Biodegradable hematite depressants for green flotation separation – An overview

Due to environmental issues and the restrictions imposed on mineral flotation separation


Introduction
Nowadays, due to depleting high-quality ores and rich resources, the importance of processing low-grade and complicated iron ores, specifically those with high silicate gangue minerals, in a cost-effective and environmentally friendly manner is becoming increasingly evident (Birinci and Gök, 2021;Fang et al., 2019;Kumar and Luukkanen, 2019;Mehrabani et al., 2010).Processing iron oxides could be varied based on their deposit types and associated gangue phases (Yellishetty et al., 2010).Magnetite (Fe 3 O 4 ), hematite (Fe 2 O 3 ), goethite (FeO[OH]), limonite (FeO[OH].nH 2 O), and siderite (FeCO 3 ) are the main iron oxide minerals (Filippova et al., 2014;Gao et al., 2020;Chang Liu et al., 2022;Liu et al., 2014Liu et al., , 2019;;Montes and Montes Atenas, 2005;D. Wang et al., 2022).Magnetite is particularly important due to its occurrence in large quantities and its ability to be economically mined, while it has straightforward processing via mainly low-intensity magnetic separation.However, a significant proportion of the iron reserves in Earth's crust is completely oxidized iron, mainly hematite.Nowadays, with the decrease in the grade of magnetite reserves in shallow deposits, it is necessary to extract it from greater depths (Nakhaei and Irannajad, 2018).Thus, extracting and beneficiating other complicated iron ores, more specifically hematite, goethite, and ultrafine magnetite, is essential to completely meet the needs of industries (Arvidson et al., 2013;De Moraes and Kawatra, 2010;Eisele and Kawatra, 2010;Haselhuhn and Kawatra, 2015;Kawatra et al., 2004;Shen and Huang, 2005;Zhang et al., 2022Zhang et al., , 2017;;Zhu et al., 2022).
Several separation methods can process these complex iron oxides and reject their associated gangue content, including washing, magnetic-based techniques, gravity-based methods, and flotation separation (most practically a combination of these separation techniques) (David et al., 2011;Svoboda, 1989;Zeng and Dahe, 2003).However, the fine association of gangue minerals with valuable iron oxides makes physical separation methods somehow inefficient (Filippov et al., 2014), especially for ultrafine silicates, which can be mentioned as the prevalent gangue phase in iron oxide deposits.Others can be added to this list as phosphorous, alumina, and sulfur (Fig. 1) (Pattanaik and Venugopal, 2018).
Flotation is the most widely used and effective processing method for fine and ultrafine liberated iron oxide mineral separation, which can be carried out directly or indirectly (reverse flotation).Currently, there has been a growing trend toward reverse flotation, in which the surface of iron oxide minerals is deactivated by adding depressants (Li et al., 2019;Yang and Wang, 2019).Various reasons can be considered for such a trend, such as the high iron oxide minerals/silicate ratio in the ores, being more cost-effective than direct process, less sensitive to slimes, and enhancing the selectivity (Lu et al., 2017;Shrimali et al., 2017).Moreover, silicates can be strongly activated by the abundant Fe 3+ present in the silicate-iron oxide mineral flotation system due to the highly reactive Fe 3+ resulting from mineral dissolution.Thus, their separation could be quite challenging in practice through direct iron oxide flotation (Yuan et al., 2018;Ng et al., 2015;Peng et al., 2017;Vidyadhar et al., 2012).
Several different depressants have been used for iron oxide depression during its reverse flotation (mostly polysaccharide-based and polyphenolic-based), which some review articles have named them through the general iron oxide flotation conditions (Araujo et al., 2005;Filippov et al., 2014;Houot, 1983;Ma, 2012;Rath and Sahoo, 2020;Uwadiale, 2010).However, no review has comprehensively investigated the mechanisms and conditions of biodegradable depressants during the reverse flotation of iron oxide minerals.Since it is essential to understand the behavior of iron oxide minerals in the presence of these depressants, the present review aims to robustly explore the performance of various environmentally friendly depressants used in the effective separation of iron oxides from their accompanying troublesome gangue phases.Numerous factors that have to be considered for their applications have been reviewed.Adsorption mechanisms were analyzed, and gaps within each area were highlighted.

Iron oxides flotation routes
Iron oxide minerals and their associated gangues are varied in their physicochemical properties, which results in different flotation behavior (Montes and Montes Atenas, 2005).As mentioned, the direct flotation (DF) route could be a way to separate iron oxide minerals from their gangues.However, the presence of hydrolyzable cations such as Ca 2+ , Mg 2+ , and Fe 3+ ions in the flotation environment significantly decreases the selectivity of the direct flotation route by activating gangue surfaces that also float along with iron oxide minerals (Ma, 2012).Thus, the DF process seems to be a potential technique for concentrating iron oxides from tailings.In this route, although fatty acids have been reported as appropriate collectors, the depression of gangue minerals is challenging (Marins et al., 2020;Matiolo et al., 2020;Pan et al., 2019;Tang and Tong, 2020;N. Wang et al., 2022).
To tackle problems involved in DF, reverse flotation has been considered for processing iron oxide ores, which can be divided into two categories: reverse anionic flotation (RAF) and reverse cationic flotation (RCF) (Han et al., 2022b).Through reverse flotation, iron oxide minerals would be deactivated by different depressants (Fig. 2) to minimize their collector adsorption, and gangue phases will be floated with various collectors (Table 1).RCF has been the most extensively used route for upgrading iron oxide minerals (Marins et al., 2020).As the most common impurity, quartz is usually floated with ether amines in RCF.When mineral processors had no access to amines, early quartz flotation was conducted using the RAF method, which can be employed for activated quartz (Araujo et al., 2020;Lima et al., 2013;Safari et al., 2020;Silva et al., 2021;Yehia et al., 2021).
In the reverse flotation of iron oxides, sodium silicate, and sodium co-silicate have been reported as promising depressants by which an iron concentrate with high recovery could be resulted (Arantes and Lima, 2013;Tohry and Dehghani, 2016).However, efficient and selective inorganic depressants can cause serious environmental issues and increase the costs associated with their handling (Mu et al., 2016).Thus, depressants derived from natural resources have recently gained considerable attention in various flotation systems.Their comparatively low price and biodegradable, environmentally friendly, and green nature make them ideal alternative reagents (Bicak et al., 2007;Dong et al., 2021;Cheng Liu et al., 2022;Liu et al., 2021;Mu et al., 2016;Sanwani et al., 2021;Valdivieso et al., 2004).

Natural depressants
Biodegradable chemicals refer to naturally-occurring substances that can be decomposed into simpler harmless components to humans and the environment.Since microorganisms (bacteria and fungi) carry out the biodegradability process, using these organic chemicals can greatly help move towards green and environmentally friendly processes (Hyvönen et al., 2005).Different eco-friendly depressants have been investigated for iron ore oxides depression flotation, including polysaccharides polymers (starch and guar gum), wood extracts (lignosulfonate-based biopolymers), and tannin-based depressants (tannic acid and quebracho).These substances display a high degree of complexity in terms of chemistry and interaction characteristics.Additionally, raw material composition and flotation route can significantly affect the obtained results in the separation process (Bulatovic, 2010(Bulatovic, , 1999)).However, these reagents usually have similar structural characteristics as follows: (I) They have a hydrocarbon-based backbone that can easily adsorb on the mineral surfaces, driving them hydrophilic; (II) In most cases, a great number of hydroxyl (OH − ) groups have been distributed throughout the structures, making them able to develop hydrogen bonding or ionization; (III) They contain various strongly hydrated polar groups, such as SO 3 2− , COO − , etc. which are also frequently dispersed through the molecule (Pugh, 1989).It is generally believed that these reagents can adsorb onto the mineral surface through various interactions.Their hydrophilic functionalities improve the surface's hydrophilic properties, reduce bubble-particle attachment, and facilitate iron oxide mineral depression (Beaussart et al., 2009;Kar et al., 2013).

Starch
Starch is a colorless semi-crystalline solid substance that can be found in various sources such as plants, cereals, and fruits.Starch has been extensively employed in various industrial applications since it is an inexpensive, available, and environmentally friendly polymer (Rath and Sahoo, 2022).This nonionic polymer has been categorized as a polysaccharide composed of glucose monomers bound to each other Fig. 1.Gangue minerals associated with iron ores (Pattanaik and Venugopal, 2018).
Each starch polymer contains three hydroxyl groups free on the cyclic glucose units, and they may rotate to one side of the glucose ring, increasing the hydrophilicity of that side.The other side is somewhat hydrophobic because of -CH groups.It is known that, in aqueous solutions, AM creates a helix through six glucose monomers per turn.The internal helix is hydrophobic, while the outer shell is mostly hydrophilic (Pavlovic and Brandao, 2003).AP can be a flotation or flocculation reagent, while AM has a low affinity for reacting with minerals (Kar et al., 2013).
Since fine particles in the flotation process have high specific surface areas, the consumption of natural starch as the depressant is too high.Starch is usually modified through different methods, including chemical, physical, and compound modifications, to improve its adsorption on the surface of iron oxides.In this way, starch consumption in the froth flotation process is reduced (Tharanathan, 2005).Among them, chemical techniques are the most common modification (synthesizing) method, which produces modified products, namely carboxyl methyl starch, dextrin, phosphate ester starch, and oxidized starch (Cao et al., 2015;Fletcher et al., 2020;Yue and Wu, 2018;Zhou et al., 2019).There have been reports of effectively using modified starches in froth flotation, but only for laboratory studies.In mineral processing plants, caustic starch has been extensively employed in RAF and RCF for beneficiating iron oxide minerals.New forms of starches have been produced in recent years to improve their depression efficiency.It can also depress other minerals, such as separating copper sulfides from carbonaceous materials (Chimonyo et al., 2020a(Chimonyo et al., , 2020b) ) and fluorite from calcite (Sun et al., 2022).

Starch types.
Starch can be produced from various sources, such as rice, corn, potato, etc. (Table 2).Corn starch is commonly used as a depression reagent for iron-containing minerals because of its effectiveness, affordability, and availability (Kar et al., 2013).Soluble starches (soluble only in hot water) are frequently used in flotation (Quast, 2017).When parameters such as solution pH, temperature, or mass ratio of AP/AM are increased, the solubility of starch increases (Yang et al. (2017) exhibited that the more the percentage of AP in starch structure, the easier its dissolution).However, using alkali or thermal techniques, the gelatinization process can also improve starch's solubility.Starch dissolution consists of three phases: swelling, rupturing, and releasing starch molecules/ghosts (Yang et al., 2017).Corn starches are inexpensive and green depressants for depressing iron oxide minerals, but their solubilities in water with natural pH and at ambient temperature are low (Yang et al., 2017).After the dissolution of starch in an aqueous environment, a complex system is created, which consists of two parts.A portion of the reagent is dispersed, while the rest is associated with molecules.The main characteristics of an aqueous starch system are pH and the electrolytes present in the aqueous environment.Although starches generally react with alkali solutions, the reactions could be complex (Quast, 2017).
One of the new forms of chemically modified starches is metallicstarch complex (MSC) solutions.To synthesize MSCs, an ionic solution of relevant metal is mixed with a caustic starch solution.The MSCs are nano-sized colloidal complexes arranged by hydrophilic metal hydroxide as the colloidal nucleus on which hydroxyl and starch complex is adsorbed.This process leads to the creation of a bigger molecule than caustic starch.Yue and Wu (2018) exhibited that the depression effect of some MSCs (Fe 3+ , Zn 2+ , Pb 2+ , and Mg 2+ ) on the surface of iron oxides such as hematite was more than the caustic starch, and the order of their depression effect is as follows: Zn 2+ -starch > Pb 2+ -starch > Fe 3+ -starch > Mg 2+ -starch > Caustic starch.However, it was noted that the Fe 3+ -MSC showed the highest selectivity among MSC depressants in flotation experiments (Yue and Wu, 2018).
As mentioned, based on their chemical structures, starch types may lead to different metallurgical responses (Table 3).It was reported that maize starch does not modify hematite wettability during RCF; however, it can effectively limit the adsorption of a cationic collector (mono ether amine) on the hematite surface.Higher concentrations of amine may have an adverse effect on the separation process; thus, it should be avoided (Shrimali et al., 2018).A comparison impact study of four distinct starches (including soluble starch (SS), corn starch (CS), potato starch (PS), and rice starch (RS)) for hematite depression in RCF utilizing dodecyl amine as the collector showed that SS could lead to the maximum hematite recovery.The performance of SS was shown to be superior to that of other starches in all circumstances (Kar et al., 2013).Hematite can be clearly depressed by soluble starch during apatite flotation by sodium oleate, and apatite can be floated satisfactorily at a pulp pH of 10.However, using a high amount of soluble starch would be reduced the process efficiency (Bai et al., 2019).
The effects of various starches (corn, wheat, potato, rice, and dextrin) were examined for hematite depression in a mixture containing 80% hematite and 20% quartz using cetyl trimethyl ammonium bromide (CTAB) as a cationic collector for quartz.It has been reported that corn and wheat starch performed better than the other types (Abdel-Khalek et al., 2012).Yang and Wang examined different corn starches varying in ratios of AM to AP (including waxy starch (WS, 0:100), normal starch (NS, 27:73), and Gelose 50 (G50, 50:50)) for column RAF separation of hematite from quartz.They indicated that corn starch with a higher AP content decreased froth and collection zone of Fe more than corn starch with a lower AP content.According to their findings, the overall performance of RAF followed the order of WS > NS > G50 (Fig. 4) (Yang and Wang, 2019).
Corn, sorghum flour, and sorghum starch were examined for hematite depression in the CRF.Sorghum starch showed great potential as an alternative to corn starch.In alkaline pH (9-10), both sorghum and corn starches can be effective depressants for hematite depression.However, sorghum flour requires greater dosages to perform similarly (Silva et al., 2019a).In all pH levels examined (3-12), sorghum starch doses less than 40 mg.L − 1 recovered more quartz than corn starch.Quartz floatability is intensely reduced by adding sorghum or corn starches; however, it is hardly affected by the presence of sorghum flour.It also should be noted that high dosages of starches as a depressant can result in poor separation efficiency.The best quartz floatability was obtained using sorghum starch as a hematite depressant, especially at low dosages and pH 9 and 10 (Silva et al., 2019a(Silva et al., , 2019b)).
demonstrated that hydroxyl ions on starch molecules and hydroxylated metal locations on the surface of minerals interact together strongly, which is responsible for the chemical adsorption of starch.(Filippov et al., 2013;Laskowski et al., 2007;Moreira et al., 2017;Partridge and Smith, 1971).In general, the main adsorption parameters are the adsorbent mineral, starch type, starch concentration, and other soluble substances, pH (Schulz and Cooke, 1953).
The hydrogen bonding between numerous (-OH) groups present in the starch structure, and the hematite surface was thought to be responsible for the hematite depression by starch.However, starch selectively adsorbs on the hematite surface mainly because of numerous hydroxylated Fe-sites on the hematite particles (Rohem Peçanha et al., 2019).It has also been reported that polysaccharides such as starch have a low affinity for some other minerals (quartz, for instance).However, starch as a macromolecular polymer can bind particles together during flocculation, leading to slight depression of quartz (Chen et al., 2007;Yang et al., 2017).Starch adsorbs onto only silicon surface sites on the quartz, predominantly at pH values between 1.8 and 5.0 (Rohem Peçanha et al., 2019).In RCF of iron ore, using an amine-based cationic collector (alkyl ether amine) can increase starch adsorption on the quartz particles, leading to hydrophobic surfaces (Lima et al., 2013).Sodium oleate (C 18 H 33 NaO 2 ), another well-known collector of activated quartz (usually by metal ions such as Ca 2+ ), would adsorb on the hematite surface without starch.While investigations revealed that when using starch, the distance between the hematite surface and oleate ion is too wide to allow oleate adsorb (Li et al., 2019).
It was reported that the iso-electric point (iep) of pure hematite (in the absence of flotation reagents) varied from 4.5 to 7.1 (Table 4).The factors influencing this variation are sample preparation methods, conditioning time, zeta-potential measurement techniques, and mineralogy.The general assessment indicated that various starches affect the hematite iep.No noticeable shift was documented with the addition of soluble starch (from 6.2 to 6) and rice starch (from 6.2 to 6.1) on hematite.However, its iep was changed to 6.8 and 7.1 when potato and corn starch was adsorbed on its surface, respectively.The caustic starch and Fe 3+ -starch also could decrease hematite's iep from 6.5 to 4.5 and 6.2, respectively (Yue and Wu, 2018).Starch generally drives the hematite surface charge more negatively (Table 4) (except in some reports
A. Asimi Neisiani et al. (Abdel-Khalek et al., 2012;Kar et al., 2013)).Lima et al. (2013) examined the impact of ether amine (as a quartz collector) and gelatinized corn starch (as depressant) dosages during RCF for three size fractions of hematite, including fine (− 45 μm), coarse (− 150 +45 μm), and global (− 150 μm).Since the coarse particles possess a lower specific surface area (Table 5), they need a lower starch concentration to be depressed.The proper starch dosage for the coarse fraction was between 250 and 500 g.ton − 1 .In higher dosages, there was the possibility of unbound starch molecules onto the mineral surface in the solution.The increase in starch dosage from 500 to 1000 g.ton − 1 could result in a modest rise in SiO 2 content in the concentrate.It was also concluded that increased amine dosages might cause clathrate formation between amine and starch molecules, explaining the rise in SiO 2 concentration in coarse-size concentrates.For effective flotation, the total amine consumption for the coarse fraction was five times higher than for the entire fraction and seven times higher for the fine fraction (Lima et al., 2013).
Unlike usual depressants that are mainly used at froth flotation, starch may not prevent surfactant adsorption.During calcite flotation, for instance, starch adsorption increased by oleate (collector) presence, indicating a significant cooperative interaction between the adsorbed starch and oleate ions (Somasundaran, 1969).In the hematite RCF using dodecyl amine as a collector, the presence of starch increases amine adsorption in acidic solutions but decreases its adsorption density in alkaline pH.Additionally, starch adsorption can be improved by increasing amine concentration up to 10 − 4 mol.L − 1 and then decreasing it (Partridge and Smith, 1971).It has also been proven that branching can enhance starch adsorption's surface activity and selectivity.Branching may happen by altering the alkali or acid properties of the solution, temperature, or through oxidation (Quast, 2017).
Along with the depressing effect of starch on hematite, it has been demonstrated that corn starch can also flocculate fine particles in the flotation separation of hematite.The flocculation effect is based on particle size, with more flocculation for smaller particles.Thus, starch serves as both a depressant and a flocculant in the reverse flotation of iron oxide ores.It can act as a depression reagent by not allowing the adsorption of a collector (amine) on the surface of hematite, maintaining it hydrophilic, and as a flocculation agent, aggregating fine particles of hematite.The flocculation effect, in turn, is significant.If the fine hematite particles are not flocculated, the flotation efficiency may be considerably reduced due to their presence in the froth zone during the reverse flotation.The results have also indicated that the flocculation effect is because the corn starch depends on the hematite particle size, and a higher flocculation effect occurs for a − 5 µm size fraction (Shrimali et al., 2018).The flocculation of fine particles using starch was also confirmed by Liu et al. (2006).It was shown that moderately high molecular weight polymer depressants such as corn starch and carboxymethyl cellulose (CMC) with a molecular weight of about 700,000 render the hematite and hydroxyapatite particles hydrophilic.Therefore, these particles flocculated.This phenomenon depresses the genuine flotation of the hematite particles and reduces their mechanical entrainment (Liu et al., 2006).
3.1.1.3.Mineral's impact.Generally, the recovery of quartz flotation can be adversely affected when carbonate-bearing gangues, siderite (FeCO 3 ), and dolomite (CaMg[CO 3 ] 2 ) are present in the flotation system.Under these conditions, iron oxides cannot be selectively separated from quartz (Faramarzpour et al., 2022;Wonyen et al., 2018).This phenomenon occurs because of the CaCO 3 formation, which can precipitate on the quartz particles and interact with starch molecules (Luo et al., 2016).A molecular simulation study assessed the performance of various reagents in a flotation system containing hematite, quartz, and siderite.Results demonstrated that both hematite and siderite could react with starch.While calcium ions (in CaCl 2 solution) did not interact with the siderite particles, and hydroxyl radicals could locate there.The formation of Ca(OH) + and its connection to -COO − led to quartz separation.However, the activation of quartz surfaces by Ca 2+ could not occur when fine particles of siderite covered the quartz surface.Therefore, quartz particles were unfavorably inhibited in iron reverse flotation.It should be noted that oleate ions (collector) could penetrate the layer of starch adsorbed on the surface of siderite particles and enhance its floatability.In other words, the high content of fine siderite negatively affects the flotation separation efficiency (Hao et al., 2018).It was also reported that siderite could affect RAF of hematite from quartz considerably more than dolomite (Luo et al., 2016).
In addition to hematite, other iron-bearing oxide minerals, such as magnetite and pargasite, can be depressed using starch in the reverse flotation process.Pargasite, a complex inosilicate mineral of the

Table 5
Specific surface area and amine dosages for the three hematite size fractions (Lima et al., 2013).amphibole group with formula NaCa 2 (Mg, Fe 2+ ) 4 (Fe 3+ , Al)Si 6 Al 2 O 22 (OH) 2 , is present in some iron deposits.Some research has investigated the possibility of using starch in systems containing this Fe-Mg-Al-bearing amphibole (Filippov et al., 2010).After using starch, significant changes in pargasite surface characteristics imply that starch can bind to the amphibole particles.The reason for this reaction is both H-bonding with hydroxyl groups present on the surface (as in the case of quartz) and the development of a surface chemical complex with metal atoms (as in the case of magnetite) (Filippov et al., 2013).The substantial depression impact of starch during magnetite and pargasite flotation with an amine relative to quartz could be explained by a starch adsorption process based on chemical complex formation.A significant starch adsorption affinity was reported for the surfaces of Fe-Mg-Albearing silicates.Various flotation experiments revealed that pargasite did not float with the DDA after adding starch to the flotation system, and its recovery was negligible.
Additionally, starch can completely depress magnetite floatability (Filippov et al., 2013).As mentioned, it was approximately confirmed that starch molecules could not efficiently affect the pure quartz flotation when it is not activated (Balajee, S. R., 1969;Cooke et al., 1952).However, it was documented that the flotation recovery of nonactivated pure quartz with alkyl ether amine or DDA can be decreased by over 40% in the presence of starch.In this case, the processes of starch binding and depression effect on the flotation of the silicates are likely to be similar to the oxidized iron, particularly magnetite (Filippov et al., 2013).
These days, rare-earth elements (REE) have gained considerable economic and scientific attention in various high-technology industries (Marion et al., 2020).Hematite is one of the common gangue REE minerals (REM) (Jordens and Zhiyong, 2016; Li et al., 1989).However, using green depression reagents for hematite depression in the REEs flotation is a surprising gap that needs to be addressed more.In fact, sodium silicate is now used in the mentioned conditions, and it is essential to investigate the performance of biodegradable alternatives (Qi, 1993;Satur et al., 2016).Abaka-Wood et al. (2018, 2019) compared the performance of starch and sodium silicate as depressants in a flotation system containing artificially mixed REM, hematite, and quartz using sodium oleate and hydroxamic acid as collectors.The obtained results demonstrated that starch could be considered a green alternative depressant in this system (Abaka-wood et al., 2019;Abaka-Wood et al., 2018).However, there is still an evident lack of published research to investigate other environmentally friendly depressants and their applicability in the industry for the flotation of REM.

Guar gum
Guar gum is a naturally derived nonionic polysaccharide with an average molecular weight between 100,000 and 2,000,000.The guar gum macromolecules comprise 1 to 4 linked β-D-Mannopyranose components with α-D-Galactopyranose components randomly attached to the core mannose chain via 1 to 6 glycosidic linkages (Fig. 5).The mannose-to-galactose ratio is between 1.8 and 1.0 (Painter et al., 1979;Whistler and Hymowitz, 1979).Guar gum has nine OH groups in each unit of its structure.These OH groups are accessible for the guar gum molecule to attach via hydrogen bonding to mineral surfaces (Wang et al., 2005).As a result, froth flotation using guar gum as a depressant is more successful in separating minerals, more specifically sulfide ore from gangue minerals.Guar gum is a depressant for various minerals, such as potash, talc, and chromite; even promising results have been reported for sulfide minerals like pyrite.Additionally, it has been reported as a useful depressant in the flotation of iron ore oxides (Laskowski et al., 2007;Ma andPawlik, 2007a, 2006;Shortridge et al., 2000).Nanthakumar et al. (2009) used batch flotation experiments in the presence of soda ash to investigate guar gum impacts as a hematite depressant from a difficult-to-float igneous phosphate ore containing 68% apatite.The obtained result exhibited that guar gum satisfactorily could depress hematite.They compared its performance with starch and concluded that guar gum is a much more efficient depressant with lower Fe 2 O 3 in reverse flotation (Nanthakumar et al., 2009).Ma and Pawlik (2007) investigated the adsorption of guar gum onto various minerals, including hematite.They indicated that NaCl and KCl saturation causes guar gum to adsorb less onto hematite, alumina, and titania.Overall, the adsorption density of the guar gum is quite steady on these three minerals over a wide ionic strength range.It has also been reported that regardless of the electrolyte type present in the flotation environment, guar gum's interactions with oxide minerals are not pHdependent (at pH 3-11), which suggests that acid-base-type chemical interactions do not influence adsorption.In fact, it appears that guar gum, as a polysaccharide, shows pH-independent adsorption behavior on experimented mineral oxides as an inherent feature.Hydrogen bonds were introduced as the major adsorption mechanism of guar gum (Ma and Pawlik, 2007a).Although the promising results of using guar gum in the flotation separation of various minerals, such as smithsonite from calcite, were reported (Yang et al., 2022), the lack of comprehensive studies on its effect on various iron oxide minerals and a detail of its adsorption mechanism shall be considered a fundamental gap.

Polyphenolic (tannins)-based depressants
Tannins are polyphenols with high molecular weight, present in numerous wood species, particularly in the southern hemisphere.Principally, they protect plants from insects and fungi.Furthermore, the astringent nature of tannin-rich vegetables can limit herbivorous animals' consumption of these vegetables.From an industry perspective, tannins have various applications in flotation, petroleum process, agriculture, medicine, and leather production (Fraga-Corral et al., 2020;Koopmann et al., 2020;Nagaraj and Farinato, 2016;Raja et al., 2014;Rutledge and Anderson, 2015).This macromolecule organic compound can be divided into two major groups, namely hydrolyzable and condensed (nonhydrolyzable) complex tannins (Fig. 6) (Raja et al., 2014).
Based on the origin and molecular structure, there are several species of tannins, such as acacia, bayberry, mangium, valonea, emblic, and larch (Chen et al., 2011;Daniel et al., 1989;Kraus et al., 2003).However, all species contain hydroxyl groups (-OH), which cause tannins to create bonds with various molecular structures to reach a stable crosslink association (Das et al., 2020;Haslam, 1996Haslam, , 1979)).Because of this unique feature, they can be distinguished from other common polyphenols.Furthermore, the diversity of polyphenol structures explains the weight of tannin molecules, ranging from 500 to 20,000 g. mol − 1 .This feature and their chemical properties make them suitable as a coagulant, adhesive, and depressant agent in froth flotation and other applications (Fraga-Corral et al., 2020;Rutledge and Anderson, 2016).Tannins can be used as an agent in the plastic and various mineral flotation process.The ability of tannins to form complexes with some ions, specifically iron ions and noble metal salts, along with their environmental friendliness and affordability, make them suitable for flotation modifying agents.The industrial application of tannins in mineral processing started in 1920 when naturally derived reagents were employed as modifiers in froth flotation.Between 1921 and 1930, tannins were being used as a depressant in coal and non-sulfide flotation systems.Subsequently, between 1933 and 1949, they began to be employed as a depressant agent in the flotation of sulfides, barite, Mnbearing, and iron ores (Matveeva et al., 2016;Nagaraj and Farinato, 2016;Rutledge and Anderson, 2015;Wang et al., 2015).

Tannic acid
Tannic acid (C 76 H 52 O 46 ) is the specific liquid form of tannin.This biodegradable liquid can be obtained from various parts of plants (Zhang et al., 2013).The chemical structure of tannic acid (Fig. 7) possesses weak acidity due to several phenol functional groups (Dbira et al., 2019).Tannic acid is a potential green depressant for metallic minerals due to the numerous hydroxyls in its molecule, leading to the affinity for metal ions (Han et al., 2020).It can also be used in water purification to remove fine particles through flocculation.Tannic acid can also inhibit the separation of waste plastics (Negari et al., 2018;Pita and Castilho, 2017).Furthermore, this natural macromolecule can make it a common flotation depression reagent in mineral beneficiation (Ozcan and Bulutcu, 1993;Zhang et al., 2018).
Few investigations have addressed the application of tannic acid (TA) as an iron oxide depressant.Tohry et al. (2021a) investigated the possibility of using TA to depress hematite in an RCF of a quartzhematite system.They employed Flotigam®EDA (commercial-grade ether amine 50%, neutralized by acetic acid) as the cationic collector (Tohry et al., 2021).Zeta potential assessments (Fig. 8) indicated TA enhanced negative charge on both quartz and hematite surface.However, with an increase in TA dosage, quartz zeta potential moved toward positive while hematite kept decreasing (Fig. 9).It has been reported that if the functional groups in polyphenols (including hydroxyl, carboxyl, and phenolic hydroxyl) are ionized in an aqueous environment, alkaline condition of the system enhances.For this reason, it is expected that after ionization, the particle surface gains a negative charge, resulting in a sharp decrease in zeta potential (Fraga-Corral et al., 2020;Han et al., 2020;Zhang et al., 2012).The adsorption of TA molecules is much stronger on the surface of hematite in comparison to quartz.That is the reason for the greater negative shifting of hematite's zeta potential after exposure to TA, compared to the TA-treated quartz (Tohry et al., 2021).
A powerful non-electrostatic/chemical interaction can be responsible for the strong adsorption between TA molecules and hematite particles.However, in the case of quartz, physical adsorption takes place.Additionally, the results of zeta potential measurement on the pure minerals indicated that quartz possesses a greater negative charge.Fig. 6.Structure of (A) hydrolyzable and (B) condensed tannins (Lochab et al., 2014).This phenomenon results in a remarkable electrostatic repulsion force between pure quartz and the anionic species in the TA structure, decreasing the adsorption (Tohry et al., 2021).Also, the polyphenol's strong affinity for metallic ions has been proven in some investigations.Because of their excellent ability to adsorb metallic minerals, polyphenols are promising depressants for minerals containing metallic ions (Fraga-Corral et al., 2020;Mahiuddin et al., 1992;Malan, 2015).Therefore, hematite, as an iron-containing mineral, can easily accommodate the molecules of TA on the surface compared to quartz.Exploring the metallurgical responses, 100 mg/L TA could selectively depress hematite (more than 90%) at pH 9.2 in a reverse flotation system containing 30 mg/L ether amine as a cationic collector.However, the floatability of quartz was decreased by 8% (Tohry et al., 2021).

Quebracho
Quebracho, the most well-known form of tannins, is a nonhydrolyzable (condensed) tannin.It is not broken down into other chemical compounds in the acidic and alkaline environment or when enzymes are introduced (J.Iskra et al., 1980;Rutledge and Anderson, 2015).Quebracho is derived from TA and obtained from the internal core (heartwood) of two types of trees (Schinopsis Balansae and Schinopsis Lorentzii) that grow in southeastern South America (Bulatovic, 2007;Rutledge and Anderson, 2015).Generally, the quebracho structure (Fig. 10) is formed of phenol and carboxylic groups with primary phenolic nuclei (Giesekke, 1983;Kupka and Rudolph, 2018).
Quebracho can be absorbed onto the mineral surface to prevent them from floating.The di-hydric and tri-hydric phenol groups exist in the structure of quebracho and can interact with the mineral surface (Giesekke, 1983).During quebracho adsorption on mineral surfaces, its high molecular weight plays an important role (Mehdilo and Irannajad, 2014).There are various mechanisms by which quebracho can adsorb on mineral surfaces.The most important of them are; H-bonding, the formation of complexes between hydroxyl functional groups and various cations, the neutralization of electrical charge when OH − reacts with positively-charged particles, and electrostatic reaction between quebracho micelles (negatively-charged) and positively charged mineral surfaces (J.Iskra et al., 1980).When carboxylic and phenol groups in the quebracho structure are ionized, they can attach to cationic mineral surface sites.This phenomenon results in strong hydrogen bonding and the production of tannate salts, making the mineral surfaces negatively charged.The negative surface would prevent the adsorption of the collector and make the particle hydrophilic (Leja and Roa, 2006).It should also be mentioned that there is a competition between quebracho and collector on the surface of minerals.Consequently, quebracho can be displaced if the collector is at an extremely high concentration (Hernáinz Bermúdez de Castro and Calero de Hoces, 1993; Kupka and Rudolph, 2018).Quebracho has been rarely applied for depressing iron oxide minerals.Yuhua and Jianwei used a combined quaternary ammonium salt (CS-22) as a collector to separate artificially mixed samples of quartz, magnetite, and specularite (various hematite).According to microflotation results, CS-22 acts as a suitable cationic collector in an RCF Fig. 9.The results of zeta potential measurement of quartz and hematite at different TA dosages (pH ~ 9.2) (Tohry et al., 2021).system containing magnetite and quartz.However, in the case of the quartz-specularite flotation system, selective separation is quite difficult.Thus, it is necessary to use quebracho as a depression reagent for preventing specularite flotation.After adding 16 mg/l quebrachos to a flotation system containing quartz and specularite (ratio 1:4), an iron concentrate with 67.79% total Fe, 5.45% SiO 2, and the total Fe recovery of 76.61% was reported (Yuhua and Jianwei, 2005).

Humic acid
Humic compounds are huge, structurally complex macromolecules that exist in soils and natural waterways due to microbial activity breaking down plant and animal wastes.Humic substances make up a significant percentage of the earth's carbon, accounting for around half of all carbon on the planet.Fulvic and humic acid (HA) are two types of acids found in soil.In other words, non-humic material, such as protein, polysaccharides, nucleic acids, tiny molecules such as sugars and amino acids, and humic substances, are the two main types of organic matter found in soils, sediments and natural waterways (Weber et al., 2018;Wong and Laskowski, 1984).Humic compounds are difficult to characterize because of their structure, size variability, and inclination to associate with solutions as their concentration rises (Christman and Gjessing, 1983;Kononova, 2013).Although the structures of humic compounds are still unknown despite decades of investigation, several speculative structures have been presented (Fig. 11) (H-R Schulten and Schnitzer, 1997;Hans-Rolf Schulten and Schnitzer, 1997;Swain, 1970;Weber et al., 2018).
Humic compounds absorb on various mineral surfaces, making them excellent froth flotation reagents.Several scholars have noticed and researched the adsorption of humic compounds on iron oxides (Table 6).Unlike corn starch, the adsorption mechanism of HA on hematite appears to be associated with an ionic interaction among minerals and reagents, with a hydrophobic interaction character (Veloso et al., 2018).Ramos-Tejada et al. (2003) indicated HA adsorption enhances hematite hydrophilicity, and this fact, along with the low HA adsorption on silicate surfaces, might indicate the presence of a selective depressant (Ramos-Tejada et al., 2003).
Turrer and Peres (2010) studied various iron oxide depressants through RCF.They indicated that HA was not a selective depressant, producing poor iron recovery in concentrates with high silica grades (Turrer and Peres, 2010).Contrastingly, other studies reported that HA might be an alternative to corn starch due to its high depression efficiency.Tohry et al. (2021) demonstrated that when using HA as a depressant, DDA (cationic collector) might alter the electrical charge of the hornblende surface more than that of augite.They indicated that both chemisorption and H-bonding were active in adsorbing HA onto the hematite surface.On the silicate surfaces, a similar adsorption mechanism can be identified; however, hornblende has a lower adsorption strength.These adsorption rates can be explained by the fact that hornblende has more free Si sites on its surface than augite and has a greater capacity to resist the anionic functional groups of HA.Generally, when pH 10, the inhibition impact of HA can be observed on augite, hornblende, and hematite.The kind and availability of metallic sites, as well as the availability of Si-sites on the minerals' surface are the influencing parameters in this inhibition impact.According to Fig. 12, the highest depression strength of HA was observed in the hematite mineral, followed by augite and hornblende (Tohry et al., 2021).
It was reported that even at low concentrations (10-20 mg/L), HA might considerably depress hematite.At low HA concentrations, contact angle analyses and flotation studies revealed that HA did not influence the surface of the investigated silicates (particularly hornblende).However, raising the HA content can also enhance the depression impact on these silicate gangue minerals.Cationic batch flotation experiments at low HA concentrations revealed that more than 35% of the silica content could be eliminated, and iron recovery of over 92% could be achieved.In the final flotation concentrate, the grade of iron and SiO 2 were about 67.3 and 2.8%, respectively, under ideal conditions using dodecyl amine (DDA) as a collector (Tohry et al., 2021).
HA has shown a good recovery of iron and a low silica content in the concentrate.Dos Santos and Oliveira (2007) studied the depression effect of HA in the RCF of hematite from quartz.The results exhibited that at an ideal dosage of HA and dodecyl amine, more than 90% of quartz could be floated, and 61% of the hematite was depressed.They also investigated the performance of flotation separation on an artificial mixture of hematite and quartz (hematite: quartz = 3:1).In the depressed concentrate, which included 86.0%Fe 2 O 3 , hematite recovery was greater than 90%.The findings imply that HA might be used instead of starch in iron ore flotation.The most efficient results were observed at an alkaline pH of 9.5 to 11.5 and 40 mg/L of HA (dos Santos and Oliveira, 2007).

Lignosulfonate-based depressants
Lignosulfonates, which are water-soluble polyelectrolyte polymers, can be produced as a by-product during the wood pulp process.Even though these strongly anionic natural products have no distinct  molecule structure, it is generally agreed that some functional groups are found in the structure of all lignosulfonates.Hydroxyl, carboxylic, aliphatic, sulfonic, and aromatic functional groups are the most common constituents of lignosulfonates.Various lignosulfonate salts (calcium, ammonium, potassium, and sodium) are available and widely used in different industries (Ma and Pawlik, 2007b;Yan et al., 2010).Lignocellulosic biomass is primarily utilized for various industrial purposes due to its ability to extract lignin (Fig. 13) (Zakzeski and Weckhuysen, 2011).
Only a few investigations have considered lignosulfonates as a depressant for iron oxide minerals.It was indicated that lignosulphonate derived more negative charges on the hematite surface (Table 7).It was indicated that the adsorption density of lignosulfonates on hematite only

Table 7
Iso-electric point of hematite in the absence and presence of lignosulphonates.enhanced when the hematite surface was positively charged (at pH values below the iep) (Nanthakumar et al., 2010).Wang et al. (2021) claimed that calcium lignosulphonate could be considered for separating hematite from Fe 3+ -activated quartz (pH 9).Lignosulfonates could adsorb on both minerals; however, it hindered the adsorption of NaOl on the Fe 3+ -activated quartz than hematite and depressed it significantly.However, NaOl could still adsorb on the hematite surface, and hematite could be floated.Thus, the process was not selective, and the floatability of both minerals dropped significantly (L.Wang et al., 2021).

Summary
Reverse flotation is the most extensively utilized beneficiation technique for separating iron oxide minerals (mostly hematite) from their associated gangue minerals.In reverse flotation, depressant plays a significant role, where iron oxide is depressed.The current change in the world's perspective and the tendency towards environmentally friendly activities have made the use of green and natural flotation reagents in the process more critical.Various investigations indicated polysaccharides (starch and guar gum), tannin-based products (tannic acid and quebracho), and humic acid had shown promising performance in selectively separating iron oxide minerals from their accompanying minerals, while lignosulfonate showed a low selectivity.
These organic chemicals are mostly polymeric heavy molecules derived from plants and wood and therefore are compatible with the environment.One common characteristic that almost all these depressants have is the presence of a hydrocarbon-based backbone, making them able to easily be adsorbed on the mineral surfaces and driving them hydrophilic.In addition, many hydroxyl groups are distributed throughout their structures, making hydrogen bonding the most common type of adsorption mechanism between them and mineral surfaces.Their adsorption on hematite mostly rendered its surface charge more negative and hindered collector adsorption.Even though these biodegradable depressants showed an efficient depression effect on iron oxide minerals (mostly hematite), detailed and comprehensive studies on their applications are limited except for starch.There is still a significant gap in their fundamental studies of some critical processes, such as separating iron oxide minerals from REE-bearing minerals.
From a mineralogical point of view, investigations mostly focused on hematite, not other iron oxides such as magnetite, goethite, and siderite.In comparison to the experiments on floatability and zeta potential, research on surface analyses to highlight surface interactions and adsorption mechanisms was quite limited (mainly about starch), and there is a need to conduct more detail investigations to basically understand the reaction of these eco-friendly depressants with iron oxides and their associated minerals.Furthermore, it should be noted that reducing the reagent dosage used in flotation and using natural chemicals is a practical step in moving towards green flotation.Reducing the use of depressants requires sufficient knowledge about the effect of various parameters, such as particle size, pulp chemistry, and synergistic reagent interactions, which need to be explored when using biodegradable depressants.This is because large quantities of reagents are required for efficient depression of the fine particles.The lack of scientific approaches in this field is another gap, and addressing it paves the way toward green flotation.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 4 .
Fig. 4. The effect of various starches on the iron recovery and grade in the hematite-quartz flotation system (Yang and Wang, 2019).

Fig. 8 .
Fig. 8.The results of zeta potentials measurement of quartz and hematite as a function of pH without and with using 100 mg.L − 1 TA (Tohry et al., 2021).

Table 1
Common anionic and cationic collectors in iron ore reverse flotation. .

Table 3
Application of various starches in iron oxide flotation.

Table 4
Iso-electric point of hematite with and without the addition of various starches.

Table 6
Application of humic acid as a depression reagent in the flotation of iron oxides.