MechChem Africa

MechChem Africa visits the Constantia Kloof premises of FLSmidth to talk to Terence Osborn, commercial manager of minerals in sub-Saharan Africa, and Roy Hazell, the region’s head of sales, about the company’s counter-current decantation (CCD) thickeners, its P-Duc feedwell innovation and the use of LDX stainless steel for highly acidic application.

FLSmidth CCD Thickeners P Duc“A few years ago, our business was about helping mines to increase production. Today we have an added dimension and the focus is on operational efficiencies, such as reducing clean water consumption, using less flocculant, improving reliability and extending life. Our current role is to use what mines already have and do more with it,” begins Hazell.

“Along with our REFLUXTM Classifier (RC™) technology, another driver of our customer vision towards sustainable success is our CCD thickener technology,” he says, adding that a definite market for thickener optimisation has been identified and several retrofits have already been done.

Thickeners are an essential component of mineral processing plants. Following production of the final mineral concentrate, solid concentrate needs to be dewatered before being dried and shipped for smelting. For solution-based mineral processing, such as the liquor needed to produce copper or uranium in electro winning, ion exchange or solvent extraction processes, the liquor solution needs to be concentrated to extract the maximum possible amount of the dissolved metal from a mineral slurry.

In the above scenarios, FLSmidth offers retrofit thickener solutions that can improve recoveries, as well as purpose-designed new-build systems that give optimal thickener performance.

“Thickeners rely on gravity to separate liquids and solids from a slurry. The clear supernatant is taken off at the top as a clear liquid overflow, while the solids are allowed to settle before being extracted from the underflow of the thickener,” explains Osborn.

The solid particles in the slurry may be relatively course or very fine, depending on the mineral and the treatment process being used. The finer the particle, the more likely agents will be required to aid settling, and two common additives are used: flocculants and coagulants.

“Flocculants are a long chain polymer with active charge sites that make them act like ‘sticky string’, attracting and clumping small suspended particles into bigger aggregates that can more easily be settled out.

“Coagulants, on the other hand, work on a surface charge effect, binding particles via electrostatic forces to enable them to coagulate and settle,” Osborn tells MechChem Africa.

“And some complex slurries might need both to be used at the same time,” adds Hazell.

“There are many applications for FLSmidth thickeners: sometimes a dilute slurry needs to be concentrated up, so the mineral product is in the underflow; sometimes simple dewatering is required to recover process water for reuse in the plant, in which case a very high underflow density will be targeted; then there is the third option, where the liquor from the overflow is the product required,” Osborn continues.

Typical solid to liquid densities of underflow slurries are around 56% solids by weight, but if recovering water or liquor, a taller paste thickener can sometimes be used, which can achieve densities of up to 70% solids.

Counter-current decantation CCD

For hydrometallurgical circuits where the mineral product is already dissolved in an acid solution, the role of the thickener is to remove the solids, while recovering a clear liquid solution with the highest possible concentration of the dissolved mineral. “Our CCD solution is about optimising this concentration while minimising the amount of water used,” says Hazell.

Osborn explains how this works:

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