Question 1 – Narrative Process Description of Furnaces 5 and 6

Furnaces 5 and 6 are the final step in the pellet-making process, which consists of converting crude ore 

into a final product. In this stage, the green balls that are delivered from the balling circuit are dried, 

fire-hardened, and cooled before being sent to the yard or boat loading. The furnace converts the 

green balls that consist of magnetite (Fe3O4) and additives into a hardened hematite (Fe2O3) pellet 

capable of enduring the handling required during stockpiling, reclaiming, boat loading, and shipping. 

Furnaces 5 and 6, which consists of approximately 200 6-foot by 2-foot pallets that have 15-inch side 

plates, are continuous traveling grate furnaces. Each pallet has approximately 40 grate bars with air 

spaces between the bars. The pallets move along the top strand through the furnace zones from the 

feed, or drive end, of the furnace. A motor with reduction gears is the bull gear for moving the pallets. 

The weight of the pallets on the return strand along with a gravity-sloped rail feed the empty pallets 

into the feed end.

The upper strand of pallets accepts layer of green pellets via a roll feeder that are produced in the 

balling drums. The green pellets dry, fire, harden, cool, and discharge while the pallets ride over 28 6-

foot square windboxes in Furnace 6 while Furnace 5 has one less windbox. The windboxes are 

connected by a series of downcomers to the main furnace air ducts, breechings, and six process fans. 

Each windbox has a dust leg with a dump valve that allows the dumping of dust, fines, or pellets that 

infiltrate the system through the grate bars on the pallets. A conveyor that runs the entire length of the 

underside of the furnace, reclaims this product. Dripping hoppers under the return grate strand also 

collect spillage and discharge onto the conveyor.

Roll feeder – Location B1
Pellets entering furnace – Location B2

Firing of pellets – Location B3

Pellets after firing – Location B4.1
Windbox – Location B5

The energy source that ignites and hardens the green pellets as they travel through the furnace comes 

from natural gas. The furnace has a natural gas system with 22 natural gas pilot burners. The furnace 

has three combustion sections that operate in the range of 1,600°; 1,900°; and 2,400° Fahrenheit (F). 

Burner controls – Location B6

The furnace has a Fluid Ionics emission control system that consists of three electrostatic precipitators. 

The precipitators remove particulate matter and scrub the emissions from the hood exhaust and waste 

gas fans.

The discharge ends of Furnaces 5 and 6 each are equipped with a vibrating feeder that conveys pellets 

to a discharge screen.
Discharge end of Furnace – Location B7

Pellet feeder – Location B8

Discharge screen – Location B9
PROCESS FAN SYSTEM

All process fans are located on the bottom floor, east of the furnace. The forced draft fan is the only 

updraft fan. The other five fans are downdraft fans. During normal operation of the process fan 

system, ambient air is drafted down through the pellet bed at windboxes Nos. 15, 16, 17, 18, 19, 20, 21, 

22, and 23 by the recuperation fan. This air passes through a duct system to the forced draft fan intake 

and also supplies roof air to the furnace. The recuperation fan has a main inlet damper and a tempering 

air damper and supplies combustion air for the pilot burners and the main burners. This fan also 

supplies roof air to the ignition zone through the 4E damper. The 4E damper is also called the 

recuperation-air-to-ignition-furnace damper. 

The forced draft fan receives a volume of air from the recuperation fan and forces it upwards against 

and through the wet pellet bed at windboxes Nos. 1, 2, 3, 4, and 5. The forced draft fan has a main inlet 

damper (10E).

Ambient air is drafted down and through the pellet bed at windboxes Nos. 24, 25, and 26 by the first 

stage cooling fan. This air passes through a duct system that has an intake damper (5E) called the firststage-cooling-air-to-preheat-and-downdraft damper. The first stage cooling fan has a main fan damper 

(3E) and a tempering air damper.

Ambient air is drafted down through the pellet bed at windboxes Nos. 27 and 28 by the second stage 

cooling fan. This air passes through a duct system and is forced down on top of the pellet bed at 

windboxes Nos. 6 and 7. This is the downdraft drying section, and the air is drafted into the waste gas 

fan. The second stage cooling fan has a main inlet damper (9E) and a tempering air damper.

The volume of air that is supplied to the firebox from the recuperation fan, the first stage cooling fan, 

and the second stage cooling fan is drawn down through the pellet bed at windboxes Nos. 6, 7, 8, 9, 10, 

11, 12, 13, and 14 by the waste gas fan. This air exhausts to the atmosphere through the Fluid Ionics 

system. The waste gas fan has a main damper (1E) and a tempering air damper.

The hood exhaust fan accepts the volume of air that is discharged from the forced draft fan and has 

been forced up and through the wet pellet bed through windboxes Nos. 1, 2, 3, 4, and 5. The hood 

exhaust fan does not have a damper.

Atomizing air is used to supply air for the gas mixture to the pilot burner, cool the flame monitors and 

the pilot burners and gas burners.