Extraction Methods for Oxidized Gold Ore

First, why is oxidized ore considered “easier to process”?

Engineers and geologists often classify oxidized ore as ‘free-milling,’ a term that implies simpler processing. This classification, however, requires careful definition to avoid misinterpreting the metallurgical challenges involved.

Oxidized ores are termed ‘easier’ because the gold particles are liberated and chemically accessible. They can be leached directly with a cyanide solution, bypassing the costly and energy-intensive pre-treatment (e.g., roasting, pressure oxidation) required for refractory sulfide ores where gold is encapsulated.

The term ‘oxidized’ describes a zone of supergene weathering where atmospheric oxygen and water have altered the primary ore mineralogy. Sulfide minerals, such as pyrite, which commonly host or encapsulate gold, are decomposed into oxides and hydroxides (e.g., goethite, hematite). In refractory sulfide ores, this gold is physically locked and chemically inaccessible. Extracting it requires destroying the sulfide matrix. In oxidized ores, this natural decomposition has already occurred. The gold particles, though often microscopic, are physically liberated and available for direct contact with a lixiviant like cyanide. This geological pre-treatment significantly simplifies the metallurgical flowsheet for gold ore processing, reducing complexity and cost compared to refractory ore treatment.

What are the methods for oxidized gold extraction?

With the gold being chemically accessible, the next step is determining the most effective extraction method. For commercial-scale oxidized gold ore extraction, the options condense to a few primary processes.

The primary methods are Heap Leaching, Carbon-in-Leach (CIL), and Carbon-in-Pulp (CIP). Gravity concentration is frequently integrated as a pre-concentration step to recover coarse, free gold. These processes can be implemented individually or in combination to optimize recovery.

Here is a technical overview of the primary methods, based on our experience in equipment supply and process design.

Heap Leaching

This is a primary method for large, low-grade gold ore deposits (typically 0.5 – 1.5 g/t). The process involves crushing the ore, stacking it on an impermeable liner, and irrigating the stack with a dilute cyanide solution. The solution percolates through the heap, dissolving the gold, and the gold-bearing solution is collected for recovery.

CIL (Carbon-in-Leach) / CIP (Carbon-in-Pulp)

For higher-grade or metallurgically complex ores, CIL/CIP is employed. In these processes, the ore is ground to a fine slurry. Leaching and adsorption occur in a series of large, agitated tanks where activated carbon is used to recover the dissolved gold from the slurry.

Gravity Concentration

This method leverages the high specific gravity of gold to separate it from lighter gangue minerals. While rarely a complete solution, integrating a gravity circuit with equipment like a Centrifugal Concentrator can recover a significant portion (20-40%) of free gold at a low cost before the leaching stage.

What equipment is needed for each gold extraction method?

A process flowsheet is defined by its required equipment. The capital cost and operational complexity of a project are directly tied to the machinery that must be procured, installed, and maintained.

A Heap Leach circuit primarily consists of crushers, conveying systems, and potentially an agglomeration drum. A CIL plant is significantly more capital-intensive, requiring a complete grinding circuit with ball mills and a train of large, agitated leaching tanks.

The equipment scope defines the project’s capital cost and operational complexity. As a manufacturer of this full range of minerals processing machines, we design circuits based on these distinct equipment requirements. The following table provides a typical equipment breakdown for each process:

Process	Key Equipment Required	Primary Function
Heap Leaching	Jaw Crusher, Cone Crusher	Ore size reduction to a target crush size.
	Conveyors & Stacker	Material transport and placement on the leach pad.
	Agglomeration Drum	Pelletizing fine particles with a binder (e.g., cement).
	Irrigation System & Pumps	Application and circulation of the lixiviant solution.
CIL / CIP Plant	Full Crushing Plant	Multi-stage crushing to prepare ore for grinding.
	Ball Mill & Cyclones	(High-Cost Item) Liberation of gold through fine grinding.
	Leaching/Agitation Tanks	(High-Cost Item) Vessels for leaching and adsorption.
	Thickeners, Pumps	Slurry density control and water management.
	Carbon Screens	Separation of loaded carbon from the slurry.

What factors determine which process I should choose?

This is the most critical technical decision in project development. The selection must be based on empirical data from the ore itself to ensure a positive gold recovery rate and economic outcome.

The decision is driven by four key factors: gold grade, clay content, gold particle size, and the presence of ‘preg-robbing’ carbonaceous material. These factors, determined by metallurgical testing, dictate the technical and economic viability of each process.

Based on our extensive experience, the process selection is a technical trade-off informed by a detailed orebody analysis.

Critical Decision Factors:

• Gold Grade: This is a primary economic driver. Heap leaching is typically applied to large, low-grade deposits where the lower unit operating cost can offset a lower recovery. Higher grades are required to justify the significant capital investment of a CIL plant.
• Clay Content: High clay content is a critical challenge. In heap leaching, it reduces permeability. For CIL, it increases slurry viscosity. This factor alone can render heap leaching unviable without agglomeration or force the selection of a CIL circuit.
• “Preg-Robbing” Carbon: If the ore contains natural carbonaceous material, it will compete with the activated carbon for dissolved gold, reducing recovery. This makes heap leaching infeasible and requires a Carbon-in-Leach (CIL) circuit to ensure the industrial carbon has preferential access to the gold.
• Gold Particle Size: The presence of coarse, gravity-recoverable gold necessitates a gravity circuit. Very fine, disseminated gold requires the intensive grinding and agitated leaching of a CIL process to achieve sufficient liberation and recovery.

How different is the upfront investment (CAPEX) between a heap leach and a CIL plant?

The project’s financial structure depends on the balance between initial investment and long-term returns. The capital cost differential between these two methods is substantial and is a key determinant of project feasibility.

The capital expenditure (CAPEX) for a CIL plant is substantially higher than for a heap leach operation. The primary cost drivers are the grinding circuit, specifically the ball mills, and the series of agitated tanks, which are not required for heap leaching.

An analysis of major cost centers clarifies the difference.

• Heap Leach CAPEX: Major costs include the crushing equipment, material handling systems (conveyors), leach pad construction (impermeable liner), and solution management infrastructure (ponds, pumps).

• CIL Plant CAPEX: This includes the entire crushing circuit, plus the most capital-intensive components in mineral processing:
  ◦ The Grinding Circuit: One or more large ball mills or SAG mills and associated classification systems are typically the single largest equipment cost.
  ◦ The Leaching Circuit: A train of large, mechanically agitated mixer tanks represents another significant capital investment.
  ◦ Support Infrastructure: Thickeners, complex piping networks, and more substantial civil and structural works are required.
  The final decision is an economic trade-off: does the superior gold recovery rate (90-97%) of a CIL plant justify its high CAPEX, or is the lower recovery (60-80%) of a heap leach the more profitable route for the specific mining project ROI?

What fatal impact will high clay content have on my process?

Among the gangue minerals present in an ore, clays are particularly deleterious to hydrometallurgical processes. Underestimating their impact is a common cause of operational failure in oxidized gold ore extraction.

High clay content severely compromises heap leach permeability by blocking solution pathways. In a CIL circuit, it increases slurry viscosity, leading to higher power consumption, accelerated equipment wear, and potential mass transfer limitations that inhibit gold adsorption.

The specific detrimental effects of clay differ by process.

In Heap Leaching

Clay particles, due to their fine size and plate-like structure, migrate with the solution flow and block the interstitial voids between larger rock fragments. This loss of permeability results in solution “channeling” and the creation of large unsaturated zones within the heap, preventing the lixiviant from contacting the gold in those regions.

In a CIL Plant

In a milled circuit, high-clay ore generates high-viscosity slurries. This has several deleterious effects:

• Increased Power Draw: Agitators in the leaching tanks require significantly more power to maintain solids in suspension.
• Increased Wear and Pumping Cost: The abrasive and viscous nature of the slurry accelerates wear on pumps, pipes, and agitators.
• Reduced Adsorption Kinetics: The viscous slurry can create a boundary layer around activated carbon particles, slowing the rate of mass transfer of the gold-cyanide complex and reducing overall adsorption efficiency.

How do I definitively determine the best gold extraction method through testing?

Process selection for a multi-million dollar investment must not be based on assumptions. The final decision for your gold ore processing flowsheet must be validated by scientific evidence derived from your specific ore.

A definitive process selection requires comprehensive metallurgical ore amenability testing. This is a multi-stage investigation designed to simulate process options at lab and pilot scales, generating quantitative data on recovery, reagent consumption, and identified processing risks.

A structured test program is the most effective method for de-risking a project. At ZONEDING, we maintain that equipment selection is a direct outcome of these test results. A typical program includes:

1. Ore Characterization: Detailed chemical, mineralogical, and geotechnical analysis to quantify gold occurrence, gangue mineralogy, and clay content.
2. Gravity Recoverable Gold (GRG) Test: Quantifies the percentage of gold that can be recovered by gravity concentration methods.
3. Bottle Roll Leach Tests: Small-scale agitated leach tests that determine the ultimate achievable gold recovery rate, optimal grind size, leach kinetics, and reagent consumptions for a CIL-type process.
4. Column Leach Tests: Larger-scale tests that simulate heap leaching conditions to determine expected recovery, leach cycle time, and the effect of crush size on permeability and extraction.
   This comprehensive ore amenability testing provides the necessary engineering data to develop a robust financial model and confidently select the most profitable process.

Conclusion

Selecting the optimal oxidized gold ore extraction method is a data-driven engineering decision. The ore’s specific mineralogical characteristics, not generalized assumptions, determine whether heap leaching or CIL represents the most profitable process.