CA1056108A - Thixotropic mixture and method of making same - Google Patents

Abstract

THIXOTROFIC MIXTURE AND METHOD OF MAKING SAME
Abstract of the Disclosure A mixture for the storage of heat energy utilizing a heat of fusion material includes sodium sulfate decahy-drate, borax as a nucleating agent, and hydrous magnesium aluminum silicate (attapulgus clay) as a homogenizing agent.
This composition maintains the salt-hydrate in suspension during repeated heating and cooling cycles.
A method is also disclosed for preparing the mic-ture which includes the steps of mixing water with the attapulgus clay, adding a nucleating agent to the initial mixture, and mixing the initial mixture, including the nucleating agent, with a heat of fusion material such as sodium sulfate decahydrate.

Description

:L~ ,ac~k~round o~ the Inventlon With the increased use o~ solar enerOy heating, it becomes almost an cbsolute necessi~y that thermal ener be tored in orcler to spread the e};ces solar heat avail-able clurin~ the daytime for use at night ancl on cloudy da~. The use Or heat Gf rusi n mater_al ~or this heat sta~ e has ~ained in^reasin~ or in recent times due to ts lo~l cost and hi~h heat of ~uC~ion per unit wel ht.
Such heat o~ fusion rnaterials declrably should meet the criteria o~ low cost~ availability in lar~e quant:1ties, and simplicity of preparation. In addition, it is pre-~erred that they be non-to~ic, non-flamma~le, non-com-bustible and non-corrosi~re. Ihe lowes~ cost materiQls ~or use are lar~e volume chemicQls based on compounds o sod~um, potassium, magnesiur.rl, al~lminum and iron. Pre-~.

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.~erably, the mate:ri.ala are ln the form of salt-hydrates and the:l.r eutectics. The type of low cost compouncls are re~.3t;rlcted to ~hlor:l.cle~s, n:il;ral;es, sul:fates, phosphates a~l~l carbonate~ ila aclditives or mo(llf`lers may include ~j bo.ratQu, ~Iyclrox;i.cles and sLll.cates. Arnong ~hose low cost salt-hydratQ~ havlng h:lgh heat o:E`.fusion, low cost and lowest ~ncompatlbil:Lty due to undes:Lrable properkies are included:

Heat of Fusion BTU
Chemical per Dens if3y Compound Point,F Pound lb/ft Calcium chloride hexahydrate12 2 84-102 75 102 Sodlum carbonate decahydrateNa2C3 1H2 90-97 106 90 Disodium phosphate dodecahydrate Na2HPOL~ 12H O 97 114 95 Calcium nitrate tetrahydrateCa(N03)2 4H20 102-108 60 114 Sodium sulfate decahydrateNa SO lOH O 88-go 108 97 Sodium thiosul:~ate pentahydrate Na2S2~3-5H20118-120 90 loL~
In use these materials are usually placed in sealed containers together with a nucleatin~ agent andl.:
subjected by the system to successive heating and cooling cycles above and below the melting point of the heat o~
fusion material selected to use the "stored" heat or cold.
The need for nucleating agents is déscribed in .~
U.S. Patent 2,667,664 issued May 4, 1954 to Maria Telkes. ~ :-As is described in the Telkes patent, a suitable hetero-genous nucleating agent may be borax (sodium tetraborate decahydrate) in small quantities, about 2 to about 5~
These nucleating agents provide for the necessary seeding . . . .. . . . .

to Lnitlate the rormatlorl of crystals ancl thereby avoid supercoolinp, wh:!ch can occur ln llquid solutlons at rest.
Othcr known nucleatlng technlques may be used to prornote crys~allLzat:lon. Crystallization, Or course, :Is necessary to make use Or the heat o~ fusion o~ the mater:Lal. With supercoollng, only the specific heak of the materlal is used. The spec:Lflc heat of a material is far less than its heat of fusion - hence the need for nucleation. When using sodium tetraborate decahydrate (a near-isomorphous nucleating agent) in combination with Na2SO~ lOH20, it is possible to obtain complete crystallization in a melt by inverting or occasionally shaking the container after the crystals start to form. When used for the storage of heat energy, unfortunately, it is not always convenlent, or for that matter, possible to shake the containers.
Another problem encountered in utilizing heat of fusion materials has been that after several cycles of heating and cooling, liquid tends to separate from the salt-hydrate and form salt crystals of lower hydration or for that matter, anhydrous salt, with a corresponding -loss of available heat of fusion.
Stated di~ferently, the melting of sodium sulfate decahydrate and many other salt-hydrates is found to be `~
partly incongruent. That is, during melting some anhy-drous sodium sulfate remains undissolved in its water of crystallization which is released in the melting. Due to its higher density, sodium sulfate sinks in the sat-urated solution. When the mixture solidifies again without mechanical mixing or stirring, dissolved sodium - 3 - ` ~

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sul~ate combines w:ith water of crystalllzation, but those heavy cryst~ls of sod:l~ sul~ake on or near khe bottom of l;he con~;~lner recomb:~ne only with waker molecules ln thelr In~medlate v:lcln:~y formltlg solld sodlum sulfate ~ecahydrate S cry~tals rJ'h:ls solid layer prevents further recomblnatlon Or the remalning sodium sulfate wlth the balance of the water of crystallization. Due to this effect, molten sodium sulfate decahydrate, when lt solidifles without stlrring or without additlves, forms three distinct layers - a bottom layer of white anhydrous sodium sulfate crystals, some embedded into crystals of sodlum sulfake decahydrate, then a larger lntermediate layer o~ trans-lucent sodlum sulfate decahydrate crystals, and on top, a layer of liquid saturated solution. The heat of fusion required to melt thls salt ls 108 B~U's per pound which could be released again if the salt could be homogenized during solidification by stirring or by sultable additives.
During coollng, (without homogenlzing or stirrlng), the heat release is less, because part of the sediment cannot regain its water of crystallization. Some saturated solution remains in this case when the mixture is cooled depending upon the solubility of the salt. Separation and settling of the salt-hydrate must be prevented. ~-Over the years various thickening agents have been included in heat storage mixtures as additives with the aim of producing a gel in which the salt-hydrate does not setkle out even over successive heating/cooling cycles.
Many different thickening agents have been kried including wood shavings, wood pulp, sawdust, various types Or - 4 ~

:~5~8 cellulosic mixtures, and an organic material sold under the tradename *METHOCEL", starch and organic alginates.
Inorganic thickening agents were also used, such as silica cJel, d~atomaceous earth, and other finely divided silica products. Many of these materials perform quite well but only Eor a limite~ number of cycles. Some of the organic materials become slowly hydrolyzed or decomposed by bacteria or by enzyme action. In many cases such action can be prevented or slowed by adding small quantities of formaldehyde or other suitable agents. Wood shavings, wood pulp and the like were not found to be durable enough. Silica gel formed in the mixture itself proved to be a hindrance to filling the mixture into containers because it thickened too quickly.
Eutectics of the salt-hydrates are used to modify the freezing point of the various hydrates. For the most part, the eutectics are based on low cost compounds such as sodium chloride, ammonium chloride, potassium chloride and other kno~n types. Most eutectics also require a nucleating agent as ~ell as a homogenizing or thickening agent since they tend to melt partly incongruently. The homogenizing agent prevents the settling of the higher density anhydrous components.
Ac¢ordingly, it is an object o this invention to ~ -obviate many f the disadvantages of the prior art heat of fusion mixtures.
Another object of this invention is to prov~de an `~
improved heat of fusion material in which water and * Trademark ~ S~8 the salt hydratc have a re~uced tendency to separate dur:ing free~:Lng ancl melting.
A rurther ob~ect of thls lrlventLon ls to prov:icle an ~ provecl ~et;hotl ror making heat of ~uslon mlxtures wh~ch mell; the same way as congruent materlals do.
~rle~ Descrlptlon o~ the Inventlon In accordance with a preferred embodiment o~ thls lnventlon a mixture f`or the storage o~ heat energy utlllzlng the heat O:r fuslon o~ a material comprises salt-hydrate, a nucleating agent and a homogenizlng agent, wherein the homogeniæing agent ls a clay-type substance which exhlbits thixotropy. The clay-type substance is made up o~ particles that are lath-lilce in appearance. Preferable, an attapulgus-type clay is used.
A pre~erred method of preparing a mixture ~or the storage o~ heat energy, as described above, includes the steps o~ mixing water with a clay-type substance which exhibits thlxotropy by vlrtue o~ lts partlcles being lath- -llke to form an lnitlal mlxture, and mlxing the initial mixture with a salt to ~orm the hydrate. When the com-posltlon ls thus prepared, lt provldes a stable suspensoid exhibiting thixotropic properties whlch encapsulates all o~ the crystals of the salt-hydrate and prevents them ~rom dropping to the bottom o~ the contalner and thereby dimin-ishing the heat of fuslon. In short, such mixture preventsthe salt-hydrate ~rom meltlng incongruently such that the salt-hydrate does not separate ~rom the solution.
Detailed Description o~ the Invention ~-The mlxture of this lnventlon acts as a homogen-izlng or thiclcenlng agent ~or salt-hydrate materials used ~v~

for the storage of thertnal energy. The thickening agent preven-ts water solut:Lon from separating out of the salt cr~stclls due lo partly :l.ncongruent meltln,r of the salt cl~!ts~lLs durLng l,he successive heatinr and coolin~ cycles S wh:l.ch typlcally occurs in such materials when used for the storare o~ thermal energy. Salt-hyclrates ~hat may be used for the storage Or thermal energy include those set forkh herelnbefore. These preferred salt-hydrates, which practically should have a heat of fusion greater than 50 BTU per pound, include calcium chloride hexahydrate, sodium carbonate decahydrate, disodium phosphate dodeca-hydrate, and sodium thiosulfate pentahydrate. These are selected because of their rela~tively high heat of fusion (more than 50 BTU per pound) and low cost as described previously. The mixture, including such salt-hydrates together with a suitable nucleating agent, if desired, of known type, ls made up to prevent the solution from -supercoollng instead of crystallizing during the cooling phase. Preferably, a salt-hydrate having a heat of fusion of more than 100 BTU per pound is used (see above table).
According to this invention, a unique homogen-izing agent is used to prevent the incongruent melting of the salt-hydrates during successive heating and cooling cycles. The homogenizing agent in accordance with this ;
lnvention is a clay-type substance which exhibits thixotropy and whose particles are lath-like in structure. Any of ;;
the known nucleating agents or devices may be used in this mixture. Such nucleating agents include those agents disclosed in the said Telkes patent, and for sodium sulfate ;

decahydrates a heterogeneous nucleating agent such as sodium tetraborate decahydrate in small ~uantities, usually about 2~ to about 5~, with the preferred ran~e being about 3~ to about 4% based on the weight of the total salts. Other nucleatin~ agents may be used as well as the nucleating devices describedand claimed in my Canadian Patent Application Serial No. 260,947 filed September 10, 1976.
The thixotropic or homogenizing agents which are used in this mixture should have a relatively low cost. As is known, thixotropic agents form highly fluid suspensions with water (or other solvents) while the mixture is stirted or agitated. On the other hand, when it rests, the mixture thickens forming a gel usually after a short period o~ time.
In accordance with this invention, a clay-type thixotropic agent is used whose particles are lath-like in structure and which provide a high colloidal stabili~yi~n the presence of salt solutions and other electrolytes. Clays of this type which are suitable for use with this invention are known as attapulgite, polygorskite or sepiolite. Hereinafter such clays will be referred to as attapulgite-type clays.
One such attapulgite-type is available commercially under the tradename 'JMin-U-Gel 200" manufa`ctured by the Floridin Company of Berkely Springs, West Virginia. This particular material is a calcium silicate hydrate which is finer than 200 mesh.
Attapulgite clay has the chemical formula (OH2)4(OH~2Mg5Si 020.4H20. Based upon this theoretical formula, attapulgite ;~

* Trademark ls a hydrou~ magneslum sillcate, or more speclfically, a hydrous alumin~ magnesium silicate, slnce aluminum ions carl be subst:ltuted for ma~neslum and sllicon ions in the cr~yst;al cltructure. Thus, the actual chemlcal analysls typl~all~ shows thc presence of an alumln~ oxlde (A1203).
A ~pical chemlcal analysls of t~lls clay ls as follows:
Oxlde Attapulglte sio A12~ 7.89 Fe 03 2.82 ~e~ 3 __ MgO 13 . 41~
CaO ~ 3 K O o.o8 N~ 0 0.53 Ti~
H20_ ~2+ 16.95 .
TOTAL 99.86 Structurally, attapulgite consists of a double chain of tetrahedrons o~ silicon and oxygen parallel to the long axls. The double chain is linked by a spaced layer o~
magnesium atoms ln six~old combination. In turn, the chains form a network of strips whlch are ~oined together along the edges. Eight water molecules are contained in each crystal unit. Attapulgite clay can be visualized as a bundle o~ lath-shaped units held together at their long edges. Because of this unique structure, i.s., three dimensional chains, attapulgite clay cannot swell as do clays such as montmorillonite clays which are sheet or plate-like. In addition, the cleavage parallel to the 110 plane along the Si--O--Si bonds gives the particles a lath-like appearance. -~
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~Ihen the clay i9 d:lspersed :ln water, the lath-shaped un:Lts te~nd to separate into smaller bundles by cleavage along these edges where the laths are ~olned together. Ttle degree Or spl:Lt-up :Ls a func-t~on O:r the amount of wor~c that; cn~ers into ~lc d1saggre~ation. Ttle lndlvitlual laths may separate but they tencl to remaln ~undles not unllke brush heaps or haystacks. It is th:~s tendency to form a haystack-type structure whlch is believed to give attapulgite clays their unusual properties which render them particularly suitable for use with heat of fusion mlxtures. The haystack structure maintalns the surface support of the crystals. It is the unusually high surface area which give attapulgite such a high adsorption. This large surface area together with the bundling tendenc~ gives attapulgite its properties. The surface area of commercial grades of attapulgite varies from 210 m2/g down to 125 m2/g. Attapulgite can take up water to 200~ of its own weight. Specifically, it is the surface area which attracts water molecules and which in turn permits the clay to retain its colloidal properties even in the presence of electroly~es.
1ath-like clays have many advantageous rheological properties which render them particularly useful with this invention. When the lath-like crystals dissociate to ~ '~
~orm a random lattice, it entraps liquid to increase the viscosity o~ the system. They can thicken both fresh and salt water. Their suspensions are thixotropic and have a high viscosity even at low concentrations, The viscosity of their suspensions can be modified by additives, : .:
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dispersants in aqueous systems, and surfactants in nonaqueous `
media.
A typical attapulgite lath has a length of about 1 micron, a width of about 0.01 micron and a thickness of about 50-100 Angstroms. Stated difPerently, the length to thickness ratio oE a lath is about 1000 whereas the length to width ratio of a lath is about 100.
Chemically, attapulgite clays in colloidal suspension, as noted, are usually unaffected by salts. They are not flocculated. Many other electrolytes, particularly those salt-hydrates used as heat of fusion materials, also have little effect.
According to the preferred method of this invention, ~`
a heat of fusion material is prepared which is stable and melts congruently even over many-cycles of heating and cooling (melting and crystallizing) such that the heat of fusion of the material may be used to store heat or cold. As a first step o~ the method, water is mixed vigorously with attapulgite clay, the thixotropic agent, to form an initial mixture. It -is often desirable that the initial mixture be allowed to stand for some hours and repeatedly mixed at intervals. Next, i a nucleating agent such as borax in fine crystalline form, is `
added to the mixture, stirred thoroughly and the resu~ant product is mixed with the re~iuired amount of a salt-hydrate by way of example, sodium sulfate decahydrate. A nucleating device such as that des~ribed in my abovementioned Canadian Application Serial No. 260,947 filed September 10, 1976 may be used and the agent omitted if desired.

.: . . . -5~ 8 The resulting mixture can be easily poured ~rom a tank or mixing container while agltated into the desired storage tube~ or other storage system. The ~illed storage contalners a~e sealed. The m:Lxture sets to a gel rather rapiclly a~ter 1~ Ls no longer agltated. Tlle containers are now reacly ~or use Ln a heat and/or cold storage system O:r known type. As noted they are capable o~ many cycles o~ heating and cooling with congruent melting of the salt-hydrate so that ef~lcient production and use o~ the high 0 heat of fusion of the salt-hydrate is attained.
EXAMPLE
A thermal storage mixture was prepared as described in accordance with this invention having the ~ollowing compositlon by welght: water 56 parts; attapulgite clay (Min-U-Gel 200) 7 to lO parts; borax 3 parts; sodium sul-fate decahydrate 44 parts. This mixture has been sub-Jected to more than lO0 successive heating and cooling cycles, the equivalent of more than 5 years use, without any apparent water separation. All o~ the heat o~ fusion of the salt-hydrate was used rendering this mixture a very effective thermal storage material.
The ~ollowing experiments were made by way of contrast with the above Example of the invention.
a) The salt-hydrate composition used in the Example was tested with other thickening additives, sub~ecting ;~ ~
the mixture to alternate heating and cooling cycles. ~ -In one experiment khe additive was bentonite This clay wa~ used in up to 10% by weight. Liquid separa-tion could be observed a~ter 8 cycles.

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b) In another experiment, asbestos fiber-pulp was employed in amounts up to lO~ by weight. Some segreg-ation could be observed after 5 cycles.
c) :[n slill another case, a clay not having lath-llke part:L(les was tested in amounts up to 10% by we~ght. Although the clay was an excellent thickenlng a~en~ :Lt was unable to prevent segregatlon to some extent a~ter l~ cycles.
d) In ~urther testing, use o~ some o~ the more expensive organlc agents based on Kelp extracts made ~`
lt possible to obtain nearly 100 cycles~ but their use was unreliable if the material was kept in the molten state ~or longer periods.
Eutectlc mixtures can also be used wlth the same thixotropic additive, in accordance wlth this lnvention~
to store cold. Eutectic mixtures of the salt-hydrates -;
have a lower freezing point than the typlcal salt-hydrate, and hence are capable of storing cold at low enough temper- ` -atures to be e~ective in alr condltionlng appllcations. ~ `
In any event, dispersants, wetting agents and the like desirably may be used with either the salt-hydrates or eutectlc mlxtures of the salt-hydrates. Such use reduces the amount o~ the thixotropic agent required and can in ` ~--many cases reduce the cost considerably.
The additive to Min-U-Gel 200, a grade attapulgite clay avallable ~rom the Floridln Co., may be dispersed using tetrasodium pyrophosphate (TSPP), an inexpensive, ~-readily avallable materlal. The dispersant is used in amounts of 2 to 3~ by weight o~ Mln-U-Gel used. The ;; ~' ., ~s~
recommended process consists of dissolving TSPP in the re-quired amount of water and then adding Min-U-Gel and stirring rapidly for a short period. Dispersion occurs rapidly and the maximum viscosity is obtained promptly, without the need of allow~ g several hours of mixing and standing to obtain the same r~ult w.ithout the use of TSPP.
~ s to eutectic mixtures, the following table shows some salts used with sodium sulfate decahydrate to produce thickened mixtures within this invention, the table showing the molar ratios employed. Eutsctic mixtures that can be used with the thickened sodium sulfate decahydrate mixture contain the following compounds:

Melting Point of Added Eutectic Component the.Eute.ctics Mols per mol Na2SO4.10H20 ~== . _ _ . . .
70 to 75 F 0.5 to 1.0 KNO3 (Potassium nitrate) 65F 1.0 NaCl (Sodium chloride) : ,.
50 to 55F 0.5 to 0.75 each of NaCl + NH4Cl (Sodium chloride + Ammonium .
Chloride) 50F 1.0 NH4Cl (Ammonium chloride) 40~ 1.0 KCl (Potassium chloride) -~
In general, it is preferred to use about 92% to about 95% of the salt-hydrate or eutectic mixture with about 5% to about 8% of the thixotropic agent. The nucleating agent may . ~.
be either borax or a nucleating device as disclosed in my ~.
Canadian Patent Application Serial No. 260,947 filed September 10, 1976. It is also to be appreciated that the thixotropic ~ .
agents used in this invention can be used with most other salt- ~ -hydrates and that sodium sulfate decahydrate and the others ; :
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specifically he~rein are given only as illustrative and to enumerate preferred embodlments.
There has thus been described the use of attapulgite-type~ clay as a homo~enlzlng agent wlth salt-hydrates and 5 eutectic m:t.x~ures thereof` ~or use as heat Or fus:ton materlals ln the storage of solar energy. In addltlon, unlque methocls of' formlng such mlxtures are disclosed.
Many embodiments may be made of this inventive con-cept~ and many modifications may be made in the embodiment herelnbefore described. There~ore, it is to be understood that all descriptive material herein is to be interp~eted ;
merely as illustrativeJ exemplary and not in a limited sense. It is intended that various modifications which might readily suggest themselves to those skilled in the art be covered by the following claims~ as far as the prior art permits. ; `

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Claims (18)

WHAT IS CLAIMED IS:

1. A mixture for the storage of heat energy utilizing the heat of fusion of the composition com-prising:
a salt-hydrate having a heat of fusion of more than 50 BTU per pound, a nucleating agent, and a thixotropic agent, said thixotropic agent being a clay-type substance that has lath-like particles.

2. A composition according to Claim 1 wherein the thixotropic agent is a calcium silicate hydrate finer than 200 mesh.

3 A composition according to Claim 1 wherein said salt-hydrate is a salt-hydrate eutectic for the storage of cold.

4. A composition according to Claim 1 wherein said thixotropic agent is a hydrous magnesium silicate.

5. A composition according to Claim 4 wherein said thixotropic agent includes an aluminum oxide.

6. A composition according to Claim 1 wherein said thixotropic agent is attapulgite clay.

7. A composition according to Claim 1 where said mixture consists of about 56 parts water, 7 to 10 parts attapulgite clay, 3 parts borax, 44 parts sodium sulfate decahydrate, all by weight.

8. A composition according to Claim 1 wherein said lath-like particles are about 1 micron in length, 0.01 microns in width, and 50-100 Angstroms in thickness.

9. A composition according to Claim 1 wherein said lath-like particles have a length to thickness ratio of about 1000.

10. A composition according to Claim 1 wherein said lath-like particles have a length to width ratio of about 100.

11. A composition according to Claim 1 wherein said salt-hydrate has a heat of fusion of more than 100 BTU per pound.

12. A method of preparing a mixture for the storage of heat energy comprising the steps of:
mixing water with a clay-type substance having lath-like particles which exhibits thixotropy to form an initial mixture, with the clay-type substance dispersed in water, and mixing said initial mixture with a salt to form a salt-hydrate having a heat of fusion of more than 50 BTU per pound.

13. A method according to Claim 12 wherein said clay-type substance is a hydrous magnesium silicate.

14. A method according to Claim 12 wherein said clay-type substance is a hydrous magnesium aluminum silicate.

15. A method according to Claim 12 wherein a dis-persant is initially dissolved in said water prior to mixing the water with said clay-type substance.

16. A method according to Claim 12 wherein said clay-type substance is attapulgite clay.

17. A method according to Claim 12 which includes the additional step of adding a nucleating agent to said mixture.

18. A method according to Claim 12 wherein said salt-hydrate has a heat of fusion of more than 100 BTU
per pound.