Transformation Mechanism And Engineering Application Of Extracting Heavy Metals From Wastewater And Chromium Residue Based On Phase Change Control

Metal resources is exploited severely on the cost of leaving out a multitude of waste water and solids.At present,prevailing waste management targeted at risk minimization suffers from secondary pollution as well as resource dissipation,therefore,effectively recovering metal resource from waste water and solids is recognized as an ideal solution to the abovementioned dilemma.It is a crucial difficulty for separation and recovery that heavy metals interact intensely with nanoparticles contained in waste water treatment sludges and other solid wastes.One of the promising solutions is to suppress nanoparticles during waste water treatment and eliminate nanoparticle phases inside solid wastes.In terms of that,this paper dedicates to reduce sludge formation and recover metal resources through CO₂-assisted nano-Mg（OH）₂cycling phase transformation with differential solubility of its carbonated products,featured by its outstanding capture ability for metals and unique chemical property（phase transformation cycles as Mg（OH）₂-MgHCO₃-MgCO₃–MgO-Mg（OH）₂under carbonization）.Furthermore,to eliminate nanoparticle phase in solid wastes,chloride is brought in for selective separation and recovery,making use of its high electronegativity and reactive feature to access metals in solid wastes to form soluble and volatile salts.Based on the aforementioned theory,this paper takes electroplating waste water and chromium-contained hazardous waste as research models,to explore the new principles and new technologies underlying CO₂-assisted Mg（OH）₂cycling phase transformation for separation and recovery of heavy metals in wastewater,and of chlorination phase regulation of volatilization and separation of heavy metals in solid wastes.The specific research contents are as follows:（1）Study on the construction and enrichment rule of Mg（OH）₂nanomaterials with strong affinity to different types of heavy metals:Mg（OH）₂materials with the best enrichment effects for different types of cations and anions were constructed.The results indicate that flower-like Mg（OH）₂by precipitation has excellent removal ability for various cationic heavy metals（C₀=100 mg/L,C_e<0.07 mg/L）;the defect-rich Mg（OH）₂materials prepared by quenching method are highly effective for anions such as HAsO₄^2-and CrO₄^2-.After the reaction,crystalline and amorphous phases such as metal hydroxides and oxides are formed on the material with various reaction paths.This research provides basic theoretical support for strategy of heavy metal recovery in waste water through CO₂-assisted Mg（OH）₂phase transformation.（2）Research on cyclic enrichment and recovery of heavy metals in electroplating wastewater by Mg（OH）₂phase transformation:First and foremost,taking Pb-containing wastewater as an example,the process and separation mechanism of enrichment and recovery of Pb²⁺through Mg（OH）₂phase transformation under the assistance of CO₂were investigated.The results show that molar ratio of Mg:Pb above 2 is conducive to the thorough removal of Pb²⁺where leftover Pb²⁺is lower than 0.0205 mg/L;subsequent CO₂treatment converts unreacted Mg（OH）₂into Mg（HCO₃）₂,while Pb₃（CO₃）₂（OH）₂into nano-PbCO₃with 99.21%purity.More than 90%of magnesium salts are converted into nano-flaky MgO for recycling.This strategy possesses certain universal effect on metal recovery from electroplating wastewater containing Cu²⁺,Eu³⁺,Cr₂O₄^2-,Pb²⁺/Zn²⁺,and CrO₄^2-/Cu²⁺.（3）Selective recovery of hexavalent chromium from hazardous wastes through mineral phases regulation by chlorination roasting:This work systematically looks into the microstructure of typical chromium-containing hazardous wastes such as chromium ore processing residue,chromium-containing electroplating sludge,leather sludge,and stainless steel slag.Moreover,it determines the optimal chlorination formula and process parameters,and unravels the regulation method and reaction mechanism of phase transformation and chromium volatilization in chromium ore processing residue at low temperature.To be specific,the best chlorinated volatilization process is conducted at 350℃under N₂ atmosphere using NH₄Cl as a chlorinating agent,so that Cr⁶⁺is reduced to CrCl₃with more than 99%reduction rate and volatilization rate,which provides data support for development ton-scale equipment and engineering experiments.（4）Development of complete sets of equipment for chlorination roasting method and ton-scale engineering practice:Based on the above process,a cage stirrer and an electromagnetic heating chlorination reaction tank were developed according to special process requirements,and a complete set of chromium residue chlorination and volatilization equipment for separation was designed.Also,select a historically stockpiled chromium residue in a chemical industry region in Guangdong for pilot-scale experiment（the processing capacity is about 100 kg each time）.The test results illustrate that the extraction rate of Cr⁶⁺can reach more than 95%,and the total chromium content in the leaching solution of treated residue is less than 0.3 mg/L,which meets the disposal standard of non-hazardous industrial solid wastes.