The process of agglomeration is quantitatively described by the use of two factors: (i) the hydrophobic compatibility between the solid and the liquid, and (ii) an agglomeration coefficient. If in an agglomeration system, the gap between the hydrophobicities of the agglomerants (i.e. the solid and the liquid) exceeds certain tolerances, it is difficult for the agglomeration process to proceed properly, especially to produce appropriately sized, dense, agglomerates. By the use of certain organic surfactants, it is possible to adjust agglomerant compatibilities and make the agglomeration system work well. This promotion action of surfactants is explained in terms of the interfacial interactions and the immersion energy of the system. In addition, the agglomeration efficiency of different agglomerants can be evaluated by the 'agglomeration coefficient', which is measured by the capillary rise method and reflects the physical properties of the liquid and the mutual interactions between the solids and the liquid. (c) 2004 Elsevier Ltd. All rights reserved.