Effect And Mechanism Of Fractional Crystallization To Remove Calcium Hardness In Drinking Water

Higher hardness of drinking water will not only affect human health,but also bring certain harm to industrial applications The traditional chemical crystallization precipitation method has the advantages of simple operation,strong adaptability and low cost,but it also has defects such as high effluent pH,fine granules,difficulty in sedimentation,and generation of a large amount of precipitated substances.The stepwise crystallization method was used to remove the calcium hardness in drinking water,The mechanism and removal efficiency of calcium hardness was investigated.The main conclusions are as follows:In the one-time dosing mode of the precipitant,the pH value and conductivity of the solution system increased rapidly in the first min,and the removal of Ca2+ also occurred in the first minutes.In the continuous dosing mode of precipitant,the change of pH value and conductivity was.obviously staged:firstly,it increased rapidly,then decreased rapidly and gradually stabilized and finally rised slowly.The significant difference in the physicochemical properties of the solution system of the two modes was mainly due to the difference in supersaturation of the CaCO3 system.In a single dosing mode,the supersaturation of CaCo3 increased rapidly,while the continuous dosing increased gradually.Since the supersaturation of the system occured when the crystallization reaction occured in the two dosing modes,the particle size of the crystallized product also had a significant difference.Among them,the addition of the inducing agent kaolin in the continuous dosing mode can also reduced the supersaturation required for crystal nueleation and shortend the time required for nucleation,thereby significantly increasing the Ca2+ removal rate at the initial stage and the precipitation in the later stage.With the continuous addition of the agent,the removal rate is limited.When the total dosage of the precipitant was constant,increasing the dosage of the precipitant can increase the removal rate of Ca2+ and reduce the turbidity of the supernatant,but has no significant effect on the pH of the supernatant.Under the experimental conditions,after the crystallization reaction was completed,it was allowed to stand for 30 minuts,and the corresponding Ca2+ removal rates were 56.0%,63.5%and 65.9%,respectively,once,twice and three times.The supernatant turbidity was 200.1 NTU,112.35 NTU and 72.78 NTU,respectively.The optimized crystallization reaction scheme of fractional crystallization includes:two additions of a precipitant,a stirring intensity of 200 rpm,and a reaction time of"2 min+10 min".The SEM and XRD results of the crystalline product of CaCO3 indicated that the dosage mode of the precipitant affected the crystal form and crystal practice of the crystalline product.In the one-time dosing mode,due to the high degree of supersaturation of the system,the crystalline products are mainly calcite,and the continuous dosing mode is all vaterite.In the step-by-step mode,the first step is ball meteorite,and the second step is calcite,In the one-time dosing mode,since the nucleation rate is fast and the number density of crystal nuclei is high,the particle size of the crystal product is difficult to be further improved.In the continuous dosing mode,the growth rate of the crystal nucleus was also slow,and even secondary crystallization occurs.In the distributed feeding of precipitation mode,since the crystallization of each step occurs at a lower supersaturation,the number density of crystal nuclei was lower,and the particle size was correspondingly higher,above 15?m.