What is a bulk cylinder primarily used for?

A bulk cylinder is primarily used for transporting compressed gas. These cylinders are designed to safely hold gases at high pressures, which is crucial for transporting materials such as oxygen, propane, or natural gas. The construction and design of bulk cylinders ensure that they maintain the integrity and safety of the compressed gas during transit, preventing leaks and minimizing the risk of accidents.

Additionally, the other options refer to different types of handling materials or situations that do not align with the specific purpose of a bulk cylinder. For example, air transportation of hazardous cargo typically involves specialized containers and packaging, not a bulk cylinder. Storing solid materials does not relate to the function of a cylinder which is specifically designed for gases. Similarly, collecting liquid waste usually involves different containment systems, as liquids require different handling methods compared to gases.