Back to the bench: water treatment optimization through innovation
Ore processing is water intensive and results in the production of tailings. Tailings are a mixture of gangue material, trace metals or other minerals, residual processing aids, and water. For most mine tailings, the water readily separates from the solid fraction once the tailings are deposited in a tailings management facility (TMF). The recovery of water from tailings is essential to mining operations, especially in arid climates with scarce water sources.
In most modern mines, all of the available water in a TMF is recycled. The direct recycling of water from a TMF without treatment is preferred, given the large volumes of water in circulation. If treatment is required, steps should be taken to decrease water usage as much as possible and to optimize water treatment systems to manage operating costs. Bench-scale testing is one tool that can achieve this objective. Smart application of bench-scale testing is a cost-effective way to evaluate various technologies in support of new capital projects, or troubleshoot and optimize existing treatment systems. Bench-scale investigations should not replace long-term pilot programs at scale; instead, bench-scale tests should be viewed as complementary and used to inform the piloting phase.
Test at the source. When dealing with complex mine waters, variations in water quality often occur. These variations may be seasonal—for example, an increase in turbidity due to run-off—or they can be process-related and result in significant changes to water quality in a short timeframe. Testing should be conducted in the field to get the most applicable results. This way, changes to water chemistry that result from shipping samples off-site and sample storage conditions are eliminated. This is particularly important for water that may be saturated with dissolved salts, where fluctuations in temperature can cause precipitation to occur prior to testing.
Challenge the status quo. Typically the standard "jar test" method is the go-to for coagulation and flocculation tests. Jar tests are best used to evaluate the performance of a small number of chemicals when precedent exists for treating a particular water stream. Each individual jar test requires one litre of sample, so there are practical limitations to the number of tests that can be performed in a reasonable amount of time for a given water sample. The volume of raw sample required to run numerous jar tests also limits the number of tests that can be reasonably performed.
At Hatch, we have successfully developed and applied high-throughput methods to test mining waters and tailings. The methodology involves simultaneous testing of up to 48 treatment conditions using small sample volumes (15 mL or 50 mL). The result is a very efficient method to screen multiple chemicals, oxidants, or pH conditions, all while minimizing the size of the raw water sample required and maximizing the number of tests and data collected. This method has proven to be very effective when testing mine waters having complex water chemistry.
Respond rapidly. Portable battery-operated instruments for water testing allow for in-field measurements and timely feedback to guide bench-scale tests. Further, water analytics are benefiting from miniaturization as chip-based sensors are becoming smaller and thus more portable. Innovative solutions are also available for measuring very low concentrations of select contaminants using biological-based sensors. All of this supports the ability to quickly mobilize water-testing equipment and analytical instruments for site-based testing globally.
A case study in reclaim water treatment optimization
Hatch recently completed a high-throughput testing study to evaluate and compare several coagulants and flocculants and combinations thereof against the chemical recipe used at an existing tailings reclaim water-treatment plant in Ontario. The testing program was developed to screen the performance of a wide range of chemicals at various dosages due to the limited data that was available for treating this particular reclaim water. Using a high-throughput testing method, 425 distinct tests using various chemicals at different dosages were conducted in three days. The results of this study were used to guide larger volume tests conducted at the mine site and to evaluate treated water toxicity. The testing demonstrated that more conventional chemicals could be used at reasonable doses, and that switching to the new recipe would result in significant savings in annual operating costs. While not guaranteed to have the same results at other sites, this high-throughput bench-testing method could be used at existing plants to quickly confirm whether dosing levels are optimal and whether costs could potentially be lowered.