Handling cement poses a range of flow challenges that are unique due to the material’s properties and the demanding processes involved in its production. Whether dealing with raw materials like limestone or managing finished cement in storage, issues like arching, ratholing, erratic flow, and flooding frequently disrupt efficient handling. Understanding and addressing these flow challenges is essential to ensure smooth operations in cement production.
Raw Material Flow Problems
The production of cement relies on a steady and consistent feed of raw materials. The most common raw materials include limestone, clay and sand (for silica). If necessary, mill scale (for iron), bauxite (for aluminum), gypsum and fly ash are added to the cement. However, these raw materials often exhibit poor flow behavior, particularly during reclaim from stockpiles or when stored in silos or hoppers.
Arching and Ratholing
Two common flow issues in cement handling are arching (also known as bridging) and ratholing. Arching occurs when material forms an arch-like obstruction above the hopper outlet, effectively blocking material flow. This can happen for several reasons, including the interlocking of larger particles where the particles mechanically lock to form obstructions or when particles pack together. In contrast, ratholing happens when material only flows through a central channel while the material outside the channels remains stagnant, forming a vertical hole. Both phenomena severely limit the efficiency of material flow and often require manual intervention to rectify.
Erratic Flow
Cohesive materials such as limestone, gypsum and clay can exhibit erratic flow when handled in an improperly designed silo or hopper. This flow behavior results from obstructions in the silo alternating between an arch and a rathole. A rathole may fail due to external forces, e.g., air cannons, resulting in some material discharge as the rathole collapses. The falling material often compacts over the outlet to form an arch, which may similarly fail due to external forces. Once the arch fails, material flow will resume until the flow channel is empty and a rathole forms again. This flow condition can lead to significant dynamic loads on the silo that can lead to structural failure and downstream inefficiencies if continuous feed is critical.
Flooding
If a rathole fails and the collapsing material is fine (typically less than 150 micrometers), such as portland cement or fly ash, flooding may result. A flooding material will exhibit fluid-like behavior, due to the aerated state of the material, and potentially discharge uncontrollably from a silo at high rates of discharge. This flow issue may result in process spillage if the downstream equipment is not equipped to handle this flow condition. Additionally, flooding may lead to bag-filling issues due to the low and variable bulk density of the discharge.
Dust Generation and Segregation
Cement handling generates a significant amount of dust, leading to various operational challenges:
- Fluidization segregation: Fine particles can retain air in their void space during handling, leading to separation from larger particles.
- Dusting segregation: Fine particles remain suspended in an air stream longer than coarser particles and may be displaced from the fill point by secondary air current, leading to inconsistent material compositions.
- Health and safety concerns: Dust is a major safety risk for workers and can cause wear and tear on equipment, increasing maintenance costs.
Controlling dust not only ensures better material flow but also improves the working environment and reduces equipment downtime. Specialized equipment, such as dust collection systems, can mitigate these risks, but they must be integrated into the overall material handling process.
Temperature Effects
High temperatures involved in cement production, particularly during the handling of clinker (the primary component of cement), can alter the flow properties of the material. Hot clinker flows differently compared to cooled cement. Additionally, fluctuations in temperature during the production or storage process can cause changes in material behavior, sometimes leading to flow inconsistencies.
Segregation Issues
Segregation can be a major issue in cement handling, where materials separate based on particle size or density. Common types of segregation include:
- Sifting segregation: This occurs when smaller particles sift down through larger ones during material transfer.
- Trajectory segregation: During discharge, larger particles travel further, while smaller particles tend to fall near the outlet.
Both types of segregation can lead to inconsistent material quality, requiring operators to adopt specific handling strategies to avoid such outcomes. Implementing optimized storage methods and controlling material flow can help mitigate segregation.
Equipment Design Considerations
Addressing these flow challenges requires specialized equipment designed to handle cement’s unique properties. Mass flow bin designs are essential to prevent common issues like ratholing and ensure consistent material discharge. The outlet size must also be carefully designed to prevent arching, and certain materials, such as sticky limestone, may require specially designed feeders like apron feeders with mass flow interfaces.
Case Study: Cohesive Clay and Limestone Raw Material Handling at Cemex
Moreover, bulk material testing is a proactive measure that can identify potential flow issues before they cause disruptions. Testing flow properties like shear strength and wall friction allows cement producers to modify equipment designs and implement corrective measures.
Closing Thoughts
Cement handling presents several flow challenges due to the material’s inherent properties and the production process. Arching, ratholing, erratic flow, dust generation, temperature effects, and segregation are common issues that must be addressed to maintain efficient production. Solutions often involve optimizing equipment design, controlling moisture and dust, and conducting regular testing to understand the material’s behavior. By focusing on these factors, cement producers can ensure smoother operations, reduce downtime, and improve overall material handling efficiency.
