Blending/Homogenizing Silos – All They’re Cracked up to Be?

Abstract

Consistency in raw material is vital for the production of cement with reliable characteristics. As the process of cement production has developed, so has the preparation and storage of the blend of materials used to produce clinker. This paper will discuss some of the history of how raw material has been produced, stored, and transported to the kiln. The effects of the changes on the storage of the material, more recently in storage silos, will be a major focus of the paper.From vertical kilns to horizontal, then the wet process to the dry process, to current day multi stage preheater towers, with silos of varying arrangements where material is aerated by twelfths, quarters, flat bottoms or inverted cones that use less power and air, the sophistication of the process has, in some cases, left the structural engineer responsible for the design of the storage of the raw material in the dark, sometimes with disastrous effects.Based on past experience and observations of dozens of existing homogenizing and blending silos, the structural engineering performed for the design of the silo storing the raw material prior to being taken to the preheater has been largely insufficient. Why? Has the structural engineer taking this responsibility not understood the principles of the process in the silo? Or is it a simple communication problem, where the process engineer has not communicated what is really happening inside the silo when the material is blended / homogenized. The answer is likely "both".It should be stated explicitly that the homogenizing / blending silos of recent years differ from most storage spaces. In most cases, material placed in a silo is in a static condition: material is placed inside, and stored until it is ready to be reclaimed. While that is a gross overstatement, compare this with the demands on a blending / homogenizing silo. These structures are not just expected to hold material, but they instead are now part of the process. Material is placed inside them and manipulated while in the storage space, with the desire to make them more homogenous.This blending / homogenizing process fluidizes or moves the material in segments. This causes the material in the silo to take on different properties in these different segments. Depending on the process being implemented (i.e. homogenization / blending, number of segments used, inverted cones, flat bottoms, eccentric or concentric discharge, etc.), the fluidized / activated material in the silo will be less stiff and exert less pressure outward than the remaining static material. This difference in stiffness and pressure results in bending in the silo walls, causing stresses that nearly always exceed those originally expected for uniform material pressure. This bending causes the overstress of the silo walls. This overstress first manifests itself in vertical cracking, then delaminated and spalled concrete, and in worst cases, catastrophic failure.Why are there so many homogenizing / blending silos with problems? Based on first hand observation, historical information, and discussions with those in the cement industry, the issue is one of miscommunication and misunderstanding of the principles. Personnel responsible for determining needed air pressures and geometry to homogenize the material did not properly communicate the process to the structural engineers responsible for designing the silos. Or, structural engineers did not fully understand material flow and differing material pressures when designing the silos. It should be noted that the concept of pressures of the same material changing based on its being at rest or active was not realized until the mid-1970s, with some inexperienced engineers still not being fully aware of its impact.This paper will discuss some of the different homogenizing / blending silo arrangements, with a discussion on historical trends, common issues, and options for repair.