Table 7414 Chrome Reduction And Precipitation Reactions

Ferrous sulfate 2H2CrO4 + 6FeSO4 + 6H2SO4 ®

Cr2(SO4)3 + 3Ca(OH)2 ® 2Cr(OH)3 + 3CaSO4 Sodium metabisulfite Na2S2O5 + H2O ® 2NaHSO3

Cr2(SO4)3 + 3NaHSO4 + 5H2O Cr2(SO4)3 + 3Ca(OH)2 ® 2Cr(OH)3 + 3CaSO4 Sulfur dioxide SO2 + H2O ® H2SO3

most wastewater treatment facilities conduct Cr reduction at pH levels less than 3.0; however, if the tank capacity is available, higher operating pH levels can reduce the consumption of acid reagents.

The chromium in the treated waste is generally in the form of chromic acid, chromate, or dichromate. Table 7.41.4 shows the reduction from the Cr6+ to the Cr3+ form with ferrous sulfate, sodium metabisulfite, and sulfur dioxide. The table also shows precipitation of the Cr3+ form is with Ca(OH)2.

Of the three reagents in Table 7.41.4, sulfur dioxide has economic and handling advantages. The first two reagents require auxiliary acid to hold the pH at the proper level for a low reduction time. If auxiliary acid is not used,

FIG. 7.41.4 Approximate path for Cr6+ to Cr3+ reduction. (Reprinted, with permission, from G.B. Hill, 1969, Complete removal of chronic acid waste with the aid of instrumentation, Plating 172 [February].)

an excess of reagent chemicals is usually required to complete the reaction. For ferrous sulfate, about 250% excess is required; for sodium metabisulfate about 75% excess is required. The sulfurous acid formed when sulfur dioxide is hydrolized is normally sufficient to maintain the process at a low pH value, thereby precluding the need for excess acid.

Additionally, sulfur dioxide is easier to handle and feed due to its availability in bulk or large cylinders and standard feed-regulating equipment. Ferrous sulfate and sodium metabisulfite are dry powders that require feeders and mix tanks. Wastewater treatment facilities can require from 10 to 200% excess reducing reagent to overcome the quasi-buffering effect of oxidants in the process stream, especially if the stream is well aerated.

Figures 7.41.4 and 7.41.5 show titration curves for the Cr6+ reduction process in which the reducing reagents are sodium sulfite and sodium bisulfite, respectively. The reaction associated with the sodium sulfite reagent is as follows:

FIG. 7.41.4 Approximate path for Cr6+ to Cr3+ reduction. (Reprinted, with permission, from G.B. Hill, 1969, Complete removal of chronic acid waste with the aid of instrumentation, Plating 172 [February].)

FIG. 7.41.5 Typical titration curve for Cr6+ to Cr3+ reduction at 2.5 pH. (Reprinted, with permission, from Hill, 1969.)
0 0

Post a comment