Hydrometallurgy MINE 292 Introduction to Mineral Processing Lecture 21 John A Meech
Hydrometallurgical Processing • Comminution(Grinding) • Leaching Metal (Quantity - %Recovery) • Removal of Metal from Pulp a. Solid/Liquid Separation - CCD thickeners - Staged-washing filtration b. Adsorption (Carbon-in-Pulp and/or Resin-in-Pulp) (CIP/RIP or CIL/RIL) - granular carbon or coarse resin beads
Hydrometallurgical Processing 4. Purification (Quality - g/L and removing other ions) - Clarification and Deaeration (vacuum) - Precipitation (Gold: Zn or Al dust) (Copper: H2S or scrap Fe or lime) (Uranium: yellow cake) (Zinc: lime) - Solvent Extraction (adsorption into organic liquid) - Ion Exchange (resin elution columns) - Elution (contact carbon or resin with an electrolyte)
Hydrometallurgical Processing 5. Electrowinning or Precipitation followed by Smelting
Hydrometallurgical Processing Classifier
Hydrometallurgical Processing Feed Grade = 5 g Au/t Ore %Recovery during Grinding = 60% >>> solids content = 2.00 g/t %Recovery during Leaching = 35% >>> solids content = 0.25 g/t %Recovery during CCD = 0% %Recovery Total = 95% Underflow Densities = 50%solids Leach Density = 40% solids Classifier O/F Density = 40%solids Pregnant Solution Flowrate = 300% Barren Bleed Flowrate = 25% Gold in Barren Solution = 0.05 g/t Calculate the gold content of the Pregnant Solution and the U/F water from each thickener. What is the actual mill recovery? What difference would occur if fresh solution was added to Thickener E rather than Thickener B?
Metal Recovery by Dissolution • Primary extraction from ores • Used with ores that can't be treated physically • Secondary extraction from concentrates • Used with ores that can be beneficiated to a low-grade level
Metal Recovery by Dissolution • Applied to • Copper (both acid and alkali) CuO + H2SO4 → CuSO4 + H2O Cu+2 + 4NH4OH → Cu(NH3)4+2 + 4H2O • Zinc (acid) ZnO + H2SO4 → ZnSO4 + H2O • Nickel (acid and alkali) – Nickel Laterite Ores NiO + H2SO4 → NiSO4 + H2O NiO+ 6NH4OH → Ni(NH3)62+ + H2O
Ammonia Leaching of Malachite NH4Cl → NH4+ + Cl– (1) NH4+ + H2O → H3O+ + NH3 (2) CuCO3·Cu(OH)2 + 2H3O+ → Cu2+ + CO2 + 3H2O + Cu(OH)2 (3) Cu(OH)2 + 2H3O+ → Cu2+ + 2H2O (4) Overall Leaching Reaction CuCO3·Cu(OH)2 + 4 NH4Cl → 2Cu2+ + 4Cl– + CO2 +3H2O +4NH3(5) Formation of complex amine ions Cu2+ + 2NH3 → Cu(NH3)22+ (6) Cu(NH3)22+ + 2NH3 → Cu(NH3)42+ (7)
Nickel Lateritic Ores • acid heap leaching method similar to copper • H2SO4 much higher than for copper (1,000 kg/t) • patented by BHP Billiton • being commercialized by • Cerro Matoso S.A. in Columbia • Vale in Brazil • European Nickel Plc in Turkey, Balkans, Philippines
Metal Recovery by Dissolution • Applied to • Aluminum (alkali) Al2O3 + 3H2O + 2NaOH → 2NaAl(OH)4 • Gold and Silver (cyanidation / alkali) • Uranium (acid and alkali)
Aluminum Smelting • Fused Salt Electrolysis – Hall-Herault Process
Aluminum Smelting • Fused Salt Electrolysis – Hall-Herault Process
Uranium Acid Leaching • Oxidize tetravalent uranium ion (U4+) to hexa-valenturanyl ion (UO22+) using MnO2 or NaClO4 • About 5 kg/t of MnO2 or 1.5 kg/t of NaClO4 • UO22+ reacts with H2SO4 to form a uranyl sulfate complex anion, [UO2(SO4)3]4-.
Leaching Processes • Tank Leaching (Agitation) • Vat Leaching • Pressure Leaching (high temperature/pressure) • Biological Leaching (Bacteria) • Heap Leaching • In-situ Leaching (solution mining)
Lixiviants • Lixiviant is a liquid medium used to selectively extract a desired metal from a bulk material. It must achieve rapid and complete leaching. • The metal is recovered from the pregnant (or loaded) solution after leaching. The lixiviant in a solution may be acidic or basic in nature. - H2SO4 - NH4OH - HCl - NH4Cl or NH4CO3 - HNO3 - NaOH/KOH - HCN >> NaCN/KCN
Tank versus Vat Leaching • Tank leaching is differentiated from vat leaching as follows: Tank Leaching • Fine grind (almost full liberation) • Pulp flows from one tank to the next Vat Leaching • Coarse material placed in a stationary vessel • No agitation except for fluid movement
Tank versus Vat Leaching • Tanks are generally equipped with • agitators, • baffles, • gas nozzles, • Pachuca tanks do not use agitators • Tank equipment maintains solids in suspension and speeds-up leaching • Tank leaching continuous / Vat leaching batch
Tank versus Vat Leaching • Some novel vat leach processes are semi-continuous with the lixiviant being pumped through beds of solids in different stages • Retention (or residence) time for vat leaching is much longer than tank leaching to achieve the same recovery level
Important Efficiency Factors Retention time = total volume of tanks / slurry volumetric flow - normally measured in hours - gold: 24 to 72 hours - copper: 12 to 36 hours - sequence of tanks called a leach "train" - mineralization &feed grade changes may need higher retention times
Important Efficiency Factors Particle Size - material ground to size to expose desired mineral to the leaching agent (“liberation”), tank leach >>> size must be suspendableby an agitation vat leach >>> size must be most economically viable - high recovery achieved as liberation increases or kinetics faster is balanced against increased cost of processing the material. Pulp density - percent solids determines retention time - determines settling rate and viscosity - viscosity controls gas mass transfer and leaching rate
Important Efficiency Factors Pulp density - percent solids determines retention time - determines settling rate and viscosity - viscosity controls gas mass transfer and leaching rate
Important Efficiency Factors Numbers of tanks - Tank leach circuits typically designed with 4 tanks Dissolved gases - Gas is injected below the agitator or into the vat bottom to achieve the desired dissolved gas levels - Typically, oxygen or air, or, in some base metal plants, SO2 is used.
Important Efficiency Factors Reagents - Adding/maintaining appropriate lixiviant level is critical - Insufficient reagents reduces metal recovery - Excess reagents increases operating costs and may lead to lower recovery due to dissolution of other metals - recycling spent (barren) solution reduces need for fresh reagents, but deleterious compounds may build-up leadingto reduced kinetics
Pressure Leaching • Sulfide Leaching more complex than Oxide Leaching • Refractory nature of sulfide ores • Presence of competing metal reactions • Pressurized vessels (autoclaves) are used • For example, metallurgical recovery of zinc: 2ZnS + O2 + 2H2SO4 → 2ZnSO4 + 2H2O + 2S • Reaction proceeds at temperatures above B.P. of water (100 °C) • This creates water vapor under pressure inside the vessel. • Oxygen is injected under pressure • Total pressure in the autoclave over 0.6 MPa.
Sulfide Heap Leaching • Ni recovery much more complex than Cu • Requires stages to remove Fe and Mg • Process produces residue and precipitates from recovery plant (iron oxides/Mg-Ca sulfates) • Final product – Ni(OH)2precipitates (NHP) or mixed metal hydroxide precipitates (MHP) that are smelted conventionally
Bio-Leaching • Thiobacillus ferrooxidans used to control ratio of ferric to ferrous ions in solution (Tf acts as a catalyst) 4Fe2+(aq) + O2(g) + 4H3O+ → 4Fe3+(aq) + 4H2O • Ferric sulfate used to leach sulfide copper ores • Basic process is acceleration of ARD • Typical plant leach times for refractory gold ore is about 24 hours
Bio-Leaching at Snow Lake, Manitoba • BacTech to use bio-leaching to deal with As and recover gold from an arsenic-bearing waste dump • Two products • Chemically-stable ferric arsenate precipitate • Gold-rich Residue Concentrate • 110 tpd of concentrate for 10 years • Annual production = 10,400 oz plus some Ag • CAPEX = $21,400,000 OPEX = $973/oz • Gold Recovery after toll-smelting = 88.6%
SX - Solvent Extraction • Pregnant (or loaded) leach solution is emulsified with a stripped organic liquid and then separated • Metal is exchanged from pregnant solution to organic • Resulting streams are loaded organic and raffinate(spent solution) • Loaded organic is emulsified with a spent electrolyte and then separated • Metal is exchanged from the organic to the electrolyte • Resulting streams are stripped organic and rich electrolyte
Ion Exchange Resins • AMn = synthetic ion-exchange resin (class A - 0.6–1.6 mm) • Phenyl tri-methyl ammonium functional groups • Macro-porous void structure • Similar to strong base anion exchange resins • Zeolite MPF (GB) • Amberlite IRA (USA) • Levatite MP-500 (FRG) • Deion PA (JPN)
In-situ Leaching • In 2011, 45% of world uranium production was by ISL • Over 80% of uranium mining in the USand Kazakhstan • In US, ISL is seen to be most cost effective and environmentally acceptable method of mining • Some ISLsadd H2O2 as oxidant with H2SO4 as lixiviant • US ISL mines use an alkali leach due to presence of significant quantities of gypsum and limestone • Even a few percent of carbonate minerals means that alkali leach must be used although recovery does suffer
In-situ Leaching Average grades of sandstone-hosted deposits range between 0.05% to 0.40% U3O8.
In-situ Leaching • Acid consumption varies depending on operating philosophy and geological conditions • In Australia, it is only a fraction of that used in Kazakhstan • In Kazakh , about 40 kg acid per kg U (ranging from 20-80) • Beverley mine in Australia in 2007 was 7.7 kg/kg U. • Power consumption is about 19 kWh/kg U (16 kWh/kg U3O8) in Australia and around 33 kWh/kg U in Kazakhstan • www.world-nuclear.org/info/Nuclear-Fuel-Cycle/Mining-of-Uranium/In-Situ-Leach-Mining-of-Uranium/#.UUihT1fQhLo