Sulfur and pastilles
New high-speed solidification system at Econova with output of 12 tons per hour

The engineers from Sandvik Process Systems have every right to feel satisfied with themselves: The test phase of the Rotoform HS has been brought to a successful close. What is more, the new plant has twice the pastillation capacity of the standard Rotoform series. PROCESS Worldwide was already able to view the new Rotoform HS under production conditions in Econova, Sicily.

The high capacity Rotoform HS was installed at the Sicilian sulfur processor Econova, Melilli. It has undergone performance tests under real production conditions and was successfully operated at a production capacity of 12 tons per hour, 24 hours a day during several months. “We’ve been working in close cooperation with Sandvik since the early 1990s,” explains Dr. Massimo Andaloro, technical and plant manager at Econova, “so we know the advantages of this system. The decision to remain with Rotoform was made on the basis of the proven quality of pastilles and reliability of performance.”

By 1992, a complete turnkey installation had been completed at Melilli. This system, consisting of sulfur pit, preconditioner, four steel belt coolers with Rotoformers and the cooling water tower, was designed for 24 hour a day operation, 365 days a year, and has been processing up to 300 tons a day ever since (and is now about to be refurbished/upgraded as part of the company’s overall investment program).
As Econova’s operation expanded over the next few years due to demand from other companies in the region—including the likes of Exxon Mobil Chemical, Agip Petroli and Isab Energy—it became clear that the original plant would be unable to provide the capacity required to meet anticipated growth and a second plant was commissioned. Completed in 1998, this plant is also based on four Rotoform lines and, being completely automated, was among the most advanced in the world with system improvements delivering almost 50% more capacity than the original installation. Now, with the third plant Econova is able to reach a total production of 50.5 t/hr—24 hrs/day, 365 days/year.
The main difference between the Rotoform HS and the base model Rotoform 3000 is the much bigger diameter of the rotating outer shell that actually deposits the molten product onto the steel cooling belt. This means the influence of centrifugal force on the droplet shape is reduced. The system can be operated at a higher speed while maintaining control of end product quality—still delivering a consistent pastille with a regular, hemispherical shape. This involved a tremendous feat of engineering, as sales manager Ulrich Nanz explains. “The engineering that goes into producing a dropforming device with much bigger diameter, 1500 mm wide, complete with anything up to more than 30,000 accurately positioned holes, is impressive enough. Making it capable of depositing high quality droplets on a steel belt running at anything up to 120 meters a minute, and making it simpler in design, easier to service and safer to use is what really makes this system stand out.” The new Rotoform HS is suitable for use with low and medium viscosity, fast cooling products (< 20 seconds) . For sulfur this equates to a 15 m long belt. However, products will differ according to their cooling times. Pastille sizes are usually 3–10 mm in diameter but can be up to 15 mm if required. Sulfur is processed at 130°C; the system however, is suitable to handle products at a much higher temperature. “The principle is the same,” says Nanz, “all we’ve done is to redesign a successful dropformer so customers can process twice as much in the same amount of time. The arrival of the Rotoform HS is another milestone in the history of this system and we’re sure that it will have a major impact on sulfur solidification.”
Looking at the system in more detail, the process starts with the delivery of sulfur melt by pipeline or in special trucks, and its storage—ready for processing—in heated tanks. Heated pumps then supply the liquid sulfur, at a constant pressure, to the Rotoform units. The Rotoformer itself consists of a heated, cylindrical stator and a perforated rotating shell that turns concentrically around the stator, depositing sulfur drops across the whole operating width of the steel belt. The circumferential speed of the Rotoformer is synchronized with the speed of the belt, ensuring that drops are deposited accurately, consistently and without deformation. Heat released during solidification and cooling is transferred by the stainless steel belt to the cooling water. This is sprayed against the belt underside, collected in tanks and returned to the recooling plant. At no stage does the cooling water come into contact with the sulfur. At the cooler end, pastilles are taken off with a discharge knife and pass, via a chute, to a collecting belt for further processing.
To eliminate the possibility of damage to the pastilles during discharge, a silicon-based release agent is applied to the steel belt as a thin film by means of a roller system. An automatic filling system ensures that sufficient quantity of the release agent is always held in the tank. The Rotoformer is equipped with an effective exhaust system, with a tight exhaust hood, an air exhaust and throttle flap positioned above each Rotoformer. The solid pastilles are transferred to the warehouses via belt conveyors or a bucket elevator.
Benefit of pastillation
Considering the fact that solidified sulfur may be moved as many as 10 times between production and re-use (i.e. the various stages of handling, transport, storage, etc.), there are a number of prerequisites of a premium pastille:
low friability and impact abrasion resistance (low visual dust generation);
good flow characteristics, but high angle of repose;
uniform size and therefore good characteristics for blending;
low moisture content;
stable properties over time;
consistent quality.
With indirect but highly efficient heat transfer, clearly defined cooling times and controllable quality through accurate crystallization, the Rotoform system satisfies all necessary technical requirements and the consistent, hemispherical shape of the pastilles themselves ensures excellent handling and storage characteristics. In short, the Sandvik Rotoform lines here at Econova deliver all the requirements of this product form: easy product flow and high bulk weight through a pastille that’s uniform, stable, thermally unstressed and of high purity and quality. While this in itself is a key advantage of the system, Sandvik’s Rotoform also provides three other major benefits for Econova: easy operation and maintenance, minimal environmental impact and maximum versatility.
The experience gained over the last
10–12 years has enabled Econova to establish itself not only as Italy’s No. 1 company in sulfur solidification, but also as a major supplier of consultancy, design and installation services to other companies. This involves a great deal of research, development and system testing—carried out in partnership with Sandvik—and the company now sees major opportunities for growth through diversification into other solidification applications.
“Together with Sandvik we have ex-
tensive expertise in the Rotoform system having worked on everything from feasibility studies to turnkey construction and commissioning of sulfur solidifica-
tion plants,” says Massimo Andaloro, “over the next few years, we want to put this knowledge to use in other applications such as the solidification of paraffins, sulfur bentonite and other chemical products.”