Why does 930°C fluidized-bed sphalerite roasting form insoluble zinc ferrite (ZnFe2O4), and how is it leached?

Commercial sphalerite concentrates ([Zn,Fe]S) contain 50% to 55% Zn and 6% to 11% lattice-substituted iron. During exothermic fluidized-bed dead roasting at 910°C–950°C (2ZnS + 3O2 -> 2ZnO + 2SO2), iron oxidizes and reacts in the solid state with ZnO to form refractory spinel zinc ferrite (ZnO·Fe2O3, franklinite), which locks up 10% to 15% of total zinc alongside trace Indium (In) and Germanium (Ge). While neutral leaching (pH 4.5–5.2) dissolves free ZnO, a subsequent 95°C hot-acid leach (80–120 g/L H2SO4) is required to crack ZnFe2O4, boosting zinc recovery above 96% and releasing In3+ and Ge4+ into solution.

How are dissolved iron, indium, and germanium separated after hot-acid zinc ferrite leaching?

Cracking zinc ferrite in hot sulfuric acid dissolves 15 to 30 g/L of ferric iron (Fe3+) alongside Indium and Germanium. Before the solution can return to the neutral leach and cellhouse, Indium is extracted from the acidic liquor via solvent extraction using di-(2-ethylhexyl)phosphoric acid (D2EHPA) and stripped with HCl to produce 4N5 Indium metal, while Germanium is precipitated with tannic acid or extracted via chelating resins. Ferric iron is then precipitated at 95°C as crystalline ammonium or sodium jarosite (NH4Fe3(SO4)2(OH)6), goethite (FeOOH), or autoclave hematite (Fe2O3).

How much electricity is required to electrowin 1 tonne of 99.995% Special High Grade (SHG) zinc?

By Faraday's Law, depositing 1 mole of divalent zinc (65.38 g) requires 2 moles of electrons (2 × 96,485 C = 53.60 Ah), giving a theoretical electrochemical equivalent of 819.85 kAh per tonne of zinc. At an industrial cell voltage of 3.20 to 3.30 V (using Pb-0.75%Ag oxygen-evolving anodes and aluminum cathodes at 450 A/m2) and 90% to 92% cathodic current efficiency, direct-current (DC) electrolysis consumes ~2,900 to 3,000 kWh/t SHG Zn, or ~3,150 to 3,350 kWh AC/t Zn including rectifier losses and induction melting.

How do sulfuric acid, indium, and roaster steam cogeneration credits reduce the Net AISC of a zinc refinery?

For every tonne of sphalerite concentrate roasted, ~0.95 to 1.0 tonne of 98% sulfuric acid (H2SO4) is captured in the double-contact acid plant, while the waste-heat boiler recovers ~0.88 tonnes of high-pressure superheated steam (~0.18 MWh/t concentrate electrical equivalent) from the exothermic oxidation reaction. Combined with 4N5 Indium ($680/kg), Germanium ($2,150/kg), and Cu-Cd cementate cake sales, byproduct credits offset $450 to $800 per tonne of SHG zinc operating cost.

Zinc Refining, RLE Electrowinning & Indium Lab

Q: Why is zinc-dust cementation purification critical before ZnSO4 electrolyte enters the electrowinning cellhouse?

Zinc has a standard reduction potential of E° = -0.76 V vs SHE, which is far more negative than hydrogen evolution (0.00 V). Zinc can only be electrowon from acidic aqueous solution because hydrogen evolution has a very high kinetic overpotential on pure zinc and aluminum cathodes. If electronegative impurities such as cobalt, nickel, antimony, germanium, copper, or cadmium exceed parts-per-billion thresholds (>10–50 ppb), they deposit on the cathode and act as low-overpotential catalytic sites that trigger violent hydrogen evolution, current efficiency collapse, and catastrophic re-dissolution of the zinc sheet. Multi-stage cementation with atomized metallic zinc dust reduces Cu, Cd, Co, and Ni below 0.02 mg/L.

Saleable SHG Zinc

210,425 t/yr

99.995% SHG | Enables 5.54 Mt HDG Steel

Cellhouse Specific Energy

2,944 kWh/t

DC Cellhouse | Net Plant: 626 GWh/yr

4N5 Indium & Byproducts

72.1 t/yr

4N5 In + 11,995 kg Ge + 403 kt H2SO4

Annual Revenue

$775.09M

EBITDA: $466.50M (60.2% Margin)

20-Yr Project NPV

$2,835.79M

IRR: 51.5% | Payback: 2.0 yrs

Net SHG Zinc AISC

$949/t

Net of +$123.82M/yr In/Ge/H2SO4

RLE Smelter & Cellhouse Controls

Sphalerite (ZnS) Concentrate Feed 420 kt/yr

Dry sphalerite flotation concentrate throughput

Concentrate Zinc Grade (% Zn) 52.5% Zn

Contained zinc in marmatitic sphalerite concentrate

Neutral + 95°C Hot-Acid Leach Recovery 96.2%

Overall Zn extraction after cracking ZnFe₂O₄ spinels

Cathodic Current Efficiency (%) 90.5%

Faraday current efficiency on Al cathodes (vs H₂ evolution)

Cellhouse Operating Voltage (V) 3.25 V

Cell voltage across Pb-0.75%Ag anodes & Al cathodes at 450 A/m²

LME SHG Zinc Market Price ($/t) $2,950/t

Base LME 99.995% SHG zinc price (+$145/t physical slab premium)

Concentrate Indium Content (ppm In) 220 ppm In

Trace indium in sphalerite recovered via D2EHPA SX ($680/kg 4N5 In)

Integrated RLE Refinery CAPEX ($M) $680M

Fluidized roaster, acid plant, hot-acid leach, In/Ge SX & jumbo cellhouse

Discount Rate / WACC (%) 8.5%

Real weighted average cost of capital for 20-year DCF

Industrial Electricity Tariff ($/MWh) $58/MWh

Power tariff for cellhouse rectifiers and induction melting

Interactive 930°C Roaster, Hot-Acid Ferrite Leach, In/Ge SX & SHG Cellhouse Flowsheet

ZnSO₄ + H₂O → Zn° + H₂SO₄ + ½O₂↑

Stage-by-stage zinc RLE mass balance, sulfuric acid and indium/germanium byproduct credits, and operating cash flow breakdown
Unit Operation / Process Stage	Feed / Reaction Basis	Mass Output / Yield	Financial Contribution

20-Year discounted cash flow schedule for integrated zinc roast-leach-electrowinning and indium recovery
Year	SHG Zinc (t)	H₂SO₄ (kt)	4N5 Indium (t)	Net Power (GWh)	Revenue	OPEX	EBITDA	Taxes	Free Cash Flow	Cum. FCF

20-Year Project Net Present Value (NPV) across Industrial Electricity Tariff ($\pm \$20/\text{MWh}$) vs. LME $99.995\%$ SHG Zinc Market Price ($\pm 20\%$). Base case cell is outlined in bold.

5 by 5 sensitivity matrix of 20-year project NPV across electricity tariff and SHG zinc market prices
Power Tariff \ Zn Price	-20% Price	-10% Price	Base Price	+10% Price	+20% Price

RLE Hydrometallurgy, Ferrite Leaching & Electrowinning Mastery Quiz

Score: 0 / 4

First-Principles RLE Hydrometallurgy, Zinc Ferrite Chemistry & Cellhouse Electrochemistry

1. 930°C Dead Roasting, Zinc Ferrite Cracking & Jarosite Precipitation

Fluidized-bed dead roasting oxidizes sphalerite ($\text{ZnS}$) to calcine ($\text{ZnO}$), while iron impurities form refractory spinel zinc ferrite ($\text{ZnFe}_2\text{O}_4$). Cracking $\text{ZnFe}_2\text{O}_4$ in $95^\circ\text{C}$ spent acid liberates $\text{Zn}^{2+}$, $\text{In}^{3+}$, and $\text{Ge}^{4+}$:

{ZnFe}_{2} O_{4} + 4 H_{2} {SO}_{4} \overset{95 ° C}{\to} {ZnSO}_{4} + {Fe}_{2} {({SO}_{4})}_{3} + 4 H_{2} O

After extracting $\text{In}^{3+}$ into $\text{D2EHPA}$ organic solvent, dissolved $\text{Fe}^{3+}$ is precipitated as filterable crystalline ammonium jarosite ($\text{NH}_4\text{Fe}_3(\text{SO}_4)_2(\text{OH})_6$) or goethite ($\alpha\text{-FeOOH}$).

2. Faraday Cellhouse Electrowinning & Hydrogen Overpotential

Purified neutral $\text{ZnSO}_4$ solution ($<10\text{ ppb}$ $\text{Co, Ni, Sb, Ge}$ after zinc-dust cementation) is electrolyzed between $\text{Pb-0.75\%Ag}$ anodes and aluminum cathodes at $400\text{–}500\text{ A/m}^2$:

E_{DC} = \frac{2 F \cdot V_{cell}}{M_{Zn} \cdot η_{CE}} = \frac{819.85 kAh/t \times V_{cell}}{η_{CE}}

At $V_{\text{cell}} = 3.25\text{ V}$ and $\eta_{\text{CE}} = 90.5\%$, specific DC power is $2,944\text{ kWh/t}$ of $99.995\%$ Special High Grade ($\text{SHG}$) zinc, while regenerated $\text{H}_2\text{SO}_4$ at the anode recycles in a closed loop back to the leaching tanks.

Frequently Asked Questions: Zinc RLE Refining, Jarosite & Indium Recovery

Commercial sphalerite concentrates ($\text{[Zn,Fe]S}$) contain $50\%$ to $55\%\ \text{Zn}$ and $6\%$ to $11\%$ lattice-substituted iron. During exothermic fluidized-bed dead roasting at $910^\circ\text{C–}950^\circ\text{C}$ ($2\text{ZnS} + 3\text{O}_2 \rightarrow 2\text{ZnO} + 2\text{SO}_2$), iron oxidizes and reacts in the solid state with $\text{ZnO}$ to form refractory spinel zinc ferrite ($\text{ZnO}\cdot\text{Fe}_2\text{O}_3$, franklinite), which locks up $10\%$ to $15\%$ of total zinc alongside trace Indium ($\text{In}$) and Germanium ($\text{Ge}$). While neutral leaching ($\text{pH } 4.5\text{–}5.2$) dissolves free $\text{ZnO}$, a subsequent $95^\circ\text{C}$ hot-acid leach ($80\text{–}120\text{ g/L H}_2\text{SO}_4$) is required to crack $\text{ZnFe}_2\text{O}_4$, boosting zinc recovery above $96\%$ and releasing $\text{In}^{3+}$ and $\text{Ge}^{4+}$ into solution.

Cracking zinc ferrite in hot sulfuric acid dissolves $15$ to $30\text{ g/L}$ of ferric iron ($\text{Fe}^{3+}$) alongside Indium and Germanium. Before the solution can return to the neutral leach and cellhouse, Indium is extracted from the acidic liquor via solvent extraction using di-(2-ethylhexyl)phosphoric acid ($\text{D2EHPA}$) and stripped with $\text{HCl}$ to produce $4\text{N5}$ Indium metal, while Germanium is precipitated with tannic acid or extracted via chelating resins. Ferric iron is then precipitated at $95^\circ\text{C}$ as crystalline ammonium or sodium jarosite ($\text{NH}_4\text{Fe}_3(\text{SO}_4)_2(\text{OH})_6$), goethite ($\text{FeOOH}$), or autoclave hematite ($\text{Fe}_2\text{O}_3$).

Zinc has a standard reduction potential of $E^\circ = -0.76\text{ V vs SHE}$, which is far more negative than hydrogen evolution ($0.00\text{ V}$). Zinc can only be electrowon from acidic aqueous solution because hydrogen evolution has a very high kinetic overpotential on pure zinc and aluminum cathodes. If electronegative impurities such as cobalt, nickel, antimony, germanium, copper, or cadmium exceed parts-per-billion thresholds ($>10\text{–}50\text{ ppb}$), they deposit on the cathode and act as low-overpotential catalytic sites that trigger violent hydrogen evolution, current efficiency collapse, and catastrophic re-dissolution of the zinc sheet. Multi-stage cementation with atomized metallic zinc dust reduces $\text{Cu, Cd, Co,}$ and $\text{Ni}$ below $0.02\text{ mg/L}$.

By Faraday's Law, depositing $1\text{ mole}$ of divalent zinc ($65.38\text{ g}$) requires $2\text{ moles}$ of electrons ($2 \times 96,485\text{ C} = 53.60\text{ Ah}$), giving a theoretical electrochemical equivalent of $819.85\text{ kAh}$ per tonne of zinc. At an industrial cell voltage of $3.20$ to $3.30\text{ V}$ (using $\text{Pb-0.75\%Ag}$ oxygen-evolving anodes and aluminum cathodes at $450\text{ A/m}^2$) and $90\%$ to $92\%$ cathodic current efficiency, direct-current (DC) electrolysis consumes $\sim 2,900$ to $3,000\text{ kWh/t SHG Zn}$, or $\sim 3,150$ to $3,350\text{ kWh AC/t Zn}$ including rectifier losses and induction melting.

For every tonne of sphalerite concentrate roasted, $\sim 0.95$ to $1.0\text{ tonne}$ of $98\%$ sulfuric acid ($\text{H}_2\text{SO}_4$) is captured in the double-contact acid plant, while the waste-heat boiler recovers $\sim 0.88\text{ tonnes}$ of high-pressure superheated steam ($\sim 0.18\text{ MWh/t}$ concentrate electrical equivalent) from the exothermic oxidation reaction. Combined with $4\text{N5}$ Indium ($\$680/\text{kg}$), Germanium ($\$2,150/\text{kg}$), and $\text{Cu-Cd}$ cementate cake sales, byproduct credits offset $\$450$ to $\$800$ per tonne of SHG zinc operating cost.