Coal washing wastewater treatment: Coal washeries discharge wastewater with about 3% suspended solids (clay, slate, stone). Which primary unit is most appropriate for removing these solids at scale?

Introduction / Context:
Coal washing produces large volumes of slurry containing dense mineral particles and clays. Designing cost-effective primary treatment requires distinguishing bulk solids separation (settling/thickening) from polishing or dewatering steps.

Given Data / Assumptions:

• Suspended solids ≈ 3% by mass, dominated by mineral fines.
• High flow rates common in washeries.
• Goal: primary removal and thickening for recycle/disposal.

Concept / Approach:
Gravity settling in circular thickeners or sedimentation tanks with rotating scrapers is the industry standard to concentrate underflow to high solids, while producing overflow for further treatment or reuse. Scrapers move settled sludge to a central sump for pumping, ensuring continuous operation.

Step-by-Step Solution:
Select a robust, continuous primary separator: a thickener/clarifier with scrapers.Mechanics: hindered settling followed by sludge consolidation; rake mechanism transports solids.Downstream steps (e.g., vacuum filters) can further dewater the thickener underflow if needed.

Verification / Alternative check:
Process flowsheets from washeries show thickeners as the mainstay for bulk solids removal, with flocculant addition when fines are high to improve settling.

Why Other Options Are Wrong:

• Chemical coagulation as sole step: often used as an aid, but not a standalone substitute for the primary gravity unit.
• Vacuum filtration first: uneconomical for dilute slurries; used after thickening.
• Clarifiers without scrapers: impractical at these loadings; sludge removal is essential.
• Membrane ultrafiltration: cost-prohibitive and prone to fouling for 3% solids.

Common Pitfalls:
Sending dilute slurry directly to filters; ignoring need for rake torque and underflow density control in thickener design.

Final Answer:
Sedimentation tanks with mechanical scrapers (thickeners)