9.3 Precious Metal Recovery: CIP, CIL, Merrill-Crowe & Electrowinning

Gold and silver extraction relies heavily on alkaline cyanidation to form soluble dicyanoaurate $\text{[Au(CN)}_2\text{]}^-$ and dicyanoargentate $\text{[Ag(CN)}_2\text{]}^-$ complex ions, followed by solution purification, concentration, and reduction to metallic bullion.

Cyanidation Chemistry & Elsner's Equation

The oxidative dissolution of metallic gold in alkaline sodium cyanide solution is defined by Elsner's Equation:

$4\text{Au} + 8\text{NaCN} + \text{O}_2 + 2\text{H}_2\text{O} \rightarrow 4\text{Na}[\text{Au}(\text{CN})_2] + 4\text{NaOH}$

Key Process Parameters

• Cyanide Concentration: Set free-cyanide concentration from ore demand, solution chemistry, kinetics, recovery, detoxification, safety, and permit conditions; a plant range is not universal.
• Dissolved Oxygen (DO): Oxygen is an electron acceptor in gold dissolution. Air or oxygen addition and any lead-nitrate use must be justified by diagnostic leach tests and plant response; excessive reagent can add cost, hazards, or downstream effects.
• Protective Alkalinity: Lime or another alkali provides protective alkalinity. The operating pH must suppress HCN while balancing cyanide consumption, scaling, leach kinetics, and the approved safety system; 10 to 10.5 is a common reference range, not a universal strict setpoint. Maintaining alkaline conditions prevents the hydrolysis of cyanide ions ($\text{CN}^-$) into volatile, lethal hydrogen cyanide gas ($\text{HCN}$):

$\text{CN}^- + \text{H}_2\text{O} \rightleftharpoons \text{HCN} + \text{OH}^- \quad (pK_a = 9.31)$

If slurry $\text{pH}$ drops below $9.31$, the equilibrium shifts rapidly toward $\text{HCN}$ gas evolution, posing severe health hazards and consuming expensive reagent.

Carbon Adsorption Technologies: CIP vs. CIL & Carbon Elution

Once gold is dissolved into solution, granular activated carbon manufactured from coconut shells ($6 \times 16 \text{ mesh}$, surface area $1,000 - 1,200 \text{ m}^2/\text{g}$) is utilized to adsorb the gold-cyanide complex.

1. Carbon-in-Pulp (CIP) vs. Carbon-in-Leach (CIL)

• Carbon-in-Pulp (CIP): Slurry flows through a series of leach tanks (typically 4 to 6 tanks) to achieve complete gold dissolution before entering a separate cascade of adsorption tanks containing $10 - 25 \text{ g/L}$ of activated carbon. Carbon moves counter-current to slurry flow via air-lift pumps, and interstage vibrating or equalized screens retain coarse carbon while fine slurry passes through.
• Carbon-in-Leach (CIL): Cyanidation leaching and carbon adsorption occur simultaneously in the same tank cascade. CIL is often considered for preg-robbing ores—ores containing natural active carbonaceous matter or reactive clays that adsorb dissolved gold cyanide before recovery. By adding fresh activated carbon directly into the leaching tanks, activated carbon outcompetes native preg-robbing components for gold uptake.

2. Carbon Elution (Desorption) Methods

Loaded carbon carrying $3,000 - 8,000 \text{ g/t Au}$ is screened, washed with dilute hydrochloric acid ($\text{HCl}$) to remove calcium carbonate scale, and transferred to an elution column for desorption:

• Zadra Elution Process: Hot solution containing $1% \text{ NaOH}$ and $0.1 - 0.2% \text{ NaCN}$ is passed through the carbon bed at $95^\circ\text{C} - 100^\circ\text{C}$ (atmospheric Zadra) or $130^\circ\text{C} - 140^\circ\text{C}$ (pressurized Zadra, $300 - 400 \text{ kPa}$). Cycle time ranges from $8 - 16 \text{ hours}$.
• Anglo American Research Laboratory (AARL) Process: Carbon is pre-soaked with a strong caustic/cyanide solution ($3% \text{ NaOH} + 3% \text{ NaCN}$) at $90^\circ\text{C}$ for 2 hours, then eluted with high-purity deionized water at $110^\circ\text{C}$. AARL offers rapid elution ($4 - 6 \text{ hours}$).

Stripped carbon is thermally regenerated in a rotary kiln at $650^\circ\text{C} - 750^\circ\text{C}$ in a steam atmosphere to reactivate fouled pores before returning to the adsorption circuit.

Merrill-Crowe Zinc Dust Precipitation Process

The Merrill-Crowe process is a cementation technology used to recover gold and silver from clear pregnant solutions. It is preferred over carbon processes when treating high-silver ores (silver-rich feed, subject to comparative testwork and economics) or high-grade solutions where carbon loading capacity would require impractically large carbon inventories.

The core Merrill-Crowe sequence commonly includes the following controlled operations:

1. Clarification: Filters remove suspended solids to a circuit-specific target so solids do not consume zinc, coat particles, or blind the precipitate filter.
2. Vacuum Deaeration: A Crowe tower lowers dissolved oxygen to the validated cementation target because oxygen can re-oxidize precipitated metal and passivate zinc surfaces.
3. Zinc Dust Addition & Cementation: Fine zinc dust ($\text{Zn}^0$) and lead nitrate solution ($\text{Pb(NO}_3)_2$) are injected into the deaerated solution. Zinc displaces precious metals via a reduction reaction:

$2\text{Na[Au(CN)}_2] + \text{Zn} \rightarrow 2\text{Au} \downarrow + \text{Na}_2[\text{Zn(CN)}_4]$

Lead nitrate forms a catalytic lead-zinc galvanic couple on zinc particle surfaces, accelerating gold precipitation kinetics. The resulting gold-silver-zinc precipitate is filtered using plate-and-frame filter presses.

Electrowinning, Calcination & Doré Bullion Smelting

Pregnant eluate from carbon stripping (or dissolved precipitate) passes into electrowinning cells containing stainless steel mesh anodes and cathode baskets filled with knitted steel wool. An electric current ($2 - 4 \text{ volts}$) deposits metallic gold and silver onto the steel wool cathodes:

$\text{Cathode: } \text{Au(CN)}_2^- + e^- \rightarrow \text{Au}^0 + 2\text{CN}^-$

$\text{Anode: } 4\text{OH}^- \rightarrow \text{O}_2 + 2\text{H}_2\text{O} + 4e^-$

After isolation from the cyanide circuit, controlled washing, and verification under the site procedure that acid-reactive cyanide hazard has been removed, gold-laden steel wool or filter cake may be digested with dilute sulfuric acid in engineered, ventilated equipment to dissolve base metals or excess zinc. The residue is then washed and calcined in a furnace at $600^\circ\text{C} - 700^\circ\text{C}$ to oxidize residual base metals.

The calcined sludge is mixed with fluxing agents—borax ($\text{Na}_2\text{B}_4\text{O}_7$), silica ($\text{SiO}_2$), nitre ($\text{KNO}_3$), and soda ash ($\text{Na}_2\text{CO}_3$)—and smelted in a tilting induction furnace at $1,100^\circ\text{C} - 1,200^\circ\text{C}$. Impurities form a glassy liquid slag, while molten precious metals settle at the bottom. The liquid metal is poured into molds to cool, yielding unrefined gold-silver doré bullion bars (whose precious-metal content depends on feed and circuit performance) ready for final refinery purification.

Circuit Selection, Control and Reconciliation

A gold-recovery route must be selected from representative mineralogy and variability tests. Compare free-milling response, preg-robbing index, sulfide encapsulation, cyanide-soluble copper, clay behavior, oxygen demand, carbon activity, silver content, solution clarification, residue detoxification, water balance, and product security. CIL can reduce dissolved-gold exposure to preg-robbing material, but severe preg-robbing may require blinding, flotation, roasting, pressure oxidation, resin, or another tested route. Merrill-Crowe can be attractive for silver-rich clear solutions, while carbon adsorption can be simpler for other feeds; neither route wins from head grade or a silver-to-gold ratio alone.

Control the circuit through a metal balance. Reconcile dry feed tonnes and head grade with solution inventory, carbon inventory, precipitate or cathode production, residue grade, spills, cleanup material, and gold-room work in process. A high solution assay is not automatically good performance: it may indicate slow adsorption or cementation. Track extraction, solution loss, carbon loading and advance, elution efficiency, carbon attrition, electrowinning efficiency, barren-solution tenor, residue grade, and unaccounted metal. Use simultaneous, representative samples and consistent dry-mass and volume bases.

Cyanide service demands engineered containment and disciplined work. Maintain compatible materials, secondary containment, ventilation, gas monitoring where the risk assessment requires it, controlled reagent mixing, interlocks, eyewash and showers, emergency response, access control, and trained operators. Never acidify cyanide-bearing solution outside an engineered destruction or analytical procedure: lower pH shifts cyanide toward volatile HCN. Verify instrumentation independently and define actions for low pH, low oxygen, screen failure, carbon loss, high barren tenor, filter breakthrough, vacuum loss, power failure, spill, and abnormal gas indication.

For an exam scenario, first identify where the metal is, then identify the limiting mechanism. If residue grade rises while dissolved gold remains low, investigate leach chemistry, liberation, residence time, oxygen, passivation, and sampling. If dissolved gold rises but residue extraction remains normal, investigate adsorption or precipitation capacity. If calculated recovery exceeds 100%, stop and reconcile moisture, flow, assays, inventory, timing, and sampling before changing reagents.