Coal-water slurry (CWS) is a pumpable fuel made of fine coal in water. To pump it at high coal content, the slurry needs a dispersant that lowers its viscosity and keeps the coal suspended. Sodium lignosulfonate is one of the dispersants used for this, either alone or blended with naphthalene-based products.
How Sodium Lignosulfonate Lowers Slurry Viscosity
The lignosulfonate adsorbs on the coal surface. Its sulfonic and carboxyl groups give the particles a negative charge (measured as zeta potential), and the adsorbed layer keeps them apart, so water is released and the slurry flows more easily.
A study by Yang, Qiu, Zhou and Lou (2007) fractionated sodium lignosulfonate into five molecular-weight fractions by ultrafiltration and dialysis. It found that the amount adsorbed on coal and the zeta potential had the largest effect on viscosity, and that the 10,000 – 30,000 molecular-weight fraction gave the largest viscosity reduction in their tests. Results depend on the coal, so treat this as a guide for screening, not a specification.
Choosing a Grade
| Option | When to consider it |
|---|---|
| Grade One powder (GAC-NaLS-1) | General starting point; the TDS lists it as a dispersant for insoluble solids and a binder for coal |
| Liquid Grade Two (GAC-NaLS-L-2) | When dosing from tanks; alkaline pH 9.0 – 13.0 |
| Blend with naphthalene sulfonate | When lignosulfonate alone does not reach the target viscosity at your coal loading |
How to Test a CWS Dispersant
- Prepare slurries at your target coal content with two or three dispersant dosages.
- Measure apparent viscosity with a rotational viscometer at a fixed shear rate and temperature.
- Check static stability: leave samples standing and record settling and hard sediment over several days.
- Choose the dosage that meets viscosity and stability at the lowest cost per tonne of slurry.
Grade data: sodium lignosulfonate product page. Related: sodium lignosulfonate applications.
Frequently Asked Questions
Why is sodium lignosulfonate used in coal-water slurry?
It adsorbs on coal particles and gives them a negative charge, which lowers slurry viscosity and helps keep the coal suspended.
Which molecular weight is most effective?
In a 2007 study by Yang and co-workers, the 10,000 – 30,000 fraction gave the largest viscosity reduction. The most effective fraction depends on the coal.
Can it be blended with naphthalene dispersants?
Yes. Blends are common when lignosulfonate alone does not reach the target viscosity.
How is a CWS dispersant tested?
By measuring apparent viscosity at a fixed shear rate and the static stability of slurries at your coal content.
