Chemistry: The Solvay Process

Hello, readers, welcome to another episode of 'The HSC with AB," after a somewhat belated hiatus (read: procrastination). Today marks the first post in almost two weeks, as well as the first one in the three weeks of study leading up to the HSC. This one is about Chemistry, since someone asked me to do one on the Solvay process (apparently I take requests now?), and so here it is.

The Solvay process is the process which is used to manufacture sodium carbonate, a weakly basic salt used in the manufacture of glass, ceramics, and paper, as well as a water softener and to remove sulfur dioxide. It is produced from the raw materials of sodium chloride, calcium carbonate and ammonia, although ammonia is recycled (more on that later). If you want to get down to it, the overall reaction is CaCO_{3 (s)} + 2NaCl _(aq) → Na₂CO_{3 (aq)} + CaCl_{2 (aq)}, but it's not as simple as that. Put calcium carbonate and sodium chloride in a beaker together and nothing will happen, so you need some intermediate steps. And that's where the tricky bit comes in.

There are four steps involved in the Solvay process, and a whole mess of equations. The syllabus states that you have to analyse a flow chart, and what I've found is that all flow charts for the Solvay process are either a) messy or b) incomplete, just due to the sheer amount of recycling that goes on. So I'll put up my flow chart, then go through the steps. This one might take a while to sink in, so don't be troubled if you don't get it straight away. It's not too difficult, but it's a fair amount to remember.

There are four steps to remember: Brine Purification, Hydrogen Carbonate Formation, Formation of Sodium Carbonate and Ammonia Recovery. Now I've never been a huge fan of mnemonics, but someone taught me this one and I haven't forgotten it, so obviously it must be effective, so when you're in the exam, just remember: Both Paige Hawkins and Corinne Fulford Failed Senior Chemistry and Are Raging. That's one that just stuck with me, and hopefully it'll do likewise with you. Anyway. I'll put up the flow chart now and then go step by step.

Brine purification pretty much just involves removing impurities from sea water to leave water and sodium chloride, also known as brine. Calcium salts are precipitated out by adding sodium carbonate, as per Ca²⁺_(aq) + CO₃^2–_(aq) → CaCO_{3 (s)}, and magnesium salts by adding sodium hydroxide, with Mg²⁺_(aq) + 2OH^–_(aq) → Mg(OH)_{2 (s)}.

So now we move on to step 2: Hydrogen carbonate formation, which if we look at our diagram is at the carbonating tower. But first, a small diversion in the form of our lime kiln, where CaCO_{3 (s)} → CaO_(s) + CO_{2 (g)}. This will become more important later, but just know for now that we get some of our carbon dioxide from here. But anyway. We collect our brine, our ammonia, and our carbon dioxide, and we form two chemicals: sodium hydrogen carbonate and ammonium chloride, which can be expressed as NaCl_(aq) + NH_{3 (g)} + H₂O_(l) + CO_{2 (g)} → NH₄Cl_(aq) + NaHCO_{3 (aq)}. A fairly long equation, yes, but it does have to be memorised. With that being said, we move onto Step 3 (I am trying to make this as interesting as I can, but this topic is a tad dry).

Now what we came here for: Step 3: Formation of sodium carbonate. We take our sodium hydrogen carbonate for step 3 and we heat it up (in the diagram, it's in the heater; that's probably not it's real name. Work with me here.). So we heat it up and we form sodium carbonate, water and carbon dioxide, the latter of two which we recycle. Our equation, because we need it, is 2NaHCO_{3 (s)} → Na₂CO_{3 (s)} + CO_{2 (g)} + H₂O_(l).

So now we have our sodium carbonate, are we finished? No! We have Step 4: Ammonia recovery. Remember our CaO from Step 2? We take that and we dissolve it in water, as per CaO_(s) + H₂O_(l) → Ca(OH)_{2 (aq)}. We then take this calcium hydroxide and we react it with the ammonium chloride from Step 2, giving us our ammonia back, as well as water and waste calcium chloride, with our magical chemical equation being 2NH₄Cl_(aq) + Ca(OH)_{2 (aq)} → 2NH_{3 (g)} + CaCl_{2 (aq)} + 2H₂O_(l). We recycle our ammonia and our water and we're left with that equation from the beginning: CaCO_{3 (s)} + 2NaCl _(aq) → Na₂CO_{3 (aq)} + CaCl_{2 (aq)}.

So now we've done the actual process, we're left with some final bits that the syllabus demands. Firstly, environmental problems. The main problem is the waste calcium chloride, which can be sold as a desiccant, or a drying agent (you know, those little packets you sometimes get in food packets that say do not eat?), but are mainly thrown away just because of the sheer amount that is produced. It can be discarded into the ocean without ill effects though, since all it would do is put salt into the ocean which really doesn't do much. If you want another environmental problem, then the process requires heat (see Step 3), which means burning coal, global warming: the standard drill. Really, all you need is the calcium chloride bit.

Two bits left: Locating a chemical industry for the Solvay process and the "carrying out of a chemical step involved in the Solvay process, identifying any difficulties associated with the laboratory modelling of the step," to quote the syllabus. For the first one: put it near the ocean. There is salt water there (seriously, that's it). Finally, the second bit, which is a little trickier to do in a blog considering it requires a prac, but that's why you have Youtube videos. This one does both the heating of calcium carbonate (Step 2) and the formation of calcium hydroxide (Step 4).

Well, that seems to be it for the Solvay process. I do hope soon to an English post soon, maybe one on Wilfred Owen's poetry, or one of his poems. We'll see. I also take requests, so if you want me to do a topic, I'll be happy to give it a shot. Finally, you'll see I have several sets of notes ready for download: all but Chemistry and English. Chemistry is hopefully coming soon, English is difficult to make study notes for; there's no real syllabus to work on, especially for related texts. That being said, post is now officially done.

Good luck with study,

AB