System and method for producing electric arc silica and silica produced thereby

Abstract

A method and system is disclosed for producing electric arc silica comprising the following method steps: providing (301) a plasma reactor configured to generate at least one electric arc by establishing an electric field between the at least two electrodes; receiving (302) a feedstock comprising silica, preferably quartz, in the plasma reactor; introducing gas (303) between the at least two electrodes through a first gas inlet; feeding (305) between the electrodes through a powder feeder a silica containing powder, preferably quartz, comprising a chloride as additive; and quenching (306) oxidic silica compounds to produce electric arc silica. Further the present invention refers to electric arc silica (124) produced according to the above mentioned method steps as well as a system comprising a plasma reactor configured to perform above mentioned method.

Description

[0001] 202400278 Foreign Filing 1

[0002] SYSTEM AND METHOD FOR PRODUCING ELECRTRIC ARC SILICA AND SILICA PRODUCED THEREBY

[0003] Field of the invention

[0004] The present invention relates to a method and a system for producing high surface silicon dioxide (SiO2). More particularly electric arc silica is produced with a plasma or electric arc in a plasma reactor. Further, a system is provided configured to perform the method steps of the present invention. Furthermore, electric arc silica produced according to the method steps of the present invention is provided, wherein the form is preferably amorphous silica powder having a chain-like structure.

[0005] Background

[0006] The present disclosure relates generally to high surface silicon dioxide (SiO2) and the production thereof. These SiO2-products are in powder form and are primarily produced in a flame and to a lesser extent in thermal plasma processes. The thermal plasma technology may use an electric arc and the product may be defined as either fumed silica or electric arc silica, respectively.

[0007] Already in the early 1940s a chemist at Degussa AG invented a process for producing high surface and extremely fine silica. This kind of silica (SiO2) may also be called “fumed silica”, “pyrogenic” or “flammable” silica because it may be produced in a flame. Fumed silica or pyrogenic silica is industrially formed at high temperature by flame oxidation-hydolysis of silicon halides (SiCk) as disclosed by Degussa in EP 0 015 315 or Florke et al in Ullmann’s Encyclopedia of Industrial Chemistry, Silica, Volume 32, pages 421- 507, Wiley-VCH Verlag GmbH & Co KGaA, Weinheim, DOI:

[0008] 10.1002 / 14356007.a23_583.pub3 (2012).

[0009] The industrial hydrolysis of silicon tetrachloride converts SiCkor other chlorosilanes (e.g. methyltrichlorsilanes, MTCS) in the gas phase and reacts within an oxyhydrogen gas flame above 1000°C to form the desired silicon dioxide. Known commercially available fumed silica produced by this process is AEROSIL® which was formerly produced by Degussa and now by Evonik Industries AG. The ASEROSIL® fumed silica have primary particles with average diameters between 7 and 40 nm, while the specific surface areas according to BRUNAUER-EMMET-TELLER (BET) range between 50 and 380 m2 / g (Technical Information 1411 , “AEROSIL® fumed silica and SIPERNAT® specialty silica in Silane Terminated Polymers (STP)”).

[0010] There are numerous target applications for silica produced by the present invention such as reinforcing fillers, e.g. for mechanical enhancement of tires, thickening and thixotropic agent, anti-caking for powders and drying agent. Other uses include:

[0011] Adsorbents, antiblocking agents for plastic films, coatings, catalyst supports, matting / grining or polishing agents, raw material for silica glasses, thermal insulation and additive carriers (See Ullmann’s Encyclopedia of industrial chemistry, Silica Volume 32, section 6.1.7 Uses; pages 483- 485).

[0012] For fumed silica the average primary particle diameters is ranging usually between 5-100 nm. Numerous methods have been developed to characterize the porosity, structure and chemistry of silicon dioxide.202400278 Foreign Filing 2

[0013] The specific surface area is generally determined by the BET (BRUNAUER, EMMETT und TELLER) method in accordance with DIN 66 131; ISO 57494-1, Annex D). Ullmann’s Encyclopedia of industrial chemistry, Silica, Volume 32, page 487 discloses that the BET surface measures both the outer surface and the surface of accessible pores.

[0014] The primary particle size and aggregate structure of fumed silica can further be characterized using transmission electron microscopy (TEM).

[0015] There is the need for processes to produce fumed silica, which do not involve silicon halides (SiCk) as in the industrial scale hydrolysis in order to avoid the byproduct HCI (g). Further there is the object to use a process wherein the raw products are cheaper than SiCk. Another object is to achieve a product of high purity.

[0016] A known thermal plasma method decomposing as raw silica containing material quartz sand is applied in an electric arc furnace, wherein quartz sand is reduced with coke at high temperatures and subsequently the SiO that is formed is oxidized. As product electric arc silica is formed. Quartzite and coke are described, e.g., by Mishra et al., “Fine silica powder preparation by use of a transferred arc thermal plasma reactor”, Material Letters 23 (1995) 153-156 for the preparation of silica powder using a plasma reactor. However, it needs to be taken care that the combustion is complete in order to gain fumed silica of high purity. Further, in order to produce high purity fumed silica very pure raw material of silica as well as carbon needs to be provided.

[0017] Another way of thermal plasma production of fumed silica is disclosed by Pristavita et al., “Transferred Arc Production of Fumed Silica: Rheological Properties”, Ind. Eng. Chem. Res., 2008, 47, 6790-6795. For this thermal plasma production of fumed silica also the relatively cheap raw material quartz sand was used. Pristavita et al. assumes that the production process comprises a decomposition of quartz to gaseous silicon monoxide (SiO (g)) and oxygen followed by an oxidizing quench back to SiO2.

[0018] Pristavita et al., 2008, rely partially on the results of Addona et al, “The Study of a Novel Thermal Plasma Process for the Production of Fumed Silica”, 1998, wherein fumed silica could be produced but the expected rheological properties of the produced powder could not be demonstrated. In the study of Pristavita et al., 2008 plasma experiments analyses the effect of quenching gases (steam and / or air) and as commercial comparative example AEROSIL® 200 was analysed. The results of both Pristavita et al, 2008 and Addona 1998 showed lower viscosities values of the plasma produced fumed silica than the commercial AEROSIL® material, although quenching gas was used and the obtained particles were agglomerated. Pristavita et al. found that even with greater specific surface area the viscosity of plasma produced samples was lower (see Pristavita et al page 6792, last paragraph pages 6790-6795, Ind. Eng. Chem. Res., 2008, 47). Pristavita concluded that the inferior rheological properties of the plasma produced fumed silica were due to the lack of free hydroxyl groups on the surface.

[0019] Thus, there is the need to find a way of producing fumed silica using thermal plasma processes with better thickening and thixotropic properties and having a comparable surface chemistry as the comparative commercial example such as Aerosil® fumed silica. One desired characteristic of the surface chemistry of fumed silica is to have so-called active silica that is to say silica with surface hydroxyl (OH)202400278 Foreign Filing 3

[0020] groups or silanol groups to improve the viscosity. FIG. 1 shows typical silanol groups on the surface of commercial AEROSIL® fumed silica (Fig. 4 of Technical Information 1411, “AEROSIL® fumed silica and SIPERNAT® specialty silica in Silane Terminated Polymers (STP)). According to FIG. 1 at the surface of the fumed silica AEROSIL® the structure terminates in either a siloxane group (=Si-O-Si=) with the oxygen on the surface (see FIG. 1(d)), or one of several forms of silanol groups (=-Si-OH). The silanol groups can be divided into the following subgroups:

[0021] (a) isolated groups (see “free” silanol groups or hydroxyl (OH) groups in FIG. 1 (a)), where the surface silicon atom has three bonds into the bulk structure and the fourth bond attached to a single OH group;

[0022] (b) vicinal silanols (or bridged silanols), where two single silanol groups, attached to different silicon atoms, are close enough to hydrogen bond.

[0023] (c) geminal silanols — consists of two hydroxyl groups attached to one silicon atom.

[0024] The geminal silanols are too close to hydrogen bond each other (see FIG. 1(c)), whereas the free hydroxyl groups (see FIG. 1(a)) are too far separated. A Fourier transform infrared (FTIR) analysis may be used to identify the presence of isolated or bridged silanol groups via their typical absorption spectra. If the FTIR spectra analysis shows a peak at around 3750 cm1this is due to the presence of “free” or isolated hydroxyl groups. FTIR analysis is a generally known method in the art to measure isolated silanol groups (SiOH). The silanol (Si-OH) groups on the surface of the silica can be qualitatively measured using Fourier transform Infrared spectroscopy (FTIR) by detecting the respective absorption bands. An important absorbance band in the IR spectrum is a potential peak at about 3750 cm-1indicating isolated OH groups. Further at 3000-3800 cm-1absorbance for bridged OH groups can be detected (see Pristavita et al 2008, Table 2).

[0025] For quantitatively determining silanol groups on the surface of silica the lithium aluminium hydride (UAIH4) method can be used. For this analysis method the reducing agent LiAIH4 reacts with dried silica such as AEROSIL® (Drying process e.g.: 1 h, 100 °C, < 10-2mbar, to remove any free or bound water) and the hydrogen evolved by this reaction of LiAIH4can be measured. The reaction of 4 SiOH-groups with LiAIH4 results in 4 H2 as follows:

[0026] 4 =-Si-OH + Li AIH4 =-Si-O-Li + ( -Si-O)3Al + 4 H2

[0027] Accordingly, the resulting hydrogen corresponds to the concentration of SiOH-groups on the surface of the silica. That is to say, the lithiumalanate method can reliably determine the amount of formed hydrogen by measuring the hydrogen partial pressure. This method detemines all Si-OH groups i.e. the free isolated or „free“ hydroxyl groups and the bridged silanols.

[0028] One object of the present invention is to produce electric arc silica, which is rich in “free” silanol or hydroxyl group. Such “free” or isolated hydroxyl (OH) groups play a key role for functionalities of fumed silica or electric arc silica. In particular, the presence of isolated OH groups defines the viscosity of compounds containing fumed silica. In other words, the viscosity can be increased if the fumed silica surface has more free hydroxyl groups (see structure (a) designated “free” in FIG. 1). Conversely, the viscosity values will be low if the isolated hydroxyl groups are absent from the silica surface or if they are202400278 Foreign Filing 4

[0029] already involved in hydrogen bonding with another molecule. Accordingly, it is another object to provide electric arc silica having comparable viscosity values as commercial fumed silica products.

[0030] If fumed silica is produced according to the common industrial hydrolysis process the surface of the freshly produced silica particles show mainly isolated or “free” silanol groups whereof the hydroxyl group is sufficiently remote from a neighbouring hydroxyl group such that hydrogen bonding cannot occur. Since the OH group can act as hydrogen donor the surface may alter during aging. During storage water may be adsorbed and react with strained siloxane groups to form bridged silanol groups such that the content of isolated silanol groups or free hydroxyl groups decreases with time. For AERSOSIL® there are no significant changes observed in the SiOH groups for a storage period of at least about 2 years. It is noted that any measurements regarding the surface chemistry in the below experimental examples have been made shortly after the production or after a drying process. For instance, for measuring the SiOH group density with the lithiumalanate (UAIH4) method the samples of the produced silica have been dried in order to remove any potential free or bound water (H2O).

[0031] Pristavita et al. concluded that inferior rheological properties of the plasma produced fumed silica were due to the absence of the free hydroxyl groups from the surface of the particles (page 6794 second last paragraph, pages 6790-6795, Ind. Eng. Chem. Res., 2008, 47). Accordingly, there is need to improve the thermal plasma process to produce electric arc silica in order to achieve a product having isolated hydroxyl groups at the surface.

[0032] One object of the present invention is to provide a thermal plasma process using a plasma arc or electric arc to convert crystalline quartz with low BET surface area into a high surface area silica with isolated silanol groups. Another object is to avoid post treatment processes such as HCI impregnation in order to achieve free silanol groups as described below in comparative example 3 (403). Further it is another object to provide a process that does not generate waste or byproducts.

[0033] Summary of the invention

[0034] It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative. In particular it is an object to provide an improved thermal plasma process for producing electric arc silica. For instance, impurities as the residual contents of commonly used reducing agents such as carbon compounds as well as additional method steps such as post treatment to achieve free silanol groups on the surface of the produced silica should be avoided.

[0035] One or more of said objects is / are achieved by a method according to the independent claim, a corresponding system claim and electric arc silica produced by said method. Various features and advantages of the invention will be apparent from the dependent claims and are set forth in part in the following description and figures, respectively. Further, a system comprising a plasma reactor is provided configured to perform the method steps according to the independent claim and claims dependent thereon.202400278 Foreign Filing 5

[0036] In a first aspect of the present invention a method is provided for producing electric arc silica comprising the following method steps:

[0037] providing a plasma reactor configured to generate at least one electric arc by establishing an electric field between the at least two electrodes; receiving a feedstock comprising silica, preferably quartz, in the plasma reactor;

[0038] introducing gas between the at least two electrodes through a first gas inlet; feeding between the electrodes through a powder feeder a silica containing powder, preferably quartz, comprising a chloride as additive; and

[0039] quenching oxidic silica compounds to produce electric arc silica.

[0040] In this way electric arc silica i.e. silicon dioxide (SiO2) with high specific surface area and of fine size can be formed. The electric arc silica has not only a high surface but also improved rheological properties, in particular an increase of viscosity compared to electric silica wherein no chloride is added to the fed raw material i.e. the silica containing powder.

[0041] The plasma reactor provides at least two electrodes. Alternatively four electrodes can be provided in order to generate two electric arcs. The feedstock material is comprising a major part of silica in the form of silicon dioxide (SiO2) preferably in an amount of at least 95% SiO2, preferably at least 99 % SiO2. As preferred feedstock material crystalline quartz is used, which becomes conductive once molten by the heat of the electric arc or another heat source separate from the electric arc.

[0042] After a stable electric arc is generated the silica containing powder is fed between the electrodes in order to be injected into the zone of the hot plasma. Due to the high temperatures the fed silica powder can be vaporized to form oxidic silica compounds in the gas phase. The obtained silica vapour comprises at least oxidic silica compounds that can at least partially be in the form of gaseous silicon monoxide i.e. SiO (g). Other disproportion products of silica might be present in the vapour cloud. The oxidic silica compounds can then be quenched, preferably by steam, air or mixtures thereof in order to produce electric arc silica. The electric arc silica produced is amorphous and can be separated off in a common manner such as with a filter unit made e.g. of ceramic or a cyclone separator or combinations thereof.

[0043] Preferably after the generation of an electric arc further a reducing agent or reaction gas is introduced through the first gas inlet.

[0044] A vapour of oxidic silica compounds can be formed with the very high temperature of the plasma using the plasma process gases and / or with the aid of a reducing agent. A common reducing agent is hydogen (H2). The addition of H2 can reduce the energy required of silica decomposition. The presence of the reducing agent may at least partially support the formation thermally decomposed products of silica such as silica monoxide, SiO (g), which by means of quenching with an suited oxidizing agent such as H2O or O2 can be re-oxidized to SiO2.

[0045] According to a preferred embodiment, the surface of the produced electric arc silica comprises isolated silanol (SiOH) groups.202400278 Foreign Filing 6

[0046] In this way the viscosity can be improved with regard to electric arc silica which does not have isolated silanol groups i.e. free hydroxyl groups on its surface. By way of experiments it was found that neither adding hydrogen e.g. in the form of hydrogen gas (see comparative example 1 below) nor further adding hydrogen as quenching gas in the form of water steam as discussed below with regard to comparative example 2 resulted in the formation of isolated hydroxyl groups. Surprisingly only the presence of the additive containing a chloride in combination such as NaCI in the fed silica powder with a hydrogen source produced detectable free hydroxyl groups at the surface which indicate a high silanol group density.

[0047] That is to say in order to generate hydroxyl groups at the surface of the electric arc silica not only a hydrogen containing gas needs to be introduced through the first and / or the second gas inlet but also a chloride containing additive in the raw material silica is required. Alternatively to adding hydrogen as reaction gas a hydrogen containing compound may be added e.g. via the powder feeder, which then decomposes and releases hydrogen if entering the electric arc.

[0048] According to a preferred embodiment, the additive is selected from the group of chloride salts consisting of:

[0049] sodium chloride (NaCI), ammonium chloride (NH4CI), potassium chloride (KCI), magnesium chloride (MgCh), calcium chloride (CaCh), other salts of the hydrochloric acid (HCI), and any combination thereof. It was surprisingly found if a chloride salt is added the desired surface chemistry could be reproducibly produced. Sodium chloride and ammonium chloride are preferred chloride salts of the salts noted above. According to a further preferred embodiment the additive is added in an amount of smaller than about 10 w%, preferably about 4 w%.

[0050] According to a further preferred embodiment the powder feeder feeds the silica containing powder, preferably quartz, with an average particle size of micrometric size or smaller. The fine sized quartz particles are preferably smaller than 0,8 mm. The dosage rate depends on the scale of the plasma reactor. For experiments at laboratory scale a feeding rate of about 400g powder / h may be used.

[0051] According to another preferred embodiment, the produced electric arc silica is hydrophilic.

[0052] According to a further preferred embodiment the produced electric arc silica has a silanol group density between about 0,5 to 8 SiOH / nm2, preferably of at least 1 ,0-3,0 SiOH / nm2, more preferably about 1 ,2 -2,5 SiOH / nm2and / or a BRUNAUER-EMETT and TELLER (BET) surface of the electric arc silica is at least 15 m2 / g, preferably at least 30 m2 / g. It was observed that when the specific surface is increased that the silanol density does not increase but stays in the same range. For instance, for a BET surface of 30 m2 / g the silanol density is between 1 ,5 - 1 ,5 SiOH / nm2. For instance, for a BET surface of 30 m2 / g the silanol density is between 1 ,0 - 1 ,5 SiOH / nm2.

[0053] The hydrophilic property is attributed to the presence of free silanol groups. The silanol density of 8 SiOH / nm2corresponds to a fully hydroxylated surface. The viscosity increases if the density of SiOH per nm2increases. Another unique property of the produced electric arc silica is the large specific surface area of the product, wherein the thickening effect increases with the BET surface area.202400278 Foreign Filing 7

[0054] It is noted that it is assumed that the silanol group density is independent of the BET surface area. The viscosity values of the produced powder depend on the properties such as specific surface area and surface hydroxyl groups density. When the electric arc silica is dispersed in a liquid, the silanol (Si-OH) groups interact with each other either directly or indirectly with the molecules of the liquid. The predominant mechanism is hydrogen bonding and results in a reversible, three-dimensional network structure that is visible as thickening.

[0055] According to another preferred embodiment, the diameter of primary particles of the produced electric arc silica are nano-scale, preferably ranges between 10 to 50 nm, and wherein said primary particles form aggregates, preferably in a chain-like structure. The structure can be determined by using, e.g., transmission electron microscopy (TEM), as generally known in the art.

[0056] The chain-like aggregate structure is another advantageous specialty of the produced electric arc silica. It is noted that the primary nano-scale particles do not exist as isolate primary particles but are joint with one or more adjacent particles to primarily form irregular chains which may also occur as branched chains. These chain -like aggregates cannot be broken down to individual particles. These aggregates may form larger agglomerates by means of e.g. hydrogen bonds or Van-der-Waals-forces. In general, the higher the specific surface area, the greater the potential degree of agglomeration in liquids. Under mechanical stress the agglomerated aggregates may be dispersed to the original chain-like aggregates. Such rheological properties can be measured by standardized analysis using viscometers or other common methods. Due to the chain-like structure of aggregates and ability to create hydrogen bonds the thickening and thixotropic effect of the produced electric arc silica is very pronounced.

[0057] According to another preferred embodiment, the gas comprises at least one inert gas and / or a reducing agent, wherein as preferred reducing agent hydrogen is used.

[0058] The process gases introduced into the plasma reactor may be selected from the group of inert gases consisting of:

[0059] Argon (Ar), nitrogen (N2), Helium (He), Neon (Ne) or mixtures thereof.

[0060] The advantage to use inert noble gases in plasma technology is that they form ions. However, noble gas as Helium is more cost intensive than using for example nitrogen or Argon. Accordingly, a preferred plasma process gas is Argon (Ar) or a mixture of Ar and N2.

[0061] The reducing agent is preferably introduced through the first gas inlet after generating an electric in order to increase the rate of decomposition of the silica containing powder such as quartz to oxidic silica compounds. As preferred reactive gas hydrogen (H2) is introduced, which can advantageously be added to the plasma reactor as part of the plasma process gas. The volume ratio of reducing agent to process gas may range from 1 :10 to 10:1 , preferably 1 : 5 to 5:1. Usually it is sufficient to add the reducing agent at the same or a lower amount than the plasma process gas i.e. at a ratio of smaller than or equal 1 :1. A preferred ratio of Ar / N2 is 3:1. Adding Argon or nitrogen to the reactor dilutes the formed vapor cloud of oxidic silica compounds comprising mainly SiO(g). By dilution the amount of silica monoxide SiO (g) condensing in the plasma chamber can be reduced. This is of interest because the hot vapor cloud within the plasma zone should optimally contain only gaseous compounds.202400278 Foreign Filing 8

[0062] According to another preferred embodiment, the method step quenching introduces a quenching gas through a second gas inlet into a reaction chamber of the plasma reactor and / or into a gas outlet of the plasma reactor.

[0063] According to yet another preferred embodiment the quenching gas is selected from the group consisting of:

[0064] air, water steam, preferably at temperatures greater than about 110°C provided by a heating (135) around the gas inlet, nitrogen (N2) and any combinations thereof.

[0065] Preferably the quenching gas comprises at least water steam (H2O(g)).

[0066] The quenching gas may either be introduced through a second gas inlet into the reaction chamber or into the gas outlet of the plasma reactor. If water steam is injected a heating around the external part of the second gas inlet avoids condensation of the steam. The quenching gas or mixture of quenching gases are used to oxidate the vaporized oxidic silica compounds and depending on the location of the gas inlet may also be used to cool down the oxidation zone to generate electric arc silica. After the oxidic silica compounds such as silicon monoxide reacts with oxygen of the quenching gas, nano-sized amorphous silica particles are formed, which aggregate to form a chain-like structure.

[0067] According to another preferred embodiment, a conductivity enhancing agent, preferably sodium hydroxide (NaOH) or Aluminium oxide (AI2O3), is added to the feedstock.

[0068] Since silica (SiO2) has a low electrical conductivity and the melting point is about 1710°C it is advantageous to enhance the electrical conductivity of the silica melt and / or lower its melting temperature. In order to lower the melting point NaOH or other additives may be added, which have a low melting point. Using such conductivity enhancing additives reduces the time until the feedstock becomes conductive in the crucible prior to initiating the electric arc. Alternatively, an additive with intrinsic conductivity of the additive as e.g. graphite can be used in the feedstock. In this way the difficulty is addressed that there is need to provide a conductive material in or near the SiO2 in order to start the electric arc.

[0069] Preferably the range of plasma generating power is between about 3,5-20 kW and the generating current applied to the electrodes is preferably between 80 A and 120 A, more preferably 100A. These ranges apply if the plasma reactor is used on a laboratory scale and can differ if the plasma reactor is scaled up for higher production rates. Then the generating power may be increased from several tens of kW up to several MW. In general increasing the arc power will enhance the silica vaporization rate.

[0070] Alternatively to increase the arc power the use of multiple torches that is to say the use of two or more upper electrodes may be used. In this way each electrode pair (anode and cathode, respectively) may only supply a fraction of the total power which prolongs the lifetime of each electrode pair. Another advantage of the use of two electrode pairs or more is that maintenance can be done at one upper electrode whereas the other electric field of the other electrode pair can still be operated at an increased power. Thus, shutdown times of the plasma reactor can be reduced.202400278 Foreign Filing 9

[0071] According to another preferred embodiment, the feedstock is received by a crucible, preferably made of graphite or tungsten or other fire resistant materials based on magnesite, siliconcarbide or corundum or other suited fire resistant materials, wherein the feedstock, preferably quartz, is molten at a temperature equal or more than the melting point of the feedstock.

[0072] According to another preferred embodiment, the crucible is arranged above the first electrode of the at least two electrodes, preferably comprising a sacrificial electrode, preferably made of graphite, in contact with the first electrode and / or with the bottom of the crucible.

[0073] Typical temperatures at the crucible to melt the feedstock and keep the silica in the crucible in a molten state are above 1710° C, preferably above 1750 °C. Instead of giving additives to lower the melting point of the feedstock a sacrificial electrode made of graphite or other conductive material can be provided in the crucible, which sacrificial electrode is consumed during the heating process until a stable and electrically conductive molten silica is generated in the crucible. The disadvantage of the use of such sacrificial electrode is that this leads to contamination of the produced electric arc silica at the beginning of the manufacturing process. If high purity electric silica is needed the contaminated powder (C-based contamination if graphite is used) needs to be discarded. Thereafter pure electric arc silica can be collected.

[0074] According to another preferred embodiment, the first electrode is an anode and the second electrode is a moveable cathode and configured to contact the sacrificial electrode of the crucible; and wherein after establishing the electric field between the electrodes the cathode is raised such that the electric arc is built between the lower tip of the cathode and the upper surface of the molten feedstock.

[0075] In this way the distance to the molten feedstock can be reduced and thus simplify the start of the electric arc.

[0076] Preferably the upper or second electrode of the two electrodes is configured to be hollow and part of the powder feeder, wherein optionally the hollow elcetrode has an inner diameter of minimum 50 mm.

[0077] In this way the second electrode, which is located at the top of the plasma reactor, can be used as inlet either for the plasma process gas or the powder to be fed. If the upper electrode is used in combination with a powder feeder the inner diameter of the hollow electrode is chosen such that it allows the fine particles of the silica powerderto be easily transported to the crucible by gravity.

[0078] Preferably the electric arc silica is collected in a filter unit in the gas outlet of the plasma reactor.

[0079] Such filter may be a ceramic filter or a baghouse filter, wherein the powder collected is periodically removed. Further cyclones can be used as separators.

[0080] According to another aspect of the invention a system is provided configured to perform any of the afore mentioned method steps. The system according to the present invetion comprises at least the following: A plasma reactor comprising at least two electrodes configured to generate at least one electric arc;

[0081] a first gas inlet for introducing gas between the at least two electrodes;202400278 Foreign Filing 10

[0082] a powder feeder to feed a silica containing powder comprising a chloride as additive, wherein the powder is preferably quartz,

[0083] a second gas inlet to introduce a quenching gas to produce electric arc silica.

[0084] According to a preferred embodiment of the system comprises an electric field generator to apply an electric field between the electrodes. Further a gas outlet is provided and downstream of the gas outlet a separation unit to separate the produced powder from the offgas such as a filter unit is provided.

[0085] The method according to the present invention comprises at least the following steps:

[0086] First providing a plasma reactor. Further receiving a feedstock therein and then introducing gas in the plasma reactor. The next method step comprises generating an electric arc between the at least two electrodes. After a stable electric arc has been established silica comprising a chloride containing additive is fed into the reaction chamber of the plasma reactor. By the method step quenching electric arc silica is produced.

[0087] According to another aspect, an electric arc silica is produced according to the afore mentioned method steps. This produced electric arc silica is characterized by a high specific surface and the presence of free silanol groups on its surface.

[0088] Brief Description of the Figures

[0089] The accompanying figures illustrate exemplary embodiments of the disclosure and serve to explain, by way of example the principles of the disclosure and are not intended to be drawn to scale or to restrict the disclosure to the embodiment illustrated in the figures. Where technical features in the figures or detailed description are followed by references signs, the reference signs have been included for the sole purpose of increasing the intelligibility of the figures and description. For purposes of clarity, not every component may be labelled in every figure. As used herein, the terms “top,” “bottom,” “upper,” “lower,” and “above” are used to provide a relative relationship between structures. The use of these terms does not indicate or require that a particular structure must be located at a particular location in the apparatus or plasma reactor or a respective system.

[0090] FIG. 1 shows typical groups on the surface of fumed silica: (a) free hydroxyl groups, (b) hydrogen-bonded I bridged hydroxyl groups, (c) geminal hydroxyl groups and (d) siloxane groups;

[0091] FIG. 2 shows an exemplary embodiment of a plasma reactor with two electrodes and a separate powder feeder comprising silica with a chloride additive;

[0092] FIG. 3 shows another exemplary embodiment of a plasma reactor feeding silica powder with additive through one electrode;

[0093] FIG. 4 method for producing electric arc silica;

[0094] FIG. 5 IR-spectrum of silica produced according to comparative example 1;

[0095] FIG. 6 IR-spectrum of silica produced according to comparative example 2;

[0096] FIG. 7 IR-spectrum of silica produced according to comparative example 3;202400278 Foreign Filing 11

[0097] FIG. 8 IR-spectrum of silica produced according to comparative example 4;

[0098] FIG. 9 IR-spectrum of silica produced according to an example in accordance with the present invention (inventive example 5);

[0099] FIG. 10 TEM picture of plasma produced electric arc silica according to the present invention; and FIG. 11 IR-spectrum of silica produced according to another inventive example (6).

[0100] FIG. 12 IR-spectrum of silica produced according to another inventive example (7).

[0101] Detailed description

[0102] FIG. 1 shows typical groups on the surface of fumed silica as taken from Fig. 4 of Technical Information 1411, “AEROSIL® fumed silica and SIPERNAT® specialty silica in Silane Terminated Polymers (STP)”, 2016).:

[0103] (a) free hydroxyl groups,

[0104] (b) hydrogen-bonded / bridged hydroxyl groups,

[0105] (c) geminal hydroxyl groups and

[0106] (d) siloxane groups.

[0107] The picture (a) shows free hydroxyl groups which are isolated silanol groups (Si-OH) which allow hydrogen bonding networks to form between aggregates. A hydrogen bonded structure of vicinal hydroxyl groups also called bridged hydroxyl groups is shown in picture (b), and (c) shows geminal hydroxyl groups. In (d) the surface structure terminates in a siloxane group (=Si-O-Si=) with the oxygen on the surface. With regard to the functionality of the hydroxyl groups it is also referred to the above mentioned description in the background section.

[0108] FIG. 2 shows a schematic illustration of an exemplary embodiment of a plasma reactor 105 with two electrodes and a separate powder feeder 120 comprising silica with a chloride additive 150.

[0109] The transferred arc reactor or plasma reactor 105 is made of stainless steel and is isolated with an insulation 118 made of refractory material to improve the heat efficiency. Fireproof insulation 118 is provided in the lower housing 113 in the bottom 116 and side walls as well as in the upper housing lid 114. For operation of the plasma reactor 105 the housing 115 is closed and at least one plasma process gas 141 such as inert gas as well as a reaction gas as a reducing agent 143 are entered by first gas inlet 131 into the reaction chamber 110. An electric arc 117 is established between the first electrode 101 and the second electrode 102 by an electric field generator 170. On top of the first electrode 101 , which is preferably a metallic anode, a graphitic crucible 111 is arranged, which is filled with feedstock 121 comprising silica, wherein preferably crystalline quartz is used as precursor material.

[0110] The second electrode 102 is arranged in the top of the plasma reactor 105 and is preferably movable in the vertical direction (see vertical double arrow). Before initiating the electric arc 117 the second electrode 102 can be positioned close to the upper surface of the feedstock 121 received by the crucible 111. The202400278 Foreign Filing 12

[0111] use of the movable upper electrode 102, which is lifted after the silica in the crucible is in a molten state and an electric arc 117 is established, addresses the problem of the high resistivity of silica at low temperatures and may ease or accelerate the start of the arc transfer.

[0112] The heat of the electric arc 117 is melting the surrounding silica which initially becomes conductive. Optionally additives to increase the conductivity (such as NaOH) and the product properties can be added to the feedstock 121 , which is preferably quartz. During operation the internal temperature of the reaction chamber 110 can be maintained above the melting point of the silica, which is about 1710°C. Typical temperatures at the crucible can be at least 1750°C or upto 1800°C or more. The temperatures in and close to the electric arc 117 are significantly higher and suited to vaporize silica. If the current applied between the at least two electrodes 101 , 102 is increased the heat input and thus the decomposition rate of the fed silica powder 122, in the form of SiO2, preferably quartz, to oxidic silica compounds such as for example silica monoxide, SiO (g), can be increased.

[0113] Once a stable electric arc 117 is established, additional silica 122, preferably quartz in the form of powder, is fed by means of a powder feeder 120. In the plasma reactor 105 the powder feeder is located separately of the upper electrode 102 and guides the silica powder 122 between the second electrode 102 and the crucible 111.

[0114] The generated exhaust gas, which partially contains SiO is continuously flushed out of the plasma reactor 105 via the gas outlet 134. This exhaust gas can be optionally quenched (with quenching gas 144 such as air, N2 and / or water) via a second gas inlet 132 outside of the reactor chamber 110 and the produced electric arc silica 124 is subsequently collected at a filter unit (not shown) such as a ceramic filter.

[0115] FIG. 3 shows another embodiment of a plasma reactor 100 which was used for all electric arc experiments which are described in the following as comparative examples 1 to 3 (401-403) and inventive example 5 (see 501 and results shown in FIGs 5-7 and 9-10, respectively). The general design is similar to the plasma reactor 105 above, except that a hollow graphitic cathode is used as second electrode 102 and configured to be connected to a powder feeder 120. In this way the powder silica 122, preferably quartz with the chloride containing additive 150 can accurately be injected into the electric arc 117, which is formed at the lower tip of the second electrode 102.

[0116] In comparison to the plasma reactor 105 of FIG. 2 further a sacrificial electrode 112 is provided in the embodiment of FIG. 3. The sacrificial graphitic electrode (112) is arranged in the middle of the graphite crucible 111 and has contact to the crucible bottom, which is located above the first electrode 101 (e.g. a metal anode). The crucible 111 is preferably filled with quartz as feedstock 121 outside of the plasma reactor and then inserted into the plasma reactor 100 on the first electrode 101. Alternatively, the first filling of the crucible 111 can be done by the powder feeder 120 or other suited means. The sacrificial electrode 112 can produce an electric field in close proximity to the field produced by the tip of the upper electrode 102. The use of such sacrificial electrode 112, optionally in combination with a movable upper electrode 102, addresses the problem of the high resistivity of silica at low temperatures and may ease the start of the arc transfer.202400278 Foreign Filing 13

[0117] In order to produce electric arc silica the reaction chamber 110 is flushed with inert gases as Argon (Ar, 141) and nitrogen (N2, 142) and the reaction gas 143, which is in the shown example the reducing agent H2. To start the electric arc, plasma generating power is applied to the electrodes by the electric field generator 170. Then the upper electrode 102 (in FIG. 3 the second electrode 102 is the cathode) contacts the sacrificial electrode 112 and the cathode 102 is gradually lifted upwards so that the electric arc 117 is generated.

[0118] The plasma reactor 100 is equipped with two gas inlets 132, 132a, wherein each can be used as quenching port for quenching gas 144 such as water steam. It is noted that each gas inlet 132 and 132a, respectively have a heating 135, to heat the steam above 110°C, in order to avoid condensation of the water steam 144 before entering the reactor chamber 110 or exhaust gas stream in the gas outlet 134. Each of the second gas inlets 132, 132a can be used but not simultaneously.

[0119] In case the second inlet 132 is used, then most of the powder or aerosol formation occurs in the region close to the reactor exit where the gas inlet 132 introduces the quenching gas 144 into the gas outlet 134. In this way the injection of the quenching gas 144 does not disturb the streams in the reaction chamber 110. However, the distance of the quenching zone having a lower temperature might be long enough that condensation is partially initiated prior to reaching the quenching or oxidizing zone, which should be avoided. Thus, the gas streams and power applied from the field generator 170 should be adapted to ensure that too early condensation processes can be avoided.

[0120] In the other configuration, wherein the quenching gas is introduced into the reaction chamber 110 via gas inlet 132 a, it can be ensured that nearly no undesired condensation occurs because the oxidizing zone is closer to the plasma zone. Then the compounds are quenched in the gaseous phase. For the following comparative examples 1 to 3 and inventive example 5 the plasma reactor 100 according to FIG. 3 and the gas inlet 132a was used.

[0121] FIG. 3 shows further an option how the produced electric arc silica 124 can be collected. Downstream of the gas outlet 134 a filter unit 130 is provided which separates particles of the electric arc silica 124 from the offgas 146 using a filter 147 as a ceramic filter.

[0122] FIG. 4 refers to a flow diagram of the method 300 for producing electric arc silica 124 according to the invention and comprising the following method steps:

[0123] providing 301 a plasma reactor 100, which plasma reactor is configured to generate at least one electric arc 117 by establishing an electric field between the at least two electrodes;

[0124] receiving 302 a feedstock comprising silica, preferably quartz, in the plasma reactor 100;

[0125] introducing 303 gas between the at least two electrodes through a first gas inlet 131 ;

[0126] generating 304 an electric arc 117;

[0127] optionally introducing 313 a reducing agent 143;202400278 Foreign Filing 14

[0128] feeding 305 between the electrodes through a powder feeder 120 a silica containing powder comprising a chloride as additive 150; wherein the fed silica containing powder is preferably quartz; and

[0129] quenching 306 oxidic silica compounds to produce electric arc silica 124.

[0130] The present invention is also characterized by the following items:

[0131] 1. Method for producing electric arc silica (124) comprising the following method steps:

[0132] providing (301) a plasma reactor (100) configured to generate at least one electric arc by establishing an electric field between the at least two electrodes;

[0133] receiving (302) a feedstock comprising silica, preferably quartz, in the plasma reactor; introducing (303) gas between the at least two electrodes through a first gas inlet;

[0134] generating (304) an electric arc further;

[0135] feeding (305) between the electrodes through a powder feeder a silica containing powder, preferably quartz, comprising a chloride as additive (150); and

[0136] quenching (306) oxidic silica compounds to produce electric arc silica (124).

[0137] 2. Method according to item 1 ,

[0138] wherein the surface of the produced electric arc silica comprises isolated silanol (SiOH) groups.

[0139] 3. Method according to item 1 or 2, wherein the additive is selected from the group of chloride salts consisting of:

[0140] sodium chloride (NaCI), ammonium chloride (NH4CI), potassium chloride (KCI), magnesium chloride (MgCh), calcium chloride (CaCh), other salts of the hydrochloric acid (HCI), and any combination thereof.

[0141] 4. Method according to any of the preceding items, wherein the additive is added in an amout of smaller than about 10 w%, preferably about 4 w%; and / or

[0142] wherein the powder feeder(120) feeds the silica containing powder, preferably quartz, with an average particle size of micrometric size or smaller.

[0143] 5. Method according to any of the preceding items,

[0144] wherein the produced electric arc silica is hydrophilic and / or has a silanol density between about 0,5 to 8 SiOH / nm2, preferably 1-2,5 SiOH / nm2and / or

[0145] a BRUNAUER-EMETT and TELLER (BET) surface of the electric arc silica is at least 15 m2 / g, preferably at least 30 m2 / g.

[0146] 6. Method according to any of the preceding items, wherein the diameter of primary particles of the produced electric arc silica are nano-scale, preferably ranges between 10 to 50 nm, and wherein said primary particles form aggregates, preferably in a chain-like structure.202400278 Foreign Filing 15

[0147] 7. Method according to any of the preceding items, wherein the gas comprises at least one inert gas and / or a reducing agent, wherein the reducing agent (143) is preferably hydrogen (H2) and / or introduced through the first gas inlet (131) after generating (304) an electric arc (117).

[0148] 8. Method according to any of the preceding items, wherein the method step quenching introduces a quenching gas through a second gas inlet into the plasma reactor and / or into a gas outlet of the plasma reactor; and / or

[0149] wherein the quenching gas is selected from the group consisting of:

[0150] air, water steam (H2O(g).), preferably at temperatures greater than 110°C, nitrogen (N2) and any combinations thereof.

[0151] 9. The method according to any of the preceding items, wherein an conductivity enhancing agent, preferably sodium hydroxide (NaOH) or Aluminium oxide (AI2O3), is added to the feedstock.

[0152] 10. The method according to any of the preceding items,

[0153] wherein the feedstock is received by a crucible, preferably made of graphite, wherein the feedstock (121), preferably quartz, is molten at a temperature equal or more than the melting point of the feedstock; and / or

[0154] wherein the crucible (111) is arranged above the first electrode (101) of the at least two electrodes, preferably comprising a sacrificial electrode (112), preferably made of graphite, in contact with the first electrode (101) and / or with the bottom of the crucible (111).

[0155] 11. Method according to item 10, wherein the first electrode (101) is an anode and the second electrode (102) is a moveable cathode and configured to contact the sacrificial electrode (112) of the crucible (111); and

[0156] wherein, after establishing the electric field between the electrodes (101, 102), the cathode is raised such that the electric arc (117) is built between the lower tip of the cathode and the upper surface of the molten feedstock (121).

[0157] 12. The method according to any of the preceding items, wherein the first electrode (101) of the two electrodes is configured to be hollow and part of the powder feeder (120), wherein optionally the hollow elcetrode has an inner diameter of minimum 50 mm.

[0158] 13. The method according to any of the preceding items, wherein the electric arc silica is collected in a filter unit (130) in the gas outlet (134) of the plasma reactor (100, 105).

[0159] 14. A system configured to perform the method steps according to any of the preceding items comprising at least:

[0160] a plasma reactor (100, 105) with at least two electrodes (101, 102) configured to generate at least one electric arc (117);

[0161] a first gas inlet (131) for introducing gas between the at least two electrodes (101 , 102);202400278 Foreign Filing 16

[0162] a powder feeder (120) to feed a silica containing powder (122) comprising a chloride as additive (150); wherein the silica containing powder is preferably quartz; and

[0163] a second gas inlet (132) to introduce a quenching gas to produce electric arc silica.

[0164] 15. An electric arc silica produced according to the method steps according to any of the items 1 to 13.

[0165] EXAMPLES

[0166] Next, a detailed description will be given below of the results of comparative examples 401 - 404 as well as of an inventive example 501 and 502. The Comparative examples 1 to 3 as well as the inventive examples 5 and 6 are conducted with the plasma reactor as shown in FIG. 3 and using the second gas inlet 132 a.

[0167] For the quantitative determination of hydroxyl (OH) group density the lithiumalanate (UAIH4) method was used as generally described above. This method is generally known in the art for the quantitative determination of the hydroxyl (OH) group density. For the following results according to this method a sample of 2g of the fumed silica are placed in a flask and are then evacuated at 120°C for 1h and a pressure < 2 x 101- 1 hPa. In this way free or bound water can be removed. After the sample has been cooled down to 20°C, 40 mL of a 2% LiAIH4 are added to the sample. The pressure increase after 10 min at 20°C can be correlated with the hydrogen (H2), which is formed upon the reaction of OH with LAIH4. From this quantitative amount of H2 it is possible to derive the total amount of OH groups. By using the BET surface area, the OH group density can be calculated.

[0168] For the qualitative measurements of OH groups FTIR measurements are used, wherein a powder sample is placed between two NaCI windows. This method is generally known in the art for the qualitative measurements of OH groups. The measurement is performed by a FTIR spectrometer Vertex 70 (Fa. Bruker) in transmission mode. One important SI-OH band is at 3750 cm-1indicating the presence of isolated or “free” Si-OH. The results of these IR measurements are converted to extinction values and are shown in FIG. 5. Analogue measurements and data processing apply to the data shown in all of the following comparative examples (FIG. 6- 8) and inventive examples (FIG. 9 and 11), respectively.

[0169] Comparative Example 1:

[0170] Firstly, according to comparative example 1 (401) as feedstock 60g of fine sized quartz (<0.8 mm) containing 4wt% NaOH as additive to improve the conductivity of the feedstock 121 is placed in the crucible 111 in the plasma reactor 100 shown in FIG. 3. The plasma reactor 100 is flushed with Argon at a mean flow of 3L / min using the first gas inlet 131. Furthermore, the top or upper electrode 102 is contacted with the sacrificial electrode 112 in the crucible 111.

[0171] In order to start an electric arc 117 and to create a thermal plasma associated therewith 6kW with an average current of 100A are applied to the two electrodes 101, 102. After the electric current was stable, the second electrode 102 i.e. the cathode is lifted such that the electric arc 117 generates.202400278 Foreign Filing 17

[0172] Once the electric arc 117 between the tip of the second electrode 102 is stable a hydrogen flow of 1 L / min is added to the process gas to form a volume ratio of Argon to H2 of 3 / 1. After 2min (for generating the liquid silica melt mainly by means of the sacrificial electrode) silica 122 is fed in the form of fine sized quartz powder by means of a powder feeder 120 and through the hollow cathode with a feed rate of 400 g / h. The resulting produced electric arc silica 124 powder is collected at a filter unit 146.

[0173] Specific surface area measurements were made on samples of the produced electric arc silica using BRUNAUER-EMETT and TELLER (BET) in accordance with DIN 66 131; ISO 57494-1 , Annex D). The BET surface area of the powder produce in accordance with comparative example 1 was 62 m2 / g. FIG. 5 plots the wavenumber per cm [cm'1] (x-axis) versus the extinction (y-axis). A Fourier transform infrared (FTIR) analysis is used to identify if isolated SiOH or hydroxyl groups are present. As indicated by the IR measurements and the absence of a signal at a wavenumber of 3750cm-1(see arrow 400) no isolated silanol (SiOH) groups could be observed in the sample of comparative example 1. The SiOH group density derived by the LiAIH4 method was 0,2 SiOH / nm2. Thus, the rheological properties are expected to be below average and that the viscosity is lower than the viscosity of a commercially available AEROSI L® having a significantly higher density of SiOH groups on the surface.

[0174] Comparative Example 2 + quenching:

[0175] As in comparative example 1 (401), according to comparative example 2 (402) 60g of fine sized quartz (<0.8 mm) containing 4wt% NaOH as additive to improve the conductivity is placed in the crucible 111 to form the initial feedstock 121 in the plasma reactor 100 shown in FIG. 3. Then the plasma reactor 100 is flushed with Argon at a mean flow of 3L / min using the first gas inlet 131. The top electrode 102 is lowered to contact the sacrificial electrode 112 in the crucible 111.

[0176] The same electric field conditions have been chosen as in comparative example 1 , that is to say 6kW with an average current of 100A is applied to the two electrodes 101, 102. After the electric current is stable, the second electrode 102 i.e. the cathode is lifted in order to establish the electric arc 117.

[0177] Once the electric arc 117 is stable a hydrogen flow of 1 L / min is added to the process gas and thus to the system. After 2 min (for generating the liquid silica melt mainly by means of the sacrificial electrode) silica 122 without any additives is fed in the form of fine sized quartz powder by means of a powder feeder 120 and through the hollow cathode with a feed rate of 400 g / h.

[0178] In addition to the method used in comparative example 1 quenching is performed using the gas inlet 132a shown in FIG. 3. After a stable operation of the electric arc 117, a quenching gas mixture of water steam and nitrogen is introduced into the reaction chamber 110, wherein water vapor (H2O) is injected with a volume flow of 50 L / h mixed with a volume flow N2 of 60 L / h.

[0179] The resulting produced electric arc silica 124 powder is collected using a filter 147. Specific surface area measurements were made on samples of the collected electric arc silica using BRUNAUER-EMETT and TELLER (BET) in accordance with DIN 66131; ISO 57494-1, Annex D). The BET surface area of the powder produced in accordance with comparative example 2 was 28 m2 / g.202400278 Foreign Filing 18

[0180] FIG. 6 shows a measured IR-spectrum of silica produced according to comparative example 2. The FTIR spectra analysis results of FIG. 6 show at arrow 400 that at the wavenumber of about 3750 cm-1there is no significant signal of extinction which indicates the absence of free silanol groups. The OH group density derived by the LiAIH4 method was 0,3 SiOH / nm2. This example 402 shows that the addition of a hydrogen containing quenching gas (here H2O as hydrogen source) does not result in the formation of isolated hydroxyl groups. As noted above the absence of isolated silanol groups on the surface of the produced electric arc silica results in significant lower viscosity, when this material is used as thickening agent.

[0181] Accordingly, there is the need to find a process where the lack of thickening and thixotropic ability due to the absence of free hydrolyzed groups or SiOH groups is to overcome. Therefore, a post treatment in the following comparative example 3 (403) has been tested in order find if a formation of SiOH groups on the surface of a post-treated powder can be observed.

[0182] Comparative Example 3- HCI impregnation of silica of example 1:

[0183] The comparative example 3 (403) refers to a post treatment of the electric arc silica produced according to the method of comparative example 1 (401). As post treatment an impregnation with HCI is used, which is the chlorine form present in the quenching process of the industrial flame hydrolysis to generate AEROSIL® from silicon tetrachloride.

[0184] 10g of electric arc silica produced according to comparative example 1 is placed in a tubular oven. A nitrogen gas flow of 100L / h N2 is passed through a bubbling system containing 1M HCI at room temperature. The resulting gas mixture is passed through the tubular oven at 500°C for 3h with subsequent cooling of the sample.

[0185] The resulting impregnated electric arc silica powder has been analyzed. The surface area of this material is 58 m2 / g.

[0186] The FTIR spectra analysis shows for this example 403 a peak at about 3750 cm-1(see arrow 460). This is due to the presence of “free” or isolated hydroxyl groups. The OH group density derived by the LiAIH4 method was 0,9 SiOH / nm2which is significantly higher than the results of the comparative example 1 showing a density of 0,2 SiOH / nm2.

[0187] One result of this comparative example using an additional post treatment with HCI is that the desired surface chemistry in particular the presence of SiOH groups could be achieved. However, this treatment is time consuming and there is the need to provide a process using a transferred electric arc without any post-treatment.

[0188] Comparative example 4

[0189] The use of HCI for the post processing step in Comparative example 3 was derived from the form of chlorine present in the industrial flame hydrolysis process starting from Chlorosilane SiCk. The established manufacturing process for known silica products as AEROSIL® uses flame hydrolysis and oxidizes SiCk under controlled conditions with an H2 / O2 burner to obtain high surface amorphous SiO2202400278 Foreign Filing 19

[0190] and HCI. Accordingly, the following comparative example produces fumed silica as in said known AEROSIL® manufacturing method.

[0191] This known method is described for example in EP0015315 A1 and uses a burner as described in US 2 990249. These burners are designed to provide a homogeneous mixture of the compound to be decomposed (SiCk) and the gas such as air or oxygen forming the water for hydrolysis and then supply this mixture in the gaseous phase to the flame in a uniform and laminar way. For this purpose, these burners have a mixing chamber. In this way the components taking part in the reaction such as O2 or air and SiCk can be added as mixture to the burner for the flame oxidation-hydolysis.

[0192] According to the comparative example 4 (404) SiCk is vaporized and the vapors are transferred into a mixing chamber of a common burner for flame oxidation-hydrolysis. Separately from the raw material, the combustion gas H2 and primary air enriched with oxygen are introduced into the mixing chamber. The combustion gas has an excess of 1.05 (hydrogen) compared to the theoretical needed combustion gas which is required for the complete hydrolysis of SiCk following the equations:

[0193] 2H2+ O2^ 2H2O

[0194] SiCk+ 2 H2O -> SiO2+ 4 HCI

[0195] The oxygen excess which is required for the above reaction for full hydrolysis of SiCk is 1.04.

[0196] The reaction mixture is fed in a central tube to a burner and ignited. The exit speed of the reaction mixture from the burner is 33,7 m / s. The flame burns in a water-cooled reaction chamber. The powder formed in the flame hydrolysis process is deposited in a downstream cyclone and filter. The separated powder is subsequently treated with air and steam in counter-current at approx. 700°C.

[0197] The obtained powder has a BET surface area of 31 m2 / g. Aggregates are formed that consists of primary particles that form chain-like structure.

[0198] FIG. 8 shows the results of the IR measurements of comparative example 404 and a significant signal 470 at 3750cm-1. As previously discussed, this peak 470 in FIG. 8 shows that isolated SiOH groups are present.

[0199] The OH group density derived by the LiAIH4 method was 1 ,8 SiOH / nm2.

[0200] The byproduct HCI of this AEROSILO-process can be recycled as schematically shown with the reaction equation 480 in FIG. 8. However, there is the need to produce fumed silica which does not involve silicon halides in order to avoid the byproduct HCI.

[0201] Inventive example 5

[0202] The following example is intended to illustrate the invention in more detail, but not to limit the scope of protection as set forth in the claims. In contrast to all preceding examples the following inventive examples 5 and 6 use an additive in the silica of the powder feeder, wherein the additive comprises chloride and is selected from chloride salts. The following method steps lead to the high surface electric arc silica with desired properties.202400278 Foreign Filing 20

[0203] As in comparative example 1 (401) and 2 (402) the plasma reactor in accordance with FIG. 3 is provided for the inventive example 5 (405). First, 60 g of fine sized quartz (<0.8 mm) containing 4wt% NaOH as additive to improve the conductivity is placed in the crucible 111 to form the initial feedstock 121 in the plasma reactor 100 shown in FIG. 3.

[0204] To initiate the thermal plasma process first the plasma process gas is injected into the reaction chamber 110. In particular, the reactor is flushed with an Argon (Ar) volume flow rate of 3 L / min using the first gas inlet 131. The top electrode 102 is lowered to contact the sacrificial electrode 112 in the crucible 111.

[0205] The same electric field conditions are chosen as in comparative examples 1 (401) and 2 (402), that is to say a power of about 6 kW with an average current of 100A is applied to the two electrodes 101, 102. After the electric current was stable, the second electrode 102 i.e. the cathode is lifted in order to establish the electric arc 117.

[0206] Once the transferred arc is stable 1 L / min of the reaction gas comprising the reducing agent H2 is added to the system and after 2 min (for generating the liquid silica melt) fine sized quartz powder is added through the second electrode 102, which is designed as a hollow cathode, with a feed rate of 400 g / h. In contrast to the examples 401 and 402, which contained no additives in the silica of the powder feeder, the fine sized quartz according to the inventive example 501 contained 4wt% sodium chloride (NaCI).

[0207] After a stable operation of the electric arc 117, a quenching gas mixture of water steam and nitrogen is introduced into the reaction chamber 110, wherein water vapor (H2O) is injected with a volume flow of 50 L / h mixed with a volume flow of nitrogen (N2) of 60 L / h.

[0208] The resulting produced electric arc silica 124 in form of a fine powder is collected at a filter unit 130. Specific surface area measurements were made on samples of the collected electric arc silica using BRUNAUER-EMETT and TELLER (BET) in accordance with DIN 66 131; ISO 57494-1 , Annex D). The BET surface area of the powder produced in accordance with inventive example 501 was 31 m2 / g.

[0209] FIG. 9 shows a measured IR-spectrum of silica produced according to the inventive example 501. The FTIR spectra analysis shows at arrow 510 that at the wavenumber of about 3750 cm-1a significant signal of extinction was detected, which indicates the presence of free silanol groups. This example 501 shows that the addition of a chloride containing quartz in combination with a quenching gas results in the formation of isolated hydroxyl groups. The presence of isolated silanol groups on the surface of the produced electric arc silica results in a higher viscosity than the silicas obtained in examples 401 and 402, respectively. Accordingly, the obtained BET surface area of the inventive example 501 together with the presence of SiOH groups on the surface results makes the product suited to be used as thickening and thixotropic agent.

[0210] FIG. 10 shows a transmission electron microscopy (TEM) picture of silica produced according to the inventive example 5 (501). The plasma produced electric arc silica in accordance with the present invention has a chain-like structure which is partially branched. There are exemplary measurements shown of primary particles having the size of 12 nm (black double arrow 212), 15 nm (double arrow 215), 18 nm (double arrow 218) and 38 nm (white double arrow 238), respectively. Thus, the produced electric202400278 Foreign Filing 21

[0211] arc silica are nano-scale, preferably ranges between 10 to 50 nm, preferably up to 40 nm, wherein said primary particles form advantageously aggregates in a chain-like structure. These aggregates are suited to form agglomerates enhancing the thixotropic properties. Furthermore, these aggregates may form larger agglomerates by means of e.g. hydrogen bonds orVan-der-Waals-forces. Hydrogen bonds may be formed by silanol groups, which are present according to the FTIR measurements.

[0212] The silanol density as determined with the lithiumalanate method as described above is 1 ,2 SiOH / nm2. This silanol density is higher than the comparative example having a density value of about 0,9 SiOH / nm2. Accordingly, the inventive method is an advantageous alternative to the time- and costconsuming post processing with HCI. The viscosity is expected to be comparable to AEROSIL® fumed silica.

[0213] By the inventive process 501 cheaper raw materials, namely quartz, as needed e.g. for the flame hydrolysis process starting from SiCk (see comparative example 4, 404 in FIG. 8) can be used and at the same time post-treatment with HCI (see comparative example 3, 403 in FIG. 7) can be avoided.

[0214] Inventive example 6

[0215] The following inventive example 6 uses another chloride as additive in the silica of the powder feeder, namely ammonium chloride. The following method steps using the plasma reactor on a laboratory scale led to the high surface electric arc silica with desired properties.

[0216] As in inventive example 5 (501) the plasma reactor in accordance with FIG. 3 is provided for the inventive example 6 (502). First, 60g of fine sized quartz (<0.8 mm) containing 4wt% NaOH as additive to improve the conductivity is placed in the crucible 111 to form the initial feedstock 121 in the plasma reactor 100 shown in FIG. 3.

[0217] To initiate the thermal plasma process first the plasma process gas is injected into the reaction chamber 110. In particular, the reactor is flushed with an Argon (Ar) volume flow rate of 3 L / min using the first gas inlet 131. The top electrode 102 is contacted with the sacrificial electrode 112 in the crucible 111.

[0218] The same electric field conditions are chosen as in comparative examples 1 (401) and 2 (402) and inventive example 5 (501), that is to say a power of about 6 kWwith an average current of 100A is applied to the two electrodes 101, 102. After the electric current was stable, the second electrode 102 i.e. the cathode is lifted in order to establish the electric arc 117.

[0219] Once the transferred arc is stable 1 L / min of the reaction gas hydrogen (H2) is added to the system and after 2 min (for generating the liquid silica melt) fine sized quartz powder is added through the hollow cathode 102 with a feed rate of 400 g / h. In contrast to the inventive example 501, which uses NaCI as additive, the silica of the powder feeder according to the inventive example 502 comprises 4wt% ammonium chloride (NH4CI).

[0220] After a stable operation of the electric arc 117, a quenching gas mixture of water steam and nitrogen is introduced into the reaction chamber 110, wherein water vapor (H2O) is injected with a volume flow of 50 L / h mixed with a volume flow of nitrogen (N2) of 60 L / h.202400278 Foreign Filing 22

[0221] The resulting produced electric arc silica 124 in form of a fine powder is collected at a filter unit 130. Specific surface area measurements were made on samples of the collected electric arc silica using BRUNAUER-EMETT and TELLER (BET) in accordance with DIN 66 131 ; ISO 57494-1 , Annex D). The BET surface area of the powder produced in accordance with inventive example 502 was 33 m2 / g.

[0222] FIG. 11 shows a measured IR-spectrum of silica produced according to the inventive example 502. The FTIR spectra analysis shows at arrow 510 that at the wavenumber of about 3750 cm1a significant signal of extinction was detected, which indicates the presence of free silanol groups. This example 502 shows another example for a chloride containing quartz, which in combination with a quenching gas results in the formation of isolated hydroxyl groups. The presence of isolated silanol groups on the surface of the produced electric arc silica results in a higher viscosity than the silicas obtained in examples 401 and 402, respectively. Accordingly, the obtained BET surface area of the inventive example 502 together with the presence of SiOH groups on the surface results makes the product suited to be used as thickening and thixotropic agent.

[0223] Inventive example 7

[0224] The following inventive example 7 also uses ammonium chloride as additive in the silica of the powder feeder. The following method steps using the plasma reactor on a laboratory scale led to the high surface electric arc silica with desired properties.

[0225] As in inventive example 5 (501) the plasma reactor in accordance with FIG. 3 is provided for the inventive example 7 (503). First, 60g of fine sized quartz (<0.8 mm) containing 4wt% NaOH as additive to improve the conductivity is placed in the crucible 111 to form the initial feedstock 121 in the plasma reactor 100 shown in FIG. 3.

[0226] To initiate the thermal plasma process first the plasma process gas is injected into the reaction chamber 110. In particular, the reactor is flushed with an Argon (Ar) volume flow rate of 3 L / min using the first gas inlet 131. The top electrode 102 is contacted with the sacrificial electrode 112 in the crucible 111.

[0227] The same electric field conditions are chosen as in comparative examples 1 (401) and 2 (402) and inventive example 5 (501), that is to say a power of about 6 kWwith an average current of 100A is applied to the two electrodes 101, 102. After the electric current was stable, the second electrode 102 i.e. the cathode is lifted in order to establish the electric arc 117.

[0228] Once the transferred arc is stable 1 L / min of the reaction gas hydrogen (H2) is added to the system and after 2 min (for generating the liquid silica melt) fine sized quartz powder is added through the hollow cathode 102 with a feed rate of 400 g / h.

[0229] After a stable operation of the electric arc 117, a quenching gas mixture of water steam and nitrogen is introduced into the reaction chamber 110, wherein water vapor (H2O) is injected with a volume flow of 100 L / h mixed with a volume flow of nitrogen (N2) of 100 L / h. In contrast to the inventive example 502, the quench gas volume flow has been increased.202400278 Foreign Filing 23

[0230] The resulting produced electric arc silica 124 in form of a fine powder is collected at a filter unit 130. Specific surface area measurements were made on samples of the collected electric arc silica using BRUNAUER-EMETT and TELLER (BET) in accordance with DIN 66 131 ; ISO 57494-1 , Annex D). The BET surface area of the powder produced in accordance with inventive example 503 was 77 m2 / g.

[0231] FIG. 12 shows a measured IR-spectrum of silica produced according to the inventive example 503. The FTIR spectra analysis shows at arrow 510 that at the wavenumber of about 3750 cm-1 a significant signal of extinction was detected, which indicates the presence of free silanol groups. This example 503 shows another example for a chloride containing quartz, which in combination with a quenching gas results in the formation of isolated hydroxyl groups. The presence of isolated silanol groups on the surface of the produced electric arc silica results in a higher viscosity than the silicas obtained in examples 401 and 402, respectively. Accordingly, the obtained BET surface area of the inventive example 503 together with the presence of SiOH groups on the surface results makes the product suited to be used as thickening and thixotropic agent.

[0232] The plasma reactor used for the inventive examples are of laboratory scale. However, it is believed that the results are also representative for the production of new electric arc silica with high surface and desired silanol densities in plasma reactors of larger scales and that this electric arc silica can also be industrially produced. The increasingly stringent requirements for high purity fumed silica and the need for hazardous or waste free processes makes the novel and inventive method based in particular on the addition of a chloride containing additive into the silica comprising powder, which is fed into an electric arc of a plasma reactor, an attractive alternative to conventional industrial technologies as the flame hydrolysis. One advantage is also that the plasma reactor can be rapidly switched off. Using the methods according to inventive examples 5 and 6 and 7, respectively are examples of efficient and reliable production processes of electric arc silica with the desired rheological properties.

[0233] Reference list

[0234] 100 plasma reactor with hollow top electrode for silica feed

[0235] 101 first electrode

[0236] 102 second or upper electrode

[0237] 105 other embodiment of plasma reactor

[0238] 110 reactor chamber

[0239] 111 crucible

[0240] 112 sacrificial electrode

[0241] 113 lower housing with bottom and sidewalls

[0242] 114 upper housing lid

[0243] 115 housing

[0244] 116 bottom of lower housing

[0245] 117 electric arc

[0246] 118 insulation of reactor

[0247] 120 powder feeder202400278 Foreign Filing 24

[0248] 121 feedstock in crucible in molten state

[0249] 122 silica powder

[0250] 124 product: electric arc silica

[0251] 125 dosing system

[0252] 130 filter unit

[0253] 131 first gas inlet

[0254] 132 second gas inlet

[0255] 132a alternative second gas inlet

[0256] 134 gas outlet

[0257] 135 heating of gas inlet

[0258] 141 first inert gas

[0259] 142 second inert gas

[0260] 143 reducing agent

[0261] 144 quenching gas

[0262] 145 dosing valve(s)

[0263] 146 off gas

[0264] 147 filter

[0265] 150 chloride containing additive

[0266] 170 electric field generator or plasma generating power

[0267] 212 diameter of primary particle (12 nm)

[0268] 215 diameter of primary particle (15 nm)

[0269] 218 diameter of primary particle (18 nm)

[0270] 238 diameter of another primary particle (38 nm)

[0271] 300 method for producing electric arc silica

[0272] 301 method step providing a plasma reactor

[0273] 302 method step receiving a feedstock

[0274] 303 method step introducing gas

[0275] 304 method step generating an electric arc

[0276] 305 method step feeding silica comprising a chloride containing additive 306 method step quenching to produce electric arc silica

[0277] 313 further introducing a reducing agent

[0278] 400 no signal at 3750 cm-1indicating absence of free silanol groups

[0279] 401 comparative example 1

[0280] 402 comparative example 2

[0281] 403 comparative example 3

[0282] 404 comparative example 4

[0283] 470 signal for free silanol groups at the surface at 3750 cm-1

[0284] 460 signal at 3750 cm-1indicating presence of free silanol groups at example 3 470 signal at 3750 cm-1indicating presence of free silanol groups at example 4202400278 Foreign Filing 25

[0285] 501 examplatory embodiment of the present invention - example 5

[0286] 502 examplatory embodiment of the present invention - example 6

[0287] 503 examplatory embodiment of the present invention - example 7

[0288] 510 signal at 3750 cm1indicating presence of free silanol groups on the surface of the electric arc silica produced according to inventive examples 5 and 6

Claims

202400278 Foreign Filing 26CLAIMS1. Method for producing electric arc silica (124) comprising the following method steps:providing (301) a plasma reactor (100) configured to generate at least one electric arc by establishing an electric field between the at least two electrodes;receiving (302) by a crucible a feedstock comprising silica, preferably quartz, in the plasma reactor;introducing (303) gas between the at least two electrodes through a first gas inlet; generating (304) an electric arc;feeding (305) between the electrodes through a powder feeder a silica containing powder, preferably quartz, comprising a chloride as additive (150); andquenching (306) oxidic silica compounds to produce electric arc silica (124).

2. Method according to claim 1 ,wherein the surface of the produced electric arc silica comprises isolated silanol (SiOH) groups.

3. Method according to claim 1 or 2, wherein the additive is selected from the group of chloride salts consisting of:sodium chloride (NaCI), ammonium chloride (NH4CI), potassium chloride (KCI), magnesium chloride (MgCL), calcium chloride (CaCL), other salts of the hydrochloric acid (HCI), and any combination thereof.

4. Method according to any of the preceding claims, wherein the additive is added in an amout of smaller than about 10 w%, preferably about 4 w%; and / orwherein the powder feeder(120) feeds the silica containing powder, preferably quartz, with an average particle size of micrometric size or smaller.

5. Method according to any of the preceding claims,wherein the produced electric arc silica is hydrophilic and / or has a silanol density between about 0,5 to 8 SiOH / nm2, preferably 1-2,5 SiOH / nm2and / ora BRUNAUER-EMETT and TELLER (BET) surface of the electric arc silica is at least 15 m2 / g, preferably at least 30 m2 / g.

6. Method according to any of the preceding claims, wherein the diameter of primary particles of the produced electric arc silica are nano-scale, preferably ranges between 10 to 50 nm, and wherein said primary particles form aggregates, preferably in a chain-like structure.

7. Method according to any of the preceding claims, wherein the gas comprises at least one inert gas and / or a reducing agent, wherein the reducing agent (143) is preferably hydrogen (H2) and / or introduced through the first gas inlet (131) after generating (304) an electric arc (117).

8. Method according to any of the preceding claims, wherein the method step quenching introduces a quenching gas through a second gas inlet into the plasma reactor and / or into a gas outlet of the plasma reactor; and / or202400278 Foreign Filing 27wherein the quenching gas is selected from the group consisting of:air, water steam (H2O(g).), preferably at temperatures greater than 110°C, nitrogen (N2) and any combinations thereof.

9. The method according to any of the preceding claims, wherein an conductivity enhancing agent, preferably sodium hydroxide (NaOH) or Aluminum oxide (AI2O3), is added to the feedstock.

10. The method according to any of the preceding claims,wherein the feedstock is received by a crucible, preferably made of graphite, wherein the feedstock (121), preferably quartz, is molten at a temperature equal or more than the melting point of the feedstock; and / orwherein the crucible (111) is arranged above the first electrode (101) of the at least two electrodes, preferably comprising a sacrificial electrode (112), preferably made of graphite, in contact with the first electrode (101) and / or with the bottom of the crucible (111).

11. Method according to claim 10, wherein the first electrode (101) is an anode and the second electrode (102) is a moveable cathode and configured to contact the sacrificial electrode (112) of the crucible (111); andwherein, after establishing the electric field between the electrodes (101 , 102), the cathode is raised such that the electric arc (117) is built between the lower tip of the cathode and the upper surface of the molten feedstock (121).

12. The method according to any of the preceding claims, wherein the first electrode (101) of the two electrodes is configured to be hollow and part of the powder feeder (120), wherein optionally the hollow elcetrode has an inner diameter of minimum 50 mm.

13. The method according to any of the preceding claims, wherein the electric arc silica is collected in a filter unit (130) in the gas outlet (134) of the plasma reactor (100, 105).

14. A system configured to perform the method steps according to any of the preceding claims comprising at least:a plasma reactor (100, 105) with at least two electrodes (101 , 102) configured to generate at least one electric arc (117);a first gas inlet (131) for introducing gas between the at least two electrodes (101 , 102);a powder feeder (120) to feed a silica containing powder (122) comprising a chloride as additive (150); wherein the silica containing powder is preferably quartz; anda second gas inlet (132) to introduce a quenching gas to produce electric arc silica.

15. An electric arc silica produced according to the method steps according to any of the claims 1 to

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