### THE HIBBING TACONITE COMPANY STORY

B. 

## w. Stunkard E. R. Tyler 

### K. F. Jopke 0. 

### Borgeson 

## c. L. Matson A. E. Englund G. N. Carlson R. A. Bauer Hibbing Taconite Co.

Hibbing, Minnesota

For presentation at the 1979 AIME Annual Meeting New Orleans, Louisiana, February 18-22, 1979 

# PREPRINT NUMBER

79-119HIBBING TACONITE COMPANY INTRODUCTION

Hibbing Taconite Company is located by the cities of Hibbing and Chisholmnear the center of the Mesabi Range, approximately 169 km (105 miles) from the Canadian Border. The plant site is equidistant from these two communities, about 6.4 km {4 miles) from each. A 169 km (105 mile) railroad connects us with theport of Superior, Wisconsin, where thepelletproduction is loaded into lake carriers for lower lakes destinations. Inunediately to the south of the mine area is the world famous HullRust-Mahoning Mine whichproducedmore than 700 000 000 million tons of red ore between the years 1893 and 1973. During the period inwhich this countrywas being sustainedby red ore**mines~** such as the.Pickands Mather &co. managed MahoningMine, men were already thinking about future sources of rawmaterials for the steel mills. Beginning in the early 1930's, Pickands Mather, Bethlehem Steel and Youngstown Sheet and Tube Companies recognized that processing of the low grade iron ores, or namely the ferruginous silicate called taconite, would be required in order toprovide for the future needs of the steel industry. Extensive research and exploration followed during the next decade. This work**reve~l~d** the magnitude of the available low grade ores and some of the requirements to develop these reserves. In 1941, the Minnesota StateLegislature passed a taconite tax lawwhich established that taconite would be taxed upon a productionbasis and therebymade it economicallypossible to acquire the extensive reserves for future operations.
In 1942, a laboratorywas established in Hibbing for the purpose of treating.taconite ores and to develop a flow sheet for a commercial operation. work in this laboratory led to the development of a pilot plant situated in the eastern end of the Mesabi Range whichwas ca.lled Erie Preliminary Taconite Plant. Work in this plant developedmany refinements in the processing of the low grade taconite ores. It was in this plant that the commercial scale flow sheet for Erie Mining Company was developed. As a matter of reference, the time frame for this preliminary work was 1948 thru 1952.
The next step was the announcement in 1953 of the Erie Commercial Plantwhichwas completed in September, 1957, as a 7 600 000 MT (7.5 million long tqns) pellet facility and later expanded to 10 800 000 MT (10.6 million long tons). With the knowledge and experience acquired during the construction and operation of Erie Mining Company, Pickands Mather participated in the development of the HiltonMine at Shawville, Quebec, the Wabush Mines 

### in Labrador and Quebec, the Griffith

### Mine in Ontario and the Savage River Mine in Tasmania, Australia. The last three of these properties were developed utilizing autogenous grinding in the flow sheet. Each of thesepelletproducing facilities contributed to the design andplanning**~or** the Hibbingproject.- 2

- 

### Starting in the late 1960's and continuing intothe early 1970's,, extensive feasibility and design studies wereundertakenby Bethlehem Steel and PickandsMath.er toward developing the taconitereserves inthe Hibbing area. InJuly 1973, plans toproceedwith the projectwere announcedby the two participants of thejointventure, whichwere at that time Bethlehem steef Corporation and Pickands Mather. The initial announcement ata press conferencebyMinnesota's Governor and the Presidents ofPickands Mather andBethlehemSteelwas for the development and constructionof 5 500 000 MT (5.4million long tons) peryearpelletplant. Designof this new third.generation taconite facilitywas layed out to incorporate the largest and most efficientmining andplant equipment inuse on the Mesabi Range today. In addition, a new andunique flow sheet for theplantwas developed inwhich a single stage of crushing and single stage of autogenous grindingwas to beused. In the indurating section, four-meter wide straightgratemachines utilizing external coinbustion chambers capable o.f firing either coal or oilwere tobe installed. EvalQation.of the Hibbing TaconiteFlow Sheetbegan in 1967. Testing to develop this flow**she~t** 

### was conductedjointlyby Pickands .Mather and BethlehemSteel Corporation atthe PickandsMather Research Laboratory atHibbing, Minnesota. Final design, engineering and construction for Hibbing Taconite Company was performedby the Bethlehem SteelCompany Engineering Department in cooperationwith Bethlehem's and**Pickand~** 

### Mather's OperatingDepartment. During cpnstruction, thepurchasing andprocurement for Hibbing Taconite Company was the responsibilityof the BethlehemPurchasingDepartment. Constructionbeganwith land clearing and grading on September 1, 1973. Anew 6.4 km 

### (4 mile) railroad spurwas broughtinto the plant siteby the BurlingtonNorthern Railroad and thepublic utility company broughtpower into theplant sitewhichwas, until this time, an almost roadless forest area. All marketable timberwas harvestedbyprivate contract loggers inorder to salvage alluseable trees. More than 19 937 m3 

### (5 500 cords) of timberwere taken**~rom** 

### Hibbing Taconite lands priorto thebeginning of construction. Mine strippingwas c01mnenced in 2.6 km

### 2 (one squaremile) area to the south of the plant

# and foundation construction for allmajorbuildings was underway. During the course of construction, 63 subcontracting companies involving a total of 167 contractswere involved. Over 1,400 workerswere employed at theplant site atthepeak of construction in April 1976. During construction in 1974 and early 1975, the owners ofHibbing Taconite dec:1-ded to expand construction to increase its size by 50 percent and itsproduction capacityfrom 5.5 to 8.2millionmetric tons ( 5.4 

### to 8.1million long tons) peryear.- 3 -

Pelletproduction at Hibbing Taconitebegan on August 28 of 1976. Our first train load of pellets leftthe plant onJanuary 20, 1977 via BurlingtonNorthernRailroad

## to the Lake Superiorport of Superior,**Wi~consin.** During the interim, the pellets were stockpiled at the plantsite. The general layout of the project includes the main plant area containing theMine Shopwhere mobile mine equipment ismaintained, the CentralShops and Warehouse, Crusher, Concentrator and Pelletizer. To the southof

..the plant is located theMiningArea connectedby the main haul road and to the north, the ReclaimWater Pump Stationand Tailing B13.sin. Bordering on the west is the Burlington Northern Railroad and to the east, the Administration Building and the main entrance**road~** The owners of Hibbing Taconite Company are Bethlehem Steel Corporation with 62.3%,**R~public** Steel Corporation at 16%, Pickands Mather at 15% and the Steel Company ofCanada at 6.7%. Pickands Mather serves as manager ofHibbing Taconite Company. The total investment.in theproject is $360 million. Employment currently at 1,160willpeak out atabout 1,250 upon completion of the present expansion·. The operati.D.g level now at 5 500 000 MT (5.4million long.tons) per year.will increase as the first of the new mill lines begins operation near the end of March 1979.**~he** year of 1978·wasthe first full year of operation, withproduction

.slightly over the rated 5 500 000MT (5.4 million long tons) per year

capacity. With the completion of construction set forJuly of this year, expected**productio~** will be 6 600 000 MT (6.5 million long tons).- 4 - lUNING

The Nesabi Range extends from west of Grand Rapids to the east of Babbitt, a distance of approximately 209 km {130 miles). The commonly used term range is derived from the line of high hills formed for the most part by the Giants Ridge granite. The Biwabik iron formation lies on the south slope and itdips at approximately 8 percent in a southeasterly direction. The Hibbing Taconite Company mine lies about mid-waybetween the two extremities of the Hesabi Range.
Mineral leases, surface lands and permits required to make the Hibbing project viable were started in the early thirties and completed by 1970.

Inpreparation for a long range**m~n~ng** estimate and preproduction miningplans, a series of diamond drill holes was completed in Section 36. Also, itwas decided that a bulk samplingprogramwould be undertaken whereby representative fractions of the mineable taconite in Section 36 would be obtained for test work in the Pickands Mather &Co. Research Laboratory.

In order to obtain representative samples, itwas determined that itwould be necessary tg_make crude taconite ore available from four test pits. The trench locations were selected on thewest side of Section 36 for three reasons. 1. Close geological control of the sub-units from previous churn ·drilling.
2. An adjacent diamond drill hole sectionwhich couldbe used to correlate and compare the testpit and drill.hole analysis.
3. Utilization of topography to minimize the stripping of surface and broken taconite.
Two hundred fifty-four t-lT ofmagnetic taconite were taken from each of four test pits. From this composite, bulk sample laboratory tests were conducted which led to theplant flow sheet selection.

In 1970, long range operating estimates and preproductionmining plans were formalized. Following the construction announcement in July, 1973, the 1970 mining plans and estimates provided the details required to get the mine development underway.
The MahoningMine whichwas managed and operatedby Pickands Mather &Co. closed December 31, 1973. Hibbing Taconite Company acquired the equipment and facilities from Mahoning to start the Hibbingproject.

In April of 1974 1 land clearingbegan.

### - 5 -



### Other thanland clearing, themine forces during the summer of 1974 confJ.ned their activities to the overhaul of two 5m3 {6-1/2 cu. yd.) shovels; a fleet of five 65 ton trucks and eight 34 tonhaulage trucks. The initial strippingbegan in the fall of 1974using that equipment. The preproductionstripping called for the removal of 3 800 000 m3 (5 000 000 cu. yds.)·of surface overburden on a portion of Section 36. As_ection is one squaremile, 2.6k:m2 (640 acres) or 16 forties. The

.surface strippingprogramuncovered taconite on 7 forties. Sub-grade taconite, defined as less than 10% magnetic iron content, scheduled for movement·inpreproductionwas limited to 380 000m3 (500 000 cu. yds.). This was the minimum volume necessary tomeet the road constructionprogram. Stripping of surface and, inparticular, the sub-grade taconiteprior 

### to theplant operationwas limited to reduce startup costs. ·._Two miningcutsweredevelopedprior 

### to theplantoperation. These 2 

### cutsmade 2 

### 100 000 tons of taconite ore available to start.theplant operation. 

### Allpreproduction stripping and development cuts were confined to an areato the south.andwestof that fault tominimizemetallurgical problems in the plantoperation. Oxidationof the taconite along the fault is quite extensive. Prior to stripping, magnetometer surveyswere run to setthe surface stripping limits•. They were rerun after the surface was stripped and rotary drillhole sampleswere taken before the final alignmentof the taconite development cutrunningparallel to the fault was established•.**I~** 

### was decidedtobuildupmaintenance forces in early 1975 and erect shovels, haulage trucks and drills withHibbingTaconite Company personJ?,el to.familiarize::themwith the equipment to assure a trained work force. The erection.of·shovels, haulage trucks of 170 ton capacity and rotary drills has·heen an ongoingproject thatnears completion at this date.The first drill was_moved from the erection site to the mine in August of.1975. The distance from the erection site to the mine development areawas**~-4** km 

### j4miles). The trucks were driven as·theybecame operative. The.first 12.2m (16 cu. yd.) shovel was walked from the erection site to 'the mine in October of 1975. Major equipment acquired for mine operations consistedof: l. Seven 12.2m3 {16 cu. yd.) capacity electric shovels.
2. 

### One 7.6 m3 ·(10 cu. yd.) capacity electric shovel. 3. Nineteen 17Q ton diesel electric driven haulage trucks.- 6 -

4. 

### Four 120 

### ton electricwheel haulage trucks. 5. Seven rotary drills. Auxiliary eqUipment consisted of:
1.

. 

### EightrUbber tired dozers.
2. 

### Eight crawler tractors.
3. 

### Fourroadgraders.
4. 

### Two 70 

### ton truck mounted cranes. 5. 

### Two sprinkler trucks with

### an 30 283 

### liter (8000 

### gallon) capac;ity tank.
6. 

### Fourfront end loaders of 7.6 

### to 11.5

### m 3 (10 

### to 15 cu. 

### yd.)· capacity.
7. Atirehandlingunit.**~s** 

### previously stated, preproduction strippingofwaste taconite was restricted**~o** 

### that amount required tomeet the requirements of the road constructionprogram. From the onset, construction ofpermanentmain haulage roads was conducted as outlinedby the following guidelines: If the original top of groundwas void ofmuskeg, pitrun surface was placed to designed grade elevation. As the roadprogressed, a •.9 to 1.2

### m (3 

### to 4') 

### liftofwaste taconite was placed and 203 

### rnrn (8") 

### ofpit run gravel was·kept abreast of the waste taconite fill. After a period of time, as the voids in the rock lift filled, additionalpit·run gravel wouldbeplaced tomaintain the road elevations. At· this time1 

### the road crown wouldbe established. At a laterdate, sections of theroadwould be coveredwith a 102 

### to 152

### mrn (4 

### to 6") liftof top dressing. Ifmuskeg was encountered, itwouldbe excavated, back filled with blocky waste taconite to former elevation of the muskeg thatwas excavatede Thereafter, constructionwouldbe as detailed. The permanenthaulage roads are constructed'with a minimum width of 30.5 

### m·{1001 ) • 

### The roads require constant grading, re-application of top dressing and re-establishment of the crown. However, the oftenobserved rolls and swells of heavy duty truck haulageroads havenotbeen experienced. Drilling the relatively soft taconite on the CentralMesabi Range withrotary drills**ha~** 

### presented no problems. Drillingrates and tons per foot ofhole drilledhaveboth exceeded expectations.
For the year of 1978, 

### drilling rates averaged 50 

### m (165') 

### per shift. Bitlife in the 311

### rnrn (12-1/4") 

### size averaged 549 

### m {1800'). 

### The 381

### rnrn {15") 

### averaged 64{) 

### m (2100') 

### perbit.- 7 -

There are no significant**~vents** to report in the drilling and blasting of taconite other than the problemof over**fragrr~nting** of the taconite ore. To elaborate on the last statement, the fully autogenous mills at Hibbing Taconite require a uniformblend from the mine for weight recovery, silica and size distribution. 

### Size distribution is essential to obtain designed tonnage rates in the autogenous mills. Ithadbeen established that a feed whereby 40% is +152 nim (+6"} material, 40% is minus 76 mm (3") material and the remaining 20% between a minus 152 mm (6") and a plus 76 mm (3") size rangeprovides maximum throughput. It is to this end the mining operation is gearedwhen itcomes to drill andblastpattern design.
Pattern layouts are simple. 381 mm (15") rotary blast holes are up to a 11by 11 m (36 x 36') burden and spacing. 311 mm (12-1/4") holes are from 7.3 by 7.3 m {24 by 24') and**upward~** Initial detonation is usually at the north end. After the first four holes are pulled, the succeeding rows are controlled by 17 millisecond delays. With the tiein around the perimeter using 25 millisecond delays. The powder factor ·in 1978 was .213 kg (.47 of a pound) of explosiveper ton blasted. The powder :!;actor will he lowered in 1979.

.. A 11.7 m (45')**bla~~** hole on a 9.8 m (32') burden and 9.8 m (32') spacing would be loaded with 227 kg**(~00** lbs.} of slurry and 431 kg {950 lbs.) of anfo. Stemmingwould be a minimum of 6.1 m (20') and 38 mm (1- 1/2") crushed rock is used. The slurry is primed with two .45 kg (1 lb.) primers and the anfo.with .45 kg (1 lb.) Individual down lines are used.
The heavy bottom load of slurry is to pulverize the taconite and produce the minus 7.6 em (3") material requirement. The top, with 6.1 rn (20' or more) of stemming is to produce the +6 inch material for feed to the autogenous mills. Powder factors of less than .18 kg (.4 of a lb.) per ton are probably necessary to provide a better size distribution for the**p~ant** feed. This remains one of the mine operator's major objectives at this time.
Prior to the detonation of the first blast at Hibbing Taconite

. Company, a blastmonitoring network was established. Eight monitoring stations are occupied for eachblast. Eight sound level meters and two seismographs are used. Testblasts using two 45 kg (1 

### lb.) cast boosters and an electric cap are the means used to provide the data as to go .or no go. The main blast is not shot unless a**re~ording** of less than 100 decibels on the linear scale is recorded at all 8 monitoring stations.
As with most mine**op~rators,** problems have been experienced with some trucks of the 170 ton capacity, shovels of the 12.2 rn3 (16 cu. yd.) capacity and the rotary drills capable of drilling 311 mrn to 381 rnrn (12- l/4 to 15") diameter holes. The problems with the drills were insignificant.- 8 -

The shovel and truckproblems experiencedhave been correctedwith the exception of**th~** frame problem on some of the 170 ton haulage trucks. Aframe modification repairprogram is underway at this time and are optimistic that the problemhas been corrected.
In 1978, 33.5MT (33 millionL T) of allmaterial was moved as the operationproduced the designed tonnage of 5.5 MT (5.4 million L T) of qualitypellets. The Mining Department currently employs 325 persons in Operations. and 147 in Maintenance. During 1978, the production rate was 556 MT per man shift, excludingmaintenance.
In 1979, the mining operation is scheduled to handle 38 600 000 MT (38 million L T) of all material. Among mandatory permits secured for the projectwas one from the State ofMinnesota that grantedpermission to pump makeup water required for the**~peratiori** from any 1 ormore of 5 abandoned red oremines. Basically, these mines are a ground water source. Initially, two of the these mines located southof the mining area have been tapped to supply makeup water. A 508mm (20") diameterpipeline runs to theplantsite from these two mines. Additionally, the permitprovided for the construction of a tailing basin north of the plant: Only one small lake of approximately .16 km2 (40 acres) was within the confines of the entire tailingbasin area. Reclaim water is pumped from thebasin.to the plant through two, 1.98m {78") diameterpipelines. Of special mention is the fact that the entire system is closed as specified in thepermits. Hibbing Taconite Company, bypermit requirement, established 3 stations tomonitor water quantity on 3 streams north of the tailing basin. In addition, water quality is monitored. ·This monitoring is on a weeklybasis with reports required onweekly, monthly, quarterly and yearbasis dependent upon theparticular requirement that isbeing monitored.
While on the subject of monitoring, as requiredby the State of Minnesota, an ambient airmonitoring network has been set up. Itconsists of 7 stations. The published EPA schedule isbeing followed. At this station, the·setup for monitoring the 24 hour EPA sample of suspended particulate canbe observed. The value of the network, established in January of 1975, whichwas 18months prior to plant operations, has alreadybeenjustified.- 9 -

ORE DRESSING

The Ore Dressing Department at Hibbing Taconite Company is responsible for the crushing bf 28 052 713 dryMT (28 200 000 dry long tons) per year of crude taconite ore and the subsequent grinding, magnetic separation, tailing disposal anddelivery of concentrate topermit theproduction of 8 200 000 dryMT (8.1 millionDLT) per year of iron oxide pellets.
The flow sheet represents the latest in taconiteplantdesign and currentmetallurgical**technology~** Its simplicity is unparalleled in the taconite industry. Bri·efly, it consists of only one stage of crushing and one stage of grinding. Eachmill line has only one conveyorbelt, a single autogenous mill, two slurrypumps, two stages ofmagnetic separation and one stage of cyclone classification. In addition, the tailing flow by gravityvia a launder into a basinhas eliminated the need for tailing pumps and thickeners• Test·pitting and initialpilotplant testing to develop the Ore Dressing flow sheetwas conductedjointlyby Pickands Mather and Bethlehem Steel Corporation at the Pickands Mather Research Laboratory atHibbing, Minnesota. The owners of the Hibbingprojectrecognized the potential that autogenous grinding offered due to the competency of the ore. A total of 53 pilot scale tests from early 1968 to mid 1970 led to single stage autogenous grinding that is described 

### in this paper.

CRUSHING AND STOCKPILING:
The processing of crude ore allbegins in the crushing and stockpiling system. One system serves 6 mill lines and the other serves 3 lines. TWo parallel 1.5 m (60") x 2.8 m (109") gyratory crushers are usually operated at a nominal 229 mrn (9") open side setting. Control of truck dumping, crushing and stockpiling is accomplished from a crusher central control room. In each crushing system, crushed crude ore drops into a 254 MT {250 L T) surgebinbeneath the crusher. The material from the surge bin is then conveyedby a 2.1 m (84") wide by 6.1m (20') long two-speed apron feeder to a 1.5 m (60") wide steel cableprimary ore conveyor belt. This·in turn transfers the crude through a moveable stone box or chute to a traversing 1.5 m (60") wide steel cable shuttle conveyor positioned nearlyperpendicular to the primary conveyor. The shuttle conveyors distribute the crushed crude to a stockpilewith a combined capacity of 350 539 MT (345 000 L T) which is a four day supply of feed for the mills. Live capacity of the stockpile is approximately 61 000 MT 60 000 L T).

MILL FEED SYSTEM:
In each mill line is a 1.4 m (54") t..,ride by 5.5 m (18') long variable speed apron feeder located under the center of the crude ore stockpile. Located near the toe of the pile is a 1.4 m (54") \...ide by 2.4 m (8') long vibratory feeder. The primary source of feed to the mills is the apron feeder. The vibratory feeder is used as a standby unit and as a- lO """'

source of coarse grinding media from the edge of the stockpile when insufficient coarse material is being delivered to the crusher. The feeders deposit the mill feed on a 1.2 m (48") wide conveyor.beltwhich delivers it to the mill feed chute. Each conveyor is equipped with a 2 idler-load cell

- electronic scale which operates at a range of 0 to 762 MTPH (750 LTPH). Awide range is necessary to record peak loads on the belt createdbyuneven loadingby the apron feeder.

AUTOGENOUS MILLS:
There are nine autogenous mills each 11m (36') diameter by 4.6 m (15') long. Each of the mills is powered by two 6 084 metric h.p. (6000 h.p.) wound rotor motors utilizing liquid rheostats for controlled torque start. Six of the**mil~s** operate at 78% of critical speed or 10.06RP.t-1 ahd 3 operate at 74% of critical or 9.55 RPM. The mill is fed at a rate of approximately 40'6 MT (400 L T) per hour. Anormal charge is 35% of total volume. Pulp density ispresentlymanually controlled to achieve maximum mill throughput. Tests indicate the millpercent solids tobe in the 65 to 70% range. Slurry is discharged from the mill through 9.53 rmn (3/8") slotted grates located at the outerperiphery of the discharge head. Dischargers behind the discharge wear plates carry the slurry into a 4.1m (13'-6") diameterby 4.4m {14'-3 1/2") long attached cylindrical trammel. The trammel is equipped with rubber**~creen** panels wi.th 3.2 It!ID_. (1/8") by 25.4 mm (1") openings. The -3.2 mm {-1/8") material drops into a rougher sump below the tronmiel. The +3.2 mm (+1/8") is discharged into a bucket wheel at the end of the trammel. The bucketwheel elevates and deposits the material into an oversize returnbox, from which it is waterjetted directlyback into the mill through a rubber lined return tube. Highpressure water is the transport vehicle. The amount of trammel oversize circulating load is unmeasurable.

SEPARATION AND CLASSIFICATION:

In the separation and classification circuit, the 3.2 mm (-1/8") trammel undersize collected in the rougher sump is pumped by a 457 mm (18") x 457 rom {18") steelpump to the rougher magnetic separator feed distributor. The pump is poweredby a 406 metric h.p. (400 h.p.) motor. The rougher distributor divides the slurry to feed 14 single drumrougher magnetic separators in eachmill line. Eachof the counter current tank design rougher magnetic separators is equippedwith a 914 mm (3') diameterby 3m (10') long drum. Field strength is 800 gauss at 51 mm (2"). The drum is driven at 21 RPM by a 7.6 metric h.p. (7 1/2"·h.p.) gearmotor through a chain and sprockets. Tailing from each of the machines flows by gravity into the main tailing launder

..The concentrate is laundered to the cyclone feed sump andpump. The 406 mm (16") x 356 mm (14") 608 metric h.p.(600 h.p.) variable speed steel cyclone pump delivers feed to a cluster of ten, 381mm (15") diameter cyclones for classification. The coarse underflow is returned by gravityback to the feed end of the autogenous mill. The fine overflow fraction flows to the finisher distributorwhich divides it to feed 5 triple drum concurrent tank finisher magnetic separators for final concentration. The finisher drums are the same size and powered similarily to the rougher separator drums. The strength of the finisher magnets is- 11

-

500 gauss. -Finisher tailing also flows by gravity to the main tailing launder. The final concentrate flows by gravity in a launder to the Pellet Plant.. Tailing from eachmill line is collected in a trench lined with a high alumina brick. The trench discharges into a 290 m (950') long, 3m (10') diameter semi-circularwood .launder linedwith**rubb~r** and sloped at 3/4-of 1%.

RECLAIMING WATER: Tailing flows directly into a 28.3 millionm 2 (7 000 acre) basin where process water is separated from the waste material and impounded behind dikes. This is a closed system as required under state permits. ·water is·taken into a reclaimpumphouse through two 2A m (96") diameter intake pipes•.The pumphouse contains ten, 914 

### mm (36") x 610

### mm (24") horizontal centrifugal 3 549 metric h.p. (3500 h.p.) pumps rated at 108 765 J;.PM (29 000 GPM)**each~** Water is recycled to the plant at a norma,l rate of 757 000

### LPM (200 ODO GPM) through two 2 m (78") diameter buriedpipelines for a distance of 6.5

### k.m (4 miles}.

CONTROL:
The.control of allmill**~ines** is accomplished from a central control room. All mill**equ~pment** except the magnetic separators and a portion bf the cyclones**i~** controlled from here. The 2 major control circuits govern mill power draw.·and the classification loop.
Mill powerdraw canbe controlled in three separate modes which are manual, cascade or.computer. In the manual mode, the mill power draw signal is sent to the control room as indication only. The control room operator manually adjusts the apron feeder speedbased on the indicated power signal. In the analog cascade mode, the power setpoint is set on the controller. The analog signal is sent to the power controller via the power sensor. Thepower controller and weight controller determine the feeder speed, based on the rate of deviation and the actual deviation from setpoint of the 2 process variables, which are power and weight. In computer control, the computer receives the mill power analog signal and converts it to a digital signal. The computer then operates on the signal via a pre-programmed set of operations. The resultant control signal is converted to analog and sent to the apron feeder. The computer controls power 

### in a much tighter band whichpermits an increase in the power setpoint. The second major control circuit is the classification control loop around the cyclones. Density of the cyclone feed is thepr,imary control to achieve the desired silica level in the mill product. Classification control is based on the silica results obtained from concentrate samples which are gatheredhi-hourly. 

### Silica and -325 mesh grind results are relayed to the control room operator from the lab. The control room operator raises or lowers the cyclone feed density controller setpoint to staywithin a silica range of 4.3 to 4.6%. Anuclear density gauge senses variation from setpoint and controls water addition to the cyclone feed sump to maintain the pre-set density. Avariable speed cyclone feed pump maintains sump level through a level sensor. Variations in

### -.12

-

pump speed result in**pres~ure** 

### changes inthe cycloneheader. The control room operator adds or subtracts cyclones to staywithin a givenpressure range. There are four remotely controlled cyclones permill line. Control of grade has been good and haspermittedmeeting targeted specifications.

METALLURGICAL AND OPERATING**RESULTS~** In 

### 1978 theplant averagemill feed rates were 395 MT (389 LT} per hour as compared with a designed capacity of 406 MT (400 LT) perhour. Rates in excess of 406 MT (400 L T) were reached in 4 months dtiring the year•.Milling rates varywithchanging ore characteristics and size. Swings as great as a'minus 76MTPH (75 LTPH) andplus 51 MTPH {50 LTPH) from rated capacity have beenexperienced. Earlypilottesting and subsequentbulk sampling afterplant startup indicated a properly sized·. crude f1=ed as essential to attaining grinding capacity. As previously noted, the desiredblend contains 40% plus 152mm (6") material, 20% minus 152mm (6") plus.76 mm (3

..) ai::td 40% minus 76 mm (311). 

### The primary crusher is also adjusted to avoid over-reduction of the crude ore. The autogenous mill grindingpower averaged 23.o0 KWH perMT (23.38**~VH/L** 

### T) in 1978. Since this includes inefficiencies through the pinion, gear reducer, millmotor and electrical system, itcompares favorably with a pilotplantprediction of 21.4 KWH per,MT (21.7 :K'WH/L T) at the mill shell.

-- '
Mill availability was 83.6% in 1978. The 1979 target is 87%. Mill lining is responsible for themajority ofmilldoWn. time. Improvements

. in this areawillbe discussed later in this paper. The percentmagnetic iron in 

### the crude ore averaged 21.48% and total iron 31.40%. The plantproduced a concentrate containing 67.95% iron ar1d 4.57% silica. The silica target for 1978was 4.70%. Carefulblending of crude oredelivered to the crusheris.requiredto meet these specifications with a flow sheet thatdoes notpermitregrinding of off specification concentrate. The targeted silica content in the concentratorwas reached at a fairly coarse grind level of 78.2% minus 325 mesh. The Blaine surface area of 1420 cm2 per gramprovides high filtering capacity in 

### the Pellet Plant. The average recirculating load from the cyclones to the mills was 135%. This does not·include the mill trammel oversize fraction internally returned to the millwhichwas previously noted as unmeasurable.
Asteady improvement throughout the yearresulted in an average magnetic iron recovery of 95.38%. 

### An ongoingprogrambetween operations and staffmetallurgists detects and corrects off targetmag ironlosses. Conversion of all roughermagnetic separators from double to sin.gledrum is 

### now underway andwill further improvemagnetic ir.on recovery.- 13 ....;

CRUSHER WEAR LINERS:
The two major users ofwear liners 

### in ore dressing are the 1.5 m**~60")** crusherand the autogenous mills. Wear rates of the crusher**mangan~se** concaves and mantles havebeen exceptional to date. Rather than the 3.6 to 4.1 millionMT (3.5 to 4 million L T) life orginally anticipated, we are experiencing an average 

### life of 9.1 to 10.2million MT (9 to 

### 10 million L T)• Ourmost recent concave change was made after 15.2

### I'(lillion MT (15 mil:lion L T)• Consequently, only 3 concave changes were necessary since startup in 1976. Prime contributors to the extended life are a wide crusher setting and closelybedded and fractured taconite 

### in the upperpart.of the orebody.

MILL WEAR LINERS: 

### Mill linermaintenance 

### is presentlybeing done on a 15 shiftbasis with three liner crews 5 days perweek. The originalmill lining consisting of a plate-lifterbar design on the feed, shell anddischarge ends of the mill. soon after startup, itbecame apparent that itwould be difficult to meet availability requirements with the large number of pieces necessary in a mill of this size. Atestingprogramwas started to investigate the use of fewer pieces and improving the wear qualities. Five mills have nowbeen converted to a one-piece single wave design of shell liners. The number of individual shell liners has been reduced by 75%•.A similarreductionis anticipated on the feed end of themills where testing willbe done with the onepiece design.

on. the discharge end, theplate-lifterbar design has been retained. However,·largerplates and lifterbars now in use are giving longer · life.The original chrome moly plate-lifter.bars of 370 Brinnel hardness were soon replacedby 540 Brinnel chrome moly material. High chrome white irons of 600 Brinnel hardness have also been tested and shown to greatly improve wear life. Their use in other areas will be. evaluated. Te?tirig and evaluating of new designs and metallurgywill continue as we seek higher and highermill availabilities.
Our single stage autogenous mill steel consumption is considerably less than is being experienced in rodmill-ball mill circuits and semiautogenous mill-ball mill circuits. Total steel consumption rates are .about 15% of the semi-autogenous mill and rod mill-ball mill rates. The autogenous mill steel consumption rates should improve when the present standard'chromemoly liners are replacedwith white irons.

_PUMP WEAR LIFE: Pump impeller and shell life.has exceeded expectations. Since startupno rougher pump casings have beep changed out. Projected life of the casings is 25 000 hours. Rougher pump impellers now have an average life of 14 000 hours. Similar experience with the cyclone pumps shows 6000 hours average casing life and 4600 hours impeller life.- 14

-

MILL GEARS AND PINIONS:
As with any new plant, some difficulties were experienced during startup. The major area in the mill was a concern about uneven load distribution across the gear andpinion tooth faces. This was corrected by a program of hand grinding. Extensive testing of tooth loading, mill and gear deflection, gear hardness, tooth straightness and tooth profile and spacing showed that all were within specified design limits. We are optimistic that gear and pinion problems have been resolved.

CONCLUSION:
In conclusion, the Ore Dressing flow sheet incorporates single stage crushing and single stage autogenous grinding with the largest autogenous mills in the world. Its simplicitypermits good housekeeping practices, requires a lowmanpower level and lends itself to an effective preventive maintenance program. Continued improvements are being accomplished in areas such as wear material, mill throughput, power control and magnetic iron recovery. Major problem areas have been encountered and corrected. Experience to date has demonstrated that all design criteria can be met or exceeded.- 15

-

AGGLOMERATING DEPARTMENT

The Agglomerating Department is responsible for the thickening, storage andblending of the high grade magnetic concentrate; dewatering .formoisture control on**filters~** green ball preparation in theballing section and indurating to the desired physical quality on the travelling grate furnaces. The final product is conveyed to the loadoutbins where it is sUbsequently loaded into unit trains.
The flow sheet of the original 5 500 000 MT {5.4millionL T) plant is divided into four major sections. The first is thickening and dewatering**wh~re** the concentrate is thickened in one thickener, stored and blended in two slurry storage tanks. Haterial is withdrawn from the storage tanks withpumps and directed to eight filters for dewatering and conveying directly to the balling section or to an outside stockpile. The next major section is balling, where filter cake is stored in eight storage bins. There are two lines with four balling modules in each line. The greenballs are conveyed to two travelling grate furnaces in the indurating section. The lastmajor section is material handling where the final product is collected and conveyed to the pellet loadout bins, or diverted to an emergency stockpile. In the expanded plant, one additional·thickener and storage tank will be provided to complement four disc filters. One line of four modules inballing will prepare green balls for indurating on one additional furnace.
In the thickening and dewatering section, the concentrate is received by gravity directly from the Ore DressingDepartment at 45% solids in two 33.5 m (110'} diameter thickeners.
The feed to the thickener is normally a 1420 cm2

## ;gm Blaine surface and will average 22.3% ferrous iron. The thickener overflow is pumped to the concentratorwhere itis utilized as process make-up water. The concentrate is thickened to 70% and the underflow is pumped by rubber linedpumps to three slurry storage tanks. Each tank stores up to 4064 drymetric tons (4000 DLT) of concentrate. An interesting feature.in the design of this section is the fact that the equipment is located outside and exposed to the harsh environment of the NorthernMinnesota winters.
Concentrate is withdrawn from the storage tanks and pumped to the filters through pressure distributors.· The 12 filters are 2.7 m (8' 10") in diameter, twelve disc, and operate at.610rran (24"} of vacuum while producing a filter cake with 9.0% moisture.
A series of conveyors normally transport the filter cake to the balling section. The last conveyor is 1.8 m (72"} wide and equipped with air operatedplows.· The plows are automatically controlled by load cells on the filter cake bins and divert the material to the bins preferentially. As an alternate, the filter cake can be stored outside. The material in stockpile can be reclaimed with a front end loader and a reclaim systemwhich slurrifies the material and returns it to the concentrate thickener.- 16

-

In a typical balling circuit, filter cake is withdrawn from a surge bin, bentonite is introduced and the two products are**mix~d** in a rotary mixer. The mixed product is conveyed to a balling drum as new feed. The product from the drum is distributed to a vibrating screen. The seedpellet, or -9.5 rnrn (-3/8") screen undersize, is collected on

### a wide ·conveyor and is recirculated as drum feed. Agrizzly with 15.9rnrn

(5/8") openingproduces a final product. The+15.9 rnrn (+5/8u) material is discharged to a shredder and the shredder product is returned as drum feed.Material is withdrawn from a 76 MT (75 ton) filter cake bin with a variable speed table feeder. The bentonite feeder, drawing from a 15 MT (15 ton) bentonitebin, is a vertical, gravimetric, load cell type. The filter cake andbentonite is conveyed on a belt to rotarymixer. This conveyor is equippedwith a process control scalewhichmaintains the rate atwhich the filter cake is withdrawn from the bin and provides the signal to the bentonite-feeder to regulate the additive rate. Bentonite consumption rates average 8.9 kg/MT (20 lbs/LT) of pellets. Experience indicates a goal of 7.1 kg/MT (16 lbs/L T) is realistic.
The mixer is a top feed, horizontal rotary type. The mixed product is combinedwith recycle from the seed pellet screen and conveyed to the balling drum. Eachballing drum is 3.7m (12') in diameter and 9.8m (32') long, driven with a 100 h.p. motor. Each drum is equipped with a rotary cutterbar. The vibrating seedpellet screens in the original'plant are 2.4m (8') wide by 6.1m (20') long.
The final product from four balling circuits is collected on a belt and conveyed to the indurating section. This conveyor is equipped with a dual idler, load cell type process control scale which provides a rate signal to the furnace control loop. The green balls discharge onto a 1.4m (54") wide oscillating conveyor, which distributes the product on a variable speed 4.4m (172") wide belt. The wide belt provides an even feed distribution to the roller conveyor ahead of the pelletizing furnaces. Each roller conveyor assembly consists of twenty-three, 152rnrn (6") diameter chrome plated rolls. They are spaced with a 6.4 rnrn (l/4") gap and serve to re-roll the green pellets as well as rerttove any entrapped fines. The roller conveyor undersize is collected by_two transverse conveyors and conveyedback to the-filtering section where it is mixed with the new filter cake for reprocessing. The roller conveyor discharges greenballs onto a reverse roll whichprovides a more permeable bed of green balls on the pelletizing grate. ' There are three horizontal continuous travelling grate machines which are 4 meters (13') wide and 76 meters {249') long. Each furnace has 38 windboxes, 2 meters**(6~')** in length; 124 individual pallet cars. The total grate area is 304 square meters (3271 square feet) which results in a grate factor of 27.3MT (26.9 L T) per square meter (square foot) per day at the designed production rate of 346 drymetric tons (346 DLT) perhour of fired product. With a 305 rnrn (12") green ball bed depth, the retention time is 32 minutes.- 17

-

Each furnace is divided into 7 zones. The green balls are initially

.dried in the up-draft drying zone at a gas temperature of 315.6°C

(600°F).. The gases leaving the top c:;>f the bed are normally saturated at 82.2°C (180°F} and are removed from the process with the hood·exhaust fan. · Further drying is accomplished in the downdraft drying zone at a gas temperature of 232.2°C (4500p). The drying process is followed by preheat starting at a temperature of 648.goc (1200°F)• In the preheat zone, the oxidation of the magnetite begins and the hood temperature is raised to prevent thermal shock and too rapid oxidation of the pellets on the top of the bedbefore entering the firing zone. The pellets are heated to the final induration temperature in the firing zones with downdraft "on gas" temperature of 1343.3°c (2450°F.) .This temperature·is usually sufficient to producea high strength abrasion resistantpellet. After-firing, where gas temperatures are normally l037.8°C (1900°F), is the zone where heat is recuperated from the top of thebedby lowering the on gas temperature and using the 1343.3°C (2450°F) pellets to heat the downdraft gases. The pellets are subsequently cooled to a final temperature of 93.3°C (200°F) in two zones. Cooling is accomplished by the.introduction of air at ambient temperatures by the cooling air fan. Each furnace is equipped with 5 process fans with a total connected metric h.p. of 15 464 (15 250 h.p.). The two waste gas fans, hood exhaust and windbox exhaust, are discharged to atmosphere thru 5664 A.C.M.M. (200 000 A.C.F.M.) wet process scrubbers.
The heating of the pellets to 1343.3°C (2450°F) is accomplished by burningBunker "C" oil in two external, side mounted, vertical combustion chambers which are top fired. Each chamber if 14.8 m (48'-6") high and 4 m (13'-3") outside diameter. A 25.2

### million Kcal/hour (100,000,000 BTu/Hr.) burner is mounted .tn the top of each chamber which is the only source of external heat applied to the furnace. This furnace design will facilitate coal firing in the future. Anotherunique feature of the indurating section is the fact that all of themain process fans are located outside of the main building. This design reduced the initial capital cost, facilitates maintenance andminimizes the problems of heat, fumes and noise inherent with fans. The final product from each furnace is conveyed to a transfer ·house•. At this point, a transfer binpreferentiallyprovides approximately 30% recirculation of fired pellets for hearth layer that are conveyedback to a screening station in the mainbuilding. The 1.8m (6') wide by 4.3.m (14') long hearth layer screens provide 100% +9.5 rom

- .25.4mm (+3/8"

- 1") screenedpellets for hearth layer to protect the

·hardware on the furnace grates. Throughout the entire pellet handling system, dust collectors are providedto maintain clean in-plant conditions.
From the transfer house, the final product is conveyed to the pellet loadout bins, each of which has a capacity of 12 193MT (12 000 long tons). In addition, a small 25 400 MT (85 000 L T) emergency stockpile facility 

### is provided near the loadoutbins. Unit trains of- 18

-

180-91MT (90 L T} capacity cars are loaded uniformly as'the trains move under the bins. Average loading time is two hours. The trains are pulled a distance of 168.9

### km (105 miles} to a transhipping terminal in Superior, Wisconsin.
The entire pelletplant is monitored from a .central control room which is located in the indurating area. With the exception of the controls for individual main balling circuits and-the filters, the operator has control of the process 24 hours a day.

OPERATING EXPERIENCE: Since the initial start-up in 1976, the PelletPlant has experienced numerous highlights:in its short history. On June 21, 1976, the first filter cake was produced that was stockpiled. The flow sheet and equipment arrangement in the balling and indurating sections provide for a limitedproduction of green balls from one drum whichcan also be stockpiledwith the filter cake. This allowed trainingof operators in the critical filtering and-ballingphases as well' as debug the equipment· prior to starting the first furnace. The first pellets were produced on August 28.
Ten days later, when the furnace was on hold for·corrective work, a fire broke out in the area above the combustion chamber on the roof. What could have been a major disaster was confined to the immediate area above the No. 1 furnace with effective control by trained fire brigades. In the initial design, the fuel trains to the burners were located immediately above the hot chamber roof. It is believed that a leak from a pilot oil line directed oil on top of the hot chamber which·ignited the entire fuel train. As a result of this, the fuel train was relocated to a new control house which is an appendage to the main building and isolated from the chambers. The new piping arrangement to the burners on top of the combustion chambers was up-graded to heavierpiping, welded fittings and highpressure flexible connections to eliminate the hazard of oil leaks.
On the cool down of the No. 1 furnace, following the fire, refractory problems were observed that dictated a change in design and materials in the combustion chambers. Rebuild work was carried outwhile the fire damage was repaired.
In themeantime, constructionwork on the No. 2 'furnace was completed and itwas brought on 

### line in October of 1976. Following repairs and**modifica~ions,** No. 1 furnace was returned to production inFebruary df 1977. By June of that year, both furnaces were producing at rated capacity. In 1978, the first full year of un- -interruptedproduction, tonnage slightly in excess of our 5 500 000 MT

(5.4 million ton} design capacitywas produced.- 19

-

DEWATERING SECTION:
In the early operations of the filtering section, extreme wear was**·experie~ced** on filter bags, sector$ and the barrels.

· In cooperationwith themanufacturer, on a warrantybasis, a significant modificationprogramwas undertaken to eliminate highwear rates. This work was completed in time tomeet plant requirements. Original design.called for 102 DMTPH (100DLTPH) and are now operating at 153 DMTPH (151 DLTPH) andproducing a 9.0% moisture filter cake.

·BALLING SECTION:
.Some operatingproblems have occurred in the balling section.
The original cutter bars were 406 rom (16"0) in diameter. Vibration was experienced with the 406 rom (16"0) bar and they failed. These bars were repaired and reinforced. The first failure was replaced with a large 572 mm (22 1/2") diameterbar. In addition to the larger diameter, the design has been changed to eliminatewelding of the flutes to the main bar. The flutes are welded to a separate tube which is clamped to themainbar. This larger unithas been operating since April of 1978 with excellent results.
Mechanical**problems~have** been experienced with the 2.4m {8') wide by 6 m (20') long vibrating screens. Failures were experienced of the main side body·plates, excitermounting plate and cross strutmembers. Using a spare screen assembly, itwas possible to keep ahead of the operating requirements bymaintenance procedures developed to facilitate a complete changeout in less than 8 hours. As a result of these problems, roll screens are being.provided in the expanded plant.**Improvern~nts** have been achieved in our green ball**s~z~ng** program over the past 12 months. The +12.7 m-15.9m (+1/2" -5/8") has been reduced from 44.5% to 35.3% and the +9.5 m-12.7 m (+3/8" -1/2") has been increased from 48% to 55.6%.

INDURATING SECTION: Pioneering of the.large, external, side mounted combustion chambers is an important highlight in the indurating section. Earlier furnaces were equippedwith multiple burner arrangements mounted in the side walls of the furnace hood which do not lend themselves to coal firing. The·original chambers were equipped with a large vertical burner located in the center of the roof. The brick in the side wall of the chambers was not supported. During the initial operation of the furnaces, hot spots developed in the wall and roof of the chambers. When the first furnace came down to ambient, an inspection revealed that the brick had slumped. The chambers were redesigned to include 2 water cooled annular support rings for the brick. The burners were relocated off center to achieve better control of the flame pattern in the chamber. Improved operating procedures have been adapted to ensure that the- 20

-

chambers never operate under positive pressure conditions. Since these changes, the operation ofthe chamber and refractory experiencehas**b~en** very satisfactory. Several significant performance factors in the indurating section were experienced in the first full operating year of 1978. During 1978·, fuei consumption averaged 75 000 Kcal per MT (306 000 BTU per ton). During the year, 4 months were operated at capacitywith Kcal (BTU) consumption under the 74 400 (300 000) mark.

Additional areas being considered for fuel reduction include: Improved greenball sizingwhich will provide increased void area in the bed.
Reducedhearth layer on the grate and sidewalls. Higherproduction·rates made possible by an adequate grate factor design. Improvedprocess control with an "on line" computer.
In 1978, availability of the indurating section at 94.2% exceeded the design of 90% as a result of a totally committedpreventative maintenance program. From tlie beginning, a scheduledmaintenance program throughout the entire plantwas followed. Each furnace is scheduled down every sixweeks for a 12 hourmaintenance turn. Common equipment, or life-line systems such as hearth layer and filter cake units are · scheduled down every 4 weeks for 8 hours or less. The furnaces are taken to ambient temperature every 12 months for refractoryrepairs and majormechanical machine repairs. This majormaintenance shutdown has

.been accomplished in5 days withminimal refractory repairs since startup.

PELLET QUALITY:
Thepellet quality of the furnace product in 1978 is as follows: The +6.35 mm (+1/4") before tumble averaged 99.4%•. The +6.35 mm (+1/4"} after tumble was 95.5%. The minus 28 mesh after tumble averaged 3.9%

. and the cold compressive strengthwas.210 kg (462.lbs.). Quality improvement goals are aimed at increasing the +6.35 mrn (+1/4") after-tumble and reducing the variance. Probably one of the most desirable features of the Hibbing pellet is the high reducibility at 82.6% as compared to the international goal of 60%. The percent swelling at 12.5% is below the maximum goal of 15%.
The pellet sizing in 1978was 35.3% +12.7 mrn -15.9 mm (+1/2" -5/8") and 55.6% +9.5 mm -12.7 mm (+3/8"-1/2"). Recent operating experience in the +9•.5 mm -12.7mm (+3/8'' -1/2") range is running in excess of 70%.- 21

-
The chemistry of the pell·ets produced in 1978 was as follows: The total iron averaged 65.86%, the ferrous iron was 1.0% and the silicawas 4.94% on a dry basis.