FI60582C - MALNINGSSKIVA ELLER STYCKE - Google Patents

Description

® FINLAND — FINLAND patent publication — patentskrift 60582 <g> Kv.lk ^ / Int.CI.3 D 21 D 1/30 ® (g) Patent application - Patentansöknlng 79h.jkk • & 8P ® (g) Application date - Ansökningsdag 21.12.79 ( g) Starting date — Giltighetsdag 21.12.79 (g) Made public - Blivit offentlig 22.06.8l @ Date of display and publication——

Ansökan utlagd och utl.skriften publicerad 30.10.81

National Board of Patents and Registration ® Patent granted ~ Patent meddelat 05.0U.83

Patents and registries

Privilege claimed - Begärd prloritet (73) Oittivalu Oy, 12100 Oitti, Finland-Finland (FI) (72) Pentti 0. Kettunen, Nattari, Tapio Mäntylä, Tampere,

Heikki Ristolainen, Tampere, Finland-Finland (FI) (7U) Berggren Oy Ab (5U) Grinding disc or piece - Malningsskiva eller -stycke This invention relates to a grinding disc or piece. The object of the invention is to provide a refining plate or body for use in various grinding, the wear resistance of which is superior to that used hitherto.

The grinder plate of a grinder used in the grinding of wood pulp or cellulose is the most crucial machine part for the final product, the grinder pulp. In order to obtain an economically competitive pulp, the grinding wheel must be as long-lasting as possible, so that the production interruptions caused by the replacement of the grinding wheels are kept to a minimum and, on the other hand, the costs of the grinding wheels themselves are minimized.

In the current era of ever-increasing energy costs, the competitiveness of pulp is increasingly dependent on the useful life of the grinding wheels.

Long service life requires good wear resistance and reasonable corrosion resistance of the grinder plate. However, the grinding wheel must not be polished during use, as this will reduce the quality of the pulp. The refining plate should thus remain to a certain degree of roughness despite its wear and corrosion resistance requirements. In general, this roughness requirement conflicts with very good wear resistance (and to some extent also sufficient corrosion resistance).

In addition, based on the safety and usability of the grinding equipment, a certain impact toughness is also required from the grinding plate in order for the grinding plates to withstand the impact-like stresses applied to them during grinding grinding.

i

Finally, it should be mentioned that in faster mill grinders or mill grinders operating at elevated temperature (T ^ 80 C), the grinder plate is subjected to considerable cavitation stresses, so that the grinder plate must also be cavitation-resistant. This requirement is usually always also in conflict with the wear resistance requirement.

Because the basic requirements for a grinding plate are in conflict with each other, finding a workable solution is laborious and based on different trade-offs in the mutual weighting of different properties.

In the following, these trade-offs will be illustrated first on the basis of the prior art and then within the limits allowed by the new solution now presented.

Ores, minerals (limestone, cement, quartz, feldspar, etc.), etc., are generally ground in mills, where the grinding bodies, as they rotate among the pulp to be ground, perform grinding. The grinding bodies are generally spherical, cylindrical or other simple geometric bodies. Since the grinding effect is largely based on the impact effect of the grinding body, the grinding bodies must have a certain mass in order to maintain good grinding ability. These shocks, as well as the erosion caused by the pulp to be ground, also consume the grinding pieces and therefore the grinding pieces have to be added at certain intervals to keep the grinding capacity of the mills at the desired level. This wear of the grinding balls is indeed one of the most important cost factors in this type of grinding process. When grinding hard ores and quartz, the wear of the 3 60582 grinding bodies is very strong, because the hardness of the grinding bodies is then generally lower than that of the mass to be ground.

It has been typical of prior art grinding pad manufacturing techniques that the grinding pads are made entirely of the same material, usually either cast iron or steel, but plastic grinding pads have also been used in very low consistency grinders. The cast irons used have been either ordinary white cast irons (e.g. Cleva), high chromium white cast irons (e.g. OV-alloy) or martensitic-bainitic cast irons (e.g. various Nihard grades). Of the cast steels, various modifications of martensitic stainless steel have been used for grinding plates, either conventionally or specially (e.g., Sunds Ti alloy) alloyed. In addition to martensitic stainless steels, maraging steels, controlled phase change steels and precipitation hardening austenitic or ferritic austenitic steels have also been used to a small extent. A kind of pinnacle in grinding plate development to date is represented by Sunds titanium alloy steel, where titanium carbide distributed in the structure as a highly wear-resistant component ensures that the roughness of the grinding plate is maintained, while also increasing the wear resistance of the martensitic base structure.

In the development of grinding discs to date, the main focus has been on increasing wear resistance. First, the macrohardness of the material was generally added (Cleva, Nihard, martensitic stainless, etc.), but when abrasion resistance was found to be more correlated with microhardness, development has shifted more toward dispersion-reinforced grades (Sunds, etc.). The great disadvantage of the former group, polishing during use, has diminished with the latter stage of development. Yet the materials of both stages of development are characterized by their poor impact strength and almost non-existent cavitation resistance. Thus, such grinding plates are suitable for situations where cavitation is generally not present. Austenite-based control transformation steels, as well as some maraging steels, have shown reasonable cavitation resistance while having good impact resistance of 4,60582. However, the wear resistance has been limited, clearly weaker than, for example, Sunds' Ti-alloy quality.

Finally, no matter what material the grinding wheels are made of, they have always been disposable. It has not been possible to reprocess the grinding wheels in such a way that reprocessing would have been economically viable. Commonly used grinding wheels have gone to scrap. Due to polishing, poor impact toughness, or cavitation, the life of the grinding disks has generally been quite short.

Until now, grinding pieces have mostly been made of cast iron or steel. In cases where iron is not allowed to enter according to the mass to be ground (e.g. grinding of quartz used in the manufacture of glass), solid ceramic bodies have also been used as grinding pieces. In the majority of cases, however, the grinding pieces are cast iron and steel. To increase wear resistance, these are made by doping or heat treating the entire surface layer or throughout the body to martensitic, bainitic, carbide-rich, or various combinations thereof.

Because grinders are mass-produced, they have generally been considered reasonably priced; it is more economical to compensate for the wear of the grinding pieces by adding new pieces to the mill than to use slightly more durable but more expensive grinding pieces.

It has been typical of grinders to date that they - are the same material throughout the body, although the surface layer has been made harder than the core by heat treatment or rapid cooling - the wear resistance has been increased by increasing the 900 HV. While the hardness of the pulp to be ground with ores and quartz often rises to the order of 1500 HV, the grinders have at their best been clearly 5 60582 softer than such a hard mass - low or at most moderate impact toughness, which has been one of the reasons for using simple geometric shapes.

The main features of the novel type of grinding disc or body according to the present invention appear from the appended claim 1.

The technology of the new refining plate or body presented here is based on the following basic principle: - The set of properties required of a refining plate or body is not achieved by the properties of one and the same permeable material, but by combining different materials into a suitable composite, each material participating in the composite. The main emphasis is on composites formed of ceramic and metallic materials.

The surface of a well-functioning grinding wheel is required to be 1) wear-resistant, 2) roughness-retaining, and 3) reasonably corrosion-resistant. These property requirements are limited to surface layers only. Since metals cannot satisfactorily meet these requirements at the same time, they must be implemented with a ceramic layer applied to the surface of the grinding plate. At temperatures below 200 ° C, diamond has the best wear resistance, but due to its high cost and manufacturing difficulties, it is not economically suitable for a grinding wheel. Instead, the titanium carbide following from the diamond, as well as the titanium ninitride and alumina close to it, are within economical limits when the coating layer is kept sufficiently thin, e.g. in the order of less than 25 .mu.m. In addition to being wear-resistant, these ceramics are reasonably corrosion-resistant under grit process conditions. In the said thickness class, even their toughness is 6 60582 sufficient for grinding plate purposes. If the surface of the material under the coating is initially suitably rough, the ceramic layer coming to the surface retains this roughness and thus the roughness requirement is also met.

Adequate impact toughness and cavitation resistance are required for a well-functioning grinder plate base material, i.e., a material below the surface. The impact toughness requirement is general, whereas the requirement for cavitation resistance depends on the conditions of the grinding. Achieving these requirements to the desired level is accomplished with a ferritic, ferritic perlite, perlite, austenitic, or other phase structure typical of steel and cast iron, where the phase structure may also contain various precipitates or inclusions (e.g., graphite) and the like. castable.

When the wear-resistant and roughness-protective ceramic layer wears so as to weaken the protective effect, the refining plate can be recoated with said ceramic layer, whereby the refining plate is suitable for re-use. The treatment can be repeated, so that the nature of the grinding plate becomes more or less permanent.

When the base material for impact resistance and cavitation resistance is made of low-cost steel or cast iron, the price of the entire grinding plate is made competitive. Competitiveness is enhanced by the fact that the refining plate lasts longer than previous refining discs, so new production discs have fewer mandatory downtime.

Thus, the advantages of the new composite grinding disc compared to the former grinding discs can be summarized as follows: the cavitation resistance of the grinding plates can be increased, which allows faster milling processes or higher operating temperatures - in certain cases, the manufacturing costs of the grinding plate can be reduced from the current level.

A well-functioning grinding body, on the other hand, is required to be wear-resistant at least in its surface area.

A well-functioning grinding body is required to have a certain impact strength. The abrasion-resistant hard surface layer must be able to withstand as such or transmit impacts to the part to any tougher structure below. In the case of very hard ceramic materials, both cases require a very thin material thickness, usually of the order of <25 .mu.m.

The aim is to make the refining plate or body permanent in that, after undergoing a certain reprocessing, it can be returned to use one or more times.

Simultaneous implementation of these three basic requirements is possible when - the interior of the refining body, i.e. the base part, is made of cheap, unalloyed or slightly alloyed steel or cast iron, and - the surface, layer thickness <25 μm, is made of titanium carbide, titanium nitride, alumina or in addition, there may be different layers between the base layers which promote mutual adaptation.

8 60582- • <The advantages of such a composite refiner compared to previous refiners are: 1) The surface hardness of the refiner is of the order of 2000-4500 HV, whereas until now steel or cast iron refiner pieces have only been of the order of & goo HV. Thus, the hardness of the refining body is made higher than that of quartz or hard ores.

2) The service life of the grinders is longer than before, so the need to add grinders to the mills is reduced.

3) Worn grinders can be reprocessed and returned for use. The need to cast new grinding pieces is thus reduced.

4) The impact toughness of the grinding pieces can be widely varied by appropriately selecting the inner material.

5) The price of a refiner can be considered reasonable by choosing cheap, poorly made steel or cast iron as the interior material.

In order to obtain a ceramic layer on the surface of a steel or cast iron grinder plate or piece which is well adhered to the substrate material, with a uniform thickness of only 25 μm or less, there are essentially two suitable coating methods: - chemical vapor deposition and - sputtering.

Both methods are known in their basic form. Difficulties usually occur in the mutual adaptation of the ceramic layers and the metal below. For example, titanium carbide has hitherto been difficult to adhere to low-carbon steel or graphite-containing cast iron. In the present technique, it is realized by means of suitable thin intermediate layers.

The same principle can be applied in the case of titanium nitride or alumina layers.