Fluoride and silicon removal from spent glass etching solution by chemical treatment

Abstract

The effects of chemical treatment for spent silicon etching solution using a combination of two precipitants, K/NaOH, and calcium salts, have been investigated. The impact of the precipitant dose and the initial and final pH value on fluoride and fluorosilicate removal efficiency has also been evaluated. Fluoride and silicon were removed simultaneously in the form of insoluble (K/Na)2SiF6 using the precipitant K/NaOH at pH 5, while the residual concentration of soluble fluoride was in the range of 0.013 to 0.016 M. According to the XRD and wet-chemical analysis the purity of the K2SiF6 precipitate was 98%. The residual fluoride concentration in the solution was additionally reduced in the form of insoluble CaF2 using the precipitant Ca(NO3)2 with [Ca]:[F] = 1:1 at pH 6.7. The basic ingredients in the precipitate were CaF2 and Ca(NO3)2, while F and Si concentrations in the supernatant were undetectable. The combination of K/NaOH and Ca(NO3)2 makes it the best precipitating agent for fluoride and silicon removal from the spent chemical etching solution and production of precipitates suitable for industrial application.