Metallurgy and Pulp Production
Metallurgy
Mineral Concentration/Preparation
- Flotation: Enriches ore by removing worthless mineral species.
- Amalgamation: Creates an alloy of mercury with other metal(s).
- Magnetic Concentration: Uses electromagnets to attract magnetic minerals, separating them from non-magnetic materials.
Pretreatments
- Roasting: Converts sulfides into oxides by heating in air.
- Calcination: Converts carbonates or hydroxides into more easily reducible compounds.
Reduction
- Chemical Reduction: Employs a more electropositive metal as a reducing agent.
- Electrolytic Reduction: Used for highly electropositive metals.
Purification
- Distillation: Purifies metals with volatile impurities.
- Electrolysis: Electrochemical purification method.
- Zone Refining: Dissolves impurities, allowing the molten metal to crystallize.
Metal Production
- Iron: Sources include pyrite, siderite, hematite, and magnetite. Pig iron is produced in a blast furnace using iron ore, limestone, and coke.
- Steel: Produced by removing impurities from pig iron and adding carbon and other elements. Types include carbon steel, alloy steel, stainless steel, tool steel, and low-alloy ultra-high-strength steel. Treatments include galvanization, plating, bluing, and painting.
- Copper: Sources include chalcopyrite, cuprite, and azurite. High-purity copper is obtained through electrolysis.
- Aluminum: Extracted from bauxite using the Bayer process and the Hall-Héroult process (electrolysis).
- Lead: Obtained from galena.
- Zinc: Extracted from sphalerite.
- Magnesium: Obtained from magnesite.
Pulp Production
Wood
Wood is composed of cellulose, hemicellulose, and lignin.
Debarking
- Drum Barker: Bark is removed by friction between logs and water jets in a rotating drum.
- Ring Barker: Logs are debarked individually in a ring with scrapers, using minimal or no water.
- Hydraulic Barker: High-pressure water jets remove bark from large logs in a chamber.
Pulp Production Methods
- Mechanical Pulping: Cellulose fibers are separated physically using grinders. This method offers good yield but produces weaker pulp.
- Chemical Pulping: Delignification is achieved using acidic or alkaline chemicals. This method results in easier bleaching, higher quality, and stronger pulp, but with lower yield.
Chemical Pulping Types
- Acid Pulping: Offers better cellulose separation and is more energy-efficient but is limited by certain resins, particularly those with phenolic compounds.
- Sulphite Pulping: Provides good yield, low cost, and high whiteness, with easy bleaching. However, it is suitable only for certain wood types and produces mechanically weak pulp.
- Alkali Pulping: Phenols and resin acids are removed in residual liquors.
- Soda Pulping: The oldest and simplest alkali process.
- Kraft/Sulphate Pulping: The most widely used method, offering high delignification speed, high yield, low cost, suitability for any wood type, and shorter cooking time.
Bleaching
- Chlorine-Based Bleaching:
- Molecular Chlorine: Older method, highly polluting but low cost.
- Chlorine Dioxide: More commonly used, selective but unstable, toxic, and corrosive.
- Chlorine-Free Bleaching:
- Alkaline Extraction (NaOH):
- Oxygen Bleaching: Eliminates much of the residual lignin in an alkaline medium.
- Ozone Bleaching: A good oxidant, but less selective than chlorine.
- Hydrogen Peroxide: Cleaner, with few drawbacks.
- Peracids: Obtained by reacting hydrogen peroxide with acids.
- Xylanase: Removes non-cellulose polysaccharides, non-polluting but expensive.
Pulp Quality
Pulp quality is determined by factors such as extracts content, lignin content, polysaccharide content, ash content, kappa number, alkali resistance, viscosity, and unsaponifiable groups.