AKM Rainbow, BSc, PhD, CEng, FICE, FIMM, FIHT, FIPHE, FGS. This paper was first published in GE’s May 1987 edition.
What are they?
Minestone (burnt and unburnt) are by-products of mining coal and of its preparation for markets. They are derived from the rocks – mainly siltstones and mudstones, with seat-earths and sometimes sandstones, limestones and other rock types – lying above, below and sometimes within, the coal seams in the coal measures. During mining operations quantities of these rocks unavoidably extracted with the coal, or in driving the tunnels (roads) which give access to the coal faces, are brought to the surface with the coal.
It is then generally necessary to remove some or all of this material to yield a coal product of the required quality. This separation is carried out with other operations, such as crushing, sizing and dewatering, in the coal preparation plants, generally by making use of the higher density of the Minestone – roughly between 11/2 and 2 times that of the coal. There is usually no immediate use for the separated non-coal material, so it is tipped onto a spoil heap, familiar – although far less so in recent years – around the coalfields. This is Minestone, or unburnt colliery spoil.
Until about twenty years ago, and more particularly before the post-World War 2 advance in mining technology, nearly all colliery spoil heaps were constructed by tipping the spoil loose from transporting containers – aerial flights, skips, wagons or trams. The resulting spoil heaps were nearly always loose and open-textured so that air could readily penetrate. If, for any reason, the residual coal in parts of the spoil heaps became ignited, air could readily reach the burning section and sustain and extend the heating. Before World War 2, nearly all collieries were steam-powered and had to dispose of hot boiler ashes. There was then no well-established market for this clinker as lightweight aggregate, so it was often tipped on the spoil heaps, which were ignited and eventually burned through. The residues of calcined rocks left in these spoil heaps are burnt.