Steel mill's solution reduces water treatment costs

Paul Boughton

Today, strict environmental regulations require companies in Northern Europe to clean spent process water before release into rivers and sewers. Stephen Hinton reports.

Transporting mill scale with water to create a continuous recycling loop reduces costs by eliminating much manual handling and allowing continuous operation in less space than traditional solutions. Mills in Northern Europe like Outokumpu’s plant in Avesta, Sweden have been turning to Nordic Water for solutions based on screw dewaterers, lamella separators and Dynasand filters.

Today, strict environmental regulations require companies in Northern Europe to clean spent process water before release into rivers and sewers. Ideally, all process water should be recycled in a closed loop; in practice it needs to be “bled” out and treated.

In steel production, hot milling and casting uses water at rates of between 100 m3/h and 10,000m3h depending on the scale of the operation.  As steel leaves the casting operations, cooling and cutting with water produces mill scale (iron oxides).

Mill scale particles range from a few microns up to the size of a fist. They are also heavy and abrasive. (Density 4.9 – 5.2 g/cm3). Traditionally, this coarse material is collected in pits to be emptied by cranes or chain-scrapers. Huge settling basins then separate fines and oil. Periodically back-washed batteries of sand filters finally remove the micron size material and residual oil.[Page Break]

This approach however has several disadvantages that increase the overall costs of water treatment and thereby cost of operation of the mill. Firstly, the operation requires manual handling and cannot be automated. Secondly, basins and pits require good deal of space. Thirdly, maintenance costs are high. Finally, when retrofitting an existing plant to upgrade water treatment, this approach requires finding additional space.

Collection and processing presents environmental challenges

Mill Scale collection can be particularly troublesome for the work environment. As late as the  early 1970s, workers collected scale in buckets standing on rails and move them by hand. This was an unpleasant task in cramped, noisy and dusty conditions.

As demands on steel mills increase to improve the working environment, reduce overall costs and comply with more restrictive environmental legislation, several mills have turned to the water treatment specialists Nordic Water.  Nordic Water, based in Sweden, offer advanced solids-liquid separation products as well as turn-key solutions for industrial process water and waste water treatment.

New technologies have been pioneered by Nordic Water (earlier Axel Johnson Engineering) and Metso Minerals (earlier Sala International). Ovako Steel AB, part of the SKF group in Hofors Sweden, called on these companies to help remove the scale in a continuous flow of water. This eliminated the need for a scale pit close to the main production and reduced dust whilst improving the work environment. However, the scale and water slurry needed transporting. [Page Break]

Scale slurry transport with water

A pump and sump arrangement at floor level was designed to provide the means to transport the slurry away. Two semi submersible pumps (one for standby) move the water and scale mix. A scrap basket at the inlet traps scale and foreign objects over 2.5 – 5 cm. The sump is designed to avoid build up of walls, it is small with steeply sloping walls with a bottom area about twice the width of the pump house. To resist wear, industrial rubber hoses connect the pump line and the pump, although steel pipes can be used on straight sections.

High chromium alloyed steel is used in the pump house and impeller. Despite the harshness of the slurry, the wear parts last more than a year even on hard roughing duties. On finer scale the wear parts may last over ten years.[Page Break]

Finding the right pump critical

Choice of pump was crucial and the following criteria were identified:

• Rigid shaft and bearings to withstand  the shock of the heaviest objects
• Flow variation insensitive
• Air blockage insensitive
• Ability to run dry
• Ability to release sump deposits with a bottom impeller or return spray-holes.

Nordic Water chose the semi-submersible pump type Metso VT for this task. The entire pump can be lifted for inspection and it does not require sealing water.

The largest pumps of this type handles a flow of 1500 m3/h (4400 gpm) at pressures up to 3 bar(45 psi). A fully submersible variant can be used when the shaft length is insufficient for the depth of the sump.

Pumping the scale away from the point where it falls eliminates the need for a scale pit in the milling area and opens up for alternatives in scale removal.

Removing coarse scale from the slurry

Three options present themselves for slurry treatment: a screw dewaterer, magnetic separator or cyclone. The disadvantages of the cyclone include performance degradation due to wear and that it is less tolerant to large foreign objects. Furthermore, the effluent requires additional processing. The magnetic separator has a short contact distance between the magnet and the scale which either calls for large separators (suitable only where economies of scale allow) or results in increased wear.

Screw dewaterer preferred

Nordic Water’s preferred solution is a large screw dewaterer. This is basically a settling tank where the material settles out and a screw conveyor removes and dewaters the settled scale. Although used for many years in mineral treatment, application in steel mills is fairly recent.

Continuous operation also during maintenance is possible as the screw can be lifted for service with the feed flow still going for several hours.

The screw removes about 100 per cent of the scale material above 100 microns carrying up to 10t/h.

The dewatered scale is a drip dry material with less than 2-8% moisture depending on particle size distribution.

A mud guard traps the oil and other floating material which is removed with the oil skimmer supplied with the dewaterer for removal of floating products. Oil is still present in the overflow but the water quality is sufficient to be reused as transport. For cooling purposes further treatment is needed.

The first screw dewaterer, and also the first DynaSand filters, for scale was installed at Avesta (today Outokumpu Stainless) in 1980. It was rapidly followed by installations at Ovako Steel, Fundia, SSAB and later at a number of mills in Germany, Austria and France. Said Project Manager Nils Albertsson at Avesta: “the use of the Nordic Water/Metso system saved  35 per cent in the overall water treatment investment."

A cross current lamella pack mounted in the dewaterer increases the pool area to give a total settling area (or pit equivalent) of  100m2 allowing for a feed flow of close to 2000m3/h (8800)gpm in one unit. Seven units of this size operates at Voest Alpine, Linz, Austria.

The screw dewaterer delivers solid concentrations in the range of 100 – 300 ppm. This is too high to feed the static filters in the traditional scale effluent systems with acceptable back-wash frequency, but within the capacity of the Dynasand continuous filter manufactured by Nordic Water.[Page Break]

Continuous filtration

In the DynaSand filter fouled sand is continuously removed from the filter bed, washed and recycled without interruption to the filtration process. The DynaSand filter is based on the counterflow principle (see diagram). The water to be treated is admitted through the inlet distributor (1) in the lower section of the unit and is cleaned as it flows upward through the sand bed, prior to discharge through the filtrate outlet (2) at the top. The sand containing the entrapped impurities is conveyed from the tapered bottom section of the unit (3), by means of an air-lift pump (4), to the sand washer (5) at the top. Cleaning of the sand commences in the pump itself, in which particles of dirt are separated from the sand grains by the turbulent mixing action. The contaminated sand spills from the pump outlet into the washer labyrinth (6), in which it is washed by a small flow of clean water. The impurities are discharged through the wash water outlet (7), while the grains of clean sand (which are heavier) are retained in the sand bed (8). As a result, the bed is in constant downward motion through the unit. Thus, water purification and sand washing both take place continuously, enabling the filter to remain in service without interruption.

Thus, the filter operates with no moving parts, no controlled valves and at a low pressure drop, less than 0.1 bar(1.5 psi). The rinse water flow operates at 5-7 per cent of the feed flow.

The filtered water contains less than 5-10ppm of residual solids and oil, satisfying most demands for recycling or bleed. For polishing purposes the filter may be operated with the support of coagulants, with a lower feed load or with activated carbon as filter bed, which will produce filtrates of just about any desired purity.

Operation on mill scale allows for a filtration rate of 25m/h (10 gpm/sq ft) through the filter bed. Thus the largest filter unit, the DST 50 with 5m2 (54ft2) filter area, provides a capacity of 123m3/h (550gpm). Dynasand filters can be arranged in a battery layout. Sixteen filters together will serve a flow of 2000m3/h (8800gpm) matching the capacity of screw dewaterer model SD 60-200 from Metso Minerals.

Rinse water from the Dynasand filters is normally concentrated in a conventional thickener operating with a surface load of about 2m/h (0.8 gpm/sq ft) The thickener overflow is returned to the filter feed pumps. The underflow is discharged through a sluice valve system to allow for a high solids concentration of 45-65 per cent solids. Polymers have to be used in this thickener but in no other parts of the system of pumps, screw dewaterer or sand filters. Lamella separators have been successfully used for the same purpose in some recent installations.[Page Break]

Outokumpu Stainless (previously Avesta Sheffield) commissioned Nordic Water to design and implement a complete process water handling solution for the cold rolling plant in Avesta, Sweden.

The diagram shows a schematic of the solution.

1) Mill-scale slurry is pumped to the screw dewaterer (2). Sump – pumps transport process water to sand filters (4). Water from the sand filters is pumped (5) back to the process via cooling towers. Wash from the continuous washing of the sand enters the thickener.
Oil is (7) is skimmed off  as a residue, normally sent for burning.

The instrumentation includes level controls for stand-by pumps, rotation guards on rotating shafts, turbidity indication on filtrate and timer or torque signals for the operation of the sludge discharge. The plant operates with a feed flow of 320 m3/h(1 400gpm).

The feed pumps are situated below floor level and the remaining equipment is erected on a concrete foundation. The floor slopes slightly with a central canal for general clean up water which ends up in a scrap basket. The rinse water is then pumped to the screw dewaterer to be added as make up to the process flow.

The plant does not require permanent attendance but is visited once per shift to check on operation. Removal of scale and sludge is a daily operation. The plant is clean and silent.

Both Project Manager Torbjörn Gustafsson and Mill Operator Thomas Nordin have expressed their satisfaction with the high availability and trouble free operation of the system.

Sedimentation basins and back-washed filters are often proposed as a matter of course when retrofitting an existing plant or planning a new one. It may prove advantageous to compare space investment, operation costs and maintenance with a continuous flow and recycling solution similar to the above.

Fig. 1. The Dynasand process.

Fig. 2. Side elevation drawing of pump and sump arrangement.

Fig. 3. The screw dewaterer installed in Outokumpu Stainless' plant in Avesta, Sweden.

Fig. 4. Flowsheet.

Fig. 5. Mill scale particles range from a few microns up to the size of a fist.

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