Ask the Expert Question-and-Answer Archive (Wastewater Treatment)
by Mike McGinness, EcoShield Environmental Systems, Inc.
Treating Concentrated Cyanide Waste
Most literature that I read says that the most effective way to treat concentrated cyanide waste is by electrolytic oxidation at elevated temperature and with extraction until you reach 1 g/l when you can use hypochlorite oxidation. I have seen references to using ozone generators to treat concentrated wastes. My questions are: Is this in common use? Is it expensive? Are there difficulties? Are there more recent developments for treating cyanide waste?
Excellent question. Since you are asking about concentrated cyanides versus dilute rinse waters I will focus my answer on concentrated cyanide. The American Electroplaters Society funded some research on this back in the 1980's. As I recall they demonstrated a thermal process that takes advantage of the disassociation of H2O into OH and H free radicals when water is heated to elevated temperatures (above 400 F). Without going into the details, research revealed that if you safely seal the vessel (to withstand the resulting steam pressure safely) and heat it up to a temperature greater than 400 and up to 527 degrees F, and hold it for 2 hours they were able to take 50,000 mg/l of Cyanide contaminated water solution down to 0.02 mg/l of total cyanide. I have seen periodic magazine advertisements from a Canadian firm that operates a disposal facility using this system and they also (at least at one time) built and sold systems based on a license (as I recall) from the AESF for the thermal oxidation systems.
There are many concerns about side reactions and possible unwanted byproducts from the chlorination of cyanide, along with the known safety concerns of handling chlorine gas or compounds. The ever increasing concern about chlorinated organic byproducts (there are many possible nitrogen, carbon, chlorine combination by products that can form in small concentrations that can be persistent in the environment and that have related health effect issues) is one reason to consider alternatives. Electro wining and electro-oxidation have also been used, but can be maintenance intensive depending on electrode choice, and can produce reusable metal or tons of sludge depending on electrode choices, and finally can consume a fair amount of electricity. I have not seen a power comparison of thermal oxidation, to electrolytic (heated) oxidation to ozone oxidation.
I do know that current ozone generator designs are currently 5000% (50 times) more energy efficient that they were 20 years ago. Considerable progress has been made in reducing their capital and operating (mostly electrical usage - 95+ %) costs in recent years. I have used ozone in the past (circa 1990) for cyanide destruction as well as electrolysis (with various electrodes) for up to 5,000 mg/l on small quantities. I have not seen or read about ozone use for high concentration cyanide solutions. The capital cost of a complete system to produce about 1 lb of ozone per day would cost about $10,000 to $12,000. At best 1 lb of ozone might treat 1 lb of cyanide, if there are no competing organics to be oxidized in the solution. So the capital cost and or operating time required to treat a quantity of liquid may be a problem.
Another option (I think the patent has expired) is called singlet oxygen. There was a European patent issued in the 1980's for using hydrogen peroxide and sodium hypochlorite. The hydrogen peroxide was added first and the sodium hypochlorite was added second. This had to done slowly and carefully while the mixture was stirred. The presence of some chelated metal such as copper acted as a catalyst to the hydrogen peroxide and some OH free radicals would form and oxidize some of the cyanide to cyanate. When the sodium hypochlorite was added it reacted with the dispersed hydrogen peroxide and formed an extremely active O2 molecule known as singlet oxygen with a highly active unstable electron spin state. It is reportedly even more active than ozone or OH free radicals. I tried it out once on 10% gold-copper cyanide stripping solution (used in the gold jewelry industry) and got the total cyanide down below 1.0 mg/l.
It was extremely cost effective but the procedure I used was tested very carefully in small glass beakers in a hood before trying it out in a larger quantity (five gallon bucket) and was never set up or operated in an automatic mode. It was possible to add the 2 reactants too fast. When that happened it foamed out of the container due to the excess formation of gases (O2, CO2). Although we never attempted to automate it, it was extremely cost effective and required almost no capital equipment. It did not even get hot. I suspect that manually setting a standard feed rate of dilute solutions (H2O2 and NaOCl) to a batch mixed tank, as opposed to manually pouring from a jug, and ventilating the operation would be what I would try. As I recall the amount of chemical needed varied from batch to batch which meant that either temperature, or feeding and mixing differences from batch to batch in the batch tank (reactor) affected the stoichiometry. Also lab analysis and volume measurements may have added to calculation errors. Sometimes it took 50% more of the reagents to complete the job, but even then it was still almost zero cost from a capital standpoint, and minimal from a chamical reagent standpoint.
Finally we diluted the 5 gallons (remember it was 10% cyanide) with 35 gallons of water in a 55 gallon drum, making about a 1.4% solution, or about 14,000 mg/l. Then we added a small amount of defoamer. Both were done for safety and to minimize the foaming problem.
Just as a point of interest, apparently cyanide occures naturally (cherry pits) in nature, in small quantities and is biodegradable. If you removed the bio (bacteria) toxic metal ions (silver, copper, nickel, zinc) below toxic levels first (sodium ion exchanger?) it may be possible to set up an activated sludge bio-reactor and feed the cyanide solution slowly over time and let nature (aerobic bateria and an aereator) oxidize the cyanide.
If you want more details on ozone technology or activated sludge bioreactors please let me know. I have been building specialized ozone systems and bioreactors since 1988. Also if you want more details on the singlet oxygen process such as the ratios, feed rates, etc., please let me know.