Many industrial processes involve the use of furnaces, reactor vessels, and the like, which operate at elevated pressures and to which particulate material involved in the process is fed. With some arrangements the particulate material is injected with a carrier gas under pressure. Also, slurry feeding methods are known in which a liquid-solid mixture is pumped into the high pressure vessel. With either arrangement the fluid (either gas or liquid) required to transport the particulate material may not be required in the process or, if required, the desired ratio of particulate material to fluid may be difficult to provide. Also, lock hoppers, or the like, are employed where relatively large quantities of particulate material are to be supplied. Batch, and not continuous, feeding of the particulate material is provided by such arrangements.