metal and ore preparation

7/30/2019 metal and ore preparation

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Metals and Ores

sourav ch. Cep-17th


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Earth is composed of 112 elements mainly.

They are found in atmosphere and

lithosphere. lithosphere constitutes the main

source of most of the elements. The elements

have been broadly divided into metals and

non-metals on the basis of their physical and

chemical properties


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Occurrence of metals

Metals and their compounds are found in earth as natural elementsknown as minerals.Metals occur in nature as two kinds of state mainly:

1. In the Free State: Element which have low chemical reactivity generallyoccur native or free or metallic state. e.g. Au, Pt noble gas etc. Such

metals are not acted upon by air or water.

2. In the Combined State : Element which are chemically reactive,generally occur in the combined form as compounds such as oxides,carbonates, sulphides, sulphates, silicates, chlorides, nitrates, phosphatesetc.


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ORE

The mineral from which the metal is conveniently and economicallyextracted is called an ore.All the ores are minerals but all mineralscannot be ores. Ores may be divided into four groups,

(1) Metallic core (siderophile) of the earth crust contains (Mn, Fe, Co, Ni,Cu, Ru, Rb, Pd, Ag, Re, Os, Ir, Pt, Au). Entire composition of metals in earthcrust may be given as,

Al (8.3%); Ca(3.6%); Na (2.8%); K (2.6%); Mg (2.1%); Ti (0.4%); Mn (0.1%);Fe (5.1%) other metals (0.1%).

(i) Native ores : These ores contain metals in free state, e.g., silver, gold,platinum, mercury, copper, etc. These are found usually associated withrock or alluvial materials like clay, sand, etc. sometimes lumps of puremetals are also found. These are termed nuggets. Iron is found in free

state as meteroites which also have 20 to 30% nickel. (ii) Sulphurised and arsenical ores : These ores consist of sulphides and

arsenides in simple and complex forms of metals. Important ores of thisgroup are


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ORE PREPARATION

Ores straight from the mine are seldom directlysuited for metal extraction. Quite often theirmetal content is too low for processing in the

furnace, or they may be composed of mineralscontaining different metals requiring differentkinds of metallurgical treatment.

For these reasons, must ores have to undergo

variety of preparatory processes


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The process start witha)comminution (crushing and grinding);b) sizing (by screening);c)classifying (e.g., the grading of finely divided material

by rates of settling);d)separation (e.g., by magnetism, electricalconductivity, specific gravity, etc.).The degree of comminution (size reduction) to be

applied will depend on the size of the ore lumps and on

the requirements of the subsequent treatment to beapplied. Sizing and classifying i.e., grading thecomminuted material according to particle size areimportant operations in ore dressing


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2) Further purification and reduction to obtain the metal in itselementary state:

a)Hydrometallurgy leaching : Metal is refined from oreusing aqueous reactions

b)Pyrometallurgy roasting, smelting: using hightemperatures to obtain the free metal.

c)Electrometallurgy.

3) Final purification and refining of the

metal.


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Steel and Wrought Iron

Steel: Steel is an alloy of iron and carbon

(varying upto 2%) Other alloying elements

sometimes used

are manganese,chromium,vanadium,tungsten.

Varying the amount of alloying elements we

can controls qualities such as

the hardness,ductility, and tensile strength ofthe resulting steel.
http://en.wikipedia.org/wiki/Hardness_(materials_science)http://en.wikipedia.org/wiki/Ductilityhttp://en.wikipedia.org/wiki/Tensile_strengthhttp://en.wikipedia.org/wiki/Tensile_strengthhttp://en.wikipedia.org/wiki/Ductilityhttp://en.wikipedia.org/wiki/Hardness_(materials_science)


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Wrought Iron : Wrought iron is an iron alloy witha very low carbon content in contrast to steel,and has fibrous inclusions, known as slag.

Wrought iron is tough, malleable, ductile andeasily welded. Wrought iron is no longerproduced commercially, however, because itcan be effectively replaced in nearly all

applications by low-carbon steel, which isless expensive to produce and is typically ofmore uniform quality than wrought iron


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Production of Steel

THE MANUFACTURING PROCESS :

Iron ore is converted to steel via two main

steps. The first involves the production of

molten iron and the second is that of actual

steel manufacture.


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Step 1 - The production of molten iron:

The Primary Concentrate is mixed with

limestone and coal and heated. The iron

oxides are reduced in the solid state to

metallic iron, which then melts, and the

impurities are removed either as slag or gas.

The flow diagram for this process is shown inFigure 1.


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The multi-hearth furnaces :

There are four multi-hearth furnaces. The furnaces preheatthe materials fed into the rotary kiln and reduce theamount of volatile matter present in the coal from about44% to about 9%. There are 12 hearths in each furnace .Thetemperature of furnaces is 490C to 620C. The totalresidence time in the multi-hearth furnace is 30 - 40minutes. The waste gas from the multi-hearth furnacecontains water vapour and other volatile compounds fromthe coal (e.g. carbon dioxide, carbon monoxide and other

combustion products) as well as suspended coal andprimary concentrate dust particles. These solids are

removed and returned to the furnace


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The rotary kilns :

There are four rotary kilns. Here about 80% of theiron of the primary concentrate is reduced tometallic iron over period of 12 hour. The pre-heated coal char and primary concentrate fromthe furnaces is mixed with limestone and fed intothe kiln. In the first third of the kiln, known as thepre-heating zone, the feed from the multi-hearth

furnace is further heated to 900 C- 1000C as aresult of the combustion of remaining volatilematter of coal.


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The last two-thirds of the kiln is known as the reductionzone, and here the solid iron oxides are reduced tometallic iron. In this region the air reacts with thecarbon from the coal to produce carbon dioxide and

heat: C + O2 CO2 H= -393 kJ molThe carbon dioxide then reacts with more carbon to

produce carbon monoxide, the principal reductant, inan exothermic reaction which then reduce iron oxide

to iron:C + CO2 2CO H= +171 kJ mol

Fe3O4+ 4CO 3Fe + 4CO2 H= -10 kJmol


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The melters :

The hot reduced primary concentrate from the

kilns is fed into two melters. The lime reacts

predominantly with sulfur from the coal. The

temperature in the melters rises to 1500C,

and this causes the reduced primary

concentrate to melt and form two layers. Thelower layer is of molten iron.


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Step 2 - Steel making


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Vanadium recovery :

Before conversion into steel, vanadium is

recovered from the molten iron. This is done

firstly because vanadium rich slag produced

(15% vanadium as V2O5) has a high value and

secondly because a high vanadium content

can make the steel too hard.


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The Klockner Oxygen Blown Maxhutte process :The KOBM steel making process involves oxidizingdissolved impurities by blowing oxygen through themolten metal. The oxygen oxidizes the elements other

than iron (including any free carbon) to their oxides. Inthis way contaminants are removed as the oxides forma slag which floats on the surface of the molten metal.Powdered lime is blown to help slag formation and thisparticularly reduces the levels of sulfur and

phosphorous by combining with their acidic oxides.now we get molten steel.


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Ladle treatment:

The final stage of steel making is the ladle treatment.The bulk of the alloying elements are added in thefurnace to adjust composition and, after blowing argon

through the molten metal to ensure homogeneity , thetemperature is measured and a sample removed foranalysis after stirring. If the metal requires cooling,scrap steel is added. If the temperature is too low,aluminum is added and oxygen blown through. When

all adjustments are complete argon is blown throughonce again to ensure mixing and the ladle taken to thecontinuous casting machine to shape the steel.


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Cement Steel :Steel produced from wrought iron by cementation, knownas blister steel. In order to convert wrought iron into steelthat is, increase the carbon contenta carburizationprocess was used. Metal Iron are heated with charcoal in

sealed clay pots that were placed in large bottle-shapedkilns holding about 10 to 14 tons of metal and about 2 tonsof charcoal. When the kiln was heated, carbon from thecharcoal diffused into the iron. In an attempt to achievehomogeneity, the initial product was removed from the

kiln, forged, and again reheated with charcoal in the kiln.During the reheating process, carbon monoxide gas wasformed internally at the nonmetallic inclusions


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crucible steel :

A high-quality steel made by melting wrought

iron, charcoal, and other additives in a

crucible. It is formed by slowly heating and

cooling pure iron and carbon (typically in the

form of charcoal) in a crucible . It was used to

produce sword blade ,knife etc.


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Carbon Steel :Carbon steel is an alloy made up of mainly

iron and carbon but still other elements do

exists in this alloy as shown in table 2 below:


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Out of these elements, Phosphorus, Sulphur andSilicon are considered as minor elements as theyhave some negative impacts on the steel.

Effects of minor elements on steel:1) Phosphorus: Phosphorus is an element, whichaffects primarily the ductility and the toughnessof steel. It react with the iron to form iron

phosphide (Fe3P) which has the speciality ofbeing brittle. Hence, phosphorus renders steelless tough and ductile while it increasesbrittleness.


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2)Sulphur:Sulphur is a trace element, which has a great tendencyto segregate (that is to isolate itself in the structure).Trace amounts of sulfur in particular make the

steel red-short(brittle at a red heat). Sulphur plays agreat role also in the drop in weldability, impacttoughness and ductility of steel.3)Silicon:

silicon has the tendency of making hot and coldworkability and machinability less effective. It alsoaffects the surface quality of the steel.


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Wrought iron manufacturing process:The modern technique of making wrought ironuses molten iron from a Bessemer converter andmolten slag, which is usually prepared by melting

iron ore, mill scale, and sand in an open-hearthfurnace. The molten slag is maintained in a ladleat a temperature several hundred degrees belowthe temperature of the molten iron. When themolten iron, which carries a large amount of gas in

solution, is poured into the ladle containing themolten slag, the metal solidifies almost instantly,releasing the dissolved gas.


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The force exerted by the gas shatters themetal into minute particles that are heavierthan the slag and that accumulate in thebottom of the ladle, agglomerating into a

spongy mass with impurities. After the slaghas been poured off the top of the ladle, theball of iron is removed and squeezed androlled by subjecting the impure metal to intense

heat and frequent stirring inside a furnace, and inthe presence of oxidizing substances. This processfrees the metal from some of its impurities, suchas sulfur and carbon.

metal and ore preparation

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