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Utilisation of Acid Mine Drainage Sludge

9.7.1 Introduction

As stated in a number of chapters in this book, AMD has always been one of the mining problems that is difficult to avoid. The AMD that has not been treated before it is discarded into the water bodies can cause serious negative impacts to the environment because of its low pH values and higher content of heavy metals (Amanda and Moersidik, 2019) as discussed in Chapter 5. As extensively discussed in Chapter 7, the treatment of AMD can be carried out by active and passive methods. Unfortunately, both AMD treatment methods produce sludge with various compositions depending on the treatment type, the use of lime, and the quality of the water treated (Amanda and Moersidik, 2019). The objective of this section of the chapter is to discuss and illustrate the potential of AMD sludge as a valuable material. One of the applications of sludge - adsorbents - has already been discussed in great detail in Section 9.6.1 of this chapter; and the rest of the other pertinent applications of AMD sludge are discussed in Section 9.7.2. Simate and Ndlovu (2014), Ndlovu et al. (2017), and Rakotonimaro et al. (2017) also highlighted some of these applications in their publications.

  • 9.7.2 Selected Typical Studies of the Reuse of Acid Mine Drainage Sludge
  • 9.7.2.1 Production of Fertiliser

Fertiliser is considered as a material that is produced in order to supply elements, in a readily available form, that are known to be essential for plant growth and development. Zinck (2006) argues that low metal content sludges, such as sludges from coal mining operations, which have been found to have excess alkalinity present in the sludge, can be utilised to raise soil pH. A study by Dobbie et al. (2005) investigated the use of phosphorus- enriched ochre (hydrous iron oxide sludge) as a phosphorus fertiliser. The AMD sludge according to Sibrell et al. (2009) is a waste product produced by the neutralisation of AMD and consists mainly of the same metal hydroxides used in traditional wastewater treatment for the removal of phosphorus. Ideally, when ochre (or simply AMD sludge) is used for remediation of wastewaters, it adsorbs phosphorus (in the form of inorganic phosphate) from the solution (Heal et al., 2003; Shepherd, 2017) effectively which makes the resulting phosphate-enriched ochre a potential phosphorus fertiliser (Dobbie et al., 2005). In the study by Dobbie et al. (2005), pot and field experiments were set up to assess performance and environmental acceptability of ochre as a fertiliser, using grass and barley as test crops, as well as birch and spruce tree seedlings. It was noted from the study that applying phosphate-saturated ochre as a fertiliser increases the phosphorus status of soils and has a useful liming effect. Phosphate-saturated ochre is also less water-soluble than conventional phosphorus fertiliser, thus reducing the potential for diffuse pollution from agricultural land. The results in the study by Dobbie et al. (2005) also showed that the ochre caused no metal contamination, but other AMD sludge sources would need to be monitored to ensure that they do not contain undesirable concentrations of metals. The slow release of phosphorus from phosphate-saturated ochre means that less frequent applications would be required than when using conventional phosphorus fertiliser. It was found that all crops studied grew well comparatively when using fertiliser with phosphate-saturated ochre or with conventional phosphorus fertiliser.

Heal et al. (2004) state that when the phosphorus removal capacity of ochre is finally exhausted after removal of phosphorus from wastewater such as agricultural runoff and sewage effluent, the "spent" material will require removal and disposal, and a more sustainable alternative to landfill disposal is to recycle the phosphorus as a fertiliser. Pot experiments and field trials comparing barley and grass grown in soils amended with phosphorus-saturated ochre with the plants grown with conventional phosphorus fertiliser studied by Heal et al. (2004) showed that ochre additions improved soil fertility and increased the pH of the soil whilst the same crop yields were maintained similar to conventional fertiliser. At the end of the growing season, the results also showed that there was more phosphorus available in the ochre-amended soil than in soil treated with conventional fertiliser, indicating that phosphorus-saturated ochre had a further desirable property of acting as a slow-release fertiliser, thus reducing the need for future phosphorus fertiliser applications.

Several other studies also suggested that AMD sludge has the potential to adsorb phosphorus from agricultural wastewaters for possible use as a fertiliser (Adler and Sibrell, 2003; Fenton et al., 2009).

9.7.2.2 Production of Iron Pigments

Studies have shown that the sludge obtained from AMD can be considered for the production of inorganic pigments (Hedin, 1998; Hedin, 2003; Marcello et al., 2008; Michalkova et al., 2013) and magnetic particles like ferrites (Wang et al., 1996). Indeed, a range of products with various purities, phase compositions, and properties, including surface properties, can be synthesised from AMD depending on the reaction conditions (Michalkova et al., 2013). To produce commercially usable iron oxides as raw material for production of pigments, additives to ceramics, etc., treatment of AMD using a two-step selective precipitation process was developed (Hedin, 1998; Hedin, 2003). The two-step process that uses magnesium oxide and sodium hydroxide resulted in the ferrous and ferric oxyhydroxide sludge that can be thermally transformed to basic ferric pigment. A study by Hedin (2003), however, indicated that while the end product was of high quality, the costs associated with the processing made the materials more costly to produce than mined oxides although this may be offset when considering the high cost of hydrous ferric oxide disposal.

A study by Bernardin et al. (2006) used acid drainage mud from a coal mine to produce ceramic pigments. The raw material was collected at an effluent treatment station, and the mud was dried (105°C, 8 h), ground (250 pm) and calcined (~1 250°C). The calcined pigment was then micronised (Dw~2 pm). After calcination and micronisation, mineralogical analyses (XRD) were used to determine the pigment structure at 1 250°C. Finally, the pigments were mixed with transparent glaze and fired in a laboratory roller kiln (1130°C, 5 min). The results showed that the drainage residue can be used as a pigment only when mixed with pure oxides or as part of a commercial pigment. When used alone, the residue pigment presented a faded brown colour, inadequate for ceramic glazes. However, when mixed with transparent glaze, the residue pigment had better results, but was still poor compared with a commercial pigment. Nevertheless, the AMD residue could be used in other ceramic applications, as filler for brick pastes and other ceramic products. Most importantly, the study has shown that the residue can be eliminated from the environment through the production of pigments. A similar study was carried out by Marcello et al. (2008) who investigated the use of hydrous ferric oxides from active coal mine drainage treatment as pigment within ceramic tile glaze. Favourable results similar to the ones obtained by Bernardin et al. (2006) were also obtained when the ferrous hydrous oxides were blended with an industrial standard pigment.

Research by Cheng et al. (2007) and Cheng et al. (2011) has also shown that fuel cell technologies are not only used for simultaneous AMD treatment and power generation, but also generate iron oxide particles having sizes appropriate for use as pigments and other applications. As already discussed in Section 9.5, a fuel cell called an AMD fuel cell based on an MFC was developed and used during the studies. During the AMD treatment process in the studies, ferrous iron was oxidised in the anode chamber under anoxic conditions, while oxygen was reduced to water at the cathode. Ferrous iron was completely removed through oxidation to insoluble ferric iron and precipitated at the bottom of the anode chamber. The particle diameter of the iron oxides could be controlled by varying the conditions in the fuel cell, especially current density, pH and initial ferrous iron concentration. Upon drying, the iron oxide particles were then transformed to goethite (a-FeOOH).

Silva et al. (2019) evaluated several processes for purifying iron sludge from AMD so as to obtain a yellow pigment (goethite) of good quality. The study optimised the process for precipitating iron (III) selectively by assessing three variables (the reagent, the number of washes, and the separation method). Two alkaline agents (sodium hydroxide or sodium bicarbonate) with different neutralisation powers and two processes for solid-liquid separation (filtration or centrifugation) were used. In other words, the experiments were carried out by causing precipitation with strong (NaOH) and weak (NaHC03) bases and removing other metals from the sludge by washing and filtering the sludge or by centrifugation. The results of the study found that high quality goethite can be produced from AMD effluent provided that the process for recovering iron can remove the contaminants, particularly aluminium which adversely affects the growth of crystals, thereby preventing goethite from taking an acicular form, which is characteristic of pigment goethite. Basically, the results showed that the colour, type, and morphology of the compounds formed depended on the number of contaminants; and that the removal of various contaminants was strongly dependent on the type of reagent used and less dependent on the separation process and the number of washes. In other words, the purification results indicated that it was the kind of reagent which was mainly responsible for separating iron and aluminium during neutralisation process. When the reagent was NaHC03, 67% of the samples produced yellow pigment; whereas when the reagent was NaOH, 33% of the samples produced yellow pigment. The results clearly show that the weak base (NaHC03) prevents aluminium from contaminating the sludge during the precipitation process.

Lottermoser (2011) states that reuse of mine wastes allows their beneficial application, whereas recycling extracts resource ingredients or converts wastes into valuable products. In the study by Lottermoser (2011) various reuse and recycling options that have been proposed for mine wastes by numerous researchers were listed. Extraction of hydrous ferric oxides for paint pigments and extraction of manganese for pottery glaze were considered as two of the reuse and recycling options for AMD sludge.

9.7.2.3 Building and Construction Related Materials

When AMD is treated, dewatered and dried, the resulting sludge is composed largely of inorganic components that are suitable for use in building materials such as in the manufacture of cement (Simate and Ndlovu, 2014;

Michael, 2016; Ndlovu et al., 2017; Rakotonimaro et al., 2017). Basically, many of the constituents of sludge are the same as those used in cement manufacturing (Simate and Ndlovu, 2014; Michael, 2016; Ndlovu et al., 2017). For example, calcite, gypsum, silica, Al, Fe, and Mn are common raw materials for cement (Simate and Ndlovu, 2014; Ndlovu et al., 2017). Therefore, the components that make up AMD treatment sludge such as gypsum, calcite, and ferrihydrite can be utilised as raw materials in the manufacture of construction materials and other products (Simate and Ndlovu, 2014; Ndlovu et al., 2017). According to Michael (2016), calcium, iron, and aluminium are three of the four principal components of Portland cement; therefore, the use of AMD sludge as a feedstock for the manufacture of cement could have both economical and environmental benefits. Some studies have actually suggested that sludge can replace up to as much as 30% Portland cement in blended cement (Tay and Show, 1994); thus in such cases, sludge is expected to lower the use of binders (Rakotonimaro et al., 2017).

In some studies it was observed that the high aluminium content in sludge produced from the treatment of acidic drainage at coal and gold mines could be used for the production of aluminous cement (Lubarski et al., 1996). The production of bricks by adding sludges of various compositions of inorganic components has also been studied by several researchers (Benzaazoua et al., 1999; Rouf and Hossain, 2003; Weng et al., 2003; Benzaazoua et al., 2006; Mahzuz et al., 2009; Hassan et al., 2014). The studies showed that the addition of sludge coupled with high curing temperatures produced bricks of high quality. Some of the studies found that the bricks manufactured with the addition of sludge had high comprehensive strength compared to normal clay bricks (Rouf and Hossain, 2003). However, though there was less arsenic release by leaching from bricks, the presence of arsenic in sludge does not produce good quality bricks and hence sludge that contains arsenic in large quantities is not preferable for manufacturing bricks (Mahzuz et al., 2009). There is no doubt that sludge containing inorganic materials in reasonable quantities can be utilised in building and construction related materials and thus reduce mining of raw materials for production of building material and construction materials (Simate and Ndlovu, 2014; Ndlovu et al., 2017).

9.7.2A Material for Carbon Dioxide Sequestration

The increasing C02 concentration in the Earth's atmosphere, mainly caused by fossil fuel combustion, has led to concerns about global warming (Montes- Hernandez et al., 2008). Without drastic market, technological, and societal changes, C02 concentrations are projected to increase to alarming levels in the near future (Feely et al., 2004; Olajire, 2013). It is, therefore, paramount that carbon capture and sequestration are instituted if meaningful C02 reduction is to be achieved.

Among various technologies for capturing and storing C02, mineral carbonation technology has been found to be a potentially attractive sequestration technology for permanent and safe storage of C02 (Olajire, 2013). Mineral carbonation technology is a process whereby C02 is chemically reacted with calcium and/or magnesium containing minerals to form stable carbonate materials which do not incur any long-term liability or monitoring commitments (Olajire, 2013). In other words, mineral carbonation technology stores C02 by reacting natural minerals and industrial by-products containing a lot of calcium or magnesium with C02 and subsequently forming carbonate minerals (Lee et al., 2016). In general, the mineral carbonation process consists of extracting reactive calcium or magnesium from the raw materials, and, thereafter, the leached calcium or magnesium ions react with C02 to form the carbonate minerals in high pH conditions (Lee et al., 2016).

According to Zinck (2006), the same mechanism that generates CO, during the production of lime can be utilised to sequester CO,. In this regard, C02 gas can react with AMD treatment sludges and iron-rich metallurgical residues to produce solid calcium, magnesium, and iron carbonates while stabilizing the sludge/residue and its impurities. Furthermore, the extraction of calcium ions from the raw materials will not be necessary if neutralised mine drainage is utilised because calcium ions are already present in the AMD solution (or sludge) through the reaction between AMD and dissolved lime that is added during the AMD neutralisation process. In comparison with other common mineral carbonation processes, carbonation utilizing the neutralisation process of mine drainage does not need pre-treatment and any additional facilities to sequester C02. There is also an additional advantage of short treatment time and the process can be carried out at ambient temperature and pressure.

A study by Lee et al. (2016) demonstrated the concept of using AMD sludge for C02 sequestration at laboratory scale on both synthetic and real AMD. In the study, hydrated lime, as used in the process of neutralisation, was added to adjust the pH of AMD solution and evaluated its feasibility as a probable technology for C02 sequestration through carbonation. In the first step, hydrated lime was added to the mine drainage, and then the mine drainage was stirred for 5 min in order to increase its pH. Thereafter, C02 gas was injected into the mine drainage until the pH reached 8.3, which is the minimum pH level for the production of carbonate ions. To evaluate the efficiency of C02 injection, two sets of experiments were carried out: (1) a carbonation experiment in which C02 was injected, and (2) a non-carbonation experiment without any C02 injection. The results showed that as hydrated lime was added into the mine drainage the overall pH increased up to about 12. In the carbonation treatment, the pH decreased after CO, injection because C02 generates H+ ions when dissolved in water (i.e., C02 + H20 H+ + HCCL). In the non-carbonated study in which there was no C02 injection, the pH remained high at about 12. In the case of real AMD, 1 kg of mine drainage could retain CO, of up to 0.54 g through carbonation treatment using the neutralisation process. Undoubtedly, C02 sequestration using the neutralisation process of mine drainage can be considered as a positive technique in terms of sustainable development.

Merkel et al. (2005) developed a sustainable low risk concept - CDEAL - on how to incorporate C02 into the subsurface and thus exclude it from the atmosphere. However, the results of the concept do not seem to have been published. The overall goal of CDEAL was twofold: (1) reduce the C02 emissions into the atmosphere, and (2) rehabilitate contaminated, acidic mine waters using carbonation. As the storage of C02 would be in the form of carbonate, it would, therefore, be sustainable. Thus, in this case, CDEAL would have positively contributed to the reduction of greenhouse gas emissions by C02 sequestration. The details of the concept indicate that CDEAL would only use pre-treated mine water and mine water with elevated CaO- contents, where an excess of CaO existed and can be used to react with C02 to form CaC03. In addition, where iron-hydroxide sludge is available in great amounts in some parts of the open pit lakes and the waste rock piles, it could be used as a reacting material as well.

Unger-Lindig et al. (2010) conducted a study that investigated whether alkaline cations in both the deposited sludge and the pore water can be used to improve the alkalinity in lake water, when CO, was added. The batch test results showed that addition of low-density sludge to acidic water (from mining lake) increased the pH of the water and the injected CO, could be captured mainly in the form of metal-bicarbonate complexes. According to Rakotonimaro et al. (2017), this was associated with the availability of oxy- hydroxides contained in the AMD sludge. Rakotonimaro et al. (2017) state further that the advantage of sludge incorporation in an acidic pit lake, for C02 sequestration, is its capacity to be employed as a neutraliser because it still contains unreacted hydrated lime (or calcite if dried), thus reducing the acidity of the lake by up to 30% and stabilise the sludge itself. However, depending on the concentration of the elements and mineral solubility in the AMD sludge, the possibility of contaminant release into the surrounding environment is a serious risk (Rakotonimaro et al., 2017).

9.7.2.5 Stabilisation of Contaminated Soil

Soils may become contaminated by the accumulation of heavy metals and metalloids through emissions from the rapidly expanding industrial areas, mine tailings, disposal of high metal wastes, leaded gasoline and paints, application of fertilisers on land, animal manures, sewage sludge, pesticides, wastewater irrigation, coal combustion residues, spillage of petrochemicals, and atmospheric deposition (Khan et al., 2008; Zhang et al., 2010; Wuana and Okieimen,

2011). Immobilisation, soil washing, and phytoremediation techniques are frequently listed among the best demonstrated available technologies for remediation of heavy metal-contaminated soils (Wuana and Okieimen, 2011).

At the moment, research has shown that AMD sludge which is found in abundance contains lots of metal oxides (or hydroxides) that may be useful for heavy metal stabilisation in soils (Kim et al., 2014). For example, Tsang et al. (2013) explored the potential use of AMD sludge and carbonaceous materials (green waste compost, manure compost, and lignite) for minimizing the environmental risks of As and Cu in the soil. After 9-month soil incubation, significant sequestration of As and Cu in soil solution was accomplished by AMD sludge, on which adsorption and co-precipitation could take place. However, in a moderately aggressive environment, AMD sludge only suppressed the leachability of As, but not Cu. Therefore, the provision of compost and lignite augmented the simultaneous reduction of Cu leachability, probably via surface complexation with oxygen-containing functional groups. Under continuous acid leaching in column experiments, combined application of AMD sludge with compost proved more effective than AMD sludge with lignite. This was attributed to the larger amount of dissolved organic matter with aromatic moieties from lignite, which may have enhanced Cu and As mobility.

The main objective of a study by Lee et al. (2013) was to investigate the effectiveness of soil stabilisation treatments using waste resources such as calcined oyster shell and coal mine drainage sludge. The study focused, particularly, on the feasibility of the simultaneous stabilisation of As and other heavy metals using mixed stabilizing agents. Both batch and column-leaching tests were used as evaluation methods, and the free and easy movement (or mobility) of As and other heavy metals after stabilisation treatments was compared to that of the control samples. The overall results of both the batch and column tests indicated that a combination of calcined oyster shell and coal mine drainage sludge was effective for stabilizing As and the other heavy metals. More specifically, in the acid extraction experiments, after the batch wet-curing process, the stabilisation efficiencies of As, Pb, and Cu were more than 90%, compared to the control experiments. In addition, in the column tests, the stabilisation process successfully prevented the migration of contaminants by leachate infiltration into the lower part of the soil samples, which suggested the feasible application of oyster shell and coal mine drainage sludge waste resources for stabilisation of As and other heavy metals in the soil. A similar study to that of Lee et al. (2013) in which calcined oyster shell was combined with coal mine drainage sludge was performed by Moon et al. (2016). The results also showed a good retention of As (>93%), Cu (>99%), and Pb (>99%). However, it is noted that the calcined oyster shell wastes mixed with mine sludge might raise calcium content in the soil, thus inducing a potential increase of hardness in the surrounding water in case of any leaching (Rakotonimaro et al., 2017).

Kim et al. (2014) studied stabilisation of heavy metals in agricultural soils affected by the abandoned mine sites nearby using AMD sludge. The results indicated that AMD sludge could be applied to soil contaminated with heavy metals as an alternative for reducing heavy metal mobility and bioavailability. Ко et al. (2012) investigated the stability of arsenic in solution and soil using various additives, such as limestone, steel mill slag, granular ferric hydroxide, and AMD sludge. The total concentrations of arsenic in the area where soil used in the study was collected ranged up to 145 mg/kg. After the stabilisation tests, the removal percentages of dissolved As(III) and As(V) were found to differ depending on the additives employed. Approximately 80% and 40% of the As(V) and As(III), respectively, were removed with the use of steel mill slag. The addition of limestone had a lesser effect on the removal of arsenic from the solution. However, more than 99% of arsenic was removed from solution within 24 h when using granular ferric hydroxide and AMD sludge, and similar results were observed when the contaminated soils were stabilised using granular ferric hydroxide and AMD sludge. These results suggested that granular ferric hydroxide and AMD sludge may play a significant role on the arsenic stabilisation. Moreover, the result in the study by Ко et al. (2012) showed that AMD sludge can be used as a suitable additive for the stabilisation of arsenic.

9.7.2.6 Covers in the Prevention of Acid Mine Drainage

In Chapter 6, several techniques for preventing the generation of AMD have been discussed. One of the methods involves the creation of physical separation barriers for water and oxygen such as the use of dry covers that have a number of roles (Pozo-Antonio et al., 2014). The AMD sludge was given as an example of alternatives for covering materials in place of natural soil in the prevention of AMD generation (Demers et al., 2017). This section gives an in-depth discussion of studies where AMD sludges have been used as covers for acid-generating waste materials.

A number of studies have been performed by Demers et al. with the view of using AMD sludge to control AMD produced by tailings and waste rocks (Demers et al., 2015a,b; Demers et al., 2017). In the first study by Demers et al. (2015a), the possibility of using AMD treatment sludge as a cover component for controlling AMD generation by tailings and waste rocks was investigated in the laboratory. Column experiments were conducted in order to identify the potential mixtures that could reduce acid generation when placed over acid-generating tailings and waste rock. The sludge-waste rock mixtures, when placed on waste rock, were not able to limit the transport of gaseous oxygen. However, the tests on the use of sludge-waste rock mixtures placed over waste rock demonstrated the capacity of the sludge to neutralise part of acid generated by waste rocks. In addition, the sludge waste rock mixtures significantly reduced metal loading in the effluents. Covers made of sludge and tailings mixture were able to reduce the generation of copper, zinc, calcium, and sulphur from the tailings; and acidic conditions were not observed for any test that was conducted, including the control conditions. The second study by Demers et al. (2015b) involved field work in order to evaluate the effectiveness of waste rock-sludge and tailings-sludge mixtures. The field results showed that waste rock-sludge mixture placed over waste rock was able to reduce the generation of AMD from the waste rock, therefore, confirming laboratory results, and was able to produce a neutral effluent with low concentrations of dissolved metals. The tailings-sludge mixture placed over tailings, with an evaporation protection layer, maintained a high volumetric water content and reduced sulphide oxidation from the tailings as exhibited by a neutral effluent. The two studies by Demers et al. (2015a,b) highlighted two sludge valorisation options as follows: (1) the use of waste rock-sludge mixture to reduce, at least temporarily, acidity and metal loads from a waste rock pile effluent, and (2) the use of tailings-sludge mixtures as cover to limit oxygen transport towards acid-generating tailings.

Mbonimpa et al. (2016) investigated the geotechnical properties of silty soil-sludge mixtures as possible components for covers with capillary barrier effects to prevent AMD generation from mine waste. It must be noted that both the soil and sludge used in the study were non-acid-generating. The silty soil-sludge mixtures with (3 values of 10%, 15%, 20%, and 25% sludge (p = wet sludge mass/wet soil mass) were studied. Two water contents were considered for each of the mixture components: 175% and 200% for the sludge and 7.5% and 12.5% for the soil. The results indicated that adding up to 25% of sludge to a silty soil can provide a mixture with appropriate saturated hydraulic conductivity (about 10-5 cm/s) and water retention properties (air-entry value about 30 kPa) for the mixture to be used in the moisture retention layer for covers with capillary barrier effects. This understanding was based on the comparison with existing efficient covers with capillary barrier effects. The impact of sludge addition to the silty soil on freeze-thaw behaviour was relatively limited. Volumetric shrinkage at complete drying of the mixtures (worst-case scenario) increased with the sludge content, but shrinkage can be reduced by covering the mixture with a layer of coarse material (the drainage and protection layer of the covers with capillary barrier effect) to control evaporation. The study indicated that the sludge mass that can be reused in the moisture retention layer of covers with capillary barrier effects could be significant, which consequently reduces the mass (or volume) of soil required as well.

Previous research, as indicated already, showed that AMD sludge has geotechnical and geochemical properties that can be used in combination with silty soil to be part of covers (oxygen barriers) that can prevent AMD generation from waste rock and tailings impoundments (Mbonimpa et al., 2016). On this basis, Demers et al. (2017) studied the use of sludge as a replacement for a portion of natural soil used for cover systems. Mixtures of sludge and a natural silty soil were tested in the laboratory (for over 500 days) and in field experiments (4 years) as an oxygen barrier cover placed over acid-generating tailings and waste rock. Field cell experiments were conducted in order to validate the results obtained in the laboratory and to evaluate the effect of a protective sand-gravel layer placed over the sludge-soil mixture. Data from the test work included monitoring of leachate geochemical parameters (e.g., pH, conductivity, metal and sulphate content) and hydrogeological parameters (water content, suction, effluent flow rate). The results of the laboratory column experiments showed that the sludge-soil mixture was efficient to prevent AMD generation, as long as the cover maintains its integrity. The results for the four monitoring seasons, in the field experiments, indicated that the sludge-soil mixture covers are effective in limiting AMD generation and reduce dissolved metal concentration in the cells effluents. Volumetric water content and suction measurements confirmed that the covers maintained a high degree of saturation, which made them efficient oxygen barriers. The presence of the sand-gravel layer possibly helped to reduce evaporation.

It is well known that Portland cement is an effective, but expensive option for source control of AMD (Sephton and Webb, 2017); but the cost could be reduced by blending the cement with cheap waste materials such as fly ash from coal combustion and sludge produced from neutralisation of AMD with lime (Sephton et al., 2019). To test how the two additives affect cement performance in reducing AMD generation, a study was carried out by Sephton et al. (2019). In the study, blended cement slurries were applied to sulphidic waste rocks in leaching columns that were monitored for about a year. The study found that applications of fly ash-blended cements and AMD sludge-blended cements to acid-producing sulphidic waste rocks in leaching columns considerably reduced AMD generation, decreased acidity, metal and sulphate loads in column leachates by 80-95%, similar to the effects of unblended cements. The AMD sludge showed no evidence of releasing its adsorbed heavy metals, but the fly ash released some silicon, indicating that it is not chemically stable in the cement. The overall analysis of the long-term effectiveness of the blended cement applications shows that cement placed as a surface cap on top of the waste rock provides more value, because the slower cement dissolution rates ensure continued effectiveness for many years.

 
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