Method for purifying waste water from a stainless steel slag treatment process

Abstract

The present invention relates to a method for purifying highly alkaline waste water from a stainless steel slag treatment process. This waste water contains between 0.5 and 5 mg / l of trivalent chromium (Cr(III)) and between 0.5 and 10 mg / l of molybdenum. To reduce the chromium and molybdenum content a trivalent iron is added to the waste water in the form of a water soluble ferric salt and the trivalent iron is allowed to co-precipitate with said chromium and molybdenum by lowering the pH of the waste water. It was found that by performing the co-precipitation at a pH of 3.5 to 5.7, both the chromium and the molybdenum content could be effectively reduced to a level of less than 0.5 mg / l, and this without having to provide large amounts of trivalent iron in the waste water, without having to repeat the purification process for several times, without having to add sulphide to the waste water, and without having to lower and raise the pH of the waste water for more than one time.

Description

technical field [0001] The present invention relates to a method for purifying waste water such that its chromium and molybdenum content is reduced, the chromium content is reduced to a concentration below 0.5 g / l, and the molybdenum content is reduced to a concentration below 4 mg / l, said waste water It is waste water from stainless steel slag treatment processes in which stainless steel slag is brought into contact with water, in particular to neutralize the free lime contained therein, thus generating said waste water. The waste water thus obtained has an initial pH greater than 10, in particular higher than 11, and more particularly higher than 11.5. It contains chromium, contains 0.5 to 5 mg / l trivalent chromium (Cr(III)), and also contains 0.5 to 10 mg / l molybdenum. Typically, it also contains 200 to 2500 mg / l of chloride. During the purification process, the chromium and often molybdenum content has to be reduced in order to be able to discharge the purification waste...

AI Technical Summary

Problems solved by technology

[0008]Therefore, the purification process has the disadvantage of having to add a large amount of ferric iron sludge and remove it again from the wastewater, although there is a large amount of ferric iron in the wastewater , the molybdenum was removed so little that the process had to be repeated three times

Also, the Japanese patent application does not give any teaching on the removal efficiency of trivalent chromium when the wastewater also contains chromium

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