Remediation of metal-bearing aqueous waste streams via direct carbonation

Direct carbonation using liquid carbon dioxide can be used for the remediat ion: of aqueous streams or slurries while sequestering carbon dioxide in th e form of metal carbonates. Carbonic acid (pH = 2.9), which formed when the aqueous phase was contacted with excess liquid carbon dioxide at ambient t emperature (295 K) and elevated pressure (6.89 MPa), reacted with the metal cations to form metal carbonates in the agitated vessel. These metal carbo nates precipitated out of solution as the pH returned to neutral when the s ystem was depressurized. Electric are furnace K061 dust, red mud from the B ayer process of alumina manufacture, and metal-bearing wastewater streams w ere amenable to this treatment. The treatment of a K061-dust slurry from a steel plant was semi-continuous. The dust particles were retained in the hi gh-pressure reaction vessel as fresh water was continuously injected and hi gh-pressure, metal carbonate-bearing water was withdrawn. The water residen ce time in the reactor was 12 min. About 30% of the metal in the K061 dust was extracted into a metal carbonate product, and 98% of the metal in the c arbonate product was zinc. Unfortunately, lead was not selectively extracte d from the dust. Red mud was neutralized in batch experiments that lasted 5 -15 min. The pH of a 45 wt % red mud/55 wt % water slurry was reduced from 12.5 to 7. A post-treatment pH elevation to 9.5 was attributed to slow desi lication reactions that occurred over 1-2 weeks at ambient temperature and pressure. A plating bath wastewater stream containing aluminum (666 ppm) an d zinc (40 ppm) was contacted with excess liquid carbon dioxide for 5 min. The aluminum and zinc concentrations were reduced by 89% and 90%, respectiv ely, and the metal carbonate precipitate was easily filtered. Although the combined sequestration potential of these wastes is small, the ability to e ffectively remediate waste streams could lead to an industrial interest in the development of direct carbonation technology.