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Tailings & water are tightly related. A full recycle of water is a prerequisite for future tailing utilization. Water should be thought as an enabler to give the highest recoveries
About
Imagine a future where tailings are stacked geotechnically stable with minimal environmental effects and in such a way that future utilization of any of the minerals, including gangue, can be performed with reasonable ease. What does it need?
We need to have a change in thinking; we cannot think tailings and water separately. They are parts of the same grand environmental challenge the mining industry faces.
We worked through ore-types and their mineral surface chemistry. We have also conducted many plant studies with Cu-Ni ore to validate the approach we propose.
Water must become an enabler. It is the key to start solving the grand environmental problem. Each sulphide mineral has its own sweet spot where the flotation response is at its best. We realised that in a single water circuit it was not possible to obtain the sweet spots of all the minerals we wanted to float or vice versa stay out from the sweet spot if we did not want that mineral to float. The solution is a distributed, compartmentalized flotation circuit.
This gives huge benefits!
For tailings, the distributed water circuitry is the prerequisite for any beneficiation of the tailings. Any process to treat the tailings can be conceived without any effects on the main operation!
The distributed system eliminates the seasonal variations in the process. It also allows to design a cleaning operation in any of the sub-systems to just keep the effecting impurities at levels where their negative economic effect is the same the cleaning cost. The cleaning need is always the same as rate of impurity accumulation during one circulating pass of the water, usually quite small.
The use of freshwater intake will reduce substantially and the need for effluent release to nothing at its best.
The environmental impact will be huge!
Key Benefits
Our link to tailings is fundamental. We see it an enabler to develop techniques to store tailings in geotechnically stable TSF’s. No tailings challenge can be solved if water is not part of the solution. The environmental impacts of an TSF are mainly related to water effluent effects. In dry climates our solution will diminish water evaporation losses as all water is recycled inside the plant. In climates where the TSF experiences rainfall and thus seepage the water can be collected irrespective of its quality to be used to float f. ex. residual sulphides. For climates where the TSF water balance is positive, technologies can be developed to prevent major rainwater flows to enter deeper into the TSF and thus reduce the need for water cleaning when restoring the water balance at the site. In such cases it is possible to obtain a situation with zero freshwater intake.
The distributed water stewardship protocol is an important prerequisite for any future new possibilities for tailings management. Our approach will allow any kind of residual sulphide removal steps to be performed without affecting the flotation processes of the valuable minerals. It opens even possibilities to perform any other separation step to separate gangue minerals from each other; say for example removal of mica and clays to produce artificial sand or even separating some Al and Mg containing minerals to produce geopolymers.
Our approach will also have other benefits. As an example, gold containing porphyry ore where copper and gold are floated in a basic pH 8, 5-9 circuit and the residual pyrite with sulphuric acid-controlled pH 5 circuit will benefit from an exchange of water after the copper float. In the traditional case water from the TSF with a low pH needs to be taken back to pH 8,5 by adding milk of lime. The sulphate entering from the TSF depresses Au recovery and causes gypsum precipitation problems. By water exchange the consumption all the reagents will diminish and the recovery of gold increases. Similar benefits can be seen in cases like chalcopyrite-sphalerite where the circulation of Cu2+ ions need to be controlled.
Applications
We conducted a study of more than 10000 mining properties. Of them, many of the Porphyry ores, especially the Au variety, the volcanic massive sulphides with Chalcopyrite-Sphalerite-Galena-Pyrite combination, the Layered Mafic Complex ore with Chalcopyrite-pentlandite-Pyrrhotite +PGM´s and the IOCG ores with Magnetite-Chalcopyrite-gold- pyrrhotite –uraninite are ore types which will benefit from our proposal. Also, the South-African Bushveld complex ores with pyrrhotite-PGM may benefit from our ideas.
We have identified several thousand sites possible, where the process will benefit from our ideas both economically and environmentally.
As water does not travel well, we need to pilot the idea at mill sites.
Our approach concerns sulphide ores. All the mining companies have promised to reduce the use of fresh water and increase the safety and environmental impact of their tailings. A challenge is how to handle the outcome of the existing water-tailings stewardship. One single water circuit from comminution to final discharge to TSF causes several adverse effects. These effects become more prominent when the companies try to reduce the intake of fresh water. The main solution has been to increase the water recycling rate. This approach causes the water quality to have marked seasonal variations (hot-cold; wet-dry). For sulphides this means that the pH-Eh-Temp. regime for any of the exploitable minerals may not be close to the regime where flotation is optimal (see f. ex. Lotter et al 2016). The water being in contact with solids during several cycles will change the quality of the water. This again causes the water quality to drift outside the said pH-Eh-T regime. The drift leads either to an increased need for water cleaning or to a decrease in valuable mineral recovery. Both alternatives decrease the economy of the process.
The existing way of studying tailings and water separately misses the need to control sulphur chemistry and sulphide surface oxidation in the process. The balance of oxidative-reductive properties of water in the various phases of the process have not been studied.
Our approach aims to decouple the overall oxidative-reductive cycle into smaller parts, which can be measured and controlled to maintain an optimal pH-Eh-T regime. A single water circuit cannot be chemically modified to suit all different optimal operation regimes the mineral to exploited require. One of our main targets is to get rid of water effluent streams from the plant.