4.1. Material Variation
To enable the reuse of GLD, the effect of varying material compositions on the desired function should be understood to identify possible limitations. The dry content of GLD monitored at the Billerud mill ranges between 29% and 72%, commonly within the range 36% and 54%. Increasing dry content is a consequence of larger amounts of lime mud scraped off from the filter coating. This decreases the relative concentration of non-process elements.
The XRD analysis indicated the presence of calcite and small amounts of brucite, which is in agreement with Martins et al.
], while no other buffering minerals were detected. Calcite crystals were detected by SEM analysis. The chemical analysis showed large amounts of sulphur but no crystalline forms of this element was detected. However, Martins et al.
] found gipsite in GLD, not detectible by XRD. Ettringite and gypsum are other solid forms of sulphate that may be present in small amounts and therefore not picked up by XRD due to the instruments detection limit. It is likely that mainly amorphous and liquid forms of sulphate are present in GLD and can therefore not be detected with XRD. Based on the ANC, up to 75% of the GLD content is CaCO3
, assuming that the source of alkalinity that is buffering around pH 8.3 is only CaCO3
. The higher content of Ca in batches A and B (Table 1
) explains the higher acid buffering capacity observed compared to the two other batches (Figure 3
). The observed variation in the chemical and mineralogical composition of the GLD can be caused both by the origin of the wood and the efficiency of the GLD retrieval. The variation between batches had no strong effect regarding the hydraulic conductivity, buffering capacity, pH and particle size of the studied materials compared to the natural variation observed in the replicates. However, the surface area, the density and the porosity varied significantly between the batches. The observed variation may complicate the use of the residual product on a large scale. Therefore potential negative effects of the quality variation on the intended application of the material should be addressed.
Acid neutralization capacity for GLD batch A, B, C and D.
Acid neutralization capacity for GLD batch A, B, C and D.
4.2. Use of GLD in Sealing Layers
A sealing layer applied on reactive mine waste should function as a barrier for oxygen transport. Pore size and degree of saturation affect the effective diffusion coefficient, De
, which in turn affects the oxygen flux through the material. The porosity forms channels for oxygen diffusion if the pores are not saturated. De
increases with increasing pore size and quantity [29
] and the oxygen penetration decreases when the particles are irregularly shaped compared to a more regular structure [30
]. Decreasing particle size is correlated to increasing total porosity [29
]. GLD is a fine-grained material. Comparing the particle size distribution of GLD with sealing layers made of clayey till [1
] shows that GLD particles are significantly smaller with a d50 of 12 μm compared to 60 μm for the till. A sealing layer should be homogeneous to achieve uniform water saturation and GLD has a small particle size throughout the material. As the porosity of GLD is high, >70%, oxygen diffusion may occur if it dries. Oxygen diffusion decreases sharply as the degree of saturation increases above 85% [1
]. WRC is a hydraulic property affecting the transport of fluids in the material. GLD has very high water retention potential, much higher compared to other cover materials (Figure 2
) such as clayey till. The high WRC is assumed to be due to the grain structure together with the ionic charge of the particles. Although the particle size falls into the range of silt, the WRC is more comparable to clayey materials than silty materials, especially at high under-pressures. Due to the high WRC, GLD has good potential to reduce the oxygen flux through a sealing layer by maintaining saturated or close to saturated conditions. The risk of shrinkage and cracking due to desiccation is reduced, which is otherwise a common problem in clay and may have a significant impact on the sealing layer performance.
A sealing layer should consist of a material that decreases water infiltration. Decreasing the particle size of a material is usually correlated to decreasing its hydraulic conductivity [31
]. Hydraulic conductivity is negatively correlated to porosity in the case of fine-grained material [33
] and the specific surface area [32
]. A high porosity enables water to enter the material, but the large surface area also enhances chemical interactions binding water to particles, thereby limiting water transport, resulting in high grade of saturation. Our results showed that GLD has adequate characteristics, such as a small particle size, high porosity and average surface area of 18 m2
/g, to limit the water flow through the material. The tests confirmed a relatively low hydraulic conductivity, between 1 × 10−8
and 7 × 10−9
m/s. However, it did not reach the recommended minimum hydraulic conductivity [1
] of 1 × 10−9
There are several advantages of using GLD over virgin material such as till and other natural soils for construction of sealing layers. Opening quarries is not only costly but also has a significant environmental impact. A shortage of fine-grained cover material close to the mines is often an issue [2
]. However, because the quality of GLD varies, there is a risk that not all of the GLD produced would have the appropriate characteristics for use in sealing layer applications. Based on the results, GLD with low hydraulic conductivity would be most suitable to use for construction of sealing layers. However, the small variations observed between the batches of GLD from Billerud are not expected to lead to complications if used for barrier applications. GLD has previously been used in applications such as a stabilization agent for road construction [15
Ideally, the material used as a sealing layer should both decrease oxygen penetration, water infiltration and simultaneously function as a chemically reactive barrier retaining heavy metal ions originating from the mine waste. A major concern when using residual products is assessing their potentially negative impact on the environment. Both undesired alteration of the material, which may alter its sealing function, and side effects on underlying mine waste caused by GLD leachates are potential issues to be addressed. The GLD from Billerud, however, has low content of most potentially harmful metals. The leaching data indicated that a large amount of the K and S content was leached. It is possible that the sulphate leached from the GLD may increase the chance of gypsum precipitation and could decrease the chance of jarosite formation in the underlying sulphidic waste [35
]. It can be concluded that calcite was not dissolved despite of the repeated leaching test. The pH remained at a high level (pH 11.6). The leaching behavior is likely to change if the material is acidified resulting in a lower pH. Due to the production process of GLD (burned at high temperatures) it does not contain organic matter meaning that GLD is not susceptible to organic matter degradation that may harm the integrity of the sealing layer [28
]. The material also lacks elements that are likely to interact and transform into harmful substances or oxidize easily, thereby changing its initial structure and properties. The ζ-potential is similar to those of kaolinite and montmorillonite at lower pH, but the clay minerals show a larger negative charge at high pH [36
]. The ζ-potential had a negative correlation with pH which can be related to its high content of CaCO3
and is consistent with other electrokinetic studies on calcite summarized by Guichet et al.
]. For pH below 8, the presence of CaCO3
decreases leading to more free Ca2+
in solution [37
]. The measured ζ-potential should therefore be less negative for pH below 8, which is consistent with the result. The electrical properties of calcite are a controversial topic. Electrokinetic measurements have shown to be dependent on CO2
partial pressure and have yielded different results on calcites of various origins [37
]. The calcite in the GLD has a negative ζ-potential at all pH levels, which is not the case with kaolinite. This indicates that the GLD particles are relatively stable and resist the formation of aggregates [38
]. This, together with the high WRC, suggests that crack formation due to aggregation, providing a pathway for water and oxygen, would be limited. The function can be maintained independent of the pH.
The buffering capacity of GLD is high, which is consistent with its high carbonate content. To maintain pH > 6, the average neutralization capacity was 18.5 mmol H+
/g dry weight for the four batches of GLD. The high buffering capacity indicates that GLD could act as an alkaline barrier. The high grade of saturation will minimize oxygen intrusion into the underlying sulphidic waste, slowing down oxidation. Only small amounts of alkalinity would therefore be consumed by the mine waste, resulting in a long lasting sealing layer. This is in agreement with Jia et al.
] who showed that it is feasible to use GLD as a cover material to immobilize trace metals in tailings. Leaching experiments have shown that the amount of elements released was below the leaching limits for non-hazardous waste regulated by the European Union [25
], making it a good candidate for sealing layer applications. In addition, it has been shown that addition of 10% green liquor dregs to reactive, sulphide-rich tailings reduced leaching of metals such as Cu, Co, Cd and Ni [39
]. A probable explanation is that GLD increases pH, thereby immobilizing metal ions by precipitation of secondary minerals and absorption to mineral surfaces [1
The results indicate that the application of GLD may be effective in the Swedish subarctic climate since the neutralization capacity is expected to last for a long time because the dissolution of buffering compounds such as CaO, Ca(OH)2 and CaCO3 will be slow due to the low hydraulic conductivity of the material and low precipitation. However, the applicability of a GLD sealing layer may not be suitable in all environments. Waste rock piles have to be carefully engineered diverting the water away from the pyritic waste. In climate zones with very high annual precipitation, there is a risk of water flow on top of the sealing layer. This may harm the integrity of the sealing layer or slowly dissolve the chemical compounds compromising its long-term function. The function may also be compromised if the material dries, which is a risk arid regions.
Although GLD has shown desirable properties to function as a sealing layer, some practical issues have to be addressed beforehand. The material is very sticky, making it difficult to handle on a large scale. In addition, the total amount of GLD annually produced is small in comparison with the amount of mine waste. In Sweden, as an example, the annual production of GLD is ca
. 240,000 t produced in many mills spread over the country, many situated far away from the mining areas. Logistics and transport economics must be considered. During 2010, 89 million tons of mine waste was produced, most of it from mines where sulphide-bearing ores were mined [40
The shear strength of GLD was assessed at 11.8 ± 1.6 kPa after one month curing time, which is low compared to the shear strength of bentonite and clayey till. Such low shear strength is insufficient for engineering applications [26
]. It is important that the material exhibit sufficient compressive strength to support the weight of a protective layer but also exhibit plasticity to resist cracking that can be caused by differential settlements in the landfill. If GLD is applied as a sealing layer in a slope, there is a risk of sliding due to the low shear strength and the high water content. Improving the mechanical properties is therefore necessary. A mixture with the proportions 7:2:1 of tailings: GLD: fly ash was found to be geotechnically satisfactory but its function as oxygen barrier remains to be proven [41
]. GLD may also be frost sensitive, which is a common problem with silty soils, causing damage to the cover (i.e.
, frost heave) when used at shallow depths in cold regions.