## 1 Introduction  
During the last decade, there has been great interest in converting some hardwood kraft pulp mills to dissolving pulp (DP) operations (Figure 1). It has been reported that about 14 pulp mills have announced an increase in DP production or conversion from kraft to DP [1]. In kraft dissolving pulp mills, a step of pre-hydrolysis (using steam or hot water) is required to remove a significant part of hemicelluloses and lignin in order to produce purer cellulose pulp during subsequent steps. By subjecting wood chips to hot water treatment at about 170 \({}^{\circ}\)C and high pressure, the hemicelluloses are hydrolyzed to sugar oligomers and monomers. As a result, the hemicelluloses and part of the lignin present in wood chips are released into solution to form the pre-hydrolysate. This solution is then mixed with black liquor generated at the end of the cooking process. Black liquor has a high calorific value; it is sent to the recovery boilers for burning to produce energy and steam and to recycle chemicals used throughout the process to meet the internal needs of the plant. This high calorific value of black liquor is mainly due to the presence of lignin. The excess lignin can be extracted from black liquor using various techniques, namely acid precipitation, ultrafiltration, and use of organic solvents. It is well known that most kraft pulp and paper mills in North America have recovery boilers with limited capacity [2; 3]. To increase pulp production, either the capacity of the boiler should be increased or a new boiler must be purchased and installed. The latter option would be very expensive for a pulp mill. One less expensive option would be to reduce the load of the boilers by extracting a portion of the lignin from black liquor [4; 5; 6] or by extracting and valorizing the pre-hydrolysate. The pre-hydrolysate generated during the Kraft-based (PHK) dissolving pulp process can be a potential source for the production of biochemicals. Organic components such as lignin, monomeric and oligomeric sugars, and furfural and organic acids are present in the pre-hydrolysate and can be used to produce high value-added products. Several studies have been conducted on pre-hydrolysate valorization such as furfural production from pre-hydrolysate [7, 8, 9, 10], furfural production from a pre-hydrolysate generated using SO\({}_{2}\)[11], lignin removal by acidification [12], acidification and flocculation [13, 14], surfactant and calcium oxide [15] or increase treatments [16, 17], as well as lignin recovery using membrane technology [18] and pre-hydrolysate detoxification [19]. However, the focus of all these studies was the valorization of hemicelluloses and organic acids to produce biochemicals and biofuels such as furfural, bio-ethanol, bio-butanol, acetic acid, formic acid, and succinic acid. Lignin in the pre-hydrolysate is considered a fermentation inhibitor and is removed prior to the conversion of sugars to biochemicals [13, 15, 19]. To date, very few works have been done on the extraction and characterization of lignin from wood pre-hydrolysate. In addition, no studies have been done on its recovery from the pre-hydrolysate. Unlike other types of lignin (kraft, lignocellulosic, and organosolv), this lignin has never been exposed to chemicals. Thus, its purity as well as chemical and physical properties are expected to be different from other lignins. However, it has not yet attracted attention from researchers. Numerous studies have been conducted to extract lignin from black liquor and various applications have been discussed in the literature such as the use of kraft lignin in rigid polyurethane foams [20], in flexible polyurethane foams [21], in phenol formaldehyde resins [22], and in activated carbon [23] as well as the use of lignin as an antioxidant and antibacterial [24]. Therefore, the recovery and characterization of lignin from the pre-hydrolysate will provide valuable information about its potential use and commercial applications.