Study On The Crystallization Kinetics And Thermodynamics Of Scorodite Under Atmospheric Pressure

The disposal problem of high concentration arsenic containing wastewater is focused and the crystallization kinetics and thermodynamics research of scorodite?Fe As O₄×2H₂O?which has weak solubility and high chemical stability in an acidic medium has been studied.The thermodynamically stable region of scorodite has been achieved and the reaction mechanism of crystallization process has been cleared.The theoretical support has been provided for the improvement of the process flow of arsenic harmless and technology integration.The main subjects of this dissertation are as follows:Based on the thermodynamic analysis,the thermodynamic stability region of each precipitation component in Fe-As-H₂ O system under room temperature is cleared.The results show that the boundary pH value of the transformation process from scorodite to iron hydroxide is 2.3,which means the pH value close to 2.3 will be more conducive to the synthesis of scorodite.In addition,with the increasing of the concentration of iron in the solution,the stability region of scorodite will be reduced.The preferable synthesis conditions of scorodite via Fe?III?and As?V?are determined as follows: reaction temperature of 80 ?,initial pH of 2,iron to arensic mole ratio of 1 and the initial arensic concentration of 20 g·L^-1.Under the above condition,the scorodite can be synthesised with the reaction time of 6 h,and the precipitation rate can be reached as 94.8%.The XRD and SEM pictures suggest that the crystallization has a high crystallinity and the crystal grain is clear in the shape of two-dimensional rod.The nucleation and growth mechanism of the crystallization process of the scorodite is analyzed,and the crystallization process can be divided into two steps: precipitation and transformation.The poorly crystalline ferric arsenate is firstly precipitated with the mixture of high concentration of Fe3+ and AsO₄^3-,due to the high ionization degree of Fe As O₄ in aqueous,the amorphous ferric arsenate will transferred to crystalline ferric arsenate through the process of reverse ionic dissociation after aging.Considering the precipitation process happens very soon,the transformation process will be the controlling step of the crystallization process of scorodite.The crystallization kinetics of scorodite at different iron to arsenic mole ratio in the range of 60⁹0 ? has been researched,the results show that the crystallization process follows the mechanism of action of Johnson-Mehl-Avrami equation.The controlling step of the crystallization process is interface reaction while the iron to arsenic mole ratio is 0.8,and the activation energy is 52.01 k J·mol^-1.While the iron to arsenic mole ratio is above 1,the nucleation process of scorodite is the controlling step of crystallization reaction,the activation energy of the crystallization reaction with the iron to arsenic mole ratio of 1 and 1.2 are 72.94 k J·mol^-1 and 171.92 k J·mol^-1,respectively.