When is sand not 'just' sand?
And why the details matter at the cooling stage
Author: Jamie Zachary
If you think sand is “just” sand, then think again.
In the metal casting industry, this granular material can be quite the chameleon. From size, shape and moisture content to flowability, strength and thermal conductivity, the varying properties of foundry (e.g. molding) sand can have a significant impact on the process solutions provided – particularly at the cooling stage.
Devon Robinson, Applications Engineer at Solex Thermal Science, points to green sand, as an example.
Green sand castings are made using wet sand molds for everything from ship parts to engine blocks. They are a mixture of silica sand with 18 to 30 per cent bentonite clay that helps make the sand stick when creating molds. In order to bind, it requires a higher moisture content — typically around eight per cent — which is much higher than common resin-bound (or no-bake) sand, for example, which generally has a near-zero moisture content.
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“That creates a different set of considerations when we cool it down during the reclamation process. If it still has a high moisture content when it comes into the heat exchanger, the sand will likely cake on the plates,” says Robinson, referring to Solex’s moving bed heat exchangers (MBHEs) in which the sand flows by gravity between a series of tightly packed plates, while a heat transfer fluid (e.g. hot water) flows counter-currently through the plate’s internal channels.
In other cases, it’s something happening upstream in the casting process that’s impacting the incoming sand – particularly when reclaiming resin sand.
After the casting has cooled and it is broken out of the mold, the sand is literally shaken off the casting — most commonly through agitation tables or rotary tumblers. Once shaken off though, the sand may still contain remnants of the resin.
“The typical process is a thermal reclaim where the sand is heated to around 800°C in a specially designed fluidized bed combustor that burns off any remaining resin and moisture that might still be in there,” says Robinson.
“However, if that process is different or the binder is different, then that incoming sand can end up exhibiting quite different moisture contents, thermal properties, etc.”
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In many cases, Robinson notes lab testing, or even pilot testing, can often play a determining role in providing technology partners such as Solex Thermal Science with a better understanding of a customer’s type of sand.
“There are times where we will want to look a little more closely because of a tight constraint in the process. Other times, it’s because of some application difference or difference in the inlet parameters for the MBHE. Or it could be the fact that it’s an olivine sand or chromite sand or something slightly different, and we have to double check the properties to ensure it doesn’t flow super different.”
Does that mean Solex tests every type of sand? Not necessarily, says Robinson.
“While the sand used and the process conditions can be unique, some types of sand will exhibit similar characteristics and generally fall within certain range of particle sizes, certain range of moisture contents,” he says. “After working extensively with foundry sand, we often know the property values already and how it will flow and cool within one of our heat exchangers.”
Did you know? Solex Thermal Science has worked with many of the world’s largest foundries. Our collaboration with Waupaca Foundry and MT Systems in the U.S. on a more efficient heat exchange solution resulted in significantly lower scrap/salvage rates and sand-additive costs. Read the full story.
Want to learn more about Solex’s heating and cooling solutions? Visit our Foundry Sand page for videos, handouts and more.
Ready to talk specifics? Contact a Solex team member today.
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