Developed between 1979 and 1981, Dos Santos Sandwich Belt high-angle conveyors began commercial operation in 1983. Since then hundreds of systems have gone into successful operation throughout the world with most still in productive operation many years later.
They have been successful with many, varied bulk materials from light, friable wood chips and grains to heavy, coarse copper and iron ores. Throughput rates have varied widely, from 272 kg/h to 6000 t/h. These systems have been especially successful in coal handling applications from the mine to preparation to transfer and export. Sandwich Belt high-angle conveyors also found use in the disposal of refuse from coal preparation plants.
This article will present the technical aspects of the system then describe the important Sandwich Belt high-angle conveyor installations including vertical continuous haulage from underground mining and direct haulage from open pit mines, yard handling and prep plant feed, coal prep refuse disposal, and coal transfer to market, domestic and export.
Also, this article places special emphasis on noting the systems suitability for handling large size materials which are typical in haulage from underground and open pit mines, as part of IPCC (in-pit crushing and conveying) systems.
Development of the Sandwich Belt high-angle conveyor concept has come a long way since its first introduction in the early 1950s. Over the approximate 30-year period until 1979, significant advances were few and only came in spurts. Such advances did not build on previous developments rather they were independent developments which soon reached their technical limitations.
The latest significant development of this technology, beginning in 1979, is the first to take a broad view of the industries to benefit from high-angle conveying and of all previous developments. As a result, these latest developments know few technical limitations, address a broad range of applications and offer a forum for further logical development or evolution.
Sandwich Belt Principle
Dos Santos Sandwich Belt high-angle conveyors represent logical evolution and optimization of the sandwich belt concept. The sandwich belt approach employs two ordinary rubber belts, which sandwich the conveyed material.
Additional distributed force on the belt provides hugging pressure to the conveyed material in order to develop sufficient friction at the material to belt and material to material interface to prevent sliding back at the design conveying angle.
Here we see a realistic model of the belt sandwich. An ample belt edge distance assures a sealed material package during operation even if belt misalignment occurs. This model also illustrates the interaction of forces within the sandwich. The applied or induced hugging load is distributed across and along the carrying belt sandwich.
Of that hugging pressure only the middle pressure hugs the material load while the outer pressure merely bears against the material free edges of the belt. Both belt surfaces apply their frictional traction on the material. From this model, one can calculate the required material hugging pressure that will ensure the material does not slide back due to the tangential gravity loads.
This is expressed by this equation:
Equn. 1. Nm ≥ Wm/2 ( sinα/µ-cos〖α )〗
Where: µ = µm or µ = µb, whichever is the smaller
Dos Santos Sandwich Belt High-Angle Conveyors
When investigated anew in the late 1970s it was clear that the Sandwich Belt concept offered the greatest potential for a cost-effective, operationally appropriate high angle conveying system to address the broad needs of the mining and materials handling industries.
Following the extensive study of past Sandwich Belt conveyors, the governing theory and constraints, and development of the governing design criteria, a broader scope effort was undertaken in 1982 to develop the first sandwich belt high angle conveyor to meet these needs. The resulting DSI Sandwich Belt high angle conveyors are truly evolutionary in judiciously selecting and advancing desirable features while avoiding the pitfalls of the past. They conform entirely to the governing theory, to the constraint equations and to the development criteria.
Dos Santos Sandwich Belts fulfill all established operational requirements. The profiles can conform to a wide variety of applications.
Advantages of Dos Santos Sandwich Belts
Dos Santos Sandwich Belt high-angle conveyors offer many advantages over other systems including:
- Simplicity of Approach – The use of all conventional conveyor hardware, for very high availability and low maintenance costs.
- Virtually Unlimited in Capacity – The use of all conventional conveyor components permits high conveying speeds. Available belts and hardware up to 3000 mm wide make capacities greater than 10000 t/h possible.
- High Lifts and High Conveying Angles – High lifts to 300 m are possible with standard fabric. Much higher single run lifts are possible with steel cord or aramid fiber belts. High angles up to 90 degrees are possible.
- Flexibility in Planning and in Operation – Dos Santos Sandwich Belts lend themselves to multi-flight conveying systems with self-contained units or to single run systems using externally anchored high angle conveyors. The system may be shortened or lengthened or the angle may be altered for the requirements of a new location.
- Belts are Easily Cleaned and Quickly Repaired – Smooth surfaced rubber belts allow continuous cleaning by belt scrapers or plows. Smooth surfaced belts present no obstruction to quick repair by hot or cold vulcanizing.
- Spillage Free Operation – During operation the material is contained within the belt sandwich from loading to discharge. Well centered loading and ample belt edge distance result in no spillage along the conveyor length.
Dos Santos Sandwich Belt high-angle conveyors are well established in the industry with the first commercial unit beginning operation in 1984. Since then hundreds of units have gone into operation throughout the world.
- Materials handled vary from various grades of coal to coal refuse to coarse copper ore, excavated earth, dewatered sludge, wood chips to blast furnace slag, gypsum, sulfur to various grains.
- Conveying rates vary from a low of 272 kg/h to a high of 6000 t/h.
- Conveying angles vary from 35 degrees to 90 degrees.
- Elevating heights are as low as 3.66 metres and as high as 175 metres. These same units are respectively the shortest and the longest, at 8.6 metres and 455 metres, respectively.
Mine to Prep Plant and Beyond
Having noted the versatility of Dos Santos Sandwich Belt systems we now consider applications from coal mining to prep plant and beyond, to clean coal load-out then to market and to refuse disposal. Dos Santos Sandwich Belt installations have fulfilled the elevating functions at all of the stages en route.
Haulage From Underground
The wide use of longwall systems in the 1980s required upgrading all of the underground conveyors to larger belt widths that could keep up with the longwall production.
In deep coal mines this resulted in choking the flow at the main haulage shafts where skip hoist systems could not meet the increased production requirements. This created opportunities for vertical high angle conveyors in the 1990s. Studies of that time developed single and multi-flight systems as alternates to the traditional skip hoist systems.
These proved to easily handle the large throughput rates continuously through a mere conveyor to conveyor transfer chute precluding the large terminal storage and feeding systems that are required for the skip hoist batch haulage systems. The economics are overwhelmingly in favor of the continuous haulage systems with Sandwich Belt high-angle conveyors along the vertical shafts from underground.
Figure 2 shows two variations of continuous vertical haulage from underground, Scheme-A and Scheme-B. The original basis for these schemes is an underground nickel/copper mine with a net vertical lift to surface of 1381 metres.
Scheme-A consists of eight main sandwich conveyors along the main vertical shaft and seven small connecting sandwich conveyors at excavated pockets. A main shaft of 7 metres finished diameter is able to accommodate the haulage system as well as all mine equipment access. The continuous vertical haulage system occupies half of the main shaft while the travel path of the 1.8- x 4.9-m mine equipment cage occupies the other half.
Scheme-B consists of only eight main sandwich conveyors, four along the main vertical shaft and four along independent vertical shafts that are solely dedicated to each sandwich conveyor. The four independent shafts are of 3.7 metres finished diameter as this accommodates the haulage system as well as a 1.1- x 1.7-m service cage. Scheme-B requires the additional local excavation between the main shaft and the ends of each independent shaft in order to accommodate the transfers between the alternating sandwich belt conveyors.
Such ambitious multi-flight systems as described above are yet to be realized but single flight vertical sandwich belt high-angle conveyors, the basis for these systems, were commercially utilized at vertical shafts from underground coal mining, gypsum mining and tunneling projects. The most impressive these went into operation at a United States Midwestern coal mine. It featured 1524-mm wide belts running at 4.57 m/s and elevated 1361 t/h of coal vertically along the mine shaft 102 metres.
To Prep Plant Feed and Clean Coal Storage
Numerous Dos Santos Sandwich Belt high-angle conveyors are in operation at storage facilities that feed the coal preparation plants and at clean coal storage facilities after the prep plants. Vertical systems were built in North America to storage silos of up to 76 metres height.
A second product of coal preparation plants, mine refuse must be hauled to waste dumps. In the states of Virginia and West Virginia in the United States, often the disposal sites are at high elevations between mountain ridges. The refuse haulage system must scale steep mountain slopes.
The Sandwich Belt high-angle conveyor system here served such a function for 22 years in Virginia. This system is of major proportions. Though modest in tonnage rate at 454 t/h the 914-mm wide belts elevate the coal refuse 175 metres of lift, 455 metres along the mountain slope to a 272 t truck loading bin at the top. Trucks then haul the refuse into the valley where it is spread and compacted.
To dramatize the dimensions the illustration with the photo shows a to-scale depiction of the Washington monument, a well-known U.S. landmark. The system replaced two skip type aerial tramways which were supplemented by truck haulage. The original economic study compared the options of constructing another identical dual tramway to achieve the needed haulage rate against complete replacement by the Sandwich Belt high-angle conveyor. The economics was strongly in favor of the high angle conveyor even in this case.
Coal Transfer to Market
Numerous Dos Santos Sandwich Belt high-angle conveyors are employed at the various transfer terminals en route to market and at the storage and transfer facilities of the coal market; the power plants, steel mills and chemical plants. These units have included a trailing high angle elevating conveyor to a ship loader in England in order to reduce the length of the costly dock structure and a complete high angle mobile ship loader at the Port of Adelaide in Australia.
Most interesting is the latest commercial system as it dramatizes the high angle advantage from a practical, environmental and cost standpoint.
The major expansion of a lower Mississippi River transfer terminal in the United States resulted in a short conveying path from the down-river CBU (continuous barge unloader) to a new transfer complex over parallel yard conveyors that can alternately direct the coal flow to the storage yard or to ship loading.
This most direct path subtends an incline angle that far exceeds the capability of any conventional open-troughed belt conveyor. The reflexive solution was to use two conventional conveyor flights in a switch-back arrangement. A specified 9-degree maximum incline limitation and the location over water made this a costly proposition. The large (environmental) footprint was also a negative.
Dos Santos Sandwich Belt high-angle conveyors have found wide use in the marketplace. They have proven to be versatile with widely varying applications. Furthermore, they have proven their suitability for applications from underground mines to prep plants and beyond. The possibilities with Dos Santos Sandwich Belt high-angle conveyors are far from being fully exploited. This continues to make for a bright and exciting future.
Dos Santos International, https://dossantosintl.com