Promoting material circularity in pulp and paper mills: organic-inorganic cellulosic composites for slow-release of nutrients

The pulp and paper industry generates large quantities of waste, while the forestry sector experiences economic and environmental challenges with commercial fertilisers. Slow/controlled-release fertilisers (SCRFs) offer an opportunity to repurpose waste-stream materials and improve nutrient retention in forests. The aim of this study was to evaluate the potential of transforming pulp and paper waste components into functional elements of a SCRF product. Two primary objectives were defined: the immobilisation of urea onto synthesised hydroxyapatite, for a SCRF pellet core, and the screening and optimisation of the coated pellet’s formulation conditions. Synthesis of hydroxyapatite was performed via wet coprecipitation with two different calcium salts from the pulp and paper mill system: calcium hydroxide (Ca(OH)₂) and calcium carbonate (CaCO₃). The immobilisation of urea was conducted via in-situ wet coprecipitation using Ca(OH)₂, forming a urea-hydroxyapatite (UHA) composite. A similar method was attempted with CaCO₃ as well as a two-step adsorption method. Pellet development progressed in three experimental phases, each using compressive strength and urea release as response variables. One-factor-at-a-time (OFAT) screening determined the best binder and coating type for the pellet. Thereafter, a 2(⁵¯²) fractional factorial design (FFD) screened five factors, including binder content, initial water content, hydraulic press pressure, CaCO₃ filler content, and coating solution concentration. A central composite design (CCD) was conducted using the significant factors identified from the FFD. Initial water content (14.3 – 25.7 %) and coating solution concentration (7.93 – 22.1 %) were used as independent factors. Quadratic response models were generated with statistical software, and optimisation was performed using a desirability analysis that targeted high compressive strength and lower short-term urea release. The hydroxyapatite yield for Ca(OH)₂ and CaCO₃ was 84.9 % and 60.9 %, respectively. Urea immobilisation was only successful for the Ca(OH)₂-derived product, achieving a urea loading of 25.0 ± 8.73 %. No detectable urea was incorporated into either CaCO₃-derived product. The OFAT experimentation yielded a lignosulphonate binder and cellulose acetate coating to be the best performing combination with a compressive strength of 685 ± 65.0 kPa and a 78.3 ± 2.90 % urea release after 24 hours. The FFD identified initial water content and coating solution concentration to be the significant factors (p < 0.05) for the compressive strength and release at 2 hours response, respectively. CCD optimisation indicated optimal formulation conditions of 22.8 % initial water content and 22.1 % coating solution concentration. Pellets prepared at these conditions achieved a compressive strength of 4.50 ± 0.23 MPa and released only 50.3 ± 2.30 % urea after 24 hours, outperforming plain urea and approaching commercial SCRFs. The results present the promising technical feasibility of SCRF pellet production from pulp and paper mill waste components. The formulation process demonstrated promising scalability although coating uniformity may require stricter control with drum coating technologies. Limitations include the lack of raw materials directly recovered from mill waste streams and their associated, variable compositions. However, the study does outline a potential avenue for incorporating circular economy principles into mill and forestry sectors.

Grantor dc:publisher

Stellenbosch : Stellenbosch University

Year dc:date.issued

2026

Author dc:creator

• Carzola, Gabriela

Advisors dc:contributor.advisor

• Chimphango, A. F. A.
• Mapholi, Zwonaka

Language dc:language.iso

en

Repository record dc:identifier.uri

https://scholar.sun.ac.za/handle/10019.1/135670

OAI identifier oai:identifier

oai:scholar.sun.ac.za:10019.1/135670