Re-examining the pitch/coke wetting and penetration test

To produce structurally soundcarbon anodes for use in aluminum smelting, a strong bond between filler and binder coke is necessary. Bond strength results from mechanical interlocking and adhesion of the binder coke to the filler coke. Critical for creating such bonds is the ability of the pitch to wet the coke surface and penetrate the coke porosity during mixing and forming. Wettability is normally assessed from the pitch behavior during the initial stages of a penetration test. In the test, the observed contact angle between a pitch droplet and a bed of fine coke particles is recorded as the temperature is increased. The temperature at which this contact angle becomes 90° is referred to as the wetting temperature of the pitch. The penetration test may be useful to identify pitch and coke combinations that are unlikely to produce baked anodes of acceptable quality with standard paste preparation conditions. It does not, however, provide a measure of the true wettability of a coke by a pitch. The isothermal penetration experiments reported here demonstrate that the observed contact angle of a pitch against a coke bed changes continuously from >90° to <90°, even to 0‡, at a temperature much lower than the wetting temperature derived from the penetration test. The requirements for the measurement of a true contact angle and the difference between the concepts of adhesion and wetting are discussed.