Metal Extraction

An ore body is a commercially viable concentration of one or usually several metals. This concentration reflects the amount of an element in the Earth's crust; for a common element like iron an ore body could be ~50% iron whereas for a rare element like gold it could be much less than 1% (Table 9.2). However, digging an ore out of the ground is but the first step in the release of the metal and its conversion into commercially useful forms. Whereas some common metals are recovered by very simple redox reactions that do not involve coordination chemistry to any marked extent, some metals do rely heavily on coordination chemistry for their extraction and recovery.

Pure titanium metal is made by chlorination of the oxide TiO₂ to form the tetrahedral complex TiCl₄. This is then reduced in a redox reaction with magnesium metal to yield free titanium metal as a powder. To provide a continuous loop of reagents, the MgCl₂ also formed in this reduction step is electrolyzed to produce chlorine and magnesium metal. With the chlorine and magnesium re-used fully, this is a good example of an industrial process with negligible waste products; of course energy is consumed so it still carries an environmental impact.

Gold recovery is a simple example where complexation plays a key role. Gold ore where the gold is present as finely-divided metal that cannot be recovered mechanically is slurried in the presence of air and added cyanide ion. The elemental gold is oxidized by dioxygen and complexed by cyanide to form a gold(I) cyanide complex (9.1).

This water-soluble complex is then captured following filtration by adsorption onto carbon and subsequently recovered by further processing. The linear two-coordinate coordination complex of Au(I) is at the core of the process. Other ligands are being developed to replace the environmentally dangerous cyanide, such as thiourea (S=C(NH2)2) which also acts as a good monodentate ligand and forms a linear two-coordinate complex.

This latter chemistry is an example of what is called hydrometallurgy or the treatment of ores in aqueous solution. It is in this medium that most of the coordination chemistry of metallurgical processes can be found. A decrease in the grade and increase in the complexity of available ores of some metals has increased the need to develop new hydrometallurgical processes. The two key steps for ore treatment are leaching to dissolve the target metal and recovery (often involving a precipitation reaction) of the purified dissolved target metal. Although simple acid or base leaching can be successful some ores require the addition of complexing agents (ligands) to assist, such as the dissolution of copper oxide in ammonia/ammonium chloride (9.2).

CuO) s) +4NH4⁺+2-OH→ [Cu (NH3)4]²⁺_aq + +3 H₂O (9.2)

This is only a complexation reaction, as the copper is already in its desired oxidation state. The gold recovery process described above involves both oxidation and complexation. Another example of such a reaction is nickel sulfide pressure leaching, where it is the sulfur anion that is oxidized, the nickel remaining in its Ni (II) form throughout (9.3).

NiS (s) + 2O₂ +6 NH₃ → [Ni (NH₃)²⁺_aq+(SO₄)₂ (9.3)

Complexation also plays a role in the commercial process for the separation of the platinum group metals, involving formation of chloride and ammonia complexes; this separation of so many related elements is a demanding procedure. Platinum metal concentrates are usually dissolved in the strong mixed acid aqua regia as the first step to separate the soluble fraction (Au. Pd. Pt species) from the insoluble fraction (Rh, Ir, Ru, Os and Ag species). Solvent extraction of the soluble fraction with dibutyl carbitol separates gold from palladium/platinum species. The latter soluble complexes are the square planar H2[PdC14] and the octahedral H₂[PtCl₆]. Addition of ammonium chloride precipitates sparingly soluble (NH₄)₂ [PtCl₆] leaving the palladium complex in solution. From the separated complexes, the free metals are recovered through different redox reactions.

Solvent extraction of a metal ion in aqueous solution with another immiscible solvent containing ligands allows the target metal ion to be separated from other unwanted metal ions in the original leach solution; it is a process sometimes met in hydrometallurgy. One example is the recovery of uranyl ion (UO₂²⁺) which uses a phosphate diester dissolved in kerosene to complex and extract the uranyl ion into the organic phase, from where it can be recovered. The extraction chemistry involves reaction (9.4) where two R₂PO^2- chelates bind to the uranyl cation to form a neutral complex that is highly soluble in the organic phase, into which it shifts.

UO²⁺(aq) +2 R₂PO₂H_(org) → [UO₂(O₂PR₂)²] _(org) +2H+_aq Similar chemistry, but with a different chelate ligand, is used to purify copper (II). The copper complex is loaded into the organic phase at a pH between 2 and 4 then back- extracted into water as the Cu2+_aq ion using dilute sulfuric acid at pH 0. It is recovered as either copper sulfate or with following electrochemical reduction copper metal. Solvent extraction involving complex formation is also employed in the separation of cobalt-nickel mixtures.

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مواضيع ذات صلة

Molecular Symmetry: The Point Group

Organometallic Compounds

Bridging Ligands

Organic Molecules as Ligands

Multipliers

Polydenticity

Crystal Ball Gazing

Taking Stock

Complex Futures

Better Ways to Synthesize Fine Organic Chemicals

Industrial Roles for Ligands and Coordination Complexes

Fluoroionophores