Amonyum pentaborat ve borik asitin kalsinasyonu ile bor oksit üretimi
Boron oxide by calcination of ammonium pentaborate and boric acid
Yazar: ÜLKER BEKER(GÜRBÜZ)
Danışman: PROF.DR. OĞUZ RECEPOĞLU
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Kimya Mühendisliği Ana Bilim Dalı
Derece: Doktora, 1992
Konu: Kimya = Chemistry
Dizin: Amonyum pentaborat = Ammonium pentaborate ; Bor oksit = Boron oxide ; Borik asit = Boric acid ; Kalsinasyon = Calcination ; Üretim = Production

Özet:
ÖZET Bu çalışma, sabit ve akışkan yatakta amonyum pentaborat ve borik asitin termal bozundurulması ile bor- oksit eldesini incelemek amacıyla yapılmıştır. Borik asit, özellikle 1 5D C den sonra hızlı bir şekilde aşırı bir köpürme ile dehidrate olduğundan, daha stabil bir bor bileşiği olan amonyum pentaboratın da boroksit üre timinde kullanımı incelenmiştir. Amonyum pentaborat sabit yatakta kalsine edildiğinde, 350 C de kuvvetli sinterleşme sonucu partiküller birbirine yapışmakta ve geri kalan su ve amonyağın tamamen giderilmesi ancak 450 C 'nin üzerindeki sıcaklıklar ve uzun sürelerde gerçekleşmektedir. Bu nedenlerle partiküllerin serbestçe hareket edebileceği, akışkan yataklı bir kalsinatör kullanmanın daha uygun olacağı sonucuna varılmıştır. Akışkan yatakta sabit yatağa nazaran daha hızlı bir dönüşümle elde edilen bor oksitin amorf yapıda olması ve 350 C de sinterleşmesi sebebiyle, akışkanlaşan karışımın ergime noktasını düşürmek ve kalan son amonyak miktarını gidermek üzere akışkan yataktaki tanelerin üzerini yüksek sıcaklıkta ergiyen ince bir tabaka ile kaplanması koşulları incelenmiştir. Buna göre, akışkanlaşan kütleye boraks ilavesi ile partiküllerin yüzeyinde Na-0. 9B-0., yüzeysel reaksiyonu sonucu ergime noktası yüksek bir zarf oluşarak ergime ve sinterleşme sıcaklığının yükseltilebileceği belirlenmiş ve prosesin her kademesi incelenerek, amonyum pentaborat ve borik asitten % 99.5 saflıkta boroksit ve % 0-.1 5-0. 20 oranın da Na"0 içeren bir ürün elde edilebileceği saptanmıştır, Borik asitten bor oksit üretimini sürekli bir proses haline getirmek üzere, uygun tane dağılımı ve oranındaki boraks ve borik asit karışımlarının iki kademeli kalsinasyonu da incelenmiştir. Bu işleme göre, karışım birinci kademede 150 C a ısıtılmış akışkan yatak ta 30 dakika süreyle kalsine edildikten sonra 375 C 'a ısıtılmış akışkan yatağa beslenerek 1 saat süreyle kal sine edilmiştir. Böyle bir proses ile % 99. B saflıkta bor oksit üretimi mümkün görülmektedir. vı

Abstract:
BORON OXIDE BY CALCINATION OF AMMONIUM PENTABORATE AND BORIC ACID SUMMARY In this study, production methods of boric oxide from boric acid and ammonium pentaborate are investigated. Turkey, an important boron-producing country of great signifiance besides the United States of America, is likely to gain importance as a competitor for international markets. Although boric oxide is a well known subtance,its potentially rich chemistry has been discovered recently, This is certainly due to its high viscosity of the molten form. Boric oxide exists in three distinct forms. The glassy or vitreous farm, prepared by dehydration of boric acid at high temperatures, is a clear and very hard glass. It has no definite melting or bailing point. It shows noticeable volatility around 1 ODD C and its temperature of vaporization lies above 1500°C. Crystalline boric oxide is prepared by gradually heating of glassy boron oxide at low temperatures. It is very fine. crystalline lump or cake. Its melting point has been reported to be between 45G- 460DC. A reactive form of boron oxide can be made by slowly heating finely ground boric acid in vacuo to a temperature of 26D-27G C. The product formed is nearly anhydrous and dissolves very rapidly in water. Boric oxide finds many uses in industry, especially in applications in which a high boric oxide content material, containing little or no oxides of alkaline Vllearth metals, is desired. Thus, the oxide is used in the production of many special glass compositions, enamels, alloys, in the preparation of fluxes, and as a catalyst in organic reactions. The production methods of boric oxide can be classified in four main groups: 1- Dehydration of boric acid: Bacceradde et. all, produced boric oxide by azeotropic distillation of boric acid in organic medium. In other method Fedyuskin et. all, boric acid was dehydrated in a stream of air or other gases, in three stages, each at various temperature ranges. 2- Boric oxide production from metal borate: The main reaction of metal borates with sulfuric acid and seperation at high temperatures with sodium tetraborate pentahydrate as a metal borate the reaction is a follows: Na2BD4.5H20 + HgSG^ - 2 BgOg + Na2S0^ + 6 H2G The resultant mixture of this reaction is fed in a furnace and heated to obtain two molten layers. The upper layer which is boric oxide, and lower layer which is sodium sulfate, are separated and cooled to provide the granular products. 3- Boric oxide from ammonium pentaborate: The aim is to develop a process of converting boron ares to baric oxide without using sulfuric acid. Wilson et.all, have seperated ammonium pentaborate [ (NH, ) "0. 5B"0_.BH"DJ from reaction of borax and aquous NH.C1, and heated ix to 800-90DDC. Thus, NH, and H2Ü. is liberated and boron oxide left as a residue. *f- Another method consist of esterif ication of boric acid with ethanol followed by rectification, hydrolysis, and vacuum dehidration. PREPARATION AND PROPERTIES OF EXPERIMENTAL SUSBTANCES Ammonium pentaborate: Ammonium pentaborate is a very stable compound, having no ammonia odor. When ammonium pentaborate is heated to 150°c, three-fourts of the its water content and less then 1 % of its ammonia content are liberated. Ammonium pentaborate Vlllused in this investigation is prepared by introduction of ammonia solution in a saturated boric acid solution at 9G C and by cooling the reaction mixture to 20 C. Boric Acid: Technical and analytical grade boric acid are used in experiments. CALCINATION BEHAVIOR DF AMMONIUM PENTABORATE In order to investigate the conversion of ammonium pentaborate to boric oxide in a fixed bed, an equipment consisting of a temperature controlled furnace, a computer, and a computer competible digital top loading single pan balance is developed. The platinum crucible containing the sample are placed on the top of a bar inserted into the furnace through the hole at the bottom of the furnace. Samples are treated at constant temperature for several hours. Ueight lasses versus definite time intervals are determined continuously by means of the computer. Thermal decomposition of ammonium pentaborate to boric oxide is investigated at 150°C, 20DDC, 25DDC, 3DDDC, 35G°C, 400°C, 450°C, 50D°C. In orderto follow dehydration and convertion to boric oxide, the samples are retained for 5, 10,15,20,30,40,50,60 minutes at above mentioned temperatures. According to the results obtained from these experiments, dehydration rates increased above 300 C. Boric oxide content of the samples increase rapidly with increasing calcination temperature. Sintering begins at 300°C and at 450°C the melting is completed. At 400 C some boric oxide loss also occurs. The first 5 moles of crystal water is liberated between 150-200DC. The results of DTA measurements show that the water and ammonia left are released between 270-300 C and decomposition is completed at 450 C. The height of the bed also effects the decomposition of ammonium pentaborate. The sintering reduce the reaction surface and so decomposition is hindered. IXCALCINATION BEHAVIOR OF AMMONIUM PENTABORATE IN FLUIDIZED BED Fluidized bed shown in Figure 10 a is made of a heat resistant 70 cm high glass column with 30 mm inner diameter. The f luidiziation air is heated by a temperature controlled furnace and then fed to the fluidized bed at a constant flow rates. The calcination behavior of ammonium pentaborate o ıs investigated for temperatures lying between 1 50-350 C. Samples are fed directly in the preheated bed and treated for B hours. Results show that the calcination in a fluidized bed developes more rapidly than in a fixed bed. The product of 85.62 % boric oxide content which is obtained in one hour at 350 C in the fixed bed can be withdrawn at 250 C after treatment of one hour from fluidized bed. The operation can not be run over 350 C, as fluidized bed sintered in 20 minutes. An adheration of the fluidized particles on the glass column wall is also observed. The precalcination of the ammonium o pentaborate between 200-400 C in the fixed bed furnace, before the calcination in fluidized bed, can avoid the occurance of this adherence. According to the obtained results, precalcination accelerates the calcination without varying the sintering temperature. The best result obtained by this treatment is a product containing 96 % boric oxide and 2 % ammonia [as (NH4)20]. Introduction of NaCl to the fluidized bed and HC1 vapour or C0" gas in fluidizing air, show no effect on the liberation of left ammonia. CALCINATION BEHAVIOR OF BORAX ADDED BORIC ACID AND AMMONIUM PENTABORATE Na20-Bp0_ phase diagram of Figure 23 shows that a compound (Na?0. 9B"0, ) with a high boric oxide percentage and a high melting point of approximately 700-800 C, can be obtained. If this compound produced in situcovers the preformed boric oxide particles, the sintering can be avoided. Fluidized bed experiments with an addition of 2, ^t-,5,10 % borax dihydrate results in the elevation of sintering temperatures of the bed to values lying between 440-540 C. With an addition of 5 % borax dihydrate the sintering temperature is approximately 525 C and under this temperature, fluidizing can be carried out properly. An important result of this experiment is that the added borax forms some popcorn like particles which can be easily removed by simply blowing. The end product contains G.5 % ammonia and D.5 % Na"0, the rest being boric oxide. Similar experiments with boric acid as starting substance, show better results. For this purpose, a series of experiments are carried out by adding various amounts of borax as borax pentahydrate and borax dihydrate. The addition of 5.5 % borax pentahydrate to the fluidized system system rises the sintering point to 450 C. In the case of borax dihydrate fluidized bed can be operated up to 525 C resulting in a product with 99.5 % boric oxide. Particle size distribution of borax and boric acid crystals show also a marked effect on the sintering behavior of the fluidized bed. Experiments carried out with mixtures of particles at various sizes show that the ratio of the mean particle diameters of borax and boric acid crystals must be between 1 and 2. A two stages process is also investigated to minimize the calcination time and to make the process continuous. In the first stage the mixture consisting of boric acid and 5.5% borax pentahydrate is fed to fluidized bed and dehydrated at 150 C for 30 minutes, and in the second stage, dehydration is completed at 375-400 C. it is determined that the first stage dehydration must be 30 minutes long in order to provide a smooth fluidization without sintering in the second stage dehydration. In the second stage, after a treatment of 1 hour the obtained product contains 99.5% boric oxide. The tendency- of sintering increases only at temperatures above 400 C. XIRESULTS The results obtained from the experiments are as follows: 1- In fixed bed, the decomposition of ammonium o o pentaborate is incompleted upto 5D0 C. Above 400 C, some sintering and elevated boric oxide lass is observed. 2- In. fluidized bed, the maximum temperature at which the ammonium pentaborate works without sintering, is about 35D C. The product obtained under these conditions contains 96 % baric oxide. 3- The addition of 5% borax dihydrate to ammonium pentaborate decreases the sintering tendency of the fluidized mixture and the working temperature can be raised up to 5QD C. 4- After fluidized bed treatment, the added borax pentahydrate forms large popcorn like spherical particles which can be easily removed by blowing. The products contain only a small percentage Na"0 ranging at approximately 0.15-0.20 %. 5- The same procedure can also be applied to boric acid. In this case, 5.5 % borax pentahydrate addition to boric acid increases the sintering temperature of the fluidized bed upta 475 C and a product with 99.5% boric oxide and 0.15-0.20 % Na"0 can be obtained. 6- The addition of borax in the form of readily dispersible dihydrate instead of pentahydrate incerases the sintering point of the bed from 450 C to 525 C and well formed fluidized bed can be operated upto 500 C. 7- A solid phase reaction occurs between boric acid and borax particles and a cover with high melting point is formed on the surfaces of boric acid or boric oxide which hinders the sintering of the system. Particle size of boric acid and borax play an important role in the sintering tendency of the bed. The ratio of mean particle sizes of borax and boric acid crystals must be between 1 and 2. B- Dehydration of boric acid and borax mixture can be best performed in a two stages operation. In the first stage, the mixture containing 5.5 % borax pentahydrate will be treated at 150 C to form metaboric acid and in the second the dehydration will be completed at 375 C in 1 hour. Xll