Blast furnace gas cleaning wastewater: Which unit process is typically used to remove suspended solids from BF gas cooling/cleaning effluent?

Introduction / Context:
Blast furnace (BF) gas cleaning uses scrubbers and separators that produce wastewater laden with fine solids (flue dust, iron oxides). Effective primary treatment is needed to remove these suspended solids before downstream polishing or reuse.

Given Data / Assumptions:

• Wastewater contains high suspended solids and colloidal fines.
• Objective: primary solid-liquid separation.
• Coagulant options available to enhance settling.

Concept / Approach:
Radial settling tanks (clarifiers) and thickeners provide quiescent zones for particle settling. Lime and ferrous sulphate destabilize colloids, increase floc size, and accelerate sedimentation. The dense underflow can be thickened and handled, while clarified overflow proceeds to further treatment if needed.

Step-by-Step Solution:
1) Characterize solids: iron-rich fines with poor natural settling.2) Apply coagulation/flocculation: lime raises pH and aids coagulation; ferrous salts form hydroxide flocs.3) Use radial settlers/thickeners to separate flocculated solids efficiently.4) Produce clarified effluent and concentrated sludge for disposal or recovery.

Verification / Alternative check:
Steel plant water circuits commonly show BF scrubber effluents routed to thickeners with coagulant dosing before reuse or discharge.

Why Other Options Are Wrong:
Biological pond: Targets biodegradable organics, not primarily suspended mineral solids.Lagooning: Too slow and land-intensive for heavy solids loads.Filtration only: Rapid blinding and impractical without prior clarification.

Common Pitfalls:
Skipping coagulation, leading to carryover and poor clarity.Undersized settlers that cannot handle peak turbidities.

Final Answer:
Radial settling tank (thickener) with coagulant addition (lime and ferrous sulphate)