Exfoliation/delamination of kaolinite by low-temperature washing of kaolinite–urea intercalates

The effects of short milling (2 × 10 min) of two fractions of kaolinite (< 5 μm (S5) and 5–45 μm (S)), previously expanded by intercalation with 20 wt.% and 40 wt.% urea (U) and exfoliated/delaminated using low-temperature procedures, were studied. Untreated and treated kaolinite samples were examined by X-ray powder diffraction (XRPD), and their specific surface area (SSA), particle size distribution (PSD) and Fourier-transform infrared (FTIR) spectra were measured and interpreted. Samples intercalated with different concentrations of urea and subsequently exfoliated during thermal or low-temperature washing exhibit a variable intensity of the 001 diffractions of kaolinite (K) and interstratified intermediate kaolinite (H) with randomly interstratified layers of grafted urea and water. Low-temperature washing procedures yield H structures interstratified with layers grafted with urea and water with a mean d(001)= 8.168(50) A. After thermal washing procedures, H structures form with a mean d(001)= 8.322(60) A, apparently as a result of prevailing interstratifications of water over grafted layers of urea. The H/K peak intensity ratio for exfoliated/delaminated samples S and S5 previously intercalated with 20% U is significantly higher than for those intercalated with 40% U. After all washing procedures, fine fraction S5 increases its SSA from 20.2 m(2)/g to 25–35 m(2)/g, whereas the grain-size diameter remains approximately constant, with a mean MD = 2.80(20) μm. Exfoliated/delaminated coarse kaolinite of the S fraction not only increases its SSA from 9.4 m(2)/g to SSA between 12.5 m(2)/g and 20.5 m(2)/g, but also decreases its MD from 12 μm to between 8.73 μm and 5.60 μm due to the breaking of particles. This procedure is quicker than milling and thermal washing operations reported and used so far, and, moreover, it yields SSA values comparable to those obtained for exfoliated/delaminated kaolinite.