Patent Application
20080245912
The present invention relates to the grinding of mineral or organic substances, and in particular to spheroid fritted-ceramic balls used in agitator mills, of the attrition type or others, for the reduction, dispersion and regeneration of particles in a dry and/or wet process.
The equipment and methods of fine grinding and the dispersion with dry and wet processes are well known to those skilled in the art and developed in industries such as:
In most cases, these equipment and methods use dispersion or grinding materials of an essentially spherical shape and of reduced diameters (generally less than 10 mm).
Fine and ultra-fine grinding has recently become an essential development path in the mining industries, with the need to supply economical grinding materials.
The agitator mill can be described as follows: it comprises a cylindrical chamber positioned horizontally or vertically and containing small balls with a diameter of 0.5 mm to 12 mm depending on the desired fineness for the finished product. An axis with discs rotates in this chamber. These discs transmit the motion to the grinding medium and to the material to be ground. The motion of the whole assembly allows the dispersion of the material to be ground between the grinding balls that have the function of reducing said material to a predetermined fineness. The particle size obtained is thus a function of the energy put into the machine.
This grinding method can be achieved continuously in wet and dry processes, with the supply and discharge of the material to be ground, or in batches, i.e. in an entirely closed cylindrical chamber.
The grinding media themselves are clearly subjected to wear and their selection will depend on the following criteria:
A limited number of media used in agitator mills or in dispersers can be found on the market:
Sand with rounded grains is a natural and cheap product. Its poor resistance to mechanical impacts, its low density, its variations in quality (inconsistency of deposits), its abrasiveness towards the internal equipment of agitation mills and dispersers constitute its limitations in application.
Glass balls that are widely used to overcome the drawbacks of sand with rounded grains turn out to be less effective in grinding or dispersion applications that require balls with high mechanical resistance and high wear resistance. Their low density of 2.5 is also a feature that limits their use when grinding efficiency is an important element of the process.
Metal balls turn out less effective for the following reasons:
Ceramic balls on the other hand have better mechanical resistance than glass balls, a density that is intermediate between that of glass balls and that of metal balls as well as good chemical inertness relative to the ground or dispersed products.
Depending on the manufacturing methods, ceramic balls are classified into two groups:
Depending on the ceramic compounds shaped, the fritted-ceramic balls are classified into four groups:
The compositions of mullite/zirconia as well as those of mullite/alumina/zirconia have generally been studied in the context of fireproof bricks to be used in glass furnaces.
In their publication “High temperature mechanical properties of reaction-sintered mullite/zirconia and mullite/alumina/zirconia composites” (Journal of Materials Science 20 (1985), pp. 2533-2540), G. Orange and F. Cambier studied the break resistance of such compounds as well as their impact resistance at high temperature (1000° C.) for using these compounds in structural applications such as glass furnaces.
The problem with grinding balls is however very different. These balls must be much smoother than fireproof bricks in as much as the mere presence of angles on the grinding media may reduce by 50% the useful life of the internal equipment of attrition mills, leading to maintenance costs that make the use of very rough grinding media impossible.
The surface finish of grinding balls is thus very important since it directly affects the internal wear of the equipment and the grinding quality. Said surface finish is also directly affected by the chemical composition and the manufacturing method of said balls.
Grinding balls in electrically fused alumina and zirconia are disclosed in the American patents U.S. Pat. No. 3,486,706 and U.S. Pat. No. 5,502,012, respectively. These documents claim specific vitreous phases.
Patent application EP-0 662 461 A1 discloses balls of ceramic material formed by the fusion of a mixture of zirconia and silica and studies the effect of the presence of yttrium and cerium oxides.
Document EP 1 167 320 A1 describes low-cost products made of alumina-zirconia-silica, melted and cast into blocks, to be used in glass furnace regenerators or in superstructures.
All grinding balls comprising silica, alumina or zirconia have the common feature that they have been produced by fusion, which requires access to temperatures above 2000° C., which is technically difficult and hence expensive. By contrast, none of these documents discloses grinding balls made of fritted ceramic, comprising at the same time silica, alumina and zirconia. This process can be achieved at about 1500° C., which is much easier in terms of technology and hence less expensive.
The present invention aims to provide grinding balls made of fritted ceramic with a particular composition having high durability and good resistance to cold wear, to be used in grinding mills for mineral or organic substances. In addition, it aims to provide a method for manufacturing such balls.
The present invention discloses grinding balls made of fritted ceramic comprising a compound of alumina silicate (αAl2O3.βSiO2), zirconia (ZrO2+HfO2) and alumina (Al2O3).
Preferably, the ceramic moreover comprises 1 to 5% by weight of oxides selected from the group of Na2O, MgO, CaO and BaO.
In addition, said alumina silicate compound preferably comprises mullite (3Al2O3.2SiO2).
In a particularly advantageous way, said ceramic moreover comprises a stabilizing agent selected from the rare earth oxides.
The invention also presents grinding balls made of fritted ceramic and produced from a mixture comprising the following ingredients (in weight %):
Preferably, the grinding balls made of fritted ceramic are produced from a mixture comprising the following ingredients (in weight %):
Still according to the invention, the grinding balls made of fritted ceramic are obtained by a method for fritting a mixture comprising the following ingredients (in weight %):
According to the invention, the grinding balls have a diameter between 0.1 mm and 100 mm, preferably between 0.5 mm and 50 mm, and in particular preferably between 0.5 mm and 10 mm.
The present invention further discloses a method for manufacturing grinding balls made of fritted ceramic, comprising the following steps:
Moreover, the invention specifies that the granulation means comprise fluidised-bed granulators and granulation discs.
Moreover, the granules may also be obtained by gelation methods or by injection moulding methods.
The invention also specifies that said organic binding agents are chosen from the group of polysaccharides, thermoplastic polymers, thermosetting polymers or polymers based on aqueous or organic solvents.
As an advantage, said surfactants are chosen from the group of carboxylic acids such as stearic acid or oleic acid and/or polyelectrolytes such as ammonium polymethylacrylate.
Moreover, the invention discloses the use of the grinding balls made of fritted ceramic for grinding mineral or organic materials.
The invention relates to balls made of fritted ceramic comprising alumina-zirconia-alumina silicate, and in particular alumina-zirconia-mullite. The quality of these balls is higher than that of balls made of alumina, of alumina silicate or of alumina-zirconia, and their cost is significantly lower than that of zirconia balls, which are very expensive and used in particular as grinding and/or dispersion medium.
More precisely, the invention relates to balls made of fritted ceramic with the following chemical composition, by weight %:
These balls may be formed by shaping from a slurry and/or a paste of ceramic oxides, dried and fritted at temperatures between 1400° C. and 1700° C., preferably between 1500° C. and 1600° C.
In the following description, where ZrO2 (zirconia) is mentioned, the total of (ZrO2+HfO2) is to be taken into account. Indeed, some HfO2, that is chemically inseparable from ZrO2 and has similar properties, is always present in addition to ZrO2, which is well known to those skilled in the art.
The invention is based on the dissociation reaction of zircon at high temperature in the presence of alumina. This reaction is also well known to those skilled in the art:
Varying x allows to adjust the relative volume proportions of alumina (Al2O3) (if x≠0), of alumina silicate in general and of mullite (3Al2O3.2SiO2) and in particular, of zirconia (ZrO2), and thus to modify the properties of the final fritted ceramic composite such as hardness, durability and wear resistance.
The reaction at high temperature between zircon and alumina allows to obtain, by fritting pulverulent powders, very homogeneous composites, the synthesised phases of which are finely dispersed.
In a preferred embodiment of the invention, yttrium oxide (Y2O3) is added to the basic reagent of the preceding reaction, which allows to stabilise the crystallographic forms (cubic or tetragonal) of zirconia obtained at high temperature.
Zirconia remains mainly in tetragonal and sometimes in cubic shape in low proportions. These states depend on the amount of Y2O3 introduced. The tetragonal form is the densest of the three allotropic variants of zirconia: density of 6.1 g/cc versus 5.9 g/cc for zirconia in the cubic shape and 5.8 g/cc for zirconia in the monoclinic shape.
The tetragonal shape also gives a mechanical reinforcement effect to the matrix in which it is located. The total quantity of Y2O3, i.e. the portion entering the zirconia in order to stabilise it and the excess portion entering the composition of the silicate phase (mullite), leads to the production of denser balls with greater resistance to high impact forces and to wear.
Stabilisation of the zirconia also allows to improve the durability of the composite and to increase the dissociation speed of zircon, hence to reduce the temperature or the duration of fritting. This reduction in the temperature or duration of fritting allows to obtain a finer microstructure which is a major advantage in terms of wear resistance.
The pelletisation method for obtaining balls can be diagrammatically represented as follows:
The dry ceramic ingredients of the balls of this invention are intimately mixed in a mixer, some water may possibly be added to the mixture, for example in order to obtain a pasty consistency or in order to form a slurry.
The mixed constituents, particularly in a dry process, may be transformed in spheres by means of a rotating pelletising disk or of a granulation disc. The powder of the ceramic ingredients turns into spheres by fogging of water with one or several organic binding agents added, on the rotary bed of the solid components. Fogging is adjusted so that the balls coming out of the pelletising disc comprise a humidity between 18 and 22% by weight of water.
After the spheres are formed, they undergo a sieving process according to three grain size fractions:
The fraction with the desired grain size (a) is dried in a conventional drying oven at a temperature of about 110° C. until the balls have a humidity level lower than 1% by weight.
The dried balls are then loaded into a fritting oven. The balls are fritted at a temperature between 1400° C. and 1600° C. according to a well-defined heating speed programme.
After fritting and cooling of the oven, the balls are conditioned in their packaging and are ready for dispatch.
Granulation by a fluidised bed dryer is shown in the following diagram:
The ceramic ingredients that make up the balls of this invention with water and a dispersant added, are finely ground in a proportion of dry material within the range of 50 to 75%. After grinding, the extracted slurry receives organic binding agents (5 to 7%) and is finely mixed.
The slurry is injected at room temperature through injectors into a granulation fluidised-bed dryer where it comes into contact with the fluidised solid particles (seeds) having the same composition as that of the invention.
The hot air (140° C. max.) entering the dryer evaporates the water contained in the slurry, thus causing the deposition of the dry materials on the seeds. The successive depositions of dry material make the granules grow in layers and transform them into balls. Once the balls reach the desired size, they are removed from the dryer.
After extraction from the dryer, the balls undergo a sieving operation:
Shaping of the balls by gelation reaction:
Shaping of the balls by the injection moulding process:
The performance of the balls of the invention (alumina-zirconia-mullite) by comparison with zirconia balls stabilised or partially stabilised with yttrium oxide.
All conditions being identical, the grinding performance of the two kinds of balls are equivalent. If the wear performance are in favour of the zirconia balls, 3.32 times better, the cost of those balls is ±10 time higher than that of the balls of the invention. This makes the alumina-zirconia-mullite balls thus very attractive on the economical side.
Gains in wear and in grinding efficiency of the balls of the invention relative to the commercially available alumina balls.
The balls of the invention in the packaging phase are more wear efficient (2 to 2.89 times) than the alumina balls. This test shows the very good durability of the balls of the invention.
C) comments
The balls of the invention are 1.3 times more efficient in terms of grinding time. This performance is partly due to the higher density of those balls. The wear performance is of the order of 1.4 times better than the alumina balls. This confirms the inner feature of those balls in terms of durability.
| Number | Date | Country | Kind |
|---|---|---|---|
| 04447035.9 | Feb 2004 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/BE2005/000016 | 2/7/2005 | WO | 00 | 7/24/2006 |
