Washing train design recommendations

The conclusions drawn from the mass balances presented in Table 15.3 are further discussed in the next section. 15.6 Washing train design recommendations The countercurrent washing trains installed in industry often represent the most complex parts of the overall processes and are usually designed and Table 15.3 Three cases of mass balance calculations for liquid flows and solubles mass rates for comparison. Note that all three cases have the same washing efficiency, marked, and the point of thickest underflow, marked, has moved to the end of the train for case 3  

The following recommendations may be drawn from the mass balance calculations performed on different possible scenarios encountered in industry. Consulting experience with several working systems in industry confirms the conclusions. 

The washing efficiency of a countercurrent washing train is primarily determined by the values and order of the flow ratios Rf used in the train, and by the number of stages employed. It is not affected by the supply rate of wash liquid, irrespective of whether the Uquid comes in with the solids or through the wash liquid feed at the end of the train. The wash liquid supply only determines the dilution of the solids in the underflows in the different stages and overall. 

It is clear from equation 15.2 that the washing efficiency is maximized by minimizing the flow ratios in all stages. The solubles/wash system cannot, however, be considered in isolation. The transport of the solids along the washing train and the solids separation efficiencies in the individual stages and overall also have to be taken into account. The problem is, therefore, one of optimization of a three-component separation system. 

In order to be able to optimize the system, we have to know how the flow ratio affects the separation of the solids. In the case of hydrocyclones, the effect of the ratio is two-fold increasing Rf leads to improvements to separation efficiency through the contribution of dead flux and a further improvement is caused by the reduction in the crowding of the underflow orifice. Both of the effects can be described analytically for certain hydrocyclone geometries and the above-mentioned optimization is therefore possible, using the entropy index as a general criterion for the optimization. 

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The use of countercurrent washing is spreading in many areas of chemical, mineral and food processing. Very little is known, however, about the optimum operating conditions and design of such washing trains, and many installations can be much improved if some basic rules and recommendations are followed. 

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