Enhancing Precious Metals Recovery Through Advanced Wet Chlorination Processes

Advanced and Sustainable Innovations in Precious Metals Recovery

Copper anode slimes contain a massive wealth of gold and silver, but recovering them through wet chlorination requires a difficult balance between chemical precision and environmental rigor. A fully hydrometallurgical process has been developed to recover gold, silver, platinum group metals, and other by-products from these copper electrorefining residues.

This article presents sustainable developments in slimes processing, specifically focusing on the optimization of leaching precious metals and selenium. By utilizing continuous, in-line Oxidation-Reduction Potential (ORP) monitoring, operations can determine the exact end-point of the leach reaction. This precision results in a significant reduction in costly reagents, increased yields, and improved overall efficiency.

Figure 1: Material flow diagram of slimes processing in the precious metal plant; Mining Wet Chlorination

Strategic Resource Recovery and Sustainable Metallurgy

Sustainable development has become an integral facet of modern mineral processing. To remain competitive, mining enterprises must focus on maximizing resource recovery and adopting technological advancements that enhance profitability, safety, and operating efficiency. A key area for such improvement is the treatment of copper anode slimes—the insoluble material released during electrorefining. These slimes are a complex mixture of copper, gold, silver, selenium, tellurium, and platinum group metals. Traditional recovery methods are being replaced by advanced hydrometallurgical routes, such as wet chlorination, which offer a safer and more precise alternative to elemental chlorine gas.

This article describes the process chemistry and technical innovations, developed by Dr. Shijie Wang at Rio Tinto Kennecott Copper Refinery, that allow for a leaner, more productive gold leaching operation.

Figure 1: Material flow diagram of slimes processing in the precious metal plant

Principles of Sustainable Hydrometallurgy

Modern mineral processing is defined by the pursuit of "Total Resource Recovery." Beyond primary copper production, the ability to efficiently isolate high-value elements like gold and selenium determines the long-term profitability and environmental footprint of a refinery. Sustainable development in this context focuses on:

Maximizing Resource Value

Capturing trace PGMs and selenium that were previously lost to residue.

Operating Efficiency

Eliminating bottlenecks such as tank foaming and over-chlorination.

Safety and Stewardship

Utilizing safer oxidant alternatives and reducing chemical effluent.

The Anode Slimes Process Flow

In the electrorefining process, insoluble materials—slimes—accumulate at the bottom of the cells. These slimes contain varying concentrations of Au, Ag, Se, Te, Pb, and PGMs. A modern, optimized recovery circuit typically follows these stages:

Thickening and Filtration

Slimes are collected as a slurry, thickened, and filtered to recover electrolyte.

Decopperization

Slimes are leached with sulfuric acid and oxygen in autoclaves to remove copper.

Wet Chlorination

The core stage where gold and selenium are solubilized.

Solvent Extraction

Gold is isolated using organic extractants (e.g., dibutyl carbitol) and subsequently reduced.

Residue Processing

Final leaching with aqueous ammonia or sodium carbonate to recover lead and silver.

Wet chlorination is preferred for slimes with high gold-to-silver ratios. This process utilizes the metered addition of hydrogen peroxide (H₂O) into an agitated slurry of slimes in hydrochloric acid (HCl).

The reaction environment oxidizes and solubilizes gold and selenium while converting lead sulfates into chlorides. However, side reactions can lead to the catalytic decomposition of reagents if not monitored closely. The primary chemical reactions include:

Conversion of lead sulfate to lead chloride: PbSO₄ + 2HCl → PbCl₂ + H₂SO₄

Oxidation and solubilization of silver selenide: Ag₂Se + 2HCl + 3H₂O₂ → H₂SeO₃ + 2AgCl + 3H₂O

Chlorination of gold metal: 2Au + 8HCl + 3H₂O₂ → 2HAuCl₄ + 6H₂O

Generation of chlorine from hydrogen peroxide and hydrochloric acid: 2HCl + H₂O2 → Cl2 + 2H₂O

Decomposition of hydrogen peroxide to oxygen and water: H₂O₂ → ½ O₂ + H₂O

To prevent operational bottlenecks like rapid foaming (often caused by the oxygen evolution in reaction 5) and to minimize the generation of chlorine gas, reagent dosage must be carefully controlled based on real-time data.

Figure 2: Control chart of wet chlorination process

The wet chlorination step is the primary value-driver of the precious metals plant. Because the reaction is highly exothermic and prone to rapid gas evolution, manual sampling is insufficient for maintaining safety and efficiency. Precise control is required to prevent two extremes: under-leaching, which leaves high-value gold in the solid residue, and over-chlorination, which leads to excessive reagent costs and the generation of hazardous chlorine gas.

ORP monitoring provides a continuous, real-time indicator of the solution's oxidative state. In a chlorination environment, the "oxidative strength" correlates directly to the concentration of dissolved oxidants and the progress of the metal dissolution.

The measurement is performed using a combined pH/ORP sensor. While the pH glass electrode monitors acidity, the platinum (Pt) measurement electrode acts as an electron donor or acceptor.

In an oxidizing environment, the solution "pulls" electrons from the platinum, creating a positive millivolt (mV) potential.

In a reducing environment, the solution "pushes" electrons toward the platinum, creating a negative potential.

This voltage difference against a stable reference electrode provides the granular data needed to automate the H₂O₂feed. As the gold is solubilized, the mV reading shifts predictably, allowing for a digitized "fingerprint" of the leach cycle.

The conditions inside a wet chlorination tank are notoriously hostile to sensitive instrumentation. High concentrations of hydrochloric acid, abrasive suspended solids, and the tendency for lead chlorides to form scales can quickly lead to sensor fouling or reference junction poisoning. To ensure 24/7 measurement reliability without constant manual intervention, modern facilities utilize automated sensor maintenance systems:

Retractable Housing

These pneumatic assemblies allow the sensor to be extracted from the pressurized process stream without stopping the leach cycle.

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Integrated Cleaning

Once retracted into a flushing chamber, the sensor is washed with water or cleaning agents to remove mineral scaling and bio-fouling.

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Solid Polymer Electrolytes

Unlike traditional liquid-filled sensors, solid electrolytes resist the ingress of process chemicals, preventing the "poisoning" of the reference system.

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The diagram in Figure 4 shows the integration of the EasyClean™ cleaning system into the measuring point. A real-time recording of the ORP measurements vs H₂O₂dosing is shown in Figure 5.

Figure 4: Illustration of the ORP measurement system

The ultimate goal of in-line monitoring is to identify the kinetic end-point of the reaction. As H₂O₂ is added, the ORP value rises. A plateau in the mV curve—typically observed at or above 700mV—indicates that the gold has been fully complexed into HAuCl_4.

By tracking this curve in real-time, operators can transition to the "Ending Check" phase immediately upon completion.

This data-driven approach eliminates the "safety margin" of over-adding chemicals, significantly reducing reagent spend while ensuring that gold losses in the raffinate are kept to an absolute minimum.

Figure 5: ORP measurement in-line

The transition from traditional, manual leaching methods to automated, fully hydrometallurgical recovery represents a significant step-change in refinery evolution. The recovery of precious metals from copper anode slimes is no longer a matter of "brute force" chemical addition, but a discipline of precision control.

By integrating advanced technology—specifically intelligent, in-line ORP monitoring and automated sensor maintenance—refineries can overcome the inherent challenges of harsh process environments.

The ability to pinpoint the kinetic end-point of the gold leach reaction in real-time delivers a triple-bottom-line benefit:

Economic	Operational	Environmental
Maximized yields of gold, silver, and PGMs with a significant reduction in the consumption of costly reagents like H₂O₂ and HCl.	The elimination of process bottlenecks, such as tank foaming and manual sampling delays, through digitized logic and predictive diagnostics.	Enhanced safety through the use of liquid oxidants and a reduced chemical footprint, aligning metallurgical performance with global sustainability frameworks.

Ultimately, the shift toward data-driven hydrometallurgy ensures that refineries remain competitive in a landscape of fluctuating commodity prices and tightening regulations. Through the adoption of these intelligent sensor systems, "Total Resource Recovery" moves from a conceptual goal to an operational reality, securing a sustainable and profitable future for precious metal production.

Enhancing Precious Metals Recovery Through Advanced Wet Chlorination Processes

Advanced and Sustainable Innovations in Precious Metals Recovery

Strategic Resource Recovery and Sustainable Metallurgy