text: These processes involve high-temperature blending of iron ore and coal powder, with a little limestone to reduce the acidity of the ore. Processes such as Carl Wilhelm Siemens', based on the use of a short drum, first appeared at the end of the 19th century. The tool used then evolved into a long tubular rotary kiln, inspired by those used in cement works, as in the Basset process, developed in the 1930s.
  A process of historic importance is the Krupp-Renn. Developed in the 1930s, there were as many as 38 furnaces in 1945 which, although they only had a capacity of 1 Mt/year at the time, were installed all over the world. This process was improved and inspired the German Krupp-CODIR furnaces and the Japanese Kawasaki and Koho processes. Both Japanese processes integrate a pelletizing unit for steel by-products upstream of the rotary furnaces. Two units of each process were built between 1968 (Kawasaki) and 1975 (Koho).
  The ACCAR process, developed in the late 1960s and used confidentially until 1987, uses a mixture of 80% coal and 20% oil or gas: the hydrocarbons, although more expensive, enrich the reducing gas with hydrogen. The German Krupp-CODIR process, operational since 1974, has had little more success: only three units have been commissioned. Finally, Indian steelmakers are behind the SIIL, Popurri, Jindal, TDR and OSIL processes, which are simply variants developed to meet specific technical and economic constraints.
  Other processes, built on the same principle, failed to develop, such as the Strategic-Udy, consisting of a single plant commissioned in 1963 and shut down in 1964.
  The SL/RN process, developed in 1964, dominated coal-fired processes in 2013. In 1997, it accounted for 45% of pre-reduced coal production. In 2012, however, production capacity for this process had fallen to just 1.8 Mt/year, out of a total of 17.06 Mt attributed to coal-fired processes.