Tuesday 26th May, 2020

Taking it to 11

No matter what your ‘10’ equates to in terms of throughput, there are times when you need to go to ‘11’. Corin Holmes, Operations Manager at Jenike & Johanson, explains what you need to know before fiddling with your plant’s throughput dial.

No matter what your ‘10’ equates to in terms of throughput, there are times when you need to go to ‘11’. Corin Holmes, Operations Manager at Jenike & Johanson, explains what you need to know before fiddling with your plant’s throughput dial.

If your plant features bulk solids within its value chain, uprating throughput is not as easy as turning
the dial.

The inconvenient truth is that bulk solids do not flow like liquids. Bulk solid flow properties are typically very sensitive to rate/velocity, time, processing history as well as what happens upstream. All this makes for a complex system where performance at each step depends on others, in often mysterious and sensitive ways.

They frequently run balanced on the proverbial knife edge with a very small sweet spot, where even a modest 10 per cent increase in rate is enough to push it over the edge resulting with a non-reversible loss of controlled flow.

The recent COVID-19 pandemic has jolted markets globally and the world is witnessing vast market disruption which may potentially have long-term effects. As an example, the pandemic and ancillary lockdowns and restrictions are expected to reduce the supply of iron ore by around 18 million tonnes. For most bulk solids handling operations, revenue is directly proportional to the ability to maintain bulk solid flow and squeezing every ounce of production from your supply chain will be critical to business success.

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Producers in all industries will want to ‘go to 11’ on the throughput dial, but this will come with increases in bottlenecks as a result of material handling issues that were considered or accounted for.

Bottlenecks are defined as any point in the value chain where the desired flow rate cannot be achieved. They can occur for various reasons like increasing throughput in a value chain system originally designed for a lower rate, changing material characteristics (finer material or increased moisture content), changing to a new type of material or blend which alters handleability, or changes in operating conditions due to equipment availability or process utilisation.

Unlike in fluid systems, bottlenecks in value chains featuring bulk solids depend on the properties of the material flowing causing them to alternate frequently. This situation creates a moving target and gives rise to a characteristically noisy throughput run chart (see Figure 1) and downtime whack-a-mole. While all value chains by definition, have a bottleneck somewhere, the trick is to prevent them from affecting/throttling revenue.

Figure 1: Daily operation feed run-chart.

Potential sources of bottlenecks

Consider a typical iron ore handling value chain which may consist of car dumpers, conveyors, stackers, stockpiles, reclaim hoppers, bins, belt/apron feeders, and transfer chutes. Bottlenecks will result in various problems throughout this value chain.

Stockpiles: Material may discharge through a central flow channel resulting in the formation of a stable rathole. This can cause insufficient discharge one moment but then, due to collapse, cause material to flood the downstream system. Material feed may be segregated, causing downstream processes expecting a uniform distribution to fall out of control.

Bins and hoppers: If material flow properties are not considered in the context of increased throughput the geometry of the equipment may be incorrect. The geometry of the bin needs to be tied to the material via the underpinning science. In doing so, proper outlet sizes and adequate wall angles can be defined to ensure reliable flow. In addition, if material has a high fines component then rate restrictions may occur as a result of two-phase solids/air flow.

Feeders: Consideration of the feeder type, size, and interface with downstream equipment is needed. Figure 2 shows how an improperly designed feeder interface can affect material flow from the hopper above. Activating the entire cross section of the hopper above is critical and knowing the material flow properties will ensure that rate increases are aligned with the feeder and interface thereby ensuring reliable flow.

Figure 2: Belt feeder interfaces.

Transfer chutes: An example of what can happen when increasing throughput is not considered is shown in Figure 3. This transfer chute was designed to reliably handle a sticky/cohesive ore. At its design tonnage rate it worked well but by simply dialling up the throughput rate resulted in plugging because the cross-sectional area of the chute was not large enough to allow flow.

Figure 3: DEM output showing plugging at increased throughput rate.

Wear: Increased throughput will result in increased abrasive wear. In some cases, this may be the result of improper design or incorrect wear liner type selection. Maintenance schedules are routinely based on ‘life of liner’ and increasing throughput without consideration to the effect of wear liner life could result in skyrocketing maintenance costs or increased downtime required for the change-out of liners.

Matching test work to handling problems

The ability to increase throughput and avoid bottlenecks will hinge on reliable feed of bulk solids into and out of the unit processes by considering ‘flowability’. Defined as the ability of a bulk solid to flow through a given piece of equipment reliably, it considers the relationship of the material itself to the equipment in which it is to be handled. A poor-flowing, difficult-to-handle material may be reliably handled in properly designed equipment while an easy-flowing material may exhibit flow problems in incorrectly designed equipment. The most critical flow properties to aid assessing a material’s flowability in relation to the handling equipment are:

• Cohesive strength (the ability of particles to pack together and form arches and ratholes)

• Wall friction (measure of friction between particles and flow surfaces)

• Chute angle (critical angles in a chute to maintain flow)

• Bulk density (changes in density as a function of consolidation)

• Abrasive wear (progressive loss of a solid surface caused by sliding contact).

Cohesive strength, wall friction, and compressibility are commonly measured at bench scale using a direct shear tester. These flow properties are affected by many material and operational characteristics and parameters. Having current flow properties information is critical to success when you are looking to increase throughput. Reliable flow is contingent upon the equipment design being matched to all of the bulk materials it will handle over the full range of conditions to which they will be exposed. Figure 4 shows the typical effect of moisture on material flow characteristics. Where the moisture content is low a low cohesive strength is observed, however, the material may be more frictional. If the material is fine, then there is potential for unreliable flow caused by either a flow rate limitation, a result of low permeability of the material, or flooding, caused by aeration of the material. As the moisture content of the material increases, so does the strength of the material leading to greater arching and ratholing potential, requiring larger hopper outlet sizes or steeper transfer chutes. As the moisture continues to increase, it approaches its greatest strength. At this point it may not be possible to discharge material out of a bin via gravity alone resulting in a no flow condition. As the moisture content increases further, it gets to the point of saturation after which it begins to lose strength as the water begins to act as a lubricant.

Figure 4: Typical material’s cohesive strength with respect to moisture content (illustrative purposes only).

Understanding flow patterns

To understand where and how bottlenecks may occur in the transportation value chain, it is necessary to clearly define the required flow pattern and ensure the equipment’s design is appropriate. There are two main discharge patterns for material flowing from storage vessels, known as funnel flow and mass flow (see Figure 5). Funnel flow occurs when the hopper is not sufficiently steep and smooth to force material to slide along its walls or when the outlet is partially blocked.

Figure 5: Funnel flow pattern.
Figure 5: Mass flow pattern.

In funnel flow, material flows towards the outlet in a channel that is surrounded by stagnant material. When the storage vessel is emptied faster than it is filled, the level of solids within the flow channel drops, causing layers of the solid to slough off the top of the stagnant material and fall into the channel. With cohesive materials the resulting impact pressures may increase the possibility of arching, but if it is sufficiently cohesive, the sloughing of material will not occur, and the channel will empty completely forming a stable rathole. In general, funnel-flow bins are usually suitable only for coarse, free-flowing, non-degrading, and abrasive solids in applications where segregation is unimportant. Funnel flow is generally the default flow pattern when little or no consideration is given to the design of the storage vessel and/or interface.

Mass flow occurs when the bulk solid slides along the walls of the storage vessel and all material is in motion during discharge. Mass flow occurs when hopper walls are sufficiently steep and smooth to allow the material to slide on them. In mass flow bins material density at the discharge is both low (because in flow it is dilated), independent of the head of solids in the bin, and is consistent. Ratholes cannot form because stagnant regions are eliminated.

In mass flow the first-in-first-out flow sequence minimises segregation of the discharged material, ensures uniform residence time, and allows time for de-aeration of fine powders. Mass flow bins are usually recommended for applications in which the downstream process cannot handle a segregated feed and for handling cohesive materials, fine powders, and any material that may degrade with time. A properly designed mass flow hopper must have appropriate geometry that results in a fully active outlet without ledges or protrusions.

Predicting bottlenecks using correct methodology and analysis

Correctly quantifying and understanding material flow properties of the storage handling and transportation equipment, bottlenecks can then be predicted by analysis.

Continuum methods

With today’s high-speed computers, it is possible to quickly calculate velocity and displacement profiles in many standard silo geometries using continuum mechanics models. This provides greater insight into material flow behaviours. Numerical studies are also used to analyse stress and flow behaviour of particulate solids in many bulk solids applications. Well-established numerical methods, such as the finite element method, have been used to investigate macroscopic behaviour of bulk granular solids. However, difficulties of representation have frequently been found because the models are based on continuum and homogeneity assumptions, and because it is difficult to establish robust constitutive equations for granular bulk solids.

Discrete Element Method

In recent years, Discrete Element Method (DEM) modelling has been developed in its theories and computational speed to become an advanced simulation technique. As a result, it is increasingly used to study the physics of industrial particulate problems with the goal being to develop a general theory that links the discrete and continuum methods. From this, particle scale information, generated from DEM simulation, can be quantified in terms of governing equations, constitutive relations and boundary conditions that can be implemented in continuum-based process modelling.

Using flow properties to calibrate the particles and wall properties, DEM models are quickly becoming mainstream technology for belt-to-belt transfer chute design and are ideal for situations where large scale, parametric experimental studies are difficult to perform or cost prohibitive. In our transfer chute bottleneck example above, by performing DEM simulations in the virtual world our client was able to converge on a solution prior to increasing the throughput rate, avoiding the costly creation of a bottleneck in their process.

As tonnage rates undoubtedly increase, following the pandemic, so too will the associated costs due to bottlenecks. To correctly identify and solve bottlenecks in advance, a comprehensive set of flow properties is necessary to determine cohesion, wall friction, bulk density, and abrasive wear properties. Analysis, using appropriate methodologies, empirical methods, and advanced technology such as DEM should be contemplated with consideration given of the value chain equipment itself.

Either way simply dialling up the gauge to ‘11’ without considered analysis to inform the decision can have a significant effect on your operation and revenue stream.

As we collectively wait for the “accelerate out of the crisis” flag to come up, you should check your bulk material flow properties are up to date, or if you don’t have them, commission some testing. Flow characterisation is a specialised task that takes time it can be undertaken now ensuring you can move off the grid smoothly and efficiently to 11 and beyond.