Field of the invention

The invention relates to a method for preparing a lining element of a grinding mill and to the use of a lining element prepared by the method.

Background of the invention

The linings used in grinding mills are mainly prepared from cast and rolled steels which have been cast by using traditional melting methods. It is known to use various steel grades and compositions, hi addition to steels, different elastomers and in certain cases combinations of elastomers and steels, or so-called hybrid materials, are used.

The lining may consist of a variety of lining elements. Examples of lining elements used in the prior art include a lifting element, a wear protection used in a lifting element and a shell plate. A lifting element may consist entirely of a wear protection, or it may also include a flexible matrix, i.e. a body, in addition to the wear protection.

In the flexible body matrix it is known to use elastomeric materials which can be attached to the wear protection either mechanically or by means of a bond formed as a result of vulcanization of the elastomer, i.e. heating the elastomer with sulphur. Elastomers are cross-linked polymers which are capable of reversible deformation of hun- dreds of per cents. This behavior is achieved by vulcanization, prior to which the behavior of the elastomer is plastic, or irreversible.

When the manufacturing of steel materials is based on casting technology, naturally the cast structure, the casting pores and the impurities of sand cast materials remain in them, whereby there are many constraints on developing the properties of the materials. Rolled materials are typically prepared by hot rolling either a chilled cast material or a continuous cast material. Because the cast structure of the structure contains solidification-induced segregation, which is not completely removed even by hot rolling, there is non-homogeneity in the structure although rolling removes the residual po- rosity. Because the manufacturing is based on conventional melting technology, the purity level and mechanical properties of ordinary steel remain in the structure.

From the publication US 6,036,127 is known a single-piece lining element comprising a lifting element prepared from a wear-resistant material and a support element pre- pared from an elastomeric material. The material of the lifting element can be for example a high-chrome white cast steel, steel or aluminum oxide. It is possible to cover the entire inner surface of the mill with lining elements as disclosed in the publication US 6,036,127, and separate bottom lining elements need not be used.

A large part of the lining materials of the prior art are only suitable for a particular field of application, such as for a particular mill size or for a certain type of material to be ground. As the diameter of the mill increases, the intensity of the impacts caused by the steel balls or rods used as the grinding medium increases. This imposes constraints on the materials used in mill linings, which materials shall not be too brittle but shall have a sufficient toughness and an ability to receive impacts without being damaged. The use of too brittle materials causes damage to the linings and unscheduled interruptions in the process. In addition to impact loads, mill lining material shall also tolerate fatigue loads. Fatigue loads are caused i.a. by the mass of the material to be ground.

The non-homogeneity of the lining material and the inclusions and impurities contained therein, such as nitrogen, oxygen, hydrogen and sulfur, deteriorate the toughness and fatigue properties of the material especially in long-term use. The toughness properties may be emphasized for example in service interruption situations, in which the grinding mill operates but no mineral has yet been charged for grinding. Initially small cracks formed in service interruption situations may grow to critical dimensions as a result of mechanical loads caused by normal operation. Fatigue problems are emphasized in high-alloy, wear-resistant materials, because in them the wearing of the material will not have time to eliminate fatigue cracks or breaks before they grow to critical dimensions. In high-alloy materials the wear-resistance has typically been increased by raising the carbon content and the amount of carbide-forming alloying substances, such as Cr, Mo, Nb, V and W. Steel with a total content of alloying sub- stances of 5 % or more is regarded as a high-alloy steel.

The amount of impurities cannot be minimized in a satisfactory way in the lining elements prepared by traditional melting techniques.

The object of the invention is to provide such a wear-resistant mill lining material in which the above mentioned fatigue and toughness problems due to impurities and non-homogeneities have been solved.

Disclosure of the invention

According to the invention it has been observed that the object can be achieved by hot working a solid metal preform to a working degree of at least 3, and by carrying out said hot working in at least two directions.

By carrying out sufficient working to at least the degree of 3, a sufficient degree of working is achieved for proper toughness. The degree of working is defined as the ratio of the sectional areas measured before and after working.

Particularly advantageous properties in the steel are obtained when steel of good quality is worked to a sufficient working degree. In addition it is of importance that the working is carried out in more than one direction. When working is carried out in a single direction, the toughness measured perpendicularly (short transverse) to the working direction remains weak. Preferably, the working is carried out in three directions perpendicular to each other. Particularly advantageous toughness properties are obtained when the steel is worked to at least working degree 4; preferably to working degree 5; and particularly preferably by carrying out the working to working degree 6. A high degree of working may be required to obtain sufficient toughness in all directions.

Preferably, the metal preform is prepared using casting technology or electro slag re- melting (e.g. ESR, Electro Slag Remelting, or ESRR, Electro Slag Rapid Remelting), or vacuum arc remelting (e.g. VAR).

The ESR, ESRR and VAR techniques are described in more detail for example in the publications Metals Handbook, 10th edition, p. 970 (ESR and VAR), and D. Alghisi, M. Milano, L. Pazienza, "From ESR to continuous CC-ESRR process: development in remelting technology towards better products and productivity" (ESRR).

It is known that by means of the ESR, ESRR and VAR methods it is possible to lower the impurity levels in metal materials. Typical impurities include metallic oxides and sulfides, non-metallic sulfur and nitrogen oxides and carbon monoxide, and impurities due to segregation, hi addition, by means of the VAR method it is possible to remove also gaseous impurities. By means of the ESR, ESRR and VAR methods it is also possible to improve the homogeneity of metallic materials by reducing segregation.

The toughness, fatigue resistance and many other properties of the ESR, ESRR and VAR remelted materials are clearly superior to those of the materials prepared by the traditional methods. As an example, the impact toughness of an alloyed tool steel was observed to be increased from a value of 8 J to a value of 16J. In practice this implies a doubling of the performance of the steel in an environment with impact load. However, the hardness and wear-resistance of steel prepared by both techniques can be brought to the same level, for example the hardness value may be 40HRC independent of the impact toughness value.

In the solution according to the invention the homogeneity and slag purity of a metallic material can be improved. The increase in the purity level of remelted steel is sub- stantial. Particularly the amount of such inclusions and impurities in the structure can be decreased that would deteriorate the fatigue properties of the material. The purity level of metals is measured by means of the DIN 50602 standard. By means of the invention a slag purity that is superior to K3 = 5 after remelting, as defined in accor- dance with the standard DIN 50602, can be obtained for a mill lining material. The increase of the purity level has been observed to substantially improve the fatigue properties of the material. In addition, the deviation of the fatigue properties and the toughness values is very small in comparison to materials prepared by other tech- niques. The deviation of the fatigue properties of remelted steels has been observed to decrease by 50% in some cases in comparison to steels prepared by a conventional casting technique.

In one embodiment, the metal preform is manufactured using spray forming (SF) technology.

The invention enables the use of more wear-resistant, high-alloy materials in mill linings and the increase of the service life of mill linings. With regard to the invention it is essential that concurrently with increasing the wear-resistance of mill lining mate- rials, also the fatigue properties of the material are improved, because a more wear- resistant component has to withstand more load changes and possible abnormal loads during its longer service life than a faster wearing component with a shorter service life.

By using the method according to the invention, advantages that had not been achieved previously are achieved in grinding mill applications. The invention enables the use of steel with an alloying level higher than conventional in applications in which this was previously impossible due to the poor fatigue properties of alloyed steels.

By means of the method according to the invention, high-alloy materials can be used in demanding applications. Similar materials prepared by conventional methods could not be used in similar applications.

In an embodiment of the invention, the metal preform is hot worked by forging, rolling or extrusion. In an embodiment of the invention the metallic material is iron-based, containing at least about 50 wt-% Fe.

In an embodiment of the invention the carbon content of the metallic material is at least about 0.20 wt-%, and additionally it contains one or more of the group Cr, Mn, V, W, Mo and Ni to a total of at least about 0.25 wt-%.

In an embodiment of the invention the carbon content of the metallic material is at least about 1.0 wt-%, and additionally it contains one or more of the group Cr, Mn, V, W, Mo and Ni to a total of at least about 10.0 wt-%.

In an embodiment of the invention the microstructure of the metallic material is mar- tensitic or bainitic.

In an embodiment of the invention the metallic material is a high-alloy steel.

In an embodiment of the invention the metallic material is a tool steel.

The performance of a lining element manufactured according to the invention is measured using the following index: The index is the product of the hardness (Rockwell hardness C) and the impact strength (Charpy V). Preferably, this index shall be greater than about 180. The measurement of the impact strength used in the calculation of this index is to be allowed in any direction, i.e. the worst possible value of impact strength is used.

Preferably, the value of the aforementioned performance index is at least about 200, and more preferably at least about 220.

In an embodiment of the invention the lining element is a lifting element.

In another embodiment of the invention the lining element is a wear protection used in a lifting element. In a further embodiment of the invention the lining element is a shell plate.

A further object of the invention is the use of a lining element prepared by the method in linings of grinding mills.

Brief description of the drawings

Figure Ia shows the implementation of the method according to the invention by means of an ESR apparatus.

Figure Ib shows the implementation of the method according to the invention by means of a VAR apparatus.

Figure 2 shows a three-dimensional view of an exemplary lining of a grinding mill.

Figures 3a-3c show cross-sectional views of exemplary grinding mill lining elements, in the manufacturing of which a method according to the invention was used.

Detailed description of embodiments

In the following, the invention is described in detail by referring to the appended drawings in which corresponding parts are always referred to by the same reference numbers.

The method according to the invention is intended for preparing a lining element of a grinding mill. According to the invention, a metallic material which can be a high- alloy steel grade is used as the material of the lining element. In steel lining elements, alloying substances that affect particularly the mechanical properties and wear resistance of the component are i.a. carbon, chrome, tungsten, vanadium and molybdenum. The alloying degree of the steel grade used in the invention may vary from medium- carbon to high-carbon. Higher alloying generally implies higher wear-resistance. In practice this means that the carbon content of the steel varies in the range of about 0.2 % to about 1.4 %. According to the invention, for example an iron-based metallic material can be used as the metallic material, whereby the Fe content is at least about 50 wt-%. According to an embodiment, the C content of the iron-based metallic material is at least about 0.20 wt-%, and the Cr, Mn, V, W, Mo and Ni contents are at least about 0.25 wt-% in total. According to another embodiment, the C content of the iron-based metallic material is at least about 1.0 wt-% and the Cr, Mn, V, W, Mo and Ni contents are at least about 10.0 wt-% in total. Preferably the microstructure of the iron-based metallic material is martensitic or bainitic.

According to the invention, the metallic material is remelted by means of electro slag remelting or vacuum arc remelting to improve the slag purity and/or homogeneity of the material. For example the ESR electro slag remelting technique (Electro Slag Remelting), the ESRR electro slag remelting technique (Electro Slag Rapid Remelting) or the VAR vacuum arc remelting technique (Vacuum Arc Remelting) can be used. In addition to the remelting techniques, also the use of primary melting techniques, such as the VIM technique (Vacuum Induction Melting), can be combined with the method according to the invention.

According to the invention, the hardness of the remelted material can be for example 40HRC and its impact strength can be 7J (Joule). Then the performance index becomes 280. If the steel had been prepared by a conventional casting technique and its impact strength was for example 4J, then the steel would not satisfy the requirements, as the performance index would become 40(HRC) x 4(J) = 160.

Figure Ia shows the implementation of the method according to the invention by means of an ESR apparatus. Figure Ia illustrates an electrode holder 1, an electrode 2, a water-cooled mold 3, a slag bath 4, molten metal 5, water cooling 6, a solidified billet 7 and a molten metal droplet 8.

In the embodiment according to Figure 1 a a prefabricated electrode 2 is melted through a slag layer 4 as small metal droplets 8 while the tip of the electrode is in contact with the molten slag 4. As the metal droplets react with the slag, the purity level of the metallic material is increased. The composition of the slag usually contains one or more of the following: CaF ₂ , Al ₂ O ₃ , MgO and CaO. Also different additives, such as MgF ₂ , BaF ₂ , BaO, SiO ₂ , CrO ₃ , FeO and TiO, can be used. Because the melting and transition from electrode 2 to the remelted and solidified billet 7 takes place one drop- let at a time, the homogeneity of the metallic material to be remelted is improved.

Figure Ib shows the implementation of the method according to the invention by means of a VAR apparatus. Figure Ib illustrates an electrically insulated vacuum seal 10, an electrode 2, a water-cooled mold 3, water cooling 6, molten metal 5 and a soli- dified billet 7.

In the embodiment according to Figure 1 b, a prefabricated electrode 2 is melted one droplet at a time by means of an electric arc formed at the tip of the electrode in vacuum, whereby gaseous impurities, such as hydrogen and nitrogen, are removed and simultaneously the slag purity is improved. Also in this embodiment the remelting taking place one droplet at a time substantially improves the homogeneity of the metallic material to be remelted.

Figure 2 shows an exemplary mill lining structure, which consists of lifting elements 21 and shell plates 22. Shell plates are used between the lifting elements and they are an essential part of the lining of a grinding mill. The purpose of the lining is to protect the sheath 23 of the grinding mill.

The lining element prepared by the method according to the invention may be for ex- ample a lifting element or a wear protection used in a lifting element. In certain components it is possible to use inserts prepared by the method according to the invention, which inserts are attached to a polymeric or metallic structure.

Figure 3a shows a lifting element with a hybrid structure, in which lifting element a wear protection 31 is attached to a flexible body 32. The flexible body is prepared for example from a polymeric material. The wear protection 31 is prepared by a method according to the invention. The location of the wear protection in the lifting element is preferably such that the wear protection covers most of the surface that is exposed to the heaviest wear. In the embodiment of Figure 3a a wear protection 31 with a rectangular cross-section covers the front wall of a lifting element.

In alternative embodiments the wear protection may extend over the region of the front wall and the upper part of the lifting element. The cross-section of the wear protection may be for example a rectangle, a parallelogram, a pentagon, or a hexagon.

Figure 3b illustrates a lifting element which consists only of a metallic wear protection 31 which has been prepared using a method according to the invention.

The lining element prepared by the method according to the invention may also be a shell plate. The structure of a shell plate may be a hybrid structure comprising a flexi- ble body and a wear protection, or a monolithic structure consisting only of a wear protection and not comprising a flexible body.

Figure 3c shows an exemplary monolithic shell plate 33 of a mill lining, which shell plate has been prepared by a method according to the invention. Shell plates can be used together with lifting elements in linings, as shown in Figure 2.

Example

This example shows the manufacturing of a wear protection of a grinding mill from WRl 2 ^® tool steel using the method according to the invention.

WR 12 ^® tool steel is a high-alloy steel and therefore the impact strength of the steel is low and its indentation sensitivity for breaking is high. Particularly the fatigue resistance of the steel is improved by using the method according to the invention. The chemical composition of WRl 2 ^® steel is shown in Table 1. Table 1. Chemical composition of WR 12 ^® steel

The following steps are performed:

1. An electrode is cast from WRl 2 ^® steel.

2. The electrode is remelted and cast to a preform with a desired shaped using either the ESR method or the VAR method.

3. The cooled preform is hot worked according to the invention by e.g. forging or rolling, to a desired shape.

4. The hot worked object is machined and annealed to a desired hardness.

5. The component is ready to be used as such or it can be attached to another material, such as a flexible polymeric structure.