Trans. Nonferrous Met. Soc. China 31(2021) 2081-2101

Typical roles of metal ions in mineral flotation: A review

Zhi-yong GAO^1,2, Zhe-yi JIANG^1,2, Wei SUN^1,2, Yue-sheng GAO³

1. School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China;

2. Key Laboratory of Hunan Province for Clean and Efficient Utilization of Strategic Calcium-containing Mineral Resources, Central South University, Changsha 410083, China;

3. Department of Chemical Engineering, Michigan Technological University, Houghton 49931, United States

Received 24 July 2020; accepted 20 January 2021

Abstract: In flotation, metal ions possess significant roles that are usually fulfilled by either selectively activating or depressing the target minerals. Despite that tremendous efforts have been made to address the roles of metal ions in flotation, it still lacks a comprehensive review, especially to compare various ions instead of focusing on a specific one. This review begins by elaborately categorizing the factors involved in affecting the roles of metal ions in flotation. After that, well-accepted mechanisms are updated and discussed from the ore type. Furthermore, typical approaches to explore the underlying mechanisms are emphasized, including traditional techniques such as micro-flotation, contact angle measurement, zeta potential measurement, and other recent prevailing methodologies, like computational method, solution chemistry calculation, and cyclic voltammetry. This work will pave the way to promote flotations via activities like selectively adding/reducing metal ions, choosing reagents, and regulating the slurry chemistry.

Key words: metal ions; flotation; oxide mineral; silicate mineral; sulfide mineral

1 Introduction

Flotation is a versatile method extensively used in mineral processing fields to separate valuable minerals from gangue minerals selectively, which is based on the wettability differences among mineral surfaces [1,2]. Metal ions, mainly originated from grinding media, dissolution of minerals, and the use of the process water, etc, can make a substantial impact on the flotation. For instance, the presence of Ca²⁺ and Mg²⁺ takes different effects on flotations of galena, pyrite, and sphalerite [3-5]. Besides, it is hard to separate zircon from cassiterite when using fatty acid as a single collector, since both recoveries maintain relatively low levels. Nevertheless, zircon is proven to be selectively activated by Fe³⁺, while cassiterite is not [6]. On the other hand, some inadvertent activations are harmful to the flotation separation. It is well known that some divalent ions, like Zn²⁺, Pb²⁺ and Ni²⁺, may accidentally activate quartz, leading to a low-quality concentrate product [7].

The effects of metal ions on flotations have been intensively reported since the 1950s [8]. However, the previous reports have mainly focused on the influence of a specific metal ion on the flotation of a particular mineral. Hence, there is still a lack of a systematic summary on topics such as categorizing the related factors, updating the latest understandings of the underlying mechanisms, as well as the research approaches. Specifically, the well-accepted mechanisms have been proposed for decades. They can be divided into two hypotheses, i.e., metal ion hydroxyl complexes [9] and adsorption of hydrolysis products of the parent metal ion [10]. Both mechanisms can explain most of the flotation results. However, their limitation starts to appear when it comes to unconventional flotation phenomena, urging for new explanations. Under this circumstance, more and more theoretical and practical mechanisms have been suggested to solve the predicament. For instance, from the liquid film stability point of view, metal ions in the slurries might affect the drainage of liquid film between minerals and bubbles prior to the attachment [11].

In previous work, a systematic review on this topic was published [12]. However, with the continuous efforts from researchers worldwide, more results and analyses have been reported. Thus, an updated review is in desperate need. Furthermore, powerful research techniques, contributing a lot to the understanding of governing mechanisms, will be presented in this work.

2 Direct and indirect factors

The key factors influencing the roles of metal ions in flotation can be divided into two categories: direct and indirect, as shown in Fig. 1. The direct factors mainly refer to the intrinsic properties of metal ions, such as ion type, radius, valence, and concentration. On the other hand, the indirect factors relate to the ambient environment where metal ions are located, including the operations, pH, reagents, and minerals. Table 1 summarizes metal ions of interest in flotation, as well as their properties. More details are provided in the following paragraphs.

Fig. 1 Factors influencing roles of metal ions in flotation

Table 1 Basic parameters of metal ions commonly involved in flotation

2.1 Direct factors

2.1.1 Type of metal ions

Flotation results can vary with the types of involved metal ions. FORNASIERO and RALSTON [20] found that quartz could be well floated when using xanthate as the collector in the presence of either Cu²⁺ or Ni²⁺. However, in the pH range of 7-10, Cu²⁺ performed more efficiently as an activator ion than Ni²⁺. FUERSTENAU et al [7] found that quartz could be activated by Pb²⁺ in the pH range of 5.8-8.5, but Mg²⁺ was not an excellent activator under the same conditions. Besides, metal ions in a flotation system may exhibit mutual influences. For example, it is challenging to separate sillimanite and microcline in the presence of Al³⁺. Because Al³⁺ will serve as an activator and lead to high recoveries of both minerals. However, its activation was significantly weakened in the presence of Fe³⁺ in the slurry [21].

Explanations of the phenomena have been suggested to lie in the interactions between metal ions and mineral surfaces. FENG et al [22] reported both Cu²⁺ and Ca²⁺ could promote the adsorptions of carboxymethyl cellulose (CMC) on chlorite surfaces, but the interactions involved are not the same. Copper ions facilitated CMC adsorptions by being adsorbed onto the mineral surface, and consequently weakening the electrostatic repulsion between the CMC and minerals. In contrast, calcium ions enhanced the CMC adsorption by interacting with the CMC molecules and forming complexes adsorbed on mineral surfaces.

Besides, different valence states of metal ions engender different flotation results. Generally, monovalent metal ions are less active compared with divalent and trivalent metal ions. Meanwhile, divalent and trivalent cations are not identical in terms of affecting the flotations, either. For  instance, Al³⁺ and Fe³⁺ can strongly inhibit the flotation of cyanite, while Ca²⁺ and Mg²⁺ can activate cyanite in the meantime [23].

2.1.2 Radius and valence state of metal ions

The flotation results are closely associated with the radius and valence states of involved metal ions. Indeed, different metal ions have different radii, even if they have the same valences. ZACHARA et al [24] summarized the order of the adsorption capacity of divalent ions on the calcite surfaces: Cd²⁺>Zn²⁺≥Mn²⁺>Co²⁺>Ni²⁺>>Ba²⁺=Sr²⁺, which is more or less consistent with the atom radius order. Theoretically, the adsorption capacity of one certain metal ion is closely related to the hydration energy, which is calculated by the following equation:

E_hyd=E_{metal ion-water}–(E_{metal ion}+E_water)            (1)

where E_{metal ion-water} refers to the energy of the metal ion-water cluster; E_{metal ion} and E_water denote the energy of the metal ion and water system, respectively [25].

Metal ions with high hydration energies exert relatively small adsorption capabilities on calcite surfaces. In contrast, metal ions with low hydration energies tend to form hydroxyl complexes and precipitations on the calcite surface, causing mineral surfaces hydrophobic. Furthermore, it is generally considered that larger atoms possess higher hydration energy, which can be used to illuminate the order mentioned above.

In addition, the ionic radius is an essential factor influencing the ion-exchange in mineral surface lattices. DEMIR et al [26] studied the flotation separation of potassium feldspar and sodium feldspar, elaborating that Ca²⁺ can replace Na⁺ in the crystal lattice of Na-feldspar, while Ba²⁺ can replace K⁺ in the crystal lattice of K-feldspar. The primary reason is that the ionic radii of Na and Ca cations (0.98 and 0.99 ) and those of K and Ba cations (1.33 and 1.38 ) are incredibly close. After the replacement of Na⁺ by Ca²⁺ on the Na-feldspar surface, its electronegativity will decrease, rendering a weak flotation recovery.

2.1.3 Concentration of metal ions

Metal ion concentration is another crucial factor. MENG et al [27] studied the activation by lead ions in the ilmenite flotation using benzyl hydroxamic acid (BHA) as the collector. They found that when using 120 mg/L BHA at pH 8.0, the adsorption amount of BHA on ilmenite surfaces increased with increased lead ion concentration. When the Pb²⁺ concentration was higher than 2.0×10^-4 mol/L, both recovery and BHA adsorption amount increased slowly prior to reaching a plateau.

Metal ions can also cause an inhibitory effect on minerals, especially at high concentrations. It is reported that Mg²⁺, Ca²⁺ and K⁺ significantly enhance the adsorption of CMC onto talc by changing the adsorption morphology [28]. The increase of Mg²⁺, Ca²⁺ and K⁺ concentrations caused the coiling of the carbon chain in CMC, leaving more space for further CMC adsorption, and consequently enhancing the CMC coverage on the talc surface.

2.2 Indirect factors

2.2.1 Minerals in slurry

When different minerals exist in the slurry, the roles of metal ions in flotation may be altered, even reversed. For example, in the flotation separation of zircon and cassiterite using NaOL as the collector, Fe³⁺ can depress the cassiterite flotation while it can activate the zircon flotation at the same time [6].

The underlying mechanism might lie in the mineral crystal structure, which is recently attracting intense attention. Different crystal faces of the same mineral will result in difference in polarity, electrical properties, and solubility, which is known as anisotropy [29-32]. For instance, the activation of sphalerite (ZnS) flotation is commonly seen using Cu²⁺ [33,34]. Still, the activation of smithsonite, another Zn-containing mineral, by Cu²⁺ has yet been reported, which is related to the bond types and coordination numbers of Zn in crystal structures of mineral surfaces. Additionally, element substitutions can cause structural changes in minerals and then flotation behaviors. For example, BOULTON et al [35] found that iron ions substituted in the mineral lattice could weaken the activation of sphalerite. Besides, the presence of other minerals may affect the impact of metal ions on flotation. FINCH et al [36] investigated the pyrite floatability using Cu²⁺, Fe³⁺, and Ca²⁺ ions as activators, and they concluded that these metal ions could activate pyrite flotation. However, the system is required to be free of sphalerite.

2.2.2 Solution pH

The pH of the slurry represents the relative content of H⁺ and OH^- in the solution, which can affect the existing forms of metal ions and the surface electrical properties of minerals. To achieve the best out of metal ions in flotations, there are specific optimal pH ranges, as summarized in Table 2.

Table 2 Preferred pH range for mineral flotation in the presence of metal ions

Generally, pH can affect the impact of metal ions on flotation in two ways. For one thing, the metal ion species in solution varies at different pH values. Under the optimal pH range, the active metal ion species will distribute in their appropriate forms. Take the activation of sphalerite by Pb²⁺ as an example, the activating entity is lead ion on mildly acidic conditions while that changes to lead hydroxide on mildly alkaline conditions. RASHCHI et al [52] found that lead activation on sphalerite flotation is dependent on pH values. Specifically, Pb²⁺ is the dominant species when pH≤7, where sphalerite can be effectively activated. While at pH 10 and 11, the dominant species shifts to Pb(OH)_2,s, where the activation ceases.

It is suggested that activation reactions can vary with pH values. Table 3 shows the comparison of activation steps in sphalerite flotation at different pH values. The activation is remarkable in acidic media, as metal ions tend to be fixed on sphalerites surface by lattice substitution with Zn²⁺. While in a weak alkaline medium, metal ions adsorb on sphalerite surfaces in the form of sulfide precipitate, which is easily redissolved into the solution. Therefore, the optimum pH for metal ion activation of sphalerite is not in the weak alkaline range.

Table 3 Comparison of activation steps of sphalerite flotation by metal ion at different pH

2.2.3 Flotation agents

The joint agents, mainly including collectors, regulators, depressants, or inhibitors in flotation, also affect the roles of metal ions. Because of the interactions between agents and metal ions, metal ions can promote the adsorption of anionic collectors on the mineral surfaces but weaken the interaction between cationic collectors and minerals [37]. Apart from these, metal ions alter the interaction between inhibitors and minerals. Dextrin, a standard inhibitor, was barely adsorbed on pyrite surfaces. However, in the presence of Fe²⁺ and Pb²⁺, its adsorption amount reached 0.45 and 0.3 mg/g, respectively, indicating that some metal ions could promote the adsorption of dextrin, probably via the formation of metal ion-dextrin complexes [55].

The collectors can also modify the distribution of metal ion species in slurries. Spectral analysis shows that flotation agents can form chelates with metal ions [56]. For instance, BHA can chelate with Pb²⁺ to form a five-membered ring structure, which in turn reduces the concentration of metal ions in the slurry [57,58]. Besides, in the pyrite flotation system, the total iron concentration detected in the slurry was doubled with the addition of xanthate [59], indicating the collector might facilitate the extraction of metal ions from the minerals.

2.2.4 Operations

Operations like the adding sequence of metal ions and flotation reagents have been recently found to have a significant impact on flotation. Typically, the metal ions are added into the flotation system prior to the addition of reagents. However, when premixing metal ions and reagents to form complexes, a more vital collecting ability couple with a higher selectivity have been frequently reported in the past few years.

TIAN et al [56,57] found a soluble lead-BHA complex, obtained by premixing lead nitrate and BHA in a separated container, performed much better in the cassiterite flotation, compared with the traditional sequential addition of lead nitrate (LN) and BHA. HAN et al [60] observed the adsorption amount and intensity of BHA on scheelite surfaces were more considerable when adding the lead-BHA complex. Other researchers followed similar tracks and confirmed conclusions by launching the flotation of scheelite, ilmenite, and titanaugite [44,61,62].

3 Underlying mechanisms

3.1 Oxide and silicate minerals

Oxide and silicate minerals are widely distributed in the Earth’s crust. When it comes to revealing the interactions between metal ions and minerals, they can be discussed using the same electrostatic model [63,64]. In this area, two classic mechanisms of interactions between metal ions and minerals have been well-appreciated.

The first traditional mechanism comprises two steps. Its schematic is provided in Fig. 2. Firstly, metal ions react with OH^- on mineral surfaces, releasing water molecules and forming hydroxide species, namely active sites. Subsequently, collectors adsorb the active sites to render the surface hydrophobic.

FUERSTENAU and RAGHAVAN [9] suggested that metal ions could react with OH ^- in the pulp and adsorb on minerals surface as a metal cation complex, forming an active site that can interact with the flotation agents, consequently adjusting the hydrophobicity of the mineral surface.

However, some researchers [65] hold a different view and proposed an alternative mechanism: the adsorption of metal ions on mineral surfaces and the subsequent activation of flotation cannot be solely attributed to the formation of a hydroxy complex. Instead, the formation of metal hydroxide surface precipitation should be a more effective way, as shown in Fig. 3.

JAMES and HEALY [66] reported that the solubility of the metal hydroxide produced at the mineral-water interface is smaller than that in water due to a more modest change of Gibbs free energy. Therefore, metal hydroxides are easily formed at the mineral-water interface. A summary of commonly-seen metal hydroxides is given in Table 4.

Both mechanisms can explain the roles of metal ions in most of the flotation processes; nevertheless, with observations of some exceptional phenomena, other means have been proposed. For instance, FUERSTENAU et al [74,75] reported cation sulfonates, functioning as the collector, precipitated immediately in the flotation of beryl and quartz. Recent researches [56,76] demonstrated that a soluble metal-organic complex (lead-BHA complex), produced by premixing lead nitrate and BHA, exhibited an enhanced collecting ability and selectivity in the flotation of cassiterite. A novel activation model was then proposed: metal ions can directly react with the collector and form a cation-collector complex or precipitation, which serves as the “real” collector in the flotation, as illustrated in Fig. 4, where cassiterite is taken as an example. This mechanism is in good agreement with the results in other reports [43,57,77].

Fig. 2 Formation of metal ion-hydroxy complex on mineral surface

Fig. 3 Formation of metal hydroxide precipitation on mineral surface

Table 4 Active components of different metal ions in flotation system of oxide and silicate minerals

3.2 Sulfide minerals

Fig. 4 Formation of metal ion-collector complex as actual collector on mineral surface [57]

The flotation of sulfide minerals has been a research hotspot for more than a century. It is widely known that most of the sulfide minerals, like pyrite, chalcopyrite, and galena, are readily floated in the presence of thiol collectors [78]. However, sphalerite responds poorly to the short-chain thiol collectors. To enhance its flotation, some metal ions, such as Cu²⁺, Pb²⁺, Cd²⁺, Fe²⁺, are applied to activating sphalerite. Simultaneously, some sulfide minerals can be inadvertently activated by metal ions, which might be detrimental to the selective separation of sphalerite from other sulfide minerals, especially pyrite. The activation often occurs in either acidic or alkaline circuits, seldomly at a neutral pH. The formation of ternary surface complexes (M-OH-X) at the mineral surface is probably the main reason for the inhibition at neutral pH [79-81]. Other researchers reported similar results. LASKOWSKI et al [82] attributed the low recovery at nearly neutral pH to the formation of CuOH⁺. The activation mechanism generally consists of two processes, i.e., ion-exchange reaction and electrochemical reaction, based on some comprehensive reviews [83-86]. In acidic pH pulp, the ion-exchange reaction can be expressed as

MeS+M²⁺→MS+Me²⁺                              (2)

where MeS is the sulfide on the mineral surface to be activated, and MS is the product of the ion exchange. It has been well documented that the exchange ratio, defined as R=[M²⁺]_exchanged/ [Me²⁺]_exchanged [53,54], is close to unity [34,85-87]. The activation of sphalerite in flotation has been well investigated and will be taken as an example in this section. It is generally agreed that the ion-exchange reaction between Cu²⁺ and sphalerite can be written as

ZnS_(surf)+→CuS_(surf)+            (3)

which is thermodynamically favorable [82]. However, it is a different scenario for the alkaline pulp, where precipitation would appear in the form of the activator-metal hydroxide on sphalerite surfaces. Then, precipitation would be converted to the thermodynamically stable substance through an ion-exchange reaction [83,85], as

ZnS_s++2H₂O→ZnS_sCu(OH)_2surf +2H⁺       (4)

Cu(OH)_2,ads+ZnS_surf→CuS_surf+Zn(OH)_2,ads       (5)

or,

yZnS+xCu²⁺+2x(OH^-)yZnS+xCu(OH)₂

yZnS·xCu(OH)₂                                    (6)

yZnS·xCu(OH)₂(y-x)(ZnS)xCu²⁺S²⁺xZn(OH)₂  (7)

Apart from ion-exchange, electrochemical reactions are also taking effect. For example, divalent Cu²⁺ ions adsorbed on the sphalerite surface tend to be reduced to monovalent Cu⁺. It has been continuously debated what the final copper activation products are. Here we provide a comprehensive summary for the readers’ information, as shown in Table 5.

Table 5 Products of copper activation on sphalerite surface

These activation products then create the complex of Cu-xanthate after interacting with xanthate, leaving sphalerite surfaces hydrophobic. However, some researchers believe that Cu(OH)₂ directly interacts with xanthate and decomposes to form Cu-xanthate and dixanthogen [34]. More pieces of evidence should be collected to settle the debate with confidence.

4 Research approaches

In this part, various research methods are summarized, which can help to attain a comprehensive and profound understanding of how metal ions take effects in flotation in both macroscopical and microcosmic aspects.

4.1 Micro-flotation tests

Prior to the industrial applications, micro- flotations, or lab-scale flotations, dealing with a feed of 0.002-1 kg, are usually launched to determine a reasonable flowsheet. In this section, key messages are provided considering variables like metal ion dosages, types, valence states, as well as reagents like collectors and depressants.

Micro-flotations show that metal ions can promote the adsorption of inhibitors on the mineral surface, causing a more hydrophilic surface [28,91]. For instance, Ca²⁺ can efficiently enhance the inhibition of talc by CMC in flotation [92]. Figure 5 presents the talc flotation recoveries applying Ca²⁺concentrations of 0, 1×10^-4, 1×10^-3, 1×10^-2 mol/L as a function of CMC dosage at pH 8.5. With the increase of CMC concentration from 0 to 50 mg/L, the flotation recoveries decreased from 93% to 30% when the Ca²⁺ concentration was above 1×10^-3 mol/L. The results elucidate that CMC has a limited inhibitory effect on talc  flotation; however, the depression can be effectively reinforced in the presence of Ca²⁺.

Fig. 5 Recovery of talc in the presence of various Ca²⁺ concentrations as function of CMC dosage at pH 8.5 [92]

On the other hand, the metal ions can  facilitate the adsorption of collectors on the mineral surfaces [93-95]. A series of single-mineral flotation experiments were conducted by TIAN et al [43] to investigate the effect of Fe³⁺ on the flotation of cassiterite using BHA as the collector. As shown in Fig. 6, the recovery of cassiterite reached the highest at pH 7, only 30%, in the presence of BHA. In contrast, it increased significantly after adding 10 mg/L ferric chloride, and the highest recovery turned into 80%. It shows that the floatability of cassiterite can be improved by Fe³⁺.

Fig. 6 Flotation recovery of cassiterite as function of pH [43]

4.2 Contact angle measurement

Contact angle measurement is a technique for characterizing solid surface hydrophobicity [96]. To quantify the change of silica surface hydrophobicity, LIU et al [42] measured contact angles of the silica surface on the conditions of different lead ion and potassium amyl xanthate concentrations, and the results are shown in Table 6. It can be seen that a higher contact angle appeared with the increase of lead ion concentration, which means that lead ions may facilitate the xanthate adsorption on the silica surface so that the silica surface becomes more hydrophobicity.

Table 6 Sessile drop contact angles on silica surface for various lead and potassium amyl xanthate concentrations at different pH values (lead concentration equals the amyl xanthate concentration) [42]

4.3 Zeta potential measurement

The zeta potential measurement is a conventional method to determine the electrokinetic properties on mineral surfaces. Zeta potential is an excellent indicator of the distribution of metal ions near mineral surfaces since it will vary with electrolyte concentrations. For instance, the experimental zeta potential and theoretical surface charge of TiO₂ change with KNO₃ concentrations, as depicted in Fig. 7.

Fig. 7 Comparison of experimental data for zeta potential and surface charge with theoretical predictions of TiO₂ in KNO₃ solutions with different concentrations [97]

To fundamentally understand the relations between metal ions and zeta potentials, one should first understand the establishment of the surface charge. There are two well-known mechanisms.

(1) Hydrogen or hydroxyl ions adsorbing towards mineral surfaces

     (8)

(2) Forming hydroxylated metal species and depositing on mineral surfaces

                     (9)

          (10)

    (11)

In both mechanisms, concentrations of metal ions, hydrogen, or hydroxyl ions will affect the surface potential’s final state, consequently influencing the flotation. Zeta potentials were usually obtained as a function of pH [98], and their differences before and after adding metal ions or reagents hold the key to interactions, especially the electrostatic interaction.

Explicitly, the interactions between metal ions, reagents, and mineral surfaces can be represented by the shifts of zeta potentials [99-101]. For example, in order to study the influence of Fe³⁺ on the cassiterite flotation, TIAN et al [43] carried out a series of zeta potential measurements, and the results are shown in Fig. 8. In the presence of 10 mg/L ferric chloride, the negative shift of zeta potential of cassiterite sharply increased, indicating that Fe³⁺ can actively enhance the adsorption of BHA on the cassiterite surface. And zeta potential results were also consistent with flotation experiments in Fig. 6 [43] .

Fig. 8 Zeta potential of cassiterite in the presence and absence of Fe³⁺ as function of pH in the absence and presence of BHA [43]

The zeta potential measurements by FENG et al [102] suggested that the addition of lead ion negatively shifted the zeta potential of cassiterite in the presence of SHA, as shown in Fig. 9. The finding confirms the enhanced adsorption density of SHA on the cassiterite surfaces preoccupied by lead ions.

4.4 Adsorption amount of measurement

Some researchers attempted to quantitively characterize the interaction intensity between collectors and mineral surfaces by launching the adsorption tests [101,103]. The adsorption mechanisms are proposed to be hydrolysis and precipitation on mineral surfaces, referring to the zeta potential section. Some features in the adsorptions are worth being highlighted in this review.

Fig. 9 Zeta potential of cassiterite in the presence and absence of lead ions as function of pH in the absence and presence of SHA [102]

(1) The adsorption rate increases from zero to 100% in a critical pH range, which is usually less than 100%. The reason is due to the change of free energy of adsorption, written as

          (12)

where i notes species type, △G⁰_ads is the overall adsorption free energy, △G⁰_coul is the free energy of Coulomb’s interaction (electrostatic interaction), △G⁰_solv is the free energy of solvation, and △G⁰_chem is the free energy of the chemical reaction.

The △G⁰_ads has to be negative for the adsorption to be spontaneous to occur. It is necessary to calculate the individual item to obtain the △G⁰_ads.

Take the adsorption of Co²⁺ on the SiO₂ surface as an example:

                 (13)

            (14)

    (15)

Clearly, both △G⁰_coul and △G⁰_chem are negative; thus, the sign of △G⁰_solv will govern the overall free energy, and △G⁰_solv can be expressed as

  (16)

where ψ_δ is obtained by the Gouy-Chapman expression with a surface potential given by the Nernst equation, ε is the permittivity, r is atom radius, z is the valence, and e is the electric charge unit, equal to 1.602176634×10^-19 C.

Successful adsorptions of metal ions occur at the inner Helmholtz plane (IHP), as shown in Fig. 10. For the highly charged species, the solvation energy needed for the ion to penetrate to IHP is so enormous that the adsorption will not occur. In other words, the adsorption of metal ions will not start until a pH is reached where the lower charged hydroxylated species M(OH)^(n-1)+ begins to appear.

Fig. 10 Schematic of hydrated cation adsorbed at IHP [104]

Fig. 11 Experimental adsorption isotherm for Co²⁺ adsorption on silica surface [66]

(2) Species types vary with changing pH, and an example is provided in Fig. 11.

(3) The adsorption is not correlated with ion concentration. In contrast, the amount of adsorption for the surface to be saturated corresponds to an ion layer forming at least one hydration sheath.

(4) Adsorption of ions is affected by the surface charges of the mineral; however, the adsorption behavior varies for different minerals even with the same charge. For instance, cobalt ion can adsorb towards TiO₂ even when the surface is slightly positively charged, while it can only adsorb on SiO₂ when its surface is highly negatively charged. On the other hand, the surface charge can be modified and even reversed due to the metal ion adsorption.

(5) The isoelectric point (IEP) of oxide is not influenced by metal ions, indicating that the adsorption type is not chemisorption.

In addition, metal ions are proven to promote the adsorption of collectors on mineral surfaces. TIAN et al [43] found that Fe³⁺ can benefit the adsorption of agents on cassiterite surfaces. As depicted in Fig. 12, the adsorption amount of BHA on the cassiterite surface in the presence of ferric chloride was almost three times as high as that in the absence of ferric chloride, which is in line with the results of flotations and zeta potential measurements in Figs. 6 and 8, respectively.

Fig. 12 Adsorption amount of BHA on cassiterite surface as function of BHA concentration in the absence and presence of ferric chloride [43]

Metal ions in the optimum concentration range could facilitate the flotation. SONG et al [105] found that the activation of Ba²⁺ at a low concentration on the flotation of K-feldspar using dodecyl amine chloride as the collector was caused by the ion exchange, which increased the lattice vacancies and adsorption sites on the mineral surface. Their results are provided in Fig. 13. The recovery of K-feldspar increases from 86.73% to 92.21% as the Ba²⁺ concentration is increased to 1.1×10^-2 mol/L. After that, the recovery of K-feldspar decreases. The optimum Ba²⁺ concentration to enhance the flotation of K-feldspar is 1.1×10^-2 mol/L. Once it is above or below this concentration, flotation will be inhibited due to the decreased adsorption sites.

Fig. 13 Recovery rate of K-feldspar versus BaCl₂ concentration at 10 mg/L of dodecyl amine chloride addition (a) and effect of BaCl₂ concentration on adsorption capacity of Ba²⁺ on surface of K-feldspar (b) [105]

4.5 Imaging analysis techniques

To further extract details of the morphology mineral surfaces in the absence and presence of metal ions, researchers utilize imaging analysis approaches like atomic force microscopy (AFM) and scanning electron microscope (SEM) to visualize the dynamic interaction.

SEM image can display a two-dimensional morphology of the sample surface, via which one can ascertain crucial information, such as the particle size and low dimensional morphology of the mineral surface. YANG et al [106] investigated how the flotation of molybdenite was significantly depressed by Cu²⁺ and Fe³⁺. They found that Cu and Fe elements evenly and intensively distributed on the molybdenite surface from the EDX mapping in Fig. 14. This is a good piece of evidence proving that a large amount of Cu and Fe species were adsorbed on the molybdenite surface, leading to depression of molybdenite flotation.

Additionally, three-dimensional morphology and flatness of the surface can be obtained by AFM. JIN et al [69,92] revealed that the Ca²⁺ concentration had a significant impact on the morphology of CMC adsorbed on the talc surface. Height and phase are typical messages when analyzing the AFM results: The former represents the straightforward topography of the adsorbed layers, and the latter indicates the hard or soft nature. The AFM images of CMC in the presence and absence of different Ca²⁺ ions concentrations can be seen in Figs. 15-17.

From the images above, when CMC was added alone, the morphology of the talc surface presented the random distribution of salient points, occupying a low coverage. In contrast, salient points were combined, and reticulated adsorption appeared in the presence of 10^-4 mol/L Ca²⁺. Moreover, reticulate multilayer adsorption with a high coverage occurred when increasing Ca²⁺ concentration to 10^-3 and 10^-2 mol/L Ca²⁺. The results prove that CMC adsorption on the talc surface can be facilitated when pretreated with Ca²⁺, consequently enhancing the depression of talc by CMC.

Fig. 14 SEM images of molybdenite sample conditioned in Cu²⁺(a) and Fe³⁺(b) solution [106]

Fig. 15 AFM images of bare talc

Fig. 16 AFM images of adsorbed CMC (100 mg/L) on talc surface in the absence of ions

4.6 Surface composition analysis techniques

Surface analysis techniques are also applied to identifying the active metal ion species on mineral surfaces. Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS) are the most widely used methods [107]. Besides, time-of-flight secondary ion mass spectrometry (ToF-SIMS), extended X-ray absorption fine structure (EXAFS), near edge X-ray absorption fine structure (NEXAFS), and the high-resolution AFM have been intensively utilized to visualize the surface components [108].

LIU et al [109] investigated the effects of Ca²⁺ and Mg²⁺ ions on the flotation of spodumene using NaOL as the collector by FT-IR. The results are shown in Figs. 18 and 19.

As shown in the results, new absorption peaks at 1634.75, 2928.49, and 2857.20 cm^-1 were observed in the spectra of activated spodumene after the addition of Ca²⁺ and Mg²⁺ ions, which manifested the appearance of new species on the mineral surfaces. The peaks were assigned to hydroxy complexes (CaOH⁺ and MgOH⁺) and precipitates (Ca(OH)₂ and Mg(OH)₂).

Mineral surfaces can be elementarily detected by measuring the change in chemical bonds under the FT-IR analysis. But in order to get a better understanding of the reaction mechanism, XPS could be a more powerful tool. CHEN et al [110] detected oxygen and iron on the ilmenite surface and sequentially obtained the status and the relative abundance of the elements. The results are shown in Tables 7 and 8.

In Table 7, a Fe-O-Pb complex was observed at the new peak at about 712.4 eV, which will serve as the activated sites. Similarly, in Table 8, the new peak was obtained at 532.3 eV, which was also contributed to Fe-O-Pb complex. Both results illustrate that the lead ions adsorbed onto the ilmenite surface by reacting with Fe-O-H, imparting active sites for sodium oleate and rendering a surface hydrophobic.

4.7 Computational method

Fig. 17 AFM height and phase images of adsorbed CMC (100 mg/L) on talc surface in the presence of 10^-4 mol/L (a), 10^-3 mol/L (b), and 10^-2mol/L (c) Ca²⁺, at pH 8.5 [69]

Fig. 18 FT-IR spectra of spodumene (a) and NaOL (b) [109]

In order to further explore the activation mechanism of metal ions, researchers have built the adsorption models for the interaction of metal ions and agents, calculating adsorption energies by the first principle calculations based on the density functional theory (DFT).

Fig. 19 FT-IR spectra of spodumene with Ca (II) ions at pH 12.5 (a), with Ca (II) ions and NaOL at pH 12.5 (b), with Mg (II) ions at pH 10.0 (c), and with Mg (II) ions and NaOL at pH 10.0 (d) [109]

DFT calculations performed by YU et al [111] proved that the precipitation of calcium hydroxide (Ca(OH)₂) on the spodumene surface is the dominant active species. They positioned Ca²⁺, CaOH⁺ and Ca (OH)₂ on the spodumene (110) surface after geometry optimizations and then calculated relevant parameters by Cambridge Serial Total Energy Package (CASTEP). The calculation results can be found in Table 9. It shows that the adsorption energies of Ca²⁺, CaOH⁺, and Ca(OH)₂ on the spodumene surface are -187.14, -350.81, and -523.31 kJ/mol, respectively, confirming that Ca(OH)₂ is the steadiest and the most active species.

Table 7 Parameters of elemental Fe 2p and chemical status on ilmenite surface treated with various conditions [110]

Table 8 Parameters of elemental O 1s and chemical status on ilmenite surface treated with various conditions [110]

Table 9 Adsorption energy of three adsorbates on spodumene surface and formed Ca-O bond length [111]

Besides, researchers compared interactions between collector and mineral surface in the absence and presence of metal ions. In terms of the simulations of the interactions among sphalerite (110) surface, copper, and ethyl xanthate (EX), LIU et al [112] summarized four types of stable interaction models, as outlined in Fig. 20 and  Table 10. The function of Cu²⁺ as a bridge is vividly visualized in the schematics in Fig. 20. More importantly, Fig. 20(d) shows that Cu(OH)₂ can adsorb on the ZnS surface prior to serving as the bridge. From Table 10, the interaction energies (△E_int) of EX and activated sphalerite surfaces are more negative than that of EX and the bare sphalerite surface (-87.63 kJ/mol), indicating that it is easier for EX to adsorb onto the Cu-activated sphalerite surface than the bare surface.

4.8 Solution chemistry calculation

It is well-known that the species distribution of metal ions and flotation reagents in pulp is dissimilar at different pH values and concentrations. Therefore, solution chemical calculation method is needed to describe the chemical species, especially the significant active components of metal ions and reagents in the flotation system. The calculation results are usually presented as species distribution diagrams.

TIAN et al [57] found that a high flotation recovery could be obtained when using lead-BHA complex as collector at pH 8-9, and the species diagrams of lead ions and BHA were depicted in Fig. 21. Pb²⁺ ions usually interact with surrounding water molecules to form hydrated lead ions. At pH 6-8, Pb(H₂O)₆²⁺ is the dominant species in the lead nitrate solution. While Pb(OH)(H₂O) ₅⁺ becomes the dominant species at pH 8-10. As for BHA, molecular BHA and BHA^- are the predominant forms at pH 6-8 and 8-9, respectively. Based on the chemistry calculations, the primary active components of the lead-BHA complex at pH 8-9 could be HO-Pb-BHA.

4.9 Cyclic voltammetry

Due to the electrical semi-conductivity, redox reactions frequently occur on the surfaces of many sulfide minerals in an oxygenated environment. In some cases, the sulfide minerals are poorly collected by short-chain thiol collectors without the treatment of activation by metal ions. The cyclic voltammetry, an electrochemical method for quantitative analysis of the parameters like voltage and current during electrolysis, is introduced to reveal the electrical characteristics during the redox reactions involving metal ions.

Fig. 20 Schematic of EX interaction with Cu-activated ZnS (110) surface

Table 10 Interaction energy and bond length of Cu—S for four interaction models [112]

The activation products by metal ions can be deduced from the anodic and cathodic waves. CHEN and YOON [113] found that a CuS-like activation product was formed on the surface of sphalerite activated by 1×10^-4 mol/L CuSO₄, as shown in Fig. 22.

The anodic wave (A₁) shown in Fig. 22(a) represents the oxidation of the activation products. And the cathodic wave (C₂) shown in Fig. 22(b) represents the reduction of the activation product. The detailed reactions are shown in Table 11. Activation reactions of metal ions on sulfide surface can be quantitatively revealed: compared to a thermodynamic potential of 0.303 V for CuS to be oxidized to S⁰, it is simultaneously oxidized to CuS_n as the intermediate step. This cyclic voltammetry provides a straightforward way to understand the activation mechanism of metal ions in sulfide flotations.

Fig. 21 Species distribution diagrams of lead (a) and BHA (b) in aqueous solution [57]

Fig. 22 Voltammograms of CMC

Table 11 Activation reactions occurring under anodic and cathodic wave [113]

5 Conclusions and future directions

For the mining industry, understanding and utilizing the roles of metal ions in flotation are of both fundamental and practical importance. Thanks to substantial progress that has been accomplished, we are able to comprehensively address this topic here.

The main factors affecting the roles of metal ions in flotations fall into two categories: direct and indirect. Direct factors include metal ion type and valence, radius and valence state, concentration. On the other hand, minerals in the slurry, solution pH, flotation agents, and operations of adding metal ions are placed in the category of indirect factors.

Mechanisms of interactions between metal ions and minerals vary with mineral types. For silicate and oxide minerals, the first possible mechanism contains two steps: the cation and its hydroxide species react with hydroxy on the mineral surface to form activated sites initially; subsequently, the collectors adsorb the active sites to render the surface hydrophobic. An updated assumptive mechanism was proposed in recent years: Metal ions can react directly with the collector and form a cation-collector complex or precipitation, which functions as the real collector in the flotation. As for sulfide minerals, ion-exchange and oxidation-reduction govern the interactions between metal ions and minerals.

Research approaches like micro-flotation tests, contact angle measurement, zeta potential measurement, adsorption measurement, AFM, SEM, FT-IR, XPS, DFT, calculation of chemical solution, and cyclic voltammetry have been used to investigate the roles of metal ions from both macroscopic and microcosmic perspectives. And findings from one method are in good agreement with others.

However, there is still a lack of technical means to detect specific active sites and forces on mineral surfaces in actual flotation, which is prerequired to quantify interactions between metal ions and minerals as well as flotation agents. Besides, in order to better capitalize the roles of metal ions in flotation and benefit the mining industry, further explorations, notably the real-time detection of metal ions, are in urgent need. Furthermore, endeavors in building simulation models with high accuracies are required to conveniently reconstruct the relevant reaction processes.

Acknowledgments

The authors acknowledge financial supports from the National Natural Science Foundation of China (Nos. U2067201, 51774328), the Key Program for International S & T Cooperation Projects of China (No. 2021YFE0106800), the Science Fund for Distinguished Young Scholars of Hunan Province, China (No. 2020JJ2044), the Young Elite Scientists Sponsorship Program by Hunan province of China (No. 2018RS3011), and the National 111 Project of China (No. B14034). The authors also wish to thank Dr. JOHN RALSTON for the early guidance on this manuscript. And the authors also much appreciate the help from Mr. HASSAN RAZA for improving the language.

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金属离子在矿物浮选中的典型作用

高志勇^1,2，江哲伊^1,2，孙 伟^1,2，高跃升³

1. 中南大学 资源加工与生物工程学院，长沙 410083；

2. 中南大学 战略含钙矿物资源清洁高效利用湖南省重点实验室，长沙 410083；

3. Department of Chemical Engineering, Michigan Technological University, Houghton 49931, United States

摘  要：在矿物浮选中，金属离子可以通过选择性活化或抑制矿物表面，从而发挥重要的作用。尽管此前大量工作报道了金属离子在浮选中的作用，但大多仅是针对某一特定类型金属离子或目的矿物进行研究分析，缺乏一个全面的综述。本文对金属离子在矿物浮选中的影响因素进行详细分类，总结金属离子在不同种类矿物浮选中的作用机理。此外，还对常见探索作用机理的研究方法，包括经典的浮选、接触角测量、动电位测量，以及近年来流行的模拟计算、溶液化学计算和循环伏安等，进行梳理和分析。本文全面综述了金属离子在矿物浮选中的作用，发现金属离子的添加与去除、溶液化学环境改变、浮选药剂的选择等都可调控金属离子对矿物浮选的影响，对于精确调控矿物浮选具有重要指导意义。

关键词：金属离子；浮选；氧化矿；硅酸盐矿；硫化矿

(Edited by Xiang-qun LI)

Corresponding author: Yue-sheng GAO, E-mail: ygao4@mtu.edu

DOI: 10.1016/S1003-6326(21)65640-6

1003-6326/ 2021 The Nonferrous Metals Society of China. Published by Elsevier Ltd & Science Press