The Strength of Interfaces in Metallurgical Coke and its Influence on Coke Abrasion

This project applied tribological and scratch testing techniques to metallurgical coke samples to determine:

• The abrasive strengths of the coke textural constituents; and
• The strength of the interfaces between the inertinite maceral derived components (IMDC) and the reactive maceral derived components (RMDC), as a function of the properties of the parent coal(s).

These parameters were quantified via the application of advanced microscopy techniques and then related to fundamental coal properties, including rank, measure, petrographic composition, and grind characteristics.

During tribological testing, a stationary pin or ball indenter is under a controlled load in contact with a rotating polished block of the material being tested. The wear track is then analysed to determine the degree and nature of the damage to the surface. The wear that occurs in rotational tribology tests is due to the progressive loss of surface material at the points at which the two surfaces (the polished block and the indenter) come into contact as they rub against each other.

One of the key measurements that can be obtained from tribological testing is the coefficient of friction (COF). The frictional force between two opposing surfaces, i.e. the polished coke block and the indenter, shows their resistance to relative motion, and indicates the susceptibility of the coke to tribological wear. A COF value of zero indicates a frictionless surface. The higher the COF, the greater the efficiency in transferring mechanical energy to the coke that can weaken or break it up.

The key outcomes of this project are:

• The coefficient of friction (COF) was contrasted between cokes of different coal origin. The amount of ultrafine material produced by the cokes during continuous rotational tribological testing increased as the rank of the parent coal or coal blend increased. We hypothesise that these ultrafines are mostly graphitic in nature. Importantly, the COF for the coke samples from coals with a mean maximum vitrinite reflectance above or equal to 1.38% began to decrease slightly beyond the first 60 seconds of testing, whereas it continued to increase for the remainder of the cokes tested. We speculate that this reduction in COF is due to the graphitic ultrafines acting as a lubricant. By acting as a lubricant, these graphitic ultrafines would change the nature of coke surface as the tribological test progresses.
• The peaks of acoustic emission profiles generated during coke scratch testing were linked, via their relationship with frictional force, to the energy release, dispersal or absorption on coke fracture. Characterisation of the acoustic emission peaks led to the identification of four distinct “signatures” which were used to classify these peaks primarily by their relationship with frictional force.
• A robust quantitative approach was developed to link the acoustic emissions to the texture or textural interface at which they occurred, the main mechanism of damage at that location, the severity of the damage, and the loading force. This was achieved via the application of high resolution microscopy techniques, including scanning electron microscopy, 3D laser scanning microscopy, petrographic imaging, and optical microscopy.

The strength attributes of cokes were related to coal properties, demonstrating that tribological and scratch testing techniques can be used to distinguish between cokes of different coal origin. The key findings are:

• IMDC abrasion is insensitive to parent coal rank.
• RMDC abrasion is sensitive to parent coal rank.
• RMDC fracture mechanisms are insensitive to parent coal petrographic composition.
• Sole-heated oven cokes provide a reasonable analogue to pilot-scale oven cokes. They showed slightly more damage in tribological and scratch tests.
• Coal blending was found in most circumstances to produce stronger RMDC-IMDC interfaces than were obtained in the cokes formed from the constituent single coals.
• Tribological and scratch test results replicated the different dependence of coke strength (measured by tumble drum indices) on IRF grind observed between high rank Moranbah and Rangal coals in previous ACARP studies, providing a potential new probe to understand the reasons behind the effect.

The next step would be to use these findings to identify a path to help improve coke strength prediction and coke resistance to abrasion in the blast furnace. This would help to improve the accuracy of models used by the coal technical marketing industry to predict the value of their coal and coke products.