Vacuum Salt Refinery

In vacuum crystallization process, raw salt is dissolved in water to make a saturated solution and sent to clarifier wherein bulk of insoluble impurities settles down and is removed from the bottom. Solution transferred from the clarifier is filtered and pumped to treatment vessels for precipitation to the extent required by means of chemical reaction. Precipitated mass is filtered out and the solution thus purified is pre-heated and subjected to continuous evaporation on being heated with steam. For economizing the steam consumption, multiple units are used in series, so that the steam coming out from one unit, be used in the next effect, thus eliminating the need for application of fresh steam at each stage. Vapour from the last effect is condensed in a condenser using water from cooling tower. The entire plant initially is under vacuum, and thereafter the pressure in the system is thermodynamically stabilized. Vacuum is generated by using a suitable Vacuum Pump. The number of effects depends upon the capacity of the plant, so that optimized economy is achieved at minimum investment.

Depending upon the requirement, the system can be of forced circulation type or falling film type. Each effect consists of an External Calendria, circulation pump, Vapour Separator (Flash Chamber) and entrainment separator. The salt solution is continuously circulated through external calendria and the superheated liquid is flashed in flash chambers for separating vapour. The vapour is freed from entrainment in the separator and then fed to next effect which serves as heating medium. The vapour from last effect is sent to a direct contact Steam Condenser where it gets mixed up with water from cooling tower, and serves as make up water. Vapour from earlier effects are condensed, and collected as pure water. Hence, a properly designed vacuum crystallizer is capable of simultaneously producing pure vacuum salt and pure water. This makes the whole process much more economically viable.

As the evaporation proceeds, salt starts crystallizing, and the slurry formation takes place. Slurry is continuously withdrawn from the evaporators and fed to centrifuge through a thickener. Mother liquor from centrifuge is recycled to evaporator, and centrifuged salt is blended with crystal habitat modifiers, dried, graded, blended with additives including suitable quantity of a free flowing agent, and then finally packed.