Study On Mechanism Of Releasing Alkalinity And Control Methods Of Its Process Of Calcium Silicate Filter Material

Calcium silicate materials （e.g. Calcium Silicate Hydrate, Xontlite, Hydrowollastonite,Tobermorite, etc.） with porous structure release calcium ions and hydroxide ions to adsorbphosphate in the solution and to generate microenvironment which is productive for theformation of hydroxyapatite and then to recovery of high-quality phosphorus. The preliminarystudy of the mechanism of micro-porous calcium silicate for phosphorus removal shows that thekey of the phosphorus removal is good performance of releasing alkalinity and calcium ionsfrom itself. However there aren’t any researches involving the material to keep basicatmosphere or mechanism of providing calcium ions and alkalinity and so on.The points of thisresearch were to prepare new type of calcium silicate filter materials（CSFM）, to analysecharacteristics and mechanism of alkali release of the filter materials, to clarify mechanism andeffects of alkaline atmosphere of filter material internal forming, and to explore the effectivecontrol methods of calcium and hydroxyl ions dissolved out of filter materials.Through the above research contents, conclusions are as follows:Taking calcium oxide and silicon dioxide as raw materials, adding binder （cement） andpore forming agent （aluminum） as follow, a porous CSFM was prepared by means of steamcuring at high temperature in a reaction kettle. During the single factor experiment, the optimumprocess conditions of the filter material determined by investigating the amount and persistenceof releasing alkalinity and calcium ions are calcium silicon ratio （molar ratio） of3:4, water-solidratio of0.6（mass percent）, and aluminium powder added quantity of0.8%with steamingtemperature at180°C in the reaction kettle for4h. The results also show that the changerelationship between pH value and concentration of Ca²⁺dissolution of leaching solution isgood positive correlations under different conditions of the preparation of CSFMs.Performances of releasing alkalinity and calcium ions impacted on four factors of particlesize, velocity, temperature and additive quantity in static leaching experiment and factors offlow type and volume of accumulation in flow dynamic leaching experiment within watermedium were investgated of the CSFMs which prepared under the optimum conditions. Theresults show that the effect of particle size on the quantity of releasing alkalinity and calciumions of filter materials is more than on the alkalinity and the calcium dissolution rate, velocityhas the influences of both, the effect of temperature is not obvious, and the more of dosingquantity the smaller growth rate of dissolution quantity. The results of dynamic conditions showthe process of filter materials for releasing alkalinty and calcium ions is rising rapidly in short-term to smoothly.With the flow type of feeding water intermittently, within the uniform phaseof releasing, pH values of dissolution of filter materials have dropped from11to10.22for16 days with decreasing amplutide of0.8pH unit and the average drop in0.05pH unit daily. Andthe concentrations of Ca2+have fallen from44mg/L to14mg/L with the average drop less than1.88mg/L daily. Both of the decreasing amplutides are about half of the type of flowing watercontinuely and its dissolution regulation is basically the same to volume of accumulation in half.According to the results of static test of dosing quantity, chemical ionization equations ofCa（OH）₂and formulas of the ionic strength and ion activity combined with, concentrations ofCa²⁺were conversed into pH and compared with the measured pH. The results show that bothof them fit in and verify the significant positive correlation between the alkalinity and calciumions release of CSFMs caused by precipitated Ca（OH）₂from them.The calcium dissolution dynamics behaviors of CSFMs are in line with first-orderdiffusion equation and Elovich equation. It shows the process of calcium dissolution from filtermaterials fits to the slow reaction diffusion mechanism.The release rate is not only controlled byparticle diffusion within but also other factors including. And based on the characterizationresults of filter materials soaked in water or not by scanning electron microscopy （SEM）, X-rayspectrometer （EDS） and XRD to analyse the alkalinity release process of filter materials includeproduction base and the precipitation process. The alkalinity and calcium release rates of filtermaterials are mainly controlled by hydration speed of the calcium silicate minerals composed offilter materials and the structure of filter materials controlled on its dissolution quantity. Theinfluence of the dual function makes CSFMs have the characteristics of the slow release ofcalcium ions and hydroxyl ions. The feasibility analysis of control technology alternatives,preparation in the form of filter material of CSFMs could be belonged to the technologies ofcontrolled release.