Finex Process for Liquid Iron Production
FINEX Process for Liquid Iron Production
The FINEX smelting-reduction process was developed by Primetals Technologies, Austria and the South Korean steel manufacturer Posco. FINEX process is a commercial proven alternative iron making process for the production of hot metal (HM) in addition to the blast furnace (BF) process route, consisting of BF, sinter plant, and coke oven. This process is based on the direct use of non-coking coal. The FINEX process can directly use iron ore fines without any kind of agglomeration.
In the FINEX process, fine iron ore is preheated and reduced to fine DRI (direct reduced iron) in three stage fluidized bed reactor system with reduction gas produced from melter gasifier. The fluidized bed reactors enable the FINEX process to use fine ores instead of lump ore or pellets.
As a result the process requires neither coke making nor ore agglomeration. Briquetting of the pre-reduced ore and the coal, pulverized coal injection, and controlled charging of the melter gasifier (MG) results into improved fuel rate for the process. The fine DRI produced in the fluidized bed reactor system is compacted and then charged in the form of HCI (hot compacted iron) into the melter gasifier to produce hot metal (HM). The charged HCI is reduced to metallic iron and melted. The heat needed for the metallurgical reduction and melting is supplied by coal gasification with high purity oxygen (O2). The FINEX process is an environmental friendly process which uses low-cost fine iron ore and coal.
The FINEX process uses high purity O2, resulting in an export gas with only low amounts of nitrogen (N2). As its net calorific value (CV) is more than two times of the BF top gas, it can be partially recycled for reduction work or can be used for heat or energy generation.
The basic laboratory scale research had been done with a 15 tons/day bench scale unit from 1992 to 1996. The results of this unit were used for the test operations of a 150 tons/day pilot plant in 1999. The FINEX demonstration plant of 0.6 million tons per annum (Mtpa) was built in Pohang works of Posco and started production in June 2003. This plant has three fluidized bed reactors. Since February 2004, the demonstrated plant steadily produced at a rate of more than 0.7 million tons per annum of hot metal. Posco commissioned the first commercial FINEX plant of capacity 1.5 Mtpa in April 2007. Based on the successful results of this plant, Posco and Primetals Technologies decided to develop the FINEX plant with a capacity of 2.0 Mtpa at Pohang. The plant has been put into operation in January 2014.
Main raw materials
Coal and iron ore are two main raw materials. The major criteria for an initial evaluation of coals or coal blends suitable for the FINEX process are (i) fix C (carbon) content at a minimum of 55 % , (ii) ash content at a level of maximum 25 %, (iii) VM (volatile matter) content less than 35 %, and (iv) S (sulphur) content less than 1 %. In addition to these general characteristic, the coal is required to meet certain requirements related to thermal stability in order to allow for the formation of a stable char bed in the melter-gasifier. The thermal stability of potential coals for the FINEX process is checked using special testing procedures in laboratories.
FINEX process can operate without coke due to the lower burden load in the melter- gasifier char bed and the use of O2. In case of changing coal briquette quality and reduction degree fluctuations, some coke breeze (less than 30 mm) typically is used before and after a shutdown or in case of a decreasing HM temperature to maintain productivity and decrease fuel ratio. The current operation provides a constant level of coke breeze to minimize the above described effects. The quality of coke breeze used in the FINEX process is not suitable for the BF operation and has strength of around 60 % of BF coke. For achieving a zero coke breeze operation, several operation optimizations are essential such as binder optimization, and development of a coal briquette pre-heating technology. Coal characteristics for the FINEX process and its comparison with the coal characteristics for BF ironmaking are given in Fig 1.
Fig 1 Coal characteristics or FINEX and BF processes
In case of iron ore, in general, 100 % of sinter feed fine ore is charged into fluidized bed reactors. 30 % to 50 % of pellet feed can also be used. Types and mix of iron ore are decided based on the chemical and physical properties such as total iron (Fe) content, composition structure, and grain size etc. As in the case HM production by BF process, the Fe content of iron ore determines the productivity. The mixing ratio is to be decided both by considering the quality of the ore and the cost. Since higher alumina (Al2O3) slag tapping is more tolerable in the FINEX process than in BF process, iron ores with higher Al2O3 content can also be used. Normally, there is no limitation in feeding material structure of hematite and goethite for fluidized bed reactors. Flexibility of iron ores suitable for FINEX process is shown in Fig 2.
Fig 2 Flexibility of iron ores for FINEX process
The FINEX process is distinguished by the production of high quality HM on the basis of directly charged iron ore fines, and coal as the reductant and energy source. The key feature of the FINEX process is that iron production is carried out in two separate process steps. In a series of three fluidized-bed reactors, fine iron ore is reduced to DRI, which is then compacted (HCI) and transported to a melter-gasifier by a hot metal conveyor. Coal and coal briquettes charged to the melter-gasifier are gasified, providing the necessary energy for the melting in addition to the reduction gas. The process flowsheet for the FINEX process is given in Fig 3.
Fig 3 Flowsheet of the FINEX process
The liquid iron is produced in the FINEX process in two steps. In the first step iron ore fines is preheated and reduced to fine DRI in fluidized bed reactors in three stages. The first reactor (R3) serves primarily as a reactor for preheating of iron ore fines. Iron ore fines are charged into the series of fluidized bed reactors together with fluxes such as limestone and/or dolomite. The charged ore fines travels in a downward direction through the three reactors where the ores are heated and reduced to DRI by means of the reducing gas which is obtained from the gasification of the coal in the melter gasifier. This reducing gas flows in the counter current direction to the movement of ore.
As per the process ore route, a pneumatic conveying system transports the ore fines to the fluidized-bed reactor tower. The fine ore is then charged to the fluidized bed reactor series. The reduction gas generated in the melter-gasifier flows through each of the fluidized-bed reactors in counter flow to the ore direction (From R1 to R3). The typical temperature and the composition of the reduction gas in the three fluidized bed reactors are given in Tab 1.
|Tab 1: Typical gas atmosphere for a three-stage fluidized bed reactor for FINEX process
|Fluidized bed reactor
The fine iron ore is fluidized by the gas stream and the ore is increasingly reduced in each reactor step. Following the exit of the reduced iron from the final fluidized-bed reactor, it is then compacted to produce HCI. The HCI is subsequently transported via a hot-transport system to the top of the melter gasifier where it is directly charged together with coal into the melter gasifier. Final reduction and melting of the HCI then takes place.
As per the process coal route, non-coking coals and coal briquettes are directly charged into the melter gasifier through a lock-hopper system. After the coal drops onto the char bed, degassing takes place. The released hydrocarbons, which are environmentally harmful, are immediately dissociated to CO (carbon monoxide) and H2 (hydrogen). This is due to the high prevailing temperatures exceeding 1,000 deg C in the dome of the melter-gasifier. O2 injected into the lower part of the melter-gasifier gasifies the coal, generating heat for melting work as well as producing a highly valuable reduction gas comprised mainly of CO and H2. This gas, which leaves from the dome of the melter-gasifier, is first cleaned in a hot-gas cyclone before entering the fluidized bed reactors. Following melting of the DRI, the tapping procedure is carried out exactly in the same manner as in standard BF practice. The quality of HM from the FINEX process is similar to the HM produced in BF.
The FINEX export gas is a valuable by-product of the FINEX process. The clean export gas leaving from the top of the fluidized-bed reactors can be used for a wide variety of applications. These include production of DRI, power generation and the generation of synthesis gas for the chemical industry. The typical composition of different gases produced in the FINEX process is given in Tab 2.
|Tab 2 Typical composition of the Gases
The gas flow in the FINEX process is given in Fig 4.
Fig 4 Flow of gas in the FINEX process
Typical specific consumption values for the materials and the utilities in the FINEX process are (i) dry fuel around 720 kg/tHM, (ii) iron ore around 1,600 kg/tHM, (iii) additives (limestone and dolomite) around 285 kg/tHM, (iv) O2 around 460 N cum, (v) N2 around 270 N cum, (vi) power around 190 kWh/tHM, and (vii) refractories around 1.5 kg/tHM.
The characteristics of the HM produced by the FINEX process consist of (i) C around 4.5 %, (ii) silicon (Si) around 0.7 %, (iii) manganese (Mn) around 0.07 %, (iv) phosphorus (P) around 0.07 %, (v) sulphur (S) around 0.04 %, and (vi) temperature around 1,500 deg C.
The characteristics of the export gas of the FINEX process consist of (i) CO around 34 %, (ii) CO2 around 43 %, (iii) H2 around 13 %, (iv) H2O around 3 %, (v) CH4 less than 1 %, (vi) N2/Ar around 6%, (vii) H2S less than 100 ppm (parts per million), (viii) dust 5 mg (miiligrams)/N cum, (ix) pressure 0.1 kg/sq cm, (x) temperature around 40 deg C, and (xi) CV in the range of 1,300 kcal/N cum to 1,500 kcal/N cum. Around 1.9 giga calories of the export gas is produces per ton of HM.
Environmental aspects of the process
The FINEX process has the possibility) to recover high purity CO2 for the CO2 capture and storage (CCS). Besides storage, the recovered CO2 can also be used for oil recovery enhancement as well as for other economical uses. This is possible because of the use of high purity O2 in the melter gasifier for gasification of coal and hence, the export gas contains only low amounts of N2. This allows removal of the CO2 in high concentration from the recycling gas and to generate after further purification high purity CO2 with CO2 percent higher than 95%. The CO2 emission rates for the FINEX process without CCS and with CCS are 99 % and 55 % respectively when compared with the average CO2 emission rate in case of BF ironmaking process.
The FINEX process is a coal based process for the reduction of iron ore to iron, which is subsequently melted into HM. A certain amount of environmentally harmful substances are inevitable based on the raw material mix. Since the FINEX process captures most of the pollutants in an inert state in the slag and the released hydrocarbons are destroyed in the dome of the melter gasifier, the emissions of harmful substances are very low. The values of emissions per ton of HM for dust, SOx, and NOx are around 58 grams per ton (g/t), around 32 g/t, and around 94 g/ton respectively.
Advantages of the FINEX process
The various advantages of the FINEX process include (i) utilization of the low grade fine iron ores as oxide feed, (ii) use of the non-coking coals as reducing agent, (iii) independent control of reduction and melting processes, (iv) advantageous economics because of significantly decreased capital and operational costs, (v) environmental benefits, (vi) flexibility in raw materials selection and in the operation such as utilization of lower grade iron ores possible (e.g. iron ores with higher Al2O3 content), (vii) production of HM which is similar to the quality of the HM from the BF, (viii) export gas with higher CV which can be utilized for different purposes (e.g. power generation, DRI production, and production of chemical products), (ix) commercial proven alternative iron making process, and (x) brown field application at integrated steel plant gives synergies with the BF.