How does Wohlwill chloroauric acid (HAuCl4) electrorefining produce 4N (99.99%) and 5N (99.999%) gold?

The Wohlwill cell uses 99.5% Miller gold anodes and pure titanium or 4N gold starter-sheet cathodes submerged in an aqueous electrolyte of 80-100 g/L gold as chloroauric acid (HAuCl4) and 1.5-2.5 M hydrochloric acid (HCl) at 65-70 degrees Celsius. Applying 1.1 to 1.5 V DC dissolves gold at the anode primarily as [AuCl4]- (E0 = +1.002 V vs SHE, 3-electron transfer requiring 407.77 kAh per tonne of Au). Platinum and palladium dissolve into the electrolyte without plating at the cathode, while insoluble silver chloride (AgCl), osmium, and iridium report to the anode slime, yielding 99.99% (4N) to 99.999% (5N) gold cathodes at ~565 kWh DC per tonne.

Gold Refining, CIL Cyanidation & Wohlwill Lab

Q: How does Elsner cyanidation and Carbon-in-Leach (CIL) recover gold from low-grade ore?

Finely ground gold ore (P80 = 75 microns) is agitated in aerated alkaline sodium cyanide solution (pH 10.5 to 11.0 maintained with lime to prevent toxic HCN gas volatilization). According to Elsner's equation (4Au + 8NaCN + O2 + 2H2O -> 4Na[Au(CN)2] + 4NaOH), dissolved oxygen oxidizes metallic gold while cyanide ions stabilize Au(I) as the soluble dicyanoaurate complex [Au(CN)2]-. Activated coconut-shell carbon granules added counter-currently adsorb the gold complex, which is subsequently stripped in a Zadra or AARL pressure elution column at 140 degrees Celsius and electrowon onto stainless-steel wool cathodes before smelting into Doré bullion.

Q: Why does the Miller chlorination process selectively remove silver and base metals without chlorinating gold?

In the Miller process, gaseous chlorine (Cl2) is sparged through molten Doré bullion at 1,150 degrees Celsius. At this temperature, gold chlorides (AuCl3 and AuCl) are thermodynamically unstable (positive Gibbs free energy of formation, Delta G > 0) and decompose immediately into metallic gold and chlorine gas. Conversely, Fe, Zn, Pb, Cu, and Ag have strongly negative free energies of chloride formation: zinc, lead, and iron chlorides volatilize into the fume scrubber first, followed by molten copper(I) chloride (CuCl) and silver chloride (AgCl) slag, leaving 99.5% to 99.7% pure gold ready for LBMA Good Delivery bars or Wohlwill anodes.

Q: Why must Miller chlorination precede Wohlwill electrorefining when Doré bullion contains more than 5% silver?

If Doré bullion containing more than 5% to 8% silver is placed directly into a Wohlwill chloride electrolyte, anodic oxidation coats the anode surface with an adherent, electrically insulating crust of insoluble silver chloride (AgCl). This AgCl passivation layer rapidly blocks chloride ion diffusion, spikes cell voltage, and halts gold dissolution unless superimposed AC current is applied. Pre-refining Doré via the Miller Cl2 process strips silver down to <0.4% (producing 99.5% Au anodes) and recovers the AgCl slag separately for reduction and Moebius electrorefining.

Industrial Presets:

Circuit & Economic Parameters

1. Mill Ore Throughput (Mtpa) 4.5 Mtpa

Annual SAG/ball mill ore feed (

P_{80} = 75 μm

2. Ore Gold Head Grade (g/t Au) 3.2 g/t Au

Grams of gold per metric tonne of run-of-mine (ROM) ore.

3. Gravity + CIL Leach Recovery (%) 91.5%

Elsner

NaCN

leaching + activated carbon adsorption efficiency.

4. Miller

{Cl}_{2}

Anode Purity (% Au) 99.50% Au

1,150 °C

chlorine sparging output feeding Wohlwill cells.

5. 5N Semiconductor Wire Allocation (%) 45%

Share of Wohlwill cathodes drawn into

20 μm

5 N

IC bonding wire.

6. LBMA / COMEX Gold Spot Price ($/oz) $2,650/oz

Benchmark 4N LBMA Good Delivery bullion price per troy oz.

7. 5N Semiconductor Wire Premium (+$/oz) +$85/oz

Value-add premium for

99.999 %

zone-refined

20 μm

spool wire.

8. Integrated Mine & Refinery CAPEX ($M) $420M

Total capital expenditure for mill, CIL tanks, Miller & Wohlwill refinery.

9. Discount Rate / WACC (%) 8.5%

Weighted Average Cost of Capital for 20-year DCF valuation.

Total Refined Gold (4N + 5N)

419.6 koz/yr

13.1 t/yr Au ($1,128M gross Au)

5N Semiconductor Bonding Wire

188.8 koz/yr

5.9 t/yr 5N Wire (~2.2B IC packages)

Refined 4N Silver & PGM Credits

303.8 koz/yr Ag

+ 5.04 koz/yr Pt/Pd ($15.2M credits)

Annual Revenue & EBITDA

$1,143.25M

EBITDA: $931.71M (81.5%)

20-Year Project NPV & IRR

$6,645.12M

IRR: 147.2% | Payback: 0.7 yrs

All-In Sustaining Cost (AISC)

$468/oz Au

564.6 kWh DC/t Au | 128.3 GWh/yr mill

Interactive Process Flowsheet: CIL Cyanidation → Miller ${Cl}_{2}$ → Wohlwill ${HAuCl}_{4}$

Live Telemetry

Stage-by-Stage Hydrometallurgical, Pyrometallurgical & Wohlwill Mass Balance

Stage-by-stage mass and electrochemical balance for gold CIL cyanidation, Miller chlorination, and Wohlwill electrorefining
Unit Operation Stage	Governing Reaction / Mechanism	Primary Feed / Reagent	Recoverable Output	Key Process Metric

5×5 Project NPV Sensitivity Matrix (Head Grade vs. Gold Price)

Evaluates 20-year after-tax Net Present Value ( $NPV$ ) and Internal Rate of Return ( $IRR$ ) across simultaneous variations in ROM gold head grade ( $\pm 25 %$ ) and LBMA spot gold price ( $\pm $ 400 /oz$ ).

20-Year Discounted Cash Flow (DCF) Pro-Forma Schedule

Includes Year 1 commissioning ramp-up ( $85 %$ utilization), 20-year straight-line depreciation, $25 %$ corporate income tax, and silver/PGM byproduct credits.

20-year discounted cash flow schedule for the integrated gold CIL and Wohlwill refinery
Year	Gold Output	Revenue	OPEX	EBITDA	Tax (25%)	Free Cash Flow	PV of FCF	Cumulative FCF

First-Principles Gold Hydrometallurgy, Miller Chlorination & Wohlwill Electrorefining

1. Elsner Cyanidation, CIL Adsorption & Zadra Elution

Metallic gold is noble in aqueous solution ( $E^{\circ} ({Au}^{+} / Au) = + 1.69 V$ vs. SHE), preventing dissolution by non-oxidizing mineral acids. However, strong coordination by cyanide ions ( $β_{2} = 10^{38.3}$ for ${[Au {(CN)}_{2}]}^{-}$ ) shifts the anodic oxidation potential to $- 0.60 V$ , enabling spontaneous dissolution in aerated alkaline slurry via Elsner's stoichiometry:

4 Au (s) + 8 {CN}^{-} (aq) + O_{2} (g) + 2 H_{2} O (l) ⟶ 4 {[Au {(CN)}_{2}]}^{-} (aq) + 4 {OH}^{-} (aq)

Slurry $pH$ is strictly controlled between $10.5$ and $11.0$ using slaked lime ( $Ca {(OH)}_{2}$ ) because $HCN$ has a $p K_{a} = 9.21$ . Loaded activated carbon ( $4,000 – 8,000 g Au/t C$ ) is stripped in a pressurized Zadra or Anglo American Research Laboratories (AARL) elution column at $135 – 145 °C$ and $350 – 500 kPa$ , followed by electrowinning and induction smelting into Doré bars.

2. Miller Chlorination & Wohlwill ${HAuCl}_{4}$ Electrorefining

Mine Doré ( $50 – 90 % Au$ , $10 – 45 % Ag$ ) is first refined in the Miller furnace at $1,150 °C$ by sparging ${Cl}_{2}$ gas through ceramic lances, converting base metals and silver into molten/volatile chlorides ( $2 Ag + {Cl}_{2} \to 2 AgCl$ ) to yield $99.5 % Au$ . To reach $4 N$ ( $99.99 %$ ) and $5 N$ ( $99.999 %$ ) purity, Miller anodes undergo Wohlwill electrorefining in ${HAuCl}_{4} + HCl$ :

Anode: Au + 4 {Cl}^{-} ⟶ {[{AuCl}_{4}]}^{-} + 3 e^{-} (E^{\circ} = + 1.002 V)

Q_{Faraday, Au(III)} = \frac{3 \times 96,485 C/mol}{196.967 g/mol \times 3.6 C/mAh} = 407.77 kAh/t Au

At $1.35 V DC$ and $97.5 %$ current efficiency, specific electrical energy consumption is only $\sim 564.6 kWh DC/t Au$ . Ultra-pure $5 N$ gold cathodes are vacuum-melted and drawn through diamond dies into $20 μm$ ( $0.8 mil$ ) semiconductor bonding wire ( $6.07 mg/m$ ).

Frequently Asked Questions: Gold Cyanidation, Miller Chlorination & Wohlwill Refining

Finely ground gold ore (

P_{80} = 75 μm

) is agitated in aerated alkaline sodium cyanide solution (

pH

10.5 to 11.0 maintained with lime to prevent toxic

HCN

gas volatilization). According to Elsner's equation (

4 Au + 8 NaCN + O_{2} + 2 H_{2} O \to 4 Na [Au {(CN)}_{2}] + 4 NaOH

), dissolved oxygen oxidizes metallic gold while cyanide ions stabilize

Au(I)

as the soluble dicyanoaurate complex

{[Au {(CN)}_{2}]}^{-}

. Activated coconut-shell carbon granules added counter-currently adsorb the gold complex, which is subsequently stripped in a Zadra or AARL pressure elution column at

140 °C

and electrowon onto stainless-steel wool cathodes before smelting into Doré bullion.

In the Miller process, gaseous chlorine (

{Cl}_{2}

) is sparged through molten Doré bullion at

1,150 °C

. At this temperature, gold chlorides (

{AuCl}_{3}

and

AuCl

) are thermodynamically unstable (positive Gibbs free energy of formation,

Δ G^{\circ} > 0

) and decompose immediately into metallic gold and chlorine gas. Conversely,

Fe

Zn

Pb

Cu

, and

Ag

have strongly negative free energies of chloride formation: zinc, lead, and iron chlorides volatilize into the fume scrubber first, followed by molten copper(I) chloride (

CuCl

) and silver chloride (

AgCl

) slag, leaving

99.5 %

99.7 %

pure gold ready for LBMA Good Delivery bars or Wohlwill anodes.

The Wohlwill cell uses

99.5 %

Miller gold anodes and pure titanium or

4 N

gold starter-sheet cathodes submerged in an aqueous electrolyte of

80 – 100 g/L

gold as chloroauric acid (

{HAuCl}_{4}

) and

1.5 – 2.5 M HCl

65 – 70 °C

. Applying

1.1 – 1.5 V DC

dissolves gold at the anode primarily as

{[{AuCl}_{4}]}^{-}

(

E^{\circ} = + 1.002 V

vs. SHE, 3-electron transfer requiring

407.77 kAh/t Au

). Platinum and palladium dissolve into the electrolyte without plating at the cathode, while insoluble silver chloride (

AgCl

), osmium, and iridium report to the anode slime, yielding

99.99 %

(

4 N

) to

99.999 %

(

5 N

) gold cathodes at

\sim 565 kWh DC/t

If Doré bullion containing more than

5 %

8 %

silver is placed directly into a Wohlwill chloride electrolyte, anodic oxidation coats the anode surface with an adherent, electrically insulating crust of insoluble silver chloride (

AgCl

). This

AgCl

passivation layer rapidly blocks chloride ion diffusion, spikes cell voltage, and halts gold dissolution unless superimposed AC current is applied. Pre-refining Doré via the Miller

{Cl}_{2}

process strips silver down to

< 0.4 %

(producing

99.5 %

Au

anodes) and recovers the

AgCl

slag separately for reduction and Moebius electrorefining.

High-density IC packages, RF power amplifiers, automotive sensors, and radiation-hardened aerospace electronics rely on thermosonic ball-and-wedge bonding using ultra-fine gold wire drawn through diamond dies to

15 – 25 μm

(

0.6 – 1.0 mil

) diameter. Trace metallic impurities above

10 ppm

(such as

Fe

Cu

Pb

, or

Bi

4 N

gold) segregate to grain boundaries, causing wire breakage during high-speed capillary drawing, erratic free-air ball (FAB) hardness during spark formation, and intermetallic Kirkendall voiding at the

Au-Al

bond pad interface.

5 N

(

99.999 %

) purity doped with controlled trace

Ca/Be

(

3 – 8 ppm

) ensures uniform loop stability and commands a

$ 60

$ 150 /oz

fabrication premium.

Related Precious Metals, Semiconductor Materials & Hydrometallurgy Simulators

Silver Refining & TOPCon PV Lab

Parkes Ag₂Zn₃ cupellation, Moebius AgNO₃ electrorefining, Wohlwill gold recovery & PV paste.

PGM Refining & PEM Catalyst Lab

Matte pressure leaching, Pt/Pd/Rh/Ir solvent extraction & PEM electrolyzer catalysts.

Copper Bioleaching & SX-EW Lab

Chalcopyrite heap bio-oxidation, LIX oxime solvent extraction & 99.99% LME Grade A cathodes.

Zinc Refining, RLE & Indium Lab

930°C sphalerite roasting, 95°C ferrite leaching, In/Ge SX & 99.995% SHG zinc cellhouse.

Tellurium Refining & CdTe Solar Lab

Copper anode slime pressure decopperizing, Na₂TeO₃ electrowinning & 5N CdTe thin-film PV.

High-Purity Quartz (4N8/5N HPQ) Lab

Alaskite flotation, 1,000°C quench decrepitation, Cl₂/HCl roast & CZ silicon crucibles.

Tin Refining, Solder & EUV Lab

Cassiterite TSL smelting, vacuum distillation, PV ribbon SAC305 solder & 6N EUV tin.

Bismuth Refining & Thermoelectrics Lab

Kroll-Betterton debismuthizing, Betts Bi₂SiF₆ electrorefining & Bi₂Te₃ Peltier modules.

Lead Refining, KIVCET & Betts Lab

Galena KIVCET oxygen flash smelting, Betts H₂SiF₆ electrorefining, Ag/Bi slimes & 4N5 BESS lead.

Gold Refining, CIL/CIP Cyanidation, Miller Chlorination & Wohlwill 4N/5N Lab

Circuit & Economic Parameters

Interactive Process Flowsheet: CIL Cyanidation → Miller ${Cl}_{2}$ → Wohlwill ${HAuCl}_{4}$

Stage-by-Stage Hydrometallurgical, Pyrometallurgical & Wohlwill Mass Balance

5×5 Project NPV Sensitivity Matrix (Head Grade vs. Gold Price)

20-Year Discounted Cash Flow (DCF) Pro-Forma Schedule

First-Principles Gold Hydrometallurgy, Miller Chlorination & Wohlwill Electrorefining

1. Elsner Cyanidation, CIL Adsorption & Zadra Elution

2. Miller Chlorination & Wohlwill ${HAuCl}_{4}$ Electrorefining

Precious Metals Hydrometallurgy & Electrorefining Mastery Quiz

Frequently Asked Questions: Gold Cyanidation, Miller Chlorination & Wohlwill Refining

Related Precious Metals, Semiconductor Materials & Hydrometallurgy Simulators

Circuit & Economic Parameters

Interactive Process Flowsheet: CIL Cyanidation → Miller Cl2 → Wohlwill HAuCl4

Stage-by-Stage Hydrometallurgical, Pyrometallurgical & Wohlwill Mass Balance

5×5 Project NPV Sensitivity Matrix (Head Grade vs. Gold Price)

20-Year Discounted Cash Flow (DCF) Pro-Forma Schedule

First-Principles Gold Hydrometallurgy, Miller Chlorination & Wohlwill Electrorefining

1. Elsner Cyanidation, CIL Adsorption & Zadra Elution

2. Miller Chlorination & Wohlwill HAuCl4 Electrorefining

Precious Metals Hydrometallurgy & Electrorefining Mastery Quiz

Frequently Asked Questions: Gold Cyanidation, Miller Chlorination & Wohlwill Refining

How does Elsner cyanidation and Carbon-in-Leach (CIL) recover gold from low-grade ore?

Why does the Miller chlorination process selectively remove silver and base metals without chlorinating gold?

How does Wohlwill chloroauric acid (HAuCl4) electrorefining produce 4N (99.99%) and 5N (99.999%) gold?

Why must Miller chlorination precede Wohlwill electrorefining when Doré bullion contains more than 5% silver?

What makes 5N (99.999%) gold essential for 20-micron semiconductor wire bonding and aerospace interconnects?

Related Precious Metals, Semiconductor Materials & Hydrometallurgy Simulators

Interactive Process Flowsheet: CIL Cyanidation → Miller ${Cl}_{2}$ → Wohlwill ${HAuCl}_{4}$

2. Miller Chlorination & Wohlwill ${HAuCl}_{4}$ Electrorefining