Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~9~
BOND STABILIZATION OF SILICATE BONDED SANDS
BACKGROUND OF THE INVENTION
Silicate bonded foundry sands are widely known.
They have been used extensively in making molds and cores
for the casting of steel and iron. Such foundry sand
mixtures are composed of a major proportion of a suitable
refractory sand, an alkali metal silicate binder and
sometimes a small amount of other materials such as clay
and finely divided coal, or other organic matter, to
improve certain properties of the foundry sand. The
silicate bond is ohtained by precipitating a silica gel
from the alkali metal silicate by (1) the addition of an
organic ester or an acid producing gas such as carbon
dioxide; or (2) by dehydrating the alkali metal silicate
such as by evaporation, pulling a vacuum, or a heating
reaction.
Silicate is usually added in the form of a
solution to enable the binder material to regularly and
uniformly coat the grains of sand, forming a network of
interengaged silicate films. Thus, when the mixture is
dehydrated, water is removed from the silicate solution
reverting the binder to a water soluble glass which is
rigid. The new form or shape of the glass~ which it has
assumed as a result of coating the sand grains, envelopes
the sand grains and acts as a bridge tying the mass
together mechanically rather than by adhesion. The effect
is similar, but not as strong, when chemical additions,
- such as C02 gas, are used to cure the silicate solution
which coats the sand grains. The CO2 chemically reacts
with water to change the acid content of the solution,
resulting in the precipitation of silica glass or a
silicate with much lower sodium content and a sodium
carbonate~ Again~ droplets of glass (or silicate and
carbonate) remain as interconnected coatings or films
on the sand grains which when stiffened by sufficient time
promote an enveloping network that binds the sand grains of
the mixture. The specific chemical bonding reactions are:
Cure by Dehydration: [nSiO2:Na20:Bound Water~ aq
H ~t [nSiO2:Na2O: Bound Water] Solid Amorphous Glass
Cure by CO2 Gas: [nSiO2:Na2O:Bound Water]water solution
[C2^H2]water solution-~ SiO2 amorphous or
[n'SiO2:Na20:Bound Water] + Na~C03 where n'~ n.
Among the chemical addition processes, the use of
carbon dioxide has become the most widely used because it
possess certain advantages over other methods of producing
molds and cores. Nonetheless, the CO2 process has certain
disadvantages which include the generation of a harmful
carbonate of soda (Na2CO3), the resistance to breakdown
upon solidification of the casting so that the core sand or
mold sand is not easily removed, and the deterioration of
the strength of the reused sand due to the build up of
alkali metal compounds. These disadvantages have been
overcome in part by certain developments in the art, but
one significant disadvantaye remains. The water content of
the silicate sand mixture remains in the material after the
C2 processing. This results in bonded sand with low
tensile strengths.
Sand bound by dehydrated alkali metal silicates is
stronger than sand bound with a comparable amount of
silicate gassed with CO2. The tensile strength obtained
from dehydration generally ranges from 150-600 psi. Dehy-
dration by CO2 gas curing results in tensile strengths of
about 20-50 psi or at least one-third the level of that
obtained by dehydration. However, when the sand mass is
prepared by either curing method and exposed to a humid
environment for at least 24 hours, the tensile strength
L6
3 --
drops to 10 psi or lower and the mass completely disin-
tegrates if left in the same environment for longer than 24
hoursc The hygroscopic nature of water glass is sub-
stantial. The ability to pick up moisture during storage
must be reduced or eliminated if silicate molding processes
are to become viable commercial methods.
SUMMARY OF THE INVENTION
The invention is a method of making bonded sand
masses and particularly sand masses which may be used as
1~ molds or cores in the casting of metals. Large scale
production foundries must store prepared molds or cores for
ready use, which storage may be for several daysO High
humidity conditions in such foundries will cause the bond
strength of such sand masses to deteriorate significantly
during such storage. To obtain and stabilize high bond
strength for short or long term storage, the present
invention has discovered that the problem can be obviated
by following the sequence of:
(a) substantially dehydrating a body of sand
mixed with an alkaline metal silicate solution to provide a
strengthened rigidized sand body bonded by water glass; and
(b) subjecting the strengthened rigidized sand
body to humid reactive gas to reduce the water glass to
polysilicic acid glass, thereby eliminating the water
sensitivity of said sand body.
- More particularly, the method comprises mixing an
aqueous alkaline metal silicate solution and sand to form a
substantially continuous solution film about each of the
sand grains, the solid content of the aqueous alkaline
metal silicate solution constituting at least ~75% by
weight of the mixturec Substantially all of said solution
of said mixture is dehydrated by heating above 100C to
form substantially solid water glass and to rigidize the
o~
mixture to a shape having a tensile strength of at least
150 psi (1. MPa~. Finally, the sand mixture is exposed to
- a humid reactive gas until substantially all of the water
glass is converted to silica glass or substantially all of
the alkaline metal in said mixture has reacted with said
reactive gas. A humid gas is defined herein to mean a gas
having a vapor pressure exceeding the vapor pressure
(usually 23 Torr) of water vapor at ambient conditions.
It is advantageous if the alkaline metal silicate
solution is comprised of an aqueous sodium silicate solu-
tion, the mole ratio of silica to soda being in the range
of 2.0-4.0:1, and optimally about 2.75-3.3 1. It is pre-
ferable if the sand is comprised of a type which has an
average particle size of 30-90 AFS, said particle size
standard being defined in The Foundry Sand Handbook,
published by The American Foundrymen's Society, 7th Edition
(1963). The mixture may also include small amounts t5% or
less by weight) of other materials such as organic resins
or cereals introduced for shake-out and other physical
properties.
In carrying out the second step of dehydration, it
is preferable if heating is carried out by use of microwave
energy so that the aqueous alkaline metal silicate solution
is heated to a temperature in excess of 100C, advanta-
geously 110-175C. The energy level required of the
microwave depends on the water content of the silicate
usedO
The humid reactive gas may preferably be comprised
f C2 in an amount of at least 25% when mixed with air and
water Yapor. However, the reactive gas can be any acid gas
such as CO2, SO2, HCl, and CH3COOH, that will affect the
chemical conversion. Using a substantially pure CO2 humid
gas at ambient will permit the resulting sand body to have
a tensile strength of at least 150 psi (lo MPa) after the
conclusion of such exposure.
To reduce the time for exposure to the humid
reactive gas, it is preferable to increase the temperature
- of said reactive gas during such exposure to the range of
45-60C which reduces the exposure time to as little as
3-1/2 hours, with a similar tensile strength. A still
further reduction of the time of treatment with reactive
gas can be achiev2d by increasing the temperature of the
humid reactive gas to the range of 100-125C while in-
creasing the pressure of said gas from ambient to in e~cess
of 20 psi; the period of treatment is reduced to 15 minutes
or less with the resultant sand body having again a similar
tensile strength.
DETAILED DESCRIPTION
A major characteristic of sodium silicate bonded
sands is the change in properties of the bond as a function
of humidity. With cure carried out by the use of CO2 gas,
both low and high humidity ènvironments have shown to cause
a reduction in the bond strength. In the former case,
water is lost over a period of time and sand composites
become very fragile. In the latter case, the essential
hygroscopy of the material produces a water pick-up and the
composite can essentially disintegrate.
The dehydrated strength of an alkaline metal
silicate bonded core depends upon three factors: the
number of necks formed or the number of grain/grain contact
points (where the binder forms a neck or junction), the
thickness of the binder at the necked junctions, and the
strength of the binder film. It is the strength of the
binder film that is primarily affected by moisture during
storage periods in foundries. Alkaline metal silicate
solutions tend to form very good grain/grain contact points
as well as relatively thick-necked junctions when properly
mixed with the sand and dehydrated.
, . . .
9~LQ~L6
~ preferred method in accordance with this
invention to obviate the problem of humidity degradation of
sand masses mixed with alkaline metal silicate solutions is
as followsc
(1) An aqueous alkaline metal silicate solution
is mixed with sand to form a substantially continuous
liquid film about each of the sand grains, the solid
content of the alkaline metal silicate constituting at
least .75% by weight of the mixture.
(2) The wet sand mixture is then dehydrated by
various ways such as by heating said solution above 100C
to form water glass and thereby rigidize the mixture to a
predetermined shape having a tensile strength of at least
1. MPa.
(3) Exposing the rigidized sand mixture to a
humid reactive gas until substantially all of the alkali
metal content of the water glass is converted to carbon-
ates, thus converting the water glass to polysilicic acid
glass.
Mixing
It is preferable that the alkaline metal silicate
be sodium silicate. Other operable silicates included
within the group are potassium silicates, mixed sodium and
potassium silicates, and mixed sodium and lithium sili-
cates. The method works best when the proportion ofalkaline metal silicate solution is arranged so that the
silica to alkali metal occupies a mole ratio of between
- 2.0-4Ø Lower mole ratio silicate solutions tend to
increase the alkali metal, which promotes water pick-up in
the final bonded sand mass. Although a higi?er mole ratio
alkali metal to silicate solution may be employed, being in
excess of 4.0 is disadvantageous because such silicates
form discontinuous films on sand and thin-necked junctions
when dehydratedO When working with mole ratios below 2.0,
1~9~6
it is necessary to increase the length of time at which the
~:eactive gas is exposed to the sand mass; the time changes
linearly in proportion to the decrease in mole ratio.
It is desirable that the amount of solid content
of the alkali metal silicate solution be within the range
of .75~4.0 weight percent of the sand mixtureO Such weight
percent is defined as including the weight of the soda and
silica in the formulation as well as the weight of bound
water. The bound water after dehydration will be present
1~ as hydroxyl ions and protons. It is possible, within the
operable limits of this method, to use weight percent
alkaline metal silicate above 4.0%, but such amounts add
considerably to the cost of the method without a propor-
tionate justification for it in terms of increased useful-
ness of the product. The strength of the sand massaccordingly increases with increasing amounts of silicate.
However, the shake-out characteristic of the sand mass is
significantly reduced. There is a production of rock
particles within the sand as a result of metal heating
which prohibits the sand from being easily removed if the
weight percent of the silicate is excessive. The optimum
weight percent of silicate has been found to be in the
range of 1.0-1.50% of the sand mass.
The particular sand employed for the method need
not be of any special kind. However, the average particle
size should preferably be in the range of 30-90 AFS. Such
size identification is defined in The Foundry Sand Hand-
book, published by The American Foundrymen's Society, 7th
Edition (1963). The sand need not be dry to be employed in
this process.
,- " ,,
~191~
-- 8
Although other additives may be made to the basic mixture
to improve other physical characteristics, such as the
addition of organic resins to facilitate shake-out, such
additions should be limited to 5~. Fly ash, as an
additive, should not be used in this method since it adds
alkali metal compounds which are undesirable.
The sodium silicates required of this invention
are produced by melting sodium carbonate with silica (SiO2)
at silica:soda ratios varying from 1:1 to 3.75:1, adjust-
able by adding sodium hydroxide (NaOH) to the moltenmaterial. The molten glass is then quenched and dissolved
in water. While most silicates used in foundries are
purchased in liquid form, solid hydrous products produced
by flash evaporation are also available.
lS The composition of solid hydrous silicates thus
produced are identical to those formed by dehydration of
silicate solutions within a sand mass as described in step
(2). It is possible that solid alkaline metal silicates
- may be added to the mixture in a dry form and water added
to the sand in a predetermined amount to create the
solution efect.
Mixing can be appropriately carried out in a
mulling device or other equivalent mixer for a period of
time until the liquid solution substantially uniformly
coats each of the sand grains. The mulled mixture is then
preferably and carefully tucked into a molding device for
either forming a casting mold or a core, which body is then
treated in the remainder of the process.
-
Dehydration
It is preferable to employ microwave energy for
dehydration or by heating in an oven or in a hot core box,
although other forms of water removal are acceptable~ The
dehydration step might also be accomplished by application
of a vacuum to the sand mass, Microwave heating or curing
~19~
works when an electromagnetic wave of microwave dimensionsis propagated in a heatable dielectric material, its energy
being converted to heat. Water is the major dielectric
material in the method herein that is heated by microwave
energy; the dielectric is more accurately a sodium
silicate/water solution. Water molecules consist of
hydrogen and oxygen atoms arranged so that each molecule is
electrically neutral. Because of this arrangement, the
electrical charges within the molecule have a dipole moment
and are said to be polar. Different molecules have
different degrees of polarity. A microwave field exerts a
twisting force on a polar molecule that attempts to align
the molecule with the field. When the direction of the
field is reversed, the molecule attempts to reverse its
orientation. However, in doing so, frictional forces
created by the molecules rubbing together have to~ be over-
come. Energy is thereby dissipated as heat. Friction
generates heat and the dielectric becomes hot. Electrical
energy that should be stored in the dielectric material is
in part lost as heat, often called dielectric lossiness.
Sodium silicate/water solutions are particularly
dielectrically active or lossy in this regard and heat up
quickly when exposed to a microwave field. It is typical
to employ an energy source which is in the range of 2-5
kilowatts, the frequency for said energy being typically
about 2450 MHz. However, the level of energy employed
should be that which is necessary to raise the temperature
of the water to exceed 100C and advantageously 110-175C.
Typically, this will occur after a two minute
exposure to microwave energy at such frequencies; the final
temperature achieved is usually around 115C. The amount
of microwave energy employed may be regulated in a ratio of
about .71 killowatt for each 100 grams of sand mixture.
~19i~16
~ 10 --
The water glass (silicate formed as a result of
dehydration~ is not sticky or adhesive~ It carries out a
bonding function by forming a film about each of the sand
grains, which films are interconnected at necked portions
to adjacent films~ Glass, which is rigid, forms an inter-
connected structure enveloping the sand grains, such net-
work forms a very strong intersupporting mehanism. The
actual chemical change that takes place during this dehy-
dration may be thought of as follows:
10OH OH OH 2-
OH ¦ OH ¦ OH I
HO -Si O- --S~ O - S~ - OH , 2Na+
¦ ~ ¦b H ¦ OH (aq)
OH OH ~ (aq~
1 dehydration
. __
. q-Na+ O~H ~~Na
HO~ Si O Si - O---Si OH
. OH dH OH
. solid
.... . ...
~19~
It $s typical to obtain strength levels of at
least 450 psi (3.1 MPa) when employing alkali metal
silicate in amounts of at least 2~ by weight of the sand
mixture. Typically, dehydrated sand/silicate strengths can
range from 150-600 psi.
sure to Humid Reactive Gas
The reactive gas employed may be any acid gas such
as CO2, SO-21 HCl, CH3COOH 5acetic acid), etc. CO2 gas is
preferred because it provides no noxious emissions and
10 provides optimum strengths. The CO2 gas, under humid
conditions, forms carbonic acid, which in turn reacts to
draw the soda out of the water glass to form a polysilicic
acid glass. The polysilicic acid glass operates similar to
the water glass, being an interconnected glass network
enveloping each of the sand grains to provide a strong
mechanical binding network. Howeverl the strength of the
polysilicic acid glass network is slightly lower than the
strength of the water glass network. The actual chemical
change that takes place during this process may be thought
~f as follows:
r O~Na+ OH l~Na+
HO~ O ~ O - Si - OH + C02oH~O
H OH OH
_ solid
2 5OlH Ol O~
~0--S~l--O--S~i--O ~ S i--OH + Na2 C3
OH OH OH :.
: polysilicic acid
gla3s
119~
- 12 ~
It is typical when working with a humid CO2
reactive gas at amb;ent temperature conditions for the
dehydrated sand mass to be exposed for at least 14 hours to
obtain substantially full reaction of all of the sodium in
the sand mixture. It is not necessary to use pure CO2 gas,
al~hough this is desirable; other mixtures usinq CO~ in the
range of 25 to 100% may be employed, with the remainder of
the reactive gas consisting of air and water vapor. The
important aspect of the reactive gas is that it be a humid
gas with a CO2/H2O ratio greater than unity~ It has been
found that the optimum proportion of CO2, air and water
vapor at ambient temperature (25C) conditions and pressure
is CO2, 25 Torr; H2O, 24 Torr (saturated), air remainder.
The stabilizing reaction with the reactive gas produces a
strength in the resulting sand mass which is typically one-
third of the dehydrated strength, giving values that can
range from 50 to 200 psi.
The amount of time at which the reactive gas is
exposed can be reduced to a period of 3 to 3-1/2 hours if
the temperature of the humid reactive gas is increased to
the range of 45-60C, still at ambient pressure conditions.
It is surprising that temperatures above 60QC, notably 80C
while at ambient pressure, do not further reduce the time
required for stabilization. However, stabilization times
can be reduced by blowing the humid CO2 gas through the
sand body~ The times required for this gassing are
dependent on the configuration of the sand body. Yet still
further decreases in the amount of time at such exposure
- ~an be obtained by increasing the pressure of the reactive
gas ~o be in excess of ~0 psi, while also increasing the
temperature of the reactive gas to a range of 100-125C.
In this case, the exposure time can be reduced to no
greater than 15 minutes and as little as 7 minutes.
3~9~6
- 13 ~
Certain test examples were prepared and examined
to substantiate the above method procedures. Specimens
were prepared by mixing 1050 grams of wedron sand with
49.50 grams of 3:1 mole ratio sodium silicate (which is
42.4% solids)O Ten standard tensile samples were prepared
following specifications of the American Foundrymen Society
which after dehydration weighed 101.88 grams. Humidity
stabilization or treatment was accomplished in a pressure
chamber flushed with carbon dioxide. The samples were
treated with (.14 MPa) 20 psi carbon dioxide and 1 cc
liquid water at ambient temperature. The temperature of
the chamber was increased to 115C for 15 minutes.
In order to test the humidity resistance of
treated samples, they were exposed for at least 24 hours to
97% relative humidity at 25C without added carbon dioxide.
The tensile strength of the treated samples exposed to such
humidity remained at 1.2 MPa, 170 psi, while the untreated
samples exposed to humidity had less than OOS MPa, 10 psi,
tensile strength.
..... . ... .
