Monday, October 11, 2010

Where does "toxic red sludge" come from???

As you may know, on October 4th a reservoir retaining wall collapsed near Kolontar Hungary, releasing over 180 million gallons of what is being called "toxic red sludge".  Before we delve into the nature of this sludge and why it is produced, here is an insane video of the sludge flowing through Kolontar:

The reservoir in question is part of a plant that produces a substance called alumina, which is basically aluminum metal with a bunch of oxygen atoms bonded to it (chemical formula Al2O3, also known as aluminum oxide).  This white powdery stuff is then turned into aluminum metal by a process I will describe in a later post.  I have often used alumina in lab for purifying chemicals, and it works pretty well.  In this Hungarian plant, alumina is isolated from the ore bauxite using a process known as the Bayer process invented in 1887 (so old school!) by German chemist Karl J. Bayer.  Over 100 years later, this is still the principal means of refining bauxite to produce alumina.  Even though bauxite contains 50-65% alumina, the hard part is separating it from the other metal oxides, which are mainly iron oxide (goethite & hematite), titanium dioxide, and a crazy aluminum/silicon oxide known as kaolinite (the earth's crust is composed almost exclusively of the oxides of various elements).

The Bayer process works by treating bauxite ore with the chemical sodium hydroxide (NaOH, also known as lye or caustic soda) in water.  Sodium hydroxide looks like this:

Once you dissolve it in water, it looks like this:

After you mix it with bauxite to extract the alumina, put it in a reservoir with the bauxite remnants, and the reservoir wall collapses, it looks like this:
Photo courtesy Associated Press.
Back to the Bayer process.  The thing about the aluminum atoms in alumina is that they looooooove to form bonds with oxygen.  Sodium hydroxide provides those oxygen atoms, because when you put sodium hydroxide in water, the ionic bond that holds the molecule together break, and they form sodium (Na+) and hydroxide (OH-) ions.  So, when you treat alumina with aqueous sodium hydroxide and add some heat it forms aluminum hydroxide, which, importantly, is dissolved by water:

Al2O3 + 2 OH- + 3 H2O => 2 [Al(OH)4]-

The other metal oxides in bauxite don't react with the sodium hydroxide in this way and so are not able to be dissolved in water.  Filtering this mixture then gives you the aluminum hydroxide solution and leaves behind the "toxic red sludge" containing the other metal oxides.  It is the high concentration of iron oxide (aka rust) that gives the sludge its red color (iron is also what makes your blood red).  The residual sodium hydroxide left in the red sludge is one of the reasons this "toxic red sludge" is so dangerous, because in addition to reacting with alumina as described above, sodium hydroxide can also react with things in your body like the phospholipids that hold your cells together (that would huuuuuurt!).  The up-side is that for a water-soluble pollutant like sodium hydroxide, "dilution is the solution", and it will become less harmful as it is washed away, though that fact certainly doesn't prevent the immense ecological and human health hazard that sodium hydroxide poses in the interim.

The real pain-in-the-butt in the longer term is what to do with all those metal oxides (mainly iron, calcium, and titanium oxides).  I'm not sure how toxic they are, but they certainly don't wash away like sodium hydroxide does.  As for claims by greenpeace that the sludge contains high levels of the heavy metals mercury and arsenic (those are definitely toxic), that seems a reasonable claim, but the fact that they are the only ones claiming this makes me skeptical.  I am made even more skeptical by their claim that the sludge contains high levels of "chrome", as "chrome" is not an element (see chromium).

The big question is, what were they planning on doing with this reservoir full of "toxic red sludge" anyway?  Unless they had some sort of plan for emptying the reservoir, the 2nd law of thermodynamics (disorder always increases) dictates that it would escape eventually.  If you have any info about what alumina plants do with this sludge when they don't dump it on villages I would love to find out, thanks!

Finally, to complete the Bayer process, the aluminum hydroxide solution that was filtered away from the red sludge can be cooled off, which results in white crystals of aluminum hydroxide precipitating out (crystallization is SO COOL).  Aluminum hydroxide can then be turned into alumina by heating the crap out of it to drive off water:

2 Al(OH)3 => Al2O3 + 3 H2O

Stay tuned to find out how to turn alumina into aluminum, and why the Washington monument is topped with a solid chunk of aluminum.