AARL process as applied at SDGM. • Identify the key elution circuit components. • Demonstrate an understanding of carbon reactivation theory. • Demonstrate an. Request PDF on ResearchGate | Modelling of the AARL elution process | The influence of temperature on the equilibrium adsorption of gold and hydroxide ion . successful methods is the carbon-in-pulp (CIP) process. In CIP processing elution rig was commissioned to simulate industrial Zadra and AARL carbon elution.
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Many methods are used commercially for recovering gold and silver from loaded activated carbon. The major processes include:.
Each process is briefly described. Variations to and combinations of wlution basic methods are also highlighted. The advantages and disadvantages of each procedure are discussed. Activated carbon has long been known to exhibit a strong affinity for the extraction of gold from cyanide solutions.
It was not, however, until methods for chemically desorbing gold from loaded carbon were developed, that the process came into widespread use for gold recovery from ore. These procedures allowed the carbon to be recycled for further gold recovery. Today, pdocess options are available to the recovery plant designer procfss operator for the stripping eltion gold from carbon.
Each method has advantages and disadvantages, which should be evaluated when deciding which process to use. Atmospheric pressure Zadra stripping was the first commercially successful process developed for stripping gold from carbon.
The process was developed by J. Zadra, and others, at the U. This publication is still in print and is actually the foundation for the other stripping processes. The process is still widely used today because of its simplicity. Gold is recovered from the pregnant strip solution by electrowinning onto steel wool.
The gold depleted solution is then reheated and recycled to the carbon bed for reuse see Figure 1. The process generally takes about 48 to 72 hours.
The Zadra process is characterized by simplicity of system design and operation. Mild steel orocess is normally used. Manual control is the standard. Fluctuations in flow and temperatures aadl reduce stripping efficiency but the only adverse effect is an extension of the required stripping cycle time. After the desorption aqrl has been filled with loaded carbon and solution flow is started, the only operator attention required is periodic system checks typical of any process plant operation.
The main disadvantage of the original Zadra process is its low rate of desorption. It is much slower than the alternatives. This necessitates larger carbon inventories and larger equipment than other faster processes. Stripping temperature is the most elutiion operating parameter so solutions are kept as close to boiling temperature as is practical.
Since many mines are at high elevations, with resultant epution boiling points, the reduction in stripping rate can be significant when compared with operations at near sea level altitudes.
Buildup of miscellaneous ions in solution after continued recycling also reduces stripping efficiency. To alleviate this problem, most operations routinely bleed a fraction procsss their strip liquor inventory and replenish with fresh solution.
The efficiency of the electrowinning cells is also significant to stripping efficiency. High levels of gold in recycled eluant result in a reduction in stripping rate as illustrated typically by Figure 2 3.
Continued research at the USBM revealed that the Zadra process stripping rate could be increased greatly by stripping at higher temperatures 4. A comparison of the increase in stripping rate with temperature is shown in Figure 3. To operate at higher temperatures, the process must operate at pressures higher than the vapor pressure of the solution. High pressure operation is accomplished by means of a high pressure solution pump and a stripping column pressure control regulator.
The time required for pressure stripping is generally from 10 to 14 hours. Barren strip solution is typically pumped through a heat recovery heat exchanger and a solution heater. The solution then flows up through the bed of carbon and overflows near the top of elutino stripping vessel.
The solution is cooled by exchanging heat with barren solution and flows through a back pressure control valve, to the pregnant solution holding tank. Pregnant solution is pumped from the pregnant solution tank through electrowinning cells where gold is recovered procesx electrolysis.
Barren solution is then returned to the barren solution tank for recycle see Figure 4. High temperature limits are generally constrained by pressure and temperature limitations of system components, such as vessel design pressures and gasket temperature limits. Porcess research indicated that increases in stripping efficiency could be achieved up to deg-F.
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Above deg-F cyanide was decomposed and metallic gold precipitated in the carbon. Plant practice generally indicates that about deg-F is the upper limit for maximum stripping efficiency.
Pressure stripping columns are normally sized with a height to diameter ratio of about 4 to 1. Internal solution distributors and collectors are used to provide even flow of solution throughout the carbon bed.
The majority of pressure strip vessels are constructed from stainless steel, but many carbon steel vessels are providing satisfactory service. Solution flow rate has little effect on stripping efficiency in the range of 1 to 4 bed volumes per hour.
Low solution flow rates produce slightly higher efficiencies in most cases, but the increase is not significant. Stripping efficiency decreases as flow rates are increased above 3 to 4 bed volumes per hour. The design solution flow rate is generally based on a compromise between reduced elution time and increased equipment costs at higher flow rates.
Most columns are operated with upflow of solution, but some plants have selected to elute by downflow. The advantage to downflow is reduced potential for binding of flow distribution screens by tramp material in the carbon. Upflow operation means that the carbon bed is always flooded, and insures that the carbon is continually contacted by strip solution.
The extent of instrumentation is generally determined by operator preference. Automatic solution temperature control and column back pressure control are the minimum automation required. Solution bleeding is required to prevent the buildup of contaminants, which reduce stripping efficiency. Control of the amount of solution purged from the system is done either on a routine scheduled basis or by monitoring stripping efficiency and bleeding as efficiency drops.
Since that time, its application has become standard practice in South Africa and Australia. The process involves a series of procedures generally starting with an acid wash followed by a water wash to remove residual acid.
High quality fresh water at about deg-F is then pumped through the pressurized stripping vessel to produce the pregnant eluant. Gold is recovered from the pregnant eluant by electrowinning and the barren eluant is discarded See Figure 3.
It is interesting to note that the Zadra stripping procedures researched by the USBM, originally envisioned presoaking carbon with a caustic cyanide solution followed by elution with deionized water.
This idea was discarded in practice, in favor of the simpler one step caustic cyanide elution.
AARL Carbon Stripping System Operation: Elution & Electrowinning
processs The main advantage of the AARL process is the ability to strip a batch of carbon to low gold residuals in an 8 hour shift. This offers the potential of either designing for multiple stripping cycles each day and reducing equipment sizes in new plant design, or increasing daily capacity in existing mills by stripping on additional shifts each day.
Disadvantages of the AARL process include the requirement ;rocess high quality water, high water consumption, the potential for mixing acid with cyanide, and the necessity for automated controls. With the AARL process, elution is normally proceeded by acid washing the carbon. Acid washing is used with all of the other stripping systems, but it is mentioned specifically with the AARL procedure, because AARL systems generally use the stripping vessel to acid wash and acid washing is, therefore, controlled as part of the stripping sequence.
Acid washing has been shown to typically increase the efficiency of AARL stripping.
Acid washing is currently being done both in a separate vessel from the stripping column and in the elution vessel. Factors favoring acid washing in the elution vessel include: Factors favoring installation of a separate acid wash vessel include: Following acid washing, the carbon is rinsed with fresh water to prevent acid and chlorides from entering the strip circuit. The presoak step is started by preheating the carbon with hot water. Changes in the duration of soaking time, for most carbons, has little effect on stripping efficiency.
The quality of water used has a substantial effect on stripping efficiency with the AARL procedure. The implementation of a hot acid wash step has been shown to reduce water quality requirements to a certain degree. The stripping efficiency is virtually independent of eluant water flow rate in the range of 1 to 5 bed volumes per hour.
Carbon Stripping – Denver Mineral Engineers
Selection of design and operating flows is made on the basis of equipment costs and time constraints. Eluant water temperature has a very significant effect on stripping efficiency. Operation at deg-F requires operating pressures of 10 to 15 PSIG to prevent flashing steam in the system. Operating temperature limits of deg-F are widely specified because of temperature limitations of the butyl rubber lining material utilized to line the strip vessel.
Higher temperatures also accelerate the decomposition of cyanide. The last bed volume of eluant water is generally introduced at ambient temperature to cool the carbon for transfer out of the column.
Due to the timed cyclical nature of the procedure, a programmable logic controller PLCautomatic pump starting and automatic valves are generally used to time and sequence the system. Electrowinning of gold from the pregnant solution is done on a batch basis. The solution pH is increased to 12 by the addition of sodium hydroxide and electrowinning is started. Solution is circulated through electrowinning and back to the pregnant solution tank until acceptable barren levels are achieved.
The solution is then discarded. Further research at the USBM showed that the atmospheric pressure Zadra stripping cycle can be made to operate much faster by the addition of alcohol to the strip solution 6.
Several different alcohols were investigated. Methanol, ethanol, and Isopropanol were all found to increase the gold desorption rate. Ethanol and methanol were found to perform almost equally, but were substantially better that Isopropanol. In plant operation alcohol stripping normally requires about 12 to 16 hours to strip carbon to less than 3 oz.
Au per ton of carbon. This is achieved at flow rates in the range of 2 bed volumes per hour operating in series flow with electrowinning cells.