Naked Engineering - Mass Chemical Manufacture
How do manufacturers make chemicals on seriously large industrial scales? We sent Meera Senthilingam and Dave Ansell to find out...
Meera - For this week's Naked Engineering, Dave and I have come along to this site of Industrial Chemicals Limited based in Grays in Essex. Now as their name might suggest, they make chemicals for industry and they do make hundreds of thousands of tons of chemicals here each year. Now Dave, why have we come along today?
Dave - Well chemicals are such an important part of our everyday life. They're the fundamentals of everything from plastics to hair products to washing up liquid, and my only exposure to making chemicals is in a lab where you take a few grams and if you're feeling extravagant, mix them up and you get something else at the end. But on an industrial scale, it gets entirely different because you're talking of thousands and thousands of tons of these chemicals made every year which is a completely different proposition.
Meera - So to find out just how chemicals are made on such a large scale, we're here with Chief Chemist for Industrial Chemicals, David Compton. Now firstly I guess David, what range of chemicals are made here?
David C. - On this particular site, we make inorganic chemicals that are used for either the water treatment industry or for soaps and detergents. The basis for our technology is aluminium chemistry and we're actually standing inside the incoming bay for our aluminium hydroxide material which is one of our prime starting materials.
Meera - There are just mountains of it here, over 10 meters in height, and we're just surrounded by a powder of it.
David C. - This building will actually take 4 or 5,000 ton shipments. It's a very useful material for us because aluminium as a metal is amphoteric which means that it can either be reacted with acidic substances or with caustic basic substances, and we do both on this site. We can use hydrochloric acid to dissolve the aluminium hydroxide and form aluminium chloride, which is an important chemical for the water treatment business as a coagulant, or we can dissolve it in sodium hydroxide and make sodium aluminate, and this is the starting material for one of our key products which is called zeolite.
Meera - What is zeolite? Why is that an important compound?David C. - Zeolite is a very useful material because it's a framework of atoms, aluminium and silicon atoms, built up to form a structure which can actually have spaces in the middle, voids, and those voids can hold different atoms. Depending on which zeolite you use, you can hold anything from water molecules right up to large organic molecules in these structures. The one we're using is called zeolite-A and that's very important in the manufacture of washing powder. It can actually absorb calcium and magnesium from the water. It acts as a softening agent.
Meera - So you've mentioned to make this you need sodium aluminate which we've made with the mountains of aluminium hydroxide around us here, but what's the second ingredient?
David C. - The chemical name for zeolite is sodium aluminosilicate, so we need some silicate as well, and we make sodium silicate and mix that with the sodium aluminate to form zeolite. The sodium silicate is made from a similar mountain of sand, just straightforward silica, again dissolved in caustic soda to make sodium silicate.
Meera - So there are mountains of aluminium hydroxide around us and this is being moved on conveyor belts, 10 meters or so above us, over to another building where there are reaction vessels, large ones in fact.
David C. - Inside the reactor, first we charge sodium hydroxide solution into the bottom of the vessel, start the stirrer and then we start adding the aluminium hydroxide. We then add steam to the vessel to increase the temperature up to the point at which it can react. To make the full solution takes time, temperature, and stirring.
Meera - So we've moved around to the reaction vessels where these two core ingredients for the zeolite are actually combined.
David C. - We take the sodium aluminate and the sodium silicate, mix them together, and they form a gel like a great big jelly pudding. If kept at a high enough temperature for long enough, that actually crystalizes and little tiny cubic crystals of zeolite fall to the bottom of the reaction vessel. It takes about a four-hour cycle for the reaction to go to completion.
Meera - The reaction vessels themselves are just gigantic. They're about 30 meters high - I'm craning my neck here to look up at the top of it! How much of these ingredients, and how much zeolite is being made here?
David C. - We're putting about 80 tons of the starting materials into one of these reactors and dropping out about 20 tons of zeolite products. So, 60 tons, which is what we call the mother liquor, then goes back and is recycled back into the process.
Meera - So once these have combined here and you've got these crystals forming, and so on, what happens next to get your zeolite?
David C. - So at the end of the reaction, you've got something like 20 tons of powder but it's mixed up with the rest of the solution, so we empty the reactor straight over to what's called a filter press which we're going to have a look at next.
Meera - So we've come through now to where the filter press is located. So this is really where the zeolite you've made so far is essentially pressed together to get your final product.
David C. - Yes. The filter press is a large hydraulically-driven ram which squeezes the material between cloths. The cloth lets the water out and zeolite powder stays in.
Meera - So essentially, there are just, say, 50 sheets of this cloth all parallel to each other and that's what's really being pushed together with this 'slurry' in it to dry it.
David C. - Yes. At that point, it's probably got 40% of water still in it so it's a wet cake. This is dropped out from the bottom of the press at the end of the procedure and then passed via conveyors to drying ovens.
Meera - There's a really interesting point to remember about zeolite in that there are different forms of it that could be made and this is what you really need to control.
David C. - We control the actual type of zeolite that's made by the formulation, by the actual ratio of the sodium to silicon to aluminium and that's very carefully controlled throughout the process. Otherwise, we end up with the wrong zeolite which doesn't have the correct function.
Dave - So essentially you end up with a structure with the wrong sized holes?
David C. - That's correct: if we end up with the wrong sized holes, they won't absorb the calcium and magnesium and won't function as a water softener.