In this paper Aumund, recent winner of the Global Slag Equipment Innovation Award 2007, showcases its broad range of GBFS and GGBFS handling, conveying and storage options.
During recent years the cement industry has suffered significant increases in operating costs driven in particular by spiralling energy prices plus pressure from environmental lobbies to reduce dependence on fossil fuels and reduce CO2 emissions overall. Considering that the production of a ton of conventional Portland cement generates almost a ton of CO2, and in terms of total greenhouse gas emissions for all industries worldwide, this places cement second only to the power industry in the scale of global polluters.
These factors have generated an increased awareness of alternative fuel possibilities for kiln firing and substitute raw material options for cement production, also reflecting an increased demand for blended cements particularly including ground granulated blast furnace slag (GGBFS). This last point is particularly important; for every ton of GGBFS included in the final blended cement the total CO2 production is reduced by around 1t.
Whilst the processes behind the use of GGBFS as a substitute raw material is well understood, the application of the material represents a considerable challenge for the plant designer from the intake of the granulated slag to the introduction of the ground material to the blended cement. In general these materials are difficult to handle, reliably varying from extremely free flowing (GGBFS) to very sluggish and prone to bridging and blockage and liable to agglomerate in storage (GBFS). GBFS is also extremely abrasive, with wear rates some five times greater than cement clinker in comparable installations.
The Aumund Group, including B&W and Schade, recognises these challenges. Over many years it has developed innovative solutions for many of these handling problems, including using special adaptations of existing designs that are tailored to suit the specific demands of these new applications.
Combining the traditional strengths of the Aumund Group products with new concepts to improve plant design flexibility and reduce project costs, Aumund offers fast-track effective solutions allowing clients to capitalise on short term market positions and maximise plant profitability.
Production
Granulated blast furnace slag is a byproduct of the steel industry and is produced by rapid quenching in cold water of the molten slag recovered from the blast furnace; this process produces a granular, glassy, homogeneous, noncrystalline material that has cementitious properties.
Depending upon the process, around 25% or more of the raw material used in the blast furnace is tapped off as slag. This represents a vast quantity of material produced worldwide and the market is effectively limited only by the availability of granulation equipment at the steel plant and the logistics of transportation to the cement plant.
The granular material can then be dried and ground in a mill at a prescribed rate. The resulting powder, known as ground granulated blast furnace slag may be incorporated into concrete materials. GGBFS has been used for many years as a supplementary cementitious material in Portland cement concrete, either as a mineral admixture or as a component of blended cement.GGBFS may replace 35–85% Portland cement in concrete with corresponding reductions in CO2 emissions resulting in obvious environmental benefits.
The use of GGBFS as a partial Portland cement replacement takes advantage of the energy invested in the slag-making process and its corresponding benefits with respect to the enhanced cementitious properties of the slag. Grinding slag for cement replacement requires only about 25% of the energy needed to manufacture normal Portland cement.
Without the use of granulated material the total volume of slag produced could not be viably or economically utilised in other processes, such as aggregate for example, and would inevitably go to landfill as in the past. Therefore by maximising the appropriate use of GGBFS in cement, the industry is not only significantly reducing greenhouse gas production and energy consumption but also reducing landfill. This constitutes a significant reduction in total environmental pollution levels, in line with the general objectives of the Cement Sustainability Initiative.
Market conditions
In today's rapidly changing international market the ability to react quickly to market volatility and take advantage of short-term market positions is essential if operators are to maximise profitability and return on capital invested. In this respect the use of substitute raw materials such as GBFS should be considered as part of an overall plan to reduce costs and improve the environment. Often these are mutually exclusive demands but in the cement industry the combination of substitute raw materials and alternative fuels benefits all and is truly a win-win situation.
The use of these materials is also very much dependent upon geography. In certain regions the cost of fuel and access to raw materials, as well as the demand for particular properties of the finish blended cement may significantly affect the economic mix. This situation demands a flexible and fast-trackable new plant solution. Similarly, other materials such as flue gas desulphurised gypsum (FGD), often known as synthetic gypsum, is widely used in cement and other industries. As such, pricing is market driven and the relative economies of use vary accordingly.
Where all of these conditions are factored into the overall equation, market volatility may dramatically change the relative economy of specific solutions in a short time. Under these conditions the cement manufacturer has a difficult calculation to make when evaluating the payback period of any new plant investments dedicated to the handling of alternative fuels or substitute materials. As such reduced installation cost and flexibility are key issues when making these decisions. Similarly, the lead times to commissioning new plants once these decisions are resolved is critical in order to maximise the financial benefits within a predicted cost and time-frame.
In this paper the handling requirements of granulated and ground materials in the cement industry will be discussed, but similar solutions are also applicable for independent grinding stations or grinding facilities installed directly at the steel production plant.
Key plant design issues
In addition to the suitability of the new plant for the intended application, there are other issues that can influence decision making. To maximise the financial benefits relating to substitute raw materials or alternative fuels, the plant design criteria and budget constraints are clearly substantially different to what would be considered for a mainline process demand. In addition, flexibility in location and suitability for other materials and applications is an attractive combination. Should the relative economies of the intended project change significantly it is always useful if the plant may be reused elsewhere and therefore the investment risk mitigated.Clearly civil works costs are also an important factor in any new plant and are not only expensive but obviously not re-usable and certainly incur delays in construction where extensive design, planning and excavation may be required.
With few exceptions the costs associated with rail car delivery of these alternative materials is prohibitive, meaning that delivery by road is generally the only option. Intake by a tipping truck is always sensitive to fugitive dust pollution and necessary control measures are generally required to minimise any environmental impact. A successful and economic new plant installation must address these key issues if the financial rewards, environmental benefits and process demands are to be realised.
Aumund Group experience
With over 80 years experience in designing specialised bulk materials handling solutions for the steel and cement industries, the Aumund Group offers mechanical conveying systems for GBFS both in its raw state and after grinding. This experience extends to the handling of very hot materials in cement, sinter and pellet plants using the long established Aumund pan conveyor concept, as well as the use of highly developed designs specifically tailored for these aggressive and demanding applications.
As an example of the depth of engineering expertise available, Aumund has executed the largest bucket conveyor installation delivered thus far for handling hot compacted iron with a lift height of 110m (150m centre distance) at a handling rate of 210t/h. These conveyors are designed to handle material at 900°C in an inert atmosphere to maintain product quality.
Within the cement industry the name of Aumund is synonymous with quality and reliability, with over 10,000 installations worldwide in over 100 countries. Aumund clinker transports and vertical bucket elevators are market leaders in this industry with clear technological benefits derived from continuous detailed development.
Developed from the pan conveyor concept, Aumund has delivered special cooling conveyors for pellet plants incorporating perforated pans using ambient air drawn through the pans and exhausted through comprehensive ducting systems to cool the pellets down from 250°C to 100°C. In addition to the pan conveyors and vertical elevators Aumund also delivers heavy duty feeder equipment and silo extractors, plus, with the addition of the Schade Group in 2001, a comprehensive range of stackers and chain scraper reclaimers bringing 125 years of continuous experience in this market. In 2002 also Aumund acquired B&W Mechanical Handling Ltd of England, adding the Samson Surface Feeder and more flexible solutions based on mobile stacking and ship loading equipment to an already comprehensive portfolio.
The experience gained in continuous process applications operating under the most arduous working conditions has been brought to bear on the detailed design of equipment for the handling of GBFS, as will be discussed herein.
From the standard Aumund range there exist a range of solutions from feeders for the intake of the raw or granulated slag through to stacking and storage, recovery from storage and feeding of the material to the mill and the raising of the finished ground material to the final silo storage.
Tailored solution driven from experience:
Intake and elevation
The B&W range of Samson Surface Feeders offer the ideal solution for the intake of both raw and granulated slag. The Samson feeder receives raw slag direct from the steel works for processing at an offsite crushing plant. The Samson discharges directly into a belt conveyor mounted at 90° allowing an ideal transfer onto the belt, minimising the risk of blockage as there is a high risk of oversize and tramp materials. From the belt conveyor the slag is first screened to remove the tramp and then crushed down to a particle size suitable for use as aggregate in road building.
Slag samples vary significantly in their properties. In order to maximise the output of the system under any conditions the Samson output is automatically adjusted relative to the actual handling rate (measured by belt weigher) and crusher current demand. The same Samson Surface Feeder design may also be utilised for the intake of granulated glag at a cement works or grinding plant, either directly from tipping trucks or from a loading shovel.
The new installation at the Holcim Carboneras plant in Spain receives imported granulated slag from tipping trucks which discharge to a ground stockpile. From the stockpile the material is recovered by loading shovel using the Samson as a buffer hopper with feed regulation (by levelling blade) to the inclined belt conveyor which raises the slag to the mill bunker at a high level.
Since the granulated slag is liable to agglomerate in a ground stockpile creating large hard lumps, the Samson must be fitted with a reject grill either at the entry or at the discharge to prevent these large lumps entering the mill bunker causing potentially catastrophic damage to the mill.
In some plants there simply is not sufficient space to store large volumes of material and therefore the intake of granulated slag must be managed using road tipping trucks. This acts as a suitable alternative to the ground stockpile solution. In the road tipping scenario the Samson surface feeder is the ideal answer providing an intake, buffer storage and feed rate control in a single machine, whilst at the same time eliminating the need for deep pits and hoppers, thereby reducing civil works costs and improving flexibility.
To overcome the problem of oversized agglomerated lumps a Samson with integral twin shaft rotary sizer can be mounted at the discharge. The rotary sizer is compact and may be accommodated between the Samson and the following conveyor. From the following conveyor the material may be raised to the mill silo using an Aumund BWZ central chain bucket elevator as shall be describe later in this paper.
This particular plant layout is illustrated in the following description of the Lafarge cement plant at Villaluenga in Spain. In this particular case the equipment is handling gypsum rock but the same basic equipment may be used to size agglomerated granulated slag lumps. Clearly this represents a considerably more arduous duty showing the local material stockpile. In addition to the integral rotary sizer other options are available to reject the oversize and tramp material after the Samson.
A Samson surface feeder can also be supplied with an inclined vibratory screen at the discharge arranged to allow only the material passing the screen to be transferred to the mill bunkers. When the slag is recovered from a ground stockpile by shovel there is a strong possiblity of tramp material being picked up and loaded to the feeder. To overcome this problem a magnetic separator plus metal detection systems is normally mandatory, mounted to the following belt conveyor system.
Belt conveying options
The combination of Samson and sizer is not new and naturally there are alternative solutions and as an example the Samson may be paired with a 'side wall belt' vertical conveyor system as used in the new installation for Holcim Germany handling granulated slag.
In this project the Samson receives the granulated slag direct from tipping trucks in the usual way with the output regulated using the standard levelling blade system but the material is discharged direct to the horizontal loading section of the 'S' shaped side wall belt conveyor.
As shown in the figure on the left, a typical exposed side wall belt of the vertical section supported only from the head. However, these conveyors are notoriously difficult to clean, but the spillage returns conveyor is designed to pick up any spilt material and transfer this to the main conveyor feed boot.
As an alternative the Aumund Group offers the type BWZ central chain bucket elevator. This offers a totally enclosed and spillage-free handling system, raising the granulated slag direct to storage silos. The vertical bucket elevator may be combined with any type of transfer or feeder conveyor system. When integrated to the Samson surface feeder the whole installation may be made compact and clean.
The Aumund chain bucket elevator is the ideal solution for handling GBFS to silo storage, suitable for lift heights from 10-60m and handling rates of more than 600t/h. In other applications where required material temperatures up to 400°C may also be accommodated. Utilising a single central forged chain these machines are ideal for handling aggressive materials. However, for slag handling special features are included to ensure long term reliability. These measures include:
- Rubber lined head and boot;
- Flexible rubber casing panels;
- Fully lined buckets;
- Toothed buckets (at 10th pitch);
- Additional inspection access;
- Seal pre-lubricated chains.
The features are all geared to resist wear and provided extended machine life with minimum maintenance and minimum downtime. Developed from our extensive experience in the cement industry, handling hot cement clinker, these elevators offer proven reliability worldwide.
Rubber linings are the proven solution at the inlet and outlet of the machine, but for the vertical casing the conventional pressed steel sheet sections are replaced by a fabricated lattice structure with flexible rubber panels which resist wear and material adhesion. Each bucket is fully lined in wear-resisting steel plates applied to every plane of the bucket structure. In addition, on each 10th bucket, digging teeth are applied which create a clear path through any residual material to minimise wear on the conveying strand as a whole.
However, the major feature of these elevators is the massive forged conveyor chain with sealed and prelubricated pins which forms the backbone of the unit and is the key to long term reliability. The labyrinth seals formed into chain-side plates effectively prevent material ingress to the pin and bush and, when combined with effective pre-lubrication, extends the chain life by a massive 60% compared to the traditional unsealed design without prelubrication. The chains are not relubricated in service and require no special routine maintenance but the choice of preinstalled lubricant is critical if the benefit of this design is to be fully realised.
The chart below demonstrates the improvement in wear performance achievable with this design, effectively more than doubling the actual chain life. The blue line represents a standard chain without a seal and without pre-lubrication; the red line represents a sealed chain with premium grade lubrication. The intermediate lines represent results obtained with other grades of lubricant.
When in operation minute traces of the conveyed material pass the seal and combine with the lubricant to form a mild grinding medium which polishes the pin/bush interface. It is this high level of surface finish that improves the long term wear life. This is the advantage of an effective research and development facility, a clear and demonstrable benefit of the technological know-how of Aumund Group's product where attention to detail is rigorously pursued with the goal of long term performance and reliability for the benefit of its clients.
Silo discharge
Having raised the granulated slag to the storage silo or mill bunker, the stored material must be extracted and perhaps weighed for dosing. For the dosing of slag the Aumund weigh feeder combines accuracy with performance and reliability. It utilises a weigh rail mounted to load cells supporting the close pitch apron, or pan conveyor rollers to accurately measure the applied load. By measuring the material load over a known length, and by monitoring the chain speed, the desired output rate may be continuously adjusted in real time to suit the process demands.
Based on the proven deep drawn pan conveyor principle, these weigh feeders offer long term reliability while working under the most difficult conditions.
For handling cohesive materials the BPB-type arched plate conveyor is the preferred solution, and is also available with weigh rails for proportional dosing or controlled rate feeding applications. With truly massive construction these feeders are suitable for the most extreme duty applications.
For extracting cohesive materials from silo storage the Aumund Centrex® provides an effective solution eliminating bridging and blockage by recovering material from the silo periphery using logarithmically shaped arms mounted to a rotating carrier. The Centrex® is offered with a range of mounting options including the possibility of a rotating cone to ensure reliable discharge and first in-first out material flow. Since the discharge arm sweeps right out to the periphery of the silo, silo discharge is not impeded resulting in reliable extraction.
For larger silos the 'rotating discharge machine' is also an option comprising a multi-arm discharge wheel mounted to a rotating carrier. This arrangement caters for silo diameter up to 12m, enabling the handling of a wide range of cohesive materials. This has found a wide set of applications handling FGD gypsum, both in power plants and in cement plant as this type of gypsum may be utilised as a direct replacement for natural gypsum required to modify the cement properties at the final grinding stage.
Stacking and reclaim
For larger projects where substantial volumes of granulated slag are to be handled the Schade range of rail-mounted boom stackers and chain scraper-type portal reclaimers are an ideal solution. It receives the slag by a belt conveyor discharged by a tripper car to the travelling stacker boom at a handling rate of 600t/h to generate a stockpile capacity of 35,000m3.
Reclaim from the stockpile is achieved by a Schade portal frame reclaimer fitted with a special scraper chain. The material is fed to a belt conveyer running parallel to the stockpile at a rate of 300t/h to match the process requirements. Stockpiling and reclaim are fully automatic using onboard control systems with extensive field and instrumentation and protection systems allowing unattended operation.
Recognising the potential wear problems handling this extremely abrasive material, Schade has developed the outboard bearing chain specifically for operation under these arduous conditions. The outboard roller is carried on an elongated chain pin using double precision ball bearings plus a multipath labyrinth grease seal. The roller may be lubricated for life requiring no further attention in service. Outboard roller chains also significantly reduced the machine noise level and are often used in applications located in sensitive areas close to urban developments. In addition to the special outboard bearing rollers special scraper shovels are also employed with wear-resisting digging face.
Schade offers an extensive range of reclaimer designs and has over 50 years experience in this market. Schade was the first company to deploy the chain scraper technology in this manner. Schade has produced some of the world's largest installations of this type with reclaim rates of 2400t/h and rail spans of 65m. In addition to the longitudinal portal reclaimer configuration Schade also offers the circular storage system comprising integrated stacker and reclaim boom mounted to a common column.
The Schade storage and reclaim systems may be supplied complete with rail or road intake equipment based on the Samson surface feeder or type BPB plate feeder as a complete handling package. Similarly the Schade storage system may be linked to the cement plant using, for example, Aumund bucket elevators and silo storage and extraction equipment and as such a complete package from intake to the mill.
In addition to the specific example illustrated herein Schade offers many alternative designs such as the cantilevered boom as described below. The semi-portal design involves one side of the portal being supported on a raised building wall, which also acts as load-bearing containment to increase the effective storage capacity per metre of building length.
For the homogeneous blending of materials such as limestone at a cement plant, Schade also offers the bridge reclaimer design with a reciprocating harrow ensuring material is recovered from the full face of the stockpile and thereby mixing samples for every layer. Bridge reclaim systems may be either longitudinal replacing the portal design or may be based on a circular format. Schade produces some of the largest circular storage systems ever built. At 120m diameter these designs represent the pinnacle of the market and demonstrate the capability and engineering prowess of Schade in this market.
Raising the ground slag
The intake and storage of granulated slag as it is received from the steel works is not the full story. After the slag is ground, typically in a vertical mill, the finished material generally known as GGBFS must be raised to silo storage before blending or direct dispatch. The handling characteristics of the ground slag are very similar to that of finished cement and in this situation Aumund is able to use the BWG-type belt bucket elevators. Units are installed to over 140m lift height and conveying rates reach 1500t/h, with up to 450kW installed motor power.
As with the chain elevator, the belt elevator has been the subject of continuous development over many years culminating in a design of proven reliability on a 24/7 operational basis. Attention to detail is the key to success and every aspect of the design is the subject of ongoing development to maximise the benefit of technological advances in materials and construction, typified by the unique encapsulated clamping connection.
The Aumund belt bucket elevator uses a steel cord conveyor belt with close pitch buckets to reduce the individual bucket size and therefore minimise the stress at the bucket fixing. By minimising the bucket size the belt stress is also reduced at the terminal pulleys resulting in much improved belt life.
A typical installation at an independent grinding plant with adjacent storage silos and truck loading facility includes a bucket elevator, mounted within the tower structure next to the silo. This raises the GGFBS from the grinding station. To convey the material to the elevator and from the elevator to the storage silos, air slide conveyors are generally employed in a similar manner to that seen in a cement plant. Naturally there are alternative solutions to raise the slag generally based on pneumatic handling but this is extremely energy intensive and therefore expensive to operate.
Typically in a direct comparison the belt bucket elevator has demonstrated a reduction of 75% in energy consumption, as shown in Graph 1. Not only is the bucket elevator energy efficient it is also less prone to wear in comparison to pneumatic handling and offers excellent long term reliability.
In the UK market the slag is typically ground by an independent company and supplied to the concrete manufacture, for final blending in the concrete mix. There are several such plants located close to the steel works to minimise the logistics costs associated with the granulated slag delivery which is done invariably by tipping truck.
The ground slag is invariably delivered to the user by tanker truck using pneumatic discharge equipment which will generally 'blow' the ground slag directly to a local storage silo before blending to the concrete mix. For blended cements the slag grinding operation is generally located at the cement plant and the ground slag is blended directly with Portland cement to obtain the desired characteristics and cement specification.
Generally the integration of the slag grinding to the cement plant for the production of blended cements is the most popular solution internationally, and eliminates the need for separate independent plants with a clear economy of integration to an established operation. Clearly logistics costs are important along with the expanded CO2 footprint that such transportation operations bring to the total pollution envelope of the cement manufacturer as a whole. This issue must be considered alongside other sustainability issues. The location of such integrated plants must be market-driven but respecting the environmental cost of transportation.
Flexible mobile solutions
Where budget constraints will not permit the installation of a dedicated fixed handling system the mobile solutions offered by B&W represent an economical answer to the storage and even export of slag. The Stormajor receives granulated slag direct from the Corus steel works in South Wales (UK) delivered by tipping truck for a new grinding plant operated by Civil and Marine Slag Cement. The Stormajor comprises a Samson Surface Feeder to receive the material and a radial stacking boom to discharge direct to stockpile, all mounted to a common mobile chassis.
The machine, with a boom length of 23m, will produce a stockpile height up to 12m allowing a large quantity of material to be stored with the minimum of machine movements. First delivered in 1998, this particular machine has handed many thousands of tons of slag and has proven an effective solution. The Stormajor may also be effectively used for loading barges and railcars and as such represents a flexible investment.
In addition to mobile stackers B&W also offers mobile ship loading equipment incorporating the Samson surface feeder for direct loading from truck to ship. The use of mobile handling solutions eliminates costly civil works and provides maximum flexibility, allowing operators to take full advantage of short-term market conditions, therefore maximising financial resources and minimising the investment risk without sacrificing performance or reliability.
Retrofits and upgrades
In addition to the design and execution of new plant and installations Aumund also offers a unique upgrading service. This is not only the case for Aumund Group products but also for compatible machinery from other suppliers. When using the upgrade service clients have the option to convert existing machines using, for example, Aumund chains, buckets, shafts, pulleys, drives and other key components with minimal cost and plant disruption. Aumund offers fast track availability of spare parts from its service centres worldwide guaranteeing the highest level of after-sales service and support.
Conclusion
This paper has discussed a series of detailed innovations, resulting from Aumund's commitment to ongoing R&D, which is specifically related to the handling of raw, granulated and ground blast furnace slag. These products may be integrated into plants, either as individual machines or as part of an integrated handling system where Aumund also offers engineering support for the design and local manufacture of associated structures, conveyor bridges, transfer towers and similar ancillary services.
Aumund offers a comprehensive service to its clients from the conceptual plant layout through the design stage to the final commissioning of the plant. The ongoing service backup supported from its head office in Germany, plus daughter companies and representative offices in France, Switzerland, Poland, India, China, Brazil, Hong Kong, the US and the UK. Outside of these territories Aumund has an extensive network of agents, fully trained in Aumund Group products, providing local support and direct access to Aumund's field sales and service network.
Aumund is a truly international group with vast experience and resources dedicated to the delivery of effective and economical bulk materials handling solutions tailored to meet specific demands worldwide.
