Refining refractory materials

Materials World magazine
,
1 Feb 2010

Improving the oxidation resistance of carbon-containing refractory bricks and the
wettability of castables is the focus of research at the University of Sheffield, UK. This could enhance furnace lining life and performance to benefit the metallurgical processing industries.

Dr Shaowei Zhang at Sheffield proposes graphite deposited with high quality carbide coatings using a molten salt method. He explains, ‘The technique is based on the principle that a metal
partially dissolves in a molten salt, whereas graphite does not’.

Graphite and metal powders (such as titanium or silicon) are mixed with alkali chloride salts, heated to about 1,000ºC and then held for several hours in an argon-protecting furnace. The dissolved metal diffuses onto the graphite’s surface. Uniform nanostructured titanium carbide (TiC) layers form.

According to Zhang’s tests, the carbide coatings provide a more effective thermal barrier against oxygen than the uncoated materials traditionally used – for example, graphite’s oxidation
finishing temperature increases by 100-120ºC. Meanwhile, silicon or titanium are said to result in castables with improved wettability. The contact angles between water and graphite drop from 101º (for uncoated substrates) to about 58º (for TiC-coated material).

‘In addition, we have examined and compared rheological behaviour,’ says Zhang. ‘The viscosity of water suspension containing coated graphite was found to be about 100 times lower than that containing uncoated graphite’.

Work is ongoing to characterise and compare the microstructures and properties of model MgO-C bricks and Al2O3-C castables made from the two different materials.

Zhang believes carbon-containing bricks and castables made from coated graphite could have long-term benefits for sectors such as steelmaking.

He says, ‘Conventional carbon-containing bricks suffer from poor oxidation resistance. Due to the low density of graphite and addition of antioxidants in powdered form, a high
antioxidant level has to be used, which means for 100 portions of graphite, 15-50 portions of antioxidants. In addition to increasing production costs, such high level addition has shown negative effects’. For example, it ‘severely acclerates the oxidation-reduction reaction between magnesium oxide and carbon, consuming [them] and weakening the structure’.

Meanwhile, Zhang argues that carbon-containing castable development and commercial-isation ‘is hindered by several technical problems, including poor water-wettability of graphite’. This results in inadequate flow and the need for higher water content (he suggests about 18wt% for a castable containing five weight per cent graphite). This, in turn, increases porosity to more than 30% after drying, impacting on mechanical strength and corrosion resistance.

‘The steel industry requires better bricks to further reduce refractory consumption and
overall production cost, the inclusions/carbon pick-up in steel (which negatively effect the
quality) and greenhouse gas emissions,’ notes Zhang. ‘[Also] carbon-containing castables are in high demand, owing to their quicker and cheaper installation, and properties approaching those of shaped bricks.’

David Jarvis, an independent specialist on refractory materials and a member of the associated committee at IOM3, agrees on the advantages this research could offer. He says, ‘Improved resistance of carbon compounds against attack by metals and slags would have considerable commercial implications for the cost effective use of refractories’.br />

Zhang is working with refractory and steel manufacturers to scale up and optimise the
synthesis conditions, and has also started work on research projects into nanostructured
carbon containing refractories and low-carbon structures for ‘clean steelmaking’.