Feasibility study on the removal and recovery of ionic mercury from waste water using tannery hair

Removal of ionic mercury from aqueous solutions using untreated, shampooed and solvent degreased hair was investigated. Considering the 20 percent weight losses due to degreasing, the rate of removal for solvent degreased hair was ten percent higher than untreated hair. The diffusion of ionic mercury into hair was found to be particle diffusion controlled, and the diffusion coefficient calculated was in the range of 2.77 x 10⁻¹² to 8.68 x 10⁻¹² cm²/sec.

Regeneration of mercury loaded solvent degreased hair was possible with acids and sodium chloride solutions. Acids of high concentrations recovered all the mercury on hair, however, the loading capacity of hair went down drastically due to degradation. Only 67 percent of the mercury was recovered in the first regeneration using 0.2 N NaCl, but there was no loss in the loading capacity. The equilibrium recovery of mercury from hair with 0.2 N NaCl was 38 percent, reached at fifth regeneration. The diffusion of desorption process was particle diffusion controlling also, and the diffusion coefficient calculated from the experimental data was 2.11 x 10 ⁻¹² cm²/sec.

A preliminary process design study was completed for treating a waste stream with 100 ppm mercury. The process lowers the mercury concentration to one ppb before discharge and the hair is regenerated for further use. The NaCl-HgCl₂ liquor is treated with NaOH to precipitate mercury as HgO at a pH of 10.5 - 12.0. The results of preliminary cost analysis showed that the break-even cost of mercuric oxide produced was 5.45 dollars per pound.