Metso's traveling grate pelletizing process consists of a green balling section where, after the mixing stage, green pellets are formed either by rolling on discs or in drums. The green pellets are then screened and heat-hardened in the indurating furnace.
The process comprises four steps:
- Raw material preparation and mixing
- Green pelletizing
- Pellet hardening (indurating)
- Hearth layer and product screening
In order to adjust the moisture content, water is added in the mixing step. The ore is then mixed with small amounts of binding agents. Fluxes such as limestone, olivine, and dolomite give the pellets the necessary physical and metallurgical properties for further processing. The green pelletizing step is where the pellets are formed using pelletizing discs or drums. Drums use a two-stage process where undersized pellets are returned to the drum via a roller screen. This is in contrast to discs using a single process step to discharge pellets from the disc rim within a very narrow size range. Pellet size can be precisely adjusted by varying the disc inclination, circumferential speed, and feed or water addition rates.
During induration, the green pellets are distributed evenly across the traveling grate and then hardened in a furnace where they pass through updraft drying, downdraft drying, preheating, firing, afterfiring, and cooling zones. Our unique updraft and downdraft drying sequence significantly reduces fuel consumption. The homogeneous pellet charge on the grate reduces the pressure drop within the furnace, which further reduces energy consumption and enables an even heat treatment, resulting in high-quality pellets.
ECONOMICAL AND ENVIRONMENTALLY FRIENDLY GAS FLOW AND GAS CLEANING SYSTEM
A range of recuperation techniques are applied to the grate's gas flow in order to reduce both fuel consumption and atmospheric emissions:
- Direct recuperation of heated process gas (850-1,000°C) from the first cooling zone, where the gases are transferred on to the pre-heating and firing zones
- Windbox recuperation of exhaust gases (330-380°C) from the windboxes to the downdraft drying and pre-heating zones
- Exhaust gases from the second cooling zone (330-380°C) are used for updraft drying
ADDITIONAL IMPROVEMENT IN COOLING ZONE TO FURTHER SAVE FUEL
- Exhaust gases from the final cooling zone (approx. 250°C) are used in the first cooling zone
Metso has developed and continuously improved both, proprietary equipment and process control facilities.
A traveling grate pelletizing plant typically comprises the following equipment:
- Grinding (wet or dry) and storage facilities
- Dosing equipment
- Pan-type mixers or mixing drums
- Pelletizing discs or drums
- Roller screens
- Feeding facilities
- Traveling grate indurating machine with burners and process fans
- Product and hearth layer screens
- Waste gas cleaning facilities
- Waste reclamation facilities
