Research On The Effect Of Normal Dissolved Gases As Impurities On Crystallization Process

The existence of impurities is unavoidable during crystallization process,which has a great influence not only on thermodynamic aspects of solutions but also on the kinetic parts.The concept of impurities is not strict.Normally speaking,it refers to the solid or liquid compounds.A few researches took advantage of gases as special seeds to induce nucleation or phase transformation.And other researchers used degassed solvents to investigate the influence on crystal defect.However,few of them treat the dissolved gases as special impuritries and do systematic research on their effect on thermodynamic and kinetic parts of crystallization process.From this point of view,four types of normal gases were choosed as special impurities and were investigated their effect on thethermodynamic and kinetics aspects of epsomite.The main work is summarized as below.Firstly,four normal gases,i.e.N2,O2,CO2 and air,were selected as special impurities to investigate their effect on the crystal defect of epsomite.Through the obversation of single crystals,it was found the surface of epsomite was easily to form cavities or holes.Besides,the crystals had high tendency to form liquid inclusions,some of which contain gases inside.These gases could be divided into two types according to their behavious during heating.When the solutions were dissolved with CO2epsomite crystals tend to form more liquid inclusions.While the other three gas impurities have no much effect on the liquid inclusions of epsomite.Secondly,the effect of four dissolved gases on the solubility and metastable zone width of epsomite was investigated.The ultrasound technology was applied,and the result of air dissolved groups was chosed as comparison.When temperature was higher than 303.15 K,the solublity of CO2 dissolved groups were increased,and vice verse.The MSZW of epsomite was wider with the existence of gas impurities,and this phenomenon was more obvious in CO2 dissolved groups.Then the growth and dissolution experiments were designed to investigate the effect of dissolved gases on kinetic parts.Ultrasoud technology and thermal controlled microscopy were utilized to observe the growth and dissolution processes.The growth kinetic model was adopted to fit the growth curve.The microscopic image showed that at room temperature the dissolved CO2 was more likely to adsorbe on the(111)and(1~-1~-1~-)faces,which leads to the growth inhibition of these faces.After the elimination of thermodynamic effect,it was found that dissolved CO2 could surpress the growth and dissolution rate when temperature was around 298.15 K.This has relationship with the derease of pH value when CO2 was dissolved in aqueous solutions.And when the temperature rised to 308.15 K,dissolved CO2 could increase the dissolution and growth rate.This phenomenon may due to the nucleation of dissolved gases and the bubbles started to grow which could adsorp on the the crystal surfaces of epsomite.The last part of the research was focused on the solution induced phase transformation under the effect of dissolved gases.The on-line monitoring technologies were employed to record the phase transformation process and the phase induced time were measured.During the phase transformation experiments of glycine,the dissolved gases,namely N2、O2、CO2 could extend the transformation time.