1 Introduction

Bunkers are mostly used for storage or buffering. Particle size segregation effects at the bunker outflow lead to fluctuations in particle size distribution. In most applications and for their following processes, an evenly distributed bunker discharge is desired in terms of particle size distribution. This especially applies to blast furnace sinter bunkers as a constant particle size distribution for blast furnace operation is needed to ensure a sufficient gas flow.

This study was conducted within the RFCS-project MinSiDeg, which received fundings from the European Union and was performed by the University of Leoben and several industry partners from Austria and Germany. One objective in MinSiDeg was to develop or further develop innovative conveying and storage equipment in order to reduce degradation and segregation effects during conveying and storage processes of blast furnace sinter. One of these devices is the solid state material turbine, which was originally developed and patented in [1] and was described in [2,3,4] for energy recovery. In this contribution the solid state material driven turbine was further developed and optimized to reduce segregation effects. Most of the content in this contribution was also presented in [5].

2 Segregation Effects during Bunker Filling

Significant segregation effects during bunker filling occur, mainly due to the following two effects. Due to vibrations during transport by conveyor belt, small particles accumulate at the bottom and large particles at the top of the bulk material heap in the conveyor belt (Fig. 1). Thus, the large particles have a different trajectory than the small particles when discharged. Depending on the belt incline and speed, the large particles at discharge could have a higher translational velocity than the small particles due to their greater distance to the center of the discharge pulley (Fig. 1). This could lead to an accumulation of large particles in the conveying direction and an accumulation of small particles against the conveying direction of the discharging conveyor belt [6].

Fig. 1
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Segregation effects at conveyor belt discharge [6]

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The second effect for segregation is noticed every time a bulk material pile is formed. As larger particles have a greater forward momentum than smaller particles, the coarse material continues moving down the side of the pile more than the fine material. The material that tumbles down the slope of a pile is called overrun. Larger particles tend to roll down the entire length of the slope and fine particles tend to settle into the side of the pile. This effect of overrun causes the outer and bottom zones of the pile to consist of coarser material, while the inner and upper zones of the pile consist of more fine material [6].

3 Current State

The current state at a steel manufacturer in Austria was investigated. Sinter is stored in square-shaped bunkers with filling levels of up to 600 t, which are filled by conveyor belts with a mass flow of 300 t/h and an almost constant particle size distribution (Fig. 2). Low filling levels lead to high drop heights, which is assumed to be one of the main reasons for material degradation.

Fig. 2
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Blast furnace sinter bunker filled by conveyor belt

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The current state was simulated by means of the discrete element method (DEM) with EDEM (Figs. 3 and 4; [7]). Parameters for simulations were determined in angle of repose and slip tests in [8], rebound tests in [9] and further investigations regarding contact processes in [10]. In the simulations the bunker was filled with 350 t with the original particle size distribution listed in Table 1. The small fractions 6, 10, and 16 mm were combined in the simulation (52.245% in total), and all particles were up-scaled by factor 3 for computational efficiency.

Fig. 3
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DE-Simulation of the bunker at current state, side view [7]

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Fig. 4
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DE-Simulation of the bunker at current state, top view [7]

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TABLE 1 Particle size distribution for bunker filling
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At a filling level of 350 t, the bunker was discharged and the particle size distribution at the outflow was determined in the simulation. In Fig. 5 the mass fraction for each particle size in connection to the bunker filling level is shown. Small particles (6 + 10 + 16 mm) contribute with approximately 80% to the total mass flow at the beginning and decrease to around 10% at the end of the discharging process. As shown in Table 1, the optimum would be a constant flow of 52.245% for the small particles during the whole discharging process. In contrast to this, the mass fraction of large particles is relatively small at the beginning of the discharging process and starts to increase at approximately 50% of the filling level. This effect can be explained by the core flow effect, whereby a core flow forms during discharging. This leads to an early discharge of material in the inner zones of the bunker, which are the smaller particles. The larger particles (overrun) are accumulated at the outer zones.

Fig. 5
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Particle size distribution at bunker discharge, current state

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4 Measures to Reduce Segregation Effects

As the core flow effect is one of the main reasons for fluctuations in particle size distribution during bunker discharge, the simplest measure to reduce these is to reduce the wall friction between the bunker and the material. A friction coefficient of µ = 0.89 between the steel wall and sinter was measured in experiments [8]. Simulations were repeated with a fictively reduced friction coefficient of µ = 0.4. Figure 6 shows the mass fraction for each particle size during bunker discharge in connection with the bunker filling level with a reduced friction coefficient between wall and sinter. In comparison to Fig. 5, the mass fraction of the small particles (6 + 10 + 16 mm) is significantly reduced to approximately 60% at the beginning. However, such a low friction coefficient is fictitious at the time of writing as a material with a friction coefficient of µ = 0.4 and sufficient wear resistance is hardly provided with the current state of technology.

Fig. 6
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Particle size distribution at bunker discharge with reduced wall friction

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Another simple approach to reduce segregation effects would be a smart filling and discharging procedure. When the bunker is not completely discharged but filled again when a filling level of 50% is reached, also a more constant particle size distribution during the discharge is noticed in simulations. Due to the core flow effect, the material near the walls will never be discharged, which would lead to undesired material aging near the walls.

Various types of installations to reduce segregation effects during bunker filling were tested in simulations (Fig. 7). The discharge from a conveyor belt was simulated with and without different installations. To determine segregation, the top view of the bulk material pile with only one particle size faded in was evaluated. A significant improvement in terms of segregation was noticed with a pitchback turbine at low rotation speeds. The disadvantage is that a pitchback turbine needs a guide plate, which is an additional part exposed to wear. Good results were also noticed with an overshot turbine at low rotation speeds in this case, but this significantly worsened when tested with the given bunker geometry. The best results were achieved with a cross flow turbine at low rotation speeds.

Fig. 7
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Evaluation of various installations to reduce segregation effects during bunker filling

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5 Cross Flow Turbine

Compared to all other devices and types of solid state material turbines, the best results regarding a reduction of segregation effects were obtained with the cross flow turbine, shown in Fig. 8. The cross flow turbine consists of 10 segmented blades, which form an ideal curvature for material flow. The center of the cross flow turbine is hollow, which allows the material to flow through the turbine. The material flow through the turbine is shown in Fig. 14a, which was simulated with the particle size distribution listed in Table 1. The color scale represents the translational velocity of the sinter particles. Simulations with different rotation speeds were performed and an optimum in terms of segregation was found at 5 rpm. Depending on the mass flow, this low rotation speed can be achieved by regenerative braking or by an electric drive at low mass flows.

Fig. 8
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CAD model of the cross flow turbine

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The bunker filling process was simulated again with the cross flow turbine (Fig. 9). The 100 mm particles were represented by 50 mm particles, otherwise they would not pass the turbine due to the upscaling by a factor of 3. A significant reduction of segregation effects is noticed at bunker discharge when the bunker is filled with the cross flow turbine (Fig. 10). An almost constant particle size distribution till a filling level of 100 t is noticed, which is a great improvement compared to Fig. 5.

Fig. 9
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DE-Simulation of the bunker filled with the cross flow turbine

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Fig. 10
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Particle size distribution at bunker discharge, bunker filled with cross flow turbine

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6 Wear

As sinter is a very abrasive material, the wear at the cross flow turbine was investigated by means of DEM with EDEM. A triangular mesh of 10 mm was used to evaluate wear after 50 s of operation with the normal cumulative contact energy. The wear maxima were noticed at the blade edges (Fig. 11). Also for the tangential cumulative contact energy, the wear maxima were noticed at the blade edges. With wear data from iron ore, the wear was roughly estimated for the turbine blade edges. For the bunker with a capacity of 600 t filled with 300 t/h, the blade edges would resist 126 full bunker fillings until a 10 mm surface erosion, if equipped with Hardox 400. If equipped with Kalmetall W100, the blade edges would resist 265 full bunker fillings until a 5 mm surface erosion. As worn blade edges do not lead to operating failure of the turbine, the actual lifetime of the turbine is much higher.

Fig. 11
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Wear maxima at the cross flow turbine’s blade edges

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7 Power Output

In this contribution the cross flow turbine was optimized to reduce segregation effects, but generally it could also be used for energy recuperation as described in [2, 4]. To calculate the power output of the cross flow turbine in this case, the torque at the turbine was evaluated using DEM. For power output evaluations, the original particle size distribution without upscaling was used. The torque was plotted over a duration of 60 s of operation (Fig. 12). An average torque of 375 Nm, with significant fluctuations of up to 1400 Nm, was determined. The fluctuations are due to impacts of large particles. This results in an average power output of about 200 W at 5 rpm. The optimum rotating speed for the power output for solid state material driven turbines would be 30–40 rpm. Then a power output of about 1 kW could be provided in this case, but then no improvements in terms of segregation would be achieved. However, the power output is negligible in this case, but could be of interest for other applications.

Fig. 12
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Torque at the cross flow turbine at 5 rpm with a mass flow of 300 t/h iron ore sinter

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8 Material Degradation

The cross flow turbine significantly reduces segregation effects, but could eventually also cause damaging effects on particles. As a minimum grain size of 6.3 mm is required for blast furnace operation, fines generation is especially critical for blast furnace sinter and is one main focus in conveying and storage processes, apart from equipment wear [7]. According to [11], 6% of the total mass flow of sinter provided at the blast furnace are fines due to conveying and storage processes in EU average. These fines have to be screened out before reaching the blast furnace and have to be re-sintered (return fines), which is quite an energy consuming process causing high emissions and costs [7].

On the one hand, the cross flow turbine reduces drop height into the bunker by its diameter, but on the other hand, it causes additional impacts against the turbine and among the particles. To quantify the material degradation and fines generation caused by the turbine, a detailed investigation was necessary. Therefore, a recently developed breakage model for DEM was used [12,13,14].

The novel breakage model is based on a probabilistic particle replacement with voronoi-tesselated fragments. Depending on the stress on the individual particle, the initial particle is probabilistically replaced by different breakage patterns (Fig. 13). Depending on the resulting fragment sizes, the breakage pattern is generated by means of the voronoi algorithm [15,16,17] with a certain amount of seeds. In contrast to [18] and other particle replacement models [19,20,21,22], where the particle is replaced by a number of spheres, in this model the initial particle is replaced by an exact copy of the initial particle, which has been previously tessellated. This ensures a mass and volume consistency. Polyhedral particles of any shape can be used. As the model is based on probabilities, a high amount of particles is necessary, but then the model delivers high accuracy in terms of fragment size distribution. Further breakage of fragments can also be simulated, which allows simulation of long and complex conveying systems with multiple breakage. The breakage model was programmed in C++ using an API (Application Programming Interface) in the simulation software ThreeParticle from Becker3D. A detailed description of the concept is provided in [12]. The probability for each breakage pattern is dependent on the mechanical stress and is determined by single particle impact tests in [23,24,25]. The impact tests were performed with a specially developed automated single particle impact tester, which is described in detail in [23] and allows rapid analysis of breakage behavior of bulk materials. The model was verified and validated with a trial of shatter tests and trials with two different transfer systems in [12] using different batches of sinter from two different manufacturers.

Fig. 13
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Voronoi-tesselated polyhedral particle of spherical shape [12]

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To quantify particle breakage by using the cross flow turbine at 5 rpm, a comparison of the drop with and without the turbine was performed in ThreeParticle (Fig. 14). For computational efficiency, a simplified model with fewer particles in the bunker was used. For that, a fragment collecting box was implemented. Very soft material properties were assigned to the collecting box so that no breakage occurs due to impacts with the collecting box. To simulate the drop without the turbine, only the pile peak was simulated, as the drop into a bulk material bed has a damping effect. The bulk material bed was placed on the same height as the bottom of the turbine, which allows a direct comparison. The turbine reduces the drop height for the discharged material from h to hT but causes additional impacts with the turbine and within the material.

Fig. 14
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DEM breakage simulation of bunker filling a with cross flow turbine at 5 rpm b without turbine

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The results of the particle breakage evaluation are shown in Fig. 15, which illustrates the increase of mass fractions for each particle size due to the filling process. Slightly more breakage occurs at the drop with the cross flow turbine at 5 rpm and about 1% more fines are produced in this case. It is assumed that volume breakage occurs mainly due to impacts with the blade edges, where also the most wear occurs, and generation of fines is mainly due to abrasion among sinter particles inside the turbine. Furthermore, it is assumed that significantly less particle breakage occurs when the turbine is operated with higher rotation speeds (30–40 rpm) for energy recovery, which will be investigated in future investigations.

Fig. 15
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Particle breakage due to the cross flow turbine at 5 rpm and drop without turbine

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Various studies confirm that replacing a drop by several smaller drops leads to less particle breakage [26]. Thus, material degradation is dependent on the mounting height of the turbine and bunker filling levels. The reduction of particle breakage due to splitting the drop height into two smaller drops could not be evaluated with the simulations described in this contribution. Overall, it is assumed that the turbine leads to less particle breakage during a whole bunker filling process, which needs to be confirmed by further investigations.

9 Summary and Outlook

The current state of filling and discharging processes at an existing bunker for blast furnace sinter at a steel manufacturer was simulated using DEM. Segregation due to conveyor belt discharging and overrun at the bulk material pile leads to significant size segregation during bunker filling. This causes fluctuations during bunker discharging in terms of particle size distribution. At the beginning of the bunker discharge process, more small particles exit the bunker than large particles and vice versa at the end of the discharge process. This can be explained with the core flow effect.

A simple way to reduce the core flow effect and thus achieve a more evenly distributed bunker outflow would be a reduction of friction between bunker walls and bulk material. Considering sufficient wear resistance, this is hardly possible with the technology provided at the time of writing. Another simple way would be a smart filling and discharging procedure, but this would lead to undesired material aging near bunker walls.

Various devices to reduce segregation effects during bunker filling were tested in discrete element simulations. The best results were achieved with solid state material driven turbines, especially with the cross flow turbine at low rotation speeds. Bunker filling simulations were repeated with the cross flow turbine at 5 rpm. A significant reduction of segregation effects and thus a more constant bunker outflow was noticed.

According to additional wear investigations with DEM, wear mainly occurs on the blade edges of the cross flow turbine. Rough estimations with data from iron ore and two commonly used equipment materials predict 126–265 full bunker fillings for a bunker with 600 t before wear blade edges show an erosion of 5–10 mm.

Generally, solid state material turbines can also be used for energy recovery. An evaluation of power output was conducted with the cross flow turbine. At low rotation speeds, in this case 5 rpm, which is the optimum to reduce segregation effects, the power output is negligible. The optimum rotation speed for energy recovery is 30–40 rpm, which could be of interest for other applications.

Additionally, bulk material degradation due to a drop into the bunker with the cross flow turbine was investigated with a novel breakage model for DEM. The model is based on a probabilistic particle replacement with voronoi-tesselated fragments and is verified and validated. Simplified simulations reveal that slightly more particle breakage occurs due to a drop with the cross flow turbine at 5 rpm than a drop into a bulk material bed at the same height. This is assumed to be caused by impacts at the blade edges and by abrasion among the material in the turbine. However, it is assumed that overall the cross flow turbine reduces particle breakage during a whole bunker filling process by splitting one big drop into two smaller drops. This effect depends on the mounting height and bunker filling levels but needs to be confirmed by further investigations. Furthermore, it is assumed that significantly less particle breakage occurs when the turbine is operated with higher rotation speeds (30–40 rpm) for energy recovery, which will be investigated in future investigations.

A trial at a steel manufacturing plant is planned in the near future in order to test the cross flow turbine under real conditions and to confirm segregation and wear predictions.