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.
Mill scale is coarse, heavy and abrasive
In steel production, hot milling and casting uses water at rates of between 100 cubic meters per hour and 10,000 cubic meters per hour, 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 to 5.2 grams per cubic centimeter). 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.
This approach, however, has several disadvantages that increase the overall costs of water treatment and, thereby, cost of operation of the mill. First, the operation requires manual handling and cannot be automated. Second, basins and pits require good deal of space. Third, 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 moved 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 specialist Nordic Water. Based in Sweden, this company offers advanced solids-liquid separation products as well as turn-key solutions for industrial process water and wastewater treatment.
New approaches
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 while improving the work environment. However, the scale and water slurry needed transporting.
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 to 5 centimeters. 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 more than 10 years.
Finding the right pump critical
Choice of pump was crucial and the following criteria was 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 handle a flow of 1,500 cubic meters per hour (4,400 gallons per minute) at pressures up to 3 bar (45 pounds per square inch). 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 and 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 nearly 100 percent of the scale material above 100 microns. The dewatered scale is a drip-dry material with less than 2 to 8 percent 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.
“The use of the Nordic Water/Metso system saved 35 percent in the overall water treatment investment,” said Avesta project manager Nils Albertsson.
A cross-current lamella pack mounted in the dewaterer increases the pool area to give a total settling area (or pit equivalent) of 100 square meters, allowing for a feed flow of close to 2,000 cubic meters per hour (8,800 gallons per minute) in one unit. Seven units of this size operate at Voest Alpine in Linz, Austria.
The screw dewaterer delivers solid concentrations in the range of 100 to 300 parts per million. This is too high to feed the static filters in the traditional scale effluent systems with acceptable back-wash frequency, but it is within the capacity of the Dynasand continuous filter manufactured by Nordic Water.
Continuous filtration with DynaSand
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. The water to be treated is admitted through the inlet distributor 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 at the top. The sand containing the entrapped impurities is conveyed from the tapered bottom section of the unit by means of an air-lift pump to the sand washer 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, in which it is washed by a small flow of clean water. The impurities are discharged through the wash water outlet, while the grains of clean sand (which are heavier) are retained in the sand bed. 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 to 7 percent of the feed flow. The filtered water contains less than 5 to 10 parts per million 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 25 meters per hour (10 gallons per minute per square foot) through the filter bed. Thus the largest filter unit, the DST 50 with a 5-square-meter (54-square-foot) filter area, provides a capacity of 123 m3/h (550 gpm). Dynasand filters can be arranged in a battery layout. Sixteen filters together will serve a flow of 2,000 m3/h (8,800 gpm), 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 2 meters per hour (0.8 gallons per minute per square foot). 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 to 65 percent 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.