An evaluation of sulfuric acid manufacturers by route: Sulfur-Burning, Smelter and Pyrite Compared begins with the recognition that all three routes converge on the same contact-process converter and absorb SO₃ in circulating sulfuric acid, but diverge upstream in SO₂ generation, gas-cleaning burden, waste-heat recovery, and trace-metal rejection. Industrial plants releasing 93.0 wt% to 99.2 wt% H₂SO₄ under ISO 910:1977 total acidity titration and ASTM D4052-22 density testing operate V₂O₅-catalyzed SO₂ oxidation at first-bed inlet temperatures between 410°C and 430°C. The true route boundary lies in the gas preparation island, not the converter.

Sulfur-Burning Acid Plants and the Double-Absorption Configuration

Elemental sulfur feed is specified above 99.5 wt% sulfur, with arsenic below <1 ppm and chloride below <10 ppm. Molten sulfur is maintained at 135°C to 145°C and filtered through steam-jacketed leaf filters before atomization into a refractory-lined sulfur furnace. Dried combustion air from a packed drying tower reduces moisture to <50 mg/Nm³ H₂O to prevent sulfuric acid condensation in the cold end of the waste-heat boiler. The furnace outlet gas reaches 1,000°C to 1,200°C; a waste-heat boiler recovers 2.5 GJ to 3.0 GJ per tonne of H₂SO₄ as high-pressure steam at 4.0 MPa to 6.0 MPa. The cooled gas is diluted to 10 vol% to 12 vol% SO₂ and 9 vol% to 11 vol% O₂ before the converter.

Double absorption is the standard configuration for sulfur-burning plants above 500 t/d nameplate capacity. Gas passes through three catalyst beds, then through an intermediate absorption tower using 98.5 wt% to 99.5 wt% acid at 70°C to 90°C. After the fourth bed and final absorption, total conversion is 99.7% to 99.9%, producing tail gas with 200 ppmv to 450 ppmv SO₂ before tail-gas scrubbing. Where 40 CFR Part 60 Subpart H applies, tail gas SO₂ is controlled by a gas scrubber using sodium hydroxide or hydrogen peroxide; emission values below 75 ppmv are achievable with a packed tower and mist eliminator. Acid coolers are typically anodically protected stainless steel shell-and-tube units and must maintain outlet acid below 60°C to avoid accelerated corrosion.

Operational failure modes on sulfur-burning lines include sulfur furnace refractory spalling from rapid turndown, acid cooler leakage after low-concentration upset, and mist eliminator plugging from sulfate solids. These plants have the lowest capital cost per tonne of acid among the three routes because the gas cleaning train is essentially absent beyond drying and mist removal.

What Distinguishes Smelter Off-Gas Acid Duty from Dedicated Sulfur Burning?

Metallurgical acid plants are coupled to the gas output of primary copper, nickel, lead, or zinc smelting. The SO₂ concentration reaching the acid plant depends on furnace type, oxygen enrichment, and batch converter cycles; after gas cleaning and dilution, design levels are commonly 6 vol% to 16 vol%. Transient dips below 4 vol% during charging may require natural gas or light fuel oil preheat to hold the first catalyst bed at autothermal temperature. Because the upstream smelter cannot guarantee constant SO₂ flow, the acid plant must operate with a wider turndown range, typically 40% to 100% of design gas flow.

The gas-cleaning train is the defining route burden. Hot gas from a flash smelter or bath smelter carries dust loadings of 20 g/Nm³ to 200 g/Nm³, arsenic, fluoride, chloride, and mercury. A typical sequence includes a waste-heat boiler, hot electrostatic precipitator, quench scrubber, wet electrostatic precipitator, and washing towers. Mercury is removed where needed in a Boliden Norzink selenium-based reactor, with treated gas mercury target below 0.1 mg/Nm³. Fluoride is absorbed in a water wash; residual fluoride below <1 mg/Nm³ is required to prevent attack of silica packing in the drying tower and converter catalyst support. Published design values for specific smelter feeds vary with concentrate composition.

Because the smelter generates SO₂ without a dedicated sulfur furnace, elemental sulfur storage and melting are eliminated. The trade-off is operational subordination: an acid plant outage may force cleaned SO₂ bypass to a limestone or lime scrubber, while a smelter outage idles the acid plant unless a backup sulfur burner is installed. This coupling is the main process conflict observed on integrated metallurgical sites. Acid quality from smelting is generally acceptable for phosphate fertilizer and leaching applications, but mercury, selenium, and nitrosylsulfuric acid may require post-treatment for higher grades.

Pyrite roasting for sulfuric acid production differs fundamentally because the feedstock is a solid sulfide concentrate. After drying and ball-mill grinding to 80% passing 75 µm, the concentrate is fed to a fluidized-bed roaster operating at 850°C to 950°C. The exothermic roasting reaction 4FeS₂ + 11O₂ → 2Fe₂O₃ + 8SO₂ produces roaster gas containing 10 vol% to 14 vol% SO₂ and dust loadings of 100 g/Nm³ to 400 g/Nm³. Cyclone trains and a waste-heat boiler recover dust and steam before wet gas cleaning. Roaster superficial gas velocity is held between 0.3 m/s and 1.0 m/s; bed temperature is controlled by water-cooled tube bundles and off-gas recirculation. Calcine, primarily hematite, is discharged at 900°C and cooled in fluidized-bed coolers before sale as cement iron correction material or heavy-media aggregate when impurity levels permit.

The wet cleaning train must remove arsenic, fluoride, mercury, and residual dust before the drying tower. Pyrite concentrates typically carry 0.1 wt% to 1.5 wt% arsenic and 100 ppm to 1,000 ppm mercury. Arsenic oxides are absorbed in dilute acid wash towers; the resulting arsenic sludge requires dedicated impoundment. Without this step, arsenic trioxide deposits on the V₂O₅ catalyst surface and reduces SO₂ conversion by 2% to 5% over 6 months, with published site data for specific decay rates limited by concentrate variation.

This route is selected where elemental sulfur is unavailable or high-priced and where pyrite concentrate is low-cost and its calcine byproduct can be sold. Relative capital cost is 1.5 to 2.5 times a sulfur-burning plant of equal acid capacity, excluding arsenic residue disposal. The solids-handling equipment—roaster, calciners, grinding mills, and wet gas cleaning—creates the largest maintenance burden among the three routes.

When Pyrite Replaces Sulfur as Feedstock in Regenerative SO₂ Oxidation

In route replacement studies, the contact section is often assumed common; this assumption fails under impurity and thermal duty mismatches. Sulfur-burning converters are designed for a first-bed outlet temperature of 600°C to 625°C, while pyrite-based converters may operate with a lower temperature rise due to wet gas cleaning losses. The interpass coolers and steam superheater surfaces must be resized when switching from sulfur-burning to pyrite roasting because the roaster itself supplies saturated steam from the raw gas, while the converter off-gas cools through economizers and superheaters.

Catalyst deactivation mechanisms also differ. Sulfur-burning feeds primarily age V₂O₅ catalysts through potassium migration and sintering; smelter and pyrite feeds add arsenic, fluoride, and dust blinding. For pyrite gas after wet cleaning, residual As₂O₃ below 1 mg/Nm³ is the typical design target to maintain conversion above 99.5% over a 36-month catalyst cycle. Filtered acid clarity and mercury content are controlled through selenium-based mercury removal or sulfide precipitation, depending on local discharge limits. Commercial acid strength and density are verified by ISO 910:1977 and ASTM D4052-22, but trace metal limits require ICP-OES or cold vapour atomic absorption methods commonly referenced in buyer-supplier quality plans.

The selection matrix below summarizes the upstream variables that control converter feed quality and capital cost. All three routes share the same absorption section: a four-bed or five-bed converter with V₂O₅ catalyst, interpass cooling, and double absorption in 98.5 wt% to 99.5 wt% acid.