Study On Comprehensive Recovery And Utilization Of Rare Earths And Aluminum From FCC Catalyst Waste Slag

Fluid catalytic cracking catalysts(FCC catalyst)are widely used in the process of cracking heavy gas oil into gasoline blending compounds.In China,the annual FCC catalyst output is estimated to 0.23 million tons.For the each ton production of FCC catalyst,about 350 kg of FCC catalyst waste slag(FCC waste slag)containing rare earths(REEs)and aluminum(Al)is generated.And about 80 thousand tons of FCC waste slag is discharged each year.Furthermore,most of them were disposed by landfill,which leads to valuable metal resource waste.financial burden and serious environmental pollution.Therefore,it is urgent to develop an economic method for FCC waste slag treatment that achieves efficient recycling of valuable elements and FCC slag ultilization.Based on the characteristics of FCC waste slag composition and the property difference between Al and REEs.Fundamental studies of thermodynamic and kinetic.leaching characteristics of Al and REEs from FCC waste slag,control of silicon gels polymerization and selective precipitation process were conducted.And then a method including acid leaching and selective precipitation was proposed to achieve the recycle of REEs and Al from FCC waste slag and reduce the discharge of FCC waste slag.Listed below are the main results:(1)Fundamental study of acid leaching processThe main component of FCC waste slag was amorphous hydroxides.During production,a small amount of NaY zeolite,ZSM zeolite and FCC catalyst were brought into the FCC waste slag and the quantity depends on the characteristics of various batches of FCC waste slags.Eh-pH graph indicated that both of Al and REEs can be leached in the case of pH<3.0,which confirmed the possibility of extraction of REEs and Al form FCC waste slag via acid leaching.Leaching studies indicated that NaY zeolite can be completely decomposed by mineral acids and the leaching efficiencies of REEs and Al up to 99%were achieved.However,ZSM zeolite,kaolinite and FCC catalyst were stable in HCl acid.which illustrated that Al and REEs in these substances were difficult to leach.Based on this,the leaching characteristics of REEs and Al existed in different phases were discovered.The leaching kinetic investigations demonstrated that the leaching process of Al,La and Ce from FCC waste slag followed the shrinking core model and governed by the surface chemical reaction.The activation energies for the leaching process of Al,La and Ce were 6.24,9.71 and 9.10 kJ·mol-1,respectively.Furthermore,the leaching process was effected more significant by acid concentration than that of temperature in the range of 20?80 0C.The kinetic equations of Al,La and Ce were obtained as follows:1-(1-x)1/3=A/r0?C0.129exp(-6240/8.3147)t1-(1-x)1/3=A/r0?C0.320exp(-9710/8.3147)t1-(1-x)1/3=A/r0?C0.645exp(-9100/8.3147)tThese three metals would be leached simultaneously due to their similar and relative low value of the activation energies.Guided by the above theoretical findings,a leaching method with control of acid concentration was proposed to extracted REEs and Al form FCC waste slag.The leaching conditions were optimized and the results showed that more than 90%of REEs and Al can be leached under optimum conditions and the acid leaching slurry can be filtered smoothly.In addition,the SiO2 concentration in leaching residue was>85%,which can be used as a silicon source.(2)Study on control of silica gelation in acid leaching processSoluble silicates in the FCC waste slag were dissolved and then fromed silica gel during acid leaching process.which leads to difficult filtration of leaching slurry.It was confirmed by experiments that silicate polymerization rate curve was V-type with 0.5<pH<2.0 and the slowest polymerization rate appears at pH=1.0.Al3+,La3+and Ce3+promoted the polymerization of silica gel by reducing the thickness of the double layer.Among them.Al3+is more effective than La3+and Ce3+for promoting the polymerization of silica gel in terms of its greater charge density.XRD,IR and XPS analysis of silica gel containing metal ions proved that metal ions polymerized with silicate and entered the skeleton of silica gel caused the loss of valuable metals.Moreover,XRD patterns indicated that Al3+and low concentration RE3+promoted the crystallization of silica colloid significantly.Therefore,in order to ensure the smooth filtration of slurry and guarantee the high leaching efficiency,the acidity of leaching system should be controlled accurately.(3)Technical studies of valuable metal selective precipitation processIn the acid leachate,the concentration of Al was higher than REEs.That makes it difficult to separate REEs from the leaching solution via traditional solvent extraction.Based on different precipitation properties of REEs and Al,stepwise hydrolysis precipitation method and double salt selective precipitation process were designed and studied to separate REEs and Al form FCC waste slag acid leaching solution.In the case of stepwise hydrolysis precipitation method,pH of acid leachate was adjusted by NaOH solution to obtain Al(OH)3 and RE(OH)3 in succession.However,the operation window of this method was so narrow that it was difficult to apply practically and REEs loss during precipitation process was significant.For the double salt precipitation process,firstly,REEs can be precipitated in the form of double salt while A1 was still present in liquid by adding Na2SO4 to leaching solution.Subsequently,A1 in the solution was precipitated by adjusting the pH of leaching solution.The obtained REEs and Al products after purification and dissolution can be used for zeolite preparation,which simplified the recycle process and reduced the loss of valuable metals.(4)Semi-industrial experimentA semi-industrial experiment using Qilu FCC waste slag and Changling FCC waste slag was conduted to verify the universality and feasibility of the method.The results indicated that the recovery of REEs and Al from Qilu FCC waste slag were 87,22%and 80.50%,respectively.93.32%and 88.91%of REEs and Al were recycled from Changling FCC waste slag.Concentration(REO)of RECl3 solutions prepared by recovery products was>100g·L-1 and concentration(Al2O3)of Al2(SO4)3 solution was>90g·L-1.The quality of solutions have meet the requirements for zeolite preparation.This method has beens apprvoved with a good industrialization prospect.