High demanding waste water purification through ultrafiltration
ArcelorMittal Gent processes considerable quantities of difficult to treat oil-bearing waste waters in an economically responsible way. They mainly consist of emulsions used for the lubrication and cooling of the rollers. In technical terminology a stable dispersion between oil and water is called an emulsion. By separating the emulsion compound into its components, we can reuse the water and oil. This is ArcelorMittal Gent's environmentally friendly solution to a problem that is not so simple. We have therefore abandoned the traditional ways of processing of incineration or dumping. The environment benefits, and production takes place in an economically responsible way using innovative membrane technology!
The waste water treatment plant fully automatically treats the oil-bearing waste waters from the rolling mills. It consists of three parts:
- pretreatment where the suspended particulate matter in the water settles and where the first free oil is skimmed off
- ultrafiltration that separates the water fraction and recovers it for the cooling water system
- oil treatment that separates the oil fraction for injection in the blast furnace
Pretreatment involves 5 vats of 150 m³ to 250 m³. The long flow time of the waste water that is to be treated ensures a first separation of the free oil that floats to the top. Skimmers remove this oil and take it to the oil treatment facility. The suspended particulate matter settles because of the slow flow rate. Periodic cleaning of the vats ensures the removal of formed sludge. The initial removal of the free oil and the drainage of the sediments are required for an optimal performance of the ultrafiltration membranes. The last vat in the flow direction is also the buffer vat to supply the ultrafiltration work tanks. Previously, a self-cleaning filter added with a filter containing filter candles removes the still remaining suspended particles.
The ultrafiltration work tanks feed the ultrafiltration skids.
Each skid consists of a frame on which 34 parallel rows of 6 membranes are assembled in series. This construction provides a membrane surface area of 48 m² per skid. A sufficiently high liquid speed of up to 6 m/s with a pressure of 5 barg pushes the water fraction through the diaphragm and feeds the permeate flow. The oil particles and substances with a higher molecular weight do not pass through the diaphragm, and remain behind in the concentrate flow. Particles large than 10 to 100 Å (1 Ångström is 0.1 nanometer) do not pass through the membrane.
Membrane section and membrane schematic diagram
From a technical construction perspective the membranes function at temperatures up to 50°C. The operating temperature is set at 40°C because the yield increases with the temperature. The average yield amounts to 3 m³/h clean water per ultrafiltration skid. Oil-bearing waste water with an oil concentration of up to 10% is purified in this way. The produced water fraction disappears through the recovery network in the cooling water network for further use. We use turbidity measurements to monitor the purity of the water.
The membranes must be periodically cleaned. For a first rinse, we use a detergent diluted with a base for higher alkalinity. The increased alkalinity improves the tensio-active effect and contributes to the more efficient removal of the organic soiling. This is followed by acid rinsing to remove inorganic soiling.
The waste water circulates over the ultrafiltration skids at a flow rate of 330 m³/h to obtain the specified liquid speed. The content of the work tanks supplements the recirculation volume with a flow rate of 70 m³/h. The oil concentration in the work tanks increases accordingly: clean water is drained away and replaced by oil-bearing waste water. The work tanks are periodically blown out to neutralise the increasing oil concentrations. The oil treatment is calculated on the basis of the further processing of this blown out waste water with increased oil concentration.
The oil treatment consists, in essence, of three 60 m³ process tanks. All waste oil to be processed arrives in these tanks. A thermo-chemical treatment processes the volume arriving. Ninety-eight per cent sulphuric acid is dosed with the tank content to be processed. The built-in steam heat exchanger then heats the tank to 90°C. The tank content remains at this temperature for 24 hours and then decants for 48 hours. Sampling and laboratory analyses are used to check the separation of the water phase and oil phase. The measured oil concentration determines which fraction is removed to where. All oil with a water content of less than 20% is processed by injection in the blast furnace. The remaining water fraction soiled by oil disappears to the above-mentioned pretreatment facility for further purification.
This installation is the work of a project team that consists of chemical engineers and technicians, all permanent employees at ArcelorMittal Gent. Five years of adaptations to the control units and numerous experiments with varying results were needed to achieve the required effect. Thorough research in combination with innovative technologies gave us the intended result: a fully automatic installation that recovers the constituent components from problematic waste waters as efficiently as possible and makes them reusable.