Kalsiyum bazlı minerallerin boraks çözeltilerindeki çözünme mekanizması ve kinetiği /
Yazar: BEYHAN EREN
Danışman: PROF. DR. A. NUSRET BULUTÇU
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Kimya Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 1996
Konu: Kimya Mühendisliği = Chemical Engineering
Dizin: Boraks = Borax ; Kalsiyum = Calcium ; Çözeltiler = Solutions

Özet:
Bu çalışma, tinkal minerali ile birlikte bulunan kalsiyum ve magnezyum bazlı minerallerin komplekslerime ve çözünme kinetik ve mekanizmalarının sürekli karıştırmalı bir reaktörde incelenmesi ve komplekslenme olayı sırasında mineral yapısında ve özelliklerinde herhangi bir değişikliğin olup olmadığım araştırmak amacıyla yapılmıştır. Bu amaçla, çalışmanın ilk kademesinde tinkal cevheri ile birlikte bulunması muhtemel dolomit, kalsit, uleksit, kolemanit, inderit, inyoit, hidroborasit ve tünellit minerallerinin sıcaklık ve boraks konsantrasyonuna bağlı olarak boraks çözeltilerindeki çözünürlükleri incelenmiştir. Bu minerallerden boraks üretim çözeltilerine geçene en önemli safsızlıklar kalsiyum, magnezyum ve silistir. Bu safsızlıklar özellikle rafine boraks üretiminde önem kazanmaktadır. Çözünebilen safsızlıklar çözme kademesinde üretim çözeltisine geçmekte ve kristalizasyon sırasında borlu bileşikler olarak ürünle birlikte kristalize olmakta ve ürün kalitesini bozmaktadır. Çalışmanın ikinci kademesinde komplekslenme olayı sırasında mineral yapısında ve özelliklerinde herhangi bir değişikliğin olup olmadığı, farklı boraks konsantrasyonu ve sıcaklıklarda yapılan çözünme deneylerinden kalan katı numunelerin karakterizasyonu, kimyasal analiz, X-ışınlan difraktometresi ve FTIR sepktrometresi ile incelenmiştir.

Abstract:
Tincal (Na2B407. 10H2O) is the most important water-soluble boron mineral which is used in the production of borax decahydrate and borax pentahydrate. Although the production proces of borax salts is not complex, the impurity behaviour is not very simple. Saturated borax solution disolves the minerals such as dolomite, calcite, ulexite, colemanite, inyoite in tincal ore. The solubility of these minerals in the ore is higher in borax solutions than in water. Therefore borax production solutions contain calcium, magnesium, silica and aluminium ions. The main impurity in borax solutions is calcium and these calcium ions finally deposit from the hot solutions, usually in the form of probertite NaCaBjOg.SKbO. Since calcium is the main creating problem in the production of borax decahydrate and borax pentahydrate from tincal ore, the possibility mat borate acts a chealeting ion may be prove to be of considerable importance in this production. The borate ion, being the conjugate base of a weak acid, is excepted to form a variety of complex salts with suitable metal accepter ions. Although the calcium borates are quite insoluble in water, hot concentrated borax solutions, containing calcium ions in excess of its equilibrium value can be prepared. In a previous study, it was observed that the dissolution mechanisms of calcium minerals in borax solutions are quite different from the normal diffusion controlled dissolution mechanism It was proposed, from long term dissolution experiments in a shaker-bath, that the overall dissolution of calcium minerals should be controlled by two different mechanisms: one is the complexation and the other is the normal dissolution. In a diffusion controlled dissolution, the following equation is valid: 1 dm 4" ?a1T=KdAC (1) where, A is the surface area, m is the mass of the substance, t is the time, AC is the concentration driving force (C*-C0), C* is the equilibrium saturation concentration at the given temperature, Q is the solution concentration and KD, is the diffusion coefficient. If the solubility is low and the amount of the substance in the solvent is high, change of the surface area of the substance due to the dissolution can be ignored for a closed isothermal system and following equations can easily be derived: IXdC dm^V.pt- (2) where V is the volume of the solution, dC is the variation of the concentration, pk is the solid density and f, is the proportion of the following ion in dissolved substance. V.Pk dC ix-t=KdAC (3) where pk, V, A and fare constants dC --V.AC (4) V.Pk If tiie equation (4) was solved for the following boundary conditions: C=Co at t=0 C=Ci at t=t where, C0 is the complexation concentration. ln|^=k'.t (5) Equation (5) can be reordered and one get: lnc4=k't+,nc^c: (6) According to Eq.(6), in the case of no diflussion controlled dissolution occurs, the plot of ln(C /(C*-Ci)) versus time should give a straight line, the slope should give k', and the intercept should give ln(C*/(C*-C0)). The aim of this study is to determine dissolution mechanisms and kinetics of calcium and magnesium based minerals such as calcite, dolomite, ulexite, colemanite, inyoite, inderite, hydroboracide and tunellite in borax solutions. In order to investigate the effects of borax concentration and temperature on the dissolution behaviours of calcium and magnesium minerals, dissolution experiments were carried out in a thermostated, double jacketed one liter, reactor at a constant stirring rate of 750 rpm and a constant mineral to borax weight ratio of 0.01. After addition of the mineral to the borax solution at a given concentration and temperature the suspensionsamples were withdrawn at small time intervals and filtered through a glass filter (porosity 4) by suction. Total calcium and magnesium concentraitons of the clear solutions were determined by the well known complexometric titration with EDTA at pH:10 and pH:12.5, respectively. The characterization of the solid phase after the experiments was determined with chemical analysis, X-ray diffraction and FTIR spectrometer in order to see weather or not any change occurs in the original structure of mineral. The analysis of the examined minerals are given in Table 1. The chemical compositions of the minerals fit to their theoretical compositions within the analytical errors, except dolomite. Their mineralogical structures were also proved by X-ray diffraction analysis. Table 1. Analysis of the original and extracted minerals. * In Na2B407 %30 wt., at 95°C. Figure 1 shows dissolution behaviour of dolomite in borax solutions. As can be seen from Fig.l.a there are two different steps, one is dissolution and the other is precipitation in to form of probertite. XI0 20 40 60 80 100 120 140 160 180 200 Time, min. Figure l.a. Variation of Ca concentration with time as a function of borax concentration and temperature for dolomite. It is not possible to determine the saturation concentration of calcium from Fig. 1.a directly because of the competition of these two mechanism. Therefore, it was determined by using trial and error method to get the highest correlation coefficient in respect to Eq.(6). ln(C*/(C*-Ci) 40 60 Time, min. 100 Figure l.b. Lineerized dissolution rate at 95°C and in 30% borax solution according to Eq.(6). XllFigure l.b. is an example for the best fit obtained for the experiment carried out at 95°G and 30% Na2B407 concentration. Probertite precipitation can be clearly determined from the deviation from this line. Since the value obtained for 90 minutes does not fit to the dissolution line, this point is excluded from the straight line different from zero implies that the calcium ion transfer from the mineral can not derive from the diffusion controlled dissolution. This intercept indicates that the calcium transfer is due to complexation rather than spontaneous dissolution. As it seen from Fig. l.b. that the value obtained for 5 minutes is very close to the dissolution line. In other experiments the values obtained for 5 minutes fit well to the dissolution line. This indicates that the complexation rate is very high and complexation is completed is less than 5 minutes. Therefore, the complexation concentration C0 was calculated from the intercept. If this assumption is true, the complexation concentration should be the function of borax concentration and temperature. Figure l.c shows the complexation and saturation concentrations as a function of borax concentration and temperature. 700 600 500 400 300 200 100 5 10 15 20 25 30 Borax concentration, % Na2B4 O, 35 Figure l.c. Dependence of saturation C*, and complexation concentrations C0 on borax concentration and temperature. It is "seen from Fig. l.c that both saturation and complexation concentrations increase with increasing borax concentration. On the other hand, the saturation concentration does not depend on the temperature, whereas the complexation concentration increases slightly with increasing temperature. As can be seen from Table 1, the dolomite samples used in the present study contain some free calcite : the chemical analysis and X-ray diffraction patterns of the extracted dolomite samples showed that their composition change to the composition of pure dolomite during dissolution. These results and the very low magnesium ion concentrations (< 5 ppm) in the clear solutions indicate that the dissolution of dolomite is only due to the dissolution of the free calcite in dolomite. It is also observed that the dissolution of pure dolomite in borax solution is very low. xmSimilar experimental results were obtained for calcite. The results show that the complexation concentration changes linearly with the borax concentration and that it does not depend on the temperature. The saturation concentration of calcite was also independent of temperature and sharply increases with increasing borax concentration. Small concentration changes were observed after complexation, except at 95°C and 30% Na2B407. Chemical analysis of the extraction residues given in Table 1, showed that the chemical composition of calcite is partly changing by forming an acid insoluble part. It is confirmed by X-ray diffraction analysis that a new solid phase formed in a small amount is amorphous. The experiments carried out with ulexite show also similar results. After reaching the complexation concentration, only minor concentration changes were observed. In this case the effect of the temperature on complexation and saturation concentrations is higher than in the cases of calcite and dolomite. The solubility of ulexite is depressed by the existence of borax ion up to about 10% NazB^ and then increases. It is determined by chemical analysis that, by increasing the temperature tramformation of ulexite to amorphous propertite is caused. By increasing the borax concentration this transformation is prevented. The highest transformation was observed at 70°C and 10% N&2J&4O7 concentration. As can be seen from Table 1 that the compositions of the solids remaining after dissolution are not exactly identical to the composition of ulexite since a part of ulexite is transformed to an acid insolubled material similar to in this case of calcite. The experiment was carried out with colemanite only conducted at 95°C and 30% Na2B407 solution since colemanite has very low solubility. The saturation concentration with respect to calcium was found to be 135 ppm and the complexation concentration was 52 ppm at this condition. The experiment was only conducted at 95°C and 30% Na2B407 solution for inyoite, inderite and hydroboracide. The saturation concentration with respect to calcium for inyoite mineral was found to be 1420 ppm and the complexation concentration was 1273,5 ppm at this condition. The mineral seems inyoite but it was determined that it was not being inyoite. It is mixed mineral based calcium and magnesium mineral according to by X-ray diffraction patterns. As can be seen from Table 1, the chemical analysis and X- ray diffraction patterns of the extracted inyoite samples showed that their composition completely change to the composition of the mixed mineral during dissolution. It showed that the structure is similar to inderite. This part of the mixed mineral dissolved completely in borax solution and the residue contains less B203, high calcium and magnesium. The saturation and complexation concentration of inderite with respect to calcium could not determined at this condition. The chemical analysis of solid phase was shown at Table 1. Similar experiment was carried out for hydroboracide at the same conditions. The complexation concentration with respect to calcium was found 102.6 ppm. The chemical analysis and X-ray diffraction patterns of the extracted hydroboracide samples showed that xivtheir composition has no change during extraction. The magnesium ion concentration was found to be 34 ppm in the clear solution when hydroboracide dissolved. In this case of tunellite, the saturation concentration of Sr was found to be 717 ppm and the complexation concentration was 58.7 ppm at this condition. Similar experimental results were obtained as can be seen from Table 1. The chemical analysis and X-ray diffraction pattern of the extracted tunellite samples showed that their composition has no change during extraction. The proportions of B2O3 and SrO are the same within the analytical errors and acid insoluble part increase. It is concluded that borax solutions show some peculiar dissolution behaviour with calcium and magnesium bearing minerals. The dissolution of calcium from these minerals occurs in two steps. In the first step, calcium reacts very rapidly with borax to form a complex. The second step is difiiision controlled dissolution and proceeds at a slower rate. Both steps are function of the type of mineral.