Flash dryers commissioned at the Polokwane smelter

13th December 2002 By: vanessa vasques

Two flash dryers, each with a capacity to process around 70 t/h of bone-dry concentrate, have been installed at Anglo Platinum's (AngloPlat's) Polokwane platinum smelter in the Limpopo Province.

The dryers have been designed and supplied by local dryer specialist Drytech – based in Johannesburg – and commissioning of the two dryers will take place at the beginning of this month. The commissioning will conclude what has been a very satisfactory project in conjunction with overall project manager Greenblatt & Hutton, a Johannesburg-based project management and engineering consultancy.

Each of the dryers is comprised of approximately 700 t of steel, stands 53 m high and they are the two largest supplied by Drytech, being the seventh and eighth flash dryers supplied to AngloPlat. "We believe these dryers are unique in that they have been developed to be reliable and easily controllable in a totally integrated process – all of which is managed by a programmable logic controller (PLC)," says Drytech MD Harry Traub. They embody the concepts of being easy to operate and are user-friendly, he adds. Moist concentrate feed and dried recycled feed are mixed in the dryer and then fed into the dryer's disintegrator. This is a highly-sophisticated piece of rotating equipment which is used to break the concentrate up into smaller particles.

From the disintegrator, the product is forced up the drying column with the hot air which is now being introduced into the system.

When the product is in the air stream, any tramp material drops out and the product then goes through a three-stage dust air separation system to separate the fine product from the air stream.

The three stages consist, firstly, of triple cyclones which drop out more than 90% of the product, which is then sent to drop-out boxes and then to the hopper.

The second of the three stages is to take the air stream, which now contains a small percentage of particulate matter, through a multiclone. This is a structure with a number of smaller cyclones to further separate the product from the air stream.

Finally, the air stream, containing very fine particles, is sent to the bag- filter system, which Traub says is unsual because of its low emission rates. The bag system is made up of four separate off-line pulsing components which are necessary for the fine matter coming into the bag system which contains rhodium, the most valuable of the platinum-group metals (PGMs).

The dust-laden air is passed through the outside of the bag filters, resulting in the emission of clean air and the dust or fine particles remaining on the outside of the bags.

In order to retain as much of the fine material as possible, to increase the PGM findings and keep dust emission as low as possible, the dust is removed with a pulsing motion, while the bags are off-line. "We are running the baghouse system at less than ten milligrams of particulate emission per cubic metre of air going through the system. "This is below the statutory limits and is actually about a third of the best standards in the industry at the moment," Traub notes.

Another advantage of this specially conceived baghouse system is that the plant can continue operating with only three bag compartments on line, which means the plant does not need to be shut down when maintenance on one of the bag compartments is required. A major leap forward in technology has been the integration into the flash dryers of the Fire Sand hot-gas generator. This provides a controlled stream of hot gas for the provision of heat to a downstream process. Drytech has the world rights for the Fire Sand and currently there are more than 25 installations working satisfactorily worldwide.

The company's flash-drying process was developed more than ten years ago with AngloPlat, due to the need for a low-maintenance and easily-operable system. Previous dryers, which were either rotary or Buttner dryers, were labour-intensive and maintenance requirements were high.

"We came up with the idea, which we developed with AngloPlat, using facilities at the University of the Witwatersrand and the first installation was erected at the mining house's Waterval smelter," he adds. That first installation was, in fact, the world's first PGM concentrate flash dryer. The result was a system which is environment-friendly, thermally efficient and reliable. Traub believes the success with the Polokwane smelter and other installations for AngloPlats will provide the breakthrough the company has been looking for in trying to establish itself in the South American market.

The company is currently tendering for a project on that continent and has been trying for a number of years to break into that market, which Traub says the company is now on the verge of doing.

The difficulty in establishing itself in that market has been due to the fact that the flash dryer process has never been used there before. "We have developed a process which – I believe – is a world breakthrough and I think, over the next ten years, we are going to see this process have an impact on the international market," Traub says.

Traub established Drytech 20 years ago and he says the company is unusual in that, rather than specialising in one type of dryer, it specialises in the thermal process of moisture removal using whatever drying process and technology is required.

The company is equipped to design and supply a wide range of dryers including cabinet dryers, conveyor dryers, fluid bed dryers, infrared dryers, rotary cascade dryers, spray dryers, indirect dryers and, of course, flash dryers. The manufacture of all components, other than the critical items, is contracted out.

"We supply and design the best type of dryer for the process, which means we cover the whole range of drying equipment," Traub concludes.