The Rise and Advantages of External-Circuit Vertical Roller Mill Systems

Vertical Roller Mills (VRMs), integrating grinding and drying, have been the backbone of global cement raw material preparation for decades. They’re prized for their low energy consumption, simple layout, and operational ease. In China, they dominate approximately 80% of existing cement production lines. However, the relentless drive for lower energy consumption and enhanced sustainability in cement manufacturing demands continuous technological advancement. The development and application of the External-Circuit Vertical Roller Mill system represent a significant evolutionary leap. It offers a powerful solution for modernizing grinding circuits and achieving substantial energy savings.

Demystifying External-Circuit VRM Technology

To understand the innovation, let’s first introduce the standard VRM. Material is fed onto a rotating grinding table and crushed by rollers. The ground material is then carried by a hot gas stream (primary air) upwards into an integrated dynamic classifier. Fine product exits with the gas for collection, while coarse material falls back to the table for further grinding. This internal circulation relies heavily on pneumatic lifting, which is energy-intensive.

The External-Circuit VRM system re-engineers this process to drastically cut power use. It retains VRM’s core grinding principle but replaces a significant portion, or even all, of the pneumatic material circulation with more efficient mechanical conveying.

Two main configurations exist:

1. The Partial External-Circuit System

This is often the most attractive upgrade path for existing mills. After initial grinding, a portion of the material exiting the grinding bed is diverted out of the mill housing. This material is then lifted by a bucket elevator and returned to the mill feed, creating a mechanical recirculation loop. The remaining fine material is still pneumatically conveyed to the internal classifier. This hybrid approach immediately reduces the load and pressure requirement on the system’s main circulating fan, leading to direct fan power savings of 10-25%. It also alleviates over-grinding within the mill bed, improving overall grinding efficiency.

2. The Full External-Circuit System

This represents a complete paradigm shift. All material ground by the VRM is discharged and transported via bucket elevator to an external separation stage, typically involving a V-Separator for initial classification and a high-efficiency dynamic separator for final product sizing. The grinding and classification functions are entirely decoupled. This design eliminates the mill’s internal pneumatic lift, allowing the mill fan to operate at minimal pressure solely for drying and conveying the finest fractions. The result is a system whose power profile and layout become strikingly similar to a Roller Press (RP) system, but with the inherent drying and material flexibility advantages of a VRM.

The Compelling Case: Energy Savings Breakdown

Energy analysis reveals the core benefit. In a traditional 400 t/h raw meal VRM system, the main circulating fan can consume 30-40% of the total grinding circuit’s power, primarily due to the work of lifting and circulating all material internally. By shifting material transport from pneumatic to mechanical means, External-Circuit systems attack this major energy drain. Mechanical conveying (e.g., bucket elevators) is inherently more energy-efficient than high-pressure air conveying for dense-phase flows.

Comparative estimates show that implementing an External-Circuit system can reduce the specific power consumption of a raw meal VRM by 2 to 6 kWh/t. For a 5,000 tpd production line, this translates to annual electricity savings in the range of millions of kilowatt-hours, offering a rapid return on investment and a drastic cut in carbon footprint.

VRM vs. Roller Press

For years, the Roller Press-based grinding system has been touted as the benchmark for lowest specific power consumption in raw meal preparation, often beating traditional VRMs by 3-5 kWh/t. This made it the preferred choice for many new plants. However, the advent of the Full External-Circuit VRM changes this competitive dynamic.

Power Consumption: A Full External-Circuit VRM system achieves a power consumption profile virtually on par with a Roller Press system. The key differentiator is no longer sheer energy use.
Process Simplicity & Flexibility: The Partial External-Circuit VRM retains a simpler layout than a full RP circuit. More importantly, VRMs maintain a decisive advantage in material adaptability. They excel at handling sticky, moist, or highly abrasive materials and are less sensitive to feed material flowability. Roller Press systems require a stable, compacted material bed between the rollers, which can be challenging with very fine or dry, fluid materials. This makes the External-Circuit VRM a more versatile and robust solution for plants with variable or challenging raw material compositions.
Inherent Drying: VRMs, including external-circuit versions, maintain their superior integrated drying capability using kiln exhaust gases, a feature where Roller Press systems are more limited.

The Strategic Upgrade Path for Existing Plants

Facing stricter national energy consumption standards (like GB 16780-2021) and the need to improve plant efficiency, many cement producers with older VRM systems are seeking retrofit solutions.

The External-Circuit technology provides a compelling, tailored answer:

Partial External-Circuit Retrofit: This is often the optimal first-step strategy. It requires moderate investment, utilizes most of the existing mill and foundation, and can be implemented with minimal disruption. It delivers immediate, significant energy savings (typically 2-4 kWh/t) and can also increase output by optimizing in-mill conditions. It’s the go-to solution for balancing cost, benefit, and technical risk.
Full External-Circuit Retrofit: This is a more comprehensive overhaul, suitable for plants needing maximum energy savings equivalent to a new RP system or those seeking to greatly increase capacity. While requiring more investment than the partial circuit, it is still generally more economical than a full switch to a Roller Press, as the core VRM structure is reused.
Synergistic Retrofits: The benefits can be amplified by combining the external-circuit modification with other optimizations, such as:
- Cyclone & Ductwork Optimization: Reducing system pressure drop directly lowers fan power.
- High-Efficiency Fan Upgrade: Modernizing the main circulating fan to match the new, lower-pressure operating profile can yield additional savings of 0.5-1.0 kWh/t.

The External-Circuit Vertical Roller Mills are not merely an incremental improvement. It is a transformative technology that redefines the value proposition of VRM systems. By strategically replacing energy-intensive pneumatic circulation with efficient mechanical transport, it delivers Roller Press-level energy efficiency while preserving the VRM’s legendary reliability, drying capacity, and material flexibility.

For new plant designs, it presents a highly efficient and versatile alternative. For the vast installed base of existing Vertical Roller Mills, the Partial External-Circuit retrofit emerges as a particularly powerful and cost-effective pathway to achieve mandatory energy savings, boost production, and extend the competitive life of the grinding plant. As the industry marches towards net-zero, adopting and refining External-Circuit VRM technology will be a cornerstone of sustainable cement manufacturing.

Epic Powder

Epic Powder is specialized in fine powder processing technology for mineral industry, chemical industry, food industry, pharama industry, etc. We are a most professional supplier of powder processing projects, especially powder milling, powder classifying, powder dispersing, powder classifying, powder surface treatment and waste recycling. Our team has more than 20 years experience in various powders processing. We supply consultancy, testing, project design, machines, commissioning and training. Contac us today to have your tailor-made powder solution.

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— Posted by Jason Wang, Senior Engineer