NaOH (Caustic Soda) in Kraft Paper Production — White Liquor Cooking Process & Standard Consumption

TL;DR: NaOH (sodium hydroxide) is the main component of white liquor used in Kraft pulping, combined with Na₂S at a 3:1 to 4:1 ratio, temperature 160–170°C. NaOH consumption ranges from 300–500 kg/ton of pulp depending on wood type and technology. This article compares NaOH 32%, 45%, and 50% for paper mills, analyzing the pros and cons of each concentration to help procurement choose the right grade.

What Is NaOH and Its Role in Kraft Paper Production?

NaOH (sodium hydroxide, CAS 1310-73-2), also known as caustic soda, is a strong alkali chemical used as the primary cooking agent in the Kraft process (sulfate process) — the pulping method that accounts for over 80% of global pulp production.

In the Kraft process, NaOH performs two key roles:

• Breaking down lignin structure — NaOH combined with Na₂S hydrolyzes ether bonds in lignin, releasing intact cellulose fibers
• Maintaining alkalinity — keeps pH >12 throughout the cooking cycle, preventing dissolved lignin from re-precipitating

Without NaOH, the Kraft process cannot separate lignin from cellulose fibers at industrial scale. This is why paper mills nationwide — from Phu My Industrial Park (Ba Ria-Vung Tau) to Tra Noc Industrial Park (Can Tho) — all maintain long-term NaOH supply contracts.

White Liquor Cooking Process: NaOH + Na₂S

Cooking Liquor Composition

White liquor is the standard cooking liquor in the Kraft process, consisting of two main active components:

Recommended NaOH:Na₂S ratio: 3:1 to 4:1 (by active weight). This ratio ensures sufficient alkalinity to dissolve lignin while Na₂S protects cellulose fibers from alkaline degradation.

Standard Operating Parameters

• Cooking temperature: 160–170°C (heated by high-pressure steam)
• Pressure: 7–10 bar (in closed digesters)
• Residence time: 2–4 hours depending on wood type (hardwood faster than softwood)
• Liquor-to-wood ratio: 3:1 to 5:1 (L liquor/kg dry wood)
• Effective alkali charge: 10–15% based on oven-dry wood weight

Chemical Reactions

Main reactions in the digester:

1. Lignin cleavage: NaOH + Na₂S attack β-O-4 bonds in lignin → lignin fragments dissolve into the liquor
2. Cellulose release: Cellulose fibers are released intact, preserving fiber length
3. Acid neutralization: NaOH neutralizes acetic and uronic acids generated from hemicellulose

After cooking, the mixture consists of pulp + black liquor (containing dissolved lignin and residual chemicals). Black liquor is sent to the recovery boiler for combustion — recovering heat and regenerating chemicals. Ca(OH)₂ is added to convert Na₂CO₃ into fresh NaOH, regenerating white liquor for the next cooking cycle. This closed-loop system enables paper mills to recover 90–95% of cooking chemicals.

NaOH Consumption: 300–500 kg/Ton of Pulp

NaOH consumption depends on three main factors:

1. Wood Raw Material Type

2. Cooking Technology

• Batch digester: Higher consumption due to difficulty controlling temperature — 380–500 kg NaOH/ton
• Continuous digester: Lower consumption through optimization — 300–400 kg NaOH/ton
• Modified Kraft (RDH, Lo-Solids): 280–350 kg NaOH/ton — newest and most efficient technology

3. Desired Pulp Quality

• Unbleached Kraft: Kappa number 30–50 → lower NaOH
• Bleached Kraft: Kappa number 15–25 → higher NaOH, requires deeper cooking

Note for procurement: When signing NaOH supply contracts for paper mills, clearly specify the wood type, cooking technology, and output pulp quality to budget the correct volume. Large-volume supply with batch COA/MSDS commitment — no “short delivery” or “concentration variance” between shipments.

Comparing NaOH 32% vs 45% vs 50% for Paper Mills

Choosing the right NaOH concentration directly affects logistics costs, operations, and cooking efficiency. Below is a detailed comparison:

Recommendations by Mill Scale

Small mills (≤ 50 tons pulp/day):

• NaOH 32% is the optimal choice — easy to pump, little or no tank heating needed in warm climates, no dilution equipment investment required. Small mills typically lack steam heating for storage tanks; NaOH 32% suits them where winter temperatures stay well above its crystallization point (about 5°C; a 31–33% batch can start anywhere from about 2 to 8°C), and in colder locations tanks and piping should be insulated. Reference figures follow the NaOH–H₂O phase diagram and technical literature from caustic soda producers; the actual crystallization point depends on each batch’s concentration per its COA/TDS.
• Consumption: ~3.1 tons of 32% solution/ton pure NaOH.

Medium mills (50–200 tons pulp/day):

• NaOH 45% is the balanced choice — saves 29% on shipping costs compared to 32%, while its crystallization point (about 6–7°C, close to that of NaOH 32%) remains manageable at Southern industrial parks. One mill at Long Hau Industrial Park (Long An) cut annual shipping costs by about 29% after switching from NaOH 32% to 45%, while investing in a 50m³ insulated stainless steel tank.
• Consumption: ~2.2 tons of 45% solution/ton pure NaOH.

Large mills (> 200 tons pulp/day):

• NaOH 50% is the most logistics-optimal choice — only 2 tons of solution/ton pure NaOH, minimizing tanker truck movements in and out of the mill. However, it requires an insulated stainless steel storage tank (typically maintained at 15–20°C to prevent crystallization — follow the guidance of the producer or your system designer).
• Consumption: ~2.0 tons of 50% solution/ton pure NaOH.
• Case study: a large paper mill at Phu My Industrial Park (BR-VT) uses NaOH 50%, receiving 30-ton tanker loads, stored in two 100m³ stainless steel tanks with heating jackets.

Storage and Transport of NaOH for Paper Mills

Storage Tanks

• NaOH 32%: Carbon steel or HDPE tank, insulation recommended in Northern winter (crystallization point close to that of 45%)
• NaOH 45%: Stainless steel 304 or HDPE tank, insulation recommended (Northern winter)
• NaOH 50%: Stainless steel 316 or 304 tank with heating jacket or coil, maintain ≥15°C

Transport

• Tanker trucks 5–30 tons — contact the hotline 0979 891 929 to discuss suitable transport for cold-season deliveries
• Requirements: safety valves, sealed caps, hazardous chemical placards (UN 1824)
• Delivery time: Southern and western industrial parks 24–48 hours, northern and central Vietnam 2–3 days

Safety

• Mandatory PPE when handling: safety goggles, nitrile gloves, PVC apron, rubber boots
• Eyewash station and emergency shower within a 10m radius
• Neutralize spills with dilute acid (HCl 5% or CH₃COOH) before disposal

QC Department and Documentation

Each batch of NaOH supplied to paper mills includes:

• COA (Certificate of Analysis): specifying NaOH content, density at 20°C, impurity levels (NaCl, Na₂CO₃, Fe₂O₃) — tested by QC Technician Phan Cam Thuy
• MSDS (Material Safety Data Sheet): complete per GHS standards, current year update
• Batch records: documents for each batch are controlled by Vo Thi Nhu Hoa, Loc Thien QC Department

Each batch comes with a COA and MSDS; documents can be provided on request for customer audits. Loc Thien’s NaOH holds an official ZDHC MRSL Version 3.1 Conformance Level 1 Certificate issued by TÜV SÜD (#ZDHC1 127736 0001).

Frequently Asked Questions

Should a small paper mill use NaOH 32% or 50%?

NaOH 32% is more suitable for small mills because it needs far less tank heating than 50% in milder regions (in cold Northern winters keep tank and piping temperatures away from the crystallization point of about 5°C; a 31–33% batch can start at about 2–8°C), is easy to pump and operate, and needs no dilution before cooking.

Can NaOH consumption in the Kraft process be reduced?

Yes. Adopting continuous digester technology or Modified Kraft (RDH, Lo-Solids) can reduce consumption from 500 down to 300–350 kg NaOH/ton of pulp.

Does NaOH affect cellulose fiber quality?

If the NaOH:Na₂S ratio is correct (3:1 to 4:1) and cooking temperature does not exceed 170°C, cellulose fibers remain intact. Na₂S is the key factor protecting cellulose from alkaline degradation.

Does a paper mill need COA for each NaOH delivery?

Yes. QC departments require batch COA to control input quality. Each batch of NaOH from Loc Thien comes with COA and MSDS.

What is the NaOH price and how to order?

Contact hotline 0979 891 929 for updated pricing based on volume and concentration. Tanker delivery 5–30 tons to industrial parks nationwide.

Chemical Recovery Process in Paper Mills — Optimizing NaOH Consumption

A major advantage of the Kraft process over other pulping methods is the ability to recover and regenerate cooking chemicals. The recovery cycle enables paper mills to save up to 90–95% of the NaOH and Na₂S used.

Chemical Recovery Cycle

1. Black liquor from the digester contains dissolved lignin + residual chemicals (NaOH, Na₂S, Na₂CO₃, Na₂SO₄)
2. Concentration in multiple-effect evaporators — from 15% solids up to 65–80%
3. Combustion in the recovery boiler — lignin burns generating heat (producing high-pressure steam for power generation), Na₂SO₄ is reduced to Na₂S
4. Green liquor — bottom ash dissolved in water to form Na₂CO₃ + Na₂S solution
5. Causticizing — adding Ca(OH)₂ to green liquor: Na₂CO₃ + Ca(OH)₂ → 2 NaOH + CaCO₃↓
6. Fresh white liquor — NaOH + Na₂S regenerated; CaCO₃ (lime mud) is calcined into CaO (lime) and reused

A paper mill at Bac Ninh Industrial Park saved 22% on annual chemical costs after upgrading its evaporator system and strictly controlling the NaOH:Na₂S ratio at 3.5:1. With a capacity of 150 tons of pulp/day, this 22% reduction represents a major saving on the annual chemical budget.

White Liquor Quality Control

Paper mill QC departments typically check 5 key white liquor parameters each production shift:

Note: If the sulfidity index exceeds 35%, the mill should consider supplementing Na₂SO₄ or adjusting the reduction process in the recovery boiler.

Conclusion

NaOH plays a vital role in Kraft paper production — from pulping and chemical recovery to wastewater treatment. Choosing the right NaOH concentration (32%, 45%, or 50%) and a reliable supply partner helps paper mills optimize operating costs, ensure pulp quality, and meet export standards.

With 6 warehouses nationwide (Ba Ria – Vung Tau, Dong Nai, Binh Duong, Da Nang, Can Tho and Bac Ninh) and a fleet of tanker trucks (5–30 tons), Loc Thien Chemical meets all NaOH supply needs for paper mills — with delivery within 4 hours in inner HCMC and 24–48 hours to Southern and Western industrial parks.

Request a Quote — Contact

Need NaOH for your paper mill? Get a quote for NaOH 32%, 45%, 50% — batch COA/MSDS, delivered to industrial parks nationwide, tanker 5–30 tons. Hotline 0979 891 929.

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