Centrate quality

Imperforate Bowl Tests The amount of supernant hquid from spin tubes is usually too small to warrant accurate gravimetric analysis. A fixed amount of shiny is introduced at a controlled rate into a rotating imperforate bowl to simulate a continuous sedimentation centrifuge. The liquid is collected as it overflows the ring weir. The test is stopped when the solids in the bowl build up to a thickness which affects centrate quality. The solid concentration of the centrate is determined similarly to that of the spin tube. 

Some modem decanter centrifuge designs are claimed to eliminate the need for polymer flocculant, notably in the water treatment industries, but centrate quality usually suffers. During flocculation the very fine particles can become adsorbed on the larger agglomerates, enhancing centrate quality. Some polymers marketed for use in centrifuges claim to increase the solid content of the cake. 

Required results (filtrate/centrate quality and cake dry solid content). Quantity of dilution water available for polymer. 

Basic decanter performance (in terms of solids recovery, centrate quality and solids dryness) remained relatively little changed in this period, but improving engineering and materials of construction enabled the use of longer bowls to give greater feed capacity. 

An alternative use for the variable pitch is to improve centrate quality. For this the pitch of the flight adjacent to the front hub is increased, as much as double that for the rest of the conveyor. This design is particularly useful for applications in which the decanter can become heavily loaded with solids. The increased pitch tends to keep the centrate discharge region of the bowl relatively free of solids, thus reducing re-enlrainment of solids as the centrate streams towards the weir. 

With main drive and back-drive systems under control, it remains to control centrate quality. For this, a good centrate monitor is required, capable of assessing the level of suspended solids in the centrate. This has been difficult but there are a few reliable devices now available on the market [20]. The monitor is then coupled via a PID controller to the polymer pump speed control. 

Feed rate is most often the first parameter to be investigated. The capacity of the decanter is of fundamental interest to the user. The pond level and bowl speed will have been fixed during the preliminary testing. If flocculanl is to be used, then a safe dosing level will have been determined and used during the preliminary tests, after bench evaluation of likely polymers to be used. The safe dosing level will be such as to permit a wide range of feed rates and conveyor differentials, without any appreciable deterioration in centrate quality. A series of tests at. say. five different feed rates will be conducted and samples of feed, centrate and cake will be taken for solids analyses. 

Each test run will be conducted with, as near as can be judged, the same flocculant dosage level. The setting of differential speed and/or torque for each run will depend upon the type of test being conducted. For a simple dewatering lest, a fixed differential speed may be cho.sen. However, if it is thought that the decanter may be solids capacity limited, then the same feed rale/differential speed ratio could be chosen for each test. For dry solids operation, a fixed torque would be more likely to be chosen. Alternatively, for each test the minimum differential is found, where the best dryness is achieved without deterioration of centrate quality. 

Once the first test series has been completed, the data should be graphed and examined to see whether an improvement in performance is required, or is likely to be achieved by altering one of the other parameters. For instance, if better dryness were required, the test series could be repeated with a lower differential, higher torque or higher bowl speed. For better centrate quality, a deeper pond might be chosen. 

Once a set of data has been obtained which correlates, the flocculant consumption, if used, needs to be optimised. Alternative polymers might be examined if centrate quality had been difficult to maintain or if the quantity needed was considered excessive. The relative flow rates of polymer solution, and feed, would be assessed to see whether the polymer concentration needs to be adjusted to make it minimum strength, without causing it to be a large fraction of the total flow. This should not be more than, say, 10 or 15%. Polymer tends to be most efficient when it is most dilute. Moreover it is easier to get a uniform mix of two liquids when they are both of comparable size. However the larger the volume the flocculant is, then the greater is the clarification capacity lost unnecessarily to the clean flocculant. The location for admitting the polymer may be questioned, and considered for introduction further upstream, if flocculation in the centrate has been observed, or if extra dryness is required at the expense of extra polymer in dry solids work. 

A centrate clarity limiting application is characterised by a fall off in centrate quality, when feed rate is increased, independent of conveyor differential, once pond depth has been optimised. Spent wash dewatering, discussed in the previous chapter, is one such example. 

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