Almost every tonne of sulphur in world trade today was never mined. It was stripped out of crude oil and natural gas because refiners and gas processors are obliged to remove it, and it arrives on the market as an unavoidable co-product rather than a deliberately produced mineral.
Hydrotreating units in a refinery convert organic sulphur compounds in the crude fractions into hydrogen sulphide. Amine treating units concentrate that hydrogen sulphide into an acid gas stream. That acid gas is then fed to a Claus sulphur recovery unit, where roughly one third of the H2S is burned to sulphur dioxide and the remainder reacts with it over alumina or titania catalyst to produce elemental sulphur and water.
Recovery is staged. On US EPA figures a two-bed catalytic Claus plant typically recovers 92 to 95 % of the inlet sulphur and a three-bed plant 95 to 96 %, with the Claus reaction itself thermodynamically limited to roughly 97 to 98 %. A tail gas treatment unit placed downstream lifts total recovery into the 99 to 99.9 % band. Sour gas processing plants run the same chemistry on a much larger scale, which is why the biggest export streams sit next to sour gas fields rather than next to refineries.
The sulphur leaves the recovery unit as a liquid at roughly 130 to 140 °C. From that point it takes one of two commercial paths: it is kept molten and moved in insulated heated tanks to a nearby consumer, or it is solidified into granules, prills, pastilles, slate or blocks for storage and long-distance shipment.
Because the feedstock is a gas stream rather than an ore body, recovered sulphur starts out very clean. There is no silicate gangue to remove and no beneficiation step. Typical commercial purity of 99.8 to 99.9 % is not an achievement of refining — it is simply what falls out of the process.
That changes which impurities matter. The contaminants a buyer checks are not minerals but the specific residues the process leaves behind:
Older literature and some national standards still refer to Frasch sulphur, mined by injecting superheated water into underground deposits and pumping the melted sulphur to surface, and to native sulphur from volcanic deposits. Frasch production has almost entirely ceased on economic grounds. If a project specification or an import regulation names a sulphur type by production route rather than by property, raise it before contracting — the material available in the export market is recovered sulphur, and the practical answer is to specify properties rather than provenance.
Chemically these are the same product. Commercially they are not interchangeable, because the form decides dust generation, crush strength, transport classification and how much of the cargo survives handling as saleable material rather than fines.
| Form | Typical size | How it is produced | Handling character | Where it normally goes |
|---|---|---|---|---|
| Granular (globular) | 2–4 mm core range, 1–6 mm overall | Layer-built onto a seed particle in a wet or dry granulator fed with molten sulphur | Lowest dust of the bulk forms, free flowing, high crush strength, resists degradation through multiple transfers | The export benchmark: bulk vessels, jumbo bags, sulphuric acid plants |
| Prilled | roughly 1–4 mm | Molten sulphur droplets solidified in a water bath or an air prilling tower | Rounder but more friable than granular; produces more fines on repeated transfer and long sea legs | Regional trade and acid plants close to the production point |
| Pastilles / hemispheres | uniform hemispheres, a few mm across; diameter set by the drop die | Molten sulphur dropped onto a moving cooled steel belt | Very uniform and very low dust; the cleanest presentation for bagged trade | Bagged trade, agricultural blends, bentonite sulphur feed |
| Slate / flake | irregular flakes, roughly 1–10 mm | Molten sulphur cooled on a drum or belt and broken | Angular with a higher fines fraction; bulk density varies with the break | Local consumption and chemical processing |
| Crushed lump / coarse grained | coarse and irregular; top size agreed per contract | Block sulphur poured into vats or slabs, then reclaimed and crushed | Highest dust and fines generation; carried as an IMSBC Group B cargo in bulk | Bulk shipment from sites without forming capacity |
| Powder / micronised | roughly 100 micron down to 10 micron and finer | Milling of solid sulphur; the very finest grades are made by sublimation | Highest dust explosion hazard of any form; requires full dust control and inerting where applicable | Rubber compounding, agrochemical formulation, chemical synthesis |
The values below are the typical commercial limits quoted for granular and prilled export sulphur, with the published test method that produces each one. Limits vary meaningfully between producers and between destinations, and an acid plant will write tighter trace-element limits than a fertiliser blender. One unit note before you read it: every ppm in this table is parts per million by mass, which is the same quantity as mg/kg. The ppm figures in the exposure-limit table further down are parts per million by volume in air, and the two must not be compared with each other.
| Property | Test method | Unit | Typical limit |
|---|---|---|---|
| Purity, elemental sulphur (dry basis) | By difference from ash, moisture, acidity and carbon | % by mass | min 99.8–99.9 |
| Moisture, loss in mass at 80 °C | ISO 3426 | % by mass | max 0.5 |
| Ash at 850–900 °C | ISO 3425 | % by mass | max 0.05 |
| Residue at 200 °C | ISO 3425 | % by mass | reported |
| Acidity, expressed as H2SO4 | ISO 3704, titrimetric | % by mass | max 0.01–0.02 |
| Total carbon / organic matter | ISO 2866, titrimetric (withdrawn, still widely cited) | % by mass | max 0.02–0.05 |
| Arsenic | ISO 3705, silver diethyldithiocarbamate photometric (stated applicable at 0.5 mg/kg and above) | ppm (mg/kg) | max 0.5–1 |
| Selenium | Producer method, commonly hydride-generation AAS or ICP | ppm (mg/kg) | max 1–2 |
| Tellurium | Producer method, commonly ICP | ppm (mg/kg) | max 1–2 |
| Residual hydrogen sulphide, after degassing | Producer method on the melt | ppm by mass | max 10 (15 in some contracts) |
| Colour | Visual | — | Bright yellow |
A sulphur certificate is short, which makes it easy to skim and easy to misread. Each line maps to a specific commercial failure, and one of them is not a quality line at all.
This is the single most important thing to understand about a sulphur COA. Nobody measures 99.9 % sulphur directly. Purity is calculated by difference: everything that is not sulphur is measured, summed and subtracted from 100. The consequence is unavoidable — a purity figure is only as trustworthy as the impurity lines that produced it. If a certificate reports 99.9 % purity but does not report carbon, then the carbon was either not measured or not subtracted, and the purity figure is unsupported. Treat a purity claim with fewer than four supporting impurity lines as an assertion rather than a result.
Sulphur is practically insoluble in water, so buyers sometimes dismiss moisture as a weight question. It is not. Free water on the surface of the granules combines with the free acid already present to form dilute sulphuric acid, and that is what attacks ship holds, hoppers, conveyor structures and the woven polypropylene of a jumbo bag. Water is also what makes a cargo cake: wet granules bridge in a silo and set into a solid mass in a bag stack, and breaking out a caked cargo generates exactly the fines and airborne dust that the granular form was chosen to avoid. ISO 3426 measures this as loss in mass at 80 °C, and a max of 0.5 % is a normal commercial ceiling.
ISO 3425 determines ash by slow combustion in air followed by ignition in a furnace held at 850 to 900 °C, and it also reports the residue remaining at 200 °C. Ash matters most to sulphur burners: in an acid plant the non-combustible fraction ends up as deposits in the burner and the waste heat boiler, and abrasive fines contribute to catalyst bed fouling and pressure drop. A max of 0.05 % is a normal commercial figure and acid plants often ask for tighter.
ISO 3704 extracts the acidic material with a water and propan-2-ol mixture and titrates the extract against standard sodium hydroxide to a phenolphthalein end point. The method is applicable from about 0.01 % (m/m) upward, which is worth knowing: if your contract specifies a limit below the method's working range, you have specified something the standard method cannot verify. Free acidity is the corrosion driver in storage and in transit, it degrades FIBC fabric, and combined with moisture it accelerates caking.
Carbon carried over from the Claus reaction furnace is the impurity most visible to the naked eye — it is what turns a bright yellow cargo dull, grey or green. Commercially it does three things. In a sulphuric acid plant it burns to carbon dioxide and can form deposits and contribute to catalyst fouling. In agricultural sulphur it is inert but drives colour rejection, because buyers and end farmers use colour as a proxy for quality. And in the purity calculation it is a term that is frequently omitted. ISO 2866 is the classical titrimetric method; it has been withdrawn as a live ISO standard but is still cited on commercial specifications, so check which method your supplier's laboratory actually runs.
These three appear on a sulphur specification for one reason, and it is not general toxicity. In a contact sulphuric acid plant, arsenic is a poison for the vanadium pentoxide catalyst, shortening bed life and reducing conversion. Selenium and tellurium largely report into the finished acid, where they are unacceptable for food-contact, pharmaceutical, electronics and battery-grade acid. This is why the limits are set by the destination rather than by the sulphur itself. In agricultural use the driver is different again: arsenic is a regulated contaminant in fertiliser inputs in a number of importing markets, so a blender may write the limit against the destination's fertiliser contaminant regulation rather than against catalyst life. Ask which of the two a stated limit came from, because the regulatory route and the process route produce different numbers and often call for different test methods. ISO 3705 gives the classical silver diethyldithiocarbamate photometric method for arsenic, and it carries the same trap as ISO 3704: the method is stated as applicable to arsenic contents equal to or greater than 0.5 mg/kg. A contract limit of 0.2 or 0.1 ppm therefore cannot be verified by ISO 3705 at all, and must be written against an instrumental method such as hydride-generation AAS or ICP-MS if it is to mean anything. Selenium and tellurium are normally run by hydride-generation atomic absorption or by ICP, and the method should be named on the certificate because sensitivity at the one-part-per-million level depends on it.
The H2S figure on a sulphur certificate tells you nothing about how the product will perform. It tells you how dangerous it is to handle. It belongs on the certificate for the same reason a flash point belongs on a fuel certificate, and it should be read by the people who will open the tank, not by the people who will run the plant.
Sulphur has one property that surprises everyone who handles it for the first time: its liquid viscosity does not fall smoothly with temperature. Above a threshold just above 159 °C it rises catastrophically, and every rule about molten sulphur storage exists because of that single fact.
| Property | Value | Handling consequence |
|---|---|---|
| CAS number | 7704-34-9 | The identifier that ties the SDS, the customs entry and the purchase order together |
| Melting point | 115.2 °C | Solidification and remelting point; sets the minimum jacket temperature on any line that must stay liquid |
| Boiling point | 444.6 °C | Never approached in commercial handling |
| Density, solid | 2.07 g/cm³ | Orthorhombic alpha sulphur, the stable form at ambient temperature |
| Bulk density, formed solid (granules, prills, pastilles) | 900–1,350 kg/m³ per the IMSBC schedule | Stowage factor 0.74–1.11 m³/t; drives hold capacity and jumbo bag fill volume |
| Flash point, closed cup | approx. 207 °C for pure sulphur; reported as low as approx. 168 °C where hydrocarbon is present | Applies to the bulk liquid, and contamination lowers it. It says nothing about dust cloud ignition, which occurs far below it |
| Autoignition temperature | approx. 232 °C | Bulk material in air. Sets the ceiling on any hot surface in contact with sulphur |
| Viscosity minimum, liquid | approx. 7 cP (7 mPa·s) near 157 °C | The lowest-viscosity condition, and a tempting but dangerous place to operate |
| Lambda transition | approx. 159 °C | S8 rings open and polymerise into long chains; the practical hard limit for molten handling |
| Viscosity at 187 °C | approx. 93,000 cP (93 Pa·s) | A rise of about four orders of magnitude above the minimum. A local hot spot will stall a pump and plug a line |
| Solubility in water | Practically insoluble | Soluble in carbon disulphide; water contact is a corrosion and caking problem, not a dissolution problem |
| Combustion product | Sulphur dioxide (SO2) | Burns with a pale blue flame that is very hard to see in daylight; SO2 is the acute inhalation hazard in a sulphur fire |
One application dominates the tonnage so completely that the sulphur market is effectively a derivative of the sulphuric acid market, which in turn is largely a derivative of phosphate fertiliser demand.
On USGS figures about 90 % of all sulphur consumed is consumed in the form of sulphuric acid: the sulphur is burned to sulphur dioxide, converted to sulphur trioxide over vanadium pentoxide catalyst and absorbed into acid. This is the application that writes the arsenic, selenium and tellurium limits into the specification.
Sulphuric acid digests phosphate rock to phosphoric acid, the base material for most phosphatic fertilisers. On USGS figures close to 60 % of all sulphur consumption ends up in phosphate fertiliser production. That single number is why sulphur demand tracks the phosphate fertiliser cycle rather than the refining cycle that produces the sulphur.
Elemental sulphur applied as a nutrient, most commonly as bentonite sulphur pastilles at roughly 90 % sulphur and 10 % bentonite. Soil moisture swells the clay, breaking the pastille into particles in the 20 to 500 micron range so that Thiobacillus bacteria can oxidise it to plant-available sulphate. Also used to lower pH on alkaline soils.
The crosslinking agent for natural and synthetic rubber. A conventional cure system uses roughly 2.0 to 3.5 phr sulphur with 0.5 to 1.0 phr accelerator; semi-efficient systems use 1.0 to 2.0 phr; efficient systems use 0.3 to 1.0 phr with 2.0 to 6.0 phr accelerator. Tyre building uses insoluble grades to prevent surface bloom before cure.
Modified molten sulphur replaces Portland cement as the binder, mixed with graded aggregate at roughly 130 to 140 °C. It develops most of its ultimate strength within about a day rather than over weeks, because the set is a thermoplastic freeze and not a hydration reaction. ACI 548.2R sets minimum one-day values of 27.6 MPa (4,000 psi) compressive and 5.2 MPa (750 psi) flexural; reported unmodified sulphur concretes reach roughly 50 to 60 MPa compressive and 8 to 10 MPa flexural. It resists acids and salts that destroy conventional concrete. ASTM C1159 and ACI 548.2R-93 are the reference documents; both have been withdrawn but remain the technical basis in practice.
Carbon disulphide production, sulphite pulping in paper manufacture, sulphur dyes, agrochemical actives, and acid supply for copper and uranium leach circuits. Individually small against acid demand, collectively the reason powdered and micronised grades exist.
Sulphur is chemically benign in the sense that it is not acutely toxic to touch or swallow, and this is exactly why it is underestimated. Every serious sulphur incident traces back to one of three mechanisms, and all three are controllable with measures that are well documented and inexpensive relative to the consequence.
Sulphur dust suspended in air forms an explosible cloud. It ignites more easily than most industrial dusts, and the fines are generated by the handling itself — every belt transfer, ship loader drop, silo fill and bag discharge produces them, and repeated handling of a friable form produces a great deal of them.
The governing standard in the United States was NFPA 655, Standard for Prevention of Sulfur Fires and Explosions, which covered the crushing, grinding and pulverising of sulphur and the handling of sulphur in any form. As of the 2025 edition, effective 6 December 2024, NFPA consolidated six combustible dust standards — NFPA 61, 484, 652, 654, 655 and 664 — into a single document, NFPA 660, Standard for Combustible Dusts and Particulate Solids. If your site procedures still reference NFPA 655 by number, that is the change to pick up.
The explosion characteristics are measured, not assumed, by four ASTM methods: ASTM E1226 for maximum explosion pressure and the deflagration index Kst, ASTM E1515 for minimum explosible concentration, ASTM E2019 for minimum ignition energy of a dust cloud, and ASTM E1491 for minimum autoignition temperature. Published 20 litre sphere work on sulphur reports a maximum explosion pressure of roughly 5 to 8 bar and a Kst of roughly 86 to 251 bar·m/s, which straddles dust explosion classes St1 and St2 depending on particle size distribution and moisture content. That spread is the point: your material's numbers depend on your material, and a dust hazard analysis should be run on the actual product rather than on a table.
Two practical consequences follow. First, any enclosure where a suspendable cloud can form — a bag house, a bucket elevator leg, a silo headspace, a bagging station, a grinding mill — needs explosion protection designed against tested data, not a general assumption. Second, housekeeping is an engineering control and not a courtesy. The catastrophic event in a dust explosion is almost never the primary ignition; it is the pressure wave from the primary lifting a settled dust layer off beams, ledges and cable trays into suspension and igniting it. A layer thin enough to write your name in is enough.
Sulphur has extremely high electrical resistivity. Charge generated by pneumatic conveying, by free fall into a bag or a hold, by belt transfer and by sliding contact does not drain away — it accumulates, on the product and on anything ungrounded near it. Combine that with a dust cloud whose minimum ignition energy is in the low millijoule range — published 20 litre sphere work on sulphur reports roughly 2 mJ, against tens or hundreds of millijoules for many industrial dusts — and you have the classic sulphur ignition scenario. An energy that low is within reach of an ordinary human static discharge.
The controls are specific. Bond and earth all conveying, filling and receiving equipment, and verify continuity rather than assuming it. Where flexible intermediate bulk containers are used, specify the electrostatic type deliberately: Type C bags contain conductive threads and are only safe when actually bonded to earth at filling and discharge, Type D bags dissipate charge without a bonding connection, and Type A bags offer no protection at all and have no place in a sulphur dust atmosphere. Do not add an unrated plastic liner to a rated bag, because the liner can defeat the bag's classification. Keep free-fall filling heights short, and never use compressed air to clear a blockage or to sweep a dusty area.
Sulphur leaving a Claus unit carries dissolved hydrogen sulphide and hydrogen polysulphides, typically around 250 to 300 ppm by mass and up to roughly 400 ppm depending on operating conditions. Degassing units strip it out, and the usual commercial ceiling for degassed product is 10 ppm by mass, with 15 ppm written into some contracts. Critically, H2S continues to evolve from the liquid even after degassing to that level, and it accumulates in the vapour space of a molten storage tank, a road tanker, a rail car or a ship tank. Freshly formed solid sulphur also continues to release gas into the headspace of a closed container.
The exposure numbers are unforgiving. The OSHA permissible exposure limit is a 20 ppm ceiling, with a peak of 50 ppm permitted for a single period of up to 10 minutes in a shift only where no other measurable exposure occurs (29 CFR 1910.1000 Table Z-2). The ACGIH threshold limit value is 1 ppm as an 8-hour time-weighted average with a 5 ppm short-term exposure limit, and this is the value most modern safety data sheets and site rules adopt. The NIOSH recommended limit is a 10 ppm 10-minute ceiling, and the NIOSH IDLH is 100 ppm — the concentration above which the atmosphere is immediately dangerous to life and health.
The odour threshold is around 0.01 to 0.1 ppm, which sounds protective and is not. At high concentration H2S rapidly deadens the sense of smell, so the warning disappears precisely when the hazard becomes lethal. Smell is not a gas detector. H2S is heavier than air and collects in pits, sumps, tank bottoms and low points. It is lethal at several hundred ppm with very short exposure, and it is flammable, with a lower flammable limit in air of roughly 4 % by volume.
The operational rule that follows is absolute: the vapour space of any molten sulphur tank, tanker or hold is a permit-required confined space. Entry demands continuous personal and area gas monitoring, forced ventilation, a standby attendant and supplied-air respiratory protection. A filtering cartridge respirator is not adequate protection against H2S in an oxygen-uncertain confined space and must not be substituted.
Burning sulphur produces a pale blue flame that is extremely difficult to see in daylight, so a fire can be walked into. The combustion product is sulphur dioxide, and SO2 is the acute hazard to anyone responding: the OSHA permissible exposure limit is 5 ppm as an 8-hour average and the NIOSH IDLH is 100 ppm. Firefighting is normally by fine water spray or foam to cool the surface below the ignition temperature; a hard jet on molten or burning sulphur splashes burning material and spreads the fire. Keep sulphur segregated from strong oxidising agents and from finely divided metals. The supplier safety data sheet and the site emergency plan govern in every case, and both should be read before the first cargo arrives rather than after.
These are the published numbers a site safety case is normally built against. They are United States values because they are the most widely cited internationally; national limits in the destination country may be lower and take precedence. Read the units carefully: every airborne figure below is parts per million by volume in air, while the residual H2S line and the trace-element limits in the specification table are parts per million by mass (mg/kg). They are different quantities and cannot be compared with one another.
| Substance or hazard | Value | Source | What it governs |
|---|---|---|---|
| Hydrogen sulphide (H2S) | 20 ppm, ceiling | OSHA 29 CFR 1910.1000 Table Z-2 | The enforceable US general industry ceiling |
| Hydrogen sulphide (H2S) | 50 ppm peak, one period up to 10 min per shift | OSHA 29 CFR 1910.1000 Table Z-2 | Permitted only where no other measurable exposure occurs in the shift |
| Hydrogen sulphide (H2S) | 1 ppm TWA, 5 ppm STEL | ACGIH TLV | The value most safety data sheets and modern site rules adopt |
| Hydrogen sulphide (H2S) | 10 ppm, 10-minute ceiling | NIOSH REL | Recommended rather than enforceable |
| Hydrogen sulphide (H2S) | 100 ppm | NIOSH IDLH | Escape-only threshold; drives respiratory protection selection and confined space entry rules |
| Hydrogen sulphide (H2S) | approx. 4 % by volume | Lower flammable limit in air | Why vapour spaces are treated as flammable as well as toxic atmospheres |
| Sulphur dioxide (SO2) | 5 ppm, 8-hour TWA | OSHA PEL | The combustion product in any sulphur fire |
| Sulphur dioxide (SO2) | 100 ppm | NIOSH IDLH | Escape-only threshold for fire response planning |
| Residual H2S in degassed sulphur | max 10 ppm by mass, usual commercial ceiling | Commercial practice | Compare against roughly 250–400 ppm by mass in undegassed Claus sulphur |
| Sulphur dust explosion severity | Pmax approx. 5–8 bar; Kst approx. 86–251 bar·m/s | 20 litre sphere testing per ASTM E1226 | Straddles dust classes St1 and St2; explosion protection must be sized on your own tested data |
Sulphur is one of the few commodities where the cargo can arrive perfectly on specification and still be commercially useless, because the two things that ruin it in transit — moisture pickup and caking — do not change its purity at all.
The standard export package is a flexible intermediate bulk container, most commonly filled to 1,000 kg, with 800 kg and 1,250 kg fills also traded. Three things should be specified rather than assumed. The liner, because the liner is what keeps water out and keeps free acid off the woven fabric; a bag without a liner will take up moisture through a long humid sea leg and will lose fabric strength where acid concentrates. The safe working load ratio, normally 5:1 for single-trip and 6:1 for multi-trip service, because a bag that fails on the crane is both a cargo loss and a serious dust release. And the electrostatic type, for the reasons set out in the safety section: sulphur dust plus an ungrounded Type A bag is a documented ignition scenario, not a theoretical one.
Bulk carriage is the default for large parcels moving to acid plants and fertiliser complexes. Two entries in the IMO International Maritime Solid Bulk Cargoes (IMSBC) Code apply, and the distinction is commercially significant. Sulphur (crushed lump and coarse grained), UN 1350, is a Group B cargo, which possesses a chemical hazard and requires the carrying vessel to hold a Document of Compliance for the carriage of dangerous goods. Formed solid sulphur — prills, granules, pellets, pastilles and flakes — is carried as a Group C cargo, which is neither liable to liquefy nor chemically hazardous, with correspondingly simpler carriage requirements. Hold condition matters in both cases: wet sulphur is corrosive to steel, and holds should be clean, dry and appropriately coated before loading.
This is the failure mode that generates most sulphur claims. Free surface water combines with the free acidity already present on the granules and forms dilute sulphuric acid. That acid corrodes the hold, the hopper and the bag fabric, and the water bridges the granules so that the cargo sets. A caked bag stack has to be broken out mechanically, which shatters granules into fines and creates precisely the dust cloud the granular form was chosen to avoid. Discharge slows, cargo is lost as unsaleable fines, and the receiving plant's dust controls are loaded beyond design.
The controls are unglamorous and effective: buy at the low end of the moisture specification rather than at the limit; insist on intact liners and reject bags with liner damage at loading rather than at discharge; keep bags under cover and off wet ground; avoid long periods with heavy stack pressure on the lower tiers, since compaction and moisture together are worse than either alone; and consider specifying a moisture figure at discharge as well as at load, so that responsibility for transit conditions is allocated in the contract instead of argued afterwards.
Solid sulphur is UN 1350, SULPHUR, Class 4.1 flammable solid, packing group III. However, Special Provision 242, which applies to this entry in the dangerous goods regulations, exempts sulphur that has been formed to a specific shape — prills, granules, pellets, pastilles or flakes — from those regulations. This exemption is a substantial part of the commercial reason the granular form dominates export trade: the same chemical product moves under materially simpler paperwork purely because of how it was solidified. Confirm the current wording in the edition of the regulations applicable to your route, and note that the IMSBC Code still applies to bulk carriage by sea irrespective of the road and packaged-goods exemption.
Molten sulphur is a separate entry: UN 2448, SULPHUR, MOLTEN, Class 4.1, packing group III, carried in insulated, steam-traced tanks at elevated temperature and not carried in bulk holds. For customs, HS 2503.00 covers sulphur of all kinds other than sublimed, precipitated and colloidal sulphur, and that is where recovered export sulphur classifies. Sublimed, precipitated and colloidal sulphur classify separately under HS 2802.00. Several importing countries carry a national subheading beneath 2503 specifically for sulphur recovered as a by-product of crude oil refining, so the operative code at the border is longer than six digits and is country-specific. Confirm both the heading and the national subheading before the entry is filed, since duty and inspection treatment can differ.
Typically a minimum of 99.8 to 99.9 % by mass on a dry basis. The important point is that purity is calculated by difference rather than measured directly: moisture (ISO 3426), ash (ISO 3425), acidity as H2SO4 (ISO 3704) and carbon or organic matter are measured and subtracted from 100. A purity claim on a certificate that does not report those impurity lines is unsupported, because there is nothing behind the subtraction.
Both are chemically identical and broadly overlap in size, with granular usually quoted on a 2 to 4 mm core range and prills running somewhat finer, roughly 1 to 4 mm. Granular sulphur is built up in layers onto a seed particle in a granulator, which gives it high crush strength and low dust generation, and it is the export benchmark for that reason. Prilled sulphur is formed by solidifying molten droplets in a water bath or air tower; it is rounder but more friable, so it produces more fines through repeated transfer and a long sea leg. For a cargo that will be transhipped or handled several times, granular usually delivers more saleable tonnes at destination.
Because the damage is not dissolution. Surface water combines with the free acidity already present on the granules to form dilute sulphuric acid, which corrodes ship holds, hoppers and conveyor structures and degrades the woven polypropylene of jumbo bags. Water also bridges the granules so the cargo cakes into a solid mass, and breaking out a caked cargo shatters granules into fines and generates airborne dust. A maximum of about 0.5 % is a normal commercial ceiling, measured as loss in mass at 80 °C under ISO 3426.
Solid sulphur is UN 1350, Class 4.1 flammable solid, packing group III. However, Special Provision 242 exempts sulphur that has been formed to a specific shape — prills, granules, pellets, pastilles or flakes — from the dangerous goods regulations, which is one reason formed sulphur dominates export trade. Bulk sea carriage is separately governed by the IMSBC Code, where crushed lump and coarse grained sulphur is a Group B cargo requiring a Document of Compliance and formed solid sulphur is carried as Group C. Molten sulphur is UN 2448, Class 4.1, packing group III.
It applies to both, though the severity differs. Molten sulphur is the acute case: undegassed Claus sulphur carries roughly 250 to 400 ppm by mass of dissolved H2S, degassing brings that down to a usual commercial ceiling of 10 ppm, and gas continues to evolve even after degassing, accumulating in the vapour space of tanks, tankers and ship tanks. Freshly formed solid sulphur also releases gas into the headspace of a closed or poorly ventilated container. Any enclosed sulphur headspace should be treated as a permit-required confined space with gas monitoring and supplied air, and never entered on the assumption that solid product is inert.
Moisture combined with compaction. Surface water bridges the granules, the free acidity accelerates the bonding, and stack pressure on the lower tiers of a bag stack or the base of a silo consolidates the mass. Prevention is straightforward: buy at the low end of the moisture specification rather than at the limit, insist on intact bag liners and reject liner damage at loading, store under cover and off wet ground, limit stack height and stack duration, and consider specifying a moisture figure at discharge as well as at load so transit responsibility is allocated in the contract.
That depends entirely on what the sulphur is for, which is why suppliers quote ranges. Commercial limits are commonly seen around 0.5 to 1 ppm by mass for arsenic and 1 to 2 ppm for selenium and tellurium, and the method matters as much as the number: ISO 3705, the classical photometric method for arsenic, is stated as applicable at 0.5 mg/kg and above, so a tighter limit has to be written against an instrumental method such as hydride-generation AAS or ICP-MS. These exist because of sulphuric acid manufacture: arsenic poisons the vanadium pentoxide catalyst in a contact plant, while selenium and tellurium report into the finished acid and are unacceptable in food-contact, pharmaceutical, electronics and battery-grade acid. A producer of those acid grades will specify far tighter than the ranges above. A fertiliser blender may not test for selenium and tellurium at all, but may still hold a hard arsenic limit, because arsenic is a regulated contaminant in fertiliser inputs in a number of importing markets and the limit there comes from the destination's fertiliser regulation rather than from catalyst life. Agree the limit list, the basis for each limit and the test methods before contracting, not after the certificate arrives.
HS 2503.00 covers sulphur of all kinds other than sublimed, precipitated and colloidal sulphur, and that is where recovered export sulphur classifies. Sublimed, precipitated and colloidal sulphur sit separately under HS 2802.00. Several importing countries add a national subheading beneath 2503 specifically for sulphur recovered as a by-product of crude oil refining, so the code that actually governs at the border is longer than six digits and differs by country. Have the broker in the destination confirm the national code against the form and grade actually being shipped before the declaration is lodged, because duty and inspection treatment can follow the subheading rather than the heading.
Send the form you need — granular, prilled, pastilles, lump or powder — along with quantity, packing, destination port and Incoterm. If the material is going to a sulphuric acid plant or into an agricultural blend, state the required purity and trace-element limits with the test methods, and the offer will be checked against them before pricing. Enquiries are handled on WhatsApp.