Dehydration and Dehydroxylation Process

Dehydration and Calcination

• Water can be present in materials as free water, water of crystallization, or combined water.
• Dehydration refers to the removal of water of crystallization, while dehydroxylation is the removal of water, resulting in the breakdown of hydroxyl groups to form water vapor.
• Calcination is a thermal decomposition process that can be expressed as Solid 1 → Solid 2 + Gas.
• Calcination is often used in process metallurgy, where the compound Solid 1 is usually a carbonate, and Solid 2 is an oxide.
• Example: CaCO3 (s) → CaO (s) + CO2 (g)
• Calcination is more endothermic than drying, requiring a relatively high temperature.

Objectives of Calcination

• Bring about thermal decomposition
• Induce phase transition
• Remove volatile fractions such as CO2 and H2O

Types of Furnaces for Calcination

• Shaft furnace: suited for calcining coarse limestone
• Rotary kiln: used for handling materials of mixed particle sizes and lumps
• Fluidized bed reactor: used for materials of small and uniform particle size

Pyrometallurgy

• A branch of metallurgy that uses heat as the primary means of extracting metals from ores or other materials.
• Pyrometallurgic techniques include:
  • Ore-dressing
  • Sintering
  • Roasting
  • Calcination
  • Smelting
  • Refining
• Normally, the final product is in the molten form.

Pyrometallurgic Techniques

• Ore-dressing: preparation of ore using crushing, grinding, washing, drying, flotation, etc.
• Roasting: heating a sulfide ore in air to convert it to a metal oxide or free metal.
• Calcination: decomposition of carbonates and other compounds, causing loss of moisture, reduction or oxidation using high temperature.
• Sintering: process that produces agglomerated fine powders through atomic diffusion.
• Smelting: chemical process to isolate an element from its ore using heat and a reduction agent.
• Refining: purification of an impure material.

Reactor Design

• A metallurgical reactor is a piece of equipment designed to carry out a certain type of reaction or unit process.

Drying

• Removal of water is necessary to reduce the weight of the material handled.
• Water removal prevents charge blow off in electric furnaces.

Roasting

• Mostly used for sulfidic sources of metals.
• Oxidation of metal sulfides to produce metal oxides and sulfur dioxide.
• Carried out by heating the sulfides in air or oxygen below the melting points of the sulfides and oxides.

Objectives of Roasting

• Partial oxidation causing loss of part of the sulfur content.
• Oxidation to sulfates (sulfation roasting).
• Complete removal of sulfur, i.e., conversion to oxides (dead roasting).