WO1995029872A1

WO1995029872A1 - Method of producing fumed silica

Info

Publication number: WO1995029872A1
Application number: PCT/US1995/005100
Authority: WO
Inventors: Gael Ulrich
Original assignee: Ulrich Research & Consulting, Inc.
Priority date: 1994-05-03
Filing date: 1995-04-27
Publication date: 1995-11-09
Also published as: AU2364195A

Abstract

A method of producing fumed silica by replacing a portion of the required fuel with superheated steam (8). The steam replaces a portion of the hydrogen fuel required for hydrolysis of SiC14 (12). This results in an increase in the powder producing capacity of existing fumed silica plants without expanding most of the process equipment. This also enables fuel costs to be substantially reduced since 40 to 70 % of the required hydrogen and oxygen is obtained from steam. This process also eliminates the expensive step of drying the combustion air.

Description

METHOD OF PRODUCING FUMED SILICA

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to methods for making fumed silica.

2. Description of the Related Art

Flame-generated particles have found practical use ever since prehistoric cave

dwellers collected soot from their fires to paint scenes on cave walls. Today a wide

variety of particulate commodities are made by flame technology. Now, too, as

understanding of flame processes improves, flame-generated particles promise to

become even more significant, not only as commodity products but as specialty chemicals as well.

One of the distinguishing characteristics of flame-synthesized particles is the

variation in size possible with the elemental or primary particle - that is, the particle

that, cemented to others, forms the clustered frameworks that are known as

aggregates. In commercial products, these primary particles can range from as small

as 6 nm to as large as 600 nm. Experimental materials have been made in which

the particles are even smaller.

The size of the aggregates, moreover, also can vary because they may be

formed from one to as many as 1000 primary particles, depending on the base

substance and manufacturing conditions.

Such wide variations in two properties suggest great product flexibility.

Versatility is limited, unfortunately, because the two properties are somewhat

interdependent. Conditions that produce small primary particles, for instance, also

yield aggregates with large populations (and vice versa). Thus, there are no small primary particles that are not aggregated or any aggregates having a large population of big primary particles.

Additional valuable characteristics - such as purity, uniformity, and flexibility -

are consequences of flame synthesis technology. This unique combination of properties contributes to the strength, beauty, stability, and other desirable properties of many consumer goods.

Fumed silica was developed as a substitute for carbon black in hydrocarbon- limited Germany during World War II. It is formed by the combustion of silicon tetrachloride or some other silane with a hydrogen containing fuel and oxygen. Minuscule liquid droplets, precipitated by the reaction, form a "smoke" that resembles carbon black in particle size and structure. In context with its more ancient relative, it has, rather carelessly, been called "white soot."

Fumed silica is an amorphous (lacking defined crystalline structure) anhydrous (free of water) form of Silicon Dioxide (Si0₂). From a macroscopic perspective, it appears as an extremely light fluffy powder. Electron micrographs reveal its structure to be clusters of small (1 to 100 nanometer diameter) particles. These small particles are referred to as primary particles, while the clusters, which can range from .005 to 1 microns in size, are known as aggregates. These aggregates often have irregular chain-like shapes. Fumed silica's have very high purity (99.7% or better). An easily measured indicator of primary particle size of fumed silica is surface area as measured by nitrogen adsorption. Specific surface areas of fumed silica's can range from 15 to 800 square meters per gram. Different applications of fumed silica have different optimal size ranges.

Fumed silica is also sometimes referred to as pyrogenic silica. Fumed silica is added to rubbers, silicones, epoxy resins, and plastics during

their manufacture to reinforce them or otherwise enhance their properties. It is also used as a thickening and gelling agent. The world-wide market for fumed silica

exceeds 100 million kilograms per year. Most commercial fumed silica's are produced by the combustion of silicon tetrachloride (SiCI vapor, along with hydrogen or methane fuel, in air. The ingredients are pre-mixed and fed to a tube-like burner with one end open to the

atmosphere. The burners, which are two to ten centimeters in diameter, produce a

steady cone-shaped flame. There is often a thin concentric flow of pure fuel

(hydrogen or methane) around the outer diameter to anchor the flame, keep the burner from fouling, and to ensure complete reaction.

The reaction products, an aerosol of silica in HCI, C0₂, N₂, H₂0, and excess oxygen are pulled (along with ambient air, which quickly quenches the products) by

a vacuum to bag filters, where the silica is collected. The gases are then recycled,

scrubbed, or discharged. Depending on the end use of the silica, various after- treatments may be performed on the powder before it is sold.

Product qualities, such as surface area and aggregate size are controlled by changing the stoichiometry. More dilute flames (with greater excesses of fuel or air) generally produce smaller primary particles and, thus, materials having higher surface

areas. Stoichiometries are described in terms of per cent theoretical hydrogen (100% theoretical hydrogen would be enough hydrogen or methane to convert all the chlorine contained in the SiCI₄ to HCI; 150% theoretical hydrogen would be one and

a half times this amount) and percent theoretical oxidant (100% theoretical oxidant would be enough oxygen to form Si0₂ with all the Si present, C0₂ with all carbon present, and H₂0 with any excess hydrogen that has not gone to HCI).

It is well known in the art that silanes react prematurely with moisture to

produce a hydrated silicon oxide of poor quality. Because products of this reaction

are sticky, corrosive, and tend to foul equipment, moisture is carefully avoided in the manufacture of fumed silica. Combustion air is scrupulously dried (at added expense) to prevent this premature reaction from occurring in a production line.

For example, using water vapor and gaseous silicon tetrachloride heated separately to a temperature of 300° to 400° C, mixing them together and then reacting the mixture at 600° to 1200° C with a dwell time greater than 5 seconds produces massive, non-crystalline particles of spherical hydrophillic silica as disclosed in U.S. Patent No. 4,967,943, issued to Pauli et al. on December 11 , 1990. This method is not a flame process and it is unsuitable for the production of fine sized particles required for most applications using fumed silica.

SUMMARY OF THE INVENTION It is an object of the invention to provide a method of manufacture of fumed silica that can increase the powder producing capacity of existing fumed silica plants without expanding most of the process equipment.

It is another object of the invention to provide a method of manufacture of fumed silica that increases particle loading. It is still another object of the invention to provide a method of manufacture of fumed silica that uses steam as a "fuel" in SiCI₄ hydrolysis.

Another object of the invention is to provide a method of manufacture of fumed silica that reduces fuel costs by enabling a portion of the required hydrogen and oxygen to be obtained from steam. It is still another object of the invention to provide a method of manufacture of

fumed silica that eliminates the need for drying the combustion air.

Finally, it is an object of the invention to provide a

method of manufacture of fumed silica that uses superheated steam as a reactant

constituent.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic diagram of an apparatus used to practice the method of

manufacture of fumed silica in accordance with the invention.

Fig. 2 is a detailed schematic diagram of the burning section of the apparatus

used to practice the method of manufacture of fumed silica in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

The inventor has discovered that superheated steam and silicon tetrachloride

can be mixed without premature reaction occurring. This contradicts the well-known

fact cited above that spontaneous reaction occurs with moist air. However, it is

postulated that an aqueous hydrochloric acid mist condenses at ambient

temperatures to drive the hydrolysis reaction with an extra heat and free-energy of

solution. If the steam is superheated to the point that a condensed phase cannot

form, it has been found that SiCI₄ can be mixed with it, and they will not react.

Given that SiCI₄ and superheated steam can be mixed without reacting

prematurely, one can devise a stoichiometry where steam provides some of the

hydrogen and oxygen needed for combustion in the manufacture of fumed silica.

The invention replaces a portion (greater than 0% and possibly as much as

100% of the methane or hydrogen) with an amount of superheated steam that provides equivalent moles of hydrogen. At the same time, the oxygen in the water molecule serves as part of the oxidant need. In order to maintain a 2000 K or higher

flame temperature (which produces a quality product), the amount of nitrogen in the

oxidant mix (air is 79% nitrogen) is reduced as more fuel is replaced by superheated steam. Thus, these adjustments to the stoichiometry require shifting from air as the oxidant in the flame reaction to enriched air (less nitrogen) and eventually to pure oxygen as the fraction of hydrogen from steam is increased.

When considering ranges of stoichiometries, four variables need to be considered: % theoretical oxidant, % theoretical hydrogen, % fuel replaced by superheated steam, and % nitrogen removed from air. In the inventor's experimental work, the first two variables were fixed at 110%. It is believed that the invention is valid at any combination of % oxidant and % hydrogen levels which are used commercially today.

In laboratory experiments, up to 70% of the fuel was replaced with superheated steam, and it is believed that as much as 80% or more could be replaced before product quality is seriously compromised. Replacement beyond the 80% level is possible, however, the pre-heat temperature of the raw material flows (including the steam) would have to be increased. These high levels of fuel replacement (70% or greater) require pure oxygen (100% nitrogen removal from air) as the oxidant. As the percentage of fuel replaced by superheated steam decreases, the amount of nitrogen that can enter the process is increased.

In terms of product quality, there is no single optimal stoichiometry. Silica of different surface area and aggregate size are called for in different applications and it is by changing stoichiometries that these properties are controlled. From an economic standpoint, raw material cost decreases as fuel is replaced by superheated steam, therefore optimal stoichiometries are those which provide the desired product characteristics with the highest level of fuel replacement. More important is the fact that silica concentrations (grams of silica per cubic meter of process gas) become greater with water addition. This means that plant capacity can

be increased without enlarging the equipment. Also, air no longer needs to be dried if it is superheated to the conditions described in the patent. This saves in both

capital and operating expense.

Table 1 shows the stoichiometries used in three laboratory experiments

conducted to test this invention. The first is a base composition typical of existing commercial processes. Samples of silica were made at this condition as basis for comparison. Ingredients were SiCI₄, air, and methane fuel. Hydrogen in the fuel equaled 110% of that needed to convert all chlorine to HCI (110% theoretical hydrogen). Air was added at 10% above that needed to convert all silicon to Si0₂, all carbon to C0₂, and all excess hydrogen to H₂0 (110% theoretical oxygen). The adiabatic flame temperature for this stoichiometry is approximately 2100 K, and the silica loading is 180 grams per standard cubic meter of combustion products. The premixed reagent temperature should preferably range from 200 to 220°C.

Two stoichiometries containing superheated steam were considered. One included pure oxygen and the other, oxygen-enriched air as oxidant. These stoichiometries were selected to yield adiabatic flame temperatures near 2100 K,

matching the base case. (Adiabatic flame temperatures of at least 1800 K are desired to perpetuate stable combustion in the laboratory burner selected.) Both stoichiometries are based on total oxygen and total hydrogen at 110% theoretical as Run Preparation: Silica with H20 addition All feeds at 220 C

Methane fuel

Core Diameter: 0.9 inches = 2.286 cm; area = 4.10 cm^Λ2

Adiabatic

% Water Flame Core Fiame

Condition %H Oxideήt mg- mol / second Rate Temp. Velocity

SiCl_d H,Od CH₄ N, o_? ml/hr K ^' std cm/s c

Reference Condition 110 110 0.46 0.0000 0.506 4.135 1.113 0.00 2098 33.9 18

40% H from H20, 50% N2 Reduction 110 110 0.46 0.4048 0.304 1.240 0.668 26.25 2188 16.8 34

70% H from H20, 100% N2 Reduction 110 110 0.46 0.7084 0.152 0.000 0.334 45.94 2032 9.0 57

TABLE I

in the base case.

The first stoichiometry that involves superheated steam employs enriched air containing 50% of its normal nitrogen. In this case, the methane rate can be reduced

by 40% with the equivalent amount of hydrogen coming from steam. The second stoichiometry uses pure oxygen with methane reduced by 70%. The adiabatic flame

temperature remains near 2100 K for both these conditions, whereas silica loadings are increased to 340 and 580 grams per standard cubic meter, respectively

(approximately double and triple that of the base case). Fumed silica with a surface

area of approximately 200 m²/g was produced in all 3 cases.

Experiments were conducted using the apparatus described in Figure 1.

However, the process described herein would also work in burners used to produce fumed silica in commercial plants. Such burners differ from the one described here. Those burners have a larger diameter core tube, and instead of a guard flame (as described below), commercial burners often provide a thin annular flow of hydrogen around the core flame. Also, the typical commercial reactor is open to the

atmosphere, and lacks a residence chamber as provided here.

The apparatus for generating superheated steam described below has a noteworthy advantage. Certain compounds added to the flame in small amounts may have beneficial effects on the properties of the fumed silica. The instant apparatus has the ability to add any water-soluble compound to the flame at a uniform, controlled rate. If the additive compound was volatile, it would be vaporized along with the water. If it was not volatile, it would be carried along as a fine aerosol dust after the water droplets from the atomizer have been vaporized. The instant

apparatus requires a low flow of nitrogen or oxygen around the atomizer tip to carry the water droplets to the heated vaporizer coil. If no additives are desired in the flame, superheated steam could be generated using any conventional means, such

as a commercial boiler.

The raw material flow rates provided in the following example are for a

5 stoichiometry with 110% theoretical oxygen and 110% theoretical hydrogen, with 40% of the hydrogen coming from superheated steam, and the nitrogen/oxygen ratio providing half as much nitrogen as standard air (second condition described in Table

1).

Syringe pump 2 is used to feed water plus any desired additives to ultrasonic lo atomizer 6. Pump 2 is preferably Harvard Apparatus Model 22 or an equivalent metered source of water. In this example, water was fed to atomizer 6 at the rate of approximately 26 ml/hour. Atomizer 6 is preferably SonoTek model 8700-60MS. A metered stream of nitrogen is introduced into port 4. The nitrogen acts as a carrier for the atomized water droplets. In this example, the nitrogen flow rate was 0.10 mg- i5 mol/sec.

From there, the stream of nitrogen with the atomized water droplets plus additives, if any, is fed into vaporizer/superheater 8. This consists of a coil of copper or stainless steel tubing that has been wrapped with an electrical resistance heating element surrounded by mineral fiber insulation. In this example, 1.27 cm diameter

20 copper tubing having a length of approximately 60 cm was used. The tubing was wrapped with a 140 watt heating element and maintained at 220°C. However, temperature as low as 150°C may be suitable under some conditions.

Through port 12, a pre-mixed metered flow of Silicon Tetrachloride (0.46 mg-

mol/sec), Oxygen (0.668 mg-mol/sec), methane (0.304 mg-mol/sec), and Nitrogen (1.14 mg-mol/sec) is introduced. These gases are fed through heated (220° C)

stainless steel lines. Noted that the above rates must be varied in accordance with desired output and necessary stoichiometries to obtain it.

In stainless steel mixing chamber 14, the flow from port 12 and the flow from vaporizer/superheater 8 are mixed and are fed to stainless steel concentric flame

laboratory burner 16. Burner 16 will be discussed in detail in Fig. 2. As noted above, existing commercial silica flame burner geometry could also be used. Dotted line 10

indicates that all structures located within are heated by electrical resistance heaters

and wrapped with mineral fiber insulation. Thermocouples are placed in or on the

equipment and are connected to programmable temperature controllers. The temperature controllers maintain the equipment at a predetermined temperature. In this case, that temperature is 220° C.

Through port 18, oxygen, carbon monoxide, and hydrogen is fed through

heated (220° C) stainless lines which serves a premixed guard flame reagent. After leaving the flame, the reaction products enter the residence chamber which is

preferably a 46 cm long by 4.1 cm inside diameter tube of fused quartz. The core and guard flames can be observed as the reactions proceed. From there, the reactant products are introduced into open air quench area 22. At this point, the reaction products along with a great deal of room air are pulled via a vacuum in the powder collection device 24. This structure could be either an electrostatic precipitator, bag filter, cyclone, or some combination thereof. In this example, an electrostatic precipitator, 10 cm in diameter by 1 meter long, was used. Fumed silica is collected via exit 26, in this example, at the rate of 1.66 grams per minute. Combustion product gases 28 are pulled by vacuum to a scrubber and exhaust. Fig. 2 is a detailed schematic diagram of the burning section of the apparatus used to practice the method of manufacture of fumed silica in accordance with the

invention.

Coil 40 carries the steam, nitrogen, and additives, if any, to the reaction. Areas 42 are preferably controlled by three separate controllers. Preferably, thermocouples are installed in or on the surface of the apparatus and, then, wrapped with electrical resistance heaters and mineral wool insulation. Each temperature is

maintained at 220° C, however, other temperatures would also be suitable. Silicon tetrachloride, methane, oxygen and nitrogen is introduced via inlet 44 which, along with the steam, nitrogen and additives via coil 40 are fed into mixing chamber 46. Preferably, in this laboratory model, the chamber should have a volume of approximately 60 cc to ensure that thorough mixing occurs. Commercial sized units would be scaled accordingly.

Burner 48 is preferably fabricated from stainless steel components sealed with with Viton O-rings 70. Inlet 50 is used to introduce the guard flame fuel and oxidant, carbon monoxide, hydrogen and oxygen. Commercial burners typically use an annular flow of H₂. In the laboratory burner used for these experiments, other fuel oxidant combinations, e.g., propane-air were utilized.

Burner core tube 52 is a uniform stainless steel tube, 2.29 cm inside diameter. The reactants flow from the mixing chamber to the burner face through tube 52. Guard flame distributor 54 is a ring of porous sintered stainless steel having an outside diameter of 2.91 cm and an inside diameter of 2.37 cm and having a 20 micron pore size. The seals 72 between the guard flame distributor 54 and tube 52 are press fits between tapered surfaces. Teflon tape (not shown) is used to ensure the integrity of the seals.

Guard flame 56 is a clean-burning flame that surrounds the core flame 60 to serve as an anchor and to isolate and insulate the core flame. Guard flame combustion products 58 (carbon dioxide, water vapor, and excess oxygen) flow as a laminar cylindrical column through the quartz residence chamber 64. This column

is centered within residence chamber 64. Core flame 60 is a luminous cone, ranging

from 4 to 8 cm long. The products produced by core flame 60 are a powder-laden

stream of silica, hydrogen chloride, carbon dioxide, nitrogen, and excess oxygen.

This stream has the appearance of white smoke.

In the laboratory burner, residence chamber 64 is a 4.1 cm inside diameter tube of clear fused quartz. The flame properties of the powder-laden core flame 60 surrounded by the clean guard flame 56 keep tube 64 clean and transparent. Thus, through tube 64, the core flame 60 and the plume 62 of reaction products can be

monitored while the apparatus is running. As noted above, some commercial burners also employ an anchor flame fueled by a thin "mantle" layer of hydrogen gas. There is no porous distributor, just a thin circular gap through which the hydrogen passes. This hydrogen combines (burns) with both oxygen in the surrounding air and excess oxygen from the core combustion zone. Its purpose is essentially the same as the guard flame's in the laboratory

burner: to anchor the flame, keep the burner from fouling, and to help the reaction proceed to completion.

Guard flame 56 reagents were carbon monoxide and oxygen, moderated with hydrogen. Velocity and composition of these gases were adjusted to provide a velocity and adiabatic flame temperature to match those of the core flame. With this arrangement, mixing of guard flame products 58 and core products 62 is prevented, and guard products provide a clean sheath of hot gases to isolate the particle-laden

core plume from the residence chamber wall.

In the examples, there was no evidence of fouling anywhere in the system during more than two hours of operation. The luminous flame cone that one associates with complete combustion was present, similar to that observed with our methane/air/SiCI₄ reference flame. Flame length with steam added was about 5.5 cm, near the 6 cm noted for the reference flame. (Since gas flow dropped with steam addition, the fact that flame length stays almost constant indicates a drop in burning velocity.) Product surface areas were in the vicinity of 200 m²/g, similar to that made at the reference condition.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention and it is, therefore, aimed to cover all such changes and modifications as. fall within the true spirit and scope of the invention.

Claims

What is claimed is:

1. A method of preparing fumed silica which comprises the steps of: superheating water to form steam at a temperature where a condensed phase

cannot form: mixing the steam with SiCI₄, a hydrogen containing fuel, nitrogen and oxygen;

reacting said mixture in a flame having a temperature of at least 1800 K to produce fumed silica.

2. The method of claim 1 wherein at least a portion of the required hydrogen

for hydrolysis of SiCI₄ is obtained from the steam.

3. The method of claim 2 wherein the nitrogen introduced in said mixing step is reduced corresponding to the mol equivalent of hydrogen present in the steam.

4. The method of claim 3 wherein the steam in said superheating step is

heated to at least 150° C.