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LOW CEMENT CASTABLE WEAR MECHANISMS IN DRIROTARY KILNS

Chris Parr, Genine Assis *,

* Kerneos, France

Presented at St Louis Section Meeting, St Louis, USA, March 2008

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ABSTRACT

Production of sponge iron, via the direct reduction route using rotary kilns, continues to experiencesignificant growth in countries such as South Africa and India. The vast majority of these DRI kilnsare lined with monolithic castables and there is a need to improve the durability of low cementcastables for this specific application. To a large extent, the refractory life of a DRI kiln isdetermined by the build up of accretions within the kiln. The accretions must be removed tomaintain production levels, but removing them not only results in lost production due to downtimebut the refractory lining is often also damaged in the removal process. A post-mortem analysis ofused DRI kiln refractories has been performed in order to provide a clearer understanding of theunderpinning refractory wear mechanisms. Used samples of a low cement castable have beenrecovered from two direct reduction rotary kilns. Samples were taken from areas where severeaccretion formation was observed as well as areas where there was no sign of build up. Chemicaland physical analysis has been carried out using XRF and XRD and this data has been coupledwith an analysis of microstructures by SEM and EDS. The investigation reveals a number ofmechanisms which account for the accretion build up and lead to premature castable wear throughthe removal of the refractory hot face during removal of the accretions from inside the kiln.

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1 Introduction

DRI (Direction Reduced Iron or Sponge Iron)is being produced in increased quantities innumerous countries around the world. WorldDRI production in 2007 for the countriesreporting to the International Iron and SteelInstitute is estimated to be 54.433 milliontonnes, up by 9.1% YoY as compared to49.460 million tonnes in 2006.

The largest national producer of DRI is India.India maintained the position of global leaderby producing 17.820 million tonnes of DRI in2007 and accounting for 32.7% of total globalDRI production. Indian production of DRI hasgrown rapidly over the last 10 years jumpingfrom 4.8 millions tonnes in 1996 to 15.4 milliontonnes in 2006 at an annual growth rate of33% per annum. The growth of productioncontinues on a monthly basis as evidenced byfigures for January 2008. The InternationalIron and Steel Institute production figures fordirect reduced iron for the month of January2008 show the global production of DRI inJanuary 2008 was 4.966 million tonnes, up by10.9% YoY and up by 3.8% MoM ascompared to December 2007. And Indiaretained the top slot, producing 1.6 milliontonnes and accounting for 32.2% of the totalglobal DRI production.

The national steel plan in India foresees DRIincreasing to 30 million tonnes by 2010, anapproximate two fold increase over volumesproduced in 2007. This assumes an annualgrowth rate of 20% in DRI production. SinceDRI production consumes a significantamount of refractories, growth in DRIproduction should stimulate refractorydemand. A survey [1] of this market wasundertaken to understand the impact of thisgrowth in DRI production on refractories and,more specifically, the refractory demandlinked to monolithics and the consequential

demand for calcium aluminate cement. Aspart of this survey it was necessary to studyrefractory wear mechanisms within DRIproduction units to better understand potentialopportunities for technological progress andways to reduce the specific refractoryconsumption through improved refractoryperformance.

2DRI Production and MarketStatistics

The primary use of DRI is as an alternativesource of metallics for steel making. There isalso a second and allied product, namely hotbriquetted iron (HBI), which does not generatefines as readily and is less susceptible tooxidation, making HBI the preferred product toexport around the world. DRI is primarily usedas a cheaper substitute for HBI and is addedto supplement the scrap charge for steelmaking via the EAF route. This is especiallyrelevant in India due to scrap sourcing issues- both in terms of scrap quantity and scrapquality. The chemical composition of DRI is farmore stable than that of steel scrap, the latterbeing prone to high and varying amounts oftramp elements e.g. Cu, Pb, Zn etc. Inaddition, DRI is generally predictablyavailable, although this is not always true inIndia!.

Other applications for DRI are as a coolant inthe BOF steel making route (replaces scrap),and as charge mix to Blast Furnaces toeconomise coke consumption and increasefurnace productivity.

DRI is produced by the direct reduction of Ironore by CO gas. The reduction reactions areperformed in either a rotary kiln or verticalshaft furnace and the energy required issupplied by either gas or coal as shownschematically in Figure 1 (which illustrates therotary kiln process).

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Figure 1: Reactions in the DRI process

There is a plethora of DRI productionprocesses, with more than 15 differentprocess types existing in the world. Themajority of the worlds DRI is produced byMidrex/Hyl processes using gas fired verticalfurnaces. 60% of world production is throughMidrex units and an additional 20% of worldsproduction is via other vertical shaft furnaces.

Notable exceptions to producing DRI by gasfired vertical furnaces can be found wherethere is currently limited gas supply e.g. inIndia and in South Africa. This has led to 60%of DRI production in India employing a varietyof indigenous and imported technologies thatare coal fired - essentially rotary kilns with anaverage throughput of 100t.p.d. Theremaining 40% of DRI production in India isgas fired, and both shaft kilns and rotary kilnsare in operation. Shaft kilns tend to be largeunits with an average of 1500 to 4000 t.p.d.

It is thought that DRI production in coal firedrotary kilns will remain dominant and continueto grow in the future in India largely due tolimited gas availability (gas is really onlyavailable in Western India). The reasons for apreponderance of coal fired rotary kilnsproducing DRI in India is believed to berelated to:

1. Low investment cost using localtechnology I lakh (1800) per tonneproduced which is half the worldstandard

2. Coal is readily available low gradelocal coals can be used

3. Iron ore is locally available.4. Low cost alternative to the Blast

Furnace integrated steel making route relatively low capital investment

5. Coal fired DRI plants can beconfigured for a wide range ofproduction throughputs - from smallscale of

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Thus the target of 30 million tonnes based onan installed capacity of 44 millions over thenext 5 years appears achievable. Mediumterm plans show an estimated 56 million tonnecapacity with more than 40 millions tonnesproduced by 2020.

3DRI Production and MarketStatistics

A market survey [1] of DRI producers andrefractory producers showed that therefractory environment depends upon theprocess type as well as the size of the unit.Generally speaking the refractory life span ofgas fired plants (e.g. Midrex) is much longerthan coal fired units. This is largely becausethere is less wear in gas fired furnaces sincegas contains a low level of elements whichcause melt formation at low temperature (e.g.SiO2, alkalis) unlike coal. Abrasion is alsolower since gas fired units are static furnaces.

Coal fired rotary kiln units tend to have twobasic refractory lining configurations: Combination linings of phosphate bonded

+ 80% Alumina brick combined withbauxite/mullite based LCC Monolithics.This is quite commonly found in very large+500 t.p.d. units.

100% LCC Monolithic is the liningconfiguration adopted mainly by Indianproducers. The linings are zoned with60%, 70% and 80% Alumina material andLCCs are used in the entire length of thekiln.

Rotary kilns are an ideal application formonolithics as these refractories are usuallyquicker to install than bricks in this case andless complicated (no need for jacking andspecialised equipment). In addition, partialrepair of sections can be carried out moreeasily with monolithics than with bricks.

Refractory wear is very dependant upon kilnsize, iron ore feedstock quality and coalquality. The coal fired units suffer from thebuild up of accretions which can grow to sucha size that they impede kiln throughput and

force the kiln to be shut down so they can beto removed. The accretions themselvesdamage the refractory lining as they lead tothe lining tearing out during the mechanicalaccretion removal and lining cleaning.

The basic usage characteristics and qualitiesrequired for Refractories for DRI Rotary kilnsare estimated to be: Limit the build up of accretions (iron

aluminate spinels/CAS glasses) inside thekiln due to adhesion between charge andrefractory [3]

o Limit CaO% which formsCAS2/C2AS low me