The present invention relates to a method of repairing a roller and/or a table used in a roller mill and to a roller and/or a table so repaired and relates particularly, but not exclusively, to a method of repairing a roller and/or a table used in a vertical roller mill as part of a cement production process mainly but in many other industries too.

The use of roller mills and in particular vertical roller mills and Roller Presses for the creation of fine powders in processes such as cement production has been well known for many years. An example of such a vertical roller mill is disclosed in US patent US6193176 with particular reference to figure 24. The rollers (indicated at 5 in that figure) crush material against a table (indicated at 3) which rotates relative to the rollers. Over time the grinding parts (this is the rollers and the table) become worn with material being lost from their respective surfaces.

Repair of the rollers can be undertaken by the deposition of replacement material onto a worn section of the grinding parts (rollers and table) . This is typically undertaken by a process such as open arc welding where a source of material, in the form of a wire such as a chromium carbide wire, is deposited onto a roller to replace worn sections and bring back the original dimensions and roller profile. Skilled technicians use a template to determine the locations at which replacement material is required. They also ensure that the refurbished roller is provided with the required shape of outer surface and that the milling surface of the roller is rotationally symmetrical . Repairing a roller is significantly less expensive than replacing with a new roller. In addition, the wear characteristics of a refurbished roller are not the same as an original roller as the grinding surface is now the weld overlay rather than the original cast material of the roller. The material that is chosen as a casting for the roller is a compromise choice balancing the mechanical strength required for the roller body for the application and the wear resistance on the outer grinding surface. Typically, no cracks can be tolerated in such a casting, and this limits the hardness and wear resistance of the grinding surface. Consequently, there are a limited number of cast materials which are suitable for the production of the rollers. There is a significant cost associated with the amount of time that the roller is either not working (because it is being refurbished) or is working inefficiently (because the original optimal roller profile has been changed due to wear) . Consequently, the wear performance of the overlay is a critical cost issue for the operator. The longer the roller lasts in service, the few stoppages are needed to refurbish or replace the worn roller, and the fewer lost days of production. The longer that the roller keeps its optimal grinding profile, the longer grinding will occur at optimal efficiency. Loss of profile means that regrinding is necessary and this has an extra cost and loss of grinding capacity for the plant. A homogeneous cast material does not have a homogeneous wear loss of the grinding surface profile. Some areas on the grinding surface wear more than others, and the optimal profile is lost. Increasing the wear resistance of the surface material by refurbishment with more wear resistant material slows down the rate, but loss of optimal profile still occurs.

Preferred embodiments of the present invention seek to overcome or alleviate the above described disadvantages of the prior art. According to an aspect of the present invention there is provided a method of repairing a grinding part used in a roller mill, comprising the steps :- measuring a grinding part to be repaired compared to predetermined dimensions to map material loss across at least a portion of at least one surface of said grinding part; and adding material to build said grinding part to said predetermined dimensions characterised by varying the wear resistance characteristics of the added material depending on the mapped material loss.

By mapping the material loss across the grinding surface of a roller (or other grinding part) and refurbishing the roller by varying the wear resistance of the repair material depending on the mapped material loss significant advantages are provided. In particular, where more wear resistant material is applied to the areas of greatest material loss the advantage is provided that the wear characteristics, that is the amount of material lost from the outer surface, of the refurbished roller is significantly improved compared to refurbished rollers of the prior art. Furthermore, because the most wear resistant materials are located where previously wear had occurred most severely, the refurbished roller can wear more evenly than a brand-new roller and maintain the designed optimal grinding profile for longer. Even though the process of refurbishment is more expensive when using materials or techniques to vary the wear characteristics of the added material, the advantage of even wear across the surface of the roller significantly outweighs the additional cost. In particular, the even wear across the surface ensures that the roller works at its maximum efficiency for the longest possible time. This does not only increase the time between required refurbishments but also means that the vertical roller mill works at its optimum efficiency for as long as possible. In a preferred embodiment the wear resistance is varied by using materials of different wear resistance to the target material to be ground on at least the outermost portion of the repaired grinding part. In another preferred embodiment the materials of different wear resistance comprise substantially the same components in different proportions.

In a further preferred embodiment the materials of different wear resistance comprise different components Such as completely difference materials, chemistries, microstructures if required to achieve the target performance.

In a preferred embodiment the wear resistance is varied by using different material application techniques on at least the outermost portion of the repaired grinding part. These techniques include, but are not limited to Open arc Welding, Plasma welding, Submerged arc welding, Gas shielded welding, brazing, laser cladding and the like.

In a further preferred embodiment material having the greatest wear resistance is located where the greatest material loss is mapped.

The method may further comprise removing further material from said grinding part after the step of measuring and before the step of adding material.

According to another aspect of the present invention there is provided a repaired grinding part for use in a roller mill, the grinding part comprising: - a base portion comprising an unrepaired grinding part; added material coating at least a portion of said base portion creating a repaired grinding part to predetermined dimensions said added material having varied wear characteristics depending on material loss mapped across said unrepaired grinding part. In a preferred embodiment the added material coating comprises a plurality of materials with those having higher wear resistance characteristics being located at positions of greatest material loss as mapped. Preferred embodiments of the present invention will now be described, by way of example only, and not in any limitative sense with reference to the accompanying drawings in which: -

Figure 1 is a schematic representation of a cross-section of a roller used in the present invention; and Figure 2 is a flowchart showing the steps of a method of the present invention.

The process of the present invention is the refurbishment of grinding parts used in roller mills and therefore the creation of such refurbished parts. Throughout this application reference to grinding parts includes but is not limited to rollers and tables used in vertical roller mills and rollers used in roller presses. These rollers and tables, both refurbished and non-refurbished, are used in roller milling/grinding machines for the creation of fine powdered materials such as cement, clinker, raw meal, sinter, coal, coke, several other minerals and these milling/grinding machines include vertical roller mills, coal pulverizing mills, steel sinter grinding mills, raw meal and cement finishing mills. In the embodiment of the invention set out below, the process of refurbishment of a roller mill roller has been described in detail and a similar process can be used on similar grinding components including, but not limited to, roller mill tables and roller press rollers.

An example of such a roller mill roller is schematically shown in figure 1 and indicated with reference numeral 10. In this example the roller 10 is frustoconical with the circular side surfaces 12 and 14 not directly involved in the milling process but with the curved milling surface, generally indicated at 16, engaging a plate surface of the mill. The dotted line indicated at 18 represents the original outer surface of the curved milling surface 16 of the roller 10 and the solid line 20 represents the worn down after use version of this same curved milling surface 16. When the outer curved milling surface 16 of roller 10 becomes worn, as schematically indicated at 20, the efficiency and effectiveness of the milling process is significantly decreased by uneven nature of that worn surface. In order to allow refurbishment of the roller 10 it is unmounted and removed from the roller mill, as indicated at step SI in figure 2. In another example, the roller and table are measured and repaired in place without removing the roller or table . Once the roller has been removed from the mill it is mapped

(at step S2) in order to identify variation in the wear, that is the material loss, on the milling surface. A variety of techniques can be used to undertake this mapping process including laser linear or 3D scanning, mechanical measurement, optical measurement but in each case the map produced indicates the depth of material lost compared to the roller as originally formed. From this mapping portions of the milling surface 16 of roller 10 can be identified as having suffered similar material loss and these portions categorised and grouped together accordingly (at step S3) . In other words, the areas suffering the greatest material loss can be grouped together as can the areas suffering least material loss and further groups can be created relating to material losses therebetween. The number of groups to be identified depends on how many varieties of wear resistant coatings can be applied to the roller surface. The more different coatings that can be applied the more groups should be identified. These groups are used to determine which material or application technique (both of which can be used to vary the wear characteristics of the material applied) should be used on which portion of the milling surface 16 of roller 10. Specifically, the areas which have suffered the greatest material loss are replaced with material having the greatest wear resistance whilst areas which have suffered the least material loss are replaced with material having the least wear resistance . Before the application of new material, the worn roller has the outermost surface treated to ensure good adhesion between the new material and the original roller material. This treatment can include removal of an outer layer of the surface material (at step S4) by mechanical means or gouging by using carbon electrodes, continuous wire, plasma or similar systems.

Once the outer surface has been prepared, material can be applied to that surface to recreate the original profile, or a similar profile, of the roller 10. Depending on the removal system the part may be machined down to a certain depth. The refurbished profile 20 is recreated by adding material to build the roller to the predetermined dimensions of that profile whilst varying the wear resistance characteristics of the material being added depending upon the material loss as mapped in step S2 and identified in step S3. In particular, portions of the profile which have suffered the greatest material loss are replaced with material having the greatest wear resistance characteristics, typically the greatest hardness.

There are three main techniques for varying the wear resistance characteristics. The first example is to use a single material application process, such as open arc welding, and change the material used in the process. Open arc welding uses a stock feed of wire and therefore the wear resistance characteristics are varied by changing the wire which is being fed into the open arc welding apparatus. It is envisaged that this happens in a single machine which has three or more wire feeds going into it or multiple machine heads having their own dedicated wire. The application process allows for all the material of one wear resistance to be applied followed by the next material. Alternatively, the material can be applied by a process of working around the roller varying the wire being used to apply material depending upon where the material is being applied. For example, for the areas requiring the least wear resistance, a conventional cored wire can be used which is primarily composed of a tough, iron-based matrix with precipitated primary chrome carbide grains included which act as the hard and wear resistant material. To increase the wear resistance, this wire can be substituted for a wire containing an additional amount of complex carbides and borides. These are typically based on niobium and vanadium carbides and borides which have a higher hardness than simple chrome carbide. For the areas requiring the greatest wear resistance, those which had suffered the greatest material loss, an even more wear resistant wire is used which contains, for example, harder wearing grains such as tungsten carbide, ceramics and metal/ceramics composites.

The second example is to remain with a single material application process and to have a series of wires formed from the same mixture of components but with varying quantities of the harder grain particles creating the variety in the wear resistance characteristics of the material applied.

The third example is to use one wire of a consistent chemistry, for example a wire containing tungsten carbide but changing the application method to give the different wear characteristics. For example, for the least wear resistant areas open arc welding is used. The process of open arc welding generates significant amounts of heat in the wire to melt it and to create a good bond to the roller substrate. However, these high temperatures have a negative effect on the tungsten carbide grains in the centre of the cored wire which causes them to dissolve into the metal matrix. As a result, there are less tungsten carbide wear resistant grains and they are smaller. However, it should be noted that open arc welding with tungsten carbide still gives better wear resistant than complex carbide or simple chrome carbide alloys. However, by swapping to an alternative process but using the same wire chemistry different wear resistances can be achieved. For example, less heat is generated by Plasma Transferred Arc, Gas Shielded Welding, Laser Welding, Cold Metal Arc, submerged arc, and the like. These methods have been shown to give an improved wear performance over conventional open arc welding. These material application processes are shown in figure 2 at steps S5 and S6 and these iterative steps are repeated until all of the roller profile has been recreated using all of the different techniques or materials. Once the materials have been applied sufficiently to recreate the required roller profile 20 the roller can be returned to the vertical roller mill (step S7) and milling/grinding operations recommenced.

It will be appreciated by persons skilled in the art that the above embodiments have been described by way of example only and not in any limitative sense, and that various alterations and modifications are possible without departure from the scope of the protection which is defined by the appended claims. For example, the more wear resistant coatings may be only applied to the outer layers of the roller to recreate the original profile. That is, lower cost materials can be used to fill significant volumes of the refurbished rollers and the more expensive wear resistant materials, which are generally more expensive, can be used on the outermost layer. The example described above explains in detail how the present invention is applicable to the repair of rollers in a roller mill. However, the same principles can be applied to the repair of other similar components including but not limited to the table in a roller mill and the rollers in a roller press used for milling.