Inorganic Waste Generated in Kraft Pulp Mills: The Transition from Landfill to Industrial Applications

applied sciences Review Inorganic Waste Generated in Kraft Pulp Mills: The Transition from Landfill to Industrial Applications Margarida J. Quina * and Carolina T. Pinheiro CIEPQPF, Chemical Process Engineering and Forest Products Research Centre, Department of Chemical Engineering, Faculty of Sciences and Technology, University of Coimbra, Rua Sílvio Lima, Polo II, 3030-790 Coimbra, Portugal; [email protected] *Correspondence: [email protected]; Tel.: +351-239798700 Received: 29 February 2020; Accepted: 26 March 2020; Published: 28 March 2020   Abstract: Kraft pulp mills produce the main raw material for paper, while several waste products are generated in large quantities in the process. This review study addresses four of the main inorganic wastes formed by this industry, namely green liquor dregs (GLD), slaker grits (SG), lime mud (LM) and boiler fly ash (BFA), which are still mostly discarded in landfills. A brief overview of a typical industrial process was included to outline the waste generation points. The main chemical and physical properties are indicated for highlighting the most relevant characteristics to determine which applications may be considered in each case. An in-depth literature review allowed the identification of the main applications that have been tested mainly at the laboratory scale and some at an industrial scale. The applications are grouped into construction materials, geotechnical, environmental, agricultural and others. This assessment shows that the circular economy and the sustainable development goals of the UN are important issues for organizations in general, and the pulp mill in particular. In fact, this industry has managed to close the chemicals loops, recover energy and reduce water consumption in the process. However, the current situation of inorganic waste can still be improved if industrial applications are developed to avoid landfill. Keywords: green liquor dregs; slaker grits; lime mud; boiler fly ash; recycling; kraft pulp mills 1. Introduction According to the Confederation of European Paper Industries (CEPI), which represents nearly 151 pulp mills from Europe, in 2018, about 154.9 Mt of wood (42.8 Mt of hardwood and 112.1 Mt of softwood) was consumed and 38.3 Mt of pulp was produced [ 1 ]. Sweden, Finland, and Portugal are the top three countries, accounting for 68.6% of the pulp production. The European countries produce 25.3% of world pulp production (184.4 Mt in 2017), which accounts for a total paper and board production of 92.2 Mt. Indeed, the pulp and paper industry is one of the largest industries in the world, with a very significant contribution to the economy of many countries (e.g., Sweden, Finland, Portugal, Germany, Spain, France, and Poland). Although the European Commission adopted an ambitious Circular Economy Action Plan in 2015 [ 2 ], there is potential for more recycling of paper, with both environmental and economic benefits, including less wood utilization for pulp production. The main raw materials for pulp production are wood, various chemicals, and water, while significant amounts of waste are also generated. Indeed, besides the impact associated with wood consumption, pulp mills generate large amounts of solid, liquid and gaseous emissions, requiring treatment before being released into the environment [ 3 ]. This study is focused on the main inorganic solid wastes that require an environmentally friendly solution: green liquor dregs (GLD), slaker Appl. Sci. 2020,10, 2317; doi:10.3390/app10072317 www.mdpi.com/journal/applsci Appl. Sci. 2020,10, 2317 2 of 20 grits (SG), lime mud (LM) and boiler fly ash (BFA). These wastes have been produced in high quantities, and landfill has been the main disposal method [ 4 , 5 ]. According to the EU List of Waste (Commission Decision 2014/955/EU), the assignment of codes to GLD is 03 03 02, SG 03 03 09, LM 03 03 09 and BFA 10 01 01, all of them classified as non-hazardous waste. The current environmental policies (Waste Framework Directive 2008/98/EC) suggest that the disposal of materials in landfill must be minimized and a clear recommendation is made for certain waste to cease to be waste. For that, end-of-waste (EoW) criteria should be developed, in particular, if—i) material is commonly used for specific purposes; ii) there is an existing market or demand; iii) the material fulfills the technical requirements for the specific purposes; iv) the use does not lead to adverse impacts on the environment or human health. Thus, it would be of interest to develop EoW criteria for these wastes to minimize the loss of that anthropogenic resources and to promote the development of applications at the industrial scale. Furthermore, taking into consideration the large quantities of GLD, SG, LM, and BFA formed, the development of practical applications could make a valuable contribution to a circular economy agenda, because it ensures that the resources are kept in the economy for as long as possible. The literature review to identify the main works in the field was carried out mainly on Web of Science (WoS), searching for “green liquor dregs” or “slaker grits” or “lime mud” or “boiler fly ash”, combined with “pulp mills” in the title or topic. Other references were collected based on those of the main works identified in the WoS. Overall, 90 references were considered in this study, with 75% of those sources published from 2010 onwards. In this context, the main objective of this review is to summarize the properties of the main inorganic wastes generated in kraft pulp mills, highlighting which resources can be recovered and the applications which can be implemented on an industrial scale to reduce or eliminate the current disposal in landfills. For each waste (GLD, SG, LM, and BFA) the main applications tested/used at the laboratory or industrial scale were identified. The study ends with future perspectives for the use of these resources, emphasizing the main factors for selecting the best paths for an emerging circular economy. 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 ◦ 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 1shows a simplified diagram of the phases to obtain pulp from wood, highlighting the cycles Appl. Sci. 2020,10, 2317 3 of 20 for 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. Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 21 Figure 1. The chemical recovery loops and energy generation. 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 byproduct 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]. 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 wastes under analysis, specifically GLD, SG, and LM are of great importance for the elimination of many NPE. The specific production of GLD, SG, LM and BFA in kraft pulp mills is variable depending on the technology and other specific factors in each site. Even though, Table 1 shows an overview of the specific production of each residue. Table 1. Specific waste generation in kraft pulp mills (kg/t AD). Figure 1. The chemical recovery loops and energy generation. 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 ]. 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 wastes under analysis, specifically GLD, SG, and LM are of great importance for the elimination of many NPE. Appl. Sci. 2020,10, 2317 4 of 20 The specific production of GLD, SG, LM and BFA in kraft pulp mills is variable depending on the technology and other specific factors in each site. Even though, Table 1shows an overview of the specific production of each residue. Table 1. Specific waste generation in kraft pulp mills (kg/t AD). Wastes Industrial Information (a) [3] [20] [21] [22] [23] GLD 12 10–20 (b) 12 15 4–20 12.8 SG 10 7 16 LM 25 10–20 15 13 BFA 30 9(c) 20 5 (a) Data provided by the Portuguese Industry (The Navigator Company); (b) includes green liquor dregs (GLD) and slaker grits (SG); (c) may be higher if biomass from external sources is also used; AD – air dried pulp. Data in Table 1demonstrate that there are variations in the flows of each residue, depending on the technology and the operating conditions at each site. However, considering the pulp production worldwide, these wastes 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 [ 22 ], and the world production can rise from 0.5 to 1.3 Mt [23]. 3. Main Properties of the Inorganic Wastes In this section, the main physical and chemical properties of GLD, SG, LM, and BFA are highlighted to reveal the main pros and cons of a specific application. 3.1. Chemical composition Table 2shows the composition expressed in oxides, determined by XRF, and also the loss on ignition (LOI). Table 2. Chemical composition of green liquor dregs (GLD), slaker grits (SG), lime mud (LM), and boiler fly ash (BFA) determined by XRF (wt %). GLD SG LM BFA [4] [24] [25] [5] [24] [26] [27] [28] [5] [23] [26] [29] CaO 34.3 33.0 34.9 49.45 55.8 44.4–52.0 57.12 54.1 16.7 34.9 0.8–10.4 16.5 MgO 10.9 4.65 5.94 0.45 0.47 0.6–3.4 0.91 0.86 3.44 4.4 0.7–1.9 3.07 SiO20.23 2.35 ni 0.47 1.31 3.4–11.0 3.58 0.34 38.5 11.6 33.9–59.7 34.0 Al 2 O 31.94 0.69 0.47 0.29 0.42 0.5–1.4 0.07 0.07 14.8 4.4 16.5–35.4 13.5 Fe 2 O 30.61 0.65 0.59 0.05 <0.1 0.2–1.2 0.20 0.15 5.94 2.6 1.5–19.7 4.95 Na 2 O 2.10 11.7 9.49 4.52 0.60 ni 2.32 0.91 1.53 1.4 ni 1.52 K2O<0.1 1.03 0.37 0.27 <0.1 ni 0.26 0.06 5.97 6.5 ni 5.49 P2O5ni 0.33 0.37 0.38 0.65 ni 0.03 0.96 1.12 1.6 ni 1.11 TiO2ni <0.1 ni ni <0.1 ni ni ni 0.76 0.25 ni 0.65 MnO 4.21 0.37 0.06 ni <0.1 ni ni 0.09 0.50 1.4 ni 0.45 SO33.6 2.82 ni 1.86 0.11 ni 0.4 ni 2.66 11.4 ni 2.77 LOI ni 42.10 * ni 41.1 40.10 31.5–43.5 ni ni 6.38 15.8 1.2–33.6 14.3 LOI–Loss on ignition; * Determined at 1200 ◦C; ni–not indicated. These data demonstrate that GLD, SG, and LM are particularly rich in Ca, while BFA contains less of this element. Indeed, BFA is rich in Si with noteworthy content in Al and Fe. In addition to Ca, GLD also contains noticeable Na and Mg content. The concentration of K and P in BFA can be interesting for the production of fertilizers. The high LOI associated with these wastes is normally due to the decarbonation of calcite instead of organic matter, which generally is low [30]. Appl. Sci. 2020,10, 2317 5 of 20 3.2. Mineral phases To select the best utilization option in each case, it is meaningful to identify the mineral phases that compose each material. For that purpose, XRD could be a valuable technique, in particular, to identify the crystalline phases, as indicated in Table 3. Accordingly, it can be concluded that calcite (CaCO 3 ) is present in all wastes, whereas in a much more expressive quantity in GLD, SG, and LM. Besides calcite, GLD also contains dolomite (CaMg(CO 3 ) 2 ), cesanite (Ca 2 Na 3 (SO 4 ) 3 (OH)) and pirssonite (Na 2 Ca(CO 3 ) 2 .2H 2 O). Some references indicate that pirssonite is the dominant mineral [ 21 , 29 ] in GLD, while others [ 31 ] indicate calcite as the most relevant phase. Sodium might be present in pirssonite, cesanite, natrite (Na 2 CO 3 ) as well as sodium sesquicarbonate (Na 3 H(CO 3 ) 2 ). Sodium and sulfur content result from the green liquor composition (mainly composed of Na 2 CO 3 and Na 2 S). In addition, brucite (Mg(OH)2) is commonly found in GLD. Regarding SG, besides CaCO 3, it may contain dolomite (CaMg(CO 3 ) 2 ), pirssonite (Na 2 Ca(CO 3 ) 2 .2H 2 O), portlandite (Ca(OH) 2 ), larnite (Ca 2 SiO 4 ). The high concentration of calcium-bearing mineral phases in SG results from the slaking reaction of quick lime with green liquor (aqueous solution of sodium hydroxide, sodium sulfide, and sodium carbonate) in the slaker unit. Moreover, quartz (SiO 2 ), wustite (FeO) and brucite (Mg(OH) 2 ) are also detected. Mg concentration in the SG can be related to the use of magnesium sulfate in the delignification process in some mills [ 19 ]. As aforementioned, LM is formed in the causticizing reaction, and thus the main phase (more than 90%) is calcite [ 30 ], while some minor or trace phases may contain Mg, Si, Al, Fe, Na, K, P, and S, but normally not detected through XRD. Besides calcite, which is clearly identified in X-ray diffraction spectra, in the literature some minerals containing Ca and Mg carbonates have been identified (Ca(1−x)MgxCO3) [31]. With respect to BFA, the major minerals are quartz (SiO 2 ), calcium-bearing minerals such as calcite (CaCO 3 ), dolomite (CaMg(CO 3 )), anhydrite (CaSO 4 ) and portlandite (Ca(OH) 2 ). Moreover, alumina (Al 2 O 3 ), iron oxide (Fe 2 O 3 ), periclase (MgO) and sylvite (KCl) also constitutes the BFA in variable amounts. In fact, BFA from coniferous trees tends to contain a higher amount of Si and Ca when compared to hardwood trees and relatively low K and S content [ 32 ]. The high calcite and aluminosilicate contents seem suitable as a cement replacement material and as aggregates for road construction [ 32 ]. Attention should be paid to sulfate and chloride content, which can be harmful in concrete applications. In particular, sulfate may lead to ettringite salts, which can cause cracks in the final material. Chlorides can provoke, for example, corrosion in reinforced concrete. Regarding the nutrients content, BFA is poor in nitrogen due to its volatilization during combustion and loss as gaseous compounds. On the contrary, the amount of K and P can be interesting for agronomic applications. Table 3. Mineral phases in GLD, SG, LM, and BFA identified by XRD. GLD SG LM BFA Calcite (CaCO3) [4,24,31] Calcite (CaCO3) [5,24,31] Calcite (CaCO3) [31] Calcite (CaCO3) [5,23] Dolomite (CaMg(CO3)2) [23] Dolomite (CaMg(CO3)2) [23] Ca(1−x)MgxCO3[31] Dolomite (CaMg(CO 3 ) 2 ) [ 23 ] Cesanite (Ca2Na3(SO4)3(OH)) [23] Quartz (SiO2) [24] Halite (NaCl) [23] Natrite (Na2CO3) [23,24,33,34] Pirssonite (Na 2 Ca(CO 3 ) 2 .2H 2 O) [ 31 ] Quartz (SiO2) [5,23,31] Pirssonite (Na2Ca(CO3)2.2H2O) [4,21,30] Portlandite (Ca(OH)2) [31] Sylvite (KCl) [23] Manganite (Mn4O8H4) [4] Wustite (FeO) [31] Anhydrite (CaSO4) [23] Sodium sesquicarbonate (Na3H(CO3)2) [35] Larnite (Ca2SiO4) [31] Portlandite (Ca(OH)2) [23] Brucite (Mg(OH)2) [35] Brucite (Mg(OH)2) [31] Periclase (MgO) [23] 3.3. Physico-chemical properties In order to find the best applications for the wastes considered, in addition to the elemental composition and minerals indicated in Tables 2and 3, the knowledge of other physical and chemical properties is fundamental, such as those reported in Table 4. The moisture content can vary in GLD, SG, and LM, depending on the technology and operating conditions in the pulp mill. High moisture content can be advantageous depending on the application, namely to decrease the release of dust in handling operations. However, transportation costs can be Appl. Sci. 2020,10, 2317 6 of 20 higher, as well as the difficulties in mixing with other powdered materials (for example, clay, and cement). BFA is usually generated with very low moisture, while showing high hydrophilic properties and easily forming agglomerates [32]. Table 4. Other relevant physicochemical properties to select technological applications. Property GLD SG LM BFA Moisture (%) 50.8 [36]; 48.0–57.0 [17], 54.0 [37] 15.7 [36]; 28.4 [19], 7.0–16.0 [17], 41.1 [38], 1.1–45.6 [17], 28.0 [39], 39–60 [26] 0.30–0.80 [40] pH 12.8 [36]; 12.2 [25], 12.9 [4]13.1 [36]; 12.6 [25], 13.1 [19]12.6 [41] 11 [32]; 12.8 [42]; 13.3 [43] EC (mS/cm) 26.2 [36]; 9.76 [4] 20,8 [36]; 94.3 [19] 7.3 [41] 13.6 [42]; 11.63 [43] VS (% TS) 8.3 [36] 2.4 [19] ANC (% CaCO3)64.4–95.6 [25], 8.3 mmol H+/g [4]69.4–100 [25] 106 [41] 54.3–77.7 [25] D50 (µm) 11.6 [24], 8.97 [4]; 6 [29] 24.1 [24] 49.3 [5]; 150–250 [32] Density (g/cm3)2.498 [24], 2.47–2.60 [37] 2.703 [24] 2.83 [44], 2.43 [39] 2.4–2.8 [32], 2.615 [23] Bulk density (g/cm3)1.2–1.64 [4], 0.44–0.67 [37] 0.15–1.3 [32] Sa (m2/g) 72.08 [24]; 12–21 [37] 2.901 [24] 5.17 [44]3.03 [5]; 4.2–101 [32], 3.25 [23] HC (m/s) 8.8 ×10−9−1×10−8[4] Kjeldahl N (%) 0.07 [25] 0.05 [25] 0.17 [25] Chlorides (%) 0.30 [29]; 0.8 [36]0.1 [12]; 0.02 [19]; 0.1 [36] 0.08 [44]; <LQ [45]; 0.06 [46] 1.5 [29]; 2.7 [23]; 1.2 [45]; 1.2 [43]; 0.10 [46] EC–electrical conductivity; VS–volatile solids; ANC–acid neutralization capacity; D 50 –median size; S a –specific area; HC–hydraulic conductivity. A key property to define the best application of a specific material is the pH. In this case, the four wastes are alkaline, and very high values (>13) can be observed in certain cases (e.g., for SG). Thus, if not properly managed, they can cause corrosion and ecotoxicity problems in natural ecosystems. However, this property may also be positive in certain applications, for example, to be used as neutralizing agents, liming applications or as operating supply in applications that normally use alkaline raw materials (e.g., cement). BFA reveals an average pH of 11, but can vary between 8 and 13 [32]. From the electrical conductivity (EC) it is possible to infer the total concentration of dissolved electrolytes (or total dissolved solids, TDS) in aqueous suspension. In this context, it is important to note that some applications do not allow high ionic strength (applications as liming agent), but if the waste is used in bound materials (e.g., in cement formulations) this would not be a problem. Watkins et al. [19] measured a concentration of TDS equal to 88.5 g/kg in SG leachates, which exceed both the EU limits for inert waste landfill (4 g/kg) and the non-hazardous waste landfill (60 g/kg dw). The organic matter or volatile solids (VS determined at 550 ◦ C for 2–4 h until constant weight) is in general low (much less than 8%) in all these wastes, which are then classified as inorganic. The acid neutralization capacity (ANC) or alkalinity or buffering capacity of GLD, SG, and LM is typically high. This property can be measured as calcium carbonate equivalents and as can be seen in Table 4, values close to 100% can be obtained for GLD, SG, and LM, while lower values are common for BFA [ 25 ]. Mäkitalo et al. [ 37 ] reported that to keep pH >6, the average ANC was 18.5 mmol H+/g for GLD. This property is particularly important for assessing the potential applications as a liming agent of soil or as neutralization material for acid wastewaters or in alkaline barriers. All of those materials are generated in granular form. BFA shows, in general, a coarser particle size distribution, while GLD is a very fine powder with a mean particle size of a few micrometers [ 29 ]. Some of them are generated in agglomerates (SG and LM) but could be easily disintegrated if required. Thus, the bulk density is lower or close to 1 g/cm 3 , whereas the real (or skeletal) density may reach values higher than 2.4 g/cm 3 for all of those wastes. Indeed, for instance for GLD, although the bulk density ranges between 0.44 and 0.67 g/cm 3 , the real density may achieve from 2.47 to 2.60 g/cm 3 [ 37 ]. The specific surface area, S a , may vary within each waste, but expected values are from a few m 2 /g to about 100 m 2 /g. For using these materials in geotechnical applications such as sealing layer for water and oxygen, the hydraulic conductivity is a relevant property. This parameter has not been Appl. Sci. 2020,10, 2317 7 of 20 reported much in the literature. Nevertheless, the hydraulic conductivity measured for GLD vary from 3.7 ×10−9–4.6 ×10−8m/s, which is a low value and similar to those of silt or muddy moraine [4]. It is important to note that these wastes are definitely not nitrogen sources (Kjeldahl N less than 0.05%). The content of chlorides could limit specific applications, namely for clinker production or in concrete and mortar applications. Data from the literature shows that the content of chlorides in GLD, SG, and LM is normally low, whilst BFA can contain a slightly higher amount. 3.4. Potentially toxic metals To find reliable applications while protecting the environment, the total content of potentially toxic metals (PTM) and their leaching behavior are of high importance. Table 5summarizes the total quantities of diverse PTM commonly found in the four inorganic wastes. For comparison purposes, three additional references are also included. In particular, the Finnish limits for the use of ash as a forest fertilizer, the Finnish limits in respect to the maximum allowable element concentrations in ashes (e.g., from coal, peat, and biomass) used as earth construction material [ 19 ] and the average crustal abundance of these elements. Table 5. The total content of potentially toxic elements (mg/kg). GLD SG LM BFA Limit FF Limit CM Crust * [4] [25] [30] [19] [25] [28] [41] [25] [42] [47] [47] [19] [48] Pb 6.12 46.8 13 <3 34.1 6.79 <3 44.3 28.7 31 150 300 12.5 Cd 3.81 5.19 9.4 0.3 4.75 0.91 <0.3 4.7 2.9 3.3 25 15 0.2 Cu 229 80.9 102 <10 4.6 0.73 4.1 25.8 63.6 72 700 400 55 Cr 295 56.0 118 12.6 12.4 16.7 7.0 24.1 66.9 74 300 400 100 Ni 233 189 84 23.9 25.2 ni 4.0 97.4 32.4 33 150 ni 75 Zn 3197 160 1000 9.9 15.0 ni 36 68.9 295.3 320 4500 2000 70 Hg <0.05 ni ni <0.03 ni <0.04 <0.03 ni 0.03 0.1 1.0 ni ni Vni ni 1.9 39.0 ni ni ni ni 92.7 ni ni 400 135 Mo 0.29 ni 1.7 <1 ni ni 2 ni 3.8 ni ni 50 1.5 As <0.1 ni 0.3 <3 ni 0.38 2.7 ni 13.0 14 40 50 1.8 ni–not indicated; Limit FF–current Finnish limit values for ash used as a forest fertilizer; Limit CM–limits in Finnish legislation for ashes use as an earth construction material; * Average crustal abundance Table 5reveals low concentrations for PTM in GLD, SG, LM, and BFA. The legal Limit FF is fulfilled for all elements, except for Ni in the GLD, whereas the Limit CM is exceeded only once for Zn (concentrations marked in bold in Table 5). The concentrations of PTM can also be compared to average crustal abundances and it is possible to conclude that some of the elements can be significantly enriched in these anthropogenic materials. Indeed, the concentrations of Pb, Cd, Cu, Cr, Ni, and Zn may be enriched in all wastes. However, it is possible to support the beneficial use of inorganic wastes instead of landfilling [ 19 ]. The discussion presented in the next section will demonstrate that among the possibilities of utilization, some of them encapsulate the waste in a matrix, and thus reduce the leaching processes. Indeed, from an environmental point of view, the leaching behavior of each PTM is more important than the total elemental content. It is well known that the leaching behavior of each element may present a specific pattern: amphoteric leaching (high leaching in acidic and basic pH conditions, with a U shape), cationic leaching (with high leaching in acidic pH), and a leaching pattern not dependent on the eluate pH. Moreover, the leaching can be controlled by solubility restrictions or availability. Thus, for each application, the leaching behavior must be considered to assess the environmental impact. In the literature, diverse studies addressed the leaching of these wastes. Jia et al. [ 4 ] analyzed the leaching of GLD regarding Ca, Fe, K, Mg, Na, Si, Al, As, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sn, and SO 42− [ 4 ]. The leaching solutions of GLD revealed low Eh, high pH and EC, and thus low mobility of metals. The legal limits for inert landfills were exceeded only in a few samples for As, Cr, and Zn. Watkins et al. [ 19 ] highlighted that the very low metal concentrations in SG supports utilization instead of disposal in landfill, indicating that the concentrations of As, Ba, Cd, Cr, Cu, Mo, Pb, V, and Zn are clearly lower than the allowable concentrations in other materials used as an earth Appl. Sci. 2020,10, 2317 8 of 20 construction agent. In addition, the leached concentrations of heavy metals, chloride, fluoride from SG were lower than the EU limits for disposed waste in inert waste landfills. Nevertheless, sulfate can exceed the limit for inert waste landfills (1000 mg/kg dw), since the value found was 5250 mg/kg. Additionally, Cherian and Siddiqua [ 32 ] concluded that fly ash from pulp and paper mills is in general considered non-hazardous, since PTM are mainly held in the amorphous aluminosilicate phases, and thus with low solubility. Ribeiro et al. [ 49 ] produced a glass from BFA and the material exhibited a satisfactory leaching behavior. Moreover, Alvarenga et al. [ 43 ] highlighted that biomass ashes produced in Portugal contain very low metals concentration. Cabral et al. [ 25 ] tested GLD, SG, and LM as alternative liming materials and concluded that metals do not appear to be a limiting factor for their use in the soil. 4. Potential Applications Although kraft pulp mills have developed sophisticated integrated waste management plans, with the objective of minimizing the generation of waste and landfilled quantities, possibilities for reuse, recycling or recovery are still lacking. The main objective of waste management in the pulp industry includes its use on-site or in other industries following an industrial ecology approach. Figure 2 provides an overview of the potential categories of applications found in the literature for GLD, SG, LM, and BFA. Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 21 Figure 2. Categories of the potential applications for inorganic wastes from the pulp industry. 4.1. Green Liquor Dregs Chemical characteristics of GLD such as the strongly alkaline pH (>10), the presence of alkaline and alkaline-earth oxides, and the lack of knowledge of its long-term chemical stability, technical performance and environmental impact of several applications can hinder its potential incorporation in materials. This may explain why this industrial by-product remains unexplored on an industrial scale. Despite that, several studies have attempted to search for alternative management routes, as listed in Table 6. Reducing disposal in landfills can result not only in significant environmental and economic benefits, but also in reducing the need for natural raw materials. Table 6. Applications of GLD reported in the literature. Application Highlights Scale Construction materials Concrete GLD as a replacement of part of the cement in concrete is not suitable since the loss in mechanical properties is significant [23]. Laboratory Cement Substitution of clinker up to 10% is feasible to obtain Portland cement (CP I-S and CP II-F) [50]. Laboratory Clinker production The mixture of GLD (0.13 wt %) with standard materials is technically viable and does not present noticeable environmental effects [34]. Industrial Geopolymer mortars GLD can be used as a fine filler up to 25 wt % incorporation. The obtained mortars exhibited enhanced tensile and compressive strength [29]. Laboratory Geotechnical Landfill cover Alternating layers of 0.15 m of GLD (70 wt %) and SG (30 wt %) covered by 1 m of MSW have the potential for replacing soil as intermediate covering in landfills [27]. Industrial Figure 2. Categories of the potential applications for inorganic wastes from the pulp industry. 4.1. Green Liquor Dregs Chemical characteristics of GLD such as the strongly alkaline pH (>10), the presence of alkaline and alkaline-earth oxides, and the lack of knowledge of its long-term chemical stability, technical performance and environmental impact of several applications can hinder its potential incorporation in materials. This may explain why this industrial by-product remains unexplored on an industrial scale. Despite that, several studies have attempted to search for alternative management routes, as Appl. Sci. 2020,10, 2317 9 of 20 listed in Table 6. Reducing disposal in landfills can result not only in significant environmental and economic benefits, but also in reducing the need for natural raw materials. Table 6. Applications of GLD reported in the literature. Application Highlights Scale Construction materials Concrete GLD as a replacement of part of the cement in concrete is not suitable since the loss in mechanical properties is significant [23]. Laboratory Cement Substitution of clinker up to 10% is feasible to obtain Portland cement (CP I-S and CP II-F) [50]. Laboratory Clinker production The mixture of GLD (0.13 wt %) with standard materials is technically viable and does not present noticeable environmental effects [34]. Industrial Geopolymer mortars GLD can be used as a fine filler up to 25 wt % incorporation. The obtained mortars exhibited enhanced tensile and compressive strength [29]. Laboratory Geotechnical Landfill cover Alternating layers of 0.15 m of GLD (70 wt %) and SG (30 wt %) covered by 1 m of MSW have the potential for replacing soil as intermediate covering in landfills [27]. Industrial Road pavement construction GLD require washing before incorporation as aggregates in bituminous mixtures to guarantee stability in terms of water sensitivity [36]. Laboratory Sealing layer in mines A mixture with the proportions 7:2:1 of tailings: GLD: fly ash was found to be geotechnically satisfactory to be used as a sealing layer in dry covers on mine [37]. GLD showed high water retention capacity and low hydraulic conductivity, which prevents water percolation and oxygen transport [51]. Laboratory Hot-mix asphalt GLD used as filler in hot-mix asphalt leads to poor water resistance, despite displaying adequate mechanical properties (stiffness and permanent deformation) [52]. Laboratory Environmental Neutralize acidic wastewaters The liming effect of GLD (39.6% Ca eq) is similar to commercial limestone (38%). The pH of 10.7 indicates a strong liming effect [53,54]. Industrial Acid mine drainage remediation GLD exhibited high buffering capacity [4] at low dosages of 1 g/L [55] for remediation of acid mine drainage. Laboratory Soil amendment Doses up to 20 t/ha achieved neutralization of acidic soil, not causing deterioration of soil properties or depressing crop yields [56]. After 5.5 years since application, positive effects were observed on the soil chemical attributes [57]. Laboratory Drying adjuvant of sewage sludge A dose of 0.15 g GLD per g of sewage sludge reduced by 8% the energy required for the evaporation of humidity at 130 ◦C. A reduction in phytotoxicity was also observed in tests with garden cress, revealing a good potential for agricultural applications [34]. Laboratory Agricultural Liming material Valid option to substitute commercial agricultural limestone [ 25 , 41 ]. Industrial Co-composting The addition of a moderate amount of GLD (5–8 wt %) with kraft mill sludge did not show a negative effect on the biological activities during the composting process [58]. Laboratory Despite the interest in developing applications for GLD, some studies have revealed its properties inadequate for the intended purpose. For example, the incorporation of GLD as cement replacement in concrete yielded products with inappropriate quality [ 23 ]. GLD also disclosed inadequate water resistance for hot-mix asphalt for road pavement in geotechnical applications [ 52 ]. However, a few Appl. 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