Comparing Hydrate-Based Method With Freezing/Thawing Method For Chromium Hydroxide Sulfate Removal Close To The Melting Point Of Ice

In this work, both freezing/thawing method and hydrate-based method for chromium hydroxide sulfate removal with the temperature closing to melting point of ice have been conducted and compared in consideration of stirring speed, freezing time (or hydration-time), temperature, and chromium hydroxide sulfate concentration. The results suggested that chromium hydroxide sulfate removal efficiency of 93.9% was optimal with 85.6% of yield of melted water for freezing/thawing method. However, optimal chromium hydroxide sulfate removal efficiency and yield of melted water was 71.2% and 53.4% for hydrate-based method respectively. The optimal temperature for chromium hydroxide sulfate removal was different between the freezing/thawing method and the hydrate-based method. The optimal stirring speed freezing time, temperature, and chromium hydroxide sulfate concentration for freezing/thawing method were 800 r/min, 3 h, 271.6 K, and 100 ppm, respectively. However, the optimal stirring speed, hydration time, temperature, and chromium hydroxide sulfate concentration for hydrate-based method were 400 r/min, 1 h, 271.1 K, and 100 ppm, respectively. The results also suggested that the freezing/thawing method seems to be more effective than the hydrate-based method at 5.50 MPa for chromium hydroxide sulfate removal at the same temperature in this work. Therefore, the freezing/ thawing method should be the first choice to remove chromium hydroxide sulfate at the temperature ranged from 269.2 K to 271.2 K when the concentration of chromium hydroxide sulfate was less than 800 ppm.