[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Spray Cabinet Sludge

These two salts are common alkaline builders used along with organic surfactants
in aqueous cleaners.  The silicate is needed to protect soft white metals (
aluminum and zinc) from alkaline attack, is  good for suspending soils and
preventing redeposition of soils.   I suspect the initial operating pH of this
solution is around 10-11.  The pH tends to drop over time because of acid soils
and CO2 absorbtion.  Silicates become less soluble at the lower -but still
alkaline- pH and drop out.  In hard water areas, calcium from the incoming water
can also form calcium silicate salts that are insoluble.  Calcium tends to cycle
up from make up water.  Check out your water source too.  Some municipalities
treat water with calcium (lime).  Some of it remains in the water.

Tetra sodium pyrophosphate (TSPP) is excellent for buffering, sequestering metal
and hard water salts( calcium and magnesium), and removing particulate soils.
TSPP solubility is less pH sensitive, but may revert to the ortho sodium
phosphate salts at lower alkaline pHs.  Again, calcium  from the water can also
form calcium phosphate salts that are insoluble.

The sludges likely are a mixture of insoluble soils from the work being cleaned,
calcium salts of silicate and phosphate.   If hard water is used for initial
charging and makeup, then calcium salts could build up as sludge.  Using soft
water may help reduce sludge because it has no calcium.  Also, maintaining a pH
of 10 or so will help prevent silicate precipation.   The use of an oil
splitting surfactant in the cleaner formula and a oil skimming devise can help
extend the useful life of the cleaner and extend dumping cycles.  If the system
is large enough to warrant the investment, closed loop filtration equipment will
greatly extend cleaner life and remove the insolubles - a little at a time.

You may want to have a lab check out the salt content in the sludge before and
after improvements.  I think some silicate and phosphate will be present, but
not as pyro or metasilicate.  I can't predict the percentages.

mgaither@pprc.org on 08/10/2000 02:57:23 PM

Please respond to mgaither@pprc.org

To:   p2tech@great-lakes.net

Subject:  Spray Cabinet Sludge

Client has historically paid attention to heavy metals content and pH of the
aqueous sludge generated from cabinet spray ops.

Now wondering if this sludge designates as toxic due to 2 detergents used -
Sodium Metasilicate and Tetrasodium Pyrophosphate (TSP).  Both are salts, are
toxic, and would only be allowed in the sludge at less than 10% for solid waste
disposal (per local/state designations).

As they are salts, these substances would not be expected to "leave the

Has any one heard of the fates of these chemicals in an oily environment? Can
they be expected to be found in the sludge at the same volume they were added?
Since they lose there cleaning effectiveness, are they some how "used up"?  Any
insights would be appreciated.

Michelle Gaither
Pollution Prevention Resource Center