On a worldwide basis there exist large stocks of by-products from the production of ferrous and non-ferrous metals. For example, in Scandinavia there is a site which currently has 60000 tons of stainless steel trapped in slag waste. Even at current market prices this is a valuable resource. However, current technological approaches, such as ball milling, are uneconomic. High power ultrasound (HPU) is a novel approach to this problem to allow recovery of the stainless steel and a recycling path for the silicate slag as a building material. At the University of Strathclyde, pulsed power (the compression of electrical energy with time) has been used to generate HPU shock waves from spark discharges in water. Trials using a prototype HPU system have demonstrated that stainless steel metal can be separated from the slag waste by-product rapidly and with low power consumption. Glass may also be comminuted for recycling using the HPU system. The results of the trials are presented and proposed methods for industrial scale-up are discussed. (C) 2005 Elsevier Ltd. All rights reserved.