Pilot scale evaluation of mine water (MW) as a cooling medium

Sasol One abstracts large volumes of water from various sources, such as the Zuikerbosch & Vaal River for various applications including make-up to the cooling water systems. In an attempt to evaluate the use of underground MW as an alternative cooling medium, two pilot-scale cooling towers (CTs) were used. Data of five runs (3 weeks each) at different linear flow velocities (LFVs) and cycles of concentration (COC) were obtained. A prescribed chemical treatment program from a local supplier was also evaluated. Mild steel corrosion coupons and heat exchanger tubes were used to monitor the fouling, scaling and corrosion rates. An experimental design program was followed to determine the layout of the different experimental runs. The respective indices indicate the presence of low scaling and corrosion rates in the CTs which correspond well with that obtained from the analyses of the coupons and heat exchanger tubes (1.5 to 11.0 mg/dm2/d and 0.04 to 0.21 mm/y, respectively). In addition, the fouling rates (2.8 to 27 mg/dm2/d) obtained during the five runs indicate the presence of sessile bacteria in the system. According to the supplier, the total cost of the chemical treatment program used during the pilot-scale test work is comparable to the current cost for the commercial cooling systems at Sasol One. It is evident from the results obtained during this study that the use of underground MW as a cooling medium is a viable option. However, the work conducted in this study was only a first effort and it is possible that the high levels of sulphate (up to 966 mg/ l) in the make-up water could result in concrete corrosion, which could have a detrimental effect on the integrity of the cooling tower structures. In addition, the variability in the MW quality, which was not considered during the experimental period, could be problematic since it will complicate the chemical treatment of this water in a full scale cooling tower system.