CA1191016A - Bond stabilization of silicate bonded sands - Google Patents
Bond stabilization of silicate bonded sandsInfo
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- CA1191016A CA1191016A CA000396904A CA396904A CA1191016A CA 1191016 A CA1191016 A CA 1191016A CA 000396904 A CA000396904 A CA 000396904A CA 396904 A CA396904 A CA 396904A CA 1191016 A CA1191016 A CA 1191016A
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Abstract
ABSTRACT OF THE DISCLOSURE
A method is disclosed of making bonded sand masses which retain strength under prolonged high humidity conditions. A body of sand, mixed with an alkaline metal silicate solution, is substantially dehydrated to provide a strengthened rigidized sand body bonded by water glass.
The dehydrated body is exposed to humid reactive gas to convert the water glass to polysilicic acid glass.
A method is disclosed of making bonded sand masses which retain strength under prolonged high humidity conditions. A body of sand, mixed with an alkaline metal silicate solution, is substantially dehydrated to provide a strengthened rigidized sand body bonded by water glass.
The dehydrated body is exposed to humid reactive gas to convert the water glass to polysilicic acid glass.
Description
~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 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.
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 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.
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~
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:
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.
..... . ... .
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~
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:
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.
..... . ... .
Claims (23)
1. A method making bonded sand masses, comprising:
(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) exposing the dehydrated body to humid reactive gas to convert said water glass to polysilicic acid glass.
(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) exposing the dehydrated body to humid reactive gas to convert said water glass to polysilicic acid glass.
2. The method as in Claim 1, in which said dehydration is carried out by heating said solution to above 100°C.
3. The method as in Claim 1, in which said reactive humid gas is comprised of CO2 in an amount of at least 25% of said reactive gas.
4. The method as in Claim 1, in which said mixture has an average particle size of between 30-90 AFS.
5. The method as in Claim 1, in which said mixture of step (a) contains organic materials limited to an amount no greater than 5% by weight.
6. The method as in Claim 1, in which said dehydration is carried out by the use of microwave energy for a period of time until the temperature of said solution exceeds 100°C.
7. The method as in Claim 6, in which the microwave energy dehydration is carried out until such time as the sand body is heated to 110-175°C.
8. The method as in Claim 1, in which dehydration is carried out in an internally or externally heated oven.
9. The method as in Claim 1, in which dehydration is carried out by heating said mixture while contained and molded within a core box.
10. The method as in Claim 1, in which dehydration is carried out by pulling a vacuum on said mixture.
11. The method as in Claim 1, in which said reactive gas is selected from the group consisting of CO2, SO2, HCl, and CH3COOH.
12. The method as in Claim 1, in which exposing the mixture to said reactive gas comprises blowing said reactive gas through said dehydrated mixture while porous.
13. A method of making bonded sand masses, comprising:
(a) substantially uniformly mixing an aqueous alkaline metal silicate solution with sand, the solid content of said solution constituting at least .75% by weight of said mixture;
(b) dehydrating substantially all of said solution of said mixture to form solid water glass; and (c) exposing said dehydrated mixture to humid reactive gas to convert said water glass to polysilicic acid glass.
(a) substantially uniformly mixing an aqueous alkaline metal silicate solution with sand, the solid content of said solution constituting at least .75% by weight of said mixture;
(b) dehydrating substantially all of said solution of said mixture to form solid water glass; and (c) exposing said dehydrated mixture to humid reactive gas to convert said water glass to polysilicic acid glass.
14. The method as in Claim 13, in which the solid content of said solution is 1-1.5% by weight of the mixture.
15. The method as in Claim 13, in which said alkali metal silicate is selected from the group consisting of sodium silicate, potassium silicate, or mixtures thereof.
16. A method of making bonded sand masses, comprising.
(a) 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 said aqueous alkaline metal silicate solution constituting at least .75% by weight of said mixture;
(b) dehydrating substantially all of said aqueous alkaline metal silicate solution of said mixture to form substantially solid water glass and to rigidize said mixture to a predetermined shape having a tensile strength of at least 150 psi (1.0 MPa); and (c) exposing said rigidized sand mixture to a humid reactive gas until substantially all of the alkaline metal in said mixture has reacted with said reactive gas to form polysilicic acid glass.
(a) 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 said aqueous alkaline metal silicate solution constituting at least .75% by weight of said mixture;
(b) dehydrating substantially all of said aqueous alkaline metal silicate solution of said mixture to form substantially solid water glass and to rigidize said mixture to a predetermined shape having a tensile strength of at least 150 psi (1.0 MPa); and (c) exposing said rigidized sand mixture to a humid reactive gas until substantially all of the alkaline metal in said mixture has reacted with said reactive gas to form polysilicic acid glass.
17. The method as in Claim 16, in which said reactive humid gas is comprised of CO2 in an amount by volume of at least 25% of said reactive gas, the remainder being air and water vapor.
18. The method as in Claim 1, in which said step (c) is carried out with a reactive gas maintained at a temperature in the range of 50-60°C and is exposed for a period of about 3-1/2 hours, the resulting sand mass having a stabilized strength of about 100 pSi.
19. The method as in Claim 1, in which said step (c) is carried out with a reactive gas consisting substan-tially of humid CO2 at a pressure in excess of ambient pressure and at a temperature in the range of 100-150°C, said exposure being carried out for a period of no greater than 15 minutes.
20. The method as in Claim 16, in which said aqueous alkaline metal silicate solution is comprised of sodium silicate having a silica to soda mole ratio of 2.0-4.0:1.
21. A method of preparing silicate bonded sand masses, comprising:
(a) uniformly mixing an aqueous sodium silicate solution having a silica to soda mole ratio of 2.0-4.0:1, with sand, the solid content of said solution constituting at least .75% by weight of said mixture;
(b) thermally dehydrating said mixture by subjecting to microwave energy effective to raise the temperature of said solution above 100°C for a period of time to convert said solution to substantially solid water glass; and (c) subjecting said dehydrated mixture to warm humid gas comprising at least 25%.by volume CO2 to convert substantially all of said water glass to poiysilicic acid glass.
(a) uniformly mixing an aqueous sodium silicate solution having a silica to soda mole ratio of 2.0-4.0:1, with sand, the solid content of said solution constituting at least .75% by weight of said mixture;
(b) thermally dehydrating said mixture by subjecting to microwave energy effective to raise the temperature of said solution above 100°C for a period of time to convert said solution to substantially solid water glass; and (c) subjecting said dehydrated mixture to warm humid gas comprising at least 25%.by volume CO2 to convert substantially all of said water glass to poiysilicic acid glass.
22. The method as in Claim 21, in which the time for step (c) is reduced by increasing the pressure of said warm humid gas or by blowing said gas at ambient pressure conditions through said mixture.
23. The method as in Claim 1, in which the content of said alkaline metal silicate in said mixture is in the range of .75-4.0% by weight.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US24208081A | 1981-03-09 | 1981-03-09 | |
| US242,080 | 1981-03-09 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1191016A true CA1191016A (en) | 1985-07-30 |
Family
ID=22913366
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000396904A Expired CA1191016A (en) | 1981-03-09 | 1982-02-23 | Bond stabilization of silicate bonded sands |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1191016A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE102013114581A1 (en) * | 2013-12-19 | 2015-06-25 | Ask Chemicals Gmbh | A method of producing molds and cores for metal casting using a carbonyl compound, and molds and cores produced by this method |
| CN114080284A (en) * | 2020-04-27 | 2022-02-22 | 雅马哈发动机株式会社 | Regeneration method of foundry sand |
-
1982
- 1982-02-23 CA CA000396904A patent/CA1191016A/en not_active Expired
Cited By (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE102013114581A1 (en) * | 2013-12-19 | 2015-06-25 | Ask Chemicals Gmbh | A method of producing molds and cores for metal casting using a carbonyl compound, and molds and cores produced by this method |
| US10773297B2 (en) | 2013-12-19 | 2020-09-15 | Ask Chemicals Gmbh | Method, using a carbonyl compound, for producing moulds and cores for metal casting, and the moulds and cores produced thereby |
| EP3092092B1 (en) * | 2013-12-19 | 2021-04-28 | ASK Chemicals GmbH | Method for producing moulds and cores for metal casting, using a carbonyl compound, and moulds and cores produced according to said method |
| CN114080284A (en) * | 2020-04-27 | 2022-02-22 | 雅马哈发动机株式会社 | Regeneration method of foundry sand |
| CN114080284B (en) * | 2020-04-27 | 2023-08-11 | 雅马哈发动机株式会社 | Foundry sand regeneration method |
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