# 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 willbe reviewed. Both ferrous and non-ferrous materialswillbe covered. The influence of environmental considerations on the decision to build, the process selection, and design of the plantswillbe discussed. Use of coal in iron ore pellet indurationwill also be reviewed as the availability and price structure of oil and gasbecomes a consideration to the plant operators. INTRODUCTION The greatest utilization of agglomeration in themetallic 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 prepareburdenmaterial for blast furnace consumption. Iron ore pelletizing, whichwas cornmerciallized on a large scale starting only 25 years ago, is now rapidly approaching 300million tons per year installed capacity.

1494INDUSTRIALTRENDSINPELLETIZINGANDSINTERING 1495

The industry has seen the development of very largemachines in the past decade, with the latest concepts in computer control and environmentalprotectionbeing applied. Sinteringhasbeenpracticed on lead and lead/zinc concentrates from the earliest days of theDwight-Lloydmachines to remove the sulfur and producea suitableburden for blast furnaces and smelting. Updraft sinteringwas introduced in 1955with approximately 30 suchmachines now in existence. Pelletizing of other concentrates and metal bearing reverts for easierhandling and processing is finding increasing application. Briquetting, compacting, extrusion etc. have long been practiced in the metallic industries. These technologies arenot included in the scope of this paper. IRONMAKING The advantages of an agglomerated burden for blast furnace consumptionwerewell established 25 years ago. However, to this day the debate continues as to thebetter burden, pellets vs. sinter. The factors involved are complex, and may often be obscured by political considerations. One would assume that thepeoplewho import all of their ironores are guided by purely economic considerations. Most Japanese steel companies have concluded that at this point in time their least cost alternative is tobuy sinter feed and make sinter in Japan. However, Kobe Steel has severalpellet plants. Hoogovens, alreadypossessing sintering capacity, built a pelletplant on expansion of facilities. British Steel at Redcar in recent years built both a large sinterplant and pellet plant. NorthAmericanblast furnacepracticehas tended to a mixed burden of acid pellets and superflux sinter. This is dictated in large part by thenecessity of pelletizing the available fine grained concentrates produced domestically, and at some distance from the blast furnace operations. Thebottom line, in any case, should be the production of hot metal at the lowest costper ton. Sintering The size of the grate available for iron ore sintering has increased dramaticallyover the past decade. Anumber of machines 5 meters wide andwith a grate area of between 500-600 sq mare operating with a capacity of 20,000tons ormoreper day of sized sinter product. Even largermachines are possible in the future.1496 FINEPARTICLESPROCESSING



### Utilization of high suction fans 11,250-15,000pa (45-60 inches W.G.) along with the closely controlled burdens and well sealedmachines has resulted in sinterproduction rates in the range of 35 to 45 tons sinter per sq meter per operating day. The blast furnace requirement of uniformburdenmaterials has resulted in the production of closely sized sinter products. A30mmby 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 separateblast furnaces. In the United States, the installation of new sintering facilities came to a virtual standstill in the early sixties; the role of the sinteringprocesswas considered to be one of diminishing importance. That general opinion is no longer held. Severalhighly automated new plants have been installed, and other olderplants have beenmodernized. In addition, severalmachines have been moved and re-installed incorporatingmodern technology. TheNorthAmerican practice of using superfluxed sinterwith 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. Afew of the more recent ormost controversial subjects are covered here. (1-5) BurdenPreparation. The decision to use bins vs. a bedding pile for sintermix 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 associatedwith a bedding pile often cannot bejustified by the improved performance of the sintering plant, although the improved burden uniformity has only advantages. The major justificationusually must come from theblast furnace operator and his requirement for uniform chemical and physical properties of the sinter. Modern sinter plants often operatewith 12 to 15 ormore components in themix, many varying individually in chemistry and 

### particle size, aswell 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 systemmust thus be designed considering the available feed materials, the requirements of the blast furnace operator, and the capital.available.INDUSTRIALTRENDSINPELLETIZINGANDSINTERING 1497

Conventionalvs. On-Strand Cooling. The subject of on-strand cooling is as controversialtoday as itwas several decades ago. Agreat deal of laboratory pot-grate testwork in the past decade along with recent commercial experience has confirmed that the on-strand cooling concept canbe successfully applied to selectedburdenmixes. To be successful, the flame front in the sinterbed mustbenarrow. Hot-burningmixes, thosewithwide 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 sintermix to have acceptable characteristics for strand cooling, the decision as to which styleplant tobuild oftenbecomes one of economics. Some of themajor considerations are listed. Plant Costs: Cost of cooler and associated hardware vs. longer grate. Local air qualitycodes: cost of cleaning air from on-strand cooling sectionvs. cleaning air from separate cooler. Maintenace: Reduced wear on breaker etc. with strand cooling. Energy: Power consumption of the two process, including housekeeping andprocess gas cleaning. Depending on the individual local situation, we have seen the economics favor either case. Environmental Considerations. Virtually all sinter plants now must practiceboth housekeeping and waste gas cleaning. The selection of the cleaning equipment is dictated by the contaminents in thewaste gas streamand the codes to bemet. 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 lowpressure drop and high pressure drop type, andwet electrostatic scrubbers and precipators. Where wet cleaning is applied, the blowdown from the systemmust be treated tomeetwater pollution codes. Additionally, the sludgemust oftenbe dewatered and disposed of as the impurities are not acceptable for recycle back to the sintering system.1498 FINEPARTICLESPROCESSING

Also, the recyclewater in the systemmay require treatment to prevent corrosion of the system. The cost of housekeeping plus process gas cleaningmay approach or exceed 30 percent of the capital cost of the sinter plant. The operatingmaintenance 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 additionalhydrocarbons. It is possible that a sinter plant could not be constructed in an otherwise logical location, evenwith themost 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 normallybeen 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 constructionwhere 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 portionINDUSTRIALTRENDSINPELLETIZINGANDSINTERING 

### 1499

of the cooler. Computer Control. The objective of control is to optimize the important sinterproperties suchas size consist, strength, reducibility, chemistry andmineralogy and tomaintain the uniformity of these characteristicswhile achievingproductionrequirements. Adigital 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 canbevaried to keep the proportions necessary to maintain the optimum sinterquality and return fines balance. The desired moisture content and burn throughpoint canbe controlled tomaintain themaximum production rate and insure that sintering is completed on themachine. Theentire computer process canbe designed to allow the operator to trim controlvariables; allowing him to enjoy the benefits of computer controlwithout burdeninghimwith the details of the actual execution of the changes. We feel the digital computer is more versatile than an analog computer as ithas 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 controlmay be utilized. Analog inputs includeprocess signals from controllers, controller setpoint feedback, temperatures, pressures, ignition furnace flows and temperatures and fan power. Digital inputs include feeder motor mode (manual/computer) indication, controller setpointmode 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 digitaloutputswhich are controlledby 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 intervalswere selected because of the variability of individual signals and the inertia of the process. Itwas felt, and operation supports the premise, that averageswould givemore accurate input and, since the analog controllerswouldmaintain the computer setpoints, the frequency of control action couldbe reduced.1500 FINEPARTICLESPROCESSING

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-kilnand straight grate have predominated the field innew orders. The size of the induration equipment, as with sintering, has increased greatly during the past decade. The standardwidth of the travelling grate machines is now up to 4meters, 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 7.5meters diameter and 48.5 meters length. The rating of these largemachines 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 individualmachines with the capacity to indurate from 7 to 10million tons per year. Machines of this size have been designed. The hardware for greenballing has also increased in size over the last decade. Pelletizing discs of 7.5 meters diameter and drums of 3.7 meters diameter x 10meters long arenow in common use, with the ability to produce 120 tons of green pellets ormore per hour. The specific capacity of each type of balling devicemay vary with type of pellet feedstock. Some of the industry personnel feel we are approaching the limit inballing equipment capacity, keeping inmind that the green pellets oftenhave 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 inAgglomeration 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. Initialproduction andblast furnace trials using fluxed pellets of various types were performed 15 ormore years ago. It has been shown that fluxed pellets, oftenwith 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 1980AIME Annual Meeting. The controversy over the benefits vs. added cost of fluxed pellet production continues, with severalblast furnace trials recentlyINDUSTRIALTRENDSINPELLETIZINGANDSINTERING 1501



### scheduledand run inNorthAmerica. Until now, theblast furnace/ operators have generallynot been ready topay extra for contained CaO andMgO in thepellets, which displaces iron units. Additionally, the added calcinationload on the pellet indurator requires a higher energy load per ton for the pellet producer. It has been claimedby some that wheremixed burdens of superflux sinter and acidpellets are used, the blast furnaceburdenmay be supported by the high strength characteristics of the sinter in the lower portions of the reduction zone. Several recentpellet plants have been specified and built for fluxed pellets of either limestone or dolomite types. The interest and debate over this subjectwill certainly continue. Environmental Considerations. Plant housekeeping dedusting has long been practicedwith the use of baghouses, wet venturi scrubbers and cyclones. Cyclones have also beenused to remove coarse dust particles for process fan protection. In recent years, electrostatic precipitators andventuri scrubbers havebeen utilized on process gas streams. Considerations forwater treatment and blowdown processing must be taken on thewet systems to handle thevarious impurities and chlorine, florine and sulfur in the gases. Fine ore and concentrate producers who export pellet feed may face loss inmarkets if potential clients arenot able to economically handle the impurities in the feed. Use of SolidFuel. Use of coal, cokebreeze, and various other fuel containingmaterials as a prime source or supplement fuel was practiced in the earliest days of the shaft furnaces and straight grates. Considerable effortswere devoted in the 1950's and early 1960's to eliminate theuse of coal in the pellet feed. We have come full cycle and are devoting considerable effort to use coal inboth 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 thePyrolysis 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 fromburners. 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 FINEPARTICLESPROCESSING

Depending on the individual situation, internal fuel equivalent to asmuch as 1 to 1.5percent carbon canbe added without product qualitydeterioration, provided the appropriate process set-point changes aremade. In fact, the product may have porosity 2-4percent 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 beenburned in the Grate-Kiln System. Anumber of this type plants havebeen equippedwith coal handling and combustion equipment. Computer Control. Dravo is currently involved ina project to use a digital computer to control a straight grate induratingmachine. This development program is aimed at reducing fuel and power consumption, stabilizing the process to reduce product variability and developing empirical measures of qualitywhich canbe 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 pelletizingmachine, feed forward control of drying and firing, on-line empirical measurement of permeability, decrepitation and tumble. Using the computer to collectprocessmeasurements from the entiremachinewill allow control functions tobalance conditions throughout theprocess 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, greenpellet quality, drying and firing rates and overall controlphilosophy. These factors willbe 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 haveplaced increasinglymore importance on a closely sized pelletburden. One typical recently commissioned plantmustmeet an 80 percent plus 9.5mm (3/8") by 12.7 (1/2") size specification. Ausual specification allows 85% 9.5mm x 16mm. To achieve this type of size control, we are seeing increased use of roller screens onboth drum and disc circuits to recycle undersize pellets, and to remove for shredding and thus recycle the oversizepellets. Well engineered roller screens, while requiring a greater capital investment, require less maintenance than traditional vibrating screens.INDUSTRIALTRENDSINPELLETIZINGANDSINTERING 1503



### Electrical SystemControl. Inmany newer applications, programmable logic controllers (or PLC's) are beingused to replace relay and solid state logic systems. The PLC, a small dedicated computer, can be programmed tohandle all switching and interlock functions in the plant. Priced competitivelywith relay systems for a medium sized plant, PLC's offer savings inwiring costs by distributing the switching components throughout theplant instead of in central equipment rooms. PLC's offer advantages in system checkout and startup by allowing logic changes through the computer portionwithout requiring rewiring as in the case of relays. DirectReduction

### Pellets. As direct reduction of 

### pellets for feed to electric furnace steelmaking becomes more popular, we are finding pelletsbeing customproduced for this market. Aprime consideration is very low silica. Some of theDirect Reduction processes also prefer some CaO orMgO content in the pellets, which alsomay aid the electric furnace operation. Hydrated limemay in selected cases be used as a pelletbinder inplace 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 Theproblemof utilizing steelmill reverts has provided generations of researchers and promoterswith continuing opportunity. 

### An untold number of coldbonded processes have been invented and tested, semiworks productionruns have beenmade and blast furnace trials have beenrunwith results generally indicating that 5 or 10percent of a blast furnaceburden, at least on smaller furnaces at nominal capacity, might be tolerated. Another possibility is roasting followed by hot briquetting, which also has been testedwithblast furnace trials. The bottom line again is cost. We find many sintering plants using mixes containingpredominant amounts of reverts as the lowest cost alternative for waste oxide recycle. Old sinter plants have been modernized andnew plants considered for this application. The plant sizingmay bemodest, often in the 2,500 tonper day class to handle material from single or several local mills. However, the zinc containing steelmaking dusts must be kept separate. Hydrocarbon containingmill scale is also a problem. Afew operations are producing greenpellets from steelmaking dusts for direct charging back to the respective furnaces. InbothJapan andEurope, where economy and availability are somewhat1504 FINEPARTICLESPROCESSING

different than the U.S., direct reductionprocesses havebeenbuilt to remove these undesirable elements and provide a blast furnace feed of high quality. LEAD AND LEAD/ZINC InitialHuntington-Heberlein and Dwight-Lloydmachines were downdraft grates. The new technology of up-draft sinteringwas first introduced commercially in 1955. Most of theupdraft lead and lead/zinc sinter machines have been in the 30 to 130m2 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 bothmetal and sulfer values, and safe and hygenic working conditions. Environmental consideratiomplay 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 Anumber ofmetallic minerals and concentrates havebeen successfully agglomerated by various means over the years. Modern sintering and pelletizing technology can be applied to many of these materials. Afew examples are givenbelow. Manganese Manganese ores and concentrates may be pelletized, sintered, modulized in kilns, or briquetted. The operations of a recentmanganese pellet plant inBrazil are described inAgglomeration 77. The process and equipmentwere adaptations from the iron ore industries. Chromites Aprocess for producing and indurating pellets from chromiteswas 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 vanadiumINDUSTRIALTRENDSINPELLETIZINGANDSINTERING



### produced in the indurated agglomerates are thenwater leached for further processing of thevalues. Copper 1505



### Kennecott Copper at its Noranda smelter isballing copper sulfide concentrates to facilitate handling and processing. Kennecott have also developed, patentedand run pilot trials on a process for pelletizing cement copper followed by reduction heat hardening ina shaft furnace. SUMMARY 

### Current sinter and pellet plants are being designed using themost modern process and environmental control technology. The products from these plants are custom tailored to optimize their utilization in themelting and smelting processes. Energy consumption has influenced the designof recent plants, with coal based technology receiving increasedemphasis. The available sizeof the agglomerating and indurating hardware has increased significantly in the past decade, resulting in capital, energy andmanpower 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.