Nordic Steel Mill Solution Cuts Water Treatment Costs by 35% with Innovative Dewatering & Filtration Systems

By transporting mill scale together with water, steel mills establish a continuous recycling loop that cuts labor costs, reduces footprint, and supports nonstop operation.

Northern European mills—including Outokumpu’s Avesta, Sweden facility—have adopted Nordic Water’s solutions, featuring screw dewaterers, lamella separators, and DynaSand filters.

Current environmental regulations mandate that Northern European steel mills clean and treat process water before discharge. While a closed‑loop system is ideal, practical operations still require bleeding and treatment.

Mill scale is coarse, heavy and abrasive
In steel production, hot milling and casting use water at rates ranging from 100 cubic meters per hour to 10,000 cubic meters per hour, depending on plant size. As steel leaves the casting operations, cooling and cutting with water produces mill scale—iron oxides whose particles span from a few microns up to fist‑size. They are dense (4.9–5.2 g cm⁻³) and abrasive.

Traditionally, this coarse material is collected in pits, emptied by cranes or chain‑scrapers, and processed in large settling basins that separate fines and oil. Periodic back‑washed sand filters then remove the micron‑size material and residual oil. The approach is labor‑intensive, space‑consuming, and maintenance‑heavy, making retrofits costly.

Collection and processing present environmental challenges
Historically, workers manually hauled mill scale in buckets on rails, a hazardous task in cramped, dusty environments. Modern mills must improve safety, cut costs, and meet stricter environmental laws, driving the shift to automated solutions.

New approaches
Nordic Water (formerly Axel Johnson Engineering) and Metso Minerals (formerly Sala International) pioneered a continuous‑flow system that eliminates the need for a scale pit adjacent to the main production area. This reduces dust, improves working conditions, and frees up valuable plant space.

To transport the slurry, a floor‑level pump and sump assembly was designed. Two semi‑submersible pumps—one in standby—move the water‑scale mixture. A scrap basket at the inlet traps foreign objects between 2.5 cm and 5 cm. The sump features steep walls and a low profile to prevent buildup. Industrial rubber hoses protect the pump line, while straight sections may use steel piping.

High‑chromium alloy steel constructs the pump house and impeller. Despite the abrasive slurry, wear parts last over a year on heavy roughing duties and more than a decade on finer scale.

Finding the right pump is critical
Key selection criteria included a rigid shaft and bearings to absorb shock, flow‑variation insensitivity, air‑blockage tolerance, dry‑run capability, and a bottom impeller that can release sump deposits.

Nordic Water chose Metso’s VT semi‑submersible pump. The unit is fully liftable for inspection and does not require sealing water. The largest models handle 1,500 m³ h⁻¹ (4,400 gpm) at up to 3 bar (45 psi). A fully submersible variant is available for shallow sumps.

By pumping scale away from the fall point, the system eliminates the need for a milling‑area pit and unlocks alternative scale‑removal strategies.

Removing coarse scale from the slurry
Three main options exist: screw dewaterers, magnetic separators, and cyclones. Cyclones suffer from wear‑induced performance loss and are less tolerant of large foreign objects, requiring additional downstream treatment. Magnetic separators need either large units (unsuitable for many sites) or suffer increased wear due to short contact distances.

Screw dewaterer is preferred
Nordic Water’s preferred solution is a large screw dewaterer—a settling tank coupled with a screw conveyor that dewaters settled material. Though common in mineral processing, its use in steel mills is relatively new.

The design allows continuous operation during maintenance: the screw can be lifted while feed continues for several hours.

The screw removes almost 100 % of particles above 100 µm. The dewatered scale emerges as a drip‑dry material with 2–8 % moisture, depending on particle size distribution.

A mud guard traps oil and other floating materials; an oil skimmer removes the oil. While some oil remains in the overflow, the water quality is suitable for reuse as transport water. Cooling‑specific treatment may be required afterward.

The first screw dewaterer—and the first DynaSand filter—for scale was installed at Avesta (now Outokumpu Stainless) in 1980, followed by Ovako Steel, Fundia, SSAB, and several mills across Germany, Austria, and France.

“The Nordic Water/Metso system saved 35 % in overall water‑treatment investment,” said Avesta project manager Nils Albertsson.

A cross‑current lamella pack mounted in the dewaterer increases the settling area to 100 m², supporting a feed flow of nearly 2,000 m³ h⁻¹ (8,800 gpm) in a single unit. Voest Alpine in Linz, Austria, operates seven such units.

The screw dewaterer delivers solid concentrations of 100–300 ppm, too high for conventional static filters but well within the capacity of Nordic Water’s DynaSand continuous filter.

Continuous filtration with DynaSand
The DynaSand filter employs a counter‑flow principle: water enters the lower distributor, travels upward through a sand bed, and exits at the top. An air‑lift pump extracts fouled sand from the bottom, washes it in a small clean‑water stream, and returns clean sand to the bed, keeping the filter operational without interruption.

Key advantages include no moving parts, negligible pressure drop (<0.1 bar), and a rinse water flow of only 5–7 % of the feed. The filtrate contains <5–10 ppm residual solids and oil, meeting most recycling or bleed requirements. For polishing, coagulants, lower feed loads, or activated‑carbon beds can be used to achieve higher purity.

With mill scale, filtration rates reach 25 m h⁻¹ (10 gpm ft⁻²) through the sand bed. The largest unit, DST 50, has a 5 m² (54 ft²) area and processes 123 m³ h⁻¹ (550 gpm). Filters can be arranged in a battery; sixteen units handle 2,000 m³ h⁻¹ (8,800 gpm), matching the capacity of Metso’s SD 60‑200 screw dewaterer.

The rinse water from DynaSand is usually sent to a conventional thickener operating at 2 m h⁻¹ (0.8 gpm ft⁻²). The thickener’s overflow returns to the filter feed pumps; the underflow, discharged via a sluice valve, achieves 45–65 % solids. Polymers are used only in the thickener, not in pumps, screw dewaterers, or sand filters. Lamella separators have successfully performed the same function in recent installations.