The Utilization of Fly Ash in the Mining Sector

Abstract

Fly ash, as a kind of hazardous by-product at coal-fired power stations, has been produced on a tremendous scale worldwide. Still, the utilization of fly ash is far from 100% despite some industrial sectors consuming a considerable amount. The top three industries that have successfully applied fly ash on a large scale are cement production, backfill mining, and civil engineering construction. However, compared to the other two fly ash disposal methods, the use of fly ash in backfill mining is still being extensively researched. Not only is fly ash a perfect substitute for cement due to its cementitious properties bringing the cost of backfill mining down to an affordable level for enterprises, the carbon sequestration capacity it possesses is generating new research enthusiasm. This chapter provides a comparative analysis of the current status of the use of fly ash in different mining methods and the role it plays in the corresponding mining method, with an emphasis on the mechanisms by which fly ash content affects the rheological properties of the paste and the strength of the fill. Therefore, this chapter can contribute to understanding the potential of fly ash in mining applications and exploring innovative applications of fly ash.

1. Introduction

The electricity production from coal-fired power plants currently accounts for 37% of all electricity produced worldwide. This percentage is substantially more significant in developing nations that are rapidly urbanizing. A poll estimates that more than 62% of the total electrical energy consumed in 2020 was produced by coal-fired power plants, which have long held the primary position in China’s power supply.

In India, coal is also the primary source of electricity production due to its accessibility and affordable price [1]. However, the pulverized coal’s impurities will produce a significant quantity of coal combustion leftovers, typically making up 30–50% of the powdered coal burned. The precise residue composition depends on the type of coal used ([2], p. 942). In addition, fly ash is the component with the most significant volume fraction of all the solid residues [3].

According to Huang’s review, the total amount of coal ash produced worldwide is estimated to be around 600 million tons, with fly ash making up roughly 500 million tons, 75–80% of the total ([4], p. 1). Nevertheless, coal ash production is predicted to reach 1000 million tons in 2031 [1]. The lack of knowledge about fly ash has created a void regarding the best way to utilize this priceless resource.

Fly ash used to be disposed of on the surface as a terrible waste that took up much space and polluted the ecosystem around it. Researchers have undertaken extensive research on fly ash’s physical, chemical, and practical applications since realizing its potential value for use. As a result, fly ash has been systematically categorized according to its performance and quality. Fly ash was partially recycled and used in various valuable uses, such as asphalt concrete, ground improvement, agricultural sector, roller compacted concrete, brick, road, and mining, based on all the extensive studies undertaken by engineers and academics [1].

However, different countries have varied percentages of recycled fly ash (Figures 1 and 2). There is a noticeable difference between the generation and use of fly ash in some countries. For example, the output of fly ash in Russia alone is no less than 25 million tons per year, whereas the utilization of fly ash is not higher than 7–8%. Although the overall recycling rate of fly ash in China reached about 78% in 2020 (Figure 3), only 2% of the fly ash was used in the mining industry (Figure 2). The main applications of fly ash in the mining industry are generally in two ways, one is to use its small particle characteristics as fine particles to adjust the gradation of the filling material and increase the compactness of the filling material [5], and the other is to use its pozzolanic properties to replace cement as a cementitious material [6] to enhance the compressive strength of the filling body. However, the operational complexity and higher capital cost of the backfill mining method compared to traditional mining methods has made it not widely accepted by mining companies at present, which has directly led to the use of fly ash in mining remaining at a low level. It is likely that as the potential of fly ash for carbon sequestration is progressively exploited, the proportion of its use in functional infill mining will explode [7].

Figure 1.

Top countries with best utilization of fly ash.

Figure 2.

Fly ash application and percentage in China, 2019.

Figure 3.

Demonstration of the generation and the utilization of fly ash in China.

Those pioneers who have explored the application of fly ash for decades have nearly achieved 100% beneficially re-use ([8], p. 107). Therefore, identifying the area where fly ash can be increased and increasing application techniques with considerable practical significance can be identified. For example, in China and other comparable regions, using fly ash in the mining industry can enhance the utilization rate of fly ash as it is too low.

The present chapter contributes a state-of-the-art review on using fly ash in mining practices. In addition, it also discusses the significant performance of fly ash to facilitate better application and specifically expounds on its utilization in the mining sector, which still has a great deal of potential.

2. Types of backfill mining with fly ash admixture

Backfill mining is a relatively newly emerged extraction method compared with traditional caving mining. Generally, this mining technology mixes aggregates and binders and then delivers the mixture by gravity force or pump to the underground stopes and goafs. The backfill mass in underground voids provides a variety of beneficial financial and environmental effects on mining activities, including increased sill pillar recovery, providing a secure working environment for mine operators ([9], p. 772).

Nevertheless, underground mining with backfill minimizes ore dilution, provides a working floor, controls subsidence, and facilitates subsequent excavation and ore removal nearby [10, 11]. Those numerous advantages of the backfill mining method make it a competitive alternative to traditional ways of excavation and also lead its subdivision into different subsystems to meet the diverse requirements of different mine sites.

The backfill subsystems currently in operation are solid backfill, cemented backfill, and paste backfill, and the role played by fly ash differs from backfill system to system.

2.1 Solid backfill

During excavation and afterwards processing, a large number of solid rejections are left over in mine sites. For instance, the volume of coal gangue in China is about 500 million tons and is still increasing at a speed of 70 million tons per year [12]. Therefore, how to dispose of those harmful residues in an eco-friendly way is a significant concern for the mining industry. Other than this, surface subsidence prevention is another vital driver for developing the solid backfill mining system. After coal or ore is mined out, the spaces left underground will lead to the overlying strata subsidence and further cause damage to surface buildings. The solid backfill method directly delivers dry solid wastes by scraper conveyor belts to underground caves, supporting the top strata, and surrounding rocks.

As illustrated in Figure 4, the solid backfill system is similar to longwall caving mining except for the tamping devices mounted at the rear part of the hydraulic supporter, and the tamping device is specially designed for the compaction of the filling materials. With the dumping of solid wastes from the conveyor, the tamping arm exerts pressure that can reduce the voids within the filling piles. Considering that the filling quality is mainly determined by the compressibility of the filling piles [11], the tamping arm is a genius initiative to improve the filling effect. However, the large size and poor particle gradation of the coal gangue dramatically drop its effectiveness. Therefore, fly ash is added to fill the voids between giant solid rock or particles to further enhance the backfill mass’s compactness and minimize subsidence.

Figure 4.

Schematic of solid backfill.

Due to no water added, chemical reactions between backfill materials are absent, i.e. no hydrates are generated, which can bind solid particles together and form a solidified filling mass. Hence, the function played by fly ash in the solid backfill mining method is to improve particle gradation physically.

Compared with cemented backfill mining and paste backfill mining, solid backfill exhibits numerous noticeable advantages, and its most significant merit comes from a financial perspective. Firstly, the backfill system is easy to set up and requires fewer auxiliary facilities. For example, since the backfill mixtures do not include water, thus, the costs on building an underground reservoir and drainage pump for bleeding water can be avoided. Furthermore, solid backfill has a high tolerance to the size or type of backfill materials, so pre-treatment fees on backfill materials can be removed from the budget. Besides, filling material can be delivered to the design site in a very timely manner, as filling operations and mineral extraction can be carried out simultaneously, which is essential to reduce the amount of subsidence ([13], p. 2670).

2.2 Cemented backfill mining

The binding property of fly ash is utilized in cemented backfill mining, as opposed to the solid backfill that uses fly ash as a type of particle and can only fill the spaces between huge rocks. Backfill materials used for cemented backfill include large-size rocks and coal gangues from coal washing and processing, and those large-size aggregates work as the skeleton of the filling body.

Fly ash and Portland cement are mixed with water on the surface before the filling operation and then pump through pipelines to the designed sites. At the discharging point, the slurry contains fly ash and cement is poured on the aggregate pile delivered by belt or conveyor (see Figure 5). A dam must be built at the lower opening of the branch to store the filled materials from working areas and mining operations and to guarantee that the materials may fill the mined-out spaces. Once the coarse coal gangues, fine fly ash, and cement are put into the sealed branch, the filling materials are evenly mixed and stable.

Figure 5.

Discharging scene of the cemented backfill mining.

The stable state creates a possible condition for the hydration reaction of the fly ash and cement. Compounds with cementing capacity resulting from hydration as a cementing agent hold all the filling together as a unit. Therefore, when overlying strata above the backfilled goaf continue to subside, it should break the friction force between particles and the bonding force of binders. The reasons mentioned above make the cemented backfill much more effective at preventing subsidence than the dry solid backfill method.

Although the hydration reaction of fly ash and cement results in the filling mixture’s self-stabilizing capacity and the support strength’s growth to a considerable degree, the excessive water washes more fly ash and cement down to the bottom. Therefore, in this backfill method, the cementitious properties of fly ash are not thoroughly exploited.

2.3 Paste backfill

The use of paste as an underground backfill started in 1979 when it was used at Preussag’s Bad Grund Mine in Germany [10]. Paste backfill is mixing grounded coal gangue powder or classified tailings with fly ash and cement, making a homogenous paste slurry, and then transporting it through a pipeline to gob, providing confinement to the surrounding rocks and roof.

Two reasons mainly drive the commencement of paste backfill. One is that the primary purpose of using fly ash in the mining sector has changed from disposing of harmful residues to beneficially applied as scientists and engineers become more knowledgeable about fly ash’s physical and chemical properties. Another contributing factor is subsidence control in some areas calls for a better filling solution. The main components of paste backfill are dewatered mine tailings and grounded coal gangue (70–85% solids by weight), binders (3–7% by dry paste weight), and mixing water (fresh or processed) ([14], p. 2). The most commonly used binders are fly ash, cement or a mixture of those two ([15], p. 4). The solid fraction in the paste slurry is considerably high, generally in the range of 65–75%, in some practical cases, it can reach 80%, and the properties of paste flurry change significantly even if the solid fraction is slightly changed [16]. The high solid concentration in paste slurry increases the number of particle-particle contacts, thereby increasing the gel formation rate ([4], p. 10). However, it also contributes to water retention, thus eliminating the need for drainage (Figure 6).

Figure 6.

Paste backfills.

Therefore, to maintain that all the solid particles in the slurry are in a suspension state, ensuring the highly dense backfill slurry can be delivered through the pipeline without clogging or segregation, strict standards for particle size and grading are applied ([17], p. 116871; [18], p. 1443). With its physical and chemical characteristics, fly ash has become an indispensable additive for paste filling. For example, its large specific surface makes the fly ash particles more easily grafted by the surfactant, making it more suitable to be modified. In addition, the high specific surface area of fly ash will also result in a robust adsorptive capacity of fly ash to surfactant suspensions.

As the setting time of backfilled paste is mainly associated with the fineness of the binder ([19], p. 101), the addition of fly ash paste can provide support to adjacent rocks very soon. Furthermore, due to their pozzolanic reaction, the fly ash blended pastes have a lower early reactivity than cement. Thus, it raises the amount of C-S-H and C-A-S-H and creates a compact microstructure free of cracks, boosting the material’s long-term strength and durability ([19], p. 105).

In addition to filling efficiency, filling costs are a significant consideration for the coal sector. In terms of filling effectiveness, paste filling has a marked advantage over the previously described mining methods, but the complexity of the operation and high cost makes it less attractive. The cost of filling materials with paste backfills is 10 and 20%, whilst cement accounts for up to 75% of that cost ([20], p. 284). So, maximizing inexpensive fly ash as a replacement for cement in paste filling is key to further reducing filling costs and promoting more widespread use of paste backfill mining.

3. The influence of fly ash on the rheology of backfill slurry

When the backfill mining method was adopted, especially paste backfill, many materials treated at surface processing plants needed to be delivered to discharge sites via pipelines under gravitational force or pumping pressure ([21], p. 443).

Furthermore, it is advisable to assess the available gravitational force and the pumping capacity before the operation because insufficient power will not only fail to deliver the filling material to the specified location but also cause severe blockages in the pipeline. However, if too much redundancy is provided, energy wastage and wear on the pipes will increase ([22], p. 925). Given that, the rheology of backfill slurry becomes a concern of interest both for its influence on transportation expense and pipeline maintenance.

The rheology of a slurry is a mathematical description of its motion in response to shear stress. As illustrated in Figure 7, due to the different intrinsic properties of each slurry, they exhibit distinct differences in rheological behavior and are classified into different types. Moreover, the most common model is Newtonian, amongst those models describing the relation between shear rate and shear stress. This type of fluid is characterized by its viscosity being constant at a given temperature and not changing with the force applied to it. On the other hand, although the dense backfill slurry obeys different flow laws ([23], p. 1181), the commencement of its flow demands a force that exceeds the yield stress [24].

Figure 7.

Rheological model.

Figure 8 demonstrates the relation between the shear rate and shear stress of a series of high concentration slurries with different dosages of fly ash. Generally, all the rheological curves depicted in this figure are inconsistent with Newtonian fluid. This kind of backfill slurry with a changing kinematic viscosity was identified as a Herschel-Bulkley fluid. In the Figure 8, with the increasing replacement of fly ash to cement, the shear stress declines rapidly, especially when the shear rate goes higher, which proves the significant positive effects of fly ash on the fluidity of the backfill slurry ([25], p. 223). Moreover, the root of this phenomenon has been attributed to the unique physical structure of fly ash. Fly ash has more spherical particles than the other binder in use. This feature can create a lubricating effect, known as the ball-bearing phenomena, resulting in a frictionless flow in stowing pipelines ([26], p. 7).

Figure 8.

Rheogram of slurry with different fly ash content.

In addition to the physical analysis of fly ash’s role in changing the filling slurry’s rheological properties, some tests were conducted to explain it from a chemical perspective (Figure 9). This figure illustrates that when the curing temperature is different, the rheological characteristics become distinctly different, although the fly ash content remains the same. It is generally accepted in the scientific community that the rate of hydration reaction of fly ash and the amount of polymer produced by hydration is dominated by temperature.

Figure 9.

Rheogram of slurry with a 10% fly ash addition.

In practice, fly ash’s effect on slurry rheology is directly reflected in the extraordinary pressure drop reduction during pipeline transport [27]. Pressure drop is produced mainly by the mechanical friction between the pipes and the transported slurry and the collision between solid particles dispersed in the slurry ([28], pp. 9–18).

It can be seen from Figure 10 that the pressure drop per meter of paste slurry experiences a shape decrease with the increased fly ash-to-cement ratio. Therefore, the increasing ratio between fly ash and Portland cement means more cement was replaced by fly ash when preparing paste slurry. In the above narrative, we have mentioned the lubrication effects possessed by fly ash due to its spherical shape and stiffness, and that explanation applies here. Furthermore, the early hydration of fly ash is much slower than cement, so more cement is replaced by fly ash. These fewer hydration products possess cementing properties and can bond the aggregate together.

Figure 10.

Effect of fly ash on the backfill paste slurry through loop pipe.

When the ratio of coal gangue to fly ash increases, the pressure drop value also increases dramatically. The lubrication theory can explain this phenomenon. Less fly ash in paste slurry means more collision between the large aggregate, such as coal gangue, and more friction between slurry and pipes.

4. The influence of fly ash on the strength of backfill mass

Several pieces of research show that adding fly ash improves the aggregate formation strength of coal gangue-fly ash backfill. In a study, the response surface method, multi-objective multi-verse optimization, and the desirability function approach were used [29]. The target of the study was to improve aggregate formation strength. The ideal combination was produced at mass concentrations of 79.65%, 57.19% for fine gangue, and 15.67% for fly ash as a percentage of the total mass. In addition, findings showed that the newly developed mixing method increases the fly ash’s activity to encourage the early synthesis of calcium silicate gel and calcium silicate hydrate gel, strengthening aggregate formation.

Paste backfilling’s rising popularity necessitates alternative binder optimization and bulk waste disposal. The effectiveness of fly ash as a partial replacement for regular Portland cement (OPC) for paste backfill application in underground mines is examined by Behera et al. [30]. The impact of fly ash addition on paste backfill’s cohesion and uniaxial compressive strength (UCS) is demonstrated. According to the study, when fly ash was used in place of OPC, paste backfill’s rate of strength development slowed down. However, with binder dosages of 8 wt% OPC, 7 wt% OPC, 6 wt% OPC, 7 wt% OPC +1 wt% fly ash, and 6 wt% OPC +2 wt% fly ash, the desired 28 days UCS of 1.1 MPa for the backfilling stope of the lead-zinc mine was reached. Therefore, fly ash is suitable for binders and can replace up to 25% of OPC (8 wt%). In the early curing stages, calcium silicate hydrate (C-S-H) did not form in paste backfills based on the OPC-FA binder. Gypsum was only identified in samples with OPC as the only binder, according to the analysis of microstructural evolution in paste backfill. The UCS development is more sensitive to fly ash replacement, according to the results of the multiple linear regression analysis on the interaction effects of OPC, curing time, and fly ash replacement percentage for 8 wt% and 5 wt% binder groups. The results would aid a better comprehension and paste backfill design for underground lead-zinc mines.

Fly ash for underground mining backfilling can effectively utilize solid waste, increase the backfill’s strength, and lower cost, resulting in positive social and economic outcomes. Using a scanning electron microscope and the hydration properties of cement and fly ash, the causes of the variance in backfill strength were examined by Chang et al. [31]. When the concentration of the filling slurry was 74%, the cement content was 5%, the mass ratio of waste rock-tailings-fly ash was 6:2:3, and the CaO substance was 6:3, the strength of the backfill was substantially more significant than the existing strength of the mine’s backfill. When the filling slurry contains tailings, however, the excessive amount of fly ash is likely to cause many fine particles to obstruct the hydration. The hardening of the slurry also reduces the porosity of the backfill.

The backfill strength steadily increased with the rise in fly ash to cement mass ratio, and both grew about linearly, when the filling aggregate was completely waste rock. The strength increased less from 56 to 90 days during the maintenance phase, yet it was 2.24 times as strong as at 56 days. The maintenance period from 28 to 56 days was when the strength development of backfill containing fly ash was mainly concentrated, and the growing tendency slowed. The backfill strength abruptly decreased after the fly ash-cement mass ratio exceeded 3 (the fly ash-tailings mass ratio at this time was about 3:2). Fly ash had a significant negative impact on the strength at this time. As tailing particles were fine, the continuous increase in fly ash content resulted in more fine particles in the backfill, hampered the hydration of cementitious materials, and affected the growth of the backfill strength. In general, the increase of fly ash content promotes the strength of backfill in the middle and late stages. Fly ash has a two-way impact on the backfill’s late strength when tailings are present. Therefore, determining the proper mixture of fly ash and tailings is crucial.

5. Conclusion

As electricity generation using coal-fired power shows an increasing trend, dealing with fly ash is an important issue. However, some developing, and developed countries are not utilizing fly ash effectively. On the other hand, backfill mining shows a considerable scope where fly ash can be utilized in a win-win situation. In three types of backfill materials, fly ash can be an essential component and can enhance the effectiveness and minimize the cost.

Fly ash has a considerable positive impact on the fluidity of the backfill slurry, as shown by the quick drop in shear stress that occurs with increasing fly ash replacement of cement. This property may produce the ball-bearing phenomenon, a lubricating effect that results in a frictionless flow in stowing tubes. Even whilst the fly ash content stays the same, the rheological qualities change noticeably when the curing temperature changes. The amount of polymer formed by hydration and the rate at which fly ash reacts to hydrate are generally acknowledged in the scientific community to be strongly influenced by temperature.

Fly ash positively influences the backfill’s strength in mining, with the strength improvement focusing primarily on 28–56 d. However, too much fly ash may result in many fine particles in the backfill when the filling material comprises tailings, which will impede the hydration of cementitious material and impact the growth of backfill strength. Workability is affected by the solid mass concentration, fine gangue ratio, and fly ash content. When the mass ratio of the waste rock-tailings-fly ash mixture is 6:2:3, the paste backfill material exhibits greater strength.

Acknowledgments

“We acknowledge financial support for the publication of this book by the State Digitization Program for Science and Culture of Saxony”.