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 CaF 2 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 , CaCl 2 and NH 4 H 2 PO 4 could restrain fluoride elution concentration of CaF 2 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 MgCl 2 could restrain fluoride elution concentration of CaF 2 to 0.47 mg/L in alkaline region. It was only 0.16% of 288.5mg/L which was the fluoride elution concentration from CaF 2 at pH 12.2. The elution of fluoride ions was restrained by the coprecipitating of CaF 2 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 2+ . 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.

A Suppression Method for Elution of F−, [B(OH)4]−, AsO43−, and CrO42− from Industrial Wastes Using Some Inhibitors and Crushed Stone Powder

“…In addition, ettringite [Ca 6 Al 2 (SO 4 ) 3 (OH) 12 ·26H 2 O], which is formed during the hydration of ordinary Portland cement (OPC), showed a high removal preference for F [9][10][11], B [11][12][13][14][15], As [16,17], and Cr [12,[18][19][20][21] due to its ion exchange capacity by replacing SO 4 2− of ettringite. In our precious study, F concentration eluted from CaF 2 pure reagent could be reduced from 288.5 mg/L to 0.47 mg/L in alkaline regions with the addition of Ca(OH) 2 , MgCl 2 , and OPC [22]. The coprecipitation effect of Ca 2+ , ion exchange effect of ettringite, and solidification effect of Ca-bearing hydrates contribute to the decrease in the elution concentration of F. This has been successfully applied for the inhibition of F eluted from paper sludge and coal ash.…”

“…Portlandite (Ca(OH) 2 [9] due to the high content of Ca 2+ , which originates from cement hydrates. Our previous studies have shown that the solubility of CaF 2 increases with an increase in pH in the alkaline range due to the competition between F − and OH − [22]. However, ettringite with a high ion exchange capacity was formed during the hydration of OPC, and therefore, the elution of F was inhibited due to the replacement of the exchangeable anion, such as SO 4 2− in ettringite by F. The coprecipitation effect of Ca 2+ , ion exchange effect of ettringite, and solidification effect of Ca-bearing hydrates contribute to the inhibition of the elution of F. Additionally, under the conditions of the same added amount of OPC, the elution concentrations of F were lower than those without additions of CSP.…”