Air movement on and around tailings storage facilities on the Highveld of South Africa

Abstract

Dust from gold tailings storage facilities (TSFs) is problematic in certain areas of the South African Highveld. Many of these localities around TSFs are densely populated, which often result in tailings dust being a nuisance and health hazard. Mines spend large sums of money to mitigate dust from TSFs due to the legal requirements and responsible conduct towards people and the environment. Implementation of these dust suppression measures is often not entirely successful, that may cause financial resource loss and legal liability for mines. The focus of this study was to acquire data that would contribute towards illuminating the understanding of airflow dynamics over a TSF. By understanding the fundamental concepts of wind movement on and around these facilities, environmental managers can optimise dust management efforts with the limited resources available to them. A review of literature in this field indicated that very little research has been done on airflow patterns on specifically gold TSFs. This study also revealed that airflow dynamics are inherently complex and difficult to predict. A study site was selected on the South African Highveld that presented suitable conditions for this research. The selected facility was the Chemwest 5 TSF near Klerksdorp in South Africa. In order to investigate airflow over this TSF, twelve wind-monitoring units (WMU) were constructed and placed on the facility for a period of eleven months. The WMUs were composed of RM Young (model 05103) wind monitors and Campbell Scientific CR200X data loggers. Wind speed and direction data were continuously collected on the facility. The WMUs were placed at different locations on the TSF, which encompassed different levels of wind exposure and elevation differences. The data was analysed for wind speed correlation between TSF WMUs and a reference site, wind velocity profiles at each WMU and the statistical significance of wind direction classes measured during the study period. Data from this study was further used to develop a decision-tree analysis (DTA) system that can be used to predict high wind speed events, before such an event occurs. Data analysis illustrated that significant wind speed and direction variations occurred for different locations on the Chemwest 5 TSF, relative to a reference wind profile. Wind roses and wind direction class frequencies of each WMU illustrated that, for much of the research site, there was an over representation (greater frequency that for the reference WMU) of wind from the north. This likely indicates that the geometry of the facility redirects airflow from a range of natural incoming wind directions. Chi-square analyses of the possible 256 wind direction combinations that could take place between a reference site and the TSF indicated that for only eight combination classes, the cause was not associated with the location of the WMU on the TSF. The Decision Tree Analysis (DTA) software assessed wind speed (mean and maximum), direction, air temperature and relative humidity. The model was tested against a dataset from the research site and found to be able to predict high wind speed events in 63% of cases. The model accuracy is therefore not adequate for implementation. Monitoring of additional atmospheric variables could improve the accuracy of the model. Susceptibility of the tailings material to wind erosion was also assessed by means of wind tunnel tests. Structureless samples (from three different slopes) with low moisture content were placed in a wind tunnel and exposed to different wind speeds. Mass-loss measurements were made for the samples at different wind speeds and a minimum wind speed threshold velocity (ut) for the tailings material was calculated. The analysis identified the ut-value to be 3 m.s-1. Tailings crusts were identified on the Chemwest 5 TSF. Since these structures offer resistance to wind erosion, their compositions were investigated. Genesis processes by which the crusts originated were also investigated by means of a scanning electron microscope (SEM). Results found that three different crusts were present on the site: physical crusts that originated from the settling of fine particles in basins; erosion-induced crusts that originated by the removal of the erodible fraction of the material; and a chemical crust that resulted from the precipitation of secondary minerals. The structure of these crusts, especially the crust that resulted from the settlement of fines, indicated that the crusts should offer resistance to wind shear. The study concluded that the geometry of the Chemwest 5 TSF influenced the air movement characteristics on the facility, but that it was not significant enough to ignore the natural wind profile of the area in wind mitigation planning. The study also identified the presence of crust structures on the TSF that could increase surface stability and decrease dust. If these crusts were not present, the ut–value of the TSF would have been 3 m.s-1, as identified by the wind tunnel study. Dust would therefore have been more prevalent, given the frequency with which the wind speed of the study site exceeded the stated threshold value (41.5% of the observations)