid concentration was maintained. The effects of different operating parameters on separation performances of selective flocculation were evaluated in terms of Fe content (%) and Fe recovery (%).

3.1 Effect of pH

For evaluation of the effect of pH on separation performance of the selective flocculation process, experiments were conducted by varying pH values, whereas flocculant dose and dispersant dose were kept constant i.e. 208.33 g/t and 444.58 g/t, respectively. The plot is shown in Figure 6. From the plot, it was noted that at pH 7, 62.39% Fe grade of Fe was achieved whereas at pH 11, 63.48% Fe was achieved. It was also observed that at the same operating condition at pH 7, 53.9% Fe recovery and at pH 11, 31.2% Fe recovery was achieved. Better grade (%) was achieved at higher alkaline pH. This result confirms literature [6]. At higher alkaline condition and at higher dispersant dose, surface charges of the particles acquire higher zeta potential values, which cause repulsion of the particles and decrease Fe recovery (%) of the concentrate. The schematic diagram of a particular experimental condition is presented in Figure 7. The detailed comparative assay analysis of feed and products i.e. concentrate and tailings in terms of Fe (%), SiO2 (%) and Al2O3 (%) is presented in Figure 8. From the plot, it is clearly observed that there is a difference of the assay (%) of the feed and test products i.e., concentrate and tailings. From the plot, it was inferred that in the concentrate 63.48% Fe, 2.86% SiO2, 2.55% Al2O3 were achieved from a feed value of 50.98% Fe, 9.64% SiO2, 8.86% Al2O3 (%) whereas 48.37 Fe (%), 10.59 SiO2 (%) and 9.82 Al2O3 (%) assay was achieved in the tailings. Result of selective flocculation shows significant difference in the Fe (%) grade of the concentrate and the tailings. In the concentrate higher Fe (%) with less Al2O3 and SiO2 (%) achieved compared to the tailings assay values.

To validate the experimental test results, the products were characterized with SEM analysis. It was noted that clear flocculated structures were obtained in the concentrate in comparison to the feed samples (Fig. 9). Thermogravimetric analysis of the feed sample along with the products for concentrate and tailing products are shown in Figure 10. From the plot, it can be deduced that approximately 63% of LOI in the feed sample is split into ∼ 28% of LOI in the concentrate and ∼ 90% in the tailing. Similarly, it was observed that in the feed sample approximately 39% of goethite is present whereas approximately 19% of goethite is present in the concentrate and ∼ 60% of goethite is present in the tailing sample.

thumbnail Fig. 6

Effect of pH on Fe (%) and recovery (%) on 208.33 g/t of flocculant dose and 444.58 g/t of dispersant dose.

thumbnail Fig. 7

Schematic diagram of the selective flocculation process.

thumbnail Fig. 8

Assay (%) values of the feed, concentrate and tailings.

thumbnail Fig. 9

SEM analysis of the (A) feed and (B) concentrate.

thumbnail Fig. 10

TGA Analysis of the feed and products: (a) concentrate; (b) feed; (c) tailings.

3.2 Effect of flocculant dose

For evaluating the effects of flocculant dose on separation performance of selective flocculation, experiments were conducted by only varying flocculant dose while dispersant dose and pH were kept constant. The experiments were conducted at dispersant dose 361.25 g/t and pH 9. The plot is shown in Figure 11. Figure 11 shows at flocculant dose 41.67 g/t, 61.1% Fe grade was achieved whereas at flocculant dose 125 g/t, 60.77% grade of Fe (%) was achieved. With an increase in the flocculant dose from 41.67 g/t to 125 g/t, Fe recovery (%) increases from 14.67% to 16.6%. With an increase in the flocculant dose, recovery (%) increases, because of the entrapment of gangue particles along with agglomerated desired mineral particles [6,1114].

thumbnail Fig. 11

Effect of flocculant dose on Fe (%) and Fe recovery (%) at pH 9 and dispersant dose 361.25 g/t.

3.3 Effect of dispersant dose

Few experiments were conducted by varying the dispersant dose only and keeping flocculant dose 208.33 g/t and pH 9 constant. Figure 12 shows that with increasing dispersant dose from 361.25 g/t to 527.92 g/t, grade (%) in the concentrate increases from 60.48% to 61.2%. Similarly, it was observed that with an increase in dispersant dose from 361.25 g/t to 527.91 g/t, Fe recovery (%) of the concentrate decreases from 21.98% to 18.69 %.

This happens because at higher dispersant dose and at higher alkaline pH, the repulsion among all the particles increases which increases the zeta potential value. Also the recovery (%) of the concentrate decreases.

thumbnail Fig. 12

Effect of dispersant dose at 208.33 g/t of flocculant dose and at pH 9.

4 Mechanism investigation

The adsorption mechanism was investigated by zeta potential and FTIR analysis.

4.1 Zeta potential analysis

The zeta potential plot is shown in Figure 13. The plot depicted that after adsorption of the flocculant dose, the point of zero charge of the feed material shifts to 7.68, which raises clear indication that the flocculant is selective to the haematitic minerals [5]. Shifting of the pzc value to 8 indicates better selectivity can be obtained at the higher alkaline pH range, supports the experimental results.

thumbnail Fig. 13

Variation of zeta potential of the iron ore tailing sample as a function of pH, as received feed sample; feed sample with flocculant.

4.2 FTIR analysis

FTIR analysis was carried out at wave numbers between 4000 cm−1 to 400 cm−1 (Fig. 14). It was observed that a lot of peak difference was observed in the peaks of the wave numbers for feed, concentrate, and tailings. For clear identifications of the peak difference the wave numbers were shown in three parts: (a) 3700 to 2500 cm−1; (b) 2600 to 1200 cm−1; (c) 1100 to 400 cm−1. In the range of 1100 to 400 cm−1, a huge peak difference was observed between the feed samples and the products (concentrate and tailing). One major peak was present at 1631.5 cm−1 which shifted to 1632.4 cm−1 in the concentrate, representing the deformation of H-O-H bond [10]. One new peak was observed at 1531.4 cm−1 in the concentrate, it is due to attachment of hydrogen atom in the sample, because of which floc formation occurs selectively on desired minerals [24,25]. Similarly, it was observed that another new peak was observed at 1038.5 cm−1, which represents adsorption of water molecules on the desired mineral surfaces.

One major peak was observed at the concentrate in the wave number at 910.4, 641.1 and 496.4 cm−1 which shows the presence of Fe-O stretching in the concentrate [2527]. One intense band was observed at 1038.5 cm−1 indicating adsorption of the polysaccharide flocculant on hematite mineral surfaces [28]. In the concentrate, the peak at 1035.9 cm−1, which shifts to 1038.5 cm−1 which represents adsorption of −OH band on hematite surface [28]. The peak at 915.2 cm−1 indicating hematite shifts to 910.4 cm−1 after adsorption of the flocculant.

thumbnail Fig. 14

(a) FTIR analysis of the feed and the products at 3700 to 2500 cm−1; (b) FTIR analysis of the feed and the products at 2600 to 1200 cm−1; (c) FTIR analysis of the feed and the products at 1100 to 400 cm−1.

5 Conclusion

Selective flocculation is a promising technique for beneficiation of ultrafine iron ore particles. Following the results of this study, it was observed at higher alkaline pH, Fe (%) grade was enriched to more than 63% with 31% Fe recovery from the feed assay value of 50% Fe. For removal of Al2O3 (%), higher alkaline condition is favourable. The concentrate products can be used directly for making pellets. The adsorption mechanism was investigated using zeta potential and FTIR analysis. Shifting of the point of zero charge value near to 8 is an indication of strong adsorption of flocculant on hematite mineral surfaces. Higher selectivity can be achieved at higher alkaline pH range.

Acknowledgements

Authors are thankful to the Tata Steel management for their support, giving permission to publish this manuscript.

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Cite this article as: Lopamudra Panda, Surendra Kumar Biswal, Rayasam Venugopal, Narayan R. Mandre, Investigation of the mechanism for selective flocculation process using natural iron ore tailings, Metall. Res. Technol. 117, 102 (2020)

All Tables

Table 1a

Size analysis of as received Joda tailings.

Table 1b

Assay analysis of the iron ore tailings.

All Figures

thumbnail Fig. 1

Optical image analysis of the as received iron ore tailing.

In the text
thumbnail Fig. 2

XRD analysis of the as received iron ore tailing.

In the text
thumbnail Fig. 3

SEM analysis of iron ore tailings.

In the text
thumbnail Fig. 4

Zeta potential of the iron ore tailings.

In the text
thumbnail Fig. 5

FTIR of natural iron ore tailings.

In the text
thumbnail Fig. 6

Effect of pH on Fe (%) and recovery (%) on 208.33 g/t of flocculant dose and 444.58 g/t of dispersant dose.

In the text
thumbnail Fig. 7

Schematic diagram of the selective flocculation process.

In the text
thumbnail Fig. 8

Assay (%) values of the feed, concentrate and tailings.

In the text
thumbnail Fig. 9

SEM analysis of the (A) feed and (B) concentrate.

In the text
thumbnail Fig. 10

TGA Analysis of the feed and products: (a) concentrate; (b) feed; (c) tailings.

In the text
thumbnail Fig. 11

Effect of flocculant dose on Fe (%) and Fe recovery (%) at pH 9 and dispersant dose 361.25 g/t.

In the text
thumbnail Fig. 12

Effect of dispersant dose at 208.33 g/t of flocculant dose and at pH 9.

In the text
thumbnail Fig. 13

Variation of zeta potential of the iron ore tailing sample as a function of pH, as received feed sample; feed sample with flocculant.

In the text
thumbnail Fig. 14

(a) FTIR analysis of the feed and the products at 3700 to 2500 cm−1; (b) FTIR analysis of the feed and the products at 2600 to 1200 cm−1; (c) FTIR analysis of the feed and the products at 1100 to 400 cm−1.

In the text

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