Metal matrix composite grinding balls with structural reinforcement.

Abstract

The present invention relates to a composite grinding ball comprising: a ferroalloy metal matrix; and a reinforcing shell of an openwork ceramic structure of agglomerated ceramic-metal composite particles, the agglomerated particles comprising micrometer ceramic particles fixed in a binder metal matrix, the agglomerated ceramic-metal composite particles being embedded in the ferroalloy metal matrix.

Description

[Technical Field]

[0001] The present invention discloses a metal matrix composite grinding ball obtained by conventional casting techniques and reinforced with a precast ceramic openwork shell, the shell being a precast body placed in a mold cavity before injecting the casting metal matrix, the resulting grinding ball having improved resistance to combined wear and impact stresses.

[0002] The openwork reinforcement structure of the disclosed grinding ball is obtained by additive manufacturing using a 3D printer and consists of a microporous agglomerate of ceramic-metal composite and metal particles. The ceramic-metal composite particles include fixed micrometer particles of borides, nitrides, carbonitrides, or carbides, such as TiC, TiCN, NbC, TaC, ZrC, HfC, Mo2C, and WC, preferably titanium carbide or titanium carbonitride, fixed in a metal binder matrix. The micropores of the agglomerate are infiltrated by the casting metal during the pouring operation, and the agglomerated ceramic-metal composite particles are ultimately completely embedded in the casting metal matrix. [Background technology]

[0003] The present invention relates to grinding balls used in tumbling mills in the grinding and crushing industry, such as for example, ore grinding in cement plants or mines. Grinding balls are subject to high mechanical stresses in bulk and high surface wear due to abrasion or corrosion. It is therefore desirable for grinding balls to exhibit high abrasion or corrosion resistance and some degree of ductility so that they can withstand ball-to-ball or ball-to-liner impacts.

[0004] Considering the difficulty of matching these two properties in the same material composition, composite grinding balls have been proposed in the past, using a matrix made of a relatively ductile alloy with ceramic particles that have good wear resistance embedded in it.

[0005] Chinese Patent Application Publication No. 106914620 (2017) discloses a method for preparing a composite grinding ball with a precast body openwork structure placed in a mold cavity before casting, using selective laser cladding combined with three-dimensional digital modeling technology.

[0006] Chinese Patent Application Publication No. 103357854 (2013) and Chinese Patent Application Publication No. 113564511 (2021) disclose ceramic reinforced grinding balls in which the reinforcing layer comprises nanometer-grade ceramic particles embedded on and below the surface of the grinding ball, and the inner wall of the mold is coated with nanometer-grade ceramic particles.

[0007] WO 2022 / 122393 (Magotteaux 2022) discloses a layered composite wear part reinforced by a triple periodic minimal curve ceramic lattice structure having a plurality of cell units, the ceramic lattice structure being embedded in a bicontinuous structure with a cast metal matrix. This document does not disclose an openwork ceramic structure of agglomerated ceramic metal composite particles comprising micrometer ceramic particles fixed in a binder metal matrix embedded in a cast metal matrix.

[0008] The grinding ball market is cost-sensitive, and therefore must target optimal wear performance and price, placing considerable importance on the manufacturing process. Therefore, the economical and safe production of precast ceramic spherical openwork structures by 3D additive manufacturing, their robustness during the casting operation, and their ability to be infiltrated by the casting metal without damage are of great importance.

[0009] For example, fine, non-oxide microcrystalline ceramic powders such as TiC, TiCN, NbC, TaC, and WC with diameters less than 53 μm can be difficult to handle because they are reactive (potentially flammable and / or explosive) and require an inert atmosphere for safe use in additive manufacturing. While larger diameter non-oxide ceramic particles (greater than 100 μm) can potentially be used without a controlled atmosphere, such coarse particles perform less well due to the well-known inherent brittleness of ceramic particles. Pure ceramic coarse particles in additively manufactured precast bodies are too brittle to withstand the abrasive conditions. From a manufacturing perspective, fine particles typically have little flowability, while coarse particles have much higher flowability. Coarse particles also pose fewer health concerns than fine particles. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Chinese Patent Application Publication No. 106914620 [Patent Document 2] Chinese Patent Application Publication No. 103357854 [Patent Document 3] Chinese Patent Application Publication No. 113564511 [Patent Document 4] International Publication No. 2022 / 122393 [Patent Document 5] U.S. Patent No. 6,036,777 [Patent Document 6] U.S. Patent Application Publication No. 2015 / 0069649 [Patent Document 7] International Publication No. 2020 / 146452 [Patent Document 8] U.S. Patent Application Publication No. 2019 / 0111618 [Non-patent literature]

[0011] [Non-Patent Document 1] ScienceDirect:http: / / www.sciencedirect.com / topics / engineering / binder-jetting [Non-patent document 2] "Binder jetting of ceramics: Powders, binders, printing parameters, equipment, and post-treatment" (2019) Xinyuan Lv, Fang Ye, Laifei Cheng*, Shangwu Fan, Yongsheng Liu Science and Technology on Thermostructural Composite Materials Laboratory, Northwestern Polytechnical University, Xi'an, 710072, PR China [Non-patent document 3] Massoud Malaki et al. “Wettability in Metal Matrix Composites” (2021) [Non-patent document 4] Banerji et al. “Role of wettability in the preparation of metal-matrix composites” (1984) [Non-patent document 5] “Overview of grinding media consumption in communution” (Slabbert, Paton, Moema and Zimba) World Gold Conference Summary of the Invention [Problem to be solved by the invention]

[0012] The present invention aims to provide a grinding ball obtained by conventional casting techniques, reinforced with a precast ceramic openwork shell placed in a mold cavity before the grinding ball is poured in. The openwork structure contains a suitable concentration of ceramic-metal composite particles embedded in a cast metal matrix, and is substantially free of unfilled micropores. [Means for solving the problem]

[0013] The present invention provides a ferroalloy metal matrix; - a reinforcing shell of an openwork ceramic structure of agglomerated ceramic metal composite particles, said agglomerated ceramic metal composite particles comprising micrometer ceramic particles fixed in a binder metal matrix; A composite grinding ball comprising: A composite grinding ball is disclosed in which the agglomerated ceramic metal composite particles are embedded in the ferroalloy metal matrix.

[0014] Preferred embodiments of the present invention disclose at least one, or a suitable combination of, the following features: - the micrometric ceramic particles are selected from the group consisting of metal borides, metal nitrides, metal carbides, and metal carbonitrides; - the micrometric ceramic particles are selected from the group consisting of titanium carbide, titanium carbonitride, niobium carbide, tantalum carbide, zirconium carbide, hafnium carbide, vanadium carbide, molybdenum carbide, and tungsten carbide, or mixtures thereof; - the micrometric ceramic particles are selected from the group consisting of titanium carbide or titanium carbonitride, or mixtures thereof; - the ceramic composite particles have an average particle size D of less than 500 μm, preferably less than 400 μm 50 having; - the micrometer ceramic particles fixed in a binder metal matrix have an average particle size D of less than 30 μm, preferably less than 20 μm, most preferably less than 10 μm 50 having; - the binder metal matrix fixing the micrometer ceramic particles is selected from the group consisting of ferromanganese-based alloys, ferrochromium-based alloys, and nickel-based alloys, and the binder metal matrix and the ferroalloy metal matrix have different compositions; - the cast ferroalloy metal matrix comprises steel or chromium cast iron; - the concentration of ceramic particles in the openwork structure is 30-55% by volume, preferably 35-50% by volume; - The openings in the reinforcing shell of the openwork ceramic structure represent 10-70%, preferably 25-70%, of the surface of the grinding ball.

[0015] The present invention further provides a method for making the grinding balls of the present invention, comprising the steps of: - a) additive manufacturing of a ceramic openwork shell from a powder mixture containing ceramic-metal composite particles; - b) placing the ceramic openwork shell into the mold cavity; - c) pouring the ferroalloy casting metal into a mold to obtain the reinforced grinding ball of claim 1; A method is disclosed, comprising:

[0016] Preferred embodiments of the method of the present invention disclose at least one or a suitable combination of the following features: - an additional step of at least partially sintering the shell of the ceramic openwork structure; - the step of at least partially sintering the shell of the ceramic openwork structure comprises partial impregnation of the voids between the ceramic-metal composite particles of said structure with a metal selected from the group consisting of steel and chromium cast iron, or a combination thereof, prior to placement in the mold cavity and final casting; - a step of at least partial sintering followed by a hot isostatic pressing step or post-infiltration; - step a) comprises the addition of 2 to 20% by weight, preferably 5 to 20% by weight, most preferably 10 to 18% by weight, of carbide-forming metal particles selected from the group consisting of tungsten, vanadium, molybdenum, titanium, niobium, hafnium and zirconium, or mixtures thereof; - the ceramic composite particles of the powder mixture of step a) comprise at least 95%, preferably 98%, of particles larger than 150 μm; - step a) is carried out by binder jetting technique followed by binder curing at temperatures above 100°C; The process of injecting the casting metal to obtain reinforced grinding balls is carried out in a grinding ball cluster mold. [Brief explanation of the drawings]

[0017] [Figure 1] 1A-1D represent various 3D models of precast ceramic openwork shells that can be produced by additive manufacturing and are designed to be placed in a mold cavity prior to pouring. [Figure 2] 1A-1D represent various 3D models of precast ceramic openwork shells that can be produced by additive manufacturing and are designed to be placed in a mold cavity prior to pouring. [Figure 3] 10A-10C show examples of precast ceramic openwork additively manufactured shells as single or double spheres containing agglomerated ceramic metal composite particles. The spheres are 80 mm in diameter with 2 mm diameter holes. [Figure 4] 10A-10C show examples of precast ceramic openwork additively manufactured shells as single or double spheres containing agglomerated ceramic metal composite particles. The spheres are 80 mm in diameter with 2 mm diameter holes. [Figure 5] FIG. 10 illustrates the placement of a precast openwork shell in a mold cavity prior to pouring the casting metal. [Figure 6] 1 is a schematic representation of a typical mold configuration for grinding balls cast in a cluster mold. FIG. [Figure 7]7A and 7B depict a 3D model of the precast ceramic openwork structure of Example 1 used to illustrate the reinforced grinding balls of the present disclosure. In particular (ignoring the inlet holes), FIG. 7 depicts the openwork structure for an 80 mm diameter grinding ball with 6 mm diameter holes / openings for Example 1. [Figure 8] 8A and 8B depict a 3D model of the precast ceramic openwork structure of Example 2 used to illustrate the reinforced grinding balls of the present disclosure. Specifically (ignoring the inlet holes), FIG. 8 depicts the openwork structure for a 90 mm diameter grinding ball with 8 mm diameter holes / openings for Example 2. [Figure 9] 9A and 9B depict a 3D model of the precast ceramic openwork structure of Example 3 used to illustrate the reinforced grinding balls of the present disclosure. Specifically (ignoring the inlet holes), FIG. 9 depicts the openwork structure for a 70 mm diameter grinding ball with 6 mm diameter holes / openings for Example 3. [Figure 10] 10 depicts a 3D model of the precast ceramic openwork structure of Example 4 used to illustrate the reinforced grinding balls of the present disclosure. Specifically (ignoring the inlet holes), FIG. 10 depicts the openwork structure for a 70 mm diameter grinding ball with 10 mm diameter holes / openings for Example 4. [Figure 11] FIG. 1 is a diagram showing the openwork structures of Examples 1 to 4 as a group in a real ratio format. [Figure 12] FIG. 10 depicts a precast openwork structure placed in a mold. DETAILED DESCRIPTION OF THE INVENTION

[0018] In the following description, the expressions "holes" or "openings" in the openwork structure are used interchangeably.

[0019] Explanation of the additive manufacturing process for openwork shells To manufacture the precast openwork structure of the present disclosure, it is necessary to create a digital 3D model structure and build it with a powder of ceramic metal composite and metal particles in a 3D printing (additive manufacturing) device, the technique used in this case being preferably, but not limited to, binder jetting.

[0020] A general overview of various ASTM standards related to 3D printing technologies and characterization and methods is published on ScienceDirect: http: / / www.sciencedirect.com / topics / engineering / binder-jetting. This overview summarizes the contents of several papers related to 3D printing technologies that represent the knowledge of those skilled in the art. For general information purposes, this publication is incorporated herein by reference.

[0021] Binder jetting technology is disclosed, inter alia, in U.S. Pat. No. 6,036,777 (2000) and U.S. Patent Application Publication No. 2015 / 0069649.

[0022] Recent publications provide a thorough overview of the relevant parameters of ceramic binder jetting technology: "Binder jetting of ceramics: Powders, binders, printing parameters, equipment, and post-treatment" (2019) Xinyuan Lv, Fang Ye, Laifei Cheng*, Shangwu Fan, Yongsheng Liu Science and Technology on Thermostructural Composite Materials Laboratory, Northwestern Polytechnical University, Xi'an, 710072, PR China.

[0023] This publication explores the process and applications of binder jet printing of ceramics and discusses important factors such as powder properties, binders, printing parameters, equipment, and post-treatment processes, as well as the influence of ceramic powder particle shape and size distribution. The influence of additives such as binder droplet formation mechanism and droplet infiltration kinetics is also described. Furthermore, this document discusses printing parameters such as layer thickness, saturation, printing direction, equipment, and post-treatment. For the purposes of describing binder jet technology, this document is incorporated by reference into this application.

[0024] One key element of binder jetting technology is the selection of an appropriate type of binder, taking into account its compatibility with the associated ceramic, ceramic-metal composite, or metal powder. Various prior art documents have investigated different types of binders and ceramic powders.

[0025] WO 2020 / 146452 discloses a method for binder jetting additive manufacturing of a particular amine-containing adhesive polymer and an object, comprising the steps of separately supplying a powder from which the object is to be manufactured and a solution containing an adhesive polymer dissolved in a solvent to an additive manufacturing apparatus, the adhesive polymer being an amine-containing polymer having a molecular weight of at least 200 g / mol, and dispensing selectively positioned droplets of the adhesive polymer from a print head of the additive manufacturing apparatus onto a powder bed to bond particles and generate a preform of the object to be manufactured.

[0026] U.S. Patent Application Publication No. 2019 / 0111618 discloses a method for indirectly additively manufacturing an object by separately supplying a powder from which the object is to be manufactured and either a difunctional curable monomer or an adhesive polymer binder to an additive manufacturing apparatus, dispensing selectively positioned droplets of the difunctional curable monomer or adhesive polymer binder from a print head of the additive manufacturing apparatus onto the powder bed to bond particles of the powder with the difunctional curable monomer or adhesive polymer binder to produce a curable preform having the shape of the object to be manufactured; and, in the case of a difunctional curable monomer, curing the curable preform to form a crosslinked object. These documents list a range of available curable monomers along with their curing temperatures. This document is incorporated herein by reference.

[0027] Methodology for Making the Grinding Balls of the Present Invention A preferred method for manufacturing a grinding ball reinforced by a ceramic-metal openwork shell is to create a digital 3D model structure (see Figures 1 and 2, which show various possibilities) and build it in a 3D printing device, optionally partially sintering the structure to improve its robustness. The 3D model is then placed in a cluster sand mold, which is subsequently injected with a hot liquid matrix metal (e.g., high-chromium cast iron or steel containing more than 11% by weight of Cr) to infiltrate the micro-porosity of the precast body and obtain a grinding ball substantially free of unfilled micropores in its reinforced ceramic-metal composite structure. The manufacturing of a precast openwork structure includes the following steps:

[0028] Digital 3D model structure - Numerical 3D models of the openwork shells are created using computer-aided design (CAD) software (e.g., nTopology). https: / / ntopology.com / generative-design-software / and converted into a format that can be handled by a 3D printing device, for example, STL (stereolithography) format. nTopology uses field-driven design to spatially vary parameters of the openwork structure, such as shell thickness, mesh size, or openwork geometry. - The file is then processed by slicing software which slices the model into printable 2D layers of a predetermined thickness.

[0029] Additive Manufacturing (AM) Process Small particles (less than 100 μm) are usually dangerous to handle for health and / or safety reasons, therefore larger ceramic metal composite particles, in which fine ceramic particles are fixed in a metal binder matrix, are used in the present additive manufacturing process. - In this process, ceramic metal powder is fed through a hopper to build up one monolayer at a time, for example, about 200-500 μm thick, depending on the powder particle size. The ceramic metal powder particles can be selected from nitrides, borides, or carbonitrides, such as TiC, TiCN, NbC, TaC, WC, ZrC, HfC, VC, Mo2C, etc., fixed in a metal binder matrix, preferably titanium carbide or titanium carbonitride, and possibly some other carbide-forming metal particles, to enhance the wetting of the ceramic metal composite particles by the casting alloy, while forming additional carbides in situ. The average particle size (D 50 ) is usually comprised between 200 and 500 μm, preferably between 200 and 400 μm, most preferably between 200 and 300 μm (measured by dry sieving according to ISO 4497:2020). - If the powder particle size analysis does not meet the particle size distribution target, a sieving step may be required to remove the particle size fraction below 150 μm (conditioned by safety, flowability, or print resolution). - Ceramic metal composite powders that can be bonded to metal powders are introduced into the 3D printing device to build the openwork structure layer by layer, which is deposited in the build box (mainly for fine or poorly flowing powders, vibration and re-coating rollers can be used to increase the packing density of the layers). Depending on the additive manufacturing technology, a moving head generates powder agglomerates in specific areas of a layer, for example, by binder jetting. Agglomerations also occur in previous layers beneath the current layer. In binder jetting, a liquid binder is deposited by a moving head as droplets in specific areas of a layer according to a 2D file. A key parameter is the appropriate saturation level to ensure proper bonding between the particles of the layer and between previous printed layers. Preferred binders are water-dispersible glycol acrylic binders, such as tetraethylene glycol dimethacrylate, tetraethylene glycol diacrylate, triethylene glycol dimethacrylate, diethylene glycol dimethacrylate, and methoxyethanol, or, preferably, mixtures suitable for forming interconnected molecular networks that can be dispersed in water and cured at 130-200°C for approximately 2 hours per cm of material thickness of the box to be cured. - The next layer is then deposited and the above steps are repeated until the entire insert is incorporated into the powder bed.

[0030] Curing and optional sintering - If a cross-linkable monomer binder is selected, it must be cured. The finished box is heated in a curing oven at a temperature of approximately 130-200°C to impart strength to the part (via polymerization, cross-linking, solvent evaporation, or some other mechanism) for a period depending on the box volume and printed material, ensuring temperature uniformity (e.g., approximately 2 hours per cm of thickness). The box is then left to cool completely and excess powder is removed from the box, for example by brushing, vacuuming or blowing with compressed air. The green part can now be handled safely without risk of breakage. - If the green precast openwork body needs to be sintered, it is placed in a furnace and heated to high temperatures (usually above 900°C) in a controlled atmosphere (usually argon or vacuum) to carry out the sintering process. The sintering rate achieved depends on the desired mechanical properties and the final porosity of the openwork structure.

[0031] Grinding ball manufacturing - The prepared precast body (shell with openwork structure) is placed into the preformed cavity of the cluster sand mold. - The mold is closed and the liquid metal is poured into the mold. - The pore size of the agglomerated ceramic metal composite powder is generally observed to be in the range of 10-700 μm, but most of the pores are in the range of 50-500 μm, with an average pore size of 150-300 μm. - 3D openwork structures have uniform open porosity throughout the volume, and in the case of a partially sintered shell of a ceramic openwork structure, the liquid casting metal infiltrates the remaining microporosity of the structure, resulting in an extremely tight ceramic / metal bond. - The liquid metal is allowed to cool until the grinding ball is completely solidified. The sand mold is then removed, the grinding ball is cleaned of any remaining sand, and can be subjected to the usual finishing casting process steps known to those skilled in the art (knockout, shot blasting, grinding, additional heat treatment such as annealing, hardening, tempering...).

[0032] Grinding ball example This example is illustrative for the present disclosure and should not be considered limiting.

[0033] Preparation of ceramic metal composite particles The following powders were used for three different types of ceramic metal composite particles (CMP1-3): TiC, titanium carbonitride, WC, NbC, MoC, iron, manganese, chromium, and nickel; all powders had a particle size of less than 325 mesh (less than 44 μm).

[0034] [Table 1] The composition of CMP 1 is particularly suitable for impact resistance due to its low content of ceramic particles (45 wt. % TiC) fixed in a tough manganese steel binder. The composition of CMP 2 is particularly suited for wear resistance due to its high content of titanium carbide (85 wt. % TiC) fixed in a hard, high chromium white iron wear-resistant binder. The composition of CMP 3 is high in a composite mixture of finely tuned ceramic particles (TiC) fixed in a corrosion-resistant nickel binder. 0.5 N 0.5 +WC+NbC+Mo2C=total 85% by mass), particularly suitable for corrosion resistance and wear resistance.

[0035] Other ceramic particles can be added to create composite solid solution particles and to control or fine-tune the grain size, morphology, and / or core-rim structure of the ceramic particles.

[0036] Powders according to the composition in Table 1 were mixed and ground in a ball mill containing isopropyl alcohol and metal grinding balls for 24 hours to obtain an average particle size D 50 was set to about 3 μm.

[0037] An organic wax binder is added at 2% by weight of the powder and mixed with the powder obtained from the grinding process. The alcohol is removed using a vacuum dryer with rotating blades (the alcohol is condensed and reused). The resulting agglomerated powder is then sieved through a 600 μm sieve. Strips of 60% of the theoretical density of the ceramic / metal powder mixture are produced by compaction between the rotating rolls of a roller compactor granulator. The strips are then crushed into irregularly shaped particles by forcing them through a sieve of appropriate mesh size. After crushing, the granules are sieved to obtain dimensions of approximately 200-600 μm. These irregularly shaped porous particles are then sintered at high temperatures (1300-1500 °C for approximately 2 hours) in a high-vacuum furnace with a low partial pressure of argon until a minimum porosity (less than 5% by volume, preferably less than 3% by volume, and more preferably even less than 1% by volume) is reached. After sintering, a further grinding and final screening step is usually required to de-agglomerate any ceramic metal composite particles that may have agglomerated during sintering.

[0038] Powder blend preparation for 3D printing: The composition and size distribution of the powder blend are conditioned by the infiltrability of the additively manufactured shell of the openwork precast body structure, which depends in particular on the following parameters: - Thickness and microporosity of openwork precast body, - the proportion of openings in the openwork structure, - The composition of the alloy being poured into the mold, which influences the liquidus temperature and the superheat that may be required based on the liquidus-solidus range (high chromium casting alloys are easier to infiltrate into openwork precast body structures than steel at the same temperature...).

[0039] Some metals, such as magnesium, titanium, or niobium, have previously been shown to have a positive effect on the wettability of the ceramic particles by the cast metal matrix and thus on the infiltrability of the openwork shell. Some of these metals (such as Ti, Nb, and other carbide-forming metals) also have the positive effect of forming additional carbides in combination with the carbon present in the cast iron, as mentioned below.

[0040] The wettability of ceramic metal particles has been particularly investigated by Massoud Malaki et al., "Wettability in Metal Matrix Composites" (2021) and Banerji et al., "Role of wettability in the preparation of metal-matrix composites" (1984).

[0041] In the present disclosure, the titanium and niobium metal particles serve a dual role as they improve the infiltrability of the openwork structure and react with the carbon always present in the iron casting alloy to build additional carbides in situ.

[0042] Three different types of powder blends 1 to 3 were prepared by combining ceramic metal composite particles CMP1 to CMP3 with titanium and niobium powders. All powders contained less than 5% by weight, preferably 2% by weight, of particles with a size less than 150 μm.

[0043] The blend according to the table below is mixed in a blender for 15 minutes.

[0044] [Table 2]

[0045] Casting alloys

[0046] [Table 3]

[0047] Cast Alloy 1 contains 2.2% by weight of C and 16.5% by weight of Cr and is particularly suitable for impact conditions with appropriate heat treatment to improve impact resistance. Blend 1 powder is used to create openwork structures. Cast Alloy 1 composite grinding balls of the present invention and reference metal grinding balls are compared together in the same ball mill processing the same ore under significant impact conditions for the same period of time. Both grinding balls are subjected to the same heat treatment.

[0048] Cast Alloy 2 contains 2.85% by weight C and 14.5% by weight Cr and is particularly suited to abrasive conditions with appropriate heat treatment to improve wear resistance. Blend 2 powder is used to make shells. Cast Alloy 2 composite balls of the present invention and reference metal balls are compared in the same ball mill containing copper ore under significant abrasive conditions.

[0049] Cast Alloy 3 contains 2.3% by weight C and 29% by weight Cr, making it particularly suitable for highly corrosive conditions. Blend 3 powder is used to make shells. Cast Alloy 3 composite balls of the present invention and reference metal balls are compared in the same ball mill containing magnetite under significant corrosive conditions.

[0050] Cast alloys 1, 2 and 3 also contain less than 2% by weight of other alloying elements (Si, Mn, Mo, Ni...) known to those skilled in the art, depending on the specific properties and the intended heat treatment. [Example]

[0051] Blend 1 + Casting Alloy 1 Blend 1 was used to print 80 mm diameter ceramic openwork shells with a shell thickness of approximately 5 mm and 6 mm diameter openings / holes in an EXone X25 Pro 3D binder jet printer, resulting in 57% surface coverage (shown below and in Figure 7). Parts were printed using a water-based binder based on a mixture of diethylene glycol as a dispersion in an aqueous solution of 2-butoxyethanol (Aquafuse BA 005 EXone).

[0052] [ka]

[0053] The key parameters of the AM process were: - Each printed layer was approximately 300 μm thick; - Printing speed was 60 seconds per layer; - the binder saturation of the powder pores was 30%; and - The powder packing density was approximately 43%.

[0054] Once completed, the entire printed box was cured in an oven at approximately 200°C for 2 hours per cm of part height, with dwell times depending on the structure, as several items can be produced in one run. After cooling, the printed box was vacuumed to remove any dust and brushed to obtain the green openwork structure.

[0055] The resulting green openwork precast structure was placed in a furnace and heated to approximately 1100°C under high vacuum for 2 hours.

[0056] The shell of the openwork precast structure contains a total of about 83 vol% open space available for ferroalloy infiltration (about 61 vol% due to the holes / openings in the openwork structure and an additional 22 vol% of micrometer pores), with the remaining 17 vol% in the openwork structure being ceramic-metal composite particles + titanium metal particles.

[0057] The resulting openwork precast structure is then placed in the cavity of a cluster sand mold.

[0058] Hot liquid high-chromium white iron (having the composition of Cast Alloy 1) is then injected into the mold, filling 61% by volume through the openings of the openwork precast structure and infiltrating the 22% by volume micrometer porosity between the particles of the aggregate structure while forming additional carbides from the reaction of the carbon contained in the cast alloy with the metal powder.

[0059] After injection, 39% by volume of the reinforcement volume (1 minus the volume percentage of the openwork pores) contains a high concentration of titanium carbide, about 30% by volume. Therefore, the overall volume content of titanium carbide particles in the reinforcement part of the grinding ball is about 12% by volume. [Example]

[0060] Blend 2 + Casting Alloy 2 Example 2 is carried out in the same manner as Example 1, but using Blend 2 and a different shell of ceramic openwork structure 90 mm in diameter with a shell thickness of 8 mm and openings / holes 8 mm in diameter, resulting in a surface coverage of 69% as shown below and in Figure 8.

[0061] [ka]

[0062] Using the homogeneous powder mixture of Blend 2, ceramic openwork shells were printed using the same equipment and printer configuration as used in Example 1. Parts were printed using an aqueous binder based on a mixture of diethylene glycol as a dispersion in an aqueous solution of 2-butoxyethanol (Aquafuse BA 005 Exone).

[0063] The main parameters of the AM process were the same as in Example 1. The resulting green shell of the openwork ceramic precast structure was placed in a furnace and heated to about 1000°C under an argon atmosphere for 2 hours.

[0064] This structure, again obtained with a powder packing density of 46%, contains a total of about 74 vol% of free space available for ferroalloy infiltration (about 42 vol% due to openings in the structure and an additional 32 vol% of micrometer pores between the ceramic-metal composite particles), with the remaining 27 vol% representing the openwork ceramic-metal and metallic titanium structure.

[0065] The resulting ceramic openwork shell is then placed into a cavity in a sand mold.

[0066] High-temperature liquid high-chromium white iron (with the composition of Casting Alloy 2) is then injected into the mold, filling the 42 vol% openings / pores of the openwork structure and infiltrating the 32 vol% micrometer porosity between the ceramic-metal composite particles of the structure.

[0067] After injection, 58% by volume of the reinforcement volume contains a high content of titanium carbide, about 42% by volume, so the overall volume content of titanium carbide particles in the reinforcement portion of the grinding ball is about 25% by volume. [Example]

[0068] Blend 3 + Casting Alloy 3 Example 3 is carried out using Blend 3 and a different shell with an openwork structure of 70 mm diameter with a shell thickness of 3 mm and openings / holes of 6 mm diameter, resulting in a surface coverage of 64% as shown below and in Figure 9.

[0069] [ka]

[0070] Parts were printed using an alcohol binder (Cleanfuse EXone).

[0071] The key parameters of the AM process were: - Each printed layer was approximately 450 μm thick; - Printing speed was 45 seconds per layer; - the binder saturation of the powder pores was 35%; and - The powder packing density was approximately 47%.

[0072] The main parameters of the AM process were the same as in the previous example (curing at 200°C for 2 hours, followed by furnace heating at 1050°C for 2 hours under argon atmosphere).

[0073] The shell of the ceramic openwork structure is obtained by a powder packing density of 47% and contains a total of about 77% by volume of free space available for ferroalloy infiltration (about 51% by volume due to the openings / pores in the openwork structure and an additional 26% by volume of micrometer pores within the structure), with the remaining 23% by volume representing the structure of the ceramic-metal composite grains + metallic niobium particles in the openwork structure.

[0074] The resulting structure is then placed in a cavity of a sand mold, and then hot liquid high-chromium white iron (with the composition of Casting Alloy 3) is injected into the mold, filling 51 vol.% of the openwork structure openings / pores and infiltrating the 26 vol.% micrometric porosity between the particles present in the openwork structure.

[0075] After injection, 49% by volume of the reinforcement volume contains a high content of ceramic particles, such as titanium carbonitride, tungsten carbide, molybdenum carbide, and niobium carbide, or mixtures thereof, with about 43% by volume, or even mixed carbides, such as (Ti, Nb, W, Mo) C. The overall volume content of ceramic particles in the reinforcement portion of the grinding ball is therefore about 21% by volume. [Example]

[0076] Blend 3 + Casting Alloy 2 Example 4 is carried out using Blend 3 and a different shell with a 70 mm diameter openwork structure with a shell thickness of 3 mm and an opening of 10 mm diameter, resulting in a surface coverage of 56% as shown below and in Figure 10.

[0077] [ka]

[0078] Parts were printed using Alcohol Binder I (Cleanfuse EXone).

[0079] The key parameters of the AM process were: - Each printed layer was approximately 450 μm thick; - Printing speed was 45 seconds per layer; - the binder saturation of the powder pores was 35%; and - The powder packing density was approximately 47%.

[0080] The main parameters of the AM process were the same as in the previous example (curing at 200°C for 2 hours, followed by furnace heating at 1150°C for 2 hours under argon atmosphere).

[0081] The shell of the ceramic openwork structure is obtained by a powder packing density of 47% and contains a total of about 77% by volume of free space available for ferroalloy infiltration (about 51% by volume due to the openings in the openwork structure and an additional 26% by volume of micrometer pores within the structure), with the remaining 23% by volume in the openwork structure being ceramic-metal composite grains + metallic niobium particles.

[0082] The resulting structure is then placed in a cavity of a sand mold, and then hot liquid high-chromium white iron (with the composition of Casting Alloy 3) is injected into the mold, filling the pores representing 50% by volume of the openwork structure and infiltrating the 31% by volume micrometer porosity between the ceramic-metal composite particles of the openwork structure.

[0083] After injection, 49% by volume of the reinforcement volume contains ceramic particles such as titanium carbonitride, tungsten carbide, molybdenum carbide, and niobium carbide, or mixtures thereof, with a high concentration of about 43% by volume, or even mixed carbides such as (Ti, Nb, W, Mo) C. Thus, the overall volume content of carbides or carbonitrides in the reinforcement portion of the grinding ball is about 21% by volume.

[0084] [Table 4]

[0085] The expressions "shell", "reinforced shell", "openwork shell" and "openwork" are used interchangeably. The thickness of the shell is the thickness of the openwork, ignoring the holes.

[0086] Performance index calculated in comparison with conventional grinding balls The paper "Overview of grinding media consumption in comminution" (Slabbert, Paton, Moema and Zimba) from the World Gold Conference provides an overview of grinding media testing.

[0087] Real-life testing of grinding balls on industrial grinders is particularly difficult to manage due to the low percentage of test balls compared to the conventional grinding ball load. Because grinders contain grinding balls with a wide diameter size distribution (e.g., diameters from 20 to 70 mm) that are worn at various stages, the test sample typically represents less than 0.1% (400 balls out of 400,000 to 800,000 balls). The main difficulty here is to identify at least a significant number of composite grinding balls and reference metal grinding balls that have been subjected to testing among the thousands of conventional grinding balls after several days or weeks of grinding, so that a representative average mass loss rate can be measured for the composite test balls compared to the reference metal grinding balls.

[0088] The key to solving this problem is to use grinding balls with a diameter slightly larger (approximately 10 mm larger) than those regularly added and already present in the conventional grinding machine. For example, if the diameter of the fresh conventional grinding balls regularly added to an industrial grinding machine is 60 mm, a diameter of 70 mm is selected for the test grinding balls (composite grinding balls and reference grinding balls) to make it easier to retrieve the marked balls. Because these balls are larger, they are not only easier to find visually, but also have a natural tendency to "float" on the surface of the conventional grinding machine.

[0089] The wear performance of the grinding balls of the present disclosure was evaluated under real-life conditions in the grinding of copper ore, magnetite, and platinum ore in three mills A; B; C, with the following characteristics and conditions:

[0090] Grinding of copper ore using the grinding ball (diameter 90 mm) of Example 2 - Mill A, diameter 7.31m and length 10.97m - Contains 660 tons of conventional grinding balls (33% volume fill rate) - Several tons per day of conventional 76mm diameter grinding balls are added periodically to offset wear - Rotation speed 11.9 rpm - Produces 10,500kW of power - Processing 950T / h of copper ore - Wear conditions - Test duration: 8 days

[0091] Grinding of magnetite with grinding balls of Example 3 followed by testing with grinding balls (70 mm) of Example 4 - Mill B, 4.6m diameter and 6.7m length - 165 tons of conventional grinding ball filling (35% volume filling rate) - Periodically adding several tons per day of 60mm diameter conventional grinding balls to offset wear - Rotates at a speed of 15.1 rpm - Produces 2300kW of power - Processing 420T / h of magnetite - Using corrosive conditions - Test duration: 20 days

[0092] Grinding of platinum ore in the presence of grinding balls (80 mm) of Example 1 - Mill C, 7.925m diameter and 11.735m length - 800 tons of conventional grinding ball filling (32% volume filling rate) - Periodically adding several tons per day of 70mm diameter conventional grinding balls to offset wear - Rotates at a speed of 10.5-11.5 rpm - Produces 15,000kW of power - Processing 1000T / h of platinum ore - Use impact conditions - Test duration: 10 days

[0093] Preparation of test samples For performance testing of the composite grinding balls of Examples 1-4 and the metal reference ball mold, 200 grinding balls of all Examples were manufactured, with each grinding ball of the same alloy and reinforcement marked with the same identification means (e.g., one or two drill holes of defined diameters and locations for all grinding balls of Example 1; two holes for all grinding balls of Example 2, etc.). All grinding balls of the same composition were further machined or ground to have the same mass (+ / - 2 kg for 80 mm diameter grinding balls) with a tolerance of + / - 5 grams, preferably + / - 2 grams. The composite grinding balls of the present disclosure and the reference metal grinding ball were manufactured from the same ferroalloy and subjected to the same heat treatment so that the effect of ceramic reinforcement as the only variable could be evaluated.

[0094] In the next step, 200 composite test grinding balls and 200 reference metal grinding balls were loaded together into an industrial mill (Mill C for the composite balls of Example 1, Mill A for the composite balls of Example 2, and Mill B for the composite balls of Example 3). The industrial mills already contained a typical load of conventional grinding balls at a fill factor of approximately 35% by volume. This represented 600 tons of conventional grinding balls for Mill A, 165 tons for Mill B, and 800 tons for Mill C. However, because the combined weight of the test and reference grinding balls (200 test and 200 reference balls in each mill) only represented approximately 600-900 kg, the impact on grinding capacity was negligible. Therefore, the addition of the test grinding balls did not significantly affect the overall mill fill factor.

[0095] The mill must then be operated for a sufficient period of time to observe sufficient wear to be measurable, which is generally on the order of several days or weeks under 24 hour / day conditions.

[0096] Depending on the type of ore being ground, the wear of conventional grinding balls on magnetite is known to be about 0.5 mm in diameter per 100 hours (about 2.4 mm after 20 days), while for copper ore it has been observed to be about 1.5 mm in diameter per 100 hours (about 3 mm after 8 days). For platinum ore, wear is expected to be about 1.3 mm per 100 hours (about 3 mm after 10 days). This indication of the wear rate of conventional grinding balls was considered to allow for the selection of a test duration that ensured that the wear remained within the reinforcing shell thickness in order to properly interpret the effect of the reinforcing shell itself.

[0097] Finally, the mill was stopped and the found and identified test grinding balls (usually less than 10% of the starting amount) were weighted to assess the average mass loss and calculate a figure of merit shown in the results table below.

[0098] figure of merit calculation For each example of grinding ball, the average mass loss is measured before and after the test duration, and the average mass loss percentage is calculated. Mass loss% / 100=(Initial mass - Final mass) / Initial mass The figure of merit is defined as follows, where the reference mass loss is the average mass loss of the reference metal grinding balls: PI = mass loss of reference metal grinding ball / mass loss of test composite grinding ball

[0099] A figure of merit greater than 1 means that the test composite grinding ball according to the invention wears less than the reference, while a figure of merit less than 1 means that the test composite grinding ball wears more than the reference, which is a conventional grinding ball made of the same cast alloy but without the ceramic reinforcement.

[0100] As shown in the table below, the grinding balls of the present invention, on average, perform better with the reinforced shell.

[0101] [Table 5]

[0102] The performance index for Example 3 shows the best performance of the composite shell for the smaller diameter holes compared to Example 4, which has larger holes. Examples 1-4 show significantly superior performance of the composite core-shell grinding balls using various particle, cast alloy compositions and properties, and test conditions.

[0103] When comparing the performance of the composite grinding balls of the present disclosure with conventional grinding balls when the reinforcing shell is not completely worn or completely lost, as described above, the thickness of the reinforcing shell naturally affects the overall performance of the grinding ball. A grinding ball with a 10 mm thick composite reinforcing shell has better long-term operating performance than an equivalent 5 mm thick reinforcing shell.

[0104] The influence of performance index on overall grinding performance The table below compares the wear of Examples 1-4 (composite grinding balls) with their respective reference grinding balls in terms of average mass loss. Life extension is calculated taking into account the performance index and its effect on the life of the grinding ball.

[0105] The life of a grinding ball is usually measured as the time required to wear the ball down to 20 mm from its initial diameter. Once the ball reaches 20 mm in diameter, it is considered small enough to exit the mill with the grinding material through the exit trunnion. The measured mass loss is converted to diameter reduction per unit time, allowing calculation of extended lifespan.

[0106] A significant increase in lifespan is observed due to the reinforced openwork structure, which allows the grinding ball to maintain a diameter close to its original value for a longer period of time.

[0107] [Table 6]

[0108] Examples 1-4 show significantly superior performance of the composite core-shell grinding balls using various compositions and properties of granules, cast alloys, and test conditions.

[0109] findings Ignoring the influence of various ceramic-metal compositions and cast alloy compositions on the wear behavior in a specific grinding environment, the influence of the geometry of the openwork structure disclosed herein plays an important role. Parameters to be considered are, for example, the surface coverage ratio of the ceramic reinforcement, the diameter of the openings, the ratio of the opening thickness to the diameter, and the distance between the openings (see Table 1).

[0110] The ease of infiltration of openwork structures with cast ferroalloys has been observed to be generally inversely proportional to the surface coverage ratio and reinforcement thickness, and proportional to the diameter of the openings and the distance between the openings. However, the wear resistance of a particular CMP / cast alloy composition depends on the surface coverage ratio and reinforcement thickness, and is inversely proportional to the diameter and distance between the openings.

[0111] The difficulties of infiltrability should not be underestimated, so a compromise must be found between these parameters, keeping in mind that the combination of high opening thickness (more than 10 mm) and very small openings (less than 2 mm) that are very far from each other should be avoided, while low openwork thickness (less than 10 mm) and medium openings (more than 2 mm) that are close to each other (less than 10 mm) are preferred.

[0112] Advantages of the Invention The present invention has the following advantages over conventional grinding balls: - Good wear performance efficiency due to the good wear performance of the shell layer, which occupies the majority of the wear volume (reinforcements throughout the ball are useless). - Superior wear performance and / or mechanical properties of composite grinding balls by adjusting the nature, size and volume content of the ceramic particles and using a high mechanical manganese steel combined with a suitable metal binder matrix for the ceramic metal composite granules, e.g. a suitable iron cast alloy such as high chromium white iron for wear parts (the binder metal matrix possibly being different from but complementary to the cast ferroalloy metal matrix). - Superior wear performance and / or mechanical properties of the grinding balls due to controlled porosity and / or crack defects in the granules before injection. - Easy / simple process.

Claims

1. a ferroalloy metal matrix, a reinforcing shell of an openwork ceramic structure of agglomerated ceramic metal composite particles, said agglomerated ceramic metal composite particles comprising micrometric ceramic particles fixed in a binder metal matrix; A composite grinding ball comprising: The composite grinding ball, wherein the agglomerated ceramic metal composite particles are embedded in the ferroalloy metal matrix.

2. 2. The composite grinding ball of claim 1, wherein the micrometer ceramic particles are selected from the group consisting of metal borides, metal nitrides, metal carbides, and metal carbonitrides.

3. 3. The composite grinding ball according to claim 1, wherein the micrometer ceramic particles are selected from the group consisting of titanium carbide, titanium carbonitride, niobium carbide, tantalum carbide, zirconium carbide, hafnium carbide, vanadium carbide, molybdenum carbide, and tungsten carbide, or mixtures thereof.

4. 4. The composite grinding ball according to claim 1, wherein the micrometric ceramic particles are selected from the group consisting of titanium carbide or titanium carbonitride, or mixtures thereof.

5. The ceramic composite particles have an average particle size D of less than 500 μm, preferably less than 400 μm ５０ 5. The composite grinding ball of claim 1, wherein

6. The micrometer ceramic particles fixed in the binder metal matrix have an average particle size D of less than 30 μm, preferably less than 20 μm, and most preferably less than 10 μm. ５０ 6. The composite grinding ball of claim 1, wherein

7. 7. The composite grinding ball according to claim 1, wherein the binder metal matrix that fixes the micrometer ceramic particles is selected from the group consisting of a ferromanganese-based alloy, a ferrochromium-based alloy, and a nickel-based alloy, and the binder metal matrix and the ferroalloy metal matrix have different compositions.

8. 8. The composite grinding ball of claim 1, wherein the cast ferroalloy metal matrix comprises steel or chromium cast iron.

9. 9. Composite grinding ball according to any one of claims 1 to 8, wherein the content of the ceramic particles in the openwork structure is 30-55% by volume, preferably 35-50% by volume.

10. 10. The composite grinding ball of claim 1, wherein the agglomerated ceramic metal composite particles comprise a sintered metal filling voids between the ceramic metal composite particles of the openwork structure, the sintered metal being selected from the group consisting of steel and cast iron, or a combination thereof.

11. 11. A composite grinding ball according to claim 1, wherein the openings in the reinforcing shell of the openwork ceramic structure represent 10 to 80%, preferably 20 to 70%, most preferably 25 to 65% of the surface of the grinding ball.

12. 12. A method for producing a composite grinding ball according to any one of claims 1 to 11, comprising the steps of: a) additive manufacturing of a ceramic openwork shell from a powder mixture containing ceramic metal composite particles; b) placing said shell of said ceramic openwork structure in a cavity of a mould; c) pouring a ferroalloy casting metal into the mold to obtain the reinforced grinding ball of claim 1; A method comprising:

13. The method of claim 12 including the additional step of at least partially sintering the ceramic openwork structure.

14. 14. The method of claim 13, wherein the step of at least partially sintering the shell of the ceramic openwork structure comprises partially impregnating voids between the ceramic-metal composite particles of the structure with a metal selected from the group consisting of steel and chromium cast iron, or a combination thereof, prior to the placement into the cavity of the mold and final casting.

15. 15. The method of claim 13 or 14, wherein the step of at least partially sintering the shell of the ceramic openwork structure is followed by a hot isostatic pressing step or post-infiltration.

16. 13. The method of claim 12, wherein step a) comprises the addition of 2 to 20%, preferably 5 to 20%, most preferably 10 to 18%, by weight, of carbide-forming metal particles selected from the group consisting of tungsten, vanadium, molybdenum, titanium, niobium, hafnium, and zirconium, or mixtures thereof.

17. 13. The method according to claim 12, wherein the ceramic composite particles of the powder mixture of step a) comprise at least 95%, preferably 98%, of particles larger than 150 μm.

18. 13. The method of claim 12, wherein step a) is carried out by binder jetting technique followed by binder curing at temperatures above 100°C.

19. 13. The method of claim 12, wherein the step of injecting the casting metal to obtain the reinforced grinding balls is performed in a grinding ball cluster mold.

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