The boundary layer drum mixer is an apparatus which can pass through the narrow top port of a standard chemical drum or bulk container without the need to disassemble any part of the mixer or the vessel. The invention is an enclosed system with an internal arrangement of impellers and stators which draw liquid-based media from the adjacent vessel and strategically distributes it throughout the vessel through an array of outlet ports. The strategic mixing requires low energy input, minimizes the risk of media aeration and works with a wide range of fluid levels. The apparatus imparts no net external torque, and therefore, requires no rigid mounting. The upper impeller flow positively entrains particles or liquid phases that tend to float. The lower impeller flow positively entrains particles or liquid phases that tend to sink and/or remain static in boundary layers against or near the vessel walls and base.

Embodiments of the present disclosure generally relate to apparatus for solvent extraction of coal and to processes for solvent extraction of coal. In an embodiment, an apparatus for solvent extraction of coal is provided. The apparatus includes a plurality of stationary members coupled to a stationary shaft, each stationary member having a stationary member opening. The apparatus further includes a rotatable shaft extending through the plurality of stationary members. The apparatus further includes a plurality of rotatable members coupled to the rotatable shaft, each rotatable member paired with a stationary member, and each rotatable member has a rotatable member opening configured to rotate relative to the stationary member opening. The apparatus further includes a plurality of members for agitating materials passing through the apparatus, the members for agitating materials coupled to the rotatable shaft, each member for agitating materials positioned between the paired stationary members and rotatable members.