Failure of pump systems operating in highly corrosive mine water at Otjihase mine
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Date
2018
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Abstract
In the Namibian Otjihase underground mine, water is pumped from natural underground reservoirs to the surface using mild steel pumps that have cast iron valves and shaft sleeves coated with a Ni-Cr-Fe alloy. As these components failed very frequently in the highly corrosive mine water environment, it became necessary to provide recommendations for alternative pump materials. The Ni-Cr-Fe coated, carbon steel shaft sleeves were mechanically worn by abrasion in contact with debris trapped in packing glands. The highly corrosive mine water contained solid soil particles, which contributed to internal erosion-corrosion of the pump components. Once the protective coating was breached, the exposed steel corroded rapidly when reacting with the corrosive mine water, resulting in leakages. The main objective of this work was therefore to characterise the steel and cast iron components used in the pump system, determine methods to improve the tribocorrosion resistance of the pump components and recommend a hardfacing material with improved performance in the tribocorrosive mine environment. To simulate and study the synergistic effect of electrochemical and mechanical interaction between the pump components and highly corrosive mine water, the hardness and electrochemical response in synthetic mine water of the following proposed bulk materials were tested: Hastelloy G30, ULTIMET, Stellite 6B and ToughMet 3. Hastelloy G30 demonstrated good corrosion resistance, but had low hardness and poor abrasion resistance. ToughMet 3 had high hardness, but low corrosion resistance. As ULTIMET and Stellite 6B both had high hardness and good corrosion resistance, they were selected for further investigation to assess sliding abrasive wear and tribocorrosion behaviour in synthetic mine water. The possibility of enhancing the corrosion resistance of ULTIMET and Stellite 6 (not 6B) alloys as protective coatings by adding minor amounts of ruthenium was investigated. ULTIMET and Stellite 6 powders were each mixed with nominal 0.3 wt% Ru and nominal 0.6 wt% Ru additions. The powders with no Ru, nominal 0.3 wt% Ru and nominal 0.6 wt% Ru were then thermally sprayed by the high velocity oxy-fuel flame (HVOF) process onto a carbon steel substrate, and compared to a Cr2O3 coated steel as a benchmark. The powders and the coatings were characterised using optical and scanning electron microscopy with energy dispersive X-ray spectroscopy, and X-ray diffraction. Hardness, sliding abrasive wear, and corrosion and tribocorrosion behaviour of the coatings and the substrate in synthetic mine water were then determined. Comparison of the hardness showed that the nominal 0.3 wt% Ru ULTIMET coating had higher hardness than the same coatings with no Ru and nominal 0.6 wt% Ru, the nominal 0.6 wt% Ru Stellite 6 coating had the highest hardness and overall, the Stellite 6 coatings had higher hardness values than both ULTIMET and Cr2O3 coatings. At pH 6, the ULTIMET and Stellite 6 coatings with and without Ru additions had low corrosion current densities and consequently low corrosion rates in synthetic mine water. For the ULTIMET coatings, the corrosion rates decreased as the Ru content increased. The Stellite 6 coating had slightly lower corrosion current densities and corrosion rates than ULTIMET under all tested conditions. Stellite 6 coatings had lower abrasive wear rates at the tested loads than the other materials. The lowest abrasive wear rates were recorded with additions of nominal 0.3 wt% Ru (5 N), and nominal 0.6 wt% Ru (10 N). As expected, tribocorrosive wear rates increased with increasing load for all alloys. Ruthenium additions to ULTIMET and Stellite 6 coatings decreased the tribocorrosive wear rate. The best tribocorrosion resistance was achieved by the Stellite 6 coatings. Stellite 6B bulk samples and Stellite 6 coatings with Ru had higher hardnesses, lower corrosion rates, lower wear rates and lower tribocorrosion rates than the carbon steel substrate, Cr2O3 coating, and ULTIMET bulk material and coatings. Stellite 6 coating with nominal 0.6 wt% Ru exhibited lower corrosion rates at pH 6 and 3 than the ULTIMET coating. Therefore, the Stellite 6 coating were recommended for use in pump shaft sleeves and the Stellite 6B bulk alloy in valves at Otjihase Underground Mine. The cost saving for using the proposed alternative materials was calculated as US$ 8 546.68 (R 128 200.20) per year. This is a major economic incentive for Otjihase Underground Mine
Description
A thesis submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Doctor of Philosophy
Keywords
Pumping system, Corrosive mine wate, Otjihase mine