The Counter Current Decantation thickener circuit at Swakop Uranium’s Husab Project demonstrates the potential of an FLSmidth package solution. Designed and supplied by FLSmidth, this CCD circuit has the largest diameter thickeners of its kind to be installed on the African continent in this type of application. Located near Swakopmund on the west coast of Namibia, Swakop Uranium will be the largest open pit uranium mine in the world, and will mine 150 million tonnes of rock per year and produce over 15 million pounds of uranium oxide.

The Husab Mine will propel Namibia to become the world’s third largest producer of uranium, with the country justifiably claiming to cleanly empowering the globe. The shareholders of Swakop Uranium are the Republic of Namibia, through state owned mining company Epangelo, and the People’s Republic of China, through state owned CGNPC.

The scope of the FLSmidth order for the Husab CCD circuit comprises eight 40 metre diameter thickeners, one of which is a high density pre-leach feed thickener and the other seven are high rate CCD thickeners. FLSmidth is also supplying a 25 metre diameter ADU (ammonium diuranate) high rate thickener for installation further down the process. In addition to this 38 Krebs® centrifugal pumps will also be installed in all slurry pumping applications on the project.

Significantly, such is FLSmidth’s confidence is the CCD thickener solution provided that the company has offered the customer a process guarantee in this regard.

Describing a CCD thickener circuit, Terence Osborn, sales & market manager, base and precious metal for sub-Saharan Africa at FLSmidth, says that this technology is used to recover soluble metal as pregnant liquor solution from ore leach residue.

The basis of CCD operation is to concentrate the leached solids, thereby minimising liquor content in the underflow slurry that flows in one direction. The underflow slurry is then diluted with wash liquor that flows in the opposite direction, while the leached solids are concentrated repeatedly. The amount of liquor in the thickener underflow contributes to determining the number of CCD stages required to recover the desired amount of soluble metal.

The nature of the process means that pumps are integral to the CCD circuit as these transfer the solution and the slurry from one stage to the next. Slurry being transferred has a very high density which maximises the wash efficiency making this type of application specifically suited to Krebs® pumping technology. “The Krebs® pumps are well matched from a process performance perspective and the ideal pump for the transfer of material between the stages within the CCD circuit. Centrifugal pumps are capable of achieving the high flow rates required in this application and the Krebs® pump has the added benefit of its patented wear ring design which minimises wear enhancing pump performance and life,” Osborn says.

It is significant that Krebs® pumps were selected for the transfer of slurry from the underflow. The majority of pumps installed Krebs® slurryMax™ units with a smaller number of Krebs® millMAX™ pumps being used. The pumps supplied range in suction internal diameters 80 mm through to 500 mm. These pumps will be handling flow rates from 35 m3/hr to 4 000 m3/hr.

One of the most important features of the Krebs® slurryMax™ is its patented suction side sealing system. This feature, unique to Krebs®, has an externally adjustable wear ring that closes the suction side impeller clearance between the suction liner and the eye of the impeller, whilst maintaining the primary clearance between the impeller shroud and the suction side liner. This primary clearance eliminates the problem conventional rubber lined pumps experience; that of pressure pulsations caused by the close proximity of the rotating raised expelling vanes to the suction liner, necessary to maintain flow, and subsequently generating heat in the liner and rubber devulcanisation failure.

Osborn says this feature makes it ideal for high density pumping applications such as the Husab CCD thickener underflow one. “The adjustment can be done while the pump is operational which eliminates the need to stop the plant during this activity.”

The ability to control the suction side clearance reduces the hydraulic recirculation and therefore contributes to maintaining the design flow over the life of the pump, increasing the life of the impeller. The extra thick rubber lining has rigid reinforcement to prevent collapse onto the rotating impeller under upset vacuum conditions. Impellers are available in metal or elastomers.

The tailings pumps, also supplied by FLSmidth, have been installed in series. These Krebs® slurryMAX™ pumps will facilitate the transfer of material over the extended distance from the process plant to the tailings storage facility.

FLSmidth has delivered a number of thickener underflow installations in the southern African market in the past ten years, both in uranium and in copper processing plants. Osborn attributes this to the quality and unique technological advantages possible with this equipment package from the company.