Washing thickeners

The overflow product contains 100 kg solute/m3 solution. This concentration is the same as the underflow from the first thickener and hence the material fed to the washing thickeners contains 82 kg inerts and 0.0608 m3 solution containing (100 x 0.0608) = 6.08 kg solute. 

The calculation may be made on the basis of this material entering the washing thickeners together with 60 Mg water. If xi,x2, and x3 are the Na2S/water ratio in the respective thickeners, then the mass balances are ... 

In the production of caustic soda by the action of calcium hydroxide on sodium carbonate, 1 kg/s of sodium carbonate is treated with the theoretical quantity of lime. The sodium carbonate is made up as a 20 per cent solution. The material from the extractors is fed to a countercurrent washing system where it is treated with 2 kg/s of clean water. The washing thickeners are so arranged that the ratio of the volume of liquid discharged in... 

The solvent in the underflow from the final washing thickener = 50 kg/s. 

Thus 4 washing thickeners or a total of 5 thickeners are required. 

First thickener First washing Second washing thickener thickener... 

The compositions of the various streams may be calculated in terms of three unknowns x[ and 2, the ratios of solute to solvent in the first and second washing thickeners respectively, and qP, the amount of insoluble solid B in the underflow streams. An overall material balance is made and then a balance is made on the agitator and its thickener combined, the first washing thickener and the second washing thickener. The procedure is as follows. 

It may be noted that these relations apply only to the washing thickeners and not, in general, to the primary thickener in which the product from the agitators is first separated. 

A system is now considered consisting of n washing thickeners arranged for counter-current washing of a solid from a leaching plant, in which the whole of the soluble material is dissolved. The suspension is separated in a thickener and the underflow from this thickener is fed to the washing system as shown in Figure 10.12. 

S0 and Wq are the amounts of solute and solution with the solids which are fed to the system for washing. The solvent associated with these solids is taken as the same as that in the underflows from the washing thickeners. 

In this equation, (Sn/S0) represents the fraction of the solute fed to the washing system which remains associated with the washed solids. If in a given case it is required that this fraction should not exceed a value /, the minimum number of washing thickeners required is given by ... 

In the systems considered so far, the quantity of solvent, or of solution, removed in association with the insoluble solids has been assumed to be constant and independent of the concentration of solution in the thickener. A similar countercurrent system is now considered in which the amount of solvent or solution in the underflow is a function of the concentration of the solution. This treatment which is equally applicable to the washing thickeners alone, or to the whole system involving agitator and thickeners, is attributable to Ruth 7 ... 

Revision of the nomenclature for the underflow streams of washing thickeners to bring it into line with that used for other stagewise processes, including distillation and absorption (Chapter 10). 

Clarification Classification Dewatering Extraction Pnrification Rinsing Washing Thickening... 

In washing, thickening and recovery applications we try to set the cut size of the system below the finest particle size present in the feed, so that any loss of particles into the system overflow is minimized. When the system cut size cannot be set sufficiently low it may be necessary to use an additional separator on the system overflow to return the escaping solids back in the feed—refer to the case study on washing of wheat starch later in this chapter, for an example. 

In these process alternatives that are completely dedicated to the unit operation crystallization, one can find remarkably often, though, some other unit operations used to intensify the crystallization in separating components and to make the entire process more feasible (process intensification). Best examples are the discoloration with activated carbon or the use of a washing thickener for replacing the mother liquor (Figure 16.4). In the case of washing thickener, one uses the countercurrent washing principle to treat the suspension of the crystals from second crop with the purest solution available in this system, which is the feed solution. Even a solution prepared by redissolution of product could be taken. 

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