Last updated on: Apr 05 2007

An easy way to model a Dorr thickener

You can call it a Dorr thickener, clarifier or sedimentation tank. Whatever name you choose, it is a common fixture in steel mills, metals processing plants, water treatment facilities, and many other heavy industries.

The principle is simple. Water contaminated with heavy particulate matter flows into the center of the tank and the sediment drops to the bottom while the cleaner water rises to the top and flows out over the edge of the inner tank and is collected and sent on for further purification. The sediment is then removed from the inner tank and in the case of steel mills, can be recycled.

In Weirton, WV, there is a Dorr thickener that sits next to the BOF and is easily photographed from the public highway bridge.

I wanted to model a tank of this type for Magarac Iron & Steel, but I didn’t want to spend a lot of time scratch building. The basic structure is a steel cylinder with a cylindrical center column, and an inner tank that held the dirty water. Add to this a truss structure, some valves, pipes and a concrete base.

I needed a large tube with a thin wall for the basic tank, so I was in a quandary on how to get started for large tubing tends to have a thick wall.

The answer to my dilemma simply dropped into my lap. I went to Office Depot to purchase some blank CDs and there it was, the CD case that held 50 blank disks. It was round, thin walled, had the right proportions, and even had the center post and a lip around the edge that formed the separation between the inner and outer tank. Just about perfect.

It was a bit taller than I wanted, and it had a base with a cover locking mechanism, but that these could easily be handled so I bought 100 disks, and took my two CD cases home. I set about cutting the height down with a Zona saw and cutting off the protruding locking mechanism on the base. I probably could have left the case full height, for Dorr thickeners come in a variety of diameters and heights, but I wanted something that seemed to have better proportions when installed in the mill. My height selection was purely arbitrary. I have seen prototypes that sat close to the ground and others that stood better than 20’ high.

Next came the water. From the times I have seen the prototype, it always looked brackish and opaque. Since the water was always flowing the surface would look rippled or mottled. The CD case came with a plastic disk over the CDs that acted as a protective cover. I use this for the water surface painting it black with Poly Scale gloss black and then, after it dried thoroughly, dobbed on several coats of Clear gloss. By dobbing the gloss onto the surface I got the mottled, churning look of the water.

The plastic used for the CD case was thin and semi-flexible. I couldn’t get Poly scale to reliably stick to it so I purchased a spray can of Krylon black “Fusion” paint. This is a spray paint made to paint outdoor plastic furniture and kids play equipment. It really sticks well to these semi-rigid plastics. I then painted the Poly-Scale red oxide onto this surface and everything worked fine.

The rest of the construction process was pretty standard. I made the truss structure from Plastruct girders, added the walkway and railing using Tichy components and added the piping and valves.

The Dorr thickener now sits on the Magarac Steel & Iron mill site and completes the gas cleaning portion of the blast furnace.

Photo #1Door thickener at Weirton. You can clearly see the separation of the inner and outer tank and the center post and truss structure.	Photo #2 Here is another view of the same Dorr thickener which shows the height and physical arrangement of parts from another angle.
Photo #3 Here is the basic CD rom case before any modifications. The thin outer edge, the recessed center and the center post all seem to be designed to make the ideal Dorr thickener.	Photo #4 Here the case has been painted with the “Fusion” paint and the top filled with coffee to get the effect of water.
Photo #5 The transformation is beginning to take place. Several details remain to be added but the basic appearance is there.	Photo #6 The completed model installed at the mill. Magarac Steel & Iron will now make the EPA happy!

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