Mining activities can generate large volumes of effluents due to the extensive areas they occupy and the large volume of water required for mineral processing.
Mining effluents can be acidic, neutral or basic, contain high levels of suspended solids and sulphates, as well as being characterized by the presence of metals such as arsenic, iron, manganese, mercury, lead and even radioactive metals such as uranium.
The chemical composition of mining effluents can be quite varied and contain various components, which requires the search for appropriate strategies to ensure that they are returned correctly to the environment. This search usually involves a number of stages and different processes, bearing in mind that each mine and processing plant has its own particularities and there is no ready-made solution. The best solution for the correct management of effluents will be a set of integrated actions and the first stage in the search for these solutions is a detailed diagnosis.
This stage involves understanding all the transformation processes that take place within the boundaries of the enterprise and all the internal and external flows of solids and liquids, as well as evaluating monitoring data on water, effluents and solid waste and gathering existing information, including operational routines, raw materials and inputs used.
The monitoring data must be associated with the plant’s activities, so that all intervention needs and opportunities for improvement are identified. Also in the diagnostic stage, it is necessary to identify future scenarios and changes that will be carried out and that should be considered, such as the relocation of tailings, other planned or ongoing projects, among others. In addition, this stage may identify the need to carry out additional analyses to better understand the effluents and tailings.
If situations that will change over time have been identified in the diagnostic stage, future scenarios must be predicted.
For example: depending on the complexity of the scenario and the types of tailings to be disposed of, it may be necessary to foresee the effluent that will be formed from the leaching of the piles.
Another point would be to understand and consider future projects that are underway and will impact on effluent generation in some way.
After understanding the current and future scenarios, we can then identify the points that need intervention and think about solutions. The solutions package includes any actions that can help solve problems or are opportunities for improvement, such as: identifying possibilities for reusing and redirecting water and material flows, actions that can reduce the volume of effluents, optimize the use of water, segregate effluents and others.
After the search for rearrangements and minimization of effluent generation, we reached the ideal point for assessing the need to install an Effluent Treatment Plant (ETP).
With the flow rates, the well-sampled chemical composition, the route and the points of contact between the effluent and the environment, it is possible to know which parameters need to be treated and at which points this effluent should be treated.
The search then begins for appropriate treatment technologies depending on the final quality of the water to be obtained. In addition to the parameters that need to be treated in the effluent, which are the parameters of interest, the concentrations present in the raw effluent are also extremely important when choosing technologies.
Treatment methods are also listed according to the area available for treatment, the expected time of use of the WWTP, accessibility of the plant or site, costs, type of labor required, among others.
Based on this, treatment routes are proposed that can be tested in order to simulate the treatment of that effluent in the laboratory and/or on a pilot scale.
Treatability tests are important to verify the efficiency of the proposed treatment routes in reducing the concentrations of the parameters of interest, defining the best treatment route to achieve the desired final quality.
Treatability tests can also be used to optimize the performance of already installed wastewater treatment plants, improve operating conditions, make it possible to replace chemical products and so on.
In this way, several technologies can be tested in series, simulating a complete treatment route.
In addition to the different technologies, different chemical products, dosages, operating times, application rates, speeds, etc. can also be tested.
Generally, treatability tests involving physical-chemical treatment stages are associated with reactor tests, most commonly jar tests, which allow several tests to be carried out simultaneously.
Reactors with a controlled agitation system are needed, preferably transparent ones, because visual observation is also important.
As previously mentioned, mining effluent is rich in metals in relatively high concentrations that most often require at least one stage of physical-chemical treatment to be removed.
Among the most common treatments tested in the laboratory for mining effluents are precipitation, with or without the addition of an oxidizing agent, coagulation, flocculation, flotation, sedimentation and various types of filtration, including sand and zeolite filtration.
Treatability tests of various other techniques can also be carried out, such as: limestone beds, wetlands, membrane filtration, adsorption in various media, as well as aerobic and anaerobic biological treatments, such as activated sludge, biodisc, biological filters, among others.
At the end of the tests, it is possible to identify the most efficient treatment route in terms of removing the parameters of interest. This way, in addition to defining the route, information will also be available on the operating parameters, such as reaction, settling and hydraulic detention times, speed, dosage of chemical products, temperature, volume of sludge formed and its concentration, among others.
The time needed to complete the treatability tests and analyze the results lasts an average of 60 days, which can be shorter or longer depending on the technologies used. This is a very important stage of the project, which will form the basis for all subsequent stages.
The gains are notable, since after analyzing the results there is a greater likelihood that the proposed route is efficient in removing the parameters of interest and is aligned with the desired end objectives. This is an indication that when the WWTP is installed, it will have a good chance of adequately treating the effluent in question.
See other related Water Services and Technologies webinars
Author
Raquel Annoni Martins
A chemical engineer with a master’s degree in metallurgical and mining engineering and a doctorate in sanitation, environment and water resources, she has 20 years’ experience in environmental projects, R&D and industrial effluent treatment. She is currently a member of the Geochemistry team atΒ Water Services and Technologies.
