The steel caster’s water fix: a scale‑pit‑to‑filter train that turns dirty spray into reusable cooling water

Continuous casting (CC) of steel gulps water — on the order of 250–300 m³ per tonne of hot metal — yet more than 90% typically circulates in closed loops (mheavytechnology.com) (samcotech.com) (mattech‑journal.org). The dirty outlier is the spray‑water effluent from the secondary cooling zone, a relatively small but nasty stream — on the order of 20–30 m³/hour per caster strand — loaded with mill scale and oils (mheavytechnology.com).

The contaminant profile is stark: suspended solids at 5–7 g/L (93% Fe₂O₃ mill scale; Fe₂O₃ is iron oxide) and ≈30 mg/L oil, so a single 20 m³/h strand can shed ~500–700 kg of solids per hour (mheavytechnology.com). Most solids are coarse flakes that settle fast, while oil forms surface films. Regulators, including Indonesian MoEF, typically require post‑treatment effluent with TSS (total suspended solids) ≪50 mg/L and oil & grease under 5–20 mg/L. That steers plants to a multistage train: a vinyl‑lined steel scale pit (a primary gravity settler), an oil‑water separator, and multimedia filtration. Treated water is hot but manageable, with a temperature rise ~5–7 °C above makeup.

Below is the per‑strand influent snapshot the design must tame: flow 20 m³/h (≈480 m³/day), TSS 5,000–7,000 mg/L, oil & grease ~30 mg/L, and the ~5–7 °C temperature delta (mheavytechnology.com) (westechwater.com).

Primary gravity settling design

The heart of the system is the scale pit sized for gentle hydraulics. A loading rate near ~1.0–1.2 m³/h·m² is effective for this effluent (mheavytechnology.com). At 20 m³/h, that implies ~17–20 m² of basin area. With 3–4 m depth, the pit volume is ≈50–80 m³, giving ~2–4 hours of retention — enough for the heavy scale to drop out.

The payoff is immediate. In primary settling, TSS can fall from ~6,000 mg/L to ~300 mg/L (≈95% removal) (mheavytechnology.com) (mheavytechnology.com). Sludge management matters: iron‑oxide solids (often 70–80% moisture) collect at the bottom; plants use sloped floors, scrapers, or conveyors to concentrate them, then dewater with a filter press. For compact footprints, many mills pair pits with clarifiers; a second clarifier can be added if needed, and compact modules such as a lamella design align with that role (mdpi.com). Where space drives the choice, a packaged unit like a lamella settler or a conventional clarifier can deliver the required detention time.

Oil–water separation parameters

Next up is oil control. The scale pit itself should skim floating oil (westechwater.com). Downstream, API‑style separators or coalescers are typically sized for ~15–20 minutes retention — ≈5–7 m³ at 20 m³/h — which promotes droplet coalescence and rise. Combined, skimming plus separation routinely removes >90–95% of oil, cutting ~30 mg/L inflow to ≲5 mg/L (designs often target <5–10 mg/L for reuse) (westechwater.com). For free‑oil capture, coalescing modules such as an oil removal separator fit the duty.

Emulsified oils and fine droplets are tougher. Many steel mills add a dissolved‑air flotation (DAF) or induced gas flotation step as a polish; performance improves further with flocculant dosing (westechwater.com) (trea.com). Where this is required, a compact DAF unit with polymer addition — metered via a dosing pump and supported by tested flocculants — is standard practice.

Fine filtration and polishing

Finally, the neutralized effluent is filtered. Industrial practice favors “sedimentation and sand pressure filtration” for steel process water (mdpi.com). A duplex set of pressure media filters — for instance, two parallel vessels each ~2 m² operating ~10 m³/m²·h — comfortably handles 20 m³/h while allowing backwash rotation. Dual‑media beds using sand media and anthracite capture fines; if trace hydrocarbons must be minimized, packed activated carbon or organophilic resin can adsorb residuals.

Plants often house these in robust pressure shells; for elevated pressures, steel filter housings are common. Typical finished water quality from this train is turbidity <10 NTU (NTU: turbidity unit based on light scatter), TSS <20–30 mg/L, and oil ≲2 mg/L — clean enough for cooling reuse in most mills.

Optional coagulation step

Where very fine colloidal iron oxide challenges filters, coagulation/flocculation (e.g., ferric salts or polymers) ahead of the clarifier or DAF deepens purification, as noted in a case study (mdpi.com). If added, pH‑sensitive coagulants and appropriate polymers are dosed sparingly to improve settling and reduce filter loading.

Performance and mass balances

Using a single strand at 20 m³/h (~480 m³/day) as the design case, influent solids at 5–6 g/L translate to ≈110 kg/h. The scale pit captures roughly ~100–110 kg/h (assuming ~300 mg/L in the overflow, ≈6 kg/h), so ~2,400–2,600 kg per day lands in the pit and will need dewatering — a filter press or vacuum belt typically reduces this to ~200–300 kg of dry cake per day. Oil and grease at 20–30 mg/L (~0.6 kg/h) are cut by >90% across separation and filtration, leaving 1–2 mg/L residual (≈20–40 g/h out of 480 m³).

The polished effluent typically runs 5–20 mg/L TSS and <5 mg/L oil. Multi‑stage plants (sedimentators + media filters) routinely achieve effluent with turbidity ≪50 NTU and oil ≪10 mg/L (westechwater.com) (trea.com) (mdpi.com). Overall removals are on the order of 95%+ for both solids and oil.

Water reuse and blowdown control

With those numbers, the CC plant can loop the treated water back into the cooling circuit. Globally, closed‑loop recirculation often exceeds 80%–90% of water input (samcotech.com) (mattech‑journal.org), and one study reports China’s mills cut freshwater use per tonne from ~36 m³ to ~2.5 m³ by recycling 98% of their water (iwaponline.com). In a closed‑loop CC circuit, only makeup is added to cover evaporation and blowdown; evaporation losses are modest (only a few percent of flow), and periodic partial bleed‑off (say 5–10% of flow) plus fresh or deionized makeup maintains conductivity in‑spec (mdpi.com) (mdpi.com).

Many plants install a small reverse‑osmosis skid on condensate or blowdown to desalinate reuse water (mdpi.com) (mdpi.com). Where that route is selected, compact RO packages such as a brackish water RO can be paired with simple security filtration (the MDPI case study notes “cartridge security filters”) — an application for a rugged cartridge filter — to protect membranes. For broader standardization, integrated membrane systems can dovetail with existing recirculating cooling infrastructure.

Compliance and savings

Indonesia’s environmental policy (GR 22/2021) explicitly encourages reuse over discharge (enviliance.com) (enviliance.com). Translate that to the plant floor: reusing even 80% of a 20 m³/h stream saves ~384 m³/day — roughly ~140,000 m³/year per strand — with corresponding cuts in raw‑water intake and discharge (samcotech.com) (mattech‑journal.org).

What the full train delivers

Step by step: the scale pit strips ≈95% of solids; the oil separator removes >90–95% of hydrocarbons; filters mop up the rest. The result is a treated stream with turbidity <10 NTU, TSS <20–30 mg/L, and oil ≲2–5 mg/L — routinely good enough for recirculation, with achievable reuse on the order of 80–95% of treated volume (westechwater.com) (trea.com) (mdpi.com) (samcotech.com) (mattech‑journal.org).