Why Two Sulfuric Acid Quotes Differ: Route, Concentration and Freight

Two sulfuric acid quotations that appear identical on a wet-tonne basis can diverge by more than 15% after the production route, concentration-normalized acid basis, and freight classification are reconciled. A quote for 98.0 wt% acid from a sulfur burning unit does not automatically equal a quote for 93.0 wt% smelter acid, even when both meet UN1830. The delivered cost is a function of the route-specific cost structure, the water or free SO3 content, and the dangerous goods freight tariff applied to the wet mass.

Sulfuric acid is listed under CAS 7664-93-9 and EC No 231-639-5. Analytical verification for quote comparison is typically anchored to ASTM E223-23. Density-based mass correction for tank volume measurement is handled under ISO 910:1977. The transport classification is established in 49 CFR §172.101. A defensible quote comparison therefore requires extracting the f.o.b. price, certified concentration, and freight charge separately.

What Makes Sulfur Burning Acid Structurally Different from Smelter Acid?

Sulfur burning acid produced by the contact process has a cost structure that tracks molten sulfur, energy, and catalyst replacement. Dry air is compressed and sent to a sulfur furnace; the resulting gas contains 10 vol% to 12 vol% SO2 before passing through a waste heat boiler and a vanadium pentoxide converter. In a double-contact double-absorption train, conversion of SO2 to SO3 exceeds 99.8% when the converter uses a 4-bed arrangement with intermediate absorption. The SO3 is absorbed into circulating sulfuric acid maintained at 98 wt% to 99 wt%; water addition controls concentration. Because the sulfur furnace uses filtered air and merchant sulfur, the resulting acid has a low metallic impurity burden, which makes it suitable for more demanding derivative streams. The quote therefore carries a raw-material index and often a fuel adjustment, not a disposal credit.

In smelter byproduct acid, the sulfur arrives in the metal sulfide feed. Off-gas from copper, lead, or zinc smelting typically enters the contact plant at 6 vol% to 10 vol% SO2 after electrostatic precipitation, wet gas cleaning, and mercury removal. The gas is dilute, variable, and contaminated with trace elements including arsenic, selenium, mercury, and chlorides. These impurities require additional gas cleaning and produce an acid that may be discounted for phosphate fertilizer or mining end uses. Such acid can still be suitable for many chemical processes if a certificate of analysis is attached, but the quote should not be compared against a sulfur burning grade without normalizing impurity specifications.

Regenerated acid and pyrite roasting occupy a different route category. Spent acid regeneration uses a decomposition furnace at 1,000 °C to 1,200 °C; the feedstock may carry organic contamination that increases fuel consumption and waste gas cleanup. The quote often contains a disposal fee credit, which is regional and tied to hazardous waste incineration capacity. Pyrite roasting yields an SO2 gas stream of 8 vol% to 12 vol% and a solid iron oxide residue. Neither route responds to sulfur price indices, so their quotes are driven by fuel, co-product credits, and environmental compliance. Published plant-specific data for older pyrite roasters in merchant acid markets is limited; the quote impact is inferred from gas strength and residue handling mass balances.

Concentration Normalization Exposes the Water Freight Penalty

A quotation is directly comparable only after the total H2SO4 equivalent is fixed. Commercial acid is sold as 93.0 wt%, 98.0 wt%, or as oleum; the wet-tonne price must be divided by the equivalent acid mass fraction. A 98.0% grade has 0.980 tonne of H2SO4 per wet tonne before the analytical tolerance is applied. A 93.0% grade has 0.930 tonne of H2SO4 per wet tonne. At the same wet-tonne price of $100, the 93.0% material costs $107.53 per tonne of contained H2SO4, while the 98.0% material costs $102.04. The 93.0% quote must be $5.10 lower per wet tonne before it equals the 98.0% quote on an acid basis.

Oleum quotations are routinely misread because the free SO3 is not already counted as H2SO4. A 20% oleum contains 20 g of free SO3 per 100 g of product plus 80 g of H2SO4; upon water addition the free SO3 forms 24.5 g of H2SO4, yielding 104.5 g total acid equivalent per 100 g. The total acid equivalent mass fraction is calculated as the weight percent H2SO4 plus 1.225 times the weight percent free SO3. Thus a 20% oleum quote can carry more acid per wet tonne than a 98.0% acid quote, but it has a different dangerous goods classification and requires heated or insulated storage depending on ambient conditions.

Analytical verification of each quote is anchored to ASTM E223-23. The standard covers titration and calculation of total acidity in sulfuric acid and oleum. For density, ISO 910:1977 is used to convert volume measurements to mass because tank gauging is often volumetric. The certificate of analysis should state the method designation, lot number, and sampling port. Without this, the concentration basis is not technically defensible.

Freight is quoted on a wet-mass tariff in most regional truck and rail contracts. A 93.0 wt% acid shipment carries 7.0 kg of water per 100 kg; a 98.0 wt% acid shipment carries 2.0 kg of water per 100 kg. Because the freight rate is charged per gross ton, the delivered cost per unit of H2SO4 is obtained by dividing the freight rate by the mass fraction. The freight cost per tonne of contained H2SO4 for 93.0% grade is 5.4% higher than for 98.0% grade at the same wet-tonne freight rate. This concentration correction should be applied before comparing two supplier quotes.

For volume-limited truck tanks, the specific gravity determines whether a lane is weight-limited or volume-limited. At ambient temperature, 98.0 wt% sulfuric acid has a specific gravity near 1.84; 93.0 wt% acid is near 1.83. The difference in density is too small to compensate for the water dilution. In cargo tanks, the acid is often weight-limited because the material density exceeds the capacity of a standard tank trailer at the legal gross vehicle weight. Bulk acid shipped in DOT 412 trailers cannot be cross-loaded with spent acid or organic chemicals without a wash certificate. Rail tank cars for sulfuric acid are insulated and lined to prevent iron contamination; railcar availability is tighter on routes with low backhaul, so a freight rate may be low but the cycle time high.

Transport classification determines freight access and emergency response cost. Sulfuric acid with more than 51% acid is UN1830, Class 8, Packing Group II; oleum is UN1831, Class 8, Packing Group I; and sulfuric acid not exceeding 51% acid is UN2796, Packing Group II. Under 49 CFR §172.101, the proper shipping name and hazard class determine tank specification and emergency response requirements. A supplier offering freight included may be quoting a route with a transload point rather than direct delivery, or a rail route that uses a different tank car class with higher demurrage risk. Transloading raises handling charges and requires a terminal with secondary containment and rail spur access; the buyer should require the freight tariff to state the route, transfer point, and tank specification.

If the Quote Includes Freight, Validate the Packing Group and Basing Dry Mass

A delivered quote is normalized by assigning the freight cost to the contained H2SO4, not the wet tonne. For a truck move, the payload is limited by either tank capacity or gross vehicle weight. If a 98.0 wt% acid trailer fills by weight at 24 short tons, a 93.0 wt% acid trailer at the same weight limit contains 1.20 short tons less H2SO4. The freight cost per tonne of H2SO4 is therefore higher for the 93.0% grade by the inverse mass fraction shift. Rail movements behave similarly over longer hauls, but tankcar cycle time and demurrage add another layer: a supplier with a low freight rate may assume a two-day unload window and then apply demurrage after that.

The same normalization applies to oleum. Because 20% oleum carries an equivalent mass fraction of 1.045, the freight charge per wet tonne is spread over more contained H2SO4; however, the UN1831 Packing Group I classification can restrict carriers and raise the freight rate. A buyer comparing delivered quotes should request the f.o.b. price, freight charge, and certified equivalent mass fraction separately, then divide the sum of f.o.b. and freight by the certified equivalent mass fraction.

In a delivered quote, freight charges are often equal to or greater than the f.o.b. production cost in regional short-haul truck moves. The minimum safeguard is to compare all quotes on a contained H2SO4 basis, verify the proper shipping name, and inspect the freight tariff for transloading, demurrage, and tank wash assumptions. A quote that appears lower on a wet-tonne basis can carry a higher delivered acid cost if the route uses a higher water content, a longer hazardous-material lane, or a more restrictive packing group.