Published industry surveys covering Sulfuric Acid Production Capacity in China: Plants, Routes and Turnarounds indicate that installed nameplate capacity exceeds 140 million metric tons per year (140 Mt/a), with annual output generally in the 100 million to 110 million metric ton range during recent campaigns. The capacity base consists of sulfur-burning contact acid units, nonferrous smelter off-gas acid plants, pyrite roasting operations, and smaller spent acid regeneration and wet-gas sulfuric acid systems. Route economics, sulfur import prices, nonferrous metal output, product standards such as GB/T 534-2014, and emission rules such as GB 26132-2010 influence operating rates across these plants.

Sulfur burning remains the largest production route in most published capacity tabulations, with a reported share of 45% to 55% of national capacity. Smelter off-gas acid production accounts for 25% to 35%, while pyrite roasting supplies 15% to 25%. Spent acid regeneration, wet-gas processing, and other routes together represent less than 10%. Capacity is geographically concentrated in inland provinces tied to phosphate fertilizer complexes and in mining regions tied to nonferrous smelting, with major clusters in Hubei, Sichuan, Yunnan, Guizhou, Anhui, Jiangxi, and Gansu.

Among the largest nonferrous smelter off-gas acid installations are those operated in conjunction with copper and nickel smelters in Gansu, Anhui, Jiangxi, and Yunnan. Published plant descriptions identify large acid units at Jinchuan, Tongling, Guixi, and Yunnan Copper, where flash smelter and converter gases are cleaned, dried, and sent to contact acid trains. These units frequently operate at single-train capacities above 3,000 tpd because the upstream smelter gas volume cannot be stored or flared for extended periods during acid plant outages. Sulfur-burning plants in Hubei, Sichuan, and Guizhou commonly operate in the 1,000 to 2,500 tpd range per train, with capacity additions designed around phosphate fertilizer demand and waste heat recovery from sulfur combustion.

Route selection controls the gas cleaning intensity and the turnaround exposure of a sulfuric acid train. Sulfur-burning plants start with a clean feedstock and therefore focus maintenance effort on the sulfur furnace, waste heat boiler, converter, and acid towers. Smelter off-gas plants must remove dust, arsenic, fluoride, and acid mist in a wet gas cleaning system before the SO₂-bearing gas enters the drying tower. Pyrite roasting plants add a fluidized bed roaster, cinder cooler, cyclones, and arsenic purge equipment upstream of the contact section.

What Determines Nameplate Capacity in Sulfur-Burning Acid Trains?

A 2,000 tpd sulfur-burning train combusts approximately 27,000 kg/h of elemental sulfur, producing a gas stream with 10.5 to 12.0 vol% SO₂ after dry air dilution. Sulfur furnace outlet gas temperatures are maintained at 950 to 1,100 °C, and waste heat boiler outlet gas is cooled to converter inlet conditions of 410 to 430 °C for the first catalyst bed. The first bed outlet can exceed 600 °C. Interpass cooling between beds reduces downstream inlet temperatures to 410 to 450 °C to protect the vanadium catalyst from thermal ageing.

Across Chinese sulfur-burning plants, converter design is typically a 4-bed double-contact arrangement with intermediate absorption after the third bed. Published design bases indicate that the total vanadium pentoxide catalyst volume for a 2,000 tpd train is generally 150 to 220 L per daily metric ton of acid capacity, with cesium-promoted catalyst in the fourth bed to improve final conversion. The fourth bed inlet temperature is set near the SO₂ oxidation equilibrium optimum, commonly 380 to 410 °C. Total converter pressure drop is monitored across the campaign; an increase above 15 kPa often triggers catalyst screening during the next turnaround.

Converter performance is tracked through continuous SO₂ analyzers, catalyst bed thermocouples, and tail-gas SO₂ monitors. In a 4-bed double-absorption train, conversion across the first three beds typically reaches 93% to 95% before intermediate absorption. Final conversion after the fourth bed is designed for 99.7% or higher. A fall in final conversion below 99.5% during the campaign indicates catalyst ageing, bed channeling, or heat exchanger leakage, and becomes a turnaround trigger if associated with higher tail-gas SO₂ concentration.

In the absorption section, drying and absorption towers generally use ceramic packing and distribute 93% sulfuric acid for gas drying and 98.5% sulfuric acid for SO₃ absorption. Inlet acid temperatures are controlled between 60 and 90 °C, with coolers holding acid return temperatures low enough to maintain absorption efficiency and limit acid mist. Anodically protected shell-and-tube acid coolers are standard on large units. Tube-to-tubesheet welds and water-side fouling are routine turnaround inspection items because acid leaks can force an unplanned shutdown.

Unlike sulfur-burning plants, smelter off-gas acid units operate with variable SO₂ concentration and gas saturated with water after wet gas cleaning. The gas cleaning train typically includes a quench tower, a wet electrostatic precipitator, and two-stage mist eliminators. Conversion capacity is often less stable than sulfur-burning capacity because upstream Peirce-Smith converter cycles can swing SO₂ strength from below 8 vol% to above 14 vol% within a single converting cycle. To limit catalyst poisoning, gas cleaning must reduce arsenic, fluoride, and dust to levels specified by the catalyst supplier. In Chinese smelter acid plants, the acid train is usually interlocked with the smelter gas handling system, and any acid plant trip can curtail smelter production unless bypass abatement systems are available.

For pyrite roasting capacity, fluidized bed roasters typically support 300 to 800 tpd of acid production. The roaster operates at 650 to 900 °C, and the resulting cinder must be cooled and handled separately. Roaster gas contains dust, arsenic, and residual unconverted pyrite, requiring cyclones and wet gas cleaning before the contact section. The pyrite route has declined relative to sulfur burning because of higher solid handling cost, lower SO₂ strength, and the requirement to manage cinder byproduct disposal under environmental restrictions.

Turnaround Scope, Catalyst Screening, and Acid Tower Inspections at Chinese Plants

Turnaround scheduling in Chinese sulfuric acid plants is determined by converter catalyst activity, gas cleaning system condition, acid cooler integrity, and statutory inspection of pressure parts. Sulfur-burning single-train units commonly operate 24 months between major catalyst screening and converter entry outages. Smelter off-gas units on dirty or fluctuating feed may align acid plant maintenance with flash furnace or converter rebuilds at 12 to 18 months. Pyrite roaster plants often require 12 to 24 month intervals for roaster refractory and cinder cooler maintenance. Shorter outage windows of 5 to 10 days are used for acid pump seal replacement, acid cooler cleaning, and instrument calibration.

During a major sulfur-burning plant turnaround, the converter is opened for catalyst screening. This involves removing the catalyst from each bed, screening out dust and broken pellets, and recharging with the same or supplemental catalyst. Simultaneously, the sulfur furnace refractory and waste heat boiler tubes are inspected for hot spots, thinning, and acid dew point corrosion. Drying and absorption tower internal inspections focus on ceramic packing, distributor troughs, and mist eliminator elements. Acid coolers are hydrotested or eddy-current tested. An anodically protected acid cooler leak on the water side can produce dilute acid corrosion failures in the cooling water circuit if not detected.

A major turnaround for a large sulfur-burning train ranges from 15 to 30 days when catalyst changeout is required. Smelter off-gas acid plants often require 20 to 35 days because the wet gas cleaning system, quench tower spray nozzles, wet electrostatic precipitator tubes, and acid coolers represent additional inspection volume. A 3,000 tpd train out of service for 30 days removes 90,000 metric tons of acid production from the market between planned turnarounds. That volume is sufficient to tighten regional merchant acid supply in phosphate-producing provinces when multiple plants in Hubei or Yunnan schedule overlapping turnarounds.

When a 3,000 tpd Off-Gas Acid Train Is Forced Out by Wet Gas Cleaning Issues

Wet gas cleaning malfunctions are a major cause of unplanned downtime in smelter off-gas sulfuric acid plants. If the wet electrostatic precipitator loses voltage or the mist eliminator pressure drop exceeds design limits, moisture and particulate carryover can enter the drying tower and subsequently corrode converter insulation and catalyst supports. The acid plant may then be kept online only if the upstream smelter gas volume is reduced. Published operating guidance for Chinese off-gas acid units specifies that gas cleaning outlet dust be maintained below 5 mg/Nm³ and mist below 20 mg/Nm³ before the drying tower. Sustained excursions above these thresholds can lead to catalyst bed plugging, acid tower fouling, and acid quality defects requiring an early turnaround.

Acid quality during turnarounds and route transitions is assessed against GB/T 534-2014 limits for industrial sulfuric acid, including concentration, residue on ignition, iron, arsenic, and chloride parameters. Plants producing acid for fertilizer or chemical use additionally monitor free SO₃ and acid mist in the tail gas. Emission compliance is verified against the relevant requirements of GB 26132-2010 for sulfuric acid industry tail gas. Turnaround planning therefore includes tail-gas scrubber inspections. Ammonia or hydrogen peroxide scrubbers are checked for packing integrity, spray nozzle plugging, and byproduct salt deposition.

Concentrated sulfuric acid storage is performed in carbon steel tanks at 93% to 98.5% acid strength, with internal corrosion allowances and hydrogen gas venting. Piping velocities are maintained below 0.6 to 0.9 m/s to minimize erosion-corrosion. Transfer pumps are typically ductile iron or alloy units, with seal-less magnetic drive pumps used in smaller metering applications. These material constraints limit how quickly a plant can load out acid during a turnaround and influence the minimum storage volume required to avoid curtailing downstream phosphate or titanium dioxide production.

At production complexes where sulfuric acid is consumed on-site for phosphate fertilizer or titanium dioxide production, turnaround timing is coordinated with downstream unit maintenance. A major acid plant turnaround without downstream coordination can force phosphate granulation to curtail rates within hours because sulfuric acid storage capacity at Chinese fertilizer complexes is typically limited to 3 to 7 days of normal consumption. This storage limitation is a key operational boundary governing acid plant scheduling. Published data for this specific configuration is limited, but site-level technical bulletins indicate that turnarounds are usually phased across linked plants to avoid simultaneous loss of acid supply and steam from waste heat recovery.