An insight into the criteria for the design of the plants for raw materials handling, storage and homogenisation in the cement industry.
Raw materials used in cement plants have a great influence on the investment for a cement plant as the design and selection of the handling equipment is based on their physical and chemical properties. The important factor to note is that the cement process requires the raw materials to be prepared in a certain condition which could be in variance to the available materials.
For the cement process, the chemical composition of the raw materials is of importance, whereas the bulk handling facility designer is also interested to have the information on sieve analysis, repose angle, flow characteristic, abrasion, bulk density, moisture content and other relevant physical properties. The knowledge of variations in chemical and physical properties of the mined materials is critical to the optimally designed plant. It should also be recognised that facility to handle and blend the raw materials should cater round the clock under different climatic conditions. In a cement plant there are several raw materials, which require mechanical handling and storage (Fig 1). TenovaTAKRAF has engineered solutions to cement plants around the world. For instance, a cement plant´s most important raw material ´limestone´ may have vastly varying physical and chemical properties depending on the mine. In such cases the material demands to be blended together (often referred to as Homogenisation) over a time so that the variation of properties has minimal effect on production. It is imperative for the system designer to come up with tailor made solution for each and every application.
There are other bulk materials in a cement plant like coal, pet coke, bauxite, gypsum, iron ore, etc. The requirement of handling and storage of these materials is dependent on cement process requirements.
Handling and storage without homogenisation
In cases where homogenisation is of less importance, material storagemay consist of a simple stacking arrangement either by overhead travelling tripper or stacker on rails. The simplest storage system envisages coneshell type stacking, where smaller cones are automatically formed over the first fully formed cone (Fig 2).
Since blending is not a requirement, the scraper reclaimers operate on the principle of lateral reclaiming. The scraper arm makes a small cut on the sloping face of pile and the reclaimer travels over the length of the stockpile. This process can be fully automatic with active controls for depth of cut and travel speed to regulate the output from the equipment. The type of scraper can be either side scraper (cantilever), semi portal or full portal type.
These scraper reclaimers do provide a minor blending effect since the cones are being sliced uniformly during the travel. However, the larger benefit comes out of the flexibility of the equipment, since the travel of the machines in the store is not hindered by the presence of stockpile. This is useful for the cement plant when handling different materials from the same set of stacker and scraper reclaimer or when the process demands different grade of the same material (stocked as different piles).
Handling and storage with homogenisation
A good homogenisation is provided by the Chevron method (Fig 4) of stacking enabled by a fully automatic control system. The linear stacker moves over the length of the stockpile forming several layers (maybe hundreds), one over another, thus initiating the blending in the stacking process itself.The end cones with predetermined stacking procedure give correction to the maximum extent possible. To reduce the effect of end cones on the product, it is prescribed to have a pile configuration having length to width of minimum 4:1. The end cone effect can be completely eliminated in case of continuous stockpile in the circular pre-blending system.
A good stacking procedure is a job half done for the purpose of homogenisation. To ensure that the product coming out of the stockpile is fully blended, TAKRAF recommends their dependable workhorse, the bridge type scraper reclaimer. TAKRAF bridge scraper has a large harrow covers almost the entire width of the stockpile. The bridge moving into the pile and harrow travelling across gives a true representation of the cross section. Being large sized, the harrow also needs a short traverse across the pile by means of a hydraulic cylinder, which guarantees minimum wear and maintenance.
Storages can be arranged in both longitudinal and circular form. The homogenization process is similar in both trapezoidal and circular piles. However, the end-cone effect is eliminated in case of circular stocking method, thus ensuring uniform material properties in the reclaimed material stream. Circular storages being compactly arranged, need lower capital investment in cover (dome). The Case-2 and Case-3 below show typical example of longitudinal and circular systems.