INTRODUCTION TO
HYDROMETALLURGY
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Hydrometallurgy
Extraction, recovery and purification of metals, through
processes in aqueous solutions. Metals are also
recovered in the other forms such as oxides,
hydroxides.
Electrometallurgy
Recovery and purification of metals through electrolytic
processes by using electrical energy.
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Traditionally, hydrometallurgy is emphasized for metals extraction from
low grade ores.
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Hydrometallurgical processing may be used for the following
purposes:
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Production of pure solutions from which high purity metals can be
produced by electrolysis, e.g., copper, zinc, nickel, gold, and silver.
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Production of pure compounds which can be subsequently used for
producing the pure metals by other methods. For example, pure
alumina to produce smelter grade aluminium.
Hydrometallurgy principles can also be applied to a variety of areas
such as metals recycling from scrap, slag, sludge, anode slime, waste
processing, etc.
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General outline of hydrometallurgical processes
Ore/concentrate
Leaching agent
Oxidant
leaching
Solid-liquid separation
Solid residue to waste
Pregnant Solution
Precipitant or electric
current
Solution puification
Precipitation
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Pure compound
Metals
Unit Processes in Hydrometallurgy
1. Ore/Feed Preparation
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This can be by physical or chemical processes i.e. traditional mineral processing
techniques
(i) Comminution
(ii) Concentration
(iii) Flotation
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This is done to improve the efficiency of the leaching process.
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The extent of feed preparation is dependent on the grade of ore. Why?
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Low-grade ores are hardly ever beneficiated.
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Direct leach ores go through size reduction before further processing.
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High-grade ores go through sizing and concentration before further processing.
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2. Chemical Treatment
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This is done in order to render the values soluble in the leaching reagent or to prevent the
leaching reagent from being consumed by impurities.
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Oxidative Alteration ---involves the alteration of the oxidation state of a mineral to make it
more soluble e.g. Uranium U (2,4) insoluble → U (6) soluble
Ways of alteration of the oxidation state:
(i) Roasting / Thermal energy employment
(ii) Chemical oxidation - reacting substances with salts e.g. Fe3+
(iii) Aerial oxidation ( very slow)
(iv) Bio-oxidation
(v) Reductive smelting
Qn: Use examples to explain how the oxidation state is altered.
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3. Leaching
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This is selective solubilisation of valuable metals.
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The process must be selective, however this is rare because of co-dissolution of impurities.
4. Solution Purification/Enrichment
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Value and impurity metal are both present in aqueous solution hence solution purification
arises from the need to separate the contaminants from the values.
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If this is not done the product will be of poor quality and this affects revenue/returns, also
affects the performance of product.
5. Product Recovery and Refining
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This is involves the deposition of pure metal on the cathode of an electrolytic cell. During
the deposition process there is removal of minute impurities by .
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Scope and Future of Hydrometallurgy
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Hydrometallurgy has gone through tremendous development in the past 40+
years however full potential is yet to be reached.
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The technology has a lot of room for improvement or development.
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There is a general unavailability of techniques required for low grade ores,
complex ores and more environmentally friendly methods.
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Treatment of old tailings and mine dumps has not been possible for a very
long time.
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ADVANTAGES OF
HYDROMETALLURGICAL PROCESS
(i) Hydrometallurgical methods are ideally suited for lean
and complex ores.
(ii) Hydrometallurgical operation ensures great control over
every step in the processing of ore. Resulting in the recovery
of valuable by- products.
(iii) It is environmentally friendly, it does not create much
pollution like pyro-metallurgical operation.
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(iv) Processes are generally carried out at room temperature or slightly
elevated temperature.
(v) The waste liquor from the final recovery step can be recycled to the
initial leaching operation.
(vi) Besides meeting the mounting demand for both the quantity and
quality of metals hydrometallurgy can produce metals in a variety of
physical forms such as powders, nodules, and coherent cathode
deposits.
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DISADVANTAGES OF
HYDROMETALLURGICAL PROCESS
(i) Handling of large amount of chemicals becomes difficult
and also a large amount of space is required.
(ii) The cost of the reagents and equipment is high.
(iii) Corrosion and erosion of tanks and ducts used for
handling concentrated acidic/alkaline solutions.
(iv) In hydrometallurgy, the disposal of effluent without
causing pollution posses a serious problem.
(v) Slow processes
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