MechChem Africa February 2017

February 2017

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MechChem Africa

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Water and wastewater processing

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When a cationic resin is brought into contact with contaminated water containing Ca

ions, two

ions are discharged into the water for each Ca

ion that attaches to a bead.

Multotec employees operating the DeSAlx test rig currently

installed at the company’s premises in Spartan.

Multotec’s 1.0 m

/h DeSAlx test rig used to do on site test

work at customer sites.

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

ions fromthe resin

and replaces themwith two H

ions from the

acid. Once in solution, these ions immediately

reactwith SO

ions to formCaSO

(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

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.

“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.