Powder Behaviour and the Nature of Powders

About powders Predicting powder flowability How many numbers do we need to describe a cup of coffee? Powder testing and the need for innovation Classification of powders

About powders

We think of powders as a mass of solid particles or granules. In fact these particles are usually surrounded by air (or other fluid) and it is the solids plus fluid combination that largely determines the bulk properties of the powder. It is perhaps the most complicating characteristic because the amount of fluid can be so variable.

Powders are probably the least predictable of all materials in relation to flowability because of the large number of factors that can change their rheological properties. Physical characteristics of the particles, like size, shape, angularity, surface texture, porosity and hardness will all affect flow properties. External factors such as humidity, conveying environment, vibration and perhaps most

importantly, aeration, will compound the problem. The more common variables would include:

Powder or Particle variables External Factors influencing Powder Behaviour • Particle size • Size distribution • Shape • Surface texture • Cohesivity • Surface coating • Particle interaction • Wear or attrition characteristic • Propensity to electro-static charge • Hardness • Stiffness • Strength • Fracture toughness

• Flow rate • Compaction condition • Vibration • Temperature • Humidity • Electro-static charge • Aeration • Transportation experience • Container surface effects • Storage time

Another characteristic of powders is that they are often inherently unstable in relation to their flow performance. This instability is most obvious when a free flowing material ceases to flow. This transition may be initiated by the formation of a bridge in a bin, by adhesion to surfaces or by any event that may promote compaction of the powder. The tendency to switch in this way varies greatly from one powder to another, but can even be pronounced between batches of the same material.

Powder behaviour will be very dependent upon particle size, the variation of size and the shape of the particles. In general powders with large particles (>100µm) will be non-cohesive, permeable and will probably fluidise and will have low compressibility and relatively low shear strength.

Conversely, fine powders <10µm say, are likely to be cohesive, compressible, contain much entrained air and yet have poor aeration characteristics. Generally they have high shear strength, high flow energy, low permeability and are very affected by being consolidated when entrained air is excluded.

There are many exceptions to the above – for example toner used in printers and copying machines are fine powders with an outstanding fluidisation characteristic. A small amount of aeration is sufficient to transform a consolidated powder into one with fluid like rheology. Another broad generalisation is that under forced flow conditions, where powders are made to move other than by gravity, fine powders can behave more like a fluid. They are able to extrude round corners or through holes, unlike coarse powders that are more likely to become solid like as particles realign and lock together and become very resistant to flow.

The nature of powders therefore is such that an adverse combination of environmental factors can cause an otherwise

free flowing powder to block or flow with difficulty. Conversely, a very cohesive powder may be processed satisfactorily if the handling conditions are optimised.

Because processing is largely a matter of making things work, it is often not known how marginal a set up may be. This often results in a system operating well for hours, and suffering from stoppages from time to time. What is needed is knowledge of both the flow properties of the material and the processing characteristics of the machinery to be used.

Aluminium, D50 134µm Tungsten, D50 4µm

Predicting powder flowability

Given the complex nature of powders, it is not surprising that processing difficulties are commonplace. Being able to predict flow performance would bring many operational advantages such as reducing stoppages and improving product quality.

To achieve this, we need to know how a given powder is affected by the variables mentioned above and also to have a reliable indicator of the potential instability of the powder. These are the primary functions of the FT4 Powder Rheometer.

The difficulties are well illustrated if we look at how transportation can affect the flow properties of a material. A powder could become highly aerated or even fluidised under some conditions of transportation. Alternatively, it might be gently agitated in a different situation, or as often happens, highly consolidated by being continuously vibrated. The energy needed to establish flow of a fluidised powder might be only 0.3% of that needed if that powder becomes consolidated by vibration or direct compaction. This energy requirement would reduce to 15% if the powder were in a lightly packed state, free of excess air and residual compaction.

Processing Powders and predicting flowability performance in a particular plant therefore requires knowledge of the handling and processing conditions as well as the flowability characteristics of the material under these conditions. It means that the process conditions relevant to flowability need to be determined. These might include the level of static and dynamic head produced in a storage bin or hopper, the amount of aeration that occurs, the opportunity to adsorb moisture, become electrostatically charged or be consolidated due to vibration.

Other factors could be segregation and attrition that may cause fines to collect, rounding of particles and so on - all potentially affecting the flow characteristics of the material. All, or at least the most important of these factors then needs to be quantified regarding how they affect flowability. Again, this is the function of the FT4 Powder Rheometer and the methodology that has been developed.

How many numbers do we need to describe a cup of coffee?

Arguably the answer is at least 4 or 5, probably more! So how many do we need to describe powders which are very much more complex? How do they flow under gravity when consolidated, unconsolidated, aerated or even fluidised? How readily will a powder entrain air and release it again? How is it affected by moisture, vibration, storage, the accumulation of fines or flow additives? How compressible is it, can it be forced to flow, will it extrude? Is it prone to segregation or attrition? How does the variation of particle size affect flowability? What are its shear properties, compression strength, internal angle of friction and cohesion values?

Yes it is complex and we cannot ignore this reality. Fortunately it is possible to identify those parameters that are

almost always important from those that maybe sometimes. For example key parameters are usually:

• Flow energy when consolidated, non-consolidated, aerated and fluidised • Cohesivity – or Specific flow energy • Permeability • Sensitivity to flow rate • Stability • How flow energy is affected by aerating the powder

Although there might be 100 or more factors that influence powder behaviour, there are 10 or 20 that can give a reasonable indication of how a powder will flow if we assume it is stable and not affected by segregation, moisture or fines variability. What is certain is that the traditional single number description is certainly inadequate and probably misleading.

Powder testing and the need for innovation

We have seen that a powder is a blend of particles and air and that the amount of entrained air can transform the flow properties of the powder bulk. Typically, the difference in energy needed to establish flow in a compacted powder may be 100 times that needed when the powder is aerated and possibly fluidised. For some powders, this ratio can be more than 1000 and in the most extreme case, 5000.

Slight compaction, a small vibration, or the smallest amount of aeration can significantly affect flowability. This is the main reason why traditional methods of flowability measurement have not been suitable as a basis for repetitive testing. In all traditional techniques, the packing condition and the air content, are largely unknown quantities and so the results will vary accordingly. When making an assessment it is essential to know what was tested and the condition of the powder when tested.

The first innovation needed, therefore, is a way of producing a standardised packing state as a preliminary to testing (Conditioning). It is then possible for measurements to be compared - whether between different batches, dates or sites.

In addition to the packing problem, traditional flowability measurements are prone to operator error, have poor repeatability and, for the most part, are very time-consuming. An automated test and analysis system is needed that takes only minutes, is very repeatable and is independent of the operator.

A further need is to measure flowability performance at flow conditions that are representative of real processing situations. Hence a dynamic flow condition is required that may be established and maintained for a period of time, during which measurements and observations are made.

Matching Powder and Plant characteristics What we do to powders Powder Processing issues Improving plant efficiency Matching powder and plant characteristics

What we do to powders In everyday processing powders can have a tough time. They may be stored for long periods, compressed, vibrated, affected by moisture, aerated during discharge and transfer, even fluidised, and then subjected to specific processeslike granulation, drying, milling, lubricating, blending, dosing and compression.Mostly they flow under gravity, but sometimes an applied force is needed andhere powders usually object since, unlike gases and liquids, they generallycannot be pushed around.

The typical process line subjects the material to a great deal of change. Screw feeders, conveyors, chutes and

pneumatic conveying using solid or dilute phase methods will all have an impact. In storage and transportation, bulkstresses may be high, in motion, particle impact and abrasion stresses can cause fracture and fines generation.Agglomeration may occur for finer materials and segregation or de-blending in freer flowing powders with air content changing continuously whatever is happening to the powder.

Another important variable can be the human operator. A good example is the sequence and precise loading methodof adding ingredients to a bin or blender which can be critical.

If we consider the variability of the process plant and human operators, the environment, the changes imposed uponthe powders during the process and then take into account how key properties like flowability are affected by aircontent, particle size, coating, fines, moisture etc, then it is not surprising that powder processing is challenging.There are few industrial processes where the material can change so dramatically during processing.

Powder Processing issues Most companies processing powders have learned how to confront the inherent variability of powders so thatsatisfactory productivity and quality are achieved. Even so, batch to batch variability is commonplace as areexpensive trials of new product launches.

Typical powder processing problems are:

Consolidation resulting from storage, transportation or high levels of compaction applied in a silo or hopper. Long term storage can result in chemical bonds being formed between adjacent particles resulting in ‘caking’. Transportation often vibrates and consolidates powders making them resistant to flow.

Blockages – especially hopper or bin discharge. The cause is usually the formation of a bridge near the hopper outlet which prevents flow. It occurs because of the high compression strength of the powder.

First in – last out due to poor flow in hoppers. Occurs when the material flows only down the centre of the bin – the material near the walls remaining relatively still. Cause – lack of compatibility of powder and plant characteristics.

Aeration – the addition of air to the powder bulk that can transform bulk density and flowability. Can occur naturally during discharge and cause problems.

De-aeration – the slow release of entrained air can transform a powder from free flowing to almost a solid state in which it will never flow.

Entrained air – can be a problem in many dosing operations causing weight variability. Fluidisation – aeration of fluidisable powders can lead to fluidisation when the material flows like a fluid and

may be difficult to control, as well as being potentially hazardous. Segregation or de-blending – especially where blends of relatively non-cohesive powders have different bulk

densities and particle size. Attrition – the process of wear and tear of particles that occurs in processing, especially in dilute phase

pneumatic transfer, which can make the particles rounder, reducing size and shape and often generating fines. Extrusion or lack of extrudability – relevant in die or mould filling where recesses need to be filled. Coarse

grained powders have little ability to flow or extrude under pressure. Agglomeration - formed by combinations of individual particles (usually very fine) and may hinder flowability or

further processing operations. Effect of moisture – can dramatically change flowability and other properties.

The above processing issues can exist individually or in combination. Each can be the subject of separate study.Many of these are investigated in the Applications section.

Improving plant efficiency Companies dealing with many powder types will know that the processability of their powders can vary greatly.Some are easy to process at high production rate while maintaining the required high quality, and others will be problematic and may require constant monitoring in order to maintain flow and quality. Some will process wellprovided an expert operator is on hand to fine tune – for example by adjusting feed rates, using a vibrator or injecting air.

Inefficiencies come from stoppages, equipment breakdowns or reduced operating speeds necessary to avoidproblems occurring. Batch to batch variability, mostly due to changes of powder properties or start up issues on newproduct are not unusual.

But there is something quite new that could significantly improve processing efficiency. It requires acceptance of thefollowing:

It makes good sense to understand the properties of your materials Powders cannot be adequately described with the traditional single number Adopting recent technology to measure powder properties is a good idea QC standards are viable, useful and necessary Making the most of your expertise and processing experience is good sense Efficient processing requires powder and plant compatibility

Most would agree that adopting modern technology to gain more understanding of their materials and making themost of their extensive in-house experience makes a lot of sense. How do we achieve this? The key is to understandfor each piece of equipment or each process line, the material characteristics that suit it best, as described below.

Matching powder and plant characteristics What suits one plant in terms of powder properties, may not suit another. This is just as true for individual items of equipment like a bin or chute, as it is for a complete process line such as a bag filling or tablet making process.

Describing powders as ‘good’ or ‘bad’ is not appropriate unless it relates to a specific process – for example, how well it flows through this hopper or how well it makes tablets or fills bags on that line. Powders are not ‘bad’ because theyare too cohesive or too easily fluidised, since these properties may or may not suit the process. For example:

Low cohesion and high flowability are desirable but a level of cohesivity may be necessary to avoid segregation, attrition and dust generation. Hence there is often an optimum level of cohesivity or flowability for a given powder / plant combination.

Permeability or the ease with which air can pass through a powder bed can be too high – hence releasing air and reducing flowability, or too low causing unwanted air retention.

Fluidisation that produces a very free flowing powder, may be good if the system is designed for it, or a serious problem and even a major hazard, if it is unexpected. For example fluidised powder would flow straight through a stationary screw feeder.

For most plant there will be optimum values of key powder properties like permeability, flowability and aeratability. For a hopper or a chute, the optimum properties that allow consistent flow and avoid potential problems like rat-holing or segregation, are easily determined. If this hopper is part of a more complex process line, quite different powderproperties may be desirable elsewhere such as a certain level of permeability required to achieve rapid and accuratebag filling. This means that an ‘optimum’ set of powder properties is what suits the entire process line. For example,high permeability may suit the bag filling operation, but result in over-consolidation in the feed hopper due to lack of entrained air. In practice the compromise required might be achievable by reducing the filling level in the hopper to

achieve consistent, high throughput.

Continuous fine tuning of a process line to achieve an optimum performance is commonplace. What is unusual is tothen determine the powder characteristics required to achieve this. Determining optimum powder properties for agiven plant and monitoring powders on a regular basis allows continuous improvement as the causes of stoppagesare investigated and understood. New product can be introduced with greatly reduced risk and productivity andquality are both enhanced.

Classification of powders

The most important innovation required in relation to traditional techniques, is a way of classifying powders so that flowability performance of each powder can be measured and recorded along with processing experience. Eventually, such a database of information could remove much of the uncertainty from the Processing of Powders and provide a reference-base for the development of new powders. It would allow each piece of production equipment to be classified in terms of the powders that could be efficiently processed.

Ideally, the classification of powders would provide more than just flowability data, such as flow rate and compaction indices. It would also include data describing the robustness and stability of the powder - for example, vulnerability to segregation, attrition and vibration. Given this, then the two key issues of powder processing could be addressed. Firstly, will the powder flow satisfactorily - does it have flowability properties that suit the process? And secondly, is the powder robust - will it be adversely affected by being processed?

Classification is now possible and by providing answers to these questions is set to revolutionise the design, processing and quality control of powders.