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A steel mill's scale solution reduces water treatment costs

21st February 2013

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. 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.

Environmental regulations require companies 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 100m3/h and 10000m3h 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 heavy and abrasive. (Density 4.9-5.2g/cm3).

New technologies have been pioneered by Nordic Water and Metso Minerals. 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.

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 2.5-5cm. 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.

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. The pump can be lifted for inspection and does not require sealing water. The largest pumps handle a flow of 1500m3/h (4400gpm) at pressures up to 3bar(45psi). A 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.

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.

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 100microns carrying up to 10t/h The dewatered scale is a drip dry material with less than 2-8 per cent 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.

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..

The screw dewaterer delivers solid concentrations in the range of 100-300ppm. 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.

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 (see diagram) is based on the counterflow principle. 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. Water purification and sand washing both take place continuously, enabling the filter to remain in service without interruption. The filter operates with no moving parts, no controlled valves and at a low pressure drop, less than 0.1bar(1.5psi). 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 almost any purity.

Operation on mill scale allows for a filtration rate of 25m/h (10gpm/sqft) through the filter bed. Thus the largest filter unit, the DST50 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 SD60-200 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 (0.8gpm/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.

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