Processing sand ready for a batch of green sand

TLDR: Going to try coarser green sand for aluminium castings.

A few days ago I scored some free sand of random grain size, it looks like it's raw un-graded random particle size, freshly dredged from the river. All local sand mines are dredging from the river or river delta deposits so it's mined wet from underwater and pumped into piles. Particle size is up to 6-7mm or 1/4" and down to dust in effect. I picked up 100Kg or so for testing and there's more where it came from.

My last batch of green sand was made from fine beach sand and I've had difficulties packing it into deep patterns and had to resort to ramming in layers, not to mention having to up the bentonite content to get enough strength. With the new sand I'll have a larger average particle size to make ramming up easier to do (I'd better get the Navy handbook out and read up on the topic).

I sieved the dried sand on a tarp with with a flyscreen riddle and quickly sieved maybe 40% coarse to 60% fines. The coarse sand went onto the pathway between the house and shed and the fines about 1.5mm and smaller I saved in a bin for processing today. I spread a tarp the next day and taking note of the breeze I put a shop fan on one corner and sprinkled the fines into the wind blowing over the tarp: the dust blew a fair distance and didn't settle at all, the fines settled on the tarp and the large particles right in front of the fan.

The coarse sand and some of the fines closest to the coarse went back into the bin and the the level wasn't much lower than before. The fines spread over 2/3rs of the tarp were about 1/5th of a bucket, so not much of them. Compared to my earlier sand it's the same range of grain sizes but there's a lot more of the larger particle.

When I make some green sand up I'll report back on how it behaves.


Raw sand, it has mica and basalt in it.


Close up of the raw sand after drying.


After sieving and wind blowing the fines:


Sieve on the left was used as the larger mesh foundry riddle broke.


All of the wind blown fines:


Remaining sand, level in the bin is not much lower.

 

I had a similar experience with flyscreen and the inexpensive bagged sands from the home improvement store, but perhaps with more culled for being too large.
If I want nicer sand from there, I go for the paint texture or mortar sand that is uniform grade and kept dry in the store.
For K-Bond, I have been getting a 130 mesh product from a supply house that caters to sand blasting, which is right down the street from the firebrick and ceramic fiber blanket supply and a shop that carries fiberglassing supplies for my boat stuff so a 'trip to town' can be quite productive if not wallet-draining...

 

I looked up GFS after seeing your reply, makes sense that particle size is important. Sand for phenolic urethane resin super fine, I'd estimate 200 mesh or finer, it tamps down with minimal force. Interestingly when offered to people who make a sand/cement "render" for plastering concrete walls they said after trying it, that it's too "sharp" whatever that means. I'm on the lookout for fine screens, all I have is a stretchy weave fabric that gets the fines out but it slow and inconsistent to use because of the stretchiness, sort of like cloth over speakers.