### 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 rotarykiln, 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’shad 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 ashigh 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 layerdo 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 theTinfoss 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 directreduction, 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 movethrough 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 SMEdesigns 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 theRefractory 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 pressurerequirements. 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, wasadded 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 notwork, 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. Chart1 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 alongthe 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 toMinntac’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 pelletsin 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. 

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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 DRYI NG

DOWNDRAFT DRYI NG

TEMPERE D PREHEAT

PREHEAT ZONE

ROTARY KILN ZONE 1 ZONE 2

# ~ PELLETS

### SME Annual Meeting Feb. 23-25, Denver, Colorado

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 BrickFigure 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 

### SME Annual Meeting Feb. 23-25, Denver, Colorado

5 Copyright © 2004 by SMEFigure 13: In operation.

Table 1 Operational Comparison Chart 1: Oxidation vs Airflow

Chart 2: 10 meter Drop Test Comparison 

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6 Copyright © 2004 by SME