Leaching of Rare Earth Elements from the Residue Generated by the Lixiviation of Waste Phosphor with Sulphuric Acid

La, Ce and Tb were leached from a residue generated by lixiviating phosphor with 3.3 N H2SO4. Model equations to predict leaching extent of La, Ce and Tb were generated by factorial designing. At the same strength, HCl could leach more La, Ce and Tb than HNO3. Parameters considered for two-level full factorial design were HCl strength, temperature and pulp concentration. Dissolutions achieved at design conditions were entered into Minitab to formulate equation for leaching period of 0.5, 1, 1.5 and 2 h. Leaching achieved at random experimental conditions were within ± 5% of predicted values which established the validity of model equations. Conditions obtained by solving these equations for 99.9–100% leaching were provided in the form of surface plots. Ranges of HCl strength, temperature and pulp concentration for leaching more than 99% of La, Ce and Tb in 2 h were 46–63% v/v, 80–95 °C and 5–20% w/v, respectively. The equations were also utilized for generating data needed for kinetic studies which suggested that leaching was chemical reaction controlled.