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Application Note Cyanide Leach Process - pH Mining: Gold Processing While there are many different processes to remove gold High grade ore is typically reserved for agitated leaching
   Application NoteCyanide Leach Process - pH Mining: Gold Processing While there are many different processes to remove gold from its encapsulating ore, cyanide leaching (cyanidation) is perhaps the most common and well known. A cyanide solution (NaCN or KCN) is elevated to a high pH level (>10.5pH) so that free cyanide (CN - ) will dissolve the gold in the ore. The chemical reaction is as follows:4Au + 8NaCN + O 2  + 2H 2 O ►  4Na[Au(CN) 2 ] + 4NaOHIn this paper we will look at the various mining processes and the role that pH control plays in the gold extraction process. Gold Processing Gold ore comes in a wide variety of formations. The type of ore will in fl uence the grinding and milling steps along with the extraction strategy. It is a common belief that gold is only found in veins in the rock. This type of formation accounts for only 20-25% of the world’s production. More common is microscopic gold particles mixed with silver and metal su fi des of lead, copper, and zinc. The latter formations are referred to as “refractory” ores and must have pretreatment with roasting or oxidation done prior to cyanide leaching. These processes also have pH measurements but will be reviewed in a separate paper. Low grade ore is often processed using a heap leach process. Heap leaching involves crushing the ore to <3/4” (<19mm) diameter chunks and piling on a puncture-proof liner. The liquid cyanide solution is poured over the heap and allowed to percolate down through the ore. The gold containing solution (pregnant leach) is collected from the liner into a leach pond and sent off to carbon adsorption. The leaching process will take from 60-90 days. High grade ore is typically reserved for agitated leaching. fi nely ground ore (< 200 mesh) is combined with the cyanide solution in a series of tanks. This slurry is referred to as “pulp”. Agitators and air / oxygen are added at each tank. The oxygen reacts with the sodium cyanide to create the desired aurocyanide gold complex. Typically lime (CaO) or sodium hydroxide (NaOH) is added during this stage to keep >10.5pH so that toxic hydrogen cyanide (HCN) is not produced. Cyanide leaching is typically followed by an extraction step to remove the solubilized gold from solution. Carbon adsorption is often used to isolate the aurocyanide complex. This is typically referred to as CIP (Carbon In Pulp). Sometimes the leaching and carbon adsorption steps are combined. This is referred to as CIL (Carbon In Leach). The pulp from the leach circuit is cascaded through 4-6 tanks via gravity fl ow. Activated carbon is added at the opposite end and is pumped upstream through the tanks. Tank-to-tank screening separates the increasingly loaded carbon from the pulp in solution. The fi nal loaded carbon is sent to a stripping process. “Elution” or desorption of the gold complex from the carbon is the next step. This is accomplished by moving the loaded carbon through a stripping vessel at elevated pH and high temperature (95ºC or higher if pressurized). The pregnant solution is pumped to electrowinning cells while the regenerated carbon is reused in the carbon adsorption process. Electrowinning involves passing a current through the solution to break the chemical bond between the gold and cyanide. pH at this point in the process is critical to Crushing &GrindingFlotation(Ores with Sul󿬁des)Roasting or AutoclavingBio - OxidationPressure OxidationCyanide Leaching(Heap Leach, Agitated Leach)Extraction(Carbon in Pulp)Carbon Stripping(Eluation)ElectrowinningSmeltingWaste Treatment(Tailings and Cyanide Detox)End product Gold Processing - Block Diagram Figure 1   Application NoteCyanide Leach Process - pH 2 prevent corrosion of the anodes and limit the creation of hydrogen gas. Caustic (NaOH) is used to elevate the level to >12.5pH. The fi nal barren cyanide eluate solution is pumped back to the leach circuit. Gold collected on the electrowinning cathodes is melted off in the smelter for fi nal processing. pH Measurement Challenges Gold processing operations are highly dependent on online pH measurements for process control as well as safety requirements. All processes involving cyanide must be kept >10.5pH (typically 11-12pH) to avoid the conversion to dangerous HCN gas. Lime (CaO) and caustic (NaOH) are most commonly used to keep the pH elevated. Lime is added to water to create “milk of lime”. In these applications the excess calcium compounds from the milk of lime are well known to coat pH sensors. Beyond chemical addition, fine ground ore and carbon particulates can also coat as well as plug up the porous reference  junction used in most pH sensors. Gold Processing - Details Figure 2 The most common installations found in mining processes will be submersible sensors mounted vertically into tanks and vats. Proper agitation of the fl uid helps to reduce some of the coating issues. If the sensor is installed in-line then fl owrate past the electrode at 3 - 6 ft/second (1 - 2M/second) helps to prevent build-up. In some applications a  jet cleaner can be used to help blast off coating on the tip of the sensor (see fi gure 6). For most submersible and sample line installations we recommend the Barben Performance Series 546 threaded sensor. The 546 uses Barben’s patented Axial Ion Path ®  reference design (Figure 3). The reference half cell uses multiple fi ltering chambers to greatly slow the ingress of process chemicals from poisoning the sensor internals. The large annular Te fl on junction provides a much greater surface area then conventional pH sensors. The increased area is effective in reducing the plugging due to particulate matter and simpli fi es cleaning of sensor. Crusher  HeapLeaching NaCNPregnantLeach PondBallMillVarious Pre-treatment Steps:- Flotation- Roasting- OxidationCyanide,Lime &OxygenAddition AgitatedLeaching Cyanide,Lime &OxygenAddition Carbon InPulp Extraction Spent Ore to Tailingsand Cyanide Detox(Dashed Line)     C   a   r    b   o   n    S   t   r    i   p    C   o    l   u   m   n Regenerated ActivatedCarbon Return (Blue Line)ElectrowinningPregnant EluateGold to SmeltingCausticor Lime Addition pH / ORP Measurement Challenges Problem Solution- Process Coating - Increase fl ow or agitation by sensor (>3 to 6 ft/sec) Sensor - Coating Resistant glass electrode - Axial Ion Path Reference (large surface area) - Jet Cleaner Accessory- Process Poisoning - Axial Ion Path Reference ( fi ltering design) Sensor   Application NoteCyanide Leach Process - pH 3 H H  + H  + H  + H  + H  + H  +  H  + Multiple Axial Ion Paths Plug free communicationSeal individual fi ltering chambers  Annul ar   Filtering Junction Maintains measurement signal   Wood slows process ingress   Highly resistant to strong chemicals Te fl on Junction Interface Initial protection against processLarge surface area ION PATH ® Figure 3  A typical installation of the 546 sensor would use the 3/4” Male NPT threads for installation on a submersible dip tube. For slurry installlations such as shown below, “CR” coat resistant hemispherical glass electrode should be specified with a dual notch tip configuration. The notched tip provides additional protection against accidental breakage of the electrode. In high velocity flow streams or abrasive applications >10% solids the “CF” coat resistant flat glass electrode can be specified. A Kynar (PVDF) body and Viton ® Extreme seals provide chemical compatibility with most mining chemicals.In severe coating applications the 546 pH sensor can be used in conjunction with our B37 /C37 Jet Cleaner. The  jet cleaner uses either water or air pressure to blast off soft particulate coatings that may occur in some processes. The jet cleaner can be automated with our Distribution Valve Assembly to provide cleaning via relay logic from a remote controller, PLC, or DCS system (figure 6). pH / ORP Electrode Examples The top electrode shows a “CR” hemispherical glass pH electrode. The dome shape provides a large surface area and high resistance to cracking when process pressures and tem-peratures fl uctuate. The notches on each side provide added insurance to protect the electrode against impact.The bottom electrode is Barben’s “CF” fl at glass pH electrode. The fl at and fl ush design is best suited for abrasive, high particulate applications (>10%) or when fl ow velocities surpass 6 ft/second (2 M/second). Figure 5  Automated Cleaning Accessories Figure 6 The slurry seen in this picture shows the type of coating that can occur in cyanide leach applications. The pH sensor is mounted on the end of the submersible dip tube in the center of the stilling well leading into an open vat. The recirculated leach solution is entering from the piping on the right. Figure 4  A submersible installation (shown below) can have automated clean-ing by using the Jet Spray Cleaner (left). The 546 pH / ORP sensor with 1.5” insertion depth is threaded into the Jet Spray Cleaner which is installed into a user supplied 2” diameter dip tube.The Distribution Valve Assembly (right) can be used to control the fl ow of air or water to clean the sensor.  Cert. No. 43271 ISO 9001:2008 © 201 5 , by AMETEK, Inc. All rights reserved ã Gold_Cyanide_AN_RevA ã A ugust , 201 5 USA ã BELGIUM ã CHINA ã SINGAPORE Toll Free  +1(800)993-9309   ã Phone +1(775)883-2500 ã Fax  +1(775)297-4740  S ales. Barben @  A ã  Application NoteCyanide Leach Process - pH Barben Analytical reserves the right to make technical changes or modify the contents of this document without prior notice. We reserve all rights in this document and in the subject matter and illustrations contained within. Viton ®  are registered trademarks of DuPont Dow Elastomers L.L.C.Kynar  ®  is a registered trademark of Elf Atochem North America Inc.Te fl on ®  is a registered trademark of E.I. DuPont de Nemours Company Inc. Contact Us Barben Analytical is a leading supplier of analytical mea-surement technology targeting the industrialmarketplace. It is a wholly owned subsidiary of Ametek. Ametek has nearly 14,000 colleagues at over 120 manu-facturing locations around the world. Supporting thoseoperations are more than 80 sales and service locationsacross the United States and in more than 30 other coun-tries around the world.Barben Analytical5200 Convair DriveCarson City, NV 89706 USAToll Free: Phone: Fax: Email: Web: +1 (800) 993-9309+1 (775) 883-2500+1 (775) 297-4740 S ales. Barbem @  A Summary Barben Performance Series pH sensors offer many advantages for gold processing facilities including the following:ãLess frequent cleaning and calibration intervalsãHigh pH / ORP measurement accuracyãIncreased pH / ORP sensor lifespanãSimpli fi ed sensor speci fi cationMaintenance expenses are decreased as spares inventory is reduced and fewer calibration hours are required to keep measurements accurate.
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