What is hydrometallurgy?

Hydrometallurgy is a branch of extractive metallurgy that uses aqueous solutions to recover metals from ores, concentrates, and recycled materials. It’s commonly used for metals such as copper, gold, nickel, and uranium, particularly in cases where traditional smelting methods aren’t feasible due to low metal concentrations or environmental concerns.

Key steps in metal recovery from raw materials

There are three main stages of hydrometallurgy:

1. Leaching
2. Solution concentration and purification
3. Metal recovery.

Leaching

Leaching is the first step of hydrometallurgy. It involves dissolving metal-bearing ores into a liquid solution using chemical solvents. This process separates the target metals from waste rock (known as gangue) and prepares them for further purification.

Different leaching methods are used depending on the metal being extract:

• Heap leaching: This involves stacking crushed ore in a heap and spraying it with a leaching solution that percolates through the material, dissolving the metal. Heap leaching is often used for copper, gold, and uranium.
• In-situ leaching: Also known as solution mining, this involves injecting the leaching solution directly into an underground ore deposit through boreholes. Metals suitable for this technique include uranium and copper.
• Tank leaching: Otherwise known as agitation leaching, this involves placing finely ground ore in large tanks where the leaching solution is mechanically agitated to speed up metal dissolution. This method is used for precious metals such as gold and silver, and certain copper ores.
• Autoclave (pressure) leaching: Here, ore is processed in high-temperature, high-pressure vessels to accelerate the leaching reaction. It’s commonly used for nickel and refractory gold ores.

The choice of leaching solution varies depending on the type of metal:

• Sulfuric acid is common for copper, zinc, and uranium extraction
• Ammonia solutions are often used for nickel and cobalt
• Cyanide solutions are used for gold and silver recovery.

The result of leaching is a pregnant leach solution (PLS), a liquid containing dissolved metal ions that will undergo further processing in the next stage.

Solution concentration and purification

Once the metal is dissolved in solution, it must be separated from impurities. This step involves several techniques designed to selectively concentrate the desired metal while removing unwanted elements.

Common purification techniques include:

• Solvent extraction (SX): This involves mixing the metal-rich solution with organic solvents that selectively bind to the desired metal. SX is widely used in copper and uranium processing.
• Ion exchange: Here, resins or synthetic materials exchange unwanted ions with metal ions in the solution. This is used when working with gold, uranium, and rare earth metals.
• Precipitation: In this process, chemical reagents are added to selectively remove undesirable metal ions from the solution. For example, iron and aluminum impurities can be precipitated before copper is recovered.
• Cementation: This involves using a redox reaction, where a more reactive metal (e.g. iron) is added to replace and recover a less reactive metal (e.g. copper) from the solution.
• Gas reduction: Here, the solution is treated with hydrogen gas to convert metal ions into solid metal form. This method is used for nickel production.

By the end of this stage, the purified solution contains concentrated metal ions, ready for final recovery and processing.

Metal recovery

In the final stage of hydrometallurgy, the purified metal is recovered from the solution in a solid form, ready for industrial use. Again, different methods are used depending on the metal and desired purity level.

Common metal recovery processes include:

• Electrowinning: Also known as electrodeposition, this involves applying a direct current to the solution, causing metal ions to deposit onto a cathode as solid metal. Electrowinning is highly effective when working with copper, zinc, and nickel.
• Electrorefining: This further purifies metals by dissolving impure metal at the anode and redepositing it as high-purity metal at the cathode.
• Precipitation (chemical or thermal): In this process, metals are forced out of solution by changing pH temperature or adding precipitating agents. Precipitation is often used for gold refining, helping achieve the purity levels needed to produce good delivery gold.
• Gaseous reduction: This is a more cost-effective method where a more reactive metal is used to replace another metal in solution. It’s commonly used for copper recovery.

Why hydrometallurgy is more sustainable than traditional methods

Hydrometallurgy is considered a more sustainable alternative to traditional methods like smelting and flotation. Because it uses aqueous solutions instead of high-temperature processes, it can help reduce energy consumption, lower emissions, and improve resource efficiency.

Lower energy consumption

Traditional flotation and smelting methods require finely grinding ores, a process which consumes large amounts of energy. Hydrometallurgy reduces the need for such extensive grinding, helping decrease energy-intensive operations and reduce power demand.

Smelting also requires extreme temperatures to separate metals from ores, which often relies on fossil fuels. In contrast, hydrometallurgic methods work at relatively lower temperatures using electricity, which may be sourced from renewable energy. This can reduce the carbon footprint associated with the metal extraction process.

Reduced greenhouse gas and pollutant emissions

Smelting emits significant amounts of carbon dioxide (CO2), sulfur dioxide (SO2), and nitrogen oxides (NOx), which can contribute to air pollution, acid rain, and climate change. Hydrometallurgy, on the other hand, doesn’t involve combustion and therefore produces fewer direct emissions.

Since hydrometallurgical processes rely on chemical reactions in aqueous solutions, they produce fewer airborne pollutants than traditional smelting. This can make them a cleaner option for metal recovery.

Hydrometallurgy vs pyrometallurgy: a comparative approach

Hydrometallurgy and pyrometallurgy are the two main methods used in metal extraction and metal refining processes. Each approach has its own distinct advantages, efficiencies, and environmental impacts.

Let’s explore the differences between hydrometallurgy and pyrometallurgy.

What is pyrometallurgy?

Pyrometallurgy uses high temperatures to extract metals from ores or concentrates. The process involves smelting, roasting, or calcination, where heat and chemical reactions separate the metal from impurities. It’s commonly used for high-grade ores and is particularly effective for materials with stable mineral compositions.

The basic process of pyrometallurgy involves:

1. Preparing the ore: The ore is crushed and mixed with fluxes (e.g. limestone or silica) to aid in separation.
2. Heating in a furnace: The mixture is heated at extremely high temperatures to melt the materials and separate impurities (known as slag).
3. Collecting the metal: The molten metal settles at the base of the furnace and is removed for further refining.

What is hydrometallurgy?

As we’ve covered earlier, hydrometallurgy is a solution-based metal extraction method that uses aqueous chemical reactions to dissolve and recover metals. It’s well-suited for low-grade ores and complex mineral compositions where direct smelting may be inefficient or not environmentally viable.

Differences between pyrometallurgy and hydrometallurgy

The table below outlines the key differences between pyrometallurgy vs hydrometallurgy:

Industrial applications of hydrometallurgy in B2B markets

Hydrometallurgy is widely used in B2B markets to extract, refine, and recycle metal across various industries. Some industrial applications of hydrometallurgy include:

• Mining & metal extraction: Hydrometallurgy is used to recover copper, nickel, uranium, and gold from ores.
• Battery recycling: Hydrometallurgy is essential in recovering lithium, cobalt, and nickel from used batteries.
• E-waste processing: Hydrometallurgy is used to extract gold, silver, and rare earth metals from circuit boards and electronic scrap.
• Aerospace & automotive: Hydrometallurgy can be used to refine titanium and aluminum.
• Steel & manufacturing: Hydrometallurgy is used to purify zinc and lead to create corrosion-resistant coatings.

Challenges of solvent extraction techniques

Solvent extraction is used to separate and purify metals in hydrometallurgy, but this process can have several challenges including:

• Selection of extractants: Choosing the right extractant is essential for effective solvent extraction. If an unsuitable extractant is used, it can lead to inefficiencies like rapid degradation. Extractants must be carefully selected based on the ore type and metal composition, impurity content, and leaching process.
• Selection of diluents: Diluents impact the extractant’s solubility. Choosing the wrong diluent can lead to inefficient metal recovery and higher environmental contamination from impurities. Diluents should have low volatility, minimal impurities, and be highly compatible with the extractant used.
• Emulsification and flocculation: Emulsification and flocculations are two common solvent extraction challenges that can reduce metal recovery rates, lower product quality, and lead to equipment issues. Mitigating these challenges requires controlling operating conditions and choosing the right combination of extractant and diluent.

This material is for informational purposes only and should not be considered as an investment recommendation or a personal recommendation.