| Summary: | This study examines four aspects of the use of ultrasonics in steel strip pickling: the cavitation process, the kinetics of scale removal, the relative significance of variables in ultrasonic pickling and the application of ultrasound in a pilot-scale operation. To ensure optimum location of transducers in relation to their cavitational effects, studies were carried out in water tanks using two techniques to map cavitational activity: resistance thermometry and quantified mechanical damage to coupons of lead or aluminium. Both techniques gave similar results. Thermometric mapping had the advantage of speed but the use of a panel of lead coupons provided more tangible evidence of the mechanical influence of ultrasound in water. A cylindrical bar transducer was the most effective of those tested and could readily be adapted for use in wide strip pickling. The kinetics of pickling were analysed using a Johnson-Mehl type equation. Time exponents 'n' were in the range 2.03 - 2.56 with activation energies in the range 42.18 - 50.54 kJ/mol. The process both in normal and ultrasonic pickling was envisaged to occur by progressive undercutting and fracture of the scale. The application of ultrasonics reduced completion times considerably. To explore the individual and linked influences of variables on the pickling process, and the effect of applying ultrasonics, a statistical factorial analysis was carried out. Acid temperature was found to be the most important single variable while the most significant interaction was that between acid temperature and acid concentration. An equation has been derived reflecting the influence of all the pickling variables and their interactions. Finally, to assess better the applicability of ultrasonics to full-scale commercial operations, tests were carried out on a large pilot plant funded as part of a European Coal and Steel Community project between British Steel plc and Hoogovens Staal. The results of the smaller tests were confirmed and, furthermore, the influence of ultrasound on inhibitor efficiency was found to be insignificant.
|
|---|
