Study On The Control Of Chlorate By-products In The Preparation Of Sodium Hypochlorite

Sodium hypochlorite disinfection instead of liquid chlorine disinfection,reduce safety risk,has become a global trend of drinking water disinfection.Sodium hypochlorite is a strong oxidant for bactericidal algae killing with broad spectrum,and its bactericidal efficiency is similar to that of chlorine.And easy to store,use,no residue,non-toxic to human body,avoid the risk of liquid chlorine high pressure storage,is a safe and ideal chemical disinfection method.But sodium hypochlorite disinfection will produce new disinfection by-products,harmful to human health,should be avoided as far as possible.There are seven valence states and corresponding compounds of chlorine,namely,Cl-ion with-1 valence,free C12 molecule with 0 valence,ClO-/HClO with+1 valence,ClO2 with+3 valence,ClO2 with+4 valence,ClO3 with+5 valence and ClO4 with+7 valence.Under certain conditions,the forms are transformed into each other by hydrolysis,oxidation and reduction.The main body of sodium hypochlorite disinfectant is ClO-/HClO,which can play a bactericidal role and is basically harmless to human body.There are also a small amount of ClO2-chlorite,ClO3-chlorite and ClO4-perchlorite that are harmful to human body,collectively referred to as sodium hypochlorite disinfection by-products,among which the proportion of chlorite by-products is the highest,which needs special attention.Sodium hypochlorite preparation methods include alkaline absorption method,electrolytic salt water method,will produce chlorate by-product.Therefore,this paper simulates the on-site preparation of sodium hypochlorite in waterworks,improves and optimizes the existing technical methods,and explores how to reduce the generation of chlorate in the preparation process.（1）The single-parameter static electrolysis experiment mainly studied the process of producing hypochlorite（ClO-）and chlorite（ClO3-）in the electrolysis of NaCl solution under different parameters.Under the condition of constant power supply voltage（voltage regulator）or current（current regulator）of 1%～5%salt water,the current density is controlled in the range of 10A/dm2～20A/dm2,and the experiment of preparing sodium hypochlorite by electrolytic salt is carried out.Effective chlorine（ClO-）and chlorate（ClO3-）concentrations were sampled at 25-30 min intervals until higher chlorate concentrations were produced.The experiment was repeated by changing brine concentration,voltage,current density,pH value,electrode plate spacing and electrolyte circulating flow rate.And calculate salt consumption and electricity consumption,get unit production cost,compare chlorate/residual chlorine ratio and unit production cost.It is concluded that the best electrolysis parameters are voltage 5.5V,salt water concentration 50g/L,current density 15A/dm2,temperature 20℃,PH=7,pole spacing 1cm,flow rate 0.2L/min,which has the best effect on the control of toxic by-product chlorate.（2）cascade variable condition electrolysis experiment electrolysis is mainly divided into variable voltage cascade electrolysis experiment and variable current cascade electrolysis experiment,which variable voltage cascade control mode is divided into buck cascade and boost cascade,series is divided into three cascade,four cascade and five cascade;The control mode of variable current cascade is divided into downflow cascade and upflow cascade,and the cascade series is divided into three cascade,four cascade and five cascade.The experiment of preparing sodium hypochlorite by cascade electrolysis is carried out.Effective chlorine（ClO-）and chlorate（ClO3-）concentrations were measured at 25-30min intervals until higher chlorate concentrations were produced.And calculate the salt consumption,power consumption,unit production cost,and compare the ratio of chlorate/residual chlorine and unit production cost,it is concluded that the cascade variable condition electrolysis method to choose five series of step-up voltage method（5.22V-5.5V-5.8V-6V-6.2V）,or（10A-12.5A-15A-17.5A-20A）five series of step-up flow method,It is beneficial to control the formation of by-product chlorate and save the operation cost of electrolysis.