NaOH (Caustic Soda) in Kraft Paper Production — White Liquor Cooking Process & Standard Consumption | Hóa Chất Lộc Thiên
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 Phú Mỹ Industrial Park (Bà Rịa-Vũng Tàu) to Trà Nóc Industrial Park (Cần Thơ) — 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:
| Component | Content (g/L as Na₂O) | Role |
|---|---|---|
| NaOH | 81–120 | Alkalization, lignin breakdown |
| Na₂S | 30–40 | Accelerates lignin removal, protects cellulose |
| Na₂CO₃ | 11–44 | Dead load (does not participate in cooking reactions) |
| Na₂SO₄ | 2–7 | Dead load, residual from recovery boiler |
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:
- Lignin cleavage: NaOH + Na₂S attack β-O-4 bonds in lignin → lignin fragments dissolve into the liquor
- Cellulose release: Cellulose fibers are released intact, preserving fiber length
- 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
| Wood Type | NaOH Consumption (kg/ton pulp) | Characteristics |
|---|---|---|
| Softwood (pine, eucalyptus) | 400–500 | Lignin content 25–30%, harder to cook |
| Hardwood (acacia, Australian eucalyptus) | 300–400 | Lignin 20–25%, easier to cook |
| Bamboo, rice straw | 350–450 | Short fiber structure, medium consumption |
| Recycled pulp (deinking) | 20–50 | Only NaOH for deinking, no cooking |
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:
| Parameter | NaOH 32% | NaOH 45% | NaOH 50% |
|---|---|---|---|
| Density (20°C) | 1.348 | 1.478 | 1.52 |
| NaOH content (g/L) | ~432 | ~665 | ~760 |
| Shipping weight/ton pure NaOH | 3.13 tons | 2.22 tons | 2.0 tons |
| Freezing point | -5°C | 5°C | 12°C |
| Transport cost | Highest | 29% lower | Lowest |
| Storage tank requirement | Standard steel | Stainless steel/PE | Stainless steel/PE (with heating) |
| Viscosity (pump operation) | Easy to pump | Moderate | High (requires metering pump) |
| Dilution before cooking | Little or none | Required | Extensive dilution required |
| Unit cost per pure NaOH | Lowest (pre-diluted) | Moderate | Highest (concentration cost) |
Recommendations by Mill Scale
Small mills (≤ 50 tons pulp/day):
- NaOH 32% is the optimal choice — easy to pump, no tank heating needed, no dilution equipment investment required. Small mills typically lack steam heating for storage tanks, making NaOH 32% with its -5°C freezing point very suitable.
- 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 the 5°C freezing point remains manageable at Southern industrial parks. One mill at Long Hậu Industrial Park (Long An) saved 180 million VND/year in shipping costs 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 (maintained at 15–20°C to prevent crystallization).
- Consumption: ~2.0 tons of 50% solution/ton pure NaOH.
- Case study: a large paper mill at Phú Mỹ 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, no heating required
- 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, stainless steel body, insulated
- Requirements: safety valves, sealed caps, hazardous chemical placards (UN 1824)
- Delivery time: Southern industrial parks 4–24 hours, Northern/Bắc Ninh as per order schedule
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 — ZDHC Level 1
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 Cẩm Thủy
- MSDS (Material Safety Data Sheet): complete per GHS standards, current year update
- ZDHC MRSL Level 1 (TÜV Rheinland): certifying the chemical contains no prohibited substances in the textile and paper export supply chain — verified by Võ Thị Như Hòa, Lộc Thiên QC Department
ZDHC Level 1 certification is mandatory for paper mills exporting to the EU and North America. Hóa Chất Lộc Thiên is one of the few NaOH suppliers in Vietnam with ZDHC MRSL Level 1 issued by TÜV Rheinland, ensuring international-standard inputs for export Kraft paper production.
Frequently Asked Questions
Should a small paper mill use NaOH 32% or 50%?
NaOH 32% is more suitable for small mills because it requires no tank heating, 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 Lộc Thiên comes with COA, MSDS, and ZDHC Level 1 certification.
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
- Black liquor from the digester contains dissolved lignin + residual chemicals (NaOH, Na₂S, Na₂CO₃, Na₂SO₄)
- Concentration in multiple-effect evaporators — from 15% solids up to 65–80%
- Combustion in the recovery boiler — lignin burns generating heat (producing high-pressure steam for power generation), Na₂SO₄ is reduced to Na₂S
- Green liquor — bottom ash dissolved in water to form Na₂CO₃ + Na₂S solution
- Causticizing — adding Ca(OH)₂ to green liquor: Na₂CO₃ + Ca(OH)₂ → 2 NaOH + CaCO₃↓
- Fresh white liquor — NaOH + Na₂S regenerated; CaCO₃ (lime mud) is calcined into CaO (lime) and reused
A paper mill at Bắc 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, the savings amounted to approximately 1.2 billion VND/year.
White Liquor Quality Control
Paper mill QC departments typically check 5 key white liquor parameters each production shift:
| Parameter | Method | Standard Range |
|---|---|---|
| Total Titratable Alkali (TTA) | ABC titration | 110–130 g/L (as Na₂O) |
| Active Alkali (AA) | ABC titration | 90–110 g/L (as Na₂O) |
| Effective Alkali (EA) | Calculated from AA - ½ Na₂S | 70–90 g/L |
| Sulfidity | Calculated from titration results | 20–35% |
| Residual NaOH | Phenolphthalein titration | 55–85 g/L |
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 (HCMC, Đồng Nai, Bà Rịa-Vũng Tàu, Cần Thơ, Đà Nẵng, Bắc Ninh) and a fleet of tanker trucks (5–30 tons), Hóa Chất Lộc Thiên meets all NaOH supply needs for paper mills — from emergency delivery within 4 hours in HCMC to delivery within 24 hours for Southern industrial parks.
Request a Quote — Contact
Need NaOH for your paper mill? Get a quote for NaOH 32%, 45%, 50% — batch COA/MSDS/ZDHC, delivered to industrial parks nationwide, tanker 5–30 tons. Hotline 0979 891 929.
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