Title of Invention

HIGH CARBON CONTENT STEEL OR CAST IRON GRINDING MEDIUM AND ITS MANUFACTURING PROCESS

Abstract

The invention relates to steel or cast iron balls with high carbon content obtained by centrifugal atomization in a granulometry range preferably situated between 3 mm and 12 mm, and with an apparent density greater than 4.3 g/cc, making them suitable for use as grinding media for ore grinding mills. The speed of rotation of the centrifugation table 12 can be adjusted between 300 rpm and 1,500 rpm. The minimum cooling rate in the mass a a ball is greater than 10°C/second and depends on the height of fall (hi) and on the depth of the tank (h2).This invention also relates to a process for manufacturing the steel or cast iron balls with high carbon content.

Full Text

High carbon content steel or cast iron grinding medium, and its manufacturing process Background of the invention This invention relates to a cast steel shot with a relatively high carbon content that is atomized by centrifugation to make it suitable for fine and extremely fine grinding in the mining and mineral industry. State of the art In the mining industry, valuable elements have to be liberated from the rock in which they are embedded. One means of liberating these elements is fracturing (crushing-grinding), consisting in progressively fragmenting the rock until the liberation size of the valuable elements is reached. Depending on this size, coarse, fine or extremely fine grinding will be referred to. The grinding operations, in particular where fine grinding is concerned, may be dictated not only by a liberation size but also by constraints connected with the downstream operations or with the subsequent use of the product thus treated, whether it be in the mining industry (for example production of iron ore pellets where the iron ore concentrate must have a size of about 60m before pelletization), or in the mineral industry (for example the cement industry). To perform fine grinding of the ore, it is known to use metallic grinding media in grinding mills to reduce the granulometry of the previously crushed iron ore particles. The types of grinding mills using grinding media are variable, but are all based on the principle of bringing the grinding media into movement on the product to be ground, a movement which seeks to favour, among the possible fragmentation modes (impact, attrition), the one which will enable the best possible fragmentation efficiency to be achieved. Impact will consist in projecting or dropping grinding media onto the product to be ground in order to achieve in-depth breaking, attrition will consist in friction of the grinding media on the product to be ground in order to achieve superficial scalping. Grinding media of large sizes will be preferred for impact so as to have a high unit weight. Small size grinding media will preferably be used for attrition to have a large contact surface with the product to be ground. It is known that below a certain fragmentation size proper to the product involved, the product can not be further reduced efficiently by impact. The defects present in the coarse particles of the product (cracks, holes,...) are in fact less and less numerous as the particles are fragmented, and grinding by impact becomes less and less effective. In this case, preference should be given to attrition. Thus it is admitted that a correlation exists between the liberation size sought for and the size of the grinding media used: the smaller the size of the grinding media, the larger the contact surface with the products to be ground and the greater the efficiency of grinding by attrition. Apart from the grinding efficiency proper, which can amount simply to the energy spent to achieve the required grinding size or to the speed of fragmentation obtained, another element has a certain importance in the performance of grinding operations involving the use of grinding media: wear of these media. The grinding media are generally in the form of balls, truncated cones, or cylinders. The materials used for the grinding media are either inexpensive steels with low strength, or high-alloy cast irons that are very expensive or low-alloy cast irons with low wear resistance. These materials can be classified as follows: - low-alloy steels of martensitic structure (0.7%-l% of carbon, alloys less than 1%), manufactured from rolling or forging by heat treatments to obtain a surface hardness of about 60-65 HRC; - cast irons alloyed with chromium of martensitic structure (L7%-3.5% carbon, 9%-30%> chromium), manufactured by casting and heat treatment to obtain a hardness of 60-68 HRC on all the sections; - low-alloy white cast irons of pearlite structure (3%>-4.2%> of carbon, alloys less than 2%), not heat treated and having a hardness of 45-55 HRC obtained when casting is performed. The document FR 2,405,749 describes a grinding medium forged from chromium alloy white cast iron, whose structure is composed of a solid martensitic or austenitic solution containing secondary chromium carbides and primary or eutectic M7C3 type chromium carbides, finely divided and distributed homogeneously in the matrix. The grinding media contain between 1% and 3% weight of carbon, between 5% and 15%o weight of M7C3 type chromium carbides, i.e. between 2%> and 8%> weight of chromium, from 0 to 2%> weight of molybdenum, from 0.5% to 1.5% of silicon, from 0.1% to 2% of manganese, from 0 to 5%o of vanadium, and from 0 to 1% of copper. Other elements can be added such as tungsten, nickel, boron, niobium, tantalum and zirconium. The manufacturing process of these grinding media consists in heating a white cast iron bar up to a first temperature of about 900°C to 1000°C enabling hot shearing to be performed in the plastic state, of cutting the bar into several slugs, of reheating the slugs up to a second temperature comprised between 1000°C and 1150°C causing reaustenitizing and complete solution heat treatment of the carbides in the austenite region, of forging the slugs at the second temperature, and of cooling the slugs at a controlled rate down to a temperature comprised between 600°C and 800°C. Forged parts of pearlite structure subjected to an annealing treatment to adapt their structure to the conditions of use are thus obtained. The document WO 95/28506 refers to a manufacturing process of high-carbon steel grinding media (from 1.1% to 2% of C) containing in addition manganese (from 0.5% to 3.5% of Mn), chromium (from 1% to 4% of Cr), and silicon (from 0.6% to 1.2% of Si) in order to give a fine pearlite structure having a hardness situated in the 47-54 Rockwell range. The grinding media are melted into balls from 70 to 100 mm in diameter, and the pearlite structure is obtained by extracting the part from the casting mould while it is still hot. One of the drawbacks of the above-mentioned material manufacturing processes, whether it be forging, rolling or casting, is the significant increase of the cost price when the size of the grinding media sought for decreases. It is thus very rare to see grinding media resulting from the foregoing processes smaller than 12 mm in size used today in the mining and mineral industry. However as indicated above, the efficiency of the grinding in particular in the field of fine and extremely fine grinding will depend to a great extent on the size of the grinding media used. The applicant has already manufactured shot for grinding applications, shot of sizes situated between 3.5 and 6.5 mm. The manufacturing process used consists in subjecting the molten steel to atomization with water to obtain appreciably spherical balls. The shot thus formed has a very broad granulometric distribution (from 0.3 mm to more than 8 mm), but above 3 mm the quality of the particles presents major density defects, notably cavities, defects which may prove to be a radical impediment in grinding applications - they can not only give rise to excessive wear, but they can also even prevent this shot from being used in grinding mills where the flows of product to be ground are so large that they would tend to lift the shot up and carry it with them when passing through the grinding mill. The apparent density of the shot larger than 3 mm obtained by atomization with water and sorted in order to keep only the appreciably spherical shapes is smaller than 4.3 g/cc, the measurement being obtained by weighing in a broad burette with a capacity of at least 1 litre. Atomization by centrifugation is currently used to manufacture steel shot of small dimensions in particular smaller than 1.5 mm. The applicant thus proposes to use this method of atomization for manufacturing shot intended for grinding applications, enabling in particular an apparent density greater than 4.3 g/cc to be achieved for particles larger than 3 mm. Object of the invention A first object of the invention is to achieve fine grinding media with a high-carbon steel shot base, having a good wear resistance and a high density. The steel or cast iron particles or balls according to the invention are formed by centrifugal atomization so as to obtain an apparent density greater than 4.3 g/cc in particular for particles larger than 3 mm. The granulometry of the particles is preferably situated between 3 mm and 12 mm. The steel or cast iron particles have a carbon content comprised between 0.6% and 3.5% and can be alloyed with chromium with a content comprised between 1% and 30%. other elements, in particular manganese, silicon, tungsten, or molybdenum, can be integrated in the composition with a global content of less than 10%. A second object of the invention is to achieve a manufacturing process of steel or cast iron particles able to be used as grinding media, said particles being obtained by atomization from a steel or cast iron casting coming from a melting furnace. The process is characterized by the following steps: - atomization of the particles is performed by centrifugation by means of a revolving table revolving at a speed comprised between 250 rpm and 1,500 rpm to obtain a granulometry comprised between 0.5 mm and 15 mm and an apparent density greater than 4.3 g/cc, - the particles are made to fall by gravity into a tank filled with water, - the particles are recovered and heat treatment is then performed for core hardenings designed to obtain a homogeneous structure and a predetermined hardness. According to one feature of the invention, the hardness of the particles after heat treatment is about 64 Rockwell. According to another feature of the invention, the granulometry of the particles is inversely proportional to the speed of rotation and diameter of the table. The minimum cooling rate in the mass of a particle is greater than 10°C/second and depends mainly on the size of the particles, the height of fall, and the depth of the tank. Other advantages and features will become more clearly apparent from the following description of an embodiment of the invention given as a non-restrictive example only and represented in the accompanying drawing which illustrates a schematic view of a device of the atomization process according to the invention. Description of a preferred embodiment Figure 1 shows an atomization device 10 by centrifugation making use of a revolving table 12, above which there is placed a channel 14 connected with a steel or cast iron melting furnace 16. The revolving table 12 is made of refractory material and the melting temperature of the metal inside the furnace 16 is about 1500°C to 1650°C. The furnace 16 is of the induction or electric arc type. Under the revolving table 12 there is located a tank 18 into which the particles fall by gravity to undergo quenching. The tank 18 is filled with water for cooling the particles and a pump 20 performs regular agitation of the water inside the tank 18. The steel or cast iron particles with high carbon content (0.6% to 3.5% of C) atomized by centrifugation can be alloyed in particular with chromium (1% to 30% of Cr) and with other elements such as manganese, silicon, tungsten, and molybdenum with a content of less than 10%. It has been observed that the granulometry, density and shape of the particles obtained by centrifugation depend on the viscosity and flow rate of the liquid metal and also on the speed of rotation of the table 12. Other parameters also have an influence, in particular the diameter of the revolving table 12, the height of fall hi of the particles between the table 12 and the top level of the water, the temperature of the cooling water, and the depth h2 of the tank 18. The granulometry of the grinding media is situated in a range from 0.5 mm to 15 mm, preferably between 3 mm and 12 mm. This granulometry is inversely proportional to the speed of rotation of the table 12 and to the diameter of the table 12. The grinding media present appreciably spherical shapes for dimensions of up to about 6 mm, and are shaped as pellets for larger sizes. The technique of atomization by centrifugation enables an apparent density of more than 4.3 g/cc to be obtained for particles larger than 3 mm, i.e. greater than the density accessible by atomization with water. The speed of rotation of the table 12 can be chosen between 200 r.p.m. and 1500 r.p.m. according to the dimensions required. The minimum cooling rate in the mass of a ball is preferably greater than 10°C/second. This value depends in particular on the height of fall hi and on the nature of the ambient medium. After the atomization and cooling phase, the balls are removed from the tank 18 and the following operations are performed: - drying of the balls, - selecting the shape and sorting by sizes, - heat treatments to perform core hardenings to obtain a homogeneous structure and a predetermined hardness of about 64 Rockwell. According to one embodiment, a steel is used having a carbon content of 1%, alloyed with 0.8% Mn, 0.1% Al, and 1% Si. The table 12 has a diameter of 300 mm, and rotates with a speed of rotation of 1,000 r.p.m. The casting temperature of the molten steel is situated between 1600°C and 1460°C at the end of atomization. The height of fall hi is 500 mm, and the depth h2 of the tank 18 is 4 metres. The casting rate is adjusted to 340 kg/mn with a subdivision of three jets arranged angularly 120° from one another. The three-dimensional distribution of the shots atomized by centrifugation is as follows: - 36% for balls ranging from 4 mm to 8 mm in diameter; - 31 % for pellets larger than 8 mm; - 33% for balls smaller than 4 mm. The apparent density for the whole granulometry range is 4.6 g/cc. The process by centrifugal atomization enables steel or iron grinding balls or pellets with a high carbon content to be obtained, having dimensions comprised between 0.5 mm and 15 mm, and an apparent density greater than 4.3 g/cc for particles larger than 3 mm, ensuring efficient grinding and a good wear resistance. CLAIMS 1. Grinding media constituted by steel or cast iron particles with a high carbon content, characterized in that the steel or cast iron particles are formed by centrifugal atomization in a granulometry range comprised between 0.5 mm and 15 mm, so as to obtain an apparent density greater than 4.3 g/cc. 2. Grinding media according to claim 1, characterized in that the granulometry of the particles is preferably situated between 3 mm and 12 mm. 3. Grinding media according to claim 1 or 2, characterized in that the steel or cast iron particles have a carbon content comprised between 0.6% and 3.5%. 4. Grinding media according to claim 3, characterized in that the particles can be alloyed with chromium with a content comprised between 1% and 30%. 5. Grinding media according to claim 4, characterized in that the particles can be alloyed with other elements, in particular manganese, silicon, tungsten, or molybdenum, with a global content of less than 10%. 6. Manufacturing process of steel or cast iron particles able to be used as grinding media, said particles being obtained by atomization from a steel or cast iron casting coming from a melting furnace (16), characterized by the following steps: - atomization of the particles is performed by centrifugation by means of a revolving table (12) revolving at a speed comprised between 250 rpm and 1,500 rpm to obtain a granulometry comprised between 0.5 mm and 15 mm and an apparent density greater than 4.3 g/cc for particles larger than 3 mm, - the particles are made to fall from the table (12) by gravity into a tank (18) filled with water, - the particles are recovered and heat treatment is then performed for core hardenings designed to obtain a homogeneous structure and a predetermined hardness 7. Manufacturing process according to claim 6, characterized in that the hardness of the particles after heat treatment is about 64 Rockwell. 8. Manufacturing process according to claim 6 or 7, characterized in that the particles recovered in the tank (18) are first dried and then sorted by size and shape. 9. Manufacturing process according to one of the claims 6 to 8, characterized in that the granulometry of the particles is inversely proportional to the speed of rotation and diameter of the table (12). 10.Manufacturing process according to claim 9, characterized in that the minimum cooling rate in the mass of a particle is greater than 10°C/second and depends on the height of fall (hi) and on the nature of the ambient medium. 11. Manufacturing process according to claim 9, characterized in that the ball-pellet transition size depends on the nature of the ambient medium and on the depth of the tank (h2). Grinding media substantially as hereinabove described with reference to the accompanying drawing. Manufacturing process of steel or cast iron particles substantially as hereinabove described with reference to the accompanying drawing.

Documents:

0483-chenp-2004 abstract-duplicate.pdf

0483-chenp-2004 claims-duplicate.pdf

0483-chenp-2004 descripition(completed)-duplicate.pdf

0483-chenp-2004 drawings-duplicate.pdf

483-chenp-2004-abstract.pdf

483-chenp-2004-claims-.pdf

483-chenp-2004-claims.pdf

483-chenp-2004-correspondnece-others.pdf

483-chenp-2004-correspondnece-po.pdf

483-chenp-2004-description(complete).pdf

483-chenp-2004-drawings.pdf

483-chenp-2004-form 1.pdf

483-chenp-2004-form 3.pdf

483-chenp-2004-form 5.pdf

483-chenp-2004-form18.pdf

483-chenp-2004-pct.pdf