Principles of Extractive Metallurgy - · PDF fileCore Sub-assemblies for Breeders Activities...

Principles of Extractive Metallurgy

Dr. Sunil Tonpe

Nuclear Fuel Complex

Hyderabad

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What is Extractive Metallurgy ?•Deals with extraction of metals from its naturally existing ore/minerals and refining them•Minerals: Inorganic compounds with more than one metal in association with non-metals like S,O,N etc.•Naturally existing minerals are sulphides, oxides, halides like:Hematite (Fe2O3), Magnetite (Fe3O4), Chalcopyrite (CuFeS2), Dolomite (CaCO3.MgCO3) ..list is endless.


What are the sources of metals ?•Earth Crust: (Aluminum: 8.1%, Iron 5.1%, Calcium: 3.6%, Sodium: 2.8%, Potassium: 2.6%, Magnesium: 2.1%, Titanium: 2.1%, Manganese: 0.10%)•Ocean water: ( Na: 10500 g/ton, Mg: 1270 g/ton, Ca: 400 g/ton, K: 380 g/ton) ; Ocean nodules (Mn: 23.86%, Mg 1.66%, Al 2.86%, Fe 13.80%..)•Recycled scrap (at the end of metals' life)

•Abundant:

Al, Be, Cr, Fe, Mn, Mg, Ti, Zr, Th,

Pb and Zn, raw earth metals

•Very small:

Co, Ni,Cu,Sn, Au,V,Ni, Cd and U.

•Poor or not found:

Sb, Bi, Co, Hg, Mo, Nb, Ta, Sr, Se,

Ag, W, Pt

Resources of metal containing minerals in India

Types of ores

Oxide ores: Examples: Fe2O3, Fe3O4

Apart from Fe, other heavy metals which are produced from oxide ores are: Manganese, Chromium, Titanium, Tungston, uranium and Tin.

Sulphide ores: Copper ore (CuFeS2, Chalcopyrite), sphalerite (Zn,Fe)S, Galena PbS, Pyrite FeS2.

Others: Nickel, Zinc, Mercury and MolybdenumHalide ores: Rock salts of Sodium, Magnesium chloride in sea water


Commercial production of metals:

Availability of ore deposits

Concentration of metal in the ore

Availability of technology of extraction and refining of

that metal

Physical and chemical properties of the metal

Market demand of that metal

Economy of the process:Readily available, Easily produced and available at low processing cost with desired properties


Extraction of metals

Only some unreactive metals such as silver, gold and platinum can occur freely in nature. Most metals

react with other elements to form ores.

Major steps in extraction of metal

• Ore concentration• Ore is purified and concentrated, unwanted rocks

removed

• Reduction to crude metal• Metal oxides to be reduced to metals, resulting in a

mixture of metals collected

• Refining to obtain pure metal• To obtain a specific metal, purify and remove unwanted

metal impurities


Extraction of metal involves:

o Getting rid of the unwanted rock to obtain concentrated form of the mineral

o Obtaining pure metal from the mineral by chemical reactions

Method of extraction depends on the position of the metal in the reactivity series.


Metals at the top of the reactivity series are very reactive:

bonds in their compounds are very strong

must be extracted by decomposing their compounds with electricity in an expensive process called electrolysis

aluminum is extracted from aluminum oxide by passing an electric current through it

2Al2O3 4Al + 3O2

extraction of metals


Ways of Extraction

• Potassium K

• Sodium Na

• Calcium Ca

• Magnesium Mg

• Aluminium Al

• Zinc Zn

• Iron Fe

• Tin Sn

• Lead Pb

• Copper Cu

• Mercury Hg

• Silver Ag

• Gold Au

• Platinum Pt

Extracted by electrolysis of molten

chlorides

Extraction by electrolysis of molten

Al2O3 dissolved in cryolite

Extraction by reduction of oxides

using carbon

Roasting ore by heating alone

Unit processes and Unit operations

• Any metal extraction process is the combination of similar and unique kind of steps known as Unit processes/unit operations.

• Unit operations: Physical operations like crushing, grinding, sizing, mixing through agitation, filteration, distillation, comminution

• Unit processes: Chemical processes like leaching, smelting, roasting, Electrolysis, decarburization, Dephosphorization, Degassing, Deoxidation etc.

Combination of all unit steps/processes are resulting in Flow-Sheets

According to phases involved:

• Gas-Solid: Roasting, Gas reduction • Gas-liquid: steelmaking blowing/refining, Distillation • Liquid-Liquid: Slag metal reactions • Solid-solid: Leaching, precipitation etc.

Classification of unit processes/ operation by different criteria

According to equipments involved:

• Fixed bed: Sintering, percolation leaching • Fluidized bed: Fluidized roasting and reduction • Shaft furnace: Iron blast furnace, lime calcination kiln • Rotary kiln: Drying and calcination• Retort: Coke open, carbothermic zinc production• Reverberatory furnace: Matte smelting (Cu etc.), open hearth steelmaking • Electric arc furnace: Steelmaking, matte smelting, ferro alloy production • Cell for salt fuse electrolysis: Production and refining of aluminium• Cell for aqueous electrolysis: Electrolytic reduction and refining

• Oxidation: Roasting, sintering, LD steelmaking

• Reduction: Blast furnace ironmaking

• Slag metal reactions: Steelmaking, matte smelting

• Chlorination: Titanium (converting to tetrachloride)

• Electrolytic reduction: Zinc and Aluminium production

• Electrolyte refining: Refining of Copper and Nickel

Classification according to chemical reactions

Physical seperation/Mineral processing :

The objective is to concentrate the metallic content in the ore, achieved by a series of comminition (crushing and grinding), screening and separation process

Pyrometallurgy : It involves the smelting, converting and refining of metal

concentrate.

Hydrometallurgy : It involves the precipitation of metal in an aqueous

solution.

Electrometallurgy : Electrolysis process to extract metal.

• Electrowinning: Extraction of the metal from electrolyte;

• Electrorefining: Refining of impure metals in the form of an anode.

Majority of metals are extracted by pyro-metallurgical route because it is fast, easily adaptable and cheaper


Principles you must know ?

• Heat and mass balance : to know the material requirement

• Thermodynamics : Feasibility criteria

• Kinetics and rate of process: How long it take to complete the process

• Heat transfer: For improving the thermal efficiency of the process

• Fluid dynamics: To know the mixing of the reactor

• High temperature properties of metals/slag: To know the physical properties of various phases, their mobility and role in metal refining processes.

• Electrochemistry: To estimate, over-potential, current efficiency

• Hydrometallurgy: Eh-pH diagram, rate estimation of leaching process


(e) Used as a tertiary crusher (f) Used as a grinder

(d) Used as a secondary crusher

Physical separation/mineral processing

Comminution process: Size reduction of mineral

By crushing/grindinga) Jaw crusher

b) Roll crusher

c) Gyratory crusher

d) Cone crusher

e) Hammer mill

f) Ball mill

Classification process: Due to different size, shape

and densities, materials are classified in fluids/water. It

depends upon following factors:

1. Smaller particles fall more slowly in fluids than do

larger ones (stokes law)

2. In cyclonic movement (hydro-cyclone), centrifugal

force have larger influence on larger size particles

than smaller ones.

3. Small particles having low inertia behaves like

suspended medium.

4. Larger particles require higher velocity for separation

Classifiers:

I. simple Box Classifier

II. Bowl/rake classifier III. Hydro-cyclones

Feed

Vortex finder]

Primary vortex

Secondary vortex

Slurry of ore fines in water

Water pressure control

Coarse particles Fine particles

Overflow containing the very fine particles and gongue (tailings)

Hydrocyclone

a) Simple sluice box classifier

b) Bowl/rake classifier

Separation process (Froth Floatation)

Due to different surface free energies of the different minerals,

there is selective adsorption on to the air bubbles

• Frothers: To stabilize the air bubbles

• Collectors: Selective adsorption by lowering interfacial

energies.• Modifying agents: Intensify the collector performance

Agglomeration process: Example : Sintering of iron ores

Moving bed of fine iron ore (<6 mm), mixed with coal fines (5-6%, as a

fuel and water (10-12%, for permeability) is ignited for agglomeration of oxide and sulphide fines.

Extraction MetallurgyCase studies

• Copper – Pyrometallurgy route and hydrometallurgical alternative.

• Hydrometallurgical processes – ion exchange processes,

solvent extraction, and bacterial leaching.

• Iron – Pyrometallurgy and the blast furnace.

• Aluminium – Electrolytic reduction.

Pyro-metallurgy of copper

Pyrometallurgy is the use of heat to reduce the mineral

to the free metal, and usually involves 4 main steps:

1. Calcination: thermal decomposition of the ore with

associated elimination of a volatile product.

2. Roasting: a metallurgical treatment involving gas-

solids reactions at elevated temperatures.

3. Smelting: a melting process which separates the

chemical reaction products into 2 or more layers.

4. Refining: treatment of a crude metal product to

improve its purity.

Pyrometallurgy of copper

Cu ore usually associated with sulphide minerals.

Most common source of Cu ore is the mineral chalcopyrite

(CuFeS2), which accounts for ± 50% of Cu production.

Other important ores include:

chalcocite [Cu2S],

malachite [CuCO3 • Cu(OH)2],

azurite [2CuCO3 • Cu(OH)2],

bornite (3Cu2S • Fe2S3),

covellite (CuS).

Pyro-metallurgy of copper

The following steps are involved in Cu extraction:

1.Concentration

2.Roasting

3.Smelting

4.Conversion

5.Refining

Pyro-metallurgy of copper : Concentration

Finely crushed ore concentrated by the froth-flotation process:

•Ground ore mixed with xanthates (salts & esters of xanthic acid),

dithiophosphates, or thionocarbamates. These make the ore

surface hydrophobic.

•Ore then introduced into a water bath where air is bubbled through

the suspension.

•Finely divided hydrophobic ore particles latch on to the air bubbles

and travel to the surface where a froth is formed.

•The froth containing the Cu ore is skimmed off and reprocessed.

•The remaining material (sand particles & other impurities) sink to

the bottom & is discarded or reprocessed to extract other elements

Pyro-metallurgy of copper :Roasting

• Involves partial oxidation of the sulphide mineral with air at

between 500C and 700C.

• For chalcopyrite, the main reactions are:

CuFeS2(s) + 4O2(g) → CuSO4(s) + FeSO4(s)

4CuFeS2(s) + 13O2(g) → 4CuO(s) + 2Fe2O3(s) + 8SO2(g)

• Reactions are exothermic, roasting is an autogenous

process requiring little or no additional fuel.

• Not all the sulphides are oxidised, only around 1/3. Rest remain

as sulphide minerals.

• The gases produced contain around 5 – 15% SO2, which is

used for sulphuric acid production.Objectives of roasting:1)Remove part of the sulphur.2)Convert iron sulphides into iron oxide and iron sulphate to facilitate removal during smelting.3)To pre-heat the concentrate to reduce amount of energy needed by the smelter.

Pyro-metallurgy of copper : Smelting

• Smelting consists of melting the roasted concentrate

to form 2 molten phases:1) a sulphide “matte”, which contains the iron-copper

sulphide mixture.

2) an oxide slag, which is insoluble in the matte, and

contains iron oxides, silicates, and other impurities.

• Smelting is carried out at around 1200C, usually with a

silica flux to make the slag more fluid.

• The matte layer sinks to the bottom, and the slag layer

floats on top of the matte & is tapped off & disposed of.

• The main reaction is the reduction of copper oxides

(formed during roasting) back into copper sulphide to

ensure that they migrate into the matte phase:

Pyro-metallurgy of copper : Conversion

• After smelting, matte contains from between 30 to 80% Cu in

the form of copper sulphide.

• The sulphur is removed by selective oxidation of the matte with

O2 to produce SO2 from S, but leave Cu metal.

• Converting is carried out in two stages:

1) an iron removal stage

2) a copper-making stage.

Iron removal

• A silica flux is added to keep the slag (see below) molten.

• Air is blown into the converter to oxidize the iron sulphide

•The oxidized Fe and Si form a slag (insoluble in matte) that is

skimmed off & disposed off.

Pyrometallurgy of copper : Refining

• The copper is refined by electrolysis.

• The anodes (cast from blister copper) are placed into an

aqueous CuSO4/H2SO4 solution.

• Thin sheets of highly pure Cu serve as the cathodes.

• Application of a suitable voltage causes oxidation of Cu metal

at the anode.

• Cu2+ ions migrate through the electrolyte to the cathode, where

Cu metal plates out.

• Metallic impurities more active then Cu are oxidized at the

anode, but don’t plate out at the cathode.

• Less active metals are not oxidized at the anode, but collect at

the bottom of the cell as a sludge.

Hydrometallurgy of copper

Advantages

• Much more environmentally friendly than

pyrometallurgy.

• Compared to pyrometallurgy, only a fraction of the

gases liberated into the atmosphere.

• Emissions of solid particles comparatively non-

existent.

Disadvantages

• Large amount of water used, greater potential for

contamination.

• Waste waters contain soluble metal compounds,

chelating compounds & organic solvents.


The following steps are involved:

1.Ore preparation

2.Leaching

3.Solution purification

4.Metal recovery

Hydrometallurgy of copper : Ore preparation

• Ore undergoes some degree of comminution (crushing &

pulverisation) to expose the Cu oxides & sulphides to leaching

solution.

Hydrometallurgy of copper : Leaching

Definition : The dissolution of a mineral in a solvent, while

leaving the gangue (rock or mineral matter of no value)

behind as undissolved solids.

Dump leaching

Heap leaching

Hydrometallurgy of copper: Solution Purification

Ion exchange chromatography

• DEFINITION: a solution containing a mixture of metal

ions is contacted with a resin that is insoluble in the

metal-ion solution.

• Ion-exchange resin consists of an inert solid phase to

which labile functional groups are chemically bonded.

• Functional groups can either be acidic (H+) or basic

(OH–) groups that exchange with cations (M+) or

anions (M–), respectively.

• The ion-exchange process is reversible.

Hydrometallurgy of copper : Solvent extraction

• DEFINITION: a method to separate compounds

based on their relative solubilities in 2 different

immiscible liquids.

• In industry, this is usually set up as a continuous

process

Hydrometallurgy of copper Solvent extraction

• Organic + aqueous stream pumped into a mixer.

• Organic (containing an extractant) and aqueous

components mix, and ion transfer occurs between them.

• Once ion transfer is complete (equilibrium), mixture is

allowed to separate.

• Aqueous solution is removed & the organic phase

(containing the Cu2+) is mixed with an aqueous stripping

solution.

• Cu2+ moves back into the aqueous phase, and the two

phases are again allowed to separate.

• The aqueous phase (containing the Cu2+) is removed &

the organic phase is recycled back into the first mixer.

Solution Purification:

Hydrometallurgy of copper Electrochemical recovery

• An electrochemical process for precipitating metals from

solution.

Electrowinning

•The anodes consist of unrefined impure metal.

•Current passes through the acidic electrolyte corroding the anode into the solution.

•Refined pure metal deposited onto the cathodes.

•Metals with a greater Ered than Cu (such as Zn and Fe) remain in solution.

•Metals with a lower Ered than Cu (Au, Ag) accumulate as an “anode sludge”

collected & sold for further refining.


Metal Recovery: Electrochemical recovery

Electrorefining


Summary:

Pyrometallurgy of iron

• The most important sources of iron are hematite

(Fe2O3) and magnetite (Fe3O4).

• Prehistorically, iron was prepared by simply heating it

with charcoal in a fired clay pot.

The reduction of iron oxides to the

metal is accomplished in a blast

furnace.


• Molten iron is produced lower

down the furnace & removed.

• Slag is less dense than iron &

can be drained away.

• The iron formed (called pig iron)

still contains around 4-5% C, 0.6-

1.2% Si, 0.4-2.0% Mn + S and P

and needs to be further

processed.


• Cast iron is made by remelting pig iron & removing

impurities such as phosphorous and sulphur.

• The viscosity of cast iron is very low, & it doesn’t

shrink much when it solidifies.

• Ideal for making castings.

• BUT, it is very impure, containing up to 4% carbon.

This makes it very hard, but also very brittle.

• Shatters rather than deforms when struck hard.

• These days cast iron is quite rare, often being

replaced by other materials.

Cast iron


• Pig iron is brittle, and not directly very useful as a

material.

• Typically, pig iron is drained directly from the blast

furnace (referred to as hot metal), and transported to

a steelmaking plant while still hot.

• The impurities are removed by oxidation in a vessel

called a converter.

• The oxidising agent is pure O2 or O2 mixed with Ar.

• Air can’t be used as N2 reacts with iron to form iron

nitride which is brittle.

Steelmaking


Steelmaking

Iron converter

• O2 blown directly into molten

metal.

• Reacts exothermically with

C, Si + other impurities.

• C & S expelled as CO and

SO2 gas.

• Si oxidised to SiO2 &

incorporates into the slag

layer.

• Once oxidation complete,

contents poured out &

various alloying elements

added to produce steels.

• Wrought iron – iron with all the C removed. Soft &

easily worked with little structural strength. No longer

produced commercially.

• Mild steel – iron containing around 0.25% C. Stronger

& harder than pure iron. Has many uses including

nails, wire, car bodies, girders & bridges, etc.

• High carbon steel – contains around 1.5% C. Very

hard, but brittle. Used for things like cutting tools, and

masonry nails.

Types of iron & steel


• Stainless steel – iron mixed with chromium and nickel.

Resistant to corrosion. Uses include cutlery, cooking

utensils, kitchen sinks, etc.

• Titanium steel – iron mixed with titanium. Withstands

high temperatures. Uses include gas turbines,

spacecraft parts, etc.

• Manganese steel – iron mixed with manganese. Very

hard. Uses include rock-breaking machinery, military

helmets, etc.

Types of iron & steel


The thermite reaction

• Aluminium metal can reduce Iron(III) oxide (Fe2O3) in

a highly exothermic reaction.

• Molten iron is produced at around 3000C.

• Reaction used for thermite welding, often used to join

railway tracks.

Fe2O3(s) + 2Al(s) 2Fe(s) + Al2O3(s)


Oxygen Steelmaking process for refining of pig iron

Selective oxidation of C,Si,Mn,P,Fe with the help of high speed

oxygen blow. Process done in a basic lined vessel and Lime

added as slag former to combine SiO2. Oxidizing and enough

volume of slag is formed to promote phosphorous removal.

Process generates a lot of heat of oxidation which is compensated by iron ore/scrap additions as a coolant.

Extraction of Titanium

• Titanium ore exists in the form of oxide (Rutile,TiO2) or

Ilmenite (FeO.TiO2).

• As a first step TiO2 is chlorinated at 900ᵒ C:

TiO2 (s) +C(s) + 2Cl2(g) = TiCl4 (g) + CO2 (g)

• TiCl4 is removed by selective distillation, followed by

reduction of TiCl4 by Magnesium, known as Kroll’s process.

TiCl4(l) + 2Mg(l) = 2MgCl2(l) + Ti (s)

Extraction of reactive metal by halide route

• Oxidation to remove C,Si,P,Mn etc (steelmaking)

• Sulphidation to remove :Cu,Ni.Co from lead; Cu from

tin etc.

• Chlorination to remove Zn from lead; Zn,Cu and Pb

from bismuth

• Electrochemical method by cathodic deposition; examples: Cu,Ag,Au,Ni,Co,Pb,Sb,Bi etc.

Refining process

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Zircon Sand

ZirconiumOxide

ZirconiumSponge

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IREL

55

TBP & KEROSENE

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FRIT LEACHING

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WET CAKE

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DISSOLUTION

SOLVENT EXTRACTION

PURE SOLUTION

PRECIPITATION

DRYING AND

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FUSION

ZrO2

AMMONIUM

NITRATE

SULPHATE

EFFLUENT

ACIDIC

RAFFINATE

EFFLUENT

HNO3

ALKALINE

EFFLUENTWATER

NaOH

PRODUCTION OF Hf FREE ZrO2

Zirconium Sponge Production Flow Sheet

Feed preparation

Coking

Carbo oxide chlorination

Magnesio thermic Reduction

Pyro vacuum distillation

Scrap chlorination

2T ZrO2

0.4T C

60Nm3

Temp:10000 C3.5T Cl2

1.0T Mg

Argon purging 80Nm3

Briquettes

Coked briquettes

Temp: 650°C

Starch 50Kg

MgCl2 Gr-I waste 2.3T

& Mg Scrap

ZrCl4 waste 0.3T

Magnesium scrap 0.46TTemp: 990°C

Vacuum < 100 microns

Zr Sponge

Zircalloy scrap

Zr + Mg + MgCl2

Magnesium Di-Uranate (MDU) Solvent Extraction Unit Rotary Calcination Furnace

Powder Compaction PressPusher type Continuous Sintering Furnace

Fuel Bundle

Dissolution, Solvent Extraction & Precipitation

Calcination,Reduction & Stabilisation

Granulation &

Pelletisation

Centreless Grinding

End-cap Welding Machine

Major Activities in Fuel Fabrication

Sintering

Stacking,Loading &

End-closure weldingAppendageWelding &Assembly

End-plate Welding Machine

QC/QS & Dcoumen-tation

Zinc metallurgy at Zawar mines

Zinc

Indian metallurgists were familiar several other metals, of which zinc deserves aspecial mention because, having a low boiling point (907°C), it tends tovaporize while its ore is smelted.

Zinc, a silvery-white metal, is precious in combination with copper, resulting inbrass of superior quality. Sometimes part of copper ore, pure zinc could beproduced only after a sophisticated ‘downward’ distillation technique in whichthe vapour was captured and condensed in a lower container. This technique,which was also applied to mercury, is described in Sanskrit texts such as the14th century Rasaratnasamuccaya.


Text books:

Principles of Extractive Metallurgy, Terkel Rosenqvist, McGraw-Hill Book Company

Principles of Extractive Metallurgy, H. S. Ray and A. Ghosh, WEL Publishing

Extractive Metallurgy of Copper, W.G. Davenport, A.K. Biswas, PERGAMON publishing company

Handbook of Extractive Metallurgy: Fathi Habashi; Wiley-VCH

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