1 Introduction

The mining industry is the main generator of waste worldwide. In Sweden, 116-million-ton mine waste were generated in year 2020, accounting for two thirds of the annual waste produced [1]. Sulfide minerals, in Sweden 70% of the mine waste, in contact with oxygen and oxidizing bacteria, such as acidophiles, are in risk to generate acid rock drainage (ARD) [2,3,4]. ARD is a multifactor pollutant that can become a major long-term threat to the environment, physically, chemically, and ecologically [3,4,5,6,7,8,9]. There are different methods to prevent ARD formation after closure [10], where the two main categories are engineered barriers and water covers. Dry covers are engineered covers with the purpose of limiting the infiltration of water and oxygen into the underlying waste and by that reducing the ARD formation [10,11,12,13]. In Sweden, soil covers are generally used as dry covers and consist of a sealing layer placed on top of the mine waste and above this, a protective layer that protects the underlying layers from erosion and frost-/root penetration. The sealing layer is usually made of a compacted fine grained material, such as a clayey till, that is kept close to saturation to prevent oxygen diffusion. There is however a great shortage of available sealing layer material close to mines and the need for alternative materials is prominent. Bentonite amendments to a local till are one solution to improve the sealing layer qualities of the till [14,15,16]. Nonetheless, bentonite production is both time- and resource consuming, which makes it expensive both economically and environmentally. Fine grained industrial residues might be a better solution for an amendment to the local till. Green liquor dregs (GLD), an inert residue of pulp production, have shown to have properties suitable as a sealing layer [17,18,19], i.e., it is fine-grained (d100 < 63 µm) and has a low hydraulic conductivity (10−8–10−9 m/s), a high water retention capacity (WRC; compared to materials with similar particle size) [17], and a low oxygen diffusion coefficient [18]. According to the Confederation of European Paper Industries (CEPI), 38 Mt of pulp was produced in year 2018 [20]. Sweden, Finland, and Portugal are the top three countries accounting for 69% of the pulp production [21]. In Sweden, about 200,000 tons of GLD were generated annually according to a survey made in 2003 [22], and production has increased based on a survey made in 2017 (unpublished data). Most of the GLD is today landfilled and there is a great drive to find use of the waste product. Using an industrial residue in a mine remediation program can be economically and environmentally beneficial for the mining industry and the industry providing the residue. The downsides of GLD are its low shear strength and high water content, which makes it unstable from a soil mechanical point to use on its own [17;19]. Mixing a locally available till with approximately 10 wt.% GLD has in previous studies shown to keep the GLD positive properties and improve its soil mechanical properties [18, 23, 24]. The specific mixing recipe is however highly dependent on the properties of the till and the GLD that is used in the mixture, mainly their initial water content [24, 25]. There is much work done on the field application of mine waste covers and the use of different industrial residues in these covers, e.g., sewage sludge, fly ash, desulfurized tailings, coal combustion by-products (CCB), and steel slag [26,27,28,29,30,31,32,33,34], but none for the authors known published work is yet done on the field application of GLD as a sealing layer material in mine waste remediation.

In this study, GLD and a local silty till were characterized individually and as mixtures with the objective (i) to find the optimal recipe, with the lowest possible hydraulic conductivity, to be used in a field application of a sealing layer on top of a waste rock dump at Näsliden Mine in northern Sweden. Another objective was (ii) to perform an application of the optimal GLD-till mixture in the sealing layer of the Näsliden mine waste. The final objective was (iii) to characterize 15 GLD from different paper mills to valorize the GLD as a potential product.

2 Methods

Laboratory studies were conducted during year 2016 and forward, and the field application of the sealing layer made of GLD amended till in the summer of year 2016.

2.1 Materials

The till that was used in the sealing layer was a local silty till from the vicinity of the Näsliden mine. The GLD that was used in the field application of the Näsliden mine derived from BillerudKorsnäs paper mill in Karlsborg. The GLDs that were used for characterization derived from 15 different paper mills in Sweden: BillerudKorsnäs AB, Frövi/Rockhammar; BillerudKorsnäs AB, Gruvön; BillerudKorsnäs AB, Gävle; BillerudKorsnäs AB, Karlsborg; BillerudKorsnäs AB, Skärblacka; Domsjö Fabriker AB; Holmen AB, Iggesund; Mondi Dynäs AB, Munksjö; Aspa Bruk AB; Munksjö Paper AB; Billingsfors; Rottneros AB, Vallvik; SCA Munksund AB; SCA Obbola AB; SCA Östrand; Smurfit Kappa Kraftliner Piteå AB; Stora Enso AB Skutskär; Stora Enso Skoghall AB. Four of these GLD are pictured in Fig. 1 to illustrate their differences.

Fig. 1
figure 1

Picture of GLD from Smurfit Kappa (left) and from four different paper mills in northern Sweden. A Iggesund. B Smurfit Kappa. C Vallvik. D BillerudKorsnäs (Karlsborg)

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2.2 Sampling of GLD

Sampling of the GLD was conducted during spring and fall of year 2017 and 2018 and spring of year 2019. One to three subsamples was taken in sealed buckets of approximately 10L. The sampling was made by the mills at the end of the GLD processing line and sent to Luleå University of Technology for chemical and physical analysis (total solid content, particle size distribution, plasticity index, yield point, proctor compaction, and hydraulic conductivity).

2.3 Particle Size Distribution

The till was washed and dry sieved according to SS-EN 933–1:2012 to obtain the weight percentage of fines in the material. A mechanical sieve tower with cut-off sizes 12.5, 10, 8, 5, 4, 2, 1, 0.5, 0.25, 0.125, and 0.063 mm was used.

PSD for the GLD was done by laser diffraction analysis on triplicate samples of each material by a CILAS Granulometer 1064 (CILAS, Orléans, France). The PSD was then calculated using the CILAS software.

2.4 Total Solid Content (TSC), Plasticity Index, and Yield Point

The TSC was performed according to the SIS standard SS-EN 14346:2007. Plasticity index was decided using standard SS 27121:1990 and yield point using standard SS 27120:1990.

2.5 Proctor Compaction and Density Measurements

The till and the till-GLD mixture were characterized with the modified- (PC) and standard- (LC) compaction method, SS-EN 13286–2:2010 to obtain the optimal water content and the maximum dry- and bulk density.

2.6 Hydraulic Conductivity

For hydraulic conductivity, the constant head-method [35] in airtight cylinders was used. The samples were compacted in the cylinders with standard proctor compaction method (LC) and modified compaction method (PC; for more information, see Sect. 2.5). Water was led to the bottom of the cylinder with the hydraulic gradients, sample areas, and sample heights presented in Table 1. The hydraulic conductivity was calculated using Darcy’s law.

Table 1 Hydraulic heads, sample areas, and sample height (cm) for the hydraulic conductivity measurements in till and its mixtures with GLD (BillerudKorsnäs, Karlsborg). LC, a lighter proctor compaction
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2.7 Chemical Analysis

The GLDs were chemically analyzed by an accredited commercial laboratory (ALS Scandinavia AB in Luleå, Sweden) by semi-quantitative screening analysis, ICP-SMS (HR-ICP-MS).

2.8 Field Measurements and Application

The field application of the GLD-amended till in Näsliden mine, northern Sweden (Fig. 2) was conducted during summer 2016. The GLD-till mixture was used as part of a 0.5-m thick sealing layer (Fig. 2) that later was instrumented and monitored [36, 37]. The plan was to use the GLD-amended till in the sealing layer of the whole cover system on top of the Näsliden waste rock dump. However, due to problems with the GLD production and its variability within the paper mill, 4 wt.% of bentonite was used as an amendment to the till in the rest of the sealing layer (see Sect. 4.1 for more discussion).

Fig. 2
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A map over the Näsliden mine and the waste rock dump that was covered (right) and a schematic profile picture of the cover application (left)

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The field application of the till-GLD mixture is summarized in Fig. 3 and can be divided in to six different steps:

  1. (1)

    Extraction of the till from the local till quarry. Sorting it, i.e., removing material with a particle size above 20 mm. Transporting and storing the material to the place where mixing is to be conducted.

  2. (2)

    Transporting the GLD from the paper mill to the place where the mixing is to be conducted (Fig. 4A and B).

  3. (3)

    Dosage of the quantities of till and GLD that is to be used in the mixtures (Fig. 4A and B).

  4. (4)

    Mixing the till and the GLD with an ALLU excavator bucket DH 4-17-X75 (Fig. 4C and D). Premixing was performed by a shovel (Fig. 4B).

  5. (5)

    Exposition of the sealing layer consisting of till and 10 wet weight percentage of GLD and compaction of the layer with a road roller (Fig. 5A).

  6. (6)

    Exposition of the protection layer consisting of the local till (Fig. 5B).

Fig. 3
figure 3

A schematic picture of the different steps conducted during field application. TSC, total solid content; w, water content; PSD, particle size distribution; ρ, bulk density

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Fig. 4
figure 4

A GLD storage seen in the background and Till-GLD mixture being produced by a forced mixer in the forefront. B Storage of GLD covered by tarpaulin in the background. In the forefront, pre-mixing of Till-GLD by shovel is performed. C Mixing performed by a DH 4–17-X75 ALLU shovel (D)

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Fig. 5
figure 5

A Compaction of the sealing layer in one step by a road roller (final thickness 0.5 m). B Distribution of the protection layer by a bulldozer (year 2016)

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Density in field was measured on the surface of the sealing layer by MCV (Moisture Condition Value) and with a Troxler nuclear density gauge at a depth of 50, 100, and 300 mm.

3 Results

3.1 Characterization of the Sealing Layer Materials

3.1.1 GLD

There was a great chemical variation between the different GLD, up to approximately 2500 order of magnitude difference (Zn; Table 2). Other elements that differed significantly between the different GLD were Cd, Co, Cu, and W (Table 2).

Table 2 Chemical elements (mg/kg) in the GLD from BillerudKorsnäs in Karlsborg (B), in variation of GLD from 15 paper mills (15) and the Swedish guidelines for contaminated ground (*) [38]
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The GLD from BillerudKorsnäs paper mill in Karlsborg was chosen to be used in the field application of a sealing layer on top of the waste rock dump at the closed Näsliden mine (See Sect. 4.1 for more discussion). The main elements of the Billerud GLD were Ca > Na > Mg > S > Mn > Si (Table 2). Lime mud added in the process is the source of the high Ca concentrations.

The typical production of GLD for the 15 Swedish paper mills is between 1200 and 35,000 ton per year. Physical characterization of GLD from the paper mills is presented in Table 3. The TSC of the GLD differs between 33 and 75%, clay content between 3 and 28%, plasticity index (wP) between 46 and 170%, the yield point (wL) between 60 and 330%, and the hydraulic conductivity between 1 × 10−7 and 4 × 10−9 m/s (Table 3).

Table 3 Physical characterization of the BillerudKorsnäs GLD and a variation of GLD from 15 different paper mills. Total solid content, TSC (%); particle size distribution, PSD: % clay (< 2 µm), silt (< 63 µm), and sand (< 2000 µm); compact density (ρs; g/cm3); hydraulic conductivity (k; m/s); plasticity point (wP); yield point (wL); and plasticity index (Ip)
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The TSC in the GLD from 15 different paper mills differed between 33 and 75% (Table 3), indicating a high variability. The TSC in the GLD used in this study (58%; BillerudKorsnäs, Karlsborg paper mill) is somewhere in the middle of that range. The PSD shows a material that mainly consists of silt, 90% in the GLD from BillerudKorsnäs, Karlsborg (Table 3). The hydraulic conductivity in the Billerud GLD is around 4 × 10−9 m/s, and the variation in hydraulic conductivity between the different GLDs is great; 1 × 10−7 to 4 × 10−9 m/s (Table 3). The plasticity point (wP) and the yield point (wL) vary significantly in the different GLDs (wP between 46 and 170, wL between 60 and 330; Table 3).

3.1.2 Amended Till

With an addition of GLD to the till, the hydraulic conductivity decreases from 2 × 10−7 m/s to 3 × 10−8 m/s (Table 4).

Table 4 Physical characterization of the materials. Optimum molding water content, omw; N, number of samples; LC, lighter proctor compaction
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There is no significant difference in hydraulic conductivity between 10 and 15 wt.% GLD addition (Table 4; 2.7, resp. 3.1 × 10−8 m/s), but a slight decrease in hydraulic conductivity and dry density can be detected with increasing amount of GLD added to the till. Water content increases with increasing amount of GLD added to the silty till (Table 4). There is no significant difference in either hydraulic conductivity or dry density comparing modified proctor compaction to lighter proctor compaction (Table 4).

3.2 Field Application

The TSC of the incoming materials averaged between 41 and 49% for GLD, 92–94% for till, and 89–92% for the till-GLD-mixtures (Table 5). Dry density at 300 mm depth in the sealing layer varied between 1.67 and 2.05 g/cm3 (Table 5 and Fig. 6).

Table 5 A summary of the results from the different steps (described further in Sect. 2.8) when conducting the field application of the till-GLD mixture (three sets of measurements). Total solid content, TSC; dry density, ρd
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Fig. 6
figure 6

Dry density (ρd; g/cm3) and water content (w; %) measured in the sealing layer at 36 different measuring points at 50-, 100-, and 300-mm depth. The target values are presented as MIN and MAX

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4 Discussion

4.1 Characterization of the GLDs

The GLD from BillerudKorsnäs paper mill in Karlsborg was used in the field application of a sealing layer on top of the waste rock dump at the closed Näsliden mine. It was chosen due to its proximity to the mine (low transports costs), and because there were enough amounts of GLD at the industry ready to be used in the application. It also met the criteria of a high TSC, making it easy to handle. A previous study by Nigéus et al. [25] has shown that the initial water content of the material has a significant impact on the final hydraulic conductivity of the mixtures. With a drier GLD, more of the material can be used in the mixture which enhances the desired sealing layer properties, such as a high WRC [25].

When comparing the values to the Swedish Environmental Protection Agency’s limits for contaminated soil [38], Zn, Cu, Cr, and Ba had concentrations above those limits (Table 2). These enrichments are consistent with a study by Moyo et al. [19], on GLD (among other industrial residues). To be considered is that the GLD in this application was mixed with till, which means that the concentrations from the GLD will be diluted and are therefore not expected to reach over the limit values. Sequential extraction has in a previous study been performed on GLD and indicates low bioavailability of metals in general [39]. However, in contradiction to this and other studies [40] and [19], a study made by Bandarra et al. [41] indicated GLD as a “possible hazard” according to the chemical analysis and the biotests made in the study. The experimental study indicated high ecotoxic effects for three out of five organisms. The purpose of this study is however to use the GLD as an amendment to till, requiring only a small amount of GLD in the mixtures (10 wt.%). It is also to be used in the sealing layer which purpose is to retain the water reaching it, not flushing it out. This means that there should not be much chemical leachate from the GLD reaching out to the biosphere. Further future studies are necessary to confirm this.

In the BillerudKorsnäs, Karlsborg GLD, the Cd concentration (1 µg/l) was slightly above the Swedish Environmental Protection Agency general guidelines for contaminated soil (> 0.8 mg/kg; Table 2), but other trace metals are under these guidelines [38].

The TSC of the 15 GLD differs between 33 and 75%, clay content between 3 and 28%, plasticity index (wP) between 46 and 170%, the yield point (wL) between 60 and 330%, and the hydraulic conductivity between 1 × 10−7 and 4 × 10−9 m/s (Table 3). TSC and hydraulic conductivity are consistent with a study by Mäkitalo et al. [17] that characterized four different GLDs. The hydraulic conductivity is also consistent with the GLD studied by Moyo et al. [19]. However, the plasticity index in the BillerudKorsnäs, Karlsborg GLD (Table 3) is significantly higher than the plasticity index in the GLD studied by Moyo et al. (24/62 compared to 7–17) [19], further indicating the high variability of the GLD as a material.

Generally, GLD has a high water content due to its high water retention capacity, it also has a particle size distribution consisting mainly of particles in the silt fraction [17, 42]. This can be seen in this study as well (Table 3), where the TSC in the GLD used in this field application (BillerudKorsnäs, Karlsborg paper mill) is 58%. The PSD, with 90% of the GLD is in the silt fraction, is also consistent with the study by Mäkitalo et al. [17] (Table 4). The compact density of the Billerud GLD is 2.63 g/cm3 and hydraulic conductivity is around 4 × 10−9 m/s (Table 3), which is also consistent with other studies [17, 42]. According to the Swedish Geotechnical Societies classification of the plasticity limits [43], the GLD, with a plasticity index (IP) of 24–65 (Table 3), can be classified as a plastic to a very highly plastic material. This is a higher plasticity that is seen in the GLD in Moyo et al. [19].

This study demonstrated that there is a great chemical and physical variation in GLD, between different paper mills. GLD also varies between different batches within the same paper mill, demonstrated in a study by Mäkitalo et al. [17]. The great variability creates great challenges when GLD is to be used in a sealing layer. There is no guarantee that the properties of the GLD in the planning phase of a project are the same as when the GLD is delivered to the mine site, even though a characterization of the materials has been made. This makes valorization of the material difficult. The great variability in the GLD (i.e., dry density, initial water content, porosity) affects the properties of the GLD-till mixture (i.e., compaction degree, hydraulic conductivity, water retention capacity), and in the end, the effectiveness of the sealing layer.

4.2 Recipe of GLD Amendment to a Silty Till

The locally available silty till had a higher hydraulic conductivity than the requirements for a sealing layer material in Sweden (< 10−8 m/s). With an addition of GLD to the till, the hydraulic conductivity decreases from 2 × 10−7 m/s to 3 × 10−8 m/s (Table 4). This hydraulic conductivity is still not below the recommendations, which is consistent with other studies where hydraulic conductivity in till-GLD mixtures has been studied [24, 25]. The hydraulic conductivity after compaction might have been lower if the GLD amended till had a molding water content (11%; Table 4) closer or slightly above the optimum water content of 13.9% (Table 4). Compaction at optimum water content leads to higher degree of compaction and thereby higher dry density [25, 44,45,46]. It has been shown that the hydraulic conductivity is sensitive to the molding water-content, where the lowest hydraulic conductivity can be reached at a molding water content of 1–2% wet of the optimum water content [25, 47]. However, previous studies by Nigéus et al. [25] on till-GLD mixtures indicate that it is not likely that the saturated hydraulic conductivity would decrease to below 10−8 m/s with a drier mixture. However, the high water retention capacity of the GLD and its mixtures with till [17, 18, 25] usually corresponds to a high saturation in the sealing layer which decreases the oxygen diffusion through the layer to the underlying reactive wastes, and by that reduces ARD [11,12,13].

A slight decrease in hydraulic conductivity and dry density can be detected with increasing amount of GLD added to the till (Table 4; 3.1 to 2.7 × 10−8 m/s). This was expected and consistent with previous studies by Nigéus et al. [24, 25] that studied hydraulic conductivity in till amended with 5–20% GLD. The increase in hydraulic conductivity is likely due to an increase in clay size particles leading to a decrease in micro porosity and an increase in the water retention capacity [24, 25].

The paradox decrease in dry density, that is seen with an increasing amount of GLD added (Table 4), is also consistent with previous studies [24, 25] and might be explained by the increasing water content beyond the optimal water content (Table 4) and the lower particle density of the GLD. There is no significant difference in either hydraulic conductivity or dry density comparing modified proctor compaction to standard proctor compaction (Table 4), this as well, is consistent with the study performed by Nigéus et al. [25], where different compaction efforts were studied. This is positive results considering that the material is to be applicated and compacted in field, and laboratory tests tend to under-predict hydraulic conductivity compared to actual values in the field [48]. Despite the lack of significant differences between compaction effort in this study, lighter compaction effort might still be recommended due to the properties of the GLD (low shear strength) [17] and the aim to mimic field conditions.

The laboratory study concluded that for the field application 10% of GLD was the best recipe to be amended to the local till. As no significant improvement in hydraulic conductivity with higher amounts of amendments could be detected, and the cost for transportation increases, the more amendments are used. It also gets more and more difficult to handle and compact the more GLD that is added to the mixture. The GLD from BillerudKorsnäs paper mill in Karlsborg, northern Sweden was chosen as it is relatively close to the mine (low transport costs), it was not too wet (easier to handle), and there were enough amounts of it at the industry ready to be used in the application. The characterization of the 15 different GLDs concludes a vast variation in both physical and chemical properties. This makes a potential valorisation difficult as the properties of the GLD in the planning/laboratory phase of a project might be significantly different from the GLD that is delivered to be used as a product.

4.3 Field Application of 10 wt.% GLD-Amended Till

The aim was to reach a water content between 10 and 14% in the mixtures and to compact the sealing layer to a dry density between 1.78 and 1.95 g/cm3, which was reached in most measuring points (Fig. 6). These targets values were chosen with consideration of the compaction properties, i.e., the relation between the water content and the compaction degree, which are the results of previous unpublished laboratory work with the materials. Considering the optimal water content of the mixture and the results from the studies by Benson and Trast [47] and Nigéus et al. [25], where a water content 1–2% wet of the optimal molding water content, it might have been more beneficial to strive for a water content of 15–16% (Table 4). When looking at the field measurements in Fig. 4, not many measuring points reached 15–16%, and a water content below this might lead to a mixture that is too dry to reach the lowest hydraulic conductivity possible. This in mind, previous studies [24] indicate that the difference in hydraulic conductivity might not be significant enough to make a difference in the effectiveness of the sealing layer.

The goals of the field application, i.e., a water content of 10–14% and a dry density of 1.78–1.95 g/cm3 where mostly fulfilled and the field application, can at that point be seen as successful. However, due to problems with deliveries and production of GLD from the paper mill and time pressure, only a small area of the waste rock dump was covered with GLD-amended till. For the rest of the sealing layer, a till amended with 4 wt.% of bentonite was used.

5 Conclusions

The laboratory study concludes that 10 wt.% of GLD is the optimal quantity to be amended to the local till (that on its own does not meet the by the mining company required hydraulic conductivity of < 10−8 m/s) that is to be used as a sealing layer on top of the Näsliden waste rock dump. Higher quantities of amendments did not significantly improve the hydraulic conductivity and the cost for transportation increases, the higher the quantities of amendment. Even if the hydraulic conductivity did not improve significantly with GLD addition, previous studies have shown that the water retention capacity increases with GLD addition to a silty till [24] and by that deterring ARD formation by keeping the sealing layer close to saturation. The goals of the field application were to reach water contents of 10–14% and dry densities of 1.78–1.95 g/cm3 where mostly fulfilled and the field application can be seen as successful. However, considering a previous study by Nigéus et al. [25] where the lowest hydraulic conductivity was reached at 1–2% wet of the optimum molding water content, the sealing layer applied at Näsliden was too dry. Worth regarding is however that the increase in hydraulic conductivity seen in the study [25] was not that significant that it might affect the function of the sealing layer considerably.

This, as well as other studies [17] can conclude that there is a great variability in physical and chemical characteristics of GLD. Both between different paper mills but also in GLD within the same paper mill. This great variability creates challenges when GLD is to be used in a sealing layer. There is a risk that the properties of the GLD in the planning phase of a project are completely different than when the GLD is delivered to the mine site, even though a characterization of the materials has been made. In future studies, it would be interesting to monitor the different materials used in the sealing layer and to compare their effectiveness as sealing layer material.