The location in Mexico has a very limited water supply, so the challenge was to prepare and concentrate the 30% solution of sodium sulfate while minimizing the amount of water that the plant uses. Further, the raw chemical is from ore which requires dissolving the sodium sulfate, removing inert solids by clarifying and purifying prior to crystallization, and production of an “on-spec” size distribution of crystals.
The approach was to concentrate the crystal magma in a 4 effect system with steam being the primary heat source for the first effect and each subsequent effect heated by vapor evolved from the prior effect.
In this way, four pounds of water could be removed for each pound of steam used. The process condensate for these other 3 effects was recycled for reuse in dissolving the ore and purification.
A plant wide energy and mass balance from the initial step of receiving the ore, through making the slurry, concentrating it and finally drying the product was carefully developed to minimize water use.
With this system, most of the energy content of the steam and its condensate is recovered and the resultant condensate is recycled back through the process. The final result was that the guaranteed amount of dry sodium sulfate was produced using only 20% of water required for processing the ore. The total evaporation of water from the solution for production of the crystal product was 136,000 lb/hr yet only 38,000 lb/hr of steam was required. All of the steam condensate was returned to the boiler for reuse.