Calcium lignosulfonate is recovered from the waste liquor of the calcium-base acid sulfite wood-pulping process. In the conventional route, lime is added to the spent liquor to precipitate an insoluble, basic calcium lignosulfonate salt, separating the higher-molecular-weight lignosulfonate fraction from lower-molecular-weight material and carbohydrates also present in the liquor.
Converting the insoluble precipitate to a soluble product
The basic calcium lignosulfonate precipitate formed by this lime treatment is not itself water-soluble, so further processing is needed to reach a usable soluble grade. Historically, this has been done in a few different ways, each with trade-offs:
- Treating the precipitate with sulfuric acid to split off the excess lime and convert the product to a soluble form. This introduces calcium sulfate into the solution, which can act as a scale-forming contaminant in downstream use.
- Using sodium bicarbonate in place of sulfuric acid, which reduces calcium sulfate content but introduces sodium, so the resulting product is effectively a mixed calcium/sodium lignosulfonate rather than a calcium-only grade.
- Treating a slurry of the insoluble salt with carbon dioxide to precipitate and remove calcium carbonate. This avoids the sulfate and sodium issues above but adds the cost of a clean carbon dioxide supply.
An alternative approach uses an ion-exchange resin to adsorb the excess calcium directly from a slurry of the insoluble salt, producing soluble calcium lignosulfonate without introducing sulfate or sodium contaminants. The loaded resin can then be regenerated using sulfurous acid, which also yields a calcium bisulfite/sulfurous acid stream that can be reused in sulfite pulping liquor preparation — an approach that keeps the overall process closer to a closed loop.
Why grade and production route matter for buyers
Different production routes leave different residual contaminant profiles (calcium sulfate, sodium, or none, depending on the route used), which can matter for applications sensitive to scale formation or ion content. Buyers evaluating a calcium lignosulfonate grade for a specific application should request the current TDS/COA for the exact grade rather than assume a particular production route or purity level.
Product Documentation and Supply Enquiries
Batch COA, TDS, SDS/MSDS, samples, packaging options and commercial quotations are available on request. Contact info@greenagrochem.com or use the contact form.
