Mechanochemical Preparation Of Ca-Al Mineral Precursors And The Improved Adsorption Properties

It has recently been found that high-energy ball milling of starting samples results in the formation of a nearly amorphous phase recognized as a precursor,which crystallizes into corresponding mineral product by only agitating it inwater.During the crystallization,it is more interesting to note that theprecursor can incorporate much more specific compositions from the solution thanthe formed mineral products.Based on the newlydiscovered phenomena,in this research subject,basic researches will be systematicallyperformed to prepare mineral materials and application evaluations through precursor formation mainly by mechanochemical approach.Research work involves several minerals of hydrotalcite,ettringite.During the dry grinding for preparing precursor,the necessary conditions such as chemical bonding and crystalstructures of the used samples will be clarified and the adsorption capacity of the prepared precursor will be evaluated to develop a new and effective technique for the purification of polluted water and the recovery of valuable compositions.The current method of synthesizing mineral materials is hydrothermal one,which may have some problems such as the emission of liquid waste,high energy consumption,environmental pollution.As a new technology for synthesizing mineral materials,mechanochemical milling due to its advantages of high efficiency and low pollution,is getting more and more attention.The research content of this paper puts the focuses on the synthesis of ettringite precursor?hydrotalcite precursor and the barium saltes involved Ca-Al precursor,and subsequent characterizations of the adsorption of phosphate and chromate by the precursors.This paper mainly includes the following aspects:?1?Co-grinding of calcium hydroxide,aluminum hydroxide and gypsum wasperformed to prepare an activated precursor for synthesizing ettringite?simple formula as 3CaO·Al₂O₃·3CaSO₄·32H₂O?.Agitation of the precursor in aqueous solution with a target adsorbate even at room temperature allowed not only the synthesis of ettringite but also the adsorbate incorporation at nearly stoichiometric amounts accompanying the synthesis reaction.Potassium phosphate was used as target adsorbate to evaluate the possible incorporation with both sulfate and phosphate coexisting in ettringite structure,confirming that this novel idea by applying the reaction-accompanying incorporation of pollution components may allow removal efficiency much higher than that of the traditional adsorption operation.?2?Al?OH?₃ and Ca?OH?₂ powders are co-ground to prepare a precursor which hydrates into a layered double hydroxide?LDH?phase by agitation in aqueous solution with target hexavalent chromium?Cr?VI??at room temperature,to achieve an obvious improvement in the removal efficiency of Cr?VI?anion through an easy incorporation into the structure.The prepared precursor transforms into LDH phases also when agitated in the solutions of calcium and magnesium chlorides,it incorporates Cr?VI?preferentially to the chloride salts when they coexist.The adsorption isotherm and kinetic studies show that the phenomena occurring on the Al-Ca precursor fit a pseudo-second-order kinetics with a Langmuir adsorption capacity of 59.45 mg/g.?3?Based on the known ability of BaCO₃ to precipitate chromate anion,Al?OH?₃,Ca?OH?₂ and BaCO₃ powders were ground together to prepare a precursor to promote the capacity of BaCO₃ for the removal ofchromate anion.The adsorption product of Ca-Al-BaCO₃precursor at different molar ratio in chromate solution 200mg/l was?3CaO·Al₂O₃·CaCO₃·11H₂O?and BaCrO₄.In high concentration of chromate solution with a stoichiometric molar ratio,the formed phasesfrom 3:2:1 Ca-Al-BaCO₃precursorwere mainly chromate hydrotalcite?3CaO·Al₂O₃·CaCrO₄·14H₂O?and BaCrO₄,?3CaO·Al₂O₃·CaCO₃·11H₂O?and BaCrO₄ from 3:2:2Ca-Al-BaCO₃precursor,respectively.A new concept of applying precursor was first introduced.Dry grinding Ca and Al hydroxides with other specific compound produces fine powders which serve as precursor because it is possible to transform them into mineral products as ettringite and hydrotalcite by simple agitation in water or wastewater with target pollutants.It is more interesting to note the ability to incorporate the pollutants by the precursor is much higher than that of the final product.Much contributation is highly expected of the mineral material for purification of environmental pollution through the use of precursors.