*3.3. Alkaline Lignin Extraction*

### 3.3.1. Fiber and Lignin Yields

The extraction of lignin with 8% NaOH (based on raw material input) was conducted for the fiber fraction of all severity grades. The loading of 8% NaOH is reported as the optimum for alkaline extractions after steam refining [9,11] and represents at a consistency of 10% a thin 0.8% *w*/*w* NaOH solution. The total recovery rate was calculated with the lignin and fiber recovery after extraction and is illustrated in Figure 6.

**Figure 6.** Illustration of the fiber and lignin yield and the total recovery rate, containing both mentioned fractions, after alkaline lignin extraction.

The fiber yield is slightly, but continuously decreasing with increasing severity (Figure 6). Simultaneously the lignin yield increases with the severity. Surprisingly, the reduction of fiber yield is significantly lower compared to the increase of lignin yield. This might be due to the condensation

reactions of the lignin at high temperatures. The lignin with higher molar masses can be precipitated easier or more efficiently resulting in higher yields. However, when there is no covalent binding to the lignin, the precipitation is not sufficiently possible, resulting in lower yields.

As further illustrated, the combined recovery of lignin and fibers after extraction is strongly increasing with increasing severity of the pretreatment (Figure 6) from 75% up to 95% of the original raw material input. The increase in the total recovery rate can mainly be attributed to the higher lignin precipitation at severities above 4.
