DE1667609C3 - Process for the production of a SiO deep 2 sol in an alcoholic dispersant - Google Patents

Description

The present invention relates to a method for producing an SiO., - Sol in an alcoholic Dispersant.

There are so far various manufacturing processes for silica sols in which the colloidal oxide is in an aqueous or non-aqueous liquid medium has been described, and it is a large number of possible uses for the sols obtained in this way are known, for example as binders for the production of refractory molds for metal casting. Silica sols are actually special suitable for this latter purpose. The gels are used as fillers or thickeners in one Variety of preparations usable.

The German Auslegeschrifl 1 072 602 describes a method for producing a SiOvSoIs. But here the source for the SiGySoI is sodium silicate, which is electrolyzed in an aqueous alkaline solution.

From the work of Duffek and colleagues (Journal Elektro Chemical Soc, 1964, III, p. 1042), Besson and Mathe (Journal Chemical Phys., 1955, No. 52, p. 473) and Hannibal (Zeitschrift für Naturforschung, 1960, No. 15 a, p. 837), electrolytic processes have become known in which silicon-containing electrodes are used. The conditions described by Duffek and coworkers and Besson and Mathe result in a cohesive deposition of silicon dioxide on the electrode. On the other hand, nothing is said about the formation of a dispersion of colloidal silicon dioxide in the electrolyte. Hannibal describes the visualization of grain boundaries, with iron being etched away from iron silicon ^{in the form of Fe + +.} However, it does not emerge from this work that an attack also takes place on the silicon present as a constituent.

The invention now relates to a process for the production of an SiO ₂ sol in an alcoholic dispersant, as it is characterized in that a liquid containing a monohydric alcohol with 1 to 3 carbon atoms per molecule contains 0.1 to 5 percent by weight of water , based on the alcohol, and contains sulfuric acid or hydrochloric acid as an additional electrolyte, an electric current is passed, a silicon-containing anode contacting the liquid is used and - if necessary - water is added to the liquid during the electr >>'yse to maintain the water concentration.

The production of an SiO ₂ sol by the method according to the invention was completely surprising and does not result as obvious from the prior art described above.

It is believed that the reaction by which the silica is formed is essentially is an anodic oxidation of silicon, the formation conditions being such that the colloidal Particles of the silica are dispersed in the liquid medium. The invention Gels are formed by gelling the brine.

According to a preferred embodiment of the invention In the process, the electrolyte in the liquid medium does not exceed 5 percent by weight.

According to a further preferred embodiment of the method according to the invention, the anode is im essentially pure silicon or ferro-silicon.

A preferred variant of the method according to the invention is that there is a preceding The duration of the "anode activation" includes during which an electric current flows through the liquid medium with the electrode, which is used at the anode in the manufacture of the silica sol, as the cathode is passed through.

The new colloidal dispersions of silicon dioxide created according to the present invention contain from 1 to 30 percent by weight of the total preparation silicon dioxide in a dispersion medium which is a monohydric alcohol having 1 to 3 carbon atoms per molecule, 0.1 to 5 percent by weight, based on the alcohol, Water and from 0.1 to 10 percent by weight of the total preparation hydrochloric acid (expressed as 100 "/ oHCl) or 0.1 to 15 percent by weight of the total preparation sulfuric acid (expressed as 100% R ₁ SO ₄ ).

The alcohols useful in the process include methanol, ethanol, propanol and isopropanol and mixtures of these alcohols. Electrolysis progresses fastest when the alcohol is methanol is, however, the sols made with it tend to be somewhat less stable than those made under Using ethanol were obtained. The sol formation is less rapid in propanol and isopropanol than in ethanol, so that ethanol is the alcohol that is generally most satisfactory is working.

As indicated above, the upper limit is the amount of water present in the liquid medium at 5% by weight of the alcohol. With regard to the lower limit, difficulties arise when the amount of water less than 0.1% of the weight of the

Alcohol is. The preferred concentration range is 0.25 to 5%, preferably 0.5 to 2.5%. And most preferably 0.5 to 1%. When determining the optimal proportion, the fact " thing must be taken into account that water is consumed during the anodic oxidation, so that it becomes necessary to maintain its concentration within a given range can. Add water while the electric kettle is running.

Of the two acids that can be used as the electrolyte in the bonding, "generally sulfuric acid is the more suitable for use because of its lower volatility, and is further preferred because it gives a higher current yield of silicon dioxide. The electrolyte is essentially only a current carrier, and the amount used is therefore usually the minimum necessary to produce practical conductivity, this minimum of course depending on a number of factors, but typically it can be about 0.1 ^o 'o des It is preferred that the concentration of the electrolyte in the liquid medium not exceed 5 weight percent.

The anode used can consist of essentially pure silicon, and there can be several Semiconductor qualities of the material, e.g. B. silicon, in which essentially the only impurity a substance is d <e in a controlled amount /. to give conductivity to one of a certain type and size will be used successfully. However, it is impractical to use silicon with a resistance which is significantly higher than 10 ohm-cm, to be used in the present process. Silicon types resistivities in the range of 0.005 to 1.0 ohm-cm are generally most satisfactory. For a given resistance, silicon with n-type conductivity gives better ones Results as the p-type material with the same toughness. It is an effect which is more pronounced when one approaches the practical resistance limit of K) ohm-cm and is of little importance in the range of the lower resistivities given above.

For economic reasons, the anode used in the method of the present invention is made from a compound or alloy of silicon rather than a semiconductor grade of the element. Ferrosilicon types have been found to be particularly useful. The silicon content of the ferrosilicon can be, for example, in the range from 60 to 99 percent by weight and in typical cases in the ranges from 70 to 80, 90 to 95 or 97 to 99 percent by weight. Any inert, conductive material can serve as the cathode. Graphite is a useful cathode material that has significant cost advantages over other functionally suitable materials such as platinum or silver. The electrical current used in the method according to the invention can be a simple direct current or rectified alternating current with or without smoothing. The current density at the anode can typically be in the range from 10 to 20 A / dm ² , although it is possible to work at current densities over a wide range, greater than that specified. The preferred working voltages are no more than 70 volts above the decay voltage, because at higher voltages there is a tendency (which increases with the voltage) to form oxide

tion of the anode, forming a contiguous Layer on the anode surface exists as colloidal particles in place of dispersing the liquid medium. The formation of a hanging oxide layer is accompanied by a rapid

IQ len increase in cell resistance. For economic The reason should be that which exceeds the disintegration tension Voltage should actually be kept as low as possible. In practice, the working,; annung determined by such factors as cell design and electrolyte concentration, and there are further changes possible. The experiments made gave good results, using ' of voltages ranging from about 10 to 50 volts above the decay voltage of the system achieved. Where an unsmoothed or partially smoothed rectified alternating current is used the square root of the mean square of the voltage to determine the preferred working voltages, as stated above.

The electrolysis can be carried out over a wide temperature range. Generally the most suitable The way of working is to equip the electrolysis line with a reflux condenser and the procedure at the boiling point of the liquid medium perform. The process can also be carried out at lower temperatures, however it is then generally necessary, for example from 10 C to the boiling point of the liquid medium. Use coolant.

Improved results are obtained where the process has a previous "anode activation" stage contains while an electric current flows through the cell with the electrode that is in the method according to the invention serves as an anode, is conducted as a cathode.

Sols with a range of silica concentrations, for example from 1 to 15 percent by weight the brine, can be prepared for a first use by the method according to the invention depending on the amount of anode consumed in relation to the quantity of the liquid medium, and these can be further concentrated into brines that are up to 30 percent by weight or less contain more silica by evaporating the solvent.

The sols show an increased tendency to gel when stored under normal atmospheric conditions Conditions when silica concentrations and acidity increase. At a given Acid concentration, there is a minimum silicon oxide concentration, below which the brine is usually unlimited can remain stable and vice versa.

The new, colloidal dispersions of silicon dioxide, which have the highest practical value, are those containing 4 to 30 weight percent silica and no more than 5 weight percent hydrochloric acid or contain sulfuric acid. Within these wide limits are preparations with the content for example from 5 to 15% or from 10 to

25 percent by weight silica and from 0.2 to 1.0 percent by weight hydrochloric acid or from 3 to 5 percent by weight Sulfuric acid is particularly useful in certain cases. According to the above

gene is the common form of dispersion in which both silica and acid concentrations approach the upper limits, a gel, while preparations in which the two concentrations are relatively low, are stable brines. Accordingly, there is a wide range of sols with intermediate stabilities possible.

The following examples illustrate the invention.

example 1

5 ml of 95% by weight sulfuric acid became 500 ml of technical grade ethanol which was 0.4% by weight Containing water, given to form a solution with 0.6 weight percent water content. This Solution was placed in a multi-necked flask fitted with a graphite electrode, a ferrosilicon (98 ° ο Si) electrode, a water-cooled reflux condenser and a gas inlet pipe.

The electrical connections were made in such a way that the graphite electrode is the anode and the Ferrosilicon electrode was the cathode and a current of 2.0 A was passed through it for 10 minutes. The polarities of the electrodes were then changed and the electrolysis at 15 hours a current of 2.0 A and an equilibrium potential of 34 to 50 volts. While during this period nitrogen was bubbled into the space above the liquid. The warmth formed was sufficient to reflux the solvent cause.

The product was a clear, _{pale yellow SiO 2} sol. which weighed 404.8 g and contained 16.2 g (4 weight percent) silica. The total electricity consumption during the process was 29.6 A / h. and corresponded to a yield of 0.55 g of silicon dioxide per A / hour. The decay voltage of the system was 4 ^ 1 volts at reflux.

Example 2

This example describes the preparation of a silica-in-methanol sol.

Fine solution of 2.0 ml of 95 percent by weight Sulfuric acid in 500 ml of methanol (p. A.) Was introduced into the system described in Example 1. There was a potential difference of 20 volts with a positive graphite electrode and a negative ferrosilicon corner electrode applied and the solution heated to boiling point.

The initial current of 0.05 A rose to 2.0 A over a period of 68 minutes during which the electrolysis was carried out in this form, with further additions of 95 weight percent Sulfuric acid (one of 2.0 ml after 5 minutes and one of 0.5 ml after Gl minutes) during this period of time.

The polarities of the electrodes were then changed and the electrolysis for 15 hours at one Current of 2.0A continued for 1 hour at 1.6A and finally for 37 minutes at 1.2A. the Equilibrium voltage alternated from 20 to 48 volts.

The whole process was carried out under a nitrogen atmosphere and apart from initially heating the solution to its boiling point the resistance heating of the electric current acid sol with a weight of 391 g and a content of 17.2 g (4.4 percent by weight) of silicon dioxide. The total electricity consumption during the process was 32.8 A / h, which is a yield of 0.52 ς silicon dioxide per A / hour. corresponded.

Example 3

This example describes the preparation of a silica-in-ethanol sol using Hydrochloric acid as an electrolyte.

ίο A solution of 3 ml of 32 weight percent aqueous Hydrochloric acid in 500 ml of technical grade ethanol was introduced into the system described in Example 1. An initial current was through the graphite positive electrode and ferrosilicon negative electrode with

0.4 A at 50 volts potential difference. After 4 minutes, a further 5 ml of 32% strength hydrochloric acid were added admitted. The resistance heating caused the boil after 20 minutes and kept that afterwards Back nut at.

The electrode polarities were then reversed and a current which varied between 1.18 and 1.75 A was passed through for 14.5 hours with a potential of 49 to 78 Volts. 0.5 ml of hydrochloric acid additions were made at hourly intervals and the entire procedure was carried out in a nitrogen atmosphere. The product was a _{pale green SiO 2} sol weighing 395.2 g and containing 4.74 g (1.2 weight percent) silica. The total consumption of electricity was 20.3 Åh, which corresponds to a yield of 0.24 g of silicon dioxide per Åh.

Example 4

This example describes how to make a

Silica-in-ethanol sols using a ferro-silicon electrode with 70 to 80% silicon content in contrast to the 98% silicon content in the ferro-silicon electrode, which was described in the preceding Examples was used.

A solution of 8 ml of 95 percent strength by weight sulfuric acid in 500 ml of technical grade ethanol was used placed in a plant which was essentially the same as that described in Example 1, but with a different ferro silicon electrode. as already stated above. With the graphite positive electrode and ferrosilicon negative electrode were energized for 10 minutes passed through a potential of 30 to 40 volts. During this period of time the resistance heating brought about the solution to the boil and then refluxed. The amperage increased also from 0.90 to 1.70 A. The electrode polarity was changed and a current of 2.0 A (with small fluctuations ·) passed through for 6 hours with a potential of 40 to 82 volts. The procedure was carried out under a stream of nitrogen.

The product obtained was a clear, pale straw-colored silica sol weighing 421.5 g and 6.52 g (1.55 weight percent) silica content. The total electricity consumption at the Process was 12.0 A / h. and corresponded to a yield of 0.54 g per A / hour.

Example5

This example describes the preparation of a silica-in-methanol sol using of hydrochloric acid as an electrolyte.

^ 8

sufficient to keep the solution permanently at this temperature. The product was 441 g of silica sol with

to keep pcratur. 1.55 weight percent silica content Die

The product was a clear, pale green silica. Current yield was 0.53 g silica / A / hour.

Using those used in Example 1. -17
Plant was a potential of 30 volts between 5 Example 7
a ferrosilicon (98% Si) cathode and a graph of 8 ml of 95 weight percent sulfuric acid were phitanode in a solution of 3 ml of 32 gcwichtspro- to 500 ml of technical ethanol in a Mehrhalszcntiger aqueous hydrochloric acid in 500 ml of methanol in the flask with a Graphite cathode, a ter formation of a current of 0.95 A is applied. 95% ferrosilicon anode, a reflux condenser, by increasing the potential to 50 volts, a stirrer and a gas inlet tube were equipped with a current of 1.6 A, which was tet, after 2 minutes. A current of 2 A was increased 22.5 hours to 2.0 A after a further 3 minutes. The current passed through at a potential of 38 to 50 volts was maintained at 2.0 A for 5 minutes, with conducting, whereby a clear, pale green colourcnd during this time the potential dropped to 43 volts. loidal solution of silicon dioxide with 24.1 g of SiIi-At this point in time, the graphite electrode had a silicon dioxide content of 15 at a concentration of cathodic and the ferro-silicon electrode anodic of 5.6 percent by weight. The current efficiency of the made. The electrolysis was continued at 2.0 A, process was 0.54 g silica / A / h. After a further 5 minutes, boiling occurred and the silica brine was concentrated by distillation. The electrolysis was continued for 14 hours with 20 brines containing 11.2, 15.3 and 20.4 percent by weight SiIilang at 2.0 A amperage, with sodium dioxide. The 15.2 and 20.4 weight trends of this time fluctuated in a range of percent brine gelled -37 to 66 volts after 5 days of standing. From time to time it was left at room temperature, and the 11.2 weight increase in potential with continued electrosol gelled after 16 days if it was left to stand at room temperature by adding small amounts of hydrochloric acid during cymysis. The gels were soft, reversed. During 14 hours, all of the materials became transparent. The 5.6 weight percent amount of 9 ml of acid was added in this way. Sol was still stable after 5 months.
The product was 356.7 g of clear green silica. .
sol with 6.77 g silicon dioxide content. In this example 8
Reaction, current consumed was 28 A / h, which corresponds to 30 8 ml of 95 weight percent sulfuric acid and a current efficiency of 0.24 g silicon dioxide / A / h. 500 ml of technical grade ethanol were put into the plant. of Example 7 introduced and with an amperage. . of 2 A and a potential of 30 to 40 volts ^Example "treated electrolytically. During electrolysis

This example describes the preparation of a 35 became the volume of liquid by adding

Silicic acid-in-ethanol sols kept constant after essentially ethanol. Had after 34 hours

The same procedure as described in Example 1 was used to maintain a current strength of 2 A

bcn was increased to 50 volts, but using a ferro- required voltage during

silicon anode with a lower silicon content. over the next 13 hours a further 5 ml of 95-

500 ml of ethanol were added to a standard facility of 40 weight percent sulfuric acid, and the

ment, which in this case alternated with a voltage from 48 to 58 volts. In addition

phitelcktrode and a ferrosilicon electrode with time at which the formation of a small

a silicon content of about 92 weight percent white precipitate was observed , the

Electrolysis was interrupted under a stream of nitrogen. The low precipitation

keep. 6 ml of 95 percent strength by weight sulfuric acid 45 settled out, and a blue-green, clear color remained;

were added to the ethanol and a stream of silica sol with 13 percent by weight silicon

0.8 A at an applied potential of 30 volts. The current efficiency of the reactor

obtained, the electrical lines being 0.51 g silicon dioxide / A / hour. as FiI. That so

were arranged that the graphite electrode formed as an anode after standing at room temperature for 2 days and the ferrosilicon electrode as the cathode uses a clear, soft gel.

det were. There were a further 2 ml of 95 weight percent. -in

percent sulfuric acid was added and the Poten- P

tial held at 30 volts while the current strength was 4 ml of 95 weight percent sulfuric acid

gradually over 15 minutes to 1.10A in the 500 ml of methanol in the device of Bei The extent to which the solution warmed up. 55 given game 7 and with a current of 2 A be

At this point in time, the graphite electrode was exposed to a potential of 17 to 22 volts for 19 hours

The cathode and the ferrosilicon electrode as the anode are electrolyzed for 45 minutes

At 30 volts, a current of 1.65 A loose sol containing 4.9 weight percent silica was used

The current efficiency was 0.40 g of silicon dioxide A current of 2.0 A was increased. 60 / A / h

The potential was in the range of 42 until samples of the sol were at 7.4 and 10.2 Ge

72 volts during a period of 6Vz hours weight percent silicon dioxide by distillation cor

set, creating a constant amperage from centered. After 6 days, both samples were gelled

2, OA was obtained. During this period of time, the initial sol at 4.9 percent by weight remained b Another 4 ml of acid were stable to reduce the temperature for 65 to 2 months, after which a Teiigelierun was performed

Maintaining the current strength of the necessary potential observed

Claims (4)

Patent claims: 1. Process for the production of a SiO., - Sol in an alcoholic dispersant, thereby characterized in that by a liquid containing a monohydric alcohol 1 to 3 carbon atoms per molecule, 0.1 to 5 percent by weight of water, based on the alcohol, and contains sulfuric acid or hydrochloric acid as an additional electrolyte, conducts an electric current, a silicon-containing one contacting the liquid Anode used and - if necessary - during electrolysis for maintenance the water concentration water is added to the liquid. 2. The method according to claim 1, characterized in that the concentration of the Eüekuolyts in the liquid medium does not exceed 5 percent by weight. 3. The method according to claim 1 and 2, characterized in that the anode is substantially pure silicon or ferrosilicon. 4. The method according to claim 1 to 3, characterized in that to activate the silicon-containing Before being used as an anode, an electric current flows through the liquid electrode Medium is passed through with this electrode as cathode. 30th