Green Planet Technologies

There are two principal sources of fines generation during the direct reduction process. In an ideal world these fines would be reprocessed and recycled back through the direct reduction process - indeed there are various technologies for doing so, such as cold briquetting and pelletising. However, this is not always the case and there is a significant trade in these fines. It is inevitable that during the discharge of DRI (B) from the furnace and subsequent handling, including briquetting to produce DRI (A) and cooling, there will be some generation of fines through abrasion, etc. Users of DRI (A) and (B) require minimal content of fines in the product. Fines are undesirablein the EAF process as they have a high loss rate to the fume system (where they add unnecessarily to dust volume) and are also trapped in the slag which reduces the yield. Therefore DRI (A) and (B) are screened before shipment, thus generating Metallised DRI fines. In the case of DRI (A), newly formed hot briquettes are dropped into a quench tank for cooling, a process which also generates some fines (so-called “quench tank fines”) which are included with the Metallised DRI fines. In the shaft furnace it is inevitable that there will be some generation of dust due to decrepitation of iron ore during the reduction process and this dust is carried off with the off-gas and has to be separated from the gas stream via the off-gas scrubber(s). The dust particles are trapped by the water droplets in the scrubber and channelled as suspended particles, first to a classifier (which separates the larger agglomerated particles) and then to circular clarifiers (for pellet/lump based processes) or directly to settling ponds and/or filter presses (for fines-based processes). In the case of clarifier equipped plants, flocculants are typically used to lump the finely suspended particles into larger and denser particles which settle more quickly and stably to the bottom of the clarifier and are then pumped to settling ponds or filter presses. This material, known variously as scrubber fines, slurry fines, pond fines, top gas fines, etc. is hereinafter referred to as “Off-gas fines.” Depending on where in the furnace it arises, off-gas fines can range from virtually unreduced oxide fines from the top of the shaft to metallised fines from lower down in the shaft - its metallic iron content thus has a lower and wider range than Metallised DRI fines