# SOCIETY-OF**MIN~N ENG~NEERS'**  OF 

# AI

f

# E

CALLERNO.D,LITILETON,COLORADO80127



## RECENT DESIGN IMPROVEMENTS IN THE GRATE-KILN SYSTEH

George J. 

## Skoronski

Leonard J. 

## I.Joida

Allis-Chalmers Corporation Hilwaukee, Wisconsin 

# PREPRINT NUMBER**f~f** 83-355

For presentation at the**SME-AI~lli** 

## Fall Meeting and Exhibit Salt Lake City, Utah

- October 19-21, 1983



## Permissionisherebygiventopublishwithappropriateacknowledgments, excerptsorsummariesnottoexceedone-fourthoftheentiretextofthepaper. PermissiontoprintinmoreextendedformsubsequenttopublicationbytheInstitute mustbeobtainedfromtheExecutiveDirectoroftheSocietyofMiningEngineers ofAIME. If

## andwhenthispaperispublishedbytheSocietyofMiningEngineersofAIME,

## it mayembodycertainchangesmadebyagreementbetweentheTechnicalPublications Committeeandtheauthor,sothattheforminwhichitappearshereisnotnecessarily thatinwhich

## itmaybepublishedlater. Thesepreprintsareavailableforsale.MailorderstoPREPRINTS,SocietyofMining Engineers,CallerNo.D,Littleton,Colorado80127.



## PREPRINTAVAILABILITYLISTISPUBLISHEDPERIODICALLYIN MININGENGINEERING"RECENT DESIGN IMPROVEMENTS IN
THE GRATE-KILN SYSTEM" by George J. Skoronski Senior ApplicationEngineer
and
Leonard J. IToida Senior ApplicationEngineer
of the
Allis-Chalmers Corporation P. 0. Box 512 Milwaukee, Wisconsin 53201 

### (414) 475-4802

Introduction
The economics of pellet plant operation, in terms of fuel and power consumption, flexibility in the selection of the cheapest fuel source, system availability, maintenance costs, and the simplicity of process design, contribute to the determination of building and profitably operating a pellet plant. Design innovations, supported by actual field data, have been initiated into theGRATE-KILN System design to positively address these factors. This paper presents the recent state of the art design concepts which have helped the GRATEKILN System assume worldwide acceptance status.

I. Process Design Improvements
Heat Recuperation System

a. Conventional Plant Design
The GRATE-KILN Systemhas changed significantly indesign concept since the first systems were commissioned for Cleveland Cliffs Iron Company in the early 1960's through the most recent startup of the LKAB KP-79 plant in Kiruna, Sweden in 1981.
Initial GRATE-KILN plants were designed with no heat recuperation systems and typically consisted of a two pass, downdraft drying and downdraft preheat grate cycle. Apreheat bypass systemwas installed to complement the preheat off-gas supply so that these two streams delivered the required downdraft drying heat supply. Figure 1depicts a conventional plant design employing a two pass cycle.
From the two pass downdraft cycle, the next innovation of the process cycle appeared with an updraft drying zone. This updraft drying zone was initiated to circumvent problems associated with collapse of the pellet bed encountered with certain types of green balls. 83-355 The updraft drying zone was followed by a shorter downdraft drying zone which prepares the pellet bed for entry into the hotter preheat zone. b. Induced Heat Recuperation

c. The first GRATE-KILN System with heat recuperationwas supplied to SydvarangerAB in Kirkenes, Norway. This facilitywas commissioned in 1972. The heat recuperation design consisted of a duct from the annular cooler which was introduced into each preheat fan via the outlet chamber of the preheat cyclone dust collectors. The intent of this designwas to replace the process gas normally bypassed from the preheat furnace which, when combined with the preheat off-gas streams, comprised the drying requirement. The plant would start up in the bypass mode, and once the cooler was filled with a hot pellet charge, the recoup flow was incrementally introduced**t~** replace the bypass flow. In the recoupmode, the total drying heat and mass requirements were then composed of recoup gas and preheat off-gases. The recoup temperature was adjusted to maintain a maximum drying temperature compatible with the preheat fan construction and with the green**h~ll** characteristics.

Figure 2 depicts a preheat fan induced type heat recuperation system.
This type of recoup design induces the required process gas flow via the pressure head available at the preheat**fa~** inlet ducts. Since Sydvaranger, th1s type of heat recuperation system was installed on three new plants and as a retrofit on onemachine.

Dedicated FanHeat Recuperation
A conservative approach to heat recuperation design came about with the installation of a dedicated heat recuperation fan. This fan induces flow from the annular cooler into the recoup fan and subsequently pumps process gas directly to a process zone of the grate. Process gas flow represented by this recoup tan complemented the preheat off-gas stream to deliver the required mass and heat to the drying zone. In specific**circu~stances,** this recoup fan delivered process gas to an air heacer, for use in a dehydration zone. The dedicated fan approach was installed in three magnetite plants and six hematite plants both as original design and retroEit equipment.

Figure J depicts a dedicated fan type heat recuperation system.d.**Hi~h Teooer~ture**  Induced Heat Recuoer.:ltioo Ao.:!lysis of both the induced and dedicated fan approaches to heat recuperation revealed deviations betweendesign conditions and actual field data. Paramount among the deviations was the temperature of the heat recuperation gas. Calculation technique at the time of the aforementioned designs predicted 

### anominal4oo•c recoup gas temperature. Field experience consistently produced a source teaperature in the range of 500 to Sls•c. As
a result of this difference, the recoup gas required tempering with ambient air especially in those plants with dedicated recoup fans. This tempering premise was recognized as inefficient since the hot gas had to be cooled, and in certain instances reheated in plants equipped with air heaters for dehydration. In plants**utilizin~** the induced mode of heat recuperati;n into the preheat fans, it was found that due to a lower than exoected average preheat off-gas temper-**~ture** predicted from calculation and test data, the recoup gas temperature had to be higher thandesign level to produce a downstreamblend at the desired drying temperature.
Since the high heat value of the recuperation gases was consistently evident in our GRAIE-KILN pelletizing plants, using these gases directly**i~to** the process evolved. This idea was implemented by Cleveland Cliffs Iron Company in theirEmpire IIIunit. This unit originallyhad a dedicated fan built for 

### 42s•c operation. In lieu of installing a fan to operate at 

## 6oo•c, the original rotor was removed, a refractory lining installed where -requi-red, and the recoup gas was •Jsed directly on the grate by inducing flow with the existing waste gas fan. Figure 4 depicts a high temperature heat recuperation systemvia a single duct from the cooler.
The LKAB KP-79 plant offered another refinement to heat recuperation with four ducts from the cooler which feed updraft drying, downdraft drying, tempered preheat, and preheat via the kiln respectively. The objective of this designwas to provide theminimum quantity of high quality heat for each zone on the traveling grate. Results of this designwere reflected in low fuel consumption and low process fan power consumption.

Figure 5 depicts the schematic of a high temperature heat recuperation system withmultiple ducts from the cooler. 2

e. 

### 83-355
'lariations 'stemcing from the basic high temperature heat recuperaation system are contingent upon the temperature distribution of gases leaving the bed of pellets in the cooler. Figure 6 shows a typical curve of offgas temperature versus mass flow of cooling air. Adecrease in the amount of gas flowing into the kiln increases the gas temperature slightly. Such a change, however, causes a more distinct increase in the recoup gas temoerature. As a result, decreasing'the flow of gases from the cooler/kiln/preheat zone route can increase the overall heat efficiency of the process.

Advantages ofHigh TemoeratureHeat Recuperation
The high temperature recoup system offers the following advantages over earlier recoup designs:
1. Significant fuel reduction by taking advantage of the heat transfer capability of recoup high temperature gases.
2.

3.



### 4.

5.



### 6.

7.

8.

9.

10. Fan power nation of fan. reduction by a dedicated elimirecoup

Fan power and size reduction in the preheat fans by eliminating the requirement for these fans to handle the entire dryingheat and mass load requirement.
Ashorter kilndue to the increased strength level of pellets entering the kiln resulting from improved thermal processing in the grate. Approximately 90% oxidation rate on the grate for magnetite which impacts positively on process fuel consumption and preheat fan power draw. Lowering the cooler mass ratio requirement for magnetite plants which reduces cooling fan power consumption. Complete process recovery of all sensibleheat in the cooler.
Reduced process gas flow in the waste gas streams due to thehigh gas temperature/low mass ratio relationship for a required kilocalorie input level.
Minimum number of process fans for an induration system.
Avery simple process design for the operator to understand.3 83-355

f. Results of Heat RecuperationDevelopment on Fuel Consumption

PlantDesign
No Recoup

- Magnetite No Recoup

- Hematite Induced Recoup

- Magnetite Induced**R~coup** - Hematite Fuel Consumption 137-162,000**KCAL/~IT** 225-275,000**KCAL/~**

88-115,000**KCAL/~** 200-225,000**KCAL/~** High Temperature Induced Recoup

- Magnetite High Temperature Induced Recoup

- Hematite 50-75,000 KCAL/MT 175-200,000 KCAL/}!T
Fuel Consumptionbased on gross heating value of fuel.

g. Results of Heat RecuperationDevelopment on Fan Power ConsumPtion

Plant Design
No Recoup

- Magnetite Induced Recoup

- Magnetite Fan Power Consumption

High Temperature Induced Recoup

- Magnetite 10-14 KW-HR/MT 8-12**KI{-HR/~** 6-8**K'..T-HR/~**

One must bear inmind that a variety of factors can affect fuel consumption from plant to plant even though the ore type is similar. In addition to the intrinsic value of the heat recuperation system, such factors as:

1. magnetite content of the ore,
2. L.O.I. content of the ore,
3. green ball moisture and temperature,


### 4. greenball quality as it relates to recovery rate, 

### 5. green ball bed depth as lates to retention time grate,
6. plant production rate, it reon the

7. presence of additives or ore constituents which create endothermic or exothermic reactions in the process,
8. general physical condition of the operating plant in terms of air seal condition on the grate and kiln, wear condition of grate components, and cooler pallet alignment,
9. kiln burner stoichiometric air design, l. 10. refractory mix throughout the system,
11. circulation of warm air into the primary cooling fan inlet,
12. operator understanding of the process, all combine to enhance or detract from a plant's fuel consumption.
Mechanical Design Improvements Traveling Grate
The main purposes of the design improvements on the traveling grate were:

a.

b.

c. to control the transverse movement and the gapping of the chain components,

to improve sealing to minimize ambient air inleakage, and

to improve machine availability and the ease ofmaintenance.

a. Control of Transverse and Axial Gapping in the Conveyor Chain
Control of the transverse and axial gapping was gained by close control of tolerances on the various grate plate and chain link castings which comprise a pitch assembly. Control of the gapping is important because minimumgapping minimizes material fall through between adjacent grate plates and between chain castings and grate plates. By eliminating significant fall through of material, weight recovery in the process is improved and lese dust and chips arehandled in the recycle system.
Another important advantage mizing material fall through grate is that**m~nunum** gaps uniform heating of the pellet ofminion the promote bed and a more even temperature gradient through the conveyor chain. Any gaps which form on the chain allow channeling of nominal 1000•cprocess gases which can overheat specific areas and necessitate frequent parts replacement and reduce plant availability. Channeling of hot process gas can also attack interior parts of the chain so that the inductionhardened areas of the chain link and through rod are tempered and become soft. Once this softening action occurs, wear between the chain link/through rod interface accelerates which requires more frequent replacement of these components. The inductionhardened area of the chain linkbore and through rod which interface is 600 Brinnell. Maintaining the integrity of the wearing parts of the chain is therefore very important for chain life, plant availability,'.and minumum maintenance.
As mentioned above, primary control of the axial gapping is controlled by close casting tolerance of the chain links and grate plates. If all chain castings and grate plates are moved to form a single gap across the width of the chain, the total accumulated gap would be only 13.2 millimeters on a 4.6 meter wide grate. In addition to casting tolerance control, the pipe spacer which fits or slides on the through rod is now a thicker wall pipe. The present design has a 77% increase in contact area between the pipe spacer and chain link over the previous design. This larger contact area prevents mushrooming of the pipe spacer wall so that predetermined gaps in the pitch assembly are maintained. Maintaining the integrity of the pipe spacer eliminates its chance to move or slide on the through rod so that gaps cannot accumulate in one area which in turn promotes leakage of pellets through the chain, and the resultant problems with overheating and replacement of the affected parts.

New through rods are threaded on either end so that the axial movement is better controlled since a nut replaces che original clip design. 4

b.

c. 83-355
Transverse gapping is improved by supplying a larger diameter through rod which increases the bearing area of the through rod/chain link interface by 18% over the previous design. This feature along with inductionhardening of the contact areas thus controls the transverse gapping more effectively.
The net result of the improvements to chain design is to increase chain life to a projected44 to 52 month schedule versus the 22 to 26 months schedule of the original design.
Seal Improvements of the Traveling Grate
Ambient air inleakage into the process gas stream is undesirable because ambient air depresses the system temperatures. Lower process gas temperatures reduce heat transfer effectiveness which can promotehigher fuel rates and higher process fan power due to the larger quantity of denser process gas to be handled.
Two areas of grate sealing have been improved to reduce the detrimental effect of leakage. The upper support shafts (i.e., thosewhich have carrying rollers which actually support the conveyor chain) now have double ring carbon seals on either side which replace the original metal labyrinth seals. The carbon rings are split into 

## 1so• segments and are held in place by circular springs so that contact between the shaft and seal is always maintained. This positive seal between the windbox and atmosphere minimizes leakage.
Side castings have an improved gravity type air seal which floats on the upper surface of the side casting. The seal width.has been increased to allow some movement of the side casting. The seal and its housing have facility tomove in thevertical and horizontal direction so that proper contact between the seal bar and moving side casting is maintained. This side casting seal reduces air infiltration between the atmosphere and furnace (overbed chamber), and between the furnace and the windbox (underbed chamber). In addition, the side casting has a wear shoe attached to the underside. This wear shoe rides on a lubricated stationary sealbar to form an air seal. This seal eliminates ambient air leakage into the process gases.
Imorove Machine Availability and Ease of Maintenance
The important features which contribute to transverse gapping control

### have beendescribed above. 

### As previouslymentioned, the increase inthe through roddiameter coupled with induction hardening of the through rod chain link interface will be the prime factors for a longer chain life. The life is now projected to be in the range of 44 

### to 52

### months. This projectionis based on dataobtained from operating plants with similardesign improvements.
Grate plates supplied on the traveling grate are one**p~ece,** 

### cast, high grade stainless steel units. These units are extremelyheat resistant and replacement is usually dictated only if one shouldbreakprbecome exposed to a direct flow ofhot preheat zone ongas. Side castings are equipped with cast iron thrustbuttons which are used in conjunctionWith thrust brackets to correctly trackor guide the grate chain. Thrustbrackets are located on each furnace steel column and are easily accessible and adjustable should the grate chain require tracking adjustment. Using this thrust bracket/thrust button approach to guide the chain, the requirement for guide sprockets on the upper support roller shafts and for thrust rollers on the returnassembly are eliminated. Less hardware and less maintenance is required.
Side castings have a replaceable wear shoe on the bottom of the casting. This small shoe eliminates the need to replace the entire lower casting of the sidecasting assembly and this piece is replaced very easily in the repair zone. This feature again saves hardware and time required for repair versus the previous design.

The drive for the traveling grate is is a headshaft mounted unit wherein misalignment between a bullgear and pinion is eliminated. This new drive assembly is self-contained and has a built in circulating lube system, thus eliminating the previous spray lubrication system for the gear and pinion.

Headshaft sprockets of the traveling grate now have replaceable stellite inserts in each tooth. On the previous design, the entire sprockethad to be replaced when worn as only a 

### welded hardened overlap on the tooth was present to take the wear action between tooth and chain. The new designed inserts are replaced without sprocket removal and in a fraction of the time versus the previous design. 5

2. 

### 83-355
Ahydraulic tailshaft takeup assecbly is now designed into the traveling grate. The tailshaftwas historically fLxed to the frame of the grate.**w~enever** 

### the grate chain required tightening, due to progression ofwear with time, the gratehad to be shut down for this purpose. Now pitch removal, when required, can thenbe performed during a scheduledmaintenance period.

The traveling gratebleeder stack cap has beenredesigned such that a pivot principle is now employed, The previous stackcap assembly required a series of chains and heat shields to lift the cap. The new designhas the pivot point away from the heat source and fewer parts are required to lift 

### the cap itself.

Headshaft bearings on the traveling gratehave beenextended to the outboard side of the headshaft. This movement permits easiermaintenance of these bearings, while the cooler environment prolongs bearing life.
The net result of the above described design improvements on the traveling grate is to provide longer machine life, easier maintenance, fewer replacement parts, and improved process thermal efficiency.

Rotary Kiln
Kiln feed and discharge end air seals have been improved to further minimize ambient air inleakage to the kiln.
In order to improve the kiln feed end**a~r** 

### seal, the discharge housing of the grate, upon which the feed end air seal assembly is located, will be canted so that its face is parallel to the sloping face of the kiln. The seal assembly seating against the entire circumference of the rotattng flange of the kiln will, therefore, be improved. In order to insure contact between the stationary and rotating seal faces, air cylinders will be utilized to assist 

### in positioning the stationary seal face against the kiln. Air cylinders will be located in each 90° quadrant of the seal. This design improvement along with lubricated, machined surfaces of the seal faces, will further reduce ambient air leakage into the kiln. These design changes combine to further improve the already**L~pres** 

### sive fuel efficiency of the GRATE-KILN Systern.). 

### Annular Cooler
The 

### primary purposes of the 

### design changes 

### in che annular cooler are:

a.



### b.**cin~ize ~bienc**  

### air inleakage to the process; improve machine availability and ease of

### maintenance.

a.**Mini~ize** 

### Ambient Air lnleakage to the Process

b. 

### The sand seal between therotating and stationary wall has beenrepiaced by 

### a water seal. The water tank is attached to the rotatingwall with the seal plate being part of the stationary assembly above the rotating structure. Designing a seal withminimum maintenance and operating longevity is imperative from the standpoint of process efficiency. The water seal allows the primary and final cooling fans to deliver the required gas flow to the process without any detrimental effects to the equipment. The nature of the cooling process**~parts** 

### a slightly positive pressure slightly positive pressure above the pellet bed near the area of the seal. The seal design intent is to prevent the**flo~** 

### of ambient air into the process, which dilutes process gas temperatures and restricts the flow of cooling air through the pellet bed. Restricting the flow of cooling air through the bed retards the completion of oxidation and achievement of requisite product strength for magnetite ores. Rate of process gas flow through the cooler bed has been demonstrated to affect Low TemPerature Breakdown characteristics of certain pellets. The water seal, which oromotes control of chis flow rate,**c~n** 

### therefore impact favorably on this metallu,gical**cha~­** 

### acteristic.
Rate of process gas flow through the cooler bed has been demonstrated to affect Low Temperature Breakdown characteristics of**cer~ain** 

### pellets. The water seal, which promotes controlof this flow rate, can therefore impact favorably on this metallurgical characteristic.

Improve**~achine** 

### Availability and Ease of**~laintenance** 

### 7newater seal design, in addition to its process merits as outlined above, has also improved cooler availability. An automatic trough fill systemmaintains wacer level to compensate for evaporative losses. In addition, a pneumatic trough pump is employed**~o** 

### 6

remove pellet**b~ildup** 

### in the trough tank.

Cooler pallets are now fabricated steel pieces which replace the 

### original cast iron pallets. Fabricated pallets are easily repaired in place**~~ich** 

### has reduced maintenance time and expense.

CoalFiring
ProvenField**Perfo~ance** 

### The GRATE-KILN System significantly reduces pellet production costs by utilizing coal in place ofmore expensive liquid or gaseous fuels. Coal is directly fired in theGRATE-KILN System eliminating need for external combustion

### cham~ bers, slag tapholes, auxiliaryburners or special high priced refractories. Faulkner, Losin, and Cnare 

### (1977) presented a historical sketch of coal firing in the GRATEKILN System that dated back to a 

### 1962 commercial scale test. 1ney also presented laboratory combustor test results that related coal and coal ash properties to plantperformance. Combustor testwork and commercial coal firing have continued providing a stronger statistical base for coal characterization. The number of coals tested approaches fifty. At least ten GRATE-KILN plants are now equipped to fire coal typically at 

### 100% of the fuel requirement

### durin~ normal operation. Presentations by: Murr 

### (1982); Johnson 

### (1982); Krollman 

### (1982); Lopez, Honkonen, and Potts (1982) have described coal firing systems at various GRATE-KIL!i Systems in considerable depth. Typical size and moisture control of kiln burnercoal feed is SO to 90% passing 

### 200 mesh at one to**t~o** 

### percent free moisture. Burner tip velocity ranges from 

### 2750 to 

### 4575 meters per**U~inute.** 

### Recent field improvements which boost

Statistical Analysis of Laboratory Results
Table 

### 1 shows the relationships**bet~een** 

### various coal and coal ash variables and laboratory deposition rates at three temperature levels. The relationships areexpressed in terms of correlation coefficients that describe the statistical significance of a given variable relative to the deoosition rate obtained at a particular

### temperatu~e. The closer the correlation coefficient is to one, the better the fit of data to a straight line plot. The most si;nificant variables are coal heating value, and fixed carbon and ash contents. It should be noted that each of thesevariables is interrelated to the other. Basically, as ash content rises the fixed carbon content and heating value decrease. Coal heating value increases directly with increase in fixed carbon content. Correlationcoefficients for ash to heating value and fixed carbon to heating value were found to be in the 

### 0.73 to 

### 0.77 range. Basic ash depositionbehavior, therefore, is de-7 

### 83-355 pendent on these interrelated coal characteristics. The term Coal CharacterizationValue, or CCV, has, therefore, beendeveloped to express this relationshiprelative to deposition rates. It is defined as; for any singularvariable or any of the ten other parameters studied. This was true over the range of temperatures investigated. Acoal having a CCV of 0.8 or less is recommended for GRATE-KILN Systems operating with fuel consumptions of 125,000kcal/tonne or less. Recommended CCV increases ordecreases depending on the particular plant fuel rating. For example, a coal with a 0.55 CCV should burnvery well ina plant processing magnetite at 75,000 kcal/tonne, but its relative performance may well be expected to decrease ina similar plant processing hematite at 200,000kcal/tonne. CCV (Coal Ash Content inPercent byDryWeight) X 

### (11.6 x 

### 107) (Dry Coal Heat Content inJ/kg) (Coal Fixed

Carbon Content inPercent by Dry Weight) Correlationcoefficients obtained when relating CCV to ashdepositionrates are higher than

TABLE 1**CORREL~TION** 

### COEFFICIENTS RELATING COAL**~~D** 

### COAL ASH**COMPONE~1S** 

### TO LABORATORY DEPOSITION RATES Laboratory DepositionRate at:

Coal Related Variables
Fixed CarbonContent Volatile Content Heating Value Ash Content Sulphur Content

Ash Related Variables
Ash CaO Content Ash MgO Content Ash Na20 Content Ash K20 Content Ash Si02 Content Ash Al203 Content Ash Ti02Content Ash Fe2

## o 3 Content

Ash FusionLevels
Initial DeformationTemperature Softening Temperature Hemispherical**Te~perature** 

### Fluid Temperature

Parameter
Coal CharacterizationValue (IT) 

### (ST) (HT) (FT)

TABLE 2 925•c



### 0.62 0.17 0.75 0.57 0.31

0.67 0.34 0.31 0.35 0.15 0.43 0.21 0.12

0.20 0.21 0.23 0.26

0.79 104o•c

0.69 0.17 0.82 0.78 0.35

0.56 0.19 0.07 0.32 0.02 0.40 0.04 0.04



## o.so 0.46 0.48 0.10

0.86

THE EFFECT OF DUST LEVEL AND TYPE 011 

### OXIDIZING ASH FUSION TEMPERATURES <

## •cJ
Ash Fusion Ash 

### Ash Plus Ash Plus 

### Ash Plus

Leve1 

### Alone 

## s::: PH Dust 

## s: Pellet Dust 

### 25% PH Dust 

## zs:::

!T 

### 1218 1171 1182 1482+ 

### ST 1285 1371 1427 1482+ HT 1307 1410 1460 1482+ 

## rr 13t.6 

### 1482+ li.82+ 1482+ 115o•c



### 0.71 0.17 0.81 

## o.so 0.37

0.46 0.13 

## o.os 0.23 0.11 0.40 0.07 0.05

0.44 0.43 0.43 0.02

0.88



### Ash Plus

Pe11ec Du3t

1482+ 1482+ 1482+ 1482+Significance of Coal Ash FusionTemoerature
CCV also describes plant performance rather**~ell.** The sole exception to its indicator status is a 1975 test run in which the coal tested was found to be contaminated with millscale and other debris. Such unoxidized iron is known to depress coal ash fusion temperatures dramatically thereby contributing to ash buildup potential. On the otherhand, additions of as little as five percentpreheated or fired pellet dust to coal ash canraise the ash fluid temperature as much as lJs•c. The effect of preheated and fired pellet dusts on ash fusion temperatures is shown inTable 2. This effect is very important as the relative coal ash to iron ore material input is typically at the one-tenth of one percent or lower level. Airborne pellet dust concentrations are estimated to be at least twenty-five percent of the coal ash content. It is, therefore, doubtful ifpure coal ash fusion data bears any significance in terms of**nrP~i~~­** ing buildup tendencies.

When coals with high CCV terms must be used as GRATE-KILN System fuels, it is possible to utilize a coal cleaning circuit to upgrade that coal to an acceptable level,

DEPOSITION RATE (Mg/Min) 

### 40



### 30

20

10 

# Coal Component AsReceived 83-355
Table 3 shows how laboratory ash deposition rates canbe lowered by utilizing coal cleaning· circuits. CCV has been cutbymore than half and ash fluid temperature increased by about lJs•c. The change inbulk ash fusion tempera-· ture is explained by considering that coal ashes contain a variety ofminerals, some more easily·. removed by cleaning than others. This further' indicates that pure coal ash fusion data cannot be relied upon to predict coal performance in. the GRATE-KILN System. There have been cases**'>~here** coals with ·ash: initial deformation temperatures of 115o•chave**lo~er** deposition rates than coals with greater than 1480°C ash initial deformation temperatures. Coals with ash fluid temperatures in excess of 1480°C have had four times the deposition rates of coals with ash fluid temperatures less than 1370°C. Auxiliary**Syst~ms** Auxiliary Eouipment Utilization and Design Improvement**AS~de** from the primary thrust of innovation into basic process and mechanical design improvements, attentionhas also been directed' toward updating peripheral equipment which im'- pacts in plant efficiency.

Ash 

### 11.50 wt.pet. 

### 6.00 CCV 

### 1.21 

### 0.54
AshFT. 

### 1346 1482+

1150 1100 

### 1040 

### 985
DEPOSITIONTEMPERATURE(0

### C)

Table3. LaboratoryDepositionRatesForLowAshFluidTemperatureCoalBlending High intensity**m~xLng** bymuliers or cylindrical shaft and plow type mixers has been shown to decrease the amount of binder required for proper greenball strength development. Decreased binder additionrates tend to reduce greenball plasticity and, therefore, increase permeability of the traveling grate bed leading to improved fuel and power consumption rates. Pellet ironcontent favorably increases. Pellet alkali and silica levels are decreased inplants using bentonitebinders.
KilnBurners
The most significant design improvement relative to kiln burners has been the reduction in stoichiometric air required for combustion in oil and gas burners. Present designs utilize 4 to 6 percent combustion air relative to 25 to 30 percent- in previous applications. This design improvement decreases ambient air input to the kiln and allows substitution of high temperature secondary air into the process. Reduction in fuel consumption and increased flame temperature, which promotes improved heat transfer, result from this design improvement. Petroleumcoke, with a nominal 11 percent volatile content, has been successfully introduced to burn simultaneouslywith coal in an operating plant. Burner suppliers contend that state of the art burners cannow effectively utilize 100 percent low volatile solid fuels. This design improvement opens up markets for more cost efficient fuel relative toEastern type coals most commonly used at the present time.
Rubber Lined Balling Drums
Rubber lined balling drums are being used in at least seven pellet plants with somewhat mixed results in terms of performance. These vessels operate without cutterbars. The drum interior is lined with a series of parallel, longitudinal, rubber belts which are anchored to the shell along the belt edging with metal strips. Flexing or "droop" of the belt during drum rotation purges concentrate from accumulation on the lining, Benefits derived from a rubber lined drum circuit are lower drum power consumption, decreased maintenance cost and lower caoital cost expenditure. The verdict on**bentonit~** consumption and greenball quality is not yet definitely established. Efficient greenball screening, more difficult in a rubber lined due to the lower drum speed, which however, is drum circuit affects distribution of the drum discharge off the scroll onto the screen. Some operators report that more screening area is preferred in a rubber lined drum circuit, compared to drum circuit, to maintainmaximum

Programmable Controllers a conventional capacity.

Solid state programmable control technology is being employed in pelletizing plants and it has the following advantages over the relay type systems: 9
1.

2.
3.


### 4.
5.
6. 83-355
reduced cabling ahd associated labor costs; simpler logic changeout; higher operating reliability;
lower power consumption;
streamlined troubleshooting capability; reapplication potential; and
7. reduced area requirements. Improved process stability, in terms of a tighter range of system variation, can be achievedwith this type of instrumentationwhich contributes to improved overall fuel consumption, especially at lower plant capacity and in periods of process upset.
Variable Speed Process Fans
Utilizationof- variable speed fans in place of constant speed dampered fans results in several operating advantages. These include; reduced power consumption, lower fan liner and wheel wear, lower noise levels, softer starts with less stress on associated equipment, unlimited motor starts, and no systemvoltage drop problems onmotor starting. These systems have higher installed costs in comparison to the constant speed dampered fan systems. Decrease in plant process fan power consumption, however, candevelop a payback in additional invested capital of less than two years in a typically sized pellet plant.

Future Considerations
New Applications
One of themost interesting potentials for the GRATE-KILN System is the ability to produce reduced pellets using the oxide grate in conjunction with a reduction kiln. Conventional direct reduction feedstock pellets are produced by high temperature induration, at about 

## 13oo•c, followed by a cooling step that lowers pellet temperatures to near ambient. These pellets are then reheated in the subsequent rotary reductionkiln at temperatures near the

## Iooo•c level. By coupling an oxide traveling grate preheater to a direct reduction kiln this situation canbe averted. Also, the oxide pellet kiln and annular cooler are removed as mandatory equipment, making this coupling very attractive economically. Allis-Chalmers has performed extensive research on this process concept. One of themost beneficial results of this research was that preheated pellets survived transport through reduction kiln simulations with equal or less degradation than fully indurated pellets. This was true for three different feed materials as shown inTable 4, Reduction characteristics of preheated pellets were found to be very similar to those of fully indurated pellets.10


### Lover Cost Fuels**~ith** 

### the advent of improved burner design has come the opportunity to use fuels previously not suitable for the GRATE-KILN System. One example is petroleum coke vith a formerly prohibitive lovvolatile content of about eleven percent. These fuels, vith extremely lov ash levels hold significant promise, at least as a partial replacement for coal. Successful in-plant tests have been completed using 25 and 30 percent petroleum coke addition to the coal mill circuit. The limit of substitutionvill most often be predicated on the sulphur level of the particular petroleum coke used. Economic justification for petroleum coke utilization is significant. Delivered fuel cost at the burner can be reduced by as much as forty percent relative to an Eastern

## u.s. coal. Atypical comparison or petroleum coke and bothEastern and**~estern** 

### U.S. type coals is presented inTable 

### 5. Improved "Pellet" Shaoe for Blast Furnace Burdening
Pellets tend to migrate tovard the center of blast furnace burdens due to relative lov angle of repose in comparison to other charge materials. This can be corrected somewhat by 83-355 charging methods and procedures, but potential :or additional improvement exists. One method or 1ncreasing "pellet" angle of repose is to crush large pellets into smaller pieces as described by Taguchi et al (1983). This method has tvo potential problems associatedvith it. The energy required to indurate large diameter pellets should be greater than for more typical 

### -15 +6 mm 

### diameter pellets. Also, the crushed pellet fines require additional processing, perhaps as sinter feed orrecycled pellet feed material. An alternatemethod of improving "pellet" angle of repose is to feed formed flakes to the pellet induration process in some limited proportion. Flakes have beenprocessed in phosphate GRATE-KILN Systems for many years. They have also been fed directly to kilns processing other feed materials such as alumina. It is estimated that only 10% 

### flakes are required to increase mixture angle of repose to that of sinteror coke. No increase in induration energy is anticipated for flake production. In fact, energy consumptionmight even decrease since flakes can be formed vith coarser feed than greenballs. They also have lovermoisture contents, typically in the six to sevenpercent, area. Avery slight increase in pellet recycle fines level is anticipated.

TABLE 4



### I I I I Strength C:<g) Start 

### I Final 

### I Degradation (pet. -3 mm)

### I Metallization 180 min
.. 450

### min.

Percent Fixed Carbon Percent Volatiles Percent Ash 

### I I

Percent Moisture Percent Sulphur Heating Value J/kg Delivered Cost S/109 J Magnetite Preheated Fired
36 254 41 59 2.4 3.2

88 92

TABLE 5
Petroleum Coke
82.8 11.0 0.2 6.0 5.0 33.3 X 

### 106 1.58 Hematite Ilmenite Preheated Fired Preheated
64 445 48 68 39 33 4.3 17.8 0.6

94 94 79.5

Eastern Coal Western Coal
50.0 33.0

8.0 9.0 1.0 28.1 x 

### 106 2.86 36.0 29.0

9.0 26.0

1.0 19•8 X 

### 106 2.27 Fired
41 22 0.6
79.8

### 11


### REFERENCES

1. 

### Faulkner, 

### B. P.' Losin, 

### E. T,' 

### and Cna.re, 

### R. F.' 

### 1977. "AStudyof Coal Firing in the Grate-Kiln System", Proceedin!:ls Annual Meet**in~** 

### of the Minnesota. SectionAH!E 

### and 38thAnnualMining Synroosium, Duluth, Jan. 1977.
2. Murr, D. L., 1982, "Coal Firing for Pelletizing-Butler Taconite", Proceedings Annual Meetin!j of the Minnesota Seccion AIM:E and 43rd Annual Mining Symnosium, Duluth, Jan. 1982.
3. Johnson, A. R., 1982, "CoalFiring for Pelletizing-Experiences at CCI's TildenMine", Proceedings Annual Meeting of theMinnesota SectionAIME and 43rdAnnual Mining Svmposium, Duluth, Jan. 1982.

WASTEGASFAN 

### 4.



## s.

6. 83-355

Krollman, C., 1982, "Coal Firing a GrateKilnSystemat: EvelethMines", Proceedings AnnualMeeting of the Minnesota Section ALHE and 43rd Annual Mining Symposium, Duluth, Jan. 1982. Lopez,. R. D., Honkonen, R. C., and Potts, R., 1982, "Coal Firing at Hinntac, U.S. Steel", Proceedings Annual Meeting of the Minnesota SectionAIHE and 43rd Annual Mining Sywposium, Duluth, Jan. 1982.

Taguchi, K., Uenaka, T., Inaba, S., Isako, H., Mizuguchi, I., 

### and Umeji, K., 1983, "Development of Crushed Pellets inKakogawa Works", SME-AIME Annual Meeting, Atlanta, Georgia, March, 1983.



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