Production of high-purity quartz for optical applications

The paper investigated the production of high-grade quartz for optical devices, starting with mined quartz. This material contained 668 mg/kg of Fe2O3, and the aim was to reduce the content to below 100 mg/kg through an environmentally and economically sustainable process. It was found that the sequential leaching water-oxalic-sulphuric acid is very efficient using two sequential stages; both acids can be used two times. The best conditions were: water leaching S/L 100%, temperature 90°C, time 45 min; oxalic acid stage: OA concentration 17 g/L, leaching time 2.5 h, temperature 90°C, S/L 20% wt/vol; sulphuric acid stage: SA concentration 2 mol/L, leaching time 1.5 h, S/L 20% wt/vol. OA can be recovered from the spent solution and recycled. SA recycling was tested using an NF membrane module: the rejection of iron sulfate and silica (silicic acid) was greater than 85%. The permeate can then be used for a further leaching stage. NF removes most of the significant elements, which avoids further iron leaching from quartz. The final high-grade product contained less than 100 ppm of Fe2O3, with a Whiteness Index (D65/10°) greater than 88, the minimum threshold required for optical applications.