Problem definition and objectives: In this study, the possibility of producing pulp from wheat straw using the carbonate process was investigated. Given the increasing demand for alternative sources of cellulose fibers in the paper industry and the declining availability of wood resources, utilizing agricultural residues such as wheat straw (WS) can be considered a sustainable solution. In this regard, the sodium carbonate cooking process was evaluated as an environmentally friendly alternative method for fiber extraction.
Methodology: For this study, wheat straw was selected as the raw material and subjected to a cooking process with a sodium carbonate solution under various conditions. The maximum cooking temperatures were set at 150 and 170°C, with cooking times of 2, 3, and 5 hours. Additionally, the alkalinity of the cooking solution, based on Na2O, was adjusted to four levels: 20, 22, 24, and 26%. To improve lignin separation and enhance the cooking yield, anthraquinone was added at a concentration of 0.1% by dry weight of WS. The liquor-to-straw ratio (L/S) was varied at 4, 6, 8, and 10:1 to determine its effect on the process performance. After the cooking process, the produced pulp was washed and separated using 20 and 200 mesh screens. The parameters of kappa number, total yield, screened yield, and reject content were measured to identify the optimal cooking conditions. Subsequently, the optimized pulp was converted into handmade paper sheets at grammages of 60 and 120 g/m² in both refined and unrefined states. Mechanical tests, including tensile, bursting, and tearing strength indices, were conducted on the produced paper to evaluate the quality of the pulp obtained from wheat straw.
Results: The results indicated that the sodium carbonate cooking process under optimal conditions could produce pulp with desirable properties. Based on the obtained data, the best cooking conditions were determined as an alkalinity of 22%, a temperature of 170°C, a cooking time of 2 hours, and an L/S ratio of 8:1. The pulp produced under these conditions had a kappa number of 37.72, a total yield of 63.52%, a screened yield of 51.18%, and a reject content of 1.45%, demonstrating the effectiveness of the cooking process. The conversion of optimized pulp into handmade paper and the evaluation of its mechanical properties showed that in both 60 and 120 g/m² grammages, the refined samples exhibited higher mechanical strength indices compared to unrefined ones. The highest tensile, bursting, and tearing strength indices were achieved in the refined paper samples, indicating improved mechanical properties with fiber refining. Considering the decline in mechanical properties and the degradation of liner and test liner papers during repeated recycling processes, incorporating wheat straw carbonate pulp as a reinforcing fiber source can help improve the quality of such papers. This aspect is crucial in the packaging industry, as using recycled materials along with suitable reinforcing fibers can enhance the strength and durability of the produced paper.
Conclusion: Overall, this study demonstrated that carbonate pulp produced from wheat straw possesses suitable characteristics for use in the paper industry. This method can serve as a sustainable alternative to virgin fibers, reducing dependency on wood resources and enhancing efficiency in the production of packaging papers. Given the increasing demand for sustainable raw materials in this industry, further development and optimization of this process can significantly contribute to improving the performance and sustainability of the paper supply chain.