sponge iron review

Rotary Kiln Processof Making Sponge Iron

2.1 HISTORICAL BACKGROUND

The production of steel began in ancienttimes; but because of the complexity and slow

speed of the ancient process, they could not

be carried out on a very large scale. Conse-

quently, they were replaced by the high

production rate ‘indirect process,’ and the

development of modern DR Process did not

begin until the middle of 19th century.

Perhaps the very first patent in U.K. for

sponge iron making was in 1792 presumably

using a rotary kiln. More than 100 DR

processes have been invented and operated

since 1920. Most of these have died down. But

some of them have re-emerged in slightly

different form.

As touched upon earlier, sponge iron is

mainly produced from ore by two different

routes – (a) by reducing gases (CO and H2)

in a shaft furnace, and (b) through direct

treatment with coal in a rotary kiln.

2.2 IMPORTANT FEATURES

The coal based rotary kiln process of makingsponge iron is the focus of the present writeup. Although many different processes and

CHAPTER

2

process concepts have been emerging in thisarea, there were rapid births and deaths ofthese processes and process concepts in themiddle of the twentieth century. But thoseoperating successfully at present have manyfeatures in common. Some of the common orslightly differing features are:

(i) System of sealing to prevent air ingressinto the reactor,

(ii) System of throwing or slinging coalfrom discharge end of reactor,

(iii) System of weigh feeding andproportioning of raw materials

(iv) System of introducing controlledamount of air at regular intervals oflength in such a way that it does notoxidise the reduced product in the bed,

(v) System of temperature sensing atregular intervals of length of thereactor and recording of the same.

(vi) System of indirect cooling of spongeiron-char mixture in a rotary steelcylindrical shell using water from theoutside.

(vii) System of treating waste gases andmaintaining desired flow profilethrough pressure control.

10 // Advances in Rotary Kiln Sponge Iron Plant

A typical process scheme for makingsponge iron in a rotary kiln is presented inFig. 2.1. While Fig. 2.1 shows only the keysteps, a more detailed scheme, as it wouldappear for a typical operating plant, ispresented in Fig. 2.2.

2.3 SPONGE IRON PILOT PLANT OF

RDCIS SAIL

The sponge iron Pilot Plant (SIPP) of RDCIS,SAIL, which would be mentioned a numberof times, was set up in 1980-82 with almostall the features of a commercial rotary kilnsponge iron plant. The know-how statuswas, however, slightly different at thattime. Figure 2.3 represents more appro-priately the SIPP of RDCIS SAIL.

Fig. 2.2 A concise schematic representation of a rotary kiln sponge iron plant

Fig. 2.1 Key steps in sponge ironmaking in rotary kiln

ID fan

Dry ESP/bag filter

Waste gas

Iron orecoalflux

Inlet hood

Afterburning &coolingchamber

Reduction kiln

Shell-mountedair fans

Cooler

Pro

du

ct

Water sprayAirblower

Finecoal

Du

st

Sta

ck

Du

st

Du

st

Ash

Magnetic separator

Total time of materials in the rotary kiln(Residence time)

Rotary Kiln Process of Making Sponge Iron // 11

2

Water

Air

12

13

4

][ ][ ][

7 7 7 7 7

66

5

6 101198

3

1

1. Raw material bins2. Belt conveyor3. Bucket elevator

4. Surge bin5. Vibratory screen6. Magnetic separator

7. Product storage bins8. After burning chamber9. Radial flow scrubber

10. Induced draught fan11. Waste gas stack12. Rotary kiln

13. Cooler

1 1 1 1 Waste gas

Fig. 2.3 A schematic of the sponge iron Pilot Plant of RDCIS, SAIL

This Plant of capacity 5 to 9 tonnes perday of sponge iron was commissioned inMarch 1982 within the premises of M/s HECat Ranchi, with the objective of adapting andassimilating coal based sponge irontechnology in India. The Pilot Plant was inregular operation since its commissioning till1992-93, with 4 to 5 Campaigns each year.

48 campaigns were carried out in thePilot Plant with various ore and coalcombinations from different deposits in thecountry. The two longest campaigns lasted62 days each. A total of 26 ore-coalcombinations were processed in the PilotPlant. Most of the iron ores tested in Pilot

Plant were found suitable for sponge iron

making. When operated with a good qualitycoal, metallisation level was consistentlyabove 90%. On the other hand three of thecoals tested in Pilot Plant were either notsuitable or were only marginally acceptable.In such cases obviously metallisation levelswere reduced – in extreme cases upto 70%.Other major results include:

• Development of ore-coal compositepellets technology, which improveskiln productivity and reduces energyconsumption (patented)

• Pre-heating system of ore (patented)

• Simultaneous injection of under-bedhydrocarbon fuel and over-bed air in a

rotary reactor

Fig. 2.4 Essential features of a reduction kiln in a rotary kiln sponge iron plant

Air

Fine coal

Spongeiron

Char

Air

Rotary kiln

Thermocouples

Waste gas

Coal

Flux

Iron ore

tubes


Fig. 2.5 Material balance in a rotary kiln sponge iron plant

2.4 FEATURES OF A ROTARY KILN

SPONGE IRON PLANT

Figure 2.4 indicates the essential featureswhich are needed in the reduction kiln of asponge iron rotary kiln plant. However, theair tubes indicated in this diagram can besubstituted by a ported kiln design, which isdiscussed later in this book.

A typical material balance for the spongeiron making process is presented in Fig. 2.5.Here coal is assumed to contain about 20%ash, something which is hardly available nowa days. Product is shown to be screened intothree fractions. But due to difficulty inscreening the -1 mm fraction, it is usual nowa days not to separate out this fraction. Useof 6 to 20 mm iron ore is indicated. Presentlyit is more common to use 5 to 18 mm fraction.A typical energy balance in the form ofSankey diagram is presented in Fig. 2.6.

Rotary kiln processes have had tocompete with gas-based processes. Gas

based processes use relatively costlier

input such as pellets and reformed natural

gas and conversion cost at similar capacities

are higher. But even then, gas-based

processes have generally found favour due

to better and more consistent quality,

lower energy consumption and higher

module size. It was realised early that

rotary kiln processes can be up-scaled only

to a limited extent and bigger module size

does not mean a higher economy of scale.

Modules bigger than 500 tpd were

continuously plagued by problems of

accretion formation and were maintenance

prone. All such modules have now been

phased out.

India has been the largest producer of

sponge iron since last few years. Its

contribution to world DRI production is in

excess of 25% at present. India has been

increasing the gap with the next country

Venezuela. Trends indicate that this gap

would continue to increase in the foreseeable

future. Iran is third on the line and Mexico,

who was once the world leader, is presently

placed fourth. One thing may be noted though


that the entire production by number 2, 3 and

4 countries is through gas based route while

in India, more than half the production is by

the coal based route.

2.5 THE INDIAN SCENE

Situation in India is different from the rest ofthe world. Local conditions here havefavoured coal based rotary kiln units andpresently India has more such modules thanrest of the world put together. Over 300modules are presently in production in India.And there is another trend in India ofdownscaling. Only in India it has been foundprofitable to operate 100 tpd and 50 tpdmodule (even 25 tpd modules), while else-where 250 tpd module is considered as theminimum economic size. Table 2.1 and Fig. 2.7present the production figures of sponge ironin India vis-à-vis world, over the years.

Fig. 2.6 Energy balance in a conventional rotary kiln sponge iron plant

1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

Year

60

50

40

30

20

20

0

30%

20%

10%

0%

World DRIproduction

India’s

share, %

DRIProduction

in India

Share

ofIn

dia

'spro

duction

DR

Ipro

duction

million

tonnes

Figure 2.8 presents the scene with respect tocoal and gas based processes.

Fig. 2.7 A comparison of sponge ironproduction of India and World over the years

Input energy

6.0, coal

1.7Spongeiron(Chemicalenergy)

1.15, Char(Chemicalenergy)*

0.3, Coolinglosses(Sensible heat insolid product)

0.6, Radiative &Unaccounted losses

2.25,Wastegases(Chemicalenergy +Sensible heat)

Unit GCal 10 calories9

=


The Krupp-Renn process, about whichwe mentioned in chapter 1, was probablythe last of the rotary kiln processes, whichattempted to produce iron in fused orsemi-fused mass. All of the currentprocesses attempt to prevent any type offusion during production.

The processes, which are currently invogue, are Jindal Process (50, 350 and 500 tpd),SIIL Process (100 tpd), OSIL Process (300 and500 tpd), SL/RN (Lurgi) Process (100 and 500tpd), Krupp-Codir Process (400 and 500 tpd),DRC Process (250 to 350 tpd) and TDR Process(400 tpd). OSIL Process has evolved from theported kiln concept of ACCAR Process, whileSIIL Process has been based on the Lurgi orSL/RN Process. Presently, of the total spongeiron produced in the world, coal based rotarykiln processes contribute only about 15%, butconsidering India alone this percentage is

about 65%. Trends point to substantial increase

in the latter, in spite of the fact that a 2 mtpa

gas based module has been commissioned

recently in western India.

As mentioned earlier, nearly 35% of

India’s sponge iron production is accounted

for by the three gas based plants located near

the western coast. M/s Essar Steel Limited

located in Hazira in the state of Gujarat has

five operating modules with a total capacity

of 5.5 mtpy and claim to be the largest sponge

iron plant in the world. Ispat Industries

Limited and Vikram Ispat Limited both

located in the state of Maharashtra have one

module each of capacities 1.4 and 0.9 mtpy.

On the coal based front, the plant of M/sJindal Steel and Power Limited (JSPL) inRaigarh in the central Indian state ofChhattisgarh is the largest in India andprobably in the world. It has ten modulestotalling 1.2 mtpy capacity.

Fig. 2.8 Another comparison of sponge iron production of India and World indicatingthe dominant position of coal based route in India

Sp

on

ge

iro

np

rod

uc

tio

nin

the

ye

ar

20

06

,

mil

lio

nto

nn

es

60.00 –

50.00 –

40.00 –

30.00 –

20.00 –

10.00 –

0.00 –

15.00

9.50

5.50

Total

Coal basedGas based

India

India

World

World

49.10

59.80

10.70


Table 2.1 DRI Production: India and World*

Million Tonnes

Year India World

1970 0 0.79

1975 0 2.81

1978 0 5.00

1979 0 6.64

1980 0.01 7.14

1981 0.02 7.92

1982 0.03 7.28

1983 0.04 7.90

1984 0.08 9.34

1985 0.09 11.17

1986 0.17 12.53

1987 0.19 13.52

1988 0.19 14.09

1989 0.26 15.63

1990 0.61 17.68

1991 1.15 19.32

1992 1.44 20.51

1993 2.21 23.65

1994 3.12 27.37

1995 4.28 30.67

1996 4.84 33.30

1997 5.26 36.19

1998 5.26 36.96

1999 5.22 38.59

2000 5.44 43.78

2001 5.59 40.51

2002 6.59 45.10

2003 7.67 49.45

2004 9.37 54.60

2005 11.10 55.96

2006 15.00 59.80

*Data taken mainly direct from Midrex

Thus we see that the largest coal based

plant in the world barely stands up to the

smallest gas based module in India. But then

the coal based plants make it up in numbers.

As mentioned earlier, over 300 modules of

coal based plants are operating in India. And

situation is changing so fast, almost on daily

basis, that not much point is served by

describing these plants here.

Apart from JSPL, some of the plants

which operate large size modules (300 to 500

tpd) are Bihar Sponge Iron in Jharkhand,

Prakash Industries, Nova Iron and Steel,

Monnet Ispat, Godavari Ispat and Power all

in Chhattisgarh, Sunflag Iron and Steel

Company, Lloyds Metals and Engineers, in

Maharashtra, Orissa Sponge Iron, Tata

Sponge Iron, in Orissa, GSAL India in Andhra

Pradesh, etc. We must make special mention

of Sponge Iron India Limited in Paloncha in

Andhra Pradesh, which was the first of the

commercial plants (originally called a

demonstration plant) commissioned in 1980,

and which started the race for the installation

of the 100 tpd modules, the total number now

being well over one hundred. 50 tpd modules

could also have exceeded 100 in number. But

another special mention must be made of the

25 tpd modules. One is operating in Ramgarh

in Jharkhand (Palash Sponge Iron), while at

least two modules are operating in Raipur in

Chhattisgarh.

2.6 WHY SHOULD WE SELECT A

ROTARY KILN?

The rotary kiln direct reduction (RKDR)

processes have been looked upon with

apprehension, mainly because there have been

rapid births and deaths of processes in this

group. But the fact that it has re-emerged,


points to certain strengths of this process. Letus examine some of them.

2.6.1 Process Strengths

Rotary kiln process has to compete mainlywith the shaft process of making sponge ironand in some cases with iron making blastfurnace. As compared to them, the rotary kilnhas some advantages, as also some limitations,both with respect to the process and theproduct it makes. The major processstrengths of rotary kiln are:

(i) A rotary kiln can mix the solid chargeas it heats and reduces it. Simultaneousmixing helps in the dilution of CO

2

concentration formed around the ironore/sponge iron particles – which isnecessary for the reduction reactionto proceed.

(ii) As a large freeboard volume isavailable above the solid charge (about85%), the rotary kiln can tolerateheavily dust-laden gas. When the kilnis suitably designed, it would be bestsuited for utilising the Indian high ashnon-cooking coals. In shaft reactors,generation of such dust leads tochoking and channelling which leadsfinally to disruption of the process.

(iii) Rotary kiln can serve the dual purposeof a coal gasifier as well as an orereducer. Preparation of reducing gasfrom coal is an expensive step, whichis coming in the way of commer-cialisation of coal gasification basedDR process. Therefore, rotary kiln DRprocess has proved commerciallyviable, even with low productivity per

Fig. 2.9 Delicate balance of oxidising and reducing conditions in a sponge iron rotary kiln

Air (4N +O ) introduced

through ir ubes2 2

a tCO + CO2 + H + H O + N2 2 2

CO + ½O = CO2[Partial]

FeO + CO Fe + CO2

CO +C = CO + CO2 2

↑


unit volume, because of this capabilityto perform two different functionssimultaneously.

Figure 2.9 schematically represents thesituation inside a rotary kiln where a delicatebalance of reducing zone within thechargebed and an oxidising zone in thefreeboard is always maintained.

(iv) In comparison to blast furnace, thetemperature of reduction of iron oxideis much lower in rotary kiln (about1000oC as against 1500 to 2000oC in blastfurnace). This means that much lessenergy is required for bringing thereactants to the temperature of reaction.

2.6.2 Product Strengths

Additionally the strengths of the productmade by rotary kiln are:

(i) It is easy to desulphurise iron ore whilemaking sponge iron. Consequently thesponge iron of much lower sulphurcontent can be produced as comparedto blast furnace hot metal. For shaftprocess of sponge iron making, priorand meticulous de-sulphurisation ofnatural gas is necessary to preventpoisoning of catalyst used forreforming.

(ii) Sponge iron produced from rotary kilnis obtained in close granular sizerange. This permits charging in electricor other steel making furnaces in acontinuous manner, obviating the needfor opening and closing of roof.Continuous charging permits partialrefining during melting stage as theparticle passes through the slag layerinto the mixed layer. If adequatemelting energy is available, refiningtime, and consequently, operation timecan be considerably reduced.

2.6.3 Weaknesses of the Process

Notwithstanding the above, rotary kiln hasa number of weaknesses. These are comingin the way of its wide acceptability. The mainprocess related weaknesses of rotary kiln are:

(i) It has very low productivity. Shaftfurnaces, which make sponge iron,give upto five times more output thanrotary kilns of same inner volume.Productivity in rotary kiln isconsequently much lower.

(ii) The rotating reactor makes it difficultto incorporate process control andquality control systems. Energy savingmeasures, such as use of pre-heatedair, are difficult to incorporate. Toprevent ingress of atmospheric air anelaborate sealing system is required,which has made the reactor very“engineering intensive”.

(iii) The RKDR process has low energyefficiency. The stored energy insponge iron is about 1.7 GCal pertonne, while energy usually spent inmaking it in rotary kiln is about 6GCal per tonne. Among other things,a lot of energy goes out in waste gases(over 2 GCal per tonne).

(iv) The RKDR process produces somesponge iron in fine form (-3 mm) whichis a little difficult to utilise in electricfurnaces. While much of the fines aregenerated due to the nature of oreused, the situation is aggravated bythe tumcbling action within the rotarykiln, which forces softer particles tobreak down further.

2.6.4 Weaknesses of the Product

In addition the sponge iron made by rotarykiln has the following weaknesses:


(i) For charging in electric furnaces in

substantial quantities, a system ofcontinuous charging needs to be

installed. This would mean anadditional investment for the existing

units, which are not having thisfacility.

(ii) The sponge iron from rotary kiln hasmuch lower carbon content (usually

0.2%) than either the sponge iron fromshaft furnace (0.7 to 2%) or the hot

metal from blast furnace. Carbon insponge iron not only helps in adding to

the opening carbon in molten bath, italso carries in chemical energy, which

helps in reducing the consumption ofelectric power. Too low a carbon content

comes in the way of a healthy carbonboil and, therefore, bath carburisers

need to be added. Clean carburisers arecostly while coke, char or pig iron carries

with it undesirable elements like sulphurand phosphorous.

(iii) Sponge iron from rotary kiln carrieswith it more gangue and phosphorous

than those from shaft furnace, mainlybecause shaft furnace uses cleaner

inputs. Gangue and phosphorouscontents are much higher than they

are in iron and steel scrap, which meansextra inputs of phosphorous and slag

in electric furnaces.

(iv) When we compare with scrap and pigiron, all sponge irons are prone to re-

oxidation and the product from rotarykiln is no exception. However, this

rotary kiln sponge iron is much lesssusceptible to re-oxidation ascompared to sponge iron from shaftunits using reformed gases.

Those who have ventured into spongeiron have to endeavour to exploit thestrengths of RKDR to the fullest extent andwould have to try to mitigate the effects ofits weaknesses suitably. Those whocontemplate venturing into sponge iron haveto make a thorough analysis as to whetherthe strengths outweigh the disadvantages ornot in the scenario they are finding themselvesin. It becomes the duty of the processdevelopers to put in innovations, which makegreater use of the strengths and minimise tothe extent possible the weaknesses of RKDR.

There are many basic aspects, which needto be considered for making sponge iron inrotary kiln, the important ones being:

(i) Thermodynamics of reduction andgasification reactions

(ii) Characteristics of raw materials andtheir role in the process

(iii) Reaction kinetics, roles of reducibilityof iron ore and reactivity of coal charand thereby the basis of selection ofiron ore and coal

(iv) Movement of solids in the rotary kilnand its residence time

(v) Gas evolution and flow rate

(vi) Heat transfer, temperature profile andprocess model

These would be dealt in the subsequentchapters.

sponge iron review

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