sA novel method of producing a product of low intra-particle porosity (< 1 mum pore radius) from highly porous Latrobe Valley raw brown coals uses a combination of hydrothermal and evaporative drying. Low porosity coal was made in three different batch autoclave systems at 320 degreesC for residence times as low as 10 trim. Higher temperatures (up to 350 degreesC) increased porosity slightly but the water vapour pressure and the loss of organic material were significantly increased. Although the low and high porosity products differed dramatically in appearance and hardness, other chemical and spectroscopic properties were similar with the exception of pyrolysis-gas chromatography- mass spectrometry patterns. The relationship between intra-particle porosity and the maximum wt% dry solids concentration of aqueous slurries (for a viscosity of less than 1000 mPa s), rho(max) established by earlier workers for hydrothermally treated brown coals was found to hold for the new products and was extended to a wider range of porosities and a range of mean particle sizes (mps) (20-100 mum). A range of surfactants (anionic, cationic and neutral), which led to an increase of up to 7% in rho(max) for a bituminous, Blair Athol coal, increased rho(max) for products of hydrothermal or the new treatment by only 2-4%. This small increase resulted, however, in the formation of slurries of the low porosity products with rho(max)'s of up to 64%, similar to those obtained with high rank coals, and considered to be of commercial interest. (C) 2002 Elsevier Science Ltd. All rights reserved.