cil gold plant design technology

The Modern Alchemy: A Comprehensive Guide to CarboninLeach (CIL) Gold Plant Design and Technology

1. Industry Background: The Evolution of Gold Extraction

The quest for gold, one of humanity's most coveted metals, has driven technological innovation for centuries. From the rudimentary panning of placer deposits to the mercuryamalgamation processes of the past, the methods have evolved significantly. The late 19th and early 20th centuries saw a revolution with the introduction of cyanide leaching, which allowed for the economic recovery of gold from lowgrade ores that were previously uneconomical.

The initial method, CarboninPulp (CIP), involved leaching gold from a slurry of ground ore and cyanide, followed by introducing activated carbon to adsorb the dissolved gold. While effective, a more efficient process was developed: CarboninLeach (CIL).

The CIL process emerged as the superior technology, particularly for ores containing pregrobbing materials—natural carbonaceous matter that would prematurely adsorb the goldcyanide complex, drastically reducing recovery rates. By adding activated carbon directly into the leach tanks, CIL allows adsorption to occur simultaneously with leaching, effectively "scavenging" the gold as it dissolves and outcompeting any pregrobbing constituents. Today, CIL is the industrystandard process for most new gold greenfield and expansion projects due to its robustness, efficiency, and lower capital cost compared to traditional CIP or MerrillCrowe processes.

2. The Core of the Process: Deconstructing CIL Plant Design

A CIL plant is a complex integration of unit operations designed to transform runofmine (ROM) ore into gold doré bars. The design philosophy centers on maximizing gold recovery while minimizing operating costs and environmental footprint.

Key Stages in a CIL Circuit:

1. Comminution (Crushing and Grinding):
Purpose: To liberate gold particles from the host rock by reducing ore size.
Crushing: Typically a threestage system (Jaw Crusher → Cone Crusher → Cone Crusher) to reduce ROM ore to a fine gravel size (~1020 mm).
Grinding: The crushed ore is fed into a ball mill or SAG mill with water to produce a slurry. The target grind size (often 80% passing 75 microns) is critical and determined by metallurgical test work to optimize gold liberation.

2. Leaching and Adsorption (The Heart of CIL):
Purpose: To dissolve gold using a cyanide solution and immediately adsorb it onto activated carbon.
Process: The ground slurry flows into a series of mechanically agitated tanks (6 to 8 in series). Key reagents are added:
Lime (CaO): Added early in the circuit to maintain a high pH (~10.511.0), which prevents the formation of deadly hydrogen cyanide gas (HCN) and optimizes leaching kinetics.
Sodium Cyanide (NaCN): The lixiviant that forms a soluble goldcyanide complex (Au(CN)2⁻).
Activated Carbon: Coconutshellbased carbon is continuously added to the last tank in the cascade and moved countercurrently to the slurry flow using interstage screens (e.g., Derrick screens). This ensures that the "hungriest" carbon (fresh or regenerated) meets the "leanest" solution, maximizing recovery efficiency.

3. Carbon Handling and Elution:
Purpose: To strip the loaded carbon of its gold and regenerate the carbon for reuse.
Loaded Carbon Stripping: Goldloaded carbon from the first adsorption tank is transferred to an elution column. A hot (~110130°C), highpH solution of caustic soda and cyanide (the Zadra or AARL process) desorbs the gold from the carbon.
Carbon Regeneration: The stripped carbon is thermally regenerated in a rotary kiln (~700°C) under a steam atmosphere to burn off organic foulants and restore its adsorption activity before being returned to the CIL circuit.

4. Electrowinning and Smelting:
Purpose: To recover metallic gold from the pregnant eluate solution.
Electrowinning: The rich eluate is pumped through electrowinning cells containing steel wool cathodes. An electrical current passes through the solution, causing metallic gold (and silver) to deposit onto the steel wool.
Smelting: The steel wool cathodes are mixed with fluxes (e.g., borax, silica) and smelted in a furnace at ~1200°C. This process separates impurities into a slag layer, leaving behind a molten doré bar of ~90% purity, which is cast into molds for shipment to a refinery.

5. Tailings Disposal:
The processed slurry exiting the final CIL tank is termed tailings. It must be treated to destroy residual cyanide (using INCO SO₂/Air or hydrogen peroxide) before being pumped to a secure Tailings Storage Facility (TSF).

3. Market Dynamics & Application Scope

CIL technology dominates the global gold processing market due to its versatility.

Market Drivers: High gold prices make lowergrade deposits economical, fueling demand for costeffective technologies like CIL. Its scalability makes it suitable for both largescale mining operations (>5 Mtpa) and smaller artisanal setups.
Application Scope:
FreeMilling Ores: Ideal for ores where cyanide can easily access and dissolve native gold.
PregRobbing Ores: As mentioned, this is CIL's primary advantage over CIP.
Transitional Ores/Oxidized Ores: Handles variability well.
It is less effective for refractory ores, where gold is locked inside sulfide minerals; these require pretreatment like pressure oxidation or biooxidation before being fed to a CIL circuit.

4. Future Outlook & Technological Advancements

The future of CIL plant design is focused on efficiency, sustainability, and digitalization:

1. Process Intensification & Optimization:
Advanced process control using AI and machine learning algorithms to dynamically adjust reagent dosing based on realtime sensor data.
Use of highintensity preconditioning tanks ("Leachox") that use oxygen sparging ahead of CIL can significantly enhance leaching kinetics.

2. Sustainability & Environmental Stewardship:
Development of noncyanide lixiviants like thiosulfate or glycine for use in specific deposit types where cyanide use is problematic.
"Dry Stack" tailings technologies are becoming more prevalent as they reduce water consumption and eliminate catastrophic TSF failure risks associated with wet tailings dams.
Improved water recycling circuits within plants minimize freshwater intake.

3. Equipment Innovation:
More efficient vertical plate interstage screens for carbon transfer with lower screen blinding rates.
Advanced instrumentation like online cyanide analyzers coupled with automated control valves ensure optimal reagent consumption.

5.Frequently Asked Questions (FAQ)

Q1: What is the fundamental difference between CIP and CIL?
A1: In CIP, leaching is completed first in dedicated tanks; adsorption onto carbon happens afterward in separate tanks ("Leach then Adsorb"). In CIL, leaching and adsorption occur simultaneously in same tank series ("Leach & Adsorb Simultaneously").

Q2: Why use activated carbon? Why not just filter out dissolved metal?
A2: Activated carbon has an enormous surface area per gram (>1000 m²/g), allowing it selectively adsorb trace amounts (<10 ppm) from vast volumes efficiently—something filtration cannot achieve economically at this scale

Q3 Is cyanide safe? How do you manage risks associated with its usage?
A3 When handled correctly at high pH levels (>10), sodium cyanide forms stable complexes making risk manageable through strict protocols including realtime monitoring systems , secondary containment areas around storage facilities ,and comprehensive emergency response plans

Q4 What factors determine number stages required within my particular operation’s design ?
A4 Number stages depends primarily upon metallurgical characteristics such as : Head grade ,Pregrobbing potential ,Leaching kinetics .Typically six eight stages provide sufficient residence time ensure maximum economic recovery

Engineering Case Study Example

Project Name : Ahafi North Expansion Project
Location : Ghana
Commissioning Year : 2022
Ore Type : Transitional oxidesulfide with moderate pregrobbing tendency

Challenge
Existing CIP circuit experienced significant losses due presence natural carbonaceous material New deposit required processing facility capable handling variable feed while maintaining >92% overall recovery rate

Solution Implemented
New standalone greenfield plant designed around robust seven stage configuration featuring :

• Primary crushing followed SAG Ball mill comminution circuit achieving target grind P80=106µm
• Seven mechanically agitated thickener underflow feed maintaining density ~45% solids
• Counter current loaded carbon movement utilizing Derrick interstage screens minimize attrition losses
• Integrated elution facility utilizing pressurized Zadra system coupled acid wash thermal regeneration kiln

Results Achieved
Plant consistently achieved design throughput within first six months operation Final recovery rates stabilized at approximately93%, exceeding original feasibility study estimates by overcoming pregrobbing effect through simultaneous leachadsorption mechanism inherent specifically engineered flowsheet thereby validating initial selection over alternative options considered during planning phase

In conclusion,CarboninLeach remains cornerstone modern hydrometallurgical extraction Its continued evolution through integration advanced controls sustainable practices ensures will remain dominant force global industry foreseeable future enabling safe efficient profitable transformation precious mineral resources societal value