Chapter75 

INDUSTRIAL TRENDS IN THE PELLETIZING AND SINTERING OF 

METALLIC ORES, CONCENTRATES AND REVERTS 

D. W. Kestner 

Dravo Corporation 

Pittsburgh, PA 15222 

ABSTRACT 

Recent developments and trends in the state of the art as being 

commercially applied to the pelletizing and sintering of metallic ores, 

concentrates and reverts will be reviewed. Both ferrous and non-ferrous 

materials will be covered. 

The influence of environmental considerations on the decision to 

build, the process selection, and design of the plants will be discussed. 

Use of coal in iron ore pellet induration will also be reviewed as 

the availability and price structure of oil and gas becomes a consideration to the plant operators. 

INTRODUCTION 

The greatest utilization of agglomeration in the metallic minerals 

field has been for ironmaking, with a very rapid capacity expansion 

occurring since 1950. Worldwide sinter capacity in excess of 450 millio 

tons per year is now in existence to prepare burden material for blast 

furnace consumption. Iron ore pelletizing, which was cornmerciallized 

on a large scale starting only 25 years ago, is now rapidly approaching 

300 million tons per year installed capacity. 

1494 
INDUSTRIAL TRENDS IN PELLETIZING AND SINTERING 1495 

The industry has seen the development of very large machines in the 

past decade, with the latest concepts in computer control and environmental protection being applied. 

Sintering has been practiced on lead and lead/zinc concentrates from 

the earliest days of the Dwight-Lloyd machines to remove the sulfur and 

produce a suitable burden for blast furnaces and smelting. Updraft 

sintering was introduced in 1955 with approximately 30 such machines 

now in existence. 

Pelletizing of other concentrates and metal bearing reverts for 

easier handling and processing is finding increasing application. 

Briquetting, compacting, extrusion etc. have long been practiced in the 

metallic industries. These technologies are not included in the scope 

of this paper. 

IRONMAKING 

The advantages of an agglomerated burden for blast furnace consumption were well established 25 years ago. However, to this day the 

debate continues as to the better burden, pellets vs. sinter. 

The factors involved are complex, and may often be obscured by 

political considerations. One would assume that the people who import 

all of their iron ores are guided by purely economic considerations. 

Most Japanese steel companies have concluded that at this point in 

time their least cost alternative is to buy sinter feed and make sinter 

in Japan. However, Kobe Steel has several pellet plants. Hoogovens, 

already possessing sintering capacity, built a pellet plant on expansion 

of facilities. British Steel at Redcar in recent years built both a 

large sinter plant and pellet plant. 

North American blast furnace practice has tended to a mixed burden of 

acid pellets and superflux sinter. This is dictated in large part by 

the necessity of pelletizing the available fine grained concentrates 

produced domestically, and at some distance from the blast furnace 

operations. 

The bottom line, in any case, should be the production of hot metal 

at the lowest cost per ton. 

Sintering 

The size of the grate available for iron ore sintering has increased 

dramatically over the past decade. A number of machines 5 meters wide 

and with a grate area of between 500-600 sq m are operating with a 

capacity of 20,000 tons or more per day of sized sinter product. Even 

larger machines are possible in the future. 
1496 FINE PARTICLES PROCESSING 

Utilization of high suction fans 11,250-15,000 pa (45-60 inches W.G.) 

along with the closely controlled burdens and well sealed machines has 

resulted in sinter production rates in the range of 35 to 45 tons 

sinter per sq meter per operating day. 

The blast furnace requirement of uniform burden materials has resulted in the production of closely sized sinter products. A 30mm by 

6mm size is common. Some plants are making a further separation at an 

intermediate size, around 12.5mm and sending the two sizes of sinter to 

separate blast furnaces. 

In the United States, the installation of new sintering facilities 

came to a virtual standstill in the early sixties; the role of the 

sintering process was considered to be one of diminishing importance. 

That general opinion is no longer held. Several highly automated new 

plants have been installed, and other older plants have been modernized. 

In addition, several machines have been moved and re-installed incorporating modern technology. 

The North American practice of using superfluxed sinter with acid 

pellets has resulted in the development of techniques for producing 

sinter with a basicity of up to 5.0 and higher (CaO + MgO/SiOz + Al203). 

Many plants are currently running at a basicity in the range 2.0 to 3.5 

with a sinter product analyzing 45-55 percent iron. 

Many changes and improvements have occurred over the past decade in 

sintering technology. These are well covered in the proceedings of the 

Annual Ironmaking Conference of the ISS, AIME, and in the two volume 

Agglomeration 77 proceedings of the 2nd International Symposium on 

Agglomeration, AIME. A few of the more recent or most controversial 

subjects are covered here. (1-5) 

Burden Preparation. The decision to use bins vs. a bedding pile for 

sinter mix proportioning control is often a decision of plant construction economics vs. control of product uniformity for blast furnace 

optimization. Increases in capital and operating costs associated with 

a bedding pile often cannot be justified by the improved performance of 

the sintering plant, although the improved burden uniformity has only 

advantages. The major justification usually must come from the blast 

furnace operator and his requirement for uniform chemical and physical 

properties of the sinter. Modern sinter plants often operate with 12 to 

15 or more components in the mix, many varying individually in chemistry 

and particle size, as well as intermittent or fluctuating availability. 

Statistical analysis discloses that the degree of homogenity achieved by 

proportioning out of bins invariably results in higher deviations in 

product chemistry compared to a bedding system. The burden preparation 

system must thus be designed considering the available feed materials, 

the requirements of the blast furnace operator, and the capital .available. 
INDUSTRIAL TRENDS IN PELLETIZING AND SINTERING 1497 

Conventional vs. On-Strand Cooling. The subject of on-strand cooling is 

as controversial today as it was several decades ago. A great deal of 

laboratory pot-grate testwork in the past decade along with recent 

commercial experience has confirmed that the on-strand cooling concept 

can be successfully applied to selected burden mixes. To be successful, 

the flame front in the sinter bed must be narrow. Hot-burning mixes, 

those with wide flame fronts, cannot be economically strand cooled. 

The ratio of cooling area to sintering area for those mixes with 

acceptable characteristics is generally 0.5 to 0.7. 

Providing laboratory pot-grate trials confirm a sinter mix to have 

acceptable characteristics for strand cooling, the decision as to which 

style plant to build often becomes one of economics. Some of the major 

considerations are listed. 

Plant Costs: Cost of cooler and associated hardware vs. longer grate. 

Local air quality codes: cost of cleaning air from on-strand cooling 

section vs. cleaning air from separate cooler. 

Maintenace: Reduced wear on breaker etc. with strand cooling. 

Energy: Power consumption of the two process, including housekeeping 

and process gas cleaning. 

Depending on the individual local situation, we have seen the economics favor either case. 

Environmental Considerations. Virtually all sinter plants now must 

practice both housekeeping and waste gas cleaning. The selection of the 

cleaning equipment is dictated by the contaminents in the waste gas 

stream and the codes to be met. The sinter basicity and the presence of 

hydrocarbons, alkalai vapors, so2, chlorine and florine all require 

appropriate considerations. 

Types of gas cleaning devices range from simple dry cyclones for 

coarser particulate removal to baghouses, electrostatic precipitators, 

wet venturi scrubbers of both low pressure drop and high pressure drop 

type, and wet electrostatic scrubbers and precipators. 

Where wet cleaning is applied, the blowdown from the system must be 

treated to meet water pollution codes. Additionally, the sludge must 

often be dewatered and disposed of as the impurities are not acceptable 

for recycle back to the sintering system. 
1498 FINE PARTICLES PROCESSING 

Also, the recycle water in the system may require treatment to 

prevent corrosion of the system. 

The cost of housekeeping plus process gas cleaning may approach or 

exceed 30 percent of the capital cost of the sinter plant. The operating maintenance and energy requirements could even exceed this 

percentage under extreme conditions. 

The requirement for gas cleaning has resulted in the development of 

gas recycle systems on the sinter machines· 

to reduce the amount of gas to be cleaned. 

to burn off additional hydrocarbons. 

It is possible that a sinter plant could not be constructed in an 

otherwise logical location, even with the most modern pollution abatement controls. 

The gas would be cleaned to meet standards as to mg dust/m3 air 

in the plant discharge. 

Due to local air factors, the proposed plant has a maximum discharge, kg per hour and per day. 

This allowable discharge is less than the possibilities afforded on a 

mg/m3 basis due to large gas volumes being processed. 

Hot Screening. In conventional sinter plants, with separate coolers, 

hot screening has normally been practiced following the breakers. The 

hot fines are fed to the return system and the hot screened sinter is 

cooled. 

Several plants are operating and under construction where the hot 

screen is eliminated. 

Advantages 

Reduced capital and maintenance costs 

Reduced housekeeping dedusting equipment. 

Either a lower profile plant or the possibility of placing the 

cooler under the discharge end of the strand. 

Disadvantages 

Higher pressure cooling air fan required. 

Additional dedusting capacity may be required on the first portion 
INDUSTRIAL TRENDS IN PELLETIZING AND SINTERING 1499 

of the cooler. 

Computer Control. The objective of control is to optimize the important 

sinter properties such as size consist, strength, reducibility, chemistry and mineralogy and to maintain the uniformity of these characteristics while achieving production requirements. 

A digital computer is the deal tool for monitoring and controlling 

the large number of variables in the sinter process. Mix chemistry can 

be continuously adjusted to produce a consistent product. Coke and 

return fines can be varied to keep the proportions necessary to maintain 

the optimum sinter quality and return fines balance. The desired 

moisture content and burn through point can be controlled to maintain 

the maximum production rate and insure that sintering is completed on 

the machine. The entire computer process can be designed to allow the 

operator to trim control variables; allowing him to enjoy the benefits 

of computer control without burdening him with the details of the actual 

execution of the changes. 

We feel the digital computer is more versatile than an analog computer as it has the ability to modify control algorithms. It is more 

flexible in that new input sensors and control loops can be added to an 

existing computer easily and economically. 

With the computer control, both feed foreward and feed back systems 

of control may be utilized. 

Analog inputs include process signals from controllers, controller 

set point feedback, temperatures, pressures, ignition furnace flows 

and temperatures and fan power. Digital inputs include feeder motor 

mode (manual/computer) indication, controller set point mode indication, low level alarms and feeder on/off contacts. Digital outputs are 

feeder on/off and feeder start contacts and annunciator points. Set 

point changes are sent to the controllers as a pulse train using digital outputs which are controlled by a software timing routine. 

In a typical system analog points are read once every five seconds 

and averaged once a minute. Most control programs run once a minute. 

Return fines and burn through control programs run once every five 

minutes. These intervals were selected because of the variability 

of individual signals and the inertia of the process. It was felt, 

and operation supports the premise, that averages would give more 

accurate input and, since the analog controllers would maintain the 

computer set points, the frequency of control action could be reduced. 
1500 FINE PARTICLES PROCESSING 

Pelletizing 

Almost all of the iron ore pellets are fired with one of three types 

of induration furnace: namely shaft furnace, grate-kiln, and travelling 

straight grate. In recent years the grate-kiln and straight grate have 

predominated the field in new orders. 

The size of the induration equipment, as with sintering, has 

increased greatly during the past decade. The standard width of the 

travelling grate machines is now up to 4 meters, with a machine 

operating having 724 sq meter active process area. 

Likewise, the grate-kilns have increased in size, with grates up 

to 5.9 meters wide and 380 sq meters area and kilns of 7.5 meters 

diameter and 48.5 meters length. The rating of these large machines 

is in the 4 to 6 million-tons-per-year range, depending on the process 

and type of pellet feed-hematite, mixed ore, or magnetite. Over 80% 

of the pellet capacity commissioned since 1976 has been on machines 

rated at 2.5 million tons or more per annum. In the next decade 

we may well see individual machines with the capacity to indurate from 

7 to 10 million tons per year. Machines of this size have been designed. 

The hardware for green balling has also increased in size over 

the last decade. Pelletizing discs of 7.5 meters diameter and drums 

of 3.7 meters diameter x 10 meters long are now in common use, 

with the ability to produce 120 tons of green pellets or more per hour. 

The specific capacity of each type of balling device may vary with type 

of pellet feedstock. Some of the industry personnel feel we are 

approaching the limit in balling equipment capacity, keeping in mind 

that the green pellets often have a_wet compression strength in the 

order of one to one and a half kg. 

The history and development of current pellet technology is well 

covered in Agglomeration 77, and in recent proceedings of the Ironmaking Conference. As with sintering, only a few of the more 

recent and controversial subjects are covered here. 

Fluxed Pellets. Initial production and blast furnace trials using 

fluxed pellets of various types were performed 15 or more years ago. 

It has been shown that fluxed pellets, often with 1.0 to 1.5 percent 

MgO content, exhibit enhanced performance on selected laboratory high 

temperature reduction under load tests. Higher strength at temperature, 

reduced deformation at temperature, and greater reducibility are all 

claimed. Several papers on the characteristics of fluxed pellets 

are being read in the Pyrolysis and Agglomeration Sessions of this 

1980 AIME Annual Meeting. 

The controversy over the benefits vs. added cost of fluxed pellet 

production continues, with several blast furnace trials recently 
INDUSTRIAL TRENDS IN PELLETIZING AND SINTERING 1501 

scheduled and run in North America. Until now, the blast furnace/ 

operators have generally not been ready to pay extra for contained CaO 

and MgO in the pellets, which displaces iron units. Additionally, the 

added calcination load on the pellet indurator requires a higher energy 

load per ton for the pellet producer. 

It has been claimed by some that where mixed burdens of superflux 

sinter and acid pellets are used, the blast furnace burden may be 

supported by the high strength characteristics of the sinter in the 

lower portions of the reduction zone. 

Several recent pellet plants have been specified and built for fluxed 

pellets of either limestone or dolomite types. The interest and debate 

over this subject will certainly continue. 

Environmental Considerations. Plant housekeeping dedusting has long 

been practiced with the use of baghouses, wet venturi scrubbers and 

cyclones. Cyclones have also been used to remove coarse dust particles 

for process fan protection. In recent years, electrostatic precipitators and venturi scrubbers have been utilized on process gas streams. 

Considerations for water treatment and blowdown processing must be taken 

on the wet systems to handle the various impurities and chlorine, 

florine and sulfur in the gases. Fine ore and concentrate producers 

who export pellet feed may face loss in markets if potential clients 

are not able to economically handle the impurities in the feed. 

Use of Solid Fuel. Use of coal, coke breeze, and various other fuel 

containing materials as a prime source or supplement fuel was practiced 

in the earliest days of the shaft furnaces and straight grates. Considerable efforts were devoted in the 1950's and early 1960's to eliminate the use of coal in the pellet feed. We have come full cycle and 

are devoting considerable effort to use coal in both old and new pellet 

plants. 

External combustion chambers have been developed for the travelling 

grate process to burn coal of either high ot low ash fusion 

temperature. Solid fuels can also be used internally in the pellets 

to replace oil or gas aa fuel. Papers dealing with this subject are 

being read in the Pyrolysis and Agglomeration sessions at this annual 

meeting. 

It has been demonstrated that fuel can be supplied from blast 

furnace flue dust, mill scale, coke breeze, and coal as internal 

additive to hematite or mixed hematite/magnetite ores. On the 

straight grate system, this can result in the direct replacement 

of fuel from burners. The firing process, with these burners turned 

off, more closely resembles a magnetite process. Some have claimed 

an overall reduction in fuel, and a reduction in fan power. 
1502 FINE PARTICLES PROCESSING 

Depending on the individual situation, internal fuel equivalent 

to as much as 1 to 1.5 percent carbon can be added without product 

quality deterioration, provided the appropriate process set-point 

changes are made. In fact, the product may have porosity 2-4 percent 

greater, with corresponding potential for enhanced reducibility. 

Other alternatives include coal-oil-mixture often referred to as 

COM and low Btu gas. 

High ash fusion temperature coal has been burned in the Grate-Kiln 

System. A number of this type plants have been equipped with coal 

handling and combustion equipment. 

Computer Control. Dravo is currently involved in a project to use 

a digital computer to control a straight grate indurating machine. 

This development program is aimed at reducing fuel and power 

consumption, stabilizing the process to reduce product variability and 

developing empirical measures of quality which can be used for on-line 

control. These features will permit higher production of specification quality pellets at a reduced cost. 

Specific areas to be studied include smoothing of green feed 

flow to the pelletizing machine, feed forward control of drying and 

firing, on-line empirical measurement of permeability, decrepitation 

and tumble. Using the computer to collect process measurements from 

the entire machine will allow control functions to balance conditions 

throughout the process as a whole, rather than on single loop basis. 

Optimization studies will include evaluation of effects of 

changes in bed height, ratio of green feed to total feed, green pellet 

quality, drying and firing rates and overall control philosophy. These 

factors will be evaluated in terms of their effect on fuel consumption, 

fan power required, mix additives, product tumble index and variability 

in product quality. 

Pellet Sizing. In recent years some of the pellet consumers have placed 

increasingly more importance on a closely sized pellet burden. 

One typical recently commissioned plant must meet an 80 percent plus 

9.5mm (3/8") by 12.7 (1/2") size specification. A usual specification 

allows 85% 9.5mm x 16mm. 

To achieve this type of size control, we are seeing increased 

use of roller screens on both drum and disc circuits to recycle 

undersize pellets, and to remove for shredding and thus recycle 

the oversize pellets. Well engineered roller screens, while 

requiring a greater capital investment, require less maintenance 

than traditional vibrating screens. 
INDUSTRIAL TRENDS IN PELLETIZING AND SINTERING 1503 

Electrical System Control. In many newer applications, programmable 

logic controllers (or PLC's) are being used to replace relay and solid 

state logic systems. The PLC, a small dedicated computer, can be programmed to handle all switching and interlock functions in the plant. 

Priced competitively with relay systems for a medium sized plant, PLC's 

offer savings in wiring costs by distributing the switching components 

throughout the plant instead of in central equipment rooms. PLC's offer 

advantages in system checkout and startup by allowing logic changes 

through the computer portion without requiring rewiring as in the case 

of relays. 

Direct Reduction Pellets. As direct reduction of pellets for feed to 

electric furnace steelmaking becomes more popular, we are finding 

pellets being custom produced for this market. A prime consideration is 

very low silica. Some of the Direct Reduction processes also prefer 

some CaO or MgO content in the pellets, which also may aid the electric 

furnace operation. Hydrated lime may in selected cases be used as a 

pellet binder in place of the more commonly used bentonite. The pellet 

induration and additive schedules are often custom tailored to produce 

optimum quality for the specialized reduction processes. 

AGGLOMERATION OF STEEL MILL WASTE OXIDES 

The problem of utilizing steel mill reverts has provided generations 

of researchers and promoters with continuing opportunity. An untold 

number of cold bonded processes have been invented and tested, semiworks production runs have been made and blast furnace trials have 

been run with results generally indicating that 5 or 10 percent of a 

blast furnace burden, at least on smaller furnaces at nominal capacity, 

might be tolerated. 

Another possibility is roasting followed by hot briquetting, which 

also has been tested with blast furnace trials. 

The bottom line again is cost. We find many sintering plants using 

mixes containing predominant amounts of reverts as the lowest cost 

alternative for waste oxide recycle. Old sinter plants have been 

modernized and new plants considered for this application. The plant 

sizing may be modest, often in the 2,500 ton per day class to handle 

material from single or several local mills. 

However, the zinc containing steelmaking dusts must be kept separate. 

Hydrocarbon containing mill scale is also a problem. A few operations 

are producing green pellets from steelmaking dusts for direct charging 

back to the respective furnaces. 

In both Japan and Europe, where economy and availability are somewhat 
1504 FINE PARTICLES PROCESSING 

different than the U.S., direct reduction processes have been built to 

remove these undesirable elements and provide a blast furnace feed of 

high quality. 

LEAD AND LEAD/ZINC 

Initial Huntington-Heberlein and Dwight-Lloyd machines were downdraft grates. The new technology of up-draft sintering was first introduced commercially in 1955. Most of the updraft lead and lead/zinc 

sinter machines have been in the 30 to 130 m2 size, with approximately 

30 machines and 2000 m2 total area install~d. Advantages claimed for 

updraft sintering include high capacity, uniformity of product, 

utmost utilization of both metal and sulfer values, and safe and hygenic 

working conditions. 

Environmental consideratiom play a large part for the plant design 

in the housekeeping and process gas cleaning areas. This technology, 

and the limitations and restrictions to plant construction and operation 

were described recently. As with iron ore sintering, it is entirely 

possible that a gas cleaning system could meet dust loading requirements/m3 of volume, but be out of compliance on a kg per hour or day 

basis, thus not permitting the siting of the plant. (5) 

OTHER APPLICATIONS 

A number of metallic minerals and concentrates have been successfully 

agglomerated by various means over the years. Modern sintering and 

pelletizing technology can be applied to many of these materials. A few 

examples are given below. 

Manganese 

Manganese ores and concentrates may be pelletized, sintered, modulized in kilns, or briquetted. The operations of a recent manganese 

pellet plant in Brazil are described in Agglomeration 77. The process 

and equipment were adaptations from the iron ore industries. 

Chromites 

A process for producing and indurating pellets from chromites was 

developed and piloted in South Africa. The technology is quite similar 

to that of iron ore. 

Vanadium 

Various shaft furnace and kiln processes have been developed and 

built to react vanadium ore pellets, extrusions, briquettes etc. with 

salts at induration temperatures. Water soluble compounds of vanadium 
INDUSTRIAL TRENDS IN PELLETIZING AND SINTERING 

produced in the indurated agglomerates are then water leached for 

further processing of the values. 

Copper 

1505 

Kennecott Copper at its Noranda smelter is balling copper sulfide 

concentrates to facilitate handling and processing. Kennecott have also 

developed, patented and run pilot trials on a process for pelletizing 

cement copper followed by reduction heat hardening in a shaft furnace. 

SUMMARY 

Current sinter and pellet plants are being designed using the most 

modern process and environmental control technology. The products 

from these plants are custom tailored to optimize their utilization 

in the melting and smelting processes. Energy consumption has influenced the design of recent plants, with coal based technology 

receiving increased emphasis. 

The available size of the agglomerating and indurating hardware 

has increased significantly in the past decade, resulting in capital, 

energy and manpower savings. 

REFERENCES 

1. Sastry, K.V.S., 1977, Agglomeration 77, AIME, New York, NY. 

2. Myron, T.L., 1976, Ironmaking Proceedings, Vol. 35, The Iron and 

Steel Society, AIME, New York, NY. 

3. Kusner, R.E., 1977, Ironmaking Proceedings, Vol. 36, The Iron and 

Steel Society, AIME, New York, NY. 

4. Maloney, L.G., 1978, Ironmaking Proceedings, Vol. 37, The Iron and 

Steel Society, AIME, New York, NY. 

5. Morgan, J.P., 1979, Ironmaking Proceedings, Vol. 38, The Iron and 

Steel Society, AIME, Warrendale, FA.