EP2716387B1

EP2716387B1 - Preparation method of a coated cobalt powder

Info

Publication number: EP2716387B1
Application number: EP11866957.1A
Authority: EP
Inventors: Miaomiao Guo; Zhebo MA; Xianda He; Jifeng ZHOU; Yue Fu; Cong Liu
Original assignee: GEM Co Ltd China; Jingmen GEM New Material Co Ltd
Current assignee: GEM Co Ltd China; Jingmen GEM New Material Co Ltd
Priority date: 2011-05-30
Filing date: 2011-07-28
Publication date: 2019-05-29
Anticipated expiration: 2031-07-28
Also published as: EP2716387A1; EP2716387A4; CN102303114B; CN102303114A; WO2012162936A1; US20140113140A1

Description

FIELD OF THE INVENTION

The present invention relates to primary cobalt powder in the field of hard alloys, particularly relates to coated cobalt powder and preparation method thereof.

BACKGROUND OF THE INVENTION

Hard alloys are commonly composed of refractory metal carbide and metallic binder (e.g., cobalt and nickel), the preparation method thereof comprises steps of: mixing metal carbide and metallic binder, ball milling (stirring and mixing), forming (adding forming agent, pelletizing, pressing, removing forming agent) and sintering.
Forming herein is an important process of the preparation of hard alloys. Because hard alloys mainly consist of refractory metal carbides having high hardness, high elastic modulus and high compressive strength, so it is difficult for the powders to deform plastically and form. And there still are slight defects (holes, inclusions, flakes and cracks) existing in the preform, such slight defects in the sintering process are not easy to be eliminated, may seriously affect product quality. Forming agent is added to help improve the performance of the blank and the sintered product. Meanwhile, forming agent must be able to be completely removed; any residual may threaten the sintered product quality.
At present, paraffin wax is a main forming agent commonly used at home and abroad (PEG polyethylene glycol and polyethylene PE and other forming agent have also been reported).Normally, the hard alloys are wet milled and dried, then mixed with paraffin wax in the form of wax-gasoline while stirring, the stirring makes paraffin wax evenly disperse in hard alloys. It is also reported that refractory metal carbides, metallic bonder and paraffin wax can be ball milled directly. However, in these methods, before doping wax, the metallic bonder (e.g., cobalt and nickel) tends to be oxidized by oxygen or water vapor in the air, especially fine particles and ultrafine particles. It is difficult to control the quality of hard alloys particles. In addition, before adding forming agent, the processes of mixing and ball milling are easy to generate lots of respirable dust, pollute the environment in a hard alloy production shop, and seriously threaten the health of workers.
WO 00/30787 discloses a deagglomerating cobalt powder with an FSSS average grain size of < 0.5 µm.
EP 0 437 972 discloses a non-aqueous coating composition comprising a urethane resin a paraffin wax and an organic solvent.

SUMMARY OF THE INVENTION

To solve the problem, the present invention provides methods for preparing coated cobalt powder. The cobalt powder consists of cobalt powder and paraffin wax, polyethylene glycol or polyethylene coated on the outer surface of the cobalt powder. Using the coated cobalt powder to replace traditional cobalt powder as the raw material for preparing hard alloys, the dusts generated during production can be reduced, and the environment in a production shop can be improved; simultaneously, the paraffin wax, the polyethylene glycol or the polyethylene coated on the surface of the cobalt powder can effectively prevent the cobalt powder from being oxidized by the oxygen or water vapor in the air, thus improving the quality of the cobalt powder.
The disclosure herein thus relates to coated cobalt powder, consisting of cobalt powder, and paraffin wax, polyethylene glycol or polyethylene coated on the outer surface of the cobalt powder, said paraffin wax, polyethylene glycol or polyethylene is in an amount of 1.5% to 2.5% by weight of said cobalt powder.
Herein, paraffin wax which is saturated straight-chain hydrocarbons has advantages such as: no mechanical impurities, low molecular weight, and being able to volatilize completely at a high temperature without leaving any residues; polyethylene glycol (PEG) is completely soluble in water and compatible with many substances, showing great compatibility with polar substances, non-toxic and non-irritating; polyethylene (PE) is crystalline material, with flexible molecular chain, small bond forces, and having low hygroscopicity, low melt viscosity and excellent mobility. Paraffin wax, polyethylene glycol or polyethylene served as a forming agent is commonly added and mixed with cobalt powder in the forming process during the preparation of hard alloys.
The cobalt powder has an outer surface that is coated with appropriate amount of paraffin wax, polyethylene glycol or polyethylene. The cobalt powder can replace traditional cobalt powder as the raw material for preparing hard alloys. The particle size of the coated cobalt powder of the present invention is larger than the primary cobalt powder. Paraffin wax, polyethylene glycol or polyethylene in the coated cobalt powder uniformly coats on the surface of the primary cobalt powder. Preferably, paraffin wax, polyethylene glycol or polyethylene is in an amount of 2% by weight of cobalt powder.
Using the coated cobalt powder disclosed herein to replace traditional cobalt powder as the raw material for preparing hard alloys, the dusts generated during production can be reduced, and the environment in a production shop can be improved; simultaneously, the paraffin wax, the polyethylene glycol or the polyethylene coated on the surface of the cobalt powder can effectively prevent the cobalt powder from being oxidized by the oxygen or water vapor in the air, thus improving the quality of the cobalt powder.
One aspect of the present invention provides a method for preparing coated cobalt powder, comprising: providing cobalt powder and paraffin wax in an amount of 1.5% to 2.5% by weight of cobalt powder; pouring paraffin wax into organic solvent then dissolving and forming paraffin wax-organic solvent mixture; adding paraffin wax-organic solvent mixture into cobalt powder, placing into planetary mixer for stirring and mixing, vacuum drying to obtain waxed cobalt powder.
The organic solvent herein is organic solvent having low boiling point, and in which paraffin wax has great solubility. Preferably, the organic solvent is n-hexane, n-heptane, methylbenzene or ethyl acetate. More preferably, the organic solvent is ethyl acetate because ethyl acetate, by contrast, has advantages that: ethyl acetate has low boiling point, high flashing point, high burning point and relatively inexpensive price; ethyl acetate is an organic solvent in which paraffin wax has moderate solubility; ethyl acetate is easy to dry and relatively safe. Organic solvent together with condensate is recycled in the reaction, it is suitable for industrial production. Different grades of paraffin wax have different dissolving temperatures and mixing temperatures. Preferably, solid-liquid ratio of ethyl acetate to cobalt powder (g / mL) is in the range of 20 to 50:1. More preferably, when dissolving No. 52 paraffin wax using ethyl acetate, the solid-liquid ratio of ethyl acetate to cobalt powder (g / mL) is in the range of 20 to 25:1. Preferably, the temperature of organic solvent is in the range of 2 to 50°C.Preferably, paraffin wax is in an amount of 2% by weight of cobalt powder.
Too short the mixing time leads to uneven coating, too long the mixing time leads to increased time costs of industrial production. Preferably, the stirring and mixing is carried out at a stirring speed in the range of 40 to 100 r/min for 1 to 5 h. More preferably, the stirring and mixing is carried out for 1 to 2 h. There is no specific requirement to the temperature and pressure during the stirring and mixing process, low temperature (2 to 50°C) and normal pressure will be allowed. Subsequently preferably, the vacuum drying temperature is in the range of 40 to 55 °C. After obtaining coated cobalt powder, organic solvent is recycled together with condensate.
Also discloses herein is an unclaimed method for preparing coated cobalt powder, comprising: providing cobalt powder and paraffin wax in an amount of 1.5% to 2.5% by weight of cobalt powder; placing into planetary mixer, stirring and mixing under the protection of vacuum or nitrogen to obtain waxed cobalt powder. Preferably, paraffin wax is in an amount of 2% by weight of cobalt powder.
This method does not involve the process of dissolving paraffin wax with organic solvent, but melting paraffin wax under a certain mixing temperature in the process of stirring and mixing. Preferably, the stirring and mixing is carried out at a temperature in the range of 55 to 60°CPreferably, the stirring and mixing is carried out at a stirring speed in the range of 40 to 100 r/min for 1 to 5 h. More preferably, the stirring and mixing is carried out for 1 to 2 h. Also disclosed herein is an unclaimed method for preparing coated cobalt powder, comprising: providing cobalt powder, and polyethylene glycol or polyethylene in an amount of 1.5% to 2.5% by weight of cobalt powder; placing into planetary mixer, vacuum drying to obtain cobalt powder coated with polyethylene glycol, or cobalt powder coated with polyethylene.
Preferably, the stirring and mixing is carried out at a stirring speed in the range of 40 to 100 r/min for 1 to 5 h. More preferably, the stirring and mixing is carried out for 1 to 2 h. There is no specific requirement to the temperature and pressure during the stirring and mixing process, low temperature (2 to 50°C) and normal pressure will be allowed. Subsequently preferably, the vacuum drying is carried out at a temperature in the range of 40 to 55 °C.
The present invention firstly providing coated cobalt powder and preparation method thereof has the following advantages: Using the coated cobalt powder to replace traditional cobalt powder as the raw material for preparing hard alloys, the dusts generated during production can be reduced, and the environment in a production shop can be improved; simultaneously, the paraffin wax, the polyethylene glycol or the polyethylene coated on the surface of the cobalt powder can effectively prevent the cobalt powder from being oxidized by the oxygen or water vapor in the air, thus improving the quality of the cobalt powder. The preparation process of the coated cobalt powder is free from contamination and the coated cobalt powder is suitable for industrialized production.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a picture of dust experiment of primary cobalt powder and waxed cobalt powder;
Fig. 2 is an SEM image of waxed cobalt powder (organic solvent is ethyl acetate);
Fig. 3 is an SEM image of waxed cobalt powder (organic solvent is n-hexane);
Fig. 4 is an SEM image of primary cobalt powder.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

Further description of the present invention will be illustrated, which combined with embodiments in the drawings, in order to make the purpose, the technical solution and the advantages clearer. While the present invention has been described with reference to particular embodiments, it will be understood that the embodiments are illustrative and that the invention scope is not so limited.

Example 1

A method for preparing coated cobalt powder, comprising: forming paraffin wax-ethyl acetate mixture by dissolving 9.6 g of paraffin wax into 20 mL of ethyl acetate at 25°C; adding paraffin wax-ethyl acetate mixture into 480 g of cobalt powder; placing into planetary mixer; stirring and mixing at a speed of 60 r/min for 1.5 h ; then vacuum drying at 50°C to obtain waxed cobalt powder.
The waxed cobalt powder can be used for preparing hard alloys. The method for preparing hard alloys comprises the steps of: mixing tungsten carbide with waxed cobalt powder; adding a small amount of paraffin wax as a forming agent; stirring and mixing; pressing and forming; dewaxing at a low temperature; and sintering at a high temperature.
By reasonably controlling the total addition amount of paraffin wax, the paraffin wax in the waxed cobalt powder and the paraffin wax served as forming agent which is added subsequently can be completely removed in the step of dewaxing, also, it will not affect the sintering of hard alloys. Using the waxed cobalt powder, the dusts generated during production can be reduced, and the environment in a production shop can be improved; simultaneously, the cobalt powder can be effectively prevented from being oxidized by the oxygen or water vapor in the air, thus improving the quality of the cobalt powder, and then creating additional value to cobalt powder, making the cobalt powder more competitive.
After removal of paraffin wax, cobalt powders served as a binder during the sintering process intersperse with tungsten lattices by sintering at a high temperature, the morphology has little effect on the quality of hard alloys.

Example 2

A method for preparing coated cobalt powder, comprising: providing 500 g of cobalt powder and 10 g of paraffin wax separately; placing into planetary mixer; stirring and mixing under the protection of nitrogen to obtain waxed cobalt powder. The stirring and mixing is carried out at 58°C and at a speed of 60 r /min for 3 h .
The method involves in melting paraffin wax directly at the mixing temperature of 58°C, the stirring and mixing makes the paraffin wax uniformly coat on the outer surface of cobalt powder.

Example 3

A method for preparing coated cobalt powder, comprising: providing 500 g of cobalt powder and 10 g of polyethylene glycol separately; placing into planetary mixer for stirring and mixing, the stirring and mixing is carried out at a speed of 50 r /min for 4 h; then vacuum drying at 45°C to obtain cobalt powder coated with polyethylene glycol.
Paraffin wax, polyethylene glycol and polyethylene are well known forming agent used in the preparation of hard alloys. An Example illustrating the effects of typical waxed cobalt powder is therefore provided in the present invention.

An Example illustrating the effects

I Experimental method

Paraffin wax in an amount of 2% by weight of cobalt powder was weighed and dissolved separately in 20 to 30 mL organic solvents at 2 to 50°C (n-hexane, n-heptane, methylbenzene and ethyl acetate). Then 600 g of cobalt powder (normal cobalt powder, which can be purchased in the market, hereinafter referred to as primary cobalt powder) was added. The mixture was stirred in planetary mixer at a speed of 100 r/min and 0.6∼1 atm for 1 to 5 h, then vacuum dried at 50°C and labeled. Specific optimized process was as follows: waxed cobalt powders obtained in different experiments were labeled with different letters (for example, the label "ethyl acetate A" refers to the waxed cobalt powder obtained by using ethyl acetate as organic solvent and under the conditions corresponding to the Table 1).

II Results

1. Effects of different organic solvents on waxing

Paraffin wax has different solubility in different organic solvents, thus it is important to choose proper organic solvent. This experiment aims to investigate the effects of different organic solvents (n-hexane, methylbenzene, ethyl acetate and n-heptane) on waxing. The results are shown in Table 1 (wherein paraffin wax refers to No. 52 paraffin wax, FSSS refers to Fisher particle size, paraffin wax% refers to the amount of paraffin wax by weight of cobalt powder, similarly hereinafter; moreover, n-hexane A and n-hexane B refer to that n-hexane reacts with primary cobalt powder 1; methylbenzene, ethyl acetate A, ethyl acetate B and n-heptane reacts with primary cobalt powder 2, respectively).

Table 1. Conditions and results of the experiments in preparing waxed cobalt powder by using different organic solvents

Number	Experimental conditions						results
	paraffin wax	dissolving temperature/°C	mixing temperature/°C	pressure /atm.	solid-liquid ratio	stirring time/h	FSSS µm	hydrogen loss %	paraffin wax%
primary cobalt powder 1							1.33	0.48
n-hexane A	52	50	50	0.6	20	5	1.49	0.3	2.04
n-hexane B	52	50	2	1	20	2	1.30	0.21	2.18
primary cobalt powder 2							1.2	0.45
methylbenzen e	52	50	2	1	20	5	1.27	0.15	2.05
ethyl acetate A	52	50	2	1	20	5	1.3	0.32	2.02
ethyl acetate B	52	50	50	0.6	20	5	1.37	0.36	2.28
n-heptane	52	50	50	0.6	20	5	1.41	0.34	2.09

Conclusion: n-hexane, methylbenzene, ethyl acetate and n-heptane can be used as a solvent to dissolve the paraffin wax in waxed cobalt powder. Ethyl acetate is preferred in the present invention, considering that the first choice for industrial production is an organic solvent having low boiling point, high flashing point, high burning point and relatively inexpensive price, also the organic solvent is an organic solvent in which paraffin wax has moderate solubility, and the organic solvent is easy to dry and relatively safe.

2. Effects of different mixing conditions on waxing

This experiment investigates different mixing conditions (mixing time, mixing temperature, mixing pressure and mixing solid-liquid ratio) on waxing by using ethyl acetate as organic solvent to dissolve No. 52 paraffin wax. The results are shown in Table 2 to 4.

Table 2. Conditions and results of the experiments in preparing waxed cobalt powder for different mixing time

Number	Experimental conditions					results
Organic solvent	dissolving temperature/°C	mixing temperature/°C	pressure/atm.	solid-liquid ratio	stirring time/h	Fsss µm	hydrogen loss %	paraffin wax%
primary cobalt powder 2						1.2	0.45
ethyl acetate C	50	2	1	20	0.5	1.29	0.62	2.34
ethyl acetate D	50	2	1	20	1	1.29	0.44	2.13
ethyl acetate E	50	2	1	20	2	1.28	0.25	2.09
ethyl acetate F	50	2	1	20	3	1.30	0.3	2.14
ethyl acetate G	50	2	1	20	4	1.3	0.27	2.32
ethyl acetate H	50	2	1	20	5	1.3	0.32	2.02

Analysis: It can be seen from Table 2 that when stirring for 0.5 h, the mixture is mixed unevenly, leading to 2% higher amount of paraffin wax than the theoretical value, and higher hydrogen loss. When stirring for 1h or more, the mixture is mixed evenly, content of paraffin wax fluctuated around 2%, so that the mixing time is preferably in the range of 1 to 2h.

Table 3. Conditions and results of the experiments in preparing waxed cobalt powder at different mixing temperatures

Number	Experimental conditions					results
Organic solvent	dissolving temperature/°C	mixing temperature/°C	pressure/atm.	solid-liquid ratio	stirring time/h	Fsss µm	hydrogen loss %	paraffin wax%
primary cobalt powder2						1.2	0.45
ethyl acetate I	50	2	1	20	2	1.28	0.25	2.09
ethyl acetate J	50	50	1	20	2	1.33	0.3	2.13

Analysis: It can be seen from Table 3 that there is no big difference in the experiment results between the coated cobalt powders obtained under the conditions of mixing temperature in the range of 2 to 50°C, 1 atm mixing pressure and 2 hours mixing time. So the experiment has low requirement to environment (temperature and pressure). Low temperature (2 to 50°C) and normal pressure will be allowed.

Table 4 Conditions and results of the experiments in preparing waxed cobalt powder at different solid-liquid ratios

Number	Experimental conditions					results
Organic solvent	dissolving temperature/°C	mixing temperature/°C	pressure/atm.	solid-liquid ratio	stirring time/h	Fsss µm	hydrogen loss %	paraffin wax%
primary cobalt powder2						1.2	0.45
ethyl acetate K	50	2	1	20	2	1.28	0.25	2.09
ethyl acetate L	50	2	1	25	2	1.25	0.26	1.89
ethyl acetate M	50	2	1	30	2	Paraffin wax separated out

Analysis: Solubility of No. 52 paraffin wax in ethyl acetate is 450 g/L, the theoretical solid-liquid ratio can be figured out is 22.2. It can be seen from Table 4 that the solution is supersaturated, so that the paraffin wax dissolves completely even the solid-liquid ratio is increased to 25. Paraffin wax does not separate from cobalt powder at 2 to 50°C, so it can coat uniformly on the surface of solid cobalt powder. Solid-liquid ratio of ethyl acetate to cobalt powder (g/mL) is therefore preferably in the range of 20 to 25:1. Considering that different grades of paraffin wax have different dissolving temperature and mixing temperature, when employing other grade of paraffin wax, solid-liquid ratio of ethyl acetate to cobalt powder (g/mL) is in the range of 20 to 50:1.
Conclusion: when using ethyl acetate as organic solvent, the mixing is preferably carried out for 1 to 2 h, solid-liquid ratio (g/mL) of ethyl acetate to cobalt powder is preferably in the range of 20 to 50:1, low temperature (2 to 50°C) and normal pressure will be allowed.

III Characterization results

Primary cobalt powder 2 was provided to prepare waxed cobalt powder samples under the preferred conditions set forth above, the samples were used in the characterization experiment below. See Table 5.

Table 5 Conditions for the preparation of waxed cobalt powder samples

Number	Experimental conditions
	dissolving temperature/°C	mixing temperature/°C	pressure/atm.	solid-liquid ratio	stirring time/h
waxed cobalt powder sample (ethyl acetate N)	50	2	1	25	2

1. Resistance against oxidation

Hydrogen loss of the waxed cobalt powder sample (using ethyl acetate as organic solvent) and primary cobalt powder was simultaneously measured over time. The results are shown in Table 6.

Table 6 changes of hydrogen loss of cobalt powders over time at different temperatures

Time	20°Cprimary cobalt powder hydrogen loss	20°Cwaxed cobalt powder hydrogen loss	40°Cprimary cobalt powder hydrogen loss	40°Cwaxed cobalt powder hydrogen loss
0	0.48	0.25	0.48	0.25
2h	0.48	0.22	0.48	0.28
4h	0.41	0.18	0.41	0.23
6h	0.39	0.21	0.35	0.24

Analysis: paraffin wax mixed with the waxed cobalt powder sample may affect oxygen & sulfur analyzer, so oxygen content is replaced with hydrogen loss. It can be seen from Table 6 that in 20°C and 40°C environments, hydrogen loss of waxed cobalt powder samples is lower than that of primary cobalt powder, which indicates that waxed cobalt powder samples have resistance against oxidation to a certain extent.

2. Dust experiment

100 g of primary cobalt powder and waxed cobalt powder samples (using ethyl acetate as organic solvent) were placed into wild-mouth bottle separately, then the bottle was shaken for 1 min. Primary cobalt powder was on the left, waxed cobalt powder sample was on the right. The results are shown in Fig. 1. Fig. 1 is a picture of dust experiment of primary cobalt powder and waxed cobalt powder.
Analysis: It can be seen from Fig. 1 that there is lots of dust, 1.1 g, sticking to the wall of the bottle containing primary cobalt powder, whereas there is less dust, 0.8 g, sticking to the wall of the bottle containing waxed cobalt powder. The dust is reduced by 36% after waxing.

3. Dispersity

The dispersity is a measure of the heterogeneity of sizes of powders in the air. The determination method involves dropping 10 g of primary cobalt powder and waxed cobalt powder from a certain height, determining amount of the samples which fail to fall into the receiver, calculating the percentage of the samples by weight of the total amount of primary cobalt powder and waxed cobalt powder. The results are shown in Table 7. Dispersity is related to dispersibility, floatability and splash ability of the samples.
Analysis: it can be seen from the data that, dispersity of cobalt powder decreases apparently after adding paraffin wax, dispersity decreases from 5.3% to 1.6% even lower to 0.9%, indicating that waxed cobalt powder generates less dusts than primary cobalt powder.

4. SEM

Fig. 2, Fig. 3 and Fig. 4 are SEM images. Fig. 2 is an SEM image of waxed cobalt powder sample (using ethyl acetate as organic solvent), Fig. 3 is an SEM image of waxed cobalt powder (using n-hexane A as organic solvent), Fig. 4 is an SEM image of primary cobalt powder.
Analysis: it can be seen from Fig. 2, Fig. 3 and Fig. 4 that particle size of waxed cobalt powder is greater than primary cobalt powder. The paraffin wax in waxed cobalt powder uniformly coats on the surface of primary cobalt powder.

Claims

A method for preparing coated cobalt powder, comprising:
providing cobalt powder and paraffin wax in an amount of 1.5% to 2.5% by weight of cobalt powder;

pouring paraffin wax into organic solvent then dissolving and forming paraffin wax-organic solvent mixture;

adding paraffin wax-organic solvent mixture into cobalt powder, placing into planetary mixer for stirring and mixing, vacuum drying to obtain waxed cobalt powder.
The method for preparing coated cobalt powder according to claim 1, wherein said organic solvent is n-hexane, n-heptane, methylbenzene or ethyl acetate.
The method for preparing coated cobalt powder according to claim 2, wherein solid-liquid ratio of said cobalt powder to ethyl acetate is in the range of 20 to 50:1.
The method for preparing coated cobalt powder according to claim 1, wherein the stirring and mixing is carried out at a stirring speed in the range of 40 to 100 r/min for 1 to 5 h.