1. Introduction

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) [^[1][2]]. This process can be divided into four parts: raw material handling, chemical delignification of wood with efficient chemical and energy recovery systems, bleaching with high water consumption and the wastewater treatment system. In the 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 °C [^[3]]. Among the several advantages compared to other chemical processes, it can be highlighted the following [^[4]]:

• 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.

2. Kraft Pulp Mill Process: Chemical Recovery and the Generation of Inorganic Waste

The main active chemicals employed in the kraft process are sodium hydroxide (NaOH) and sodium sulfide (Na₂S), 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 [^[5]]. 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 cycles for recovering sodium, sulphur and calcium as well as for energy recovery. These cycles aim not only at the recovery of inorganic chemicals but also to obtain energy from the combustion of organic matter and prevent pollution to the environment. The main purpose of the sodium cycle (or causticizing process) is to convert the main components of green liquor (Na₂CO₃  and Na₂S), in particular the inactive sodium carbonate (Na₂CO₃) from the burning process, into sodium hydroxide (NaOH), according to the Eq. (1), which is a central cooking chemical in white liquor:

Na₂CO₃ + Ca(OH)₂  → 2 NaOH + CaCO₃                                                   (1)

During the causticizing process, the green liquor is clarified and transformed into white liquor, by removing insoluble compounds (green liquor dregs - GLD), which are then washed and removed out from the process. The CaCO₃ from the causticizing reaction (lime mud – LM) is separated from the white liquor, washed and calcined at high temperature into CaO, according to Eq. (2) in the calcium cycle:

CaCO₃  → CaO + CO₂                                                                              (2)

This reaction is endothermic and releases CO₂ to the atmosphere. For producing hydrated lime, CaO reacts with water through Eq. (3), which will be used as a reactant in Eq.(1)

 CaO + H₂O → Ca(OH)₂                                                                                                   (3)

The insoluble fraction in the slaking operation is commonly named slaker grits (SG).  

An important point related to the recovery of chemicals is a third cycle associated with the recovery boiler, which is commonly equipped with an electrostatic precipitator to capture the dust from the flue-gas. The particles composed mainly of Na₂SO₄ are returned to the furnace by mixing them with the black liquor.

The generation points of the main four inorganic wastes produced in large quantities in the kraft pulp mill are shown in Figure 1, namely green liquor dregs (GLD), slaker grits (SG), lime mud (LM), and boiler fly ash (BFA). These wastes are of particular concern because despite several applications have been tested on a laboratory scale, few have reached the industrial scale, and thus the landfill is the main management method. GLD is 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 [^[6][7]]. 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 CaCO₃. 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 [^[8]]. 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 [^[9]]. The solids content of SG is typically about 75% and the pH is usually higher than 12.5 [^[9][10]]. 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% [^[11]]. The fly ash consists of fine particulates and precipitated volatiles, typically with a high specific surface area [^[12]]. The pH of BFA is typically alkaline but lower than the observed for lime residues [^[13]].

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 enters the pulping process through the main raw materials, namely wood, make-up chemicals, water or may arise from the equipment corrosion [^[14]]. In addition, the trend of closing the water cycle accumulates NPE such as Ca and K in the recovery cycle. Their accumulation may lead to filtration difficulties, precipitate formation, or even in undesirable catalytic effects. The purge of NPE from the recovery cycles is essential to maintain normal operating conditions. The process purges associated with GLD, SG, and LM are of great importance for the elimination of many NPE.

Data in Table 1 shows an overview of the specific production of GLD, SG, LM and BFA in kraft pulp mills, which is variable depending on the technology and other specific factors in each site.

Considering the pulp production worldwide, the wastes mentioned in Table 1 represent a huge amount. For example, according to the literature, in Finland (one of the main European pulp producers), about 100 kt of GLD were produced per year [^[18]], and the world production can rise from 0.5 to 1.3 Mt [^[19]]. Although kraft pulp mills have managed to close the chemicals loops, recover energy and reduce water consumption in the process, the current situation of inorganic waste can still be improved if industrial applications are developed to avoid landfill. In the future, the circular economy approaches will be fundamental to operate the transition from landfill to develop industrial applications for GLD, SG, LM and BFA.