TW201343552A - Process for preparing high-purity SiO2 - Google Patents

Abstract

The present invention relates to a process for preparing high-purity SiO2, comprising the steps of a. preparing an initial charge from an acidifier having a pH of less than 2; b. providing a silicate solution; c. adding the silicate solution from step b. to the initial charge from step a. such that the pH of the resulting precipitate suspension always remains at a value less than 2; and d. removing and washing the resulting silicon dioxide; wherein the silicon dioxide obtained by the precipitation is removed continuously or semicontinuously from the precipitation region and washed in a wash region separate from the precipitation region. The present invention further describes a plant for performing the present process.

Description

High purity SiO 2 method

The present invention relates to a process for producing high purity SiO ₂ . The invention further relates to a plant for carrying out the method of the invention.

An important cost factor in the manufacture of electronic components, particularly photovoltaic cells, is the cost of the high purity helium required. Therefore, every effort has been made to produce crucibles of the required purity without being expensive. One of the relatively inexpensive methods is detailed in WO 2010/037694. In this method, SiO _{2 is} reduced to metal ruthenium by carbon in a photo-arc furnace. The starting materials used are typically SiO ₂ moulds combined with a carbon source.

For this purpose, SiO ₂ can be purified by a washing process. The purified SiO _{2 is} typically ground and subsequently blended with a carbon source, such as sugar, and pressed into the mold. The sugar present in the molding can then be charred to produce carbon to give a molding that can be reduced to ruthenium in a photo-arc furnace.

The high purity tanning process known from prior art has shown a good trend in nature. However, there is still a need to improve these methods. For one of the problems detailed above, the preparation of high purity cerium oxide is a particular challenge.

To the prior art, the problem for which the present invention is to provide a simple and cheap way to perform the production of high purity silicon dioxide (SiO ₂₎ method. More specifically, the process is performed in a long-term process in a plant for purifying cerium oxide without any need to interrupt the system.

A particular problem is to provide a method that can be performed at very high speeds. Moreover, the method can be performed with a minimum number of process steps, and such steps need to be simple and reproducible.

In addition, the implementation of the method does not endanger the environment or human health, so that substances that are harmful to health or compounds that may be harmful to the environment can be physically deactivated.

In addition, the feed used should be made or obtained at very low cost.

Further, a cerium oxide molding having a particularly high strength will be provided.

Other problems that are not mentioned in detail are clear from the general description of the invention, the examples and the scope of the patent application.

This problem can be solved by applying the method described in item 1 of the patent scope. And other matters that are not explicitly stated but can be readily apparent from the context of the present invention. Appropriate modifications to this method are protected by an affiliate and are referred back to the first item of the patent application.

The invention thus relates to a process for the preparation of a high purity SiO ₂ comprising the steps of: a. preparing an initial charge from an acidulant having a pH of less than 2; b providing a citrate solution; c. administering a citrate solution from step b Adding to the initial feed from step a such that the pH of the formed precipitate suspension is maintained at a value less than 2; and d. removing and washing the formed cerium oxide; characterized by: continuous or semi-continuous self The cerium oxide obtained by the precipitation is taken out in the precipitation zone and washed in a washing zone separated from the precipitation zone.

The method of the invention can be performed simply and at a low cost. At the same time, impurities caused by purification can be minimized.

In addition, the process can be carried out in a long-term purification of cerium oxide plants without any need to interrupt the system.

Moreover, the method can be performed at very high speeds. Moreover, the method can be performed with very few process steps, and the steps are simple and reproducible.

In addition, the implementation of the method does not endanger the environment or human health, so that substances that are harmful to health or compounds that may be harmful to the environment can be physically deactivated.

In addition, the feeds used are typically made or obtained at affordable prices.

In particular embodiments of the invention, cerium oxide is provided which produces a particularly stable molding.

The process of the invention is used to prepare high purity cerium oxide (SiO ₂ ). In particular, cerium oxide prepared by the method of the present invention can be used as a raw material for preparing metal ruthenium. In addition, the cerium oxide produced by the process of the present invention can be used to produce SiO ₂ moldings which facilitate the manufacture of metal bismuth and further processing and are well known to those skilled in the art.

The term "cerium oxide" is also used in the context of the present invention to mean that it can include free and / Or an aqueous composition that binds water. In addition, a small amount of material derived from the source of the preparation or purification may be present in such compositions. In the context of the present invention, particular reference should be made to the requirements that must be met for high purity cerium oxide so that it can be used for the intended use.

The cerium oxide to be purified according to the present invention can be obtained, for example, by a precipitation reaction from a cerium-containing solution (for example, water glass).

A preferred mode of precipitation of the cerium oxide (especially fully dissolved cerium oxide) dissolved in the aqueous phase is preferably carried out with an acidifying agent. After the cerium oxide dissolved in the aqueous phase is reacted with the acidifying agent (the cerium oxide dissolved in the aqueous phase is preferably added to the acidifying agent) to obtain a precipitate suspension.

An important process characteristic is the control of the pH of the cerium oxide and the cerium oxide containing reaction medium during various process steps in the preparation of cerium oxide.

In this preferred embodiment, the initial charge and addition (preferably dropwise addition) of the precipitate suspension of the cerium oxide (especially water glass) dissolved in the aqueous phase must always be acidic. "Acid" is understood to mean a pH below 6.5, especially below 5.0, preferably below 3.5, more preferably below 2.5, and below 2.0 to below 0.5 according to the invention. In this regard, the pH is controlled such that the pH does not change too much to achieve a reproducible precipitate suspension. If the fixed or substantially fixed pH is the target, then the pH should only exhibit a range of positive/negative 1.0, especially positive/negative 0.5, preferably positive/negative 0.2.

In a particularly preferred embodiment of the invention, the pH of the initial feed and the suspension of the precipitate is maintained below 2, preferably below 1, and more preferably below 0.5. Also preferred is the acid relative to the alkali metal citrate solution. The amount present is always a significant excess so that the pH of the precipitate suspension is below 2 at any time.

Without being bound by a particular theory, it can be assumed that an extremely low pH ensures that the surface of the ceria is effectively free of negatively charged free SiO groups, and that troublesome metal ions may bind to the ceria surface.

At very low pH, the surface is unexpectedly positively charged, causing the metal cation to be repelled by the cerium oxide surface. If such metal ions are subsequently washed out, as long as the pH is extremely low, it can thereby be prevented from accumulating on the surface of the ceria of the present invention. If the surface of the cerium oxide is positively charged, it is possible to additionally prevent agglomeration of the cerium oxide particles to form pores or gaps in which impurities may accumulate.

In this case, the nature of such charge separation allows the ruthenium dioxide (if it is moved) to generate a current signal according to Maxwell's electrodynamic rules, so that the cerium oxide can be detected and/or measured externally (without contact) in a simple manner. quality.

Particularly preferred is a process for preparing purified cerium oxide (especially high purity cerium oxide) comprising the steps of: a. preparing an initial charge from an acidulant having a pH of less than 2, preferably less than 1.5, More preferably less than 1, preferably less than 0.5; b. providing a citrate solution, it is particularly advantageous to set the viscosity within a specific viscosity range for preparing cerium oxide purified by precipitation, preferably especially 0.001 Up to 1000 Pas, the viscosity range can be further broadened by other process parameters according to the process - as detailed below; c. Adding the citrate solution from step b to step a The initial feed is such that the pH of the formed precipitate suspension is maintained below a value of 2, preferably less than 1.5, more preferably less than 1, and the optimum is less than 0.5; and d. The cerium oxide, the washing medium has a pH below 2, preferably less than 1.5, more preferably less than 1, and the optimum is less than 0.5.

In a first particularly preferred variant of the method, the preferred method for preparing purified cerium oxide (especially high purity cerium oxide) is carried out in a low viscosity to medium viscosity citrate solution such that the steps are b can be modified as follows: b. providing a citrate solution having a viscosity of 0.001 to 0.3 Pas. In a second particularly preferred variant of the method, preferably used to prepare purified cerium oxide (especially high purity cerium oxide) The precipitation method is carried out with a high viscosity to very high viscosity citrate solution, so that step b can be modified as follows: b. providing a citrate solution having a viscosity of 0.2 to 10000 Pas, among the different types of changes detailed above, in step a The initial charge is prepared from the acidifying agent or acidulant and water in a precipitation vessel. It is preferred that the water be distilled water or demineralized water. Quality can be detected or determined by a simple conductivity measurement method based on known standards such as DIN.

In all variants of the process according to the invention, not only the preferred embodiments described in detail above, but also the acidifying agent used may be an organic or inorganic acid, preferably a mineral acid, more preferably a concentrated or diluted form of hydrochloric acid. , phosphoric acid, nitric acid, sulfuric acid, chlorosulfonic acid, thioindigo chloride, perchloric acid, formic acid and/or acetic acid or a mixture of the foregoing acids. Particularly preferred are the aforementioned inorganic acids. Very preferred is hydrochloric acid, preferably 2 to 14N, more preferably 2 to 12N, still more preferably 2 to 10N, particularly preferably 2 to 7N, and very excellent 3 to 6N; phosphoric acid, preferably 2 to 59N, more preferably 2 to 50N, still more preferably 3 to 40N, particularly preferably 3 to 30N, and extremely excellent 4 to 20N; nitric acid, preferably 1 to 24N, more preferably 1 to 20N, more preferably 1 to 15N, particularly preferably 2 to 10N; sulfuric acid, preferably 1 to 37N, more preferably 1 to 30N, still more preferably 2 to 20N, and particularly preferably 2 to 10N. Very good people use concentrated sulfuric acid.

The acidulant can be used in a purity generally referred to as "technical grade". It will be apparent to those skilled in the art that the diluted or undiluted acidifier or mixture of acidifiers employed should carry a minimum amount of impurities that do not remain soluble in the aqueous phase of the precipitate suspension entering the process. In either case, the acidulant should not have any impurities that would precipitate with the cerium oxide during the acidic precipitation unless it can be retained in the precipitate suspension by the addition of the wrong agent or by controlling the pH, or The washing medium is washed out later.

The acidulant which has been used for precipitation can be the same as that which is also used, for example, in step d to wash the filter.

In a preferred variant of this process, in step a, not only the acidulant, but also the peroxide is added to the initial charge, which causes the titanium (IV) ions to be yellow/orange under acidic conditions. More preferably hydrogen peroxide or potassium peroxodisulfate. The yellow/orange color of the reaction solution allows a very clear degree of purification to be detected during the washing step d.

This reason has been found that the clear composition of titanium tends to adhere to the second when the pH is higher than 2. Extremely long lasting impurities of cerium oxide. It has been found that when yellow disappears in stage d, the purified cerium oxide (especially cerium oxide) usually has reached the desired purity, from which it can be washed with distilled water or demineralized water until neutral pH is reached. Ceria. To achieve such a peroxide indicator function, peroxide can also be added to the water glass in step b instead of step a, or in a third stream in step c. Basically, it is also possible to add peroxide only after step c and before step d or during step d.

Preference is given in particular to the addition of peroxides in step a or b, since in this case other functions can be provided in addition to the function of the indicator. Without being bound by a particular theory, it is assumed that certain impurities - especially those containing carbon - are oxidized by reaction with peroxide and removed from the reaction solution. Other impurities are converted by oxidation into a form having a better solubility and thus being washable. The precipitation process of the invention therefore has the advantage that no calcination step is required, but this step can of course be an option.

In all variations of the process of the present invention, the cerium oxide dissolved in the aqueous phase may preferably be an aqueous solution of citrate, more preferably an alkali metal and/or alkaline earth metal citrate solution, preferably water glass. These solutions are commercially available, obtained by liquefying solid citrate, prepared from cerium oxide and sodium carbonate, or, for example, directly from cerium oxide and sodium hydroxide and water at elevated temperatures via water heat. Made by law. The hydrothermal process may be superior to the sodium-base process because it can result in cleaner precipitation of ceria. One of the disadvantages of hydrothermal methods is that the range of available modules is limited; for example, SiO ₂ has a modulus of up to 2 for Na ₂ O, preferably 3 to 4; in addition, water after hydrothermal The glass usually needs to be concentrated before precipitation. In general, those skilled in the art will be aware of the manufacture of water glass itself.

In an alternative version, an alkali metal water glass (especially sodium water glass or potassium water glass) is optionally filtered and, if desired, concentrated. Filtration of the water glass or dissolved aqueous citrate solution for removing solid insoluble components can be carried out by a method known per se to those skilled in the art and using a device known to those skilled in the art.

The citrate solution used preferably has a modulus of from 1.5 to 4.5 (i.e., a weight ratio of metal oxide to cerium oxide), preferably from 1.7 to 4.2, more preferably from 2 to 4.0.

The precipitation method which can be used for the SiO ₂ composition of the present invention does not require the use of a chelating agent or an ion exchanger column. The calcination step for calcining the purified cerium oxide can also be dispensed with. Therefore, the precipitation method of the present invention is much simpler and less expensive than prior art methods. Another advantage of the precipitation process of the present invention is that it can be performed using conventional devices.

It is not necessary to use an ion exchanger and/or an acidulant for purifying the citrate solution prior to precipitation, but may be suitable for the quality of the aqueous citrate solution. Thus, the alkaline citrate solution can also be pretreated according to WO 2007/106860 to minimize boron and/or phosphorus content in advance. To this end, an alkali metal ruthenate solution (an aqueous phase in which cerium oxide is dissolved) may be treated with a transition metal, calcium or magnesium, a molybdenum salt or a molybdate-modified ion exchanger to minimize the phosphorus content. Prior to precipitation, an alkali metal citrate solution can be supplied to the precipitation operation of the present invention under acidic conditions (especially pH below 2) according to the method of WO 2007/106860. However, it is desirable to use an acidifying agent and a phthalate solution which have not been treated with an ion exchanger prior to precipitation in the process of the present invention.

In a particular embodiment, according to the method of EP 0 504 467 B1, the citrate solution can be pretreated in the form of a cerium sol prior to the actual acidic precipitation of the invention. For this purpose, the entire disclosure of EP 0 504 467 B1 is expressly incorporated into the present document. The oxime sol prepared by the process disclosed in EP 0 504 467 B1 is preferably completely dissolved again after treatment according to the method of EP 0 504 467 B1, and then supplied to the acidic precipitate of the invention to obtain the purified cerium oxide of the invention.

The citrate solution preferably has a cerium oxide content of about at least 10% by weight or greater prior to acidic precipitation.

Preferably, the citrate solution for acidic precipitation, especially sodium water glass, has a viscosity of 0.001 to 1000 Pas, preferably 0.002 to 500 Pas, especially 0.01 to 300 Pas, and particularly preferably 0.04 to 100 Pas (at room temperature, 20 ° C). The viscosity of the citrate solution can be preferably measured at a shear rate of 10 l/s, and the temperature is preferably 20 °C.

The step b and/or c of the first preferred variation of the precipitation method provides a viscosity of from 0.001 to 0.3 Pas, preferably from 0.001 to 0.2 Pas, more preferably from 0.002 to 0.19 Pas, especially from 0.01 to 0.18 Pas. It is a citrate solution of 0.04 to 0.16 Pas and extremely excellent 0.05 to 0.15 Pas. The viscosity of the citrate solution can be preferably measured at a shear rate of 10 l/s, and the temperature is preferably 20 °C. Mixtures of several citrate solutions can also be used.

In step b and/or c of the first preferred variant of the precipitation method, a viscosity of from 0.2 to 1000 Pas, preferably from 0.3 to 700 Pas, more preferably from 0.4 to 600 Pas, especially from 0.4 to 100 Pas and particularly preferably from 0.4 to 10 is provided. Pas is extremely excellent in a 0.5 to 5 Pas citrate solution. The viscosity of the citrate solution can be preferably measured at a shear rate of 10 l/s, and the temperature is preferably 20 °C.

In step c of the main aspect of the two preferred variants of the precipitation process, the citrate solution from step b is added to the initial charge, and the cerium oxide is thus precipitated. It should be determined here that there is always an excess of acidifier. The citrate solution is thus added in such a manner that the pH of the reaction solution is always less than 2, preferably less than 1.5, more preferably less than 1, more preferably less than 0.5 and particularly preferably from 0.01 to 0.5. An acidifying agent may be additionally added if necessary. The temperature of the reaction solution is maintained at 20 to 95 ° C, preferably 30 to 90 ° C, more preferably 40 to 80 ° C, by heating or cooling the precipitation vessel during the addition of the niobate solution.

When the citrate solution enters the initial feed and/or the precipitate suspension as droplets, a precipitate with excellent filterability is obtained. In a preferred embodiment, it is therefore prudent to have the citrate solution enter the initial charge and/or precipitate suspension in the form of droplets. This can be achieved, for example, by introducing the citrate solution into the initial charge in a dropwise addition. This may include providing a metering device on the outside of the initial feed/precipitate suspension and/or immersing in the initial feed/precipitate suspension.

In a first particularly preferred variant, i.e. a method using low viscosity water glass, it has been found to be particularly advantageous to set the initial feed/precipitate suspension to movement, for example by stirring or pumping cycles. The flow rate measured in the region defined by the half of the radius of the precipitation vessel ± 5 cm and the surface of the reaction solution 10 cm below the reaction surface is 0.001 to 10 m/s, Preferably, it is from 0.005 to 8 m/s, more preferably from 0.01 to 5 m/s, very particularly from 0.01 to 4 m/s, particularly preferably from 0.01 to 2 m/s and very particularly preferably from 0.01 to 1 m/s.

Without being bound by a particular theory, it can be assumed that all of the incoming citrate solution is only disturbed to a relatively low degree at the instant after entering the initial feed/precipitate suspension due to the low flow rate. This phenomenon causes the outer layer of the citrate solution droplet or the citrate solution stream to gel rapidly before the impurities can be encapsulated inside the particle. The optimum choice of flow rate for the initial feed/precipitate suspension thus results in improved purity of the resulting product.

This effect can be re-enhanced by the combination of the optimum flow rate and the ceric acid solution in the form of a substantially droplet of the introduction electrode, so a preferred embodiment of one of the precipitation methods is one in which the droplet form is tannic acid at a flow rate. The salt solution is introduced into the initial feed/precipitate suspension at a flow rate measured in the region "d" defined by the surface of the reaction vessel by half ± 5 cm of the radius of the precipitation vessel and 10 cm below the reaction surface. It is 0.001 to 10 m/s, preferably 0.005 to 8 m/s, more preferably 0.01 to 5 m/s, very particularly 0.01 to 4 m/s, particularly preferably 0.01 to 2 m/s, and extremely excellent 0.01 to 1 m/ s. In this way, cerium oxide particles having excellent filterability can also be obtained. Conversely, in the method of high flow rate in the initial feed/precipitate suspension, very fine particles are additionally formed.

In another preferred embodiment of the precipitation process, i.e., when high viscosity water glass is used, the result of the dropwise addition of the citrate solution is also a particularly pure precipitate having good filterability. Without being bound by special theory, it can be assumed that the high viscosity of the citrate solution, together with the pH, results in good filtration after step c. Sediment, and only very low concentrations (if any) of impurities are incorporated into the internal cavity of the cerium oxide particles, because the high viscosity makes the dropwise addition of the citrate solution substantially in the form of droplets, expressed in droplets The droplets are not finely distributed before the surface begins to gel/crystallize. The citrate solution used may preferably be an alkali metal and/or alkaline earth metal citrate solution as defined in the foregoing, preferably an alkali metal citrate solution, and particularly preferably sodium citrate (water glass). And / or potassium citrate solution. Mixtures of two or more citrate solutions can also be used. The alkali metal ruthenate solution has the advantage that the alkali metal ions can be easily removed by washing. Viscosity can be adjusted, for example, by concentrating a commercially available citrate solution or by dissolving citrate in water.

As described above, the viscosity and/or the stirring speed of the citrate solution are appropriately selected because the particles having a specific shape are obtained, so that the filterability of the particles is improved. Accordingly, it is preferred that the purified cerium oxide particles, especially the cerium oxide particles, preferably have an outer diameter of from 0.1 to 10 mm, more preferably from 0.3 to 9 mm, and most preferably from 2 to 8 mm. In a first particular embodiment of the invention, the cerium oxide particles are annular, i.e., have a hole in the middle, and thus have a shape equivalent to a small receptacle, also referred to herein as a "doughnut." The annular particles can assume a substantially circular or more elliptical shape.

In a second specific embodiment of the precipitation method of the present invention, the cerium oxide particles have a shape equivalent to "champignon head" or "jellyfish". In other words, not the pores of the aforementioned "doughnut"-like particles, in the middle of the annular basic structure, there is a layer of cerium oxide (thin thin, that is, thinner than the annular portion), one side is curved, and traverses the "" Ring "internal opening. If the particle surfaces are placed face down on the ground and viewed vertically from above, the particles will correspond to having a bend A curved base, a solider (ie thicker) upper edge and a slightly thinner base in the curved zone.

Without being bound by a particular theory, it can be assumed that the acidic conditions in the initial feed/reaction solution, along with the dropwise addition of the citrate solution, not only cause the viscosity and flow rate of the initial feed/precipitate suspension, but also the citrate. The surface of the solution droplets in contact with the acid immediately begins to gel/precipitate, and as a result of the droplets moving in the reaction solution/initial feed, the droplets are deformed. According to the reaction conditions, in the case where the droplet movement is slow, the "spice mushroom"-shaped particles are clearly formed; in contrast, if the droplet moves faster, a "doughnut"-shaped particle is formed.

The cerium oxide obtained after precipitation was taken out from the remaining components of the precipitate suspension. According to the invention, this is carried out continuously or semi-continuously.

"Semi-continuous" means that the precipitated cerium oxide is transported to a washing zone at regular intervals where it is washed. In this case, the precipitation reaction can be continued.

For example, the precipitation can be carried out until the precipitated cerium oxide has reached a minimum height in the precipitation vessel, after which a portion of the precipitated cerium oxide is transported to the washing zone.

In the continuous process, cerium oxide is continuously transported to the scrubbing zone, in which case cerium oxide precipitation can also be carried out until the cerium oxide precipitates in the precipitation vessel to a minimum height. At the same time, cerium oxide can be preferably added at a fixed rate.

Here, the transport can be carried out by means of a mechanical device, for example by means of a doctor blade, in which case the precipitated cerium oxide is fed into the washing zone via an arbitrarily closed orifice. Optionally, a conventional centrifuge can be used for removal.

In the precipitation vessel, in this case a supernatant can be formed which can be arbitrarily removed by overflow.

After the precipitate is transported to the washing zone, it is washed, and the appropriate washing medium should be used to ensure that the pH of the washing medium at the start of washing is less than 2, preferably less than 1.5, more preferably less than 1, and even more preferably 0.5. Good is 0.01 to 0.5.

The washing medium may preferably comprise an organic and/or inorganic water-soluble acid, such as the aforementioned acid or fumaric acid, oxalic acid, formic acid, acetic acid or other organic acids known to those skilled in the art, if it is not completely purified in high purity water. Removal will not cause contamination of the purified cerium oxide by itself. Thus, in general, it is preferred that all of the organic water-soluble acids, especially consisting of the elements C, H and O, act both as an acidifying agent and a washing medium, and thus do not themselves cause contamination of the subsequent reduction step. Preferably, the acidifying agent or mixtures thereof used in steps a and c are used in diluted or undiluted form.

The washing medium may also comprise a mixture of water and an organic solvent, if desired. Suitable solvents are high purity alcohols such as methanol or ethanol. Any possible esterification does not interfere with subsequent reduction to hydrazine.

The aqueous phase is preferably free of any organic solvents such as alcohols and/or any organic polymeric materials.

In the process of the invention, it is generally not absolutely necessary to add a chelating agent in the suspension of the precipitate or during the purification. Having said that, the present invention also contemplates a method of adding a metal complexing agent such as EDTA to a precipitate suspension or to a washing medium to stabilize the acid soluble metal complex. Therefore, the clamping reagent can be arbitrarily added to the washing medium, or the precipitated cerium oxide can be Stirring is carried out in a wash medium comprising a suitable reagent having a pH of less than 2, preferably less than 1.5, more preferably less than 1, more preferably 0.5 and particularly preferably from 0.01 to 0.5. However, the washing using an acidic washing medium is preferably carried out immediately after removal of the ceria precipitate and without performing any further steps.

Peroxides can also be added for color labeling as an "indicator" for unwanted metal impurities. For example, a hydroperoxide can be added to the precipitate suspension or wash medium to confirm the presence of titanium impurities by color. Markers can also generally use other organic binders that do not cause trouble during subsequent reductions. These are generally all complexing agents based on the elements C, H and O; the element N may suitably also be present in the tweaking agent, for example to form tantalum nitride, which is advantageously decomposed again in subsequent processes.

The scrubbing system continues until the cerium oxide has the desired purity. This can be confirmed, for example, by the washing suspension containing peroxide and visually no longer exhibiting any yellow color. If the precipitation method of the present invention is carried out without the addition of peroxide (forming a yellow/orange compound with Ti(IV) ions), a small sample of the wash suspension can be taken at each wash step and mixed with a suitable peroxide. Hehe. This operation was continued until the obtained sample no longer exhibited yellow/orange color under visual conditions after the addition of the peroxide. In this case, it should be determined that the pH of the washing medium should be less than 2, preferably less than 1.5, more preferably less than 1, more preferably 0.5 and particularly preferably 0.01 to 0.5, and purified cerium oxide, especially Ceria is also the same.

The cerium oxide washed and purified in this manner is preferably further washed with distilled water or demineralized water until the pH of the obtained cerium oxide is in the range of 0 to 7.5, and/or the conductance coefficient of the washing suspension is less than or equal to 100 μS/cm, preferably less than or equal to 10 μS/cm, more preferably less than or equal to 5 μS/cm. The pH at this time may more preferably be in the range of 0.5 to 4.0, preferably 0.6 to 3.5, especially from 0.7 to 3.0, more preferably 1.0 to 2.5. Washing media containing organic acids can also be used herein. This confirms that any troublesome acid residue adhering to the cerium oxide has been sufficiently removed.

This washing operation can preferably be carried out in a second or further washing zone. Therefore, when the process is performed, at least two washing zones may be provided, and the cerium oxide to be washed is continuously or semi-continuously passed into the washing zones, and the washing medium used in the first washing zone has a second washing. The washing medium used in the zone has a low pH. In the context of the present invention, the cerium oxide to be washed can be delivered to the second or further washing zone by the method detailed above.

In a preferred embodiment, the washing medium for washing the cerium oxide is introduced, and the flow rate is preferably 0.001 to 100 m/s, preferably 0.001 to 30 m/s, and particularly preferably 0.005 to 20 m/s. It is particularly preferably 0.01 to 15 m/s, very particular, preferably 0.1 to 10 m/s, and specifically, preferably 0.5 to 7 m/s and very particularly preferably 2 to 5 m/s. In this way, the cerium oxide particles can also have excellent filterability. Conversely, at high flow rates, very fine particles are additionally formed in the initial charge, and such particles cause a bimodal curve which is particularly advantageous for molding in the subsequent step.

Moreover, the ratio of the volume flow rate of the washing medium for washing the cerium oxide to the volume of the cerium oxide to be washed may preferably be in the range of 1 to 1000, more preferably 2 to 100 and particularly preferably 3 To the range of 10.

In addition, washing is preferably carried out using high shear, in which case precipitation The average particle size of the obtained cerium oxide particles is reduced by at least 10%, preferably by 30%, more preferably by at least 50%. In the context of the present invention, the average particle size can be determined, for example, according to ISO 13320-1 (1999), Particle size analysis - Laser diffraction methods - Part 1: General principles, preferably expressed by volume average.

When the process is performed, the washing zone may have a height of preferably up to 3 m, more preferably up to 2 m. This configuration is particularly advantageous in the case where the precipitated cerium oxide is washed at a high rate, and it is preferred to achieve a reduction in particle size.

The preferred parameters of the washing method detailed above may be applied (if this step is performed in cascade), for example, 2, 3 or more stages, a single stage, preferably a later washing stage, and more preferably a final washing stage, at least 2 Or more washing steps or all washing stages.

In a preferred embodiment, the washing can be carried out by flowing the washing medium from below towards the sediment in the narrow screen basket.

Removal can be carried out by customary measures, which are well known to those skilled in the art, such as filtration, decantation, centrifugation, and/or sedimentation, with limitations such that such measures do not allow acid precipitation to be purified by oxidation. The purity of hydrazine is again low.

The factory for carrying out the method of the present invention is hereinafter described with reference to the schematic drawings, but does not limit the invention.

10‧‧‧Second wash area

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1‧‧‧Precipitation area

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3‧‧‧ Feeding tube

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6‧‧‧First wash area

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Figure 1 shows a factory for carrying out the process of the invention.

Figure 1 shows a plant with a zone of precipitation 1 into which the precipitate is introduced via inlet 2. In the configuration shown, the precipitate is introduced into the precipitation zone 1 from the bottom, and the precipitate has a pH of less than 2. The flow rate as detailed above can be established by introducing a citrate solution from the top via another feed tube 3, for example, by adding a precipitate to the precipitation zone 1. At the same time, solid cerium oxide particles are formed as detailed above. Excess precipitate was taken from the precipitation zone via outlet 4. The precipitated cerium oxide particles are continuously or semi-continuously transported from the precipitation zone 1 to the first washing zone 6 via the outlet 5. Thus, the precipitation zone 1 may contain squeegees or similar internal components, but such components must not cause contamination of the precipitated cerium oxide. Therefore, these internal members, especially the squeegee, are made of a material taken out from the process of preparing bismuth from cerium oxide, and thus can be accepted. These internal components are not shown in the schematic 1.

The cerium oxide particles delivered to the first washing zone 6 are washed with a washing medium, and the flow rate and volume of the washing medium are preferably selected according to the parameters detailed above. In the context of the present invention, the washing medium preferably has a pH of less than 2, preferably less than 1. In addition, peroxide can be added as a color marker to measure the purity of cerium oxide. In the context of the present invention, the residence time of the cerium oxide can be adjusted such that the cerium oxide does not contain the titanium content which can be measured by the peroxide before being transported to the second or third washing zone. In the particular embodiment illustrated in Figure 1, the wash medium is introduced into the first wash zone 6 from the bottom via inlet 7 and exits from the upper region of the first wash zone 6 via outlet 8.

The cerium oxide particles purified in the first washing zone 6 are conveyed through the outlet 9. Within the second wash zone 10, the first wash zone 6 of the plant of Figure 1 may also contain internal components for this purpose, not shown. In the particular embodiment illustrated in Figure 1, the wash medium is introduced into the second wash zone 10 from the bottom via inlet 11 and exits from the upper region of the second wash zone 10 via outlet 12. The pH of the washing medium in the second washing zone 10 may preferably be higher than the pH of the washing medium used in the first washing zone 6. At the same time, the flow rate and volume of the washing medium can preferably be selected according to the parameters detailed above.

The cerium oxide particles purified in the second washing zone 10 are conveyed through the outlet 13 to the third washing zone 14, and the second washing zone 10 of the factory shown in Fig. 1 may also contain internal components for this purpose, not shown. In the picture. In the particular embodiment illustrated in Figure 1, the wash medium is introduced into the third wash zone 14 from the bottom via inlet 15 and exits from the upper region of the third wash zone 14 via outlet 16. The pH of the scrubbing medium in the third scrubbing zone 14 may preferably be higher than the pH of the scrubbing media used in the second scrubbing zone 10. At the same time, the flow rate and volume of the washing medium can preferably be selected according to the parameters detailed above.

The purified cerium oxide can be taken out of the factory via the outlet 17. To transport the particles, the third wash zone 14 can likewise comprise internal components.

The washing media and precipitates detailed above can be purified and reused by conventional methods. In a particular configuration, the outlet 16 of the third wash zone 14 can be coupled to the inlet 11 of the second wash zone 10. Furthermore, the outlet 12 of the second washing zone 10 can be connected to the inlet 7 of the first washing zone 6. This configuration allows the washing medium to be used in a particularly economical manner.

The resulting purified cerium oxide, especially high purity cerium oxide, can be dried and further processed to adjust the self-assembled SiO ₂ composition to water having a preferred ratio as detailed below.

Surprisingly, the process of the present invention produces a cerium oxide composition which can be removed in a high proportion of water without any significant complexity. For example, aqueous SiO ₂ can be left in place to achieve partial removal of water. The water content can surprisingly present the value of a sufficiently self-assembled SiO ₂ material as described below.

Further drying can be carried out by all processes and equipment known to those skilled in the art, such as belt dryers, step dryers, drum dryers and the like.

The plants used to carry out the process of the invention are novel and thus form part of the subject matter of the invention.

Preferably, the plant has at least one precipitation zone connected to the storage vessel of the citrate solution such that the citrate solution present in the storage vessel can be introduced into the precipitation zone from above, and at least one washing zone containing internal components Preferably, at least one squeegee is used to deliver solids to the precipitation zone, and the solids can pass from the zone into the wash zone in a controlled manner.

In addition, the plant may comprise at least two wash zones arranged in a cascade form that are connected to each other such that solids can be transported from the first wash zone to the second wash zone.

Surprisingly, by means of the process according to the invention, cerium oxide can be produced in a particularly simple and economical manner, and this cerium oxide can be converted into SiO ₂ mouldings of any shape in a particularly simple manner. The cerium oxide which can be obtained by the process of the invention also forms part of the subject matter of the invention.

In the context of the present invention, it is preferred that the cerium oxide has a broad particle size distribution. Further, the cerium oxide may have a bimodal or multimodal particle size distribution. The particle size distribution can preferably be according to ISO 13320-1 (1999), Particle size analysis-Laser diffraction methods-Part 1: General principles).

The cerium oxide of the present invention can be surprisingly shaped into a particularly strong, stable shaped body.

In the context of the present invention, cerium oxide is available as a binder and is conventionally compressed into a molding. Surprisingly, the cerium oxide can also be a self-assembled system whereby the free-flowing aqueous SiO ₂ composition of self-assembled cerium oxide can be poured into the mold. The aqueous self-assembled SiO ₂ composition preferably has a pH in the range of from 0.1 to 4.0, preferably from 0.2 to 3.5, especially from 0.5 to 3.0, more preferably from 1.0 to 2.5.

In this case, the free flowing aqueous SiO ₂ composition can be introduced into a mold of the desired size and distributed in any desired manner. For example, the import can be done manually or by machine using a distributor unit. The filled mold can be subjected to vibration to achieve a rapid and uniform distribution of the aqueous SiO ₂ composition in the mold.

The term "SiO ₂ composition" means a composition comprising SiO ₂ in different proportions of free and/or bound water, the degree of condensation of the cerium oxide itself is not critical to this composition. Thus, the term "SiO ₂ composition" also includes compounds having SiOH groups, which are also commonly referred to as polysilicic acids.

The aqueous SiO ₂ composition which is more advantageous for use is a self-assembled composition. The term "self-set type" that applies to the method of the present invention an aqueous composition SiO ₂ reversible self-cured state is converted into a free-flowing state. At the same time, it is preferred that there is no significant degree of sustained phase separation, and thus the water in the macroscopic evaluation is substantially uniformly distributed in the SiO ₂ phase. However, it should be emphasized in this paper that there are of course two phases at the micro level. The free-flowing state means that in the context of the present invention, the aqueous SiO ₂ composition has a viscosity of preferably up to 30 Pas, more preferably up to 20 Pas and particularly preferably up to 7 Pas, which is immediately after the preparation of the composition (about 2 after sampling) Minutes) were measured using a rotary rheometer (operating at a shear rate between 1 and 200 [l/s]) at about 23 °C. The introduction was carried out at a shear rate of 10 [l/s] for about 3 minutes. The blade rotor 22 (diameter 22 mm, 5 blades) was then measured using a Rheostress viscometer from Thermo Haake at a measurement range of 1 to 2.2 10 ⁶ Pas with a viscosity of about 5 Pas. The measurement was carried out at a shear rate of 1 [l/s] and the other set values were the same, and the measured viscosity was 25 Pas.

The aqueous SiO ₂ composition is in a cured state at a starting viscosity of preferably at least 30 Pas, more preferably at least 100 Pas. This value is the viscosity value measured using a rheometer 1 second after the start of the blade rotor of the rotary rheometer at about 23 ° C and a shear rate of 10 [l/s].

Preferably, the cured aqueous SiO ₂ composition can be liquefied by forming shear forces. For this purpose, conventional methods and devices well known to those skilled in the art can be used, such as mixers, agitating units or mills having suitable tool geometries for introducing shear forces. Preferred means include an intensive mixer (Eirich), a continuous mixer or a toroidal bed mixer, such as from Lödige; a mixing vessel with a mixing unit, preferably with a slanted or toothed disc; but also a mill, especially It is a colloid mill or other rotor-stator system that uses annular gaps of different widths and different speeds. Also suitable for use are ultrasonic-based devices and tools, especially ultrasonic generators, preferably with a supersonic source of curved actuators, allowing shear forces to be introduced into SiO ₂ -water in a particularly simple or defined manner. Inside the composition, causing it to liquefy. It is particularly advantageous that the tool here does not produce a particular abrasion. This ultrasonic configuration is preferably operated in a non-linear range. The apparatus used in the liquefaction of the aqueous SiO ₂ composition in this aspect of the invention is generally determined by the shearing force required for liquefaction. The surprising advantages are achieved in particular by means of a shear rate (recorded at the peripheral speed of the tool) in the range from 0.01 to 50 m/s, in particular in the range from 0.1 to 20 m/s, more preferably from 1 to 10 m. /s range. In the case of ultrasonic liquefaction, this rate is quite likely to reach the range of the speed of sound. The time elapsed for shearing depends on the shear rate of the continuous process, and is preferably in the range of 0.01 to 90 minutes, more preferably in the range of 0.1 to 30 minutes.

For curing the aqueous SiO ₂ composition, it is preferably retained for at least 0.1 minutes, preferably at least 2 minutes, especially 20 minutes, more preferably at least 1 hour. As used herein, the term "indwelling" preferably means that the composition is not exposed to any shear forces. Furthermore, the curing can be carried out or accelerated, for example, by energy input, preferably by heating or adding an additive. The additives in this case may be all crosslinkers well known to those skilled in the art, such as decane, especially functional decane, in which case, without limiting the invention, for example, TEOS (Si(OC ₂ H _{5) 4} ; tetraethoxydecane), which can advantageously be obtained in a pure and ultra-high purity. The additive may also be a substance which brings about an increase in pH, for example, a value which is preferably in the range of 2.5 to 6.5, more preferably 2.5 to 4, such as a basic compound, which can advantageously use an aqueous ammonia solution, which is advantageous. The best is added after the mold is cast.

Preferably, the cured aqueous SiO ₂ composition may have a water content in the range of from 2 to 98% by weight, especially from 20 to 85% by weight, preferably from 30 to 75% by weight, more preferably from 40 to 65% by weight. The water content of the free flowing SiO ₂ composition can be in the same range.

In certain configurations, having a relatively low water content of SiO ₂ composition can be mixed with the composition having a higher water content of SiO _2, in order to achieve the foregoing detailed description of the water content. The SiO ₂ composition used for this purpose is not necessarily an self-assembled composition, but it may have this property individually.

Further, it is noted that the cured aqueous SiO ₂ composition preferably has a pH of less than 5.0, preferably less than 4.0, especially less than 3.5, preferably less than 3.0, more preferably less than 2.5.

A cured aqueous SiO ₂ composition which has a pH greater than zero, preferably greater than 0.5, more preferably greater than 1.0, provides surprising advantages. The pH of the cured aqueous SiO ₂ composition can be determined by liquefying it using the resulting free flowing SiO ₂ composition. Conventional measurements can be used here, such as those suitable for measuring H ⁺ ion concentrations.

Can be prepared by the method of the present invention and in particular from the group of SiO ₂ type of purified forms of silicon dioxide in the preferred composition aspect, with extremely high purity can.

The preferred pure cerium oxide is characterized by IPC-MS and the following contents are determined by sample preparation known to those skilled in the art: a. The aluminum content is less than or equal to 1000 ppm, preferably between 100 ppm and 0.0001 ppm. , better system is between 10ppm and 0.0001ppm; b. boron content is less than 10ppm to 0.0001ppm; c. calcium content is less than 8ppm, preferably between 2ppm and 0.0001ppm; d. iron content is less than or equal to 50ppm, preferably between 10ppm and 0.0001ppm; e. The nickel content is less than or equal to 10 ppm, preferably between 5 ppm and 0.0001 ppm; f. the phosphorus content is less than 10 ppm to 0.0001 ppm; g. the titanium content is less than or equal to 10 ppm, preferably less than or equal to 1 ppm to 0.0001 ppm. ; h. The zinc content is less than or equal to 8 ppm, preferably less than or equal to 1 ppm to 0.0001 ppm; i. The tin content is less than or equal to 20 ppm, preferably less than or equal to 3 ppm to 0.0001 ppm. Preferred high purity cerium oxide It is characterized in that the sum of the aforementioned impurities (ai) is less than 1000 ppm, preferably less than 100 ppm, more preferably less than 10 ppm, still more preferably less than 5 ppm, particularly preferably between 0.5 and 3 ppm and extremely excellent between 1 and Between 3 ppm, and the purity in the detection limit zone can be the target of each element, especially the metal element. The value of ppm is by weight.

The determination of impurities was performed by ICP-MS/OES (inductively coupled spectroscopy-mass spectrometry/optical electron spectroscopy) and AAS (atomic absorption spectroscopy).

To produce a SiO ₂ molding which can be contacted with a carbon compound to obtain a metal ruthenium from one another, for example, a pellet shape suitable for use in a photo-arc furnace can be cast. These granules preferably do not have any turns and edges to minimize erosion. Suitable pellets may (especially) have a rounded cylindrical shape, preferably have a diameter of 25 to 80 mm, more preferably 35 to 60 mm, and a length to diameter (L/D) of preferably 0.01 to 100, especially 0.1 to 2, more preferably 0.5 to 1.2. Further, preferred pellets may be in the form of a truncated cone having a rounded edge, or a hemisphere. The size of the SiO ₂ molding is preferably in the range of 0.001 to 100 000 cm ³ , especially 0.01 to 10 000 cm ³ , more preferably 0.1 to 1000 cm ³ , particularly preferably 1 to 100 cm ³ , especially for a 500 kW furnace. Words. The size is directly dependent on the process system. The mold can be adjusted according to the process and technical aspects, for example, in the form of gravel or gravel, when it is fed through the pipeline, it is preferably a grit-type coal. Gravel may be advantageous when added directly.

There is no particular requirement for the casting mold used to make the molding, but it should be used so that no impurities enter the SiO ₂ molding. For example, an appropriate temperature of the casting mold may be from a pure metal polymer (poly-silicon oxide, PTFE, POM, PEEK) ceramic _{(SiC, Si 3 N 4)} , any form of graphite having high purity suitable coating and / Or made of quartz glass. In a particularly preferred embodiment, the mold is segmented to make demolding particularly easy.

In a preferred embodiment, the large mold can be made from a variety of materials at this time, such as a fabric-like braid or sieve composed of a high temperature resistant pure polymer (polyoxymethylene, PTFE, POM, PEEK). Mesh, ceramic (SiC, Si ₃ N ₄ ), carbon fiber, all forms of graphite, metal fibers with appropriate high purity coatings and/or quartz glass and/or combinations with glass fibers and/or carbon fibers. In a particularly preferred embodiment, the mold segments are particularly easy to demold by disassembly. Elastomeric heat resistant materials (for example in the manner of nylon socks) are particularly acceptable for continuous processes.

The high permeability of the water vapor and/or liquid water through the braid substantially improves the drying characteristics of the moulding. The fabric characteristics of the mold unexpectedly also impart some properties. For example, in the case of a tubular mold, there may be a stress-free shrinkage of the cast molding during the drying operation, making it particularly easy to demold without cracks.

After molding, the cured aqueous SiO ₂ composition is stabilized by an alkaline additive and/or by drying. To this end, the filled casting mold can be transferred to the dryer without additives or additives, and heated using a combination of, for example, electricity, hot air, steam, IR rays, microwaves or the like. In this case, conventional devices such as belt dryers, step dryers, drum dryers, and continuous or batch drying can be used.

Advantageously, the SiO ₂ molding can be dried to a water content that allows for non-destructive demolding of the mold from the casting mold. Therefore, when dried in a casting mold, it can be carried out until the water content falls below 60% by weight, especially below 50% by weight, more preferably below 40% by weight.

The operation of drying to a water content lower than the stated value is preferably after the SiO ₂ molding is demolded, in which case the dryer as described above may be used.

An amazing advantage, in particular after drying, the SiO ₂ moulding has a water content in the range from 0.0001 to 50% by weight, preferably from 0.0005 to 50% by weight, in particular from 0.001 to 10% by weight, more preferably from 0.005 to 5% by weight, by thermogravimetric method (IR moisture meter) known to those skilled in the art.

The cured aqueous SiO ₂ composition may preferably be dried at a temperature in the range of from 50 ° C to 350 ° C under standard conditions (ie, standard pressure), preferably from 80 to 300 ° C, especially from 90 to 250 ° C. More preferably, it is 100 to 200 °C.

The pressure for drying can be in a wide range, so the drying can be carried out under reduced pressure or under high pressure. Due to economic factors, it is preferred to dry at ambient pressure or standard pressure (950 to 1050 mbar).

To increase the hardness of the dried SiO ₂ molding, it can be thermally consolidated or sintered. This can be carried out, for example, in batches in a conventional industrial furnace (for example, a shaft furnace or a microwave sintering furnace), or continuously in, for example, a push furnace or a shaft furnace.

The sintered thermal consolidation is carried out at a temperature in the range of from 400 to 1700 ° C, especially from 500 to 1500 ° C, preferably from 600 to 1200 ° C, more preferably from 700 to 1100 ° C.

The duration of thermal consolidation or sintering depends on the temperature, the desired density of the SiO ₂ molding, and, if appropriate, the desired hardness. Thermal consolidation or sintering can preferably be carried out for 5 hours, preferably 2 hours, more preferably 1 hour.

The dried and/or sintered SiO ₂ molding having the above general dimensions may have a compressive strength (in terms of breaking force) of, for example, at least 10 N/cm ² , preferably more than 20 N/cm ² , and especially The sintered SiO ₂ molding may exhibit a compressive strength of at least 50 or even at least 150 N/cm ² , measured in each case using a pressure test of the configuration for the compressive strength test.

The density of the SiO ₂ molding can be used in conjunction with the end use. Generally, the SiO ₂ molding may have a density in the range of 0.6 to 2.5 g/cm ³ . If it is sintered at a high temperature, a density of 2.65 g/cm ³ (quartz glass density) can be achieved. In the case of a SiO ₂ molding used to prepare metal ruthenium, in a possible embodiment, in order to ensure good and uniform contact of a carbon source (e.g., cerium oxide) introduced later, the purpose is preferably in vivo. An amorphous structure with a high surface area. In this aspect of the invention, it is preferred that the SiO ₂ molding has a density in the range of 0.7 to 2.65 g/cm ³ , especially 0.8 to 2.0 g/cm ³ , preferably 0.9 to 1.9 g/cm ^{3 .} More preferably, it is 1.0 to 1.8 g/cm ³ . As indicated, the density is based on the density of the molding, so the test also includes the pore volume of the molding.

Further, a preferred SiO ₂ molding for preparing a metal ruthenium may have a specific surface area before compression of from 20 to 1000 m ² /g. After the thermal compression, preferably the specific surface area of 50 to 800m ² / g range, more preferably in line 100 to 500m ² / g range, and particularly preferred is 120 to 350m ² / g range, the above system by BET Method measurement. The nitrogen specific surface area (hereinafter referred to as BET surface area) of the SiO ₂ molding is determined by the multi-point surface area according to ISO 9277. The measuring instrument used was Micromeritics' TriStar 3000 surface area measuring instrument. The BET surface area is generally determined within the partial pressure range of the liquid nitrogen saturated vapor pressure of 0.05-0.20. The sample was prepared by heating the sample at 160 ° C for one hour at a low pressure, for example, in a Vacerp 061 degasser from Micromeritics.

In another embodiment, SiO ₂ moldings having a higher density may be preferred, preferably based at least 2.2g / cm ³ of density, more preferably based at least 2.4g / cm ^3. This embodiment can be used, for example, to produce crucibles in which the metal rhodium is purified by directed solidification.

The density and specific surface area of the dried moldings (e.g., pellets) can be controlled via, inter alia, the shear input, pH, temperature and/or water content of the SiO ₂ casting material. At a peer water content, the pellet density can be increased, for example, as the shear input increases. Further, the density can be adjusted by the pH and solid content of the SiO ₂ composition, and the decrease in the solid content is related to the decrease in density. A more pronounced effect on the density or porosity of the molding can be achieved in a subsequent sintering step. In this regard, the highest sintering temperature is of particular importance, as is the hold time at this temperature. As the sintering temperature and/or holding time increases, the molding may reach a higher density.

The SiO ₂ moulding can be further processed depending on the end use. In a preferred embodiment, the sintered SiO ₂ molding can be contacted with a carbon compound. For this purpose, the pure carbon source used may be one or more pure carbon sources, any mixture, natural organic compounds, sugar, graphite (activated carbon), coke, charcoal, soot, carbon black, hot black. , hot solubilized sugar, especially fumed sugar. The carbon source, especially the tablet type, can be purified, for example, by treatment with a hot hydrochloric acid solution. Additionally, an activator can be added to the process of the invention. The activator can be used for the purpose of a reaction initiator, a reaction accelerator or a carbon source. The activator is pure tantalum carbide, tantalum carbide tantalum and pure tantalum carbide having a carbon and/or tantalum oxide matrix, such as tantalum carbide containing carbon fibers.

In terms of loading, the SiO ₂ molding may have the mentioned carbon compound, preferably carbon black (technical grade carbon black; industrial grade carbon black), especially for hot black, lamp black or carbon black experts. It is known that the carbon black treated by the Kværner process; and/or sugar is preferably one or more mono- or disaccharides. These carbon compounds can be introduced via solutions and/or dispersions of such carbon compounds. Preferably, the porous SiO ₂ molding (preferably having a density and/or specific surface area of the aforementioned numerical range) may be impregnated with an aqueous composition comprising at least one sugar and/or carbon black. To improve the absorption of the composition into the porous body, it may be pre-exposed to a reduced pressure or exposed to a vacuum to remove the gas present in the pores. Thereafter, the resulting SiO ₂ moldings (all with at least one carbon compound) can be adjusted to a temperature higher than 500 ° C to pyrolyze the carbon compound.

In another aspect of the invention, the SiO ₂ molding can be used to make tantalum, wherein the metal tantalum can be purified by directional solidification. These crucibles generally have a multi-layer structure, and the outermost layer ensures mechanical stability. This layer can be formed, for example, from graphite. The other layer provides a chemical separation between the metal crucible and the support layer. This further layer is preferably formed from silicon dioxide-based, which may have better ready Si ₃ N ₄ layer.

The moldings which can be made by the method of the invention as detailed above are novel and can also form part of the subject matter of the invention.

The SiO ₂ moldings detailed above are preferably used in the method of preparing metal ruthenium, and can be used, for example, in the manufacture of solar cells.

The definition of metallurgy and solar energy is common sense. For example, solar enthalpy has a cerium content of greater than or equal to 99.999 weight percent.

Further steps and features of the method for preparing metal ruthenium are described in detail in WO 2010/037694. In this method, SiO _{2 is} reduced by carbon in a photo-arc furnace to produce a metal ruthenium. The starting materials used are generally a combination of a SiO ₂ moulding and a carbon source. </ RTI><RTIgt;</RTI><RTIgt;</RTI><RTIgt;</RTI><RTIgt;</RTI><RTIgt;</RTI><RTIgt;

10‧‧‧Second wash area

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14‧‧‧ third wash area

15‧‧‧ entrance

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1‧‧‧Precipitation area

2‧‧‧ entrance

3‧‧‧ Feeding tube

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6‧‧‧First wash area

7‧‧‧ entrance

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Claims (15)

A process for the preparation of high purity SiO ₂ comprising the steps of: a. preparing an initial charge from an acidulant having a pH of less than 2; b. providing a citrate solution; c. adding a citrate solution from step b to the step The initial feed of a is such that the pH of the formed precipitate suspension is maintained at a value less than 2; and d. the formed cerium oxide is removed and washed; the method is characterized by continuous or semi-continuous self-precipitation zone The cerium oxide obtained by the precipitation is taken out and washed in a washing zone separated from the precipitation zone. The method of claim 1, wherein the washing is carried out under high shear, and the average particle size of the cerium oxide particles prepared by the precipitation method is reduced by at least 30%. The method of claim 1 or 2, wherein the washing zone is in a cascade form such that at least two washing zones are provided, and the cerium oxide to be washed is passed continuously or semi-continuously into the washing zones. A method of claim 1 or 2 wherein a portion of the wash medium has a pH of less than 2 for at least a short period of time. The method of claim 1 or 2, wherein at least two washing zones are provided, the cerium oxide to be washed is continuously or semi-continuously passed into the washing zones, and the washing medium used in the first washing zone has A lower pH than the wash medium used in the second wash zone. For example, the method of claim 5, wherein the second washing zone The washing medium used has a pH in the range of 0.5 to 4. The method of claim 1 or 2, wherein the washing zone has a height of at most 2 m. The method of claim 1 or 2, wherein the washing medium for washing the ceria is passed through the ceria at a flow rate of 0.001 to 100 m/s. The method of claim 1 or 2, wherein the ratio of the volumetric flow rate of the washing medium used to wash the cerium oxide to the volume of the cerium oxide to be washed is in the range of 1 to 1000. The method of claim 1 or 2, wherein the washed aqueous cerium oxide is left to stand still to achieve partial removal of water. A plant for carrying out the method of any one of claims 1 to 10, wherein the plant has at least one precipitation zone connected to a storage vessel of a citrate solution such that it is present in the storage vessel The citrate solution can be introduced into the precipitation zone from above, and at least one washing zone comprising internal components, preferably at least one scraper capable of transporting solids into the precipitation zone, the solids being self-contained in a controlled manner The zone passes through the wash zone. A plant according to claim 11, wherein the plant comprises at least two washing zones arranged in a cascade form, the zones being connected to each other such that solids can be transported from the first washing zone to the second washing zone. A cerium oxide obtainable by the method of any one of claims 1 to 10. For example, the cerium oxide of claim 13 of the patent scope, wherein the two The lanthanum oxide system has a broad particle size distribution. The cerium oxide according to claim 13 or 14, wherein the cerium oxide has a bimodal or multimodal particle size distribution.