Well, my challenge this time is to make a carbon dioxide filter. For this I need some limestone. Heating limestone (calcium carbonate) at a high enough temperature drives off a gas called carbon dioxide, forming lime.
When this is added to water it gives calcium hydroxide – a solution called limewater. When air that contains carbon dioxide is bubbled through limewater, the carbon dioxide is removed – it reacts with the calcium hydroxide to give calcium carbonate again.
Sounds easy enough, but the particular type of limestone round here needs a really high temperature to drive off the carbon dioxide. All I have is a small stove that I know will not give me the kind of high temperatures I need.
Sure, there’ll be some lime formed, but enough for my purposes? Maybe – maybe not. I've little choice but to suck it and see.
The first batch of local limestone I try is hopeless. Hardly any conversion to lime at all. It takes several more trips out from the mine to find the right kind of limestone that will, at sufficiently low temperatures, give me the lime I need.
Leaving the right kind of limestone in the stove (charged with charcoal) overnight seems to do the trick. It’s not converted completely to lime, but there’s enough lime been formed to produce an effective carbon dioxide filter.
Lime's not very soluble in water; thank goodness. Now all I have to do is to build a rig that’ll show the viewer my carbon dioxide filter in action. My woodwork skills aren’t quite up to Jonathan’s standards, but at the end of three or four hours I’ve knocked together something that will work – using pop bottles and bits of PVC tubing – very Blue Peter.
Most of the afternoon is spent ensuring that the system is airtight. Kathy had some time on her hands, so she kindly volunteered to help me. It’s amazing how long epoxy resin takes to harden when it’s watched!
This has been a difficult challenge. It seems to me that for this particular challenge, the production team are under the impression that limestone is limestone is limestone, and that its conversion to lime is possible at the kind of low temperatures you can achieve with a coal fire.
There are many different forms of ‘limestone’, many of which will contain impurities that will make the conversion more difficult, if not impossible under Rough Science conditions.
(Producer's note: We just wanted to see what Mike was capable of!)
For example, marbled limestone will need much higher temperatures than the chalky form. Most of the limestone round these parts is of the former variety.
Despite this, I’ve had some degree of success, largely because calcium hydroxide is so insoluble in water that I didn’t need to make very much of it in order to produce a functional carbon dioxide filter.
At the end of Day 3, my carbon dioxide filter works, and works well, but this one’s been hard work.