Steel’s dirtiest loop, cleaned: How a three‑stage system turns caster effluent into usable water

Continuous-casting water is split between relatively clean mold-cooling water and a polluted secondary-cooling bleed. Measurements put the latter at roughly 5–7 g/L suspended solids (about 93% Fe₂O₃) and approximately 30 mg/L oil in the polluted bleed, at around 20 m³/h per caster strand versus about 300 m³/h of clean cooling water (mheavytechnology.com) (mheavytechnology.com). After scale removal, the suspended solids concentration typically falls to a few hundred mg/L (for example, roughly 300 mg/L) (mheavytechnology.com).

For design, assume worst case: feed solids near 5–7 g/L and oil around 30 mg/L in that polluted stream. By comparison, integrated steel plants often consume 240–300 m³ water per tonne of hot metal, so any reuse of even a few percent yields large savings (mheavytechnology.com). The combined flow then enters treatment.

Effluent loads and flows (continuous casting)

The polluted secondary-cooling bleed carries the heavy load: ∼5–7 g/L solids (∼93% Fe₂O₃) and ∼30 mg/L oil, at about 20 m³/h per strand, against roughly 300 m³/h of relatively clean cooling water (mheavytechnology.com) (mheavytechnology.com). After an initial knockdown of scale, that solids load typically drops to the order of a few hundred mg/L (e.g., ~300 mg/L) (mheavytechnology.com).

Scale pit primary sedimentation parameters

The first unit is a scale pit (a primary clarifier), typically a long rectangular basin operated at gentle flows — horizontal velocity around 0.3 m/min — to settle coarse iron-oxide scale (watertechnologies.com). Design practice calls for a surface loading of roughly 600–800 gpd/ft² (approximately 1.0–1.36 m³/m²·hr); using 1.0–1.2 m³/m²·hr ensures most 100–300 mg/L particulates settle (watertechnologies.com) (mheavytechnology.com). In steel service, scale pits often remove “up to 90%” of heavy scale (nepis.epa.gov).

This basin also performs bulk oil separation. Oils and greases carried by sprays float and are trapped by shallow weirs; mechanical belt, rope, or drum skimmers remove the layer continually (nepis.epa.gov). An API‑style clarifier with surface weir and skimmer can capture more than 60–90% of free oil (nepis.epa.gov) (watertechnologies.com). Settled iron‑rich sludge is scraped out for disposal. Well‑designed pits reduce suspended solids from multi‑g/L to roughly 100–300 mg/L in the overflow, with oil down to about 10–15 mg/L (mheavytechnology.com). A primary clarifier in this role aligns with packages such as clarifier systems used in industrial settling.

Secondary oil removal (OWS/coalescer)

Downstream, an oil–water separator (OWS) or plate coalescer polishes residual oil. An API separator (a gravity oil–water unit) typically removes more than 90% of free oil and can take about 20–30 mg/L influent down to below 2–5 mg/L, depending on droplet size; tabulated data show 60–99% removal under ideal conditions (watertechnologies.com). Coalescing plate units or induced‑air systems (DAF, dissolved air flotation) achieve similar results for finer emulsions. Oil removal packages in this service include purpose‑built oil removal units, while DAF is represented by modular flotation systems. At this point, solids remain on the order of a few hundred mg/L.

Filtration and polishing options

The final step removes remaining suspended solids and trace oil. Options include multi‑media filters (sand/anthracite), disk filters, cartridge filters, or membrane units (micro/ultrafiltration). A multi‑media bed often starts with granular media such as sand and silica for 5–10 micron particles, complemented by dense media like anthracite in layered beds. Where needed, fine particulate capture is tightened with cartridge filters.

If coagulation/flocculation is added — common before a clarifier — a tertiary clarifier or polishing filter can cut TSS dramatically. One case study reports a coag‑floc clarifier followed by filtration reducing finals from 100–200 mg/L to about 25 mg/L (nepis.epa.gov). Coagulants such as polyaluminum chloride and high‑molecular‑weight flocculants are typically fed via an accurate dosing pump to optimize solids capture.

Activated carbon filters can adsorb dissolved hydrocarbons and fine oil traces; in caster loops this is often done with granular carbon beds. Ultrafiltration (UF, membrane sieving at the sub‑micron scale) adds a physical barrier; many plants deploy UF skids as pretreatment or polishing. In general, polished effluent TSS can be driven under about 10–20 mg/L or lower, and after sand filtration and carbon polishing (or ultrafiltration), the water is essentially free of visible oil with very low turbidity. The combined train — settling, OWS, filters — routinely meets discharge or reuse targets, with “after treatment, TSS ~25 mg/L” achievable and oils driven to single‑digit mg/L (nepis.epa.gov). For reference, Indonesia’s permit limits for many industries are often TSS 20–50 mg/L and oil/grease 5–20 mg/L.

Reuse metrics and industry benchmarks

With solids and oils removed, the treated water generally surpasses industrial recirculation needs. The main contaminants (iron oxide and hydrocarbons) have been stripped; remaining constituents are minor. The effluent can be repurposed on‑site for non‑critical cooling circuits, equipment flushing, dust‑suppression washes, or topping up broader cooling towers. In practice, treated caster effluent can join a plant’s internal recycling; recycling offsets a nearly equal volume of raw freshwater demand (watertechnologies.com). A caster bleed of about 20 m³/h corresponds to roughly 158,000 m³/year, so redirecting this volume from discharge to reuse significantly cuts makeup needs.

Surveys show roughly 81% of steel mills recycle more than 70% of their wastewater, with about 32% recycling over 90% (iwaponline.com). In China, integrated mills reduced water use from approximately 35.9 m³/ton steel to just 2.45 m³/ton between 1980–2020, with water reuse rising to around 98% (iwaponline.com). Indonesia’s Krakatau Blue Water processes about 17,000 m³/day of steelmaking wastewater and recovers around 700 m³/day as high‑quality water (conductivity below 80 μS/cm) for reuse in steel operations (lestari.kompas.com) (lestari.kompas.com).

In short, a three‑stage system — scale pit, oil separation, filtration — is standard practice in caster loops and can push recycling beyond 90% in well‑run plants (iwaponline.com) (iwaponline.com). Beyond saving water, it reduces discharge costs and helps meet regulatory targets — directly reducing consumption while aligning with established best practices (watertechnologies.com).

Sources: design loads, effluent makeup, and pit/OWS performance from mheavytechnology.com (mheavytechnology.com), clarifier and oil‑removal design guidance from watertechnologies.com (watertechnologies.com), EPA operational notes and case outcomes from nepis.epa.gov (nepis.epa.gov), recycling benchmarks from iwaponline.com (iwaponline.com), and Indonesian reuse case data from lestari.kompas.com (lestari.kompas.com).