Chapter-3. Raw Materials Used in Steel Plant

RAW MATERIALS USED IN STEEL PLANT

3.1. INTRODUCTION The raw materials required for the production of iron and steel can be grouped as follows:



Ferrous materials (Iron ores, Scrap).



Fuels and reducing agents (Coke, coal, oil, gas)



Fluxes (Lime, Alloying agents)



Refractories

The most important raw materials for melting iron and steel are the iron ores. As iron and steel is produced at high temperatures, refractory refractory for for

lini lining ng the the

prod produc ucti tion on inst instal alla lati tion ons s

are are

also also of cons consid ider erab able le

significance. 3.2. •

FERROUS MATERIALS IRON IR ON OR ORES ES : Mainly compounds of iron and oxygen (= iron oxides) mixed with

impurities (gangue) are called ores of Iron . Among the elements in the earth’s crust, iron, Fe, with an occurrence of about 5.6 percent ranks fourth, after oxygen, silicon and aluminum. Iron does not occur in a pure form in nature but only in compounds. The most frequent are the iron-oxygen compounds (iron oxides). Iron oxides are always mixed with impurities which are called the gangue. This mixture of iron oxides and gangue is classified as iron ore, provided melting is economically feasible. Gangue plays an important melting is economically feasible. Gang Gangue ue play plays s an impo import rtan antt part part in the the melt meltin ing g of iron iron ores ores.. If the the gangue contains mainly lime (CaCo 3) the ore is “basic”, if silicic acid (SiO2)

pre predom dominat inates es,,

the the

ore ore

is

“ac “acid” id”.

Fur Further ther

impur puritie ities s

are are

aluminums and phosphates. The appearance of the iron ores varies 16

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considerably according to geographical origin. Iron ores mostly take the from of a hard rocky mass. The colour varies generally between reddish and brownish shades and a metallic black. The weight is determined by the iron content. (IS-5842, 5843, 11093, 11373) (16)

Grades of iron ores: Corresponding to the varying iron compounds we find a great number of different grades of iron ore. They are characterized by the type type of iron iron oxyg oxygen en comp compou ound nd as well well as by the the tram tramp p elem elemen ents ts,, gang gangue ue and and othe otherr nonnon-me meta tall llic ic and and nonnon-fe ferr rrou ous s oxid oxides es.. The The most most important grades of iron ores are briefly described. (IS-7438) 

Magnetite, magnetic iron ore, Fe 3O4Magnetite is an iron ore which possesses a large share of iron (60

to 70 percent) and which is to a high degree free from undesired tramp elements. The gangue is of a siliceous nature (acid). Iron and oxygen atoms are very closely combined with each other in magnetite, thus making making magnet magnetite ite “d “dif iffi ficu cult lt to redu reduce ce” ”. As the name indicates magnetite is very magnetic. 

Hematite, red iron ore, Fe2O3Hematite possesses likewise a high Fe content and has mostly

low phosphorus and sulphur contents with a siliceous and clay gangue (acid). The typical reddish colour is caused by the iron (III) oxide. In the case of red iron ore, the compound of iron and oxygen is not so “tight” and and so the the hemat ematiite is rega egarded rded as “ easily easily reducible reducible”. ”. Workable deposits are found in all parts of the world. Special forms of the red iron ore are taconites, itaborites and laterites. 

Limonite, brown iron ore, Fe 2O3. H2O-






deposits are fairly large. Special forms of brown hematities are the ooli ooliti tic c or bean bean ores ores,, lake lake iron iron ores ores and and bog bog iron iron ores ores.. Impo Import rtan antt deposi deposits ts have have been been the Lorr Lorrain ainese ese minet minette te and the Salzgi Salzgitte tterr ores. ores. Both grades of the ore have lost their economic importance. 

Siderite, spathic iron ore, FeCO 3 Containing 30 to 49 percent Fe, siderites are relatively easy to

reduce: mostly they contain lime and manganese and a low amount of phosphorus. Variations of spathic iron ore originate from clay and coal (bla (black ck band band)) cont contam amin inat atio ion. n. Depi Depisi sits ts of side sideri rite tes s are are no long longer er of economic significance. (3, 16)

Composition of different ferroalloys: 1. High carbon ferro-manganese- (IS-4763) Analysis

Mn Mn

S

75-80 75-80% %

C

<0.02 <0.025% 5%

Pb

6-8% 6-8%

Si

0.1% 0.1%

P

<1% <1%

<0.1% <0.1%

2. Low Carbon Ferro-manganese- (IS 4763) Analysis

C

Si Si

0.4-1.5%

Mn

<1%

P

85-90%

Pb

<0.1%

S

<0.04% <0.02%

3. Ferro-Saudamet (Medium carbon ferromanganese)- (IS 4763) Analysis

Mn

C

75-85%

2.5- 3.5%

4. Silicon Manganese- (IS-13164) Analysis

Si

Mn

20-25%

65-70%

C 0.501%

S <0.04%

Pb <0.04%

5. Ferro-Silicon There are several grades of this alloy available depending upon





RAW MATERIALS USED IN STEEL PLANT b)

Silicon 45-50%. This grade may be used for either increasing the silicon content of the steel or increasing the silicon content of the steel or blocking the bath. (P< 0.05%). (IS-1110)

6. Calcium silicide (CaSi)Analysis

Ca

Si

30-33%

C

60-63%

A1

0.05%

Calcium silicate

1%

0.5%

7. Ferro-chrome high carbon- (IS-1170) Analysis Cr 65-75%

C

Si

Mn

6-8%

0.2-1%

0.1-0.5% 0.1-0.5%

S

P

<0.05% <0.05%

<0.05% <0.05%

S

P

8. Low Carbon Ferro-chrome- (IS-13452) Analysis Cr

C

62-67%

Si

1.58 max

Mn

0.2-1 0.1-0.5% <0.05%

<0.05%

9. Ferro-molybdenum- (IS-1469) Analysis

C

Mo

0.5-1 0.5-1.4% .4%

65%

Si <0.5 <0.5

P <0.05 <0.05% %

S <0.1% <0.1%

Mn traces traces

10. Ferro-titanium- (IS-1468) Analysis

Ti 60-65%

11. Pure nickel This

is usually in the form of shot and is of over 90 per cent purity.

0=<0.05%, Ni+Cr=>99%, Si=<0.03%, Mn, S&P= trace, Co=0.1%. (IS4409)

12. Ferro-tungsten- (IS-1467) Analysis

C 1% Maximum80%

W






0.01% 0.01%

50-60% 50-60%

0.5-1.5% 0.5-1.5%

<0.05% <0.05%

<0.2% <0.2%

1%

traces 15. Ferro-niobium- (IS-3014) Analysis

Nb 40

+ Ta

(Niobium + tantalum)

30%

OTHER FERROUS MINERALS 

Manganese ores: Apart from their main content of various manganese oxides, they

also contain considerable amounts of iron oxides. Ferrous manganese used in alloying is melted from manganese ores. (IS-4763 & 11895) 

Iron silicates: These ores are found in very large quantities, but for economic

reason reasons s have have rarely rarely been been melted melted,, with with the contin continuin uing g deplet depletio ion n of remaining iron ore deposits, iron silicates may gain insignificance. 

Titanium ironstone: Also found in large quantities, this represents, just as the iron

silicates, until now rarely used reserves. (IS-1468) •

Scrap: Worldwide, scrap participates 40 percent in steel production input

and must therefore be considered as an important raw material. Its use in steelmaking varies, depending on the production process applied. In BOF steelmaking, scrap accounts for about 20 percent while in electric steel production, it is 100 percent. In contrast, the use of scrap in blast furnaces is of minor significance nowadays. Scrap occurs, in particular, during rolled steel production at the steel plants (ends, edges, etc).






and the impur impuriti ities es and undesi undesired red tramp tramp elemen elements, ts, e.g. e.g. non-fe non-ferr rrous ous metals, removed in special dressing plants. Nowadays this is frequently done in modern pressing and crushing or shredding equipment. Using the various kinds of commercial scrap (e.g. packets, loose scrap, chips, shredding) helps to promote p romote recycling. (16)

3.3.

(16): FUELS AND REDUCING AGENTS (16):

Th The

fuel uels

and and

reduc educiing

agen agentts

requi equirred

for for

metal etalllurgi urgica call

processes enter the blast furnace as coke, coal, oil or gas. These fuels are mainly used for reducing the iron oxides into metallic iron and for carburizing the iron. The most important reducing agent is coke. The coking process converts coal into this valuable agent. The coal has a low content of sulphur (coking coal, gas coal), good baking qualities and and a not not too too high high amou amount nt of vola volati tile le comp compon onen ents ts.. Coki Coking ng is the the heating up of the coal with the exclusion of air (dry distillation). In the process, the volatile components, coke-oven gas, tar, benzol, sulphuric hydr hydrog ogen en,, ammo ammoni nia a are are driv driven en out, out, coll collec ecte ted d and and us used ed for for othe otherr purposes. The ground coal mixture is coked in large horizontal chamber ovens at temperatures of 900 to 1,200 0C. Many such units combine to form form a batt batter ery y of coke coke oven ovens. s. Coki Coking ng take takes s 14 to 20 hour hours. s. Then Then scrapers push the carbonized coke on to a wagon which travels to a quenching quenching tower where the coke is cooled cooled down-quen down-quenchedched- as quickly quickly as possible. This is followed by screening. (IS-1353, 5052, 5451, 9949). Now processes for producing coke use briquetted, lower grade coal, which is continuously coked. The “formed coke” derived from this process meets the requirements of the blast furnace operation for a








The coke has several jobs to perform in the blast furnace. It is the reduction agent, supplies the heat and carries the whole bulk column in the blast furnace. furnace . The size of the coke grain, larger compared to that of the ore, sinter and pellet, and its porosity increase the voids fraction and and all allow goo good per permeabi eabili lity ty of the the colu olumn. mn.

Character Characteristic istic and

continuously tested features of the coke are its strength and grain size. The share of ash, sulphur and water should be as low as possible. The consistency of these features i.e. their variation within limits as narrow as poss possib ible le,, are are of sp spec ecia iall sign signif ific ican ance ce for for trou troubl ble e free free furn furnac ace e operation, high output and low coke consumption. Some of the coke in the blast furnace can usefully be replaced by coal, oil or gas. Complete displacement of coke, however, will not be possible, as the mechanical functions cannot be taken over by other reducing agents. (IS-7189 for B.F. Coke, 1752, 5018 & 5081))

3.4.

FLUXES

(19)

The gangue of the ores and the ashes of the coke have a high melt meltin ing g poin pointt of abou aboutt 1,70 1,700 0 to 2,000 2,000 0C depe depend ndin ing g on the the gang gangue ue constituents. These constituents would melt only with difficulty or not at all at the working temperature of the blast furnace and so fluxes have to be added which lower the gangue melting point to 1,300 to






3.5.

REFRACTORIESReactions during the production of iron and steel take place at

temper temperatu atures res from from 1,000 1,000 to 2,500 2,500 0C. Thes These e temp temper erat atur ures es and and the the additional mechanical demands made on the equipment used in iron and steelm steelmaki aking ng can only only be match matched ed bylini bylining ng the reacti reaction on vessel vessels s with refractory material. These are mainly non-metallic (i.e. metallic oxide) heavy duty ceramics with a heat resistance of at least 1,580 0C (pyr (pyrom omet etri ric c cone cone equi equiva vale lent nt). ). Besi Beside des s temp temper erat atur ure e

stab stabil ilit ity y

and and

resist resistanc ance e to mechan mechanica icall loads, loads, these these refrac refractor tories ies should should under under no circ circum umst stan ance ces s part partic icip ipat ate e or inte interv rven ene e in the the chem chemic ical al reac reacti tion ons. s. Refrac Refractor tory y lining linings s consis consistt of moulde moulded, d, burnt burnt or unbru unbrunt nt (chemi (chemical cally ly compou compounde nded) d) or else else cast cast produc products ts (bric (bricks) ks) or unshap unshaped ed refrac refractor tory y materi materials als.. They They are dresse dressed d with with a liqui liquid d to develo develop p their their bondin bonding g capacity (mortar, (mortar, mass flux, concretes). In iron and steel steel industry, the foll follow owin ing g grou groups ps of refr refrac acto tory ry are are us used ed:: sili silica ca prod produc ucts ts,, fire fire clay clay produc products, ts, mullit mullite e and corund corundum um produ products cts,, magnes magnesia ia and chromi chromium um products, dolomite products, carbon bricks and compounds. (IS-14852 – Carbon Bricks, IS – 3305-For chrome Magnesite bricks, IS14296- for Dolomite bricks, IS- 195, for- Fireclay bricks, IS- 1527, 2043for Silica bricks). (19)

Chapter-3. Raw Materials Used in Steel Plant

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