Bor minerallerinin flotasyonunda şlamın etki mekanizması

Abstract

Bu çalışmada literatürde ilk defa olarak bor minerallerinin flotasyonunda kilin etki mekanizması incelenmiştir. Otomatik kumandalı bir mikroflotasyon cihazı kullanılarak bir dizi mikroflotasyon deneyleri ve kilin etki mekanizmasını incelemek amacıyla aynı şartlarda zeta potansiyel ölçümleri yapılmıştır. Bu verilerin ışığı aranda kilin etkisini azaltmak için çeşitli stratejiler önerilmektedir. Çalışmalarda kullanılan kolemanit, üleksit ve kil numuneleri Bigadiç bor yatağından el ile saf olarak toplanmıştır. Deneyler genel olarak mineral (kolemanit veya üleksit), kil ve mineral + kil sistemleri ile incelenmiştir. Elektrokinetik çalışmalarda kolemanitin sıfir yük noktası (SYN) 10.5 olarak bulunmasına rağmen üleksit ve kil için bulunamamıştır. Bigadiç kili diğer bor killeri ile karıştırıldığında; yaklaşık olarak aynı yüzey yüküne sahiptir. Kil ilavesiyle pozitif olan kolemanit yüzeyi negatife dönüşmekte, üleksitte ise etkili olamamaktadır. DAH konsantrasyonu arttıkça zeta potansiyel değerleri artmasına rağmen SDS konsantrasyonlarında ise düşmektedir. pHya bağlı olarak yapılan zeta potansiyel deneylerde; kolemanit her iki reaktif varlığında da düşük pHlarda daha az etkilenirken, üleksit ise DAH varlığında daha fazla etkilenmektedir. Ayrıca Ba+2 ve Ca+2 iyonları varlığında flotasyon verimleri tabii pHnın (pH=9.3) altında elektrostatik çekimin azalmasından dolayı düşerken; üzerinde ise artmaktadır. Ba+2 iyonu Ca+2 iyonuna göre daha etkili olmaktadır. Mikroflotasyon deneyleri sonucunda aynı zincir uzunluğuna sahip olan katyonik reaktif (DAH) ve anyonik reaktif (SDS) kolemaniti ve üleksiti tabii pHda (9.3) yuzdürebilmektedir. Üleksit 2.5x10-4 M DAH ve SDS konsantrasyonlarında maksimum verimle yüzerken, kolemanitin 1x10-4 M SDS ve 5x10-4 M DAH konsantrasyonlarında yüzdüğü bulunmuştur. Kil mikatannın artması hem kolemanit hem de üleksitin flotasyon verimlerini %20 ile %60 oranında düşürdüğü bulunmuştur, özellikle kilin varlığında kolemanitin flotasyon verimi SDS konsantrasyonuna bağlı olarak en fazla etkilenmektedir. Kolemanit ve üleksit yüzeyine negatif yüklü olan kilin özellikle düşük pHlarda elektrostatik çekim mekanizması ile adsorplandığı ve flotasyon verimlerini düşürdüğü bulunmuştur. SEM (Scanning Electron Microscop)' de çekilen fotoğraflar da bunu kanıtlamaktadır. Ayrıca çok değerlikli iyonların heterokoogülasyon sonucu üleksiti çok iyi yüzdürmelerine karşılık kil varlığında çökeleğin yapışmasını engellediğinden aynı verimleri elde etmek mümkün olamamıştır. Elde edilen tüm sonuçlar, negatif yüklü kilin pozitif yüklü bor minerallerine karşı kuvvetli ilgisinin olduğu ve hatta reaktif ile de borun yüzeyine yapışmak için rekabet ettiği anlaşılmaktadır.Bu yüzden flotasyonun fizikokimyasal açıdan kilin yapışmasını engelleyecek şartlarda seçilmesi önerilmektedir.

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. Borates and their products have become essential ingredients used in various industries with an annual world consumption of over one million ton of equivalent B2O3. 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 (Na2B4.O7.10H2O), colemanite (Ca2B60n.5H20), ulexite (NaCaB5C>7.8H20) and kernite (Na2B407.4H20). 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. Recovery of 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, borax is reportedly floated from sylvite with oleic and naphtenic acids plus xylene, turpentine and kerosine. Knickerbocker and Shelton 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 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 diffuculty 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 minerals. For example, while colemanite displays very little solubility (0.8 g/1), borax is highly soluble (25.8 g/1 at 20 oC) and thus requires flotation in its saturated solutions. The flotation properties of highly soluble minerals by itself is a subject of considerably importance. 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. 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. The most notable contribution in this area is that of Celik and his associates. They meticulously examined the surface and flotation chemistry of boron minerals and derived fundamental knowledge on electrokinetic and adsorption behavior, reagent strategies and heterocoagulation of particulates. Microflotation and electrokinetic tests on pure minerals have been conducted to elucidate the interfacial mechanisms governing the flotation of boron minerals. Flotation studies conducted previously on boron minerals revealed that the major impurity associated with boron ores, montmorillonite type clays, adversely affect flotation recoveries. The objective of this study is XI therefore to understand the mechanism of clay action in the flotation of important boron minerals, colemanite and ulexite, and to formulate conditions by which boron minerals can be efficiently floated. Towards this aim, pure boron and clay minerals will be systematically subjected to microflotation, electrokinetic measurements and Scanning Electrone Microscope (SEM) photomicrographs in the presence of anionic and cationic reagents in order to delineate the role of slime coating and to formulate the optimum conditions in flotation. In the light of these findings, a floation scheme for boron ores will be proposed for testing in laboratory scale floation cells. 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 electrokinetic 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. 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 oC) and converted to 25 oC 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. While at concentrations approximately below 4 g/1 the surfaces of colemanite are negatively charged and above this value, acquires a positive charge. In contrast to this, ulexite does not undergo any change at all solids XII concentrations. The charge reversal observed in the case of colemanite results from the release of Ca2+ 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. 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 Ca^4" 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 al. and Yarar. The lattice ions Ca-+ and B4O7-", and the carbonate derivatives are clearly the pdi's for colemanite and ulexite.. The behavior of Na+ as a lattice ion in the case of ulexite, 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. Bigadiç clay sample is characterized by a montmorillonite type clay mineral. 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. While colemanite yields an iep of 10.5, ulexite and boron clays carry a negative charge at all pH values and thus exhibit no iep value. Colemanite+clay and ulexite+clay mixtures acquire a charge profile similar to xni the clay itself, i.e. negative charges persist in the entire pH range. This is attribute to the negatively charged clay mineral adsorbing onto positive sites of boron minerals in the form of slime coating. Even the addition of 10 mg clay into a 1000-mg colemanite suspension was able to cause a charge reversal of the mixture from positive to negative. In the case of adding 30 mg clay into the same suspension, the charge of the mixture became constant and took the profile of clay itself. Ulexite, on the other hand, as it was invariably negative, it is not affected with the addition of clay as much. A similar effect of clay is observed in the microflotation of colemanite and ulexite in the presence of cationic reagent, dodecylamine hydrochloride (DAH), and anionic reagent, sodium dodecylsulfate (SDS). The flotation recoveries of colemanite with DAH decrease down to 60 % with the addition of 20 mg clay whereas a decrease of only 20 % is observed with ulexite. Contrary to the greater effect of DAH, since SDS carries a charge similar to that of clay, the flotation recoveries are more prone to the slime coating thus result in larger reductions. The microflotation studies reveal that both colemanite and ulexite float at natural pH in the presence of DAH and SDS. In contrast to this, Bigadiç clay does not float with SDS at low concentrations up to 1X10"4 M neither does with DAH. Above this concentration however, clay floats up to 50 % with DAH due to precipiation of DAH and the subsequent coagulation of DAH/clay species. A parallel behavior is observed with zeta potential measurements where clay, and clay+boron mineral mixtures in the presence of DAH undergo charge reversal above about 2X1 0"4 M DAH concentration. Colemanite flotation recoveries increase upon increasing pH with DAH and decreases with SDS. Ulexite on the other hand, exhibit a different bahavior with DAH in which recoveries decrease with increasing pH. In the presence of clay, the affinity of the surface to clay mineral depends upon whether the mineral has excess positive charges on which negatively charged slimes can adhere. Both zeta potantial and floation results demonstrate that colemanite is affected from slime coating minimally in particular at high pH's and high DAH and SDS concentrations. Ulexite in the presence of DAH is little affected at low pH's while greatly affected in the presence of SDS. Zeta potential mesurements conducted in the presence of multivalent ions such as Ba-+ have shown that the surface of ulexite becomes more XTV positive and in turn more ameneable to adverse effect of slime coating. The formation of barium dodecylsulfate collector colloids coats the surface of ulexite through hetrocoagulation leading to improved flotation at pH values above 9.3 and subsequently reducing the effect of slime coating. The foregoing discusuins along with the photographs taken by scarmning electrone microscope studies indicate that particularly at low pH values where the surface of boron minerals exhibits more positive charge, the effect of slime coating is more serious. This can be minimized by either employing high DAH concentrations, or SDS in the presence of multivalent ions such as barium and calcium at high pH values.