ABSTRACT

In 2001, the Minntac Division of US-Steel retrofitted one of their

Grate-Kiln Iron Ore Pelletizng lines with a Ported-Kiln Air Injection

System (PKAIS) the first of it’s kind in the world for oxide pellets. The

Ported Kiln System injects air under the bed of pellets in the rotary

kiln, causing rapid oxidation of the magnetite pellets. This rapid oxidation allows for lower kiln temperatures, reducing fuel demand; less

heat load on the annular cooler; and higher pellet quality. Both the

process and the equipment are patented developments by Metso

Minerals. This paper discusses the process, the key equipment, and

the benefits to the pellet plant and the downstream blast furnace

operations. 

HISTORY

The Grate-Kiln System for Iron Ore pelletizing was developed

by Allis-Chalmers (now Metso Minerals) and first installed at the

Humbolt Mine in Michigan in 1960. In the basic flowsheet (Figure 1)

green pellets are first dried and preheated on a traveling grate, then

indurated in a rotary kiln and finally cooled in an annular cooler.

Between 1960 and 2000 improvement in heat recovery, materials of

construction and process control had significantly improved fuel and

power consumption, product quality and increased capacity.

However, the search for continued production improvements continued to be the goal of Metso. 

Testing done by Metso Minerals and others in the early 1990’s

had given a general agreement that the complete oxidation of magnetite pellets in the rotary kiln rather than the annular cooler would

result in benefits to the process. Heat and mass balance models,

along with field tests and sampling indicated a reduction in fuel use

and increases in capacity. Laboratory tests using the pot-grate and

batch kiln indicated improved pellet quality. 

Why the benefits? In the pelletizing operation with magnetite,

Fe3O4 is oxidized to Fe2O3 in an exothermic reaction. Typically, the

oxidation ratios are 70% completed on the traveling grate, 5% in the

rotary kiln and the final 25% in the annular cooler. By completing as

high as 98% oxidation at the discharge of the rotary kiln, the following benefits were expected:

• The heat load on the annular cooler is reduced because

pellets are no longer oxidized in the cooler. Therefore,

the annular cooler can cool a higher tonnage of pellets

while producing a cool product.

• The temperature in the kiln can be reduced. Since oxidation, an exothermic reaction, occurs in the pellet bed

of the kiln, there is more uniform and efficient heating of

the bed to induration temperature. The lower kiln temperature makes it possible to have pellets with high

reducibility and good compression strength without

compromising one or the other. 

• The bed in the annular cooler is normally 30 inches

deep. As air is introduced, the bottom layer of un-oxidized pellets are cooled too rapidly for oxidation to

occur. So a percentage of pellets on the bottom layer

do not reach final strength. The PKAIS uniformly completes the oxidation in the kiln, so the cooler is only for

cooling. The distribution of pellet compression strength,

and other physical attributes, is tighter. 

The design of the system proved to be an engineering challenge, particularly concerns about how to actually install a system on

a commercial scale plant, one that required over 95% availability.

DESIGN

In 1998, Minntac and Metso Minerals entered into an engineering contract to take the PKAIS from concept to practical design. The

initial concept was to utilize the standard gridded port (Figure 2) and

air distribution system used by Metso Minerals in direct reduction

plants. This was a well proven design, having been in use at the

Tinfoss facility in Norway for over 10 years. Unfortunately, during preliminary engineering serious problems were discovered making the

direct use of this design questionable. 

The maximum operating temperature of a direct reduction kiln is

approximately 1,100C. For an acid pellet pelletizing operation, the

operating temperature is above 1,300C. After investigation, no metal

could be found that would survive this high of temperature. Additional

investigations into ceramics were not successful because of the

impact duty requirements for the grid. Therefore, a different method

of introducing air into the kiln had to be considered. 

Another problem that was identified was pressure. In direct

reduction, the air is supplied over the bed of material to assist in combustion, so only low pressure air supplied by a fan is required. For

pellet oxidation, the air needs to be injected under the bed of pellets

resulting in a significant pressure requirement for the air to move

through the bed of pellets. As the air enters the bed of pellets, it rapidly increases in temperature and therefore volume. Laboratory testing and CFD modeling quickly confirmed that a blower, not a fan,

would be required to achieve the necessary pressure for a commercial plant. The number and size of openings in the kiln to meet the

pressure requirements, limited by the pressure capabilities of the

rotating manifold, continued to increase. 

Because of the problems with temperature and pressure (not to

mention concerns with fines and slagging) a number of different

SME Annual Meeting

Feb. 23-25, Denver, Colorado

Preprint 04-27

APPLICATION OF PORTED KILN TECHNOLOGY TO THE GRATE-KILN PROCESS AT MINNTAC.

J. Trescot, D. Connor, B. Faulkner 

Metso Minerals

Danville, PA

F. Beloy, S. Vogely

USS/Minntac

Iron Mountain, MN

1 Copyright © 2004 by SME
designs were tried. Even so, by early 2000, no design had met

Minntac’s requirements and cancellation of the project was being

considered. 

THE REFRACTORY CHANNEL PORT 

Metso at this point developed what has since been termed the

Refractory Channel Port design. The concept was to use the refractory in the kiln as the carrying mechanism for the air, venting into the

bed through a series of slots between bricks. This solved 2 problems.

First, the refractory already was designed for the high temperatures

and abrasive environment in the kiln.

Second, the design allowed for a large number of slots, increasing the cross-section area into the kiln and reducing the pressure

requirements. Figures 3, and 4 show the actual bricks. Figure 5 illustrates the concept. 

Metso did further CFD modeling on the Refractory Channel Port

Design to complete the estimation of performance (Figure 6 ).

After discussions with Minntac, the following ideas were added

to the design:

• The slots between the bricks was to be wider at the bottom than at the top, with a small angle, to reduce fines

entering the channel. 

• A fines return port, which fills with pellets when under

the bed and then empties at the top of the arc, was

added at the suggestion of Minntac’s operation personnel. 

• The total ported area in the kiln was increased approximately 30% more than was deemed necessary by the

model. This served several purposes; it allowed the first

port to be as far uphill on the kiln as possible, starting

the oxidation early; it further reduced the pressure

requirements. Also, the additional slot area provided

extra capacity in case the slots plugged more than

expected. Hand valves make it possible to shut off air

to different parts of the ported kiln for experimentation

with optimum air injection locations. 

• In the ported section, each ring of refractory would be

“locked” in position circumferentially by adding precast

refractory anchors welded to the kiln shell. 

• All the components had to be designed for continued

operation, if the air to the ports was shut-off the kiln had

to continue to operate. Therefore, if the design did not

work, there could be no interruption to Minntac’s operation. A “failsafe” design. 

With the above design considerations, in the summer of 2000,

Minntac placed an order with Metso for a Refractory Channel Ported

kiln supply to be installed on the Line 7

INSTALLATION AND START-UP

The rotary kiln on Line #7 of Minntac is 6.4m in diameter and

39.6m long. 12 rows, with 8 ports per row, were installed in a 14.6m

section of the rotary kiln. Each row has a tripper valve to allow or shut

off the air flow to that row. Air is supplied by a rotary lobe blower, one

operating and one standby, installed in a separate blower building. 

Installation of the ported kiln took place during an extended 21-

day annual maintenance shutdown. Actual work required 19 days,

plus 1 day for cool down and 1 day for heat up. Figures 7, 8, 9 and

10 shows the various phases of installation. Figure 11 illustrates the

air injection sequence. Figure 12 shows the completed installation.

Figure 13 shows the inside of the kiln during operation. 

RESULTS

The installation of the Ported Kiln System was predicated on

improvement to the operation. Baseline testing of Line 7 to determine

the target for the Ported Kiln improvement was done in the Fall of

2000. A wide variety of measurements were done around the entire

induration system including green ball analysis, temperatures and

gas flows, equipment speed, utility consumption and product quality.

This information was modeled in Metso Minerals proprietary

Heat and Mass balance program for simulation of operation with the

ported kiln to set expected performance. These measurements were

also used to set the same operating conditions for ported operation.

Table 1 compares the Baseline operation with the actual performance of the Ported Kiln system under similar operating conditions. 

In all areas, there was improvement. The reduction in fuel use

and the increase in reducibility are related to the lower burning zone

temperature. The improved LTD and compression strength relates to

the complete oxidation occurring in the kiln, not in the cooler. Chart

1 shows the oxidation level and air requirements for each rate of production.

One key target was not reached, and that was an increase in

average compression strength shown in Chart 2. The improvement

was not as great as expected from the original testing. The problem

became clear when the comparison was made that the testing was

done with acid pellets, versus the fluxed pellets currently produced at

Minntac. 

Since start-up, Minntac has done testing varying the amount of

air injected under the bed and the location of the air injection along

the length of the ported kiln to determine the effect on pellet oxidation.

Over the past 1-1/2 years of operation, Minntac has seen other,

less quantifiable benefits to Ported Kiln operation. These include:

• More stable operation

• Greater operator control of product quality

• Cool pellets from annular cooler, less maintenance on

the annular cooler and discharge belts

• Reduction or elimination of kiln ring formation. The ports

create areas that are free from slag build-up, thereby

not allowing a complete ring to form. This makes the

slag that does form structurally unstable. 

There are also expected benefits downstream from the pellet

plant because of the higher quality pellets being produced. Higher

R40 Reducibility means that less coke is required per ton of hot

metal. Higher LTD means fewer fines in the Blast Furnace. 

CONCLUSION

The Line 7 Ported Kiln operation proved to be beneficial to

Minntac’s goal to reduce operating costs and to improving pellet quality. This successful application of the PKAIS to Minntac’s Line 7 has

resulted in an order being placed with Metso Minerals for a second

Ported Kiln System that was installed on Line 6 in the spring of 2003.

Further investigations are also underway on installing the Ported Kiln

System on Lines 4 and 5 in the future. 

The concept of injecting air under the bed of magnetite pellets

in a Grate-Kiln pelletizing system to improve performance and product quality has been verified. A unique system for getting the air into

the bed, with high reliability, has also been proven. A proposed next

step in the technology is to evaluate the admixture of carbon in the

pellets. This could improve performance in a magnetite operation and

also make it possible to apply the Ported Kiln to hematite pelletizing

operations. 

SME Annual Meeting

Feb. 23-25, Denver, Colorado

2 Copyright © 2004 by SME
BIBLIOGRAPHY

R. Potts, “Induration of Taconite Pellets using Grate-Kiln System

at Minntac”, Skillings Mining Review, USA, August 1991.

D. Rierson, “The Ported Kiln Concept as Applied to the GrateKiln System”, SME Conference, Duluth, USA, 1993.

J. Trescot, D. Connor, B. Faulkner, “Optimization of the Iron Ore

Pelletizing Process”, ISS Ironmaking Conference, Pittsburgh, PA,

2000.

L. Hass, J. Nigro, R. Moe, “Use of Oxygen-Enriched Gas for the

Oxidation of Acid and Fluxed Magnetite Pellets” 

State of Minnestoa Department of Natural Resources, 1996.

C. Urban, “Summary of CFD Study- Refractory Channels in

Ported Kiln”, Internal Metso Report, 2000. 

David W. Rierson, US Patent No. 5,248,330 September 28,

1993”Method of Pyroprocessing Mineral Ore Material”

Neil R. Dock, Edward T. Maciejewski, US Patent No. 6,309,211

October 30, 2001”Port Air Conveying System for Rotary Kiln”

James D. Foresman, US Patent Application No. 10/053,196

January 15, 2002 “Butterfly By-Valve Actuation System for Ported

Kiln”

SME Annual Meeting

Feb. 23-25, Denver, Colorado

3 Copyright © 2004 by SME

Figure 1: Grate-Kiln Flowsheet

Figure 2 Tinfoss Ilmenite Reduction Kiln

DOWNDRAFT DRYING 

DOWNDRAFT DRYING 

TEMPERED PREHEAT 

PREHEAT ZONE 

ROTARY KILN ZONE 1 ZONE 2 

PEL

~ LETS 
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4 Copyright © 2004 by SME

Figure 3: Refractory Channel Brick Figure 4: Refractory Channel Brick

Figure 5: Port Section

Figure 6: Temperature gradient in Refractory Channel Brick
Figure 7: Preparation

Figure 8: Installing Ducting and Port Castings

Figure 9: Installing Refractory

Figure 10: Slots and Fines Return Brick

Figure 11: Air Injection Sequence

Figure 12: Installed Ported Kiln Assembly

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Figure 13: In operation.

Table 1 Operational Comparison

Chart 1: Oxidation vs Airflow

Chart 2: 10 meter Drop Test Comparison

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Feb. 23-25, Denver, Colorado

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