Introduction

Calcite is one of the most abundant simple salts in the earth’s surface and in sedimentary rocks; it occurs in the form of armours and skeletal frames of marine creatures. It is the prime constituent of limestone, chalk etc. In igneous rocks, calcites are present in the form of fine-to-coarse grains. Carbonatites, kimberlites, and lamprophyres are some of the alkaline igneous rocks, which contain more than 50% of carbonates, especially calcites (Chang et al. 1996). It is therefore said that calcite is one of the most abundant simple carbonate salts on the earth’s surface as it is found in almost all kinds of rock samples. The most important carbonate minerals belonging to the calcite group include magnesite, rhodochrosite, siderite, and smithsonite.

Calcite has the chemical formula CaCO3, and the chemical structure is shown in Fig. 1. To be specific, calcite is composed of a basic structure of one atom of calcium, one atom of carbon and three atoms of oxygen. Calcite is colourless or white when pure but may contain any colour, i.e. red, pink, yellow, green, blue, lavender, black, or brown, which owing to the presence of diverse impurities. Thus, it implies calcite may be transparent, translucent, or opaque. Its lustre ranges from vitreous to dull; many crystals, especially the colourless ones, are vitreous, whereas granular masses, especially those that are fine-grained, tend to be dull. Additionally, the specific gravity of calcite is approximately 2.71.

Fig. 1
figure 1

Chemical structure of calcite (redrawn from Pubchem 1977)

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The three important crystal occurrences (distinctive shapes of the mineral) of calcite are (1) prismatic (both short and long), (2) rhombohedral, and (3) scalenohedral. Twinning is a common property in crystalline limestones. Some calcites fluoresce under ultraviolet light; some are also triboluminescent (luminescent when scratched); and some other samples of calcite show birefringence (Dietrich 2013).

Regarding applications of these calcium-containing minerals, calcium carbonate, silica, wollastonite, kaolin, and talc, etc. are used as fillers in various industrial sectors with specific quality requirements and cost. Calcite is a valuable industrial mineral used in the production of value-added products such as paper, polymers, steel, and paint. The calcite quality requirements for these industries are presented below.

The Paper Industry

Precipitated or beneficiated calcium carbonate is widely used as a filler and coating material in the manufacture of paper. For coating applications, the desired product should have a brightness of 93 to 95%. An Einlehner abrasiveness number in the range of 18 to 23 is required for use as a filler and for coating purposes. The acid-insoluble content should be below 0.5 wt% to be commercially acceptable. Paper production requires calcite possessing as uniform particle size distribution as is possible. A minimum of 75 wt% of the particles should have an equivalent spherical diameter of less than 1 μm or 5 μm, for coating and filler purposes, respectively. This prerequisite emphasizes the importance of fine-grinding techniques for successful utilization of calcite in the paper market.

Polymer Composites

Minerals are used as fillers in polyolefinic composites. They are added to a polymer matrix to improve the properties of the host polymer such as stiffness, opacity, environmental stability, gas permeability, and electrical conductivity. Calcium carbonate is the most widely used filler for thermoplastics and composite materials. In mineral-filled polymer composites, it is important to disperse particles individually in the matrix. Separation of particles from aggregates and promoting the wetting of particles by the polymer are the most important factors. This is achieved by matching the calcite surface properties to those of the matrix into which the dispersion is to take place. Stearic acid or calcium stearate is commonly used for surface modification of calcium carbonate.

Paint Industry

Calcium carbonate is the most widely used extender pigment by the surface coating industry; other large users are the rubber and plastic industries. It is used throughout the range of water- and solvent-based paints for interior and exterior applications. The properties exhibited vary with the type of extender used and depend on individual physical characteristics and, in the case of naturally occurring grades, on the crystal form of the minerals from which they are derived, the presence of impurities, and the degree of refinement. The pigment particles’ crystal form, size, shape, and size distribution affect the pigment’s film-packing properties; oil absorption; film strength and permeability; and surface texture as well as the dispersion, settling, and rheology characteristics.

The Steel Industry

Iron ore pellets are stable and homogenous and therefore easy to transport and load into the blast furnace. Limestone (both low and high grade) and lime are employed in the preparation of these iron ore pellets. An alternative raw material for the blast furnace is iron ore sinter. Limestone and lime added to the sinter react with impurities and balance the slag composition of pig iron production.

Calcite occurs in nature either as high-grade (with > 75% calcite) or low-grade calcite ores (with < 30% calcite). Generally other carbonate, phosphate, silicate, and iron sulphide minerals are associated with the calcite ores. Gravity and/or magnetic separation and flotation are the beneficiation techniques most commonly applied to calcite ores. However, flotation is the most effective solution in industries, both technologically and economically, when upgrading calcite ores. The reasons are (1) flotation is highly efficient as compared with the physical separation processes. (2) Magnetic separation proved potential for removal of iron sulphide impurities; however, cases, the iron sulphide grains are deeply disseminated within the calcite matrix, cases, magnetic separation may not be employed as the sole separation process.

Flotation is primarily based on differences in physicochemical properties of the minerals. Surface properties play an important role in establishing the electrical characteristics of interfaces. Every pure mineral in water carries a certain surface charge due to hydroxylation or protonation of surface ions based on the pH of the solution. In flotation, specific reagents, i.e. collectors, activators, depressants, modifiers, frothers, etc., are employed in order to alter the surface properties of minerals in an aerated slurry. If the desired mineral is not naturally hydrophobic, it is essential to modify its surface properties in order that it becomes hydrophobic and will report to the float fraction, separating it from gangue waste minerals (unless reverse flotation is required, in which case gangue waste minerals report to the float fraction).

The charged surface ions of a particle and the oppositely charged counter-ions diffused into the bulk of the solution constitute an electric double layer. The value of the electric potential at the shear plane of the particle when it is moving in an applied electric field is termed as zeta potential. There are various ways of calculating the zeta potential such as electrophoresis, dispersion method, streaming potential method, electroosmosis, and sedimentation potential method. Electrophoresis is most frequently used in studies related to minerals (Leja 1982; Douglas and Walker 1950). Numerous papers were published defining the calcite surface charge considering the electrolyte concentration, experimental conditions, the origin of materials, and the various ions of solution (Quast 2016).

With respect to flotation, the adsorption mechanism of the reagent on the calcite mineral was studied by using different techniques such as FTIR (Fourier transform infrared), UV (ultraviolet), and XPS (X-ray photoelectron spectroscopy). Generally, the interaction is due to electrostatic, physisorption, chemisorption, ion exchange, or surface precipitation methods; however, it can also occur because of indirect activation processes (Leja 1982). The adsorption of various reagents at calcite and impurity mineral surfaces is reviewed in this paper since different mechanisms are reported in the literature to explain adsorption phenomena.

Reagent selection plays an important role in the flotation processes. The different reagent systems used can be classified into two fields: (1) cationic reagents, when calcite was the major component and the impurity (gangue) minerals were floated, and (2) anionic reagents, when calcite occurred in moderate quantities in the ore and was being directly floated. The development in molecular structures of reagents led to a comprehensive understanding in the expansion in selection of reagents, improvement in productivity, and efficiency of industrial minerals (Nagaraj and Farinato 2016).

Fundamental research relevant to flotation has been well-presented in the literature and it is crucial for selection of reagents. Recent literature reports on the zeta potential studies of calcite (Dawoud et al. 2017), evolution of calcite flotation chemistry and chemicals (review, Nagaraj and Farinato 2016) have been summarized and discussed in this paper. Additionally, there are several review articles on the reagent adsorption on surface flotation behaviour of salt type minerals and their interaction with reagents, specifically their separation among calcium-containing minerals (Somasundaran 1969; Pradip and Fuerstenau 1985; Rao et al. 1991a, b; Quast 2016; Dawoud et al. 2017). However, until now, no comprehensive study has been presented on the purification of calcite ores from silicates and sulphides by flotation. Thus, this paper summarizes the understanding of the surface properties of minerals and their interaction with reagents which is currently to be found in the literature. The aim of this paper is therefore to discuss the parameters of calcite ore that affect the beneficiation process which are presented as follows: (1) introduction: structure and properties of calcite; (2) reagent systems used for calcite ore flotation: (a) anionic, (b) cationic, (c) non-ionic, and (d) mixed reagent systems; (3) collation and review of the calcite-reagent interaction which includes zeta potential, adsorption, and FTIR studies; (4) flotation studies on different kinds of calcite ores; and (5) patents filed on calcite beneficiation.

Collectors in Calcite Flotation

There are various kinds of calcite ores in nature and based on the quality of calcite in the ore, they can be regarded as high-grade and low-grade calcite ores. When calcite is present in a very high quantity with a minor amount of associated impurities, it is economically feasible to float the impurities and this process is known as reverse flotation. In some cases, calcite is present in low quantities within significant deposit masses that may be utilized for industrial purposes, in which case they are floated by direct flotation. In most such cases, calcite is accompanied with other Ca-bearing minerals and the beneficiation process is difficult. In the following section, various reagent systems that have been used are presented.

Cationic Collectors

Cationic reagents are generally used for flotation of silicate impurities from calcite ore. However, there are few Nordic high-grade calcite ore deposits with sulphides and graphite as impurities. Generally, different kinds of amines were used as cationic reagents to float the undesirable impurity minerals from this calcite ore.

From the literature, it can be deduced that amines have already been employed industrially for sulphide flotation from calcite ore. Amines are classified into primary, secondary, tertiary, and quaternary amines on the basis of a number of organic substituents for hydrogen atoms on the nitrogen atom. In the case of quaternary ammonium surfactants, a 4 alkyl or aryl group is attached with one nitrogen at the centre, NR4+ (usually called quats) which binds to an anion, chiefly a chloride. Thus, it explains that although the quats are permanently charged, this charge is independent of pH in solution (Bulatovic 2007). Recently, Bunkholt and Kleiv (2015) have presented results showing that the pure iron sulphide minerals were optimally floated in the presence of a quaternary amine. There are few studies on sulphide flotation by primary di-amines in literature; Kydros et al. (1993) showed that pyrite can be floated with cetyltrimethylammonium bromide (CTMAB) resulting in maximum recoveries at pH 3–4 and decreasing recoveries with increasing pH although floating at pH 12. Sirkeci (2000) observed a different trend, where HTA (hexyl thioethylamine hydrochloride, a derivative of primary amine) was employed as a collector. Pyrite was not floating until pH 9.3, and flotation recovery was at a maximum between pH 10.5 and 11.5. Few studies have been performed on the comparison of amines and xanthates. Nanthakumar and Kelebek (2007) conducted flotation experiments using EDTA (ethylenediaminetetraacetic acid) and TETA (triethylenetetraamine) and compared it with potassium amylxanthate (KAX) and found that EDTA produced best results at alkaline pH. Similarly, Kongolo et al. (2004) compared Armac C (cocoalkyl amine thioacetate) and KAX to float pyrites and found the recovery was higher with Armac C at pH 11.

The species distribution diagram of diamine collector as a function of pH at total initial concentrations of 1 × 10−5 M employed in flotation studies is redrawn from Scott and Smith (1991) and presented in Fig. 2. The average alkyl chain of the diamine part of the collector molecule used in this case is about C16. However, the pKa1 and pKa2 values of 6.8 and 9.3, respectively, for C12 diaminopropane were used since the pKa values change marginally with chain length. The distribution diagram shows that the amine dissociates mainly into either singly or doubly charged species at and below pH 8 and forms precipitate in the alkaline pH region above pH 8.5.

Fig. 2
figure 2

Species distribution diagram of amine at collector concentration 1 × 10−5 M (redrawn from Scott and Smith 1991)

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In some ores, calcite is present in moderate quantities along with silicates. In such cases, silicates or sulphides associated with calcite are floated using amine surfactants (Hanna 1975). All these experiments were performed at alkaline pH; if silicates or sulphides were present in the ores, these were floated before the separation of scheelite and calcite was performed.

Overall, the flotation of sulphides and silicates with amines from calcite ore showed that at alkaline and neutral pH region, there is a possibility to float sulphides using amines. The interaction mechanism of these reagents is explained in a later section (“Flotation Studies”).

Anionic Collectors

Generally, calcium-containing minerals are best floated with fatty acids. Sodium oleate was studied broadly to specifically float calcite in several cases. The species distribution diagrams of oleate collector as a function of pH at total initial concentrations of 5 × 10−4 M employed in flotation studies are redrawn from Han et al. (2017) and presented in Fig. 3. The distribution diagram shows that oleate is in neutral acidic form or colloidal precipitate in the acidic pH region below pH 8.5. The pH at which calcite was floated was around 9–11, and in this region, both anionic oleate monomer and dimer species exist. The other flotation conditions depend on the mineral composition in the ore. The various depressants used for calcite depression are presented in a later section (“Flotation Studies”).

Fig. 3
figure 3

Species distribution diagram of sodium oleate (5 × 10−4 mol L−1) (redrawn from Han et al. 2017)

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Mixed Collectors

Studies on the synergistic effects of collectors were chiefly performed using anionic collectors in combination with non-ionic reagents in the past. The presence of nonyl phenol (non-ionic surfactant) shows a synergistic effect with oleate reagent by increasing the hydrophobicity at relatively low concentrations of both the reagents (Giesekke and Harris 1994; Sis and Chander 2003; Filippova et al. 2014).

Calcite-Reagent Interactions

Fundamental studies in the field of flotation have proved to be crucial in the selection of flotation reagents and flotation conditions. The surface chemistry of reagent interaction with minerals is usually comprised of zeta potential experiments, adsorption studies, and flotation tests. In this section, we will summarize the zeta potential and adsorption studies of calcite/impurity minerals with collectors.

Zeta Potential Studies

Calcite-Oleate Interactions

The zeta potential of calcite in the aqueous system and in the presence of oleate has already been thoroughly studied by several researchers (Quast 2016; Andersen et al. 1991; Moulin and Roques 2003). Generally, the isoelectric point (IEP) of calcite was approximately pH 7–8.5. The most important factor controlling the surface potential of calcite is the dissolution equilibrium of Ca2+ versus CO32− species from the calcite surface. Continuous dissolution and re-precipitation of ions are taking place, and protons and hydroxide ions play a dual role in the determination of the surface potential of calcite. One of the major breakthrough studies in the zeta potential values of calcite was performed by Somasundaran et al. (1996) which was that the isoelectric point of calcite deviated towards a higher value after mixing for 2 weeks with the water from an equilibrated apatite solution. This showed that the isoelectric point of calcite is influenced by the presence of other salt-type minerals (apatite specifically) in the ore. Surface conversions due to reactions of the dissolved species with the mineral surface were studied in parallel using thermodynamic stability diagrams for heterogeneous mineral systems. Both the zeta potential and theoretical calculation results corroborated with each other. Addition of oleate has a major influence on the surface properties of calcite. A brief description of zeta potential results after calcite-oleate interaction is presented below.

Sodium oleate is primarily used as a collector for calcite-apatite or calcite-scheelite separation. Calcite, scheelite, and apatite are sparingly soluble minerals and due to its solubility, it is difficult to measure the zeta potential below pH 7. Almost all the researchers indicated that after increasing sodium oleate concentration, there is an increase in negative values of zeta potential for calcite (Somasundaran 1969; Mishra 1979; Somasundaran et al. 1985; Amankonah and Somasundaran 1985; Rao et al. 1989; Zimmels and Lin 1974; Smith and Shonnard 1986); a few peculiarities are explained below.

In 1996, Somasundaran et al. found that at pH 8.2 and in the presence of 10−6 M oleate concentration, the calcite zeta potential was zero (Fig. 4). Below this pH, the values of zeta potential significantly increased with respect to collector concentration, which was attributed to the electrostatic attraction between the negatively charged oleate ions and positive surface sites of calcite. Above pH 8.6, there were continuous changes in the zeta potential values which indicate specific adsorption of oleate.

Fig. 4
figure 4

Zeta potential of calcite as a function of collector concentration (redrawn from Somasundaran 1996)

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In 1974 and 1975, Zimmels et al. presented two articles about the effect on adding sodium oleate on the zeta potential of synthetic calcite; the results are shown in Fig. 5. The figure represents variations in the zeta potential of calcite with oleate as a function of initial and equilibrium concentrations. Below 2 × 10−4 M, initial concentration, there was little increase in the negative value of the zeta potential, which was similar to the results of Somasundaran (1969). In the second region between 2 × 10−4 and 8 × 10−4 M, the zeta potential gradually changed to a steep increase. In the third region (8 × 10−4–1 × 10−3 M), the potential was constant at − 50 mV and above at which concentration it gradually decreases. The constant zeta potential at − 50 mV refers to the precipitation of Ca-oleate, which was later substantiated by several researchers (Bulaovic 2007).

Fig. 5
figure 5

Variation of zeta potential of calcite with sodium oleate. a Initial concentration. b Equilibrium concentration (redrawn: Zimmels and Lin 1974)

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Figure 6 shows the various results produced by several researchers on zeta potential values of calcite as a function of initial concentration. This figure was presented by Rao (1992), where the trend is similar for all the studies. At low concentrations, there is a decrease in the zeta potential values referring to chemisorption, but between oleate concentration 1 × 10−5 and 1 × 10−4 M, the potential remains constant. As the concentration increased, the negative value of zeta potential dramatically increased and reached a minimum potential of − 50 mV, which was attributed to Ca-oleate precipitate.

Fig. 6
figure 6

Comparison of zeta potential of calcite as a function of oleate concentration at pH 10 (redrawn from Rao 1992)

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Sayan (2005) measured the effect of oleate on calcite in alkaline pH range and reported that the potential was negative and became more negative with increasing sodium oleate addition. The authors explained that this is due to the increasing concentration of negative oleate ions around the calcite particles and this caused an exchange between the oleate ions and carbonate ions on the surface of calcite. Interaction of anionic oleate on the negative calcite surface was due to chemisorption of oleate on surface calcium sites. This interaction led to a more negative surface charge, and therefore, the zeta potential remained negative at all the pH ranges.

Sulphide-Amine Interactions

Little literature is available about the interaction of sulphides and calcite with amine collectors. Kydros et al. (1993) studied pyrite interaction with amines and found that the zeta potential was zero for pure pyrite mineral at pH 2.5 which was similar to the previous results of Leja (1982). However, after the addition of amine collector, the IEP shifts to around pH 6–7, showing electrostatic adsorption of positive amine on negative sulphide minerals. Sirkeci (2000) indicated that the IEP of pure pyrite was around 6.3 which was shifted to pH 6.9 on the addition of amines. These results were very different from those of Kydros et al. (1993); however, they also observed that the zeta potential values were highly negative at pH 11.

Bunkholt and Kleiv (2014) reported that the zeta potential of pyrrhotite in quaternary amine solution remained positive throughout, although there is a steep increase at pH 11 due to adsorption of amine precipitate species. In this study, the IEP of pyrrhotite was extrapolated and found to be at less than pH 7. Similarly, for pyrite, the zeta potential in aqueous solution was reported to be negative before pH 10.5 which had a steep rise after the IEP (pH 10.5). Pyrite, when conditioned with amines, was negative throughout the range 7–11, but became positive just after that, becoming completely positive at pH 11. Thus, this result shows that amine precipitate species adsorb on the pyrite particles. This was different from the results of Sirkeci (2000) who observed negative zeta potential values at pH 11. Overall, it is well-understood from the above studies that sulphides are positive in the alkaline range from 7 to 11, whereas calcite is negative in the same pH range which clarifies that it is possible to float sulphides using amines in the alkaline pH range. However, as amine protonated to positively charged ions, it will preferably attach to the negatively charged sulphides in the alkaline pH range.

Adsorption Studies

The anionic oleate and cationic amine collector adsorption theories proposed by various researchers are briefly summarized in this section. The adsorption mechanism of these surfactants on calcite surfaces differed and was explained as columbic, chemical, and surface precipitation reactions. Several theories were proposed to explain the nature of adsorption of the long-chain surfactants on mineral surfaces.

Gaudin and Fuerstenau (1955) suggested that the long-chain surfactants are adsorbed as individual counter-ions at low concentrations and hemimicelle structures due to chain-chain bonds that are formed between the adsorbed long-chain molecule in the stern layer which takes place above a critical equilibrium concentration, termed as the critical hemimicelle concentration. Later, Cases (1979) and Cases et al. (1986) introduced a stepwise adsorption model based on thermodynamic considerations such as potential energy and entropy of the adsorbed layer as well as the heterogeneity of adsorbent surfaces. Due to the surface heterogeneities, the two-dimensional condensation takes place because of lateral hydrophobic chain-chain bonds on the most energetic homogeneous domains initially at lower concentrations. Successive two-dimensional condensation of surfactant occurs on the following lesser energetic domains of the surface with increasing concentration. Thus, within the monolayer, the isotherm shows a stepwise increase in adsorption, i.e. a series of small vertical steps, or a smooth curve depending on the distribution of surface energetic domains (Fig. 7). The second layer is always characterized by a vertical step since the surface is rendered homogeneous due to the formation of a first layer. Harwell et al. (1985) proposed an admicelle hypothesis, which states that the surfactant aggregation producing the admicelle structures forms locally on the surface due to heterogeneity in the same manner as micelle formation. These admicelles form on a given patch of the heterogeneous surface at a critical solution concentration, and for concentrations, less than this value, aggregates do not exist on that patch. However, Cases (1989) later explained that this admicelle hypothesis is just an extension to their two-dimensional condensation model. Later, Somasundaran et al. (1985) stated that the adsorption of oleate on semi-soluble minerals is in fact a surface precipitation which they have substantiated by calculating the concentration of the counter-ions at the surface region whereby the solubility product for precipitation reached at the surface region first before in the bulk solution.

Fig. 7
figure 7

Adsorption isotherms at 25 °C and pH 8.3 of alkyldodecylammonium chloride on synthetic calcite, before (curve I) and after fine-grinding (curve II) isotherms of adsorption at 25 °C. and pH 8.3 of alkyldodecylamine chloride on a synthetic calcite (Cases et al. 1986; Goujon and Mutaftschie 1976)

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For studying the adsorption model of oleate on calcite, concentration-pH factors were chosen and tested by various researchers. Jung (1976) and Jung et al. (1988) stated that when the solution conditions devoid of liquid oleic acid and NaH(OL)2 prevailed, calcium oleate is the adsorbed species. Oleate ions are the predominant species at basic pH values, and therefore, it is not worth experimenting at lower pH values where undissociated oleic acid exists in the form of colloidal precipitate. The concentration range of oleate studied in most cases exceeds the solubility limit of calcium oleate considering the dissolved calcium ions from the minerals; the solubility product of calcium oleate is reported to fall in the pK range of 12.4–15.74. However, Cases et al. (1986) showed that there will be no confusion between the adsorption and precipitation since the isotherms indicate an infinite vertical step following precipitation under conditions of excess ions in solution.

As presented in the above section, the deviations in zeta potential and ambiguities with the shape of adsorption isotherms have attracted many researchers to investigate the calcite-oleate system. Somasundaran (1969) proposed hemimicelle formation at pH 9.6 (oleate concentration around 2 to 3 × 10−6 M). He observed a sharp increase in the slope of the adsorption isotherm above 3 × 10−5 M, and this increase was attributed to the precipitation of calcium oleate. The concentration of calcium at the interfacial region was calculated using the Boltzmann equation and the pKa value of calcium oleate for precipitation satisfaction from the beginning of low oleate concentrations, thereby showing that the abstraction of oleate by calcite is not adsorption but surface precipitation.

Iskra et al. (1973) observed the adsorption of oleate remains constant in the pH range 8.6–12.0. On the other hand, Gutierrez (1979) claimed that adsorption was pH-dependent and at maximum between pH 10.2 and 11.5. The author also reported a steep increase in the slope of isotherm above 4 × 10−5 M equilibrium oleate concentration, which was similar to that reported by Somasundaran (1969). The reason stated was also the same, i.e. precipitation of calcium oleate.

Other works presented by Zimmels and Lin (1974) and Zimmels et al. (1975) describing a three-stage adsorption model of oleate on calcite consist of direct monomer, hemimicelle, and micelle adsorption based on their affinities for adsorption at different oleate concentrations (Fig. 8). Physical adsorption and chemisorption were considered for the adsorption of either monomer or aggregate of oleate species. This model of physical and chemisorption of oleate on calcite was similar to the results of Szezypa et al. (1979) and Fuerstenau et al. (1968) for sodium laurate.

Fig. 8
figure 8

Adsorption isotherm of sodium oleate on calcite (Zimmels and Lin 1974)

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Another important conclusion drawn by Marinakis and Shergold (1985) was the abstraction of oleate by calcite with the precipitation of calcium oleate for the range of oleate concentration studied. The results are shown in Fig. 9. The experimental and theoretical oleate concentrations for calcium oleate precipitation match in the entire concentration range studied, and thereby, the authors claim that the adsorption is only due to calcium oleate precipitation. However, Rao et al. (1989, 1991a, b, c) showed such a correlation between the experimental and theoretical concentration happens only after 5 μmol m−2 corresponding to a monolayer coverage in a liquid crystal state.

Fig. 9
figure 9

Amount of oleate abstracted from solution in equilibrium with 0.1 g calcite at pH 10. (redrawn from Marinakis and Shergold 1985)

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Kulkarni (1975) explained the governing role of the oleic acid-oleate complex (later termed as iono-molecular complex/acid-soap complex, Somasundaran 1975) in determining the pH dependence of oxide flotation using oleate. The pH of the maximum flotation response was found to be identical to the pH of the maximum concentration of the acid-soap complex. It was also shown that the maximum in both surface tension decay rate, as well as surface tension lowering, was obtained for oleate solutions at the same pH which suggested that the higher surface activity of the acid-soap complex compared with that of the acid or the soap to be one major reason for the observed pH effect on flotation response. Later, many researchers also reported the influence of acid-soap complex species on various minerals, including carbonates (Young and Miller 2007). However, Chernyshova et al. (2011) performed spectral investigations and showed that acid-soap dimers in the bulk solution did not directly associate with mineral hydrophobicity.

Later in 1989(b), Antti and Forssberg reported the formation of a new species CaOHOl at pH greater than 10.35, for the reduction in calcite recovery at pH 10 and 11. Additionally, the mineral surface became less hydrophobic only if it was covered with a layer of CaOHOl instead of Ca(Ol)2 which was obtained slightly at lower pH. They also stated that even at lower pH value, CaOHOl is formed first and deposited in a monolayer on the mineral surface. Precipitated Ca(Ol)2 can then be adsorbed to the mineral surface by physisorption, making the surface more hydrophobic.

According to the series of papers published by Rao et al. (1989, 1991a), the interaction of oleate on calcite corresponded to the adsorption of oleate until monolayer coverage and physical adsorption of precipitated calcium oleate at a higher concentration similar to previous studies. Young and Miller (2007) studied the adsorption of oleate on calcite at pH 9.2. The incomplete monolayer was observed at equilibrium oleate concentrations below 1 × 10−5 M, and multilayer coverage was observed at higher concentrations (> 10−4 M). According to these authors, chemisorption at low oleate concentrations and surface precipitation of calcium oleate at higher concentrations occurred.

Recently, Sayan (2005) performed adsorption studies and concluded that oleate adsorption increased with increasing addition up to critical micelle concentration, after which increasing oleate addition caused lower adsorption due to the formation of oleate micelles in the solution rather than surface oleate compounds.

FTIR Studies

The qualitative adsorption of oleate on calcite was assessed by several FTIR investigations. Generally, oleate adsorbs chemically on calcite by the carbonyl adsorption band; however, the exact position of the band was stated to be uncertain owing to the highly interfering carbonate absorptions. Peck (1963) and Peck and Wadsworth (1964) indicated a shift in the peak from 1562 to 1555 cm−1 after washing and assigning the peak at 1555 cm −1 to surface calcium oleate which is thought to be obscured by the peak at 1562 cm−1 for sodium oleate after adsorption. The bands at 1562 and 1555 cm−1 were assigned to physically adsorbed sodium oleate and surface calcium oleate, respectively. However, the same authors in a subsequent investigation considered the peak at 1562 cm−1 for the surface calcium oleate and the physically adsorbed sodium oleate and suggested that it is not possible to distinguish between them. Kellar et al. (1990) characterized the mono-coordinated calcium oleate (chemisorbed) by an absorption band at 1549 cm−1 by using in situ internal reflection FTIR spectroscopy. Double peaks corresponding to calcium oleate were observed in other studies.

Antti and Forssberg (1989) studied the calcite-oleate system by FTIR studies and found that the composition and surface structure of the mineral influence the adsorption process. They also concluded that the adsorption density was higher at pH 9 and 10 than at pH 11; however, precipitation of calcium oleate after the monolayer was attributed at all the pH values 9, 10, and 11. The monolayer was formed with 1:1 coordination of oleate with calcium. Thus, the result agreed with the previous adsorption studies.

Young and Miller (2007) recorded the in situ and ex situ FT-NIR/IRS spectra of oleate adsorption on the calcite surface. The spectral quantitative absorption data was presented as the adsorption isotherms (Fig. 10) and its dependence on temperature and pH. The isotherm is basically divided into two regions. Region 1 is below 1 × 10−5M oleate concentration at sub-monolayer, and region 2 corresponds to a multilayer coverage at higher oleate concentrations. The concentration of oleate below 1 × 10−4 M dimers in solution is formed which slowed down the reaction kinetics. At a concentration of around 1 × 10−3 M, the adsorption density decreases due to micelle formation and below 2 × 10−3 M was attributed as CMC and many dimers were formed between 1 × 10−4 and 2 × 10−3 M. Region 1 represented oleate chemisorption and 2 calcium oleate precipitation. The precipitation reaction is as follows:

$${\mathrm{Ca}}^{2+}+{2\mathrm{Ol}}^{-}=\mathrm{Ca}{\left(\mathrm{Ol}\right)}_2\left(\mathrm{ppt}\right)$$
Fig. 10
figure 10

Oleate adsorption isotherms determined for calcite at 20 °C and 60 °C and at pH 9.2 by in situ FT-NIR/IRS with overtone and combination bands of the aliphatic stretch from Young and Miller (2007) (calcite at pH 9.2)

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Oleate chemisorption has given an equation which was later used by Young and Miller (2007):

$$\mathrm{Ca}\left[\mathrm{X}{\left({\mathrm{H}}_20\right)}_{\mathrm{n}}+\mathrm{NaO}\right]=\mathrm{Ca}\Big[\mathrm{Ol}{\left({\mathrm{H}}_2\mathrm{O}\right)}_{\mathrm{n}-\mathrm{m}}+\mathrm{m}\ {\mathrm{H}}_2\mathrm{O}+\mathrm{NaX}$$

They concluded that in situ FT-NIR/IRS spectra have shown that D2O restructures at the calcite surface upon wetting and upon addition of oleate. The isotherm is presented below.

Sayan (2005) suggested that Na-oleate chemically adsorbs on the calcite surface, which interacts with oleate ions, giving the surface calcium oleate product. Ca-oleate is water-insoluble and remains on the calcite surface as 1:1 coordination species.

Sirkeci (2000) studied the amine-sulphide adsorption system and reported that a weak peak was observed for pyrite that interacted with HTA at pH 5, which means HTA adsorbs on to the surface of pyrite to a much lesser degree at acidic pH values compared with an alkaline range. There is a need to perform a detailed study in order to develop a better understanding of the calcite- amine system.

High-Grade Calcite Ore Deposits

Our study is primarily focussed on the high purity calcium carbonate deposits in the world, comprising a high-quality raw material for GCC production. Many such deposits are located in the Nordic region of Europe, which are listed below.

Akselberg Calcite Deposit

The Northern-Norway marble deposit is one of the major high purity calcium carbonate deposits with minor inclusions of graphite and sulphides in the world, comprising a high-quality raw material for GCC production (Watne 2001). It was later demonstrated by Bunkholt and Kleiv (2014, 2015) that the deposit contains a substantial volume of sulphide-bearing marble, where the main contaminant phases are pyrite and pyrrhotite, the latter being the most abundant contaminant. The presence of sulphides was responsible for the colouring effect, which reduces the brightness of marble.

Kurikka Calcite Marble Deposit

A high-grade calcite deposit, which was mainly composed of calcite, silicates (quartz, tremolite, feldspars, diopside, mica, and serpentine), and graphite from sub-microscopic size to about 0.2–1.5 mm, and rarely, hydrated iron oxide and sulphides (Chernet et al. 2006). The authors mentioned that the presence of fine-grained graphite in the deposit was responsible for the uneven grey colour of the calcite marble and it also lowers the deposits’ grade as a source for GCC (Pekkala 1985).

Gåsgruvan Deposit

Sweden has a high-grade calcite deposit with minor impurities of silicates (quartz), which reduces its industrial applications. It was studied by Rao and Forssberg (1991) and Rao and Forssberg (1993).

A low-grade calcite deposit located in Tamil Nadu, India, was studied by Srinivas et al. (2011). The feed material analysis showed that the average particle size was 37 μm and that d80 was 63 μm. Petrological studies indicated the presence of quartz, feldspar, pyroxene, and mica as the major silicate gangue minerals. Liberation studies were performed and found that the majority of these silicates are liberated from the selected feed size material (− 63 μm). A few silicate grains, in the size range from 5 to 15 μm, remained locked within the calcite grains as inclusions and appeared in the concentrate. Another important calcite deposit (Nigde region in Turkey), with scheelite and quartz as impurities, was also studied by Ozcan et al. (1994).

Flotation Studies

Pure Mineral Flotation

Cationic Reagents

Table 1 presents and discusses research studies on the flotation of various calcium-bearing pure minerals, silicates, sulphides, and mixed sulphide-silicate minerals.

Table 1 Pure mineral flotation overview in the presence of cationic reagents
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The flotation of sulphides and quartz from calcite is possible while using an amine collector at neutral or mildly alkaline pH values where the amines are present in mono and doubly charged ions. Optimum results were also observed above pH 10, where the neutral amine molecules are in colloidal precipitate form interacting with the sulphides (explained in the previous section). However, a few studies claimed that amines are capable of floating calcite at and above pH 10. Thus, it can be summarized that pH 7–8 is the most suitable pH for sulphide/silicate flotation from calcite. Collector concentration, flotation and conditioning times are other parameters, having a higher impact on these flotation results. Considering the above results and the species distribution diagram of amine, it could be stated that below pH 8, sulphide-amine interaction is due to electrostatic Coulombic interaction, whereas above pH 8, neutral amine precipitate species physically adsorb on calcite. The pure mineral flotation results correspond well with zeta potential and adsorption studies.

Anionic Reagents

The floatability of calcite in the presence of anionic collectors is studied in this section. Sodium oleate is generally used as the collector for separation of calcite and other calcium-containing minerals. Hallimond flotation tests and single-mineral flotation tests were performed using pure minerals of calcite, apatite, scheelite, fluorite etc. (Ananthapadmanabhan and Somasundaran 1984; O’Connor et al. 1987; Mishra 1982; Yongxin and Changgen 1983; Atak et al. 1986; Baldauf et al. 1986; Finkelstein 1998; Gallios and Matis 1989; Zhang et al. 1999; Feng et al. 2015; Laskowski et al. 1991; Morozov et al. 1992; Valdivieso et al. 2000; Shi et al. 2012). Usually, a modifier or depressant (sodium metasilicate, Alizarin S, cashew gum, starch, etc.) is used to depress selectively one of the minerals (Fu and Somasundaran 1986; Mishra 1982; O’Connor et al. 1987). Several studies were conducted on the calcite-apatite system and several chemicals were tested as depressants. For example, sodium metasilicate and Acrol LG-21 are calcite depressants (Mishra 1982; O’Connor et al. 1987; Orthgiess and Dobiáš 1994; Sadowski 1990).

Generally, the results showed that in the presence of oleate, effective separation among salt-type minerals is observed in between pH 8 and 10. In most cases, it was noted that there is a steep rise in the recovery of calcite at about 1 × 10−5 M concentration, which is again constant until 1 × 10−4 M (Mishra et al. 1980; Rao et al. 1989). The results are in agreement with zeta potential and adsorption studies.

Recently, a study on tailor-made reagent derivatives with two functional groups that are spaced with increasing distance was performed by Karlkvist et al. (2015) where the carboxylate surfactants, disodium N-dodecanoylaminomalonate (C12MalNa2), disodium N-dodecanoylaspartate (C12AspNa2), and disodium N-dodecanoylglutamate (C12GluNa2) having one, two, or three methylene groups between the two anionic groups, respectively, were tested for flotation of pure calcite and apatite. A monocarboxylate surfactant, sodium N-dodecanoylglycine (C12GlyNa) floated calcite with no effect on apatite flotation, whereas with C12MalNa2, apatite was seen to float well and no effect on calcite flotation. These results illustrate a geometric, electrostatic, and stereochemical compatibility between the mineral surface and anchoring surfactant molecule to achieve selective flotation between two calcium minerals.

Overall, most of the studies reported optimum flotation conditions of calcite recovery above pH 8 with anionic collectors. A few researchers have claimed that calcite flotation is independent of pH and some other studies showed that the depression of calcite is possible between pH 8 and 10 while separating it from other salt-type minerals. The IEP of calcite was at pH 8 and the microflotation studies corroborate with the zeta potential and adsorption results either with anionic or cationic collector. Generally, sodium silicate or metasilicate was used as a depressant in most of the separations. There were some non-traditional reagents such as hydroxamic acid (Ni and Liu 2013; Yu et al. 2014; Wu and Zhu 2006); amphoteric collectors like alpha-benzol amino benzyl phosphoric acid, BABP (Hu and Xu 2003) and N-hydroxyamino-alkyl-amide (Deng et al. 2016) used at alkaline pH to separate from one calcium mineral to the other.

Mixed Reagents

Mixed reagents were used for flotation of various mineral combinations, and our prime concern is if one among those minerals is calcite. Valdiviezo and Oliveira (1993) performed a few tests with cationic-anionic reagents while most of the other studies were performed on anionic-non-ionic reagent pairs (Giesekke and Harris 1994; Sis and Chander 2003; Filippov et al. 2012; Filippov and Filippova 2006; Filippova et al. 2014). The adsorption mechanism was primarily synergistic; however, a few instances of competitive interaction were also observed. The basis was mainly the interaction of reagent constituents where the polar reagent initially adsorbs on the mineral surface and, subsequently, the non-ionic reagents adsorb via hydrophobic interactions. Overall, it can be stated that by addition of non-ionic surfactants, the concentration of traditional surfactant usage can be controlled as all these studies indicate that total chemical content used in the reagent mixture was lower than that of single reagent systems (Gao et al. 2015).

Bench-Scale Flotation Results

Cationic Reagent

After the zeta potential, adsorption, and pure mineral flotation studies, a detailed overview about the flotation of calcite ores is presented below. High-grade calcite ores generally contain silicates or sulphides as impurities and are beneficiated by reverse flotation using cationic reagents. Table 3 presents the bench-scale flotation results of calcite ores when cationic reagents were used for floating the impurity minerals.

In general, the bench-scale flotation results are dependent on two major parameters: (1) collector concentration and (2) pH. Srinivas et al. (2011) showed that collector concentration is the most important parameter to control the grade and recovery. Their study concludes that with increasing collector concentration, there is an increase in calcite grade, which is highest at 400 g/tonne and decreases beyond this dosage. Bunkholt and Kleiv (2015) claimed that there is not any significant increase in the brightness or grade with increasing collector concentration; however, they stated that pH variation above pH 9 decreases the grade and brightness of the product. They also showed that adding more collector concentration can solve the problem as the decrease in the results was attributed to oxidation of sulphides. Earlier Rao and Forrsberg (1993) suggested that quartz can be floated from calcite efficiently at pH 6 and variation of pH has a strong influence on the recovery. Thus, it can be concluded that at optimum amine concentration and pH 7–8, it is possible to float sulphides from calcite ore. The results corroborate with the pure mineral studies, as the concentration of positively charged amine ions is greater in the pH region 6 to 8 or 9. At alkaline pH region, the amine precipitate could interact with the silicates or sulphide minerals, which are negatively charged.

A detailed overview of cationic reagents is presented in Table 2, which contains information regarding the cationic reagents used in past years for direct flotation of impurity minerals from calcite ores.

Table 2 Overview of cationic reagents used for beneficiation of high-grade calcite ore
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Anionic Reagents

Sodium oleate was generally employed as a reagent for calcite direct flotation from calcite-apatite or calcite-scheelite ores (Abouzeid and Elgillani 2009; Elgillani 1978; Prasad et al. 1994; Castro and Calero 1993; Castro et al. 1996; Ribeiro et al. 2003; Abdel-Khalek 2000; Rao et al. 1985; Jena et al. 2013; Santos et al. 2015). Later, due to the complexity of several ores, few modifiers and depressants were employed to facilitate the flotation process. The commonly used modifier was sodium silicate (Prasad et al. 1994; Jena et al. 2013), sodium hexa-metaphosphate (Schubert et al. 1990; Gibson et al. 2015), and hydrated/acidified sodium silicate (Zhou and Lu 1992; Martinez et al. 2000). A few studies have also reported the unconventional depressants, such as cashew gum (Ribeiro et al. 2003), when it was used as a depressant for calcite, and 70–80% calcite was recovered as tailings. Similarly, Ozcan et al. (1994) used quebracho as a depressant for separation of scheelite and calcite at pH 8–8.5, which also catalyzed in the presence of the collector oleoyl sarcosine. Although, conventionally, calcite was directly floated using oleate or hydroxamic acid as collector, the process has a significant disadvantage: As mentioned earlier, chemical treatments as collector agents are used to render the surface of the calcite hydrophobic to separate these particles from the impurity minerals. These collector agents are adsorbed on the surface of the calcite and, therefore, modify the properties of the final product (for example GCC feed).

However, these alterations may be undesirable in the following uses of the white pigments in paper, plastics, paint, coatings, concrete, cement, cosmetic, water treatment, food, pharma, ink, and/or agriculture applications. Furthermore, the direct flotation of the desired calcite is also disadvantageous due to quality and economic reasons in these industries. Thus, in the next section, we will focus on the patents filed for high-grade beneficiation of calcite mineral.

Patents in High-Grade Calcite Beneficiation

The patents published on high-grade calcite beneficiation are either based on the development of the grinding process flowsheet or the reagent scheme used in flotation. This study is based on collector systems; thus, the different reagent schemes patented for calcite ore flotation are discussed below (Table 3). In most of the work presented in the table, the flotation results are assessed by grade/recovery, acid-insoluble tests, and brightness analysis. Table 3 indicates that environmentally friendly reagents such as quaternary amines, ester quats, and microbial-based collectors have been used as the surfactants in the past decade. However, limited studies related to the interaction mechanism of such collectors with metal sulphides have been performed. Such studies could lead to better development of non-toxic collectors for impurity mineral flotation from calcite ores.

Table 3 High-grade calcite ore flotation patents
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Conclusion and Future Work

Extensive research has been done on improving the reagent scheme for direct/reverse flotation processing of calcite ores. This paper reviews the fundamental research and bench-scale flotation of calcite ore constituents. The advantages and disadvantages of anionic direct flotation, cationic reverse flotation, and mixed collector direct flotation are summarized in this paper. A large variety of reagents commonly used or in development for flotation, for example, collectors, and depressants are also described. The aim of this paper is to provide a well-detailed source for the current status of calcite ore flotation and thus provide a useful guide to its future development of flotation performance for calcite ore.

Amines are the major cationic reagent used for reverse flotation of silicate/sulphide impurities. Pure mineral studies have shown that amines adsorb on the sulphide minerals via electrostatic interactions. Below pH 8, sulphide/silicate-amine interaction is due to 1:1 coordination, whereas above pH 8, silicate/sulphide-amine interaction is attributed to 1:2 coordination. Primary and secondary amines also adsorb on calcite above pH 8; thus, effective separation is possible at neutral or near-neutral pH values.

However, if calcite is present in the ore in minor quantities, it is usually floated out by using anionic collectors. Calcite displays good flotation performance in the presence of sodium oleate as the collector and a few studies were reported that used unconventional reagents. This article also presents various depressants and modifiers used for efficient separation of calcite from other calcium minerals. Extensive information regarding the interaction/adsorption mechanism of these anionic reagents has been reported in the past which is summarized in the text.

For the beneficiation of high-grade calcite ores with sulphides or silicates as impurities, reverse flotation has an edge over the normal flotation technique in industries. The reason is that the directly floated calcite is usually influenced by the reagent employed for flotation. However, these alterations may be undesirable in the end-use of the paper, plastics, paint, and coating industries. Thus, the major patents filed in recent or past years are based on the reverse flotation of calcite ore. However, few studies were conducted in the past on the interaction mechanism of these tailor-made reagents or the novel reagents with calcite/impurity minerals. In conclusion, it can be stated that there is considerable scope for studying the detailed interaction or adsorption mechanism of quaternary amines/esterquats/novel biosurfactants with calcite and the other gangue minerals.