With the increased focus on environmental legislation and the need for companies to comply with ISO 14001 it has become critical for process plants to deal effectively with effluent streams. This typically involves the introduction of the Zero Liquid Discharge process at the back end of an existing process plant. ZLD is used across a number of industries including mining, pulp and paper, chemical processing and the petrochemical sector. Its application is typically to evaporate and concentrate an effluent or waste stream; however it is also sometimes used to produce a saleable product. The process involves a suite of equipment from Multotec including dewatering centrifuges and CleanTeq solvent extraction.

Gerrit du Plessis, product specialist - solid/liquid separation at Multotec Process Equipment, explains that because ZLD is always at the end of a process flow the equipment used is generally subjected to extremes such as the chemical composition concentration in the slurry and how its flow rate fluctuates. “Due to both these issues, it is essential that the equipment selected for the ZLD process is capable of dealing with these varying factors, while still effectively producing the desired end result,” he says.

“Often, where engineers have only previously known and used pusher centrifuges they may not be aware that there is an option which is far more suited to the ZLD process. And more importantly, one that will provide significant benefits including more reliable performance with associated cost savings,” du Plessis adds. He recommends that those implementing the ZLD process ensure that they have access to an OEM that understands the entire process and is able to take all factors including flow rate, solids concentration, temperature, particle size distribution, chemical composition and corrosive properties into account.

There is an extensive global reference base of successful Siebtechnik Conturbex centrifuge installations in ZLD applications, with some 7 000 machines in more than 1 000 applications across a spectrum of industries. Multotec markets this range of centrifuges in southern Africa, and du Plessis says it has the requisite depth of minerals processing knowledge and ability to assess any given application providing a customised solution capable of meeting process expectations.

Du Plessis says the Siebtechnik Conturbex centrifuge offers a number of major advantages in the ZLD process. “Chief amongst these is significantly increased reliability as the machine is designed for a short product residence time, which means that its performance is not impacted by the fluctuating flow rate or fluctuating concentration of solids.”

The Siebtechnik Conturbex centrifuge has been engineered with a scrolled product transporting mechanism that distributes the solids product evenly across the basket. Du Plessis says this allows vibration free operation ensuring mechanical integrity of the equipment. “This is an important feature as uneven distribution during this process would result in increased stresses on the components, with eventual mechanical failure. Unplanned downtime as a consequence of this sort of stress would have serious knock on effects for the plant, as well as a significant environmental impact,” he explains.

A third major benefit is that the machine is self-cleaning. This feature is particularly relevant during a power outage or stoppage on the plant. When this occurs, the rotating components slow down while simultaneously cleaning the basket. This then allows the machine to be restarted without the need to do any form of manual cleaning. Significantly, this feature is unique to the Conturbex machine.

Materials of construction are always important when equipment operates in extreme conditions, as this ensures durability and optimum component life. Where extremely high temperatures and highly corrosive elements are found in the effluent being processed, specialised materials of construction are recommended. And in applications subjected to harsh abrasion, abrasion resistant materials are applied to the flight of the scroll to ensure long life of both the rotating components and the centrifuge itself. The double drive functionality of the Siebtechnik Conturbex centrifuge is an important feature which enables independent adjustment of the scroll and drum speed. This flexibility is important as it allows adjustment where extreme process conditions exist, and this will facilitate optimum performance. The machine is engineered in such a manner so as to enable easy maintenance, and one of the features relevant to this is the single bearing overhang design, which allows easy access to the internal rotating components. This saves time, leading to greater cost effectiveness during planned maintenance activities. Another important time saving feature is that cleaning nozzles, strategically positioned within the machine, facilitate Cleaning-in-Place during scheduled maintenance shutdowns.

Pointing to applications where Conturbex centrifuges are operating successfully, du Plessis says that in the petrochemical sector these centrifuges replaced pusher centrifuges as part of a de-bottle necking project with the added benefit of less frequent washing required. Also in the petrochemical industry, Conturbex centrifuges are used to produce a sodium sulphate by-product from a sulphur effluent stream.

Du Plessis says that there are definitely applications where the features of pusher centrifuges will outweigh those of the Conturbex centrifuge; however the ZLD process is not one of these. “We manufacture all types of centrifuges, including pusher centrifuges, and will recommend the correct type of centrifuge for the application at hand. Pusher centrifuges are suitable for applications where the need for crystalline purification is essential,” du Plessis says.

These machines are also used for water treatment and de-watering of mixed salts, including sodium sulphate in a colliery plant, and in a crystalliser plant de-watering mixed salts at a platinum operation.