Bor minerallerinin elektrostatik ve elektrokinetik özellikleri

Abstract

Bor mineralleri, teknolojinin gelişmesi ile günlük yaşantımızın ayrılmaz bir parçası haline gelmiştir. Bu yüzden borun dünya pazarlarındaki önemi gittikçe artmaktadır. Dünya bor rezervleri bakımından ilk sırada yer alan ülkemiz için bor çok önemli bir hammaddedir. Bu. sebeple ülkemizin bor teknolojisinde de dünyada ilk sırada yer alması gerekmektedir. Saf kolemanit, üleksit, boraks, indent ve tünelit gibi karakteristik bor mineralleri ve bunların başlıca empüriteleri. olan killerin elektrostatik ve elektrokinetik özellikleri ilk defa bu çalışmada incelenmiştir. Çalışmada kullanılan bütün numuneler Bigadiç, Kırka ve Kesîelek'bor yataklarından el ile saf olarak toplanmışlardır. Yapılan kimyasal analizlerde numunelerin Sr ve Li içerdiği tesbit edilmiştir. Elde edilen sonuçlar elektrostatik ve elektrokinetik olmak üzere iki bölüm halinde sunulmaktadır. Elektrostatik deneylerde kullanılan mineraller beyaz, bej ve siyah renkli kolemanit, üleksit, boraks, beyaz ve gri renkli boraks killeri ile Bigadiç kilidir. -2 + 0.84 mm boyutları arasına indirilen numunelerin elektrostatik özellikleri Dings Elektrodinamik ayırıcıda incelenmiştir. Elektrokinetik deneylerde bor mineralleri olarak saf kolemanit, üleksit, inderit ve tünelit, kil minerali olarak da beyaz ve gri renkli Kırka kili, Bigadiç kili ve Wyoming kili kullanılmıştır. -105 jım boyutuna indirilen numunelerin farklı ortamlardaki yüzey yükleri Zeta Meter 3.0 cihazı ile ölçülmüştür. Bor elementi literatürde yalıtkan olarak bilinmesine rağmen elektrostatik deneyler sonucunda beyaz kolemanit hariç diğer bor minerallerinin iletken davranış gösterdiği bulunmuştur. Sıcaklık değişiminin boraks ve üleksit numuneleri üzerinde büyük bir etkisi olduğu, sıcaklıktaki küçük değişimlerin bile bu iki numunenin büyük ölçüde yalıtkan davranmasına sebep olduğu tesbit edilmiştir. Aynca bor minerallerinin iletken özelliğinin bünyelerindeki oldukça büyük miktarda Sr ve Li gibi toprak alkalilerden.kaynaklandığı bulunmuştur. Bor minerallerine refakat eden killerin genellikle iletken özellik gösterdikleri ve sıcaklıkla iletkenliklerinin değişmediği saptanmıştır. Elektrokinetik ölçümlerde ise katı konsantrasyonunun bor mineralleri için tane yükünü belirleyici bir özellik olduğu tesbit edilmiştir. Kolemanitin sıfir yük noktası (syn) pH 10.5 olarak bulunurken, diğer bor mineralleri için belirli bir syn bulunamamıştır. Aynca bor minerallerinin kafes yapılanndaki kaîyonlann (Ca2+, Mg2+, Sr2+) ve B4072_,ün potansiyel tayin edici iyon olduklan saptanmıştır. Elde edilen bu sonuçlar mikroflotasyon sonuçlan ile uyum içindedir. Bor killerinin ise montmorillonit tipi kil özelliği gösterdikleri ve tüm pHlarda negatif yük sergiledikleri görülmüştür. Bu da bor killerinin tabaka yüzeyleri ve kenarlanndaki net yükün negatif olmasından kaynaklanmaktadır.

Although about 70 % of the total boron reserves are in Türkiye, there is very little literature data available on the fundamental properties of boron minerals. The aim of this study is to investigate the basic properties of boron minerals related to such concentration technologies as electrostatic and flotation separation. The electrostatic separation tests have been conducted by a Dings electrostatic separator. The conditions under which boron minerals become conductive have been investigated and the governing mechanisms delineated. The electrokinetic properties, a basic tool interperting the flotation phenomenon, have been investigated by a Zeta Meter using pure boron minerals such as colemanite, ulexite, inderite and tunellite in the presence of various electrolytes. The zero point of charge (zpc) and the potential determining ions (pdi) for these minerals have been determined. Electrostatic separation of solid particles is based upon forces acting on charged or polarized bodies under the influence of an electric field. If a particle in a mixture receives surface charge on entering an electrostatic field, it will be repelled from one of the electrodes and attracted towards the other depending on the sign of the charge on the particle. The charging mechanisms can be generally classified into three categories: induction charging, charging by ion bombardment (corona charging), and contact charging or tribocharging. If a particle is placed on a grounded conductor under an electric field, it will develop a surface charge by induction. While both conductor and insulator particles will become polarized, a conductor particle will exhibit a equipotential surface through its contact with the grounded conductor. The insulator particle will remain polarized due to its inability to redistribute electrons and subsequently become uncharged [27, 28, 29]. Electrostatic separation has been applied to a variety of materials but notably in the processing of beach sands and alluvial deposits containing titanium minerals. Boron element (at 0°C 1.8x1 0*2 microohm-cm) is reported virtually to be a nonconductor. Borax and colemanite are also listed as insulators in a compilation by Carpco. Our preliminary experiments, however, were contrary to this. Research on electrostatatic separation of boron minerals is rather scarce in the literature. The only study, to our knowledge, has found that electrostatic separation can concentrate colemanite from its gangue mineral with some success. In particular, increasing the temperature has been found to enhance the separation. However, no mechanistic interpretation has been afforded to explain whether the boron mineral or its gangue clay mineral itself is conductive [3 1, 32], VII It is the objective of the electrostatic study to examine whether the pure boron minerals are conductive or the conductivity observed is imparted by the impurities in the ore. Towards this aim, the boron minerals of commercial importance, i.e., colemanite, ulexite and borax and their accompanying clay minerals have been subjected to a set of systematic electrostatic separation tests. The fundamental reasons governing the absence or presence of conductivity are interpreted with the help of the micro impurities associated with the boron minerals. The boron minerals used in electrostatic investigation are ultra purity. White, beige and black colemanites (Ca^On.SHjO), ulexite (NaCaB509.8H20) and borax (Na2B407 İOH2O) were hand picked from Kestelek, Bigadiç and Kirka boron deposits, respectively. The chemical analyses revealed 51.31 %, 42.50 % and 36.30 % B203, respectively. The corresponding montmorillonite type clay minerals which are Bigadiç clay, white and gray Kırka clays were received from Bigadiç and Kirka deposits. The former clay sample exhibited a layered structure. All the selected boron and clay samples were first crushed to a size of about minus 1 cm by a hand hammer followed by a agate mortar. The size fraction of -2.00+0.84 mm was used for electrostatic separation tests. The boron minerals used in electrokinetic investigation which are colemanite, ulexite, inderite and tuneîliîe were received from the Bigadiç boron deposit. A small amount of Wyoming clay has been had for standardization. The hand picked samples of ultra purity were ground in an agate mortar below approximately 38 urn. The samples were kept in plastic zipper bags The electrostatic separation tests were carried out by a rotor type Dings Electrodynamic separator. A sample of 100 g was placed into the feeding chute which was operated at constant conditions of 70 rpm speed and 20 Hz frequency such that all the material reached the rotor in one minute. The product splitter height was kept at the vertical position (0°) when using the beam type electrode (corona charging) and 7° away from the rotor for the static electrode (induction charging). The beam electrode is composed of a series of needle points positioned parallel to a ground rotor of a separating machine. The position of the splitter was adjusted with the reference materials (quartz and pyrite) such that the recovery was either nil or full in the absence of any electric field. Under the applied voltage, the material collected at the front of the splitter was considered as conductor. At the end of each experiment, the material was kept in a grounded container for a period of 12 hours to discharge the possible static electric charge. This period was found to be sufficient to reuse the same material for another experiment. It should be noted that in calculating the recoveries, the material collected in the front of the splitter was taken as the basis for the static electrode while the opposite was taken for calculating recoveries for the beam type electrode. Experiments at temperatures higher than the ambient were performed through a heating coil wrapped around the feeding chute. The material was preheated to a desired temperature in an oven and then fed to the feeding chute The material used at temperatures higher than ambient were used only once. VIII Quartz and pyrite were selected as the reference materials to represent the insulator and conductor materials, respectively. The optimum electrode position was determined at IS kV constant applied voltage for both static and beam type electrodes. The electrode position is defined as the horizontal angle between the intersection of center of the rotor and that of the electrode. An electrode position of 80°, the uppermost position, for both materials was found appropriate and thus used in the subsequent experiments for boron minerals. Conductor and insulator materials behave rather differently depending on the type of electrode used. While a conductor material, e.g. pyrite yields increasing recoveries with a static electrode upon an increase in the applied voltage, it exhibits almost no recovery with the beam type electrode. In contrast to this, an insulator materia], e.g. quartz responds to the electrodes in exactly opposite manner. The electrostatic behavior of white, beige and black colemanites have been tested with both static and beam type electrodes. The ability of white colemanite to accept (reversible positive) or donate (reversible negative) electron has been tested with the both polarities. Evidently, with the static electrode, regardless of the polarity, the conductivity of white colemanite becomes slightly enhanced with increasing the applied voltage This behavior is characteristic of dielectric materials. The corona charging, on the other hand, is pronounced by an increasing tendency of colemanite to be pinned on the rotor. This is again indicative of the insulating feature of colemanite. All colemanites responds indifferently to changes in polarity. Under the conductive induction ulexite (NaCaB509 8H2O) becomes increasingly conductive with increasing the applied voltage at ambient temperature (20 °C). The conductivity is found to decrease at negative polarity. Ulexite thus results in good recoveries with the static electrode but yields marginal recoveries with the beam electrode. This behavior is a typical of conductive materials The Variation of applied voltage on the recovery of borax (^28407. IOH2O) have been determined by using static and beam type electrodes. The results obtained are similar to those of ulexite in that borax also exhibits features common to a conductor mineral. Borax shows a marginal increase in recoveries upon increasing the applied voltage with the corona charging electrode. Borax becomes increasingly conductive upon increasing the voltage with the induction charging system. This also indicates that borax shows the properties of a conductor mineral. Since the borax particles immediately redistribute their charge, they freely leave the surface. Temperature has interestingly a pronounced influence on the conductivity of ulexite. While the mineral is conductive at ambient temperature, it becomes insulator at 80 °C. This temperature apparently coincides with the onset of loss of water of crystallization. It should be noted this experiment was done whale the temperature at the chute of the separator was maintained at 80 °C. However, ulexite heated at 80 °C in an oven followed by cooling to the ambient temperature (20 °C) resumed to its original conductivity. This interesting phenomenon reveals that the conductivity observed in the case of ulexite is reversible and the acquired conductivity is largely dependent on the state of crystal lattice ions, i.e. the loss of conductivity occurs at IX temperatures where the loss of water starts. Similarly, ulexite exhibits a behavior characteristic of a conductive mineral with the beam electrode, i.e., all the material freely flows. It is interesting to note that the preliminary tests show that heating borax to temperatures above 30 °C leads a decrease in conductivity up to 50 °C followed by an increase in conductivity again at higher temperatures. These results will be reported in the future. Bigadiç clay sample is characterized by a montmorillonite type clay mineral. Bigadiç clay sample acts as a conductor sample both with static and beam electrodes. Interestingly, switching of polarity from positive to negative decreases the conductivity of the sample in the case of the induction charge. Both of Kirka borax clays are also a montmorillonite type clay mineral. Their behaiour are very much similar to those of the Bigadiç sample, i.e., the sample is conductor. Both change in polarity and in temperature does not cause any difference in the conductivity of the sample. The conductivity acquired by the boron minerals except colemanite and also the Bigadiç and Kirka montmorillonites leads to an interesting question. Although boron minerals are of high purity with no visible impurities. They are expected to contain some alkali earth elements such as lithium and strontium in the order of several hundreds ppm. The resistivity values reported for some elements in ohm-cm are: boron, 1.8x1012 at 0°C, lithium, 8.55 at 0°C, strontium; 23.0 at 20°C, and highly conductive copper, 1.673 at 20 oC. In particular, the boron clays are known to comprise lithium and strontium elements. Normally kaolinitic clays do not exhibit conductivity. The analyses of boron and clay minerals in terms of Strontium and lithium contents are presented in Table 3.1 [25, 30]. 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. The flotation and electrokinetic properties of boron minerals have been sparingly reported in the literature. However, recently there has been an upsurge of interest to compile a voluminous set of data on the flotation chemistry of boron minerals [25, 29, 31, 32]. A successful separation of boron minerals from the gangue or from each other requires the development of suitable reagent strategies. This emphasizes the need for fundamental knowledge on the elektrokinetic behavior of hydrated boron minerals. The electrokinetic behavior is an indicator for the ability of ions and in particular for the flotation reagents to be incorporated in the double layer. It also reveals the extent of specific interactions between ions at the solid/liquid interface. Studies on the electrokinetic behavior of boron minerals have been hitherto limited to colemanite only. Recently, the electrochemical behavior of borax has been studied by doppler laser electrophoresis technique. Borax, ulexite, inderite, colemanite and tunellite form a series of hydrated boron minerals of different cations X in the order of their solubilities. It is the objective of the electrokinetic study to determine the isoelectric point (iep) and potential determining ions (pdi) for the above hydrated boron minerals [25, 30, 31]. Zeta Meter 3.0 equipped with a microprocessor unit was used to measure the zeta potential of boron minerals. The unit automatically calculates the electrophoretic mobility of the particles and converts it to the zeta potential. One gram of mineral was conditioned in 100 cc of distilled water for 10 minutes. The suspension was kept still for 5 minutes to let larger particles settle. Each data point is an average of approximately 10 measurements. All measurements were made at ambient temperature (22-26 °C) and converted to 25 °C by the correction factors provided in the instruction manual. Salt-type minerals such as borates when dissolved in water will release a number of species into solution. These ionic species will be produced at the solid- liquid interface or may form in solution and subsequently adsorb on the solid in amounts proportional to their concentrations. Therefore, solids concentration in solution is a major parameter governing the surface charge generation. At concentrations approximately below 4 g/1 the surfaces of colemanite, inderite and tunetlite are negatively charged. Above this value, however, the surface acquires a positive charge In contrast to this, ulexite does not undergo any change at all solids concentrations. The charge reversal observed in the case of colemanite, inderite and tunellite respectively results from the release of Ca2+ and Mg2" and Sr2+ ions upon increasing the solids concentration. Eventually the colemanite surface is saturated with the cation above 4g/l solids concentration. Interestingly, ulexite remains indifferent to increasing solids concentrations. This reveals that the surface of ulexite is deficient in cation concentration. A similar result can be obtained by increasing the equilibrium time instead of the solids concentration. However, it is more practical to monitor the solids concentration rather than the conditioning time to achieve the same effect. A similar effect of solids concentration on zeta potential of calcite has been reported in the literature. It should be noted that using inadequate solids concentrations can lead to erroneous conclusions in the interpretation of adsorption and zeta potential measurements. Colemanite is a boron mineral with a Ca2+ cation in its lattice structure. Colemanite, as other hydrated boron minerals, undergoes acid-base reactions in the vicinity of minimum solubility which corresponds to pH 9.3. The iep of colemanite is found to occur at pH 10.5 in agreement with the previously reported values by Celik et ai. and Yarar. It should be noted that the iep of colemanite is in accord with the flotation results reported earlier considering the electrostatic interaction of anionic and cationic surfactants on the surface [30, 3 1], For a cation to be a potential determining ion (pdi), it should render the surface more positive with increasing the concentration of the cation. Similarly, a potential determining anion should make the surface more negative with an increase in its concentration. Various candidate ions, i.e., the lattice components and carbonate related species were tested. While colemanite exhibits a positive zeta potential value at natural pH, ulexite yields a negative charge This is indicative of a cation deficiency during the dissolution of ulexite. XI The constituent lattice ions and those of carbonates have been tested to find out whether they are the pdi. As colemanite, the lattice ions Ca2+ and B4O72', and the carbonate derivatives are clearly the pdi's. The behavior of Na+ as a lattice ion, however, is interesting. While NaCl is expected to render the surface of ulexite less negative either by compressing the double layer as an indifferent electrolyte or at least acting as a potential determining lattice ion. In either case, the ulexite surface exhibits an increasingly negative potentials with increasing the NaCl concentration. The above behavior has been observed with montmorillonite type clay minerals. The negative charge on these minerals predominate in the entire pH region with usually no iep value present. The addition of an indifferent cation such as NaCl makes the surface more negative. The sign of the charge and the absence of iep in montmorillonite are analogous to that of ulexite. However, in contrast to montmorillonite, the zeta potential of ulexite cannot be practically measured below pH 7 due to strong dissolution of the mineral. Inderite is a hydrated boron mineral with magnesium as the lattice cation. The iep of inderite is practically not measurable. The positive charges persist even above pH 10. Inderite is an interesting borate mineral in that it gives very low zeta potentials, e.g. less than 5 mV in the pH range of 7 to 11. Such low zeta potentials make inderite very much conducive to coagulation. Strong tendency of inderite, particularly in the pH range of 9 to 11, indeed made the measurements extremely difficult and sometimes impossible. A snow-like precipitation was commonly observed in majority of the measurements. As in the case of colemanite, in addition to the lattice ions, i.e. B4O72- and Mg2~, H* and OH" which control the ratio of HCO3VCO32' are the pdi's for inderite NaCl appears to act as an indifferent electrolyte and to compress the double layer. Tunellite is a hydrated boron mineral with strontium as the lattice cation. The iep of tunellite is similar to that of inderite in which the potentials are all positive in the pH region of 8 to 12. However, compared to inderite, the zeta potentials of tunellite are at least two to three times higher in the absolute value. The coagualation tendency of the particles and also polarization of the electrode made the measurements difficult above pH 11. As in the case of inderite, in addition to the constituent lattice ions, i.e. Sr2+ and B4O72-, H+ and OH" which control the ratio of HC03"/C032" are the pdi's for tunellite. It should be noted that the contribution of C02 on zeta potential measurements was tested with suspensions purged with nitrogen gas did not produce a significant change. The fundamental information regarding electrostatic properties of a series of boron minerals, colemanite, ulexite and borax can be recapitulated as follows: Colemanite behaves as an insulator material both with static and beam type electrodes. Increasing the applied voltage to high levels marginally enhances the conductivity. The switch of polarity does not induce any change to the conductivity to mineral. Ulexite under the conductive induction becomes increasingly conductive with increasing the applied voltage at ambient temperature. Borax also exhibits features common to a conductor mineral. Borax shows a marginal increase in XII recoveries upon increasing the applied voltage with the corona charging electrode. Borax becomes increasingly conductive upon increasing the voltage with the induction charging system. Bigadiç and Kirka clay samples characterized by the montmorillonite type clay show that they act as conductor and increasing the temperature does not cause any difference in the conductivity of the minerals. Temperature has interestingly a pronounced influence on the conductivity of ulexite. While the mineral is conductive at ambient temperature, it becomes insulator at 80 °C. The loss of conductivity occurs at temperatures where loss of water starts. The preliminary tests show that heating borax to temperatures above 30 °C leads a decrease in conductivity up to 50 °C followed by an increase in conductivity again at higher temperatures. The conductivity acquired by the boron minerals except colemanite and also the Bigadiç and Kirka montmorillonites is puzzling as both type of minerals are pure and are listed as insulator materials. The present study has clearly demostrated that alkali earth elements such as lithium and strontium in large amounts are present boht in boron minerals and clays. The conductivity response of boron minerals and clays stems from the presence of these conductive elements. The implication of these associated impurities must be carefully considered in the design of electrostatic separation strategies for boron minerals. The following salient points have been resulted out of electrokinetic investigation: Solids concentration is an important parameter in the charge generation of boron minerals. Colemanite, inderite, ulexite and tunellite exhibit a charge reversal from negative to positive at solids concentrations of above 4 g/1 whereas ulexite remain negatively charged in the entire practical pH range, i.e. 8 to 11. Colemanite yields an iep at pH 10.5. Ulexite, inderite and tunellite show virtually no iep, if at all, may have iep values below pH 7 and above pH 11, respectively. The pdi's for the boron minerals studied are the lattice cations, Ca2% Mg2" and Sr2+, and the lattice anion B4072". The addition of monovalent salts such as NaCl was found to change the zeta potential of ulexite in the opposite direction. This was ascribed to the dissolution of cations from the surface and rendering the surface deficient in cation. The charge generation of boron minerals investigated in this study agrees with the adsorption of collectors on oppositely charged surfaces through electrostatic attraction mechanism.