ABRASIVE ZIRCONIUM CORUNDUM GRAINS HAVING A HIGH SIO2 CONTENT

Abstract

The present invention relates to abrasive zirconium corundum grains based on Al.sub.2O.sub.3 and ZrO.sub.2 that have been melted in an electrical arc furnace and have a content ofAl.sub.2O.sub.3 between 52% and 62% by weight,ZrO.sub.2 (+HfO.sub.2) between 35.0% and 45.0% by weight, where a total of at least 80% by weight of the ZrO.sub.2, based on the total content of ZrO.sub.2, is in the tetragonal and/or cubic high-temperature modification,Si compounds of more than 0.8% by weight, expressed as SiO.sub.2,carbon between 0.03% and 0.5% by weight,additives between 0.5% and 10% by weight andraw material-related impurities of less than 3% by weight, where the raw material base for the abrasive grains comprises aluminum oxide, baddeleyite and zircon sand.

Claims

1. Abrasive grains based on Al.sub.2O.sub.3 and ZrO.sub.2 that have been melted in an electrical arc furnace and have a content of Al.sub.2O.sub.3 between 52% and 62% by weight, ZrO.sub.2 (+HfO.sub.2) between 35.0% and 45.0% by weight, where a total of at least 80% by weight of the ZrO.sub.2, based on the total content of ZrO.sub.2, is in the tetragonal and/or cubic high-temperature modification, carbon between 0.03% and 0.5% by weight, reduced titanium oxide, expressed as TiO.sub.2, between 1.0% and 4.0% by weight, Y.sub.2O.sub.3 between 0.2% and 1.5% by weight, Si compounds, expressed as SiO.sub.2, of more than 0.8% by weight, raw material-related impurities of less than 3% by weight, characterized in that the raw material base for the abrasive grains comprises aluminum oxide, baddeleyite and zircon sand, the ratio of baddeleyite to zircon sand being 3:1 to 1:2.

2. The abrasive grains according to claim 1, characterized in that the raw material base for the abrasive grains comprises aluminum oxide, baddeleyite and zircon sand, the ratio of baddeleyite to zircon sand being from 1.5:1 to 1:1.5.

3. The abrasive grains according to claim 1 or 2, characterized in that the proportion of Si compounds in the abrasive grains, expressed as SiO.sub.2, is more than 1.0% by weight.

4. The abrasive grains according to claim 3, characterized in that the proportion of Si compounds in the abrasive grains, expressed as SiO.sub.2, is 1.1% to 1.5% by weight.

5. The abrasive grains according to any one of claims 1 to 4, characterized in that the ratio of TiO.sub.2 to Y.sub.2O.sub.3 is 2:1 to 6:1.

6. A method for the production of abrasive grains according to any one of claims 1 to 5, comprising the steps of: mixing the starting materials for abrasive grains with a content of a) Al.sub.2O.sub.3 between 52% and 62% by weight, b) ZrO.sub.2 (+HfO.sub.2) between 35.0% and 45.0% by weight, wherein a total of at least 80% by weight of the ZrO.sub.2, based on the total content of ZrO.sub.2, is in the tetragonal and/or cubic high-temperature modification, c) Si compounds, expressed as SiO.sub.2, of more than 0.8% by weight, d) carbon between 0.03 and 0.5% by weight, e) reduced titanium oxide, expressed as TiO.sub.2, between 1.0% and 4.0% by weight, f) Y.sub.2O.sub.3 between 0.2 and 1.5% by weight, the ratio of TiO.sub.2 to Y.sub.2O.sub.3 being from 2:1 to 6:1, and g) raw material-related impurities of less than 3% by weight, melting the mixture in an electrical arc furnace, quenching the molten mixture to obtain a solid product, and crushing of the solid product and subsequent sieving to obtain abrasive grains, characterized in that the raw material basis for the abrasive grains comprises aluminum oxide, baddeleyite and zircon sand, the ratio of baddeleyite to zircon sand being 3:1 to 1:2.

7. The method according to claim 6, characterized in that the ratio of baddeleyite to zircon sand is 1.5:1 to 1:1.5.

Description

DESCRIPTION OF THE INVENTION

[0010] Faced with the task of further optimizing the production of eutectic abrasive zirconium corundum grains, attempts were also made to use less expensive raw materials. For example, series of tests were run in which, in addition to baddeleyite, zircon sand, which up to now was usually only used as a source of SiO.sub.2 in the production of zirconium corundum, was used directly as a raw material for the ZrO.sub.2 in zirconium corundum. As expected, the proportion of SiO.sub.2 in the product increased, which in most cases also led to the product deterioration that was also to be expected. Surprisingly, however, the amount of zircon sand could be tripled without deteriorating the product quality when stabilizing with a combination of TiO.sub.2 and Y.sub.2O.sub.3. Since high proportions of SiO.sub.2 were previously synonymous with a deterioration in the product for those skilled in the art, the tests were repeated frequently and varied accordingly with changing raw material composition for the melt and using quartz as the source of SiO.sub.2 in addition to zircon sand. It was shown that when the ZrO.sub.2 was stabilized with a mixture of TiO.sub.2 and Y.sub.2O.sub.3 in a ratio of 2:1 to 4:1 with the increased use of zircon sand as a raw material source, the SiO.sub.2 proportions in the product increased, however, even with a proportion of more than 1% by weight of SiO.sub.2 in the product, no negative influence on the product quality was found. In contrast, comparative tests in which the SiO.sub.2 proportion was increased by adding quartz sand, while standard formulations were used as raw materials, always showed a significant deterioration in product quality as soon as the SiO.sub.2 proportion in the product was more than 0.6% by weight. Even when the high-temperature modification of zirconium oxide was stabilized with TiO.sub.2 or Y.sub.2O.sub.3 alone or in a ratio of TiO.sub.2 to Y.sub.2O.sub.3 other than 2:1 to 4:1, product deterioration was always found.

[0011] The subject matter of the present invention thus relates to abrasive grains based on Al.sub.2O.sub.3 and ZrO.sub.2 that have been melted in an electrical arc furnace and have a content of Al.sub.2O.sub.3 between 52% and 62% by weight and ZrO.sub.2 (+HfO.sub.2) between 35.0% and 45.0% by weight. In the abrasive grains a total of at least 80% by weight of the ZrO.sub.2, based on the total content of ZrO.sub.2, is in the tetragonal and/or cubic high-temperature modification. Since the abrasive grains are produced under reducing conditions, with carbon being used as the reducing agent, the abrasive grains contain between 0.03 and 0.5% by weight of carbon. The high-temperature modifications of zirconium oxide are stabilized by adding rutile (TiO.sub.2) and yttrium oxide so that the abrasive grains have a reduced titanium oxide content, expressed as TiO.sub.2, between 1.0% and 4.0% by weight and Y.sub.2O.sub.3 between 0.2 and 1.5% by weight, wherein the ratio of TiO.sub.2 to Y.sub.2O.sub.3 is 2:1 to 6:1. In addition, the abrasive grains have less than 3% by weight of raw material-related impurities. The proportion of Si compounds in the abrasive grains according to the invention, expressed as SiO.sub.2, is more than 0.8% by weight, preferably more than 1.0% by weight. In an advantageous embodiment of the present invention, the content of SiO.sub.2 is 1.1% to 1.5% by weight. The raw material basis for the abrasive grains comprises aluminum oxide, baddeleyite and zircon sand, the ratio of baddeleyite to zircon sand being 3:1 to 1:2, preferably 1.5:1 to 1:1.5.

[0012] Grinding tests are usually carried out to assess the quality of abrasive grains. These grinding tests are relatively complex and time-consuming. It is therefore common in the abrasives industry to assess the quality of abrasive grains in advance based on mechanical properties, which are more easily accessible and serve as indicators for later behavior in the grinding test. In addition to the structure already mentioned above and the proportions of high-temperature modifications, micrograin disintegration in particular during grinding in a ball mill is used to assess the quality of abrasive grains.

Micrograin Disintegration (MKZ)

[0013] To measure microparticle disintegration, 10 g of corundum (grain size 36) are ground in a ball mill filled with 12 steel balls (diameter 15 mm, weight 330-332 g) at 188 revolutions per minute for a specific period of time. The ground abrasive grains are then sieved for 5 minutes through a 250 ?m sieve in a (Haver B?cker EML 200 sieve) and the fines are weighed.

The MKZ value is calculated as follows:

MKZ(%)=(Sieve passage 250 ?m/total weight)?100

[0014] In the present case, the proportion of high-temperature phases of the zirconium oxide was determined as a further criterion for the product quality, although no distinction was made between the cubic and tetragonal phase, rather only a T factor encompassing both phases was determined.

T Factor

[0015] The quantitative measurement of the proportion of high-temperature modifications of ZrO.sub.2, based on the total proportion of ZrO.sub.2, is carried out using an X-ray diffractometer in a 2-theta measuring range between 27.5? and 32.5?. The proportions of high-temperature phases (T factor) are determined according to the equation:

[00001]$T\left(\%\mathrm{by}\mathrm{weight}\right)=\frac{2t?100}{2t+m_1+m_2}$

t=intensity of the tetragonal peak at 2 theta of 30.3?
m.sub.1=intensity of monoclinic peak at 2 theta of 28.3?
m.sub.2=intensity of monoclinic peak at 2 theta of 31.5?

DESCRIPTION OF PREFERRED EMBODIMENTS

[0016] The invention is explained in detail below, without limitation, using a few selected examples. These examples are used to demonstrate some general relationships in the melting system Al.sub.2O.sub.3/ZrO.sub.2 with the stabilizers TiO.sub.2 and Y.sub.2O.sub.3 in the presence of SiO.sub.2 as a flux, which provide the person skilled in the art with clues as to how to optimize the production of abrasive grains on the basis of eutectic zirconium corundum that has been melted in an electrical arc furnace, without the products deteriorating.

[0017] The samples for the investigations were produced in a conventional manner by melting a mixture of alumina, baddeleyite concentrate, zircon sand and petroleum coke with the addition of rutile sand and/or Y.sub.2O.sub.3 in an electrical arc furnace. After the entire raw material mixture had melted completely, the melt was poured according to EP 0 593 977 into a gap of approx. 3 to 5 mm between metal plates. After complete cooling, the zirconium corundum sheets quenched in this way were crushed in the usual manner using jaw crushers, roller crushers, roller mills or cone crushers and sieved to give the desired grain size fractions. In comparative Example H, quartz was used as the SiO.sub.2 source instead of zircon sand.

[0018] The raw materials are listed first in Table 1 below, with the proportion of coal, which essentially burns in the melt, not being included in the sum of the raw material mixture. In the product composition, the remainder to 100% is the value for Al.sub.2O.sub.3. In addition to the MKZ values and the T values, the proportion of Zr sand in percent in the raw material mixture, which is crucial for the desired product optimization, are listed again separately.

TABLE-US-00001 TABLE 1 Example A B C D E f G H Al.sub.2O.sub.3 kg 350 350 350 350 350 350 350 350 baddeleyite 230 230 190 190 150 230 190 250 Zr sand 50 50 100 100 150 50 100 rutile 20 20 20 20 20 20 Y.sub.2O.sub.3 6 6 6 6 6 6 carbon 20 20 20 20 20 20 20 20 quartz 30 total 650 656 660 666 676 636 646 656 Al.sub.2O.sub.3 % rest rest rest rest rest rest rest rest ZrO.sub.2 41.5 41.1 40.8 40.5 39.8 42.4 41.8 39.5 TiO.sub.2 3.0 3.2 3.1 2.9 3.2 2.9 SiO.sub.2 0.4 0.37 1.1 1.1 1.3 0.3 0.72 1.2 Y.sub.2O.sub.3 0.9 0.82 0.9 1.0 0.98 0.80 MKZ value % 5.0 4.4 7.0 4.6 5.2 5.9 6.0 8.4 T factor % 94 98 90 96 97 96 95 88 Zr sand % 8 8 15 15 22 8 15 0

[0019] Examples A and B are comparative examples and correspond to commercially available products, the zirconium oxide in comparative Example A being stabilized only with reduced titanium oxide, while in comparative Example B the high-temperature modifications were stabilized with a combination of titanium oxide and yttrium oxide. The different stabilization is noticeable in comparative Example B primarily in the increased T value. At the same time, compared to comparative Example A, an improved MKZ value is apparent, which means that improved grinding performance can be expected, which was then confirmed in the subsequent grinding tests. For both comparative examples, the usual amounts of zircon sand were used as the SiO.sub.2 source, at 8% by weight, resulting in an SiO.sub.2 content of 0.4% and 0.37% by weight, respectively, in the products.

[0020] Example C corresponds to Example A in terms of stabilization, but the proportion of zircon sand has been doubled, while the proportion of baddeleyite concentrate has been correspondingly reduced in order to keep the overall zirconium oxide content in the product at a constant level. Doubling the zircon sand increases the SiO.sub.2 proportion in the product to 1.1% by weight. At 6.9, the MKZ value is in the range of the value for product A. Like Example B, Example D is stabilized with a combination of TiO.sub.2 and Y.sub.2O.sub.3, with the proportion of zircon sand being increased as in Example C. A corresponding SiO.sub.2 proportion of 1.1% by weight was measured in product D. At 4.6, the MKZ value is surprisingly low and an attractive grinding performance can be expected.

[0021] A further increase in the proportion of zircon sand in the raw material mixture was implemented in Example E, with baddeleyite and zircon sand being used in a ratio of 1:1. In order to keep the zirconium oxide content in product E at a comparable level, the baddeleyite content was reduced accordingly. The raw material mixture thus contained a total of 22% by weight of zircon sand. Product E had an SiO.sub.2 proportion of 1.3% by weight and had an MKZ value of 5.0.

[0022] Example F is another comparative example with a conventional proportion of zircon sand, the high-temperature modifications of the zirconium oxide being stabilized solely with Y.sub.2O.sub.3. The T factor and the MKZ value are comparable to the values found for product A, which can possibly be seen as an indication that the type of stabilizer plays a minor role when only one type of stabilizer is used. Example G, in which the proportion of zircon sand has now been doubled compared to Example F, shows a deterioration in the key figures, which, however, is relatively small compared to the individual stabilization with TiO.sub.2 in Example C. Comparative Example H corresponds to Examples B, D and E in terms of stabilization, however, only quartz was used as the SiO.sub.2 source. The proportion of zirconium oxide in the product was adjusted by increasing the amount of baddeleyite used. In the case of product H, the negative influence of the high proportion of SiO.sub.2 on the product quality can be clearly seen in the key figures (MKZ value, T factor), which then also manifests itself in the grinding tests. In addition, a significantly increased porosity with a high proportion of micro and macropores was found in polished sections of product H in the scanning electron microscope (SEM), which can be seen as another reason for the poor results in the grinding tests on polished sections.

Grinding Test 1 (Cutting Disk Test)

[0023] Cutting disks of specification R-T1 180?3?22.23 were chosen for the cutting disk test. For this purpose, a pressing mixture consisting of 75% by weight zirconium corundum, 5% by weight liquid resin and 12% by weight powdered resin from HEXION specialty chemicals GmbH, 4% by weight pyrite and 4% by weight cryolite. To produce the disks, 160 g of the press mixture was molded onto commercially available fabric and pressed at 200 bar and then cured according to the resin manufacturer's instructions.

[0024] For the cutting test itself, round rods made of stainless steel (CrNi) with a diameter of 20 mm were used. The cutting operations were carried out with a disk speed of 8,000 revolutions per minute with a cutting time of 3 seconds. After 20 cuts, disk loss was determined from the decrease in diameter of the disks. The G ratio was then determined from the quotient of material removal and disk loss.

Cutting disks: 180?3?22.23 mm

Grain: F24 (40%); F30 (40%); F36 (30%)

[0025] Material: CrNi stainless steel rods, diameter 20 mm
Grinding machine: Fein WSB 25-180 X, speed 8000 rpm

Test Method:

[0026] To condition the system, 3 cuts were made beforehand. The starting diameter of the disks was then determined. After 40 additional cuts, the final diameter of the disks was determined. The performance of the grains was determined by determining the decrease in disk diameter after the 40 cuts.

[0027] 3 disks of each grain were produced and tested.

[0028] Table 2 below shows the average values for each of the 3 cutting disks.

TABLE-US-00002 TABLE 2 Test grain, Diameter Diameter Diameter according start end difference Performance to Example (mm) (mm) (mm) (%) A 179.4 175.2 4.2 100% B 179.8 176.3 3.5 120% C 179.3 174.7 4.6 91% D 179.5 176.0 3.5 120% E 179.4 175.8 3.6 117% f 179.4 175.1 4.3 98% G 179.2 174.9 4.4 96% H 179.0 173.9 5.1 82%

Grinding Test 2 (Cutting Disk Test)

[0029] Commercially available synthetic resin-bonded and glass fiber-reinforced cutting disks were produced and tested with the grains produced (Examples A to H according to Table 1):

Cutting disks: 125?1.2?22.23 mm

Grain: F46 (60%); F60 (40%)

[0030] Material: stainless steel rods, diameter 20 mm
Grinding machine: Fein WS 14 1, 2 kW, speed approx. 10,000 rpm

Test Method:

[0031] To condition the system, 3 cuts were made beforehand. The starting diameter of the disks was then determined. After 25 additional cuts, the final diameter of the discs was determined.

[0032] The performance of the grains was determined by determining the decrease in disk diameter after the 25 cuts.

[0033] 3 disks of each grain were produced and tested.

[0034] Table 3 below shows the average values of the 3 measurements

TABLE-US-00003 TABLE 3 Test grain, Diameter Diameter Diameter according start end difference Performance to Example (mm) (mm) (mm) (%) A 124.0 112.9 11.1 100% B 124.1 114.8 9.3 119% C 123.9 111.8 12.1 92% D 124.2 114.9 9.3 119% E 124.3 114.9 9.4 118% f 124.2 113.2 11.0 101% G 123.9 112.3 11.6 96% H 123.6 110.4 13.2 84%

Grinding Test 3 (Abrasive Belt)

[0035] Commercially available abrasive belts were produced on an impregnated polyester/cotton blend fabric by means of electrostatic scattering with the grains produced (Examples A to H according to Table 1).

Abrasive belt: length 2000 mm width 50 mm
Grain size: NP 40
Grain coating: see below

[0036] The abrasive belts were used to grind the face of stainless steel rods.

Workpiece: stainless steel rod (CrNi steel) diameter 20 mm
Contact disk: diameter 250 mm, hardness 90 shore
Contact force: 68.7 Newton
Cutting speed: 30 m/s
Grinding cycle: 10 seconds grinding phase?20 seconds cooling phase

[0037] The performance of the grains in the belts was determined by the total removal rate after 24 grinding cycles with a total grinding time of 12 min. The results are summarized in Table 4 below.

TABLE-US-00004 TABLE 4 Test grain, Spreading Material according density removal Performance to Example (g/m.sup.2) (g) (%) A 593 582.3 100% B 588 696.3 119% C 596 541.6 93% D 591 694.3 119% E 582 687.2 118% f 589 583.9 100% G 592 580.0 99% H 594 474.8 81%

[0038] As can be seen from the grinding tests in Tables 2-4, no performance increase is achieved with Examples D and E compared to Comparative Example B. Rather, the product optimization consists in the fact that with the selection made, the ratio and the amounts of stabilizers, the proportion of inexpensive zircon sand in the raw material mixture can be increased compared to the prior art (Comparative Example B) without there being any loss of performance. Inexpensive zircon sand partly replaces the expensive and scarce raw material baddeleyite or the artificial ZrO.sub.2 concentrates. The cost savings are approx. 30% in relation to the raw materials baddeleyite and zircon sand and approx. 8 to 10% in relation to the finished product (abrasive grains), which means an enormous competitive advantage in view of the highly competitive abrasives market.