Flotation Chemistry of Boron Minerals - Science Direct

Journal of Colloid and Interface Science 256, 121–131 (2002) doi:10.1006/jcis.2001.8138

Flotation Chemistry of Boron Minerals M. S. Celik,∗,1 M. Hancer,† and J. D. Miller† ∗ Istanbul Technical University, Mining Faculty, Minerals and Coal Processing Section, Ayazaga 80626 Istanbul, Turkey; and †Department of Metallurgical Engineering, University of Utah, Salt Lake City, Utah 84102 Received May 18, 2001; accepted November 24, 2001; published online February 21, 2002

Boron minerals exhibit an array of solubilities depending upon the cations in the lattice structure. A classification of semisoluble (colemanite and ulexite) and soluble salts (borax) may be appropriate as each group of minerals behaves differently in flotation. While borax has to be concentrated from its saturated brine, colemanite can be recovered by conventional flotation as other semisoluble salttype minerals. A common problem encountered in both classes of boron minerals is the presence of significant amounts of clay minerals, which adversely affect flotation recoveries in the form of slime coatings. Flotation is typically accomplished by means of both anionic and cationic collectors. Pure boron and clay minerals and their mixtures have therefore been subjected to a series of microflotation, electrokinetic, adsorption, and atomic force microscopy studies with cationic and anionic collectors. The effect of mono- and multivalent ions in the absence and presence of clay is presented to elucidate their effect as activators in certain pH regions. Flotation chemistry studies on the interaction of boron minerals with anionic and cationic surfactants over the last 10 years together with new findings are reviewed to illustrate the role of electrostatic interactions even in saturated brine solutions. C 2002 Elsevier Science (USA) Key Words: boron minerals; flotation; colemanite; borax; ulexite; borates.

INTRODUCTION

Boron compounds are used in the manufacture of a variety of industrial products including advanced materials. The total world boron ore reserves as of 1999 are estimated to be a minimum of 2.79 billion tons equivalent to 1.19 billion tons of B2 O3 (1). The United States and Turkey are the world’s two largest producers of boron compounds. Together, these two countries make up about 90% of the world’s boron reserves (2, 3). Borates are primarily produced from boron minerals and boron-rich lake brines by different mining and mineral processing technologies. Although there are over 150 boron minerals identified, the most important boron minerals of commercial importance are borax (Na2 B4 O7 · 10H2 O), colemanite (Ca2 B6 O11 · 5H2 O), ulexite (NaCaB5 O9 · 8H2 O), and kernite (Na2 B4 O7 · 1

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4H2 O). Apart from these minerals, a considerable amount of boron compounds is also produced from boron-rich lakes (4). In most boron ores, the major accompanying gangue minerals are montmorillonite-type clays and carbonate minerals. Selective recovery of boron minerals usually requires a preconcentration step in some form of scrubbing to disintegrate undesirable clay minerals followed by classification and then crystallization. The recovery processes for boron minerals vary according to the solubility of the minerals. While borax has to be concentrated from its saturated brine, for the semisoluble colemanite and ulexite minerals, different recovery strategies need to be employed. The friable nature of boron minerals, however, tends to produce a large amount of fines mostly below 0.1 mm, which are usually discarded as waste. A plausible technique to recover these fines appears to be flotation. Surprisingly, despite the successful application of flotation technology in the potash industry, flotation has not yet been well developed for borax recovery. This may be attributed to inherent difficulties such as high ionic strengths, high viscosity brines, and particularly the presence of clay minerals, which act as persistent slimes. Nevertheless, significant progress has been achieved in the areas of soluble salt flotation chemistry (5–8) and abatement of slime coating in the flotation of boron minerals (9–11). The structure-making and -breaking tendency of ions in water proposed by Miller and his co-workers provided new insight into explaining the selective flotation of several soluble salt minerals from their saturated brines (8). On the other hand, Celik and his associates have identified some particular characteristics of the flotation behavior of boron minerals with cationic and anionic collectors (9, 10). Also, flotation studies conducted previously on boron minerals revealed that clay minerals adversely affect recoveries (12). The objective of the present investigation is therefore to understand the mechanism of slime-coating action in the flotation of boron minerals and to formulate conditions under which boron minerals can be efficiently floated. Toward this aim, pure boron and clay minerals have been subjected to a series of systematic microflotation, electrokinetics, adsorption, and atomic force microscopy studies in the presence of anionic and cationic reagents. 0021-9797/02 $35.00

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CELIK, HANCER, AND MILLER

EXPERIMENTAL

Materials Ultrapure colemanite, ulexite, and borax crystals were handpicked from the Bigadic and Kestelek colemanite/ulexite and Kirka borax deposits of Turkey, respectively. The lump-sized crystals were crushed by a hammer and then ground in an agate mortar to produce a sample of 150 × 74 µm in size for microflotation studies. The fine fraction (−74 µm) was further ground to obtain a sample less than 38 µm in size for ζ potential and adsorption measurements. High-purity dodedcylamine hydrochloride (DAH) purchased from Eastman Kodak Co. and sodium dodecyl sulfate (SDS) purchased from Fluka were used as collectors throughout this study. The pH was adjusted by HCl and NaOH. Borax decahydrate of 99.8% purity used in preparing the synthetic borate solutions were received from the Bandirma Acid Boric Plant of Turkey and also purchased from Sigma Chemical Co. Other inorganic chemicals, NaCl, KCl, CsCl, LiCl, CaCl2 · 2H2 O, BaCl2 · 2H2 O, MgCl2 · 6H2 O, and Al(NO3 )3 · 2H2 O were all Fluka made with more than 99% purity. Distilled and deionized water of 1–2 × 106 µ/cm conductivity was used in all experiments. Methods Microflotation tests were carried out in a 150-ml column cell (25 × 220 mm) with a 15-µm frit and magnetic stirrer. The samples of 1 g of boron mineral were conditioned in a 150-ml solution containing the desired collector for 10 min and then floated for 1 min with nitrogen gas at a flow rate of 50 cm3 /min. An automatically controlled microflotation apparatus was used to control nitrogen flow rate and flotation time (13). The electrokinetic measurements at intermediate ionic strengths were conducted by a Zeta Meter 3.0 equipped with a microprocessor unit to directly calculate the ζ potential. One gram of colemanite in 100 ml of solution was conditioned for 10 min. The suspension was kept still for 5 min to let larger particles settle. Each data point is an average of approximately 10 measurements. All measurements were made at ambient temperature and converted to 22 ± 1◦ C temperature at which flotation tests were performed. At high ionic strengths where the conventional Zeta Meters failed to function, the phase analysis light-scattering technique (PALS) was used. In this technique, phase modulation is applied so that the Doppler frequency for a particular zeromobility particle is equal to the modulation frequency ωo . It is possible then to measure the deviation of the actual frequency present in the scattered light by performing a phase comparison of the detected signal with the imposed modulator frequency. If the mobility is truly zero, the relative phase of the two will be constant: if a small mobility is present, the relative phase will be shifted, and a small phase shift can be detected by a phase comparator. Adsorption tests were carried out in 40-ml Teflon-capped glass vials. Two grams of colemanite sample were mixed in 20 ml of surfactant solution at a solid to liquid ratio of 0.2. The

vials were shaken for 2 h on a shaker and centrifuged for 15 min. The supernatant was analyzed for its amine or sulfate content by a modified two-phase titration technique (14). The adsorption density was calculated by = (Ci − Cr )V /1000 S A,

[1]

where Ci and Cr represent the initial and residual surfactant concentrations in mol/L, S denotes the amount of solid in grams, V is the volume of the solution in ml, A is the specific surface area of the sample, and the adsorption density in M/m2 . Light transmission measurements were performed by a Shimadzu spectrophotometer at a wavelength of 600 nm. The solutions were shaken for 15 s and then left intact for 24 h before the measurements. Force measurements were conducted using a Nanoscope E atomic force microscope (Digital Instruments, Inc.). The interaction of a micro particle tip (silica and clay) with the colemanite surface was determined. The silica microspheres were a mean diameter of 5.0 µm. The microspheres suspended in distilled water were recovered by drying a drop of suspension on a glass plate. Then, a single spherical particle was mounted on the AFM cantilever tip using a speed bonder and an activator by means of a micropositioner and a CCD camera/monitor system. All the force measurements were conducted in a liquid cell in an aqueous environment. RESULTS AND DISCUSSION

Dissolution of Boron Minerals Boron minerals exhibit a spectrum of different chemical compositions with cations ranging from monovalent to multivalent ions. The type and valency of the cation dictate the solubility of the mineral and in turn its electrokinetic behavior. Salt-type minerals such as borates release a number of species upon dissolution in water. Colemanite is a boron mineral with Ca2+ ion in its structure and undergoes acid–base reactions in the vicinity of pH 9.3. The following overall dissolution process for colemanite occurs in a system open to the atmosphere (15): 2CaO · 3B2 O3 · 5H2 O + 4CO2 + 6H2 O ⇔ 2Ca2+ + 6H3 BO3 + 4HCO− 3.

[2]

Similarly, ulexite dissolves in water in the presence of CO2 : NaCaB5 O9 · 8H2 O + 3CO2 + 6H2 O − ⇔ Na+ + Ca2+ + 5H3 BO− 3 + 3HCO3 + 5H2 O.

[3]

On the other hand, the highly soluble boron mineral, borax, will dissolve according to the following reaction: 2Na2 B4 O7 · 10H2 O − − ⇔ 4Na+ + B4 O2− 7 + HB4 O7 + OH + 9H2 O.

[4]

FLOTATION CHEMISTRY OF BORON MINERALS

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The solubilities of colemanite, ulexite, and borax at 20◦ C are 5 × 10−3 , 1.5 × 10−2 , and 0.11 M, respectively (11, 15–18). Electrokinetic Behavior of Boron Minerals The electrokinetic behavior is an indicator of the ability of ions to be incorporated in the double layer and, in particular, may reveal the expected flotation response for a certain collector scheme. The electrokinetic behavior of boron minerals has been characterized in terms of the pH of their isoelectric points and also on the basis of potential-determining ions. The solids concentration has been found to be an important parameter in controlling the magnitude and even the sign of the ζ potential measurements (9). Figure 1 shows the ζ potential of a series of boron minerals in water as a function of pH (15). While the isoelectric point of Bigadic colemanite is found to occur at pH 10.5, inderite (magnesium borate) and tunellite (strontium borate) show positive ζ potentials throughout the pH range with no clear iep. Similarly, ulexite (Fig. 1) and borax (Fig. 3) yield no iep down to about pH 7. The potentialdetermining ions (pdi) for boron minerals are found to be the constituent lattice cations, i.e., Ca2+ (for colemanite) and the + − anion B4 O2− 7 , and the H and OH ions, which control the ratio 2− of HCO3 /CO3 (15, 16). These are exemplified for colemanite in Fig. 2. Figure 3 illustrates the ζ potential of borax as a function of pH. It should be noted that these equilibrium ζ potential measurements for borax have been determined for the first time in the literature using the phase analysis light-scattering technique (PALS). As described in the Experimental section, this technique is capable of measuring the ζ potential of particles in electrolyte solutions with concentrations higher than 0.1 M

FIG. 1.

Variation of pH with ζ potential for different boron minerals (15).

FIG. 2. Effect of added ions on ζ potential of colemanite at the natural pH of 9.3 except for H3 BO3 and Na2 CO3 (15).

depending upon the type and nature of the ions in solution (19). The nonequilibrium laser-Doppler electrophoresis (LDE) measurements reported earlier (20) now reveal their inaccuracy and therefore Fig. 3 represents a breakthrough in this area. The iep of borax, similar to ulexite, appears to be absent of impossible within the pH range of stability. However, borax is found to exhibit a negative charge in the pH range of interest. Further studies were conducted to determine the pdi’s for borax. The borate anion (B4 O2− 7 ) was verified to be the pdi for all boron minerals studied except borax due to the impossibility of testing the borate anion beyond the saturation limit of borax (21).

FIG. 3. ζ potential of borax as a function of pH as determined by ZETA PALS technique (21).

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Flotation of Boron Minerals with Monovalent Salts Monovalent salts are also prevalent in the lattice structure of a number of boron minerals. The most prominent one is sodium in the structure of borax. Apart from the effect of monovalent ions in compressing the double layers of minerals, they also modify the bulk water structure and micellization of collectors and consequently enhance surface activity of collectors. There are controversial opinions about the role of monovalent ions in the floatability of minerals. While some assert that monovalent cations and anions depending on their water-breaking and -making structure modify the bulk structure of water and extrapolate the same effect all the way to the surface, others emphasize adsorption of monovalent ions at solid surfaces and their subsequent interaction with water (8, 22, 23). A series of systematic flotation, ζ potential, and turbidity measurements were performed to test which mechanism may be operative in the case of boron minerals. NaCl and KCl, typical structure maker and breakers, and CsCl, a strong structure breaker, were used to test the above mechanisms (24). It should be noted that in this section we exclusively used a colemanite sample from Kestelek colemanite deposit, which was found to exhibit totally different ζ potential behavior. The sample exhibited a negative charge at the natural pH of 9.3, as opposed to the Bigadic colemanite used throughout this study, which yields a positive charge with an iep at pH 10.5. Figure 4 illustrates the effect of monovalent salts, i.e., NaCl, KCl, and CsCl, on the floatability of colemanite at 4 × 10−5 M collector concentration. The percent floated is plotted as a function of salt concentration for both SDS and DAH. The highest salt concentration, i.e., 4 M, used in the experiments is close to the saturation concentrations of the respective salts. (Saturation concentrations of NaCl, KCl, and CsCl are respectively 5.2, 4.1, and 9.6 M at room temperature.) The data show that

FIG. 4. Effect of monovalent salts on the floatability of colemanite with 4 × 10−5 M of cationic and anionic collectors, pH 9.3 ± 0.1.

both surfactants enhance the floatability of colemanite. While DAH exhibits a plateau with increasing salt concentration, SDS undergoes a maximum at salt concentrations corresponding to the precipitation/dissolution of the surfactant salt, particularly at high salt concentrations where precipitation of Ca(DS)2 is inevitable (25–27). The first question arising under such high ionic strength conditions is the occurrence of precipitation in the presence of surfactant. For turbidity tests colemanite supernatants have been used to mimic the test conditions. The desired salt was dissolved in colemanite supernatant and then mixed with as much concentrated surfactant solution as possible. Since colemanite contains 5 × 10−3 M of Ca2+ and probably an equal amount of 2+ B4 O2− with dodecyl sulfate anion 7 ions, the interaction of Ca is expected to lead to precipitation of Ca(DS)2 . However, since the surfactant concentration is low (4 × 10−5 M), the solution turbidity is expected to be rather insignificant. Indeed, the results indicate that there is almost no precipitation of DAH with both NaCl and KCl within the detection limits of the spectrophotometer together with visual observations. However, in the case of SDS there are indications that precipitation and dissolution of Ca(DS)2 and possibly reprecipitation of SDS are all taking place in the system. Addition of monovalent salts is known to reduce the CMC and consequently dissolve salts of multivalent ion precipitates at lower SDS levels (28). The precipitation appears to start at 0.1 M salt concentration and undergoes dissolution with increasing of the salt concentration. The role of electrostatic interactions in the flotation of most boron minerals and particularly on that of colemanite and ulexite was clearly illustrated elsewhere (10, 26). It is generally accepted that monovalent ions are indifferent electrolytes and thus only function in the compression of an electrical double layer; this compression ceases at about 1 M monovalent salt addition where the thickness of the electrical double layer is on the order of ˚ Some researchers believe that the electrostatic interactions 1 A. sharply decay above 0.1 M of salt addition. At monovalent salt levels higher than 0.1 M and especially at 1 M, the electrostatic mechanism may be conveniently ruled out. The effect of added salt on the surface forces between two mica plates immersed in water revealed a short-range oscillatory force at high ionic strength in addition to the expected van der Waals and electrostatic double-layer forces (29–31). This was called the hydration force and originated from the dehydration of cations at the mica surface. Accordingly, smaller, more strongly hydrated ions would produce a larger short-range repulsive force because of the greater energy required to dehydrate the smaller cations (23). The only plausible mechanisms that can be proposed under such high ionic strength conditions are ion exchange or hydrogen bonding. The hydrogen-bonding mechanism can be tested by floating colemanite with a quaternary amine (dodecyltrimethylammonium chloride, DTAC), which has little affinity to form hydrogen bonding with anions. A series of systematic flotation tests with monovalent salts were carried out to determine the response of


FIG. 5. Comparison of the floatability of colemanite with primary and quaternary amines at pH 9.3 ± 0.1.

DTAC to colemanite both in the absence and in the presence of NaCl, KCl, CsCl, and LiCl (21). The results in the absence of any salt are shown in Fig. 5 where for the sake of comparison the data for the primary amine (DAH) are also included. These results indicate that the primary amines are capable of floating colemanite at collector levels lower than the quaternary amines. This interestingly reveals that in the case of primary amine collectors, in addition to electrostatic interactions, the possibility of hydrogen bonding exists. Crystallographic studies have shown that the anion [B3 O4 (OH)3 ]2− is equivalent to (B4 O7 )2− (32). Colemanite (Ca[B3 O4 (OH)3 ]H2 O) forms a monoclinic prismatic class of 2-m crystals. Its structure contains infinite boron–oxygen chains parallel to the a axis. The chain element consists of two BO4 tetrahedra and a ring of [B3 O4 (OH)3 ]2− composition. The chains are joined to each other laterally by ionic bonds through the Ca2+ ions to form sheets extending parallel to 010. A system of hydrogen bonds involving the hydroxyl groups in the chain and the water molecules tie the structure together (32–34). In this structure, hydrogen bonding between the borate anion and amine cation is always possible.

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and activity coefficients. The adsorption of these ions on the surface of colemanite was verified by adsorption measurements (35). As apparent from Fig. 6, the activation of colemanite with multivalent ions is marked by three distinct regions. Region I is characterized by the adsorption of multivalent ions onto the colemanite surface followed by the adsorption of anionic surfactant. Activation in Regions II and III is ascribed to the surface and bulk precipitation of metal dodecyl sulfate onto the colemanite surface. The presence of surface precipitation prior to bulk precipitation has been demonstrated in a number of systems (36, 61). Contrary to the activation phenomena observed in the flotation of colemanite, the effect of multivalent ions on the floatability of ulexite at a natural pH of 9.3 was found to either depress or have little influence on ulexite flotation (26). However, the same ions resulted in enhanced floatability at higher pH values. Irrespective of the type of ion, the flotation recoveries exhibited approximately the same trend. This indicates that the system is governed by a similar mechanism in terms of the effectiveness of ions. Supporting evidence comes from Fig. 7 where the ζ potential of various particulates in the presence of aluminum ion is presented. In the pH range of 4.5–8.5, the Al(OH)3 precipitate remains positively charged whereas above this pH the precipitate undergoes a charge reversal: the iep of the precipitate is 8.5. The ζ potential measurements in aluminum-containing solutions versus pH reported by Li and Somasundaran (37, 38) reveal a striking similarity to those of the precipitate in Fig. 7. Accordingly, it was proposed that bubbles, particles, and precipitates, which all become hydrophobic particulates in the region of collector-metal ion precipitation, coagulate due to hydrophobic interactions in the form of structural forces. The charge character of the particulate seems to be of no relevance to the coagulation process (26).

Flotation Behavior of Boron Minerals in the Presence of Multivalent Ions Most boron minerals contain a multivalent ion as a constituent lattice ion. For instance, multivalent ions such as Ba, Ca, and Mg were shown to activate the flotation of colemanite in the presence of an anionic surfactant (SDS). As shown in Fig. 6, the least soluble Ba(DS)2 activates colemanite most; the most soluble Mg(DS)2 activates colemanite the least. Metal ions depending on their solubility products with dodecyl sulfate are expected to activate colemanite (35). This is evident in the solubility products of collector salts in the order of Ba(DS)2 (4.8 × 10−12 ) < Ca(DS)2 (2.1 × 10−10 ) < Mg(DS)2 (3.1 × 10−9 ). The solubility products for each ion were determined by mixing salts of multivalent ions with SDS and calculating the solubility products from the onset of precipitation using the concentrations

FIG. 6. Effect of multivalent ion concentration on the activation of colemanite at pH 9.3 ± 0.1 (35).

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for the same system indicated a sharp break at about the same DAH concentration, indicating both the presence of the onset of precipitation and that of micellization (44). When amine is dissolved in water, it undergoes the following reactions (45, 46): − RNH3 Cl(s) ⇔ RNH+ 3 (aq) + Cl

[5]

+ RNH+ 3 (aq) ⇔ RNH2 (aq) + H (aq)

pK a = 10.63

[6]

RNH2 (aq) ⇔ RNH2 (s)

pK sp = 4.69.

[7]

On the other hand, the dissolution of borax in water is envisaged to occur as in Eq. [4]. In an earlier paper, the precipitation of amine was proposed to occur in the form of aminehydroborate complex formation (20). Accordingly, the formation of amine borate and hydro borate can be written as follows: FIG. 7. ζ potential of various particulates in the presence of 10−3 M aluminum ion and 2 × 10−4 M SDS: () Al(OH)3 precipitate; () aluminum dodecyl sulfate precipitate; () ulexite particle coated with precipitate (26).

Interaction of Amine with Borate Species in Boron Flotation Pulps ζ potential of amine adsorbed onto colemanite (9) revealed an abrupt increase in ζ potentials above a critical DAH concentration. Such a rise in ζ potential in the colemanite/DAH and ulexite/DAH systems was earlier ascribed to the presence of precipitate uptake on the mineral (9, 10). It is reasonable to expect that the same increase in ζ potential be induced as a result of both the precipitate and the colemanite acquiring the same charge. The colloidal precipitates composed of long-chain surfactants have been reported to be generally hydrophobic (39, 40). The presence of DAH precipitate observed in boron supernatants is crucial in the interpretation of flotation results, particularly those of colemanite. Therefore, a series of systematic tests were initiated to determine the nature of the precipitate. Figure 8 presents percent transmission of DAH/borate solutions as a function of initial surfactant concentration at two levels of borax (Na–borate) addition and in the presence of colemanite supernatant. The effect of pH on amine hydrolysis and the colloidal behavior of amine have been well discussed in a number of publications (41–43). The pH of solutions was also recorded to follow the effect of pH on precipitate formation. It is clear that the percent transmission starts decreasing at a DAH concentration of 10−4 M and then decreases further with increasing initial DAH concentration. At both levels of Na–borate additions, the transmissions exhibit a minimum; while at 10−3 M Na–borate addition, the precipitates dissolve due to a decrease in pH from 9.3 down to 6.6, the precipitate remains undissolved at 10−2 M of borax addition at the highest amine addition because the pH is lowered to 7.8 only. Apparently, this is not sufficient to induce the redissolution of the precipitate (44). The surface tension data

2− 2RNH+ 3 + B4 O7 ⇔ (RNH3 )2 B4 O7 (s)

[8]

− RNH+ 3 + HB4 O7 ⇔ RNH3 HB4 O7 (s)

[9]

or

The formation of amineborate or aminehydroborate was tested by spectrophotometric and infrared measurements and found that the precipitate is dodecylamine rather than the dodecylamine hydro borate complex (44, 47, 48).

FIG. 8. Percent transmission of DAH/borate solutions as a function of initial surfactant concentration at two levels of borax addition and in the presence of colemanite supernatant.


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Flotation Behavior of Boron Minerals in the Presence of Clay Slimes Flotation studies conducted in the past with boron ores using a bench-type Denver cell have shown that borax recovery generally does not exceed 35% (49). In comparison to the semisoluble boron minerals (colemanite and ulexite), borax is a relatively soluble boron mineral and unique in that it easily floats in its hydrated state but shows almost no flotation in its anhydrous form (Na2 B4 O7 ) with either cationic or anionic collectors at room temperature. To find out the reasons for such low recoveries, microflotation studies were carried out with boron minerals in the presence and the absence of clay slimes. While all boron minerals floated with both anionic and cationic surfactants in the absence of clay, they exhibited a dramatic decrease in recovery with the addition of clay. This behavior is illustrated for colemanite in Fig. 9. To study the mechanism of this process, ζ potential measurements were made for colemanite, clay, and their mixtures. The results are presented in Fig. 10. Here, the mixture is composed of 1 g of colemanite and 0.1 g of clay. It is clear that colemanite reverses its charge at pH 10.5, whereas the clay remains negatively charged over the entire pH range. Interestingly, the colemanite + clay mixture acquires a charge profile similar to that of clay alone. This suggests that the negatively charged clay slimes have a strong affinity for colemanite, which induces electrostatic interactions between negatively charged clay and positively charged colemanite particles at the natural pH of 9.3. It has been found that as low as only 1% clay addition can reverse the charge of colemanite surface from positive to negative. Similar behavior was also found for other boron minerals under

FIG. 9. Floatability of colemanite + clay mixture as a function of percent clay added in the presence of anionic and cationic collectors at the natural pH of 9.3.

FIG. 10. ζ potential of colemanite, clay, and the mixture of colemanite + clay (10/1) as a function of pH.

different conditions (10, 49). However, since ulexite and borax are negatively charged in the pH range of practical interest, the electrostatic interactions with clay are diminished. Therefore, only the results with colemanite are shown here. Various methodologies were examined to enhance the flotation of boron minerals in the presence of clay. Figure 11 illustrates the floatability of colemanite as a function of DAH for individual colemanite and clay minerals and colemanite + clay mixture. While colemanite alone floats well with dodecylamine hydrochloride (DAH), the colemanite + clay mixture and to a lesser extent the clay alone starts floating at concentrations several times higher than that of colemanite alone. Interestingly, the onset of flotation for both systems starts at about 10−4 M DAH concentration, which corresponds to the onset of collector

FIG. 11. Floatability of colemanite, clay, and colemanite + clay mixture as a function of DAH concentration at pH 9.3 ± 0.1.

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precipitation also. A similar phenomenon was also observed in the ulexite/clay/DAH system. In both systems it appears that the presence of colloidal precipitates facilitates the floatability of boron minerals in the presence of slimes. These results may be explained on the basis of heterocoagulation between colemanite, clay, and the precipitate. This explanation is supported by the ζ potential measurements in Fig. 10 where a drastic increase in charge from negative to positive is seen. Similar results were also obtained with ulexite. The anionic surfactant (SDS) floats colemanite and borax well but does not float ulexite very well. The colemanite + clay mixture is found to float well with SDS particularly in the region of Ca(DS)2 formation. Similar results were also obtained with the ulexite/clay/SDS system (49).

in the isotherms correlates well with the onset of precipitate formation. Adsorption isotherms in the presence of clay for the colemanite/clay/SDS system with all precipitate contributions taken into account were given elsewhere (12). It is shown that the colemanite and colemanite + clay systems yield similar adsorption values in the rising branch of the isotherm, which is of interest for flotation. The uptake of clay, on the other hand, appears to be much less in the same region compared to colemanite. These adsorption results support the flotation results presented earlier in that SDS is probably adsorbed onto colemanite rather than the slime-coated clay surface due to the higher affinity of surfactant toward colemanite.

Adsorption of Collectors on Colemanite

The uptake of negatively charged clay mineral by positive sites of boron minerals through electrostatic attraction forms the basis of slime coating in flotation. Although ζ potential and flotation results clearly demonstrate the presence of slime coating, the mechanism of restoring flotation at higher surfactant concentrations is not well established. There are at least two possible mechanisms operating in the system. In the first mechanism, at high surfactant concentrations anionic collector molecules are exceedingly higher in number and compete successfully with negatively charged clay particles; this competition favors the uptake of surfactant molecules by boron minerals and the clay particles are replaced from the surface. In the second mechanism, part of the collector molecules adsorb onto the clay particles and induce hydrophobicity to clay-coated colemanite and make the clay–colemanite aggregate floatable. Recently, a major breakthrough has been achieved in quantifying the forces involved between interacting bodies. Such forces are now measured by atomic force microscopy (AFM), which can determine the nature of interaction forces in boron mineral flotation. Standard AFM technique allows the interaction forces between a smooth flat surface and a pyramidal tip, usually silicon nitride. However, the resulting force–distance curves cannot be analyzed theoretically due to ill-defined geometry that does not allow normalization of the measured forces with respect to the radii of the objects involved. Only when the force measurements are conducted between two crossed cylinders, two spheres, or between a sphere and a plate (50–54) can the data be analyzed theoretically and converted to energy or disjoining pressure using the Derjaguin approximation (55). Even though the surface force apparatus (SFA) has been used for force measurements between surfaces with these geometries, AFM has only recently been used to measure interaction forces with the sphere–plate geometry (56–58). These investigators used microspheres of radius 1–10 µm in place of the pyramidal tip on the AFM cantilever. AFM measurements conducted with a naturally cleaved colemanite plate and a spherical aminopolystyrene particle as a function of pH indicated that electrostatic forces prevail in the system (59). A typical set of data for colemanite/silica/SDS and colemanite/clay/SDS systems at 5 × 10−5 M SDS concentration

Boron minerals release significant quantities of mono- and multivalent cations and borate species. These ions have been found to interact with both anionic SDS and cationic DAH collectors to form insoluble precipitates. While the interaction of Ca2+ with SDS results in the formation of calcium dodecyl sulfate [Ca(DS)2 ], that of borate with amine is found to lead to the formation of dodecylamine precipitate itself. Although in the absence of clay flotation of boron minerals is complete before the formation of collector precipitates, in the presence of clay, flotation is generally restored only after the onset of collector precipitate formation. To fully understand the contribution of the precipitate in overcoming the slime coating, adsorption measurements have been carried out in the absence and the presence of clay. Figure 12 illustrates the adsorption density as a function of residual SDS concentration for the colemanite/SDS system. It is seen that subtracting the effect of precipitate from the abstraction isotherm yields a plateau. Calculations from the solubility product of Ca(DS)2 (K sp = 1.3 × 10−10 ) show that the sharp rise

FIG. 12. Adsorption isotherm for the colemanite/SDS system with all precipitate contributions taken into account at pH 9.3 ± 0.1.

AFM Studies on Slimes Coated by Silica and Clay


FIG. 13. Force/radius vs separation distance profiles for colemanite plate with silica or clay tips in the presence of colemanite supernatant at 5 × 10−5 M SDS concentration.

are shown in Fig. 13. In both systems the attractive forces are evident. In the case of the silica tip, the attractive forces were found to increase with increasing surfactant concentration. However, the distance of separation appeared to shift upon increasing surfactant concentration and reached a maximum distance of about 70 nm at 5 × 10−3 M of SDS concentration (49). It should be noted that these tests were conducted in the presence of colemanite supernatants. Therefore, the presence of approximately 5 × 10−3 M of calcium ions will exert a considerable influence on the interaction of silica with colemanite. Since a silica surface at pH 9.3 is negatively charged, calcium ions will adsorb onto silica and induce the adsorption of SDS onto the positively charged surface through electrostatic attraction. As the surfactant concentration was increased, the attractive forces became long range, indicating the existence of hydrophobic interactions at higher surfactant additions. The colemanite/clay system again illustrated in Fig. 13 exhibits similarities to the colemanite/silica system. It should be noted that the clay particle, which represented a typical platetype bentonite particle, was about the same as the silica particle (49). Only the force/radius curve in the case of a colemanite/clay/SDS system is less deep than the colemanite/silica system with a shorter distance of separation of about 25 nm. Although not quantitative, all these results generally reveal that it is qualitatively possible to mimic the interaction of clay with boron minerals by using silica microspheres. This is indeed a major breakthrough in contributing to the interaction of clay particles with various solid surfaces.

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ultrasonic bath by employing two different methods. In the first one, the conditioning was done by magnetic stirring for 9 min followed by a minute of ultrasonic conditioning. The sample was then transferred to the microflotation cell and floated. No difference was observed between this and that floated using 10 min of magnetic stirring. On the other hand, when the sample conditioned for 9 min by magnetic stirring followed by in situ ultrasonic conditioning (while the microflotation cell was placed in the ultrasonic bath), a remarkable increase in recovery was obtained. The flotation recoveries versus ultrasonic treatment time presented in Fig. 14 vividly show that while in the absence of ultrasonic treatment the recovery is only 5%, after 60 s of sonication full recoveries over 90% were obtained. The mechanism of this process is merely attributed to the effect of an ultrasonic field, which helps in detaching clay particles from the surface and in turn facilitates the uptake of SDS molecules by colemanite. A detailed analysis of this process shows that ultrasonic treatment not only enhances the dispersion of clay but also improves the solubility of colemanite (60). Similar enhancement was found with the addition of calcium ions. This observed improvement was earlier explained on the basis of the formation of the surface precipitate of Ca(DS)2 followed by its bulk precipitate at higher SDS or CaCl2 concentrations. The synergistic effect of calcium and sonication can be explained on the basis of the oppositely charged clay and calcium that do not compete for adsorption sites. While clay and SDS compete for sites over colemanite, calcium has no such drawback and thus directly adsorbs onto colemanite over which more SDS can adsorb. Ultrasonication, on the other hand, cleans the colemanite surface from clays and makes it amenable

Ultrasonic Treatment for Detachment of Clay Particles from Colemanite Surfaces Since the attachment of clay particles at the colemanite surface is the major driving force for the decrease in colemanite flotation, a set of flotation experiments was carried out to test the ability of ultrasonic treatment to disperse the clay particles and thereby restore flotation. Sonication tests were done in an

FIG. 14. In situ ultrasonic flotation for the colemanite/clay/SDS system as a function of sonication and flotation time (60).

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to the adsorption of SDS. The mechanism of enhanced flotation results observed under sonication was further elaborated elsewhere (60). CONCLUSIONS

1. Electrokinetic studies indicate that while colemanite yields an iep value of pH 10.5, ulexite, borax, and boron clays carry negative charges at all practical pH values. For all boron minerals the potential-determining ions are the constituent lattice ions, + − i.e., B4 O2− 7 and the counterion, as well as the H and OH ions, − 2− which control the ratio of HCO3 /CO3 . 2. Flotation of colemanite with both anionic (SDS) and cationic (DAH) collectors in the absence of salt is attributed to electrostatic interactions in the system. Flotation of colemanite in the presence of monovalent ions is enhanced. ζ potential and turbidity measurements reveal that electrostatic interactions are possibly prevalent, even up to 0.5 M of salt concentration beyond which electrostatic interactions cease. It appears that, for semisoluble minerals, i.e., colemanite, whose pdis are the constituent lattice ions, the effect of monovalent salt addition is to improve the salting out of collectors and enhance their consequent surface activity and micellization. 3. Multivalent cations have also been shown to act as activators in boron flotation systems. The precipitation of collector colloids in flotation pulps followed by their attachment to oppositely charged surfaces through heterocoagulation has been found to lead to improved recoveries both in the presence and in the absence of clay slimes. This phenomenon is particularly enhanced at pH values where the multivalent cations undergo hydrolysis. 4. A common problem encountered in all types of boron minerals is the presence of significant amounts of clay-type minerals, which adversely affect flotation recoveries due to slime coatings. It is found that even as little as 1% of clay addition can reduce the flotation recoveries considerably. Boron minerals plus clay mixtures acquire a charge profile similar to that of clay itself. This is ascribed to the uptake of negatively charged clay mineral onto positive sites at the surface of boron minerals as slime coating. Flotation recoveries of colemantie upon increasing pH increases with DAH and decreases with SDS. It is shown that, regardless of the system, the flotation recoveries are generally restored at high collector concentrations. 5. The restoration of flotation can be explained in terms of two different collector adsorption mechanisms. One explanation considers that, at high concentrations of the anionic collector, the collector species compete well and the clay particles are displaced from the surface. In the second explanation, a portion of the collector molecules adsorb onto clay particles and induce hydrophobicity to clay-coated colemanite and consequently make the clay–colemanite aggregate floatable. 6. Atomic force microscopy (AFM) has been used to reveal the extent of interactions between particles (boron–clay) and particle and collector molecules to quantitatively determine the

interaction of clay particles with colemanite as a model boron mineral. AFM nicely corroborates the results of both adsorption and flotation and quantify the magnitude of interactions between the clay particle and two boron mineral surfaces. 7. One of the physical ways to enhance adsorption of flotation chemicals is to condition the flotation pulp by ultrasonic treatment. In situ sonication involves the simultaneous action of an ultrasonic field during conditioning and appears to be most promising for reducing the dosage of chemical reagents and more importantly detaching clay particles from the colemanite surface. Conditioning with in situ sonication requires a collector addition several times lower than that prepared by conventional conditioning. ACKNOWLEDGMENTS Financial support for this work during 1996–2000 was provided by NSF (Grant No. INT-9512813) and TUBITAK of Turkey.

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