High-wear-resistance bimetal composite grinding ball and preparation method thereof

Abstract

This invention belongs to the field of metal matrix composite materials and grinding media production technology, specifically relating to a high wear-resistant bimetallic composite grinding ball and its preparation method, including the following steps: S1: Batching and vacuum purification melting of the inner core alloy; S2: Thixotropic preparation of the semi-solid core blank; S3: Melting of the outer wear-resistant alloy and preparation of in-situ self-generated TiC-VC reinforcing phase; S4: Assembly and preheating of the vacuum negative pressure mold cavity; S5: Negative pressure composite casting and instantaneous high-pressure die forging: the outer wear-resistant alloy liquid obtained in S3 is injected into the mold to wrap the core blank, and after casting, high-pressure die forging is applied instantaneously and held under pressure, and the composite grinding ball blank is removed after mold opening; S6: Gradient temperature controlled heat treatment: the blank obtained in S5 is subjected to gradient quenching-tempering treatment; S7: Shot blasting finishing to obtain the finished product. This invention achieves a dual combination of interfacial metallurgical diffusion and mechanical interlocking through vacuum negative pressure, semi-solid composite and high-pressure die forging operations, thereby improving the interfacial shear strength and completely eliminating the peeling of the wear-resistant layer.

Description

Technical Field

[0001] This invention belongs to the field of metal matrix composite materials and grinding media production technology, specifically relating to a high wear-resistant bimetallic composite grinding ball and its preparation method. Background Technology

[0002] High wear-resistant bimetallic composite grinding balls, as the core grinding media of ball mills, directly affect grinding efficiency, production costs, and service life. Existing technologies for preparing bimetallic composite grinding balls mainly include centrifugal casting, dual-liquid gravity casting, surface welding / laser cladding, and powder metallurgy, but all have insurmountable technical drawbacks.

[0003] 1. Centrifugal casting and dual-liquid gravity casting both employ liquid-liquid composite molding, relying on centrifugal force or density difference to achieve metal stratification. This easily leads to component segregation, interfacial oxidation inclusions, porosity, and shrinkage defects, resulting in low interfacial bonding strength (typically 120-180MPa). Small-sized grinding balls (Φ<30mm) exhibit poor composite stability, while large-sized grinding balls (Φ>150mm) are prone to cracking and out-of-roundness.

[0004] 2. Surface modification processes such as surface welding and laser cladding are solid-liquid coatings, which have problems such as uneven wear-resistant layer thickness, low production efficiency, and high cost. In addition, the wear-resistant layer is mechanically bonded to the core, which is prone to peeling off during service.

[0005] 3. Powder metallurgy involves pressing and sintering to form products with low density (usually <95%), insufficient mechanical properties, high raw material costs, long production cycles, and limited grinding ball specifications (Φ<80mm), making it difficult to adapt to large-scale industrial production.

[0006] In view of this, and to address the above-mentioned shortcomings, the present invention provides a method for preparing a highly wear-resistant bimetallic composite grinding ball. Summary of the Invention

[0007] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing a high wear-resistant bimetallic composite grinding ball, comprising the following steps:

[0008] S1: Inner core alloy batching and vacuum purification smelting: low alloy raw materials containing Fe, C, Mn, Si and Ni are prepared according to mass fraction, grain refiner and deoxidizer are added, and smelting is carried out in a vacuum environment and heat preservation is carried out to remove gas.

[0009] S2: Thixotropic preparation of semi-solid tough core blank: The inner layer alloy liquid obtained in S1 is cooled to the semi-solid temperature range, and then injected into the pre-forming mold after electromagnetic stirring to obtain a semi-solid tough core blank.

[0010] S3: Smelting of outer wear-resistant alloy and preparation of in-situ self-generated TiC-VC reinforcing phase: Wear-resistant alloy raw materials containing Fe, C, Cr, Mo and W are prepared according to mass fraction, and in-situ reinforcing precursors are added. The alloy is smelted and kept warm in a vacuum environment so that Ti, V and C react in-situ to generate nano-TiC-VC dual hard phase.

[0011] S4: Assembly and preheating of vacuum negative pressure mold cavity: Place the semi-solid tough core blank obtained in S2 into a closed spherical forging mold, preheat the mold and then evacuate to the set negative pressure;

[0012] S5: Negative pressure composite casting and instantaneous high pressure die forging: The outer layer wear-resistant alloy liquid obtained in S3 is injected into the mold to wrap the tough core blank. After casting, high pressure die forging is applied instantaneously and pressure is maintained. The composite grinding ball blank is then taken out after the mold is opened.

[0013] S6: Gradient temperature controlled heat treatment: Gradient quenching-tempering treatment is performed on the blank obtained in S5.

[0014] S7: Shot blasting finishing: The heat-treated grinding balls are shot blasted and finely ground to obtain the finished product.

[0015] As a preferred embodiment of the high wear-resistant bimetallic composite grinding ball and its preparation method of the present invention, the mass composition of the low alloy raw material in S1 is as follows: C 0.18-0.25%, Mn 1.4-1.8%, Si 0.7-1.0%, Ni 0.4-0.7%, with the balance being Fe; the grain refiner is Mg, with an addition amount of 0.02-0.04%; and the deoxidizer is pure Ti, with an addition amount of 0.08-0.15%.

[0016] In a preferred embodiment of the high wear-resistant bimetallic composite grinding ball and its preparation method of the present invention, in step S1, the vacuum degree is ≤6×10⁻⁶. -3 Pa, melting temperature is 1470-1500℃, and holding and degassing time is 28-35min.

[0017] As a preferred embodiment of the high wear-resistant bimetallic composite grinding ball and its preparation method of the present invention, in step S2, the semi-solid temperature range is 1280-1350℃, the solid phase rate is 50-60%, the electromagnetic stirring speed is 280-350 r / min, and the stirring time is 9-12 min.

[0018] As a preferred embodiment of the high wear-resistant bimetallic composite grinding ball and its preparation method of the present invention, in step S3, the mass composition of the wear-resistant alloy raw material is: C 2.6-3.0%, Cr 24-28%, Mo 1.3-1.8%, W 0.7-1.0%, with the balance being Fe; the in-situ reinforcing precursor is Fe-Ti50 master alloy, Fe-V50 master alloy and ultrafine graphite powder, which are added in an atomic ratio of C:Ti:V=1:1:1.

[0019] In a preferred embodiment of the high wear-resistant bimetallic composite grinding ball and its preparation method of the present invention, the vacuum degree in step S3 is ≤1.2×10⁻⁶. -3 Pa, melting temperature is 1500-1550℃, holding time is 18-25min.

[0020] In a preferred embodiment of the high wear-resistant bimetallic composite grinding ball and its preparation method according to the present invention, in step S4, the mold preheating temperature is 430-480℃, and the vacuum degree is ≤1.5×10⁻⁶. -2 Pa.

[0021] As a preferred embodiment of the high wear-resistant bimetallic composite grinding ball and its preparation method of the present invention, in step S5, the pouring temperature of the outer wear-resistant alloy liquid is 1480-1500℃, the forging pressure is 120-160MPa, the holding time is 4-8min, and the time interval from the completion of pouring to the application of the forging pressure is ≤4s.

[0022] As a preferred embodiment of the high wear-resistant bimetallic composite grinding ball and its preparation method of the present invention, in step S6, the gradient heat treatment parameters are: outer layer quenching temperature 970-1000℃, core tempering temperature 710-740℃, and oil quenching cooling method is adopted.

[0023] A highly wear-resistant bimetallic composite grinding ball is prepared using the above-described method.

[0024] Compared with the prior art, the beneficial effects of the present invention are:

[0025] 1. This invention achieves a dual combination of interfacial metallurgical diffusion and mechanical interlocking through vacuum negative pressure, semi-solid composite and high-pressure die forging operations, thereby improving the interfacial shear strength and completely eliminating the peeling of the wear-resistant layer;

[0026] 2. This invention achieves a wear rate of ≤0.08g / t through the synergistic effect of an outer layer of in-situ self-generated nano-TiC-VC dual hard phase and M7C3 carbide, which is more than 60% lower than that of traditional high-chromium balls;

[0027] 3. The fine-grained tough core prepared by semi-solid electromagnetic stirring in this invention has an impact energy AKV≥28J and does not crack or lose its roundness under high impact conditions;

[0028] 4. This invention overcomes the limitations of centrifugal force and mold size, and can stably produce grinding balls of all sizes from Φ10 to Φ200mm to meet the needs of different working conditions. Attached Figure Description

[0029] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings:

[0030] Figure 1 This is a schematic diagram of the process of the present invention. Detailed Implementation

[0031] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] This invention relates to a method for preparing a highly wear-resistant bimetallic composite grinding ball, comprising the following steps:

[0033] S1: Inner core alloy batching and vacuum purification smelting: low alloy raw materials containing Fe, C, Mn, Si and Ni are prepared according to mass fraction, grain refiner and deoxidizer are added, and smelting is carried out in a vacuum environment and heat preservation is carried out to remove gas.

[0034] S2: Thixotropic preparation of semi-solid tough core blank: The inner layer alloy liquid obtained in S1 is cooled to the semi-solid temperature range, and then injected into the pre-forming mold after electromagnetic stirring to obtain a semi-solid tough core blank.

[0035] S3: Smelting of outer wear-resistant alloy and preparation of in-situ self-generated TiC-VC reinforcing phase: Wear-resistant alloy raw materials containing Fe, C, Cr, Mo and W are prepared according to mass fraction, and in-situ reinforcing precursors are added. The alloy is smelted and kept warm in a vacuum environment so that Ti, V and C react in-situ to generate nano-TiC-VC dual hard phase.

[0036] S4: Assembly and preheating of vacuum negative pressure mold cavity: Place the semi-solid tough core blank obtained in S2 into a closed spherical forging mold, preheat the mold and then evacuate to the set negative pressure;

[0037] S5: Negative pressure composite casting and instantaneous high pressure die forging: The outer layer wear-resistant alloy liquid obtained in S3 is injected into the mold to wrap the tough core blank. After casting, high pressure die forging is applied instantaneously and pressure is maintained. The composite grinding ball blank is then taken out after the mold is opened.

[0038] S6: Gradient temperature controlled heat treatment: Gradient quenching-tempering treatment is performed on the blank obtained in S5.

[0039] S7: Shot blasting finishing: The heat-treated grinding balls are shot blasted and finely ground to obtain the finished product.

[0040] Example 1:

[0041] Taking the preparation of Φ30mm high wear-resistant bimetallic composite grinding balls as an example:

[0042] Inner core alloy formulation and vacuum purification smelting: Raw materials were prepared according to the following mass fractions: Fe-0.18C-1.4Mn-0.7Si-0.4Ni, with the addition of 0.02% Mg and 0.08% pure Ti; the mixture was then placed in a vacuum induction furnace and evacuated to a vacuum level of 4×10⁻⁶. -3 Pa is heated to 1470℃ to melt, held at that temperature for 28 minutes to degas, and then allowed to stand after slag removal. Among them, C, Mn, and Si ensure the core strength and plasticity; Ni improves the low-temperature impact toughness; Mg is a grain refiner to eliminate coarse columnar crystals; Ti deoxidizes and purifies grain boundaries; and a vacuum environment eliminates oxide inclusions and pores to ensure the purity of the core blank.

[0043] Thixotropic preparation of semi-solid tough core blank: The molten steel is cooled to 1300℃ (solidity ≈ 52%), electromagnetic stirring is turned on (speed 280 r / min), and the mixture is stirred at this temperature for 9 min. The mixture is then rapidly poured into a Φ30mm spherical preforming mold to obtain a semi-solid tough core blank. In the semi-solid state, there is no violent liquid flow, and subsequent composite processing does not result in slag entrapment or damage to the core blank. Electromagnetic stirring produces an equiaxed fine-grained structure, which significantly improves the toughness of the core. Preforming ensures the matching of inner and outer layer dimensions and uniform thickness of the wear-resistant layer.

[0044] Outer layer wear-resistant alloy smelting and in-situ self-generated TiC-VC reinforcing phase preparation: Raw materials were prepared according to the following mass fraction: Fe-2.6C-24Cr-1.3Mo-0.7W; Fe-Ti50 master alloy, Fe-V50 master alloy, and ultrafine graphite powder were added according to the atomic ratio C:Ti:V=1:1:1; the mixture was placed in a vacuum furnace and evacuated to 8×10⁻⁶. -4 Pa is heated to 1500℃ and melted, and held for 18 minutes to generate nano-TiC-VC dual hard phases in situ; 25% Cr forms high-hardness M7C3 carbide, providing basic wear resistance; Mo and W stabilize the carbide and prevent softening during high-temperature tempering; Ti, V, and ultrafine graphite generate nano-dual hard phases in situ, dispersing and strengthening the wear-resistant layer without the problems of external particle agglomeration and weak bonding.

[0045] Vacuum negative pressure mold cavity assembly and preheating: Place the semi-solid core blank into the center of the closed spherical forging mold, preheat the mold to 430℃, and then evacuate the mold to 8×10℃. -3 Pa, maintaining negative pressure; among which, vacuum negative pressure completely eliminates interfacial pores, oxide films and inclusions; mold preheating reduces the temperature difference between the inner and outer layers to prevent cracking during compounding; closed mold ensures that the forging pressure is effectively applied to the entire grinding ball;

[0046] Negative pressure composite casting and instantaneous high-pressure die forging: 1480℃ outer layer wear-resistant alloy liquid is injected into the mold, completely encapsulating the core blank; within 2.5 seconds of casting completion, 120MPa axial die forging pressure is applied, held for 4 minutes, and the blank is removed from the mold; The outer liquid phase slightly fuses with the surface of the semi-solid core blank, forming a diffusion metallurgical bond; the 120MPa die forging pressure forcibly compacts the product, eliminating shrinkage cavities and porosity, and increasing density; achieving a triple combination of interfacial mechanical interlocking, metallurgical diffusion, and forging;

[0047] Gradient temperature controlled heat treatment: The residual heat after forging is used to send the material into a gradient heat treatment furnace. The outer layer is oil quenched at 970℃, and the core is tempered at 710℃. The temperature is then gradually reduced to room temperature to achieve decoupling of the inner and outer layer properties. The hardness of the outer layer is HRC63~66, and the hardness of the core is HB240~260. This eliminates internal stress and prevents the grinding ball from cracking and becoming out of round.

[0048] Shot blasting finishing: Shot blasting removes oxide scale, and fine grinding ensures roundness, producing Φ30mm finished grinding balls.

[0049] Example 2:

[0050] Taking the preparation of Φ80mm high wear-resistant bimetallic composite grinding balls as an example:

[0051] The inner layer raw material was prepared according to the following mass fraction: Fe-0.22C-1.6Mn-0.9Si-0.6Ni, with 0.03% Mg and 0.12% pure Ti added. It was then placed in a vacuum induction furnace, evacuated to 5×10⁻³ Pa, heated to 1490℃ for melting, held at this temperature for 32 min to remove gas, and allowed to stand after slag removal. The molten steel was cooled to 1330℃ (solidity ≈ 58%), and electromagnetic stirring was started (320 r / min). It was held at this temperature for 11 min and then quickly poured into an 80mm spherical preform mold to obtain a semi-solid tough core blank. The outer layer raw material was prepared according to the following mass fraction: Fe-2.9C-26Cr-1.6Mo-0.9W. Fe-Ti50 master alloy, Fe-V50 master alloy, and ultrafine graphite powder were added according to the atomic ratio C:Ti:V = 1:1:1. It was then placed in a vacuum furnace and evacuated to 1×10⁻³ Pa. -3 Pa, heated to 1530℃ to melt, held for 22 min, in situ generating nano-TiC-VC dual hard phase; the semi-solid core blank was placed in the center of a closed spherical forging die, the die was preheated to 460℃, and after sealing the die, a vacuum of 1.2×10⁻⁶ was drawn. -2Pa; maintain negative pressure; inject the outer layer of wear-resistant alloy liquid at 1495℃ into the mold, completely encapsulating the core blank; within 3.5s after pouring, apply 140MPa axial forging pressure, hold for 6min, open the mold and remove the blank; use the residual heat after forging to send it to a gradient heat treatment furnace, oil quench the outer layer at 990℃, temper the core at 730℃, and then gradually cool it to room temperature; shot blast to remove oxide scale, and fine grinding to ensure roundness, to obtain a Φ80mm finished grinding ball.

[0052] Example 3:

[0053] Taking the preparation of Φ180mm high wear-resistant bimetallic composite grinding balls as an example:

[0054] Prepare the raw materials according to the following mass fractions: Fe-0.25C-1.8Mn-1.0Si-0.7Ni, with the addition of 0.04% Mg and 0.15% pure Ti; place them into a vacuum induction furnace and evacuate to 6×10⁻⁶. -3 The solution is heated to 1500℃ and melted, held at that temperature for 35 minutes to remove gas, and then allowed to stand after slag removal. The molten steel is then cooled to 1350℃ (solidity ≈ 60%), and electromagnetic stirring is started (350 r / min). The mixture is held at that temperature and stirred for 12 minutes, and then quickly poured into a Φ180mm spherical preforming mold to obtain a semi-solid tough core blank. Raw materials are prepared according to the following mass fraction: Fe-3.0C-28Cr-1.8Mo-1.0W. Fe-Ti50 master alloy, Fe-V50 master alloy, and ultrafine graphite powder are added according to the atomic ratio C:Ti:V=1:1:1. The mixture is then placed in a vacuum furnace and evacuated to a vacuum level of 1.2 × 10⁻⁶. -3 Pa, heated to 1550℃ to melt, held for 25 min, in-situ generation of nano-TiC-VC dual hard phase. Vacuum negative pressure mold cavity assembly and preheating: The semi-solid core blank is placed in the center of a closed spherical forging mold, the mold is preheated to 480℃, and after sealing the mold, a vacuum of 1.5×10 is drawn. -2 Pa, maintaining negative pressure. Negative pressure composite casting and instantaneous high-pressure die forging: 1500℃ outer layer wear-resistant alloy liquid is injected into the mold, completely encapsulating the core blank; within 4 seconds of casting completion, an axial die forging pressure of 160MPa is applied, held for 8 minutes, and the blank is removed from the mold. Gradient temperature controlled heat treatment: Utilizing the residual heat after forging, the blank is sent to a gradient heat treatment furnace, with the outer layer oil quenched at 1000℃, the core tempered at 740℃, and then gradually cooled to room temperature. Shot blasting removes oxide scale, and precision grinding ensures roundness, yielding a Φ180mm finished grinding ball.

[0055] Comparative Example 1:

[0056] Taking the preparation of Φ80mm bimetallic composite grinding balls using the traditional centrifugal casting method as an example from existing technologies:

[0057] The outer layer of high-chromium cast iron is prepared according to the following mass fraction: Fe-3.2C-20Cr-0.5Mo, and the inner layer of low-alloy steel is prepared: Fe-0.3C-1.0Mn-0.5Si. The centrifugal casting mold is preheated to 250℃, and the centrifuge speed is started at 800r / min. The outer layer of high-chromium cast iron liquid is poured first (1450℃) to form a semi-solid shell, and then the inner layer of low-alloy steel liquid is poured (1520℃). After natural cooling, the mold is demolded, and the mixture is quenched at 950℃ and tempered at 680℃. The mixture is shot blasted and finely ground to obtain Φ80mm grinding balls.

[0058] The grinding balls prepared above were tested and compared, including the interfacial shear strength tested using the lap shear test method in GB / T 16777-2008 "Tension Properties of Fiber Reinforced Plastics"; the wear rate was tested by grinding in cement clinker media for 100 hours using an MQ-800 ball mill according to GB / T 17445-2009 "Casting Grinding Balls" standard; and the density was tested using the drainage method. The test results are shown in Table 1 below.

[0059] Table 1

[0060] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a high wear-resistant bimetallic composite grinding ball, characterized in that, Includes the following steps: S1: Inner core alloy batching and vacuum purification smelting: low alloy raw materials containing Fe, C, Mn, Si and Ni are prepared according to mass fraction, grain refiner and deoxidizer are added, and smelting is carried out in a vacuum environment and heat preservation is carried out to remove gas. S2: Thixotropic preparation of semi-solid tough core blank: The inner layer alloy liquid obtained in S1 is cooled to the semi-solid temperature range, and then injected into the pre-forming mold after electromagnetic stirring to obtain a semi-solid tough core blank. S3: Smelting of outer wear-resistant alloy and preparation of in-situ self-generated TiC-VC reinforcing phase: Wear-resistant alloy raw materials containing Fe, C, Cr, Mo and W are prepared according to mass fraction, and in-situ reinforcing precursors are added. The alloy is smelted and kept warm in a vacuum environment so that Ti, V and C react in-situ to generate nano-TiC-VC dual hard phase. S4: Assembly and preheating of vacuum negative pressure mold cavity: Place the semi-solid tough core blank obtained in S2 into a closed spherical forging mold, preheat the mold and then evacuate to the set negative pressure; S5: Negative pressure composite casting and instantaneous high pressure die forging: The outer layer wear-resistant alloy liquid obtained in S3 is injected into the mold to wrap the tough core blank. After casting, high pressure die forging is applied instantaneously and pressure is maintained. The composite grinding ball blank is then taken out after the mold is opened. S6: Gradient temperature controlled heat treatment: Gradient quenching-tempering treatment is performed on the blank obtained in S5. S7: Shot blasting finishing: The heat-treated grinding balls are shot blasted and finely ground to obtain the finished product.

2. The method for preparing the high wear-resistant bimetallic composite grinding ball according to claim 1, characterized in that: The mass composition of the low alloy raw material in S1 is as follows: C 0.18-0.25%, Mn 1.4-1.8%, Si 0.7-1.0%, Ni 0.4-0.7%, with the balance being Fe; the grain refiner is Mg, added at 0.02-0.04%; the deoxidizer is pure Ti, added at 0.08-0.15%.

3. The method for preparing high wear-resistant bimetallic composite grinding balls according to claim 1, characterized in that: In S1, the vacuum degree is ≤6×10 -3 Pa, melting temperature is 1470-1500℃, and holding and degassing time is 28-35min.

4. The method for preparing the high wear-resistant bimetallic composite grinding ball according to claim 1, characterized in that: In S2, the semi-solid temperature range is 1280-1350℃, the solid phase rate is 50-60%, the electromagnetic stirring speed is 280-350 r / min, and the stirring time is 9-12 min.

5. The method for preparing high wear-resistant bimetallic composite grinding balls according to claim 1, characterized in that: In S3, the mass composition of the wear-resistant alloy raw material is as follows: C 2.6-3.0%, Cr 24-28%, Mo 1.3-1.8%, W 0.7-1.0%, with the balance being Fe; the in-situ reinforcing precursor is Fe-Ti50 master alloy, Fe-V50 master alloy and ultrafine graphite powder, which are added in an atomic ratio of C:Ti:V=1:1:

1.

6. The method for preparing the high wear-resistant bimetallic composite grinding ball according to claim 1, characterized in that: In S3, the vacuum degree is ≤1.2×10 -3 Pa, melting temperature is 1500-1550℃, holding time is 18-25min.

7. The method for preparing high wear-resistant bimetallic composite grinding balls according to claim 1, characterized in that: In step S4, the mold preheating temperature is 430-480℃, and the vacuum degree is ≤1.5×10⁻⁶. -2 Pa.

8. The method for preparing high wear-resistant bimetallic composite grinding balls according to claim 1, characterized in that: In S5, the pouring temperature of the outer wear-resistant alloy liquid is 1480-1500℃, the forging pressure is 120-160MPa, the holding time is 4-8min, and the time interval from the completion of pouring to the application of forging pressure is ≤4s.

9. The method for preparing high wear-resistant bimetallic composite grinding balls according to claim 1, characterized in that: In S6, the gradient heat treatment parameters are: outer layer quenching temperature 970-1000℃, core tempering temperature 710-740℃, and oil quenching cooling method is adopted.

10. A high wear-resistant bimetallic composite grinding ball, characterized in that: It is prepared by the preparation method described in any one of claims 1-9.

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