Evaluation of Alkaline Extraction for Silica Removal from Wheat Straw and Wheat Straw Pulp

Sammanfattning

Wheat straw is an abundant lignocellulosic agricultural residue with potential as a raw material for paper products, and higher-value cellulose applications. However, its high inorganic content, particularly silica, can limit pulp quality and create processing problems. This thesis evaluated alkaline extraction as a method for reducing ash and silica from wheat straw and wheat-straw derived pulp. Sodium hydroxide extraction was studied using Design of Experiments, with screening and optimization stages for both straw and pulp. The treatment effects were evaluated using total ash content, acid-insoluble ash content, dissolved silica in the extraction liquid, and X-ray fluorescence analysis of untreated ash. The results showed that alkaline extraction substantially reduced the inorganic content of both materials compared with the untreated straw and pulp. The ash content of the optimization-treated straw samples was reduced by 88.6–94.2% and acid-insoluble ash by 94.3–100.6% respectively relative to untreated straw. In the optimization-treated pulp samples, ash content was reduced by 63.4–92.4% and acid-insoluble ash by 65.6–99.7% relative to untreated pulp. XRF analysis showed that untreated straw and pulp ash had different inorganic compositions, meaning that the two materials should mainly be interpreted relative to their own untreated state rather than with each other. The pulp optimization model gave the clearest process interpretation, indicating that temperature was the most important factor for ash and silica removal within the investigated range, while treatment time and NaOH concentration had smaller effects. For straw, the optimization model was less reliable, but the results showed that even low NaOH concentration could substantially reduce ash content compared with untreated straw. The study therefore supports alkaline extraction as an effective ash- and silica-reduction method, while suggesting that the most practical route is likely a staged process combining physical removal of silica rich straw fractions, mild alkaline pre-extraction before pulping, and postpulping alkaline extraction or bleaching.