Final Report: Microwave-Assisted Oxidative Recovery of Cyanide From the Thiocyanate-Containing SolutionsEPA Contract Number: 68D99034
Title: Microwave-Assisted Oxidative Recovery of Cyanide From the Thiocyanate-Containing Solutions
Investigators: Milosavljevic, Emil
Small Business: BioQuest
EPA Contact: Manager, SBIR Program
Project Period: September 1, 1999 through March 1, 2000
Project Amount: $69,480
RFA: Small Business Innovation Research (SBIR) - Phase I (1999) RFA Text | Recipients Lists
Research Category: Water and Watersheds , SBIR - Water and Wastewater , Small Business Innovation Research (SBIR)
Description:Precious metals operations use a large excess of cyanide to extract gold/silver from the ore. Cyanide consumption is often the most important component of the total operating cost. The greatest cyanide losses occur through the formation of thiocyanate and metal-cyano complexes. Currently, there are no commercially available processes for recovering cyanide from thiocyanate and thermodynamically stable cyano complexes. The purpose of this Phase I SBIR project was to: (1) Identify the most promising oxidation system for the recovery of cyanide from thiocyanate; (2) Investigate the effect that microwave irradiation has on the oxidative recovery process, and (3) Establish the effectiveness of the microwave-assisted recovery of cyanide form thermodynamically stable cyanide complexes such as [Fe(CN)6]4-/[Fe(CN)6]3-. If successfully developed, this process will reduce cyanide consumption and thereby reduce cyanide production. This, in turn, will substantially lower the environmental impact of various cyanide species.
The following oxidation systems were investigated: (1) HNO3/H2SO4; (2) H2O2/HNO3; (3) H2O2/H2SO4; (4) Br-/BrO3-/H2SO4; and (5) KHSO5/H2SO4. For most of the systems investigated, the experiments in the presence and absence of microwave irradiation were conducted. Two methods for continuous removal of generated hydrogen cyanide were examined: (1) AVR and (2) membrane separation.
All oxidation systems investigated, except Br-/BrO3-/H2SO4, can recover cyanide from the thiocyanate containing solution. The HNO3/H2SO4 system can regenerate cyanide from thiocyanate only in the presence of microwave energy. Peroxide and monopersulfate function in the absence and presence of microwave radiation.
Based on the results obtained, KHSO5/H2SO4 system (model system for Caro's acid) was identified as the optimal oxidant to effect the recovery of cyanide from thiocyanate. The advantages of the monopersulfate oxidant are: (1) many potentially interested companies already have Caro's acid production and delivery systems installed, (2) the rate of the reaction of interest is almost instantaneous and (3) this system is probably environmentally friendly since, besides relatively benign sulfate ion, it should not generate significant amounts of other species.
The effects of the following parameters were investigated for this oxidation system: (1) oxidant (KHSO5) concentration and its source; (2) acid (H2SO4) concentration; (3) reaction time; (4) presence and absence of microwave irradiation; (5) SCN- levels in the treated samples; (6) presence of excess cyanide added as free cyanide and/or complexed cyanide; (7) the influence of the continuous removal of generated HCN using AVR or membrane gas-diffusion processes.
Independent of the source of the oxidant (RENEW or OXONE), the highest cyanide recoveries for the test solutions containing 100 ppm SCN- were obtained when the oxidant levels were 6.0 mM. One of the interesting exceptions was the system that contained 10.0 ppm cyanide added as [Cu(CN)4]3-. For example, the recoveries with 4.0 and 6.0 mM monopersulfate were 102 and 83.4 %, respectively. Of the two sulfuric acid levels tested, 0.1 M produced higher cyanide yields.
The tests conducted to investigate the room temperature kinetics of cyanide formation due to the oxidation of thiocyanate by monopersulfate indicate that the reaction is over or reaches equilibrium within a minute. Of course, this finding may have important ramifications for process economics since only short residence times will be needed to obtain satisfactory conversion efficiencies. The short residence time needed for this process may lower the capital costs for applying this technology.
The microwave tests (pressure control at 5 psi) showed slight improvement in cyanide yields when compared to the room temperature experiments. For the microwave experiments controlled at higher pressures (20 psi) lower cyanide yields were obtained. Probable reason is that at higher pressures (temperatures) enhanced cyanide decomposition due to its hydrolysis might be operational.
The tests performed to establish the effects that increased thiocyanate levels have on cyanide conversion efficiency indicate, as expected, that lower cyanide yields found at lower oxidant levels are offset by increasing the KHSO5 concentration (conversion efficiencies were actually greater in the latter case).
The addition of free cyanide as well as the complexed cyano species ([Fe(CN)6]4- and [Cu(CN)4]3-) did not have any detrimental effects on the recovery of cyanide from thiocyanate in the absence of microwave irradiation. This is, of course, very important because these are the ubiquitous cyanide species likely to be present in most of the thiocyanate containing effluents. In addition, this is contrary to what was observed previously when ozone was used as an oxidant. U.S. Patent No. 5,482,694 teaches that copper-cyano species catalyze the oxidation of cyanide when ozone was used as an oxidant which decrease significantly cyanide yields from thiocyanate. Hence, with ozone as an oxidant, schemes had to be devised to pre-oxidize copper-cyano complexes using peroxide which significantly complicated the process.
In microwave experiments when cyanide was added as [Fe(CN)6]4- at 10.0 ppm levels no detrimental effects were observed for the cyanide recoveries. If anything, slight increase in the cyanide yields from thiocyanate were found. Similar results (maximum recovery ~90%) were observed even when 50.0 ppm cyanide was added as the hexacyanoferrate(II) species. However, at very high [Fe(CN)6]4- levels (100 ppm as CN- ion), the total recoveries from thiocyanate and added cyanide decreased to about 70%. The tests designed to determine cyanide recoveries from [Fe(CN)6]4- in the absence of thiocyanate were also performed using a low-pressure (controlled at 5 psi) microwave method. Excellent cyanide recoveries (~100%) were observed in the absence of KHSO5 oxidant. However, in the presence of the oxidant yields decreased substantially. Hence, for treatment of the effluents containing high hexacyanoferrate and low thiocyanate levels it is important to strictly control the oxidant amounts to achieve maximum cyanide yields.
The tests conducted to establish whether AVR process may be used for continuous removal of generated cyanide were successful. The overall recoveries for all samples were about 80%. Exceptionally encouraging were the hydrophobic TEFLON membrane separation tests conducted in the flow configuration with continuous separation of generated hydrocyanic acid. The cyanide recovery of over 95% was obtained in the scrubber solution even with the very small membrane gas-diffusion unit utilized.
We believe that the use of gas-diffusion membranes to recover generated HCN may have significant advantages when compared to the AVR process. In the AVR process large amount of air are needed to strip HCN from the treated solution and volatilize it so that it can be absorbed in high pH scrubber solution. This necessitates long residence times. When membrane separation is used the high chemical potential (concentration gradient) for HCN exists which accelerates the transfer of this species across the membrane. In addition, the membrane manifold can be a part of the continuous microwave process (it may even be positioned in the microwave cavity) which would, due to the higher temperatures, ensure even better hydrogen cyanide transfer across the membrane.
The data obtained show that two different modifications of a potentially viable industrial process may be possible. For the effluents containing high levels of complexed cyanide and thiocyanate, microwave-assisted process would be beneficial. On the other hand, if a particular effluent contains high levels of free cyanide and thiocyanate, a less expensive system would be the non-microwave approach (combination of peroxide or monopersulfate release of cyanide from thiocyanate in combination with the AVR or membrane separation processes).
The preliminary cost estimates show that, at current sodium cyanide prices, recovery of cyanide from thiocyanate can only be profitable if the costs of thiocyanate detoxification is included in the operating costs and/or cyanide from thermodynamically stable species can be recovered. Hence, the process can have niche application, depending on the cyanide speciation, in the precious metals cyanidation operations. In addition, we believe that microwave-assisted release of cyanide from stable metal-cyano species in combination with membrane cyanide recovery process can find applications in the electroplating industry.