As a leading supplier of vertical roller mills (VRMs) and ball mills for cement, mining, and industrial minerals, we have partnered with hundreds of clients worldwide. One recurring challenge we help our customers solve is unstable mill operation, particularly excessive reject discharge (often called “spilling” or “airlock”) that reduces throughput and increases maintenance costs.
While every mill system is unique, certain root causes are pervasive. In this article, we share our engineering perspective on the most common causes of excessive rejects in VRMs, and – more importantly – the design-backed countermeasures we recommend. This knowledge comes from our own R&D and from on-site support provided to clients in Asia, Europe, Africa, and the Americas.
In a well-tuned vertical roller mill, only coarse impurities (e.g., shale, tramp metal) should be discharged from the reject port. When large amounts of partially ground material are continuously expelled, the grinding efficiency drops dramatically. Our diagnostics show that this almost always stems from:
Uneven wear between grinding rollers and table – leading to loss of contact geometry.
Airflow and pressure imbalances – causing poor material lift and internal recirculation.
Mechanical degradation of internal components – such as dam rings, nozzle rings, or scraper blades.
Below, we break down these areas and present both operator-level adjustments and factory-recommended engineering improvements.
In many plants, rollers and the grinding table are replaced at different intervals, or only one roller is renewed. This creates mismatched profiles, reducing grinding pressure effectiveness and triggering rejects.
Our approach:
We supply matched wear-resistant liners (high-chromium or composite materials) with extended service life.
For clients who cannot replace all components simultaneously, we offer on-site profile restoration (e.g., hardfacing) to re-establish the correct grinding track, eliminating the need for full replacement.
We also provide wear monitoring systems that predict uneven wear patterns, allowing planned maintenance rather than reactive fixes.
A dam ring that is partially worn or broken (even 200–300 mm gap) dramatically increases rejects because it fails to maintain the proper material bed thickness.
Nozzle ring wear enlarges the air passage, reducing the air velocity needed to carry fines upward.
Our engineered solutions:
We design modular, abrasion-resistant dam rings of the vertical roller mill with replaceable segments – no welding needed, reducing downtime.
For nozzle rings, we supply wear-resistant welded overlay or cast inserts that can be easily fitted to restore the original air velocity, often improving drying efficiency at the same time.
Operators often see rejects when the system pressure differential deviates from the norm (e.g., ±3500 Pa). High differential usually points to baghouse issues or air leaks; low differential indicates fan damper misalignment.
Our value-added support:
We include comprehensive commissioning checklists and remote troubleshooting assistance to quickly pinpoint such imbalances.
For long-term stability, we offer automated control upgrades that stabilise the grinding pressure and hot gas flow, adapting to feed variations without operator intervention.
| Issue | Symptoms | Our Recommended Fix (as a Manufacturer) |
| Scraper blades fallen off | Material accumulates in the reject chamber, choking airflow. | We supply heavy duty, bolt on scraper blades made from AR400 steel, with a design that prevents loosening under vibration. |
| Classifier (separator) vane loss or coupling failure | High rejects and increased product fineness variability. | Our latest dynamic classifiers feature wear resistant vanes with quick change mounting, plus a torque limiting coupling that alerts the control system before breakage. |
| Hydraulic tension system instability | Roller pressure cannot be maintained; rejects rise. | We use proven hydraulic cartridge valves and offer a remote diagnostic service to identify leaking accumulators or pumps without lengthy downtime. |
Based on our global service records, we advise customers to:
Schedule regular wear-profile inspections – every 1,000 operating hours – using our tablet-based measurement tool.
Keep a critical spare parts kit (dam ring segments, classifier vanes, scraper blades) to minimise unplanned stops.
Consider our wear-package upgrade – a combination of high-performance liners, optimised dam ring height, and adjusted nozzle ring geometry – which can increase throughput by 5-10% while reducing rejects to near zero.
Excessive reject discharge is not an inevitable fate of any vertical roller mill – it is a sign that the grinding system has drifted from its design equilibrium. As a manufacturer with decades of experience and a global clientele, we don’t just sell mills; we provide total process solutions. From initial design to retrofits and remote support, our goal is to help you achieve maximum availability and lowest cost per tonne.
If you are facing similar challenges or planning a new grinding line, please contact our engineering team. We are happy to share case studies and conduct a free preliminary audit of your current operation.
Submit Request
