Coal Handling and Preparation Plant (CHPP) Corrosion Control and Management

The primary objective of this work was to contribute to a reduction in the maintenance costs and production downtime that can be directly attributed to corrosion problems found at the Coal Handling and Preparation Plants (CHPP's). The report provides an insight into the factors that are driving corrosion problems at the CHPP's and recommends some of the methods and strategies that can be used to reduce/eliminate corrosion related damage/deterioration of CHPP assets.

The project investigation established corrosion maintenance costs for key equipment/structures at two Bowen Basin and two Hunter Valley mines.

The maintenance cost analysis for the four selected sites revealed the average costs due to corrosion for three of the plants to be approximately $2.0M/annum and about $0.7M/annum for the fourth plant. The main driver of corrosion cost is the structural steelwork used in buildings and mechanical equipment support framework. The majority of these costs are derived from the coal preparation building. The second corrosion costs driver was the replacement of water and air pipework.

The major factors driving corrosion problems at the CHPP's were investigated and have been identified to be as follows:

1. Time/degree of wetness present at the plant. The proportion of total time during which the surface is wet is considered to be the most important factor driving corrosion at the CHPP's.
2. Plant maintenance. The practice of regular washing the build-up of deposits from the plant structures, and the assurance of minimal spillage of process water was found to substantially increase the service life of plant structurals (by up to 100%).
3. Corrosivity of CHPP water supply . This was found to vary significantly subject to mine site geology, hydrology and water management practices. The key findings derived from the mine site water samples analysis are as follows:
• Increased recirculation of mine water and shortage of fresh water supply due to low rainfall has led to increased corrosiveness of the CHPP water supply at the Bowen Basin mines.
• Recirculation of CHPP water without any treatments or dilution by the water from other mine site water storage dams resulted in sulphate and chloride concentration increases of up to 150% when compared with the water stored in the main storage dam.
• The water supplied to the CHPP's at the four selected sites is largely alkaline; however low pH (2.5 ? 4) acid waters have been reported at a number of other sites.
• The highest recorded chloride concentration in the CHPP supply water was about 3,500 ppm. This is about 1/6th the chloride concentration of the sea-water.
• In general, low pH water was found to have elevated concentrations of sulphates.

4. Corrosivity of localised environment . The analysis of water and sludge deposit samples collected across the plant has shown significant increase in the corrosiveness of CHPP spillage water and sludge deposits compared with the main water supply. Chloride and sulphate concentrations of up to 11,000 mg/l have been reported (about 10 ? 15 times that of the raw water supply system) with water conductivity readings rising to levels similar to that for sea water. These results clearly indicate that past practices of describing the plant corrosive environment by sampling wash water/raw water supply have been grossly underestimating the corrosive conditions present at the plants.
• Plant design . The extent of the design issues observed varied between the plants. The older sections of the plant were found to have the most numerous design and fabrication related problems.

Improving any one of the above factors can contribute to an improvement in the service life of the CHPP structurals, but optimal results may be obtained by improving all five.

The control of corrosion at the CHPP's has been investigated under the four basic corrosion control methodologies. These are:

1. Modification of operating environment.
2. Minimising the impact of corrosion through design.
3. Use of cathodic protection/electrochemical control methods.
4. Use of protective coatings.

In addition to recommendations of potential technical solutions and experience in other industries, this report also provides sets of guidelines that combine good practice in design, surface preparation and coating application in order to achieve the optimum corrosion protection of structural steel. Other resources of expertise and reference to relevant standards have also been provided, from which further advice can be obtained to minimise corrosion problems.

The project has significantly improved the knowledge and understanding of CHPP corrosion issues through a series of systematic and detailed investigations. The recommendations given were formulated based on this new knowledge and understanding. The implementation of these recommendations should significantly reduce the maintenance and downtime cost of the CHPP structures.