New Developments in Transfer Chute Design

by Colin Benjamin

B.E. (Chem.) FAIM

GULF Conveyor Systems, Australia

Modern transfer chute design can arguably be dated to the development of the “hood and spoon” transfer chute in the mid 1980s as up until then most transfers were drawn by draftsman based on the collective experience of site work. This period also corresponded to the industry demand for higher capacity conveyor systems and faster conveyor belt speeds particularly in the coal industry where the low specific gravity of the material being handled meant material volumes were very large.

“Hood and Spoon” transfers evolved by applying many of the principles of fluid flow to the flow of solids along with friction assumptions based on the Coulomb friction model. It is this and the relative free flowing characteristics of coal that in many ways has misled researchers over the last 20 or so years. Coupled with this is that a number of Universities and companies, specialising in materials handling, looked at the research done over many of the preceding years on bins and hoppers and argued that much of this work was applicable to transfer chute design given we were handling identical materials in similar circumstances. In particular it has become the norm to characterise ore properties using tests evolved for bin and hopper designs as relevant to transfer chute design. We have known this not to be accurate for quite some time but like many others have struggled to develop alternative design models that could be universally applied.

In publishing our latest findings it is important to emphasise the evolution of our work sequentially;

•In the mid 1990s it became very clear that complex ores that is those containing a diverse particle size, variable moisture content and cohesive or adhesive materials could not be easily evaluated using any known techniques. It was the pioneering work done by Peter Donecker (see Ref. 1), developing scale modelling techniques (DSM) based on the scaling principles of Froude and then extending them to cohesive ores that opened up a methodology of accurately assessing complex material flow.

•In the mid 1990s we became aware that the trajectory models published and generally being used as part of the design of transfers were not universally accurate. Through sponsored research and reverse engineering over many years Dr Shams Huque and Colin Benjamin have developed very accurate trajectory calculation models that we can now apply universally (see Ref. 2).

•At the same time we realised that sizing ore to obtain physical properties that can be used for transfer design is a waste of time particularly when dealing with complex materials. There are a number of simple tests that can easily illustrate this, none simpler than trying to build a sand castle with coarse sand and fine sand with identical moisture content.

•The significance of the differences between quasi-static and dense granular flow became increasingly apparent. This is well summarised in a paper “On Dense Granular Flows” by G.D.R. Midi (see Ref. 3) that characterises various flow regimes and clearly distinguishes between the flow regime in bins and hoppers and that in transfer chutes. In bins and hoppers where the flow is not continuous, the flow is described as quasi-static flow. Flow within transfer chutes, i.e. where the flow is continuous, is described as dense granular flow and we have continued to use this description when describing flow in the transfers we generally design and do research on. This paper confined its work to materials that were not cohesive in nature, acknowledging that such materials represented even more complex issues that were beyond current research.

Extending the above to the design of continuous flow (i.e. ones that meet the demand for high flow volumes) transfer chutes follows:

•The key to the design of such transfer chute starts with having an accurate material trajectory. This is the key to controlling the flow down the body of the transfer. Any trajectory calculation must allow for the complexity of the ore flow for instance the affect moisture has on the trajectory of ore flow.

•Once we have an accurate idea of the material trajectory into the transfer the design of these transfers is all about flow control.

•The flow within these transfers is controlled by the “effective friction” where the effective friction can be described as an aggregate of all the many forces and interactions that may occur in the dense granular flow regime that is characteristic of the flow in transfers. We do not believe that Coulomb Friction has any relevance to transfer design despite the many publications that describe its relevance by suggesting that the wall friction of the chute lining should be included in any flow model.

• The effective friction of ores varies with the ore composition and moisture content. It is also affected by particle size, particle size distribution, material angularity, the resistance of the material particles to shear forces, material hardness, crystalline structure of the particles, electrostatic forces, chemical and physical bonding between the particles and the space between each particle. To calculate the effective friction mathematically is impossible, there are simply far too many variables so any mathematical method that purports to show the flow of a complex ore in a transfer is at best an approximation.

•Our work through re-evaluating transfer designs we have done, scale modelling and doing a series of simple tests on how material flows in transfers has shown us that we can use the angles at which the material “sets” up in transfers as a direct correlation to the maximum effective friction of the ores we are dealing with in the dense granular flow regime. We have termed this angle the “stall angle” and have used it to establish a series of design parameters from which we can create transfer designs.

•In qualitative terms the stall angle follows the same trends that one would expect with the same angle of repose and these directions can be used to make minor modifications to the design values. We also need to emphasise that the stall angle is a characteristic of continuous flow so transfers that are designed using ledges or based on the material overflow principle (cascade chutes) where flow alternates between quasi static flow and dense granular (or continuous) flow will set up at much higher angles especially when handling cohesive or adhesive materials.

•The Stall Angle varies significantly with variations in material properties. Our primary interest has been with materials exhibiting cohesive and/or adhesive properties as these have represented the most difficult materials to manage in a predictable manner. For these materials we have now established reliable stall angles that allow us to design transfers with very predictable and reliable performances.

•In applying these angles we also have to consider the worst case scenario in our design i.e. the highest stall angle, and then manage the stall angle changes that will occur should the ore characteristics change. Simple examples are when the ore is mined from below the water table as is the case in many instances in the Pilbara Iron ore province or above the water table. Another is variations between wet and dry seasons.

•We can evaluate our designs using scale model theory as developed by Peter Donecker (see Ref. 1).

We have been working on these principles for some time, initially conceptually and eventually in the field. Our first application of these principle in the field was in a transfer we did on copper/ore where the material variability was extreme. The transfer worked considerably better than its predecessors but importantly we were able to take many videos and through studying these videos, reverse engineering and utilising DSM we were able to develop our transfer design principles considerably. Importantly we were able to very accurately represent what we saw in the field in our scale models (see Fig. 1).

Our next opportunity was 18 months later with the Fortescue Metals Group. This transfer was handling minus 400mm iron ore but on occasions the ore was highly cohesive so there was extreme variability in the ore characteristics. The performance of this transfer was considerably more controlled than the previous transfer we designed and after we were allowed to make some minor adjustments the transfer worked to our expectations. Today it is described as the best transfer on the mine site.

Once again our ability to accurately scale model the outcomes was compelling reassurance that we had an accurate and very reliable method of designing transfers for even the most complex of materials. We also used the opportunity to further develop our designs through additional reverse engineering and evaluation of the issues identified utilising DSM.

This led to the next challenge and that was to design a transfer for Fortescue’s Port Operations. The location and critical importance of the transfer was such that it represented a very high profile application.

•Previous transfers developed using the latest DEM software had failed to deliver reliable, high volume transfers at the Port so FMG was looking for a solution.

•The Transfer we were asked to design was on a parallel system to the new transfers that Fortescue were dissatisfied with so any performance comparisons would be quite apparent.

•The current transfers despite being ceramic lined, wore out quickly, occasionally blocked, required cleaning out between each shift, could not load the receiving belt above about 13,000 tph as they failed to develop a reasonable surcharge angle, created dust and spillage around the transfers and regular failures of the skirting system and finally created tracking and wear problems for the receiving belt. None of these issues by the way are unique, in fact they are common issues/complaints with a great many of the transfers throughout the Pilbara iron ore mining area.

•The transfer we designed was based on an input belt speed of 5.5 m/s, peak tonnages of 20,000 tph, normal 17,000 tph and a belt to belt height of 17 metres (as they needed to have a sampler installed). Ore was minus 8 mm but most of the ore contained high levels of micro-fines and ultra-fines and the ore at times was known to be highly cohesive.

The outcome achieved was a very reliable transfer that does not require impact idlers at the load point, profiles the ore excellently, does not block regardless of ore types, loads centrally and after 6 months of operation there is no significant wear or the need for any maintenance on the transfer.

The way we have applied our research and design experience utilising accurate trajectory models and assessing the effective friction empirically using the stall angle is subject to a patent in that we have developed a unique transfer design that achieves the above outcomes by filling the body of the transfer in a totally controlled way with ore such that all the flow surfaces are embedded in ore. This is why wear is not a significant issue.

Some mistake this transfer as a typical micro-ledged or variation of a cascade chute (WEBA being the most common cascade chute) but these chutes rely on ore building up through an overflow process. The Transfer design concept we have patented actually develops the build-up of ore initially at the outlet of the chute and then on this base re-creating within the chute the natural stall angle of the ore. The build-up in the chute never exceeds the stall angle of the worst case ore being handled and this method of building up material in the chute both ensures the chute does not block and that the angles within the chute are different to what has been the industry norm when handling such cohesive materials.

We have also applied the above design principles to other types of transfers such as “hood and spoon” transfers and know that the same principles work. Simply it is an empirical method that we believe is far more accurate than any computational options. There is more research to be done which is now being assisted by Fortescue but the benefits are very obvious to those that have had to suffer from the current constraints that transfers represent in the hard rock industry.

In conclusion:

1.The design principles we have been using to design transfers over the last 20 or so years in most cases have questionable logic. Flow in transfer chutes varies significantly from flow in bins and hoppers.

2.The stall angle is conceptual characteristic that will give designers a better indication of the material flow within continuous flow transfers than any theoretical calculation of the effective friction especially when designing transfers for complex ores.

3.Computational evaluation methods are at best an approximation that lacks the accuracy to be usable with complex ores.

4.We can accurately scale model what we have designed and therefore create very predictable outcomes before we commit to the major cost of building and installing a transfer.

References:

1. Donecker P. Dynamic Scale Modelling (DSM) of Transfer Chutes – Australian Bulk Handling Review Sept/Oct 2011

2. C. Benjamin, P. Donecker, S .Huque, J Rozentals The Transfer Chute Design Manual for Conveyor Systems www.conveyorsystemstechnology.com

3. G.D.R. Midi On Dense Granular Flow – Eur.Phys.J.E,14,341-365

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