In this example, we examine a case where solar evaporation is combined with a modern salt crystallization system to improve product salt quality. Fig. 9 shows the flowsheet of such a crystallization unit, which is operated with feed liquor concentrated in a solar pond.
Along with standard FC type crystallizers, there is installed one fluidized-bed crystallizer for producing a coarse product as well. To improve its energy efficiency, the plant is operated as a multiple-effect evaporation unit. The Messo/Oslo crystallizer is the first effect, ensuring optimum conditions for crystal growth (high temperature). The plant is fed with concentrated liquor from the solar pond into a tank from which the Messo/Oslo crystallizer is fed. This tank has unsaturated liquor, ensuring crystal-free feed (feeding a crystallizer with suspended fines usually reduces its product size). Solution not used for the Mes-so/Oslo unit overflows into a second tank from where the FC crystallizers are fed. The slurry in the FC crystallizer train is advanced from effect to effect and from the last effect to a slurry tank.
Figure 9 Multiple-effect evaporation crystallization for NaCl
From there the slurry is fed to the solids separation section, which consists of a hydro cyclone, a washing thickener and a centrifuge. In the washing thickener, the impure mother liquor is exchanged with fresh feed liquor, which has lower impurity levels. The slurry from the washing thickener is fed to the centrifuge where the salt is dewatered and washed with clean water. The wet centrifuge cake is then dried and the crystals sent to the packaging section. The dewatering of the granular product of the Messo/Oslo unit is made separately in a special centrifuge, ensuring lower breakage of the crystals.
A plant of this type was erected for the treatment of 60 t/h concentrated liquor from a solar pond on the Mediterranean Sea coast. It is producing 2.5 t/h of granular salt (average size of about 2 mm) and 6.5 t/h of standard vacuum salt. It consumes 11 t/h of steam to evaporate 34 t/h of water. By storing extra concentrated solar pond liquor over the summer period, the plant is able to be operated at full rate for the entire year.
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