Selective Removal Of Silicic Acid By Modified Resin

Silica scale is a kind of precipitation which has very low solubility.The formation of silica scales proceeds by the polymerization between silicic acid molecules or reaction with metal ions.Silica scale is easy to precipitate in the inner wall of the equipment and the membrane surface.it will block the pipe and membrane,reduce the water flow,result in uneven heat conduction,hinder the normal operation of the system seriously and bring huge economic losses and security risks to industrial production.Reducing the content of silicic acid in the water is the most effective way to prevent the formation of silica scales.Ion exchange method is widely used to remove silicic acid.However,ion exchange can only remove silicate ion but the molecules which is a predominant form in water.In this study,gallic acid will be grafted onto the resin to eliminate different forms of silicic acid.This study made GA grafted on the ordinary ion exchange resin and determined the best graft conditions by investigating the effects of pH,ionic strength and GA concentration on the grafting amount.Compared the adsorption effect of GA resin and OH resin for silicic acid,discussed the mechanism of silicic acid and GA preliminarily,and studied the influence of anions on adsorption to discuss the adsorption process.Fitted the experimental data by different adsorption isotherm models,adsorption kinetics and adsorption thermodynamics to determine the adsorption mechanism.Studied the desorption effect of different eluent on GA resin adsorpted silicic acid and determine the best eluent.Then,make a comprehensive evaluation about the effect of GA resin on the cyclic adsorption of silicic acid.The main conclusions are as follows:(1)The ordinary OH resin can be grafted by GA,and the optimal grafting conditions are as follows:the pH of solution was from 6 to7,the ionic strength was zero and the concentration was 2.5 g/L.The maximum grafting amount was 340.3 mg/g.(2)GA resin can adsorpt molecular and ionic silicic acid,the change of pH value had no obvious effect on GA resin adsorb silicic acid.Under different pH conditions,the removal rate and adsorption capacity of GA resin on silicic acid were 30 times larger than that of OH resin.A chemical reaction may occur between GA and silicic acid and produce new substances.(3)GA resin had high selectivity to silicic acid and the effect of adsorption didn’t affected by low concentration of coexisting anions.When the anion concentration increased,the adsorption capacity would decreases,and the effect of multiple anions on adsorption was much greater than that of single anion.(4)By the adsorption isotherm,it could be know that the saturated adsorption capacity of GA resin to silicic acid was 6.471 mg/g(Si meter),environment media had little effect on the adsorption.The adsorption process fitted the Temkin and pseudo-second-order kinetics modes,particle diffusion and chemical reaction were the main steps to control the reaction rate,which indicated that the adsorption type was chemical adsorption.Thermodynamic results show that the reaction is spontaneous,endothermic physical and chemical adsorption process.Combing the results of dynamics and thermodynamics,the adsorption of silicic acid by GA resin is the result of physical adsorption and chemical adsorption.(5)Desorption effect was best when put the GA resin adsorpted silicic acid under alkaline conditions,that means the alkaline solution was the best eluent.The GA resin could adsorb silicic acid twice and maintain the removal efficiency of two were more than 90%on the condition of no re-grafting of the GA and desorbed with alkaline solution.It is possible to consider a graft treatment after adsorpting twice.