Borik asit büyüme kinetiğine elektriksel alan ve safsızlıkların etkisi

Abstract

Bu çalışmada, borik asitin kristalizayon kinetiğine etki edebi leceği düşünülen safsızlıkların ve elektriksel alanın etkileri ince lenmiştir. Bununla ilgili denemeler laboratuvar ölçüsünde akışkan ya taklı bir kristali zörde gerçekleştirildi. Borik asitin üretiminde kar şılaşılan bir başka problem de dentiritik büyümedir. Bu yüzden denti- ritik büyüme çalışmamızda devamlı gözönünde tutularak, safsızlıkların ve elektriksel alanın dentiritik büyüme üzerine etkileri mikroskopik ölçüde gözlemlenmiştir.

In order to solve the problems encountered in the production of boric acid from kolemanite mineral the crystallization of boric acid and the impurity effects on the crystallization, have been investigated The main problem encountered in the crystallization of boric acid is the formation of (fentritical crystal growth. This causes some indus trial problems such as a broad range of crystal size distribution, low washing characteristics, difficulty in drying and caking tendency. Therefore in this study the rate of growth and dissolution is deter mined and then crystal habit is controlled in pure boric acid solution, in the presence of some impurities such as d-mannit and polyacryl amide (PAA) and in the different level of electrical fields. The main reason for trying electrical field, a charged chemical such as PAA and a chemical such as d-mannit which can cause surface character is the different behaviour seen in boric acid crystals. These crystals adhese to the any surface very strongly. This phenome na shows that boric acid crystals have surface charge. Changing the charge either by supressing or depressing can cause some different effect on crystal growth. In this study, effects of impurities and electrical fields on the kinetics of the crystallization of boric acid are investigated. A laboratory scale fluidized bed crystal! izer, have been used in the experiments as it is shown in Figure 5-1 and 5-2. Crystal! izer which has a curculating flow system, is made of glass and has a total capa city of approximetly 10 liters. It is observed that temperature changing during any particular experiment was less than *0.1°C. The height of bed is 20 cm. and its diameter is 1.6 cm. All fluidized experiments were carried out by using of 3 grs seed crystals and for a period of 12 mins. The particle size used are 500-600 ym and 425-500 ym. At the end of the experiments crystal habits were also observed under the microscope if there are any changing in pure and impure medium. As the impurities, d-mannit (1000 ppm, 5000 ppm) and polyacryl- amide (1 ppm, 5 ppm, 100 ppm) is fed directly to the solution. To obtain the electrical fields in the system, DC supply has been used. The electrical charges were 100 mV, 1000 mV, 15000 mV and 48000 mV. In 1000 mV experiments, (+) and (-) sides has been changed to obtain different directions of current. At the end of the experiments, boric acid crystals which can readly be grown by the cooling of supersaturated solutions are easily seperated from the mother liquor by vacuum filtration and washed by di ethyl ether and then these crystals are dried at the room tempera ture. Growth rate is determined by following equation for each ex periments, L2-h =_h_ (A)17'.! t t Gl Where; L,, L~ : Crystal sizes, at the beginning and at the end of the experiment respectively. G-|, Go : Crystal we'ighths, at the beginning and at the end of the experiment respectively. At the end of the experiments, the following results had been found: -For the 500-600 urn, dissolution rate is written as follows, D=3. 189+48. 86. AC (AC<-0.1) -In growing area, the growth rate of crystals is not linear with super saturation. Figure 6-4 shows this situation for the 425-500 ym fraction. The dentritic growth is observed in pure solution.