Thermal properties of sodium-exchanged palabora vermiculite

Abstract

Palabora represents an interstratified vermiculite-biotite containing less than 50% vermiculite. The sodium-exchanged form was prepared by immersion in saturated brine at ambient conditions. The sodium content reached the equilibrium level within the first month of exposure. However, XRD spectra continued to evolve and sharpen over a six month period. The thermal-activated exfoliation was studied using thermo-mechanical analysis. The expansion of the neat "vermiculite" commenced at temperatures above 420 C. In some applications a lower exfoliation temperature is desirable, i.e. in the range 200 to 350°C. This study revealed that simple sodium exchange is effective towards this goal as it lowered the onset temperature by about 120 C. The linear expansion reached a maximum of about eight-fold at temperatures around 700 C and decreased again at higher temperatures.

Characterization of urea-modified Palabora vermiculite

“…Selected metal-urea complexes were prepared in a closed 500 mL vessel under constant agitation (200 rpm in an orbital shaker) for 2 h. The metal chloride to urea molar ratio was set at 1:6. vermiculite was added and the ion exchange reaction (between the cationic complex and the interlayer cation) was allowed to proceed for 15 days, based on the slow rate of exchange previously observed (Inoue, 1984;Kodama and Komarncni, 1999;Muiambo et al, 2010;Skipper et al, 1991). The complexes, in the reaction mixture, amounted to 150% CEC.…”

“…Thermomechanical analysis of neat and sodium-exchanged vermiculite revealed that the expansion behaviour varied considerably from flake to flake (Muiambo et al, 2010). Hillier et al (2013) recently offered a plausible and convincing mechanism for the thermal exfoliation of vermiculite.…”

“…The degree of expansion depends on the flake size, the nature of interlayer cations, the presence or absence of interstratified phases, the heating rate and the terminal temperature (Huo et al, 2012;Justo et al, 1993;Marcos et al, 2009;Muiambo et al, 2010). For some 4 applications, e.g.…”

“…For some 4 applications, e.g. intumescent fire barriers, lower exfoliation onset temperatures are desirable (Baumeister and Hahn, 1976;Langer and Marlor, 1981;Muiambo et al, 2010;Wada, 1973a, b).…”

Ion Exchanged Vermiculites with Lower Expansion Onset Temperatures

“…In intumescent fire barriers applications it is desirable that the vermiculite exfoliates at relatively lower temperatures [11][12][13][14][15].…”