The Process Research On Resource Treatment Of Light Rare Earth Extraction Wastewater

With the successive introduction of environmental protection policies and the regulation of the rare earth processing industry,the requirements for emissions in the rare earth processing process have been increasing.In the extraction stage of the concentrated sulfuric acid roasting process of light rare earths,a large amount of Mg SO₄ is present in the extraction wastewater due to the addition of the saponification agent Mg O,and it is important to recycle the Mg resources in the high-salt wastewater and achieve"near-zero discharge"of the wastewater for the environment-friendly development of the rare earth smelting industry.In this study,chemical precipitation was used to treat the wastewater,and the p H range was controlled by adding Na OH and Na₂CO₃ to enable the precipitation of Mg²⁺and Ca²⁺.Meanwhile,Mg O was prepared by calcination of Mg（OH）₂ and characterized for different calcination durations to determine the optimal preparation conditions.The Na₂SO₄ present in the remaining Mg²⁺-Ca²⁺-free wastewater was recovered by concentration and freezing crystallization.In this paper,the performance of the two processes was analyzed and compared by concentrating Na₂SO₄through DCMD and ED,and the main research contents and results were as follows:（1）99.8% of Mg²⁺was precipitated as Mg（OH）₂ by chemical precipitation with 44.6 kg/m³Na OH to regulate the p H of the wastewater,and the remaining Ca²⁺in the water sample was removed by adding 2.1 kg/m³ Na₂CO₃.SEM,XRD,TG/DTA,BET and activity characterization results of Mg O showed that the Mg O prepared under 2 h calcination duration condition was of high quality with 98.5%purity and 20.1 kg/m³ yield.（2）The performance and membrane fouling of DCMD concentrating Mg²⁺-free and Mg²⁺-Ca²⁺-free wastewater were investigated.The flux during DCMD decreased from 16.1 kg/m²·h to 2.6 kg/m²·h due to the presence of Ca²⁺.The performance of DCMD was improved after the treatment with Na₂CO₃ injection.SEM,EDS and XRD results of the membrane surface showed that the decrease in DCMD performance was mainly caused by Ca SO₄·10H₂O.After freezing crystallization,Na₂SO₄ with 99.2%purity,49.7 kg/m³ yield and 43.3μm average particle size was obtained.The total energy consumption of chemical precipitation method and DCMD treatment of wastewater was 6343.1 k Wh/m³,and the water recovery rate was 72.6%.（3）Parameter optimization experiments were conducted for ECO and ED treatment of Mg²⁺-Ca²⁺-free wastewater.The COD removal rate,energy consumption and current efficiency of ECO during the degradation of organic matter at different p H and current densities were monitored,as well as the conductivity,salt ion concentration,water transport,and energy consumption of ED in the dilute and concentrate chambers during the concentration process at different voltages,volume ratios,and flow rates.Under the conditions of p H value of 3 and current density of 30 m A/cm²,ECO achieved more than 90%COD removal in 90 min with an energy consumption of 21.5 k Wh/m³;ED achieved a concentration factor of 1.8 for salt at an operating voltage of 9 V,a volume ratio of 1:1.5 for the dilute and concentrated chambers and a flow rate of 600 m L/min,and maintained a low water transfer rate of 23.7%,and an energy consumption of 70.7 k Wh/m³.After freezing crystallization,the yield of Na₂SO₄ was 45.7kg/m³.The total energy consumption for chemical precipitation,ECO and ED was 139.9k Wh/m³,and the water recovery was 40.3%.