For the past two months, weve talked about cyanide tests, what they mean and how they are performed. This time, our topic is operating a typical cyanide destruct system, including some simple techniques for improving its performance.

Cyanide oxidation is typically performed in two separate tanks, each providing one "stage" of the destruction reaction. The first stage utilizes sodium hypochlorite (or chlorine gas) to convert cyanide to cyanate at elevated pH (10+). The cyanide and chlorine react to form cyanogen chloride, which is then mixed with more chlorine to form cyanate.

The first reaction between chlorine and cyanide-bearing wastes is conversion of the simple cyanide in excess of that required to solubilize the metallo-cyanide complexes. The reaction then proceeds to form varying amounts of insoluble heavy metals containing cyanide. The heavy metals actually trap some cyanide as they are formed. Over time and with a large excess of chlorine, they may further react with chlorine to form heavy metal hydroxides and the reaction products from cyanide chlorination.

This last reaction proceeds very slowly. In fact, it is so slow that complete destruction of all cyanide by chlorination at high pH is not possible in a flow-through system. By lowering the pH below 10, the ionization of cyanides increases and the activity of the chlorine accelerates, resulting in more efficient cyanide and cyanate destruction. In other words, at pH values above 10, the cyanide/chlorine reaction almost completely stops at the cyanate stage and usually leaves some residual of untreated cyanide. At pH values lower than 10, the process favors conversion of cyanate to carbon dioxide and nitrogen. Since these go off as gases, there is more likelihood that the reaction will go to completion.

Cyanide wastes are additionally treated in a clarifier, where insoluble metallo-cyanides are precipitated and removed in the sludge.

Operating Details

The oxidation of cyanide in each chlorination tank is controlled by an oxidation-reduction potential (ORP) meter. The meter reads millivolt (mV) potential by means of a probe in the rinsewater being treated. The mV reading is not critical and will vary from one plant to the next, depending on the amount of oxidizable chemicals in the rinsewater. The meter cannot distinguish between cyanide and cuprous copper, for example.

The meter is functioning properly when it turns on the hypochlorite pump when there is less than 10 ppm of chlorine in the oxidation tank and shuts the pump off when 50 ppm is present. Whatever setpoint (in mV) this condition corresponds to is the correct one. Operators should not go by a given mV reading provided in the literature.

The easiest way to find this setpoint is to use potassium iodide/starch test papers, which turn varying shades of blue if immersed in rinse waters containing differing levels of chlorine. Some even have a chart on the vial of papers that lets the technician estimate what the chlorine concentration is. To set the ORP meter, the operator dips a test paper in the liquid in the oxidation tank and watches to see if it turns blue. If not, the hypochlorite pump is turned on by adjusting the setpoint of the ORP meter and the test is repeated every minute or so until a deep blue color is obtained. At this point, the ORP meter is readjusted so that it turns off the hypochlorite pump.

The paper should never test so strong in chlorine that the blue color is washed out and only a blue stripe is present at the liquid/air interface. Too much chlorine can cause severe problems in the clarifier (floating sludge)! The next step is to see if the ORP meter properly cycles around the setpoint. The pump should go on just as the papers turn faint blue and within a few seconds after they test white. The pump should go off when the papers test deep blue, but before they test white with a blue stripe.

One note of caution on interpreting the test papers: If they are dipped directly in the sodium hypochlorite, they will not turn blue at all because the chlorine bleaches the paper instantly. On a rare occasion, too much hypochlorite might be added to the cyanide destruct tank by mistake (e.g., due to a broken probe or stuck pump) . If the operator suspects that this is the case (the scent of chlorine in the air usually is strong), some of the waste should be diluted with water in a 5-gal bucket before testing with the papers. If the papers turn blue, the sample is way over-chlorinated. In such a case, the hypochlorite pump should be left off until the first-stage cyanide destruct tank tests a true "white." The clarifier may also have to be treated for excess chlorine by adding sodium thiosulfate dissolved in water (a few 50-lb bags should be kept handy).

Second Stage

Many waste-treatment operators assume that because everything is "controlled" by meters that they dont have to monitor the system very often. This is a big mistake. The operator should check both chlorination compartments with test papers at least twice a shift.

As stated earlier, the pH of the wastewater in the first stage is not important as long as it is above 10. In the second stage, however, a pH of 8 to 9 is required, so control is very important here. At least twice-a-shift pH checks are essential in this compartment. The reading of the pH meter/controller should not be trusted because it can go awry when dirt or oil gets on the probe, leading to drift under normal use. Also, pH is temperature sensitive, so a calibration The setting that was okay last summer when the rinsewater was 50 F will not be satisfactory in the winter when the water is 35 F. (Some meters automatically compensate for temperature.)

The ORP meted In the second stage of the chlorination system should be adjusted and sat so that the starch test papers yield a sky-blue color and so that the waste entering the clarifier is either not blue when tested or a very faint (almost impossible to see) blue. The ORP and pH meter probes should be inspected once per shift and cleaned, if necessary.

A rigorous program of probe maintenance must be implemented and followed. The pH and ORP meters are only as good as the probes they use to "senses the status of the system. A daily wipe with alcohol to remove oil and grease from the probe is highly beneficial. The operator should be sure that he wipes the right part of the probe. The pH probe has a glass bulb that senses pH, whereas the ORP probe has a metallic disc/strip. These need to be cleaned. Should there be a stubborn film of metal hydroxide on the probes, it can be removed by immersion in 10 to 20 percent hydrochloric acid. A typical waste-treatment operator will check all probes (and clean when necessary) at least twice per 8-hr shift. Some check hourly.

Keeping spare probes on hand for each meter serves two purposes. First, it keeps the system operating when a probe is damaged or wears out. Second, it allows the operator to determine if a meter is defective by changing to a probe he knows is sound.

High Cyanide

It is quite possible that even with a cyanide chlorination system working at peak efficiency, the total cyanide in the discharge will still be too high. This is because some cyanides are only partially chlorinated or not chlorinated at all.

If cyanide plating is carried out on steel parts, the waste will always have some iron cyanide in it. Worse yet, the iron cyanide has a tendency to settle out in the clarifier, leading to cyanide in the clarifier sludge. However, there are many different iron cyanide compounds and some variations do not settle out in the clarifier. These yield high-total-cyanide test results.

Experiments have shown that cyanide removal in the clarifier can be improved by balancing the zinc and iron concentration in the pH-adjust tank.

If there is a lot of zinc and a lot less iron, ferrous sulfate can be added to the pH-adjust tank. The procedure is to dissolve 10 lb of ferrous sulfate to 55 gal of water and feed with a chemical pump at 1 gal/hr. If there is a lot more iron than zinc in the pH-adjust tank, zinc sulfate (at a similar feed rate) can be added instead.

Recently, the above change was made for a zinc plater who had a hard time meeting the limit of 1.9 mg/L total cyanide on a routine basis. He meets the limit routinely now by adding ferrous sulfate to his pH-adjust tank. Experimentation may be required with feed rates and concentrations because every plant has a slightly different waste.

Reference

1. K. Tanihara et al., Met. Fin., 85,131 (June 1987).