September 2004

When it comes to making drinking water germ-free, certain disinfecting chlorine compounds, including chlorine gas, sodium hypochlorite and calcium hypochlorite, are public health champions. But there is another chlorine compound that deserves recognition for its role in helping make water clean and clear. That compound is ferric chloride (FeCl3).

When first piped from reservoirs, lakes and rivers, water is not only full of germs, but also full of STUFF: leaves, dirt, insects, animal droppings, you name it. As you can imagine, water makes a very important detour in its path from the reservoir to your residence--and that detour is to the water treatment plant. There it undergoes processes that remove the STUFF and destroy disease-causing germs, turning germ- and STUFF-filled water into safe, clean drinking water.

Flocculant Facts

Ferric chloride is known as a flocculant (FLOCK-u-lent). Flocculants are chemicals that help make tiny particles clump together so they can be removed from water. Even after large pieces of debris are removed using filters, natural surface water still may be full of tiny particles that are too small to sink. These simply float through the water, giving water a cloudy appearance. Adding ferric chloride to a tank of brown, cloudy water causes tiny pieces of STUFF to come together. Eventually, the clumps grow large enough to sink down to the bottom of the tank, clearing the water above.

Ferric chloride (FeCl3) is a chemical compound consisting of two elements--iron (Fe) and chlorine (Cl). "Fe" comes from the Latin word for iron which is ferrum. The small "3" next to Cl in the chemical formula means that for every atom of iron, there are three atoms of chlorine.

Ferric Chloride: Recycled From Steel Pickling

Steel is a mixture of iron and other metals (an alloy) that is very strong; it is used to construct buildings, bridges, trucks and automobiles. Much of the ferric chloride that is used today is recycled from a liquid left over from steel-making. During a process called steel pickling, liquid hydrochloric acid (HCl) is used to remove rust from the surface of steel. Removing rust prepares the surface for a protective coating.

If you think of steel as the "pickle" and hydrochloric acid as the "pickle juice," it's in the pickle juice that ferric chloride forms. That's where iron (Fe) in rust combines with chlorine in HCl, making FeCl3. Instead of discarding this product, today more than 40 percent of steel mills recycle it.

Ferric Chloride Uses

The pie chart below shows the most important uses of ferric chloride in the United States1. Notice that 60 percent of all ferric chloride is used for wastewater treatment. Wastewater is the water that flows down our drains after we use it for washing, bathing and flushing. Before it is released to rivers, lakes and oceans, wastewater takes a detour of its own--to the wastewater treatment plant. There, ferric chloride is used to clear the water of particles. And chlorine and other disinfectants destroy harmful germs.



1Kirschner, M. (Sept. 8, 2003). "Ferric chloride: September 8, 2003 - Chemical Profile - Industry Overview." Chemical Market Reporter.

For a list of previous "Chlorine Compound of the Month" features, click here.

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