Felix: Iron Chloride - Three friends on an incredible journey

Papa, that's a beautiful stone – such a lovely red! I'm going to take it home!
Not another one, my rucksack is getting heavier and heavier all the time!

Just this one, please! What kind of stone is this, by the way?
The red colour indicates that it contains iron. Here in the region it could be one of the iron carbonates known as siderite or ankerite. The layer of rust on the surface is likely to be limonite, an iron hydroxide that is formed by weathering. Incidentally, this type of ores has been mined not far from here in the Hüttenberg area already since Roman times.

Ah – that's close to Donau Chemie. That's why you produce ferric chloride in Brückl!
Unfortunately, siderite would not be a good raw material for us. We use iron metal, iron oxides or even solutions containing iron. Most of these are by-products. We process them into high-quality precipitants in the sense of a circular economy.

But how does the ironstone become iron metal and then ferric chloride?
Let me tell you a little story: Once upon a time there were three iron atoms, Rusty, Sid and Anka. They spent millions of years in the rock of Erzberg, the iron mountain in Styria – until they were catapulted to the surface with a huge bang. After processing the ore, the three friends were taken by train to the steelworks and thrown into a giant blast furnace. There it was so hot that the friends had to say goodbye to their companions – the oxygen atoms, the carbonates, the calcium or even the acid silicates. The heat caused the three iron friends to melt away.

Like yesterday, the ice cream in the sun?
Yes, exactly, but at temperatures between 1.300 and 1.800°C.

That would be too hot for me!
For me, too. The molten pig iron was then turned into steel in another pot, the converter, by burning off the carbon in it. And so it was that the three friends finally found themselves in a block of steel. But soon it was time for a farewell: Rusty was lost in the rolling of the steel when he combined with oxygen on the glowing surface of the block to form scale. During the subsequent cleaning process, pickling with hydrochloric acid, Sid was removed by the acid. Anka remained in the steel.

Oh dear, the poor must have been sad!
Sure, but the story continues: one day Anka ended up in a big pot of acid liquid. The piece of metal dissolved. Suddenly she felt free, like a fish in water. That's when she spotted Sid and Rusty. What a joy it was! Rusty had come here with the scale and Sid with the pickling acid. The pot was part of the Donau Chemie iron chloride plant and the liquid was called...

Iron Chloride!
Exactly! In the plant, some other unpleasant companions – heavy metals, carbon and silicates – were first removed and chlorine gas converted the iron (II) into iron (III) chloride. They were then reunited in a huge tank.

They certainly had a reunion party there!
Of course they had. But just a few hours later, they had to leave the factory in a truck and ended up in a storage tank at a sewage treatment plant. No sooner had they arrived than they were sucked in by a pump, squeezed through a thin tube and plunged into a tank of dirty water. They clung desperately to each other, but Rusty was caught by a phosphate ion, Sid by a sulphide ion and Anka was surrounded by water molecules, forming iron hydroxide.

And now they are separated from each other again!
Yes, but they fulfil important functions: Phosphate precipitation to keep rivers from being too heavily polluted and to remove sulphides, namely toxic hydrogen sulphide.

That's right - the rotten egg gas!
Exactly. The iron reacts with the sulphides, preventing them from entering the digester gas from the anaerobic stage – an important prerequisite for the sustainable use of biogas. As a result, wastewater treatment plants can cover a significant proportion of their own energy requirements. Skilful use of coagulants not only optimises biogas production but also reduces the load on the biological stage. This improves the energy balance and also contributes to the reduction of greenhouse gas emissions such as carbon dioxide and laughing gas.

Why is the gas laughing?
Laughing gas is nitrous oxide, produced during nitrification, with a greenhouse potential 298 times that of CO₂. When it is inhaled, it can actually cause a state of euphoria and therefore make some people laugh.

Cool – too bad it's a greenhouse gas, otherwise people would laugh more in the future. Does the hydroxeride also have a function?
Of course it does. Iron hydroxides, for instance, help to improve sludge coagulation. This means that the sludge sinks more quickly to the bottom of the secondary settling tank – which is important for the reliable separation of the sludge from the treated waste water.

Why?
The sludge contains the micro-organisms that purify the wastewater. They must be retained in the biological stage. The sludge also contains substances that should not be discharged into the river: Nutrients such as phosphorus, organic matter and heavy metals. Proper flocculation ensures that almost all of these substances end up in the sludge.

Even the nasty bacteria and viruses?
Yes, indeed. Studies have shown that biological treatment removes at least one log₁₀, or 90%, of the germs in the water. In the case of bacteria, even higher removal rates can be achieved. Studies with precipitants have shown that coagulation can remove up to 99.9% of viruses from water, which is a log₁₀ reduction of up to three. All iron chlorides or polyaluminium chlorides, such as we produce, can do this. It is therefore not necessary to use expensive special products for this purpose. However, it is important to bear in mind that a reduction of three log₁₀ levels will not result in waste water that is free of germs. To achieve this, you need a reduction of at least four to seven log₁₀ levels. But then you are already talking about disinfection. This would require products classified and registered as biocides. We also produce this type of products – hypochlorite or chlorine gas, for example. Disinfection of waste water is rather unusual and, in most cases, not justified – but for drinking water and swimming pool water it is.

What happens to the viruses in the sewage treatment plant afterwards?
The coagulants do not destroy the viruses, but fix them to the sludge flocs.

Then it will be a reunion in the sludge?
Yes, exactly. That's how it went. The three friends celebrated a reunion in the digestion tower. Once the sludge had been dewatered, they continued on their journey. This took them to the thermal treatment plant. There it got really hot again. Many of the pollutants – Including the viruses – were destroyed and the three iron friends ended up as oxides and phosphates in the ash.

And what happened next?
Eventually, someone will come along, process the ash, extract the valuable materials and make new products from them. And then Rusty, Sid and Anka will be back on the road again – but that's another story.
 

Literature

Coagulation and Flocculation in Water and Wastewater Treatment. (n.d.). Retrieved 10 9, 2020, from IWA Publishing: https://www.iwapublishing.com/news/coagulation-and-flocculation-water-and-wastewater-treatment
Oakley, S., & Mihelcic, J. (2019). Pathogen Reduction and Survival in Complete Treatment Works. Global Water Pathogen Project.
Rahmberg, M. (2020). LCA of different WWTP process. ivl - Swedish Environmental Research Institute.