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Blending Silos: Advanced Technology for New and Existing Installations

February 27, 2012
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Abstract
The cement industry is part of an ever-changing industry where new technologies are constantly being introduced. The past 50 years have produced great innovations in this industry. The driving forces behind these changes are environment and efficiency requirements. This paper will focus on the efficiency within the area of raw material blending. Today’s cement market is highly competitive, more so than ever before, requiring proficient system operation.

Cement plant management has placed a priority on system modifications designed to increase production at a lower cost. One of the areas affected is the raw meal blending system. It is essential today to reduce operating costs, while maintaining the highest raw material quality. Most modifications in this area center around the reduction of consumed power. It is a fact that this area of cement production is pretty much ignored with regard to power consumption and maintenance. However, this area provides the means for the greatest savings in production cost.

Fluidized blending of raw material was considered the acceptable method for many years. When power costs increased in the 1970s, alternate methods of blending became necessary. Gravity blending of raw materials coupled with on-line raw mix control has become the answer to reduced blending power consumption. Even the modernization of existing fluidized blending systems shows a great deal of improvement in operating efficiencies.

Utilizing gravity blending innovations in technology for plant modifications can result in a power savings of up to 10 times the original system design. Results obtained through these modifications can also reduce fuel consumption in kiln operation due to a more uniform kiln feed and a reduced air flow.
Conversion of existing fluidized blend silos to a gravity method provides the opportunity for existing cement production plants to reduce operating costs while maintaining or increasing the efficiency of the blending system.
This article will provide a historical look at the blending process in the Cement Industry and discuss the new techniques available to improve existing system operation.

Introduction
The types of modifications that shall be discussed involve changing present homogenization methods. Understanding the philosophy of current plant operation will allow for a look into new innovations that can be adapted to present systems.

The blending and kiln feed systems work together forming a major part in the cement production process. New plant production lines along with modifications to existing systems can take advantage of new cost-effective methods for material blending.

Gravity blending of cement raw meal combined with the latest on-line computer chemistry correction technology allows for improvement of existing systems and the streamlining of new production lines. A favorable return on investment will clearly show that conversions are economically sound and competitively necessary.

Blending Alternatives
Disregarding the slurry type blending process used in wet process plants, only three types of homogenizing systems exist:

1. Mechanical Systems
The mechanical type application consists of multiple raw meal storage silos. Each silo is equipped with a regulated withdrawal system. Blending is achieved by systematic withdrawal of material at variable rates from all silos. This type of system, while low in power consumption for homogenization, requires a great deal of material handling which increases power consumption. Also, the number of storage silos required consumes a great deal of real estate.

2. Air Fluidized Systems
From 1950 through 1980 the most common homogenization system was the fluidized method. Air introduced through a permeable media on the silo floor causes the cement raw meal to react as a liquid. Agitation of each individual material particle provides the highest blending efficiency capability. While being the most efficient system, the fluidized method is the highest consumer of power. Agitation of the material is accomplished by varying the velocity of the air flow through the material bed. Figure 1 illustrates the velocity profile within a fluidized blend silo. The blender bottom is divided into segmented areas. The velocity of air directed into the selected blending section is greater than that of the aeration air, thus creating an extremely active, light-density column of material. The less dense material over a blending column continuously spills onto the more dense material over the aeration sections, thus creating active internal circulation and mixing of the material.

3. Gravity Type Systems
The gravity approach to homogenization was conceived out of a need for reduced power consumption in the late 1970s to early 1980s. This application is similar to the mechanical method for mixing raw materials. While the mechanical system utilizes multiple silos, the gravity method can achieve the same blending with as few as one single silo. Multiple discharge points within one silo, acting similar to multiple silos, operate on a timed cycle, withdrawing raw material at controlled variable rates. Chemical variations in the raw feed are in the form of material layers within the silo. As the material is discharged, these layers decay forming a funnel type pattern at the discharge point. The decaying layers create a shearing effect between each layer which enhances mixing as the material moves through the silo. Varying velocities of the material as it discharges creates the desired blending effect. Figure 2 shows a combination of gravity and fluidization to achieve the required blending effect.
Power consumption of a blending system is relatively low compared to the milling systems within the plant. However, savings in energy consumption in the blending area has some of the greatest impact on production costs. Table 1 shows the wide range in power usage in relation to raw meal production.

Blending Power Consumption
The mechanical system utilizes the most power. Most plants with this type of system use a complex matrix of screw conveyors for material transfer. It is not unusual to find as many as six to eight storage silos involved in this type of homogenization system. While this type of system provides a fair blending efficiency, it is the most labor intensive as well as the highest consumer of power.

The fluidized blender, while being the system with the greatest blending potential, is also a high consumer of power. The major advantage is the reduced amount of material handling equipment and the need for fewer silos.

Gravity type blending systems have the greatest potential for power savings. Because of this potential, the gravity system is the most popular application today. While this has the best potential for power savings, it has drawbacks. Blending efficiency potential is reduced as compared with fluidized blending methods. However, this is manageable with improved methods of raw mix control. The second drawback is in relation to multiple type of cement production. Large single gravity flow silos make it difficult to make fast changes in alternative types of cement.

Table No. 1 indicates the two types of gravity systems. The inverted cone silo refers to the type utilizing a homogenizing chamber within the central cone. The fluidization required beneath the central cone increases the power consumption required for this option. The multi-outlet silo refers to those silos with multiple discharge points located about the silo floor. Figure 3 shows an illustration of the multiple-outlet type silo. This silo uses seven discharge points equally spaced about the silo floor. Each of the discharge points is divided into six fluidized zones giving the effect of 42 discharging points. By sequencing fluidization air on a predetermined pattern, material is withdrawn from three zones simultaneously. This procedure results in a high degree of blending. This approach also results in the lowest power consumption because of the selected method of fluidization.

Silo Conversions
The time for silo conversions is now. With the industry emphasis on production cost reduction, ways are being investigated to accomplish this task. The majority of older blending systems in operation are of the fluidized type. Conversion of these silos to a gravity type system will result in power savings of up to 90 percent.

Conversions to existing fluidized blenders can be accomplished with little change to the silo structure. Feed and discharge equipment modifications can be minimal. Those silos equipped with a bottom discharge can be utilized as they exist. A silo operated as an overflow type silo requires a change to bottom discharge.

The method of withdrawal of a converted fluidized type silo is similar to the multiple outlet silo discussed above. Instead of creating additional openings in the existing silo floor slab, we utilize a collecting Airslide in each of six discharge zones. Each discharge zone has six pickup points where material is transferred to the discharge outlet. This approach has 36 effective blending points which provide the same blending results as seen with the multi-outlet silo described above. Figure 4 shows one typical discharge zone.

Selective aeration is the means by which material is withdrawn and mixed in a converted silo. Three regions, Figure 5, each with a dedicated air supply, combine to provide a timed discharge pattern.
When an individual aeration zone is activated, aeration air causes material in that zone to flow into its dedicated opening in the closed type gathering Airslide. The gathering Airslide conveyor transports the material to the discharge of the silo. When the zone is de-activated, there is no air flow to the Airslide plenums and the material flow stops. The varying time sequences promote a differential velocity of material as it is withdrawn. This movement produces mixing of the various layers of material, Figure 6, as it is withdrawn. This movement produces mixing of the various layers of material within the silo. The sequence of aeration is designed so that it does not repeat the pattern for nearly 13 days, which is attractive for systems with a long-term deviation in feed chemistry. Each region contributes 1/3 of the total discharge rate.

Blender dust collection equipment operated in conjunction with a converted silo has a greater efficiency. The reduced volume of air flow lowers wear and tear on the existing collectors. This reduces maintenance time and required spare parts.

Conclusion
History shows that the modern cement plant is a constantly evolving operation. Improvements are constantly required to reduce operating costs. We have seen the implementation of computerized quarry and raw mill feed control which results in a better control of raw feed quality. This improved quality allows the plant operator to maintain a homogenization system that required less blending efficiency. The gravity system is ideally suited for this type operation. However, we still need to achieve the best blending capability to handle those upset conditions that cannot be predicted.

This article describes the evolution of raw meal blending systems from the wet process to the modern dry operation. Gravity systems are the way of the future providing efficient operation while operating at very low power consumption. It is possible for your system to be operated more efficiently using this latest technology.

Les Bartholomew is the technical application manager for pneumatic transport systems at FLSmidth Inc. He has been with the FLSmidth group for 39 years providing material handling systems including blending and storage. A graduate from Penn State University in mechanical engineering, Bartholomew holds a patent for material blending and a patent for cement storage. For more information on FLSmidth, call 610-264-6011 or visit www.flsmidth.com.

FLSmidth