Process innovations using mechanical activation of minerals and wastes

Abstract

This article is a critical review of recent research pursued in India on mechanical activation of bauxite and industrial wastes, namely blast furnace slag and fly ash. The emphasis is on unique findings in terms of basic understanding and process innovations. Importance of pure minerals and their mixtures is brought to focus to unravel the intricacies involved during the activation of multi-mineralic ore such as bauxite. In the gibbsitehematite mixture, hematite which is a hard mineral, can enhance the activation of gibbsite. Use of mechanical activation as a pretreatment, or simultaneously during alkali leaching of bauxite, results in moderation in leaching condition and significant reduction in soda and alumina losses in the red mud, and thus, paving the way for a newer technology. Judicious selection of milling device is the key to achieve hydration of blast furnace slag without any chemical activation. Quite interestingly, complete hydration of the slag which takes several years is possible in about a month time for an attrition milled slag. The hydration product of slag shows unique features in terms of crystallinity and phases present. Portland slag cements containing upto 80-85% slag can be produced using the activated slag. The results on fly ash revolve around mechanically induced and glass content induced reactivity in the context of geopolymerisation of fly ash. Mechanically induced reactivity is exploited to produce very high strength (up to 120 MPa) geopolymer cements. Lastly, the article touches upon the process innovations commercialization prospects and the barriers.