Manipulating the nature of slurry flows

to overcome production and maintenance challenges

Dr Steve Algie

Consultant, FloCEP Pty Ltd

This presentation is about PROmotion™ Technology, which forms the basis for a family of devices that solve problems arising from the inherent tendency for segregation of the solids within slurry flows

This patented technology has been developed by Scott Doig over the past six years. Over the last couple of years I have been helping with the introduction of a range of new devices based on this patented technology, which is licensed to FloCEP Pty Ltd.

But, we have found ourselves running into a persistent challenge in getting the message across. When we talk about the benefits, we find many people aren’t even aware that they might have problems. When we talk about a device, we find many people think that it couldn’t possibly be what we say it is! They tend to assume that it’s something else; something else that they believe could not possibly work.

However, your interest in this presentation suggests that you are a different audience from most people we talk to! You are here because you know that pumping a slurry is not just like pumping water – not everyone appreciates this. Therefore, I am going to start this presentation by describing the core technology. Then, I will give examples of devices based on this technology, and show where they can be profitably applied.

So, first I will describe the core technology, then the devices, and how they can improve users’ bottom line.

A couple of points:

The technology applies to slurries in which the solids are transported by the kinetic energy of the fluid – we are not dealing with pastes transported by positive displacement pumps.

The focus for this presentation is on slurries, but the principles are applicable to multi-phase flows in general.

PROmotion™ Technology solves problems – what are these problems?

The problems that PROmotion™ Technology solve are typically ones that people don’t see, or even think about. This is because when we design or operate a slurry transport system we typically, and usually unconsciously, think in terms of engineering models. These let us treat the flow as if it were a uniform fluid.

Most of the time this is a very good model; for example, we can represent a complex network of multi-phase flows by single lines on a Process Flow Diagram, as shown in Figure 1, where slurry flows are reduced to lines.

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Figure 1. Engineering models (usefully) simplify reality

Going into more detail, by characterising the flows by, say, percentage solids and flow rates we can complete a mass balance for the system – for both solids and fluids. If we add information on density and particle size we can calculate deposition velocity, which combined with flow rates and distances, leads to pipe diameters, to pressure drops, pump sizes and power requirements.

Of course this glosses over some complex calculations (and some uncertainty), but nevertheless, we can proceed from a schematic to the basis of a physical design, just by representing flows by a few simple parameters. All without having to think much about what is going on inside the pipe.

Treating a slurry flow as if it were a uniform fluid is a very useful model for designing slurry transport systems. However, we should not confuse the model with reality, which in this case is what’s happening inside the pipe.

The clue to what’s really happening is hidden in the term deposition velocity. Deposition velocity reminds us that all but the smallest of solid particles tend to segregate within the slurry. This is due to the influence of gravity or centrifugal force. Slurry flows are only homogeneous (strictly, pseudo-homogeneous) under quite limited circumstances, which are not usually economically achieved (e.g. very fine particles, high velocities, and low solids loading).

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Figure 2 Newitt’s (1955) slurry flow classification (single particle size)

We all know this, and most of the time we can safely ignore it. However, there are some situations where ignoring reality comes at a significant cost. These are situations where PROmotion™ Technology is best applied.

Cases where ignoring reality can hurt

Here are three common objectives that are quite difficult to achieve with a non-homogeneous slurry flow

Measuring, or sampling, the flow – where we want the result to be representative of the average flow overall.

Adding of a chemical to the slurry – where we want the addition to be mixed evenly throughout the solids.

Splitting a slurry flow, where we want the percentage solids to be the same in each downstream line, and preferably with the ability to control the total flow in each outlet.

In practice, these actions are performed on non-homogeneous slurry flows all the time. However, often they are done sub-optimally. What is happening is typically hidden from view – inside a pipe. Sub-optimal and inconsistent results are accepted because of lack of practical alternatives.

Of course, one could achieve accurate results by dropping the slurry into a tank, giving it a good stir, taking the samples, making the additions and pumping it out again, in one or more streams, but this requires a tank, stirrers and pumps, takes space, and wastes energy.

A better way would be to homogenise the flow inside the pipe before taking these actions. Then all these actions become a lot easier!

This is what PROmotion™ Technology achieves.

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What PROmotion™ Technology does

PROmotion™ Technology manipulates the flow velocity profile to homogenise the slurry as it flows. The desired sampling, addition or flow splitting is then performed on the pseudo-homogeneous slurry.

PROmotion™ Technology targets the root cause of the difficulties.

PROmotion™ Technology makes very clever use of the principles of hydrodynamics. The principles call to my mind those that operate in a fluidic control switch – a small change in a flow at one critical point can re-direct a much larger flow. It makes sense, but no one had done it before Scott Doig came up with the idea, and if it had been obvious it couldn’t have been patented!

The features that makes PROmotion™ Technology a practical option have to do with the way it achieves homogenisation:

It makes use of the kinetic energy in the slurry flow. It does not require additional energy input – it does not increase pressure drop. It has no moving parts, and is subject to very little wear. It is a short in-line system. It can be manufactured to match standard pipes and flanges, and from a wide range of

materials.

Figure 3 shows an installed FLOprep unit.

Figure 3 FLOprep unit

So, to sum up, the patented core PROmotion™ Technology is applied in the FLOprep in-line, static, passive device that homogenises multiphase flows.

Now I will describe a couple of FloCEP’s devices based on its core PROmotion™ Technology technology.

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Putting the core PROmotion™ Technology to work

The forces that lead to segregation continue to operate as the slurry flows downstream, just as in as they did upstream of the intervention. The point of homogenising a slurry is to do something useful with it while it is in the homogeneous state. This is what FloCEP’s range of devices achieve – this presentation highlights two of these applications:

FLOdistributor (separately patented) – for in-line splitting of a slurry flow. MAXbend® – for reducing wear in bends.

FLOdistributor – splitting a slurry flow

There are many reasons for splitting a slurry flow; most commonly because of capacity limitations in individual pieces of downstream processing equipment. Hydrocyclones, as used for dewatering or particle classification provide a good example.

The key point about the way a hydrocyclone works is that it is mechanically a very simple device, and its performance is directly governed by three things: its physical dimensions; the input flow rate, and; the percentage solids in the inflow. This means that a hydrocyclone can only achieve a specified performance over a limited range of flow rates.

In order to achieve the same separation at higher flow rates, hydrocyclones have to be operated in parallel, typically in the form of a cluster of identical units.

It is very easy to draw this on a Process Flow Diagram: the cluster is represented by a single (imaginary) hydrocyclone with the required capacity. However, for a real cluster to operate like a larger single hydrocyclone, which is the whole idea, the slurry feed into each parallel unit must have the same solids loading, and the same flow rate. Designing a manifold to achieve this is easier said than done.

There is nothing new about pointing out that uneven distribution is a problem with hydrocyclone clusters. Manufacturers advise avoiding having bends just before a cluster, for this very reason, and there have been several patents issued for manifold devices. So is it really still a problem? Hydrocyclone clusters are everywhere, and they do work, but do they work optimally? More to the point, even if they didn’t work optimally, would it matter anyway?

I will give an example where sub-optimal operation causes significant costs.

If you were to take the trouble to measure the performance of a typical hydrocyclone cluster and compare this with that of a single hydrocyclone unit in the cluster, you would be likely to find that:

1. the separation is not as sharp, and if you were to look in more detail you would find that;2. the flow rate and percentage solids in the underflows are not identical, and this is because;

a) the flows (rate and percentage solids) into each unit are not identical, which directly affects separation, and furthermore;

b) the different input flows cause the individual units to wear at different rates, and this also directly affects the separation

3. In short, there is a vicious circle where sub-optimal performance further gets worse over time.

Fortunately, it is not necessary to make such detailed measurements because there are some simple visible signs of uneven feeding that are evident from observing how the cluster is operated:

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Is there uneven wear in the individual units? – look at maintenance practices. Are operators making frequent adjustments made to inlet valves? – attempting to equalise

the performance of individual units. If these practices are observed, the most likely cause is uneven distribution, and the

consequence is sub-optimal separation performance.

There are four main factors that are likely to determine how much this matters:

1. Percentage solids in the slurry:a) The performance of a hydrocyclone becomes increasingly sensitive to any variations as

solids loading increases, and;b) Wear rates increase with increased solids loading;

2. Flow velocity – higher velocity results in higher wear rates;3. Abrasiveness of the solid particles velocity – higher hardness results in higher wear rates;4. Value of the products – the quality of the separation directly affects recovered value.

All four of these factors come into play in a typical gold processing plant. This is the very application that spurred the development of the FLOdistributor.

Figure 4 FLOdistributor at the Edna May operations

Apart from having a FLOprep unit upstream, which ensures that the incoming slurry is homogeneous, the FLOdistributor looks much like a conventional manifold, so how is it that patents have been granted for this invention?

While it might look much the same on the outside, the FLOdistributor is very different inside! The patented feature of the FLOdistributor is the way it is designed to preserve homogeneity while the flow is distributed. Without this feature the radial acceleration resulting from re-directing the flow into the different outlets would cause internal segregation.

The FLOdistributor is a fully engineered solution. The complete assembly consisting of a PROmotion™ flow homogeniser and the FLOdistributor was fabricated off-site and retrofitted to the

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existing plant in 12 hours during scheduled maintenance shut-down. Design parameters are fully documented so individual units can be designed without trial and error testing.

The installation is fitted with valves to each hydrocyclone unit in the cluster; these are not for adjusting flows performance – they are just there to turn flows to individual units on and off. Even distribution is achieved regardless of the number of units in operation and their location in the cluster.

Figure 5 summarises performance.

Figure 5 Improved performance due to FLOdistributor at the Edna May operations

As a direct result of installing the FLOdistributor, hydrocyclone maintenance has been reduced by 95%. The operators aren’t constantly fiddling with the valves, and there is no unscheduled downtime to replace bits and pieces.

And what about the similarity in the underflow densities? The previous spread of about 4 percentage units between individual hydrocyclones in the cluster has been reduced to practically uniform performance. The increased stability has been instrumental in allowing better control of grinding and on leaching, leading to a pay-off in the form of a 3-4% increase in gold recovery.

I’ll now move on to another example of an application based on PROmotion™ Technology.

MAXbend® – reducing wear in bends

Pipes carrying slurries experience localised wear at bends. In most cases the solution is to make the bend from a suitably wear-resistant material. Selection is often simply a matter of balancing the cost of material against the cost of maintenance and potential lost production in periodic, or even unscheduled, replacement of worn bends.

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However, in some situations there is no satisfactory material solution and an operation is stuck with high material costs, and high operating costs, typically where:

The solid particles are highly abrasive. Flow rates are high. The percentage of solids in the slurry is high. Abrasive wear exacerbates corrosion.

However, apart from materials selection, there is another way of controlling wear. This is to change the way the solid particles interact with the wearing surface – in this case the pipe. In a segregated slurry, particles are concentrated towards the outside of the bend, where they impinge upon the interior surface of the pipe bend. In contrast, in a homogenous slurry, the particles are dispersed within the fluid, and relatively few of them contact the surface.

The MAXbend® solution to wear in pipe bends is to combine a PROmotion™ FLOprep device with the entry part of the bend. This is shown in Figure 6, where the MAXbend® unit is installed in a very demanding application in an alumina refinery. This location is where a high concentration of abrasive particles is being carried in superheated caustic, on the point of flashing.

Figure 6 MAXbend® installation

Note that as with the FLOdistributor, MAXbend® is provided as a fully engineered system.

It can be produced to any piping specification, from any material. It can be often be fitted as a direct replacement for existing bends, and for bends as tight as

a 3D radius.

The performance is summarised in Figure 7.

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Figure 7 MAXbend® performance in service

This is a spectacular result!

120 day service life extended to five years (see Figure 8). No change in bend material (Ni-Hard lined).

Figure 8 MAXbend® life compared with best previous option

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Similarly spectacular results have been achieved, in a more conventional high-wear situation at Sandfire Resources’ DeGrussa copper-gold operations.

Conclusion

Now that I have described the core technology and a couple of applications, I’d like to finish with the ‘take home’ message to remind you where you might use PROmotion™ Technology.

When working with a slurry transport system, think about the places where the simple engineering model of homogeneous flow, which works so well most of the time, might get you into trouble:

Measurement, or sampling. Addition of a chemical. Splitting a slurry flow. Where there’s a bend in the pipe.

Think how much easier these operations would be if the slurry were actually homogeneous at these points.

Then think of PROmotion™ Technology!

Contact details

FloCEP information contacts:

• www.flocep.com.au (under construction)

• scott@maxbend.com.au

• Phone: +61 8 9353 3929

Speaker contacts:

• Dr Steve Algie• algie@multiline.com.au • +61 8 9385 9969

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