Specialized metallurgical processes – compositions for use therei – Processes – Producing or purifying free metal powder or producing or...
Reexamination Certificate
2002-11-22
2004-05-11
Mai, Ngoclan T. (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or purifying free metal powder or producing or...
C075S230000, C419S010000, C419S036000, C419S040000
Reexamination Certificate
active
06733562
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention lies in the metallurgy processing field. More specifically, the invention relates to a method of producing a hard metal refractory hard metal grade powder consisting of hard material, binding metal and non-water-soluble pressing aid components by drying a slurry containing the components using pure water as a liquid phase.
Molded parts made of hard metal alloys are produced by pressing and sintering powdered base materials. This is accomplished by milling the hard material and binding metal components in a liquid medium to a form a finely dispersed mixture which takes the form of a slurry. When coarser-grained starting powders are used, this step also involves milling the starting powders, whereas the slurry is merely homogenized when fine-grained starting powders are used. The liquid protects the powder particles against fusion and prevents them from oxidizing during the milling process.
Suitable milling systems used almost exclusively today are agitator ball mills known as attritors, in which the material to be milled is set in motion together with hard metal balls by a multiple-blade agitator arm inside a cylindrical container. A pressing aid, e.g. paraffin, can be introduced to the slurry produced through the liquid-enhanced milling process. The addition of a pressing aid facilitates the compression of the hard metal grade powder during the pressing process and also enhances its green strength, which facilitates the handling of the pressed molded parts. The slurry is then dried to produce a finished hard metal grade powder that is ready for subsequent processing involving pressing and sintering.
A commonly used drying method is spray drying. In this process, slurry with a sprayable consistency is sprayed through a nozzle positioned inside the spray tower. A stream of hot gas dries the airborne spray droplets, which then precipitate as granulate in the form of small granules or beads in the lower conical segment of the spray tower, from where it can then be removed. The great advantage of producing a hard metal grade powder in granular form is that the flow characteristics of the hard metal grade powder are substantially improved, which facilitates the process of filling in compacting dies.
Spray towers in spray drying systems used in the hard metal industry are designed with a cylindrical upper segment and a conical, downward pointing lower segment and ordinarily operate in a countercurrent mode in accordance with the fountain principle, i.e. the sprayer lance is positioned in the center of the lower segment of the spray tower and sprays the slurry under high pressure (12-24 bar) upward in the form of a fountain. The gas stream which dries the sprayed droplets flows into the drying chamber from above, against the direction of travel of the sprayed droplets, and escapes from the spray tower in the upper third portion of the conical, downward pointing segment below the spray lance. In this way, the droplets are first conveyed upward and then pulled downward by the force of gravity and the opposing stream of gas. In the course of the drying cycle, the droplets are transformed into a compact granulate with a low residual moisture content. As they fall to the floor of the spray tower, they automatically trickle down through the conical, downward pointing lower segment to the central discharge outlet.
Because the flight pattern of the sprayed droplets takes them first upward and then down, the distance traveled by the droplets during drying is equivalent to that of spray towers that operate with co-current downward streams of sprayed slurry and drying gas, but the process requires almost fifty percent less tower height.
Spray towers in practical use which operate with countercurrents on the basis of the fountain principle have a cylindrical segment measuring between 2 and 9 m in height with a height-diameter ratio of between 0.9 and 1.7, whereas spray towers which operate in a co-current mode with top-down gas and sludge flow are equipped with a cylindrical segment measuring between 5 and 25 m in height with height-diameter ratio ranging from 1:1 to 5:1.
In the hard metal industry, such organic solvents as acetone, alcohol, hexane or heptane are still used almost exclusively in the milling and pressing of slurries today. These solvents are used in concentrated form or diluted only slightly with water. As the wax-based pressing aids, such as paraffin, frequently used in practical applications are generally readily soluble in these solvents, no problems arise in the milling and spraying of the hard metal grade powder.
The great disadvantage is that all of these solvents are highly flammable and volatile. Therefore, attritors and spray drying systems must be designed as explosion-resistant units, which requires considerable engineering design input and thus generates high costs. In addition, the materials must be dried in an inert gas atmosphere, ordinarily nitrogen, in the spray tower.
All of the above-mentioned solvents are also environmental pollutants and are subject to substantial evaporation loss, despite the use of recycling measures, due to their high volatility.
In view of the significant disadvantages involved in the use of these organic solvents, attempts have been made to replace the organic solvents with water. The difficulty involved is that the most commonly used pressing aids—such as paraffin, for example—are not water-soluble, which means that special measures must be taken in producing the slurry in order to ensure satisfactory quality of the finished hard metal grade powder.
The general term “hard metal” as used in this specification also encompasses so-called cermets, a special group of hard metals which ordinarily contain hard materials with nitrogen. U.S. Pat. No. 4,397,889 describes a method of producing a hard metal grade powder in which a pressing aid that is not soluble in the liquid milling medium is used. As examples, the patent mentions paraffin as a pressing aid and water as a milling medium. To achieve a suitable hard metal grade powder with uniform distribution of the pressing aid despite the insolubility of the pressing aid in the milling medium, the patent specification proposes heating the hard material powder components first, with or without metal binder particles, to a temperature above the melting point of the pressing aid and then mixing them with the pressing aid. The powder mixture is then cooled as rapidly as possible in order to limit oxidation of the powder. In order to prevent excessive lump formation of the powder mixture during cooling, the mixture is kneaded during the cooling phase. After cooling, metal binder components are added, if not already contained in the powder mixture, and the powder mixture is milled in water. The slurry produced in this manner is then sprayed and dried, e.g. in a spray drying system. A disadvantage of this process is that the mixing units in which the hard metal powder and the pressing aid are mixed are heavily soiled by lumpy, adhesive deposits of the powder-pressing-aid mixture and must be cleaned to remove all residues at considerable effort and cost before each new hard metal powder production run.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method of producing a hard metal grade powder which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of producing a refractory hard metal grade powder of hard material, metal binder, and non-water-soluble pressing aid components, the method which comprises:
milling the hard material and the metal binder with water as a liquid phase to prepare a slurry;
subsequently mixing the pressing aid components in the form of an emulsion produced with an emulsifier and an addition of water with the slurry; and
drying the slurry with the hard material, the metal binder, and the pressing aid components to for
Beirer Helmut
Glätzle Wolfgang
Hartlmayr Erwin
Knünz Gerhard
Lackner Andreas
Ceratizit Austria GmbH
Mai Ngoclan T.
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