Bulk solids handling — selecting a conveyor:\nFor transporting a pasty or sticky material over short-to-moderate distances in a plant, which mechanical conveyor is most suitable?

Introduction / Context:
Pasty, sticky, or semi-solid materials pose challenges for bulk handling because they may adhere to belts, leak from buckets, or bridge in hoppers. Process engineers therefore choose conveying equipment that positively moves the material with an enclosed trough and rotating element that wipes and advances the mass. This question checks your ability to match material behavior with the most appropriate conveyor type used in chemical and process plants.

Given Data / Assumptions:

• Material is pasty or sticky (non-free-flowing).
• Distance is plant-scale (not long overland), with moderate throughput.
• Cleanability and containment are desirable to avoid spills.

Concept / Approach:
Screw conveyors employ a helical flight on a rotating shaft inside a U-trough or tube. Rotation drags and pushes cohesive materials forward while the flights continuously scrape the trough surface, limiting build-up. Enclosure reduces spillage and odor, which is critical for sludges, filter cakes, and dough-like feeds. Belt conveyors need free-flowing solids to avoid carryback; buckets suit granular or lumpy solids, not pastes; apron conveyors target hot, coarse, and abrasive lumps rather than sticky pastes.

Step-by-Step Solution:

Verification / Alternative check:
Practical references list screw conveyors for sludge, wet filter cake, and dough-like feeds; belts and bucket elevators are reserved for granular, free-flowing materials.

Why Other Options Are Wrong:

• Apron conveyor: suited to hot, coarse, abrasive lumps; inefficient for pastes.
• Belt conveyor: suffers from carryback and spillage with sticky pastes.
• Bucket elevator: meant for vertical lift of free-flowing particles, not cohesive pastes.
• Pneumatic pipeline: requires discrete particles, not cohesive pastes that can plug lines.

Common Pitfalls:
Assuming any enclosed conveyor works; without a rotating flight to shear and move the paste, build-up and blockages occur.

Final Answer:
Screw conveyor