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 ancient rivers to the massive industrial complexes of today, the goal has remained the same: to efficiently and economically separate fine gold from vast quantities of ore.

The pivotal moment in modern gold processing came with the advent of cyanide leaching in the late 19th century. The MacArthurForrest Process (1887) allowed for the dissolution of gold into a cyanide solution, a far more effective method than mercury amalgamation. Initially, this involved leaching the ore in tanks or vats (agitation leaching) and then precipitating the gold onto zinc dust (the MerrillCrowe process).

While effective, MerrillCrowe had limitations, particularly with ores containing clays, carbonaceous material, or "pregrobbing" constituents. This led to the development of CarboninPulp (CIP) in the 1970s, where activated carbon was used to adsorb the dissolved gold directly from the slurry, bypassing the need for costly filtration.

CarboninLeach (CIL) emerged as a direct evolution of CIP. The key difference is integration: in CIL, leaching and adsorption occur simultaneously in the same tank series. This is critically important for ores where pregrobbing occurs—where naturally occurring carbon in the ore steals the dissolved gold before it can be processed. By introducing activated carbon at the start of the leaching circuit, CIL ensures that the gold is captured by the introduced carbon almost as soon as it is dissolved, rendering the native carbon ineffective.

Today, CIL is the industrystandard process for most new greenfield gold projects due to its robustness, lower operating costs compared to MerrillCrowe, and superior performance on a wide range of ore types.

2. Core Technology: Deconstructing the CIL Process

A CIL plant is a sophisticated system of interlinked unit operations. Understanding its core components is essential to appreciating its design.

The CIL Circuit StepbyStep:

1. Comminution (Crushing & Grinding): RunofMine (ROM) ore is crushed and ground into a fine slurry (typically 8090% passing 75 microns). The objective is to liberate the microscopic gold particles from the host rock, making them accessible to the cyanide solution. This circuit often includes a SemiAutogenous Grinding (SAG) Mill and a Ball Mill.

2. Leaching & Adsorption (The Heart of CIL): The ground slurry is pumped into a series of large, agitated tanks (typically 6 to 10 in series).
Sodium Cyanide (NaCN) and lime (for pH control to ~10.511.0) are added to the first tank.
Granular Activated Carbon is added directly into the slurry in each subsequent tank.
As the slurry flows from one tank to the next, two key processes occur simultaneously:
Leaching: Cyanide dissolves the gold: 4Au + 8NaCN + O2 + 2H2O → 4Na[Au(CN)2] + 4NaOH
Adsorption: The dissolved goldcyanide complex ([Au(CN)₂]⁻) is adsorbed onto the porous surface of the activated carbon.
The carbon moves countercurrently to the slurry; fresh or regenerated carbon is added to the last tank and transferred forward towards the first tank, while barren slurry exits from the last tank to tailings. This ensures that nearly barren slurry meets fresh carbon for maximum recovery.

3. Carbon Handling & Elution: The loaded carbon from Tank 1 ("pregnant carbon"), now rich with gold, is screened out of slurry.
It undergoes an acid wash to remove inorganic foulants like calcium carbonate.
It is then transferred to an elution column where a hot (~110130°C), caustic cyanide solution strips (elutes) th e gold from th e carbon.
The now "pregnant eluate" solution, containing a high concentration of gold , proceeds t o electrowinning.

4. Gold Recovery (Electrowinning & Smelting):
The pregnant eluate is passed through electrowinning cells containing steel wool cathodes . An electric current causes th e g old t o plate ont o th e steel wool .
Th e loaded steel wool i s then mixed with fluxes and melted in an induction furnace (smelting) at temperatures exceeding 1 ,100°C . Th e result i s a doré bar o f impure g old an d silver , ready for refining at a specialized refinery .

5. Carbon Regeneration: Th e stripped ("barren") carbon i s thermally regenerated i n a rotary kiln (~700°C ) t o burn off any organic foulants an d reactivate it s pores . It i s then quenched an d returned t o th e CIL circuit , closing th e loop .

3. Key Design Considerations & Technological Innovations

Designing a CIL plant i s a complex balancing act between metallurgical efficiency , capital expenditure (CAPEX) , an d operating expenditure (OPEX).

Ore Characterization: Th e single most critical factor . Extensive test work—including comminution , leach , an d pregrobbing tests—determines grind size , reagent consumptions , an d residence times .
Residence Time: Typically ranges from 24 t o 48 hours across th e entire CIL train . Thi s i s determined by th e leach kinetics o f th e specific ore .
Tank Design: Tanks are typically cylindrical with flat bottoms . Agitation i s provided by lowspeed , highefficiency impellers t o keep solids in suspension while minimizing carbon attrition .
Interstage Screening: Robust interstage screens are vital t o separate carbon from slurry as it moves between tanks . Derrick screens or Sweco screens are commonly used .
Safety & Environmental Controls:
Cyanide Detoxification: Incoming tailings are treated with SO₂/Air (INCO process ) or hydrogen peroxide t o destroy residual cyanide before discharge t o th e tailings storage facility .
Containment: All process areas are designed with bunds an d containment sumps t o prevent environmental spills .

Recent Innovations:
GravityRecoverableGold (GRG) circuits ahead of CIL can recover coarse g old early , reducing g old lockup i n th e circuit an d improving overall cash flow .
Intelligent Control Systems: Using online analyzers an d AIdriven models t o optimize reagent addition realtime , reducing costs .
HighEfficiency Elution: Pressure Zadra systems offer faster elution cycles with lower water consumption .

4. Market Applications & Advantages

CIL technology dominates th e global g old mining industry due t o its compelling advantages:

Versatility: Handles freemilling oxide an d sulphide ores effectively . I s particularly advantageous for pregrobbing or clayrich ores that challenge other processes .
Lower CAPEX/OPEX: Eliminates th e need for expensive filtration units required by MerrillCrowe . Lower operating costs due t o less intensive labor an d maintenance .
Simplicity & Robustness: Th e process i s relatively simple t o operate an d control compared t o alternative methods .
High Recovery Rates: Consistently achieves recoveries above 90% , often reaching >95% for amenable oxide ores .

Its primary application i s in mediumt o largescale g old mining operations processing >1 million tonnes per annum .

(5). Future Outlook

Th e future trends shaping CIL plant design focus on sustainability , efficiency , an d digitalization:

1. Sustainability:
Water Recycling: Designs now emphasize >80% water recirculation from tailings facilities.
Alternative Lixiviants: Research into noncyanide lixiviants like thiourea or thiosulphate continues but cyanide remains dominant due t costeffectiveness.
2 Efficiency:
Equipment optimization continues with more efficient pumps motors thickeners etc reducing energy footprint per tonne processed
3 Digitalization Smart Plants
Integration IoT sensors AI machine learning create digital twins plants enabling predictive maintenance dynamic process optimization autonomous operation

(6). Frequently Asked Questions FAQ

Q What main difference between CIP
A Both use activated carbon recover dissolved CIP involves separate sequential stages Leaching completed first followed adsorption pulp In both leaching adsorption occur simultaneously same tanks

Q Why so important control around
Cyanidation only effective alkaline conditions pH prevents formation highly toxic hydrogen cyanide gas HCN ensures optimal dissolution kinetics

Q How much typically lost circuit
Welldesigned welloperated modern plant experiences low attrition rates around grams tonne processed Carbon makeup routine part operation

Q Can used recover other metals
Yes principle can adsorb other metals like silver copper However presence high concentrations these metals can foul reduce efficiency specific recovery requiring specialized elution regimes

Q What biggest operational challenge
Managing pregrobbing materials maintaining lowcarbon inventory attrition handling difficult rheology clayey slurries can plug screens

(7). Engineering Case Study Hypothetical Example

Project Name Liberty Gold Mine Expansion
Location West Africa
Ore Type Transitional oxide sulphide moderate pregrobbing characteristics
Design Capacity Million Tonnes Per Annum Mtpa

Challenge Existing MerrillCrowe circuit struggling declining recovery increasing operating costs due rising copper content ore pregrobbing

Solution Greenfield plant designed replace existing facility Key design features included
Eighttank cascade total residence hours based test work results
Dedicated GRG circuit Knelson concentrators recover coarse first improving project economics
Advanced interstage screening minimize carbon losses fines reporting tailings
Integrated SO Air cyanide destruction circuit meet stringent environmental standards

Results Commissioned achieved recovery compared previous Significant reduction reagent consumption operating costs overall Payback period new plant estimated years based increased production reduced costs demonstrating compelling business case adoption technology