Effects of different tailings handling methods on water chemistry changes

Abstract

In mineral processing plants, water is returned to the process either after a quick solid-liquid separation by thickening and filtration, or after a long-term solid-liquid separation by the natural settling of solids at the tailings dam. Each one of these water circulations and recycling are fundamentally different from the solid contact/content and retention time perspectives. As a result, water chemistry experiences changes from different tailings handling systems and it is important to know how different dewatering scenarios affect the water chemistry or properties. To achieve this goal, two tailing slurry samples (one from the final tailing and another one from the intermediate tailing) were prepared from the flotation process of a mineral processing plant in Finland where the final tailing is directly transferred to the tailing storage facilities for water recovery which is a type of dewatering based on the long-term and natural settling of solid particles at the tailings dam. Different four subsamples were prepared from these two samples to simulate short and long circulation loops where water is returned to the process either by the thickening process and some suspended solids within it or by the filtration process where the water is separated from solids immediately and less suspended solids compared to the thickening are present in it or as an aqueous phase at the tailing storage dams with a direct contact with solids and higher retention time. After that, subsamples were kept under two different ambient temperatures (5 oC as the water temperature in the tailing dams in winters and 35 oC as the water temperature in short loops or in hot summers in long loops) for a two-month period to monitor and measure water chemistry changes over time. Various parameters such as pH, SPC, ORP, and DO, TSS, elements and ions concentration, alkalinity, hydrocarbonate, chlorine, nitrate, and sulfate were measured during this project.

The results showed that water samples related to short loops in the high ambient temperature have higher SPC, lower pH, higher ORP, and higher DO values over time. Furthermore, the contact of water with solids in long loops leads to the same pH value (about 7) for both temperature conditions after two months. On the other hand, the pH value of water samples in short loops (thickener overflow and filtrated water) was dependent to the ambient temperature and it was reached to about 3 at the high ambient temperature while, it was about 7 at low ambient temperature. The variations of ORP values for all water samples was the same at the beginning of the experiments but the changes appeared after about 10 days, and the water samples of long loops (final and intermediate tailing slurries without any change) had lower ORP values after two months. It was concluded that water samples related to the short recycling loops and the high ambient temperature conditions experienced the highest ORP variations over time. Water samples in short loops had lower DO and higher COD levels over time.

The variations of total and soluble concentrations of various elements in different water samples were measured over two months. It was found that there are the same trends for the variations of the total and soluble concentration of K, Ca, Na, S, Mg elements in different water samples and their concentration did not change significantly for about 41 days but they experienced some reductions after a longer period (two months) which depended on the type of water samples and the ambient temperature. After two months, the filtrated water samples had the highest reduction in the total concentration of K, Ca, Na, S, Mg elements while, the water in tailing slurries thickener underflow samples where it stays in contact with solids experienced less reduction over time. The variations of total concentrations of Al, As, Fe, and Si were the same over time and their concentration was low at the beginning and after an increase over time, their concentrations finally dropped. However, the soluble concentrations of these elements had different patterns and the water samples of short loops (i.e. the filtrated water and thickener overflow) experienced an increase on the soluble concentrations of these elements after 41 days while, other water samples had the same and low concentrations over time.

The alkalinity and hydrocarbonate levels of all water samples at the beginning were similar to each other but it was realized that they increased after two months at samples that the water has direct contact with solids (i.e. the tailing slurries and thickener underflow at the high ambient temperature). The nitrate and chloride levels of all water samples reduced after two months.

It can be concluded that the type of water circulation loops in the mineral processing plants could have huge effects on the water chemistry and its properties. As it was shown in this research, some changes in the water parameters such as the total and soluble concentration of different elements, pH, SPC, alkalinity level, and hydrocarbonate level could happen in a long-term process and it is crucial to be aware of their impacts on the process.