Boksit ve rAl2 O3'in sodyum borat çözeltilerinde çözündürülmesi
Boksit ve rAl2 O3'in sodyum borat çözeltilerinde çözündürülmesi
Dosyalar
Tarih
1997
Yazarlar
Çatalbaş, Sema
Süreli Yayın başlığı
Süreli Yayın ISSN
Cilt Başlığı
Yayınevi
Fen Bilimleri Enstitüsü
Özet
Bu çalışmada boksit cevherinden alümina üretimi için Bayer prosesine alternatif bir prosesin uygulanabilirliği araştırılmıştır. Önerilen proses, boksitin sodyum hidroksit çözeltisi yerine, sodyum borat çözeltilerinde ekstraktif çözündürülmesinden sonra, çözünmeyen katıların çökeltme ve filtrasyonla ayrılmasını, elde edilen berrak çözeltinin soğutulmasıyla sodyum boratların kristalizasyonu sonucu Al(OH):,'in çöktürülmesini ve ince partiküller halinde çöken Al(OH)3'in iri sodyum borat kristallerinden hidrolik olarak ayrılmasını içermektedir. Kristalizasyonla elde edilen sodyum borat kristalleri, boksitin ekstraktif çözündürülmesinde tekrar kullanılmak üzere, yeniden çözülerek çözünürleştirme kademesine beslenecektir. Önerilen proseste, sodyum borat kristallerinin yeniden çözülmesi için gereken enerji Bayer Prosesi'nde sodyum hidroksit çözeltisinin evaporasyonu için gerekli enerjiden daha az olacağından, prosesin ekonomik olarak daha avantajlı olacağı düşünülmüştür. Ancak böyle bir prosesin teknolojik olarak geliştirilebilmesi için sodyum borat çözeltilerinde boksitin çözünme veriminin yüksek olması gereklidir. Bu nedenle bu çalışmada, y-AKO? ve böhmitik yapıdaki boksitin çeşitli sodyum borat çözeltilerindeki çözünürlükleri farklı çözelti derişimlerinde ve farklı sıcaklıklarda incelenmiştir. Düşük sıcaklıkta yürütülen deneyler, 1000 cm3 hacimli, karıştırmak ceketli bir cam reaktörde gerçekleştirilmiş, sıcaklık kontrolü ise bir termostat vasıtasıyla sağlanmıştır. Yüksek sıcaklıklardaki çözündürme deneyleri için ise, 40 cm3 kapasiteli teflon hazneli çelik otoklavlar kullanılmış ve çalışma sıcaklığı 185-230 ve 240°C olarak değiştirilmiştir. Deney sonuçlarının kinetik değerlendirilmesi, difuzyon kontrollü çözünme hızı eşitliğinden türetilen İn- - =k'.t C*-C eşitliğine göre yapılmıştır. Deney sonuçlan hem, y-Al^O?, hem boksit için çözünürlüğün sodyum borat bileşiğinin yapısına ve çözeltinin derişimine bağlı olduğunu göstermiştir. Ayrıca kalsiyum minerallerinin boraks çözeltilerinde çözünmesine benzer olarak, çözünmenin iki farklı mekanizmayı ( komplekslenme ve difîizyon kontrollü çözünme) içerdiği belirlenmiştir. Y-AI2O3 ve boksit için çözünürlük sodyum pentaborat - boraks - sodyum metaborat sırasında artmakta ve hem doygunluk hem komplekslenme derişimleri çözelti derişiminin artmasıyla önce artmakta sonra azalmaktadır. Ancak bu etki sodyum pentaborat için çok düşük seviyededir. IX Her üç sodyum borat bileşiği için de, çözünürlük verimleri çalışılan koşullarda önerilen proses için yeterli olmamakla birlikte, özellikle sodyum metaborat çözeltilerinde çözünürlük verimlerinin birtakım önlemlerle artırılabileceği düşünülmektedir.
Aluminium composes eight percent of the earth's mineable layer. Aluminium minerals present in the earth's crust generally consist of aluminium oxide or combined oxides. Bauxite is the most important aluminious ore used for the manufacture of aluminium. It contains around 40 to 60 percent alumina. The Bayer Process is the most important extractive hydro-chemical process producing nearly all the alumina from bauxite. In the Bayer Process, bauxite is ground to finer than 1 mm. It is slurried in the circulating solution and digested at an elevated temperature (150-200°C). The next step is to remove the insoluble impurities by sedimentation and filtration. The clean solution obtained from filtration is cooled and seeded with Al(OH)3 to precipitate relatively pure Al(OH)3. The coarse product fraction is washed and heated above 1000°C to obtain A1203. The amphoteric nature of aluminium permits both acid and alkaline processes for the recovery of alumina from the aluminious ores. Technology has been developed to produce alumina from other minerals such as clays, anorthosite, nepheline, alunite and leucite. However, these technologies are costly and more energy intensive than the Bayer Process. Bauxite are classified according to their main minerological constituents as gibbsitic, boehmitic and diasporic. The minerological composition has a profound effect on digestion. Generally, gibbsite (aluminium trihydrate) is the most economical to process since it is the most soluble in caustic solution. Boehnite (aluminium monohydrate) is less soluble and requires higher concentration of caustic or higher temperatures to process. Diaspor is almost insoluble in caustic solutions and treatment with the Bayer Process is uneconomical. The degree of solubility of the aluminium bearing minerals in bauxites is determined by the strength of the hydrogen bond and by the crystal lattice. Considerable deviations can be observed in the digestibility of bauxites having the same minerological composition due to their different morphology and degree of crystallinity. The largest market of aluminium containing compound is metal grade aluminium. However, the market for other applications such as additive filler, other chemical manufacture, refractory materials and for ceramics show a dynamic development. Therefore, it would be logical to produce relatively pure aluminium hydroxide from various aluminious ores without the increase of the manufacturing costs. The goal of the present study is to develop an alternative process for the production of aluminium hydroxide from bauxite ores, ilie suggested process is consisted of extractive digestion of bauxite with sodium borate solutions, seperation of insoluble XI impurities by sedimentation and filtration, precipitation of aluminium hydroxide through crystallization of sodium borates by cooling. The next step is the seperation of fine aluminium hydroxide particles from coarse crystals hydrolically, and dissolution of sodium borates and recyling of the solution. Because the energy requirements of dissolution of sodium borates is less than the energy requirements of caustic liquor evaporation in the Bayer Process, the suggested process could be further improved for technological application. However, this applicability depends on the yield of the extractive digestion of bauxite with sodium borate solutions. Therefore, in the present study, dissolution behaviour of y-Al203 and bauxite in various sodium borate solutions were examined. Experiments at 95°C were carried out in a thermostated and mechanically stirred 1000 cm3 glass reactor. Experiments above 180°C were carried out in a 40 ml bomb digester. The boehmitic sample containing 59.3 % wt. AI2O3 was provided from Etibank Seydişehir Aluminium Plant. Y-AI2O3 is reagent grade. Dissolution behaviour of Y-AI2O3 and bauxite were followed by determination of aluminium contents of the solution against time. In the all the experiments the solid / solution weight ratio was kept constant as 5/100. Sodium borate solutions were prepared from pure borax decahydrate, sodium metaborate and, sodium pentaborate. Dissolution curves for y-Al203 and bauxite are given below. Figure 1. Solubility of y-AkOs in different concentration of borax solutions versus time (t = 95°C) Xll e1-.s § 1 § 0.200 0.180 0.160 0.140 0.120 0.100 0.080 0.060 0.040 50 100 tw "^ 150 lime, mm. 200 250 Figure 2. Solubility of y-A1203 in different concentration of sodium metaborat solutions versus time ( t = 95°C ) 0.016 0^ «8 § 1 § a 0.014 -'-. 0.012 -: 0.010 -: 0.008 -: 0.006 i 0.004 0.002 0.000 A O d 6 o D A O D D A D A O % 10 Pentaborate D % 15 Pentaborate A % 20 Pentaborate 0 50 100 150 Time, miıı. 200 250 Figure 3. Solubility of Y-AI2O3 in different concentration of sodium pentaborat solutions versus time ( t - 95°C ) Xlll The effect of concentration on the solubility of Y-AI2O3 in the sodium pentaborate solutions is less distinguishable than the effect in the other two sodium borate solutions. The result of dissolution of y-Al203 in autoclaves at higher temperatures than 95°C show that the temperature increase has a positive effect on the solubility. XVI The effect of concentration on the solubility of Y-AI2O3 in the sodium pentaborate solutions is less distinguishable than the effect in the other two sodium borate solutions. The result of dissolution of y-Al203 in autoclaves at higher temperatures than 95°C show that the temperature increase has a positive effect on the solubility. XVI Figure 4. Solubility of bauxite in different concentration of borax solutions versus time ( t = 95°C ) In the kinetics evaluation of experimental results the following equation was used. C* In c*-c = k'.t 0) Equation (1) was obtained from the diffusion controlled dissolution equation given below. A dt (2) where, A is the surface area of the disolved substance, m is the mass of the dissolved substance, t is the dissolution time, AC is the concentration driving force (C*-C), C* is the equilibrium saturation concentration at the given conditions, C is the solution concentration and k is the diffusion coefficient. According to Equation (1), the plot of ln- - against time should give a line C*-C having a slope k' and no intercept in the diffusion controlled dissolution. An example of the kinetics evaluation according to Equation (1) is given below.
Aluminium composes eight percent of the earth's mineable layer. Aluminium minerals present in the earth's crust generally consist of aluminium oxide or combined oxides. Bauxite is the most important aluminious ore used for the manufacture of aluminium. It contains around 40 to 60 percent alumina. The Bayer Process is the most important extractive hydro-chemical process producing nearly all the alumina from bauxite. In the Bayer Process, bauxite is ground to finer than 1 mm. It is slurried in the circulating solution and digested at an elevated temperature (150-200°C). The next step is to remove the insoluble impurities by sedimentation and filtration. The clean solution obtained from filtration is cooled and seeded with Al(OH)3 to precipitate relatively pure Al(OH)3. The coarse product fraction is washed and heated above 1000°C to obtain A1203. The amphoteric nature of aluminium permits both acid and alkaline processes for the recovery of alumina from the aluminious ores. Technology has been developed to produce alumina from other minerals such as clays, anorthosite, nepheline, alunite and leucite. However, these technologies are costly and more energy intensive than the Bayer Process. Bauxite are classified according to their main minerological constituents as gibbsitic, boehmitic and diasporic. The minerological composition has a profound effect on digestion. Generally, gibbsite (aluminium trihydrate) is the most economical to process since it is the most soluble in caustic solution. Boehnite (aluminium monohydrate) is less soluble and requires higher concentration of caustic or higher temperatures to process. Diaspor is almost insoluble in caustic solutions and treatment with the Bayer Process is uneconomical. The degree of solubility of the aluminium bearing minerals in bauxites is determined by the strength of the hydrogen bond and by the crystal lattice. Considerable deviations can be observed in the digestibility of bauxites having the same minerological composition due to their different morphology and degree of crystallinity. The largest market of aluminium containing compound is metal grade aluminium. However, the market for other applications such as additive filler, other chemical manufacture, refractory materials and for ceramics show a dynamic development. Therefore, it would be logical to produce relatively pure aluminium hydroxide from various aluminious ores without the increase of the manufacturing costs. The goal of the present study is to develop an alternative process for the production of aluminium hydroxide from bauxite ores, ilie suggested process is consisted of extractive digestion of bauxite with sodium borate solutions, seperation of insoluble XI impurities by sedimentation and filtration, precipitation of aluminium hydroxide through crystallization of sodium borates by cooling. The next step is the seperation of fine aluminium hydroxide particles from coarse crystals hydrolically, and dissolution of sodium borates and recyling of the solution. Because the energy requirements of dissolution of sodium borates is less than the energy requirements of caustic liquor evaporation in the Bayer Process, the suggested process could be further improved for technological application. However, this applicability depends on the yield of the extractive digestion of bauxite with sodium borate solutions. Therefore, in the present study, dissolution behaviour of y-Al203 and bauxite in various sodium borate solutions were examined. Experiments at 95°C were carried out in a thermostated and mechanically stirred 1000 cm3 glass reactor. Experiments above 180°C were carried out in a 40 ml bomb digester. The boehmitic sample containing 59.3 % wt. AI2O3 was provided from Etibank Seydişehir Aluminium Plant. Y-AI2O3 is reagent grade. Dissolution behaviour of Y-AI2O3 and bauxite were followed by determination of aluminium contents of the solution against time. In the all the experiments the solid / solution weight ratio was kept constant as 5/100. Sodium borate solutions were prepared from pure borax decahydrate, sodium metaborate and, sodium pentaborate. Dissolution curves for y-Al203 and bauxite are given below. Figure 1. Solubility of y-AkOs in different concentration of borax solutions versus time (t = 95°C) Xll e1-.s § 1 § 0.200 0.180 0.160 0.140 0.120 0.100 0.080 0.060 0.040 50 100 tw "^ 150 lime, mm. 200 250 Figure 2. Solubility of y-A1203 in different concentration of sodium metaborat solutions versus time ( t = 95°C ) 0.016 0^ «8 § 1 § a 0.014 -'-. 0.012 -: 0.010 -: 0.008 -: 0.006 i 0.004 0.002 0.000 A O d 6 o D A O D D A D A O % 10 Pentaborate D % 15 Pentaborate A % 20 Pentaborate 0 50 100 150 Time, miıı. 200 250 Figure 3. Solubility of Y-AI2O3 in different concentration of sodium pentaborat solutions versus time ( t - 95°C ) Xlll The effect of concentration on the solubility of Y-AI2O3 in the sodium pentaborate solutions is less distinguishable than the effect in the other two sodium borate solutions. The result of dissolution of y-Al203 in autoclaves at higher temperatures than 95°C show that the temperature increase has a positive effect on the solubility. XVI The effect of concentration on the solubility of Y-AI2O3 in the sodium pentaborate solutions is less distinguishable than the effect in the other two sodium borate solutions. The result of dissolution of y-Al203 in autoclaves at higher temperatures than 95°C show that the temperature increase has a positive effect on the solubility. XVI Figure 4. Solubility of bauxite in different concentration of borax solutions versus time ( t = 95°C ) In the kinetics evaluation of experimental results the following equation was used. C* In c*-c = k'.t 0) Equation (1) was obtained from the diffusion controlled dissolution equation given below. A dt (2) where, A is the surface area of the disolved substance, m is the mass of the dissolved substance, t is the dissolution time, AC is the concentration driving force (C*-C), C* is the equilibrium saturation concentration at the given conditions, C is the solution concentration and k is the diffusion coefficient. According to Equation (1), the plot of ln- - against time should give a line C*-C having a slope k' and no intercept in the diffusion controlled dissolution. An example of the kinetics evaluation according to Equation (1) is given below.
Açıklama
Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 1997
Anahtar kelimeler
Alüminyum hidroksit,
Boksit,
Sodyum borat,
Çözeltiler,
Çözünme,
Aluminum hydroxide,
Bouxite,
Sodium borate,
Solutions,
Dissolution