Knowledge for the Sulphuric Acid Industry
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Acid Plant Database
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DKL Engineering, Inc.
June 19, 2002
Precipitation of Impurities
All acid plants produce some form of liquid effluent either from the process such as weak acid from a gas cleaning system, wash down water, accidental spills, boiler blow down, cooling water blow down, precipitation collected in a containment area, etc. Some effluents are not treated prior to release to the environment while others must be treat extensively to remove contaminants before it can be safely discharged.
The purpose of an effluent treatment system is to treat the sulphuric acid plant effluents to generate a solids stream containing the impurities in the effluent and a liquid stream that is suitable for discharge to the environment or reuse elsewhere in the process. The level and nature of the impurities in the weak acid varies considerably from plant to plant and as a result so does the treatment methods employed.
In general, all effluent treatment methods employ the following steps:
Impurities are generally metals which can be removed from solution by precipitation. Precipitation can take place in low pH solutions (i.e. arsenic) or in high pH solutions (zinc, lead, copper, etc.). The choice of precipitation methods is based on the solubility of the salt in solution, the subsequent disposal method and/or further processing of the salt. Impurities are generally precipitated as hydroxides, sulphides, carbonates or chlorides with hydroxides being by far the most common.
Once the impurities are precipitated from the solution they are separated from the solution so they can be disposed of or sent for further processing and treatment. The method used for solids/liquid separation will depend on the nature of the solids. Initial de-watering of the stream can be done in a clarifier followed by filtration. In some cases the stream is suitable for direct filtration to remove the solids. Filtration may require the use of a filter aid to achieve the desired removal efficiency.
During the processing of the waste stream the pH of the solution may be adjusted many times. The final pH of the solution may not be in the range required for disposal of reuse. As such one final adjustment of the pH may be required.
Neutralization is the simplest way to treat a weak acid effluent. As the pH of the solution increases, the solubility of various impurities changes causing them to precipitate from solution.
Sulphuric acid can be neutralized by the addition of calcium hydroxide (lime) to form calcium sulphate or gypsum according to the following reaction:
H2SO4 + Ca(OH) 2 => CaSO4·2H2O
Calcium hydroxide is generally the least expensive and simplest to use neutralizing agent available for treatment of an acidic effluent.
Metal impurities will react to for metal hydroxides.
|Impurity||Form in Acidic Solution||Metal Hydroxide|
Where heavy metals remain in the stream after hydroxide precipitation, the addition of a chemical oxidant may be helpful to further enhance the removal of the metals. In general, those metals with many oxidation states (i.e. iron, manganese, etc.) will precipitate more readily if they are in their highest oxidation state. The use of hydrogen peroxide is effective in oxidizing metals to their higher oxidation state.