Composite grinding ball with metal matrix having structural reinforcement

Abstract

FIELD: grinding; crushing industry.SUBSTANCE: present invention relates to grinding balls used in drum mills such as in cement plants or in ore grinding in mines. The composite grinding ball comprises a ferroalloy metal matrix and a reinforcing shell in the form of an openwork structure made of aggregated particles of a ceramic-metal composite, wherein said aggregated particles of a ceramic-metal composite comprise micrometric ceramic particles cemented in a binding metal matrix. The specified aggregated particles of the ceramic-metal composite are embedded in the specified ferroalloy metal matrix, and the openings of the reinforcing shell of the openwork structure make up from 10 to 80% of the surface of the grinding ball. The micrometric ceramic particles are selected from a group comprising metal borides, metal nitrides, metal carbides, and metal carbonitrides. To manufacture the ball, particles of a ceramic-metal composite are obtained, a mixture for 3D printing is prepared, an openwork structure of the ball shell is printed, cured, and infiltrated with a ferroalloy metal in a cluster sand mould.EFFECT: improvement in the operational characteristics of the ball in terms of wear due to an improvement in the wear resistance of the shell layer.19 cl, 4 ex, 6 tbl, 12 dwg

Description

Field of technology

[0001] The present invention relates to a composite grinding ball with a metal matrix, produced by conventional casting technology and reinforced with a shell made of a pre-cast ceramic body with an openwork structure. The shell is a pre-cast body placed in a mold cavity before the cast metal matrix is ​​poured. The resulting grinding ball has improved resistance to combined abrasive and impact loads.

[0002] The openwork reinforcing structure of the grinding ball according to the present invention is obtained by additive manufacturing technology in a 3D printer and consists of a microporous aggregate of a ceramic-metal composite and metal particles. The particles of the ceramic-metal composite contain cemented micrometric particles of borides, nitrides, carbonitrides or carbides, such as TiC, TiCN, NbC, TaC, ZrC, HfC, Mo2C, WC, preferably titanium carbide or titanium carbonitride, cemented in a binder metal matrix. During the casting operation, the casting metal penetrates into the micropores of the aggregate, and the aggregated particles of the ceramic-metal composite are finally completely embedded in the casting metal matrix. Prior Art

[0003] The present invention relates to grinding balls used in drum mills in the grinding and crushing industry, such as in cement plants or for ore grinding in mines. Grinding balls are subject to high mechanical loads within their mass and high surface wear due to abrasion or corrosion. Therefore, it is desirable for grinding balls to have high resistance to abrasion and corrosion and some ductility to withstand the mechanical loads that occur during ball-to-ball or ball-to-liner impacts.

[0004] Considering that these two properties are difficult to obtain with the same material composition, composite grinding balls with a matrix made of a relatively ductile alloy in which ceramic particles with good wear resistance are embedded have been proposed previously.

[0005] Document CN 106914620 A (2017) discloses a method for producing a composite grinding ball using selective laser cladding in combination with a 3D digital modeling technology with an openwork structure of a pre-cast body placed in a mold cavity before casting.

[0006] Documents CN 103357854 (2013) and CN 113564511 A (2021) disclose ceramic reinforced grinding balls, wherein the reinforcing layer comprises an overlay of nanometer-sized ceramic particles on the surface and in the near-surface zone of the grinding ball. The casting mold is coated with nanometer-sized ceramic particles on the inner wall.

[0007] Document WO 2022 / 122393 (Magotteaux 2022) discloses a hierarchical composite wear component reinforced with a triple periodic minimal surface ceramic mesh structure with a plurality of cells, wherein the ceramic mesh structure is embedded in a dual continuous structure with a cast metal matrix. This document does not disclose an openwork ceramic structure of aggregated particles of a ceramic-metal composite containing micrometric ceramic particles cemented in a binder metal matrix embedded in a cast metal matrix.

[0008] The grinding ball market is cost-sensitive, requiring optimization of wear characteristics and cost, making the manufacturing process crucial. Therefore, the cost-effective and safe production of pre-cast ceramic spherical openwork structures using 3D additive manufacturing technology, their robustness during casting, and their ability to penetrate the cast metal without damage are of paramount importance.

[0009] Handling finely ground micron-sized non-oxide ceramic powders such as TiC, TiCN, NbC, TaC, and WC (less than 53 μm) can be challenging because they are reactive (potentially flammable and / or explosive) and require an inert atmosphere for safe use in additive manufacturing. On the other hand, larger non-oxide ceramic particles (>100 μm) can potentially be used without a controlled atmosphere, but such coarse particles are less effective due to the well-known brittleness of ceramic particles. Pure ceramic coarse particles from pre-cast bodies are too brittle to withstand wear conditions in additive manufacturing. From a manufacturing perspective, fine particles typically have virtually no flow, while coarse particles have much higher flowability. Coarse particles also pose fewer health concerns than fine particles.Objectives of the invention.

[0010] The present invention is directed to providing a grinding ball produced by conventional casting technology, reinforced with a shell of a pre-cast ceramic openwork structure placed in the mold cavity before casting the grinding balls. The openwork structure contains a suitable concentration of ceramic-metal composite particles embedded in a cast metal matrix and substantially free of unfilled micropores. Disclosure of the invention

[0011] The present invention relates to a composite grinding ball comprising: a ferroalloy metal matrix, a reinforcing shell of an openwork ceramic structure made of aggregated particles of a ceramic-metal composite, wherein said aggregated particles of a ceramic-metal composite contain micrometric ceramic particles cemented in a binder metal matrix; wherein said aggregated particles of a ceramic-metal composite are embedded in said ferroalloy metal matrix.

[0012] In preferred embodiments of the present invention, at least one of the following features or a suitable combination thereof is disclosed: 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 size D 50 particles less than 500 µm, preferably less than 400 µm; micrometric ceramic particles cemented in a binder metal matrix have an average size D 50particles less than 30 μm, preferably less than 20 μm, most preferably less than 10 μm; the binder metal matrix cementing the micrometric ceramic particles is selected from the group consisting of a ferromanganese-based alloy, a ferrochromium-based alloy and a nickel-based alloy, wherein the binder metal matrix and the ferroalloy metal matrix have different compositions; the ferroalloy cast metal matrix comprises steel or chromium cast iron; the content of ceramic particles in the openwork structure is from 30 to 55 vol. %, preferably from 35 to 50 vol. %; the openings of the reinforcing shell of the openwork ceramic structure are from 10 to 70%, preferably from 25 to 70% of the surface of the grinding ball.

[0013] The present invention also relates to a method for producing a composite grinding ball according to the present invention, comprising the steps of: a) additively manufacturing a shell of a ceramic openwork structure with a powder mixture containing particles of a ceramic-metal composite; b) placing the shell of the ceramic openwork structure in a mold cavity; c) pouring ferroalloy cast metal into the mold to obtain a reinforced grinding ball according to claim 1.

[0014] In preferred embodiments of the present invention, at least one of the following features or a suitable combination thereof is disclosed: 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 includes partially impregnating the spaces between the particles of the ceramic-metal composite of said structure with a metal selected from the group consisting of steel and chromium cast iron or combinations thereof, before placing in the mold cavity and final casting; the step of at least partially sintering the shell of the ceramic openwork structure is followed by a step of hot isostatic pressing or subsequent infiltration; step a) includes adding from 2 to 20 wt. %, preferably from 5 to 20 wt. %, most preferably from 10 to 18 wt.% of carbide-forming metal particles selected from the group consisting of tungsten, vanadium, molybdenum, titanium, niobium, hafnium and zirconium or mixtures thereof; the particles of the powder mixture of the ceramic-metal composite of step a) contain at least 95%, preferably 98%, of particles larger than 150 μm; step a) is carried out using a binder jet application technology followed by curing of the binder at a temperature above 100°C; the step of pouring cast metal to obtain a reinforced grinding ball is carried out in a cluster form of the grinding ball. Brief description of the drawings.

[0015] Fig. 1 and 2 show various 3D models of pre-cast ceramic openwork shells manufactured using additive manufacturing and intended for placement in a mold cavity prior to pouring.

[0016] Figures 3 and 4 show examples of shells manufactured by additive manufacturing from pre-cast ceramic openwork structures, in the form of a single or double sphere containing aggregated particles of a ceramic-metal composite. The spheres have a diameter of 80 mm and openings of 2 mm.

[0017] Fig. 5 shows the placement of pre-cast openwork shells in mold cavities before pouring the cast metal.

[0018] Fig. 6 is a schematic diagram of a typical configuration of a mold for grinding balls cast in a cluster mold.

[0019] Fig. 7-10 show three-dimensional models of pre-cast ceramic openwork structures from Examples 1-4 used to illustrate the reinforced grinding balls of the present invention. In particular, (excluding the inlet): Fig. 7 shows an openwork structure for a grinding ball with a diameter of 80 mm with holes / slots with a diameter of 6 mm, for Example 1. Fig. 8 shows an openwork structure for a grinding ball with a diameter of 90 mm with holes / slots with a diameter of 8 mm, for Example 2. Fig. 9 shows an openwork structure for a grinding ball with a diameter of 70 mm with holes / slots with a diameter of 6 mm, for Example 1. Fig. 10 shows an openwork structure for a grinding ball with a diameter of 70 mm with holes / slots with a diameter of 8 mm, for Example 4.

[0020] Fig. 11 shows the openwork structures from Examples 1-4 as a group in the format of real proportion.

[0021] Fig. 12 shows a pre-cast openwork structure located in a casting mold. In the following description, the expressions "holes" or "slots" in the openwork structure are used interchangeably. Carrying out the invention Description of the stages of additive manufacturing of the shell of the openwork structure

[0022] In order to produce a pre-cast openwork structure according to the present invention, it is necessary to create a digital structure of a 3D model and manufacture it using a ceramic-metal composite powder and metal particles in a 3D printing (additive manufacturing) device, the technology used in this case preferably involves inkjet application of a binder, but is not limited to this.

[0023] A general overview of 3D printing methods and various ASTM (American Association for Testing and Materials) standards related to characteristics and methods was published on ScienceDirect: http: / / www.sciencedirect.com / topics / engineering / binder-jetting. This overview summarizes the contents of several articles related to 3D printing methods, representing the knowledge of specialists in this field. For general information purposes, this publication is incorporated into this application by reference.

[0024] The technology of jetting a binder is disclosed, in particular, in documents US 6,036,777 (2000) and in US2015 / 0069649 A1.

[0025] A recent publication provides a comprehensive overview of the relevant parameters of binder jetting ceramics processing 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.

[0026] This publication examines the stages and applications of binder jetting ceramic printing and discusses key factors such as powder properties, binders, printing parameters, equipment, and post-processing, as well as the influence of the particle shape and size distribution of ceramic powders. The influence of additives, such as the droplet formation mechanism and the droplet infiltration kinetics of binders, is also described. Furthermore, this paper discusses printing parameters such as layer thickness, saturation, printing orientation, equipment, and post-processing. For the purpose of explaining binder jetting technology, this article is incorporated by reference into this application.

[0027] One of the key elements of binder jetting technology is the selection of the appropriate binder type, taking into account its compatibility with the corresponding ceramic, ceramic-metal composite, or metal powder. Various prior art documents have examined various types of binders and ceramic powders.

[0028] WO 2020 / 146452 A1 discloses a specific amine-containing adhesive polymer and a method for manufacturing an object by additive manufacturing using a binder jet. The method comprises separately feeding a powder from which said object is to be manufactured and a solution containing an adhesive polymer dissolved in a solvent into an additive manufacturing device, wherein said adhesive polymer is an amine-containing polymer having a molecular weight of at least 200 g / mol, dispensing selectively located droplets of said adhesive polymer from a print head of said additive manufacturing device into a powder layer to bind the particles and obtain a preform of the object to be manufactured.

[0029] US 2019 / 0111618 A1 discloses a method for indirect additive manufacturing of an object by separately feeding a powder from which said object is to be manufactured and either a difunctional curable monomer or an adhesive polymeric binder into an additive manufacturing device and dispensing selectively positioned droplets of said difunctional curable monomer or adhesive polymeric binder from a print head of said additive manufacturing device into a layer of said powder to bond particles of said powder with said difunctional curable monomer or adhesive polymeric 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 said curable preform to form a cross-linked object. These documents list a number of available curable monomers with their curing temperatures.This document is incorporated into this application by reference. A method for manufacturing a grinding ball according to the present invention.

[0030] A preferred method for producing a grinding ball reinforced with a shell having an openwork structure made of cermet is to create a digital 3D model of the structure (see Figs. 1 and 2, showing various possibilities) and build it in a 3D printing device, optionally the structure is partially sintered to increase its strength. Then, said 3D model is placed in a cluster sand mold, followed by the step of pouring hot liquid matrix metal (e.g., high chromium cast iron or steel containing more than 11 wt.% Cr) to penetrate into the micropores of the pre-cast element and obtain a grinding ball that is substantially free of unfilled micropores in its reinforced structure made of cermet composite. The production of the pre-cast openwork structure includes the following steps:Digital 3D model of the structure

[0031] A 3D numerical model of the openwork shell is created using computer-aided design (CAD) software (such as nTopology) https: / / ntopology.com / generative-design-software / and converted into a format that can be processed by a 3D printing device, such as the STL (stereolithography) format. In nTopology, Field-Driven Design technology can be used to spatially modify the parameters of the openwork structures, such as the shell thickness, mesh size, or the geometry of the openwork structure. The file is then processed by slicing software, which cuts the model into 2D layers of a specified thickness that can be printed. Additive Manufacturing (AM) Process

[0032] Small particles (<100 µm) are generally hazardous to process for health and / or safety reasons, and therefore the present additive manufacturing process uses larger ceramic-metal composite particles in which small ceramic particles are cemented in a binder metal matrix. In this process, the ceramic-metal powder is fed through a hopper, for example, to form a single layer with a thickness of about 200 to 500 µm, depending on the powder particle size. The ceramic-metallic powder particles can be selected from nitrides, borides or carbonitrides, such as, for example, TiC, TiCN, NbC, TaC, WC, ZrC, HfC, VC, Mo2C, preferably titanium carbide or titanium carbonitride cemented in a binder metal matrix, and possibly some other carbide-forming metal particles to improve the wettability of the ceramic-metallic composite particles with the casting alloy while forming additional carbides in situ. The average particle size (D 50) is typically between 200 and 500 µm, preferably between 200 and 400 µm, most preferably between 200 and 300 µm (when measured by dry sieving according to ISO 4497:2020). If the powder granulometry does not meet the target particle size distribution, a sieving step may be required to eliminate the fraction of particles with a size below 150 µm (due to safety, flowability or printing resolution). The ceramic-metal composite powder, possibly associated with metal powder, is introduced into the 3D printing device to build up the openwork structure layer by layer deposited in the build chamber (for repeated coating, vibrations and a roller can be used to increase the layer packing density, mainly in the case of fine or poorly flowing powders). Depending on the additive manufacturing technology, a moving head creates cohesion of the powder in certain areas of the layer, for example by jetting a binder.Agglomeration with the previous layers beneath the current layer also occurs. In the case of inkjet application of the binder, the liquid binder is deposited by a moving head in the form of droplets on specific areas of the layer according to a 2D file. An important parameter is the determination of the appropriate saturation level to achieve suitable adhesion between the layer particles and between the previous printed layer. The preferred binder is, for example, a water-dispersible binder based on glycol and acrylic acid, such as tetraethylene glycol dimethacrylate; tetraethylene glycol diacrylate; triethylene glycol dimethacrylate, diethylene glycol 2, methoxyethanol or, preferably, mixtures suitable for the formation of interconnected molecular networks that can be dispersed in water and cured at 130-200°C for about 2 hours per cm of the thickness of the material of the cured block.The next layer is then applied and the above steps are repeated until the entire insert is embedded in the powder bed. Curing and further sintering.

[0033] If a crosslinkable monomer binder has been selected, it must be cured. The entire block is heated at a temperature of approximately 130 to 200°C in a curing oven to impart strength (through polymerization, crosslinking, solvent evaporation, or some other mechanism) to the part. This time period depends on the block volume and the printed material to ensure temperature uniformity (e.g., about 2 hours per cm of thickness). The block is then allowed to cool completely, and excess powder is removed from the block, for example, by brushing, vacuuming, or blowing with compressed air. The "green" part can be safely processed without the risk of breakage. If a "green" pre-cast openwork body needs to be sintered, it is placed in an oven and heated at a high temperature (usually above 900°C) in a controlled atmosphere (usually argon or vacuum) to perform the sintering step.The sintering rate to be achieved depends on the desired mechanical properties and the final porosity of the openwork structure. Manufacturing of a grinding ball.

[0034] Prepared pre-cast elements (openwork structure shells) are placed in a pre-formed cavity of a cluster sand mold. The mold is closed and liquid metal is poured into the mold. It is observed that the pores of the aggregated ceramic-metal composite powder generally range from 10 to 700 μm, with the majority of pores ranging from 50 to 500 μm with an average pore size of 150 to 300 μm. Three-dimensional openwork structures have uniform open porosity throughout the volume, and in the case of partially sintered shells of ceramic openwork structures, the liquid cast metal penetrates into the remaining micropores of the structure, resulting in a very strong ceramic / metal bond. The liquid metal is left to cool until the grinding balls are completely solidified.The sand mold is then removed and the grinding balls are cleaned of any remaining sand, after which the usual finishing process steps known to those skilled in the art may follow (ejection, shot blasting, grinding, additional heat treatments such as annealing, hardening, soaking, etc.). Examples of grinding balls.

[0035] These examples are illustrative and should not be construed as limiting.Preparation of ceramic-metal composite particlesThe following powders of TiC, titanium carbonitride, WC, NbC, Mo2C, iron, manganese, chromium and nickel were used for 3 different types of ceramic-metal composite particles (CMP 1-3), all powders had a particle size of less than 325 mesh (<44 μm). Composition CMP1 is particularly suitable for providing impact resistance due to its lower content of ceramic particles (45 wt.% TiC) cemented in a tough manganese steel binder. Composition CMP 2 is particularly suitable for providing abrasion resistance due to its high content of titanium carbide (85 wt.% TiC) cemented in a tough wear-resistant binder with a high content of chromium and white iron. Composition CMP 3 is particularly suitable for providing resistance to corrosion and wear due to its high content of a complex mixture of ceramic particles (TiC 0.5 N 0.5 + WC + NbC + Mo2C = 85 wt.% in total), precisely selected for cementation in a corrosion-resistant nickel binder.

[0036] Other ceramic particles can be added to create complex solid solution particles, control or fine-tune the grain size, morphology and / or core structure of the solid ceramic particles.

[0037] Powders according to the compositions in Table 1 were mixed and ground in a ball mill with isopropyl alcohol and metal grinding balls for 24 hours until an average size of D was achieved 50 particles of about 3 microns.

[0038] An organic wax binder (2% by weight) is added and mixed with the resulting powders during the milling stage. The alcohol is removed using a vacuum dryer with rotating blades (the alcohol is condensed for reuse). The resulting agglomerated powder is then sieved through a 600-μm sieve. By compacting between the rotating rollers of a roller compactor granulator, strips of 60% of the theoretical density of the ceramic-metallic powder mixtures are produced. The strips are then crushed into uneven particles by forcing them through a sieve with an appropriate mesh size. After crushing, the granules are sieved to obtain a size of approximately 200 to 600 μm. These non-uniform porous particles are then sintered at high temperature (1300-1500°C for about 2 hours) in a vacuum furnace with a low partial pressure of argon until a minimum porosity is achieved (less than 5 vol.%, preferably less than 3 vol.% and more preferably even less than 1 vol.%).After sintering, an additional step of crushing and final sieving is usually required to deagglomerate any ceramic-metal composite particles that might otherwise agglomerate during sintering. Preparing a powder mixture for 3D printing.

[0039] The composition and size distribution of the powder mixture are determined by the infiltration capacity of the shell of the openwork precast housing structure manufactured by additive manufacturing. This infiltration capacity depends, in particular, on the following parameters: the thickness and microporosity of the openwork precast housing, the proportion of holes in the openwork structure, and the composition of the alloy to be poured into the mold, which will affect the possible superheating required depending on the liquidus temperature and the liquidus-solidus range (an alloy with a high chromium content penetrates openwork precast housing structures more easily than steel at the same temperature).

[0040] In the past, certain metals, such as magnesium, titanium, and niobium, have been shown to have a positive effect on the wettability of ceramic particles in the casting metal matrix and, consequently, on the infiltration capacity of the openwork shell. Some of these metals (e.g., Ti, Nb, and other carbide-forming metals) also have the beneficial effect of additional carbide formation in combination with the carbon present in the cast iron, as discussed below.

[0041] The wettability of ceramic-metallic particles has been investigated, among others, in the paper “Wettability in Metal Matrix Composites” (2021) by Massoud Malaki et al. and in the paper “Role of wettability in the preparation of metal-matrix composites” by Banerji et al. (1984).

[0042] In the present invention, the titanium and niobium metal particles play a dual role in that they improve the infiltration ability of the openwork structure, and they react with the carbon always present in the cast ferroalloy to produce additional carbides in situ.

[0043] Three different types of powder mixtures 1-3 were prepared, bonding particles of the ceramic composite CMP 1-3 with titanium and niobium powders. All powders contained less than 5 wt.%, preferably 2 wt.%, of particles smaller than 150 μm. The mixture was mixed for 15 minutes in a blender according to the table below. Casting alloys

[0044] Cast alloy 1 contains 2.2 wt% C and 16.5 wt% Cr and is particularly suitable for impact loading conditions with suitable heat treatment to improve impact resistance. The powder of mixture 1 is used to manufacture an openwork structure. The composite grinding balls of the invention and the control metal grinding balls are compared with each other over the same period in the same ball mill processing the same platinum ore under significant impact loading conditions. Both grinding balls are subjected to the same heat treatment.

[0045] Cast alloy 2 contains 2.85 wt% C and 14.5 wt% Cr and is particularly suitable for abrasion conditions with suitable heat treatment to improve abrasion resistance. The powder of mixture 2 is used to manufacture shells. The composite balls of the invention and control metal balls of cast alloy 2 are compared in the same ball mill with copper ore under severe abrasion conditions.

[0046] Cast alloy 3 contains 2.3 wt.% C and 29 wt.% Cr and is particularly suitable for highly corrosive environments. The powder of mixture 3 is used to manufacture shells. The composite balls of the invention and control metal balls of cast alloy 3 are compared in the same ball mill with magnetic ore under severely corrosive conditions.

[0047] Cast alloys 1, 2 and 3 also contain other alloy components <2% by weight (Si, Mn, Mo, Ni...) known to those skilled in the art, depending on the specific properties and directional heat treatment. Example 1 (Mixture 1 + Cast Alloy 1)Mixture 1 was used to print a ceramic openwork shell with a diameter of 80 mm, a shell thickness of about 5 mm and holes / openings with a diameter of 6 mm, providing a surface coverage of 57% (as shown in Fig. 7), on an X25 Pro 3D Binder inkjet printer from EXone. An aqueous binder based on a mixture of diethylene glycol in the form of a dispersion in an aqueous solution of 2-butoxyethanol was used to print the part (Aquafuse BA005 EXone).

[0048] The key parameters of the additive manufacturing process were as follows: each printed layer was about 300 μm thick; the printing speed was 60 seconds per layer; the binder saturation of the powder pores was 30%; and the powder packing density was about 43%.

[0049] After completion, the entire print block was cured in an oven at approximately 200°C for 2 hours per cm of part height. The curing time depends on the design, as multiple parts can be produced in a single pass. After cooling, the print block was vacuumed and brushed to create a "green" openwork design.

[0050] The resulting “green” openwork pre-cast structure was placed in a furnace and heated to about 1100°C for 2 hours in a high vacuum.

[0051] The shell of the openwork pre-cast structure contains a total of about 83 vol.% of the void space available for ferroalloy infiltration (about 61 vol.% due to the holes / openings of the openwork structure and an additional 22 vol.% of the micrometric pores), the remaining 17 vol.% in the openwork structure are ceramic-metal composite particles + titanium metal particles.

[0052] The resulting openwork pre-cast structure is then placed into the cavity of a cluster sand mold.

[0053] Then hot liquid high chromium white iron (with composite casting alloy 1) is poured into the mold, filling 61 vol.% due to the holes of the openwork pre-cast structure and infiltrating 22 vol.% micrometric porosity between the particles of the aggregate structure to form additional carbides from the reaction of the carbon contained in the casting alloy with the metal powder.

[0054] After casting, 39 vol.% of the reinforced volume (1 - vol.% of the openwork structure holes) contains a high concentration of about 30 vol.% of titanium carbide. Thus, the total volume content of titanium carbide particles in the reinforced part of the grinding ball is about 12 vol.%. Example 2 (Mixture 2 + Casting Alloy 2) Example 2 is made in the same way as Example 1, but with Mixture 2 and another ceramic openwork structure shell with a diameter of 90 mm, a shell thickness of 8 mm and holes / openings with a diameter of 8 mm, providing a surface coverage of 69%, shown in Fig. 8.

[0055] Homogeneous powder mixture 2 was used to print the shell of a ceramic openwork structure with the same equipment and printer settings as in Example 1. An aqueous binder based on a mixture of diethylene glycol in the form of a dispersion in an aqueous solution of 2-butoxyethanol was used to print the part (Aquafuse BA005 EXone).

[0056] The key parameters of the AM process were the same as in Example 1. The resulting “green” shell of the openwork ceramic pre-cast structure was placed in a furnace and heated to about 1000°C for 2 hours in an argon atmosphere.

[0057] This structure was again obtained with a powder packing density of 46% and contains a total of about 74 vol.% void space available for ferroalloy infiltration (about 42 vol.% due to holes in the structure and an additional 32 vol.% micrometric pores between the particles of the ceramic-metal composite), the remaining 27 vol.% is the structure of the ceramic-metal and metallic titanium of the openwork structure.

[0058] The resulting shell of the ceramic openwork structure is then placed into the cavity of the sand mold.

[0059] Then hot liquid high chromium white iron (with casting alloy composition 2) is poured into the mold, filling 42 vol.% of the holes / openings of the openwork structure and obtaining 32 vol.% infiltration of micrometric porosity between the ceramic metal composite particles of the structure.

[0060] After casting, 58 vol.% of the reinforced volume has a high titanium carbide content of about 42 vol.%, thus the total volume content of titanium carbide particles in the reinforced part of the grinding ball is about 25 vol.%. Example 3 (Mixture 3 + Casting Alloy 3)

[0061] Example 3 was made with Mixture 3 and another openwork shell with a diameter of 70 mm, a shell thickness of 3 mm, and holes / openings of 6 mm in diameter, providing a surface coverage of 64%, as shown in Fig. 9. An alcohol binder was used to print the part (Cleanfuse EXone). The key parameters of the AM process were as follows: each printed layer had a thickness of about 450 μm; the printing speed was 45 seconds per layer; the binder saturation of the powder pores was 30%; and the powder packing density was about 47%.

[0062] The key parameters of the AM process were the same as for the previous examples (curing at 200°C for 2 hours, and then the oven was heated at 1050°C for 2 hours under argon atmosphere).

[0063] The ceramic openwork shell was obtained with a powder packing density of 47% and contains a total of about 77 vol.% of empty space available for ferroalloy infiltration (about 51 vol.% due to the holes / openings of the openwork structure and an additional 26 vol.% of micrometric pores inside the structure), the remaining 23 vol.% is the structure of metal-ceramic composite granules + niobium metal particles of the openwork structure.

[0064] The resulting structure is then placed into the cavity of a sand mold and hot liquid high chromium white iron (with composite casting alloy 3) is then poured into the mold, filling 51 vol.% of the holes / openings of the openwork structure and infiltrating 26 vol.% of the micrometric pores between the particles present in the openwork structure.

[0065] After casting, 49 vol.% of the reinforced volume has a high content of about 43 vol.% of ceramic particles, such as titanium carbonitride, tungsten carbide, molybdenum carbide and niobium carbide or mixtures thereof, or even mixed carbides such as (Ti, Nb, W, Mo)C. Therefore, the total volume content of ceramic particles in the reinforced part of the grinding ball is about 21 vol.%. Example 4 (Mixture 3 + Casting Alloy 2)

[0066] Example 4 was made with Mixture 3 and another openwork shell with a diameter of 70 mm, a shell thickness of 3 mm, and holes / openings of 10 mm in diameter, providing a surface coverage of 56%, as shown in Fig. 10. An alcohol binder was used to print the part (Cleanfuse EXone). The key parameters of the AM process were as follows: each printed layer had a thickness of about 450 μm; the printing speed was 45 seconds per layer; the binder saturation of the powder pores was 35%; and the powder packing density was about 47%.

[0067] The key parameters of the AM process were the same as for the previous examples (curing at 200°C for 2 hours, and then the oven was heated at 1150°C for 2 hours under argon atmosphere).

[0068] The ceramic openwork shell was obtained with a powder packing density of 47% and contains a total of about 77 vol.% of empty space available for ferroalloy infiltration (about 51 vol.% due to the holes of the openwork structure and an additional 26 vol.% of micrometric pores inside the structure), the remaining 23 vol.% is the structure of ceramic-metal composite granules + niobium metal particles.

[0069] The resulting structure is then placed into the cavities of the sand mold and then hot liquid high chromium white iron (with composite casting alloy 3) is poured into the mold, filling the holes with 50 vol.% of the openwork structure and infiltrating 31 vol.% of the micrometric pores between the particles of the ceramic-metal composite of the openwork structure.

[0070] After pouring, 49 vol.% of the reinforced volume contains a high concentration of about 43 vol.% of ceramic particles such as titanium carbonitride, tungsten carbide, molybdenum carbide and niobium carbide or their mixtures or even mixed carbides such as (Ti, Nb, W, Mo)C. Therefore, the total volume content of carbides or carbonitrides in the reinforced part of the grinding ball is about 21 vol.%. The terms "shell," "reinforcing shell," "openwork shell," and "openwork structure" are used interchangeably. Shell thickness is the thickness of the openwork structure excluding openings. Performance index calculated compared to conventional grinding balls

[0072] The article "Overview of grinding media consumption in communication" (Slabbert, Paton, Moema and Zimba) in "World Gold Conference") provides an overview of grinding media testing.

[0073] Real-life grinding ball testing on industrial grinding machines is particularly challenging due to the small proportion of test grinding balls compared to the conventional grinding ball load. Test samples generally represent less than 0.1% (400 balls out of 400,000-800,000 balls), as a grinding machine contains grinding balls worn at various stages with a wide diameter size distribution (e.g., from 20 to 70 mm in diameter). The main difficulty is to find at least a significant number of test and control composite grinding balls from thousands of conventional grinding balls after several days or weeks of grinding, so that a representative average mass loss percentage of the composite test balls can be measured for comparison with the control metal grinding balls.

[0074] A solution to this problem is to use grinding balls with a slightly larger diameter (approximately 10 mm larger) than those regularly added and already present in a conventional grinding machine. For example, if fresh conventional grinding balls regularly added to an industrial grinding machine have a diameter of 60 mm, a diameter of 70 mm is selected for the test grinding balls (composite and control grinding balls) to facilitate the removal of the marked balls. Because these balls are larger, they are not only visually easier to find, but they also have a natural tendency to "float" on the surface of the conventional grinding media charge.

[0075] The evaluation of the performance characteristics with respect to wear of the grinding balls of the present invention was carried out under the actual grinding conditions of copper ore, magnetite iron ore and platinum ore in three industrial mills A; B; C (with the following characteristics and conditions).[00761 Grinding copper ore with the grinding balls of Example 2 (diameter 90 mm)Mill A with a diameter of 7.31 m and a length of 10.97 m;containing 660 tons of ordinary grinding balls (33 vol.% filling);regularly adding several tons per day of ordinary grinding balls with a diameter of 76 mm to compensate for wear;rotational speed of 11.9 rpm;power consumption of 10500 kW;processing 950 t / h of copper ore;under abrasion conditions;test duration: 8 days.

[0077] Grinding magnetite iron ore with the grinding balls of Example 3, then testing with the grinding balls of Example 4 (70mm)Mill B with a diameter of 4.6m and a length of 6.7m;filling with 165 tons of ordinary grinding balls (filling rate 35%);regularly adding several tons a day of ordinary grinding balls with a diameter of 60mm to compensate for wear;rotating at a speed of 15.1 rpm;power consumption 2300kW;processing 420t / h of magnetite iron ore; under corrosive conditions;test duration: 20 days.

[0078] Platinum ore grinding using grinding balls from Example 1 (80 mm)Mill with a diameter of 7.925 m and a length of 11.735 m; filled with 800 tons of ordinary grinding balls (32% by volume filling); regularly adding several tons of ordinary grinding balls with a diameter of 70 mm per day to compensate for wear; rotating at a speed of 10.5 to 11.5 rpm; power consumption of 15,000 kW; processing 1,000 tons / h of platinum ore; under impact load conditions; test duration: 10 days. Preparation of test samples

[0079] To test the performance of the composite grinding balls of Examples 1-4, as well as the control ball, 200 grinding balls from each example were manufactured, and each grinding ball made of the same alloy and reinforcement was marked with the same identification means (e.g., one or two drilled holes of a certain diameter and position in all grinding balls of Example 1; 2 holes in all grinding balls of Example 2, etc.). All grinding balls of the same composition were further machined or ground to obtain the same weight (+ / - 2 kg for grinding balls with a diameter of 80 mm) with a tolerance of + / - 5 grams, preferably + / - 2 grams. The composite grinding balls of the present invention and the control metal grinding balls are made of the same ferroalloy and subjected to the same heat treatment in order to be able to evaluate the effect of the ceramic reinforcement as a single variable.

[0080] In the next step, 200 composite test grinding balls and 200 control metal grinding balls are loaded together into an industrial mill (Ex 1 composite balls in Mill C, Ex 2 composite balls in Mill A, and Ex 3 composite balls in Mill B). The industrial mills described above already contain a normal charge of ordinary grinding balls with a filling rate of about 35 vol%. This amounts to 660 tons of ordinary grinding balls for Mill A, 165 tons for Mill B, and 800 tons for Mill C, while the amount of test and control grinding balls (200 test grinding balls and 200 control balls in each mill) is only about 600-900 kg, which has little effect on the grinding performance. Therefore, the addition of test grinding balls does not significantly affect the overall filling of the mill.

[0081] The mill must then be operated for a sufficient period of time to observe wear sufficient to be measurable. This period of time is generally around several days or weeks under 24-hour operation.

[0082] Depending on the type of ore being ground, the wear rate of conventional grinding balls on magnetite iron ore is known to be approximately 0.5 mm / 100 h (approximately 2.4 mm after 20 days) in diameter, while for copper ore it is approximately 1.5 mm / 100 h (approximately 3 mm after 8 days) in diameter. The expected wear rate on platinum ore is approximately 1.3 mm / 100 h (approximately 3 mm after 10 days). This indication, related to the wear rate of conventional grinding balls, was considered when selecting the test duration to ensure that the wear remains within the thickness of the reinforcing shell, allowing for an appropriate interpretation of the effect of the reinforcing shell itself.

[0083] Finally, the mill is stopped, and the detected and identified test grinding balls (generally less than 10% of the initial amount) are weighed to evaluate the average mass loss and calculate the service index, as shown in the result table below. Calculation of service index

[0084] For each sample grinding ball, the average weight loss is measured before and after the test, and the average percentage weight loss is calculated. % weight loss / 100 = (initial weight - final weight) / initial weight. The performance index is defined as below, where the weight loss of the control ball is the average weight loss of the control metal grinding balls. PI = weight loss of the control metal grinding balls / weight loss of the tested composite grinding balls

[0085] A performance index greater than 1 indicates that the tested composite grinding ball according to the invention is less worn than the control ball; a performance index less than 1 indicates that the tested composite grinding ball is more worn than the control ball. The control grinding ball is a conventional grinding ball made of the same cast alloy but without any ceramic reinforcement. As shown in the table below, the grinding balls according to the present invention exhibit, on average, better performance due to the reinforced shell.

[0086] The performance index of Example 3 shows the superior performance of the composite shell for smaller-diameter holes compared to Example 4 with larger holes. Examples 1 through 4 demonstrate significantly better performance characteristics for core-shell composite grinding balls with different particle compositions and properties, casting alloys, and test conditions.

[0087] The performance of the composite grinding balls of the present invention is compared with that of conventional grinding balls as mentioned above. If the reinforcing shell is not completely worn out or completely disappeared, the thickness of the reinforcing shell naturally affects the overall performance of the grinding ball. A grinding ball with a 10mm thick composite reinforcing shell will have better long-term performance than its equivalent with a 5mm thick reinforcing shell. The effect of the performance index on overall grinding performance

[0088] The following table compares the wear, as an average percentage of mass loss, of Examples 1-4 (composite grinding balls) with their corresponding control grinding balls. The increased service life is calculated based on their service index and its effect on grinding ball service life.

[0089] The service life of a grinding ball is generally estimated as the time it takes for the ball to wear from its original diameter to 20 mm. Once the grinding ball diameter reaches 20 mm, it is considered small enough to exit the mill along with the ground material through the outlet chute. The measured weight loss is converted into a reduction in diameter per unit time, allowing for the calculation of extended service life.

[0090] The service life is significantly increased due to the reinforced openwork design, which allows the grinding ball to maintain a diameter close to its original value for a longer period of time.

[0091] Examples 1-4 demonstrate significantly improved performance of core-shell composite grinding balls with different granule compositions and properties, casting alloys, and testing conditions. Observations

[0092] Besides considering the influence of different cermet and cast alloy compositions on wear characteristics in a specific grinding environment, the influence of the openwork structure geometry disclosed in this document plays an important role. Parameters to be considered include, for example, the surface coverage ratio of the ceramic reinforcement, the hole size, the thickness-to-hole ratio, and the hole spacing (see Table 1).

[0093] It has been observed that the ease of infiltration of an openwork structure with a cast ferroalloy is generally inversely proportional to the surface coverage ratio and the reinforcement thickness, while it is proportional to the hole size and spacing. However, the wear resistance of a specific CMP / cast alloy composition depends on the surface coverage ratio and the reinforcement thickness, while it is inversely proportional to the hole size and spacing.

[0094] Since the difficulty of infiltration should not be underestimated, it is necessary to find a compromise between these parameters, keeping in mind that the combination of high thicknesses of the openwork structure (>10 mm) with very small holes (<2 mm) located very far from each other should be avoided, while preference should be given to lower thicknesses of the openwork structure (<10 mm) with medium holes (>2 mm) located close to each other (<10 mm). Advantages of the present invention

[0095] The present invention has the following advantages over conventional grinding balls. Improvement of wear performance due to better wear resistance characteristics of the shell layer representing most of the wear volume (it is useless to have reinforcement everywhere inside the ball). Improvement of wear performance and / or mechanical properties of composite grinding balls by adapting the nature, size and volume content of ceramic particles and using a suitable binder metal matrix for the ceramic-metal composite particles, such as, for example, manganese steel with high mechanical properties in combination with a suitable cast ferroalloy, such as, for example, high-chromium white cast iron for the wear part, wherein the binder metal matrix can be different from and complementary to the cast ferroalloy metal matrix.Improved performance in terms of wear and / or mechanical properties of grinding balls due to controlled porosity and / or crack defects in granules before casting. Easy / simple process / .

Claims

1. A composite grinding ball comprising: ferroalloy metal matrix, a reinforcing shell in the form of an openwork structure made of aggregated particles of a ceramic-metal composite, wherein said aggregated particles of a ceramic-metal composite contain micrometric ceramic particles cemented in a binding metal matrix; wherein said aggregated particles of the ceramic-metal composite are embedded in said ferroalloy metal matrix, the openings of the reinforcing shell of the openwork ceramic structure make up from 10 to 80% of the surface of the grinding ball.

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

3. A composite grinding ball according to claim 1 or 2, wherein 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.

4. A composite grinding ball according to any one of the preceding claims, wherein the micrometric ceramic particles are selected from the group consisting of titanium carbide or titanium carbonitride or mixtures thereof.

5. A composite grinding ball according to any one of the preceding claims, wherein the ceramic-metal composite particles have an average particle size D 50 particles less than 500 µm, preferably less than 400 µm.

6. A composite grinding ball according to any one of the preceding claims, wherein the micrometric ceramic particles cemented in the binder metal matrix have an average size D 50particles less than 30 µm, preferably less than 20 µm, most preferably less than 10 µm.

7. A composite grinding ball according to any one of the preceding claims, wherein the binder metal matrix cementing the micrometric ceramic particles is selected from the group consisting of a ferromanganese-based alloy, a ferrochromium-based alloy, and a nickel-based alloy, wherein the binder metal matrix and the ferroalloy metal matrix have different compositions.

8. A composite grinding ball according to any one of the preceding claims, wherein the ferroalloy cast metal matrix comprises steel or chromium cast iron.

9. A composite grinding ball according to any one of the preceding claims, wherein the content of ceramic particles in the openwork structure is from 30 to 55 vol.%, preferably from 35 to 50 vol.%.

10. A composite grinding ball according to any one of the preceding claims, wherein the aggregated particles of the ceramic-metal composite comprise a sintering metal filling the spaces between the particles of the ceramic-metal composite of an openwork design, selected from the group consisting of steel and cast iron or combinations thereof.

11. A composite grinding ball according to any one of the preceding claims, wherein the openings of the reinforcing shell of the openwork structure make up from 20 to 70%, preferably from 25 to 65% of the surface of the grinding ball.

12. A method for manufacturing a composite grinding ball according to any one of paragraphs 1-11, comprising the steps of: a) manufacturing a shell in the form of an openwork structure using additive manufacturing from a powder mixture containing particles of a ceramic-metal composite; b) placing the shell of the openwork structure in the cavity of the mold; c) pouring ferroalloy cast metal into a mold to produce a reinforced grinding ball according to paragraph 1.

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

14. The method according to claim 13, in which the step of at least partially sintering the shell of the openwork structure includes partially impregnating the spaces between the particles of the ceramic-metal composite of said structure with a metal selected from the group including steel and chromium cast iron or combinations thereof, before placing in the mold cavity and final casting.

15. The method according to any one of claims 13-14, in which the step of at least partially sintering the shell of the openwork structure is followed by a step of hot isostatic pressing or subsequent infiltration.

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

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

18. The method according to claim 12, in which step a) is carried out using a technology of jet application of a binder followed by curing of the binder at a temperature above 100°C.

19. The method according to claim 12, in which the step of pouring the cast metal to obtain the reinforced grinding ball is carried out in a cluster mold of the grinding ball.

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