Question 1 – Narrative Process Description of Furnaces 11 and 12

The No. 11 and No. 12 indurating furnaces provide the final step in the pellet-making process of 

converting crude ore into a finished product. In this process stage, the green balls that are delivered 

from the balling circuit are dried, fire-hardened, and cooled before being sent to the yard or loaded into 

a boat.

The furnace converts the green balls consisting of magnetite (Fe3O4) and additives into a porous, 

hardened hematite (Fe2O3) pellet capable of enduring the handling required during stockpiling, 

reclaiming, boat loading, and shipping. Daily pellet quality controls on furnace production ensure good 

performance of the product in the customer’s blast furnaces. 

The No. 11 and No. 12 grate furnaces, that consist of approximately 280 8-foot by 24-inch pallets that 

have 20-inch side plates, are continuous traveling conveyor-type furnaces that have upper and lower 

return strands of pallets. The pallets ride on the top and bottom rails and in double rails at the feed and 

discharge ends of the furnaces. Each pallet has approximately 57 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 that is equipped with reduction gears is the driving force 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.

Pellets on grate leaving firing zone – Location B4

 
Turning gear – Location B10

The upper strand of pallets accepts layer of green pellets that are produced in the balling drums. The 

upper strand of pallets also accepts a hearth layer of fired pellets. Pellets are added to the furnace via a 

roll feeder. The green pellets dry, fire, harden, cool, and discharge while the pallets ride over 27 8-footsquare windboxes. The hearth layer of pellets helps to protect the grate bars from excessive heat and 

ensures good quality pellets in the bottom layer of green balls by reducing the influence of the heat 

transfer effect.
Roll feeder – Location B1

Green balls entering furnace – Location B2

 
Hearth layer pellets in furnace – Location B3

The windboxes are connected by a series of 27 downcomers to the main furnace air ducts, breechings, 

and six process fans. Each windbox has a dust leg with a dump valve that allows dust, fines, or pellets 

that infiltrate the system through the grate bars on the pallets to be dumped. 

Windbox – Location B5

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

from natural gas (though No. 2 fuel oil is also permitted). The furnace has a fuel oil system and a 
natural gas system with 28 natural gas pilot burners and 28 main oil or gas burners. The furnace has 

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

Each furnace has a Fluid Ionics emission control system that consists of five electrostatic precipitators. 

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

gas fans. There are two precipitators for the waste gas fan and three precipitators for the hood exhaust 

fan.

After passing through the furnace, the pellets enter the discharge end system. The fired pellets are 

discharged onto a vibrating feeder, which in turn feeds the vibrating fired-pellet screen. This screen 

allows pellets that have broken up or are undersize to fall through. Pellets that do not fall through the 

screen are considered the correct size and are directed to the No. 60A conveyor for screening or to the 

No. 160 conveyor for boat loading. 

Furnace discharge – Location B7
Pellet screening – Location B8

PROCESS FAN SYSTEM

The process fan system consists of the recuperation fan, the updraft drying fan, the hood exhaust fan, 

the updraft cooling fan, the furnace fan, and the waste gas fan.

The recuperation fan receives its air volume from the recirculation hood. The air is drafted down 

through windbox Nos. 13, 14, 15, 16, 17, 18, l9, 20, and 21 on the bottom side of the pallet strand for 

the No. 11 and No. 12 furnaces. This air, which is pulled down through the bed, burns the pellet bed. 

After passing through the pellet bed, the air discharges from the recuperation fan at an average 

temperature of 800° F. The air is discharged through duct systems to the updraft drying fan intake.

The updraft drying fan receives its air volume from the recuperation fan through the duct system. The 

air volume leaving the updraft drying fan discharges through a duct system to windbox Nos. 1, 2, 3, 4, 

and 5 to the updraft drying section of machine.
The hood exhaust fan removes the moisture and waste gases that are discharged from the top of the 

pellet bed and discharges them through the Fluid Ionics dust control system. This is the updraft drying 

section of the machine and is over windbox Nos. l, 2, 3, 4, and 5.

The updraft cooling fan receives its air volume through ductwork, the inlet of which is located 

underneath the roof of the pelletizing building. For furnaces No. 11 and No. 12, the updraft cooling air 

discharges through the duct system to Nos. 22, 23, 24, 25, 26, and 27 windboxes. The air cools the 

pellet bed and supplies air to the recirculation hood. This air, in turn, is supplied to the furnace fan to 

maintain furnace combustion and pressure and to the recuperation fan for burning and drying the 

pellets. The cooling fan supplies all the process air to the furnace.

The furnace fan receives air from the top of the recirculation hood and maintains 860° F automatically, 

using tempering air from the atmosphere. This fan supplies combustion air to the top of the furnace and 

operates the automatic pressure control system for the furnace.

The waste gas fan drafts air from beneath the furnace firebox through windbox Nos. 6 through 12 and 

exhausts the combustion wastes and downdraft drying air. The fan exhausts through the Fluid Ionics 

dust control system.