SOCIETY-OF MIN~N ENG~NEERS' OF 

AI f E 

CALLER NO. D, LITILETON, COLORADO 80127 

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 

Permission is hereby given to publish with appropriate acknowledgments, 

excerpts or summaries not to exceed one-fourth of the entire text of the paper. 

Permission to print in more extended form subsequent to publication by the Institute 

must be obtained from the Executive Director of the Society of Mining Engineers 

of AIME. 

If and when this paper is published by the Society of Mining Engineers of AIME, it 

may embody certain changes made by agreement between the Technical Publications 

Committee and the author, so that the form in which it appears here is not necessarily 

that in which it may be published later. 

These preprints are available for sale. Mail orders to PREPRINTS, Society of Mining 

Engineers, Caller No. D, Littleton, Colorado 80127. 

PREPRINT AVAILABILITY LIST IS PUBLISHED PERIODICALLY IN 

MINING ENGINEERING 
"RECENT DESIGN IMPROVEMENTS IN 

THE GRATE-KILN SYSTEM" 

by 

George J. Skoronski 

Senior Application Engineer 

and 

Leonard J. ITo ida 

Senior Application Engineer 

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 the GRATE-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 System has changed significantly in design 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. A preheat bypass 

system was installed to complement the 

preheat off-gas supply so that these 

two streams delivered the required 

downdraft drying heat supply. 

Figure 1 depicts 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 

recuperation was supplied to Sydvaranger AB in Kirkenes, Norway. This facility was 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 design was 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 recoup mode, 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 one machine. 

Dedicated Fan Heat 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 between design 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 a nominal 4oo•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 than design level to 

produce a downstream blend 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 their Empire III unit. 

This unit originally had 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 system via 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 design was to provide the minimum quantity of high quality heat for 

each zone on the traveling grate. Results of this design were reflected in 

low fuel consumption and low process 

fan power consumption. 

Figure 5 depicts the schematic of a 

high temperature heat recuperation 

system with multiple 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. A decrease 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 of High Temoerature Heat 

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 drying heat and 

mass load requirement. 

A shorter kiln due 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 

sensible heat in the cooler. 

Reduced process gas flow in the 

waste gas streams due to the high 

gas temperature/low mass ratio 

relationship for a required kilocalorie input level. 

Minimum number of process fans 

for an induration system. 

A very simple process design for 

the operator to understand. 
3 83-355 

f. Results of Heat Recuperation Development on Fuel Consumption 

Plant Design 

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 Consumption based on gross heating value of fuel. 

g. Results of Heat Recuperation Development 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 in mind 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. green ball 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 of maintenance. 

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 minimum 
gapping 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 are handled in the 

recycle system. 

Another important advantage 

mizing material fall through 

grate is that m~nunum gaps 

uniform heating of the pellet 

of minion 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•c process 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 induction hardened 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 induction hardened 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 induction hardening 

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 projected 44 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 promote higher 

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., those which 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 

to move in the vertical 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 seal bar 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 been described above. As previously mentioned, the increase in the 

through rod diameter 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 projection is based on data obtained from 

operating plants with similar design 

improvements. 

Grate plates supplied on the traveling 

grate are one p~ece, cast, high grade 

stainless steel units. These units 

are extremely heat resistant and replacement is usually dictated only if 

one should break pr become exposed to 

a direct flow of hot preheat zone ongas. 

Side castings are equipped with cast 

iron thrust buttons which are used in 

conjunction With thrust brackets to 

correctly track or guide the grate 

chain. Thrust brackets 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 return assembly 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 side casting 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 sprocket had 

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 

A hydraulic tailshaft takeup assecbly 

is now designed into the traveling 

grate. The tailshaft was historically 

fLxed to the frame of the grate. 

w~enever the grate chain required 

tightening, due to progression of wear 

with time, the grate had to be shut 

down for this purpose. Now pitch removal, when required, can then be performed during a scheduled maintenance 

period. 

The traveling grate bleeder stack cap 

has been redesigned such that a pivot 

principle is now employed, The previous stack cap assembly required a 

series of chains and heat shields to 

lift the cap. The new design has the 

pivot point away from the heat source 

and fewer parts are required to lift 

the cap itself. 

Headshaft bearings on the traveling 

grate have been extended to the outboard side of the headshaft. This 

movement permits easier maintenance 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~pressive fuel efficiency of the GRATE-KILN 

Sys tern. 
). 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 the rotating and 

stationary wall has been repiaced by a 

water seal. The water tank is attached to the rotating wall with the 

seal plate being part of the stationary assembly above the rotating structure. Designing a seal with minimum 

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 control of 

this flow rate, can therefore impact 

favorably on this metallurgical characteristic. 

Improve ~achine Availability and Ease 

of ~laintenance 

7ne water seal design, in addition to 

its process merits as outlined above, 

has also improved cooler availability. 

An automatic trough fill system maintains 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. 

Coal Firing 

Proven Field Perfo~ance 

The GRATE-KILN System significantly reduces 

pellet production costs by utilizing coal in 

place of more expensive liquid or gaseous fuels. 

Coal is directly fired in the GRATE-KILN System 

eliminating need for external combustion cham~ bers, slag tapholes, auxiliary burners 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 plant performance. 

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 

burner coal 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 are expressed 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 these variables 

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. Correlation coefficients 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 deposition behavior, therefore, is de-
7 83-355 

pendent on these interrelated coal characteristics. The term Coal Characterization Value, 

or CCV, has, therefore, been developed to express this relationship relative to deposition 

rates. It is defined as; 

for any singular variable or any of the ten 

other parameters studied. This was true over 

the range of temperatures investigated. 

A coal having a CCV of 0.8 or less is recommended for GRATE-KILN Systems operating with 

fuel consumptions of 125,000 kcal/tonne or less. 

Recommended CCV increases or decreases depending 

on the particular plant fuel rating. For example, a coal with a 0.55 CCV should burn very 

well in a plant processing magnetite at 75,000 

kcal/tonne, but its relative performance may 

well be expected to decrease in a similar plant 

processing hematite at 200,000 kcal/tonne. 

CCV 

(Coal Ash Content in Percent 

by Dry Weight) X (11.6 x 107) 

(Dry Coal Heat Content in J/kg) (Coal Fixed 

Carbon Content in Percent by Dry Weight) 

Correlation coefficients obtained when relating CCV to ash deposition rates are higher than 

TABLE 1 

CORREL~TION COEFFICIENTS RELATING COAL ~~D 

COAL ASH COMPONE~1S TO LABORATORY DEPOSITION RATES 

Laboratory Deposition Rate at: 

Coal Related Variables 

Fixed Carbon Content 

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 Ti02 Content 

Ash Fe2o3 Content 

Ash Fusion Levels 

Initial Deformation Temperature 

Softening Temperature 

Hemispherical Te~perature 

Fluid Temperature 

Parameter 

Coal Characterization Value 

(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 

Leve 1 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 

Pe 11ec Du3t 

1482+ 

1482+ 

1482+ 

1482+ 
Significance of Coal Ash Fusion Temoerature 

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 other hand, additions of as 

little as five percent preheated or fired pellet 

dust to coal ash can raise the ash fluid temperature as much as lJs•c. The effect of preheated 

and fired pellet dusts on ash fusion temperatures is shown in Table 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 if pure 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 As Received 

83-355 

Table 3 shows how laboratory ash deposition 

rates can be lowered by utilizing coal cleaning· 

circuits. CCV has been cut by more than half 

and ash fluid temperature increased by about 

lJs•c. The change in bulk ash fusion tempera-· 

ture is explained by considering that coal ashes 

contain a variety of minerals, 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•c have 

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, attention has 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 

Ash FT. 1346 1482+ 

1150 1100 1040 985 

DEPOSITION TEMPERATURE (

0 C) 

Table 3. Laboratory Deposition Rates For Low Ash Fluid Temperature Coal 
Blending 

High intensity m~xLng by muliers or cylindrical shaft and plow type mixers has been shown to 

decrease the amount of binder required for 

proper greenball strength development. Decreased binder addition rates tend to reduce 

greenball plasticity and, therefore, increase 

permeability of the traveling grate bed leading 

to improved fuel and power consumption rates. 

Pellet iron content favorably increases. Pellet 

alkali and silica levels are decreased in plants 

using bentonite binders. 

Kiln Burners 

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. 

Petroleum coke, with a nominal 11 percent volatile content, has been successfully introduced 

to burn simultaneously with coal in an operating 

plant. Burner suppliers contend that state of 

the art burners can now effectively utilize 100 

percent low volatile solid fuels. This design 

improvement opens up markets for more cost efficient fuel relative to Eastern 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 cutter bars. 

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 maintain maximum 

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 

achieved with this type of instrumentation which 

contributes to improved overall fuel consumption, especially at lower plant capacity and in 

periods of process upset. 

Variable Speed Process Fans 

Utilization of- 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 system voltage drop 

problems on motor starting. 

These systems have higher installed costs in 

comparison to the constant speed dampered fan 

systems. Decrease in plant process fan power 

consumption, however, can develop a payback in 

additional invested capital of less than two 

years in a typically sized pellet plant. 

Future Considerations 

New Applications 

One of the most 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 reduction kiln at temperatures 

near the Iooo•c level. By coupling an oxide 

traveling grate preheater to a direct reduction 

kiln this situation can be 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 the most 

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 in Table 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 

lov volatile 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 substitution vill 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. A typical comparison 

or petroleum coke and both Eastern and ~estern 

U.S. type coals is presented in Table 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 associated vith 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 or recycled pellet feed material. 

An alternate method of improving "pellet" 

angle of repose is to feed formed flakes to the 

pellet induration process in some limited proportion. Flakes have been processed 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 sinter or coke. No increase in induration energy is anticipated for flake production. 

In fact, energy consumption might even decrease 

since flakes can be formed vith coarser feed 

than greenballs. They also have lover moisture 

contents, typically in the six to seven percent, 

area. A very 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.' Los in, E. T,' and Cna.re, 

R. F.' 1977. "A Study of Coal Firing in the 

Grate-Kiln System", Proceedin!:ls Annual 

Meet in~ of the Minnesota. Section AH!E and 

38th Annual Mining Synroosium, Duluth, Jan. 

1977. 

2. Murr, D. L., 1982, "Coal Firing for 

Pelletizing-Butler Taconite", Proceedings 

Annual Meetin!j of the Minnesota Sec cion 

AIM:E and 43rd Annual Mining Symnosium, 

Duluth, Jan. 1982. 

3. Johnson, A. R., 1982, "Coal Firing for Pelletizing-Experiences at CCI's Tilden Mine", 

Proceedings Annual Meeting of the Minnesota 

Section AIME and 43rd Annual Mining Svmposium, Duluth, Jan. 1982. 

WASTE GAS FAN 

4. 

s. 

6. 

83-355 

Krollman, C., 1982, "Coal Firing a GrateKiln System at: Eveleth Mines", Proceedings 

Annual Meeting 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 Section AIHE 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 in Kakogawa 

Works", SME-AIME Annual Meeting, Atlanta, 

Georgia, March, 1983. 

Figure 1 - Grate-Kiln System Perspecive With No Recoup System and Two-Pass Grate Cycle 
I 

i 

I i 

Figure 2 - Grate-Kiln System Perspective With Fanless Recoup System 

..... 

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(X) 

w 

I 

w 

t..n 

t..n 
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WASTE GAS FAN 

Figure 3- Grate-Kiln Systom Perspective With Dedicated Fan Type Recoup System 

.-

w 

o:> 

w 

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83-355 
lKAB GRATE-KILN PLANT 

UDD WASTE HEAT FAtl RECOUP STREAM 2 

RECOUP STREAM I 

RECOUP STREAM 3 

UDD- UPDRAFT DRYING TPH -.TEMPERED PREHEAT COOLING FAN 2 COOWlG FAt~ I 

DOD- DOWNDRAFT DRYING PH- PREHEAT 

Figure 5 - Grate-Kiln System Schematic With Multiple Duct Heat Recuperation System 

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Mass Ratio, l<g Air/Kg Pellets 

% Magnetite Into Cooler 

0 50% 

® 25% 

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" 762 MM Bed Height 

" 30 Minute Cooling Cycle 

., Typical One Duct Recoup 

System Profile 

·, 

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1.5 

Figure 6- Cooler Off-Gas Temperature Vs. Mass Ratio (1093°C Solids Inlet Temperature) 

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