Roll compaction and wet granulation both turn fine powder into larger, free-flowing granules. Roll compaction does it dry, by pressing the powder between two rolls, while wet granulation adds a liquid binder and then dries the granules. If your material is sensitive to water or heat, or you want fewer process steps, roll compaction is usually the place to start.
How does roll compaction work?
Roll compaction is a dry granulation process that presses powder between two counter-rotating rolls into a dense sheet or ribbon. The ribbon is then broken up in a mill and screened to the target granule size. No water, solvent or binder solution is added, and there is no drying step afterwards.
In more detail: powder is fed, often by a screw, into the narrowing gap between the rolls, where it is put under high pressure. That densifies the material and bonds the particles together. The ribbon goes through a mill with a screen sized to the target, and the milled granules are screened to remove fines and oversize (PCI Pharma Services). Roll compaction is also called chilsonation. At D&A it is the core of our dry granulation service.
How does wet granulation work?
Wet granulation is a process that binds powder particles together with a liquid, usually water or a solvent carrying a binder, then dries and sizes the wet mass. It typically involves mixing, wetting, wet massing, drying and milling, so it takes more equipment, time and energy than dry granulation.
A review in Pharmaceutical Technology Europe notes that wet granulation "typically requires five or more process steps," while the main roll compaction step takes only a few minutes (Boswell and Smith, PharmTech). Wet granulation is widely used and produces good granules, but the liquid and the drying step are exactly what some materials can't tolerate.
D&A runs dry processes only, so we don't offer wet granulation. If a material genuinely needs it, we'll tell you.
Roll compaction vs. wet granulation at a glance
| Roll compaction (dry) | Wet granulation | |
|---|---|---|
| How particles bond | Pressure between two rolls | Liquid binder, then drying |
| Liquid or solvent | None | Water or solvent required |
| Drying step | No | Yes |
| Process steps | Fewer: feed, compact, mill, screen | More: mix, wet, mass, dry, mill |
| Moisture- or heat-sensitive materials | Well suited | Can degrade them |
| Fines | More, recycled back to the compactor | Fewer |
| Energy and cleanup | Lower | Higher, with drying and wet cleanup |
Which powders suit roll compaction?
Roll compaction suits powders that are moisture-sensitive, heat-sensitive, or simply dusty and hard to handle. Because no liquid is added and nothing is dried, it is described as "especially suitable for moisture- and/or heat-sensitive compounds" (Jeon, PharmTech).
Good candidates tend to include:
- Hygroscopic salts and minerals that cake when they pick up moisture
- Materials that degrade or change when wetted or heated
- Low-density, dusty powders that need to be densified for handling, shipping or blending
- Fertilizer and mineral fines that need to become a spreadable granular product (see agriculture and fertilizer)
- Plastics additives that need a dust-free, free-flowing form for compounding lines (see chemicals and plastic additives)
For materials with a low melting point, the heat generated by compaction can still matter. We use liquid nitrogen injection to keep temperatures down on low-melting materials.
When might wet granulation be the better choice?
Wet granulation can be the better fit when a powder will not bond under pressure alone, or when the product needs granule properties that only a liquid binder gives. The same review notes that roll-compacted tablets can show lower tensile strength than wet-granulated ones, an effect called loss of reworkability (Jeon, PharmTech).
Very free-flowing or very cohesive powders can also be harder to feed and compact. Fast-flowing materials can slip through the rolls and leave a high share of uncompacted fines (Boswell and Smith, PharmTech). That doesn't rule roll compaction out, but it is the kind of thing a trial will show quickly.
What about fines and yield?
Fines are the main trade-off with roll compaction. Milling the ribbon always makes some particles smaller than the target, and those are screened out after milling. In a well-run process they are fed back to the compactor rather than lost, so the real question is how many passes it takes to get the size range you need.
A tighter size range usually means more screening and more recycle. When you set a spec, say what the granule is for (feeding a hopper, blending with other granules, spreading, dissolving), so the size range is no tighter than it needs to be. Our screening runs on Sweco and Kason screeners, and milling can bring oversize back into range.
How do I decide for my material?
The fastest way to decide is a trial on your actual material. Bring three things: the powder (with an SDS), the target granule size range and bulk density, and what happens to the product next. Those answers decide whether roll compaction works and how the run should be set up.
Most of our projects start that way. Our how toll processing works page explains the steps, and granulation batches start at around 750 lbs.
Frequently asked questions
Is roll compaction the same as dry granulation?
Roll compaction is the most common way to dry granulate. Powder is pressed between two rolls into a dense ribbon, which is then milled and screened into granules. Chilsonation is another name for the same process.
Do I need to add a binder for roll compaction?
Usually not. Many powders bond under roll pressure on their own. Some formulations need a dry additive to compact well, which is something a trial run will show.
What happens to the fines from roll compaction?
Granules are screened after milling. Fines that pass through the bottom screen are normally fed back to the compactor rather than thrown away, so the final yield depends on how well the material compacts.
Can a material that was wet granulated be switched to roll compaction?
Often, yes, but it should be tested. The granules will not be identical, so it is worth checking particle size, bulk density, hardness and how the product performs downstream before switching production.