Recovery Of Lanthanum And Cerium From Waste Rare Earth Polishing Powder By Hydrochloric Acid Leaching And Roasting With Sodium Hydroxide

With the wide application of rare earth polishing powder in glass polishing and other industries,the amount of waste and invalid rare earth polishing powder produced increases year by year.Rare earth is an indispensable and non-renewable resource,so it has become inevitable to recover rare earth from waste and invalid rare earth polishing powder.At present,the recovery of waste rare earth polishing powder is in the experimental stage,and the commercial production has not been realized.The main constraints include the low price of rare earth polishing powder and the high recovery cost.In order to solve this problem,a low acid reduction leach-alkali roasting process was adopted to recover waste rare earth polishing powder,and the leaching process conditions were studied.The roasting reaction mechanism of rare earth fluoride was determined,and the goal of obtaining high rare earth leaching rate and low residual acid concentration in rare earth mother liquor under low acid condition was achieved.In this paper,thermodynamic analysis was carried out in the process of low acid reduction leaching,and La-Ce-Al-Cl-H₂O E-p H diagram of different temperatures and acidity was drawn.It can be seen that Ce O₂does not react directly with hydrochloric acid to form Ce⁴⁺,but is first reduced to Ce³⁺by Cl-in hydrochloric acid,and then into aqueous solution.The mechanism of non-leaching of Ce O₂in low temperature and low acid environment and leaching in high temperature and high acid environment were revealed.Gibbs free calculation was carried out for the reaction of each substance in alkali roasting,TG-DSC analysis and phase structure analysis were carried out during roasting,and the phase transformation in the conversion process of rare earth fluoride alkali was revealed.The optimal conditions of hydrochloric acid leaching of waste rare earth polishing powder were as follows:in direct hydrochloric acid leaching,the concentration of hydrochloric acid was 6mol/L,the leaching temperature was 90℃,the ratio of liquid to solid was 10ml:1g,and the leaching time was 120min.In this case,the total leaching rate of rare earth reached 93.94%and the residual acid concentration was 3.42mol/L.When the reducing agent thiourea assisted hydrochloric acid leaching of waste rare earth polishing powder,the concentration of hydrochloric acid was 2.5mol/L,the dosage of thiourea was 2.0g,the leaching temperature was 90℃,and the liquid-solid ratio was 10ml:With 1g leaching time of40min and stirring speed of 100r/min,the total leaching rate of rare earth reached 94.37%and the residual acid concentration was 0.984mol/L.After the response surface optimization,the concentration of hydrochloric acid was 2.17mol/L,the reaction temperature was 93.7℃,the reaction time was 45min,the dosage of thiourea was 1.6g,the ratio of liquid to solid was10ml:1g,and the stirring speed was 100r/min.The total extraction rate of rare earth elements reached 94.11%,and the residual acid concentration of rare earth leaching solution was0.6598mol/L.The leaching kinetics of La and Ce were controlled by internal diffusion and chemical reaction,respectively.The apparent activation energies（Ea）were 57.95k J/mol and23.69k J/mol,respectively.The optimum conditions for rare-earth fluoride alkali conversion were as follows:roasting temperature 450℃,mass ratio（alkali ratio）of leaching slag to sodium hydroxide 1.5,roasting time 60min;The immersion temperature was 40℃,the liquid-solid ratio was 25ml:1g,and the immersion time was 15min.When the acid volume fraction of hydrochloric acid was 10%,the ratio of liquid to solid was 10ml:1g,and the acidification temperature was 40℃,the leaching rates of Al and Si were 12.88%and 3.54%.The recoveries of lanthanum and cerium fluoride were 87.8%and 98.6%,and the total rare earth recoveries were 88.04%.After acid leaching and alkali conversion,the final rare earth recovery reached 98.53%.