EP1373586B1 - Method for producing hard metal granulated material - Google Patents

Abstract

The invention relates to a method for producing a hard metal granulated material by carrying out wet milling and spray drying inside a spray tower -1- while using pure water as the liquid phase. According to the invention, the spray tower -1- is configured and operated in such a manner that the ratio of the amount of water, which is supplied over the wet slurry, in liters per hour ranges from 0.5 to 1.8 with regard to the tower volume in m3, and that a maximum of 0.17 kg wet slurry is supplied per m3 drying gas, whereby the wet slurry has a solids content ranging from 65 to 85 wt. %. Under these conditions, the usually necessary addition of a water-soluble, long-chain polyglycol to the wet slurry before spraying in order to prevent the hard metal granulated material from oxidizing is eliminated.

Description

The invention relates to a method for producing a hard metal granules by wet grinding the desired in the finished granules hard material and binder metal and forming a sprayable wet sludge using pure water as a liquid phase, wherein the wet sludge by spray drying in a gas stream with a gas inlet temperature in the range of about 160 to 220 ° C and a gas outlet temperature in the range of about 85 to 130 ° C in a spray tower in granular form, wherein the spray tower consists of a cylindrical portion and a conical portion.

Carbide-alloy shaped parts are produced by pressing and sintering the powdery starting materials. For improved processability, the finely divided starting powders of the hard metal alloys with a mean grain size in the range of a few microns and often even lower, in granular form, i. in as ideal spherical shape with a mean granule size of at least about 90 microns brought. For this purpose, the individual hard material and binding metal powders are first brought into the form of a wet sludge by grinding with the addition of liquid in finely disperse mixture. When using coarser starting powders, this step is associated with a comminution of the starting powder, while in fine-grained starting powders mainly a homogenization 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 grinding units 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 optionally added. The addition of a pressing aid is particularly necessary when the finished granules are compacted by die pressing and brought into the desired shape.

The pressing aid gives the granules in addition to the better compression during the pressing process also a better flowability, which facilitates the filling of the press dies. If the finished carbide granules are further processed by extrusion, the wet sludge is usually added no pressing aid. The wet sludge is then brought to a sprayable consistency and dried in a spray drying plant and granulated at the same time. For this purpose, the wet sludge 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 are deposited in the lower conical part of the spray tower in the form of small pellets as granules, where it can then be removed. 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.

Since all these solvents are highly flammable and easily volatilized, the attritors and the spray-drying plant must be explosion-proof, which entails a high constructive effort and thus high investment costs. In addition, the drying in the spray tower under a protective gas atmosphere, usually nitrogen, must be performed.

In addition, all the solvents mentioned are harmful to the environment and lead to high evaporation losses due to their ease of volatilization, despite the implementation of recycling measures.

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 arranged 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 from above against the Spraying the droplets out and leaves the spray tower in the upper third of the tapered section 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 is first up and then down is compared to spray towers operating in cocurrent, where both the spray of the wet sludge, as well as the flow of drying air from the top of the spray tower directed downwards is the same drying path for the drying of the droplets reached with almost half the spray tower height, resulting in a compact design.

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 designed with a cylindrical section having a height in the range of about 5 - 25 m with a numerical ratio of height to diameter in the range of about 1 to 5.

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.

The US 4,070,184 describes a process for the production of a cemented carbide granulate by milling and spray-drying, in which pure water is used instead of organic solvents for grinding and producing the sprayable wet sludge. By use pure water as a liquid phase makes it possible to produce the attritors and the spray-drying plant in an open, no longer explosion-proof construction, which brings significant cost savings. In spray drying, instead of inert gas, air can be used as the dry medium. In addition, the complete replacement of organic solvents avoids any health hazard from solvent vapors.

The major disadvantage of this method is that the use of pure water and air leads to a strong deterioration of the powder by oxidation. Extremely fine-grained carbide powders with a mean particle size between 0.5 and 0.6 μm, which according to the BET measurement corresponds to a surface area of 1.6 to 3.2 m ² / g, as used today for many carbide grades, are due to their large surface particularly sensitive to oxidation and can not be produced by this method. Even with carbide powders with a larger, average grain size of 1 micron and just below and thus a much smaller surface area, which at the time of the application of the US patent were the smallest usual grain sizes, it was forced to the oxidation susceptibility by the addition of a long-chain polyglycol in to limit wet sludge immediately before spray drying. The polyglycols, which also entail improved compressibility of the granules, envelop the powder particles completely and thus largely prevent oxidation of the powder particles during spray drying.

A disadvantage is that such polyglycols show an unfavorable evaporation behavior during sintering of the pressed powder. It comes only at temperatures of about 250 to 300 ° C to a complete evaporation, which can cause bursting of the part or to form cracks especially for large parts due to the evaporation over a wide temperature range.

The object of the present invention is therefore to provide a method for producing a hard metal granules by grinding and spray drying using pure water as the liquid phase, in which also extremely fine-grained hard metal powder can be ground and sprayed and in which the disadvantages mentioned in the prior art are avoided during sintering.

According to the method mentioned above, this is inventively achieved in that the wet sludge is sprayed and dried without using a water-soluble, long-chain polyglycol in the spray tower and that 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 , based on the tower volume in m ³ in the range between 0.5 and 1.8 and that a maximum of 0.17 kg wet sludge per m ³ supplied drying gas are atomized, 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 the method according to the invention, therefore, is to keep the amount of water supplied via the wet sludge 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 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.

A solids content of the wet sludge in the range from 70 to 80% by weight has proven to be particularly advantageous.

Oxidation of even extremely fine-grained starting powder is largely prevented under the process conditions mentioned, so that the waiver on polyglycols also in the granule production is associated with no disadvantages.

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 granules produced 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 method according to the invention is applicable both to spray-drying plants operating in the cocurrent principle and those operating in countercurrent flow, it has proven particularly useful in countercurrent systems using the fountain principle, whereby the spray-drying plant can 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.

A particular advantage of the method according to the invention is that air can be used as the drying gas, which in turn makes the method extremely cost-effective.

In order to keep the oxidation of the particles as low as possible during spray drying, it is advantageous to use a single-substance nozzle for spraying the wet sludge. In the case of single-substance nozzles, in contrast to two-component nozzles where the wet sludge to be atomized is fed simultaneously with a gas stream to the nozzle, only the wet sludge is supplied under pressure, so that contact with a possibly oxidizing gas stream is further shortened.

It is particularly advantageous for producing the hard metal granules according to the invention if the milling preferably takes place in the attritor with a viscosity of the wet sludge in the range from 2,500 to 8,000 mPas (measured in a rheometer of the RC20 type from Europhysics at a shear rate of 5.2 [1 / s ]) is carried out at least 4 to 8 times 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 on the order of significantly less than 1 micron so short meals are achieved that excessive oxidation of the particles is avoided.

In extreme cases, if longer meals are required within the specific viscosity range for producing small grain sizes, it is advantageous to add to the water before grinding and / or spray-drying an antioxidant, for example based on amine compounds, e.g. Add aminoxethylate or resorcinol, whereby even at longer meals and during subsequent spraying excessive oxidation of the particles can be prevented.

When the process according to the invention is carried out using a countercurrent spray-drying plant according to the fountain principle, 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.

Furthermore, it is advantageous to carry out the process according to the invention such that the granules are cooled in the outlet region of the spray tower to a maximum temperature of 75 ° C and is cooled rapidly to room temperature immediately after removal from the cooling tower. This rapid cooling of the finished carbide granules to room temperature also severely limits further oxidation of the granules.

The cooling of the granules in the outlet region of the cooling tower is most expedient in that the conically tapered portion of the spray tower is double-walled and is cooled by a suitable cooling medium. The rapid cooling to room temperature, for example, take place in that the granules after removal from the spray tower passes through a cooling trough.

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 carrying out the method according to the invention.

The spray tower -1- consists of a cylindrical portion -2- and an adjoining, tapered downwardly tapered section -3-. The spray tower -1- works in countercurrent to the fountain principle, i. the gas stream for drying the granules is fed at the upper end -11- of the cylindrical portion -2- and blown down, while to be atomized wet sludge at the lower end of the cylindrical portion -2- via a spray lance -4- with a nozzle opening -5 - Is sprayed upward against the direction of the gas flow -6- on the principle of a fountain upwards.

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 160 to 220 ° C and an exit temperature when 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 130 ° 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 tapered portion -3- of the spray tower is double-walled for passing a cooling liquid, e.g. Water, running.

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 trough -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 tungsten carbide granulate having an average particle size of 135 μm, consisting of 6% by weight of cobalt, 0.4% by weight of vanadium carbide, remainder tungsten carbide, 36 kg of cobalt powder having an average particle size of about 0.63 μm FSSS and an oxygen content of 0.56 wt.%, 2.4 kg vanadium carbide powder with an average particle size of about 1.2 μm FSSS and an oxygen content of 0.25 wt.% And 563.5 kg tungsten carbide powder with a BET surface area of 1.78 m ² / g, which corresponds to an average grain size of about 0.6 microns and an oxygen content of 0.28 wt.% Milled with 150 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 sludge during grinding was kept constant at about 40 ° C. The finished ground wet sludge was adjusted by further addition of water to a solids content of 75 wt.% With a viscosity of 3000 mPas.

For granulating the wet sludge thus prepared, a spray tower -1- was used with a cylindrical portion -2- with a height of 6 m and a diameter of 4 m and with a tapered portion -3- with a cone angle of 50 °, 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 85 ° 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 an air feed of 4000 m ³ per hour 320 kg of wet sludge were sprayed per hour. Since the wet sludge was adjusted to 75 wt.% Solids content, the 320 kg wet sludge per hour correspond to an hourly supplied amount of water of 80 liters.

The ratio of the amount of water supplied in liters per hour, based on the tower volume, was therefore 80 l / h = 0.86 1 . 93 m ³ m ³ .h

The oxygen content of the granules produced was 0.53 wt.%.

FIG. 2 shows an SEM image of the carbide granules produced according to the example with an average particle size of 135 microns in 100-fold magnification.

Claims (9)

1. Method for producing a hard metal granulate by wet grinding of the hard material and binder metal components desired in the final granulate and forming a sprayable wet slurry by using pure water as a liquid phase, the wet slurry being brought into granulate form in a spray tower -1- by spray drying in a gas stream having a gas inlet temperature in the range of about 160 to 220°C and a gas outlet temperature in the range of about 85 to 130°C, the spray tower -1- consisting of a cylindrical section -2- and a conical section -3-, characterised in that
   the wet slurry is sprayed and dried in the spray tower -1- without using a water-soluble long-chained polyglycol, and in that the spray tower -1- is configured and operated so that the numerical ratio of the amount of water delivered via the wet slurry in litres per hour in relation to the tower volume in m³ lies in the range of between 0.5 and 1.8, and in that at most 0.17 kg of wet slurry is sprayed per m³ of drying gas delivered, the wet slurry having a solids content in the range of from 65 to 85 wt.%.
2. Method for producing a hard metal granulate according to Claim 1, characterised in that the wet slurry has a solids content in the range of from 70 to 80 wt.%.
3. Method for producing a hard metal granulate according to Claim 1 or 2, characterised in that the spray drying is carried out in countercurrent flow according to the fountain principle.
4. Method for producing a hard metal granulate according to Claim 3, characterised in that the gas inlet and gas outlet temperatures are adapted to one another so that a temperature of between about 70 and 120°C is set up at the geometrical centroid -S- of the spray tower -1-.
5. Method for producing a hard metal granulate according to Claims 1 to 4, characterised in that air is used as the gas.
6. Method for producing a hard metal granulate according to one of Claims 1 to 5, characterised in that a single-substance nozzle is used for spraying the wet slurry.
7. Method for producing a hard metal granulate according to one of Claims 1 to 6, characterised in that the grinding is preferably carried out in an attritor with a wet slurry viscosity in the range of between 2500 and 8000 mPas with at least 4- two 8-fold volume turnover per hour.
8. Method for producing a hard metal granulate according to one of Claims 1 to 7, characterised in that an antioxidant based on amine compounds is added to the water before the wet grinding and/or spray drying.
9. Method for producing a hard metal granulate according to one of Claims 1 to 8, characterised in that the granulate is cooled to a temperature of at most 75°C in the outlet region -3- of the spray tower -1- and is rapidly cooled to room temperature immediately after extraction from the cooling tower.

Images