Bulk solids handling: bucket elevators are least suitable for vertically conveying which class of materials in process plants? Choose the most problematic feed type for filling, carry-up, and discharge.

Introduction / Context:
Bucket elevators lift bulk solids vertically using spaced buckets on a belt or chain. Suitability depends on flowability and the ability of the material to discharge by gravity from the bucket. Understanding the worst-case feed type prevents chronic plugging and hygiene problems.

Given Data / Assumptions:

• Typical elevator relies on gravity discharge at the head pulley.
• Materials range from free-flowing granules to cohesive pastes.
• Equipment design changes (bucket shape, speed) help, but cannot overcome severely sticky rheology.

Concept / Approach:
Sticky pastes adhere to bucket walls, carry back on the return run, and clog chutes, which disrupts capacity and causes maintenance and safety issues. Conversely, free-flowing materials fill and discharge cleanly with minimal carryback.

Step-by-Step Solution:
Evaluate the discharge mechanism: gravity-driven; needs non-cohesive flow.Assess each material: powders (manageable with dust control), small lumps (common), free-flowing granules (ideal), sticky pastes (problematic).Select the least suitable: sticky pastes.

Verification / Alternative check:
OEM catalogs and CEMA guidance flag sticky, smeary, or adhesive materials as poor candidates unless special continuous-bucket or scraper arrangements are used (often with reduced reliability).

Why Other Options Are Wrong:
Fine powders: can be elevated with venting, enclosures, and proper buckets.Small lumpy and free-flowing solids: routinely handled with standard elevator designs.

Common Pitfalls:
Assuming “very fine” automatically means “unsuitable.” Cohesiveness and stickiness matter more than particle size for bucket discharge behavior.

Final Answer:
Sticky pastes (e.g., clay paste)