CA1039758A - Lightweight inorganic material - Google Patents

Abstract

TITLE: LIGHTWEIGHT INORGANIC MATERIAL
Abstract of the Disclosure A lightweight inorganic material containing fly ash cenospheres, phosphoric acid and metal phosphates. The phosphoric acid and metal phosphates are initially contained in a liquid binder solution. When combined with fly ash cenospheres and after heating of the combined mass to a temperature below the sintering temperature of the cenospheres, a rigid, lightweight inorganic material is produced which has particular value as a construction insulating material.

Description

~odern utilization of rigid plastic foams as insulating materials in building and construction presents serious hazard in case of fire.
Fire tests in the laboratory, particularly with respect to plas~ic materials, are inaccurate and tell little about how a material will perform in a real building fire. The fact that a plastic material is not flammable has lulled the consumer into a false sense of security. The toxicity of the fumes developed by plastics in a building fire forms an additional hazard. These uncertainties and the potential danger of unrestricted use of plastics, particularly foamed plastics in building and construction, have been recently publicly denounced. The quick spread of recent building fires has frequently been blamed on the use of plastic as a component of walls or partitions.
While the plastics industry is aware of this flammability crisis and the danger of plastics in the fire, its attempts to counter the problem have been mainly directed into the incorporation of increasing amounts of fire-retardant chemicals into plastics. One industry source predicts that close to nine billion pounds of plastics would consume over :
one billion pounds of fire-retardant chemicals by 19~0. However, toxic fumes evolved from these fire-retardant chemicals will only be added to those resulting from the plastics themselves when decomposing l~der heat. , Even strongly "fireproofed" plastics will eventually become ignited, adding to the conflagration and flame spread by burning drippings. It may well be that the plastic industry is fighting a losing battle and that no "safe" plastic foam will ever be developed at a reasonable cost for building purposes.
This situat~on is particularly serious in the case of rigid -~
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plastic foams when used as wall or door insulation materials or as ceiling tiles in homes or in vehicles. In these cases, an inorganic and non-.. .
flammable lightweight material ou~h~ to be used instead. This need for a rigid, lightweight inorganic insulating material Ied to the development ~

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~39758 of the method and composition of matter described and claimed below.
This disclosure is concerned with material made entirely of fly ash cenospheres held together by an inorganic, heat-stable cement.
Cenospheres are hollow microspheres found in fly ash, the ash formed when pulverized coal is burned. Large quantities of cenospheres are produced as a waste product at coal-fired electricity generating stations. The fly ash is disposed of at such stations by mixing it into a slurry with water and pumping the slurry into a manmade lagoon or pond.
The cenospheres then float to the surface and may be readily collected.
~uch ponds are known to be located in Kentucky, Michigan, Missouri, the Tennessee Valley and West Virginia. They are found as well in many other parts of the world.
The quantity of cenospheres in fly ash varies from less than 0.01 to 4.8 percent by weight, but by volume the quantity of cenospheres can reach as much as 20~. They have a density of about 0.6 g/cm3 (37.5 lbs/cu.ft.). Fillite Ltd. in England has developed a separation ~` process which results in commercial quantities of cenospheres having a density of 0.4 gm/cm3 (25.0 lbsicu.ft.).
Chemically, cenospheres consist essentially of SiO2 (55-61%) and 2Q Al2O3 (26-30%), with smaller quantities o~ Fe203 ~2-10%), CaO (0.2 0.6%)~
MgO (1-4%), and Na20, K20 (0.5-4.0%). By their high content of A12O3, they dlffer distinctly in composition from commercial glass microspheres.
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Prior researchers have studied the behaviour of syntactic foams comprising epoxy resin-bonded cenosphere structures under hydrostatic pressure. This material was proposed for use as buoyancy material in ;
deep submersible vehicles. Some preliminary pressurization experiments onl:.these syntactic foams were carried out and suggested that cenospheres will~proba~ly perform as weIl as the manufactured microspheres for at ~ least some applications.
~ Although epoxy resi-n-bonded cenospheres had yielded structures - ;~

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~ 2--1~)397S8 which combined low density (0.45 to .83 g/cm3) with high compressive strength, they contained three parts by weight of the epoxy resin for two parts by weight of the cenospheres. When subjected to intense heat, composites with such high resin contents will still give up toxic fumes and are likely to disintegrate.
In search for an inorganic cement to replace the organic polymer, we found that phosphoric acid would serve as this binder when mixed with cenospheres. After gradual heating of these premolds the mixed mass yields lightweight, rigid inorganic specimens of good structural integrity.
It is believed that the cementing of the cenospheres is effected by formation of chemical bonds between the phosphoric acid and the aluminum oxide in the cenospheres under elimination of water mol~cules, viz... . . .. .
2H3PO3 + Al203 2AlPO3 + 3H20 ....
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This scheme is substantiated by the fact that only natural `
cenospheres, but no sy~thetic microballoons can be effectively welded together by phosphoric acid, since the synthetic microballoons do not , contain Al203 in their chemical composition.
In preliminary tests, structures with a density as low as :
0.24 c/cm3 ~15 lbs/cu.ft.) could be made with Fillite* cenospheres, and ;
with a density of 0.48 g/cm3 (30 lbs/cu.t.) with wator-floated ceno-spheres from West Virginia. Data on compressive strength of a few samples, together with the temperatures to which they had been heated, i~
are shown in the following table:
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1~)39758 SAMPLE PREPARED DENSITY COMBINED STRENGTH
AT C g!cm3 lb/cu.ft. Kg/cm2 lbs/sq.in.
. _ _ Fillite * Cenospheres 200 .40 25 41 583 Fillite * Cenospheres 950 .37 23 23 323 Cenospheres from West Virginia, l~ater-floated 200 .49 31 29 414 Cenospheres from l~est Virginia, Water-floated 950 .48 30 17 237 Cenospheres from l~est Virginia, Untreated 200 .72 45 64 911 Cenospheres from West Virginia, Untreated 950 .66 41 31 435 ::. .
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A new composition of matter is disclosed herein which is capable of forming a lightweight rigid monolithic inorganic mass after application of heat. The composition of matter consits essentially of cenospheres, phosphoric acid and a source of metal ions. As discussed below, where hygroscopic properties are no problem, cenospheres and phosphoric acid alone may be used to produce the composition. The metal ions produce metal phosphate in the binder liquid. Metal phosphate can be produced in situ during the preparation of the binder liquid or can be obtained , from conventional sources of con~mercial metal phosphates.
The method of bonding the cenospheres to produce the material comprises the step of combining the cenospheres with a liquid binder containing phosphoric acid, producing a moldable paste. The paste is then molded into the desired configuration of the final mass. It is finally heated to a temperature below the temperature at which the ceno- ;
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spheres sinter and cure.
It has been known for years that a small proportion of the particles in pulverized-coal ash consists of thin-walled hollow spheres commonly termed "cenospheres". The apparent density of these fly ash *Trademark -,:

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1(~39758 cenospheres is less than that of water. They separate from the dense ash in settlement lagoons. Different amounts of the light~eight cenospheres are produced by the boilers at different power stations. In some cases the quantity of ash cenospheres is negligible. Other boilers discharge sufficient amounts of cenospheres to form a thick layer of floating material on lagoons. The amount of cenospheres is influenced by the nature of the mineral matter in the coal being fired and the method of fly ash selection and disposal. ' ' Ihe following constitutes a brief summary of the properties of ' ,' cenospheres: ' a. ,Cenospheres average 20 to 200 microns in diameter and are ~ - , regular spheres with coherent, non-porous shells of silicate ', glass. The thickness of shell is about 10% of the radius.
The true particle density of the individual spheres is in the ~' , range of 0.4 to 0.6 g/cc and the bulk density 0.25 to 0.40 g/cc.
b. The floating spheres collected from lagoons are free from , soluble matter. ~ , , c. The hollow spheres start to sinter at 1200 C. and collapse , above 1300 C.
d. The gas in the cenospheres consists mainly of CO2 and l-l2.
At room temperature, the internal pressure is 0.2 atmos. ,' e. The cenospheres are folmed at an estimated temperature of 1400 C. and the formation and size are governed by the viscosit~ and surface tension of the fused silicate glass, by rate of change in particle temperature and the rate of ~ diffusion of gases in the silicate. The molten spheres freeze -~ at 1000 C. trapping the gases ~hich are formed internally through the catalytic action of ferric oxide or carbonaceous material prese~t.
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1~)39758 f. The chemical composition of cenospheres is somewhat variable but the principal constituents are aluminum, silicon and oxygen. T~e range of chemical composi~ion (percent) by weight is as follows:

Silica ~as SiO2) 55 to 61 Alumina ~as A12O3) 26 to 30 Iron Oxides ~as Fe2O3) 2~.~0 to 10 ~ -Calcium ~as CaO) 0.2 to 0.6 Magnesium (as MgO) 1.0 to 4.0 Alkalis (as Na20, K20) 0.5 to 4.0 Loss on ignition 0.01 to 2.0 It is obvious from the foregoing composition that a cenosphere is a glassy matrix of calcined clay in which the gases have had no opportunity to escape.
Some generating plants discharge sufficient quantities of fly ash cenospheres to form a thick layer of floating material on lagoons.
The floaters may create a significant air pollution problem, since on a warm day the top surface can dry and be blown away. Given favorable raw material and environmental conditions during combustion, one can expect as much as 4-5% by we.ight, or, on a ~olume basis, as much as 15-20% of the fly ash to consist of cenospheres.
In comparison to the dense fly ash materlal, the silica content of the cenospheres is higher, but the calcium oxide content is lower.
Cenospheres also contain a small amount (0 to 2~) of soluble material, in contrast with two to five percent of soluble material in precipitated fly ash.
Observations under optical and electron microscopes have shown that fly ash cenospheres are colorless glass spheres of sizes ranging from 20 to 200 microns. There is a noticeable absence of small particles, less than 10 microns in diameter, commonly found in dense fly ash, excluding - ~
:~ ~ '',,:' : -1~397~8the usual small amount of debris from broken sphere~. The cenospheres do have a demonstrated absence of pores on their surfaces. Blisters are occasionally seen on large particles.
The separatdon of dried cenospheres into different size fractions by sieving is much easier than with precipitated fly ash. The absence of the sub-micron particles prevents the formation of agglomerates and clogging of sieves. Cenospheres are much larger than the particles of dense precipitated fly ash from a given station. For example, floating cenospheres contain only five percent by weight of particles that are less than 50 microns in diameter, wherein dense precipitated fly ash might have more than 80% below this size.
The behavior of cenospheres at high temperatures has been observed by heating in a microscope. Published reports state that no change in size or in shape of the cenosphere occurs up to approximately 1250 C. Above this temperature the size of the particles slowly decrease, and at 1300 C.
they collapsed to a dense blob. Cenospheres from several sources were tested and it was found that they all collapsed at approximately the same temperature.
The collapse without expansion or sudden gas release suggests that either there was a partial vacuum in the particles or that the diffusion of gas in the particle shell was suficiently rapid to prevent increase of internal pressure on heating. A rapid collapse of particles took~place when these were inserted in a furnace at 1400 C.
On sintering there has been observed a marked difference in ceno-spheres in comparison with dense precipitated fly ash. Pellets made of ~` cenospheres require heating to 1200 C. before a bond by sintering occurs.
ense precipitated fly ash starts to sinter at 10Q0 C. to 1100 C. and " there is no significant change of volume during sintering. The high sintering temperature of fly ash cenospheres îs probably due to the , ~ , .
absence of sub-micron particles~and perhaps also to the fact that some of ~ ~ 7 ~

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9'758 the low melting glass is dissolved during separation of cenospheres from the dense ash by flotation in water. Another possible explanation is that the stable cenospheres form only from the silicates of a higher softening temperature.
These cenospheres are the starting material for a lightweight and fireproof material which is capable of replacing plastic foam in many applications, particularly as an insulating material for building and construction purposes. Tests of this material have shown it to be stable at temperatures as high as 2000 F. It will simply not burn because of its totally inorganic nature. Furthermore, the material can be produced inexpensively from the waste by-product cenospheres. The material is -believed to be an economic substitute for plastic in the manufacture of ceiling and wall tiles, door cores, insulation, trims and molding.
We have found that we can bond cenospheres with phosphoric acid plus various metal phosphates. The ones tested and shown to be satisfactory are aluminum, zinc, magnesium and chromium. Other metal phosphates are also satisfactory. The cenospheres can be bonded by using only phosphoric acid but this produces a very hygroscopic product. The metal phosphates seem to prevent this hygroscopicity.
The optimum amount of the acid-phosphate mix appears to be in the range of 1 part by welght acid mix to 0.5 to 3.5 parts of cenospheres.
The acid-phosphate mix composition depends upon the individual metal phosphate as some of them are not very soluble in phosphoric acid. In all , cases, the metal phosphates have been made in situ by adding a suitable oxide, hydroxide or carbonate to the acid. There is no reason however, to preclude the use of commerclal metal phosphates if they are available.
The range of molar ratios of metal phosphates to free phosphoric acid is from 0.1/l to 1/1.
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The times and temperatures of curing the premolds of binder and cen spher-s can be varied from 3 hours at 500 C. to 48 hours at 600 C.

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1~)397S8 and even to 3 hours at 700 C. but ~he optimum time and temperature seems to lie between 8 and 24 hours at 600 C. Heating the premolds to about 200 to 950 yields specimens of great structural integrity. Compressive strengths generall~ are about 750-800 pounds per square inch but values ~
up to 1500 psi have been obtained. ' ' Example 1 ~ ' The binder liquid was prepared by adding Al(OH)3 and H20 to 85~ H3PO4 with heating to cause solution.
Grams H3PO4 (85%) 384 ~' Al(OH)3 78 Thirty grams of the above binder solution was added to 90 grams of cenospheres. After careful mixing, the material was pressed into a mold, removed from the mold, and dried overnight at 80-90 C. The dried, moIded material was then heated to 600 C. for 8 hours and then cooled slowly.
Example 2 A binder liquid of the following composition was prepared:
H3PO4 (85%) 354 Grams Zn~OH)2 185 Grams ~l2 126 Grams ' ' The cenospheres were moistened with the binder liquid in the ratio of 3 parts by weight of cenospheres to 1 part by weight of binder liquid. The material was molded and dried as in Example 1. It was then heated for curing to 600 C. for 24 hours.
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'~ ''Binder L~q ''Grams H3PO4 ~85~) 317 MgO 30 ~ . -:~ ~ 9-~ " .'' ., .~ ~ . . ........ . , . ,~ . , .
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1~39758 The cenospheres and binder were mixéd in the weight ratio of 3:1, molded and cured as in Example 1.
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Binder L~ Grams 3PO4 (85~) 384 Al(OH)3 78 Mg3(po4)2 -5 H2O 30 The cenospheres and binder were mixed in the weight ratio of 3 molded, dried and cured as in Example 1.
The following chart shows the compressive strengths obtained by varying temperature and time of cure and cenosphere-binder liquid ratio.
The binder is that shown in Example 1.
Tem~rature of Cure 500 C.600 C. 700 C.
Time of Cure, Hours 3 24 48 3 8 24 48 3 Cenosphere/
Liquid Ratio Compressive strengths, psi.

2.5/1 400 600 485 765 703 700 500 535

3/l 350 384 407 608 676 770 600~ 599 3.5/1 300 400 316 405 544 448 325 463

4/1 350 300 400 391 548 651 400 494 With the binder used in Example 4 a compressive stren~th o 856 psi was obtained at 600 C. and 8 hours curing.
The binder in Example 3 gave a compressive strength of 1500 psi at 600 C. and 24 hours curing. However, this material was hygroscopic so it seems necessary to add Al(OH)3 to the binder and take the resultant decrease in strength.
The cenospheres that we have been using have a bulk density of 0.5 lbs. ~`
, ~pe~ . ~ t i~p~s~i~eto obta~n commercially cenospheres of varying densities from 0.2 to 0.5 lbs. per cubic foot. The density of the final , ~ ,., -10 ,,, ~ ""

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1~39~758 prod~ct thus can be lowered from the approximately 35 lbs. per cu. ft.
of the material produced in Example 1 to about 20 lbs. per cu. ft.
Any density between 20 and 35 lbs. per cu. ft. can be prepared by the proper choice of starting cenosphere materials.
The formulations can be adjusted so that the cured material is not hygroscopic, i.e., it will absorb less than 0.5 percent of its weight of water when kept at 100~ relative humidity for 12 weeks. All formu-lations, however, absorb water readily when immersed. They can be water-proofed by surface treatment with silane or other compounds of the type used to waterproof brick and concrete.
It is possible to accurately control the density of the lightweight resulting material. Density can be controlled by choice of the cenospheres.
Cenospheres vary in density and size from one source to another, and can be effectively segregated by size, using known sieve procedures. One ccan also blend cenospheres of various sizes to develop the required density in the final product.
The addition of a foaming agent in the acid also tends to produce porosity in the final product. The metal carbonates previously discussed as a source of metal ions are an example of such a foaming agent. Metal powders, such as aluminum and magnesium can also be used as a foaming agent.
It is also possible to use an organic filler in the composition which volatilizes when heated. '~xamples would be hollow phenolic beads or solid polystyrene or polyethylene powders. To make a heavier material, a~ailable fly ash can be utilized as a filler, although obviously other inorganic fillers can be added as desired.
The method of bonding the cenospheres comprises the combining of cenospheres with a liquid binder containing phosphoric acid. The binder and cenospheres are preferably mixed to produce a moldable paste. The paste is then molded into a mass that has the desired physical configuration in th~product being produced. The premold is preferably allowed to dry. `
Ovcrnight drying is normally sufficient when usir~ ccmpositions as discusscd :

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in detail above. Finally, the molded mass is heated to an elevated curing temperature below that at which sintering of the cenospheres occurs. Sintering is undesirable because of the notable loss of volume that results.
The liquid binder should preferably include a source of metal ions. Suitable sources of metal ions for in situ production of metal phosphates are metal oxides, hydroxides or carbonates. The liquid binder is in the form of solution of metal phosphates, free phosphoric acid and sufficient water to assure solubility of the metal phosphate in the solution. The solution should be a saturated solution of the metal phosphate in phisphoric acid.

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Claims (19)

The embodiment of the invention, in which an exclusive property or privilege is claimed is defined as follows:

1. A method of bonding cenospheres separated from the fly ash formed as pulverized coal is burned to produce a rigid, lightweight material, comprising:
combining cenospheres with a liquid binder containing phosphoric acid to produce a moldable paste;
molding the paste into a mass having a desired physical con-figuration;
and heating the molded mass to a temperature below that tem-perature at which sintering of the cenospheres occurs.

2. A method as set out in claim 1 wherein the liquid binder further comprises a source of metal ions.

3. A method as set out in claim 1 wherein the liquid binder is produced by adding to phosphoric acid a metal oxide, hydroxide or car-bonate as a source of metal phosphate in the liquid binder, the ratio of metal phosphate to free phosphoric acid in the liquid binder being between 0.1/1 and 1/1.

4. A method as set out in claim 1 wherein the liquid binder further comprises a source of metal ions;
the liquid binder comprising a solution of metal phosphate in free phosphoric acid, the ratio of metal phosphate to free phosphoric acid in the liquid binder being between 0.1/1 and 1/1.

5. A method as set out in claim 4 wherein the liquid binder is combined with the cenospheres in a proportion by weight of one part liquid binder to 0.5 to 3.5 parts cenospheres.

6. A method as set out in claim 1 wherein the molded mass is further produced by combining with the liquid binder and cenospheres, an organic powder or beads of a material that volatizes during the heating step.

7. A method as set out in claim 1 wherein the molded mass further comprises a material that foams during the heating step.

8. A method as set out in claim 1 wherein the molded mass further includes fly ash as an inorganic filler.

9. A method as set out in claim 1 wherein the temperature to which the molded mass is heated is in the range of 200-950° C.

10. An inorganic composition that forms a rigid monolithic lightweight mass upon application of external heat; said composition consisting essentially of:
cenospheres separated from the fly ash formed as pulverized coal is burned; and phosphoric acid.

11. A composition as set out in claim 10, further comprising:
a metal phosphate.

12. A composition as set out in claim 10, further comprising:
a source of metal phosphate selected from materials reactive with phosphoric acid and containing aluminum, zinc, magnesium or chromium.

13. A composition as set out in claim 10, further comprising:
a metal phosphate, the range of molar ratios of metallic phos-phate to free phosphoric acid being 0.1/1 to 1/1.

14. A composition as set out in claim 13 wherein the ratio by weight of metal phosphate and free phosphoric acid to cenospheres is in the range of 1/0.5 to 3.5 parts.

15. A composition as set out in claim 14 as results after heating thereof to a temperature in the range of 200-950° C. for a periodof 3 to 48 hours.

16. An inorganic composition that forms a rigid monolithic lightweight mass upon application of heat, said composition consisting essentially of:
a liquid binder comprising phosphoric acid, a source of metal ions and sufficient water to form a solution of metal phosphate and free phosphoric acid; and cenospheres separated from the fly ash formed as pulverized coal is burned.

17. A composition as set out in claim 16 wherein the molar ratio of metal phosphate to free phosphoric acid is between 0.1/1 and 1/1.

18. A composition as set out in claim 17 wherein the ratio by weight of liquid binder to cenospheres is in the range of 1/2.5 to 3.5 parts.

19. A composition as set out in claim 18 as results after heating thereof to a temperature in the range of 200-950° C. for a periodof 3 to 48 hours.