Increasing Evaporation Rate For Cyanide-Zinc

Question:

We have been trying to use an atmospheric evaporator on our cyanide-zinc plating operation and have not succeeded in evaporating enough solution. How can we increase the evaporation rate?

Answer:

Atmospheric evaporators present wonderful, low-cost opportunities for solution recovery on many processes. One fact that many forget is that it takes approximately 800 BTUs per gallon for evaporation. The process is driven by the principle of evaporative cooling. This means that you must provide solution that can lose BTUs, so it can be evaporated. For best results, the temperature drop within the process must be wide, to drive evaporation. This works best when the solution to be evaporated is hot.

If the process solution is at 140 F and it is pumped into the evaporator at the rate of 900 gal/hr, exiting at 120 F, there is a temperature loss of 20 degrees. The BTU loss is approximately 149,760 BTUs. This would mean that, if everything is perfect, the evaporation rate would approach 18.72 gal/hr. Add to this the fact that an average humidity on the U.S. East Coast is 50 percent, the maximum evaporation rate would approach only 9.36 gal/hr. Next, considering the fact that evaporator systems are not perfect, you will realize an actual rate lower than the predicted 9.36 gal/hr. Translating this knowledge to a cyanide-zinc process, which operates at not greater than 110 F, you see that the temperature drop inside the evaporator will be negligible, as will the evaporation rate.

You might use the evaporator on a heated side tank, adjacent to the process tank, relying on the extra heat applied to increase the evaporation rate.

In considering the problem, you should be aware that it doesnt really matter where you remove water from the system. It is equally efficient for you to remove it from the rinse tanks as from the plating bath.

Another approach to evaporation is the use of a system that is not atmospheric in its action. These alternative systems evaporate solutions by adding energy into the process and are generally self-contained. They are, however, much more costly to acquire.