The Study Of Selective Separation Of Heavy Metals From Typical Industrial Sludge Via Crystal Phase Control

Industrial hazardous wastes containing heavy metals have wide sources,large output and serious environmental hazards.Among them,the industrial sludge from industrial wastewater treatment is very difficult to treat due to its fine particles and high heavy metal activity,which is easy to slow release of heavy metals.If the selective separation of heavy metals in sludge can be realized,it is expected to achieve environmental protection and resource recovery simultaneously.Acid leaching is the most commonly used method to recover heavy metals.Although it is easy to operate and has strong applicability,the simultaneous dissolution of multi metals or the difficult release of valuabe metal result in poor selectivity or low extraction rate during acid leaching.Recent studies have shown that the crystal phase control of the key mineral phases can achieve the selective and efficient separation of heavy metals by changing their properties or adjusting the force of the coexisting minerals on the heavy metals.Based on our team’s previous understanding of the crystal phase control principle and significance of key mineral phases,this dissertation explores the separation behavior of heavy metals through the study of key mineral phases in complex industrial sludge by combining a variety of characterization methods and simulation calculation research methods.The main conclusions are as follows:1.Taking a variety of electroplating sludges as an example,the occurrence characteristics of heavy metals in industrial sludge and the law of separation of coexisting mineral phases in acid leaching were systematically explored,and their common characteristics were summarized.The results show that besides heavy metal elements,there are also a large number of impurity elements（such as Fe,Al,Ca,C,S,P,etc.）in industrial sludge.Many elements are mixed and coexisted in fine agglomerated particles,resulting in the simultaneous dissolution of valuable metals Cu,Ni,Zn and impurity elements during acid leaching without selectivity.The common phases in hazardous sludge are calcium salts（Ca SO₄·2H₂O,Ca SO₄·0.5H₂O and Ca CO₃）and a large number of amorphous phases.Sulfuric acid leaching can control the transformation of calcium salt into gypsum phase,thus significantly reducing the concentration of Ca²⁺in the leaching solution.Experimental follow-up analysis and simulation calculation show that gypsum,ferrite are formed during acid leaching,and anhydrous calcium sulfate,hematite,boehmite and other minerals are potential minerals.Regulating their crystal growth can reduce the concentration of impurity ions in the solution,which is conducive to the resource utilization of the extraction solution.This work expanded the understanding of the relationship between the separation behavior of heavy metals with impurities and mineral phase transformation during acid leaching of electroplating sludge,and provides a theoretical basis for the follow-up study on the cascade separation of heavy metals under the coordination of acid leaching schemes.2.For the problem that it is difficult to selectively separate mixed heavy metals in hazardous sludge,taking Cr-containing electroplating sludge as an example,a method for selective separation of Cr by differential crystallization control was developed.The results showed that the chromium leaching solution with purity up to 99.9%could be obtained by roasting-dilute acid leaching with Na OH as molten salt regulator.Both energy calculation and experimental results show that Gibbs free energy of Cr（III）oxidation by hydroxide in alkaline additives is the lowest,and most of Cr（III）can be oxidized to Cr（VI）and exists as soluble chromate.On the other hand,calcination regulates the conversion of amorphous Ni and Cu hydroxides into inert nano oxide crystals,which have lower acid dissolution kinetics.This work confirmed the feasibility of differential crystallization regulation in the selective separation of heavy metals,and provided a reference idea for solving the problem of metal co-dissolution in acid leaching.3.For the problem that it is difficult to separate the doped heavy metals in hazardous sludge,taking gypsum（the key mineral phase）,as an example,the efficient separation method of heavy metals by lattice reconstruction based on calcium sulfate phase transformation was studied.The heavy metal anions(Cr O₄^2-)and cations(Cd²⁺)doped in gypsum crystal structure are stable and difficult to extract and separate.Gypsum is dehydrated into hemihydrate or anhydrous calcium sulfate by low-temperature roasting（<400℃）,and then the phase transformation control is realized by spontaneous rehydration in solution.DFT results show that heavy metals tend to separate in the process of dehydration and rehydration.However,the dehydration regulation based on"solid-solid phase transition"can not release heavy metals because the main structure of calcium sulfate crystal is stable and the atoms are difficult to be fully adjusted;However,rehydration based on"dissolution-redeposition"mechanism realizes lattice reconstruction and heavy metals are completely released.It is found that large solubility product difference and acidic conditions can promote the hydration of calcium sulfate,and then promote the release of heavy metals.The calculation results of doping energy of various heavy metals in gypsum show a general energy barrier for redoping into the lattice during the recrystallization of gypsum.This work provides a universal method for high-efficiency separation of heavy metals by regulating the phase transition of calcium sulfate.