Metso is one of the original technology suppliers for straight grate systems having acquired the technology from Dravo and Jacobs.
These plants typically produce pellets using less fuel than other types of pelletizing systems.
Metso has designed and supplied some of the most modern and largest straight grate systems in the world. These plants have the highest outputs with lowest fuel usage and lowest emissions.
Straight grate design usually has a higher bed depth compared to a grate kiln but due to excellent refractory and recycling of air have a lower fuel usage.
Straight grate systems consist of one major piece of equipment. The complete process is done on the grate. The object of the process is to transform the pelletized concentrate into hardened pellets that can be used as blast furnace feed or direct reduction furnace feed.
When pellets are fed onto the grate, they are dried and then pre-heated up to a temperature of about 800-900 deg C. The heat used to dry and preheat the pellets is typically hot air pulled from the indurating section and cooling zone. The recycling of the the hot air from the different zones increases energy efficiencies.
In the indurating zone, pellets are brought up to final indurating temps. Hot gas recycled from the cooling zone is further heated up to 1200-1340 °C through a set of burners and pulled through the bed of pellets to complete the slag bonding and mineral bridging to form pellets.
In the cooling zone, pellets are brought down to a suitable temperature for downstream material handling equipment. The gases from the cooling zone are recycled to the indurating area and the pre-heat and drying zone, resulting in the straight grate being the most energy efficient system for producing indurated pellets.
How it works
The straight grate consists of a stationary furnace with a moving set of pallet cars travelling through the furnace on a set of rails. The pellets are fed onto the cars and travel along the furnace where they are dried, heated and cooled.
Because the refractory is stationary and not subjected to rotation and abrasion as the rotary kilns refractory’s are, it can be much thicker and keep more heat in the furnace.
The air flow and machine layout is designed to properly indurate all the green pellets and still protect the pallet cars from the high temps of the furnace. To do this, the cars are lined with high temperature stainless steel bars and the sidewalls are also made of high temperature stainless steel. Then the cars are lined with a 70-100mm thick layer of already fired pellets called the hearth layer. These features allow the full indurating temperature to reach all the unfired pellets even the bottom layer without damaging the pallet cars.
The pallet cars are pushed through the furnace by a large sprocket where they are loaded with pellets, then after the pellets are discharged, the pallet cars go around a sprocket at the end of the machine and return to the drive sprocket on a return set of rails.
The cars travel through different temperature zones where the pellets are dried, then pre-heated, then indurated, then finally cooled. There can be multiple burners in some of the zones to allow control of the temperature profile as the cars travel along.