WO1999007905A1

WO1999007905A1 - Method of disintegrating sintered hard metal carbide bodies and recovering hard metal carbide powder

Info

Publication number: WO1999007905A1
Application number: PCT/IL1998/000360
Authority: WO
Inventors: Nely Kopylov; Boris Polyakov
Original assignee: Refin Ceramic Powders Ltd.
Priority date: 1997-08-05
Filing date: 1998-07-31
Publication date: 1999-02-18
Also published as: AU8558698A; IL121475A0

Abstract

An improved method for disintegrating sintered hard metal carbide bodies with comparatively low energy expenditure and recovering hard metal carbide powder, having about the same granulometric composition as the original metal carbide particles and a high degree of purity which is at least the same as that of the original sintered metal carbide material. The method comprises treating the carbide bodies in a strong hydrochloric acid solution for a sufficient time to weaken the surface layer and mechanically disintegrating the so treated bodies by crushing or milling.

Description

METHOD OF DISINTEGRATING SINTERED HARD METAL CARBBOE BODIES AND RECOVERING HARD METAL CARBIDE POWDER

FIELD OF THE INVENTION

This invention relates to a method of disintegrating sintered hard metal carbide bodies cemented by a binder phase and recovering metal carbide powder, having the same granulometric composition as the hard metal carbide making up the original bodies and a degree of purity comparable and even higher than that of the original metal carbide.

BACKGROUND OF THE INVENTION

Sintered or cemented hard carbides are widely used in various fields of technology, such as in metal cutting tools, dyes, etc. They are generally composed of tungsten carbide particles, often with additional carbides such as TiC, TaC, NbC and HfC, cemented together with a binder phase, in most cases cobalt. These carbides are comparatively expensive materials, and therefore numerous methods have been developed for recovering them from cemented carbide scrap, e.g. cemented carbide products which have become worn or broken. The interest in such methods of recovery dates back to approximately 1925, soon after the industrial production of sintered hard carbide products had started. Sintered hard metal is very hard and strong, and therefore the mechanical disintegration of its scrap demands large energy expenses and causes intensive wear of the equipment. At first, cemented hard metal scrap was used as an alloying material for the production of high-speed cutting steels, but this did not satisfy the need to recover f om cemented carbide scrap the original hard metal carbide particles of the same pre-sintered sizes and purity which can be re-used in the production of high quality sintered carbide products. The main techniques which have been developed to attain this purpose are the following:

In accordance with U.S. Patent No. 2, 485, 175, sintered tungsten carbide bodies are heated to a temperature of at least 1800° C (i.e. above the melting point of cobalt which is 1495° C) in a non-oxidizing atmosphere, whereupon some of the cobalt binder is exuded and the mass becomes swollen, cracked and porous. After cooling, the sintered carbide mass is treated with acid to remove the cobalt, and the residue is crushed to a powder.

In the cold stream process developed by Metallurgical Industries, re-crushed cemented carbide scrap is pulverized in a high velocity air jet by impact on a massive carbide anvil. While the resultant powder has approximately the same size as the original carbide particles, it is contaminated, not only by the impurities contained in the original scrap, but also by added impurities derived from the equipment. The powder also has structural defects due to the great dislocation density in the surface layer of the carbide particles, which causes grain growth during sintering of the this recovered powder. These drawbacks seriously limit the use of the metal carbide powder obtained by this method.

In accordance with U.S. Patent 3,438,730, sintered hard metal carbide masses cemented by cobalt are immersed in relatively weak phosphoric acid at relatively low temperatures, thereby dissolving the cobalt binder and freeing the metal carbide particles which remain in suspension in the phosphoric acid solution and must be separated therefrom mechanically by centrifuging or filtering. These separation operations are technologically cumbersome. Furthermore, this method has the drawback that leaching by phosphoric acid gives rise to partial oxidation of the carbide particles, as well as precipitation of phosphates on the carbide particles. Also it is very difficult to achieve complete extraction of the cobalt binder.

In the process disclosed in U.S. Patent 4,234,333, pieces of cemented carbide are submerged in an aqueous electrolyte, having an anode and a cathode therein, and comprising a corrosive agent which, electrochemically and selectively dissolves the binder phase without significant dissolution of the metal carbide grains. This method, besides requiring expensive equipment, has the above mentioned drawback in that the obtained metal carbide grains must be separated mechanically by centrifugation or filtration.

In the zinc process described in U.S. Patent 3,595, 484, the cemented carbide scrap is treated with molten zinc at a temperature above 900 °C. The cobalt binder is alloyed with the zinc (the alloy may contain 40% of Zn at 925°C). The zinc is subsequently removed by vacuum distillation at 950°C, and the porous residue is crushed to a carbide-cobalt powder.

In the Metek process (Reviews in Chemical Engineering, Vol. 9, Nos. 3-4, 1993), tungsten carbide scrap is chemically decomposed by mixing with inorganic salts (mainly sodium nitrate), and heating in a smelter (revolving furnace), preheated to 600°C. An exothermic reaction develops, and the mixture is then poured into water and treated with calcium chloride to obtain calcium tungstate. This can subsequently be converted into tungsten carbide powder by conventional methods of calcination reduction and carburisation. It is well known that for its most important applications, the starting metal carbide powder must conform with rather stringent requirements as regards its granulometric composition and purity. Thus, while some of the above described known methods for the recovery of metal carbide material are satisfactory in that the grain size distribution of the reclaimed metal carbide powder is close to that of the starting carbide scrap, these known methods leave much to be desired as regards the purity of the recovered metal carbide. As explained above, the recovered metal carbide powder contains not only the impurities originally present in the virgin metal carbide (mostly inside the metal carbide particles), but also residues of the binder phase metal (e.g. cobalt) which was not completely removed during the process of recovery, as well as "technological impurities" arising from the wear of the equipment used in the process and from the environment. A further drawback of some of the known processes for recovering metal carbide powder where great energy expenditure is required for crushing the scrap, resides in the great dislocation density resulting in the surface layer of the carbide particles which causes undesirable grain growth in the course of sintering of such recovered metal carbide material.

OBJECT OF THE INVENTION

It is an object of this invention to provide an improved method for disintegrating sintered hard metal carbide bodies with comparatively low energy expenditure. It is a further object of the invention to provide a process for recovery of hard metal carbide powder, having about the same granulometric composition as the original metal carbide particles in the scrap material, and a high degree of purity which is at least the same as that of the original sintered metal carbide material.

SUMMARY OF THE INVENTION

The above object is attained by one aspect of the present invention, which provides a method of disintegrating sintered hard metal carbide bodies cemented by a binder phase, except for such bodies having a ceramic surface coating, which method comprises immersing said bodies in 28-37% aqueous hydrochloric acid for about 10 to 48 hours at a temperature of up to about 90° C, separating the thus treated bodies from the hydrochloric acid solution, drying them and mechanically disintegrating said bodies by crushing or milling to obtain metal carbide powder. In accordance with another aspect thereof, the invention also provides a method of recovering high purity hard metal carbide powder substantially free of binder phase, which comprises treating the metal carbide powder obtained in accordance with the above aspect of the invention, with aqueous 28-37% hydrochloric acid for about 4 to 6 hours at a temperature of up to about 90°C, separating the powder from the aqueous solution by decantation and washing the powder with water and then with aqueous ammonia, separating the powder from the aqueous ammonia solution by decantation and drying it at an elevated temperature.

The method of the invention is applicable, both to uncoated sintered hard metal scrap bodies, and to such bodies having a surface coating of TiC and/or TiN. The method of the invention is, however, unapplicable to hard metal bodies having a ceramic surface coating, e.g. of alumina

DETAILED DESCRIPTION OF THE INVENTION

The invention is based on the surprising finding that the energy expenditure required to disintegrate sintered metal carbide scrap bodies can be sharply decreased by subjecting such bodies to a preliminary treatment with 28-37% aqueous hydrochloric acid, at temperatures up to about 90°C, for a time sufficient to cause the dissolution of the binder phase metal in the surface layer of the sintered metal carbide bodies, to a depth of at least half the size of a carbide particle in these bodies. This preliminary treatment is continued preferably, for at least 48 hours. It is assumed that as a result of this preliminary treatment, there are formed in the surface layer of the sintered metal carbide bodies, so-called "stress concentrators", i.e. points of decreased mechanical strength, which considerably facilitate the fracture by impact of the sintered carbide bodies.

After the preliminary treatment with the aqueous hydrochloric acid, the aqueous solution is decanted off, and the metal carbide bodies are preferably washed several times with water and dried, e.g. by a stream of compressed air. The aqueous hydrochloric acid solution can be re-used for the preliminary treatment of the next batch of metal carbide scrap, or in the subsequent stage for dissolving the binder phase metal.

The metal carbide bodies obtained from the preliminary hydrochloric acid treatment are then crushed, e.g. in a planetary ball mill, and the resultant material is then classified by screening, e.g. by passing through a 10- mesh sieve to obtain a powder and a fraction of coarser particles which is recycled to the preliminary hydrochloric acid treatment. The relative amount of the powder obtained by the crushing is a measure of the crushing efficiency, and is dependent on the crushing time. As will be shown in the examples hereinafter, a tungsten carbide scrap, which was subjected to a preliminary hydrochloric treatment for about 48 hours, yielded about 67% of metal carbide powder, after crushing in a planetary ball mill for 25 minutes (48.5 % after 15 minutes). A comparison sample, which was submitted to the same crushing operation without preliminary hydrochloric acid treatment, afforded only about 16.7% of powder after a crushing time of 15 minutes, and crushing it for 25 minutes resulted in strong heating of the scrap material, and extensive sticking of powder to the bowl and balls of the apparatus. For the recovery of metal carbide powder of high purity and the same granulometric composition as the original sintered metal carbide material, the metal carbide powder obtained in the crushing stage is reacted again with 28-37% aqueous hydrochloric acid with stirring for about 4 to 6 hours at a temperature of up to about 90°C. For control of complete dissolution of the binder phase metal (in most cases cobalt), aliquot samples of the powder are taken out periodically and tested for the presence of the binder phase metal, and the treatment with the hydrochloric acid is continued until a sample of the powder shows a negative reaction for the binder phase metal. Thereafter, the powder is separated by decantation and washed several times with water, followed by washing by aqueous ammonia, preferably 25% concentration, in order to dissolve any oxides of the hard metal constituent of the carbide, and to facilitate the sedimentation of the metal carbide powder. The powder could then be easily separated by decantation, and is dried in an oven at a temperature of above 100°C. The recovered hard metal carbide powder has the same granulometric composition as the original metal carbide scrap material, and a very low content of impurities. Thus, tungsten carbide powder recovered by the method of the invention from cemented carbide scrap material containing 94% of WC and 6% of Co, had a cobalt content of only 0.03%.

Examples

The invention will be illustrated in more detail by the following non-limiting examples:

Example 1

Disintegration of Sintered Tungsten Carbide Bodies

15 kg of pieces of sintered tungsten carbide scrap (average size 12 x 12 x 3 mm) obtained from cutting inserts K10 (ISO application code), were divided into three equal portions of 5 kg each. The sintered tungsten carbide material consisted of 94% WC, 2% Ta(Nb)C and 6% Co. A first portion (hereinafter "Sample 1"), was subjected to preliminary treatment by immersion in 1.2 L of extra pure aqueous hydrochloric acid (about 32%), in a 5 L glass vessel, heated on an electric plate to a temperature of 80 ± 5°C for 48 hours. A second portion of the scrap ("Sample 2") was subjected to the same preliminary treatment as sample 1 above, but was left in the hydrochloric acid solution for 72 hours.

Each of samples 1 and 2 was then treated by the same procedure as follows:

The hydrochloric acid was decanted off and the undissolved sintered tungsten carbide bodies were washed thrice by 0.4 L portions of deionized water, dried by a stream of compressed air, and crushed in a planetary ball mill type PM 400 (Retsch Company) having a 250 ml WC bowl containing three WC balls 30mm in diameter and six WC balls 20mm in diameter. The planetary ball mill was charged with 40-45 ml portions of the tungsten carbide scrap bodies, (about 300-350g). Different portions of each of samples 1 and 2 were crushed for periods of 5, 15 and 25 minutes, at a speed of 300 rpm.

After the crushing, each portion of the scrap was passed through a 10- mesh sieve (nominal sieve opening-2mm). The third portion of the sintered tungsten carbide scrap (Sample 3), was subjected directly to the same crushing regime, without the preliminary hydrochloric acid treatment.

The proportion of the powder fraction obtained after the crushing is shown in the following Table 1 (crushing efficiency). TABLE 1

balls.

As evident from table 1, a preliminary hydrochloric acid treatment for 48 hours increases the crushing efficiency by a factor of 2.5 to 3.0 as compared to the untreated "Sample 3". Continuing the preliminary hydrochloric acid treatment for 72 hours, does not significantly increase the crushing efficiency, especially if the crushing time is 25 minutes.

Example 2

Purification and Recovery of Tungsten Carbide Powder from Disintegrated Scrap

2.4 kg of the disintegrated tungsten carbide powder obtained by the procedure of Example 1, were charged to a glass crystallization dish

(190 mm diameter), containing 1.2 L of 32% aqueous hydrochloric acid, recycled from the preliminary treatment stage of Example 1. The mixture was thoroughly stirred and heated on an electric heating plate to 85 ± 5° C for 4-5 hours with stirring by a glass rod. Aliquots of about 1 gram of the metal carbide powder were taken out every hour, placed in a test tube and washed repeatedly with deionized water, until the disappearance of the cobalt coloring. Some fresh 32% aqueous hydrochloric acid was added to the sample in the test tube, whereupon the appearance of a blue coloring is evidence of the presence of residual cobalt in the powder sample. After a powder sample showed no color upon treatment with hydrochloric acid, i.e. the carbide powder contained no more cobalt, the aqueous hydrochloric acid solution was separated from the powder by decantation. The residual powder was washed with 0.4 L of distilled water, with stirring. After about 10 minutes, the carbide powder was allowed to sediment and was separated from the water by decantation. This washing operation was repeated six times until the water no longer showed the cobalt coloring. The presence of Co ions was controlled by a qualitative analytical method (reaction with diethyldithiocarbamic acid sodium salt trihydrate). The carbide powder was washed three more times with water, bringing the total amount of water used for the washing to 3.5 L. The metal carbide powder was then treated twice with 0.3 L portions of 25% aqueous ammonia, in order to dissolve any tungsten oxides and to facilitate the sedimentation of the tungsten carbide powder. The ammonia solution was decanted off and the resultant metal carbide powder was dried in a drying oven at 115° C for 12 hours.

The analysis of residual impurities in the recovered tungsten carbide powder, is shown in Table 2 (Sample 1, recycled). TABLE 2

Example 3

Disintegration of Tungsten Carbide Scrap Coated With TiC-TiN and Recovery of WC Powder

2.0 kg of scrap cutting inserts K10 (ISO application code), coated with a 5-6 μ thick layer of TiC-TiN coating, were submitted to the preliminary acid treatment by the procedure of Example 1 for 48 hours. The resulting scrap was crushed as described in Example 2, exhibiting crushing efficiency of 58.5%. The tungsten carbide powder obtained was refined by the procedure described in example 2, and dried to yield a tungsten powder, the analysis of which is also shown in Table 2 (Sample 2, coated).

Claims

CLAIMS:

1. A method of disintegrating sintered hard metal carbide bodies cemented by a binder phase, except for such bodies having a ceramic surface coating, which method comprises immersing said bodies in 28-37% aqueous hydrochloric acid for about 10 to 48 hours at a temperature of up to about 90° C, separating the thus treated bodies from the hydrochloric acid solution and mechanically disintegrating said bodies by crushing or milling to obtain metal carbide powder.

2. A method of recovering high purity hard metal carbide powder substantially free of binder phase, which comprises treating the metal carbide powder obtained by the method of claim 1, with aqueous 28-37%) hydrochloric acid for about 4 to 6 hours at a temperature of up to about 90 C, separating the powder from the aqueous hydrochloric acid solution by decantation, washing the powder with distilled water, washing the powder with aqueous ammonia solution, separating it by decantation and drying it at an elevated temperature.

3. A method according to claim 1, wherein after the hydrochloric acid treatment, the hard metal carbide bodies are washed several times with water.

4. A method according to claim 1, wherein the hard metal carbide bodies are dried by a stream of compressed air.

5. A method according to claim 2, wherein the metal carbide powder is washed with about 25 % aqueous ammonia.

6. A method according to claim 2, wherein the metal carbide powder is dried at a temperature of about 105 to 115° C for about 12 hours.

7. A method according to any one of the preceding claims wherein the hard metal carbide bodies consist substantially of about 92-99% WC and about 6% Co.

8. A method according to claim 7 wherein the hard metal carbide bodies are uncoated.

9. A method according to claim 7 wherein the hard metal carbide bodies have a thin surface coating layer of TiC-TiN.

AMENDED CLAIMS

[received by the International Bureau on 04 January 1999 (04.01.99); original claim 7 amended; remaining claims unchanged (1 page)]

1. A method of disintegrating sintered hard metal carbide bodies cemented by a binder phase, except for such bodies having a c«rarnic surface coating, which method comprises immersing said bodies in 28-37% aqueous hydrochloric acid for about 10 to 48 hours at a temperature of up to about 90° C, separating the thus treated bodies from the hydrochloric acid solution and mechanically disintegrating said bodies by crushing or n illing to obtain metal carbide powder.

2. A method of recovering high purity hard metal carbide powder substantially free of binder phase, which comprises treating the metal carbide powder obtained by the method of claim 1, with aqueous 28-37% hydrochloric acid for about 4 to 6 hours at a temperature of up to about 90°C. separating the powder from the aqueous hydrochloric acid solution by decantation, washing the powder with distilled water, washing the powder with aqueous ammonia solution, separating it by decantation and drying it at an elev.ted temperature.

4. A method according to claim 1, wherein the hard metal carbide bodies are dried by a stream of compressed air. 5. A method according to claim 2, wherein the metal carbide powder is washed with about 25% aqueous ammonia.

7. A method according to any one of the preceding claims wherein the hard metal carbide bodies consist substantially of about 80-97% of WC, Tic and Ta (Nb)C and about 3-16% of Co.