Leaching of yttrium, europium and accompanying elements from phosphor coatings

The aim of this work was to study and describe leaching behaviour of Y, Eu and accompanying elements (Zn, Cd) from the phosphors of waste cathode ray tube (CRT) screens in low concentrated sulphuric acid solutions under both non-oxidizing and oxidizing conditions (H2O2). Thermodynamic study showed that non-oxidizing acid leaching leads to elemental sulphur and hydrogen generation followed by formation of H2S(g). In the presence of oxidant in the leaching system SO2(g) or H2SO4 formation is possible. PbSO4 was formed during leaching independently from the presence of the oxidant. Y, Eu and Zn dissolution without using H2O2 in 0.4 M H2SO4 at 80 °C attained 6, 9 and 8% respectively. The appropriate content of H2O2 in leaching solution was determined as 3.9 v/v %. The highest leaching efficiency of Y and Eu (around 90-95% after 20 min) was achieved by using the leaching solution (400 ml 0.4 M H2SO4, 60 ml 30 v/v % H2O2) at 80 °C and L/S ratio 23. Under these conditions Zn extraction was relatively low (20%) in comparison with Cd extraction (80%). The study also reveals that high leaching efficiency of both Y and Eu (around 90%) was achieved with even more diluted leaching solution (400 ml 0.05 M H2SO4, 60 ml of 30 v/v % H2O2) at 60 °C, L/S ratio 23 after 20 min of leaching. Zn and Cd extractions reached 20% and 55% respectively. Activation energy values calculated for Y and Eu leaching in the solution (400 ml 0.4 M H2SO4 with 60 ml of 30 v/v % H2O2) refer to diffusion mechanism in the range 40-80 °C (Ea = 14.85 kJ/mol and 16.47 kJ/mol respectively). In contrast, Ea values for Y and Eu leaching in the solution (400 ml 1 M H2SO4, 60 ml of 30 v/v % H2O2) suggest a mechanism change at the temperature around 40 °C. The leaching of Y as well as Eu up to 40 °C is controlled by diffusion and at higher temperatures by chemical reaction.