From screen to furnace: protect and distribute the green feed
Connect screening, transfers and bed loading before assigning a performance benefit.
Define quality at the final feed point
The accepted stream at a roller screen is an intermediate result. A later transfer can generate fines, a surge can create a deeper layer on the next screen, and a distribution device can load one part of the furnace more heavily than another. The useful boundary is the material condition and bed arrangement actually entering induration.
Draw every transfer between balling discharge and furnace feed. Mark where fines are removed, where streams combine and where a representative sample can be taken safely. A proposed screen upgrade should state whether its acceptance boundary ends at discharge or includes these downstream interfaces.
Separate cushioning the belt from protecting the pellet
An impact support system may reduce belt damage without preventing fracture of fragile green pellets. The material sees a combination of drop height, impact angle, velocity change, contact surface and repeated impacts. Evaluate the trajectory and pellet condition rather than specifying belt protection alone.
| Interface | What to compare | Failure that can remain hidden |
|---|---|---|
| Screen discharge to belt | Matched size distributions before and after transfer | New fines despite a compliant screen outlet. |
| Belt-to-belt or chute transfer | Relative motion, drop geometry and local accumulation | Pellet deformation, concentrated impact or blockage. |
| Distribution equipment | Mass delivered across width over repeated cycles | A satisfactory time average concealing cyclic overload. |
| Final feeder to bed | Widthwise loading, fines and bed-depth variation | Nonuniform gas resistance entering induration. |
A retrofit review should include access for cleaning and wear replacement. A low-drop layout is not successful if it introduces persistent buildup or an inaccessible blockage. Compare the complete material route under normal, reduced-rate and credible upset conditions.
Transverse uniformity and vertical layering are different objectives
Across the width, unwanted variations in mass loading and bed depth can create different flow resistance and thermal histories. Through the depth, some plant-specific feed systems deliberately arrange size layers to suit a particular thermal profile. Controlled layering should not be confused with accidental segregation.
The 2017 UFRJ roller-screen thesis describes a proposed arrangement intended to place larger pellets above smaller ones. That configuration was evaluated for a particular plant and screening system. It is not a universal instruction for all straight-grate or grate-kiln installations. The furnace flow direction, heating programme, bed depth and resulting material distribution must be considered together.
A drawing of two screen decks does not demonstrate the final bed arrangement. Verify how material actually travels through the intervening belts and feeders. Classification, conveying and charging must preserve the intended distribution for the proposed mechanism to be credible.
Evidence: Benito Barbabela e Silva, UFRJ thesis (2017) ↗ — methodology section 4.4. The proposed arrangement is a modelling case, not a measured CVIC performance result.
Do not confuse pellet porosity with bed voidage
| Quantity | Physical meaning | Why the distinction matters |
|---|---|---|
| Internal pellet porosity | Pore space inside a pellet; connectivity also matters | Relates to internal transport and reaction as well as strength. |
| Bed voidage | Space between pellets in the packed bed | Influences the pathways available to gas through the burden. |
| Bed pressure drop | Measured resistance at a stated gas flow and condition | Also depends on bed depth, size distribution, fines and loading arrangement. |
Removing fines can change a bed’s resistance, but a predicted voidage change is not itself a measured pressure-drop improvement. A lower pressure drop is also not automatically a fixed fuel saving. Fan operation, gas flow, heat recovery, furnace control and product requirements determine how a physical change is used.
When reviewing a proposal, identify each step in the evidence chain: measured separation, measured bed distribution, measured gas response and measured energy per accepted tonne. A calculation linking these steps is valuable when its assumptions are visible; it should remain labelled as a calculation.
Set up a matched before-and-after campaign
Compare representative periods with recorded ore blend, moisture, green-size distribution and production rate. Use time-aligned flow and size measurements at the screen inlet, each outlet and final furnace feed. Note transfer inventory and residence time when matching samples. Do not treat a startup period and a stable baseline as directly comparable.
For distribution, examine variation across width and through operating cycles, rather than a single photograph. For thermal response, record bed loading and the available plant gas and temperature measurements. Retain individual quality results or their distribution, because a satisfactory average can hide a persistently weak region.
Judge the project against the original objective: cleaner feed, recovered on-size material, lower recirculation, more uniform loading or better availability. An improvement in one measure should be reported alongside any penalty in the others.
Use this guide to frame testwork and engineering reviews. Actual settings and acceptance limits belong to the approved plant-specific procedures.