Coal Preparation » General
This project gives details of a study of the wear and performance of DMCs from the Hunter Valley and the Bowen Basin. The project consisted of information gathering, in-situ monitoring of DMCs, and ex-situ failure analysis of DMCs.
Due to cost factors there is a limited number of wear materials used for DMCs, namely Ni-hard cast iron or alumina tiles.
The service lifetime of DMCs is variable, with complete change out, due to wear, occurring at anytime from 12 months to more than 8 years. Alumina lined DMCs have a lower wear rate and less change in internal dimensions than Ni-hard DMCs for the conditions examined here (Hunter Valley and Bowen Basin). The longer lifetime and consistent wear translate to cost savings and efficiency, respectively. However, there is a reluctance to use alumina due to its catastrophic failure mode and the inability to patch/repair. Catastrophic failure due to tramp material dramatically reduces the lifetime, with the worst case reported at 3 days.
Three failure mechanisms were identified:
- Catastrophic failure due to overload (Ni hard and alumina)
- Erosion-corrosion and cracking (Ni-hard cyclones)
- Abrasive wear (Ni-hard and alumina)
Catastrophic failure comprised less than 5% of the cases studied; however, catastrophic failure caused many tens of thousands of dollars of lost revenue production time. It has been concluded that the conditions for catastrophic failure can be avoided.
Erosion-corrosion due to inclusions (large inhomogeneities in the material) was observed in greater than 50% of the Ni-hard DMCs indicating poor metallurgical control by the manufacturer. Better control over the composition, the casting cooling rate, and the subsequent heat treatment are necessary for optimum wear life.
Improvements in DMC lifetime and performance can be made in several ways.
- The DMC manufacturers can optimise the wear materials for the mineral impurity (ash) content of the coal. The wear rates can be minimised by controlling the processing and composition of Ni-hard and alumina at the manufacturing end. The alumina tile wear rates can also be minimised by reducing the angle between the tiles (smoothing the joint between tiles).
- Plant managers can specify the materials characteristics to the manufacturer (e.g. Ni hard cast iron with uniform fine dispersion of carbides less than 20mm grain size) instead of specifying just the material (e.g. Ni-hard). In this way, the microstructure of the Ni-hard is matched with the ash size, reducing the wear rate and increasing the corrosion resistance.
- Non-destructive evaluation techniques such as ultrasound or X-ray may be used to detect large inclusions in Ni-hard cyclones prior to installation.
- Plant managers can screen to control the size of tramp entering the cyclone.
- Plant managers can monitor and control feed pressure at the inlet.
A trend that was evident from our interviews was that cyclone performance and wear monitoring was not carried out as regularly as may be needed to ensure that optimum operating conditions of the DMC are maintained. Another area of concern identified was the possible inaccuracies in measurement of feed inlet pressure into the DMC. Pressure gauges are located up to 4m away from cyclone inlet, so actual inlet pressure could be higher than that measured. This increase in pressure would translate to particles of higher velocity entering the cyclone body and increasing the amount of wear and changing the cut-off condition.
For properly manufactured wear materials, the main cause of corrective or emergency maintenance downtime is due to tramp. For the plant of the future, which will utilise predictive maintenance with one 12 hr maintenance period occurring every three months, both Ni-hard and alumina wear materials are adequate if the materials are properly manufactured and the operating conditions are controlled. If one assumes that a 20% change in internal dimensions causes an unacceptable loss of material to discard [ACARP C10045], then properly manufactured alumina DMCs will perform at least 5 times longer than properly manufactured Ni-hard DMCs. A purchasing decision based on whole of life costing can then be made by the plant manager.
It is recommended that:
- DMC manufacturers be approached to discuss the optimisation of existing wear materials, both alumina and Ni-hard
- DMC manufacturers include microstructure information, wear rates by standard test method, and fracture toughness of the wear materials in their specifications
- Non-destructive evaluation methods for the detection of inclusions be developed in collaboration with the manufacturers such that the manufacturers can use the method for quality control
- Methods to eliminate or at least control the size of tramp be investigated and implemented
- New composite wear materials that may extend wear life whilst eliminating the catastrophic failure mechanism be investigated.