Industrial sulfuric acid is traded as a concentrated mineral acid with grade designations spanning 93 wt%, 96 wt%, and 98 wt% H₂SO₄, together with oleum grades containing 20% to 65% free SO₃. The CAS registry number is 7664-93-9, EC number 231-639-5, and transport identity UN 1830, class 8, packing group II. At 20°C the density of 98 wt% acid is approximately 1.84 g/cm³, and the normal boiling point is approximately 337°C. The dominant production route remains the contact process, which oxidizes sulfur-derived SO₂ over a vanadium pentoxide catalyst and absorbs SO₃ in circulating 98.5 wt% acid. Consumption is concentrated in phosphate fertilizer acidulation, hydrometallurgical leaching of copper and uranium ores, petroleum alkylation, titanium dioxide pigment production, lead-acid battery electrolyte preparation, and metal finishing. Merchant acid quality is not a single property set: the same 98 wt% material may pass one application and fail another due to trace halide, iron, nitrate, or arsenic content.

What Drives Purity-Related Rejection Rates in Electrolyte-Grade Sulfuric Acid?

Electrolyte-grade and reagent-grade sulfuric acid are specified less by total acidity than by impurities that affect electrode kinetics or downstream catalyst poisoning. Total acidity is determined by titration per ASTM E223-23; color is reported as APHA using ASTM D1209; trace metals are measured by ICP-MS after closed-vessel dilution, with reporting limits at or below 0.01 mg/kg for most transition elements. Lead-acid battery electrolyte produced from merchant 98 wt% acid is diluted to approximately 37 wt% H₂SO₄. Commercial rejections frequently involve chloride above 1 mg/kg, nitrate above 1 mg/kg, or iron above 0.5 mg/kg. Arsenic and antimony receive separate speciation because they can affect hydrogen overvoltage and gas evolution on lead anodes. In semiconductor etchants, merchant acid often requires additional sub-ppb metal purification at point of use; published data for specific commercial rejection rates in this niche is limited because most high-purity acid is supplied under confidential quality agreements.

Pressure Drop and Acid Mist Control in Double-Absorption Trains

Pressure drop across drying and absorption towers in double-absorption contact plants interacts with SO₂ conversion and energy consumption. In a typical sulfur-burning train, dried air and sulfur react in a furnace at 1000°C–1200°C; the cooled SO₂ stream enters a multi-bed converter with inlet temperatures between 400°C and 620°C. Catalysts are cesium-promoted or potassium-promoted V₂O₅ extrudates, and interpass absorption removes SO₃ before the final conversion stage. Double absorption reduces tail gas SO₂ below 150 ppmv corrected to 7% O₂ on many permitted units; single absorption without tail gas scrubbing is generally above 400 ppmv. Acid mist in the final absorber is controlled by candlestick or knitted mesh demisters; pressure drop across a clean mist eliminator typically adds 100–250 mm H₂O to the absorber train. Elevated pressure drop above 300 mm H₂O indicates sulfate carryover, broken elements, or metal sulfate fouling from upstream corrosion products. Operators use differential pressure transmitters across each bed and absorber to schedule catalyst screening and demister replacement.

When Delivery Volume Shifts from ISO Containers to 25,000 t Coastal Barges

A 20 t ISO tank frame allows shipment under ADR/RID and IMDG Code as UN 1830, class 8, PG II. Rail tank cars of 90–100 t capacity are frequently specified for 93–98 wt% acid; unloading rates through 50 mm or 75 mm connections vary from 10 L/s for portable pumps to over 30 L/s for terminal transfer pumps. A 2,500 t river barge can discharge at 100–150 m³/h with a pump discharge pressure of 2.5–4.0 bar. Coastal and ocean chemical tankers of 20,000–25,000 t deadweight require mooring, vapor return or scrubber connection, and inert gas blanketing if acid is carried in coated cargo tanks. Demurrage is commonly charged after 24 h free time for barges and after 36 h for coastal vessels; pressure transmitters at the manifold and radar-level gauges on shore tanks are used to calculate discharge rate for demurrage evidence.

Bulk storage terminals handling 98 wt% sulfuric acid typically specify vertical cylindrical carbon steel tanks with a minimum shell corrosion allowance of 3 mm and floor plates of 6 mm; carbon steel is acceptable only when acid concentration is kept above 93 wt% and metal temperature below 30°C. Inlet nozzles receive dip pipes to prevent sidewall splashing, and vents are routed through caustic scrubbers or sulfuric acid scrubbers to prevent atmospheric acid mist. Tank level is monitored with radar or guided-wave radar, independent of air purge level systems that plug in acid service. Secondary containment is sized to 110% of the largest tank where applicable under 40 CFR 112; emergency showers and eyewashes are placed within 10 s travel and maintained per ANSI/ISEA Z358.1. Spill pumps are specified with Hastelloy C-276 or PTFE-lined casings, and acid-resistant concrete coatings using silica or furan resin binders minimize substrate attack. Flow velocity in carbon steel transfer piping is limited to 0.8 m/s because higher shear can strip the passive iron sulfate film and increase localized corrosion. Transfer hoses use EPDM or PTFE-lined corrugated stainless steel to resist acid permeation; routine inspection includes hydrostatic testing at 1.5 times working pressure per local dangerous goods regulation.

Payment Terms for Sulfuric Acid Orders: T/T, L/C at Sight and Open Account

Commercial contracts for sulfuric acid link the payment instrument to cargo size, buyer credit, delivered grade, and Incoterms 2020 risk transfer. Three common instruments are used: telegraphic transfer wire payment, sight documentary credit, and open account.

For T/T, producers of ISO-tank or containerized loads below 500 t often require 30% advance by SWIFT MT103 and the remaining 70% against scanned copy of the bill of lading or prior to release at destination. T/T is also used for spot truck and rail shipments where title documents are not routed through banks. The seller retains no bank guarantee after payment; pre-delivery payments expose the buyer to insolvency risk, while post-delivery payments expose the seller to collection delay.

For L/C at sight, the issuing bank substitutes its payment undertaking under UCP 600. The documentary requirement commonly includes commercial invoice, full set of clean on board bill of lading marked freight prepaid for CIF/CIP deliveries, certificate of quality and weight issued by an independent inspection company such as SGS or Intertek at load port, certificate of origin, dangerous goods declaration under IMDG Code, and packing list for containerized shipments. For bulk cargo, the bill of lading quantity is determined from vessel ullage survey, and inspection at load port uses composite sampling per ISO 3165. Banks have 5 banking days after presentation to examine documents; payment is released at sight after a conforming presentation. Non-documentary issues, such as an out-of-spec arsenic result, are handled between buyer and seller under contract quality remedy clauses.

Open account is used mainly where the buyer has an audited purchasing history of at least 24 months and a credit line approved by the seller’s export credit insurer. It is commonly documented as net 30–60 days from bill of lading date, with interest at a contractually defined rate after overdue date. The seller releases original documents directly to the buyer; the buyer pays by SWIFT wire on maturity. This method minimizes bank charges but shifts default and country transfer risk to the seller, so credit limits are typically capped and reviewed quarterly.

Evaluating Corrosion Rate Curves for Transfer Piping Alloys

Material selection for sulfuric acid transfer depends on concentration, temperature, velocity, and aeration. Carbon steel is frequently used for 93–98 wt% acid at ambient temperature because the surface forms a protective iron sulfate film. The film is not stable below 65 wt% or above 30°C under flow; dilute acid at pump suction during water flushing after maintenance is a common cause of accelerated corrosion. 316L stainless steel is not a general-purpose upgrade: it resists 98 wt% acid at ambient temperature but may corrode rapidly at intermediate concentrations. Alloy 20 (UNS N08020) provides service in 80–98 wt% acid up to approximately 50°C. Hastelloy C-276 (UNS N10276) is reserved for high-temperature acid transfer and acid with oxidizing contaminants. PTFE-lined carbon steel is used for loading arms and flexible connections where concentration can drop below 65 wt%; the liner is subject to permeation and mechanical damage, and published long-term field failure data is limited because failures are typically recorded in confidential terminal maintenance records.

Certificate of analysis documentation for bulk sulfuric acid shipments should align with the inspection clauses in the payment instrument. At load port, an independent inspector draws composite samples from shore tanks, vessel tanks, or ISO containers using ISO 3165 sampling procedures; split samples are sealed and retained by seller, buyer, and inspector for at least 90 days. Total acidity is reported by ASTM E223-23, and trace impurities are reported by ICP-MS with method detection limits stated on the COA. Color may be reported as APHA according to ASTM D1209. For oleum, free SO₃ is calculated from total acidity and included on the same certificate. Documentation discrepancies, such as a bill of lading quantity differing from the inspector’s ullage report by more than 0.1%, frequently trigger refusal under letters of credit even when the chemical quality is in specification. Electronic documents submitted under eUCP or approved platforms must carry the bank’s required security identifiers; otherwise, originals are presented through the nominated bank.