These processes consist of an annular furnace in which iron ore mixed with coal is circulated. Hot reducing gases flow over, and sometimes through, the charge. The ore is deposited on a tray, or carts, rotating slowly in the furnace. After one rotation, the ore is reduced; it is then discharged and replaced by oxidized ore.
A number of processes have been developed based on this principle. In the 1970s-1980s, the INMETCO process demonstrated only the validity of the idea, with no industrial application. The MAUMEE (or DryIron) process came to fruition in the US with the construction of two small industrial units in the 1990s. Similarly, in Europe, a consortium of Benelux steelmakers developed the COMET process in the laboratory from 1996 to 1998. Despite the consortium's withdrawal from the research program in 1998, a single industrial demonstrator was extrapolated from it, the SIDCOMET, which was discontinued in 2002. RedIron, whose only operational unit was inaugurated in Italy in 2010, also benefits from this research. Japan has adopted the FASTMET process, with the commissioning of three units dedicated to the recovery of iron-rich powders, and is proposing an improved version, the ITmk3 process, with one unit in operation in the United States.
This non-exhaustive list shows that, despite the keen interest shown by steelmakers in developed countries during the 1990s, none of these processes met with commercial success.