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 processes like granulation, drying, milling, lubricating, blending, dosing and compression. Mostly they flow under gravity, but sometimes an applied force is needed and here powders usually object since, unlike gases and liquids, they generally cannot 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, bulk stresses 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 method of 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 upon the powders during the process and then take into account how key properties like flowability are affected by air content, 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 that satisfactory productivity and quality are achieved. Even so, batch to batch variability is commonplace as are expensive 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 well provided 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 avoid problems occurring. Batch to batch variability, mostly due to changes of powder properties or start up issues on new product are not unusual.
But there is something quite new that could significantly improve processing efficiency. It requires acceptance of the following:
- 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 the most of their extensive in-house experience makes a lot of sense. How do we achieve this? The key is to understand for 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 they are 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 powder properties may be desirable elsewhere such as a certain level of permeability required to achieve rapid and accurate bag 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 to then determine the powder characteristics required to achieve this. Determining optimum powder properties for a given plant and monitoring powders on a regular basis allows continuous improvement as the causes of stoppages are investigated and understood. New product can be introduced with greatly reduced risk and productivity and quality are both enhanced.
