Dr Daniel Grasser, a consulting engineer with TUNRA Bulk Solids, explains to ABHR the influence of wear on mining equipment and modern modelling techniques to understand wear mechanisms.
Mining plays a major role in society by providing raw materials. Inevitably, this requires the processing of solid particles which cause severe wear on mining equipment. On a global scale, economic losses resulting from wear in mineral mining are $340 billion AUD annually [1].
The energy consumption of global mining activities is 6.2 per cent of the total global energy consumption, where 17.4 per cent of the consumed energy in mineral mining is used to remanufacture and replace parts needed due to wear failures[1]. In Australia, the mining industry is an industrial sector contributing multi-billions of dollars to the economy every year. Materials transport and handling of the solid minerals is one of the main operational categories in mining, beside extraction and processing.
For example, chute wear is a critical issue that affects cost and productivity in materials transport and handling. Most importantly, the costs of the wear liner and development of a good design are often relatively low, while the economical loses due to chute downtime caused by the replacement of worn chute liners can be significantly higher.
An introduction to wear
Wear resistance is an important property in the mining industry. Firstly, wear resistance of a material is not a mechanical property. Unlike material properties, for example the hardness of a wear liner, wear resistance depends on the occurring mechanisms caused by the system it is exposed to.
It is commonly agreed that wear is a system property rather than a material property. Wear can be described as the “progressive loss of substance from the operating surface of a body occurring as a result of relative motion at the surface” [2].
Interestingly, the wear process of many applications can be distinguished into three separate stages: Break-in, steady state and critical wear. Increased wear rates are often observed in the initial break-in period, followed by a constant wear rate at steady state [3]. This is eventually followed by the critical wear stage where wear liners require replacement.
Abrasive wear is an important wear mechanism occurring, for example, in chutes. Here, solid particles shear onto the wear liner surface, causing a progressive loss of wear liner. Abrasive wear can be sub-divided into high-stress abrasion and low-stress abrasion. In high-stress abrasion, the abrasive particles fracture into smaller sub-particles. In low-stress abrasion, the abrasive particles do not fracture.
Increasing the hardness of a wear liner often leads to increasing abrasion wear resistance. However, high hardness generally leads to lower impact toughness. This can be interpreted as a trade-off and optimum wear liners depend on the targeted application. Related to this, the hardness of the abrasive particles is important. Generally, the hardness of the abrasive medium needs to be higher than the abraded surface to create significant abrasive wear.
It was stated that the wear loss significantly increases when the ratio between the hardness of the abrasive particle and the hardness of the wearing material increases from 1.0 to 1.2 [4]. In general, harder abrasives cause more wear than relatively soft abrasives. Moreover, the impact angle of the abrasive particles on the wear surface is important, and the amount of wear depends on the type of impacted material. Ductile and brittle materials possess different relationships between the angle of attack of the solid particle and the wear of the material (Figure 1). For ductile materials, most wear occurs at 30. For brittle materials, the lowest amount of wear is found at this angle, and the maximum wear is found at 90 (Figure 1) [5].
Additionally, another contributor to wear is the particle motion, such as sliding or rolling (Figure 2). The relationship between the wear volume (w) of a liner and the sliding velocity (v) can be described by a power law (w~vn) [4]. For ductile materials, the power law exponent is reported between to be 2<n<3, while for brittle materials it is 3<n<4. Hence, an equipment design allowing a reduction of the particle sliding velocity, while increasing the rolling motion, is favourable especially when hard and brittle wear liners are used.
Wear testing
Predicting wear of liners is important, especially when comparing wear liners for different applications. To achieve this, the working conditions must be simulated as close as possible. These conditions should be well controlled and close to reality.
Field tests possess realistic test scenarios; however, these are costly, and the conditions are difficult to control. In contrast, for over-simplified wear tests, the acting wear mechanism can be different from the ones occurring in the industry application. Additionally, to speed up the development process, accelerated wear rates on a lab scale test compared to the industry application are desirable.
For example, Archard’s wear model is commonly applied to predict abrasive wear of liner [6]. Archard’s wear law considers the sliding distance, load induced by the abrading particles and the hardness of the wearing material to estimate the amount of wear. Additionally, all other effects of the complex wear system are represented by an empirical constant, sometimes referred to as the Volume/ Shear work ratio, i.e. wear volume per induced shear work.
This empirical constant can be difficult to determine and scaling it from laboratory scale tests to industry scale applications can be a challenge. For example, the Dry Sand Rubber Wheel (DSRW) test can be utilised to apply Archard’s wear law on a laboratory scale.
The DSRW test can be used to compare and rank the wear performance of different wear liners using small abrasive particles (<0.5 mm) [7]. In addition to the experimental DSRW, the test can be implemented as a numerical simulation (Figure 3) using Discrete Element Method (DEM) modelling [8].
The DEM model is often calibrated using particle flow tests, such as the Angle of Repose (AOR) test (Figure 4) and calibrated Volume/ Shear work ratios.
Even widely used, the DSRW test is mainly used for a relative comparison between different liners. However, this laboratory wear test can make it difficult to predict the service life of a liner. To tackle this, wear tests using abrasive particles similar to these occurring in the industry application can be applied. For example, the TUNRA abrasive wear test utilises particle sizes up to approximately 15 mm (Figure 5). This increased realism is important when estimations regarding the service life of wear liners is required.
Concluding remarks
Wear is a complex system, where multiple factors affect each other. There seldom is a one-fits-all solution. Advanced laboratory scale wear tests and numerical techniques, such as DEM modelling, can provide important insights and predictions of the service life. To achieve an optimum design, combined with a good choice of wear liner, a good understanding of the complex system wear is required.
References
[1] K. Holmberg, P. Kivikytö-Reponen, P. Härkisaari, K. Valtonen, A. Erdemir, Global energy consumption due to friction and wear in the mining industry, Tribology International 115 (2017) 116-139.
[2] K.-H. Zum Gahr, Microstructure and wear of materials, Elsevier, Germany, 1987.
[3] R.D. Wilson, J.A. Hawk, Impeller wear impact-abrasive wear test, Wear 225-229 (1999) 1248-1257.
[4] K.-H. Zum Gahr, Wear by hard particles, Tribology International 31(10) (1998) 587-596.
[5] B. Bhushan, Principles and Applications of Tribology, John Wiley & Sons, Ltd.2013.
[6] J.F. Archard, W. Hirst, The Wear of Metals under Unlubricated Conditions, Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences 236 (1206) (1956) 397.
[7] Standard test method for measuring abrasion using the dry sand/ rubber wheel apparatus, ASTM International, USA, 2017
[8] D. Grasser, S. Corujeira Gallo, M. Pereira, M. Barnett, Wear simulation and validation of composites (insert-reinforced matrix) in the dry sand rubber wheel test, Minerals Engineering 207 (2024) 108583.