Occurrence and Preparation of d-Block Elements

Natural Occurrence of d-Block Elements

d-Block elements (transition metals) occupy groups 3–12 of the periodic table. They are relatively dense, often form colorful compounds, and many are technologically important. In this chapter, the focus is on where they are found in nature and how they are obtained on an industrial scale, not on their detailed properties or uses.

General Features of Occurrence

Most d-block elements are not found as pure metals in nature; they occur as:

• Sulfides: e.g. chalcopyrite (CuFeS$_2$), galena (PbS with Ag), sphalerite (ZnS)
• Oxides and hydroxides: e.g. hematite (Fe$_2$O$_3$), magnetite (Fe$_3$O$_4$), cassiterite (SnO$_2$), chromite (FeCr$_2$O$_4$)
• Carbonates: e.g. siderite (FeCO$_3$), malachite (Cu$_2$(OH)$_2$CO$_3$)
• Silicates or complex minerals: e.g. garnets and pyroxenes containing Fe, Mn, Ti; monazite containing rare earths with some d-blocks
• Native metals (rarely): e.g. native copper, silver, gold, platinum

Their occurrence is controlled by:

• Geological processes (magmatic, hydrothermal, sedimentary)
• Redox conditions (oxidizing vs. reducing environments)
• Complex formation (e.g. with chloride, cyanide, sulfate, carbonate)

Only a few (like Au, Pt) are “noble” enough to exist in the metallic state; most are found in oxidized or sulfide forms.

Major Ore Minerals of Selected d-Block Elements

Below are some important examples of how specific d-block elements occur in nature.

Iron (Fe)

• Abundance: One of the most abundant metals in the Earth’s crust.
• Common ores:
• Hematite: Fe$_2$O$_3$ (red–brown)
• Magnetite: Fe$_3$O$_4$ (magnetic, black)
• Goethite: FeO(OH) and limonite (hydrated mixture)
• Siderite: FeCO$_3$
• Geological settings:
• Banded iron formations (BIFs): ancient marine sediments with alternating Fe-rich and silica-rich layers.
• Laterites: weathering products in tropical climates.

Copper (Cu)

• Abundance: Moderately abundant.
• Common ores:
• Sulfides: chalcopyrite (CuFeS$_2$), bornite (Cu$_5$FeS$_4$), chalcocite (Cu$_2$S)
• Carbonates/hydroxides: malachite (Cu$_2$(OH)$_2$CO$_3$), azurite (Cu$_3$(OH)$_2$(CO$_3$)$_2$)
• Oxides: cuprite (Cu$_2$O)
• Geological settings:
• Porphyry copper deposits (large, low-grade, associated with intrusive igneous rocks).
• Volcanogenic massive sulfide (VMS) deposits.
• Oxidized caps of sulfide deposits (secondary enrichment zones).

Nickel (Ni) and Cobalt (Co)

• Nickel ores:
• Sulfides: pentlandite ((Fe,Ni)$_9$S$_8$), pyrrhotite (Fe$_{1-x}$S) with Ni.
• Laterites: silicate and oxide minerals (e.g. garnierite, Ni-rich serpentine and goethite) formed by tropical weathering of ultramafic rocks.
• Cobalt occurrence:
• Often a by-product in Ni and Cu ores.
• Cobaltite (CoAsS) and other arsenides.
• Settings:
• Magmatic sulfide deposits in ultramafic–mafic intrusions.
• Lateritic weathering profiles above such rocks.

Chromium (Cr)

• Main ore:
• Chromite: FeCr$_2$O$_4$ (spinel-type oxide).
• Settings:
• Layered mafic and ultramafic intrusions (e.g. Bushveld Complex).
• Podiform chromite deposits in ophiolites (fragments of oceanic crust thrust onto land).

Manganese (Mn)

• Main ores:
• Pyrolusite: MnO$_2$
• Manganite: MnO(OH)
• Rhodochrosite: MnCO$_3$
• Settings:
• Sedimentary deposits, often as marine crusts and nodules.
• Supergene enrichment (weathering zones).

Titanium (Ti)

• Main minerals:
• Ilmenite: FeTiO$_3$
• Rutile: TiO$_2$
• Settings:
• Igneous rocks (ilmenite-bearing).
• Heavy mineral sand deposits (placer deposits) where dense rutile and ilmenite accumulate.

Vanadium (V)

• Occurrence:
• Substitution in minerals (e.g. vanadiferous magnetite).
• Vanadate minerals: e.g. carnotite K$_2$(UO$_2$)$_2$V$_2$O$_8$, patronite (VS$_4$).
• Shales, tar sands, and some coals.

Zinc (Zn) and Lead (Pb)

• Zinc ores:
• Sphalerite: ZnS (often with Fe, and accompanied by Cd, In, Ga).
• Lead ores:
• Galena: PbS (often with Ag).
• Cerussite: PbCO$_3$; anglesite: PbSO$_4$.
• Settings:
• Mississippi Valley-type (MVT) deposits (carbonate-hosted).
• Sedimentary-exhalative (SEDEX) deposits.

Silver (Ag), Gold (Au) and Platinum Group Metals (Pt, Pd, Rh, etc.)

• Gold:
• Native Au (often alloyed with Ag).
• Tellurides and sulfides.
• Placer deposits in river gravels (mechanically concentrated).
• Silver:
• Native Ag.
• Argentite/acanthite (Ag$_2$S).
• Often as an impurity in galena (PbS) and other sulfides.
• Platinum group elements (PGEs):
• Native alloys and sulfides in layered mafic intrusions and placer deposits.
• Associated with Ni–Cu sulfide ores.

Molybdenum (Mo) and Tungsten (W)

• Molybdenum:
• Molybdenite: MoS$_2$ (main ore).
• Tungsten:
• Wolframite: (Fe,Mn)WO$_4$.
• Scheelite: CaWO$_4$.
• Settings:
• Granitic and skarn deposits linked to magmatic activity.

Many other d-block elements (e.g. Nb, Ta, Re, Hf) occur in complex or rare minerals and are usually recovered as by-products.

General Strategy of Metal Extraction from Ores

To obtain d-block metals from their ores, industrial processes follow some basic steps. Details differ by metal and ore type, but the overall pattern is:

1. Mining
• Open-pit or underground mining.
• Removal of overburden, blasting, and transport of ore.
2. Ore Dressing (Mineral Processing)
• Crushing and grinding to liberate valuable minerals.
• Physical separation methods:
• Gravity separation: based on density.
• Magnetic separation: for magnetic ores (magnetite, some chromites).
• Flotation: attaches hydrophobic mineral particles to air bubbles to separate sulfide minerals from gangue.
3. Concentration
• The result of ore dressing is a concentrate enriched in the desired mineral(s), reducing transport and processing costs.
4. Chemical Extraction
• Conversion of the mineral to a form suitable for metal recovery:
• Pyrometallurgy: High-temperature processes (roasting, smelting, converting, refining).
• Hydrometallurgy: Aqueous solution processes (leaching, purification, precipitation, electrowinning).
• Electrometallurgy: Use of electrical energy (electrolysis, electrical refining).
5. Metal Refining
• Removal of remaining impurities to reach desired purity.
• Often involves combinations of thermal treatments, electrorefining, and chemical refining.

Energy consumption, environmental impact, and the value of by-products heavily influence which route is chosen.

Pyrometallurgical Extraction of d-Block Elements

Pyrometallurgy uses high temperatures to transform and reduce metal compounds to metals. It is especially common for abundant and structurally simple ores (oxides and sulfides).

Typical Steps

1. Roasting
• Heating ore in the presence of air or controlled atmosphere to:
• Convert sulfides to oxides:
  $$ 2\,\text{MS} + 3\,\text{O}_2 \rightarrow 2\,\text{MO} + 2\,\text{SO}_2 $$
• Remove volatile components (e.g. S, As).
• Applied to many sulfide ores (Cu, Zn, Ni, Pb, Mo).
2. Smelting
• Melting the roasted ore with a reducing agent and a flux.
• Reducing agents:
• Most commonly carbon (coke):
  $$ \text{MO} + \text{C} \rightarrow \text{M} + \text{CO} $$
• Carbon monoxide:
  $$ \text{MO} + \text{CO} \rightarrow \text{M} + \text{CO}_2 $$
• Fluxes (e.g. CaCO$_3$) form a fluid slag with impurities (SiO$_2$, Al$_2$O$_3$, etc.), facilitating separation from the molten metal.
3. Converting and Refining
• Further removal of impurities by controlled oxidation or reduction, decarburization, and slagging.
• Refining steps may be repeated to achieve high purity.

Example: Iron from Iron Ores (Blast Furnace)

Iron is typically produced from Fe$_2$O$_3$ or Fe$_3$O$_4$ in a blast furnace:

• Charge: Iron ore, coke (C), and limestone (CaCO$_3$).
• Key reactions:
• Decomposition of limestone:
  $$ \text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2 $$
• Formation of reducing agent:
  $$ \text{C} + \text{O}_2 \rightarrow \text{CO}_2 $$
  $$ \text{CO}_2 + \text{C} \rightarrow 2\,\text{CO} $$
• Reduction of iron oxide:
  $$ \text{Fe}_2\text{O}_3 + 3\,\text{CO} \rightarrow 2\,\text{Fe} + 3\,\text{CO}_2 $$
• Formation of slag:
  $$ \text{CaO} + \text{SiO}_2 \rightarrow \text{CaSiO}_3 $$
• Products:
• Pig iron (liquid Fe with C, Si, Mn, etc.)
• Slag (CaSiO$_3$ and other silicates).

Further refining (e.g. in basic oxygen furnaces, electric arc furnaces) produces steels and cast irons (discussed elsewhere in more detail).

Example: Copper from Sulfide Ores

From chalcopyrite (CuFeS$_2$), the classical copper extraction route is:

1. Concentration by flotation to obtain a Cu–Fe sulfide concentrate.
2. Roasting (partial):
• Converts part of CuFeS$_2$ to oxides and removes some S:
  $$ 2\,\text{CuFeS}_2 + 5\,\text{O}_2 \rightarrow \text{Cu}_2\text{S} + 2\,\text{FeO} + 4\,\text{SO}_2 $$
3. Smelting:
• Melting with silica flux to produce:
• Matte: liquid mixture of Cu$_2$S and FeS.
• Slag: FeSiO$_3$ formed from FeO and SiO$_2$:
  $$ \text{FeO} + \text{SiO}_2 \rightarrow \text{FeSiO}_3 $$
4. Converting (Bessemer-type process):
• Air blown through matte oxidizes FeS and remaining S:
  $$ 2\,\text{FeS} + 3\,\text{O}_2 \rightarrow 2\,\text{FeO} + 2\,\text{SO}_2 $$
• FeO again forms slag with silica.
• Further oxidation:
  $$ \text{Cu}_2\text{S} + \text{O}_2 \rightarrow 2\,\text{Cu} + \text{SO}_2 $$
• Result: blister copper (~98–99% Cu, containing gas bubbles).
5. Fire refining and Electrorefining
• Fire refining adjusts impurity levels.
• Electrorefining in an aqueous electrolyte yields very high purity Cu and recovers precious metals from anode slimes.

Example: Zinc from Sulfide Ores (Pyrometallurgical Route)

• Roasting:
  $$ 2\,\text{ZnS} + 3\,\text{O}_2 \rightarrow 2\,\text{ZnO} + 2\,\text{SO}_2 $$
• Reduction (e.g. in a retort or furnace):
  $$ \text{ZnO} + \text{C} \rightarrow \text{Zn} + \text{CO} $$
• Because Zn has a relatively low boiling point, the metal vapor is condensed to yield liquid zinc.

Example: Ferroalloys (Chromium, Manganese)

Chromium and manganese are often produced as ferroalloys (Fe–Cr, Fe–Mn) directly used in steelmaking.

• Chromium:
• From chromite FeCr$_2$O$_4$ by carbothermic reduction in electric furnaces:
  $$ \text{FeCr}_2\text{O}_4 + 4\,\text{C} \rightarrow \text{Fe} + 2\,\text{Cr} + 4\,\text{CO} $$
• Manganese:
• From Mn ores by similar high-temperature reduction with C.

Given the high melting points and stability of these oxides, large amounts of energy (often electric) are required.

Hydrometallurgical and Electrochemical Extraction

Hydrometallurgy uses aqueous chemistry to dissolve and recover metals, often at lower temperatures than pyrometallurgy. It is especially important for low-grade ores, complex ores, and secondary resources (scrap, tailings).

General Sequence

1. Leaching: Selective dissolution of the metal into a liquid (usually water with added reagents).
2. Solution Purification/Concentration:
• Precipitation (e.g. of impurities as hydroxides or sulfides).
• Solvent extraction or ion exchange (selective separation of metal ions).
3. Metal Recovery:
• Precipitation as compounds (e.g. hydroxides, carbonates, sulfides).
• Cementation (displacement with a more reactive metal).
• Electrowinning (electrodeposition of the metal from solution).

Leaching Types

• Acid leaching:
• Sulfuric acid (H$_2$SO$_4$) for many oxidic ores (Cu, Zn, Ni, Co).
• Hydrochloric and nitric acids in more specialized processes.
• Alkaline leaching:
• Sodium hydroxide (NaOH) for amphoteric oxides (e.g. Al, some Zn).
• Complexing agents:
• Cyanide (CN$^-$) for Au and Ag.
• Ammonia or ammonium salts for certain Cu, Ni.

Leaching can be performed in stirred tanks, heaps, dumps, or in situ (injecting solution underground).

Example: Copper from Oxide Ores (Acid Leaching and Electrowinning)

Oxidized copper ores (malachite, azurite, cuprite) and secondary copper sources are often processed hydrometallurgically.

1. Leaching:
• Copper(II) oxide with sulfuric acid:
  $$ \text{CuO} + \text{H}_2\text{SO}_4 \rightarrow \text{CuSO}_4 + \text{H}_2\text{O} $$
• Copper(II) carbonate hydroxide (malachite) with acid:
  $$ \text{Cu}_2(\text{OH})_2\text{CO}_3 + 2\,\text{H}_2\text{SO}_4 \rightarrow 2\,\text{CuSO}_4 + \text{CO}_2 + 3\,\text{H}_2\text{O} $$
2. Purification:
• Removal of Fe, Al, and other impurities by pH adjustment and precipitation.
3. Electrowinning:
• Aqueous CuSO$_4$ solution used as electrolyte:
• At cathode (metal deposition):
  $$ \text{Cu}^{2+} + 2\,\text{e}^- \rightarrow \text{Cu} $$
• At anode (oxygen evolution from water):
  $$ 2\,\text{H}_2\text{O} \rightarrow \text{O}_2 + 4\,\text{H}^+ + 4\,\text{e}^- $$

Result: high-purity copper plates, often ≥ 99.9% Cu.

Example: Zinc from Oxide Ores (Hydrometallurgical Route)

This route is now dominant for Zn.

1. Roasting of ZnS to ZnO:
   $$ 2\,\text{ZnS} + 3\,\text{O}_2 \rightarrow 2\,\text{ZnO} + 2\,\text{SO}_2 $$
2. Leaching with sulfuric acid:
   $$ \text{ZnO} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2\text{O} $$
3. Purification:
• Removal of Cd, Cu, Co, Ni by cementation with Zn dust:
  $$ \text{Cu}^{2+} + \text{Zn} \rightarrow \text{Cu} + \text{Zn}^{2+} $$
4. Electrowinning:
• Deposition of Zn:
  $$ \text{Zn}^{2+} + 2\,\text{e}^- \rightarrow \text{Zn} $$

Example: Nickel and Cobalt from Laterite Ores

Laterite Ni ores are difficult to process pyrometallurgically; hydrometallurgical routes include:

• Acid pressure leaching with H$_2$SO$_4$ at elevated temperature and pressure.
• Ammonia leaching where Ni and Co form soluble ammine complexes.
• Subsequent solvent extraction and electrowinning or precipitation to recover Ni and Co separately.

Example: Gold and Silver – Cyanidation

Gold and often silver are extracted by forming soluble cyanide complexes.

1. Leaching (cyanidation):
• Gold:
  $$ 4\,\text{Au} + 8\,\text{CN}^- + \text{O}_2 + 2\,\text{H}_2\text{O} \rightarrow 4\,[\text{Au}(\text{CN})_2]^- + 4\,\text{OH}^- $$
2. Recovery:
• Adsorption onto activated carbon or precipitation with zinc (Merrill–Crowe process):
  $$ 2\,[\text{Au}(\text{CN})_2]^- + \text{Zn} \rightarrow 2\,\text{Au} + [\text{Zn}(\text{CN})_4]^{2-} $$

Due to the high toxicity of cyanide, strict process control and waste treatment are required.

Biohydrometallurgy (Biomining)

Certain microorganisms accelerate leaching by oxidizing sulfide minerals and Fe$^{2+}$ to Fe$^{3+}$, which acts as an oxidant for metal sulfides.

Example (simplified for Cu from CuFeS$_2$):

• Bacteria oxidize Fe$^{2+}$ to Fe$^{3+}$:
  $$ \text{Fe}^{2+} \rightarrow \text{Fe}^{3+} + \text{e}^- $$
• Fe$^{3+}$ dissolves Cu from sulfides:
  $$ \text{CuFeS}_2 + 4\,\text{Fe}^{3+} \rightarrow \text{Cu}^{2+} + 5\,\text{Fe}^{2+} + 2\,\text{S} $$

This is especially useful for low-grade ores in heap or dump leaching.

Special Extraction Routes for Selected d-Block Elements

Some d-block elements require specialized processes due to very stable oxides, high boiling points, or the need for extremely high purity.

Titanium (Ti)

Titanium dioxide (TiO$_2$) from ilmenite or rutile must be reduced without contamination by carbon (to avoid TiC formation).

Kroll process:

1. Chlorination:
   $$ \text{TiO}_2 + 2\,\text{Cl}_2 + 2\,\text{C} \rightarrow \text{TiCl}_4 + 2\,\text{CO} $$
2. Purification:
• TiCl$_4$ is purified by distillation.
3. Reduction with Mg:
   $$ \text{TiCl}_4 + 2\,\text{Mg} \rightarrow \text{Ti} + 2\,\text{MgCl}_2 $$
4. Separation:
• MgCl$_2$ and excess Mg are removed (e.g. by vacuum distillation or acid leaching), leaving porous “titanium sponge”.

A related route uses Na instead of Mg (Hunter process).

Vanadium (V)

Example route from vanadium-bearing ores:

1. Salt roasting with NaCl or Na$_2$CO$_3$ and oxygen forms soluble NaVO$_3$ (sodium metavanadate).
2. Leaching dissolves NaVO$_3$.
3. Precipitation of ammonium metavanadate:
   $$ \text{NaVO}_3 + \text{NH}_4\text{Cl} \rightarrow \text{NH}_4\text{VO}_3 + \text{NaCl} $$
4. Calcination to V$_2$O$_5$:
   $$ 2\,\text{NH}_4\text{VO}_3 \rightarrow \text{V}_2\text{O}_5 + 2\,\text{NH}_3 + \text{H}_2\text{O} $$
5. Reduction of V$_2$O$_5$ (e.g. with Ca or Al) to yield metallic V.

Tungsten (W) and Molybdenum (Mo)

From wolframite/scheelite (W) and molybdenite (Mo):

1. Concentration of ores.
2. Roasting of MoS$_2$:
   $$ 2\,\text{MoS}_2 + 7\,\text{O}_2 \rightarrow 2\,\text{MoO}_3 + 4\,\text{SO}_2 $$
3. Alkaline leaching (for W from scheelite):
   $$ \text{CaWO}_4 + \text{Na}_2\text{CO}_3 \rightarrow \text{Na}_2\text{WO}_4 + \text{CaCO}_3 $$
4. Purification and precipitation as ammonium paratungstate or paramolybdate.
5. Calcination to WO$_3$ or MoO$_3$ and reduction with hydrogen:
   $$ \text{WO}_3 + 3\,\text{H}_2 \rightarrow \text{W} + 3\,\text{H}_2\text{O} $$

Platinum Group Metals

PGEs are usually recovered as minor constituents during Ni–Cu sulfide processing:

• Concentration and smelting produce a matte.
• Chemical treatment and complexation separate PGEs.
• Individual metals are then purified by precipitation, solvent extraction, and redox reactions.

Because of their high value, refining processes are intricate and must achieve extremely high recovery efficiencies.

Environmental and Resource Aspects

The occurrence and extraction of d-block elements are strongly linked to environmental and sustainability questions.

• Waste rock and tailings:
• Large volumes produced; can contain residual metals, acid-generating sulfides, and processing chemicals.
• Air emissions:
• SO$_2$ from roasting and smelting of sulfide ores.
• Dust and metal vapors in high-temperature processes.
• Water pollution:
• Acid mine drainage from oxidation of sulfide minerals.
• Leaching residues containing metals and reagents (e.g. cyanide, acids).
• Energy consumption and CO$_2$ emissions:
• Particularly high for ferrous and ferroalloy production, and high-temperature reduction of stable oxides.
• Recycling:
• Many d-block metals (Fe, Cu, Ni, Co, Pt-group metals) are widely recycled.
• Recycling reduces the need for primary mining and lowers environmental impact.

Modern metallurgy aims to:

• Improve process efficiency (higher recovery yields, lower energy use).
• Increase use of hydrometallurgy and recycling where appropriate.
• Implement pollution control (gas scrubbing, water treatment, tailings management).

Understanding natural occurrence and extraction methods of d-block elements provides the foundation for later discussions of their properties, coordination chemistry, and technological applications.