Valorization of apatite mining flotation residues by the manufacture of artificial aggregates

Abstract

Mining residues are usually stockpiled because of their understudied physico-chemical properties and of their abundance, which make their valorization challenging. In this work, a cold-bonding granulation apparatus was built in order to transform a mining residue sludge from an apatite ore, into artificial aggregates suitable for use in civil engineering. A 25 rpm rotation speed, 15° angle relative to horizontal, and an ideal feeding amount of the granulation drum, as such as the liquid (supernatant) distribution method, were preliminary determined. Effects of rotation speed, liquid content, and granulation time were then studied to evaluate the best combination for appropriate aggregate size distribution. Box-Behnken experimental designs were used to investigate the evolution of compressive strength of the individual aggregates when Portland cement is added to the mix, and when granulation time varies, around the critical liquid content of 14% of total weight. Thus, the possible parameters combinations which allow a maximization of compressive strength without exclusively increasing cement content, were determined and used to confirm the Box-Behnken predictions. Spherical aggregates were thus manufactured, with diameters mainly comprised between 5 and 40 mm, and compressive strength comparable to those of commercialized aggregates, up to 7 MPa and to 3 MPa for aggregates with diameters respectively below and above 10 mm. Paths for greater consolidation as prior homogenization, use of humid cure conditions, and use of other additives emerge from literature and from the results presented.