Sodium Sulfide in Pulp and Paper Industry — White Liquor, Lignin Removal & Supply
Sodium Sulfide in the Pulp and Paper Industry — Kraft Process, White Liquor & Bulk Na₂S Supply
Sodium sulfide is one of the two chemicals that define Kraft pulping. This guide explains what Na₂S actually does inside the digester — how it forms white liquor with sodium hydroxide, how it strips lignin without destroying cellulose fibers, and what governs consumption and quality in a working mill. SUHA International Trading supplies and exports sodium sulfide 60% yellow flakes, low iron, to pulp mills and their distributors from Jebel Ali (UAE) and Mersin (Turkey), with a batch Certificate of Analysis and Safety Data Sheet on every consignment.
Quick answer — Sodium sulfide (Na₂S) is a core pulping chemical in the Kraft process. Dissolved in white liquor together with sodium hydroxide, it supplies hydrosulfide ions that accelerate lignin removal from wood chips while limiting damage to cellulose fibers. The result is faster delignification, higher pulp yield and stronger paper than alkali-only pulping.
One commercial point is often missed: in an operating mill the sulfide circulates through the chemical recovery cycle rather than being consumed once. Purchased Na₂S is therefore makeup chemical, bought to replace sodium and sulfur lost from the loop — which is why requirements follow from the mill's own balance and not from a generic dosage figure.
What Is Sodium Sulfide (Na₂S)?
Sodium sulfide is an inorganic salt of sodium and sulfur, formula Na₂S. In international trade it is supplied mainly as flakes, chips, crystals or solution, and almost always in hydrated form — which is why the stated assay must always be read together with its basis. A "60%" flake and a "30%" crystal can describe very different tonnages of contained sulfur.
Identifiers and Classification
| Item | Value | Note |
|---|---|---|
| Chemical formula | Na₂S | Supplied commercially in hydrated form |
| CAS number | 1313-82-2 | Sodium sulfide |
| HS code | 2813.90 | Used on our export documentation; confirm the import heading with your broker |
| UN number | UN 1849 | Hydrated grade. Anhydrous and low-hydration material is classified differently |
| IMDG class | Class 8 | Corrosive |
| Appearance | Yellow to light-red flakes | Colour broadly tracks iron content; yellow grades are the low-iron material |
| Not to be confused with | NaHS | Sodium hydrosulfide — different sodium-to-sulfur ratio, different effect on the liquor balance |
Sodium sulfide (Na₂S) is not the same product as sodium hydrosulfide (NaHS). Both deliver sulfur to a pulping system, but they carry different amounts of sodium alkalinity with that sulfur, and in a Kraft mill that difference decides which one you should be buying. The comparison is set out further down this page, and in more depth in our guide on sodium sulfide vs sodium hydrosulfide.
Why Sodium Sulfide Matters in Pulp and Paper
Wood is not fiber alone. It is a composite of three materials, and chemical pulping is essentially an exercise in selective destruction: dissolve one of them and keep the other two intact. The commercial value of sodium sulfide comes entirely from how well it supports that selectivity.
Cellulose
The long-chain polysaccharide that forms the papermaking fiber. This is the material the mill wants to keep intact, because chain length translates directly into strength.
Hemicellulose
Shorter branched polysaccharides. They contribute to yield and to inter-fiber bonding, and they are more vulnerable to alkaline degradation than cellulose.
Lignin
The aromatic polymer that binds fibers together and gives wood rigidity. It is what must largely be removed to produce chemical pulp — and it is what Na₂S attacks.
Kraft Pulping vs Soda Pulping — the Difference Is Sulfide
In alkali-only pulping, the older soda process, sodium hydroxide removes lignin but also degrades a significant share of the carbohydrates alongside it. Yield falls and fiber strength suffers. Adding sodium sulfide changes the reaction pathway: delignification speeds up sharply while carbohydrate damage does not rise in proportion.
That single change is the reason the Kraft process became the dominant chemical pulping technology worldwide — Kraft being the German and Swedish word for strength. It is also why sodium sulfide is not an additive at the margins of papermaking but one of the two chemicals the process is built on.
Stated plainly: sodium sulfide does not replace sodium hydroxide, and sodium hydroxide cannot replace sodium sulfide. The hydroxide creates the alkaline environment; the sulfide makes that environment fast and selective. Mills buy both, and we supply both — sodium sulfide and caustic soda (NaOH) — under one contract where that simplifies procurement.
Role of Sodium Sulfide in Kraft Pulping
The sequence below is the backbone of every Kraft mill. Equipment, cooking schedules and recovery configurations differ from site to site, but the chemistry and the order of operations do not.
Chipping
Logs are reduced to wood chips of controlled size and thickness so that cooking liquor can penetrate evenly. Oversized or ragged chips cook unevenly and raise both rejects and chemical demand.
Impregnation with white liquor
Chips are wetted with white liquor — the alkaline cooking solution containing NaOH and Na₂S. Uniform impregnation before the temperature rises is what makes the cook uniform.
Cooking in the digester
Chips and liquor are heated under pressure in a batch or continuous digester. Published cooking temperatures for Kraft pulping generally sit in the region of 160–170 °C, but temperature, pressure and time are set by each mill's own cooking schedule and wood furnish.
Delignification
Hydrosulfide ions cleave the linkages holding the lignin network together. The fragments dissolve into the liquor, which — now loaded with dissolved organics and spent inorganics — becomes black liquor.
Washing and screening
Pulp is separated from black liquor and washed. Washing efficiency matters twice over: it determines pulp cleanliness and it determines how much sodium and sulfur is recovered rather than lost.
Chemical recovery
Black liquor is concentrated, burned for energy, and the inorganic chemicals are regenerated back into white liquor. This loop is why an operating mill buys makeup Na₂S rather than the full cooking charge.
The Vocabulary Mill Chemists Actually Use
Cooking liquor is described with a specific set of terms. Getting them right matters commercially: an enquiry that specifies sulfidity and assay basis gets a quotable answer, while one that specifies only "sodium sulfide, 100 tons" does not.
| Term | Definition | Why it matters |
|---|---|---|
| Active alkali (AA) | NaOH + Na₂S, expressed on a common basis, usually as Na₂O | Total chemically active alkali charged to the cook |
| Effective alkali (EA) | NaOH + ½ Na₂S | Reflects the hydroxide genuinely available for delignification |
| Sulfidity | Na₂S ÷ (NaOH + Na₂S), as a percentage on the same basis | The single best indicator of how sulfide-rich the cook is |
| Kappa number | Standard test indicating residual lignin in the pulp | The target that cooking conditions and alkali charge are set to reach |
| Liquor-to-wood ratio | Volume of liquor per unit mass of wood | Sets chemical concentration inside the digester |
Sulfidity is chosen by each mill according to wood furnish, digester design, recovery balance and product requirement. Levels commonly reported in the pulping literature sit in a broad band of roughly 20–35%, but the correct figure for any given mill is an operating decision, not a specification anyone can supply from outside.
How Sodium Sulfide Removes Lignin
Delignification in the Kraft digester is a competition between two reactions running at the same time. Hydroxide drives both of them. Hydrosulfide strongly accelerates only the one you want.
At white liquor pH the sulfide exists as the hydrosulfide ion (HS⁻), a powerful nucleophile — a species that attacks electron-poor sites in the lignin structure. That attack cleaves the ether linkages holding the lignin network together far faster than hydroxide alone manages, and it does so at reaction sites the carbohydrates largely do not share. The sulfur involved is not locked into the fiber: it is largely released again as the reaction proceeds and remains available in the liquor system.
What This Buys the Mill
- Faster delignification at a given temperature and alkali charge, which shortens cooking time or allows a lower cooking temperature for the same Kappa target.
- Better selectivity — more lignin removed per unit of carbohydrate lost.
- Higher screened yield than soda pulping on the same wood.
- Stronger fiber, because cellulose chains survive the cook better. This is the property that makes Kraft pulp the standard for liner, sack, packaging and other strength-critical grades.
Two limits worth stating plainly. First, sodium sulfide improves delignification but does not brighten pulp — Kraft pulp leaves the digester dark and needs a separate bleaching sequence for white grades. Second, sulfide chemistry is the origin of the reduced sulfur compounds behind the characteristic odour of a Kraft mill. Both are managed, not eliminated, and any supplier telling you otherwise is selling rather than advising.
Sodium Sulfide and White Liquor
White liquor is the active cooking liquor of the Kraft process. Its two functional components are sodium hydroxide and sodium sulfide; everything else in it is either residue from the regeneration step or accumulated dead load.
| Component | Formula | Role in the liquor |
|---|---|---|
| Sodium hydroxide | NaOH | Primary alkali — maintains high pH, hydrolyses lignin bonds, keeps lignin fragments dissolved |
| Sodium sulfide | Na₂S | Sulfur source — supplies HS⁻ for rapid selective delignification, and adds alkalinity through hydrolysis |
| Sodium carbonate | Na₂CO₃ | Largely inactive residue of incomplete causticizing |
| Sodium sulfate | Na₂SO₄ | Inactive dead load, and the traditional makeup chemical known as salt cake |
| Thiosulfate, chloride, others | Na₂S₂O₃, NaCl | Minor species that accumulate in the loop and are purged when they build up |
The Liquor Cycle — Why Sulfide Is Recycled, Not Consumed
In an operating Kraft mill the sodium and sulfur are not bought fresh for every cook. They circulate:
White liquor cooks the chips
NaOH and Na₂S do the work in the digester and leave with the spent liquor.
Black liquor is evaporated
The spent liquor, carrying dissolved lignin plus the inorganic chemicals, is concentrated in the evaporator train.
Recovery boiler
The organic fraction is burned for steam and power. Under the reducing conditions of the char bed the sulfur compounds are reduced to sulfide, leaving a molten smelt of mainly Na₂S and Na₂CO₃.
Green liquor
The smelt is dissolved to form green liquor, and dregs are removed before causticizing.
Causticizing
Slaked lime converts the carbonate back to hydroxide. The sulfide passes through this step essentially unchanged, so what comes out is white liquor again.
Lime kiln
Calcium carbonate from causticizing — lime mud — is calcined back to lime, closing the calcium loop alongside the sodium and sulfur one.
Where Purchased Sodium Sulfide Actually Fits
No recovery cycle is perfectly closed. Sodium and sulfur leave with washing losses, spills, purge streams and stack emissions, and what leaves has to be replaced with makeup chemicals. Depending on whether the mill is short of sodium, of sulfur, or of both, that makeup is supplied as sodium sulfate, sodium hydroxide, sodium sulfide or sodium hydrosulfide.
This is the commercial entry point for purchased Na₂S: it balances the loop; it does not feed the whole cook. In practice the most frequent buyers are non-integrated and smaller mills, specialty operations, and mills whose recovery cycle runs sulfur-deficient — plus distributors serving them.
Na₂S vs NaOH — and Na₂S vs NaHS
Two comparisons come up in almost every technical conversation with a pulp mill. The first is conceptual and settles what each chemical is for. The second is commercial and decides which sulfur source belongs on your purchase order.
Sodium Sulfide vs Sodium Hydroxide in Kraft Pulping
| Aspect | Sodium sulfide (Na₂S) | Sodium hydroxide (NaOH) |
|---|---|---|
| Function in the cook | Sulfur source; supplies HS⁻ for nucleophilic cleavage of lignin | Primary alkali; maintains high pH and hydrolyses lignin |
| Effect on delignification rate | Strong accelerating effect | Necessary, but slower acting on its own |
| Effect on selectivity | Improves it — protects carbohydrates relative to alkali-only cooking | Attacks lignin and carbohydrates less selectively |
| Effect on yield and strength | Supports higher yield and stronger fiber | Essential, but alone gives soda pulping and lower yield |
| Contribution to alkalinity | Indirect, through hydrolysis to NaOH and NaHS | Direct and primary |
| Behaviour in recovery | Recirculates through the sulfur cycle | Regenerated in causticizing |
| Typical reason to purchase | Sulfur and sodium makeup | Sodium and alkali makeup, plus other mill uses |
| Handling profile | Corrosive; releases H₂S on contact with acid; classification varies by grade | Corrosive; strongly exothermic on dilution |
Bottom line: sodium hydroxide sets the alkaline environment and sodium sulfide makes that environment fast and selective. Remove the sulfide and the process reverts to soda pulping, with lower yield and weaker pulp. Remove the hydroxide and there is no functional cook at all. If you buy both, our caustic soda flakes 98% and liquid caustic soda ship from the same two loading ports.
Sodium Sulfide vs Sodium Hydrosulfide (NaHS)
| Aspect | Sodium sulfide (Na₂S) | Sodium hydrosulfide (NaHS) |
|---|---|---|
| Sodium per unit of sulfur | Higher | Lower |
| Effect on the mill balance | Adds sodium and sulfur together | Adds mainly sulfur |
| Typical selection driver | Mill is short of both sodium and sulfur | Mill needs sulfur without the extra sodium load |
| Common supply forms | Flakes, chips, crystals, solution | Flakes and solution |
How we handle this at enquiry stage. A useful supplier conversation starts with which element your cycle is actually short of, not with a product name. Send us the sodium and sulfur position you are trying to correct and we will tell you plainly whether Na₂S is the right makeup for it — including when it is not. Background reading: Sodium Sulfide vs Sodium Hydrosulfide (NaHS).
Benefits of Sodium Sulfide in Pulp Production
These are relative advantages measured against alkali-only cooking. Their size depends on wood furnish, cooking conditions, equipment and the target Kappa number — they are directional, not guaranteed numbers.
Higher delignification rate
Faster lignin removal at comparable cooking conditions, which improves digester throughput or allows a milder cook.
Improved selectivity
Cellulose and hemicellulose are better preserved during the cook, which is where the yield advantage comes from.
Higher pulp yield
More usable fiber per tonne of wood than soda pulping on equivalent furnish.
Superior strength
Longer surviving cellulose chains support tear, tensile and burst strength — the reason Kraft dominates packaging and sack grades.
Wood species flexibility
Kraft cooking handles both softwood and hardwood and tolerates resinous species that other processes struggle with.
Recovery compatibility
The sulfur can be regenerated and recirculated rather than discharged, which is both an economic and an environmental advantage.
Factors Affecting Sodium Sulfide Consumption
Sodium sulfide demand is not a fixed figure per tonne of pulp, and any supplier who quotes one without seeing your process is guessing. It is the outcome of several interacting variables.
| Factor | Influence on sulfide requirement |
|---|---|
| Wood species | Softwood, with higher and more condensed lignin content, generally needs a more severe cook than hardwood |
| Wood composition and condition | Chip moisture, extractives, bark content, storage time and season all shift chemical demand |
| Chip quality and size distribution | Uneven or oversized chips impair liquor penetration and raise both rejects and chemical use |
| Alkali charge and sulfidity | The chosen AA/EA charge and sulfidity level directly set how much Na₂S is present in the cook |
| Liquor-to-wood ratio | Determines chemical concentration inside the digester |
| Cooking temperature | Higher temperature raises reaction rate; the trade-off is paid in selectivity |
| Cooking time / H-factor | Time and temperature are traded against each other to hit the same target |
| Target Kappa number | A lower Kappa — more delignification in the digester — demands a more severe cook |
| Mill configuration | Batch versus continuous digesters, modified cooking technologies and washing efficiency all change the balance |
| Recovery cycle performance | Reduction efficiency in the recovery boiler, causticizing efficiency and washing losses decide how much makeup must actually be purchased |
Practical consequence for procurement: makeup sodium sulfide requirements should be derived from the mill's own sodium and sulfur balance, not from a generic dosage figure found online. If you send us the balance position and the assay basis you work to, the quotation can be built against real tonnage of contained Na₂S instead of nominal product weight.
Sodium Sulfide Quality Requirements for Pulp Mills
There is no single universal specification for pulp-grade sodium sulfide. Requirements are set by the buyer's process and recovery cycle. What follows is the parameter set that normally appears in a serious technical negotiation — and what each parameter is actually protecting.
| Parameter | Why it is specified |
|---|---|
| Na₂S assay and stated basis | Determines the sulfur and sodium actually delivered per tonne. The hydration basis must be explicit or two offers are not comparable |
| Iron (Fe) content | Iron can affect downstream brightness and contribute to deposits; low-iron yellow grades are generally preferred over red flake |
| Free alkali (NaOH) | Contributes to the alkali balance and must be accounted for in liquor calculations |
| Sulfite, carbonate, thiosulfate | Oxidation and carbonation products; they represent dead load in the liquor cycle and lost active content |
| Sodium chloride | Chloride accumulation in the recovery cycle is a recognised corrosion and plugging concern |
| Water-insoluble matter | Affects dissolving equipment, filtration and residue handling |
| Physical form and particle size | Flakes, chips, crystals or solution — determines dissolving and dosing arrangements at the mill |
| Packaging and moisture protection | Sealed moisture-barrier packing protects assay through transit and storage; air exposure oxidises active Na₂S |
Our Export Grade — Na₂S 60% Yellow Flakes
The specification below is the grade we ship as standard. It is indicative: the batch Certificate of Analysis issued with each consignment governs, and where your process needs a different basis or tighter limit, tell us at enquiry stage rather than after the contract.
| Parameter | Specification | Note |
|---|---|---|
| Na₂S active content | 60 ± 2% | Confirmed per batch by titration |
| Iron (Fe) | ≤ 10 ppm | Low-iron yellow flake |
| Sodium sulfite (Na₂SO₃) | ≤ 2.0% | Oxidation product; indicator of handling and storage discipline |
| Sodium carbonate (Na₂CO₃) | ≤ 0.5% | Dead load in the liquor cycle |
| Water-insoluble matter | ≤ 12% | Affects dissolving and residue handling |
| Appearance | Yellow to light-red flakes | Colour broadly tracks iron content |
| pH, 1% solution | 11–13 | Strongly alkaline |
| Shelf life, sealed | 12–24 months | Protect from air and moisture; prolonged exposure oxidises Na₂S toward sulfite and thiosulfate |
Sodium Sulfide Handling and Storage
The guidance below is general and industrial. Site-specific procedures must follow the Safety Data Sheet for the exact grade supplied and all applicable national and local regulations. Nothing on this page replaces your own risk assessment.
Storage
Cool, dry, well-ventilated area. Tightly closed moisture-resistant packaging, off the floor, segregated from acids, oxidising agents and foodstuffs. Avoid direct sunlight and heat.
Hydrogen sulfide risk
Contact with acid liberates H₂S — toxic, and dangerous precisely because olfactory fatigue removes the warning at high concentration. Ventilation, gas detection and confined-space procedure are essential.
Personal protection
Chemical-resistant gloves, splash goggles, full protective clothing, and respiratory protection in enclosed or poorly ventilated areas. Control dust when handling flakes.
Dissolving
Add sodium sulfide to water, never water to the solid, and manage the heat released. Confirm materials of construction are compatible with alkaline sulfide solutions before commissioning a line.
Emergency provision
Eyewash and emergency shower within reach of handling points, plus training specific to H₂S exposure rather than generic corrosive-chemical training.
Transport classification
The hydrated grade ships as UN 1849, IMDG Class 8. Anhydrous and low-hydration grades are classified differently — the declaration must match the grade actually shipped.
Never store sodium sulfide near acids. This is the single most common serious incident with Na₂S in industrial storage, and it is entirely preventable through segregation. Confirm the UN number and hazard class from the Safety Data Sheet for the delivered product before every shipment — not from a previous consignment of a different grade.
Environmental Considerations in Sulfide Pulping
Odour and TRS emissions
Sulfide chemistry in the digester produces total reduced sulfur (TRS) compounds — hydrogen sulfide, methyl mercaptan, dimethyl sulfide and dimethyl disulfide. They are detectable at extremely low concentrations, which is why a Kraft mill has a characteristic smell even when emissions are well within permit. Modern mills collect and incinerate concentrated and dilute non-condensable gases; TRS is regulated in most jurisdictions, with limits and monitoring obligations set locally.
Wastewater
Sulfide-bearing effluent is toxic to aquatic life and adds to oxygen demand. Mill practice centres on containment and recovery of spills and washing losses first, and treatment of what remains second. Discharge limits are jurisdiction-specific and should be confirmed against your own permit rather than against general industry description.
Chemical recovery as environmental control
The recovery cycle is not only an economic system — it is the primary environmental control in a Kraft mill. Efficient evaporation, combustion, causticizing and washing keep sulfur inside the loop instead of releasing it to air or water, and simultaneously reduce the volume of makeup chemical that has to be purchased, transported and handled.
Sulfur balance management
Sulfur enters with makeup chemicals and leaves through emissions, effluent and purge streams, so mills actively manage the balance in both directions. Excess sulfur raises odour and emission risk; a deficit pulls sulfidity down and degrades pulping performance. Purchased sodium sulfide should be specified with that balance in view, which is another reason a bare tonnage figure is rarely the right way to start an enquiry.
Sodium Sulfide Supply for Pulp and Paper Producers
SUHA International Trading L.L.C. is an exporter and supplier of sodium sulfide and related industrial chemicals, operating from Dubai and Ankara and loading FOB Jebel Ali (UAE) and FOB Mersin (Turkey). We serve pulp mills, chemical distributors and industrial buyers who need dependable Na₂S quality and a documentation file that clears customs without argument.
Packing Options
25 kg PP/PE BagsPalletised, stretch-wrapped
Multi-layer polypropylene/polyethylene with an inner vapour-barrier liner. Around 800 bags to a 20-foot container at 20 MT. Suited to distributors and to mills dosing in smaller batches.
Quote this packing →150 kg Steel DrumsUN-approved, airtight
Sealed steel drums for extended transit, humid climates or remote storage where bag integrity is harder to protect. Roughly 133 drums to a 20-foot container at 20 MT.
Quote this packing →1,000 kg FIBCJumbo bags, PE liner
Moisture-sealed flexible intermediate bulk containers, 20 to a 20-foot container. Usually the lowest cost per tonne for mills with the equipment to handle jumbo bags.
Quote this packing →How to Order
Send the enquiry
Grade and assay basis, quantity in MT per order and per month, packing, destination port and preferred Incoterm. If you know your sulfidity target or sulfur balance position, include it — it changes what we recommend.
Receive a dated offer within 24 business hours
Unit price, total value, payment terms, packing, loading port and estimated shipment schedule, with a stated validity period.
Review the technical file
Specification, a recent Certificate of Analysis and the Safety Data Sheet, checked against your process requirement and your own liquor calculation before anything is signed.
Confirm order and payment instrument
T/T, irrevocable L/C at sight, or D/P. For new buyers the standard structure is a deposit with the balance against a copy of the Bill of Lading.
Optional pre-shipment inspection
SGS or Bureau Veritas inspection at the warehouse before loading, with the report and COA in your hands before the balance is released.
Loading, documents and tracking
For material in stock, loading within seven to ten working days of confirmation and deposit, from Jebel Ali or Mersin. Full document set issued after sailing, with container and B/L details for tracking.
On price: we do not publish a sodium sulfide price on this page. Na₂S re-prices with sulfur, freight and regional demand, so a number here would be stale within days and useless for a purchasing decision. Send the specification, tonnage, port and Incoterm and you will get a firm, dated quotation instead — including, where relevant, the FOB Jebel Ali and FOB Mersin numbers side by side so you can compare landed cost properly.
Request a Sodium Sulfide Quotation, COA or SDS
Send six data points and you will receive a firm, dated offer — FOB Jebel Ali, FOB Mersin, or CIF to your discharge port — with packing, net tonnage and documentation costed in.
Speak With Our Chemical Export Desk
We respond to sodium sulfide enquiries within 24 business hours, usually faster. For an urgent requirement or a tender deadline, WhatsApp is the quickest channel. Certificate of Analysis, Technical Data Sheet and Safety Data Sheet are available at no cost and with no obligation.
💬 WhatsApp — fastest response channel+971 50 720 9246 📧 Email — enquiries and formal quotationsinfo@causticsodaco.comInclude This for the Fastest Quote
FAQ — Sodium Sulfide in Pulp and Paper
Process & chemistry
What is sodium sulfide used for in the pulp and paper industry?
Sodium sulfide is used as a pulping chemical in the Kraft process. Dissolved in white liquor together with sodium hydroxide, it supplies hydrosulfide ions that accelerate lignin removal from wood chips while limiting damage to cellulose fibers, which raises pulp yield and fiber strength. Purchased sodium sulfide is normally used as makeup chemical to replace sodium and sulfur lost from the mill's chemical recovery cycle, rather than as the sole source of cooking chemical.
Is sodium sulfide used in the Kraft process?
Yes. Sodium sulfide is one of the two defining chemicals of Kraft pulping, alongside sodium hydroxide. The Kraft process is sometimes called the sulfate process because sodium sulfate was traditionally the makeup chemical, which is then reduced to sodium sulfide in the recovery boiler. The active pulping species in the digester, however, is sulfide and not sulfate.
What does sodium sulfide do to lignin?
It accelerates the breakdown of lignin. At the high pH of white liquor, sulfide is present as the hydrosulfide ion, a strong nucleophile that attacks and cleaves the ether linkages holding the lignin network together. The fragments dissolve into the cooking liquor and leave the digester with the black liquor. The attack is selective: it degrades lignin far more aggressively than it degrades cellulose.
What is the role of Na₂S in white liquor?
Na₂S is the sulfur-bearing component of white liquor. It provides the hydrosulfide ions responsible for rapid, selective delignification, and through hydrolysis it also contributes alkalinity to the cooking liquor. Its share of the total active alkali is expressed as the sulfidity of the liquor.
Is sodium sulfide necessary for Kraft pulping?
Yes. Without sulfide the process is no longer Kraft pulping but soda pulping, which uses alkali alone and gives lower yield and weaker fiber. The presence of sulfide is precisely what distinguishes the Kraft process and gives Kraft pulp its strength advantage.
Quality & comparison
How does sodium sulfide affect pulp quality?
By improving selectivity it preserves cellulose chain length and fiber integrity during cooking, which supports tear, tensile and burst strength in the finished pulp. It does not brighten pulp: Kraft pulp leaves the digester dark and requires a separate bleaching sequence for white grades. The size of the effect depends on wood species, sulfidity, alkali charge and cooking conditions.
What is the difference between sodium sulfide and sodium hydroxide in Kraft pulping?
Sodium hydroxide is the primary alkali: it maintains high pH and hydrolyses lignin. Sodium sulfide is the sulfur source: it supplies hydrosulfide ions that make delignification faster and more selective. Both are required. Sodium hydroxide alone gives soda pulping, and sodium sulfide cannot function without an alkaline medium.
What is the difference between sodium sulfide (Na₂S) and sodium hydrosulfide (NaHS)?
Both supply sulfur, but Na₂S carries more sodium per unit of sulfur than NaHS. Mills short of both sodium and sulfur usually prefer Na₂S, while mills that need sulfur without adding sodium load usually prefer NaHS. The correct choice follows from the mill's own sodium and sulfur balance rather than from price per tonne alone.
How much sodium sulfide does a pulp mill consume?
There is no universal figure. In an operating Kraft mill the sulfide circulates through the recovery cycle, so purchased volume covers losses rather than the full cooking charge. Requirements depend on wood species, alkali charge and sulfidity, cooking conditions, target Kappa number, washing efficiency and recovery boiler reduction efficiency. Makeup requirements should be derived from the mill's own sodium and sulfur balance.
Sourcing & supply
Which grade of sodium sulfide do you supply for pulp mills?
We export sodium sulfide as 60% active-content yellow flakes with iron guaranteed at 10 ppm or below, shipped from Jebel Ali in the UAE and Mersin in Turkey. Every consignment carries a batch Certificate of Analysis stating Na₂S content, iron, sulfite, carbonate and insoluble matter, together with the Safety Data Sheet. Tell us the assay basis your liquor balance uses and we will confirm against it before quoting.
What is the minimum order quantity and how is sodium sulfide packed?
The standard minimum is 20 metric tons, one 20-foot full container load. Packing options are 25 kg multi-layer PP/PE bags with an inner vapour-barrier liner, 150 kg sealed steel drums for long transit or humid storage, and 1,000 kg FIBC jumbo bags, which is usually the lowest cost per tonne for mills with bulk handling equipment.
How long does shipment take and which documents are provided?
For material in stock we load within seven to ten working days of order confirmation and deposit, from Jebel Ali or Mersin. Each shipment is supplied with the Certificate of Analysis, Safety Data Sheet, Commercial Invoice, Packing List, Bill of Lading and Certificate of Origin, and SGS or Bureau Veritas pre-shipment inspection can be arranged on request.
What is the HS code and transport classification for sodium sulfide?
Sodium sulfide is exported under HS code 2813.90 and carries CAS number 1313-82-2. The hydrated grade ships as UN 1849, IMDG Class 8 corrosive. Anhydrous and low-hydration grades are classified differently for transport, so the UN number on the dangerous goods declaration must match the grade actually shipped, and the Safety Data Sheet for the delivered product governs.
Sodium Sulfide in Pulp and Paper — What Actually Matters
Sodium sulfide is one of the two chemicals that define chemical pulping as it is practised worldwide. By supplying hydrosulfide ions to the white liquor, it turns a slow and indiscriminate alkaline cook into a fast and selective delignification process — which is where the yield and the strength that made Kraft pulp the industry standard actually come from.
For a procurement team, three points carry the practical weight. Quality parameters — assay basis, iron content and dead-load salts — have real process consequences, not just contractual ones. Makeup requirements follow from the mill's own sodium and sulfur balance, never from a generic dosage figure. And handling demands disciplined attention to alkalinity and hydrogen sulfide risk, governed by the Safety Data Sheet for the specific grade delivered.
If you are sourcing Na₂S — tell us the grade, the assay basis, the iron limit, the tonnage, the port and the Incoterm, and we will come back with a dated offer, a recent Certificate of Analysis and the Safety Data Sheet within 24 business hours. If Na₂S is not the right makeup chemical for your balance, we will say so — start the enquiry here.
