The choice of pneumatic conveyor depends on the size, shape and density of the material. If attrition is not an issue, an inexpensive dilute phase conveyor would work for most materials. Dense phase conveyors would depend on the fluidization characteristics of the material. There are hundreds of manufactures and most have experience in both types. It sounds like you would want a vacuum conveyor for unloading of bags. Some companies are Delta/Ducon, Smoot, Dynamic Air, Cyclonaire, Hapman, Hardy Systems, Vac-U-Max, Young Industries and Claudius Peters (in the Americas). Please note that you need to specify the amount of time you can allow for the unloading, but you should be able to unload 300 kg in 5-10 minutes with most small dilute phase vacuum conveyors.