Sulfuric acid capacity is frequently compared using The Three Acid Routes: Sulfur-Burned vs Smelter vs Pyrite Cost Breakdown, because feedstock, energy recovery, gas cleaning and residue management are allocated differently in each route. Elemental sulfur combustion generates a clean 10–12 vol% SO2 gas with minimal dust and metals. Smelter off-gas acid plants monetise a by-product stream that is also a pollution-control obligation. Pyrite roasting utilises domestic sulfide mineral but adds solids handling and gas-cleaning load. A plant-level comparison therefore requires a boundary that includes sulfur preparation, gas cleaning, converter yield, tail gas abatement, steam export and solid residue logistics rather than a single variable cost per tonne.

Production-scale behaviour of these routes differs sharply. Sulfur-burning trains with 2,000–4,000 t/d capacities are the benchmark for merchant acid because the feed gas is clean and the process energy balance is highly favourable. The oxidation of sulfur to SO2 and catalytic conversion to SO3 release enough heat to produce 1.25–1.35 t of high-pressure steam per tonne of acid. In smelter gas service, acid output is determined by the upstream furnace campaign and off-gas SO2 concentration, which can vary from 5 vol% to 18 vol% during a converting cycle. Pyrite roast gas is typically 8–11 vol% SO2 after wet cleaning, but the cleaning train itself consumes water, pressure drop and maintenance. These differences are not marginal; they change the capital cost, operating flexibility and environmental compliance burden.

What Distinguishes a Sulfur-Burning Acid Plant from a Smelter Gas Train?

In sulfur-burning acid plants, molten sulfur is filtered, atomized into a refractory-lined furnace, and combusted with dried air. The furnace outlet gas passes through a waste heat boiler and gas cooler before entering a 3+2 or 4+1 double-contact double-absorption converter. The stoichiometric cascade is S + O2 → SO2, SO2 + 1/2 O2 → SO3, and SO3 + H2O → H2SO4. The first converter bed uses a cesium-promoted vanadium pentoxide catalyst to handle the high inlet SO2 concentration without excessive equilibrium losses. Bed temperatures are controlled between 420 °C and 630 °C; lower temperatures risk condensation and catalyst wetting, while higher temperatures reduce equilibrium SO2 conversion. Drying towers upstream of the blower maintain air moisture below 20–50 mg/Nm3 in most designs to prevent acid condensation and corrosion.

Smelter off-gas acid plants must handle contaminants that sulfur burners never see: dust, halides, mercury, arsenic, and fluctuating SO2 strength. A hot electrostatic precipitator removes particulates at 300–350 °C, followed by quench and wet gas cleaning, wet electrostatic precipitators, and a drying tower. The variable SO2 load requires converter beds with wider turndown and often a pre-treatment step to remove fluorine because fluoride attacks the silica support of vanadium catalysts. Capital cost per tonne of acid is higher than for sulfur burning; published engineering estimates for smelter gas acid plants of comparable capacity place the installed cost at 1.4–2.0 times that of a clean sulfur-burning plant, mainly because of wet gas cleaning, weak acid handling and materials selection. This differential is not a fixed multiplier; site factors such as gas volume swings, mercury removal and available cooling water can push it higher.

In smelter acid service, the absence of elemental sulfur cost does not automatically produce lower cash costs. The acid plant is a captive environmental control unit, and its output is coupled to metal production. When furnace feed grade drops or matte converting slows, SO2 supply falls and the acid plant operates at partial load, raising fixed cost per tonne. The weak acid stream from wet gas cleaning must be neutralized or concentrated, and mercury or selenium in the gas must be captured to avoid catalyst poisoning and stack emissions. These costs are assigned to the smelter or to acid cost depending on the accounting boundary, which is why smelter acid often appears as low cash cost but carries higher complexity.

For a sulfur-burning plant at merchant scale, elemental sulfur typically represents 60–75% of the variable cost, while energy credit from high-pressure steam can offset 10–20% of total production cost when steam is sold or used internally. The sulfur requirement of 0.327 t/t means a $100/t delivered sulfur price yields $32.70/t feedstock cost, excluding losses. Because sulfur price is volatile, long-term acid tolling contracts often index the acid price to sulfur plus a conversion fee.

The following table summarises the principal comparative parameters used in cost breakdowns for the three routes.

Acid storage and loading equipment is standardized across routes.

Pyrite Roasting Economics at 1,000–1,400 t/d Feed Rates

Pyrite roasting is structurally different because the sulfur carrier is a sulfide mineral that must be oxidized in a fluidized-bed reactor. The principal reaction is 2 FeS2 + 5.5 O2 → Fe2O3 + 4 SO2, although pyrrhotite and other sulfides may be present. Roaster beds operate at 850–950 °C, and freeboard residence times of 3–6 s are used to complete sulfur release. Heat removal is achieved through immersed cooling coils and a waste heat boiler; steam production is possible, but the gas cleaning train consumes more pressure drop and water than a sulfur-burning plant. If bed temperature exceeds 1,000 °C, defluidization and agglomeration can occur, particularly when low-melting arsenic or antimony phases are present. This is a critical processing window; operators control feed particle size distribution and roaster temperature to avoid defluidization.

The off-gas from a pyrite roaster is heavily loaded with dust and volatile impurities. A typical cleaning sequence includes high-efficiency cyclones, a hot electrostatic precipitator, a quench and scrubbing system, wet electrostatic precipitators, and a drying tower. Arsenic, fluorine, and mercury are removed before the converter because arsenic can deactivate vanadium catalysts and fluorine attacks catalyst support structure. Cleaned gas strength is generally 8–11 vol% SO2, which is sufficient for double-contact double-absorption but provides less conversion driving force than sulfur-burning gas. The iron oxide cinder, if sufficiently low in arsenic and heavy metals, is sold to cement plants or steel sinter operations; otherwise it must be landfilled. Cinder mass is approximately 0.4–0.5 t/t acid, so logistics and disposal can erode the feedstock cost advantage. Published data for cinder sale value is site-specific and depends on the ore source; no universal credit should be assumed.

Capital cost for a pyrite roasting acid plant is high relative to sulfur burning because the roaster and gas cleaning islands are additional units. At feed rates of 1,000–1,400 t/d pyrite concentrate, installed costs are often 1.5–2.5 times those of a sulfur-burning plant at equivalent acid capacity. However, the pyrite route may be selected when domestic pyrite concentrate is available at a delivered cost per tonne of sulfur below elemental sulfur. The feedstock requirement of 0.65–0.75 t concentrate per tonne acid means that a $40/t concentrate price translates to roughly $26–$30/t of acid sulfur feedstock, before residue credits or disposal costs. This calculation defines the lower bound, not the full production cost. Published data for this specific configuration is limited because small pyrite acid plants are often site-integrated and do not disclose disaggregated cost sheets.

Heat recovery from pyrite roasting is less standardized than sulfur burning. The roasting reaction is exothermic, but about 20–30% of the heat may be lost in gas cleaning and quench water. High-pressure steam from a pyrite plant can be 0.8–1.2 t/t acid, but the actual value depends on the roaster gas outlet temperature and the extent of waste heat recovery. Cinder cooling and handling adds equipment such as fluidized-bed coolers or rotary coolers, which are not present in sulfur-burning plants.

When Tail Gas Regulations Drive Acid Route Selection

The environmental compliance boundary differs by route and can dominate route selection. Sulfur-burning plants have a clean feed, so the main compliance issues are tail gas SO2 and acid mist. Smelter off-gas and pyrite roasting plants add mercury, arsenic, fluorine, weak acid and solid residue streams. In the United States, sulfuric acid plants are subject to 40 CFR Part 60 Subpart H, and stack testing for acid mist typically follows US EPA Method 8. In the European Union, non-ferrous smelter acid plants fall under the BAT Conclusions for the Non-Ferrous Metals Industries, 2016/1032/EU, which specify monitoring and abatement requirements for fugitive gas and weak acid. Life-cycle inventories for route comparisons use ISO 14040:2006 and ISO 14044:2006 to define system boundaries.

Alkali scrubbing of tail gas may be required to meet SO2 emission limits if the DCDA converter underperforms or if inlet SO2 to the tail gas scrubber exceeds 300–500 ppmv. The scrubber uses sodium hydroxide or ammonia and produces a sulfate salt stream that must be disposed of or sold. At this slip concentration, alkali consumption is not the main cost; the larger burdens are reagent handling, scrubber pressure drop, liquid bleed treatment and continuous monitoring. Smelter acid plants may also require mercury removal by selenium scrubber or activated carbon, weak acid neutralization, and arsenic precipitation. These environmental unit operations can add 10–25% to the acid plant operating budget, depending on impurity load and discharge limit.

The compliance and analytical standards matrix below identifies where each route carries distinct measurement and reporting obligations.

Product quality for merchant acid is specified by ASTM E223 for total acidity and impurities; grades range from 93 wt% to 98 wt% H2SO4, with oleum at 20–40% free SO3. Sulfur-burning acid has low iron and heavy metals, making it preferred for catalyst and battery applications. Smelter acid may contain trace arsenic and selenium, limiting use in food-grade or high-purity applications unless purified. Pyrite acid may have higher iron if drying tower carryover is not controlled. These end-use constraints affect netback pricing, especially for smelter acid sold into the merchant market.

For site selection, the decision between sulfur-burned, smelter and pyrite routes is a boundary-specific exercise. A project that uses smelter off-gas must accept that acid capacity is a dependent variable tied to metal converter cycles. A pyrite project must demonstrate that the cinder has a viable outlet and that arsenic and fluorine removal can meet catalyst protection limits. A sulfur-burning project must manage elemental sulfur price volatility and ensure that high-pressure steam has a host. These operational constraints, rather than the raw stoichiometric yield, determine the actual production cost and plant availability.