Fly Ash and Fly Ash Leachate Treatment

Abstract

The present invention is directed to a process of treating fly ash and / or fly ash leachate to immobilize heavy metals contained in such fly ash and / or fly ash leachate, which process comprises treating such fly ash and / or fly ash leachate with a soluble ferrous compound under alkaline conditions. This process may be conducted in the absence of any pH modification, mixing (in the sense of a physical blending with a solid material), drying or heating steps, making it practical for treatment of alkaline fly ash (and other coal combustion by-products) which is currently stored in landfills or wet ash lagoons, particularly fly ash which has been recovered from flue gas streams treated with highly alkaline materials such as trona, bicarbonate or limestone and the like.

Description

FIELD OF THE INVENTION[0001]The present invention is directed to a process of treating fly ash and / or fly ash leachate to immobilize heavy metals contained in such fly ash and / or fly ash leachate, which process comprises treating such fly ash and / or fly ash leachate with a soluble ferrous compound under alkaline conditions.BACKGROUND[0002]The generation of power from coal can result in a number of undesirable pollutants being placed into the environment. These pollutants may be released in a number of forms, including flue gases and fly ash. Many of the gases which may be produced as a result of coal combustion, such as oxides of nitrogen and sulfur, will react with water in the environment to produce acid rain. Such gases may also contain oxides of heavy metals including selenium, arsenic, vanadium and chromium which can cause problems in the environment. Fly ash, which constitutes fine solid particles which rise with such flue gas, typically contains oxides of such heavy metals as...

AI Technical Summary

Benefits of technology

The present patent text describes a process for immobilizing heavy metals in fly ash and fly ash leachate. The process involves treating the fly ash and fly ash leachate with a soluble ferrous compound under alkaline conditions. This process can be applied to alkaline fly ash that is currently stored in landfills or wet ash lagoons, particularly fly ash that has been recovered from flue gas streams treated with highly alkaline materials. The technical effect of this process is to reduce the risk of heavy metals leaching into the environment, which can be a potential health and safety risk.

Problems solved by technology

The generation of power from coal can result in a number of undesirable pollutants being placed into the environment.

Such gases may also contain oxides of heavy metals including selenium, arsenic, vanadium and chromium which can cause problems in the environment.

Therefore, heavy metals contained in such combustion products can create environmental concerns if it is leached through contact with rain water or other similar means.

While such systems are effective for removing sulfur oxides from flue gas, they can result in the production of fly ash which has increased amounts of heavy metals, particularly selenium, due to the extraction of such material from the flue gas stream as a consequence of the use of such an alkaline sorbent material.

Thus, processes which are effective to reduce selenate to selenite under the acidic pH conditions and high temperatures at which FGD processes are employed to treat flue gas may not be practical to treat alkaline fly ash deposits, particularly those which are highly alkaline due to treatment with trona or similar high carbonate materials.

Specifically, such processes would require the addition of large amounts of acid and heat to such fly ash deposits, which additions themselves are expensive and environmentally unfavorable.

Although details of this treatment are not provided, it is apparent the addition of insoluble FeS would require substantial physical mixing; while the addition of strongly basic materials to fly ash stored in landfills or ponds would be environmentally undesirable.

Such a drying step is not practical in treating fly ash stored in wet ash lagoons or in landfills which are subject to periodic rainfall, dew condensation or other forms of moisture addition.