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

Excessive hardness of drinking water affects people's daily life and physical health.The traditional chemical crystallization precipitation method has the advantages of low operating cost and simple operation,and is one of the commonly used hardness removal techniques.However,the chemical precipitation method has disadvantages such as high pH value of the effluent,unstable turbidity,and large amount of sludge.In order to solve the above problems,this paper attempts to remove the hardness by induced crystallization method.The removal efficiency of calcium hardness by induced crystallization technology was investigated,and the mechanism of induced crystallization to remove calcium hardness was discussed.The main findings are as follows:The conventional chemical precipitation method increased the Ca2+ removal rate with the addition of the precipitant Na2CO3,when the precipitant dosage is 1.0 time(compared with the theoretical value required for complete reaction,the same below),the Ca2+ removal rate was close to 100%.However,an increase in the amount of precipitant added resulted in a significant increase in the pH value and turbidity of the supernatant.The kaolin-induced crystallization transforms the homogeneous crystallization process into a heterogeneous crystallization process,which reduces the CaCO3 critical supersaturation S*required for the crystallization reaction,increases the crystallization reaction rate,and eventually significantly improves the Ca2+ removal efficiency.When the precipitant dosage was 0.4 times,the Ca2+ removal efficiency was even higher than the theoretical value(40%),which was 44.8%.The addition of kaolin had no significant effect on the pH value of the supernatant,but had a significant effect on the turbidity.The effect of kaolin-induced crystallization reaction conditions on the removal efficiency of calcium hardness is mainly concentrated in the turbidity of the supernatant.Self-induced crystallization with CaCO3 as an inducer can increase the pH value of the supermatant and the removal efficiency of Ca2+.When the dosage of Na2CO3 was 0.4?0.6 times,the removal rate of Ca2+ was higher than the theoretical value,and the pH value of effluent was lower than 8.5.The optimization of crystallization conditions shows that the smaller particle size and the lower dosage of the inducer were more conducive to the improvement of calcium hardness removal efficiency.Properly reducing the agitation intensity also reduced the turbidity of the supernatant.Preferred operating conditions under the experimental conditions included CaCO3 mesh number 11000 mesh,dosage of 0.01 g/L,and agitation intensity of 50 rpm.The crystal forms of kaolin-induced knot crystal products are mainly calcite and vaterite,and the self-induced crystal products are calcite.The reaction mechanism of the two is similar:due to the adsorption of the inducing agent,the supersaturation of CaCO3 in the water film at the solid-liquid interface is higher than that in the liquid phase,and the crystallization reaction is rapidly completed in the water film,thereby achieving the effect of reducing the critical supersaturation.The agitation speed has an effect on the kaolin-induced crystal product habits,it was spherical and hexahedral at 200 rpm;while irregular masses and agglomerates at 50 rpm.In the self-induced crystallization mode,the agglomeration of the crystal product is more obvious when the particle size of the inducer is reduced,which contributes to the decrease of the turbidity of the supernatant;appropriately reducing the rotation speed of the agitation is beneficial to suppress the secondary nucleation,which thereby achieved the effect of reducing the turbidity of the supernatant.