Study On Treatment Of Low-concentration Heavy Metal Wastewater With Activated Calcium Carbonate By Wet Stirred Ball Milling Process

With the rapid development of industry,the discharge of heavy metal wastewater is also increasing day by day.Unreasonable treatment and disposal of heavy metal wastewater will not only result in secondary pollution of environment but also waste of resources and energy.Therefore,how to reduce the concentration of heavy metal ions in industrial wastewater,achieving the direct discharge of purified water and effective separation,recycling and re-useing of various heavy metals at the same time has become very important to the dayly life of the people and the sustainable development of the society.Considering the problems such as large amount of reagents,post-neutralization treatment,difficulty in separation and recovery of precipitated metals,and poor treatment of low-concentration heavy metal wastewaters,with the conventional chemical alkali precipitation method,this paper deals with the treatment of heavy metal wastewater containing Pb²⁺,Cu²⁺,Ni²⁺with CaCO₃ activated by wet stirred ball milling process.During the wet stirred ball milling process,with actions such as collision,extrusion and shearing of the ball milling medium,the particle size of CaCO₃ decreased,the surface wettability increased,the solubility and the chemical activity increased.The effects of calcium carbonate dosage,ball milling time,ball to material ratio,the amount of other aditive,initial concentration and other conditions of wet stirred ball milling process were systematically studied to activate CaCO₃ for the treatment of Pb²⁺,Cu²⁺,Ni²⁺solution.A set of analytical methods of XRD,TG-DSC,FTIR and etc.were used to analyze the heavy metal precipitates to understand the mechanism for Pb²⁺,Cu²⁺,Ni²⁺precipitation from the solutions.In case of Pb,at moral ratio of CaCO₃/Pb²⁺of 1,milling time of 120 min,ball material ratio of 10%,and rotation speed of 200 rpm,PbCO₃ was formed in the precipitate with the removal rate up to 99.82%at the initial concentration of 0.5 mM.The final solution pH of 6.51 and the remaining Pb²⁺concentration of 0.13mg/L after the treatment would allow a direct discharge of the treated wastewater.In case of Cu,at the moral ratio of CaCO₃/Cu²⁺of 1,milling time of 90 min,ball to material ratio of10%,and rotation speed of 200 rpm,Cu₄（SO₄）（OH）₆·2H₂O was formed in the precipitate with the removal rate up to 99.62%at the initial concentration at 1 mM.The final solution pH at 8.25 and the remaining Cu²⁺concentration of dissolved of 0.35mg/L after the treatment met the national emission standards.In case of Ni,at moral ratio of CaCO₃/Al³⁺/Ni²⁺of 1.5:1:1,milling time of 90 min,ball material ratio of 10%,and rotation speed of 200 rpm,Ni₁₄Al₉（SO₄,CO₃）₆（OH）₄₃·7H₂O was formed in the precipitate with the removal rate up to 99.79%at the initial concentration of 1 mM.The final solution pH of 7.94 and the remaining Ni²⁺concentration of 0.12mg/L after the treatment could allow the direct discharge.In summary,activation of CaCO₃ by wet stirred ball milling to treat heavy metal not only realized the goal of replacing Ca（OH）₂ used in the traditional alkali precipitation,but also achieved the chemical reaction of CaCO₃ with heavy metal ions at nearly molar equivalent to give different chemical compounds in the precipitates,depending on the type of the heavy metal,suggesting possible separation among the metals.At the same time,this process provided new ideas and methods for the sequential separation and recovery of valuable metals in multi-metal mixed solution,as well as a new way for the development and utilization of calcium carbonate.