By James Joyce
Bauxite residue has a number of aliases. It’s also known as bauxite tailings, alumina refinery residue and a number of other technical sounding terms. But perhaps the most common one, which is descriptive if not very flattering, is red mud. In addition to its unflattering nickname, bauxite residue often gets described in the press (and even in some academic journals) in somewhat pejorative terms. One common label that gets wrongly attached to it is ‘hazardous waste’.
Bauxite residue is NOT a hazardous waste
In the EU, the term ‘hazardous waste’ means something. The EU has a document known as the ’List of Waste’. This, as the name suggests, is a list of all of the types of waste that are produced in the EU, and each waste has a code. For example, your aluminium beer can that you throw in the recycling has the code 20 01 40, while acid tars produced during petroleum refining have the code 05 01 07*. The * that the acid tar code has means that this, in the EU, is a hazardous waste.
Bauxite residue is in the EU List of Waste too. Its code is 01 03 09. Note the absence of an asterisk. Here’s the full section:
You’ll notice that the full description of bauxite residue says ‘red mud from alumina production other than the wastes mentioned in 01 03 07’ and that 01 03 07 has an asterisk. This is important. Red mud contains residual sodium hydroxide from the Bayer process and sodium hydroxide is nasty stuff. If a waste contains more than 1% sodium hydroxide (or any other corrosive chemical that can cause burns) it is indeed classified as a hazardous waste. Red mud however typically contains between 0.2 – 0.6% residual sodium hydroxide1, below the 1% threshold.
So what have we learned? Bauxite residue is not, repeat, NOT a hazardous waste.
Is it a toxic waste?
Another term often used to describe bauxite residue is ‘toxic waste’. Toxic waste is a less well defined term than hazardous waste, and generally is applied to something that is harmful if ingested or inhaled. I want to make very clear at this point that under no circumstances should you ever attempt to ingest or inhale bauxite residue. Remember the 0.2 – 0.6% residual sodium hydroxide left over from the Bayer process? That gives red mud a pH of around 12.5. That’s about the same as household bleach. It may not be a hazardous waste, but it’s still pretty nasty stuff. It’s this sodium hydroxide that means that spills of bauxite residue, rare as they are, are a serious business. This stuff can and will cause severe burns.
When it comes to toxicity though, the sodium hydroxide in the bauxite residue is pretty much the only thing to worry about. Yes, bauxite residue contains some heavy metals, but studies following the Ajka spill concluded that the effects seen on plants in the area were as a result of the sodium toxicity, not the trace metals 2.
It’s worth noting that the sodium hydroxide is actually a contaminant of the bauxite residue. The alumina refiners do not want it to be there and go to great lengths to recover as much of the sodium hydroxide as possible. Why? Because it’s expensive! The sodium hydroxide is there in the Bayer process to dissolve the alumina from the bauxite. The more that’s lost to the residue, the more they have to buy to replace it…
So is bauxite residue a toxic waste? Well, you certainly wouldn’t want to eat it. But then you wouldn’t want to eat that gone off chicken you threw out last week. Vague terms like toxic waste are unhelpful. Caustic waste certainly, but I’d hesitate before calling bauxite residue a toxic waste.
So what is it?
Bauxite residue is a secondary resource.
A what now? A secondary resource is essentially a second hand primary resource. So ores, including bauxite are primary resources: things you can extract from nature and process to get something you want from them – in this case alumina. Bauxite residue is what’s left over, but there’s a lot of good stuff left in there. Firstly there’s all of the alumina which you couldn’t extract using the Bayer process – up to 20% of that. Then there’s a load of iron in there too – up to 50% iron oxide. Then there’s all the little stuff – the rare earth metals, the scandium, the titanium dioxide. Small concentrations yes, but that stuff is valuable.
If you think of bauxite as the turkey you have for Christmas dinner (at least in the UK!), bauxite residue is what’s left over after dinner. Dinner (the Bayer process) has taken away most of the nice white juicy breast meat (alumina), but there’s still a bit of that left – enough to make some sandwiches surely. Definitely worth keeping! Then you’ve got all of the dark meat from the legs and various other places around the carcass (iron), that’s not quite as good as the breast meat, but lovely in a turkey curry on boxing day (again a UK thing3…). You’ve still got your nice surprises like the wishbone4 (REE’s/scandium/titanium). And then you can use all of the bones and the rest of the carcass to make a nice stock for soup (like you can use the remaining residue to make geopolymers).
No-one in the UK would ever dream of throwing away their Christmas turkey after dinner, which is why it’s such a tragedy that so much of the bauxite residue being produced worldwide is sitting in disposal areas. We here at the MSCA-ETN REDMUD project are working to come up with ways to make tasty things out of the alumina industry’s leftovers. Waste not want not, after all!
- Ruyters, S. et al. The red mud accident in Ajka (Hungary): Plant toxicity and trace metal bioavailability in red mud contaminated soil. Environ. Sci. Technol. 45, 1616–1622 (2011).