FR2829405A1 - Grinding material used for fine and ultrafine grinding of minerals in mining and mineral industries is made from high carbon steel or iron particles formed by centrifugal atomization to a specified granulometry - Google Patents

Abstract

A grinding material is made from steel or iron particles with an elevated carbon content. The steel or iron particles are formed by centrifugal atomization in a granulometry range of 0.5 mm and 15 mm, in a manner to obtain an apparent density greater than 4.5 g/cm<3>. The carbon content of the steel or iron particles is between 0.6 and 3.5% and may be alloyed with between 1 and 30% of chromium, with other additions of manganese, silicon, tungsten, and molybdenum up to a total of less than 10%. An Independent claim is also included for a method for the fabrication of the steel or iron particles by centrifugal atomization.

Description

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Grinding body made of steel or cast iron with a high carbon content, and its manufacturing process.

Technical Field of the Invention This invention relates to a relatively high carbon content cast steel shot which is atomized by centrifugation to make it suitable for fine and ultra-fine grinding in the mining and mineral industry.

STATE OF THE ART In the mining industry, it is necessary to free valuable elements from the rock in which they are embedded. One means of releasing these elements is fracturing (crushing-grinding), which consists in gradually fragmenting the rock until it reaches the size of release of the valuable elements. Depending on this size, we will speak of coarse, fine or ultra-fine grinding.

It is also possible that the grinding operations, in particular at the level of fine grinding, are not only dictated by a release size, but also by constraints linked to the operations of the downstream process or to the subsequent use of the product thus treated, whether in the mining industry (for example the production of iron ore pellets where the iron ore concentrate must reach a size of the order of 60um before pelletizing), or in the mineral industry (for example the cement industry).

To achieve the fine grinding of the ore, it is known to use metal grinding bodies in grinders to reduce the particle size of the mineral particles crushed beforehand. The types of grinders using grinding bodies are variable but all are based on the principle of setting the grinding bodies in motion on the product to be ground, setting in motion which will seek to promote, among the possible modes of fragmentation (impact, attrition) the one that will achieve the optimal fragmentation efficiency. The impact will consist in projecting or dropping grinding bodies on the product to be ground in order to allow a deep break,

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attrition to rub the grinding bodies on the product to be ground in order to allow a superficial scalping.

The impact will favor grinding bodies of large sizes, to have a significant unit weight. Attrition will favor small grinding bodies, to have a large contact surface with the product to be ground.

It is known that below a certain size of fragmentation, specific to the product in question, the product can no longer be reduced effectively by impact. In fact, the defects present in the coarse particles of the product (cracks, holes, etc.) are less and less numerous as the particles are fragmented, and impact grinding becomes less and less effective. In this case, we will seek more to favor attrition.

This is how it is accepted that there is a correspondence between the target release size and the size of the grinding body used: the smaller the grinding bodies, the greater the contact surface with the products to be ground. the higher the crushing efficiency in attrition.

Apart from the actual grinding efficiency, which can be summed up as the energy expended to achieve the desired grinding size or the speed of fragmentation obtained, another element is involved in the performance of grinding operations involving the use. of grinding bodies: the wear of the latter.

The grinding bodies are generally shaped as balls, truncated cones, or cylinders.

The materials used for the grinding bodies are either inexpensive steels with low mechanical strength, or highly alloyed and very expensive or low alloyed and low wear resistance cast irons.

These materials can be listed as follows: - low alloy steels with a martensitic structure (0.7% -1% carbon, alloys less than 1%), made from rolling or forging followed by heat treatments to obtain a surface hardness of the order of 60-65 HRC;

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- cast iron alloyed with chromium of martensitic structure (1.7% -3.5% carbon, 9% - 30% chromium), manufactured by molding and heat treatment to obtain a hardness of 60-68 HRC on all sections ; - weakly alloyed white cast iron with a pearlitic structure (3% -4.2% carbon, alloys less than 2%), not heat-treated, and having a hardness of 45-55 H RC obtained during casting.

Document FR 2 405 749 describes a forged grinding body of white chromium cast iron, the structure of which is composed of a martensitic or austenitic solid solution containing secondary chromium carbides and primary or eutectic chromium carbides of the M7C3 type, finely divided. and distributed homogeneously in the matrix. The grinding bodies contain between 1% and 3% by weight of carbon, between 5% and 15% by weight of chromium carbides of the M7C3 type, that is to say between 2% and 8% by weight of chromium, from 0 to 2% by weight of molybdenum, 0.5% to 1.5% silicon, 0.1% to 2% manganese, 0 to 5% vanadium, and 0 to 1% copper. Other elements can be added such as tungsten, nickel, boron, niobium, tantalum and zirconium. The manufacturing process of these grinding bodies consists in heating a white cast iron bar to a first temperature of the order of 900 C to 10,000 C allowing hot shearing in the plastic state, in cutting the bar into several sections, heating the sections to a second temperature between 1000 C and 1150 C causing re-austenization and complete solution of the carbides in the austenitic domain, forging the sections at the second temperature, and cooling the sections at high speed controlled up to a temperature between 6000C and 800C. Perlitic structure forgings are thus obtained, subjected to a reheating treatment to adapt their structure to the conditions of use.

Document WO 95/28506 relates to a process for manufacturing high carbon steel grinding bodies (from 1.1% to 2% of C), additionally containing manganese (from 0.5% to 3.5% of Mn), chromium (from 1% to 4% of Cr), silicon (from 0.6% to 1.2% of Si) in order to give a fine perlite structure having a hardness between 47-54 Rockwell. The grinding bodies are melted into balls 70 to 100 mm in diameter, and

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the pearlitic structure is obtained by extracting the still hot part from the foundry mold.

One of the drawbacks of the processes for manufacturing the aforementioned materials, whether forging, rolling or casting, is the significant increase in the cost price when the size of the target grinding bodies decreases.

It is therefore very rare to see the use in the mining and mineral industry today of grinding bodies resulting from previous processes less than 12mm in size.

However, as indicated above, the efficiency of the grinding, in particular in the field of fine and ultra-fine grinding, will strongly depend on the size of the grinding bodies used.

The Applicant has already manufactured shots for grinding applications, shots of sizes between 3.5 and 6.5mm. The manufacturing process used consists in subjecting the molten steel to water atomization to obtain substantially spherical balls.

The pellets thus formed have a very wide particle size distribution (from 0.3 mm to more than 8 mm) but beyond 3 mm, the quality of the particles have major density defects, in particular hollows, defects which can prove to be prohibitive in grinding applications: they can not only cause too much wear, but they can also prevent the use of these shots even in grinders where the flows of product to be ground are so great that they would tend to lift and carry the shot in their passage inside the crusher.

The apparent density of the shots greater than 3 mm obtained by atomization with water and sorted in order to retain only the substantially spherical shapes, is less than 4.5 g / cc, the measurement being obtained by weighing in a large burette of at least 1 liter of capacity.

Atomization by centrifugation is currently used to manufacture steel shot of small dimensions, in particular less than 1.5mm. The applicant thus proposes to use this method of atomization for the manufacture of shot intended for

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for grinding applications, making it possible in particular to ensure a real density greater than 4.5 g / cc for particles greater than 3 mm.

OBJECT OF THE INVENTION A first object of the invention consists in producing fine grinding bodies based on high carbon steel shots, having good wear resistance and high density.

The particles or balls of steel or cast iron according to the invention are formed by centrifugal atomization, so as to obtain an apparent density greater than 4.5, in particular for particles greater than 3 mm.

The particle size distribution is preferably between 3 mm and 12 mm. The particles of steel or cast iron have a carbon content of between 0.6% and 3.5%, and can be alloyed with chromium with a content of between 1% and 30%.

Other elements, in particular manganese, silicon, tungsten, molybdenum, can be integrated into the composition with an overall content of less than 10%.

A second object of the invention consists in developing a process for manufacturing steel or cast iron particles which can be used as a grinding body, said particles being obtained by atomization from a steel or cast iron casting coming from of a melting furnace.

The process is characterized by the following steps: - the atomization of the particles is carried out by centrifugation by means of a rotating plate at a speed of between 250 rpm and 1500 rpm to obtain a particle size of between 0.5 mm and 15 mm, and an apparent density greater than 4.5, - the particles are dropped by gravity into a pit filled with water, - the particles are recovered, and then a heat treatment is carried out for core quenching intended to obtain a homogeneous structure, and a predetermined hardness.

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According to one characteristic of the invention, the hardness of the particles after heat treatment is of the order of 64 Rockwell.

According to another characteristic of the invention, the particle size distribution of the particles is inversely proportional to the speed of rotation and to the diameter of the plate. 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 pit.

Brief description of the drawings Other advantages and characteristics will emerge more clearly from the following description of an embodiment of the invention given by way of non-limiting example, and shown 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.

FIG. 1 shows a device 10 for atomizing by centrifugation making use of a turntable 12, above which is placed a channel 14 in connection with a furnace 16 for melting steel or cast iron. The turntable 12 is made of refractory material, and the melting temperature of the metal inside the furnace 16 is of the order of 1500 ° C. to 1650 ° C. The furnace 16 is of the induction or electric arc type.

Under the turntable 12 is a pit 18 into which the particles fall by gravity to be quenched therein. The pit 18 is filled with water for cooling the particles, and a pump 20 ensures regular mixing of the water inside the pit 18.

The particles of steel or cast iron with a 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 to other elements, such as manganese, silicon, tungsten, molybdenum with a content of less than 10%.

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et le niveau supérieur de l'eau, la température de l'eau de refroidissement, et la profondeur h2 de la fosse 18. It has been observed that the particle size, density and shape of the particles obtained by centrifugation depend on the melting temperature of the metal, and on the speed of rotation of the plate 12. Other parameters are also involved, in particular the diameter of the turntable 12. , the height of fall h1 of the particles between the plate 12

and the upper water level, the temperature of the cooling water, and the depth h2 of pit 18.

The particle size of the grinding bodies is within a range of 0.5 mm to 15 mm, and preferably between 3 mm and 12 mm. This particle size is inversely proportional to the speed of rotation of the plate 12, and to the diameter of the plate 12.

The grinding bodies have substantially spherical shapes for sizes up to about 8mm, and are pelletized for larger sizes. The atomization technique by centrifugation makes it possible to obtain a real density greater than 7.5 for particles greater than 3 mm, ie greater than the density accessible by atomization with water. The speed of rotation of the plate 12 can be chosen between 200 rpm and 1500 rpm depending on the desired dimensions.

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 h1 and the nature of the ambient environment.

After the atomization and cooling phase, the balls are evacuated from the pit 18, and the following operations are carried out: - drying of the balls, - selection of the shape and sorting by size, - heat treatments to carry out quenching at core to obtain a homogeneous structure and a predetermined hardness of the order of 64 Rockwell.

According to an exemplary embodiment, a steel is used having a content of 1% C, and alloyed with 0.8% Mn, 0.1% Al, and 1% Si. The plate 12 has a diameter of 300 mm, which rotates with a rotation speed of 1000 rpm. The casting temperature of molten steel is between 1600 C and 1460 C at the end of atomization. The drop height h1 is 500 mm, and the depth h2 of the pit 18 is 4 meters. The casting rate is

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adjusted to 340 kg / min with a subdivision of three jets arranged angularly at 1200 from each other.

The dimensional distribution of the beads atomized by centrifugation is as follows: - 36% for beads ranging from 4 mm to 8 mm in diameter; - 31% for pellets greater than 8 mm; - 33% for balls less than 4 mm.

The bulk density for the entire particle size range is 4.6.

The centrifugal atomization process makes it possible to obtain steel or cast iron grinding balls or balls with a high carbon content, having dimensions between 0.5 mm and 15 mm, and a real density greater than 7.5 ensuring a good resistance to wear and corrosion, as well as efficient grinding.

Claims (11)

Claims 1. Grinding body constituted by particles of steel or cast iron with a high carbon content, characterized in that the particles of steel or cast iron are formed by centrifugal atomization in a range of particle size between 0.5 mm and 15 mm, so as to obtain a bulk density greater than 4.5 g / cc. 2. Grinding body according to claim 1, characterized in that the particle size distribution is preferably between 3 mm and 12 mm. 3. Grinding body according to claim 1 or 2, characterized in that the steel or cast iron particles have a carbon content of between 0.6% and 3.5%. 4. Grinding body according to claim 3, characterized in that the particles can be alloyed with chromium with a content of between 1% and 30%. 5. Grinding body according to claim 4, characterized in that the particles can be alloyed with other elements, in particular manganese, silicon, tungsten, molybdenum, with an overall content of less than 10%. 6. A method of manufacturing steel or cast iron particles which can be used as a grinding body, said particles being obtained by atomization from a cast of steel or cast iron from a melting furnace (16). , characterized by the following steps: - the atomization of the particles is carried out by centrifugation by means of a plate (12) rotating at a speed of between 200 rpm and 1500 rpm to obtain a particle size of between 0, 5 mm and 15 mm, and a real density greater than 7.5, - from the plate (12), the particles are dropped by gravity into a pit (18) filled with water, - the particles are recovered, and a heat treatment is then carried out for through quenching intended to obtain a homogeneous structure, and a predetermined hardness. <Desc / Clms Page number 10> 7. The manufacturing method according to claim 6, characterized in that the hardness of the particles after heat treatment is of the order of 64 Rockwell. 8. The manufacturing method according to claim 6 or 7, characterized in that the particles recovered in the pit (18) are first dried, and sorted by size and shape. 9. Manufacturing process according to one of claims 6 to 8, characterized in that the particle size distribution of the particles is inversely proportional to the speed of rotation and to the diameter of the plate (12). 10. The manufacturing method 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 (h1), and the nature of the ambient. 11. The method of claim 9, characterized in that the billepastille transition size of the particles depends on the nature of the ambient medium and the depth of the pit (h2).