# Inorganic Waste Generated in Kraft Pulp Mills:
## 2 Kraft Pulp Mill Process and the Recovery of Chemicals  
Chemical processes are the most common for obtaining pulp of cellulose fibers, among which stands out the'sulfate process', commonly known as 'kraft process' due to the high physical-mechanical resistance of the pulps produced (kraft means strength in German) and is currently the most widely used process in the world (80% of total chemical pulp) [6; 7]. In this cooking process, water-solubilized reagents (liquor) are added to the wood chips in a reaction vessel (digester) for 1 to 3 h at 150-170 \({}^{\circ}\)C [8]. Among the several advantages compared to other chemical processes, it can be highlighted the following [9]:  
* Higher strength and flexibility of the produced pulps;
* Applicability to various wood species, regardless of their physico-chemical characteristics;
* The wide range of pulp applications;
* The efficient recovery of chemicals used in cooking, off-setting the high capital costs, which makes it economically more viable and competitive. The main active chemicals employed in the kraft process are sodium hydroxide (NaOH) and sodium sulfide (Na\({}_{2}\)S), commonly known as white liquor. Indeed, the designation _sulfate process_ is due to the addition of sodium sulfate to replace lost chemical reagents. This mixture leads to lignin fragmentation and dissolution, while cellulose fibers are released [10]. The cooking reagents are not completely selective for lignin and there are also undesirable reactions of polysaccharides, mainly hemicelluloses, which due to their chemical structure are very susceptible to chemical attack. The cellulose fibers are recovered from the black liquor, which contains lignin and valuable chemicals. Figure 1 shows a simplified diagram of the phases to obtain pulp from wood, highlighting the cyclesfor recovering sodium and calcium as well as for energy recovery. In addition, the generation points of the wastes under consideration (GLD, SK, LM, and BFA) are also shown in the diagram. Green liquor dregs (GLD) are generated during the clarification of green liquor and contain the insoluble material of the recovery boiler inorganic flux (smelt). The reported GLD suspended solids content is about 600-2000 mg/L and the pH is strongly alkaline [11; 12]. Lime mud (LM) is a by-product formed during the causticizing reaction, which is separated from the white liquor, washed and calcined in the lime kiln, while a fraction is purged as waste and replaced by fresh CaCO\({}_{3}\). Higher LM production occurs when there are differences between the production of white liquor and the production capacity of the lime kiln. The pH of LM can vary slightly but is often strongly alkaline [13]. Slaker grits (SG) are the coarse material removed from the discharge of the lime slaker to avoid build-up on causticizers and mechanical wear on filter components [14]. The solids content of SG is typically about 75% and the pH is usually higher than 12.5 [14; 15]. The boiler fly ash (BFA) is generated in a biomass fluidized bed boiler, as a result of the combustion of wood bark and other wood residues for energy recovery. The fused particles are carried upwards along with the flue gas. As the flue gas approaches the low-temperature zones, the fused substances solidify to form fly ash, which is captured by cyclones, fabric filters and/or electrostatic precipitators (ESP) with cleaning efficiency above 99% [16]. The fly ash consists of fine particulates and precipitated volatiles, typically with a high specific surface area [17]. The pH of BFA is typically alkaline but lower than the observed for lime residues [18]. Closing the loops in kraft mills has environmental advantages but leads to the build-up in the liquor cycle of non-process elements (NPE), such as Ca, Mg, K, Mn, Ba, Fe, Al, Ni, Cu, Zn, etc., which may hinder the pulping, bleaching or chemicals recovery process. NPE enter the pulping process through the main raw materials, namely wood, make-up chemicals, water or may arise from the equipment corrosion [19].