The Inhibition of Fluoride Elution from Industrial Wastes with Portland Cement, Calcium and Magnesium Salts in Alkaline Region

Abstract

Recycling of fluoride industrial wastes is difficult to dispose, since the fluoride ions eluted again easily. In this study, newly effective inhibition method in both neutral and alkaline region for fluoride elution was investigated. Fluoride elution concentration from CaF2 was 9.8 mg/L at pH 8.1 and 288.5 mg/L at pH 12.2 with water/solid ratio of 10. Additions of Ca(OH)2, CaCl2 and NH4H2PO4 could restrain fluoride elution concentration of CaF2 to 0.22 mg/L in the neutral region. This inhibition of fluoride elution was due to a generation of hydroxyapatite (HAp) or chlorapatite (ClAp) which has high ion exchange capacity. Additions of Portland cement, Ca(OH)2 and MgCl2 could restrain fluoride elution concentration of CaF2 to 0.47 mg/L in alkaline region. It was only 0.16% of 288.5mg/L which was the fluoride elution concentration from CaF2 at pH 12.2. The elution of fluoride ions was restrained by the coprecipitating of CaF2 with the high content of Ca 2+ provided from Ca(OH)2 and cement hydrates. The carbonation of Ca 2+ was prevented by the addition of Mg. Moreover, the forming of ettringite with a high ion-exchange capacity and the solidification effect of Ca-bearing hydrates contributed to the inhibition of the elution of fluoride. These additives for the practical industrial wastes, such as paper sludge and coal ash, with high concentration of fluoride were conducted. The result indicates that the fluoride elution could be restrained to meet the environmental standard (0.8 mg/L) in Japan in alkaline region. This inhibition method for fluoride elution would be able to contribute to promotion of recycling of fluoride industrial wastes.