Research On Recovery Technology Of Ca And Cu From Municipal Solid Waste Incineration Fly Ash

In recent years,the production of fly ash from municipal solid waste incineration has been rising.In addition to the landfill disposal mainly after solidification/stabilization,resource exploration such as collaborative disposal in cement kilns is also being carried out.Fly ash is rich in calcium compound and heavy metals,which has a certain recovery value.In this paper,the NH₄Cl was used as leaching agent,the optimal process parameters of calcium elution by salt were determined by single factor and response surface experiments,and the process conditions for recovering CaCO₃ from the leaching solution were studied by carbonization method.The PAN-PEI material was prepared,and the heavy metal Cu was recovered from the leaching solution by acid pickling desorption and alkali precipitation.The unit contents of Ca and Cu in the municipal solid waste incineration fly ash used in the experiment are 415.94 g/kg and 387.05 mg/kg respectively.The mineral phase of Ca is composed of Ca（OH）₂,CaCO₃,Ca SO₄,etc.The occurrence form of Cu is mainly residual state and reducible state.Firstly,the effects of reaction time,NH₄Cl concentration and liquid-solid ratio on decalcification efficiency were investigated by single factor experiments,and the optimal process conditions were further obtained by response surface experiments:NH₄Cl concentration was 3.9 mol/L,reaction time was 64min and the liquid-solid ratio was 5.6m L/g.Under these conditions,leaching rate of Ca²⁺was 58.9%,and the concentrations of Ca and Cu in the leaching solution were 61.85 g/L and 51.375 mg/L respectively.After decalcification,the mineral composition of the fly ash residue after decalcification is mainly CaCO₃,Ca SO₄,Si S₂,and the toxic leaching of heavy metals in the residue meets the relevant requirements of comprehensive utilization in article 6.3 of“Technical Specification for Pollution Control of Fly Ash Pollution from Domestic Waste Incineration（Trial）”（HJ1134-2020）.Through single factor experiment,the suitable conditions for preparing CaCO₃ in the CaCl₂-NH₃.H₂O-CO₂ system were determined as follows:CO₂ flow rate was 70min/L,NH₃H₂O addition amount was 8%,reaction termination time was 50min.Under these conditions,the conversion rate of Ca²⁺is 99.85%.If CaCO₃ is prepared directly from the leaching solution,the contents of Pb and Cd in the product do not meet the requirements of“Industrial Fine Precipitated Calcium Carbonate and Industrial Fine Precipitated Calcium Carbonate”（HG/T 2226-2010）due to the influence of CaCO₃ adsorption and the step-by-step precipitation of heavy metals.However,CaCO₃ is prepared after pretreatment with PAN-PEI.The content of heavy metals in the product meets the limit requirements of（HG/T 2226-2010）,and the microscopic morphology is spherical,and the crystal form is mainly vaterite.PAN-PEI was obtained by grafting PEI on PAN.Through single factor experiment,the suitable conditions for the adsorption of Cu on PAN-PEI material were determined as follows:the addition amount of PAN-PEI was 0.08 g,the adsorption time was 70 min,and the initial p H.Under these conditions,the adsorption rate of Cu was 99.37%,the adsorption behavior accords with Langmuir model and the first-order kinetic equation;PAN-PEI still maintains good adsorption after using 6 times.The Cu adsorbed by PAN-PEI was decomposed by HCl,and precipitated and recovery by Na OH solution.The recovery efficiency of Cu in leaching solution can reach 98.03%,so as to realize a large recovery of Cu in fly ash（about 72.86%）.