Abstract
Effective boron removal from coal fly ash containing various elements was studied. Dilute HCl is used to leach Al, Ca, and Mg in addition to boron from coal fly ash. The dissolved boron forms borate-ion pair, CaH2BO3+ and B (OH) 4-, under high pH conditions in the presence of Ca. The positive borate-ion pair CaH2BO3+may form coordinative bonds with Al (OH) 4- and finally be removed by coprecipitation. The negative borate-ion pair B (OH) 4- may form a complex with the diol group of polysaccharides. In this study, the effects of pH and the addition of Al on boron removal from the acid leachate of coal fly ash were investigated. The pH of the leachate was adjusted to 12, 11, 10.5 and 10. Then AlCl3aq was added to give Al/B molar ratios of 1, 10 and 50. After CaH2BO3+ was coprecipitated with Al (OH) 4-, the dissolved B (OH) 4- was adsorbed at pH12 using glucomannan semigel. The highest removal ratio of CaH2BO3+ was obtained under the condition of pH10 and Al/B molar ratio of 50, and that of B (OH) 4- at pH12. About 78% of the boron leached from the coal fly ash was finally removed by the combination of coprecipitation and adsorption.
