The Cyclone Converter Furnace Process (CCF)
Jointly developed in the years 1986-1989, at British Steel, the CCF process was born from work carried out surrounding the Converted Blast Furnace.
In the CCF process pre-reduction and final smelting is carried out in a single reactor in combination with the application of a melting cyclone.
The most significant production route for steel from ore is the Blast Furnace (BF) and associated Basic Oxygen Furnace (BOF) production processes that cover approximately 2/3 of total worldwide and European steel production. Only a small amount of primary steel was produced via alternative routes, direct reduction processes with subsequent processing of produced sponge iron in an Electric Arc Furnace (EAF).
Despite the fact that many alternative methods to reduce iron ores have been developed, some of which are also commercially available, the blast furnace continues to be the dominant process to produce iron. This is due to the cost and energy efficiency of it, partly as a result of the large size and high production rate combined with a high and uniform quality of the hot metal. The blast furnace is a combined counter current heat exchanger and chemical reactor, where the reduction of iron oxides takes place.
The main product of the blast furnace is hot metal, which typically consist of about 94% Fe, 4.5% C, 0.5% Si and small amounts of Mn, P, S and other metals. The temperature of the tapped hot metal is usually between 1400°C and 1500°C. Other main outputs from the process are slag and top gas. Slag consists of oxides of Ca, Mg, Al and Si, and its temperature is 50-100°C higher than that of the hot metal. Slag can be used by other industries, for example, in cement making. A typical composition of the blast furnace top gas is 20-25% CO2, 20-25% CO, 50% N2, 1-4% H2 and some H2O. With a relatively low heating value (of about 3.5 MJ/m3) a part of the top gas is commonly used to preheat the blast at the hot stoves while the remaining portion is used to produce heat and power at the power plant.
During the past century, many efforts were made to develop processes for producing iron for steelmaking that could serve as alternatives and/or supplements to the conventional blast furnace. Many of these projects were stimulated by a desire or necessity to use lower grade ores and available fuels that are unsuitable for the blast furnace.
The Cyclone Converter Furnace (CCF) process originated from the Converted Blast Furnace (CBF) jointly developed by Hoogovens, British Steel and Ilva in the years 1986 to 1989. In the CBF process, lumpy ore is highly pre-reduced in a shaft with final reduction and melting taking place in an iron bath in which fine coal is gasified. The process can avoid coke making but not ore agglomeration and related environmental problems. To further eliminate ore agglomeration in the process, the Cyclone Converter Furnace (CCF) is developed, in which a melting cyclone is applied for pre-reduction and pre-melting of fine ore. A small CCF pilot plant was built in Taranto, Italy.
A schematic diagram of the CCF process is shown in Figure 1. In the CCF process, the pre-reduction and the final smelting reduction stages take place in a single reactor. Fine ores and coal are injected tangentially by carrier gas into the melting cyclone, which is mounted directly on top of a vertical type converter. The pre-reduced molten ore is collected on the water cooled wall of the cyclone and falls into the iron bath for its gravity, where final smelting reduction of the ore and gasification of coal take place.
The gases arising from the smelter are further combusted in the melting cyclone in order to generate heat required by melting and pre-reduction. The off-gas of the CCF leaves the process at a temperature of 1800°C and the final combustion ratio of the off-gas is about 75%. As can be seen from the process, the CCF process requires a minimum amount of equipment. Gas conditioning steps such as cooling, de-dusting and reforming are not required.
The gas off-take resembles a closed hood of a BOF gas cleaning system and includes a boiler for the utilization of the sensible heat in the off-gas. The steam generated in the boiler can be used for the production of oxygen and to generate electricity. Furthermore, the use of a melting cyclone is the unique feature of the CCF process. Due to high pre-reduction and smelting intensity in the melting cyclone, the size of the melting cyclone is much smaller than that of a conventional shaft reduction furnace, and thus a very high volumetric production rate and required pre-reduction degree can be achieved.
In addition, as a high degree of pre-reduction and melting occur in the melting cyclone, the final reduction and smelting taking place in the converter bath is relatively moderate, which should result in moderate slag foaming and post combustion requirements. Moderate slag foaming and post combustion are important for the successful operation and maintenance requirements of the converter vessel.
Figure 1: The Schematic diagram of the CCF process