One area of the aluminium industry that doesn’t get talked about much is, what happens to a dead pot?
In the normal scheme of things, the steel furnace in which pure aluminium is made will last somewhere between 5 and 10 years before the protective linings wear down too far. At that time, the entire shell is lifted out of its position and transported to a special workshop.
It takes a couple of days to let it cool, then another couple of days to jackhammer out what’s left in the shell so that new linings can be installed and the shell put back into the line.
But what exactly is left in a cold dead pot?
There are four main elements. There is some pure aluminium that missed being tapped out of the furnace. There is some steel – the steel bars at the bottom of the pot that act as part of the electrical circuit, and are called collector bars or cathode bars. There is the remainder of the cathode, and the various levels of refractory and protective bricks that go under the cathode.
The pure aluminium can be retrieved and recycled, and a strong magnet will pull out any pieces of steel from what the jackhammers dig up.
But that leaves you with the two worst parts – the brick portion and the carbon portion. The brick portion can be used as road fill, if you can sort it from the carbon portion. But the carbon portion contains some nasty impurities, namely arsenic, cyanide and hydrogen. These two portions, the brick and carbon pieces, are what is called Spent Pot Linings, or SPL.
In Australia, there were a variety of processes for disposing of SPL. One was a method developed by Alcoa that produced aluminium fluoride, but the operating cost and yield made it impractical for commercial use. Another method, which we used at Tomago, was to ship the SPL to a company in Italy, whom we had to pay to take it off our hands. Another way is to let the stockpile build up, in hope of someone finally coming up with a foolproof commercially viable system to treat and dispose of the SPL without upsetting the environment or the environmentalists.
But what do they do in China? They bury it.
If we take a 5-year average of total China aluminium production, and use 2.5 tons per pot per day output, we can roughly estimate the amount of SPL that China produces each year. Let’s assume for this calculation that the average metal production per year has been 20 million tons. That means 55,000t per day, so approximately 22,000 pots in operation at any one time over the last 5 years. If the average life is 5 years, then we can expect that 4,400 pots have been delined and relined per year in that time. Assuming 20t of materials per pot to come out (cathode blocks, refractory materials, collector bars, sidewalls, etc) then that means that China is generating roughly (very roughly) about 88,000t of SPL per year.
Even if I am 50% out in my calculations, that’s still 44,000t of nasty material that’s going into landfill every year.
I don’t want my local water supply getting infected by leached cyanide and arsenic. And I don’t want an explosion happening when the trapped hydrogen reacts. But that’s exactly what is happening in China.
At least in most of the rest of the world, SPL is being treated somewhat responsibly. Nobody wants to make a big deal of it, because it only reminds people of the less environmentally-friendly side of aluminium. It’s up there with the red mud ponds scattered all over the world that come about because of the bauxite-to-alumina process. Provided aluminium companies accept their responsibilities and dispose of their waste correctly, there is no problem.
But China is not part of the rest of the world.
Mr. Paul Adkins is co-founder and Managing Director of AZ China Ltd, a leading consultancy on China’s primary aluminium industry. Paul has more than 30 years of experience, including almost 14 years with Alcoa, 5 years with the former Alcan (now part of Rio Tinto) and 4 years with Tomago Aluminium in Australia.
To read more from Paul Adkins you can go to http://az-china.com/blackchinablog