In 2001 Sasol investigated the possibility to convert a Sasol-Lurgi MK III fixed bed dry bottom coal gasifier at the former Sasolburg coal-to-liquids plant to a slagging gasification process and selected the Lurgi Multi Purpose Gasification (MPG) process for this purpose. At the time the MPG process was considered as a possible technology option suitable for the gasification of feedstocks which are difficult to manage, for example solid-liquid mixtures which are unsuitable for conventional solids or liquids processes and furthermore tars with a large variety of properties and high solids content have been gasified successfully. The most obvious differences between the feedstocks previously gasified, compared to the Sasol dusty tar, were found to be the viscosity and melting point of the dusty tar. The viscosity of the Sasol dusty tar mixture (as received) was higher than a factor of 10 of the previously used feedstocks. Another important feedstock property is the ash melting point of the feed within the gasifier. Ash particles fed with the tar melt in the high temperature zone of the flame. Molten ash particles, which hit the gasifier wall, will solidify and stick to the wall, if the wall temperature is below the melting point of the ash. The melting point of the dusty tar ash is 1380 degrees C and a fluxing agent has to be added to the dusty in order to reduce the melting temperature below 1250 degrees C to limit excessive wear of the refractory lining. Two approaches were evaluated in order to lower the viscosity, i.e. milling of the dusty tar and addition of low viscosity feedstocks to the dusty tar. It was concluded from this study that the viscosity of dusty tar can be decreased with the addition of specific waste solvent streams. The ash fusion temperatures of dusty tar can be lowered with the addition of a fluxing agent. The addition of spent Fe-catalyst as fluxing agent was also investigated, but was found to be not as effective as limestone. In some cases the addition of Fe can cause the acid/base ratio to change in such a way that the ash fusion temperature increases. In conclusion it can be highlighted that the ash flow temperature and viscosity of the Sasol dusty tar stream, and possibly other carbonaceous sources as well, can be decreased to acceptable values for slagging gasification applications. The results clearly show in both oxidizing and reducing atmospheres the Fe-catalyst was transformed into the slag melt as either Fe(2)O(3) under oxidizing conditions and FeO under reducing conditions. The slag showed no sign of metallic Fe and was very homogeneous under oxidizing and reducing conditions. Spent Fe-catalyst can be discarded in a safe and environmentally acceptable way during high temperature gasification. (c) 2009 Published by Elsevier Ltd.