DROSS TREATMENT AT ALCAN

To a metallurgist, it’s “dross’ . . . the grey, spongy mixture that forms on the surface of molten aluminum. Others might call it scum.

As unpleasant as the stuff sounds, don’t dismiss it as waste. Apart from oxides, dross is largely aluminum (up to 80% by weight), making it a valuable commodity. Alcan has just developed an advanced technique for retrieving the metal.

Alcan Smelters and Chemicals’ C$13-million dross treatment plant, opened last year in Jonquiere, Que., is pioneering the use of plasma arc heating technology (see accompanying story on page 49) in the aluminum industry. The high-temperature process breaks down the dross into aluminum and other reusable materials and eliminates the salt fluxes used in the conventional processes.

The Guillaume-Tremblay Works, as the plant is known, is modelled after a pilot plant built about two years ago at Alcan’s Arvida Research and Development Centre (ARDC), where the safety and reliability of the plasma technology were demonstrated. The plant, named after a former Alcan manager in Quebec, will provide superior environmental protection and higher aluminum recovery rates and is expected to cost less to operate than traditional facilities.

THE ARVIDA CENTRE WAS BUILT TO ACCOMMODATE THE ALUMINUM AND ENVIRONMENTAL NEES OF THE NEXT CENTURY.

The classic dross processing technique involves the use of large quantities of salt to separate aluminum from the oxides. But the salt remains trapped in the residue and can leach into the soil if disposed of in landfills. The salt can be removed from the residue and made reusable, but only at significant expense.

“Our process eliminates the need for salt,” says ARDC’s Serge Lavoie, who, along with Ghyslain Dube, Jean-Paul Huni and Wes Stevens, developed the technology. Additionally, the process reduces gaseous emissions.

“We use an electric arc to heat compressed air to about 5,000degC, ionizing it and turning it into a plasma “flame” to heat the dross. Because the atmosphere within the airtight furnace is virtually inert, we have more control over its reaction with the dross than we do with other methods.”

Alcan and Plasma Processing Corp., also a subsidiary of First Mississippi Corp., have entered into a cross-licensing arrangement to exploit the technology. PPC has the responsibility for sub-licensing the technology in the U.S. and Canada.

The Jonquiere processing centre is about midway between Arvida Works and Laterriere Works. When fully operational, the plant will employ up to 16 people, with a capacity to treat 15,000 tonnes of dross per year — sufficient to handle all of the dross generated by Alcan’s Quebec operations.

Dross from the smelters and fabricating plants will be classified according to alloy and source and shipped to the new centre for processing in one of its two rotary furnaces. The dross will be heated above aluminum’s melting point (700deg to 800degC) by the plasma torch and the aluminum will be drained from the furnace. Metal recovery efficiency will range from 85% to 95%.

The recovered metal will be cast into ingots called “sows” to become part of Alcan’s metal pool, while the residue will be put aside for secondary applications. Alcan is discussing with various parties the possibility of using the environmentally safe residue in their processes.

“This is an environment-friendly recycling centre,” says Roger Giasson, project director of Alcan Smelters & Chemicals, who was responsible for overseeing the engineering, construction and commissioning of the planned centre. “It was built to accommodate the aluminum and environmental needs of the next century. One of our main concerns is to reduce the dust that circulates in the air when handling the dross.”

To ensure the health and safety of employees exposed to the powdery material, engineers are commissioning atmospheric controls that will eliminate all traces of dust within the plant.

The advent of plasma technology will also provide Alcan with additional cost-effective supplies of aluminum in the future. “On average,” says Serge Lavoie, “for every nine tonnes of dross processed, we can recover five tonnes of metal.” The proportion of recovered aluminum varies only slightly according to the various alloys cast.

Transport trucks will be dispatched to the plants to collect the dross in specially constructed steel containers with a capacity of nine tonnes. Depending on the availability of the plasma furnaces, the dross is either stored for later processing or immediately charged into a furnace.

Alcan’s leading-edge efforts in plasma technology are due to the emphasis of recent years on research and development in the processing of molten metal. ARDC’s successful application of the technology is a first for the aluminum industry.

“Our work with plasma is an outgrowth of research we conducted on alternative forms of heating during the energy crisis,” says Serge Lavoie. “And, because Alcan’s business is so closely entwined with electricity, we were interested in exploring all kinds of electrical systems.”

The initial findings of ARDC encouraged Alcan to commit the necessary funding to move the project along. In 1988, a pilot plant was built to test the plasma torch and the specially designed, rotating and tilting 3-tonne furnace. In that year, the pilot plant treated more than 325 tonnes of dross, with exceptional results.

Alcan’s advance in dross-processing demonstrates how innovation and technological development can answer both economic and environmental concerns with profitable results.

SIDEBAR

THE PRINCIPLES OF PLASMA

When heated to extremes, gas molecules break down into atoms, which themselves separate into ions and electrons. This high-temperature mixture of eelctrically charged particles is caused a “plasma” — the fourth state of matter after solid, liquid and gas.

Plasma occurs naturally in such phenomena as lightning and the aurora borealis (northern lights). It is also the state of the matter that makes up the stars, including our sun.

In industrial plasma systems, a gas such as air or nitrogen is made to flow through an electric are within a tube called a plasma torch. The direct contact with the electric are instantly heats the gas, transforming it into plasma for use as a heat source.

The first industrial plasma systems were developed in the 1960s to simulate the blistering temperatures experienced by space vehicles during re-entry into the Earth’s atmosphere. Since then, the technology has been demonstrated in a variety of laboratory and industrial applications.

DROSS + PLASMA = ALUMINUM

1. Drosscharging — Cold dross, received from Alcan plants in special steel containers, is loaded into one of two rotary furnaces via a mobile charging machine.

2. Plasma heating — While the furance rotates, the plasma torch in the furnace door is ignited, heating the dross beyond the melting point of aluminum.

3. Aluminum recovery — The molten aluminum settles to the bottom of the furnace and is drained into ingot moulds. After cooling, it is returned to Alcan’s metal system.

4. Residue removal — The furance tilts forward and the remaining solid residues are dumped into containers, then stored in one-tonne bags.

The preceding was adapted from Alcan’s Compass magazine, No. 2, 1990.