EP1373585B2 - Method for producing a hard metal projection - Google Patents

Abstract

The invention relates to a method for producing a hard metal projection made of hard material portions, binding metal portions and water-soluble pressing auxiliary agent portions by drying a wet slurry, which contains the constituents and pure water as the liquid phase. According to the invention, the hard material and binding metal portions are ground with water while forming a wet slurry and, after grinding, the pressing auxiliary agent portions are admixed in the form of an emulsion to the wet slurry. The emulsion is produced with the aid of an emulsifier while adding water.

Description

The invention relates to a process for producing a hard metal batch from hard material fractions, binder metal fractions and water-insoluble compression assistant fractions by drying a wet sludge containing the constituents with pure water as the liquid phase.

Carbide-alloy shaped parts are produced by pressing and sintering a mixture of the powdery starting materials, the so-called hard metal batch. For the preparation of the hard metal approach, the individual hard and binder metal powders are first brought by grinding with the addition of liquid in finely disperse mixture in the form of a wet sludge. When using coarser-bodied starting powders, this step is associated with comminution of the starting powders, while with fine-grained starting bulbs, mainly homogenisation of the wet sludge takes place. The liquid is intended on the one hand to prevent the caking of the powder particles and on the other hand their oxidation during the grinding.
As a suitable Mahtaggregate today almost exclusively agitator ball mills so-called attritors are used, in which the ground material is mixed in a cylindrical container together with carbide balls by a multi-bladed stirring in motion. In the wet sludge produced by grinding with the addition of liquid, a pressing aid, eg in the form of paraffin, is added in most cases. The addition of a pressing aid facilitates the compression of the hard metal approach during the pressing process and results in a better green strength and thus improved handling of the pressed moldings. The wet sludge is then dried to form the finished hard metal batch which is further processed by pressing and sintering.

A commonly used method of drying is spray drying. For this purpose, the wet slurry brought to a sprayable consistency is sprayed through a nozzle, which is located inside a spray tower. A hot gas stream dries the sprayed droplets on the flight path and these separate in the lower conical part of the spray tower in the form of small beads as so-called hard metal granules, where it can then be removed. If the cemented carbide approach is in granular form, this has the great advantage that the flowability of the hard metal approach is significantly improved, whereby the filling of the press dies is facilitated.

The spray towers of spray drying in the cemented carbide industry are designed with a cylindrical upper portion and a tapered, lower portion and work in countercurrent to the fountain principle, ie located in the lower portion of the spray tower is centrally located the spray lance the wet sludge from high pressure about 12 to 24 bar sprayed in the form of a fountain upwards. The gas stream for drying the sprayed droplets is directed from above against the spray direction of the droplets and leaves the spray tower in the upper third of the tapered portion below the spray lance.
In this way, the droplets are first pushed upwards and then diverted downwards due to gravity and the oppositely directed gas flow. In the course of running this drying path, the droplets are converted into a compact granules with a low residual moisture, which then trickles after hitting the bottom of the spray tower by its tapered course automatically to the central discharge opening.
The fact that the trajectory of the sprayed droplets goes first up and then down, results in a compact design compared to spray towers, which work in DC. In the DC method both the spray of the wet sludge, as well as the flow of drying air run from the top of the spray tower down. In the countercurrent process, the same drying path for the drying of the droplets is achieved at about half the spray tower height.

Spray towers operating countercurrently according to the fountain principle are in practice designed with a cylindrical section having a height in the range of about 2 to 9 meters with a height to diameter number ratio in the range of about 0.9 to 1.7 while Spray towers operating cocurrently with supply from above are made with a cylindrical section having a height in the range of about 5 to 25 meters with a height to diameter number ratio in the range of about 1 to 5.

In the cemented carbide industry, to date only solvents such as acetone, alcohol, hexane or heptane in concentrated or at best only slightly diluted with water form are used as solvents for grinding and forming the wet sludge almost without exception.
In these solvents, the wax-based pressing aids such as paraffin, which are frequently used in practice, are generally readily soluble, so that there are no problems with grinding and spraying of the cemented carbide mixture.

The big disadvantage is that all of these solvents are highly flammable and easily volatilizable. Therefore, the attritors and the spray-drying plant must be designed to be explosion-proof, which entails a high design effort and thus high investment costs.
In addition, the drying in the spray tower under a protective gas atmosphere, usually nitrogen, must be performed.

All solvents mentioned are also polluting and lead due to their easy volatilization despite implementation of recycling measures to high evaporation losses.

Due to the serious disadvantages of these organic solvents, attempts have been made to replace the organic solvents with water. The difficulty is that the most commonly used pressing aids - such as paraffin - are not soluble in water and special measures must be taken to produce the wet sludge to obtain a satisfactory hard metal approach.

For clarification, it should be noted in particular that the general term carbide includes, of course, so-called cermets, a special group of hard metals with usually nitrogen-containing hard materials.

US-A-5922978 discloses a process in which hard material components, binder metal components and pressing assistant components in the water are mixed to form a wet sludge, e.g. B. by grinding. The auxiliary ingredients are preferably added as a paraffin water emulsion. The order of addition of hard material components, binder metal components and compression assistant components is not fixed.

The US 4,397,889 describes a method for producing a hard metal batch in which an insoluble in the liquid grinding medium used pressing aid is used. For example, paraffin is also mentioned as a pressing aid and water as a grinding medium. In order to obtain a usable cemented carbide approach with evenly distributed pressing aid despite the insolubility of the pressing aid in the grinding medium, it is proposed according to the US Patent first to heat the hard material powder fractions with or without binding metal content to a temperature above the melting point of the pressing aid and then to mix with the pressing aid. Then the powder mixture is cooled as quickly as possible to keep oxidation of the powder within limits. In order to avoid excessive clumping of the powder mixture during cooling, the powder mixture is kneaded. After cooling, if not present in the powder mixture, the binder metal components are added and the powder mixture is ground in water. The resulting wet sludge is then sprayed, for example, in a spray-drying plant and dried.
A disadvantage of this method is that the mixing devices in which the mixing of the hard metal powder is carried out with the pressing aids, are heavily contaminated by lumpy, sticking residues of the powder-Presshilfsmittelmischung and must be removed with large, costly cleaning effort before each new production of a hard metal approach.

The object of the present invention is therefore to provide a method for producing a carbide approach, in which the disadvantages mentioned in the prior art are avoided.
This is achieved by the method according to claim 1 according to the invention.

By this measure, a uniform distribution of the pressing aid in the hard metal approach is achieved in a simple manner. The preparation of the emulsion takes place without problems in a commercially available emulsifier with a heatable double-walled kettle with a stirrer and a high-dispersion machine. In this case, the desired amount of water is added after melting the pressing aid and the emulsifier. Only when the temperatures of the two immiscible phases (pressing aids and water) are adjusted, with the aid of an extremely fast-running high dispersing (for example, about 6000 rpm), the pressing aid phase is dispersed in water. As emulsifiers usually commercially available emulsifiers, as they are used in the food industry, can be used. The emulsifier must be adapted to the specific composition of the pressing aid to be emulsified. It is important that the emulsifier does not contain any harmful substances for further Hartrnetallherstellung, such as alkali, alkaline earth, or sulfur compounds that can form fracture-initiating phases after sintering. In addition, no emulsion-stabilizing additives, such as pH-increasing agents, should be included, since these additives may not evaporate completely without residue during dewaxing and may cause problems in the subsequent sintering of the carbide approach. Even without such stabilizing additives, the emulsion is stable for at least 5 days when stored at room temperature, which ensures a trouble-free production process in the production of hard metal approach.

Particularly advantageous is the use of an emulsifier which allows the preparation of an emulsion with single droplets having an average droplet diameter of less than 1.5 microns.

A particularly frequently used pressing aid for the production of a carbide approach is paraffin.

When paraffin is used, a mixture of fatty alcohol polyglycol ethers with monodiglycerides has proved suitable as an emulsifier for the preparation of the emulsion.

For the preparation of the hard metal approach, the grinding of the hard material and binder metal parts in the attritor with a viscosity of the wet sludge in the range between 2.500 to 8,000 mPas (measured in a rheometer of type RC 20 from Europhysics at a shear rate of 5.2 [1 / s]) is carried out with at least 4 to 8 times the volume exchange per hour.

In this way, even in the production of a wet sludge with hard material and binder metal particles of very small particle sizes of the order of magnitude less than 1 μm, meals as short as possible are achieved so that an excessive oxidation of the particles is avoided.

Particularly interesting is the application of the method according to the invention for the production of a hard metal batch when a hard metal granulate is produced by the drying of the wet sludge in a spray-drying plant. For spray drying, advantageously, a spray tower with a cylindrical section and a conical section is used, in which the gas stream for drying the wet sludge has an inlet temperature in the range of 130 to 195 ° C and an outlet temperature in the range of 85 to 117 ° C and wherein the Spray tower is designed and operated so that the numerical ratio of the supplied via the wet sludge amount of water in liters per hour to the tower volume in m ³ in the range between 0.5 and 1.8 and that atomizes a maximum of 0.17 kg wet sludge per m ³ supplied drying gas wherein the wet sludge has a solids content in the range of 65 to 85 wt.%.

It goes without saying that the available amount of energy resulting from the amount and temperature of the supplied gas stream must be sufficient to easily evaporate the amount of water supplied.

The essence of this special spray drying is therefore to keep the amount of water supplied in relation to the tower volume significantly smaller than is usual in spray towers and adjust the amount of air supplied to the sprayed wet sludge so that at least 1 m ³ air per 0, 17 kg of wet sludge is available. As a result, on the one hand a gentle drying and on the other hand a maximum residual moisture content of 0.3% by weight, based on the finished granules, is achieved under the prevailing conditions.
Oxidation of even extremely fine-grained starting powder is largely prevented under the process conditions mentioned.
Of course, in this method, as in the production of cemented carbide generally common, the carbon balance, taking into account the chemical analysis of the starting powder used and the oxygen uptake during milling and spray drying is adjusted, possibly by adding carbon before milling, that with the hard metal granules the production of a finished sintered carbide without eta-phase and without free carbon is guaranteed.

The average particle size of the granulate produced by the special spray drying is generally between 90 and 250 microns and can be adjusted by the size of the spray nozzle opening, the viscosity of the wet sludge to be sprayed and the spray pressure. The smaller the nozzle opening, the lower the viscosity and the higher the spray pressure, the smaller the mean grain size. The amount of wet sludge supplied via the spray nozzle is in turn regulated by the spray pressure and by the size of the swirl chamber and nozzle opening of the spray nozzle.

Although the special spray-drying is applicable to both co-current and countercurrent spray-drying equipment, it has been found to be particularly effective in countercurrent systems using the fountain principle, which allows the spray-drying plant to be manufactured in a compact design.
It is advantageous to carry out the cylindrical, upper section of the spray tower with about 6 m in height and about 4 - 5 m in diameter. For the adjoining, conical lower section, a cone angle of approximately 45 ° - 50 ° has been proven.

It is also a particular advantage of the special spray-drying that air can be used as the drying gas, which in turn makes spray-drying extremely cost-effective.

If a spray-drying plant operating countercurrently according to the fountain principle is used for spray-drying, it is advantageous to determine the temperature of the incoming drying air at the upper end of the cylindrical section and the temperature of the exiting drying air in the region of the conical section of the spray tower within the stated ranges to agree that sets a temperature between about 70 and 120 ° C in the geometric center of gravity of the spray tower. Under these conditions, the lowest possible oxidation of the hard metal granules is achieved.

In the following the invention will be explained in more detail with reference to a drawing and to a production example.

FIG. 1 shows the schematic diagram of a spray tower for the particularly advantageous production of hard metal granules from a wet sludge according to the invention.

The spray tower -1- consists of a cylindrical section -2- and an adjoining, conically tapering down section -3-. The spray tower -1- works in countercurrent to the fountain principle, ie the gas stream for drying the wet sludge is fed at the upper end -11- of the cylindrical portion -2- and blown down, while the wet sludge to be atomized at the lower end of the cylindrical portion - 2- over a spray lance -4- with a nozzle opening -5- on the principle of a fountain up against the direction of the gas stream is sprayed -6.
The sprayed liquid droplets -7- are thus first directed upward and then change due to the opposite gas flow and due to gravity their direction and fall down. Before hitting the bottom of the spray tower -1-, the tapered portion -3-, the liquid droplets -7- must be converted into the dried granules.
Through the conically tapered portion -3- of the spray tower, the granules to the discharge opening -8- passed. The gas stream -6- has an inlet temperature in the range of 130 to 195 ° C and an exit temperature on leaving the spray tower through the outlet pipe -9- below the spray lance -4- in the upper third of the conical section -3-, ranging from 85 to 117 ° C. The gas inlet and outlet gas temperature are advantageously coordinated so that sets a temperature between about 70 and 120 ° C in the geometric center of gravity -S- of the spray tower. It is important that the ratio of the amount of water supplied via the wet sludge in liters per hour, based on the tower volume in m ³ in the range between 0.5 and 1.8 and that per m ^{3 of} supplied drying gas atomizes a maximum of 0.17 kg wet sludge wherein the wet sludge has a solids content in the range of 65 to 85 wt.%. Of course, it must be ensured that the amount of energy provided by the temperature conditions and the amount of the supplied drying gas, which is sufficient for easy evaporation of the supplied via the wet sludge amount of water.
It is advantageous if the conically tapering section -3- of the spray tower is double-walled for passing a cooling liquid, eg water.
With this measure, the granules are cooled in this area to at least 75 ° C.
After leaving the spray tower -1- through the outlet opening -8- the granules pass to a cooling channel -10- where it is then cooled to room temperature.

In the following the invention will be explained in more detail with reference to a production example.

example

To produce a guarded cemented carbide granules with a mean particle size of 125 microns, consisting of, apart from the 2% wax content (paraffin), 6 wt.% Cobalt, 0.4 wt.% Vanadium carbide, remainder tungsten carbide, 36 kg of cobalt powder having an average particle size of about 0.8 microns FSSS and an oxygen content of 0.56 wt.%, 2.4 kg of vanadium carbide powder having an average particle size of about 1.2 microns FSSS and an oxygen content of 0.25 wt.% And 561.6 kg tungsten carbide powder with a BET surface area of 1.78 m ² / g, which corresponds to an average particle size of about 0.6 μm and an oxygen content of 0.28% by weight, with 148 liters of water in an attritor for 5 hours. As grinding media, 2000 kg of carbide balls with 9 mm diameter were used, the Attritordrehzahl was 78 U / min, the Umpumpleistung the wet sludge 1000 liters / hour. The temperature of the wet slurry during milling was kept constant at about 40 ° C. The finished ground wet sludge was cooled to 30.6 ° C and mixed homogeneously with 24 kg of a paraffin emulsion (48.8 wt.% Water, 48.8 wt.% Paraffin, balance emulsifier), by further addition of water to a solids content of 75% adjusted with a viscosity of 3000 mPas.
The emulsion was prepared in a commercially available emulsifier from IKA, Germany. In this case, 40 kg of paraffin wax were mixed with 2 kg of a commercial emulsifier, essentially a mixture of fatty alcohol polyglycol ether and monodiglyceride and melted at 85 ° C. (The exact composition of the emulsifier is to be adjusted empirically to the exact composition of the paraffin wax used). After melting, 40 kg of water were added and brought to the same temperature. Then the Hochdispergiergerät was switched on for Emutsionsherstellung for 60 minutes. This was followed by a controlled cooling of the emulsion at 2 ° C per minute to room temperature with the aid of a stirrer. A check of the droplet size distribution in a laser granulometer revealed a mean diameter d ₅₀ of 1.16 μm.
FIG. 2 shows a KRYO-SEM image of the finished emulsion in 7.500x magnification.

.For granulating the wet sludge thus prepared, a spray tower -1- having a cylindrical portion -2- with a height of 6 m and a diameter of 4 m and a tapered portion -3- with a cone angle of 50 ° was used, resulting in a Tower volume of 93 m ³ corresponds. The spray tower was designed to operate in countercurrent to the fountain principle. As the gas for drying the wet sludge, air was used, which was supplied to the spray tower at 4000 m ³ / h.
The slurry was fed to the spray tower through a spray lance 4 with a single fluid nozzle -5- with an outlet opening of 1.12 mm diameter at a pressure of 15 bar, resulting in a slurry feed of 0.08 kg wet sludge per m ³ of drying air revealed. The air outlet temperature was set to a constant value of 88 ° C, which was achieved under the given conditions by an air inlet temperature of 145 ° C. The atomization of 0.08 kg of wet sludge per m ^{3 of} supplied drying air means that at one Air supply of 4000 m ³ per hour 320 kg of wet sludge were sprayed per hour. Since the wet sludge was adjusted to 75% solids, the 320 kg of wet sludge per hour correspond to an hourly supplied amount of 80 liters.
The ratio of the amount of water supplied in liters per hour, based on the tower volume, was therefore $\frac{80\mathrm{l}/\mathrm{H}}{93{\mathrm{m}}^3}=0.86\frac{\mathrm{l}}{{\mathrm{m}}^3\mathrm{,}\mathrm{H}}\mathrm{,}$

,

The oxygen content of the granules produced was 0.51% by weight.

FIG. 3 shows a recording of the carbide granules produced according to the example with a mean grain size of 125 microns in 50-fold magnification.

Claims (8)

1. Method for producing a hard metal grade powder from hard material components, binding metal components and water-insoluble pressing aid components by drying a slurry containing said components with pure water as a liquid phase, characterised in that the hard material components and binding metal components are first milled with water to form a slurry and in that after milling the pressing aid components, in the form of an emulsion which is produced with the aid of an emulsifying agent with the addition of water, are admixed to the slurry, and in that the milling is carried out in the attritor with a viscosity of the slurry within the range from 2500 to 8000 mPas and with at least a four-fold to eight-fold volume exchange per hour.
2. Method for producing a hard metal grade powder according to claim 1, characterised in that an emulsifying agent is used which makes possible the production of an emulsion with individual droplets having a mean diameter of less than 1.5 µm.
3. Method for producing a hard metal grade powder according to either of claims 1 and 2, characterised in that paraffin is used as the pressing aid.
4. Method for producing a hard metal grade powder according to claim 3, characterised in that a mixture of fatty alcohol polyglycolether with monodiglycerides is used as the emulsifying agent.
5. Method for producing a hard metal grade powder according to any one of claims 1 to 4 in the form of a hard metal granulate, characterised in that the slurry is dried in a spray-drying apparatus.
6. Method for producing a hard metal granulate according to claim 5, characterised in that for the spray drying a spray tower -1- having a cylindrical segment -2- and a conical segment -3- is used in which the gas stream for drying the slurry has an inlet temperature within the range from 130 to 195°C and an exit temperature within the range from 85 to 117°C, the spray tower (1) being so designed and operated that the ratio of the quantity of water supplied via the slurry in litres per hour to the volume of the tower in cubic metres is within the range from 0.5:1 to 1.8:1, and in that not more than 0.17 kg of slurry per cubic metre of incoming drying gas is atomised, the slurry having a solid content within the range from 65 to 85 wt.%.
7. Method for producing a hard metal granulate according to claim 6, characterised in that the spray drying takes place in the countercurrent on the fountain principle and in that air is used as the drying gas.
8. Method for producing a hard metal granulate according to claim 7, characterised in that the gas inlet and gas exit temperatures are so coordinated that a temperature from approximately 70 to 120°C is established at the geometrical centre of gravity - S- of the spray tower -1-.

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