Stackers and reclaimers play a large part in achieving the reliability that helps deliver a constant supply of homogenised raw materials to the mills. The storage solution must cope with quarry operations and logistics, as well as different raw material parameters. To ensure the reliability of stackers and reclaimers and to optimise their total costs, accurate raw materials characterisation is essential.
At Holcim Brazil's Barroso plant, the clay excavator was installed in the exiting storages’rails, allowing free access during installation and a very quick changeover with minimum production downtime
Equipment reliability is largely the result of two factors: the equipment selected to handle the materials and the design of the project to integrate the equipment into the plant. When designing the optimal raw material equipment set-up, raw material specifications provide the key to achieving maximum reliability.
Raw materials characterisation
Raw materials can be characterised according to four basic parameters: feed and product size, abrasiveness, compressive strength and stickiness. Understanding these parameters will allow the supplier or design institute to make the right decisions to achieve maximum reliability, for example, regarding equipment size and type, storage size, power calculations and belt conveyor design rules.
It requires in-depth analysis and key know-how to characterise raw materials accurately. Accurate results are most likely to be achieved by extensive laboratory testing by experienced personnel who have conducted hundreds, if not thousands, of tests. Relevant tests include angle of repose, wear tests, shear tests and point loads. It is not only personal experience but also extensive data accumulated over time that helps to accurately predict outcomes at any given quarry.
It is useful to categorise raw materials into different types based on their flow properties, for example free-flowing, moderately sticky, sticky and extremely sticky. Therefore the material’s stickiness is a critical parameter, which is mainly a factor of material type, particle size distribution and moisture content.
In addition, the prevalent local climate can have a profound influence on this indicator. Therefore, cement plant operators find stickiness particularly challenging, especially in regions that appear to be exposed to changing rainfall patterns.
There are two types of stickiness: cohesion and adhesion stickiness.
Cohesion stickiness is the tendency or property of similar particles to stick together. This is a fixed material property that can only be changed by modifying the raw material.
This type of stickiness has an impact on power calculations and loads where the materials are sheared, such as in crushing, reclaiming and feeding. It also affects the material flow and extraction behaviour in storages.
Adhesion stickiness refers to the tendency or property of dissimilar particles to stick together. This property can be influenced by equipment design and material type.
Adhesion has an impact on the material used in the equipment as well as the equipment’s size and design.
Why is stickiness so important?
Stickiness has a major impact on the flowability of raw materials, which influences the stacker and reclaimer options as well as other aspects of materials handling. As it becomes stickier, flowability is reduced, which can often cause a build-up of raw materials leading to blockages. Blockages cause stoppages in the system, which will of course decrease the production efficiency.
Managing the issue
Stickiness can be managed in different ways. First, it can be managed in the quarry management process through draining, so that the raw material becomes less sticky before it enters the production system. This can contribute significantly to solving the issue. Second, the plant can be designed to manage the material optimally, which highlights the importance of understanding the raw materials’ characterisation.
Planning in this way will not only reduce the likelihood of downtime, but also reduce the need for manual intervention. Many cement plants are faced with having to manually clear blockages, which is a highly labour-intensive way of managing the problem. Reducing the need for people to clear blockages improves safety and reduces labour costs.
Reducing civil costs
Civil costs are clearly an important selection factor as they are a major part of the total installed cost. In fact, the civil investment can be as much as three times that of the equipment. Here it is necessary to calculate total costs accurately, using advanced calculation tools.
Several factors are at play. Costs depend largely on the building quantities and design parameters. The size of the equipment affects the building dimensions, so adjusting the equipment to make best use of the space available will lower civil costs, as will optimising the pile cross-sections, which is another reason that intimate knowledge of the raw material is so important.
When Holcim Brazil decided to build new production lines at its Barroso works, plant management realised that the site’s existing stacker and reclaimer system would not provide the necessary raw materials capacity. A new system was needed that would support the expanded capacity of the plant.
The knowledge of the raw materials to be handled at the works proved key to making the correct decision in terms of the stacker and reclaimer system required. Faced with a clay with a high stickiness factor, the 100tph clay bucket excavator supplied by FLSmidth was the only possible option for Barroso’s clay storage needs. An additional benefit was that the existing storage and rails could be used, significantly reducing civil costs. Therefore the clay excavator was manufactured to fit the existing storage by reducing the height of the standard equipment.
In terms the dry limestone used in the production process, Barroso needed to expand its limestone storage to accommodate the new plant line. The new 900tph stacker and bridge scraper reclaimer was also designed to fit the existing rails. This allowed free access during installation and a very quick changeover with minimum production downtime.
During installation Barroso was assisted by a team of supervisors from FLSmidth. The entire system was commissioned within two weeks, with handover certifications finalised in November 2015. Since then Barroso has started up its second new production line, which is now also fed by the new stacker and reclaimer system.
In both Holcim Brazil and PT Semen Indonesia’s case, raw material knowledge has allowed the cement producers to make significant cost savings and improve operational efficiency.
Semen Gresik boosts blended cement capacity with new raw material storage
Due to the characteristics of the raw materials a bucket chain excavator storage was chosen at PT Semen Gresik’s Tuban plant
As the demand for blended cement increased in Indonesia, PT Semen Gresik’s Tuban plant needed a new stacker and reclaimer system as part of its efforts to increase capacity. Trass is one of the most readily available raw materials in the region, so Semen Gresik decided to incorporate more trass into its cement production. This required changes to the plant’s production process and its equipment would need to handle greater amounts of additives.
Trass has a high degree of stickiness and moisture content.
Achmed Vaival Istiadi, general manager at Semen Gresik, says that a closed storage system was chosen mainly due to these raw material characteristics. The solution was a bucket chain excavator storage from FLSmidth. In addition to allowing windrow stacking, it could deliver 720tph of trass to the five cement mill feed bins, ensuring a constant feed to the production line. It had the added advantage of minimising the fuel consumption of heavy equipment.
With commissioning of Semen Gresik’s new stacker and reclaimer system completed in 2014, Achmed Vaival Istiadi gave a highly satisfied comment on the system’s performance:
"The system has reached its design capacity and we have not experienced any major issues. It helps us to ensure the stability of the trass feed to our cement mill, while also contributing to reducing the total moisture of the trass, because it has been stored in closed storage to anticipate the Indonesian rainy season". - Achmed Vaival Istiadi, General Manager at Semen Gresik
A unique element of the Semen Gresik solution was the introduction of the ‘floating pile principle’, as illustrated here. This involves stacking in predetermined segments in front of and behind the reclaimer. The result is one large pile, which increases the storage capacity and reduces the number of pile shifts required by the reclaimer.
*This article was first published in International Cement Review, August 2016.