Executive summary
The contractor (Fumio Saito) has conducted his group to investigate his work on mechanochemistry of materials supported financially by IFPRI Inc. for six-years since 1998 to 2003. The first year’s work has been devoted to study the effect of dry grinding of a CaO-SiO2 mixture on synthesis of para-wollastonite by heating, followed by synthesis of tricalcium aluminate hydrate (3CaO·Al2O3·6H2O (C3AH6)) by dry grinding a mixture of calcium hydroxide and boehmite. He has also dealt with mechanochemical (MC) direct synthesis of CaTiO3 from a CaO-TiO2 mixture and its high-resolution transmission electron microscope (HR-TEM) observation. These were the work done in 1998, and, the main results are as below:
The contractor has attempted to synthesize CaTiO3 from a mixture of TiO2 and CaO. It has been known that there are three polymorphs in TiO2, i.e., anatase (TiO2, tetragonal), rutile (TiO2, tetragonal) and brookite (TiO2, orthorhombic). The use of anatase leads to MC synthesis of crystalline CaTiO3 from the mixture with CaO easier than from the CaO-rutile system. Grinding the CaO-anatase mixture for 2hours or more enables us to produce very fine particles of about 20nm in the first order mean size. The amount of CaTiO3 in the ground product increases with improving its crystallinity as the grinding progresses. Many CaTiO3 crystal grains of about 5nm are formed in the mixture ground for 2hours, and they grow up in the prolonged grinding. The grain size of the CaTiO3 crystals reaches about 20nm by about 5hours of grinding, and the grain boundary and lattice fringe become clear as the grinding progresses.
The second material is 3CaO·Al2O3·6H2O (C3AH6), which has been synthesized mechanochemically from the mixture composed of calcium hydroxide (Ca(OH)2) andpseudo-boehmite ( -AlO(OH)) powders by room temperature grinding using a planetary ball mill. Use of -AlO(OH) sample with inferior crystallinity is more favorable for the mechanochemical synthesis rather than that with well crystalline one. The time required to form C3AH6 from the Ca(OH)2 - -AlO(OH) mixture is much longer than that from the Ca(OH)2-gibbsite (Al(OH)3) one. Adsorbed water from air during grinding plays a significant role in the formation of C3AH6 from the former mixture. After water addition to the Ca(OH)2 - -AlO(OH) mixtures ground for various times, excess hydrated calcium aluminates such as C2AH8, C3AH8-12 and C2A0.5H6.5 are formed in the starting and the short time ground mixtures, while a few amount of these compounds is formed in these hydrated mixtures after prolonged grinding. Formation of these excess hydrated compounds, which belong to layered structural materials, is enhanced in the presence of free Ca, Al compounds and water. A mixture of CaO and silica-gel (SiO2) was subjected to grinding using a planetary ball mill, followed by heating to investigate the temperature for synthesizing para-wollastonite (CaO·SiO2). The MC treatment of the mixture brings about amorphous aggregates with almost homogeneous chemical composition. 2-hours MC treatment enables us to synthesize para-wollastonite by heating for 2-hours at 1273K, which is significantly lower by about 130K as usual. Heating the 5-hour ground mixture at 923K gives us to form a precursor of wollastonite, leading to its easy crystallization at higher temperature than about 1273K. Thus, it is found that the MC treatment for the mixture before heating is quite effective for synthesizing para-wollastonite.
The suggestion from the IFPRI members has come to the contractor for the second year’s work in 1999 on MC interaction between organic and inorganic materials. This is the initiation to start the solid-state reaction between polyvinyl chloride (PVC, [CH2CHCl]n) and CaO and/or Ca(OH)2 powders. The mixture was subjected to grinding using a planetary ball mill under different conditions, to investigate their MC reactions. The grinding causes dehydrochlorinating reaction, forming CaOHCl and [CH=CH]x. The reactivity against PVC of CaO is superior to that of Ca(OH)2, but all the same, the reaction yield is advanced as the grinding progresses. Furthermore, the yield and rate of the reaction are improved with an increase in the molar ratio of (CaO/PVC) as well as the rotational speed of the mill. Impact energy of balls would be also an important operational parameter governing the MC reaction.
In the third year (2000), the contractor has made the work for estimating the yield of MC processes by the use of ball mill simulation work based on the Discrete Element Method (DEM). This work enables us to find out the optimum condition of the MC process as well as the scale-up role of a MC reactor (a mill). The present study has been composed of three examples; 1) MC treatment of EP dust, forming soluble vanadium (V) compound in water, 2) MC treatment of fluorescent powder, accelerating its structure change, and 3) dechlorination of polymers with halogen by its MC treatment with inorganic material such as CaO. Regarding the first example, the yield of vanadium extracted by water leaching is well correlated with impact energy of balls in the mill calculated from the result obtained by. As for the second example, the dry grinding the EP dust enables us to form a water soluble vanadium compound. The well correlation between the V-yield and the impact energy of balls is obtained, suggesting that the ball impact energy plays a significant role to control the formation of vanadium compound. The third one is dechlorination of polymers such as PVC (poly-vinyl chloride), PVDF (poly-vinylidene fluoride) and PTFE (poly-tetra fluoro-ethane) by their MC treatment with inorganic material such as CaO. The report described only the dechlorination of PVC and its correlation with the impact energy of balls in the mill calculated from the result simulated. All the same, the impact energy of balls in a mill is a significant key to control MC effect and reaction. In such sense, the computer simulation regarding the ball motion during milling is a quite useful tool for determining the optimum operational parameters, mill design with scaling-up.