BEAD MADE OF A FUSED PRODUCT

Abstract

A bead having a sphericity greater than or equal 0.6 and made of a fused product having the following chemical composition, in percentages by weight on the basis of the oxides and for a total of 100%: 20%(ZrO.sub.2+HfO.sub.2), with HfO.sub.22%, 5%SiO.sub.2, 0%Al.sub.2O.sub.320%, 8.5%MgO20%, 0.5%TiO.sub.220%, and oxides other than ZrO.sub.2, HfO.sub.2, SiO.sub.2, Al.sub.2O.sub.3, MgO and TiO.sub.2, or other oxides: 5% provided that the (ZrO.sub.2+HfO.sub.2)/SiO.sub.2 weight ratio is greater than 1, and provided that Al.sub.2O.sub.3+TiO.sub.226% if MgO>17%.

Claims

1. A bead having sphericity greater than or equal to 0.6 and made of a fused product having the following chemical composition, in percentages by weight based on the oxides and for a total of 100%: 20%(ZrO.sub.2+HfO.sub.2), with HfO.sub.22%, 5%SiO.sub.2, 0%Al.sub.2O.sub.320%, 8.5%MgO20%, 0.5%TiO.sub.220%, and oxides other than ZrO.sub.2, HfO.sub.2, SiO.sub.2, Al.sub.2O.sub.3, MgO and TiO.sub.2, or other oxides: 5% provided that the weight ratio (ZrO.sub.2+HfO.sub.2)/SiO.sub.2 is greater than 1, and provided that Al.sub.2O.sub.3+TiO.sub.226% if MgO>17%.

2. The bead as claimed in claim 1, in which MgO17%.

3. The bead as claimed in claim 1, in which ZrO.sub.2+HfO.sub.2+SiO.sub.240%.

4. The bead as claimed in claim 1, in which the weight ratio MgO/SiO.sub.2 is greater than 0.1 and below 1.

5. The bead as claimed in claim 1, in which the Al.sub.2O.sub.3 content, in percentage by weight based on the oxides, is greater than or equal to 0.5%.

6. The bead as claimed in claim 5, in which the Al.sub.2O.sub.3 content, in percentage by weight based on the oxides, is greater than or equal to 4% and less than or equal to 18%.

7. The bead as claimed in claim 1, in which the TiO.sub.2 content, in percentage by weight based on the oxides, is greater than or equal to 1% and less than or equal to 18%.

8. The bead as claimed in claim 7, in which the TiO.sub.2 content, in percentage by weight based on the oxides, is greater than or equal to 4% and less than or equal to 13%.

9. The bead as claimed in claim 1, in which ZrO.sub.2+HfO.sub.2+SiO.sub.280%.

10. The bead as claimed in claim 1, in which the ZrO.sub.2 content, in percentage by weight based on the oxides, is greater than or equal to 30% and less than or equal to 60%.

11. The bead as claimed in claim 1, in which the SiO.sub.2 content, in percentage by weight based on the oxides, is greater than or equal to 10%.

12. The bead as claimed in claim 1, in which the SiO.sub.2 content, in percentage by weight based on the oxides, is greater than or equal to 13% and less than or equal to 30%.

13. The bead as claimed in claim 1, in which the MgO content, in percentage by weight based on the oxides, is greater than or equal to 9%.

14. The bead as claimed in claim 1, in which the weight ratio ZrO.sub.2/SiO.sub.2 is greater than or equal to 1.3 and less than or equal to 5.

15. The bead as claimed in claim 1, in which the content of other oxides is less than or equal to 2%.

16. The bead as claimed in claim 1, having a sphericity above 0.8.

17. A powder consisting of beads as claimed in claim 1 to more than 90% of its weight.

18. The use of the powder as claimed in claim 18 as a grinding agent, agent for dispersion in a wet medium, propping agent, heat exchange agent, or for surface treatment.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0061] Other features and advantages of the invention will become clearer on reading the detailed description given below and on examining the appended drawing, in which FIG. 1 shows a photograph of the beads according to the invention.

DEFINITIONS

[0062] Particle means an individual solid product in a powder. [0063] Bead means a particle having sphericity, i.e. a ratio of its smallest to its largest diameter greater than or equal to 0.6, regardless of how this sphericity was obtained. [0064] The median size of a powder of particles, generally denoted D.sub.50, is the size that divides the particles of this powder into first and second populations of equal weight, these first and second populations only comprising particles having a size above or below the median size, respectively. The median size may for example be evaluated using a laser granulometer. [0065] Fused product means a product obtained by solidification of a fused material by cooling. [0066] A fused material is a mass which, to maintain its shape, must be contained in a vessel. A fused material is generally liquid. However, it may contain solid particles, but in an insufficient amount for them to form a structure in said mass. [0067] Precursor of an oxide means a constituent that is able to supply said oxide during the manufacture of a bead according to the invention. As an example, magnesium carbonate MgCO.sub.3 is a precursor of MgO. [0068] Impurities means the inevitable constituents, introduced necessarily with the raw materials. In particular, the compounds in the group of oxides, nitrides, oxynitrides, carbides, oxycarbides, carbonitrides and metallic species of sodium and other alkali metals, iron, vanadium and chromium are impurities. As examples, we may mention CaO, Fe.sub.2O.sub.3, Y.sub.2O.sub.3 or Na.sub.2O. Residual carbon forms part of the impurities in the composition of the products according to the invention. [0069] When reference is made to zirconia or ZrO.sub.2, this is to be understood as (ZrO.sub.2+HfO.sub.2). In fact, a small amount of HfO.sub.2, chemically inseparable from ZrO.sub.2 in a fusion process and having similar properties, is always present naturally in the sources of zirconia at contents generally less than or equal to 2%. Hafnium oxide is not then regarded as an impurity.

[0070] All the percentages in the present description are percentages by weight based on the oxides, unless stated otherwise.

[0071] Other features and advantages will become evident on reading the description given below.

DETAILED DESCRIPTION

[0072] Method

[0073] According to one embodiment of the invention, a product may be made by the method comprising the following successive steps: [0074] a) mixing raw materials to form a starting charge; [0075] b) melting the starting charge until fused material is obtained, [0076] c) dispersing said fused material in the form of liquid droplets and solidification of these liquid droplets in the form of solid beads,
the raw materials being selected in step a) in such a way that the beads obtained in step c) are according to the invention.

[0077] These steps are conventional, except with respect to the composition of the starting charge, and a person skilled in the art knows how to adapt them as a function of the intended application.

[0078] A preferred embodiment of this method will now be described.

[0079] In step a), the starting charge is formed from the oxides indicated or from their precursors. Preferably, sand of natural zircon ZrSiO.sub.4 is used, having about 66% of ZrO.sub.2 and 33% of SiO.sub.2, plus impurities. Supplying ZrO.sub.2 via zircon is in fact much more economical than adding ZrO.sub.2.

[0080] The compositions may be adjusted by adding pure oxides, mixtures of oxides or mixtures of precursors of these oxides, notably by adding ZrO.sub.2, SiO.sub.2, MgO, TiO.sub.2 and Al.sub.2O.sub.3.

[0081] According to the invention, a person skilled in the art adjusts the composition of the starting charge so as to obtain, at the end of step c), beads according to the invention. The chemical analysis of the beads according to the invention is generally roughly identical to that of the starting charge. Furthermore, if applicable, for example to take account of the presence of volatile oxides, or to take account of the loss of SiO.sub.2 when melting is carried out in reducing conditions, a person skilled in the art knows how to adapt the composition of the starting charge accordingly.

[0082] Preferably, no raw material other than ZrO.sub.2+HfO.sub.2, SiO.sub.2, MgO, Al.sub.2O.sub.3, TiO.sub.2 and their precursors is introduced deliberately in the starting charge, the other oxides present being impurities.

[0083] In step b), the starting charge is melted, preferably in an arc furnace. In fact, electric melting makes it possible to produce large amounts of beads with advantageous yields. However, all known furnaces are conceivable, such as an induction furnace or a plasma furnace, provided that they make it possible to melt the starting charge more or less completely.

[0084] In step c), a thin stream of the molten liquid is dispersed as small liquid droplets, most of which, owing to surface tension, assume an approximately spherical shape. This dispersion may be effected by blowing, notably with air and/or steam, or by any other method of atomizing a fused material, known by a person skilled in the art. A bead with a size from 0.005 to 10 mm may be produced in this way.

[0085] The resultant cooling of the dispersion leads to solidification of the liquid droplets. Solid beads according to the invention are then obtained.

[0086] Any conventional method of producing beads made of a fused product may be employed, provided that the composition of the starting charge allows beads to be obtained having a composition according to that of the beads according to the invention.

Examples

[0087] Measurement Protocols

[0088] The following methods provide an excellent simulation of the real behavior in service in grinding applications.

[0089] To determine the so-called planetary wear resistance, a charge of test beads is sieved between 0.8 and 1 mm on square-mesh sieves. 20 ml (volume measured using a graduated measuring cylinder) of said test beads are weighed (weight m.sub.0) and are placed in one of the 4 bowls coated with dense sintered alumina, with a capacity of 125 ml, of a high-speed planetary mill of the PM400 type from RETSCH. 2.2 g of silicon carbide made by Presi (having a median size D50 of 23 m) and 40 ml of water are added to one of the bowls. The bowl is closed and set rotating (planetary motion) at 400 rev/min, reversing the sense of rotation every minute for 1.5 h. The contents of the bowl are then washed on a 100-m sieve to remove the residual silicon carbide as well as the fragments of material due to wear during grinding. After sieving on a square-mesh sieve with mesh with side of 100 m, the beads are then dried in a stove at 100 C. for 3 h and then weighed (weight m).

[0090] The planetary wear (PW), expressed as a percentage, is given by the following formula:

100(m.sub.0m)/m.sub.0

[0091] The result PW is given in Table 1.

[0092] The results are considered to be particularly satisfactory if the beads display an improvement of planetary wear resistance (PW) of at least 10% relative to that of the example Reference 1.

[0093] To determine the so-called wear in basic medium. i.e. in media having a pH above 8, a charge of test beads is sieved between 0.8 and 1 mm on square-mesh sieves. An apparent volume of 1.04 liter of beads is weighed (weight m.sub.0). The beads are then put in a horizontal mill of the Netzsch LME1 type (useful volume of 1.2 L) with eccentric disks made of steel. An aqueous suspension of calcium carbonate CaCO.sub.3 with a pH equal to 8.2, containing 70% of dry matter, of which 40% of the grains by volume are smaller than 1 m and whose viscosity is adjusted to a value between 100 and 250 centipoise, passes through the mill continuously, at a flow rate of 4 liters per hour. The mill is started gradually until a linear velocity at disk end of 10 m/s is reached. The mill continues in operation for a time t equal to 24 hours, and then is stopped. The beads are rinsed with water, cautiously removed from the mill and then washed and dried. They are then weighed (weight m). The wear rate V in grams/hour is determined as follows:

V=(m0m)/t

[0094] The charge of beads is taken and supplemented with (m0m) grams of fresh beads so as to repeat the grinding operation as many times as necessary (n times) for the cumulative grinding time to be at least 100 hours and for the difference between the wear rate calculated in step n and in step n1 to be below 15 rel %. Typically, the total grinding time is between 100 hours and 140 hours. The wear in a basic medium is the wear rate measured for the last grinding operation n.

[0095] The percentage improvement relative to comparative example 1 is defined by the following formula: 100*(wear of the product in comparative example 1wear of the product in question)/wear of the product in comparative example 1. The results are regarded as particularly satisfactory if the products have an improvement in wear resistance of at least 10% relative to that in comparative example 1.

[0096] The total porosity, in %, is evaluated from the following formula:

Total porosity=100.Math.(1(d.sub.beads/d.sub.ground beads)), with [0097] d.sub.beads, the density for beads that have not been ground, obtained using a helium pycnometer (AccuPyc 1330 from the company Micromeritics), according to a method based on measuring the volume of gas displaced (helium, in the present case), and [0098] d.sub.ground beads is the density measured as for d.sub.beads, but on a powder resulting from grinding the beads in a dry grinder of the annular type from Aurec for 40 s and followed by sieving, only keeping for measurement the powder passing through a square-mesh sieve with mesh with side of 160 m.

[0099] Manufacturing Protocol

[0100] A pulverulent starting charge consisting of zircon sand, alumina, magnesia and titania in the form of rutile are placed in an arc furnace of the Heroult type, in order to melt it.

[0101] The fused material is poured as a thin stream, then dispersed as beads by blowing with compressed air.

[0102] Several melting/pouring cycles are performed, adjusting the contents of titania, alumina, magnesia and zircon.

[0103] This technique provides several batches of beads of various compositions, which can then be characterized.

[0104] Results

[0105] The results obtained are summarized in the following Table 1:

TABLE-US-00001 TABLE 1 Wear in ZrO.sub.2 + Other (ZrO.sub.2 + Al.sub.2O.sub.3 + Planetary basic HfO.sub.2 SiO.sub.2 Al.sub.2O.sub.3 MgO TiO.sub.2 oxides HfO.sub.2)/ Al.sub.2O.sub.3/ MgO/ TiO.sub.2 wear medium Ex (%) (%) (%) (%) (%) (%) SiO.sub.2 SiO.sub.2 SiO.sub.2 (%) (PW) (%) (g/h) 1(*) 67 31 1 <0.05 <0.05 <1 2.16 0.03 <0.01 <1.05 6 3.7 2 40.2 20.3 15.1 16 6.9 1.5 1.98 0.74 0.79 22.0 2.0 1.5 3 36.1 18.1 14.7 15.6 14.3 1.2 1.99 0.81 0.86 29.0 2.6 4 61.2 26.4 1.0 8.9 1.1 1.4 2.32 0.04 0.34 2.1 3.4 1.4 5 37.8 19.0 6.2 17.3 18.5 1.2 1.99 0.33 0.91 24.7 3.3 6(*) 36.2 18.4 8.4 17.7 18.6 0.7 1.97 0.45 0.96 27.0 5.3 7(*) 29.0 14.1 19.2 18.3 18.5 0.9 2.06 1.36 1.29 37.7 6.3 3.5 (*)example not according to the invention

[0106] The beads obtained according to the invention have a total porosity less than or equal to 2%.

[0107] The beads are considered to have particularly good performance when they display, simultaneously, planetary wear less than or equal to 4%, preferably below 3%, and wear in a basic medium below 3 g/h, preferably below 2.5 g/h, preferably below 2 g/h, preferably below 1.9 g/h, preferably below 1.8 g/h, preferably below 1.7 g/h, these types of wear being measured following the above protocols.

[0108] Table 1 shows that the examples according to the invention have planetary wear two to three times lower than that of the comparative examples.

[0109] Example 1 shows that in the absence of TiO.sub.2, or with very low contents of TiO.sub.2, wear in a basic medium is high, typically above 2 g/h.

[0110] Example 2 according to the invention, which is the most preferred and is shown in FIG. 1, on the contrary displays wear in a basic medium well below 2 g/h, as well as planetary wear equal to 2%, i.e. well below 6%.

[0111] Example 4 shows that in the presence of 1% of TiO.sub.2 and 1% of Al.sub.2O.sub.3, planetary wear is equal to 3.4%, i.e. below 4%, and wear in a basic medium is equal to 1.4 g/h.

[0112] Comparison of example 5 according to the invention and example 6 not according to the invention shows, for roughly constant MgO contents above 17%, the effect of the sum TiO.sub.2+Al.sub.2O.sub.3: example 5, with said sum equal to 24.7%, shows planetary wear equal to 3.3%, in contrast to example 6, with the sum TiO.sub.2+Al.sub.2O.sub.3 equal to 27%, for which planetary wear is high and equal to 5.3%. Example 7, not according to the invention, with an even higher content of TiO.sub.2+Al.sub.2O.sub.3, equal to 37.7%, has high planetary wear and wear in a basic medium, equal to 6.3% and 4 g/h, respectively.

[0113] Of course, the present invention is not limited to the embodiments described, which are supplied as illustrative examples and are nonlimiting.