A Critical Review on the Flotation Reagents for Phosphate Ore Beneficiation

Abstract

Phosphate ore is the dominating natural resource for the production of fertilizers and phosphorous chemical products. Flotation is the most widely employed technique to enrich apatite and remove the impurities for the separation of the phosphate ore. The flotation reagents play an important role in this efficient separation of phosphate ores. In the last few decades, great progress has been achieved in the flotation reagents for phosphate ores. However, a critical review on this theme has rarely been reported in recent years. Thus, the goal of this paper is to critically analyze the published literature on the flotation reagents for phosphate ores, mainly ranging from 2010 to 2024, including the regulators, depressors, collectors, and frothers. Additionally, the interaction mechanisms between the reagents and mineral surfaces were analyzed. It is concluded that sulfuric acids and its derivatives play a leading role in the depression of phosphate minerals. Highly selective biobased depressants have become potential carbonate inhibitors. Additionally, the derivatization and synthesis of multi-functional compounds and identifying the biobased frothers are the main development trends of collectors and frothers. Overall, a high-quality concentrate with a 31.05% grade and 98.21% recovery at pH 5 was achieved using lecithin as an ecofriendly amphoteric collector and sulfuric acid as the depressor when the feed contained 24.49% P₂O_5, which was superior to other flotation indexes of similar phosphate ores. This review will help researchers to document knowledge gaps and provide a reference for the efficient and green beneficiation of phosphate ores in the future.

1. Introduction

Phosphate ore is a crucial non-metallic mineral resource with non-renewable and irreplaceable characteristics. It is ultimately used in agriculture, food, medicine, military, new energy, electronics, and other fields [1,2]. According to the statistics, the total reserves of the world’s phosphate exceed 70 billion tons. Notably, Morocco, Western Sahara, China, and Syria have abundant phosphate resources, accounting for more than 80% of the total amount in the world [3]. Apatite minerals, which mainly involve fluorine apatite, hydroxyapatite, carbon hydroxyapatite, fine crystal apatite, and kurskite, are the leading phosphate-contained minerals in phosphate ore [4]. According to the mineralization classification, sedimentary deposits, igneous deposits, resources metamorphic deposits, and biogenic deposits are the major types of phosphate resources. Among them, the sedimentary deposits occupy 75% of phosphate [5].

Phosphate ore is the dominating natural resource for the production of fertilizers and phosphorous chemical products. The separation of low-grade phosphate ores is a vital part for phosphate concentrate production. Eventually, the phosphate concentrate used in the fertilizer industry should meet the criteria. Specifically, the content of P₂O₅ is no lower than 30%, the CaO/P₂O₅ mass ratio is below 1.6, and the MgO content is below 1% [6]. At present, reverse flotation, which floats gangue minerals into the foam product and keeps useful minerals in the pulp, has been widely used to separate gangue minerals (such as, dolomite, calcite, quartz, clay, feldspar, mica, etc.) with the depression of apatite, while direct flotation, which floats useful minerals into the foam product and leaves gangue minerals in the pulp, is frequently used to enrich apatite with the impurities’ depression. Additionally, the combined utilization of reverse and direct flotation has been widely reported to beneficiate the complex phosphate rock [7,8,9,10]. Thus, flotation crafts and reagents play an important role in this efficient separation of phosphate ores. Among them, collectors and depressants exert an important role in achieving the desired hydrophobic or hydrophilic state of the interesting fractions [6,11]. For instance, phosphate reverse flotation involves the usage of apatite depressors, including H₃PO₄ and H₂SO₄, as well as their derivatives, which can significantly increase the hydrophilia of apatite surfaces. However, the aforementioned reagents may cause some environmental and health issues [12]. Similarly, the direct flotation of apatite relies on the collector, such as fatty acids that can considerably increase the hydrophobicity of apatite surfaces, but fatty acids usually face problems of poor solubility and low selectivity [11].

With the continuous consumption of high-grade phosphate ores, the green and efficient beneficiation of the low-grade phosphate ores has gained soaring interesting. Meanwhile, great progress has been achieved in the flotation reagents for phosphate ores [5,13,14,15]. However, a critical review on this theme has been rarely reported in recent years. This lack of critical review is not conducive to the efficient and green development of phosphate flotation. In lieu of this, the goal of this paper is to critically analyze the published literature on the flotation reagents for phosphate ores. Moreover, the interaction mechanisms between the reagents and mineral surfaces are analyzed. This review will help researchers to document knowledge gaps that can facilitate shape the trend of future research in this area.

2. Flotation Reagents for Apatite

At present, flotation is widely used for phosphate beneficiation. The flotation reagent exerts a crucial role in the regulation of mineral surface features and the gas–liquid interface, facilitating the separation behavior. Usually, regulators, depressors, and collectors are used to control the wettability of the mineral surface, while frothers are added to make fine bubbles and also to aid froth stability [16,17]. Assiduous efforts on this aspect have been ongoing during the last decade.

2.1. Regulators

Acid and alkaline are frequently cited for regulating the pH of the flotation medium. The pH regulator will directly alter the electrical properties of mineral surfaces and the activity of regent molecules existing in the solution, which can create a desirable condition for mineral flotation. Phosphoric acid, sulfuric acid, and organic acids with small molecules are commonly employed in phosphate ore reverse flotation. Additionally, waste acid or slag acid from the phosphorus chemical industry has been comprehensively utilized to replace phosphoric acid and sulfuric acid. Furthermore, oxalic acid and citric acid were employed to regulate the pH [6,18]. The aforementioned acid could interact with the phosphate mineral surface to reinforce its hydrophilicity; thus, the anionic collectors were expected to interact selectively with calcium and magnesium carbonate minerals. Manar Derhy et al. [19] employed phosphoric acid to adjust to a pH = 5 and enhanced the selective flotation of calcite and dolomite using fatty acids and phosphate esters. It was possible that H₂PO₄^- stemming from the hydrolysis of phosphoric acid could depress the apatite, and the anionic collector could interact selectively with carbonate minerals. Ruan et al. [20] reported that a weak acidic or neutral pH was necessary for silica reverse flotation with cationic collectors. It was inferred that cationic collectors preferentially adsorbed onto the more negatively charged surfaces of silica. Yin et al. [21] found that the presence of fluoride in the flotation systems prompted the formation of CaF₂ and MgF₂, which covered on the dolomite surface when the pH was approximately 5.5, with HCl, H₂SO₄ or H₃PO₄ regulation, both of which impaired NaOL collection on the dolomite.

During the direct flotation process, sodium carbonate, sodium hydroxide, and sodium bicarbonate are frequently used to regulate the pH values [22,23]. For instance, 0.6 kg/t NaOH and 0.1 kg/t Na₂CO₃ were used in the flotation of siliceous ores from Senegal [24]. Meanwhile, the phosphate minerals from Florida phosphate ore were floated with a fatty acid collector when soda ash was used to adjust pH values ≥ 9 [25]. For the flotation of phosphate minerals with a common fatty acid collector, the pulp pH value determines the distribution of fatty acid components (as illustrated in Figure 1). Specifically, fatty acids are principally in molecular form in weak acid pulp systems, displaying low intensity physical adsorption on mineral particles. Conversely, fatty acids are mainly in the ionic form and able to interact with the Ca²⁺-activated sites on the mineral surface via chemical adsorption under weak alkaline conditions [26,27,28]. The combination of molecular and ionic species of fatty acid is generated under neutral conditions and facilitates the hydrophobization of particles, because the structure is more stable and higher surface activity [29,30,31]. Additionally, pH values exert an essential influence on the surface properties of apatite, such as zeta potentials. Camilla L. Owens et al. [32] conducted a detailed analysis on the zeta potentials of apatite in various pH conditions after reviewing the related literature. The results indicated that the highly pure apatite of had an IEP ranging from pH 3 to pH 6.5. This variation was mainly attributed to a multitude of factors [33,34,35,36,37]. For example, the contamination within the apatite crystal lattice with concentrations of Si, Fe, and Mg inevitably altered the IEP [33]. H⁺ and OH^– ions are commonly accepted as the ions that determine the potential of apatite. The pH values determine the electronegativity of the apatite surface [38]. When the pH is greater than pH_iep, the surface of apatite presents as electronegative and is beneficial to the electrostatic adsorption of the cation collector. However, a higher pH value would not be conducive to the adsorption of anionic collectors due to the competitive adsorption of OH^– ions. When the pH is lower than pH_iep, the surface of apatite presents as electropositive, which will repel the adsorption of the cation collector. Meanwhile, the H⁺ will promote the dissolution of the apatite surface, decreasing the Ca-active sites of minerals. Thus, it is very important to identify the relationship between the zeta potentials of apatite surfaces and pH for the selection of collectors.

As is well known, apatite may present different positive and negative surface electrokinetic properties with the addition of different regulators and exhibit different interactions with the collectors [30]. Moreover, the regulators can eliminate the negative effects of unavoidable ions (such as Ca²⁺ and Mg²⁺ ions) on the apatite surface properties to some extent [39]. Additionally, the activity of collector molecules existing in the solution is determined by the pH regulator. Herein, the role of regulators in the complex apatite flotation system is multifaceted. Nevertheless, the development of a regulator needs to closely integrate mineral surface properties (such as electrokinetic potential and solubility) and collector solution chemistry. In view of the operation cost, sulfuric acid is widely used compared to other acids during the reverse flotation of phosphate ores. On the other hand, the excessive use of sulfuric acid often leads to the generation of acidic wastewater as well as the corrosion of flotation equipment [15]. Moreover, the application of sulfuric acid is only limited to a narrow pH range (pH = 4~5) [30]. Sodium carbonate acts as a desirable pH regulator during direct flotation because it plays positive effects on the dispersion of pulp and the elimination of the adverse influences of calcium and magnesium ions [40,41]. The choice of pH regulator depends on the flotation craft. Overall, new pH regulators, as well as the related mechanism studies on phosphate ore flotation, have rarely been reported.

2.1.1. Depressors

Depressors play an important role in the flotation of phosphate ores, i.e., the decrease in mineral hydrophobicity, the change in the wettability of target minerals, the prevention of bubble attachment, and the decrease of the receding contact angle [42]. A basic rule for depressor selection lies in that the hydrophobicity in the corresponding collector for the particular mineral is replaced with hydrophilicity and retains the same functional group. According to the flotation craft, the depressors used in phosphate ores flotation mainly involve phosphate depressors, carbonate depressors, and silicate depressors.

Depressors of Phosphate Minerals

Several phosphate depressants, including H₂SO₄, H₃PO₄, and their derivatives have commonly been used for phosphate reverse flotation [43]. As mentioned earlier, the inhibition of apatite can be realized by regulating the pulp pH to an acidic medium (usually pH < 5.5). The underlying depression mechanism is attributed to the fact that H₂PO₄⁻ and HSO₄⁻ prefer to interact with the Ca²⁺ exposed on the apatite surface, forming a hydrophilic film and causing the decrease of available active sites [44,45]. It is consequently inferred that inorganic acids (H₃PO₄ [46,47,48] and H₂SO₄ [11,49,50]) and various phosphate/sulfate salts, such as KH₂PO₄ [51], Na₄P₂O₇·10H₂O [52], and Na₂SO₄ [53], can efficiently depress the apatite. It emphasizes that the formation of the CaHPO₄/gypsum is the main reason behind apatite hydrophilicity and depression. Calcite flotation from a carbonate phosphate ore by CO₂ as the apatite depressant has also been reported [54]. The results demonstrated that the injection of CO₂ could replace inorganic acids completely. Additionally, the presence of sulfate or oxalate salts could reinforce the suppression of apatite in acid conditions [53,55]. It has been found that oxalate or sulfate ions would react with dissolved Ca²⁺ spontaneously, resulting in the formation of calcium salt precipitate. This activity inevitably promotes the presence of phosphate ions in aqueous solution due to apatite dissolution, which would be responsible for the apatite depression [55]. Notably, sulfuric acid and its derivatives demonstrate an intense depression on phosphate minerals with the merit of being relatively low-cost, though they would give rise to pipe-clogging because of gypsum formation.

Zhang and Snow evaluated multiple depressants including sodium tripolyphosphate, fluosilicic acid, diphosphonic acid, and starch for phosphate minerals during cationic reverse flotation [56]. The results confirmed that starch showed a desirable depression during the separation between −35 mesh silica and apatite, while Na₅P₃O₁₀ could serve as a first-class depressant for the beneficiation of +35 materials. Nagaraj et al. [57] investigated the feasibility of polyacrylamide, which contains both –OH and –COOH functional groups, for the depression of apatite and the flotation of siliceous gangue in the cationic collector system. The results showed the polyacrylamide could effectively inhibit apatite flotation and, therefore, had a good application prospect in the beneficiation of siliceous phosphate ores.

Zhang et al. [58] found that the addition of hydroxyethylidene-1,1-diphosphonic acid (HEDP) could realize the separation between fluorapatite and dolomite because the interaction between HEDP and the fluorapatite surface was stronger than that of dolomite, and the interaction between sodium oleate (NaOL) and the fluorapatite surface decreased significantly. He et al. [59] demonstrated that amino-trimethylphosphonic acid (ATMP) could inhibit the floating of fluorapatite and had a feeble inhibitory effect on dolomite. It is possible that ATMP is more inclined to bond with Ca sites on the fluorapatite surface, which possesses a higher calcium density, which in turn reduces the adsorption of NaOL.

Depressors of Carbonate Minerals

Calcite (CaCO₃) and dolomite (CaMg (CO₃)₂) are the typical carbonate gangue minerals in calcareous phosphate. Herein, it is extremely important to study the depressant for carbonate minerals. Common depressors for carbonate minerals mainly include lignin sulfonate, sulfonated phenol tar, sodium nitrohumate, sulfonated naphthalene (S711), sulfonated crude phenanthrene (S808), etc. [60]. In recent years, new inhibitors have attracted much attention. Highly selective biobased depressors such as sodium alginate, carboxymethyl cellulose, pectin, gum arabic (GA), xanthan gum (XG), and carboxymethyl chitosan have become potential carbonate inhibitors due to their merits, including nontoxicity, biodegradability, eco-friendliness, sustainability, and cost-effectiveness [61,62,63].

Dong et al. [62] evaluated the selective inhibition of polyepoxysuccinic acid (PESA) on calcite during apatite flotation. Eventually, a concentrate with 32.74% P₂O₅ and 84.82% recovery was obtained when 80 g/t PESA was added. Mechanism analysis confirmed that PESA presented a stronger chemical adsorption on calcite than apatite via the chelation between –COO– and calcium atoms and hindered the adsorption of NaOL (as illustrated in Figure 2). Thus, PESA could serve as a green reagent for the beneficiation of phosphate ores. Zhong et al. [64] using gum arabic (GA) as a depressant for the flotation separation of apatite and calcite. The multitude free –OH and –COOH groups in the GA molecule preferentially adsorbed onto calcite surface, giving birth to the steric hindrance and electrostatic repulsion effect of GA on the NaOl molecule. Eventually, an apatite recovery of 88.84% and P₂O₅ grade of 26.86% were obtained by one roughing during the apatite-calcite artificial mixed mineral (mass ratio 1:1). Obviously, flotation separation efficiency needs to be improved, and a cleaning process is indispensable for producing a higher-grade phosphorus concentrate. The selective adsorption of xanthan gum (XG) on dolomite reported by Zeng et al. [65] indicated that –COOH or –OH groups within XG occurred chemisorption with Ca sites on dolomite, while they weakly interacted with apatite by hydrogen bonding between –OH groups within XG and F atoms on the apatite surface. The flotation tests of artificial mixed minerals (dolomite/apatite = 2:8) showed that the MgO content decreased dramatically from 3.32% to 0.94% while that of P₂O₅ increased considerably by 5% in the presence of XG (10 mg/L) at pH 10. Due to its smaller amount and green reagent properties, XG presents as a good application prospect for the flotation of calcareous phosphate.

Carboxymethyl cellulose, citric acid, and naphtyl anthyl sulfonates could effectively depress dolomite based on single and mixed mineral experiments, as reported by Zheng and Smith [66]. Li et al. [67] reported that sodium lignin sulfonate chemically adsorbed on the dolomite surface, and hydrogen bonded occurred on the apatite surface, resulting in the selective depression on dolomite with NaOL as the collector. Yu et al. [68] investigated the effect of β-naphthyl sulfonate formaldehyde condensate (NSFC) on the selective separation of collophane from dolomite. The results demonstrated that NSFC interacted with dolomite surfaces by chemical adsorption, while NSFC exhibited weak hydrogen bond adsorption on collophane surfaces. Although NSFC could selectively depress the dolomite, its application in production still faces many challenges due to its chemical toxicity.

The flotation separation of apatite from dolomite with acrylic acid-2-acrylamido-2-methylpropanesulfonic acid copolymer (P (AA-AMPS), Figure 3a) and poly (acrylic acid-co-maleic acid) sodium salt (PAMS, Figure 3b) was conducted by Yang al. [69,70]. The results confirmed that P (AA-AMPS) and PAMS could selectively interact with Mg²⁺ on the dolomite surface, preventing NaOL adsorption on the surface of dolomite, but almost had no effect on apatite flotation; both could be used as green depressors for directly floating apatite from dolomite.

In recent years, biobased inhibitors have received soaring attention (Figure 4). For example, the molecular structure of sodium alginate (SA) (Figure 4a) contains many -COO- and -OH groups and can bond with metal active sites on the surface of minerals. Studies indicated that SA had a selective inhibition on calcite and dolomite but no obvious inhibition on apatite [73,74,75,76]. Carboxymethyl cellulose is an anionic polymer (Figure 4b) that can inhibit calcite [77]. Pectin has also been used to inhibit calcite (Figure 4c) and significantly reduced its floatability in the pH 7–9 range [78]. Carboxymethyl chitosan (Figure 4d) displays a selective depression impact on calcite but a feeble depression impact on apatite [79].

It is well accepted that polysaccharides have been the topic of widespread research as biobased depressors. Since polysaccharides usually implicate multiple –OH and –COO– groups, they could chelate metal ions (for instance, Ca²⁺, Mg²⁺) that presented on the surfaces of apatite, calcite, and dolomite in phosphate ores [76]. Although these minerals have similar surface chemical characteristics, polysaccharides may have different interactions with different mineral surfaces due to the differences between the chemical structure of the minerals and the metal active site. As described in Figure 5, polysaccharides could interact with calcite, dolomite, and apatite surfaces through a variety of possible interactions, providing efficient access to depress carbonate gangue minerals and float apatite [80,81,82]. Additionally, researchers focused on quantifying the adsorption amount of the biobased inhibitor on mineral surfaces to evaluate its effect on the collector’s adsorption. As can been seen from

Table 1. The adsorption dosage of biobased depressors onto apatite, calcite, and dolomite surfaces using the TOC technique.

Mineral System	Size/μm	Biobased Depressors	Dosage /mg/L	pH	Adsorption Dosage /mg/m2	Selectivity	Refs
Apatite Calcite	−74 + 34	CMC	50	9	0.329 0.844	Larger adsorption onto calcite surface	[79]
Apatite Dolomite	−74 + 37	SA	50	9	0.86 1.73	Larger adsorption onto dolomite surface	[74]
Dolomite Apatite	−74 + 37	XG	30	9	0.75 1.8	Larger adsorption onto dolomite surface	[59]
Calcite Fluorite	−74 + 37	PSG	200	7	14.9 12.14	Similar adsorption onto both surfaces	[83]

, these biobased depressants possess good, promising applicability for phosphate ore beneficiation. However, more effort should be made to improve the performance (selectivity and effectiveness) of biobased depressors.

Depressors of Silicate Minerals

Sodium silicate is a conventional depressor of oxidized ore flotation and has good dispersion, which can effectively depress silicate minerals and aluminosilicate minerals. Wang Qun and K. Heiskanen [84] reported that apatite could be enriched from siliceous phosphate ores using fatty acids with sodium silicate as a depressant in direct flotation, but it was difficult to obtain a low iron-containing apatite concentrate. Silva et al. [85] demonstrated that the most efficient depression pH range of sodium silicate was observed between 5 and 8. Rao et al. [86] showed that the modulus and dosage of sodium silicate played a key role in the depression effectiveness of quartz during the flotation process.

The effects of carbonate alkalinity on the flotation behavior of quartz were investigated by Sayilgan and Arol [87]. The result indicated that HCO₃⁻, CO₃²⁻, and OH⁻ exerted a depression effect on the quartz in both amine flotation and Ca²⁺-NaOL flotation. Firstly, carbonate alkalinity compressed the double layer or caused a competitive adsorption with amine ions on the quartz surface. Secondly, the spontaneous reactions between carbonate or bicarbonate ions and the calcium ion component (Ca²⁺ and Ca(OH)⁺) promoted the formation of precipitates, considerably decreasing the concentration of calcium ions and weakening the quartz activation. Wang et al. [88] found that citric acid (CA) had a significant inhibition on calcium-activated quartz and proposed two possible inhibition mechanisms (as depicted in Figure 6). Firstly, Ca²⁺ on the surface of quartz could be desorbed by CA³⁻. Secondly, the pre-adsorption of citric acid impaired the Ca²⁺ adsorption and hindered the interaction between quartz surfaces and the NaOL collector.

Recently, biobased depressants for quartz depression have been reported. As shown in

Table 2. Some reported biobased depressants for quartz depression.

Biobased Depressant	Quartz	Refs	Biobased Depressant	Quartz	Refs
Sodium alginate	*	[92]	Starch	***	[89,93,94]
Carboxymethyl cellulose	*	[95]	Caustic Cassava starch	*	[96]
Carboxymethyl chitosan	*	[97]	Calcium lignosulphonate	*	[98]
Dextran	*	[99]	Tannic acid	*	[100]
Hydroxypropyl starch	*	[101]	Tannin	*	[91]
Sorghum starch	*	[102]	Humic acid/salt	***	[103,104,105]

*: One study was conducted on a biobased depressant for a specified mineral. ***: Three studies, at least, were conducted on a biobased depressant for a specified mineral.

, starch and humic acid/salt have been more widely studied compared to other biobased depressants. A previous study discovered that [89] the flocculation property of starch caused a mild effect in decreasing the floatability of quartz using the amine collector. Consequently, this property also depressed apatite and dolomite, leading to a nonselective inhibition on the minerals. Even so, starch has been used as the gangue depressant in the Brazilian igneous phosphate ore concentrators, considering the cost [90]. Additionally, Tannin (TA) as a natural and eco-friendly organic reagent has shown lower affinity and efficacity toward quartz compared to that of hematite, limiting their application in phosphate ore flotation [91].

In summary, the selective adsorption of hydrophilic substances, or the occupation of active sites on the quartz surface, due to the addition of depressants, mainly contributed to the underlying depressant mechanism. Notably, it is more difficult to suppress the silicate minerals effectively compared to phosphate minerals and carbonate minerals in the biobased depressants system.

2.2. Collectors

Collectors play an essential role in increasing the hydrophobicity of mineral surfaces and the bubble attachment probability with rapid three-phase contact formation. Fatty acids, amines, and amphoteric collectors are the main types of phosphate ore collectors. Even though fatty acids and amines are the most commonly used collectors in phosphate ore flotation, other types of collectors, such as disodium dodecyl phosphate, sodium dodecyl sulfate, hydroxamates, and so on, are also available for the flotation of phosphate ores. In recent years, derivatization has become an important method to improve the physicochemical properties of collectors (such as water solubility, selectivity. and collecting efficiency). Moreover, the synthesis of multi-functional compounds is the main development trend of collectors [94].

2.2.1. Anionic Collectors

Long chain fatty acids are widely used anionic collectors in the flotation of phosphate ores. Oleic acid and its soap are frequently cited as examples for this class of collector. Paiva et al. [106] indicated that NaOL adsorption occurred through the formation of clusters of Ca(OL)₂ on the apatite surface. This activity made the apatite surface rougher and heterogeneous, thus improving its floatability. Meanwhile, Yang et al. [107] indicated that the hydrophobic agglomeration of apatite fines could be induced by sodium oleate in aqueous solutions. This activity increased the apparent particle size of fine apatite and improved the recovery of the mineral. Additionally, Bai et al. [108] reported that sodium oleate (NaOL) displayed desirable collecting properties on Ca²⁺-activated quartz. The Ca-OOCR colloid precipitates facilitated and dominated the flotation of quartz, thus promoting the application of NaOL in the reverse flotation of siliceous phosphate ores (as depicted in Figure 7).

During the past decades, the shift from tall oil and oxidized petroleum to vegetable oil as the dominant manufacturing material of anionic surfactants for apatite flotation improved the properties of fatty acid collectors and decreased production costs. Nevertheless, the current fatty acid collectors still have many shortcomings, such as low temperature resistance, low solubility, high consumption, and high sensitivity to slimes [109,110]. To further improve the physicochemical properties of fatty acid soaps at low temperatures, many efforts have been made toward the modification of fatty acids through chlorination, hydroxylation, and etherification. For example, hydroxamate surfactants were found to be very efficient in phosphate ore flotation from Florida [111], while modified soybean oil displayed a superior performance compared to a conventional fatty acid collector for the beneficiation of a refractory collophanite [112]. Zheng et al. [113] investigated the effect of sodium dodecyl benzene sulfonate (SDBS) collectors on the separation between fluorapatite and dolomite. The result confirmed that SDBS was superior to sodium oleate for fluorapatite enrichment at low temperatures.

Herein, after the modification of the fatty acid collector, the selectivity and adaptability of the reagent are significantly improved, and the flotation process is simplified to a certain extent. For some minerals with complex properties, the modified fatty acid collector needs to be mixed with other collectors or synergists to obtain the ideal index. Oliveira et al. [14] used sulfosuccinate (ROOC–CH₂–CH–COO⁻–SO₃⁻) as an auxiliary reagent to improve the separation efficiency of rice bran oil soap during the recovery of apatite from flotation tailings. Meanwhile, the synergetic effects of fatty acids in amazon oil-based collectors for phosphate flotation have been reported [114], which provided the idea for developing new green collectors. Ding et al. [115] employed oleamide-sodium and SDBS as combined collectors for apatite flotation. The results revealed that the active components of C₁₇H₃₃CONH⁻, (C₁₇H₃₃CONH₂)_m, and RSO₃⁻ in the combined collector were chemically adsorbed with the calcium active-sites on apatite surfaces. More importantly, oleamide solubility might be improved in the presence of SDBS. The reaction mechanism is shown in Figure 8. Accordingly, the research of mixed collectors will become an important trend in the development of flotation collectors for phosphate ores.

2.2.2. Cationic Collectors

Amine collectors are the most common cationic collectors for silicate mineral removal from phosphate minerals. Dodecylamine (DDA) is the predominant amine collector, which exhibits a desirable collecting capacity towards quartz [116]. On the other hand, the excessive stability of foam produced by DDA led to poor selectivity. To solve this dilemma, a multitude of efforts has been made toward the modification of amines. Zhang et al. [117] studied a novel amine collector N, N-bis (2-hydroxy-3-chloropropyl) dodecylamine (BHCPDA) via the introduction of two 2-hydroxy-3-chloropropyl groups. The results demonstrated that BHCPDA preferentially adsorbed on quartz surfaces rather than on apatite surfaces and was superior to DDA in collector selectivity. Figure 9 shows the optimized adsorption configuration of a BHCPDA⁺ ion on the (101) surface of quartz.

Zhao et al. synthesized a novel cationic collector 2-[2-(Tetradecylamino) ethoxy] ethanol (14-2G) by introducing the –CH₂CH₂OCH₂CH₂OH group to tetradecylamine. The results confirmed that 14-2G enhanced the hydrophobicity of the quartz surface more effectively compared to DDA [118]. Zhang et al. indicated that DDAIP (1-(dodecylamino)-2-propanol) displayed a better selectivity in the flotation separation of apatite/quartz than DDA because DDAIP chemically interacted with the Si atoms more intensively, and more hydrogen bonds were formed on the quartz surface compared to DDA [119]. Liu et al. [120] claimed that the novel polyhydroxy amine collector N-(2,3-Propanediol)-N-dodecylamine (PDDA) possessed better flotation efficiency and superior selectivity compared to DDA. Furthermore, Peng et al. [121] applied a new surfactant-N-(2-hydroxy-1, 1-dimethylethyl) dodecylamine (HDMEA) by introducing an additional hydrogen bond functional group to improve the separation of apatite from quartz, which mainly attributed to the significant adsorption of hydrogen bonds on the quartz surface and the steric-hindrance effect on apatite (as depicted in Figure 10).

In recent years, plenty of research has been conducted on the development of mixed collectors in flotation. Xu et al. [122] reported the adsorption of three most widely used mixed surfactant systems (anionic–cationic, anionic–nonionic, and cationic–nonionic) to understand the interactions between these collectors on the basis of reviewing the oxide and silicate mineral flotation. Vidyadhar et al. [123] claimed that the head-to-head electrostatic repulsion between the cations decreased and the hydrophobic tail–tail interactions increased, which reinforced the adsorption of the cationic collectors in the presence of anionic collectors. Filippov et al. [124] employed a mixed collector consisting of etherdiamine, C13 alcohol, and dodecylamine to remove silicates from magnetic concentrates. As a result, a denser and more stable adsorption layer could be formed because of the decrease in electrostatic repulsion caused by etherdiamine cations. Nogueira et al. [125] reported that a mixture of ethermonoamine and etherdiamine exerted a synergistic effect on quartz floatability with no addition of a depressant. Recently, Monte et al. [126] investigated the synergism of combined cationic collectors in the flotation of quartz. The result indicated that the mixed collectors (etherdiamine and ethermonoamine) with a mass ratio of 3:1 showed a synergistic effect and obtained the greatest recovery and selectivity. The underlying reason is attributed to the minimization of ether–ether and cation–cation repulsion and the maximization of ether–amine or amine–amine neighbors in the reorganization of the collectors (as illustrated in Figure 11).

In summary, the traditional amine collectors have a strong collecting ability for silicate minerals, and the separation of useful minerals from gangue can be achieved with less consumption of the reagent at ambient temperatures. On the other hand, amine collectors are extremely sensitive to slimes, and the high viscosity and long-life foam will decrease the separation efficiency significantly, even making the flotation process infeasible. This has limited the application of cationic collectors in the separation of fine-grained materials. In any case, the modification of amines and mixed collectors play an essential role in the reverse flotation of silicate minerals from phosphate minerals.

2.2.3. Amphoteric Collector

The amphoteric collector molecule has both anionic and cationic functional groups with a desirable low temperature resistance, water solubility, and adaptability. It can interact with the mineral surface through various adsorption methods, such as electrostatic adsorption, chemical adsorption, and chelation. Usually, the amphoteric collector has excellent flotation selectivity in a wide pH range and is feebly affected by the dissolved species and temperature. Therefore, this kind of collector has great application potential in phosphate rock flotation.

Amphoteric collectors mainly involve amino carboxylic acid types, amino phosphonic acid types, amino sulfonic acid types, amino ester group types, and amide carboxylic group types. Among them, amino carboxylic acid types and amino phosphonic acid types are often used in phosphate ore dressing. Cao et al. [127] reported that N-hexadecanoylglycine (C16Gly) preferentially adsorbed onto the fluorapatite surface through tridentate binding, and this adsorption configuration was much more stable than the corresponding adsorption configuration between C16Gly and dolomite (as depicted in Figure 12), which provided support for the selective flotation of fluorapatite from dolomite by C16Gly. Li et al. [128] proposed that N-dodecyl-β-amino propionate (C12Giy) had a strong collective capacity for dolomite and quartz rather than collophane. C12Giy could selectively adsorb to the quartz surface in the form of –RNH₂⁺ at pH < 4.3 and adsorb to the surface of dolomite and collophane in the form of –RCOO⁻ at pH > 4.3. Figure 13 shows the possible adsorption mechanism of C12Giy on the mineral surfaces. Therefore, C12Giy presents as a potential collector for the reverse flotation of silica-calcareous phosphate rocks.

An amphoteric surfactant (dodecyl-N-carboxyethyl-N-hydroxyethyl-imidazoline) was used as collector for the separation of dolomite and phosphate; the experimental results of artificial mixed ores confirmed that a phosphate concentrate containing 32% P₂O₅ was obtained, and the recovery was about 90% when the feed contained 26.2% P₂O₅ [129]. Elmahdy et al. [130] used a similar amphoteric collector to remove dolomite from a phosphate ore. Ultimately, a concentrate of 0.53% MgO and 31% P₂O₅ with a recovery of 90% was obtained under the recommend flotation parameters. Yang et al. [131] synthesized an amphoteric collector SB-116 that contained both amino and carboxyl functional groups. The result confirmed that SB-116 had good selectivity in weakly acidic media and could generate stable hydrophobic chelates on the mineral surface to collect dolomite and sesquioxide. The interactions of benzol amino benzyl phosphoric acid (BABP) with calcium-containing minerals studied by Hu et al. [132] indicated that the chemisorption between the monovalent anionic species of BABP and Ca-activated sites of minerals was responsible for the flotation of calcium-containing minerals, and the flotation separation of calcite from apatite could be achieved in a weak alkaline medium due to the different Ca densities on the mineral surfaces. Recently, lecithin was used as a new amphoteric collector for the reverse flotation of phosphate ores [133]. As displayed in Figure 14, phosphatidylcholines (PC) and phosphatidylethanolamines (PE) preferentially interacted with the surface of carbonate gangues and quartz by careful control of the flotation pH. For the beneficiation of Khouribga region rock phosphate ore, a phosphate concentrate containing 31.05% of P₂O₅ was obtained with a recovery of 98.21% at pH 5. Similarly, a concentrate assaying at 32.39%P₂O₅ with a recovery of 96% was obtained at pH 8.5.

Notably, the development of more widely available, effective, and environmentally friendly amphoteric collectors is one of the major prospects of the phosphate industry. However, complex synthesis processes and high production costs have limited the real industrial application of amphoteric collectors. In the future, it is advisable to develop a new amphoteric collector based on molecular design theory and process mineralogy to overcome the above-mentioned dilemmas.

2.3. Frothers

The formation of a froth phase, in which the target minerals are adhered for further upgrading, is one of the key factors of flotation [134]. Small and well-dispersed bubbles (diameter < 1.0 mm) are more efficient in flotation due to a lower bubble rise velocity and high surface-to-volume ratios [135,136]. The commonly used frothers in concentrators are non-ionic frothers. According to their chemical structures, non-ionic frothers are classified into the following categories [137]: aliphatic, cyclic, aromatic alcohols, alkoxy paraffins, and polymers consisting of polyglycols. Due to a lack of affinity for solid–water interfaces, these non-ionic frothers display negligible collecting properties and merely help to disperse bubbles and form a froth.

On the other hand, frothers have some synergistic effects via collector–frother interactions in phosphate flotation. The addition of nonionic surfactants has been found to improve flotation performance by interacting with fatty acids [138]. Lu et al. [139] confirmed that the tolerance of oleate to pH variation and Ca²⁺ ions increased with the addition of a nonionic polymer. Lovell [140] showed that the presence of solid particles (e.g., apatite and calcite) prevented froth formation when fatty acids are used alone, but the addition of a nonionic surfactant (nonylphenol tetraglycol ether) improved froth formation. Although the positive effects of nonionic surfactants on the phosphate flotation have been well recognized, some difficulties in the process control should be expected. For instance, the flotation indexes are prone to fluctuate due to the compositional heterogeneity of naturally occurring cresylic acid or pine oil. Moreover, because all surfactants in this class exhibit high interfacial activity, even the smallest overdose may result in overly stable and persistent froth formation and/or effects, hindering the collection efficiency [141].

Similar to the trends in the aforementioned collectors and depressors, some biobased frothers have been reported for copper ore and in coal flotation processes [142,143]. The results indicated that the presence of rhamnolipid surfactant remarkably increased the flotation rate. Additionally, it was more efficient in decreasing interfacial tension than commercial frothers, such as pine oil, Dowfroth-250, and Aerofroth-65. Moreover, the rhamnolipid surfactant produced a very viscous and elastic foam, which resulted in a higher dynamic frothability index than these commercial frothers [144]. So far, limited data are available on the successful use of biobased frothers for phosphate ore beneficiation. These findings would provide new paths for the green and efficient flotation of phosphate ores, and biobased frothers may be important aspects of the future research.

3. Summary

Phosphate with a low and middle P₂O₅ grade is commonly beneficiated through the flotation technique, in which different kinds of reagents, such as pH regulators, depressors and collectors, are involved. The choice of pH regulator depends on the flotation craft. Sulfuric acid is widely used compared to other acids during the reverse flotation of phosphate ores. Meanwhile, Na₂CO₃ acts as a desirable pH regulator during direct flotation. According to the flotation craft, depressants used in phosphate ore flotation mainly include phosphate inhibitors, carbonate inhibitors, and silicate inhibitors.

Table 3. Main flotation reagents for phosphate ores beneficiation and the corresponding flotation index.

Separation System	Depressant Dosage	Collector Dosage	pH	Flotation Index	Refs
Depressors of phosphate minerals
Apatite/Dolomite	H3PO4: 80 mg/L	NaOL: 25 mg/L	5.5	GP2O5: 27.32% RP2O5: 66.60%	[47]
Apatite/Dolomite in collophanite	H2SO4: 17 kg/t H3Cit: 500 g/t	Gutter oil fatty acid: 1.5 kg/t	6.0	GP2O5: 28.46% RP2O5: 87.20%	[49]
Apatite/Dolomite	NaPP: 100 m/L	NaOL: 60 mg/L	7.0	GP2O5: 34.1% RP2O5: 96.5%	[52]
A low-grade phosphate ore	Na2SO4: 7.5 kg/t	Oleic acid: 1.5~2.0 kg/t	4.5	GP2O5: >32% RP2O5: 84%~87%	[53]
Fluorapatite/Dolomite	ATMP: 1.5 mmol/L	NaOL: 80 mg/L	9~10	Rdolomite: 66.9% Rapatite: 15.3%	[59]
Depressors of carbonate minerals
Apatite/Calcite	PESA: 7 mg/L	NaOL: 0.1 mmol/L	8	GP2O5: 29.26% RP2O5: 72.03%	[62]
Apatite/Calcite	GA: 50 mg/L	NaOL: 0.05 mmol/L	9	GP2O5: 26.86% RP2O5: 88.84%	[64]
Apatite/Dolomite	XG: 10 mg/L	NaOL: 0.3 mmol/L	10	GP2O5: 36.77% RP2O5: 83.6%	[65]
collophane/dolomite	NSFC: 30 mg/L	NaOL: 15 mg/L	9	GP2O5: 32.58% RP2O5: 86.01%	[68]
Dolomite-bearing phosphate ore	P(AA-AMPS): 0.24 kg/t	NaOL: 1 kg/t	8.5	GP2O5: 38.53% RP2O5: 82.56%	[69]
Apatite/Dolomite	PAMS: 14 mg/L	NaOL: 100 mg/L	9	GP2O5: 35.43% RP2O5: 88.40%	[70]
Apatite/Dolomite	SA: 20 mg/L	NaOL: 0.1 mmol/L	9	Rdolomite: 0.05% Rapatite: 88%	[74]
Apatite/Calcite	Carboxymethyl chitosan: 10 mg/L	NaOL: 0.1 mmol/L	9	Rapatite: 87.8% Rcalcite: 10.1%	[79]
Apatite/Dolomite	Flaxseed gum: 40 mg/L	NaOL: 40 mg/L	8	GP2O5: 39.39% RP2O5: 76.43%	[80]
Apatite/Dolomite	TSG: 10 mg/L	NaOL: 40 mg/L	9	GP2O5: 35.86% RP2O5: 78.76%	[81]
Depressors of silicate minerals
Ca2+ activated quartz	Citric acid: 1 mmol/L	NaOL: 0.5 mmol/L	12	Rquartz decreased by 92.15%	[88]
Ca2+ activated quartz	NaAl: 20 mg/L	NaOL: 60 mg/L	11.5	Rquartz: <10%	[92]
Quartz/Hematite	CMCS: 12 mg/L	DDA: 60 mg/L	7.5	Rquartz: >90% Rhematite: <5%	[97]
Anionic Collectors
Quartz	-	NaOL: 0.5 mmol/L	12.5	Rquartz: 84.50%	[108]
Phosphate flotation tailings	Corn starch: 500 g/t	ROS/KE: 80 g/t	11.5	GP2O5: 29.4% RP2O5: 46.2%	[113]
Apatite	-	Oleamide/sodium dodecyl benzene sulfonate	9	Rapatite improved by 4.87%	[115]
Cationic collectors
Apatite/Quartz	-	BHCPDA: 0.1 mmol/L	6	Rquartz: ~100% Rapatite: ~0%	[117]
Apatite/Quartz	-	DDAIP: 0.01 mmol/L	7	Rquartz: 90% Rapatite: 10%	[119]
Quartz/Hematite	Starch	PDDA: 20 mg/L	7.38	Rquartz: 94.9% RHematite: 62.4%	[120]
Apatite/Quartz	-	HDMEA: 20 mg/L	6.43	GP2O5: 38.57% RP2O5: 95.8%	[121]
Amphoteric collector
Apatite/Dolomite	-	C16Gly: 0.1 mmol/L	9	Rgap was 80%	[127]
Collophane/Quartz /Dolomite	-	C12Giy: 30 mg/L	3.5	Rgap of collophane and dolomite was 82.26% at pH 3; Rgap of dolomite and collophane was 74.57% at pH 5	[128]
Francolite/Dolomite	-	Dodecyl-N-carboxyethyl-N-hydroxyethyl-imidazoline: 290 g/t	10.5	GP2O5: 35.2% RP2O5: 90%	[129]
Apatite/Calcite	-	BABP: 0.36 mM	9	Rapatite: >90% Rcalcite: <20%	[132]
Phosphate ore	H2SO4	Lecithin: 40 mg/L	5	GP2O5: 31.05% RP2O5: 98.21%	[133]
Phosphate ore	-	Lecithin: 40 mg/L	8.5	GP2O5: 32.39% RP2O5: 96%	[134]

Note: G denotes grade; R denotes recovery.

displays the main flotation reagents for phosphate ore beneficiation and the corresponding flotation indexes based on the published literature. As compared to other depressors of phosphate minerals, sulfuric acids demonstrate an intense depression on phosphate minerals and is widely used, although its dosage is large. Notably, ATMP could depress phosphate minerals in an alkaline solution system, which improves the flotation environment.

Common depressors for carbonate minerals mainly include lignin sulfonate, sulfonated phenol tar, sodium nitrohumate, sulfonated naphthalene, and sulfonated crude phenanthrene. Notably, highly selective biobased depressants such as sodium alginate, carboxymethyl cellulose, pectin, and carboxymethyl chitosan have become potential carbonate inhibitors. Moreover, the flotation index of carbonate inhibitors focused on the NaOL collector system. These green reagents can be selectively adsorbed onto carbonate mineral surfaces and increase their hydrophilia, facilitating the separation of apatite from the carbonate minerals. However, it is more difficult to suppress the silicate minerals effectively as compared to phosphate minerals and carbonate minerals in biobased depressant systems.

Collectors used in phosphate flotation can be classified into anionic, cationic, amphoteric, and mixed collectors according to the reagent type. Fatty acids and amines are the most commonly used collectors in phosphate ore flotation. Furthermore, other types of collectors, such as disodium dodecyl phosphate, sodium dodecyl sulfate, hydroxamates, and so on, are also available for the flotation of phosphate ores. Moreover, derivatization and the synthesis of multi-functional compounds are the main development trends of collectors. Specifically, the amphoteric collector has excellent flotation selectivity in a wide pH range even without the depressor. Overall, a high-quality concentrate with 31.05% grade and 98.21% recovery at pH 5 was achieved using lecithin as an ecofriendly amphoteric collector and sulfuric acid as the depressor when the feed contained 24.49% P₂O₅, which was superior to other flotation indexes of similar phosphate ores. Finally, the commonly used frothers in the concentrator are non-ionic frothers. Although the positive effects of nonionic surfactants on the phosphate flotation are well recognized, some difficulties in process control should be expected.

4. Future Perspective

Great progress has been obtained in flotation reagents for phosphate ores. Nonetheless, it is important to note that, firstly, further study is needed to develop potential green depressors with high selectivity on phosphate minerals for phosphate reverse flotation. Secondly, there is an urgent need for efforts to find selective depressors for silicate minerals. Thirdly, basic research on the four minerals (apatite, calcite, dolomite, and quartz) in the same system that contains the Ca–Mg–Si multicomponent should be reinforced when depressants and collectors are involved. In any case, continued studies should be conducted on the interaction between biobased depressants and mineral surfaces to improve their performance and selectivity. Finally, further studies are needed to improve the performance of collectors for the derivatization and synthesis effects of collectors. Similar to the trends for the aforementioned collectors and depressors, biobased frothers may be important subjects of future research.