Conveying Equipment for Extremely Hot Bulk Materials
Matthias Moritz, 1. April 2020
In the current situation in world steel, with overcapacity, turbulent markets and geopolitical conflicts, many producers have to focus on their short range outlook, solving today’s problems to get rapid results in the immediate. This often results in a preference for simple and traditional technical solutions. However, there could be alternatives, as something which looks good in the short term may bring disadvantages long-term. Two examples of technological solutions which could fall into this category are described, and they illustrate the benefits of tailor-made solutions.
In metallurgical plants there are a small number of vital areas where hot bulk materials have to be conveyed.
In the traditional blast furnace route there are basically three of these areas; where the hot coke comes out of the coke battery, the hot sinter from the sintering machine or hot Fe pellets in pelletizing plants.
In Direct Reduction Plants it is the charging of hot DRI from Direct Reduction facilities into the EAF, or in some processes, for example at Direct Reduction by using the fluidized bed process, inside the direct reduction plant from one process stage to the next.
In almost all traditional blast furnace facilities the plant has typically been in operation for several decades. When we visit such plants we often see situations where existing installations have reached the end of their useful life. In these cases the most common approach has been to replace the existing conveyor systems, but following exactly the same concept. This usually follows recommendations produced by conservative feasibility studies. However, with the technological advances of recent years, innovative ideas are gaining ground.
Adapting and optimizing existing machinery, rather than just replacing like for like, has in some plants led to significant increases in capacity, and large improvements over previous design concepts.
For example, the temperatures of sinter or pellets at the outlet of the cooler have been rising continuously over the years. The classical solution, used for decades, was to install rubber belt conveyors after the cooler. With the increased temperatures the service life of the rubber decreases and belts have to be changed more and more often. Our customers report that the cost of replacing the belts is only one aspect. A more important consideration is the downtime when production is hindered. On the one hand, there are personnel costs for dismantling and assembly, and on the other hand there is a great deal of effort involved in maintaining the necessary material flows. The sinter material for charging the blast furnaces has to be produced in advance and kept in stock, which creates another set of logistical challenges.
The rubber belt conveyor, as a classic conveyor unit, is comparatively inexpensive to purchase, but the operating costs are increasing considerably. Depending on established decision-making structures, the upgrade or retrofit has traditionally been attributed either to the CAPEX or to the OPEX budget, i.e. either to new investment or to maintenance and service. Increasingly, however, an overall view is being considered.
These days, steel plate conveyors are available which can transport very hot and abrasive sinter material without problems. In some plants this type of conveyor has been operating successfully for around 10 years, needing very little repair work during that time. Over the same period these conveyors and their components have been constantly improved and increasingly adapted to the requirements, with conveying capacities up to 1,500 tph currently in use.
Replacing existing belt conveyors with more modern metallic plate conveyors for the transportation of very hot bulk materials does not, however, involve just a simple exchange of machines. Dimensions such as height and width will vary due to differing conveying speeds, as belt conveyors operate at considerably higher velocities than metallic conveyors. Material will be layered at a greater height on a steel plate conveyor than on a belt conveyor. Plate widths are also greater than belt widths. Therefore a metallic plate conveyor will not necessarily fit into the space vacated by a belt conveyor without some modification to the surroundings, and the machinery manufacturer has to work closely with the plant project team so that a suitable installation situation for the metallic plate conveyor is created, by making adaptations at the existing location. This can often be achieved by modifying platforms, supporting bridges and steel structures, but in some cases the foundations also have to be adjusted. The solution is always tailor-made for a specific situation. There is never a “standard” situation, and the equipment is always customised. Once the solution has been found and the metallic plate conveyors installed, plant operators have consistently been pleased with the reliability and low running costs.
Another area for improvement in the conveying of hot sinter material was identified directly after the sinter machine. Here it had been common practice for some time to arrange one, two or more vibratory conveyors in a row. But in many countries requirements have changed considerably in recent years, in terms if noise protection and health and safety. Another challenge is to limit the wear of the troughs, which can be done to a certain extent by using improved wear-resistant materials in the fabrication. However this increases the operating costs. The replacement of worn troughs also requires downtime to cool down and dismantle the equipment.
It can also be viable to use a metallic plate conveyor at this location in the plant, even if this is not the classic solution. An additional and significant advantage is the great saving on energy. As well as that, the conditions for workers are improved by a considerably lower noise level. Downtimes and costs for maintenance and repair have been reduced in all cases where vibratory conveyors have been replaced by steel plate conveyors. The overlapping design of the metallic plates prevents the formation of the fine particles produced by vibration conveying, which in turn reduces the amount of fines to be returned, leading to increased productivity at lower costs.
As we have been designing and developing this innovative type of metallic plate conveyor and its components for more than 10 years now, it is a solution which is optimised for this application. Since no water is required to cool the sinter and the conveying is vibration-free, neither ageing nor chemical changes of the sinter are caused. Nevertheless, we are constantly improving the design based on practical experience gathered.
We find a similar scenario in pelletizing plants with regard to the comparative age and condition of the plants. When planning a pelletizing plant, dimensioning the cooler is a decisive criterion for the overall costs of the plant. Even if the cooling capacity is sufficiently calibrated at the time of commissioning, an increase in the temperature of the pellets at the outlet of the cooler can be achieved by continually optimising the plant and consequently increasing productivity. Some plants have even been able to double their projected capacity. Belt conveyors are also provided as standard after the cooler. These are belt conveyors which were originally designed for material temperatures up to 100°C but are installed in situations requiring them to operate with temperatures up to 600°C and more. Depending on the local situation, pellets are usually transported either to a storage area, to a logistics centre or to a port. Instead of a rubber belt conveyor, we recommend using a metallic plate conveyor from the outset in a new plant.
In our experience, there are parallel situations at many plants, and when replacing or modifying an existing belt conveyor, similar problems will be encountered to those in a sintering plant; the steel plate conveyor must fit into the space which was designed for a belt conveyor.
Some customers consider this conveyor to be the most critical part of the whole plant as it is the last place they want a bottleneck. Specific cases have been reported of belts having to be renewed every 40 days because they were burned by the hot pellets. This was then the bottleneck which made an increase in production impossible. Resources were permanently required for the belt changes, both in terms of the cost of the belts themselves, the personnel required and the time needed for the belt change operations.
Successful applications of metallic plate conveyors are described. The slightly higher costs compared to the rubber belt conveyors or vibratory conveyors are compensated by the reduction of downtimes.
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