Synthesis of Ca Bearing Compounds by Mechanochemical Treatment Under Dry Atmospheric Conditions

Publication Reference: 
31-04
Author Last Name: 
Saito
Authors: 
F Saito
Report Type: 
ARR
Research Area: 
Wet Systems
Publication Year: 
1998
Publication Month: 
11
Country: 
Japan

A mixture of CaO and silica-gel (SiO,) was heated to investigate the temperature for synthesizing para-wollastonite (CaO * SiO,) after dry grinding the mixture using a planetary ball mill. Mechanochemical treatment of the mixture brings about amorphous aggregates with almost homogeneous chemical composition. Para-wollastonite can be synthesized from the 2hours ground mixture by heating at 1273K for 2 hours, while it is normally synthesized from the mixture by heating at constant temperature of around 1400K for about one week. Heating the Shour ground mixture at 923K enables us to form a precursor of wollastonite, leading to its easy crystallization at higher temperature than about 1273K. Thus, dry mechanochemical grinding for the mixture before heating is quite effective operation for thermal synthesis of para-wollastonite.

Tricalcium aluminum hydrate (3CaO a AIZO, * 6H20 : C&H,) is synthesized mechanochemically from mixtures composed of calcium hydroxide (Ca(OH),) and pseudo-boehmite ( y - A l O ( powders by room temperature grinding using a planetary ball mill. Use of the boehmite sample with inferior crystallinity is more favorable for the mechanochemical synthesis rather than that with well crystalline one. The time required to form C&E& from the Ca(OH),- y - A l O mixture is much longer than that from the Ca(OH),-gibbsite (AI(O one. Adsorbed water from air during grinding plays a significant role in the formation of C,AH, from the former mixture. After water addition to Ca(OH),- y - A l O mixtures ground for various times, excess hydrated calcium aluminates such as &AH,, C&H,.,, and C,A, gH6 .i 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.

Anatase (TiO,, tetragonal) is transformed into rutile (Ti02, tetragonal) via brookite (TiO,, ot-thorhombic) within lhour by room temperature grinding using a planetary ball mill. This polymorphic transformation of anatase leads to mechanochemical synthesis of crystalline CaTiO, from the mixture with CaO easier than from the CaO-t-utile system. Grinding the CaO-anatase mixture for 2 hours or more enables us to produce very fine particles of about 20nm in the first order mean size. The amount of CaTiO, in the ground product increases with improving its crystallinity as the i grinding progresses. Many CaTiO, crystal grains of about km are formed in the mixture ground for Zhours, and they grow up in the prolonged grinding. The grain size of the CaTiO, crystals reaches about 20nm by about Shours of grinding, and the grain boundary and lattice fringe become clear as the grinding progresses.