4 Processes in Absence of O2
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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
[email protected]@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.