Revisiting gouging abrasion test for jaw crushers

The use of a gouging abrasion test to evaluate the wear of jaw crushers is revised in terms of its procedures, considering the effects of the most significant variables, such as the minimum amount of crushed material and the minimum opening between the jaws during the crushing cycle (minimum discharge aperture). A correlation between the work hardening of jaws and the amount of crushed material is presented. The wear of stationary and movable jaws is compared, being the results dependent on the jaw’s material and the discharge aperture. The abrasiveness of several rocks was evaluated, showing a good correlation with their Mohs hardness.


Introduction
Around 40 per cent of the total energy used in mineral processing operations is due to the crushing and grinding steps. The wear of crushing and grinding media corresponds approximately to 50 per cent of process costs in these operations (Radziszewski, 2002;Aldrich, 2013).
In this context, it is worthwhile to mention that there only exists relatively scarce literature on the wear of jaw crushers. Basically, most of wear data was produced by two research groups: the US Bureau of Mines (closed in March 1996) in EUA (Blickensderfer et al., 1985;Tylczak et al., 1999;Hawk et al., 1999), and Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia (Sare et al., 1980;Sare and Arnold, 1989;Sare and Constantine, 1991;Sare and Arnold, 1995;Sare and Constantine, 1997).
Before these publications, the Climax Molybdenum Company presented significant investigations for the interpretation of results obtained in tests to estimate the wear of jaw crushers (Borik and Sponseller, 1971a;Borik and Sponseller, 1971b).
The contributions of these research groups promoted a consolidation of an ASTM standard for a gouging abrasion test using jaw crushers, edited for first time in 1997 and reapproved in 2002, 2007 and finally in 2013.
More recently, a research group from Chalmers University (Lindqvist and Evertsson, 2003) studied the wear of jaw crushers, but their goal was to transfer a developed wear model for cone crushers.
As austenitic manganese steel is widely used as material to manufacture jaws. Some recent investigations de-served attention for the microstructural aspects of this alloy and its performance being tested in terms of wear (Silva, 2004;Olawale et al., 2013;Magdaluyo et al., 2016). Considering the wear performance of a wide range of materials and hardfacing deposits during gouging abrasion test, the findings described by Llewellyn et al. (2006) can be considered a very useful guide.
In Brazil, academic studies were conducted at the University of São Paulo, being the first one a thesis (Siriani, 1973), whose detailed data was extracted directly from mining operations, and two Master's Dissertations (Pintaúde, 1998;Bartalini, 1999), which used a small machine, a lab prototype of a jaw crusher. This article is a historical summary of the main contributions given by these researches for understanding wear of jaw crushers.

Materials and methods
Three sets of experiments were performed using a small lab jaw crusher, with sample dimensions of 135 mm X 75 mm X 25 mm. For the first and second ones, the same crushed material, granite with particle size between 1" and ¾" was used. Details of its mineralogical characterization can be found in Pintaude et al. (2001). In these experiments, the work hardening of worn surfaces was evaluated for different amounts of crushed material, to establish a minimum of cycles to measure wear in the steady-state regime. The procedures to determine hardness were described by Pintaude et al. (2003). In the second set, two materials were used for this, a high silicon (2Cr-1.5Si-0.5Mo) and a manganese cast steel, evaluating the effect of the closed side set (CSS) -the minimum opening between the jaws during the crushing cycle (minimum discharge aperture). Finally, the third set of experiments was carried out varying only the crushed material, to verify the abrasiveness of rocks on the wearing of the manganese cast steel. Mohs hardness of each rock was calculated using a law of mixtures, considering their mineral contents. Table 1 shows the main parameters in each set of experiments.   One can clearly observe that the wear reached a steady-state regime when the values of microhardnessa at the worn surfaces did not present any variation. In this fashion, the description made by Blickensderfer et al. (1985) for a minimum crushed material to reach a steady-state regime of wear in a similar lab equipment can be considered as suitable. They recommended a minimum of 90 kg, while ASTM G81-97(2013) requires 900 kg, but in this case the dimensions of the equipment are higher.

First set of experiments
The extension of plastic defor-mation was also evaluated, as shown in Figure 2. One can observe that the level of deformation is sufficient to alter up to 1 mm below the worn surface. Similar results were reported by Borik and Sponseller (1971) for austenitic manganese steel.

Figure 2
Extension of plastic deformation of a stationary jaw (high Si cast steel) after tests crushing granite for CSS = 3.2 mm. Figure 3 shows the variation of wear rates in terms of CCS (mm). In a general way, the smaller the CSS, the higher the wear. This is an expected result (Borik and Sponseller, 1971), since the compression forces increase with the reduction of discharge aperture.

Second set of experiments
In addition to the effect of CSS on the wear rates, other observations can be discussed from this set of experiments. The first one is related to the tested materials, which were dependent on the position of jaw. High Mn cast steel performed much better when a stationary jaw is considered, but for a movable one, the high Si cast steel presented a better behavior in terms of wear resistance. This difference explains, from the numerical point of view, the wear of stationary jaws in relation to those determined for the movables, which was much superior in case of the experiments conducted using high Si cast steel.
The wear of a stationary jaw tends to be increasingly higher than that determined for movable side as the CSS diminished. In an indirect way, one can conclude that the increase of compression forces can cause an increase on the gouging action on the stationary jaw, but this extension will depend on the tested material. More accurate conclusions with this issue could be treated in future investigations, helping to select in a better way for the materials with different jaws.  Figure 4 shows the relationship between the wear rates and the Mohs hardness of several rocks, tested under the same conditions. Good correlations are found (R2 ~ 0.85), being the increase of hardness, consequently the abrasiveness, attributed to the increase of quartz content. This relationship was pointed out previously by Siriani (1973) and confirmed here in another way.

Conclusions
This article summarizes results obtained in a gouging abrasion test obtained with a lab jaw crusher machine. The following conclusions can be presented: -T he minimum amount of crushed material to achieve the steadystate regime of wear was closely related to the work hardening of the worn surface; -The wear rate of jaws increases for smaller close sided sets, but this variation depends on the material of the jaws; and -A gouging abrasion test can be used to verify the abrasiveness of rocks, and the quartz content is an important factor for this issue.