These are exclusively coal-fired processes, with the reducing gases generated inside the reduction vessel. The ore is charged with coal into a closed container. This is then heated until the oxygen present in the ore combines with the carbon before being discharged, mainly in the form of CO or CO2. This production of gas by heating a solid material means that the reactor belongs to the retort category.
The principle is an ancient one: in northern China, the shortage of charcoal led to the development of processes using hard coal before the 4th century. To avoid any contact between iron and sulfur, the brittle element provided by coal, China developed a process that involved placing iron ore in batteries of elongated tubular crucibles and covering them with a mass of coal, which was then burned. This process survived into the 20th century.
More recently, other historic processes have come to the fore, such as that of Adrien Chenot, operational in the 1850s in a number of plants in France and Spain. Successive improvements by Blair, Yutes, Renton, and Verdié are not significant. Among the processes developed is the HOGANAS process, perfected in 1908. Three small units are still operational (as of 2010). Not very productive, it is limited to the production of powdered iron, but as it is slow and operates in closed retorts, it easily achieves the purities required by powder metallurgy.
Other retort processes were developed, such as KINGLOR-METOR, perfected in 1973. Two small units were built in 1978 (closed) and 1981 (probably closed).