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To recover potassium from feldspar, a biowaste, i.e., eggshell, was used. The chief composition of eggshells is calcite. As it is a rich source of Ca, hence it is used with HCl to produce calcium chloride. Feldspar is an aluminosilicate mineral that bears potassium in the interstitial sites. To unlock the potassium from the interstitial sites, it was roasted with calcium chloride prepared by mixing eggshell and hydrochloric acid. At the roasting temperature, CaCl2 melts and penetrates into the aluminosilicate matrix to replace K with Ca. Potassium ion released from the silicate matrix combines with chloride ions to form potassium chloride, which solubilized in water during the leaching process of the roasted feldspar. For elucidation of the mechanism of the roasting process, the shrinking core model was applied to the roast–leach data, and diffusion through the product layer was inferred as the rate-determining step. The order of the roasting process was found to be 2.158 and activation energy calculated to be 155.3 kJ/mol. Apart from potassium, sodium and excess calcium also got co-leached. To recover potassium from the leach liquor selectively, sodium perchlorate was added to precipitate potassium as KClO4. Further, potassium perchlorate was thermally decomposed to give fertilizer grade potassium chloride (purity: 99.81%).

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Factorial design has been implemented to develop regression equations for leaching of Y and Eu from phosphor coating of spent fluorescent lamps (FLs). On the basis of leaching extent, H 2 SO 4 was the most suitable lixiviant. Parameter chosen for 2 3 full factorial design were temperature, H 2 SO 4 concentration and pulp density. Leaching extent obtained at design conditions were fed to Minitab and regression equations for time intervals namely 1, 2, 3 and 4 h were generated. Experiments were carried out to validate equations and results were within ±5% w/w of calculated values. These equations were solved for 99.9-100% leaching and obtained conditions have been represented as response surfaces. More than 99% of Y and Eu can be leached with H 2 SO 4 of concentration 6-9.3% v/v at temperature 70-95°C and pulp density 5-20% w/v in 3 h.Leaching extents required for kinetic studies were also determined from regression equations.