Tinkal mineralinden boraks tuzları üretiminde safsızlık davranışları ve giderilmesi

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Tarih
1993
Yazarlar
Yavaşoğlu, Nergül
Süreli Yayın başlığı
Süreli Yayın ISSN
Cilt Başlığı
Yayınevi
Fen Bilimleri Enstitüsü
Institute of Science and Technology
Özet
Bu çalışma, tinkal cevherinden boraks dekahidrat ve boraks pen- tahidrat tuzlarının üretiminde safsızlıkların hangi kaynaklardan ne şekilde geldiği ve bu safsızlıkların giderilme yollarının araştırılması amacıyla yapılmıştır. Bu amaçla, çalışmanın ilk kademesinde tinkal cevheri ile birlikte bulunması muhtemel uleksıt, kolemanit gibi bor minerallerinin, yanısıra kalsit, dolomit ve değişik kil minerallerinin sıcaklığa ve boraks konsantrasyonuna bağlı olarak boraks üretim çözeltilerindeki çözünürlükleri incelenmiştir. Bu minerallerden boraks üretim çözeltilerine geçen en önemli safsızlıklar ise kalsiyum, magnezyum ve silistir. Bu safsızlıklar özellikle rafine boraks üretiminde önem kazanmaktadır. Boraks tuzları üretim prosesi temelde basit olmasına rağmen, çözünebilen safsızlıklar çözme kademesinde üretim çözeltisine geçmekte ve kristalizasyon sırasında borlu bileşikler olarak ürünle beraber kristalize olmakta ve ürün kalitesini bozmaktadır. Çalışmanın ikinci kademesinde, çözme kademesinde üretim çözeltisine geçmesi kaçınılmaz olan bu safsızlıkların giderilme yolları araştırılmıştır. Boraks tuzlarındaki en önemli safsızlık olan kalsiyumu gidermek için üç farklı yöntem uygulanmıştır. Çalışmanın son kademesinde ise önemli safsızlıkların değişik konsantrasyonlarının boraks pentahidratın kristalizasyon kinetiğine etkisi, MSMPR tipi kristalizörde incelenmiştir. 
 Tincal (borax decahydrate) is the main mineral among the various boron minerals for the production of borax decahydrate and pentahyd- rate salts since it 1S water soluble. Tincal ore contains some clay minerals (mainly montmorillonite), dolomite and ulexite as the major accompanying minerals and other calcium borate minerals at a lesser degree. Although the production processes of borax salts are very simple, the impurities coming from the raw material (tincal ore) cause many problems in the processes. These impurities exist especially in the final salt products and consist mostly of calcium, magnesium and si lica ions originating from the dissolution of ulexite, calcite, cal cium borates and clay minerals. The greater part of the impurities in the product was observed as in them form of probe'rtite (NaCaBcOg.. 5H20). The aim of this study is to determine the source, dissolution and precipitation behaviour of the impurities under different condi tions and to develop methods for removing of them. Calcium concentration in a borax solution can be decreased by one of the following ways: -Precipitation of calcium ion as a calcium salt having a very low solubility. -Accel arating the probertite precipitation. -Removal of calcium by ion exchange. Any source of carbonate, such as sodium carbonate, CÛ2j sodium bicarbonate, and sodium sesqui carbonate can be used in the first met hod to precipitate calcium ions as CaCQ3 in the borax solutions. Carbonate ion concentration must be 0.5-2.5% of the solution by weight. Under these conditions, calcium is precipitated as calcium carbonate and it is possible to reduce the calcium concentration to 150-90 ppm. Vll The more effective method for the removal of calcium from borax production solutions is to use an ion exchanger. However, this is an expensive method because of the initial investment costs and has the temperature problem when the solution at 95-l00°C is contacted with ion exchanger hawing lower temperature stability limits. The way of accelarating the probertite precipitation is based on the addition of the propertite seed crystals to the borox product ion solutions containing calcium ions. In the first step of the study, the solubility of some minerals present in tincal ore was examined. For this purpose, calcite, dolo mite, colemanite, ulexite and raw cliy minerals were chosen. To see the effect of concentration and temperature on the solubility of im purities, borax solutions were prepared in different concentrations and different temperatures. The chosen borate concentrations were 3%, 8%, 18% and 30% ^B^.which are at rooi.i temperature, mother liquour concentration of borax decahydrate production, mother liquour concentration of borax pentahydrate production and solution concentration of borate pentahydrate production, respectively. The impurities were added to these prepared solutions and they were placed in a shaker equipped with a thermostate. The calcium concentration changes' versus time were examined in these solutions. From these experiments, the following results were found: 1) Calcium ions in the borax production solutions form calcium borate complexes having high solubilities first. Then these complexes reach a maximum concentration. finally, calcium startsto precipitate very slowly in the form of probertite (NaCaBsQg.B^O). 2) Calcite and dolomite represent higher solubilities than the other minerals in borax solutions. They reach the highest solubility in the 30% ^B^ solution concentration and the precipitation of probertite is very slow in this concentration. For this reason, it is very difficult to control the probertite precipitation in the borax production processes and this causes some problems in the pipe.lines. Therefore, it is necessary to precipitate probertite in a shorter time or calcium must be removed at the beginning, in a way. In the.case of raw clay minerals, on the other hand, it must be taken in to account both calcium and magnesium. The clay minerals are named as blue, green, white and green according to their seems and contain montmorillonite (Al, cjM% ız^tp-in^lo) > dolomite (CaCÛ3.MgC03) and sometimes CaC03. The changes ör calcium and magnesium concentra tions (as calcium) in 30% Na2B4Û7 borax solutions at 92°C are very similar to the results of former minerals. The solubility and the Vlll precipitation rates of calcium and magnesium ions in these solutions depend on the type of the raw clay mineral. Blue clay mineral has the lowest dissolution rate and the white clay mineral has the highest dissolution rate. As the clay mineral ratio increases in the raw clay mineral, the calcium concentration increases in the solutions. When the calcium concentration reaches a maximum value, magnesium concent ration becomes minimum in the solution. As the calcium ion precipita tes in the form of propertite, then magnesium ion concentration begins to increase. According to these results, it is possible to say that, dolomite in clay minerals dissolves in borax solutions and clay mineral gives ion exchange reaction as in the following equation: X-clay + Y+ s± Y-clay + X+ In conclusion, dolomite, calcium borates, calcium carbonate and raw clay minerals in tinea! ore give calcium ions in to the borax solution in two different ways: Dissolution of the minerals or ion exchange mechanism. Silica is one of the other important impurities in borax produc tion solutions. This impurity comes from the dissolution of silica in borax solutions. Dissolution of silica depends on the concentration of borax solution, temperature and the clay mineral ratio in the raw clay mineral. Increasing of the clay mineral ratio in the raw clay mineral increases the silica concentration in borax solutions. In the second step of the study, the methods of calcium removal were examined. Firstly, a synthetic ion exchanger", dualite ES 467 and natural zeolite were tried to remove calcium ion in borax solu tions. It was not possible to remove calcium ion with natural zeolite because of the high sodium ion concentration in the solution. Dualite ES 467 was used in the 80°C borax solution because of its physical properties and it was possible to reduce the calcium concentration under the probertite precipitation limit. But the temperature (80°C) was lower then the temperature of borax production solutions; Secondly, sodium carbonate was used to reduce calcium concentra tion in the borax production solutions. For this purpose, different amounts of sodium carbonate (from 0.5% to 3%) was added into the dif ferent concentrations of borax solutions containing calcium ions. Calcium concentration was reduced to very low levels with 2 and 3% sodium carbonate concentrations in borax solutions which contain 18% and 30% Na2B407, respectively. But these amounts of sodium carbonate are much more higher than the stoichiometric ratios and cause extra costs for the process. In order to see the other ways of the reducing of the calcium ix ion concentration, NaF, Na3PC>4 and sodium oleate reagents were added individually into the borax solutions containing calcium ion. The addition of these reagents into the solution did not cause any new precipitation and probertite precipitation has. continiued. At the third step, probertite and ulexite were used to reduce cal cium ion concentration in borax production solution. Both of them reduced the calcium ion concentration in the solution. In the case of probertite, on the other hand, calcium removal is much more effective. When calcium and magnesium are present at the same time in borax so lution, probertite addition effects only calcium ion concentration and it has not any effect on the magnesium ion concentration. The mechanism in the calcium ion removal by the probertite addition can be explained "the seed crystal role" of the added probertite crystals in the system. For this aim, probertite requirement is at least 3% of the solution and this amount is yery high because of the filtration problems in the process. To investigate the probertite precipitation mechanism, the sur face charge of probertite was measured in different borax concentrati ons.using a zeta-meter (Zeta-Meter 3.0+). The surface change (zeta- potensial) of probertite in borax solutions was found as negative (about -20 mV). Therefore, this negative surface charge migth be neutralized by using a cationic polyeletroT/yte. For this aim, non- ionic and cationie polyelectroTytes were tried. Non-ionic polyelec- tralyte did not effect the probertite surface charge but cationic polyelectrolyte (WT 2479) neutralized the negative surface charge of probertite. Another cationic polyelectrolyte (Cat Floe TL) was also used in the experiments and it is found that, Cat Floe TL neutralizes the surface charge of probertite with the much more lower concentra tions than the WT 2479 does. This polyelectrolyte (Cat Floe TL) is completely stable at high pH's and its lower viscosity is also an advantage compared to WT 2479 polyelectrolyte. Only 5 ppm concentra tion of Cat Floe TL is sufficient to charge the negative surface charge of probertite to a positive value. It is well known that, lower than 200 ppm calcium concentration in borax solution has no harmful! effect in the process and it is possible to reduce the calcium concentration lower than 200 ppm when using 5 ppm Cat Floe TL with only 0.5% probertite seeds. From these results, the most important factor for probertite precipitation was found as the surface charge neutralization and the addition of very low percentage of probertite seeds is a good advantage for the filt ration process. At the last step of the study, the effects of the impurities on the nucleation and crystal growth kinetics of borax crystallisation were investigated. The experiments were carried out in a MSMPR (Mixed Suspension Mixed Product Removal) type crystallizer with a 2 1 volume. The crystallizer was made of cylindrical plexyglass and equipped with three flow breakers. There was a spiral acting as a heat ex changer and an internal draft tube in the crystallizer. Temperature inside the crystallizer was controlled by a contact thermometer calibrated with ASTM thermometer either permitted the flow of cool ing water to above mentioned spiral, or light on the infrared heat ing lamp basing on the signals. Feed solution was pumped from 25 1 closed tank jacketed with steam and made of 316 stainless stell. This tank was stirred with a- propeller. A peristaltic pump was used for feeding. In order to attain a steady state in the crystal! izer, the system was- run for a period of 8 residence time which is 1 hour and then the characteristic sample was fed to graduated flask. To measure the volume of the suspension, crystals were filtered and chemical analysis of the mother liquor, was immediately performed. Crystals on the filter were washed with aceton (saturated with borax pentahydrate) and left remain to air for drying and then sieved and impurity analysis was done. In order to examine the effects of calcium concentration to borax pentahydrate crystallization, 102, 440 and 1250 ppm calcium were added to the feed solution which contains about 23% Na2B407. After the par ticle size distributions of the product from the MSMPR crystallizer were evaluated according to the population density theory, it was pos sible to say that, the ideal population behaviour is not valid for borax pentahydrate crystallization and crystal growth depends on par ticle size. As a result, calcium ions accelarate nucleation rate and retard the growth rate of borax pentahydrate. Calcium distribution in the borax pentahydrate crystals was higher in the small crystals than the laFger ones. However, calcium retards the aglomeration and causesthe shape changes of borax pentahydrate crystals. Similar experiments were done in the presence of silica. 18, 55, 148, 520 and 960 ppm silica were used in the feed solutions at the same conditions. Growth depends on the particle size in the presence of silica, as well. On the other hand, there is not any effect of silica on growth and nucleation rates of borax pentahydrate. In addi tion, crystal shape of borax pentahydrate was not affected by silica addition.
Açıklama
Tez (Doktora)-- İTÜ Fen Bil.Enst., 1993
Anahtar kelimeler
Boraks, Mineraller, Tinkal, Tuz, Borax, Minerals, Tincal, Salt
Alıntı