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                      Carbon Stripping

                       

                      CARBON STRIPPING THE PRACTICAL ALTERNATIVES

                      Presented at the? International Gold Expo
                      September 7, 1989
                      Reno. Nevada
                      John L. Fast, P.E.
                      Consulting Process Engineer
                      Denver Mineral Engineers, Inc.

                      Many methods are used commercially for recovering gold and silver from loaded activated
                      carbon. The major processes include:

                      (1) Atmospheric Zadra Stripping

                      (2) Pressurized Zadra Stripping

                      (3) Anglo American Research Laboratory (AARL) Method

                      (4) Alcohol Stripping

                      (5) Micron Elution Procedure

                      Each process is briefly described. Variations to and combinations of the basic methods
                      are also highlighted. The advantages and disadvantages of each procedure are discussed.

                      I. INTRODUCTION

                      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, many options are available to the recovery plant designer and operator for the
                      stripping of gold from carbon. Each method has advantages and disadvantages, which should
                      be evaluated when deciding which process to use.

                      II. ATMOSPHERIC ZADRA STRIPPING

                      Atmospheric pressure Zadra stripping was the first commercially successful process
                      developed for stripping gold from carbon. The process was developed by J. B. Zadra, and
                      others, at the U.S. Bureau of Mines (USBM) in the early 1950’s. The results of their
                      research were first applied at Golden Cycle Gold Corporation’s Carlton Mill at Victor,
                      Colorado in 1951. The results of Zadra’s work were published by the USBM as RI #4843 (1).
                      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.

                      The Zadra process consists of circulating a 1% sodium hydroxide and 0.1% sodium cyanide
                      water based solution upflow through a stationary bed of loaded carbon at a flow rate of
                      about 2 bed volumes per hour at about 200 deg-F. Gold that was previously adsorbed on the
                      carbon as a sodium or calcium/gold cyanide ion pair (2) is desorbed from the carbon by a
                      reversal of the adsorption kinetics. 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. Typically the gold content of carbon
                      is reduced from 150 oz Au/ton of carbon to less than 3 oz. Au/ton of carbon.

                      The Zadra process is characterized by simplicity of system design and operation. Mild
                      steel equipment is normally used. Manual control is the standard. Fluctuations in flow and temperatures can reduce stripping efficiency but the only adverse effect is an extension of the required stripping cycle time. After the desorption vessel 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 significant operating parameter so solutions are kept
                      as close to boiling temperature as is practical. Since many mines are at high elevations,
                      with resultant low 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 of 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).

                      III. PRESSURE ZADRA STRIPPING

                      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.

                      In practice, a solution containing about 1% sodium hydroxide and 0.1% sodium cyanide at
                      about 280 deg-F and 65 PSIG, is circulated through a pressure vessel filled with loaded
                      carbon at a flow rate of 2.0 bed volumes per hour. 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 the 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 by 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. USBM research indicated that increases in stripping efficiency could be achieved
                      up to 356 deg-F. Above 356 deg-F cyanide was decomposed and metallic gold precipitated in
                      the carbon. Plant practice generally indicates that about 300 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. The amount of solution bleed required varies from about 1/3 of the
                      eluant volume per cycle, to as low as the residual eluant on the carbon during carbon
                      transfer. 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.

                      OPERATING SCHEDULE:

                      The following is a typical operating schedule for a Pressure Zadra stripping cycle:

                       

                      SOLUTION TIME
                      Load Column Transfer Water 90 minutes
                      Elution 0.1% NaCN, 1% NaOH 480 minutes
                      Carbon Cooling Fresh Water 60 minutes
                      Unload Column Transfer Water 30 minutes
                      TOTAL 11 hours

                       

                      IV. AARL STRIPPING

                      The Anglo-American Research Laboratories (AARL) stripping procedure (5) was first used
                      on a large scale in 1980 at the President Brand Gold Mine in South Africa. 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. The carbon is then soaked for about 30
                      minutes in a solution containing about 3% sodium cyanide and 1% sodium hydroxide. High
                      quality fresh water at about 230 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.

                      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 for high quality water, high
                      water consumption, the potential for mixing acid with cyanide, and the necessity for
                      automated controls.

                      ACID WASHING

                      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.
                      Hydrochloric acid is always used in AARL systems with concentrations generally around 3%
                      by volume.

                      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: (1) eliminating a carbon transfer which reduces gold losses from fine carbon
                      breakage of loaded carbon, (2) conservation of heat if hot acid washing is employed and
                      (3) reduction in stripping cycle time by eliminating a carbon transfer. Factors favoring
                      installation of a separate acid wash vessel include: (1) less potential for mixing acid
                      and. cyanide in the event of operator error or equipment malfunction, and (2) less
                      rigorous requirements for materials of construction in the stripping circuit since acid
                      proof equipment is not required.

                      Following acid washing, the carbon is rinsed with fresh water to prevent acid and
                      chlorides from entering the strip circuit.

                      PRESOAK

                      The presoak step is started by preheating the carbon with hot water. This is followed
                      by soaking the carbon bed with hot (90 deg-C) 3 WT% NACN/1 WT% NAOH solution for about 30
                      minutes. Reduced elution efficiency is experienced if soak solutions concentration are
                      less than 3% NACN but elution rates remain relatively constant with soak solution
                      concentrations above 3% NACN. Changes in the duration of soaking time, for most carbons,
                      has little effect on stripping efficiency.

                      ELUTION

                      Elution is generally performed using about 6 bed volumes of good quality water at a
                      rate of about 2 bed volumes/hour.

                      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. 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 236 deg-F requires operating pressures of 10 to 15 PSIG to prevent flashing
                      steam in the system. Operating temperature limits of 236 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.

                      PROCESS CONTROL

                      Due to the timed cyclical nature of the procedure, a programmable logic controller
                      (PLC), automatic pump starting and automatic valves are generally used to time and
                      sequence the system. Carbon is loaded and transferred manually, but the sequencing of’
                      valves and pumps during the strip cycle is controlled by the PLC.

                      ELECTROWINNING

                      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.

                      OPERATING SCHEDULE:

                      The following is a typical operating schedule for an AARL stripping cycle:

                       

                      SOLUTION TIME
                      Load column Carbon 90 minutes
                      Acid Wash 3% HCl 20 minutes
                      Water Rinse Potable Water 90 minutes
                      Pre Heat ?Potable Water 30 minutes
                      Pre Soak 3%NaCN,1%NaOH 30 minutes
                      Elution Potable Water 180 minutes
                      Cooling Potable Water 30 minutes
                      Carbon Transfer Transfer Water 30 minutes
                      TOTAL 7 hours 50 minutes

                       

                      V. ALCOHOL STRIPPING

                      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).

                      Figure 6 shows the dramatic laboratory results obtained by adding 20% ethyl alcohol to
                      a Zadra solution. 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.

                      The main drawback to the alcohol stripping process is the potential for fires. Fires
                      have been reported at several alcohol stripping operations. The electrowinning section is
                      especially vulnerable to fires because of the potential for sparks.

                      Ethanol is generally used rather than methanol. This is due to ethanol’s greatly lower
                      health risks from exposure to vapors. There are, however, isolated examples of operations
                      using methanol.

                      Ethylene or propylene glycol are frequently used, rather than alcohol, to increase the
                      speed of atmospheric pressure Zadra stripping (7). Typical strip times with glycol are 24
                      to 36 hours.

                      Glycols are generally used, rather than alcohols, because they are virtually
                      uninflammable. The disadvantages of glycols are their inferior strip rate increase and
                      higher costs.

                      A typical glycol stripping solution contains 20 to 25 wt% ethylene or propylene glycol,
                      and 2 wt% sodium hydroxide. Sodium cyanide is sometimes added to the solution but it is
                      frequently unnecessary. The solution is heated to about 190 deg-F and pumped through the
                      carbon stripping vessel at a flow rate of about 2 Bed Volumes per hour. Gold and silver
                      values are recovered from the pregnant solution by electrowinning and the barren solution
                      is reheated and recycled through the stripping vessel. Glycol consumption is typically in
                      the range of 20 to 40 gallons, per ton of carbon stripped.

                      VI. MICRON STRIPPING

                      The most recently developed stripping procedure being used commercially was developed
                      at Micron Research, in Australia (8). The Micron method involves pretreatment of loaded
                      carbon, with a caustic cyanide solution followed by elution with an alcohol mixture.

                      The Micron elution procedure takes advantage of the enhanced stripping rate achieved
                      with alcohol, but confines the alcohol to the closed stripping unit. Fire dangers are
                      reduced quite substantially, as the pregnant eluant that is subsequently processed for
                      gold recovery does not contain alcohol.

                      The elution unit is configured like a packed bed distillation tower with a heater on
                      the base of the column, an overhead condenser, a reflux pump and the loaded carbon
                      functioning as the tower packing.

                      Loaded carbon is first presoaked with sodium cyanide/sodium hydroxide solution. The
                      presoak solution is drained from the carbon bed and an alcohol solution is added to the
                      vessel.

                      The unit is then switched to the batch distillation mode. Within a few hours, the
                      alcohol is concentrated in the overhead condenser tank. The tower bottoms solution is then
                      free of alcohol and loaded with gold solution which has been stripped by the refluxing
                      action in the column.

                      The Micron process consists of the following operations (See Figure 7):

                      (1) Presoak

                      The carbon is first soaked in a solution of 1 to 2 % sodium hydroxide and 5 to 10 %
                      sodium cyanide at ambient temperature. Carbons with particularly high concentrations of
                      gold, silver, or copper may require solutions containing up to 20% sodium cyanide. The
                      solution is then drained from the carbon until free of excess moisture.

                      (2) Desorption

                      About 0.5 Bed Volume of alcohol is added after the carbon bed has drained. Methyl
                      alcohol is used in the majority of applications, but ethanol is occasionally applied.
                      Acetonitrile may be substituted for the alcohol, but its higher cost generally discourages
                      its use.

                      Heat is then applied to the base of the desorption vessel. Organic vapors rise through
                      the carbon bed and are condensed in the overhead condenser. The condensate is pumped back
                      to the top of the carbon bed and is sprayed on the carbon. The downflowingcondensate
                      washes the gold values from the carbon particles into the boiler section below.

                      (3) Alcohol Recovery

                      When desorption is completed, as indicated by gold solution concentration reaching a
                      constant level in the column boiler, alcohol recovery commences. The condensate recycle
                      spray is stopped and the alcohol is allowed to boil out of the pregnant solution. The
                      boiling is terminated when the temperature in the boiler rises to the boiling point of the
                      water solution. The pregnant liquor is then drained from the desorption vessel. The carbon
                      is then steam stripped to recover residual alcohol.

                      The Micron process produces a very concentrated eluant free of alcohol, with gold and
                      silver values two or three times higher than those in the loaded carbon. This is in direct
                      contrast with the Zadra and AARL procedures, which produce eluant concentrations one or
                      two orders of magnitude lower. The high solution grades make recovery methods such as
                      chemical precipitation and aluminum foil electro-deposition very attractive.

                      The micron eluted carbon also appears to have a somewhat higher level of activity than
                      carbon eluted by other methods. This may reduce the need to reactivate carbon as
                      frequently at some operations.

                      The entire stripping cycle takes about 8 hours. Over 20 licenses for this process have
                      been issued, but none are in the United States.

                      VII. VARIATIONS

                      There are numerous variations to, and combinations of, the basic processes as
                      illustrated by the following:

                      (1) Glycol or alcohol are sometimes added to pressure strip or AARL strip operations to
                      increase stripping rates.

                      (2) A caustic/cyanide presoak may be used in a Zadra system.

                      (3) A hot water wash is sometimes used at the end of a pressurized Zadra strip to gain
                      some of the advantage of the? water elution used in AARL stripping.

                      (4) In situations where large amounts of copper load onto carbon with the gold and
                      silver, a two stage strip may be
                      beneficial. Copper may often be selectively eluted with a cold caustic/cyanide solution.
                      This is then followed by one of ? ? ? the standard stripping methods.

                      (5) Carbon is normally stripped batch wise, but moving bed continuous elution systems
                      have occasionally been used with ? ? ? ? both Zadra and AARL
                      procedures.

                      (6) Electrowinning may be done under pressure to avoid repressuring solution on each
                      pass through the carbon.

                      (7) Zinc precipitation may be substituted for electrowinning.

                      VIII. CONCLUSIONS & RECOMMENDATIONS

                      Each new project should be evaluated individually to determine the best procedure for
                      the particular ore and site specific circumstances.

                      In general atmospheric pressure Zadra stripping is favored for smaller projects where
                      the increased size of equipment can be justified by a simplification of the system. It may
                      also be preferred for areas where extreme ease of maintenance and operability are a
                      priority due to a lack of skilled manpower.

                      The pressurized Zadra system has recently been the preferred process in the United
                      States for most medium to large sized projects. This is due to its significant cost
                      advantage over atmospheric pressure Zadra systems.

                      The AARL process is the preferred process in Australia and South Africa, except where
                      water balance or water quality problems exist. Several recent United States projects have
                      also elected to use AARL systems. The AARL process should definitely be considered for
                      large projects with sophisticated operators.

                      Alcohol stripping has generally fallen out of favor due to the flammability concerns.
                      Glycol stripping frequently is used to increase capacity in existing Zadra operations, but
                      economics usually favor conversion to pressure stripping if continuing operation is
                      planned.

                       

                       


                      Fast & Associates, LLC

                      Denver Mineral Engineers, Inc.
                      10641 Flatiron Rd.
                      Littleton, CO 80124 USA
                      sales@denvermineral.com

                       Posted by at 2:27 am
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