Success beyond expectation

Materials World magazine
1 Feb 2019

The festive season is over, but looking for something unusual next year? Black snow, for example? Here’s how to do it, and how its discovery came about. I warn you, it has to be an outdoor display. It started with my visit to see CR Kennedy and his slag test rig at the Argonne National Labs, a suburban train trip and cab ride a few miles out of Chicago, USA. 

I knew that Kennedy’s rig was of the rotating barrel type, in which refractory bricks of various types were used to line the interior of the barrel. The barrel, mounted horizontally, contained a pool of molten slag, which through the rotation, continually washed over the bricks. Temperatures were around 1,500°C, with a burner nozzle poking in one end of the barrel and the exhaust fumes exiting from the other.

It was an ideal way to test refractories for slagging gasifiers, where molten coal ash runs down and erodes the walls of the gasifier. In Kennedy’s rig, as well as subjecting the bricks to erosion, there was a temperature gradient through the bricks, as in reality. At a certain depth, depending on the thermal conductivity of the brick, slag freezes solid, the attack then stopping. 
From the description, I had surmised that the barrel was 4ft across and 6ft long.  We might just be able to squeeze in something like that into our labs in London, UK, I hoped. Any such possibility vanished when I saw the network of pipes around the apparatus, and the size of the dust and fume extraction system. Then there was the huge insulated tank. ‘What’s that?’, I asked. ‘Oh,’ Kennedy replied, with the greatest possible enthusiasm, ‘it’s for liquid oxygen. An oxy-gas burner is the only way we can get the temperature.’ As we walked back, I mused that, along with steelplant metallurgists, like myself, some ceramics people must also suffer from latent pyromania.

Expert Jeremiahs 

Back in London, rejecting the gas burner approach, I toyed with the idea using an electric arc to supply the heat. But droplets of molten slag would short out the electrodes, so that wouldn’t work. Then the penny dropped – if slag was conductive, couldn’t electricity passing through the pool of slag supply the heat? 

Metaphorically, I looked around to see if the idea had legs. In Smithell’s Metals Reference book, there were pages on the electrical conductivity of molten salts. Wasn’t slag ionic too? It led me to contact a London-based manufacturer of molten salt heat treatment baths, eventually buying power transformers and control equipment off them. I also sought the advice of two of the most distinguished experts in Britain on slag, Ken Mills at the NPL, and Professor Paul Grieveson at Imperial College. Neither thought there was much hope of success. The slag would not be conductive enough. My response was that I would just turn up the voltage. We would get power through, somehow.

Anyway, by that time, we knew we would be okay. The picture shows a section through an alumina tube in which molten slag had been kept molten, the current being supplied by molybdenum electrodes, which had also melted. In practice, contrary to what we had been told, slag had so little resistance that we kept burning out the salt bath control equipment. At today’s equivalent of £40,000 a time, this was not on. It was an electrical engineer, seemingly well past his prime, who told us a welding set would do what we needed. 

Don’t try this at home

We hit 1,600°C on every test. But we needed heavy-duty graphite electrodes to carry the current. Then we got a new request. The British Gas slagging gasifier is a kind of pressurised blast furnace, and like that, potassium and its compounds reflux and recirculate between the hearth and stack areas. It causes alkali attack on the stack walls. Related to this is the formation of a scaffold – a huge mass of potassium carbonate, cyanide and ‘coke’. Could we use our rig to investigate?

In consequence, we filled up the slag pool area with potassium carbonate, lowered in the electrodes and started up. As expected, wisps of white vapour appeared, then, quite suddenly, a huge and unconstrained mass of fluffy black snowflakes poured out of the rig. The potassium carbonate had reacted with the graphite electrodes, forming potassium vapour and carbon monoxide. But as soon as they were away from the heat, the back reaction occurred. And just as real snowflakes form directly from water vapour, so did the flakes of carbon emerge from the gaseous mixture of carbonate and monoxide. Accordingly, you now have all the info for next year’s display. But please, do it outdoors, when your neighbours and the HSE safety zealots are on holiday.