Exploring Ore Grindability Tests with the Steel Wheel Abrasion Test Machine

Motivation:
The Comminution Dynamics Laboratory in the Department of Mechanical Engineering at McGill University is dedicated to the better understanding of the breakage process of rocks and ore and how this breakage occurs in existing mining equipment. From there, the optimization of existing mining equipment can be studied and implemented allowing for the efficient use of resources for the energy intensive comminution processes. Work done previously in the lab has resulted in a better understanding of charge motion and better prediction of media wear in tumbling mills. Expanding on the latter, one of the decoupled wear model’s tests is aptly suited for studying ore breakage. Thus, allowing for a number of valuable results being extrapolated from a single test.

Combining steel media wear determination with that of the Bond Work Index, operators will be able to understand the power needs of their mills for breakage as well as optimizing their machines for the more efficient use of their resources. Currently this work is being performed in a laboratory in Montreal, QC. The companies in need of these tests are thousands of kilometers from this site; it is the hope of the author that this body of work will, in the future, allow for on-site testing. This machine could easily be used in open pits to determine energy requirements for each of the blocks to be processed, or it could be used on mill circuit feed for quality control, process monitoring, etc.

Introduction
Abrasive wear plays an important role in the mining industry; it can be a sizeable portion of maintenance budgets. Another major cost for the mining industry is the power consumption of the mill. Efficient size reduction, or breakage, of ore and hard rock for future steps in processing can be a daunting task. When combining these requirements with the need to limit abrasive wear of the machinery, one can see that any relevant assistance would be tremendously valuable.
The goal of this work is to explore the possibility of combining two pre-existing test  procedures in order to create a single test that would generate this required information for operators. This will be achieved by:
1. Understanding abrasive wear and how it is to be studied and predicted
2. Investigating how ore breakage can be studied with similar test methodologies.
3. Examining and understanding abrasive wear and breakage under various conditions in an effort to better understand their behavior.
4. Proposing a methodology for concurrently testing abrasive wear and ore breakage.

ABRASIVE WEAR:
2.1 Introduction
The subject of wear will be covered briefly in the following chapter. It will begin with an overview of the phenomenon of wear, how wear is quantified, and its role in mineral processing. Efforts made to better understand and minimize wear will round out the discussion.
2.2 Wear Overview
Wear is an interaction between a surface and its environment. The end result is a quantifiable mass lost from the surface. It can be described in terms of the number of interacting species involved (two-body or three-body are common) and how this interaction occurs (physical, chemical, etc). Three-body abrasive wear will be further elaborated upon in this work; however, many other types of wear exist and are the topic of other research (Chenje 2007) (Radziszewski 2002) (Hawk, Wilson et al. 1999).
2.3 Abrasive Wear
The prevalent form of wear in this research, and economically significant to many industries, (Hawk, Wilson et al. 1999; Radziszewski 2002; Chenje 2007) abrasive wear occurs when forces exerted on particles, harder than the surface they are in contact with, cut into the surface and create grooves or troughs. The surface material displaced by this action is quantified as the mass lost due to abrasive wear. Three-body abrasive wear involves two hard surfaces with an abrasive media forced between them. See Figure 2.1 below. This type of wear is present in abrasive wear testing as performed with the ASTM G-65 apparatus, discussed in greater detail shortly (ASTM 2006).

Depending on the hardness of the surfaces involved and the amount of force applied, one surface should wear preferentially. This wear mechanism is of great importance in mineral processing because of the nature of the processes performed (Radziszewski 2002). Ore, a hard rock, is broken by mechanical means such as crushing and grinding; it is transported by conveyors and chutes or pumped through pipelines. The ore can be harder than the surfaces it contacts throughout these processes, and therefore will abrade, gouge and/or cut these surfaces, no matter the particle size. The replacement of worn parts represents a significant cost to companies (Hawk, Wilson et al. 1999)

2.4 Quantification of Wear
Since it has been established that wear is an issue, it must now be measured and then somehow minimized. Previous work in the laboratory has demonstrated that the modified G-65 test, further referred to as the SWAT, can perform such tasks (Chenje, Radziszewski et al. 2009; Radziszewski 2009). With the use of a strain gauge on the drive shaft, the energy input into the system can be measured. The mass loss of the steel media sample is simply the differential mass readings of the sample before and after performing the test. These two measurements create a value for the media sample’s wear rate. This value is used to rank material performance (abrasion resistance), discussed shortly, as well as being part of the total media wear model (Hewitt, Allard et al. ; Chenje 2007).

2.5 SWAT Machine
The test apparatus used for this research is a variation of the apparatus used in the ASTM G65–04 “Standard Test Method for Measuring Abrasion Using the Dry Sand/Rubber Wheel Apparatus,” (ASTM 2006), as seen in Figure 2.2A. The RWAT apparatus consists of an abrasive hopper, rubber-lined wheel driven by a 1 hp motor and a sample holder fixed to a lever arm, Figure 2.2B. This lever arm is loaded with weights in order to transmit the required applied force to the sample-wheel interface (ASTM 2006).
Differences between the two apparatus’ are as follows: The abrasive feed for the standard test is always a standard Ottawa Foundry Sand, while the SWAT can be operated with any abrasive preferably of that size fraction (Hewitt, Allard et al.).

The wheel diameter of the standard test is fixed at 228.6 mm (9 inches) while the SWAT typically operates with a wheel diameter of 285.8 mm (11.25 inches). Wheel material also varies, the standard wheel is chlorobutyl rubber-lined steel, while the SWAT is entirely steel. The standard test also has specific operating parameters including: wheel speed, applied force and test time (or lineal abrasion). The SWAT machine has a variable operating speed, determined by the motor control unit, it also has a range of applied forces used for testing. Finally, testing is usually only performed for 2 minutes, but this has been amended as required under certain circumstances. As well, the SWAT machine’s drive shaft has been equipped with a strain gauge which is used to calculate the energy input to the system. The SWAT machine can be seen in Figure 2.3 below.

The strain gauge used on the SWAT machine is supplied by Binsfeld Engineering, it is a full bridge strain gauge, meaning that there is only one gauge required for strain and torque measurement. The gauge is precisely bonded to the drive shaft of the SWAT machine, meaning that any strain felt by the shaft will be picked up by the gauge. A DC signal is sent through the strain gauge at all times. In its relaxed state, there is no resistance to this signal; however, as the shaft is strained or torqued, this resistance will vary, see Figure 2.4 below, creating a change in the signal picked up by the receiver. These different signals are then converted into useful information with the use of a computer and calibration or conversion factors.

2.6 Abrasion Resistance Testing (and Ranking)
Work performed in the lab has been used to determine comparative abrasion resistance between pipe lining materials for a paste backfill mining operation in Northern Ontario. The objectives of these tests were to compare the current pipe material with that of new, potential replacement pipe linings. Of the nine competitive samples tested, only one performed worse than the current pipe material. Tests were run with standard Ottawa Foundry Sand, seen in Figure 2.5 below, and, attempts were made to run further tests with the mine tailings as the abrasive. Unfortunately, the size distribution of the tailings fell outside of the typical abrasive size distribution, testing required modified procedures. The modified procedures required the fine tailing particles to be transported in a slurry suspension from the hopper to the test chamber. The slurry mixture was approximately 40% solids by mass. This abrasive tailing slurry was tested on four of the 10 samples. These results are listed in Figure 2.6 below. It is evident that the slurries, both current and proposed tailings, abraded the samples much less than the dry abrasives. This trend appears consistent for all four abrasives tested. This demonstrates the possibility of further expanding testing procedures from dry abrasive testing into the areas of wet testing and slurry testing.

2.8 Conclusion
Three-body abrasive wear plays a very important role in mineral processing; recreating this wear mode in a laboratory setting has been successfully achieved by adapting the ASTM G65 test apparatus, creating the SWAT machine. This machine is currently used not only to comparatively test metallic samples for their wear resistance, but also for the prediction of steel media wear inside tumbling mills with impressive accuracy. With the value this machine and testing procedure hold, it would be beneficial to examine further uses for this laboratory setup.

Prev: Working principle of Rubber wheel abrasive wear tester