Silicon that will be used to make everything from solar cells to sealants is drawn off from huge furnaces in Thamshavn and other Norwegian ferroalloy industry smelters. The white-hot material poured from the furnace is followed through the tap-hole by a cloud of greyish-white dust, which until now has been released into the smelting hall itself.
“Dust from the discharge stage has no immediate effects on health, but according to doctors it can be a long-term hazard, which is why we are looking for solutions that can reduce the amount of dust inhaled by furnace operators,” says Halvard Tveit, Elkem Group specialist and adjunct professor at NTNU.
A research programme led by SINTEF (see fact-box) has generated a new extraction system that captures more of the dust from the tap-hole than older systems.
Exploiting natural updraft forces
Elkem’s plant in Thamshavn in the County of Sør-Trøndelag was the first to adopt the new solution. Measurements have shown that the new extraction system has reduced the concentration of dust in the work-zone around furnaces by 75 percent.
“Our success lies in the way that we have managed to exploit the natural updraft forces in the dust and gas leaving the oven. We have quite simply played along with nature’s own laws,” says project manager Bernd Wittgens of SINTEF.
More efficient hood
The innovation is that the hood that collects the dust has been given a new design and a better location than those that have been the norm in silicon metal production.
The new system has also been adopted by Wacker’s smelter in Holla in Sør-Trøndelag, and it will soon also be installed at Elkem’s plants in Salten and Bremanger. Although the principle itself has been developed and tested, some development work is still being done in order to optimise the solution.
Filter masks were the first stage
Halvard Tveit emphasises that the short-term dust-reduction measure in this industry has been the requirement that everyone in the vicinity of the tap-holes must wear a filter mask.
“We are also trying to automate the discharge process, in the hope that in the future we will be able to move the operators out of the area when metal is being tapped. But in any case, it will always be important to have an efficient dust-removal system,” he points out.
The FUME research program
- The dust extraction system discussed in this article was developed through the FUME (Fugitive Emissions of Materials and Energy) research programme, which is being carried out by SINTEF and NTNU (2000 – 2015) with funding from the Reserach Council of Norway and Ferroalloy Industry’s Research Association (FFF).
- The ferroalloy industry comprises smelters that extract manganese, chromium and silicon from crushed stone.
- Norway has a total of 16 furnaces that are fed with silicon-rich stone. In six of these furnaces, iron is added to the raw material. What emerges from the furnace is ferrosilicon, a product that ends up in steel alloys and is used as an additive in steel production.
- The other ten furnaces produce silicon, which is used to make solar cells, electronic circuits and silicon metal, but is also used as an additive in aluminium alloys.
Like a storm
The dust that is emitted during tapping originates from the gas that forms inside the charge of the fucrnace. In addition to the smelt itself, the oven transforms every litre of solid raw material into 4000 litres of silicon-rich gas. When the gas emerges from the tap-hole, it is immediately combusted as it encounters the oxygen in the air of the smelting hall. The dust particles (silicon dust; see fact-box) is a result of this combustion process)
The less gas emitted by the oven, the less dust will be formed. According to Tveit, Norwegian research results have already made it possible to reduce the emissions of gas, and thus of dust too, from the tap-hole. The gas that still escapes may do so at a speed of up to storm strength, according to estimates made by SINTEF. The high flow velocity involved has been one of the challenges of the extractor project.
Underlying measurements and simulations
The idea for the new extraction system came from Elkem itself, and the subsequent research programme followed it up through a series of steps. First, it developed methods for making the necessary measurements at the tapping point. Then the measurement results were used as input for computer simulations, which in turn formed the basis of a prototype version. Early prototypes were tested at Elkem’s works in Salten and Thamshavn.
“If we had proceeded by trial and error, this could have become the world’s most expensive extraction system. Instead, we ended up within acceptable time and cost limits,” says Halvard Tveit.
Dust from silicon smelting
- The atoms in the dust that escapes when silicon is tapped do not form crystals and are therefore less harmful than the materials that make up many other types of industrial dust.
- Silicon is the most common element in the Earth’s crust. In nature, it reacts with oxygen. But at 1500–2000 degrees Celsius in a smelting furnace, the two elements separate, enabling the pure silicon to be incorporated into useful products.
- A furnace produces silicon smelt plus the gases SiO (silicon oxide) and CO (carbon monoxide), which burn when they encounter oxygen, producing a fine dust. This is formed on the route of the gases trough the furnace as well as at the tap-hole.
- Filter bags collect the dust from the furnace, as well as what is extracted from the tap-hole. The dust is used as a binder in concrete, and in refractory (fire-resistant) materials.
- The dust consists of almost perfectly round particles that are 100 times smaller than grains of cement, and can therefore be used to fill the cavities in ordinary cement mixtures, making them stronger and more durable.