Caustic Soda in the Pulp and Paper Industry: Kraft Pulping, Bleaching, Deinking and Bulk Supply

Caustic soda in the pulp and paper industry

How Is Caustic Soda Used in the Pulp and Paper Industry?

Caustic soda in the pulp and paper industry is the main alkali of the mill. Sodium hydroxide (NaOH) cooks wood chips as part of kraft white liquor, cooks straw and bagasse on its own in soda pulping, dissolves oxidised lignin in the oxygen, extraction and peroxide bleaching stages, and swells fibres to release ink when recovered paper is deinked. Kraft mills regenerate most cooking alkali in the recovery cycle, so purchased caustic soda goes mainly to sodium make-up and the bleach plant.

Chemical
Sodium hydroxide, NaOH
CAS
1310-73-2
Average use
≈ 35 kg per t market pulp[3]
Main users
Kraft fibre line and bleach plant
Grade mills specify
Membrane, low chloride and iron
Forms we supply
Flakes · pearls · liquid
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Mill map

7 Uses of Caustic Soda in the Pulp and Paper Industry

Caustic soda in the pulp and paper industry touches almost every department of an integrated mill, but in very different amounts. ResourceWise estimates that chemical pulp accounts for about 80% of caustic soda consumed across pulp grades, and that kraft takes over 90% of the chemical-pulp share[3].

Uses of caustic soda in pulp and paper production
Mill areaWhat NaOH doesWhy the mill buys it
Kraft digesterPart of white liquor with sodium sulfide; dissolves ligninMake-up of sodium lost from the recovery cycle
Soda pulping (non-wood)The cooking chemical on its own for straw, bagasse and bambooMain cooking chemical
Oxygen delignificationProvides the caustic environment, about 1.5–3.5% NaOH on pulp[2]Bleach plant alkali
Extraction (E, EO, EOP)Dissolves lignin fragments oxidised in the previous stage[2]Bleach plant alkali
Peroxide stage (P)Forms the perhydroxyl anion that brightens the pulp[2]Bleach plant alkali
Deinking of recovered paperSwells fibres so ink detaches in the pulper[5]Recycled-fibre line
Wet end, effluent and cleaningpH adjustment, acid effluent neutralisation, equipment boil-outUtilities and environmental compliance
Chemical pulping

Caustic Soda in Kraft Pulping: White Liquor and the Recovery Cycle

The kraft process accounts for over 80% of the chemical pulp produced in the United States[1]. Its cooking liquor, white liquor, is a water solution of sodium hydroxide and sodium sulfide[1].

  1. CookingWood chips are cooked in white liquor in the digester. Sodium hydroxide dissolves the lignin that binds the fibres; sodium sulfide speeds delignification and protects cellulose, which is why kraft pulp is strong.
  2. Black liquorThe spent liquor, now carrying dissolved lignin, is washed out of the pulp, concentrated in evaporators and burned in a recovery boiler.
  3. Green liquorThe smelt from the boiler is dissolved to make green liquor.
  4. CausticisingQuicklime is added in a causticising tank to turn green liquor back into white liquor; the lime mud is calcined in a lime kiln to regenerate quicklime[1].
  5. Make-upThe cycle is not closed: sodium leaves with the pulp, wash water and effluent. Purchased caustic soda replaces the alkali that is lost, and sodium sulfide or sodium sulfate replaces lost sulfur.

What this means for buying. A kraft mill’s caustic soda demand is set less by digester size than by its sodium losses and its bleach plant. Two mills of the same capacity can buy very different tonnages, so a supply contract should be sized from the mill’s own consumption records. On the sulfur side of the liquor, see sodium sulfide in the pulp and paper industry.

Kraft, soda and recycled fibre compared

Role of caustic soda in each fibre route
Fibre routeCooking chemicalsTypical raw materialRole of caustic soda
Kraft (sulfate)NaOH + Na₂S (white liquor)Softwood and hardwoodMake-up alkali, then bleaching
SodaNaOH aloneWheat and rice straw, bagasse, bambooPrincipal cooking chemical
Recovered paperNo cookingOld newsprint, magazines, office paperAlkali for deinking and peroxide brightening
Bleach plant

Caustic Soda in Pulp Bleaching: O, E and P Stages

Modern elemental-chlorine-free (ECF) bleach plants run sequences such as OD(EO)DD or D(EOP)DD[2]. Chlorine dioxide (D) stages are acidic; the stages between them depend on caustic soda.

Bleaching stages that use sodium hydroxide, with typical conditions from Kirk-Othmer
StageWhat NaOH doesTypical conditions
O — oxygen delignificationSupplies the caustic environment in which oxygen removes ligninAbout 1.5–3.5% NaOH and 2.0–2.4% O₂ on pulp
E — alkaline extractionDissolves oxidised lignin fragments so they diffuse out of the fibre; neutralises organic acids1–2 h at 60–90 °C, 10–12% consistency, terminal pH 10.5 or higher at 25 °C
EO / EOP — reinforced extractionExtraction reinforced with oxygen, or oxygen and peroxideE stage alkali plus O₂ and/or H₂O₂
P — peroxideIonises hydrogen peroxide to the perhydroxyl anion, the main brightening speciesAlkali charged with the peroxide

Source for all stage data: Hart, “Pulp Bleaching”, Kirk-Othmer Encyclopedia of Chemical Technology, 2019[2]. Each mill sets its own charges.

The E-stage rule of thumb. Kirk-Othmer gives the optimum sodium hydroxide charge, as per cent on pulp, as roughly 0.085–0.11 × the entering kappa number for softwood and about 0.075 × for hardwood[2]. The second tool in the calculator below applies it. Hydrogen peroxide for P and EOP stages: hydrogen peroxide supply.

Recycled fibre

Caustic Soda in Deinking Recycled Paper

Conventional alkaline deinking uses three chemicals together in the pulper[5]:

Alkali

Caustic soda

Swells the fibres so the ink detaches from them. Sodium hydroxide is the usual alkali.

Brightness

Hydrogen peroxide

Prevents the yellowing (alkali darkening) that alkali alone would cause.

Buffer

Sodium silicate

Buffers the system, stabilises the peroxide, collects ink and stops it settling back onto fibres.

The detached ink is then removed by flotation and washing. Because alkali also darkens mechanical fibre, the caustic charge in deinking is balanced against the peroxide charge rather than maximised.

Planning tool

How Much Caustic Soda Does a Pulp Mill Need?

Two quick estimates for sizing an enquiry. Both run in your browser only; replace the defaults with your mill’s own figures.

1 · Annual caustic soda demand

The default 35 kg/t is the ResourceWise average for market pulp[3]. Use your own consumption per tonne if you know it.

NaOH, 100% basis
10,500 t/yr
As flakes 98%
10,714 t/yr≈ 429 × 20ft FCL
As liquid 50%
21,000 t/yr≈ 955 ISO tanks
Per month, flakes
893 t≈ 36 × 20ft FCL

2 · E-stage sodium hydroxide charge

Rule of thumb from Kirk-Othmer[2]: charge (% on pulp) ≈ factor × entering kappa number.

NaOH charge
1.7–2.2%on oven-dry pulp
Per tonne of pulp
17–22 kg

Estimates only. Container figures assume about 25 t of bagged flakes per 20ft and about 22 t of solution per ISO tank.

Quality

Caustic Soda Specification for Paper Mills: What to Check on the COA

Buyers of caustic soda in the pulp and paper industry judge a lot on four lines of the certificate of analysis. The values in the last column come from our published technical data sheets; the COA issued with your lot governs.

Certificate of analysis lines that matter to pulp and paper mills
COA lineWhy the mill caresOur TDS values
NaOH assaySets the tonnage actually bought as alkaliFlakes 98% ± 1% · pearls 99% min · liquid 48% min
Sodium chloride (NaCl)Chloride accumulates in the kraft liquor cycleFlakes 0.0085% · pearls 0.05% max · liquid 0.03% max
Iron and transition metalsDecompose hydrogen peroxide in P and EOP stages and cost brightnessFlakes Fe 10 ppm · liquid Fe 10 mg/kg max · pearls Fe₂O₃ 50 ppm max
Sodium carbonate (Na₂CO₃)Inactive load in the liquorFlakes 0.43% · pearls 0.8% max · liquid 0.5% max
Production routeMercury-free, low-salt productMembrane cell only

How to test a delivery yourself: how to check caustic soda quality.

Form choice

Caustic Soda Flakes or Liquid for a Paper Mill?

The chemistry is identical; the logistics are not. A 50% solution carries one tonne of water with every tonne of NaOH, so the right form depends on how far the mill is from the source and how it stores alkali.

Caustic soda forms compared for pulp and paper mills
CriterionFlakes 98–99%Pearls 99%Liquid 32–50%
Best forMills far from the source, buying by the containerAutomated make-down systems, low-dust handlingMills with bulk tanks near a producer or port
On-site workDissolving station; heat releaseDissolving, faster and more evenPump straight to the day tank
Packing25 kg bags, palletised or in jumbo bags25 kg bags, palletised or in jumbo bagsISO tank
NaOH per 20ft unit≈ 25 t≈ 25 t≈ 11 t in a 50% ISO tank
Cold climateKeep dryKeep dry50% solution crystallises near 12 °C; heat tanks and lines

Full comparison: caustic soda flakes vs liquid caustic soda. Storage and dissolving: how to store caustic soda safely.

Supply for mills

Caustic Soda Supplier for the Pulp and Paper Industry

SUHA International Trading is a caustic soda supplier and exporter, not a producer. We supply membrane-grade caustic soda flakes, pearls and liquid caustic soda to pulp, paper, board and tissue mills, recycled-fibre plants and non-wood pulp mills.

Who we serve best

Container-scale mills

Non-wood and recycled-fibre mills, tissue and board plants, and mills that buy solid caustic by the container.

Second source

Back-up supply

Large integrated mills that buy liquid caustic by vessel from a producer can hold us as a second source for flakes or ISO-tank liquid during outages and shipping disruption.

One order cycle

Other mill chemicals

Caustic soda can ship with sodium sulfide, hydrogen peroxide and soda ash for the paper industry.

Supply terms for caustic soda to pulp and paper mills
ItemDetail
GradesFlakes 98–99% · pearls 99% · liquid 32–50% · membrane grade only
Minimum orderOne full container load, 20ft or 40ft · one ISO tank for liquid
Packing25 kg PP bags with PE liner, palletised or inside jumbo bags · ISO tank
LoadingMersin, Türkiye · Jebel Ali, UAE · by truck from Türkiye
TermsFOB · CFR · CIF · payment by T/T or L/C at sight
DocumentsCOA per lot, SDS, invoice, packing list, B/L or CMR, certificate of origin, Class 8 dangerous-goods declaration

Send these six facts for a firm quotation within 24 hours

  1. Form and grade: flakes, pearls or liquid at a stated strength
  2. Use in the mill: make-up, bleach plant, deinking or utilities
  3. Quantity: tonnes per shipment and per year
  4. Packing: 25 kg bags palletised, in jumbo bags, or ISO tank
  5. Destination and Incoterm: port or mill address; FOB, CFR or CIF
  6. COA limits: any chloride, iron or carbonate limits in your specification
Send an enquiry WhatsApp info@causticsodaco.com

Indicative prices are kept on the caustic soda price and caustic soda flakes price pages. Overview of every grade: caustic soda supplier.

Handling

Handling Caustic Soda Safely in a Mill

Dissolving

Always add caustic to water, never water to caustic: dissolving releases strong heat. Use a dissolving tank with agitation and cooling.

Protection

Splash goggles and face shield, alkali-resistant gloves, apron and boots; eyewash and safety shower close to make-down and unloading points.

First aid

On skin or eyes, flush with running water for at least 30 minutes and get medical care. Read the safety data sheet before handling.

Common questions

Caustic Soda in the Pulp and Paper Industry: FAQ

Caustic soda in the pulp and paper industry is used in four main places: as part of the white liquor that cooks wood chips in kraft pulping, as the only cooking chemical in soda pulping of straw, bagasse and other non-wood fibres, as the alkali in oxygen, extraction and peroxide bleaching stages, and as the alkali that swells fibres and releases ink when recovered paper is deinked. Smaller amounts go to pH control, effluent neutralisation and equipment cleaning.

White liquor is the cooking liquor of the kraft process: a water solution of sodium hydroxide and sodium sulfide. In the digester it dissolves the lignin that binds wood fibres together, leaving cellulose pulp. The spent liquor, called black liquor, is burned in a recovery boiler and the resulting green liquor is causticised with lime to make white liquor again.

The recovery cycle regenerates most of the cooking alkali, but not all of it. Sodium is lost with the pulp, in washing and in effluent, and the bleach plant needs its own alkali for oxygen delignification, extraction and peroxide stages. Mills therefore buy caustic soda to make up sodium losses and to run the bleach plant, and many also use it for pH control and cleaning.

An industry analysis by ResourceWise puts the average at approximately 35 kg of caustic soda per tonne of market pulp. The real figure for a given mill depends on whether the pulp is bleached, the bleaching sequence, the wood species, the efficiency of the recovery cycle and how much caustic is used outside the fibre line, so a mill should size its contract from its own consumption records.

In an extraction stage, sodium hydroxide dissolves the lignin fragments that the previous chlorine dioxide stage has oxidised, so they can be washed out of the fibre. In oxygen delignification the reaction runs in a caustic environment, typically at about 1.5 to 3.5 per cent NaOH on pulp. In peroxide stages sodium hydroxide ionises hydrogen peroxide to the perhydroxyl anion, which is the main brightening species.

Kirk-Othmer gives the optimum sodium hydroxide charge, as per cent on pulp, as roughly 0.085 to 0.11 times the entering kappa number for softwood and about 0.075 times for hardwood. Typical E stage conditions are 1 to 2 hours at 60 to 90 °C and 10 to 12 per cent consistency, with a terminal pH of 10.5 or higher measured at 25 °C.

Soda pulping is a chemical pulping process that cooks the raw material in sodium hydroxide alone, without the sodium sulfide used in kraft pulping. It is used mainly for non-wood fibres such as wheat straw, rice straw, bagasse and bamboo, where caustic soda is the principal cooking chemical rather than a make-up chemical.

In alkaline deinking the alkali swells the fibres so that ink detaches from them in the pulper. Hydrogen peroxide is added to prevent the yellowing that alkali alone causes, and sodium silicate buffers the system, stabilises the peroxide and helps stop ink particles settling back onto the fibres before flotation removes them. Sodium hydroxide is the usual alkali in this system.

Liquid caustic soda suits mills with bulk tank storage that are close to a producer or a port, because it needs no dissolving. Flakes or pearls suit mills far from the source or buying by the container, because a 50 per cent solution ships one tonne of water with every tonne of alkali. Compare the cost per tonne of NaOH delivered to the mill, not the cost per tonne of product.

Most mills specify membrane-grade sodium hydroxide and check four lines on the certificate of analysis: the NaOH assay, sodium chloride, because chloride builds up in the kraft liquor cycle, iron and other transition metals, because they decompose hydrogen peroxide in bleaching, and sodium carbonate, which is dead load in the liquor. Mercury-free origin is standard.

Both are in kraft white liquor but they do different jobs. Sodium hydroxide supplies the alkali that dissolves lignin, while sodium sulfide speeds up delignification and protects the cellulose, which gives kraft pulp its strength. Soda pulping uses sodium hydroxide alone. Mills buy sodium sulfide or sodium sulfate to make up sulfur losses and caustic soda to make up alkali losses.

Yes. SUHA International Trading supplies membrane-grade caustic soda flakes 98 to 99 per cent, pearls 99 per cent and liquid caustic soda 32 to 50 per cent to paper mills. The minimum order is one full container load, 20ft or 40ft, or one ISO tank for liquid. Solids are packed in 25 kg bags, palletised or inside jumbo bags. We ship from Mersin and Jebel Ali and by truck from Türkiye, on FOB, CFR or CIF terms, with payment by T/T or L/C at sight and a certificate of analysis for every lot.

No price is published on this page, because the price for a mill depends on grade, form, packing, volume, loading port and destination. Current indicative levels are on the caustic soda price and caustic soda flakes price pages, and a firm FOB, CFR or CIF quotation is sent within 24 hours of a complete enquiry.