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Utilising starches from sugarcane and cassava residues as hematite depressants
Minerals Engineering 145 (2020) 106090
Contents lists available at ScienceDirect
Minerals Engineering journal homepage: www.elsevier.com/locate/mineng
Utilising starches from sugarcane and cassava residues as hematite depressants
T
⁎
Tatiana Fernandes Marins , Otávia Martins Silva Rodrigues, Érica Linhares Reis, Jessica Goulart Beltrão School of Mines at the Federal University of Ouro Preto, Campus Morro do Cruzeiro, Ouro Preto, MG 35400–000, Brazil
A R T I C LE I N FO
A B S T R A C T
Keywords: Food residues Alternative depressants Starch Microflotation Hematite
The aim of this study is to investigate the actions of sugarcane bagasse and cassava wastewater (two residues from the food industry) when used as hematite depressants. Additionally, the effect of each residue preparation method on the flotation results was analysed. The efficiencies of the depressors were investigated through microflotation tests utilising samples of pure hematite and quartz from Quadrilátero Ferrífero, which is in the state of Minas Gerais (Brazil). The pH of 10.5 was selected due to its frequent use in the concentration of iron ore via flotation in Brazil. Three residue gelatinisation methods (natural, heated, and conventional) were evaluated to determine the most efficient method for the flotation process. Further, zeta potential measurements were performed on the hematite to ascertain the performances of the residues. Microflotation test results showed that hematite recovery decreased with increasing depressant concentration, for all reagents. Finally, the actions of residues gelatinised with the addition of sodium hydroxide (i.e. the conventional method) were shown to be similar to that obtained from corn starch, which indicates their potential application in the flotation process.
1. Introduction Flotation is widely used in the beneficiation of ores that require comminution below 150 μm to achieve mineral extraction. For iron ores, reverse cationic flotation is the most common method, because the combination of reagents (typically starch and amine) promotes effective mineral selectivity at a relatively low cost. Each year in Brazil, more than 300 million tons of iron ore concentrate are obtained through flotation using a combination of starch and amine (Shrimali et al., 2017). Depressants modify the flotation process by either inhibiting the collector's action on the mineral(s), or by rendering the surface of the mineral(s) hydrophilic character (Laskowski et al., 2007). Corn starch is widely used as a hematite depressant in iron ore flotation, because starch molecules are hydrophilic due to the presence of many eOHe groups in the constituent glucose monomers. Most starches are composed primarily of the polysaccharides amylose and amylopectin. Amylose is a predominantly linear macromolecule formed of α-ᴅ-glucose monomers linked at C-1,4. Amylopectin is formed of α-ᴅ-glucose monomers linked at C-1,4 and C1,6 (Laskowski et al., 2007), with branching occurring every 20–25
glucose molecules (Liu, 2005). In its naturally occurring state, starch takes the form of insoluble grains in water at room temperature (20–30 °C). Therefore, for use as a reagent, starch grains must undergo a solubilisation process known as gelatinisation. Starch gelatinisation may be conducted by either heating a mixture of water and starch, or by adding a strongly alkaline solution. During gelatinisation, the hydrogen bonds of the amylose molecules rupture, which weakens the original structure of the grains and results in the formation of a clear and viscous paste that is water-soluble. In industrial flotation, sodium hydroxide (NaOH) is often added to gelatinise starch due to its lower consumption in the gelatinisation process when compared with other alkaline bases (Araujo, 1988; Araujo et al., 2005). Pavlovic and Brandão (2003) investigated the actions of starch, amylose, amylopectin, glucose, and maltose on the depression of hematite. The results obtained via Fourier transform infrared spectroscopy indicated a new band in the mineral spectrum at a frequency of 810 cm−1, which suggests the formation of a chemical complex on the surface of the hematite after its interactions with the compounds studied. Several studies have sought reagents to replace corn starch in the flotation of iron ores, such as starches extracted from cassava, potato,
⁎
Corresponding author. E-mail addresses: [email protected] (T.F. Marins), [email protected] (O.M.S. Rodrigues), [email protected] (É.L. Reis), [email protected] (J.G. Beltrão). https://doi.org/10.1016/j.mineng.2019.106090 Received 18 March 2019; Received in revised form 4 October 2019; Accepted 12 October 2019 Available online 04 November 2019 0892-6875/ © 2019 Elsevier Ltd. All rights reserved.
Minerals Engineering 145 (2020) 106090
T.F. Marins, et al.
software, the obtained diffractograms were interpreted using HighScore Plus software, and a Powder Diffraction File (PDF-4) database provided by the International Centre for Diffraction Data. Qualitative analysis and mineralogical quantification used the Rietveld refinement method. The densities of the samples were obtained with an Ultrapyc 1200e helium pycnometer, and each obtained specific mass corresponded to the average of three measurements.
rice, and sorghum (Kar et al., 2013; Rocha et al., 2018; Shrimali and Miller, 2016; Silva et al., 2018). However, studies investigating the extraction of starch from residues are rare in the literature. Pereira (2013) performed microflotation tests on quartz and hematite samples using a food industry residue named BR. The results showed that gelatinising the residue with heat and the addition of NaOH produces a potential selective collector for quartz/hematite systems that can obtain a quartz recovery of approximately 93% and a hematite recovery of below 20%. Further, Resende (2013) tested a biopolymer derived from sugarcane bagasse (named XMC) as a substitute for starch in the flotation of itabirite ore and obtained higher selectivity results than those for corn starch. The author also found that amine consumption was reduced by approximately 70% in the presence of the novel reagent. Because starch extraction from commercial sources rarely reaches 100%, starches can remain in plant residues after processing. Therefore, the aim of this study is to evaluate the actions of two different residues from the food industry as hematite depressants: sugarcane bagasse and cassava wastewater. Sugarcane is the raw material in the production of several products, such as sugar and ethanol. As the world's largest producer of sugarcane, Brazil produces approximately 25% of the global total (CONAB, 2018). According to the Brazilian National Supply Company (2018), sugarcane production in Brazil for the 2018–2019 crop is predicted to be 625.96 million tons. Sugarcane bagasse is the residue obtained after the extraction of the liquor (from which sugar and ethanol are produced) and is composed primarily of cellulose, hemicellulose, and lignin. However, the utilisation of sugarcane bagasse in the mineral industry would result in added value. Currently, sugarcane bagasse is utilised primarily in energy production and animal nutrition. According to the Brazilian Institute of Geography and Statistics (2019), Brazil is expected to produce approximately 20 million tons of cassava in 2019. Cassava wastewater, which is popularly known as manipueira, is a residue from the production of cassava flour or tapioca (i.e. cassava starch) after the cassava tubers are grated and processed with water to extract the starch (John, 2009). The wastewater is usually discarded as an effluent and treated to extract the remaining starch that cannot be discharged directly into the watercourse. Similar to sugarcane bagasse, the utilisation of cassava wastewater in the mineral industry would result in added value. The performances of the residues were evaluated through microflotation tests and compared with that of corn starch. Various gelatinisation methods and depressant concentrations were tested and zetapotential measurements were performed to ascertain the actions of the depressants on the surface of the hematite.
2.2. Microflotation tests Microflotation tests were performed in a modified Hallimond tube with a total internal volume of 300 mL and an extender to minimise the hydraulic entrainment of the particles. A mineral mass of 1.0 g was used in each test, and duplicate tests were performed to ascertain experimental error. The chosen pH was 10.5, which is typical in iron ore cationic flotation. Nitrogen was used for flotation at a flow rate of 80 cm3/min. Flotigam EDA, a commercial amine by Clariant, was used as a collector at a concentration of 20 mg/L for both minerals. Corn starch gelatinised with the addition of 5 mL of NaOH solution at 5% (w/ v) was used as the standard depressor for comparison with the residues. The sugarcane bagasse was obtained from a company in Cláudio city in Minas Gerais as the residue of cachaça production. This fibrous bagasse was comminuted and screened to 1 mm. The cassava wastewater was obtained from the production of homemade tapioca flour in Nova Era city in Minas Gerais. The wastewater was obtained in a dry powder form, possessed a grey colour, and was also screened with 1 mm mesh. Based on the characterisations of the residues, the amounts of starch present in the sugarcane bagasse and cassava wastewater were 17% (w/ w) and 73% (w/w), respectively. The residues were gelatinised according to three methods to identify which would provide the best results. The first method uses only distilled water and stirring (i.e. natural gelatinisation). The second method uses a combination of distilled water and heating at 90 °C for 20 min to gelatinise the starch completely (i.e. heated gelatinisation). The third method uses a NaOH solution (i.e. conventional gelatinisation). In all gelatinisation methods, the depressor solutions were allowed to stand for 1 h to ensure sufficient interaction time between the substances. The concentration of the initial solutions of depressants contained 0.5% (w/w) or 1% (w/w) starch and these were diluted for use in the microflotation tests. Solutions of 1% (v/v) and 5% (v/v) HCl and 1% (w/v) and 5% (w/v) NaOH were used as pH modifiers. The conditioning times were 5 min for the depressants and 1 min for the collector. The flotation time was 1 min.
2. Materials and methods 2.3. Zeta potential measurements 2.1. Preparation and characterisation of mineral samples Zeta potential measurements were performed to elucidate the interactions between the depressants and the hematite surface. The most effective gelatinised depressants in the microflotation tests were chosen for the zeta potential measurements. The zeta potential measurements were performed using a ZetaMeter 4.0 system. Mineral suspensions of 0.1% (w/w) were prepared with a particle size of below 38 μm and a NaNO3 solution was used as the indifferent electrolyte. According to Stokes’ law, a 10 μm hematite particle has a settling velocity of 13.7 cm/min, and therefore the suspensions were allowed to stand for 10 min. After sedimentation, 200 mL of the suspension was collected, divided equally between four 50 mL beakers, and pH adjustments ranging from 4 to 12 were applied to each beaker. A HCl solution of 0.5% (v/v) and a NaOH solution of 0.5% (w/ v) were used as pH modifiers. Finally, the zeta potential curves of the indifferent electrolyte and the depressors were obtained.
A hematite sample from Quadrilátero Ferrífero was comminuted in a ball mill and passed through Tyler sieves to obtain the size range chosen for the microflotation tests (−212 µm + 74 μm). Due to the presence of impurities, the sample was subjected to a Carpco Wet HighIntensity Magnetic Separator, model 3X4L. The obtained magnetic fraction was then dried and homogenised for use. The quartz sample, also from Quadrilátero Ferrífero, was comminuted in a porcelain mill and sieved to obtain a grain size of 212–74 μm. Next, the sample was leached to remove possible impurities and cleaned with concentrated hydrochloric acid (37% HCl) for 12 h. The sample was then washed successively with water to attain a neutral pH, dried, and homogenised for use. The compositions of the samples were evaluated using a PANalytical X'Pert3 Powder X-ray diffractometer equipped with a copper tube and a Cu-Kα radiation wavelength of 1.5406 Å. The system has a 2θ range of 5° to 90°, an operating voltage of 45 kV, a current of 40 mA, and a collection time of 15 min. Data was obtained with data collection 2
Minerals Engineering 145 (2020) 106090
T.F. Marins, et al.
100
100 90
Hematite recovery (%)
Recovery (%)
80
60 Quartz without depressant Hematite without depressant
40
Cane natural
80
Cane heated
70
Cane conventional
60
Cassava natural
50
Cassava heated
40
Cassava conventional
30 20
20
10 0
0 0
20
40
60
80
100
120
3.1. Characterisation of mineral samples The obtained diffractograms of the mineral samples indicated that the predominant phases were hematite and quartz. The measured densities of the hematite and quartz samples were 5.18 g/cm3 and 2.63 g/cm3, respectively, which are consistent with those found in the literature. 3.2. Microflotation tests Fig. 1 shows the results of the microflotation tests for the hematite and quartz samples with increasing amine concentration. A high quartz recovery of 85% for 2 mg/L of collector indicates the preferential adsorption of amine on the quartz surface. Conversely, an amine concentration of 100 mg/L was required to achieve a hematite recovery percentage comparable to that of quartz. Fig. 2 shows the results of the microflotation tests of the hematite and quartz samples in the presence of corn starch. These results indicate that, even at a low concentration of 10 mg/L, the corn starch is a highly effective hematite depressant that decreased recovery from 57% to 8.5%. Conversely, quartz recovery remained above 90% in the presence of 50 mg/L of depressant. To analyse the performances of the residues, a maximum hematite recovery limit of 10% was established, and depressant concentrations yielding recovery values within this limit were deemed ideal reagent 100 92,4
Recovery (%)
60 Hematite with 20 mg/L amine and starch Quartz with 20 mg/L amine and starch
20
0
10
20
30
40
300
400
500
600
700
concentrations. Fig. 3 shows the results of the hematite microflotation tests in the presence of sugarcane bagasse and cassava wastewater prepared according to the three gelatinisation methods (natural, heated, and conventional). As can be seen in Fig. 3, all three methods produced decreases in hematite recovery with increasing depressant concentrations. For the sugarcane bagasse, the ideal concentrations were 400 mg/L, 200 mg/L, and 25 mg/L for the natural, heated, and conventional preparations, respectively, and the corresponding hematite recoveries were 4.9%, 6.3%, and 7.5%. Therefore, gelatinisation with NaOH was more efficient in the flotation process when compared with the other gelatinisation methods proposed herein. Further, sugarcane bagasse prepared using the natural method depressed hematite at 400 mg/L, which was unexpected because starch is insoluble in distilled room temperature water. However, as shown by Pavlovic and Brandão (2003), both the monomer glucose and the dimer maltose exhibit depressant actions on hematite at high concentrations and do not affect quartz recovery. Thus, the combined actions of starch and sugars probably contributed to the depressant effect of the sugarcane bagasse. Sugarcane has an average grade of 10% to 12% soluble sugars (Food and Agriculture Organization of the United Nations, 2019), while cassava tubers possess a sugar content lower than 2% of its total composition (United States Department of Agriculture, 2019). Therefore, the sugarcane bagasse likely contained more sugar than the cassava wastewater. The cassava prepared using only distilled room temperature water produced a slight decrease in hematite recovery with increasing depressant concentration and failed to reach the 10% hematite recovery limit; its lowest hematite recovery was 16%, which was achieved at a depressant concentration of 600 mg/L. Because starch is insoluble in room temperature water, the poor performance of the cassava wastewater prepared according to the natural gelatinisation method could therefore be due to the absence of gelatinised starch. Further, and as mentioned before, the lack of soluble sugars in the cassava wastewater suggests that only the presence of starch influenced the recovery of hematite by this residue. The heated and conventional gelatinisation preparations exhibited similar performances and the ideal concentration for both was 10 mg/L, which is the same concentration reported herein for corn starch. The hematite recovery of corn starch at the ideal concentration (10 mg/L) was 8.5%, while the ideal concentration of the heated and conventional gelatinisation preparations of cassava wastewater achieved hematite recoveries of 2.7% and 7.2%, respectively, which demonstrates the potential of this residue. Another interesting observation based on Fig. 3 is that the heated method was effective in gelatinising the starch content of the cassava wastewater but not the starch content of the sugarcane bagasse. Further, the data for the conventional and heated preparations overlap only for the cassava wastewater. The fibre content of a starch source
3. Results and discussion
0
200
Fig. 3. Hematite flotation results with sugarcane bagasse and cassava wastewater.
Fig. 1. Hematite and quartz flotation results with increasing amine concentration.
40
100
Residue starch concentration [mg/L]
Amine concentration [mg/L]
80
0
50
Starch concentration [mg/L] Fig. 2. Hematite and quartz flotation results with amine and increasing corn starch concentration. 3
Minerals Engineering 145 (2020) 106090
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other studies, such as pH values of 6.8 (Alexandrino et al., 2016), 6.7 (Rocha et al., 2018), and 6.95 (Silva et al., 2018). In the presence of the depressants, the IEP of the hematite shifted and no positive zeta potential value was found within the tested pH range. Zeta potential analyses were performed on the depressants prepared with the addition of NaOH (i.e. the conventional gelatinisation method) because their performances were similar to that of corn starch. The occurrence of adsorption at pH 10.5 is evidenced by the microflotation curves in Fig. 4b, which indicate hematite depression in the presence of the depressants prepared with NaOH. However, Fig. 4a indicates that adsorption of the depressants on the hematite surface occurred in a pH range of 4–8, because the inversion of the zeta potential of the mineral occurred under these conditions. Further, Fig. 4a indicates that the residues influenced the zeta potential of the hematite in a manner similar to that of corn starch, which confirms that the residues acted as hematite depressants due to their constituent starch. According to Pugh (1989), although polymers such as starch are normally considered as non-ionic, they may exhibit anionic characteristics, and the charges of these molecules increases with increasing pH. This can be due to the oxidation and hydrolysis of starch molecules; the ionisation of fatty acids, phosphates, and other minor constituents; or the ionisation of hydroxyls, especially those linked to the C-2 and C-6 carbons of the glucose monomer. Thus, for a pH range of 4–6.9, in which the zeta potential of hematite is positive, the electrostatic attraction contributes to the adsorption of starch on the mineral surface. In a pH above 6.9, the starch and mineral charges are both negative. Nevertheless, their mutual interaction exceeds the electrostatic repulsion force, which indicates chemical adsorption.
Table 1 Quartz flotation results with depressants at ideal concentrations. Depressant
Gelatinisation
Concentration (mg/ L)
Quartz recovery (%)
Corn starch
Conventional
50
92.4
Sugarcane bagasse
Natural Heated Conventional
400 200 25
91.1 93.9 94.1
Cassava wastewater
Natural Heated Conventional
600 10 10
93.1 90.4 92.2
4. Conclusions Microflotation experiments indicated that cassava wastewater presented satisfactory results when gelatinised using both heating and conventional methods, and that the ideal depressant concentration for both preparations was 10 mg/L. However, cassava wastewater gelatinised with distilled water did not significantly depress hematite. Further, of the three preparations of sugarcane bagasse, the one gelatinised according to the natural method was found to be the least effective hematite depressant, while the sugarcane bagasse gelatinised with the addition of NaOH was the most efficient and possessed an ideal concentration of 25 mg/L. Microflotation tests verified that none of the residues had a depressant effect on the quartz, indicating that all reagents possessed good selectivity in relation to the mineral species studied. Zeta potential measurements demonstrated that the most efficient residues for hematite depression (sugarcane bagasse and cassava wastewater prepared with the addition of NaOH) each had a low impact on the zeta potential values of hematite in the pH range above the IEP of the mineral. Finally, these residues influenced the zeta potential of hematite in a manner similar to the commonly used reagent corn starch.
Fig. 4. (a) Zeta potential results of hematite without reagents and with 10 mg/L of depressants gelatinised with sodium hydroxide and (b) microflotation results.
affects starch extraction, and therefore the extraction and solubilisation of the starch contained in the fibrous sugarcane bagasse was more difficult than the cassava wastewater, which was a powder residue. Therefore, the heated method was inefficient at degrading the fibres and solubilising all of the starch contained in the sugarcane bagasse. The quartz recoveries were evaluated only for the ideal depressant concentrations to ascertain any depressant actions on the mineral. Table 1 presents the results, from which it can be concluded that the depressors did not affect quartz recovery in the evaluated conditions because the recoveries remained above 90% in all cases.
Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgements The authors would like to thank the Coordination for the Improvement of Higher Education Personnel (CAPES) and the National Council for Scientific and Technological Development (CNPq) for all funds provided for this research. The authors, also would like to thank the Federal University of Ouro Preto.
3.3. Zeta potential measurements Fig. 4a shows the zeta potential measurements of hematite in the absence of reagents and in the presence of the depressants (10 mg/L) corn starch, sugarcane bagasse, and cassava wastewater at 10 mg/L concentrations. Fig. 4b shows the hematite recoveries of the three depressants. The isoelectric point (IEP) of the hematite in the absence of depressants was determined to be pH 6.9. Similar results were found in
References Alexandrino, J.S., Peres, A.E.C., Lopes, G.M., Rodrigues, O.M.S., 2016. Dispersion degree
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Minerals Engineering 145 (2020) 106090
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Departamento de Engenharia de Minas, Ouro Preto, pp. 45. Pugh, R.J., 1989. Macromolecular organic depressants in sulphide flotation – a review, 1. Principles, types and applications. Int. J. Miner. Process. 25, 101–130. Resende, T.M.de, 2013. Sustainable use of biopolymer as a depressant in the selective flotation of hematite iron ore (pt). Master's Thesis. Universidade Federal de Viçosa, Viçosa, pp. 93. Rocha, G.M., Machado, N.R.De S., Pereira, C.A., 2018. Effect of ground corn and cassava flour on the flotation of iron ore tailings. J. Mater. Res. Technol. 1, 1–5. John, R.P., 2009. Biotechnologial Potentials of Cassava Bagasse. In: Nigam, P.S.nee', Pandey, A. (Eds.), Biotechnology for Agro-Industrial Residues Utilisation: Utilisation of Agro-Residues. Springer Science+Business Media B.V., pp. 225–236. Shrimali, K., Miller, J.D., 2016. Polysaccharide depressants for the reverse flotation of iron ore. Indian Institute of Metals – IMM 6983 (1). Shrimali, K., Yin, X., Wang, X., Miller, J.D., 2017. Fundamental issues on the influence of starch in amine adsorption by quartz. Colloids Surf., A 522, 642–651. Silva, E.M.S., Peres, A.E.C., Silva, A.C., Florêncio, D.L., Caixeta, V.H., 2018. Sorghum starch as depressant in mineral flotation: part 2 – flotation tests. J. Mater. Res. Technol. 1, 1–8. https://doi.org/10.1016/j.jmrt.2018.04.002. United States Department of Agriculture (USDA). 2019. Food Composition Databases – Cassava raw. Available online at < https://ndb.nal.usda.gov/ndb/foods/show/ 11134? > . Food and Agriculture Organization of the United Nations, 2019. Sugarcane: Land Water. Databases and Software (accessed 01 aug 2019).
and zeta potential of hematite. Rem Revista Escola de Minas 69 (2), 193–198. https://doi.org/10.1590/0370-44672014690073. Araujo, A.C., 1988. Starch Modification of the flocculation and flotation of apatite. PhD Thesis. Universityof British Columbia. Department of Mining and Mineral ProcessEngineering, Vancouver, pp. 379. Araujo, A.C., Viana, P.R.M., Peres, A.E.C., 2005. Reagents in iron ores flotation. Minerals Eng. 18, 219–224. CONAB. Companhia Nacional de Abastecimento, 2018. Follow-up of the Brazilian crop – sugarcane: First survey, May 2018. Crop 2018/2019. (pt). Companhia Nacional de Abastecimento, Brasília. Kar, B., Sahoo, H., Rath, S.S., Das, B., 2013. Investigations on different starches as depressants for iron ore flotation. Minerals Eng. 49, 1–6. Laskowski, J.S., Liu, Q., O'connor, C.T., 2007. Current understanding of the mechanism of polysaccharide adsorption at the mineral/aqueous solution interface. Int. J. Miner. Process. 84, 59–68. Liu, Q., 2005. Understanding starches and their role in foods. In: Cui, S.W. (Ed.), Food Carbohydrates: Chemistry, physical properties, and applications. CRC Press, United States of America, pp. 309–355. Pavlovic, S., Brandão, P.R.G., 2003. Adsorption of starch, amylose, amylopectin and glucose monomer and their effecton the flotation of hematite and quartz. Miner. Eng. 16, 1117–1122. Pereira, V.B., 2013. Use of food industry residue in quartz and hematite flotation (pt). Undergraduate Thesis. Universidade Federal de Ouro Preto. Escola de Minas.
5
Contents lists available at ScienceDirect
Minerals Engineering journal homepage: www.elsevier.com/locate/mineng
Utilising starches from sugarcane and cassava residues as hematite depressants
T
⁎
Tatiana Fernandes Marins , Otávia Martins Silva Rodrigues, Érica Linhares Reis, Jessica Goulart Beltrão School of Mines at the Federal University of Ouro Preto, Campus Morro do Cruzeiro, Ouro Preto, MG 35400–000, Brazil
A R T I C LE I N FO
A B S T R A C T
Keywords: Food residues Alternative depressants Starch Microflotation Hematite
The aim of this study is to investigate the actions of sugarcane bagasse and cassava wastewater (two residues from the food industry) when used as hematite depressants. Additionally, the effect of each residue preparation method on the flotation results was analysed. The efficiencies of the depressors were investigated through microflotation tests utilising samples of pure hematite and quartz from Quadrilátero Ferrífero, which is in the state of Minas Gerais (Brazil). The pH of 10.5 was selected due to its frequent use in the concentration of iron ore via flotation in Brazil. Three residue gelatinisation methods (natural, heated, and conventional) were evaluated to determine the most efficient method for the flotation process. Further, zeta potential measurements were performed on the hematite to ascertain the performances of the residues. Microflotation test results showed that hematite recovery decreased with increasing depressant concentration, for all reagents. Finally, the actions of residues gelatinised with the addition of sodium hydroxide (i.e. the conventional method) were shown to be similar to that obtained from corn starch, which indicates their potential application in the flotation process.
1. Introduction Flotation is widely used in the beneficiation of ores that require comminution below 150 μm to achieve mineral extraction. For iron ores, reverse cationic flotation is the most common method, because the combination of reagents (typically starch and amine) promotes effective mineral selectivity at a relatively low cost. Each year in Brazil, more than 300 million tons of iron ore concentrate are obtained through flotation using a combination of starch and amine (Shrimali et al., 2017). Depressants modify the flotation process by either inhibiting the collector's action on the mineral(s), or by rendering the surface of the mineral(s) hydrophilic character (Laskowski et al., 2007). Corn starch is widely used as a hematite depressant in iron ore flotation, because starch molecules are hydrophilic due to the presence of many eOHe groups in the constituent glucose monomers. Most starches are composed primarily of the polysaccharides amylose and amylopectin. Amylose is a predominantly linear macromolecule formed of α-ᴅ-glucose monomers linked at C-1,4. Amylopectin is formed of α-ᴅ-glucose monomers linked at C-1,4 and C1,6 (Laskowski et al., 2007), with branching occurring every 20–25
glucose molecules (Liu, 2005). In its naturally occurring state, starch takes the form of insoluble grains in water at room temperature (20–30 °C). Therefore, for use as a reagent, starch grains must undergo a solubilisation process known as gelatinisation. Starch gelatinisation may be conducted by either heating a mixture of water and starch, or by adding a strongly alkaline solution. During gelatinisation, the hydrogen bonds of the amylose molecules rupture, which weakens the original structure of the grains and results in the formation of a clear and viscous paste that is water-soluble. In industrial flotation, sodium hydroxide (NaOH) is often added to gelatinise starch due to its lower consumption in the gelatinisation process when compared with other alkaline bases (Araujo, 1988; Araujo et al., 2005). Pavlovic and Brandão (2003) investigated the actions of starch, amylose, amylopectin, glucose, and maltose on the depression of hematite. The results obtained via Fourier transform infrared spectroscopy indicated a new band in the mineral spectrum at a frequency of 810 cm−1, which suggests the formation of a chemical complex on the surface of the hematite after its interactions with the compounds studied. Several studies have sought reagents to replace corn starch in the flotation of iron ores, such as starches extracted from cassava, potato,
⁎
Corresponding author. E-mail addresses: [email protected] (T.F. Marins), [email protected] (O.M.S. Rodrigues), [email protected] (É.L. Reis), [email protected] (J.G. Beltrão). https://doi.org/10.1016/j.mineng.2019.106090 Received 18 March 2019; Received in revised form 4 October 2019; Accepted 12 October 2019 Available online 04 November 2019 0892-6875/ © 2019 Elsevier Ltd. All rights reserved.
Minerals Engineering 145 (2020) 106090
T.F. Marins, et al.
software, the obtained diffractograms were interpreted using HighScore Plus software, and a Powder Diffraction File (PDF-4) database provided by the International Centre for Diffraction Data. Qualitative analysis and mineralogical quantification used the Rietveld refinement method. The densities of the samples were obtained with an Ultrapyc 1200e helium pycnometer, and each obtained specific mass corresponded to the average of three measurements.
rice, and sorghum (Kar et al., 2013; Rocha et al., 2018; Shrimali and Miller, 2016; Silva et al., 2018). However, studies investigating the extraction of starch from residues are rare in the literature. Pereira (2013) performed microflotation tests on quartz and hematite samples using a food industry residue named BR. The results showed that gelatinising the residue with heat and the addition of NaOH produces a potential selective collector for quartz/hematite systems that can obtain a quartz recovery of approximately 93% and a hematite recovery of below 20%. Further, Resende (2013) tested a biopolymer derived from sugarcane bagasse (named XMC) as a substitute for starch in the flotation of itabirite ore and obtained higher selectivity results than those for corn starch. The author also found that amine consumption was reduced by approximately 70% in the presence of the novel reagent. Because starch extraction from commercial sources rarely reaches 100%, starches can remain in plant residues after processing. Therefore, the aim of this study is to evaluate the actions of two different residues from the food industry as hematite depressants: sugarcane bagasse and cassava wastewater. Sugarcane is the raw material in the production of several products, such as sugar and ethanol. As the world's largest producer of sugarcane, Brazil produces approximately 25% of the global total (CONAB, 2018). According to the Brazilian National Supply Company (2018), sugarcane production in Brazil for the 2018–2019 crop is predicted to be 625.96 million tons. Sugarcane bagasse is the residue obtained after the extraction of the liquor (from which sugar and ethanol are produced) and is composed primarily of cellulose, hemicellulose, and lignin. However, the utilisation of sugarcane bagasse in the mineral industry would result in added value. Currently, sugarcane bagasse is utilised primarily in energy production and animal nutrition. According to the Brazilian Institute of Geography and Statistics (2019), Brazil is expected to produce approximately 20 million tons of cassava in 2019. Cassava wastewater, which is popularly known as manipueira, is a residue from the production of cassava flour or tapioca (i.e. cassava starch) after the cassava tubers are grated and processed with water to extract the starch (John, 2009). The wastewater is usually discarded as an effluent and treated to extract the remaining starch that cannot be discharged directly into the watercourse. Similar to sugarcane bagasse, the utilisation of cassava wastewater in the mineral industry would result in added value. The performances of the residues were evaluated through microflotation tests and compared with that of corn starch. Various gelatinisation methods and depressant concentrations were tested and zetapotential measurements were performed to ascertain the actions of the depressants on the surface of the hematite.
2.2. Microflotation tests Microflotation tests were performed in a modified Hallimond tube with a total internal volume of 300 mL and an extender to minimise the hydraulic entrainment of the particles. A mineral mass of 1.0 g was used in each test, and duplicate tests were performed to ascertain experimental error. The chosen pH was 10.5, which is typical in iron ore cationic flotation. Nitrogen was used for flotation at a flow rate of 80 cm3/min. Flotigam EDA, a commercial amine by Clariant, was used as a collector at a concentration of 20 mg/L for both minerals. Corn starch gelatinised with the addition of 5 mL of NaOH solution at 5% (w/ v) was used as the standard depressor for comparison with the residues. The sugarcane bagasse was obtained from a company in Cláudio city in Minas Gerais as the residue of cachaça production. This fibrous bagasse was comminuted and screened to 1 mm. The cassava wastewater was obtained from the production of homemade tapioca flour in Nova Era city in Minas Gerais. The wastewater was obtained in a dry powder form, possessed a grey colour, and was also screened with 1 mm mesh. Based on the characterisations of the residues, the amounts of starch present in the sugarcane bagasse and cassava wastewater were 17% (w/ w) and 73% (w/w), respectively. The residues were gelatinised according to three methods to identify which would provide the best results. The first method uses only distilled water and stirring (i.e. natural gelatinisation). The second method uses a combination of distilled water and heating at 90 °C for 20 min to gelatinise the starch completely (i.e. heated gelatinisation). The third method uses a NaOH solution (i.e. conventional gelatinisation). In all gelatinisation methods, the depressor solutions were allowed to stand for 1 h to ensure sufficient interaction time between the substances. The concentration of the initial solutions of depressants contained 0.5% (w/w) or 1% (w/w) starch and these were diluted for use in the microflotation tests. Solutions of 1% (v/v) and 5% (v/v) HCl and 1% (w/v) and 5% (w/v) NaOH were used as pH modifiers. The conditioning times were 5 min for the depressants and 1 min for the collector. The flotation time was 1 min.
2. Materials and methods 2.3. Zeta potential measurements 2.1. Preparation and characterisation of mineral samples Zeta potential measurements were performed to elucidate the interactions between the depressants and the hematite surface. The most effective gelatinised depressants in the microflotation tests were chosen for the zeta potential measurements. The zeta potential measurements were performed using a ZetaMeter 4.0 system. Mineral suspensions of 0.1% (w/w) were prepared with a particle size of below 38 μm and a NaNO3 solution was used as the indifferent electrolyte. According to Stokes’ law, a 10 μm hematite particle has a settling velocity of 13.7 cm/min, and therefore the suspensions were allowed to stand for 10 min. After sedimentation, 200 mL of the suspension was collected, divided equally between four 50 mL beakers, and pH adjustments ranging from 4 to 12 were applied to each beaker. A HCl solution of 0.5% (v/v) and a NaOH solution of 0.5% (w/ v) were used as pH modifiers. Finally, the zeta potential curves of the indifferent electrolyte and the depressors were obtained.
A hematite sample from Quadrilátero Ferrífero was comminuted in a ball mill and passed through Tyler sieves to obtain the size range chosen for the microflotation tests (−212 µm + 74 μm). Due to the presence of impurities, the sample was subjected to a Carpco Wet HighIntensity Magnetic Separator, model 3X4L. The obtained magnetic fraction was then dried and homogenised for use. The quartz sample, also from Quadrilátero Ferrífero, was comminuted in a porcelain mill and sieved to obtain a grain size of 212–74 μm. Next, the sample was leached to remove possible impurities and cleaned with concentrated hydrochloric acid (37% HCl) for 12 h. The sample was then washed successively with water to attain a neutral pH, dried, and homogenised for use. The compositions of the samples were evaluated using a PANalytical X'Pert3 Powder X-ray diffractometer equipped with a copper tube and a Cu-Kα radiation wavelength of 1.5406 Å. The system has a 2θ range of 5° to 90°, an operating voltage of 45 kV, a current of 40 mA, and a collection time of 15 min. Data was obtained with data collection 2
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100
100 90
Hematite recovery (%)
Recovery (%)
80
60 Quartz without depressant Hematite without depressant
40
Cane natural
80
Cane heated
70
Cane conventional
60
Cassava natural
50
Cassava heated
40
Cassava conventional
30 20
20
10 0
0 0
20
40
60
80
100
120
3.1. Characterisation of mineral samples The obtained diffractograms of the mineral samples indicated that the predominant phases were hematite and quartz. The measured densities of the hematite and quartz samples were 5.18 g/cm3 and 2.63 g/cm3, respectively, which are consistent with those found in the literature. 3.2. Microflotation tests Fig. 1 shows the results of the microflotation tests for the hematite and quartz samples with increasing amine concentration. A high quartz recovery of 85% for 2 mg/L of collector indicates the preferential adsorption of amine on the quartz surface. Conversely, an amine concentration of 100 mg/L was required to achieve a hematite recovery percentage comparable to that of quartz. Fig. 2 shows the results of the microflotation tests of the hematite and quartz samples in the presence of corn starch. These results indicate that, even at a low concentration of 10 mg/L, the corn starch is a highly effective hematite depressant that decreased recovery from 57% to 8.5%. Conversely, quartz recovery remained above 90% in the presence of 50 mg/L of depressant. To analyse the performances of the residues, a maximum hematite recovery limit of 10% was established, and depressant concentrations yielding recovery values within this limit were deemed ideal reagent 100 92,4
Recovery (%)
60 Hematite with 20 mg/L amine and starch Quartz with 20 mg/L amine and starch
20
0
10
20
30
40
300
400
500
600
700
concentrations. Fig. 3 shows the results of the hematite microflotation tests in the presence of sugarcane bagasse and cassava wastewater prepared according to the three gelatinisation methods (natural, heated, and conventional). As can be seen in Fig. 3, all three methods produced decreases in hematite recovery with increasing depressant concentrations. For the sugarcane bagasse, the ideal concentrations were 400 mg/L, 200 mg/L, and 25 mg/L for the natural, heated, and conventional preparations, respectively, and the corresponding hematite recoveries were 4.9%, 6.3%, and 7.5%. Therefore, gelatinisation with NaOH was more efficient in the flotation process when compared with the other gelatinisation methods proposed herein. Further, sugarcane bagasse prepared using the natural method depressed hematite at 400 mg/L, which was unexpected because starch is insoluble in distilled room temperature water. However, as shown by Pavlovic and Brandão (2003), both the monomer glucose and the dimer maltose exhibit depressant actions on hematite at high concentrations and do not affect quartz recovery. Thus, the combined actions of starch and sugars probably contributed to the depressant effect of the sugarcane bagasse. Sugarcane has an average grade of 10% to 12% soluble sugars (Food and Agriculture Organization of the United Nations, 2019), while cassava tubers possess a sugar content lower than 2% of its total composition (United States Department of Agriculture, 2019). Therefore, the sugarcane bagasse likely contained more sugar than the cassava wastewater. The cassava prepared using only distilled room temperature water produced a slight decrease in hematite recovery with increasing depressant concentration and failed to reach the 10% hematite recovery limit; its lowest hematite recovery was 16%, which was achieved at a depressant concentration of 600 mg/L. Because starch is insoluble in room temperature water, the poor performance of the cassava wastewater prepared according to the natural gelatinisation method could therefore be due to the absence of gelatinised starch. Further, and as mentioned before, the lack of soluble sugars in the cassava wastewater suggests that only the presence of starch influenced the recovery of hematite by this residue. The heated and conventional gelatinisation preparations exhibited similar performances and the ideal concentration for both was 10 mg/L, which is the same concentration reported herein for corn starch. The hematite recovery of corn starch at the ideal concentration (10 mg/L) was 8.5%, while the ideal concentration of the heated and conventional gelatinisation preparations of cassava wastewater achieved hematite recoveries of 2.7% and 7.2%, respectively, which demonstrates the potential of this residue. Another interesting observation based on Fig. 3 is that the heated method was effective in gelatinising the starch content of the cassava wastewater but not the starch content of the sugarcane bagasse. Further, the data for the conventional and heated preparations overlap only for the cassava wastewater. The fibre content of a starch source
3. Results and discussion
0
200
Fig. 3. Hematite flotation results with sugarcane bagasse and cassava wastewater.
Fig. 1. Hematite and quartz flotation results with increasing amine concentration.
40
100
Residue starch concentration [mg/L]
Amine concentration [mg/L]
80
0
50
Starch concentration [mg/L] Fig. 2. Hematite and quartz flotation results with amine and increasing corn starch concentration. 3
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other studies, such as pH values of 6.8 (Alexandrino et al., 2016), 6.7 (Rocha et al., 2018), and 6.95 (Silva et al., 2018). In the presence of the depressants, the IEP of the hematite shifted and no positive zeta potential value was found within the tested pH range. Zeta potential analyses were performed on the depressants prepared with the addition of NaOH (i.e. the conventional gelatinisation method) because their performances were similar to that of corn starch. The occurrence of adsorption at pH 10.5 is evidenced by the microflotation curves in Fig. 4b, which indicate hematite depression in the presence of the depressants prepared with NaOH. However, Fig. 4a indicates that adsorption of the depressants on the hematite surface occurred in a pH range of 4–8, because the inversion of the zeta potential of the mineral occurred under these conditions. Further, Fig. 4a indicates that the residues influenced the zeta potential of the hematite in a manner similar to that of corn starch, which confirms that the residues acted as hematite depressants due to their constituent starch. According to Pugh (1989), although polymers such as starch are normally considered as non-ionic, they may exhibit anionic characteristics, and the charges of these molecules increases with increasing pH. This can be due to the oxidation and hydrolysis of starch molecules; the ionisation of fatty acids, phosphates, and other minor constituents; or the ionisation of hydroxyls, especially those linked to the C-2 and C-6 carbons of the glucose monomer. Thus, for a pH range of 4–6.9, in which the zeta potential of hematite is positive, the electrostatic attraction contributes to the adsorption of starch on the mineral surface. In a pH above 6.9, the starch and mineral charges are both negative. Nevertheless, their mutual interaction exceeds the electrostatic repulsion force, which indicates chemical adsorption.
Table 1 Quartz flotation results with depressants at ideal concentrations. Depressant
Gelatinisation
Concentration (mg/ L)
Quartz recovery (%)
Corn starch
Conventional
50
92.4
Sugarcane bagasse
Natural Heated Conventional
400 200 25
91.1 93.9 94.1
Cassava wastewater
Natural Heated Conventional
600 10 10
93.1 90.4 92.2
4. Conclusions Microflotation experiments indicated that cassava wastewater presented satisfactory results when gelatinised using both heating and conventional methods, and that the ideal depressant concentration for both preparations was 10 mg/L. However, cassava wastewater gelatinised with distilled water did not significantly depress hematite. Further, of the three preparations of sugarcane bagasse, the one gelatinised according to the natural method was found to be the least effective hematite depressant, while the sugarcane bagasse gelatinised with the addition of NaOH was the most efficient and possessed an ideal concentration of 25 mg/L. Microflotation tests verified that none of the residues had a depressant effect on the quartz, indicating that all reagents possessed good selectivity in relation to the mineral species studied. Zeta potential measurements demonstrated that the most efficient residues for hematite depression (sugarcane bagasse and cassava wastewater prepared with the addition of NaOH) each had a low impact on the zeta potential values of hematite in the pH range above the IEP of the mineral. Finally, these residues influenced the zeta potential of hematite in a manner similar to the commonly used reagent corn starch.
Fig. 4. (a) Zeta potential results of hematite without reagents and with 10 mg/L of depressants gelatinised with sodium hydroxide and (b) microflotation results.
affects starch extraction, and therefore the extraction and solubilisation of the starch contained in the fibrous sugarcane bagasse was more difficult than the cassava wastewater, which was a powder residue. Therefore, the heated method was inefficient at degrading the fibres and solubilising all of the starch contained in the sugarcane bagasse. The quartz recoveries were evaluated only for the ideal depressant concentrations to ascertain any depressant actions on the mineral. Table 1 presents the results, from which it can be concluded that the depressors did not affect quartz recovery in the evaluated conditions because the recoveries remained above 90% in all cases.
Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgements The authors would like to thank the Coordination for the Improvement of Higher Education Personnel (CAPES) and the National Council for Scientific and Technological Development (CNPq) for all funds provided for this research. The authors, also would like to thank the Federal University of Ouro Preto.
3.3. Zeta potential measurements Fig. 4a shows the zeta potential measurements of hematite in the absence of reagents and in the presence of the depressants (10 mg/L) corn starch, sugarcane bagasse, and cassava wastewater at 10 mg/L concentrations. Fig. 4b shows the hematite recoveries of the three depressants. The isoelectric point (IEP) of the hematite in the absence of depressants was determined to be pH 6.9. Similar results were found in
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