| The current mainstream zero-discharge route for desulfurization wastewater is:pretreatment softening—concentration reduction—solidification.The pretreatment softening process is the basis for zero-discharge technology of desulfurization wastewater.Its function is to soften wastewater and to ensure the safe operation of subsequent equipment.The pretreatment softening process of desulfurization wastewater encounters the following problems:Some coal-fired power plant desulfurization wastewatercontains abundant magnesium ions,thus a large amount of sodium hydroxide needs to be added during the softening process to control p H to precipitate magnesium ions,and the desulfurization wastewater may not be softened completely,leading to hightconcentration of calcium and magnesium ions in the effluent;The dosage of the drug is increased,which results in high cost of dosing treatment of desulfurization wastewater.Large amount of sludgecould be generate,which contains high concertration of magnesium hydroxide and calcium carbonate.In view of the above problems,this paper proposes a softening method that involving calcium and magnesium separation,stepwise softening,and magnesium hydroxide recovery.In this study,theexperiments were carried out on the desulfurization wastewater of a coal-fired power plant in Baoding City,including（1）Quicklime-sodium carbonate softening wastewater experiment,in which the effect of sodium carbonate dosage on desulfurization wastewater softening was investigated;（2）Hydrogen Sodium oxide-sodium carbonate softening wastewater experiment:explored the effect of sodium carbonate dosage and p H on desulfurization wastewater softening;（3）Sodium hydroxide-sodium oxalate softening wastewater experiment:studied the impact of sodium oxalate dosage and p H on desulfurization wastewater softening;（4）Sodium carbonate-sodium oxalate compound softening wastewater experiment:investigated the effect of sodium carbonate,sodium oxalate dosage and p H on desulfurization wastewater softening;（5）Magnesium ion recovery experiment in desulfurization wastewater:analyzed the effect of p H to magnesium ion recovery in desulfurization wastewater;（6）The experiment of preparing magnesium hydroxide by hydrothermal method:explored the effects of reaction temperature,sodium hydroxide concentration and reaction time on the purity and micro-morphology of magnesium hydroxide samples.The experimental results show that when the dosage of the agent is 1.2 times Na2CO3and 0.2 times Na2C2O4,and the reaction p H is controlled to 8.7,the calcium and magnesium ion separation performance is the optimum,and one-step softening can be achieved.In the magnesium ion recovery experiment,it is necessary to control the p H of reaction environment to 11.5,the total recovery rate of magnesium ions is93.5%,and the purity of the prepared magnesium hydroxide sample is above 97.2%.This technology could solve the difficulties in softening process of high-magnesium content desulfurization wastewater,and lay the foundation for zero-discharge of wastewater. |
