We have a captive, acid zinc plating operation at our facility. For many, many years, we had been utilizing hexavalent chrome-based chromates for post treatment after zinc plating. During this time, we used ferrous sulfate to reduce hexavalent chrome to trivalent chrome. Over recent years, we have replaced our hexavalent chrome-based chromates with trivalent-based chromates in order to meet customers’ needs. As we have made this conversion, the need for chrome reduction has been eliminated. In fact, we abandoned the chrome reduction module about six months ago.
For many, many years, the filter press cake from our wastewater pretreatment system has been a non-hazardous waste which we could dispose as a special waste at our local sanitary landfill. We have been testing the filter press cake about every six months for all of these years. However, our most recent filter press cake analysis showed TCLP chrome well above USEPA’s threshold of 5.0 mg/liter; to make sure the lab did not make a mistake, they reran the sample and came up with about the same result. We then took another sample and sent it to another lab; they, too, found TCLP chrome well above threshold. We then thought, maybe, we had some hex chrome in the filter press cake, but lab analysis could not detect any hex chrome.
Needless to say, we are confused. We thought by switching to trivalent chrome we would be more environmentally friendly, but now our non-hazardous waste has, apparently, become hazardous waste with a significantly higher disposal cost. Do you have any ideas what could have caused this and how to correct? S.E.
We did run into a similar situation some years ago, but first things first. Since filter press cakes from metal finishing wastewater pretreatment systems are usually NOT homogeneous and the laboratory uses a fairly small sample size (minimum of 100 grams or 3.5 ounces) for the Toxicity Characteristic Leaching Procedure (TCLP), it is imperative to obtain a representative sample of the material. We utilize several possible procedures. One is to collect at least three (3) samples from each filter press dump over a period of at least three (3) days. If the filter press cake is in drums, we typically take at least one (1) sample from as many drums as we can get access. If the filter press cake is in an open top, roll-off container, we typically obtain at least twelve (12) samples from around the roll-off when it is nearly full.
Then, in order to prepare the sample for laboratory analysis, we composite the samples together and place into a heavy gage plastic bag and proceed to “knead” the sample as if it were a loaf of bread for at least ten (10) minutes; we have found that this much preparation is needed to produce a sample with consistent color and texture, since color and texture are good indicators of the type of metal present and its distribution throughout sample.
Before proceeding any further, we would recommend that you re-sample using the above procedures. If the TCLP results of this sample, again, are above the Chrome TCLP threshold of 5.0 mg/liter, then let’s take a look at possible cause and solution.
Some years ago, we ran into a similar situation where an electroplater was utilizing an iron based chrome reduction system and after their conversion to trivalent chromate, their filter press cake also flunked TCLP chromium. To understand the likely cause, we need to go back to the chrome reduction process itself.
If one is using ferrous sulfate, as you did, for chrome reduction, it has the following reaction equation:
6FeSO4 + 2H2CrO4 + 6H2SO4 >>> Cr2(SO4)3 + 8H2O + 3Fe2(SO4)3.
If one is using scrap iron or steel for chrome reduction, it has the following reaction equation:
2Fe + 2H2CrO4 + 6H2SO4 >>> Cr2(SO4)3 + Fe2(SO4)3 + 8H2O.
In each case, the hexavalent chromium (Cr+6) oxidizes the iron to its ferric (Fe+3) state as it is reduced to its trivalent state (Cr+3). Apparently, the ferric iron is key to this issue.
Many articles have been written over the last few decades of the benefits of “co-precipitation” of heavy metals with iron, that is, when heavy metals, such as zinc and chrome, are precipitated out of wastewater as metal hydroxides, the remaining soluble metals are at a lower concentration as compared to precipitation without iron. One possible explanation is that the iron creates a matrix around the heavy metal, thus aiding in the heavy metal’s removal as the iron is removed. Secondly, ferric iron, such as in the form of ferric hydroxide (FeOH3), is significantly more insoluble than ferrous hydroxide (FeOH2). In the iron and steel making industries, the oxidation of ferrous to ferric iron, usually by aeration, has been part of its conventional wastewater technology for many, many decades in order to remove iron and other metals from wastewater discharges.
Lastly, because ferric hydroxide is more insoluble and contains an extra hydroxyl ion (OH-), it is able to resist the acid’s attack during the TCLP procedure.
Putting these all together, here is our likely scenario of why your filter press cake is now hazardous. Utilizing ferrous sulfate as reducing agent for the hex chrome chromates, the chromates created ferric hydroxide as they were reduced and pH raised above 8 with caustic. The ferric hydroxide then co-precipitated with the chrome hydroxide (CrOH3) to form a tight matrix. When your past filter press cake samples were subjected to the acid extraction fluid of the TCLP test, the ferric hydroxide was able to sufficiently resist the acid’s attack and hold enough chrome so that the sample would pass TCLP. Since you are no longer performing chrome reduction, the trivalent chrome still precipitates as chrome hydroxide, but it no longer has the tight ferric hydroxide matrix, therefore, it flunks the TCLP test.
We suggest two possible modifications to your wastewater pretreatment system. Fortunately, in your acid pickle tanks, ferrous iron is generated. If you have sufficient holding time in your wastewater sumps or holding tanks, you can consider aeration using low pressure, regenerative blowers; the oxygen in the air can oxidize the ferrous iron to ferric iron as evidence by color change to rust color.
If aeration is not practical or if you have insufficient ferrous iron in your wastewater, you can install a ferric chloride feed before pH adjustment; each gallon of 42% ferric chloride contains about 1.75 pounds of iron.
In our case, we installed a ferric chloride feed. After about one week of operation and a ferric chloride dose rate of 2–3 gallons per day, we observed that the filter press cake color changed from a light, olive brown color to a more rust, olive brown color, and the filter press cake sample passed the TCLP test; in fact, a follow-up sample taken two (2) weeks later showed a TCLP chrome at non-detectable concentration. The small additional cost for the ferric chloride feed had a very, very quick payback.
Let me know what results you achieve.
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