Recycling of steel from grinding scraps: Reclamation plant design and cost analysis

Abstract About 10,000 t of grinding scraps are produced in and around Turin each year. Grinding scraps result from high speed tool cutting processes in the automobile and aerospace industries in which a fluid, called lubrorefrigerant oil, is used to cool the cutting tool. At present, grinding scraps are landfilled with an involved cost of 60 €/t, because the oil content (equal to 6.40% by weight, b.w.) does not permit recovery in secondary smelt furnaces. Secondary smelt foundries in fact only accept turnings, borings, scraps and oily machinery waste with an oil content no higher than 1% b.w. The lubrorefrigerant fluid and heavy metal content of some grinding scrap samples have been determined. The characterized waste was then subjected to several laboratory tests in order to single out the most convenient operating conditions; these resulted to be: solid–liquid ratio 1:20 b.w., one washing phase, surfactant amount equal to 0.15% b.w. (of the washing water amount), solid–rinsing water ratio 1:20 b.w., employment of docusate sodium salt. The obtained results allowed a wet treatment to be arranged aimed at removing the cooling/cutting fluid from the waste and at a subsequent recovery or recycling the steel. The wet treatment involves a washing phase in a stirred reactor in the presence of docusate sodium salt, a filtration in a plate and frame filter for the solid separation and a final rinse in order to eliminate the residual surfactant amount. The treated material is finally pressed to minimize the residual moisture content. The oil is separated from the exhausted washing water using a cloth filter. The oily sludge is eventually landfilled. A cost analysis, carried out on the proposed reclamation plant, highlighted the economic advantage of the treatment (total annual plant operating costs are equal to about 40 €/t of treated grinding scrap) in comparison to the landfilling operation.