Physicochemical Problems of Mineral Processing, 42 (2008), 107118 Fizykochemiczne Problemy Mineralurgii, 42 (2008), 107118 Eisa S. Mosa*, AbdelHady M. Saleh*, Taha A. Taha.*, Anas M. ElMolla**           Received February 10, 2008; reviewed; accepted July 31, 2008 In the present paper, the effect of chemical additives or reagents on rheological characteristics of coal water slurry (CWS) was investigated. The powerlaw model was applied to determine the non Newtonian properties of coal slurries. Three types of dispersants namely, sulphonic acid, sodium tri polyphosphate and sodium carbonate were studied and tested at different concentrations ranging from 0.5 to 1.5% by weight from total solids. Sodium salt of carboxymethyl cellulose (NaCMC) and xan than gum were tested as stabilizers at concentrations in the range of 0.05 to 0.25 % by weight from total solids. It was found that apparent viscosity and flow properties of CWS are sensitive to the use of chemical additives (dispersants and stabilizers). Among studied dispersing agents, sulphonic acid recorded the best performance in modification and reducing CWS viscosity. The best dosage of all tested dispersants was found to be as 0.75 % by wt of solids. With regard to studied stabilizers, Na CMC recorded better performance than xanthan gum. The best dosage of investigated stabilizers was found to be as 0.1 % by wt. from total solids. key words: coal slurry, viscosity, dispersants, carboxymethylcellulose, gum guar, sulphonic acid, tri polyphoshate INTRODUCTION Xiaoan et al.,(1994) stated that the surface nature of coal is very important in the preparation of high solid/liquid ratio coalwater suspensions (CWS). In this investiga tion, two kinds of additives were used. The first was a nonionic surfactant while the second was a mixture of two anionic polyelectrolytes. The first was a polyalkylene glycol ether derivative prepared in the laboratory and the second was a mixture of so dium humate and naphthalenesulphonic acid formaldehyde condensate. By proper _________ * Mining & Petroleum Eng. Dept., Faculty of Engineering, AlAzhar University, Cairo, Egypt ** Civil Eng. Dept., Faculty of Engineering, AlAzhar University, Cairo, Egypt