Surface and chemical properties of chlorite in relation to its flotation and depression

Abstract

The chlorite group of minerals exhibit a wide range of surface charging properties in aqueous suspensions. A systematic review of the literature as it relates to the flotation, depression and surface properties of chlorite and related minerals was undertaken, with a view to using this information to help develop suitable reagent schemes which might allow the selective removal of chlorite from sulphide and oxide flotation systems. The surface charge on chlorite mineral particles originates from permanent charges on basal planes of the crystallites and pH dependent (amphoteric) charge on crystallite edges, typical of clay minerals. The net charge on chlorite particles depends upon the edge/face plane ratio (crystallite size), the solution pH and the presence of specifically adsorbing ions (principally Ca²⁺). Both cationic and anionic surfactants (collectors) are observed to adsorb on chlorite particles, over a wide range of pH conditions. This apparently contradictory behaviour is most likely due to the dual modes of surface charging and the effects of cation substitution into the chlorite lattice. In the case of anionic collectors, some specific (chemical) adsorption energy may also contribute to the adsorption properties. Chlorite depressants used in oxide and sulphide flotation are typical of those used for other silicate minerals. The most common of these depressants are sodium silicate (Na₂SiO₃), carboxymethylcellulose and fluoro compounds (hydrogen fluoride and sodium hexafluorosilicate). There have been no studies of the effects of the chlorite surface charging properties upon depressant selection, although it appears likely that the edge/face plane ratio will strongly influence the depressant adsorption behaviour.

Keywords

Chlorite flotation Chlorite depression Chlorite surface properties

Introduction

Aluminosilicate minerals are gangue minerals which are present in many base metal sulphide and oxide flotation systems and their removal is critical in achieving grades acceptable to further processing of the concentrates. Hence their flotation, depression and surface chemical properties are of great interest in mineral processing operations.

While there are many references in the literature to the flotation of aluminosilicates, including kaolin, illite, kyanite and mica minerals, talc and pyrophyllite (Miller et al., 2007), and on conditions to float kaolinite, pyrophyllite and illite from Chinese diasporic bauxites (Xu et al., 2004), there are fewer references to the specific flotation and surface chemistry properties of chlorite which might be helpful in developing selective separation techniques and reagent schemes (collectors, modifiers and pH values) for this aluminosilicate which could be helpful in achieving acceptable metallurgy. Consequently, a systematic review of the literature has been undertaken as it relates to the flotation, depression and surface properties of chlorite and related minerals, with a view to using this information to help develop suitable reagent schemes which might allow the selective removal of chlorite in sulphide and oxide flotation systems.

Chlorite surface chemistry

Adsorption processes on silicate minerals are strongly influenced by the surface charge properties of the mineral, as controlled by the structure of the mineral as well as the solution chemical conditions.

Chlorite crystal structure and surface charging mechanisms

The chlorite group of minerals, with a general formula (Mg, Fe²⁺, Al)₆(Si, Al)₄O₁₀(OH)₈, are 2∶1 phyllosilicates with an interlayer sheet. The 2∶1 ‘talc-like’ layer has two sheets of SiO₄ tetrahedra separated by an interlayer ‘brucite-like’ sheet containing Mg, Al and Fe octahedrally coordinated. The crystal chemistry of chlorites has been reviewed by Bailey (1988). More recently, the atomic structures of the talc-like and brucite-like layers have been imaged by atomic force microscopy (Vrdoljak et al., 1994; Henderson et al., 1994).

In general, the talc-like layer in chlorite exhibits a permanent negative charge due to isomorphous substitution of Al³⁺ for Si⁴⁺. This excess charge is partially compensated by the brucite layer due to Al³⁺, and in some cases, Fe³⁺ substitution for Mg²⁺. In aqueous suspensions, the permanent charge is unaffected by solution pH, such that in the absence of any other charging effects, chlorite would be negatively charged across the pH range of 2–12. However, modification of the basal plane charge is possible via cation adsorption onto the siloxane surface. This mechanism of charging, and the associated effects upon adsorption processes, are considered in the following sections.

In addition to the charge on the basal plane, the edges of the talc-like layer ionise in aqueous solutions. The charge sites on the edge planes of chlorite crystals are amphoteric in nature, allowing the formation of both positive and negative charges, as controlled by the solution pH. The surface charging characteristics of the edge planes are similar to metal oxide minerals, with a characteristic pH at which the surface exhibits zero net charge. The pH at which this occurs is termed pH_pzc (point of zero charge). At pH conditions below the pH_pzc, the surface will be positively charged, while above the pH_pzc, the surface will be negatively charged. In the case of clay minerals, the pH_pzc corresponds to the pH at which the sum of both the permanent charge on the basal planes and the pH dependent charge on plane edges is zero.

The electrokinetic properties of chlorite minerals have been reported in a number of studies (Alvarez-Silva et al., 2010; Edwards et al., 1980; Hussain et al., 1996; Sondi and Pravdic, 1998; Sondi and Velimir, 1996; Sondi et al., 1996, 1997; Sysila et al., 1996). Values of pH_pzc reported in these studies vary in the range of 3<pH_pzc<6 dependent upon the particular chlorite mineral species, the prevailing solution conditions and the method of mineral preparation. In all cases, the zeta potentials determined for these minerals by electrokinetic studies are quite low, rarely exceeding −30 mV.

It has been reported by Sondi and Pravdic (1998) and Sondi and Velimir (1996) for the chlorite mineral ripidolite that the pH_pzc depends upon the relative ratio of edge to face surfaces. They observed that dry milling of ripidolite in an agate mill changed the pH_pzc from <2 to 6. This difference was attributed to the relative dominance of edge planes over basal planes in the milled material, allowing the generation of a net positive charge on the mineral particles. This dramatic effect of mineral preparation offers a partial explanation for the wide range of pH_pzc values recorded for this group of minerals. The other explanation may be related to cation substitution into the chlorite lattice.

In a separate study, Sondi et al. (1996) considered the effect of solution Ca²⁺ upon the surface charge of chlorite mineral species. Using a chlorite species with a natural pH_pzc of ∼5, it was found that a change in surface charge, from negative to positive, could be induced as a result of Ca²⁺ adsorption. While this result was interpreted as evidence for the strong adsorption of Ca²⁺ onto chlorite, the issue of whether adsorption occurred on the siloxane basal surfaces, or on edge planes, was not resolved. On the basis of the hydrolysis behaviour of Ca²⁺ (Baes and Mesmer, 1976), and the known interaction of the base cations with clay mineral surfaces (Stumm and Morgan, 1995), it is most likely that adsorption occurred on the siloxane planes. Neither Na⁺ nor Mg²⁺ induced a charge reversal on chlorite, consistent with their lower affinities for the siloxane plane (Stumm and Morgan, 1995).

Anionic surfactant and anionic polymer adsorption on chlorite

Studies on the adsorption of anionic surfactants or anionic polymers onto chlorite have not been directed towards mineral processing applications; however, the general chemical observations may have direct application to pulp chemistry approaches in chlorite flotation and depression.

The adsorption of octylbenzene sulphonate (OBS⁻), added as the sodium salt, onto chlorite and other clay minerals was investigated by Siffert and Zundel (1985). Strong adsorption of the sulphonate surfactant onto a high Ca²⁺ chlorite was observed, which was attributed to both the precipitation of Ca(OBS)₂ as well as the adsorption of monomeric OBS⁻ onto exchangeable sites and edge planes. Surprisingly, these adsorption processes occurred at pH 8, a pH at which it would be expected that the chlorite mineral would be net negatively charged, although it is not obvious that the pH was well controlled. This strong adsorption could be interpreted as evidence for surface charge reversal by Ca²⁺, although the electrokinetic properties of the clays were not measured in this study.

The adsorption of the natural polymeric material, fulvic acid, onto chlorite was studied by Sondi and Pravdic (1998), Sondi and Velimir (1996) and Sondi et al. (1997) in order to understand the electrokinetic behaviour of natural clay particles in estuarine systems. Strong adsorption of fulvic acids was observed at pH 6·5, which was attributed to the coordination of carboxylate functional groups to metal ions on edge surfaces. Similar adsorption behaviour was observed for the model polymer polyacrylic acid (PAA). In both cases, the adsorption of these polymeric molecules led to an increased negative surface charge on the chlorite particles over the entire pH range studied (2–12). Accordingly, below the pH_pzc, the adsorption of these polymers resulted in surface charge reversal.

Chlorite flotation

Flotation of chlorite can be achieved with cationic collectors, anionic collectors or non-ionic collectors. These are discussed in turn below.

Cationic collectors

Cationic collectors, including alkyl amines, alkyl ether amines and quarternary ammonium salts have been used for the flotation of chlorite.

Alkyl amines

A number of reports deal with the flotation of chlorite by amine based collectors, either as a valuable product, or in terms of an unwanted gangue mineral in the upgrading of another mineral. The pK_a values of amines are in the vicinity of 9–10 and are such that at pH<9 amine collectors are cationic (positive). Tertiary amines are positively charged across the entire pH range. Given that many chlorite mineral samples exhibit a relatively low pH_pzc value, the flotation of chlorite with amine collectors is not surprising that pH values exist for adsorption of the cationic collector on the negative mineral.

In studies with a single chlorite mineral, Zheng et al. (2009) showed that lauryl amine floated chlorite at pH values between 7 and 9 with the highest recovery (50%) obtained at the natural pH of 7·7. Addition of calcium ions at the natural pH increased recovery to 58%.

The flotation of chlorite (in association with biotite) has been reported using a straight chain primary amine (Armac 12D, dodecylamine) at pH 3 and a reagent dose level of 200 g t⁻¹. Although the purpose of this separation was to achieve an upgraded biotite product, for mineral age determination, the selective rejection of chlorite was not achieved (Harris et al., 1967).

The flotation of chlorite from quartz has also been reported using straight chain primary amines (Houot et al., 1995). In that report, it was concluded that the shorter chain primary amine, octylamine (C₈), was a more selective collector for chlorite than longer chain amines with carbon numbers greater than 12. This effect can be attributed to the stronger flotation of quartz with increasing alkyl chain length. Strong flotation of chlorite was reported with octylamine over a wide pH range (2–12) and at a dose level of 200 g t⁻¹. A strong flotation response for chlorite in this system was also observed using the quaternary amine Flotigam K2C over a similar pH range and dose levels. The exact structure of the Flotigam K2C reagent was not ascertained; however, this molecule is similar to tetrabutylammonium chloride except that the alkyl chains may not be identical and are probably longer than C₄.

The secondary amine (HOE-F-2642), used in the flotation of pyrochlore, (Na, Ca)₂(Nb, Ti, Ta)₂O₆(OH, F, O), also acts as a collector for chlorite over the pH range of 2–6 and at a dose level of 1 kg t⁻¹ (Rao et al., 1988). Interestingly, it was found that Ca²⁺ had no appreciable effect upon the flotation performance of chlorite with this collector, suggesting that any surface charging effects caused by Ca²⁺ adsorption were minor in this system.

The flotation of chlorite has also been observed in the upgrading of talc using tributylamine at pH 8·5 with dose levels in the range of 50–200 g t⁻¹ (Belardi et al., 1991, 1995). Upgrading of the talc product was achieved due to the stronger flotation response of talc over chlorite. Note that talc is a naturally floating mineral and may well have floated in the absence of the amine collector.

In none of the mineral systems in which amines were used as collectors were depressants identified which would provide an enhanced selectivity. Both carboxymethylcellulose (CMC) and Na₂SiO₃ (discussed in a later section) were examined in the flotation of chlorite from quartz although neither improved selectivity in this system (Houot et al., 1995). In fact, the only variable which provided selectivity was pH due to the poor floatability of quartz at low pH with amine collectors. Similarly, in the flotation of talc from chlorite, no selective depressants were identified, although the depressants examined were not listed by Belardi et al. (1995).

Alkyl ether amines

Alkyl ether amines are generally suitable for the flotation of silicate minerals; however, only one specific reference was found to the flotation of chlorite with this chemical class. This reference (Hancock and Wang, 1993) was a patent dealing with the removal of chlorite and quartz impurities from calcite using either alkoxy alkyl amines or alkoxy alkyl guanidines. Optimum chain lengths, dose levels and pH were not specified in the abstract for this patent. Alkyl ether amines used in flotation typically have 12 carbon atoms in the alkyl chain.

Quarternary ammonium salts

Chlorite can be a troublesome impurity in diasporic bauxites, which are major feedstocks for the Chinese aluminium industry. Reverse flotation of these ores is being practised to remove silicates, including chlorite, as well as pyrophyllite, illite and kaolinite. Wang et al. (2004) have reported that chlorite can be effectively removed from diasporic bauxites that have been deslimed, using an unspecified quarternary ammonium salt (DTAL) as a collector in the pH range of 6–7. An unspecified inorganic reagent (SFL) was used as a depressant and dispersant. Methyl isobutyl carbinol was the frother, and Na₂CO₃ was added during desliming to regulate pH and aid dispersion. It is claimed that the DTAL reagent is more selective than other conventional amine collectors such as dodecylamine. The mechanism by which enhanced flotation and selectivity is achieved was not specified.

Anionic collectors

Anionic collectors reportedly used for the flotation of chlorite include alkyl phosphonic acids, alkyl sulphonates, alkyl imino-bis-methylene phosphonic acids, oleic acids and xanthates.

Alkyl phosphonates

In cassiterite (SnO₂) flotation, chlorite as a gangue mineral was observed to float similarly to cassiterite with alkyl phosphonates (C₆–C₈) in the pH range of 5–6·5 at a dose level of up to 300 g t⁻¹ (Collins, 1967; Collins et al., 1968). Chlorite could be selectively depressed in this system by scrubbing with HF or and then conditioning with Na₂SiO₃ before flotation. The molecular mechanism by which HF and/or operate was not identified, although it is possible that these reagents modify the chlorite surface charge, either by removal of Ca²⁺ or by direct adsorption, inhibiting the adsorption of the anionic phosphonate collector.

Alkyl sulphonates

Alkyl sulphonates have been observed to float chlorite in cassiterite flotation (Collins, 1967), with selectivity achieved using Na₂SiO₃. The use of alkyl sulphonate collectors has also been reported in the removal of chlorite, biotite and hornblende from feldspar under acidic conditions (Hill et al., 1969). In contrast, poor flotation response by chlorite was observed in the flotation of dolomite–chlorite mixtures using petroleum sulphonates, even at a high collector concentration of 500 g t⁻¹ (2·5<pH<5·5) (Houot et al., 1995). One might expect difficulties in holding dolomite–chlorite mixtures at such low pH values.

Alkyl imino-bis-methylene phosphonic acids

The alkyl imino-bis-methylene phosphonic acids were shown to float chlorite and other silicate minerals (Collins et al., 1984). The maximum flotation response was observed in the pH range of 5–6, although moderate flotation was observed over the pH range of 3–9. Required dose levels for flotation varied between 50 ppm (pH 5) to 100 ppm (pH 3 and pH 8). The 2-ethyl-hexyl-imino-bis-methylene phosphonic acid was studied in the case of chlorite, although it appears likely that other alkyl groups also would provide a similar flotation response. In the selective flotation of oxide minerals from silicates, it was speculated that Na₂SiF₆ or Na₂SiO₃ would provide selective depression of chlorite and other silicate minerals, although this was not demonstrated. It should be noted that flotation tests with this reagent were conducted in a glass test tube, using an unrealistically high collector/surface area ratio, from a commercial flotation point of view.

Oleic acids

It has been noted (Harris et al., 1967) that chlorite reports to the concentrate in the flotation of pyroxenes and amphibole using oleic acid collectors at pH 6·8. A similar observation has been made in the flotation of scheelite (CaWO₄) from chlorite using sodium oleate (Shin et al., 1987, 1988). In this study, the oleate collector was found to adsorb on both minerals, with maximum adsorption at pH 8. The adsorption of oleate onto chlorite at pH conditions under which chlorite would be expected to be negatively charged is consistent with a chemical (as opposed to electrostatic) contribution to the adsorption energy, characteristic of oleates. Selective flotation of scheelite in this system was achieved by the use of Na₂SiO₃ and quebracho. The latter depressant has a strong affinity for iron and may have had an effect in this system. Sodium oleate has also reportedly been used in the reverse flotation of sericite and chlorite from scheelite and wolframite [(Fe, Mn)WO₄] (Koval, 1982).

The action of oleates in the flotation of chlorite is further complicated by the successful flotation of chromite away from gangue containing chlorite using oleate collectors at pH<3 (Sysila et al., 1996). The selectivity which was observed in this system was attributed to the positive surface charge of chromite at pH<3, in contrast to the gangue minerals which were all negatively charged under these conditions. These results imply that the specific adsorption of oleate onto chlorite is weak, although it should be noted that the oleate used in this study was a sulphonated form, complicating any interpretation of the surface adsorption processes.

Zheng et al. (2009) reported increasing recovery of chlorite (single mineral) with increasing pH up to 12 with sodium oleate as collector. This was proposed to be due to increasing adsorption of the collector on the surface of the chlorite with increasing pH. Addition of calcium ions was detrimental to flotation of the chlorite due to the formation of insoluble calcium salts with the oleate collector.

The adsorption of Ca²⁺ onto the chlorite surface may activate chlorite towards oleate adsorption in a similar way to that observed for silica, but care must be exercised with the calcium dose level as collector precipitation, as seen by Zheng et al. (2009), may occur. It should however be kept in mind that oleic acids are generally non-specific collectors and the control of a commercial separation procedure using oleic acids would probably be difficult.

Xanthates

Fornasiero and Ralston (2005) have reported that chlorite present in sulphide ores can be activated with Cu (II) and Ni (II) and floated with xanthate in the pH range of 7–10. They postulated that copper or nickel species dissolved from copper or nickel minerals present, or from copper sulphate added as an activator for slow floating nickel sulphides, can form positively charged copper and nickel hydroxides in this pH range. These species can then adsorb or precipitate on negatively charged chlorite surfaces and this can promote xanthate adsorption and genuine flotation.

In their study, flotation tests were conducted in a microflotation cell, and zeta potential measurements and infrared spectroscopy were used to identify the surface copper, nickel and xanthate species responsible for mineral flotation. That chlorite flotation in this manner is possible with xanthate collectors is contrary to conventional flotation theory and it would seem sensible to validate these results in larger scale batch flotation tests.

Non-ionic collectors

The use of isoamyl alcohol in the flotation of talc and chlorite has been reported (Cases and Piga, 1992). Interestingly, significant recovery of both minerals was observed in this study in the presence of frother alone. Although this result is not surprising in the case of talc, it indicates that chlorite might exhibit some natural floatability. Tall oil has also been reported to act as a collector for chlorite, with an optimum pH range of 6–7 (Lyubimov and Shokhin, 1965).

Chlorite depression

Depression of chlorite can be achieved with sodium silicate, hydrogen fluoride, sodium hexafluorosilicate, polysaccharides and a variety of other depressants.

Sodium silicate

Sodium silicate (Na₂SiO₃) appears to be the most common depressant for chlorite, employed in both oxide and sulphide separations. There appears to be no scope for the use of Na₂SiO₃ as a selective depressant in the separation of different silicate minerals.

Na₂SiO₃ (dose level of 250 g t⁻¹) has been used for chlorite depression in cassiterite flotation using phosphonate collectors at pH 5·5 (Andrys et al., 1981; Collins, 1967; Collins et al., 1968). In two of these studies, it was not clear that Na₂SiO₃ was the sole depressant species since HF and/or Na₂SiF₆ were added before Na₂SiO₃ addition (Collins, 1967; Collins et al., 1968).

Na₂SiO₃ (dose level of 200 g t⁻¹) has been reported as a siliceous gangue depressant, with chlorite as a dominant component, in molybdenum flotation with cresylic acid collector (Prasad et al., 1975). The molybdenum tail was further floated to recover copper sulphide minerals using a xanthate collector, with apparently no further depressant addition. The use of Na₂SiO₃ has also been reported in other studies of the separation of copper sulphide minerals from quartz and chlorite gangue with a xanthate collector (Raja, 1974; Kovachev et al., 1970). In one of these studies, a relatively high Na₂SiO₃ dose level of 1·4 kg t⁻¹ was employed (Raja, 1974).

The use of Na₂SiO₃ in the separations of scheelite (Shin et al., 1988) and apatite (Beraldo, 1981) from chlorite using oleate collector has been reported. In both of these separations, Na₂SiO₃ was added with another depressant; quebracho in the case of scheelite and polysaccharide in the case of apatite. It is not clear whether the addition of these depressant mixtures results in synergistic effects.

Hydrogen fluoride

Hydrogen fluoride (HF) has been used at a dose level of 500 g t⁻¹ in a ‘scrubbing’ step at pH 2·5–3·0 before cassiterite flotation with phosphonates. The mode of action was not clear since HF was employed in conjunction with Na₂SiO₃ (Collins, 1967; Collins et al., 1968). It is possible that the fluoride ion modifies the surface charge of chlorite, decreasing the adsorption of the anionic collector or increasing the adsorption of a cationic collector. It was noted that HF could be replaced by Na₂SiF₆ without loss of performance. From an occupational health and safety point of view, however, the use of large quantities of HF would seem to be undesirable.

Sodium hexafluorosilicate

Sodium hexafluorosilicate (Na₂SiF₆) has been used as part of a suite of reagents in the depression of chlorite in cassiterite flotation (Collins, 1967; Collins et al., 1968; Isakov and Matsuev, 1972). It has also been suggested as a suitable selective depressant in the flotation of oxide minerals from silicates using alkyl imino-bis-methylene phosphonate collectors (Collins et al., 1984).

Na₂SiF₆ has been used effectively for the depression of chlorite and sericite in the flotation of gold with butyl xanthate collector (Sazhin et al., 1974). The maximum flotation of chlorite was observed to be in the pH range of 6–8, although the mechanisms by which the flotation of chlorite occurred are not clear. The possibilities of a significant entrainment contribution were not discussed. It was found that neither starch nor CMC were as effective in the suppression of silicate minerals in this system.

The use of Na₂SiF₆ has also been reported for the suppression of chlorite in a single mineral system with oleic acid collector, and to a lesser extent with lauric acid collector (Zheng et al., 2009). It was also used in the flotation of hubnerite (MnWO₄) with oleic acid (Myasnikova and Krasnikova, 1971).

Polysaccharides

Carboxymethylcellulose is a generic silicate depressant and not surprisingly has been used in separating sulphide minerals from gangue containing chlorite. Selective depression of chlorite in nickel sulphide flotation using amyl xanthate collector (pH 8·2–9·2) has been reported by Mashanyare and Storey (1986). In a separate study in which CMC was used as a depressant in nickel sulphide flotation, it was found that the total addition level of depressant was the most important control parameter, rather than the degree of polymerisation (Rhodes, 1979). Although the mechanism of CMC depression of chlorite does not appear to have been identified, there are similarities between the molecular structures of CMC and PAA. It has been speculated that PAA adsorbs exclusively on edge planes on chlorite (Sondi et al., 1997). If a similar preferential adsorption occurs in the case of CMC, then it is likely that the edge/face ratio of the chlorite particles is critical in the effectiveness of this depressant.

Polysaccharides have been used in conjunction with Na₂SiO₃ for the suppression of chlorite in the flotation of apatite with oleate collector (Beraldo, 1981).

The depression of chlorite flotation by CMC has been reported in chlorite–quartz separations, although quartz was equally affected, with the result that selectivity was not improved between these two silicate minerals (Houot et al., 1995).

Other depressants

Other depressants used for selective flotation from ores containing chlorite are as follows:

modified guar gum, which has been used in place of CMC for the flotation of nickel sulphides from chlorite containing gangue (Mashanyare and Storey, 1986)

quebracho, used in scheelite flotation from chlorite with oleate (Shin et al., 1987, 1988)

iii

tripolyphosphate, used in the selective depression of chlorite in talc–chlorite mixtures (collector not specified) (Chizhevskii et al., 1981). The specific interaction of tripolyphosphate with the chlorite surface was supported by infrared analysis

an alginic acid (‘Sobragene’) has been used as an effective depressant of chlorite in the flotation of CaF₂ with oleate (Dominique, 1973).

Conclusion

The ‘chlorite group of minerals’ can be successfully floated with either cationic (amine or ether amine) or anionic based collectors. This apparent contradictory behaviour is most likely due to both the wide variation in chlorite surface charging properties as well as specific adsorption of some anionic functional groups, especially oleates. In the case of amine based flotation, selectivity over other silicate minerals is obtained by pH control. With anionic collectors, specific depressants, most commonly Na₂SiO₃, are used to control selectivity in oxide separations.

Chlorite depression in oxide and sulphide flotation can be achieved using reagents typically used for the depression of silicate minerals, in particular, Na₂SiO₃, CMC and fluorocompounds. The dose levels required for chlorite depression with these depressants are typical for silicate minerals.

The appropriate chlorite depressant for a given mineral separation application is likely to depend upon the surface charging characteristics of the particular chlorite. The surface charging characteristics in turn depend upon the edge/face ratio of the chlorite particles and the solution chemical composition of the suspending medium.

Footnotes

Acknowledgements

The authors wish to acknowledge Dr Ivan Adair, formerly of Rio Tinto Research and Technical Development, for financial support of this work, and Dr Graham Sparrow of CSIRO Process Science and Engineering for his assistance with editing of the manuscript.

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