Electrometallurgy and Industries

Electrometallurgy

• It involves the extraction and refining of metals through electrolysis from aqueous and molten solutions.
• It relies on electrochemistry principles.
• Most reactions are endothermic, thus needing electrical energy.
• Its use is limited, mostly in countries where electrical energy costs are high.

Electrometallurgical Industries

• Industries deal with electrometallurgy.
• Electrolytic industries include:
  • Electro separation, such as Chlor-alkali industries
  • Electro winning producing Copper, Zinc, Aluminum and Gold
  • Electro refining of Copper, Nickel, and Aluminum
  • Electro deposition like Electroplating and Electro reforming
  • Oxidation-Reduction of Chlorates, Peroxides, Metallic Oxides and Organic Materials
• Fused electrolyte groups use Aluminum, Magnesium and Alkali metals.
• Electro thermal groups produce Calcium carbide, cyan amides, graphite, and synthetic abrasives.
• Includes electric furnaces for Ferro alloys, Electric Steel and Special Alloys
• Uses electric current to create reactions in the gaseous phase like lightning-reaction of O2 and N2 to produce NO2.
• Also includes battery and corrosion production.

Prerequisites for Electrometallurgy

• Operations require constant conditions.
• Changes in solution concentration cause current fluctuations, following Ohm's law (V=IR), affecting mass deposition.
• Raw materials must be pure, removing impurities prevents side reactions and ensures efficiency.
• Raw material must be consistently available with a constant composition.
• There must be a constant, uninterrupted, well regulated and cheap power supply.

Electrolytes

• Electrolytes are substances dissociating into positive and negative ions when in solution or a fused state thus becoming ionized.
• Due to this dissociation, the solution can conduct electric current.

Ionic Theory

• Electrolytes contain free ions.
• Current passage in an electrolyte depends on the number and speed of ions present.
• In strong electrolytes, ionization is complete, but ion movement is hindered; ionic interference diminishes with dilution.
• In weak electrolytes, ionization is partial; the degree of ionization increases with dilution, becoming complete at infinite dilution

Electrolyte Properties

• Electrolytes have an equivalent number of positively and negatively charged ions.
• The system contains S number of different ions
• System has concentration Cj
• System has molality mj, or mole fraction Xj
• System has Zj elementary charges

Ionic Mobility

• Ionic mobility (uB) refers to the ease with which ions migrate in an electrolyte.
• It's the rate of motion of a particular ion type under an electric field.

Conductance

• Conductance (G) of a solution is the reciprocal of resistance (R).
• Conductance is measured in Siemens (S), where 1S = 1Ω-1.
• Electrolytic solutions conduct electricity through multiple pathways.
• Conductance is additive, shown as GT = G1 + G2 + G3 +...
• Conductance of an electrolyte is directly proportional to the number of ions or charge carriers.
• Overall conductance G = GT – G0 where the conductance of the medium/solvent is Go

Molar Conductivity

• Conductivity (k) is approximately a linear function of electrolyte concentration (C).
• Molar conductivity (Λ) is defined by Λ = k/C.
• Molar conductivity has units of Ω-1cm2 mol-1.
• For a hypothetical salt MaXb ionizing into aM+ + bX-, its molar conductivity is ΛMaXb = aΛM+ + bΛX−.
• The "Law of Independent Ion Migration" suggests that cations and anions move independently.

Salt Melts

• Solid salts usually have low conductance at room temperature.
• Conductance rises rapidly with temperature due to increased ion mobility and lattice defects.
• Conductance is lower than aqueous solutions below the melting point.
• Conductance increases significantly (3000x for KCl) at the melting point.
• Above the melting point, conductance increases with temperature, following the Arrhenius equation: K = Ko exp(-E/(R·T)).

Molten Slags

• Molten slags are ionic conductors.
• Conductance in silica (SiO2) containing slags increases inversely with cation size and silica concentration.
• Cooled slags forming glass retain conductance.
• Ordinary soda-glass exhibits conductance carried by Na+ ions.

Solid Electrolytes

• Certain solids have high ionic conductivity.
• ZrO2 with 5-10% CaO in solid solution is an example.
• Ca2+ replaces Zr4+, creating vacancies in the anion lattice for O2- movement, making ZrO2 + CaO an oxygen ion conductor.
• Solid zirconia electrolytes are used in metallurgical research and measurements.
• A measuring cell can measure oxygen potentials down to 10-20 atm at 1000 °C and about 10-10 atm at 1600 °C using reference electrode.

Electrolyte Choice

• Electrolyte selection depends on several factors.
• Aqueous electrolysis requires a working temperature of 0-100°C.
• High decomposition voltage is needed.
• Molten salt electrolysis requires the salt to be molten and its temperature attainable.
• High electrical conductivity of the melt is essential.
• Fused salt must be stable over a wide temperature range above its melting point with a reasonable vapor pressure.

Electrode Potential

• Electrochemical reactions are considered, such as A+X- becoming A + X.
• This reaction consists of half-reactions, A+ + e- becoming A, and X + e- becoming X-.
• E1 represents the electrode potential of the first half-reaction
• E2 that of the second
• The electromotive force (e.m.f) = E2 – E1.
• Electrode potential can be expressed as E1 = E1o - (RT/nF) ln(aA/aA+) and E2 = E2o - (RT/nF) ln(aX-/aX).
• E0 values from electrochemical series show standard reduction potential relative to hydrogen.
• Negative values indicate stable cations/unstable anions while positive values mean opposite.

Transport Numbers

• Current in aqueous lithium chloride (LiCl) solution is carried by Li+ and Cl- ions.
• Molar conductivities are ΛLi+ = 38.7 S cm2 mol-1 and ΛCl- = 76.3 S cm2 mol-1.
• Chloride ions carry the majority of current.
• Transport numbers, t+ and t-, represent the fraction of total current carried by cation and anion respectively.
• Equations are t+ = Λ+ / (Λ+ + Λ-) and t- = Λ- / (Λ+ + Λ-).
• Note: t+ / t- = Λ+ / Λ- and t+ + t- = 1.
• For aqueous lithium chloride, t+ = 0.34 and t- = 0.66.
• For potassium chloride (KCl) solution, t+ = 0.49 and t- = 0.51.

Factors affecting Transport Number

• As temperature increases, diffusion and ion migration increase.
• Rising temperature decreases resistance and increases diffusion/migration.
• Smaller ions are more mobile and easily transported.
• Transport number increases to a maximum with concentration before decreasing due to an increase in ionic interference.

Electro-Processes

• Electro-processes are heterogeneous chemical reactions where charge transfers to or from electrodes.
• In cathodic processes (reduction), electrons transfer from the electrode.
• With anodic processes (oxidation), charge transfer happens from the removal of electrons from the electrodes.
• A simple electro-reaction is Ox + n e- = Re.
  • Ox is the Oxidized product
  • Re is the Reduced product
• The overall reaction involves mass transport, electron transfer, and diffusion.
• Cathodic processes include equations for reactions like 2H2O + 2e- = H2 + 2OH- and Cu2+ + 2e- = Cu0.
• Anodic processes are for reactions like 2H2O = O2 + 4H+ + 4e- and Pb + SO4 = PbSO4 + 2e-.

Steps involved with Electro-Processes

• Ions migrate due to the electric field.
• Diffusion occurs due to concentration differences.
• Convection happens through mixing from local temperature differences.
• Ions are adsorbed on electrodes and solvated.
• Neutralization of ions.
• Product adsorption on the electrode surface is next.
• The product then desorbs from the surface.
• Products undergo secondary transformations to form molecules.
• Finally, products move away from electrode via diffusion.
• The rate-determining step dictates the process kinetics which is usually the fifth step.

Kinetics of Electro-Processes

• The electrodes must connect externally for electron flow.
• Flow rate of current determines total reaction amount.
• Alternatively, the charge passed gauges the reaction's extent.
• Free energy change (∆G) should be negative for a reaction to occur spontaneously.
• Reaction rate is assessed through current density i = nFv
• n = change in valence
• F = Faraday's constant

Overpotential

• It also discusses various equations such as the Butler-Volmer equations, Tafel equation etc