Lignosulfonates are complex polymeric materials obtained as co-products of wood pulping. The end use markets for the co-product mixtures are functionally determined by the surface activity, binding, chelation, reactive group and chemical intermediate properties of the mixtures. This article will be limited to binding and chelation and, specifically, their application in fertilizers.

Lignin based products - crude, slightly modified and specialty lignosulfonates - are used in many chelation, binder or adhesive applications. Some of these binder markets use a mixture of lignosulfonate polymers, wood sugars or sugar acids plus polysaccharides; others use specially formulated lignosulfonate systems. The volume used in some of these markets is as much a function of low cost / performance and availability as it is the specific performance characteristics. The chelation markets use lignosulfonate products resulting from complex chemical processing. Molecular size control as well as the degree of sulfonation and carboxylation have led to optimization of properties for chelation.

Some examples of large volume binder use are road binder, animal feed binder, ceramics, brick and foundry cores. Almost any inert powder may be granulated, pelletized or briquetted with lignosulfonate powdered or liquid solids when processed through appropriate equipment. Carbon black pellets in rubber compounding, light weight fly ash aggregates for concrete, coal pellets for utility boiler furnaces, charcoal briquettes and sawdust fireplace logs and briquettes are among the products that use lignosulfonate as the bonding agent. A relatively new potentially large volume application which utilizes the binding and anti-caking properties of lignosulfonates is in the granulation and prilling of fertilizers, specifically urea in the phosphate product family of fertilizers.

Cost / Performance
Lignosulfonate vs. Formaldehyde - Urea Granulation

Extensive laboratory and field evaluations by the Tennessee Valley Authority showed specific lignosulfonate systems could replace formaldehyde and/or urea-formaldehyde resins as conditioner for urea granules and prills. This could be done with improved perfomance for granule strength and at a considerable economic advantage, as shown in Tables 1 and 2.

The tables show a 96% fines reduction on attrition at a $1.95/ton urea savings using lignosulfonate to replace formaldehyde or urea-formaldehyde resin. This equates to the following: a plant making slightly over 500,000 tons of urea granules per year would save $1,000,000 with better product quality.

Lignosulfonate systems in urea have been in commercial use for several years. Use is not universal due to initial product quality problems (now solved) and the perceived resistance to color change. Commercial producers and retail users have found the following advantages:

Benefits to Urea Producers Lignosulfonate can be added directly to the melt at the same addition rate as formaldehyde and provides anti-caking properties equal to or better than that of formaldehyde or urea-formaldehyde resins. Lignosulfonate has been found to provide increased hardness and strength versus formaldehyde resulting in less dusting in-plant and during subsequent handling throughout distribution. In-plant use has yielded less dust to the scrubber system, thereby reducing costs. Lignosulfonate provides a manufacturing cost savings of $0.75 to $1.95 per ton of urea at the same addition rate as formaldehyde. This equates to an annual saving of $75,000 to $195,000 per 100,000 tons of urea produced. Lignosulfonate requires no capital expense. The product can be used in the same tanks, lines and feed points as formaldehyde or urea-formaldehyde resins. Lignosulfonate is classified by the EPA/FDA as a non-toxic liquid. The EPA has classified formaldehyde as a "probable carcinogen" and the OSHA has issued regulations limiting exposure to formaldehyde.

 

Benefits to Farmers Stronger granules mean less dust and minimal product loss. This allows for a more uniform distribution to crops with typical application systems. Light tan color of urea made with lignosulfonate provides product differentiation between domestic and imported urea.

 

Slow release urea systems have been developed through graft copolymerization modifications of lignosulfonate. These modifications have allowed a slow release capability of urea granules and prills. The copolymer at only 5% concentration in urea forms a matrix in the granule or prill that slows the solubility of urea in water, especially under dew precipitation conditions. This allows a more uniform availability of nitrogen to the plant.

Extensive trials for lignosulfonate conditioning of phosphate granules have been carried out on many members of the phosphate fertilizer family. Commercial continuous use on diammonium phosphate and selective use on monoammonium phosphate provides the following benefits:

Benefits to Phosphate Producers Lignosulfonate is classified by the EPA/FDA as a non-toxic liquid. Lignosulfonate can be introduced in slurry feed or acid before granulator; safe for use and storage in standard carbon steel equipment. Where strengths are low, lignosulfonate increases granule strength over 100% resulting in improved handling / anti-caking properties during production, storage and shipping. Lignosulfonate gives significant dust reduction during production, lowering cost of operating dust collectors and improving air quality in the plant. There is less chute plugging and general transfer improvement. Plant trials have shown lignosulfonate can allow the producer to lower his recycle rate and significantly improve granulation quality and efficiency before screening. With the use of lignosulfonate, coating oil often can be eliminated. Lignosulfonate does not harm transfer belts as does oil. Oil environmental concerns are eliminated. Addition rate of lignosulfonate is normally 0.5 gal./ton to 0.75gal./ton of phosphate. Treating cost per ton is less than cost of oil. Lignosulfonate offers price stability to the phosphate producer when compared to oil. Being a natural polymer derived from wood, it is not priced on crude oil value which is subject to fluctuation and sudden increases.

 

Use in DiCal, triple super phosphate, etc. also is applicable, in many cases alleviating dust in the plant completely and showing good granule strength improvement. Competitive spraying of environmentally sensitive oil to coat the outside of the granules remains as a traditional dust alleviation treatment. In contrast, environmentally safe lignosulfonates are added to the acid or to the granulator feed and become an integral part of the fertilizer granule, binding it much like it did originally to cellulose so effectively in the tree. Being an integral part of the granule, dust preventative and anti-caking properties are preserved - even on fracturing.

Micronutrients trace metal chelates such as zinc, iron, copper, magnesium and manganese are widely used in the fertilizer industry. Areas where micronutrient fertilization is needed are increasing; however, they are still quite small compared to acreage where major food nutrients of nitrogen, phosphorus and potassium are used extensively. Increases in acre yields in many cases have depleted micronutrient soil supply. When prepared using specialty lignosulfonate chelation products, the micronutrient trace metals are easily applied and readily available to the plant. Because lignins come from plants, micronutrients based on them can be applied to leaves of plants without any damage to the plant (lack of phytotoxicity).

Research on continued improvement of lignosulfonate for use in fertilizer is on-going and includes many other fertilizer products, fertilizer blends and application areas.

Selected References

1. Chemical Economics Handbook, Lignosulfonate Review, 1985.

2. Lignins - Sarkanen & Ludwig, Wiley Interscience, 1971.