Doygun çözeltilerde boraksın flotasyon kimyasının incelenmesi

Abstract

Bu çalışmada literatürde ilk defa olarak boraksın doygun çözeltiler içindeki yüzey kimyası ve özellikle flotasyon kimyasını belirlenmeye yönelik çalışmalar yapılmıştır. Boraksın pH ' ya bağlı olarak çözünürlüğü tesbit edilerek minimum çözünürlüğün 9-10 arasında olduğu gösterilmiştir. Boraksın flotasyonunu incelemek amacı ile katyonik dodesil aminhidroklorür (DAH) ve anyonik sodyum dodesil sülfat (SDS) kullanılmıştır. DAH ' m tabii pH ' da boraksı SDS ' ye göre 10 misli daha düşük konsantrasyonlarda yüzdürdüğü saptanmıştır. pH ' ya bağlı olarak yapılan deneylerde SDS ' nin düşük pH ' larda daha iyi yüzdüğü bulunmuştur. Bu durum pH düştükçe SDS ' nin karşıt yüke (+) sahip boraks yüzeyine elektrostatik çekim mekanizması ile artan miktarlarda adsorplanması ile açıklanmıştır. DAH ise amin gurupları ile, (-) yüklü olan boraks yüzeyine yapışarak yüzeyi hidrofob yapmaktadır. Bu durum pH ' 10 a kadar devam ederken 10 dan daha yüksek pH larda iyon moleküler komplekslerinin azalması nedeni ile verim düşmektedir. Boraks cevherinde mevcut başlıca empürite olan kilin flotasyon özellikleri yine aynı reaktiflerle incelenmiş ve killerin aynı şartlarda yüzemediği bulunmuştur. Boraks killeri ile yapılan elektrokinetik ölçümler sonucu bu killerin tıpkı boraks çözeltileri gibi pH=9.3 ' de kuvvetli tampon özellik gösterdikleri ve pH ' 2 nin altoda bile sıfır yük noktasına ulaşamadıkları tesbit edilmiştir. Saf boraksa % 1 oranında kil ilavesinin bile flotasyon verimlerini önemli ölçüde düşürdüğü bulunmuştur. Reaktif konsantrasyonu ve pH 'ya bağlı olarak yapılan adsorpsiyon deneylerinde SDS ve DAH ' m değişik sıcaklıklarda adsorpsiyon izotermleri ve adsorpsiyon enerjileri AGo tesbit edilmiştir. Adsorpsiyon verileri Frumkin, Uyarlanmış Frumkin, Langmuir ve Flory Huggins modellerine uygulanmış ve bunlardan Uyarlanmış Frumkin modelinin SDS/boraks adsorpsiyon sistemini en uygun temsil ettiği tesbit edilmiştir.

Borates and their products have become essential ingredients used in various industries with an annual world consumption of over one million ton of equivalent B90-. Although more than 150 boron minerals have been identified, only about a dozen of them are found in commercial deposits and less than half of these are considered ore. The most important boron minerals of commercial importance are borax (Na^B.O-.lOHLO), colemanite (Ca^BgOj r5H20), ulexsite (NaCaB50?.8H70) and kernite (Ns^B^AUJD). Turkey has the largest borate deposits in the world, with reserves of over one billion tons. Development activities in the last 30 years has also made Turkey the second largest borate producer and the premier exporter. The boron deposits in Turkey are located in the western part of the country. Beneficiation of these minerals usually involves a preconcentration step to separate the gangue minerals in same form of scrubbing operation. However, since boron minerals are rather friable, they tend to get into finer fractions as tailings. The finer fractions, mostly below 0.2 mm in size, are often discarded as waste from commercial operations even though they often contain large amounts of valuable boron minerals. Beneficiation of boron minerals by flotation with well defined systems has been very little investigated in the literature. The diffuculty arises from the fact that boron minerals cover a spectrum of solubilities. Borax is a highly soluble boron mineral. Recovery of these requires an advance beneficiation methods such as flotation. Although flotation is applied in the USA, very little information exists on its technology and " know how ". As noted by M. E. Defoe (1985), borax is reportedly floated from sylvite with oleic and naphtenic acids plus xylene, turpentine and kerosine. R. G. Knickerbocker and F. K. Shelton VI (1940), in an earlier study showed that colemanite decomposed to boric acid by sulfuric acid can be made naturally hydrophobic with frother alone B. Yarar (1973) carried out a series of tests to investigated the floatability of colemanite in the different presence of combination alkyl sulfonates, naphtenic acid and kerosene in different combinations. B. Yarar (1985) later discussed the mechanisms of calcite - colemanite separation by anionic surfactants. Ayok and Tolun (1975) studied the role of various complexing agents e.g. polyols mixed with alkyl sulfonates and were found to improve the flotation recoveries. The flotation chemistry of soluble boron minerals has been very little investigated in the literature. The difficulty arises from the fact that the boron minerals exhibits a spectrum of solubilities. The presence of different ions, e. g. Na, Ca, Mg in their lattice structure impart different charecteristics to these minarals. For example, while colemanite displays very little solubility (0.8 g/1), borax is highly soluble (25.8 g/1 at 20 °C) and thus requires flotation in its saturated solutions. The flotation properties of highly soluble minerals by itself is a subject of considerably importance. A succesful separation of boron minerals from the gangue or from each other necessitates devolepment of suitable reagent strategies. This per se reiterates the need for fundamental knowledge on flotation chemistry of boron minerals. It is the aim of this study to understand the flotation properties of a typical highly soluble boron mineral, borax in the presence of anionic and cationic reagents in the saturated brines. Solubility, microflotation, electrokinetic and adsorption tests on pure minerals have been conducted to elucidate the interfacial mechanisms governing the flotation of boron minerals. Flotation of boron minerals were determined using a 125- ml column cell (20x220 mm) with a fine frit and magnetic stirrer. In each test, one gram of sample was used in the microflotation tests. The samples were conditioned in 125 ml of solution containing the desired reagent for 10 min. and floated for one min. using nitrogene at a flow rate of 50 cm3/min. In the case of borax mineral the saturated solutions were prepared by dissolving sufficient quantity of pure borax. The zeta potantial of clay minerals was measured by means of Zeta Meter 3.0 equipped with a microprocessor unit. The surface charge of particulates in suspension is of particular significance in phonomena such as adsorption, crystallization and coagulation /dispersion. Generally the surface charge of such particulates is described by measurement of their vn electrophoretic mobility and the calculation of the corresponding zeta potantial, the potential in the region of shear some distance away from the particle surface. Electrophoretic mobility measurements are limited to relatively low ionic strength solution ( I<10-2 M) due to electrode polarization, conductivity limitation, and the generation of thermally induced convective flow in the sample capillary, for these reasons and because of complete double-layer collapse in high ionic strenght solution, experimental electrophoretic mobility measurements for soluble salts such as alkali halide particles have not been reported in the literature. The isoelectric point of borax was found at pH 9.3 by Miller et al. with a new technique, Laser-Doppler. The solubility of borax versus pH also shows a minimum solubility to occur at pH 9.3. Both cationic and anionic reagents float borax at concentration above 1(H M. interestingly, cationic reagent floats borax at concentrations one order of magnitude lower than the anionic reagent. Flotation of borax with Sodium Dodecyl Sulfate (SDS) shows a decrease with increasing pH indicating the role of electrostatic interactions in the system. Similar resuls were found in the colemanite/SDS system. Flotation of borax with Dodecylamine Hydrochloride (DAH) at concentration of 6.7x 10"6 M exhibits a 50 % of recovery at pH 9 where ion molecular complexes form at this pH. Flotation of borax decreases above this pH due to the increase in the ion molecular complexes then increases above pH 12 due to the amine precipitation on the borax surface. The results provide clear evidence that borax carries a negative surface charge in its saturated brine (pH 9.3) and the sign of the surface charge of borax reverses and becomes positive if the pH is reduced to 8. The equilibrium between borax and its saturated brine can be describe by the following reaction : 2 Na2B4O7.10H2O=> 4 Na+ + B4O7-2 + HB4O7" + OH"+ 19 H20 It appears that the oxy - anions of the borax lattice provide weak acid sites at the surface which readily undergo protonation. The floatability of borax has also been investigated in the absence and and presence of a pure clay minerals. Clay minerals are the major source of vui slime and cause problems in selective separation of minerals. The effect of clay is investigated using pure borax and clay minerals their mixtures with a cationic and anionic reagents in a microflotation cell. Zeta potantial measurements on clay minerals reveal that they exhibits a negetive charge even at pH 2 and adsorbs on borax through electrostatic attraction. It is also found that while borax floats with both collectors, it fails to float in the presence of even as low as 5 % clay addition. This deleterious effect is explained in terms of the slime coating effect. The adsorption behaviour of sulfate and amine on borax were also analyzed according to the Frumkin, Modified Frumkin, Langmuir and Flory- Huggins equations. The free enery of adsorption (AG0), lateral interaction coefficient (a) and the isoteric heats of adsorption values have been calculated from these equation and compared with each other. The effect of solution temperature on adsortion in general is determined by the type of adsorption, whereas any increase in temperature is expected to decrease the extent of fhysical adsorption, the reverse is found to be generally true for the case of chemisorption. Thus adsorption of sodium dodecly sulphate on borax is found to decrease with an increase in temperature. On the other hand the changing of the isoteric heats of adsorption values indicate that the existing of the fhysical adsorption. Frumkin and Modified Frumkin isotherms show a lower free energy changing than Langmuir and Flory-Huggins. The value of "a" is a measure of the strength of the lateral interaction between the sulphate ions which are adsorbed on the mineral surface. Positive "a" values represent repulsive interaction, in contrast negative "a" values represent attractive interaction. The observed differences between the " a " values is attributed to differences in the orientation of adsorbed sulphate with respect to the surface ; which is certainly related to the structure of the surface. Larger " a " values correspond to more perpendicular orientation than lower " a " values. Examination of the results obtain in this study indicate that, due to the value "n" which is a measure of the size ratio and the lateral interaction coefficient (a) Modified Frumkin equation yields the most resonable free energy of adsorption and the isoteric heat of adsorption and the isoteric heat of adsorption (AH°). It is well known that the free energy of adsorption is a sum of two contributions arising from enthalpy is considered, the enthalpic contribution is found to be small whereas the entropic contribution turns out to high and negative. IX These flotation and adsorption results reveal that in borax/Sodium Dodecyl Sulfate (SDS) system the adsorption of SDS on borax is governed by electrostatic interaction driven by entrophy. In particular in saturated solution systems such as borax the electrical double layer is expected to become highly compressed. Such compression will lead to reduced electrostatic interactions between the solid and the sorbent. In addition, due to the high ionic strength, SDS molecules will undergo salting out phenomenon and in turn will be forced to approach closer to the interface. The adsorption conducted with the cationic reagent (Dodecylamine Hydrochloride) indicates that DAH is very much sensitive to precipitation due to the formation of amine hydroborate. Particularly, at pH 9.3, the formation of precipitate masks the true adsorption. Thus, abstraction which is the sum of adsorption + precipitation was taken into account to understand the nature of cationic surfactant adsorption. It is found that the amine adsorption decreases with increasing pH above 9.3. Under such conditions, if we assume the electrostatic forces to be operative, the heat of adsorption is expected to be highly small. In fact the Modified Frumkin model seems to be reasonable in this respect. It should be noted that the sign of the enthalpic contribution is only indicative of whether the reaction is endothermic i.e., the isothermic heat of adsorption need to be negative for adsorption to proceed.