CA1048209A

CA1048209A - Ceramic binder

Info

Publication number: CA1048209A
Application number: CA75226198A
Authority: CA
Inventors: Frederick V. Reven; Edwin T. Sortwell
Original assignee: Nalco Chemical Co
Current assignee: ChampionX LLC
Priority date: 1974-11-19
Filing date: 1975-05-05
Publication date: 1979-02-13
Also published as: JPS5159013A; ZA753220B; BE829458A; FR2291809B1; FR2291809A1; NL7509821A; ES438143A1

Abstract

ABSTRACT OF THE DISCLOSURE
This invention relates to an improved method for preparing a coating composition suitable for coating cast iron mold bases by preventing the adherence of these mold bases to subsequently formed solidified ingots. More specifically, this invention discloses a new hybrid binder for use in the coating composition. This new hybrid binder is prepared by the alkalinization of acid silica sol with an aqueous alkali metal silicate solution.

Description

~1 1048~9 X RODUCTION

hll met~l ingotsjare cast from molds. One popular type is ~ ~big end down" mold. These molds rest on bases commonly known as nstools. n The stools are merely large, normally rectangular, flat slabs of metal, commonly made of cast iron, which are used as support for the mold sides and also, of course, form the bottom portion of the mold. The mold sides generally taper up in diameter ~rom bottom to top. Another type of mold is known as the 'ibig end up" mold. These are ladle-like receivers for the molten metal, the bottom portion of which molds are integral, non-removable parts of the entire mold.
Various problems commonly occur in use of these molds and particularly with respect to the surface of their base por~ions.
First, the unprotected metal surface quickly erodes and pits in the presence of molten metals which are cascaded upon their surface.
Large gouges in the base portions are produced due to the force and high temperature developed by the ~lowing molten metal which con-tacts the surface of the stool. Since many molds are generally approxima~ely 5-10 fee~ in height, the metal must be poured from ~ height at least equal to that distance a~d quite often is poured fxom even greate~ heights. A cons~derable pressure head is thereby de~eloped. Thus, the hot molten metal easily gouges gaping de-pre~sions in the base members under such force and at a temperature of the molten metal. Moreover, the problem of creation of pits or gouges in the base portions of the molds, caused by the above fac-tors is aggravated due to the fact that the molten metal, especially near the bottom of the mold, remains in its erosi~e hot liquid st~tes for a considerable amount of time subseguent to pouring.
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11 1(~14~9 The molten metal upon solidification to an ingot thereby has a bottom form conforming to the undesirable eroded surface configuration of the s~ool or base member of the mold. Thus, a considerable amount of the ingot, when withdrawn from the mold and subsequently processed into slabs or blooms, is lost through a cropping of the irregul~rly formed end of the slab. This, of ~ourse, is highly undesirable, sînce it results in undue loss of usable meta} and increase in scrap, which must be subsequently repocessed.
Ano~her extremely serious and cos~ly problem results after the ingot in the mold has solidiied to a point where it can be removed from both the mold sides and its base platform member or ætool. In many casesl if ~he surface of the stool is unprotected, or inadequately pro~ected, and erosion OCCUX5 as described above, the ingot has a greater tendency to remain tightly adherent to the stool. Thus, ater the mold sides are removed from around the ingot, which process can normàlly be efficiently achieved with a minimal f~lm of coating of the type to be described, - ¦
the ingot must be forcibly removed rom the s~ool. This is nor-mally achieved by raising both ingot and adherent stool, and thrusting them against some other larger object whereby the ingot i8 jarred loose. In many cases the stool and ingot are merely ~ropped on the floor from some suit.ible height. In such a situa-tion, the stool is often broken into two or more smaller pieces and cannot be subsequently reused in casting other ingots. Again, replacement cost of these stools is high, making this aspect of the I
overall casting process somewhat disad~antageous. ~he same problem j exists with respect to big end up molds wherein sticking of ingots particularly occu~s a~ their base portion. New molds of this type are especially vulnerable to sticking due to their smooth surfa~e unprotec-ted by any layers of metal oxides or scale. A tight ; metal-to-metal bond between mold botkoms and lngots then occurs.

- 2 o )4t~2~9 Cracking of molds and par~icularly their base poxtions due to ~he above discllssed rough handling occasioned by "stickers" be-tween the base portions and ingots ~s also enhanced by thermal ~hock during ingot formation. Unprotected or inadequately protec-ted bottom suraces of mold are especially susseptible to such destructive shockO
Nany pxior art coating materials have failed to give ade-guate protection to the mold base members, and in some cases caused ancillary process difficulties. For example, some inferior coa~ings were washed off the ~urface of the base portion of the mold and were thereby inGluded as an unwanted impurity in the ingots. Such inclusions often~imes deleterious1y a~fected de-3irab1e metal properties. `
V.S. Patents 3,184,813 (Re 26,969) and 3,184,815 describe excellent methods of combatting the above described problems using a slurry containing a refractory and binder.
V.S. Patent 3,184,813 teaches the use of a variety of re-~ractories and specifically fused silica in combination with aqueous colloiaal silica sol binders to produce satisfactory stool coatings. Fused silica in combi~ation with colloidal silica has been employed for many years to protect stools used as the ba~e member of ingot molds in the production of steel ingots. While these coatings have proven success~uI in numerous applications in the area of stool coating, they have one disadvantage with respeot to the difficulty in ~roducing and ~anufacturin~ costs involved in manufacturing concentxated aqueous silica sols which when used commercially contain between 15-35g by weight or more of ~olloidal silica. In order to produce silica sols with such silica concentrations, the manufactuxing te~hniques involved normally require that large volumes of water be evaporated under carefully controlled conditions. This ~vaporation procedure, ~ ~ 3 - ,~
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whlle producing ~pod binders for stool coating form~as, tend~ to make these binders relatively expensive.
U.S. Patent 3,184,815 teaches the use of sodium sillcate as a binder for refractory containing stool coating formulas. qhe one disad~ant-age that has been found with the use of sodium silicate binders is that they must be very carefully formulated with respect to sodium concentration and the particular refractory used. If such careful formulation is not main-tained, the formulas in certain instances of commercial use will tend to glass on exposure to hi~h temperatures and not provide a good refractory coating. Due to the criticalities of formulation, sodium silicate has been a~oided as of binder in many commercial applications.
It would be of beneflt to the art if that component of the ~` stool coating which is known as the binder would be made of a new material producible by a more efficient means than binders presently being produced and at least as effective as presently known binders. Such an improved binder would enhance the casting process and would find ready acceptance in the art.
OBJECTS 0~ I~E INVEMTION
This invention seeks to provide a method of casting metal -;;~
ingots into metal molds whereby adherence of the base me~ber of the mold to the formed ingot and erosion of the same base member during ingot formation are substantially prevented. `~
The invention more particularly seek~ to inhibit such erosion and adherence by applying a film of coating material to the surface of the base member whereby these problems ar~ overcome.
~- Alternatively~ this invention seeks to provide improved metal molds used for casting metal ingots which have at least the base member of the mold protected by a film of material so that the above described problems of erosion of the base member and adherence of same to metal ingots no -longer exist to any appreciable degree. ;
THE INVENTION
In accordance with the invention a method of casting metal '~: ' ', ' obJects from metal molds has been dlscovered, whereby adherence of the base ~.ember of those molds to ingots formed therefrom, and erosion of these same base members during such ingot formation are substantially inhibited.
Thus~ this invention seeks to provide a method of inhibiting erosion of the surface of the base member of a metal mold used to cast ingots, which erosion normally occurs during contact of said surface with flowing molten metal, while substantially preventing adherence of said ingots to said base member upon ingot ~ormation; which comprlses the steps of applying to said surface a slu~ry comprising a refractory suspended in a binder, said slurry being applied in an amount sufficient to form a coating of sufficient thickness to inhibit said erosion and prevent said a~lerence and allowing said slurry to dry whereby a protective solid coating is formed upon said base member~ said slurry comprising at least one refractory selected from the group consisting of vitreous silica, crystalline silica, aluminum silicate, alumina, graphite, zirconium silicate, magnesium silicate and chromite suspended in a binder comprising a small particle size silica sol having an initial pH of not greater than pH 3.5 and a silica content, ex-pressed as SiO2, of at least 3% by weight, said sol having a sufficient quantity of an aIkali metal silicate added to it to ad~ust the pH thereof -~ 20 to between 8-11 with said binder being present in an amount sufficient to bind the refractory particles together to thereby form a tightly adherent coating which is bonded to said surface.
In a second e~hodiment this invention seeks to provide a composition comprising a sma11 particle size sllica sol having an inltial pH of not greater than pH 3.5 and a silica content, expressed as SiO2, of at least 3% by weight, said sol having a sufficient quantity of an aIkali metal silicate added to it to ad~ust the pH thereof to between 8~
In its broadest aspects, the invention comprises the steps of applying a slurry consisting of a binder and refractory to the surface of the base menber of a metal mold. In order to best achieve dual purposes of prevention of erosion of base member and non-adherence to for~.ed ingots, the slurry should consist of at least one refractory of the following types:
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vitreous silica, crystalline silica~ magnesium ~ilicate, aluminum silicate, alumina, graphite, zirconium silicate and chromite. These materials are all well-kno~n substances and are all commercially available. Typical aluminum silicates, for example, may include mica, a lamina~ed type of aluminum silicate and mullite, an orthorhonbic alumlnum silicate available from the Island of Mull or artificially made by heating andalusite, sillimanite or lyanite. Excellent magnesium silicates are forsterite or talc~ while a useful zirconium silicate is zircon. A typical crystalline silica is quartz. The most preferred refractory is vitreous silica.
; 10 The binder, as further discussed herein, is a small particle size silica sol having an initial pH of not greater than pH 3.5 and a silica content, Or at least 3% by weight, alkalinized to a pH between 8-12.5 by the addition of a sufficient quantity of an alkali metal silicate.
In a preferred embodiment of the invention, the small particle size silica 901 has a pH within the range of 2.5-3.5. Preferably, it does not exceed pH 3.

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Th~ silica content o ~he small particl~ si~ed silica sol may b~
as low as 3~ by weight expressed as SiO2 and in certain instancos it may be as hi~l as 18%.
'l~o convenient methods for preparing relatively concentrated small particle sized silica sols are further stated in Canadian Patent 623,562 and in United States Patent 3,468,813. These patents relate t~ producing con-centrated small particle sized silica sols by passing relatively concentrated ~ solutions of alkali metal silicates in contact with a hydrogen form cation ; resin. The cation resin removes the alkali metal from the silica~e and produces a finished small particle sized silica sol, sometimes hereinafter referred to as an acid sol, which meets the characteristics and specifications set forth above.
Canadian Pa~ent 623,562 discloses a technique for producing con-centrated acid sols which involves the use of rapid flow ion exchange - techniques in conjunction with other parameters.
United States Patent 3~468,813 deals with using rapid flow techniques of the type described in the Canadian Patent in conjunction with low tempera-tures to produce relatively concentrated acid sols.
Our invention comprises not merely the formation and use of an alkali stabilized silica sol, as discussed in Canadian Patent 623,562, but rather the ~ormation and use of a new binder material. The alkali metal silicate acts not only as a stabilizing influence on the SO1J but also contributes additional binder not present in the acid sol prior to its addition.rIhe slurry is providod in an amount adequate to form a coating of sufficient thickness to prevent the above mentioned adherence and erosion from occurring. After the slurry is allowed to dry with or without application of heat, whereby the liquid ' ' , ' ':

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phase is drlv~n rom the surface of the base member, leaving a thin fllm s:f solid refractory coating, the molten metal ~s thereafter poured into the mold arld on ts:~p of the now coated base men-ber. The liqui~ me~al is allowed to sol~di~y into an ingot and then removed from the coated base member and mold sides~ ¦
e slurry coating reagen~c is simply applied to the stool or bottom of the mold by a wide variety of methods. ~br example~
the slurry may be applied by flowing it over the stool, by spray techniques, by coating the stool wi~h some type of applicator, etc.
Spray application is belie~ed to be the most e~ficient and practical way of slurry application. The slurry may be applied to the stool portion of big end down molds before the metal mold sides are placed thereon or to the stool portion and the lower part of the mold wall ~imultaneously when the metal mold is in place on the stool.
Effecting removal of the liquid phase of the s}urry from the I
~olia refractory material may likewise be carried out in a variety !
o methods. For example, the bottom portion o~ the mold may be coated with the slurry and allowed to dry gradually. Another method of laying down a thin protective coating is ~o apply the sl~rry to an already hea~ed stool or mold. This is particularly preferred in tha~ the s~ools and their metal`mold sides or big end up molds are generally already hot beforeintroduction o~ the molten metal due to the residual heat from the previous casting run, and ln such a method drying time i~ a very minimum period.
Another way o applying the protective film i9 to coat the base member, and then heat it as slowly or rapidly as desired to drive off the liquid portion o the slurry. In any event all that i~ necessary is that the coating be laid down in some manner after contact of the base member with the ~lurry.
In the most..preferred method the coating slurry is applied to stools or mold base m-mbers having a temperature ranging fxom ,.
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that uf room temperature to l,~00P, and more preferably from 200F to 800F.
Best adherellce of solid coating to stools is achieved by slurry application to the stools at a temperature range of 200-500~. ~'or best results, films should measure in thickness from 0.01" to 3" and most preferably from 0.011' to 1/8".
After the coating has been formed preparation for casting is complete except when bîg end down molds are used. In those cases the mold sides should be placed on the base member fol~owing coating, if not already present during coating.
A~ter coating operations, molten metal is flowed into the mold, allowed to solidify into an ingot and the ingot is then separated from mold surfaces. The invention is not limited to use with specific mold sides or any particular metal mold bottom or to use with any particular molten metal.
However, it has been found specially preferred use in coating cast iron ,. ~
metal base members for molds which are used in forming ingots of steel.
As mentioned above, the most preferred refractory materials are those generally referred to as vitreous silicas. These are glassy modifica-tions of silica, obtained by the fusion of selected low temperature crystal-line forms, and are ~requently reerred to QS quartz glass or silica glass.
Specific vitreous silicas include those particles made from fused quartz glasses, silicate glasses, silica glasses such as the well-known Vycor*
materials and fused silica glasses. With respect to all of these materials, the thermal expansion coefficients are relatively small in proportion to ;~ other ceramics such as those of the soda-lime and lead glass types. Generally, ! they have thermal expansion coefficients smaller than 5xlO 6 cm/cm/C. Also, ~ the silica content of these granular siliceous refractory materials is gen-'~ erally greater than 96% silica expresse as SiO2 and may range as high as 99.8% SiO2. Thus, by the term "vitreous silica" is meant a refractory ~ comprising a silica ,''' * Trade Mark ~` - 8 -.
;' :1048Z~9 ¦glass having a thermal coef~iclent of expanslon and sio2 content ¦within the above rang ¦ It has been determined that for best results in coating ¦ stools the r~ractory used in the silica slurry should be able to ¦withstand severe heat shocks. Due ~o ~he ex~reme hot temperature t ¦of the molten metal as compared to that of the stool even when the ¦latter is heated, an exceedingly abrupt change in temperature ¦occurs when the metal contacts the stool. The coating must itself ¦be able to withstand this heat shock to impart necessary protection ¦to the stool base. It has been theorized that failure of some ¦prior art materials was due, at least in part, to their inability ¦to withstand this sudden increase in heat, thereby resulting in ¦cracking of the coating and subse~uent exposure of the metal 3ur-¦face to ~he cascading molten m.etal poured in~o the mold.
¦ In view of the above, i~ i9 generally thought that the ¦most preferred refractories are those which ha~e the highes~
purities concomitant with the lowest thexmal coefficient of expan-¦~on. Thesq properties are particularly possessed by vi~reous ¦s~licas and more particularly those of the used silica types. The ¦latter materials have a silica content greater than 97% silica ex-¦pressed as SiO2 and a thermal coefficient of expansion not ~reater ¦than about 6xlO 7 cm./cm. p C.
¦ A typical fused silica of the type described above which is ¦extremely useful in the practice of the invention, having a thermal ¦coefficient of expansion of about 5xlO 7 cm~/cm. ~ C., has the following t pical analysis:

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In~redients: Percen~ By Weight 2 ------------------------------ 97.3 123 ------------------___________ __ 1.7 Sub-oxides of silica ~ 1.0 ,:~' The above type silica products are readily pxepared ~y grinding very pure fused silica glasses. ~ikewise, the boro-silicate glasses and Vycor silica gLasses may be also ground to produce extremely useful refractories.
The particle size of the reractory may vary over a wide ra~ge. It is pxeferred, however, that the refractory particles be sufficiently small so that a uniform dispersion of refractory and binder may be made. The smaller the particle size the longer a slurry made up o binder and refractory remains in a homogeneous state. It ~as been determined that particles ranging in size rom 150 mesh to as low as a fraction of a micron may be employed.
Preferred refractory materials have an average particle size rang-ing from 20 to 500 microns in particle diameter, wi~h particles corresponding to the lower ranye diameters being most preferred.
Specific vitreous silica substances, marketed under the name, "Nalcast"*~all within the above pre~erred particle size range and have been employed with much success in preventing erosion of base portions of molds and adherence of same to the formed ingots.
Prior to this invention, among ~he binders commonly con-s~dered effective in the art were colloidal silica sols, as discussed in U.S DPatent 3,184,813 and alkali metal silicates, such as sodium silicate and potassium silicate, as set forth in U.S.
Paten~ 3,184,815. We h~ve found, however, a hybrid binder more :~ . .
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THE BINI)fiR
Tha hybrid binder used in this invention is produced by treat-ing an acid sol with an alkali metal silicate. The acid sol is produced, as indicated, by passing an alkali metal silicate Eeed solu~ion having an 5iO2 concentration of at least 3% by weight through a cation exchange material bed in the hydrogen form. As the acid sol comes off the bed, it should be at a pH between 2.5 and 3.5. Immediately upon coming off the bed, the acid sol is ~reated with a sufficient quantity of an alkali metal silicate to adjust its pH to between 8 and 12.5. The silica concentration, expressed as SiO2, of the sodium silicate may be as little as 5% although concentra- ~ .
tions of 8% or moreJ e.g. up to saturation solubility may be used. The particle size of the hybrid binder at this time will lie in the range of ~` 3-8 millimicrons and the intrinsic viscosity of the solution will be between

3 and 8 centipoise.
t~: In a preferred embodiment of the invention, the alkali metal ~ silicate used to alkalinize the acid sol will be sodium sili.cate at a ratio t',~ of SiO2 to Na2O of between l and 3.2, the pH of the acid sol as it comes off the column will be pH 3.0, the pH after the addition of the sodium silicate , will be pH 9-11 and the particle size will be between 5 and 7 millimicrons.
This binder may be used as either part o a "two-component"
; system, in which case the binder is mixed with the refractory just prior to !~ ~ use, or it may be used as part of a "one-component" system, in which case the i binder and the re~ractory are mixed will before use and stored.
i In order to prepare "one-component" stool coating formulations, it is necessary that a suspending agent be used which prevents the refract-ory material from settling out from the binder.

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An exc01lent suspending agent for this purpose is a Xanthomonas hydrophilic colloid. This colloid is a polymer containing mannose, glucose, potassi~
glucoronate and acetyl in the approximate molar ratio of X 2~
lhe use of these colloids for preparing "one-component" stool coating formulas is set forth in United States Patent 3J428J464. In the one-component ~ystem, in addition to the refractory and the binder, suspension aids and biocides are used in propor~ions as set forth in Table II below.

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`- ~ 3 , The amount of binder making up a portion of the coating slurry must be such that it is present in an amount sufficient to bind the refractory particles together to thereby form a tightly adherent, continuous and unbroken coating which is securely bonded to the surface of the s*ool. Without proper amount of binder in relation to refractory, the resultant coating, after application and drying o~ slurry, exhibits a "pan-cake" effect with numerous undesirable holes appearing in the coating, thereby exposing por-tions of the stool or mold base surface. To achieve this, it has been determined that the slurr~ is preferabl~ composed of from 10 to 70% by weight of refractory~ and from 30 to 90% by weight of binder. The hybrid binders of the invention may be treated with water soluble alcohols such as methanol, ethanol, isopropanol ethy-lene and propylene glycols in amounts ranging between 2-50% by weight to sufficiently depress the free3ing point of the binder to make it usable during ~inter months in colder cl;mates.
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The experiments described below were carri.ed out in order to test our in~ention. The variable parameter .in each example is the percentage by weight of sodium silicate added to the acid sol.
Since we were only interested in stool coatings which pre~ented : any erosion of the stools during a single ingot formation, e~alua- tion of the test data is straight forward: each of the concentrations of sodium silicate tested is rated as either pass or fail~
EXL9MPIE 1 ~:
A solution of sodium silicate having an Na20 to SiO2 ratio of 1:3.2 was diluted to a SiO2 concentration of 7.5% by weight - This diluted silicate solution was then passed through an ion ex-c~ange column containing a cation exchange resin in the hydrogen .

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¦ fo~m. The sol ~hus produccd at a silica concentr~tion of approxi~
mat~ly 7.5~ and a pl~ of 3~0. The cation exchange rcsinue was a commercial material sold under the trade name of Nalcite }ICR, a sulfonatcd copolymer of styrene and divinylbenzene. A more com-plete description of this cation exchange resin is given in U.S .
2,366,007.
As the sol came off the column, varying amounts of a 38% by weight total solids aqueous solution of sodium silicate were added.
The percentages by weight of this sodium silicate solution which were added to the sol appear in the left-hand column in Table III.
Once these test binders were made, they were used to make up a slurry composed of 50~ of fused silica, as a refractory and 50 of the test binder. A coating of 17~20 microns thickness of the (dry bases) slurry was then sprayed on a test cast iron stool heate~
to between 200-400 F. ~fter the stool coating dried, 30 pounds of molten steel at a temperature of about 2900 F. was cast onto khe coating from a distance of 12.5". After it cooled, the steel was I `
removed from the test stool and the stool coating was examined. The results for each of the test binders are summarized in Table III
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T~BLE III

Test Binder Test Results -Binder- 95% Column Sol Pass 5% Sodium Silicate ~3~%) Binder- 15% Sodium Silicate Fail (as Sodium Silicate Solids) Binder-3 0% Colloidal Silica Pass (~alcoag *1030) t ., . ~
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CONCLUSION
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The invention thus involves the utiliza~ion of a new and im-proved stool coa~ing binder. This acid sol - alkali silicate product is very advan~ageous ~or commercial production because its manuacture is straightforward and does not require sophisticated ~ntermediate steps.
In addition to i~s use as a stool coating binder, the acid sol - alkali silica~e product discussed herein may be useful in a wide vaxiety of industrial applications, including: surface fric-tionizing; surface modification; inorganic bonding and reinforcing agent; production of refractory shells for investment casting; bind-er in refractory gunning, ramming ana tamping mixes.

P~IOR ART - ~
1~ U.S. Patent Re. 26,969 ME~AL CASTING PROCESS
This patent discloses a stool coating comprising a refractory 9US--pendea in a binder. The refractory can be selected from the ~roup consisting of: crystalline silica, aiuminum silicate, alumina graphite, zirconium silicate, magnesium silicate, clay, and vitreous or fused silica. The binder Gan be colloidal silica sol, aluminum phosphate or ethyl silîcate.
2. U.S. Patent 3,184,815 METAL CASTING PROCESS
mi8 patent discloses a stool coating comprising a r~ractory sus-pended in a binder. The refractory can be selected ~rom the group consisting o~: vitreous silica, cry~talline silica, magnesium sili-ca e, aluminum silicate, alumina, graphite, zirconium silicate and d ay. The binder ca~ be an aqueous alkali metal silicate.
3. U.S. Patent 3,468,813 METHOD OF PRODUCING ACID SILICA
:'' . SO~S , Thi~ invention i concerned with producing concentrated silica sols by a proces~ comprising passîng a concentrated alkali metal silicate solution through a bed of hydrogen form ca~ion ex~hange resin at a reduced temperature and a high flow rate.
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4. Canadian Patent 634,562 SILICA SOL COMPOSITIONS AND PROCESS
FOR PREPARATION TIIEREOF. 'Ihis invention is concerned with producing con-centrated silica sols by a process comprising passing a concentrated alkali metal silicate solution through a bed of hydrogen form cation exchange res.in at a high flow rateO ~ .

5. United Sta~es Patent 3,428,464 RE~RACTORY COATING CO~'OSITIONS.
This invention is concerned with producing a suspension of finely divided microscopic refractory solid particles in an aqueous liquid containing a binder stabilized with an Xanthomonas hydrophobic colloid.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of inhibiting erosion of the surface of the base member of a metal mold used to cast ingots, which erosion normally occurs during contact of said surface with flowing molten metal, while substantially preventing adherence of said ingots to said base member upon ingot formation;
which comprises the steps of applying to said surface a slurry comprising a refractory suspended in a binder, said slurry being applied in an amount sufficient to form a coating of sufficient thickness to inhibit said erosion and prevent said adherence and allowing said slurry to dry whereby a protect-ive solid coating is formed upon said base member, said slurry comprising at least one refractory selected from the group consisting of vitreous silica, crystalline silica, aluminum silicate, alumina, graphite, zirconium silicate, magnesium silicate and chromite suspended in a binder comprising a small particle size silica sol having an initial pH of not greater than pH 3.5 and a silica content, expressed as SiO2, of at least 3% by weight, said sol having a sufficient quantity of an alkali metal silicate added to it to adjust the pH thereof to between 8-11 with said binder being present in an amount sufficient to bind the refractory particles together to thereby form a tightly adherent coating which is bonded to said surface.

2. The method of claim 1 wherein said base member is cast iron, and said ingots cast are composed of steel.

3. The method of claim 1 wherein said slurry comprises 10-70 parts by weight of vitreous silica and 30-90 parts by weight of said binder.

4. The method of Claim 3 wherein said vitreous silica is fused silica which is characterized as having a silica content not less than 96% silica, expressed as SiO2, and a thermal coefficient of expansion less than 5x10-6 cm./cm./°C.

5. In a method of casting metal ingots from a metal mold whereby ad-herence of the base member of said mold to said formed ingots and erosion of said base member during formation of said ingots are substantially prevented;
which comprises the steps of applying to the surface of said base member, a slurry comprising at least one refractory selected from the group consisting of vitreous silica, crystalline silica, aluminum silicate, alumina, graphite, zirconium silicate, magnesium silicate and chromite suspended in a binder com-prising a small particle size silica sol having an initial pH of not greater than 3.5 and a silica content, expressed as SiO2, of at least 3% by weight, said sol having a sufficient quantity of an alkali metal silicate added to it to adjust the pH thereof to between 8-11, said slurry being applied in an amount adequate to form a solid coating of sufficient thickness to inhibit said erosion and prevent said adherence, allowing said slurry to dry whereby the liquid phase of said slurry is driven from the surface of said base mem-ber leaving a thin film of refractory, pouring molten metal into said mold, allowing said metal to solidify into an ingot, and removing said ingot from said coated base member in said mold, said binder being present in an amount sufficient to bind the refractory particles together to thereby form a tightly adherent coating which is bonded to said surface.

6. The method of Claim 5 wherein said coated base member is composed of cast iron and said ingots are steel.

7. The method of Claim 5 wherein said slurry comprises 10-70 parts by weight of vitreous silica and 30-90 parts by weight of said binder.

8. The method of Claim 7 wherein said vitreous silica is fused silica which has a silica content not less than 96% silica, expressed as SiO2, and a thermal coefficient of expansion less than 5x10-6 cm./cm./°C.

9. A composition comprising a small particle size silica sol having an initial pH of not greater than pH 3.5 and a silica content, expressed as SiO2, of at least 3% by weight, said sol having a sufficient quantity of an alkali metal silicate added to it to adjust the pH thereof to between 8-11.