DE3617687A1 - Process for preparing oxide-ceramic powder mixtures suitable for preparing pressable granular material (granulate) for the manufacture of sintered ceramic materials - Google Patents

Abstract

Process for preparing oxide-ceramic powder mixture suitable for preparing pressable granular materials for the manufacture of sintered ceramic materials of high homogeneity, in particular of ferrites for inductive components, of hot conductors, cold conductors, ceramic resistors, varistors and ceramic capacitors, in which a mixture having a viscosity of a few poise and comprising at least 60, preferably from 75 to 85, % by weight of solids, the remainder being water and substantially inert fluidiser, is prepared, and the wet mixture thus obtained is milled and mixed to make it homogeneous. The solids reactivity required for the sintering process and the required level of bulk density of the granular material to be prepared which is desired for pressing is here determined by the following parameters: solids components, solids content, fluidiser content, fluidiser viscosity, milling time and intensity of milling.

Description

The invention relates to a method for manufacturing of oxide ceramic powder mixtures, suitable for Manufacture of pressable granules for manufacturing sintered ceramic materials of high homogeneity, in particular of ferrites for inductive components, of Thermistors, PTC thermistors, ceramic resistors, Varistors and ceramic capacitors.

The production of oxide ceramic powder mixtures, e.g. B. from ferrite powder mixtures to the pressable So far, granules have been made by dry or wet mixing of the powdered raw material components that are pre-sintered, finely ground and then by spraying or mixing granulation. When wet mixing must be the powder mixture is dried before presintering and in addition, depending on the type of presintering, in Sintered boats filled or dry mixed in Rotary kilns are formed into pellets. The pressable Granules are finally sprayed with one Binder offset, sufficiently liquid mass, as Spray granules, or by adding a binder solution, usually a polyvinyl alcohol solution, to which pre-dried regrind, obtained as mixed granulate.

All of the procedural steps mentioned require numerous control measurements and extremely difficult control measurements mechanisms.  

The number of process steps and associated with it ne cost and especially energy expenditure reduce considerably by not presintering gladly. However, a prerequisite for this is a corresponding one designed mixing process, as that required for sintering high homogeneity of the starting or raw material components depends solely on the mixing intensity, which according to the current technology level only by a wet mixing process in sufficient quality is guaranteed. A dry mixing process that be of sufficient mixing quality especially for those with complex morphological properties The "cheap raw materials" provided is still not known.

In this wet mixing process, the granules must also be added Filling density of the powder can be defined as defined the determining size for those with permissible pressure achievable raw part density is. Must be controlled at this wet mixing process also the sinter activity, the shrinkage behavior, the sintered density and finally a large part of the electromagnetic properties, also the dimensional accuracy of the manufactured and sintered by pressing the granulate Products have a decisive influence.

As experience shows, the granulate fill density and the sinter reactivity in the usual wet mixing process without additional pre-sintering and grinding processes only in a small with a larger range of raw materials or with fluctuating ones Raw material properties even insufficient controllable. This low controllability also means usual wet mixing process very long and therefore inn socially unacceptable mixing times and thus associated high iron abrasion with all its disadvantages and  excludes the use of particularly cheaper, inactive Raw materials.

The mix behaves similarly in other liquids as water, e.g. Acetone or ether, where one sufficient miscibility, but hardly any worth mentioning Grinding effect, at least not economically represented times can be reached.

The present invention therefore has the task ge represents the previously common technology, which is under other by the process features mentioned at the beginning, so z. B. by the pre-sintering and subsequent fine grinding and if presintering is dispensed with, by Uncontrollability of reactivity and granulate filling characterized by the disadvantage in the wet mixing process, by to replace a new technology that is waived the pre-sintering and despite the sole wet mixing of the Raw material components the controllability of the above sizes up to high quality materials in enabled economically.

To achieve this object, the invention provides one Method according to the preamble of the claim, that a mixture of a viscosity of a few poise, be consisting of at least 60, preferably 75 to 85% by weight oxide ceramic solid content rest water and far inert liquefier is produced, and that the wet mixture thus obtained is homogeneously ground and ge is mixed, the one required for the sintering process Reactivity of solids and those for pressing aspired degree of granule fill density to be manufactured Granules is determined by the following sizes:  Solid components, solids content, liquefaction medium content, plasticizer viscosity, grinding time and grinding intensity.

In this wet mixing process, one is untrue Lich high solids worked, with a not agile, but precisely sufficient liquefaction of the Powder mixture of the raw material components and thus one desired low viscosity through doped additions of Water and a liquefier is reached. Suitable as a liquefier e.g. an ammonium salt a polyacrylic acid in an amount of about 3% by weight is added to water.

The sufficiently good ver liquid ensures a high mixing quality of the Raw material components and therefore an excellent meal effect in the mixing process, via which the granulate filling density and sinter reactivity to a large extent and is controllable. In contrast to the usual wet grinding comes in water with solids content of about 50% by weight under the action of the liquefier usually high adhesion of the mix to the metal Grinding balls of the grinding device, which is why Grist is located between the grinding media. So can be early mixing, compression and activation of the Powder batches take place.

The desired high granule filling density and the sinterre activity of the mix, as already mentioned was, by the choice of raw material or solid compo due to the amount of solids, the liquefiers amount of detergent, the grinding time and grinding intensity which among other things the grinding ball size and quantity contributes, controlled. The parameters mentioned are included  depending on the type of raw material combination and the desired one Specify powder data individually or in combination.

The suitable organic liquefiers evaporate even at relatively low temperatures - the ammonium salt of a polyacrylic acid evaporates, for example, at 50 ° C in the form of ammonia vapors - which is why care must be taken during the grinding process that the critical temperature range of the liquefier is not exceeded . Suitable liquefiers that evaporate only above 60 ° C are also on the market.

After expelling the liquefier is the Zu the ground material with a pasty, kneadable mass comparable. You get the same state if e.g. when using the above ammonium salt the ammonia in the regrind by adding acetic acid neutra siert.

The method according to the invention is made possible by Fort case of pre-sintering and subsequent fine grinding one significant reduction in the number of process steps and thus a reduction in the equipment and analysis effort. It is also characterized by an improved Controllability of the desired properties of the ground material, through a higher degree of reproduction and Lich a wet grinding process is carried out by complete Dust-free, which is particularly advantageous in the Production of mixtures with harmful to health Fabrics such as Nickel affects.

The high solids content decreases after the completion of the Wet mixing process also the usual great effort like squeezing and drying in the oven, for drying process, which pre-production of mixed granules is switched.  

Further advantages of this method are the ones required less effort when cleaning the grinding containers if mixed granulate is produced, better maintainable Granulate quality and the achievable, considerably better magnetic properties of the granules pressed and sintered materials.

To summarize, this procedure compared to the conventional with reduced personnel wall, space requirements, equipment costs, energy requirements and with lower powder losses is feasible.

The wet grinding process described above without Intermediate addition of other procedural steps Production of the binder-containing, pressable granulate can be done in different ways depending on whether spraying granules or mixed granules to be produced.

If you want to produce spray granules, you can on a ge separated, on the expulsion of the liquefier ge completely do without the drying process. One must only the required amount of water, binder, e.g. Polyvinyl alcohol, and optionally lubricants, e.g. Zinc stearate, add to the ground goods and the like atomize the mixture obtained in a spray oven to obtain Spray granules to arrive.

Manufacturing is more economical because it saves energy of mixed granulate. For the wet-ground mixture of the starting or raw material components, i.e. the so-called mixed ground material you first give a binder dissolved in water, e.g. the already mentioned polyvinyl alcohol and possibly also a lubricant, e.g. again zinc stearate, and briefly mixes these agents into the mixed ground. The mass thus obtained is e.g. pressed through a sieve and into  granulated with an agitator.

The addition of the binder to the mixed ground material can be the reason additionally happen in every phase of the grinding process, not however without influencing the grinding effect. One intends therefore no influence on the addition of the binder To take mixed grinding process, then you can use the binders solution to the grinding vessel after completion of the grinding process to intervene briefly. Because of this the adhesion of the ground material to the balls and the container of the Grinding vessel is increased, it is more advantageous to use the bandage medium solution only after emptying the grinding container Add regrind.

Because of the strong adhesion of the regrind to the meal container and the balls is provided that the Grinding container to be completely emptied, the Ent empty by suction or squeeze using gas pressure. This measure can be applied to the last one in batch operation Batch of a total mix can be limited.

The binder is only set after the meal has been emptied container, so the addition before or after drying and expelling the liquefier. Essentials advantages of one or the other procedure don't stand here.

Claims (6)

1. Process for the production of oxide ceramic powder mixtures, suitable for the production of pressable granules for the production of sintered ceramic materials of high homogeneity, in particular of ferrites for inductive construction elements, thermistors, PTC thermistors, ceramic resistors, varistors and ceramic capacitors, characterized in that a mixture with a viscosity of a few poise, consisting of at least 60, preferably 75 to 85% by weight of oxide-ceramic solids content, the rest of the water and largely inert liquid, is produced, and that the wet mixture thus obtained is homogeneously ground and mixed, the for The reactivity of the solids required for the sintering process and the degree of granule filling density of the granulate to be produced, which is required for the pressing, is determined by the following variables: solid components, solids content, liquefying agent content, liquefying agent viscosity, grinding time and grinding intensity. 2. The method according to claim 1, characterized ge indicates that the homogeneously ground and mixed wet mix, the maximum viscosity of which 2.5 piose, a preferably dissolved in water Binder, especially polyvinyl alcohol, and given if a lubricant, especially zinc stearate, is added and the mixture thus obtained is granulated by spray drying is gated. 3. The method according to claim 1, characterized ge indicates that the wet mix, whose Viscoaität is a maximum of about 2.5 poise, with preference  binder dissolved in water, especially poly vinyl alcohol, and optionally with a lubricant, especially zinc stearate, and then homo is ground and mixed, after which the thus obtained Mixture is granulated by spray drying. 4. The method according to claim 1, characterized ge indicates that the homogeneously ground and mixed wet mix, the maximum viscosity of which 4 poise, one preferably dissolved in water Binder, especially polyvinyl alcohol, and given if a lubricant, especially zinc stearate, is added after which the mixture thus obtained is driven off of the liquefier at least partially dried and then z. B. is granulated in an agitator. 5. The method according to claim 1, characterized ge indicates that the wet mix, whose Viscosity is at most about 4 poise, preferably binder dissolved in water, especially polyvinyl alcohol, and if necessary with a lubricant, esp special zinc stearate, mixed and then homogeneously ge grind and mix, after which the mixture thus obtained at least partially while expelling the liquefier dried wise and then z. B. in an agitator is granulated. 6. The method according to claim 1 and one of the preceding Claims, characterized, that the binder already contains the solid components of the Material is admixed.