B is for Blinding

Therese Dunphy

February 1, 2010

An enterprising operator finds a solution to a geologic problem that had been in the making for 1.4 million years.
An enterprising operator finds a solution to a geologic problem that had been in the making for 1.4 million years.

An enterprising operator finds a solution to a geologic problem that had been in the making for 1.4 million years.

by Bill Langer

ˈblīn-diŋ (verb) – The blocking of screen apertures by the agglomeration of damp fine material, this results in a reduction of the effective area of the screen (Institute of Quarrying, 2006).

When aggregate operators are confronted with the problem of screen blinding, they may ask themselves, “Can we fix this problem?” (See the September 2006 issue of Aggregates Manager for some solutions to screening problems.) As a geologist, my curiosity goes in a different direction. I ask myself, “What is it about this particular deposit that causes the blinding?” A very interesting example of a super–sticky, gooey, screen-blinding deposit occurs about 15 miles from my home near Denver, Colo. I can view it from afar as I drive to work.

It all started during the cooler, wetter climate of the Ice Age, about two million years ago. Over many tens of thousands of years, flash floods of water carrying sand, gravel, and boulders sporadically roared down a steep canyon in the Rocky Mountains west of Denver. When the floods left the confines of the canyon and spread out onto the plains east of the Rockies, the water lost the ability to transport the material it was carrying and spread outward into a network of shallow braided streams, depositing the sand, gravel, and boulders as a huge alluvial fan referred to today as Rocky Flats. It must have been a pretty exciting time and would have been fun to watch — from a safe distance.

But if you weren’t there early enough, you would have missed all the action. About 1.4 million years ago, the flooding ceased, and the stream that deposited the fan was diverted around the north side of the fan. From that time on, life was pretty boring on Rocky Flats. Rainfall and snowmelt trickled through the fan. The ground would freeze and thaw, fracturing some of the gravel through thermal expansion and contraction. Plant roots and burrowing animals made even bigger pathways for water movement through the fan. Slowly, time took its toll: carbonate minerals were dissolved; iron-rich minerals were oxidized; and feldspars were altered into clay.

The pebbles, cobbles, and boulders in the fan are quartzite mixed with a smaller amount of gneiss and other rock types. The quartzite is very resistant to weathering, but the other rocks did not fare as well. What is left is a deposit rich in quartzite, which makes an excellent aggregate.

No surprise, then, that some enterprising operator would develop a gravel pit on Rocky Flats. Unfortunately, although the quartzite remained intact, 1.4 million years of weathering created a lot of clay — sticky, gooey clay — from the remainder of the deposit. When the alluvial fan deposit was processed for aggregate, this clay caused severe blinding of the screens.

The Rocky Flats operation changed ownership a number of times as producers successively failed to overcome the blinding issue. Finally, the problem was solved through the application of a flexible screen where parallel sides of the screen were moved in opposing directions, thus creating a shearing action that popped out the clay plugs.

Cool! Saved from another sticky situation.

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