CA1167234A - Process and apparatus for the production of finely- divided metal and metalloid oxides - Google Patents

Abstract

ABSTRACT
There is disclosed an improved process and apparatus for the production of finely-divided metal and metalloid oxides by flame hydrolysis of corresponding metal and metalloid halides whereby fouling of burner apparatus is substantially completely avoided.

Description

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The present invention xelates generally to the production of f.inely-divided metal and metalloid oxides by high temperature decomposition of corresponding metal or ; metalloid feedstocks and is more particularly concerned with the production of finely-divided metal or metalloid oxides by flame hydrolysis of corresponding metal or metalloid hal-ide feedstocks in th~ vapor phase.

Flame hydrolysis of vaporized metal or metalloid halide feedstocks to produce corresponding finely-divided oxide products is, broadly, a well-known and ex~ensively practiced art. In such processes, a vaporized or,gaseous hydrolyzable metal or metalloid halide feedstock is co-mingled with a flame fvrmed by combustion of a water-producing, hydro-gen-containing fuel and an oxygen containing gas. The prin-cipal roles of-the eombustion flame are to provide water for hydrolysis of the halide feedstock, to provide su~ficient auxiliary heat to support the normally endothermic nature of the hydrolysis rPaction and to promote the particular thermal environment necessary to produce the desired oxide product.

2~ The resulting reaction products, csmprising finely-divided particulate oxide entrained in reac~ion off-~ases, are subjected to conventional cooling and solid pxoduct separation ~echniques, ~he separated off~gases, including hydrogen halide, thereafter ~` being recycled and/or treated ~o as to recover valuable com-~- 25 pon~nts therefrom and/or suitably disposed.

The ~nely-divided metal or metalloid oxide pro-duct~ produci~le by flame hydrolysi~ of corresponding metal

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or metalloid halide eedstocks find utility in various applications. For instance, finely-divided titania, van-adia and zirconia are useful as ~illers and pigments in diverse polymers ~nd elastomers and as catalysts and cata-lyst supports. Finely-divided alumina is useful as a filler for various matrices and finds additional utility as an anti-static and anti-soil agent when applied to textiles and as a frictionizing or anti-slip agent when applied to paper pro-ducts or to textile fibers prior to spinning thereof. Co-formed oxides produced by way of ~he flame hydrolysis process,such as silica/alumina or titania/alumina, also find utility in catalytic applicationsO

Finely-divided silicas presently represent a sub stanti~l portion of the metal or metalloid oxides commercially produced by flame hydrolysis techniques. These silicas are characterized by their relatively high purity, amorphous struc-; ture, small particle size and ~endency ~o form loosely heid gel-forming networks when dispersed in various liquids. The flame hydrolysis silicas are used, inter alia, as reinforcing fillers in elastomers, particularly silicone elastomers; as rheology control and thickening agents in organic and inorgani~
liguids; as flow and sag control agents in caulk, sealant and adhesive compositions; as anti-blocking agents for plastics, rubb2rs and adhesive coatings; and as free flow agents in vari-ous powdered pxoducts.

O~e of the pr~blems faced by manufacturers of fl~me ~:~ hydrolysis metal and metalloid oxides resides in the tendency 3~
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of presently practiced processes to deposit solid oxide pro-duct on the discharge end or mouth of the burner through which the hydrolysis reactant mixture is intxoduced into the flame hydrolysis reaction zone. This deposition phenomenon is known variously as "whiskering", "bearding" or, simply, "burner fQuling". Such burner fouling can be detrimental since, if sufficiently extensive, it can adversely affect the geometry and smoothness of the hydrolysis flame and thereby lessen the facility by which the process is carried out and can render the finely-divided metal or metalloid oxide product non-uniform. Thus, substantial efforts which have heretofore generally been met by only limited success have been made to minimize burner fouling or at least to limit the extent to whi~h it occurs. For instancet burner fouling can generally be periodically mechanically removed from the burner mouth prior to deleterious build-up thereof. Preferably, however, the burner and the process stream(s) are designed so as to minimi~e ~he rate at which such fouling occurs. As an example of this latter, for instance, reference is made to U. S. Pat-ent No. 2,990,249, Wagner, June 27, 1361, wherein there is disclosed a technique by which burner fouling is minimized.
Said technique broadly comprises the introduction of a purge gas stream adjacent the mou~h of the burner and at about the - point of discharge of the hydrolysis reactant mixtur stream therefrom. This is accomplished by charging the purge gas, which may be air, through an annular slit which completely sur-rounds the burner mouth. The technique is aid to mi~igate against burner fouling by ~erving tG meshanically impede the the formation of solid reaction products at ~he burner mouth ~nd by loca~ized dilution ~f the hydrolysis reactants such that the rate of ignition o~ the reactant ~tream is depressed Z3~
to the point that the oxide-producing hydrolysis reac-tion initiates only at some point physically removed from the burner mouth. In short, the technique disclosed by Wagner îs intended to prevent anchoring of the oxide-producing hydrolysis flame directly on the burner mouth. In a subsequent disclosure, referring now to U.S.
Patent No. 3,954,945, Iallge et al, May 4, 1976, hydrogen is disclosed as a suitable purge gas for use in the general technique originated by ~agner. The anti-fouling method disclosed in the ~agner patent mentioned above is not normally performed without difficulty since the projection o~ the purge gas into the reactant mixture stream at tlle burner mouth tends to result in unstable operations of the hydrolysis flame. T~is is believed to occur because the purge gas, when injected through the annular slit at a rate sufficient to beneficially affect the fouling problem, can also physically disturb the boundary of the hydrolysis reactant gas stream emanating from the burner mouth. In addition, the technique of the '~agner patent does not provide a stable site for propagation of the hydrolysis flame.
A~cordingly burner operations employing the anti-fouling technique of Wagner are normally found to require rigid control of the flcws through the burner to within relatively narrow limits in order to avoid flamR-outs and flam~ sputtering and, even if due attention is paid to these parameters~ the aforementioned instability problems may nevertheless arise.
~ n U. S. 4,048,290, K. Bo Lee, September 13, 1977, there is disclosed a substantial modification of the apparatus and method of r~agner.
Replacing Wagner's annular slit is a sintered porous diffusion m~mber surrounding each halide-containing reactant stream at the point(s) of dis-charge thereof frQm the burner. A hydrogen-con-taining w rge gas or vapor is - 5 - ;

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diffused ox transpired through the porous diffusion mem-bers defining the boundaries of such halide-containing streams at flow rates sufficient to prevent fouling. It has been found that the apparatus and method of Lee ~oes tend to result in lesser disturbance of the ~eometry of the react-: . ant stream burner efflux than that resulting from the practice of the Wagner invention and that the resulting hydrolysis flame tends to be somewhat more stable. However, the method and apparatus of UO S. 4,048,290 are also possessed of certain di~advantages. For instance, the mass flow rate of the hydro-g~n-containing purge gas required to maintain clean burner conditions is usually quite substantial and can represent a j substantial deficiency in terms of process economics. Secondly, in part due to the substantial mass flow rates required of the purge gas, it is not normally possible to replace the hydrogen-~; containing gas of Lee with purge gases of lesser cost, such as air or recycle process off-gases since the latter tend to excessively dilute the periphery of the reactant mixture stream ;~ effluxing from the burner mouth.

In accordance with the present invention, the above problems and deficiencies of the prior art methods and appar-~:
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atuses have been substantially overcome.

OBJECTS OF THE INVENTION

It is a principal object of the invention to pro-vide a novel improved process for the production of finely-divided metal or metalloid oxides by vapor phase flame hydrol-ysis of corresponding metal or metalloid halide feedstocks.

It is another object of the invention to provide an improved process of the foregoing type whereby the problem of burner fouling can be substantially completely avoided.

It is another object of the invention to provide an improved process of the foregoing type wherein, in addition to the benefit of freedom from the problem of burner fouling, said process is additionally characteri2ed by good stability of ~he hydrolysls flame.

;~ 15 It is another object of the invention to provide an improved process of the foregoing type wherein, for a given burner apparatus, the overall consumption of hydrogen-~ontaining fuel gas for a given rate of production of a metal or metalloid oxide product of a given surface area i5 substan-~ tially reduced.

It is a~other object of the inven~ion to provide an împroved process of the foregoing type chaxacteri~ed by ~mproved burner capacity.

It is another object ~f ~he invention to provide a novel fouling-free burner system for the production of ~inely-, dlvided metal or metalloid oxides by vapor phase flame , r~ hydrolysis of corresponding metal or metalloid halide feed-, 7' stocks.
Other objects of the invention will, in part, appear hereinafter and will, in part, ~e obvious.
~; In accordance with the present invention, the above and other objects and advantages are generally realized by performing a metal or metalloid oxide-producing flame hydrolysis reactant mixture within a burner means; continuously ~,j7 10 discharginy the resulting reactant mixture as a stream pro-jected from the mouth of said burner means/ continuously dis~
charging a purge gas at a fouling preventive rate along the boundary of said reactant mixture stream as it ~s discharged from the burner mouth; and, at a plar.esubstantially normal ~` to the axis of said projected reactant mixture stream and physically spaced downstream from the burner mouth, contact-- ing substantially only the periphery of said reactant mixture ~J,,' stream with a plurality of continuous pilot flames arranged ~ substantially tangentially thereabout.
'~r~`~ 20 The reactant mixture suitably comprises a va~orous or gaseous metal or metalloid halide, or mixtures thereof, with a hydrogen-containing water-producing fuel-and an r, oxygen-containing oxidant therefor.
' D~, The i~proved burner system;of the~invention com~
~- prises burner means~adàpted to continuously receive, mix ir !~ and enclose vapor-ous or gaseous metal or metalloid oxide-~r~ producing flame hydrolysis reactants therein, and to con-~,~ tinuously discharge the resulting reactant mixture as a ~;` substantially linearly projected stream from the mouth thereof;
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means to continuously discharge a purge gas a,long the bound-ary of the discharging reactant mixture stream in the region of the mouth of the burner means, a plurality of pilot flame burner spuds arranged about the circumference of the reactant mixture stream, each such spud being disposed so as to direct a pilot flame therefrom into substantially tangential contact with the periphery of said reactant mixture stream at a common plane spaced downstream of the burner mouth; and means to continuously supply a fuel gas to each said pilot flame burner spud.
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9~ The continuous discharge of the purge gas impedes t anchoring of the hydrolysis flame on the burner mouth.
,, The burner spuds suitably direct continuous , pilot flames.
'~ The plane at which the substantially tangential contact is formed, defines an anchor point for the hydrolysis flame which is removed from the burner mouth.
The invention is illustrated in particular and preferred embodiments by reference to the accompanying drawings in which:
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FIGURE 1 forming part hereof is a schematic, - diagrammatic, longitudinal section of a burner system representative of an embodiment of the invention.
:~ FIGURE 2 is a schematict diagra~matic, bottom view of the burner system of Figure 1.
FIGURE 3 is a schematic, diagrammatic, partially ~ sectional side view showing the burner .~. ..
system of Figures 1 and 2 in con-, 10 junction with a suitable reaction ~, .
chamber arrangement therefor.
Metal or metalloid halide feedstocks-useful in the practice of the invention include substantially any vapourizable , .

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6t7~3~a or gaseous metal or metalloid halide capable of undergoing hydrolysis to the corresponding oxide under the conditions imposed thereonin the hydrolysis flame. Exemplary metal and metalloid halides are: vanadium tetrachloride, titanium S tetrachloride, titanium tetrabromide, zirconium tetrachloride, aluminum trichloride, zinc chloride, antimony trichloride and the like. Included among suitable silicon halides are silicon tetrachloride, silicon tetrafluoride, methyltrichlorosilane, trichlorosilane, dimethyldichlorosilane, methyldichlorosilane, methyldichlorofluorosilane, dichlorosilane, dibutyldichloro-silane, ethyltrichlorosilane, propyltrichlorosilane and mix-tures thereof. Where co-formed oxides of different metals or metalloids are desired it is, of couxse~ apparent that the feedstock ~an comprise compatible mixtures of the correspond-15 ing metal or metalloid halides.

Substantially any vaporizable or gaseous water-; producing hydrogen-containing fuel may be employed in the pre-paration of the hydrolysis reactant mixture, it being of im-portance that the selected fuel produce water as a by-product of i~s combustion with an oxygen-containing gas. Exemplary suitable fuels are hydrogen and the hydrocarbons such as methane, natural gas, refinery gas, ethane, propane, acetylene, butane, butylene, ethylene, pentane or propylene as well as normally liquid but vaporizable fuels ~uch as aliphatic, aromatic, or ~licyclic hydrocarbons. Generally, hydrogen will represent the preferred water-producing fuel ~ince it burnc cleanly with-out the ~ormation of carbonaceous by-products.

~ ~y~en represents the oxidant for the combustion of `~ the hydroge~-~o~aining water-producing fuel in the process of ;~ 30 the invention and may b~ employed in its pure state or admixed ~ L6~;~3~
with other gases. Thus, oxygen, air, or oxygen-enriched air may be co*v~niently employed as the oxidant gas in the present process. However, i~ desired, it is also within the ambit of the invention to employ oxygen admixed with such gases as nitrogen, argon, helium,or carbon dioxide or hydroyen halide.
The hydrolysis reactant mixtures of interest will usually comprise an at least stoichiometric quantity of the hydrogen-containing fuel and, preferably, an at least stoichiometric 1~ quantity of the oxygen-containing oxidant gas. In other words, the hydrolysis reactant mixture performed within the burner and projected as a stream from the burner mouth will normally : contain a sufficient concentration of hydrogen-containing fuel as to provide, upon combustion thereof, sufficient water ~o convert substantially all of the metal or metalloid halide vapor component to the corresponding oxide. Desirably, although not mandatorily, the concentration of oxygen-containing oxidant gas forming part of said mixture will be at least sufficient to burn all of the hydrogen-containing fuel component con~ained therein. Preferably, the concentrations of the hydrogen-con-taining fuel and oxygen-containing oxidant components contained in the reactant mixture will each be at least slightly in excess of the stoichiometric requirements described above there-for.

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. The purge gas or vapor discharged in the region of the burner ~outh along ~he boundary of the reactant mixture tream can be ~ubstantially any ga~ or vapor which does not : react deleteriously with the components of the hydrolysis reactant mixture or with ~he products of the hydrolysis reac-tion. In accoraance ~ith this broad requirement, therefore, . it is apparent ~hat the inert gases of Group O Df the Mende-~i723~

leev Periodic System, such as argon, neon, helium and xenon, are all ~enerally suitable purge gases in the practice of the invention. So too, however, are other elemental and chemica combined gases such as nitrogen, carbon dioxide, and fuel or oxidant gases as hereinbefore described with respect to the reactants forming part of the reaction mixture stream.
Where the atmosphere surrounding the discharged hydrolysis reactant mixture stream in the region of the burner mouth is air or recycled off-gases of the process, the preferred purge gas is air or hydrogen.

The rate at which the purge gas is discharged along the boundary of the hydrolysis reactant mixture stream as said stream is projected from the burner mouth is subject to c~nsiderable variation, the principal requirement being that said rate, at the minimum, be sufficient to prevent fouling of the burner mouth. Obviously, therefore, the minimum rate of purge gas flow necessary to achieve this r~sult will be dependent upon such parameters as the particular composition of the hydrolysis reactant mixture stream, the velocity of said stream at the point of discharge thereof frsm the ~urner mouth, the design and size of the burner apparatus, the precise manner i~ which the purge gas is discharged along ~he boundary ~f the reactant mixture stream, the location and number of pilot flames positioned downstream of the burner mouth and the like. Suffice it to ~ay, there~ore~ that said minimum rate of purge gas ~low ~an nonmally be readily ascertained in the practi~e o~ the ~nvention.

, The continuous pilot flames of the invention are produced by the combustion of a fuel gas wi~h a suitable oxi-dant therefor. Conveniently, but not necessarily, the fuel gas and the oxidant supplied ko the pilot flames are_of the ~ame types as ~hose employed in the performance of the react-ant mixture stream. Where the region surrounding the pilo~
flames contains sufficient oxygen to stably support the burn-ing of the pilot fuel gas 3F vapor, such as air, it is generally sufficient to supply fuel gas, alone, through the pilot flame burner spuds. Where, however, said region does not contain sufficient oxidant for the stable combustion of the pilot flames, such as when the atmosphere surrounding the reactant mixture stream is constituted entirely of recycled reaction off-gases, the pilot burner spuds should be supplied with a combustible mixture o~ a fuel gas and an oxidant. It is norm-ally preferred to supply the pilot burner spuds with a combus-tible fuel/oxidant mixture, especially a hydrogen/air mixture, since this relie~es the practitioner of the invention with need to consider the nature of the specific atmosphere sur-rounding the pilot flames while assuring that the pilot flameswill burn smoothly and continuously without especial regard to that external atmosphere.

A better understanding of the performance and work-ings of the m vention can be had by reference to the drawing :`

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hereof wherein, in Figures 1 ~hrough 3, like reference numerals refer to like structures. ~urner 1 co~prises an inlet end 2 and a discharge end or mouth 3. The inlet end 2 defines a receiving and mixing chamber 4 wherein the gas-S eous or vaporous hydrolysis reactants are introduced andmixed. Thus, for instance, the hydrogen-containing water-producing fuel can be introduced through conduit 15, the metal or me'alloid halide vapor or gas feedstock through con-duit 16 and the oxygen-containing oxidant through conduit 17.
As shown, the oxidant and metal or metalloid halide feedstock reactants can, if desired, be at least partially co-mingled in common conduit 18 prior to entry thereof into the mixing chamber 4 of burner 1. Due to the turbulent mixing of the gaseous reactants within the mixing cha~b~r 4, it is desirable 15 that a substantially linear mdss flow of the resulting react-ant mixture be established within the burner 1 prior to its discharge from the mouth 3 thereof. This can be conveniently achieved, for instance, by the presence of a plurality ~f stages 5 of flow xectifying baf~les 6 arranged longitudinally in star-shaped patterns within the interior of the bore of the burner 1. The reactant mixture mass is then discharged from the mouth 3 of burner 1 and is preferably, but not necessarily, projected into a suitable encl~sed reaction space 100 therefor.

Another essential element in the burner syste~ of he invention comprises means to continuously discharge a purge gas along ~he boundary of the reactant mixture stream as the latter is discharged from the burner mouth 3. In the 6pecific embodiment shown in the drawing he-eof, said means compri~es a plenum 7 affixed to the exterior of burner 1 and defini~g an a~nular ~pace 8 therebetween. Plenum 7 extends dowmwardly o~er the exterior ~f burner 1 and narrows to define 3~
an annular slit 9 located in the region of and surrounding the burner mouth 3. Purge gas is introduced into the annular space 8 through conduit 10, flows downwardly through said space and is discharged through annular slit 9 along the boundary of the reactant mixture stream as the latter is dis-charged from the mouth 3 of burner 1. It will be understood, of course, that while the annular slit arrangement described above represents a preferred embodiment of the invention, the invention is not to be limited to the provision of an annular slit surrounding the burner mouth. For instance, a suitable alternative to the specific apparatus shown herein is that disclosed by Lee in U. S. 4,048,290 wherein a purge gas is transpired along the boundary of the discharging reactant mix-ture stream through porous diffusion means surrounding the burner mouth.

.~ The burner system described thus far is broadly sim-~ ilar to those disclosed in U. S. Patent Nos. 2,990,249;
: 3,954,945 or 4,048,290. These patents, however, do not cQn-templàte the presence of plural continuous pilot flames ~ta-tioned at a spaced distance downstream of the burner mouth and : it is this pilot flame arrangemen~ which represents a critical and essential component of the method and apparatus combinations ~: of the present in~ention. Thus, the burner system of the pres-~:~ ent inYention also comprises a plurality of pilot burner spuds ~` 25 20 whieh spuds: (1) are arranged on a plane normal to the axis ~ of the reactant mixture stream; (~) are arranged ~uch as to ;~ project the pilot flames therefrom to a plane which is spaced ~own~tream from the burner mou~h 3; (3) surround the reactant mixture ~tream; and (4) are disposed to pro~ect pilot flames therefro~ i~o su~stant~ally ~angential contact with the per-iphery of the reactant mixture ~tream. In accordance with these general criteria it will be seen from the drawiny that ~7~3~

the burner spuds 20 are disposed such as to project their pilot flames to a plane ~ich is spaced downstream from the plane of the burner mouth 3. This min~m~,l spacing of the pilot flames at the points of contact thereof with the reactant mixture stream should normally be at least 1/8 inch ~0.3175 cm~ dc~lstream fro~ the plane of burner mouth 3 and the maximum of such spacing being at about that plane in ~: the flow of the discharged reactant mixture stream at which the physical integrity of said stream begins to bre~lk do~m. Generally speaking, however, it has usually been found sufficient to arrange the burner spuds 20 on a plane such that the issuing pilot flames are directed to contact the reactant muxture stream at a plane spaced fram about 1/4 to about 1/2 inch (0.635 - 1.27 cm) downstream from the burner mouth 3.
Also of importance, the pilot flame burner spuds 20 are disposed non-radially with.respect to the centerline of the reactant mixture stream such that the pilot flames issuing therefrom contact said reactant mixture stream substantially only at the periphery thereof and in a substantially tangential manner. This is improtant since it assures that the flow of the reactant nLL~ture mass will be disturbed little, if at all, by contact of the periphery thereof with the pilot flames.
: As will be rec~ nized by those of skill in the art, it is : in the nature of things that the reactant mixture stream, as it is projected from the burner mouth 3, will ten~ to expand or "blcam."
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This expansion can be controlled, to s~me extent, by control of the purge gas flow within judicious lImits, the minimum flow, of course~
being that which av~ids fouling of the burner mouth 3. In any event, the :~ cross-sectional dimension of the reaction muxture stream as it courses ~: - 17 r ' 39~

through the plane defined by the burner spuàs 20 should be taXen into account and the ~puds 20 spaced sufficiently from the periphery ~f ~aid reaction mixture stream as to avoid physical contact ~herewith.

As mentioned previously, the burner spuds 20 are ~upplied with a fuel gas ~r, prefexably, with a stably com-bustible mixture of fuel gas and oxidant. In this connection, this function can be readily achieved along with convenient mechanical fixation of the spuds 20 by the arrangement shown most clearly in Figure 1. Therein, it will be noted, each spud 20 is supplied thr~ugh a oonduit 21 having its origin at a manifold 22 which is slidably affixed to the exterior of burner 1. ~hen it is desired to alter the planar spacing of the pilot flames downstream from the mouth 3 of burner 1: it is only necessary to slide ~he manifold 22 up or down relative to the burner 1. Fuel gas or a combustible mixture of fuel gas and oxidant is supplied to the manifold 22 thr~ugh ~onduit 23. If desired, of course, the fuel gas and the oxi-dan$ may be separately charged into ~h~ manifold 22 for ad-~;: 20 mixture thereof and distribution of the resulting fuel/oxidantmixture to the burner spuds ~0.

: ~he number of burner ~puds 20 employed is ~ubject to considerable v~riation ~nd will depend, in large measure, upon -the cx~ss-sectional dimension o~ ~he reactant mixture 25 Btream a~ it cour~es through the plane de~ined by ~h~ pilot flPmes. Desirably~ ~ubstantially ~he entire periphery of the reacta~t ~ixture ~tream will be cloaked in pilot flames, ~hereby tD aYoid the formation of Wdead ~pots" ab~ut ~he cir--~ Gumference o th~ reactant mix~ure stre~m. ~or instance, for ~ burner h~ving ~ mouth 3 diameter ~f between about 1-1/2 and 23~
a~out 2 1/2 inches (3.81 - 6.35 cms) and whose burner spuds 20 are located on a plane spaced from l/4 to l/2 inch (0.635 - 1.27 cms) below the burner mouth 3, this desirable subst~mtially cc~lete cloaking of the periphery of the reactant mixture stream in pilot flames can generally be achieved when six pilot Q ~mes are utilized, employing six burner spuds 20 equiangularly spaced about the circum-ference of the reactant mixture stream. In the case of burner mans 1 having mouth 3 diameters of substantially greater than about 2-l/2 inches (6.35 cms), or in the case where the plane of the reactant mixture stream contacted by the pilot flames is located at a substantial distance below the burner mouth 3, the number of burner spuds 20 is desirably substantially greater, for example eight, ten, or even twelve.
e vertical angle of projection of the pilot flames from the burner spuds 20 (relative to the orientation of the apparatus shown in Figures l or 3) is s~bject -to considerable variation and will usually range from essentially cocurrent with respect to the flow of the reactant mixture stream to somewhat countercurrent thereto.
Desirably, this angle, shown as angle ~ in Figure l, will be akout normal to the longitudinal axis of the reactant mixture stream, the preferred range being between about 85 and 95 and the range of greatest preference being between about 90 and 94. Where the angle of projection of the pilot flames lies substantially outside the broad ~; limits set forth above there is usually developed a tende~cy of the pilot flames to course in a helical fashion about the circumference of the reactant mixture stream, thereby mitigatlng against the desired substantially complete cloaking of the periphery of the reactant stream with pilot flames and tending to create "dead spots" about the surface of the reactant mixture stream. If of suf~icientimagnitude, these dead spots can result in hydrolysis flame instabilities or in the tendency of the hydrolysis flame to snap back and periodically anchor on the burner mouth.

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, As can be appreciated from the abovel the pilot ! flhmes of the invention serve to con~inuously initiate the hydxolysis flame r~action, to stabilize the hydrolysis flame and to define an anchoring point for ~he hydrolysis flame 5 which is physically removed from the burner mouth 3. Thus, in accordance wi~h the practice of the present invention, hy-drolysis o~ the metal or metalloid halide feedstock is avoided in the region adjacent the burner mouth 3 and the burden of the purge gas flow to maintain a fouling-free environment about the burner mouth is thereby much reduced.

Having thus formed an initiated and stabilized hy-drolysis reactant mixture flame, the rea~ant mixture mass is then provided with a salubrious envirsnment wi~hin which the oxide-producing reaction can be brought to completion. Pref-erably, this environment is defined by an enclosed reactionspace and, as shown in Figure 3, the reac~an~ miYture is pro-~ected from burner 1 into a suitably sized and enclosed reac-: tion space 100. The reaction space ~00 is defined by a reac-tion chamber 101 having a cooling jacket 102 annularly spaced about the exterior surface thereof. Cooling air is introduced unto the annular ~pace of jacket 102 throush conduit 103 and is exhausted through conduit 104. In the so-called "open-quench" system ~hown in Figure 3, the xeaction is cooled by air which i~ inducted throu~h chamber inlet 105 ~nd which cloak~
the hydroly~is reacti~n flame ~nd cools ghe reaction products ~; therefrom to b~low the ~intering temperature of ghe metal or ~etalloid oxide reaction product. ~he partially cooled reac-tion ~~-~a~e~, coDtaining the particul~te oxide product en-~rained th~rein, are ~hen withdrawn from reaction chamber 101 ~hrough ~utl~ 106 and ~re ~ubjected to the usual ~urther ~ooling and ~olid product ~eparation steps conventional in -~0--;Z3~

the art. The process economics of flame hydrolysis metal or metal-loid oxide producing processes can often be beneficially affected by ernploying aS the reaction quench process off-gases which have been cooled and from which at least most (e.g., greater than about 95%) of the particulate oxide product has been removed. While this specific embodiment is not explicitly disclosed in Figure 3 hereof, it should ke borne in mind that the only ~difications of -the Figure 3 arrange-ment required to perform this of~-gas quench scheme resides in the additional provision of a manifold sealingly interposed between the reaction char~ber 101 and the burner l and a supply conduit to said manifold through which conduit cooled recycle off-gases are supplied to the rnanifold. '~he rEanifold, of course, opens into the reaction space lO0.
There follow an illustrative, non-l~niting example.

A burner system and reaction char~ber of the general types shown in the drawing were employed hawing the following essential dirnensions.
Burner 1 .
Diameter of rnouth 3 -2.5 inches (6.35 cms) ~- Width of annular slit 9 --0.005 inch (0.0127 cm) Pilot Flame Burner S~uds 20 : ....
Construction - 3/16 inch (0.48 cm) O.D. steel Number ~ 6 Angle ~ 92 Spuds 20 oriented to tangentially contact pilot flames with periphery of reacta~t ~o`xture stream.
Plane spuds 20 located about 1.5 inch (3.81 cms) downstream from burner rnouth 3.

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Reaction Chamber 101 _ .
Length ~ 101.5 inches (257.8 cms) I.D. at inlet 105 ~- 5.25 inches (13.34 cms) I.D. at shoulder 107 - 15.25 inches (38.74 cms) I.D. at outlet 106 -- 8.69 inches (22.07 c~s) Diameter inlet 105 ~ 7.75 inches (19.68 cms) Cooling jacket 102 spaced from chamber 101 at a nominal spacing of about 2.5 inches (6.35 cms).
Dial thermometer located at cooling air outlet 104.
Burner system stationed coaxially above inlet 105, the pilot flame burner spuds 20 thereof being located about 2.375 inches (6.03 cms) above said inlet 105.
The specific starting materials employed in this example were as follows.

Feedstock ~ silicon tetrachloride preheated to about 325 F
(162.8C).

Hydrogen-containing fuel -- dry hydrogen prehea-ted to about 160 F (71.1C).
Oxidant - dried air at about ambient temperature.

Purge gas ~ either air (Runs 2, 3) or hydrogen (Run 1) as noted in Table.
Pilot flame fuel - hydrogen.
Pilot flame oxidant -~ air.
A series of finely-divided silica-producing runs were made, throughout which runs cooling air was flowed through cooling jacket 102 by means of a blower operated as to maintain a su'ostantially constant volume flow rate through inlet 103. At the start-up of each run the system was first placed on heat load in order to dry -the appara-tus and in or~er to bring it up to ahout operating temperature. The heat load cycle comprises operation of the pilot flames and burner 1, the latter being operated without supply of feedstock thereto. Switchover to the ; silica-producing run was then achieved by adjustment of the feed streams to the~-valuès stated in t-he.;Ta~le appeari~g hereinafter. It should be ~:`

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noted that the control run, Run 1, was under-taken a-t what was con-sidered to be the m~ximum capacity of the burner apparatus for the particular feedstock employed and goal silica product obta med.
The surface areas of the silica samples were determined in accordance with the well-known BET techr~que utilizing nitrogen isotherms. The EET (Brunauer-Em~et-Teller~ method is completely descriked in an article appearing in the Journal of the American Chemical Society, Vol. 60, page 309 (1938~.
The thickening efficiencies of the collected silica samples were determined by comparison of their individual thickening performances in a standard polyester resin liquid against that of one o~ the other of two standard flame hydrolysis silicasr C~B-O-SIL ~ fumed silica M~S, a silica produced by Cabot Corporation, Boston, MA., having a BæT-N2 surface æ ea of 200 r 25 m /g or C~B-0-SIL PIG, a silica having a BET N2 surface æea of 220 m /g + 15. In this test, six and one-half grams of the silica standard ~ld the silica under ~ test were each dispersed in sep æate 394 gram batches of an unpromoted ; - polyester resin. Polylite 31007, P~eichhold Chemicals, Inc., White Plains, N.Y~ The dispersion was carried out in a Premier Dispersator, Premier Mill Corporation, New York, N.Y for a period of 5 minutes and at a shaft speed of 3000 r.p.m. '~he resulting silica/polyester samples w~re then transferred into sepæate glass jars which were capped and placed in a constant temperature water bath for a period of . out

4 hours, the bath being maintained at a temperature of 77F (25&).
Next, the silica/polyester samples were subjected to viscometric analyses by m~ans of a Brookfield ~odel L~T Viscometer, Brookfield ` En~ineer~in~ Lakoratories, Inc.~ Stoughton, M~. 'rhe thickening ef-: .

~67~3~
.
; ficiency of the tes-t silica was then expressed as follows:

Thickening Efficiency (~) =

; Test silica/polyester resin (c~s) x 100 =
' Standard silica/polyester ''' (c~ps) ., Test silica/polyester resin (Pa.5) x 100 Standard silic ~
During the course of each of the runs accomplished in accor-dance with the present invention, Runs 2 and 3, the hydrolysis flame was periodically visually inspected and, in each instance, i* was noted , ~ that said flame was of smooth stable geometry and was firmly anchored at ~ ~ 10 a point physically removed from the burner mouth 3 ky the continuous i pilot flames. Upon ccmpletion of each of the runs in accordance with the invention, the burner was inspected and the mouth 3 thereof was ~ ~ found to have only a light and negligible dusting of silica product

5 ~ thereon.
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Referring now to the ~able, it is apparent there-from that~ for a given goal silica product and a fixed through-put of feedstock through a given burner apparatus~ the prac-tice of the present invention provides for substantial reduc-tions in consumption of both hydrogen-containing fuel and ~xidant. In t~rn, there also results a substantial reduction in heat released per weight unit of product oxide. In this connection, for instance, comparisvn is made between Control Run 1 and Invention Run 2. These reduced fuel and oxidant rates obviously represent substantial operating economies and, moreover, can also be reasonably expected to result : ~ in lower operating costs, redueed upkeep and longer sPr~icelife of equipment. Moreover, these experienced reductions appeared to give cause to believe that the rated capaci~y of the burner apparatus, when operated under conventional onditions, might no longer be valid when operated in accordance with the invention. Accordingly, Invention Run 3 was made in order to explore a ~eedstock throughput ra~e which was substantially in excess of the rated ~urner capacity for the particular feedstock employed and for the given goal ; silica product when produced under conventional operating conditions. As will be noted, Invention Run 3 successfully produced the goal ~ilica product at an increas~ in throughput of about 33~ over Control Run 1. Moreover, despi~e this ~5 improved feedstork rate, the overcapacity run of Run 3 utilized ss total hydrogen-containing fuel than ~heir corresponding ntrol run of Run 1 ~nd did not result in ~ co~ling air temperature exceeding that o the control run.

~ Further benefit~ re5ulting from the practice ~f `~ 30 the ~Yen~ion are ~ee~ t~ reside in reduced ~ff-gas volume:
oxide produ~t ratio~ ich reduced ratios serve to redu~e .

~;7~3~

the off-~as handling burden on equipment downstream of o~ltlet 106 such ~s eooling, handling and c~llection equipment. More-over, as a further corollary to such reduced xatios, a higher concentration of hydrogen halide i~ experienced in the reaction S off-gas composition, thereby reducing the gas handlIng burden on downstream recovery system components adapted to recover the valuable hydrogen halide by-product of the reaction.
~, .
While this invention has been described in the foregoing specification in connection with certain preferred embodiments thereof, obviotlsly many additional variations and modifications will suggest themselves to those skilled in the art. Thus, it i5 to be understood that the foregoing specification, taken in conjunction with the drawing, is intended to be illustrative in nature and ~hat the scope of the invention is to be Clrcumscribed only by the scoPe of the claims appended hereto.

~:~ The embodiments of the invention in which an exclu-~ive property or privilege is cleimed are defined =s follows:

:

.

Claims (25)

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

1. In a process for the preparation of finely-divided particulate metal and metalloid oxides comprising mixing, within a burner means, a vaporous of gaseous metal or metalloid halide, or mixture thereof, with a hydrogen-containing water-producing fuel and an oxidant therefor; projecting the resulting reactant mixture as a continuous stream from the mouth of said burner means into a reaction space; continuously discharging a purge gas at a fouling preventative rate along the boundary of said reactant mixture stream in the region of the burner mouth and continuously combusting the reactant mixture stream within said reaction space to produce said finely-divided particulate oxide therein; the improvement which comprises:
substantially tangentially contacting the periphery of said reactant mixture stream, at a plane substantially normal thereto and spaced down-stream from said burner mouth, with a plurality of continuous pilot flames.

2. The process of Claim 1 wherein said purge gas is air.

3. The process of Claim 1 wherein said purge gas is hydrogen.

4. The process of Claim 1 wherein said fuel is hydrogen.

5. The process of Claim 1 wherein said metal or metalloid halide comprises silicon tetrachloride.

6. The process of Claim 1 wherein said plane is located at between about 1/4 and about 1/2 inch (0.635 - 1.27 cms) downstream of said burner mouth.

7. The process of Claim 1, 5 or 6 wherein the fuel employed for performance of said pilot flames is of similar type to the employed in the performance of said reactant mixture stream.

8. The process of Claim 1, 5 or 6 wherein the fuel employed for performance of said pilot flames is hydrogen.

9. The process of Claim 1, 5 or 6 wherein said pilot flames are supplied with a combustible fuel/oxidant premixture.

10. The process of Claim 1 wherein the angle of said pilot flames with respect to the longitudinal axis of said reactant mxiture stream, is about normal thereto.

11. The process of Claim 10 wherein said angle is between about 85° and about 95°.

12. The process of Claim 11 wherein said angle is between about 90° and about 94°.

13. The process of Claim 1, 2 or 3 wherein the number of said pilot flames is sufficient to substantially completely cloak the periphery of said reactant mixture stream.

14. A burner system for the production of flame hydrolysis metal or metalloid oxides comprising burner means adapted to continuously receive, mix and enclose vaporous or gaseous metal or metalloid reactants therein and to continuously discharge the resulting reactant mixture as a substan-tially linearly projected stream fram the mouth thereof; means to contin-uously discharge a purge gas along the boundary of the discharging reactant mixture stream in the region of the burner mouth; a plurality of pilot flames burner spuds arranged about the circumference of the reactant mixture stream projected from the burner mouth, each said spud being disposed to project a pilot flame therefrom into substantially tangential contact with the periphery of said reactant mixture stream at a plane located at a spaced distance downstream of said burner mouth; and means to contin-uously supply a fuel gas to each said burner spud.

15. The burner system of Claim 14 wherein said means to supply said fuel gas to said burner spuds comprises a manifold to receive fuel gas therein; a plurality of depending supply conduits affixed to and in open communication with said manifold the end of each said supply conduit having a burner spud affixed thereto in open communication therewith.

16. The burner system of Claim 15 wherein said manifold is slidably affixed to said burner means.

17. The burner system of Claim 14, 15 or 16 wherein the diameter of said burner mouth is no greater than about 2-1/2 inches (6.35 cms), the number of said burner spuds is six and the disposition of said spuds-is such as to project the pilot flames therefrom into substantially tangential contact with the periphery of said reactant mixture stream at a plane located at between about 1/4 and about 1/2 inch (0.635 - 1.27 cms) down-stream of said burner mouth.

18. The burner system of Claim 14, 15 or 16 wherein the diameter of said burner mouth is greater than about 2-1/2 inches (6.35 cms) and the number of burner spuds employed is greater than six.

19. The burner system of Claim 14, 15 or 16 wherein said means to discharge said purge gas comprises a plenum exterior of and surrounding said burner means, said plenum defining an annular space which terminates in the form of a thin annular slot in the region of burner mouth, and means to supply purge gas into said plenum.

20. The burner system of Claim 14 wherein each said burner spud is disposed as to project the pilot flame therefrom at an angle which is about normal relative to the longitudinal axis of said projected reactant mixture stream.

21. The burner system of Claim 20 wherein said angle is between about 85° and 95°.

22. The burner system of Claim 21 wherein said angle is between about 90° and about 94°.

23. The burner system of Claim 14, 15 or 16 wherein said burner means includes means to rectify the flow of the reactant mixture stream prior to discharge thereof from said burner mouth.

24. The burner system of Claim 14, 15 or 16 wherein said means to supply a fuel gas to said burner spuds includes means to supply said fuel gas in combustible admixture with an oxidant therefor.

25, A burner system for the production of flame hydrolysis metal or metalloid oxides comprising burner means adapted to continuously receive, mix and enclose vaporous or gaseous metal or metalloid reactants therein and to continuously discharge the resulting reactant mixture as a substantially linearly projected stream from the mouth thereof; means to continuously discharge a purge gas along the boundary of the discharging reactant mixture stream in the region of the burner mouth, thereby to impede anchoring of the hydrolysis flame on said burner mouth; a plurality of pilot flame burner spuds arranged about the circumference of the reactant mixture stream projected from the burner mouth, each said spud being disposed to project a pilot flame therefrom into substantially tangential contact with the periphery of said reactant mixture stream at a plane located at a spaced distance downstream of said burner mouth; said plane thereby defining an anchor point for the hydrolysis flame which is removed from the burner mouth and means to continuously supply a fuel gas to each burner spud.