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introduction , rake design , and underflow withdrawal method . However , the two key parameters investigated during dynamic test work include optimising the flocculant dosage and determining the thickener sizing scale-up factor . The unit for flocculant / coagulant dosage determined in test work is typically expressed as grams of dry flocculant per tonne of solids ( g / t ). Two different thickener scale-up units are referenced by different thickener vendors however , both are valid and essentially equivalent .
The two thickener sizing units referenced include the solids loading rate (( t / h )/ m 2 ) and the unit area loading ( m 2 / t / d ). They are the inverse of each other with the time factor converted to either hours or days . It is essentially the amount of solids settling across the thickener cross sectional area . Table 1 compares the solids loading rates and unit area loading for a typical 20 m thickener at four different solids feed tonnages . As the solids loading rate increases , the unit area loading decreases due to the units being the inversed .
During dynamic thickener test work , relationships between the flocculant dosage and underflow and overflow solids concentrations are produced . Typical results see underflow solids mass concentration attaining an optimum flocculant dosage at a point below which the material is under-flocculated and above which the material is overflocculated . The overflow solids concentration ( clarity ) improves with increasing the flocculant dosage due to more fines being captured . The overflow is measured either as overflow solids concentrations or as turbidity ( NTU ).
The other relationship determined in test work is the thickener sizing parameter ( solids loading rate or unit area loading ). Typically , as the solids loading rate increases , the underflow solids concentration decreases , and the overflow solids concentration then increases . However , if the process targets are known , the resulting solids loading rate / unit area loading can be taken from these graphs and used for sizing the diameter of a full scale thickener .
Laboratory scale test results are typically found to be conservative in high-rate thickening , so a slight increase in underflow solids concentrations and an improvement in overflow clarities is usually observed in full-scale thickeners . The reasons for these improvements include increased retention time in the settled bed , increased selfcompression of the material in the thickener , and further dewatering from increased raking efficiency in a full-scale thickener .
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Paste and High-Compression Thickener Test Work The sizing of paste and high-compression thickeners is slightly more involved than simply applying a solids loading rate or a unit area loading . Bed retention times and bed depth become issues since a higher underflow solids concentration is the target and must be considered to predict dewatering characteristics . Various vendor test work approaches exist however , usually involves the bed of solids being allowed to compact on itself over a period of time ( typically about 24 hours ) in a bench scale thickener unit . The bed height is recorded at certain time intervals to track the solids concentration of the bed of solids similar to a typical static cylinder settling test . When the underflow solids concentration achieves the underflow target , the time is recorded and used as the parameter for solids bed retention time to achieve the underflow process target . In doing so , the full-scale thickener solids bed volume is established , and the thickener sidewall height can be calculated and used in the final thickener design .
Larger thickener pilot units ( up to 1 m in diameter & about 5 m in height ) also exist and are operated in continuous mode to simulate longer bed retention times and deeper solids beds . Logistically , these paste / highcompression test work campaigns are more involved including significantly more material , a considerable amount of equipment , set-up on site , 24- hour continuous operation , and additional manpower . Slightly more
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