Industrial-grade sulfuric acid (CAS 7664-93-9) is produced in Chinese chemical parks primarily through contact-process oxidation of sulfur dioxide, with three dominant sulfur carriers: elemental sulfur, nonferrous smelter gas, and pyrite concentrate. The acid molecule has a molar mass of 98.079 g/mol and a density of 1.8305 g/cm³ at 98.0 wt% and 20°C. Wet-process phosphoric acid consumes the largest single share of Chinese output, followed by sulfate-route titanium dioxide, caprolactam, hydrofluoric acid, ammonium sulfate, and lithium-ion battery precursor refining. The contact process remains the technical backbone because it permits SO₂ conversion above 99.7% in double-contact double-absorption (DCDA) trains, with tail gas SO₂ controlled to 200 mg/Nm³ for new plants under GB 26132-2010.

Converter trains in sulfur-burning plants typically use four catalyst beds with promoted vanadium pentoxide; first-bed outlet temperatures are limited to 600–630°C to avoid catalyst deactivation. Interpass coolers maintain second and third bed inlet temperatures at 420–450°C. Absorption towers operate at 60–80°C with acid circulation rates of 15–20 m³/h per tonne per day of acid product. Drying towers reduce air moisture below 0.1 g/Nm³ before the sulfur furnace to prevent downstream corrosion and acid mist. Sulfur guns atomize liquid sulfur into a refractory-lined furnace at 1,050–1,150°C; the gas leaving the waste heat boiler contains 10.5–11.5 vol% SO₂, with the balance nitrogen.

Sulfur Burning, Smelter Off-Gas, and Pyrite Roasting as Competing SO₂ Feedstock Routes

Chinese capacity is not a single-process monolith. It is split among sulfur-burning acid units, metallurgical off-gas acid plants attached to copper, lead, and nickel smelters, pyrite roasting installations, and a smaller number of wet gas sulfuric acid units processing dilute hydrogen sulfide or spent alkylation acid streams. The route mix matters for export behavior because by-product smelter acid is inelastic to market price, while sulfur-burning merchant acid can adjust operating rates to sulfur cost and domestic demand.

Published disaggregated data for the current year is limited; route shares are broad estimates from Chinese sulfuric acid industry survey data and should be treated as indicative rather than audited capacity figures. Pyrite-based plants are increasingly constrained by arsenic and lead disposal. Roaster off-gas passes through cyclone and hot electrostatic precipitators; dust loading after cleaning is typically 10–50 mg/Nm³ before the wet purification train. Wet purification uses dilute acid scrubbers to remove fluorine and arsenic, followed by electrostatic mist precipitators. Sulfur-burning systems avoid wet gas cleaning entirely, which is one reason they dominate coastal capacity additions.

China's Position in Global Sulfuric Acid: Huge Output, Tiny Export Share

China’s sulfuric acid output exceeded 100 million metric tons in recent annual series, representing approximately 30% of global production. The export share, by contrast, has remained below 1.0% of output in reported customs data. The divergence is not an inventory failure but a direct consequence of acid’s low unit value and the country’s internal demand profile. A metric ton of 98.0 wt% sulfuric acid carries a normal ex-works value far below the freight and regulatory compliance cost for a seaborne parcel; therefore the exportable surplus disappears when benchmark prices undercut delivered costs. The phrase “huge output, tiny export share” should be understood as a structural effect of the fertilizer and chemical downstream chain rather than an export restriction.

The Chinese merchant market is segmented into smelter acid, sulfur-burning merchant acid, and refinery alkylation acid. Smelter acid plants in copper-producing provinces operate as by-product units and their production is inelastic to acid market prices. Sulfur-burning merchant plants adjust operating rates to sulfur price and downstream demand. Because many coastal sulfur-burning units receive sulfur through Jiangsu, Zhejiang, and Shandong terminals, their acid is consumed by nearby chemical fibre, titanium dioxide, and phosphate complexes. Rail and road tanker distribution limits the practical inland radius to 300–500 km before freight exceeds the acid’s margin.

Why Does Seaborne Export Volume Remain Marginal Despite Surplus Capacity?

Seaborne sulfuric acid is dominated by dedicated chemical tankers or stainless-steel parcel tankers with lined integral tanks. The cargo is classified as UN 1830, Packing Group II, for acid above 51 wt%, and requires emergency response protocols under the IMDG Code for packaged shipments. For bulk liquids, the International Code for the Construction and Equipment of Ships Carrying Dangerous Chemicals in Bulk (IBC Code) requires cargo tank ventilation, stripping, and material compatibility assessments. At 98.0 wt%, carbon steel tanks are usable because the acid passivates the surface with an iron sulfate film; however, linings and tank cleanliness are critical if the concentration falls below 93 wt% or if temperature exceeds 50°C during cleaning. Flow velocity in carbon steel piping is maintained below 0.9 m/s to avoid erosion of the passive film.

Terminal storage at Chinese coastal export berths must satisfy hazardous chemical handling rules under GB 15603 and pollution control under GB 26132-2010; acid mist and SO₂ fugitive emissions are subject to local environmental permits. Loading a 5,000 metric ton parcel requires outage, stripping, and vapour balancing; the jetty and tank-farm allocation often has higher opportunity cost than the acid cargo's net margin. Furthermore, sea freight rates for corrosive liquids are quoted as a function of distance, parcel size, and tank coating condition, and the per-tonne freight cost frequently exceeds the ex-works price spread between regional ports. Consequently, exports are limited to regional Indonesian, Vietnamese, or Philippine buyers with a direct receiving terminal and no local smelter acid source. Published data for specific freight rates is limited.

Within the domestic market, phosphate fertilizer production consumes the largest fraction of sulfuric acid by converting phosphate rock to wet-process phosphoric acid. The attack section operates at 70–80°C with acid diluted from 98.0 wt% to approximately 30–40 wt% in the reactor slurry. A typical dihydrate wet-process phosphoric acid plant uses 2.6–2.8 metric tons of 98.0 wt% sulfuric acid per metric ton of P₂O₅. This ratio anchors the domestic market and explains why fertilizer seasonality drives acid demand more than export opportunities. Acid dilution follows acid-to-water addition under agitation to manage the exothermic hydration heat.

Sulfate-route titanium dioxide production digests ilmenite with concentrated acid at 150–220°C, producing titanyl sulfate liquors; the subsequent hydrolysis step requires controlled ferrous iron oxidation and acid strength between 80–85 wt% to govern particle nucleation. Caprolactam plants use oleum or concentrated acid in the Beckmann rearrangement of cyclohexanone oxime; the acid is recovered by recycling spent acid through decomposition and wet gas cleaning. Hydrofluoric acid production from fluorspar uses oleum at 20–30% free SO₃ in rotary kilns, and the resulting calcium sulfate is discharged as a solid by-product.

Lithium-ion battery precursor refining consumes sulfuric acid for nickel, cobalt, and manganese sulfate production; leach solutions are typically maintained at pH 1.5–2.0 and acid concentration is controlled by metering pumps to avoid nickel hydroxide precipitation. The quality requirements for these non-fertilizer uses are stricter than agricultural grade. Iron, arsenic, and chloride limits are tested under GB/T 534-2014 for industrial acid, and titrimetric total acidity is cross-checked to ASTM E223-23 where export or joint-venture specifications apply.

When Smelter Off-Gas Variability Creates Acid Quality Conflicts

In copper smelters, acid plants are oxygen-coupled; flash furnace off-gas and converter off-gas are combined after waste heat recovery and hot electrostatic precipitation. The SO₂ content swings from less than 5 vol% to above 15 vol% during converter blow cycles. Such fluctuation forces the DCDA converter to operate with a variable first-bed temperature profile. Operators maintain first-bed inlet temperature above the vanadium catalyst light-off threshold of approximately 410–430°C. If the inlet temperature falls below this range, SO₂ oxidation collapses and unconverted SO₂ reaches the tail gas scrubber. The acid produced from smelter gas may carry trace arsenic, selenium, and mercury depending on the concentrate; hence the acid quality is monitored against GB/T 534-2014 and buyers may request additional mercury analysis.

Smelter acid plants with less than 8 vol% SO₂ feed cannot sustain autothermal operation and require supplemental sulfur or hot gas bypass to keep the converter stable. This is the central process conflict for by-product acid: gas volume is set by metal reduction, not acid demand. During smelter maintenance shutdowns, downstream acid consumers must switch to merchant sulfur-burning acid, creating batch-to-batch variation in acid trace metals. This variation is managed by blending tanks of 500–1,000 m³ capacity at distribution terminals, where acid is homogenized before tanker loading.

Sulfuric acid must not be mixed with water in reverse sequence during dilution; concentrated acid should be added to water under agitation because the hydration enthalpy can generate localized boiling. Contact with chlorates, permanganates, or alkali hydroxides must be controlled to avoid violent reactions. Carbon steel storage tanks for 98.0 wt% acid require inert gas blanketing or dry air to prevent atmospheric moisture absorption and surface acid dilution; at relative humidity above 60%, tank vent desiccants or nitrogen blankets are used to maintain passivation. These operational boundaries are inseparable from the low export share: every additional day in storage or transport adds moisture ingress risk and quality management cost.