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The slurry pipeline is setup or deeply buried in underground mines and is mostly made of steel. It is difficult to determine the coarse particle trajectory and flow velocity distribution during slurry pipeline transportation procedure. Relying on testing equipment and having a long testing cycle, the cost of a single test is high, and it is urgent to explore new methods for slurry pipeline transportation testing. To better visualize the slurry patterns during pipeline transportation, the numerical simulation and modelling methods are utilized into pipeline flow prediction and 3D visualization. Kumar et al. discuss the slurry flow pattern of two-phase solid–liquid flow in 90° bend pipelines and straight pipelines by using computational fluid dynamics (CFD), the results show that the pipeline pressure drop significantly increases with the increase of slurry transportation velocity and its solid concentration. The main models include the Lagrangian model based on the discrete element method, the Euler model using solid particles as quasi fluids, the algebraic sliding flow hybrid model and so on. The rheology theory of slurry hydro-transportation is essential basis for flow behaviour modelling and visualization. The common Newtonian flow and non-Newtonian flow model is listed in Table 2. The commonly used flow models can be divided into three categories based on the relationship between key parameters: 1) shear stress shear rate model (includes Newtonian, Power Law, Bingham, Herschel-Bulkley, and Casson), 2) viscosity to shear rate model (includes Williamson, Sisko, Carreau, Carreau-Yasuda, and Cross), and 3) viscosity to shear stress model (Ellis).
Among them, the Bingham model and Hershel-Bulkley model (H-B model) are commonly utilized in define the rheology features of slurry flow. 1) Bingham model is simply and regards that the Bingham viscosity is liner relationship with the shear stress and shear rate. 2) H-B model further define the power law between the shear stress and rate. The apparent viscosity (_η_) of H-B model is calculated by Eq. (1) and Eq. (2). When the flow behaviour index n=1 in the model, the H-B model will degenerate into the Bingham model, which can be used to describe Bingham fluids. Some revised and experienced model are established.
Where _k_ is the consistency index, also known as the consistency coefficient; _n_ is a power law index, a flow behaviour index and non-Newtonian index, which is a temperature dependent parameter. The greater the deviation of _n_ from 1, the stronger the non-Newtonian features of slurry; _τ 0_ is the yield stress, and flow occurs only when the external force exceeds this stress; γ̇0 is the critical shear rate, which is the shear rate at the beginning of flow. For instance, relied on pipeline transportation test of Jinchuan Nickel Mine, the conditions for the boring mixture ratio of the leaching paste were derived, and the calculation formula for the flow resistance of the cemented/non cemented paste pipeline was obtained, as shown in Eq. (3) and Eq. (4). The resistance coefficient reflecting the settling characteristics of particles is shown in Eq. (5).
Where _J m_ is pipeline resistance loss, _U m_ is average flow rate of paste, _D_ is inner diameter of slurry pipeline, _L_ is the total distance of pipeline transportation, _C w_ is mass concentration of non-cemented paste slurry or cemented paste slurry. On premise of obtaining initial inlet and outlet, slurry suspension rheology properties (viscosity, plasticity, etc.), slip wall boundary and other key conditions, the numerical simulation is an important means for slurry pipeline transportation and wear prediction. For instance, the force diagram and shear stress in pipeline transport velocity is shown in Fig. 11. The flow structure includes plug flow layer and shear flow layer. At the centra of pipeline flow, the velocity increase obviously, however, the flow velocity is lower at the edge of slurry pipeline flow (Fig. 11 a). The flow regimes are similar with the laminar flow above-mentioned in this paper (Fig. 11 b). The flow velocity is almost higher in the plunger flow regions. Based on Fig. 11 c, as the slurry temperature increases, the viscosity of the slurry significantly decreases, leading to a gradual expansion of the low-velocity areas of the slip layer and a gradual reduction of the high-velocity areas in the middle regions of slurry inside pipelines. Under such high temperature conditions, the slurry pipeline flow can cause many suspended particles to migrate at low speeds, deposit, and even bond with the pipeline, which to some extent compresses the diameter of the pipeline that can pass through and increases the difficulty of subsequent flushing and cleaning procedure.
In the pipeline transportation of industrial-scale slurry, it can be observed that the slurry velocity of the pipeline cross-section does not decrease linearly. The particle velocity distribution is mainly affected by the concentration distribution, and the asymmetry of particle velocity distribution in vertical direction improves with the increase of particle concentration. Fig. 12 shows the force diagram and shear stress in slurry pipeline transportation driving by gravity forces. The experimental system is composed of pipes with different shapes, diameter, orientations, and inclinations. It shows that the granular flow motion is formed by quasi-static mode (slow flow, strain rate independent), intermediate mode, and intertial mode (rapid flow, strain rate dependent). The maximum shear stress of slurry flow will improve from 700 Pa to around 1250 Pa when volume increase from 50 m/h to 90 m/h (Fig. 12 c). Increasing the concentration of the slurry will increase the resistance loss along the route and strengthen the wear and corrosion effect during slurry transportation. Regarding the phenomenon of mortar segregation. Some studies have used Euler-Lagrange simulations to investigate the segregation of solid–liquid two-phase turbulent slurries and found that particle segregation occurs in the vertical cross-section of the pipeline, with smaller particles predominantly located in the upper section and larger particles in the lower section. Similarly, another study used a real-time resistivity monitoring method to assess the static segregation of coal gangue cemented paste backfill (GCPB) in pipelines. It was observed that the coarse aggregate content was higher in the lower section of the pipeline, indicating segregation of the samples within the pipeline. Some solutions have also been proposed. It is proposed that segregation can be prevented by increasing the viscosity and shape coefficient of the mortar, and reducing the density difference and aggregate diameter. This will effectively prevent segregation. To further understand the particle segregation caused by different bulk density and particle. Fig. 13 shows FLUENT modelling results of undesirable settling that the coarse particle sediment in slurry pipeline flow transportation by using Eulerian model and RNG _k-ε_ turbulence model. The non-uniformity and instability of the flow in the pipeline are obvious (Fig. 13 c). As the slurry concentration increases from 40% to 80%, the gross pressure at the bottom of the pipeline significantly increases. Combined with modelling results of wall shear stress, it indicates that the shear stress is higher at the bottom of pipeline and generates many adverse disturbances. It suggests that this is due to the strengthening of momentum exchange through collisions between coarse particles, which in turn enhances friction, leading to an increase in particle pressure and shear stress. Moreover, with the increase of solid concentration and coarse particle size, the particle collision and settlement in the pipeline are more obvious and harder controlled.
Slurry transport has been a progressive technology for transporting a huge amount and variety of solid materials in both long distance pipelines and short commodity pipelines. Many slurry pipelines already exist, and there will be more to build in all pipeline orientations. In order to ensure safe transport, optimized operation, and reduction of financial costs by avoiding blockage, the operational engineer requires a reliable technique that suits all the conditions in industry. Clearly, the complex nature of settling slurry flow forces the operator to continuously measure the local parameters governing the flow, such as solids concentration and solids velocity, and visualize the internal structure of the flow within the pipeline. Undoubtedly, the measurement of slurry flow parameters and monitoring the motion of solids within the pipe, in fast-evolving processes, requires a fast-responding instrument (i.e., high frame rates of milliseconds). ERT has shown its capability to rapidly interrogate the internal structure of complex and dynamic structure of slurry flow in a nonintrusive fashion. The development of fast-responding ERT system of high-speed imaging in excess of 1000 frames per second, along with the transition of the technology from solids concentration mapping technology to solids velocity mapping technology, can be considered as a direct response to today's slurry flow engineering requirements and a step forward for better controlling the processes and reducing unnecessary financial costs. The current commercially available image reconstruction technique for ERT, LBP, is widely used and provides excellent time resolution for generating qualitative images, which may be sufficient for rapid monitoring slurry flow. Further work on the development of quantitative tomography techniques and efficient strategies of image reconstruction for ERT will be beneficial for detailed characterization of slurry flow and providing a novel insight over the mechanisms of solid–liquid flow. Application of ERT in a wider spectrum of slurry flow, including complex fluid, will further reveal the capabilities of the ERT and pave the way to a reliable and robust technique for characterization of a wide range of slurries and optimization of operation of settling slurry pipelines in industry. Integration of ERT with nonintrusive techniques such as Electrical Capacitance Tomography or ultrasound method will yield an enhanced and versatile multimodality sensor that can be beneficial for a wide range of chemical and allied industries dealing with solid–liquid flow.
To better meet the backfilling requirements, the slurry filling pipeline consists of over 95% fixed pipelines and 5% movable soft pipelines suitable for mining room in underground mining of metal mine. Due to good wear resistance and structural strength, the steel pipelines are mostly utilized as fixed conveying devices in slurry flow transportation, but the pipeline wear and rupture phenomenon caused by paste slurry transportation is still inevitable and require frequent replacements, and it is a key factor affecting mining, cost, and backfilling efficiency in the metal mine. In the actual mining procedure, the undesirable poor wear resistance, localized tearing, and even bursting phenomenon of slurry pipelines has been widely verified by underground mines worldwide that use backfill mining methods (Fig. 14). Thus, the pipeline wall erosion, wear detection and it’s controlling always regarded as one of important optimization measures.
Different from the erosion effect of solution flow on porous seepage path under free interface conditions (such as surface ditch runoff, leaching solution flow in heap leaching system, etc.), the slurry pipeline erosion is jointly affected by various factors such as pipeline system layout, pipeline material, therefore, there are many erosion forms co-existed during the slurry pumping procedures. Based on existing understanding of pipeline erosion behaviour, it mainly includes three types: 1) Impact erosion in slurry pipeline transportation: the suspended coarse particles randomly settle and continuously impact the pipelines during slurry transportation. 2) Rubbing abrasion in water pipeline scouring: the need for water injection to wash the pipeline after the slurry stops pumping, causing friction between the particles and the pipe wall under the erosion effect; 3) Electrochemical erosion in slurry pipeline transportation. Due to the certain pH value (alkaline or acidic) of the slurry and the presence of dissolved oxygen inside, it is easy to form galvanic effects except for physical wear on steel pipelines, and there is still some electrochemical corrosion phenomenon. When the slurry is acidic medium, the hydrogen evolution corrosion gradually occurs, producing ferrous ions (Fe 2+) and hydrogen (H 2); When the slurry is alkaline medium, the oxygen absorption corrosion occurs, producing Fe(OH)3, which is one type of loose and porous substances. What should be noted that the latter two types of pipeline damage modes also occur, but their impact is significantly smaller than long-term wear of the pipeline caused by coarse and fine particles in the slurry during backfill slurry transportation. Therefore, the impact erosion occurs in slurry pipeline flow transportation is the key part discussed, the other two modes are not considered them temporarily.
The slurry is one of typical multiphase flow, which contains coarse particles, fine particles, nano-scale chemical agent powders, and liquid basis medium, it easily promotes the undesirable particle segregation and concentration stratification in pipeline transportation process if the slurry thickening (commonly using deep cone thickener) and stirring condition (commonly using biaxial horizontal mixer) is not satisfied (Fig. 14 b). Moreover, this slurry pipeline transportation process is also influenced by pipeline parameters (total pipeline length, pipeline height difference, inclined angle of pipeline, etc.), environmental parameters (temperature, pipeline vibration, etc.), and pumping parameters (initial pumping pressure, etc.). The coarse particles or crushed waste rocks in the slurry will deposit to the bottom of pipelines, accompanied by the transportation of the slurry in the pipeline, which will cause direct, strong, intermittent, or continuous impact erosion of the coarse particles on the pipeline. It presents different degrees of abrasion at the same cross-section inside the pipeline (Fig. 14 a). This differential abrasion can easily lead to weakening of the slurry pipeline strength, bursting or even failure of the pipeline (Fig. 14 c). This pipeline rupture can easily lead to large-scale leakage of slurry and tunnels blocking in the underground mining. Once the slurry pumping is stopped, a large amount of water solution is required to scour the inside of slurry pipeline, otherwise it may lead to slurry pipeline blockage or even discard of filling pipeline.
In the cemented/paste backfilling procedure of metal mines, the tailings or slurry fluid is mainly transported through inclined pipelines, vertical pipelines, and horizontal pipelines. Combining the existing research by Jiandong Wang, Xiaolin Wang et al. recently, we basically obtained pipeline wearing regulation: 1) For vertical slurry backfilling pipelines, the main stages of wear are slurry impacting the pipeline wall, free fall, slurry and air interface impact stage, and full pipeline transportation stage, and the main rupture stage of the pipeline is the slurry and air interface impact stage Therefore, different pipelines can be installed according to the full-pipeline flow height to control the pipeline rupture. 2) For horizontal slurry backfilling pipelines, there are three kinds of slurry movement forms: suspended mass, jumping mass, and pushing mass. The wear of pipeline is serious, so the flow rate of the slurry should be reasonably controlled to make the motion state of suspended matter and jumping matter, to minimize wear on pipeline. 3) For the inclined slurry backfilling pipelines, especially for not full-pipeline slurry transportation, whether it is a downward or upward inclined pipeline, the slurry is in a saturated mode for the lower part of the pipeline, and the pipeline abrasion is more serious; for the upward inclined pipeline. For the downward inclined pipeline, there are often jumping masses when the slurry is started to be transported, and it will change to the thrusting mass after it stabilizes.
The flow regimes of backfilling slurry in pipeline transportation can be divided into the laminar flow and turbulent flow. Among them, when the filling slurry flows in a laminar flow state, the filling slurry presents a plug flow, specifically showing that the particles do not produce relative motion and particle exchange, while the filling slurry flowing in a turbulent flow state is regarded as a shear flow. Generally, when the Reynolds number exceeds 2100∼2300, the filling slurry will transform from plug flow (laminar flow) to shear flow (turbulent flow). The friction loss increases with the increase of mass fraction or flow rate, and the mass fraction has a significant effect on friction loss, while the impact of flow rate is small. Besides, the friction loss decreases significantly with the increase of pipe diameter. The slurry viscosity has a serious impact on the closure erosion wear of pipelines. The high viscosity fluids have high viscous resistance, which to some extent prevents particle sedimentation and enables particles to move forward in the same direction as the fluid. However, the higher the viscosity, the greater the pipeline resistance, and the higher the required pumping requirements and energy consumption. When coarse particles are transported with high concentration full-tailings slurry in a pipeline in a steady state, except for gravity forces, coarse particles are also usually subjected to pressure gradient forces, shear resistance, Magnus lift forces, and Saffman lift forces. Except for influences of the slurry pipeline angle, the wear of the slurry pipeline is also easy to appear at the variable pipeline diameter and the corner of the pipeline.
Fig. 15 shows the influencing factors and schematic illustration of slurry pipeline erosions. As pointed out in Fig. 15 a, the slurry pipelines erosion caused by filling slurry is mainly affected by four factors, including slurry characteristics (solid/slurry concentration, viscosity, temperature, corrosivity, carrier fluid and turbulence), target material properties (corrosion resistance, hardness, toughness, microstructure, and work hardening), solid particle properties (particle shape, size, hardness, and density), and impingement (particle velocity, impact angle, impact level, and wet/dry condition). Generally, the main composition and ratio of the slurry, as well as the environmental condition are fixed in a specific metal mine. The impact erosion of slurry on pipelines is mainly affected by the transportation velocity, which could be easily controlled at surface pumping station. As shown in Fig. 15 b, the different slurry pipeline velocity can lead to different erosion effects. In specific, at low slurry flow velocity, the coarse & fine particles suspended in slurry interact with the pipeline as the slurry flows, gradually wearing down the ridges of the pipeline and creating weak areas and microcracks. Under the impact of the solution, these microcracks can be susceptible to damage. At high slurry flow velocity, these abrasive particles suspended in slurry further impact and erose the weak surface of pipeline structure and accelerate the formation of erosion scars on the pipeline surface. As a result, some free particles attached and embedded in pits of the pipeline surface, some debris stripped from pipelines and bound into slurry. With the erosion of filling slurry and suspended particle continuous impact, the structural weak surface of inner wall of pipelines is becoming increasingly apparent, and its strength is significantly reduced.
For non-full pipeline slurry, there are always unsaturated areas present inside pipelines. Except for the uneven impact wear caused suspended particles in the horizontal and vertical pipeline transportation, the structural erosion of pipeline inner surface mainly occurs at the joints, corners, or bends of the pipeline. To better predict this undesirable erosion, the computational fluid dynamic (CFD) simulation methods are usually used to predict the wear and impact inside the pipeline.
For instance, as Fig. 16 a shows, under the effects of hydraulic dragging forces, the massive suspended particle impact and smash the inner wall when the slurry pumped into the horizonal curved elbow pipelines. Based on velocity legend, it shows that the velocity of slurry flow rapidly decreases when it meets the inner walls of pipelines because fluid is obstructed by the pipe wall and changes its flow direction along the adjacent connected pipelines. The maximum wear position of pipelines is observed in the intersections of lateral and oblique position. Considering the facts that curved elbow pipelines are susceptible to impact, the simulation studies have also been conducted on multiple impacts and collisions at the same position of slurry pipelines (Fig. 16 b). It can be seen that because of the first collision, the damage to the pipeline in the second collision became more severe and significant, manifested as extending towards the periphery with the collision point as the centra. It is predictable that under this continuous collision for a sufficient long time, the slurry pipeline will experience pitted corrosion or even damage, ultimately leading to structural damage and failure.
The transportation process of filling slurry is influenced by various factors such as its own flow characteristics, pipeline layout and length, filling line, pipeline inner wall material, transportation control system, etc. In the past, the design of filling transportation system could only rely on empirical analogy, and its rationality cannot be evaluated before the system is put into on-site use. To better reveal the phenomenon of pipeline erosion caused by multiphase slurry flow and propose measures to reduce pipeline wear, some scholars have summarized that the erosion wear modelling, erosion wear detecting, resistance to erosion wear, and erosion wear risk assessment are mainly four research areas (Fig. 17). Based on these results, the pipeline erosion could be retarded by following four suggestions: 1) the accuracy and breadth of prediction models (CFD, etc.) should be further improved based on mathematical model and wear detecting data; 2) the novel acoustic, electromagnetic detecting technique and single/integrated detecting methods (such as ERT, MRI, micro-CT, etc.) should be introduced into the in-situ undisturbed detections; 3) the inner, structural morphology from the micro/meso/macro scales and erosion resistance materials of slurry pipelines should be updates based on multiscale measurement and modelling prediction; 4) the RUL prediction and fault diagnosis of pipeline transportation based on the modelling, detection and morphology examination of multiphase slurry flow. These potential operation and methods will delay the erosion procedure of slurry pipeline flow transportation, which contribute the safe, environmental-friendly, and efficient tailing backfilling in metal mine.
The backfilling slurry of metal mine, as a multiphase fluid composed of liquid, cement, cementitious chemistry, tailings, crushed particles, and other solid wastes, is transported to fill the underground goaf, which is considered as an important method to control the surface collapse, reduce mine tailing ponds, and achieve environmental-friendly mining of mineral resources. This particle stratification and segregation is prone to be appeared in multiphase slurry flow during the long-term, high drop, and high-vibration pipeline transportation, leading to severe pipeline wear, low-strength filling body, and high-cost operations. Thus, a deep understanding of multiphase slurry flow behaviour in pipeline transportation is an important premise to improve underground mining efficiency.
In this paper, four main aspects tightly related to the slurry pipeline flow transportations is carefully discussed: 1) Multiphase features and understanding of fluid flow behaviour. It is necessary to distinguish whether it is full pipeline or non-full slurry pipeline flow. The slurry flow regime and its features present vastly different under various pipeline filling condition. The full pipeline flow can to some extent reduce negative wear and transportation resistance, while non full pipeline flow can easily lead to coarse/fine particle segregation inside slurry and the resulting pipeline undesirable wear and impact. 2) Experimental detection and evaluation of slurry pipelines transportation. Except for the understanding of theoretical flow regimes, some of laboratory slurry flow rheology tests, pilot loop-pipeline transportation tests and other studies have been carried out. Many novel undisturbed detection methods (includes MRI, ERT, Micro-CT etc.) are also introduced into meso- and micro- scale slurry flow regime and wear behaviour of pipeline transportation; 3) Characterization and modelling in pipeline transportation of cemented slurry. To supplement the findings of actual pipeline transportation experiments, the mathematical fluid flow models (e.g. Bingham model, H-B model) and computational fluid dynamics (CFD) simulation of slurry pipeline flow are applied to the multiphase slurry flow morphology inside the pipeline, as well as the effect of coarse particles on pipeline erosion, providing a good anticipatory effect for controlling pipeline erosion. 4) Wall erosion, wearing reduction and controlling in slurry transportation. The erosion patterns of inclined pipeline, vertical pipeline, and horizontal pipeline in slurry flow transportation is totally different. Based on the previous research results on fluid regimes and slurry pipeline flow numerical simulations, it is not difficult to find that pipeline wear seems inevitable. However, its wear procedure can be artificially intervened by controlling pipeline transportation parameters (slurry features, initial pumping parameter settings, etc.), which is a good method to reduce pipeline wear and replacement frequency. Currently, there are issues with detection accuracy and a lack of predictive models for multiphase flow pipeline transportation research. It is still necessary to combine various new technologies and equipment to improve the accuracy of detection in pipeline inspection. In terms of slurry transportation, it is necessary to establish a multi-field coupling model that considers the influence of various factors on the flow process of the slurry. By establishing a fully coupled mathematical model, the accurate identification and prediction of the flow state of multiphase fluids in pipelines can be achieved. It should be pointed out that many specific scenarios, such as the discussion between different solid fluxes, pumping modes, ash sand ratio, flow rate, slurry flow distance, and other key parameters, are insufficient or limited in this paper. All in all, the paper critically reveals main research interest areas, breakthroughs, problems, and challenges faced in slurry pipeline flow transportation of underground backfilling, which offers a good reference and inspirations of future related researches.
The underground cemented backfilling of tailings slurry in metal mines is an important guarantee for controlling goaf, reducing tailings storage, and ensuring mining safety. As a typical multiphase flow, the slurry and its pipeline flow transportation are important factors that affects mining efficiency and leads to poor pipeline wear. To better understand the flow regime and its pipeline transportation characteristics, this paper systemically reviews following aspects: 1) the non-full/full pipeline flow regimes, patterns, and key evaluating parameters (distribution of pressure drop, velocity, etc. of slurry flow); 2) the breakthroughs, findings and limitations by using the slurry rheology experimental (loop pipeline test, etc.), or undisturbed detections methods (electrical resistance tomography, magnetic resonance imaging, Micro-CT, etc.) of slurry pipeline transportation; 3) the computational fluid dynamic models and numerical simulations, and 4) the generating mechanism, major modes and its controlling methods of slurry pipeline wear and undesirable erosion are critically summarized. It conducts that the non-full pipeline transportation, slurry composition heterogeneity, coarse particles segregation/sedimentation, pipeline shapes and its layout, and even electrochemical corrosion caused by acidic/alkaline slurry could be key causes for the high resistance and poor effectiveness of slurry pipeline transportation. These discussions in this review paper will contribute to offering good supports to understand slurry pipeline flow behaviour and potential wearing controlling methods.
In a typical slurry pipeline design situation, the flowrates and solids concentrations are fixed by process material balances and equipment performance specifications. In these circumstances, a primary goal in design is selection of the optimum pipe diameter. For slurries in turbulent flow, the optimum transport condition almost invariably occurs when all the particles are suspended but moving at the lowest possible mean velocity. By operating the pipeline at the slurry deposition velocity, the frictional energy losses and wear are minimized and the whole of the pipe cross-section is available for flow.
Because of its importance, the deposition velocity has been the subject of innumerable experimental investigations, some of which have had a theoretical component. Rather than attempt to summarize all of these, the present communication is intended to provide a guide to the designer. In addition to presenting correlations for use in estimating the deposition velocity, the limitations of these correlations are described so that experimental tests may be considered for particular slurries.
Before 1900s, the multiphase and rheological behaviour of slurry fluid was not paid sufficient attention to and not systematically analysed, it was not until around 1930 that the viscosity of slurry flow was measured with Marsh funnel and Storm viscometer. Exploring the slurry flow regimes in pipelines, the solid particles dynamic characteristics, resistance characteristics, and optimizing transportation parameters, is of great significance for increasing slurry pipeline transportation concentration, reducing pump resistance, avoiding undesirable pipeline blockage, and extending transportation distance. Furthermore, the transportation of the slurry in the filling pipeline plays an important role in the process of slurry preparation. For example, the addition of flocculants during the deep cone thickening process can result in flocculation and sedimentation effects, which have both advantages and disadvantages for pipeline transportation. On one hand, flocculation and sedimentation can cause the solid suspended particles in the slurry to aggregate into larger particles, reducing the frictional resistance generated by solid particles during pipeline transportation. Additionally, flocculation and sedimentation increase the viscosity and yield stress of the slurry, making it more stable during pipeline transportation. On the other hand, the formation of larger particles due to flocculation and sedimentation increases the viscosity of the slurry, leading to increased pipeline transportation resistance. Moreover, the formed solid particles are prone to settle at the bottom of the pipeline, causing blockage issues.
The slurry that’s composited by cemented paste backfill is a fluid with polydisperse phase, and its deformation and flow under external force are determined by internal resistance, such as the size of internal friction, strength of internal network structure, action time, etc., which can be expressed by viscosity (plastic viscosity, structural viscosity, apparent viscosity), shear stress (static shear, dynamic shear), thixotropy, consistency coefficient, liquidity index, shear dilution and other parameters. The key parameters can be represented by rheological mathematical equation, which is represented by shear rate (dv/dx), plane Cartesian coordinate system of shear stress, rheological curve in logarithmic coordinates. The rheological parameter relationships of single-phase homogeneous systems such as clear water, solution, and glycerol can be described by Newton's equations, hence these fluids are called Newtonian liquids. Heterogeneous two-phase or multiphase systems, such as slurry fluid, quasi plastic fluids, and expansive fluids such as slurry, emulsion, polymer solution and foam slurry, are subject to Bingham equation, power law equation or Carson equation and their rheological diagrams. The rheological properties of slurry flow are important parameters for calculating the pressure loss of slurry circulating in the hole, selecting pump pressure, pump power, exciting pressure during tripping, the upward and backward flow rate of rock cuttings, and the hydraulic power of the drill bit.
To better understand the slurry flow regimes, this paper systemically reviews and lists the horizontal flow and vertical flow in Table 1. In particular, the horizontal slurry flow is divided into dense phase (including full pipeline flow, slug flow, bed flow) and lean phase (dilute flow), similar but different, the vertical flow is bubbly flow, slug flow, chum flow, annular flow, and dispersed flow. The regime of slurry flows is not immutable, it will transform continuously under the different conditions of pipeline time, pump flow velocity, feed particle size distribution and so on. When the slurry properties and pumping conditions change, the flow regimes of the pipeline slurry will change and the transformation among them will occur. Taking the horizontal flow, the full pipeline flow (dense flow) nearly fully slurry backfill pipelines, which is regarded as ideal flow regime in underground backfill, named paste backfill. On this understanding, its pump pressure is higher while the slurry flow velocity is lowest. However, caused by multiphase composition in slurry pipeline, the slug flow (dune flow) and bed flow (segregated flow) are more easily observed in industrial pipeline transportation of slurry flow. In the slug flow, the slurry moving dunes as it’s conveyed and velocity is also quite low. In the bed flow, the velocity is mixed and bed slurry moves slow. The slurry above moving bed is flowing above saltation velocity caused by the solid–fluid multiphase segregation. The flow state of slurry in a pipeline is influenced by various parameters such as solid content, pressure, filling ratio, and travel distance. When the slurry has a higher solid content and density, it typically exhibits a dense phase flow. Conversely, when the slurry has a lower density, it is considered to be in a lean phase flow. The filling ratio of the pipeline also plays a role in the flow behaviour of the slurry. If the pipeline is completely filled with the slurry, it is referred to as a full pipeline condition. However, if the pipeline is not completely filled, the slurry may exhibit different flow forms. The travel distance of the slurry also affects its flow behaviour. In some cases, the solid particles in the slurry may undergo separation and segregation, leading to changes in the flow pattern.
To simply understand slurry pipeline flow, Fig. 3 the slurry flow regimes to four types. Under the controlling of slurry velocity and rheological features, the slurry flow regimes are divided into four types regimes (stable turbulent, unstable turbulent, stable laminar, and unstable laminar) by three deposition boundaries (critical deposition boundary, transitional deposition boundary, laminar deposition boundary). Accompanied with the increase of flow velocity, the unstable turbulent transfer to stable turbulent; similarly, the unstable laminar transfer the stable laminar (Fig. 3 a). Based on Fig. 3 b, for the stable turbulent, the turbulent eddy drag forces are sufficient to suspend the particle buoyant mass, while it is not sufficient in unstable turbulent. For the unstable laminar, the turbulent eddy forces are dissipated by viscous forces. From the stable laminar, the yield stress force dominates the unsheared core region of pipe flow and supports particles in transfer. The wall shear stress pushes particles deposited along pipe wall.
According to the differences of velocity profiles at steady state, the slurry pipeline flow regions can be roughly divided into laminar flow (Fig. 4 a) and turbulent flow (Fig. 4 b). It shows that the flow structure includes viscid/irrotational flow region and viscous/rotational flow region. The viscous fluid has two flow regimes of laminar flow and turbulent flow, and can also be divided into Newtonian fluid and non-Newtonian fluid according to whether Newton's law of internal friction is satisfied. For paste slurry, it belongs to a class of typical non-Newtonian fluid, and it is also typical viscoelastic materials that show the rheology behaviour with both viscous and elastic properties. The viscosity of the slurry paste is due to the friction that occurs when the adjacent layers move at different speeds. The resistance at the centre of the slurry pipelines is the smallest, and the flow velocity of the slurry layer is the largest; the slurry layer near the pipeline wall is simultaneously affected by the viscous resistance of slurry flow and the friction of the pipeline wall, and the velocity is the smallest. As Fig. 4 c shows, the full-pipeline slurry flow is ideal and expected in paste backfill of metal mine, the flow structure is composited of slip-wall flow, shear flow and plunger flow. Meanwhile, the positive displacement pumps have a certain impact on the flow structure of the slurry in the pipeline. When the pump pressure is high, the range of piston flow may be larger, while the range of shear flow may be smaller. Conversely, when the pump pressure is low, the situation is reversed. Moreover, the volumetric pump also affects the uniformity, stability, and flow velocity of the slurry, thereby impacting the flow pattern of the slurry. The main flow includes of shear flow and the central plunger flow. Among them, the slip flow near the pipeline wall has aroused the interest of scholars, and has been proved by experiments to help reduce the pipeline transportation resistance. Several paste pipeline transportation resistance models considering the wall slip effect have been gradually established to describe the rheology behaviour of filling slurry in the pipeline. At low speeds, the molecules in the Laminar fluid move in the same direction, with the size and direction of V being equal. The fluid tends to flow without lateral mixing, without transverse flow perpendicular to the flow direction, and without vortices or eddies of the fluid. Fluid, there may be shear thinning (the apparent viscosity decreases with the increase of the shear velocity) and shear thickening (the apparent viscosity decreases with the increase of the shear velocity). Increases with increasing speed).
For paste tailing in metal mine, as Fig. 5 shows, Richard Jewell and Andy Fourie carefully summarized flow regime, deposition, thickener, and pump modes under the different yield stress (_τ y_) and the ration of solid concentration and packed solid concentration (C/C max). Based on Fig. 5 shows, its flow regime presents the slurry or suspension when _τ y_ is less than 100 s. When the C/C max is large than 1.0, the _τ y_ is much large than 1000 s because liquid content is low and solid concentration is high. Under different concentration, the deposition is different, the slurry or suspension is commonly dumped above ground, the ideal paste slurry is pumped into underground and utilized in backfill procedure, the tailing cake is stacked into waste rock pile or idle industrial site.
Before the industrial-scale utilization, the laboratory-scale, pilot-scale experimental system such as pipe-loop test, L-type pipeline test, horizonal pipeline test, and vertical pipeline test are setup to explore backfilling slurry rheology and flow behaviour. Besides the pipeline erosion experimental system are also developed to quantify the slurry pipeline erosion by direct and indirect measurement. The direct method is measuring changes of the total weight or wall thickness of pipelines before and after slurry transportation procedure, which is inaccurate and with significant errors.
The indirect methods include loop-pipe wear testing system, rotating wear testing system, drum wear testing system, tank wear testing system, etc. Relied on current research findings, the wear rate of pipeline decreases with the increase of the density (concentration) of the filling slurry. Increasing the density (concentration) of the slurry can reduce the wear rate of the pipeline. For instance, the schematic of pilot-loop test is shown in Fig. 6 a. By comparing microscopic images of slurry flow at 60 s, 3600 s, it infers that the homogeneity of the slurry gradually improves with the flocculation and pipeline transportation processes (Fig. 6 b). In recent years, the undisturbed detections have made progress in detection accuracy and sensitivity. The regimes of slurry flow change from laminar flow to turbulent flow if the power law Reynolds number (Re pl) larger than 2200. The Re pl is negatively related to the friction factor (Fig. 6 c).
Similarly, as Fig. 7 shows, the pipe-loop test, matched sensors and central control and data collection system are fabricated. The slurry transportation is pumped by piston pump in this loop test system. At the elbow of the pipeline or every other length of the pipeline, a certain amount of pressure, flow, and temperature sensors will be installed. By providing feedback on these data, it can be obtained whether there is any phenomenon of slow flow rate, rapid increase in flow rate, or even blockage between adjacent sensors (Fig. 7 a). The effect of different solid concentration (77.1%, 75%, and 73%) on pipeline pressure and flow velocity (flow rates) are carefully quantified (Fig. 7 b). There will be significant fluctuations in flow velocity and pressure drops, especially at the diameter-changed, or elbow bending position of pipeline transportation. The results of pilot loop tests provide the basic reference data for actual mining.
To better verify and understand the slurry flow behaviour in actual pipeline transportation, some undisturbed detection, chromatography analysis methods such as the electrical resistance tomography (ERT), electrical capacitance tomography (ECT), micro- computer tomography (micro-CT), magnetic resonance imaging (MRI), High-speed photography (HP), and
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