Sodium Sulfide in Pulp and Paper Production: Its Role in Kraft Pulping

Its contribution must be understood within the complete mill system. Cooking performance, pulp quality, chemical recovery and the balance between sodium and sulfur are closely connected.

For manufacturers considering sodium sulfide supply, the central questions are how the chemical supports the process, which specification is required and whether external addition is justified by the mill’s material balance.

1. Where Is Sodium Sulfide Used in Pulp Production?

Sodium sulfide is used principally in the cooking stage of kraft chemical pulping.

Wood chips are treated with an alkaline solution called white liquor. Its principal active cooking chemicals are sodium hydroxide, NaOH, and sodium sulfide, Na₂S. Other components, including sodium carbonate, are also present.

TAPPI identifies white-liquor composition as an important factor in pulping operations and pulp quality.

The relevant application is chemical pulp production. A paper mill that purchases finished pulp does not necessarily operate a kraft cooking or recovery system and therefore may have no requirement for sodium sulfide.

Identifying the mill’s process is essential before discussing supply.

2. Why Must Lignin Be Removed?

Wood contains cellulose fibers within a structure that also includes hemicelluloses and lignin.

Lignin contributes to the rigidity and cohesion of wood. During chemical pulping, enough lignin must be broken down and dissolved to release usable fibers.

The manufacturing objective is selective treatment: remove the required amount of lignin while preserving sufficient carbohydrate material and fiber quality.

More severe cooking is not automatically better. Extending the treatment can change both delignification and the amount or condition of useful material retained in the pulp.

This makes chemical composition, temperature history and cooking time part of a combined process decision.

3. How Does Sodium Sulfide Support Delignification?

The Role of Dissolved Sulfide Species

In alkaline cooking liquor, sodium sulfide supplies sulfur-containing species, including hydrosulfide ions, HS⁻.

These species participate in reactions that break important linkages in lignin. Research on kraft chemistry describes their involvement in the cleavage of certain aryl-ether structures and subsequent lignin fragmentation.

Fragmentation and dissolution allow lignin-derived material to leave the fiber structure and enter the spent liquor.

This is a more precise description than saying that sodium sulfide simply “dissolves wood.” The intended result is separation of fibers through controlled chemical treatment.

Why Sodium Hydroxide Is Also Needed

Sodium hydroxide provides the strongly alkaline environment required for kraft cooking and participates in the treatment of wood components.

Sodium sulfide adds a different contribution through its sulfur chemistry. The chemicals work together, but they are not interchangeable.

Chemical	Main contribution in kraft cooking
Sodium hydroxide	Provides alkalinity and supports the reactions responsible for delignification
Sodium sulfide	Supplies sulfide species that contribute to lignin breakdown and cooking selectivity
Sodium carbonate	Forms part of the circulating chemical system but is not equivalent to active caustic alkali

TAPPI’s recausticizing guidance identifies sodium hydroxide and sodium sulfide as major white-liquor components and describes their different contributions.

4. What Is Sulfidity?

Sulfidity expresses the proportion of sulfide within a defined alkali basis. It is a liquor-composition parameter, not the purity of a bag of sodium sulfide.

On an active-alkali basis:

Sulfidity (%) = Na₂S ÷ (NaOH + Na₂S) × 100

The chemical quantities must be expressed on the same equivalent basis, conventionally as Na₂O. Actual masses of NaOH and Na₂S should not be inserted into this expression without the appropriate conversion.

TAPPI defines active-alkali-basis sulfidity in this way.

Three different purchasing and process measurements should therefore remain separate:

Measurement	What it describes
Na₂S content of the purchased product	The product’s sodium sulfide assay
Na₂S concentration in liquor	The amount present in a specified volume of solution
Liquor sulfidity	The sulfide proportion relative to the stated alkali basis

Confusing these values can lead to incorrect comparisons between raw materials and mill operating conditions.

5. Does Higher Sulfidity Always Improve Pulping?

No universal sulfidity level produces the best result for every mill.

Research on eucalyptus kraft pulping found that changing sulfidity affected the conditions needed to reach a target kappa number, as well as yield and viscosity. The response depended on the tested conditions, and the benefits were not unlimited.

The appropriate operating range depends on factors such as:

• Wood species and furnish variation.

• Chip preparation and impregnation.

• Alkali charge and liquor composition.

• Cooking temperature and time.

• Target pulp grade.

• Recovery-system performance.

• The mill’s sodium and sulfur balance.

A mill should evaluate a change against both pulp performance and the wider chemical cycle. Increasing sodium sulfide solely to obtain a higher sulfidity number is not a sufficient technical objective.

6. How Should the Effect on Pulp Quality Be Measured?

Kappa Number

Kappa number is commonly used to assess the extent of pulping and remaining oxidizable material.

It is associated with residual lignin, but it is not a direct lignin-percentage measurement. Other components, including hexenuronic acids in relevant pulps, can contribute to the result.

For this reason, a kappa-number change should be interpreted in the context of the furnish and analytical method.

Screened Yield and Rejects

Screened yield measures the usable pulp obtained after screening on the stated basis.

Rejects help identify material that has not been adequately converted into acceptable pulp. Both should be considered when evaluating a cooking change.

Pulp Viscosity and Strength

Viscosity is a useful indicator when assessing changes in cellulose condition, but it does not replace physical performance testing.

The final evaluation should include the strength properties required for the intended pulp or paper grade.

Indicator	Purpose in an evaluation
Kappa number	Track delignification and remaining oxidizable material
Screened yield	Measure usable pulp production
Reject content	Assess incomplete or uneven processing
Pulp viscosity	Help assess cellulose condition
Specified strength tests	Confirm performance for the intended use
Downstream chemical demand	Evaluate consequences for later processing

The objective is a suitable balance of these results, not the lowest possible kappa number in isolation.

7. White Liquor, Black Liquor and Green Liquor

These terms describe different stages of the kraft chemical cycle.

Liquor	Where it occurs	Main significance
White liquor	Supplied to the cooking process	Contains active cooking chemicals
Black liquor	Leaves cooking and washing operations	Contains dissolved wood material and spent inorganic chemicals
Green liquor	Produced by dissolving recovery-boiler smelt	Intermediate liquor used to regenerate white liquor

Understanding this cycle is essential to understanding sodium sulfide demand.

A kraft mill does not normally discard all cooking chemicals after one use and replace them with an equivalent amount of fresh product. Recovery is central to the process.

8. How Is Sodium Sulfide Recovered?

Recovery of Chemicals and Energy

Black liquor is concentrated and processed in the recovery system.

The recovery boiler produces a smelt containing principally sodium carbonate and sodium sulfide, with other components also present. Its chemical reactions include the reduction of sulfate to sulfide.

After the smelt is dissolved, the resulting green liquor proceeds through clarification and recausticizing.

Regeneration of White Liquor

During recausticizing, sodium carbonate is converted into sodium hydroxide. Sodium sulfide is largely carried through this stage into the regenerated white liquor.

Valmet specifically notes that the sulfide generated in the recovery boiler is carried into the final white liquor because it does not undergo the causticizing reaction.

This is why cooking, recovery-boiler operation and white-liquor preparation must be considered together.

9. Why Might a Kraft Mill Purchase Sodium Sulfide?

External chemical requirements depend on losses, operating conditions and the overall sodium–sulfur balance.

A mill may evaluate an external sulfide source when its technical assessment identifies a need to adjust chemical inventory or composition. However, sodium sulfide is not automatically the preferred make-up chemical.

Valmet describes several methods used to manage mill chemical balance, including adjustments to sulfur-rich streams and white-liquor oxidation. A TAPPI mill case study also shows that alternative sulfur make-up sources can be selected following a site-specific assessment.

For purchasing discussions, establish:

• Whether the site operates kraft pulping.

• Why an external chemical is required.

• Whether sodium, sulfur or both need adjustment.

• Which addition route the mill has approved.

• What specification and handling form the system accepts.

The importance of sodium sulfide in kraft chemistry should not be mistaken for a universal requirement to purchase commercial flakes.

10. Is Sodium Sulfide a Bleaching Chemical?

Its principal role in this article is kraft cooking and delignification.

A mill should not infer that untreated sodium sulfide can be added directly to any bleaching stage. Later stages have their own chemical requirements.

For example, Valmet describes white-liquor oxidation systems that convert sulfide into oxidized forms before the liquor is used as an alkali source in appropriate oxygen-delignification or bleaching applications.

The distinction is important: using a modified liquor in a later stage is different from using ordinary sulfide-containing cooking liquor without modification.

11. Sodium Sulfide and Polysulfide Cooking

Some mills use modified cooking liquors containing polysulfides.

ANDRITZ describes technologies that convert sulfide in white liquor into polysulfide as part of an approach intended to improve cooking yield.

This is a distinct process modification. It does not mean that adding more sodium sulfide automatically reproduces the benefits of polysulfide cooking.

The mill must evaluate the intended liquor composition, production technology and cooking strategy as an integrated system.

12. Selecting Commercial Sodium Sulfide for Mill Use

NBWCHEM supplies yellow and red sodium sulfide flakes. Their suitability for a particular mill should be established against the mill’s raw-material specification.

Parameter	Purchasing relevance
Na₂S content	Establishes the active-material input
Water content, where specified	Supports preparation and material-balance calculations
Water-insoluble matter	Helps assess additional residue and separation requirements
Other sodium and sulfur compounds	Helps characterize the chemical input to the mill
Relevant metal and mineral impurities	Supports the mill’s assessment of non-process inputs
Batch consistency	Improves predictability of supply
Packaging condition	Helps protect against moisture exposure and contamination

Color alone does not establish chemical suitability. Neither “yellow flakes” nor “red flakes” replaces an agreed assay and impurity specification.

The effect of impurities should be evaluated within the circulating system, particularly where the mill has limited opportunities to remove them.

13. How Should a New Supply Grade Be Evaluated?

A useful qualification process links the purchased material to the mill’s operating and pulp-quality requirements.

Review the Specification

Confirm the Na₂S assay, reporting basis, impurity limits and analytical methods.

Check the Chemical Balance

Calculate how the proposed material affects sodium, sulfur, water and other relevant inputs.

Use Comparable Testing Conditions

When comparing grades, account for differences in active content. Equal masses of two products may not provide the same amount of Na₂S.

Evaluate the Complete Result

Assess liquor composition, preparation behavior, cooking performance and relevant pulp properties.

For production trials, retain batch records and review the recovery implications as well as the immediate cooking result.

A supplier change should be justified by consistent performance against the agreed specification.

14. Common Problems and Investigation Priorities

Observation	What to review
Unexpected kappa variation	Furnish, chip condition, impregnation, liquor analysis and cooking history
Increased rejects	Chip distribution, chemical penetration and cooking uniformity
Reduced viscosity or strength	Overall cooking severity and subsequent processing
Unstable sulfidity	Analytical basis, sampling, recovery performance and make-up inputs
Increased preparation residue	Product insolubles, contamination and preparation conditions
Rising chemical cost	Recovery losses, material balance, purchasing basis and operating changes

These are investigation priorities rather than automatic diagnoses.

An unfavorable pulp result does not, by itself, prove that purchased sodium sulfide is the cause.

15. Safety, Storage and Sulfur-Gas Management

Commercial sodium sulfide is corrosive, and contact with acids can release toxic hydrogen sulfide. Keep it sealed and follow the product SDS for storage and handling.

Kraft mills also manage sulfur-containing process gases through dedicated collection and treatment systems. Equipment suppliers describe non-condensable-gas handling as part of mill odor and sulfur management.

Changes to chemical inputs should be reviewed for their effect on the existing process and gas-handling arrangements.

Hydrogen sulfide must not be assessed by smell alone. Appropriate monitoring and trained response are required wherever exposure is possible.

16. Sodium Sulfide Supply for Pulp Manufacturers

Gansu Nabowang Chemical Products Co., Ltd. (NBWCHEM) manufactures sodium sulfide in Gansu, China, and supplies yellow and red flakes for industrial customers.

Our minimum order quantity is 1 metric ton. Third-party inspection can be arranged before shipment, and export documentation can include customs declarations and certificates of origin.

For a pulp-mill inquiry, provide your required Na₂S content, impurity limits, packaging, quantity and destination. Include the intended use and the specification of any currently approved external supply.

We recommend confirming both product suitability and the mill’s chemical-balance requirement before introducing a new grade.

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