Silica and Green Sand

I would not do that. If you adulterate the green sand too much, the quality of your castings will deteriorate very fast and that will cost you many times what you save. The local bronze foundry has a very complete recycling operation and is able to recover much of their green sand. The feasibility, of course, depends on a number of factors, in particular the resin you use and the metal(s) you're casting. Here's a link to a big GM operation that might give you some guide.

If you're using the Green Sand for water filtration you will need to conduct an Acid Solubility, a sieze size analysis and compare the glass to the green stuff, and a hardness test as starters.......

Why on earth do you want to contaminate the green sand in the first place with silica sand???? I get a Bad JUJU feeling about this idea.....

I am assuming that you are utilizing a clay bonded water activated silica based green sand system but you have not specified what metal you are pouring.

I will direct my comments toward an non-ferrous system, especially aluminum, and add a few comments toward a brass/bronze operation.

The major assumptions that I make is that you are reusing the sand, that it is NOT an olivine based system (to which you NEVER-EVER purposely add silica sand) and that it is not a proprietary additive system like Petro-bond or a Thiem Airdry J-82.

Basically you should figure on adding at least 100# of new pre-mixed green sand to your system for every ton (2000# or 1000 kg) of aluminum poured . Could be 150# for every ton if you're pouring a lot of heavier bronze.

I always recommend using a wheel and plow muller the first time you mix up new sand. A S-blade muller is useless mixing up new green sand but acceptable for re-mulling to just add water. Take 100# silica and add 4 to 5# water and mull for 1-2 minutes. For light aluminum castings then add 6# southern (calcium) bentonite to the muller as it's running to distribute it better and mull for another 5 to 10 minutes. Longer mulling I found will increase your green strength quite a bit. If you're pouring a lot of heavier aluminum or light bronze make it 4# southern and 2# western bentonite, heavier bronze maybe 3# southern to 3# western even or 4# southern to 3# western.

You should be making clay additions to compensate for burnout in your sand anyway. Depending on your typical sand to metal ratio you should be adding at least 8-10 # of southern for every ton of aluminum poured. That also should be done in a wheel and plow muller. If your main muller is a wheel and plow you can add the clay bond into the muller after the water is added and ratio the amount of bond to the size of the batch (muller capacity) so it gets evenly distributed in your system.

I added a link below that you can paste into your browser for a very good article on green sand control.

http://www.thefreelibrary.com/Tips+for+auditing+your+green+sand+system.-a054350163

Actually, the green sand i was referring to is clay-bonded silica sand, and the metal to be poured on the moulds is cast iron, aluminum, and bronze. The problem that i have encountered this past few months is scabbing and other sand erosion defects, looking at the history of my sand its been 2-3 yrs since "someone" added new silica sand in the sand system, and so i researched on many books and sites and they have one thing in common "replenish base silica sand" but i dont know what factors to consider in adding new sand. btw, happy hearts day.

I would be of a mind to say that you probably have a lot of dead (inactive) clay in your system and that your moisture levels are probably too high because of it. Dead clay wants water just like your active clay but it has little to no binding power. So now your people add more water to make the sand stick together. I'm surprised that you didn't mention pinholes from the water vapor since that is also a result.

Using only one sand system for that mix of metals is not the best. Aluminum poured at 1375F does not need the same sand requirement that cast iron at 2750F needs.

The iron will burn out the clay much faster than the aluminum or even the bronze which I would assume that you're pouring at 2050 to 2350F depending on the bronze alloy.

After that long a period without any new sand your fines (270 & Pan) must be pretty high from being worked so long without replenishment, probably well over 7% combined and therefore your permeability is probably not too good.

The most important thing that you should do is find a sand testing lab if your supplier doesn't have one and send out a sample. It is best to know where you are starting from.

Keep in mind that too slow a filling of a mold with a lot of surface area can also cause expansion defects like scabbing. It is not necessarily the casting weight that's important but the rate at which the sand surface feels the heat of the metal until that surface is wetted. At too slow a rate the molten metal starts drying out the surface of the sand and by the time the metal reaches the surface the sand has lost its strength and buckles or expands. Western bentonite generally helps to prevent that since it contributes much more to the hot strength than the southern does. Cast iron mixes are typically 5% western to 2% southern while your aluminum would be 5% southern to 1 to 2% western. I found that a 4% southern with 2% western was a good all around mix for aluminum and not too heavy bronze.

As I said in the earlier response you should be adding new sand at a rate of 100# per ton of aluminum poured (including gates and risers not just shipments) and 150# per ton of CI poured. Depending on the majority of your metal poured you can't hurt your situation by immediately starting to add new silica mixes to your existing system since it hasn't been done in such a long period and estimate from those numbers how much you should have added. I'm surprised that you have any left. Do not salvage the sand really stuck to the castings since that often is in the worse condition. If you try to save the really burnt out sand you will be adding more fines and dead clay to your system.

If you pour largely iron, then use 70-75 GFN silica with 4% western and 2% southern, 4% water. If more aluminum and bronze, 4% southern and 2% western and a 80-85 GFN silica and 3 1/2% water. On the initial mix use a wheel and plow muller and mull for no less then 7 minutes after adding your clay. But most importantly find a lab to test what you do have. Mix up a batch of your system sand like you've been doing all along and send them a 5-8# sample and have them run the standard green sand tests and post it here when you get it and we can go from there.

Iron foundries ofter add iron oxide to their mix and wood flour is used by a lot of non-ferrous people to help prevent expansion problems but a proper mix generally doesn't need those.