MechChem Africa February 2017

⎪ Water and wastewater processing ⎪

When a cationic resin is brought into contact with contaminated water containing Ca 2+ ions, two H + ions are discharged into the water for each Ca 2+ ion that attaches to a bead.

column from the top and the same basic cycle is used to remove the negatively charged ions,” Van deWalt says. 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 potablewater flows out the other, without the need tohalt the process tobackwash and regener- ate fixed resin beds,” she adds. In addition to wastewater treat- ment, 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‑iXmetal recovery technol- ogy, 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 treat- ment costs can be subsidised by the added-value of the recovered metals. While the payback is dependent on the concentra- tion of the metal in the wastewater, we have found for copper, for example, that if the water containsmore than 100 ppmof 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 awide range of valuable metals present in low con- centrations, including gold, silver, platinum, nickel, copper, uraniumand rare earthmetals such as vanadium and scandium.

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 col- umn at the bottom and is then moved across to a desorption column. To prevent the resin having topass through a pump, CleanTeQhas developed and patented air lift transfer tech- nology:“Sincepumpingresindamagesthesoft polymer beads, we transfer the loaded resin back up to the top by creating an air vacuum pulse. Eachpulse causes a plug of loaded resin to shoot up the transfer pipe, where it is first passedover adewatering screenbeforebeing 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 theCa 2+ ions fromthe resin and replaces themwith two H + ions from the acid. Once in solution, these ions immediately reactwith SO 4 2- ions to formCaSO 4 (gypsum), which precipitates as a solid. After another air lift, the resinagainpasses over a screen that removes the solid particu- lates, while the resin drops into the wash col- umnwhere it iswashedviafluidisationbefore being transferredback to the loading column. It thus completes a transfer cycle. When purifying mine water to potable quality, a second anion removal stage is re- quired to remove dissolved non-metal ions and to reduce the water’s acidity. “Anion exchange resins are typically loaded with hy- droxide (OH - ) ions, whichwill 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 col- umn, the anion exchange resin enters the

Multotec employees operating the DeSAlx test rig currently installed at the company’s premises in Spartan.

Multotec’s 1.0 m 3 /h DeSAlx test rig used to do on site test work at customer sites. “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 anannoyingexpensedrivenbyenvironmental legislation, value-creating propositions can be identified. So being clean can also improve profitability,” Van der Walt concludes. q

February 2017 • MechChem Africa ¦ 35