Chemical analysis of process water from concentration tests
Anna Tornivaara & Päivi Kauppila, Geological Survey of Finland, P.O. BOX 1237, FI-70211 Kuopio, FINLAND, anna.tornivaara(at)gtk.fi
In ore processing water is one of the major and most important commodities. Groundwater or surface water is typically used as process water in mineral beneficiation and hydrometallurgical operations. The composition of the process water largely depends on the ore mineralogy, the applied process chemicals (such as flotation reagents, modifiers, flocculants/coagulants, hydrometallurgical agents, oxidants) and processing techniques (Lottermoser 2007). Common practice is to discharge process water as a slurry with tailings to the tailings pond and eventually to the decant pond. Water from the tailings pond can be recycled back for further use in processing after dilution, or after water treatment.
Laboratory scale testing (i.e. processing/beneficiation tests or pilot processing) during feasibility studies can provide valuable information for the environmental characterisation of process water and mine wastes (e.g. by studying the quality and the chemical content of the process and tailings water). These laboratory tests usually target at the selection of the most beneficial concentration method. The result of these tests can also be used to provide data for planning the design of mine waste facilities and water treatment, if sufficient characterisation of waste materials and process/waste water is included in the beneficiation tests. For example, information for waste and water management regarding liner permeability, dam structures, and seepage collection design can be produced. Characterization results can also aid in modelling of the quality of effluents or the impacts of the waste disposal on the surrounding environment in the short and long term, and in evaluating utilisation potential of the process water and the need for water treatment.
Description of the method
Process water and/or tailings water samples are collected during beneficiation tests or the test piloting using normal water sampling procedures. Vacuum filtration can be used to remove the solids from the samples prior to sampling. During sampling, it is recommended to measure pH, EC, Redox and alkalinity using portable field meters.
The recommended set of analysis for process and tailings water includes:
- Electric conductivity, pH, Redox, and alkalinity
- Chemical composition
- Total and dissolved ions (trace metals, rare earth elements (REE), major cations and anions)
- Residues of process chemicals
- Radionuclides and radioactivity (if mineralogy suggests that these measurements are needed)
- Suspended solids
Sampling equipment, measurements and methods must be suitable for these purposes. Additionally, the selected laboratory and analysis methods have to be commonly approved (e.g. certified and standardized). Sampling and water quality measurements should be conducted as soon as possible after the beneficiation test is completed to avoid any changes in water chemistry. It should be noted that the concentration tests should provide enough water for all the laboratory determinations.
It is recommended to repeat the water quality measurements if the settings and parameters of the concentration test change substantially.
When the mine is in operation, process water test results can be compared with the results of the concentration test to evaluate the benefits and accuracy of the early modelling and readjust the water treatment if needed.
Chemical analysis of the process water and tailings water from the beneficiation tests can be used to assess the drainage quality from tailings and the water quality from ore processing. The mine planning phase is the primary stage for estimating process and tailings water quality. Pilot testing results provide valuable information for mine water recycling, water treatment, and waste facility components of mine planning. Additionally, these results are necessary for the environmental risk assessments. The acidity of tailings water is affected both by the tailings compostion and the applied ore processing techniques. For example, the dissolution of gold using cyanide solutions is conducted under alkaline conditions, while the hydrometallurgical extraction of copper, nickel and uranium is based on the use of sulphuric acid under oxidizing conditions (Lottermoser 2007).
Advantages (e.g. INAP 2009):
- Water quality data which is already available at the design and construction stage of the mine can reduce the footprint of the mine and provides data for planning ARD prevention and water treatment.
- Results can be integrated into the mine waste facility design to ensure that the economic model for mine development fully considers the true costs of the whole lifecycle, including post-closure.
- Possible to save in fresh water cost as the process water quality is estimated in advance.
- Provides information about the feasibility of the direct use of the process water, as well as selection of treatment methods to remove undesirable constituents from the water.
- In some mine localities fresh water can be critical and early investigations of water quality help define water treatment/purification methods. This provides a better basis for the reuse and recycling of process water.
Disadvantages (e.g. Lottemoser 2007)
- Modelling in an early stage is only indicative; as the composition of the tailings water will change over time, especially if mineral oxidation processes are allowed to occur in the waste facility.
- It is difficult to predict long term changes in tailings water caused by natural bacterial, chemical, or photolytic degradation processes.
- Evaporative concentrations cause secondary mineral precipitation at and below the tailings surface, elevating the concentrations of various elements and compounds in the tailings water.
- Dilution by rainwater is difficult to estimate.
Recycling of the process water will gradually increase concentrations of the conservative elements/compounds (such as SO4 and Cl) in the process water.
INAP 2009. The GARDGuide. The Global Acid Rock Drainage Guide. The International Network for Acid Prevention (INAP). http://www.gardguide.com/ modified 2014. Read 17.7.2014.
Lottermoser, B.G. 2007. Mine wastes – Characterization, Treatment, Environmental Impacts 2nd ed. Springer. 304 p.