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Vol. 8. Issue 6.
Pages 5529-5535 (November - December 2019)
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Vol. 8. Issue 6.
Pages 5529-5535 (November - December 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.09.021
Open Access
Depressants in nanoemulsion systems applied to quartz and hematite microflotation
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Paula Romyne de Morais Cavalcante Neitzkea, Tereza N. Castro Dantasa, M. Carlenise P.A. Mouraa,
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, Antônio E. Clark Peresb, Afonso Avelino Dantas Netoa
a Universidade Federal do Rio Grande do Norte, Graduate Program in Chemical Engineering, Senador Salgado Filho Ave, Campus Universitário Lagoa Nova, 59.072-970 Natal, RN, Brazil
b Universidade Federal de Minas Gerais, 6627 Antônio Carlos Ave, Pampulha, 31270-901 Belo Horizonte, MG, Brazil
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Abstract

Nanoemulsion systems were investigated in the microflotation of quartz and hematite. Four different depressants were used: Maizena®, soluble starch, amidex, and amylose. The nanoemulsions comprise in a single stable phase varying amounts of water, kerosene, Flotigam EDA® (collector), and n-butyl alcohol, allowing all flotation reagents to be added at the same time, reducing interfacial tension and favoring their application in the process. Microflotation tests were performed in a modified Hallimond Tube and the zeta potential values of the minerals were determined in the presence and absence of reagents. Quartz floatability was not affected by depressants, even when concentrations of 100ppm of depressant were used in the nanoemulsion. The determined isoelectric point of quartz (pH 2). High levels of hematite depression were obtained for all depressants (80%–90% depression). Quartz floatabilities of up to 95% were achieved by using nanoemulsions in the process.

Keywords:
Iron oxide depressants
Polysaccharides
Quartz-hematite selectivity
Nanoemulsion
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1Introduction

Iron ore is the main raw material used in the steel industry [1,2]. The depletion of its reserves throughout the world requires the use finely disseminated minerals. Processing methods such as gravity separation, magnetic separation, and flotation are widely used in the production of iron ores concentrates [3].

Cationic collectors and amines or amines derivatives are used to separate silicates, such as quartz, from iron oxides [4]. In the process, organic polymers, i.e. starch, play the roles of selectively flocculating and depressing hematite particles and other iron mineral constituents, keeping them concentrated in the unfloated fraction [5].

Corn starch is largely used as a depressant for iron-containing minerals [6,7]. Starch consists mainly of amylopectin and amylose, polysaccharides of monosaccharide α-d-(+)-glucopyranose [8]. Amylose is a linear molecule of 106–107 molecular weight which has helical behavior in solution. Amylopectin is a branched molecule consisting of several thousands of crosslinked amylose short chains having a molecular weight 10–100 times greater than the amylose. Most industrial starches contain 20–30% amylose, 70–80% amylopectin [9], between 1 and 2% oil, and between 7–8% protein [7].

The adsorption of polysaccharides has been addressed in several investigations [10–14]. Initially, hydrogen bonds between hydroxyl groups were supposed to be the main interaction mechanism. Further studies, especially ones reporting advances in FTIR techniques, indicate that chemical adsorption occurs via complexation between hydroxylated metal ions on the mineral’s surface and polar groups in the polysaccharide.

Nanoemulsions are nanoscale-sized colloidal dispersions that are generally composed by surfactants, aqueous phase, and oil phase [15]. These systems can be produced by several methods, but they are typically prepared in a two-step process. Firstly, a microemulsion is obtained and, then, it is converted into a nanoemulsion in a second step [16]. They present kinetic stability and transparent or translucent appearance [17]. Jaiswal et al. [18] considered nanoemulsions those with droplet sizes ranging from 10nm to 1000nm. Comparing with microemulsions and emulsions, the size of the dispersed droplets, high interfacial area, and the use of lower amount of active matter are the main advantages of using nanoemulsion.

This investigation addresses the floatability of hematite and quartz using nanoemulsion systems. Alkyl ether monoamine (Flotigam EDA®) and apolar oil (kerosene) were used as collectors. Maizena®, high purity grade corn starch (20–30% amylose), soluble starch (25–30% amylose), amidex (2% amylose), and pure amylose were the selected depressants.

2Materials and methods2.1Materials2.1.1Mineral species

High purity samples of hematite, from Mina de Casa, Congonhas – MG, Brazil, and quartz, collected in Turmalina – MG, Brazil, were utilized. The minerals were comminuted in a porcelain mill. The microflotation tests used the fraction in the size range –150μm+75μm. The zeta potential determinations were conducted using the −38μm fraction.

2.1.2Nanoemulsion systems

To obtain the nanoemulsion systems, first microemulsion systems were obtained. They were composed by Flotigam EDA® (Clariant) as surfactant; kerosene (Petrobras) as oil phase; n-butyl alcohol (Pro-Analysis, 99%) as co-surfactant; and distilled water (Tecnal, TE-078) as aqueous phase. Four depressants were used in the experiments: modified corn starch (Maizena®), soluble starch, amidex, and amylose. They were added to the aqueous phase of the microemulsion at specific concentrations ranging from 10 to 100ppm. The pH of nanoemulsion systems was adjusted prior to the flotation tests. Solutions were used at 0.5M NaOH (CRQ, 98%) or HCl (Vetec, 37%). All reagents were used without further purification.

The titration methodology described by Bellocq and Roux [19] was used to obtain the microemulsion region inside the pseudoternary phase diagram. The maximum solubility point of the active substance (surfactant (C)+co-surfactant (S)) in the aqueous phase was determined. In a glass vial, 2g of active material were weighed using a constant C/S ratio of 1, at room temperature (27°C). The active material was titrated dropwise with the aqueous phase until the transition from cloudy to clear appearance. The flask was weighed and the amount of aqueous solution added to the system was determined. This point was called titration endpoint. To obtain the pseudoternary phase diagrams with the tested depressants, a depressant solution was used as aqueous phase (10 and 100ppm, C/S=1 ratio).

Eighteen mixtures (2g) were titrated drop wise with the titrant solution until the system became clear: nine with different ratios of active matter+oil phase (10–90% by weight) and nine with varied proportions of depressant solution+oil phase (10–90% by weight). After titration, the flasks were weighed and the amount of titrant determined. The final amounts of aqueous phase, oily phase, and surfactant/co-surfactant phase (% by weight) were plotted in pseudoternary phase diagrams to determine the microemulsion region.

The nanoemulsions were obtained by microemulsion systems (MS) dilutions, according to the methodology described by Bellocq and Roux [19] and Gupta et al. [16]. All MS were composed by (wt %): 79.5% aqueous solution, 0.5% kerosene, 10% Flotigam EDA®, and 10% n-butyl alcohol, with a C/S ratio=1. This MS composition was selected because it comprises a great amount of aqueous phase and a small amount of active matter. Firstly, microemulsion systems (100mL) were obtained in a 150mL beaker by adding the components in a two step process. In the first stage, the active material (co-surfactant+surfactant) was mixed at 500rpm for 10min. In the second, the remaining components of the microemulsion were added (500rpm, 10min). A low energy methodology was employed in the preparation of nanoemulsion systems, adding slowly the desired depressant solution to the microemulsion system, under constant stirring (500rpm). Five nanoemulsion systems with varying concentrations of surfactant were obtained (10, 25, 50, 75, and 100ppm). The choice of nanoemulsion systems was based on the relationship between surface tension and surfactant concentration, always aiming a stable system.

The surface tension was measured using a SensaDyne tensiometer (QC6000) at 27°C. Real-time stabilities of nanoemulsion systems were verified by long-term (six months) storage tests, at 27°C and natural light [20]. No phase separation was observed after long period of storage. Droplet sizes and particle size distribution (PSD) for microemulsion and nanoemulsions were obtained using a Nanotrac NPA252 (Microtac), in triplicate. Droplet sizes between 10–100nm and the presence of only a single narrow peak were expected for the microemulsion. For nanoemulsions, droplets between 20–500nm and single or multiple peaks may be observed which may be narrow or broad [21].

2.2Apparatus and calculations

Quartz and hematite microflotation tests were performed, with nanoemulsions containing the four types of depressants at different concentrations, in a 320-mL modified Hallimond tube, originally designed by Hallimond and modified by Fuerstenauet et al. [22]. A magnetic stirrer was used to promote the mixing of reactants in suspension without occurrence of hydrodynamic drag, at flow rate of 60mL/min to ensure efficient operation of the apparatus (Fig. 1). The pH of the mineral sample (1.0g) and nanoemulsion (V=100mL) was adjusted to the required value.The volume of the tube was completed, and the system was conditioned for 5min under constant stirring. After this period, air was bubbled through the porous plate, located at the base of the Hallimond tube for 1min at a constant flow rate. The floated fraction was oven dried at 100°C until constant weight. The initial weight miand that of the floated fraction mfwere used to calculate the floatability.

Fig. 1.

Schematic diagram of the flotation cell with indication of the addition of materials.

(0.17MB).

The following conditions were investigated:

  • hematite floatability as a function of depressant concentration and type using a nanoemulsion system with 30ppm of collector, at pH 10;

  • quartz floatability as a function of collector and depressant concentrations in the nanoemulsion system, at pH 10, aiming at verifying the formation of amine/starch complexes.

The samples were also analyzed by the electrophoresis technique. Zeta potential curves of the minerals were determined with use of the Zeta Meter ZD3-D-G 3.0+ equipment.

Aqueous suspensions were prepared with the mineral, electrolyte (NaCl 58.44ppm) and the reagents, collector and depressant, in the nanoemulsion. The top size of sanples was 38μm. The decantation time in solution was approximately 3h, in order to analyze only particles in the size range <10μm. The pH range from 2 to 12 was inverstigated using NaOH or HCl solutions. The reagents were used in the lowest possible amount, avoiding a significant increase in the ionic strength of the solution, which would decrease the reliability of results. Experiments were done in triplicate and results are presented as mean values. Samples results with standard deviation greater than 5% were discarded and re-analyzed, in order to obtain reliable results.

3Results and discussion3.1Pseudoternary phase diagrams

In order to obtain nanoemulsion systems with varying proportions of depressants, pseudoternary phase diagrams, for each depressant under study, were constructed to obtain the microemulsion area, verifying whether the depressant dosage modifies microemulsion stability.

The C/T ratio=1 was fixed in the process and the concentration of Maizena®, soluble starch, amidex, or amylose in aqueous solution were studied for values of 10 and 100ppm (Fig. 2). No significant change was observed in the microemulsion area, allowing concluding that these additives can, in this concentration range, be added to the system without impairing its stability.

Fig. 2.

Pseudoternary phase diagrams for systems composed by the depressor aqueous solution (10 and 100ppm), kerosene, Flotigam EDA®/n-butyl alcohol.

(0.58MB).
3.2Microflotation

Hematite floatability curves for nanoemulsion systems with 30ppm collector and varying concentrations of the four depressants tested, at pH 10, are shown in Fig. 3.

Fig. 3.

Hematite floatability as a function of depressant concentration and type in the nanoemulsion (Nanoemulsion with 30ppm of Flotigam EDA®, pH 10).

(0.06MB).

The results indicate that all four polysaccharides in the nanoemulsion acted as hematite depressants, amidex being the most effective due to its high amylopectin content. Pavlovic and Brandão [12] studied the depressant effect of corn starch and its amylose and amylopectin components on hematite and quartz surfaces by infrared spectrometry, adsorption isotherms, and microflotation tests. All the carbohydrates enhanced the hydrophilic character of hematite.

Quartz floatability curves with nanoemulsion containing collector and depressant as a function of collector concentration, at pH 10, are presented in Fig. 4.

Fig. 4.

Quartz flotability with nanoemulsion containing collector and depressant as a function of collector concentration for (a) Maizena®, (b) Soluble Starch, (c) Amidex, and (d) Amylose.

(0.26MB).

Negative effects on quartz floatability were not detected over the entire range of collector and depressant concentrations analyzed. The use of nanoemulsions, even at low collector concentrations, provided high floatability for quartz, regardless of the presence of varying amounts of amylose in the structure of the depressant.

Other authors have shown different results suggesting the formation of clathrates with the surfactant due to the helical structure of the starch. Somasundaran [23] and Takagi and Isemura [24] explained that clathrate formation occurs mainly due to the housing of the surfactant molecule inside the amylose helices, impairing the performance of the collector in the flotation.

Pinto et al. [10], in their study on the effects of amylose and amylopectin on oxyminerals, observed a trend of quartz depression at low concentrations of amine and starch (approximately 1ppm and 10ppm, respectively). However, they attributed the fact to low amine concentration.

Nanoemulsion collector and depressant systems did not show tendency to clathrate formation, even when the depressant used was constituted mainly by amylose, probably due to the stability of this compound, being able to preserve the collecting power of the surfactant.

3.3Zeta potential

The zeta potential of quartz was determined by the electrophoresis method in suspensions with 58.44ppm of NaCl. The results are shown in Fig. 5.

Fig. 5.

Zeta potential of quartz in water and in 58.44ppm NaCl solution.

(0.05MB).

Negative values of the zeta potential were obtained for quartz in the pH range of 2–11. The isoelectric point (PIE) of the quartz was determined at pH 3.0, which is consistent with the literature values for quartz pH 2–3 [25–28].

Curves of zeta potential of quartz in suspensions with Flotigam EDA® and nanoemulsion at concentrations of 30ppm and 50ppm are illustrated in Fig. 6. As expected, being NaCl an indifferent electrolyte [29], its ions are not specifically adsorbed onto quartz and the isoelectric point (IEP) coincides with the point of zero charge (PCZ). The zeta potential curve for quartz in amine nanoemulsion at 30 and 50ppm showed that the addition of amine promotes a reduction of the zeta potential modulus in the studied pH range. Increase in the pH of the isoelectric point of the quartz was also observed. In the absence of amine, the isoelectric point occurred at pH 2. Adding nanoemulsion with 30ppm of amine, the isoelectric point reached pH 5, and at 50ppm a value of pH 6 was attained, indicating the occurrence of adsorption of the collector onto the mineral surface. The positive charge of amine contributes to the decrease of the negative charge presented by the quartz in the exclusive presence of indifferent electrolyte.

Fig. 6.

Zeta potential of quartz in 30ppm nanoemulsion and in 50ppm Flotigam EDA®.

(0.06MB).

Fig. 7 shows the results of zeta potential determinations of quartz in nanoemulsion containing concentrations of 50ppm of amine and 100ppm of depressants.

Fig. 7.

Zeta potential of quartz with nanoemulsion of 50ppm amine Flotigam EDA® and 100ppm of Maisena®, soluble starch, amidex, and amylose.

(0.05MB).

The surface charge of mineral particles is greatly influenced by the pH of the solution [30,31]. The zeta potential of quartz became less negative for pH values above 6. Somasundaran [23] showed evidence of increased adsorption on the mineral surface in the presence of starch, and of starch in the presence of collector, thus, contributing to the observed decrease of the zeta potential modulus. Starch should in fact be co-adsorbed at the quartz/solution interface in association with amine.

Similar results were obtained by Yin et al. [32] in their study on the separation of quartz and hematite using magnetite. The negative zeta potential value for each of the three minerals increased nearly linearly with increasing pH from the isoelectric point up to pH 12.

Feng et al. [33], studying the mechanism of interaction of magnesium ions with cassiterite and quartz surfaces and their response to flotation separation, showed that the zeta potential of magnesium ion-treated quartz was less negative within the pH range investigated. The extent of change in negativity of zeta potential of mineral particles increased with increasing pulp pH, and the maximum value was found within the pH range from 8.6 to 11.7. The authors believe that the results may be related to the adsorption of magnesium species onto quartz surfaces.

4Conclusions

The study showed that nanoemulsions can be used in the microflotation of quartz and hematite. Some of the main conclusions are summarized below:

  • (i)

    Even at low concentrations of amine and depressants it was possible to achieve high flotabilities of quartz;

  • (ii)

    All depressants introduced into the nanoemulsion systems were efficient to promote hematite depression;

  • (iii)

    No significant effects were observed by the presence or absence of amylose in depressants showing that clathrate formation does not occur in nanoemulsion systems applied to reverse cationic flotation.

The proposed methodology is an innovation in the concentration of iron ores and opens the way for the use of nanoemulsions in this field.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

The authors would like to thank the Federal University of Rio Grande do Norte (UFRN) and the Coordination for the Improvement of Higher Education Personnel (CAPES) for the financial support.

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Journal of Materials Research and Technology

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