5 - Hydrometallurgy.pdf

LECTURE 5: Hydrometallurgy Introduction, Leaching, Enrichment This Photo by Unknown Author is licensed under CC BY-SA-NC HYDROMETALLURGY | definition Hydrometallurgy is the treatment of ores by dissolution in aqueous solutions which imitates natural leachi...

LECTURE 5: Hydrometallurgy Introduction, Leaching, Enrichment This Photo by Unknown Author is licensed under CC BY-SA-NC HYDROMETALLURGY | definition Hydrometallurgy is the treatment of ores by dissolution in aqueous solutions which imitates natural leaching processes. It involves chemical reactions carried out in aqueous solutions at ambient temperature and pressure. “Uranium may be recovered from the ore using a heap leach process. In the heap leach process, the ore is placed on an engineered barrier and sprayed with acid. This Photo by Unknown Author is licensed under CC BY-SA-NC The uranium dissolves into solution and is collected at United States Nuclear Regulatory Commission. “Uranium Recovery (Extraction) the engineered barrier. The solution undergoes Methods.” USNRC, March 29, 2018. https://www.nrc.gov/materials/uranium- additional chemical processing to produce ‘yellowcake’.” recovery/extraction-methods.html. First Semester AY 2020 - 2021 MetE 11: Introduction to Metallurgical Engineering 2 HYDROMETALLURGY | objectives To recover metals from low grade ores which are not amenable to concentration or suitable for smelting ▪ oxides, carbonates, sulfates To recover metals from high grade concentrates that are roasted, then processed in aqueous solutions ▪ providing an alternative path to pyrometallurgical extraction As a preliminary operation to mineral processing ▪ i.e. flotation by dissolving the soluble constituents that are coating the valuable minerals First Semester AY 2020 - 2021 MetE 11: Introduction to Metallurgical Engineering 3 HYDROMETALLURGY | general steps 1. Comminution/mineral 2. Roasting (optional) 3. Leaching beneficiation of the starting ▪ heating of ore with excess air ▪ to dissolve the valuable metal/mineral ▪ changes structure/composition into the leach liquor material (optional) to make metal easier to leach ▪ to control fineness of the by converting metal to a more feed material leachable form, or by modifying 4. Solid/liquid separation matrix to make metal more ▪ to separate the leach liquor from the accessible to lixiviant unwanted mineral/undissolved residue ▪ Example: roasting of refractory gold ores to make the gold more accessible to sodium cyanide This Photo by Unknown Author is licensed under CC BY-SA First Semester AY 2020 - 2021 MetE 11: Introduction to Metallurgical Engineering 4 HYDROMETALLURGY | general steps 5. Metal/Mineral Recovery 6. Solid/Waste treatment ▪ final extraction of the metal from the solution ▪ Treatment of the liquor to be recycled in the operation ▪ Neutralization prior to disposal First Semester AY 2020 - 2021 MetE 11: Introduction to Metallurgical Engineering 5 LEACHING | definition Leaching involves the selective dissolution of the valuable metal in a suitable reagent or lixiviant. Leaching Reagents/Lixiviants Liquid or reagents used to selectively extract/dissolve desired metals Lixiviants must be: readily available in quantity relatively cheap Not unduly reactive with gangue minerals non-corrosive amenable for regeneration/ recycling First Semester AY 2020 - 2021 MetE 11: Introduction to Metallurgical Engineering 6 LEACHING | leaching reagents/lixiviants ❑ Water ▪ Water can be used to leach calcines from chloridizing or sulfating roasts ❑ Acids Example: leaching of zinc oxide + 2+ ▪ The most important solvent for oxide 𝑍𝑛𝑂(𝑠) + 2𝐻(𝑎𝑞) → 𝑍𝑛(𝑎𝑞) + 𝐻2 𝑂(𝑙) ore and for dead roasted sulfide concentrates is dilute sulfuric acid. ❑ Bases Example: digestion of hydrated alumina from ▪ Alkaline leaching minimizes bauxite using sodium hydroxide solution corrosion. 𝐴𝑙3 𝑂3 ∙ 𝐻2 𝑂 𝑠 + 2𝑂𝐻 −𝑎𝑞 → 2𝐴𝑙𝑂2− (𝑎𝑞) + 4𝐻2 𝑂(𝑙) ❑ Aqueous Salts Example: leaching of gold − ▪ Can be achieved by complexation 2𝐴𝑢0 (𝑠) + 4𝐶𝑁(𝑎𝑞) + 0.5𝑂2 (𝑔) + 𝐻2 𝑂(𝑙) → − using a suitable ligand 2 𝐴𝑢 𝐶𝑁 2 − (𝑎𝑞) + 2𝑂𝐻 𝑎𝑞 First Semester AY 2020 - 2021 MetE 11: Introduction to Metallurgical Engineering 7 LEACHING | leaching reagents/lixiviants Mineral Common Lixiviants Oxides Dilute H2SO4 Sulfates Dilute H2SO4 or H2O Sulfides FeSO4 solution Alumina NaOH solution Cu/Ni compounds (NH4)2CO3 First Semester AY 2020 - 2021 MetE 11: Introduction to Metallurgical Engineering 8 LEACHING | leaching methods ❑ Selection of leaching method depends on: ▪ ore composition ▪ required leaching rate ▪ subsequent recovery and extraction methods ❑ Some leaching methods: ▪ Dump leaching ▪ Heap leaching ▪ In-Situ leaching ▪ Vat leaching ▪ Agitation leaching ▪ Pressure leaching First Semester AY 2020 - 2021 MetE 11: Introduction to Metallurgical Engineering 9 LEACHING | leaching methods Dump Leaching Heap Leaching ▪ For low grade ore (sulfide and oxide) ▪ Ore material is piled into small heaps on ▪ for stripped marginal materials adjacent to the impervious grounds or on a concreted or asphalted main ore body surface with drainage channels and pipes and a ▪ ~20 cm particles in diameter are dumped in open collecting pond pit mines and the lixiviant is fed periodically by ▪ The lixiviant is fed in the same manner as that of sprays at the top of the dump dump leaching. ▪ leaching time lasts for years ▪ Leaching periods extend for several months First Semester AY 2020 - 2021 MetE 11: Introduction to Metallurgical Engineering 10 LEACHING | leaching methods In-Situ Leaching ▪ “in place” leaching ▪ This involves the dissolution of low-grade surface deposits or worked-out underground mines without the removal of the ore from the mine. ▪ The ores are normally too low in grade to justify mining and transportation expenses. ▪ This is also used for higher grade ores that are in deeper locations. First Semester AY 2020 - 2021 MetE 11: Introduction to Metallurgical Engineering 11 LEACHING | leaching methods Percolation/Vat Leaching Agitation Leaching ▪ involves the use of tanks with false bottom, ▪ used to process particles