Coal Preparation » General
Maintenance accounts for 41% of the washing costs in coal preparation. Sieve bends and centrifuge baskets are among the most costly items in the maintenance area (ACARP project C8063), and these costs are associated with their relatively short life due to abrasive wear. In most situations, sieve bends and centrifuge baskets have to be replaced in less than 8 weeks. In addition to the large new operating expenditure and maintenance labour cost, plant designs make the sieve and basket changes difficult and therefore give rise to many OH&S issues.
The objective of this project was, therefore, to extend the lifetime of sieve bends and centrifuge baskets by using tungsten carbide HVOF coatings which provide a substantially greater wear resistance than the stainless steel substrate. The potential benefits to coal producers are reduced maintenance costs, reduced plant downtime and reducing the risk of potential injuries caused by maintenance activities on the parts.
The wear-resistant performance of 4 HVOF coating powders, WC-12Co, WC-17Co, WC- 10Co-5Cr and Cr3C2-25NiCr, were examined using small specimen testing. The small specimen wear testing indicated that WC-12Co, WC-17Co and WC-10Co-5Cr performs well for resisting abrasive sliding wear and erosion wear with small particles.
WC-12Co is the hardest and the most effective wear resistant coating. WC-17Co is considered the most suitable coating for applications involving situations where a smooth surface is required. For situations with a low pH, WC-10Co-5Cr may be the most suitable coating. WC-12Co,WC-17Co and WC-10Co-5Cr were selected for plant based trials. The WC-12Co coating remained effective after 11 weeks in service.
Approaches or measures for avoiding excessive aperture reduction and severe pegging due to HVOF spraying have been identified or developed:
- garnet grit of 30/60 grade should be used for surface preparation to avoid grit pegging in the aperture slots
- special care must be taken during handling so that grit blasted surfaces do not get contaminated before coating
- the coating thickness is preferably in the range of 200 to 250 μm to avoid excessive aperture reduction
- the HVOF operation should be carried out by a robotic hand or other equivalent equipment to ensure uniform coating and keep the spraying gun perpendicular to the surface all the time
The high hardness of WC based coatings make them brittle, and hence high impact stress or coated surface bending and flexing would cause cracks in the coatings, leading to a short coating life. Further work may be warranted to address these issues, eg:
- Design and implement a feed chute with a low particle velocity and even feed distribution to reduce impact wear
- Improve the stiffness of sieve bends and centrifuge baskets