A glass plant was receiving a granular raw material that appeared acceptable by conventional delivery sampling. The producer’s delivery composite, riffled down from grab samples taken during unloading into the storage silo, showed a reasonably centered PSD.
Samples from the storage silo told a different story.
Compared with the delivery composite, the silo samples were broader and less centered. They were enriched in the coarse/intermediate region, especially around 425–300 µm, while the delivery composite was more concentrated in the 212–106 µm range. The silo samples also showed a heavier fine tail. This pattern repeated over time, which made it unlikely to be random sampling noise. The storage silo was acting as a PSD transformer: the producer delivered one distribution, but the furnace was being fed another.
The applied constraint was clear. The silos were fixed assets. Their geometry, withdrawal pattern, and internal flow behavior were not going to be corrected quickly or cheaply. Therefore, the operating question changed:
What incoming PSD could be specified so that, after storage and withdrawal through this silo, the furnace receives the most favorable practical PSD?
The wrong answer would be to ask simply for “finer” material. Finer particles increase specific surface, but uncontrolled fines create dust, carryover, poor flow, and local segregation. Dust is not just extra surface area. In a batch house, dust often behaves as a separate process risk.
The better objective is more precise:
Increase useful specific surface without creating a dust penalty.
For this case, that means reducing excess material in the 425–300 µm band, increasing controlled material in the 150–75 µm range, and imposing a firm cap on <75 µm pan material.
In mesh terms:
reduce 40–50 mesh dominance, strengthen 100–200 mesh, cap pan.
In micron terms:
reduce excess 425–300 µm, increase controlled 150–75 µm, limit <75 µm.
This is not a request for an ideal laboratory curve. The supplier’s ability to approach that target will depend on the ore, the beneficiation equipment available and the economics of operating to the adjusted specification. Ore texture, fracture behavior, natural fines, clay content, impurity distribution, moisture sensitivity, screening efficiency, classification equipment, drying, if used, and yield loss all have a bearing. A producer with only crushing and screening has a different capability envelope than one with washing, attritioning, hydraulic classification, cyclones, fine screening, drying, or air separation.
Therefore, the revised PSD should be negotiated as a practical capability envelope, not imposed as an abstract ideal.
The specification should control both cumulative passing values and fractional bands. A cumulative curve alone can hide the problem. Two materials may show similar percent passing at 100 mesh while one carries too much mass in the 40–50 mesh region and the other places more material in the useful 100–200 mesh region.
A practical revised specification would include:
| Control | Purpose |
| Maximum above 425 µm / 40 mesh | Limits slow-reacting coarse material. |
| Control band for 425–300 µm / 40–50 mesh | Avoids reinforcing the silo-enriched fraction. |
| Target band for 150–75 µm / 100–200 mesh | Raises useful specific surface. |
| Maximum <75 µm / pan | Limits dust and fines mobility. |
| Bulk density and flowability | Confirms the material still handles properly. |
| Moisture range | Suppresses dust without causing caking. |
| Silo-discharge PSD | Confirms what the furnace actually receives. |
The last item is the most important. The producer’s delivery composite remains necessary, but acceptability should be based on the combination of delivery PSD and silo-discharge PSD.
Any trial should therefore be simple. The producer supplies a modified PSD within its realistic beneficiation capability. The plant samples the delivery composite and the silo discharge. The plant then asks three questions:
Did the incoming PSD move in the intended direction?
Did the silo-discharge PSD move closer to the desired furnace-facing envelope?
Did dusting, flow behavior, density, feeder response, batch handling or carryover get worse?
The operating conclusion follows directly from the evidence.
The silo cannot be ignored, and it cannot be treated as a neutral container. If it changes the PSD. And the silo cannot be changed, then the incoming material specification should be adjusted to the extent practical so that the transformed material is better suited to melting.
The goal in this example is not simply finer material. The goal is more useful surface area, less coarse/intermediate surplus, and no dust penalty. That turns the PSD specification from a purchasing tolerance into an operating tool.