Fines and Coarse Particle Distribution – Managing the “Tails” of Powder PSD for Flow, Quality and Safety

This topic is part of the SG Systems Global powder handling, particle engineering and dry-ingredient operations glossary.

Updated December 2025 • Particle Size Reduction & Milling Control, Powder Cohesiveness Classification, Sifter & Mesh Validation, Air Fluidization & Powder Aeration, Powder Conditioning (Temperature & Humidity Control), Dust Explosion Hazard (NFPA 652, ATEX) • Ingredients & dry mixes, bakery premix, nutraceuticals, pharma, agrochemicals, detergents, plastics, minerals

Fines and coarse particle distribution refers to the “tails” of a powder’s particle size distribution (PSD): the small fraction of very fine particles at the low end and the oversized granules or agglomerates at the high end. Most specs talk about D10, D50 and D90, but in real plants it is often the extremes – the fines and the coarses – that drive flow failures, segregation, caking, dust explosions, tablet defects and customer complaints about “grittiness” or “dustiness.” Managing those tails is therefore a core part of designing mills, sifters, mixers, silos and packaging lines that actually behave.

“You don’t run a plant on D50 – you run it on the fines that dust up your collectors and the coarse lumps that jam your feeders.”

1) What “Fines” and “Coarses” Actually Mean

In practice, most plants use “fines” and “coarses” loosely. For clarity:

• Fines: The smallest particles in the distribution – below a defined cut-point (e.g. <75 µm or <200 mesh). They contribute disproportionately to cohesion, dust and surface area.
• Coarses (oversized): The largest particles, lumps or agglomerates – above a defined cut-point (e.g. >850 µm or failing a safety screen).
• PSD tails: The ends of the size distribution (both fine and coarse) outside the main “bulk” where most volume sits.

Exactly where “fine” ends and “coarse” begins is product-specific. The point is that both tails behave differently from the main fraction and often drive the worst behaviours in storage, mixing, conveying and end-use. A specification that only quotes “D50 = 150 µm” ignores this reality and invites trouble.

2) Why Fines and Coarses Matter More Than the Median

From a process point of view, the tails often dominate:

• Fines: Drive cohesiveness, dustiness, static, caking and explosion risk – even when they are only a small mass fraction.
• Coarses: Cause attrition in mills, plugging of nozzles and feeders, “grit” in finished product and uneven dissolution or rehydration.
• Segregation: Size and density differences between fines and coarses fuel segregation during discharge and conveying.
• Sifters and screens: Overs fractions clog meshes and reduce effective capacity; fines blind meshes and reduce cut accuracy.

In Ingredients & Dry Mixes, it is common to see plants chasing D50 or D90 targets while ignoring fines percentage and overs load – then being surprised by chronic feeder issues, dust collector overloads or customer feedback about mouthfeel and dust. The fix starts by explicitly specifying and monitoring those tails, not simply hoping they behave themselves.

3) How Fines Affect Flow, Cohesion and Dust

Fine particles (<50–100 µm) have huge surface area relative to mass. Consequences:

• Increased cohesion: van der Waals forces, moisture bridges and electrostatics are stronger; flow is more like damp flour than dry sand.
• Higher Carr Index / Hausner Ratio: Fines push powders into more cohesive classes (see Powder Cohesiveness Classification).
• Dustiness: Fines detach and become airborne easily during tipping, conveying and packaging, driving respirable dust and housekeeping load.
• Explosion risk: Fines lower MEC and often increase K_st, making dust clouds more easily ignitable and violent (see Dust Explosion Hazard).

Design-wise, a powder with 3 % fines behaves very differently from one with 15 % fines even if their D50 values are identical. That difference should be reflected in hopper design, feeder choice, dust collection and powder-conditioning strategy, not just noticed post hoc when silos bridge or feeders flood.

4) How Coarse Particles Affect Flow, Segregation and Performance

Coarse particles and agglomerates (>500–1000 µm, depending on product) create their own set of issues:

• Grittiness and defects: Unpleasant mouthfeel in food, visible specks in cosmetics, tablet core or coating defects in pharma.
• Flow interruptions: Plugging of small outlets, auger inlets, valves or nozzles; jamming in scales and feeders.
• Segregation: Larger particles often roll and migrate faster, causing “rich” and “lean” zones in mixed blends.
• Inconsistent dissolution: Coarses may dissolve much more slowly than fines, affecting assay, texture and bioavailability.

Coarses typically come from upstream lumps (moisture caking, weak milling), incomplete de-lumping, or deliberate agglomeration that has aged or broken unevenly. Managing coarses usually means combining good milling control with robust sifter & mesh validation, not simply tightening product specs and hoping suppliers magically comply.

5) PSD Metrics that Capture Fines and Coarses Properly

Standard PSD metrics can be extended to capture tails better:

• D10, D50, D90: Basic descriptors; D10 reflects fines tail, D90 reflects coarse tail. Alone they may miss extreme outliers.
• Specific fines/coarse fractions: “% < 63 µm” or “% > 850 µm” measured by sieving or laser diffraction, tied to process and quality limits.
• Span and uniformity indices: (D90 − D10) / D50 as a simple width metric; narrower span often means fewer tail problems.
• Tail specs: Explicit limits like “<2 % retained on 20 mesh” or “<5 % passing 325 mesh” written into raw material and in-process specs.

For Ingredients & Dry Mixes, it’s often enough to add one or two critical tail fractions to the spec for key materials – e.g. test for fines below 63 µm and overs above 850 µm alongside the usual D50/D90 – and to link those numbers to what you actually observe in silos, feeders and customer experience.

6) Fines, Cohesiveness and Silo / Hopper Behaviour

Fines are one of the main reasons real silos behave differently than clean DEM simulations and vendor brochures suggest:

• Bridging and rat-holing: High fines content raises yield strength; see Silo Rat-Holing & Bridging for how this shows up as flow failures.
• Plugging of aeration systems: Fines can blind fluidising pads and distributor plates, making air fluidization unreliable.
• Apparent bulk-density variability: Fines-rich pockets compact differently, confusing level sensors, load cells and “calculated inventory.”

Hopper design and cohesiveness classification should therefore reference fines content explicitly – e.g. design for the worst-case fines percentage you expect from your milling and sieving operations, not the nice, clean distribution in the development lab’s laser-diffraction file.

7) Fines and Dust Explosion Hazard (NFPA 652, ATEX)

Fine tails dominate dust-explosion risk:

• They are easiest to disperse into clouds during tipping, conveying and cleaning.
• They often have the highest K_st and lowest MEC, driving worst-case conditions in Dust Explosion Hazard assessments.
• They accumulate in dust collectors, filters, rafters and ducting – classic ignition and secondary explosion fuel.

Any NFPA 652 DHA or ATEX dust zoning exercise that treats “powder” as a monolithic entity without considering its fines content is half blind. Where you have a growing fines problem (e.g. repeated milling, attrition in conveyors, aging product), explosion protection and housekeeping need to be tightened, not left static while the material slowly becomes more hazardous without anyone noticing.

8) Managing Fines – Milling, Classification and Conditioning

Fines can be controlled at multiple points:

• Milling optimisation: Avoid over-milling and excessive recirculation in milling steps; tune energy input and classification to meet PSD without over-generating fines.
• Air classification and cyclones: Dedicated removal or recycling of fines to control their proportion in key streams.
• Sieving and scalping: Using sifters not just for overs but also for fines when process conditions justify it.
• Powder conditioning: Controlled RH and temperature to prevent fines from becoming overly cohesive or electrostatically charged (see Powder Conditioning and Powder Electrostatic Charge Management).

For some products, fines are actually desirable (e.g. for rapid dissolution) – but even then, they must be controlled, not left as an uncontrolled by-product of fossilised milling and classification settings that no-one has revisited since the line was installed.

9) Managing Coarse Particles – De-lumping, Screening and Rework

Coarses typically come from:

• Caking during storage and transport (moisture, temperature swings).
• Incomplete or poorly controlled milling and de-lumping.
• Agglomeration and granulation that is not stable over time.

Practical controls include:

• De-lumping mills / comills: Gentle, low-energy size reduction prior to blending or packing.
• Validated safety screens: See Sifter & Mesh Validation for ensuring meshes capture overs reliably without bypass or damage.
• Rework loops: Controlled routes to return overs to milling or granulation; clear rules preventing endless recycles that generate even more fines.
• Improved conditioning: Temperature/RH control to minimise caking that produces hard lumps in the first place.

From a QMS perspective, coarses “found” in downstream sieves or packers should trigger FMRA, segregation and root-cause work on upstream PSD control – not just binning of overs and moving on until the next complaint about grit or plugging shows up.

10) Fines/Coarses and Blend Uniformity

PSD tails have direct implications for Blend Uniformity:

• Fines-rich components: Tend to adhere to larger carrier particles, forming “coated” granules; good for some premixes, bad for others.
• Very coarse or dense components: May segregate during blending, transport and discharge, especially in gravity feed and vibratory conveying.
• Tail drift: Over time, repeated handling and conveying can generate more fines, changing blend behaviour mid-campaign.

Tracer-based BU work should therefore be done on realistically handled material – with the fines/coarse profile you have after upstream handling – and should feed back into upstream PSD targets where poor uniformity traces directly to mismatched tails between components (e.g. micronised active in a coarse carrier vs all components milled into a comparable range).

11) Fines, Coarses and Downstream Unit Operations

PSD tails often show up as problems in downstream processes:

• Tableting and encapsulation: Excess fines may reduce flow into dies and caps; coarses cause capping, lamination or soft spots.
• Extrusion, granulation and coating: Fines overload filters and scrubbers; coarses lead to non-uniform coating and weak granules.
• Filling and packing: Fines cause weight variability and dust; coarses jam augers, nozzles and filling valves.
• End-user performance: Fines drive fast dissolution and sometimes off-flavours; coarses cause slow dissolving, sediment or gritty texture.

Tail specifications and control should therefore be written backwards from these unit operations and end-use requirements. For example, “<1 % >850 µm to avoid grittiness in beverage mixes” or “<5 % <63 µm to avoid dusting in warehouse operations.” If the tails aren’t linked to reality, they will be quietly ignored the next time someone wants to increase mill throughput by relaxing screen changes or classification settings.

12) Supplier Specs, Incoming QC and Change Control

To avoid surprises, fines/coarse expectations must flow through the supply chain:

• Raw material specs: Include tail limits (e.g. “<3 % <63 µm; <1 % >850 µm”) alongside moisture and chemical analysis.
• Supplier change control: Require notification if milling, classification or granulation changes will alter tails, not just D50.
• Incoming QC: Simple sieve or laser tests for tail fractions on high-risk materials before release to production.
• Internal change control: PSD tail changes due to internal process tweaks (e.g. new mill screens, different sifter settings) evaluated for impact on flow, safety and quality before implementation.

This is where a lot of trouble starts: a supplier “optimises” milling to reduce energy cost or a maintenance team changes a screen size, and suddenly your silos and feeders start misbehaving. Explicit tail specs and associated change-control hooks stop those changes from sliding in unnoticed.

13) Digital Integration – PSD Tails as Master Data and Design Inputs

In MES/ERP and design tools, fines/coarse distribution can be embedded as master data attributes:

• Material master: Store D10/D50/D90 and key tail percentages (e.g. % <63 µm, % >850 µm) for formulations and equipment selection.
• Equipment limits: Silos, feeders, mills and packers tagged with the PSD tail ranges they are validated to handle.
• Routing rules: WMS/MES prevent assignment of high-fines materials to dust-sensitive equipment or poorly ventilated areas.
• Dashboards: PSD tail trends displayed alongside flow incidents, dust events, BU results and customer complaints to expose correlations quickly.

That’s how you move from reactive troubleshooting (“why is silo 4 suddenly rat-holing?”) to proactive triggers (“supplier X’s last two lots show fines > spec; route them only to lines designed for cohesive class 3 powders until root cause is closed”).

14) Common Pitfalls in Managing Fines and Coarses

Classic pitfalls that show up again and again:

• Spec blindness: Only D50/D90 specified; tails ignored until there is a flow, dust or quality issue.
• One-time PSD characterisation: Lab PSD measurements during development never repeated after scale-up or supplier change.
• Over-tightening specs without capability: Writing impossible fines/overs limits into specs that mills and sifters cannot realistically achieve, leading to chronic OOS and informal waivers.
• Rework loops without tail control: Continually re-milling overs until fines dominate, making flow and dust issues worse.
• No link to safety: Fines growth not reflected in DHA / ATEX reviews, so protection and housekeeping stay tuned for a less hazardous PSD than is actually present.

Most of these are fixable with simple discipline: define what you care about in the tails, measure it periodically, and connect it to real-world behaviour in flow, safety and quality. That is enough to avoid 80 % of the surprises that currently chew up engineering and QA bandwidth in dry plants.

15) Implementation Roadmap – Getting PSD Tails Under Control

A pragmatic roadmap for an Ingredients & Dry Mixes facility might include:

• Step 1 – Identify critical products: Pick the lines and materials with the worst flow, dust, BU or customer issues.
• Step 2 – Characterise tails: For those materials, measure D10/D50/D90 plus simple fines/coarse fractions by sieving or laser diffraction.
• Step 3 – Link to behaviour: Correlate tail fractions with silo/feeder incidents, dust collector load, BU results and complaints.
• Step 4 – Set practical limits: Define tail specs that your current milling/sieving can meet and that materially reduce risk; add them to material/batch specs.
• Step 5 – Harden the process: Tune milling, sifting, conditioning and rework loops to consistently hit those limits; validate sifters; update DHAs where fines content materially changes explosion risk.
• Step 6 – Embed and review: Put tail attributes into master data, WMS/MES routing rules and QRM; review annually or when major changes occur.

The goal is simple: know how many fines and coarses you actually have, know what that does to your plant, and build just enough control around them that you don’t spend your life fighting dust, grit and erratic powder behaviour every time weather or suppliers change.

16) FAQ

Q1. Is it really necessary to measure fines and coarses separately if we already have D10/D50/D90 from the lab?
Often yes. D10 and D90 give a general sense of tails but can hide small but critical fractions (e.g. 1–2 % of very coarse lumps or ultra-fine dust) that cause outsized problems in flow, dust and product performance. Simple sieve cuts (e.g. % >850 µm, % <63 µm) are cheap and give much more actionable data for equipment design, safety and quality decisions.

Q2. How much fines is “too much” for good flow?
It depends on the material, particle shape, moisture and conditioning. Some powders tolerate high fines with little issue; others become cohesive above 5 % <63 µm. The realistic path is to correlate fines percentage with measured flow indices (Carr/Hausner, shear tests) and with real-world flow behaviour in your hoppers and feeders, then define limits that avoid known problem regimes rather than relying on generic numbers.

Q3. Can we solve fines and coarses issues entirely with better sifters?
Sifters are a big part of the solution but not the whole story. If milling is over-aggressive or storage is causing caking, sifters can get overwhelmed with overs and fines, leading to blinding, low capacity and more attrition. A balanced approach uses controlled milling/classification, validated sifters, and conditioning to prevent tails from drifting to extremes in the first place.

Q4. Do PSD tails really affect dust explosion risk that much?
Yes. Fines strongly influence minimum explosible concentration, K_st, ignition sensitivity and dustiness. As fines grow, a dust that was marginally explosible can become clearly hazardous. That’s why NFPA 652 and ATEX-aligned DHAs should be revisited when milling practices, attrition, or recycling significantly change the fine fraction in key streams.

Q5. What is a practical first step if we suspect our flow and dust problems are linked to PSD tails?
Pick one problematic product, run a simple sieve analysis to quantify fines and coarses, and compare the result with a similar product that behaves better. If tails are significantly different, run a small trial where you deliberately reduce fines or overs (via adjusted milling/sieving) and see how flow, dust and quality respond. That quick experiment often makes the case for adding tail limits to specs and tuning upstream operations accordingly.

Related Reading
• Particle Engineering & Flow: Particle Size Reduction & Milling Control | Powder Cohesiveness Classification | Sifter & Mesh Validation | Vibratory Conveying Dynamics
• Safety & Environment: Air Fluidization & Powder Aeration | Dust Explosion Hazard (NFPA 652, ATEX) | Powder Electrostatic Charge Management
• Conditioning & Governance: Powder Conditioning (Temperature & Humidity Control) | Ingredient Conditioning & Storage | Quality Risk Management (QRM) | Quality Management System (QMS)