Priscilla Freire, TUNRA Bulk Solids’ Business Development Manager, has worked with TUNRA’s experts to compile a summary of three areas TUNRA can help your materials handling plant.
Area 1: Bulk Materials Testing for Optimum Design
Flow properties testing has a multitude of applications within materials handling, from informing design parameters for coarse and fine ore plants, through processing equipment and finally for the storage and transport of product and waste fractions. These tests include mechanical tests of bulk material strength which inform chute and hopper design, wall friction and wall adhesion characterisation, particle size and density characterisation, and operational tests such as those required to inform moisture limits and time-dependent behaviour (e.g. undisturbed storage time). However, the nature and extent of testing need to be carefully considered with respect to the project or study phase and the availability and representativeness of test samples as the project progresses through conceptual, feasibility and definitive study milestones.
Designing a testing scope without firstly understanding the handling systems and the range of bulk material flow behaviours (from free-flowing to highly cohesive) can result in unnecessary expense in budget and time. It is also important to assess the nature of the bulk material samples that are available to test at the given project stage. For example, if only indicative samples are available that do not closely meet the expected characteristics of the final fractions, then a reduced testing regime may be developed to provide general guidance, with an eye to developing detailed flow property test data once more refined pilot or plant samples become available.
From a materials handling perspective, once the preliminary flowsheet has been defined and the main processing equipment selected, details such as throughputs, size fractions and expected moisture ranges can be determined. An overall idea of plant dimensions also becomes available, so that parameters for the design of belt conveyors can be defined, such as lengths, inclination angles and widths. The need for stockpiles or bins for storage or surge capacity is also defined. With the progression of the flow sheet comes the need for more detailed definition of design parameters such as critical opening dimensions and hopper half-angles and wall liner selection. These parameters are strongly governed by bulk material flow properties, particularly for fine and cohesive materials. That is when detailed handling testwork should come into play, given that such design parameters are required to inform preliminary mechanical general arrangement drawings, which will be further detailed in the feasibility stage.
Area 2: Transfer Chute Design and Troubleshooting
Transfer chute problems are by far the most common we deal with in TUNRA’s engineering projects, which involve the design or re-design of transfers and associated work, including material characterisation and calibration tests, and computational modelling. Our engineers have put together some essential considerations for transfer chute design:
Adequate attention to the bulk solid flow properties in view of the transfer configuration
According to Em Prof Alan Roberts, TUNRA’s founding director, chute design has been the object of extensive research over the past decades, but attention to the properties of materials being handled and their flow dynamics is often overlooked.
Material properties of relevance for transfer chute design include bulk material specific parameters such as the internal friction angle, unconfined yield strength, cohesion and bulk density, as well as properties concerning the interaction of the bulk material with chute surfaces such as wall friction or wall adhesion, which are of special concern for the so-called wet and sticky materials.
Material characterisation tests are usually conducted across a range of consolidation (pressure) conditions. Different pieces of equipment will be subject to different loads, but, generally in the case of transfer chutes, flow is characterised by lower stresses, usually in the order of 0-5 kPa, where wall friction and adhesion effects often dominate flow behaviour.
Also worth noting is that the material captured by the belt scraper is very different from the main stream, given the different particle sizes: it is a much finer and wetter material, often exhibiting significant cohesion. Therefore, the inclination of the dribble chute shall be determined based on the flow properties of that finer material instead of the material in the main stream. Furthermore, the dribble chute may be lined with materials with the lowest friction coefficients at lower pressures, especially considering that wear is not generally a concern in this region.
Chute symmetry and central loading as a design aim
Although not always possible, transfer chutes should be designed with the simplest configuration possible, aiming at a symmetrical geometry. This is especially challenging for brownfield designs, where structural constraints are often in place. In all cases, however, material should be centralised to the receiving belt as early as possible in the transfer. Decentralised loading conditions often cause belt mistracking, one of the most common causes of plant downtime, and a major contributor to excessive belt wear, idler failures and material spillage. Non-central discharge often also contributes to lateral size segregation on the receiving belt, which may present challenges in product quality control and downstream processing performance.
Stream velocity at the discharge point
Handling issues such as excessive abrasive wear and spillage often arise from large gradients in velocities between the incoming bulk material stream and the outgoing belt. Therefore, aiming at matching these velocities as closely as possible helps to minimise these issues, and reduces power requirements to accelerate the material to the outgoing belt velocity.
As a general guide, the chute should be designed so that the velocity component of the bulk material in the direction of travel of the outgoing belt matches that belt’s velocity as closely as possible. This will ensure that there is little relative motion between the belt surface and the material stream, which will, in turn, reduce belt wear.
Minimising normal velocity and impact angles
Minimising the normal component of the impact velocity at the loading point on the outgoing conveyor is necessary to prevent impact wear and damage to the belt and idlers. Doing so also reduces dust generation and spillage caused by material re-bounding. Additionally, maintaining low impact angles within the transfer (15-20 degrees or lower) reduces wear, minimises material dispersion and dust generation, as well as reduces any losses of momentum in the material stream which can lead to unfavourable flow patterns and build-up within the chute.
Appropriate volumetric clearance to achieve accelerated flow
Transfer chutes are volumetrically limited devices and thereby the maximum mass throughput that may be achieved is dependent on the bulk density of the material being transported. By application of a chute analysis technique such as DEM modelling, the cross section of the transfer may be designed to provide appropriate clearance for a given stream velocity. As a guideline, the ratio between chute width and material stream thickness shall be smaller than 1 to avoid flow restriction.
Precautions in the discharge zone to minimise spillage
Material spillage is a serious issue not only from a production point of view but is also a safety hazard and an environmental issue. Ensuring that the chute and receiving belt are interfaced correctly is key to avoiding a material deceleration zone with material ‘choking’ at the exit, while minimising impact is also imperative.
In addition to adequate design, the use of appropriate skirt plates can go a long way in preventing material spillage issues.
Area 3: Idler Rollers and Conveyor Accessories
Idler rollers are integral components of belt conveying systems, significantly influencing operational behaviour, power consumption, and overall system suitability.
While a single idler roll is relatively inexpensive, labour and downtime costs due to roll failure can be prohibitively high, potentially reaching hundreds of thousands of dollars per hour, with catastrophic consequential losses (e.g., damaged belts) running into the millions.
To mitigate premature failures and improve efficiency, design protocols must extend beyond basic standards to address complex, application-specific operational and manufacturing factors.
TUNRA Bulk Solids leverages advanced, non-standardised testing methodologies to overcome critical deficiencies in current industry standards, providing manufacturers with actionable insights for product development and enabling end-users to select components optimised for their specific operational environments.
Key failure mechanisms
Premature idler failure is primarily linked to rolling bearing failure, resulting in noise, seizure, or high temperature, and excessive roller shell wear. Failure mechanisms are often highly dependent on the idler roll type (carrying, impact, or return) and its location (centre or wing).
Analysis of nearly 100,000 idler replacements showed that:
- Overall, rolling bearing failure and roller shell wear are the main reasons for idler replacements.
- Seizure (a form of rolling bearing failure) occurs predominantly on wing idlers. This is often attributed to water and dust contamination as wing idlers are more exposed than centre idlers, which causes grease to lose its lubricity.
- Central idlers failures are typically linked to its higher loads, which decreases rolling bearing life and increases misalignment.
TUNRA Bulk Solids employs specialised protocols aimed at providing actionable data to help manufacturers quickly improve idler performance and enable end-users to select idlers that better fit their operations. Some of the testing offered include:
- Static misalignment checks – under load and unloaded to accurately determine the relative sources of misalignment. This helps ensure the idler selected by the end-user will operate within acceptable misalignment limits established by bearing manufacturers.
- Enhanced sealing and lubricant performance testing – dust and water ingress testing under aggressive conditions. Post-test analysis using techniques like Inductively Coupled Plasma (ICP) or X-ray fluorescence (XRF) spectroscopy to quantify the concentration of wear metals and contaminants that successfully breached the sealing system and entered the rolling bearing grease.
- Shell wear and durability – Instead of relying on pure sliding tests which represents wear of seized idler rolls, TUNRA works in collaboration with partners to support methods where the idler roll rotates to compare the wear characteristics of different composite materials, offering a more relevant assessment for predicting shell wear under normal operating conditions.
Furthermore, idler design directly influences conveying system energy efficiency. Power losses are dictated by two primary factors associate with idler rolls: indentation rolling resistance (IRR), controlled mainly by the idler’s outer diameter and belt bottom cover viscoelastic properties, and rim drag losses, determined by the lubricant, rolling bearing, and sealing system.
Standard predictive models often fail to accurately estimate friction losses. For example, current models showed differences up to 80 per cent for rolling bearings and up to 600 per cent for labyrinth seals compared to measured experimental data, highlighting the necessity of dedicated testing.
TUNRA addresses efficiency measurement through two specialised rigs:
- Indentation Rolling Resistance (IRR) – TUNRA’s one-of-a-kind IRR test facility evaluates the indentation rolling resistance as a function of key parameters, including idler roller diameter, load, belt speed, sag ratio, and temperature.
- Rim drag test facility, which can be used to provide a breakdown of friction losses into grease, bearing, labyrinth, and lip seal losses.
Final remarks
The design of effective materials handling systems requires a thorough characterisation of the bulk solid being handled and how its properties vary throughout processing. Flow properties testing techniques have been developed specifically to inform the design of critical materials handling equipment. However, the testing program should be designed with careful consideration of the project phase and sample availability.
Additionally to bulk materials testing, assessment of components such as belts and idlers is also of utmost importance for the appropriate and effective design and operations of materials handling systems, with programs including verifying wear and durability, efficiency of seals and friction.