The Krupp–Renn process was a direct reduction steelmaking process used from the 1930s to the 1970s. It used a rotary furnace and was one of the few technically and commercially successful direct reduction processes in the world, acting as an alternative to blast furnaces due to their coke consumption. The Krupp–Renn process consumed mainly hard coal and had the unique characteristic of partially melting the charge. This method is beneficial for processing low-quality or non-melting ores, as their waste material forms a protective layer that can be easily separated from the iron. It generates Luppen, nodules of pre-reduced iron ore, which can be easily melted down.
The first industrial furnaces emerged in the 1930s, firstly in Nazi Germany and then in the Japanese Empire. During the 1950s, new facilities were constructed, notably in Czechoslovakia and West Germany. The process was discontinued in the early 1970s, with a few nuances.
It was unproductive, intricate to master, and only pertinent to certain ores. In the beginning of the 21st century, Japan modernized the process to manufacture ferronickel, which is the sole surviving variant.
History
Setting up The direct reduction of iron ore principle was tested in the late 19th century using high-temperature stirring of ore powder mixed with coal and a small amount of limestone to adjust the ore's acidity. Carl Wilhelm Siemens'Alexander Lyman Holley, Siemens' direct process of making wrought iron from ore, in a rotating gas furnace, 1877, 21 p. direct reduction process, which was sporadically employed in the United States and United Kingdom in the 1880s, is particularly noteworthy. This process is based on using a 3-meter in diameter and similarly lengthy drum with a horizontal axis for blowing gases preheated by two regenerators.Greenwood, William Henry (1884). Steel and Iron: Comprising the practice and theory of the several methods pursued in their manufacture, and of their treatment in the rolling mills, the forge, and the foundry. p. 213-217.
The metallurgy industry underwent much research regarding the implementation of rotary tubular furnaces, inspired by similar equipment used in cement works. The Basset process, developed during the 1930s, is capable of even producing molten cast iron.(fr) Jacques Corbion (pref. Yvon Lamy), Le savoir... fer – Glossaire du haut-fourneau : Le langage... (savoureux, parfois) des hommes du fer et de la zone fonte, du mineur au... cokier d'hier et d'aujourd'hui, 2003, 5th ed. [detail of editions] (read online [PDF]), § Procédé Basset.. In the 1920s, German metallurgist , head of the metallurgy department at the and professor at the Clausthal University of Technology,(de) Paul Funke, "Friedrich Johannsen: Nachruf", Jahrbuch 1988 der Braunschweigischen Wissenschaftlichen Gesellschaft, Göttingen, Erich Goltze KG, 1988, p. 253-254 (DOI 10.24355/dbbs.084-201309201345-0, read online). explored the metallurgical applications of this type of furnace. He filed a series of patents for removing volatile metals from steel raw materials.(fr) Johannsen, Friedrich. "Liste de brevets de Friedrich Johannsen". worldwide.espacenet.com. Office européen des brevets. Retrieved 28-01-2021.
During the 1930s Johannsen initiated the development of direct-reduction iron production. The first installation underwent testing from 1931 to 1933 at the Gruson plant in Magdeburg.Iarkho, E. N. (1970–1979). "Krupp-Renn Process". The Great Soviet Encyclopedia. Research on the Krupp–Renn process continued until 1939 at the Krupp facility in Essen-Borbeck. The process, named after the Krupp company that created it and the , translating to "low furnace,". displayed potential. As a result, Krupp procured patents overseas to safeguard the invention after 1932.Brevet US 2900248 "Direct smelting process in rotary kilns", Friedrich Johannsen, 1959.
Adoption In 1945 there were 38 furnaces worldwide, each with a capacity of 1 Mt/year.. The process was favored in Germany due to the autarky policy of the Nazi regime, which prioritized the use of low-quality domestic iron ore.(de) "Gesang der Erzengel", Der Spiegel, no. 29, July 18, 1962, pp. 31–32 (read online). The transfer of technology between Nazi Germany and Imperial Japan led to the Japanese Empire benefiting from this process. Furnaces were installed in the co-prosperity sphere and operated by Japanese technicians. By the eve of the Pacific War, the process was being used in four steelworks in Japan.Kudo, Akira (1998). Japanese-German Business Relations : Co-operation and Rivalry in the Interwar. Routledge. p. 89-108. (read online).
After World War II all installations in Germany, China, and North Korea were dismantled,. with 29 furnaces sent to the Soviet Union as war reparations. Only the Japanese and Czechoslovak plants remained functional.
In the 1950s Krupp rebuilt several large furnaces in Spain, Greece, and Germany. The Czechoslovaks were the primary drivers, constructing 16 furnaces and increasing process efficiency.Mach, J.; Verner, B. (1963). "Czechoslovak Experience with the Krupp-Renn Process". UNIDO. The Great Soviet Encyclopedia reports that over 65 industrial plants, ranging from 60 to 110 meters in length and 3.6 to 4.6 meters in diameter, were constructed between 1930 and 1950. By 1960, 50 furnaces were producing 2 million tons per year in several countries.
Disappearance
The Soviet Union recovered 29 furnaces as war damage, but failed to gain significant profits from them. According to sources, the Red Army's destructive techniques in dismantling German industrial plants proved inappropriate and wasted valuable resources. It was also challenging for Russians to reconstruct these factories within the Soviet Union. Travelers from Berlin to Moscow reported observing German machinery scattered, largely deteriorating, along every meter of track and shoulder, suffering from the harsh climatic conditions. The Russian iron and steel industry did not heavily rely on technological input from the West.. Eventually, the Eastern Bloc only maintained this marginal technology to a limited extent in the recently sovietized European countries, where it was eventually abandoned.
Meanwhile large furnaces rebuilt in the 1950s in West Germany operated for approximately ten years before shutting down, due to the low cost of scrap and imported ore. The process then vanished from West Germany, concurrently with Western Europe.
In Japan furnaces also progressed towards increasingly bigger tools. However, the dwindling of local ferruginous sand deposits, along with the low cost of scrap and imported ores, eventually resulted in the gradual discontinuation of the process. The process was steadily improved by the Japanese, who developed it under various names for specialized products including ferroalloys and the recycling of steelmaking by-products. Currently, at the start of the 21st century, the Krupp–Renn process is exclusively used for ferronickel production in Japan.Torsner, Elisabeth (14 March 2014). "Nickel and nickel alloys in stainless steel melting " (pdf).
By 1972 most plants in Czechoslovakia, Japan, and West Germany had ceased operations. The process was widely considered obsolete and no longer garnered the attention of industrialists.
Process
General principles
The Krupp–Renn process is a direct reduction process that uses a long tubular furnace similar to those found in cement production. The most recent units constructed have a diameter of approximately 4.5 meters and a length of 110 meters. The residence time of the product is influenced by the slope and speed of rotation of the rotary kiln, which is inclined at an angle of roughly 2.5 percent..
Prior to usage, the iron ore is crushed to less than 6 mm in particle size. The iron ore is introduced into the furnace upstream and mixed with a small amount of fuel, typically hard coal. After 6 to 8 hours,(de) Friedrich Johannsen, "Das Krupp-Rennverfahren", Stahl und Eisen, no 38, 20 september 1934, p. 969-978 (read online [PDF]). it exits the furnace as pre-reduced iron ore at 1,000 °C. The amount of iron recovered ranges from 94% to 97.5% of the initial iron in the ore.
A burner located at the lower end of the furnace provides heat, transforming it into a counter-current reactor. The fuel comprises finely pulverized coal, which, upon high-temperature combustion, generates reducing gas primarily consisting of CO. Once the furnace reaches an optimal temperature, the ore-coal mixture can serve as the primary fuel source.
The fumes exiting the furnace's upper end attain temperatures ranging from 850 to 900 °C and are subsequently cooled and purged of dust by water injection before discharge through the chimney.
The process is efficient in producing ferronickel due to the proximity of its constituent elements. At 800 °C, carbon easily reduces iron and nickel oxides, while the gangue's other oxides are not significantly reduced. Specifically, iron(II) oxide (or wustite), which is the stable iron oxide at 800 °C, has a reducibility similar to that of nickel(II) oxide, making it impossible to reduce one without reducing the other..
Process characteristics The rotary kiln's maximum temperature ranges between 1,230 and 1,260 °C, which significantly exceeds the 1,000 to 1,050 °C threshold for iron oxide reduction. The main objective is to achieve a paste-like consistency of the ore gangue. The reduced iron agglomerates into 3 to 8 mm metal nodules called . If the infusibility of the gangue is high, the temperature must be increased, up to 1,400 °C for a basic charge. It is crucial to control the gangue's hot viscosity. Among rotary drum direct reduction processes, it stands out for using high temperatures. Direct reduction processes using rotating drumsJörg Grzella, Peter Sturm, Joachim Krüger, Markus A. Reuter, Carina Kögler and Thomas Probst, "Metallurgical Furnaces", John Wiley & Sons, 2005, p. 7.12345Consistency of the product obtainedsolidpastysol. (clinker) liq. (cast iron)Max. temperature (°C)600–900900–11001200–13001400–1500Reduction (% O2 removed from Fe2O3)12%20–70>90100Process examplesAir LiquideHighveld Udy LARCORN SL/RN
KruppKrupp- RennBasset Another distinctive attribute of the procedure involves introducing powdered coal to the furnace outlet. Furthermore, the process has evolved to enable terminating the supply of coal and running exclusively on the coal dust or coke dust(pl) Bogusław Barczyk, "Rozwój technologii przeróbki, wzbogacania i uzdatniania krajowych rud żelaza", Zeszyty naukowe politechniki śląskiej, seria Górnictwo no 27, 1967, p. 275-307 (read online [PDF]). introduced with the ore. In this situation, solely combustion air is injected at the furnace outlet. Thermal efficiency is improved in shaft furnaces such as blast furnaces compared to rotary furnaces due to the air absorbing some of the Luppen heat. However, the oxygen in the air partially re-oxidizes the product, meaning that the Luppen is still altered by contact with air at the end or after leaving the furnace, despite complete reduction of iron in the furnace.
The hot assembly is discharged from the furnace and then rapidly cooled and crushed. The iron is separated from the slag via magnetic separation. Magnetically intermediate fines make up 5–15% of the charge. While partial melting of the charge leads to the increased density of the products, it also requires significant energy consumption.
Load behavior as it passes through the furnace
The furnace comprises three distinct zones:
Firstly, the preheating zone heats the ore to 800 °C using the hot fumes within the furnace. Ore reduction occurs only if temperatures exceed 900-1,000 °C, while the coal releases its most volatile constituents.