Recovery of rare earth elements from CRT phosphors. Part II: Selection of optimal recovery conditions by using factorial design of experiments to develop empirical models as the main tool of a response surface methodology approach

Cathode-ray tube (CRT) phosphors contain significant amounts of critical rare earth elements (REEs). As CRT screens fall into disuse, REEs accumulate in waste deposits, waiting for efficient recovery strategies. This study revisits a hydrometallurgical route to recover REE oxides from CRT phosphors, aiming to improve process efficiency. The method involves sieving (covered in a companion paper), leaching, precipitation, and calcination, using low-cost reagents and minimal energy consumption. The experimental design followed factor selection, experimental runs, empirical model validation and response surfaces generation. Key variables (solid/liquid ratio, temperature and acid type) were tested to optimise leaching recovery of yttrium and europium, while minimising zinc dissolution. Main, interaction and quadratic effects were evaluated. A special factorial design of experiments enabled empirical modelling of REE recovery. Lower temperatures improved REE selectivity by reducing Zn recovery, while higher SL ratios slightly increased overall metal recovery. Solid/liquid ratio increased REE recovery. Acid type had no significant effect; nitric acid was chosen for compliance with the CHON principle. Oxalic acid precipitation achieved full REE recovery within 15 min without pH adjustment. Zn co-precipitation is favoured at higher pH. Final REE recoveries were 68% for Y and 56% for Eu, with 77.9% purity in the product. The process, conducted mostly at room temperature, enhances energy efficiency, safety, and cost-effectiveness. This work complements a companion study [1] to define optimal conditions for REE recovery from phosphor waste.