Inside the cold mill’s clean room: how alkaline and electrolytic stages strip rolling oils at 600 m/min

Cold-rolled steel exits reduction with a thin film of rolling lubricants and fine metal particulates that must be removed before downstream annealing, coating, or plating to prevent defects and poor adhesion (chemical.kao.com) (www.scribd.com).

Modern cleaning/degreasing lines combine mechanical and chemical stages. One industry supplier lists high‑pressure hot‑water wash, neutral‑detergent wash with brushes, alkaline degreasing baths, acid pickles, electrolytic degreasers, or combinations thereof (www.salico.net). SALICO cites lines handling strip up to 6.5 mm thick, 2.5 m wide, at speeds up to ~600 m/min (www.salico.net), so each stage must act quickly.

Alkaline degreasing bath parameters

Alkaline degreasing—using caustic detergents such as NaOH or KOH plus surfactants at high pH (usually pH 12–13)—is the workhorse for heavy rolling oil removal. Immersion baths typically run 3–10% alkali and 50–95 °C with 3–10 minutes dwell; sprays at 1.4–2.0 bar and 50–65 °C can clean in ~3 minutes (www.solverchem.com) (www.solverchem.com). The bath saponifies and emulsifies the oil (saponify: convert oils to soap‑like, water‑dispersible compounds), and formulations often include wetting agents or chelants to lift iron fines; alkaline detergents remove both the rolling oil film and fine iron particles embedded in the strip (chemical.kao.com).

Effectiveness hinges on tight control of pH, alkalinity, and temperature, and on equipment upkeep—pumps, spray bars, and filters must be designed for frequent maintenance (www.salico.net). Without upkeep, baths quickly saturate with oil and dirt and lose strength; typical bath life is ~1–2 weeks before replacement or rejuvenation is needed (chempedia.info).

To extend life, mills turn to continuous filtration or oil–water separation. Microfiltration (0.2 μm) at 40–70 °C is reported at ~250 L/m²·h and can keep oil in the treated water below ~100 ppm, extending bath life by about 5× with payback within ~2 years (chempedia.info) (chempedia.info). In routine operation, baths are kept in continuous recirculation, floating oil is skimmed, and periodic filter changes prevent particulate build‑up; SALICO stresses pump/filter access and quick‑change elements to keep contamination in check (www.salico.net).

Operating ranges at a glance

• Concentration: ~3–10% NaOH or equivalent (www.solverchem.com).
• Temperature: 50–95 °C (commonly ~60 °C) (www.solverchem.com).
• Time: 3–10 minutes immersion; sprays ~3 minutes at 50–65 °C (www.solverchem.com).
• Bath life: ~1–2 weeks before saturation; ~5× longer with microfiltration (chempedia.info) (chempedia.info).
• Oil removal: typically >95% of residual oil is emulsified/saponified in a dedicated pass.

Electrolytic alkaline cleaning (HCD)

For thin or stubborn oil films, mills add electrolytic cleaning: the strip passes through an alkaline electrolyte under DC current (direct current) so hydrogen/oxygen evolve at the surface and bubbles scrub away contamination. Many lines run HCD (high‑current‑density) cleaning, typically 5–15 A/dm² (amps per square decimeter), with aluminum or stainless electrodes generating the bubbles; China Steel Corporation describes an HCD line with an alkaline cleaning tank, brush tank, and rinse tank to remove attached rolling oil (www.csc.com.tw) (www.scribd.com).

Vendors emphasize speed and thoroughness: electrolytic cleaning “ensures [a] totally clean surface” and is more effective than conventional spray or immersion washes (www.scribd.com). It operates at line speed—SALICO cites up to ~600 m/min for cleaning lines (www.salico.net)—with electrolytes often containing NaOH (~1–4%) and anticorrosion additives. Mills may precede the electrolytic tank with a light alkali pre‑wash (NaOH ~1–3%) (patents.google.com), and the combined effect removes >99% of oil in one pass, yielding surfaces ready for annealing or coating.

Electrolytic cleaning also reshapes the waste profile. Because it avoids strong chemical detergents or solvents, it can reduce chemical costs and waste; oily effluent is primarily dilute alkali plus emulsified oil treatable with oil–water separators. Some systems are designed to recycle their electrolyte. Market adoption reflects OEM demands for clean surfaces: an industry report notes a substantial global market for steel electrolytic degreasing chemicals, with ~38% of demand from auto manufacturers in 2023 (pmarketresearch.com).

Bath cleanliness and monitoring routines

Both alkaline and electrolytic stages depend on clean, well‑controlled baths. SALICO emphasizes routine swaps of squeegee rolls, spray bars, brushes, and backup rolls, and ready access to pumps, filters, and sensors (pH, conductivity, temperature) for easy servicing (www.salico.net). Sensors typically monitor bath pH and conductivity continuously so operators can add reagents or replace solutions as needed; frequently changing filters (or backwashing) removes dispersed particles and emulsified oil, while floating skimmers capture free oil.

Once degreasing baths become “polluted” with oil they must be changed—often within a few weeks—if not supported by filtration (chempedia.info). With continuous microfiltration holding oil below ~100 ppm, bath life extends by ~5× (chempedia.info). The economic impact is tangible: cleaner, well‑maintained baths mean consistent oil removal, fewer rejects, and lower chemical consumption over time.

Environmental compliance is part of the calculus. In Indonesia, spent rolling oils and emulsions are classified as hazardous B3 wastes requiring special handling (www.researchgate.net). Discharge limits for metal finishing streams can be tight—oil and grease at 5.0 mg/L for plating effluent (www.scribd.com)—so capturing oil in‑plant matters. In practice, mills recycle rinse water and use skimmers or coalescers so effluents comply; pairing electrolytic stages with oil–water separation keeps loads in check (www.salico.net) (chempedia.info).

Line configurations and throughput

Suppliers enumerate complementary steps: High‑Pressure Hot Water (HPHW) to blast off loose oil, neutral cleaning with mild detergents and brushes, alkaline degreasing, acid cleaning for scale removal, and electrolytic degreasing (www.salico.net). The specific configuration depends on incoming oil/fines and target cleanliness, but the common denominator is speed: lines process 6.5 mm‑thick, 2.5 m‑wide strip up to ~600 m/min without sacrificing cleaning performance (www.salico.net).