Cyanide recovery
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  • 1. Full PaperEnhance Cyanide Recovery by Using Air-Sparged HydrocycloneBy JosØ R. Parga Torres* and David L. CockeHuman health and environmental concerns dictate that industrial processes be improved or replaced. Recovery or recycling is animportant activity that allows cyanide residue from the industrial processes to be re-used, reducing its production cost anddisposal problems. In this regard, the air-sparged hydrocyclone (ASH) has been used as a reactor for the treatment of cyanidesolutions for cyanide recycling by acidification/volatilization using the Mexican modification of the Mills-Crowe process.Aqueous cyanide-ion concentration can be reduced from 250 ppm to below 20 ppm in the ASH with recoveries greater than 80 %in a single stage.1 Introduction Au + 2 CN± = Au(CN)2± + e± (2) A variety of industrial effluents are known to contain O2 + 2 H2O + 2e± = 2 OH± + H2O2 (3)cyanides. These various waste streams arise from differentprocess industries, such as those wastes from manufacturing H2O2 + 2e± = 2 OH± (4)synthetic fiber (acrylonitrile), coal conversion wastes orcoking effluents (from the iron and steel industries), electro- In this mechanisms cyanide ion is the complexing agent orplating waste and wastes from the petroleum industry. The ligand, and oxygen is the oxidant [2].most significant source of hazardous cyanide waste is the After extraction and recovery of the precious metals,metal finishing industry and wastes from the processing of substantial amounts of cyanide are delivered to tailings ponds,precious metals resources by cyanidation. All these wastes which create environmental problems due to the toxicity ofhave varied characteristics and are therefore subject to cyanides. Therefore, the recycling of cyanide is a matter ofdifferent processing and treatment strategies that depend interest from both an economical aspect and to protect theupon the concentration of cyanides and the flow rate of the receiving water from potentially harmful cyanide, as waswaste stream. shown by White [3]. The cyanide process has been in large-scale use in many Due to the widespread use of cyanide in mining operations,parts of the world for more than 110 years and is likely one of the recycling or destruction of cyanide is important both fromthe most thoroughly studied and well-understood industrial the environmental aspects of wastewater and effluent treatment,chemical processes, as was shown by McNultty [1]. In mining and from the economic aspects associated with the high reagentoperations, cyanidation is the predominant method by which consumption by the process itself, for example, the use of agold and silver are recovered from their ores. In practice, the procedure to recover cyanide may be a good option since thedissolution of gold and silver in aqueous cyanide solution is market price of cyanide is between US$ 1.00 and US$ 1.50 ontypically carried out with 0.03±0.3 % NaCN and it is usually average [4]. This latter situation was the case for a cyanidationthe most significant reagent cost. Lime is added as a pH process developed at Bacís mine (in Durango, Mexico) for themodifier to increase the pH and prevent as much as possible recovery of gold and silver from a pyrite concentrate [5]. Thethe hydrolysis of the cyanide ion to hydrogen cyanide. Also process comprises the following steps: leaching the complexaeration is necessary to keep the pulp or solution saturated sulfide concentrate by a one-stage pressure oxidation in awith oxygen (> 7 ppm). The overall reaction for the dissolution highly alkaline cyanide solution (1 % cyanide), filtration andof gold and silver in dilute, aerated, and alkaline cyanide washing to separate the solid, and precipitation of gold andsolutions may be expressed by the classic Elsner equation silver with zinc dust from the filtrate. The formation of metal complexes (copper, iron and zinc) with thiocyanate during4 Au + 8 CN± + O2 + 2 H2O = 4 [Au(CN)2±] + 4 OH± (1) pressure leaching results in species, which are particularly toxic. Since ultraviolet light decomposes thiocyanate to formwhich has the following mechanism: cyanide, it is then possible that sunlight may liberate cyanide, which would be toxic to aquatic life. Also, it should be noted that cyano-complexes of moderate stability could be readily decomposed by acidification or oxidation.± The US Environmental Protection Agency has proposed[*] J. R. Parga Torres (author to whom correspondence should be addressed, e-mail: drjrparga@hotmail.com), Institute of Technology of Saltillo, a limit of 0.2 mg/l cyanide in drinking water. The German Department of Metallurgy and Materials Science, V. Carranza 2400, and Swiss regulations have set limit of 0.01 mg/l for cyanide C.P. 25000, Saltillo Coah. MØxico; D. L. Cocke, Lamar University, Chemistry and Chemical Engineering Department, Beaumont Texas for surface water and 0.5 mg/L for sewers [6]. In Mexico, 77710, USA. the Secretary of Environmental and Natural RecoursesChem. Eng. Technol. 26 (2003) 4, Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 0930-7516/03/0404-0503 $ 17.50+.50/0 0930-7516/03/0404-0503 $ 17.50+.50/0 503
  • 2. Full Paper(SEMARNAT) has set the limit for cyanide as 0.2 mg/L. Inview of these considerations, cyanide recycling is a necessaryprocessing step. HCN Air2 Air-Sparged Hydrocyclone Reactor ASH Waste Air The ASH technology was originally developed at the solution withUniversity of Utah for the fast and efficient flotation of fine cyanideparticles from suspension [7,8]. Also, recent studies indicatethat the fluid flow conditions inside the ASH system can be SO2effectively exploited for air stripping of VOCs from con- pH=2 Ca(OH)2 NaOHtaminated water [9]. Tank Underflow Overflow Results reported in the literature [10,11] indicate that thegreatest mass transfer of compounds from water to air is Figure 1. Schematic drawing of the Ash system used for stripping of cyanide.achieved for those compounds, which have high values ofHenrys law constants and are relatively insoluble in water. It issafe to say that when compound properties favor air stripping,maximum mass transfer will occur in air strippers that± maintain the greatest possible interfacial area between bulk 3 Cyanide Recycling liquid that contains the HCN(g) and the stripping air;± increase the magnitude of the liquid mass-transfer coeffi- Free cyanide exists as the uncomplexed cyanide ion, CN±, cient by providing sufficient turbulence to minimize the and molecular hydrogen cyanide, HCN. These species are boundary layer thickness. related by the acid dissociation of HCN: The ASH reactor is one of the new, emerging strippingtechnologies, which can fulfill both requirements for max- HCN(aq) = CN± + H+ (5)imum mass transfer. The ASH unit consists of two concentricright-vertical tubes and a conventional cyclone header at the The concentration of free cyanide is the sum of the CN± andtop. The porous inner tube is constructed of any suitable HCN concentrations, and the equilibrium diagram shown inmaterial, such as plastic, ceramic or stainless steel, and allows Fig. 2 illustrates the distribution. This figure shows thefor the sparging of air or any other gas or steam. The outer proportions of free cyanide as CN±, and HCN as a functionnonporous tube simply serves to establish an air jacket and of pH at 25 C. At pH values below 7, cyanide is predominantlyprovides for the even distribution of the air through the porous present as the un-ionized HCN molecule, which is easilytube. Thus, the ASH can be used for air stripping where volatilized because of its high vapor pressure. The equilibriumvolatile species, such as HCN(g) which has a high vapor is displaced in favor of cyanide ion formation at pH valuespressure and volatilizes as a gas (Henrys law of constant of above 7.6.4 atm/mole) [12], can be displaced from solution by air,which is considered in this paper. The cyanide solution is fedtangentially at the top through the cyclone header to develop aswirl flow adjacent to the inside surface of the porous tube, 100leaving an empty air core centered on the axis of the ASH unit. 90The high-velocity swirl flow shears the sparged air to produce 80a high concentration of small bubbles and intimate interaction Percentage of HCNbetween these numerous fine bubbles and the cyanide 70solution. Gaseous products are then transported radially to 60the center of the cyclone. The major portions of the gas phase 50move towards the vortex finder of the cyclone header and arevented into an appropriate post-treatment device. The specific 40capacity of the ASH system is at least 1,500 liters per minute 30per .028 cubic meter of equipment volume, 100±600 times that 20of conventional air-stripping equipment. The ASH equipmentrequires an operating space significantly less than that of a 10packed tower or other air stripping devices, which result in a 0significant savings in capital cost. A schematic drawing of the 6 7 8 9 10 11 12ASH unit as used for air stripping of cyanide is presented in pHFig. 1. Figure 2. Equilibrium distribution diagram for cyanide as a function of pH.504 Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 0930-7516/03/0404-0504 $ 17.50+.50/0 Chem. Eng. Technol. 26 (2003) 4
  • 3. Full Paper Hydrogen cyanide (HCN), also known as hydrocyanic acid, It has been almost 80 years since the Mills-Crowe processis a colorless gas or liquid with a boiling point of 25.7 C, a for cyanide regeneration was developed by the Companyvapor pressure of 100 kPa at 26 C and Henrys law constant of Beneficiadora de Pachuca, Mexico (England Pat. No. 241669,6.4 atm/mole [12], this makes HCN very volatile. Thus, low 3.9.24) [14] and until today no significant changes to thepH, high temperature, low pressure, and intimate contact with process have been made. The simplest process for cyanideair, all tend to increase the rate of dissipation of cyanide from recycling involves acidifying the clarified solution, thensolution as hydrogen cyanide. volatilizing of the HCN(g) formed and reabsorbing it from In addition to free cyanide, other complexes, such as the the air stream with a caustic or milk of lime spray to producemetal cyanide complexes formed with gold, mercury, zinc, aqueous NaCN.cadmium, silver, copper, nickel, iron and cobalt, must be Recently, the company Minera Real del Monte [15]considered. These are classified into five general categories, as acidified the clarified cyanide solution with sulfuric andshown in Tab. 1 [13]. hydrochloric acid (to avoid gypsum formation). In the volatilization stage a series of four stripping towers packedTable 1. Classification of cyanide and cyanide complexes on the basis of stability with wooden grids are used. The towers are constructed of 316[13]. stainless steel and measure about 95 cm of diameter by 8 m inClassification Compound height. A total cyanide recovery of about 95 percent isFree cyanide CN±, HCN achieved with about 50 percent removal realized in each of the four stripping stages.Simple compoundsa) readily soluble Zn(CN)2, Cd(CN)2, CuCN, Ni(CN)2, AgCNb) neutral insoluble salts NaCN , KCN, Ca(CN)2, Hg(CN)2 4 Experimental ProcedureWeak complexes Zn(CN)42±, Cd(CN)32±, Cd(CN)42±Moderately strong Cu(CN)2±, Cu(CN)32±, Ni(CN)42±, Ag(CN)2± Experiments for cyanide recycling by air stripping at thecomplexes Institute of Technology of Saltillo pilot plant includedStrong complexes Fe(CN)64±, Co(CN)64±, Au(CN)2±, Fe(CN)63± acidification of the cyanide solution by bubbling SO2 gas to the 2±7 pH range for HCN(g) formation and stripping with air in a 2-inch diameter ASH unit. Chemical analysis for cyanide The term total cyanide is used for all cyanide, (free as well as in the effluent streams was accomplished with a refluxcoordinated cyanide), present in a sample. The concentration distillation method. Important aspects of the distillation stepof free cyanide in a solution depends on the pH value of the are the elimination of interferences and the decomposition ofsolution and its content of heavy metals capable of forming stable metal-cyanide complexes. The collected cyanide wascyanide complexes. Weakly complexed metal cyanides de- quantified by titration with silver nitrate standard solutioncompose at pH values lower than 4, with the evolution of and/or the ion-selective electrodehydrogen cyanide. Strong metal-cyanide complexes are During the experiments two streams had to be delivered tousually unaffected at room temperature because these the ASH: the cyanide solution and the air. Cyanide solutioncomplexes are very stable and resist oxidation, however, was provided by a sump pump mounted on a 300 liter retentionpartial decomposition can occur with increasing temperature tank. The cyanide solution flow rate was adjusted using aand acid content. regulated return flow to the tank. Using an air compressor, The cyanide recycling process utilizes the volatility of airflow was evenly distributed between the upper and lowerHCN(g) at low pH to strip free cyanide from solution or slurry sections of ASH and all parts were sealed with gaskets.with air and recover it in a caustic solution. The simplified Cyanide solution acidified to pH = 2±7 in the tank was fed atchemistry of the process is represented by the following different flow rates to the top of the ASH. The exit pipe wasreactions: located at the bottom of the closed regenerated cyanide tank to prevent release of HCN(g). The HCN-laden air wasCN± + H+ = HCN(aq) (6) collected in the absorber where reaction with sodium hydroxide 10 % v/v regenerated the NaCN aqueous solution. Air was in closed circuit at slightly reduced pressure forHCN(aq) = HCN(g) (7) volatilization of hydrogen cyanide from the acidified waste and absorption of hydrogen cyanide in a solution of sodium hydroxide. The use of air in closed circuit prevents introduc-HCN(g) + OH± = CN± + H20 (8) tion of atmospheric carbon dioxide which would neutralize lime in the absorption solution. Operators were provided with In the final step, the HCN(g) diffuses into the stripping personal HCN gas monitor/alarm units (DrägerSensor-solution of concentrated sodium hydroxide and reacts to form XSECHCN-68 09 150 is a trademark of the Drägerwerkaqueous NaCN. Aktiengesellschaft, registered in Germany).Chem. Eng. Technol. 26 (2003) 4, Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 0930-7516/03/0404-0505 $ 17.50+.50/0 505
  • 4. Full Paper5 Results and Discussion 80 70 Cyanide recovery (%) Experiments were performed using 500 ppm of CN± 60prepared from a 50 g/L stock solution of aqueous sodium 50cyanide from plant in a 300 liter fiberglass vessel at ambient 40temperature (24 C). Fig. 3 presents the experimental 30 Air = 150 l/min.conditions and the results obtained regarding cyanide deple- 20 Air = 130 l/min.tion with the acidification of the feed solution with SO2(g). 10Also, Fig. 3 shows the variation with time of the cyanide 0 0 5 10 15 20 25 30 35concentration for the two flow rates of SO2(g). As may be seen, Time of distribution air (sec.)the formation of volatile HCN(g) is a fast reaction and changes Figure 4. Effect of time of air distribution on cyanide recovery.in the concentration of free cyanide are a function of thegradual acidification of the solution. 85 Cyanide Regeneration, % 75 Air flow rate = 210 l/min 1000 65 100 pH=2Free Cyanide (PPM) 55 10 0.5 l/min of SO2 45 pH=5 2 l/min of SO2 35 1 pH=7 25 0.1 15 0.01 15 20 25 30 35 40 45 0.001 Solution Flow rate, l/min 0 5 10 15 20 Figure 5. Cyanide recovery and regeneration with ASH. Time Of Feed SO2(Minutes)Figure 3. Variation with time of the cyanide concentration for two flow rates ofSO2(g). Based on these test results, an initial economic comparison with current Mills-Crowe processes is summarized in Tab. 2. Typical results collected at pH 2.0 for cyanide regeneration All of these processes for cyanide recovery are currentare presented in Fig. 4. All experiments were made at the same versions of the original Mills-Crowe process [12,15,16]. Withsolution flow rate of 20 liters/minute and two air flow rates. two stages, the cyanide-ion concentration can be reduced toAlso, the data in Fig. 4 show that cyanide regeneration below 0.2 mg/L with recoveries > than 99 %.increases because air flow is rate-dependent. Finally, in Tab. 3, based on the pilot plant results, estimates Also, as seen in Fig. 5, the pH and solution flow rate of the cost of cyanide recovery have been prepared perinfluence cyanide recovery. Thus, at a low pH value when the kilogram of cyanide recovery, and the performance of theconcentration of CN± is very small, a high recovery is achieved ASH compares favorably to the packed-bed stripping towerdue to the easy volatilization of HCN(g). On the other hand, at technology.pH = 5, the recovery is significantly lower (52 % at 20 liters/ The advantage of the ASH technology over packed towersminute). These tests indicate that stripping of volatile HCN(g) is the residence time. In packed towers, the residence time forwith air and regeneration of cyanide with sodium hydroxide is stripping varies from 7 to 20 minutes, whereas the ASHpH-dependent. operates with a retention time of only 4 seconds [7].Table 2. Comparison of the experimental results with results from traditional Mills-Crowe operations[12,15,16]. Mine Reactor Air/Liq % Rec. CNIN CNOUT Streams Flin Flon Mine 4 towers 521 92 560 44 solution Real del Monte (Mex.) 4 towers 340 93 220 3 solution AVR (Canmet) 2 towers 330 95 330 2 solution C. R. P. (Tasmania) ± ± 95 200 5 solution Cyanisorb (NERCO DeLamar, US) 2 towers 300 95 600 30 slurry ASH (Bacis-MØxico) 1 ASH 10 80 250 50 solution ASH (Bacis- MØxico) 1 ASH 100 90 250 25 slurry506 Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 0930-7516/03/0404-0506 $ 17.50+.50/0 Chem. Eng. Technol. 26 (2003) 4
  • 5. Full PaperTable 3. Comparison of cost and performance Cost Ratio % RecoveryMine Remarks (US$/kg CN) Air/solution Single stage Build-up gypsum andReal del Monte 1.00 200±350 64 copper thiocyanate(packed towers) precipitatesBacis Mine 0.85 20±100 80 Free of precipitates(ASH)6 Conclusions References The application of the gas-sparged hydrocyclone reactor for [1] T. McNulty, Mining Mag. 2001, 5, 256. [2] M. I. Jeffrey, I. M. Ritchie, J. Electrochem. Soc. 2000, 147, 3257.cyanide recycling is a new and potentially inexpensive [3] D. M. White, T. A. Pilon, C. Woolard, Wat. Res. 2000, 34, 2105.approach for cyanide recovery. The ASH reactor has been [4] G. L. Miltzarek, C. H. Sampaio, J. L. Cortina, Minerals Eng. 2002, 15, 75.tested in bench and pilot-plant scale applications and has been [5] J. R. Parga, H. Mercado, Precious Metals Extraction by Direct Oxidative Pressure Cyanidation of Bacís Concentrates, Proc. Randolproven effective for the recycling of cyanide in solution and Gold Forum, Beaver Creek 1993, 209.slurries. [6] J. D. Desai, C. Ramakrishna, P. S. Patel, J. Awasthl, Chem. Eng. World 1998, 33, 115. Experiments performed show that the ASH reactor is very [7] J. D. Miller, Ye Yi, Min. Proc. and Extract. Metall. Rev. 1989, 3, 307.competitive with other technologies and that single-stage [8] D. Lelinski, R. Bokotko, J. Hupka, J. D. Miller, Min. Metall. Proc. 1996,cyanide recovery exceeding 80 % can be achieved. 5, 87. [9] J. D. Miller, D. Lelinski, J. R. Parga, Final Report-CX 823711, Advance Process Technology for the Wastepaper Recycling Plants and Pulp/ Paper Plants, Southwest Center for Environmental Research and Policy,Acknowledgements [10] 1996. D. F. LaBranche, M. R. Collins, Wat. Environ. Res. 1996, 68, 348. [11] W. J. Parker, H. D. Monteith, Environ. Progress 1996, 15, 73. The authors wish to express their gratitude to CONACYT, [12] Smith, T. Mudder, The Chemistry and Treatment of Cyanidation Wastes, Mining Journal Books Ltd., London 1991, 277.COSNET (701.95-P) and Grupo Minero Bacis for financial [13] W. Hoecker, D. Muir, Res. Dev. in Extractive Metallurgy 1996, 5, 29.support and permission to publish the results. Many thanks go [14] C. W. Lawr, Cyanide Regeneration as Practiced by the Compaæíato Lamar University for support and assistance. Beneficiadora de Pachuca, Mexico, Technical Publication AIME No. 208 (06) 1929, 1±37. Received: July 31, 2002 [CET 1668] [15] Report Compaæía Minera de Real del Monte, Pachuca, Mexico, Process for the Recovery of Cyanide, 1997, 1±13. [16] M. Botz, J. Stevenson, Eng. & Mining J. 1995, 6, 44.AbbreviationsASH air-sparged hydrocycloneaq aqueous phaseg gas phase _______________________Chem. Eng. Technol. 26 (2003) 4, Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 0930-7516/03/0404-0507 $ 17.50+.50/0 507