Mineral processing specialist Multotec believes its continuous ionic filtration (CIF®) process could help change the mining sector’s outlook on wastewater treatment. Peter Middleton talks to Carien van der Walt.
According to Multotec environmental process engineer, Carien van der Walt, continuous ionic filtration technology augments existing solutions such as reverse osmosis, by achieving higher water recoveries while delivering a zero liquid discharge solution.
“CIF® is a significantly improved version of the familiar and widely accepted ion exchange methodology,” says van der Walt, “and it has been tested and proven in treating wastewater in various applications around the world.”
The technology was developed by the All Russian Research Institute: Chemical Technology (ARRICT), originally for uranium and rare earth extraction and recovery. Using ion exchange resins, the technology enables uranium ions to be loaded onto the resin from a solution, and then regenerated in order to produce a concentrated uranium solution. This solution, can be processed even further to produce a saleable uranium product, “and there are now more than 10 uranium recovery plants in Kazakhstan and Russia that have been using the technology advantageously for many years,” Van der Walt adds.
In 2000, the Australian water treatment and metals recovery specialist, Clean TeQ, which is now commercialising the process, obtained the exclusive license to the technology. Some three years ago, Multotec recognised the benefits and became the exclusive partner for the technology in Africa. “It fits in well with our range of solid-liquid separation technologies, such as our centrifuges and filter presses,” she tells MechChem Africa.
Describing how ion exchange technology works, Van der Walt says that ion exchange resins consist of polymer beads chemically engineered to suit specific ion exchange reactions. “The reactions take place on the surfaces and within the porous structure of these tiny spheres. Typically for water treatment applications, an ion exchange resin used for removing cationic elements, starts out with hydrogen ions (H+) attached to the polymeric structure of the beads. When brought into contact with contaminated water containing, for example, calcium (Ca2+) ions, two H+ ions are discharged into the water for each Ca2+ ion that attaches itself to a bead.
Most traditional ion exchange treatment systems rely on a static resin bed, which is laid out similarly to a sand filtration system, the water being passed through the bed, usually from above.
“As the ion exchange reaction proceeds, the resin in the fixed resin bed becomes saturated and then has to be regenerated. This is, therefore, an intermittent batch process that has to be halted at regular intervals while the resin bed is flushed, washed and treated to remove the accumulated Ca2+ ions and replace them with H+ ions again,” Van der Walt explains.
The difference between CIF and traditional ion exchange processes? “The key difference is that we do not use a fixed resin bed. Instead, we are moving ion exchange resins through the system in the opposite direction to the water flow,” Van der Walt responds.
Explaining how the continuous process works, she says: “By moving the resin in the counter current direction to the solution, we enable continuity and we get a chemical advantage by naturally creating a driving force for the loading and regeneration reactions to occur”.
The water being treated enters at the bottom of the first column, called the adsorption column. Cation exchange resin, that is, resin with H+ ions around its surface, enters the exchange column from the top. During a transfer cycle, the fresh resin moves downward creating a concentration gradient within the bed as soon as the contaminated water comes into contact with the resin.
“Because ion-exchange reactions are equilibrium reactions and therefore reversible, Le Chatelier’s principle of dynamic equilibrium can be used to optimise the process. As the water rises up the column and through the resin, it becomes less and less contaminated, while the resin becomes more loaded with ions as it moves down.
“So at the bottom of the column, water with a high concentration of dissolved elements comes into contact with the most Ca2+ loaded resin. As the water rises, it becomes less and less contaminated. At the same time, however, the resin becomes less and less loaded, which keeps the concentration well to the left of the equilibrium point, so decontamination occurs at an ideal condition over the full length of the column.
“Chemically speaking, we say that the concentration gradient between the ionic solution and the resin continuously drives the reaction in the direction of decontamination because it prevents the system from ever truly reaching its equilibrium point,” explains Carien van der Walt.
The loaded resin exits the adsorption column at the bottom and is then moved across to a desorption column. To prevent the resin having to pass through a pump, Clean TeQ has developed and patented air lift transfer technology: “Since pumping resin damages the soft polymer beads, we transfer the loaded resin back up to the top by creating an air vacuum pulse. Each pulse causes a plug of loaded resin to shoot up the transfer pipe, where it is first passed over a dewatering screen before being passed into the desorption column,” Van der Walt tells MechChem.
A reagent is added to the column, typically sulphuric acid for cation exchange resins, and the column is air agitated. “The acid in this example removes the Ca2+ ions from the resin and replaces them with two H+ ions from the acid. Once in solution, these ions immediately react with SO42- ions to form CaSO4 (gypsum), which precipitates as a solid.
After another air lift, the resin again passes over a screen that removes the solid particulates, while the resin drops into the wash column where it is washed via fluidisation before being transferred back to the loading column. It thus completes a transfer cycle.
When purifying mine water to potable quality, a second anion removal stage is required to remove dissolved non-metal ions and to reduce the water’s acidity. “Anion exchange resins are typically loaded with hydroxide (OH-) ions, which will go into solution in preference to other dissolved non-metal ions such as sulphates or nitrates.
“Therefore, to treat water continuously, we need a second stage, an anion removal section. The acidic water is passed into the bottom of the anion adsorption column, the anion exchange resin enters the column from the top and the same basic cycle is used to remove the negatively charged ions,” Van de Walt explains. The combined cation and anion desalination process is called dual-stage ionic desalination, or DeSALx®. “Our process is fully continuous. Contaminated water can be pumped into one end, and potable water flows out the other, without the need to halt the process to backwash and regenerate fixed resin beds,” adds Van der Walt.
In addition to wastewater treatment, by using the Clean-iX® process, “we can purposefully select resins in order to recover valuable metals”. “Hence, if on site mine water contains a commodity such as copper, for example, then we can recover that copper before purifying the water,” she suggests.
So, by combining DeSALx with the Clean-iX metal recovery technology, wastewater treatment can be used to improve profitability. “Water treatment is often seen as a grudge purchase, but by extracting value from the metal content, water treatment costs can be subsidised by the added-value of the recovered metals. While the payback is dependent on the concentration of the metal in the wastewater, we have found for copper, for example, that if the water contains more than 100 ppm of copper (100 mg/ℓ), then the payback on the initial investment can be less than one year and, in some cases, the clean water can be viewed as a free by-product of the metal recovery process. Even gypsum can have value if it is already a product being used or sold by the plant,” says Van der Walt.
Clean-iX is ideal for the recovery of a wide range of valuable metals present in low concentrations, including gold, silver, platinum, nickel, copper, uranium and rare earth metals such as vanadium and scandium.
“We are also very interested in point of use acid mine drainage (AMD), ie, treating mine water to enable it to be reused by the mine rather than allowing it to enter the public water system. This is an ideal long-term solution to AMD in South Africa. Adding a secondary solution that fits onto the backend of current treatment plants is a cost effective solution that is also much faster to implement than large purpose-built AMD plants,” she argues.
“CIF technology is changing the way we see water treatment. Now, instead of being an annoying expense driven by environmental legislation, value-creating propositions can be identified. So being clean can also improve profitability,” Van der Walt concludes.