Microwave absorption properties of spent green phosphor and enhanced extraction of rare earths

The efficient extraction of rare earth elements from spent phosphor was achieved by microwave alkali fusion -leaching. Effect of temperature on the dielectric properties was studied. The addition of alkali greatly enhanced the microwave heating ability of the phosphor with loss tangent value increasing from 0.031 to 0.084 at 800 °C, which was ascribed to the strong susceptibility of molten sodium hydroxide and roasted products to microwave interaction. Effects of process parameters of roasting and acid leaching were investigated. When the phosphor was treated at 800 °C for 30 min with mass ratio of NaOH/phosphor of 2:1 followed by acid leaching of 4 mol/L HCl with the liquid-solid ratio of 10:1 at 60 ºC for 120 min, the leaching efficiencies of Ce and Tb were 97.86% and 95.75%, respectively. Microwave roasting showed the advantages of lower temperature, shorter time and higher leaching ratio of rare earths compared with conventional roasting. Enhanced thermal motion of sodium hydroxide, improved kinetic conditions and changes of crystal structures resulting from microwave selective heating may be responsible for microwave-enhanced rare earths extraction. Kinetic studies showed that both the alkali roasting and leaching process of rare earths were controlled by product layer diffusion.