WO1999035083A1 - Method for producing homogenous and stable oxidized preliminary product suspensions - Google Patents

Abstract

The invention relates to a method for producing a homogenous suspension that can be used in the production of oxidic supraconductive powders, wherein poorly water-soluble or water insoluble oxidized preliminary products from groups Mg, Ca, Ba, Sr, Bi, Pb, Cu, Al, Zr, Hf, Sc, Y, Tl, Pt, Ag, Hg and rare earth elements are dispersed in water. The suspension thus obtained is electrostatically stabilized at a pH value ranging from 1.5 to 5.

Description

description

Process for the production of a homogeneous and stable suspension from oxide precursors

The invention relates to a method for producing a homogeneous and stable mixture of several inorganic solids by dispersing these substances in water. The resulting suspension is subsequently dried and then further processed so that a high-temperature superconducting mixed oxide consisting of the elements of these substances and oxygen is formed.

For the production of high-temperature superconductors with a high current carrying capacity, it is necessary to optimize the material with regard to chemical purity, homogeneity, phase purity, phase composition, degree of crystallization and fineness. These materials are mixed oxides which, in addition to oxygen, predominantly contain one or more earth metal, copper and one or more elements from the group bismuth, lead, yttrium, lanthanide, thallium or other oxides. The raw materials required to produce the desired chemical composition must be mixed very homogeneously and thermally converted to the mixed oxides with defined crystallographic phases in order to obtain high-quality end products.

In order to achieve the highest possible chemical homogeneity and high reactivity in the formation of the mixed oxides, very different processes are proposed in the literature for the production of the high-temperature superconducting powders.

Mixing the respective oxides or carbonates in the case of alkaline earth metals is a common method, but does not lead to high quality end products. With this method, because of the insufficient reactivity of the powder, long reaction times during the thermal treatment are required, so that relative coarse-grained mixed oxide powders are formed. In addition, the required chemical purity with regard to a low residual carbon content is difficult to achieve due to the difficult thermal decomposition of alkaline earth metal carbonates or new carbonate formation.

In spray pyrolysis, solutions containing metal nitrates are sprayed in the presence of organic substances in a high-temperature reactor and converted to the oxide (EP 0 473 621; EP 0 369 117). These processes are technically complex and are difficult to reproduce with regard to the chemical composition of the end products, since volatile metal compounds can form under these conditions. In addition, the environmental impact of waste products is high, which in turn can only be avoided with increased technical effort.

The sol-gel process in which an aqueous solution of metal salts z. B. in the form of acetates with water and acetic acid z. B. is mixed with an esterification product of citric acid and ethylene glycol, the transfer from the sol to the gel state and then the dewatering. This process is technically complex and is usually only practiced on a laboratory scale.

The mixed precipitation of nitrates or chlorides, which are dissolved in water, the solutions being mixed and precipitated with oxalic acid as water-insoluble or sparingly soluble metal oxalate mixtures, is described in the patents EP 0 117 059; EP 0 522 575; EP 0 285 392; EP 0 302 830; EP 0 912 450; US 5,298,654. In order to bring all nitrates into solution during the production of a compound of the system Bi, Pb, Sr, Ca, Cu, a pH value <1 is set at which the alkaline earth metal oxalates which subsequently precipitate are water-soluble again. It is therefore either with a strong excess of the precipitant in the form of oxalic acid or with the addition of organic solvents such as. B. ethanol or worked with ammonium oxalate or sodium oxalates as a precipitant.

The technical implementation of these processes requires a high level of technical Effort for the disposal or reuse of waste products in the form of the ammonium hydroxide or the aqueous solution of the nitrate acid residue with a pH <1, which may contain ethanol and also contains excess oxalic acid. Sodium impurities from the use of sodium oxalates as precipitants are also undesirable for these substances or the end products.

The spray drying of such mixed precipitation products, which are usually only present in a small concentration in the "mother liquor" in the order of 10% as a solid, is unfavorable from an energy technology point of view. In addition, thermal reactions can result in volatile products which are discharged with the exhaust air and influence the chemical composition of the successor products in a non-reproducible manner.

For drainage without the influence of temperature e.g. Filtration, decanting and centrifugation must prevent the precipitates from redissolving, for which only a narrow pH range can be used. To complete the precipitation, proportions of organic solvents are again added, which have to be separated from the filtrate by distillation at great technical expense. In these processes, especially when decanting and centrifuging, separation phenomena can also occur due to the different densities of the raw materials used.

The above Disadvantages are avoided in the process according to the invention by producing an aqueous suspension with the oxide precursors of the elements to be introduced into the raw material mixture in the form of oxalates, hydroxides or acetates or mixtures thereof. These oxide precursors have the high reactivity required for the later formation of mixed oxides, which is one of the prerequisites for high-quality end products.

The invention was based on the object of a method for producing a homogeneous, stable in terms of rheological properties and for the Spray drying to produce a suitable aqueous suspension in which the inorganic raw materials are dispersed according to the later chemical target composition.

The object is achieved with a method for producing a homogeneous

Suspension which can be used for the production of oxidic superconducting powders, in which precursors of the oxides from the group consisting of Mg, Ca, Ba, Sr, Bi, Pb, Cu, Al, Zr, Hf, Sc, which are sparingly water-soluble or water-insoluble, Y, Tl, Pt, Ag, Hg and rare earth elements (= SE) are dispersed in water, the suspension formed here being electrostatically stabilized at a pH in the range from 1.5 to 5.

Among the rare earth elements, lanthanides including yttrium are particularly suitable, and particularly preferred among these are yttrium, lanthanum, cerium, neodymium, samarium, ytterbium and luthetium.

This is used to produce a well-dispersed, homogeneous, long-term stable slip suitable for spray drying. The hydrolysis properties, the surface charges of the products and the control as well as the setting of a narrow pH value range of the suspension must be taken into account. A combination of electrostatic and static stabilization of the suspension is preferably selected.

The electrostatic stabilization is based on the principle that the electrical charge present on the solid surface is identified and modified in such a way that sufficiently high and similar charges are present, which thus lead to the repulsion of the solid particles. This means that the existing agglomerates caused by Van der Waals forces are eliminated and homogeneous mixing of different raw material components can take place. The surface charges are modified by using anionic surfactants as dispersants in the case of negative charges, which increase the amount of charges and thus lead to an improvement in particle repulsion. Positively charged material components are reloaded with the appropriate amount of surfactant before mixing with negatively charged substances.

For better stabilization of this dispersed, electrostatically stabilized state and especially in multi-component systems whose components have different densities, a steric (spatial) stabilizing effect is achieved by additionally adding and incorporating polymers with chain lengths of> 2000, preferably> 3000 monomer units between the solid particles .

As polymers, water-soluble substances which do not decompose in the pH range from 1.5 to 5 or in the preferred range from 2.5 to 4.5 can preferably flocculate or strongly thicken out of the solution, such as polyglycols, for example. Polyoxazolines, polyester urethane, polyvinyl alcohol and cellulose ether can be used.

In this way it is possible to disperse oxide precursor mixtures which contain so-called heavy elements such as Bi and Pb or Y in addition to “light” elements such as Sr, Ca or Ba, and to stabilize the suspension in such a way that no sedimentation of heavy components or other segregation takes place within this suspension.

It was surprisingly found that to homogenize the suspension and

Deagglomeration of the solid components no grinding aggregates with the use of grinding balls, as it corresponds to the state of the art in the ceramic industry, are required. The dispersion is preferably achieved with the aid of dissolvers, agitators or, if appropriate, mixing units which operate on the stator-rotor principle and have increased peripheral speeds, such as colloid mills. According to the invention, by selecting the raw materials, a pH value which is favorable for the dispersion behavior is set in the range from 1.5 to 5, preferably in the range from 2.5 to 4.5, at which the inorganic components of the batch are insoluble or only slightly soluble in water are. This avoids working in the area of the isoelectric points, where dispersion is difficult or impossible due to the absence of repulsive particle charges. In the method according to the invention, there are sufficiently large and similar negative surface charges on the solid particles which lead to compensation of the van der Waals adhesion forces, to their repulsion, to complete dispersion and thus to the homogeneous distribution of the various raw material components. This results in electrostatic stabilization.

If, in addition, a polymer with a number of monomer units of at least 2000, preferably at least 3000, is attached between the solid particles, the particles are kept spatially at a distance, and steric stabilization also takes place. In addition to electrostatic stabilization, steric stabilization ensures an even more homogeneous distribution of the different cations and long-term stabilization of the suspension despite the solid components, which tend to hydrolysis. Without steric stabilization, the effect of the dispersants or surfactants could decrease due to the hydrolysis.

The influencing and control of the pH value is also of particular importance for setting the slip viscosity required for spray drying and its constancy over time.

The pH of the various components of the superconductor precursor is different and is subject to a different change over time due to hydrolysis when introduced into water. Table 1 contains this information for the raw material components of a BiPbSrCaCu oxide superconductor. It follows from this that the pH is very strong, in particular due to the relatively high proportion of Bi in these substances influenced and shifted towards lower values. All other components, in particular Ca oxalate, but also bi-hydroxide act in the opposite direction.

Another measure that goes far beyond the current state of the art in the manufacture of high-temperature superconductors is the pH adjustment of a suspension by selecting the type and amount of the raw materials used for a certain chemical composition of the end product, in particular in the form of oxalates and hydrates In this way, the pH value can be kept within a narrow range even with different chemical element compositions if certain proportions of Bi-oxalate are replaced by Bi-hydroxide.

This also prevents the oxalates of Cu, Ca, Sr, which are soluble in water at pH values <2.5, from being dissolved and discharged as volatile substances or as fine particles with the exhaust air stream during spray drying. Substantial Cu fractions can also be dissolved by complex formation at pH> 5. In this regard, the suspension can only be rheologically controlled if no or only insignificant proportions of cations are dissolved. For this reason, too, the adjustment of the pH of these suspensions is of particular importance.

Table 1: Time dependence of the pH values of water-dispersed oxalates and bi-hydroxide (10%)

Surprisingly, it was found that the thickening of the slip can be avoided and the slip viscosity, as is aimed for later spray drying, can be reduced if the oxalates and the proportion of hydroxide are first introduced into the water and by hydrolysis on the surface of the solid particles OH - Groups are attached. Then a dispersant or surfactant, for example with NH ₄ ⁺ and COOH ^" groups, is introduced into the suspension, which stabilizes the suspension electrostatically. The NH ₄ ⁺ group will attach to the hydrolyzed solid surface and become the negative charge the COO ^" group against the other inorganic components in the suspension. This initially leads to an increase in viscosity, which is compensated for again after about 5 hours after the active group of the dispersant has completely deposited on the solid surface. Even after a long standing time of 24 hours and beyond, the viscosity changes only slightly and the slip is easy to spray.

With these measures according to the invention it is possible to disperse a five-component system in water, which has a strong tendency to hydrolysis, such as, for example, BiPbSrCaCu oxalates and hydrates, which contains both high-density and low-density components, and to avoid the suspension Thickening, segregation through sedimentation, flocculation and Stabilize viscosity increase. In addition, contrary to expectations, a high solids content of 50% was achieved. As a result, spray drying using the process according to the invention is considerably less expensive than in the case of a mixed precipitation with approximately 10% solids content.

With the use of oxalates and hydroxides, apart from CO ₂ and water or water vapor during spray drying and subsequent calcination, no further waste products are produced, which distinguishes the process according to the invention from the prior art described in the introduction in the field of the production of high-temperature superconductor materials .

characters

FIGS. 1 to 3 show the viscosities (η) of the suspensions as a function of the shear rate gradient (D), measured with a Rotovisko RV 20 from Haake. Due to the deviating from the spherical shape

Geometry of the solid particles results in the strong increase in viscosity for most non-spherical inorganic powders at very low shear forces or without any shear, which is referred to as the structural viscosity. The lower the viscosity at low shear forces, the more advantageous this is

Pumping and spraying behavior. A high viscosity with low shear forces can cause the dispersion to thicken in the area of the pump, associated lines and / or spray nozzle and lead to blockages. This danger also increases because - especially in the area of the spray nozzle - elevated temperatures occur. The suspensions in FIGS. 1 to 3 had a solids content of 50

% on.

The processing process of the suspensions is characterized by constant movement of the suspension by stirring or conveying by means of pumps or atomizers, so that good processing properties are available at viscosities η <100 mPa s at shear rates> 100 1 / s. Examples

The following examples are intended to explain the invention in more detail without restricting it.

Example 1 corresponds to a substitution of 12.5% of the bi-oxalate by bi-hydrate, as a result of which the pH was in the range from 3 to 4, which is particularly preferred, both immediately after preparation of the suspension and after storage for 24 hours. Example 2 describes the production of a lead-free material, with 15% of the bi-oxalate being replaced by the equivalent amount of bi-hydroxide.

If a measurement value deviating from this pH range is found in an already prepared suspension due to fluctuations in the raw material properties, the pH value can be adjusted by adding small amounts of an organic acid, e.g. Oxalic acid to lower values or by adding hydroxides such as e.g. Ammonium hydroxide to be corrected to higher values.

Comparative Example 3 describes an approach which contained no Bi (OH) ₃ for pH correction and no polymer for steric stabilization. The too low pH of approx. 2 was outside the range required for liquefaction and slip stabilization, where the solution of alkaline earth metal oxalates and copper oxalate must also be expected.

In Comparative Example 4, a batch was weighed which, after substitution of 50% of Bi ₂ (C ₂ 0 ₄ ) _{3, contained} an excessively high proportion of Bi (OH) ₃ and thus had pH values> 5 after storage for more than 3 h . At pH values> 5, water-soluble Cu complex compounds can form under the given conditions after a relatively short time. Accordingly, under the above-mentioned conditions no slip which is stable over the long term with regard to viscosity constancy can be produced.

Example 1 The following raw materials were weighed out for a 1 kg batch: 304 g Bi ₂ (C ₂ 0 ₄ ) ₃ ; 30 g Bi (OH) ₃ ; 51 g Pb (C ₂ 0 ₄ ); 220 g Sr (C ₂ 0 ₄ ); 145 g Ca (C ₂ 0 ₄ ); 250 g Cu (C ₂ 0 ₄ ).

A pH of 3 to 4 is set by the amount and type of raw materials selected.

The raw materials were stirred into 1 liter of deionized water. After one hour, 10 g of ammonium polyacrylate solution with 25% active substance content were added as a dispersing agent with further stirring. This was followed by homogenization for about 1 minute using an agitator operating on the rotor-stator principle at a peripheral speed of approximately 20 m / s.

With further stirring, 20 g of polyethylene glycol with a chain length of 20,000 monomer units PEG 20000 were then added for steric stabilization. After all components had been introduced, the pH was initially 4.0 and decreased to 3.46 after 3 h and to 3.42 after 24 h. Figure 1 shows the slip viscosity η as a function of the shear rate D after immediate measurement (V28) and after 1 h (V28-1), 3 h (V28-2) and 24 h (V28-3). Since the viscosity was below 100 mPa s after immediate measurement and after 24 h at shear rates> 100 1 / s, these suspensions are considered to be sprayable, taking into account the time effect, and can be spray dried with a spray dryer that works with a two-substance nozzle.

Example 2 (According to the Invention)

The following raw materials were weighed for a 1 kg batch of a lead-free material:

414g Bi ₂ (C ₂ 0 ₄ ) ₃ ; 48g Bi (OH) ₃ ; 254g Sr (C ₂ 0 ₄ ); 82g Ca (C ₂ 0 ₄ ); 202 g Cu (C ₂ 0 ₄ ).

The suspension was produced according to Example 1.

After all components had been introduced, the pH was initially 4.2 and decreased to 3.6 after 3 h and to 3.5 after 24 h. After an immediate measurement, the slip viscosity η was 80 mPa s (at D = 100 1 / s; in analogy to Example 1) and after 24 h η = 75 mPa s was determined. The As in Example 1, the suspension was easy to spray and dry.

Example 3 (comparative example):

The following raw materials were weighed for a 1 kg batch (without Bi (OH) ₃ for pH

Value correction):

342 g Bi ₂ (C ₂ 0 ₄ ) ₃ ; 50 g Pb (C ₂ 0 ₄ ); 217 g Sr (C ₂ 0 ₄ ); 143 g Ca (C ₂ 0 ₄ );

248 g Cu (C ₂ 0 ₄ ).

The suspension was produced according to Example 1, but without the addition of polyethylene glycol for steric stabilization.

After all components had been introduced, the pH was initially 1.9 and increased to 2.0 after 3 h and to 2.06 after 24 h.

After an immediate measurement (V27), the slip viscosity η was 130 mPa s (at D =

100 1 / s) and after 24 h (V27-3) η = 300 mPa s was determined (see Figure 2; V27-1 and V27-2 after 1 h and 3 h of idle time). The viscosity did not decrease significantly even after an even longer service life, and the suspension was therefore unsuitable for spraying.

Example 4 (comparative example):

The following raw materials are weighed for a 1 kg batch which, after substitution of 50% of Bi ₂ (C ₂ 0 ₄ ) _3, contained too much Bi (OH) ₃ and therefore too high pH values: 179 g Bi ₂ (C ₂ 0 ₄ ) ₃ ; 128 g Bi (OH) ₃ ; 53 g Pb (C ₂ 0 ₄ ); 222 g Sr (C ₂ 0 ₄ ); 149 g Ca (C ₂ 0 ₄ ); 261 g Cu (C ₂ 0 ₄ ).

The suspension contained no polymer for steric stabilization (ie no addition of polyethylene glycol) and was otherwise prepared as described in Example 1. After all components had been introduced, the pH was initially 4.8 and increased to 5.2 after 3 h and to 5.6 after 24 h. The slip viscosity η after immediate measurement (V26) was 120 mPa s (at D = 100 1 / s) and after 24 h (V26-3) η = 450 mPa s was determined (see Figure 3; V26-1 and V26-2 after 1 h and 3 h of idle time). The viscosity did not decrease significantly even after an even longer service life, and the suspension was therefore unsuitable for spraying.

The comparison of the examples according to the invention and those not according to the invention illustrated the influence of the pH adjustment by substitution of bi-oxalate with bi-hydrate. The positive influence of the combination of dispersant and a long-chain polymer has been determined on the basis of the approximately same slip viscosity after the dispersant and polymer have been deposited on the particle surface and after prolonged storage (e.g. 24 h) (see FIG. 1).

Contrary to the original expectations, the combination of pH adjustment (e.g. by combining oxalate and hydrate additives), selection of the type and amount of dispersant and addition of another polymer made it possible to solve the problem from a five-component system of oxide precursors that tended to hydrolyze to produce a long-term stable suspension for spray drying suitable for high-temperature superconductor materials.

Claims

claims 1. A process for the preparation of a homogeneous suspension which can be used for the production of oxidic superconducting powders, in the sparingly water-soluble or water-insoluble precursors of the oxides from the group of Mg, Ca, Ba, Sr, Bi, Pb, Cu, Al , Zr, Hf, Sc, Y, Tl, Pt, Ag, Hg and rare earth elements are dispersed in water, characterized in that the suspension formed at a pH in the Range from 1, 5 to 5 is electrostatically stabilized. 2. The method according to claim 1, characterized in that a steric stabilization is additionally used. 3. The method according to claim 1 or 2, characterized in that hydroxides, citrates, oxalates, tartrates and / or acetates are used as precursors. 4. The method according to any one of claims 1 to 3, characterized in that anionic surfactants are added as dispersants to the intermediate products dispersed in water. 5. The method claim 4, characterized in that the dispersed in water as a dispersant polymers from α, ß-ethylenically unsaturated carboxylic acids, preferably their salts or water-soluble copolymers with esters of ethylenically unsaturated Carboxylic acids are added. 6. The method according to any one of claims 1 to 5, characterized in that in addition to the dispersant, polymers with chain lengths> 2000, preferably polymers with chain lengths> 3000 are used. 7. The method according to claim 6, characterized in that as polymers Polyglycols, polyoxazolines, polyester urethane, polyvinyl alcohols and cellulose ethers can be used for stabilization. 8. The method according to any one of claims 1 to 7, characterized in that the pH of the suspension to avoid the dissolving of components of the inorganic solid and to reduce and long-term stabilization of the slip viscosity is kept in a fluctuation range of up to two units, the pH is set within these limits in the range from 1.5 to 5, preferably in the range from 2.5 to 4.5, very particularly preferably in the range from 3.0 to 4.0. 9. The method according to any one of claims 1 to 8, characterized in that the pH is adjusted by varying the proportions of oxalates and hydroxides of the same element and / or by adding an organic acid and / or a further hydroxide. 10. The method according to any one of claims 1 to 9, characterized in that the pH of bismuth-containing materials by substitution of portions of bismuth oxalate by bismuth hydroxide in an amount of 5 to 25 % By weight, preferably from 7.5 to 15% by weight, is set. 11. The method according to any one of claims 1 to 10, characterized in that the suspension is dispersed with stirrers, dissolvers and / or with systems which operate on the stator-rotor principle. 12. The method according to any one of claims 1 to 11, characterized in that the suspension is spray-dried, filtered and / or dewatered by means of a belt dryer or other methods. 13. The method according to any one of claims 1 to 12, characterized in that the powders are converted into mixed oxides from the group of the following compositions after the thermal decomposition of the precursors during the further thermal treatment: Bi-Ea-Cu-O, (Bi, Pb) -Ea-Cu-0, Y-Ea-Cu- O, (Y, SE) -Ea-Cu-0, Tl-Ea-Cu-O, (TI, Pb) -Ea-Cu-0 or TI- (Y, Ea) -Cu-0, where Ea for alkaline earth elements and especially for Ba, Ca or / and Sr stands. 14. The method according to claim 13, characterized in that high temperature superconducting wires, strips, rods, tubes, hollow or solid bodies are made from the mixed oxide. 15. The method according to claim 14, characterized in that the mixed oxide power cables, power lines, transformers, windings, magnets, magnetic bearings and / or power supplies are made.