Chrome wastewater

Treatment of Industrial Wastes. The alkaline nature and inexpensive price of lime make it ideal for treatment of acid waste Hquors (6), including waste pickle Hquids from steel plants, wastes from metal plating operations, eg, chrome and copper plating, acid wastes from chemical and explosives plants, and acid mine wastewaters. 

In conventional chrome tanning processes 20 to 40% of the chrome used is discharged into wastewaters. In the new process 95 to 98% of the waste Cr3+ can be recycled. 

In addition to the heavy metals stated in Table 22.10, ferro- and ferricyanide are also part of the pollutants in the wastewater generated in a chrome pigment plant. These wastes are generally combined and treated through reduction, precipitation, equalization, and neutralization to be followed by clarification and filtration processes. Most of the heavy metals are precipitated using lime or caustic soda at specific pH. Chromium is reduced by S02 to a trivalent form, wherein it is precipitated as chromium hydroxide at specific pH. Sodium bisulfide is also employed to precipitate some of the metals at a low pH. The treated water is recycled for plant use while the sludge is sent to landfills (Figure 22.7). 

FIGURE 22.7 General wastewater treatment process flow diagram at a typical chrome pigment plant. 

K002 Wastewater treatment sludge from chrome... 

Replacement of hexavalent chromium with trivalent chromium offers important environmental advantages. Trivalent chromium is considerably less toxic than hexavalent. Trivalent systems use chromium concentrations that are typically two orders of magnitude less than in hexavalent systems. Thus, far less chromium enters the waste stream. Trivalent systems also generate few toxic air emissions, while hexavalent systems involve a reaction that produces hydrogen bubbles which entrain chromium compounds and carry them out of the baths. Trivalent chromium is readily precipitated from wastewater, while hexavalent chromium solutions must go through an additional step in a treatment system in which the chromium is reduced to its trivalent form before precipitation. It has been shown that trivalent chromium systems can successfully replace hexavalent ones for decorative chrome applications. Trivalent chromium systems are not suitable for hard chrome applications. More information regarding trivalent chromium plating can be obtained from Roy (1984), Robison (1978), Chementator (1982), and Smart (1983). 

Environmental applications require detoxification of hazardous substances to a level of parts per million (ppm) and even parts per billion (ppb). These purity levels, which were rarely considered in product synthesis, are now possible for wastewater due to Fenton s reagent. Fenton s oxidant is cost effective and relatively fast in destroying many toxics (Bigda, 1996). It attacks all reactive substrate concentrations under acidic conditions. Hydrogen peroxide is used to remove such contaminants as cyanide, sulfides, sulfites, chrome, and heavy metals by varying batch conditions. With an iron catalyst, the process often oxidizes organics, as well as reducing hexavalent chrome to trivalent precipitable form. 

Continuous chromium reduction treatment requires a tank for acidification and reduction with separate tanks for precipitation and sedimentation. The retention time in the reduction tank is dependent on the pH employed but should be at least four times the theoretical time for complete reduction. In cases where the chromium content of the wastewater varies markedly, equalization should be provided prior to the reduction tank to minimize fluctuations in the chemical feed system. Successful operation of a continuous chrome reduction process requires instrumentation and automatic control. Redox and pH control should also be provided. 

The rinse water from a chrome plating operation has an average total chromium concentration (as Cr) of 20mg/L 450,000 gal/yr of this rinsewater is generated as a process wastewater. What is the value of the chromium ( /yr) as chromic acid (H2Cr04) if it is recovered from the rinse water. Chromic acid costs 3.75/lb. 

Wastewater treatment sludge from the production of chrome yellow and orange pigments. 

K007. . Wastewater treatment sludge from the production of iron blue pigments K008. . Oven residue from the production of chrome oxide green pigments... 

Another important group of acid dyes is chrome dyes, a special subclass of dyes for wool that produce a beautiful range of black and navy shades. According to Shaw [49] and to Duffield [66], about 70% of wool dyeing today uses heavy metals, mainly chrome. Conventional chrome dyeing of wool produces about 155 ppm chrome in spent baths, but special methods can be used to bring chrome levels well under 35 ppm [49-51, 66], These baths, when mixed with other wastewaters from the operation give chrome levels of 15 and 0.7 ppm, respectively [5]. 

These chrome discharges are toxic in wastewater and also can accumulate in waste treatment sludges, thus rendering the sludges hazardous and complicating the issue of sludge wasting. 

---