Patented Sept. 30, 1947 2428,178 UNITED STATES PATENT OFFICE METHOD OF MAKING FNELY DVDED WOLATZED S LCA Richard Reik, Piedmont, and Alva C. Byrns, Los Altos, Calif., assignors to The Permanente Metals Corporation, Oakland, Calif., a corpora tion of Delaware No Drawing. Application July 18, 1944, Serial No. 545,551 6 Claims, (C. 23-182) 2 This invention relates to finely divided amor the assumption slipped into the technical liter phous silicon dioxide that is in a condition of ature that silica is "volatile' in the presence of unusually high reactivity. This special kind of carbon at temperatures below 1600 C. by being silica, which shows valuable properties in a great reduced to silicon from which silica is again number of various applications is particularly formed by reoxidation (A. B. Searle, Refractory suited for use in the manufacture of refractory Materials, London, 1940, pp. 57/58 and 704). materials, Seeing that the boiling point of silica is at 2230 It is amongst the principal objects Of the inven C. and silicon boils even at 2600 C., the princi tion to prepare silica of an extraordinary degree ple underlying the above described empirical of fineness without having recourse to mechan 10 methods remained unexplained. ical trituration that requires special intensity or On the other hand it has been assumed for costly separation treatments to yield a material a long time that, analogously to the oxides of car anything similar to the Silica, resulting as a pri bon, silicon forms in addition to silicon dioxide mary product from the present process. Another a lower degree of oxidation of the formla SiO. eSSential object of the invention is to enable 5 According to the U. S. Patent No. 875,286 to amorphous Silica to be produced from naturally H. N. Potter, SiO is obtained by heating crystal Occurring Crystalline siliceous material. Both line silicon in the presence of oxygen under such these purposes are accomplished according to the conditions of oxygen supply and pressure as to present invention, without working cost being prevent the oxidation from progressing to the exceeded that are bearable in the manufacture of 20 stage of dioxide. For rendering the atmosphere a raw material which is intended for use in quan in which heating takes place adequately poor in tities like silica. To this end ordinary sorts of oxygen, Potter suggests to operate under re Silica, are caused to react with reducing agents at duced pressure or while diluting the normal at the relatively low temperature at which reduction mosphere with large quantities of an inert gas. to gaseous Silicon monoxide occurs, while con 25 Alternatively it has been proposed to prepare tinuously removing the SiO-gas from the reduc SiO by partial reduction of SiO2, for which pur ing Zone and Subjecting it to Oxidizing conditions pose different Ways have been chosen. to reform solid SiO2 therefrom which is then sep The first experiments were made in 188 with arated and collected. The dioxide appears in this fused silica while using carbon as the reducing manner in an extremely fine state of subdivision. 30 agent (American Chem. Journ, vol. 9, pp. 14/15). As early as in 1897 it has been discovered that In 1904. Potter succeeded in performing the car when fireclay is heated in the presence of car bothermic reduction. Without a melt by simply bon, part of the silica, is reduced and volatilized, heating a mixture of SiO2 and an amount of Some of the alkalis in the clay being volatilized at carbon that corresponds to the reaction the same time; subsequently the supposition was 35 made that volatilization occurs under oxidizing SiO2--C=SiO--CO conditions. No industrial application resulted to a temperature at which the monoxide volatil from these observations until in 1912, the British izes. Another investigator, Frank G. Tone, dis Thomson-Houston Co. disclosed a method of pro closed in the U. S. patent No. 993,913 a similar ducing refractory materials by highly heating 40 method of partial reduction of SiO2 by means of mixtures of silica and carbon that contain a me carbon, while leaving the question undecided tallic compound intended to yield the respective Whether the product obtained by condensation metal by reduction, and by collecting the "sub from the gaseous products of combustion leaving limate.' A treatment of Such kind of a mixture the reduction furnace, was a lower oxide of sili of silica, carbon and an aluminum silicate, with 45 con or a mixture of silica and amorphous silicon. or without one part of manganese oxide, is re - According to Tone the silica to carbon ratio in ported to result in a felted material of low thermal the furnace feed may conform to the reaction conductivity. A similar process consists in heat ing a mixture of silica, carbon and rutile in an SiO2--2C=Si--2CO electric furnace under reducing conditions and 50 or be even lower in silica and higher in carbon, collecting the sublimate of flaky, laminated respectively; but Tone adds that it is much bet ter to employ mixtures higher in silica since Structure that contains silicon, carbon, titanium Such mixtures yield the desired product in greater and oxygen (British specifications Nos. 25,370, abundance. 1912, and 29,389, 1912). Out of these disclosures 55 A second method, proposed by Potter in his 2428,178 4. 3 monoxide which is disengaged as a gas at temper specification No. 875.675, consists in performing atures far below the boiling point of silicon, and the partial reduction of SiO2 with the aid of readily reforms SiO2 on contact with air. How silico according to the equation ever, this sequence of reactions was heretofore SiO2--Sis2SiO. turned to account only for the production of coS netics such as dentifrice and face powder be for instance by heating an approximately com cause of yielding a finely divided powder of posed mixture of Si and SiO2 above the resistor of white color. Thereby it did not come to light an electric furnace of the arc type in an in that silicon dioxide directly produced by the way different atmosphere. 0. of gaseous SiO without intermediately isolating Finally according to R. Flusin (Ind. chimique solid SiO by condensation shows the character 9, 391) silicon carbide may be used instead of and function of a chemical of extremely high re carbon for the reduction by means of the re activity that is particularly valuable for the man action ufacture of refractory materials; nor was it SiOa--SiC=SiO--Si-CO recognized that this method offers the possibility Of whatever kind the reducing agent may be, s of preparing amorphous silica from naturally Oc the silicon monoxide is liberated in the state of curring crystalline raw materials without oppres brown-colored vapors which have to be condensed sive rise of the manufacturing expenses. quickly because otherwise decomposition takes The development of these discoveries of more place according to the equation 2SiO=SiO2--Si generai import may have been impeded by the (Ulmann, Enzyklopaedie der technis chen 920 fact that it was an open question whether the Chemie, 2nd ed., 1932, vol. 9, p. 505). The silicon product resulting from the partial reduction of monoxide depositing on condensation is reported SiO2 and Subsequent oxidation of the vapor to be in the form of an extremely finely divided evolved was actually a distinct substance of the powder or to appear at least principally in that formula SiO or rather a mixture of silica and form while a smaller portion is in the vitreous 25 silicon showing the composition of SiO. As al State. ready pointed out, Frank G. Tone left this ques Powders of such kind are meant for use as tion undecided in 1907; this was still the state of body for oil paints and for thermal and electrical the matter in 1941, though K. F. Bonhoeffer had insulation purposes, moreover as abrasive or pol in the meantime succeeded in establishing, by ex ishing powders (Ullmann, loc. cit.). Frank G. 80 amining the absorption spectra, that in the prod Tone mentions in the first place the use of his lucts formed by reduction of silicon dioxide with products as reducing agents. carbon actually the compound SiO is existent Apart therefrom, Potter suggests to utilize the in the gaseous state (Zeitschrift fuer physikalische finely subdivided silicon monoxide powder result Chemie, vol. 131/132, 1927 to 1928, p. 363). Zintil ing from partial reduction of SiO2 for converting 35 reports in 1940 (Zeitschrift fuer anorganische und it into silicon dioxide of equal fineness by oxida allegemeine Chemie, 245, 1 to 7) that quick con tion. Potter states that there are various ways densation of the vapor evolved in the vapor space of doing this, one of which consists in blowing results in the formation of an exceedingly light, a dust of silicon monoxide through a heated finely divided powder, but adds that the ques chamber in which it is oxidized by reaction with 40 tion whether the said powder consists of the an oxidizing gas (specification No. 886,637). A chemical individual SiO or a mixture of Si and second method consists in blowing an oxidizing SiO2 in correspondent proportions could not have gas through a furnace during the progress of re been decided until then. Likewise, H. N. Baumann action which, per se, would liberate silicon mon 45 stated in 1941 in Transactions of the Electro oxide (specification No. 875,674). Still another chemical Society, vol. 80, 95, 1941, that the ex method, that is disclosed in specification No. istence of silicon monoxide has been reported at 908,131 consists in burning the powder of silicon different times, but it has remained doubtful monoxide in the state of suspension in an oxidiz whether this suboxide is a definite compound. ing atmosphere. All these processes by which the Now, with regard to all the industrial purposes brown monoxide powder is changed into the 50 to which the condensation product had previously white dioxide have been devised according to been appointed, it matters little whether the prod Specification No. 908,131, lines 21 to 26, for the uct consists of a unitary compound or of a mix purpose of altering the color of the primary prod ture. Yet, in full contrast thereto it is prerequisite luct which, although in itself attractive and per to the present process that it is the compound fectly adapting the powder to certain uses, might 55 SiO which is being formed, and even exclusively be less serviceable for other uses. The change formed, because only this compound passes over of color to white is stated particularly to re to substantially pure SiO2 comprising a consider move any objection which may exist to the use able proportion of particle sizes within, or ap of the material as a tooth powder or as an in proaching to, colloidal ranges when being sub gredient of many other light colored powders such 80 jected to oxidizing conditions in the vapor state. as face powder for which purpose both the brown In carrying out the reduction according to the monoxide powder and the white dioxide powder present invention the SiO gas evolved is removed are reported to be easily available by reason of from the heated system, preferably drawn off by their extreme fineneSS, non-poisonous nature and Suction, So to Speak in the instant of its forma good spreading qualities (specification No. tion. Thereby the gas disengaged is protected 908,131, lines 35 to 44). from decomposition according to the equation When the industrial methods suggested by the SiO=SiO2--Si, and the equilibrium associated British Thomson-Houston Co. are considered in With the reaction is steadily displaced in the the light of the discoveries made subsequently by favor of SiO-formation. Reoxidation is then Potter and others in connection with the forma 70 preferably effected by injecting a stream of oxi tion of silicon monoxide, it may become obvious that it is not the silicon which is readily volatile dizing gas, such as air, into the SiO gas at the in the presence of carbon and then reforms SiO2 point of its leaving the heated system. under Oxidizing conditions, but that there occurs Alternatively a stream of inert gas may be used a partial reduction of silicon dioxide to silicon for continuously conducting away the SiO gas as 2,428,178 5 soon as it forms from the reduction zone instead bag house or by electrical precipitation, or other ofpressure. or while concurrently operating under reduced wise. The variety of uses is great. Amongst the reducing agents heretofore used The silica resulting from the process according in the service of silicon monoxide preparation, to the invention is suitable for every purpose to silicon is obviously best suited for the purpose which it is adapted by its physical properties, because the reaction viz., voluminous and intangibly soft nature and SiO-Si-2SiO -- large surface extension. Such purposes are for yields silicon monoxide as the only gaseous prod instance the uses as a base in pigments, printing uct. Nevertheless in this case also it is a sine O inks, paints, enamels, lakes, cleaning and polish qua non for successful tonnage production of very ing powders and preparations for fine surfaces, finely divided amorphousand relatively pure SiO2 lubricants, cosmetics, and as a filler in rubber ... that the monoxide is taken off from the reduc products, linoleum and so forth; moreover, the tion zone as soon as it is formed On purpose to use as a carrier for specifically effective sub minimize its decomposition to Si and SiO2. The 5 stances, such as catalysts or insecticidal, fungi silicon is preferably employed in the form of fer sorbent.cidal, bactericidal substances; finally as an ab rosilicon as it is the customary practice with sili On the other hand this special kind of silica is rathercon reduction. expensive. This makes the method, however, surprisingly adapted for any application where its 20 outstanding chemical reactivity, taken in con If, on the other hand, the reduction is effected junction with its other properties, can be turned by means of carbon this means not only saving to profit. A typical example is the use in the cost, but also offers the valuable advantage that manufacture of refractory material. To serve the reduction takes place at lower temperatures such purposes the silicon dioxide formed, sepa (between 1250° C. and 1450° C. under reduced rated and collected according to the invention is pressure). Yet, the situation is complicated by mixed with a component of amphoteric or basic the fact that a gaseous mixture issues from the nature (or a plurality of such components if nec reducing zone which carries equal volumes of sili essary), whereupon the mixture is shaped, or con monoxide and carbon monoxide gas according tempered and shaped, and eventually kiln-fired if to the equation SiO2-C=SiO--CO. Seeing that 30 desired. Even if firing is the final step of the in the said mixture of reduction products CO re procedure, the reduction of the primary SiO2 acts with SiO to reconvert it partly into SiO2 with the use of carbon as reducing agent ought to with the deposition of carbon, the reversion re be brought about under conditions controlled to action has necessarily to be suppressed in order suppress the deposition of carbon as a conse to enable silicon dioxide as free as possible from 35 quence of the back reaction between SiO and carbon to be recovered as the final product. To CO as far as possible; in this case also this pre this end the reduction products are kept up to cautionary measure is indispensable because the the very point of their issuing from the furnace at or closely below reduction temperature. By the gascarbon during would the firinggive rise Operation. to the formation of SiO injection of a great volume of oxidizing gas, par 40 Still another application of the silica resulting ticularly air, into the mixture of SiO and CO in a finely divided state as the original product as it leaves the furnace, the SiO is converted of the process according to the present invention, into solid finely divided SiO2 which is collected, will be found in the use of Said silica, for the whereas CO2 escapes in the gaseous state. production of magnesia, ceramics. To this end The operation may be conducted in a furnace 45 the material recovered from gaseous SiO by ox or retort of any desired type as a batch method idation and subsequent separation is blended with or carried out continuously; it is, however, a par a preponderant amount of a finely divided mag ticular advantage of the method that it offers nesia, and the mixture thus produced formed and the possibility of being readily put to practice fired. The fired material may be reground, if in a continuous manner. To this end finely di 50 desired, and then again shaped and fired. vided silica, may be made into compacted bodies, We claim: N - pellets or briquettes, with the addition of an 1. In the production of finely divided amor amount of reducing agent which only slightly phous silica by that process which comprises sub exceeds the proportion theoretically required for jecting a siliceous material in a reducing zone partial reduction to SiO, that is a ratio of one 55 to the action of a reducing agent at a temperature mole silicon dioxide to One mole reducing agent. sufficient to cause reduction of the siliceous ma In the case of carbothermic reduction a carbona terial to silicon monoxide and to produce gaseous ceous binding material such as tar or pitch may silicon monoxide, and Oxidizing the silicon mon be added in such amounts that, on preheating oxide to silicon dioxide, the improvement which the compacted bodies before the beginning of the 60 comprises withdrawing gaseous silicon monoxide actual reducing operation, a porous coke skeleton from the reducing ZOne and subjecting the silicon is left behind which firmly binds the silica and monoxide to oxidation while maintaining the said coke particles into a coherent mass and never monoxide substantially entirely in the gaseous theless allows the gas evolved freely to escape. state. Each of these compacted bodies preferably em 85 2. In the production of finely divided amor braces just the amount of reducing agent suf phous silica by that process which comprises sub ficient for partial reduction and contains besides jecting a siliceous material in a reducing zone to a quantity of tar or pitch that is equal to about the action of a reducing agent at a temperature Sufficient to cause reduction of the siliceous ma eight to twelve per cent of the total weight of the 70 terial to silicon monoxide and to produce gaseous silica and carbonaceous reducing agent. The silicon monoxide, and oxidizing the silicon non compacted bodies are fed in the furnace either oxide to silicon dioxide, the improvement which in continuous Or intermittent succession. comprises reacting the siliceous material with a The solid SiO2 is caused to separate and col carbonaceous reducing agent, wtihdrawing gase lected by any desired means, for example in a 75 ous products of reduction including silicon mon 2,428,178 8. 7 perature up to the point of issuing from the re oxide from the reducing zone and subjecting the duction furnace, and subjecting the withdrawn silicon monoxide to oxidation while maintaining gaseous products of reduction to oxidation by an said monoxide substantially entirely in the gase oxidizing gas while maintaining the silicon Ous3. state. In the production of finely divided amor monoxide component thereof substantially en phous silica by that process which comprises Sub tirely in the gaseous state. jecting a siliceous material in a reducing zone to 6. In the production of finely divided amor the action of a reducing agent at a temperature phous silica by that process which comprises sub sufficient to cause the reduction of the siliceous jecting a siliceous material in a reducing Zone to material to silicon monoxide and to produce gas O the action of a carbonaceous reducing agent at a eous silicon monoxide, and oxidizing the silicon temperature sufficient to cause reduction of the monoxide to silicon dioxide, the improvement siliceous material to silicon monoxide and to pro which comprises reacting the siliceous material duce a mixture of gaseous silicon monoxide and with silicon reducing agent, withdrawing gaseous carbon monoxide, the improvement which com silicon monoxide from the reducing Zone and Sub 5 prises continuously withdrawing said gas mixture jecting the silicon monoxide to oxidation while from the reduction zone while maintaining the maintaining the said monoxide substantially en Said gas mixture at a temperature high enough tirely in the gaseous state. to minimize the back reaction between SiO and 4. In the production of finely divided anor CO, and subjecting the said gas mixture to oxi phous silica by that process which comprises sub 20 dation while maintaining the silicon monoxide in jecting a siliceous material in a reducing Zone to the gaseous state. the action of a reducing agent at a temperature RICHARD REIK. sufficient to cause the reduction of the sliceous ALVA C. BYRNS. material to silicon monoxide and to produce REFERENCES (CITED gaseous silicon monoxide, and oxidizing the sili 25 con monoxide to silicon dioxide, the improvement The following references are of record in the which comprises withdrawing gaseous silicon file of this patent: monoxide from the reducing Zone under reduced UNITED STATES PATENTS pressure, and subjecting the silicon monoxide to oxidation while maintaining the said monoxide 30 Number Name Date substantially entirely in the gaseous state. 1,104,384 Potter ------July 21, 1914 5. In the production of finely divided anor 908,131 Potter ------Dec. 29, 1908 phous silica by that process which comprises Sub 886,637 Potter ------May 5, 1908 jecting a siliceous material in a reducing furnace 875,674. Potter------Dec. 31, 1907 to the action of a carbonaceous reducing agent at 875,675 Potter ------Dec. 31, 1907 a temperature sufficient to cause reduction of sili 886,636 Potter ------May 5, 1908 ceous material to silicon monoxide and to pro 2,095,982 Heany ------Oct. 19, 1937 duce gaseous silicon monoxide, and oxidizing the 2,313,746 Heany ------Mar. 16, 1943 silicon monoxide to silicon dioxide, the improve 2,206,131 Seil ------July 2, 1940 ment which comprises withdrawing the gaseous 40 2,358,107 Seil ------a ma am - - Sept. 12, 1944 reduction products including silicon monoxide 2,292,644 Lee ------Aug. 11, 1942 from the reduction furnace while maintaining 2,308,092 McDougal et al. ------Jan. 12, 1943 said products at approximately reduction tem 2,272,346 McDougal et al. ----- Feb. 10, 1942