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New EraNew EraPressure HydrometallurgyPressure Hydrometallurgy

4. Leaching in absence of4. Leaching in absence of oxygenoxygen

Fathi HabashiFathi HabashiLaval University, Quebec City, CanadaLaval University, Quebec City, Canada

Fathi.Habashi@arul.ulaval.caFathi.Habashi@arul.ulaval.ca

Bauxite

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Aluminum minerals in bauxite are soluble indilute H 2SO 4 but this process is not used onlarge scale for the following reasons:

• Iron minerals and to some extent titaniumminerals are also soluble; this will lead to anexcessive reagent consumption and solutionpurification problem later.

• Al(OH) 3 precipitated from acid solutions isgelatinous and difficult to filter and wash.

Acid leaching is used only on a small scale toproduce aluminum sulfate needed for watertreatment.

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Bayerprocess

Leaching of Bauxite

Al(OH) 3 + OH – → [Al(OH) 4] –

AlOOH + OH – → [AlO(OH) 2] –

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• Sometimes NaOH is formed in situ in theautoclaves by adding sodium carbonate andcalcium hydroxide:

• Na 2CO 3 + Ca(OH) 2→ 2NaOH + CaCO 3

Bayer ProcessBayer Process

BAUXITE

Sand

Grushing

Washing

Drying

Grinding

Leaching

Water

Settling

Dilution

Seed

Thickener

Filtration

Precipitation

Filtration

Evaporation

Centrifuge

make-up

NaOH

NaOH

Recycle

Red mud

Red mud

Solid impurities

Was hing Calcina tionPureAl2O 3

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Red mud

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Bauxite %Red mud

(dried at 105°C)%

CalcinedAl2O3 %

Al2O3

SiO 2

Fe2O3

TiO 2 Na 2OLoss on ignition

57.83.5

24.3

2.5 —12.5

14.07.6

57.6

5.77.47.7

99.550.050.04

—0.180.18

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g/100 g free Na 2O

Al2O3

CaOFe2O3

Ga 2O3

Na 2OSiO 2

TiO 2

32.80 0.120.050.22

100.000.60

Trace

Cl –

CO 2

F –

MgOP2O5

SO 3

V2O5

1.003.740.030.170.900.07

0.45

Silicon

• Silicon occurs as quartz, SiO 2, or as clays, e.g., kaolinite,Al2(OH) 4(Si 2O5).

• Quartz is insoluble in NaOH under the conditions of leaching but the silicates are soluble.

• During digestion, silica that goes into solution combines withalumina and sodium hydroxide forming insoluble hydratedaluminosilicates such as 2Na 2O·2Al 2O3·3SiO 2·2H 2O, whichare carried away in the red mud, thus causing losses.

• About 1 kg of NaOH is lost for each kilogram of soluble silica

in bauxite. Although most of the soluble silica in bauxite is precipitated during digestion by forming sodiumaluminosilicate, small amounts may still be found in solution,especially when concentrated NaOH solution is used.

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• To precipitate the silica completely, additionof CaO is recommended, since insolublecalcium silicate can be formed.

• Lime addition during digestion has a furtheradvantage: any Na 2CO 3 present in the solutiondue to absorption of CO 2 from the atmosphere,and which has no dissolving action on bauxite,will be converted to NaOH.

Gallium

• Gallium occurs in most bauxites, and is highestin French bauxites (0.0–0.05 % Ga 2O3).

• It dissolves completely during extraction.Recycling of NaOH in the process results ingallium enrichment up to 0.5 g/L.

• Such liquors are therefore an important source

of gallium, from which it can be recovered,e.g., by solvent extraction or electrolysis usinga mercury cathode.

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Vanadium

• Vanadium in bauxite is partly soluble duringdigestion.

• In some ores it is precipitated during evaporating theleach solution as complex salts such as2Na 3VO 4·NaF·19H 2O.

• This is especially the case for ores containing fluorinesince fluorides are dissolved during leaching.Sometimes these precipitate to form a hard scale inthe evaporators which interferes with heat transfer. Inother ores vanadium builds up in the recycle NaOH toa concentration of about 0.5 g/L V 2O5 and isrecovered.

Kaolinite• Kaolinite is the most important mineral in clays.• North America produces about 50% of the

world’s aluminum, yet must import more than90% of the raw material needed, although thereis abundant domestic resources of aluminum-bearing silicates raw materials such as clay,shale, anorthosite, nepheline, and fly ash from

power plants.• That is why there is extensive research

underway to recover alumina from these non-bauxitic sources.

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Bauxite%

Clay%

Shale%

Anorthite%

Nepheline%

Fly ash%

Al2O3 55–60 34 23 20–35 23–28 24–32

SiO 2 3.5 45 58 45.55 45.60 45–51Fe2O3 2–25 2.6 6 1–3 1–3 7–11TiO 2 1–5 2.4 1 < 1 0.2 1

CaO 2 5–15 1–3 1–5

MgO

0.2–0.6

5 1 0.1 1–4 Na 2O+ K 2O

0.02 2–15 18 4–6 10–30

H2O 10–30 13 < 1 trace nil

• Russia has the only aluminum industrybased partly on non-bauxite raw materials,namely a nepheline syenite that containsapatite in the Kola Peninsula.

• This operation was possible because ofthe large production of apatite for fertilizer,and the production of Portland cement asa co-product

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Dehydroxylation at 400°C

Al 2(Si 2O5)(OH) 4 → Al 2O3.2SiO 2 + 2H 2O

ProcessCost of

acid Decomposition reactionEnergy needed

fordecomposition

H2SO 4 Low Al 2(SO 4)3·18H 2O→ Al 2O3 + 3SO 3 + 18H 2O

High

HCl Medium 2AlCl 3.H2O→ Al 2O3 + 6HCl + 9H 2O

Medium

HNO 3 High 2Al(NO 3)3·9H 2O→ Al 2O3 + 6HNO 3 + 15H 2O

Low

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Pressure leaching of clay

• All attempts to apply this technology cannot competewith Bayer process.

• . All this may change in the future. Researchers at theformer US Bureau of Mines extracted alumina fromun-calcined kaolinitic clay in 15 minutes by heatingat 200 oC using 20-27% HCl at 20% excess to thestoichiometric amount:

A1 2O3.2SiO 2

.2H 2O + 6HCl→ 2AlCl 3 + 2SiO 2 + 5H 2O

Orbite process

Clay

separation

Silica

Aluminum chloride

crystalization

Calcination

Al2O 3

Iron chloride

crystalization

Leaching

Calcination

Solid/liquid

Fe 2O 3

HCl

HCl

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• An advantage of the new technology isthat silica is produced which can beconsidered as a byproduct while red mudfrom bauxite is a waste product.

Laterites

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Laterite leaching plants

in Western Australia

Murrin Murrin

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Ramu laterite plant in Madang inthe Papua New Guinea

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Ramu laterite plant in Madang in

the Papua New Guinea

One of the three large autoclaves being transported from

China to Madang

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Ambatovy mine in Madagascar

Autoclaves at Ambatovy

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Production ofnickel, cobalt,and ammonium

sulfatein Madagascar

Processing of Ilmenite

• Pyrometallurgical method . Partial reduction withanthracite in an electric furnace to get cast ironand a slag rich in titanium. Titanium slag inmainly iron magnesium titanate, (Fe,Mg)Ti 4O 10 ,and a small amount of silicates

• Hydrometallurgical method . Leaching of ironoxide and obtaining a residue rich in titanium(90–95% TiO 2) known as “synthetic rutile”.

• The slag and synthetic rutile can then beprocessed to TiO 2 pigment or titanium metal.

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Leaching ofLeaching of IlmeniteIlmenite

Baking

Ilmenite

Residue

Conc.H2SO 4

Leaching

Filtration

Crystallization

Centrifuge

Hydrolysis

Filtration

Drying

Calcination

H2O

H2O

DiluteH2SO 4

Seed

TiO 2

FeSO 4 · 4H 2O

FeTiO 3 + 4H + Fe 2+ + TiO 2+ + 2H 2 O

Titanium Raw Materials

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SyntheticSynthetic RutileRutile

Ilmenite

Synthetic

Digestion

Filtration

Fe 2O3

HCl

Oxyhydrolysis

rutile

FeTiO 3 + 2H + TiO 2 (impure) + Fe 2+ + H 2 O

Oxyhydrolysisof FeCl 2

2FeCl 2 + 2H 2O + ½O 2

→ Fe2O

3+ 4HCl

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Processing ofProcessing ofTitanium SlagTitanium SlagBaking

slag

Residue

Conc.H2SO4

Leaching

Filtration

Hydrolysis

Filtration

Drying

Calcination

H2O

H2O

DiluteH2SO4

Seed

TiO2

Titanium

Niobium & Tantalum

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• Columbite, tantalite, and pyrochlore are mainlyused to prepare ferroniobium and ferrotantalumby pyrometallurgical methods

• To prepare metallic niobium and tantalum apure oxide is usually prepared first by treatingthe concentrates by hydrometallurgical methods

• Tin slags, especially those from Malaysia andThailand, were at one time an important source

of niobium, tantalum, as well as tungsten.

• Pyrochlore can also be beneficiated to a product containing90–97% Nb 2O5 by reaction with 36% HCl at 200 °C and about1000 kPa for 4 hours in a pressure reactor.

• The reaction is based on the formation of the niobium ionwhich hydrolyses to Nb 2O5 at the reaction temperature. Thereaction takes place in two consecutive steps: – 3(Nb 2O5

.CaO) + 2HC1 → 2Nb 2O5 + Ca 2 Nb 2O7 + CaC1 2 + H 2O pyrochlore

– Ca 2 Nb 2O7 + 4HC1 → Nb 2O5 + 2CaC1 2 + 2H 2O

• Calcium niobate, Ca 2 Nb 2O7, is formed as a non-porous

intermediate product on the pyrochlore grains through whichthe reactant and the products must diffuse.

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WOLFRAMITE AND SCHEELITE

• Wolframite, (Fe,Mn)WO 4 or (Fe,Mn)O.WO 3

• Scheelite, CaWO 4 or CaO·WO 3

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Acid digestion

Sulfuric should not be used

Alkaline Digestion

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Plant for pressure leaching of tungsten concentrate

ARSENIDES AND ANTIMONIDES

• The presence of arsenic and antimony insulfide concentrates is undesirablebecause these metals complicate thesmelting and refining.

• As a result there is interest to removethem before smelting.

• One route is leaching the concentrate byan alkaline sodium sulfide solution at hightemperature and pressure.

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Purification of chalcopyrite

concentrate• A copper sulfide concentrate containing 4% As and7% Sb was treated in British Columbia by EquitySilver Company by this method.

• The finely divided concentrate is leached for 16 hoursat 110 °C to solubilize arsenic and antimony sulfides:

As2S3 + 3S 2– → 2AsS 33–

Sb 2S3 + 3S 2– → 2SbS 33–

• After filtration, the copper concentrate is shipped tosmelters.

• The leach solution contains 30 g/L As and 53g/L Sb. It can be treated in two ways:Electrolyzed in a diaphragm cell to getantimony and regenerate the leach solution: – SbS 3

2– + 4e – → Sb + 3S 2–

• Treated with oxygen in autoclaves at 150 °C

and 550 kPa to decompose the antimonythiocomplex: – Na 3SbS 3 + 4NaOH + H 2O + 13/2O2

→ NaSb(OH) 6 + 3Na 2SO 4

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• In a similar way complex cassiterite, SnO 2,concentrate especially those from Bolivia was

purified by boiling at 110 oC with HCl inautoclaves to remove impurities.

• This was conducted in rotating sphericalautoclaves at the Longhorn Smelter in Texas.This resulted in removing most of theimpurities and the tin oxide obtained wasamenable to conventional smelting.